Page 31 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

30 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
quence of lava flows, dividing the andesites in this fashion into two or three distinct flows, and setting the rhyolites off from the andesites by a considerable time interval. Finally, following the period of lava flows and in general contemporaneity with the Santa Fe formation in the valley, deposits of arkosic sandstone and conglomerate, detrital products of the erosion of these latest igneous extrusions, began to form as alluvial fans and piedmont plains at the base of the mountain ranges. The basal part of these beds is composed in general of grains and pebbles of rhyo- lite and rhyolite tuff, with a fine cementing matrix of rhyolitic and tuffaceous material, partly cemented with chalcedony and calcite and in part altered and decomposed to claylike material. The upper portions of the beds are composed largely of andesitic fragments and matrix, and the decomposition products contain more abundant chlorite.
QUATERNARY SYSTEM
Detrital material derived from the wearing down of the adjacent slopes fills the Rio Grande valley to a depth in places of more than 2,000 feet, as shown by well records, and covers the high plains and bolsons to varying depths. Much of this material is gravel, which consists of boulders and pebbles of quartzite, limestone, granite, rhyolite, andesite and basalt, and reflects the character of the immediately surrounding rocky com- plex. These wash deposits cover the greater part of the Rio Grande valley and the adjacent plains and bolsons. They are coarser along the valley sides, grading to finer material toward the middle of the basins. Below Hot Springs in the Rio Grande valley, the lower part of the horizontal sandy to clayey beds exposed in the terraces may be the equivalent of Santa Fe mail of late Tertiary age. In the higher regions, as in the sides of the valley between Fairview and Hillsboro and at Hermosa and else- where, beds of Quaternary age are composed of partly worn boul- ders and pebbles loosely cemented in a finer matrix of the same material into a firm hard conglomerate. These are known as Pal- omas gravel, named from the type locality in Palomas Creek. In composition and geologic age these gravels are apparently identical with and form an extension of the top part of those deposits in Arizona which have been named Gila conglomerate by Gilbert. 17 The upper part of the Palomas gravel may be some- what younger than the latest Gila conglomerate.
Younger than the Palomas gravel are the sand and gravel deposits which form some of the intermediate terraces represent- ing a filling of channels excavated in the older deposits of Santa Fe and Palomas age. In other places these intermediate terraces appear to be merely the older gravel eroded down to a new base level as a result of a former state of equilibrium in the land ele- vations of the region. Still younger are the present flood-plain
Gilbert, G. K., U. S. Geog. Surveys W. 100th Mer., Vol. 3, p. 590, 1875.