SYNSEDIMENTARY CLASTIC DIKES IN THE CONEMAUGH GROUP (PENNSYLVANIAN) OF SE OHIO: FAR-FIELD EFFECTS OF ALLEGHANIAN DEFORMATION

Three beds (2.5 - 4.2 m thick) containing clastic dikes are present in two groups that stretch 4.3 km in a northeast to southwest orientation within the middle of the Conemaugh Group in Athens, Ohio. The two groups of siltstone dikes occur in shales of similar appearance separated by 30 m of section that includes at least five unconformities related to 4th-order relative sea level changes. The abundance and size of dikes increases with the thickness of the marine shale host beds. Various types of small-scale deformation in the lower group, such as ptygmatic folding in vertical thin dikes, dip changes in oblique dikes, and upturned shale laminations surrounding thicker pillar-like dikes, are evidence of post-injection shale compaction. The contoured plots of poles for 64 dikes show that there are two sets. The most common set (n=35) trends approximately 110°-290° with a subordinate set (n= 29) trending 010°-190°.

The compaction features associated with the dikes shows that they were injected while the host shales were only partially dewatered and hence are Pennsylvanian in age. The presence of multiple unconformities separating the two groups of beds implies that both groups underwent different compaction histories and that there were at least two separate injection events. The orientation of the dikes is consistent with repeated fracturing of the shales perpendicular and parallel to the major fold axes of the Pennsylvania Salient, which we interpret to represent a response to thrust-induced compression and folding in the back-bulge zone of the Appalachian foreland basin.