COMPUTED TOMOGRAPHY SCANNING OF A TUFA DEPOSIT CORE FROM WESTERN PENNSYLVANIA

Active limestone precipitation and accretion around springs and other groundwater seeps, due primarily to the degassing of carbon dioxide and the subsequent supersaturation of calcium carbonate, results in formations variably referred to as tufa or travertine. Such deposits in eastern North America are especially well-known in the karst region of western Virginia, often associated with the discharge of thermal spring waters, but reports of tufa formations outside of this specific region are few. Recently, we have studied a perched springline tufa that is associated with the Benwood Limestone member of the Upper Pennsylvanian, Monongahela Group. Like many sites in this region, the steep elevational drop in discharged water and enhanced carbon dioxide release are related to human disturbances of natural terrain during the past 150 years, such as highway and railroad construction and the strip mining of bituminous coal. Preliminary core material was collected from the tufa site and analyzed by computed tomography (CT) scanning. The core scans showed the limestone material to be extremely vuggy and porous. The CT scanning produced 3D volume reconstructions with resolution in the 20-30 micron range making it possible to nondestructively extract various physical characteristics of the tufa samples. For each sample the porosity and pore size distribution were extracted as well as the surface area of the porosity and the connectivity of the functional pore spaces. The observed limestone structure is consistent with the active and continuous calcification of the moss that grows in dense mats on the surface of the tufa deposit. Such core scans may provide the basis for future descriptive modeling in the fields of karst geology and carbon dioxide sequestration.