Erosion Structures

Introduction

Erosion structures are sedimentary features produced by scour - the mechanical removal of sediment by currents, waves, or mass movements - followed by partial or complete filling. They record a gap in the depositional record (erosion) and then the resumption of deposition (fill), sometimes with different sediment types filling than were removed. Two of the most important erosion structures are channels and scour-and-fill structures.

Channels

Channels are structures that show a U-shape or V-shape in cross-section and cut across earlier-formed bedding and lamination. They form by erosion - principally by currents, but in some cases by mass movements. Channels may be filled with sediment that is texturally different from the beds they truncate. Those visible in outcrop range in width and depth from a few centimetres to many metres; even larger channels may be recognisable only by mapping or drilling. Channels rarely expose their full length in outcrop, but they can presumably extend for distances many times their width. ^[1]

Channels are very common in fluvial and tidal sediments. In turbidite sediments, the long dimensions of channels tend to be oriented parallel to current direction, as shown by other directional structures on the same beds. ^[1]

The erosional contact at the base of a channel is one of the most useful markers in stratigraphic analysis. Because it is a time gap - sediment was removed, then more was deposited - a channel base records a change in the energy or style of the depositional system. A sand-filled channel cutting through mudstone signals a shift from low-energy to high-energy conditions; a mud-filled channel in sandstone records the reverse.

Scour-and-Fill Structures

Scour-and-fill structures are similar to channels but are generally smaller. They consist of small, filled asymmetrical troughs a few centimetres to a few metres in size, with long axes that point downcurrent. They typically have a steep upcurrent slope and a more gentle downcurrent slope. Fill material may be either coarser-grained or finer-grained than the substrate. These structures are most common in sandy sediment and are thought to form by scour from currents followed by backfilling as current velocity decreases. Unlike channels, several scour-and-fill structures may occur together closely spaced in a row. ^[1]

The asymmetric profile of a scour-and-fill structure is a paleocurrent indicator. The steep upcurrent wall reflects the direction from which the scouring current arrived; the gently sloping downcurrent wall is where sediment was deposited as the current decelerated. This shape can therefore be used to determine flow direction even in the absence of other directional structures on the same surface.

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