INCITAP   20787
INSTITUTO DE CIENCIAS DE LA TIERRA Y AMBIENTALES DE LA PAMPA
Unidad Ejecutora - UE
artículos
Título:
Large striated burrows from fluvial deposits of the Neogene Vinchina Formation, La Rioja, Argentina: A crab origin suggested by neoichnology and sedimentology
Autor/es:
RICARDO NESTOR MELCHOR; JORGE FERNANDO GENISE; JUAN D. FARINA; MARÍA VICTORIA SÁNCHEZ; LAURA SARZETTI; GRACIELA VISCONTI
Revista:
PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY
Editorial:
ELSEVIER SCIENCE BV
Referencias:
Año: 2010 vol. 291 p. 400 - 418
ISSN:
0031-0182
Resumen:
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 threefour 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.