STRATIGRAPHIC AND PALEOENVIRONMENTAL IMPLICATIONS OF CAMBRIAN GLAUCONITE ABUNDANCE, SIZE, AND MORPHOLOGY REVEALED BY PETROGRAPHIC ANALYSES

The authigenic green clay mineral glauconite is common, yet variable, throughout the shallow marine, storm-dominated Cambrian Tunnel City Group of WI and MN. While current research on glauconite is often limited to its chemistry, kinetics, and clay mineralogy, the variability in individual glauconitic grain morphologies, sizes, and abundances may also be informative of depositional histories. The goal of this study is to constrain the paleoenvironmental conditions that drive grain-scale variations in glauconite within a stratigraphic section.

Tunnel City Group glauconite varies in morphologies, sizes, and abundances with particular sedimentary structures and surfaces. Laminated and cross-laminated beds contain 5.3%-13.5% glauconite dominated by subangular grains (up to 0.4 mm) partially surrounded by fibroradiated rims. Flat-pebble conglomerates contain the least amount of glauconite (5.5%-6.3%), with a mix of pore-filling and subangular glauconitic grains (up to 0.4 mm). Omission surfaces host 9% glauconite that is almost entirely pore-filling in morphology and of larger size (up to 1 mm).

The co-occurrence of specific glauconite grain morphologies and micro-scale characteristics within a stratigraphic context can provide greater insight into detailed paleoenvironmental conditions. For example, abundant pore-filling glauconite on omission surfaces is commonly associated with iron oxides, suggesting both sediment starvation and protracted exposure to at least periodically oxygenated waters, such as would occur along flooding surfaces in nearshore settings.