The previous chapter may be found here.
Artist's impression of Chicxulub imact.. NASA
The age of the dinosaurs ended in global catastrophe. Traces of
this event are found in New Mexico, though not in the Jemez, where
the relevant beds were eroded away during the Laramide Orogeny.
In this chapter, we will look at the Jemez area during the early Tertiary Period.
The Mesozoic Era ends, and the Cenozoic Era begins, with a mass
extinction, albeit one less catastrophic than the Great Dying at
the end of the Permian. However, what it lacked in destructiveness
was made up in drama and knowledge: We know a lot more about this
much more recent extinction. Unfortunately, no record of this is
preserved in the immediate Jemez area.
You have almost certainly heard that the age of the dinosaurs
ended with a catastrophic asteroid impact near what is now the
northern shore of the Yucatan Peninsula. This Chicxulub event has
been popularized to the point where there is even a Pixar
children's movie depicting it, Dinosaur. (Which I quite
like, at least for the depiction of the impact itself -- I do
simulations of such things for a living, and Pixar's version is
really not bad.) The global consequences of this impact included
the destruction of most of the dinosaurs and many other families
of life. Only those dinosaur families whose descendants became
modern birds survived the impact. Enough reptiles and mammals
survived that they, too, became part of the fauna of the Tertiary.
In fact, conditions became suitable for mammals to become the
dominant form of land animal, leading ultimately to the ascent of
The Cretaceous-Tertiary boundary is marked in many places in the world by a thin layer of clay that is enriched with traces of the noble metal, iridium. Iridium is extremely scarce in the universe, but even more so in the Earth's crust, since most of the small amount with which the Earth formed settled into the Earth's core along with iron and other heavy elements. Iridium is much more abundant (if still very scarce) in meteorites. This was the first clue that an asteroid impact was the main trigger for the mass extinction. Eventually the deeply buried traces of the impact crater were discovered, mostly by petroleum geologists, and there is now a broad consensus that the asteroid impact was the major cause of the extinction. (There is still a vigorous debate about what other factors may have contributed.)
Although the Cretaceous-Tertiary boundary is not found in the
Jemez, it is well exposed near Raton,
at the K-T Boundary Trail.
K-T Boundary Trail. 36 54.198N 104 27.077W
The hill is underlain by the Raton Formation, which is mostly shale with interbedded coal.
Here's a close up of the boundary layer.
K-T Boundary Trail. 36 54.198N 104 27.077W
The boundary is the thin light layer separating lighter shale
beneath from darker shale above.
Relief map of the Jemez with Ojo Alamo Formation outcroppings highlighted in red.
By the end of the Cretaceous, the Laramide Orogeny was in full
swing. Buckling and faulting of the crust had first become severe
in Wyoming and Montana but began to spread south. By the beginning
of the Tertiary, a large area was beginning to be thrown up in
southern Colorado and northern New Mexico very closer to the
location of the ancient Uncompaghre uplift. This is known as the
Brazos-Sangre de Cristo Geanticline. A geanticline is a
large-scale upwarp in the Earth's crust, and this uplift included
the entire area from the present-day Tusas Mountains to the
present-day Sangre de Cristo Mountains. This uplift began shedding
sediments into the surrounding basins.
West of the Sierra Nacimiento, there are exposures of the Ojo
Alamo Formation, whose exact age has been much debated. The
current consensus seems to be that it straddles the
Cretaceous-Tertiary boundary, with dinosaur fossils in its lowest
beds and early Eocene fossils further up. There are several small
discontinuities in the formation, and, unfortunately, the exact
Cretaceous-Tertiary boundary is missing in one of these
The Ojo Alamo shows a coarsening upwards sequence. The lower beds
tend to be shale and mudstone, while the upper beds are
increasingly coarse sandstone. This likely reflects the
progression in the erosion of the source uplifts. The first beds
exposed by uplift would include thick sequences of Lewis and
Mancos Shale, providing ample clay and silt for shale and
mudstone. As erosion progressed, the exposure of sandstone and
basement rock would provide more coarse sediment.
The upper beds of the Ojo Alamo are themselves resistant to erosion and form the cap of Portales Mesa, as we saw in the previous chapter. The formation is also exposed as a hogback in the Regina area in the far northwest of the Jemez region.
Although the period of time from 65 million to about 2.6 million years ago is still widely called the Tertiary in geologic literature, the name is no longer formally recognized. This period of time is divided into the Paleogene (65 to 23 million years ago) and the Neogene (23 to 2.6 million years ago.)
During the Paleogene, India collided with Asia and Antarctica moved towards the South Pole. This caused the hot, moist climate of the Cretaceous to give way to a cooler, drier climate. The rise of the Himalayas exposed great masses of silicate rock that absorbed carbon dioxide from the atmosphere, reducing greenhouse warming of the Earth. The rifting of Australia away from Antarctica allowed the Southern Ocean Current to became established around Antarctica, which also contributed to cooling. The ocean basins increased in volume and the seas receded off the continents. The world was beginning to take the form familiar today, although South America was not yet joined to North America and the Atlantic was narrower.The Laramide Orogen continued to affect northern New Mexico into the mid-Paleogene, raising uplifts along fault lines that often coincided with Paleozoic faulting and which remain active today.
The earth's upper crust is brittle. If the underlying ductile layers
are deformed by tectonic forces, stress will build in the brittle
upper crust. When this stress becomes great enough, the rocks beds
of the upper crust will abruptly fracture, releasing the potential
energy built up in the strained rock by shattering the rock and
generating an earthquake. The rock beds will shift along the
fracture to relieve the stress, leaving a zone of broken and
deformed rock with intact rock beds on either side displaced from
each other. This is the fault.
Relief map of the Jemez with major faults highlighted in red.
The map of major faults in the Jemez region shows that faults are not rare features. In addition to the major faults shown on the map, there are numerous minor faults crossing the entire area, most of which are relatively short and not particularly active. There are some patterns evident in this map. Faults in the Jemez region are more likely to run north to south than east to west. The faults also are not evenly distributed. There are particularly prominent chains of faults running along the west face of the Sierra Nacimiento (the Nacimiento Fault); along the eastern edge of the map (the Tusas-Picuris Fault); and from Black Mesa past the foothills of the the Sierra de los Valles and on south (the Pajarito and La Bajada Faults). These are all associated with the Laramide Orogeny. There is also a swarm of north-south faults across the southern part of the map, associated with the opening of the Rio Grande Rift.
Faults are characterized by the angle of dip, which is the angle the fault plane makes with the vertical. Thus a high-angle fault, with an angle of dip close to 90 degrees, plunges nearly straight into the ground. The side beneath the fault is called the footwall and the side above the fault is called the hanging wall, from terminology used by miners (who often mine minerals deposited along a fault.) The footwall would be under the miner's feet, while he would hang his lamp on the hanging wall, which hung over his head. The fault trace is the path where the fault intersects the surface of the earth.
Almost all the faults in the Jemez area are normal faults,
in which the hanging wall is displaced downwards relative to the
footwall. If a normal fault is active enough, the footwall will
form an escarpment above the fault trace. This will be eroded away
if the fault is inactive for a long period of time.
Normal faults are characteristic of settings where tectonic
forces are stretching the crust, such as in the Rio Grande Rift.
Where the crust is being compressed, as it was in the Jemez area
during the Laramide, one sees reverse faults where the
hanging wall is displaced upwards relative to the footwall.
A reverse fault with a shallow dip (less than 45 degrees) is
called a thrust fault. Active reverse faults of any kind
are uncommon in the Jemez area today, with the Nacimiento Fault
being the only major thrust fault in the area. However, reverse
faults were more common during the Laramide.
Both normal and reverse faults (including thrust faults) are
known as dip-slip faults. Whether normal or reverse, the
side of the fault that is displaced downwards relative to the
other side is described as the downthrown side of the
fault. A fault can also be displaced laterally; such strike-slip
faults are less common, but include such famous faults as
the San Andreas Fault in California. The Embudo Fault along the
south flank of Black Mesa is displaced both laterally and
vertically and is described as an oblique-slip faults.
Strike-slip faults are further described as right lateral or left
lateral faults. If you stand on one side of a strike-slip fault
and look across at the other side, the direction of displacement
of the far side determines which kind of strike-slip fault it is.
Thus, if the far side is displaced to the right, it is a right
lateral fault. You should be able to satisfy yourself that this is
true no matter which side of the fault you are standing on, by
picturing the geometry in your head.
Major faults tend not to occur as single isolated fractures.
Instead, there are several parallel strands making up a fault
zone. A strand can fork off the main fault and rejoin it
further along the fault zone. Major faults may also end in a set
of fault splays that branch off to spread the displacement
over a broad area, almost like forks of a river delta.
Once a fault forms, it continues to be a zone of weakness in the
crust for a very long time. If the deformation of the lower crust
continues, as it typically does, there will be repeated
earthquakes and repeated displacements along the fault. If new
rock beds are being deposited on top of the fault, either by
volcanic activity or by continued sedimentary activity, the older
beds will be displaced by a greater amount than the younger beds.
This permits geologists to learn something about the history of
activity along the fault. The fault itself will be marked by a
zone of finely crushed rock called fault gouge. If the
fault ceases to be active, the fault gouge is sometimes cemented
into hard rock by minerals carried by fluids moving along the old
One fault zone established during the Paleogene in the Jemez
region is the Canones Fault Zone, located west of Abiquiu. The
fault zone crosses the floor of Arroyo del Cobre and the highway
to the south.
Arroyo del Cobre. 36
14.740N 106 22.057W
The main strand of the Canones Fault Zone runs across the floor of the canyon from left to right. This fault zone is generally taken to mark the boundary between the Rio Grande Rift (on the near side) and the Colorado Plateau (on the far side), two major tectonic provinces. The rock beds on the near side of the fault zone are thrown down a considerable distance, so that the Jurassic rocks forming most of the rim we’re standing on are considerably younger than the Triassic rocks forming the far rim. Just how great the displacement is is demonstrated by the presence of the white beds in the center of the valley floor. The main strand of the fault zone runs just beyond them, and they are the same formation (Poleo Sandstone) as the far canyon rim on the other side of the fault.
The cliffs in the foreground in the first frame are Jurassic Entrada Sandstone, with a thin covering younger Tertiary El Rito Formation and Ritito Conglomerate. There was once a bed of Jurassic Toldilto Formation above the Entrada Sandstone, as shown by the yellowish color; this is characteristic of the uppermost Entrada Sandstone, just beneath the Todilto Formation. A few weathered remnants of the Todilto Formation lie between the Entrada Sandstone and the El Rito Formation, and on the far side of the cliffs the Todilto Formation is much better preserved.
The far canyon wall is underlain by dark red Arroyo del Agua Formation, the upper formation of the Permian Cutler Group. The lower formation, the Canon del Cobre Formation, is exposed in the valley floor and in the Canon del Cobre further north. These are capped by the tan beds of the Poleo Sandstone of the Triassic Chinle Group, with a significant discontinuity between them.
The fault trace is spectacular where it crosses a local saddle.
Fault race of main strand of Canones
Fault. Looking soutwest from near
14.787N 106 22.471W
The fault trace is directly to the right of the yellowish outcrop
of Entrada Sandstone capped with Todilto Formation. These Jurassic
formations have been thrown down until they are in contact with
Permian Arroyo del Agua red beds.
Trace of main strand of Canones Fault
14.744N 106 22.573W
The zone of fine red material is the fault trace itself, which is
filled with fault gouge apparently dominated by the soft red bed
sediments of the Permian side of the fault. You are looking here
at the closest thing to the exact boundary between two major
tectonic provinces, the Rio Grande Rift province (to the left) and
the Colorado Plateau (to the right).
There is a spring arising almost exactly on the fault trace (behind us as we’re facing here) that produces very brackish water. It must flow through the Todilto Formation, or some other formation rich in partially soluble minerals, at some point. Because the water is brackish, it does not support a local flourishing of vegetation; the spring is dominated by sparse salt grass instead.
In previous chapters, we visited the area just to the south where the main strand of the fault crosses the highway. You may recall that the fault has thrown down the contact between the Morrison Formation and the El Rito Formation. The Morrison Formation is of Jurassic age, about 150 million years old. Here it takes the form of fluvial sandstone, deposited in a river valley. All the rocks above the Morrison Formation were eroded away at this location, then buried again under the El Rito Formation some 50 to 30 million years ago.
The contact is clearly visible towards the top of the cliff,
where the beds in the two formations lie at an angle to each
other. This is a good example of what geologists call an angular
unconformity. The contact is important for another reason: Whereas
here the Tertiary El Rito Formation lies directly on Jurassic
Morrison Formation rocks, we find that further west, across the
Canones fault, the El Rito lies on top of younger sedimentary beds
of Cretaceous age. This tells us that the Canones Fault existed at
least thirty million years ago, and that at that time it was the west
side of the fault that was thrown down. Geologists believe the
Canones Fault was part of the Laramide orogeny, and has been
reactivated by the opening of the Rio Grande Rift, this time with
the east side being thrown down.
The surprising conclusion is that the San Luis Valley and Espanola Basin were part of a highland area, the San Luis Uplift, during the Paleogene. The portion between Santa Fe and Los Alamos is called the Pajarito Uplift. These areas are now deep basins of the Rio Grande Rift.
This is illustrated in the following diagram.
Here is a close up of the point of contact, where it comes close to the road level.
This contact represents a gap in the geological record of at
least 100 million years.
The Canones Fault Zone has been mostly inactive for the last
three million years, since it does not displace volcanic rocks
younger than that age.
Relief map of the Jemez with Paleocene outcroppings highlighted in red.
65 milllion years ago, northern New Mexico was beginning to look
something like it does today. The Western Interior Seaway was no
more. The San Juan Basin to the west was accumulating sediments
eroded off the newly uplifted San Luis and Pajarito Uplifts to the
east. The climate was hot and moist, as it was across almost the
entire globe. Far to the west, Europe was just beginning to
separate from Greenland and Greenland from North America, and
there was an intermittent land bridge between Asia and North
America, which would continue to open and close until almost the
present day. South America had pulled away from Africa but was
still separated from North America by a tropical sea, while Africa
was separated from Europe by the Tethys Ocean. India was still
well south of Asia but moving north. Australia was just beginning
to pull away from Antarctica, and its bulk diverted southern ocean
currents to ensure that warmth from the tropics was well mixed
across entire ocean area of the Earth.
This earliest epoch of the Paleogene has been named the Paleocene ("old recent"), and lasted from about 65 million years ago to 56 million years ago. This epoch left little trace in the Jemez proper, but Paleocene beds of the Nacimiento Formation are found west of the Sierra Nacimiento in the San Juan Basin.
Relief map of the Jemez with Nacimiento Formation outcroppings highlighted in red.
During the early Paleogene, the San Juan Basin west of the Sierra
Nacimiento was a lush, well-watered intermontane basin, and
fossils of recognizable families of modern plants typical of
semitropical climates are found in the Nacimiento Formation, along
with numerous mammals. These included examples of the first true
rodents and primates, which branched off from a common ancestor in
the Cretaceous, 80 million years ago. That's right: Of all the
common household pets, rats, mice, hamsters, and gerbils are our
There are excellent exposures of the Nacimiento Formation throughout this area, generally forming slopes under cap rock of the San Jose Formation. Mesa de Cuba is an example.
The mesa is underlain by San Jose Formation sandstones and
conglomerates forming a cap over softer shales of the Nacimiento
This sequence may be examined more closely along the main highway just northwest of Cuba.
The San Jose Formation is the sandstone cliffs, while the slope-forming sedimentary beds beneath the cliffs are Nacimiento Formation.
Relief map of the Jemez with Eocene outcroppings highlighted in red.
The Eocene, which lasted from about 56 million years ago to 38
million years ago, was the warmest epoch of the Cenozoic Era.
Ocean temperatures were at least 5 degrees Centigrade (9 degrees
Fahrenheit) warmer than today. However, global temperatures began
to decline in the middle of the epoch and plunged dramatically
towards its end.
The start of the Eocene in the Jemez was marked by uplift of the Sierra Nacimiento to the west and further uplift of the Brazos-Sangre de Cristo geanticline to the north and east. A broad river valley lay between the Sierra Nacimiento and the Pajarito Uplift, where the Jemez Mountains are located today.
The tectonic compression of the Laramide Orogeny came to a close
towards the end of the Eocene, and the uplifts began to erode down
to fill the surrounding basins with sediments. The remnants of the
Farallon Plate began to sink into the mantle, and hot
aesthenosphere rose to take their place. This caused volcanism to
break out in the Yellowstone area, and crustal extension began to
take the place of compression, beginning in what is now western
Relief map of the Jemez with San Jose Formation outcroppings highlighted in red.
The San Jose Formation is separated by a definite discontinuity from the underlying Paleocene Nacimiento Formation. It dominates the area northwest of Cuba, where it tops mesas and other hills. It can also be a beautiful formation:
Here is a gorgeous exposure in a road cut. This exposure is
heavily cross-bedded and strongly indurated.
A fairly coarse, angular, and feldspar-rich sandstone, with some organic debris. Other exposures are quite dirty. In general, this formation is a relatively immature sandstone near Cuba, reflecting its origin as sediments off of the nearby Sierra Nacimiento and Brazos-Sangre de Cristo geanticline.
Locations near Cuba have fossils of the giant carnivorous bird, Diatryma.
And if "giant carnivorous bird" seems indistinguishable from
"dinosaur", I doubt many paleontologists would argue with you.
The San Juan Basin west of the Sierra Nacimiento dips very gently to the north. As a result, successive resistant beds form nearly level mesa tops, with the youngest beds exposed on mesa tops to the north and the oldest to the south. This panorama shows the sequence of beds.
Panorama of the eastern San Juan Basin. Looking west to southwest from 36 04.289N 106 59.026W
At left in the far distance is the Mount Taylor volcanic field. The northeastern part of the field includes several volcanic plugs, of which Cabezon Peak is the largest. Just visible in front of the plugs is San Luis Mesa, which is topped by Lookout Point sandstone. The mesas at far left, which are labeled La Ventana Mesa North and La Ventana Mesa South on the geologic map, are also topped by the Lookout Point Sandstone (La Ventana Member).
In front of San Luis Mesa and to the right of Cabezon Peak is La Ventana Mesa, capped with resistant Cliff House Sandstone underlain by less resistant Menefee Formation. The next four formations, the Lewis Shale, Cliff House Sandstone, and Fruitland and Kirtland Formations, are less resistant, while the Ojo Alamo Formation forms the resistant cap of Mesa Portales near the center of the panorama.
At right is Cuba Mesa, capped with resistant San Jose Sandstone over less resistant Nacimiento Formation.
In the middle distance in the center and right of the panorama is
a hogback ridge formed by steeply dipping beds of the Mesaverde
Group (Lookout Point, Menefee, and Cliff House Formations) that
were exposed by the uplift of the Sierra Nacimiento. The flat
ground to their east is underlain by Mancos Formation (here poorly
exposed) while everything in the foreground is Dakota Formation or
Relief map of the Jemez with El Rito, Gallisteo, and Diamond Tail Formation outcroppings highlighted in green, red, and yellow, respectively.
The El Rito Formation extends from north of the Valles caldera to the Chama area in northermost New Mexico. It is somewhat difficult to precisely date, having very few fossils, but is probably in the ballpark of 40 million years old. The formation is mostly interbedded red mudstone and sandstone, tending to form slopes.
The lowest bed of the El Rito Formation is very coarse conglomerate with ample sandy matrix, as shown in the earlier photograph of the contact between the El Rito Formation and the Morrison Formation near the Canones Fault. This is even more evident in the Red Wash Canyon area.
Basal conglomerate of El Rito Formation. 36 14.428N 106 22.484W5
The clasts are mostly quartzite, of the type found in the Tusas
Mountains to the north. This suggests that there was already
a Tusas Range 40 million years ago, part of the San Luis Uplift,
and geologists interpret the El Rito Formation as alluvial fans
eroded off the early Tusas Mountains to the south and west. The
lack of fossils and other characteristics of the formation suggest
it was deposited under semiarid conditions.
Here is a sample of the El Rito Formation from its lowest bed.
Notice that there is a quartzite pebble embedded in this sample.
This was taken from near the bottom of the bed. The sandstone
consists of poorly rounded, poorly sorted clasts of quartz with
considerable feldspar and lithic clasts, and much cement in the
pore spaces — all consistent with an immature sandstone formed
close to its source rock. Because of the proximity of the Canones
Fault, which forms a path for ground water, this exposure is
unusually well cemented.
El Rito Formation forms the south rim of Arroyo del Cobre.
El Rito Formation on south rim of Arroyo
del Cobre. 36
14.754N 106 22.123W
In this area, the El Rito Formation rests unconformably on Jurassic beds.
El Rito Formation on south rim of Arroyo del Cobre. 36 14.754N 106 22.123W
This photograph nicely contrasts a conformable contact with an
unconformable contact. At left, Todilto Formation limestone beds
lie atop massive yellow beds of Entrada Formation standstone. The
two formations are bedded at the same angle. At top right, red
beds of the El Rito Formation lie atop the same massive beds of
Entrada Formation, but the beds lie at a different angle with
respect to each other. The contact at right represents an
erosional surface across which around 130 million years of
geologic history are missing.
There are also some good exposures north of the village of Canones.
El Rito Formation. Looking north from 36 11.271N 106 26.738W
A particularly accessible exposure is found in a road cut along
the old road into Canones.
El Rito Formation. 36 11.463N 106 26.989W
Towards the top, the El Rito Formation gives way to the
light-colored Ritito Conglomerate, which we'll describe later.
Here's a sample of the El Rito Formation from this location.
El Rito Formation. 36 11.463N 106 26.989W
This sample is a medium sandstone of well-rounded but
poorly-sorted quartz grains with some feldspar and a few lithics
and a fair amount of reddish cement in the pore spaces. Note the
distinctive muscovite flakes, generally aligned with the bedding
planes of the sandstone. Mica is not usually abundant in
sandstones and so serves as a distinguishing feature of the El
Rito Formation. Its presence hints at an origin in the mica-rich
pegmatites of the Tusas.
The El Rito Formation is not typically well exposed in the Jemez proper, but along Forest Road 100 in the north Jemez, the El Rito Formation is hinted at by reddish discoloration in road cuts just below outcrops of the younger Ritito Conglomerate. One can sometimes find clasts of El Rito sandstone resembling those from the Canones Area.
El Rito Formation. 36 08.616N 106 31.070W
Similar poor exposures continue west and south as far as upper Coyote
Another formation in the Jemez area from about this same time is
the Galisteo Formation. This crops out in only a few locations
between the northern Sandia
Mountains and the southern Sangre
de Cristo Mountains, because it is mostly buried under
younger sediments. However, there is an outcropping along I-25
where the highway descends from the Cerros del Rio, at La Bajada.
The outcropping shows up clearly on Google Maps as a red patch
around the highway and to the northeast.
The resemblance to the Abo Formation and other Permian red beds we looked at a couple of chapters back is striking, and in fact I long assumed that that's what this outcropping was. However, the most recent geological map of the area identifies this clearly as Galisteo Formation.
Galisteo Formation is prominent at Garden of the Gods.
Garden of the Gods. 35 26.505N 106 04.927W
Here the beds have been tilted nearly vertical by the nearby
intrusion of the Cerrillos Hills. We'll have more to say about the
Cerrillos Hills shortly.
Galisteo Formation also crops out on the east slopes of the San Miguel Mountains in the southeast Jemez, where it has been exposed by deep faulting along the Pajarito Fault Zone. This is in a relatively inaccessible area, the Bandelier National Monument Wilderness Area, but the exposures are visible from the summit of St. Peter's Dome.
Galisteo Formation from St. Peter's Dome. Looking southeast from 35 45.441N 106 22.135W
Geologists who have examined these exposures have found that the
sediments are significantly coarser here than in the exposures
further south. This supports the conclusion that the sediments
came from highlands to the north, with the coarser sediments found
closer to the source rock and the finer sediments found further
The resemblance to the Abo Formation is not accidental. Like the Abo, the Galisteo formed in a fluvial environment, where rivers descending from the Pajarito Uplift and Sierra Nacimiento meandered south across a level valley. Similar environments produce similar rocks.
The Galisteo Formation was originally defined to include older
beds that are extensively exposed south of the villages of Cerrillos
and northeast of the ghost town of Hagen.
Geologists later recognized that there was a significant
discontinuity separating the two sets of beds, and the lower beds
were split off into their own formation, the Diamond Tail
Formation. This formation is exposed mostly at the southern limits
of the Jemez region.
Diamond Tail Formation south of Los Cerrillos. 35 24.865N 106 08.435W
As with the Permian Cutler Group and Abo Formations, the question
arises whether the El Rito Formation and the Galisteo Formation
are really distinct formations. They appear to be the same age and
are similar in their characteristics. It is likely that they were
deposited on opposite ends of a river valley running south between
the Brazos-Sangre de Cristo geanticline and the Sierra Nacimiento
whose middle section is now buried under the volcanic rocks of the
Jemez. They also appear to correlate with the San Jose Formation
west of the Sierra Nacimiento.
The Galisteo and Diamond Tail Formations contains numerous petrified logs in the Cerrillos area, mostly conifers, oak, and beech. Fossil pollen includes palm pollen, suggesting a subtropical forest giving way to a mixed pine-oak forest at higher elevations. The elevation of the Cerrillos area was probably around 550 m (1800') with a climate that was frost-free throughout the year. Cerrillos presently has an elevation near 1800 m (6000'), which is an indication of how much regional uplift has taken place in the last 50 million years.
The sediments of the Diamond Tail Formation appear to have come mostly from the northeast, where the Brazos-Sangre de Cristo geanticline provided a rich source of sediments. The Sierra Nacimiento became an additional source of sediments in Galisteo time, supporting the conclusion that the Sierra Nacimiento was not uplifted until the Eocene. There appears to be no contribution from the south, in the area of the present-day Sandia Mountains, suggesting that these were not uplifted until after the Eocene.
Fossils are not abundant in the Galisteo and Diamond Tail Formation. Curiously, the lowest beds contain Paleozoic brachiopods and Cretaceous shark teeth, eroded from underlying beds and redeposited as reworked fossils. However, two locations in these formations have some beds containing bone fragments, mostly mammals. A site in the Diamond Tail Formation in the Cerrillos Hills produces fossils belonging to the Wasatchean Stage, which extended from about 55.4 to 50.3 million years ago. These include freshwater fish scales, fragments of turtle shells and vertebra, tapir teeth, Coryphodon teeth, a Hyopsodus jaw, and Hyracotherium teeth. Coryphodon was one of the earliest really large grazing mammals, roughly the size of a horse; Hyposodus was a primitive mammal resembling a shrew but more closely related to modern horses; and Hyracotherium was a dog-sized ancestor of both horses and rhinoceroses.
The other fossil site is in the Galisteo Formation at Arroyo del Tuerto, and contains numerous fossil remains of titanotheres, very large mammals related to horses and rhinos. These date to the Duchesnean Stage, about 42 to 38 million years ago.
Towards the end of the Eocene, volcanic activity picked up in
what is now the San Juan Mountains of Colorado. Ash and basalt
flows from the San Juan field reached into northern New Mexico,
though not as far as the Jemez region. This activity would peak in
the Oligocene with the formation of at least 15 calderas, and the
activity would not remain confined to southern Colorado.
34 million years ago, northern New Mexico was rocked by a series
of volcanic eruptions along a belt extending south from the
Pajarito Uplift. Magma from the upper mantle had forced its way to
near the surface, doming up the overlying sedimentary beds almost
like blisters in the Earth's skin. In some places the magma broke
through, forming volcanoes that deposited debris in a broad apron
around the volcanic belt and choked the river valleys that had
formerly run south through the area. This marked the beginning of
the Oligocene Epoch in New Mexico.
The Oligocene Epoch lasted from 34 to 23 million years ago. It was a period of severe global cooling, caused by the rifting of Australia and South America away from Antarctica. This created the Southern Ocean and established the Antarctic Circumpolar Current, which isolated Antarctica from warmer ocean water to the north and turned the continent into an icebox.
The Oligocene was a period of volcanic upheaval in much of New
Mexico. There were some truly spectacular eruptions in the area west
of Socorro, along the southeast margin of the Colorado
Plateau. These are distant enough from the Jemez that they will
not come into our story further. Oligocene volcanism also included
Mountains southeast of the Jemez. The Ortiz Mountains are
part of the Ortiz Porphyritic Belt, a chain of volcanoes extending
Mountain north to La
Cienega. The bulk of the eruption was of monzonite and
latite, which are intrusive and extrusive rocks formed from
calc-alkaline magma that is moderately rich in silica. This
particular composition yields a rock composed mostly of
plagioclase and alkaline feldspar, with traces of iron minerals
and small amounts of quartz.
It is not clear how much of the eruption actually reached the surface. The igneous rocks in this area are primarily intrusive monzonite, coarsely crystallized and interbedded with older sedimentary rocks. Most of the extrusive latite has long since eroded away. However, there are beds of volcanic debris and ash atop the Galisteo Formation along Espinaso Ridge and west of Galisteo, named (logically enough) the Espinaso Formation. These are though to have come from the pulse of volcanism that produced the Ortiz Mountains.
It seems likely that the Ortiz Porphyritic Belt was a substantial
volcanic field at its peak, comparable in size with the Jemez
volcanic field we see today. It formed just as the Rio Grande Rift
began to open, but its connection with the rift is unclear. It has
since been deeply eroded, exposing the plumbing below the
As the eruptions died down, the crystallizing monzonite sweated
fluids rich in incompatible elements, which are
chemical elements that do not easily fit into the crystal
structure of the common silicate minerals that form from magma.
Gold and tungsten are incompatible elements, and both were mined
in the Ortiz Mountains, with the last gold mine closing in 1986.
Relief map of the Jemez with Ortiz Belt and Cienegujilla basanite outcroppings highlighted in red and green respectively.
The terrain north of La Cienega is low hills underlain with Ortiz monzonite and Espinaso brecciated latite.
Monzonite outcrop. 35
34.337N 106 07.969W
I've placed my walking stick for scale on the photograph below.
The stuff weathers a bit like granite, unsurprising considering that it's mostly feldspar and it's mostly feldspar that makes granite weather the way granite weathers. Monzonite, like granite, is an intrusive rock, but has an intermediate silica content and a moderately high alkali metal content. This is a common rock along continental rifts such as the Rio Grande Rift. Here’s a sample.
This rock consists almost entirely of large grains (they look
like plagioclase) in a white groundmass (likely alkali feldspar),
with numerous small flecks of biotite and hornblende. No quartz is
This outcropping, located in the middle of La Cienega, is not mapped, but could be either Ortiz monzonite or Espinaso Formation.
Ortiz Monzonite? 35.562N
Under the geologist’s loupe, one sees smaller gray crystals, probably alkali feldspar, with larger white crystals, probably plagioclase, and lots of oxidized iron that was probably once hornblende or another iron-rich mineral. The presence of both alkali and plagioclase feldspar is characteristic of monzonite, and the absence of visible quartz keeps this from being classified as a quartz monzonite.
The Ortiz porphyry belt continue south through the Cerrillos
Hills and the Ortiz Mountains.
Cerrillos Hills. Looking north from 35 25.104N 106 07.828W
The Cerrillos Hills are mostly laccoliths, bodies of magma that pushed up the overlying rocks but did not quite break through to the surface, and cooled underground into visibly crystalline rock. The highest point is Cerro Bonanza, also known as Santa Rosa Mountain, 7088′ tall. It is underlain by augite biotite monzonite, a rock with an intermediate content of both silica and sodium and potassium.
It's a popular belief that you can rid your body of toxins by making yourself sweat. This turns out not to be the case, unless you mean the modest quantities of urea and salt found in human sweat. But bodies of magma cooling underground really do “sweat” certain trace elements, dissolved in hydrothermal fluids, which penetrate the surrounding rock to deposit valuable metal ores. The Ortiz porphyry belt has numerous old mines and mining claims, and gold was being mined here as recently as the 1980s. But the mineral that has been mined the longest in the Cerrillos Hills is turquoise.
Turquoise from Blue Bell Mine. Near 35 28.219N 106 6.751W
Turquoise is a form of copper aluminum phosphate, and it is the gemstone most closely associated with native American jewelry of the Southwest. It has been mined in the Cerrillos Hills since at least 700 A.D., the date of an archaeological site near Mexico City where turquoise has been found that has the chemical fingerprint of Cerrillos Hills turquoise. Mount Chalchihuitl in the Cerrillos Hills was the largest open pit mine of any kind in North American when the Spanish arrived in 1535. The native Americans may have mined the deposit by building large bonfires over the ore beds, then dousing the fires with cold water to fracture the rock by heat stress. The rest was done with stone tools. The Spanish subsequently mined small quantities of silver, but the big boom was between 1879 and and 1885 or so.
The boom was fueled mostly by speculators, with just enough mineral resources in the claims to keep the speculation going. Many investors bought rights to claims with less thought of profiting from the actual mineral extraction than from reselling the rights to someone else at a profit. This was not at the level of the famous tulip mania in Holland, but the same speculative mindset was involved.
The boom, and its collapse, were influenced by the laws for staking and developing mining claims. The laws governing such claims were derived from the economic theories of the Scottish Enlightenment, which strongly influenced the American founders. Under these theories, a person rightfully established ownership of previously unclaimed land by mixing his labor with the land. This could be done by clearing and tilling it for agriculture or excavating it for valuable mineral resources. This philosophy was reflected in the Homestead Act, which allowed settlers to take possession of public land they successfully cultivated for long enough, and in mining claim law. In most American jurisdictions, you could file a claim at the nearest county seat asserting that there were valuable mineral ores on a section of public land. You then marked your claim with "monuments", such as actual wooden or metal stakes, inscribed with the number of your claim. You then had exclusive rights to develop the claim and mine the ores. For the claim to remain valid, you had to do a minimum amount of development within a prescribed time frame. Eventually, if you developed the claim sufficiently, you could purchase title to the land from the government at a bargain price per acre. This was known as patenting the claim.
The claim boom in the Cerrillos Hills was fueled in part by relatively lax development requirements. You had a full year to dig just ten feet of tunnel. When this changed to a more stringent requirement — three months to dig ten feet of tunnel — the boom collapsed. The speculators weren’t interested in actually digging; they wanted to try to make money by trading the mineral rights.
This does not mean that there were not actually any valuable minerals in the Cerrillos Hills. In addition to turquoise, the Spanish mined considerable silver and lead, and iron and manganese were also produced in significant quantities. Manganese is still used in steel manufacture today. It is a powerful deoxidizer and desulfuring agent and is almost indispensable for making high-quality steel from low-grade ore
Picture Rock, also known as Lone Bluff, is a particularly picturesque isolated laccolith of the Cerrillos Hills.
Picture Rock, Looking east from 35 29.597N 106 05.523W
The monzonite is readily accessible next to the road as it passes Picture Rock.
Ortiz Monzonite near Picture Rock. 35 29.604N 106 04.855W
Notice the resemblance to the outcrop of monzonite at La Cienega. Another local landmark is The Devil's Throne, just northeast of Los Cerrillos.
The Devil's Throne. Looking northwest from 35 26.479N 106 08.011W
Back side of he Devil's Throne. 35 26.723N 106 08.084W
The geologic map for this area duly notes that one team of investigators classified the rock making up the Devil's Throne as hornblende quartz latite porphry and another as hornblende monzonite porphyry. The difference is subtle: Latite has a relatively high potassium feldspar content while monzonite has a mixture of calcium and alkali feldspar. A good petrological laboratory could settle the issue, but then it’s entirely possible the composition varies from location to location. The geologic map chooses to map most of the rock underlying the Cerrillos Hills simply as porphyritic andesite. Here's a sample from further west, along the Jane Calvin Sanchez Trail at Cerrillos Hills State Park.
Porphyritic andesite along the Jane Calvin Sanchez Trail. 35 26.910N 106 07.151W.
The porphyritic andesite of the Cerrillos Hills is itself intruded by dikes that must have formed shortly after the main body of andesite cooled. These radiate away from the hills for great distances in all directions, Many are composed of a distinctive trachytic latite porphyry -- a rock moderate in silica and rich in potassium that contains large feldspar phenocrysts. One such dike is located along the Jane Calvin Sanchez Trail.
Trachytic latite porphyry dike along the Jane Calvin Sanchez Trail. 35 26.995N 106 07.199W
Trachytic latite porphyry dike along the Jane Calvin Sanchez Trail. 35 26.995N 106 07.199W
A dike composed of similar rock extends well west of the Cerrillos Hills.
Trachytic latite porphyry dike near Rosario. 35 27.678N 106 13.781W
Trachytic latite porphyry dike near Rosario. 35 27.678N 106 13.781W
The rock here is reddish rather than gray in color, but such color differences are often among the least useful for distinguishing rocks.
Not all the dikes radiating from the Ortiz porphyry belts are
porphyritic. Some are composed of what appears to be a uniform
fine-grained basalt. One example is the El Creston dike, which
forms the core of an impressive ridge extending for miles east of
the Cerrillos Hills.
El Creston dike. 35 24.539N 105 56.910W
This suggests two systems of dikes from two eruptive pulses in the Cerrillos Hills. A third system of basalt dikes located west of the Cerrillos Hills running in a north-south direction are likely younger dikes associated with the opening of the Rio Grande RIft.
A similar dike west of the Cerrillos Hills shows hornfelsization.
Dike with hornfelsization evident. 35 24.539N 105 56.910W
The dike is the dark gray rock with redish patina on the right. The whitish zone immediately to the left of the dike is hornfels, produced by the heat of the magma baking the Niobrara Member shale into which it intruded. Close to the dike the shale layers have more or less fused together, while further out, the hornfels gradually transitions into unaltered shale.
The Ortiz Mountains are another cluster of laccoliths of the Ortiz monzonite.
Ortiz Mountains. Looking south from 35 29.787N 106 05.653W
The Ortiz Mountains are gold mining country. Early mining exploited placer deposits, where gold that erodes out of bedrock accumulates deep in stream beds because of its high density. More recently, the bedrock ores themselves have been exploited.
Ortiz Mountains. Deep telephoto shot looking south from 35 29.787N 106 05.653W
This photograph shows part of the tailings pile for the Cunningham Hill mine, which was first opened in 1828. The host ore is the breccia filling a volcanic pipe through which much of the Espinaso Formation is thought to have been erupted. The content of gold in the ore is quite low, a few grams per ton, but this can still be profitably extracted using cyanide. Though extremely toxic, cyanide is one of the few chemicals that can pull gold into solution, so it is used to leach the gold out of the crushed ore. The gold-bearing solution is then treated with powdered zinc, which replaces the gold in solution. The result is a mixture of remaining zinc powder and metallic gold, from which the zinc can be removed by treatment with sulfuric acid.
The Cunningham Hill mine was most recently operated by
Consolidated Gold Fields, which began open pit mining in 1973.
Some 7 metric tons of gold (250,000 ounces), with a value of about
$300 million at March 2017 prices, was extracted before the mine
was closed again in 1986. Subsequent attempts to reopen the mine
have foundered over environmental regulations and the fluctuating
price of gold.
Relief map of the Jemez with Espinaso Formation outcroppings highlighted in red.
The Espinaso Formation is slightly younger than the Ortiz Monzonite and is thought to be volcaniclastics from the portion of the Ortiz eruptions that made it to the surface. It is a somewhat complex formation, containing a few latite flows in its lower beds but mostly consisting of latite or andesite breccia. The type section is at Espinaso Ridge itself.
Espinaso Ridge. Looking east from near 35 25.970N 106 22.965W
Unfortunately, while Espinaso Ridge is located mainly on public
lands, the road access is very poor.
More accessible exposures are found at La Cienega and along Galisteo Creek. Here's a likely fragment of latite from the La Cienega area.
Espinaso latite. 35 34.591N 106 07.893W
My photo doesn't do it justice. It's slightly lavender on fresh
surfaces under sunlight, with a beautiful pattern of interlocking
crystals. There are larger striated crystals, possibly albite
(sodium-rich plagioclase), in a matrix of much finer crystals that
are white and grey -- probably two varieties of feldspar. There
are also some large rusty grains that are probably weathered
Here's an outcropping of Espinaso Formation breccia conglomerate north of La Cienega, formed mostly of broken fragments of andesite.
Espinaso breccia. 35.570N 106.127W
A dike runs through the center of La Cienega that has been dated at about 20 million years in age. This is mapped as part of the Espinaso Formation.
Espinaso dike. 35.564N 106.127W
The dike is composed of a very pretty hornblende trachyandesite. Trachyandesite is a rock formed from lava with an intermediate silica content, similar to andesite, but with an unusually high content of sodium and potassium. This trachyandesite contains numerous phenocrysts of hornblende.
Hornblende trachyandesite.. 35.564N
The long needlelike hornblende crystals are obvious. The clump of crystals at the left end of the sample is either a clot (the crystals forming around some nucleus as the lava cooled underground before being erupted) or a xenolith. A xenolith is a bit of country rock that is caught up in a body of magma and does not quite melt. Both are fairly common in andesite.
Closer examination of the outcrop itself shows numerous xenoliths.
Xenoliths get geologists’ attention because they sample the rocks deep below the surface, including the lower crust and upper mantle. This dike has apparently gotten a lot of geologists’ attention, because there are drill holes all over it, presumably where geologists have taken samples of the xenoliths.
The Espinaso Formation is strikingly exposed northeast of Los Cerrillos.
Espinaso Formation. 35 28.519N 106 04,199W
This is almost certainly an example of a debris flow that has been preserved as conglomerate. The original flow was a mass of rock and mud that flowed off the slopes of the nearby volcanoes to produce the beds here. These were later cemented into hard rock. We will have much more to say about debris flows later in the book.
There are few exposures of sedimentary rocks of Oligocene age in New Mexico, and fewer fossils, but the end of the epoch marked an important milestone in the geologic history of the Jemez: The birth of the Rio Grande Rift.
Around 30 million years ago there was a major shift in the tectonics of the western United States. The compression produced by the subducting Farallon Plate along the west coast was replaced by tension and uplift as North American drifted onto the northern East Pacific Rise, a mid-ocean ridge. Precisely what began to take place under the western United States is still debated by geologists, but it seems likely that the remnants of the Farallon Plate began to sink deep into the mantle and hot mantle rock rose to the level of the asthenosphere to take its place. As a result, for the last 30 million years, the western United States has been simultaneously uplifted and pulled apart, producing the Basin and Range Province so distinctive in topographic maps today. And the eastern boundary of that province runs right down the middle of New Mexico.
The change from compression to tension in western United States
produced a series of north-trending ranges separated by valleys
that characterize the Basin and Range Province. The heart of this
province is in Nevada and western Utah, but portions extend
through Arizona into southwest New Mexico. The Colorado Plateau
seems to be a block of particularly cold, rigid crust that has
held together even as it has been pushed up by the rise of hot
mantle beneath, but it has begun to split off the rest of North
America along the valley of the Rio Grande. This is the Rio Grande
Rift, which extends from near Leadville
in central Colorado down at least as far as the area west of El
Paso, where it blends into the surrounding basin and range
geology. Seismological evidence shows that the crust has thinned
significantly in the Rift, from a thickness of 50 km (30 miles)
east and west of the Rift to 30 km (20 miles) under the Rift.
There is also greater heat flow from deep in the earth through the
Rift than in the surrounding crust.
The direction in which the Rift is pulling apart is not directly
east-west. Instead, the separation is more west-southwest to
east-northeast. This means that there is some left-lateral
movement along the major faults bounding the rift. In the northern
part of the Rift, this is consistent with the Colorado Plateau
rotating slightly clockwise.
This map shows all exposures of rocks of Oligocene age or older
in the Jemez region.
Relief map of the Jemez with pre-Neogene outcroppings highlighted in red.
These older rocks are useful for tracing the outline of the Rio Grande Rift, since they all formed before rifting began. Although the young volcanic rocks of the Jemez Mountains obscure part of the western margin of the Rift, one can see that this margin runs almost directly under the Jemez. The isolated exposure of Gallisteo Formation in the San Miguel Mountains, to the right of the center of the map, is an anomaly that suggests the western margin of the Rift is not sharply defined at the latitude of the southern Jemez. This is an accommodation zone in which the Rift shifts abruptly to the east as one moves north from the Albuquerque Basin to the Espanola Basin.
Espanola Basin. Looking north from 35 51.251N 106 08.896W
The Espanola Basin began to form late in the Oligocene as a broad, shallow basin, typical of ductile extension. The crust in north central New Mexico had been heated by the burst of volcanism in the early Oligocene, and it deformed relatively smoothly as it came under tension. At this time, the western boundary of the basin was located along the Sierrita Fault, just east of the southern Sierra Nacimiento. Similar basins had formed along the entire length of the Rio Grande Rift by 26 million years ago.
South of the accomodation zone, the displacement shifts west into the Santo Domingo Basin, sometimes regarded as a northern extension of the Albuquerque Basin.
Santo Domingo Basin. Looking north from 35 18.883N 106 27.122W
North of the Jemez area, the displacement shifts east into the
San Luis Basin.
About 25 million years ago, silica-poor magma from the upper
mantle reached the surface near La
Cienega as a series of low-volume eruptions. This ultramafic
magma was also rich in alkali metal oxides. Such alkaline
magmas are thought to form from relatively undepleted
mantle that has produced little magma in the past and so has
retained most of its original alkali metal content. When
undepleted mantle experiences a low degree of partial melting, so
that only the most alkali-rich fraction of the mantle rock
contributes to the magma, the magma is greatly enriched in alkali
Low-silica alkaline magma forms an uncommon kind of igneous rock called basanite. The excess sodium converts much of the feldspar that would otherwise have crystallized from the magma to nepheline, Na3KAl4Si4O16. Other common minerals in basanite are olivine, magnetite, and clinopyroxene.
Cerro Seguro is the most accessible eruptive center for the basanite flows of the La Cieneguilla Basanite.
Cerro Seguro. 35 34.228N 106 08.263W
to the hill are covered with numerous small clasts of basanite.
The underlying rock is monzonite.
The eruption center looms above us.
The view from the top
shows many features associated with the Rio Grande Rift.
The panorama begins with a view to the southwest, towards Sandia
Crest (washed out but just visible in the distance.) Mesa
de Juanita lies just north of a bend in Interstate 25. The canyon
crossing much of the left side of the panorama and disappearing
into the horizon is the canyon of the Santa Fe River. The hills
just beyond the canyon are underlain by Ortiz monzonite, which
rises like islands out of the much younger Cerros del Rio Basalt
forming the surface of Tsinat Mesa on the other side of the Santa
Fe River. Such islands of older rock completely surrounded by
younger lava flows are described as kipukas, a Hawaiian
term adopted by geologists. The peak behind and to the right of
these hills is Tetilla
Peak, New Mexico's etymological counterpart to Grand Teton
in Wyoming, and on
the far size of the Santa Fe River canyon where it enters
the panorama you see exposures of highly weathered monzonite.
Just right of center of the panorama are Sangre
de Cristo mountains east of Santa Fe, and on the right side
of the panorama village of La Cienega. The cluster of hills in the
middle distance beyond La Cienega are the Cerrillos Hills, of
which the monzonite hills around La Cienega are geologically the
The basanite at the top of the hill is described as agglomerated
in a recent geological paper on this area, which confirms we are
near the vent location. Rock with agglomerated texture is formed
when blobs of semimolten lava are thrown out of the vent and pile
up around it.
The rock does indeed look like individual blogs of magma tightly welded together. The rock is very black, very dense, and very fine-grained, but even with a loupe I doubt I would have recognized it as different from very dark basalt. It takes laboratory analysis to confirm the lack of feldspar and the low (44%) silica fraction.
Further down the hill, a pile of basanite boulders shown a distinctive red weathering.
The magnetite in the basanite slowly oxidizes to red hematite when exposed to atmospheric oxygen. Basalt sometimes contains magnetite and gets a reddish patina, so this is not necessarily diagnostic for basanite, but there is a difference in degree. A sample from basanite here is more massive than the rock from the top of the cone.
is an example of a feldspathoid. Like feldspars,
feldspathoids are tektosilicates. Each silica or aluminum
tetrahedron is joined to four neighbors at its corners to form a
three-dimensional network. However, feldspathoids contain less
silica than feldspars, and the framework of silica-alumina
tetrahedra is more open. This makes room for the additional alkali
metal atoms needed to supply the electrons to compensate for the
Feldspathoids generally crystallize only from magma that is
enriched in alkali oxides relative to its silica content. In
particular, feldspathoid is almost never found in the same rock as
quartz, since any excess silica that might have formed quartz
would have gone into converting feldspathoid into feldspar
instead. Minerals that do not normally coexist with quartz are
described as silica-undersaturated. A rock that contains
such minerals is itself described as silica-undersaturated. A rock
containing neither quartz nor silica-undersaturated minerals is
described as silica-saturated. If the rock contains
quartz, it is described as silica-oversaturated.
Those silica-undersaturated rocks which contain feldspathoids are
described as alkaline, by contrast with the subalkaline
tholeiitic and calc-alkaline suites discussed in the second
chapter of this book. Alkaline magma is a product of a low degree
of partial melting, because minerals bearing potassium and sodium
are some of the first to melt under mantle conditions. Only if
melting proceeds further will the magma become subalkaline.
Alkaline magmas are typical of continental rifts like the Rio
Grande Rift. Here the crust is pulling apart relatively slowly,
less than 2mm per year, versus centimeters per year for the
mid-ocean ridges. In addition, the mantle under the continents is
relatively undisturbed, making it more likely to be undepleted.
The slow melting of undepleted crust can produce highly alkalic
magmas. These are also more likely to penetrate the crust than
subalkaline magmas, since alkaline magmas are more than 0.1 g/cc
less dense than tholeiitic magmas.
The rocks of the Jemez volcanic field straddle the composition
boundary between subalkalic and alkalic rocks. The older rocks
tend to be alkalic, as do flows near the periphery of the field.
Younger rocks near the center of the field tend to be subalkalic,
suggesting than enough magma has been generated here to begin
depleting the underlying mantle.
Relief map of the Jemez with Abiquiu and Ritito Formation outcroppings highlighted in red.
North of the Jemez Mountains, in the valley of the Rio Chama, lies the village of Abiquiu. This was originally founded as a community of genizaros during the Spanish era, in the mid-18th century. The Spanish word genízaro has the same roots as the English janissary, used to describe slaves (usually Christian boys) under the Ottoman Empire, who gained some measure of freedom and considerable social status by converting to Islam and becoming elite soldiers of the Turkish Army. The genízaros were native American boys (mostly Navajo) enslaved by the Comanche and sold to the Spanish as household servants or shepherds. They were indoctrinated in Catholicism by their Spanish masters, gained some measure of freedom, and eventually formed communities of their own, in which there was extensive intermarriage among tribes and with the Spanish. At Abiquiu, the genízaro community was awarded its own land grant or merced by the Spanish governor. The Merced del Pueblo de Abiquiu grant, originally awarded around 1750, is still a recognized legal entity today. The Abiquiu merced owns the northern part of Mesa Alta, which I have found is very well maintained, even in comparison with the adjacent National Forest lands. And, yes, they still raise some sheep, though cattle is more common now.
The village later became the eastern terminus of the Old Spanish
Trail, whose western terminus was Los Angeles. The trail was first
successfully traveled in 1829-1830 by Antonio Armijo, whose
caravan took 142 days to make the round trip. Bode's
General Merchandise, founded in 1890 as Grants Mercantile
and purchased by Martin Bode in 1919, is a fixture of the modern
community, which has now become somewhat gentrified. Artist
Georgia O'Keefe settled permanently in the village in 1946 and
produced several paintings of "The White Place", Plaza Blanca
Canyon, which is underlain by dramatic white beds of ash-rich
sediments now assigned to the Abiquiu Formation. This is one of
the most distinctive formations of the northern Jemez.
The La Cienega eruptions were contemporaneous with the formation of the Latir volcanic field north of Taos. This included a giant caldera eruption from the Questa caldera. The first Latir eruptions occurred 30 million years ago as the rift opened, while the Questa eruption occurred five million years later. The Questa caldera has subsequently been uplifted by tectonic activity and deeply eroded, exposing valuable deposits of ores of molybdenum and other metals.
The Latir field produced extensive ash falls, while the Questa caldera produced voluminous pyroclastic flows. The pyroclastic flows formed the Amalia Tuff, which crops out as far to the southwest of Questa as the southern Tusas Mountains.
Quite a pretty tuff, full of quartz phenocrysts. I haven't seen
anything quite like it outside of Yellowstone.
A pyroclastic flow is a kind of avalanche of red hot particles of
rock carried along by hot gases from the volcano. Because the hot
particles are buoyed up by the hot gas, the avalanche can travel
great distances (this outcrop is about 25 miles from the Questa
caldera) and destroy everything in its path. The hot rock then
settles out on the ground. I will have much more to say about
tuffs and pyroclastic flows later in this book.
The ash from the Latir field is an important component of the
Abiquiu Formation, which crops out throughout much of the northern
Jemez. This formation originally included three members: a lower
member of somewhat unremarkable, poorly consolidated light tan
sandstone and conglomerate beds; a thin layer of limestone and
chert (the Pedernal Chert); and the white upper layers, which are
rich with volcanic ash and clasts of Amalia Tuff. This division
and assignment has recently undergone revision. The lower Abiquiu
Tuff turns out to correlate with the Ritito Conglomerate, which
was originally described to the north of Abiquiu, and newer maps
and papers of this area now assign exposures that were formerly
described as lower Abiquiu Tuff to the Ritito Formation. Only the
distinctive white upper beds are now assigned to the Abiquiu
Formation, while the Pedernal Chert remains an informal unit name.
This sort of thing is neither terribly uncommon nor particularly
unhealthy in geology. Formation designations change as further
study improves our understanding.
The white beds of the Abiquiu Formation can be traced far to the west and south, cropping out in the Jemez as far to the southwest as lower Canon de San Diego. For the traveler approach Abiquiu from the south, the first dramatic exposures of the formation are visible on the southwest flank of Sierra Negra.
Sierra Negra. Looking north from 36 11.809N 106 14.396W
The white cliffs halfway down the left side of the mountain are
Abiquiu Formation. There is a fault cutting right through the
mountain, at about the location of the right side of the Abiquiu
outcrop. To the east (right) are younger beds of the Santa Fe
Group, and the top of the mountain east of the fault is capped by
a small basalt lava flow that had been dated to about 4.8 million
Sierra Negra. Looking north from 36 12.231N 106 14.805W
The fault is part of the eastern edge of the Rio Grande Rift. It
displaces the basalt flow by just a a few meters, showing that
activity has been limited on this fault in the last four million
The Abiquiu Formation is particularly well exposed in Plaza Blanca Canyon, Georgia O'Keefe's "White Place", north of Abiquiu.
The canyon is presently owned by Dar al Islam Mosque, but is open
to the public.
Abiquiu Formation at Plaza Blanca Canyon. 36 14.048N 106 18.180W
The darker bed in the cliffs in the center of the panorama contains numerous clasts and may mark an ancient stream bed or a debris flow. Here's a closer view.
Possible debris flow in Abiquiu Formation at Plaza Blanca Canyon. 36 14.048N 106 18.180W
The ridge here lies along the trace of the Plaza Colorado fault. The fault plane itself is dramatically exposed.
Fault plane of Plaza Colorado Fault. 36 14.073N 106 18.235W
Slickenside is present along the fault plane.
Slickenside at Plaza Colorado Fault. 36 14.073N 106 18.235W
Slickenside is a distinctively polished rock surface on the plane
of a fault, caused by the motion of the rock faces past each
other. The striations here are consistent with this being a normal
fault, with primarily up/down motion.
A peculiar feature of some of the Abiquiu Formation beds at Plaza Blanca is the presence of well-developed conjugate jointing.
Conjugate jointing in Abiquiu Formation at Plaza Blanca Canyon. 36 14.117N 106 18.416W\
It is not uncommon for rock to have joints (fissures) that meet at right angles, with one of the angles commonly being the bedding angle. Joints that do not meet at right angles are described as conjugate joints. They're something of a geological curiosity, for which I've not come across a good explanation.
Some surfaces show well-developed manganese dendrites.
Manganese dendrites at Plaza Blanca Canyon. 36 14.294N 106 18.388W
The black patterns are formed by unusual manganese oxides formed
by weathering of the rock.
In the area west of Abiquiu, geologists have carefully studied
the sediments making up the Abiquiu Formation, and find a
transition from sediments derived mostly from the San Juan
volcanic field to the northwest to sediments derived mostly from
the Latir volcanic field to the northeast. The former is
relatively poor in quartz while the latter is rich in quartz. The
feldspar composition also shifts, from mostly plagioclase to
mostly sanidine. The transition from one to the other suggests
changes in drainage patterns associated with the very earliest
opening of the Rio Grande Rift in the Taos area.
Further west, past the ridge of Cerrito
Blanco, the Abiquiu Formation is visible just north of the
West side of Cerrito Blanco showing fault offset of Tesuque and Abiquiu Formations.
Just west of here, across a fault boundary that is thrown down to the east, the Ritito Conglomerate is well exposed.
Ritito Conglomerate in road cut west of Abiquiu. 36 13.855N 106 22.846W
The Ritito Conglomerate contains an abundance of rock clasts,
embedded in a light ash-rich sandy matrix. This is rarely very
well cemented, except just below the Pedernal Chert at the top of
the formation. Areas underlain by the Ritito Conglomerate thus
tend to form gentle slopes and rounded hills, like this one. The
clasts are dominated by quartzite and granitoid rocks of
Precambrian age, likely weathered off the Tusas Mountains to the
Superb exposures of the Ritito Conglomerate are found along Red Wash Canyon.
Ritito Conglomerate along Red Wash
14.354N 106 22.532W
The area also features a slot canyon.
Slot canyon in Ritito Conglomerate. 36
14.103N 106 22.562W
The clasts in the conglomerate here are varied and interesting,
including epidote and garnet-bearing metamorphic rocks. These very
thick deposits of very coarse conglomerate indicate that the
source rock was very close to this location.
The Abiquiu Formation can be traced west to Canones Mesa
and the Ritito Conglomerate is beautifully exposed north of the village of Canones, where it lies atop El Rito Formation.
Ritito Conglomreate north of Canones. 36 11.369N 106 26.796W
Exposures occur on the lower slopes of Cerro Pedernal. Cliffs
of Abiquiu Formation are prominent along Forest Road 100 in the
valley of the Rio Encino as it climbs towards the La
Grulla Plateau, southwest of Cerro Pedernal.
Nearby, the Ritito Conglomerate is well exposed in the road cut.
Ritito Formation. 36 08.342N 106 30.784W
Some exposures of the Ritito Conglomerate in this area contain
large clasts of Amalia Tuff at the top of the formation, but I
found none here. The clasts were all quartzite, gneiss, and other
Precambrian rocks likely weathered from the Tusas Mountains.
The Pedernal Chert is particularly well exposed here, forming a
prominent ledge separating the Ritito Conglomerate and Abiquiu
Pedernal Chert forming the prominent ledge. Looking north from 36 08.418N 106 30.630W
It's a bit of a scramble to reach the ledge, beneath which are
unusually well cemented beds of Ritito Conglomerate. Here's the
chert layer close up.
Pedernal Chert. Car keys for scale. 36 08.486N 106 30.622W
The Pedernal Chert is a bed of amorphous silica and limestone. The limestone probably formed as a calcium-rich soil atop the Ritito Conglomerate under arid conditions. Later, the Abiquiu Formation was deposited on top of the limestone layer, and groundwater circulating through the ash-rich Abiquiu Formation picked up silica that was redeposited in the limestone layer. Deposition of amorphous silica in limestone beds is very common, though the chemistry seem not to be fully understood. The chert varies in color from milky white to reddish brown to pitch black.
Pedernal Chert. 36 08.486N 106 30.622W
Pedernal Chert. 36 08.486N 106 30.622W
Large clasts of chert can be found around the base of Cerro
Pedernal, where it was exploited by early native Americans.
Pedernal Chert artifacts have been found as far east as the
eastern slopes of the Sangre de Cristo Mountains, and Spanish
archives mention chert quarries in the mid-18th century.
Further south along Forest Road 100, the road cuts through Abiquiu Formation.
The higher elevation at which the Abiquiu Formation is exposed in
the northern Jemez, compared with the exposures in the Abiquiu
area to the northeast, reflects the displacement along the Canones
Fault Zone, which lies between the two areas.
The Ritito Formation is exposed again in the northwest wall of Valles Caldera, in upper San Antonio Canyon (a relatively inaccessible area) and in Valle San Antonio, which can be reached by passenger vehicle with a back country permit from Valles Caldera National Preserve. Viewing the exposure requires hiking some distance along San Antonio Creek.
Ritito Formation in northwest caldera
58.102N 106 36.742W
The exposure here is somewhat poor, and easily confused with nearby lacustrine beds and Bandelier Tuff. However, it establishes that the base of the topographic rim of the caldera is located here.
Spotty exposures of the Abiquiu Formation continue south along the caldera rim, of which the most accessible is in a road cut along State Road 126.
More Abiquiu Formation. 35 53.056N 106 39.781W
The Abiquiu Formation is also present in the southern Jemez, in
upper San Juan Canyon. The easiest exposures for the casual
visitor to reach are in landslide deposits along the road.
This is located at about the same elevation as in the northern Jemez. Under the loupe, this looks like a mass of small quartz grains with considerable lithic fragments and abundant white cement. This makes the rock a lithic wacke, rather than a lithic arenite.
More pristine exposures on the east side of San Juan Canyon can
be reached by a short but strenuous hike from Forest Road 10.
Abiquiu Formation on east side of San Juan Canyon. 35 45.293N 106 37.256W
The exposures here are quite thick, with a particularly spectacular exposure in a drainage into the east side of the canyon.
Abiquiu Formation on east side of San Juan Canyon. Looking east from 35 45.293N 106 37.256W
There are also exposures on the west side of the canyon, which can be reached by a less strenuous but longer hike down the canyon along an abandoned forest road branching off of Forest Road 10.
The southernmost exposures of Abiquiu Formation in the Jemez are
in southern Canon de San Diego, occurring in two isolated
erosional remnants just south of Red Rocks and in more substantial
exposures across the Jemez River on tribal lands.
Abiquiu Formation remnants near Red Rocks. 35 38.251N 106 43.429W
The exposure here is the thin layered white beds, sandwiched between red Abo Formation on which the Abiquiu was deposited in this area and dark beds of much younger terrace gravels laid down by the Jemez River before it cut down to its current level.
Also present in the southwestern Jemez is the Gilman Conglomerate. The type section is prominent in the cliffs west of the former logging camp of Gilman.
Gilman Conglomerate type section. Looking west from 35 42.977N 106 45.568W
At bottom are red beds of the Permian Abo Formation, some 300
million years old. Above are the beds of the Gilman Conglomerate.
Above those are younger gravel beds, then a mesa of Tsherige
Member, Bandelier Tuff. We know that the gravels must be older
than 1.25 million years, since that’s the age of the Tsherige
Member lying above them, but it’s very hard to date gravels. The
individual clasts in the gravel are Paleozoic but that just tells
you how old the rocks they eroded from are; it doesn’t tell you
when the beds formed.The same is true for the Gilman
Conglomerate, though it’s clearly older than the gravels. It has
the look of volcaniclastics, and is mapped as such in the
quadrangle map for this area. The clasts in its uppermost beds
have a minimum age of 25 million years, similar to the Ritito
Conglomerate. Some geologists now believe that the Gilman
Conglomerate correlates with the Ritito Formation and should
probably be regarded as a member of the Ritito Formation. Others
note that, while it is roughly the same age as the Ritito
Conglomerate, it must be derived from an Oligocene volcanic field
that is now buried under the southern Jemez. I have included it in
the map at the start of this section.
I've devoted a lot of space to the Abiquiu Formation, because its distinctive white beds nicely trace the approximate boundary of the west side of the Rio Grande Rift through the Jemez area. Further east, the Abiquiu Formation was either never deposited, was eroded off the Pajarito Uplift before the Rio Grande Rift opened, or is now buried under younger rift fill sediments of the Santa Fe Group. There is one prominent exception: an exposure along La Bajada.
Abiquiu Formation on La Bajada. 35
32.898N 106 13.971W
This is probably the finest exposure of this formation south of
the Jemez. The reasons why the formation is so well preserved here
are unclear, but may be related to its location along the La
Bajada Fault, which is unusual for the area in that the downthrown
side is to the west. This is the area where the Rio Grande Rift
jumps to the west, and this exposure likely marks the east
edge of the Rift at this latitude.
The Ritito Formation is intruded by the Cerrito de la Ventana
dike system. One dike is crossed by the highway west of Abiquiu.
The basalt in the dike is about 20 million years old, making it part of an episode of volcanic activity predating the current Jemez volcanic field. It also helps us put a lower limit on the age of the Ritito Formation, which is poor enough in fossils that its age has been hard to pin down. You can see a band of the surrounding Ritito Formation baked red by the heat of the dike though, curiously, only on one side of one part of the dike.
The basalt is a dense black rock with occasional veins of almost
pure quartz that indicate the dike has undergone some hydrothermal
Basalt is quite low in silica, while quartz is more or less pure silica. The quartz must have been injected long after the basalt cooled, probably as a solution in water formed in the very last part of the underlying magma chamber to cool.
This dike continues north past the western end of Plaza Blanca, where it splits into multiple strands.
Cerrito de la Ventana dike west of Plaza Blanca. Looking west from 36 14.178N 106 18.680W
This is but one (particularly prominent and accessible) dike of the Cerrito de la Ventana system. An example of a smaller dike is found near Red Wash Canyon.
Small dike of Cerrito de la Ventana
14.097N 106 22.257W
Another dike, or perhaps this same one in a different exposure, is found in the west wall of Arroyo del Cobre.
23 million years ago, the world was beginning to look very much like it does today, although Iraq and New Guinea were still under water and Central America was not yet a continuous ribbon of land. In the western United States, the compression of the Laramide orogeny had reversed itself, and instead of the crust being compressed, it began to be pulled apart. At the same time, the asthenosphere under the western United States became hot enough to lift the entire region as the western United States drifted onto the northernmost part of the East Pacific Rise. This was the period when rifting along the Rio Grande Rift really began to get going.
Geologists have chosen this moment in time as the start of the
Neogene. The Neogene is further divided into the Miocene and
Pliocene Epochs. During the Miocene, the severe global cooling at
the end of the Oligocene moderated somewhat, but cooling resumed
by 15 million years ago and temperatures plunged at the end of the
Miocene, about 5.33 million years ago.
Only a small part of the crustal thinning in the Rift is accounted for by lower surface elevations along the Rift. The rest is due to the mantle rising into the rift. The rift is bounded by deep faults on one or both sides, and the combination of deep faulting, a shallow, hot mantle, and the zone of fertile rock along the Jemez Lineament creates conditions for sustained volcanic activity.
The faults bounding the rift are not single large faults, but numerous parallel faults forming one or more fault zones. East of Santa Fe the Pecos-Picuris Fault Zone marks the west face of the Sangre de Cristo Mountains and plunges to the west. To the west, which is the deeper side of the Rift in the Jemez area, there are three systems of faults. These are the Pajarito Fault Zone, the Canada de Cochiti Fault Zone, and the Jemez Fault Zone. Further north, the Pajarito Fault Zone turns east to join the Embudo Fault Zone, while the Jemez and Cochiti Fault Zones disappears under the Jemez Mountains before possibly joining with the Canones Fault Zone.
The Jemez Fault Zone runs roughly along Cañon de San Diego. It is difficult to trace over much of its length, but one strand (the Sierrita Fault) is fairly obvious at Guadalupe Box.
Fault scarp at Guadalupe Box. Looking northeast from near 35 43.882N 106 45.754W
The darker rock to the left is weathered Precambrian hornblende-biotite quartz monzonitic gneiss ("granite" is close enough for most purposes.) To the right are Permian sedimentary beds. The local topography suggests, and geological mapping confirms, that this is actually a large fault. The younger beds have been thrown down by the fault a considerable distance, bringing them into contact with the Precambrian rocks.
The Canada de Cochiti Fault Zone is largely located on tribal lands in some of the less accessible areas of the southern Jemez, but one of the major strands of the fault, the Camada Fault, passes just west of the Veterans Memorial Overlook at Kashe-Katuwe Tent Rocks.
Trace of Camada Fault, Canada de Cochiti Fault Zone. 35 40.330N 106 27.430W
The white beds to the left are Peralta Tuff, while the reddish
beds to right are thin surge beds of the Tsherige Member,
Bandelier Tuff. The right side is thrown down by the fault.
Neither the Jemez nor the Canada de Cochiti fault zones have been
very active in the recent past, as shown by the lack of much
displacement of Bandelier Tuff beds overlying the faults.
The Pajarito Fault Zone is much easier to trace, probably because it is now taking up most of the extension in this part of the Rift and has been significantly displaced since the Tsherige Member of the Bandelier Tuff was erupted, 1.2 million years ago. This fault zone joins the La Bajada Fault Zone to the south and the Embudo Fault Zone to the north. The Pajarito Fault Zone is most obvious southwest of Los Alamos, along the foothills of the Sierra de los Valles, where it forms a prominent escarpment that shows up well on topographic maps:
The Pajarito Escarpment is quite obvious as one approaches it
from the east along State Road 4.
Pajarito Escarpment. 35 50.030N 106 21.351W
You can see that the highway winds up the face of the escarpment. Looking north and south from one of the hairpin turns in the road, we get an excellent view of the Pajarito Escarpment in both directions.
Pajarito Escarpment south of hairpin at 35 49.991N 106 21.804W
The view to the south shows the Pajarito Fault dividing into two
strands, one forming the escarpment at the center of the
picture, and a second strand to the east (left) forming a smaller
Further south, the Pajarito Escarpment also cuts through the San Miguel Mountains, a small outlying range of the Jemez. The fault scarp forms the east slope of St. Peter's Dome and exposes older rocks down to the Galisteo Formation.
The Pajarito Fault splits south of the San Miguel Mountains before both splays pass south out of the Jemez area. The east splay forms the La Bajada Fault, which marks the western boundary of the Cerros del Rio. This fault is thrown down to the west, unlike the main Pajarito Fault, and it marks the point where the Rio Grand Rift is displaced to the west. The fault runs roughly along the base of the La Bajada escarpment, and the exact trace is exposed in the hills west of the Galisteo dam:
Trace of La Bajada Fault west of Galisteo dam. 35 27.780N 106 14.241W
The reddish, nearly vertical, thin line on the side of the hill is the precise trace of the La Bajada Fault. The fault runs nearly at the foot of the La Bajada escarpment to the north and south, but in this area, Galisteo Creek has eroded the escarpment back so that the fault runs some distance west of it.
The pinkish beds to the left, on the downthrown side, are young Rio Grande Valley alluvium deposits. The reddish beds to the right are much older Chinle Group beds. The yellowish beds immediately to the right (east) of the fault, between the fault trace and the Chinle beds, may be a sliver of Entrada Formation. Or they may be fault gouge, the pulverized rock formed by motion along the fault, which has since been cemented by minerals brought in by groundwater flowing in the fault zone. The geologic map shows Entrada Formation at nearby locations immediately east of the fault trace.
The main Pajarito Fault, to the west of La Bajada, cuts through Eagle Canyon just west of the old Dixon apple orchard.
Eagle Canyon from north rim overlook. 35 42.491N 106 23.080W
The escarpment at right marks the trace of the Pajarito Fault.
(Click for a full resolution version.) The fault trace is well
exposed in the canyon wall -- in fact, this is the most dramatic
exposure of the Pajarito Fault I've seen. To the left (east) is
Bandelier Tuff, while to the west is an exposure of dark andesite
of the Paliza Canyon Formation capped by Bandelier Tuff. Note that
the Bandelier Tuff is significantly thicker on the left side of
the fault; there was already an escarpment 1.25 million years ago
and the pyroclastic flow that produced the tuff ponded on the east
side along the escarpment. This shows that the fault was already
active before 1.61 million years ago, when the Otowi Member of the
Bandelier Tuff was emplaced. The displacement in the Bandelier
Tuff shows that the fault has been active in the 1.25 million
years since the emplacement of the Tscherige Member of the
One of the findings of geologists that most surprises me is how
long-lived deep fault zones like the Pajarito Fault Zone and the
Jemez Fault Zone are. The area around the Sierra Nacimiento
Mountains has been repeatedly uplifted since at least the time of
the Ancestral Rocky Mountains, 300 million years ago. The
Pecos-Picuris Fault Zone has been in existence nearly as long. The
Pajarito Fault Zone was active during the Laramide Orogeny fifty
million years ago; what's more, until the Rio Grande Rift opened,
the Pajarito Fault Zone worked in reverse of how we see it today,
with the current location of the Jemez Mountains forming a low
area between the Sierra Nacimiento and a high area, the Pajarito
Uplift, between the Pajarito Fault Zone and the Pecos-Picuris
Fault Zone. The Pajarito Uplift is now the Espanola Basin of the
Rio Grande Rift. Similar basins opened up the length of the Rio
Grande Rift, including the San
Luis Basin to the north and the Albuquerque
Basin to the south. Between each basin, the Rift takes a
sharp jog to the right.
We'll return now to that hairpin turn on State Road Four where it
climbs the escarpment, and this time we'll look north.
Pajarito Escarpment north of hairpin at 35 49.991N 106 21.804W
Here the escarpment runs along the eastern foot of the Sierra de
los Valles until it joins the Santa
Clara Canyon Fault somewhere north of Los Alamos.
North of the Jemez, there is extensive rift faulting associated
with the opening of the Rio Grande Rift centered around the
village of Canones.
Deformation along this fault zone has been traced as far west as
Coyote and extends east to where the zone merges with the Embudo
Fault Zone. The main strand of the Canones Fault forms an
impressive escarpment just north of the highway where it begins to
climb up to Abiquiu Reservoir.
Canones Fault scarp. Looking northwest from 36 14.015N 106 23.294W
This is generally taken to mark the boundary between the Rio
Grande Rift and the Colorado Plateau to the northwest.
Another strand of the fault dramatically offsets exposures of the Jurassic Entrada and Todilto Formations on the north end of Canones Mesa.
Entrada and Todilto Formation offset by fault through Canones Mesa. Looking southwest from 36 14.149N 106 24.029W
Red and white beds of the Entrada Formation, capped with a thin
bed of Todilto Formation, crop out on either side of a landslide
down the north face of the mesa. On the east (left) the beds are
thrown down about 70 meters (200 feet) by the fault passing
between the two outcrops. This fault does not displace the lava on
top of the mesa, showing that this particular strand of the
Canones Fault Zone has not been active in at least three million
As our story comes closer to the present, where more geological
evidence is preserved, the growing cast of characters will tend to
break down any telling as a regular progression in time. We will
find ourselves following one story line, then backtracking to
catch up on another. The story of the Santa Fe Group extends from
the beginning of the Neogene almost to the present day, and we
will tell much of it here before returning to the middle Neogene
and picking up the story of the Jemez volcanic field.
As the rift opened, the Pajarito Uplift subsided to become the
Espanola Basin. It is estimated that the basement rock beneath
Espanola is now about 4570 meters (15,000') lower than
corresponding basement in the Sangre de Cristo Mountains to the
east. However, there is not today a yawning chasm four kilometers
deep in this area. As the Pajarito Uplift dropped to become the
Espanola Basin, the basin began to fill with sediments eroded off
the nearby highlands.
These sediments formed beds that are, in some places, thousands of meters thick. Similar thick beds of sediments accumulated in the other basins forming the Rio Grande Rift. These rift sediments are known collectively as the Santa Fe Group. For the most part, the Santa Fe Group consists of weakly consolidated muddy sandstone, though some beds are locally well indurated and the clast size varies from mudstone to coarse conglomerate. Some of the formations making up the Santa Fe Group were deposited by the ancestral Rio Grande River and its tributaries, while others were deposited by wind. Continuing regional uplift caused by the hot asthenosphere under the western United States has resulted in erosion of these thick beds of sediments, which form spectacular badlands throughout the Santa Fe and Espanola area.
The nomenclature for the Santa Fe Group is unusually complicated.
This is in part because the individual formations are not always
easy to distinguish in the field, and in part because there were
multiple sources of sediments. A sedimentary rock formation
normally represents a correlated interval in time, but the the
sediments laid down in a particular time interval may vary
systematically from place to place because they came from a
different source. A further complication is that many named
members of the group are found to interfinger with each other.
There has been much disagreeement on which formations to include
in the Santa Fe Group. The oldest formation that is sometimes
assigned to the Santa Fe Group is the Abiquiu Formation, which
partially filled the rift in its earliest stages of formation. The
next oldest formation in the Jemez area is probably the Zia
Formation of the southwest Jemez, which, like the Abiquiu
Formation, is assigned to the Santa Fe Group by some geologists
but not by others.
Relief map of the Jemez with Zia Formation outcroppings highlighted in red.
The Zia Formation is from 23 to 13 million years old, coinciding
with the earliest period of rifting along the Rio Grande Rift. It
is exposed in the southwest Jemez, in some places forming spectacular
badlands. Some of the most prominent are in lower Canon de
The Zia Formation here consists of very soft sandstone of nearly pure quartz, probably deposited as sand dunes about 13 to 20 million years ago. In this photograph, it is capped with younger gravels cemented with travertine and, as is usually the case when softer beds underlie more resistant beds, the softer Zia Formation forms cliffs and towers capped by the harder overlying sediments. Because the Zia Formation is so poorly cemented, its earliest name in the geologic literature was the Zia Sand.
The Zia Formation can be traced further east, cropping out in the banks of Vallecitos Creek southwest of the village of Ponderosa.
Zia Formation. Looking east from 35 37.693N 106 42.095W
The Zia Formation is the white area to the left. The contrasting tan beds to the right are much younger terrace deposits of Vallecitos Creek. Zia Formation also crops out in the western slopes of Borrego Mesa.
Zia Formation. Looking east from 35 37.693N 106 42.095W
These beds have been assigned to the Cerro Conejo Member of the
Zia Formation, and are relatively young. It has recently been
proposed that the Cerro Conejo Member be promoted to a formation,
but this is not yet reflected on most published maps and papers
for this area.
The Zia Formation disappears in the area west of Cochiti Pueblo,
buried under younger deposits of the Cochiti Formation. This
reflects the migration of the active western margin of the Rio
Grande Rift to the east.
Relief map of the Jemez with Tesuque Formation outcroppings highlighted in yellow (Ojo Caliente Member) or red (all other members.)
The Tesuque Formation is the chief Santa Fe Group formation in
the Espanola Basin. It lies atop the Abiquiu Formation and its age
is between about 20 million and 5 million years old. In places it
is more than 3000 meters (10,000 feet) thick. Because of its great
thickness, it was originally divided into five members, which
(from oldest to youngest) are the Nambe Member, the Skull Ridge
Member, the Pojoaque Member, the Chama-El Rito Member, and the Ojo
Caliente Sandstone. More recent classifications have added the
Cejita and Cuartales Members, both younger than the Pojoaque
There is also overlap in age. It was recognized from the start
that, while the Nambe, Skull Ridge, and Pojoaque Members form a
fairly straightforward age sequence, the Chama-El Rito Member,
with an age of about 17 to 11 million years, overlaps the Pojoaque
Member and part of the Skull Ridge Member in time. It is
identified as a distinct member because its sediments originate
from the San Juan volcanic field to the north and northwest rather
than the Sangre de Cristo Mountains to the east. The Ojo Caliente
is mostly younger than the other members, but there is some
overlap of the oldest Ojo Caliente beds with the earliest Pojoaque
and Chama-El Rito beds.
To further complicate matters, the Tesuque Formation is also
divided into Lithosome A and Lithosome B, and this division cuts
across the more traditional division into members. The two
lithosomes represent two distinct source regions for the sediments
originally thought to originate in the Sangre de Cristo Mountains.
Lithosome A is thought to be sediments eroded from the Sangre de
Cristo Mountains to the east, and it consists of distinctly
pinkish sediments rich in feldspar grains (arkose) and granite
clasts deposited by a broad network of small streams flowing to
the west. Lithosome B is thought to be sediments carried by larger
rivers from the north and northeast, and it consists of tan
sediments that contain singificant clay and a wide variety of
different clasts. Thus, geological maps distinguish, for example,
the Pojoaque Member of Lithosome A from the Pojoaque Member of
The Santa Fe Group has proven to be a good fossil hunting ground.
The American Museum of Natural History sponsored fossil digs from
1942 to 1965 that produced a vast collection of around 10,000
fossil fragments. Prehistoric mammals ranging from dire wolves
(extinct relatives of gray wolves) and horses to camels, primitive
elephants, and primitive hippopotamuses have been found in the
various members of the Tesuque Formation. This forms much of the
basis for its division into members.
Commercial camel fossil from Santa Fe area
Camel footprints were discovered in an ash bed just west of Santa
Fe Airport in the 1960s.
The Tesuque Formation is particular rich in fossils east and west
of the highway from Santa Fe to Espanola. Palm fossils north of
Santa Fe suggest that the elevation was still only 700m (2300') in
the middle Miocene, much less than the current elevation of 1800m
The Nambe Member crops out close to the Sangre de Cristo Mountains, well east of the Jemez area, and we will not have much to say about it. It is mostly soft, pink sediments that weather to low hills.
Nambe Member. Looking north from 35 54.851N 105 58.130W
Closer to the mountains, the Nambe Member starts to contain more coarse conglomerate beds.
Nambe Member. Looking north from 35 57.217N 105 55.367W
This is a very common pattern in sedimentary beds. The coarser material is found nearer the source rocks and the finer material further away. This can be a very useful clue when reconstructing paleotopography for more ancient periods in the earth’s history, where sedimentary beds are all that are left of the original landscape. Thus, the tendency for beds of the Triassic (between 200 and 250 million years ago) to be coarser north of the Jemez than south of the Jemez is a clue that the sediments originated in a highlands near the Colorado border.
The Skull Ridge Member crops out as far west as the
Pojoaque-Espanola highway and is notable for its large number of
ash beds. These date to around 15 million years old, two million
years older than the oldest known ash-producing eruptions in the
Jemez region. Either the ash came from further afield, or it came
from vents in the Jemez that are now buried under flows from later
eruptions. The base of the Skull Ridge Member is defined in much
of the region by a particularly distinctive ash bed.
A prominent feature of the Skull Ridge Member is the Red Wall, so named by paleontologists digging for fossils in the area.
The Red Wall. Looking west from 35 55.194N 105 57.965W
The Skull Ridge Member tends to form cliffs like these, probably because its numerous ash beds provide cementing silica. Similar cliffs are impressive and readily accessible in the Camel Rock area along the main highway from Santa Fe to Espanola.
Skull Ridge Member in Camel Rock area. Looking east from 35 50.019N 105 59.048W
Numerous fossils from the Skull Ridge Member have been collected,
and these help define the Barstovian faunal stage. A
faunal stage is a distinctive combination of animal fossils found
in beds of a fairly narrow range of age, not more than a few
million years. The presence of ash beds that can be
radiometrically dated has allowed paleontologists to fix the time
frame of the Barstovian stage as from about 16.3 to 13.6 million
The Tesuque Formation is prominent in the Espanola Valley east of the Rio Grande, where it forms the spectacular hogback ridges of Los Barrancos:
Los Barrancos. Looking north from near 35 52.897N 106 04.045W
Note that the beds tilt to the west (left). Geophysical data shows that the Rio Grande Rift in the Espanola area (the Espanola Basin ) is deepest to the west. These beds are mapped mostly as Lithosome A with Cuartales Member on top underlain by Pojoaque Member. To the west, the beds transition to Lithosome B, primarily of the Pojoaque Member. Numerous north-south faults cut across the beds, further complicating the geological picture.
Both of these members are exposed in road cuts along the highway from Pojoaque to Totavi.
Pojoaque Member, Lithosome A. 35 52.906N 106 06.958W
This is a thick siltstone interbedded with thin gravel layers.
Cejita Member, Lithosome B. 35 53.114N 106 06.417W
You can see the slightly redder color of Lithosome A versus the
more tan color of Lithosome B.
The contrast between lithosomes within a single member is visible in the hills along the road from Otowi to Pojoaque.
Contact between Lithosome A and LIthosome B in Pojoaque Member. 35 53.172N 106 05.896W
Notice that the top of the hill is distinctly redder than the lower slopes. The geologic map confirms that this is the contact between Lithosome A (on top) and Lithosome B (at bottom) of the Pojoaque Member.
One of the best exposures of the Tesuque Formation for close
examination is found in a road cut along Highway 285 southeast of
Road cut in Pojoaque Member, Lithosome
A, Tesuque Formation. 35.984N
The best location for appreciating the great thickness of the Tesuque Formation, though located just east of the Jemez region, is probably the south rim of the Santa Cruz river valley.
Lithosome A, Tesuque Formation, north of Santa Cruz River. Looking north from 59.832N 105 54.366W
Almost the entire thickness of the cliffs north of the river is
underlain by Lithosome A of the Tesuque Formation. The geologic
map for this area does not divide this into members.
Along the western margin of the Rio Grande Rift, at Sierra Negra
northeast of Abiquiu, the Chama-El Rito Member of the Tesuque
Formation is exposed on the flanks of the mountain. Here a major
fault has displaced the Chama-El Rito Member downwards on the
right side of the fault, placing it alongside older Abiquiu
Formation beds. We saw this picture earlier in the chapter.
Sierra Negra. Looking north from 36 12.231N 106 14.805W
The Chama-El RIto Member is the reddish beds to the right. Further west, thick beds of the Chama-El Rito Member are exposed on the flanks of Abiquiu Mesa.
The Chama-El Rito Formation overlaps the Pojoaque Formation in
age, but is distinguished by the presence of gravel beds
containing volcanic rock clasts characteristic of the San Juan
Mountains to the northwest.
North of Abiquiu, there are tuff beds interpreted as remnants of an old tuff ring formed by localized volcanic activity within the Chama-El Rito Member. The tuff ring has not been dated.
Remnants of tuff ring. Looking northeast from 36 13.361N 106 17.845W
The tuff ring remnants are the greenish-tan beds at left and right center in the photograph. Another set of tuff rings are found further to the east, on the road to El Rito.
Remnants of tuff ring. Looking west from 36 14.288N 106 11.537W
The darker rock is a basaltic tuff, formed from bits of silica-poor lava thrown out by an eruption. Basaltic tuffs rarely if ever form the vast outflow sheets you see with rhyolitic tuff, such as the Bandelier Formation. The silica-poor lava does not hold onto its gas content long enough to move far from the vent as suspended particles. This particular tuff was erupted in a small eruption some 15.3 years ago, based on potassium-argon dating of a basalt bomb embedded in the tuff.
Fifteen million years ago, continuing climate change caused the
Rio Grande Rift to begin to cool. The many small streams that had
brought sediments into the Rift started drying up and a field of
dunes began to creep across the Rift. This dune field is preserved
in the rock record as the Ojo Caliente Member of the Tesuque
I've mentioned that most of the members of the Tesuque Formation
are difficult for a non-expert to distinguish in the field.
However, the Ojo Caliente Sandstone stands out from the rest of
the Santa Fe Formation as being even less likely to be well
cemented and in being composed of relatively clean quartz grains.
It is a difficult formation to accurately date; there are no ash
beds for radiometric dating, and it is almost devoid of fossils.
However, it is thought to be between about 10 and 14 million years
The formation is exposed throughout the Medanales area.
In a number of locations, the Ojo Caliente Sandstone has been
unusually well cemented along faults that allowed mineral-rich
groundwater to circulate through the sandstone. The hardened
sandstone stands out in stark relief when the surrounding softer
sediments are eroded away. Cerrito
Blanco, west of Abiquiu, is a an excellent example of such a
Cerrito Blanco. 36 12.988N 106 19.368W
The dike lies on public land and can be reached along County Road 155.
Hills of Ojo Caliente Member are prominent east of Lobato Mesa
and west of the Rio Chama.
The foreground terrain is underlain by Chama-El Rito member. A
fault runs along the west side of the hills and may account for
the hills being better cemented than most Ojo Caliente Member.
Ash beds are visible at the southern edges of the small hills right and left of center. These are marked on the geologic map for the area, but not further identified or described.
Ojo Caliente Sandstone also underlies much of the northwestern slopes of Black Mesa.
Here the soft nature of the beds is clear. This looks a lot like the Zia Formation, doesn't it? Similar environments produce similar rocks; but the Zia Formation appears to be significantly older than the Ojo Caliente Sandstone.
Nearby is yet another sedimentary dike.
Cerrito de la Baca, a sedimentary dike north of Black Mesa. 36 7.146N 106 7.594W
This formed along a northeast trending fault, typical of this area.
There are excellent exposures of Ojo Caliente Member in Abiquiu
Canyon. One such exposure is where the road turns to ascend El
Ojo Caliente Member in Abiquiu Canyon. 36
09.992N 106 21.486W
This should be admired from a distance, as the entire canyon is
private property and visitors are restricted to the roadway.
Further down the canyon, there is another exposure of the Ojo Caliente Member, with impressive cross bedding.
Ojo Caliente Member in Abiquiu Canyon. 36
10.485N 106 20.713W
This exposure just screams eolian. Judging from the
satellite photo, this may be a borrow pit, from which sand was
extracted for road building or construction work.
Ojo Caliente Member is also found in the west rim of the Valles
caldera, where it lies atop Abiquiu Formation and beneath the
oldest volcanic rocks. These are the westernmost exposures of the
Ojo Caliente Member. 35 53.578N 106 40.262W
You can see in the photograph how deeply eroded this small outcrop is. The exposure is poorly consolidated and uncemented, with loose sand everywhere. A more substantial exposure is found further west along State Road 126 towards Fenton Lake.
Ojo Caliente Member. 35
53.074N 106 41.019W
The bulk of this road cut is light sandy sediments of the Ojo
Caliente Member. The area at upper left exposes younger formations
that we will revisit later in this book.
Similar exposures are found in many places in the caldera west rim. Some are difficult to assign to particular members of the Santa Fe Group, and are mapped simply as undivided Tertiary sediments. The "undivided" means that the Tertiary beds have not been divided into separate formations on the map. An example crops out along Forest Road 376 to San Antonio Springs.
Undivided Tertiary sediments in caldera
west rim. 35
53.757N 106 39.860W
Such sediment beds make up a fair part of the west caldera wall,
and this section must be at least five million years old. By
appearance, it could be either Ojo Caliente Member or Zia
The study of otherwise nondescript formations, such as the Tesuque Formation, is made easier by the presence of marker beds. These are beds in the formation that stand out in some way, so that they can be traced over long distances. We've seen an example of a marker bed already: the thin limestone bed near the top of the Meseta Blanca Formation, near its gradation into the Glorieta Formation. Several marker beds are found in the Tesuque Formation.
Many of these are ash beds, such as the 285 Road Ash of the Skull Ridge Member.
285 Road Ash of the Skull Ridge Member.
The ash bed is the white layer at the top of the cliff. It’s a bed rich in volcanic ash, which gives it its light color. The volcanic ash contains potassium feldspar crystals which have been radiologically dated to 15.1 million years old. This is older than the Jemez volcanic field, and the precise source of the ash is unknown. This is not the most prominent ash bed in the Skull Ridge Member, but it is probably the most accessible, being right off a major highway.
Another kind of marker bed is found between the Tesuque Formation and overlying terrace gravels throughout the area west and north of Espanola. (Terrace gravels are deposits left by a river when it cuts a deeper channel; we'll have more to say about them later in the book.) This marker bed, which is particularly well exposed at Arroyo Largo, consists of quartzite gravel.
Quartzite gravel marker bed. 36
01.457N 106 05.527W
This bed is very distinctive, consisting of well rounded clasts
of quartzite typical of the Tusas Mountains to the north. It mark
a geologically brief interval sometime in the last two million
years when the ancestral Rio Grande and its tributaries in the
Espanola area carried a heavy load of gravel from the mountains to
the north. Similar beds are found as far north as Abiquiu and as
far south as Cochiti and may be correlated.
Relief map of the Jemez with Chamita Formation outcroppings highlighted in red.
The Chamita Formation overlies the Tesuque Formation, mostly in
the area west of the Rio Grande. Based on its fossils and
radioactive dating of flows and ash beds in the formation, it is
between 10 and 5 million years old, and its makeup is sufficiently
different from the Tesuque Formation for it to be classified as a
separate formation. One of the prominent ash beds in the formation
has been accurately dated to 6.8 million years in age, and both
this age and its composition appear to be identical with those of
the Peralta Tuff, which we will learn about later in the book. The
formation is quite variable in thickness, which geologists have
attributed to widespread erosion.
The type section is located south of Black Mesa and north of the confluence of the Rio Chama and Rio Grande. This area was the location of New Mexico's first Spanish capital, San Juan de los Caballeros.
Type area of Chamita Formation south of Black Mesa. 36 03.345N 106 03.684W
There are excellent exposures along Route 291/582.
Chamita Formation. 36 03.987N 106 05.152W
The exposures here are of the Cejita Member, which is mostly sediments eroded off the Sangre de Cristo and Picuris Mountains. Further south is a contact with the Valito Member, which is thought to be flood plain deposits of the ancestral Rio Grande.
Contact between Vallito Member and Cejita Member. 36 03.825N 106 05.202W
The contact is about halfway down the hillside, separating tan
Vallito Member beds at bottom from overlying pinkish Cejita Member
The contact between the Chamita Formation and the underlying Tesuque Formation is exposed in a landslide scar on the western tip of Black Mesa. (map).
Contact between Ojo Caliente Member,
Tesuque Formation; and Chamita Formation. 36.086N
The lighter tan beds making up the lower two-thirds of the mesa
are Ojo Caliente Member, Tesuque Formation. The uppermost part of
the mesa is Chamita Formation, and is noticeably darker and
redder. The contact is quite distinct.
Note the separation in elevation between the base of the Chamita
Formation on the north side of Black Mesa and the Chamita
Formation to the south. There are major faults, part of the Embudo
fault zone, running along hte south side of Black Mesa, and the
beds to the east are thrown down. The fault zone has a
considerable left slip component, and is regarded as a continental
transform fault, similar to the San Andreas Fault in
California but much shorter. This fault transfers displacement
along the west side of the Espanola Basin, where the western edge
of the Rio Grande RIft is active, to the Sangre de Cristo Fault on
the east side of the San Luis Basin, where the eastern side of the
Rio Grande Rift is active.
The Chamita Formation is divided into four members, the Hernandez, Cejita, Vallito, and Cuartales Members. The Vallito Member is sediments deposited by the ancestral Rio Grande, while the Hernandez Member was deposited by the ancestral Rio Chama and the Cejita Member by an unnamed tributary flowing from the Penasco area to the northeast. The Cejita Member is distinctive for its content of Paleozoic limestone, sandstone, and mudstone clasts, eroded from thick beds in the Sange de Cristo. The Cuertales Member is alluvial sediments from the Sangre de Cristo Mountains and is shared with the Tesuque Formation. (As if things were not already confusing enough.)
This sharing of a member is problematic. The Chamita Formation
was originally defined from locations where it sits atop the Ojo
Caliente Member of the Tesuque Formation, with a very definite
discontinuity separating the two. Further east, the Ojo Caliente
thins out and disappears, and the Chamita Formation beds become
indistinguishable from Tesuque Formation beds. This has led some
geologists to recommend that the Chamita Formation be abandoned,
and its members assigned to the Tesuque Formation. I hate to see
the Chamita Formation go; but, as with the tightened definition of
"major planet" that reduced Pluto to dwarf planet status, the
arguments seem to be on the side of discarding the problematic
There are small exposures of the Chamita Formation in White Rock Canyon, and better exposures in lower Pueblo Canyon along the road from Pojoaque to Los Alamos. The Hernandez Member is exposed furthest west, near the Totavi gas station and store.
Hernandez Member. 35 52.810N 106 10.249W
This exposure includes some river bed sediments. Geologists have studied these closely enough to determine that the river flowed to the southeast, which makes sense if this was the ancestral Rio Chama flowing towards its confluence with the ancestral Rio Grande.
Hernandez Member. 35 52.810N 106 10.249W
This exposure shows a thick bed of river gravel sitting atop
floodplain silt beds. In addition, the rightmost part of the
exposure has been displaced downwards by a local
Further down the canyon, just before the turn to Espanola, we encounter Cejita Member on top of Cuartales Member in the road cut.
Cejita and Cuartales Members. 35 53.090N 106 09.596W
The Cuartales Member is just visible over the top of the barrier to the right in the photograph. Looking over the barrier:
Cejita and Cuartales Members. 35 53.090N 106 09.596W
The contact is very obvious now. Closer still:
Cejita and Cuartales Members. 35 53.090N 106 09.596W
You can see that the Cuartales Member is reddish muddy sandstone
consistent with this being the furthest part of an alluvial fan
from the Sangre de Cristo Range. The pink color reflects its
origin while the fine grain reflects the considerable distance
from the source. Over it is a thin layer of sandy conglomerate and
then tan sandstone, consistent with a source further north.
Between the two is a thin dark layer, also sandy, though probably
with abundant lithic grains from a volcanic source. (I neglected
to bring a loupe; I'm kind of guessing here.) A layer like this
shouts that something happened here. My guess is that the
course of the ancestral Rio Grande abruptly shifted to the east
due to a large eruption in the Jemez area.
There are extensive exposures of the Chamita Formation west of the road from Totavi to Espanola. Some of these are well exposed in road cuts just north of the turnoff at Totavi.
Cuartales Member. 35 53.351N 106 09.059W
A little further north is an exposure of Vallito Member.
Vallito Member. 35 53.492N 106 08.892W
You're looking here at a cross section of a river channel, full
of silt and gravel.
Mountain, west of the road from Otowi to Espanola, the
Chamita Formation forms the lower slopes.
The lower, pinkish part of the mesa is Chamita Formation, forming the zone below the prominent light ash-rich bed at the base of the Puye Formation, which forms most of the upper part of the mesa. There is a small cap of the Tshirege Member, Bandelier Tuff, at the very top of the mountain.
The beds at the base of the mountain have long been identified as
Ojo Caliente Member of the Tesuque Formation, but more recent work
suggests these are Vallito Member eolian beds that have been
thrown down by a fault at the base of the mountain.
North of this area, at Round
Mountain Quarry, paleontologists have discovered fossils of
short-faced, heavy-jawed ancestors of dogs, as well as horses,
pronghorn, and llamas.
The contact of the Chamita Formation with the overlying Puye
Formation is well exposed in a road cut along 30 Mile Road west of
Contact of Chamita and Puye Formations. 36 00.726N 106 08.383W
The fine pink sediments of the Chamita Formation contrast strongly with the coarse gray sediments of the overlying Puye Formation. This portion of the Puye Formation appears to be a lahar, or volcanic mud flow, which scoured clean the top part of the Chamita Formation to make this very sharp contact. We'll say much more about lahars in Chapter 8.
The Chamita Formation, like much of the Santa Fe Group to which it belongs, is often poorly cemented. However, some of the beds here are quite well cemented, and here's a sample.
Chamita Formation. 36 00.726N 106 08.383W
Under the loupe, the sandstone is revealed as medium-grained and
moderately sorted with well-rounded grains, mostly quartz but with
numerous lithic grains. The sandstone is clast-supported, though
with ample light matrix in the pores -- a lithic arenite. In other
words, a somewhat muddy and not especially mature sandstone, as
one woudl expect in rift fill sediments.
The Chamita Formation reaches its greatest thickness further north, where it is exposed beneath a cap of Lobato basalt southwest of Black Mesa.
Lobato Mesa Formation basalt flows atop a mesa of Chamita Formation northwest of Hernandez. 36 05.111N 106 08.318W
Geologists who have examined the contact between the basalt and the underlying Chamita sediments have concluded that the surface of the Chamita beds was already significantly eroded when the basalt was erupted around 10 million years ago. Regional uplift must have already become significant enough by then that deposition of sediments along this part of the Rift had reversed and net erosion of sediments had set in.
The Chamita Formation is the principal rift fill of a series of
three grabens within the Rio Grande Rift. These are the Velarde
graben, south of Black Mesa; the Santa Clara graben, north of Los
Alamos; and the Pajarito graben, underlying much of the Pajarito
Plateau. These remain the most active portions of the rift, and
there is evidence for at least 5% extension across the Pajarito
Plateau since the Valles event, 1.25 million years ago.
The formations we have just examined are all included in the
lower Santa Fe Group. A number of younger formations assigned to
the Santa Fe Group represent beds deposited as the Rio Grande
became established as a through-running river. We'll examine these
upper Santa Fe Group formations later in the book.
Next page: And, finally, we get to the birth of the Jemez volcanic field
Copyright ©2014 Kent G. Budge. All rights reserved.