for the oil and gas as well as a unique morphology of basins and domes in some areas of the slope (Figure 1.1). The GoM was, through its geologic past, an ideal environment for the generation and accumulation of recoverable hydrocarbon resources.

The entire continental margin in the northern GoM continues to be shaped by high sediment loads and the movement of salt within the strata of the margin. The salt tectonics (movement) generates hydrocarbon migration paths from source to reservoir. The Macondo well targeted hydrocarbons trapped in Miocene (~12 mya) sand strata that are bounded by several salt dome features. Frequently, the migration paths lead to the sediment surface (Roberts and Carney, 1997) resulting in the creation of cold seep communities dependent on chemically extreme conditions. Seepage from these conduits results in the natural injection of gas, liquid hydrocarbons, and brines into the deep water of the Gulf.

In addition to the impact of salt migration, typical margin-forming processes like sea-level change, erosion, and currents have also shaped the margin and impacted the creation of submarine canyons. The result is a series of seafloor ridges, minibasins, canyons, and escarpments (Jackson et al., 2010) (Figure 1.1). As topographically complex as the Gulf margin is, however, the largest portion of this system is blanketed with sediments built up from terrestrial runoff and from the remains of pelagic organisms forming a vast soft-bottom habitat.

DELTA ENVIRONMENTS AND NEARSHORE HABITATS

The Mississippi River system has long dominated the geological and biological landscape of the northern GoM. The watershed encompasses 41 percent of the lower 48 United States (~3.2 × 106 km2) surpassed in size only by the Amazon and Zaire rivers (Milliman and Meade, 1983; Meade, 1996). The river’s length and discharge of freshwater and sediment rank it among the world’s top ten rivers. The annual average freshwater discharge of 580 km3 enters the northern GoM through two main distributaries: the Mississippi River delta southeast of the city of New Orleans, Louisiana, and the Atchafalaya River delta ~200 km to the west on the central Louisiana coast (Meade, 1995).

Sediment deposition and accumulation are essential for maintaining the delta, offsetting natural subsidence, and preventing drowning of wetlands. Over tens of thousands of years, the flow of sediment-laden freshwater created a series of delta lobes that prograded (moved seaward), subsided, and switched across the northern Gulf coastal landscape, establishing a deltaic



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