Sedimentologia e Paleogeografia de depósitos piemônticos na usina de Peixotos

Abstract

In the present paper the author studies two lots of sediments laid on nonconformity over a sericite-quartzite of the pre-cambrian Minas Series near by the Peixotos electric Power Plant, State of Minas Gerais, Brazil.

            The sediment textures, in Wentworth grade scale, spread over 9 and 10 classes, and thus showing a range peculiar to fluvial piemontic deposits.

            The statistical analysis beats evidence to a turbulent fluvial transportation, followed by a rapid accumulation; such a condition is naturally found on a piemontic environment.

            On the bases of the textural maturity concept of Folk, the sediments ate immatures; this fact means a sudden loss of energy of the agent of transport, which becomes unable to segregate the textural components as clay and silt from the sand grade.

            The sample A-1 has the light minerals composed of quartz 78,67%, feldspar 8,05%, and clay 13,23% in the sizes over 0,074 mm; in the sizes under 0,074 mm there are quartz 77,13% feldspar 9,54% and clay 13,28% the sample A-2 has quartz 81,94%, feldspar 8,00%, and clay 10,06% in the sizes over 0,074 mm. The sample "B" have the light minerals distributed as follows: B-1, quartz 78,53%, and clay 21,47%; B-2, quartz 75,74% and clay 24,26%. B-3, quartz 73,74%, and clay 26,26%. The sediments "A" are comprised of feldspathic conglomeratic sandstones, and the sediments "B" ate siltestones. The first ones are massive and the second very well stratified.

            The heavy suites me comprised only by stable minerals. For better understanding the heavy minerals were separated into two fractions, one being over 0,074 mm and the other under 0,074 mm the sample "A" show tourmaline, topaz, staurolite, kyanite, rutile, zircon and biotite. The biotite occurs in the finer grades, which means good abrasion during transport as Peitijohn pointed out it (p. 431, 1952). The sample "B" show zircon, biotite, tourmaline, kyanite, corundum and rutile. With respect to the mineral composition these sediment are nature.

            The roundness values. in accordance with the Wadell formula, are 0,40 and 0,50 for the sandstones, and 0,30 for the siltstones.

            The porosity is 26% for the sandstones and 28% for the siltstones.

            The source area of these sediments was under humid and warm climate in order to provide the chemical weathering dominating over the velocity of the erosion, as witness the stabily of the heavy minerals. The topography was under peneplanation because the rate of chemical weathering of the patent rocks took advantage with reference to the velocity of the erosion. A tectonic disturbance due to epeirogenic movements associated with faulting broke this balance; then the rate of erosion was increased and it had the chance to get a great quantity of sediments rich in finer grades. Consequently the deposits ate textually immature and mineralogically mature.

            Regarding the sedimentary tectonics there was rapid erosion over a plateau under previous conditions of a peneplain, which had a thick waste mantle of weathered crystalline rocks, as granitic, pegmatites, gneiss and schists. This fact signifies that these sediments were originally fine and were released in humid and warm source area climate. The sediments were transported by rivers fed by the humid climate and deposited afterwards along a piemontic environment as the rapid accumulation and immature texture tell.

            With respect to the mineral persistence series of Pettijohn (1952), these sediments could not be more aged than the Triassic period. There is also a confirmation of that age on the number of the heavy minerals.

            Presently it is not yet possible to correlate these sediments of Peixotos to others of post-Triassic age in south Brazil. This problem is still open for future stratigraphic correlation.