Dinosaur tracksites paleoenvironment in Early Cretaceous Patagonia (Argentina).

[Ben Creisler <bcreisler@gmail.com>]

From: Ben Creisler
bcreisler@gmail.com


A new online paper:

P.J. Pazos, D.G. Lazo, M.A. Tunik, C.A. Marsicano, D.E. Fernández, b,
M.B. Aguirre-Urreta (2012)
Paleoenvironmental framework of dinosaur tracksites and other
ichnofossils in Early Cretaceous mixed siliciclastic-carbonate
deposits in the Neuquén Basin, northern Patagonia (Argentina).
Gondwana Research (advance online publication)
http://dx.doi.org/10.1016/j.gr.2012.02.003
http://www.sciencedirect.com/science/article/pii/S1342937X12000433?v=s5


The study of the uppermost section of the Early Cretaceous Agrio
Formation in northern Patagonia (Neuquén Basin) 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 ripples suggesting shoreline fluctuations
and subaerial exposure. Tidal influence is recognised by fining upward
and prograding cycles starting with subtidal carbonates and ending
with fine-grained siliciclastic deposits at the top, or rarely
laminites. Dolomitization affects subtidal deposits generated in an
alkaline media stressful for trace-makers. Intertidal facies include
abundant heterolithic deposits, coquinas composed of gastropods
encrusted by multilayered bryozoans and muddy levels with incipient
mud cracking. Invertebrate ichnofossils recognized from
tidally-dominated deposits include Arenicolites, Kouphichnium, and
Rhizocorallium. The upper interval is a transgressive-regressive cycle
that starts with dark shales, deficiently oxygenated, and covered by
prograding sandstones and finally sand flat deposits. This interval
contains Gyrochorte, Hillichnus, and Ophiomorpha documented in
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 body fossils in marginal marine deposits of southern
South America. Previous paleogeographic schemes are questioned by our
analysis, which shows evidence of extremely shallow and
tide-controlled sedimentation, sometimes with subaerial exposure, with
high cyclicity related to a marginal marine depositional setting and
lack of significant erosion by the overlying unit, as traditionally
was suggested.