Assesing the age of Los Colorados Formation, Ischigualasto–Villa Union Basin, Argentina. Temporal implications for Coloradian fauna

SANTI MALNIS, PAULA; COLOMBI, CARINA ESTER; KENT, DENNIS V.,; ALCOBER, OSCAR; MARTÍNEZ, RICARDO

San Juan, Argentina

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Instituto y Museo de Ciencias Naturales de Universidad Nacional de San Juan

The Ischigualasto – Villa Union basin was developed over the Triassic in the Southwestern quarter of Pangea. This basin hosts some globally recognized paleontological sites for the Late Triassic, highlighting not only the huge number of specimens collected, but also the diversity of taxa present in the different associations. The youngest faunal association of the basin, located in the upper section of Los Colorados Formation is composed of pseudosuquids (“rauisichids, sfenosuquids and protosuchids”), avemetatarsials (sauropodomorphal dinosaurs and teropods), icthyosaurids cynodonts and quelonids (Bonaparte 1971; 1978; Rogers et al, 1995). The unusual composition of this association made it earn the name “Coloradense” in the way of distinguishes it from others (Bonaparte, 1973). The age of Los Colorados Formation has been assigned, based in this faunal association, to the Late Triassic, possibly reaching the Jurassic (Bonaparte, 1973; Martínez, 2002; Arcucci et al., 2004). However, so far no one has produced dating with methods with high temporal resolution (e.g., radiometric dating). As a consequence, this faunal association has not been included in faunal global compilations (e.g., Simms y Ruffel, 1989; Tanner et al, 2004). We present preliminary results of a magnetostratigraphic study developed in the southern outcrops of the Los Colorados Formation. For this, 60 paleomagnetic sample sites over 600 mts were survey. A consistent and statistically reliable component was measured that preserves the primary remanent magnetization, oriented SSW and down for reverse polarity and NNE and up for normal polarity in different sites. The magnetostratigraphic column obtained was correlated with the magnetostratigraphic pattern section of the Late Triassic GPTS from the Newark basin, U.S., (Kent et al 1995; Kent & Olsen, 1999). According to the preferred polarity correlation, the Los Colorados section can be correlated with the temporal section between the magnetozones E7r and E14n from the GPTS. This means that Los Colorados Formation started its deposition nearly 227 Ma and ended around 215 Ma. Through the magnetostratigraphic correlation, the stratigraphic section which is equivalent to the fossiliferous beds of Coloradian is dated between 217 – 215 Ma. This age disagrees with the classical estimate for this paleofauna and positions it in “Medial” Norian according to GTS 2009. This result suggests that the biotic turnover cited in South America (e.g. Benton, 1986) took place earlier than predicted.