Facies and stratigraphic record of a Lower Cretaceous, tide-dominated offshore dunefield (Pilmatué Member - Agrio Formation): Sequential and palaeogeographic implications for the southern Neuquén Basin, Argentina

Abstract

The Neuquén Basin conformed, during most of the Upper Jurassic and Lower Cretaceous, an epicontinental sea in which shoreface and offshore environments were dominated by wave and storm processes. This is especially important for the Pilmatué Member as it has been described in many localities in the basin. In this context, the development of a relatively thick succession of current-dominated deposits intercalated in offshore mudstones is highly unusual and its study may reveal information regarding punctuated poalaeogeographic reconfigurations in the basin associated with periods of tectonic activity. It also provides the opportunity to describe the scarcely registered facies and stratigraphic record of tide-dominated offshore systems in the rock record. In the Cerro Mesa area (central Neuquén province, Argentina), the studied interval comprises a 30 m-thick succession of mixed (siliciclastic-carbonate) relatively coarse-grained deposits. This succession is sharply intercalated between offshore mudstones and marls. The lower section is dominated by wavy and lenticular heterolithic deposits (with Gyrochorte, Teichichnus and Chondrites) and massive mudstones, in which at least four discrete units of fine-grained bioclastic sands (up to 1.5 m thick) intercalate. The lower boundary of these coarser units is usually associated with passively filled burrows comprising a Glossifungites suite. The sandy units are mainly massive, but they may show ripple cross-lamination and locally larger cross beds. Bioturbation is present mainly associated with Ophiomorpha and in less proportion Teichichnus and Palaeophycus.In contrast, the upper portion of the succession is dominated by coarse-grained boiclastic 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. In some cases, small-scale cross-lamination is superimposed to the large-scale sets, showing an opposite migration direction. Cross-bedded units coarsen and thicken upwards, reaching up to 4 m in the upper portion, and show great continuity (for up to 1 km) although the lower boundaries may show evidence of large-scale erosion. Bioturbation is infrequent, though large Ophiomorpha has been recorded in every unit of this upper section.The abundance of deposits associated with strong unidirectional currents, evidence of reversion of these currents and the profusion of reactivation surfaces within the large scale cross-sets, suggest the development of tidally induced currents that led to the development of subtidal sand dunes in an offshore setting. The proportion of carbonate components (bioclasts and ooids) indicates a relatively close carbonate factory and the lack of dilution of the carbonates due to siliciclastic input. The overall coarsening and thickening upwards succession may reflect the gradual downcurrent migration of the dunefield until it is abandoned and covered by a thick succession of offshore fines.The development of strong tidal current in this offshore setting during the Early Cretaceous suggests unusual conditions for the Neuquén Basin. These deposits are accumulated during a low-order transgressive interval developed after a major relative sea-level fall, which has been associated with local tectonic activity and to reactivation and inversion of older structures. This tectonic activity could have modified the palaeogeography, at least locally, to promote the amplification of tidal currents, effect that could have faded away when the transgression progressed.