Mass transport deposits (MTDs) are common features that help us reconstruct the depositional environment in deepwater basins. Unlike turbidite sands that form in similar environments, MTDs only rarely form hydrocarbon reservoirs. Near the water bottom, recent MTDs can indicate the risk of future hazards to submarine platform legs, drill stems, pipelines, and communication cables. MTDs commonly exhibit an overall chaotic seismic pattern; several other associated features help to differentiate MTDs from other kinds of deposits in deep water depositional environments. MTDs have similar characteristics in intraslope basins (also called salt minibasins) but vary as a function of restricted transport direction for sediment input, limited accommodation space, and syndepositional salt movement. By coupling principles of geomorphology with seismic attributes and a depositional model, we analyze the characteristics of an MTD within an offshore Gulf of Mexico study area to determine how it differs from other deepwater architectural elements.
Weimer and Slatt (2006) consider mass transport deposits (MTDs) as one of the four main architectural elements of deep water environments. Mass transport deposits generally form due to slope failure or slumping from the shelf-slope area when sea level falls rapidly, exposing the shelf-slope area and changing sediment pore pressure. For intraslope basins, MTDs often "develop from the failure of delta front or canyon walls, have extensive erosion at their base, and overlying sediment fill" (Weimer and Slatt, 2006) Although mass transport deposits (also called mass transport complexes, or MTCs) are not prolific reservoirs, these deposits are still of great interest to industry, government safety departments, and academics. Shallow mass transport deposits are common drilling hazards due to their complex internal structure and the potential to contain local gas pockets. In some basins, including the Gulf of Mexico, individual depositional sequences may consist of more than 50% slides and/or deformed sediments (Weimer and Slatt., 2006). For this reason, MTDs are very important in setting up the sequence stratigraphic framework in a basin or minibasin. In most cases MTDs are deposited at the top of sequence boundary at early Low Stand System Tract (LST), sometimes eroding sediments at their base. In some instances, remobilized massive sands can form MTD reservoirs. Finally, the displaced water associated with the formation of MTDs is second only to undersea earthquakes in the initiation of destructive tsunamis.
Our study focuses on an MTD located within the tabular salt minibasin tectono-stratigraphic province- which covers a large area of the continental slope along the northern Gulf of Mexico margin. Salt constrains the minibasin on the eastern, western and southern sides; the main sediment fairway is from the northern side. There is a broad variability in terms of the final geometric configuration of the minibasin, depending upon the interaction of the continuously-deposited sediment load on top of the allochthonous salt, giving rise to temporally-varying lateral changes in subsea topography. Due to the constrained sediment fairway and limited accommodation space, the geomorphology of deep water deposits differ from those in open marine conditions.
