Depositional model of a tide-dominated offshore dunefield in the southern Neuquen Basin, Argentina (Pilmatue Member, Agrio Formation): sequence-stratigraphic and paleogeographic significance

Abstract

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 sets show 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. The study succession is interpreted to represent the development of a tidally dominated dunefield developed onto a previous mud-rich offshore setting. The abundance of sandy deposits associated with strong unidirectional currents, the profusion of reactivation surfaces within the large-scale cross-sets, and the burrow distribution, suggest the dominance of simple dunes and relatively mobile substrates in the center of the field, although more complex/compound dunes were likely also generated. Opposite migrating ripples could be evidence of flow reversion, but more likely, the construction of dunes large enough to develop flow separation over the lee surfaces. Cross-laminated sandstones, heterolithics, and mudstones of the lower section probably correspond to marginal and distal sectors of the field. The rippled sand patches were characterized by weaker unidirectional currents and more stable substrates. Development of firm grounds in these distal sectors could be attributed to short-term decrease in sand supply. The overall coarsening- and thickening-upward succession (lower and upper sections combined) could reflect the gradual downcurrent migration of the dunefield until it was eventually abandoned and covered by the thick succession of offshore fines. Bioclastic sand could have derived both from in situ (e.g.,well-preserved crinoids) and allochthonous material. In turn, siliciclastic sand more certainly was remobilized from nearshore settings located toward the east of the studied area, though not in great amount, because its supply was not enough to completely override the carbonate production. In any case, SW-oriented tidal currents would have been responsible to capture and transport sediment off the shore and into the offshore setting.The development of unidirectional (or strongly asymmetrical), high-energy tidal currents in the Valanginian epicontinental sea of the Neuquén Basin suggests unusual environmental conditions. As neither syn-tectonic activity nor narrow straits are supported by available data, the reason behind the existence of this offshore dunefield could be linked to an exceptional development of tidal currents within the relatively shallow interior sea, only partially connected to the open ocean. Considering that the base of the Pilmatué Member represents a low-order, high-amplitude transgression in the basin (which places marine sediments onto continental deposits of the underlying Mulichinco Formation in large regions of the basin, Schwarz et al., 2006), it could be speculated that an increasing connection with the proto-Pacific during a relative sea-level rise could have allowed for the intensification of tides. This amplification would have reached a maximum not at the begging of the transgression but towards its end. At that moment, sediment availability and strong tidal currents could have combined and triggered the formation of the dunefield, which was able to climb and migrate basinwards with time, until tides were no longer efficient. In this scenario, the tidally dominated, coarsening-upward package of the Pilmatué Member would be the depositional expression of maximum transgression.The resulting stratigraphic architecture and depositional system of this example are in marked contrast with those typically associated with offshore tidal systems developed during early stages of transgression. In the latter case, sharp-based sandstone-dominated tidal ridges with fining-upward trend seem to be more common. Thus, this case study could contribute to widen the spectrum of depositional and stratigraphic conditions where offshore tidal systems can be developed.