The Norian Mercia Mudstone Group of west Somerset (England) is dominated by red dolomitic mudstones and siltstones with thin beds of limestone, sandstone, and nodular or discoidal gypsum or celestite. Primary sedimentary structures are poorly preserved in the clastic units, but evidence of bioturbation and pedogenic features, including rootlet traces, is abundant. In contrast, many limestones contain parallel or ripple lamination. At the time of deposition, peloids seem to have been relatively common in the mudstone and carbonate facies. The sedimentary structures, plus the presence of ooids,Botryococcus, and charophyte fragments indicate that the carbonates are of subaqueous origin, but they may also show evidence of gullying, exposure, and desiccation.

Accumulation of the west Somerset Mercia Mudstone Group took place in a low-relief continental basin that had many features in common with the present arid-semiarid interior of Australia. The mudstones and siltstones are of mixed flood-plain and piaya origin. Some of the sediment was deposited from sheet floods, but material of distant, aeolian origin also made a significant contribution. Rivers entering the basin seem to have carried some of their load as peloidal mud aggregates. The carbonate units represent relatively wet climatic intervals when fresh to brackish lakes occupied shallow depressions on the alluvial/playa plain. Lakes were chiefly maintained by surface runoff, but the presence of spring-related carbonate grains implies that groundwaters may also have been of importance locally. Peloidal micritic carbonate grains in the lacustrine deposits are probably of local aeolian origin and are analogous to the clay pellets found in and around many Australian playas. The evaporites are early diagenetic, in part pedogenic precipitates that formed from saline groundwaters during arid periods. Cyclic alternation of carbonate and mudstone indicate that climatic change may have exerted an important control upon sedimentation during Norian time in southwest England.