Ichnology of Miocene hyperpycnites in the foreland Austral Basin, Tierra del Fuego, Argentina.

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

Cracovia

Congreso; The Second International Congress on Ichnology - Ichnia 2008; 2008

Polish Geological Institute

ABSTRACT:   Integrated sedimentologic and ichnologic analysis of Miocene hyperpycnites in the foreland Austral Basin, Tierra del Fuego, allows recognition and characterization of different trace-fossil assemblages from proximal to distal positions within this depositional system. In proximal and middle positions, bioturbation is mainly present in climbing-rippled and parallel-laminated sandstone facies of the hyperpycnal-channel and inner-levee deposits, being the trace-fossil suite dominated by dwelling structures of suspension-feeder organisms (e.g. Diplocraterion). In outer levee deposits, the fine-grained heterolithic intervals display a suite of opportunistic trace fossils, composed of concentrically laminated burrows of detritus feeders and locomotion traces (e.g. Protovirgularia). The low intensity of bioturbation, and the dominance of opportunistic structures suggest a stressful environment, probably due to intervals with high sedimentation rates and fluctuations in salinity conditions. Additionally, escape structures of retransported bivalves were observed in isolated sandstone bodies accumulated at the slope break. In distal positions, the trace fossils occur mainly in the heterolithic intervals at the base and top of the hyperpycnal-lobe deposits. Bioturbation intensities are commonly higher than in more proximal deposits, although the trace-fossil suites display low to moderate diversities. The sandier heterolithics show discrete intervals completely reworked by deposit-feeder structures (e.g. Scolicia and subordinately Nereites). This assemblage most likely reflects periods with normal-marine salinity and high food content. Intervals with abundant Phycosiphon, Nereites, and subordinately Tasselia, commonly occur in fine-grained heterolithics and massive mudstones accumulated in distal positions of the hyperpycnal-lobes. The presence of these ichnogenera also indicates fully marine conditions, although sporadic distribution of these trace-fossil suites suggests fluctuations in salinity. Uncommon intervals with graphoglyptids (e.g. Paleodictyon, Helicolithus), are preserved on the soles of sandstone heterolithic beds of the hyperpycnal lobes, and suggest periods of stable ecologic conditions in these settings. The trace-fossil suites of these hyperpycnite successions are commonly impoverished, showing monogeneric assemblages, poor development of the tiering structure, and dominance of opportunistic strategies. The sedimentologic and ichnologic evidence indicates that fluctuations in energy, moderate to high sedimentation rates, high food supply, and changes in salinity generated by the hyperpycnal flows were most probably the main paleoecological factors that controlled the development of these infaunal communities.