Heterolithic Bedding Formed by Froude Supercritical Flow Bedforms With Broad Grainsize Distributions

Abstract

Froude supercritical flow bedforms have complex sedimentary structures forming undulating bedding or mixed foreset and backset bedding. Field studies suggest the presence of heterolithic bedding formed under supercritical flow condition in the fluvial (Plink-Bjorklund, 2015) to deltaic (Tan pers.com. 2016) channels and deepwater (Ono and Plink-Bjorklund, 2015) channels, based on geometric similarity between sedimentary structures and sandy supercritical flow bedforms formed under the experimental conditions (Alexander et al., 2001; Spinewine et al., 2009; Cartigny et al., 2014). The heterolithic deposits consist of siltstone to fine-grained sandstone lithology with multi-scale (ten’s cm to a few m thick) scour-and-fill structures, backset bedding, lenticular bedding in long wavelength (ten’s m long), and flaser bedding like structures (a few cm thick) composed of multiple erosive lenses. Fine-grained deposits are vertically and laterally interbedded with conglomerates and sandstones that also indicate supercritical flow deposition. However, the interpretations are speculative as previous experiments have used single grain size variation.

Therefore, new experiments are presented here that focus on producing supercritical flow deposits using broad grain size distributions, ranging from silty fine-grained sands to granules. The results show that the coarse-grained sand to granule grains in fine-grained sand dominated deposition form the complex scour-and-fill structure. Individual coarse-grained to granule lenses deposit in irregular erosional pockets on the scour surfaces. These lenses then align to form a train along scour surfaces which originated from a migration of a hydraulic jump. Compared to the deposition of the coarse-grained sands to granules, the majority of fine-grained deposition occurs as the suspension in slower deposition rate and fills the scour draping on coarse-grained sand to granule lenses. This grain segregation forming coarse-gained lenses in fine-grained deposits indicates one of the depositional processes related to the heterolithic bedding under supercritical flow condition. Also, the example demonstrates that grain size variations cause more diverse sedimentary structures, (ex., normal grading, loading and mounded-shaped structures), than single-grain size supercritical flow bedforms. Geomorphology and grain size distribution studied here provides the recognition criteria for those heterolithic bedding in outcrop records.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018