Paleoenvironmental reconstruction of a Late Cretaceous dinosaur track site of Argentina: palynological evidences

PRÁMPARO, MERCEDES; GONZÁLEZ RIGA, BERNARDO J.; PREVITERA, ELENA

Bonn, Alemania

Congreso; 12th International Palynological Congress and 8th International Organisation of Palaeobotany Conference; 2008

Universidad de Bonn

“Agua del Choique” (southern Mendoza province, western Argentina) is one of the youngest (Late Cretaceous) sauropod track site, recorded in the world. The site comprises at least 160 well-preserved tracks of a middle size derived titanosaurs (14-16 m long), characterized by wide-gauge style of locomotion, located on a calcareous sandstone bed of the upper part of Loncoche Formation. This formation is related to a lake setting and river-dominated delta deposits, which represent the early stages of an Atlantic transgression that took place during the Maasstrichtian-Paleogene, in southern South America. The analyses of the palynological content of the pelitic strata, located above and below the track bed, gave new evidences to the paleoenvironmental reconstruction of the site. The palynological association above the tracks level is dominated by fresh water algae: Pediastrum sp.aff. P. boryanum var.longicorne Reinsch (67 to 80 %) and Pediastrum sp.aff.P.duplex Meyen (4 to 9%); Botryococcus spp.(5 to 9%); peridinioid dinoflagellate cysts (1 to 6%) and a few specimens of Ovoidites sp. and Tetraporina sp. The association found below the track level is dominated by Pediastrum sp.aff. P. boryanum var. brevicorne A.Br. and Botryoccocus spp. The thin-wall peridinioid cysts exhibit high degree of intraspecific morphological variation in the surface ornament, overall shape and size of the horns. They vary from granulate forms with one well developed apical and two antapical horns to smooth cysts with small horns, but all of them with a poorly delineated tabulation; probably all belong to the same genus. The samples studied yielded also scarce terrestrial spores and pollen grains varying from 3 to 20%, mainly represented by bi/ trisaccates, pteridophyte (Deltoidospora) and bryophyte spores (Zlivisporis) and only a few angiosperms pollen grains. The algae represent the autochthonous vegetation of the water body, so in this case the dominance in all the samples of the fresh water algae Pediastrum would be indicative of lakes with eutrophic to hypertrophic water, small ponds or certain lowland rivers, with alkalinity that could be above the neutral pH. The occurrence of monospecific peridinioid cysts in the palynological association and the lack of other marine taxa would indicate that these dinoflagellate cysts were deposited in a fresh to brackish water environment. In summary, a lacustrine environment dominates this portion of the section but with evidence of intermittent tidal influence.   This abstract is a contribution to CONICET-PIP Project 5132, Argentina.