GEOLOGIC FRAMEWORK FOR GROUND-WATER MODELING, MIDDLE RIO GRANDE BASIN, NEW MEXICO

The principal aquifer system that supplies most domestic, industrial, and municipal needs of the Albuquerque region consists of alluvial and fluvial sediments deposited within the Rio Grande rift during the last 20 million years or so. We have assembled a three-dimensional geologic framework model for the Middle Rio Grande ground-water basin using subsurface data from drill hole geophysical logs, detailed surface geologic mapping, and basinwide interpretations of gravity and magnetic data that define buried structure. The geometry, texture, and lithology of clastic sedimentary units in the aquifer vary widely across the basin and with age, reflecting the variability of depositional environment, faulting, subsidence/uplift, volcanism, and climate during sedimentation. Hydraulic characteristics of these units can be inferred between drillholes and in outlying areas based on conceptual understanding of the geologic processes at work during deposition and inferred trends through time. Our model reflects three discrete depositional subbasins, each recording its own pattern of early sedimentation related to local subsidence and local alluvial, volcanic, and eolian sources. Hydrostratigraphic units within these basins thicken markedly across fault zones and generally thicken toward basin centers as a result of syndepositional faulting and subsidence, especially active during middle Miocene time. Subsidence waned during the Pliocene, leading to coalesced deposits that blanketed the structural highs between the subbasins and integrated drainage. Fluvial sediments throughout the basin changed from sand-silt to coarse-sand-and-gravel beginning at 4 to 3 Ma reflecting increase in precipitation/runoff due to global climate change. The fluvial system began to incise the basin fill perhaps as early as 2.5 Ma, and the Rio Grande valley is now flanked by inset terrace-fill deposits formed by repeated incision and back-filling in response to Pleistocene glacial-climate cycles. The modern flood-plain is inset on coarse Pleistocene gravel that hydraulically links the river to the coarse Pliocene and younger fluvial deposits, the primary aquifers of the region.