AN ANALYSIS OF QUARTZ BRECCIAS ASSOCIATED WITH THE BLUE RIDGE THRUST SYSTEM: BIG MEADOWS QUADRANGLE, VIRGINIA

Recent mapping in the Big Meadows 7.5’ quadrangle, Page Valley, Virginia, documented several locations of predominately quartz arenite breccias derived from the Antietam Formation of the early Cambrian Chilhowie Group. Breccias are oriented along a NE-SW trend in the Hershberger Hill area at the western base of the Blue Ridge, and are apparently truncated to the south by the Stanley fault. Similar breccias are known to occur to the north in the southern half of the Luray quadrangle, and farther south in the northwestern corner of the Elkton East quadrangle.

There are 3 distinct varieties of the breccias; Types 1 & 2 are clearly associated with the Antietam formation, and consist of mildly fractured quartz arenite (type 1) to variably-sized fragments of silica-cemented quartz arenite (type 2). Both of these occur in multiple locations on Hershberger Hill. Near the top of the hill, type 2 breccias consist of angular pebbles to boulders of quartz arenite within a matrix of stained siliceous material. Samples from the lower outcrop on the hill are stained deep red and displaying a layered, flowing structure of clay to sand-sized particles, amid fragments of quartz arenite. Working hypotheses for the formation of these enigmatic breccias include: silica-cemented paleosols with lenses of solidified mud, altered shales, or some combination of these.

Type 3 breccias predominantly consist of orange-red stained siliceous granular aggregates that contain only occasional small fragments of quartz arenite. The type 3 location is in the centre of the Big Meadows quadrangle directly SW of Hershberger Hill. Hand samples are dominated by variably colored micro-crystalline quartz. Thin sections show interlocking quartz crystals with prevalent sub-millimeter voids. These outcrops appear to be large deposits of what were siliceous fluids, perhaps derived from the main thrust surfaces of the Blue Ridge fault system. Continuing work will compare these 3 types of breccias with other known breccia outcrops that occur at the base of the Blue Ridge, outside the Big Meadows area.