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Scolicia-Dominated Levee Deposits, Upper Cretaceous Nanaimo Group, Saltspring Island, British Columbia, Canada

Abstract

As thin-bedded turbidites occur in a wide range of submarine slope-channel and lobe sub-environments (e.g., levees, depositional terraces, channel margins, abandoned channels, and distal lobe fringes), their differentiation lends potential insights into the recognition of lateral and vertical facies trends in subsurface deposits of submarine fan systems. Further, the integration of ichnological datasets helps to refine sedimentological facies models in submarine systems, by addressing the dynamic nature of physico-chemical stresses associated with laterally heterogeneous slope environments. As part of a broader study of thin-bedded slope facies, channel-levee deposits from the Late Cretaceous Upper Nanaimo Group on Saltspring Island, British Columbia, Canada were described at a centimeter-scale through more than 300 meters of section across a lateral transect of ~12 km. Additionally, spectral gamma-ray data were collected at a 30 cm interval, and paleocurrent measurements were collected from imbricated clasts, trough-cross stratification, and current-ripple lamination. Sedimentologically, levee deposits in the study area are characterized by Tbcde and Tcde classical turbidite facies and display overall fining- and bed-thinning-upwards trends (10m- 50m) through the succession. Slope-channel facies are characterized by relatively thick (≤ 4.5 m) granule, pebble, and cobble conglomerates, as well as coarse-grained sandstone beds. Southwest paleoflow is observed in the thalwegs of channels, with paleoflow orthogonal to this in the associated levee deposits. Additionally, current ripple laminae in levee deposits show rare instances of bi-directional paleoflow. Ichnologically, levee-related thin-bedded turbidites show a unique and consistent trace fossil suite, dominated by Scolicia with subordinate Phycosiphon, Chondrites, and Zoophycos. BI ranges from 0-4, with thin (≤ 5 cm) silty sandstone beds (Tc divisions) commonly burrowed exclusively with Scolicia and showing BI 3-4. Phycosiphon is most commonly observed in mudstone interbeds (Tde divisions) with Chondrites and Zoophycos, and is present only rarely at the tops of sandstone beds. Differences in the types of traces, trace diversities, bioturbation intensities, and suite recurrence in slope deposits illustrate the utility of integrating ichnology in the predictive analysis of channel-levee units. Recognition of these ichnological criteria has important applications for characterizing thin-beds from subsurface datasets.