SEAFLOOR BRINE LAKE IMPACTED BY SUBMARINE LANDSLIDING: AN EXAMPLE FROM THE ORCA BASIN, WALKER RIDGE, GULF OF MEXICO CONTINENTAL SLOPE

The seafloor in the Orca Basin features a 13.3 km² anoxic hypersaline brine pool that has been periodically infilled by submarine landslides from the surrounding basin edges. Outcropping salt is actively dissolving to feed the brine lake. Lying at the bottom of the pool are deposits of submarine landslides that have periodically failed along the margins in the recent geologic past. We interpret a portion of this system with three-dimensional seismic data and well logs. We map the most prominent landslide scar observable on the seafloor and its correlative mass transport deposit (MTD) that now lies at the bottom of the brine pool 11.6 km away. The headwall is amphitheater-shaped with an average height of 80 meters and with little rubble remaining near the headwall. The MTD contains 8.7 km³ of material that was deposited between the lower slopes of the basin and the base of the brine pool. The bottom of the brine pool contains an amalgamation of multiple failures that, in some places, have accumulated to 450 m thickness. The MTD seismic facies is chaotic and rafted blocks are observed on the seafloor at the top of the MTD layer in the brine pool. This suggests that the post-failure mobility of the landslide was sufficient to have generated a large wave upon impact. The wave would have sloshed water along the basin walls and potentially out of the confining basin at topographic lows. Local chemosynthetic marine communities could have been affected as they were bathed in the brine, which has been previously measured to be a factor of eight higher than normal seawater salinity. Given the preponderance of submarine landslides in this basin, estimates of the salt dissolution rate and age of the brine pool that are based on the present-day brine pool volume may need to account for brine water loss induced by landsliding. The reason for slope instability in this basin is likely associated with the near-seafloor salt tectonics. Numerous faults are observed in seismic data that connect from the headwall to an underlying salt body. However, salt tectonics are likely not the only triggering or preconditioning factor for slope failures. We also observe indication of gas hydrate in this basin by a bottom-simulating reflector and elevated resistivity in well logs. Thus gas hydrate may also play a role in the slope failures in this basin.