A STRUCTURAL ANALYSIS OF DEVONIAN-AGED BEDROCK IN FOURMILE CREEK, ERIE, PA

The flat-lying bedrock of Erie, PA is locally punctuated by contractional structures. These structures appear in several of the NW-trending bedrock stream channels in the Erie area and are postulated as associated with movement along cross-strike structural discontinuities (CSDs; Wegweiser et al., 1998; see also, Harper, 1998). In this model (Wegweiser et al., 1998), Phanerozoic reactivation of Neoproterozoic fault systems that cross the structural grain of the Appalachian Mountains resulted in formation of numerous small-scale tectonic structures and likely exerted control on modern stream orientation; movement on the faults may have been horizontal, oblique, and vertical. Here we begin testing the hypothesis that small-scale structures exposed in streams could be the result of movement along CSDs. Specifically, we have examined a section of Four-mile Creek (42⁰07’05”-42⁰07’19”N, 79⁰59’24”-79⁰59’50” W) and have identified five thrust faults (strike and dips of: 000, 45W; 075, 15S; 055, 40S; 330, 30S; 040, 25N; 030, 25N), four anticlines (fold axis orientations: 295, 0; 218, 3; 300, 3; 030, 0), and two sets of broadly orthogonal joints (roughly N-S and E-W). Each thrust fault cuts through the entire exposed section within the stream (typically 2-3 m of exposure) and two faults appear to deform capping Quaternary deposits; in contrast, the anticlines typically only occur within a few shale/siltstone layers (cm’s thick) exposed in the walls of the stream. Within the study area, Four Mile Creek is consistently NW-trending. We identified five thrust faults and four anticlines. Thrust faults strike from 330 to 075 and have dips ranging from 15 to 45 degrees to the east and west. Fold axes within the study area trend between 218 and 030 . Although some variation exists in fault and fold orientation, the underlying NE trend among these structures is seemingly consistent with formation from a left lateral shear sense indicating that the course of Four-mile Creek within this stretch may represent a CSD that experienced left lateral strike slip motion. Fault and fold orientations are generally consistent with joint orientations suggesting the joints accommodated their formation. The timing of formation of these structures is difficult to constrain, but deformed Quaternary deposits suggests at least some recent activation.