Orca Basin, Anoxic Depression on the Continental Slope, Northwest Gulf of Mexico
Published:January 01, 1978
P.K. Trabant, B.J. Presley, 1978. "Orca Basin, Anoxic Depression on the Continental Slope, Northwest Gulf of Mexico", Framework, Facies, and Oil-Trapping Characteristics of the Upper Continental Margin, Arnold H. Bouma, George T. Moore, James M. Coleman
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The Orca basin is an intraslope depression on the continental slope, northwestern Gulf of Mexico, which contains an uncommon anoxic, saline brine. The 400-sq-km basin slopes at angles ranging between 3 and 14° from surrounding water depths of 1,700 to 1,900 m, to over 2,400 m. A 700-1 sparker seismic survey shows conformably dipping strata indicative of post depositional movement, as well as chaotic zones beneath the basin seafloor. The basin appears to be the result of salt tectonism. Sediments from the deep parts of the basin consist of dark anoxic clays with interstitial salinities of 250 ppt. Brine fills the bottom 200 m and provides a seismic-interface reflector at its surface similar to that of the Red Sea. Oxygen values are reduced to zero within the 260-g/kg saline brine and temperatures are slightly greater than the directly overlying bottom water. The bulk chemical composition of the brine is similar to that from the Red Sea whereas differences between the two in both heat content and geologic setting indicate different modes of origin.
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Framework, Facies, and Oil-Trapping Characteristics of the Upper Continental Margin
The Gulf of Mexico covers an area of more than 1,500,000sq km, has a maximum depth of about 3,700m, and includes many of the geomorphic features of large oceans.The continental shelf, slope, rise, and abyssal plain comprise the major physiographic provinces of the guldf and contain avariety of subprovinces distinguished by topographic character and geomorphic history.
The gulf shelf is a relatively smooth, gently sloping surface marked locally bylow-relief featuresformed by sea-level fluctuation during the Pleistocene, reef growth, near-surface movement of diapiric salt and mud, and faulting. Shelf width varies from about 280km off the Florida and Yucatan Peninsulas to less than 10km at the Mississippi Delta. The continental slope consists of a considerable variety of physiographic subprovinces and individual features that encircle the deep gulf floor.
The distinctive subprovinces of the gulf slope have evolved in response to reef building and constructional sedimentation on the Florida and Yucatan carbonate platforms; erosion, nondeposition, slumping, and fault ing in the Straits of Florida and Yucatan Channel; salt diapirism and differential sedimentation in the region off Texas and Louisiana; the largeaccumulation of mainly Pleistocene sediment on a former continental slope seaward of the Mississippi Delta; tectonic uplift and diapirism in theGolfo de Campeche; and shale mobilization of feastern Mexico. In contrast to the greatly varied, irregular topography of the continental slope,thedeep seafloor of the gulf (composed of continental rise and abyssal plainprovinces) is an almost featureless plain smoothed by turbidite and pelagic sedimentation and marked locally bylow-relief knolls, sedimentary aprons, and small-leveed channels.