Unusual shallow-marine and estuarine ichnofaunas from the Maastrichtian of Antarctica: Paradictyodora - Neonereites ichnocoenosis.

E. B. OLIVERO; J. J. PONCE; M. I. LÓPEZ CABRERA; D. R. MARTINIONI

Ushuaia

Congreso; Quinta Reunión Argentina de Icnología y Tercera Reunión de Icnología del MERCOSUR; 2007

Laboratorio de Geología Andina (CADIC-CONICET)

ABSTRACT:   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.