DIAGENETIC PATTERNS IN CAMBRIAN SANDSTONE IN WESTERN WISCONSIN: CONSTRAINING POTENTIAL SOURCES OF AIRBORNE PARTICULATE MATTER

The Wonewoc and Jordan formations of western Wisconsin are important sources of frac sand used in hydraulic fracturing. These units are valued by industry due to the ultra-pure composition, loosely consolidated nature and resistance to fracturing. Statistical analysis of raw and processed sand was undertaken to assess if grain fragmentation, as evidenced by fresh, conchoidal fracture faces, occurs during processing. Petrographic examination of both raw and processed sand grain mounts yielded no evidence of fragmentation for raw sand while > 96% of processed sand showed no evidence of fragmentation.

Frac sand mining is controversial, in part due to the potential health concerns (silicosis) caused by fine grained particulate matter. This material is likely derived from interstitial cement, and constraining the diagenetic history of these units is critical. Petrographic analysis of the Jordan and Wonewoc formations has been used to quantify the composition of both detrital grains and interstitial cement.

Interstitial material consists of void space, calcite, dolomite, sericite, authigenic orthoclase feldspar, and hematite. Silica cement is rare, and is restricted to the upper Jordan formation just below the overlying Prarie du Chien Group. Analytical results demonstrate that interstitial material within the Jordan Formation (n=15) is void space (66%), hematite (10%), calcite (8%), authigenic orthoclase feldspar (7%), authigenic quartz (6%), and dolomite (3%). Interstitial material within the Wonewoc Formation (n=17) consists of void space (70%), hematite (16%), authigenic orthoclase feldspar (9%), sericite (4%), and authigenic quartz (1%). Cement composition varies regionally within both formations. There is a noticeable increase in void space and a decrease in hematite and authigenic orthoclase feldspar from northwest to southeast in the Jordan formation. The Wonewoc Formation displays a similar increase in void space and hematite from the northwest to the southeast, and a decrease in authigenic orthoclase feldspar and sericite.

The variation in the composition and quantity of interstitial material within and between formations suggests a complex multistage diagenetic history, that consists of orthoclase feldspar, dissolution, multiple calcite events, hematite and a final dissolution event.