Paper No. 12-11
Presentation Time: 4:50 PM
LOWER SILURIAN STRATIGRAPHIC SECTION OF JOHN BRYAN STATE PARK, GREENE COUNTY, OHIO, AS A PROXY FOR THE PROBABLE INCOMPLETELY RECOGNIZED COMPLEXITY OF THE LOWER SILURIAN (LLANDOVERY) OF THE NORTH AMERICAN MIDWESTERN BASINS AND ARCHES REGION BASED ON CONODONT BIOSTRATIGRAPHY AND δ13CCARB CHEMOSTRATIGRAPHY
The Lower Silurian (Llandovery) stratigraphic section at John Bryan State Park (JBSP), Ohio, is comprised of the Brassfield Formation, Dayton Formation and lower part of the Osgood Formation. Cemex Quarry and JBSP, less than 11 km away, serve as revised type localities for the revised Dayton Formation (Brett et al. 2012) . Results from ongoing conodont biostratigraphical and δ13Ccarb chemostratigraphical study of the Llandovery section at JBSP provide evidence that the Brassfield is comprised of two separate units, the “red Brassfield,” and an unnamed limestone unit above. The former contains elements of Pranognathus tenuis, whereas the latter contains those of Ozarkodina aldridgei. δ13Ccarb records for the two units also differ, with values slightly, but consistently lower in the unnamed unit. The unnamed unit possibly correlates with the Lee Creek Formation and/or Waco Member of the Noland Formation, and is lithologically comparable with strata originally designated by Orton (1870) as the Dayton stone, a limestone immediately overlying the Clinton formation (now recognized as Brassfield) in Montgomery County, Ohio. The Dayton Formation at JBSP is a dolostone and disconformably overlies the unnamed unit. There is also evidence that the Dayton is possibly comprised of two separate units, a lower unit about 1.8 m thick and an upper unit of similar thickness. The lower unit contains few fossils, rarely yields any conodonts and records δ13Ccarb values rising slightly from the base and then remaining consistent to the top of the unit. A δ13Ccarb value (+2.6‰) from near the base of the upper unit is considerably higher than those in the lower unit, and a sample near the base of the upper unit contains elements of D. staurognathoides and possibly O. polinclinata. δ13Ccarb values in samples from above the base of the upper unit are lower, but remarkably consistent, at or just below +2.0‰. Elements of O. p. polinclinata and Pterospathodus am. amorphognathoides occur in samples from levels above the upper unit base. Lack of conodonts and any distinct δ13Ccarb excursion makes it difficult to correlate the lower unit of the Dayton, even with the revised Dayton elsewhere in its revised type area. The upper unit of the Dayton and a sample from the base of the overlying Osgood Formation are both within the Pt. am. amorphognathoides Zonal Group.