Subsurface Seepage of Seawater Across the Walvis Ridge: A Source of Water and Salt to the South Atlantic Rift Lakes During the Early Cretaceous

HR: 16:00h
AN: OS14A-01 [Abstracts]
TI: Subsurface Seepage of Seawater Across the Walvis Ridge: A Source of Water and Salt to the South Atlantic Rift Lakes During the Early Cretaceous
AU: * Nunn, J A
EM: jeff@geol.lsu.edu
AF: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803 United States
AU: Harris, N B
EM: nbharris@mines.edu
AF: Department of Geology and Geological Engineering, Colorado School of Mines 1516 Illinois Street, Golden, CO 80401 United States
AB: Barremian and Lower Aptian rift sequences in South Atlantic basins immediately north of the Walvis Ridge / Rio Grande Rise (WR / RGR) provide considerable evidence for deposition from saline lake water. Marine transgressions over the WR / RGR have been proposed as the origin for saline conditions for these deposits, yet no paleontological evidence exists for marine incursions into the lakes. We test an alternate hypothesis, that subsurface seepage of seawater through the WR / RGR was a significant source of water, influencing both lake level and water composition. Our model permits flux of water and salt into and out of the rift lake via: 1) seepage through the WR / RGR; 2) evaporation from the lake surface; 3) rainfall; and 4) incorporation into rift sediments. We test a range of parameters consistent with modern marine basalts and rift basins and inferred conditions from the West African rifts. Modeling results indicate that flux of seawater through the Walvis Ridge to the rift lake ranges from constituting the dominant component of lake water to a small fraction for reasonable values of basalt permeability and climate. The flux of salt is significant over the full range of modeled permeabilities, producing salinities ranging from 50 percent of seawater to nearly saturated brines within 5 my. This demonstrates that seepage through the Walvis Ridge can account for lithologic and organic geochemical evidence for saline lake water and that it is not necessary to call on marine transgressions across the ridge to explain the apparent high salinities.
DE: 1830 Groundwater/surface water interaction
DE: 1833 Hydroclimatology
DE: 1836 Hydrological cycles and budgets (1218, 1655)
DE: 1876 Water budgets
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005