PALEOBIOLOGY, SEDIMENTOLOGY, AND DISTRIBUTION OF CHERT IN THE TONIAN FIFTEENMILE GROUP OF YUKON-ALASKA MOUNT SLIPPER ASSEMBLAGE

Carbonate and chert deposits from the Tonian (ca. ~800 Ma) Fifteenmile Group, Yukon, Canada contain both unique ‘scale’ microfossils and a diverse microbial and eukaryotic population of organic-walled microfossils. While the organic-walled components of this fossil assemblage bear similarity to biota described from coeval strata worldwide, the scale microfossils have not been found in deposits of similar age from other sections within the Proterozoic inliers of northern Canada or elsewhere. The fossiliferous strata of the Fifteenmile Group located near Mount Slipper on the Yukon-Alaska border consist of approximately 40 meters of interbedded organic-rich parallel-laminated limestone, tabular clast or edgewise conglomerate, and calcareous black and grey shale. The absence of wave-generated bedforms, abundance of hemipelagic deposits, and presence of matrix-supported conglomerates interpreted as debrites suggests these strata were deposited well below storm-wave base in a slope environment. Interestingly, all of the autochthonous deep-water and allochthonous redeposited facies contain chert, but it is highly variable in abundance, distribution, and form. Chert deposits include nodules ranging from 1 mm to greater than 4 cm in diameter, many of which exhibit evidence of extremely early formation within sediments. Other chert deposits appear in the form of discontinuous beds and silicified debrites. Here we present a detailed stratigraphic section of the fossiliferous strata and document the distribution of chert and microfossils they contain. Detailed sub-sampling reveals patterns of fossil abundance and distribution relative to the variety of chert types present, as well as relative to organic carbon content and inorganic carbon isotope values of the host limestone. These factors shed light on why these unique fossil assemblages have not yet been found elsewhere despite extensive examination; furthermore, this deposit presents a key window into deep-water chert preservation, a taphonomic window that is quite unique among fossiliferous Proterozoic chert deposits that are generally constrained to peritidal settings.