True substrates: The exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces

Abstract

Rock outcrops of the sedimentary-stratigraphic record often reveal bedding planes that can be considered to be true substrates: preserved surfaces that demonstrably existed at the sediment-water or sediment-air interface at the time of deposition. These surfaces have high value as repositories of palaeoenvironmental information, revealing fossilized snapshots of microscale topography from deep time. Some true substrates are notable for their sedimentary, palaeontological and ichnological signatures that provide windows into key intervals of Earth history, but countless others occur routinely throughout the sedimentary-stratigraphic record. They frequently reveal patterns that are strikingly familiar from modern sedimentary environments, such as ripple marks, animal trackways, raindrop impressions or mudcracks: all phenomena that are apparently ephemeral in modern settings, and which form on recognizably human timescales. This paper sets out to explain why these short-term, transient, small-scale features are counter-intuitively abundant within a 3.8 billion year-long sedimentary-stratigraphic record that is known to be inherently time-incomplete. True substrates are fundamentally related to a state of stasis in ancient sedimentation systems, and distinguishable from other types of bedding surfaces that formed from a dominance of states of deposition or erosion. Stasis is shown to play a key role in both their formation and preservation, rendering them faithful and valuable archives of palaeoenvironmental and temporal information. Further, the intersection between the time-length scale of their formative processes and outcrop expressions can be used to explain why they are so frequently encountered in outcrop investigations. Explaining true substrates as inevitable and unexceptional by-products of the accrual of the sedimentary-stratigraphic record should shift perspectives on what can be understood about Earth history from field studies of the sedimentary-stratigraphic record. They should be recognized as providing high-definition information about the mundane day to day operation of ancient environments, and critically assuage the argument that the incomplete sedimentary-stratigraphic record is unrepresentative of the geological past.

Non‐palimpsested crowded Skolithos ichnofabrics in a Carboniferous tidal rhythmite: Disentangling ecological signatures from the spatio‐temporal bias of outcrop

“…Pickerill (1992), Keighley & Pickerill (2003) and McIlroy (2004) objected to the latter usage on the basis that it can be impossible to determine burrow synchronicity. However, in the case of the Alston Formation traces, the recognition that burrows have upper terminations on preserved true substrates (Davies & Shillito, 2018, 2021), the lack of overprinting of burrows, and the evidence for continuous daily sedimentation, implies that burrows which occur on the same bedding surfaces were produced during the same short colonization windows, forming discrete contemporaneous ichnocoenoses in the sense of Ekdale et al . (1984) and Ekdale, (1985).…”

“…These traits imply that, while the sedimentary environment in which the tidalite was deposited was clearly amenable to the tracemakers, burrowing was never pervasive everywhere. The vertical expression of intermittent burrowed laminae, colonized to their maximum possible extent, but separated by laminae that lack burrows, is explainable as a stratigraphic translation of an uneven spatial distribution of burrowed patches within the depositional environment (Miller & Smail, 1997; Dashtgard, 2011; Marenco & Hagadorn, 2019; Davies & Shillito, 2021). Supporting analogue comes from comparison with modern substrates dominantly colonized by vertically burrowing filter feeding organisms.…”

“…A further striking feature of the burrowed horizons is that some of the visible bedding planes clearly record true substrates (i.e. preserved former water–sediment interfaces; Davies & Shillito, 2018, 2021), because the burrow tops are seen to be rimmed by sand that was exhumed from antecedent heterolithic laminae during the registration of the burrow (Fig. 7C and D).…”

“…Trace fossils can be used to add definition to our understanding of sedimentation rates at outcrop, both in terms of an understanding of the time taken for burrows to be excavated (Gingras et al ., 2008) and a recognition that burrows are commonly registered during intervals of sedimentary stasis (Tipper, 2015), when the temporary cessation of local sediment accrual opens a colonization window for infaunal organisms (Pollard et al ., 1993). These attributes can reliably be combined to estimate a minimum time duration of burrowed true substrates, prior to their burial (Davies et al ., 2019; Davies & Shillito, 2021). However, estimates are commonly underdetermined by an absence of evidence for the maximum duration of time over which the burrowed substrate could have persisted as the lithic surface.…”

Enigma variations: characteristics and likely origin of the problematic surface texture Arumberia, as recognized from an exceptional bedding plane exposure and the global record

“…In order to preserve the original three-dimensional (3D) morphology, Arumberia colonized the substrate during a net interval of sedimentary stasis, with subsequent deposition (in this instance of clay) covering Arumberia's external surface, preceded by little or no scour (e.g. Davies & Shillito, 2021).…”