INVESTIGATING THE SENSITIVITY OF CROSS-STRATA SET THICKNESS STATISTICS TO SECTION ALIGNMENT WITH FLOW DIRECTION

Ancient river bedform cross strata record the motions of past dunes, and can be used to infer past flow and sediment transport conditions. Variation in outcrop orientation relative to flow direction, however, may influence measurements of cross set geometries and resultant interpretations. Abundant dune cross-strata were preserved in deposits in the Bonnet Carre Spillway, a flood control structure on the Mississippi River, following the 2019 flood and spillway opening. Set thickness from these deposits can be related to flow depth, which in turn can give helpful insight into past flow conditions. This study investigates how set thickness statistics change with section orientation to flow direction. To accomplish this, two intersecting orthogonal shallow trenches were excavated in the Bonnet Carre Spillway, one parallel to flow and one perpendicular. These trenches were imaged using digital cameras, and orthophoto mosaics of the trenched surfaces were made using a structure from motion method. The flow parallel trench largely exhibited tabular sets with nearly even and parallel bounding surfaces. However, the flow perpendicular trench contained bounding surfaces that were extremely curved and nonparallel. These bounding surfaces were digitally mapped as vector lines using QGIS software; the vector lines were then used to calculate set thicknesses along a uniformly spaced grid. To explore set thickness statistics sensitivity to section orientation, statistics from a growing window analysis will be used to determine the trench length and number of measurements required to attain steady statistical metrics. These observations will be compared to a three dimensional volume of synthetic stratigraphy produced by a numerical bedform stata-formation model.