PRECAMBRIAN ERA

Figure 1.Table of geologic time

Figure 2.New Mexico during Paleogene time

Scenery in north-central New Mexico (fig. 2) may have been similar to today’s, with mountains in the same general areas as the present-day Sangre de Cristo, Nacimiento, San Juan, and Brazos ranges. Coarse-grained gravels were stacked up at the edges of the mountains, but out in the adjoining lowlands, floodplain sands and varicolored lake-bed clays settled. Three low areas were “basins” of deposition where thick masses of sediments accumulated—the Raton and Poison Canyon formations in the Raton Basin near Raton, the Animas, Nacimiento, and San Jose formations in the San Juan Basin north and northwest of Cuba, seen alongN.M. Highway 44, with thinner deposits of the El Rito and Blanco Basin formations to the northeast of Cuba, and the Galisteo Formation in the Galisteo Basin south and southwest of Santa Fe. Volcanic rocks, the Espinaso beds, overlie the Galisteo but are not much younger in age. Reddish rocks of the Galisteo Formation crop out along U.S. Highway 85 at La Bajada Hill about twenty miles southwest of Santa Fe. The Sandia Mountains’ area appears to have been a lowland.

Silicified wood, chiefly of pines but with some oak and poplar, is abundant in the Galisteo Formation. Large logs, up to 6 feet in diameter and 135 feet long, have been found. In the great swamps of the Raton Basin, where the climate was much like that of Georgia today, tall reeds, water lilies, fig trees, palm trees, magnolias, and sycamores grew in profusion, and contributed to the thick coal beds now mined there. The early ages of the Cenozoic saw the spectacular rise of the mammals to dominance over reptiles on land; numerous remains of the early mammals are found in the Nacimiento and San Jose formations, including the famous Puerco and Torrejon faunas—as well as many clams, snails, fish, turtles, crocodiles, snakes, and birds.

Southeastern New Mexico appears to have been relatively level with only local hills and vast regions of featureless, stagnant but high plains where erosion slowly ate downward, deposition was slight, and most of the detritus was carried eastward far beyond the state’s borders. The redbeds of the Baca Formation were laid down on the north flank of low mountains that extended intermittently from somewhere near Quemado toward Socorro. Some ancient hills near present-day Sierra Blanca shed rock fragments that accumulated near Capitan as the varicolored Cub Mountain Formation. Deeply eroded uplands northwest of Elephant Butte Reservoir supplied gravels and sands that mingled with andesitic volcanic debris as the upper part of the McRae Formation in central Sierra County. Many of the weathered greenish and purplish volcanic rocks in southwestern New Mexico were extruded at this time, and beneath the surface these molten magmas (hot liquefied rocks) cut into older rocks. Vapors and hot solutions from the magmas are believed to have emplaced some of New Mexico’s vast ore deposits during this time.

The last phase of the Paleogene Period, about 25 to 40 m.y. ago, was an earth-shaking time in New Mexico—and the first explosion of an atomic bomb in 1945 on the Jornada del Muerto between Socorro and Carrizozo was a relatively low-energy-yield event compared with the late Paleogene earth movements. Almost the entire southwestern quarter of the state literally exploded, with volcanic eruptions on a grand scale. These lava flows, rock breccias, ashes, pumice, and associated intrusives (molten rocks that did not make it to the surface) form the Datil-Mogollon plateau—at least 100 miles in diameter—as part of the Datil Formation, which locally is miles thick, and made up the main mass of many other ranges near the Mexican border. Sierra Blanca (12,003 feet altitude) northeast of Alamogordo is a huge, isolated volcanic mass of late Paleogene age.

Figure 3.East-west cross section of Rio Grande graben near Santa Fe

This widespread volcanic activity continued into the Neogene Period which began about 25 m.y. ago. Rhyolites, pumice, and perlite in the southwest, as well as in other parts of the state, covered wide areas. Mount Taylor, towering up to 11,389 feet near Grants and visible on the western skyline from Albuquerque, is a Neogene volcanic pile, as are parts of the Sangre de Cristo range northeast of Taos. Shiprock and Cabezon Peak, landmarks in northwestern New Mexico, are volcanic necks—the eroded cores of ancient volcanoes.

Valle Grande caldera makes up the center of the Jemez Mountains west of Los Alamos and is a late Neogene volcanic mass with the central crater sixteen miles in diameter—one of the world’s largest calderas. Bandelier National Monument headquarters is within a canyon carved from Valle Grande’s ashes. Volcanic ash scattered over the western parts of Texas, Oklahoma, and Kansas was blown from this volcano. Capulin Mountain, east of Raton, is a huge recent cinder cone and is surrounded by numerous basaltic lava flows that cap the High Plains from Raton eastward to Clayton. The very fresh black basalt flows near Carrizozo and in the valley of Rio San Jose near Grants are probably less than 1000 years old. Numerous mesas along the Rio Grande Valley from the Colorado line to El Paso are capped by black basalt flows of late Neogene age.

Many of the present-day mountains were uplifted in early Neogene time, following the climax of the great volcanic eruptions. This uplifting, in many instances, took place along one side of huge mountain masses, forming tilted fault blocks like the Sandia, Manzano, San Andres, and Sacramento mountains. Rock beds in the Sandia Mountains, for example, dip to the east, but were uplifted along a west-bounding fault zone—a huge break in the earth’s crust—as much as four miles! This was an earth-shaking event! However, the uplifting took place slowly, and indeed is continuing today as the Albuquerque area, along with the Rio Grande Valley southward to Socorro, is one of the most active earthquake areas in the state.

Concurrent with uplift, other blocks of the earth’s crust sank, forming graben basins which were flooded with rock debris from the adjoining uplifts. A tremendous irregular graben, now followed by the Rio Grande, cut north-south across the state. Geologists label it theRio Grande structural depression(fig. 3). Mountains on the east are the Sangre deCristo, Sandia, Manzano, Los Pinos, Fra Cristobal, and Caballo ranges; those to the west include the Brazos, Jemez, Ladron, Socorro, Magdalena, and San Mateo mountains. Within this complex graben, and around the bordering ranges, the colorful sandstones and siltstones of the Santa Fe Group were deposited—these red, yellow, orange, and cream rocks are eroded in many places, such as near Santa Fe, to “badlands” characteristic of the landscapes along the Rio Grande Valley from Espanola southward to El Paso. Much brightly tinted silicified wood is found in these beds, and literally freight-car loads of mammalian remains have been shipped to museums from outcrops near Espanola.

In the basins amid the mountains of southwestern New Mexico, similar sands and gravels of the Gila Conglomerate filled low areas. East of the mountains of central New Mexico that form a north-south chain of ranges from Raton to Carlsbad, thin gravels of the Ogallala Formation were dumped onto the western edges of the High Plains. They now cap the plains as well as make picturesque bluffs east of the Pecos River and southeast of Tucumcari—the “caprock” of that area. In northwestern New Mexico, isolated mesas are topped by the Chuska and Bidahochi formations; similar sands, silts, and clays washed from adjoining highlands.

The final episodes of landscape formation occurred during the Pleistocene Epoch, the recent glacial period. Mountain valley glaciers occupied some of the higher parts of the state, as far southward as Sierra Blanca; large lakes filled many of the closed basins, such as those near Estancia and south of Lordsburg; the Carrizozo and Grants basalt flows were extruded; the final tremendous explosions of Valle Grande spread volcanic ash over large regions; sands, gravels, and clays were eroded and deposited by streams and in lakes; and sand dunes were heaped up in many areas. The glistening white gypsum dunes (fig. 4) of White Sands National Monument, built up into 50-foot-high mounds windward of gypsiferous Lake Lucero, are spectacular products of the wind.

The Rio Grande, in its present valley, probably is only as old as mid-Pleistocene, born during late uplift of its headwater mountains, the San Juan and Sangre de Cristo ranges in southern Colorado and northern New Mexico—initiated by floods of meltwaters from waning mountain glaciers. Some of the lower terraces (benches) along the Rio Grande are very young, being dated by radiocarbon methods at 2600 b.p. (before present). Until shackled by Elephant Butte Dam in 1916, and smaller dams up and down the valley, the Rio Grande switched its course with every large springtime flood. Even with these man-made controls, the Rio carves new channels during floods and covers flooded fields with silt as the high waters recede.

The highest points in New Mexico are in the north-central region. Here, along the backbone of the Sangre de Cristo Mountains, tower Wheeler Peak (13,160 feet above sea level), northeast of Taos, and South Truchas Peak (13,102 feet), northeast of Santa Fe. Snow lingers on theselofty spires all year around except during an especially hot August. Highest peaks—they must be capped by the youngest rocks. But no, the quartzites and gneisses, hard rocks made up of quartz and feldspar, that hold up these pinnacles against the attack of water and ice are among the oldest rocks known in the Southwest—perhaps as much as two billion years old. These Precambrian rocks lay deep beneath the earth’s surface from early Paleozoic time (500 m.y. ago?) until early Neogene time, then were uplifted along great breaks in the earth’s crust, uplifted slowly and intermittently during the span of time from about 20 m.y. ago to perhaps 1 m.y. ago.

Figure 4.Gypsum dunes of White Sands

How did these ancient rocks form? What did New Mexico look like during the dawn of geologic history? The record in stone is fragmentary. But about two billion or so years back, thick masses of quartz sandstone were laid down in north-central New Mexico, as well as vast lenses of mud, and some beds of feldspar-rich sandstone. Volcanic activity was intense; huge flows of rhyolite and andesite were poured out over most of the state, and these in turn, along with the muddy and sandy sediments, were intruded by enormous masses of hot granite. Mile-high mountains were formed, and during the stretch of this early geologic time, called the Precambrian Era, the high peaks were eroded by rain, wind, and sun until some were worn down to featureless plains sloping toward the ancient ancestral oceans.

Figure 5.Staurolite twin crystals in mica schist

These Precambrian rocks now make up the cores of such mountain ranges as the Sandias east of Albuquerque, the Sangre de Cristo range near Santa Fe, the Pedernal Hills south of Clines Corners, the Burro Mountains southwest of Silver City, the Brazos Range east of Tierra Amarilla, the Zuni Mountains southwest of Grants, and the San Andres Mountains west of White Sands. As seen in Tijeras Canyon east of Albuquerque, the bulk of the ancient rocks are gray to pinkish granite and granite gneiss, speckled by crystals of biotite, microcline, orthoclase, and quartz. Quartzites (hardened sandstones), greenstone, and foliated mica schists (fig. 5) are the older rocks that were intruded by granitic magmas about 1350 million years ago—as indicated by dating of radioactive isotopes, K-Ar and Rb-Sr. Locally, pegmatite dikes, a late-cooling, large-crystal stage of the granites, crisscross the granite and contain excellent crystals of quartz, feldspar, and mica, as well as less common minerals. The famous Harding pegmatite near Dixon, about forty miles north of Santa Fe, contains beryl, columbite-tantalite, lepidolite mica, spodumene, and other rare minerals. Some of these crystals are ten feet in length!

There are some primitive types of plant and animal life known from Precambrian rocks outside of New Mexico but the earliest beds that contain abundant fossils are those of Cambrian age, 500 to 600 m.y. old. Cambrian rocks in New Mexico are in the Bliss Sandstone, a reddish brown iron-rich bed, 50 to 200 feet thick, that occurs only in the southern part of the state. There it can be seen, for example, along the bold east-facing escarpment of the San Andres Mountains or the west-facing cliffs of the Caballo Mountains, as a dark band resting on the pinkish Precambrian granitic rocks. It is a shallow-sea sand, deposited on the northeastern edge of the Cambrian seas. Scattered amid the brown-stained quartz, red hematite, and green glauconite are broken shells of trilobites and primitive brachiopods. Northern New Mexico was a low, broad island during Cambrian time, a source of some of the sands in the Bliss Sandstone.

During the Ordovician Period, 425 to 500 m.y. ago, upper sands ofthe Bliss as well as overlying limestones and dolomites were deposited in the shallow warm seas of southern New Mexico; these latter rocks are the El Paso Limestone and Montoya Dolomite. The Ordovician seas teemed with invertebrate life. Fifteen-foot-long cephalopods, as much as a foot in diameter, ruled the shallow salt-water bottoms, munching on the abundant trilobites and the moss animals, the bryozoans. Numerous brachiopods, corals, snails, and clams also thrived, with many of the Ordovician carbonate-rock beds literally being made up of these fossil remains. Near El Paso, these limy fossiliferous beds are nearly 2000 feet thick, but they thin northward to a knife edge in thickness near Mockingbird Gap at the north end of the San Andres Mountains. Parts of northwestern New Mexico may have been low islands exposed to the sun and erosion during Ordovician time, but most of the state was probably within an extensive shallow ocean. Later, erosion removed the Ordovician rocks from central and northern New Mexico.

Silurian strata, the brown Fusselman Dolomite, deposited during the middle of that period (400 to 425 m.y. ago), remain only in the southern and southeastern parts of the state, thinning out northward from the 1000-foot-thick bed near El Paso. The extent of these middle Silurian seas is not known, but most of central and northern New Mexico was undergoing erosion during late Silurian time. The northward thinning of the Fusselman Dolomite is due chiefly to this erosion, evidenced by the knobby, ridged and channeled top surface of the Fusselman. Brachiopods and corals are the most abundant fossils in the Fusselman Dolomite; elsewhere, Silurian rocks are known for the sea scorpions or eurypterids, which attained a length of nine feet, and for the complete remains of primitive fishes.

During early and middle Devonian time (345 to 400 m.y. ago), most of New Mexico was a lowland rotting beneath the sun. Fossiliferous Devonian rocks are unknown in the north-central part of the state but occur beneath the surface in the Four Corners region of northwestern New Mexico and the adjoining states. These rocks are of late Devonian age and consist of lower dolomite and sandstone, middle shale and dolomite, and the upper Ouray Limestone.

In southern New Mexico, a uniform blanket of dark limy muds, called the Percha Shale, was deposited during late Devonian time. This shale marks a great change from the limestones of earlier ages. In part, it is of black muds deposited in widespread or in local stagnant basins and in part calcareous fossiliferous muds in which abundant invertebrate life was buried. The clay and quartz silt that make up the rocks were a weathered residuum that had accumulated, during the long period of late Silurian and early and middle Devonian times, on the lowland of central and northern New Mexico.

Except in the stagnant basins, invertebrate life was prolific, brachiopods, bryozoans, and corals being especially numerous. Fossil fish remnants, chiefly teeth, are abundant in some of the sandy units, and outside of New Mexico the earliest amphibians occur in upper Devonian rocks. Theoldest definitely known assemblage of land plants occurs in the Devonian, and forests containing forty-foot-high trees spread over the uplands. Such tree ferns, horsetail rushes, and lycopods (spiked-leafed trees) may have grown in profusion on the swampy lowlands near Albuquerque’s and Santa Fe’s present sites, far north of the muddy Devonian seas of southern New Mexico.

BrachiopodMarginifera.

Crinoidstem fragment.

Crinoidrestoration 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, horsetailrushes, 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.

Figure 7.New Mexico during Pennsylvanian time

The Permian Period (230 to 280 m.y. ago) dawned with renewed rising of the highlands in northern New Mexico and southern Colorado. Floods of red sand and clay, washed from the rotting hills, wiped out the seas of northern and central New Mexico, and intertongued southward with marine limestones. These early Permian rocks are the Hueco Limestone near El Paso, there 2200 feet thick, the Abo Redbeds near Albuquerque,and the upper Sangre de Cristo Redbeds southeast of Santa Fe. Wherever the redbeds crop out, their dark reddish brown hue, speckled and striped with spots and streaks of green, enlivens the drab gray-and-brown landscape. Some of the reddish coloring is from angular grains of red to orange feldspar, but most is in thin brilliantly tinted skins of hematite that coat the sand grains and saturate the clays. In northern and central New Mexico, amphibians and other primitive vertebrate animals lived amid the red soils and sands; their bones and imprints have been preserved on thin flat slabs of sandstone that now decorate sidewalks and patios.

BryozoanFenestrellina.

FusulinidFusulina.

CoralLophophyllidium.

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 theYeso 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 andred 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.

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 theOchoan 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.

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 almostto 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, thinsheets 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 andthe 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.

Thus as the Cretaceous seas withdrew from New Mexico, the Cenozoic Era dawned, and never again have marine waters shaped the landscapes. The rocks, Precambrian, Paleozoic, Mesozoic, and Cenozoic, and their ancient movements determine New Mexico’s spectacular landscapes. They tell tales of the endless war between erosion and hard rock, show the deposition of sediments, their uplift, and their eventual destruction. The result is striking scenery—volcanic mountains, as Mount Taylor, Sierra Blanca, Valle Grande—fresh lava flows near Carrizozo and Grants—volcanic necks like Shiprock and Cabezon—White Sands, the work of the wind—El Morro, Enchanted Mesa, Acoma, and badlands near Santa Fe, the result of weathering and erosion—great fault-line escarpments of the Sandia, Manzano, San Andres, and Sacramento mountains—the work of underground waters at Carlsbad Caverns. And man adds his erosive powers—the huge open-pit copper mine at Santa Rita and countless excavations for rock to build homes, to straighten highways.

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