Miocene marine transgressions: Paleoenvironments and paleobiodiversity

Abstract

Two major marine transgressions covered part of Patagonia during the Miocene and both are recorded in the Península Valdés region. The older (early Miocene) is represented by the volumetrically scarce outcrops of the Gaiman Formation, composed by shelf mudstones and fine sandstones. The late Miocene transgression is represented by the Puerto Madryn Formation, widely distributed in Península Valdés and composed of mudstones, sandstones and shell beds, being the focus of this work. Sediments of this unit were deposited in inner shelf, nearshore, tidal channel and tidal flat environments. Fossil content is abundant and diverse, including palynomorphs, foraminifers, marine invertebrates (dominated by molluscs), cetaceans, pinnipeds, marine fishes and birds, as well as continental mammals, birds, and fishes. Isotopic and biostratigraphical data suggest a late Miocene age for the Puerto Madryn Formation, although some middle Miocene biostratigraphical indicators are present. Paleoenvironmental information suggests oceanic and continental temperatures warmer than present day, evidenced by the Caribbean molluscan association and the continental vertebrate and palinological associations, respectively. Instead, cetaceans, dinoflagellates, and some marine fishes, suggest colder oceanic temperatures. Precipitations were also higher than present, evidenced by the presence of freshwater mammals, birds, fishes, and plants. This work highlighted some gaps in the geological and paleontological knowledge including geochronology, stratigraphic control of paleontological studies and the knowledge of poorly known fossil groups, which should be the focus of future investigations.