Geology of the Jemez Area, Chapter 10: Supervolcano!

The previous chapter may be found here.

Pyroclastic flows at Mount Mayon

Pyroclastic flow at Mount Mayon. Such flows occurred on a vastly greater scale in the Valles and Toledo events.

1.85 million years ago, northern New Mexico was beginning to take the shape we see today. The Sierra Nacimiento and Sangre de Cristo Mountains were in place and Sandia Crest was rising to the south. The Rio Grande and Rio Chama flowed just a little to the west of their modern courses, though White Rock Canyon was shallower and narrower. Regional uplift had led to widespread erosion, producing escarpments around La Grulla Plateau, El Alto, the Cerros del Rio, and Santa Ana Mesa and turning the Espanola Basin into a badlands.

The climate had cooled as the elevation increased and as the Quaternary ice age settled in. Most likely it was cooler then than today: We are living in an interglacial, a relatively warm period between glaciations. The palm groves of the lower elevations were long gone, replaced by pinon scrub at lower elevations, ponderosa forests at higher elevations and spruce, aspen, and fir on the peaks. Many of the animals would seem familiar, but they included large mammals such as mammoths, mastodons, horses, camels, and saber-tooth tigers, which would disappear with the arrival of humans, who most likely hunted them to extinction. But that was still well in the future; Homo erectus had only just emerged as a distinct species, was just beginning to master the use of fire, and was still largely confined to Africa.

Some things were different. Between the fault escarpment at the feet of the Sierra de los Valles and the White Rock basalt ridge lay a shallow valley. Remnants of young dacite domes may have exposed in the valley bottom, and a large cinder cone dominated the southern end of the valley. West of the Sierra de los Valles was a series of north-trending ridges and valleys, ending to the west at the broad valley of the ancestral Rio Guadaupe and Rio de las Vacas in the lee of the Sierra Nacimiento.

This green land, unspoiled by man, was about to be devastated by nature. Repeated injections of magma into the crust beneath the Jemez, going back over ten million years, had created an enormous magma chamber; and this was now primed to explode.

In this chapter, we will look at the evolution of the Jemez volcanic field into a supervolcano, and the catastrophic eruptions that produced the Bandelier Tuff.

  1. Supervolcanoes
    1. A magma chamber forms
    2. Breakout
  2. Early Leakage
  3. The Toledo Event
    1. The Guaje Pumice
    2. Otowi ignimbrites
    3. The Toledo caldera collapses
  4. The Cerro Toledo Interval
    1. Ring fracture domes
    2. Obsidian
    3. Felsic phreatomagmatism
    4. Cerro Toledo Interval pumice and sediment beds
    5. The mysterious Toledo Embayment
  5. The Valles Event
    1. The Tsankawi Pumice
    2. Surge beds
    3. Ignimbrites
      1. Cooling units
  6. The Valles Caldera
    1. Caldera fill
      1. Megabreccias
      2. Debris flows

Supervolcanoes

In Chapter 7, we discussed the various kinds of rock formations that can be produced by volcanic ash generated from a high-silica magma. These included ignimbrites, often called tuffs, which are produced by pyroclastic flows.

High-silica magma that reaches the surface disintegrates into a mixture of hot gas and tiny shards of volcanic glass.  If this mixture is not buoyant enough to rise into the atmosphere as an eruption column, then it flows across the surface as a pyroclastic flow. The pyroclastic flow is sometimes described as a nuée ardente (from the French for "glowing cloud"), since it glows bright red when seen at night.  Pyroclastic flows can travel for many kilometers, destroying everything in their path, before settling to the surface and cooling to form an ignimbrite.

Pyroclastic flows are relatively uncommon and very dangerous to observe, so many volcanologists have never seen one. However, they are common in the geological record, and many of these prehistoric pyroclastic flows completed dwarf anything witnessed in human history. These huge flows are almost always associated with calderas, huge depressions in the crust of the Earth formed when the roof of the magma chamber from the which the pyroclastic flows erupted collapsed. Volcanologists have taken to describing such volcanoes as supervolcanoes.

Supervolcanoes are not actually the largest volcanic eruptions. That honor would go to flood basalt eruptions, in which vast quantities of basaltic magma are erupted through sets of fissures that can be hundreds of kilometers long, drowning areas of up to a million square kilometers in a sea of lava. However, flood basalts are even rarer than supervolcanoes, do not produce calderas, do not produce pyroclastic flows or vast quantities of ash, and in most cases a human could have outrun the advancing lava. (Though, since the most recent flood basalts on Earth erupted some millions of years ago in British Columbia, no human has ever witnessed a flood basalt.) Perhaps it is for these reasons that flood basalts have not captured the public imagination the way that supervolcanoes have, even though the largest flood basalts are far more destructive.

That no supervolcano has erupted in historic times is something to be grateful for. Such an eruption would be a global catastrophe. Apart from the devastation in the area immediately around the volcano, significant amounts of ash would be deposited for hundreds of kilometers downwind of the volcano, and the dust and gas in the stratosphere could cause catastrophic global cooling. The eruption of Tambora in 1815, the largest in human history, killed over 70,000 persons and caused "The Year Without a Summer", but Tambora produced only 160 km3 (38 cubic miles) of ash and flows and a caldera not quite 6 km (4 miles) across, versus 600 km3 (145 cubic miles) of ash and a caldera 23 km (14 miles) across for the Valles caldera.

Though geologists have never witnesses so large an eruption, they have examined the rock record from prehistoric eruptions. These include old calderas that have been deeply eroded to expose their interiors. Geologists thus can extrapolate from smaller historic eruptions to understand how a supervolcanic eruption might behave.

A magma chamber forms

By this point in our story, nearly two million years ago, the Jemez had already been an area of high volcanic activity for twelve million years. Repeated injections of magma into the crust had heated and softened the country rock, and numerous pockets of magma accumulated at depths of around 10 km (6 miles). These gradually coalesced into a single large magma chamber full of silica-rich magma.

Computer analysis of the motion of seismic waves under today's Jemez Mountains yields a rough image of what the remains of the magma chamber look like. The most recent and sophisticated analysis shows a partially molten zone extending from 7 to 13 km (4 to 8 miles) beneath the center of the caldera and 12 to 14 km (7 to 9 miles) across. This zone is a network of molten rock, including some sill-like bodies, in which liquid magma makes up at least 13% of the total volume of the rock. More sills were detected at a depth of around 20 km (12 miles), and the crust-mantle boundary is located at a depth of 30 to 40 km (19 to 25 miles). There is some indication that the crust is underplated by primitive basalt.

The most silica-rich fraction of the magma, together with considerable quantities of dissolved gases (primarily water and carbon dioxide), accumulated at the top of the chamber. Because its density was relatively low, this gas-charged magma had considerably buoyancy and exerted enormous pressure on the solid rock forming the roof of the the magma chamber. The stage was set for a devastating eruption.

One of the mysteries of giant calderas is called the room problem. How did all that magma rising from the depths shoulder aside enough country rock to make room for itself? At great depth, the rock of the crust is hot enough to be ductile, so that it could slowly deform to make room for the rising magma. The upper crust is more brittle and is not so easily shoved aside.

One theory is that the magma didn't move at all; it formed in place, from melted country rock. This would explain why giant calderas erupt high-silica lava similar in composition to the crust. We know that there was substantial, prolonged heating of the crust in the Jemez. Perhaps the magma chamber formed and grew from repeated injection of very hot low-silica magma into its base. This is consistent with the observation that basalt flows were erupted in the early stages of the Jemez volcanic field and, later, around its periphery. The same pattern is seen at Yellowstone and other supervolcanoes. This would also explain why blobs of low-silica rock are often found in tuffs. However, as attractive as this theory is, it has some problems. The amount of heating required is enormous. The composition of the magma erupted from tuffs does not quite match that of the crust, even when mixing of some low-silica magma is taken into account. Chemical analysis of the magma of the Bandelier Tuff suggests that not more than 30% of the magma came from the crust. The remainder appears to have formed by differentiation of basalt magma from the mantle.

Another possibility is that the magma rose through stoping. This is a process in which solid pieces of rock break off the roof of the magma chamber. The denser rock sinks to the bottom of the magma chamber, and the magma rises to take its place. There is considerable evidence supporting this mechanism, coming particularly from ancient calderas whose magma chambers have been exposed by prolonged erosion. Very likely both repeated hot magma injection and stoping play a role in the formation of a giant magma chamber.

Whatever the mechanism, the long history of volcanic activity in the Jemez, extending over millions of years, would have provided plenty of opportunity for a large magma chamber to form.

Breakout

Since the catastrophic eruption of so large a magma chamber has never been witnessed, the details of how such an eruption takes place remain uncertain. Smaller eruptions, such as that of Tambora in 1815, took place from relatively small magma chambers containing intermediate-silica magma following a conventional eruption through a central vent. Geologists long assumed that giant supervolcano eruptions took place in a similar way, starting with an eruption from the central vent of a giant volcano. Some older books on the Valles volcanic field speculated that the Valles caldera was once the location of a very large volcanic peak of enormous height, perhaps as high as the highest peaks on earth today.

Geologist no longer think so. Drilling and seismic studies have shown that the floor of the Valles caldera consists of relatively flat beds of rock that collapsed into the magma chamber more or less like a piston moving in a cylinder. The terrain in the central Jemez could not have been much higher than the peaks around the rim we see today. Geologists are more inclined to think that the eruption began with the formation of a large circular fracture (a so-called ring fracture) around a giant plug of rock that began dropping into the magma chamber even as gas and ash erupted through the fracture. Such ring fractures are exposed in ancient calderas, and we have convincing evidence of the existence of a ring fracture within the Valles caldera, as we'll see in the next chapter.

Just what triggered the breakout is also not known with certainty. However, there is considerable evidence that injection of fresh magma immediately precedes breakout.

There would likely have been some warning signs, though there was no one in northern New Mexico to observe them. One of the surest signs of an impending eruption in modern volcanoes is an  increase in sulfur oxide emissions. This might have preceded the supervolcano eruption by months or even years. Another warning sign would be inflation of the volcanic field, where the surface of the ground began to rise over the magma chamber. Inflation of Mauna Loa and Kilauea in Hawaii follows so regular a pattern that eruptions can be predicted with high confidence. Finally, when breakout is imminent, the rising magma begins fracturing rock above the magma chamber, and this produces a characteristic seismic signal called harmonic tremor. Viscous magma flows into a crack, increasing the stress on the crack tip until the crack abruptly advances further into the rock. The magma then slowly flows into the new crack space. This produces a surprisingly regular pattern of seismic impulses.

Early Leakage


Digital relief map of La Cueva Member exposures in the
      Jemez Mountains
Relief map of the Jemez with La Cueva Member outcroppings highlighted in red.

In the last chapter, we examined the mafic eruptions around the perimeter of the Jemez volcano field that peaked not quite 3 million years ago. These eruptions showed that mafic magma was continuing to rise from the mantle and heat the crust under the Jemez. As a result, an enormous magma chamber was forming under the heart of the Jemez. One imagines that this must have been accompanied by considerable volcanic activity in the area. However, the collapse of the caldera destroyed most of the evidence of early activity above what would become the Toledo caldera. We do have a few hints.

The time scale for this collapse is not known with certainty, but it could not have been long. There may be preliminary low-volume eruptions that weaken the roof of the chamber and set up the main eruption. These so-called leakage events may take placed up to hundreds of thousands of years before the main event. However, the main event itself cannot last more than a few weeks, based on the character of the ash and tuff that are emplaced by the eruption. These show that individual beds of ash and tuff could not have cooled very much between flows.

Geologists mapping the rocks in San Diego Canyon discovered a small tuff bed at the very base of the Bandelier Tuff. This was originally given the sensible name of the San Diego Canyon Ignimbrite, but because it is clearly related to the later Bandelier Tuff eruptions (and is not that much older, with a radiometric date of 1.85 million years) it has been renamed the La Cueva Member of the Bandelier Tuff. This tuff is found also in the walls of the caldera, in a few places underlying Redondo Peak, and as thin beds interbedded with part of the Puye Formation north of Los Alamos and in lower Alamo Canyon in Bandelier National Monument.

One readily visible exposure is in the east face of Virgin Mesa, high above State Road 4 in Canon de San Diego.

Virgin
          Mesa
Bandelier Tuff in Virgin Mesa, showing all three members. Looking northwest from 35 45.417N 106 42.194W

All three members of the Bandelier Tuff are visible here, resting on much older red beds of the Abo Formation. The La Cueva Member shows as a thin layer at the bottom of the cliffs, with a clearly visible notch separating it from the much thicker and nearly featureless beds of the Otowi Member. Above are multiple flows of the Tsherige Member.

The best exposure for close examination of the La Cueva Member is probably in the La Cueva area itself, in the caldera wall south of the Obsidian Way subdivision. This is an area of spectacular tent rocks eroded out of the Otowi and La Cueva Members, with Tsherige Member exposed in the cliffs overhead.

Here's a tent rock showing contacts between beds.

La Cueva
          contact
Contact. Looking south from 35 52.017N 106 39.543W

It's a little challenging to tell the various contacts in this area apart. The geologic map does not have nearly the detail necessary, and while I have a road log discussing this location, it has a single photograph of only the uppermost Tshirege beds and a crude diagram of this area as a whole. But I believe this is the contact between the La Cueva Member below with the Otowi Member above.

Here's an isolated tent rock with the beds showing clearly.

La Cueva
          contact
Contact. Looking north from 35 52.017N 106 39.543W

This one seems to more closely match my road log's diagram. If so, the bottom of the tent rock is the "A" bed of the La Cueva Member; the lithic-rich layer halfway up (the thin layer full of dark pebbles) marks the transition to the "B" bed of the La Cueva Member; and the cap at the very top is Otowi Member.

Here's a view of the formation from the other side.

La Cueva
          contact
Contact. Looking west from 35 52.029N 106 39.512W

The lithic-rich bed represents a lag breccia and is an indication that the source vents for the La Cueva Member could not have been far from this location. Lag breccias represent large clasts of country rock caught up in the eruption that are too heavy to be transported far from the vent.

The La Cueva Member was clearly much less voluminous than the eruptions that followed. It has been described by some geologists as an early leakage of the magma chamber. Nevertheless, the eruption was not trivial. Drilling in the Redondo Creek area has discovered buried beds of the La Cueva Member that are up to 400m (1200') thick. This suggests that the source vents for the La Cueva Member were located in the area between Redondo Creek and the southwest caldera wall, and may even have formed a small caldera in this area.

This preliminary eruption took place some 230,000 years prior to the first truly enormous caldera eruption.

The Toledo Event


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

About 1.62 million years ago, the first truly enormous eruption burst out of the Jemez volcanic field. This eruption is known as the Toledo Event and it produced a caldera roughly coinciding with the present Valles caldera. It also emplaced the Otowi Member of the Bandelier Tuff, sometimes called the lower Bandelier Tuff in older geologic writings.

The Otowi Member covers a vast area in all directions from the caldera, although there are areas where it was clearly channeled by existing high ground. For example, none is found on the La Grulla Plateau in the northern Jemez and little on Lobato Mesa in the northeastern Jemez, and it is sparse in the highlands of the southern Jemez. On the other hand, substantial flows of Otowi Member are found in the lower ground between the La Grulla Plateau and Mesa Alta and west of the La Grulla Plateau and in Cañon de San Diego. The bulk of the Otowi Member is found in the finger mesas of the Pajarito Plateau northeast to southeast of the Sierra de los Valles and of the Jemez Plateau between the western topographic rim of the caldera and the Sierra Nacimiento Mountains. The latter are the most extensive and best preserved beds of the Otowi Formation.

One area that was not reached by the Toledo pyroclastic flows is the village of White Rock. Although there are thin flows of the later Tsherige Member north, west, and south of the village, there are none underlying the village itself, and no flows of the Otowi Formation anywhere in the immediate area. The pre-Bandelier surface beneath the Pajarito Plateau can be reconstructed in considerable detail, thanks to numerous test wells drilled to evaluate contamination from the early days of Los Alamos National Laboratory, and we now know that there is a ridge of basalt of the Cerros del Rio extending north from roughly the location of my house, on the west side of White Rock, for about three kilometers (two miles). Prior to the Toledo Event, there was a low valley west of this ridge, dominated by a large cinder cone to the south, at the present location of a LANL technical site, and the Sierra de los Valles to the west and north. The valley was underlain by beds of coarse sediments, nowhere exposed today but dubbed the Chaquehui Formation by LANL hydrologists, that could become an important aquifer for Los Alamos in the future. This valley corresponds with a region of slightly lower surface gravity that suggests it is a particularly deep part of the Rio Grande Rift.

On the Pajarito Plateau, the Otowi Member is typically found at the base of sheer mesas of the Bandelier Formation, capped with the more densely welded Tshirege Member. In the last chapter, we saw an excellent exposure in Pueblo Canyon on the main road to Los Alamos.

Cerros del Rio
          Basalt and Bandelier Tuff
Bandelier Formation sitting on top of Cerros del Rio Formation on State Road 502 west of Totavi. 35 52.099N 106 11.913W

The road cut here exposes the Otowi Member, which rests on three-million-year-old basalt of the Cerros del Rio Formation. There is a thin layer of paleosoil between the two formations that has been interpreted as shallow lake sediments from Culebra Lake. The rounded slope right of center is ignimibrite of the Otowi Member, while the gray, very level beds at the base of the Otowi Member are an air fall pumice, the Guaje Pumice.

The Guaje Pumice

The Toledo Event began when a large vent opened somewhere almost directly west of the present location of Los Alamos. A convective column of ash and pumice emerged from the vent and rose into the atmosphere. There must have been a stiff breeze blowing to the east, because the air fall pumice beds of the Guaje Pumice are thickest east of Los Alamos. This area includes the road cut along State Road 502, where the Guaje Pumice is several meters thick. North, west, and south of the caldera, the Guaje Pumice is very thin or is missing altogether.

Just how many vents were involved, and where they were located, remains controversial. The original picture was of a more or less central vent. Later, geologists began to lean towards the view that the initial vents were located at one or more points along the developing ring fracture. However, the isopach map for the Guaje Pumice, which shows the estimated thickness of the bed at various locations around the Jemez, strongly suggests the Guaje Pumice was erupted from a central vent or a few closely clustered vents. We cannot say for certain whether the vents were located near the center of what is now the caldera, or at the easternmost point of the ring fracture, or somewhere else along the same east-west line.

Thick beds of Guaje Pumice were deposited north of Guaje Canyon. Here the Copar Pumice Mine operated for many years, and there are still large beds exposed in the area.

Guaje Pumice beds north of Guaje
          Canyon

Guaje Pumice beds. Near 35.917N 106.225W

Another pumice pit is located just south of Thirtyone Mile Road.

Pumice pit along Thirtyone Mile
          Road

Pumice pit in Guaje Pumice beds. Near 36.009N 106.203W

I took the photographs without much thought about land jurisdiction, but a pumice pit will certainly be a private holding and both the Copar mines are on tribal lands. I should not have been surprised that someone materialized to ask what I was photographing and why. The correct answer in such situations is the truthful one. Tourists out looking at geology are fairly nonthreatening, though one should always be respectful of privacy on non-public lands.

The large pumice beds on the west rim of White Rock Canyon at Overlook Park are probably Guaje Pumice.

Pumice banks

Pumice bank

Pumice bank
Near 35 49.407N 106 11.089W

Though locally substantial, these beds are not mapped even on the most recent geological map of the White Rock quadrangle. Here's a sample.

Guaje
          Pumice
Near 35 49.407N 106 11.089W

Close examination shows that the pumice has abundant phenocrysts of quartz and feldspar, likely sanidine (alkali feldspar.) There are no visible phenocrysts of biotite. This is characteristic of the Bandelier Formation pumice beds and rules out the much younger El Cajete Pumice, which has obvious biotite phenocrysts. The pure white color is more characteristic of the Guaje Pumice than the Tsankawi Pumice; the latter tends to be grayish in color.

The El Cajete Pumice likely was deposited for great distances downwind. One deposit was located at Truchas, in the foothills of the Sangre de Cristo Mountains some 65 km (40 miles) downwind of the likely source vents.

Guaje
          Pumice
Near 36 02.560N 105.49.562W

This localized deposit has been entirely quarried away, but the borrow pit remains.

This is not the furthest identifiable airfall deposit of Guaje Pumice. In 1972, a team of geologists reported that an ash bed found at Mount Blanco, Texas, closely matched the Guaje Pumice in both chemical composition and radiometric age. This bed, which is 30 cm (1') thick, is nearly 500 km (300 mi) downwind of the source vents.

Some of the ash was carried far downriver by the Rio Grande, producing the Bosquecito Ash beds in the upper Sierra Ladrones Formation near Socorro.

Otowi ignimbrites

The eruption column gradually eroded the sides of the vent, widening it and increasing the volume of the eruption. Additional vents likely opened as the roof of the magma chamber lost support and a ring fracture began to form. The eruption column abruptly collapsed, producing the first of the massive pyroclastic flows of the Otowi Member. Unlike the Guaje Pumice, the pyroclastic flows extended in all directions, which is unsurprising given that pyroclastic flows move at great speed. The breeze from the west was essentially irrelevant at this point.

The most impressive exposures of Otowi Member ignimbrites are on the west side of the caldera. There seems to have been no topographic barrier on the west side of the Toledo caldera, and hot pyroclastic flows moved freely across this area, producing great thicknesses of moderately to highly welded tuff.

There is an impressive exposure of welded Otowi Member tuff north of Fenton Lake.

Otowi Member at Fenton Lake.

Otowi Member north of Fenton Lake. 35 52.905N 106 43.376W

The impressive cliffs are mostly Otowi Member with a layer of Tsherige Member at the top. The Tsherige Member caps most of the mesas of the Jemez Plateau, but is generally less thick than the underlying Otowi Member. This is likely because the caldera rim left by the Toledo event formed a modest topographic barrier to the younger flows.

Similar impressive exposures are found along State Road 126, in Bear Canyon.

Otowi Member in Bear Canyon

Otowi Member in Bear Canyon. 35 54.885N 106 42.841W

The prominent cliff at left is Otowi Member, with the Tsherige Member forming the sloping ground above the cliffs.

Another impressive exposure is found in Calaveras Canyon.

Otowi Member in Calaveras Canyon

Otowi Member in Calaveras Canyon. 35 56.249N 106 42.519W

The entire cliff here is Otowi Member, with no cap of Tsherige Member. Note what looks like a very thick sequence of surge beds making up the lower half of the exposure.

In the Pajarito Plateau on the east side of the Jemez, the Otowi Member is often difficult to distinguish from colluvium, since it is usually present only under cliffs of more durable Tshirege Member and it tends to weather to a gentle slope. However, there are some good exposures in road cuts. For example, as State Road 502 approaches Los Alamos, it climbs the north side of Los Alamos Mesa, where there is a fine exposure of the contact between the Otowi and Tshirege Members. The road here is narrow with no really safe places to pull over, so the visitor wishing to examine this exposure is well advised to park on a pullout on a side road just before SR 502 begins the climb, then walk to the outcropping. Be very careful of traffic on this busy and narrow road.

Otowi Member,
          Bandelier Tuff
Otowi Member on State Road 502 ascending Los Alamos Mesa. 35 52.233N 106 13.390W

The walking stick provides scale. The ignimbrite here is pinkish white ash with numerous small mafic clasts (probably country rock entrained in the flow) and numerous pumice clasts. The pumice clasts are completely undeformed, showing that this is an unwelded tuff.

Here's a sample of Otowi Member up close. This comes from an outcropping along 31 Mile Road.

Otowi Member,
          Bandelier Tuff
Sample of Otowi Member from Thirtyone Mile Road

The sample contains abundant phenocrysts of quartz and sanidine (alkali feldspar). The Otowi Tuff contains up to 30% of such phenocrysts.

Another exposure of the Otowi Member and its contact with the Tshirege Member is found in Ancho Canyon along State Road 4 between White Rock and Bandelier National Monument.

Road cut in
            Bandelier Tuff highlighting zeolitized zones
Road cut in Bandelier Tuff. 35 47.322N 106 15.405W

There is little evidence of the Cerro Toledo Interval here, but there is about a foot of Tsankawi Pumice at the base of the Tshirege Member.

As one hikes down the trail on the south side of Pueblo Canyon onto the talus slope, one sees that it is deeply mantled with soil.

Slope at base of cliffs in Pueblo Canyon

Colluvium on south wall of Pueblo Canyon. 35 52.890N 106 15.945W

But one of the attractions of the canyon is the hoodoos and tent rocks that are found typically at the base of the mesas.

Hoodoos

Hoodoo

Hoodoo
Hoodoos in Otowi Member, Bandelier Tuff. 35 53.072N 106 16.071W

Examination of the eroded bank of this slope shows that it's clearly solid tuff, with few signs of bedding or any other indications that this is reworked volcanic sediments rather than an original pyroclastic flow.

Unwelded
        tuff

To be sure, there are no signs of welding, but the Otowi Member of the Bandelier Tuff is generally not welded in the Los Alamos area.

Tent rocks seem to be a common feature of the Otowi Member. Other prominent outcroppings showing tent structure are found in Guaje Canyon below its confluence with Rendija Canyon

Guaje Canyon tent
          rocks
Hoodoos in Otowi Member, Bandelier Tuff. 35.904N 106.210W

and in Valle de los Indios on the southwest caldera rim.

Tent rocks

The entire thickness of the tuff beds here, including the skyline, is Otowi Member, with no overlying Tsherige Member. The flows at top appear to be more welded than those further down.

Some of the most distal surviving deposits of the Otowi Member are found to the southeast of the Jemez. These include deposits cut by the highway near the Cochiti golf course.

Otowi Member new Cochiti golf course

Otowi Member near Cochiti Golf Course. 35 39.727N 106 21.117W

The area between Aspen Ridge and the San Miguel Mountains was relatively low ground before the Toledo Event, and the pyroclastic flows of the Otowi Member are particularly thick and extensive here. One tongue of the eruption made it across the Rio Grande, where it was subsequently overlaid by the youngest basalt flows of the Cerros del Rio about 1.3 million years ago.

The Toledo caldera collapses

As the magma chamber emptied, a ring fracture formed and the roof of the magma chamber began to collapse. This led to deposition of ignimbrite beds rich in lithic clasts close to the caldera. In the exposures closest to the ring fracture, there are beds of lag breccia associated with caldera collapse. Some of the best exposures showing these lag breccias are in upper Cochiti Canyon.

Lithic beds
          of Otowi Member
Lithic beds of Otowi Member. Looking nortthwest from 35 45.999N 106 25.150W

The entire canyon wall in the center of the photograph is Otowi Member. The darker beds towards the bottom are the lithic beds. The rugged cliffs at top are more densely welded post-caldera flows. These later flows came from deep in the magma chamber, and they were both hotter and more mafic. We will see excellent examples of this later on, when we consider the Valles event.

The ring fractures from the Toledo caldera and, later, the Valles caldera are now deeply buried under younger sediments and flows. However, there are other locations in the Southwest where erosion has exposed the ring fractures of older calderas, and this allows us to study their structure. One such ring fracture zone is exposed in Red River Canyon west of Questa, New Mexico.

Rhyolite dike swarm marking Questa caldera ring fracture

Rhyolite dike swarm marking ring fracture zone of Questa caldera. 36 40.866N 105 30.985W

The jagged rocks along the cliff are rhyolite dikes marking the ring fracture zone of the Questa caldera on its south side.

With the collapse of the magma chamber, the eruption drew to a close. The steep rim of the newly-formed caldera was unstable and soon collapsed onto the caldera floor. Erosion further wore down the rim and deposited new beds of sediments in the caldera. Quite likely one or more crater lakes formed. However, the traces of all these events were obscured by the subsequent Valles eruption.

The Cerro Toledo Interval


Digital relief map of Cerro Toledo Interval exposures in
      the Jemez Mountains
Relief map of the Jemez with Cerro Toledo Rhyolite outcroppings highlighted in red and Toledo Interval sediments highlighted in yellow.

The Toledo event coincided roughly with the arrival of the first mammoths in New Mexico. These large relatives of modern elephants mostly disappeared at about the time humans arrived in North America, which most paleontologists do not see as a coincidence. The last mammoths are thought to have survived on Wrangel Island in the Russian Arctic until about four thousand years ago.

The Toledo event produced the first giant caldera in the Jemez region. However, the subsequent Valles event obscured much of the geological record of the Toledo event and the subsequent Toledo Interval, and geologists must search for clues to the size and location of this caldera. One reconstruction of the likely location of the Toledo ring fracture is shown below.

Valles Caldera resurgent dome

Reconstruction of Toledo ring fracture (dashed red) with Toledo Embayment (dashed yellow) and Cerro Toledo domes (circled in red.)

The red dotted line shows the likely location of the main Toledo Event ring fracture, while the solid circles identify ring fracture domes.

Ring fracture domes

The formation of a caldera in a supervolcanic eruption leaves a considerable amount of high-silica lava still underground, with a natural route to the surface through the ring fracture along which the caldera floor collapsed. The remaining magma is likely to be lower in gas content than before the caldera eruption, and it tends to erupt effusively rather than explosively through the ring fracture. Such eruptions localize at a single point along the fracture where the path to the surface is the most clear. The result is that a rhyolite dome forms around a central vent located on the ring fracture.

Once the energy of the eruption is spent, the vent is plugged with solidified rhyolite and overlain by the mass of the dome itself. This effectively seals the ring fracture in the immediate vicinity of the dome. If magma is still being injected into the old magma chamber, which seems to be a frequent occurrence with supervolcanoes, then the next eruption must come through a different part of the ring fracture. The result is that individual dome complexes will line the ring fracture almost like beads on a wire. We will see superb examples of this in the aftermath of the Valles event.

The ring domes produced by the Toledo event have been largely destroyed by the subsequent Valles event. However, some remnants of domes remain, and it is largely from these remnants that the location of the Toledo ring fracture has been inferred. In the previous photograph, the red circles show rhyolite domes or remnants of domes dating between the Toledo and Valles events, which have been assigned to the Cerro Toledo Formation. Those close to the ring fracture are interpreted as ring fracture domes. These are, starting from upper left, a small unnamed dome dating back 1.59 million years; the small Warm Springs dome dating back 1.26 million years; the Cerro Transquilar dome at the east margin of the Toledo Embayment, dating back 1.36 million years; the West and East Los Posos domes dating back 1.54 and 1,45 million years; and, to the south, the Rabbit Mountain dome (1.43 million years) and Paso del Norte (1.47 million years) domes. Rabbit Mountain is likely the southern remnant of a much larger dome, most of which foundered into the subsequent Valles caldera along with other Toledo ring fracture domes.

The identification of the small, unnamed dome furthest to the northwest as a Toledo ring fracture dome is questionable. Its age is nearly indistinguishable from that of the Toledo event itself, and the most recent geologic map of the area maps it as Otowi Member. The dome at Warm Springs is nearly the same age as the Valles Event, but its location well outside the Valles ring fracture supports its identification as a remnant of a Toledo ring fracture dome. The domes in and near the Toledo embayment and the Rabbit Mountain and Paso del Norte domes seem beyond question, showing that Toledo caldera extended at least across the eastern half of the present Valles caldera.

The Cerro Trasquilar dome is accessible by passenger vehicle with a Valles Preserve back country permit.

Cerro Trasquilar
Cerro Trasquilar. From near 35 58.310N 106 31.102W

This small dome is parked right in the middle of the moat, outside the ring of larger domes that mark the Valles ring fracture. Its age from radioisotope dating (1.36 million years) reveals that Cerro Trasquilar is a remnant of one of the domes that likely formed over the ring fracture of the earlier Toledo caldera. There is some disagreement over the naming here; geologists have generally referred to this small dome as Cerro Trasquilar and the much larger, younger dome complex to the south as Cerro Santa Rosa, but the Forest Service topographic map identifies the entire complex as Cerro Transquilar and the dome immediately to the south as Cerro Santa Rosa.

On the east side of Cerro Trasquilar, the visitor reaches the end of Pipeline Road, or at least of that part of Pipeline Road that a back country permit gives one permission to drive. The road continues on through the Sierra de los Valles to Los Alamos, a drive I'd very much like to take someday.  Here is a panorama:

Valle Toledo
          panorama
Valle Toledo panorama. 35 57.461N 106 28.890W

The panorama starts at the soutwest, looking towards the Cerro Santa Rosa complex. The dome known by area geologists as Santa Rosa II is prominent in the second frame while Cerro Trasquilar is on the boundary of the second and third frames, with the north caldera wall beyond. The gentle slope in the foreground is an old stream terrace.

Turkey Ridge forms the skyline across the fourth and fifth ridge. This dome complex is about 1.38 million years old and occupies the mouth of the Toledo Embayment, an odd feature of the caldera whose interpretation geologists have not been able to fully agree on. It looks like a pocket in the northeastern topographic rim of the caldera that is full of rhyolite domes, all between 1.33 and 1.45 million years old, but it is not clear what would cause this. The two leading theories are that it is a structural feature, formed by deep faulting that connects the Redondo Graben to the Escondido Fault Zone north of Espanola; or that it is an extension of the Toledo caldera to the northeast. The two are not mutually incompatible. The notion that this is the remnants of the caldera from which the Otowi Member was erupted -- the Toledo caldera -- is no longer accepted, but a few geologists think it may be the caldera from which the La Cueva Member of the Bandelier Tuff was erupted.

The dome on the left side of the sixth frame is the nearest dome of Cerros de los Posos, age 1.54 million years, which is thought to be another remnant of the domes of the Toledo ring fracture.

The last few frames show the toe of a landslide in the foreground and Cerros del Medio, from which the landslide originated, in the background. The big log in the middle of the road marks the limit of where a back country permit allows the visitor to drive.

The Rabbit Mountain and Paso del Norte dome complex is one of the more accessible of the Cerro Toledo dome complexes. Rabbit Mountain forms much of the southeast rim of the caldera and is prominent on the skyline as seen from the Valles Preserve headquarters.

Valles
        caldera south rim
Rabbit Mountain as seen from the Valles Preserve headquarters.

This is looking southeast across the Valle Grande towards the caldera rim.

The Paso del Norte Dome is accessible from Forest Road 36 south of State Road 4, from which one gets an excellent view of the dome.

Del Norte dome
Del Norte dome. Looking southwest from. 35 49.603N 106 28.551W

The dome can be climbed by a short but strenuous hike from a pullout here. Be advised that there is no trail and the dome is steep and heavily overgrown with thorny shrubs. However, the top of the dome shows excellent exposures of Del Norte rhyolite and a view (through the trees) of nearby Rabbit Mountain.

Del Norte
          summit
Paso Del Norte dome summit. Looking northeast from. 35 49.236N 106 28.735W

Paso Del Norte dome and the southern flank of Rabbit Mountain are on National Forest land just south of the preserve, so I took a sample.

Del Norte dome
        rhyolite
Del Norte dome rhyolite

No individual crystals are visible even  under the loupe; the rock is completely aphanitic.

The Del Norte dome is well outside the actual south caldera rim. Like Rabbit Mountain, it was part of a much larger dome to the northwest that foundered into the Valles caldera, leaving a small remnant overlying the precaldera rocks of the south rim.

Rabbit Mountain is accessible via the Coyote Call Trail of the Valles Preserve. The trailhead is a small pullout from State Road 4. Visitors should be aware that the parking is very limited here, though the trail requires no use fee. The hike itself is not particularly difficult nor lengthy except where downed trees force a detour. The latter seems to be a common problem throughout the Valles preserve, a consequence of recent forest fires in the area rather than any lack of diligence on the part of the Preserve staff.

Surprisingly, the rhyolite on the north flank of Rabbit Mountain is quite different in character from the rhyolite of the Paso del Norte dome. It is generally darker, with no hint of the purple discoloration, and one can understand why earlier geologists mapped the Paso del Norte dome as Bearhead Rhyolite until radioisotope dating showed it to be much younger (1.47 million years), only slightly older than the Rabbit Mountain rhyolite. I am struck by the peculiar surface texture of some of the rhyolite clasts:

Rabbit
          Mouintain rhyolite
Rabbit Mountain rhyolite. 35 50.578N 106 27.648W

The peculiar bark-like texture might be a weathering surface, but it seems more likely this is a cooling surface of a flow. Such textures are occasionally seen in basalt flows and are described as spiny pahoehoe. Here's a photograph from a very young basalt flow at Craters of the Moon National Monument in Idaho:

Spiny pahoehoe
Spiny pahoehoe. Craters of the Moon National Monument.

In both cases, these textures were probably produced here by mild stretching of the partially solidified surface.

Another feature of the mountain is occasional outcrops that show a distinctive tan color and coarser texture, almost resembling a sandstone. Examination under the loupe shows that this is crystalline rock, however. There is no sampling here on the Preserve and a large-scale photograph does not do the outcrop justice.

Obsidian

A distinctive feature of Rabbit Mountain is the presence of significant quantities of obsidian in the rhyolite flows.

Rabbit Mouintain rhyolite with obsidian
Rabbit Mountain rhyolite. 35 50.807N 106 27.190W

Obsidian is volcanic glass. It forms from magma that is cooled so rapidly that the atoms freeze into a tangled mess before they can arrange themselves into a regular crystal structure. Volcanic glass is relatively uncommon in subaerial mafic rocks, which because of their lower viscosity require very rapid chilling to form glass, but it is quite common in felsic rocks. In fact, some geologists believe that almost all extrusive felsic rocks start off as volcanic glass, which slowly devitrifies over geologic time as the atoms gradually work themselves into a regular crystalline arrangement.  If this is true, then it is possible that the entire boulder here was once obsidian, which has now devitrified to the point where only a few thin layers of obsidian remain.

Weathering has released a considerable number of small obsidian nodules from the rhyolite at some locations along the trail.

Obsidian nodules in the trail
Obsidian nodules in the trail. Click to enlarge. 35 50.765N 106 27.135W

It is not uncommon for obsidian to weather out of formations as small nodules, and these are sometimes called "Apache tears." It surprises me that obsidian would weather more slowly than devitrified rock, but it seems to be the case.

One piece of evidence for widespread devitrification of obsidian flows is the presence of spherulites. 

Obsidian spherulites
Spherulites near northeast summit of Rabbit Mountain. Click to enlarge. 35 50.771N 106 27.180W

Spherulites are characteristic of volcanic glasses, but one can see that a great many of the spherulites here have devitrified.

Rabbit Mountain was large enough to experience several episodes of dome collapse. This produces a form of pyroclastic flow described as a glowing avalanche. The steep face of the dome becomes unstable as it is pushed out from within by fresh magma entering the dome, and the face collapses in a landslide. The magma beneath, depressurized by the removal of the overlying rock, disintegrates into hot gas and volcanic ash. This mingles with the larger clasts of the original landslide to form the glowing avalanche, which can travel for miles. This is similar to a block and ash flow, but with a smaller proportion of ash, so that the flow is not as thoroughly cemented.

One such glowing avalanche deposit is found along the Dome Road south of Graduation Flats.

Pullout underlain by Rabbit Mountain debris flow
Pullout atop Rabbit Mountain debris flow. 35 47.537N 106 25.174W

The pullout and the area around it are underlain by glowing avalanche deposits from Rabbit Mountain, over five kilometers (three miles) to the northwest. Such avalanches occurred at least three times from the southeast flank of the dome, forming the surface on which my car is parked. (The dome in the background is an older dacite dome, which we visited several chapters back.) The ground surface shows numerous clasts of rhyolite and obsidian.

Obsidian and rhyolite clasts of Rabbit Mountain debris
          flow
Rabbit Mountain debris flow. Pencil for scale. 35 47.537N 106 25.174W

Here are samples of a couple of larger obsidian fragments.

Obsidian of Rabbit Mountain debris flow
Obsidian of Rabbit Mountain debris flow. Pencil for scale. 35 47.537N 106 25.174W

These chunks are about three inches long. The fragment on the right shows flow banding. Both are weapons grade, suitable for manufacturing obsidian weapon tips. There is abundant archaeological evidence of widespread trade of Jemez obsidian throughout the Southwest. 

Deposits from this avalanche are found as far as a canyon bottom northeast of St. Peter's Dome, a distance of 13 km (8 miles) from Rabbit Mountain.

A similar avalanche deposit is found south of Rabbit Mountain. This flow is thought to have come off the Del Norte dome.

Del Norte flow deposits
Del  Norte debris flow. 35 48.5785N 106 27.897W

Like Cerro Trasquilar, the Warm Springs dome is accessible by passenger vehicle with a Valles Preserve back country permit. There actually are warm springs here, with an old bath house dating from the time when the Valles Caldera was owned privately as a single large ranch. The bath house is decorated with cow skulls; I assume as a warning to other cows.

Warm Springs
Warm Springs. 35 58.328N 106 33.631W

The small hill west of the bathhouse is the dome. Its radioisotope age is just a hair more than the Tshirege Member of the Bandelier Tuff, 1.26 million years. It is interpreted as a dome on the Toledo ring fracture that was formed just prior to the Valles event. In the background is Cerro Seco, a Valles ring fracture dome.

Cerro Toledo Interval pumice and sediment beds

Dome eruptions tend to produce pumice fall beds over a significant area downwind, and the Cerro Toledo domes laid down significant pumice beds on top of the Otowi Member of the Bandelier Tuff east of the Jemez. One such series of beds is visible in the road cut of State Road 502 as it climbs the north side of Los Alamos Mesa, which we visited earlier. Let's take a look now at the Cerro Toledo beds.

Cerro Toledo
          Interval
Cerro Toledo Interval on State Road 502 ascending Los Alamos Mesa. 35 52.233N 106 13.390W

At bottom is the Otowi Member of the Bandelier Tuff. Above are a pair of pumice beds of the Cerro Toledo Interval. Above these is the basal Tsankawi Pumice and the lowest ignimbrite bed of the Tshirege Member.

The Cerro Toledo Interval becomes thicker closer to the Los Alamos town site. There is a prominent exposure in Pueblo Canyon north of Los Alamos Airport.

Pueblo Canyon north wall

Bandelier Tuff in Pueblo Canyon. 35 53.182N 106 16.262W

The top three layers of the mesa (North Mesa) are units of the Tshirege Member, Bandelier Formation. The topmost is partially obscured by houses and trees on the canyon rim. The second layer plunges into the canyon, and the third layer reaches to the sloping ground at the canyon bottom in most places. These are three flow units, erupted far enough part in time that each had cooled slightly before the next was erupted on top of it.

At the center of this photograph, you see a narrow banded layer with just a little of an eroded formation showing beneath it. (Click on the photograph to see an enlarged version.) The uppermost part of this banded layer is the Tsankawi Pumice, while the lower portion is the Cerro Toledo Interval. The boundary between the two is difficult to pin down at this distance. 

So what we're seeing is tall, resistant mesas of Tshirege Member sitting on top of low, gently eroded ridges of Otowi Member. This is an excellent example of inverted topography. The Tshirege Member settled preferentially in river channels and other low points in the existing erosional surface of the Otowi Member. Subsequent erosion preferentially eroded the Otowi Member, leaving the more durable Tshirege Member as finger mesas coinciding with the ancient river channels. Thus, the topography has been inverted: What was the high points of the old surface are now the canyons, and what was the low points are now the tall finger mesas.

Going back to the photo of the north wall of Pueblo Canyon, one can easily imagine the gentler slope showing the outline of the Otowi topography, with a paleochannel near the center of the photo which became partially filled with Cerro Toledo tephra. The sediments might well have been unusually wet. The deposit of Tshirege Member, Bandelier Tuff, on top would have vaporized the water, account for what look like vapor phase pipes in the Tshirege Member above the paleochannel. Vapor phase refers to all the fluids that percolate through the pores of a rock bed, but particularly when the fluids are very hot. Deposition of minerals from the vapor phase can indurate the rock, and if the vapor phase is mostly moving upwards along narrow channels, the result is cylinders of particularly hard rock. We'll see an even more striking example later in this chapter.

Here's another shot of the base of the Tshirege Member, where the Guaje Pumice and Cerro Toledo Formation have been deeply eroded.

Pueblo Canyon
        cav

Further up Pueblo Canyon, the Cerro Toledo Interval becomes still more prominent.

Cerro Toledo interval in Pueblo Canyon

Cerro Toledo interval in Pueblo Canyon. Looking north from 36 01.121N 106 13.867W

The Cerro Toledo Interval fills the notch in the talus slope.

The Cerro Toledo Interval is particularly thick in the area around Rendija Canyon and to its north. The area is underlain by hills rich with Cerro Toledo Interval pumice.

Cerro Toledo pumice

Soil rich in Cerro Toledo interval pumice. 35 54.766N 106 17.006W

There is a fine exposure of the Cerro Toledo Interval in the canyon wall at the end of an unnamed spur of Cabra Canyon.

Cerro Toledo exposure

Cerro Toledo interval exposure in Cabra Canyon. 35 54.766N 106 17.006W

The uppermost half of the mesa is Tsherige Member, Bandelier Tuff, with a base of light-colored Tsankawi Pumice. Below this are a mixture of pumice, tuff, fluvial sediment, and paleosoil beds of the Cerro Toledo Interval. These appear to have accumulated in a paleovalley in the underlying Puye Formation; there is no Otowi Member of the Bandelier Tuff mapped anywhere in this area.

The next photograph show the east face of this exposure.

Cerro Toledo exposure

Cerro Toledo interval exposure in Cabra Canyon. Near 35 55.157N 106 17.563W

There are a great variety of deposits here: a pumice-rich bed at bottom, followed by thin alternating ash-rich and pumice-rich beds likely reworked by streams, then a thicker pumice bed that is obviously eroded along its contact with a thick paleosol, then another pumice-rich bed at the very top of the photograph above the thick paleosol. Here's a closer view of the base of the paleosol.

Cerro Toledo exposure

Cerro Toledo interval exposure in Cabra Canyon. Near 35 55.157N 106 17.563W

There is a mixture of pumice and clasts of Tschicoma Formation dacite at the base of the paleosol, which strongly resembles the alluvium in modern drainage channels in this area. This transitions to a thick clay-rich bed with fewer clasts.

On the south side of the exposure, there is a cave eroded deeply into the paleosol layer.

Cerro Toledo exposure

Cerro Toledo interval exposure in Cabra Canyon. Near 35 55.157N 106 17.563W

The paleosol layer is particular susceptible to erosion, being very poorly cemented. It is likely that the caves seen in the cliffs in Pueblo Canyon in the photograph I showed earlier are in this paleosol layer.

This area is littered with fragments of rock from the cliff face. These include large clasts of paleosol

Cerro Toledo exposure

Cerro Toledo interval clay in Cabra Canyon. Near 35 55.157N 106 17.563W

and pumice:

Cerro Toledo pumice

Cerro Toledo interval pumice in Cabra Canyon. Near 35 55.157N 106 17.563W

Note the reddish color on the freshly fractured surface. This has the appearance of hematite cement rather than clay, and it may represent mafic minerals in the pumice that have been oxidized.

One can also see the contact between the Otowi Member and the Tshirege Member in the southwest caldera wall south of La Cueva. We saw exposures of the La Cueva Member in this area earlier in this chapter.

Bandelier contact
Contact. Looking south from 35 52.029N 106 39.512W

The tan band is probably Cerro Toledo interval sediments, while the bed of pumice is the Tsankawi Pumice that marks the lowest part of the Tshirege Member.

Finally, some purely sedimentary beds are assigned to the Cerro Toledo Interval. These include this gravel bed on 30 Mile Road, northwest of Espanola, that was deposited on top of Otowi Member.

Cerro Toledo interval gravel beds on Otowi Member

Cerro Toledo interval gravel bed. 36 01.121N 106 13.867W


These sediments have likely been reworked to an extent in geologically recent times.

The mysterious Toledo Embayment

The satellite photograph near the start of this section shows a number of Cerro Toledo Formation domes northeast of the caldera that almost fill a large embayment in the much older rocks of the Tschicoma Highlands and La Grulla Plateau. The largest of these is Cerro Toledo itself.

Cerro
        Toledo
Cerro Toledo Embayment. 36 01.136N 106 26.039W

The earliest interpretation of the Toledo Embayment is that it was the caldera from which the Otowi Member of the Bandelier Formation was erupted, and this led to this event being named the Toledo Event. However, as the previous reconstruction shows, geologists now believe the main Toledo caldera was located nearly in the same location as the subsequent Valles caldera.

Based on stratigraphy, the Toledo Embayment must have formed between 2.3 and 1.5 million years ago. One interpretation is that it formed slightly after the Toledo Event during eruption of Cerro Toledo Formation domes. Another interpretation is that it formed as part of the Toledo Event as a kind of offshoot of the main magma chamber along Rio Grande Rift faults. A few geologists have revived a version of the original interpretation, suggesting that the Toledo Embayment was the source area for the La Cueva Member of the Bandelier Tuff.

The latter interpretation gains support from gravity modeling of the caldera. Gravitational modeling begins with taking very precise measurements of the gravitational field at as many points as practical within the caldera. The structure of the caldera is then modeled on a computer, assuming one density for the basement rock, a somewhat lower density for old sedimentary beds, and a still lower density for Bandelier tuff and Valles rhyolite, which fill the interior of the caldera. The deep structure of the caldera is inferred by adjusting the depth of the various rock layers to match the gravity measurements.

In 1996, geologist D.A.G. Nowell carried out this procedure using gravity measurements published by another geologist, R.L. Segar, in 1974. The most interesting part of the model is the depth of the basement rocks relative to sea level, which presumably formed a more or less level surface before the caldera formed.

Digital relief map of depth of basement in the Jemez
        Mountains
Relief map of the Jemez with inferred depth of basement relative to sea level shown in color contours.

There are two striking aspects of this plot. First, the basement under the Valles caldera did not sink evenly; the depth of basement is much greater on the east side. I'll have more to say about this later in this chapter. The other striking feature is the indication that the Toledo Embayment coincides with a low spot in the underlying basement rocks.

The interpretation of the Toledo Embayment as a small caldera from which the Culebra Member erupted is intriguing but has not attracted much support from most geologists. The most popular interpretation is sill that it is an offshoot of the Toledo caldera that formed during or shortly after the Toledo Event.

The Valles Event


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

By 1.25 million years ago, what is now the Pajarito Plateau was a surface of relatively gentle relief. The White Rock basalt ridge still stood out, but the northern part of the valley to its west was filled with Otowi Member ignimbrites and the southern part was quite shallow. Stream valleys were typically 15 to 30 meters (50' to 100') deep, compared with over 300m (1000') for canyons today. Geologists have identified four such valleys in the Otowi surface. Because the Otowi Member is unwelded on this side of the Jemez, there were no towering cliffs on the rims of canyons.

The Jemez volcanic field as we know it today was largely shaped by the Valles Event 1.25 million years ago. This massive eruption produced the Tshirege Member of the Bandelier Formation, which is the iconic geological formation of the Los Alamos area.

Like the Toledo Event, the Valles Event emptied a magma chamber in which large quantities of gas-rich, high-silica magma had accumulated. This magma chamber likely nearly coincided with the Toledo magma chamber. As with the Toledo event, the Valles eruption was preceded by increased emissions of sulfur oxides, inflation of the volcanic field as magma rose beneath it, and, immediately before the eruption, seismic signals such as harmonic tremor. But the earliest precursor may have been the eruption of the Warm Springs dome, which we visited earlier. Eruption of this dome took place along the old Toledo ring fracture only a few thousand years before the Valles event.

The Valles Event resembled the Toledo Event, but with some significant differences. Not least of these is that much more of the Valles Event is preserved in the geological record.

The Tsankawi Pumice

Much like the Toledo Event, the Valles Event opened with the eruption of a convective column through one or more vents. However, while the Guaje Pumice is strongly concentrated directly east of the caldera center, the Tsankawi Pumice erupted early in the Valles event is spread more evenly around the caldera. It is thickest to the northwest, and ash of the Guaje Pumice has been found as far northwest as central Utah. This suggests a mild breeze blowing from the southeast, but tells us little about the source vent locations. These are thought to have been near the center of the caldera.

A moderately thick bed of Tsankawi Pumice is exposed along State Road 126 on the way to Fenton Lake.

Tsankawi Pumice

At bottom and at left is the Ojo Caliente Member of the Tesuque Formation. The darker patch of rock at top left is Otowi Member, Bandelier Tuff, as are most of the boulders in the middle layer. Atop this is a bed of Tsankawi Pumice, and above that are surge beds of the Tsherige Member.

A thin bed of Tsankawi Pumice can be found at the base of the Tsherige Member southwest of the village of White Rock.

Tsankawi
          Pumice

Tsherige Member on paleosoil above Cerros del Rio basalt. 35 48.649N 106 13.640W

The Tsankawi Pumice forms a very thin bed on the right side of the photograph and pinches out on the left. This location is almost directly southeast of the caldera, the direction from which the wind was presumably blowing during most of the eruption. We've already seen similar thin beds in Ancho Canyon and on the north side of Los Alamos Mesa.

One of the better exposures of the Tsankawi Pumice west of the caldera is in Rendija Canyon, northwest of the sportsmen's club.

Tsankawi Pumice

Tsherige Member in Rendija Canyon. 35 54.655N 106 16.990W

The pink bed at bottom is Toledo Interval sediments. Above is the Tsankawi Pumice, which consists of two beds separated by a very thin ash layer (mostly eroded at this location to form a pronounced notch.) Above are surge beds, which we'll discuss presently.  The Tsankawi Pumice beds are subtly present in the exposure along Los Alamos Mesa as well, where the upper bed is quite thin and the ash layer separating it from the lower bed is no more than a slight notch.

The Tsankawi Pumice can be distinguished from other pumices of the Jemez area by a grayish color and the presence of needles of hornblende.

Surge beds

After the initial stage of the eruption, in which the convective column deposited air-fall pumice and ash over a wide area, the eruption column collapsed to produce pyroclastic flows. Column collapse occurs when the eruptive column no longer entrains enough air to remain buoyant, but exactly what triggers this transition is uncertain. Perhaps the volume of the eruption increases to the point where enough air can no longer be entrained.

When the column collapses, a dense mixture of hot gases and ash flows out across the landscape as pyroclastic flows. These are thick with ash and move steadily across the ground with little turbulence (laminar flow). They are often preceded by pyroclastic surges, which contain less ash and are much more turbulent, and these can deposit thin beds of fine ash that have such features as crossbedding. The previous photograph shows surge beds immediately above the Tsankawi Pumice, which is a frequent but not universal occurrence in the Tsherige Member.

A road cut on Dome Road shortly before it descends the Pajarito Escarpment shows the contact between the Tsherige Member and the underlying volcaniclastic beds of the Paliza Canyon Formation.

Another contact
Contact in road cut. 35 42.800N 106 23.406W

There are surge beds in the Tshirege Member along the contact, and also near ground level to the left. The Tscherige Member is here a particularly vivid red, which may also be an indication of hydrothermal alteration.

Tscherige Member,
          Bandelier Tuff
Tshirege Member. Just north of 35 42.800N 106 23.406W

There are numerous bore holes here and in an outcrop on the other side of the road, suggesting this rock has attracted attention from geologists. I don't have any dates marked on my map for this area, though a nearby andesite outcrop is dated to 9.33 million years.

And next to the bore holes are some hash marks scratched crudely in the rock. Either this is some kind of Secret Geologist Language, or some non-geologist is less than overawed by the activities of scientists.

Ignimbrites

Pyroclastic surges were followed by pyroclastic flows. These produced the bulk of the Tsherige Member of the Bandelier Tuff. There are spectacular exposures of the Tshirege Member throughout the Jemez area, but perhaps none are better known than those visible from the Clinton P. Anderson Scenic Overlook on the south face of Pueblo Canyon.

Pueblo Canyon from Clinton P. Anderson Scenic Overlook
Clinton P. Anderson Scenic Overlook. 35 52.391N 106 14.006W

The finger mesas visible here are all Tshirege Member resting on a base of Otowi Member, with the Otowi Member partially mantled with colluvium. Geologists have identified at least five separate units in the Tshirege Member, and some of these are clearly visible here. We'll discuss these units presently.

Before doing so, we'll take a tour around the Jemez of the Bandelier Tuff. The finger mesas east of Los Alamos continue west to the foothills of the Sierra de los Valles and underlie the entire Los Alamos town site. Geology around Los Alamos largely consists of climbing down into canyons to see what is below the Bandelier Tuff or climbing hills and mountains to see what is sticking up above the Bandelier Tuff. The entire terrain in the next photograph, except for the distant Sangre de Cristo Mountains, is Tshirege Member. At this location, close to the Sierra de los Valles forming the caldera's east rim, the Tshirege Member tends to be densely welded and darker in color than further from the caldera.

Los Alamos Canyon
          and Omega Bridge
Los Alamos Canyon and Omega Bridge. 35 52.689N 106 20.615W

Tshirege Member along Camp
          May Road
Tshirege Member near Camp May Road. 35 52.689N 106 20.615W

Looking up Los Alamos Canyon from Camp May Road, we see that the Tshirege Member extends quite high up the Sierra de los Valles.

Tshirege Member in upper Los Alamos Canyon
Tshirege Member in upper Los Alamos Canyon. 35 52.689N 106 20.615W

The canyon rim in the distance, above and left of center and left of the small peak, is the upper surface of the Tshirege Member. East of where this photograph was taken (behind the photographer), the Tshirege Member is thrown down a substantial distance by the Pajarito Fault Zone.  In the foreground, across the canyon, Tshirege Member sits directly on the Pajarito Mountain Member of the Tshichoma Formation, with no Otowi Member mapped bertween the two. Apparently any Otowi Member that was deposited in this area had eroded away in the 400,000 years between the two events.

Here's the view from the opposite (north) side of Los Alamos Canyon at a different time of day. Los Alamos Reservoir is just visible at the canyon bottom in the distance.

Tshirege
          Member in upper Los Alamos Canyon
Tshirege Member in upper Los Alamos Canyon. Near 35 52.801N 106 20.292W

The section of Los Alamos Canyon between Los Alamos Reservoir and Omega Bridge is crossed by numerous strands of the Pajarito Fault Zone. Looking across the canyon to the north, one sees here two likely strands of the fault, showing as gullies with a slight displacement in the Tshirege beds.

Faults in Tshirege
          Member
Faults in Tshirege Member in  Los Alamos Canyon. 35 52.689N 106 20.615W

The fault traces are visible as two shallow gulleys in the side of the canyon. The displacement here is not particularly great.

The finger mesas of the Pajarito Plateau also extend southeast to the White Rock area and Bandelier National Monument. In fact, the Tsherige Member takes its name from Tsherige ruins, an archaeological site of the Ancestral Pueblo People located just northwest of the village. Prominent cliffs of Tshirege Member form the entire skyline north of White Rock.

Tshirege Member north of
          White Rock
Tshirege Member north of White Rock. Looking north from  35 49.452N 106 11.072W

The cliffs here are the furthest eastern extent of the Tshirege Member in this area. Here the mesas are thinner than to the north, since they are underlain by the White Rock ridge of the Cerros del Rio basalt.

Throughout the Pajarito Plateau, the Tshirege Member was erupted onto a surface of relatively gentle relief, underlain in most locations by low hills of the Otowi Member.

The furthest southeastern extent of the Bandelier Tuff is an outcropping filling an ancient meander of the Rio Grande that cuts into in the Cerros del Rio on the east side of White Rock Canyon. This outcropping may be responsible for the name of the canyon. 


White Rock

Tongue of Bandelier Tuff perched in paleocanyon on the east side of White Rock Canyon. 35 47.214N 106 11.356W

Here's the same outcropping seen from near the mouth of Potrillo Canyon, nearly to its west. We saw this in the last chapter.

White Rock Canyon

White Rock Canyon panorama showing tongue of Bandelier Tuff perched in paleocanyon. Looking east from 35.789N 106.210W

There are several large banks or hills of coarse gravel a short distance south of the White Rock Canyon Rim trail head. The gravel is well-sorted rounded tan clasts, likely of dacite. Though not mapped on any geological map, they have been identified as lake bars in New Mexico Geological Society field guides.

Gravel bank
Gravel bank. 35 49.404N 106 11.057W

There are scattered beds of similar gravel for at least a mile further down the canyon rim, and there is also a considerable quantity of this gravel on a landslide block east of this point, halfway down the canyon rim. The gravels beds appear to overlie remnants of the Tsankawi Pumice and thus must be younger than 1.25 million years in age. They were likely deposited when the Rio Grande was dammed by the Tsherige Member, creating another Culebra Lake.The dam produced by the Tsherige Member is estimated to have been 100m (180') higher than the canyon rim where these cobbles are located, and the resulting lake extended perhaps 70 km (45 miles) to the north. The Rio Grande was forced 2 km (1.25 miles) east of its former course, and had to cut through 200m (650') of basalt to reach its former level and finally drain the lake.

The Bandelier Tuff rests on basalt of the Cerros del Rio throughout the White Rock and Bandelier area. For example, there is the outcrop southwest of White Rock we saw earlier.

Contact of Tsherige Member with Cerros del Rio

Tsherige Member resting directly on Cerrros del Rio Basalt just west of White Rock. 35.811N 106.227W

This shows Tsherige Member, Bandelier Tuff, sitting on top of Cerros del Rio basalt. This is appears to be a high point of the Cerros del Rio surface. On the other side of the road, the basalt is exposed in the canyon wall descending into Potrillo canyon. There is no Otowi Tuff in this area; if it was ever deposited, it was completely eroded away before the Valles event.

Another meander of the ancestral Rio Grande is found west of the modern river, near present-day Water Canyon.

Water Canyon

Water Canyon panorama showing Bandelier Tuff in paleocanyon. Looking south from 35.791N 106.212W

Water Canyon descends from the Pajarito Plateau to the right and continues to its confluence with White Rock Canyon to the left. Part of the east rim of White Rock Canyon is visible at left, with Montoso Peak on the skyline.

The large outcropping of light pink Bandelier Tuff in the south wall of Water Canyon does not appear to be an outcropping that has slumped down the canyon wall. Instead, this is a western meander of the ancestral Rio Grande that was entirely filled by pyroclastic flows of the Tsherige Member.

As we move soutwest from White Rock, the canyons become progressively deeper and the finger mesas thicker. They reach their maximum thickness in a paleocanyon in lower Frijoles Canyon, which we saw in the last chapter.

Contact between Cerros del Rio and Bandelier Tuff in
          lower Frijoles Canyon
Contact between Cerros del Rio and Bandelier Formations in lower Frijoles Canyon.  35 45.888N 106 15.661W

The rock to the left is benmoreite of the Cerros del Rio. The steep contact with the Bandelier Tuff to the right shows that the latter filled a deep paleocanyon in the Cerros del Rio, interpreted as the previous course of the Rio Grande, prior to the Bandelier Tuff eruption.

The Tsherige Member thins out again around the San Miguel Mountains, which form a kind of island in the surrounding Tsherige Member surface. East of the San Miguel Mountains, in the southeast Jemez, the Bandelier Tuff sits on volcaniclastic beds of the Paliza Canyon Formation.

Bandelier Tuff over
          volcaniclastics of Paliza Canyon Formation in Cochiti Canyon
Paliza Canyon Formation volcaniclastics under Bandelier Tuff. 35 46.001N 106 25.159W

In this photograph, the Tshirege Member is at top, with Otowi Member exposed as white patches at two locations partway down the can yon. Below this is a jumble of heavily-eroded block and ash flows and lahars of the Paliza Canyon Formation.

On the skyline is Aspen Ridge, part of the Keres highlands that acted as a topographical barrier to the Bandelier pyroclastic flows. The Tsherige Member lapped up against Aspen Ridge, so that the whole area between Aspen Ridge and the San Miguel Mountains became a relatively level surface of Tsherige Member. This has since been cut by deep canyons and displaced by faults.

Panorama from knoll east of Aspen Ridge

Panorama from a knoll on the east side of Aspen Ridge. 35 47.888N 106 30.012W

The panorama begins to the east, where we see the San Miguel Mountains in the distance. The middle distance is a maze of mesas of Bandelier Tuff. Near the center of the panorama, to the right of distant Cochiti Reservoir, we look almost directly down Bland Canyon. To the right is the southern part of Aspen Ridge.

Though the Keres Highlands generally formed an effective barrier against the pyroclastic flows, there are isolated outcrops of Tsherige Member along Peralta Canyon and of Otowi Member in the lower canyon.

Tsherige Member in upper Peralta Canyon

Tsherige Member in upper Peralta Canyon. 35 48.113N 106 30.943W

The presence of these beds shows that Peralta Canyon already existed when the Toledo caldera collapsed 1.62 million years ago. The canyon bottom was filled with Otowi Member, which subsequently mostly eroded away, and then the canyon bottom was filled again with Tsherige Member when the Valles caldera collapsed 1.25 million years ago, which has also mostly eroded away.

Beds of Bandelier Tuff again reappear west of Cerro del Pino. These extend south to Borrego Mesa and west to Canon de San Diego in the southwest Jemez. Here the Bandelier Tuff lies directly on Permian red beds that must have already been exposed at the surface 1.65 million years ago. These contacts are sometimes quite dramatic.

Here is a case where the Permian red beds of the Yeso Group and the Glorieta Sandstone are cut by a sizable fault, which has been filled in by Bandelier Tuff. The upper beds of the Bandelier Tuff are not displaced by this fault, showing that it has not been active in at least 1.6 million years.

Fault in Permian beds buried under Bandelier Tuff
Fault in Permian red beds buried by Bandelier Tuff. Looking east from near 35 43.073N 106 43.152W

A little up the canyon, one can see hills of Yeso Group that were engulfed by the Bandelier Tuff, forming a dramatic discontinuity.

Permian hills buried under Bandelier Tuff
Permian hills engulfed by Bandelier Tuff. Looking northeast from near 35 43.073N 106 43.152W

The same contact is seen along the west side of Mesa de Guadalupe.

Mesa de Guadalupe
Mesa de Guadalupe. 35 41.718N 106 44.783W

The Tsherige Member caps most of the mesas of the Jemez Plateau west of the Caldera.

State Road 26
State Road 126 cut into Tsherige mesa top. 35 57.505N 106 42.768W

In the northwest Jemez, the Bandelier pyroclastic flows were channeled into the low ground between the Sierra Nacimiento Mountains to the west and the La Grulla Plateau to the east, forming Mesa Pinabetosa:

Mesa Pinabetosa
Mesa Pinabetosa. Looking southwest from near 36 7.650N 106 33.133W

The mesa is somewhat obscured by haze, but is the low ridge below the skyline. As in the southwest Jemez, the Bandelier Tuff here sits directly on Permian redbeds.

The Bandelier flows were channeled through the low ground between the La Grulla Plateau and the Tschicoma highlands in the northeast Jemez. Exposures of Tsherige Member begin a short distance below the caldera rim

Proximal Tsherige Member

Proximal Tsherige Member. 36 00.695N 106 29.442W

and fill the area between the two high plateaus.

Polvadera panorama
Polvadera panorama. Looking southeast from 36 06.874N 106 30.570W

The mesa across Canones Canyon is Mesa del Medio. To the left one sees the mesa underlain by Santa Fe Group with a cap of Bandelier Tuff. To the right this gives way to basalt flows of the La Grulla volcanic center. Polvadera Peak is right of center and Cerro Pelon is the smaller peak just left of center. Both are dacite domes of the Tschicoma Formation.

Bandelier
          Tuff below Polvadera Peak
Bandelier Tuff below Polvadera Peak. Looking east from near 36 7.883N 106 32.745W

Bandelier Tuff below Polvadera Peak
Bandelier Tuff below Polvadera Peak. Looking SSE from 36 13.931N 106 28.821W

The Tshicoma Highlands blocked the Tsherige flows from reaching most of El Alto, but a flow lapped onto the edge of the mesa on its west side.

Tsherige Member next to El Alto

Tsherige Member flow on west edge of El Alto. 36 06.560N 10625.029W

Tsherige Member filled much of the lower terrain in the Toledo Embayment, but it is not found again outside the caldera until we reach the north rim of Santa Clara Canyon. We are then back in the finger mesas of the Pajarito Plateau.

We finish up with the intracaldera tuffs, formed within the caldera itself. These underlie Redondo Peak and some of the hilly terrain to its north. This tuff tends to be crystal-rich and densely welded, as in this outcrop on the north side of Valle Jaramillo.

Intracaldera tuff

Intracaldera tuff near Valle Jaramillo. 35 54.857N 106 30.554W

The white specks are crystals, probably of high-temperature alkali feldspar (sanidine). The dark patches were probably pumice caught up in the flow. Notice that these have been squashed flat, so that they are systematically elongated in the horizontal direction. These are called fiamme and are characteristic of a welded tuff, which is formed from ash so hot that the individual particles are soft and weld together after settling out on the ground.

Cooling units

As we saw in San Diego Canyon, the Otowi Member is a relatively homogeneous unit. Though it probably erupted in several pulses, these were close enough together that there was no time for individual flows to cool significantly before being buried under the next flow. The Otow Member is therefore described as a simple cooling unit, with the entire thickness of the member sharing a common cooling history.

The Tsherige Member was erupted in at least five separate pulses, with enough time between pulses for each bed to cool slightly before the next was deposited. The interval between pulses was nonetheless brief enough that only a small amount of cooling took place. The Tshirege Member is thus described as a compound cooling unit. Each simple cooling unit within a compound cooling unit characteristically has a more densely welded core, with less welded upper and lower surfaces. This is because both surfaces cool faster and have less time to weld, the lower because it is in contact with the slightly cooled surface of the previous flow. Boundaries between cooling units in a compound cooling unit are sometimes marked by a vapor gap, but this is not always present, and flows can sometimes be difficult to distinguish with certainty.

The Tshirege Member shows occasional surge beds between flows. These are a good indication of a flow boundary.

Surge beds in Tshirege
          Member
Surge beds in Tshirege Member along Camp May Road. Near 35 52.689N 106 20.615W

Flows often form obvious ledges in the finger mesas, such as this ledge in upper Pueblo Canyon.

Cooling unit ledge in Pueblo Canyon
Flow unit ledge in Tsherige Member, Bandelier Tuff. 35.892N 106.316W

Further down canyon, three ledge levels are visible both in the southern canyon wall (Los Alamos Mesa):

Cooling unit
          ledge in Pueblo Canyon
Lithostratigraphic units in Tsherige Member, Bandelier Tuff, underlying Los Alamos Mesa. Looking west from near 35.873N 106.238W

A fourth unit is seen at the top the north canyon wall (Kwage Mesa):

Cooling unit
          ledge in Pueblo Canyon
LIthostratographic units in Tsherige Member, Bandelier Tuff, underlying Los Alamos Mesa. Looking northwest from near 35.873N 106.238W

These correspond to units A, B, C, and D of one classification scheme for the units making up the Bandelier Tuff. The B-C unit boundary is somewhat indistinct in Kwage Mesa, but is clearest at the right side of the photograph. Other areas in the Jemez show additional E and F units.

This scheme was developed by geologist M.A. Rogers in 1995 and emphasized lithological differences between beds that could be easily mapped throughout the Jemez Mountains. Rogers' units are not necessarily individual cooling units or flows. A competing scheme was developed at about the same time by Broxton and Reneau. Their scheme is applicable mainly on the Pajarito Plateau and does attempt to divide the Tsherige Member into cooling units. Broxton and Reneau's Unit 1 corresponds roughly to Rogers' Units A and B, 2 to C, 3 to D, and 4 to E and F, though since the Tsherige Member is quite variable across the Jemez and the two schemes were devised for different purposes using different criteria, the correspondence is not exact, particularly away from the Pajarito Plateau. In both schemes, a unit may consist of one or more flows; for example, the A unit is thought to be a single flow, while the C unit may consist of three flows. The best nomenclature for the various units of the Tsherige Member is still being worked out by geologists.

The south side of Los Alamos Mesa displays some of these unit boundaries.

Cooling units in
          Tsherige Member
South side of Los Alamos Mesa. 35.871N 106.234W

The densely welded tuff at the top of the mesa is the C unit. Immediately beneath this unit is the B unit, which is characterized by a high content of pumice in its upper beds. This produces a characteristic spongy texture.

Pumiceous
          cooling unit in Tsherige Member
South side of Los Alamos Mesa. Near 35.871N 106.234W

Each pit marks where a pumice clast was present when the tuff was laid down. The clasts exposed on the surface were subsequently preferentially eroded to produce the pits, which often contain remnants of the original pumice clast. Such zones rich in pumice beds typically mark the upper portion of a flow, while the lower portion of a flow tends to be enriched in lithic fragments and in crystals of quartz and sanidine.

The pumice content drops further down, but not abruptly. However, there is a very abrupt if subtle change in the color of the tuff, which becomes more pink. This can be inspected closely from the trail.

Possible cooling
          unit boundary
Abrupt color change, possibly marking flow boundary. Near 35.871N 106.234W\

This is also exposed on the upper surface of the mesa at a low saddle.

Possible cooling
          unit boundary
Abrupt color change, possibly marking flow boundary. Near 35.871N 106.234W

The photograph shows an area about six inches across. You can see that there is no change at all in the texture of the rock — just a color change. This may be a flow boundary within the B unit.

Two unit boundaries, probably corresponding to those dividing the A, B, and C units, can be examined at close range at Tsankawi Mesa. of the mesa on which the Tsankawi ruins sit. The more prominent is the boundary between the A and B units, which forms a very distinctive notch or shelf. Above the notch, the lower part of the B unit tends to have prominent columnar fractures. Several geologists have concluded that the A-B boundary, as distinctive as it is, was formed after the tuff was deposited through a peculiarity in the way the inner part of the flow was welded while giving off hot vapors. There is no change in the chemistry of the minerals across this vapor notch nor any change in the size and content of lithic and pumice clasts.

Tsankawi Mesa. 35 51.608N 106 13.542W

Tsankawi Mesa is the location of Tsankawi ruins, which are administered by Bandelier National Monument. The Park Service maintains a trail to the mesa top, which comes to a ladder that takes one to the upper surface of the A unit.

Upper surface of A unit. 35 51.640N 106 13.320W

This surface extends along much of the south face of the mesa, providing a natural footpath. The next photograph looks back along this natural terrace towards Pajarito Mountain.

Ledge formed on surface of A unit. 35 51.649N 106 13.300W

Here is an example of the surface becoming a vapor notch.

Boundary between A and B units. 35 51.659N 106 13.247N

You can see that the ledge becomes a notch in the side of the cliff.

South of Tsankawi Mesas, this boundary can be seen as a ledge on the nearby mesas.

Mesas south of Tsankawi showing unit boundary. Looking southwest from 35 51.663N 106 13.125N

This ledge is prominent in the cliffs north of White Rock, and can be traced to Water Canyon south of White Rock.

Water Canyon

Water Canyon. 35 48.167N 106 14.041W

The boundary is most pronounced along the south face of the canyon in frames 2 through 5.

This is the boundary between the A and B units, corresponding with Qbt 1g and Qbt 1v. In the latter scheme, the boundary is not thought to be a flow boundary, but a devitrification front. There is no change in the composition, size and number of clasts, or other obvious depositional features across the boundary. However, the A/Qbt 1g unit is composed of tuff in which the glassy shards of the ash and pumice are unchanged, while the glass shards in the B/Qbt 1v have been devitrified. The transition is remarkably sharp.

The lower part of the B/Qbt 1v unit is relatively resistant and often shows columnar cooling joints. This transitions higher in this unit to soft tuff that generally forms slopes.

The boundary between the B and C units is also accessible at Tsankawi Mesa. it is marked by a surge bed between the flows

Surge beds
              at Tsankawa B-C boundary

Surge beds at bottom of unit C on Tsankawi Mesa. 35 51.662N 106 13.001W

It is more common for the boundary to be marked by a notch.

Notch at bottom of unit C on Tsankawi Mesa. 35 51.662N 106 13.001W

Here’s a nice shot from just a little further down the trail.

Notch at bottom of unit C on Tsankawi Mesa. 35 51.671N 106 12.838W

The notch between units is clearly visible.

These units are also prominent further south, in Ancho Canyon.

Tsherige Member in Ancho Canyon

Tsherige Member in Ancho Canyon. 35 47.143N 106 15.543W

The ledge atop Unit A/Qbt 1g is very distinct. The resistant lower bed of Unit B/Qbt 1v is particularly clear at left, and above is the softer upper portion of unit B/Qbt 1v. On top are the resistant beds of Unit C/Qbt 2, which seems to form a double layer here.

Unit D (Rogers classification) or 3 (Broxton and Reneau classification) appears to consist of multiple thin flows that are particularly rich in lithic clasts. There is a remnant of this unit on the south rim of Water Canyon.

Tsherige Member
          Unit D south of Water Canyon

Unit D on south rim of Water Canyon. 35 47.434N 106 13.696W

Lithic clasts weathered out of the tuff litter the surface of the tuff.

Tsherige
          Member Unit D south of Water Canyon

Unit D on south rim of Water Canyon. 35 47.446N 106 13.632W

This unit probably marks the actual collapse of the caldera.

Units D, E, and F are visible in upper Los Alamos Canyon. We saw this photograph earlier.

Faults in Tshirege
          Member
Faults in Tshirege Member in  Los Alamos Canyon. 35 52.689N 106 20.615W

Unit F is the upper rim of the canyon. Unit E forms the slopes beneath, and the cliffs are Unit D.

Tuffs often have a distinctive chemical signature that identifies them as the product of a single batch of lava. Thus, the Peralta Tuff discussed earlier in the book is significantly poorer in iron and richer in calcium than the Bandelier Tuffs, and the Tsherige Member is significantly richer in iron than the Otowi Member. However, there are systematic differences in composition even within the Tsherige Member. Chemical analysis shows that the first flows erupted during the supereruption were less rich in mafic minerals than those erupted later in the supereruption. This compositional zoning is interpreted as the gradual emptying of a magma chamber that is richer in mafic minerals towards its bottom than its top. The high-silica magma in the chamber was strongly differentiated, and the heavier mafic minerals had crystallized and begun to settle to the bottom. (With a magma as viscous as rhyolite magma, the settling process can be extremely slow.) The later flows also tend to be more densely welded than earlier flows, indicating that the lower part of the magma chamber was also hotter.  While the Tsankawi Pumice erupted at a temperature of about 700 C, the topmost E and F units erupted at a temperature of about 850 C, and had a silica content of about 73% versus 77% for the earlier magmas.

The compositional zoning is not obvious to the casual observer in the more distant parts of the Valles outflow sheet, but it becomes more obvious near the caldera where the differences are more extreme. For example, some of the exposures of the Tsherige Member just west of the caldera rim are quite dark in color.

Mafic tuff

Mafic tuff
Mafic tuff along Forest Service Road 376 in the Jemez Plateau. 35 52.121N 106 41.964W

Likewise, the exposures of the Tsherige Member along State Road 4 where it reaches the top of the Pajarito Escarpment shows some extremely dark and densely welded tuff. This is probably part of unit E (Rogers classification) or 3 (Broxton and Reneau classification).

Tsherige
          Member on caldera rim
Highly mafic Tsherige Member from near eastern caldera rim. 35.833N 106.364W

Besides the dark color, suggesting high mafic content, the rock is much harder and noticeably denser than most samples of Tsherige Member. It is also particularly rich in crystals of quartz, feldspar, and biotite. These likely had settled towards the bottom of the magma chamber before the eruption.

The Tsherige Member is generally more densely welded than the Otowi Member, and it seems to be much less prone to forming tent rocks. However, there are some impressive vapor phase pipes in lower San Juan Canyon.

Vapor phase
          pipes
Vapor phase pipes in the Bandelier Tuff. 35.727N 106.622W

These are formed where rising vapors within the fresh pyroclastic flow followed particular paths and hardened the tuff in a cylindrical shell around each path. The pipes are hard on the surface and soft within. Or so I'm told; I didn't want to disturb them enough to confirm this for myself.

The Valles Caldera

The Valles Caldera at the heart of the Jemez Mountains is a splendid example of a giant caldera.Though not the largest caldera known, it is relatively young, exceptionally well-preserved, and show most of the characteristic features of a giant caldera.

Valles Caldera from space

This satellite photograph shows the Valles Caldera as it is today. The caldera measures about 12 miles (19 km) across and was formed by the Tsherige event. This emptied a magma chamber with a volume of 75 cubic miles (300 km3).  The caldera has a number of mountains within it, which many writers have likened to a bear claw. The pad is the large area in the center, the toes are the smaller mountains north and northeast of the pad, and the thumb is South Mountain. We'll learn about the significance of these mountains in the next chapter.

The topographic rim of the caldera is highlighted below.

Valles Caldera rim

Topograhic rim of Valles Caldera

The topographic rim is poorly defined to the northeast and southwest. In the southwest, the rim has been heavily eroded by the Jemez River in San Diego Canyon. In the northwest, erosion in upper Santa Clara Canyon has combined  with subsidence to obscure the exact caldera rim. The area of subsidence here is known as the Toledo Embayment, and, as we've seen, it is something of a puzzle. The Toledo Embayment may be part of a fault zone connecting the Jemez Fault Zone in San Diego Canyon with the Embudo Fault Zone north of Espanola.

Here is a map of precaldera rocks around the caldera.

Digital relief map of precaldera rocks in the Jemez
        region
Relief map of the Jemez with pre-Toledo exposures highlighted in red and Toledo Interval exposures highlighted in yellow.

Red areas are rocks older than the Toledo event. The exposures around the rim of the Valles caldera show the maximum possible extent of the Toledo caldera. Yellow areas are exposures older than the Valles event but no older than the Toledo event. These mark the maximum possible extent of the Valles caldera. The two are nearly identical except in the Toledo embayment.

Let's take a tour now of the caldera rim as it exists today. We'll begin with the eastern caldera rim.

The light area making up the southeast portion of the caldera in the satellite photos is the Valle Grande. It is easily accessible from State Road 4, from which the visitor sees this panorama.

Panorama of
          Valle Grande
Valle Grande. 35 51.096N 106 27.305W

Valle Grande was probably the deepest part of the caldera when it first formed. Earlier I showed a reconstruction of the basement beneath the caldera, which show that the floor of the caldera dropped considerably further on its east side than on the west, almost like a trap door hinged to the west. On the west side of the caldera, the floor dropped about 1500 meters (5000'), while the floor dropped about 4600m (15000') on the east side. The entire caldera floor was deeply blanketed by the Bandelier Tuff, which tended to level out the visible surface.

The right side of the panorama shows the eastern rim of the caldera, from the Toledo Embayment to Cerro Grande. This is the Sierra de los Valles, which also forms the western skyline of the city of Los Alamos. From left to right,the three peaks are Caballo Peak, Pajarito Mountain, and Cerro Grande.

Switching our viewpoint now to the north side of the Valle Grande, we show the southeastern rim of the caldera.

Caldera south rim

Caldera southeast rim from Cerro Pinon. 35 53.485N 106 29.538W

At left in the middle distance is Cerro del Medio, a ring fracture dome. Beyond and to the right is Cerro Grande in the southeast caldera rim. Just right of center is Sawyer Dome, the southernmost major dome of the Tschicoma Formation. Between Cerro Grande and Sawyer Dome is a low point in the rim through which State Road 4 enters the caldera, and through which pyroclastic flows of the Valles event flooded the Bandelier area and surroundings. Rabbit Mountain is the irregular mass at right, while relatively low hills form the rim between Rabbit Mountain and Conchas Peak at far right. South Mountain obscures the south rim west of Conchas Peak.

Here's this stretch of the rim viewed from the south.

Panorama east of Aspen Ridge

Panorama from east side of Aspen Ridge. 35 47.888N 106 30.012W

The panorama begins to the west, and Aspen Ridge extends across the left side of the panorama, turning east to form the south rim of the caldera. Conchas Peak is on the skyline at left. Redondo Peak forms the skyline behind the eastward leg of Aspen Ridge, and Cerros del Abrigo and Cerro del Medio are visible to the right of Redondo Peak, with the north caldera wall behind. Rabbit Mountain dominates the left center of the panorama, just left of the clump of foreground trees.

From near the Valles Preserve entrance, we get a good view further west along the south rim.

South rim

South rim of Valles caldera. Looking west southwest from near 35 49.953N 106 29.237W

The north end of Aspen Ridge dominates the left side of the panorama. This continues south into the Keres Highlands. At right, in the distance, are Las Conchas, with the pointy peak, and Los Griegos to its right. Los Griegos is the highest point on the caldera south rim. Here's a closer view from a little further west.

Las Conchas and Los Griegos

Las Conchas and Los Griegos. Looking west southwest from near 35 49.478N 106 29.994W

The rim continues west through the Valle de los Indios. This is the largest private enclave in the southern Jemez, with numerous homes and cabins in a pleasant valley making up the south moat.

South rim
          in Valle de los Indios

South rim in Valle de los Indios. Looking south from near 35 48.543N 106 35.270W

Los Griegos is at left in this panorama. The rim is relatively low to its west. Looking to the southwest:

south
          rim

South rim in Valle de los Indios. Looking south from near 35 48.529N 106 35.416W

The rim continues west towards Battleship Rock. We've seen this photograph before.Valle de los Indios

Caldera rim in Valles de los Indios.. 35 49.133N 106 36.846W

At right is the confluence of East Fork, Jemez River, with Cañon de San Diego. The canyon has cut a great cap in the caldera rim here, and we pick up the rim again further west near La Cueva.

West
          caldera rim

West caldera rim near La Cueva. 35 52.396N 106 38.644W

Notice that the west rim is as low as any part of the rim we have seen so far. This reflects the trapdoor geometry of the caldera.

Further north, the rim forms the west side of the narrow San Antonio Canyon.

West caldera rim across from San Antonio Springs

Caldera rim across from San Antonio Springs. 35 56.387N 106 38.741W

Here Paleozoic rocks of the Abo Formation are present at the base of the rim, which is capped mostly with Otowi Member with a few isolated patches of Tsherige Member.

Looking further up canyon.

Upper San Antonio Canyon

Upper San Antonio Canyon. 35 56.524N 106 38.782W

We pick up the rim again in westernmost Valle San Antonio. Here the rim is lower than any other point outside Cañon de San Diego.

Northwest caldera
          rim

Northwest caldera rim. 35 58.369N 106 35.744W

The hill in the foreground (steeper to the left) and the two rugged outcrops behind them are giant slabs of Paliza Canyon Formation andesite that originally sat on the caldera rim behind them, but which broke off and slid onto the caldera floor immediately after caldera collapse.

The remainder of the north rim is covered by this panorama from Warm Springs dome.

Warm
          Springs panorama

Warm Springs. 35 58.298N 106 33.728W

From this point, one can see almost the entire length of the northern moat of the caldera. The panorama begins looking  down the north moat to the west, and at center is the point on the caldera wall directly north of Warm Springs. The lower cliffs exposed in the rim are Paliza Canyon Formation andesites while the top of the rim is La Grulla formation andesite. Cerro de la Garita, the high point of the north rim, is to the right. The right end of the panorama looks down Pipeline Road and the north moat towards the Toledo Embayment.

We get a better view of the northwest rim and Toledo Embayment from a point further west.

Valle Toledo panorama

Valle Toledo. 35 57.461N 106 28.890W

In the middle distance at left is the northernmost part of the Cerro Santa Rosa complex and at right is Cerro Trasquilar, as known to geologists. (The Forest Service topographic map reverses these, confusingly.) Between in the distance is Cerro de la Garita. To the right of Cerro Trasquilar is more of the northeast rim, an area underlain by Santa Fe Group sediments intruded by both Paliza Canyon Formation and Bearhead Rhyolite. The right side of the panorama is dominated by Turkey Ridge, which lies across the mouth of the Toledo Embayment, while the dome at far right is Cerros de los Posos West, interpreted as a ring dome of the Toledo caldera. Cerros de los Posos is visible in the first photograph of this section, northeast of Cerro del Medio, and so we have completed our tour of the caldera rim.

Caldera fill

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

The topographic rim does not actually mark the ring fracture where the roof of the magma chamber collapsed. The steep walls left by collapse along the ring fracture were unstable, and subsequent landslides moved the rim outwards while  burying the ring fracture. These early landslides are partially buried by later sediments, but in a few places, particularly around Redondo Peak, the original caldera fill is exposed. Some of the best exposures are in the Redondo Graben, where megabreccias show where entire beds of older rock slid back into the caldera.

Megabreccias

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

The most accessible of the megabreccia blocks is located north of State Road 4 on the east side of Sulfur Creek.

Megabreccia

Megabreccia slab at Sulfur Creek. 35 52.516N 106 37.807

This low ridge is essentially a single large slab of Paliza Canyon Formation lava that broke off the caldera rim as the caldera collapsed, sliding down onto the caldera floor where we see it today.

A somewhat more spectacular megabreccia is exposed below the northwest caldera wall, at the west end of Valle San Antonio.

Collapse megabreccia
Caldera collapse landslide. Looking west from 35 58.357N 106 35.644W

We saw this photograph earlier.

Some of the largest megabreccia blocks are located near the center of the caldera, on what is now the northern slopes of Redondo Peak and Redondo Border. These form part of the ridges bounding Valle Jaramillo.

Megabreccia

Megabreccia along Valle Jaramillo. 35 54.628N 106 33.035W

The ridge on the skyline is another chunk of andesite from the caldera wall that slid down, more or less intact, into the center of the caldera almost immediately after it formed.

A spur from the ridge is underlain by a relatively small block of megabreccia, only a couple of hundred feet across.

Megabreccia

Megabreccia spur. 35 54.664N 106 32.069W

The geologic map shows the entire ridge as megabreccia, but it looks like this broke into two blocks, with a smaller block at left and a much larger block at right.

The road cut exposes part of the megabreccia block.

Megabreccia

Megabreccia in road cut. 35 54.582N 106 32.056W

Megabreccia

This rock is definitely andesite, though with some signs of hydrothermal alteration. It’s also a more or less solid block. This is just the toe end of a very large block of andesite.

Here is the main ridge, seen from the south:

Megabreccia block

Megabreccia ridge north of Valle Jaramillo. 35 54.285N 106 33.057W

Notwithstanding the foreground trees, this shows just how large the megabreccia block is. Though now faulted in several places, the original block was over 1.6 km (1 mile) in length.

There is no doubt the block is composed of the kind of andesites typical of the Paliza Canyon Formation.

Paliza Canyon
          Formation andesite on megabreccia block

Paliza Canyon Formation andesite from megabreccia block. 35 54.447N 106 33.432W

These boulders were weathered off the andesite megabreccia block.

This block is one of the two largest megabreccia blocks in the caldera. The other is located just east of Redondito.

Tschicoma megabreccia

Redondito with megabreccia block to its east (left). 35 54.093N 106 33.048W

The knob at far left is composed of Tschicoma Formation dacite.

Another large megabreccia block is found on the northern end of Redondo Border.

Megabreccia knob on
          Redondo Border

Northern part of Redondo Border. 35 54.447N 106 33.432W

The knob at top is a megabreccia block of Paliza Canyon Formation tuff and andesite.

The presence of large and seemingly intact exposures of precaldera rock in the center of the caldera contributed to the early perception that there was once a very large central volcano where the caldera is now located. The precaldera rock was interpreted as the summit  of this foundered volcano, poking through the surrounding beds of Bandelier Tuff and postcaldera sediments. However, drilling, careful mapping, and other geophysical data have conclusively shown that these are rootless beds of rock originally from the caldera rim.

Debris flows

Along with massive intact slabs of caldera rim, considerable amounts of shattered rock slid back onto the caldera floor immediately after it formed. The still-jagged caldera rim then experienced rapid erosion which often took the form of debris flows, like those we've already seen in the Puye Formation.

One such debris flow is found in lower Water Canyon west of San Antonio Campground, north of La Cueva.

Debris
          flow
Debris flow at mouth of Water Canyon. 35 53.206N 106 38.695W

The debris flow here takes the form of very large boulders of every composition from Abo sandstone to Tschicoma dacite. These form a jumble at the canyon mouth.

Debris
          flow
Lag deposit on debris flow at mouth of Water Canyon. 35 53.200N 106 38.608W

This is likely an example of lag deposit, where the larger clasts eroded out of an unsorted bed are left behind when the finer clasts are washed away.

Debris flows underlie much of the terrain north of Redondo Peak. Since these formed later, as Redondo Peak was pushed up, we'll take a look at them later in the book.

The eruption of the Tshirege Member and the collapse of the Valles Caldera were not the end of volcanism in the Jemez. In the next chapter, we'll see how volcanism resumed almost immediately after the Valles event and has continued from time to time ever since.

Next page: Resurgence &c

Copyright © 2015 Kent G. Budge. All rights reserved.