The Mesozoic Era dawned in New Mexico on extensive plains, except for a
northwest-trending range in the extreme north-central part of the state.
During this, the early part of the Triassic Period (180 to 230 m.y.
ago), sands and muds eroded from New Mexico were carried westward to
northeastern Arizona where they now form the Moenkopi Formation, the
brilliant reds and purples of the Painted Desert region. Uplands arose
in late Triassic time in southwestern New Mexico. Along with mountains
in south-central Colorado, these highlands were torn apart by water and
wind, and the detritus was swept into sheets of brightly colored sand
and shale. These beds are thickest (about 2000 feet) along the New
Mexico—Texas line east of Roswell and in west-central New Mexico (near
Grants) extending westward into northeastern Arizona. The eastern
Triassic rocks are the redbeds of the Dockum Group with the lower Santa
Rosa Sandstone and the upper Chinle beds. The northwestern rocks are the
Chinle Formation overlain by the redbeds of the Wingate Sandstone.
The Chinle Formation is of special scenic interest as its beds contain
the silicified trees so well shown at Petrified Forest National
Monument. These varicolored rocks—red, purple, green, and gray—also
decorate the Painted Desert area, the wide valley of Rio San Jose east
of Laguna, and flank Interstate 40 (U.S. Highway 66) from the Texas line
westward almost to Clines Corners. As some beds are weathered “ash”
beds, the highlands were sites of volcanoes that spread their dust over
much of the Southwest. In contrast to the underlying marine Paleozoic
rocks, these Triassic beds were deposited on land by streams and in
shallow lakes. Thus the beasts that roamed New Mexico were
amphibians—such as the thick-skulled stegocephalians—and reptiles of the
crocodilelike clan, the phytosaurs. The silicified trees in the Chinle
are mostly primitive pines; some grew to heights of more than 100 feet
and measure 7 feet in diameter.
New Mexico was featureless rolling prairie, with scattered low hills in
the northwest, during most of Jurassic time (135 to 180 m.y. ago). In
the late part of the period, the Sundance-Curtis sea and its shoreline
lagoons spread down from the north into northwestern New Mexico. Sand
dunes on its southeastern shores consolidated into the cross-bedded
Entrada Sandstone; its reddish brown cliffs rim the Rio San Jose Valley
near Grants and Gallup. In an extensive lagoon, or salt-water lake that
covered most of northwestern and north-central New Mexico, the gypsum
and limestone of the Todilto Formation were precipitated; this gypsum is
the bed mined near Rosario siding (seen between Albuquerque and Santa
Fe) by the Kaiser Gypsum Company, and near San Ysidro on White Mesa by
the American Gypsum Company. The Todilto salty basin was overwhelmed by
reddish sands and silts of the Summerville Formation, washed chiefly
from the south, and by the multicolored sands of the Zuni Sandstone
(exposed at El Morro), and then the stream and wind-blown sands and
clays of the varicolored Morrison Formation were laid down in northern
New Mexico. Petrified wood and bone fragments are abundant in the
Morrison beds—along with uranium—but no fossil finds in New Mexico equal
those in the Morrison Formation of Dinosaur National Park, Utah.
Much of North America, including southern New Mexico, was land during
the Jurassic. In the streams and lakes were many kinds of fishes,
amphibians, reptiles, snails, crustaceans, and water bugs; on dry land
were hordes of reptiles, small primitive mammals, and ants; in the air
were flying reptiles, the earliest known birds, moths, and butterflies.
The earth was ruled by the reptiles, with the dinosaurs dominant—some
being the most ponderous land animals of all earth history. Such were
Stegosaurus and Brachiosaurus, the latter 85 feet long and weighing 50
tons.
The Cretaceous Period (70 to 135 m.y. ago) was one of great contrast in
New Mexico. During Early Cretaceous time, most of the central and
northern parts of the state were low lands torn by erosion, while thick
piles of conglomerate, sandstone, and shale accumulated in depressions
in the southwestern corner. Huge volcanoes near Lordsburg added their
hot ashes, bombs, and flows to the sedimentary detritus, and thick
fossiliferous limestones were laid down in muddy and sandy waters of the
sea—the shoreline fluctuated over tens of miles in areas south of
Lordsburg, Deming, and El Paso. These Early Cretaceous beds total 20,000
feet in thickness in some areas. In eastern and northeastern New Mexico,
in contrast, thin sheets of quartz sand were deposited by streams on the
edge of a shallow sea, and black muds in local lagoons.
Rock beds thousands of feet thick were laid down in northern and central
New Mexico during Late Cretaceous time, whereas most of the southern
part of the state was above sea level and was being eroded by tireless
winds and streams. The shorelines made parallel northwest-trending bands
across the state. These are now marked by beach sands, some of which are
speckled by black minerals, high in rare elements titanium, niobium, and
zirconium. Northwestern and central New Mexico was a battleground of the
land and sea, with the beaches advancing and retreating fifty or a
hundred miles during an instant of geologic time. Stream sands and coal
beds lie landward from the beach sands which, in turn, mingle seaward
with black limy shales that were flushed into the seas. The lowest of
these rocks is the Dakota Sandstone—famous as an artesian aquifer in the
High Plains areas of states to the northeast—Colorado, the Dakotas,
Nebraska, Kansas, Wyoming, and Montana. Above is the black Mancos Shale,
which in turn is overlain by the Mesaverde Group.
The transitions from coal swamps and stream sands to beach deposits and
then into marine black shales is characteristic of the Mesaverde in
northwestern and north-central New Mexico. To the northeast, beds of the
same age were laid down in an extensive muddy sea that stretched far to
the east; the Pierre Shale and Niobrara chalky limestone that underlie
the plains northeast of Las Vegas are typical. The cliff-forming
sandstones and coal beds near Gallup are part of the Mesaverde Group and
rim the entire San Juan Basin. Above are similar rocks such as the
Kirtland Shale, Pictured Cliffs Sandstone, and Fruitland Formation that
underlie valleys cut in the shales and cliffs carved from the sandstones
in the northwest corner of New Mexico near Farmington.
Toward the end of the Cretaceous, the Laramide “revolution” began, and
New Mexico along with most of North America emerged from beneath the
seas, to be high and dry to the present. The revolution, an extensive
upheaval of the earth’s crust, saw uplift of the San Juan Mountains area
in southwestern Colorado and large volcanoes spouting fire and ashes
nearby. Fragments of the eroded mountains and debris from these
andesitic volcanoes were flushed southward by streams and steam to
settle as thick piles of mud, sandstone, and conglomerate, the McDermott
and Animas formations in the San Juan Basin. The last moment of
Cretaceous time, if we could be so precise, was ushered out almost
unnoticed—with mountains rising to the north and the andesitic-quartz
detritus being laid down to the south in the upper beds of the Animas
Formation.
Similarly, mountains arose during Late Cretaceous time in north-central
New Mexico and south-central Colorado, about on the site of the
present-day Sangre de Cristo range northeast of Taos, and shed erosional
gravels and muds into the Raton Basin area. Alluvial fan gravels and
sands grade eastward into dark muds and coals laid down in swamps and on
floodplains. These rocks now cap the rugged mesas seen northwest of the
Santa Fe Railway from Raton southward—the cliff-forming Trinidad
Sandstone and the dark siltstones, sandstones, black shales, and coal
beds of the Vermejo and Raton formations. The Kaiser Steel Corporation
mine near Koehler extracts coal from these beds. Again, the exact end of
the Cretaceous is marked only by some obscure boundary between beds, in
that area within the Raton Formation.
During the Cretaceous Period, the deciduous trees—such as the oak,
maple, poplar, and elm that dominate today’s flora—became common. The
covered-seed plants, the angiosperms, are the most notable of the
Cretaceous plants, but the development of the modern floras was an
antecedent to the great expansion of mammals and birds during the
following Cenozoic Era. Reptiles ruled the earth, led by the dinosaurs
(fig. 10) and their distinctive group, the horned large-skulled
ceratopsians such as Triceratops. The small, hairy, warm-blooded mammals
were still insignificant creatures that ran from their huge dinosaur
lords, but they ate reptile eggs, and so excelled the sluggish reptiles
in mental and physical activity that they adapted swiftly to the
changing environments of the Laramide revolution—and became dominant as
the pea-brained reptiles were unable to stand the changes.
The shallow seas of the Cretaceous swarmed with invertebrate life;
foraminifers (unicellular protozoans) in uncountable billions make up
large parts of the chalky limestones. Mollusks, particularly clams like
oysters and the heavy ribbed Inoceramus, and complexly sutured
cephalopods, the ammonites, as well as the internal-shelled belemnoids
(that look like cigars), were most numerous among larger marine animals.
Reef builders in southwestern New Mexico were the peculiarly corallike
clams, the rudistids. Widespread warm humid climates seem to have
prevailed throughout the state during most of the Cretaceous.