Understanding Early Diagenetic Silicification: Petrographic Fabrics within Proterozoic Microfossiliferous Chert

• Main Author: Dunham, Jeremy Ian
• Format: Text
• Language: unknown
• Published: TRACE: Tennessee Research and Creative Exchange 2018
• Subjects: Baffin Island; Canada; Baffin
• Online Access: https://trace.tennessee.edu/utk_gradthes/5350; https://trace.tennessee.edu/cgi/viewcontent.cgi?article=6760&context=utk_gradthes

Early diagenetic chert is abundant in Proterozoic carbonate successions and provides a critical window into organic preservation; the depositional origin of this chert, however, remains uncertain. There have been relatively few detailed petrographic analyses of Proterozoic microfossiliferous chert, which limits our understanding of both primary depositional fabrics, and of petrographic change during post-depositional recrystallization. The ~1.1 Ga Angmaat Formation, northern Baffin Island, Canada, contains abundant early diagenetic chert that contains exquisitely preserved microbial assemblages. Petrographic analysis indicates that chert is composed of four primary phases: chalcedony spherules that replace the majority of primary microbial and sedimentary elements, equigranular microcrystalline quartz which occurs as a minor component in the groundmass, chalcedony that lines primary voids within the depositional fabric, and megaquartz that occurs within chalcedony-lined voids. Here we present a detailed petrographic characterization of chert within the Angmaat Formation to better understand the mechanism of formation and potential pathway of diagenesis. Observations suggest that the silicification process began with formation of a silica gel within benthic microbial mats and associated carbonate sediment. We suggest that gel formation was intimately associated with the presence of organic matter (e.g., microbes and associated EPS), and that constructional voids (e.g., gas bubbles) within the mat remained free of silica gel. Polymeric linkages within the gel phase appear to have provided sufficient structure for precipitation of spherules as opal-CT, rather than as opal-A, thus resulting in minimal water loss associated with post-depositional recrystallization to chalcedony which favored exquisite microfossil preservation.