SOUTHERN APPALACHIANS UNDERCOVER: PERSPECTIVES ON TECTONOSTRATIGRAPHY AND EROSIONAL HISTORY FROM ITS CLASTIC COVER (Invited Presentation)

Jurassic and Cretaceous clastic strata that overlie the southernmost Appalachian orogenic belt and flanking Black Warrior foreland rocks preserve a valuable record of the distribution of underlying tectonostratigraphic units that form the basement of the eastern Gulf of Mexico coastal plain cover sequence. Jurassic strata are composed of feldspathic sands likely derived in part from first-cycle weathering and erosion of crystalline rocks. However, detrital zircon signatures in these Jurassic deposits record a variation in sediment sources from west to east. The Jurassic strata in the Mobile Bay region possess a strong Laurentian-affinity signature defined by prominent mode of 980-1170 Ma ages along with 320 and 410 Ma synorogenic populations. This signature is in contrast with the provenance signature of Jurassic strata to the east buried offshore of Apalachicola, FL, that show a peri-Gondwanan-affinity signature defined by abundant 510-610 Ma grains along with minor Paleoproterozoic ages and a prominent cluster of 260-300 Ma grains. These late Paleozoic ages are possibly derived from late to post-orogenic magmatic rocks related to deep-seated mantle processes as the convergent margin transitioned to a divergent margin. The west-to-east provenance variation is likely controlled by sediment delivery along strike NE-SW trending Mesozoic grabens and suggests a suture between Laurentian and peri-Gondwanan crust lies in the vicinity of Pensacola, FL. Cretaceous samples from southern Alabama and adjacent offshore regions retain arkosic compositions, however detrital zircon signatures show peaks at 390-410 Ma and 560-580 Ma along with a mode of ~1.4-1.6 Ga and other minor Proterozoic components consistent with a Laurentian origin. The lack of significant peri-Gondwanan components in the younger cover strata indicates these sources were buried and now lie concealed in the subsurface. The lack of Jurassic zircon in the cover strata succession implies syn-rift volcanism in the eastern Gulf of Mexico region was either limited geographically, or that the mantle beneath the region was sufficiently depleted by previous orogenic magmatism to inhibit voluminous syn-rift volcanism.