Preliminary data from multiple studies on the Santo Domingo Formation (La Rioja, Argentina): chronologic and paleogeographic implications

HAROLDO VIZÁN; SILVANA GEUNA; MARGARITA DO CAMPO; SILVIA LAGORIO; RICARDO NESTOR MELCHOR; SERGIO EDUARDO BELLOSI; JORGE FERNANDO GENISE; HÉCTOR OSTERA

Mendoza

Congreso; Gondwana 12; 2005

Centro de Investigaciones Geológicas

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 of the footprint-bearing horizons. 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 and is in fault contact, mostly by thrusts oriented NNE, with Carboniferous igneous and sedimentary rocks. The formation was considered of Late Triassic age, as suggested by the presence of the Middle-Late Triassic Gondwana wood morphogenus and a radiometric age of 212 "7 Ma carried out on a basaltic flow interbedded in the sedimentary sequence. Multiple studies in the Santo Domingo Formation coupled with detailed structural mapping and sedimentologic logging are being carried out to refine the age and determine the paleogeographic conditions during deposition of the Santo Domingo Formation. A systematic sampling that included 58 paleomagnetic sites and about 80 samples for mineralogical, petrographic, geochemical and new geocrhonological studies was developed in the middle and upper sections of the unit.   In this report, paleomagnetic analysis carried out on samples of 36 sites along with X-ray diffraction (XRD) analysis of 13 samples and petrographic observations on the basaltic flows, are presented. The siltstones and fine-grained sandstones analyzed by XRD are mainly composed by quartz and plagioclase, with subordinate zeolites, clay minerals, and in some cases minor calcite. The presence of zeolites in these samples, coming from the three facies associations identified by sedimentological studies, could be linked with the abundance of volcaniclastic detritus but also with neutral to alkaline waters during deposition or in early diagenetic stages. The interbedded basaltic flows display porphiritic texture composed of phenocrystals of olivine (strongly altered to iddingsite) set in an intergranular groundmass of labradoritic plagioclase (An63), clinopyroxene, olivine and titanomagnetite. Common calcite-bearing amygdales and oxidation of the mafic phases are related to the late subaerial deuteric process. Blocking temperatures of the paleomagnetic samples from the analyzed sedimentary rocks, suggest that the magnetic remanence is partially 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 siltstone and fine-grained sandstones with hematitic cement, interpreted as deposited in a semi-arid setting. However, the mentioned analyses suggest that some of the remnant magnetism comes essentially from detrital grains of ferrimagnetic minerals. Characteristic remnant magnetizations (ChRM) were obtained applying the principal component method to demagnetization steps above 200° C or 5 mT. These ChRM 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 ChRM directions 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 after being transferred to Africa coordinates, overlaps with the section of the path dated between 190 and 180 Ma. The agreement of the obtained preliminary PP with the interval of the path corresponding to 190-180 Ma suggests that the rotation of the area after a vertical axis proposed by Geuna and Escosteguy (2004) must have occurred prior to 190 Ma.