Evolution of Gypsum Karst in the Kirschberg Evaporite Member Near Fredericksburg, Texas

Abstract

ABSTRACT At the Fredericksburg Gypsum Quarry in Gillespie County near Fredericksburg, Texas the quarry operations expose modern karst features in a 10-m (30 ft) thick gypsum bed that is part of the Cretaceous Kirschberg Evaporite Member. The following sequence of events is documented: 1) Formation of nodular daisy-head gypsum. Replacement by gypsum was at first slow but pervasive, with phreatic fluids oozing through the calcium sulfate bed. The main permeability pathways were provided by fractures and the intercrystalline porosity in dolomite films surrounding the calcium sulfate nodules and along thin dolomite beds. Large aligned gypsum crystals nucleated off these flow paths and grew by replacement of the earlier calcium sulfate phase. A change to more active hydrologic conditions modified the rate of secondary gypsum growth, so that the remaining gypsum grew rapidly as a microcrystalline alabastrine form. This adjustment in gypsum type was due to the relative uplift of the bed into more active phreatic conditions closer to the water table. There the bed was subjected to more pronounced temperature changes and increased groundwater flow. 2) Karst Structures and Speleothems. The gypsum bed was introduced into the vadose zone and the process of karstification of the bed began. Dissolution took place forming vertical pipes or caves in the Fredericksburg gypsum bed. Gypsum and calcite speleothems, mainly in the form of popcorn and flowstone, were deposited in the pipes and caves. Extreme dissolution of the gypsum bed produced zones of collapse breccia. This collapse occurred gradually. Locally, the overlying beds were disrupted and folded; regionally, the overlying beds were minimally disrupted by widespread lowering due to complete dissolution and gentle collapse folding. Coarse, botryoidal calcite grew in void spaces in the breccia as it was collapsing. The effects of the vadose regime on the Fredericksburg gypsum bed suggest that the end result of such an unroofed evaporite bed, even if it is never subaerially exposed, is a stratiform layer that has associated with it a breccia consisting of calcite popcorn, boxwork and botryoidal calcite, mixed with fragments of the overlying strata. Little if any evidence of the gypsum remains in the breccia. Textures and structures described herein are used to interpret similar features in other ancient evaporite beds.