Dome-shaped aeolian dunes in the frodsham member of the so-called “keuper” sandstone formation (scythian-anisian: triassic) at Frodsham, Cheshire (England)

Abstract

At four localities near Frodsham, especially in the railway cutting, a wellsorted mottled soft fine-grained sandstone lithofacies lies above a cyclic fluvial pebbly red sandstone lithofacies. It is suggested that the former facies is aeolian, and comparison with the internal structures of present-day aeolian dunes in the White Sands Desert, New Mexico, enables features of dome-shaped and, to some extent transverse, dunes to be recognized. The aeolian environment is interpreted in detail in terms of local derivation of abundant sand from upwind parts of a dunefield in which dried-out river courses were winnowed by moderate breezes where transverse dune structures are initiated, rising to strong breezes and moderate gales where dome-shaped types arise. These palaeowinds were directed unidirectionally from an easterly point in low latitudes north of the palaeo-equator. In the exposures available, cross winds were most commonly directed from the south. It is suggested that the term Frodsham Facies be restricted in future to define deposits of aeolian environments where dome-shaped dunes are dominant.