REGIONAL PETROGRAPHIC ANALYSIS OF CAMBRIAN SANDSTONE IN MINNESOTA AND WISCONSIN: QUANTIFYING MINERALOGY AND POTENTIAL SOURCES OF AIRBORNE PARTICULATE MATTER WITHIN “FRAC SAND” TARGETS

Two Cambrian sandstone units in Minnesota and Wisconsin, the Wonewoc and Jordan formations, are sources of high-quality “frac sand.” The rapid increase in mining and processing of frac sand has led to concerns about potential health hazards, particularly silicosis from airborne respirable silica. A regional petrographic analysis, focused on the Wonewoc (n=17) and Jordan (n=27) fms., provides insight into detrital and matrix compositions, and constrains the types of airborne particles liberated during the production process.

Petrographic examination illustrates similarities and differences between the Wonewoc and Jordan fms., and suggests geographic variations in sandstone mineralogy. Both units primarily contain detrital monocrystalline quartz (65-70%) and void space (18-22%). Cement composition varies both by formation and geographically within formations. Cement in the Wonewoc Fm. is dominated by hematite (~18%) with lesser authigenic feldspar (Kspar, 5%) and sericite (4%). Jordan Fm. cement contains up to 20% calcite with lesser authigenic Kspar (6%) and hematite (10%). The Wonewoc Fm. displays minor geographic variations, with a slight decrease in detrital Kspar from north to south (5% to 2%), but a distinct difference in cement composition, varying from 17% authigenic Kspar cement in the north to 4% in the south. Jordan Fm. shows similar variation in authigenic Kspar (17% north vs. 4% south), but also shows a distinct geographic variation in the hematite (20% north vs. 7% south) and calcite (13% north vs. 19% south) components. Silica cement is essentially absent, with silicified zones <2m thick limited to restricted horizons in the upper Jordan Fm. in the southern region.

Textural analysis indicates both formations have undergone multiple diagenetic events, including an initial generation of authigenic Kspar, followed by hematite, then carbonate in the Jordan Fm. The high amount of pore space apparently results from late-stage dissolution.

Frac sand processing is designed to minimize crushing of the strong framework quartz grains, so fine particulate matter primarily should be derived from interstitial cement. These analyses indicate that interstices in the Wonewoc and Jordan fms. contain mainly void space, calcite, hematite, authigenic Kspar and sericite. Interstitial silica is rare.