Stratigraphic Relations of Ordovician Quartz Arenites and K-Bentonites in the Blount Molasse of the Southern Appalachians: Implications for Sequence Boundaries

Conglomeratic quartz sandstones in the Ordovician Blount molasse (Taconic foredeep, southern Appalachians) occur at different time-stratigraphic intervals as shown by their relation to the Deicke and Millbrig K-bentonite Beds, which are correlateable layers of altered tephra. The K-bentonites are isochrons; the sandstones and the disconformities beneath them are not. Our ability to parse out differences in the timing of prograding sands and gravels across the proximal Blount foredeep is significant for sequence stratigraphic models. At the over 40 exposures studied, a subtle to pronounced disconformity occurs beneath the sandstone or conglomerate. Stratigraphic relations show unequivocally that (1) the Walker Mountain Sandstone at 29 sections in southwestern Virginia, and the “middle sandstone member” at 6 sections in northeastern Tennessee, are older than the K-bentonites; (2) the Colvin Mountain Sandstone at 7 sections in Alabama and Georgia is contemporaneous with the K-bentonites; and (3) the unnamed conglomerates and pebbly sandstones at 2 sections near Dalton, Georgia are younger than the K-bentonites. The diachroneity of these gravels and coarse sands likely resulted from changes in the fluvial networks draining the Taconic highlands, with pulses of gravel and coarse sand being delivered episodically into the basin over several million years and at different depositional loci, as a result of tectonic, rather than eustatic, activity.

Petrographic, sedimentologic, and cathodoluminescent analyses show that these sediments share a similar provenance, accumulated in similar depositional settings, and had similar diagenetic histories. If the K-bentonites were not present, these coarse clastics would be correlated with each other from Alabama to Virginia, and the disconformity beneath them would likely be given regional or even global sequence stratigraphic significance. The unconformity beneath the Walker Mountain Sandstone in Virginia has already been given such significance, being labeled as the “M3-M4” sequence boundary; nonetheless, other prominent disconformities occur beneath other coarse clastics in the Blount molasse that are younger (Georgia and Alabama) or older (Tennessee) than the Walker Mountain. This shows that in active margin settings not every unconformity is of glacioeustatic origin, or has a eustatic component, especially in settings where tectonic activity governs and dominates the depositional system, as with the Taconic foredeep.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California