[Illustration: Bryozoan
_Fenestrellina._]
[Illustration: Fusulinid
_Fusulina._]
[Illustration: Coral
_Lophophyllidium._]
The early Permian seashore, where limy muds beyond the surf intermingled
with red sandy muds swept from the north, vacillated somewhere north of
Alamogordo with each sea-level change. Amid the breakers, and as
submarine banks in the shallow waters, reefs grew—moundlike masses of
shell debris and calcite mud trapped among frondlike calcareous algae.
Near Tularosa, these algal “bioherms” are sixty feet high and extend
within broad belts half a mile wide. In southeastern New Mexico, these
buried “Abo” reefs have yielded much oil.
By the middle of Permian time, the southern Colorado mountains had been
worn down to low hills that lay north of an extensive sea covering most
of New Mexico. From Santa Fe south to White Sands and southeastward
almost to Carlsbad, very shallow marine waters were alternately stifled
by pale-red sandy muds or evaporated by the sun. The results were
alternating beds of pale-red sandstone, gypsum, and silty dolomitic
limestone, called the Yeso Formation. Locally, as near Carrizozo, thick
deposits of rock salt also were precipitated, and the Yeso there is
about 4000 feet thick. At this time, the Delaware basin of southeastern
New Mexico saw the beginning of its most spectacular events, the
building of the Capitan and Goat Seep reefs. This basin—a huge oval
south of Carlsbad and east of Carlsbad Caverns—had been “deep” sea
during most of Pennsylvanian time, but it was a more distinct geographic
feature during the Permian. While the pale-red sands, gypsum, halite,
and dolomitic limestones of the Yeso Formation were laid down to the
north and northwest, the Delaware basin was rimmed by a low, broad bank
of fossil-hash calcite sand, now called the Victorio Peak Limestone. In
the basin, in deep stagnant waters, black sandy limestone and black
shale of the Bone Spring Formation were deposited.
A sheet of white quartz sand filled the late Yeso seas; the resulting
Glorieta Sandstone, about 200 feet thick, prominently caps Glorieta
Mesa. Its cliffs are a familiar sight to travelers on the Santa Fe
Railway at Glorieta Pass. The Coconino Sandstone in the Grand Canyon
area of Arizona is the western part of the Glorieta. This “clean”
sand—lacking intermixed mud—marks the continued lowering of the southern
Colorado uplands. Broad seas then spread over all but northern New
Mexico and a thick (600 to 1000 feet) persistent marine unit, the San
Andres Limestone, was laid down. Much oil is produced in southeastern
New Mexico from this dark-gray unit of limestones and dolomites. The
rich agricultural region stretching from Roswell to Artesia depends on
underground water gained from the San Andres Limestone, water that falls
as rain and snow on the Sacramento Mountains, seeps underground into the
cracks and caverns within the San Andres, and flows eastward downslope
to the Pecos Valley.
The delicate balance between land and sea swung upward at the end of San
Andres time as these late Permian seas retreated to southern New Mexico.
The deep Delaware basin was the only persistent marine body of water. It
was rimmed by magnificent towering barrier reefs, the Goat Seep and
Capitan reefs that now are host to Carlsbad Caverns. These reefs were
similar to the present-day Great Barrier Reef of Australia, except that
the Capitan and Goat Seep reefs surrounded an inland sea whereas the
Australian reef borders a continent. The Capitan reef is about 400 miles
long, and other than oceanward channels cut through to the south,
completely encircled the 10,000-square-mile Delaware basin. At its
heyday, the Capitan reef was barely awash, and teeming with life, in
contrast to the silent, stagnant deeps of the Delaware basin which were
about 2000 feet below sea level only a few miles away from the barrier
reef. On the steep slope into the basin, huge slump blocks of
fossiliferous reef limestone slid, mingling with fossil-hash sand. These
“flank” beds dip steeply from the massive reef core to interfinger with
the black sandy limestones of the basin.
The Delaware basin was a marine feature throughout Late Paleozoic time;
its northwestern border is now marked by the southeast-trending front of
the Guadalupe Mountains southeast of Carlsbad; its north edge was
east-northeast of Carlsbad, and it extended southward into West Texas.
Shallow “shelf” seas reached irregularly and intermittently northward
and northwestward from the Capitan reef and mingled with low islands
throughout all but southeastern New Mexico. Landward, away from the
Delaware basin, the rocks change from massive, thick, light-gray
limestones of the reefs into thin units of thin-bedded dolomite, then
abruptly into alternating beds of gypsum and redbeds, the Artesia Group
of rocks, and finally, marking the distant shorelines, into thin units
of red mudstone and red sandstone, the Bernal Formation. Evaporation of
sea water was excessive, and average temperatures high; the climate
varied from semiarid in northwestern New Mexico to subtropical in the
Delaware basin area—a contrast and a similarity to today’s climate.
[Illustration: Figure 9. Castile gypsum sample]
Latest Permian time saw the dramatic end of the Paleozoic Era. Most of
New Mexico was uplifted above sea level, with only the Delaware basin
remaining as a land-locked sea, much like the Caspian Sea today, but
with channels open periodically southward to the ocean. The rocks of
this waning part of the Permian are called the _Ochoan Series_; they
show an abrupt and striking change from the underlying Carlsbad reef
limestones and associated black basin-filling limestones up into the
laminated gypsum-anhydrite of the basal Ochoan rocks, the Castile
formation. Normal marine conditions ended almost instantaneously. Excess
of evaporation lowered the water level of the inland sea; the
accumulated brine (concentrated salty sea water) killed the life on and
near the Capitan reef, and thick beds of anhydrite were precipitated.
The lowest beds of the Ochoan Series, the Castile Anhydrite, and the
overlying Salado Salt, mostly filled the deep depression that was the
Delaware basin; the upper beds, the Rustler Dolomite and Dewey Lake
Redbeds, lap over the edges of the basin and in places rest irregularly
upon the Capitan limestone.
These are unusual rocks. The Castile (about 1800 feet thick) is thinly
banded, with thicker bands (laminae, thin layers) of light-gray
gypsum-anhydrite alternating with thin laminae of dark-brown calcite
(fig. 9). This lamination is believed due to annual changes, the brown
calcite being precipitated during the summer and the gray anhydrite
during the winter. On the surface, the calcium sulfate mineral is
gypsum, but at depths of about 600 feet, these laminae are anhydrite.
Addition of water to anhydrite has changed it to gypsum wherever ground
water penetrated the laminae.
The Salado Salt, about 2000 feet thick, is almost entirely of rock salt
(halite), with important interbeds of potassium-rich minerals—red
sylvite, gray langbeinite, brownish bitter-tasting carnallite, and
pale-red polyhalite. As all these salts are highly soluble in water, the
Salado Salt nowhere “crops out” at the surface. East of Carlsbad,
however, the potash-rich beds are mined underground, and supply about
ninety per cent of the United States’ production—used chiefly as
fertilizer.
The arid period of Salado Salt evaporation changed slightly as the
dolomites and anhydrites of the Rustler Dolomite were laid down in the
last drying moments (geologically speaking) of the Permian. Then as the
seas retreated to the south, the fine-grained red sands and silts of the
Dewey Lake Redbeds were spread as a thin blanket over the low lands
basking under the hot Permian sun. This was a time of dying; whole races
of vertebrate and invertebrate animals were wiped out, to be known today
only from their fossil remains. As the dim unmarked episode of latest
Permian time merged into the Triassic, an inkling of coming life was
recorded in the rocks. The amphibians were more modern types, and they
gave rise to the most striking of early land animals, the reptiles. This
was the beginning of the conquest of the land by the reptiles, which
culminated later in the dinosaurs, and was aided by the retreat of
shallow seas from the continents, a change survived chiefly by the
species adapted to living on land.