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The Santa Cruz and Pinturas Formations (SCF and PF) are two partially coeval
formations in the southern part of Santa Cruz Province, Argentina, that were
deposited during the Early to Middle Miocene. The SCF underlies the coastal
plain between 47.0° and 51.6° S and extends from the Atlantic
Coast into the Andean foothills. The PF has a more restricted distribution
centered on eastern tributaries of the Rio Pinturas along the northern perimeter
of the SCF. Both formations have abundant tephra and tuffaceous sediments with
likely sources in volcanoes associated with emplacement of the late Cenozoic
South Patagonian batholith. This study re-evaluates the age of the SCF and the
relationship of the SCF to the PF, adding some radiometric dates to those
previously published and using the methods of tephrochronology. Tephra samples
were collected from 26 localities in the SCF and 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 these analyses, in conjunction with previous studies, indicate
that there are at least 38 individual tephra layers in the SCF, while there are
likely many more tephra than the six analyzed from the PF. Of the 38 tephra
layers 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. Integrating
results of the tephra correlations and radiometric ages indicates that the SCF
spans 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 between
the PF and lower part of the SCF. With this tephrochronology in place, studies
of space-time variations such as rates of sediment accumulation, composition of
mammalian faunas, facies changes, and other aspects of the SCF and PF can be
Santa Cruz Province, Argentina, has some of the richest fossil mammal localities
in the world. However, the absolute and relative ages of its fossil localities
have long been a source of confusion and debate. In particular, there has been
longstanding disagreement about the relative ages of the fossils from the
western part of the province in deposits of the Pinturas Formation compared with
those from the numerous localities of the Santa Cruz Formation along the
Atlantic coast. Drawing on recent studies of the tuffaceous sediments in many
classic fossil localities, and studies of fossil representatives of marsupials,
rodents, and primates, we provide a synthesis of the temporal relationship among
fossil localities throughout the province. There is broad agreement between the
results of the tephrochronology and mammalian paleontology. Both tephra
correlations and paleontological comparisons indicate that the lower units of
the Pinturas Formation are older than the sections of the Santa Cruz Formation
preserved at Monte León and Cerro Observatorio, supporting Ameghino's
suggestion that part of the Pinturas Formation represents a distinct faunal
zone. However, the upper unit of the Pinturas Formation seems to correspond in
age with the lower part of the sections at Monte León and Cerro
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