MP9: Chapter 5: ‛Meniscate Burrow Ichnoguild from the Alluvial Fan Deposits of Sarzedas Basin (Upper Miocene, Portugal)
by C. Neto de Carvalho, A. Baucon and S. Canilho.
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This chapter is from GAC's Miscellaneous Publication 9: "Ichnology: Papers from Ichnia III" editted by Dr. Duncan McIlroy. To purchase the entire book on disc, please see the Miscellaneous Publications section of our bookstore at http://www.gac.ca/publications/view_pub.php?id=262.
Trace fossils from alluvial fan deposits were only rarely described worldwide. Here we document, for the first time in Portugal, an association of meniscate backfilled burrows in the alluvial cone of Sarzedas. The succession developed in response to interactions between the Rapoula Fault scarp and the flooding piedmont. The Sarzedas Basin is a ramp-valley basin related to the uplift of the Portuguese Central Cordillera. The 150-m thick fill is predominantly endorheic braided river-related sediments with episodic alluvial fan deposits that reflect major tectonic and climate events. The allostratigraphic sequence is subdivided in two groups: poorly sorted feldspathic sandstones, conglomerates and pedogenic carbonates of the Beira Baixa Group (Eocene‒lower Tortonian), and the Upper Miocene to Pliocene Murracha Group dominated by prograding alluvial fan successions of quartzite conglomerates transitioning distally into sandy-mudstone facies.
The upper Tortonian‒Messinian Torre Formation, the lower unit of the Murracha Group, 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 shows 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 Ichnofabric in its upper 5 m. This ichnofabric 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 dating back to the Middle Devonian.
Geological Association of Canada
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