Earlier this week I made a scouting trip into the Pecos Wilderness, a mountainous wilderness area northeast of Santa Fe. To do this I had to drive around the southern tip of the Sangre de Cristo Mountains, and like practically all travelers before me, I was funneled over the Glorieta Pass, the old route of the Santa Fe Trail around the Rocky Mountains, and the current route of both the Amtrak Southwest Chief, and Interstate 25, which links Denver and Albuquerque.
As soon as Glorieta Mesa begins to rise on the south side of the pass, sedimentary red beds appear in the road cuts and hillsides along the freeway.
These colorful rocks are Permian in age. The geologic record of both the Pennsylvanian and the lower Permian systems is well represented in northern New Mexico, and for much the same reason: the tectonic disturbances of the Ancestral Rocky Mountain orogeny. But unlike the rather dull grey tones of the Pennsylvanian sediments, many of our Permian beds are bright with purples, reds, oranges, and warm yellows. Why is that?
As the Pennsylvanian Period ended, a shallow sea, which had covered much of New Mexico, began to withdraw to the west and to the southeast. Vast regions of Precambrian crystalline basement rock – granite and gneiss – lay exposed in the Ancestral Rockies, deeply weathered by ages of tropical rainfall, and as the climate dried and shifted to seasonal monsoonal conditions, these weathered highlands began to release huge amounts of arkosic sand and red silt across the landscape.
The coarsest material accumulated in alluvial fans near the mountains, trapped in basins still slowly subsiding around the eroding highlands. Sand and finer material spread out in broad alluvial plains, where silt-choked streams wandered across flats alternatively steaming in monsoonal rainfall or baking in the equatorial sun, in an atmosphere charged with excess oxygen. The old Pennsylvanian basins, filled with dull grey marine limestones and black shales, were soon buried under a purplish-red blanket of mud, silt, and sand. In New Mexico these sediments have been given the name Abo Formation, after classic outcroppings near Abo Pass.
As the climate became even more arid, these streams withered away, and fine red sand was mobilized by desert winds over the alluvial plains. Vivid orange dune fields began to cover the muddy plains and sandy channels of earlier times. These dunes were bordered by the remnants of the sea in the southeast part of New Mexico, where the sand stabilized into an arid coastal plain, with blinding salt flats bordering sandy lagoons. These natural evaporating pans spiced the orange sand with dolomite and white gypsum, or yeso, in Spanish, and from this word the bright package of sediments has taken its name: the Yeso Formation.
One last assertion of the Permian Sea shaped the durable, mesa-forming rocks that cap these red beds. As the sea spread back toward northern New Mexico, away from the Permian Basin of West Texas, its waves washed the sands of the coastal plain into clean, well-sorted shallow marine bars and beaches. Much of this beautiful yellowish sand blew, on arid winds, away from the beaches and into dune fields of a much paler color than the underlying Yeso sands. Altogether this blanket of clean, and now firmly-cemented sand is called the Glorieta Sandstone, named after the Glorieta Mesa which it crowns.
Before the sea retreated it laid down a final deposit of hard limestone, thin to absent in northern New Mexico, but noticeably thicker to the west and especially to the south, where its type section in the San Andres Mountains outcrops. The San Andres Limestone is the most widely exposed Paleozoic formation in New Mexico, and, where it plunges into the subsurface, forms both important fresh water aquifers and oil reservoirs. But as far as color goes it echoes the grey marine sediments of the Pennsylvanian days, and marks the end of the vivid Permian.