HIDDEN BENEATH THE SURFACE: LITHOSTRATIGRAPHIC DIFFERENCES IN SURFICIAL AND BURIED PERMIAN STRATA ARE KEY TO UNDERSTANDING EVAPORITE KARST PROCESSES IN KANSAS AND OKLAHOMA (Invited Presentation)

Caves, sinkholes, and brackish and saline groundwater have been attributed to dissolution of near-surface evaporites in southern Kansas and northern Oklahoma. Their origin is complicated by late-stage dissolution of multiple minerals in various forms. The Permian Sumner, Nippewalla, and Quartermaster Groups comprise hundreds of meters of red-bed siliciclastics and evaporites in the subsurface, but their thickness is reduced by dissolution at, and near, the surface.

The Sumner Group contains bedded halite and gypsum (including the economically valuable Hutchinson Salt Member), some diagenetic halite and gypsum, abundant red siliciclastic, and rare carbonate and gray siliciclastic rocks. The Nippewalla Group consists of bedded halite, bedded gypsum, displacive halite, displacive gypsum and halite- and gypsum-cemented red siliciclastics. The Quartermaster Group contains red beds and bedded and diagenetic gypsum. Intergranular evaporite cements make the red beds as vulnerable as the bedded evaporites to karst processes.

Detailed comparisons of cores, salt mines, and outcrops reveal challenges in correlation of surface and subsurface beds and their stratigraphic nomenclature. More importantly, they provide details about the timing and depth of dissolution. Additional lab experiments estimate reduction in unit thicknesses and the missing rock record.

The three dominant lithofacies, bedded halite, bedded gypsum, and evaporite-cemented red beds, undergo different chemical weathering processes and rates. Bedded gypsum is the most resistant to dissolution, making it the prevalent caprock in the region, yet, on a geologic scale, it weathers and erodes quickly. A butte-and-slope landscape seems to be forming rapidly near Medicine Lodge, Kansas, as near-surface dissolution occurs in the Nippewalla Group. Differential weathering of the horizontal beds in the Group’s Flowerpot Shale and Blaine Formation has caused a westward migration of the butte-dominated landscape over the past hundred years that is substantiated by field maps and photographs. Such processes can only be fully understood when evaporite types and thicknesses are evaluated in buried, unaltered sequences of these strata. The detailed geology hidden in the subsurface provides clues to past and future karst processes and products.