Ichnology of shallow marine deposits in the Miocene Chenque Formation of Patagonia: Complex ecologic structure and niche partitioning in Neogene ecosystems

BUATOIS, L.A.; BROMLEY, R.G.; MÁNGANO, M.G.; BELLOSI, E.; CARMONA, N.B.

Tucumán

Congreso; IV Reunión Argentina de Icnología y Segunda Reunión de Icnología del MERCOSUR; 2001

Cliff exposures of the Lower Miocene Chenque Formation in central Patagonia contain superbly preserved ichnofaunas that allow detailed examination of cross-cutting relationships and tiering structure. This study is focused on lower shoreface deposits of the two lower sequences exposed in two localities near the city of Comodoro Rivadavia: Punta Delgada (south of Chubut Province) and Playa Las Cuevas (north of Santa Cruz Province). The ichnofauna is preserved in fine-grained silty sandstones, which intercalate with thin lenticular shell beds. The intervals analyzed are intensely bioturbated with discrete traces superimposed on an undifferentiated background mottling. The ichnofauna is dominated by feeding traces of infaunal deposit feeders, such as Phycosiphon, Teichichnus, Helicodromites, Asterosoma, Schaubcylindrichnus, and Taenidium, but grazing trails of deposit feeders (e.g. Planolites, Scolicia), feeding traces of chemosymbionts (Chondrites, Zoophycos?), and dwelling traces of suspension feeders (e.g. Ophiomorpha, Thalassinoides, Palaeophycus, "Terebellina") and deposit feeders (e.g. Rosselia) are present also. Associated body fossils include bivalves, echinoderms, bryozoans, serpulids, gastropods and sponges. Distinct layers showing Pinna in life position are present locally. Oysters are bioeroded displaying Entobia and Gastrochaenolites. High degree of bioturbation coupled with high ichnodiversity and variety of body fossils indicate slow or discontinuous sedimentation in well oxygenated open marine settings under normal salinity conditions. Low energy deposition was disrupted by minor storm events that produced thin coquinas and flat erosive bounding surfaces. Preliminary ichnofabric analysis allows recognition of at least ten tiers. Upper tiers are occupied by background mottling, Planolites, Asterosoma and "Terebellina". Intermediate tiers include Scolicia, Phycosiphon and Palaeophycus. Lower tiers are occupied by Ophiomorpha, Teichichnus, Helicodromites and Chondrites. The complex tiering structure suggests finely tuned, climax communities displaying vertical niche partitioning. These Miocene communities are comparable in complexity to Cretaceous chalk communities and indicate a remarkable use of the infaunal ecospace in siliciclastic environments by the early Neogene.