NEW DATA ON THE AGE AND DISTRIBUTION OF THE EARLY SILURIAN (LLANDOVERIAN) THORN HILL K-BENTONITE COMPLEX IN THE SOUTHERN APPALACHIANS

Although K-bentonites have long been known from the Ordovician and Devonian in the Southern Appalachians, such beds of Silurian age were not recognized in this region until 1995 when a complex of ash beds was discovered in the Lower Silurian Clinch Sandstone at Thorn Hill in eastern Tennessee. An apparently coeval complex of K-bentonite beds, referred to as the Thorn Hill K-bentonite Complex, was subsequently found at several other localities in the Clinch Mountain Belt as well as at one locality in Virginia and one in Georgia. Surprisingly, at all these localities and at the newly discovered ones, the ash beds occur interbedded with fairly high-energy (locally cross-bedded), shallow-water sandstones. Recent studies in northwestern Georgia and eastern Tennessee have added new localities and important new information on the age and distribution of this ash bed complex. In Georgia, where the K-bentonites appear to be restricted to the Taylor Mountain Belt, they occur in the uppermost part of the middle portion of the Red Mountain Formation that based on new conodont data and recently published brachiopod data is of late Aeronian (late Middle) Llandoverian age. Unsuccessful search at sections in the more western thrust belts in Alabama, Georgia, and the adjacent corner of Tennessee suggests that the succession having the K-bentonites may be missing at the sequence boundary at the tops of the Brassfield Limestone and the Rockwood Formation, both of which range no higher than the middle Aeronian based on conodonts. The known K-bentonite beds in eastern Tennessee are all in the Clinch Mountain Belt where they occur in the same stratigraphic interval and in a closely similar lithologic succession as in Georgia. Although these ash beds are of the same age as those in the lower Ross Brook Formation in Nova Scotia, they may have had a different source area. Both occurrences represent plate-margin volcanism during a late phase of the Taconic orogeny.