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Ichnologically Distinct Thin-Bedded Turbidite Facies in Lobe Sub-Environments, Upper Cretaceous Nanaimo Group, British Columbia, Canada


Thin-bedded turbidites are deposited in a wide range of submarine slope-channel and lobe sub-environments (e.g., levees, depositional terraces, channel margins, abandoned channels, distal lobe fringes). Correspondingly, differentiation of thin-bedded turbidite facies complements interpretations of lateral and vertical facies relationships in modern and ancient deposits of submarine fan systems. In order to develop criteria for differentiating various thin-bedded facies, lobe deposits of the Late Cretaceous Upper Nanaimo Group on Saltspring Island, BC were examined at a centimeter scale through more than 600 m of well-preserved, continuous outcrop. Additionally, spectral gamma-ray data were collected at a 30 cm interval. Overall, the stacked lobe succession shows coarsening and bed thickening upwards, interpreted to represent a regionally migrating paleo-depocenter shifting closer to the study area. Examination of thin-bedded units shows significant sedimentological, ichnological, and geochemical variability between the thin-bedded facies in the upper and lower parts of the section. Thin-bedded facies in the lower half of the succession show low sandstone-to-mudstone ratios (average 1:9), high gamma radioactivity, and both 30-50 meter-scale coarsening- and bed-thickening-upward cycles, as well as intercalated fining- and bed-thinning-upward trends. Sedimentologically, these thin beds are characterized by starved current rippled sandstone and planar parallel laminated siltstone, with low bioturbation intensities (BI 0-2) and reduced trace fossil diversities (≤ 6 ichnogenera). By contrast, thin-bedded facies in the top half of the succession are sandstone-dominated (sandstone-to-mudstone ratios average 1:1) and characterized by low gamma radioactivity. Meter-scale (1-6 m) coarsening- and bed-thickening-upward cycles are intercalated with hybrid-event beds. Sandstone beds are associated with a diversity of sedimentary structures, including planar parallel laminae, climbing current ripple laminae, loaded ripples, and soft-sediment deformation. Ichnological characteristics include greater trace fossil variability and diversity (≤ 8 ichnogenera, including complex forms), as well as a higher intensity of bioturbation locally (BI 0-4). This analysis of thin-bedded deposits provides important ichnological insights into the variability of lobe strata, with the ultimate objective to establish important predictive criteria for the recognition and analysis of lobe units in subsurface datasets.