PETROLOGY AND DETRITAL MINERAL GEOCHRONOLOGY OF THE CARBONIFEROUS SEDIMENTARY APPALACHIANS, SOUTHEASTERN USA

Orogenic events of the central and southern Appalachians culminated with the Carboniferous Alleghanian orogeny and alluvial deposition of the Pottsville Formation and associated clastic sequences that are preserved in a basin extending over 1500 km. The thickest accumulations of the Pottsville Formation occur in the Greater Black Warrior basin (GBWB) of Alabama and Mississippi. The Pottsville sandstones tend to be texturally and mineralogically immature. Laser single-crystal 40Ar/39Ar dating of many samples in the Auburn Noble Isotope Mass Analysis Lab (ANIMAL) shows that muscovite with Ordovician, Devonian, and Carboniferous ages are prominent in Pottsville Formation sandstones, with modes that correlate well with ages proposed for the Taconic, Acadian, and Alleghanian events of the southern Appalachians. The stratigraphically deeper and more eastern samples in the GBWB tend to present more variation in muscovite age distributions than we find in samples collected from more shallow and western portions of the basin. This relationship could indicate a transition from early, synorogenic sedimentation with variable and localized drainage networks to more extensive, post-orogenic drainages. Alternatively, early Alleghanian structures within the GBWB, such as the Birmingham anticlinorium, may have formed temporary barriers that limited sediment supply from local sources to deeper parts of the basin. Muscovite of Taconian age (ca. 450 Ma) is generally most prominent in samples from deeper strata, and is largely replaced by Acadian (ca. 370-380) and Alleghanian (330-280 Ma) muscovite in higher samples. The trend toward younger detrital muscovite age with decreasing age of erosion continues, as modern sediments of the Coosa, Coosawattee, and Etowah rivers (of northwest Georgia) yield muscovite grains dominated by ~ 320 Ma ages. Thus, the earlier sediment of the Carboniferous basins contains a more pronounced and complete muscovite age record of Taconian and Acadian events than would be surmised by sampling and dating muscovite from modern bedrock exposures of the southern Appalachians.