PALEOENVIRONMENTAL IMPLICATIONS OF GLAUCONY FACIES: A CASE STUDY IN THE LOWER PALEOZOIC OF THE MIDWEST, USA

One of the most distinctive and potentially informative sedimentary facies is that concentrated in green authigenic minerals, referred to as glaucony or verdine facies, depending on the composition. Despite being frequently noted in much sedimentary research, the paleoenvironmental implications of these green facies remain relatively unconstrained. This study integrates field observations with petrographic and chemical analyses to assess key characteristics and track fluctuations in paleoenvironmental conditions of glauconite grains and glaucony facies.

Within the studied interval in WI and MN, spanning the Cambrian Tunnel City Group through the Ordovician Prairie du Chien Group, abundant granular glauconite is favored in siliciclastic settings, is typically medium to dark green in color, and accumulates along laminae and in moderately bioturbated beds. Finer-grained and compositionally immature siliciclastics from lower energy shelf settings produce the highest concentrations of glauconite. Glauconite in carbonate settings is less abundant, is fine-grained to clay-sized, and varies widely in color (pale green to yellow-brown) and morphology. Glauconite associated with sandy carbonates and microbial mat deposits occurs dispersed throughout beds or concentrated on minor omission surfaces.

While the abundance of glauconite grains and likely depositional contexts of glaucony facies can be generalized and has predictive power, there is substantial variation in individual glauconite grain morphology, color, and replacement. At a large scale, glaucony facies can be used to indicate particular categories of conditions (e.g., elevated nutrients or sediment starvation can yield high concentrations of glauconite). However, individual grain types within those facies should be used to refine specific and local paleoenvironmental conditions and processes (e.g., colloform grains most likely formed from the alteration of fecal pellets, while zoned glauconite grains most likely formed as a replacement of siliciclastic or bioclastic grains due to increased porewater Fe concentration). Overall, a combination of local factors and microenvironments are shown to be particularly critical for glaucony development, in contrast to previous restrictive interpretations of formative conditions.