Abstract

The shallow subsurface of the upper slope, northeastern Gulf of Mexico, shows abrupt changes in depositional environments, sequence stratigraphy, and sedimentation rates reflecting Pleistocene to Holocene glacioeustasy. Stratigraphic change is recorded in lithologic, sedimentologic, biostratigraphic, and oxygen isotopic data from a nearly continuous 245.5 m core from Viosca Knoll Block 774, a gamma- ray log of the borehole, and high-resolution seismic-reflection data. Mud with minor sand at the base of the boring (245.5 to approximately 136 m) was deposited during several sea-level cycles corresponding to isotope stages 13 to 8, and includes a thick, probable stage 10 unit. A thick, coarsening-upward deltaic mud and sand at approximately 136-59.9 m likely was deposited during oxygen isotope stage 8 glaciation. The section upward from approximately 59.9 m contains; a deltaic silt and sand probably deposited during stage 8; a transgressive sand shoal recording the stage 8-7 transition; highstand mud and sand of stage 7; a relatively thin stage 6 lowstand mud and sand; stage 5 highstand mud and sand; stage 4-2 mud and sand; and Holocene sandy silt. Almost all contacts between systems tracts are transitional in core. Inferred lowstand deposits are lithologically and sedimentologically variable, reflecting degree of delta progradation and subsequent erosion, and consist of sand and (or) mud. Thicker, coarser intervals at the site represent shelf-margin deltas likely deposited during falling sea level. Falling sea level and lowstand sediment is characterized by mostly middle to inner neritic benthic foraminifera, cool-temperature planktonic foraminifera, and high oxygen isotopic values of the planktonic foraminifera Globigerinoides ruber. Transgressive and highstand systems tracts are thin. Rising-sea-level sections and highstand condensed intervals consist of clay or silt with carbonate concretions and with numerous pyritized microfaunal tests and small burrows. Condensed sections also contain abundant microfauna, mostly warm-temperature planktonic foraminifera, outer neritic to upper bathyal benthic foraminifera, and transitional to interglacial delta 18 O values of Globigerinoides ruber. Some sandy turbidites appear to be interbedded with highstand mud.

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