RECONSTRUCTING PALEOCURRENTS OF THE CHADOKOIN AND GIRARD FORMATIONS IN NORTHWESTERN PENNSYLVANIA

Extensive exposures of late Devonian Chadakoin and Girard Formation (Conneaut Group) are exposed in Fourmile Creek, Erie County, Pennsylvania. Both formations consist of medium-gray shale and interbedded with 5–20 cm siltstone lenses. The relatively low energy shallow marine environment of the Chadakoin formation and deep marine environment of the Girard Formation allowed for the creation and preservation of ripple marks. Ripple marks are transverse sinuous in phase and out of phase ripple crests with heights ranging from <0.5 cm to 2.25 cm and wavelengths from 7–24 cm. Ripple marks were documented throughout Fourmile Creek and mapped stratigraphically to reconstruct paleocurrent direction through time. Using rose diagrams, paleocurrent directions and changes in time were identified. In the transition from the Girard Formation to the Chadakoin Formation, asymetrical ripple crests record a shift in flow direction from the northeast to the northwest with mean flow direction at N33°W. The shift in paleocurrent direction is hypothesized to be due to a change in drainage outlet due to a drop in sea level. Asymetric ripples are interpreted as wave formed oscillation ripples due to unequal oscillatory movement of tranquil water due to passing breakers. Symmetrical ripples are interpreted as equal oscillation ripples due to storm generated wave action.