Geology of the Jemez Area, Chapter 5: The Early Tertiary

The previous chapter may be found here.

Impact event
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 a burst of mountain building known as the Laramide Orogeny.

In this chapter, we will look at the Jemez area during the early Tertiary Period.

  1. Life on Earth Takes Another Hammering
  2. The Ojo Alamo Formation
  3. The Paleogene in the Jemez
    1. Faults
    2. The Paleocene Epoch
      1. The Nacimiento Formation
    3. The Eocene Epoch
      1. The San Jose Formation
      2. El Rito Formation
      3. Galisteo Formation
    4. The Oligocene Epoch
      1. Ortiz monzonite
      2. Espinaso Formation
  4. Birth of the Rio Grande RIft
    1. La Cieneguilla Basanite
    2. Feldspathoids and silica saturation
    3. Ritito and Abiquiu Formations
      1. The Gilman Conglomerate
      2. Dikes
  5. The beginning of the Neogene and the Santa Fe Group
    1. "Every valley shall be exalted, and every mountain and hill made low"
    2. The Lower Santa Fe Group
      1. The Zia Formation
      2. The Tesuque Formation
        1. The Ojo Caliente Sandstone
        2. Marker Beds
      3. The Chamita Formation

Life on Earth Takes Another Hammering

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 destructiveness 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 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 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 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.

Meloche Mesa
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
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.

The Ojo Alamo Formation

Digital relief map of Ojo Alamo Formation exposures in
        the Jemez Mountains
Relief map of the Jemez with Ojo Alamo Formation outcroppings highlighted in red.

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 discontinuities.

The formation is resistant to erosion and forms the cap of Portales Mesa, as we saw in the previous chapter. It is also exposed as a hogback in the Regina area in the far northwest of the Jemez region.

Ojo Alamo Formation in hogback
Ojo Alamo Formation in hogback. Looking southeast from 36 14.239N 106 55.506W

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.)

The Paleogene in the Jemez


Digital relief map of Paleogene exposures in the Jemez
      Mountains
Relief map of the Jemez with Paleogene outcroppings highlighted in red.

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 east 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. However, hot aesthenosphere rising to take the place of the Farallon Plate continued to produce regional uplift to the present day.

Faults

Digital relief map of major faults in the Jemez
        Mountains
Relief map of the Jemez with major faults highlighted in red.

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.

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.


Fault diagram

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.

Normal and reverse faults

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 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 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 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

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.

Canones Fault

Fault race of main strand of Canones Fault. Looking soutwest from near 36 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.

Canones Fault

Trace of main strand of Canones Fault Zone. 36 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. 

Discontinuity
        between Morrison Formation and El Rito Formation
A geological discontinuity between Jurassic and Tertiary beds. 36 13.920N 106 23.018W

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.

Canones fault history

Here is a close up of the point of contact, where it comes close to the road level.

Contact between
        Morrison Formation and El Rito Formation
Contact between El Rito Formation and Morrison Formation. 36 13.920N 106 23.018W

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.

The Paleocene Epoch

Digital relief map of Paleocene exposures in the Jemez
        Mountains
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 Sierra Nacimiento and the San Luis and Pajarito Uplifts to the east. A broad river valley lay between the Sierra Nacimiento and the Pajarito Uplift, where the Jemez Mountains are located today. 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.

The Nacimiento Formation

Digital relief map of Nacimiento Formation exposures in
        the Jemez Mountains
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 closest kin.

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.

Mesa de Cuba
Mesa de Cuba. Looking northwest from 35 58.057N 106 59.514W

The mesa is underlain by San Jose Formation sandstones and conglomerates forming a cap over softer shales of the Nacimiento Formation. This sequence may be examined more closely along the main highway just northwest of Cuba.

Just northwest of the town is a nice outcrop of San Jose Formation over Nacimiento Formation.

San Jose and
        Nacimiento Formations
Nacimiento Formation and San Jose Formation northwest of Cuba. 36 02.076N 106 58.076W

The San Jose Formation is the sandstone cliffs, while the slope-forming sedimentary beds beneath the cliffs are Nacimiento Formation.

The Eocene Epoch

Digital relief map of Eocene exposures in the Jemez
        Mountains
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 of the Brazos-Sangre de Cristo geanticline to the north and east. 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.

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 San Jose Formation

Digital relief map of San Jose Formation exposures in
        the Jemez Mountains
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:

San Jose formation
San Jose Formation. 36 09.473N 106 58.259W

I guess I can now say that I know the way to San Jose. I don’t know that it gave me peace of mind, precisely, but I was having a pretty good day, notwithstanding the miserable weather.

Another gorgeous road cut exposure. This exposure is heavily cross-bedded and strongly indurated.


Cross beds of San Jose
        Formation
San Jose Formation. 36 04.289N 106 59.026W

Sample of San
          Jose Formation

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 a dinosaur, I doubt many paleontologists would argue with you.

El Rito Formation

Digital relief map of early Eocene exposures in the
        Jemez Mountains
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. 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.

El Rito Formation
El Rito Formation. 36 13.920N 106 23.018W

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

El Rito Formation on south rim of Arroyo del Cobre. 36 14.754N 106 22.123W

There are also some good exposures north of the village of Canones.

El
          Rito Formation
El Rito Formation. Looking north from 36 11.271N 106 26.738W

A particularly good exposure is found in a road cut along the old road into Canones.

El Rito
          Formation
El Rito Formation. 36 13.920N 106 23.018W

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
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. 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 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
El Rito Formation. 36 08.616N 106 31.070W

Similar poor exposures continue west and south as far as upper Coyote Canyon.

Galisteo Formation

Another formation in the Jemez area from about this same time period 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 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 northwest.

Gallisteo Formation on
        I-25
Galisteo Formation. 35 31.031N 106 13.552W

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.

Galisteo Formation at Garden of hte 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 Cerillos Hills. We'll have more to say about the Cerillos 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.

Gallisteo Formation east of St. Peter's Dome
Galisteo Formation from St. Peter's Dome. 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 away.

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 originally included older beds that are extensively exposed south of the villages of Cerillos and Galisteo 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
Diamond Tail Formation south of Los Cerillos. 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.

The Galisteo and Diamond Tail Formations contains numerous petrified logs in the Cerillos 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. 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 Cerillos 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.

The Oligocene Epoch

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 the Ortiz Mountains southeast of the Jemez. The Ortiz Mountains are part of the Ortiz Porphyritic Belt, a chain of volcanoes extending from South 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 in the Galisteo area, named (logically enough) the Espinaso Formation. These are though to have come from the pulse of volcanism that produced the Ortiz Mountains.

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.

Ortiz monzonite

Digital relief map of Ortiz Belt and Cieneguilla
        Basanite exposures in the Jemez Mountains
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

Monzonite outcrop. 35 34.337N 106 07.969W

I've placed my talking stick for scale on the photograph below.

Foliated rocks
Monzonite outcrop. 35 34.325N 106 08.062W

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.

Monzonite
Monzonite. 35 34.325N 106 08.062W

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 visible.

This outcropping, located in the middle of La Cienega, is not mapped, but could be either Ortiz monzonite or Espinaso Formation.

Ortiz monzonite?

Ortiz Monzonite? 35.562N 106.126W

Sample of Ortiz monzonite?

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 Cerillos Hills and the Ortiz Mountains.

Cerillos
          Hills

Cerillos Hills. Looking north from 35 25.104N 106 07.828W

The Cerillos 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 Cerillos Hills is turquoise.

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 Cerillos 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 Cerillos Hills turquoise. Mount Chalchihuitl in the Cerillos Hills was the largest open pit mine of any kind in North American when the Spanish arrived in 1535. The native Americans 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. These laws derive from the economic theories of the Scottish Enlightenment, which influenced American legislators. Under these theories, a person established a property right to previously unclaimed land by mixing his labor with the land, such as by clearing and tilling it for agriculture or excavating it for valuable mineral resources. This philosophy inspired both the Homestead Act (which allowed settlers to take possession of public land they successfully cultivated for long enough) and mining claim law. In most American jurisdictions, you could “stake a claim” to public land by literally marking off an area with wooden stakes and filing a claim asserting that there were valuable mineral ores on 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 (thus mixing your labor with the land). Eventually, if you developed the claim enough, you could purchase title to the land from the government at a bargain price.

The claim boom in the Cerillos 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 actual valuable minerals in the Cerillos 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 Cerillos Hills.

Picture Rock

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.

Picture Rock monzonite

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 Cerillos.

The Devil's
          Throne

The Devil's Throne. Looking west from 35 26.479N 106 08.011W

This is composed of porphyritic hornblende latite, the hypabyssal equivalent of monzonite.

The Ortiz Mountains are another cluster of laccoliths of the Ortiz monzonite.

Ortiz
          mountains

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 eroded out of bedrock accumulates deep in stream beds because of its great density. More recently, the bedrock ores themselves have been exploited.

Cunningham
          Hill

Ortiz Mountains. 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 to extract the gold. 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.

Espinaso Formation

Digital relief map of Espinaso Formation exposures in
        the Jemez Mountains
Relief map of the Jemez with Espinaso Formation outcroppings highlighted in red.

The Espinaso Formation is slightly younger than the Ortiz Monzonite 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.

  Monzonite
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 of monzonite.

Here's an outcropping of Espinaso Formation breccia conglomerate, formed mostly of broken fragments of andesite.

Espinaso Formation

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 Formation dike

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

Hornblende trachyandesite.. 35.564N 106.127W

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.

Xenolith

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 Cerillos.

Espinaso Formation

Espinaso Formation. 35 28.519N 106 04,199W

Espinaso Formation

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.

Birth of the Rio Grande RIft


Rio grande
        rift
Wikimedia. Click to enlarge

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, 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.

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.

Digital relief map of pre-Neogene exposures in the
        Jemez Mountains
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 a transform zone in which the Rift shifts abruptly to the east as one moves north from the Albuquerque Basin to the Espanola Basin.

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.

La Cieneguilla Basanite

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 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.

Cerro 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.

 Basanite clasts
Basanite gravel. 35 34.310N 106 08.067W

The eruption center looms above us.

Cerro Seguro

The view from the top shows many features associated with the Rio Grande Rift.

Panorama
Panorama from atop Cerro Seguro. 35 34.228N 106 08.263W

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 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 Ortiz Mountains, of which the monzonite hills around La Cienega are geologically the northern outliers.

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.

Agglomerate basanite
Agglomerated basanite from atop Cerro Seguro. 35 34.228N 106 08.263W

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.

Weathered basanite
Weathered basanite atop Cerro Seguro. 35 34.274N 106 08.167W

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.

Massive basanite
Weathered basanite from Cerro Seguro. 35 34.274N 106 08.167W

Feldspathoids and silica saturation

Nepheline, Na3KAl4Si4O16, 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 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.

Ritito and Abiquiu Formations

Digital relief map of Abiquiu and Ritito Formation
        exposures in the Jemez Mountains
Relief map of the Jemez with Abiquiu and Ritito Formation outcroppings highlighted in red.

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.

Amalia Tuff

Amalia Tuff
Amalia Tuff. 36 36.652N 105 59.000W

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
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 years old.

Sierra Negra
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 years.

The Abiquiu Formation is particularly well exposed in Plaza Blanca Canyon north of Abiquiu. 

Plaza Blanca Canyon
Abiquiu Formation at Plaza Blanca Canyon. 36 14.233N 106 18.366W

The canyon is owned by Dar al Islam Mosque, but is open to the public.

Plaza Blanca
          Canyon
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. I'll have more to say about debris flows later in the book.

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 road.

Cerrito Blanco
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
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 north.

Superb exposures of the Ritito Conglomerate are found along Red Wash Canyon.

Ritito
          Conglomerate along Red Wash Canyon

Ritito Conglomerate along Red Wash Canyon. 36 14.354N 106 22.532W

The area also features a slot canyon.

Slot
          canyon in Ritito Conglomeratemay


Slot
          canyon in Ritito Conglomerate

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

Canones Mesa
Canones Mesa

and the Ritito Conglomerate is beautifully exposed north of the village of Canones, where it lies atop El Rito Formation.

Ritito Conglomerate at Canones
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.

Cliffs of Abiquiu
        Formation
Abiquiu Formation. 36 08.678N 106 30.805W

Nearby, the Ritito Conglomerate is well exposed in the road cut.

Ritito Formation in
          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 Formation:

Pedernal
          Chert
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
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
Pedernal Chert. 36 08.486N 106 30.622W

Pedernal
          Chert
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.

Abiquiu
        Formation
More Abiquiu Formation. 36 07.850N 106 30.746W

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.

Abiquiu Formation in Valle San Antonio

Ritito Formation in northwest caldera rim. 35 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.

Abiquiu Formation
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.

Abiquiu Formation
        in landslide

Abiquiu Formation
Abiquiu Formation in the southern Jemez. 35 46.262N 106 36.697W

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
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
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
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.

The Gilman Conglomerate

Also present in the southwestern Jemez is the Gilman Conglomerate. The type section is prominent in the cliffs west of the former logging community of Gilman.

Gilman
          Conglomerate
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, 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 significantly 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 at 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.

Dikes

The Ritito Formation is intruded by the Cerrito de la Ventana dike system. One dike is crossed by the highway west of Abiquiu.

Basalt dike

Basalt dike

Basalt dike
Basalt dike of the Cerrito de la Ventana dike system. 36 13.046N 106 21.142W 

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.

Basalt dike
        showing baked zone in country rock
Basalt dike with baked zone in country rock

The basalt is a dense black rock with occasional veins of almost pure quartz that indicate the dike has undergone some hydrothermal alteration.

Close up of
        basalt

Quartz
        veins
Close up of basalt with quartz veins

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.

Cerritos
          de la Ventana dike west of Plaza Blanca
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 system

Small dike of Cerrito de la Ventana system. 35 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 beginning of the Neogene and the Santa Fe Group


Digital relief map of Santa Fe Group exposures in the
      Jemez Mountains
Relief map of the Jemez with Santa Fe Group outcroppings highlighted in red.

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 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 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 surface
          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 and has not been active in the recent past.

"Every valley shall be exalted, and every mountain and hill made low"

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. 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:

Pajarito fault
Pajarito and Bajada faults. Downthrown sides marked with red dots.

The Pajarito Escarpment is quite obvious as one approaches it from the east along State Road 4.

Pajarito Escarpment at
          ascent of 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 turn
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 escarpment.

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.

Rabbit Hill area
Pajarito Escarpment between Sierra de los Valles and San Miguel Mountains. The peak is Rabbit Hill.
 Panorama of San Miguel
        Mountains from the east
Panorama of Frijoles Canyon from Frijoles Overlook. 35 46.385N 106 15.714W

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
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 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 turn
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 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 showing Permian and Triassic beds
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.

Displacement of Jurassic beds on 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 years.

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.

The Lower Santa Fe Group

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.

The Zia Formation

Digital relief map of Zia Formation exposures in the
        Jemez Mountains
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 San Diego.

Zia Formation?
Zia Formation. Looking west from near 35 37.640N 106 43.466W

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. It is capped with younger gravels cemented with travertine 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 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 Formatio
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
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.

The Zia Formation disappears in the area west of Cochiti Pueblo, buried under younger deposits of the Cochiti Formation.

The Tesuque Formation

Digital relief map of Tesuque Formation exposures in
        the Jemez Mountains
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 Member.

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 earliest Ojo Caliente beds with the oldest 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 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. 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.

Camel bone
          fragment
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 (6000').

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
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
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.

Red Wall
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
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 years ago.

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
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 (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
Pojoaque Member, Lithosome A. 35 52.906N 106 06.958W

This is a thick siltstone interbedded with thin gravel layers.

Cejita Member,
          Lithosome B
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.

Excellent exposure of Pojoaque Member

Road cut in Pojoaque Member, Lithosome A, Tesuque Formation. 35.984N 106.053W

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
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.

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.

Tuff ring
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.

The Ojo Caliente Sandstone

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. 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 old.

The formation is exposed throughout the Medanales area.

Ojo Caliente
        Sandstone ridge
Ridge of Ojo Calienta Sandstone of the Tesuque Formation. 36 11.287N 106 13.898W

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 sedimentary dike.

Cerrito Blanco
Cerrito Blanco. 36 12.867N 106 19.583W

Cerrito
          Blanco
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.

Santa Fe sediments
          east of Lobato Mesa

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.

Hoodoos in Ojo
        Caliente beneath Black Mesa
Hoodoos in Ojo Caliente. 36 9.221N 106 5.701W

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.

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 Alto.

Ojo Caliente Member in Abiquiu Canyon

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

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 in Valles west rim
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

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 Tertiary sediments. An example crops out along Forest Road 376 to San Antonio Springs.

Undivided Tertiary sediments

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. It could be either Ojo Caliente Member or Zia Formation.

Marker Beds

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

285 Road Ash of the Skull Ridge Member. 35.937N 106.022W

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

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

Digital relief map of Chamita Formation exposures in
        the Jemez Mountains
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 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.

Chamita Formation in its type section

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 Tesuque and Chamita Formations

Contact between Ojo Caliente Member, Tesuque Formation; and Chamita Formation. 36.086N 106.140W

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 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.)

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
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
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 northwest-trending fault.

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
          Cuertales Members
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
          Cuertales Members
Cejita and Cuartales Members. 35 53.090N 106 09.596W

The contact is very obvious now. Closer still:

Cejita
          and Cuertales Members
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.

Cuertales Member
Cuartales Member. 35 53.351N 106 09.059W

A little further north is an exposure of Vallito Member.

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.

At Battleship Mountain, west of the road from Otowi to Espanola, the Chamita Formation forms the lower slopes.

Battleship
        Mountain
Battleship Mountain. 35 55.374N 106 08.843W

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 Espanola.

Chamita - Puye
          contact
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
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 atop a 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.