The mid Permian Opeche Shale of North Dakota is composed of bedded halite and red bed siliciclastics deposited in an acid lake and saline pan system. Some halite beds consist of only chevron and cumulate crystals, indicating deposition in perennial shallow saline lakes. Other halite beds contain the chevron and cumulate crystals together with dissolution and desiccation features, indicating deposition in saline pans. All red mudstones and siltstones contain paleosol features such as cracks and root features, but some also have displacive halite crystals. The red mudstones and siltstones with abundant displacive halite probably formed in saline mudflats, while those with little halite formed in dry mudflats. Textures and sedimentary structures in red sandstones and conglomerates suggest distal alluvial and aeolian deposition.

Fluid inclusions in bedded halite and displacive halite preserve ancient saline lake waters and groundwaters, respectively. Laser Raman microprobe analyses of fluid inclusions have detected bisulfate (HSO₄^-1), an indication of extremely acid waters, with pHs < 1. Fluid inclusion leachates have relatively high concentrations of aluminum, iron, and silica, a composition characteristic of modern acid waters. Small organic bodies trapped within bedded halite may be remains of bacteria that influenced the water chemistry. Sulfate minerals such as anhydrite and polyhalite exist as solid inclusions in halite and as crystals within fluid inclusions.

Fluid inclusion chemistry and other observations may serve as clues for recognizing ancient acid lake systems. Such recognition is the first step in evaluating the importance of acid lakes of the geologic past.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah