HR: 0800h
AN: H11D-1310    [Abstracts]
TI: Modeling Variable-Density Fluid Flow and Solute Transport in Glaciated Sedimentary Basins
AU: * McIntosh, J C
EM: jmcintosh@jhu.edu
AF: Johns Hopkins University, Morton K. Blaustein Dept. of Earth and Planetary Sciences 3400 N. Charles St., Baltimore, MD 21218 United States
AU: Garven, G
EM: garven@jhu.edu
AF: Johns Hopkins University, Morton K. Blaustein Dept. of Earth and Planetary Sciences 3400 N. Charles St., Baltimore, MD 21218 United States
AU: Hanor, J S
EM: hanor@lsu.edu
AF: Louisiana State University, Dept. of Geology and Geophysics, Baton Rouge, LA 70803 United States
AB: Sedimentary basins typically contain saline formation waters (35 to >250 g/L TDS) with relatively long residence times (Kyrs to Myrs). Advance and retreat of km-thick ice sheets, as recently as the Late Pleistocene (<18 ka BP), exposed regional aquifers along the margins of northern latitude basins. Overpressuring of aquifers beneath the wet-based glaciers forced meltwaters to great depths in subsurface flow systems, significantly diluting remnant saline fluids and reorganizing salinity structures. Permafrost zones outboard of the ice sheet margins further enhanced deep circulation of glacial recharge. In basins containing shallow evaporites, meltwaters dissolved large quantities of halite, generating relatively recent (<1 Ma) NaCl brines. Furthermore, glacial recharge promoted generation of economic deposits of microbial methane in shallow organic-rich sediments. To better constrain the impact of glaciation on variable-density fluid flow and solute transport in sedimentary basins, we constructed a transient 2D finite element model of the northern half of the glaciated Michigan Basin. The circular basin is relatively tectonically undeformed and contains ~4 km of Paleozoic strata, primarily composed of carbonates, clastics and bedded evaporites. Thick glacial drift deposits (up to ~300 m) form most of the topographic relief in this low-lying intracratonic region. The salinity of basin fluids sharply increases from <0.5 g/L TDS near the surface to >350 g/L at ~800 m depth. Modern groundwater flow is primarily restricted to shallow glacial drift aquifers, with discharge to the Great Lakes. During Pleistocene glaciation, however groundwater flow patterns were reversed, and meteoric waters were routed into Paleozoic carbonate and siliclastic basinal aquifer systems, depressing the freshwater-saline water mixing zones and dissolving halite. Dilute waters (<100 g/L TDS) migrated ~200 km laterally into the Devonian carbonate aquifers. Carbon-14 ages and δ¹⁸O values of confined groundwaters along the basin margins indicate they were recharged beneath the Laurentide Ice Sheet (14 to 50 ka BP). These numerical results have important implications for the residence times of fluids in sedimentary basins, sources of salinity, and natural gas accumulations.
DE: 0726 Ice sheets
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1833 Hydroclimatology
SC: Hydrology [H]
MN: Fall Meeting 2005