Jaco H. Baas

Understanding of the formative conditions of many sole structures is limited, with chevron marks and striated groove marks being particularly enigmatic. These sedimentary structures are examined here through laboratory modelling. An idealized tool, resembling an armoured mud clast, was dragged through substrates of kaolinite–seawater mixtures of different yield strengths while submerged in seawater. The experiments suggest that armoured mud clasts are the likely tools producing fine striae in striated grooves and, given the common occurrence of striated groove marks in outcrops, that these clasts are more prevalent in deep‐marine settings than previously thought. Chevron marks were observed to form over a narrow range of substrate yield stresses, likely explaining their relative rarity. Furthermore, their form is shown to be a function of substrate rheology, with chevron angle relative to the movement direction of the tool being less in weaker substrates. Moreover, the size of cut chevron marks, characterized by a narrow central cut, bears no relationship to the size of the incising tool, but rather reflects a substrate with a low yield stress that is sufficiently mobile to close behind the tool. In contrast, interrupted chevron marks, characterized by a distinct central groove, reflect greater substrate strength. Striated grooves without chevrons formed at the highest yield stresses simulated in the experiments. The relationship between tool mark type and yield stress, in combination with changes in chevron angle, enables these sole structures to be utilized as indicators of palaeosubstrate rheology. The conditions required to preserve such features include a prolonged period of bed consolidation, flow bypass and lack of bioturbation. Given changes in seafloor communities and bioturbation over time and their impact on substrate rheology, particularly during the early Palaeozoic, the present work supports the idea that the frequency of these sole structures likely changed over geological time.

Sediment gravity flows exhibit a large range of flow behaviours, making their flow dynamics hard to predict and the resulting deposits a challenge to interpret. Cohesive sediment gravity flows containing clay are particularly complex, as their behaviour is controlled by the balance of turbulent and cohesive forces. A first set of laboratory lock‐exchange experiments investigated the effect of adding 25% very fine sand by volume to high‐density cohesive sediment gravity flows with strongly suppressed turbulence. This caused these mixed clay–sand flows to become more cohesive, have shorter runout distances, and have lower head velocities than the original pure‐clay flows, despite the increase in density difference and the non‐cohesive properties of the sand. Yield stress measurements confirmed that adding the non‐cohesive very fine sand increases the cohesive strength of dense clay suspensions. This higher cohesive strength outcompetes the enhanced density difference and reduces the flow mobility. A second set of experiments across a larger range of clay concentrations showed that, for low‐density cohesive sediment gravity flows dominated by turbulent mixing, the addition of 25% very fine sand increased the head velocities because of the enhanced density difference and weak cohesive forces. Thus, the addition of very fine sand may increase or decrease the mobility of cohesive sediment gravity flows, depending on the initial type of flow and the balance between turbulent and cohesive forces. In the natural environment, this study proposes that very fine sand can only increase the cohesive strength and reduce the flow mobility of cohesive sediment gravity flows that have a sufficiently strong matrix strength to fully support the sand particles. The contribution of very fine sand to the cohesive strength of high‐density cohesive sediment gravity flows may have important implications for flow transformation on submarine fans, especially in distal regions where transient–turbulent, cohesive flows are particularly common.