ABSTRACT: Long-term time-averaging despite abrupt burial: Paleozoic obrution deposits from epeiric settings of Parana Basin, Brazil.

Simoes, M.G.; Kowalewski, M.; Torello, F.F; Anelli, L.E.

Geological Society of America Annual Meeting, Toronto, Canada.

Obrution horizons, created in abrupt depositional events, are believed to provide fossil assemblages that are exceptionally well resolved temporally (hours to days) and thus offer brief 'snapshots' of ancient seafloor conditions and communities. However, obrution horizons from the Paleozoic, epeiric deposits of Paran‡ Basin, Brazil, often mix invertebrate populations killed in an obrution mass-mortality event together with residua of older shells that were time-averaged and affected by taphonomic processes (including taphonomic feedback).

Thick sequences of marine siltstones and mudstones (the Devonian Ponta Grossa Formation) deposited at (or below) the storm wave base, include various thin obrution horizons buried by thin massive mudstone units up to 10 cm thick. One of the horizons mixes together (1) a cluster of at least 10 shells of the spiriferid brachiopod Australospirifer sp. consisting of in situ preserved specimens of uniform size and ontogenetic age; and (2) disarticulated, convex-up valves of the detritus-feeding bivalve Nuculana sp. The shell beds (proximal tempestites) of the Corumbata’ Formation (Late Permian) are also covered by thin, massive (obrutionary) mudstones. The bioclasts consist primarily of densely packed, disarticulated, and fragmented bivalve shells, but rare examples of well-preserved, in situ shells of the endobyssate, semi-infaunal bivalve Naiadopsis lamellosus can also be found.

Both examples show that taphonomically altered and substantially time-averaged skeletal remains, dominated by infaunal bivalves, were colonized by epifaunal or semi-infaunal suspension-feeding brachiopods or bivalves (Australospirifer sp. and Naiadopsis lamellosus, respectively) and then preserved together in obrution horizons abruptly buried by fine-grained, massive sediments. Clearly, much more than single benthic community, killed in a burial event can be recorded in an obrutionary horizon. Despite their catastrophic origin, obrution deposits may represent time-averaged horizons recording several background and episodic events, and thus may provide an ecological condensed record of marine benthic communities.

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