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

VEIGA, G.D.; SCHWARZ, E.

Congreso; 2015 IAS Meeting of Sedimentology; 2015

International Association of Sedimentologists

The Neuquén Basin conformed, during most of the Upper Jurassic and Lower Cretaceous, anepicontinental sea in which shoreface and offshore environments were dominated by wave andstorm processes The Valanginian-Hauterivian Pilmatué Member conforms to this generalizationwith wave/storm-influenced deposits across the basin. In this context, the development of arelatively thick succession of current-dominated deposits intercalated in offshore mudstones isunusual and its study may reveal information regarding punctuated palaeogeographicreconfigurations associated with periods of tectonic activity. It also provides the opportunity todescribe the scarcely registered facies and stratigraphic record of tide-dominated offshoresystems in the rock record.In the Cerro Mesa area (central Neuquén province, Argentina), the studied interval comprises a30 m-thick succession of mixed (siliciclastic-carbonate) relatively coarse-grained deposits. Thissuccession is sharply placed between offshore mudstones and marls. The lower section isdominated by wavy and lenticular heterolithic deposits (with Gyrochorte, Teichichnus andChondrites) and massive mudstones, in which at least four discrete units of fine-grainedbioclastic sands (up to 1.5 m thick) intercalate. The lower boundary of these coarser units isusually associated with passively filled burrows comprising a Glossifungites suite. The sandyunits are mainly massive, but they may show ripple cross-lamination and locally larger crossbeds. Bioturbation is present mainly associated with Ophiomorpha and in less proportionTeichichnus and Palaeophycus.In contrast, the upper portion of the succession is dominated by coarse-grained bioclasticsandstones showing a profusion of cross-bedded structures that range between mid-scale,trough cross-bedding to large-scale sets with tangential foresets and abundant reactivationsurfaces. 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), althoughthe lower boundaries may show evidence of erosion. Bioturbation is infrequent, though largeOphiomorpha has been recorded in every unit of this upper section.The abundance of deposits associated with strong unidirectional currents, evidence of reversionof 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 subtidalsand 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 tosiliciclastic input. The overall coarsening- and thickening-upward succession may reflect thegradual downcurrent migration of the dunefield until it is abandoned and covered by a thicksuccession of offshore fines.The development of strong tidal currents in this offshore setting during the Early Cretaceoussuggests unusual conditions for the Neuquén Basin. These deposits were likely accumulatedduring a low-order transgressive interval developed after a major relative sea-level fall, whichhas been associated with local tectonic activity and with reactivation and inversion of olderstructures. This tectonic activity could have modified the palaeogeography, at least locally, topromote the amplification of tidal currents, effect that could have faded away when thetransgression progressed.