New palynological data from the Katian of the Precordillera Basin, Argentina: a potential key for understanding the geological history of the Precordillera terrane

DE LA PUENTE, G. SUSANA; RUBINSTEIN, CLAUDIA V.

Cincinnati, Ohio

Congreso; International Geoscience Programme-IGCP Project 591, 2nd Annual Meeting and 1st Foerste Symposium, Cincinnati, 2012; 2012

Department of Geology, University of Cincinnati

Biostratigraphic data from the Precordillera Basin have traditionally been analyzed to contribute to the understanding of its biogeographic evolution from the detachment from Laurentia until the accretion to Gondwana (Benedetto et al., 1999). New palynological data are incorporated in an attempt to better constrain the timing of the collision. During the Late Ordovician, evidence of extensional tectonics are recognized in the Las Vacas Formation associated with a collision event. This unit is separated from the overlain Trapiche Formation by an angular unconformity. Research on these units has identified that the west-directed Ocloyic thrusting was coeval with the deposition of the Las Vacas Conglomerate (Thomas & Astini, 2007). Palynological data come from the shally upper part of the Las Vacas Formation and the lower part of the Trapiche Formation. Chitinozoans from the Las Vacas Formation are numerous, although poorly diversified. The assemblages mainly contain Cyathochitina macastyensis, Spinachitina bulmani, and Desmochitina cocca. The youngest sample also yields Kalochitina multispinata. These chitinozoans are typical from the Katian of Laurentia. Scolecodonts are also present. Acritarchs are poorly preserved and the few recognized genera are not biostratigraphically significant. Cryptospores represented by simple tetrads suggest the proximity to the coastline of the upper part of the unit. This cryptospore finding completes the record of spores for the Ordovician of Argentina (previously reported for the Dapingian, Darriwilian and Hirnantian). The sample from the Trapiche Formation contains a few poorly preserved chitinozoans, acritarchs and algae (cf. Botryococcus), probably due to reworking of the material. A proximal environment is also suggested by the presence of plant remains such as Nematothallus. The Las Vacas Formation has previously been assigned to the Sandbian (gracilis-bicornis Zones). Typical Katian chitinozoans from its uppermost part indicate that these deposits would have reached the Sandbian-Katian boundary, or alternatively these species may possibly have an earlier appearance in the Argentinean Precordillera. Thus, palynological studies seem to be an useful tool to constrain the geological history of controversial microplates, such as the Precordillera terrane.