THE GREAT PENNSYLVANIAN RIVER: A LINK BETWEEN PERMIAN BASIN HYDROCARBONS AND LATE PALEOZOIC COLLISION?

Great orogens spawn great rivers. Great rivers carry vast volumes of sediments and nutrients to the sea, with implications for hydrocarbon genesis. A great river likely formed in the Early Pennsylvanian (~320 Ma) in eastern and southern Laurentia when Pangea formed, and great rivers may have continued throughout the Pennsylvanian (300-320Ma). Great rivers flow away from collisional orogens in two ways: towards the colliding continent (e.g. Indus, Ganges, Brahmaputra, Tigris, Euphrates, and Po rivers) or toward and through the hinterland to the sea (e.g. Amazon, Yellow, Yangtze, Mekong, and Rhine rivers). Late Pennsylvanian continental collision in E North America reflects failed subduction of Laurussia beneath Gondwana. Collision formed foreland basins in front of the Allegheny-Ouachita-Marathon mountain belt and reactivated several midcontinent intracratonic basins. A great river(s) may have flowed along these basins from NE Laurentia (Pennsylvania or farther N) at least ~3000 km to the Permian Basin (PB) of W Texas. Such a river would have drained not only the Appalachians but also the Variscan highlands, the Tibet-like plateau in Europe and NW Africa. The orogen straddled the paleo-equator, with strong monsoonal circulation amplified by trade winds. We imagine an Amazon-scale river surrounded by rainforests, now preserved as coal deposits. Such a river would have debouched onto the Eastern Shelf of the Midland basin or the E Val Verde Basin.

One implication of the Pennsylvanian Great River hypothesis (PGRH) is that organic productivity responsible for the hydrocarbon richness of Late Paleozoic sediments in the PB may reflect nutrients supplied by the great river. The great river would have delivered abundant nutrients that triggered plankton blooms and local anoxia. Detrital zircon and monazite ages can help test the PGRH. These tests are complicated by large-magnitude sea level changes due to growth and shrinkage of the Gondwanan ice sheet. The shoreline in Pennsylvanian time would have oscillated from Arkansas to W Texas, generally regressing under the sediment flux but with rapid sea-level changes that would have flooded the river valley (times of low nutrient flux to the PB and organic productivity) and times when the river flowed into the eastern PB (times of high nutrient flux and organic productivity).