Spatial Variations in Pore Water Salinities Above and Below Allochthonous Salt in Offshore Gulf of Mexico Sediments—Implications for Mechanisms of Solute Transport and Timing of Salt Emplacement
Miles A. McCammon and Jeffrey S. Hanor
Department of Geology and Geophysics, Louisiana State University, E235 Howe-Russell Complex, Baton Rouge, Louisiana 70803
Spatial variations in salinity of pore waters in marine sediments provide useful information on processes and rates of subseafloor fluid flow and solute transport, particularly if there are evaporites in the section. Pore water salinities were determined for drill holes that penetrated allochthonous salt bodies in 12 widely-scattered protraction blocks on the Louisiana offshore continental shelf and slope. Salinities were calculated from LWD (logging while drilling) well logs using a dual-conductivity technique. Sediment ages above and below salt and approximate sedimentation rates were determined from U.S. Bureau of Ocean Energy Management paleotological data. Ages of sediment penetrated at most sites range from Miocene to Recent. Sites in seven blocks have concave- downward salinity profiles directly above salt or above superjacent Miocene sediments apparently emplaced with salt, consistent with transient, upward molecular diffusive transport with possible compaction-driven fluid flow. Solute transport calculations show that molecular diffusion alone is too slow to explain the observed salinity profiles. A better fit in space and time is obtained on the assumption that sediment compaction following deposition was also a driving force. Salinity profiles in the remaining blocks reflect probable lateral flow of brines derived from shallower salt or from seafloor brine pools. At sites where there is subsalt information, some salinity profiles are convex upward, consistent with diffusional transport of dissolved salt downward after salt emplacement. Assuming diffusion above and below salt began immediately after introduction of salt, solute transport calculations may have the potential for helping to constrain interpretation of the timing of salt emplacement.
AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012