Preliminary paleomagnetic data from the Santo Domingo Formation (La Rioja, NW Argentina): chronologic and tectonic implications

VIZÁN, H.; GEUNA, S.E.; MELCHOR, R.N.; BELLOSI, E.; GENISE, J.

LA PLATA

Congreso; XVI CONGRESO GEOLÓGICO ARGENTINO; 2005

The finding of bird-like fossil footprints in the upper section of the Late Triassic Santo Domingo Formation from northwest Argentina (Melchor et al., 2002) posed a number of questions regarding the affinity of the possible producers and the exact age dating of the footprint-bearing horizons. This finding predates the oldest record of birds by, at least, 50 My and might be important for understanding the origin and affinities of this group of vertebrates. The locality of finding is located within the Precordillera of La Rioja province (28º 32? S, 68º 45? W, Fig.1). The formation reaches a minimum thickness of circa 1950 m (Fig. 2) and is in fault contact, mostly by thrusts oriented NNE, with Carboniferous igneous and sedimentary rocks (Caminos and Fauqué, 2001). The formation is considered of Late Triassic age, as suggested by the presence of the Middle-Late Triassic Gondwana wood morphogenus Rhexoxylon (Caminos et al., 1995) and an 40Ar/39Ar step-heating analysis on albite separate of interbedded basalt flows, which yielded an age of 212.5 ± 7.0 Ma (Coughlin, 2001). Fossil wood was found in the lower part of the unit and the dated basalt flows are located about 700 m below the horizons with bird-like fossil footprints (Fig. 2). Paleomagnetic studies in the Santo Domingo Formation coupled with detailed structural mapping and sedimentologic logging were aimed to refine the age of the formation. The relatively large analytical error of the available radiometric age for the basalt flows (Coughlin, 2001) precludes the application of the magnetostratigraphic technique at this stage of the study. However, the position of the paleomagnetic pole of the formation in a properly dated apparent polar wander (APW) path may help to constrain the age of the unit.