Sedimentos clásticos em cavernas da região cárstica de Lagoa Santa (MG): origem, mecanismos de deposição e estratigrafia

Abstract

The organization, analysis and discussion of the sediments found in some caves in the karst region of Lagoa Santa was the objective of this project, which had as an initial stage the survey of those caves that contained investigated clastic sediments and their data available in the literature, in which the first models on origin, transport and deposition were suggested. In general, the main authors on the subject in the region demonstrate that the sediments found in the caves come from the metapelitic cover (siltites and claystones) of the Serra de Santa Helena Formation, which gave rise to the current pedological cover – allochthonous. This unconsolidated material reaches the caves through widened fissures and crevices – epikarstic material, and with the coupling between the openings of the caves and the slope. They have high percentages of SiO3, Al2O3 and important concentrations of FeO3, indicating longer exposure time to leaching conditions and mineral concentration – oxidation and reduction. The autochthonous clastic sediments are characterized by collapsed blocks of varied granulometry, speleothem fragments and the rock waste product, which for the limestones of the Lagoa Santa Member, is incipient. As for the transport mechanisms, mud or debris and suspension races prevail. These mechanisms promote the mixing and selection of the unconsolidated material according to the deposition environment (duct network), which impose different conditions for the circulation of water, often in the same gallery. As for the organization of sediments - stratigraphy, they present sequences of clayey sedimentary facies (often cemented by calcite), between red and yellow, with fossiliferous remains of extinct megafauna, current fauna and limestone fragments, interspersed by calcitic levels (stalagmite floors). Thus, new caves with sediment deposits that could be analyzed were selected: Gruta da Escada and Irmãos Piriás. Seizing a mixture of deposition environments influenced by fluvial/lacustrine processes (Gruta Irmãos Piriás) and vadoso (Gruta da Escada), descriptive and laboratory analyzes of the sedimentary facies surveyed in the two caves were carried out to relate the origin, transport mechanisms and the environment of deposition to which they were submitted. The models of origin, transport and deposition mechanisms presented by several authors for the deposits in caves in the region were corroborated, both in terms of particle size, chemical composition, stratigraphic organization, as well as for the deposition environment. In Gruta da Escada, the predominantly clayey texture found in the facies, the high percentages of SiO3, Al2O3 and FeO3, the stratigraphy with erosive contacts, stalagmite floor covering/sealing the deposit, demonstrate attributes common to those of the region. On the other hand, in Gruta Irmãos Piriás, the fluvial characteristics and the rock waste product (SiO3) provided new information on types of configurations for clastic sediment deposits in caves of the karst Lagoa Santa, where the particle size, stratigraphy and chemical composition of sediments suffer strong water influence, concentrating even more SiO3 and Al2O3 oxides, in addition to providing an environment for mixing materials, and organization of facies represented by laminations and lenses, associated with oscillation of the water level. Particularly the high silica concentrations may be related to the insoluble rock residue (Pedro Leopoldo member), which presents important concentrations of silica (23%), aluminum (2.95%) and iron (1.75%), higher than those observed in the limestones of the Lagoa Santa Member.