Resumen

DEL VALLE, Analía  y  MANASSERO, Marcelo J. Procedencia de las arenitas cuarzosas de la Formación Balcarce (Ordovícico inferior) en base al análisis petrográfico. Rev. Asoc. Argent. Sedimentol. [online]. 1995, vol.2, n.1-2, pp.105-112. ISSN 1853-6360.

The provenance and sedimentary petrography of the quartz arenites from the Balcarce formation (Cam-Or) were analysed using the Basu et al. (1975, 1985) discrimination model based on undulosity and policrystallinity of detrital quartz on 35 samples from different outcrops in the basin (Fig. 1). The source material are low (80% of the samples) and medium to high rank (20% of the samples) metamorphic rocks which are widespread as underlying basement rocks (see Fig. 2). The polycrystalline quartz shows low dispersion while the undulosity shows a higher one. This fact has been interpreted as the effect of an extended and areally heterogeneous dynamic metamorphism in the source areas (mainly underlying basement). The quartz with undulosity seems to be proportional to the degree of cataclasys of the basement rocks. From bottom to top of the sequence, the polycrystalline quartz and the undulose quartz decrease and the non undulose quartz increases which suggests an increasing reworking that get rid of the most unstable varieties of quartz, that is to say the ones that had suffered lattice dislocation‘ Other elements like the basin type, source material, weathering, the low sedimentation rate, the shallow platform marine environment of deposition and the effect of tidal currents play an important role in the first cycle generation of orthoquartzites. If we consider the sedimentation rate, (total thickness of the unit/time of deposition) versus the relative increase in quartz content (considering the % of quartz in the source rock) from the graph of Suttner et al., 1981 (Fig. 3), we obtain a first cycle orthoquartzite from the underlying basement rocks of the Balcarce Fm. without considering the influence of the climate, which is very important due to the intense weathering of the source rocks (Marchese & Di Paola, 1975a; Zalba et al., 1992) that form important paleosurfaces. The main conclusions in this contribution are: * The Basu et al. (1975, 1985) discrimination model for quartz provenance applied to orthocuartzites does not show coherent results in our case, due to the characteristics and heterogeneous distribution of the source rocks (granitoids affected by dynamic metamorphism, mylonitic belts and migmatic rocks). *The proportion of quartz with undulosity shows a good correlation with the undulosiy of the basement rocks. This fact suggests that the process of reworking of the source material "in situ" played an important role in the generation of the Balcarce Fm. This intense reworking works against reliable results applying this model. * The marine environment favours the compositional changes leading to the great sedimentary maturity. * The intense weathering and the reworking of the grains in a shallow and tide dominated marine environment are the main causes of this maturity. The reworking causes, also, a change in the amounts of quartz types within the sequence. * A sedimentation rate of 10.9 m/Ma for the Balcarce Fm. was inferred. * This low sedimentation rate in a cratonic basin is enough to generate first cycle orthoquartzites. * We plan to apply this statistical methodology in order to recognize and correlate other orthoquartzites along the basin.

Palabras clave : Sedimentary petrography; Quartz provenance; Sedimentation rate; Reworking; Undulosity index; Polycrystalline grains.

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