Diverse deep marine trace fossils from the Early Paleozoic metasediments of the Las Lagunitas Formation, Frontal Cordillera, Argentina

RICARDO NESTOR MELCHOR; MARÍA CRISTINA CARDONATTO; HUGO TICKJI

Santa Rosa

Simposio; Segundo Simposio Latinoamericano de Icnología (SLIC2013); 2013

INCITAP - Facultad de Ciencias Exactas y Naturales (UNLPam)

A high-diversity ichnofauna (17 ichnogenera and 20 ichnospecies) is reported from metasedimentary rocks of Late Ordovician to ?Silurian age from the Frontal Cordillera of Mendoza, Argentina. The Las Lagunitas Formation (about 800 m thick) includes four lithologic packages: 1) coarse- to medium-grained wackes interbedded with black shales containing graptolites of the Climacograptus bicornis biozone (Sandbian or Caradoc stage) overlain by 2) medium- to fine-grained sandstones. The later is in fault contact with 3) coarse- to medium-grained cross-bedded quartz sandstones and are conformably overlain by 4) a coarsening and thickening upward package composed of coarse- to fine-grained wackes with interbedded olive-green shale. The later package contains the ichnofossils described in this contribution. The specimens were collected at three localities of the Cordón del Carrizalito: Cañada Seca (34º 23? 54?S, 69º 24? 52?W), Arroyo Cortaderas (34º 21? 00?S, 69º 25´ 15?W), and an unnamed locality (34º 22? 14?S, 69º 25? 16?W). The ichnofauna is dominated by structures essentially developed in the horizontal plane including Nereites ispp., Phycodes bilix, Zoophycos isp., Lophoctenium isp. and Helminthoidichnites tenuis occurring in olive green shales interbedded with graded sandstone of turbiditic origin. Subordinate ichnotaxa in the same facies include: Palaeophycus tubularis, Protovirgularia dichotoma, Treptichnus bifurcus, Phycosiphon incertum, Chondrites isp., Glockerichnus isp., Saerichnites isp., Cosmorhaphe isp., and ? Dictyodora isp. Parallel-aminated or cross-laminated siltstone contains ?Archaeonassa isp., Teichichnus isp., and Monocraterion isp. No trace fossils were recovered from turbidite soles, which is probably a sampling bias. Nereites, the more abundant ichnogenus, is represented by four ichnospecies: N. macleayii, N. cambrensis, N. jacksoni, and Nereites isp. N. macleayii appears as submillimetre furrows with similarly sized lobes arranged perpendicular to the central furrow, which cover entire laminae and display a thigmotactic arrangement, with a curved or spiral pattern. Nereites cambrensis is usually larger, with a winding pattern and with pointed sediment lobes arranged at an acute angle with the central furrow. Nereites jacksoni is characterized by larger rounded lobes. Nereites isp. include specimens that correspond to Neonereites uniserialis and N. multiserialis preservational variations. Phycodes bilix is a bundle of horizontal, slightly curved and rarely bifurcated cylindrical burrows that converge in the proximal zone and diverge distally. The specimens assigned to Zoophycos isp. have horizontal U or J-shaped spreiten and local preservation of the central tube. Lophoctenium isp. is characterized by a horizontal lobate trace with U-shaped burrows and secondary comb-like burrows. The dominance of Nereites and rarity of graphoglyptids make the assemblage comparable with early Silurian trace fossil assemblages from New Brunswick (Canada) and Maine (USA), and also probably with Ordovician trace fossils from the Barrancos region of Portugal. Although a sedimentological framework is lacking, the trace fossil association can be compared with the Nereites Ichnosubfacies suggesting a lobe fringe setting and well-oxygenated pore waters.