The late Triassic Mercia Mudstone Group of southwest Britain, a problematic, complex arid playa-floodplain-aeolian deposit, contains a variety of Mg clays and carbonates. Recent interpretations have invoked marine waters as a source of Mg in clays. However, both the Mg-rich clays and other early diagenetic features seen in the late Triassic in Europe are directly analogous to those found in the inland drainage basins of Australia. A simple hydrogeological model is offered based on these basins where shallow groundwater evolution takes place within the basin producing a range of early diagenetic products including groundwater dolocretes and Mg clays. Such a model, as developed in Australia, is probably widely applicable to ancient arid alluvial systems.

The Mercia Mudstone Group is a problematic sequence of predominantly red siltstones and mudstones with minor sand, dolomite and limestone intercalations (Ruffell 1991). It has been extensively studied in outcrops in southwest and southern Britain, particularly in the Bristol Channel area. The siltstones and mudstones, of late Triassic age, are the distal equivalents to alluvial fan deposits and are associated, at least locally, with lake margin and spring travertine deposits (Leslie et al. 1992).

Some workers have favoured a marine-influenced setting for the deposition of the mudrocks or playa lakes (see review in Taylor 1983). Most recently they have been interpreted as the deposits of complex floodplain-playa systems with a signficant aeolian input (Talbot et al. 1994).

Extensive evaporites occur locally in the unit, including thick halite deposits and have been interpreted as evidence of marine

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