EVOLVING INTERPRETATIONS OF CAMBRO-ORDOVICIAN CRATONIC SHEET SANDSTONES

Cratonic sandstones of mid-continent North America are extremely quartz-rich (commonly >99% quartz), well-rounded and sorted, and offer little obvious mineralogical and textural evidence for their origins. Although previously interpreted as thin, shallow-marine “sheet sandstone” deposits formed in a “layer-cake” fashion during repeated Cambrian-Ordovician marine transgressions across the craton, an evolving paradigm shift developed for these rocks, based on new insights gained from flume and modern environment process studies of both subaqueous and eolian sand transport and deposition, and from associated trace fossils. These process-oriented studies have shown that Cambrian-Ordovician cratonic sandstones have more complex facies and genesis than previously held. Shallow-marine (sensu lato) sandstone deposits are now interpreted as complexly intertonguing arrays of fluvial, eolian, tidal, foreshore and marine shoreface deposits, which are developed over broad, low-lying paleolandscapes in which basement structures may have played an important role. Particular progress occurred after development of criteria for differentiating eolian from subaqueous sandstone deposits using diagnostic stratification types such as translatent (wind ripple) and adhesion ripple pseudo-cross-strata; enigmatic large-scale cross-strata could finally be either interpreted as deposited by wind (sand dunes), or deposited by water (subaqueous large ripples), but it required keen attention to subtle differences in physical structures to distinguish between the two. The origins of the quartz-rich sand itself have also been problematic, with both first-cycle and multicyclic origins proposed to explain both the high degree of textural and compositional maturity. Intense physical reworking, presumably during episodes of eolian transport, coupled with prolonged periods of chemical weathering of sediment residing on a low-relief landscape stabilized by biological crusts or microbial mats are likely based on recent studies of modern microbial mats, as well as SEM studies of grain surfaces in cratonic quartz sandstones. Recognition of increasing importance of microbially influenced sedimentary structures in shallow-marine (e.g., tidalite) deposits offers additional promise for interpreting paleoenvironments in these cryptic rocks.