Gênese, distribuição e caracterização dos espongilitos do noroeste de Minas Gerais

AUTOR(ES)
DATA DE PUBLICAÇÃO

2009

RESUMO

This study relates to the spongillites deposits that occurs in shallow ponds on a karstic planation surface developed on rocks of the Neoproterozoic São Francisco Supergroup. Cenozoic siliciclastics sediments are related to this surface and Mesozoic Areado Group rocks occur adjacent to the deposits. Field studies associated with the analysis of multispectral images have contributed to the knowledge about the geological and geomorphological context in which the lakes with the spongillites deposits occur. The Cerrado landscape evolution from the João Pinheiro region where the spongillite deposits occur have been controlled by important erosive processes after the Cretaceous that led to the formation of a disssecated high plain where 4 geomorphological domains were identified: i) domain 1, represented by plateaux related to sandstones from the Areado Group that show the highest altitudes in the area; ii) domain 2 that constitutes a dissecated area related to pelites from the Areado Group; iii) domain 3, comprising of a karstic planation surface associated to the carbonatic rocks from Bambui Group and PréBambuí unit covered by Cenozoic sediments where the lakes occur; iv) domain 4 is composed of trough shaped valleys enclosing meandering underfit streams (rivers Prata and Paracatu). These valleys cut through the karstic planation surface (domain 3) where Pleistocene sediments occur and characterize the more recent morphological domain. The preferential direction of SENW of the drainage system suggests that the sandstones of the Areado Group constituted the source area of the spongillitehosting sediments. Mineralogical and textural characterization by optical microscopic analysis, Xray diffraction (XRD), differential and gravimetric analysis (DTAGTA), infrared spectroscopy (IR), and scanning electron microscopy (SEM) of seven openpit deposits (Avião, Carvoeiro, Vânio, Preguiça, Divisa, Severino, Feijão) showed a sedimentological similarity between the deposits. They are lensshaped and are characterized at the bottom by sand facies, in the middle by spiculesrich muddysand facies and at the top by organic matterrich muddysand facies. Petrographically, the spongillitehosting sediments and the siliciclastic sediments of the Areado Group show detrital phases with similar mineralogical and textural features, such as the presence of wellsorted quartz grains and surface features of abrasion typical of Aeolian reworking that occurred in the depositional environment in which the sandstone of the Areado Group were formed. Detrital heavy minerals, such as staurolite, zircon, tourmaline, and clay minerals, such as xxiv kaolinite, low amounts of illite, scarce chlorite and muxedlayer chlorite/smectite and illite/smectite occur in the spongilitehosting sediments and in sandstones from the Areado Group. These mineralogical and textural features show that the sediments of the Areado Group constitute the main source of the pond sediments that host spongillite. The examination of the spicules by light microscopy showed spongofacies of six sponges typical of Brazilian spongillite deposits: Metania spinata (Carter, 1881), Dosilia pidanieli Volkmer Ribeiro, 1992, Trochospongilla variabilis Bonetto &Ezcurra de Drago, 1973, Corvomeyenia thumi (Traxler, 1895), Heterorotula fistula sp. n. VolkmerRibeiro &Motta (1995) e Radiospongilla amazonensis VolkmerRibeiro &Maciel 1983. The variation in composition of the assemblages and the stage of formation and preservation of the spicules in the different facies, as well as the presence of diatoms and abiotic components, suggest variation at the time of their formation and deposition. C14 radiocarbon and stable isotope ratio δ13 C analysis showed an age for the formation of the deposits between the Late Pleistocene and Holocene. The spongillite deposits show percentages between 3 to 80% of amorphous silica (opal) mainly represented by spicules. The morphological/grain size patterns of spicules as well as the associated minerals point towards a quality difference in the spongillites formed, reflecting the need for physical and mineralogical analysis in order to qualify industrially these differences. Particularly for the Preguiça and Avião deposits the intermediate facies showed the highest content of robust spicules characterizing prime quality spongillites. The grain size variations of the spicules (240 a 450gm) observed in these facies were negligible for their industrial application

ASSUNTO(S)

geologia paleontologia quaternário cerrados minas gerais

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