Trazas fósiles de la cuenca Tarija, Carbonífero Superior del norte argentino:aspectos icnológicos de la glaciación gondwánica

LUIS BUATOIS; CECILIA DEL PAPA

PUBLICACION ESPECIAL - ASOCIACION PALEONTOLOGICA ARGENTINA

ASOCIACIÓN PALEONTOLÓGICA ARGENTINA

Año: 2003

0328-347X

The Late Carboniferous Tarija Formationconsists of diamictites, sandstones and mudstones. It is present in surface andsubsurface in northern Argentina and southern Bolivia. This unit records theNamurian-Westphalian glacial event and the subsequent postglacial transgressionthat took place in Western Gondwana. In this study trace fossils are documentedfor the first time in the Tarija Formation and in the uppermost part of theunderlying Itacuamí Formation as well. The ichnofauna was recovered from twooutcrop localities, Arroyo Iquira and Arroyo Aguas Blancas, both in the Sierrade Aguaragüe, northern Salta province. The trail Diplopodichnus biformis, occasionally intergrading with thetrackway Diplichnites gouldi, occursin suspension fallout, laminated mudstones underlying diamictites in the ArroyoIquira section (Itacuamí Formation). An arthropod resting trace occurs at thissection also. The Arroyo Aguas Blancas ichnofauna (Tarija Formation) isdominated by tiny nonspecialized grazing trails (Mermia carickensis, Gordia marina,Helminthopsis tenuis, Helminthoidichnites tenuis), withsubordinate locomotion traces (Cochlichnusisp., Diplopodichnus biformis, Diplichnites gouldi). This ichnofaunaoccurs in thin layers of very fine-grained silty sandstones and siltstones,locally with rippled tops and convolute lamination, interpreted as the productof underflow currents, and thin mudstone partings covering medium- tofine-grained sandstones with wave ripples, wave ripple lamination, andmicrohummocky cross-stratification, deposited by oscillatory flows. Thesedeposits overlie diamictites and sandy turbidites and underlie a thickdiamictite package. The ichnofauna studied is assigned to the Mermia ichnofacies. Taxonomiccomposition, nature of preservation, and overall features of the ichnofauna(e.g. presence of very simple grazing patterns of epifaunal animals and absenceof trace fossils of infaunal organisms and typical ichnotaxa of marineenvironments) suggest freshwater conditions during accumulation of theichnofossil-bearing deposits. These conditions may have occurred in lakes orfjords affected by a strong discharge of freshwater due to melting of the icemasses during deglaciation. The ichnofauna analyzed is very similar to thosefrom the Agua Colorada and Guandacol formations in the Paganzo Basin,suggesting comparable paleoenvironmental and paleoecological conditions duringthe postglacial event that affected Western Gondwana during the LateCarboniferous.