VARIATIONS IN PORE WATER SALINITIES FROM THE TOP OF ALLOCHTHONOUS SALT TO THE SEAFLOOR IN SOME OFFSHORE GULF OF MEXICO SEDIMENTS - IMPLICATIONS FOR MECHANISMS OF SOLUTE TRANSPORT

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 18 drill holes which penetrated allochthonous salt bodies in 12 widely-scattered protraction blocks on the Louisiana offshore continental shelf and slope. Salinities were calculated from LAS-LWD logs using the Revil et al. dual-conductivity technique. The sites represent a wide range of field settings. Water depths range from 30 to 1990 m, depths to the top of salt from the seafloor range from 440 to 5200 m, and salt thicknesses range from 70 to 2000 m. Sediment ages above and below salt and approximate sedimentation rates were determined from BOEMRE paleo data. Sediment ages at most sites range from Miocene to Recent. Sites in 8 blocks have concave downward salinity profiles above salt, consistent with transient upward molecular diffusive transport with possible compaction-driven flow. The salinity profiles were modeled using two scenarios: 1) salt was emplaced at its present depth, and molecular diffusion upward from the top of salt has been the sole solute transport mechanism; 2) salt was emplaced near the seafloor, and there has been progressive burial of salt with compaction-driven advective flow upward since (Basin2 modeling). The time required by molecular diffusion alone tends to exceed the age of the sediments. A better fit in space and time is obtained on the assumption that sediment compaction is also a driving force. Salinities in two blocks increase upward, possibly reflecting lateral flow of brines from shallower salt. Salinities in the two remaining blocks are elevated but constant for distances of 500 to 1000 m above salt and may reflect ponding of brines derived from shallower salt. At sites where there is subsalt information, the salinity profiles are generally convex upward, reflecting probable diffusional transport downward. Our results are consistent with an earlier LSU study which concluded that dominant mechanisms of vertical solute transport in deepwater GOM sediments include diffusion and compaction-driven advection. Superimposed on this regional solute transport regime is the more localized expulsion of brines.