Tertiary and quaternary drainage of Southern High Plains
Gazdar, Muhammad Nasir
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The Southern High Plains of West Texas and eastern New Mexico appears as a large (85,000 sq km), uplifted, flat-topped crustal block. However, the conspicuous physiographic expression results from the fluvial dissection of flat-lying sedimentary beds which are capped by a massive resistant calcrete. The Southern High Plains exhibits evidence of fluvial deposition and erosion throughout late Tertiary and Quaternary time. After an extended period of erosion and incisement of streams during early Tertiary time, the streams east of the present Pecos River and across adjoining West Texas were filled with debris produced by late Tertiary uplift in the mountains of north-central New Mexico. As the immediate channels were choked with debris many streams pushed out of their original banks and, by the end of Pliocene time, the area was buried by a fluvial blanket of sands, gravels, clays and silts over 130 m thick in places. The sediments of this fluvial complex are known as the Ogallala Formation. The late Tertiary streams which deposited the Ogallala Formation flowed east-southeastward from the Sangre de Cristo and Los Pinos-Manzano Mountains of north-central New Mexico. Drainage, during Tertiary time, was originally controlled by the regional southeastward slope and by northwest-southwest trending lineaments with local north-south offsets. The initial regional drainage pattern was parallel, but was soon modifier to a trellis pattern with the development of short, localized tributaries. When the streams were no longer confined to their parallel channels, but were instead meandering across the flat aggrading plains, the number of streams increased and the drainage pattern undoubtedly changed to dendritic and subdendritic, and perhaps even an anastomotic pattern in some instances. Quaternary drainage patterns are often relict from Tertiary drainage channels, but are subdued and restricted because of non-integration of streams and sparseness of channels. The overall regional pattern is rectilinear sub-parallel to modified subdendritic, indicative of structural control. Quaternary drainage channels exhibit the significant influence of underlying Tertiary drainage channels over about half of the Southern High Plains, but only moderate to weak control occurs in other areas. The variety of Tertiary drainage patterns is absent, with most Quaternary streams exhibiting a lesser regional drainage density. The Quaternary channels lacked sustained flow throughout their reaches except during glacial stages, thus are underfit in most cases.