Ichnology and sedimentology of Miocene hyperpycnites of the Austral foreland basin (Tierra del Fuego, Argentina): trace-fossil distribution and paleoecological implications

CARMONA, N.B.; PONCE, J.J.

Sediment transfer from shelf to deepwater---revisiting the delivery mechanisms

AAPG

Lugar: Estados Unidos; Año: 2009;

Combined ichnologic and sedimentologic analysis of Miocene deep-marinehyperpycnites in the Austral foreland basin, Tierra del Fuego, allows recognition andcharacterization of different trace-fossil assemblages from proximal to distal within thisdepositional system. In proximal and middle settings, hyperpycnal-channel and innerleveedeposits contain trace-fossils in climbing-rippled and parallel-laminated sandstonefacies; dwelling structures of suspension-feeding organisms (e.g. Diplocraterion)dominate. In outer-levee deposits, fine-grained heterolithic intervals display a suite oftrace fossils related to opportunistic producers, composed of concentrically laminatedburrows of detritus/deposit feeders, and also locomotion and grazing traces (e.g.Protovirgularia, Gordia). The general low intensity of bioturbation and the dominanceof structures produced by opportunistic organisms suggest a stressful environment,probably due to episodes with high-sedimentation rates and fluctuating salinity.Additionally, in the middle slope setting escape structures of imported bivalves occur inisolated sandstone bodies that accumulated at the base of the depositional slope.In the distal setting, the ichnofossils occur mainly in heterolithic intervalsdeveloped at the base and top of the hyperpycnal-lobe deposits. Bioturbation intensity iscommonly higher than in more proximal deposits, although the ichnocoenoses displaylow to moderate diversity. The rather sandy heterolithics show discrete intervalscompletely reworked by deposit-feeders forming, for instance, Scolicia and Nereites.This assemblage probably reflects periods of normal marine salinity and high foodcontent. Intervals with abundant Phycosiphon, Nereites, and subordinate Tasseliacommonly occur in fine-grained heterolithics and massive mudstones that accumulatedat the top of the hyperpycnal-lobe deposits. The presence of these ichnogenera alsoindicates full-marine conditions, although patchy distribution of these trace-fossil suites3may suggest fluctuations in salinity and in sediment and organic input. Uncommonintervals with pre-depositional graphoglyptids (e.g. Paleodictyon, Helicolithus) arepreserved on the soles of sandstone-heterolithic beds of the hyperpycnal lobes, thusreflecting periods of rather stable ecologic conditions.The trace-fossil suites of the studied hyperpycnite successions are commonlyimpoverished, showing monogeneric assemblages, and dominance of opportunisticstrategies. Both the sedimentologic and ichnologic evidences indicate that fluctuationsin hydraulic energy, in average moderate, but nonetheless strongly fluctuatingsedimentation rates, high food supply, and changes in salinity generated by thehyperpycnal flows were probably the main paleoecological factors that heavily affectedthe development of these infaunal communities.