DEPOSITIONAL ENVIRONMENTS AND PALEOECOLOGY OF THE OIL CREEK FORMATION (MIDDLE ORDOVICIAN), ARBUCKLE MOUNTAINS AND CRINER HILLS, OKLAHOMA

The Oil Creek Formation (Whiterockian) is the second oldest of the five formations which make up the Simpson Group. Although widespread in the subsurface, it is exposed only in the study area and possibly in West Texas. The formation was deposited in a linear basin known as the Southern Oklahoma Aulacogen during a phase of apparent episodic subsidence. Deposition occurred in supratidal to shallow marine environments which were subjected to occasional tropical storms.;The transgressive phase of deposition is represented by the vertical sequence consisting of the Basal Sandstone Unit, Upper Offshore Facies, Transition Zone 1, and Lower Offshore Facies. The Basal Sandstone Unit is a massive, supermature quartzarenite, apparently deposited in the foreshore and shoreface of a broad, gently sloping beach. The overlying quartzarenite-rich Upper Offshore Facies contains a restricted fauna adapted to an unstable, sandy substrate. This facies grades through Transition Zone 1 to the Lower Offshore Facies, which is relatively sand free and contains a diverse fauna including bryozoans and echinoderms.;The regressive phase includes Transition Zone 2, the Shoal Facies, Lagoon Facies, and Tidal Flat Facies. The Shoal Facies has relatively thick limestone beds composed of skeletal debris, quartz sand, and goethitic ooids, while the Lagoon Facies is dominated by terrigenous shale. The Tidal Flat Facies is characterized by coated grains, oolites, pelsparite, and "birdseye limestone", and contains a restricted fauna dominated by ostracodes and gastropods.;The species composition and number in the different communities seem to have been largely determined by the nature of the substrate. Relatively stable, carbonate substrates supported diverse communities of suspension feeders dominated by bryozoans and echinoderms. Echinoderms include the new genera "Mandalacystis" (Eocrinoidea) and "Stellarocystis" (Diploporita).;I define six depositional facies within the shaly upper member of the formation, previously known as the "Oil Creek shale". These facies are based on the lithology and fossil content of the quartzarenite and limestone beds because the alternating shale beds are monotonous and essentially lack skeletal fossils. This facies tract shows that, as in other formations of the Simpson Group, deposition apparently took place in one major cycle of transgression and regression.