CENOZOIC ERA
The mountains, the plains, the rivers, and the lakes all are transitory features of the landscape, created during the recent part of the Cenozoic Era, and all doomed to destruction in the near future, geologically speaking. Even so, some of the plains and mountains may predate man’s evolution onto the earth scene.
Gone were the late Mesozoic seas; never again during our lifetime nor the lifetime of many future generations will marine waters roll over New Mexico, and sea-spawned creatures rule. For the first two thirds of the Cenozoic, the Paleogene Period (25 to 70 m.y. ago), New Mexico suffered from the dying effects of the great Laramide upheaval of the earth’s crust. Large areas were exposed to the harsh erosion of stream and wind. The landscape looked as parts of southwestern New Mexico do today—tall rugged mountain ranges scattered in isolated patches amid wide gravelly plains. But the climate was more humid, and while no large through rivers are known, local great swamps and lakes lay on the plains in debris-trapping lowlands.
Figure 1. Table of geologic time
| Scale of geologic time in millions of years | |||
|---|---|---|---|
| Era | Rocks | Dominant life | |
| Geologic age | |||
| 70 | CENOZOIC | ||
| Neogene | |||
| Pleistocene | Bandelier volcanic ash, basalts, sand dunes, river gravels, glacial & lake beds. | Man | |
| 1 my | |||
| Pliocene | Volcanic rocks of Mt. Taylor, early Valle Grande & Gila region; Santa Fe, Gila, and Ogallala Fms. | Mammals | |
| 11 my | |||
| Miocene | |||
| 25 my | |||
| Paleogene | |||
| Oligocene | Datil, Espinaso, and other volcanic rocks. | ||
| 40 my | |||
| Eocene | Baca, Animas, Nacimiento, San Jose, Raton, Poison Canyon, Galisteo El Rito, Blanco Basin, & Cub Mountain Fms. | ||
| 60 my | |||
| Paleocene | |||
| 160 | MESOZOIC | ||
| Cretaceous | Upper sandstone, shale, & coal. Mesaverde, Pierre, & Niobrara, Dakota Ss. and Mancos Sh. | Dinosaurs | |
| Volcanic rocks, limestone, sandstone, and conglomerate. | |||
| 135 my | |||
| Jurassic | Morrison Fm., Summerville Fm., Zuni Ss., Todilto Ls. & gypsum, Entrada Ss. | ||
| 180 my | |||
| Triassic | Wingate Ss., Dockum Fm., Chinle Fm., Santa Rosa Ss., Moenkopi Fm. | ||
| 370 | PALEOZOIC | ||
| Permian | Rustler Dolomite; redbeds Castile gypsum, Salado Salt. | Amphibians | |
| Artesia Grp.—Capitan reef San Andres Ls.—Goat Seep reef Glorieta Sandstone Yeso Fm.—Bone Spring Fm. | |||
| Abo Redbeds—Hueco Ls. | |||
| 280 my | |||
| Pennsylvanian | Mostly limestone; beds of shale & sandstone; lenses of gypsum, salt, and coal. | ||
| 310 my | |||
| Mississippian | Helms & Paradise Fms. Rancheria Ls. Lake Valley & Escabrosa Lss. Arroyo Penasco & Tererro Fms. | Fish | |
| 345 my | |||
| Devonian | Percha Shale, Ouray Ls. northern dol., ss, and sh. | ||
| erosion | |||
| 400 my | |||
| Silurian | Fusselman Dolomite | ||
| erosion | |||
| 425 my | |||
| Ordovician | Montoya Dolomite El Paso Limestone Bliss | Invertebrates | |
| 500 my | |||
| Cambrian | Sandstone | ||
| erosion | |||
| 1500+ | |||
| PRECAMBRIAN | quartzite, gneiss, rhyolite, andesite, granite, pegmatite, schist, greenstone. | Simple primitive forms | |
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 along N.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 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.