Wind-reworked carbonates, Permo-Pennsylvanian of Arizona and Nevada
Several workers have suggested that calcareous wind-blown dunes are genetically related to rapid, glacio-eustatic fluctuations of sea level. Although such Pleistocene and Holocene deposits are widespread along low-latitude coasts, older eolian limestones are either extremely rare or have been misinterpreted as subaqueous in origin. Medium- to large-scale cross-laminated limestones of the Callville Limestone (Pennsylvanian) of southern Nevada, the correlative Manakacha and Wescogame Formations (Supai Group), and the Pakoon Limestone (Permian) of western Grand Canyon, although considered marine by earlier workers, contain several diagnostic structures indicative of an eolian origin. Cross-strata are dominantly composed of thin, inverse-graded laminae produced by the migration and climb of wind ripples. Sets of cross-strata commonly have irregular upper surfaces produced by differential wind erosion of damp or lightly cemented laminae. Southward-dipping cross-strata in these rocks are the earliest record of a paleowind system that persisted in this region well into Jurassic time. Mechanical breakage of shells, fitted fabrics, and microstylolites indicate that marine beds that are cyclically interbedded with the eolian deposits did not undergo early lithification. Echinoderm grains with abraded over-growths within eolian deposits indicate, however, that some wind-transported grains were reworked from beds, that had previously undergone fresh-water phreatic diagenesis. Gravel lags composed of macrofossils and hardground lithoclasts at the contacts between marine and eolian beds and between eolian coasts give further evidence for deflation of subaerially exposed marine deposits. Where marine units underwent extensive early lithification, there are no gravel lags or interbedded eolian deposits. Subaerially exposed marine strata were the primary source of eolian sediment. During eustatic highstands, onshore winds probably drove beach sediment landward, but, in contrast to Quaternary examples, the eolian deposits of this study do not cap shallowing-upward sequences. Beach facies are rare. During lowstands, uncemented beach facies were first to be removed by deflation. Material too coarse to be wind-transported accumulated as gravel lags. Deflation was facilitated by the lack of widespread sea-floor cementation (hardgrounds) and by an arid paleoclimate, which limited the extent of meteoric diagenesis.