--> Submarine Landsliding into the Orca Basin Brine Pool, Walker Ridge, Gulf of Mexico

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Submarine Landsliding into the Orca Basin Brine Pool, Walker Ridge, Gulf of Mexico

Abstract

Submarine landslides are capable of causing tsunami and can impact seafloor infrastructure. Deposits of submarine landslides, Mass Transport Deposits (MTDs), are a significant component of deepwater basins and thus play an important role as reservoirs and/or seal elements of petroleum systems at depth as well as in near-seafloor gas hydrate systems. Here we integrate a dataset of 3D seismic, well logs, and cores, to interpret the recent history of submarine landsliding in the Orca Basin in the northern Gulf of Mexico. The seafloor in the Orca Basin features an anoxic hypersaline brine pool with salinity a factor of eight higher than normal seawater salinity. Outcropping salt is actively dissolving to feed the brine lake today. Lying at the bottom of the pool are deposits of submarine landslides that have periodically failed along the margins in the recent geologic past and have accumulated to approximately 450 meters (1475 feet) in thickness. Slope instability is related to the underlying salt tectonics in the basin where large faults can be mapped from the top of the salt directly to the near-seafloor zones. In addition, there is likely the presence of gas hydrates as in this basin as evidenced by a bottom-simulating reflector and elevated resistivity in industry well logs. We map the most prominent landslide scar observable on the seafloor and its correlative MTD that now lies at the bottom of the brine pool 12 km (7.2 miles) away from the headscarp. The headwall is amphitheater-shaped with an average height of 80 meters (262 feet) with only a small volume of rubble remaining near the headwall. The MTD contains 8.7 km3 of material, with most of this volume now at the bottom of the brine pool. The MTD seismic facies is chaotic, with rafted blocks on the seafloor at the top of the MTD. These observations suggest that the landslide may have had sufficiently high mobility to have generated a wave upon impacting the brine pool. Such a wave would have splashed high-salinity water along the basin walls, which in turn would have affected any local chemosynthetic marine communities. Waves were likely contained within the Orca Basin because the spill point in the basin is 139 meters (456 feet) above the brine pool surface. We interpret that very little, if any, brine escaped the basin due to landslide-induced waves. This study highlights the dynamic interplay of salt tectonics, gas hydrates, and submarine landslides in deepwater Gulf of Mexico basins.