Lower Devonian Psammichnites from Zenta Range (NW Argentina), associated fossils and environments in the Middle Paleozoic of the Andean Basin

CHARO MELISA ; GUILLERMO ACEÑOLAZA ; SOL NOETINGER; LUCIA ARAOZ

Montevideo

Simposio; Simposio; SLIC 2015 TERCER SIMPOSIO LATINOAMERICANO DE ICNOLOGÍA; 2015; 2015

UDELAR Facultad de Ciencias, Montevideo Uruguay

Psammichnites represents the subsurface activity of a slug-like animal, including a variety of traces of predominantly horizontal, sinuous, meandering to looping shapes with a distinct dorsal structure shown as a sinusoidal to straight ridge. This structure has been interpreted as generated by a siphon-like or snorkel device, with flatworms and mollusks - related to extinct halkieriids - among the suspected producers. Psammichnites has been referred by many ways to Olivellites, Crossopodia, Plagiogmus, Aulichnites and Palaeobullia among others, suggesting its complex taxonomic status. Mid-Paleozoic strata in the Central Andean Basin of South America is mostly represented by highly fossiliferous thick siliciclastic sequences, with a remarkable Lower Devonian section assigned to the Pescado Formation cropping out in the central sector of Zenta Range. The trace fossils are distributed all along the section, with a general trend on diversity enrichment towards the top where Psammichnites is common. Trace density is generally low, but some sectors associated to the dark-coloured fine sandstones in the lower and middle part of the sequence display increasing degrees of bioturbation with partial mottling of strata. Weathering of the outcrops, particularly of fine sandstones, facilitates the recognition of the cross-sections of ichnofossils. Sandier levels in the upper part of the sequence enhance morphological characters preserving, in some traces, delicate forms. The ichnoassemblage here presented is clearly related to the array of substrate and the palaeoenvironment of sequence. A slightly restricted shoreface setting for the lower and middle part of the section is recognized, grading to a shallower and more oxygenated facies in the upper part of the section. Archaeonassa fossulata, Cruziana isp., Helminthopsis abeli, Helminthoidichnites tenuis, Lockeia amygdaloides, Palaeophycus tubularis and Zoophycos isp. are among the associated traces to Psammichnites isp. The outstanding preservation of Psammichnites isp. highlights the presence of the ichnogenus in the late Middle Paleozoic (Lower Devonian), increasing the time span record of the genus for South America. The co-occurrences in the analyzed section of key invertebrates and palynomorphs, narrow the age of the beds to the late Pragian and early Emsian.