The NSS Bulletin - ISSN 1090-6924
Volume 57 Number 1: 43-51 - June 1995

A publication of the National Speleological Society

Helictite Bush Formation and Aquifer Cooling in Wind Cave, Wind Cave National Park, South Dakota
Edward J. LaRock and Kimberley I. Cunningham

Abstract

Limited petrographic, mineralogic, and isotopic data obtained from helictite bush and boxwork speleothems in Wind Cave, Wind Cave National Park, South Dakota, suggest a bacterial role in the subaqueous precipitation of helictite bushes, help to constrain the chemical conditions under which the bushes precipitated, and provide an independent calibration of paleotemperatures for the Wind Cave aquifer. The helictite bushes dispaly three types of growth fabrics: canal-wall crystals resembling soda-straw speleothems, fine-grained to micritic transitition-zone crystals, and coarsening-outward palisade crystals. Radial sections of canal-wall crystals contain broken rings of suspected bacterial filaments occasionally displayed as moldic porosity; bundles and clumps of filaments and iron-bearing opaque bodies are prevalent in the transition-zone crystal layers. The bushes are predominatly low-magnesium calcite containing minor amounts of dolomite, reflecting the bicarbonate-buffered chemistry of the slowly draining Wind Cave aquifer. Fluid-inclusion paleotemperatures from boxwork calcite spar provide an independent calibration of the calcite-water paleothermometer for Wind Cave speleothems by not having to assume the d18Ow of the paleowaters. The Wind Cave aquifer has cooled 28°C in the last 200,000 years according to oxygen isotopic data from the bushes.

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