Trenton and Sub-Trenton Stratigraphy of Northwest Belts of Virginia and Tennessee

ABSTRACT The Lower Cambrian Rome is the oldest formation exposed in the northwest belts of Virginia and Tennessee. The formation consists of thick clastics, the shales apparently serving as the loci of thrusting along several of the northwest thrust blocks of Tennessee. The overlying Conasauga group, largely shale northwest in Tennessee, grades through shales and limestones into dolomitic limestones and dolomite southeast and east, reflecting a general northwestward to westward source. Analyses of the more persistent sandstone units at the top of the overlying Copper Ridge dolomite suggest a similar source direction. The Copper Ridge (Upper Cambrian) and Beekmantown (Lower Ordovician) dolomites grade southeastward into limestones, the gradation isoliths closely coinciding with a structural barrier of some nature. This barrier is presumably an arch (“Tazewell arch”) quite similar to, and probably an extension of, the Adirondack arch on the northeast, and possibly is in part coincident with the Rome barrier of Ulrich. The trend of the Tazewell arch coincides closely with the present position of Clinch Mountain in Virginia and Tennessee. The Saltville thrust closely outlines the trend of the arch along the southeast flank. Middle Ordovician rocks southeast of the Tazewell arch are thick and highly clastic, indicating a rejuvenation of the landmass source on the southeast and an accompanying shift in the source direction to the southeast. The Tazewell arch was particularly effective in separating typically geosynclinal rocks from the foreland rocks on the northwest, being more effective in Tennessee than in Virginia. Pre-Trenton, post-Beekmantown units thin by convergence and overlap from approximately 4,600 feet southeast of the arch (southeast of Knoxville, Tennessee) to approximately 1,000 feet northwest of the arch near the Kentucky-Tennessee line. The same interval thins along the strike of the foreland rocks from 1,400 feet in Lee County, Virginia, to approximately 500 feet in Pendleton County, West Virginia. Isopachs of this interval are fairly closely parallel with the Tazewell-Adirondack axis trend. Recent oil developments in the area of the Cumberland thrust block in Lee County, Virginia, have created additional interest in the northwest outcrop belts. Oil has been produced from the Trenton, Eggleston, and “Lowville” limestones, with a showing from the Beekmantown dolomite. The rock porosity is not well understood. Field indications suggest fracture porosity.