Geological Survey Bulletin 1271—E Pecos National Monument New Mexico: Its Geologic Setting

Geologists can tell the environment in which the sedimentary rocks were deposited by studying various features in the rocks, such as fossils, kinds of fragments and minerals, shapes of the individual units, and depositional structures within the layers. The geologists sometimes can also tell the source of the fragments that make up the sedimentary rocks if they have a complete knowledge of the older rocks in the region.

From data such as these, geologists have concluded that the sands that make up the Santa Rosa Sandstone and the Glorieta Sandstone were deposited just offshore on the beaches of seas that were advancing over the land. The sand grains were derived from the winnowing action of waves on the sediments of the underlying formations—the Bernal and Yeso Formations.

The sediments that formed the rocks of the Bernal and Yeso Formations appear to be of shallow-water marine origin. Perhaps they were laid down on a broad, flat coastal plain. The San Andres Limestone is a marine limestone and probably was deposited during the wide advance of the Permian sea over the coastal plain after the deposition of the beach sands of the Glorieta Sandstone.

In contrast to these water-laid deposits, the Sangre de Cristo Formation consists mainly of terrestrial or land-laid sediments deposited by streams as alluvial fans, flood plains, deltas, and channel fills. Thin layers of limestone occur throughout the formation and contain fossils that indicate that the limestone layers are of both fresh-water and marine origin. Therefore, from time to time, lakes and ponds or ocean bays covered large parts of the region. Most of the fragments of the conglomerates, sandstones, siltstones, and shales of the Sangre de Cristo Formation are derived from the ancient Precambrian crystalline rocks that form the core of the high mountains a few miles to the north. From the unweathered appearance of the coarser fragments, it is evident that the source of these fragments was not very far away.