Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

Abstract:

The use of sedimentary structures as indicators of flow and sediment morphodynamics in ancient sediments is essential for reconstruction of formative flow conditions generated in a wide range of grain sizes and sedimentary environments. Yet, the vast majority of past research has concerned bedforms generated in essentially cohesionless sediments that lack the presence of mud within the flow and within the sediment bed itself. However, most sedimentary environments possess fine-grained sediments, with recent work demonstrating how the presence of such fine sediment may substantially modify the fluid dynamics of such flows. It is thus increasingly evident that the influence of mud, and the presence of cohesive forces, is essential to permit a fuller interpretation and understanding of many modern and ancient sedimentary successions.

In this paper, we summarize on the fluid dynamics of turbulence modulation generated by the presence of fine suspended sediment, and use this knowledge to propose a new extended bedform phase diagram for bedforms generated in mixtures of sand and mud under rapidly decelerated flows. This diagram provides a phase space using the variables of yield strength and grain mobility as the abscissa and ordinate axes, respectively, and defines the stability fields of a range of bedforms generated under flows that have modified fluid dynamics due to the presence of suspended sediment within the flow. We also show data on a range of bedforms generated in such flows, from laboratory experiments and examples from ancient sediments, including: i) heterolithic stratification, comprising alternating laminae or layers of sand and mud; ii) the preservation of low amplitude bed-waves, large current ripples, and bed scours with intrascour composite bedforms; iii) low angle cross-lamination and long lenses and streaks of sand and mud formed by bed-waves; iv) complex stacking of reverse bedforms, mud layers and low-angle cross-lamination on the upstream face of bed scours; and v) planar bedding comprising stacked mud-sand couplets.