Sole Markings

Introduction

Sole markings are among the most practically useful structures in sedimentary geology: they preserve a record of the currents that flowed over a muddy seafloor, sometimes with enough directional information to reconstruct paleocurrent orientation precisely. They are found on the undersides of beds - particularly sandstone beds that overlie shale - as positive-relief casts that were moulded in depressions before those depressions were buried. Understanding how they form is the foundation for interpreting every specific type: flute casts, groove casts, tool marks, and load casts.

What Sole Markings Are

Many bedding-plane markings occur on the underside of beds as positive-relief casts and irregular markings. Because of their location on the bases or soles of beds, they are called sole markings. They are preserved particularly well on the undersides of sandstones and other coarser-grained sedimentary rocks that overlie shale beds. Many sole markings show directional features that make them very useful for interpreting the flow directions of ancient currents. ^[1]

The Two-Stage Formation Process

Sole markings form by a two-stage process involving both erosion and deposition. First, a cohesive fine-sediment bottom is eroded to produce grooves or depressions. The cohesiveness of the sediment is crucial - it allows the depressions to survive long enough to be filled and buried during subsequent deposition, typically by coarser-grained sediment than the bottom mud. This coarser sediment is probably deposited very shortly after erosion - possibly in some cases by the same current that formed the depression. After burial and lithification, a positive-relief feature is left attached to the base of the overlying bed. ^[1]

The geometry of the original depression is preserved in mirror image on the underside of the overlying bed. This is why sole markings can tell us not only the shape of the original scour but also the direction it was oriented - and therefore the direction of the current.

Current-Formed vs. Tool-Formed Structures

The initial erosional event that produces depressions in a mud bottom can take two forms. Current scour - the hydraulic action of the flow itself - produces current-formed structures such as flute casts. Alternatively, depressions can result from tools carried by the current: pieces of wood, the shells of organisms, or any object that rolls or drags along the bottom. These produce tool-formed structures such as groove casts, bounce marks, brush marks, prod marks, and skip marks. The genetic classification into current-formed and tool-formed types is based on the nature of the erosional agent. ^[1]

Occurrence

Sole markings are most common on the soles of turbidite sandstones, where the same turbidity current that scoured the seafloor also deposited the sand that filled the depressions. They are also present in fluvial and shelf deposits - any setting where an erosive event is followed quickly enough by deposition that the depressions are not destroyed before they can be cast. ^[1]

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