Geology of the Jemez Area, Chapter 9: Mafic Volcanism Resumes

The previous chapter may be found here. 

Four million years ago, as the Tschicoma eruptions approached their climax, large volumes of basalt began to be erupted around the periphery of the Jemez volcanic field where it overlaps the Rio Grande Rift. In this chapter, we will examine these basalt fields and their origins.
  1. Peripheral Basalt
    1. The Servilleta Basalt
    2. The Cerros del Rio
      1. Maars
      2. Basalt flows
      3. West of the Rio Grande
      4. Andesite domes and flows
    3. Santa Ana Mesa
    4. El Alto Mesa
  2. Culebra Lake
  3. The Gravel of Lookout Park

Peripheral Basalt

Digital relief map of El Alto, Cerros del Rio, and
        Santa Ana Mesa exposures in the Jemez Mountains
Relief map of the Jemez with El Alto (north), Servilleta Basalt (northeast), Cerros del Rio (southeast), and Santa Ana Mesa (south) outcroppings highlighted in red

We've had some encounters with basalt already. It is the main rock type of the early Lobato Mesa Formation, and it is also found in the Paliza Canyon Formation. However, some of the best exposures of basalt are more recent and are located around the periphery of the Jemez volcanic field. The most voluminous of these is the Cerros del Rio field, but there are similar basalt flows at Santa Ana Mesa and at El Alto.

Basaltic lava is low in viscosity and can easily flow for miles from its source vent. As a result, it tends to form flat plateaus rather than high peaks. These are often quite distinctive in aerial photographs, as the previous Google Map links showed. The basalt flows we will be examining in this section all erupted as the higher-silica Tschicoma eruptions of the Sierra de los Valles were beginning to wind down. The Cerros del Rio dates from about 4.6 to 1.6 million years ago, peaking at around 3.2-2.4 million years ago; Santa Ana Mesa is about 2.5 million years old; and the El Alto flows are around 2.8 million years old.

The pattern of basaltic volcanism around the periphery of an existing felsic volcanic field is seen in other parts of the world, such as the Yellowstone caldera. One explanation for this pattern is that fresh basaltic lava rising from depth is blocked by the great mass of viscous magma in the magma chamber at the heart of the volcanic field. Because this high-silica magma has a relatively low density, the higher-density basaltic lava stagnates underneath it. However, the basaltic magma can reach the surface further out, where the magma chamber has not yet developed, the rock is colder and denser, and the magma can find clear paths to the surface through faults. 

The Servilleta Basalt

The Servilleta Basalt erupted in the Taos area from 4.8 to 3.7 million years ago, about the same time as the oldest Cerros del Rio basalts. Though it is not part of the Jemez volcanic field, this voluminous formation includes the massive flows underlying Black Mesa, north of Espanola, which every seasoned explorer of the Jemez area is familiar with.

Black Mesa
Black Mesa north of Espanola

A major system of faults runs along the visible south face of this mesa, which offsets the Rio Grande Rift to the east north of Espanola.

The Cerros del Rio

The most voluminous of the basaltic formations erupted during this time period is the Cerros del Rio Formation. This is exposed as a large plateau east of the Rio Grande and south of White Rock, but there are numerous exposures west of the Rio Grande that suggest that the Cerros del Rio Formation extends for some distance northwest beneath the younger Bandelier Formation. Drilling on the Pajarito Plateau has brought up basalt resembling Cerros del Rio basalt as far northwest as a well more than halfway from White Rock to the Sierra de los Valles, and it is thought the formation may extend almost to the Pajarito Fault east of the San Miguel Mountains. However, the Bandelier Formation rests directly on Paliza Canyon andesite in upper Los Alamos Canyon, showing that the formation does not reach this far.

Because the Cerros del Rio is the largest of the peripheral basalt fields, and because it is the only one that is located mostly on public lands, I will describe it in some detail.

Maars

The earliest eruptions in the Cerros del Rio field likely took the form of maars. Maars are formed when magma rising from below comes into contact with an aquifer saturated with water. The water flashes into steam, blowing the country rock and magma into small fragments that settle around the vent to form a distinctive very shallow crater. Unfortunately, none of the maars in the Jemez area are young enough to be well-preserved as surface features, but the early maars of the Cerros del Rio probably looked something like Menan Buttes in southeastern Idaho.

North Menan Butte
North Menan Butte, an example of a  young maar. 43 47.100N 111 58.607W

Beds of shattered rock and volcanic cinder produced by the interaction of groundwater with magma are described as phreatomagmatic, from the Greek phrear, "spring" or "well".  Here are some of the phreatomagmatic beds around North Menan Butte.

North Menan Butte
          beds
Cinder beds at North Menan Butte, an example of a  young maar. Near 43 47.100N 111 58.607W

Here's a sample of the phreatomagmatic beds.

Maar tuff
Maar deposit. Near 43 47.100N 111 58.607W

The presence of several maars in the Cerros del Rio suggests that this area was close to the ancestral Rio Grande three to four million years ago, so that the country rock was well saturated with groundwater. The associated phreatomagmatic beds usually lie directly on Santa Fe Group sedimentary beds and are usually buried under later Cerros del Rio lava flows.

The most accessible of these beds for the casual explorer are on the west slopes of Buckman Mesa, which is prominent as seen from State Road 502 at the northern entrance to White Rock Canyon.

Panorama of
          northern White Rock Canyon

Buckman Mesa at the northern end of White Rock Canyon. Looking south from 35.876N 106.144W

The mesa is capped with thick beds of andesite and basaltic andesite. Peeking out under these are beds mapped as phreatomagmatic deposits. Presumably the vent was Otowi Peak, which sits on top of the mesa in the last frame, and where the phreatomagmatic beds reach their greatest thickness.

If one wishes to view a maar deposit close up, there is no better place than lower Frijoles Canyon in Bandelier National Monument. The Upper Falls Trail provides a fairly easy hike from the visitors' center to some impressive maar beds.


Maar deposits
        below Upper Falls, Frijoles Canyon
Maar deposits below Upper Falls, Frijoles Canyon. 35 45.787N 106 15.578W

A closer look:

Maar
        deposits below Upper Falls, Frijoles Canyon
Maar deposits below Upper Falls, Frijoles Canyon.

The maar deposits are overlain by thick flows of benmoreite, which is sodium-rich trachyandesite. Trachyandesite, like trachybasalt, is unusually rich in sodium and potassium, and like andesite, it is slightly richer in silica than basalt. Potassium-rich trachyandesite is known as latite, and we saw examples from the Espinaso Formation earlier in the book.The benmoreite at Upper Falls has a radiometric age of about 2.75 million years.

Of course, we can't leave Upper Falls without looking at the falls!

Upper
        Falls, Frijoles Canyon
Upper Falls, Frijoles Canyon

So if there is an Upper Falls, is there a Lower Falls? Yes, here. Alas, recent forest fires in the Jemez have resulted in violent flash floods in Frijoles Canyon, and these have completely erased the trail below Upper Falls.

Trail closed
"You ... shall not ... pass!"

You may have noticed the rusty color of much of the maar deposit and the overlying basalt. This is particularly pronounced at one point on the trail just above the falls.

Hydromagmatic
        alteration
Hydrothermally altered benmoreite along Frijoles Canyon Trail

The red color comes from hydrothermal alteration of the benmoreite. The hot rock in the vent near here continued producing steam from the aquifer for some time after the initial eruption, which reacted with the iron minerals in the benmoreite to produce red hematite.

Another maar deposit is found at Cochiti Dam, right next to the main outlet channel.

Cochiti maar
Tuff ring deposit at Cochiti Dam. 35.617N 106.322W

The grayish beds to the right in the photograph are a remnant of a tuff ring, which is the deposit immediately around a maar crater. The tuff contains numerous quartzite clasts probably derived from river gravels deposited here by the ancestral Rio Grande.

A particularly large maar is located at the foot of Montoso Peak on the east rim of White Rock Canyon.

Montoso Maar
          panorama

View from north rim of Montoso Maar. 35.765N 106.211W

Montoso Maar is likely one of the oldest parts of the Cerros del Rio. The thick maar beds here were subsequently buried by more sediments and by thick basalt flows from Montoso Peak. When the Rio Grande cut the canyon to the west, the maar was exposed, and the relatively soft maar beds have since mostly eroded out.

Montoso Peak itself is on the skyline in the first frame, and its flows extend across the next two frames. In the last two frames, we see the far rim of White Rock Canyon, with Bandelier Tuff sitting on top of Cerros del Rio Basalt sitting on top of Santa Fe Group sediments. The mouths of several tributary canyons to White Rock Canyon are visible. In the next to last frame, the San Miguel Mountains are on the skyline, and the skyline in the final frame is the Sierra de los Valles west of Los Alamos. You can see one of the technical areas of Los Alamos National Laboratory on top of the mesa on the boundary of the two final frames, with a large VLA dish radio telescope. The tributary canyon to the right of the mesa is Ancho Canyon.

Note the junked appliance in the foreground in the final frame. Geologists sometimes refer to such deposits as “locally-derived anthrogenic colluvium”, or, when it consists of automobile parts, simply as “Detroitus.”

Here’s a closer look at the maar beds.

Montoso
          Maar beds

View from north rim of Montoso Maar. 35.765N 106.211W

Notice the great thickness of the beds, over a hundred meters in this area. This must have been a truly enormous maar, though not as large as the enormous Seward Peninsula maars in Alaska, which are over three hundred meters thick.

Eventually the maar eruptions transitioned to cinder code eruptions, as the supply of ground water was depleted (either through the aquifer being sealed off or the ancestral Rio Grande shifting away from the maars.) Otowi Peak, which we saw earlier, seems to have started as a maar and transitioned to a cinder cone. Another cinder cone remnant is visible in the east wall of White Rock Canyon directly across from the village of White Rock, where it has been cut in half by erosion.

Eroded cinder cone in wall of White Rock Canyon
Eroded cinder cone viewed from 35.810N 106.201W

You can see that some of the beds here are a bit lighter than the usual dark basalt beds of White Rock Canyon. The cinder cone appears as a low area in the canyon rim, probably because later flows ponded around the cinder cone, which was subsequently mostly eroded away to leave a depression. This is unmistakable in the overhead satellite view.

The cinder cone most accessible for close examination is located along Forest Road 56, where it ascends onto the Cerros del Rio Plateau from the east. Here the road cuts through this small cinder cone.

Cinder
          cone at northeast corner of Tsinat Mesa

Cinder cone in road cut. 35.617N 106.126W

The dip on the beds strongly suggests that the area in the third frame is a breach in the cinder cone wall. The breach is now filled with sediments and blocks of the overlying lava, which forms a thin bed over the cider cone.

Here are the beds close up.

Beds
          in cinder cone

Cinder cone in road cut. Car keys at lower left for scale. 35.617N 106.126W

These are very typical cinder beds, thrown up around a vent where blobs of basaltic lava were blasted into the air as gas escaped from the lava, with the blobs cooling in the air before landing around the vent. Such vents often also have a flow of degassed lava emerging from the base of the cinder cone, but here the flow likely would have been to the northwest, and is now buried under later flows.

The cinder has some commercial value. Apparently the breach here was actually excavated for cinder and is not a natural feature. Further up the road, there are two large cinder quarries, one active. The inactive quarry has a locked gate, but can be viewed from a nearby hill.

Inactive
          cinder quarry

Inactive cinder quarry. 35.620N 106.141W

The cinder is here oxidized to a deep red. The color makes coarse cinder valuable as decorative stone, and finer cinder is used on highways to improve traction.

Basalt flows

Following the maar phase of the Cerros del Rio field, basalt magma continued to rise from the depths. The magma would have shot out of the vent with considerable velocity, driven by the formation of gas bubbles in the magma as it approached the surface. Some of the bits of lava cooled while still in the air, falling to the surface as solid cinder that built up a cinder cone around the vent. However, most of the magma, still liquid but now free of gas, pooled around the vent and began flowing away from the cinder cone, often for miles. Repeated eruptions of this nature from multiple vents built up the plateau of the Cerros del Rio.

The layers of basalt flows making up most of the Cerros del Rio Formation are best examined around the perimeter of the plateau. Because the basalt is far more durable than the underlying soft sediments of the Santa Fe Group, erosion has created an escarpment around nearly the entire perimeter of the Cerros del Rio. Only along a short stretch of the eastern perimeter is the escarpment low enough for easy access by vehicle.

We'll begin our tour of the perimeter of the Cerros del Rio at Overlook Park on the west rim of White Rock Canyon.


Panorama
            of White Rock Canyon
White Rock Canyon viewed from 35 49.602N 106 10.768W

White Rock Canyon is part of the valley of the Rio Grande River, which has cut through the Cerros del Rio Formation into the softer sediments of the Tesuque and Chamita Formations underneath. This produces spectacular cliffs and large landslides where parts of the basalt plateau have broken loose and slid down into the canyon.

The first frame shows mesas of light-colored Bandelier Tuff sitting on a plateau of much darker basalt of the Cerros del Rio. Black Mesa is visible in the left of the second frame, in the far distance. It is likely a far northern outlier of the Cerros del Rio field. Centered in the second frame in the middle distance is Otowi Peak sitting atop the basaltic andesite of Buckman Mesa, which we saw from a different angle earlier in this chapter.

The third frame shows mostly Chamita and Tesuque Formation beds of the Santa Fe Group. This area contains several wells supplying water to the city of Santa Fe.

The remaining frames show upper White Rock Canyon, with its rim of basalt and basaltic andesite overlying Santa Fe Group sediments. Just left of center in the fifth frame is the cinder code we saw earlier. The skyline above the cinder cone is underlain by thick flows of andesite erupted from Ortiz Mountain, whose peak is hidden to the south. This is one of the younger flows of the Cerros del Rio, with a radiometric age of 2.32 million years.

To the left in the final frame is Montoso Peak, which is underlain by basalt with a radiometric age of about 2.59 million years.

While the eastern rim of White Rock Canyon forms the western escarpment of the Cerros del Rio Plateau, lava flows of the Cerros del Rio Formation are also present along the entire western rim of White Rock Canyon. The western boundary of the Cerros del Rio Formation is buried under younger tuffs of the Bandelier Formation west of White Rock, as we noted earlier. After we've completed our tour around the Cerros del Rio Plateau, we'll comer back and look at some of the outcroppings of Cerros del Rio Formation west of the Rio Grande.

There are two trails into White Rock Canyon from the village of White Rock, the Blue Dot Trail and the Red Dot Trail. They are so named because the trails are marked with paint spots of their respective colors. The Blue Dot Trail is a fairly steep trail from the canyon rim just south of Overlook Park down to the level of the Rio Grande. It is also well maintained, with such features as rock steps.

Blue Dot Trail steps

Achh, ssss, it's the only way yes Precious ...  35 49.430N 106 11.008W

After the initial descent down the basalt cliffs, the trail comes out on a slump block. The landslides in the area unfortunately conceal the contact between the Cerros del Rio Basalt and the underlying Santa Fe Group, but the presence of large amounts of tan sediments in the landslides shows that this slump block is already below the level of the contact.

Slump block
Slump block on Blue Dot Trail. 35 49.357N 106 10.970W

The terrain here is level enough to set up camp, and I have actually camped overnight with my son's Boy Scout troop here, at roughly the spot shown in the center of the photograph. Note the considerable amount of well-rounded clasts along the trail; these match a similar gravel bank on top of the rim adjacent to this point, deposited as a gravel bar from one of the lakes formed when eruptions in the Cerros del Rio dammed the ancestral Rio Grande River.

If one detours well off the trail, continuing south along the slump block, one sees what looks very much like an eruptive center, underlying the gravel mesa described in the previous section. This cannot be spotted from above, but is only visible from the slump block well off the trail.

Eruptive center?
Eruptive center in cliffs of White Rock Canyon. 35 49.296N 106 11.046W

Why do I think this is an eruptive center? Well, for one thing, the rock here is much lighter than the surrounding rock, and close examination suggests that this is because it is more coarsely grained than the surrounding basalt -- almost like a diabase.

Diabase?

For another, the rock is foliated in a way that looks like a subsurface dome. I'm not sure these photographs do it justice.

Foliated
        rock in dome?

Eruptive
        center?

It's a bit of a subtle thing, but the texture of the rock is different here from anything I've seen anywhere else in the canyon. However, it resembles the benmoreite outcroppings around Upper Falls in lower Frijoles Canyon. The relatively high viscosity of this magma could explain why the rock here formed a plug rather than flowing easily from the vent.

The sheer cliffs  forming the rim of White Rock Canyon are ideal for rappelling, and one finds rope anchors at scattered points along the rim, which are readily accessible from the Rim Trail.

Rappel
        anchor
Rappel anchor in Cerros del Rio basalt on White Rock Canyon rim. 35 49.285N 106 11.069W

The rock into which this anchor has been sunk is described in the most recent paper on the Cerros del Rio as hawaiite. This is a sodic trachybasalt found throughout the Cerros del Rio. A trachybasalt is a basalt that is high in sodium and potassium, and a sodic trachybasalt has significantly more sodium than potassium.

Most of the rocks of the Cerros del Rio are somewhat alkaline, so that some of the basalt is actually trachybasalt and some of the andesite is actually trachyandesite. This is typical of mafic rocks produced by continental rifting like that along the Rio Grande Rift. It reflects a low degree of partial melting of undepleted mantle.

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, which looks down on a portion of the basalt flow that has slumped into the canyon.

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

Further south along the Rim Trail a branch of the rim trail affords a view into the confluence of Pajarito Canyon and White Rock Canyon. Here an intermittent stream has carved a very deep canyon into the thick basalt flows.

Con fluence
Confluence of Pajarito Canyon and White Rock Canyon. 35 48.64N 106 11.95W

The basalt is several hundred feet thick at this location, much thicker than in most of the canyon. Here's a view from below, from along the Red Dot Trail.

Confluence from Red Dot Trail
Confluence of Pajarito Canyon and White Rock Canyon. Looking north from near 35.808N 106.198W

Most likely, the basalt here fills a paleocanyon coincident with the present-day Pajarito Canyon. This closer view shows that there are at least four distinct flow units in the basalt.

Confluence from
          Red Dot Trail
Thick basalt flows at confluence. Looking north from near 35 48.550N 106 11.919W

At the base of this very thick section of basalt is a bed of cinder.

Cinder bed

Cinder bed

Cinder bed at confluence of Pajarito Canyon and White Rock Canyon. Looking northeast from near 35.810N 106.199W

The reddish color represents oxidation, either at the time the cinders were deposited or when the overlying basalt was deposited. The darker color near the top of the bed suggests baking by the overlying basalt flow, but such baked zones are usually quite thin. Perhaps both processes were at work here.

There is a very distinct ledge in this area, formed by the boundary between two successive basalt flows. The boundary is visible in this photograph as a horizontal joint near the top of the cliff to the right of center.

Joint
        between cooling units

The ledge is formed when the top flow is eroded away to expose the flow underneath, which is apparently slightly more resistant. One can see ropy patterns on the top of the lower flow, typical of pahoehoe flows.

Ropy lava

This kind of preservation of an original flow surface is rare in White Rock Canyon, occurring only where relatively recent erosion has exposed a lower flow that was previously protected from the elements by an overlying flow. Here is a close up of the boundary between the upper and lower flows.

Flow
        boundary

This area also has a number of petroglyphs, which we'll discuss later in the book.

Here's a view from further south in White Rock Canyon, at the trailhead of the Red Dot Trail.

Panorama of
              White Rock Canyon
White Rock Canyon viewed from 35.810N 106.201W

The cinder cone in the canyon wall now appears at the boundary of the first and second frame. The third frame shows a cliff of light-colored Bandelier Tuff laid down in a paleocanyon in the Cerros del Rio. This may be the cliff from which the canyon gets its name. The final frame is again centered on Montoso Peak in the distance.

Here's a closer look at Montoso Peak and the canyon to its north.

Looking down
              White Rock Canyon
White Rock Canyon looking south from 35.810N 106.201W

In the middle distance, a large shoulder of basalt is visible on th east side of the canyon. The peak behind and to the left of Montoso Peak is Cerro Micho, which is underlain by andesite 2.76 million years in age. The cone at the extreme left in the photograph is an outlier of Cerro Micho underlain by basaltic andesite that is slightly younger, at 2.73 million years in age.

The Red Dot Trail is another well maintained from the canyon rim in the Pajarito Acres subdivision to the river. The basalt cliffs are particularly impressive here.

Cliffs above Red
          Dot Trail
Cliffs above Red Dot Trail. Near 35 48.508N 106 12.097N


Cliffs below
          Red Dot Trail
Cliffs crossed by Red Dot Trail. Looking west from near 35 48.548N 106 12.019W

The furthest point south in White Rock Canyon that is easily accessible from White Rock is near the mouth of Potrillo Canyon.

White Rock Canyon

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

The light pink outcrops in the second frame are the cliffs of Bandelier Tuff seen in the last panorama. The pyroclastic flows that produced these cliffs likely flowed down the ancestral Water Canyon and crossed the ancestral White Rock Canyon to form the beds here. These beds mark the limit of the Bandelier Tuff to the southeast.

There is a kind of natural amphitheater in the fourth frame, formed by landslides. This exposes a great thickness of Cerros del Rio lava flows lying atop beds of phreatomagmatic cinder. The basalt shoulder from the previous photograph is now visible to the left of this natural amphitheater, on the boundary of the third and fourth frame. It is identified on the geologic map as a hypabyssal plug, which cooled at some depth below ground. I am inclined to interpret this as a paleocanyon that was filled to a great depth with lava.

Near the boundary of the final two frames, some distance down the canyon, is a conical hill of basaltic andesite at the mouth of Ancho Canyon. This is likely an erosional remnant of a natural dam that once blocked the canyon at this point, where it is particularly narrow. This dam produced a large lake, which geologists have named Lake Culebra, filling the entire Espanola Valley to the north. Lake bed deposits are scattered across the mesas to the west of the Rio Grande, north of Pueblo Canyon and the main highway to Los Alamos. We'll have more to say about Lake Culebra later in this chapter.

The eroded valley in the next to last frame, at the foot of Montoso Peak, is Montoso Maar, which we saw earlier.

South of its confluence with Potrillo Canyon, White Rock Canyon is largely inaccessible except by boat along the Rio Grande. It was once possible to reach the canyon via a fairly easy hike down Frijoles Canyon from the Bandelier National Monument headquarters, but the trail is now washed out. The next point that is still accessible is the area of Cochiti.

Here the escarpment becomes particularly spectacular, in part because it coincides with a major fault. Both escarpment and fault bear the name, La Bajada.

La Bajada east of
          Cochiti

The pan begins towards the northeast, looking up the valley of the Rio Grande. At extreme left are the San Miguel Mountains, with mesas of Tsherige Member, Bandelier Tuff, at their feet. La Bajada stretches across the second through fifth frames, with Tetilla Peak just poking over the skyline in the fourth frame. Sandia Crest is on the distant skyline in the fifth frame, and Cochiti Dam crosses the last three frames.

The portion of La Bajada in the third frame is a bit unusual. Close study of the lava flows here shows that one of the flows was erupted recently enough (around 1.14 million years ago by radiometric dating) that La Bajada already existed, and the flow cascaded down the face of the escarpment and pooled on the low ground below.

La Bajada continues south to the canyon of the Santa Fe River, which bounds the southern end of the plateau. This area can be seen from atop Cerro Seguro near La Cienega. )We visited this area in Chapter Five.) This is southeast of the Cerros del Rio and provides a nice panorama of much of the plateau.


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

The panorama begins with a view to the southwest, towards Sandia Crest (washed out but just visible in the distance.) The hills in front are not named on my map, but lie just north of a bend in Interstate 25 and mark the southern limit of the Cerros del Rio. The canyon crossing much of the panorama and disappearing into the horizon in the second frame is the canyon of the Santa Fe River. The hills in the third and fourth frame are underlain by Ortiz monzonite, which rises like islands (kipukas) out of the much younger Cerros del Rio Basalt on the other side of the Santa Fe River. The peak on the left side of the fourth frame is Tetilla Peak, which dominates the western escarpment of the Cerros del Rio along the La Bajada Fault. The La Bajada Fault is a southern extension of the Pajarito Fault.

The flat mesa extending from the Santa Fe River canyon across the second, third, and fourth frames is Tsinat Mesa, underlain by Cerros del Rio basalt flows.

South of the Santa Fe River canyon is Mesa de Juanita. Though geographically detached from the Cerros del Rio, it is underlain by Cerros del Rio basalt. It is bounded by escarpments to the west and south, but to the east the basalt is buried under alluvial fans of the Cerillos Hills.

The escarpment on the east side of the Cerros del Rio is less dramatic than that on the west side, since the small rivers and intermittent streams here are not as effective as agents of erosion as the Rio Grande. The escarpment is easily scaled and there are numerous petroglyphs in the area.

Escarpment east
          of Tsinat Mesa

East escarpment of Cerros del Rio near La Cieneguilla. 35 36.526N 106 07.398W

This escarpment forms the northeastern boundary of Tsinat Mesa. The rock here is a trachybasalt dated at 2.68 million years old. My geologic map is not so specific, but this is probably hawaiite. The sediments beneath the lava cap are rich in red granite fragments, and are shown on my map as Ancha Formation. The granite came from the Sangre de Cristo Mountains to the east and was probably transported to this area by the ancestral Santa Fe River.

The escarpment is buried under sheetwash further to the north, where the two highway access points are, then becomes substantial again. A favorite destination for the adventurous in this area is Diablo Canyon, at the north end of the Cerros del Rio. This area is accessible via Buckman Road from Santa Fe.

Diablo Canyon
Diablo Canyon. 35 48.382N 106 08.562W

Notice the very thick section of basalt to the right, with pronounced columns. This area is described on the latest geological map as a former lava lake. The road in is passable to passenger vehicles, but be warned that the canyon itself is notorious for a large population of rattlesnakes. Keep a sharp eye if you choose to hike the area.

North of White Rock Canyon, there is an isolated mesa known variously as Black Mesa or Round Mesa and not to be confused with the much larger Black Mesa north of Espanola.

The other Black Mesa
The other Black Mesa. Looking southeast from near 35.932N 106.120W

Black Mesa is located on tribal lands and has religious significance to the San Ildefonso Tribe, and so it is off limits to casual visitors. This is a volcanic neck formed in an eruptive vent, probably about 2.7 million years ago.  The southwest side of the mesa has visible beds of cinders. These are visible in the panorama below.

Black Mesa
          panorama

Black Mesa (Round Mesa) seen from the west. 35.924N 106.124W

Black Mesa is the core of a volcanic vent that erupted not quite 3 million years ago. You can see the curved beds of cinders on the right side of the mesa. There are also columns of cooled lava at the base of the cliffs right on the boundary between the first and second frames (click to enlarge).

West of the Rio Grande

West of the Rio Grande, the Cerros del Rio Formation is mostly overlain by younger tuffs of the Bandelier Formation, but there are occasional exposures. Some of these are fairly extensive, as at White Rock and in Pueblo Canyon along the main road into Los Alamos from the east.

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

This particular road cut is iconic. You will see it in almost every decent book on Jemez geology. At the bottom is a layer of dark basalt of the Cerros del Rio Formation, which apparently erupted from a center somewhere to the west (left in this photograph). On top of the basalt is a very thin layer of sediment that I long assumed was a paleosol, but may in fact be lake bottom sediments. There have been a number of lakes north of White Rock Canyon as a result of volcanic eruptions damming the Rio Grande River.

Opposite this road cut is a deep canyon in the Cerros del Rio. At this point, the flows are very thick.

Cerros del Rio
Cerros del Rio on opposite side of canyon from previous photograph

It's rare to see this great a thickness of Cerros del Rio Basalt, and in fact we see a sloping contact further down the canyon with Santa Fe Formation sediments that bring the Cerros del Rio to a more typical thickness.

Contact with Santa
        Fe Group sediments
Cerros del Rio in contact with Santa Fe Group to east of previous photo. Contact indicated by white lines.

It's likely that we are seeing where the lava pooled in a deep paleocanyon.

Further west, the road takes a short turn north, and from that point the Cerros del Rio is present only on the south side of the canyon. Here it is the topmost unit.

Cerros del
        Rio in lower Pueblo Canyon
Cerros del Rio Formation in lower Pueblo Canyon. 35 52.090N 106 10.986W

Cerros
          del Rio in lower Pueblo Canyon
Cerros del Rio Formation in lower Pueblo Canyon. 35 52.090N 106 10.986W

Cerros
          del Rio in lower Pueblo Canyon
Cerros del Rio Formation in lower Pueblo Canyon

Cerros del
          Rio in lower Pueblo Canyon
Cerros del Rio Formation in lower Pueblo Canyon

The Cerros del Rio rests here on lacustrine beds (lake bottom deposits) of the upper Puye Formation. In some places, the lava shows signs of having cooled rapidly on contact with water. Note that, in this exposure, the beds seem to dip slightly to the southwest. This assumes I held the camera straight! But if we can trust my inner ear, this suggests that the Diamond Drive Graben extends this far east, and continued subsidence to the west has tilted the beds.

The last photograph shows prominent columnar fractures from the bottom of the flow up, and a more irregular pattern of fractures from the top down. This is a very common pattern in thick basalt flows, and arises from the way the flow cools rapidly from the top and much more slowly from the bottom. The lower columnar part of the flow is called the colonnade, and the irregular top part is called the entablature, by analogy with Greek temple architecture.

The next large exposure to the south and west of the Rio Grande is at White Rock. The village is built on a plateau of Cerros del Rio basalt with a thin covering of sheetwash and eolium deposits. The basalt is best exposed in the La Vista subdivision, which forms a topographic high on the Cerros del Rio surface and may have been an eruptive center. This is close to my neighborhood; my house is built on Cerros del Rio basalt on the west side of Pajarito Canyon.

Cerros del
          Rio in La Vista area
Cerros del Rio Formation in La Vista area. 35 49.359N 106 13.359W

Cerros del Rio
          underlying Chez Budge
Cerros del Rio Formation underlying Chez Budge.

Cerros del Rio basalts are also exposed in the confluence of Water Canyon and Potrillo Canyon and the lower portions of other canyons of the Pajarito Plateau south of White Rock.

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 in the third frame and continues to its confluence with White Rock Canyon in the first frame. Part of the east rim of White Rock Canyon is visible on the skyline in the first frame, with Montoso Peak on the skyline.

The feature of greatest geological interest is probably the large outcropping of light pink Bandelier Tuff in the south wall of Water Canyon in the second frame. It appears that 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 ancestral Water Canyon when it was erupted, and erosion has since re-cut the canyon, leaving this remnant on the canyon walls.

The Cerros del Rio is exposed in lower Frijoles Canyon, where some interesting features of the formation can be examined. The basalt first appears in the canyon bottom about halfway from the visitors' center to Upper Falls.

Cerros del Rio in
        lower Frijoles Canyon
Cerros del Rio not far below the visitors' center in Frijoles Canyon. 35 46.512N 106 16.111W

The Frijoles River is soon cutting deep into the basalt, exposing basal breccia at the bottom of the top flow unit.  That is, the cliff here is a thick lava flow with broken basalt fragments buried beneath. These typically form at the front of a slow-moving lava flow, crumble off the face of the flow as it advances, and are buried beneath it.

Cerros del Rio in
        Frijoles Canyon
Cerros del Rio in lower Frijoles Canyon

Further down, on the west side of the canyon, there is a spectacular contact between the benmoreite of the Cerros del Rio and the overlying Bandelier Formation.

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

Seen from the canyon rim above:

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

This illustrates how the Bandelier Tuff filled low-lying areas in the surface over which it was erupted. The Cerros del Rio was evidently already heavily eroded in this area 1.2 million years ago, and this particular paleocanyon has been interpreted as the previous course of the Rio Grande, prior to the Bandelier Tuff eruption.

Cerros del Rio basalts are common on the west side of the Rio Grande in the area north of Cochiti. These are relatively young flows. The area is on tribal lands and mostly inaccessible to visitors.

Andesite domes and flows

The basalt and basaltic andesite flows that define the Cerros del Rio Plateau, and which we have now seen in cross section in numerous locations, range in age from about 3 million to about 2.6 million years. About 2.6 million years ago, there was a particularly voluminous pulse of more silica-rich magma, mostly basaltic andesite and andesite (or their alkaline equivalents), which formed most of the high terrain on the Cerros del Rio. Additional high terrain was produced by a second pulse at around 2.4 million years. We'll now look at the plateau itself and its many hills and small mountains.

We'll begin at the southern end of the Cerros del Rio, atop Tsinat Mesa.This is mostly underlain by level flows of olivine basalt, forming a very flat surface, but the northern end of the mesa is bounded by hills and mountains of more silicic rock.

Panorama near
          Tetilla Peak

Panorama from Tsinat Mesa. Looking west to north from 35 35.052N 106 10.859W

Tetilla Peak is the shapely mountain to the left, which is a distinctive landmark visible for great distances. At 7206′, it is not actually the highest point in the Cerro del Rio, but it is the most prominent. It may also be one of the older features of the Cerros del Rio, erupted 3.04 million years ago, according to radioisotope dating. What makes this more surprising is that it has a cap of dacite that is the most evolved lava in the entire Cerros del Rio. But the radioisotope age may be off; magnetic data and field relations suggest the age is more like 2.7 million years.

Tetilla Peak is surrounded to the east by andesite flows. Because andesite lava is considerably more viscous than basalt lava, these andesite flows have steeper faces.

Face of andesite flow

Face of andesite flow at foot of Tetilla Peak.35.607N 106.198W

Compare the basalt flows surrounding the peak with the andesite flows.

Compared
          flows

Arroyo at foot of Tetilla Peak. 35.607N 106.199W

The basalt is probably younger, but has been eroded away where it made contact with the andesite flow to produce the cliffs. The andesite flow to the right is much thicker. Here is a sample taken from the right side of the arroyo.

Basalt or andesite?

I'm not sure exactly what this is. The color is within the andesite range, and the presence of plagioclase and hornblende phenocrysts (white patches and dark needles) is typical of andesite, but the rock also has green patches of what appers to be olivine. Olivine andesite is not unknown, but it’s decidedly uncommon and noteworthy, and the geologic map of the area mentions no olivine in the andesite of Tetilla Peak, only in the basalt around it. Or perhaps the green patches are an unusual pyroxene mineral, which would be more consistent with an andesite.

Most of the plateau is basalt or basaltic andesite flows, on which is superimposed several tall hills composed mostly of andesite flows and cinder. Basalt lava is relatively low in silica, relatively fluid, and flowed out freely to form the flat surface of the Cerros del Rio. Later, more silica-rich, and therefore more viscous, andesite lava was erupted to form the hills and peaks. Typical is Colorado Peak.

Colorado Peak

Colorado Peak. Looking northwest from 35.626N 106.204W

Colorado Peak is underlain by andesite about 2.6 million years old. The flat top is surrounded by a ring of dikes intruding the cinder making up most of the upper part of the mountain.

Further north we come to Cerro Ruiz.

Cerro Ruiz

Cerro Ruiz. 35 41.311N 106 11.215W

Cerro Ruiz is underlain by 2.61-million-year-old andesite.

Next is Cerro Rito.

Cerro Rito

Cerro Rito. 35 42.001N 106 11.010W

This is underlain by basaltic andesite a bit younger than the other features we’ve seen so far, at 2.53 million years old.

Incidentally, the rock under most of these peaks takes the form of cinders, while the lower slopes are solid flows. This is quite typical of cinder cones.

This is also a good spot for viewing Cerro Micho.

Cerro Micho

Cerro Micho. 35 42.552N 106 12.176W

This one is a little older, being underlain by andesite that is about 2.76 million years old.

Another turn Montoso Peak comes into view. This is very prominent on the skyline directly south of my house.

Montoso Peak

Montoso Peak. 35 44.425N 106 13.143W

Montoso Peak is underlain by basalt 2.59 million years old. The low-viscosity basalt has produced a mountain with relatively gentle slopes: A shield volcano.

The northernmost part of the Cerros del Rio Plateau is Sagebrush Flats, which resembles Tsinat Mesa at the other end of the plateau. South of Sagebrush Flats is a wild maze of steep-sided andesite flows, quite striking on the topographic map.

Ortiz andesite

Ortiz andesite flows

The andesite here all erupted from around Ortiz Peak.

Ortiz
          Peak

Ortiz Peak. 35 44.918N 106 08.911W


Ortiz
          Mountain

Ortiz Peak seen from the east, highlighted by distant rain shafts.

The Ortiz Andesite is one of the younger flows in the Cerros del Rio at 2.32 million years old. This seems to rule out the possibility that its rugged topography is the result of more extensive erosion that other parts of the Cerros del Rio. Looking at the topo map for the area, I get the sense that most of the andesite flows had a tendency towards this deeply lobed structure, but none were nearly as voluminous as the Ortiz flows and so did not fully develop it.

South of Ortiz Peak are the Twin Hills.

Twin Hills

Twin Hills. 35 42.534N 106 10.243W

These are also relatively young, being underlain by basaltic andesite with an age of about 2.53 million years.

A final pulse of magmatism came around 1.3 million years ago, almost simultaneously with the Valles event. This produced Cochiti Volcano and its associated flows. One of these flows actually overlies a bed of Otowi Member, Bandelier Tuff. Another flow is the one I mentioned earlier, which cascaded down La Bajada and pooled at the base of the escarpment. This area is all tribal lands and inaccessible to the general public.

Santa Ana Mesa

This is the second most voluminous of the basalt eruptions around the periphery of the Jemez from this time period. It is dominated by San Felipe Volcano, a classical small shield volcano, whose flows have a radiometric age of 2.5 million years.

A shield
          volcano
San Felipe Volcano, a typical small shield volcano, viewed from the south from 35 15.168N 106 33.942W

Santa Ana Mesa is cut by several north-south faults that are clear to see on the topographic map and even in the satellite photo. San Felipe Volcano erupted along one of these vents. The displacements on these faults are as much as 110 meters (350'). As with the Cerros del Rio, the oldest beds include cinder beds likely from a maar eruption early in the field's history. There is also evidence of cinder cones that were subsequently buried by a fluid lava flow, like the cinder cone exposed in the east wall of White Rock Canyon.

The southernmost part of the Santa Ana field, and thus the southernmost flow we can plausibly assign to the Jemez volcano field, is Canjilon Hill. The most prominent feature here is Octopus Plug.

Plug in Santa Ana
          field
Octopus Plug, near southern end of Santa Ana basalt field. 35 21.676N 106 32.245W

Canjjilon Hill is a diatreme, a wide vent filled with volcanic rubble that likely lay under a maar that has since been eroded away.

Finally, here is a view of Santa Ana Mesa from the southwest.

Santa Ana panorama
View of Santa Ana basalt field. Looking northeast from near 35.392N 106.639W

Santa Ana Mesa dominates the second and third frames. The first frame is centered on the Sierra Nacimiento and Chamisa Mesa and Borrego Meso are on the left edge of the second frame, with Redondo Peak as the high point of the skyline to their right.

I'm not likely to add many photos in the future. The entire mesa is on tribal lands and thus is not open to the public.

El Alto Mesa

This was the least voluminous of the basalt eruptions from this time period, consisting of just a few flows in the northeastern Jemez. However, one of the flows is quite prominent from Abiquiu, since it is the uppermost formation of Abiquiu Mesa.

Abiquiu Mesa
Abiquiu Mesa. Looking southwest from 36 12.528N 106 17.662W

A flow of El Alto basalt forms an imposing wall south of Forest Road 31 just where it turns to make the ascent to El Alto Mesa.

El Alto
          Basalt
El Alto Basalt. Looking south from 36 09.988N 106 21.330W

Note that this is on tribal land and visitors are restricted to the roadway.

The source of at least some of these flows is Cerro Pelon.This is remarkable in that the bulk of Cerro Pelon is Tschicoma dacite The El Alto flow appears to come from a vent on the north side of the dome, possibly opened by a landslide (now buried by El Alto basalt.)

Panorama from north flank of Cerro Pelon

Panorama from northwest flank of Cerro Pelon. 36 07.441N 106 24.207W

At left, smoke from a small fire, and, on the skyline, the La Grulla Plateau. Across the first two frames, Bandelier Tuff, with Cerro Pedernal on the skyline. Mesozoic beds of the Colorado Plateau across the rest of the skyline, with Abiquiu Reservoir visible.

The foreground ridge to left is Tshicoma dacite forming the west flank of Cerro Pelon. The upper part of the ridge to right is also underlain by Tshicoma Dacite, but the lower part of the ridge and the ridge in the middle distance at center is underlain by El Alto Basalt erupted from Cerro Pelon. The grassy valley may also be underlain by El Alto Basalt under its surface layer of alluvium.

Here's a sample of the basalt from this area.

El
          Alto basalt

El Alto Basalt from Cerro Pelon. 36 07.458N 106 23.802W

This is a somewhat unusual basalt, with numerous coarse phenocrysts.

From a vantage point well to the west, the level surface of the Abiquiu Mesa flow is striking.

Abiquiu Mesa from the
          west

El Alto surface looking southeast from 36 14.662N 106 22.095W

Note how the flat surface of Abiquiu Mesa lies below the level of El Alto (to the right on the skyline) and how it appears to emerge out of a canyon in the higher country. The ground at the foot of El Alto must have been fairly level at the time of eruption.

Culebra Lake

About 2.35 million years ago, an eruption in the Cerros del Rio created a lava dam across the Rio Grande south of the present location of White Rock, near Water Canyon. This created Culebra Lake, which at its maximum was over 80 meters (240') deep and covered an area greater than 1000 km2 (400 square miles) in what is now the Espanola Valley and Abiquiu areas. 

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 from Culebra Lake 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.

Culebra Lake deposited lacustrine sediments throughout its lake bed, but most of these have already eroded away. A few beds are preserved beneath Bandelier Tuff in the area east of Los Alamos. There was also for many years an outcropping of Cerros del Rio basalt next to the main highway to Los Alamos that showed ripple marks imprinted in the basalt when it flowed into the lake and made a cast of the lake floor. Regrettably, this was destroyed when the road cut was expanded some years ago. (However, the same road expansion produced the iconic road cut shown earlier in this chapter.)

Lacustrine sediment beds are still visible north of where this outcropping once existed.

Lacustrine bed
Lacustrine sediment beds. 35.877N 106.194W

The lacustrined sediment beds are located at the center of the phograph, below a hill of badly fractured Bandelier Tuff. The beds are rich in clay and form a superb slip surface for the overlying rocks to slump.

The Gravel of Lookout Park

Digital relief map of pediment gravel exposures in the
        Jemez Mountains
Relief map of the Jemez with exposures of pediment gravels highlighted in red

At about the same time the peripheral basalt eruptions peaked, erosion of the Keres Group volcanoes in the southern Jemez began to produce a pediment surface. A pediment surface is a nearly level surface cut in the bedrock at the foot of an eroding highlands. Pediment surfaces seems to be characteristic of arid regions with poor drainage, suggesting that northern New Mexico was already an arid climate two to three million years ago, but the Rio Grande did not yet fully connect the basins making up the Rio Grande Rift.

Pediment surfaces are often covered with sheet wash, which is a layer of gravel and boulders washed across the surface by infrequent but heavy runoff. The pediment surface in the southern Jemez is covered by such a sheet wash, which has been named the Gravel of Lookout Park. This is well exposed where the road climbs up to the Veterans' Memorial Overlook at Kashe-Katuwe National Monument.

Cochiti Formation

Gravel of Overlook Park near Veterans' Memorial Overlook. Near 35 39.994N 106 26.790W

The clasts are mostly Paliza Canyon Formation and Bearhead Rhyolite eroded off the mountains to the north.

The contact between the Gravel of Lookout Park and the Cochiti Formation is exposed further down the road.

Gravel of Lookout Park and Cochiti Formation

Cochiti Formation and
          Gravel of Lookout Park

Contact between Cochiti Formation and Gravel of Lookout Park. 35 40.040N 106 26.906W

The Cochiti Formation is the sandy beds at the bottom of the photographs. The beds of gravel and cobbles above are the Gravel of Lookout Park. In the second photograph, it looks like a microfault has thrown down the beds of the Gravel of Lookout Park to the right (east).

The transition is not terribly dramatic, though it is more so when seen from a distance. It is unsurprising that some recent geologic papers describe the Gravel of Lookout Park as the Lookout Park Member of the Cochiti Formation. However, since the Gravel of Lookout Park is reworked Cochiti Formation sediments mixed with additional clasts freshly eroded off the mountains to the north, and deposited on an erosional surface, I am not sure it is quite correct to make it a member of the Cochiti Formation.

The pediment surface forms the plateau on which the Veterans Memorial Overlook is located. This area has actually been thrown down a short distance by a fault just to the west of the Overlook. From a vantage point west of this fault, one can see the pediment surface beautifully displayed on the mesa to the south.

Gravel of Lookout
          Park pediment surface

Notice how remarkably flat the top of the mesa is. The flat areas further away in the first frame are also part of this pediment surface, but have been thrown down by the fault I mentioned earlier. The pediment surface extends for miles to the west and south (albeit with numerous cross-cutting arroyos) and is everywhere underlain by Gravel of Lookout Park. The Gravel of Lookout Park was deposited across this surface in a geologically short period of time, around 2.5 million years ago. Later, river drainage patterns shifted and erosion cut numerous arroyos into this pediment surface, but the surface is still discernible.

In many places, the Gravel of Lookout Park is overlain by a thin eolium, wind-deposited sediments.

Eolium

These are very young deposits, so we are getting a bit ahead of our story, but I mention them here to complete the story of the Gravel of Lookout Park.

Once, while visiting the Veterans' Overlook Memorial, I hiked along the trail east of the overlook, more out of idle curiosity than anything else. (I had not brought my own map of the area.) The trail goes some distance east, then south across the entrance road, then gradually loops around to the southwest. Where I met one of the natives:

Crotalus atrox
Crotalus atrox near the Veteran's Memorial Scenic Overlook. 35 40.138N 106 26.984W

Click for the full resolution image, and you'll clearly see the rattle and the diamond pattern. Yep, this is a fine fat specimen of Crotalux atrox, the western diamond rattlesnake. It was leisurely making its way across the trail when I happened along, and we both reared back and regarded each other with alarm. Oddly, I did not hear the snake rattle, nor did it coil in the usual rattlesnake manner.  I might wonder if this actually a bull snake (the two species look a lot alike) but the rattle is very obvious in the full-resolution photograph and the coloration is precisely correct. It was a cool day; the snake may have been torpid enough to behave oddly. Regardless, it made no move to leave the trail, and I was disinclined to try to go around. (Who knows what else was lurking off the trail?) I turned around.

The pulse of basaltic magmatism peaking 3 million years ago had largely wound down by 2 million years ago. However, big things were now cooking in the heart of the Jemez Volcanic Field.

Next page: Things gets tuff in the Jemez

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