Free-living bryozoans in the late Miocene Puerto Madryn Formation, Península Valdés: Paleoenvironmental and paleobiogeographical implications

CASADÍO, S.; HAKANSSON, E.; PARRAS, A.; GRIFFIN, M.

Puerto Madryn

Jornada; Reunión Anual de Comunicaciones de la Asociación Paleontológica Argentina; 2005

Asociación Paleontológica Argentina

Contrarily to most shallow marine bryozoans, which grow adhering to firm substrates, free-living bryozoans are associated with unconsolidated sandy substrates in warm water shelf environments, including very shallow water. The presence of the two free-living bryozoan species Cupuladria cf. biporosa and Discoporella n. sp. aff. depressa in the Puerto Madryn Formation (late Miocene) exposed at Puerto Pirámide (Chubut Province) in conjunction with interpretations based on the sediments and associated molluscan fauna suggest rapid deposition in a shallow marine setting, under tidal influence. The bryozoan bearing lens and beds, are located in the middle part of the section, between 45 and 50 m above the base of the section. They comprise units 0.4 to 2 m thick, with sharp or bioturbated bases. These units carry shell concentrations of molluscs, either transported or in life position, which are included in a sandy matrix. The beds are massive, and are intercalated with fine sandstone and mud-rock with heterolithic bedding. The fauna indicates normal marine conditions, and the grain size and sedimentary structures suggest that they accumulated in a lower shoreface to inner shelf environment often swept by tidal currents. Both, Cupuladria cf. biporosa and Discoporella n. sp. aff. depressa have unequivocal relations to the rich free-living fauna of the Panamic biogeographic province, which is in good keeping with the proposed paleogeographic origin of most of the associated molluscs, which probably originated in the tropical areas of the Americas before the definite establishment of the Isthmus of Panama