BUILDING A FACIES MODEL OF THE NEOPROTEROZOIC KONNAROCK FORMATION

The Neoproterozoic Konnarock Formation, located in the Mount Rogers area of southwestern Virginia, offers valuable insights into Earth’s climatic and tectonic evolution during a period of significant environmental change. Diamictites and varved sediments containing dropstones have both been interpreted to reflect glacial deposition. In concert with paleomagnetic values placing the Konnarock at low latitudes during deposition, these observations raise questions regarding the relevance of this unit to the Snowball Earth hypothesis, which posits global-scale glaciation during the Cryogenian period of the Neoproterozoic. The goal of this study was to reconstruct a facies model of the Konnarock formation, to allow for deeper insight into its depositional environment and the environmental conditions during deposition. We applied for the first time the geochemical paleosalinity proxies developed by Wei and Algeo (2020), which indicate that these strata were deposited in a freshwater environment, most likely a rift lake influenced by glacial meltwater and tectonic activity along the Laurentian margin during the breakup of Rodina. Based on this result, lithofacies within the Konnarock can be situated within a proximal-to-distal facies model, with diamictites representing deposition at the toe of a glacier, sandstones or siltstones displaying cross-bedding or hummocky cross stratification representing meltwater streams or shallow lake environments, and laminated siltstones representing deeper lake settings. Chemical Index of Alteration values and petrographic observations indicate minimal chemical weathering, though a proximal-to-distal weathering gradient can be discerned, suggesting a cool continental climate, potentially in a high-elevation headwater setting. While millimetric laminations superficially resemble Precambrian microbial mats, we found no evidence for microbialites in the Konnarock Formation, with fine-scale sedimentary structures reflecting grain size changes and heavy mineral lags, which can be explained via abiotic processes. These findings refine our understanding of localized depositional systems’ in the late Precambrian, challenging broad correlations with global events like Snowball Earth.