Vertical grain-size trend of mouth bar in lacustrine fan delta: Flume experiments

Field observational previously indicated a mouth bar of a fan delta could exhibit a fining-or coarsening -upward trend, which bring a new challenge to the identification of mouth bar in subsurface studies due to the lack of morphological descriptions. Previous studies have indicated that effluent behavior in river -mouth system can affect the vertical grain-size trend of mouth bar, but the drivers and magnitude of this phenomenon are not understood. We conducted flume experiments to investigate the mechanism and controlling factors of vertical grain-size trend of mouth bar. Experiment with a steeper slope of the substrate layer, greater discharge, higher sediment/water ratio, and coarser sediment induced a fining -upward trend of mouth bar, because the effluent was dominated by strong inertia. Mouth bar in the experiment with a gentler slope of the substrate layer, smaller discharge, lower sediment/water ratio, and finer sediment exhibited a coarsening-upward trend dominated by the friction-dominated effluent. The relationship between the vertical grain-size trend of mouth bar and the gradients of foreset bedding in small-scale flume models and the cut-off of 15 degrees-18 degrees are applicable in natural systems. Identifying depositional setting to infer depositional process in river-mouth system and analyzing the plane ge-ometry of sandbodies are two steps in the interpretation of ancient fan deltaic rock record. (c) 2022 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Automated summary

Experimental results showed that steeper slope, greater discharge, higher sediment/water ratio, and coarser sediment induced a fining-upward trend of mouth bar, while gentler slope, smaller discharge, lower sediment/water ratio, and finer sediment exhibited a coarsening-upward trend dominated by friction-dominated effluent.