Recognizing acid lakes and groundwaters in the rock record

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 modem 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. (C) 2002 Elsevier Science B.V. All rights reserved.