40Ar/39Ar DATING OF DETRITAL MUSCOVITE FROM THE POTTSVILLE FORMATION IN THE GREATER BLACK WARRIOR BASIN AND IMPLICATIONS FOR APPALACHIAN TECTONICS

Detrital muscovite has the potential to yield single crystal 40Ar/39Ar ages that are characteristic of the ages of tectonic activity in their source region. Peavy (2008) and Uddin et al. (in prep.) report muscovite 40Ar/39Ar age distributions based on three samples of Pottsville sandstone from the Cahaba synclinorium. The stratigraphically deepest sample has a single age mode of ca. 454 Ma, a stratigraphically higher sample has modes of ca. 374 and 328 Ma, and the highest sample has a single age mode of ca. 374 Ma. The present study focuses on a sample collected from an outcrop near Carbon Hill, AL: this sample yields a distribution of laser single crystal 40Ar/39Ar ages (n=112) with modes of ca. 440, 365, and 319 Ma. The modes identified among all of these samples are interpreted to broadly record the Taconian, Acadian, and Alleghanian tectonic events of the southern Appalachians. The results for the present sample contrast with the earlier studies of the Pottsville formation in the Cahaba synclinorium, as it is a single sample that contains a record of all three Appalachian tectonic events. We suggest the Cahaba samples reflect derivation from source terrains that were tectonically changing during sedimentation, and thus the dominant sources differ among samples. The fact that the sample from the Black Warrior basin records all three events may imply sedimentation fed by drainages that were more fully established and supplied sediment to the Black Warrior basin from multiple sources. The age modes of the Black Warrior sample seem ca. 5-10 million years younger than corresponding modes of the Cahaba basin samples. This difference may be due to erosion of deeper structural levels or erosion of different regions that provided micas that reached isotopic closure later. We are testing the hypotheses outlined above through detrital mineral studies of continuous cores and surface exposures of the Black Warrior and Cahaba basins.