DEPOSITIONAL MODEL OF A TIDE-DOMINATED OFFSHORE DUNEFIELD IN THE SOUTHERN NEUQUEN BASIN, ARGENTINA (PILMATUE MEMBER, AGRIO FORMATION): SEQUENCE-STRATIGRAPHIC AND PALEOGEOGRAPHIC SIGNIFICANCE

SCHWARZ, E.; VEIGA, G.D.

Congreso; 9th International Conference on Tidal Sedimentology; 2015

During most of the Late Jurassic and Early Cretaceous, the Neuquén Basin was occupied by an epicontinental sea connected to the proto-Pacific Ocean, with shoreface-offshore systems mostly built by wave and storm processes.The majority of the marine succession included in the Pilmatué Member of the Agrio Formation (late Valanginian - early Hauterivian) conforms to this generalization (e.g., Schwarz and Veiga, 2015). In this context, the development of a relatively thick succession of current-dominated deposits intercalated within offshore mudstones is unusual and could reflect a unique combination of palaeogeographic and sequence-stratigraphic conditions, associated to the degree of connection with the proto-Pacific Ocean. This case study also provides the opportunity to describe the facies and stratigraphic record of tide-dominated offshore systems, which have not been widely reported.In the Cerro Mesa area (central Neuquén province, Argentina), the Pilmatué Member reaches up to 500 m and is largely composed of offshore mudstones and mud-rich shell concentrations. The studied interval is located near the base of the unit and comprises a 30 m-thick succession unusually enriched in coarse-grained deposits, with variable contribution of siliciclastic (sand size) and bioclastic (sand and fine gravel size, mostly from crinoids) material. This succession is sharply intercalated between offshore mudstones and marls and is approximately dated as late Valanginian due to the presence of trigoniid bivalves belonging to the Steinmanella pehuenmapuensis Zone, which partially overlaps the Pseudofavrella angulatiformis ammonoid Zone (Lazo et al., 2014).The study succession is divided in two stratigraphic sections according to the relative proportion of sandstones and mudstones. The lower section (15 m thick) is dominated by wavy and lenticular heterolithic deposits and massive mudstones with a Cruziana-related trace fossil assemblage (Teichichnus, Planolites, Chondrites, and Gyrochorte), in which at least four discrete beds (up to 1.5 m thick) of fine-grained bioclastic sandstones occur. These sandy beds are mainly massive due to intense bioturbation, but they exhibit ripple cross-lamination and, locally, decimeter-scale cross-stratification. Bioturbation is mainly represented by Palaeophycus, Teichichnus and Ophiomorpha in these beds, and the lower bedding planes show passively filled burrows (Thalassinoides, Planolites), interpreted to represent a Glossifungites suite.In contrast, the upper section(15-20 m thick) is dominated by coarse-grained bioclastic sandstones showing a profusion of cross-bedded structures that range between mid-scale, trough cross-bedding to large-scale sets with tangential foresets and abundant reactivation surfaces. Complex internal architectures with superimposed surfaces were also observed. Paleocurrent data shows unimodal distribution toward the southwest. In some cases however, small-scale cross-lamination preferentially located near the toe of large-scale setsshow an opposite direction.Cross-bedded units coarsen and thicken upwards (reaching up to 4 m at the top of the section) and they show great lateral continuity (for up to 1 km). Their lower boundaries may show evidence of erosion. Bioturbation is infrequent, though large Ophiomorpha burrows have been recorded in every unit.