Directional sensitivity of cross-strata set thickness statistics

Cross-stratification is thought to encode the passage of bedforms and can be useful for reconstructing components of ancient sediment routing systems (e.g., include flow direction, depth, bedform height). However, observation orientation (e.g., section, outcrop, or core) relative to transport direction may affect measured stratal geometries and in turn, set thickness statistics, and geological interpretations. To investigate directional sensitivity, two intersecting orthogonal shallow trenches were excavated in the Bonnet Carre Spillway, one parallel to flow and one perpendicular. The flow parallel trench exhibited planar sets with nearly even and parallel bounding surfaces. Whereas, the flow perpendicular trench contained bounding surfaces that were extremely curved and nonparallel. These trenches were imaged using digital cameras, and orthophoto mosaics of the trenched surfaces were made using a structure from motion method. The bounding surfaces were then digitally mapped as vector lines using GIS software, the vector lines were used to calculate set thicknesses along a uniformly spaced grid. It is determined that, within the conditions of these trenches, the mean and standard deviation may be sensitive to flow-trench alignment. However, the coefficient of variation may be less sensitive. For these specific trenches, given random subpopulations of approximately 100 set thicknesses, the null hypothesis that flow parallel parallel and perpendicular measurements come from the same underlying probability distribution cannot be rejected. However, perceived anisotropy may be connected to these specific trenches, a computer simulation is run to explore set thickness statistics sensitivity to section orientation and overall bed aggradation rate. Simulations suggest that set thickness anisotropy arises from the tattered preservation of lee surfaces during low bed aggradation rates or bypass. Simulations with higher bed aggradation do not exhibit such anisotropy.