NEOGENE TEPHROSTRATIGRAPHY OF NORTHWESTERN ARGENTINA

Tephrostratigraphic analysis of Miocene/Pliocene stratigraphic sections from northwestern Argentina reveals significant potential for chronostratigraphic improvements in the region. Stratigraphic sequences of volcanic ash beds from three thick continental sections (Santa Maria Valley, Alto de San Nicolás, and Puerta de Corral Quemado) in the Sierras Pampeanas morphotectonic province have been sampled, and volcanic glass has been analyzed by electron probe microanalysis (EPMA). EPMA results serve to chemically “fingerprint” individual volcanic ashes based on concentrations of major and minor elements. EPMA has proven sufficient to distinguish all stratigraphically unique ash beds from one another. Comparison of sequences of chemically unique ash beds allows tephrostratigraphic correlation between sections, thereby establishing robust and geologically instantaneous chronostratigraphic markers. Presently, several possible correlations exist between the localities studied. These correlations are crucial to integrating isotopic records from paleosol carbonate and fossil teeth. Further work, including trace element analysis of glass separates and radioisotopic age estimates will refine these correlations and move towards a regional isotopic record from 12 Ma to present.

Incorporation of magnetostratigraphic and numerical age estimates from other localities will further strengthen the tephrostratigraphic database. Analysis of ash beds from the Argentine Puna shows promise for expanding the geographic scope of the project. Preservation of vitric material from the middle Miocene at the base of extremely thick sections (3,000 + m) is not common, but observed. Tephrostratigraphic information will be particularly valuable in coarse grained sections where magnetic polarity stratigraphy is less useful. It will also be important for time intervals where numerous geomagnetic reversals can provide sub-100 ka resolution once the record is tied to a sequence of isotopic dates.