DIAGENETIC HISTORY OF CAMBRIAN SANDSTONE UNITS IN WESTERN WISCONSIN

The Late Cambrian Wonewoc and Jordan Formations are contained within cyclic successions of cross-stratified, medium-to coarse-grained quartz arenite intercalated with thin bedded, fine grained, locally glauconitic, subfeldspathic arenite and subordinate clay laminae. These units are valued by petroleum industries for hydraulic fracturing due to the ultra-pure, monocrystalline, well-rounded grains and weak cementation.

Petrographic analysis of the Wonewoc (n=19) and Jordan (n=30) formations quantifies the composition of framework detrital grains and interstitial cement. Detrital grains are primarily monocrystalline quartz (95%); polycrystalline quartz is rare. Interstitial spaces are occupied by voids, dolomite, illite, authigenic orthoclase feldspar, and hematite. Interstices of the Jordan Fm. average void space (63%), dolomite (17%), hematite (8%), authigenic quartz (7%), authigenic kspar (4%), and illite (<1%). Interstices of the Wonewoc Fm. average void space (70%), hematite (17%), authigenic kspar (9%), illite (4%), and authigenic quartz (<1%). Void space increases as hematite and authigenic kspar cement decrease from northwest to southeast in the Jordan Fm. In the Wonewoc Fm. void space and hematite cement increase from northwest to southeast, but authigenic kspar and illite cement decrease along the same trend.

Diagenesis of these units is complex, involving multiple precipitation and dissolution events. Regional stratigraphic data suggest the units have remained under shallow diagenetic conditions since initial deposition (T < 100 C, P < 0.5 kbar). The Cambrian succession formed a porous and permeable conduit between underlying Proterozoic basement and overlying Ordovician crystalline carbonates, likely subject to multiple generations of fluid flow of variable composition and duration through time. Petrographic examination of interstitial cements constrains paragenesis: 1) authigenic kspar precipitation 2) dolomite precipitation (2 stages), 3) hematite precipitation, 4) illite precipitation. Precipitation events appear to be followed by dissolution that provides secondary porosity for next generation fluid flow. Silica cement is rare and appears to be localized along aquitards, such as the top of the Jordan Fm. along its contact with overlying Prairie du Chien Group.