Petrology of the basal middle Ordovician Blackford formation of the type belt, Russell County, Virginia

The Lower Cbazyan (Middle Ordovician) Blackford Formation of the area studied consists of dominantly carbonate rocks deposited in a shallow sea which transgressed on a subaerially eroded carbonate terrane having up to 110 feet of relief locally. The lower two-thirds was deposited in a generally supratidal environment and grades up into rocks deposited in the gradually deepening waters of the intertidal and shallow subtidal environments.

The Blackford is divisible into three parts. The lowermost part, which laterally is limited in extent, is a red to purple or yellow green breccia-conglomerate of chert and dolomite clasts in a silty to sandy argillaceous dolomite matrix.

The middle part is brownish red to purplish red locally green and mottled, dolomite to dolomitic sandy siltstone. It is locally conglomeratic. It contains algal and faunal debris locally, and varying quantities of terrigenous detritus, including chert and dolomite clasts, rounded quartz grains) quartz crystals, an illite-mica-chlorite clay mineral suite, and a heavy mineral suite characterized by hematite, magnetite and kyanite. This terrigenous detritus dominantly was derived locally, from the unconformably subjacent Canadian (Lower Ordovician) Knox dolomites.

The upper part of the Blackford is a sequence of gray to yellowish and greenish gray argillaceous calcilutites to calcareous claystones which grade into the overlying Elway Limestone. This part contains algae algal stromatolites, ostracodes, bryozoa and rare trilobites. Nodular black chert is locally present in the uppermost part. The proportion of coarse clastics and dolomite decreases upward whereas the clay content increases abruptly in the upper beds.

The new assemblage is definitely volcanic. Abundant we11 preserved conodonts found with the biotite and apatite are potentially useful for precise paleontologic dating of this volcanism.

The volcanic zone in the upper Blackford may be one previously recognized by Laurence in Lower Chazyan rockS at Douglas Dam, Tennessee. The Blackford volcanic zone is homotaxial with a bentonite recognized by Fox and Grant near Chattanooga and in Rhea County, Tennessee, and with one found by Miller and Fuller in the Rose Hill district, Lee County, Virginia, but it is somewhat older than either.

The fairly widespread extent of the argillaceous limestones-calcareous claystones of the upper Blackford, and implicitly their contained conodont population, indicates that these synchronous units may be traceable over large areas of southwest Virginia. They are therefore potentially useful as a Olazyan reference time plane in this part of the Appalachians.