MEGA-CHANNELS IN DISPERSING SEDIMENTS FROM OROGENIC SYSTEMS TO LARGE DEPOCENTERS: EXAMPLES FROM THE PALEOZOIC APPALACHIANS AND THE CENOZOIC HIMALAYAS

The Himalayas and the Appalachians are two mountain belts that stand out as classic examples of Paleozoic and Cenozoic orogenies in the geological history. The Black Warrior and Cahaba basins of the southern Appalachians represent important late Paleozoic depocenters in eastern North America that have modern analogs in the Bengal and Assam basins of the eastern Himalayas. Petrofacies evolution and systematic detrital geochronology data provide new information supporting in part long-distance dispersal by Paleozoic mega-channels in the Appalachians as being observed in modern-day Himalayas.

The Himalayas are flanked by the Ganges and the Brahmaputra, two mega-channels, which have been debouching sediments at the Assam basin during the Paleogene, at the Bengal basin during the Neogene, and at the Bengal fan during the recent years. These mega-channels carry sediments for more than thousands of kilometers following mostly orogen-parallel dispersal pathways. Both the Ganges and the Brahmaputra had ancient representatives that were active since at least the Miocene. Detrital geochronology of Miocene muscovite grains from the litharenites in the Bengal basin has documented 16 million year cooling ages in strata of essentially the same age, indicating extremely rapid uplift in the Greater Himalayas, including the Indo-Burman ranges. Compositionally more mature Paleogene sediments however suggest a diverse provenance.

Likewise, sediments from late Paleozoic clastic wedges of the southern Appalachian depocenters suggest provenance from both near and far sourced areas. Detrital geochronology of compositionally mature quartz-arenites is far-traveled as these lack an Alleghanian age whereas the compositionally immature sandstones (litharenites) with Alleghanian age seem more of a 'local' source. The quartz-arenites might have long distance dispersal from central to northern Appalachians via orogen-parallel mega-channels west of the Appalachians. Although the shift in petrofacies evolution in our study at the Appalachians does not obviously suggest orogenic unroofing as in the Himalayas, there are however strong similarity between the Himalayas and the Appalachians in terms of sediment dispersal via mega-channels reflecting variations following eustatic seal level changes.