Detrital zircon evidence of a recycled orogenic foreland provenance for Alleghanian clastic-wedge sandstones

Late Paleozoic synorogenic elastic wedges in the Appalachian foreland basin contain erosional detritus from the Alleghanian orogenic belt; however, the relative importance of detrital contributions from the frontal orogen, the orogenic hinterland, and the distal craton has not been resolved. To evaluate these components, we analyzed detrital zircons from two sandstones in the Pennington-Lee elastic wedge in eastern Tennessee: the Lower Pennsylvanian Sewanee Conglomerate and the Middle Pennsylvanian Cross Mountain Formation. Ages of detrital zircons are similar in the two sandstones, ranging from 365 to 2860 Ma. The age range spans the Acadian and Taconic orogenies, the Grenville province, and pre-Grenville rocks (Granite-Rhyolite province, midcontinent orogens, and Superior province) of the Laurentian craton. The Acadian-, Taconic-, and Grenville-age detrital zircons have sources in Alleghanian frontal crystalline thrust sheets. The pre-Grenville zircons indicate an original source in the Laurentian craton (Canadian shield), and a longitudinal river system headed in the shield might have supplied detritus directly to the Appalachian foreland basin in Tennessee. Alternatively, all of the Proterozoic and Archean ages of zircons represented in the Sewanee and Cross Mountain sandstones have been identified in the detrital zircon populations of preorogenic Iapetan synrift and passive-margin strata along the eastern Laurentian margin, suggesting a source of recycled zircons from preorogenic rocks exposed in Alleghanian thrust sheets. A lack of detrital zircons contemporaneous with the Alleghanian orogeny shows that Alleghanian igneous rocks had not been integrated into the drainage network and/ or had not been exhumed. The age distribution of the detrital zircons suggests deformation and erosion of rocks of the Laurentian margin and emphasizes the importance of preorogenic architecture of the continental margin in the construction and erosion of the orogenic belt.