Paleoenvironmental framework of dinosaur tracksites and other ichnofossils in Early Cretaceous mixed siliciclastic-carbonate deposits in the Neuquén Basin, northern Patagonia (Argentina)

PAZOS, P.; LAZO, D.; TUNIK, M.; MARSICANO, CLAUDIA; FERNADEZ, D.; AGUIRRE-URRETA, M.B.

GONDWANA RESEARCH

ELSEVIER SCIENCE BV

Lugar: The Netherlands; Año: 2012 vol. 22 p. 1125 - 1140

1342-937X

The study of the uppermost section of the Early Cretaceous Agrio Formation in northern Patagonia (NeuquénBasin) where dinosaur tracks assigned to cf. Therangospodus pandemicus are exposed (tracksites I and II)evidence mixed marginal marine siliciclastic-carbonate deposits. The succession was divided in two intervals.A lower one containing theropod tracks, recorded on top of subtidal oolithic limestones with tiny wave ripplessuggesting shoreline fluctuations and subaerial exposure. Tidal influence is recognised by fining upward and progradingcycles starting with subtidal carbonates and ending with fine-grained siliciclastic deposits at the top, orrarely laminites. Dolomitization affects subtidal deposits generated in an alkaline media stressful for tracemakers.Intertidal facies include abundant heterolithic deposits, coquinas composed of gastropods encrustedby multilayered bryozoans and muddy levels with incipient mud cracking. Invertebrate ichnofossils recognizedfrom tidally dominated deposits include Arenicolites, Kouphichnium, and Rhizocorallium. The upper interval is atransgressive?regressive cycle that starts with dark shales, deficiently oxygenated, and covered by progradingsandstones and finally sand flat deposits. This interval contains Gyrochorte, Hillichnus, and Ophiomorpha documentedin wave-influenced sandstones. Dinosaur tracks as well as Hillichnus, attributed to tellinoid bivalves,and Kouphichnium assigned to xiphosurans, imply the activity of producers rarely recorded previously as bodyfossils in marginal marine deposits of southern South America. Previous paleogeographic schemes are questionedby our analysis, which shows evidence of extremely shallow and tide-controlled sedimentation, sometimeswith subaerial exposure, with high cyclicity related to a marginal marine depositional setting and lack ofsignificant erosion by the overlying unit, as traditionally was suggested.