THE PRESENCE OF AND EXTENSIVE THRUST FAULTING WITHIN THE DEER VALLEY LIMESTONE (DVL), CASSELMAN RIVER GORGE, SUMMIT TOWNSHIP, SOMERSET COUNTY, PENNSYLVANIA

My field mapping in 1990 for a UPJ geology class has proven that the DVL outcrops along what is now the Allegheny Highlands Trail, three miles(4.8Km.) west of Garrett, Somerset County, Pa. The DVL at this location rests directly on top of the Mississippian Loyalhanna Formation and is overlain by the Mauch Chunk Clastic Interval.

The unfaulted DVL here is about 3 feet (1 Meter) thick. It consists of a sandier lower portion and an oolitic upper portion. The rock has a light gray color that contrasts with the red color of the Loyalhanna Limestone. The lower half of the DVL is an arenaceous oolitic grainstone. Sub angular quartz grains constitute about 20% of the rock. Oolites, forminifera and fossil fragments make up the rest of the rock. The upper portion of the DVL is an oolitic grainstone consisting of >95% calcite. Minor sub angular quartz grains are present. Ooids become larger toward the top of the DVL.

A distinctive red sandy limestone bed is the first prominent rock unit above the DVL. It is sometimes separated from the DVL by a thin red mudstone. Clasts and ooids of the DVL are present in the siliceous red limestone, proving the erosional nature of the upper contact of the DVL at this locality.

Extensive thrust faulting has thickened the DVL to >15 feet (5 meters) by stacking a series of thrust slices of the DVL on top of one another. This faulting does not effect the underlying Loyalhanna Limestone or the overlying Mauch Chunk clastic rocks. Faulting is negligible at the western end of the outcrop, extensive in the eastern portion of the exposure. The maximum thickness of the DVL is indeterminate as regional dip causes the DVL to dip below the erosional surface at the locality.

Almost any piece of the DVL shows evidence of faulting. Slickensides,fault gouge and shearing are readily seen. In thin section mylonization is shown by shearing of oolites. Calcite filled faults are also seen. Fracturing of quartz grains is evident in the mylonitized DVL. Quartz grains are concentrated by pressure solution along stylolites. In some samples deformation is very localized and visible only in thin section.

The intriguing aspect of this locality is that faulting only effects the DVL. Why was stress manifested in deformation of the DVL only?