Ichnology of a Storm-influenced Clastic Succession, Upper Cretaceous of Southeast James Ross Island, Antarctica.

D. R. MARTINIONI; A. LUTHER; L. A. BUATOIS

Mar del Plata

Congreso; II Congreso Latinoamericano de Sedimentología & VIII Reunión Argentina de Sedimentología; 2000

Asociación Argentina de Sedimentologìa

ABSTRACT.   The Upper Cretaceous (Campanian) succession of Rabot Point, southeast James Ross Island, Antarctica, includes shallow marine, storm-influenced deposits that contain relatively abundant trace fossils. Two main facies associations are dominated by sandstone and mudstone, respectively. The sandstone dominated facies association represents mainly nearshore-shoreface and transition-to-offshore deposits with typical storm beds and interbedded fair-weather mudstone layers. The mudstone dominated facies association reflects deposition from suspension in a fairly deeper shelf environment. The Rabot ichnofauna is subdivided into two assemblages based on its relation to storm episodes. The pre-event assemblage contains Chondrites, Palaeophycus, Phycosiphon, Planolites, Ophiomorpha, Thalassinoides, and Tissoa. This assemblage is dominated by horizontal dwelling and feeding traces of  suspension and deposit feeders, as well as predators. Both semipermanent domiciles (e.g., Ophiomorpha) and temporary structures of mobile animals (e.g., Planolites, Palaeophycus) are present. Although rare, complex spreiten feeding structures (Phycosiphon) occur. The pre-event assemblage represents a typical example of a fair-weather ichnofauna developed under stable, low energy conditions in shallow marine settings. This assemblage can be confidently ascribed to the Cruziana ichnofacies and, although not very diverse, it resembles other shallow marine, background ichnofaunas described from the stratigraphic record. These ichnofaunas typically display k-selected strategies and record the activity of the resident benthic fauna. The post-event assemblage includes Zoophycos, Rhizocorallium, and scarce Skolithos. This ichnocoenosis is dominated by feeding traces of deposit feeders that systematically mined the sediment. Dwelling structures of suspension feeders represent a very minor component of the post-storm suite. This association records opportunistic colonization of the storm-emplaced sands under unstable and stressful conditions. The post-event suite comprises organisms that typically display r-selected strategies. Colonization suites in shallow marine tempestites are very common in the stratigraphic record and, in most cases, are dominated by traces of suspension feeding animals, representing an example of the Skolithos ichnofacies. In this respect, the post-storm Rabot ichnocoenosis is somewhat anomalous because it is dominated by traces of deposit feeders, which only rarely have been recorded as components of post-storm assemblages. High abundance of deposit feeding traces in these storm beds probably records opportunistic colonization of sediment with high concentration of organic detritus eroded from the sea floor during the high energy, early stage of the storm and redeposited during the waning phase. The complex feeding structure Zoophycos is undoubtedly the most conspicuous ichnofossil in Rabot. The presence of Zoophycos in storm facies of Rabot Point is anomalous in terms of its documented onshore-offshore history, because it has been noted that this ichnogenus experienced a retreat from nearshore settings to the deep sea throughout the Phanerozoic. The occurrence of Zoophycos in the succession of Rabot Point represents one of the youngest occurrences of this ichnotaxon in nearshore environments.