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III. AFFECTED ENVIRONMENT III.A. Affected Environment Gulf of Mexico III. AFFECTED ENVIRONMENT A. Gulf of Mexico 1. Geology The marine geology of the Gulf of Mexico region includes continental shelf, slope, borderlands (transitional continental to oceanic crust), and the abyssal plain areas. Detailed geologic reports of these planning areas are in U.S. Geological Survey (USGS) (1981), Jackson and Galloway (1984), Martin (1978), Gross (1993), and Geological Society of America (GSA) (2002). This section describes the geologic features and processes associated with seafloor instabilities. Seafloor instabilities can result in damage to offshore infrastructure that could result in environmental impacts. For information on general petroleum geology, refer to The Resource Evaluation Program Structure and Mission on the Outer Continental Shelf (Dellagiarino and Meekins, 1998). For additional information on the geologic and petroleum geology of the different Outer Continental Shelf (OCS) planning areas in the Gulf of Mexico Region, refer to Minerals Management Service (MMS) (2002a, 2003c). The MMS Environmental Studies Program has conducted studies in areas where more detailed geologic information was needed for management of the OCS minerals leasing program. These studies have provided assessments of operational constraints to oil and gas exploration and production. The data and mapped information are being used on a daily basis for tract evaluation, lease stipulation development and application, protection of sensitive areas such as the Flower Garden Banks, and reviewing exploration, development, pipeline, and structure decommissioning applications. Seafloor stability and movements are often influenced by oceanographic processes acting either at the sea surface through ocean-atmosphere interactions that occur during extreme weather events, such as hurricanes and frontal cyclones, or at depth in association with currents involved in the circulation of the Gulf of Mexico, such as the Loop Current and associated eddies. The sections that follow on meteorology and physical oceanography provide more information on the interactions between the atmosphere-ocean system and geologic hazards. a. Physiography The Gulf of Mexico is composed of two broad physiographic regional provinces: the continental margin and the ocean-basin floor. The continental margin includes two sub-provinces; the continental shelf and continental slope. The continental shelf is the submarine extension of the coastal plain deposits from the shoreline to about the 200-meter (m) water depth and is characterized by a gentle slope of a few meters per kilometer (less than one degree). In the eastern part of the Gulf, the adjacent continental slope extends from the shelf edge to the Sigsbee and Florida Escarpments, in about 2,000- to 3,000-m water depths. In the northwestern Gulf, the continental slope consists of two parts: the gently sloping upper slope with its characteristic hummocky topography due to diapiric salt structures, and the relatively steep topography of the lower slope. The transitional zone between the continental shelf and slope is an area of potential geologic hazards due to differences in seafloor stability (MMS, 2002a; USGS, 1981). The deep ocean-basin floor includes the continental rise and the abyssal plain provinces. The continental rise is a gently sloping depositional feature that extends from the base of the continental slope to the abyssal plain. The sediments comprising the continental slope were transported from the continents by bottom currents, gravitational creep, and turbid flow down submarine canyons. That III-1 III.A. Affected Environment Gulf of Mexico abyssal plain is an essentially flat-lying sequence of thick sediments deposited in a deep-ocean environment where water depths are more than 3,350 m (MMS, 2002a; USGS, 1981). b. General Environmental Geology and Geologic Hazards Within the Gulf of Mexico Region, naturally-occurring processes and other surface and subsurface geologic features could cause seafloor instabilities that may become major geologic hazards to oil and gas development. Thick sequences of sediments have been deposited on the continental margin and ocean basin via bottom currents, gravitational creep, slumps, and turbidity or debris flows. Within the Gulf of Mexico, these naturally-occurring processes may become major geologic hazards to oil and gas development. Seafloor instability is considered the principle engineering constraint to the emplacement of bottom-founded structures, including pipelines, drilling rigs, and production platforms. A description of the major geologic processes that could result in seafloor instability follows. Bed Form Migration: Several types of bed forms are present in the Gulf of Mexico ranging from giant bed forms to small-scale ripples and sand waves. Bed form migration is of particular significance in the eastern Gulf of Mexico. The structural integrity of offshore facilities could be affected by the movement of these bed forms. The size, shape, and orientation of bed forms are affected by the oceanographic processes that form them. Megafurrows: Recent surveys conducted by Texas A&M University on the lower continental slope of the Gulf of Mexico have confirmed that deepwater processes have produced megafurrows parallel to the bathymetric contour lines southward of the Sigsbee Escarpment. These large bed forms, 20-30 m wide and as deep as 10 m, occur along the base of the Sigsbee Escarpment and extend to a distance of 20 kilometers (km) south of the escarpment. These megafurrows suggest swift bottom currents in water depths of over 3,000 m along the base of the escarpment (Bryant and Liu, 2000). Shallow Waterflow: Shallow waterflow, also known as geopressured sands, is the uncontrolled flow of sand and water that can create sediment accumulation at the wellhead. This process is the result of compaction, disequilibrium, or differential compaction and usually occurs at 360-530 m below the seafloor. It is more likely to occur on the upper and middle slope and less likely to occur above the salt nappe, the tabular salt blocking the escape of overpressures from below (MMS, 2000). Slope Stability: Two factors control the near-surface submarine slope stability of the continental margins offshore from Texas and Louisiana: (1) interplay between episodes of rapid shelf edge progradation and contemporaneous modification of the depositional sequence by diapirism, and (2) mass movement processes. Sediment instability is more likely to occur on the continental slope because of the relatively steep gradients. In addition, rapid deposition of sediment at the shelf edge, cyclic loading of storm waves, faulting, and vertical migration of shallow salt create further instabilities. These local high rates of deposition of unconsolidated sediments on the continental shelf edge form unstable slopes that lead to intensive soil movements such as slumping, gravitational creep, turbidity or debris flows, and mudslides. Many slope sediments have been uplifted, folded, fractured, and faulted by diapiric action creating oversteepened slopes that also lead to slope failures. Some intraslope basins contain fill sequences of repeated and stacked chaotic units that are interpreted as the products of massive failures. These deposits likely originated at or near the shelf edge during periods of lowered sea level and failed during the sediment loading process. Oversteepening on the basin flanks and resulting mass movements have created highly overconsolidated sediments underlying extremely weak pelagic sediments (MMS, 2000). III-2 III.A. Affected Environment Gulf of Mexico A wide range of failure features results from the mass movements of sediment, from massive shelf edge evacuation features (Winker and Edwards, 1983 as cited in Louisiana State University, Coastal Marine Institute [LSU CMI], 2001) to small-scale slumps along fault faces and on the sides of diapirs. Depending on scale, massive volumes of sediment can be transported downslope in association with subaqueous mass movement processes (Coleman et al. 1986 as cited in LSU CMI, 2001). The principal types of sediment mass movements that occur along the Gulf of Mexico shelf are surface mudflows and slumps. Surface mudflows most typically develop in the Mississippi Delta area off the mouths of the principal passes that empty into the Gulf. Downslope advance of these features is characterized by glacierlike flow of soft soil over the seafloor that may be either continuous or intermittent. These features are usually less than 15 m thick and form noses or scarps at their leading edges up to 15 m in height. Slumps typically occur within the upper 15 m of sediment and subparallel to the bathymetric contours. They are characterized stairstep-like faulting that decrease in steepness with depth (USGS, 1981). The upper continental slope is subdivided into several areas based on the various types of seafloor and slope instability (MMS, 1989a, b; 1990a, b; and 1991a). These areas are depicted and described in Figure III-1. Active Faulting: Faulting occurs on many scales along the continental slope, from major growth faults that cut across thousands of meters of sedimentary section to much smaller faults related primarily to salt movement in the shallow subsurface (LSU CMI, 2001). Faults are most common (1) in areas of rapid deposition, such as the Mississippi Delta, (2) in areas that are rapidly subsiding due to withdrawal of formation fluids such as water and oil, (3)

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