Exceptional preservationof Sphaerapuslarvalis in the Miocene Vinchina Formation of western Argentina

PABLO JOAQUIN ALONSO MURUAGA; VERONICA KRAPOVICKAS

Idanha a Nova

Congreso; 4th International Congress on Ichnology; 2016

INTERNATIONAL ICHNOLOGICAL ASSOCIATION

Exceptional preservationof Sphaerapuslarvalis in the Miocene Vinchina Formation of western Argentina Pablo Joaquín Alonso-Muruagaa^* and Verónica Krapovickasba Departamento de Cs. Geologicas, FCEN, Universidad de Buenos Aires, Intendente Güiraldes 2160 - Ciudad Universitaria - C1428EGA, IGEBA-CONICET, Argentina. (* corresponding author; pablojoaquin3@yahoo.com.ar )(^ presenting author)bbDepartamento de Cs. Geologicas, FCEN, Universidad de Buenos Aires, Intendente Güiraldes 2160 - Ciudad Universitaria - C1428EGA, IDEAN-CONICET, Argentina. (* corresponding author; cvkrapovickas@gl.fcen.uba.ar) (^ presenting author)Keywords:Sphaerapus, Vinchina Formation, Miocene, fluvial setting, water tableThe Miocene Vinchina Formation of western Argentinais made up of more than 5100 meters of siliciclastic sediments deposited mostly in fluvial environments, including transitions from braided and meandering, to anastomosing systems.The Upper Member of the unit is characterizedby the conspicuous occurrence of braided fluvial deposits, composed ofheterolithic multistory channels, gravelly bars, sandy bars, and very thin intercalations of floodplain fines facies, hosting invertebrate and vertebrate tracefossil assemblages. The present contribution deals with the outstanding preservation of Sphaerapuswithin these facies.In particular, the trace fossilsstudied were recorded at the sole of a massive fine to very fine-grained sandstone bed interbedded with laminated claystones and siltstones, representing sedimentation in overbank environments. The trace Sphaerapus larvalis occurs as a positive or negative hyporelief. It is characterized byseveral horizontal burrows displaying raised,often imbricated, oval blocks generatingnodular margins, and sometimes developing adiscontinuous open medial depression. Occasionally, the medialdepression is irregularly filled by smaller blocks. Individualblocks are smooth, sub-rounded to oval, 1? 4 mm in diameter, and identical in composition to the surrounding matrix. The burrow widths rangefrom 12to4mm, and their courses are mostly straight with gentle curves and decimetric lengths. Primary successive branching and intersecting of burrows are also present. Ongoing research suggests that these tracesreflect a shallow dwelling behavior in soft, fine grained substrates subaerially exposed but proximal to the water table as channel margins, interchannel areas, and marginal zones of local ponds.