Sustained and Episodic Gravity-Flow Deposits and Related Bioturbation Patterns in Paleogene Turbidites (Tierra Del Fuego, Argentina)
Published:January 01, 2007
J.J. Ponce, E.B. Olivero, D.R. Martinioni, M.I. López Cabrera, 2007. "Sustained and Episodic Gravity-Flow Deposits and Related Bioturbation Patterns in Paleogene Turbidites (Tierra Del Fuego, Argentina)", Sediment–Organism Interactions: A Multifaceted Ichnology, Richard G. Bromley, Luis A. Buatois, Gabriela Mángano, Jorge F. Genise, Ricardo N. Melchor
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Careful ichnologic analysis is an important tool for discrimination of sustained and episodic gravity-flow deposits, as shown from two stacked packages of deep-marine Upper Eocene-Lower Oligocene turbidites at eastern Tierra del Fuego. A 63-m-thick, mudstone-sandstone lower Package I consists of an upward-coarsening-thickening succession, followed by a 91-m-thick Package II with a reverse trend and an erosional lower contact, consisting of pebbly coarse- to fine-grained sandstones occasionally capped by mudstone beds. Package I represents episodic flow deposits (classical turbidites of distal lobe facies) arranged in minor (~ 3-4 m thick) upward-coarsening-thickening cycles. The trace fossils Phymatoderma Brongniart, Zoophycos Massalongo, Chondrites von Sternberg, Tasselia Heinzelin, and Paradictyodora Olivero, Buatois, and Scasso are dominant at the tops of the minor cycles in the Td-e turbidite divisions. Package II consists of sustained-flow deposits (hyperpycnites in channel fill and aggradational lobes) organized in fining- and thinning-upwards cycles of complex beds ~ 5-6 m thick capped by hemipelagic mudstones. These channelized and lobe deposits of long-duration flows record abundant interstitial fluid-escape structures (convolute bedding) indicative of rapid deposition and, as expected, are not bioturbated. Only the topmost mudstones contain trace fossils such as Phymatoderma (prevalent), Zoophycos, Chondrites, and Tasselia. During the depositional phase of each sustained flow, continuous, high sedimentation rates precluded the colonization of the substrate by organisms. During episodes between successive sustained flows, hemipelagic beds (the topmost mudstones) accumulated slowly, favoring bioturbation of the substrate. These bioturbated beds are key surfaces for sedimentologic analysis, allowing proper identification of the bounding surfaces between successive, complex deposits of sustained flows.
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Sediment–Organism Interactions: A Multifaceted Ichnology
The field of Ichnology bridges the gap between the areas of paleontology and sedimentology, but has connections to many subdisciplines within these areas. Biogenic structures record the behavior of their tracemakers and provide valuable information in paleoecologic and paleoenvironmental analysis. As in situ ethologic structures, trace fossils or ichnofossils yield valuable insights into the paleoecology of ancient benthic communities and the environmental dynamics of depositional systems. Ichnology is truly a multifaceted field, and a broad selection of its facets is represented in the 28 papers of this volume. The papers are the product of Ichnia 2004, the First International Congress on Ichnology, convened by Jorge F. Genise and held from 19 to 23 April 2004 at the Museo Paleontológico Egidio Feruglio in Trelew, Patagonia, Argentina. Seven papers connected with the congress, containing ichnotaxonomy, were published separately, in Ichnos, volume 13, issue 4. Several symposium volumes, books, and short-course notes have been published in recent years and ichnology can be considered a particularly active research area in steady growth. The 28 papers herein are arranged in five groups that reveal the broad scope of ichnology.