The relationship between organic matgrounds and sedimentary packaging: examples from the Ediacaran and early Cambrian

The late Ediacaran is characterised by the widespread occurrence of organic matgrounds in shallow marine siliciclastic settings. A key feature of matgrounds is their ability to bind sediment, thus increasing the erosional threshold of sediments exposed on the seafloor. As a result, shallow marine siliciclastic rocks of the late Ediacaran commonly preserve sedimentary signatures of matgrounds on bedding planes and in cross‐section. Cross‐sectional signatures of matgrounds reflect the stabilisation of bed tops and separation of discrete event beds by organic mats. Strikingly, this pattern includes successive stabilised and separated beds comprised of identical lithologies, which are observed continuously throughout metres of stratigraphic section in some Ediacaran settings. Features of these beds include palimpsest ripples, non‐amalgamated bed contacts, and non‐erosional bed junctions. The transition from the Ediacaran to the early Cambrian records an increase in the complexity of metazoan burrowing and grazing strategies, which led to the restriction of the once ubiquitous matgrounds to deeper and more extreme environments. As such, it is predicted that the cross‐sectional signatures of matgrounds in should persist in early Cambrian shallow marine siliciclastic settings, although rarely, having been largely replaced by amalgamated beds bound by gradational and erosional bed junctions that are more characteristic of the Phanerozoic rock record as organic stabilisation of shallow seafloor sediments declined. Comparison of evidence from the Ediacaran‐aged Ediacara Member of South Australia with the early Cambrian Wood Canyon, Prospect Mountain and Pioche formations of California and Utah (USA) confirms that the early Phanerozoic decline of matgrounds and firmgrounds was accompanied by greater reworking at bed junctions and a loss of fidelity in the shallow marine sedimentological record, although Ediacaran‐like anactualistic sedimentary structures persist into the early Cambrian.