BIOGENIC SEDIMENTARY STRUCTURES IN THE EARLY CAMBRIAN YADKIN FORMATION

During the Cambrian period, the Carolina Terrane was an offshore arc located off the coast of Laurentia, within the Iapetus Ocean. The Yadkin Formation is the uppermost unit of the Albemarle Group, a succession of Neoproterozoic through Cambrian metasediments deposited on and around this arc. They thus provide a valuable glimpse into shallow water paleoenvironments in the open ocean during early animal evolution. These sediments contain fine laminae and apparent domal stromatolites, indicating that the sedimentary substrate may have been stabilized by microbial mats. In this study, we focused on a succession of Yadkin Formation sediments exposed in Big Bear Creek, outside of Albemarle NC. The putative microbially induced sedimentary structures were analyzed with field based, petrographic, and geochemical approaches, and tested against the six criteria for biogenicity established by Noffke (2009). Symmetrical ripples are preserved on several bedding planes, indicating a tidal setting, with little to no sea level change during deposition of the studied succession and a consistent direction of water flow. This is supported by the application of newly developed geochemical proxies for salinity, which suggest that the unit was deposited in a fresh to brackish water environment, consistent with a tidal delta or estuary. The orientations of the long axes of elliptical laminated domes were compared to the water flow direction, establishing a relationship between these dome structures and the local hydraulic environment. The domes become increasingly elongate in shape with increasing overall size, suggesting that they grew by extension of the long axis perpendicular to the direction of tidal flow. While the presence of chlorite indicates metamorphism to lower greenschist facies, petrographic observations show the sediments range from silt to medium sand size, and are composed of millimetric graded laminae, consistent with deposition in the photic zone. Isotope measurements of sedimentary nitrogen are consistent with direct N-fixation. This indicates that nitrogen-fixing cyanobacteria may have been a major component of these microbial mat ecosystems, similar to extant microbial mats today. Based on these observations, we can conclude that the domes are likely to represent biogenic stromatolites.