Geologia Eclesiástica no triângulo histórico paulistano: ... ARTIGO Terræ Climate influence on the construction of a Proterozoic eolian sand sheet (Bandeirinha Formation, Minas Gerais, Brazil) FÁBIO SIMPLICIO1, GIORGIO BASILICI2 FRANCISCO ROGÉRIO DE ABREU3 1-Doutorando em Geociências, Inst. Geoc., Univ. Est. Campinas, Campinas, SP. E-mail: [email protected] 2-Associate Prof., Inst. Geoc., Univ. Est. Campinas, Campinas, SP. E-mail: basilici@ige unicamp.br 3-Consultor, Rua Teresa Mota Valadares 503/Apto 502. CEP 30575-160. Belo Horizonte - MG. Manuscrito: Abstract : Eolian sand sheet are flattened areas with sandy deposition of arid and semi-arid Recebido: 20/06/2015 environments, characterized by dunes without avalanching face. Low availability of sand is Corrigido: 16/09/2015 considered the main factor of generation of sand sheets. A detailed facies analysis of six Aceito: 15/10/2015 measured stratigraphic sections permitted to interpret the Bandeirinha Formation, a Proterozoic siliciclastic unit of the Espinhaço Range, as ancient eolian sand sheet. This formation, 250 m Citation: Simplicio F., Basilici thick, is composed of sandstone interbedded with three sandstone conglomerate beds. The G., Abreu F.R.de. 2013. Climate sandstone portion corresponds to an eolian sand sheet paleoenvironment, which is formed of influence on the construction of a sets with planar parallel laminations deposited by climbing wind ripples, interpreted as zibar Proterozoic eolian sand sheet (Ban- dunes. The sandstone conglomerate beds were deposited by high-concentrated subaqueous deirinha Formation, Minas Gerais, flows, formed within ephemeral braided river channels. The absence of typical features of Brazil). Terræ, 10(1-2):55-65. alluvial fans and the composition of the conglomerate clasts (constituted of sandstone intra- Keywords: Eolian sand sheet, clasts cannibalized from the eolian sand sheet) testify the absence of mountain structures at zibar, ephemeral channel rivers, catchment area of the fluvial system. Present and ancient desert systems record cyclic climate Proterozoic, Espinhaço Supergroup. variations that cause the alternation of eolian and fluvial sedimentary processes. Thus, the generation of sandstone conglomerate is more probably due to an increase of precipitation than attributed to tectonics pulses. Introduction with zibar dunes, and constitutes the oldest unit of the Proterozoic sedimentary succession of the Eolian sand sheets are depositional systems Espinhaço Supergroup. The boundaries with the characterized by dunes without slipface, like nab- upper unit, the paleodepositional interpretation, kha or zibar dunes. These may occur in hot or cold and the controlling factor of the sedimentation are regions, in arid or semi-arid climate environments not yet clear (Martins-Neto 1994, Silva 1998, Mar- (Kocurek 1996, Mountney and Russell 2004). The tins-Neto 2000, Lopez-Silva and Knauer 2011). absence of dunes with slipface is caused by the poor The aim of this paper is to present the results of a availability of sand for transport by wind, due to detailed facies analysis of the Bandeirinha Forma- the following controlling factors: vegetation, water tion to producing an interpretation of the deposi- table near or intersecting the depositional surface, tional processes and paleoenvironments and their relative abundance of coarse-grained sand or gran- implication for paleoclimate, paleogeographic, and ules, sheltered surface for cementation or gravel paleotectonic reconstructions. lag, and seasonal flooding (Kocurek and Nielson 1986, Basilici and Dal Bó 2014). Although there are many examples of Precambrian eolian deposits, Geological Setting few examples of Precambrian eolian sand sheets are The subject of this study is the Bandeirinha known (Eriksson and Simpson 1998, Clemmensen Formation, a Proterozoic clastic unit exposed in and Dam 1993), and still fewer studies of ancient southern portion of Espinhaço Range, nearby the zibar dunes were published (Nielson and Kocurek town of Diamantina, Minas Gerais (Fig. 1A). This 1986, Biswas 2005). The Bandeirinha Formation unit takes part of the Espinhaço Supergroup, thick, in this paper is interpreted as an eolian sand sheet which is constituted of sandstone, conglomer- 56 ISSN 1679-2297 TERRÆ 11-3,2015 ate, mudstone and igneous rocks, showing a low grade of metamorphism (Chemale et al. 2012). The Espinhaço Supergroup is divided into two groups: Diamantina and Conselheiro Mata (Dussin FÁBIO SIMPLICIO1, GIORGIO BASILICI2 FRANCISCO ROGÉRIO DE ABREU3 and Dussin 1995). The Ban- deirinha, São João da Cha- pada, Sopa-Brumadinho and Galho do Miguel formations constitute the Diamantina Group (Silva 1998, Mar- tins-Neto 2000, Chemale et al. 2012) and the Santa Rita, Córrego dos Borges, Córrego da Bandeira, Cór- rego Pereira, and Rio Pardo Grande formations form the Conselheiro Mata Group (Fig. 1). The initial stages of lithospheric stretching and subsequent breakup of the São Francisco-Congo craton (~1.73-1.5 Ga) are supposed Fig. 1. (A) Location of the study area and (B) draft of the litostratigraphy of the to have been the responsible Espinhaço Supergroup (modified by Santos et., 2013). mechanisms for creation of accommodation space of this sedimentary basin (Alkmin and Marshak 1998). The Bandeirinha Formation, based on U-Pb dating, was deposited between 1.7-1.8 Ga (Che- male et al. 2012). This unit is 250 m thick and it is composed of sandstone interbedded with three episodes of conglomerate, 1.5-18 m thick (Fig. 2). The Bandeirinha Formation unconform- ably overlies a sericite-quartz schist unit (Barão de Guaicuí Formation) and it is overlain by a sandstone unit with planar-lamination and cross- stratification beds (São João da Chapada Forma- tion). The stratigraphic contact with the Barão de Guaicuí Formation is partially deformed by a tectonic reactivation, probably due to the different mechanic behavior of the two forma- tions. This tectonic stress produced an internal stretching and metamorphism at the base of the Bandeirinha Formation, which progressively decreases from the base to disappear in 10-15 Fig. 2. Stratigraphic section of the m of section. The transition to the São João da Bandeirinha Formation. The upper Chapada Formation is considered as an angular boundary corresponds with the unconformity (Martins-Neto 1994, Silva 1998, increase of cross-stratification Martins-Neto 2000). However, field data do not suggesting a change of the permit to clearly visualize this unconformity, depositional system. TERRÆ 11-3,2015 ISSN 1679-2297 57 Geologia Eclesiástica no triângulo histórico paulistano: ... which could be confuse with the original dipping of which were used to examine in detail the texture the sandstone planar lamination of the Bandeirinha and sedimentary structures of the sediments and Formation. The strata dipping of the Bandeirinha microtexture of the grains. The low grade meta- Formation is not easy to define in field, due to the morphism of the Bandeirinha Formation did not original dipping of the planar laminations of 0-15º. affect the recognition of the sedimentary struc- However, statistical data allow defining a dipping tures and the application of the sedimentary facies N75-90/25-35. Silva (1998) interpreted the sedi- analysis method. Thus, the terminology here used mentary succession of this unit formed in coastal is typical and related to sedimentary rocks. and river systems, where the interbedded conglom- erate beds represent fluvial mass flow deposits in Architectural Elements alluvial fans. Only at the top of the formation this author described eolian deposits. Silva (1998) and Three architectural elements were identified in Martins-Neto (1998) attributed the deposition of Bandeirinha Formation: translatent climbing ripple the conglomerates to tectonics pulses of regional sandstone, cross-stratified fine-grained sandstone range, and divided the Bandeirinha Formation in and sandstone conglomerate. the three tectono-sequences (Basal, Olaria and Natureza). The depositional interpretation of the Translatent climbing ripple sandstone Bandeirinha Formation expressed in this paper is This architectural element is composed of substantially different from the previous interpreta- moderately- to well-sorted, fine- to coarse-grained tions, as also the explanation on the forcing factors sandstone. The grains of sand are rounded or well- that originated the conglomerate beds. rounded and are constituted for 95% of quartz. The grain-size distribution is bimodal, showing Methods two modes in fine and medium-coarse grain-size. The research activity has focused in the type- The sandstone is organized in low-angle planar area where the Bandeirinha Formation was defined. parallel laminations, dipping 0-15º. The laminae In this site, a complete stratigraphic section, more are 2-12 mm thick and sometimes are character- than 250 m thick, from the metamorphic bedrock ized by a rough inverse gradation due to the pres- was measured. Other five stratigraphic sections, ence of coarse-grained sand on the upper portion 5-25 m thick, were measured in nearby areas with of the lamina. In outcrop, these structures are the purpose to verify lateral variation of the lithofa- similar to the pin stripe lamination of Fryberger cies. In all the cases, the measurement of the section and Schenk (1988). The laminae are organized in was joined to a detailed facies analysis. The lithofa- sets, 0.4-1.2 m thick and laterally extended 4->9 cies and architectural elements were described and m; their