PELLETS BEFORE POOPERS: AUTHIGENIC GLAUCONITE FROM THE MESOPROTEROZOIC SHEPARD FORMATION, WESTERN MONTANA

The strikingly green mineral glauconite [K(Fe3+, Mg, Al, Fe2+)2(Si, Al)4O10](OH)2] is a marine authigenic mineral that is generally believed to form slowly (>100 ka) within the sedimentary surface mixed layer under oxygen-poor, weakly reducing conditions. The process of glauconite formation, or glauconitization, requires the prolonged exchange between a dissolved cation supply (K⁺, Mg²⁺, Fe²⁺) from porewaters and a suitable host microsystem (i.e. fecal pellet, bioclast cavities) or pre-replacement material (i.e. detrital grain, clay cement) –hereafter, “host grain,”. Recent research regarding the formation and mineralogical evolution of glauconite has begun to untangle the diverse mechanisms of glauconitization. However, intense debate persists regarding the kinds of host grain that most favor the glauconitization process. For example, glauconite grains in Phanerozoic strata are typically described as being pelletal in shape, that is rounded but elongate, leading fecal pellets to be invoked as original host grains. This theory is, however, based solely on petrographic analyses and comparisons of grain morphologies to fecal pellets. In contrast, the few known examples of glauconite grains from Precambrian strata are often granular, that is angular to sub-angular with indistinct and irregular grain boundaries, and are presumed to have formed from the replacement of K-feldspar grains, considering they predate the advent of metazoans and fecal pellets.

It is important to note the Precambrian glauconite is rarely reported in the literature, likely due to frequent chloritization and as a result, our understanding of glauconitization is restricted to Phanerozoic case studies. Here I present a petrographic analysis of glauconite-rich siliciclastic samples from the Shepard Formation of the Mesoproterozoic Belt Supergroup. Initial observations revealed pelletal glauconitic grains that closely resemble the shape, size, and color of published images of pelletal Phanerozoic glauconite presumed to be fecal in origin. This suggests that fecal pellets may not be the most common host grain favored by the glauconitization process throughout geologic time. Instead, glauconite grains may have multiple modes of formation that are dependent on the unique characteristics and abundances of host grains, which vary as a function of environment and geological age.