The deep-sea deposits of the Cuyo Group in the Río del Cobre depocenter, Mendoza, Argentina: characteristics and paleogeographic implications

MESCUA, J. F.; GIAMBIAGI, L. B.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

The Cuyo Group, of Early to Middle Jurassic age, records the first transgression in the Mesozoic Neuquén basin. This transgression was strongly diachronic, with Sinemurian to Bajocian ages, a fact which reflects the interaction of rising sea level and extensional systems in the early depocenters of the basin (Gulisano, 1981). We describe the characteristics of the Cuyo Group in a depocenter located in the Andes of southern Mendoza province, the Río del Cobre depocenter. The influence of paleotopography on the depositation of the Cuyo Group is known in the study area since the pioneering work of Gerth (1925). The Río del Cobre depocenter presents a succession of ~800 m of deep-sea deposits, and was limited to the east by a basement high, the Río Tordillo High, on top of which less than 100 m of shallow-sea Cuyo Group was deposited (Gulisano y Gutiérrez Pleimling, 1995). Further east the Valenciana depocenter has over 500 m of Cuyo deposits of deeper origin than those in the basement high (Dessanti, 1978). Segmentation of the basin by the Río Tordillo High seems to be a feature inherited from an extensional event previous to deposition, since synrift deposits do not crop out in the Río del Cobre depocenter. Vertical facies variations in the Río del Cobre depocenter were initially described by Gerth (1925); our observations support his general scheme. The lower section shows an alternance of limestones, black shales and volcaniclastic sandstones. The upper section consists of greenish sandstones and black shales with minor limestones and conglomerates. The beds are predominantly tabular, with thicknesses between a few cm and 2 m, and commonly present flute marks on the base. Occasionally lenticular beds of the same thickness range are observed. Slumps are widespread, and show a western provenance; redeposited and deformed limestone blocks are also abundant. Volcanic material becomes more abundant in the upper section; furthermore, some lavas are interbedded with the sedimentary deposits. Plant remains suggest proximity to an emerged land. Ammonites found in the shales and carbonates have allowed Gerth (1925) to assign the succession to the upper Lias and lower Dogger (Bajocian-Bathonian). These observations are consistent with the description of the Nacientes del Teno Fm., of equivalent age, located immediately to the west in Chile. This unit is composed of mostly volcaniclastic material coming from the west, and was interpreted by Davidson (1988) as deposited in proximal submarine fans.  We conclude that deposition of the Cuyo Group in the Río del Cobre depocenter took place in a turbiditic system in a narrow trough, with provenance from the western margin which provided mostly volcaniclastic and calciclastic material. Proximal facies are found to the west of the study area in the Nacientes del Teno Fm. The Río del Cobre outcrops represent distal facies, consisting of deep-sea carbonates, sandstones and shales which grade vertically to sandstones and shales with minor conglomerates, suggesting an evolution to more proximal environments.