Rochas Clásticas do Planalto de Poços de Caldas

Resumo

1 — Sandstone, siltstone and volcanic breccia occur in the region of Poços de Caldas occupying an interrupted strip near the contact alkaline rock and gneiss. On the west side of the alkaline area sandstone covers gneiss at an altitude of about 1580 m eters. Outside this area no sediment of this type has yet been found. 2 — Sandstone and siltstone are generally well sorted fg.: 2, 3, 4, 5 and 6. Coefficient of Sorting varies between 1,1 and 3,6. The median diameter of sandstone grains vary between 0,10 m illim eters and 0,54 millimeters. The m ajority has a diam eter of about 0,3 millim eters. 3 — Roundness of sandstone and siltstone components were studied in size grades of 0,5 millimeter to 0,297 m illim eter and 0,74 m illim eter to 0,044 millim eter Roundness is sim ilar in both fractions giving results always inferior to 0,5 and superior to 0,2. 4 — Analysis of sedim entary composition gave the following mineral components: 90% to 20% Quartz 10% to 30% Feldspar 1% to 80% Muscovite, bauerite and iron oxides. Its percentage is usualy lower then 30. 5 — Heavy m inerals are usualy 1% of the total composition or even less. Zircon is the most common heavy mineral in all samples. In decreasing order of frequency are the following minerals: Anatase, tourmaline, rutile, garnet, monazite, hornblende, apatite, and corundum. The greatest quantity and variability of heavy minerals appears in sedimentary rocks of Águas da Prata and Serra do Mirante at the south-western border of the alkaline complex (Table 4) Moreover, leucoxene is very common as sandstone cim ent. 6 — Frosted or dull sandstone grains are very common. As they are poorly rounded, dullness was considered as provoqued by etching. 7 — The greatest thikness of sediment is of the order of 90 meters in Águas da P ra ta . T hirty meters are of sandstone at the upper part and 60 meters of siltstone at the lower part alternating with sandstone layers. Siltstones dip never exceeds 20 degrees. Sandstones parallel or cross bedding dip does not exceed 30 degrees. In some places sandstone passes gradually to arkose, sometimes with torrential structure, sometimes with pebble intercalations. On the top of the Poços de Caldas plateau siltstone is lacking and sandstone is not so thick as on the outside region. Plateau sandstone is cut by alkaline intrusions and at certains points by basic intrusions (diabase) Sandstone intercalations in siltstone show two different types of sedimentation: one of aeolian or fluvial nature and another of lacustrine nature. 8 — Age determ ination of these sediments was not possible because no fossils were found. Correlation on lithologic basis was established between these sediments and others of known age. Comparison was tried between these sediments and fluvioglacial, Bauru and Botucatu sandstones. Some similarities between Poços de Caldas sandstone and Botucatu sandstone (triassic) were found, considering texture and heavy minerals, so correlation between these two was established. The hypothesis that Poços de Caldas sandstone belongs to Bauru series is out of question, at least in Águas da Prata and north of Andradas because there it is cut by diabase. 9 — All fragm entary rocks directly involved in the alkaline volcanism were considered as brecciae. Theses rocks follow internally the encircling mountains of the plateau. They are chiefly of two different types: a — Detritic; b — Magmatic a —* Detritic: This rock outcrops from Águas da P rata up to Poços de Caldas and isolatedly at north of Andradas at a place called Pinheirinho farm. The fragments have a chaotic distribution in a fine brown or greenish blue aphanitic mass. The most common fragments are of altered diabase and in minor quantity quartzite, lava and gneiss. Quartz and feldspar are the most common mineral fragments generaly corroded and substituted by calcite. Calcite, hematite and aegirite are very common minerals of the matrix, and in minor quantity leucoxene, apatite and biotite. These rocks show bedding in a few outcrops in Cascata and Poços de Caldas (near Rio das Antas) There is still another type of pyroclastic rock, very finely grained, containing decomposed vitreous fragments and brown mass, at the base of Serra do Paiol and in the center of the plateau in a small outcrop. b — Magmatic: The m atrix is tinguaitic containing fragments of pyroxenic rock, lava, sandstone, and foyaite, near Po¬ ços de Caldas. On the west boundary of the plateau on the Serra do Paiol, appears the same kind of rock. Its texture is coarser and the fragments are the same as in the former It outcrops in an area of aproximately five square kilometers, having at most ten meters thickness. 10 — Origin: Type a breccia is primary detritic and later on modified by movement and solution percolations. Several clues allow this conclusion. They are the following: a — Great quantity of well rounded sandgrains in the m atrix . b — Bedding of these rocks in some expositions. c — Ideomorphic sodic pyroxene ocurring in the matrix, sometimes forming amigdales or needles. d — Albitization of diabasic feldspars and sandstone grains corrosion. e — Curved shaped twinning of calcite and m ortar texture of some fragments. Type b, brecciae is magmatic for the m atrix is of igneous rock. 11 — Brecciae are younger than sandstone and diabase, as they contain fragments of these two types of rock. They must be older than the last alkaline eruption as they are cut by dykes of tinguaite. 12 — Sandstone on the plateau at 1300 meters altitude in comparison to the same rock on the outside at 820 meters hight, shows that the alkaline intrusion has probably risen. 13 — This m ust have been accomplished by faulting in block movement that kept more or less the same level all the time. This conclusions is allowed by the following: a — The sandstones either on the inside or on the outside of the plateau have a very small dip. b — Inside the plateau, the sandstone follows the alkaline circle at more or less the same level (1300 meters altitude) c — The contact of alkaline rock and gneiss is, when visible, also aproximately vertical. 14 — The following clues indicate a small subsidence of plateaus central part: a — Dipping of sandstone is generally to the center of the plateau. b — Sandstone occupies an altitude of 1580 meters on top of gneiss on the outside of the plateau (Serra do Mirante) On the inside its altitude is of 1300 meters aproxim ately c — On the center of the plateau pyroclastic rock is at the same level as sandstone occupying the internal border This shows either a small subsidence, or a long period of volcanism perm itting after erosion the deposition of pyroclastic rock at the same level as sandstone. 15 -— Considering all observations and evidence, schematic representation of geological events was tried on the drawings of fig. 12, 13, 14, 15 The first, shows the first stage, anterior to the intrusion. Sandstone was occupying all the region. Near the mountains, in the desert, siltstone and clay were beeing deposited. The second drawing 13, shows intrusion and volcanism. The third 14, shows a phase of subsidence. The draw ¬ ing is exagerated. The fourth figure 15, shows schematically the actual form of the plateau, after a long phase of erosion