CONICET | Buscador de Institutos y Recursos Humanos

The Santa Cruz and Pinturas Formations (SCF and PF) aretwo partially coeval formations in the southern part of SantaCruz Province, Argentina, that were deposited during theEarly toMiddleMiocene. The SCF underlies the coastal plainbetween 47.0 and 51.6 S and extends from the AtlanticCoast into the Andean foothills. The PF has a more restricteddistribution centered on eastern tributaries of the PinturasRiver along the northern perimeter of the SCF. Both formationshave abundant tephra and tuffaceous sediments withlikely sources in volcanoes associated with emplacement ofthe late Cenozoic South Patagonian batholith. This study reevaluatesthe age of the SCF and the relationship of the SCF tothe PF, adding some radiometric dates to those previouslypublished and using the methods of tephrochronology.Tephra samples were collected from 26 localities in the SCFand PF. Glass shards were analyzed by electron microscopy.Ten tephra samples were analyzed by the 40Ar/39Ar method:nine from the SCF and one from the PF. Results of theseanalyses, in conjunction with previous studies, indicate thatthere are at least 38 individual tephra layers in the SCF, whilethere are likely many more tephra than the six analyzed fromthe PF. Of the 38 tephra layers in the SCF, 16 are shared bytwo or more sections, with one key tephra, the Toba Blanca,present in eight and possibly nine localities from 51.6S northward to 47.0 S, over a distance of~525 km. Integratingresults of the tephra correlations and radiometric agesindicates that the SCF spans the interval ~18 Ma to 16 Main the Atlantic coastal plain and ~19 to 14 Ma in the Andeanfoothills, with a chronologic overlap between the PF andlower part of the SCF. With this tephrochronology in place,studies of space-time variations such as rates of sedimentation,composition of mammalian faunas, facies changes, andother aspects of the SCF and PF can be fruitfully pursued.

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