Geology of the Jemez Area, Chapter 12: The Recent Past

The previous chapter may be found here.

Soda Dam
Soda Dam. Looking east from 35.792N 106.687W

The resurgence of the Valles caldera and the eruption of the ring domes was not the end of geological history in the Jemez. Geology is an ongoing process. In this chapter, we will look at geological processes in the last half million years — the very recent past, in geological terms.

Potassium-argon dating becomes inaccurate for igneous rocks younger than about 100,000 years in age, because there isn't time for enough argon to accumulate to permit an accurate measurement. Carbon-14 dating is accurate only for ages of less than about 50,000 years. Other methods of dating are either inherently uncertain. As a result, events occurring between 100,000 and 50,000 years ago can be difficult to date accurately.

An important source of information on life during this time period is caves, where animal and plant remains are preserved by cool temperatures and dry conditions away from sunlight. Such remains are not actually considered fossils, since there has not been time for significant mineralization to take place. Little of the original organic material has been replaced by minerals carried by groundwater.

  1. Brrr!
  2. A final burst of volcanism
    1. The El Cajete Pumice
    2. The Battleship Rock Ignimbrite
    3. The Banco Bonito Flow
    4. El Cajete Lake
  3. Hydrothermal Activity
    1. Sulfur and volcanoes
    2. Hydrothermal alteration
      1. Epidote
  4. Erosional landforms
    1. Weathering and soil
      1. Desert soils
      2. Fire
    2. Colluvium
    3. Alluvial fans
    4. Landslides
      1. Toreva blocks in White Rock Canyon
      2. Borrego Mesa
      3. Vallecitos de los Indios
  5. A river runs through it
    1. Zeolite ledges
    2. The Sierra Ladrones Formation
    3. Old alluvium
    4. Terrace gravels
    5. Alluvium
      1. Sheetwash

Brrr!

Digital relief map of suspected rock glaciers in the
        Jemez Mountains
Relief map of the Jemez with suspected rock glaciers highlighted in red and Shinarump lag field in yellow.

The geological record indicates that the Earth's climate is usually mild and stable. However, there have been several intervals of geological time when the climate became unstable, with large ice sheets developing near the poles and periodically advancing over much of the temperate zone. These intervals of geological time are known as ice ages.

There have been at least five ice ages during the history of the Earth. Each occurred when the balance between heat arriving from the Sun and heat radiated away from the Earth was upset. Since astronomers have concluded that the Sun has steadily grown brighter over the last 4.5 billion years, decreases in the solar input are not a likely cause of ice ages (though there is evidence that shorter cold spells may be due to changes in solar activity.) However, the motion of the continents can affect solar heating, because continents reflect more of the Sun's radiation into space. When continents are covered by epicontinental seas, this effect is reduced. It is also reduced when continents drift away from the equator, where most of the solar radiation falls. But the most likely cause of ice ages is reduced levels of carbon dioxide in the atmosphere. Carbon dioxide is a greenhouse gas; it lets the relatively short-wavelength radiation from the Sun pass through to heat the Earth, but traps the longer-wavelength radiation re-emitted by the Earth. Carbon dioxide levels increase when volcanic activity is high or when vegetation dies off and decomposes. It is reduced when vegetation flourishes or when large areas of fresh silicate rock are exposed at the surface, allowing carbon dioxide to combine with sodium, calcium, and potassium in feldspar.

The first ice age occurred around 2.1 billion years ago, in the Paleoproterozoic, and likely was caused by reduced carbon dioxide levels from the proliferation of cyanobacteria. The second occurred in the Neoproterozoic, 770 to 590 million years ago, and was the worst ice age the Earth has experienced. It was likely caused by continuing decline in carbon dioxide levels and the movement of the continents towards the poles. This Varangian glaciation, as it is called, may have covered almost the entire earth in an ice sheet -- the "snowball Earth." When the ice finally retreated, an explosion of new forms of life took place, marking the beginning of the Phanerozoic Era. Two less spectacular ice ages occurred in the Ordovician-Silurian time interval, 480 to 420 million years ago, and in the  Carboniferous-Permian time interval, 330 to 250 million years ago.

All these ice ages were characterized by periodic advances and retreats of the ice sheets. This is probably because the advance of the ice reduced vegetation cover and allowed carbon dioxide to build up and trap more heat. When the ice retreated, vegetation began proliferating again and removed the carbon dioxide from the atmosphere, allowing the climate to cool again.

We are living in an ice age now, likely triggered by the collision of the subcontinent of India with Asia and the exposure of vast amounts of fresh silicate rock in the Himalayan Mountains. Beginning about 2.6 million years ago, which marks the beginning of the current Quaternary period, ice began building up in Antarctica, which has remained mostly covered with an ice sheet ever since. Fossil algae called dicoasters became extinct and, almost simultaneously, the first modern horses appeared, and this moment in time is formally defined as the start of the Pleistocene, the final epoch preceding our modern Holocene epoch. Greenland soon iced over as well, and there have been at least fourteen distinct glacial periods since then, each lasting about 100,000 years. These were separated by relatively brief interglacial periods of warmer weather in which the ice sheets retreated. We are living in such an interglacial period today, the Holocene, which began 11,700 years ago. Estimates of when the next glacial period will occur range from within centuries to over 50,000 years, depending on various assumptions about the causes of glacial periods (which are still uncertain) and the amount of carbon dioxide that will be released into the atmosphere by ourselves and our descendants.

During the Quaternary glacial periods, ice sheets have spread over much of North America and have had a profound effect in shaping the topography of the northern and more mountainous parts of the continent. The Great Lakes were produced by glaciation, as was Long Island. However, ice sheets did not extend as far south as the Jemez Mountains, and there is little evidence that large glaciers ever formed there. Valleys in the Jemez generally have the V-shape characteristic of valleys cut by rivers rather than the distinctive U-shape of valleys cut by glaciers.

However, there are several boulder fields in the higher terrain of the Jemez that have been interpreted as remnants of rock glaciers. Rock glaciers form when a boulder field is saturated with water which freezes to ice, and the ice does not completely melt during the summer months. The outer layer of boulders acts to insulate the ice. Such permanent ice is called permafrost. The ice accumulates within the boulder field to the point where the field becomes ductile, and the mixture of rock and ice can slowly flow downhill. Current conditions in the Jemez are much too warm to permit permafrost, even at the maximum elevations of around 3400 meters (11,000'), but temperatures were significantly lower during past glacial periods and may have been conducive to permafrost.

One of the likeliest candidates for an inactive rock glacier is located on the west flank of Cerros del Abrigo.



Rock glacier
Rock glacier. Looking east from 35 55.928N 106 30.163W

This boulder field shows definite flow features in the satellite photograph. It is also quite striking at close range.

For one thing, the toe of the field is quite steep.

Rock glacier toe

Toe of possible rock glacier. 35 55.867N 106 29.344W

Rock glacier toe

The flanks likewise are remarkably steep.

Rock glacier flanks

Flank of possible rock glacier. 35 55.850N 106 29.316W

Particularly striking is the smooth slope of the hillside on which this glacier sits. There are no scattered boulders beyond a few feet of the edge of the flow.

Views from atop the boulder field:

Rock glacier

35 55.864N 106 29.302W

Rock glacier

The pictures don’t quite convey the bowl shape of the glacier surface. One gets the impression that the center was significantly deflated when the ice melted away, which is probably about right.

The rocks are covered with lichen. This suggests the flow has not been active in a very long time. There is actually such a thing as lichenometric dating, based on the diameters of individual lichen colonies, though it’s very uncertain. But one can make a guess that the flow has not moved in at least two thousand years.

I did notice one patch at the south edge that looked very fresh. This small area had either moved recently, or a forest fire burned off the lichen. Some of the boulders were spalled, as if they had been subjected to fire.

You can see a logging road crossing the flow in the upper photograph. Here’s a panorama looking west from the edge of that road.

Rock glacier

Rock glacier. 35 55.878N 106 29.236W

Behind the trees at left is Rendondo Peak and Redondito. At center is the rugged terrain of debris flows, megabreccia, and low-volume Deer Canyon Rhyolite flows northeast of Redondo Peak.

For comparison, here is an active rock glacier on the flank of Mt. Lincoln, near Alma, Colorado.

dsci0108

Active rock glacier on Mt. Lincoln, Colorado. Looking west from 39 21.345N 1063.717W

The rock glacier sits in a hanging valley, which is a result of conventional glacier activity. During the height of the last glaciation, a glacier ran down this valley and joined  a larger glacier in the foreground valley. Now the valley is occupied by the rock glacier, which has the same characteristic steep front as the inactive rock glacier on Cerros del Abrigos.

Other interpretations of some of the boulder fields of the Jemez are that they are debris flows or lag deposits. However, the Cerros del Abrigo field, with its steep front and flanks, its lack of any matrix between the boulders, and the indications of pressure ridges in the satellite photograph, is best interpreted as an inactive rock glacier.

The period from 500,000 to 60,000 years ago marked a pause in volcanic activity in the Jemez. Sometime during this pause, but at least 200,000 years ago, the first bison arrived in North America. The appearance in the fossil record of Bison antiquus, the direct ancestor of the modern bison, defines the beginning of the Rancholabrean land mammal age of North America, which is named after the famous tar pits at Rancho La Brea in what is now Los Angeles.

A final burst of volcanism

The Jemez is not an extinct volcanic field, and will likely erupt again someday. There are, in fact, indications that a new cycle of volcanic activity has already commenced — though I hasten to add that there is little likelihood we will see an eruption in our lifetime.

The most recent eruptions in the Jemez are those that produced the El Cajete Pumice, the Battleship Rock Ignimbrite, and the Banco Bonito Flow, all of which have similar chemistry, seem to have originated from closely spaced vents, and are likely the result of a fresh batch of magma rising into the Valles magma chamber. Collectively they are known as the East Fork Member of the Valles Rhyolite.

The El Cajete Pumice

Digital relief map of El Cajete exposures in the Jemez
        Mountains
Relief map of the Jemez with El Cajete beds highlighted in red.

Sometime in the last 100,000 years, a vent opened south of Redondo Peak at El Cajete. The exact date is quite uncertain. This is because very young rocks are very difficult to date precisely by the usual radioisotope methods. There simply isn't time in a hundred thousand years for enough argon to accumulate for a reliable radiiometric date. Charcoal from burnt logs preserved in the volcanic beds from this eruption shows no measurable 14C, which means the eruption took place at least 42,000 years ago.  Other dating methods, such as fission track, electron spin resonance, thermoluminescence, and optically stimulated thermoluminescence, attempt to measure accumulated radiation damage to a rock to determine the time since the rock was hot enough to erase such damage. 21Ne dating is slightly different in that it measures accumulation of 21Ne in rocks from cosmic ray bombardment rather than the accumulated damage from the rock's own radioactivity. These methods yield highly discordant results, varying from 140,000 years from fission track dating to as little as 26,000 years from 21Ne dating.  The current accepted estimate, based mostly on electron spin resonance measurements, is that the eruption took place around 55,000 years ago.

The eruption produced an eruption column that blanketed much of the southern Jemez with pumice beds. The map above shows only those areas where the beds are most substantial. El Cajete pumice occurs as isolated drifts and thin beds over much of the southern Jemez and covers many of the Bandelier Tuff mesas in western Bandelier National Monument. Some beds occur as far east as the Cerros del Rio.

Today, El Cajete is a large meadow south of Redondo Peak.

El
          Cajete

El Cajete. 35 50.200N 106 33.278W

The flat area is mostly alluvium, carried in by water after the vent formed, but there is a patch towards the center that is an original deposit of El Cajete pumice. There are also some deep pits in the far side of the flats, used as barbeque pits for guests when this was part of a private ranch.

At left in the photograph is a rim of pumice thrown up around the vent. At right is Redondo Peak. The low ridge in the second and third frames is the east end of the Banco Bonito obsidian flow, which we'll have more to say about later on. The vent for the El Cajete pumice is though to be directly under El Cajete flats, but the vent for the Banco Bonito flow is further west, not far from the edge of the flow seen here.

Although the El Cajete eruption produced widespread air fall pumice beds, the area immediately around the vent has coarser eruption products. These include large clasts of snowflake obsidian.

snowflake obsidian

Snowflake obsidian is so named because of the white patches. These can vary in size from fairly small, as in these samples, to an inch across.

The white patches are typically cristobalite, a form of silica that is stable at high temperature. This poses a bit of a puzzle, as the temperatures measured in modern rhyolite magmas are not high enough to be in the stability range for cristobalite. The explanation is given by Ostwald's step rule, which says that supercooled liquids most easily crystallize to their most disordered solid form. Cristobalite is less ordered than quartz, and so a relatively cool rhyolite magma has an easier time crystallizing to cristobalite than quartz.

These clasts are likely remnants of an obsidian dome that formed on the east side of the vent.

Large clasts of vesicular rhyolite are also found close to the vent.

El Cajete vesicular rhyolite

Vesicular rhyolite. 35 50.118N 106 32.746W

Vesicular rhyolite is not bubbly enough to be classified as pumice, nor massive enough to be just plain rhyolite. The chunks I hefted felt like tuff. This chunk is larger than a basketball and is an impressive distance from the vent; given that it’s in a sheetwash deposit, and given the lay of the land, it probably originally landed even further from the vent.

Thick beds of El Cajete air fall pumice are exposed in road cuts along State Road 4 near the source vent.

El Cajete pumice
          along State Road 4 near the source vent
South Mountain Rhyolite, El Cajete Pumice, and Banco Bonito obsidian in road cut. Near 35 49.675N 106 35.413W

Here the pumice lies on top of South Mountain Rhyolite and in turn is buried under Banco Bonito obsidian. Here's a sample of the pumice.

El Cajete
          pumice
El Cajete Pumice. 35 49.675N 106 35.413W

This sample shows clumps of biotite and hornblende. This is in contrast to the Bandelier Tuff pumices (Guaje and Tsankawi) which contain no biotite but have numerous phenocrysts of quartz and sanidine. Some geologists interpret the mafic minerals as remnants of the deep crustal rocks from which this magma formed that did not quite melt.

There is an impressive bed of El Cajete pumice in a driveway cut as one begins the ascent out of the Vallecitos de los Indios onto the south caldera rim along Forest Road 10.

El Cajete pumice
El Cajete pumice. Near 35 48.194N 106 35.400W

El Cajete Pumice beds in the southern Jemez are relatively easily worked with simple agricultural implements and tend to hold moisture, and archaeologists have found that they played an important role in Native American settlements in the area. More recently, they have been extensively mined for use as abrasives, as an additive in concrete, for water filtration, and as the stone used to manufacture stone-washed jeans.

The Battleship Rock Ignimbrite

Digital relief map of Battleship Rock Flow exposures in
        the Jemez Mountains
Relief map of the Jemez with Battleship Rock Flow exposures highlighted in red.

The eruption of the El Cajete Pumice was followed by a series of pyroclastic flows that generally followed existing river valleys. These are most prominently exposed at Battleship Rock and along Cañon de San Diego to the north, where the Jemez River has cut back through the ignimbrite to reestablish its drainage. 

Battleship Rock
Battleship Rock. 35 49.694N 106 38.551W

Battleship Rock itself marks the confluence of the Jemez and East Fork Jemez rivers, which have cut through the ignimbrite deposited in their valleys to leave this dramatic erosional remnant. The rock is a welded ignimbrite showing classical fiamme:

Battleship
          Rock Ignimbrite
Battleship Rock Ignimbrite. 35 49.694N 106 38.551W

The rock is full of lithic fragments (bits of older rock caught up in the pyroclastic flow). The elongated brown patches are bits of soft rock, perhaps pumice or tuff (though the color is rather dark for that), that were softened and flattened as the very hot pyroclastic flow settled onto the surface and welded together.

The ignimbrite is quite glassy north of Battleship Rock, so that the canyon walls glisten under the right lighting conditions.

Battleship
          Rock Ignimbrite north of Battleship Rock
Battleship Rock Ignimbrite. Looking southeast from near 35 50 759N 106 38.075W

Battleship
          Rock ignimbrite
Battleship Rock Ignimbrite at a different time of day. Looking northeast from 35 50.645N 106 38.185W

Further north, the flow becomes less densely welded and resembles Bandelier Tuff.

Battleship
          Rock ignimbrite
Battleship Rock Ignimbrite at La Cueva. 35 52.289N 106 38.511W

The Banco Bonito Flow

Digital relief map of Banco Bonito exposures in the
        Jemez Mountains
Relief map of the Jemez with Banco Bonito exposures highlighted in red.

The most recent eruption in the Jemez produced the Banco Bonito Flow, which fills much of the southwest moat of the Valles caldera. This flow shows up clearly on satellite images and relief maps, because it is young enough to have experienced little erosion.

Like the El Cajete Pumice, the Banco Bonito Flow is well exposed on State Road 4 near its source vent.

Banco Bonito flow along State
          Road 4 near the source vent
Banco Bonito obsidian in road cut. Near 35 49.675N 106 35.413W

The flow is largely obsidian, though this is extensively devitrified, especially in the upper parts of the flow.

The relief map shows that the flow has pressure ridges and a number of large explosion pits. One of these is located on Forest Service land just east of State Road 4. Here's a view from the rim of the pit.

Banco Bonito explosion pit Banco Bonito explosion pit seen from rim. Near 35 51.268N 106 37.358W

Here's the view from the center of the pit itself.

Banco Bonito
          explosion pit
Banco Bonito explosion pit seen from center. Near 35 51.268N 106 37.358W

This kind of explosion pit probably does not represent a source vent, since these pits are distributed more or less randomly across the surface of the flow. Similar features are seen at Newberry Volcano, Oregon, and at other young obsidian flows. The likely explanation is that they are phreatomagmatic explosion craters, formed when groundwater accumulates within the upper part of the flow while its lower portion is still very hot. If drought lowers the water table, the decrease in pressure can allow the remaining groundwater to flash into steam and blow out very large craters like this one. 

Support for this theory comes from the presence of broken blocks of devitrified obsidian in and around the crater.

Banco Bonito
          devitrified obsidian blocks
Blocks of Banco Bonito devitrified obsidian. Near 35 51.268N 106 37.358W

Some very young explosion craters of this kind are found in Yellowstone, and the formation of one such crater was actually witnessed by geologists nearby. (Fortunately, they were not so close that they were in any serious danger.)

El Cajete Lake

The third known lake in the Valles caldera, following the early lake and the San Antonio lake, was the El Cajete lake. This was formed when the El Cajete and other East Fork Member eruptions filled the southwest moat and dammed the East Fork Jemez River. Lake deposits from this most recent lake are extensive in the Valle Grande, and sometimes contain El Cajete pumice.

An exposure of these beds is found southeast of Cerro la Jara.

Cerro la Jara rhyolite and
        tuff?
El Cajete lake beds. Near 35 51.155N 106 29.655W

El Cajete Lake also produced lake bars and terraces of loose gravel. One of these is well exposed in the road cut on the south flank of Cerro Pinon.

El
          Cajete lake terrance

Old lake terrance of El Cajete Lake. 35 53.353N 106 29.714W

The terrace is composed of loose gravel with considerable sand and clay, containing occasional bits of El Cajete pumice.

Hydrothermal Activity

The Jemez volcanic field is far from dead. It has experienced volcanism for at least the last fourteen million years, and, as we've seen, the most recent volcanic activity may have occurred just 55,000 years ago.

There is evidence that there is still a magma chamber beneath the Valles caldera. One clue is that seismic waves are seen to slow down as they pass beneath the caldera. This low-velocity zone is interpreted as a magma chamber that is only partially crystallized. Seismic waves passing through the low-velocity zone also experience higher attenuation than in normal rock.

Another clue is the continuing high rate of heat flow from deep underground, manifest both in a high temperature gradient in holes drilled in and around the caldera, and in hydrothermal activity in the same general area. Hydrothermal activity in the modern Jemez mostly takes the form of hot springs, though there are a few fumaroles in the Sulfur Springs area.

The most accessible hydrothermal feature of the Jemez is Soda Dam. This is a large natural dam of travertine, calcite deposited by hot water from nearby hot springs.

Soda Dam
Soda Dam. Looking east from 35.792N 106.687W

There are numerous small hot springs in the area, but the largest are just across State Road 4 to the west. These originally drained into a plumbing system within the dam. The calcite-rich water then emerged from a long fracture running the length of the dam, and the calcite was deposited on the dam surface. Highway engineers demolished the west end of the dam to improve State Road 4 in 1960 inadvertently destroying its plumbing system; the dam is now slowly eroding away.

Here's one of the feeder springs exposed by the demolition

Soda Dam feeder
          spring
Soda Dam feeder spring.  35.792N 106.687W

The green around the spring flow is partly algae, partly sulfur bacteria feeding on the sulfur compounds in the spring water. The smell of sulfur is noticeable throughout this area.

Here's the exposed calcite interior of the dam where it was partially demolished.

Soda Dam
          interior
Soda Dam exposed interior.  35.792N 106.687W

The interior is quite coarsely crystalline.

Soda Dam is geologically young, probably less than 5000 years old. Calcite from the spring waters has cemented together beds of eroded gneiss from nearby outcrops, which have now been cut by the road.

Soda Dam debris
          beds
Soda Dam debris beds. Near  35.792N 106.687W

There are a number of older travertine deposits in this area. Those near river level on the opposite side of the Jemez River are probably 60 to 110 thousand years old, while those  well above the level of Soda Dam in the nearby canyon walls are around 0.48 to 1.0 million years old. These show that the hydrothermal system here has been long-lived.

The location of the springs is significant. This is the area where the Jemez Fault crosses Canyon de San Diego, and the fault zone provides a natural pathway for ground water.

Old travertine is also found along the San Jose Fault, which runs along the west side of Borrego Mesa.

Travertine
          stack?
Breccia cemented with travertine. Somewhere near 35 40.618N 106 38.688W

This is one of several stacks of broken rock cemented together with travertine that are found in this area.

Further down slope there are occasional chunks of travertine in the landslide debris.

Travertine
Travertine. Near 35 40.837N 106 38.771W

The identification of this rock as travertine is confirmed both by its flow features and by the acid test for calcite, which rules out a flow-banded rhyolite (the only other real possibility.)

The age of the travertine deposits along the face of Borrego Mesa is unknown. However, stacks like those shown earlier seem to postdate the landslides, making them less than 50,000 years old.

Springs in the Jemez area fall into three categories. The most numerous are cold springs, which are recharged by precipitation that does not penetrate deeply enough to be affected by the deep magma chamber. These are similar to cold springs found in all but the most arid climates throughout the world. Their chief significance in the Jemez is that they tend to follow boundaries between volcanic flows, where the rock is more permeable, and thus can serve as clues when mapping flows.

The hottest springs in the Jemez area are found within the Valles caldera itself, such as at Sulphur Springs.

Sulfur Springs

Sulfur Springs. 35.908N 106.617W

Sulfur Springs is one of the most geothermally active areas of the Jemez, which was once mined for sulfur and later developed as a hot springs resort.  The resort failed during the Great Depression, and now all that is left are the old buildings of the resort, abandoned trailers, and the Detroitus visible in the second frame.

Here the groundwater has been superheated by the very hot rock beneath the caldera floor, and it is full of sulfur compounds that give it a very low pH. These acid-sulfate hot springs tend to rapidly alter rock in their vicinity to clays, iron oxides, and sinter (amorphous silica.) The area immediately around the springs is underlain by Deer Canyon Rhyolite tuffs, but these have been altered to light clay minerals by the hydrothermal activity in the area. The smell of sulfur is unmistakable.

West of the resort is a bank of heavily altered Deer Canyon Rhyolite tuff.

Deer Canyon Rhyolite, altered

Altered Deer Creek Rhyolite tuff. 35 54.521N 106 36.977W

Another area of hot springs is found at the mouth of Alamo Canyon.  This is a rocky area with bubbles emerging from several places. In some cases, there was what appeared to be sulfur deposits around the springs.

Alamo Springs

Springs at mouth of Alamo Canyon. 35.919N 106.604W

Sulfur
          deposits at Alamo Springs

It is hard to imagine that this is anything but sulfur deposited by the spring. However, at the time I visited the area, the water was icy cold. I tried several vents; only one was lukewarm rather than cold. I also found that the water had no noticeable taste. My visit was in May, and the hot springs may have been diluted by heavy spring runoff.

Outside the caldera, one finds warm springs that are dominated by chloride rather than sulfur compounds and have a neutral to slightly alkaline pH. The springs in the Jemez Springs area fall into this category. The water from these springs is meteoric water (water from rain and snow that has not been long underground).

The distinction between chloride and acid-sulfate springs is not a sharp one, and many springs show both chloride and acid chemistry. The springs around Soda Dam are moderately acid-sulfate, but neither as hot nor as low in pH as the springs at Sulphur Springs.

Sulfur and volcanoes

Sulfur is a moderately common element in the earth's crust, and it is particularly associated with volcanic activity.

Sulfur has a complex chemistry compared with elements like silicon, oxygen, or even iron with its two oxidation states. Sulfur has many oxidation states, ranging from donating six electrons in sulfates to accepting two electrons in some sulfides. Most magmas contain some sulfur, but because sulfur has a strong affinity for iron, the amount of sulfur in a magma is closely tied to its iron content. However, sulfur is highly incompatible with silicates in all but the sulfate state, where it forms rare sulphosilicate minerals. Because of this incompatibility, the sulfur in a magma will often separate into a distinct sulfide phase rich in iron and other chalcophile (sulfur-compatible) elements, and this phase can produce rich ore deposits.

One of the surest signs that fresh magma has risen beneath a volcano is increased emission of sulfur dioxide, SO2. This has a characteristic sharp smell and dissolves in water to produce an acid solution of sulfurous acid. When further oxidized to SO3, as is likely to occur at a slow rate when exposed to air, it dissolves in water to produce powerful sulfuric acid. Other gaseous sulfur compounds emitted by volcanoes include hydrogen sulfide and sulfur vapor. Hydrogen sulfide has a strong odor of rotten eggs, while sulfur vapor is typically deposited as solid sulfur crystals near fumaroles.

The most sulfur-rich magmas are alkaline basalt magmas, which can contain up to 5000 parts per million of sulfur. A value of 1000 parts per million is typical of more common basalts.

Hydrothermal alteration

We saw earlier that hydrothermal alteration is common in the Paliza Canyon Formation, particularly in its lower beds exposed in canyon bottoms and the base of the Valles topographic rim. Hydrothermal alteration is not easy to date, but this episode of alteration probably immediately followed the eruption of the Bearhead Rhyolite around 7 million years ago. A second period of hydrothermal alteration began with the Toledo and Valles eruptions and continues to the present day.

Here is an a rather spectacularly colored rock in an area of the Valle Jaramillo/Redondo graben saddle mapped as Deer Canyon Rhyolite.

Altered Deer Canyon rhyolite

Altered Deer Canyon Rhyolite. 35 54.159N 106 33.490W

The vivid green color is due to epidote. The rock is still very solid, which rules out certain other forms of alteration. There are visible quartz crystals, as well as transparent lath-like crystals whose identity I’m not mineralogist enough to identify. Hydrothermal alteration is pervasive in the Deer Canyon and Redondo Creek Formations.

Epidote

Epidote is a sorosilicate mineral with composition Ca2Al3(AlSiO4)(Si2O7)O(OH). It typically has a green color due to substitution of ferric iron for some of the aluminum.

Epidote
          sample

Epidote sample from Colorado.

Sorosilicates are a class of silicate minerals contains pairs of silica tetrahedra joined at one corner. Each pair thus has the formula Si2O7. Epidote also contains isolated silica tetrahedra, giving it a rather complex structure. It is a quite common mineral and can be a major component of some metamorphic rocks formed at low pressure and relatively low temperature.

Erosional landforms


Cerro Pedernal

Cerrp Pedernal. Looking west from 36 04.664N 106 25.280W

The landforms we see in the Jemez today are usually eroded, sometimes heavily. Northern New Mexico has undergone regional uplift over the last few million years, with the result that erosion has accelerated and is now destroying rock beds that, in some cases, are nearly 1.8 billion years old.

It seems kind of sad.

Weathering and soil

Erosion begins with the weathering of solid rock, which in some cases produces a layer of soil over the bedrock. Soils show great variety, even in as small a region as the Jemez Mountains, and I can only touch on a few aspects of soil formation.

Desert soils

The lower elevations around the Jemez are semiarid and produce characteristic desert soils. These are poor in organic matter and are alkaline, since the scanty rainfall is not efficient at leaching weathering products such as sodium, potassium, or calcium. Calcium, in particular, tends to accumulate in the subsurface as calcium carbonate to form a hard layer called caliche. We saw examples of this in Chapter 6, where the older basalts of the Lobato Formation are often encrusted with caliche.

Another characteristic of desert soil is desert pavement. This is well developed on the rim of White Rock Canyon.

Desert pavement

Desert pavement. Note quarter for scale. Near 35.821N 106.185W

The numerous exposed basalt clasts are an example of a lag deposit. The remaining soil around the larger clasts is often cemented, sometimes by hardy cyanobacteria that colonize the soil. This is fairly delicate terrain, and I'm sorry to say that the popularity of the area for hiking pretty much guarantees this desert pavement will eventually be pounded into oblivion. But I also can't bring myself to tell people to stay away; this is beauty I can't resist sharing. I'm proud of my home.

Even if there are creepy-crawlies along the trail.

Creepy-crawlie

This is Orthoporus ornatus, the desert millipede. Though uncommon, tarantulas are occasionally encountered in the White Rock area, along with many smaller species of spider.

This reminds me of an experience I had while out walking at twilight to the local grocery store. It was a pleasant warm evening, with the light fading fast. I had a head lamp lit so I could see my way.

I am accustomed to the occasional glint of light from the ground, from some crystal face oriented just right. But as I walked along, I saw a pair of greenish glints that rapidly flashed at me as I walked past. Curious, I moved closer, and saw that it was two spiders. Their compound eyes had given multiple green reflections as I moved by.

As I continued my walk, I spotted many more. The spiders seemed to be mostly of a single species, not large (the biggest had a leg span of perhaps an inch), brown striped, and clearly some kind of hunting spider. Of course, I was also a crepuscular hunter in search of prey, though my prey was an unwary gallon of milk with which to make my breakfast. Still.

The spiders were beautiful. It was the kind of magic moment I wish my wife or children had been along for.

Beyond the shadow of the ship,
I watched the water-snakes:
They moved in tracks of shining white
And when they reared, the elfish light
Fell off in hoary flakes.

Within the shadow of the ship
I watched their rich attire:
Blue, glossy green, and velvet black,
Then coiled and swam; and every track
Was a flash of golden fire.

O happy living things! no tongue
Their beauty might declare:
A spring of love gushed from my heart,
And I blessed them unaware:
Sure my kind saint took pity on me,
And I blessed them unaware.

The self-same moment I could pray;
And from my neck so free
The Albatross fell off, and sank
Like lead into the sea.

— Samuel Taylor Coleridge, Rime of the Ancient Mariner

Fire

A significant agent of erosion in the Southwest is wildfire.

Las Conchas
            fire
Las Conchas fire of 2011. Looking northwest from near 35 49.342N 106 13.467W

Under natural conditions, the Jemez Mountains are subject to frequent, but usually low-intensity, wildfires. Geologists have taken borings from older trees and stumps in the Jemez and matched burn scars with tree rings to find the dates of major fires. It appears that, prior to about 1890, widespread fires took place every five to fifteen years. These were mostly surface fires, which burned off grass, shrubs, and tree seedlings but rarely destroyed more mature trees. The frequency of fires dropped precipitously in the final years of the 19th century, probably due to livestock grazing that removed grassy fuels.

Human efforts since 1900 to reduce the occurrence of fires have likely had the perverse effect of making fires less frequent, but more destructive. Without the frequent burning out of ground debris and younger trees, fuel builds up until uncontrollable crown fires destroy the forest ecosystem. The trend towards a warmer and drier climate accentuates the hazard of wildfire. Destructive fires on the Pajarito Plateau include the La Mesa fire of 1977 (15,444 acres), the 1996 Dome fire (16,516 acres), the 1998 Oso Complex fire (5,185 acres), the 2000 Cerro Grande fire (48,000 acres), and the 2011 Las Conchas fire (150,000 acres).

But even before the arrival of humans in New Mexico, wildfires contributed significantly to erosion. The more intense fires are capable of speeding the weathering of some kinds of rock.

Fire-scorched boulder

Fire-scorched boulder on Las Conchas. 35 47.838N 106 31.687W

This boulder is in an area that was burned over by the 2011 Las Conchas fire, a particularly hot and destructive fire, and the rock show a thin layer of soot. There is also some indication of exfoliation, the shedding of an outer layer of rock, from the heat of the fire. It seems likely that this is a significant cause of weathering over geological time scales in an area where natural wildfires are a regular occurrence.

Fire also increases erosion by stripping the ground of vegetative cover. This allows raindrops to hit bare soil and rainfall to run downhill without the moderating influence of vegetation. This in turn leads to flash floods, in which a summer monsoon thunderstorm dropping heavy rain in a brief period can lead to channels further down the watershed being swept by sudden and violent floods. These can cause spectacular erosion in geologically insignificant intervals of time.

Colluvium

The earliest stage of erosion in mountainous areas is the formation of colluvium at the base of cliffs. In the Jemez area, this is particularly obvious around mesas of the Tsherige Member, Bandelier Tuff, which typically are surrounded by talus slopes cut into the underlying Otowi Member that are covered with clasts of Tsherige Member.

Colluvium
Colluvium at the base of a mesa of Tsherige Member, Bandelier Tuff. 35.801N 106.224W

Scree

Scree at base of Tsankawi Mesa. 35 48.055N 106 13.433W

Colluvium is unconsolidated fragments of rock and soil that accumulate at the base of a slope from relatively gentle processes, such as rainwater flowing over the surface. Colluvium does not include landslide deposits, which are not a gentle process, nor alluvial fans produced by the concentrated action of water. If the colluvium is mostly rock shards produced by weathering of the overhanging cliffs, it is sometimes called scree.

The Otowi Member is often difficult to distinguish from a talus slope at a distance, since it is usually present only under cliffs of more durable Tshirege Member and it tends to weather to a gentle slope. As one hikes down the trail on the south side of Pueblo Canyon onto the talus slope, one sees that it is indeed 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

Further complicating the picture is the fact that, because colluvium is composed of weathered fragments of nearby rocks, the two can sometimes be hard to tell apart.

Reworked Otowi
            Member

Reworked Otowi
            Member

Colluvium formed from Otowi Member in San Antonio Canyon. 35 56.572N 106 38.791W

The colluvium here looks like a tuff, but it’s very poorly consolidated. The geologic map maps this area as colluvium, but shows a bank of Otowi Member just up the hill. So this is reworked tuff, formed from sediments eroded off the original tuff bed. It is the historical use of tuff to refer both to primary pyroclastic deposits and to reworked volcanic ash that led to adoption of the term ignimbrite to describe primary pyroclastic deposits.

Alluvial fans

Digital relief map of alluvial fans in the Jemez
        Mountains
Relief map of the Jemez with alluvial fans highlighted in red.

Over time, colluvium is washed further downhill by streams and floods to produce alluvial fans. These are found along much of the inner rim of the Valles caldera, as well as other locations of high relief.

One can examine an alluvial fan on the Valles Caldera Trail, part of the Valles National Preserve. The trail descends the inner rim of the caldera on its southeast to the caldera floor, where the alluvial fan includes local debris flow deposits.

Image

Alluvial fan boulder deposits within Valles Caldera. 35 51.408N 106 26.110W

Image

Alluvial fan deposits within Valles Caldera. 35 51.432N 106 26.169W


Individual boulders are composed of the Tschicoma Formation dacite of the caldera rim.

Image

Tshicoma Dacite in landslide on Valles Caldera Trail

Debris flows continue to be a part of the modern Jemez landscape. They are particularly active in areas that have suffered recent fire damage, such as the area south of Redondo Peak (which was burned during the 2011 Las Conchas fire).

Redondo debris flow

Debris flow in drainage near El Cajete trail, south of Redondo Peak. 35 50.198N 106 33,135W

Landslides

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

The geological picture in many parts of the Jemez Mountains is of very hard, dense lava flows lying on top of soft sediments. This is a perfect recipe for what geologists call detachment events — that is, landslides.

Toreva blocks in White Rock Canyon

Some of the most spectacular landslides in the Jemez area are in White Rock Canyon, where Cerros del Rio basalt overlies poorly consolidated sediments of the Tesuque and Chamita Formations of the Santa Fe Group. Because the basalt is so durable, the mass wasting in White Rock Canyon consists of what geologists call Toreva blocks. These are named after the location they were first recognized (Toreva, Arizona.) An entire block of the canyon rim comes loose as a unit and slides partway down the canyon wall, more or less intact. A particular characteristic of these blocks is that the block rotates slightly towards the surface from which it detaches.

Slump block
Diagram of Toreva block

The Toreva blocks in White Rock Canyon include some of the best examples known.

From the west canyon rim, south of Overlook Park, one has a beautiful view of one of the most impressive Toreva blocks to be found below the canyon rim. The Google satellite image doesn't do it justice; try switching to Map Terrain view, although this also doesn't quite do it justice. Fortunately, it's highly photogenic.

Slump
          block
Toreva block on west side of White Rock Canyon Looking southwest from. 35 48.986N 106 11.422W

The broad meadow is a slice of basalt canyon rim that has broken loose and slumped partway down the canyon wall. You can see that the far end of the meadow is nearly at the level of the canyon rim, and the nearer end has slumped a greater distance, almost as if the block was hinged at the far end. You can see that there is a trail down the slump block that affords some nice views of cross sections of the basalt flows making up the canyon rim. I've hiked this trail a couple of times but apparently I took no pictures.

The Toreva block gives us a nice view from high above of hexagonal fracturing of the lava flow. We saw this picture earlier, in the chapter on the Cerros del Rio.

Hexagonal
        columns
Toreva block on west side of White Rock Canyon Looking southwest from. 35 48.986N 106 11.422W

It is very common for a thick basalt flow, as cools and contracts, to fracture  into roughly hexagonal columns. You can see the tops of such columns in this photograph.

Borrego Mesa

There are large landslides on the west side of Borrego Mesa. Here the hard cap is basalt of the Paliza Canyon Formation and the underlying poorly consolidated sediments are mostly Zia Formation of the Santa Fe Group. The landslides here do not take the form of toreva blocks, but more closely resemble a rock avalanche. 

Landslide
        near Borrego Mesa
Landslide west of Borrego Mesa. 35 38.671N 106 39.357W

The landslide flows include massive andesite landslide blocks.

Large
          landslide block
Large landslide block. Near 35 41.063N 106 39.112W

Large
          landslide block
Large landslide block. Near 35 41.063N 106 39.112W

Looking across the valley, one sees smaller but similar landslides of Bandelier Tuff on Chinle Group sediments.

Landslides in
          Banderlier Tuff
Landslides in Bandelier Tuff. Looking west from near 35 41.063N 106 39.112W

At center, a block of Bandelier Tuff has slumped across the Chinle Group, which are the red beds underneath.

Most of the landslides appear to be at least 55,000 years old, the age of the El Cajete Pumice. This is because beds of the pumice are found in many locations on top of the landslides.

El Cajete
          Pumice on top of landslide
El Cajete Pumice bed on top of landslide. Near 35 40.951N 106 38.709W

As you can see from the footprints, I crossed part of this patch before thinking to take a picture. Not all the landslides are older than the El Cajete Pumice; a few small slides in this area appear to lie on top of beds of the El Cajete.

The landslide deposits on Borrego Mesa include extensive boulder fields.

Boulder field
Boulder field. Near 35 40.837N 106 38.771W

This is a rockslide deposit below a prominent knob of andesite, of the same composition as the boulders here.

Vallecitos de los Indios

The northern slopes of Vallecitos de los Indios are covered with landslide deposits formed from Banco Bonito obsidian flows atop mudstone of the Abo Formation. These are relatively accessible, being crossed by the hiking trail to McCauley Springs. The size of some of the clasts in the landslide is impressive.

Landslide deposits in Vallecitos de los Indios
Landslide deposits along the McCauley Springs trail. Somewhere near 35.824N 106.638W

The largest bounders here are devitrified obsidian boulders of the Banco Bonito flow. The reddish soil shows its origins in the mudstones of the Abo Formation.

Because of their nature, landslides are difficult to date. Those in White Rock Canyon are less than 1.6 million years old, judging from the way that deposits of Guaje Pumice on the rim match deposits on the Toreva blocks. However, this is only a upper limit on the age, which may be much younger. On the other hand, we have already seen that the landslides at Borrego Mesa are mostly overlain by El Cajete Pumice, suggesting they are over 50,000 years old.

A river runs through it

The presence of the Totavi Lentil shows that the Rio Grande already existed 5 million years ago, when the Puye Formation began to be laid down. The history of this river thus overlaps most of the recent history of the Jemez volcanic field itself. Rather than trying to "interbed" that story with the story of the Jemez Mountains, I'm going to step back to 5 million years ago and tell that story now as a coherent narrative of its own.

The location of the Lentil, well west of the Rio Grande and at a higher elevation, suggests that the Rio Grande originally ran further to the west and was slowly forced eastward by accumulating sediments of the Puye Formation. The river was at least 225m (4760') above its current level when it first appears in the geologic record. The river may not yet have been fully integrated along its entire modern valley. The full integration of the Rio Chama and Rio Grande through the Espanola Basin is thought to have occurred sometime between 2.8 and 4.0 million years ago.

Once it had eroded through the basalt cap, the Rio Grande in White Rock Canyon rapidly cut deeply into the underlying soft sediments of the Santa Fe Group, The river reached its maximum depth 2.8 million years ago, at a level about 30 m (100') below the modern level. It left behind a series of terraces to the west, which are now deeply buried under the Pajarito Plateau and are known only from test wells. These divide Santa Fe sediments beneath from Puye Formation sediments above. The maximum depth is recorded by a flow of olivine basalt very close to the modern river level near Ancho Canyon, which has a radiometric age of 2.47 million years. This flow marked the beginning of a series of natural dams, formed by volcanic activity, that forced the river level back up in upper White Rock Canyon.

The highest of these dams was a flow of tholeiitic lava at the current location of Water Canyon that erupted around 2.46 million years ago. This drove the level of the river back up to 285m (935') above its current level. The resulting lake, Culebra Lake, covered much of the Espanola Basin. It is now believed that much of the preservation of the Puye Formation can be attributed to Culebra Lake, which raised the base level for erosion of the Puye sediments.

The Rio Grande had cut back down to within 130m (430') of its current level at the time of the Toledo event, 1.6 million years ago. Exposures of the Otowi Member are present on the east side of the river north of Cochiti, but the resulting dam must have been short-lived, and the river rapidly cut down to its former channel. The failure of this natural dam may account for deposits of Otowi Member-rich clasts far downriver, at Socorro and Las Cruces. Another short-lived dam was formed shortly afterwards by an eruption of basaltic andesite that overlies the Otowi Member. There is a shallow paleocanyon east of the river in this area that shows the river course was temporarily diverted.

The river was down to 120m (390') above its current level when the Valles event took place, 1.22 million years ago. At this time, the canyon was much narrower than it is today, with a maximum width of perhaps 600m (2000') and no evidence of the large slump blocks seen today.

The Tsherige Member completely filled White Rock Canyon between Chaquehi and Frijoles Canyon, and filled an eastern meander of the canyon further north.

Tsherige Member filling
          meander in paleochannel of Rio Grande

Tsherige Member filling eastern meander in paleochannel of Rio Grande. Looking east from 35 47.367N 106 12.630W.

Here the beds of the Bandelier Tuff overlie a thin layer of river gravel, which in turn lies atop Cerros del Rio basalts. From here the paleocanyon turned sharply west, roughly along the modern Water Canyon.

Potrillo Canyon panorama

Cerros del Rio basalts in the confluence of Water Canyon and Potrillo Canyon. 35 47.445N 106 12.712W

At far left and far right are the basalt cliffs that mark the sudden drop from the relatively shallow Potrillo Canyon into the much deeper Water Canyon. Water Canyon descends from the Pajarito Plateau at center right, and continues to its confluence with White Rock Canyon at left. Here part of the east rim of White Rock Canyon is visible, with Montoso Peak on the skyline.

The large outcropping of light pink Bandelier Tuff in the south wall of Water Canyon marks the westward meander of the paleocanyon. This outcropping has not slumped down the canyon wall; it was this thick when deposited 1.21 million years ago. Since older Cerros del Rio basalt and underlying Santa Fe Group sediments form the rest of the canyon wall, this shows that the Bandelier Tuff filled a deep westward meander of the river.

The paleocanyon was also filled in lower Frijoles Canyon.

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 damming of the Rio Grande near Frijoles Canyon raised the river level to the highest since its beginnings in the Miocene, at 325m (1070') above its current level. The Rio Grande was forced 2 km (1.25 miles) east of its former course, and while it likely rapidly cut through the tuff, it then encountered 200m (660') of solid basalt that would have taken significantly longer to cut through. Thus a second great Culebra Lake was formed that extended perhaps 70 km (45 miles) to the north.This likely formed the lake bars on the White Rock Canyon rim near Overlook Park.

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. The dam produced by the Tsherige Member is estimated to have been 100m (180') higher than the canyon rim where these cobbles are located.

The river was still probably around 150m (490') above its modern level 0.62 million years ago. This is based on the location of ash beds from the great Lava Creek eruption in Yellowstone at that date, which spread ash across the United States. The rate has been particular great in the last 100,000 years or so, at about 50 cm (20") per thousand years. White Rock Canyon is now about 250m (820') deep near the White Rock Overlook. The rate of incision probably increased significantly sometime between 0.7 and 0.3 million years ago, when the San Luis Basin of southern Colorado became integrated into the Rio Grande watershed, more that doubling the watershed area above White Rock Canyon.

There have been several smaller lakes over the last 70,000 years, mostly created by a large slump near Water Canyon that has repeatedly slid into the river channel during periods of unusually wet climate.The last such lake is thought to have formed about 12,400 years ago.

Zeolite ledges

There are a number of prominent ledges eroded into the Bandelier Tuff in Ancho Canyon.

Landslide
          deposits in Vallecitos de los Indios
Zeolitized ledges in Tsherige Member. 35 47.344N 106 15.836W

The ledges do not correspond to any flow boundaries or other bedding features in the tuff, and they can be traced into other canyons along the Pajarito Plateau. They are believed to show lake levels in nearby White Rock Canyon. The lake water would have saturated the tuff up to the lake level, and the few inches just above the water level would have produced an ideal environment for zeolites to form in the tuff. These are hydrous silicate minerals that fill pore spaces and increase the durability of the tuff. Radiometric analysis of the zeolite crystals gives an age of about 1 million years for the ledges, but this age is rather uncertain.

The Sierra Ladrones Formation

Digital relief map of old river gravel exposures in the
        Jemez Mountains
Relief map of the Jemez with old river gravel outcroppings highlighted in red

Deposits interpreted as old river gravel are present throughout the Cochiti area. These river gravels cannot be dated directly, but some beds are overlain further north by formations known to be about 2.5 million years old, showing that parts of the old river gravel must be older than this. The older portions of these gravel beds were mapped as "old gravels" in the classic Smith and Bailey map of the Jemez Mountains, but they have since been assigned to the Sierra Ladrones Formation of the upper Santa Fe Group.

Here's an example of such a river gravel from near Cochiti Dam. This is so well sorted, with a very distinct upper contact, that my first thought was that this was artificial, placed by highway workers in the road cuts for erosion control or some other purpose It was not until I saw the beds exposed in arroyos well away from the road that I was convinced they were a natural geologic feature.

terrace gravels at Cochiti

Old gravel in Cochiti area. 35 36.406N 106 19.681W

This is one of the younger old river gravels in this area, probably less than 100,000 years old. It is apparently at the fourth of five distinct terrace levels that have been mapped in the area. The silty, tan beds above the gravel are mapped as eolium, which is silt and fine sand brought in largely by the wind.

Similar old gravels are found in the Abiquiu area and probably represent gravel deposited by the ancestral Rio Chama.

Old gravel in Abiquiu area. 36 12.749N 106 16.916W\

This is apparently an old axial gravel of the Rio Chama, deposited on Chama-El Rito Member, Tesuque Formation. The flat ground above has ruins of an Ancestral Pueblo People city, Poshuinge.

Some chapters back, we saw that there is a distinctive bed of gravel between the Tesuque Formation and overlying terrace gravels throughout the area west and north of Espanola. This marker bed is particularly well exposed at Arroyo Largo.

Quartzite gravel marker bed

Quartzite gravel marker bed. 36 01.457N 106 05.527W

This bed is very distinctive, consisting of well rounded clasts of quartzite typical of the Tusas Mountains to the north. It mark a geologically brief interval sometime in the last two million years when the ancestral Rio Grande and its tributaries in the Espanola area carried a heavy load of gravel from the mountains to the north. One wonders if it correlates with the beds we've just seen at Abiquiu and Cochiti and, if so, what event it records.

Old alluvium

Digital relief map of old alluvium exposures in the
        Jemez Mountains
Relief map of the Jemez with old river gravel outcroppings highlighted in red

Closely related to the old river gravels are deposits of old alluvium north of the Cochiti Area. Smith and Bailey, in their seminal mapping of the Jemez area in 1970, treated this as a distinct unit because of its location beneath the Otowi Member, but more recent detailed quadrangle maps interpret these beds as a mixture of Cochiti Formation and of pediment gravels.

About half a million years ago, the upper and lower reaches of the Rio Grande finallly became integrated in the El Paso area. This dramatically dropped the base level of the upper Rio Grande, which had continued to slowly deposot sediments in the northern Rio Grande Rift up to that time. Thereafter rapid erosion set in, producing most of the badlands seen along the northern Rio Grande today.

Terrace gravels

Digital relief map of terrace gravels in the Jemez
        Mountains
Relief map of the Jemez with terrace gravels highlighted in red.

The ultimate fate of material eroded from the Jemez Mountains is to be carried down the Rio Grande River to the Gulf of Mexico. However, only clay and fine silt make the trip directly. Larger clasts work their way down river channels in several steps, likely driven by infrequent but powerful flood stages of the river. 

Northern New Mexico has experienced regional uplift for at least the last several million years, and as a result, its rivers and streams are slowly cutting down into the underlying rock. This results in river terraces being left behind as the river cuts below its former bed. Such terraces are particularly prominent along the Rio Grande and can be interpreted as a former flood plain. 

There are impressive contacts between terrace gravels and the underlying Abiquiu Formation west of Abiquiu. Here the terrace gravels are derived from red source rocks, making a stark contrast with the white Abiquiu Formation.

Terrace gravel
          atop Abiquiu Formation
Terrace gravel on Abiquiu Formation. Looking northwest from 36 12.751N 106 20.507W

The terrace gravel is the red bed about halfway up the hill side. The white beds beneath and in back are Abiquiu Formation. The terrace gravel thus is perched on the side of the hill, at what was once the level of a side channel of the ancestral Rio Chama.

Terrace gravels associated with the ancestral Rio Chama extend further south, and are well-exposed along Highway 84 between Abiquiu and Medanales. Here the gravels form low hills west of the highway.

Terrace gravel
Terrace gravel. 36 11.809N 106 14.396W

The terrace gravels are also well-exposed along Thirtyone Mile Road west of Espanola.

Terrace gravel along
          Thirtyone Mile Road
Upper terrace gravel along Thirtyone Mile Road. 36 00.659N 106 07.021W

Nearby the underlying Santa Fe Group sediments are exposed.

Terrace gravel along
          Thirtyone Mile Road
Upper terrace gravel along Thirtyone Mile Road. 36 00.659N 106 07.011W

The large, well-rounded clasts show that this deposit is an axial deposit, representing the major river channel in this area. The road log for this area identifies this as the ancestral Santa Cruz River. Note that the deposit in the second photograph has been heavily calichified.

There are beds of terrace gravel along the main highway to Los Alamos near the point where it crosses the Rio Grande.

Terrace gravel
Terrace gravel on top of Santa Fe Formation. 35.878N 106.144W

These can be difficult to distinguish from the Totavi Lentil of the Puye Formation, but their location close to the river, without any overlying beds, hints at their true nature.

Alluvium

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

Finally, we reach the current river beds themselves, where alluvium is being transported today or has been transported in the recent past. River channels and their floodplains are among the more obvious and recognizable of land forms.

Some of the most impressive alluvium deposits are associated with the valles of the Valles caldera. These are broad valleys winding among the ring domes that are devoid of timber but support lush grass. We've seen many photographs of these areas in previous chapters.

The lack of timber arises from two causes. First, the valles are floored with thick beds of clay in which trees have considerable difficulty putting down roots. Second, the winter climate in the valles can be brutal.

Valle Grande fogged in

Valle Grande fogged in. 35 51.103N 106 27.312W

This is a photograph of Valle Grande, the largest of the valles, on an October morning. The layer of fog is not unusual. The caldera accumulates cold air, producing a temperature inversion that often leads to the formation of fog or a low cloud layer. During the winter months, when there can be long stretches of dry and sunny weather, the snow cover evaporates off the valles floors and leaves tree seedlings exposed to bitter night cold. This is more than they can take.

Alluvium is also found along river channels, but here there is a delicate balance between deposition and erosion. Where fire has stripped cover and increased the power of water, erosion can predominate, as in the areas burned by the 2011 Las Conchas fire. Here one can find some real gullies.

Gullies near El Cajete

Some real gullies. 35 50.198N 106 33.135W

They’re real, and they’re spectacular. This section was at least ten feet deep, and it cut right across my path. I had to work upstream a modest distance to find a spot where I could cross.

These channels represent concentrated flow. If the watershed of such a channel were a square kilometer, and erosion in the watershed averaged just a centimeter per thousand years, this would correspond to a cube of debris more than two meters on a side moving down the river each year. Most likely the rate is much lower in ordinary years and much higher in years following a fire or other disturbance. There is evidence that deposition of alluvium in the canyon bottoms of the Pajarito Plateau experienced a burst of alluvium deposition between 12,000 and 8000 years ago, based on carbon dating of charcoal fragments. A borehole in Ancho Canyon found a floodplain deposit 10,000 years old five meters (16.5') below the present canyon floor.

Sheetwash

Alluvium tends to be concentrated in river channels, but in arid climates areas of low relief may become covered with sheetwash.

Sheetwash

Sheetwash deposits on valley floor. 35 50.118N 106 32.746W

Sheetwash is deposited where flood waters run across a large surface without being channeled. It is distinct from a floodplain, formed by regular seasonal overflow of a river from its channel. Sheetwash deposits occur where there is no nearby river channel.

Our story has now reached the point where it becomes our story. In the next chapter, Homo sapiens arrives on the scene.

Next page: Emerging from deep time

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