Caracterização petrográfica, geoquímica e mineralógica do silcrete de Santa Luzia, Cascalho Rico (MG)

Abstract

In the southwestern part of the Sanfranciscana Basin, near the town of Cascalho Rico, outcrops of intensely silicified sedimentary rocks are exposed. This study investigates the textural, mineralogical and chemical characteristics of silcretes developed within eolian sandstones of the Três Barras Formation (Upper Cretaceous, Sanfranciscana Basin), with a focus in identifying potential silica sources and estimate its age from the analysis of stratigraphic relationships. Field observations reveal that the silcrete exhibits a glassy appearance, conchoidal fracture, and occurs as horizontal beds with irregular boundaries. Mineralogical phases and microscopic features were characterized using optical microscopy, scanning electron microscopy (SEM/EDS) and Ramam spectroscopy. The rock framework is dominated by monocrystalline detrital quartz grains, displaying evidence of two distinct diagenetic silica phases: (i) opal and chalcedony, occasionally exhibiting botryoidal habits (coloform/geopetal texture) covering detrital grains; and (ii) microcrystalline quartz and fibrous radial chalcedony infilling residual pore spaces. Authigenic titanium oxide (anatase) and iron and aluminum oxide-hydroxides are identified at interfaces between detrital and authigenic silica phases. Macroscopic features, mineralogical composition, and microstructural analysis classify the silcrete as non-pedogenic, supported by the following criteria: (i) lateral and vertical textural homogeneity; (ii) preservation of primary sedimentary structures; (iii) bleaching features; (iv) nodular textures (glebulae); (v) corrosion features on framework grains; (vi) diagenetic sequence showing increasing cement crystallinity towards pore centers; and (vii) anatase coatings on detrital grains. These characteristics, combined with the geological context of the Três Barras Formation, suggest a groundwater-influenced origin (groundwater silcrete). Corrosion textures on detrital grains indicate their role as the primary silica source. Secondarily contributions may derive from lithoclasts weathering and water table-derived intergranular fluids. The sequential precipitation of diagenetic silica polymorphs (opal → chalcedony → microquartz) is attributed to fluctuations in silica saturation and impurities within diagenetic fluids. While groundwater silcretes are chronologically challenging to date due to silica dissolution-precipitation dynamics, stratigraphic context suggests formation of the Santa Luzia silcrete during the Campanian (Upper Cretaceous).