The previous chapter may be found here.
A modern erg, similar to those that dominated the Jemez area in the Mesozoic.. NARA
The Mesozoic was the Age of the Dinosaurs. Its beginning and end
were bookmarked by mass extinctions, in which a large
fraction of the families of fossils in the geological record
disappeared in a geological blink of an eye. Life took millions of
years to recover its diversity afterwards.
In this chapter, we will look at the Jemez area during the Mesozoic Era.
In the previous chapter, we saw that the youngest Permian formations in the Jemez area are separated from the oldest Triassic formations by a significant unconformity. Thus, there is no direct record in the Jemez of one of the greatest catastrophes in the history of the Earth.
Starting about 252 million years ago, some 96% of marine species
and 70% of land species went extinct in a geologically brief
period of time, a dramatic enough change in the fossil record that
it was chosen by geologists to mark the boundary between the
Paleozoic Era and the Mesozoic Era. It took around ten million
years for the diversity of life to recover.
Geologists are still debating just what went wrong. There is evidence that global temperatures abruptly rose, by 8C (14F), accompanied by a sharp increase in atmospheric carbon dioxide levels. There is also evidence of a surge in the ratio of biologically active carbon-12 to less biologically active carbon-13. This suggests a massive release of biological carbon into the atmosphere, which could have been caused by a global fire storm or some other massive destruction of vegetation. There is weaker evidence of repeated explosions of the fungus population, and it has been suggested that the fungus were fed by large quantities of dead vegetation. There is also evidence of an increased mutation rate in plant spores, which some geologists interpret as evidence of increased ultraviolet light reaching the Earth's surface.
The exact timing of this extinction is hotly debated, with some
geologists pointing to evidence of a single massive extinction
pulse and others pointing to repeated pulses of extinction over a
period of up to 10 million years. My own impression from my
(admittedly inexpert) reading of the evidence is that the
extinctions did not take place all at once. Perhaps the extinction
had more than one cause, giving life a one-two punch.
Among the candidate causes is an asteroid impact like the one widely believed to have caused the extinction of most dinosaurs. However, the crater from this impact has not been positively identified. It may never be identified; an asteroid is twice as likely to hit the oceans as the continents, given the relative areas, and the oceanic crust is recycled in less than 250 million years. So a crater on the ocean floor from 250 million years ago would be long erased. Some candidate craters on land have been identified, but it is not clear that the proposed candidates are actually impact craters or that they are large enough to account for the damage.
Another possibility is that the extinction was caused by the eruption of the Siberian Traps. These are colossal lava flows covering an area of northern Siberia the size of western Europe that were erupted pretty much at exactly the right time. This huge eruption may have been caused by a asteroid impact, either at the eruption location or exactly opposite it on the Earth's surface; computer models show that the shock from a major impact converges at the opposite side of the Earth and could cause significant disturbance of the crust and upper mantle. There is even some evidence of an impact crater, the Wilkes Land Crater in Antarctica, which would have been opposite Siberia at the time.
Most likely neither an impact event nor the Siberian Traps eruption alone caused the "Great Dying." The Siberian Traps erupted through rock rich in coal and carbonate, and would have released large amounts of carbon dioxide. This could have had lasting effects on the climate, as could an explosion in the population of methane-generating bacteria proposed by other geologists.
The Triassic opened with Pangaea fully assembled. A large
island arc collided with southwest North America sometime in the
late Permian or early Triassic, producing the Sonoma Orogeny. Up
to 300 km (200 miles) of crust was sutured onto the west coast.
The force of the collision produced great thrust sheets,
layers of rock driven eastward many kilometers over the existing
crust. Though these sheets did not reach as far east as the Jemez
area, the disturbance elevated the region enough to bring it well
above sea level and produce a major unconformity on the
Later in the Triassic, Africa began to split away from eastern North America, and vast red bed deposits were laid down in the Appalachian area. The Gulf of Mexico began to open late in the Triassic. The breakup of Pangaea would continue for another 150 million years.
Conifers became the dominant form of land vegetation, and the first true mammals emerged late in the Triassic from a group of mammal-like reptiles called therapsids.
We pick up our story in lower Cañon de San Diego, where the upper
Permian and lower Triassic columns are well displayed in the sides
of Meseta Blanca.
Mesita Blanca. Looking north from 35 39.979N 106 42.869W
At the bottom are the red beds of the Permian San Ysidro Formation, Yeso Group. Above these are lighter beds of the Glorieta Formation. Then comes a discontinuity and the gentler slope and darker beds of the Triassic Moenkopi Formation, and finally the steep slope and lighter beds of the Shinarump Formation, with a cap of dark ferruginous conglomerate.
Relief map of the Jemez with Moenkipi Formation outcroppings highlighted in red.
The Moenkopi Formation is the oldest Triassic formation in the
Jemez area. It is composed of thin beds of dark brown mudstone and
sandstone and tends to form slopes between cliffs of the harder
sandstones of the underlying Glorietta Formation and the overlying
Shinarump Formation. It is separated from both by unconformities.
Moenkopi Formation. 35 39.987N 106 42.866W
Moenkopi Formation. 35 39.975N 106 42.877'W
Under the loupe, the sample here is revealed as a poorly sorted coarse sandstone of well-rounded quartz and lithic grains with abundant cement filling the pore spaces -- probably enough that the rock is matrix-supported rather than clast-supported: A lithic wacke.
The Moenkopi was laid down around 240 million years ago, at a time when the Appalachians were a mighty mountain chain and great rivers flowed west from their foothills towards the Pacific. The area from the Jemez across what is now the Colorado Plateau was very flat and arid, and only the largest rivers made it clear to the ocean. The shifting rivers laid down the mudstone of the Moenkopi in what was likely a tidal flat environment.The contact with the overlying Shinarump Formation is dramatic.
Moenkopi and Shinarump Formations. 35 39.851N 106 42.844W
This transition is visible even on the satellite view of Google
Maps. This contact can be mapped throughout the Colorado Plateau,
and marked a brief period of lowered sea levels and erosion. The
entire Jemez area was high ground at this time.
Relief map of the Jemez with Shinarump Formation outcroppings highlighted in red.
The Triassic formations of the Jemez area younger than the
Moenkopi Formation belong to the Chinle Group. These rocks are
thought to have been deposited in a large river valley, comparable
with the Mississippi or other major river valleys of today. The
Chinle River ran from Texas through New Mexico, Arizona, Utah, and
Nevada before reaching the ocean. In fact, zircons have been
identified in Chinle beds that show chemical signatures identical
to granites exposed in the southern Appalachians today. The river
system was long-lived and deposited vast quantities of sediments
in its valley and delta. The Chinle River must have passed close
to the Jemez area as it threaded its way between the remnants of
the Uncompahgre Uplift to the north and the newly formed Mogollon
Highlands to the south.
The Shinarump Formation was laid down about 230 million years
ago, and it fills valleys and other low spots cut into the
Moenkopi Formation. In the southern Jemez, it takes the form of
massive sandstone beds with lenses of conglomerate.
Shinarump Formation. 35 39.844N 106 42.814W
Shinarump Formation conglomerate. 35 39.844N 106 42.814W
Under the loupe, this is seen to be a moderately well-sorted fine
conglomerate of well-rounded quartzite pebbles cemented together
with abundant tan matrix.
Shinarump Formation sandstone. 35 39.844N 106 42.814W
The sandstone is a well-sorted, coarse sandstone composed of
angular quartz grains with a modest amount of cement.
Further north, the Moenkopi is not present, and the Shinarump Formation sits directly on Permian red beds. The Shinarump continues to be a prominent formation in the western Jemez, where it caps a number of mesas. These include Eureka Mesa.
Eureka Mesa. 35
59.409N 106 52.648W
Underneath are beds of the Permian Yeso and Abo Formations.
Older papers and maps identify the Shinarump as the Agua Zarca
Sandstone in the western Jemez, while some more recent writings
attempt to reconcile terminology by designating these thick
sandstone beds as the Agua Zarca Member of the Shinarump
The west end of Eureka Mesa is the location of the Nacimiento Mine, where the Shinarump Formation was once mined for copper ore.
Nacimiento Mine showing Shinarump
Formation beds. 35
59.543N 106 54.077W
Here the Shinarump beds begin to dip steeply into the Nacimiento
Fault, located just west of the mine.
The Shinarump is locally rich in organic matter, such as
In some locations, the wood has been replaced by copper minerals
such as chalcocite, Cu2S, which is mined as copper ore.
I'll have more to say about this later in the book.
The Salitral also shows local concretions.
These are localized deposits of particularly large quantities of
cement in the sandstone. We will see many further examples in this
The unconformity dividing the Permian from the Triassic is
beautifully exposed In the Canones
fault zone along the western boundary of the Rio Grande
Rift. We visited this area in the last chapter, but it's time to
The Canones Fault runs approximately along the arroyo in front of the cliff here. The exposed units are clearer just up the road.
Permian and Triassic sedimentary beds. As with all photos here, click to enlarge. 36 14.033N 106 23.336W
The lowermost red rocks, with the many layers and the column-like
structures, belong to the Arroyo del Agua Formation of the Cutler
Group. We examined this in the last chapter.
Here is the contact between the Arroyo del Agua and the Shinarump close up:
The lower part of the Shinarump Formation is a thick bed of
This stands out clearly in the outcrop photographs earlier. The
sandstone is a well-sorted coarse quartz arenite. Somewhat to my
surprise, there is little or no calcite, the cement apparently
being additional silica.
Just above the thick sandstone is an impressive bed of
The clasts resemble those in the Shinarump conglomerate beds south of the Jemez, but there is much less tan matrix between the clasts.
Conglomerate is a clastic sedimentary rock containing a substantial number of clasts that are larger than 2mm in diameter. Thus the coarsest sandstones grade into fine conglomerate. At the other end of the size scale, conglomerates may be dominated by clasts the size of boulders, and we'll see some striking examples later in this book. The spaces between the large clasts are almost always filled with a matrix of finer sediments, such as sand or clay. A well-consolidated, highly indurated conglomerate resembles nothing so much as a slab of concrete, and concrete may be thought of as artificial conglomerate.
If the matrix is scanty enough that the clasts touch each other,
the conglomerate is described as clast-supported, because
the clasts take up the weight of the overlying rock. If the matrix
is more abundant, so that it actually separates the clasts, the
conglomerate is spoken of as matrix-supported. As with
sandstones, a conglomerate can be well-sorted (with clasts
mostly the same size) or poorly sorted (containing a
jumble of clasts of various sizes), though well-sorted
conglomerates are rather uncommon.
Geologists distinguish conglomerates having rounded, polished
clasts from breccias, which have angular, broken clasts.
Conglomerates are typical of a very high-energy environment, such
as a fast-running river bed, where very large rocks are tumbled
and polished. This is rarely found far from the source rock.
Breccias are almost always found very close to their source rock,
since they have not been transported far enough even to round and
polish the clasts. They tend to form from catastrophic
events, such as landslides, earthquakes, meteor impacts, or
The large clasts in a conglomerate can easily be examined to determine their rock type and thus identify their source. In young conglomerates, the source rock may be an obvious nearby outcropping or highland. In ancient conglomerates, the source rock may long have eroded away or been buried, and the conglomerate then provides the best information we have on an ancient uplift.
The Shinarump conglomerate sample shown above is a moderately well-sorted clast-supported conglomerate composed of bits of quartzite that weathered from the flanking uplifts of the Ancestral Rocky Mountains.
Below is a paleogeologic map of the surface on which the
Shinarump was deposited.
Paleogeologic map of the lower Shinarump Formation surface in the Jermez.region
This map shows the age of the formations immediately under the
Shinarump. Green shows areas where the Shinarump lies on the
Moenkopi Formation, the next youngest formation in the Jemez.
Magenta is the Glorietta and Bernal Formations, red is the Yeso
Formation, and yellow is the Abo Formation or Cutler Group. This
map shows that the younger formations are present under the
Shinarump to the south and east while the older formations are
present to the north, with the oldest formations of all (Abo and
Cutler Group) present directly north and to the northwest of the
The interpretation is a bit ambiguous, since this does not tell
us whether the missing beds were ever laid down or exactly when
they were eroded away if they were once present. One might suppose
that the yellow areas mark significant areas of uplifted ground
associated with the ancestral Rocky Mountains. Evidence from the
larger region, including paleocurrent analysis (in which
sedimentary structures are studied for evidence of ancient river
courses), suggests that the Sierra Nacimiento was indeed a region
of uplift but the area north of the Jemez was not. Instead, this
area likely marks the early course of the Chinle River, which,
when still young, was likely a powerful agent of erosion.
Relief map of the Jemez with Salitral Formation outcroppings highlighted in red.
The greatest thicknesses of the Salitral Formation in the Jemez occurs to the northwest, where it takes the form of red mudstone. We've seen a photograph of the Nacimiento Mine that shows Salitral Formation atop the Shinarump and along the south side of the lake filling the former open pit mine. The Shinarump here is notable for septarian nodules, formed when mud concretions in the beds dry and crack internally. The cracks are later mineralized by groundwater.
Salitral Formation is mapped in the northern Jemez, including along the Canones fault zone, but it is not prominent. I originally mistook the upper beds of the Shinarump Formation here for Salitral Formation, which is very thin in the Canones fault zone.
In the southern Jemez, the upper beds of the Shinarump Formation transition into a very coarse ferruginous conglomerate. The geologic map notes that these ferruginous beds are suggestive of the dark mudstones of the Salitral Formation, though this formation is not mapped in the southern Jemez.
A ferruginous sedimentary rock is one containing large quantities
of iron oxides. These give the rock a dark color and a high
density. Like the ferruginous Log Springs Formation further west,
these beds probably formed from prolonged leaching of soil in a
The Shinarump and Salitral Formations are thought to represent development of river deltas along the Chinle River, with the sandstone and conglomerate of the Shinarump being deposited first and the mudstone of the Salitral later as the river valley filled and the river slowed.
Relief map of the Jemez with Poleo Formation outcroppings highlighted in red.
Above the Shinarump and Salitral is the Poleo Formation, which is
very prominent north of the Jemez.
A strand of the Canones Fault is found just
west of the main fault and puts the bottom of the Poleo
Formation to the east against the middle of the Poleo Formation to
In a few places along this road cut, the Poleo Sandstone overhangs enough to provide a nice nesting spot for desert swallows.
A little further down the highway, there is a turnoff to a gravel
road leading to a bench of Poleo Sandstone along the Chama River.
Here's a sample of the sandstone from this area.
Poleo Sandstone from 36 13.965N 106 23.871W
It's a very clean sandstone composed of nearly pure quartz. The grains are fine, well-rounded, and well-sorted. This is an excellent example of a mature quartz arenite.
The mature sandstone of the Poleo Formation tells us that the
climate in the Jemez area became arid enough for windblown dunes
The view south from this bench takes in rocks of very different ages.
At the left, the Rio Chama winds around a mesa of Poleo
Sandstone. The reddish area in the drainage to the south is an
outcrop of the Painted Desert Member of the Petrified Forest
Formation of the Chinle Group, which sits atop the Poleo
Formation. To the right is more Poleo Formation. There is a strand
of the Canones Fault on the west (right) side of the drainage that
has dropped the Petrified Forest Formation down on the east.
The mesa on the skyline is Canones Mesa.
The zone of faulting and deformation along the southeast margin of the Colorado Plateau has been traced as far west as Coyote.
At Abiquiu Dam, the Poleo Sandstone is particularly thick.
The Poleo Formation grades into the overlying Painted Desert Member, without a very sharp transition. We see this just south of the dam, where the sandstone beds of the Poleo Formation begin to be interbedded with red mudstone.
The Poleo Sandstone is also prominent on the mesas west of Coyote, but begins to thin to the south. It is just a few meters thick at the Nacimiento Mine west of Cuba, where it is also strikingly thinly bedded.
Poleo Sandstone at Nacimiento Mine. 35
59.499N 106 53.988W
The Poleo Formation becomes spotty in the southwest Jemez, where the Salitral Formation pinches out and the Poleo becomes difficult to distinguish from the Shinarump Formation.
Relief map of the Jemez with upper Chinle Group outcroppings highlighted in red.
In some locations, there is a thin bed of the Mesa Montosa Member of the Petrified Forest Formation atop the Poleo Formation.. The highway cuts through such a bed here.
In this location, at least, the Mesa Montosa Member is much better consolidated than the overlying Painted Desert Member. There are numerous veins lined with calcite crystals through this bed. I picked up a particularly pretty example.
The bulk of the Petrified Forest Formation north of the Jemez is
assigned to the Painted Desert Member. This is found above the
Mesa Montosa Member where the latter is present, or else sits
directly on the Poleo Sandstone.
The Painted Desert Member is a distinctive red from all the hematite it contains. It is almost nowhere very well consolidated. The outcropping here is basically red dirt, and my sample has already almost crumbled away:
Under the loupe, the grains look very much like those in the
Poleo Sandstone, but the pore space is much more open, and the
rock is obviously much less thoroughly cemented. There are also
more feldspar grains in among the quartz, though not enough to
make this an arkose.
West of Abiquiu Dam, there is a road cut through a thick and spectacular section of the Painted Desert Member.
Painted Desert Member in roadcut south of Abiquiu Dam. 36 14.046N 106 25.824W
The formation is easily eroded to form a gullied surface. The
gray-green layers are most likely beds unusually rich in organic
matter, so that the red ferric iron was reduced and leached away
to leave whitish clay.
Here is a close up view of the formation, showing that it is a thinly bedded red shale that easily weathers into red mud.
Painted Desert Member in roadcut south of Abiquiu Dam. Quarter for scale. 36 14.046N 106 25.824W
Mudstone is actually more typical of the Painted Desert Member
than the dirty sandstone shown earlier, which may be transitional
with the underlying Mesa Montosa Member.
The road curves around south of the reservoir and at a pullout one has a nice view of the Abiquiu Reservoir.
In the distance are cliffs of Paleozoic and Mesozoic rocks of the Colorado Plateau. I won't venture a guess which formations at this distance. In the middle distance is the reservoir, whose shores are mostly Poleo Formation under rounded hills of Painted Desert Member. In the foreground, dark boulders are probably Lobato Formation basalt. The Painted Desert Member and other formations underlying the Lobato Mesa Formation basalt flows to the south lack strength, and this makes for landslides. A large landslide appears to cover the entire area immediately south and east of this point.
So the rock column in the northern Jemez for the Mesozoic that
we've seen so far is: Permian Arroyo del Agua Formation, Cutler
Group; Shinarump Formation, Chinle Group; Poleo Sandstone, Chinle
Group; Mesa Montosa Member, Petrified Forest Formation, Chinle
Group; and Painted Desert Member, Petrified Forest Formation,
of Youngsville one sees the same sequence of beds exposed,
at Mesa Naranja.
The lower red portion of Mesa Naranja is Arroyo del Agua Formation, and the thin white band near the top is the Shinarump Formation. Above this is soft shale of the Salitral Formation on which rest fractured beds of the Poleo Formation. This is the same sequence we saw back at the Canones road cut, but here the Salitral is much thicker.
Like the Shinarup and Salitral Formations, the Poleo and Painted Desert Formations are thought to represent the growth of a river delta through the area. All these Triassic formations are considered part of the Chinle Group. At this time, much of western North America was covered with flood plains and swamps. However, the ancestral Sierra Nevada, which was then located over a subduction zone, formed an island arc far to the west.
The Rock Point Formation is exposed in a few locations in the
northern Jemez, including near the entrance to "Mushroom
Canyon" along Forest Road 100 south of Youngsville.
Here the contact between the Rock Point Formation and the Jurassic
Entrada Sandstone is exposed in the canyon wall.
The Rock Point Formation is the uppermost formation in the
Triassic Chinle Group in the northern Jemez, aged about 205
million years. The overlying Jurassic Entrada Sandstone has an age
of about 160 million years. The intervening 45 million years are
missing from the geological record here.
The Rock Point Formation is fairly hard mudstone.
Exposures of the Rock Point Formation near Ghost
Ranch have yielded hundreds of fossilized skeletons of Coelophysis,
the oldest dinosaur for which complete skeletons have been
recovered in North America.The most important dinosaur quarry in
this area is the Whitaker quarry, discovered in 1948. Other fossil
quarries are found at Canjilon, Hayden, and, most recently, the
rich Snyder quarry, discovered in 1998. The Snyder quarry is rich
in carbonized wood, suggesting the fossils there were from a mass
death attributable to a forest fire, some 210 million years ago.
The Chinle Group disappears to the south under the volcanic rocks of the Jemez Mountains, but reappears south of the Jemez in the Valle de los Indios and in the area around Ponderosa. The geological column from Permian to nearly the present day is displayed on the south wall of the Valle de los Indios as seen from its the north rim, a location reached by a short hike from Jemez Falls Campground:
In the foreground, close to the camera, are outcrops of very young South Mountain Rhyolite. We'll revisit this again towards the end of this book. Across the valley, we see part of the south rim of the Valles Caldera. The prominent red to white cliffs in the lower half of the canyon wall are probably Permian Glorietta Sandstone, although there are outcrops of brighter orange Yeso Group sandstones just visible under the Glorietta Sandstone right of center. The heavily forested middle slopes, above the sandstone cliffs, are Chinle Group beds of the Monekopi, Shinarump, Salitral, and Poleo Formations. These are very poorly exposed in this location. The prominent white cliff at top center is Otowi Member, Bandelier Tuff.
Still further south, near Paliza Canyon Campground, the Painted Desert Formation underlies Bandelier Tuff on the north side of the canyon wall.
This poorly consolidated formation underlies much of the
Ponderosa area. It also underlies Borrego
Mesa, and there are very
large landslide deposits west of the mesa where the mudstone
has given way from under the lava flows capping the mesa. Chinle
mudstones are exposed in some of the landslide scarps
along the face of the mesa.
Painted Desert Formation exposed in landslide scarp. 35 40.571N 106 38.48W
A landslide scarp is a portion of an unstable slope left behind
when the lower part of the slope breaks loose as a coherent mass
and slides downslope. The scarp is typically cliff-like and
concave, as you see in this photograph.
Just north of Ponderosa there are impressive outcroppings of the Agua Zarca Sandstone of the Shinarump Formation, Chinle Group, exposed west of the road.
Agua Zarca Sandstone, Shinarump Formation. 35 40.225N 106 39.774W
Under the loupe, this rock appears almost identical with the
Across the road is a knob
topped with what my road log identifies as Poleo Sandstone.
It's on private land, so I did not take a sample.Consider the differences from the northern Jemez. There the Poleo Sandstone is thick and impressive, while the Shinarump Formation, while not insignificant, is decidedly less impressive. Here in the southern Jemez, the Shinarump has become much thicker and the Poleo Formation has become much less significant. In fact, there is some debate among geologists over whether the Agua Zarca Member and the Poleo Sandstone are really the same formation, just in different locations.
About 201 million years ago, the supercontinent of Pangaea began to rift apart to form the Atlantic Ocean. This rifting was accompanied by a pulse of intense volcanic activity that is probably responsible for an extinction event that geologists use to divide the Triassic Period from the Jurassic Period. Though not nearly as dramatic as the "Great Dying" at the end of the Permian Period, this extinction event resulted in the extinction of 20% of taxonomic families of life.
The western edge of North America continued to sit over a
subduction zone, with mountain building and volcanic activity.
From about 163 to 143 million years ago, the great batholiths of
the Sierra Nevada were formed, an episode geologists have named
the Nevadan Orogeny, but northern New Mexico was still low enough
to continue accumulating sediments. These were predominantly
deposited in ergs (sand seas) around the Sundance Sea, an arm of
Arctic Ocean whose shoreline moved back and forth across western
North American throughout the Jurassic Period. Southern New Mexico
was dominated by the high terrain of the Mogollon highlands.
There are only spotty exposures of Jurassic rocks in the southern
Jemez, but there are more extensive outcrops in the northern
Jemez. These become very extensive to the northwest, across the
Colorado Plateau. They reappear southeast of the Jemez on the east
margin of the Rio Grande Rift, and extend as far east as Oklahoma.
South of Youngsville, Forest Road 100 winds south into the mountains, past landslide deposits and outcroppings of the Painted Desert and Rock Point Formations. Ahead is Cerro Pedernal, famous from the artwork of Georgia O'Keefe, and the mouth of "Mushroom Canyon".
This photograph shows the entire Jurassic column in the northern Jemez, which is well exposed in this area. The foreground and the bank of reddish sediments at the foot of the prominent bluff are Rock Point Formation of the Triassic Chinle Group. The bluff itself is Entrada Sandstone capped by Todilto Formation, which also appear in the slopes to the left and right. The middle skyline is Summerville Formation, with a thin rim of Morrison Formation on the ridge to the right of the prominent bluff. On the far skyline is Cerro Pedernal.
Missing from the base of this column, in the 45-million-year gap
between the Rock Point Formation and the Entrada Sandstone, is the
Navajo Sandstone. Geologists believe this was laid down in a giant
dune sea that covered much of Utah, Arizona, and large parts of
Colorado and New Mexico. However, the Navajo Sandstone was
subsequently deeply eroded, and none is left in the Jemez Area.
Substantial beds still exist on the Colorado Plateau to the
Relief map of the Jemez with Entrada outcroppings highlighted in red.
Some 160 million years ago, another dune sea marched across
almost as large a region as the Navajo Sandstone. This deposited
the Entrada Sandstone, which forms many of the arches in Arches
National Monument. The Entrada Sandstone is also notable because
it finally buried the eroded remnants of the Ancestral Rocky
Mountains, some 145 million years after they first rose.
Here's a close up up of the
isolated outcropping from the mouth of "Mushroom Canyon."
The Entrada Sandstone is fairly hard when fresh, but it weathers quickly in this climate, so that exposed surfaces become so soft that they easily crumble when touched. In other parts of the Colorado Plateau, the Entrada Sandstone is thoroughly indurated and forms prominent cliffs.
The road turns between two banks of Entrada Sandstone.
You can see a considerable amount of sand that has crumbled off the face of the road cut into the road bed. There is also quite a lot of grafitti scratched into the Entrada Sandstone in and around this road cut. This illustrates how weakly indurated the sandstone is in this area.
The road cut also shows obvious cross bedding. Cross
bedding describes a formation where large beds are composed of
much smaller beds that lie at an angle to the main beds. You can
see this in the large bed at the base of the road cut. Cross
bedding is an indication that the sediments were laid down in a
strong current, either of air or water. In the case of the Entrada
Sandstone, which was laid down as sand dunes in a desert
environment, the cross bedding is a consequence of strong
prevailing winds. In this case, the tilt of the cross beds
suggests the wind was from the northeast (left in this picture)
consistent with other evidence that the Jemez area lay in the
trade wind belt during the Jurassic, at a latitude about 20 to 30
degrees north of the equator.
Here is a closer view of the butte, taken on a cloudier day, showing the Entrada beds that form its slopes.
Entrada Sandstone. 36 10.730N 106 32.615W
The lower beds of the Entrada Sandstone are pink. Above this is a
white bed, then a nearly mustard-yellow bed beneath the cap of
Todilto Formation. The pink is the original, unaltered, color of
the Entrada Sandstone, while the white and yellow zones reflect
chemical alteration from the alkaline and anoxic waters in which
the Toldilto Formation was deposited.
To the east of this bluff, the Entrada Sandstone underlies most of the hillside.
Entrada Sandstone. Looking east from 36 10.730N 106 32.615W
The soil cover and vegetation partially obscures the underlying beds, but one can make out the lower pink, middle white, and upper yellow beds of the Entrada Sandstone, the latter well exposed under a cap of grey Todilto Formation. The slopes above the Todilto forming the skyline are Summerville Formation.
The lower red beds of the Entrada Sandstone are present in the arroyo at the bottom of the canyon, and here I took a moderately fresh, well-indurated specimen from a recent rock fall.
Entrada Sandstone. 36 10.784N 106 32.481W
This is a well-sorted sandstone with considerable calcareous cement between the grains, which appear to be mostly quartz but with significant lithic fragments.
There are exposures of the Entrada Sandstone and the overlying Todilto Formation along much of the northern slopes of the Jemez Mountains, and corresponding exposures become spectacular in the Colorado Plateau country to the north. The easternmost exposures in the northern Jemez are found on the northernmost tongue of Canones Mesa, where they are dramatically offset by a strand of the Canones Fault.
Entrada and Todilto Formation offset by fault through Canones Mesa. Looking southwest from 36 14.149N 106 24.029W
Red and white beds of the Entrada Formation, capped with a thin bed of Todilto Formation, crop out on either side of a landslide down the north face of the mesa. On the east (left) the beds are thrown down at least 70 meters (200 feet) by the fault passing between the two outcrops. This fault does not displace the lava on top of the mesa, showing that this particular strand of the Canones Fault Zone has not been active in at least three million years. Further east, additional faults drop the beds beneath the surface in the Rio Grande Rift.
The Entrada Sandstone north of the Jemez occasionally shows some interesting local features.
"Bluberries" in Entrada Sandstone. 36
14.849N 106 22.358W
These are calcite concretions, sometimes called “blueberries”
(though the term is more often applied to hematite concretions.)
A few feet away:
The structures in the upper right corner may be rhizoliths, a form of fossilized plant root.
Here are some more examples of possible rhizoliths from a point further south:
Possible rhizoliths or worm tubes in
Entrada Sandstone. 36
14.376N 106 22.518W
The Jurassic beds have been entirely eroded away in the western Jemez, reappearing only on the west side of the Sierra Nacimiento. A few remnants remain in the southwestern Jemez, on the west face of Borrego Mesa. The largest exposure is visible from Paliza Canyon.
Jurassic outcrops on Borrego Mesa. Looking from 35 41.256N 106 39.031W
The white band is the Jurassic Todilto Formation, while the red slope beneath is the Entrada Formation. Beneath are landslide deposits that include large blocks of the Jurassic formations. There is also a dramatic exposure a short distance further south, visible from the village of Ponderosa.
Entrada Sandstone on Borrego Mesa. Looking northeast from 35 39.678N 106 40.038W
The outcrop is the white patch to the left. A somewhat strenuous
hike up the hills to the left gives one a closer look.
Entrada Sandstone. 35 40.623N 106 38.554W
Additional very small outcrops of Jurassic rocks are found along the mesa for some miles south.
In the southeastern Jemez the Jurassic beds disappear into the Rio Grande Rift. They reappear south of Interstate 25 in the Hagan Basin north of the Sandia Mountains. There are especially spectacular exposures east of the settlement of Puertocito.
Jurassic Park. 35 16.504N 106 17.147W
The red formation at bottom is Chinle Group, probably Correo Member of the Petrified Forest Formation. Or that's what our road log tells us; I find myself wondering if this isn't the same lower red beds of the Entrada Sandstone that appear in the northern Jemez. Above is yellow Entrada Sandstone, and at top is gray Todilto Formation. All are well-indurated here. Here's a sample of the Entrada Sandstone.
Entrada Sandstone from Hagan Basin.. 35 16.504N 106 17.147W
Relief map of the Jemez with Todilto outcroppings highlighted in red.
The Todilto Formation was laid down in a geologically brief
period of time in an embayment of the Sundance Sea. This formed
what geologists call a barred basin.
Seawater enters a barred basin across a shallow bar like that
shown in the diagram. If the climate is hot and arid, as it was in
northern New Mexico 140 million years ago, water rapidly
evaporates from the seawater in the basin, concentrating dissolved
salts and increasing the density of the seawater. This dense brine
sinks to the bottom of the basin, where it is trapped by the
bar. As more saturated brine accumulates, substantial
amounts of evaporites begin to crystallize out of the
brine onto the floor of the basin. Meanwhile, more seawater flows
across the bar to contribute its dissolved solids to the basin.
This process can produce far greater thicknesses of evaporites
than could be produced by simply evaporating a single body of
seawater one time.
Evaporites are minerals that are more or less soluble and which
are present in significant quantities in seawater. In the early
stages of a barred basin, limestone may be deposited on the basin
floor through processes little different from those that produce
limestone in other shallow marine environments. Initially, this
limestone may include marine fossils, but as brine accumulates the
basin bottom becomes inhospitable to life. As the process
continues, additional carbonate minerals are deposited from the
brine, followed by gypsum, CaSO4.2H2O, then
halite, NaCl, then potassium and magnesium minerals.
The Todilto Formation shows the early stages of such an evaporite
sequence. There is typically a limestone bed at the base of the
formation, here named the Mesa Luciano Member, which in some
places contains fossils. For example, there is a particularly rich
bed of fossils at Warm
Springs, near San Ysidro, noted for its insects and
ostrocods. The overlying gypsum is so easily dissolved that it is
missing in many locations, leaving only the limestone. There are
no halite beds; either the evaporation sequence did not progress
that far, or the halite beds have entirely dissolved away.
We've seen several photographs showing the Todilto Formation as a
grey bed on top of the Entrada Sandstone. These beds are difficult
to get to, since they tend to be found at the top of high cliffs.
However, I was able to get fairly close to the contact in Mushroom
Canyon after some scrambling around the north wall of the canyon.
Contact of Todilto and Entrada Formation. 36 10.819N 106 32.497W
The yellowish topmost beds of the Entrada Formation are visible to lower left. Above this is the basal calcareous shale of the Luciano Mesa Member. (Calcareous shale is shale containing substantial calcite.) The thick cap on top, with a more irregular texture and visible nodules of gypsum, is the Tonque Arroyo Member. Here's a sample of the Luciano Mesa Member.
Luciano Mesa Member, Todilto Formation. 36 10.819N 106 32.497W
You can see that the rock is very thinly layered. These layers
are called varves and reflect seasonal climate change, so
that a single layer corresponds to a single year. During part of
the year, clay and organic matter settled on the bottom of the
Todilto Embayment. At other times of the year, calcite
predominated. Counts of the number of varves suggest that the
Luciano Mesa Member was deposited in a geologically brief time of
about 14,000 years.
The Tonque Arroyo Member consist of shale containing numerous nodules of gypsum and limestone. Here's an example.
The translucent part of this nodule is easily carved with a knife, but does not foam under acid; the only plausible mineral identification is gypsum, CaSO4·2H2O. The more opaque sections are both harder and foam under acid; this identifies them as calcite. In some places, the gypsum forms quite large lenses. Here is an example.
Gympsum lens weathered from Todilto Formation. 36 10.784N 106 32.481W
Note the head of my walking stick, for scale. This large gypsum
lens weathered from the Todilto Formation and is located in the
arroyo beneath. Its distinctive texture is sometimes described as
chicken wire gypsum.
The gypsum weathered from the Todilto is slightly soluble in water, and repeated wetting and drying cycles in a dry climate in loose, sandy soil with abundant gypsum produces a distinctive texture.
Gypsum-rich soil showing distinctive texture. Near 36 10.784N 106 32.481W
Continuing east into Mushroom Canyon, one sees how it got its name.
Mushroom Canyon at its finest. Looking east from 36 10.798N 106 32.509W
The upper yellow beds of the Entrada Sandstone form the "stems" of the "mushrooms", the Mesa Luciano Member forms the "gills", the Tonque Arroyo Member forms the "caps", and the background hills are underlain by Summerville Formation.
The road out of Mushroom Canyon climbs a hill to the contact between the Entrada Sandstone and the Todilto Formation. Though the contact is covered with loose sediments, the Todilto Formation is partially exposed.
Approximate contact between Entrada and Todilto Formations. Near 36 10.637N 106 32.885W
Mesa Luciano Member, Todilto Formation. Near 36 10.637N 106 32.885W
This is a dense, fairly hard limestone that identifies this as an
outcrop of the the Mesa Luciano Member.
At the top of the hill one gets a nice view back at "Mushroom Canyon".
If you visit this area, please be aware that the western half of Mushroom Canyon is located on private land belonging to a nature conservancy. Only the eastern half is on Forest Service land, with a barbed wire fence marking the boundary. I was careful to collect my samples from the Forest Service side.
Todilto Formation can also be visited further north, across the main highway through Abiquiu at Red Wash Canyon. No gypsum is present here, but the limestone beds remain.
Todilto Formation near Red Wash Canyon.
14.787N 106 22.471W
The deformation being pointed to likely arises from the nearby
Canones Fault Zone.
Like the Entrada Formation, the Todilto Formation crops out in a few places along Borrego Mesa, but the first really large exposure south of the Jemez is at White Mesa. Here enough of the gypsum remains that it can be economically mined.
White Mesa. Looking northwest from near 35.48N 106.709W
The Todilto forms the white cap that gives the mesa its name and
which is mined for gypsum. Additional gypsum mines are located
south of I-25.
Impressive beds of the Todilto Formation are found southwest and west of the southern Sierra Nacimiento. The most striking of these may be Cuchilla Blanca Hill.
Cuchilla Blanca Hill. Looking east from
39.444N 106 53.547W
The hill is topped with Todilto Formation which dips sharply to the north (left). Underneath is Entrada Formation, and beneath that is Triassic upper Chinle Group red beds. The distant ridge to the right is underlain by Agua Zarca Member of the Shinarump Formation, and the ridge partially hidden behind it is underlain by more Precambrian gneiss.
Relief map of the Jemez with Summerville Formation outcroppings highlighted in red.
The Summerville Formation was laid down 155 million years ago as
tidal flats and dunes. This was a time when the major rivers of
the area began slowly shifting their courses, from the southeast
to northwest flow of the Triassic and early Jurassic to the
southwest to northeast flow that would prevail throughout the rest
of the Mesozoic. This was largely a consequence of continuing
mountain building to the west and southwest (the Nevadan Orogeny).
Bone fragments and teeth of Camarasaurus, a large
herbivorous dinosaur, have been found in the Summerville Formation
in the Hagan area.
The Summerville Formation is exposed in several locations in the northern Jemez, but none of these are easy to get to safely. Some of these have already been shown. The exposure south of Mushroom Canyon is best viewed from along the road out of the canyon.
Summerville Formation in Mushroom Canyon. Looking southeast from 36 10.662N 106 32.947W
The gray beds to the left, just peeking over the foreground ridge, are Todilto Formation. Above are the pinkish tan beds of the Summerville Formation. The very top of the ridge is a thin bed of Morrison Formation. The gray bed in the road cut to the right is Entrada Sandstone, while Cerro Pedernal looms on the skyline.
Summerville Formation is also exposed on the north rim of the canyon.
Summerville Formation in Mushroom Canyon. Looking southeast from 36 10.662N 106 32.947W
The entire skyline here is Summerville Formation. You can see that the beds dip slightly to the east (right), as do the underlying beds of Todilto Formation and Entrada Formation.
A more accessible exposure of Summerville Formation is found just north of the Jemez, along Red Wash Canyon.
Jurassic beds east of Red Wash Canyon. 36
14.789N 106 22.261W
The canyon wall is red beds of the Arroyo del Agua Formation of the Cutler Group. This is capped with a thin bed of Shinarump and Salitral Formations and a thicker bed of Poleo Sandstone. The red to yellowish beds in the foreground are Entrada Sandstone, capped with a thin layer of limestone of the Todilto Formation. Above this to the left is a pinkish exposure of the overlying Summerville Formation. Though not well exposed, my geologic map shows that the Morrison Formation is present at the top of the hill, and some thin beds of Cretaceous Burro Canyon Formation. I did not notice these at the time, but the Summerville Formation stuck out like a sore thumb.
Here's a close view of the Summerville beds.
Summerville Formation. 36
14.588N 106 22.643W
It's basically muddy sand, quite poorly cemented. It reminds me a little of the much younger Ojo Caliente Member of the Tesuque Formation, and was in fact probably a major source of sediments for the Ojo Caliente.
There are only a few small exposures of Summerville Formation in the southern Jemez, along the west face of Borrego Mesa.
The exposure map for Summerville Formation appears to show that
it is much less extensive than the underlying Jurassic formations.
This should be taken with a grain of salt. The geologic maps
showing the next formation, Morrison Formation, lying directly on
Todilto Formation are all older maps, and many describe a lower
unit of the Morrison Formation whose description is much like that
of the Summerville Formation. It is likely that this lower unit
would be mapped as Summerville Formation if these areas were
Relief map of the Jemez with Morrison Formation outcroppings highlighted in red.
The next Jurassic formation in the Jemez is the Morrison
Formation, which is a fluvial sandstone about 150 million years
old. It was laid down as the Sundance Sea began to retreat to the
north, leaving behind a swampy lowland crossed by sluggish
streams. It was also the first formation in the Jemez area
composed largely of sediments eroded off the growing highlands
along the west coast of North America.
The Morrison Formation is highly variable, with up to three
distinct members mapped in some locations in the Jemez region, and
in many places it is rich in fossils, including over 70 dinosaur
species. It is a source of uranium in other locations,
particularly in the Grants area.
The Morrison Formation is exposed around Mushroom Canyon, though the exposures are not easily reached. This is as close as I could get:
Morrison Formation in upper Mushroom Canyon. Looking east from 36 10.282N 106 32.116W
Here the Morrison Formation takes the form of easily eroded
The Morrison Formation is also exposed back at the Canones fault
zone with which we began this chapter. This exposure occurs on the
east side of the fault. Here the fault has thrown up the contact
between the Morrison Formation and the El Rito Formation.
The geology in this spot is quite interesting. All the beds above
the Morrison Formation were eroded away over 50 million years ago.
Sometime between 50 and 30 million years ago, the El Rito
Formation was laid down on the erosion surface, some time prior to
the opening of the Rio Grande Rift.
The contact is clearly visible near the top of the cliff, where the beds of the two formations lie at an angle to each other. This is a good example of what geologists call an angular unconformity.
Here is a close up of the point of contact between the Morrison and the El Rito Formations, where it comes close to the road level.
This contact represents a gap in the geological record of at
least 100 million years. Here's a sample of the Morrison Formation
from this exposure.
I mentioned earlier that the Morrison Formation is famous for its fossils. Sure enough, there are biological traces in this outcropping.
While I'm no paleontologist, I believe these are characteristic
boreholes gouged by Aspirantes philosophiae doctoris,
a late Holocene species. ;)
Here is a close up of the uppermost part of the Morrison Formation, showing some very thin bedding.
The Morrison forms extensive and well-cemented sandstone beds west of the southern Sierra Nacimiento.
Morrison Formation west of the southern
Sierra Nacimientos 35
40.134N 106 54.756W
The Morrison Formation is famous for its varied hues, which are evident in the beds to the right in the photograph.
This is not far from the area where the giant sauropod dinosaur,
Seismosaurus, was discovered. Camarasaurus fossils
have also been found in this area.
At the start of the Cretaceous, South America rifted away from
Africa, and the vigorous new mid-ocean ridge displaced enough sea
water to raise sea levels to the highest levels we know of in the
geological record. This produced coal beds rivaling those of the
Carboniferous. At the same time, vigorous subduction under the
western edge of North America began another episode of mountain
building, the Sevier Orogeny. This produced characteristic shallow
faulting in which the surface sedimentary beds detached from the
underlying basement (décollement) and rode up over younger
beds further east. The Sevier Orogeny shortened the crust in the
Nevada-Utah area by over 100 km (60 miles).
The weight of the mountains thrown up in western California and eastern Nevada caused plate flexure, where the continental lithosphere further inland was pushed down into the mantle. The flow of subducting rock downwards beneath western North America also tended to pull the crust downwards. The combined effect was to produce a deep foreland basin. As the climate warmed and sea levels rose, this basin was flooded by a shallow sea, the Western Interior Seaway, in an event called the Zuni Transgression. The Jemez area was thus in a coastal or shallow marine environment during the Cretaceous, and coal beds were laid down in many locations in northern New Mexico, including the Madrid area southeast of the Jemez. The record of oceanic transgressions and regressions is particularly complete in northwestern New Mexico, where sandstone beds in the prevalent shale shows the passage of shorelines over time. The Jemez area was above sea level at the start and end of the period, but was submerged for around 20 million years in its middle.
The Western Interior Seaway is unusual in the geologic record in
that ample runoff from the early Rocky Mountains to the west
reduced the salinity of the water significantly. The runoff also
carried considerable organic matter, which produced the many black
shale beds of the Cretaceous in the western United States.
Sediment accumulation was rapid, as much as 195 meters per million
years in some locations in southwestern Colorado.
During the Cretaceous, true flowering plants (angiosperms)
appeared and rapidly proliferated. Foraminifera and diatoms
dominated the ocean plankton and produced great beds of chalk and
chert in some parts of the world; "Cretaceous" is derived from the
German Kreide, chalk. New families of molluscs
proliferated, including the distinctive oyster-like Inoceramus.
Ammonites were also diverse and abundant, and their rapid
evolution allows beds to be correlated with high precision. The
climate was as warm as any in the geologic record, with
subtropical vegetation extending clear to the Arctic Circle.
Petrified palm stumps have been found in the upper Cretaceous near
Cuba, New Mexico, in spite of the fact that New Mexico was at
about 45 degrees north latitude, compared with 36 degrees latitude
As Forest Road 100 ascends the narrow canyon of the Rio Encino,
on the way to the La
Grulla Plateau, it passes Cretaceous rocks of the Burro
Canyon and Dakota Formations.
Cretaceous exposes in Rio Encino canyon. Looking east from 36 9.813N 106 31.990W
That's Cerro Pedernal on the skyline. The foreground cliffs are
rimmed with Dakota Formation, and the slopes beneath are exposures
of Burro Canyon Formation. Above the cliff, on the plateau, is
Mancos Formation and Tertiary El Rito Formation.
Climbing the hillside, one gets an excellent view of the opposite side of the canyon and its corresponding exposures.
The heights to the left are the northern edge of the La Grulla
Plateau. The distant mountain to their right is San Pedro Mountain,
in the Sierra Nacimiento. Mesa Montosa and Alta Mesa dominate the
right side of the panorama. The Dakota Sandstone forms the rim
visible across the entire panorama, including the distant mesas in
the final frames.
Panorama of Rio Encino canyon. 36 9.829N 106 31.878W
Relief map of the Jemez with Burro Canyon Formation outcroppings highlighted in red.
The Burro Canyon Formation was formed by braided streams along
the coastal plain of the Western Interior Seaway around 120
million years ago.
Ascending the slopes in the previous photograph, one comes to outcroppings of this formation.
Burro Canyon Formation. 36 9.833N 106 31.876W
Burro Canyon Formation. 36 9.833N 106 31.876W
Under the loupe, this is revealed as a well-sorted sandstone composed almost entirely of angular quartz grains, with significant calciferous cement in the pore spaces -- though not enough to make it a wacke. A fairly mature quartz arenite.Nearby is another outcropping.
Burro Canyon Formation. 36 9.829N 106 31.878W
This outcrop reveals thin conglomerate beds, characteristic of the Burro Canyon Formation.
The exposure maps hows that this formation is restricted to the
northerrn Jemez. Since the next youngest formation, the Dakota
Formation, is found atop Morrison Formation in almost every other
exposure of Cretaceous rocks in the Jemez Area, the area north of
the Jemez must have been relatively low ground with higher terrain
to the south. This is consistent with the picture of the Burro
Canyon Formation as fluvial sandstone deposited close to the shore
of the Western Interior Seaway while the latter was still
relatively limited in extent.
Relief map of the Jemez with Dakota Formation outcroppings highlighted in red.
The Dakota Formation was deposited around 105 million years ago
in a shallow marine or mud flat environment. Its vast extent (from
Arizona to Wisconsin) reflects the full development of the Western
Interior Seaway, which fully flooded the interior of North America
in the middle Cretaceous. This flooding came from a combination of
the highest sea levels in the geologic record and plate flexure
from the mountains thrown up along the destructive margin still
active along the west coast of North America.
Cliffs of Dakota Formation dominate the rim of Rio Encino canyon,
as we saw in the earlier photographs. Here's the formation up
Paguate Sandstone, Dakota Formatin. 36 9.859N 106 31.839W
This prominent cliff is the Paguate Sandstone, the most prominent bed of the Dakota Formation in this area. It is an extremely well-indurated sandstone, accounting for its resistance and tendency to form cliffs. Here's a sample.
Paguate Sandstone, Dakota Formatin. 36 9.859N 106 31.839W
This is similar to the Burro Canyon Formation, but the grains are smaller, better rounded, and more strongly cemented together.
The Paguate Sandstone shows significant cross bedding.
Cross bedding in Paguate Sandstone, Dakota Formation. Click to enlarge. 36 9.859N 106 31.839W
The cross bedding is evident towards the bottom of this boulder. Like many sandstones, the Paguate Sandstone also preserves some fossils.
We are looking at ichnofossils, or trace fossils. These are
fossil traces of living organisms that are not the organisms
themselves. Obvious examples are dinosaur footprints, some of which
have been found in other locations in the Dakota Formation. Because
trace fossils are often highly distinctive, but the organisms that
produced them may not always be known with certainty, ichnofossils
are given their own ichnogenera and ichnospecies names. In this
case, we are looking at trace fossils of the ichnogenus Thalassinoides,
which are believed to have been most commonly produced by
sediment-dwelling crustaceans. Sedimentary rocks that formed from
sediments that were extensively tunneled by living organisms are
described as bioturbated, and the Dakota Formation in
northern New Mexico is noted for its high degree of bioturbation.
Ichnofossils in Paguate Sandstone, Dakota Formation. Click to enlarge. 36 9.859N 106 31.839W
As one continues ascending into the Jemez along Forest Road 100, the road reaches the level of the Paguate Sandstone.
Unsurprisingly, this look identical to the previous sample under
Relief map of the Jemez with Mancos Formation outcroppings highlighted in red.
The Macos Shale was deposited in an inland sea between 95 and 80
million years ago. The area of deposition was enormous, with
equivalent shale extending well east of the Rockies (the Pierre
Shale) and to southwest Utah (the Tropic Shale.) In places it is
over 1600 meters thick. However, it rapidly weathers in exposures
to a nondescript brown mud. Areas underlain by Mancos Shale tend
to quickly erode into low relief, sparsely vegetated terrain.
Although there are exposures in the northern Jemez, they tend to be rather poor. Here is an exposure above the cliffs of Paguate Sandstone on the east side of the Rio Encino valley. The mudstone has weathered to yellowish-gray mud, mantled with basalt debris from the La Grulla beds.
Badly weathered Mancos Shale in the northern Jemez. 36 9.859N 106 31.839W
However, there are extensive exposures in the Cerrillos
area of New Mexico, southeast of the Jemez, a historically
important mining district. In this drier climate, these are better
Mancos Shale in the Cerrillos mining district. 35 26.881N 106 7.153W
The exposures extend for a considerable distance around the Cerillos area, and have been divided into a number of members. The most prominent member west of the Cerillos Hills is the Niobrara Member, which is divided into upper and lower sections by a distinctive sandstone bed.
Sandstone marker bed in Niobrara Member. Looking northeast from 35 28.521N 106 10.449W
This is well exposed in a road cut nearby.
Niobrara Member. 35 28.569N 106 10.502W
My wife and her brothers probably find this very familiar-looking, and not necessarily in a happy way. The Mancos Formation gets its name from its type location at Mancos, Colorado, and the stuff is all over the place, including the valley outside Pagosa Springs where my in-laws have owned a time share on a cabin for over forty years. The kids probably skinned a few knees on vacation by trying to scramble up slopes of this stuff. It is slippery and crumbly and just made to send you tumbling when you try to walk across it.
And, as I mentioned earlier, exposures tend to weather to nondescript piles of mud in geologically short intervals of time, so it isn't usually even much to look at. The most boring part of the drive from Los Alamos to central Utah, which my family made at least annually from the time we were kids clear through my college days, was the stretch between Green River and Price that passed over miles and miles of nondescript muddy Mancos Shale.
But I haven't really got it in for the Mancos Shale. It's rich in
fossils. It's also one of the more promising source rocks for
future oil exploration, and that counts for a lot.
The best exposures in the Jemez area are west of the Sierra Nacimiento Mountains, and these exposures include some prime fossil beds on public lands. A particularly good site is the so-called Windmill Site, a low ridge with a large windmill, where both bivalve molluscs and ammonites can be found.
This fossil is likely Inoceramus, which is very common in Cretaceous rocks of the Western Interior Seaway. It resembles a modern oyster but paleontologists now believe it is a distinct lineage of mollusc.
This is an ammonite cast.
Some of the bivalve shells are preserved at this location, but there were no examples where the actual shell of an ammonite was preserved; only casts. This may be because the shells are chemically different. The bivalve shells are composed of calcite, while ammonite shells are aragonite. Both are calcium carbonate minerals, but calcite is more stable than aragonite. Aragonite dissolves below a certain depth of ocean water, called the aragonite compensation depth, while the calcite compensation depth is considerably greater. This suggests that the floor of the Western Interior Seaway managed to get below the aragonite compensation depth at some point, dissolving all the aragonite ammonite shells while leaving the calcite bivalve shells intact.
Nearby is Sharktooth
Ridge, notable for fossil teeth of sharks and other fish.
This ridge has a rather steep escarpment to the south, caused by a
very resistant bed of sandstone that has been interpreted as a
beach deposit -- possibly corresponding to the Point Lookout
Sandstone. This marks one of the regressions of the Western
Sharktooth Ridge. Near 35
32,924N 107 7.179W
You can see the sandstone bed forming the escarpment. This
extends for a considerable distance to east and west. In the
background is Cerro Cochino, a volcanic plug marking this portion
of the Jemez Lineament.
The sandstone bed is full of fish teeth, including the occasional shark tooth. These can be hunted by digging and sifting the sediment atop the sandstone bed near the escarpment edge.
The rounded tooth at top is from a species that crushed shells for a living. The others are from large carnivorous fishes.
At top is a single calcite crystal that I dredged up. Calcite is the principal cementing mineral in the sandstone. The crystal is quite irregular, but you can tell from the uniform reflection off of cleavage planes that it is a single crystal.
Further west in the San Juan Basin, and east of the Rockies in
the San Juan Basin, there are additional Cretaceous formations
above the Mancos and Point Lookout Sandstone. These include the
Menifee Formation, which contain numerous coal beds of economic
significance, mined today in the Farmington
region and in the past in Raton
None of these formations are present in the immediate Jemez area.
The end of the Cretaceous was marked by the most famous and
second most destructive mass extinction in the fossil record.
Unfortunately, no record of this is preserved in the Jemez area,
because the upper Cretaceous and lower Triassic beds were
destroyed in the birth of the Rocky Mountains in an event known as
the Laramide Orogeny.
Next page: The end of (most of) the
dinosaurs and the rise of the mammals
Copyright ©2014-2017 Kent G. Budge. All rights reserved.