20-4 New Insights into Shallow Water Facies of the Eau Claire Formation in its Type Area

Session: Undergraduate Research

Presenting Author:

Lia Dirks

Authors:

Dirks, Lia Marin¹, Jensen, Donovan², Marchetti, Parker³, Rougvie, James R.⁴, Zambito, James Joseph⁵

(1) Department of Geology, Beloit College, Beloit, WI, USA, (2) University of Illinois, Urbana, IL, USA, (3) William & Mary, Williamsburg, VA, USA, (4) Department of Geology, Beloit College, Beloit, WI, USA, (5) Department of Geology, Beloit College, Beloit, WI, USA,

Abstract:

The Eau Claire Formation is a key hydrostratigraphic unit in the Upper Midwest U.S., serving as an important regional aquitard. Lithostratigraphically, the shaly Eau Claire Formation allows subdivision of the underlying quartz sandstone-dominated Mount Simon Formation from the overlying quartz sandstone-dominated Wonewoc Formation. Recent study of the Eau Claire Formation in the Hi-Crush Proppants Augusta OB-1 rotosonic core (OB-1 Core) has recognized a shallow water facies of this unit that is devoid of features that typify this unit. These features include glauconite, body fossils, and generally fine grain sizes. These observations enhance our understanding of the stratigraphy and facies models of the Eau Claire Formation, and since the OB-1 Core was drilled near the type area of the Eau Claire and Mount Simon formations, have the potential for recognition of a key reference succession for these units.

 

In the OB-1 Core, separating the Eau Claire Formation from the underlying Mount Simon and overlying Wonewoc formations is complicated by the overall sandstone-dominated lithology of the succession. In this study, the formation boundaries are identified through elemental composition from pXRF analysis. As typically described in its type area, the Eau Claire Formation is a mix of interbedded fine-grained, often glauconitic sandstones with siltstones and blue-grey shales commonly containing disarticulated trilobites and lingulid brachiopods in some beds. The Eau Claire Formation in the OB-1 core differs lithologically in the presence of coarse to granule-sized grains in sandstones that are interbedded with blue-green shales. Visual inspection of the core in hand sample and under a binocular microscope, in combination with XRD mineralogical analysis of various beds throughout the Eau Claire interval, has failed to identify the presence of glauconite. Likewise, visual observation of the OB-1 Core, including splitting along bedding planes and under a binocular microscope, has failed to yield evidence of body fossils though bioturbation has been observed at various horizons. 

 

Our working hypothesis is that the Eau Claire Formation in the OB-1 Core was deposited in a marine setting proximal to riverine input. Lower salinity would have precluded organisms like trilobites and brachiopods, oxygenation prevented the formation of glauconite, and fluvial input resulted in the presence of coarser clasts. This has important implications for paleogeographic and facies reconstructions during Eau Claire deposition.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

doi: 10.1130/abs/2026SE-12784

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.