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, from
which the Western Interior Seaway had already retreated.
In this chapter, we will look at the Jemez area during the early Tertiary Period.
We introduced the previous chapter with a discussion of The
Great Dying, the massive extinction at the end of the Paleozoic
Era. The Mesozoic Era likewise ends, and the Cenozoic Era begins,
with a mass extinction, albeit one less catastrophic. However,
what it lacks in catastrophe is 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 Jemez area,
because the Western Interior Seaway had already retreated from the
area and no new rock beds were being laid down.
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 are 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 Man.
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 prospectors, 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.
Although the period of time from 65 million to about 2.6 million
years ago is still widely called the Tertiary in geological
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 most important event for New Mexico was the Laramide Orogeny. This began in the late Cretaceous but reached its peak in the Paleogene. During this interval of time, oceanic crust of the Farallon Plate was being subducted under the west coast of North America at an unusually shallow angle, less than 25 degrees, which put powerful compression forces on the overlying crust. This resulted in the formation of the early Rocky Mountains (not to be confused with the Ancestral Rocky Mountains mentioned in earlier sections.) The crust was deeply faulted and buckled, throwing up high mountains separated by deep basins. Most of the mountain ranges we see in New Mexico today formed along fault lines established during the Laramide Orogeny.
The shallow subduction also produced volcanism far inland from
the continental margin. This began sweeping west around 70 million
years ago, producing volcanic fields in what is now the
Yellowstone area (the Absaroka volcanics), in southwestern
Colorado (the San Juan volcanic field) and in southwestern New
Mexico (the Mogollon-Datil volcanic field). Starting 30 million
years ago, volcanic activity retreated back to the west as the
Farallon plate began to sink deeper into the mantle.
I've mentioned faults a few times already, but it's time to look at them in greater detail.
We learned in Chapter 1 that the earth's upper crust is brittle. This means that, 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.
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 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, but were more common during
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 quite uncommon in New Mexico, but include such
famous faults elsewhere as the San Andreas Fault in California.
However, some faults in the Jemez area are displaced both
laterally and vertically and are called 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 mind.
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. 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 fault.
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 Formatino 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.
Note that the lower part of the El Rito Formation is full of clasts embedded in the sandstone. These are mostly quartzite, of the type found in the Tusas Mountains to the north. This suggests that there was already a Tusas Range 50 million years ago, part of the San Luis Uplift, and geologists believe the El Rito Formation is composed of alluvial fans eroded off the early Tusas Mountains.
Here is a sample of the El Rito Formation.
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.
El Rito Formation forms the south rim of Arroyo del Cobre, just north of this area.
El Rito Formation on south rim of Arroyo
del Cobre. 36
14.754N 106 22.123W
The El Rito Formation is somewhat difficult to date, having very few fossils, but is probably from the Eocene — in the ballpark of 40 million years old. This is older than the Rio Grande Rift, and the formation was probably laid down in valleys between highlands of the Laramide orogeny. Here and elsewhere, It is characterized by the presence of tiny flakes of muscovite mica. Mica is not usually abundant in sandstones and so serves as a distinguishing feature of the El Rito Formation.
The El Rito Formation is not typically well exposed in the Jemez, but there are some decent exposures north of the village of Canones.
El Rito Formation. Looking north from 36 11.271N 106 26.738W
El Rito Formation. 36 13.920N 106 23.018W
This is a more typical exposure than that along the Canones fault
shown earlier. The formation is interbedded red mudstone and
sandstone, tending to form slopes. 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 13.920N 106 23.018W
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.
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. Up slope, 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 from about this same time period in the Jemez
area 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 extensive outcropping
along I-25 where it 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 northwest.
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 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. 35 45.441N 106 22.135W
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.
Again, the question arises whether the El Rito Formation and the
Galisteo Formation are really distinct formations. It is likely
that they were deposited on opposite ends of a river valley whose
middle section is now buried under the volcanic rocks of the
Around 30 million years ago there was a major shift in the tectonics of the area, as the compression produced by the subducting Farallon Plate along the west coast was replaced by tension and uplift as the western United States moved onto the northern East Pacific Rise, a mid-ocean ridge. Precisely what began to take place under western North American 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 begun to be 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 the western United
States produced a series of north-trending ranges separated by
valleys, 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 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.
Rifting produces the right conditions for rock in the upper
mantle to melt and the resulting magma to rise to the surface. It
is no surprise that volcanic activity picked up 30 million years
ago, when the Rift began to form. There were some truly
spectacular eruptions in the area west of the Rift near
Socorro, where the Rift forks away from the southern margin
of the Colorado Plateau. These are distant enough from the Jemez
that they will not come into our story further. The Jemez are part
of another area of heavy volcanism along the Rift, where it
intersects another, more mysterious feature, the Jemez Lineament.
We discussed this feature in the second chapter of this
book. The key part of that discussion is that the Jemez
Lineament seems to correspond with the ancient suture where the
Yavapai and Mazatzal Precambrian provinces were fused together.
The mantle here seems to be fertile for production of magma.
The Jemez was not the site of the first volcanism in northern New
Mexico along the Rift. That honor probably belongs to the Ortiz
Mountains south of the Jemez, which are between 30 and 40
million years old. The Ortiz Mountains are part of the Ortiz
Porphyritic Belt, a chain of volcanoes extending from South
Mountain north to La
Cienega, although the vents furthest to the north are quite
small. 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 less than 5% 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 elsewhere, named (logically enough) the Espinaso Formation. These are though to have come from the pulse of volcanism that produced the Ortiz Mountains.
After the original eruptions died down, the crystallizing
monzonite sweated fluids rich in incompatible elements,
which are chemical elements that do not easily crystallize as part
of the usual silicate minerals that form from magma. Gold is an
incompatible element, and gold is still mined
in the Ortiz Mountains.
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 talking 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 Espinaso Formation is slightly younger and may represent 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. Here's a likely fragment of latite.
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
hornblende. It closely resembles the previous sample, other than
being finer grained. Latite is, in fact, the extrusive counterpart
Here's an outcropping of Espinaso Formation breccia conglomerate, 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 right 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.
About 25 million years ago, renewed faulting in this area
penetrated deeply enough to permit very silica-poor magma from the
upper mantle to reach the surface as a series of low-volume
eruptions. This ultramafic magma was also relatively 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 goes into the magma, the magma is
greatly enriched in alkali oxides.
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.
Seguro is the most accessible eruptive center for the
basanite flows of the La Cieneguilla Basanite. The approaches
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 panorama and disappearing into the horizon in
the second frame is the canyon of the Santa Fe River. The hills
in the third and fourth frame are underlain by Ortiz monzonite,
which rises like islands out of the much younger Cerros del Rio
Basalt 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 on the left side of the fourth frame is Tetilla
Peak, New Mexico's etymological counterpart to Grand Teton
in Wyoming, and on
the far size of the Santa Fe River canyon in the fourth and
fifth frames you see exposures of highly weathered monzonite.
The seventh and eighth frames show the Sangre
de Cristo mountains east of Santa Fe, while the ninth
through eleventh frames show the village of La Cienega. The
cluster of hills in the middle distance are the Ortiz Mountains,
of which the hills around La Cienega are geologically the northern
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
added aluminum. Feldspathoids generally crystallize only from
magma that is quite low in silica.
Volcanic rock with a significant feldspathoid content is described as silica-undersaturated. If the rock contains feldspar but little feldspathoid or quartz, it is described as silica-saturated. If the rock contains substantial quartz, it is described as silica-oversaturated. In other words, silica saturation is defined in terms of whether there is enough silica to maximize the amount of feldspar in the rock. Feldspathoid is almost never found in the same rock as quartz, since any excess silica that might have formed quartz should have gone into converting feldspathoid into feldspar.
Volcanic rocks that are silica-undersaturated 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. Looking at
Bowen's reaction sequence, we see that quartz, muscovite, and
alkaline feldspars are the first minerals to melt. These are rich
in silica and alkali metals, and so is the magma formed from them.
Only if melting proceeds further will the magma be 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 in excess of 0.1
g/cc less dense than tholeiitic magmas.
The rocks of the Jemez volcanic field straddle the 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 deplete
the underlying mantle, which has also experienced a high degree of
Almost contemporaneous with the La Cienega eruptions was 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 at least one voluminous pyroclastic flow. This is known as the Amalia Tuff, and it 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. We 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. This formation was first described around the
village of Abiquiu,
where it forms distinctive beds of white rock that inspired the
artist Georgia O'Keefe. The Abiquiu Formation has traditionally
been divided into 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 be essentially the same formation
as the Ritito Conglomerate that 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 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 north of Abiquiu.
The canyon is owned by Dar al Islam Mosque, but is open to the
Abiquiu Formation at Plaza Blanca Canyon. 36 14.048N 106 18.180W
The darker bed in the cliffs in third and fourth frames contains
numerous clasts and may mark an ancient stream bed.
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 round 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.
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.
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. 36 07.850N 106 30.746W
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.
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.
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 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.
The start of the Neogene (upper Tertiary) has been put at 23 million years ago. By this time the continents looked very much like they do 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 most significant geological change was that 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.
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 weakness from the Precambrian suture 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 two systems of faults, the Pajarito Fault Zone and the Jemez Fault Zone. Further north, the Pajarito Fault Zone turns east to join the Embudo Fault Zone, while the Jemez Fault Zone 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 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 Bandelier Tuff was erupted, 1.2 million years ago. We'll have much more to say about the Bandelier Tuff later, but for now, we'll focus on the fault. This 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 a zone where the Rio Grand Rift is displaced to the west. 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 crossing the boundary of the third and fourth
frame 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. 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 Bandelier Tuff.
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 town of Canones. Deformation along this fault zone has been traced as far west as Coyote and extends east to where it merges with the Embudo Fault Zone. The main strand of the Canones Fault forms the escarpment we've seen a couple of times already:
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 at least 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 the Pajarito Uplift dropped to become the Espanola Basin, it 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.
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 10 to 20 million years ago. It is capped with younger sediments that are better consolidated and, as is usually the case when softer beds underlie harder 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 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
The Zia Formation disappears in the area west of Cochiti Pueblo, buried under younger deposits of the Cochiti Formation.
The Tesuque Formation is the main 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
overlaps the Pojoaque 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
earliest Ojo Caliente beds with the oldest Pojoaque and Chama-El
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 a wide variety of different clasts. Thus,
geological maps distinguish, for example, the Pojoaque Member of
Lithosome A from the Pojoaque Member of Lithosome B.
The Santa Fe Group has proven to be a good fossil hunting ground. Prehistoric mammals ranging from dire wolves (extinct relatives of gray wolves) and horses to camels and primitive hippopotamuses have been found in the various members of the Tesuque Formation, which forms much of the basis for its division into members.
The Nambe Formation 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. 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 Tesuque Formation is prominent in the Espanola Valley east of
the Rio Grande, where it forms the spectacular hogback ridges of Los
Los Barrancos. A closer view, looking north northwest from near 35 53.44N 106 02.660W
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 (left in top photograph) 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.
One of the best exposures of the Tesuque Formation is found in a
road cut along Highway 285 southeast of Espanola.
Road cut in Pojoaque Member, Lithosome
A, Tesuque Formation. 35.984N
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.
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 enough for more
specific mention. While the other members of the Tesuque Formation
were deposited primarily by the ancestral Rio Grande and its
various tributaries, the Ojo Caliente is an eolian sandstone,
deposited by wind as dunes. It 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.
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 is 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.
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 are 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 Tertiary sediments. 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.
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.
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. The formation is quite variable in thickness,
which geologists have attributed to widespread erosion.
The type section 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 section of Chamita Formation south
of Black Mesa. 36
03.345N 106 03.684W
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.
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 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.)
There are small exposures 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.
The main exposures of the Chamita Formation are 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.
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.
The 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 youngest rift fill formation in the Jemez area is the Ancha Formation, which is exposed mostly in the area west and south of Santa Fe. Radioisotope dates from zircon grains in pumice of the Ancha Formation give it an age of around 1-3 million years. This is a time period properly belonging to a later chapter of the book, but rather than break up our discussion of the Santa Fe Group, I'm going to jump ahead of our story and discuss this formation before leaving this chapter.
Ancha Formation rests directly on much older Galisteo Formation
beds near the village of La Cienega.
Contact of Ancha and Galisteo
The Galisteo Formation is the red beds at the bottom, while the
light tan beds on top are Ancha Formation. The contact here is a
discontinuity of at least 25 million years in the geologic recod.
The Ancha Formation thins out and disappears to the north, and
becomes fairly substantial further south. Northwest of Santa Fe,
the Ancha forms the top of the higher hills and ridges, whose
lower slopes are Tesuque Formation. Here's the contact visible in
a road cut.
Contact between Ancha Formation and
35 42.239N 106 04.807W
The Tesuque Formation is the reddish beds at bottom, full of bits of red granitic rock. This has to be Lithosome A, though I can’t guess which member; the geological map for this area is not specific.The tan beds on top are Ancha Formation.
Next page: And, finally, we get to the birth of the Jemez volcanic field
Copyright ©2014 Kent G. Budge. All rights reserved.