HYDRAULIC SORTING AND SOURCE MIXING OF MUD ALONG THE ONSHORE-OFFSHORE DISPERSAL PATH: ORIGIN OF FACIES DIVERSITY IN THE EARLY PALEOZOIC BELL ISLAND GROUP, NEWFOUNDLAND, CANADA (Invited Presentation)

This study aims to investigate the origins of lithofacies variability within the exceptionally preserved shoreface mudstones of the Early Ordovician Bell Island Group, Newfoundland, Canada. More than 100 polished slabs and thin sections were characterized via transmitted light microscopy, XRD and SEM analyses, to capture the facies variability and the stratigraphic distribution of authigenic clay minerals and silica cement, across an ancient shoreline-to-offshore transect. Based on sedimentological trends, the shoreface mudstones were subdivided into two distinct facies associations: (a) well-cemented, muddy sediment gravity flow mudstone; and (b) poorly-cemented, wave-reworked mudstone. Well-cemented, muddy sediment gravity flow mudstones contain sharp grain-size breaks, commonly form beds with bi- and tripartite subdivision, and exhibit a high chlorite-illite ratio. In contrast, wave-reworked mudstones are highly variable in lateral bed thickness and are poorly cemented; the clay mineral fraction is predominantly illite. This trend of alongshore decreasing amount in chlorite can either be explained via (1) relative enrichment of detrital illite through wave- and current-driven partitioning of the initial river-sourced mineral assemblage, or alternatively, through (2) mixing of mud originating from at least two separate point sources. The sorting of muddy components by alongshore- and cross-shore directed currents and waves is inferred to separate components with different density and profoundly impact the composition of the starting composition of grain assemblages. Within this study, the ‘hydraulic sorting’ and source mixing of mud along the onshore-offshore dispersal path also has the potential to be the first-order control on the developing rock strength during burial diagenesis.