Paleohydrology of variable-density ground-water flow systems in mature sedimentary basins: example of the Palo Duro basin, Texas, USA

Abstract

The paleohydrology of mature sedimentary basins undergoing uplift and tilting, deposition, and erosion is studied by numerically modeling the flow of variable-density ground water. In regional flow systems consisting of both shallow freshwater aquifers and deep saline aquifers, complex ground-water flow patterns can exist, when buoyancy forces dominate the hydraulic head gradients arising from topographic relief. The relative magnitude of the two driving forces, hydraulic gradient and buoyancy, changes during the hydrodynamic development of a sedimentary basin that is undergoing topographic changes. The relationship between the two driving forces and their effect on flow and transport is studied through coupled flow and solute transport simulations using an idealized model scenario. Modeling shows that increases in recharge rates can rapidly change hydrodynamic conditions from buoyancy-dominated flow governed by variable density to gravity-driven flow governed by topography. Because the dissolved mass responds much slower to changes in hydrologic boundary condition than does fluid pressure, transient conditions occur whereby the solute distribution may not correspond to the simulated ground-water flow pattern. Hydrodynamic scenarios were examined in the Palo Duro basin, Texas, where observed fluid densities range between 1000 and 1150 kg m −3. The Palo Duro basin was affected by Cenozoic uplift, deposition, and erosion, causing modifications in topography and major changes in regional hydrodynamics during the last 15 Myear. Although fluid pressures have largely equilibrated with topographic changes in the recent geologic past, the distribution of dissolved mass evolves over a much longer time, and the observed solute distribution in the basin, as suggested by the geochemistry of the deep basin brines, may represent some past state of the hydrodynamic system. Paleohydrologic simulations describe buoyancy-dominated flow phenomena in the recent geologic past, when the topographic relief across the basin was much lower than it is today. Prior to maximum basin uplift, simulated flow patterns in the deep aquifers are characterized by convection cells. In the western part of the basin, where fluid densities increase with depth, convection cells show clockwise fluid motion, restricting recharge of shallow ground water to the upper part of the deep aquifers. In the eastern part of the basin, where fluid densities decrease with depth, convection cells are characterized by counterclockwise fluid motion and reflect thermohaline convection. With enhanced recharge due to continued basin uplift and subsequent erosional events, buoyancy-dominated flow phenomena diminish and the gravity-driven flow component arising from topographic relief becomes dominant. This study demonstrates that, although present ground-water flow circulation in the basin is not affected by the observed fluid densities, the flow pattern history during Cenozoic basin development is much more complex than that which would be expected from a gravity-driven flow conceptualization (assuming uniform fluid densities). More important, the model scenarios describe the interrelationship between geologic processes, regional hydrodynamics, and transport phenomena and its importance in explaining hydrologic and geochemical phenomena in sedimentary basins.