ORIGIN OF PLUMOSE STRUCTURE IN UPPER DEVONIAN SILTSTONES, CONEMAUGH RIVER WATER GAP, JOHNSTOWN, PENNSYLVANIA

Outcrops of Devonian siltstone are rare on the Appalachain Plateau of western Pennsylvania except where streams have eroded water gaps through Chestnut Ridge and Laurel Ridge. These two ridges represent small-amplitude northeast plunging anticlines with beds dipping approximately 5° to 10°. Near Johnstown, the Conemaugh River eroded a water gap that exposes a small section of the Upper Devonian Rockwell Formation in the core of the anticline. Plumose structure is present on some of the cross-fold joints that are exposed in the water gap. McConaughy and Engelder (2001) studied the origin of plumose structures on cross-fold joints in the Appalachian Plateau of central and eastern New York. They determined that structures, such as sedimentary bedforms, trace fossils, soft sediment deformation, and joints could act as potential joint initiation points. Three siltstone beds, 0.7 to 0.8 feet thick, contained numerous plumose structure on cross-fold joints. The beds occur within a 66.5-foot section exposed along State Route 403 near the border between Indiana County and Cambria County. The siltstone beds occur 23.3, 27.1, and 27.2 feet from the base of the section. Several different geometries of plumose structure were observed that depend on the location of the initiation point. When the initiation point is located at a bedform, the medial ridge of the plumose structure fans out across the surface of the joint away from the initiation point. At a greater distance from the initiation point, the medial ridge of the plumose structure straightens and is located near the center of the joint surface. When the initiation point is located at a point other than a bedform (joint or fossil) the medial ridge usually does not fan away from the initiation point. Eleven plumose structures were identified within these three beds. The initiation point of five plumose structures was traced back to bedforms such as ripple marks and one plumose structure initiated at a fossil. Two plumose structures initiated at pre-existing fractures that are perpendicular to the cross-fold joints and the initiation point of three plumose structures could not be determined. Fifty-five percent of the plumose structures initiated at bedforms. Our results are similar to the results of McConaughy and Engelder (2001).