THE EARLY MISSISSIPPIAN SYCAMORE FORMATION IN THE ARBUCKLE MOUNTAINS, SOUTHERN OKLAHOMA

The early Mississippian Sycamore Formation records the beginning of crustal mobility that marked the closure of the Laurentian and Gondwana Plates. A tectonic framework, involving localized basin and uplift, developed form the beginning of Sycamore times. The Formation, which is 250-350 ft thick, ranges in age from Kinderhookian to earliest Meramecian. It conformably overlies the upper Devonian Woodford Formation and unconformably underlies the Mississippian Caney Shales. A lower “Transition” member is composed of grey shales and argillaceous cherty limestones; an upper member consists of grey shales and tan-weathering marlstones containing up to 50% silt-sized siliciclastic grains. The shales record slow deposition in a “deep” water setting. Individual marlstone beds display a sharp or erosive base and a massive basal layer from a few inches to four feet thick. Other features include, in the uppermost part of some beds, horizontal lamination, low amplitude ripple marks and horizontal burrows. Thin lags of fossil hash occur at the base of some beds. These beds are interpreted as turbidites that reworked fine-grained distal shelf material into a hierarchical arrangement of fan packages. The near absence of typical Bouma profiles is a reflection of the fine-grained nature of the available sediment. Current movement was to the west. Well-exposed sections on I 35, located 7 miles apart on the limbs of the Arbuckle anticline, display similar, but not identical sequences. The northern section displays features that suggest it was located closer to source than the southern; this relationship can be extended to the “Transition” member where a shelf carbonate - the Welden Limestone - is represented to the south by “deep” water bedded chert and calcareous shale.