THE CLIFTY-SYLAMORE PROBLEM (MIDDLE-UPPER DEVONIAN), SOUTHERN OZARK REGION, NORTHERN ARKANSAS

The Devonian record of the southern Ozarks in northern Arkansas comprises three discreet, unconformity-bounded units referred to the Penters Chert, Clifty Formation, and Chattanooga Shale (ascending order). Exposures of the Lower Devonian Penters Chert are restricted to the type area in northeastern Arkansas, although the formation is well developed in the subsurface of the northern Arkoma Basin. The Middle Devonian Clifty Formation was described originally as a limestone from a single locality in northern Arkansas (Purdue and Miser, 1916). Continuing studies in both outcrop and the subsurface indicate that the Clifty interval is widely developed as a thin, orthoquartzitic sandstone deposited over a karst surface. Where exposed, the Clifty yields a Middle Devonian brachiopod and conodont assemblage. The Upper Devonian Chattanooga Shale is pervasive across the southern Ozarks of Arkansas and adjacent areas. Its base is marked by the Sylamore Sandstone Member, a thin, phosphatic quartzarenite, commonly with angular chert clasts that rests unconformably on strata as old as Lower Ordovician. The Sylamore produces a conodont assemblage with typical Upper Devonian forms such as Palmatolepis and Ancryodella. Typically, where the Sylamore Member rests on sandstone of the Clifty Formation, the entire sand sequence has been referred to the Sylamore, obscuring a significant part of the geologic history of the Ozarks. We also believe that natural gas production reported from immediately below the Chattanooga Shale in the shallow gas province of northern Arkansas is from the Clifty and not the Sylamore. ¹Allen, D.E., Blaylock, Matthew, Cooper, H.H., Freisenhahn, T.C., Greer, D.W., Johnson, B.J., Imrie, P.C., Moon, D.C., Nance, D.L., Rogers, T.J., Rowland, A.R.; Manger, W.L., instructor.