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Shallow Water Flows in Gulf of Mexico, Relating High Sedimentation Rates to Proglacial Lake Source and Mass Transport Deposits in Deepwater Sink

Abstract

Since 1983 industry operators in the Gulf of Mexico (GOM) have reported 163 shallow water flow (SWF) events to Bureau of Ocean Energy Management (BOEM) and predecessors. SWFs are caused by imbalanced mud weight and formation pore pressure in the shallow section as drive pipe and surface casing are installed. These hard-to-predict events tend to occur in water between 90 m (300 ft) and 460 m (1500 ft) deep and between 90 m (300 ft) and 1067 m (3500 ft) below mudline (BML). High sedimentation rates and an unstable salt substrate both contribute to differential dewatering and compaction thought to be the cause of most flows. In the GOM there are two hot spots where SWFs have occurred; one in northeast Green Canyon (GC) and the other in southwest Mississippi Canyon (MC). BOEM mapped the Mississippi Fan over all or parts of 29 3D seismic surveys (time domain) and on 2D seismic lines to tie non-contiguous seismic volumes. Fifteen mapped units were rationalized with BOEM’s Pleistocene sequence stratigraphy and chronozones and migrated into GIS for geoprocessing. Roughly half of the SWFs in MC and GC hot spots are Middle Pleistocene age and roughly one quarter Late Pleistocene (BOEM’s PLM and PLU chronozones). Middle Pleistocene SWFs occur in a relatively thin stratigraphic zone approximately 30-45 m (100-150 ft) thick. In GC this zone is ~396-427 m (1300-1400 ft) BML and in MC it is slightly deeper, ~427-457 m (1400-1500 ft) BML. A background sedimentation rate for the Mississippi Fan is ~1 m (3 ft) per 1000 years or 70 km3 (16.8 mi3) per 1000 years. The number and stratigraphic position of SWFs correspond to periods when the GOM experienced very high sedimentation rates favorably timed with breaching of, or outwash from, proglacial lakes. Timing for the breach of Appalachian Lake Tight (PLM) at approximately 700 Ka is favorable to the occurrence of mass sediment movements in the ultra-deepwater sink of the eastern Mississippi Fan. Timing for a drainage conduit into the GOM directly from northern mid-continent Lake Agassiz is favorable for a young population (18-10 Ka) of SWFs deposited as part of the western Mississippi Fan (PLU).