COMPARATIVE ANALYSIS OF CAMBRIAN GLAUCONITIC PALEOENVIRONMENTS OF THE TUNNEL CITY GROUP, WISCONSIN AND MINNESOTA

The authigenic green clay mineral glauconite is abundant and varied within the shallow marine, high-energy Cambrian Tunnel City Group of WI and MN. Glauconite abundance, grain size, and morphology are relatively understudied aspects of this mineral, and can reveal paleoenvironmental details that complement other sedimentologic data. This study evaluates variation in the physical characteristics of glauconite between specific bedding types (e.g., laminated, omission surfaces, etc.) indicative of different depositional conditions. We utilize both field and petrographic approaches. This research aims to constrain the formative conditions of glauconite using large datasets to further develop its use as a paleoenvironmental tool.

This study focuses on two key localities: Arena, WI and Red Wing, MN. Arena hosts observable glauconite grains in ~50% of beds, while Red Wing hosts observable glauconite in ~40% of beds. Glauconite is predominantly present in sandstone units. Overall, beds are thicker at Red Wing and thicken upsection at both localities. Sedimentary structures indicative of moderate to high energy are present at both localities, as are Skolithos and other trace fossils.

Glauconite is most abundant within clastic beds (mean of 8.34% glauconite vs. mean of 4.56% in carbonate beds), though the range of abundances is highly variable by sample. The maximum abundances of glauconite in particular clastic bedding types were 21.6% (bioturbated), 16.5% (flat-pebble conglomerate), and 13.5% (cross-laminated).

The most common glauconite grain morphologies were rounded and subangular, while vermiform grains were the least common. At a finer scale, ~50% of vermiform grains had phosphatic rims and ~40% had fractures. Grain size is relatively consistent across morphologies and bedding types (0.15 to 0.25 mm diameters), with the largest grains hosted in laminated beds. Glauconite on omission surfaces occurs predominantly as pore-filling cements which suggests in situ growth.

These Cambrian nearshore clastic settings supported abundant glauconite formation and deposition across high-energy and bioturbated bedding types. Furthermore, microscale grain characteristics exhibit subtle trends likely influenced by microenvironment conditions directly surrounding glauconite grains at the time of formation.