The Santa Cruz and Pinturas Formations (SCF and PF) are two partially coevalformations in the southern part of Santa Cruz Province, Argentina, that weredeposited during the Early to Middle Miocene. The SCF underlies the coastalplain between 47.0° and 51.6° S and extends from the AtlanticCoast into the Andean foothills. The PF has a more restricted distributioncentered on eastern tributaries of the Rio Pinturas along the northern perimeterof the SCF. Both formations have abundant tephra and tuffaceous sediments withlikely sources in volcanoes associated with emplacement of the late CenozoicSouth Patagonian batholith. This study re-evaluates the age of the SCF and therelationship of the SCF to the PF, adding some radiometric dates to thosepreviously published and using the methods of tephrochronology. Tephra sampleswere collected from 26 localities in the SCF and PF. Glass shards were analyzedby electron microscopy. Ten tephra samples were analyzed by the40Ar/39Ar method: nine from the SCF and one from thePF. Results of these analyses, in conjunction with previous studies, indicatethat there are at least 38 individual tephra layers in the SCF, while there arelikely many more tephra than the six analyzed from the PF. Of the 38 tephralayers in the SCF, 16 are shared by two or more sections, with one key tephra,the Toba Blanca, present in eight and possibly nine localities from 51.6°S northward to 47.0° S, over a distance of ~525 km. Integratingresults of the tephra correlations and radiometric ages indicates that the SCFspans the interval ~18 Ma to 16 Ma in the Atlantic coastal plain and~19 to 14 Ma in the Andean foothills, with a chronologic overlap betweenthe PF and lower part of the SCF. With this tephrochronology in place, studiesof space-time variations such as rates of sediment accumulation, composition ofmammalian faunas, facies changes, and other aspects of the SCF and PF can befruitfully pursued.

Santa Cruz Province, Argentina, has some of the richest fossil mammal localitiesin the world. However, the absolute and relative ages of its fossil localitieshave long been a source of confusion and debate. In particular, there has beenlongstanding disagreement about the relative ages of the fossils from thewestern part of the province in deposits of the Pinturas Formation compared withthose from the numerous localities of the Santa Cruz Formation along theAtlantic coast. Drawing on recent studies of the tuffaceous sediments in manyclassic fossil localities, and studies of fossil representatives of marsupials,rodents, and primates, we provide a synthesis of the temporal relationship amongfossil localities throughout the province. There is broad agreement between theresults of the tephrochronology and mammalian paleontology. Both tephracorrelations and paleontological comparisons indicate that the lower units ofthe Pinturas Formation are older than the sections of the Santa Cruz Formationpreserved at Monte León and Cerro Observatorio, supporting Ameghino'ssuggestion that part of the Pinturas Formation represents a distinct faunalzone. However, the upper unit of the Pinturas Formation seems to correspond inage with the lower part of the sections at Monte León and CerroObservatorio.