Petrogenesis and Petrography of Pisoliths and their Connection to Terra Rosa “Red Earth”

Adair, Christopher
Western Kentucky University, Department Geology, Bowling Green, KY

When limestone is disintegrated by solution processes, it leaves behind a clayey replacement product known as terra rosa. This terra rosa (Latin for "red earth") is characteristic of much of Kentucky’s karst terrain. Terra rosa appears to be directly connected to landscapes where similar weathering processes have been taking place for hundreds of thousands of years. Pisoliths can also be found in these soils. Pisoliths (which translated means pea like) are small, approximately spherical, pebble sized grains (0.5-2.0 cm) that appear to form during the latter stages of weathering. Mineralogical composition of pisoliths has been determined to be primarily bauxite, hematite, and goethite depending on the geographical location. In addition to these minerals, gibbsite, boehmite, quartz and kaolinite have been found using infrared microspectroscopy. There is an apparent connection between the formation of iron-rich terra rosa and pisoliths. It is known that prolonged weathering of sedimentary carbonates will cause fissures through which meteoric water transports iron and other clay particles. The question that remains to be answered is, what is the source of this iron, and, is there a possible connection between iron-rich terra rosa and pisoliths that are found in geologically similar areas?

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004