--> ABSTRACT: Trace Fossils of Marnoso-Arenacea Formation (Miocene), Northern Italy: Preliminary Data, by Earle F. McBride and M. Dane Picard; #91030 (2010)

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Trace Fossils of Marnoso-Arenacea Formation (Miocene), Northern Italy: Preliminary Data

Earle F. McBride, M. Dane Picard

Many horizons in the Marnoso-arenacea Formation contain rare to abundant trace fossils at numerous localities. Slope, fan, and basin-plain deposits have trace fossils dominated by the Nereites ichnofacies but include taxa from the Zoophycos ichnofacies plus Ophiomorpha and Thalassinoides. Slope deposits contain Chondrites, Cosmorhaphe, Desmograpton, Helminthoida, Neonereites, Paleodictyon, Pelecypodichnus, Planolites, Punctorhaphe, and Scolicia; fan-channel deposits contain Chondrites and Planolites; fan-lobe deposits contain Chondrites, Ophiomorpha, Thalassinoides, Scolicia, and Zoophycos; and basin-plain deposits contain Chondrites, Helminthoida, Planolites, and Zoophycos. The distribution of hypichnial taxa may be in part the result of selective preservation (i.e., dep ndent on the depth of erosion by turbidity currents).

Several types of Chondrites are ubiquitous in mudrocks of all facies. Vertical shafts of small Chondrites (0.3 mm, pelleted, goethite-stained) penetrate 110 cm below the top of one Bouma E layer, although no single shaft can be traced for more than 6 cm before it becomes lost in the third dimension. Ophiomorpha nodosa and Thalassinoides form galleries as much as 230 cm below the top of fan-lope turbidites. Burrow density ranges from 5% to 85% of a bed (ichnofacies indices of 2 to 4). Burrow walls are irregularly nodose in both horizontal and vertical burrows. One horizontal Ophiomorpha is 9 m long. We have noted none longer in the literature.

Two megaturbidites (beds > 5 m thick) were examined for evidence of mass mortality of trace-making animals. One bed is barren, but the other has Chondrites burrows in the upper 2 m of the Bouma E layer.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.