DÉCOLLMENT IN THE UPPER SILURIAN TO LOWER DEVONIAN ROCKS OUTCROPPING IN THE TOWN OF ROSENDALE, ULSTER COUNTY, N. Y

The Upper Silurian to Lower Devonian strata outcropping in the Town of Rosendale, N. Y. were significantly folded and faulted during the Acadian Orogeny (Ca. 416 to 359.2 million years ago). The deformed structure is characterized by highly asymmetrical to overturned anticlinal folds and imbricate thrust faulting. The folds plunge NNE. The Upper Silurian Shawangunk Conglomerate and the High Falls Shale unconformably overlie very thick deposits of Martinsburg Shale (locally known as Hudson River Shale). The Shawangunk Conglomerate, a very well indurated quartz pebble conglomerate, thins from south to north in the study area. In the northern part of the study area the Shawangunk Conglomerate does not outcrop and there the High Falls Shale is in direct, unconformable contact with the Martinsburg Shale. The authors have observed that the axis of the plunging anticline north of the northernmost extent of the Shawangunk Conglomerate is not parallel to the axes of the plunging anticlines in the southern part of the study area where the Shawangunk Conglomerate occurs. The authors have also observed rock units in the southern part of the axial area of this anticline that are younger than the rock units in the axial area of the northern part of this same anticline. Since the folds are plunging to the north, one would expect to see only older units as one looks farther south along the fold axis. As this is not what is observed, the authors hypothesize that something other than simple folding has occurred here. There are indications of crumpling in the younger rock units exposed in the center of the anticline. The evidence is indicative of décollment structure with the Shawangunk Conglomerate acting as a massif against which the other Upper Silurian and Lower Devonian strata were crumpled while sliding on the Ordovician Martinsburg Shale.