[Illustration: Brachiopod
_Marginifera._]
[Illustration: Crinoid
stem fragment.]
[Illustration: Crinoid
restoration of a crinoid.]
Mississippian rocks (310 to 345 m.y. old) probably were deposited over
most of New Mexico. Subsequent erosion removed much of the Mississippian
beds in northern New Mexico. The remnants, less than 100 feet thick in
most places, are of lower sandy and shaly beds overlain by massive
crinoidal limestones, the Arroyo Penasco Formation of the Nacimiento and
Sandia mountains and the Tererro Formation of the Sangre de Cristo range
east of Santa Fe.
In southern New Mexico, the Mississippian beds are thick and widespread,
being more than 1000 feet in thickness in the southwestern panhandle.
There the rock units are the Escabrosa Limestone of the southwest or the
Lake Valley Limestone of the south-central part of the state. These are
massive fossiliferous limestones precipitated in shallow extensive seas
abounding with invertebrate life. Huge gardens of the sea lilies,
crinoids, spread over the area, their remains mingled with those of lacy
moss animals, the bryozoans, and with brachiopods and corals. Locally,
as in the region of the Sacramento and San Andres mountains and Black
Range, moundlike fossil reefs, called bioherms, were built. Some of
these bioherms in the Sacramento Mountains are mounds of fossiliferous
limestones 350 feet high and several thousand feet in diameter. Beds on
their flanks dip as much as 35 degrees and are made up of broken “fossil
hash” calcite sands. One can stand at the base of these huge limestone
hills and almost hear the ancient waves breaking against the reef and
see the dying struggle of the brachiopod (fig. 6) that left his shell in
the reef-flank sands.
To the south, beginning near the present site of White Sands, dark
cherty limestones were laid down in stagnant waters, to become the
Rancheria Limestone. This black to reddish brown siliceous limestone is
more than 300 feet thick near El Paso. There its thin beds break down
into slabs that resemble a jumbled woodpile.
Northern New Mexico was above sea level during late Mississippian time;
in some areas caves developed in the porous limestones, and in other
places the limestones were eroded to a residuum of chert and red clay.
The land must have looked like the karst areas of Indiana, Kentucky, and
Illinois today—with lost rivers flowing into sink holes, numerous caves,
and many underground rivers. Only the southernmost part of the state was
awash in the late Mississippian seas, and in these salt waters, rocks of
the Helms and Paradise formations settled. They are typical nearshore
beds of yellowish limy sandstone, green limy shales, and brown sandy
oolitic limestones. Plant fossils occur intermingled with marine animal
remains; the plant fragments were washed into the shallow seas from the
land areas of the central and northern parts of the state.
The Pennsylvanian Period (280 to 310 m.y. ago) was a time of change.
Previously, northern New Mexico had been an emergent lowland or barely
awash in shallow waters, while to the south shallow but extensive seas
held sway, the spawning ground of the vertebrates and invertebrates that
evolved between 310 and 600 m.y. B.C. But mountains were built during
the Pennsylvanian, and the whole pattern of land and sea was altered.
The sun rose on north-south aligned ranges interspersed with
north-south-trending seas (fig. 7). Somewhere north of Albuquerque a
mighty range of mountains, the Uncompahgre Range, arose to shed rock
debris into adjoining ocean basins. To the southeast, a lower but
prominent range, the Pedernal Mountains, stretched from the present-day
Pedernal Hills southward to somewhere near Ruidoso and Piñon. Rocks
eroded from this landmass were dumped westward into the Orogrande basin
which occupied the region near the present-day White Sands; there as
much as 3000 feet of beds accumulated—impure sandstones, dark shales,
fragmental limestones, and even some gypsum during the end phase of
Pennsylvanian sedimentation.
Rocks filled the Delaware basin in southeastern New Mexico—limestones,
sandstones, and black shales that now produce oil and gas. In
northwestern New Mexico west of Grants and mostly west of the Zuni
Mountains, a low land area, the Zuni Islands, was the source of eroded
residuum released into an ocean channelway that ran north-northwest
through central New Mexico from El Paso to Farmington. And in the
northeast, granite hills of the Sierra Grande Arch stood above the
shallow Pennsylvanian seas.
In the Four Corners region, broken rock from the Uncompahgre Range was
rushed westward into the Pennsylvanian-age Paradox Basin. Amid the
clastic limestones, black shale, gypsum, and salt of this basin are
oil-bearing lenses. Today, oil wells pump this black “gold” from the
ancient rocks—wells almost in the shadow of Shiprock’s famous spire.
The Pennsylvanian Period was a time of coal making on the greatest scale
in the earth’s history. Extensive swamps and marshes, the habitat of
peat and ultimately coal, were almost lacking in New Mexico. Thus, only
thin scattered lenses of coal occur in the Pennsylvanian beds of the
state. The lands of this period were covered by tree ferns, scale trees,
horsetail rushes, and primitive conifers. In the shallow seas, the
dominant invertebrates were fusulinids (fig. 8), small-shelled
protozoans shaped like grains of wheat. Abundant cockroaches, large
dragonflies, and spiders swarmed over the land.
[Illustration: Figure 7. New Mexico during Pennsylvanian time]