Abstract

GEORGIEFF, Sergio M  and  GONZALEZ BONORINO, Gustavo. Facies and geometry of Quaternary alluvial deposits in the quebrada del Portezuelo, Sierra de Mojotoro, Salta province, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2002, vol.9, n.1, pp.31-42. ISSN 1853-6360.

Salta city is located at the northern end of the Lerma Valley, which is a Quaternary tectonic depression bounded along the east by the Mojotoro Range, lying on the eastern border of Cordillera Oriental Geological Province (Baldis et al., 1976). The Mojotoro Range, with peaks at about 1,500 m a.s.l., is underlain by Proterozoic and lower Paleozoic sedimentary rocks in an east-vergent anticline. The Portezuelo Canyon is an east-west wind gap across the Mojotoro Range (Fig. 1). The Portezuelo Canyon is a narrow valley, 500 to 1,000 m in width at a mean altitude of 1,200 m a.s.l. Quaternary deposits resting unconformably on Paleozoic and older strata are locally preserved in the Portezuelo Canyon. Recent road works on National Highway 9 have produced excellent exposures of the Quaternary deposits; these exposures are studied in the present paper. Studied Quaternary deposits in the Portezuelo Canyon occur as inclined beds (strike/slip: N350°/3°W) exposed almost continuously along approximately 350 m. These deposits are characterized by two main facies, Fig. 2: (1) conglomerate with a coarse-grained sandstone matrix, in light yellow (5Y 6/2 and 5Y 8/2; Rock-Color Chart Committee, 1991) beds showing imbricated pebble clusters, trough and low-angle planar cross-stratification, with convex-upward bases and concave-upward to flat tops; bed thickness ranges from 1 to 2 m and exposed widths are in the order of several tens of meters; and (2) siltstone to very fine-grained, light yellow red (10YR 6/4 y 10YR 4/4; Rock-Color Chart Committee, 1991), horizontally stratified sandstone beds 1 to 3 m thick, showing parallel lamination and scarce pebbles. More detailed facies subdivision is given in Table 1. Significant differences with established Quaternary formations in the Lerma Valley lead us to separate these deposits as the Portezuelo Formation. Clast source probably was mostly local from the sides of the ancient Portezuelo Canyon, fed by small alluvial fans or rock-avalanche deposits. Field data were collected from detail sedimentological logs measured along gravel bodies (Figs. 2, 4 and 6). The geometry of deposits was obtained from photomosaics of the outcrops (Fig. 3). Sedimentological logs show the real thickness of gravel bodies, its facies vertical variations and space location. Moreover, they were useful to adjust photomosaics information. Interpretations are based on Bridge (1993b), Khan et al. (1997), Bridge and Georgieff (1997), Zaleha (1997), Georgieff (1998) and Bridge et al. (2000). Most of paleocurrent directions were measured on imbricated clasts and they are toward the East and Northeast. Some measurements of channel-belt orientation were taken on the margin of gravel bodies to adjust the main direction of channel belt. Paleocurrent directions were not corrected by tilting because of the outcrop slip is only 3º toward West. Surface hierarchy (Fig. 3) used is after Allen (1983). Four orders of surfaces were identify, we do not represent the orders 0 and 1 due to scale limitations. Three gravel bodies were selected for the detail analysis of the fluvial architecture. These examples are near the base of the outcrops and therefore they are accessible to measure logs and paleocurrent directions. Besides, they represent different examples of processes and fluvial dynamics. Nine fluvial facies were recognized, six of them represent gravel channel belts and the other three represent floodplain deposits (Table 1). Gravel body 11. Description: This gravel body has concaveupward base and flat to gentle convex-upward top (Fig. 4 a; location on Fig. 3a, facies 1, Table 1). Conglomerates and very coarse sandstones show trough cross and planar stratifications from the center to the ends of the body (Fig. 4b; logs 1 to 5; Table 1: facies 1 and 2). Thickest part of body is located between logs 3 and 4. Large cross stratification (1.5 m thick) is developed from left to right associated to thickness increasing in the same direction (facies 2, Table 1). The logs show grain size decrease laterally (to right) and they only show vertical variations into stratum. Paleocurrent directions are toward North and North-Northwest. Interpretation. The Figure 5 shows an evolution model for this body (facies 1 and 2, Table 1). Six evolution stages have been recognized. The first stage corresponds to the initial erosion of the bed including the base of the channel between logs 3 and 4. The four following stages are characterized by the development of the sigmoide stratification. These stages show the episodic expansion of the bar during floods and the migration of the thalweg up and the right, developing the steps worked in the basal surface of the body, at the time the external bank of the bend is eroded. The result of this process is a topographical ascent of the bar floor and the thalweg with its consequent decrease of the depth. A possible explanation for this geometry is that the traverse section of the channel decreases as it expands the bar and also decreases the flow and the erosion capacity. Another possible explanation is that the expansion of the lateral bar bears the increment of the width of the channel, a decrease in the water depth and a smaller erosion capacity to the bottom of the bar. In this second explanation the wet perimeter stays equally due to the erosion of the external bank and it is not necessary to suppose a reduction in the flow. During the fifth stage, the bar shows a new migration and the bed is eroded, conferring to the channel body the definitive geometry. Finally, in the sixth stage the bar deposited in the fifth stage is partially eroded and the residual depression is stuffed gradually (perhaps in more than an event). In conclusion, this body is interpreted as a channel with a bar of lateral accretion (Fig. 4c). The reconstruction shown in the figure 4c corresponds at the time in that the channel had the maximum depth. Gravel body 14. Description: The base is irregular to flat and the top is gentle convex-upward (facies 3 and 4, Fig. 4, location on Fig. 3a, Table 1). Massive and planar stratifications of conglomerates form the deposit (from right to left). Very good pebble-cluster imbrications were measured in the right side (Fig. 4b, log 6; facies 3, Table 1). Trough cross and planar stratifications are main sedimentary structures on the left end (Fig. 4b, log 5; facies 1 and 4, Table 1). A fine-upward variation was recorded on the logs. The maximum thickness of this body is coincident with log 6 (Fig. 4b) and decrease to both sides. Interpretation. The convex top in the right side of the body and the good imbrication of the clasts indicate that it is a sector of a bar-head (Fig. 4c; facies 3, Table 1). The gentle inclination of the surfaces toward the left indicates a growth of the bar in the down-stream direction. The scarce dispersion in the paleocurrent directions and the low angle of the surfaces is indicative of alternate-bar (very possibly a longitudinal bar) and the migration was mainly down-stream (translation; facies 4, Table 1). The increase of the grain size associated to the surfaces of 2 nd order indicates a seasonal flow and that these deposits possibly correspond to ephemeral rivers. The reconstruction proposed in the figure 4c corresponds to the bar top for that reason it is only represented the contour of the bar, since this surface of growth covers to the previous one. Gravel body 31. Description: This body is longer than the outcrop's width (Fig. 6; location on Fig. 3b). Its base is irregular and concave-upward (logs 1, 2 and 4), flat (log 5) or gentle convex-upward (log 3); the top is flat to gentle convexupward. It is formed by conglomerates with trough cross stratifications (Fig. 6 a and b, logs 1, 2 and 4; facies 1, Table 1) and low angle inclined strata (logs 3 and 5; facies 4, Table 1). A clear lateral variation on the thickness body is associated to changes in sedimentary structures and grain size. The fining-upward arrangement of the deposit was also recorded. Interpretation. The areas of concave-upward base are interpreted as main channel deposits (logs 1, 2 and 4; facies 1, Table 1) separated by gravelly center bars, which are characterized by a convex-upward to flat base (logs 3 and 5). The paleocurrent directions and the grain size variations indicate mid and head bar sections, being also indicated the translation, main and secondary expansion (notice the difference in the inclinations in the contacts of 2nd order of the migration for expansion, Fig. 6 a and b). The body 31 is interpreted like a part of a multi-channel gravel system (braided river), with the channels separated by gravelly to sandy alternate bars. The Portezuelo Formation fluvial channels were single- (Figs. 4 and 5) and multi-threaded (Fig. 6), locally occupied by bars that migrated by downstream translation (Fig. 4c, Gravel body 14) and lateral expansion (Fig. 4c, Gravel body 11, and Fig. 4). Secondary, crevasse splay- and floodplainchannels were associated with the major channels. Steepwalled gullies are present at the base of major channels (Fig. 3). Fluvial facies in the Portezuelo Formation are attributed to a gravelly braided fluvial system that flowed onto a silty alluvial plain bounded by steep rocky margins (Fig. 7). These fluvial channels were antecedent river relative to the Mojotoro Range.

Keywords : Fluvial architecture; Upper Pleistocene; Portezuelo Canyon; Salta.

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