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

Abstract

In general, the sediments found in the caves of the Lagoa Santa karst region come from the metapelitic cover of the Serra de Santa Helena Formation, which gave rise to the current pedological cover. 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 SiO2, Al2O3 and important concentrations of Fe2O3. 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 runs prevail. As for the organization of sediments, they present sequences of clayey sedimentary facies (often cemented by calcite), between red and yellow, with fossiliferous remains of extinct megafauna and current fauna, in addition to limestone fragments, interspersed with calcitic levels. Thus, new caves with sediment deposits that could be analyzed were selected: grutas da Escada and Irmãos Piriás. Descriptive and laboratory analyzes of the sedimentary facies surveyed in the two caves were carried out to relate the origin, transport mechanisms and the deposition environment 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. In gruta da Escada, the predominantly clayey texture found in the facies, the high percentages of SiO2, Al2O3 and Fe2O3, the stratigraphy with erosive contacts, stalagmite floor covering/sealing the deposit, demonstrate attributes common to those of the region. On the other hand, at gruta Irmãos Piriás, the fluvial characteristics and the rock waste product (SiO2) provided new information on types of configurations for clastic sediment deposits in caves in the region, where the granulometry, stratigraphy and chemical composition of the sediments suffer strong water influence, concentrating even more SiO2 and Al2O3 oxides, in addition to providing an environment for mixing materials, and organization of the facies represented by laminations and lenses, associated with the oscillation of the water level.