INVESTIGADORES
PONCE Juan Jose
congresos y reuniones científicas
Título:
Unusual shallow-marine and estuarine ichnofaunas from the Maastrichtian of Antarctica: Paradictyodora -Neonereites ichnocoenosis.
Autor/es:
OLIVERO, E.B.; PONCE, J.J.; LÓPEZ CABRERA, M.I.; MARTINIONI, D.R.
Lugar:
Ushuaia, Tierra del Fuego
Reunión:
Congreso; Quinta Reunión Argentina de Icnología y Tercera Reunión de Icnología del MERCOSUR; 2007
Resumen:
In the lower
Maastrichtian strata of the James Ross Basin, Antarctica, the deltaic deposits
of the Snow Hill Island Formation and the tidal-influenced estuarine deposits
of the López de Bertodano Formation bear an unusual ichnocoenosis, dominated by
the ichnogenera Paradictyodora and Neonereites. The main objective of this
study is to document this rare ichnofauna and to discuss briefly its possible
implications.
The Snow Hill Island
Formation consists of highly bioturbated and rhythmically interbedded mudstone
and silty sandstone beds. The dominant ichnofabric consists of a dense mottling
structure that typifies thick packages, generally in excess of 10 m in thickness, where Nereites missouriensis,
Schaubcylindrichnus isp., Teichichnus rectus, Ophiomorpha isp.,
Tasselia isp., Rhizocorallium isp., and Chondrites
isp. can be recognized occasionally. Superimposed on this densely bioturbated
background, abundant burrows of Paradictyodora
isp. are clearly visible. Paradictyodora
isp. is similar to the described conical form of Paradictyodora antarctica, characterized by vertical spreite laminae
that taper downward and join in a central point.
The
basal part of the López de Bertodano Formation consists of thick mudstone
packages with large channels filled with very fine-grained sandstone and
mudstone beds. The channel fills are highly bioturbated, but ichnodiversity at
any given horizon is generally very low, and the dominant trace fossils are Neonereites biserialis and Paradictyodora isp. In some horizons Phycosiphon isp., Nereites missouriensis, Patagonichnus isp., and
Schaubcylindrichnus isp. could also be very abundant, whereas Teichichnus rectus, Ophiomorpha isp.,
and Rhizocorallium isp. are rare.
Highly bioturbated packages reveal different tiers. In some beds, the tiering
structure is clearly shown. The shallowest burrows correspond to Phycosiphon isp., the intermediate tier consists of Neonereites biserialis or
Nereites missouriensis, and the deepest tier consists of Paradictyodora isp.,
which crosscut all previous burrows.
Both Paradictyodora isp. and Neonereites biserialis are interpreted as fodinichnia structures From an
ethological point of view, their records in Antarctica
appear to be consistent with the associated trace fossils. The ichnoassemblage
in the Snow Hill Island Formation is typical for the Cruziana ichnofacies and
in the López de Bertodano Formation is typical for the low ichnodiversity and
monospecific dominance known for estuarine settings. What is unusual in the
lower Maastrichtian strata of Antarctica is the extreme density of these
burrows. It is difficult to explain this abundance, but we think that possible
controls may include a combination of exceptional preservation conditions
and/or localized geographical abundance of the trace fossil producers favored
by environmental changes that started in the early Maastrichtian in Antarctica,
particularly the known marked drop in seawater temperatures.