DUST ROCKS IN THE LATE PALEOZOIC

Quaternary loess (eolian silt) is widespread, whereas examples of pre-Quaternary loess(ite) are few. Growing documentation of pre-Quaternary loessite and associated silt-rich facies, however, suggests that the apparent dearth of loess in the pre-Quaternary record perhaps reflects more a bias of nonrecognition rather than reality. Notably, most documented pre-Quaternary loessite hails from a particular interval: the late Paleozoic. This apparent loess spike raises two key questions: (1) what mechanism(s) led to generation of such voluminous dust, and (2) what effect did this material have on the late Paleozoic Earth system?

Within western equatorial Pangaea, recognition of the prevalence of eolian silt (loess) in the late Paleozoic is becoming well developed in the Ancestral Rocky Mountains (ARM). Silt occurrences here range from thick (up to 1-2 km) massive and locally pedogenically modified siltstone interpreted as true loess deposits to anomalously silt-rich fluvial and marine units that incorporate silt of ultimate eolian origin. Conditions were apparently favorable for large-scale silt generation, but the mechanism(s) have remained enigmatic. Detrital-zircon-based provenance analysis suggests that much of the silt was derived from Precambrian basement material exposed during uplift of the ARM, and Chemical Index of Alteration (CIA) values of the loessite are typically quite low. Coupled with timing of deposition, these data are most easily reconciled with silt generation via glacial grinding, a seemingly radical hypothesis given the equatorial paleogeography, but feasible owing to recent documentation of glacial landforms and facies in the ARM.

Beyond the implications of a glacial genesis for the widespread dust, there are significant implications for the Earth system. Atmospheric dusts have a significant effect on radiative forcing, both directly, and indirectly through feedbacks that influence cloud and storm formation. Moreover, widespread generation and distribution of such a large volume of highly chemically reactive material would provide nutrients key to stimulating and sustaining both terrestrial and marine ecosystems. Dust rocks in the late Paleozoic are voluminous, and house fascinating insight into the Earth System of our most recent pre-Quaternary icehouse.