Evaporites and siliciclastics of the Permian Nippewalla Group of Kansas, USA: a case for non-marine deposition in saline lakes and saline pans



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.