The Bays formation (Middle Ordovician) and related rocks of the southern Appalachians

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Virginia Polytechnic Institute


The Bays Formation is of late Middle Ordovician (upper Wilderness) age and ls a predominantly clastic formation that crops out in 16 belts in the southeastern side of the Appalachian Valley from near Roanoke, Virginia, to northwestern Georgia. The various lithotopes in the Bays include conglomerates containing limestone cobbles and quartz pebbles more than 1 inch across, sandstones, siltstones, mudstones, shales, and impure limestones. In most strike belts much of the formation is calcareous. The characteristic color of the Bays is "red”, but olive-gray, greenish-gray, and yellowish-gray are also common. Mud cracks are common in fine-grained Bays strata, particularly in the basal portion. The maximum known thickness of the Bays is about 1095 feet, in Monroe County, Tennessee.

The Bays Formation in its maximum stratigraphic development is equivalent to the succession that includes the Bowen, Witten, Moccasin, and Eggleston formations of middle belts in the Appalachian Valley. The Bays at its type locality in the Bays Mountain synclinorium in northeastern Tennessee is underlain by the Sevier Formation and overlain by the Martinsburg Formation

Several bentonites occur in the upper part of the Boys-Moccasin sequence and augment stratigraphic control. The presence of ghosts of glass bubbles and Y-shaped shards in tuffaceous rocks associated with the bentonites indicates a volcanic origin for the bentonitic material. The bentonltes (K-bentonites of some writers) are composed chiefly of illite, mixed-layer chlorite-montmor-illonite.

Generally, clastic rocks in the Bays gradually decrease in grain size from southeast to northwest. The amount of calcium carbonate in Bays strata increases from southeast to northwest. This decrease in grain size and increase in calcium carbonate in the rocks toward the northwest also occurs in the Bowen, Witten, Moccasin, and Eggleston formations of middle belts in the Appalachian Valley. The increase in grain size of Bays sediments toward the southeast suggests that the source area of Bays sediments was in that direction. Sedimentary structures indicating current direction suggest that Bays sediments came from the south, southeast, and east.

Rock southeast, and fragments in Bays strata include limestone, sandstone, siltstone, chert, orthoquartzite, metaquartzite, amphibolite, fine-grained metamorphics such as slate and phyiltte, and vein quartz. Nonopaque heavy minerals in Bays Strata include zircon, tourmaline, apatite, hornblende, zoisite, garnet, epidote, and corundum. These clasts and minerals indicate that Bays sediments were derived from older sedimentary rocks, low-grade and high-grade metamorphic rocks, and silicic igneous intrusive rocks.

These data suggest that Bays sediments were derived from a rising crystalline land mass (or masses) to the southeast, possibly in the vicinity of the present Piedmont and adjacent Blue Ridge. A belt of Cambrian and older Ordovician sedimentary rocks, probably along the northwest side of the rising crystalline complex, was also exposed during at least part of Bays time and contributed to Bays sediments. It is believed that the great volume of Bays sediments necessitates a land mass source area larger than a volcanic island arc.

It is beleived that local troughs on the Bays sea floor subsided faster than adjacent areas, and accommodated greater thicknesses of sediments. A few of these troughs may have also attracted slightly coarser sediments that bypassed less rapidly subsiding parts of the depositional basin.

The rising of the land mass required to supply the large volume of coarse and fine Bays sediments and the volcanism that was important in the latter part of Bays time indicate considerable tectonic activity. The differential subsidence of the Bays depositional basin also suggests that Bays time was characterized by important tectonic activity in the southern Appalachian region.