The Cerro Bandera Formation is a geological formation in Neuquén Province, Argentina, in northern Patagonia, which dates to the Early Miocene, around 21 to 17.5 million years ago. It predominantly consists of pyroclastic deposits, which were deposited in a semi-arid environment. It is divided up into a number of members. The diverse fauna of the Cerro Bandera Formation include a variety of turtles and birds, also includes many mammals such as South American native ungulates (notoungulates, litopterns, astrapotheres) as well as armadillos, and caviomorph rodents.[1]
The Cerro Bandera Formation comprises a series of isolated outcrops that represent the relicts of an old alluvial filling developed on small local valleys. It is composed of a succession of reworked pyroclastic deposits alternating with primary pyroclastic and scant sandstone levels. These deposits were originally recognized at the northeast of Barda Negra, south of Cerro Bandera and northwest of Sierra del Portezuelo; new exposures are herein recognised to the northwestern slope of Cerro Bayo Mesa, Neuquén Province, Argentina. The recovered fossil remains correspond to birds of the family Falconidae and 17 families of mammals, among which are remarkable Cramauchenia, Proadinotherium, Eosteiromys, and Caviocricetus, plus a species of Protypotherium with more primitive dentition than those known from the Santacrucian SALMA. This association confirms a Colhuehuapian SALMA (Early Miocene) for this unit. This fauna is markedly different from that recorded from the lower section of the Chichinales Formation, Río Negro Province. The degree of faunistic differentiation between these probably synchronous units could be a result of local palaeoenvironmental differences.[1]
The fossiliferous localities of the Cerro Bandera Formation are latitudinally equivalent to that of the Chichinales Formation. However, there are marked differences between their known faunal compositions. The only genera shared by both units are Cladosictis, Cramauchenia, Proeutatus, Protypotherium, and Stenotatus. To this must be added the recent discovery of remains of Eoviscaccia in the Chichinales Formation. In this unit the remains of turtles, hegetotheriid and leontiniid notoungulates are extremely abundant; while that in the Cerro Bandera Formation there are still no known remains of turtles, the interatheriids predominate over the hegetotheriids and the large herbivorous ungulates are mostly represented by the astrapotheres. In addition, the presence of at least two new genera only known from the Cerro Bandera Formation belonging to the families Astrapotheriidae and Cephalomyidae, infers a certain degree of faunal differentiation with respect not only to the Chichinales Formation, but also to the typical Colhuehuapian localities of the central Patagonia.[1][2]
The Cerro Bandera Formation occurs as a succession of small isolated whitish to yellowish-brown outcrops, aligned along ancient depressions and paleo-valleys carved in Late Cretaceous and Paleocene sediments. In the study area, the deposits of this unit overlie in erosive discordance several units of the Neuquén Group, including the Huincul, Cerro Lisandro and Portezuelo Formations, from the Late Cretaceous, as well as over Danian marine sediments of the Roca Formation in the Malargüe Group.[1]