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.

AAPG Studies in Geology 61 - Sediment transfer from shelf to deepwater---revisiting the delivery mechanisms

AAPG

Lugar: Estados Unidos; Año: 2011; p. 171 - 192

Combined ichnologic and sedimentologic analysis of Miocene deep-marine hyperpycnites in the Austral foreland basin, Tierra del Fuego, Argentina, allows recognition and characterization of different trace fossil assemblages from proximal to distal within this depositional system. In proximal and middle settings, hyperpycnal channel and inner levee deposits contain trace fossils in climbing-ripple and parallel-laminated sandstone facies; dwelling structures of suspension-feeding organisms (e.g., Diplocraterion) dominate. In outer levee deposits, fine-grained heterolithic intervals display a suite of trace fossils related to opportunistic producers composed of concentrically laminated burrows of detritus or deposit feeders and also locomotion and grazing traces (e.g., Protovirgularia, Gordia). The general low intensity of bioturbation and the dominance of structures produced by opportunistic organisms suggest a stressful environment probably because of episodes with high sedimentation rates and fluctuating salinity. In addition, in the middle slope setting, escape structures of imported bivalves occur in isolated sandstone bodies that accumulated at the base of the depositional slope. In the distal setting, the ichnofossils occur mainly in heterolithic intervals developed at the base and top of the hyperpycnal lobe deposits. Bioturbation intensity is commonly higher than in more proximal deposits, although the ichnocoenoses display low to moderate diversity. The fairly sandy heteroliths show discrete intervals completely reworked by deposit feeders forming, for instance, Scolicia and Nereites. This assemblage probably reflects periods of normal marine salinity and high food content. Intervals with abundant Phycosiphon, Nereites, and subordinate Tasseliacommonly occur in fine-grained heteroliths and massive mudstones that accumulated at the top of the hyperpycnal lobe deposits. The presence of these ichnogenera also indicates full-marine conditions, although patchy distribution of these trace fossil suites may suggest fluctuations in salinity and in sediment and organic input. Uncommon intervals with predepositional graphoglyptids (e.g., Paleodictyon, Helicolithus) are preserved on the soles of sandstone-heterolithic beds of the hyperpycnal lobes, thus reflecting periods of fairly stable ecological conditions. The trace fossil suites of the studied hyperpycnite successions are commonly impoverished, showing monogeneric assemblages and dominance of opportunistic strategies. Sedimentologic and ichnologic evidences indicate that fluctuations in hydraulic energy, sedimentation rates that are moderate on average but nonetheless strongly fluctuating, high food supply, and changes in salinity generated by the hyperpycnal flows were probably the main paleoecological factors that heavily affected the development of these infaunal communities.