The previous chapter may be found here.
Artist's conception of Paleozoic swamp. Wikimedia Commons
Before the Age of the Dinosaurs came the Age of the Amphibians,
when frogs (or their larger ancient ancestors) ruled the earth. It
would have been a friendly world for amphibians, with widespread
swamps in which were formed many of the coal beds we exploit
In this chapter, we will look at the earliest fossil-bearing
formations of the
The most recent eon of time is the Phanerozoic, which dates back
to about 540 million years ago. The Phanerozoic, in turn, is
divided into the Paleozoic, Mesozoic, and Cenozoic eras. Each of
these eras is divided, in turn, into a number of periods. The
Paleozoic, which covers the time period from about 540 million
years ago to about 252 million years ago, is divided into the
Cambrian (560-485 Ma), Ordovician (485-443 Ma), Silurian (443-419
Ma), Devonian (419-359 Ma), Mississippian (359-323 Ma),
Pennsylvanian (323-299 Ma), and Permian (299-252 Ma) periods. Most
geologists outside the United States combine the Mississippian and
Pennsylvanian into a single Carboniferous period; since we are
looking at a region in the United States, we will use the
Mississippian-Pennsylvanian nomenclature. These time periods are
The Cambrian is the earliest geological period in which there were abundant hard-shelled animals to leave fossilized remains. Earlier fossils exist, but it was some time before their nature and antiquity was recognized. In addition, in many parts of the world, the Cambrian is underlain by older crystalline rocks containing no fossils. We discussed these Precambrian rocks in the Jemez area in the previous chapter.
Across most of North America, the boundary beneath the Cambrian is not a mere transition to older rock beds. The boundary is unconformable, meaning that the beds above it do not appear to be a mere continuation of the beds below it. This indicates that the rocks of Cambrian age or later were laid down on a surface eroded into the older rocks. This also means that there is a considerable length of geological time, sometimes over a billion years, between the sedimentary beds and the Precambrian rocks beneath them. This missing period of time, first recognized by the famed explorer John Westley Powell in the Grand Canyon in 1869, is so distinctive and so widespread across North America that it has been dubbed the Great Unconformity.
The Great Unconformity is beautifully visible at Cody, Wyoming, where the gap in the geological time record is over 2 billion years.
Beds of the 515 million-year-old Flatstone Sandstone sit unconformably on 2.7 billion-year-old Archean rocks. The sandstone is a tan color while the Archean rocks are mostly fine-grained gray granite intruded by ribbons of coarser pink granite. The Archean rocks date back to a time when the Wyoming-Montana area was not yet a part of North America, but instead was either part of a different continent or a small continent in itself.
Though widespread, the Great Unconformity is not identical
everywhere. For example, outcroppings of Cambrian to Devonian
rocks are exposed only in the southern part of New Mexico.
Throughout most of the rest of the state, including the Jemez
area, rocks of Mississippian age or later sit directly on
The Great Unconformity seems to roughly coincide with the existence of a great supercontinent, Pannotia, which broke up at the beginning of the Cambrian. Perhaps the supercontinent was mostly above water and subject to erosion, which not only prevented new sedimentary beds from being laid down, but destroyed many of the existing sedimentary beds. Much of this supercontinent, including the area that would someday be northern New Mexico, was very close to the South Pole, and the Varangian glaciation, the worst ice age the Earth has known, covered most of the earth in great ice sheets. These glaciers may have helped scour off younger sedimentary beds.
With the breakup of Pannotia, much of the Earth's crust became covered with shallow oceans in which the rocks of the Cambrian and succeeding periods were deposited. Northern New Mexico remained an area of higher elevation, part of a structure geologists have dubbed the Transcontinental Arch. This did not become submerged until later, and the Jemez area remained above water until the Mississippian.
A prominent exposure of the Great Unconformity is seen at the top of Sandia Crest, east of Albuquerque. Here Pennsylvanian limestone of the Sandia Formation sits atop 1.43-billion-year-old Sandia Granite.
This can be admired more closely on the road to the crest. This is one of the best exposures of the Great Unconformity I’ve seen.
Great Unconformity in the Sandia
11.466N 106 24.003W
The Sandia Granite is overlain by 310-million-year-old Sandia
Formation. The contact represents some 1.1 billion years of
missing geologic history, eroded away prior to 310 million years
There is a particularly fine example of the Great Unconformity at
Corner, along the Hyde Park Road northeast of Santa Fe.
We saw this photograph in the last chapter, where we noted that the shattered Precambrian granite at bottom may be impact breccia from the nearby Santa Fe impact structure. Here layers of dolomite of the Mississippian Arroyo Peñasco Group sit on top of the Precambrian granite. The gap in time here is probably around 1.2 to 1.3 billion years.
Since the only Precambrian outcropping in the Jemez proper is at Soda Dam, this is also the only place in the Jemez where one can see the Great Unconformity. We visited the Soda Dam area in the previous chapter, but it's time to visit the area again.
To the east of Soda Dam, the rocks sitting on top of the Precambrian gneiss are red beds of the Mississippian Log Springs Formation.
To the west of the dam, just across the highway, the geology is confused by the presence of a strand of the Jemez Fault and by extensive deposits of relatively young travertine. The fault itself is well exposed part of the way up the canyon wall.
Jemez Fault west of Soda Dam. 35 47.520N 106 41.219W
The fault is the yellowish, steeply tilted zone just above the
center of the photograph. We will have much to say about faults
later in this book, but for now it suffices to say that a fault is
a fracture in the earth's crust across which the rock beds are
displaced vertically, horizontally, or both. The Jemez Fault
displaces the rocks mostly in the vertical direction and mostly
with the downthrown side (the side where the rock beds are
displaced downwards) to the east. However, this strand or
branch of the fault is displaced downwards to the west.
To the left is Precambrian gneiss and to the right is relatively young travertine deposits of Soda Dam. The travertine sits on top of Pennsylvanian Sandia Formation, but the two are quite difficult to distinguish since they are both limestone. To the left of the fault, however, there are intact Mississippian limestone beds sitting atop the gneiss, representing another exposure of the Great Unconformity.
Great Unconformity west of Soda Dam. 35 47.520N 106 41.219W
The Mississippian beds are visible at center left, forming thin
beds tilted to the left. These are probably part of the Arroyo
Peñasco Group. To the right is Jemez Cave, and the jumbled
formations above the cave are old travertine deposits.
In the nearby Guadalupe Box, on the boundary between the Jemez and Sierra Nacimiento Mountains, there is widespread contact between Paleozoic and Precambrian rocks. These can be deceptive.
The darker rock to the left is weathered Precambrian gneiss of
the Guadalupe Box that we saw in the last chapter. To the right
are Permian sedimentary beds. However, this is not really an
example of the Great Unconformity; the local topography suggests,
and geological mapping confirms, that this is actually a large
fault, the Sierrita Fault, which is part of the Jemez Springs
fault zone that forms part of the west boundary of the Rio Grand
Rift. The younger beds have been thrown down by the fault a
considerable distance, bringing them into contact with the
However, north of the Guadalupe Box, there is a genuine exposure of the Great Unconformity.
The lower part of the photographic is Precambrian monzonitic
gneiss. Laid down on top of it is beds of Mississipian Arroyo
The gap represented by the Great Unconformity in the Jemez is from about 1.4 billion years ago, in the Mesoproterozoic Era, to about 340 million years ago, in the Mississippian Period. Some 1.06 billion years of geologic history have been erased. During this long interval, the oxygen levels in the atmosphere rose to about the level of today; eukaryotes proliferated and the first vascular plants, arthropods, fish, and amphibians appeared in the fossil record; and vascular plants and amphibians colonized dry land. This seems to have been a long period of stability for northern New Mexico, which was little touched by the formation and breakup of supercontinents and was gradually eroded to a nearly flat plain.
The Mississippian was a time when most of the Earth's continental
crust assembled into the supercontinent of Pangaea, and it was
also a time of low relief and very high sea levels. Most of the
continents were flooded with shallow seas, and New Mexico was no
exception. Shallow seas covering continental crust are called epicontinental
Geological history shows a repeated pattern of fluctuating sea
levels. Sea level today is somewhat higher than has been typical
of the Cenozoic, the most recent era of geologic time, because all
of written human history has taken place during an interglacial,
a period of relative warmth between glacial periods. The oceans
are high enough to flood part of the continents and form
continental shelves underlying shallow water in the North Sea, the
Bering Strait, and elsewhere. (The North Sea is thus a modern
example of an epicontinental sea.) The continents are almost
entirely above water, with very little submerged continental
shelf, during the peak of ice ages. But even the continental
shelves of today are small compared with what they have been in
the geological past; the Cenozoic has been a period of unusually
low sea levels. Geologists believe that during times of rapid
mid-ocean rifting, the hot, buoyant oceanic crust produced at
highly active mid-ocean ridges displaces enough ocean water to
flood the continents. The Mississippian and Pennsylvanian was a
time of such continental flooding. The advance of the shoreline
into the continental interior is called a transgression;
the subsequent retreat of the shoreline off the continent is
called a regression. The transgression that drowned the
Jemez area in the Mississippian is known as the Kaskaskian
In Europe, the Mississippian and Pennsylvanian periods are jointly called the Carboniferous Period, because there were vast and long-lived swamps in which a great deal of peat was laid down that eventually became coal. The vegetation of the swamps was dominated by lycopsids such as Lepidodendron (giant relatives of club mosses) and early seed plants such as Glossopteris. New Mexico was not one of these areas; most of the state was under salt water, in a shallow marine environment, and most of the rock laid down during this period was limestone and shale. In North America, but apparently not Europe, the seas receded briefly and allowed a short but widespread episode of erosion that separates the Mississippian from the Pennsylvanian. This unconformity is present in parts of northern New Mexico.
The Carboniferous formations of the southwest Jemez are nowhere
better seen than at Guadalupe Box. We've shown some photographs
already, to illustrate the Great Unconformity at the base of the
Carboniferous. Perhaps the best view of the whole column can be
seen from atop
the western rim of Guadalupe Box, which can be reached by a
short if strenuous hike along a fire road.
The Carboniferous at Guadalupe Box.. Looking east from 35 44.162N 106 45.954W
To the right is the mouth of Guadalupe Box, with Mesa
Garcia in the distance. Tall cliffs of Precambrian gneiss
form the Box. Atop the gneiss are Mississippian beds of the Arroyo
Peñasco Group, forming a thin layer that dips with the gneiss to
the north (left in photograph) to pinch out in the third frame.
Atop this is a spotty layer of red sediment of the Log Springs
Formation, then a a thick layer of Osha Canyon Formation. On top
of the Osha Canyon Formation is the Sandia Formation, whose
lowermost hard sandstone beds form the knob in the third and
The first frame shows an unnamed canyon descending from the north
end of Guadalupita
Mesa into the valley of the Rio Guadalupe. More Osha Canyon
Formation is visible in the wall of this canyon below center in
the first frame, overlain by more Sandia Formation. Sandia
Formation also underlies the lower part of Guadalupita Mesa on the
skyline. Above the Sandia Formation on the northern (left) slopes
of Guadalupita Mesa, and below the cap of Bandelier Tuff, are
white slopes of the Madera Formation.
The red beds on the southern (right) slopes of Guadalupita Mesa
are Permian formations that we will examine more closely later.
Relief map of the Jemez with Arroyo Penasco Group outcroppings highlighted in red.
The Mississippian Arroyo Peñasco Group is the oldest Phanerozoic
sedimentary formation in northern New Mexico. It is not found
everywhere in the region, and it shows evidence of having itself
been weathered and eroded in many places before being buried under
The Arroyo Peñasco Group was deposited on a relatively level
surface of Precambrian rock beginning around 345 million years
ago, when southern and central New Mexico was covered by a shallow
sea whose shoreline moved back and forth across the Jemez area.
Further west, this sea deposited the Redwall Limestone of the
Grand Canyon area. The Arroyo Peñasco Group is also largely
limestone. We've seen several examples earlier in this chapter,
where thin beds of the group are found atop the brecciated granite
near the Santa Fe impact structure and the gneiss at Soda Dam and
We know that the Precambrian surface was relatively level,
becauase there is little conglomerate in the lower beds of the
Arroyo Penasco Formation, which tend instead to be fairly clean
sandstone. This indicates an absence of any high ground to shed
coarse sediments into the advancing ocean.
One can more closely examine the Arroyo Peñasco Group at
Guadalupe Box by crossing the Rio Guadalupe and hiking a fire road
east of the river to the top of the
east rim of Guadalupe Box. Adventurous readers are advised
that the Rio Guadelupe has no bridges or easy fords in this area
and can be swift and deep; it should be crossed with caution.
Log Springs Formation and Arroyo Peñasco Group on east rim of Guadalupe Box. 35 43.988N 106 45.793W
This photograph shows a thick bed of limestone of the Arroyo
Peñasco Group overlain by a thin layer of Log Springs Formation.
These beds lie directly on top of Precambrian gneiss.
The Great Unconformity, up close and personal.. 35 43.988N 106 45.793W
During the Mississippian, momentous events were taking place far to the west of the Jemez. At that time, the western edge of North American was located in what is now central Nevada. The continental margin here had been passive since Australia split away from North America during the breakup of Rodinia, about 770 million years ago. Although the geological record is incomplete, the weight of evidence is that the oceanic crust beyond the edge of the continent began subducting under an island arc or small continent further out in what is now the Pacific, and in the early Mississippian the continent itself drifted into the subduction zone. This began a pattern of tectonic activity along the western edge of North America whose effects have continued to the present day.
Because continental crust is too buoyant to subduct, the arrival of North America at the subduction zone "jammed" the subduction zone. This had two effects. First, the island arc or microcontinent under which ocean crust has previously subducted to the west became fused to the western edge of North America. Second, the subduction reversed itself; oceanic crust from further west began subducting to the east under North America, whose western margin became a destructive margin. The collision threw up a substantial range of mountains, an event called an orogeny. In this case, geologists have dubbed it the Antler Orogeny, after Antler Peak, Nevada, a modern mountain whose underlying rocks provided the first clues to the event.
Limestone of the Arroyo Peñasco Group. 35 43.988N 106 45.793W
Limestone is a sedimentary rock composed mostly of calcite, CaCO3.
Most limestone consists of tiny fragments of the shells of marine
organisms, mixed with small quantities of sand, silt, and clay. It
forms primarily in shallow marine environments, where tiny shelled
organisms such as foraminifera (shelled plankton related to the
amoeba) multiply in great numbers. The Arroyo Penasco Group is
rich in fossil foraminifera and microscopic algae. Calcium
carbonate is soluble in deep, cold water, so it is not possible
for limestone to form in waters deeper than 3000 meters (10,000
feet), and while limestone sometimes forms in fresh water, this is
uncommon. Furthermore, the best shallow marine environments for
limestone formation are ones in which other sediments, such as
silt and clay, are scarce. This is most likely to be the case in
an arid climate, where there are few rivers to carry sediments
into the ocean. Thus, extensive limestone beds suggest that the
underlying tectonic plate was situated at around 30 degrees north
or south latitude when the limestone formed. These are the
so-called "horse latitudes", where most modern deserts are
located. New Mexico is thought to have been about 10 to 15 degrees
south of the equator during the Mississippian, on the edge of this
Dolostone is a close relative of limestone and likely forms from limestone exposed to magnesium-rich fluids under the right conditions of temperature, pressure, and oxygen abundance. The magnesium replaces part of the calcium to produce dolomite, a mineral with the composition CaMg(CO3)2. The details of this process are still not entirely understood, but dolostone seems to form more readily during times of high sea level, and it is rare to find a "partial" dolostone: Carbonate rocks tend to be either almost entirely calcite or almost entirely dolomite; rocks combining both minerals are very uncommon.
Limestone is easy to identify in the field. Though it is a tough rock, it is not particularly hard, so that it is easily scratched with a penknife. It sometimes displays visible calcite crystals. The surest test of all, however, is to drip some acid (such as diluted hydrochloric acid) on the rock. Limestone will foam vigorously. Dolostone resembles limestone in appearance, but bubbles feebly when exposed to acid. Rocks composed of silicate minerals generally show no reaction to acid.
Calcite, CaCO3, is one of the most common non-silicate
minerals. Its crystal structure is fairly straightforward,
consisting of of alternating layers of calcium ions and carbonate
Structure of calcite. Courtesy of Steven Dutch.
In this diagram, we are looking directly down on the alternating
layers of the crystal. Calcium ions are shown in purple and
carbonate ions as a set of three oxygen atoms, drawn in green,
blue, or yellow, surrounding a small gray carbon atom.
Well-crystallized calcite is surprisingly common, and is
popularly known as dogtooth spar.
Calcite samples. As with most images here, you may click to enlarge
Calcite crystals tend to form elongated pyramids, as in the sample at top. However, calcite has perfect cleavage in three directions, and so large calcite crystals break into cleavage fragments, like the one sitting on the ruled paper, that are boxy in shape. Calcite has the optical property of birefringence, or double refraction, in which images seen through clear calcite appear doubled. You can see in this photograph that the lines on the paper are doubled as seen through the crystal.
Calcite is also relatively soft. It is not quite soft enough to
scratch with a copper penny, but it is easily scratched with a
knife. Its softness, distinctive cleavage, optical properties, and
reaction with mild acids to produce bubbles are distinctive enough
that calcite is easily identified in the field.
Calcite is very slightly soluble in cool, acidic water. Thus, when limestone beds are exposed at the surface, rainwater (which is slightly acid from dissolved carbon dioxide, even in the absence of industrial pollution) can cut deep sinkholes and caverns in the limestone. This produces what geologists call a karst topography. Northern New Mexico is presently too arid for areas underlain by limestone to develop much karst topography, but this was not true in the distant past.
Geologists studying sedimentary formations will often attempt to construct the paleotopography of a region at various times in the past. This starts with the obvious: Wherever there is a sedimentary bed of the particular age we are interested in, we know that the conditions that produced that kind of sedimentary rock were at work at that location at that time. So wherever we see exposures of the Arroyo Peñasco Group, we know that that area was covered with a shallow sea 345 million years ago.
There is no other significant Mississippian formation in northern New Mexico. Can we conclude that these areas were high ground 345 million years ago? Unfortunately, there are three other possibilities. One that has probably occurred to you already is that most of this area is covered with beds that are younger in Mississippian age and conceal any older rocks. Unless we have reliable drilling data for these areas, we do not know whether any Mississippian rocks are located there. In principle, further drilling might someday give us that information.
But there are areas where we see bare Precambrian rocks exposed,
and other areas where we see rocks younger than the Arroyo Peñasco
Group sitting directly on Precambrian rocks. Here we are still
left with two possibilities. One is that this was high ground
during the Mississippian, so that no sedimentary beds were laid
down at that time. But there is also the possibility that beds
were laid down but subsequently eroded away. And this could have
taken place any time between the Mississippian and the age of the
The next widespread formation in northern New Mexico is the
Pennsylvanian Sandia Formation, which we'll get to shortly. There
are a couple of local formations intermediate in age between the
two, but they are not widespread enough to much affect our
reconstruction of the paleotopography, and they are usually found
above beds of the Arroyo Peñasco Group rather than directly on
Precambrian granite. if we know that areas with Arroyo Peñasco
Group were shallow seas during the Mississippian, and assume that
areas where Sandia Formation sits directly on Precambrian rocks
were high ground during the Mississippian, we get this map:
Paleotopographic map of Arroyo Penasco Group
The red areas are known to be shallow marine environments during the Mississippian. The green area areas that must have been above water during or shortly after the Mississippian. Everywhere else is unknown. This gives us a rather patchy picture of what took place. We'll see better examples with other formations later in the book.
We do have another clue that most exposures of the Arroyo Peñasco
Group were briefly above water towards the end of the
Around 320 million years ago, North America collided with Africa.
During this collision, a large area coinciding with parts of
today's Rocky Mountains was uplifted to form what are called the
Ancestral Rocky Mountains. The Ancestral Rocky Mountains included
a highlands called Uncompahgria, stretching in a great arc from
western Colorado down to roughly the current location of Espanola.
A small outlier range was located in the current location of the
Sierra Nacimiento. A deep basin, the Paradox Basin, formed
to the southwest of Uncompahgria, with its deepest point somewhere
close to the current location of Arches National Monument. The
Paradox Basin followed the curve of Uncompaghria and may have
reached as far as the present location of the Jemez Mountains.
The collision also produced a brief lowering of sea levels that
separates the Mississippian from the Pennsylvanian in northern New
Mexico. During this time, the exposed beds of the Arroyo Peñasco
Group developed a karst topography. Modern karst topography often
is mantled in a deep red iron-rich soil called terra rossa,
which tends to fill in the sinkholes and crevices in the
underlying beds. This is particularly the case in tropical areas,
and the Jemez area was located almost on the equator at this time.
The Log Springs Formation is likely a preserved terra rossa formed
on the karst topography of the Arroyo Peñasco Group, augmented
with sediments eroded off the nearby highlands. Similar thin beds
of red sediments (usually assigned to the Molas Formation) are
found at the Mississippian-Pennsylvanian boundary throughout the
western United States.
I haven't bothered with a map of the Log Springs Formation, because it is nearly identical with the map for the Arroyo Peñasco Group. The Log Springs Formation is thin and spotty at Guadalupe Box, but it is much thicker to the west, in the southern Sierra Nacimiento Mountains. The exposure of the formation at Guadalupe Box is rich in hematite, Fe2O3, which forms a thick coating on limestone clasts derived from the underlying Arroyo Peñasco Group.
Hematite-coated nodule from Log Springs Formation. 35 43.988N 106 45.793W
This sample shows the bright red color of hematite on the left,
and somewhat darker color on the right where the nodule was
partially buried. It has a high density, which I measured at
around 4.3 g/cm3, whereas normal limestone has a
maximum density of around 2.8 g/cm3. This suggests that
there is more hematite coating than limestone nodule. Hematite
makes an excellent iron ore, but the beds here are neither rich
enough nor extensive enough for economical exploitation.
Log Springs Formation nodule split in half to show calcite interior.. 35 43.988N 106 45.793W
The second sample is a somewhat larger nodule that had a
significantly lower density than the first. Split open, it shows a
highly crystalline limestone interior; the reflective patches in
the photograph are faces of large calcite crystals, as confirmed
with the acid test.
Hematite, Fe2O3, is a common minor component of sedimentary rocks. It is also common in weathered igneous and metamorphic rocks, but is rather uncommon in fresh igneous and metamorphic rocks. This is because of the chemistry of iron. An iron atom readily loses two electrons to form the ferrous ion, Fe2+, but with a bit of encouragement it can lose a third electron to form the ferric ion, Fe3+. Under conditions typical of the earth's interior, where metamorphic rock is formed and magma is generated, there is very little oxygen and iron tends to remain in the ferrous state. Thus, fresh igneous and metamorphic rocks contain most of their iron as ferrous minerals. Such ferric iron as is present is bound up with ferrous iron in magnetite. However, the high oxygen levels at the earth's surface ensure that ferrous iron is eventually oxidized to ferric iron, and most of this ferric iron exists as hematite or its hydrated counterpart, goethite, FeO(OH).
Powdered hematite has a blood-red color, which gives it its name. As the percentage of the hematite that has been hydrated to goethite increases, the color shifts to a yellowish brown. Massive hematite is dark red to black in color. Large deposits are mined as iron ore on every continent except Antarctica. However, well-crystallized hematite, which is jet black in color, is quite uncommon. Massive hematite can be identified both by its blood-red streak (produced by rubbing a sample against an unglazed porcelain plate) and by its high density. Unlike magnetite, hematite is not attracted by a magnet.
In a few locations, the iron-rich lower beds of the Log Springs
Formation are overlain by a thicker sequence of mudstones,
sandstones, and conglomerates, with the uppermost bends tending to
have more coarse material. This suggests that increasing
quantities of coarse sediments were being eroded off nearby
highlands pushed up as part of the Ancestral Rockies.
Paleotopographic map of Osha Canyon Formation
About 310 million years ago, New Mexico was a mass of mountainous
islands (the ancestral Rocky Mountains) in a shallow sea.
Limestone was again deposited over much of the region. The lowest
Pennsylvanian formation in the Jemez is the Osha Canyon Formation,
first identified at Guadalupe
Osha Canyon Formation. Looking south from 35 44.285N 106 45.830W
Here we see Osha Canyon Formation forming the south rim of an
unnamed canyon, with overlying Sandia Formation forming the hill
in the background.
Osha Canyon formation on the east rim of Guadelupe Box. 35 44.034N 106 45.793W
Here we see the formation on the east rim of Guadelupe Box, near
the spot where the formation was originally identified and
described (the type section.)
The Osha Canyon Formation is not nearly as widespread as the
other Carboniferous formations of the Jemez, being found only in a
limited area of the southwestern Jemez and southern Sierra
Nacimiento. Perhaps this was a particularly low area into which
the sea returned first. However, the Osha Canyon Formation is
noted for the presence of fossil tabulate corals and for rare
examples of a primitive giant echinoderm, Giganticlavus.
Map of Sandia Formation exposures in the Jemez area
The Osha Canyon Formation was subsequently overlain by the sandstone and thin limestone beds of the Sandia Formation, which formed from sediment eroded off the rising Uncompahgre Uplift.
We saw beds of the Sandia Formation atop Sandia Crest in the last chapter, and we've already seen several photographs in this chapter showing Sandia Formation atop Osha Canyon Formation. Here is a closer look at some of the prominent sandstone beds found near the base of the Sandia Formation.
Sandia Formation. 35 44.068N 106 45.776W
The multicolored patches on the rock are lichens, which are
communities of fungi and algae. Lichens were among the first and
most resilient forms of life to emerge from the oceans. There is
an excellent chance that their lineage is older than the rocks
they are encrusting here.
Sandia Formation sandstone. 35 44.068N 106 45.776W
The base of the Sandia Formation is a very coarse sandstone, of which a sample is shown above. The individual grains are obvious even to the naked eye. Under the loupe, these appear to be angular bits of yellowish quartz, all roughly the same size, with occasional fragments of feldspar. There is some hematite between the grains, giving the rock its reddish to tan color, but the grains are not cemented primarily with hematite. The acid test shows no calcite present, and the rock is quite tough, so the grains are most likely cemented with silica. There is considerable pore space; this rock would make an excellent aquifer or reservoir rock for petroleum.
The description I've just given of this sandstone hits on most of
the classification criteria used by geologists to describe clastic
sedimentary rock, that is, sedimentary rock composed
primarily of broken fragments (clasts) of source rock
rather than precipitated minerals. The first criterion is clast
size. Sedimentary rock whose clasts are mostly in the range of 2mm
to 0.0625mm is classified as sandstone. Rock with larger clasts
(up to boulder size) is known as conglomerate or breccia,
depending on whether the clasts are rounded (conglomerate) or
angular (breccia.) Sedimentary rocks with smaller clasts are known
as siltstones, mudstones, or shales. We'll see examples of most of
these other kinds of sedimentary rocks later in the book.
This sample is a sandstone, because its grains are all between 0.5 mm and 2mm in size.
Another classification criterion is the mineral content. Most sandstones are composed largely of quartz, because quartz is the hardest and most stable common mineral under conditions at the Earth's surface. Feldspar is less chemically stable than quartz, slowly decomposing under atmospheric conditions into clay minerals, and it tends to shatter easily along its cleavage planes. Nevertheless, it is the second most common mineral in sandstone. A sandstone containing substantial feldspar (more than 25%) is called an arkose sandstone. Our sample here contains less than 5% feldspar and so is not an arkose.
Fine-grained material between the larger clasts of a sedimentary rock, usually deposited at the same time as the larger clasts, is called matrix, while minerals deposited later from solution in groundwater are known as cement. It is the presence of strong cement that produces the hardest (most indurated) sedimentary rocks. Sandstones with considerable pore space are known as arenites. They are distinguished from wackes that contain 15% or more clay minerals between the grains. Because our sample has open pores with almost no visible clay matrix, it is a quartz arenite.
Quartz arenites are considered mature sandstones, meaning they are formed from sediment that has undergone considerable weathering that has removed almost everything but the quartz. Our Sandia Formation sandstone is a bit unusual in that its grains are large, angular, and do include some bits of feldspar. It is thus not a supermature sandstone composed of almost pure quartz grains of nearly identical size (well-sorted) that are well rounded. Our sample is not particularly well-sorted for a sandstone. It is otherwise typical of sandstones that form deep in the interior of stable continents, where well-weathered sediments can accumulate slowly over long periods of time. Most likely this sandstone was deposited in a beach environment during a marine transgression, with the large size and angular character of the grains suggesting they weathered from a nearby granitic highland -- the Uncompahgre Uplift.
As one goes higher in the Sandia Formation, the sandstone tends to give way to softer siltstone and shale and then the limestone of the Madera Formation. This is a typical fining sequence of a marine transgression. The sandstone is laid down in a near-shore environment, then, as the epicontinental sea advances, siltstone and mudstone is laid down in water further from shore. The transgression sequence ends with deposition of limestone.
Here is a paleotopographic map of the Jemez in the early Pennsylvanian, when the Sandia Formation was laid down.
Paleotopographic map of Sandia Formation
This shows that a large region north and west of the exposures of
Sandia Foramation has Madera Formation, the next youngest
formation, resting directly on Precambrian rocks. This must still
have been high ground during the early Pennsylvanian, but became
submerged in the middle Pennsylvianian, when the Madera Formation
was laid down.
Map of Sandia Formation exposures in the Jemez area
Above the Sandia Formation is the Madera Formation, the principle
Pennsylvanian formation in the Jemez Mountains (and much of the
rest of New Mexico.) It was deposited during a time when much of
New Mexico was covered with a shallow sea, though sediments
continued to be eroded off the Uncompahgre Uplift and other ranges
of the Ancestral Rocky Mountains. The Madera Formation is exposed
in the valleys of the Rio
Guadalupe and Rio de las Vacas and in Cañon
de San Diego in the southwest Jemez, north of Soda Springs,
as well as the Gallina
There is a very thick section of Madera Formation atop the Sandia Formation in Guadelupe Canyon north of Guadelupe Box. We've seen some photographs already, but here is a better look.
Madera Formation in west side of Holiday Mesa. View to the northwest from 35 44.307N 106 46.040W
Sandia Formation forms the tan beds at the base of Holiday Mesa,
and the mesa is capped with Bandelier Tuff. Almost everything
between is Madera Formation, which is hundreds of feet thick here.
This area is reputed to be a paleontologist's playground, with
some areas covered with gravel composed mostly of fossilized
mollusks and brachiopods that have weathered out of the rock.
However, the Rio Guadelupe is difficult to cross, and the climb up
to the fossil beds is strenuous.
Madera Formation continues to line the canyon walls for miles to the north, with a particularly fine exposure in the north side of Bale Canyon.
Another exposure in northern Cebolla Canyon shows the upper part of
Madera Formation in north rim of Bale Canyon. View to the north-northeast from 35 49.441N 106 49.144W
Madera Formation in northern Cebolla Canyon. 35 51.424N 106 45.595W
Such exposures continue into the northern Sierra Nacimiento and
the adjacent areas of the Jemez. This exposure is prominent in a
road cut along State Road 126.
Madera Formation in northern Sierra
59.696N 106 48.958W
Madera limestone forms cliffs along several miles of Cañon de San Diego north of Soda Dam.
Madera Formation in cliffs north of Soda Dam. View to the northwest from 35 47.911N 106 41.136W
Madera Formation. Looking north from 35 48.009N 106 41.492W
Madera Formation. Looking east from 35 48.544N 106 40.466W
The highway climbs to the level of this particular set of beds a
few miles up the road from Soda Dam, where there is a beautiful
Madera Formation along the side of State Road 4. 35 48.390N 106 40.724W
The limestone here is coarsely crystalline with occasional crinoid fossils.
Madera Formation.. 35 48.390N 106 40.724W
Not all of the Madera Formation is this coarsely crystallized. It is probably this unusually coarse crystallization that makes these particular beds so resistant to erosion and thus so prominent in the canyon wall.
There are also some large boulders of very coarse sandstone in
this area, showing that the Madera Formation is not purely
limestone, but contains some sandstone and shale as well.
Nevertheless, in the Jemez, the Madera is mostly limestone.
The uppermost Madera Formation beds are exposed along the hiking trail at the foot of Battleship Rock.
This limestone is rich in fossil brachiopods, mollusks, and
crinoids, and I have collected some fossils from a number of sites
west of State Road Four in this general area.
Fossils weathered from Madera Group limestone. Crinoid stem fragments at top; brachiopod at bottom.
It should be clear from these photographs just how extensive the Madera Formation is in the western Jemez area. It dominates the Carboniferous exposures mapped in red in the relief map at the start of this section. Some geologists promote this thick formation to group status, and divide the Madera Group into the Gray Mesa and Antrasado Formations, while others assign these beds to the Guadelupe Box Formation. However, these are not distinguished on the geological maps I have for the Jemez.
This book focuses on geology, not paleontology, and I won't have that much to say about ancient life. Part of the reason is that the Jemez is most famous for its volcanic rocks, which do not contain fossils, and part is that I simply haven't studied paleontology in great depth. But such fossils as I have collected are mostly from the Carboniferous, so I can't leave this section without saying a word or two about them.
The Mississippian marked the emergence of the first reptiles, which were able to lay eggs on dry land. The armored fish of earlier periods were largely displaced by sharks and bony fish, and arthropods (animals such as insects and spiders, having exoskeletons and jointed legs) swarmed across the landscape. Vegetation of the Carboniferous included tree ferns and other large non-seed-bearing plants, but early seed-bearing plants such as cycads were also present. Because New Mexico was under a shallow sea, these forms are not present as fossils in the Jemez. Instead, we find shallow marine fossils, dominated by bryozoans, brachiopods, mollusks, and crinoids.
Bryozoans are distant relatives of both mollusks and annelid
worms. They are microscopic animals (up to 0.5mm in size) that
form large colonies of various forms. Those found as fossils in
the Jemez created calcified "fronds" like that in one of
photographs above. These were covered with bryozoans living in
their own individual pockets, somewhat like a coral. However,
bryozoans are not close relatives of coral. There is some
functional specialization among the individual bryozoans in a
colony, so that they are not truly independent animals, like
individual polyps in a coral reef are.
Mollusk fossils in the Jemez are mostly bivalves, that is, mollusks with two symmetrical shells, resembling modern clams or scallops. I once accompanied a den of Webelos Scouts (Boy Scouts in larval form) on a fossil hunt in which one of the boys found a marine snail fossil, but they are not common in the Jemez.
More common are brachiopods, close relatives of mollusks, which
at first glance look like a variety of clam. However, brachiopods
have asymmetrical shells, as you can see in the photograph above.
This is most evident around the hinge, which lies to one side. The
edges of the shell are also typically curved in a distinctive way.
This asymmetry reflects a significant difference from bivalve
mollusks: Bivalve mollusks have shells on either side of their
soft bodies, while brachiopods have shells on the top and bottom
of their soft bodies. There are significant differences in their
internal anatomy as well. Bryozoans are found in the Arroyo
Penasco Formation, but are almost embarrassingly abundant in the
Madera Group. In the modern world, mollusks remain abundant in
many environments, but brachiopods are restricted to deeper,
colder ocean waters.
Crinoids are relatives of starfish, sea urchins, and sea cucumbers. The crinoids of the Carboniferous were mostly relatives of the modern sea lily, which attaches to the ocean floor by a long stalk and has a cluster of tentacles surrounding its mouth. Fossil fragments of crinoids are typically from the stalk and take the form of distinctive segmented cylinders. However, the tentacles or "fronds" were sometimes preserved as fossils, though I have never had the fortune to find well-preserved fronds.
Less common Carboniferous fossils found in the Jemez are
echinoderm plates (in the Arroyo Penasco Formation)
If you are thinking of hunting for fossils in the Jemez, be aware
that the regulations governing fossil collection on public lands
are a bit murky since the passage of the Paleontological Resources
Protection Act of 2009. You are perfectly fine collecting fossils
on private land with written permission of the landowner. At the
other extreme, if you collect any kind of fossil on National Park
or National Monument land, then God have mercy on your soul. If
you encounter a vertebrate fossil on any public land, my best
advice is to treat it like a rattlesnake: Do not disturb it; back
The uncertainty is over how the Act affects amateur fossil hunters engaged in casual collection of common invertebrate fossils. My best read of the current state of regulations is that you are okay collecting a reasonable number of invertebrate fossils for personal study on National Forest or BLM land. However, it's always a good idea to be mindful of those who will come after you, and interpret "reasonable" conservatively, restricting yourself to a few choice specimens that you don't have to excavate for. If your fossil collecting involves large boxes or the use of a shovel, you're probably over the line.
The paleotopography map for the Madera Formation is shown below.
Paleotopography map of Madera Formation
This map shows that there was still high terrain in the Sierra Nacimiento in the late Pennsylvanian, with a sizable ancestral range to the north and a much smaller ancestral range to the south. These are fairly unambiguous; the Madera Formation is thick and distinctive everywhere else, but in these areas the next younger formations, the Abo and Cutler Formations, rest directly on Precambrian rocks.
Another clue that sea levels were rising relative to the
Ancestral Rockies is that the Madera Foramtion shows indications
of onlap, where younger beds extend further onto the
underlying Precambrian basement.
The high terrain must have had low relief, because there is
little indication that much coarse material was shed into the
Relief map of the Jemez with Permian outcroppings highlighted in red.
Geologists put the end of the Pennsylvanian and the beginning of
the Permian period at about 299 million years ago. At the start of
this period, most of the continental crust of the Earth finished
assembling into the supercontinent of Pangaea, and animals laying
eggs that could hatch on land (early reptiles, dinosaurs, and
mammals) began to proliferate. The Permian began with an ice age,
possibly triggered by the exposure of great masses of rock in the
mountain ranges thrown up where the continents joined. The exposed
silicate rock would have absorbed carbon dioxide from the
atmosphere as it weathered, reducing the atmospheric greenhouse
effect. The same effect has occurred in geologically recent times,
when the rise of the Himalayas produced the ice ages that the
first humans were forced to adapt to. In addition, most of what
are now the southern continents (Gondwanaland) was located
very close to the south pole, which reduced reflection of sunlight
off of land. It is in the regions that were once part of
Gondwanaland that we find the geologic evidence for massive
glaciation at this time.
The Permian was also marked by lower sea levels and a drier
climate, and the vast coal swamps of the Carboniferous dried out
and were replaced by large deserts in the interior of Pangaea. It
was during this time that the last of the trilobites, which had
become abundant during the Cambrian, died out. Seed plants, such
as seed "ferns", primitive conifers, and ginkgoes, became
widespread. A single species of ginkgo, Ginkgo biloba,
A large area of west Texas and southeast New Mexico was a shallow sea, the Permian Basin, which was periodically flooded with seawater from a narrow channel to the south. Great thicknesses of sediments and salt and gypsum beds were deposited there. This is now oil country, and the salt beds are being exploited for the long-term storage of nuclear waste.
Central North America was relatively high and dry during this
time, and ocean crust was subducting under what is now the Sierra
Nevada along the California-Nevada Border. The Jemez area was a
relatively low-lying area very close to the equator, located
between the high terrain to east and west, with river valleys
meandering south to the Permian Basin. Extensive marine and
continental sedimentary beds were laid down, and Permian beds are
now exposed over more than 13% of the area of New Mexico.
The transition from Pennsylvanian to Permian time is visible in
the area around Battleship
Rock. Here in the Cañon de San Diego the Jemez River has cut
through younger rock to expose the entire geological column in
this area. Across from the Battleship Rock parking lot, the road
cuts into the upper Madera Formation.
There is no sharp transition from the Madera Formation to the lowest formation of the Permian in the Jemez, the Abo Formation. The sea was receding to the south, but the shore moved back and forth, depositing alternate layers of limestone characteristic of the upper Madera Formation (which are rich in fossils) and of sandstone and mudstone characteristic of the lower Abo Formation. The latter represents deposition of sediments eroded off the Uncompahgre Uplift in a river delta close to the shore. However, the geologic map of this area maps this road cut as Madera Formation, putting the boundary with the Abo Formation at the highest limestone bed that is more than a meter thick. Well, you have to put the line somewhere.
The vagueness of the boundary between the Pennsylvanian and the
Permian in the Jemez is not unusual.This is perhaps the most
subtle period boundary in the geologic record worldwide.
These alternating beds of fluvial mudstone and sandstone and
marine limestone may be cyclothems. Cyclothems are
alternating beds of marine and continental sedimentary beds, often
including layers of coal, that are extremely well-developed in
Appalachian rocks of late Pennsylvanian and early Permian age.
They are interpreted as resulting from fluctuating sea levels,
which in turn reflect the advance and retreat of glaciers of the
southern hemisphere during the Karoo Ice Age. When the glaciers
advance, sea levels dropped and continental sediments were
deposited; when the glaciers retreated, sea levels rose again, and
marine sediments were deposited. There were at least sixty such
episodes of glaciers advancing and retreating during the Karoo,
and each produced a cyclothem in the Appalachian area. The
cyclothems of the Madera-Abo transition of the Jemez, if that is
what they are, are rather poorly developed examples, but they do
show the basic feature of alternating marine and continental
There are some limestone beds towards the south in this road cut.
There is also gray patches of mudstone above the limestone, more
typical of the Abo Formation. As I said, it's gradational. These
patches are beds that contained enough organic matter to reduce
the bright red ferric iron to soluble ferrous iron, which was then
leached away to leave gray clay minerals.
Further down the road, as you approach Hummingbird Music Camp, it's all limestone and limy shale and definitely Madera Formation.
The limy shale here weathers into clasts that I have read described, quite accurately, as ball bearing clasts. I once tried to walk across a limestone ledge covered by these rock fragments and discovered just how slippery they can be, slipping and going down hard. I should have taken my backpack off first. Consider yourself warned.
East of Battleship Rock, in the valley of the East Fork, Jemez River, the southern valley walls are dominated by Abo Formation.
To be sure, the last shot is close enough to river level that it is ambiguous whether this is upper Madera Group mudstone or lower Abo Formation mudstone. Regardless. It's mudstone and it's in the neighborhood of 300 million years old.
Relief map of the Jemez with Abo Formation and Cutler Group outcroppings highlighted in red.
The Abo Formation marks the transition from the shallow marine
environment of the Pennsylvanian to the river delta environment of
the Permian. Limestone is replaced by numerous thin beds of
alternating red sandstone and mudstone.
The Abo Formation is widely exposed in lower Cañon de San Diego. In the next photograph, everything from the foreground hills to the lower slopes of the distant mesas is Abo Formation.
The entire lower portion of Mesa de Guadalupe is mapped as Abo
The thin sandstone layers interbedded with softer shale are characteristic of the fluvial (river delta) environment in which the Abo Formation was laid down. The lowest layers look limy from this vantage point, suggesting the gradation into the underlying Madera Group, but none of the latter is mapped here. The Abo is capped with Tshirege Member, Bandelier Tuff, a discontinuity of at least 279 million years, showing that erosion had already exposed the Abo here 1.2 million years ago when the Tshirege Member was erupted.
Near here is the Spanish Queen mine, which was once mined for
copper. The mine has also produced plant fossils and tetrapod
You may have been struck by the fact that the younger Abo
Formation is present in lower Cañon de San Diego, but older Madera
Formation is present in upper Cañon de San Diego at a higher
elevation. The explanation is that the Jemez Fault crosses Cañon
de San Diego near Soda Dam, and the area southeast of the fault
has been thrown down hundreds of feet. This has dropped the
Permian beds to the level of the Carboniferous beds to the
The Abo Formation crops out throughout the western Jemez. It is found in the western caldera rim, showing how far the caldera floor dropped during the Toledo and Valles events.
Abo Formation in the western Valles caldera rim. 35 53.212N 106 39.407W
The cliff-forming beds are partly thin limestone and partly a durable coarse red sandstone.
Abo Formation coarse red sandstone. 35 53.212N 106 39.407W
Under the loupe, this sandstone is found to consist of
well-sorted small quartz clasts, but with a considerable amount of
reddish hematite in the pore spaces. When acid is dripped on the
rock, it foams vigorously, showing that calcite is an important
component of the cement holding the grains together. This likely
came from the nearby limestone beds, transported by ground water.
The slope-forming beds in the photograph are a poorly cemented mudstone.
Particularly massive exposures of the Abo Formation are found
along Forest Road 376 in Cebolla Canyon.
Abo Formation. 35 51.282N 106 45.843W
A close look reveals that the sandstone here is unusually coarse, grading into conglomerate.
Abo Formation. 35 51.282N 106 45.843W
The northernmost exposures of the Abo Formation, mapped as such, are in upper San Antonio Canyon.
Abo Formation in upper San Antonio
56.512N 106 38.938W
These are the northernmost outcrops, not for geologic reasons, but for definitional reasons.
The Abo Formation was first identified in 1909 in Abo Canyon, in the southern Manzano Mountains. It took some time for geologists to map this extensive formation, which covers much of central New Mexico. Earlier, in 1906, a different team of geologists identified the Cutler Formation in the Silverton area. Again, it took some time for geologists to recognize that this formation extended far to the south and west, and to promote the formation to group status. Meanwhile, both names became deeply entrenched in the geologic literature.
This became a bit of a problem when it was realized that the two formations meet seamlessly at about 36 degrees north latitude in the Jemez region. They are about the same age and they're both fluvial redbed formations. It would seem that the Abo Formation and the Cutler Group are really the same formation. Geologists resolved the problem by declaring that early Permian red beds north of 36 degrees north latitude will be mapped as Cutler Group, while the corresponding beds south of 36 degrees north latitude will be mapped as Abo Formation. And now you know why the Abo exposures in upper San Antonio Canyon are the northernmost exposures of the Abo: They are located just south of 36 degrees north latitude.
The dominant Permian formation of the northern Jemez is the Arroyo Del Agua Formation of the Cutler Group. This is beautifully displayed in a number of locations, of which the most accessible is in a road cut west of Abiquiu along the Canones Fault Zone.
The Canones Fault Zone marks the boundary between the Rio Grande
Rift and the Colorado Plateau, and it is a geologist's playground.
We'll come back to this area repeatedly in the pages ahead. For
the moment, we will focus on the Permian formations in this area.
The Canones Fault runs approximately along the arroyo in front of the cliff here. The exposed units are clearer just up the road.
The lowermost red rocks, with the many layers and the column-like structures, belong to the Arroyo del Agua Formation of the Cutler Group. This is an early Permian formation, dating back about 280 million years or so. It is composed of alternating mudstone and sandstone; we'll take a close look in a moment. Sitting on top of the Arroyo del Agua Formation is the Shinarump Formation of the Triassic Chinle Group, which was laid down about 230 million years ago. Thus the contact between the two beds, where the beds are at the same angle and the contact is not necessarily obvious, is a good example of a discomformity. There is a gap of over 50 million years between the two beds.
Most of the Arroyo del Agua Formation is soft mudstone, deep red in color due to its hematite content, but with occasional thin beds of sandstone. Here's a close up shot.
Notice the white color of the fresh fracture surface (where I hammered off a sample). The old surface is coated with a thin layer of red mud from higher up in the formation. However, some of the sandstone beds do contain red minerals. Here are close ups of some samples.
This is a sample of the relatively soft mudstone making up most of the formation. There is a fair amount of sand mixed in with silt and clay.
The next sample is from the thin beds of sandstone between the layers of mudstone.
This is harder stone that stands out of the softer mudstone. In this sample, the sandstone is relatively clean of oxidized iron. Generally speaking, the Cutler Group represents sediments deposited by rivers approaching the Permian Basin south of what is now the Jemez area. The sandstone layers were probably deposited within the river channel itself, where the sand was washed clean of finer sediments. The preponderance of mudstone in the Arroyo del Agua Formation suggests that the climate had started to dry out at about this time.
Not all the sand was washed clean, however.
Some of the Arroyo del Agua mudstone becomes quite hard towards the top of the formation. Here is the contact with the Shinarump close up:
And here's a sample of the very hard mudstone just under the
Besides being strongly indurated, this stone has a
peculiar, almost waxy, feel to it.
The lower formation of the Cutler Group, the Canyon del Cobre Member, is exposed in Arroyo del Cobre and Canyon del Cobre.
Arroyo del Cobre. Looking northwest from
14.740N 106 22.057N
This shows the westernmost wall of Arroyo del Cobre. Canyon del
Cobre is located beyond the ridge in the next to the last frame.
The Canyon del Cobre Member is exposed in the lower canyon walls.
It straddles the Pennsylvanian-Permian boundary, and its lowermost
beds in Canyon del Cobre include some of the best assemblages of
fossilized Pennsylvanian tetrapods (amphibians and reptiles).
We've now seen several formations that consist at least in part of mudstone. Mudstone is clastic sedimentary rock whose clasts are less than 0.0625 mm in size. if the clasts are all between 0.0625 mm and 2 microns in size, the more precise term is siltstone. If the clasts are mostly clay particles less than 2 microns in size, the more precise term is claystone. If the rock easily breaks into thin layers, it is described as a shale; if it does not break into thin layers but contains a great deal of clay, it is probably a mudstone; and if it is fine grained but does not contain much clay, it is probably a siltstone.
Induration is the geological process of consolidated sediments being converted to solid rock. The individual clasts are cemented together, usually by additional minerals brought in by groundwater. The degree of induration depends primarily on how much and what kind of cement binds the clasts together. The most common cements are clay minerals, calcite, hematite, amorphous silica, and quartz, in order of increasing hardness. A sandstone cemented with abundant quartz is a very hard rock, while a sandstone cemented with sparse clay minerals will be so soft that it crumbles to the touch (it is friable).
Because mudstones are mostly clay minerals, they tend to be fairly poorly indurated. Some mudstone beds seem like little more than piles of dirt, albeit piles of dirt millions of years old. Shales also tend to be poorly indurated, but they have a very distinctive texture of thin layers. Individually the layers are well indurated, but they easily come apart, and the overall appearance of the shale is rather like a layered pastry such as baclava. It is uncommon to come across a mudstone that is massive, solid rock, like the Arroyo del Agua Formation mudstone sample in the photograph above. Generally, such mudstone has been deeply buried, to the point where it borders on metamorphic rock, or else has had an unusual amount of silica-rich groundwater circulate through it.
Clays form a family of minerals related to mica. All are
phyllosilicates, whose basic structure consists of sheets of
silica and aluminum tetrahedrons joined to neighbors at three of
their four corners. The sheets are glued to each other by metal
and hydroxide ions. We saw this structure earlier.
Clay minerals form from igneous or metamorphic rock that is exposed to weathering at the surface of the earth. The minerals in an igneous rock crystallized under conditions quite different than those at the earth's surface. The temperature was much higher, the pressure may have been much higher, and conditions were drier. Minerals stable under these conditions are not necessarily stable under cooler, wetter conditions. For example, potassium feldspar is susceptible to slow weathering by the traces of carbonic acid in normal surface water, which extract potassium and silica from the feldspar to produce a clay mineral called illite. Illite is quite similar in composition and structure to muscovite, but is generally found in the form of very small flakes, distinguishable only with a powerful microscope. The composition is more variable, and X-ray diffraction shows subtle differences that indicate that illite is not just very fine muscovite. It works the other way, too; one of the first indicators of low-grade metamorphism in a clay-bearing rock is that illite begins to convert back to muscovite.
If weathering is intense enough, illite can further degrade into
kaolinite by losing additional potassium. Kaolinite is a
clay mineral characterized by a structure of a single silica sheet
bonded to a sheet of alumina-hydroxide clusters. These pairs of
sheets are electrically neutral and only very weakly bonded to
Plagioclase feldspar weathers to montmorillonite with the loss of silica, sodium, and calcium. Montmorillonite is a clay mineral characterized by a mica-like structure of two silica-alumina sheets joined by alumina-hydroxide clusters. These pairs of sheets are bound together mainly by sodium or calcium ions. Montmorillonite varies considerably in its sodium to calcium ratio, like the plagioclase from which it forms, and shows other variations in composition. Clay rich in montmorillonite is known as bentonite and is particularly common in sediments weathered from volcanic ash.
The overall effect of weathering on typical igneous rock is to convert it to clay sediments that are enriched in aluminum. These are described by geologists as pelitic sediments. When magma forms from deeply buried clay beds, such as by melting of the crust by injected basaltic magma from the mantle, the resulting magma is also enriched in aluminum. Granite formed from such magma is described as peraluminous and contains unusual aluminum-rich minerals, such as corundum. Metamorphic rock formed from clay beds typically is enriched in aluminum silicates such as andalusite.
I wont' bother with the paleotopographic map of the Abo and
Cutler beds. It's identical with the exposure map. Everywhere we
find the next youngest formation, the Yeso, we find that it is
either resting on Abo or Cutler beds or the lower contact is
buried where we can't see it. Does this mean that there was no
high ground in the Jemez area 300 million years ago? Not
necessarily, because the record is always incomplete; but it does
not appear that there was much high ground in the
area. The Jemez of the early Permian was apparently mostly rather
flat river plains.
Relief map of the Jemez with Yeso Group outcroppings highlighted in red.
Not all the red rock in lower Cañon de San Diego is Abo Formation. About 280 million years ago, the continuing global drying trend dried up the rivers that deposited the Abo Formation, the Permian Basin sea receded further south, and a sea of sand dunes marched across the Jemez. This created the Yeso Group, which looks much like the Abo Formation from a distance. Some of the best exposures of this group are at the Red Rocks rest area just north of Jemez Pueblo.
The Yeso Group is distinguished from the Abo Formation by being a
less dark red, tending toward orange, and in forming thick beds of
sandstone characteristic of eolian (windswept dune)
deposits. This is in contrast with the thinner fluvial beds,
interbedded with mudstone, of the Abo.
The Yeso Group is divided into two formations, with the more massive Meseta Blanca Formation at the bottom and the thinner bedded San Ysidro Formation on top. My geological map does not make the distinction, but it looks like the two formations can be distinguished just north of Red Rocks.
San Ysidro Formation. Near 35 40.126N 106 43.062W
Under the loupe, this is seen to be a well-sorted fine sandstone of rounded quartz grains, though occasional lithic grains, with abundant hematite cement. This sample was taken further north, from Forest Service land near Mesita Blanca.
The sand grains of the Meseta Blanca Formation have a
characteristic pitting and are described as "frosted grains." This
is an indication that the grains were transported by wind at some
point, in which the grains collided with each other with enough
force to cause the pitting. Grains transported by water are
cushioned enough by the water that they do not collide violently
enough to become frosted.
I have learned over the years that lighting can make a huge difference in how a rock formation appears in a photograph. I took this photograph under lighting conditions that were perfect to show cross-bedding in the Meseta Blanca Formation.
Cross bedding in the Meseta Blanca
Formation. Click to enlarge.
Crossbedding is where larger beds are composed of much smaller beds (cross beds) at an angle to the main beds. You can see several examples in this photograph. Cross beds are usually interpreted as showing that the beds were deposited by a moving fluid, such as a stream or the wind. Since the Meseta Blanca Formation is interpreted as an eolian sandstone, deposited by wind, the beds were laid down on the downwind side of dunes. This suggests the prevailing wind was from the north or northeast, which is consistent with other geological evidence that the Jemez area was located in the trade winds belt, just north of the equator, when these beds were laid down in the Permian.
The Mesita Blanca Formation is correlated with the De Chelly
Formation (pronounced "dee shay") that was originally mapped in
Arizona. Both were deposited in the same dune sea, reaching from
central Arizona through the Four Corners region to the Jemez area.
The San Ysidro Formation marks a return to somewhat wetter conditions, with rivers once again depositing sediments on their way south to the Permian Basin.
As with the Abo Formation, the paleotopographic map for the Yeso
Group shows no instances where the next younger formation, the
Glorietta Sandstone, is underlain by anything but Yeso Group.
Relief map of the Jemez with Glorietta and Bernal Formation outcroppings highlighted in red.
The geological report for the Jemez Springs area mentions a prominent limestone bed near the top of the San Ysidro Formation. If the white band near the top of the cliff in one of the previous photograph is this bed, then the rocks above are the lower portion of the next formation, the Glorieta Sandstone, which was deposited beginning about 275 million years ago. The Glorieta Sandstone marks the return of the shallow sea to this area, since it has the characteristics of sandstone laid down in a beach environment. It is marked both by the limestone bed and by a transition to a whiter sandstone.
Looking northwest from this location, we see what is apparently the furthest southwest remaining outcrop of Bandelier Tuff.
The mesa on which this Bandelier Tuff ridge sits is called Meseta Blanca, probably because of the whiter color of the Glorietta Sandstone capping the mesa compared with the red formations underneath. There is a thin bed of Triassic Chinle Group under the Bandelier Tuff, mostly buried under talus; we'll have more to say about the Chinle Group later on. You can see the transition to redder Yeso Group sandstones with that characteristic white limestone bed halfway down the side of the mesa.
A (not very good) road climbs the bench in the foreground from the village of Canon and ascends Meseta Blanca. However, the road becomes all but impassable where it crosses a fault that brings dark red Abo Formation alongside orange Yeso Formation.
Mesita Blanca fault. Looking east from 35 40.042N 106 43.214W
The dark red Abo Formation beds in the foreground abruptly give way to more orange beds of the Yeso Group. Here's a closer view of the whitish patch in the road on the other side of the fault trace.
None shall pass! 35 40.040N 106 43.200W
The white discoloration is likely from ground water seeping from the fault. The rock beds in the road bed are so deeply gullied that vehicular travel further along the road would require great care even with an off-road vehicle, though the tire tracks show someone did so since the last rain. The road up the side of the mesa is so poor that it has been blocked with an iron gate; the adventurous geologist must travel beyond by foot.
The transition from the San Ysidro Formation to the Glorieta Formation is gradual, but this is a fair guess where it should be put.
Approximate transition from San Ysidro Formation to Glorieta Formation. 35 40.016N 106 42.862W
Further up, there is no ambiguity about the formation assignment.
Glorieta Formation. 35 40.049N 106 42.829W
Under the loupe, this is found to be a moderately well sorted medium sandstone of somewhat angular quartz grains with ample pore space that appears to be cemented with silica.
The Glorietta Sandstone is prominent in the Glorietta Pass
southeast of Santa Fe, from which the formation takes its name. It
is correlated with the Coconino Sandstone of northern Arizona,
both formations being laid down in a dune sea covering most of
northern Arizona and New Mexico.
Perhaps the most spectacular exposure of the Permian rock column in the southwest Jemez is on the western slopes of Guadelupita Mesa southeast of Guadelupe Box.
Permian red beds on the west side of Guadelupita Mesa. Looking southeast from 35 39.738N 106 42.881W
At right, at the base of the mesa, there are thin beds of the Abo
Formation. Above these are thick beds of the Meseta Blanca
Formation, Yeso Group. To the left, a fault has thrown the Meseta
Blanca Formation down by a good hundred feet or so, and thin beds
of San Ysidro Formation, Yeso Group, are visible above. Then comes
a thin lighter layer of Glorietta Sandstone, then more red beds of
the Bernal Formation.
The Bernal Formation is the youngest Permian formation present
anywhere in the southern Jemez, and the only exposure I am
familiar with is here, halfway up the sheer side of Guadelupita
Mesa. You must forgive me for not having a sample to show. I'm 54
years old, and ambitious of making it to 55 years old. Also, I
have not provided a separate exposure map for the Bernal
Formation; it is not distinguished from the Glorietta Formation on
most of the quadrangle maps for this area.
Above the Bernal Formation, and just below the cap of Bandelier Tuff, is a light sandstone formation, the Triassic Agua Zarca Member of the Shinarump Formation. We'll have more to say about this formation in the next chapter.
Another exposure of most of the Permian column is found at Mesa
de las Casas.
Mesa de las Casas. Looking southeast from 35 42.091N 106 43.873W
The foreground ridge is underlain by the Abo Formation. The sides of the mesa, working upwards, are Mesita Blanca Formation, Yeso Group (the thick, lighter beds at the base); San Ysidro Formation, Yeso Group (the thinner red beds above); Glorieta Sandstone (the lighter beds near the mesa top); Triassic Moenkopi Formation (at the very top of the mesa and under the knoll); and a knoll of Argua Zarca Sandstone.
By the end of the Permian, which is the final period of the
Paleozoic, the supercontinent of Pangaea was nearly fully
assembled. As a result, the Mesozoic would be a period of relative
tectonic quiescence in the Jemez area.
The upper Permian and lower Triassic periods are missing from the
geological record in the Jemez. This must have been a period when
the oceans had receded far enough from northern New Mexico to
permit significant erosion. As a result, there is no direct record
in the Jemez of one of the greatest catastrophes in the history of
Next page: The age of the dinosaurs
in the Jemez.
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