Abstract: Deep-Water Sedimentation and Tectonics in Middle Ordovician Sevier Shale, Eastern Tennessee

Ganapathy Shanmugam, Kenneth R. Walker

The Middle Ordovician Sevier Shale exhibits a complex mosaic of carbonate and terrigenous clastic rocks in east Tennessee. This formation comprises one of the thickest sedimentary sequences in the southern Appalachians (up to 2,000 m). The present investigation, based on bed-by-bed analysis of nearly 9,000 beds, and study of minor sedimentary features observable only in thin sections, acetate peels, and polished slabs, allows the construction of a paleoenvironmental model for the Sevier Shale. Environmentally sensitive minor sedimentary features recognizable in the calcisiltite rock types are (1) vertical accretion laminations; (2) microturbidite sequences with sharp bases, positive grading, and Bouma divisions of ^Tcde and ^Tde; (3) convolute laminati ns; (4) sinuous laminations ranging up to 4 cm thick; (5) pseudonodular zones ranging up to 4 cm thick; and (6) maximum grain size rarely exceeding 3^THgr. A deep-water basin is proposed for the Sevier Shale based on its lateral stratigraphic relation with the adjoining shelf carbonate rocks, complete absence of shallow-water fauna and algal activity, and its stratigraphic thickness of nearly 2,000 m. From primary sedimentary structures, dissipating distal turbidity currents are envisaged as probable mechanism of deposition. Analysis of columnar profiles reveals the abrupt change from shallow-water carbonate rocks at the base through the deep-water shale in the middle to the shallow-water clastic and carbonate rocks at the top. This vertically changing environmental pattern is interpreted as sudden tectonic downwarp of the basal stable shelf, and subsequent basin filling with carbonate and clastic deposits. Maximum depth of water was probably hundreds of meters.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC