Early Miocene paleoenviromental changes along the Atlantic seaboard: Evidence from the boundary between the Monte León and Santa Cruz formations

PARRAS, ANA; GRIFFIN, MIGUEL; CASADIO, SILVIO; JULIÁN, MARIA JOSÉ; MONTALVO, CLAUDIA

Puerto Madryn

Otro; Reunión Anual de la Asociación Paleontológica Argentina; 2005

CEMPAT-APA

Sedimentological and paleontological studies of the boundary between the marine Monte León and the continental Santa Cruz formations at Cerro Monte León (50º19’16.8’’S; 68º53’12.3’’W), south-eastern Santa Cruz Province, reveal paleoenvironmental changes occurring there during the early Miocene. This boundary has been deemed to be conformable or unconformable by different authors and, moreover, others suggested that the two units are separated by an erosional unconformity; a general agreement on it has not been definitely reached. However, the mollusc fauna and the sedimentologic evidence contained in the uppermost meters of the Monte León Formation suggest that, at this locality at least, the transition from one unit to the other is gradual. The lower section of the Monte León Formation comprises siltstones and fine bioturbated sandstones, with abundant pyroclastic material, and with biogenic and sedimentologic concentrations dominated by a very diverse molluscan fauna (over 130 species) deposited in a subtidal environment. Further up in the section predominate siltstones and fine sandstones with heterolithic lamination deposited in an intertidal environment. The uppermost part of the section comprises coarse to fine sandstones and siltstones with mainly sedimentologic concentrations dominated by an impoverished molluscan fauna [“Nuculana” sp., Eosolen crucis (Ihering, 1907), Mactra? sp., Corbula cf. pulchella (Ihering, 1907), and Crassostrea orbignyi (Ihering, 1897)]. At the top we recorded several reefs of Crassostrea orbignyi, an oyster closely related to Recent Crassostrea living in marginally marine environments. Together with the oysters there were mammal remains, such as a fairly complete, although disarticulated Propalaeoplophorus sp. (a glyptodont), and fragments of Protypotherium sp. (a notoungulate). Thus, the uppermost rocks and faunas of the Monte León Formation (“Suprapatagonian”) record a relative sea-level drop, which gradually gives way to the thick continental section with progradational facies of the overlying Santa Cruz Formation.