Aeolian to marine transition in Cambro—Ordovician cratonic sheet sandstones of the northern Mississippi valley, U.S.A.

ABSTRACT Cratonic quartz sandstones have presented several intractable problems. Besides their extreme textural and compositional maturity and paucity of shale, their sheet‐like geometry is particularly notable. If the sandstones were entirely marine, as long supposed, such geometry is difficult to explain in terms of modern shelf sediments, which are generally held to be either relict or only slightly reworked by the Holocene transgression (palimpsest). Re‐study of two quartz sandstones in the northern Mississippi Valley region reveals evidence for significant non‐marine deposition followed by varying degrees of marine reworking during transgressions. Facies patterns are similar in the Cambrian Wonewoc and Ordovician St Peter sandstones, both of which overlie unconformities. In both, a large‐scale cross‐stratified facies believed to represent aeolian ergs passes laterally into a planar‐and‐channelled facies inferred to represent sand plains composed of braided fluvial and aeolian sand sheet deposits. Criteria of aeolian deposition in both facies include adhesion structures, large ripple index, fine climbing translatent lamination, grainfall and grainflow stratification. Criteria of braided fluvial deposition include shallow channels containing sequences of thinning‐upward sets of trough cross‐stratification, reactivation surfaces, low‐index ripples, and polygonal cracks. Probable aeolian sand sheets contain flat bedding punctuated by small channels, adhesion structures, and coarse‐sand ripples with large index. There is a conspicuous absence of trace and body fossils from these inferred non‐marine deposits. In contrast is a burrowed and trough cross‐stratified facies characterized by medium‐scale cross‐bedding alternating with bioturbated intervals and rare brachiopod or trilobite‐mould coquinas, which is interpreted as shallow marine. In both formations, this last facies replaces laterally and overlaps the other two, reflecting transgression and variable reworking. The main areas of non‐marine deposits in both formations are capped by a thin, burrowed subfacies that represents the culmination of each transgression; that is, a stillstand during which sediment influx ceased and both physical winnowing and bioturbation were intense. It is suggested that the sheet‐like geometry of many cratonic quartz sandstones is due primarily to initial sand dispersal by aeolian and fluvial processes. That such processes must have been orders‐of‐magnitude more important on pre‐Devonian, vegetation‐free landscapes than they have been since, not only helps to explain the sheet‐like character but also the exceptional maturity of the older cratonic sandstones.