Unusual eolian dune and interdune ichnofaunas from the Eocene-Oligocene Vallecito Formation, northwest Argentina

PEREZ, M.; UMAZANO, A.M.; MELCHOR, R.N.

Puerto Madryn

Otro; XIV Reunión Argentina de Sedimentología; 2014

Asociación Argentina de Sedimentología

The Eocene-Oligocene Vallecito Formation belongs to the Vinchina Tertiary basin, which is located in northwestern Argentina. The Vallecito Formation is mainly composed of eolian sandstones with relatively thin intercalations of fluvial and lacustrine deposits. Vallecito Formation represents a large dune field developed in arid to semiarid climates during the first stages of the Andean orogeny. For this contribution we measured two partial sections denominated La Ciénaga de Huaco in San Juan province (55.45 m thick; 30° 9´ 15,66´´ S - 68° 35´ 21,6´´ W), and Quebrada de los Pozuelos in La Rioja province (27.7 m thick; 28° 37´ 53,82´´ S - 68° 6´ 6,42´´ W). Logged sedimentologic sections included trace-fossil bearing eolian deposits and associated fluvial and lacustrine settings. Four facies associations have been recognized including eolian dunes, shallow lakes, sheet-floods and low-relief eolian bedforms. The last three belong to the interdune zone. Low-relief eolian bedform deposits lack trace fossils and are composed by fine and very fine-grained sandstones with low-angle cross-bedding and plane parallel lamination, respectively. Sheet-flood deposits are represented by very fine-grained to coarse-grained sandstones mostly with trough cross-bedding and, subordinated, with low-angle cross-bedding, plane-parallel lamination or massive aspect. Trace-fossil content of sheet-flood includes Palaeophycus tubularis and Taenidium barretti. Eolian dune deposits are composed of very fine to medium-grained sandstones with tabular planar cross-bedding, and subordinated tabular-tangential cross-bedding. Cruziana problematica, Taenidium barretti, Palaeophycus tubularis, Rusophycus isp., undeterminate tetrapod footprints and avian tridactyl footprints were found in eolian dune deposits at Ciénaga de Huaco. Avian tridactyl footprints are 57 mm long, 62 mm wide and total divarication (digits II-IV) is 104°. A single trackway composed of 4 avian footprints exhibited a moderately high pace angulation (162°) and an average stride length of 465 mm Undeterminate rounded tetrapod footprints are 20 mm in diameter. All footprints display a marked marginal ridge pointing downward which is typical of tetrapod footprints in inclined surfaces like eolian foreset laminae. The ichnofauna from shallow lake interdune deposits was recorded at Quebrada de los Pozuelos. These deposits are constituted by very fine to fine-grained sandstones, siltstones and mudstones with plane-parallel lamination, mud cracks,subordinated symmetrical ripples, and rain drop imprints. Trace-fossil content of shallow lake deposits include Taenidium barretti, Skolithos isp., Palaeophycus tubularis, Spongeliomorpha carlsbergi and a single tridactyl footprint. This tridactyl footprint was preserved in a mudcracked surface, is incomplete, wider (at least 76 mm) than long (71 mm) with thick (15 mm wide) digits imprints, marked phalangeal pads and probable claw marks. Thick digit imprints and marked phalangeal pads are suggestive of a non-avian origin. The eolian dune ichnofauna of the Vallecito Formation is unique because of the presence of Cruziana problematica and avian tridactyl footprints, which have not been recorded in other eolian dune ichnofaunas. Future work is aimed to identify the potential producers of these trace fossils, what are the environmental factorsthat explain its occurrence in the Vallecito Formation and implication for eolian ichnofacies.