GEOLOGIC EVIDENCE OF TECTONIC FOLDING IN BEDROCK FORMATIONS ALONG THE BLACK RIVER IN ELYRIA AND LORAIN, OHIO

The low-relief, partially exposed anticlines along the Black River in Lorain and Elyria in northern Ohio display tectonic folding evidence within bedrock units. Ohio Shale and Bedford Shale are exposed on the banks of Black River in Lorain and southward toward Elyria. Inclined flanks of at least four low-relief anticlines with about 20 m amplitude and 300 m wavelength are present in Ohio Shale. Berea Sandstone covers upland areas flanking the valley in Cascade and Elywood Parks in Elyria. Stratigraphy of the area was studied by Edwin Wenberg in 1938, who also reported Ohio Shale anticlines along the Black River. In 1988, Judy Majoras and Neil Wells interpreted Cascade Park bedrock geometric deformations as soft-sediment deformation and diapirism of the Bedford Shale.

Microtectonic evidence at the base and within the Berea Sandstone in Cascade and Elywood Parks shows that the Berea Sandstone and underlying Bedford and Ohio Shales were folded as low-relief anticlines and synclines. The folded Ohio Shale layers are exposed at the core or flanks of the anticlines, and Bedford Shale layers are parallel to these Ohio Shale layers. In places, Bedford Shale thickness has increased as a result of local internal deformation. A low-relief, NE–SW-trending syncline in the Berea Sandstone between two falls of Cascade Park shows that the Berea layers were affected by compressional folding. The geometry of the folded Berea layers differs from the geometry of the Ohio Shale folds. The incompetent Bedford Shale has acted as a decollement zone to accommodate the folding process of the layers. Small-scale reverse faults with obvious stepping fault surface, and slickensides at the base of an eroded Berea syncline indicates that it was formed by compressional tectonic stress. The upward asymmetric drag folds in the thin sandstone beds between thicker Berea layers on the opposite flank of this syncline reveals that the Berea layers of the syncline were pushed up as a pop-up structure to accommodate the room problem of the folded layers. The overall orientation of the inclined layers of Ohio and Bedford Shales, the NE–SW trend of the syncline between two falls, orientation of the drag folds axis, and sense of motion on thrust faults under the Berea Sandstone syncline shows that the folding was the result of a northwest-directed compressional stress.