The applicability of modern tidal analogues to pre‐vegetation paralic depositional models

Abstract

In modern siliciclastic environments terrestrial and aquatic vegetation binds substrate, controls weathering and erosion rates, influences run-off, sediment supply and subsequent depositional architecture. This study assesses the applicability of modern depositional models that are impacted by vascular vegetation, as analogues for ancient pre-land plant systems. A review of pre-Devonian published literature demonstrates a paucity of described tidal successions; this is possibly due to the application of modern analogues for interpreting the record when there is a lack of tidal indicators. This paucity suggests a need for revised models of tidal deposition that consider the different environmental conditions prior to land plant evolution. This study examines the OrdovicianSilurian Tumblagooda Sandstone, which is exposed in the gorge of the Murchison River and coastal cliffs near Kalbarri, Western Australia. The Tumblagooda Sandstone comprises stacked sand-rich facies, with well-preserved bedforms and trace fossils. Previous interpretations of the depositional setting have proposed from a mixed sheet-braided fluvial and intertidal flats; to a continental setting dominated by fluvial and aeolian processes. An enigmatic element is the rarity of mud-rich facies preserved in the succession. Outcrop logging, facies and petrographic analysis record dominantly shallow water conditions with episodes of emergence. Abundant ichnotaxa indicate that marine conditions and bi-directional flow structures are evidence for an intertidal and subtidal depositional environment. A macrotidal estuary setting is proposed, with evidence for tidal channels and repeated fluvial incursions. Physical and biogenic sedimentary structures are indicative of tidal conditions. The lack of clay and silt resulted in the absence of flaser or lenticular-bedding. Instead cyclic deposition of thin beds and foreset bioturbation replaced mud drape deposits. Higher energy conditions prevailed in the absence of the binding activity of plants in the terrestrial and marine realm. This is suggestive of different weathering processes and a reduction in the preservation of some sedimentary features.

The Tumblagooda Sandstone revisited: exceptionally abundant trace fossils and geological outcrop provide a window onto Palaeozoic littoral habitats before invertebrate terrestrialization

“…Kettanah et al 2015), there is circumstantial evidence that the unit is likely Silurian. Our fieldwork supports the contention that there are no palaeosols in the Tumblagooda Sandstone, in agreement with previous work (Hocking, 1991;Trewin, 1993b;Evans et al 2007;McNamara, 2014;Kettanah et al 2015;Bradley et al 2018). As such, there is no evidence for the palaeosol 'pedostratigraphy' of Retallack (2009), who considered the Tumblagooda Sandstone to have been deposited continually throughout the entire 45 Ma duration of the Ordovician Period.…”

“…There is little evidence for fully subaerial deposition apart from the two instances of FA2e, but evidence for the subaerial reworking and emergence of subaqueously deposited sand is a common motif. In agreement with other interpretations (Hocking, 1991;Bradley et al 2018), the recognition that FA2 can only represent littoral deposition on tidal sand flats means that the fluvial-aeolian depositional model for FA2 of the Tumblagooda Sandstone (Trewin, 1993a, b;Trewin & McNamara, 1994;McNamara, 2014) must be rejected. This has crucial implications for the significance of the unit for understanding the terrestrialization process.…”

“…Twenty discrete ichnogenera have been reported from this unit, and the first papers to fully document this ichnofauna suggested that the host strata were deposited by aeolian and fluvial processes in a late Silurian non-marine environment (Trewin, 1993a, b;Trewin & McNamara, 1994). However, controversy surrounds the Tumblagooda Sandstone for two reasons: firstly, the age of the unit has subsequently been reinterpreted, from indirect evidence, to be Ordovician (Mory et al 1998;Iasky & Mory, 1999) and, secondly, an alternative explanation for the depositional environmentone in which the trace-fossil-bearing strata were deposited in shallow marine settingshas repeatedly been put forward (Hocking, 1991;Bradley et al 2018). The uncertainty that now surrounds the unit is in urgent need of clarification because: (1) examples of its trace fossils are regularly used to ascertain the stratigraphic ranges of non-marine ichnotaxa (e.g.…”

“…FA2 was described as an intertidal facies association due to the abundant bioturbation and sedimentary structures including ripple marks and various adhesion marks (Hocking, 1991). Trewin (1993b) later reinterpreted FA2 as fluvial and aeolian deposits, while the most recent interpretation of FA2 (Bradley et al 2018) considered it to have been deposited in a macrotidal estuary setting. Hocking (1991) interpreted FA3 as a low-sinuosity high-energy braided fluvial association, similar to FA1, due to its unidirectional palaeoflow indicators, coarse grain size and dominant trough cross-bedding.…”

“…Whilst Tumblagoodichnus is recognized in association with this group (T5, T6), in these instances it always originates from beds stratigraphically above those containing Heimdallia, demonstrating that it is a palimpsest upon the original ichnofabric. The association between Heimdallia and Tumblagoodichnus has previously been noted, and interpreted as both coincidental (with Tumblagoodichnus as the record of temporary subsurface refuges made by arthropods) (Trewin & McNamara, 1994) or as 'hunting' behaviour (with the animal responsible for Tumblagoodichnus scooping down through the sediment to eat the animal creating Heimdallia) (McNamara, 2014;Bradley et al 2018). The latter contention is improbable given that the Tumblagoodichnus, originating from overlying beds, would have been formed on timescales of days to years after the Heimdallia burrowing activity.…”