THE PROVENANCE AND DISPERSAL OF THE NEOGENE OGALLALA FORMATION ACROSS WEST TEXAS AND EASTERN NEW MEXICO: DETERMINING THE PALEODRAINAGE NETWORK OF THE SOUTHERN HIGH PLAINS

The provenance of the Neogene Ogallala Formation in Texas and New Mexico and the paleodrainage network of the Southern High Plains remains enigmatic. Sand and gravel from 40 sites allow for petrographic assessment of the bulk composition of Ogallala sediment, an analysis of regional variation in composition, and lithotype fingerprinting.

The composition of sand and gravel are relatively uniform over the study area, but with gradational variation between northern, central, and southern sites. The sand and gravel are rich in lithic grains predominantly derived from sedimentary rocks, with lesser amounts from metamorphic and volcanic rocks. The relative abundance of feldspar, volcanic and metamorphic lithic grains diminish from north to south. In contrast, the abundance of sedimentary lithic grains, and in particular, carbonate rock fragments increase to the south.

Of the rock types recognized in the pebbles, most or all appear to have been derived from the Sangre de Cristo Mountains and nearby areas in northern New Mexico. Pebbles comprised of volcanic and shallow intrusive rocks also appear to be mostly derived from source areas in northern New Mexico, in particular Cimarron Pluton, Sierra Grande, the Latir, Raton-Clayton, and Ocate volcanic fields. Most of the sedimentary rock types recognized in the pebbles were derived from rocks exposed directly below and adjacent to the basal Ogallala contact and transported only a short distance from their points of origin. Other pebbles of a more distant origin may be derived from units exposed along the eastern sides of the Sangre de Cristo, Sandia, and Manzano mountains, in the Cimarron valley, and the Canadian breaks in northeastern New Mexico These units include the Pennsylvanian Sandia and Sangre de Cristo formations, Pennsylvanian Madera Formation, Jurassic Exeter Sandstone and Morrison Formation, the Cretaceous Dakota and Mesa Rica Sandstone, limestones from the Cretaceous Greenhorn and Niobrara formations, and gryphaeid coquina from the lower Glencairn Formation.

These general trends are compatible with derivation of Ogallala sediment from a single primary source area north/northwest of the Southern High Plains, and transport via a major trunk stream to the south/southeast with increasing addition of lithic grains from tributary drainages to the south in transit.