Search

Casts of shrinkage cracks found within sedimentary sequence are frequently cited as being diagnostic of depositional environments periodically subjected to subaerial exposure. The term shrinkage cracks, however, encompasses a broad suite of sedimentary structures having various origins. Shrinkage cracks can form not only at the sediment-air interface by desiccation processes but also at the sediment-water interface or substratally by synaeresis processes. Shrinkage cracks are induced in muddy sediments by variations in the salinity of the depositing medium, sediment compaction, and temperature in the case of some desiccation mudcracks. Crack morphology is influenced by the interplay of numerous factors, such as sediment composition, bed thickness, and bed surface configuration. Factors controlling desiccation mudcracks also include the rate of initial drying, total exposure time, depth of the groundwater table, and direction of surface drainage. Unfortunately, because of the complex interplay of these factors, no single feature of any shrinkage crack is necessarily useful in differentiating between a desiccation or synaeresis origin. Confidence in interpreting subaerial exposure from shrinkage cracks, therefore, depends on finding othe associated sedimentary structures diagnostic of exposure.

Regularly-spaced clusters of coarse imbricate gravel in Holocene alluvial fan sediments of Depot Creek, South Australia, are interpreted as fossil transverse ribs. In grain size and morphology they are identical to modern transverse ribs in the creek and fit an extension to higher values of Koster's (1978) grain size-wavelength plot for ribs from a variety of high-gradient gravel-bed streams. Recognition of fossil transverse ribs permits determination of former antidune wavelength, and hence the calculation of paleovelocity, depth and Froude Number.