Evaluating continental channel-hosted Lockeia orientation as a paleoflow indicator: insights from the Jurassic Brushy Basin Member, Western United States

Although bivalves can orient infaunal body position to current direction, trace fossils of bivalves have rarely been corroborated against paleocurrent directions determined from physical sedimentary structures. This study compares Lockeia (bivalve-generated trace fossil) orientation and length to the three-dimensional bedform cross-strata dip direction from fluvial deposits at two localities in the Late Jurassic Brushy Basin Member (Morrison Formation) of the western United States. Lockeia mean orientation at each site are comparable, but tangential, to physical sedimentary structure mean orientations. Comparing accuracy (within one standard deviation) and precision (within two standard deviations) in-context of five macroform surfaces, the Lockeia measurements are more accurate indicators of current direction than the physical sedimentary structures. Conversely, the physical sedimentary structures are more precise than the Lockeia measurements. This dissimilarity is partly a function of the variability of cross-stratum dip orientation produced by three-dimensional bedforms resultant from the tangential nature of the bedform, its preservation, and the method by which these are measured. Lockeia has greater paleocurrent representation accuracy than physical sedimentary structures since it is a linear measurement rather than a collection of measurements that can be tangential to paleoflow. Lockeia measurements show shifts vertically and laterally with macroform changes indicating potential use for identifying local changes in paleocurrent direction.