INVESTIGADORES
MELCHOR Ricardo Nestor
congresos y reuniones científicas
Título:
Preliminary paleomagnetic data from the Santo Domingo Formation (La Rioja, NW Argentina): Chronologic and tectonic implications
Autor/es:
HAROLDO VIZÁN; SILVANA GEUNA; RICARDO NESTOR MELCHOR; SERGIO EDUARDO BELLOSI; JORGE FERNANDO GENISE
Lugar:
La Plata
Reunión:
Congreso; XVI Congreso Geológico Argentino; 2005
Institución organizadora:
Universidad Nacional de La Plata
Resumen:
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). The formation reaches a minimum thickness of c. 1950 m (Fig. 1) 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 a plateau 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 below the horizons with bird-like fossil footprints (Fig. 1). Fig. 1. Generalized lithologic log and paleonvironmental interpretation of the Santo Domingo Formation. Also indicated is the interval sampled for paleomagnetic studiesThe paleomagnetic studies in the Santo Domingo Formation coupled with detailed structural mapping and sedimentologic logging were aimed to refine the age of the formation using magnetostratigraphic techniques. The relatively large analytical error of the available radiometric dating of the basalt flows (Coughlin, 2001) precludes the application of this 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. This contribution presents the preliminary paleomagnetic data for the Santo Domingo Formation. The preliminary paleomagnetic pole (PP) obtained allows to approximate the age of deposition of the unit as well as to infer a possible block rotation after vertical poles of the region. The systematic paleomagnetic sampling included 58 sites (3 to 5 samples per site) distributed in the middle and upper sections of the unit, roughly from the dated lava flow to the top pf the fluvial section of the unit (Fig. 1). The data for this report come from 36 samples from an equal number of paleomagnetic sites. Blocking temperatures of the samples from the analyzed sedimentary rocks suggest that magnetic remanence is carried by ferrimagnetic minerals (titanomagnetites?). This inference is in agreement with the behavior of these samples during demagnetization using alternate magnetic fields and with the hysteresis cycle obtained from a single sample. The analyzed samples include red siltstones and fine-grained sandstones with hematitic cement, interpreted as deposited in a semi-arid setting. However, the mentioned analyses suggest that the remnant magnetism comes essentially from detrital minerals of ferromagnetic minerals. Characteristic remnant magnetizations were obtained applying the principal component method to demagnetization steps above 200° C or 5 mT; values of minimal angular deviation were always lower than 15°. The fold/tilt test applied to the CRM is equivocal. However, there is a slight increase in the concentration parameter (k of Fisher) of the direction population when they are corrected using the structure of the sampled beds, which is coherent with the suspicion that the CRM predate the tectonic deformation of the analyzed succession. In addition, the CRM recorded both polarities and pass the reversal test. In consequence, it is interpreted that the recorded magnetization is primary, possibly acquired shortly after deposition. The average of the CRM directions (36 samples) yielded a preliminary paleomagnetic pole (PP) with the following coordinates and confidence intervals: Lat.= 73° S, Long.= 206° E, dN= 7.5°, d(= 5°. This preliminary PP, when compared with a recently obtained APW path (Torsvik et al., 2001) using coordinates from Africa and different reconstruction parameters (Lawver and Scotese, 1988; Lottes and Rowley, 1990; Nürnberg and Müller, 1991), overlaps with the section of the path dated between 190 and 185 Ma (Fig. 1). This correspondence would indicate that the record of the CRM took place circa 190 Ma (Early Jurassic). This date is considered slightly younger that the age of sedimentation of the Santo Domingo Formation. The discrepancy between this age estimated by paleomagnetic data and the radimetric date from the basalt flows (212 Ma) could be partially explained by the time involved in more than 150 m of stacked calcrete paleosols located above the basalt flows (Fig. 1). The agreement of the obtained preliminary PP with the interval of the path corresponding to 190-185 Ma suggest that the rotation of the area after a vertical axis proposed by Geuna and Escosteguy (2004) on the basis of a paleomagnetic study of Late Paleozoic units, must have occurred prior to 190 Ma.