TECTONICALLY CONTROLLED PATTERNS OF CONTOUR-CURRENT DEPOSITION, EROSION, AND MASS TRANSPORT ALONG THE SOUTHERN MARGIN OF LAURUSSIA (S.C U.S.A.) DURING EARLY MISSISSIPPIAN TIME

Recognition of contourites in the ancient rock record is challenging because (1) outcrops are often inadequate to reveal seismic-scale stratal features associated with many drift deposits, (2) sedimentological criteria, such as facies, vertical successions, sedimentary structures, etc., are mostly non-unique, and (3) contourites, turbidites, mass transport deposits, and pelagites can be intercalated or mixed. The challenges are amplified for ancient carbonate contourites made up of fine- to coarse-grained cool-water carbonates (crinoids, bryozoans), which can be produced and deposited in a wide range of water depths.

This study addresses synchronous interaction of bottom currents, internal waves(?), and tectonic activity for Lower Mississippian carbonates deposited in a deep outer ramp, along the southern margin of Laurussia in south-central U.S.A. Methods involved field description and graphic logging of outcrops, assembling 3d photogrammetry models, and constructing stratigraphic sections and isopach maps. Bottom current-deposited facies include (1) flaser- to wavy-bedded and partially burrowed crinoid packstone with shale breaks, (2) rhythmically bedded, bioturbated crinoid wackestone with shale breaks, (3) current-rippled crinoid packstone-vc grainstone. Zoophycos and Thalassinoides are common. Mass transport features include submarine slides, slumps, and debrites. Large-scale stratal features formed by bottom currents include low-angle inclined bedsets, lateral thinning, and convergence, onlap, toplap truncation, migrating bedforms and aggradational sediment waves. A regional isopach map of the 0-30 m-thick succession of contourite deposits shows that thickening and thinning trends follow regional basement faults. Mass transport features are also most common near faults.

Results suggest that Late Devonian-Early Mississippian tectonic activity along basement weakness zones created seafloor bathymetric perturbations, which affected contour-current depositional and erosional patterns and triggered slope failure and mass transport deposition. These events may record the initial foreland tectonic, erosional, and depositional signals of the Ouachita orogeny in southern Laurussia.