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

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

Polonia

Congreso; ICHNIA 2008; 2008

Integrated sedimentologic and ichnologic analysis of Miocene hyperpycnites in theforeland Austral Basin, Tierra del Fuego, allows recognition and characterization of differenttrace-fossil assemblages from proximal to distal positions within this depositional system.In proximal and middle positions, bioturbation is mainly present in climbing-rippledand 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 asuite of opportunistic trace fossils, composed of concentrically laminated burrows of detritusfeeders and locomotion traces (e.g. Protovirgularia). The low intensity of bioturbation, andthe dominance of opportunistic structures suggest a stressful environment, probably due tointervals with high sedimentation rates and fluctuations in salinity conditions. Additionally,escape structures of retransported bivalves were observed in isolated sandstone bodiesaccumulated at the slope break.In distal positions, the trace fossils occur mainly in the heterolithic intervals at the baseand top of the hyperpycnal-lobe deposits. Bioturbation intensities are commonly higher thanin more proximal deposits, although the trace-fossil suites display low to moderate diversities.The sandier heterolithics show discrete intervals completely reworked by deposit-feederstructures (e.g. Scolicia and subordinately Nereites). This assemblage most likely reflectsperiods with normal-marine salinity and high food content. Intervals with abundantPhycosiphon, Nereites, and subordinately Tasselia, commonly occur in fine-grainedheterolithcs and massive mudstones accumulated in distal positions of the hyperpycnal-lobes.The presence of these ichnogenera also indicates fully marine conditions, although sporadicdistribution of these trace-fossil suites suggests fluctuations in salinity. Uncommon intervalswith graphoglyptids (e.g. Paleodictyon, Helicolithus), are preserved on the soles of sandstoneheterolithic beds of the hyperpycnal lobes, and suggest periods of stable ecologic conditionsin these settings.The trace-fossil suites of these hyperpycnite successions are commonly impoverished,showing monogeneric assemblages, poor development of the tiering structure, and dominanceof opportunistic strategies. The sedimentologic and ichnologic evidence indicates thatfluctuations in energy, moderate to high sedimentation rates, high food supply, and changes insalinity generated by the hyperpycnal flows were most probably the main paleoecologicalfactors that controlled the development of these infaunal communities.