Abstract: Deep-Water Sands of Gulf of Mexico: Results of DSDP Leg 96

By

James M. Coleman, A. H. Bouma, and Leg 96 Scientific Party

Sedimentologic, paleontologic, geochemical, and geotechnical studies were conducted on cores drilled at nine sites (Table 1) on the Mississippi Fan during Deep Sea Drilling Project Leg 96. Together with seismic and well log data, these studies enable us to develop a number of depositional facies within an overall fan lobe model. The central middle fan channel of the youngest Mississippi fan lobs was an effective conduit for the transport of coarse-grained material; only days and minor amounts of silt spilled over the channel margins. The channel-fill deposit is basically a fining-upward sequence, commencing with coarse-grained sand and gravels, which are overlain by sands, sandy-silty muds, and muds. The basal coarse-grained sediment interval is approximately 134 m thick. The swale deposits and underlying overbank deposits adjacent to the meandering channel, as well as the marginal overbank deposits, are characterized by fine-grained turbidites and hemipelagics. Both sites, basically, contain a minor coarsening-upward sequence.

The lower fan in the area, where the channel shifts position frequently, shows alternating sequences of channel-fill, levee, and overbank deposits. Sediments near the channel terminus have coarsening-upward sequences (i.e., channel-mouth depositional lobes) and contain 47% to 86% net sand.

Most of the sparse microfauna in both sands and muds are benthic species characteristic of inner and middle neritic origin. Traces of biogenic methane and hydrocarbon were found in underlying lobes, but not in the youngest lobe. All sediments are underconsolidated as a result of the extremely high accumulation rates of 2.0 to 12.0 m/1000 yr.

Table 1. Leg 96 drilling sites.

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