TEMPORARY OUTCROP OF GRANITE AND GRAPHITIC SCHIST IN THE PIEDMONT OF NORTHERN VIRGINIA DURING CONSTRUCTION OF POHICK BRIDGE

Between July 1999 and October 2000, the Virginia Department of Transportation constructed a bridge to carry Pohick Road over Pohick Creek in the northern Virginia Piedmont near the Fall Line. The bridge, which cuts across the local NNE-SSW structural grain of the Piedmont folds, required road cuts along more than 600 m of its east and west approaches (about 335 m across strike) and deep excavations for footings on either side of the creek. I photographed and sampled the evolving outcrop from its start to its finish. Now (Dec 03), this unusually informative outcrop exists only in those photos and samples: during construction, the rocks were excavated, built over, or vegetated, but it provided then, at no cost but the observer's time, what amounts to a brief giant channel sample across complex local geology.

The bridge and its approaches are within the mapped limits of the Occoquan Batholith, the largest granite body in northern Virginia. The excavation exposed 13 separate schists which strike sub parallel to each other and to the regional structural grain. Because these schists occupy less than six percant of the section, their spatial orientation raises the question: how did an intruding viscous magma produce the observed interleaved granite body having more than ninety percent of the section, without destroying the parallelism of the schists? At least five of the 13 schists contain graphitic beds, including one graphite layer about 20 cm thick. At least two granite/schist contacts have sedimentary bedforms delineated by graphite layers whose shape and elemental composition reproduce the shape and elemental composition of layered charcoal grains visible at some commercial sand and gravel pits. In one case, a post-folding normal fault cutting one of the graphitic schists has smeared the graphite, probably as a lubricant, along the fault surface, an occurrence which might, with further metamorphism, be mistaken for a hydrothermal deposit.