Geologic Conceptual Model of the Municipality of Sete Lagoas (MG, Brazil) and the Surroundings
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Anais da Academia Brasileira de Ciências (2016) 88(1): 35-53 (Annals of the Brazilian Academy of Sciences) Printed version ISSN 0001-3765 / Online version ISSN 1678-2690 http://dx.doi.org/10.1590/0001-3765201520140400 www.scielo.br/aabc Geologic conceptual model of the municipality of Sete Lagoas (MG, Brazil) and the surroundings PAULO GALVÃO1, RICARDO HIRATA1, ARNALDO CORDEIRO2, DANIELA BARBATI2 and JORGE PEÑARANDA3 1Universidade de São Paulo, Instituto de Geociências, Centro de Pesquisas de Águas Subterrâneas/CEPAS, Rua do Lago, 562, 05508-080 São Paulo, SP, Brasil 2Servmar Ambiental e Engenharia, Av. Ceci, 2206, 04065-004 São Paulo, SP, Brasil 3Universidade de São Paulo, Instituto de Geociências, Laboratório de Modelos Físicos/ LAMO, Rua do Lago, 562, 05508-080 São Paulo, SP, Brasil Manuscript received on August 5, 2014; accepted for publication on May 20, 2015 ABSTRACT The study area is located in the state of Minas Gerais, Brazil, among the municipalities of Pedro Leopoldo, Matozinhos, and Sete Lagoas, with Velhas River as the eastern boundary. It is located in the São Francisco Craton, where carbonated argillo-arenaceous sediments are emplaced giving origin to the Bambuí Group, in the São Francisco Basin. Despite the geological knowledge previously developed, the region needs work on integration and detailing of such information. For this reason, the main objective was to contribute to the quality of the geologic cartography, the spatial distribution, and the structural framework geometry. Thus, geologic mapping, aerial photography interpretation, and evaluation of 270 lithologic well profi les were carried out. It was possible to establish a new geologic perspective of the region by obtaining the detailed geologic map of the municipality of Sete Lagoas, 14 geologic cross sections, and a geologic conceptual model. The study showed that the area is within a basin border, presenting a geometry conditioned by horst and graben system controlled by faulting. This structural feature displaced stratigraphic sequences positioning them side by side with lithologic sequences with different ages. Key words: conceptual model, formations geometry, geology, stratigraphy. INTRODUCTION deposited in an epicontinental shelf in the São Francisco Basin. The geologic particularities The study area is located in the state of Minas Gerais, provide important economic resources to the region, Brazil, among the municipalities of Pedro Leopoldo, which have driven the economic development and Matozinhos, and Sete Lagoas, with Velhas River population increase with the municipality of Sete as the eastern boundary (Fig. 1). Geologically, Lagoas at the forefront. There is a considerable this area is located in the São Francisco Craton, industrial, mineral, and metallurgical center in where carbonated argillo-arenaceous sediments this region situated mainly along the main federal are emplaced giving origin to the Bambuí Group, highway, which is a corridor of signifi cant growth. Correspondence to: Paulo Henrique Galvão The previous studies about the Bambuí Group E-mail: [email protected] were carried out by Branco and Costa (1961), An Acad Bras Cienc (2016) 88 (1) 36 PAULO GALVÃO, RICARDO HIRATA, ARNALDO CORDEIRO, DANIELA BARBATI and JORGE PEÑARANDA who proposed the first stratigraphic column of cipality of Sete Lagoas it is estimated to 600 m the region, later modified by Oliveira (1967), thick on average (Pedrosa-Soares et al. 1994). Schöll and Fogaça (1979), Dardenne (1978) and, The sediments of the Bambuí Group were fi nally, by Ribeiro et al. (2003). In addition, other deposited in a low gradient stable epicontinental important studies related to structural (Magalhães shelf in shallow water conditions, which 1989), geochronological (Babinski and Kaufman explains a regional occurrence of the lithofacies. 2003), hydrogeological (Pessoa 1996, 2005), and (Schobbenhaus 1984). Stratigraphically, there geological evolution (Nobre-Lopes 1995, Uhlein were three regressive sedimentary mega-cycles: et al. 2004, Tuller et al. 2009) were carried out. 1) carbonated (Sete Lagoas Formation); 2) argillo- Despite the geologic knowledge being well carbonated (Serra de Santa Helena and Lagoa developed, the area needs work on integration do Jacaré formations); and 3) argillo-arenaceous and detailing of the geologic information. Thus, (Serra da Saudade and Três Marias formations). the primary goal of this paper was to contribute The Bambuí Group is represented by the to the quality of the geologic cartography, as well following formations, from the base to the as the spatial arrangement, and the structural top: Carrancas (diamictites, sandstones, and framework of the lithologies found in this region. rhythmites); Sete Lagoas (carbonate sequence); Aerial photography, geologic mapping, and the Serra de Santa Helena (siltstones, phyllites, slate, evaluation of 270 lithological well profi les located arkose, and marlstones), Lagoa do Jacaré (oolitic in Sete Lagoas and in the adjacent municipalities limestones); Serra da Saudade (siltstones, sands, were integrated with the available data to provide a and sandstones); and Três Marias (fl uvial platform new geologic perspective of the region. As a result, sandstones) (Tuller et al. 2009). the update to the geologic map of the municipality The basement is represented by gneissic rocks of Sete Lagoas, 14 geologic cross sections, and a associated with granitoids and migmatite zones geologic conceptual model were created. from the Belo Horizonte Complex. Overlying the Bambuí Group, are the unconsolidated sediments GEOLOGICAL SETTING from Cenozoic surface coverage (Tuller et al. 2009). The Bambuí Group is represented by the The study area (Fig. 1) overlies the São Francisco following formations, associated with mega-cycles Craton (Almeida 1977), which is represented by 1 and 2 (Fig. 1): crystalline rocks from the Paleoproterozoic age • Sete Lagoas Formation (Dardenne 1978): formed by the agglutination of different craton overlies the Carrancas Formation in some blocks from the Archean age. Extension events areas, or may have direct contact with the occurred between 900 and 600 Ma. (Condie basement. The ages range from 740±22 2002) allowing Neoproterozoic silicate-carbonate Ma. old, based on data from Pb-Pb analysis sediments from the Bambuí Group to be deposited. (Babinski et al. 2007), to 630 Ma., by C-O-Sr Regionally, the Bambuí Group is about 300,000 data (Caxito et al. 2012), or 540 Ma., through 2 km (Almeida 1977), including mainly the states paleontology investigations (Warren et al. of Minas Gerais, Goiás, and Bahia. The basement 2014). Three carbonate depositional systems being controlled by a faulting system causing the are recognized (Lima 1997): 1) internal thickness of the sediments to be variable (Misi et ramp; 2) intermediary ramp; and 3) external al. 2007). According to seismic surveys conducted ramp deposits; characterized by a shallowing by Petrobras S.A., the central area of this group can upward cycle. This formation is divided and have a thickness of 1,000 m, while in the muni- characterized by two members (Ribeiro et al. An Acad Bras Cienc (2016) 88 (1) GEOLOGICAL MODEL OF SETE LAGOAS AND SURROUNDINGS 37 Figure 1 - Location map of the study area and the stratigraphic column showing the lithologies found and its respective average thicknesses (modifi ed from Tuller et al. 2009). The black continuous line in the center limits the municipality of Sete Lagoas. 2003): Pedro Leopoldo, at the base, composed Regarding the structural geology, the main of fi ne grained white limestones, dolomites, planar structures are faults, foliations and fractures, marlstones, and pelites; and Lagoa Santa, on consisting of two types of faults (Danderfer Filho the top, by medium grained black limestones; 1991, Tuller et al. 2009): normal and thrust, • Serra de Santa Helena Formation (Branco and trending NNW-SSE. Kinematic indicators, such as Costa 1961): having an area greater than 7,000 stretching and mineral lineations, indicate tectonic km² and represented by three stratigraphic transport from east to west, forming interstratal members (Grossi-Sad et al. 1998): 1) lower, ramp thrust belts, accommodated in more argillous characterized by silt-clay rhythmites, generally upper planes. The fi rst generation of the structures is carbonaceous, and dark shales (about 90 m composed of normal synsedimentary faults. Based thickness); 2) medium, composed of marble on structural surveys, the Bambuí Group is divided and shales (50 m thick); and 3) upper, into four structural domains, according to the consisting of shales, siltstones, slate cleavage, magnitude of the deformation (Tuller et al. 2009): and cross bedding (around 140 m thick). 1) high intensity of deformation domain, where An Acad Bras Cienc (2016) 88 (1) 38 PAULO GALVÃO, RICARDO HIRATA, ARNALDO CORDEIRO, DANIELA BARBATI and JORGE PEÑARANDA rocks from the Espinhaço Supergroup outcrop. The (Shuttle Radar Topography Mission), Sheet SE-23- presence of mass transport commonly indicates Z-C, acquired from the EMBRAPA’s website were movement from east to west; 2) intermediate evaluated to defi ne the lithologic contacts (http:// domain, characterized by less intense deformation, www.relevobr.cnpm.embrapa.br). more deformed to the east and less deformed to the Information about lithologic well logs was west; 3) domain where the Bambuí Group rocks taken from the SIAGAS database (http://siagasweb. are presented less deformed to not deformed; and cprm.gov.br). A series of 218 lithologic well