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The eastern Gulf of Mexico is the location of a major and dramatic transition from siliciclastic to carbonate deposits and deposition. The massive lutites that make up the Mississippi Cone are juxtaposed against the equally massive carbonates intercalated with evaporites that make up the western portion of the Florida Platform. The region now occupied by the continental margin of the eastern Gulf has been cut off from clastic sedimentation since Late Jurassic or Early Cretaceous time, and over 5,000 m (16,404 ft) of predominantly carbonate sediments have built up.
Surficial sediments of the carbonate portion of the transition zone are called the West Florida Lime Mud Facies. A foraminifera-coccolith ooze is still being deposited in water depths as shallow as 200 m (656 ft); this facies covers over 40,000 km2 (15,444 mi2) and in many respects resembles a deep-sea ooze. However, in some places shelf carbonate debris, including whole valves of mid-shelf oysters, have cascaded over the shelf-slope break and are incorporated into the slope ooze. Composition, rates of accumulation, and relatively shallow deposition suggest that this sedimentary body may be an analog of some of the chalk deposits of northwestern Europe.
It has long been considered axiomatic that planktonic foraminifera and coccolith oozes are deep-sea deposits that accumulate at rates of a few tens of millimeters per thousand years. However, radiocarbon dates show that the eastern Gulf margin ooze has accumulated rapidly. Prior to the last rise in sea level, deposition rates were as high as 65 cm/1,000 years (25 in./1,000 years); and ever since sea level began to rise sedimentation rates have still been as high as 20 cm/1,000 years (8 in./1,000 years). Such a rapid buildup of the carbonate sediments has led to widespread mass wasting and all forms, from creep to massive multiple slide deposits containing thousands of cubic kilometers of material, are represented in our set of high resolution seismic profiles.
The actual contact between carbonates and siliciclastics occurs in a variety of styles. Carbonate rubble is found in a terrigenous lutite matrix at the base of the West Florida Escarpment. Farther west on the Mississippi Cone the brown and yellow-brown siliceous lutites contain carbonate turbidites with provenance the West Florida slope. Another type of contact is shown in some piston cores with carbonate ooze in normal and alternating depositional
contact with Mississippi siliceous lutite.
The position and timing of alternating carbonate and fine terrigenous deposition, as well as the loci of carbonate buildups on the slope, must be affected and perhaps even controlled by the positioning of the Loop Current, a major precursor to the Gulf Stream which, depending upon factors as yet not totally understood, irregularly advances into or retreats from the eastern Gulf. When the loop is "up" and sweeps from north to south along the West Florida slope, it blocks or deflects Mississippi sedimentation. When the loop is restricted to the area of the Florida Straits, fines derived from the Mississippi River can be deposited at the base of the escarpment and even up on the slope.
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