Sedimentary zonation of the Ogallala Aquifer

The Ogallala aquifer of the Southern High Plains, composed mainly of sands and gravels underlain by older rocks of lower permeability, is the most extensive and usable aquifer of the Great Plains province (Frye, 1970). Natural recharge of the Ogallala section in West Texas and eastern New Mexico is restricted to percolation of precipitation and playa lake seepage within a semi-arid environment (approximately 19 inches of precipitation per year). Excessive withdrawal from the Ogallala aquifer, particularly for agricultural purposes, is lowering the water table at a rate that will reduce irrigated acreage to about 60 percent of today's figure by the year 1990 (Buika, 1979).

Initial deposition of Ogallala sediments, derived from the Southern Rocky Mountains of New Mexico, occurred as valley alluviation in pre-Ogallala drainage channels. Clays and fine-grained sands were later deposited by streams meandering across the alluvial plain which formed as a result of the filled channel valleys. Fine-grained eolian sands were deposited in addition to flood plain sediments. Correlation of individual units within the Ogallala section is difficult, although some stratigraphic continuity is evident from regional maps of sand, clay, and gravel distribution.

Sand accounts for 100 percent of the Ogallala section in various localities through Quay and Curry County, but thins to the southeast. Thickness ranges from 20 to over 300 feet. Clay in the Ogallala section thickens to the north and northeast, ranging from zero in eastern New Mexico (Lea and Roosevelt counties) to over 300 feet in the northeastern section of the study area (Carson County). Thick localized clay lenses, apparently representing lacustrine environments, occur throughout the Southern High Plains. The basal gravel of the Ogallala section reveals several thick elongated accumulations indicative of pre- Ogallala drainage channel fill. Gravels throughout the remainder of the Ogallala section are thin and discontinuous, the overall amount of gravel in the section thinning to the east. Localized abnormally thick accumulations of gravel may have resulted from sink fillings.

Standard deviation values calculated for each lithologic unit are interpreted relative to geological explanations of data variance. The ratio of sand and gravel to clay was determined to assess the relation of highly porous and permeable sediments to sediments of low porosity and permeability. Sand and gravel to clay ratio values range from 1 to 4 for most of the central Southern High Plains, increasing sharply (up to 100) through eastern New Mexico.