Page 32 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 31
deposits along the bottoms of the Rio Grande and its tributaries. These are composed of unconsolidated wash material, ranging in size from clay and silt to the largest boulders that could be trans- ported during flood seasons by the various streams.
Deposits of Quaternary age are in the main the surface de- posits of the bolsons and plains of the county, although here and there Tertiary and older igneous and sedimentary rocks extend through them and are exposed just at the general level of the surrounding surface features. Close to the mountain ranges these older formations have been planed off or truncated to an even surface, which slopes toward the river and tributary bot- toms at the same angle as do the Quaternary gravels. These are the pediments studied by Bryan 18 at several places in the Rio Grande Valley. In places in Sierra County they are exposed in areas of several acres to several square miles, or as is more usual, they may be partly or wholly obscured under a mantle of a few inches to a few feet of detrital material. Closer to the river, the flat-lying partly consolidated beds of sand and silt, which have been referred in part at least to Santa Fe age, have been planed off by erosion in a similar manner and now lie at or near the surface of the younger gravels, forming what may be con- sidered as an extension of the true pediments of the higher regions.
TERTIARY AND QUATERNARY LAVAS
Late Cretaceous lava flows of considerable extent occur in the southwest portion of New Mexico, but none were noted within the boundaries of Sierra County that could be definitely classed as of this age, and in accordance with past ideas on this subject the writer has felt constrained to consider these early extrusives as being of early Tertiary, probably Oligocene, age. The extrusive rocks consist of an andesite-latite-rhyolite series, which is largely confined to the mountainous area west of the Rio Grande, but with a few scattered erosional remnants pro- jecting through the Quaternary detritus of the Jornada del Muerto, along the eastern slopes of the Fra Cristobal Range and the Sierra Caballos. The earliest extrusive rock was andesite, with associated tuffs and breccias, closely followed by intrusions and extrusions of latite and latite porphyry. Following these extrusions came a period of quiescence, during which the surface of the lavas was eroded and in many places deeply dis- sected by streams, with the accumulation locally of considerable thicknesses of sediments over the lower areas of the flows and as fanlike accumulations along their borders. Following this came a series of flows of rhyolites, tuffs and breccias of enor- mous areal extent and comprising at least half of the original total thickness of Tertiary flow rocks, which in several places
18 Bryan, Kirk, Pediments developed in basins with through drainage as illustrated by
the Socorro area, N. Mex. (abstract) : Geol. Soc. America, Bull., Vol. 43, No. 1, pp. 128, 129. 1932.