TRACE FOSSIL ZONATION IN INTERDUNE LAKES: EXAMPLE FROM THE NEOGENE OF EASTERN ARGENTINA

MANUEL ROJAS MANRÍQUEZ; FEOLA, SILVERIO F.; MELCHOR, RICARDO N.

Puerto Escondido

Simposio; V Simposio Latinoamericano de Icnología; 2023

Universidad Nacional Autónoma de México

The Neogene Río Negro Formation contains an amazing variety and abundance of trace fossils of continental and marine origin. This contribution includes a detailed sedimentological and ichnological characterization of the upper (continental) member of the Río Negro Formation (upper Miocene-lower Pliocene) near El Cóndor town (41° 03´ 05" S, 62° 51´ 01" W), Río Negro Province, Argentina. The study was focused on excellent sea cliffs exposures over a distance of about 2.5 km of the town. Recognized facies associations included ephemeral fluvial, eolian dune, dry interdune, damp interdune, and wet interdune. The wet interdune facies association was subdivided into sandy littoral, lacustrine flat, and lacustrine basinal deposits. These facies associations alternate with each other, resulting in three intervals of wet interdune, five intervals of damp interdune, and three intervals of aeolian dune. The overall environmental setting is a wet aeolian system composed of an active dune field with flooded interdune areas, where lakes of different sizes developed. Eolian flows played a central role in the sedimentary dynamics, with dominant dune migration towards the northeast. Twenty-two invertebrate, vertebrate and plant ichnotaxa were identified and grouped into four trace fossil assemblages. Eolian dune and dry interdune facies associations were devoid of ichnofossils. Trace fossil assemblage A represents interdune lake expansion and have high ichnodiversity (n=10) and abundance. It includes bivalve resting and escape trace fossils (Siphonichnus isp., Lockeia siliquaria), simple horizontal and vertical burrows (Planolites beverleyensis and Skolithos isp.), meniscate burrows (Taenidium barretti and Taenidium satanassi), avian (Fuscinapeda isp.) and mammal footprints (Macrauchenichnus cf. troyana, and Porcellusignum conculcator), and horizontal root systems. Trace fossil assemblage B represents interdune lake highstand conditions and includes essentially bivalve resting, escape, and locomotion trace fossils (including Siphonichnus isp., L. siliquaria, bilobed ridges associated with Lockeia and comparable with Protovirgularia isp., and Ptychoplasma excelsum) and rare T. satanassi. This trace fossil assemblage is comparable with the Mermia ichnofacies. Trace fossil assemblage C is linked with interdune lake contraction and also has a high ichnodiversity (n=9) and abundance, including heavily trampled grounds. It contains bivalve resting and escape trace fossils (Siphonichnus isp., L. siliquaria), meniscate burrows (T. barretti), diverse avian footprint morphotypes, some with pecking/probing traces (Aramayoichnus rheae, Fuscinapeda isp., Gruipeda isp. and tetradactyl avian footprints), mammal footprints (P. conculcator and Mylodontidichnum isp.), and ochre subvertical root casts. This trace fossil assemblage is partially comparable with the “shorebird ichnofacies” or with the Grallator ichnofacies. Trace fossil assemblage D corresponds to damp interdune facies association and exhibits a moderate ichnodiversity (n=7). The more characteristic trace fossils are Nagtuichnus meuleni (pink fairy armadillo meniscate burrows), footprints in cross-section, and subvertical calcareous rhizoliths; it also includes large didactyl footprints and tetradactyl footprints in quadrupedal trackway. Detailed sedimentologic and ichnologic studies of Neogene interdune lacustrine deposits allow to propose a detailed lacustrine zonation of trace fossils in response to changing physical conditions.