Conodont Biostratigraphy and Biofacies of Upper Knox/Beekmantown Groups and Overlying Ordovician Rocks in Appalachian Basin, New York to Alabama

John E. Repetski, Anita G. Harris

The thick, widespread, shallow-water, largely carbonate mudrocks of the Knox and Beekmantown Groups and of the immediately superjacent shallow-water carbonates have long defied precise correlation. Lithologic correlation is difficult because of facies changes, repetitive lithic sequences, sedimentologic discontinuities, and structural complexities. Moreover, these rocks are sparsely fossiliferous or contain generally biostratigraphically nondiagnostic macrofossils. These rocks contain hydrocarbons and economically significant stratabound base-metal sulfide deposits; thus their accurate dating and correlation would improve exploration strategies.

Conodont data from about 100 localities spanning the Lower to Middle Ordovician boundary provide a preliminary biostratigraphic and paleogeographic framework for the Appalachian basin. Our data show the following. (1) Deposition is nearly continuous across this boundary within the upper Beekmantown Group in the Champlain Valley. (2) Conodonts from several carbonate slices along the soles of Taconic thrust sheets on the east side of the Hudson Valley indicate the shallow-water platform extended considerably eastward. (3) South of the Champlain Valley, an unconformity separates the Beekmantown Group from younger Ordovician rocks and reaches its greatest magnitude at the northern end of the Great Valley, where latest Middle Ordovician rocks overlie Precambrian rocks. (4) Southwestward, t is unconformity decreases in magnitude. From Reading, Pennsylvania, to the western and northern limit of Ordovician outcrop along the Allegheny Front in Pennsylvania, virtually continuous deposition took place across the Lower to Middle Ordovician boundary that lies as much as 900 ft below the top of the Beekmantown Group. (5) An upper Beekmantown depocenter occurs near the Mason-Dixon Line. (6) In the Valley and Ridge province of Virginia, early Middle Ordovician discontinuities increase in magnitude rapidly southwestward and more gradually southward. (7) Across Tennessee, from the eastern Valley and Ridge to the central basin, an unconformity separates upper Lower Ordovician rocks from increasingly younger lower Middle Ordovician rocks westward. (8) An unconformity of considerable magnitude (Cambrian overlain by Silurian rocks) in the Talladega belt of Alabama gradually decreases northwestward across the Valley and Ridge province and disappears in the subsurface of the Black Warrior basin and Reelfoot rift of westernmost Tennessee.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.