PETROGRAPHIC ANALYSIS OF CAMBRIAN SANDSTONE CEMENT IN WESTERN WISCONSIN: IMPLICATIONS FOR THE COMPOSITION AND QUALITY OF FRAC SAND

Industrial sand mining has been expanding rapidly in western Wisconsin as demand for frac sand has increased. Concerns have been expressed about the potential for dust from the mining process causing non-occupational silicosis. Because the sand grains are large and hard, most respirable dust is likely derived from the sandstone cement. The purpose of this study is to use petrographic analysis to determine the cement mineralogy for major sandstone units in western Wisconsin.

Samples have been collected from the Mount Simon, Wonewoc, Jordan, and St. Peter Formations (Cambrian Period, listed from oldest to youngest) in western Wisconsin. Samples were cut, impregnated with epoxy, and made into polished thin sections. Thin sections were examined using a petrographic microscope. Standard point counting and photomicrograph techniques are used to determine the mineralogy and relative percentages of the grains, cement, and voids.

Qualitative observations (11 thin sections thus far) for each formation reveal authigenic K-spar, hematite, and quartz cements. The most abundant cement is authigenic K-spar, and the majority of samples have high porosity and little cement. Mount Simon Fm. sandstone (the oldest unit) contains large amounts of hematite and authigenic K-spar cement, with or without sericite. Wonewoc Fm. cement compositions vary, but abundant hematite and authigenic K-spar are present throughout the formation. Jordan Fm. cements differ markedly. Some rocks are primarily cemented by calcite. One sample from the upper Jordan Fm. is completely cemented with quartz, which has eliminated primary porosity and makes the rock extremely difficult to disaggregate and process. St. Peter sandstone has polycrystalline quartz grains rimmed with minor amounts of hematite and authigenic K-spar.

A more extensive suite of samples is currently being analyzed. Quantitative results will presented at the meeting. The relative abundance of K-spar and hematite cements should reduce the concentration of crystalline silica in the respirable dust fraction. Also, multiple generations of cement and the high porosity values suggest repetitive cycles of cementation and diagenesis.