Fluvial and Eolian Depositional Systems, Paleosols, and Paleoclimate of the Upper Cenozoic Ogallala and Blackwater Draw Formations

Abstract
This report summarizes results of a study based on outcrop and subsurface data that establishes a regional stratigraphic framework for assessments of the Ogallala and related aquifers. Results are broadly applicable to water resources investigations and efforts to remediate contaminated areas of the aquifer as well as to the understanding of depoisitonal environments, paleoclimate, and soil development during the late Tertiary and Quaternary. The upper Tertiary Ogallala Formation contains the Ogallala (High Plains) aquifer, which is the major source of water for agricultural and domestic use on the Southern High Plains of Texas and New Mexico. Locally, perched aquifers overlie the Ogallala aquifer. This study, based on outcrop and subsurface data, provides a regional stratigraphic framework for assessments of the Ogallala and related perched aquifers. The results of this study are broadly applicable to water resource investigations and efforts to remediate contaminated areas of the aquifer as well as to the understanding of depositional environments, paleoclimate, and soil development during the late Tertiary and Quaternary in the High Plains of Texas and New Mexico.Deposition of the basal fluvial sediments of the Ogallala Formation in northwestern Texas and eastern New Mexico was controlled by the topography of the underlying erosional surface. Paleovalley-fill facies consist of heterogeneous gravelly and sandy ephemeral-stream deposits and sandy to clayey overbank deposits interbedded with and overlain by eolian sediments deposited as sand sheets and loess. Uplands on the pre-Ogallala erosional surface are overlain by similar eolian sediments. Buried calcic soils consisting mostly of CaCO, nodules and filaments are found throughout the eolian facies.The Caprock calcrete, which represents a long period of landscape stability, separates the Ogallala Formation from eolian sediments of the overlying Blackwater Draw Formation. The Caprock calcrete in the east part of the Southern High Plains, which is buried by a thick section of Quaternary Blackwater Draw sediments containing numerous buried calcic soils, is 1.5 to 2 m thick (5 to 6.5 ft) and is not usually brecciated or pisolitic. The stage IV or V Caprock calcrete here apparently formed under a stable landscape and is probably late Pliocene in age. A thicker (4 to 10 m [13 to 33 ft]), complexly brecciated, pisolitic, stage VI Caprock calcrete is widespread beneath large areas of the western High Plains in which the Blackwater Draw is only a few decimeters thick. In these areas of thin Blackwater Draw, the accumulations of CaCO, normally found in about 10 buried calcic soils and the surface calcic soil of full sections of the Blackwater Draw have apparently been welded onto the uppermost Ogallala calcrete. Part of the Caprock calcrete is thus locally coeval with the Blackwater Draw Formation and is Quaternary in age. Elsewhere, inliers of Cretaceous limestone and Triassic mudstone that crop out on the High Plains are capped by as much as 9.6 m (32 ft) of complexly brecciated, pisolitic calcrete (stage VI). The thick calcretes that formed on these topographic highs are apparently equivalent to the CaCO, that accumulated as buried calcic soils in complete sections of the Ogallala and Blackwater Draw Formations. These isolated areas of the Caprock calcrete are probably late Miocene to late Quaternary in age. Blackwater Draw eolian sediments are similar to Ogallala eolian sediments and also contain numerous buried calcic soils. Eolian facies in both formations, which preserve numerous superposed calcic soils and calcretes and contain common fine root traces, reflect slow episodic aggradation on a stable grass-covered landscape under mostly semiarid to subhumid climatic conditions. The change from fluvial to mostly eolian sedimentation probably resulted from diversion of streams (Panhandle, Clovis, and Slayton paleorivers) that deposited fluvial sediments of the Ogallala Formation to form the Pecos and Canadian Rivers. Source areas for most of these eolian sediments initially may have been floodplains of Ogallala streams and later floodplains of the newly formed Pecos and Canadian Rivers. Buried soils in Tertiary and Quaternary sediments of the Southern High Plains reflect cycles of eolian sedimentation followed by periods of landscape stability and minimal sedimentation in which pedogenesis occurred. Cycles of sedimentation and soil development probably resulted from cyclic decreases and increases in available moisture and vegetative cover in sediment source areas to the west. Eolian sediments were eroded from source areas such as the Pecos valley during dry periods when vegetation was sparse. During more humid periods, more abundant vegetation probably protected source areas from deflation. These investigations illustrate the heterogeneity and potential for vertical and lateral compartmentalization of the Ogallala and local overlying perched aquifers. For example, at least five fine-grained low-conductivity zones, including the aquitard at the base of a contaminated perched aquifer, exist in lower Ogallala sediments beneath the Pantex Plant in western Carson County.
Authors
Thomas C. Gustavson
Citation

Gustavson, T. C., 1996, Fluvial and Eolian Depositional Systems, Paleosols, and Paleoclimate of the Upper Cenozoic Ogallala and Blackwater Draw Formations: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations No. 239, 62 p.

Code
RI239
ISSN
2475-367X
Number
239
Number of figures
42
Number of pages
62
Publisher
The University of Texas at Austin, Bureau of Economic Geology
Series
Report of Investigation
Year
1996