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 that 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 Psammich nites. The examples are unexpected because they have developed along erosional internal bounding surfaces within a succession of cross-bedded sandstones. However, their seemingly counterintuitive preservation can be explained with reference to recent advances in our understanding of the time-incomplete sedimentary-stratigraphic record (SSR). 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 processes impart a traditionally underestimated complexity to the chronostratigraphic record of geological outcrop. CHRONOSTRATIGRAPHY AND TRUE SUBSTRATES It has been recognized for over a century that unconformities and sedimentary breaks riddle Earth's stratigraphic record at a variety of scales, such that two-dimensional (2-D) stratigraphic sections are fragmentary chronicles of elapsed geological time (Barrell, 1917; Sadler, 1981; Dott, 1983). Recently, a number of largely model-driven studies have explored the previously underappreciated causes and effects of this time-deficient SSR (Miall, 2015; Paola et al., 2018). Three recurring themes are: (1) Ordinariness: The SSR preferentially records mundane rather than dramatic events (Jerolmack and Paola, 2010; Paola, 2016). (2) Sedimentary stasis: The dominant sedimentation state under which the SSR accumulated was stasis; i.e., 'neither deposition nor erosion', rather than 'either deposition or erosion' (Ganti et al., 2011; Tipper, 2015; Straub and Foreman, 2018). (3) Spatial variation: Any time gaps in one 2-D stratigraphic section of a basin fill were likely compensated by contemporaneous deposition of strata elsewhere within the same basin (

Architecture and preservation in the fluvial to marine transition zone of a mixed‐process humid‐tropical delta: Middle Miocene Lambir Formation, Baram Delta Province, north‐west Borneo

“…The facies characteristics resemble wave-influenced and tide-influenced mouth bars in the Sego Sandstone (FA 4 in Legler et al, 2014). Ichnological characteristics are consistent with some physical and/or chemical stresses during mixed-energy inshore deposition, including possible intermittent colonization of sedimentary surfaces (Davies & Shillito, 2018). Consequently, FA 4 is interpreted as storm-reworked, fluvial-influenced mouth bars with possible tide influence.…”

A new insect trackway from the Upper Jurassic—Lower Cretaceous eolian sandstones of São Paulo State, Brazil: implications for reconstructing desert paleoecology

“…Experiments have demonstrated that the combination of the two can lead to enhanced survivorship of arthropod trackways over those made in dry sand or sand with just surface moisture or the presence of clay minerals alone . Nevertheless, it has been recently argued that special conditions are not necessarily needed to preserve such trace fossils (see Davies & Shillito, 2018).…”

Death near the shoreline, not life on land: Ordovician arthropod trackways in the Borrowdale Volcanic Group, UK: REPLY

“…Further, the existence of traces on eroded bounding surfaces in the BVG (e.g., flute marks; our figure DR6 in the GSA Data Repository) proves that they had a high preservational potential, even within an erosive setting. This is not unexpected because, contrary to traditional models (e.g., Seilacher, 2008), trackway preservation is probable under many spatially heterogenous, but otherwise mundane, sedimentary conditions (Davies and Shillito, 2018).…”