OH WHERE, OH WHERE IS THE EAU CLAIRE? USING PXRF ELEMENTAL ANALYSIS TO IDENTIFY CHEMOSTRATIGRAPHIC UNITS IN A VISUALLY MONOTONOUS QUARTZ SANDSTONE SUCCESSION

The Cambrian (Marjuman-Steptoean) Eau Claire Formation is a key stratigraphic unit in the Midwestern U.S., serving as a regional aquitard as well as recording the Crepicephalus-Aphalaspis Biomere boundary and the global SPICE event onset. In offshore facies successions in western Wisconsin, the Eau Claire is a bioturbated, interbedded gray mudstone and glauconitic dolomitic sandstone containing trilobites and lingulids. In onshore successions in central Wisconsin, on the Wisconsin Arch, the Eau Claire is difficult to identify. In these shallow marine settings, the Eau Claire: 1) has significantly changed facies and is comprised of quartz sandstone with minor green mudstone stringers, and/or, 2) was erosionally removed during sea level lowstand associated with the SPICE event.

We use portable x-ray fluorescence (pXRF) elemental data to test the above hypotheses through correlating components of the Eau Claire Fm. (not just shales, which pinch out) into nearshore successions where it has not been possible to differentiate it from the surrounding sandstones using conventional techniques. Two rock cores and four well cutting sets were examined and correlated along a ~60 km offshore-onshore transect in south-central Wisconsin. An additional reference core from the type-region of the Eau Claire (western Wisconsin) was also studied. In the shallowest successions, examination of aluminum, calcium, and potassium concentrations indicate that a correlative chemostratigraphic unit can be identified, even at elemental concentrations of <1%. Aluminum is more useful than potassium, and, correspondingly, gamma shows only subtle correspondence to these correlative chemostratigraphic units. In all successions investigated, we recognize two depositional and/or chemostratigraphic sequences in the Eau Claire Fm.; neither is truncated by the SPICE lowstand. In nearshore successions the upper sequence is more likely to contain shale, likely a consequence of the Eau Claire transgression eventually overcoming the progradation of quartz sand. In nearshore settings on the Wisconsin Arch, pXRF elemental analysis is a useful tool for recognizing a chemostratigraphic equivalent to the Eau Claire Fm., thereby improving stratigraphic correlations, relative age determinations, mapping, and hydrostratigraphic models.