Grain size measurements from gravelly outcrops: Methods, uncertainties, and implications

The grain size of coarse (>2mm) grained sedimentary particles can be determined by various methods such as Wolman-point or -grid counts on photos or directly in-situ, or upon sieving the material. Among the three perpendicular axes of a grain, transportation in the water flow direction usually occurs parallel to the intermediate b-axis, thus making this specific axis essential for hydraulic applications.

Determination of the length of the b-axis in recent deposits is straightforward because the longest a-axis and intermediate b-axis are visible from a top-view, while the shortest c-axis is oriented perpendicular to the riverbed surface. However, accumulation of river sediments either forms unconsolidated gravel commonly exposed as vertical steep walls (e.g., in gravel pits), or consolidated conglomerates which are commonly analyzed along stratigraphic sections. In contrast to recent deposits, the identification and measurement of specific grain axes within such ancient sedimentary deposits bear several limitations. These outcrops usually offer a view of the deposits perpendicular or oblique to the initial bedding of sediments, where either the a-/b- or b-/c-axes are visible, depending on the paleoflow direction. Furthermore, in such deposits, the embedding and possible occlusion of individual grains still prevent measurements of the full length of a specific axis. While in unconsolidated outcrops individual grains could be extracted for sieving, lithified deposits limit such approaches. Therefore, in consolidated outcrops, the longest visible axis of individual clasts is preferably measured through photo surveys. Although such surveys have been widely applied to lithified deposits, only a few studies evaluated the accuracy of such measurements and the relation of the longest visible grain axes to the b-axis.

Here, we compare and evaluate three different measuring techniques applied to coarse grains from outcrops in a gravel pit consisting of unconsolidated sediments with architectural and morphological similarities to conglomerates. We compared grain sizes and percentiles thereof measured by i) hand using a caliper (a-, b- and c-axes), ii) mechanical sieving of the material, and iii) measurements on digital images (longest and shortest visible axis). For the data collection on images, we compared two measuring techniques and tested if image-specific factors such as distortion effects have an influence on the results.

Our results show that sieving of the material (b-axis equivalent) yields datasets that can be best compared to measurements of the longest visible axis on images. In addition, both methods (sieving and images) yield underestimates of the length of the real b-axis by c. 15% if the grain size values determined with calipers are taken as reference. Additionally, grain size datasets are independent if grains are selected randomly on images or through a Wolman-grid approach. Furthermore, measurements are not significantly biased by possible image distortion effects for short-distance surveys (c. 1-1.5 m from outcrop) with hand-held cameras.

Finally, an underestimation of c. 15% of the length of the b-axis from images influences the outcomes of further applications (e.g. paleo-hydraulic calculations) that are based on grain size data. Therefore, we recommend these uncertainties to be considered for such applications.