Cerro Bandera Formation

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]

The known fossiliferous deposits in the Cerro Bandera Formation consist of isolated concentrations of fossil remains partially exposed on the surface due to the effect of rainwater and wind action. In general, fossil remains tend to be found in greater abundance in those sectors where the sequence is less altered. Possibly, the same factors that contributed to the alteration and devitrification of the pyroclastites also influenced the degradation of the bone materials. In this sense, the dental pieces seem to be the most resistant, having been found in some cases parts of dental series in correct position within the sediment, although any bone remains were completely missing.

At the type locality of the Cerro Bandera Formation, the succession consists of dominantly chonitic (consolidated pyroclastic deposits of very fine grain size, less than 0.062 mm,) and tuffitic deposits. In this outcrop were found two fossiliferous horizons within were found within deposits corresponding to strongly pedoturbed tuffaceous chonites (paleosols). The lower fossiliferous horizon corresponds to the basal deposits of this unit, where remains of small vertebrates were found together with freshwater gastropods.[3]

On the northeastern slope of Barda Negra, the Cerro Bandera Formation exhibits a granocreccive sequence, with predominantly bentonitic (lacustrine) deposits at the base, gradually grading towards the top to deposits with higher epiclastic content (tuffites with sandy lenses), where fluvial sandstones with conglomeratic lenses bearing silicified logs dominate. In this locality, the basal bentonite deposits are commercially exploited in several mining properties, from where some dental and bone remains have been extracted, together with inadequately preserved fossil logs.

In the locality of Sierra del Portezuelo Norte, the unit is formed by an alternation of tuffites, bentonite chonites and tuffaceous chonites. At least four fossiliferous horizons have been recognized at the same locality, of which only the third horizon, located approximately 21 m from the base of the profile, has contributed fossiliferous material significant enough for this analysis. This horizon is located in pedoturbed deposits of tuffaceous chonites, which contributed numerous mammal remains along with scarce, poorly preserved beetle nests.

Finally, a fourth fossiliferous locality has been found on the northwestern slope of Cerro Bayo Mesa, a site where the Cerro Bandera Formation had not been previously recognized. This unit occurs there as a dominantly chonitic and tuffitic succession of thin thickness, with a single fossiliferous horizon, located ca. 13 m from the base of the profile, in levels of pedogenized tuffaceous chonites.[1]