POSSIBLE SEISMITES IN THE DEVONIAN LOCK HAVEN FORMATION SOUTH OF TIOGA, PA

Several road and railroad cuts through the upper Lock Haven Formation south of Tioga, PA expose soft-sediment deformation features that resemble seismites. The Lock Haven at this location consists of interbedded fine-grained, hematitic sandstone and siltstone with greenish-grey mudstone. Bed thickness ranges from a few centimeters up to 2 meters. Decimeter- to meter-scale soft-sediment deformation structures occur in two distinct horizons separated by a ~5 meter thick undeformed interval. The features include load casts, some with internal deformation of primary lamination. Numerous sandstone load casts are detached from the overlying sandstone bed to form spherical to oblong pseudonodules which lie in visibly deformed mudstone. Mudstone flame structures and plastic intrusions separate the load casts and flatten beneath overlying, undeformed sandstone layers. Flame structures show no preferred flow direction. Cohesive fragments of sandstone layers, internally undeformed or with minor warping, appear in thrusted or stacked sequences and are interpreted as sheet slumps.

Soft-sediment deformation structures must satisfy the following criteria to be considered candidates for a seismic origin: 1) occurrence in laterally continuous horizons; 2) stratigraphic separation by undeformed zones; 3) resemblance to experimental analogs generated by shaking; 4) geographic variation in intensity; and 5) development in a region of known seismicity or tectonic activity. The features observed south of Tioga satisfy criteria 1-3. Criterion 4, geographic variation in intensity, is difficult to evaluate based on the limited exposure. To satisfy criterion 5, the soft-sediment deformation structures must have developed in a region of syndepositional seismic activity. Although no Late Devonian faults have been documented in the area, tectonic activity can be inferred based on the region’s paleo-location on the margin of the Devonian Acadian orogen. To determine whether the load structures are seismogenic, future work will focus on identification, description, and delineation of soft-sediment deformation features in the upper Lock Haven Formation across a broader region.