Ordovician cosmic spherules from the cordillera oriental of nw argentina: Preliminary sem & edx investigation

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

Simposio; 11 International Symposium on the Ordovician System; 2011

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 Areniglimestone samples from the Durness Group, in Scotland. Micrometeorites are extraterrestrial particles between 10 μm and 1 mm in size recovered from the Earth´s 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 acidinsoluble 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).