Ichnology of Alluvial-Fan related sequences: The example of Sarzedas Basin (Upper Miocene, UNESCO Naturtejo Geopark)

Neto De Carvalho C.;Baucon A.

2016-01-01

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Abstract

Trace fossils from alluvial fan deposits were only rarely described worldwide. Here we document an association of meniscate backfilled burrows in the alluvial cone of Sarzedas. The upper Tortonian-Messinian Torre Formation is composed of syntectonic debris flows and sheet-flood facies that pass distally into fine-grained micaceous sandstones and mudstone facies. The section at Lomba de Sarzedas butte has fine-grained sandstones and very fine-grained greenish siltstone showing evidence for incipient edaphic processes and is characterized by a dense full-relief Beaconites coronus-Taenidium barretti-Scoyenia gracilis bioturbation. This bioturbate texture is inferred to record multiple phases of deposit feeding by arthropods (millipedes or insect larvae) following short-lived ephemeral flooding under semi-arid conditions and in response to a highly fluctuating water table. We compare the meniscate backfilled burrows ichnoguild of substrate-controlled, and moisture-related, mobile detritivores documented in this paper with similar ichnological assemblages from alluvial fan successions from few other regions recorded worldwide, dating back to the Middle Devonian.