BUILDING A NEW REGIONAL SYNTHESIS OF CAMBRIAN SILICICLASTICS: CHRONOSTRATIGRAPHY AND PALEOENVIRONMENTAL ANALYSIS OF THE EAU CLAIRE THROUGH TUNNEL CITY INTERVAL (STAGE 3-FURONGIAN) ACROSS WISCONSIN

Cambrian sandstones and shales of the US midcontinent have a reached a high level of prominence in the areas of energy extraction and byproduct sequestration—playing central roles as proppant, sink and barrier. They also play an important role as aquitard between upper contaminated aquifers and lower uncontaminated aquifers in southern to western Wisconsin. Despite a boom in exploration, exhumation and processing, detailed knowledge remains localized and restricted to narrow portions of this complex stratigraphic succession. New studies in Wisconsin seek to provide a high resolution chronostratigraphic synthesis and depositional history for the midcontinent region that will greatly reduce uncertainty associated with distribution of these natural resources.

Several recently acquired drill cores from across southern to western Wisconsin provide samples and down-hole geophysics to build a refined understanding of the Cambrian succession. Sampling for this study targeted the Eau Claire through Tunnel City interval. Analysis included handheld XRF and stable carbon isotopes, in addition to more traditional lithologic, biostratigraphic and geophysical log correlation.

This new data set, combined with archived geophysical logs, provides a three-dimensional sequence stratigraphic framework, where the distribution of lithofacies within narrowly-defined time-slices reflects migration of the late Cambrian shoreline and parallel environmental belts. Whole-rock XRF provided quantification of facies and insights to diagenetic history. Our unconventional use of C-isotopes from dolomite cements shows high-reproducibility between closely spaced sections, reflecting a chronostratigraphic record of late Cambrian carbon cycle fluctuations, including the SPICE event as documented in previous studies of brachiopods in the region. Truncation of offshore shales to midramp argillaceous sandstones below multiple high-frequency sequence boundaries removes regional aquitards in southern to central Wisconsin. Time-ordered facies and isopach maps provide a greatly refined picture of Cambrian shoreline trajectory and its evolution through multiple sea level fluctuations providing a high level of confidence in the prediction and mapping of lithofacies.