ORDOVICIAN COSMIC SPHERULES FROM THE CORDILLERA ORIENTAL OF NW ARGENTINA: PRELIMINARY SEM AND EDX INVESTIGATION

VOLDMAN, G.G.; ALBANESI, G.L.; BARNES, C.R.; ORTEGA, G.; GENGE, M.J.

Ordovician of the World, Cuadernos del Museo Geominero

Instituto Geológico y Minero de España

Lugar: Madrid; Año: 2011; p. 611 - 616

INTRODUCTION Fluctuations in the influx of extraterrestrial materials to Earth play an important role in the weak equilibrium between the oceans, atmosphere, climate, and life (e.g., Álvarez et al., 1980). Extraterrestrial flux is assumed to have been more or less constant with a few peaks in the accretion rates, such as the K/T boundary. The discovery of numerous fossil meteorites in marine limestones from southern Sweden reflects an extraordinary increase in the flux of extraterrestrial matter to Earth during the Mid-Ordovician (Schmitz et al., 2001). A Mid-Ordovician increase in the meteorite flux is further supported by an iridium anomaly, osmium isotope data and by the distribution of sediment-dispersed extraterrestrial (ordinary chondritic) chromite grains from Sweden and central China (Cronholm and Schmitz, 2010). Accordingly, Dredge et al. (2010) determined a flux of micrometeorites one to two orders of magnitude greater than at present in Arenig limestone samples from the Durness Group, in Scotland. Micrometeorites are extraterrestrial particles between 10 ƒÊm and 1 mm in size recovered from the EarthLs surface (Rubin and Grossman, 2010). Melted micrometeorites formed as molten droplets during atmospheric entry are known as cosmic spherules (Genge et al., 2008). The discovery of magnetic spherules in acid-insoluble residues from conodont samples encouraged a systematic search for Ordovician micrometeorites from Northwestern Argentina. An important depocenter containing Ordovician fossiliferous rocks from the Central Andes of Argentina is presently analyzed (Fig. 1). In the Central Andean Basin, Ordovician strata are superbly exposed at Cordillera Oriental, a thick-skinned mostly east-vergent thrust system, limited to the west by the Puna plateau and to the East by the Sierras Subandinas (Ramos, 1999). The stratigraphy of the Cordillera Oriental reflects relatively shallow environments ranging from outer shelf to shoreface rarely dominated by tidal complexes, in contrast to the deep water setting of the Puna (Astini, 2003). In particular, at the Zenta Range the Lower Ordovician strata are over 3000 m in thickness (Santa Victoria Group) (Astini, 2008). In this region, the Santa Rosita Formation is represented by a thick succession consisting of monotonous alternating series of shales and sandstones with subordinated calcareous concretions, coquinas and calcarenites. The productive samples Z4 and Z8 were obtained from two localities along the road from the Zenta path to the Santa Ana path, in both flanks of a local anticlinal structure (Fig. 1). Sample Z4 was taken from a mudstone level located in the eastern flank of the anticline at Santa Ana path, 15 m below sample Z5, a calcareous coquina that yielded conodonts of the Acodus deltatus – Paroistodus proteus Zone, and close to a shaly interval characterized by the graptolite Araneograptus murrayi (J. Hall). Sample Z8 was from a calcareous coquina located in the western flank of the anticline at ca. 4000 m altitude, which is intercalated with brown shales bearing graptolites of the Hunnegraptus copiosus Zone; i.e., late Tremadocian (Albanesi et al., in press).