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The AAPG/Datapages Combined Publications Database

Rocky Mountain Association of Geologists

Abstract


The Mountain Geologist
Vol. 50 (2013), No. 2. (April), Pages 37-63

Outcrop Control of Basin-Scale Underpressure in the Raton Basin, Colorado and New Mexico

Philip H. Nelson, Nicholas J. Gianoutsos, Lawrence O. Anna

Abstract

Hydraulic head values determined from drillstem test pressures reveal that Paleozoic, Mesozoic, and Cenozoic strata in the Raton Basin are underpressured. Pressure-depth ratios are less than hydrostatic and corresponding head values are well below surface elevation throughout most of the basin.

Geologic structure, outcrop locations, and surface topography must all be considered to understand the cause of basin-wide underpressure in this Laramide basin. The western margin of the basin is bounded by the Sangre de Cristo Mountains and is characterized by steeply dipping strata associated with Laramide folding and faulting. The Sierra Grande Uplift and Apishapa Arch form the eastern and northeastern edges of the basin, respectively. Paleozoic strata are truncated on these uplifts and gently dipping strata of Early Cretaceous age are exposed on the arches, as are windows of Jurassic, Triassic, and Permian rocks. Surface elevation rises from approximately 4,000 ft in the east to 11,000 ft in the west, so the land surface above the deepest part of the basin is topographically high.

Comparison of hydraulic-head values with outcrop elevations indicate that down-gradient exposures constitute the pressure equilibration locus for the confined aquifers. Although the sparse data set is insufficient to resolve all questions, the available data are consistent with a conceptual model of a series of confined aquifers that discharge on the eastern flank of the basin with limited recharge from the surface and the western flank. Consequently, where the surface elevation is greater than the discharge elevation, as it is over much of the basin, the aquifer pressure is less than hydrostatic.


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