Large striated burrows from fluvial deposits of the Neogene Vinchina Formation,

MELCHOR, R. N.; GENISE, J. F.; FARINA, J. L.; SANCHEZ, M. V.; SARZETTI, L. C.; VISCONTI, G.

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

ELSEVIER SCIENCE BV

Año: 2010 vol. 291 p. 400 - 418

0031-0182

This study includes three aspects: 1) the ichnotaxonomic treatment of large and dominantly vertical burrows ornamented with sharp bioglyphs from the fluvial Neogene Vinchina and Toro Negro formations (northwest Argentina); 2) the description and interpretation of the sedimentary facies where these burrows occur; and 3) neoichnologic observations on large ornamented burrows from seasonal wetlands of the Río Pilcomayo National Park (Argentina) as possible modern analogues of the trace fossils and the sedimentary environment. A new ichnogenus and ichnospecies, Capayanichnus vinchinensis, is proposed to include the large striated burrows. The new ichnotaxon is distinguished by a combination of a predominantly vertical orientation, overall “L” shape (when fully developed), absence of lining and burrow bifurcation, a distinct surface texture, and lack of burrow enlargements. The hosting sedimentary facies of the Vinchina Formation can be grouped into fluvial channel and splay (floodplain) facies associations. The rivers of that unit were of two main types: single channel rivers with frequent channel avulsion and overbank flow and multichannelised rivers with poorly defined margins. At least some of these rivers were ephemeral or with intermittent discharge. The climate was probably seasonal and semiarid, as suggested by sedimentologic evidence. C. vinchinensis was dominantly recorded from the top of fluvial channels (interpreted as abandoned channels) and proximal, intermediate and distal splay facies. The large ornamented burrows found in an extant comparable example were constructed by freshwater crabs. These burrows can be distinguished by the overall architecture, the taxonomy and sex of its occupants, and the environmental setting where they were found. The neoichnologic signatures used in the interpretation of the fossil example include the common oval burrow cross-section and the surface texture. In particular, the surface texture in the modern crab burrows includes abundant comma-shaped marks, sets of grooves oblique to burrow axis,and long segmented grooves. The predominantly oval cross-section, absence of lining, and the strongly ornamented burrow surface suggest that C. vinchinensis was produced by freshwater crabs. The key features of the surface ornament are long sets of three–four ridges whose width is in average one third of the burrow major axis. Other features that also point to a crab origin are a massive filling, common burrow ends in a casing mudstone, chevron pattern in sets of surface ridges, and possible cheliped marks. The new ichnotaxon is restricted, in the Vinchina and Toro Negro formations, to fluvial facies and is absent in shallow lacustrine facies.