DUST AND LOESS IN THE DEEP TIME RECORD OF LATE PALEOZOIC PANGAEA

Loess and dust are well recognized in Earth’s late Cenozoic record but remain poorly documented in deep time. Growing recognition of pre-Cenozoic loess and dust suggest a bias of nonrecognition, rather than reality. Vast (paleo-) loess and dust deposits occur in the Carboniferous-Permian record of western tropical Pangaea, and include the thickest loess deposits (>1 km) yet documented on Earth. These indicators of tropical atmospheric dustiness are highly unusual when compared with the demonstrably extra-tropical loess and dust of the Cenozoic, raising questions about the mechanisms of generation, and effects of this material. Within the western and central U.S., inferred eolian-sourced fine-grained (silt-sized) material occur in both continental and marine deposits. They consist of red mud/siltstone in commonly structureless units, locally overprinted by sub-aqueous (e.g., lacustrine and marginal marine) processes, and also occur as silt trapped in epeiric carbonate systems formed isolated from fluvial input. Conditions favored large-scale generation of silt, but the mechanism(s) have remained enigmatic. Provenance analysis from both detrital zircon geochronology and framework mineralogy suggests that much of the silt was derived from various basement provinces that were exposed in the cores of both the Appalachian-Ouachita and Ancestral Rocky mountains, and may reflect in part glacial genesis in these orogenic uplands. Transport pathways likely involved both fluvial systems draining piedmont regions and eolian deflation within an easterly/southeasterly and seasonal westerly/northwesterly atmospheric circulation. These vast deposits archive unusual climatic conditions, and likely acted as major agents of climate change. Atmospheric dusts affect radiative forcing directly, and indirectly-- through feedbacks that influence cloud and storm formation, as well as biogeochemical effects of seeding continental and marine ecosystems with highly chemically reactive material. Dust rocks in the late Paleozoic are vast, and house fascinating insight into the Earth System of our most recent pre-Cenozoic icehouse.