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

Description

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.

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