Abstract

SCHWARZ, Ernesto. Facies sedimentarias y modelo deposicional de la Formación Mulichinco (Valanginiano), Cuenca Neuquina Septentrional. Rev. Asoc. Argent. Sedimentol. [online]. 1999, vol.6, n.1-2, pp.37-59. ISSN 1853-6360.

A 300-m thick, mixed siliciclastic/carbonate succession accumulated during the early Cretaceous (Valanginian) in a marine shelf setting is analyzed in this contribution. These deposits belong to the Mulichinco Formation and are located in the northwest of Neuquén Province, Neuquén Basin, west central Argentina (Fig. 1). The Mendoza Group (Tithonian-Barremian), which includes the Mulichinco Formation, has mainly been studied on a regional scale in the past. Few attempts have been made in the detailed modeling of depositional systems, especially in this particular area. The analyzed unit has a clear shelf profile in seismic configuration (Mitchum & Uliana, 1985), contrary to the underlying Vaca Muerta Formation and the overlaying Agrio Formation (Fig. 3), which were accumulated in ramp settings (Legarreta & Gulisano, 1989; Legarreta & Uliana, 1991, Spalletti et al., 1999). In this shelf profile, distinguished lithosomes influenced by waves (storm and fair-weather) and tides have been formed, turning the Mulichinco Formation into a good example of an ancient mix influenced shelf environment. Thus, a detailed description of the various facies, facies relationships and stratal architecture is presented to address: 1) the facies makeup and vertical evolution 2) the accumulation processes 3) the depositional model for the Mulichinco Formation which helps, in turn, to better understand the general evolution of the Mendoza Group. The sedimentological attributes of the Mulichinco Formation are described in two detailed sections (Figs. 4 and 5): Puerta de Curacó (PUC) and Barranca or Aguada de los Loros (ADL) (locations are shown in Figure 1). The overall trend in the succession allows the unit to be subdivided into a Lower, Middle and Upper section. In the Lower and the Upper sections, siliciclastic sediments prevail, while carbonates are more abundant in the Middle section. Based on the lithology (texture and composition), mechanical structures, thickness, trace fossils and invertebrate contents, thirteen (13) siliciclastic and carbonate facies (Table 1) were recognized and grouped according to their vertical arrangement. Brief paleocurrent data are then presented, in order to assist in local coastline-orientation inferences and, finally, a depositional model is proposed. The green shales and marls (F1) together with heterolithic lentiform facies (F2) represent the more distal shelf sedimentation. Heterolithic sandstone-mudstone facies with wavy and flaser structures (F3), interbedded with planar and trough cross-laminated wackes (F4) and isolated bodies of very fine-grained sandstones with HCS (F6) are all interpreted as inner shelf deposits. Fine-grained ripple crosslaminated sandstones (F5) represent normal conditions in the lower shoreface, but thick (up to 2 m) tabular amalgamated hummocky cross-stratified sandstone deposits (F6) are also common. Fine to medium-grained sand and carbonate sands with planar cross-stratification (F7) and trough cross- stratification (F8) represent shallow marine tidal sandwaves and channels, respectively. Quartz-rich massive sandstones (F9) with conspicuous Skolithos icnofacies, and oolitic-bioclastic packstones and grainstones (F10), are often associated with the tidal deposits, and are also formed under high-energy wave conditions. In the Middle section the lithological characteristics change dramatically. Condensed levels of floatstones (F12) characterize its base. Upper parts of the section are composed of thick oyster-serpulid buildups (F13), bioclastic wackestones (F11) and marls (F1). These facies reveal scarce to null siliciclastic influx, and deposition below the fairweather wave base in an open marine environment. To the top of the Middle section, some heterolithic facies (F2 and F3) and fine sandstones (F5) show a permanent wave action, rejecting the hypothesis of a sheltered depositional environment. The general depositional model, based on the distribution of facies associations is shown in Fig. 8. Due to the dramatic change during the deposition of the Middle section, two different scenarios have been exemplified. Figure 8a illustrates conditions depicted for the Lower and Upper section, and Figure 8b, for the shelf in the Middle section, during oyster buildup accumulation. In the Lower and Upper sections (Fig. 8a) the outermost shelf conditions are represented by facies associations I, in which green shales (F1) and very occasionally turbidite-like tempestites (F2) accumulate. F3, F4, F5 and F6 composed the facies associations II and indicate the deposition from the inner shelf to the lower shoreface. This very common association usually shows upward coarsening and shallowing trends, inferring more frequent and intense oscillatory, combined and unidirectional currents. The highest-energy hydrodynamic conditions are suggested by thick amalgamated HCS sandstones (F6). Facies associations III is almost exclusive of the Upper section and groups subtidal facies (F7 and F8) and high-energy wave-formed facies (F9 and 10), which are interpreted to occur in the upper shoreface. The floatstones (F12), intercalated with marls (F1) are the major constituents of facies association IV, with subordinated wackestones (F11). This suite of facies, especially the mixing of paleoecological habitat in F12 and the low proportion of siliciclastic particles in F1 and 11 are indicative of low terrigenous input and consequent starved conditions which allowed the development of taphonomic feedback. Nevertheless, even under such conditions, occasional storms are represented by F2. Facies association V belongs to the Middle section and is entirely composed of carbonates. Green marls (F1) pass upwards to bioclastic wackestones with infaunal invertebrates (F11). Suddenly, a drastic change in substrate oxygenation may have favored the epifaunal oyster and serpulid boundstone development (F13a) forming extensive tabular buildups in the inner shelf. Occasionally, big oyster packstones (F13b) appear at the most top. This trend occurs at least three times and reveals the development of carbonate high frequency cycles. The lack of intense wave reworking suggests that the sediments forming this vertical trend have been accumulated from well below to near the fair-weather base level. In conclusion, a wave- and tidal-influenced mixed (carbonatic/siliciclastic) shelf model is proposed for the Lower and Upper sections of the Mulichinco Formation. From outer shelf to lower shoreface, wave (storm and normal) action is the main depositional process. In the upper shoreface where tidal influence is also recorded, a mixed (tidal and wave) shelf is developed. Comparing the record of both Lower and Upper section deposits (Figs. 4 and 5), deepwater sediments relatively predominate in the former. This fact may suggests a comparative deepened environment. In the Middle section, a relative sea level rise occurred. Clastic influx was drastically diminished and biogenic carbonate bodies developed in large areas of the shelf.

Keywords : Shelf; Waves; Tides; Neuquén Basin; Valanginian.

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