Preliminary investigation of the sedimentary zonation of the Ogallala aquifer, southern High Plains

The Ogallala aquifer of the Southern High Plains provides water from approximately 60,000 irrigation walls (Don Smith, personal communication, 1979), which support 68 percent of the total irrigated acreage in Texas. However, because the Southern High Plains is a topographically isolated plateau, the Ogallala aquifer receives no recharge except from infiltration of surface precipitation. The region is a semi-arid area, receiving only approximately 19 inches precipitation par year; thus a declining water table will soon reduce irrigated acreage to about 60 percent of today's figure by the year 1990.\n\nSource areas for Ogallala sediments ware the Southern Rocky Mountains of New Mexico, although the exact areas have yet to be determined. Deposition of coarse Ogallala sediments first occurred by valley alluviation in pre-Ogallala drainage channels, followed by meandering stream deposition of fine-grained sands and clays, and deposition of considerable aeolian debris. Sands, clays, and gravels are interspersed throughout the Ogallala section making correlation difficult. However, maps representing the distribution of sand, clay, and gravel units of the Ogallala Group do show soma regional continuity. \n\nClay distribution in the Ogallala section thins to the east-northeast, although several thick local clay sections occur; these may represent ancient lacustrine deposits. Clay thicknesses, which range up to 320 feet, may account for up to 70 percent of the Ogallala section. Sand lenses in the Ogallala of the Southern High Plains range up to 450 feet in thickness, and often account for 100 percent of the section. Ogallala sands are generally thicker than either clay or gravel accumulations, regionally thinning to the southeast. Gravels in the Ogallala aquifer, with the exception of the widespread basal zone, thin eastward. Elongation of two gravel trends from northwest to southwest is a product of the two major stream channels which ware active during much of the Ogallala time. A third major Ogallala-age stream channel may have existed in the area now occupied by the Canadian River Valley, as indicated by the gravels that occur in Carson County.

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