Early Paleogene brackish-water molluscs from the Caballas Formation of the East Pisco Basin (Southern Peru)

ABSTRACT

Shell beds from the Caballas Formation in the East Pisco Basin of southern Peru have yielded an assemblage of molluscs numerically dominated by oysters and cerithioid and eulimid gastropods, with lesser numbers of neritid, ampullinid, and pyramidelloid gastropods and noetiid, mytilid, anomiid, and cardiid bivalves. The shell beds mark a transition from continental to marine-influenced environments. Palaeontological evidence, including plant twig moulds on the attachment scars of oysters and a high diversity of potamidid gastropods, is consistent with an interpretation of a brackish-water setting. The majority of molluscan species from the Caballas Formation show a close affinity with Late Cretaceous species from northern Peru, but no unequivocally Cretaceous invertebrate taxa (e.g. ammonites, rudists) have been found. A morphologically distinctive bivalve species that occurs in the Caballas Formation, Carolia (Parinomya) parinensis, only occurs elsewhere in the upper lower Eocene Pariñas and Chacra formations of the Talara Basin of northern Peru. Moreover, several other Caballas Formation molluscan taxa closely resemble early Eocene species from the Talara Basin. These equivocal biostratigraphic data indicate an early Paleogene age as most likely for the Caballas Formation.

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