Part 5

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.

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 the _Rio Grande structural depression_ (fig. 3). Mountains on the east are the Sangre de Cristo, 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.

PRECAMBRIAN ERA

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 these lofty 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.

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.

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!

PALEOZOIC ERA

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 of the 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. The oldest 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.

Figure 6. Fossil specimens

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.

Permian Rocks

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.

Figure 8. Fossil specimens

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.