Deep marine, Cretaceous Zoophycos in the Fuegian Andes, Argentina: Distribution and preliminary interpretations.

D. R. MARTINIONI; E. B. OLIVERO; M. I. LÓPEZ CABRERA

Trelew

Congreso; Ichnia 2004 - First international Congress on Ichnology; 2004

Museo Paleontológico Egidio Feruglio

ABSTRACT.   The Cretaceous Fuegian rocks are largely deep-marine mudstone and, subordinate, sandstone turbidites. The Early Cretaceous wedge-shaped basin, adjacent to a southern andesitic volcanic-arc, includes across the basin-axis: a southern very coarse grained, thick slope-apron succession; a central mudstone-sandstone turbiditic basin fill; and a northern muddy slope-to-shelf succession. Prevalent Aptian-Albian outcrops are coeval with basin-wide anoxia, including scarce bioturbated horizons, mainly with Zoophycos, Phycodes, Chondrites, “Granularia”, Helminthopsis and possible Taenidium within the deep marine turbidites; and Nereites within the northern mudstones. Late Cretaceous compression caused closure of the Early Cretaceous basin, Andean uplift, and progressive foreland basin development filled with synorogenic sedimentation, mainly recorded in slope to foredeep settings comprising mudstone and localized sandstone-mudstone turbidites. The uppermost Cretaceous is characterized by strongly bioturbated horizons, being Zoophycos the most conspicuous ichnofossil. Associated ichnogenera include Chondrites, Palaeophycus, Planolites, and “Terebellina”. Zoophycos consists of large (ca. 0.5 m) lobes mining a certain tier without a significant vertical extension in muddy very fine sandstone turbidites. Spreite pattern and lobe margins are relatively simple, including rounded and continuous or radially indented, lobate margins and spreite. Zoophycos has been interpreted to have migrated to the deep-sea in the Cretaceous, and apparently particular morphologies reflect an evolutionary behavior towards more systematic and effective strip mining. Early Cretaceous Fuegian Zoophycos are only known from vertical sections, whereas Late Cretaceous Zoophycos are preserved in plan view showing entire or lobate margins, and these morphologies are coherent with the evolutionary stage of the burrowing program suggested for the Late Cretaceous.