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

PAZOS P.J.; LAZO, D. G., ; TUNIK M.; MARSICANO C.A.; DIANA ELIZABETH FERNÁNDEZ; AGUIRRE URRETA M.B.

GONDWANA RESEARCH

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2012 vol. 22 p. 1110 - 1124

1342-937X

The uppermost section of the Agrio Formation at a depocentre area of the Neuquén Basin are characterized by mixed siliciclastic-carbonate marginal marine deposits nearly devoid of body fossils, but containing a diverse trace fossil assemblage composed of vertebrate dinosaur tracks (theropod) and invertebrate ichnofossils. Here we present a detailed sedimentologic and ichnologic analyses in the Mina La Continental locality and also we describe the ichnological record at Cerro Rayoso locality. Palaentological re-cord includes body fossils as gastropods, trigonioids and bryozoans that supply crucial evidence of such marginal marine depositional setting. The interval (Late Hauteriavian- early Barremian) is composed of five facies (A-E) that are arranged in a lower prograding tide-dominated cycle arranged into high frequency sequences with a distinc-tive fining upward trend typical of progradation of tidal facies. Subtidal carbonates (oolithic packstone and grainstones) pass upward to intertidal sandy, mixed, to muddy deposits. An uppermost siliciclastic-dominated transgressive cycle includes gray to green shales an offshore prograding siliciclastic wedge. Subtidal deposits are barren of trace fossils probably because alkaline conditions related to an early stage of dolomitization and loose substrates resulted inhospitable for trace-makers. Intertidal deposits records evidences of subaerial exposure (mud cracks), pavements of Rhizocoralium jenense interpreted as dewatered and compacted substrates with firmness as Glossifungites ichnofacies and laminites, evidencing the end of depositional cycles. Ichnofossils recognized from tidal-dominated deposits include Arenicolites, Gyrochorte, Lophotecnium-like, Paleohelcura, Rhizocorallium, Skolithos, theropod tracks and Gor-dia-Treptichnus forms. The progradational clastic wedge contains Gyrochorte, Hillich-nus agrioensis, and Ophiomorpha. Dinosaur tracks as well as traces attributed to tellina-cean bivalves (Hillichnus) or scorpions (Paleohelcura) precede their body-fossil record in the Neuquén Basin. Trace fossils attributed to scorpions are an enigmatic record that agree with palaeoclimatic evidences of aridity as sand dunes, evaporates recorded in the Lower Cretaceous in the basin. Our analysis document Mermia-like components but without any evidence of freshwater influence. Food availability was previously used to explaining the looping pattern of such ichnofossils in modern tidal flats, rather than the lack of marine influence as was traditionally emphasized. Our comparison with normal marine modern analogues of tidal flats supports this interpretation. The facies and ich-nology reported in this paper is crucial to discuss the accuracy of previous paleo-geographic maps of the Neuquén Basin during the deposition that suggested permanent submerged and normal marine conditions for the studied interval in this area of the ba-sin. It is challenged by our analysis, evidencing extremely shallow, tide- controlled and sometimes with subaerial exposure into a highly cyclical depositional setting. We em-phasise the role of tidal action rather than a storm dominated ramp as traditionally ac-cepted for uppermost part of the Agrio Formation. Finally, this paper is an important base to continue exploration of the reliability of the ichnofacies models in special cases as mixed siliciciclastic- carbonate depositional systems and constitute the first integrated approach to the study of the final stages of one of the most widespread marine intervals in the Neuquén Basin, traditionally barren of biostratigraphic markers and body fossils. In this scenario, trace fossils, facies and microfacies are very useful to elucidate such complex depositional settings.