Sticky stuff: Redefining bedform prediction in modern and ancient environments

Abstract

The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments.

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

“…Although a simple relationship between suspended clay concentration in flows and the percentage mud matrix in the resultant deposits does not exist, and bed clay fractions are not available for the present experiments, it is likely that bedforms are modified at a concentration appreciably lower than 15% mud in the matrix. This was confirmed by Baas et al (2013), who showed that current ripples are almost fully suppressed at a depth-averaged flow velocity of c. 0.4 m s −1 and bed clay fractions above 13%, by Schindler et al (2015) for dunes, where even small amounts of clay in the bed showed dramatic effects on bedform size, and by Whitehouse et al (2000), whose data compilation shows that 5-15% clay is sufficient to change from bed-flow interaction dominated by non-cohesive processes to bed-flow interaction dominated by cohesive processes. It is therefore evident that in future research, even the smallest primary mud fractions in sandstones that possess primary current stratification should be considered in the reconstruction of flow dynamics and depositional processes.…”

“…Wang et al 1988;Liang et al 2007). The available data suggest that these mixedsediment bedforms are smaller than in pure sand, even at clay bed fractions below 3% (Basaniak & Verhoeven 2008;Baas et al 2013;Schindler et al 2015;Fig. 8).…”

“…This under-representation of cohesive sediments is all the more surprising as cohesive mud and clay are ubiquitous in most aquatic environments and sedimentary facies ( Fig. 1; Healy et al 2002;Schindler et al 2015). Of particular importance, cohesive clay causes flows to change their turbulence properties in a non-linear, yet predictable, manner (Baas & Best 2002), suspended sediment to change its dynamic behaviour (Winterwerp & van Kesteren 2004;Schieber et al 2007;Mehta 2013), and deposits to become more difficult to erode than pure sand (Mitchener & Torfs 1996).…”

“…This influence has been shown to be highly significant; adding even a small volume of clay particles to a flow or to a sand bed leads to the development of bedform types with shapes and dimensions that are vastly different from those in pure sand (e.g. Lowe 1988;Baas et al , 2011Baas et al , 2013Schindler et al 2015). To further demonstrate this influence, we also present a new comprehensive dataset on bedform dynamics below rapidly decelerated cohesive (mud-sand) sediment flows that extends the parameter space proposed by Baas et al (2011) that focused on current ripples, to incorporate two further flow conditions: upper-stage plane beds and washed-out ripples.…”

Biofilm origin of clay-coated sand grains

“…Biofilms have been shown to play a fundamental role in sediment dynamics and the subsequent diagenesis of marginal marine sedimentary systems (Stal, 2003), affecting grain-size heterogeneity (Garwood et al, 2015), sediment stability (Vignaga et al, 2013), sediment transport, and bedform stability Schindler et al, 2015). Intertidal biofilms typically result from the secretion of extracellular polymeric substances (EPSs) (adhesive mucilage) by microphytobenthic (MPB) communities that are composed of algae (diatoms, euglenids, crysophyceans, dinoflagellates), cyanobacteria, and other photosynthetic bacteria (Jesus et al, 2009).…”