Abstract

A. B. Leslie & M. E. Tucker write: In their recent Special paper Wright & Sandler (1994) present an interesting and convincing argument to demonstrate that the red mudstones of the late Triassic Mercia Mudstone were formed exclusively in continental waters, and that changes in stable isotopic composition and clay mineralogy do not indicate an input from marine waters during deposition of parts of the succession. Their interpretation of a carbonate δ18O signature of + 1.0 to +2.0‰ as deriving from a highly evaporated and evolved continental water is certainly plausible, as is their description of Mg-rich clays from a variety of continental environments. Their hydrogeological model involving the migration of meteoric waters through basin-margin clastics and the evolution of a hypersaline brine in more central parts of the basin deserves further study and could no doubt be refined to account for many of the sedimentary features identified in arid basins.

There are a number of points which we would like to raise, however, both generally and with specific reference to the Triassic strata of southwest Britain. Our analyses of the clay mineralogy (Leslie et al. 1993) did not include detailed structural data because we were looking for changes in the basic clay mineral assemblages, as described by other authors (Jeans 1978; Mayall 1979; Taylor 1983), which were easily identifiable without recourse to detailed studies. The formation of smectites and associated mixed-layer varieties in non-marine environments is not doubted; however Wright & Sandler apear to confuse this neoformation of clay

First Page Preview

First page PDF preview
You do not currently have access to this article.