Paper No. 140-8
Presentation Time: 10:45 AM
RECONSTRUCTING PALEOCURRENT DIRECTION IN THE CHADOKOIN AND GIRARD FORMATIONS IN NORTHWESTERN PENNSYLVANIA
Extensive exposures of late Devonian Chadakoin and Girard Formation (Conneaut Group) are exposed in Fourmile, Sixmile, Twentymile, Elk, and Walnut Creeks, Erie County, Pennsylvania. Both formations consist of medium-gray shale interbedded with 5–20 cm thick siltstone lenses. The relatively low energy marine environment of these formations allowed for the creation and preservation of ripple marks in the fine sand and silt lenses. Ripple crests are transverse sinuous in phase and out of phase. Average grain size of ripple mark sediment was 0.03 cm. Ripple heights rang from <0.5–3.25 cm (avg=0.95 cm) (std(h)=0.74 cm) and wavelengths from 6–31 cm (avg=12.88 cm) (std(λ)=5) (n=103). Ripple marks were documented throughout Fourmile, Sixmile, Twentymile, Elk, and Walnut Creeks and mapped stratigraphically to reconstruct paleocurrent direction through time. Paleocurrent directions and changes in time were identified using rose diagrams and stratigraphy. During the transition from the Girard Formation to the Chadakoin Formation, asymetrical ripple crests record a shift in current direction from the northeast to the northwest with trend in flow direction at N10°E and S90°W. The shift in paleocurrent direction is hypothesized to be due to a change in dominant current forces of deep water bottom currents flowing parallel to the continental margin of the Catskill Sea (N-NE) to basinal turbidite flows parallel to these currents (W-SW). Prograding continental slope during the transition from the Girard Shale to the Chadakoin Formation is one hypothesis explaining the observed changes in ripple current direction. Future work will consist of mapping ripple marks throughout all of Northwestern Pennsylvania along with using techniques developed by Diem (1985), Immenhauser (2006), and Okoro etal (2011) to reconstruct paleo-water depth based on ripple height, ripple spacing, and grain size.