LITHOSTRATIGRAPHIC REVISION UTILIZING PORTABLE XRF ANALYSIS: A CASE STUDY USING THE DEVONIAN MILWAUKEE FORMATION

Inconsistent descriptions and time-transgressive units are the hallmark of lithostratigraphy. In an attempt quantify map unit composition we have undertaken a case study using pXRF on Devonian strata from the western Michigan Basin. The late Givetian Milwaukee Formation was first described by Alden (1906) and subsequently revised by Raasch (1935), who subdivided it into the Berthelet (oldest), Lindwurm, and North Point (youngest) members. The Berthelet and basal Lindwurm members were described from exposures and cement quarries along the Milwaukee River that are now mostly covered, while most of the Lindwurm and the North Point member were described from well cuttings, rocks found in glacial drift, and material collected from Lake Michigan drinking water intake tunnels built by the City of Milwaukee. More recently, cores drilled during the construction of underground storm-water reservoirs for the Milwaukee Metropolitan area have provided a continuous succession through the entirety of the Milwaukee Formation (Kluessendorf et al 1981). We described the color, texture, and fabric of this unit in two of these cores, and also conducted elemental analysis using a pXRF analyzer. Non-destructive pXRF analyses of the outer surface of the I30-7-NS core were spaced at 1-foot intervals; to determine the effects of grain size and mm-scale heterogeneity on the results, we also analyzed powdered samples from the same interval in the I30-8-NS core, from ~1 mile away. This new dataset allows stratigraphic revision of the Milwaukee Formation. Using continuous core sections and quantitative elemental data we place the Lindwurm-North Point contact higher than previously defined. The Berthelet Member is a fossiliferous gray argillaceous dolostone, the Lindwurm a fossiliferous dark gray dolomitic mudstone, and the North Point a relatively clean dolomitic pack-to-grainstone with abundant pyrite. Using pXRF data as a proxy for lithologic changes, discrete transgressive-regressive cycles can be recognized in both the Berthelet and the newly revised Lindwurm members; these cycles are not apparent from lithologic descriptions alone. Comparison of pXRF results from the two closely spaced cores reveals highly similar elemental patterns and absolute values, suggesting that this is a robust tool for quantifying map units.