DETRITAL ZIRCON EVIDENCE OF PROXIMAL SOURCES FOR THE ALLEGHANIAN CLASTIC WEDGE IN EASTERN TENNESSEE

The late Paleozoic Pennington-Lee clastic wedge in the Appalachian foreland is interpreted to consist of erosional detritus from the concomitant Alleghanian orogenic belt; however, the relative importance of contributions from the frontal orogen, the orogenic hinterland, and the distal craton remains unresolved. To evaluate these components, we separated detrital zircons from two sedimentary units in the Pennington-Lee clastic wedge in eastern Tennessee: the Lower Pennsylvanian Sewanee Conglomerate (SC) and the Middle Pennsylvanian Cross Mountain Formation (CMF). Detrital zircons were separated into populations by color and morphology, and analyzed for U-Pb age by the SHRIMP II facility at the Geological Survey of Canada. Detrital zircon age populations in the SC are 385-415 Ma, 435-465 Ma, 900-1150 Ma, 1700-1820 Ma, and 2690-2755 Ma and scattered ages of 1150-1350 Ma and 1460-1540 Ma. Nearly identical age populations in the CMF are 365-430 Ma, 1000-1100 Ma, 1340-1370 Ma, 1550-1820 Ma, and 2685-2860 Ma and scattered ages of 470 Ma, 770 Ma, 900 Ma, and 1400-1480 Ma.

The Acadian-, Taconic-, and Grenville-age detrital zircons have sources in Alleghanian frontal, crystalline thrust sheets. The early Proterozoic and Archean zircons indicate an initial source in the Canadian shield. A longitudinal river system headed in the shield might have supplied detritus directly to the Appalachian foreland basin in Tennessee. However, all of the Archean and late Proterozoic ages of zircons represented in the SC and CMF have been reported in the detrital zircon populations of pre-orogenic Iapetan synrift and passive-margin strata along the eastern Laurentian margin, suggesting a supply of recycled zircons from pre-orogenic rocks exposed in Alleghanian thrust sheets. A conspicuous lack of zircons from the contemporaneous Pennsylvanian synorogenic crystalline rocks shows that the Alleghanian hinterland was not integrated into the drainage network. Detrital zircon ages from the SC and CMF are consistent with a transverse drainage in Alleghanian frontal thrust sheets comprised of Acadian and Taconic synorogenic rocks, passive-margin and synrift rocks with shield-derived zircons, and Grenville basement.