Miocene hyperpycnal cannel-levee complexes, Tierra del Fuego, Argentina: Facies associations and trace-fossil distribution.

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

Sao Leopoldo

Congreso; Simposio Latinoamericano de icnológica; 2010

UNISINOS

In the Miocene of the Austral Foreland Basin, deep-marine hyperpycnal channel-levee complexes occur at the toe of the depositional-slope in sigmoidal clinoforms. The channel fill is mainly aggradational, with its lower and upper parts characterized by distinct lithologies. The lower channel fill accumulated during the waxing period of hyperpycnal flows, consists of transitional and recurrent passages between massive sandstone with deformation structures, and pebbly sandstone with cut-and-fill structures, parallel lamination, and climbing ripples, with very well-preserved Nothofagus leaves. The upper channel fill accumulated during the waning period of hyperpycnal flows, comprises thick tabular successions of graded beds with complex internal arrangements or Bouma Ta-e like divisions, which markedly onlap the erosive channel margins. The levee deposits are wedge shaped and consist mainly of rhythmically interbedded heterolithic mudstone and fine-grained sandstone. During deposition of the mudstone-dominated packages, layers with flocculated mudstone and important concentrations of phytodetritus occur. After complete deactivation of the hyperpycnal discharge, the channel-levee complexes were draped by massive or laminated mudstone. In general, the ichnoassemblages associated to the hyperpycnites are impoverished, showing monogeneric suites, dominance of opportunistic strategies, and absence of complex behavioral strategies. In marginal zones of the channels and inner-levee areas, suspension-feeding structures (e.g. Diplocraterion) dominate, whereas in the outer-levees, the ichnoassemblages are dominated by deposit-feeding structures (mostly Scolicia and Nereites). The sedimentologic and ichnologic evidence indicates that fluctuations in hydraulic energy, strongly fluctuating sedimentation rates, high food supply, and changes in salinity generated by the hyperpycnal flows were probably the main paleoecological factors that affected the development of these infaunal communities. Furthermore, these suites differ markedly from those found in deposits accumulated during normal sedimentation periods, which show extremely abundant and diverse ichnoassemblages, with specimens of Asterosoma, Cardioichnus, Chondrites, Thalassinoides, Protovirgularia, and Scolicia, among others. Although a comprehensive characterization of the hyperpycnite bioturbation is not yet developed, ichnologic analysis is critical for the identification of hyperpycnite facies and their differentiation from facies deposited during background sedimentation.