Incomplete but intricately detailed: The inevitable preservation of true substrates in a time-deficient stratigraphic record

Abstract

True substrates are defined as sedimentary bedding planes which demonstrably existed at the sediment-water or sediment-air interface at the time of deposition, as evidenced by features such as ripple marks or trace fossils. Here we describe true substrates from the Silurian Tumblagooda Sandstone of Western Australia which have been identified by the presence of the surficial trace fossil Psammichnites. The examples are unexpected because they have developed along erosional internal bounding surfaces within a succession of cross-bedded sandstones. However, their seemingly counter-intuitive preservation can be explained with reference to recent advances in our understanding of the time-incomplete sedimentary-stratigraphic record. The preservation of true substrates seems to be an inevitable and ordinary result of deposition in environments where sedimentary stasis and spatial variability play important roles. We show that the true substrates developed during high-frequency allogenic disturbance of migrating bedforms, forcing a redistribution of the loci of sedimentation within an estuarine setting, and subsequently permitting an interval of sedimentary stasis during which the erosional bounding surfaces could be colonized. These observations provide physical evidence that supports recent contentions of how sedimentary stasis, and the interplay of allogenic and autogenic process, impart a traditionally underestimated complexity to the time-stratigraphic record of geological outcrop.