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Permian red beds and evaporites in the midcontinent of the United States were formed and altered under the influence of extremely acidic (some with pH<1) lake waters and groundwaters. Based upon geologic, geochemical, and biogeological data, as well as comparison of these Permian rocks with modern acid lake systems in Australia, we propose criteria for the recognition of the products of acid deposition and diagenesis elsewhere in the rock record. Diagnostic criteria for acidity include high bisulfate and aluminum in fluid inclusions. Other features of these Permian rocks, including possible acid minerals, acidophilic bacteria, red beds, and lack of carbonates, are considered possible, problematic, or ambiguous criteria for acidity. Regardless, recognition of this assemblage of features in other rocks may lead to the discovery of other ancient acid systems. [Copyright &y& Elsevier]

The mid-Permian Nippewalla Group of Kansas consists of bedded evaporites, red-bed siliciclastics and grey siliciclastics deposited in a non-marine environment. Lithologies and sedimentary features indicate lacustrine and aeolian deposition, subaerial exposure and palaeosol formation. Grey siliciclastic mudstones characterized by planar and convolute laminations, ostracods, peloids and plant material represent a freshwater-brackish perennial lake facies. Bedded anhydrites containing gypsum-crystal pseudomorphs, clastic anhydrite grains and grey mud drapes and partings suggest deposition in saline lakes. Bedded halites consist of chevron and cumulate crystals, dissolution surfaces and pipes and mudcracked microcrystalline salt crusts, which were deposited in saline pans dominated by flooding, evaporative concentration and desiccation. Chaotic halite, composed of red-bed mudstone and siltstone with displacive halite crystals, formed in saline mudflats. Red-bed mudstone and siltstone with little or no displacive halite, but with abundant cracking, root and plant features, suggest deposition in a dry mudflat. Red-bed sandstone, composed of well-sorted, well-rounded quartz grains cemented with halite, indicate aeolian and rare shallow-water deposition. Most deposition took place in halite-dominated ephemeral saline lakes surrounded by saline and dry mudflats, sandflats and sand dunes. Evaporation, desiccation, flooding and wind played significant roles in this environment. The Nippewalla Group siliciclastics and evaporites represent an evolution from a perennial lacustrine system to a non-marine, acidic saline pan system in the mid-continent of North America. The problem of distinguishing between ancient marine and non-marine evaporites, as well as recognizing those evaporites deposited in acid settings, with detailed field, core and petrographical study of both evaporite deposits and associated sedimentary rocks has successfully been addressed. In addition, interpretations of mid-Permian palaeoclimate data in the form of short-term air temperature proxies within longer-term wet–dry trends have been made. These data provide a new palaeogeographic and palaeoclimatic model for the mid-Permian of western Pangaea. [ABSTRACT FROM AUTHOR]

Presents research which argues for the existence of highly acidic lakes and ground waters in the North American Permian period. Where evaporites are located; Use of laser Raman microprobe analysis; Modern analogues of such systems; Traits of systems; Implications of research.