Abstract

VEIGA, Gonzalo Diego. Paleogeographic and paleoenvironmental evolution of the continental deposits for the southern margin of the Neuquén Basin during the Middle Jurassic. Rev. Asoc. Argent. Sedimentol. [online]. 2002, vol.9, n.1, pp.83-108. ISSN 1853-6360.

In the southern part of the Neuquén Basin (West-Central Argentina) the Cuyo Group (Lower to Middle Jurassic) ends with continental deposits of the Challacó Formation (De Ferraríis, 1947). Despite the numerous stratigraphic schemes proposed during the last years (Gulisano et al., 1984; Legarreta y Gulisano, 1989; Legarreta y Uliana, 1996, among others) these deposits are still poorly studied, not only in terms of depositional environments but also of their relationship with the marine facies of the Cuyo Group and the overlying Loteniano-Chacayano Cycle. The aim of this paper is to present a paleoenvironmental study of the Challacó Formation together with a paleogeographic evolution for the southern margin of the Neuquén Basin during the Middle Jurassic, in order to complete the stratigraphic evolution for the southwestern margin of the Neuquén Basin during these times. The study area is located in the central part of the Neuquén Province in west central Argentina (Fig. 2), and the outcrops of the Challacó Formation are related to a series of structures associated with the Huincul Wrench fault zone (Dorsal Neuquina). Two areas of study were defined due to the lack of a direct correlation between them. In the Eastern Sector, located 50 km south-east of Zapala (Figure 2), the Challacó Formation is more than 300 m thick and unconformably overlies marine deposits of the Lajas Formation. The basal section is characterised by conglomerates and coarse-grained sandstones (Fig. 3) and the upper section by lenticular sandstone bodies, red/green mudstones and a few conglomerates. In this part of the basin Tithonian black shales of the Vaca Muerta Formation overlie the Challacó Formation. In the Western Sector the Challacó Formation is between 40 and 120 m thick and comprises lenticular sandstone bodies and minor conglomerates intercalated with tabular red/green mudstones (Fig. 4). In this region, the Challacó Formation overlies marine deposits of the Lajas Formation and is in turn overlain unconformably by Kimmeridgian conglomerates of the Quebrada del Sapo Formation and by Tithonian black shales of the Vaca Muerta Formation (Fig. 1). In the Arroyo Picún Leufú locality (Fig. 2), a 30 m-thick succession of marine shales is observed between the Challacó Formation and the Quebrada del Sapo conglomerates. These deposits were ascribed to the Lotena Formation (Callovian-Oxfordian, Fig. 1) and some authors also included the conglomerates at the base of this succession to the Lotena Formation. Because they show a continental origin similar to that inferred for the Challacó Formation and there is no evidence of tectonic activity at its base (as suggested by Dellapé et al., 1978, 1979 for other parts of the basin), these deposits were included as Challacó Formation in this study (Veiga, 1998). Seven sections were measured (Fig. 2) for which lithology, sedimentary structures, geometry and the stratigraphic relationship between underlying and overlying units were recorded. Sedimentary structures and lithology allowed the definition of 21 sedimentary facies. Architectural element analysis (Allen, 1983; Miall, 1985, 1988), based on facies associations, geometry, orientation and main lithology was performed in order to identify the main sedimentary environments of the Challacó Formation (Veiga, 1997a, 1998) (Table 1, Fig. 5). Four different non-marine environments were defined for the Challacó Formation (Veiga, 1998): A) Mixed-load High Sinuosity Fluvial System. Characterised by a fine member that represents flooding processes, usually in coarsening-upwards sequences, and channelfill deposits (coarse member), represented by three different sedimentary bodies. Based on the proportion of fine:coarse member and the degree of amalgamation of channel deposits, two end members for this environment were defined. One is represented by a system dominated by finegrained deposits with isolated channels (composed by type V bodies exclusively, Table 1, Figure 5b). In the other endmember, floodplain deposits are less well represented due to erosion and channel-fill deposits are more complex (Table 1, Fig. 5a) building channel belts. B) Coarse-grained Braided Fluvial System. The bulk of these deposits are coarse-grained tabular bodies that represent the accumulation of transverse and longitudinal bars (type VI and VII bodies, Table 1) in a low-sinuosity braided system, composed of channels with a very high width/depth ratio and unstable banks. During periods of waning flow, small channels are developed (type I and II bodies, Table 1), cutting the previously deposited bars. Two end members were also defined for this system, according to the proportion of cross-stratified bodies that represent lower flow regime conditions (Hein y Walker, 1977). C) Floodplain to Lacustrine System. These deposits are very similar to the fine member of the high sinuosity fluvial system but they are not laterally related to channel bodies. They are composed of thick successions of red mudstones and tabular fine-grained sandstones. These deposits represent accumulation in shallow, temporary lakes with episodic floods responsible for the accumulation of the coarser material. They are characterised by pedogenic structures that represent the development of well-drained palaeosols under semiarid conditions. D) Aeolian System. Characterised by well-sorted fine- to medium-grained sandstones in tabular bodies with largescale cross-stratification and numerous reactivation surfaces. These bodies are interpreted as aeolian dunes and alternate with medium- to coarse-grained lenticular sandstones that represent periodic flooded interdunes. Due to the lack of a direct correlation between the two outcrop zones, a palaeoenvironmental evolution has been outlined separately for each one. In the Western Sector, the Challacó Formation starts with deposits of a high sinuosity river system. These deposits overlie marine facies of the Lajas Formation and are characterised by a high proportion of fine member facies and by isolated channel bodies. In the upper part the amalgamation of coarse member deposits increases substantially with only small relicts of floodplains remaining due to lateral migration of fluvial channels (Fig. 6). The pattern above described suggests a decrease in aggradation rate of the alluvial plain and a decrease in accommodation space creation, accompanied by an increase in channel sinuosity. In the Quebrada del Sapo and Picún Leufú localities (Fig. 2), a coarse-grained braided fluvial system develops above these deposits. Conglomerates overlie an erosion surface that has local relief and are succeeded in the Picún Leufú area by marine deposits of the Lotena Formation. In the Quebrada del Sapo section this conglomeratic unit is overlain by aeolian deposits and in the southern part the upper portion of the Challacó Formation is characterised by mudstones and fine-grained sandstones of a floodplain to lacustrine environment (Fig. 6). Two sequences (Sequences W1 and W2) were identified for this sector (Veiga, 1998) and their development has been related to fluvial base level oscillations related to relative sea-level changes. The characteristics of the Eastern Sector are significantly different, manly because of the presence of a ~200 m-thick coarse-grained succession at the base of the Challacó Formation that represent accumulation in a braidplain environment. These deposits overlie an important unconformity that, in the Cerro Granito area is in angular unconformity with the marine deposits of the Lajas Formation. There is also a change in the depositional environment through this section, from massive facies at the base, indicating high flow regime conditions, to crossstratified conglomerates at the top, attributed to a decrease in flow regime (Fig. 6). High sinuosity fluvial deposits dominate the upper section and, as in the western sector, there is a vertical change in channel facies amalgamation and in its internal architecture that can be correlated with changes in accommodation. As in the western sector, there are also coarse-grained intercalations that represent the development of a braided fluvial system and an increase in energy conditions (Fig. 6). The correlation of the unconformity between the continental deposits (Challacó Formation) and the marine facies (Lajas Formation) in the Eastern Sector and the one present within the Lajas Formation deposits in the Western Sector (described by Gulisano y Hinterwimmer, 1986; Zavala, 1996a, 1996b, among other authors), allowed the integration of the information available for both outcrop sectors (Fig. 7). Three stages of evolution for the southern margin of the basin were defined which were assigned tentatively to the Bajocian, Bathonian and Callovian respectively. Recently, many authors have proposed tectonic controls on deposition of the Cuyo Group. The unconformity between the Lajas and Challacó Formations observed in the Western Sector was described by Limeres (1996) and Zavala (1996a, 1996b) as the expression of tectonic movements during the Lower Bajocian. This also can be correlated with the tectonic inversion in the Huincul Wrench documented by Bettini (1984) and Vergani et al. (1995) (Fig. 8). The 200 m-thick coarse-grained succession of the lower Challacó in the east can be correlated to this period of tectonic inversion, and with the development of topographic highs that acted as local sources for proximal fluvial systems (Fig. 9a). The retrogradational sequence observed through the lower section (Retrogradational Systems Tract) is associated with the subsequent rise in base level and loss of capacity of the system due to the gradual peneplaination following the main inversion stage (Veiga, 1997b). During the Bathonian an important regression took place (Legarreta y Uliana, 1996; Uliana et al., 1999) and, in the southern part of the basin, the accumulation of the Cuyo Group is represented exclusively by continental deposits of the Challacó Formation (Figs. 7 and 9b). During this stage, the characteristics of the depositional environments were controlled by changes in base level of the fluvial system (Veiga, 1998). In this context, three different systems tracts were defined, which were related to different base level position and correlated to the classic sequence stratigraphic scheme (Fig. 10). Finally, during the Callovian, a new sequence is developed (Sequence 3, Fig. 7), which is partially preserved in the study area due to the effects of the Intermalmic Unconformity. If we consider the conglomerates of the base of the Lotena Formation as part of the Challacó Formation, then these deposits represent the Lowstand Systems Tract of this sequence (Figs. 7 and 9c). In this scheme, the marine deposits of the Lotena Formation and the aeolian deposits in the Quebrada del Sapo locality may represent the Transgressive Systems Tract of this sequence (Fig. 9d). In a broader context, the LST deposits can be correlated to the evaporite facies of the Tábanos Formation in the basin centre, because both represent lowstand conditions and it is possible to correlate the marine facies of the Lotena Formation through the entire basin.

Keywords : Palaeoenvironmental analysis; Non-marine deposits; Sequence stratigraphy; Jurassic; Neuquén Basin.

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