Objectives and summary of key geologic findings
The data collected by the USGS in the deep-water part of the Gulf of Mexico that are presented in this report originated with the goal of mapping the sea floor geology of the entire Gulf of Mexico using the GLORIA long-range sidescan sonar system. This reconnaissance study was followed by more detailed studies of escarpment erosion and deep-sea fan depositional processes. More recently, USGS studies have focused on slope processes and gas hydrates of the northern Gulf of Mexico continental slope.
The regional sea floor mapping was completed in 1982 and 1985 using the GLORIA sidescan sonar system. GLORIA (Geologic LOng-Range Inclined Asdic) is a low frequency sidescan sonar, which operates at 6.5-6.8 kHz, and for most of this survey was set to scan 22.5 km to each side of the tow vehicle (EEZ-Scan85 Scientific Staff, 1987). The purpose of this study was to improve the understanding of the surficial geology of the deep-water part of the Gulf of Mexico basin within the US EEZ (Fig. 1). The regional perspective provided by the GLORIA imagery has been used in several studies. The morphology and erosional processes that have shaped the Florida Escarpment, a carbonate cliff in the eastern Gulf of Mexico drew on the GLORIA imagery along with multibeam bathymetry, high-resolution sidescan imagery, bottom photographs, and samples (Paull and others, 1990, 1991; Twichell and others, 1990, 1991a, 1993). The recent depositional processes on the Mississippi Fan have been summarized by Twichell and others (1991b), Kenyon (1992), Wen and others (1995). Mass-wasting deposits and depositional processes have been mapped and analyzed in the salt-deformation province of the northern Gulf of Mexico based on the GLORIA data as well (Rothwell and others, 1991; McGregor and others, 1993; and Twichell and Delorey (1996).
The reconnaissance GLORIA survey was followed by a more detailed study of depositional processes on the Mississippi Fan. While the GLORIA imagery provides an unprecedented view of this entire deep-sea fan system (Fig. 1), it cannot resolve adequately the fine-scale features on the fan’s surface (Twichell and others, 1992). These features were delineated by employing the SeaMARC 1A system, a deep-towed sidescan-sonar system that operates at 30-kHz (Kosalos and Chayes, 1983). Piston and gravity cores (Fig. 2) provided ground-truth information. Results from this higher-resolution sidescan sonar and coring program included a study of the breaching of a channel levee (Twichell and others, 1996), studies of the deposits at the distal edge of the Mississippi Fan (Nelson and others, 1992; Twichell and others, 1992; Twichell and others, 1995; Bouma and others, 2000), a study of the mechanics of debris flows (Locat and others, 1996; Schwab and others, 1996), and the reworking of the fan surface by bottom currents (Kenyon and others, 2002).
Subsequent studies have addressed recent sedimentary processes of the continental slope of the northern Gulf of Mexico. This section of the continental slope is underlain by salt that has been severely deformed by subsequent sediment loading. The salt has been shaped into diapirs, salt ridges, and salt welds that are separated by small, circular basins commonly referred to as minibasins (Fig. 3). The morphologic expression of the continental slope portion of sediment pathways that linked the shelf-edge delta sources with deep-sea fans on the abyssal plain floor has been completely erased due to salt tectonics (Lee and others, 1996). Seismic data, GLORIA imagery, and short cores have been used to reconstruct the recent history of one of these pathways (Twichell and others, 2000).
The most recent USGS studies have focused on the distribution, character, and geologic setting of gas hydrates. Four studies have been conducted by the USGS that address gas hydrates. Survey approaches have varied between the four studies (Fig. 4). The first field program was conducted in 1998 and used high-resolution multichannel seismic data to image hydrates in areas where hydrates had been encountered during industry drilling (Hart and others, 2002). The second field program was conducted in 1999 and used high-resolution multichannel seismic and some sidescan sonar imagery to study the acoustic nature of the hydrate stability zone (Cooper and Hart, 2003) and whether there is a link between this zone and the overlying sea floor geology (Twichell and Cooper, 2000). The cores collected from the R/V MARION DUFRESNE in 2002 were used to address the physical properties and geochemistry of hydrates and sediment overlying presumed hydrate regions (Winters and others, 2004). The most recent field program was completed in 2003 and consisted of two detailed high-resolution multichannel seismic surveys of areas targeted for a hydrate drilling program (Hart and others, 2005).