Resumen

VEIGA, Gonzalo Diego. Influencia de factores extrínsecos en la evolución de una planicie aluvial: sedimentología de la Formación Challacó en el Cerro Lotena, provincia del Neuquén, República Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 1997, vol.4, n.2, pp.111-128. ISSN 1853-6360.

The sedimentation of the Cuyo Group ends with the continental deposits of the Challacó Formation in the southern part of the Neuquén Basin. At the vicinity of Cerro Lotena, 50 km southeast of Zapala in central Neuquén (Fig. 1), this unit is more than 300 m thick, and it is mainly composed of conglomerates at the base (Fig. 2) and lenticular sand bodies intercalated with red and green mudstones at the top (Fig. 3). These deposits were originally described as part of the Lotena Formation (Weaver, 1931), but this name is currently applied to marine sediments of the Loteniano-Chacayano Cycle, whereas the continental deposits at the top of the Cuyo Cycle, are characterized as Challacó Formation. A detailed sedimentary section was measured. This section was drawn at 1:200 scale (Fig. 4). Sedimentary body architecture, facies and facies associations were defined in order to distinguish depositional environments. Finally, a stratigraphic scheme based on the identification of sequences and changes in the depositional conditions, was developed. According to lithology and primary sedimentary structures, sixteen sedimentary facies were recognized (Table 1). These facies were identified with the classic code proposed by Miall (1977, 1978) with some modifications. Conglomerates are common in the lower part of the unit. Massive facies (Gm) is the most widespread identified, but also planar (Gp) and trough (Gt) cross-stratified conglomerates are present. Gravelly-sand facies are also important at the base of the Challacó Formation, specially those with planar and low angle cross-stratification (SGp, SGh/l) related to bidimensional and low relief subaqueous dunes. Trough cross bedded (SGt) and massive gravelly-sandstones (SGm) are also present. Sandstones and mudstones are common in the upper section. Planar, trough, inclined and low angle cross-stratified sandstones (Sp, St, Si, Sl) are present as channel deposits; massive, plane-bedded and ripple cross-laminated (Sm, Sh, Sr) facies are associated to massive and laminated mudstones (Fm, Fl) as overbank deposits. The lower section of the Challacó Formation in the studied area is characterized by the presence of two facies associations which represent the development of a braidplain, dominated by tractional flows (Fig. 5). Facies Association 1 is composed of Gm facies and represents high flow-regime deposits of longitudinal bars. Facies Association 2 is characterized  by cross-bedded conglomerates (Gp, Gt) accumulated as transverse bars and minor channel-fills. The upper section is integrated by finer sediments, mainly sandstones and mudstones. Six facies associations were identified (Fig. 6). Facies Association 1 represents the subaqueous accumulation of fine deposits in a floodplain. Associated with floods, levee (Facies Association 2) and crevasse channel deposits (Facies Association 3) are also present. These three associations were grouped into the “fine member” of a mixed-load high sinuosity river system. The “coarse member”, composed of sandy facies, is related to two different types of channel-fills: high sinuosity channels with the development of lateral accretion deposits (Facies Association 4) and less sinuous channels, with more tractional load, but with the presence of a fining-upwards trend that represents the lateral migration of the main channel. This mixed-load river system is interrupted by the occurrence of coarse-grained deposits (Facies Association 6) that represent higher flow conditions and the development of a braided river system of limited vertical extent. The abrupt facies change between the marine Lajas Formation and the coarse-grained fluvial facies of the Challacó Formation, represents a remarkable paleogeographic change. The development of 150 m of coarse-grained proximal deposits also implies a constant and considerable accommodation rate, which can be related to tectonic inversion of normal faults and the development of an Intra-Bajocian unconformity in the “Huincul Arch” zone. The transition from braided river deposits to a mixed-load, high sinuosity river system represents a gradual uplift and flattening of the base profile. This change is correlated with a base level rise associated to an important transgression. Two sequences were recognized in the upper section. They show an erosional lower boundary with basal conglomeratic deposits (Facies Association 6) of a braided river system that grades upwards to a mixed-load high sinuosity river system with isolated channel sand-bodies and a considerable proportion of fine member facies. The sequences end with similar deposits, but with more amalgamated channel-fills and thin intercalations of overbank facies. The oscillation of base level can be responsible of the above described pattern. If relative sea level is considered the base level of  the system, the portions of the sequences showing distinct accommodation conditions can be related to the Systems Tracts of the classic sequence stratigraphy framework. The Lowstand Systems Tract is characterized by coarse-grained deposits of Facies Association 6 that represent the steepening of the base profile during a base level fall. The basal erosion surface is interpreted as a sequence boundary. A Transgressive Systems Tract is defined for the middle portion, which is dominated by fluvial channel isolation. The high accommodation rate for the Transgressive Systems Tract was induced by a rapid sea level rise. Though it is quite difficult to define a maximum flooding surface in these continental deposits, the increase in coarse member deposits amalgamation suggests the transition from a Transgressive System Tract to a Highstand Systems Tract.

Palabras clave : Fluvial Systems; Extrinsic Controls; Base Level; Jurassic; Neuquén Basin.

        · resumen en Español     · texto en Español     · Español ( pdf )