UNIVERSIDADE ESTADUAL DE CAMPINAS Instituto De Geociências

UNIVERSIDADE ESTADUAL DE CAMPINAS Instituto de Geociências

FÁBIO SIMPLICIO

SISTEMAS DESÉRTICOS ANTIGOS: DISTRIBUIÇÃO DAS FÁCIES E SEQUÊNCIAS SEDIMENTARES (FORMAÇÃO BANDEIRINHA - MESOPROTEROZOICO E O GRUPO AREADO - CRETÁCEO INFERIOR)

CAMPINAS 2017

FÁBIO SIMPLICIO

SISTEMAS DESÉRTICOS ANTIGOS: DISTRIBUIÇÃO DAS FÁCIES E SEQUÊNCIAS SEDIMENTARES (FORMAÇÃO BANDEIRINHA - MESOPROTEROZOICO E O GRUPO AREADO - CRETÁCEO INFERIOR)

TESE DE DOUTORADO APRESENTADA AO INSTITUTO DE GEOCIÊNCIAS DA UNIVERSIDADE ESTADUAL DE CAMPINAS PARA OBTENÇÃO DO TÍTULO DE DOUTOR EM CIÊNCIAS, ÁREA DE GEOLOGIA E RECURSOS NATURAIS.

ORIENTADOR: PROF. DR. GIORGIO BASILICI CO-ORIENTADOR: PROF. DR. TAPAN CHAKRABORTY

ESTE EXEMPLAR CORRESPONDE À VERSÃO FINAL DA TESE DEFENDIDA PELO ALUNO FÁBIO SIMPLICIO E ORIENTADO PELO PROF. DR. GIORGIO BASILICI

CAMPINAS 2017 Agência(s) de fomento e nº(s) de processo(s): CNPq, 4742272013-8 ORCID: http://orcid.org/0000-0002-9391-122X

Ficha catalográfica Universidade Estadual de Campinas Biblioteca do Instituto de Geociências Cássia Raquel da Silva - CRB 8/5752

Simplicio, Fábio, 1985- Si57s SimSistemas desérticos antigos : distribuição das fácies e sequências sedimentares (Formação Bandeirinha - Mesoproterozoico e o Grupo Areado - Cretáceo Inferior) / Fábio Simplicio. – Campinas, SP : [s.n.], 2017.

SimOrientador: Giorgio Basilici. SimCoorientador: Tapan Chakraborty. SimTese (doutorado) – Universidade Estadual de Campinas, Instituto de Geociências.

Sim1. Sedimentos e depósitos. 2. Sedimentação eólica. 3. Bacias sedimentares. I. Basilici, Giorgio,1959-. II. Chakraborty, Tapan. III. Universidade Estadual de Campinas. Instituto de Geociências. IV. Título.

Informações para Biblioteca Digital

Título em outro idioma: Ancient desert systems : facies distribution and sedimentary sequences (Bandeirinha Formation - Mesoproterozoic and the Areado Group - Early Cretaceous) Palavras-chave em inglês: Sediments and deposits Eolian sedimentation Sedimentary basins Área de concentração: Geologia e Recursos Naturais Titulação: Doutor em Geociências Banca examinadora: Giorgio Basilici [Orientador] Geraldo Norberto Chaves Sgarbi Patrick Francisco Führ Dal' Bó Alexandre Campane Vidal Celso Dal Ré Carneiro Data de defesa: 21-08-2017 Programa de Pós-Graduação: Geociências

AUTOR: Fábio Simplicio

SISTEMAS DESÉRTICOS ANTIGOS: DISTRIBUIÇÃO DAS FÁCIES E SEQUÊNCIAS SEDIMENTARES (FORMAÇÃO BANDEIRINHA - MESOPROTEROZOICO E O GRUPO AREADO - CRETÁCEO INFERIOR)

ORIENTADOR: Prof. Dr. Giorgio Basilici COORIENTADOR: Prof. Dr. Tapan Chakraborty

Aprovado em: 21 / 08 / 2017

EXAMINADORES:

Prof. Dr. Giorgio Basilici - Presidente

Prof. Dr. Geraldo Norberto Chaves Sgarbi

Prof. Dr. Patrick Francisco Führ Dal’ Bó

Prof. Dr. Alexandre Campane Vidal

Prof. Dr. Celso Dal Ré Carneiro

A Ata de Defesa assinada pelos membros da Comissão Examinadora, consta no processo de vida acadêmica do aluno.

Campinas, 21 de agosto de 2017.

BIOGRAFIA

Fábio Simplicio é Bacharel em Geologia, formado pela Universidade Estadual de Campinas - Unicamp no ano de 2011. Na mesma Universidade posteriormente obteve o título de Mestre Geociências, no ano de 2013 e posteriormente de Doutor na mesma área de Geologia e Recursos Naturais, no ano de 2017. O autor tem experiência na área de Geociências, onde atua principalmente com análise de sistemas deposicionais de ambientes desérticos constituídos em bacias sedimentares antigas, do pre-cambriano e fanerozoico. Durante o período de trabalho da pesquisa de doutorado, o autor da tese participou de inúmeras atividades didáticas, em estágios de capacitação docente, nas disciplinas de sedimentologia, estratigrafia e geologia de campo I (mapeamento de rochas sedimentares). Além dessas atividades, Fábio Simplicio teve experiências acadêmicas na Índia, onde realizou uma conferência sobre os estudos na Formação Bandeirinha, participou de um curso de sobre análise de paleossolos e também atividades de pesquisa em campo, sobre depósitos eólicos e fluviais.

Dedico esta tese a quem nunca me faltou durante este período, a minha esposa Mayara R. Quilicone por seu amor e afeto. Também dedico aos meus pais, Sueli S. B. Simplicio e Adilson Ap. Simplicio, por todo suporte e empenho a mim dedicado, bem como aos meus irmãos, Renato e Caroline Simplicio.

AGRADECIMENTO

Agradeço a todas as pessoas que contribuíram direta ou indiretamente para a realização desse trabalho. Agradeço em primeiro lugar ao meu orientador Sir Giorgio Basilici, pois me acolheu e aconselhou ao longo de todos esses anos, e a quem devo muito da minha formação. Também agradeço ao professor Tapan Chakraborty, pelos valorosos conselhos que me dirigiu, sobretudo durante as atividades de campo na Índia e no Brasil. Deixo meu agradecimento aos Professores e Doutores Geraldo N. C. Sgarbi, Celso Dal Ré Carneiro, Alexandre Campane Vidal e Patrick Francisco Führ Dal’ Bó, por aceitarem os convites de participação como membros da banca de qualificação e defesa dessa tese de doutorado. Os apontamento e conselhos ajudaram a melhorar a tese e, sobretudo os artigos. Agradeço as agências de fomento. A Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) pela bolsa de estudos, ao Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) pelo auxilio financeiro concedido para parte da pesquisa. A PRP/FAEPEX da Unicamp, pelo subsídio a participação de evento científico e também a International Association of Sedimentologists (IAS) pela concessão de Travel Grant direcionado a participação em evento internacional. Também deixo meu obrigado, do fundo do meu coração, a minha amada Mayara Ribeiro Quilicone, pois sem seu apoio penso que teria sido difícil concluir essa etapa com sucesso. Nesse sentido, meu agradecimento se estende aos meus familiares mais próximos, aos meus pais Sueli S. B. Simplicio e Adilson Ap. Simplicio, pois sempre me apoiaram quanto as minhas decisões e toleraram todo estresse e mau humor distribuído com eles. Agradeço também aos meus irmãos Caroline e Renato Simplicio, aos meus cunhados e cunhadas, Natalia, Laura, Thiago e Arthur. A minha sogra, Ivani da Graça Ribeiro, pelo apoio e afeição, também agradeço. Agradeço imensamente. Não deixaria de fora meus colegas e amigos Mestres José H. Mattos, Rogério Araújo, Paulo Locatelli, Gustavo Melo, Luiz R. M. Fernandes, Emerson Oliveira, Thiago Araújo e Bruno Mortatti, e aos Doutores Danilo Barbuena e Patrick Dal`Bó, pela amizade e discussões enriquecedoras. Por ultimo, mas não menos importante, agradeço a todos os funcionários, especialmente Valdirene, Gorete, Cristina, Jô e Valdir, bem como a outras pessoas que compõem a comunidade do Instituto de Geociências da Unicamp, pelo apoio. A todos meus sinceros agradecimentos.

Triste de quem vive em casa, Contente com seu lar, Sem que um sonho, no erguer de asa, Faça até mais rubra a brasa Da lareira a abandonar

Fernando Pessoa

RESUMO

Os sistemas desérticos atuais são controlados por diversos fatores, dentre os quais se destaca o grau de aridez, razão entre entrada e saída de água. Os parâmetros que diferenciam os ambientes deposicionais são intrinsicamente ligados às variações congruentes a essa razão. Controlados por tectônica e/ou clima, os parâmetros são conexos às proporções de sedimentos clásticos, água e solutos dissolvidos. Os ambientes se particularizam pelos tipos de associações a que estejam envolvidos. Associados a um mesmo ambiente, podem existir leques aluviais, rios efêmeros ou perenes, lagos salinos ou de água doce, playa lakes, lençóis de areia eólica e/ou campos de dunas eólicas. A fim de compreender quais foram e de que modo atuaram os principais fatores de controle nos paleoambientes desérticos, foram selecionadas duas unidades diferentes para estudo sedimentológico, a Formação Bandeirinha e o Grupo Areado. A Formação Bandeirinha, base da Bacia Espinhaço, é datada do inicio do mesoproterozoico, e consiste em uma sucessão sedimentar composta por arenitos depositados em lençóis de areia eólica e conglomerados depositados em rios efêmeros, nos quais os eventos de enxurradas eram geralmente catastróficos e ocorriam em intervalos de deposição distintos daqueles de deposição eólica. Os fluxos de alta energia provocaram a canibalização de parte do substrato arenoso, o qual se encontrava previamente cimentado, como sugerido pelos principais registros desse processo, que consistem em intraclastos de arenitos com estruturas eólicas preservadas. Logo, entende-se que os processos que produziram os depósitos dessa unidade foram induzidos por mudanças no balanço hídrico, em intervalos de tempo distintos. O Grupo Areado, base da Bacia Sanfranciscana, é datado do Cretáceo Inferior, e consiste em uma sequência composta por depósitos de ambiente lacustre, de playa lake (i.e., planícies lamosas salinas e eventuais depósitos subaquáticos e/ou eólicos intercalados) e de campos de dunas eólicas, os quais representam o final do processo de evolução observado na sequência sedimentar. Ao longo do intervalo de deposição dessa sequência sedimentar inúmeras variações no aporte de água levaram a diversas modificações nos ambientes, que tiveram intervalos cujo balanço hídrico foi positivo, que permitiu a formação e permanência de sistemas lacustres; variável, que levou a formação de playa lakes, nos quais se alternavam períodos de exposição subaérea, com ampla distribuição de crostas salinas eflorescentes, e inundações efêmeras; e negativo, que levou ao aumento da disponibilidade de sedimentos para construção de campos de dunas eólicas. Em conclusão, a principal relação que se faz entre os depósitos estudados, da Formação Bandeirinha e do Grupo Areado, é que em ambos os casos, os processos sedimentares foram essencialmente controlados por variações no grau de aridez, no balanço hídrico. Além disso, também associado às variações no balanço hídrico, ambas as unidades têm registros de processos de cimentação por precipitação de evaporitos, que constituem evidências de clima predominantemente quente e seco, nos quais os ambientes se desenvolveram.

Palavras-chave: Ambiente desértico, Lençol de areia eólica, Playa lake, Formação Bandeirinha, Grupo Areado.

ABSTRACT

Present-day desert systems are controlled by different factors, in which the aridity is emphasized, the water input and output ratio. The parameters that differentiate depositional environments are related to the variations of this ratio. Controlled by tectonics and/or climate, the parameters are related to the proportions of clastic sediments, water and dissolved solutes. The environments are particularized by the types of associations that are involved. In the same environment may occur alluvial fans, ephemeral or perennial rivers, saline or freshwater lakes, playa lakes, eolian sand sheets and/or eolian dune fields. To understand the different factors that controlled and how they had operated in the desert paleoenvironments, were chosen two different units to sedimentological study, the Bandeirinha Formation and the Areado Group. The Bandeirinha Formation, basal unit of the Espinhaço Basin (Mesoproterozoic), consists of a sedimentary succession composed by sandstones deposited in aeolian sand sheet and conglomerates deposited in ephemeral rivers, in which the flood events were catastrophic and occurs in different intervals of eolian deposition. The high energy flows produced the cannibalization of part of the sandy substrate, which was previously cemented, such as suggested by the main records of this process, the sandstone intraclasts with eolian structures. Thus, it is understood that the depositional processes related to this unit were induced by changes in the water balance, at different intervals. The Areado Group, basal unit of the Sanfranciscana Basin (Early Cretaceous), consists in a sedimentary sequence composed of deposits from lacustrine environments, playa lake (i.e., saline mudflats and eventual subaqueous and/or eolian intercalated) and eolian dune fields, which represent the final process of evaluation observed in the sedimentary sequence. During the interval of deposition of the sedimentary sequence, numerous water input variations produced several changes in the environment, in which intervals of positive water balance allowed the formation and permanence of lacustrine systems; variable, which led to the formation of playa lakes where periods of subaerial exposure, of wide distribution of efflorescent salt crusts, alternated with ephemeral floods; and negative, which led to the increase of sediment availability to eolian dune field construction. In conclusion, the main common factor of the studied deposits, Bandeirinha Formation and Areado Group, is that the sedimentary processes were controlled by variations of aridity, in the water balance. Furthermore, also associated with the water balance, in both units there records of cementation processes by evaporite precipitation, which consists in evidence of hot and dry climate, in which the environments were developed.

Keywords: Desert environment, Aeolian sand sheet, Playa lake, Bandeirinha Formation, Areado Group.

LISTA DE ILUSTRAÇÕES

Figura 1. Localização das áreas de estudo e principais rodovias de acesso. A) Nas imagens dá- se destaque às áreas de estudo nas cidades de Diamantina e Presidente Olegário em relação a Capital do Estado de Minas Gerais, Belo Horizonte. B) A área de estudo (azul) está localizada na cidade de Diamantina, que fica próxima a Gouveia e Datas. C) A área de estudo (azul) em destaque fica na cidade de Presidente Olegário, no distrito de Galena, próximo à cidade de Varjão de Minas...... 19 Figura 2. Expressão morfológica da Serra do Espinhaço Meridional vista em imagem de satélite...... 24 Figura 3. Distribuição geográfica da Bacia Espinhaço, com destaque aos limites do Cráton do São Francisco e Faixa Móvel Araçuaí (Almeida 1977). Imagem modificada de Chaves & Brandão (2004)...... 26 Figura 4. Mapa geológico da área de estudo em Diamantina (MG), na qual se observa a distribuição das unidades depositadas na Bacia Espinhaço, com destaque a Formação Bandeirinha...... 28 Figura 5. Distribuição dos depósitos da parte sul da Bacia Sanfranciscana (sub-bacia Abaeté) e a relação com o Arco do Alto Paranaíba. Observe na figura a direita a estruturação geral da parte Sul da Bacia Sanfranciscana e sua relação com o arco do Alto do Paranaíba, fonte de sedimentos (Sgarbi et al. 2001). Modificado de Sgarbi & Dardenne (1996) e Alkmim & Martins-Neto (2001)...... 30 Figura 6. Mapa geológico da área de estudo, na cidade de Presidente Olegário (MG). A) Distribuição geográfica das unidades da Bacia Sanfranciscana, dividida em Sub-bacia Abaeté (Sul) e Sub-bacia Urucuia (Norte). B) Mapa Geológico da área estudada, com ênfase nos diferentes tipos de depósitos analisados...... 32 Figura 7. Bacia de drenagem fechada localizada no Saline Valley, California, EUA. Na ilustração são destacados os subambientes mais comuns, que se desenvolvem em condições de clima árido dentro de uma bacia de drenagem fechada. Modificado de Hardie et al. (1978)...... 36

SUMÁRIO

1 INTRODUÇÃO ...... 14

1.1 Justificativa ...... 15 1.2 Objetivos ...... 16 1.3 Organização da Tese ...... 16

2 LOCALIZAÇÃO E ACESSO ÀS ÁREAS DE PESQUISA ...... 18 3 MÉTODOS ...... 20

3.1 Etapa de organização e preparação ...... 20 3.2 Etapa de aquisição de dados ...... 21 3.2.1 Aquisição de dados em campo ...... 21 3.2.2 Aquisição de dados laboratoriais ...... 21

4 GEOLOGIA DAS ÁREAS DE ESTUDO ...... 23

4.1 A Bacia Espinhaço em Minas Gerais ...... 23 4.1.1 Aspectos litoestratigráficos na região de Diamantina ...... 25 4.1.2 O Grupo Diamantina ...... 27

4.2 Bacia Sanfranciscana em Minas Gerais ...... 29 4.2.1 O Arco do Alto Paranaíba e a evolução da sub-bacia Abaeté ...... 29 4.2.2 Litoestratigrafia da sub-bacia Abaeté ...... 31

5 REGISTROS SEDIMENTARES DE ANÁLOGOS ATUAIS ...... 33

5.1 Os principais análogos atuais ...... 33

6 PRODUTOS DA PESQUISA: OS ARTIGOS ORIGINAIS ...... 39

O apêndice I ...... 39 Título do artigo: Climate influence on the construction of a Proterozoic eolian sand sheet (Bandeirinha Formation, Minas Gerais, Brazil). (Publicado na revista Terrae) ...... 39 O apêndice II ...... 40

Título do artigo: Unusual thick eolian sand sheet sedimentary succession: Paleoproterozoic Bandeirinha Formation, Minas Gerais. (Publicado na revista Brazilian Journal of Geology) ...... 40 O apêndice III ...... 41 Título do artigo: Temporal evolution of an Early Cretaceous Playa Lake: the sedimentary record of Quiricó and Três Barras Formation (Sanfranciscana Basin, south-eastern Brazil). (Submetido à revista Terrae) ...... 41 O apêndice IV ...... 42 Título do artigo: The role of groundwater variation on a drying upward sedimentary sequence (Sanfranciscana Basin, Early Creataceous). (Em preparação para submissão em revista especializada) ...... 42

7 CONCLUSÕES ...... 43 8 REFERÊNCIAS ...... 45 APÊNDICES ...... 50 APÊNDICE I ...... 51 APÊNDICE II ...... 75 APÊNDICE III ...... 101 APÊNDICE IV ...... 128

1 INTRODUÇÃO

O principal parâmetro que controla as características geomorfológicas de ambientes desérticos atuais é o clima. A aridez, medida do balanço entre entrada e saída de água do ambiente (Parsons & Abrahams 2009), exerce influência sobre diversos outros fatores de controle, como nas variações de salinidade superficial, do tipo de cobertura vegetal (desde o Siluriano - ~444 Ma), da taxa de evaporação, da variação do nível freático, dos processos fluviais e dos processos eólicos (Goudie 2009). Os ambientes de clima árido atuais podem agregar uma variedade de ambientes ou subambientes, como leques aluviais, rios perenes ou efêmeros, campos de dunas eólicas, lençóis de areia eólica, planícies lamosas, lagoas rasas salinas e outros (Mountney 2006). Nas bacias sedimentares, o aporte de água pode ocorrer por meio de fluxos em superfície, canalizados ou não, e/ou por meio de fluxos subterrâneos, ao passo que a saída de água geralmente resulta de variações nas taxas de evaporação ou evapotranspiração (Eugster & Hardie 1975, Rosen 1994). Exemplo disso, nos ambientes em que a entrada de água é ligeiramente menor do que a saída pode-se conceber um arranjo hipotético constituído por um lago salino ou playa lake margeado por sistemas eólicos e fluviais efêmeros adjacentes. De outro modo, quando a saída de água é muito maior do que a entrada espera-se que o ambiente seja dominantemente eólico, com, por exemplo, campos de dunas, lençóis de areia e raros rios efêmeros. As várias possibilidades de arranjos sequenciais, que potencialmente podem ser preservadas no registro sedimentar, refletem, em grande parte, os aspectos climáticos supracitados. A fim de melhor compreender como os vários fatores de controle influenciaram os ambientes desérticos antigos, sobretudo no que diz respeito às variações nas proporções de água, foram selecionados dois exemplos de unidades litoestratigráficas, as quais foram submetidas a análises sedimentológicas, são elas: Formação Bandeirinha e parte do Grupo Areado. Os depósitos referentes a estas duas unidades consistem em registros de ambientes submetidos a condições de clima árido. A Formação Bandeirinha (~1,8 Ga), que pertence à Bacia Espinhaço, teve seu processo de deposição iniciado entre o final do Paleoproterozoico e inicio do Mesoproterozoico (Chemale et al. 2012, Machado et al. 1989, Santos et al. 2013), período no qual ocorreu uma grande expansão dos sistemas eólicos ao redor do mundo, com representantes preservados em todos os continentes atuais (Eriksson & Simpson 1998, 15

Eriksson et al. 2005). O Grupo Areado, constituído pelas formações Quiricó e Três Barras (~145-100 Ma), pertence à Bacia Sanfranciscana, cujo processo de deposição foi iniciado no Cretáceo Inferior (Campos & Dardenne 1997b, Carvalho & Kattah 1998, Sgarbi et al. 2001). Assim como se observa a partir do final do Paleoproterozoico e inicio do Mesoproterozoico, também o Cretáceo foi marcado por uma grande expansão dos sistemas desérticos em escala global, sobretudo no Brasil, onde importantes registros estão preservados (Almeida et al. 2012, Rodríguez-Lopez et al. 2014). As rochas que constituem a Formação Bandeirinha depositaram-se sobre uma bacia de tipo rifte (Chemale et al. 2012, Santos et al. 2013). Os depósitos empilhados compõem uma sucessão sedimentar de 250 m de espessura; são arenitos e conglomerados depositados em lençol de areia eólica submetido a intervalos com fluxos fluviais efêmeros. As rochas que constituem as formações Quiricó e Três Barras foram depositadas sobre uma bacia do tipo sag (Campos & Dardenne 1997a). As rochas se sobrepõem em uma sucessão sedimentar de aproximadamente 100 m de espessura; são pelitos e arenitos de granulometria fina depositados em ambiente lacustre e de playa lake, e arenitos de sistemas fluviais e eólicos associados (Sgarbi et al. 2001).

1.1 Justificativa

Esta tese de doutorado se propõe a elaborar modelos sedimentares de ambientes áridos, com atenção voltada as mudanças na distribuição lateral e/ou vertical dos corpos sedimentares, que podem refletir mudanças ambientais provocadas por variações no balanço hídrico. Nesta pesquisa buscou-se a compreensão sobre os fatores de controle (autóctones ou alóctones) que influenciaram os processos deposicionais, os quais se refletem nas características das rochas estudadas nas unidades. Logo, este trabalho justifica-se pela contribuição que pode trazer à compreensão de como variações nas condições hidrológicas podem modificar o ambiente sedimentar, e como estas modificações podem ser refletidas no registro geológico. Além disso, embora a Formação Bandeirinha e o Grupo Areado tenham sido objeto de diversos estudos importantes de caráter litoestratigráfico, geotectônico ou paleogeográfico, ainda restam muitas questões a serem esclarecidas, sobretudo em relação aos fatores que controlaram a evolução dos sistemas eólicos, fluviais, lacustres e de playa lake, parte das quais esta pesquisa se propôs a responder. 16

1.2 Objetivos

O objetivo geral da pesquisa é compreender quais foram e de que forma atuaram os diferentes fatores de controle sobre dois sistemas desérticos antigos (Formação Bandeirinha e Grupo Areado). A fim de alcançar este objetivo principal foram definidos os seguintes objetivos específicos: i) Interpretação dos diferentes processos físicos de sedimentação por meio da análise de fácies, ou seja, das texturas, estruturas sedimentares, geometrias das camadas, superfícies limitantes, paleofluxos, e relações entre litofácies; ii) Reconhecimento dos padrões geométricos e de organização sequencial das sucessões sedimentares; iii) Inferências sobre quais foram e como atuaram os diferentes mecanismos de construção, acumulação e preservação dos sistemas eólicos em estudo (Kocurek 1999, Kocurek & Havholm 1993). iv) Verificação de semelhanças e diferenças entre os depósitos da Formação Bandeirinha e do Grupo Areado.

1.3 Organização da Tese

Os resultados obtidos no decorrer do desenvolvimento desta tese de doutorado são apresentados por meio de quatro artigos científicos. Dois artigos são dedicados aos depósitos da Formação Bandeirinha, pertencente à Bacia Espinhaço (Mesoproterozoico). Os artigos foram submetidos e publicados em periódicos nacionais no ano de 2015, na revista Terrae e na Brazilian Journal of Geology. Os outros dois artigos discorrem sobre depósitos pertencentes ao Grupo Areado, Bacia Sanfranciscana (Cretáceo Inferior). Nenhum dos artigos foi publicado, um deles foi aceito para publicação na revista Terrae e o outro está em preparação para submissão em periódico especializado. Os trabalhos são ordenados de acordo com a idade relativa da unidade e data de publicação do artigo. Logo, a apresentação dos artigos segue a seguinte ordenação: a) Influência do clima na construção de um lençol de areia eólica no Proterozoico (Formação Bandeirinha, Minas Gerais, Brasil); b) A espessura incomum de uma sucessão sedimentar de lençol de areia eólica: Formação Bandeirinha (Paleoproterozoico), Minas Gerais; c) Evolução 17

temporal de um playa lake do Cretáceo Inferior: os registros sedimentares das formações Quiricó e Três Barras (Bacia Sanfranciscana, sudeste do Brazil); d) O papel das variações no nível do lençol freático como fator controle durante o ressecamento de uma sequência sedimentar (Bacia Sanfranciscana, Cretáceo Inferior). Esta ordem de apresentação dos artigos é condizente com o ordenamento dos apêndices, que são comentados em maior detalhe no capitulo seis desta tese.

2 LOCALIZAÇÃO E ACESSO ÀS ÁREAS DE PESQUISA

As atividades de campo foram realizadas nas regiões de Diamantina e Presidente Olegário, respectivamente a norte e noroeste da cidade de Belo Horizonte (MG) (Fig. 1A). Na região de Diamantina foram estudados os depósitos da Formação Bandeirinha, unidade basal da Bacia Espinhaço (Mesoproterozoico). O acesso é oferecido por rodovias federais e estaduais. A partir de Belo Horizonte, o caminho passa pelo percurso nas rodovias BR-040/BR-135, MG-231, BR-259 e BR-367, que passa por Gouveia e Diamantina, onde fica localizada a área de estudo (Fig. 1B). Na região de Presidente Olegário os depósitos estudados pertencem ao Grupo Areado, unidade basal da Bacia Sanfranciscana (Cretáceo Inferior). O acesso, a partir de Belo Horizonte, é também feito por rodovias federais e estaduais; pelas rodovias MG-050 que faz acesso a BR-040/BR-135, por onde se deve percorrer até a saída para BR-365, próximo a localização da área de estudo, no distrito de Galena, na cidade de Presidente Olegário (Fig. 1C). 19

3 MÉTODOS

A pesquisa se desenvolveu em duas etapas, que ocorreram, em muitos casos, de modo simultâneo. A seguir são descritas as etapas de organização e preparação, e de aquisição de dados.

3.1 Etapa de organização e preparação

Esta etapa refere-se à aquisição de informações e conhecimentos necessários à preparação para os trabalhos de campo, assim como ao posterior tratamento dos dados adquiridos. Mapas geológicos e topográficos, imagens de satélite e fotografias aéreas foram adquiridos por meio de buscas a bases de dados de órgãos governamentais, nacionais e internacionais, como Instituto Brasileiro de Geografia e Estatística (IBGE), Companhia de Pesquisa de Recursos Minerais (CPRM), Banco de Dados Geográfico do Exército Brasileiro (BDGEx) e Serviço Geológico dos Estados Unidos (United States Geological Survey - USGS). Foi realizado um trabalho de compilação e elaboração sobre as informações adquiridas; as imagens foram referenciadas geograficamente, as escalas foram ajustadas, as vias destacadas e algumas áreas foram previamente escolhidas para pesquisa de campo. Este tratamento inicial foi realizado com a utilização do software ArcGIS e editores de desenhos vetoriais. As bases topográficas, do IBGE ou Exército Brasileiro, disponíveis para as áreas de estudo (Folhas Presidente Olegário e Diamantina) possuem escala de 1:100.000, que não se considera suficientemente adequada aos estudos propostos, que de fato foram conduzidos em detalhe, em áreas de pequena extensão (< 150 km2). A fim de sanar esse problema optou-se pela utilização de bases topográficas obtidas a partir do processamento de dados de elevação (DEM – Digital Elevation Model) da missão SRTM (Shuttle Radar Topography Mission). As DEM apresentam resolução espacial de até 30 m e foram adquiridas gratuitamente no site https://earthexplorer.usgs.gov/. O processamento foi realizado com auxilio do software ArcGIS, no qual as DEM foram convertidas em mapas de contornos topográficos. Para efeito de ajustes, além das bases topográficas geradas pelo processamento, também foram utilizadas imagens de satélite, estas obtidas na base de dados online do ArcGIS. 21

3.2 Etapa de aquisição de dados

Esta etapa foi dividida em duas subetapas: (1) aquisição de dados em campo e (2) aquisição de dados laboratoriais.

3.2.1 Aquisição de dados em campo

A aquisição de dados em campo seguiu o método de análise de fácies convencional, porém sem fazer uso de códigos para as fácies, uma vez que esses poderiam limitar as possibilidades de caracterização e interpretação (Bridge 1993). As seções estratigráficas foram medidas mediante o uso de bastão de Jacob (bastão de 1.5 m de altura, com gradações de 10 cm e um clinômetro para acompanhamento preciso da inclinação das camadas) em afloramentos após se distinguir as diferentes camadas. As camadas foram analisadas e discriminadas em função das características litológicas, geométricas, de estruturas e texturas sedimentares (Walker 2006). Também foram coletadas amostras para análise petrográfica, realizada com lupa binocular de mesa e com microscópio óptico de luz transmitida. A análise arquitetural tem sido feita considerando diferentes escalas, a fim de reconhecer os processos formadores das formas de leito (bedforms). As estruturas de menor escala (como lâminas) têm sido analisadas em conjunto com sua organização nas camadas; para diferenciação entre lâminas e camadas tem-se recorrido ao critério estabelecido por McKee & Weir (1953). A realização das análises se deu a partir da produção de esquemas sobre os afloramentos, feitos tanto em anotações de campo quanto sobre as fotografias, com ajuda de um editor de desenhos vetoriais, o que facilitou a análise das relações entre os diferentes estratos (Brookfield 1977, Miall 1985).

3.2.2 Aquisição de dados laboratoriais

As análises de laboratório têm sido conduzidas com objetivo de refinar as descrições macroscópicas. A fim de cumprir esta etapa foram selecionadas e coletadas 17 amostras da Formação Bandeirinha e 43 amostras do Grupo Areado, representativas para diferentes estratos individualizados. Todas as amostras foram identificadas de acordo com sua 22

posição nas seções estratigráficas. As amostras foram cortadas e polidas para descrição visual em lupa binocular de mesa. Lâminas delgadas da Formação Bandeirinha, em um total de 11, foram confeccionadas no Laboratório de Laminação do Instituto de Geociências da Unicamp e utilizadas com objetivo de obtenção de dados complementares, obtidos pela classificação dos arenitos. Entretanto, devido às modificações texturais impostas pelo metamorfismo, as características de arredondamento, tamanho e seleção dos grãos, bem como os aspectos diagenéticos (ex.: cimentação) são pobremente preservados e, portanto inviáveis para considerações de valor estatístico.

4 GEOLOGIA DAS ÁREAS DE ESTUDO

Nesta seção são apresentados os principais elementos relativos à evolução das bacias sedimentares estudadas. A Bacia Espinhaço é interpretada como uma bacia do tipo rifte continental de idade Mesoproterozoica (Almeida-Abreu & Renger 2002, Silva 1998, Chemale et al. 2012, Santos et al. 2013), cuja estimativa de espessura preservada varia entre 3.000 a 5.000 m (Chemale et al. 2012, Martins-Neto 1998). A bacia foi submetida a processo de inversão tectônica, o que modificou parcialmente os seus depósitos em função do metamorfismo (Almeida-Abreu 1995, Dussin & Dussin 1995), que mascarou, mediante processos de dissolução e recristalização, diversas características, sobretudo em escala microscópica. A Bacia Sanfranciscana é uma bacia Mesozoica, cuja deposição iniciou-se provavelmente no Cretáceo Inferior (Sgarbi & Dardenne 1996). A bacia, interpretada como do tipo sag (Campos & Dardenne 1997a), possui uma espessura preservada que varia entre 200 e 400 m (Alkmim & Martins-Neto 2001, Campos & Dardenne 1997b). Diferentemente da Bacia Espinhaço, a Bacia Sanfranciscana não foi submetida a nenhum processo de deformação expressivo e, portanto, contém registros perfeitamente preservados em termos de suas características sedimentares. Sobre o metamorfismo ao quais as rochas da Formação Bandeirinha foram submetidas, vale salientar que as características das fácies, sobretudo em termos de estruturas sedimentares e composição mineralógica, encontram-se em grande parte preservadas, o que permitiu utilizar terminologias atribuídas ao estudo de rochas sedimentares em detrimento a aquelas utilizadas para rochas metamórficas.

4.1 A Bacia Espinhaço em Minas Gerais

A sucessão sedimentar, de preenchimento da Bacia Espinhaço, é atualmente exposta na Serra do Espinhaço (Fig. 2), que possui formato alongado com eixo principal na direção Norte-Sul, e se estende por uma distância de aproximadamente 1.200 km, entre os estados de Minas Gerais e Bahia. A Bacia Espinhaço é dividida em duas partes, uma meridional e outra setentrional (Fig. 3). A área de pesquisa, onde aflora a seção tipo da Formação Bandeirinha (Silva 1995, Santos et al. 2013), situa-se na parte meridional. A região está localizada na margem sudeste do Cráton São Francisco (Almeida 1977), no Cinturão de cavalgamentos e dobramentos Araçuaí (Almeida 1977, Alkmim et al. 2006). 24

Figura 2. Expressão morfológica da Serra do Espinhaço Meridional vista em imagem de satélite.

A Bacia Espinhaço formou-se devido a um processo de estiramento crustal que levou à implantação de um sistema de rifte, com falhamentos na direção N-S (Dussin & Dussin 1995, Silva 1998); as unidades que compõem a Bacia Espinhaço estão sobrepostas a rochas do Complexo do Embasamento e do Supergrupo Rio Paraúna. As rochas que constituem o Supergrupo Espinhaço na região da cidade de Diamantina foram depositadas entre 1,8 Ga e 0,9 Ga (Machado et al. 1989, Chemale et al. 2012, Santos et al. 2013). Posteriormente, as rochas depositadas na Bacia Espinhaço foram submetidas aos processos de tectonismo e metamorfismo impostos pelo ciclo de Orogênese Brasiliana/Pan-Africana (~700- 450 Ma). A orogênese resultou na formação do supercontinente Gondwana (Alkmim & Marshak 1998).

4.1.1 Aspectos litoestratigráficos na região de Diamantina

Os depósitos da Bacia Espinhaço (~1,8 Ga) são inseridos, do ponto de vista litoestratigráfico, no Supergrupo Espinhaço (Fig. 3), unidade que recobre o Supergrupo Rio Paraúna (Fogaça et al. 1984), que por sua vez, ocorre em contato discordante sobre os granitos, migmatitos e gnaisses do Complexo do Embasamento (Scholl & Fogaça 1979). O Supergrupo Rio Paraúna é dividido nos grupos Pedro Pereira e Costa Sena. O Grupo Pedro Pereira contém rochas meta-máficas, meta-ultramáficas e meta-ácidas, além de metassedimentares de origem química (BIF’s - Banded Iron Formations). A idade de 2.9 Ga é atribuída ao Grupo Pedro Pereira (Machado et al. 1989).

O Grupo Costa Sena, 2.4 Ga (Machado et al. 1989), até a década de 1990, era subdivido nas Formações Barão de Guaicuí e Bandeirinha (Fogaça et al. 1984). Porém, estudos focados na Formação Bandeirinha, na região da cidade de Diamantina, levaram ao reposicionamento estratigráfico da unidade (Silva 1995), que foi posta na base do Supergrupo Espinhaço (Silva 1998, Santos et al. 2013). Neste trabalho são aceitas as considerações de Silva (1995, 1998), Martins-Neto (2000), Chemale et al. (2012), Santos et al. (2013) que posicionam a Formação Bandeirinha (~1,8 Ga) como unidade inferior do Supergrupo Espinhaço. 26

O Supergrupo Espinhaço é composto pelos Grupos Diamantina e Conselheiro Mata (Dossin et al. 1985). As unidades que compõem o Grupo Diamantina são as formações

Figura 3. Distribuição geográfica da Bacia Espinhaço, com destaque aos limites do Cráton do São Francisco e Faixa Móvel Araçuaí (Almeida 1977). Imagem modificada de Chaves & Brandão (2004). 27

Bandeirinha, São João da Chapada, Sopa-Brumadinho e Galho do Miguel, enquanto que o Grupo Conselheiro Mata reúne as formações Santa Rita, Córrego dos Borges, Córrego Bandeira, Córrego Pereira e Rio Pardo Grande. Na área de estudo afloram as rochas da Formação Barão de Guaicuí e das unidades pertencentes ao Grupo Diamantino (Fig. 4), cuja ênfase é dada no item subsequente.

4.1.2 O Grupo Diamantina

As rochas do Grupo Diamantina (Dossin et al. 1990) foram depositadas sobre rochas da Formação Barão de Guaicuí, unidade a qual composta por sericita-quartzo xistos e quartzo sericita-xistos, que podem ter quantidades variadas de cianita, lazulita, turmalina e hematita (Almeida-Abreu 1995). O Grupo Diamantina é composto pelas formações Bandeirinha, São João da Chapada, Sopa-Brumadinho e Galho do Miguel (Almeida-Abreu 1995, Martins-Neto 1998, Silva 1998, Santos et al. 2013). A Formação Bandeirinha é composta por arenitos de granulometria fina a média, com laminação planas e paralelas e estratificações cruzadas de escalas centimétricas, majoritariamente produzidos por processos eólicos, como migração de marcas onduladas cavalgantes de vento. Os arenitos ocorrem intercalados a conglomerados intraformacionais, que indicam características de transporte por enxurradas efêmeras, com fluxos de alta energia. A Formação São João da Chapada é constituída por arenitos de granulometria média a grossa com estratificações cruzadas acanaladas interpretadas como depósitos de sistemas fluviais entrelaçados, os quais ocorrem intercalados a pelitos e arenitos finos depositados em sistema lacustre (Martins-Neto 1994). A Formação Sopa Brumadinho, por sua vez, é composta por pelitos, arenitos e conglomerados. Os pelitos e arenitos mais finos, que ocorrem intercalados, são interpretados como depósitos de origem lacustre, ao passo que conglomerados e arenitos grossos, como depósitos de leques aluviais a frentes deltaicas (Martins-Neto 1996). A Formação Galho do Miguel é constituída de arenitos de granulometria fina a média cujas estruturas principais consistem em camadas com estratificações cruzadas, com até 10 m de espessura (Martins-Neto 1998, Martins-Neto 2000), que ocorrem intercaladas a camadas com laminações planas e paralelas. Estes depósitos foram produzidos em um sistema eólico do tipo Erg.

Figura 4. Mapa geológico da área de estudo em Diamantina (MG), na qual se observa a distribuição das unidades depositadas na Bacia Espinhaço, com destaque a Formação Bandeirinha.

4.2 Bacia Sanfranciscana em Minas Gerais

A Bacia Sanfranciscana (Cretáceo Inferior) ocorre sobre o Cráton do São Francisco (Fragoso et al. 2011), e é interpretada como uma bacia do tipo sag (Campos & Dardenne 1997a) que possui um formato atual aproximado de uma elipse cuja direção de eixo principal é N-S. Estende-se do centro-oeste do Estado de Minas Gerais até o sul do Estado do Piauí, ocupando uma área de aproximadamente 120.000 km² (Campos & Dardenne 1997a). A Bacia Sanfranciscana é dividida em duas sub-bacias: ao sul a sub-bacia Abaeté e ao norte a sub-bacia Urucuia, que são separadas por um Alto Estrutural, o Alto do Paracatu (Campos & Dardenne 1997a). A área estudada é localizada na região central da sub-bacia Abaeté, por sua vez separada da Bacia Bauru (Riccomini 1997), a Sul, por outro Alto Estrutural, o Arco do Alto Paranaíba (Fig. 5) (Hasui & Haralyi 1991).

4.2.1 O Arco do Alto Paranaíba e a evolução da sub-bacia Abaeté

O Arco do Alto Paranaíba é importante, pois segundo autores (Hasui & Haralyi 1991, Sgarbi et al. 2001, Sgarbi & Dardenne, 1997), este foi provavelmente a principal fonte de materiais clásticos depositados na sub-bacia Abaeté (Fig. 5). O Arco do Alto Paranaíba é localizado entre os limites de borda da Bacia Bauru e da Bacia Sanfranciscana (na sub-bacia Abaeté) (Fig. 5) e corresponde a uma feição morfológica e estrutural cujos registros mais antigos são datados no mesoproterozoico (Campos & Dardenne 1997a). No Cretáceo Inferior o Arco do Alto Paranaíba foi submetido a processo de reativação positiva (Hasui & Haralyi 1991), cujos principais registros consistem em intrusões alcalinas, que segundo Campos & Dardenne (1997a) foram provocadas por plumas posicionadas no manto superior. O processo de reativação positiva, chamado de soerguimento do Alto Paranaíba (Campos & Dardenne 1997a), resultou na reativação de falhas pré-cambrianas, de direção NW-SE, associadas a estruturas menores, controlou a evolução do arcabouço estrutural da sub-bacia Abaeté durante o Cretáceo Inferior (Hasui & Haralyi 1991). 30

Figura 5. Distribuição dos depósitos da parte sul da Bacia Sanfranciscana (sub-bacia Abaeté) e a relação com o Arco do Alto Paranaíba. Observe na figura a direita a estruturação geral da parte Sul da Bacia Sanfranciscana e sua relação com o arco do Alto do Paranaíba, fonte de sedimentos (Sgarbi et al. 2001). Modificado de Sgarbi & Dardenne (1996) e Alkmim & Martins-Neto (2001).

4.2.2 Litoestratigrafia da sub-bacia Abaeté

A área de estudo na Bacia Sanfranciscana é localizada na zona central da sub- bacia Abaeté (Fig. 6A). Nesta sub-bacia, em contato discordante sobre as rochas do Grupo Bambuí, ocorrem os depósitos do Grupo Areado e do Grupo Mata da Corda; juntos compõem um pacote de até 300 m de espessura (Fragoso et al. 2011). Uma discordância angular separa o Grupo Areado do Grupo Bambuí (Sgarbi et al. 2001). O Grupo Areado, principal unidade que aflora na área de estudo, é dividido em três unidades, da base ao topo, as formações Abaeté, Quiricó e Três Barras (Fig. 6B) (Campos & Dardenne 1997, Sgarbi et al. 2001, Fragoso et al. 2011). A Formação Abaeté ocorre apenas localmente e é composta por conglomerados, brechas e arenitos, sendo interpretada como produto deposicional de leques aluviais. A Formação Quiricó é constituída por pelitos e arenitos, que foram depositados em ambiente lacustre e de playa lake. Sobreposta a essas unidades, a Formação Três Barras é composta de arenitos bem arredondados e bem selecionados depositados em contexto eólico, em campos de dunas (Sgarbi 1991). O Grupo Mata da Corda, embora não ocorra na área estudada, tem ocorrências importantes na sub-bacia Abaeté, principalmente nos arredores da cidade de Patos de Minas, vizinha a Presidente Olegário (Fig. 1C). Esta unidade é formada por rochas máficas e ultramáficas (Hasui & Haralyi 1991).

Figura 6. Mapa geológico da área de estudo, na cidade de Presidente Olegário (MG). A) Distribuição geográfica das unidades da Bacia Sanfranciscana, dividida em Sub-bacia Abaeté (Sul) e Sub-bacia Urucuia (Norte). B) Mapa Geológico da área estudada, com ênfase nos diferentes tipos de depósitos analisados. 33

5 REGISTROS SEDIMENTARES DE ANÁLOGOS ATUAIS

O objetivo deste capítulo é apresentar uma visão geral sobre os ambientes desérticos e principais subambientes associados, que se diferenciam pelos aspectos hidrológicos, químicos e sedimentológicos.

5.1 Os principais análogos atuais

Nas bacias fechadas estabelecidas em regiões de clima árido geralmente observa- se a coexistência de uma variedade de subambientes, os quais variam devido a diferentes fatores, dentre os quais destacam-se as variações de aporte hídrico (Hardie et al. 1978). Os subambientes mais comuns são os leques aluviais, rios efêmeros, planícies arenosas, planícies lamosas salinas ou não salinas, lagos efêmeros ou lagos perenes, lençóis de areia eólica e campos de dunas eólicas (Fig. 7). Os leques aluviais ocorrem nas áreas marginais das bacias sedimentares, onde muitas vezes estão associados a zonas de falha; em geral são áreas de maior altitude e às vezes tectonicamente ativas. Nas regiões de clima árido/semiárido, o grau de influência dos leques aluviais depende de sua extensão, e embora nem sempre estejam diretamente ligados aos demais subambientes, constituem fonte comum de sedimentos e água. Os leques aluviais possuem um padrão morfológico radial, são compostos por rios e/ou canais fluviais efêmeros, em geral entrelaçados. Os canais são confinados e mais profundos nas áreas proximais e se tornam mais rasos e pouco confinados, ou não confinados, nas áreas distais. Os depósitos produzidos por leques aluviais variam entre conglomerados maciços e mal selecionados, registros típicos das zonas proximais, a arenitos, estes comuns às porções distais. As zonas distais dos leques aluviais são aquelas nas quais os fluxos subaquáticos, inicialmente canalizados, se dispersam lateralmente e tornam-se não confinados. Na frente dos leques aluviais desenvolve-se a maioria dos demais subambientes, como por exemplo, as planícies arenosas (Hardie et al. 1978). As planícies arenosas são locais de predominante acumulação de areia localizados à frente dos leques aluviais, que em geral é transportada por enxurradas efêmeras. A ocorrência de prolongados períodos sem chuvas nas planícies arenosas torna comuns os processos eólicos. Logo, pode-se considerar que os registros 34

sedimentares preservados sejam compostos tanto por depósitos eólicos quanto por depósitos subaquáticos. Em geral, os depósitos eólicos consistem em arenitos com laminações planas e paralelas ou cruzadas em baixo ângulo e/ou estratificações cruzadas de médio porte, características respectivamente produzidas por marcas onduladas de vento e dunas eólicas. Os depósitos eólicos podem ocorrer em camadas intercaladas a depósitos subaquáticos, como laminações planas e paralelas produzidas em regime de fluxo superior ou estratificações cruzadas de pequeno porte produzidas por fluxos turbulentos em regime de fluxo inferior. Além disso, nas regiões das planícies arenosas é comum que, após inundações, se formem piscinas rasas e efêmeras. Os registros dessas piscinas podem ser acumulações de argila, laminações cruzadas de marcas onduladas de ondas, além de eventual acumulação de evaporitos. Localmente, estruturas erosivas, como as de canais, podem ser preservadas no registro sedimentar. Em muitos locais, os playa lakes são identificados como extensas planícies lamosas, salinas ou não, que em geral são posicionadas mais ao centro das bacias fechadas, à frente de planícies arenosas (Hardie et al. 1978). As planícies lamosas salinas são caracterizadas pela ampla presença de crostas eflorescentes salinas, que por sua vez constituem elemento de grande variação morfológica (Smoot & Castens-Seidell 1994). As crostas salinas, em geral, são mais finas nas zonas marginais dos playa lakes, onde o nível do lençol freático é mais baixo e as concentrações de solutos são menores, e mais espessas nas áreas mais internas, onde o nível do lençol freático é mais alto e as concentrações de solutos maiores (Goodall et al. 2000). Os principais minerais que formam as crostas salinas são halita, silvita, anidrita e gipsita, todos minerais sujeitos à dissolução rápida ao menor contato com a água. A dissolução pode acontecer pelo contato com água de baixa concentração de solutos (solução insaturada), que pode ser de fonte subterrânea, superficial ou até mesmo pela ação do orvalho. Assim, devido à extrema solubilidade desses minerais, o potencial de preservação das crostas eflorescentes salinas é muito baixo. Porém, embora os evaporitos que constituem as crostas eflorescentes salinas tenham baixo potencial de preservação, suas evidências indiretas podem ser observadas nos registros geológicos. Os depósitos mais comuns são pelitos arenosos quase maciços e com raras e incipientes estruturas sedimentares e pelitos arenosos com ampla distribuição de lentes de arenito com características morfológicas variadas, como formato tigela (bowl-shape), cunhas serrilhadas, estruturas do tipo tepee-like e marcas onduladas deformadas. Além disso, quando preservados, os evaporitos ocorrem na forma deslocativa (displacive) (Benison & Goldstein 2001, Smoot & Castens-Seidell 1994); os minerais crescem em fase posterior a dissolução das crostas, já em 35

subsuperfície, na diagênese. O crescimento de sal intrasedimento tem potencial de destruir qualquer estrutura sedimentar previamente formada; em geral são associados a depósitos de pelito maciço, registro comum nas partes internas de planícies lamosas salinas (Smoot & Castens-Seidell 1994).

Figura 7. Bacia de drenagem fechada localizada no Saline Valley, California, EUA. Na ilustração são destacados os subambientes mais comuns, que se desenvolvem em condições de clima árido dentro de uma bacia de drenagem fechada. Modificado de Hardie et al. (1978).

No interior dos playa lakes, margeados por planícies lamosas salinas, é comum a formação de lagos efêmeros, rasos e salinos (salt pans). Os corpos de água rasos podem alcançar até poucos metros de profundidade durante períodos com inundações. Nos períodos nos quais estes lagos recebem água, ocorre deposição de materiais clásticos, geralmente de granulometria mais fina (argila e silte), que são depositados no fundo e formam lâminas de pelitos. Os períodos de inundações se alternam aos períodos de secas, em geral prolongadas, e que podem levar ao completo ressecamento da massa de água e consequente precipitação de evaporitos, os quais se depositam no fundo dos corpos de água por mecanismo de settling (Paik & Kim 2006). Logo, os depósitos típicos produzidos nesse tipo de subambiente são camadas de pelitos intercaladas a camadas de evaporitos. No lugar de formar playa lakes, algumas bacias fechadas podem se desenvolver de modo a conter sistemas lacustres e perenes, com água doce. A formação de lagos com tais características deve-se a mudanças no clima local, de aumento de precipitação ou diminuição das taxas de evaporação. Nos lagos de água doce predomina a deposição de areia e argila, que formam camadas que podem ou não conter estruturas sedimentares; as estruturas sedimentares refletem os aspectos relativos aos fluxos, como velocidade, turbulência e tempo de duração. Os depósitos relacionados a lagos perenes de água doce, em geral, não contêm evidências de exposição subaérea, como gretas de dessecação. Vale ressaltar que são situações incomuns, e em geral, nos climas mais secos, os lagos tendem a ser mais efêmeros e salinos (Hardie et al. 1978). Os ambientes de playa lake e lagos de água doce se desenvolvem devido a maior disponibilidade de água, seja ela intermitente ou permanente. De outro modo, quando a disponibilidade de água diminui ocorre uma tendência de aumento da quantidade de sedimentos soltos e disponíveis para processos eólicos, condições esta adequada à formação de lençóis de areia ou campos de dunas eólicas. Os lençóis de areia eólica são comuns nas áreas de planície arenosa ou à margem de campos de dunas eólicas. Os lençóis de areia eólica são sistemas dominados por marcas onduladas de vento (Hunter 1977), que migram e cavalgam umas sobre as outras (Kocurek & Nielson 1986). Além disso, dependendo da disponibilidade de areia, pode ocorrer a formação de dunas sem faces de avalanches (zibar e nebkha) ou dunas com faces de avalanche, que são raras. Os depósitos sedimentares mais comuns a estes sistemas são camadas de arenitos de granulometria fina a grossa com laminações planas e paralelas ou cruzadas em baixo ângulo e separadas por superfícies de truncamento. Outros depósitos que podem se formar em lençóis de areia são as acumulações de cascalho com faces planas e às vezes de formato triangular. Os clastos com essas 38

características são produzidos pela abrasão eólica e são chamados de "ventifact". Os campos de dunas eólicas podem se formar quando há aumento de suprimento de sedimentos clásticos e/ou de disponibilidade para transporte eólico (Kocurek & Havholm 1993). A maior disponibilidade de sedimentos não necessariamente acompanha o aumento de aporte e pode resultar de outros fatores, como por exemplo, o ressecamento de superfície deposicional devido ao rebaixamento do nível do lençol freático e/ou diminuição da frequência de inundações. Além desses fatores, outro que pode contribuir para a construção eólica é a variação da capacidade de transporte pelo vento (Kocurek 1999). A acumulação de estratos eólicos se dá quando ocorre a passagem de sedimentos para baixo da superfície de acumulação (Kocurek 1999). Os sistemas eólicos se diferenciam pelos tipos de fatores que controlam a acumulação, e são classificados como sistemas eólicos secos, úmidos ou estabilizados. Os sistemas eólicos secos são aqueles nos quais o nível do lençol freático fica muito abaixo da superfície de acumulação. Nos sistemas eólicos úmidos o lençol freático permanece alto, raso e assim controlando a superfície de acumulação. Os sistemas eólicos estabilizados são aqueles nos quais outros fatores, além do nível do lençol freático, controlam a acumulação; o fator de controle mais comum é a presença de vegetação. A preservação dos estratos acumulados depende de outros fatores, sendo o principal deles a taxa de subsidência e soterramento (Kocurek 1999, Kocurek & Havholm 1993). O efeito da subsidência é refletido principalmente na espessura dos estratos preservados e não necessariamente nos tipos de estratos. As características dos registros de campos de dunas eólicas variam em função dos tipos de fatores que controlam a acumulação. Assim, os depósitos podem variar entre eólicos ou combinação com outros tipos de depósitos ou registros. Os depósitos eólicos podem ser estratificações cruzadas, que combinam estratos de avalanche (grain flow) e de queda de grãos (grain fall), e/ou laminações planas e paralelas ou cruzadas em baixo ângulo produzidas por marcas onduladas cavalgante de vento (Hunter 1977, Kocurek 1996, Kocurek & Dott 1981, Mountney 2006). Esses depósitos podem ser associados a estratos de adesão (Kocurek & Fielder 1982), que indicam lençol freático próximo à superfície de acumulação. Ainda, associados a depósitos eólicos podem ocorrer evidências de pedogênese.

6 PRODUTOS DA PESQUISA: OS ARTIGOS ORIGINAIS

Os resultados obtidos no desenvolvimento desta tese de doutorado são apontados e discutidos em um conjunto de quatro artigos científicos originais, apresentados como apêndices I, II, III e IV. No presente capítulo é feita uma discussão dos principais pontos analisados em cada um dos artigos. Os artigos completos estão anexados ao final do texto introdutório. Nas contribuições referentes aos apêndices I e II foram analisados os depósitos da Formação Bandeirinha, ao passo que nos apêndices III e IV foram analisados depósitos que abrangem partes das formações Quiricó e Três Barras.

O apêndice I Título do artigo: Climate influence on the construction of a Proterozoic eolian sand sheet (Bandeirinha Formation, Minas Gerais, Brazil). (Publicado na revista Terrae)

Neste artigo, os depósitos de arenitos e conglomerados da Formação Bandeirinha (1.8 Ga) foram reinterpretados mediante análise de fácies, com foco sobre os mecanismos deposicionais. Os arenitos da Formação Bandeirinha, segundo trabalhos anteriores, haviam sido depositados em um sistema fluvial arenoso, sujeito a alguma interação marinha (Silva 1998, Martins-Neto 2000), e os conglomerados, segundo esses mesmos trabalhos, haviam sido depositados ao pé de estruturas montanhosas, em sistemas de leques aluviais, cujos processos de deposição foram controlados por pulsos tectônicos. A nova interpretação baseia- se nas características texturais, nas estruturas sedimentares e nos aspectos arquiteturais dos depósitos desta unidade, e sugere que: os arenitos são depósitos produzidos em ambiente de lençol de areia eólica e os conglomerados são depósitos de leitos de canais fluviais efêmeros que cruzavam os lençóis de areia eólica. Em relação aos depósitos de conglomerados, embora apresentem algumas características similares aos depósitos de zonas proximais de leques aluviais, evidências, como a ausência de clastos de dimensão superior a grânulo entre depósitos de arenitos eólicos, indicam que dificilmente existiram estruturas de montanhas próximas aos lençóis de areia eólica. As características das superfícies limitantes, nos contatos basais dos conglomerados, sugerem que estes depósitos se formaram em intervalos de deposição distintos daqueles de deposição eólica; de fato, não é reconhecida uma relação de coexistência entre o lençol de areia eólica e os sistemas subaquosos. Ao contrário, os clastos dos conglomerados consistem em arenitos com algum grau de cimentação, que foram canibalizados do substrato com depósitos eólicos. Neste artigo, concluiu-se que os processos 40

sedimentares são expressões de variações climáticas, onde os depósitos de lençóis de areia eólica se formaram em intervalos de maior aridez enquanto que os conglomerados foram depositados durante intervalos de clima mais úmido.

O apêndice II Título do artigo: Unusual thick eolian sand sheet sedimentary succession: Paleoproterozoic Bandeirinha Formation, Minas Gerais. (Publicado na revista Brazilian Journal of Geology)

Neste artigo são discutidas as razões que levaram à preservação das espessuras exageradas dos depósitos de lençóis de areia eólica da Formação Bandeirinha (1.8 Ga). Os depósitos arenosos compõem pacotes de até 50 m de espessura, o que os configura como incomuns, uma vez que tanto os depósitos de lençóis de areia eólica atuais quanto os depósitos antigos não ultrapassam os 20 m de espessura (Chakraborty & Chakraborty 2001, Mountney 2006). As pequenas espessuras de depósitos de lençóis de areia eólica podem ser explicadas pelo baixo potencial de preservação dos registros sedimentares associados a esses sistemas, os quais, em geral, se formam em locais de baixo aporte de sedimentos e/ou em áreas suscetíveis ao retrabalhamento por fluxos subaquosos. Assim, neste artigo buscou-se compreender quais fatores poderiam ter permitido a preservação de pacotes espessos de depósitos de lençóis de areia eólica. O aporte clástico, diferente do que é esperado para lençóis de areia eólica, foi relativamente alto nessa área da Bacia Espinhaço (~1.8 Ga), uma vez que, obviamente, o baixo aporte dificultaria a formação de uma sucessão sedimentar tão espessa. Entende-se que alto aporte de areia pode favorecer a construção de campos de dunas eólicas, o que não foi o caso. Logo, outros fatores devem ter contribuído para a baixa disponibilidade de sedimentos para formação de dunas com faces de avalanche. Dentre os fatores de controle (Kocurek & Nielson 1986) mais comuns que atuam neste tipo de sistema, a estabilização por crescimento de vegetação terrestre foi o primeiro a ser descartado, uma vez que os depósitos são mais antigos do que a existência de vegetação. Evidências de lençol freático alto, como marcas de adesão ou registros de processos de inundação também não foram identificados. Entretanto, existe um fator de controle, responsável por diminuir a disponibilidade de sedimentos, que parece mais provável, a cimentação precoce. A hipótese de cimentação precoce é fundamentada pela presença de intraclastos de arenitos nos conglomerados, que foram depositados durante enxurradas efêmeras com fluxos de alta energia, capazes de canibalizar parte do substrato e potencialmente capazes de desagregar 41

blocos de arenito, quando pobremente consolidados. Portanto, a presença de intraclastos de arenitos preservados nos conglomerados, apesar do transporte subaquoso, constitui evidência de cimentação precoce, mesmo que o agente cimentante não seja conhecido. A combinação de fatores como aporte de sedimentos relativamente alto e constante, processos de cimentação precoce e alta taxa de criação de espaço de acomodação no rifte da Bacia Espinhaço (Martins- Neto 2000; Santos et al. 2013), levaram, portanto, à preservação de pacotes espessos de arenitos de lençóis de areia eólica.

O apêndice III Título do artigo: Temporal evolution of an Early Cretaceous Playa Lake: the sedimentary record of Quiricó and Três Barras Formation (Sanfranciscana Basin, south-eastern Brazil). (Submetido à revista Terrae)

Neste artigo é analisada a sucessão sedimentar que caracteriza a transição entre duas unidades do Grupo Areado, as formações Quiricó e Três Barras. A sucessão sedimentar estudada foi depositada durante o Cretáceo Inferior, na Bacia Sanfranciscana, e apresenta características que sugerem uma mudança paleoambiental controlada por variações no aporte de água, possivelmente por controle climático, na qual um ambiente de playa lake deu lugar a um campo de dunas eólicas. Tal interpretação deve-se a uma análise sedimentológica de detalhe. Especial atenção foi voltada à descrição e interpretação de depósitos de crostas eflorescentes salinas (efflorescent salt crusts). Neste trabalho demonstra-se que as características dos depósitos de playa lake refletem a posição do lençol freático, onde a maior profundidade resulta na formação de crostas finas (baixo relevo) e na menor profundidade a formação de crostas mais espessas (alto relevo). Além disso, nesse trabalho observou-se uma mudança nas características dos depósitos de playa lake, em que, da base ao topo, observou-se a transição de depósitos formados no interior de planícies lamosas salinas (inner saline mudflat) para depósitos formados nas zonas externas de planícies lamosas salinas (outer saline mudflat). Por fim, observou-se que os depósitos de playa lake são encobertos por depósitos de dunas eólicas. Logo, um processo de progradação entre os subambientes marginais de playa lake sobre os subambientes centrais, bem como de sistemas eólicos sobre os mesmos, é descrito neste trabalho. Esta transição entre os diferentes tipos de depósitos sugere uma diminuição progressiva no aporte de água para o sistema.

O apêndice IV Título do artigo: The role of groundwater variation on a drying upward sedimentary sequence (Sanfranciscana Basin, Early Creataceous). (Em preparação para submissão em revista especializada)

Neste artigo discutem-se as transformações paleoambientais provocadas por um processo de aridificação durante o Cretáceo Inferior na bacia Sanfranciscana. Processo que, em linhas gerais, é sugerido pela transformação de um ambiente lacustre, de água doce, para um playa lake, o qual, ao final do processo evolutivo, desapareceu, dando espaço ao estabelecimento de um campo de dunas eólicas. Seções estratigráficas foram analisadas verticalmente e correlacionadas lateralmente, o que permitiu entender como os diferentes subambientes se estabeleceram e se transformaram ao longo do intervalo temporal pesquisado. Neste trabalho, notou-se que menores mudanças na relação entre entrada e saída de água também ficaram preservadas no registro sedimentar. Na margem a sul do playa lake existia um campo de dunas eólicas que progradava em direção ao centro da bacia de drenagem toda vez que o lençol freático ficava mais profundo; o processo ficou mais frequente ao longo do tempo, como registrado pelo aumento da quantidade, espessura e distribuição lateral dos depósitos eólicos intercalados a depósitos de planície lamosa salina. O ambiente foi controlado principalmente pelas variações no nível do lençol freático, como sugerido pela preservação de diversas feições, como evaporitos deslocativos ou com estrutura nodular, depósitos relacionados a crostas eflorescentes salinas (mais ou menos espessas), e a presença de depósitos eólicos intercalados aos de playa lake.

7 CONCLUSÕES

(1) A Formação Bandeirinha (~1.8 Ga) corresponde a uma sucessão sedimentar de 250 m de espessura, cujos depósitos consistem em arenitos e conglomerados. Os arenitos são depósitos formados em um ambiente de lençol de areia eólica, onde predominavam marcas onduladas cavalgantes de vento, que durante períodos com maior disponibilidade de areia podem ter-se acumulado e formado zibars (dunas sem faces de avalanche). Os conglomerados foram depositados em sistemas fluviais entrelaçados, de fluxos efêmeros, que se desenvolveram durante intervalos específicos de tempo. (2) A alternância entre depósitos eólicos e fluviais provavelmente deve-se a mudanças das condições climáticas, respectivamente clima mais seco alternado a clima mais úmido. (3) Os depósitos de lençol de areia eólica consistem em pacotes de arenitos de espessuras anômalas (~50 m), que são explicados pela combinação de dois fatores: cimentação precoce no substrato, que diminuía a disponibilidade de sedimentos necessários para a formação de dunas com faces de avalanche, e a alta taxa de criação de espaço de acomodação, que permitiu a preservação de uma espessura exagerada. Estas evidências são, respectivamente, sugeridas pela preservação de intraclastos de arenitos depositados no lençol de areia eólica, e pela compatibilidade com os modelos de rifte continental, propostos pelos trabalhos regionais sobre a Bacia Espinhaço. (4) As unidades estudadas na Bacia Sanfranciscana (formações Quiricó e Três Barras) formam um pacote de menos de 100 m de espessura na área de estudo. Os depósitos analisados nessas unidades foram depositados em ambientes desérticos e registram um processo de aridificação. (5) Na parte inferior da sucessão sedimentar predominam depósitos formados em sistemas mais úmidos (lagos permanentes), que progressivamente mudaram para sistemas nos quais são comuns os períodos de exposição subaérea, como os subambientes de playa lake e campos de dunas eólicas. (6) Os depósitos de playa lake são dominantes na sequência sedimentar analisada. Esses depósitos apresentam evidências de controle por variações no aporte hídrico, que em maior parte evidenciou-se por variações no nível do lençol freático, que progressivamente ficou mais profundo em relação à superfície deposicional, o que se refletiu em mudanças na quantidade de evaporitos, características dos depósitos de crostas salinas eflorescentes e variação na disponibilidade de sedimentos para processos eólicos. 44

(7) Em linhas gerais, os sistemas deposicionais de ambiente desértico do Pré-cambriano e do Fanerozoico, estudados nesta tese, apresentam como principal fator de controle comum, variações na disponibilidade hídrica, controlada por variações de entrada e saída de água. As principais diferenças consistem na maior espessura e diversidade de depósitos preservados. A Formação Bandeirinha possui uma espessura exagerada em relação a outras unidades com depósitos similares, mas apresenta pouca diversidade em termos de quantidade de fácies, o que pode indicar longos períodos sem mudanças significativas no ambiente sedimentar. Diferentemente, os depósitos do Grupo Areado são pouco espessos, porém apresentam uma grande diversidade de fácies, o que pode indicar maior frequência de mudanças nas condições ambientais. Deste modo, aventa-se a ideia de que possivelmente as diferenças de frequências, em termos de mudanças nas condições ambientais (ex.: volume e frequência de precipitações), devem-se a variações climáticas tornadas mais frequentes devido a explosão na vida terrestre.

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the Statherian and Stenian Espinhaço Rift System, Brazil. Sed. Geol., 290: 47-59. Scholl W.U. & Fogaça A.C.C. 1979. Estratigrafia da Serra do Espinhaço na região de Diamantina, MG. In: Simpósio de Geologia de Minas Gerais. Diamantina, v.1, p. 55- 73. Sgarbi G.N.C. 1991. Arenitos eólicos da Formação Três Barras (Bacia cretácea do São Francisco): Caracterização, Diagênese e Aspectos Químicos. Rev. Bras. Geoc., 21: 342-354. Sgarbi G.N.C. & Dardenne M. A. 1996. Evolução climática do Gondwana na região centro-sul do Brasil e seus registros geológicos continentais durante o mesozoico, enfatizando o arco do Paranaíba, a borda NNE da Bacia do Paraná e a porção Meridional da Sanfranciscana, no oeste do Estado. Geonomos, 4: 21-49. Sgarbi G.N.C., Sgarbi P.B.A, Campos J.E.G, Dardenne M.A, Penha U.C. 2001. Bacia Sanfranciscana: registro fanerozoico da Bacia do São Francisco. In: C.P. Pinto & M.A Martins-Neto (eds.). Bacia do São Francisco: Geologia e Recursos Naturais. Belo Horizonte, Ed. SBG-MG, p.93-138. Silva R.R. 1995. Contribution to the stratigraphy and paleogeography of the lower Espinhaço Supergroup (Mesoproterozoic) between Diamantina and Gouveia, Minas Gerais, Brazil. Tese de Doutorado, Freiburger Geowiss, Universidade de Freiburg, 115 p. Silva R.R. 1998. As bacias proterozoicas do Espinhaço e São Francisco em Minas Gerais: Uma abordagem do ponto de vista da estratigrafia de sequências. Geonomos, 6: 1-12. Smoot J.P. & Castens-Seidell B. 1994. Sedimentary features produced by efflorescent salt crusts, Saline Valley and Death and Valley, California. In: Renaut, R.W., Last, W.M. (eds.). Sedimentology and Geochemistry of Modern and Ancient Saline Lakes. Soc. Econ. Paleont. Mineral, Special Publication, 50: 73-90. Walker R.G. 2006. Facies models revisited: introduction. In: H.W. Posamentier & R.G. Walker (eds.) Facies models revisited. Society for Sedimentary Geology, Special Publication, 84: 1-19.

APÊNDICES 51

APÊNDICE I

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. 52

ABSTRACT

Eolian sand sheets are flattened areas with sandy deposition of arid and semi-arid environments, characterized by dunes without avalanching face. The low availability of sand is considered the main factor of generation of sand sheets. A detailed facies analysis of six measured stratigraphic sections permitted to interpret the Bandeirinha Formation, a Proterozoic siliciclastic unit of the Espinhaço Range, as ancient eolian sand sheet. This 250 m thick formation is composed of sandstone interbedded with three sandstone conglomerate beds. The sandstone portion corresponds to an eolian sand sheet paleoenvironment, which is formed of sets with planar parallel laminations deposited by climbing wind ripples, interpreted as zibar dunes. The sandstone conglomerate beds were deposited by high- concentrated subaqueous flows, formed within ephemeral braided river channels. The absence of typical features of alluvial fans and the composition of the conglomerate clasts (constituted of sandstone intraclasts cannibalized from the eolian sand sheet) testify the absence of mountain structures at catchment area of the fluvial system. Present and ancient desert systems record cyclic climate 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.

Keywords: Eolian sand sheet, zibar, ephemeral channel rivers, Proterozoic, Espinhaço Supergroup.

RESUMO

Os lençóis de areia eólica são áreas de planície que ocorrem em ambientes áridos e semiáridos, onde predominam dunas sem faces de avalanche. A baixa disponibilidade de areia é o principal fator para geração de lençóis de areia. A análise de fácies de detalhe efetuada em seis seções estratigráficas permitiu interpretar a Formação Bandeirinha, Proterozoico da Serra do Espinhaço, como um antigo lençol de areia eólica. Esta formação, espessa 250 m, é composta por arenitos intercalados a três camadas de conglomerados. Os depósitos de arenitos são compostos por sets de laminações plano-paralelas de marcas onduladas de vento, que formam dunas do tipo zibar, e que correspondem a um paleoambiente deposicional de lençol de areia eólica. As camadas de conglomerado arenoso foram depositadas por fluxos subaquosos de alta concentração formados em canais efêmeros entrelaçados. A ausência de características de leques aluviais e a composição dos clastos dos conglomerados (intraclastos de arenitos canibalizados do sistema eólico) comprovam a ausência de estruturas relevadas nas áreas-fonte do sistema fluvial e excluem a origem por pulsos tectônicos destes conglomerados. Mais provável que a geração dos conglomerados seja devido a um aumento na taxa de precipitação e ao estabelecimento de canais fluviais efêmeros.

Palavras-Chave: Lençol de areia eólica, zibar, canais fluviais efêmeros, Proterozoico, Supergrupo Espinhaço. 54

INTRODUCTION

Eolian sand sheets are depositional systems characterized by dunes without slipface, like nabkha or zibar dunes. These may occur in hot or cold regions, in arid or semi- arid climate environments (Kocurek 1996, Mountney & Russell 2004). The absence of dunes with slipface is caused by the poor availability of sand for transport by wind, due to the following controlling factors: vegetation, water table near or intersecting the depositional surface, relative abundance of coarse-grained sand or granules, sheltered surface for cementation or gravel lag, and seasonal flooding (Kocurek & Nielson 1986, Basilici & Dal Bó 2014). Although there are many examples of Precambrian eolian deposits, few examples of Precambrian eolian sand sheets are known (Eriksson & Simpson 1998, Clemmensen & Dam 1993), and still fewer studies of ancient zibar dunes were published (Nielson & Kocurek 1986, Biswas 2005). The Bandeirinha Formation in this paper is interpreted as an eolian sand sheet with zibar dunes, and constitutes the oldest unit of the Proterozoic sedimentary succession of the Espinhaço Supergroup. The boundaries with the upper unit, the paleodepositional interpretation, and the controlling factor of the sedimentation are not yet clear (Martins-Neto 1994, Silva 1998, Martins-Neto 2000, Lopez-Silva & Knauer 2011). The aim of this paper is to present the results of a detailed facies analysis of the Bandeirinha Formation to producing an interpretation of the depositional processes and paleoenvironments and their implication for paleoclimate, paleogeographic, and paleotectonic reconstructions.

GEOLOGICAL SETTING

The subject of this study is the Bandeirinha Formation, a Proterozoic clastic unit exposed in southern portion of Espinhaço Range, nearby the town of Diamantina, Minas Gerais (Fig. 1A). This unit takes part of the Espinhaço Supergroup, ~4000 m thick, which is constituted of sandstone, conglomerate, 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 & Dussin 1995). The Bandeirinha, São João da Chapada, Sopa-Brumadinho and Galho do Miguel formations constitute the Diamantina Group (Silva 1998, Martins-Neto 2000, Chemale et al. 2012) and the Santa Rita, Córrego dos Borges, Córrego da Bandeira, Córrego 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 to have been the responsible mechanisms for creation of accommodation space for this sedimentary basin (Alkmin & Marshak 1998). The Bandeirinha Formation, based on U-Pb dating, was deposited between 1.7-1.8 Ga (Chemale 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 unconformably overlies an older 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 Formation). The stratigraphic contact with the Barão de Guaicuí Formation is partially deformed by a tectonic reactivation, probably due to the different mechanic behaviour of the two formations. The 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 m of section. The transition to the São João da Chapada Formation is considered as an angular unconformity (Martins-Neto 1994, Silva 1998, Martins-Neto 2000). However, field data do not permit to clearly visualize this unconformity which may be confused with the original dipping of the sandstone planar lamination of the Bandeirinha Formation. The strata dipping of the Bandeirinha Formation is not easy to define in field, due to the original dipping of the planar laminations of 0-15º. However, statistical data allow defining a dipping N75-90/25-35. Silva (1998) interpreted the sedimentary succession of this unit as formed in coastal and river systems, where the interbedded conglomerate beds represent fluvial mass flow deposits in alluvial fans. Only at the top of the formation this author described eolian deposits. Silva (1998) and Martins-Neto 56

Figure 1. (A) Location of the study area and (B) draft of the litostratigraphy of the Espinhaço Supergroup (modified by Santos et al., 2013).

(1998) attributed the deposition of the conglomerates to regional tectonics pulses, and divided the Bandeirinha Formation into three tectono-sequences (Basal, Olaria and Natureza). The depositional interpretation of the Bandeirinha Formation expressed in this paper is substantially different from the previous interpretations, as well as the explanation on the forcing factors that originated the conglomerate beds.

METHODS

The research activity has focused in the type-area where the Bandeirinha Formation was defined. In this site, a complete stratigraphic section, more than 250 m thick, from the metamorphic bedrock was measured. Other five stratigraphic sections, 5-25 m thick, were measured in nearby areas with the purpose to verify lateral variation of the lithofacies. In all the cases, the measurement of the section was done together to a detailed facies analysis. The lithofacies and architectural elements were described and distinguished based on grain size, sorting, type and organization of the sedimentary structures, form and dimension of the beds, characteristic of the bounding surfaces, interpretation of the depositional mechanisms. The concept herein used of lithofacies is not only descriptive (Walker 2006), but also interpretative. Indeed, this concept is based on the depositional aspects deduced by physical characteristics of the lithofacies. Moreover, in this paper lithofacies codes are not utilized (Bridge 1993a). Photos of rock expositions were arranged in photomosaics, on which bounding surfaces, relationship between the lithofacies, dimensions and shape of the architectural elements have been visualized. A large scale geological survey was executed to define the horizontal distribution of the architectural elements. Thirty-three samples were collected in the field to produce polished slabs and thin sections, which were used to examine in detail the texture and sedimentary structures of the sediments and microtexture of the grains. The low grade metamorphism of the Bandeirinha Formation did not affect the recognition of the sedimentary structures and the application of the sedimentary facies analysis method. Thus, the terminology here used is typical and related to sedimentary rocks. 58

Figure 2. Stratigraphic section of the Bandeirinha Formation. The upper boundary corresponds to the increase of cross-stratification suggesting a change of the depositional system.

ARCHITECTURAL ELEMENTS

Three architectural elements were identified in Bandeirinha Formation: translatent climbing ripple sandstone, cross-stratified fine-grained sandstone and sandstone conglomerate.

Translatent climbing ripple sandstone

This architectural element is composed of moderately- to well-sorted, fine- to coarse-grained sandstone. The grains of sand are rounded or well-rounded and are constituted for 95% of quartz. The grain-size distribution is bimodal, showing two modes in fine and medium-coarse grain-size. The sandstone is organized in low-angle planar parallel laminations, dipping 0-15º. The laminae are 2-12 mm thick and sometimes are characterized by a rough inverse gradation due to the presence of coarse-grained sand on the upper portion of the lamina. In outcrop, these structures are similar to the pin stripe lamination of Fryberger & Schenk (1988). The laminae are organized in sets, 0.4-1.2 m thick and laterally extended 4- >9 m; their bounding surfaces are erosive, dipping 0-14º, and the basal laminae of the set are parallel to the bottom surface (Fig. 3A). Locally, asymmetrical undulating laminations may be observed interbedded with the planar laminations (Fig. 3B). These undulating structures on bed surfaces correspond to asymmetrical ripples bed forms (Fig. 3C). This architectural element forms sandstone successions more than 50 m thick and with lateral exposition more than 2 km. It is interbedded with sandstone conglomerate and cross-stratified fine-grained sandstone. Interpretation Hunter (1977) and Hunter et al. (1981) named climbing translatent strata deposits produced by climbing wind ripples, which are constituted of fine to coarse-grained, well- sorted sand, bimodal in grain size distribution, arranged in horizontal or low-angle, planar and parallel laminae, commonly with inverse gradation (Fig. 3A). This architectural element corresponds to the description of the above cited authors and its deposits may be interpreted as formed by wind ripples. This interpretation is confirmed by the similarity with pin stripe lamination of Fryberger & Schenk (1988) and the presence of high rounded clasts, typical of wind transport (Mahaney 2002). Most of the observed sedimentary structures correspond to subcritically climbing wind ripples, but the asymmetrical undulating laminations can be interpreted as supercritically climbing wind ripples (Hunter 1977) (Fig. 3B and C). Climbing 60

wind ripples are typical bedforms in eolian depositional environment, they may be found on stoss side of dunes, lower portion of the lee side of dunes, dry interdune areas, or they constitute the basic structure that form dunes without slipface, as nabkha or zibar. The geometrical characteristics of the sets of this elements and the absence of high-angle cross stratification (i.e., deposits of dunes with slipface) lead to interpret the set of climbing wind ripples as deposits of nabkha or zibar. Nabkha are anchored and active dunes with main axis parallel to the wind direction; they form behind an obstacle (Tengberg & Chen 1998, Langford 2000). Zibar are dunes with main axis perpendicular to dominant wind, which do not need obstacles to be formed (Nielson & Kocurek 1986). Both are dunes with low relationship high/ short-axis length or and long-axis length compared with dune with slipface. In areas with relative supply of sediments, nabkha or zibar can superimpose and build geological bodies, characterized by sets of planar parallel or low-angle laminations separated by erosive surfaces, whose thickness and extension depend on the dimensions of the original bedforms. Nabkha and zibar cannot be distinguished by the internal structure, because they are similar in both of cases. However, nabkha dunes form in presence of an obstacle, which in present environments is manly represented by vegetation (Tengberg & Chen 1998). The Bandeirinha sedimentary succession does not display suitable physical obstacles that could have generated nabkhas, and the presence of terrestrial vegetation is obviously excluded. Thus, the more plausible interpretation is that this architectural element may represent zibar dunes.

Cross-stratified fine-grained sandstone

This architectural element is formed of well sorted, fine- and very fine to fine- grained sandstone, prevalently constituted of quartz grains, organized in lenticular sets, 0.12- 0.3 m high and 3-7 m laterally extended (Fig. 3D). Cross-stratification laminae are 3-10 mm thick, and dip 21-26º; reactivation surfaces are observed (Brookfield 1977). The foresets display thin alternation of fine- and very fine-grained laminae. This architectural element forms isolated strata of 1-3 superimposed sets within the element translatent climbing ripple sandstone, with which has erosive bounding surfaces. This element represents only 2% in thickness of the measured sections. Interpretation The restored angle of the cross-stratification indicates that are formed for avalanching processes. Thus, this architectural element corresponds to dune with slipface. The 61

very fine grained foreset laminae and the interbedding with wind-ripple strata, and the absence of sedimentary structures associated to subaqueous flows suggest that these structures are small eolian dunes (Hunter 1977, Kocurek 1996, Mountney 2006). Because the upper bounding surface of the cross-stratification sets is erosive, the real high of the dunes is unknown, but probably, due to restricted lateral extension and to be uncommon, they were small dimension dune bedforms. In other way, this foresets may be interpreted as rare slipfaces of zibar dunes, as observed by Lancaster (1982) in Namibia desert.

Figure 3. (A) Low-angle, parallel laminated sandstone, corresponding to subcritical translatent climbing strata, organized in sets separated by erosive surfaces. Hammer: 0.28 m. (B) Asymmetrical laminations (arrow), interpreted as supercritically translatent climbing strata. Pencil: 142 mm. (C) Asymmetrical laminations on the upper bed surface. Note the weak asymmetry of the bed forms. Coin: 20 mm. (D) Cross-stratified fine-grained sandstone beds (arrow) form lenticular beds, interbedded with low-angle parallel laminations. Hammer: 0.28 m. 62

Sandstone conglomerate

In study area, sandstone conglomerate constitute three sedimentary bodies, 1.5-18 m thick; their visible lateral extension is around 1 km, but a detailed geological survey showed lateral extension for more than 4 km. The bottom surface of this element is concave- up and erosive; its top surface is apparently planar. Sandstone conglomerate are constituted of three lithofacies: structureless conglomerate, sandstone conglomerate and laminated sandstone (Fig. 4A). The structureless conglomerate represents 74% of the thickness of this architectural element; it is constituted of poorly sorted, clast-supported, pebble- and cobble- grained conglomerate; the matrix is poorly sorted, on average medium-grained sandstone. The thickness of this lithofacies is <1-5 m, its bounding surfaces are erosive and concave-up, but when its upper surface is in contact with sets of translatent climbing ripples the bounding surface appears to be planar and probably non-erosive. The conglomerate clasts are constituted of laminated sandstone, whose abundance is 87%; vein-quartz, quartzite, banded iron formation, and schist clasts have an abundance of 4, 4, 3, and 2%, respectively (Fig. 4B). Laminated sandstone clasts are angular to subrounded, with prevalence of subangular; the other clasts are rounded or subrounded. The dimension of the clasts is extremely variable. Maximum particle size of laminated sandstone clasts is 0.35 m; outsized clasts up to 3 m across occur (Fig. 4C). The other clasts have maximum particle size of 0.09 m and they do not show outsized clasts. This lithofacies does not display any internal organization of the clasts. Conglomerate beds are alternated with sandstone conglomerate without evident sequential order. Sandstone conglomerate is 0.1-0.7 m thick, and it constitutes 23% of the thickness of this element (Fig. 4D). Maximum particle size of the clasts is 0.14 m; the matrix is a poorly-sorted, medium- and coarse-grained sandstone, and it is more abundant than the other lithofacies. Roundness and composition of the clasts are the same of the other lithofacies. The clasts of this lithofacies are barely more organized: discoid and bladed pebbles and cobbles display imbrication a(t) b(i) (Walker 1975) and sometimes they are aligned according a surface. The beds of this facies are commonly alternated to the laminated sandstone lithofacies. Laminated sandstone is 3% in thickness of the measured succession and is formed of medium- and fine-grained sandstone with a thickness of 0.04-0.15 cm. It forms lenticular beds laterally extended up to 3 m. The base of these sandstone beds has a gradual transition to sandstone conglomerate, but the top boundary is sharp (Fig. 4D). The sandstone displays planar parallel laminations, constituted of laminae, 3-10 mm, at times with inverse grading. 63

Figura 4. (A) Sandstone conglomerate. This bed shows structureless conglomerate on the upper portion, and sandstone conglomerate in the lower portion, which are interbedded with laminated sandstone (arrow). (B) Most of the clast of the conglomerates are constituted of intraclast of laminated sandstone. A minor quantity of quartzite (arrow), vein-quartz, banded iron formation, and schist. Pencil: 142 mm. (C) Huge intraclast (marked by dotted line) of low-angle parallel laminated sandstone. Hammer: 0.28 m. (D) Sandstone conglomerate with interbedded laminated sandstone (arrow). Hammer: 0.28 m. Interpretation The concave-up bottom surface and the planar top of this architectural element is an evidence that it is the filling of channelized morphological structures, generated by subaqueous flows. The grain-size heterogeneity and the absence of internal structures of the conglomerate lithofacies suggest that high-concentrated flows deposited it (Tooth 2000, Blair 2003). More diluted subaqueous flows deposited the sandstone conglomerate lithofacies, as suggested by imbrications and alignment of clasts (Walker 1975). The laminated sandstone may correspond to the waning flow phase deposition after the sedimentation of the sandstone conglomerate, as suggested by the gradual transition of two lithofacies. Laminae with inverse grading may testify fall of the water level, partial reworking of the sand by wind, and consequently an ephemeral regime of the fluvial channel (Cowan 1993, Tooth 2000, Jain et al. 2005). 64

Most of the clasts that constitute these deposits were originated by the erosion of the substratum, which was prevalently formed of translatent climbing ripple sandstone. Indeed, textures and sedimentary structures of the conglomeratic clasts are similar to translatent climbing ripple sandstone. Angular or subangular roundness of these clasts means that the sandstone clasts (intraclasts) did not undergo a long transport and that the translatent climbing ripple sandstone was subjected to precocious cementation. Precocious cementation has been described in arid depositional palaeonvironments by many authors (Deynoux et al. 1989, Mountney & Howell 2000, Basilici & Dal Bó 2014). Huge boulders (up to 3 m across), found in massive conglomerate beds, may be interpreted as substratum margins fallen in the channel and transported by high concentrated flows. Few rounded extraformational clasts indicate that the water drainage area was external to the depositional basin.

DISCUSSION

Conglomerate beds or clasts larger than granules were not ever found within translatent climbing ripple sandstone or cross-stratified fine-grained sandstone elements. This indicates that the conglomerate transport and deposition were processes restricted to the formation of the sandstone conglomerate element. Moreover, the geometry and the bounding surfaces of the three sandstone conglomerate beds indicate that these constitute three depositional events physically and temporally distinct from the sand sedimentation of the other two architectural elements. The subaqueous conglomerate sedimentation had not any relationship with the eolian sandy sedimentation, except to cannibalize previous cemented sandy deposits. Thus, the Bandeirinha Formation may be identified with two separated depositional systems: an eolian system, represented by translatent climbing ripple sandstone and cross-stratified fine grained sandstone elements, and a fluvial system, represented by sandstone conglomerate lenticular beds. The construction of the Bandeirinha Formation was dominated by an eolian system. This was mainly characterized by flattened and long dunes without slipface, probably correspondent to present-day zibar dunes. Smaller dunes with slipface uncommonly also occurred. Eolian systems that presently are characterized by dunes without slipface, like zibar, are called eolian sand sheet. Kocurek & Nielson (1986) attributed the formation of eolian sand sheet to an overall low rate of supply and/or availability of sand. This is mainly due to sheltered surface by conglomerate clasts or cementation, abundance of medium- to coarse- grained sand, high water table, periodic flooding, presence of vegetation, poor sandy input into the system. On the contrary, when high input and availability of sand occurs a dune field (erg) generates. Vegetation, high water table, and periodical flooding may be excluded as factors that controlled the formation of the sand sheet of the Bandeirinha Formation. Terrestrial vegetation was absent during the Proterozoic, and sedimentary features, that testify high water level, as for example adhesion structures (Kocurek & Fielder 1982), or flooding, as for example subaqueous deposits and thin mud laminae, were not observed (Tripaldi & Limarino 2008, Basilici & Dal Bó 2014). There are no pebble lags accumulations, which could have constituted potential sheltered surfaces, were found in the Bandeirinha Formation. Currently, it is possible to suppose that the absence of dunes with slipface was due to the coarser grain size than typical eolian slipfaced dunes (Kocurek & Nielson 1986) and the precocious cementation of the sand deposits. Precocious cementation in desert environments is known and associated to evaporite minerals; for example, Basilici & Dal Bó (2014) 66

recognized in a present eolian sand sheet precocious cementation due to calcium sulphate, which constitutes an important factor for the decrease of sand availability. Direct data on precocious cementation of sands of the Bandeirinha Formation do not exist, because diagenetic and metamorphic processes modified the original cementation. However, it is possible to hypothesize a precocious cementation of the sands because the deposits of the sandstone conglomerate are almost completely constituted of angular or subangular intraclasts originating from the erosion of the sandstone architectural elements. The three sandstone conglomerate beds, which are interbedded with sandstone in the typical area of the Bandeirinha Formation, constitute sedimentary bodies isolated from the sandstone succession, as can be observed by geometrical boundaries and large lateral extension of these bodies. The sandstone conglomerate is probably constituted by overlap of various fluvial channels deposits, whose textural features, absence of sedimentary structures, and sandstone beds with eolian reworking suggest high-energy and irregular subaqueous flows. Few rounded metamorphic and sedimentary clasts of the conglomerate indicate that the water flows were generated out of the depositional basin. However the water flows should not be originated in mountain areas, because most of the clasts were formed within the depositional basin, cannibalized from precociously cemented sandstone. Silva (1998) and Martins-Neto (1998) interpreted the conglomerate episodes of the Bandeirinha Formation as reactivation of alluvial fan deposits. Although these types of conglomerate deposits can be found in channel deposits of alluvial fan, the conglomerate beds of the Bandeirinha Formation do not represent alluvial fan systems. In fact, the alluvial fan systems are characterized by unchannelized deposits on intermediate and distal portion of the fan and by channel beds with low relationship width/thickness (as ribbon channel) (Friend 1983, Blair & McPherson 1994a and b), which are absent in the Bandeirinha Formation. Moreover, being the alluvial fan systems characterized by a catchment area in mountain areas, the conglomerate composition of the clasts should reflect the geology of the catchment area and should be principally composed of exotic clasts. These characteristics are not observed in sandstone conglomerate of the Bandeirinha Formation, giving another element for not consider these conglomerate beds as alluvial fan deposits. In this paper, the sandstone conglomerate beds are interpreted as ephemeral braided river deposits, which were characterized of high lateral migration and swept the eolian sand sheet eroding previous eolian sandstone deposits. High concentrated water flow deposits, eolian reworked sandstone deposits, absence of sequential organization of the channel filling, and high relationship of width/thickness are characteristics of ephemeral braided rivers (Bridge 1993b). 67

What types of forcing factors could have caused the abrupt change from an eolian sand sheet to an ephemeral braided fluvial depositional system and vice versa? According to Silva (1998) and Martins-Neto (1998) the sandstone conglomerate were originated by tectonics events that caused a topographic rejuvenation, providing the morphological conditions to generate coarser sediments. According to Santos et al. (2013) these processes are associated to depositional dynamic, although not well defined by the authors. Overall, desert depositional systems are characterized by alternating phases of more humid and drier periods in which subaqueous and eolian processes alternate (Langford & Chan 1989, Langford 1989, Gustavson & Holliday 1999, Kocurek 1999, Scherer & Lavinia 2006, Basilici et al. 2009, Basilici & Dal Bó 2010). Variations in precipitation can explain the changes of depositional systems from eolian sand sheets to braided alluvial plains (Fig. 5). A climate change characterized by a strong increase of the precipitation can modify the surface of the eolian sand sheet. Firstly, the presence of superficial water and/or high water level inhibited the wind activity, hindering the transport of grains by wind. Later, the subaqueous flows disrupted previous eolian forms and eroded the eolian deposits filling the channels with their intraclasts. The restoration of the dry period resulted with the interruption of the fluvial activity, and the burial of the fluvial deposits with a new eolian sand sheet. 68

Figure 5. Hypothetical model of generation of the Bandeirinha formation, based on the alternation of eolian sand sheet and fluvial system controlled by climate phases.

CONCLUSIONS

The Bandeirinha Formation, 1.8-1.7 Ga, is a sedimentary succession, 250 m thick, composed of sandstone interbedded with three sandstone conglomerate episodes. The main findings of the paper are as follows. (1) The Bandeirinha Formation was formed in two depositional environments: an eolian sand sheet, constituted of two architectural elements (translatent climbing ripple sandstone and cross-stratified fine-grained sandstone), and a braided fluvial systems, constituted of a sandstone conglomerate element. (2) The Bandeirinha Formation is an uncommon case of thick eolian sand sheet succession. The architectural elements, translatent climbing ripple sandstone and cross- stratified fine-grained sandstone, were formed by the overlapping of zibar dunes and uncommon small slipfaced dunes. No evidence of subaqueous processes or high water table is present in the sandstone. The construction of the sand sheet was attributed to precocious cementation of the sand deposits by evaporite minerals, which decreased the sand availability and the wind capacity to generate dunes with slipface. The sandstone conglomerate was deposited in ephemeral channel rivers, characterized of high-concentrated and sporadic subaqueous flows. The interpretation of this element as alluvial fan deposit cannot be sustained because there is a lack of the main features of an alluvial fan system (interchannel sheet flow deposits and channel deposits with low relationship width/thickness) and the composition of the conglomerate clasts does not reflect a drainage basin in mountain areas. (3) The alternation between sandstone conglomerate and sandstone is attributed to climate variations. A strong increase of precipitation has stopped the eolian processes of transport and sedimentation and formed braided fluvial channel that cannibalized previous eolian deposits and filled the channels with intraclasts. The tectonic interpretation to explain these alternations can no longer be sustained, due to the reinterpretation of the sandstone conglomerate as braided fluvial system and the fact that most of the conglomerate clasts have an intrabasinal origin. 70

ACKNOWLEDGEMENTS

The authors are grateful to Dona Carmen and Rommel of Casa da Gloria in Diamantina to be enthusiastic hosts. Funding was provided by CAPES and PROAP of post- graduation Geosciences course of University of Campinas.

REFERENCES

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APÊNDICE II

UNUSUAL THICK EOLIAN SAND SHEET SEDIMENTARY SUCCESSION: PALEOPROTEROZOIC BANDEIRINHA FORMATION, MINAS GERAIS

Insólita espessura de uma sucessão de lençol de areia: Formação Bandeirinha, Paleoproterozoico, Minas Gerais

1 1 Fábio Simplicio *, Giorgio Basilici

1 Institute of Geosciences, Universidade Estadual de Campinas – UNICAMP, Campinas (SP), Brasil. E-mails: [email protected]; [email protected] 76

ABSTRACT

Some present-day eolian sand sheets have small and width dunes, called zibars, as common type of depositional morphology. Their formation is related to different stabilizing factors, which reduce the availability of clastic materials for eolian processes. In fact, zibars are dunes which do not have time to develop a larger dune with slipface (proto-dune). Dunes in sand sheet areas generally are constructed by wind ripple laminations and commonly generate sedimentary succession less than 20 m thick, which are the consequence of low sand supply. This study deals with an uncommon eolian sand sheet sedimentary succession more than 50 m thick, Proterozoic in age, known as Bandeirinha Formation. This paper tries to explain the anomalous thickness of this sand sheet sedimentary succession. High thickness was probably the result of a high input of sand material, combined with low availability of sand, thus allowing the only construction of proto-dunes (zibars). Early cementation, due to near-surface evaporation of saline water, has been proposed as main factor that reduced the sand availability into this eolian system. Finally, the subsidence processes related to the first stage of rift Espinhaço Basin must have generated the accommodation space to preserve the sand sheet succession.

KEYWORDS: Eolian sand sheet; zibar; ephemeral channel rivers; Bandeirinha Formation; Paleoproterozoic.

RESUMO

Em alguns lençóis de areias eólica atuais existem dunas pequenas e largas, chamadas zibars, como tipo comum de morfologia deposicional. Sua formação é relacionada a diferentes fatores de estabilização, os quais reduzem a disponibilidade de materiais clásticos para os processos eólicos. De fato, os zibars são dunas que não tiveram tempo para se desenvolver como grandes dunas (protodunas). As dunas dos lençóis de areia (zibars) geralmente são construídas por laminações de marcas onduladas de vento e comumente geram sucessões com menos de 20 m de espessura, consequência da baixa oferta de sedimento. Este estudo trata de uma sucessão sedimentar com depósitos de lençóis de areia de espessuras incomuns, com mais do que 50 m de espessura, encontrados na Formação Bandeirinha, de idade Proterozoica. Este trabalho tenta explicar as espessuras anômalas de depósitos de lençol de areia. A elevada espessura deve-se provavelmente ao grande aporte de material clástico no sistema, combinado com a baixa disponibilidade de sedimentos, que não permitiu a formação de dunas com faces de deslizamento, mas somente de proto-dunas (zibars). Cimentação precoce, causada pela evaporação de água salina próxima à superfície, tem sido proposta como principal fator para a redução da disponibilidade de areia neste sistema eólico. Finalmente, os processos de subsidência relacionados aos primeiros estágios de rifteamento da Bacia Espinhaço devem ter gerado o espaço de acomodação para preservar a sucessão de lençol de areia.

PALAVRAS-CHAVE: Lençol de areia; zibar; rios efêmeros; Formação Bandeirinha; Paleoproterozoico. 78

INTRODUCTION

Eolian sand sheet depositional systems are flattened areas where the main bedforms are wind ripples, slipfaceless dunes (zibars and nabkhas) and more rarely slipfaced dunes. Poor availability of clastic material is the main responsible factor to do not form slipfaced dunes (Nielson & Kocurek 1986, Lea & Waythomas 1990). Poor availability of sand for eolian transport and deposition is controlled by (a) vegetation, which constitutes an obstacle to the wind, (b) water table, causing cohesion of the grains, (c) abundance of coarse- grained sands or granules, (d) cemented depositional surface, and (e) seasonal flooding that causes cohesive muddy covering (Kocurek & Nielson 1986, Clemmensen & Dam 1993, Chakraborty & Chakraborty 2001, Basilici & Dal Bó 2014). More recently, studies in the Arctic and Antarctic areas have pointed out that the ice can be an important factor to poor availability of sand (Lea & Waythomas 1990, Bristow et al. 2010). All these factors may be considered for Precambrian eolian sand sheet deposition with exception to vegetation factor, which appeared on the earth surface only after Silurian (Clemmensen & Dam 1993). Present-day eolian sand sheets are areas of deposition or deflation, which occur in warm or cold, arid or semi-arid climates (Kocurek & Nielson 1986, Mountney & Russell 2004, Mountney 2006, Basilici & Dal Bó 2014). The deflation areas generate deposits constituted by granule or gravel lags, whereas the depositional areas are predominantly constituted by nabkhas and/or zibars and rare slipfaced dunes. Present-day nabkhas are associated to vegetation, thus their presence in Precambrian eolian systems should be excluded. Zibars probably resulted to be the most common dunes in Precambrian eolian sand sheets depositional systems. Zibars, which are also considered proto-dunes (Nielson & Kocurek 1986), are described in present-day eolian sand sheet environments, but their recognition in the geological record is not clear. Sets of wind ripple laminations, separated by erosive truncation surfaces, are the main depositional expression of zibars and nabkhas deposits (Kocurek & Nielson 1986, Lea & Waythomas 1990, Mountney & Russel 2004, Bristow et al. 2010, Basilici & Dal Bó 2014). Ancient eolian sand sheets are typically less than 20 m thick (Mountney 2006), as these environments are characterized by low sand supply. However, the Bandeirinha Formation, a sedimentary succession c.250 m thick, is composed by four intervals of sand sheet deposits, each one more than 50 m thick. These intervals are interbedded with 1.5 - 18 m thick sandy conglomerates, interpreted as deposits of highly concentrated river flows. The 79

aim of this paper is to discuss the reasons which have induced the accumulation of this unusually thick eolian sand sheet succession. 80

GEOLOGICAL SETTING

The Bandeirinha Formation is the lowermost unit of Espinhaço Supergroup, deposited into a homonymous Paleoproterozoic Basin, located in a wide area between the São Francisco Craton and the Araçuaí fold and thrust belt (Fig. 1A) (Almeida 1977, Chaves & Brandão 2004). The Bandeirinha Formation crops out an area near the Diamantina city, in Minas Gerais (Fig. 1B). An extensional event at Statherian age (1.8 - 1.6 Ga), which generated a lithospheric stretching and subsequent break-up of the São Francisco-Congo Craton, is supposed to have been the responsible mechanism of the creation of accommodation space in the Espinhaço Basin (Alkmin & Marshak 1998). Chemale et al. (2012) and Santos et al. (2013) consider two phases of basin evolution. The first phase developed according to N-S fault-block activity that created a rift basin, and the second phase related to flexural subsidence, which was associated to thermal process (Martins-Neto 2000; Chemale et al. 2012). The Espinhaço Supergroup is subdivided in Diamantina and Conselheiro Mata groups (Dussin & Dussin 1995). The lower Diamantina Group is composed by Bandeirinha, São João da Chapada, Sopa-Brumadinho and Galho do Miguel formations (Fig. 1B) (Fogaça et al. 1984, Silva 1998, Martins-Neto 2000, Chemale et al. 2012). The Bandeirinha Formation (1.7 – 1.8 Ga; Chemale et al. 2012) is c.250 m thick mostly composed of laminated sandstone packages interbedded with three conglomerate beds (Fig. 2). This litostratigraphic unit was previously interpreted by Silva (1998) as result of coastal and braided river depositional process, where the interbedded conglomerate beds represented mainly mass flow deposits in proximal areas of alluvial fans, related to tectonic activity. Simplicio (2013) proposed a paleoenvironmental model in which sandstones were deposited in sand sheet context and conglomerates correspond to deposition in ephemeral streams. The Bandeirinha Formation is weakly to moderately deformed and metamorphized unit, with beds dipping toward N-NE of 25 – 35º. Bed geometry and primary sedimentary structures are well preserved, thus sedimentological terminologies are used.

Figura 1. Geographical location of the Espinhaço Supergroup relative to São Francisco Craton and Araçuaí fold and thrust belt. (A) The Diamantina region is located in the middle of the Southern Espinhaço Range. (B) Geological map of the study area, which is located 10 km towards SW from the Diamantina city (see Fig. 2). Modified from Chaves and Brandão (2004), Lopes-Silva (2008), Lopes-Silva and Knauer (2013). 82

METHOD

A stratigraphic section of more than 250 m thick was measured in type area of Bandeirinha Formation (Fig. 2). Detailed facies analysis, based on grain-size, sorting, type and organization of the sedimentary structures, form and dimension of the beds, characteristics of the bounding surfaces, lateral and vertical variation of lithofacies, and interpretation of the depositional mechanisms, was applied to the measured section (Miall 1985; Walker 2006). 83

Figura 2. Stratigraphic section of Bandeirinha Formation in the study area. Note the three sandy conglomerate beds interbedded with laminated sandstone. The bottom contact corresponds to the Barão de Guaicuí Formation and the upper boundary with trough cross- stratification sandstones to the São João da Chapada Formation. 84

SEDIMENTOLOGICAL ANALYSIS

The sedimentary succession of the Bandeirinha Formation is formed of eolian deposits, constituted by laminated and cross-stratified sandstone facies and fluvial deposits, constituted by sandy conglomerate facies.

Laminated sandstone

Laminated sandstone is formed of bimodal fine- to coarse-grained reddish to white sandstone, moderately to well-sorted, with rounded to well-rounded grains. Planar to low-angle inclined (up to 10º), parallel laminations, are the dominant sedimentary structures (Fig. 3A). The laminae are 2 – 12 mm thick with grain-size segregation and sometimes characterized by inverse grading; they are similar to the “pin stripe lamination” described by Fryberger and Schenk (1988) (Fig. 3B). Laminated sandstone sets, 0.4 – 2.0 m thick, laterally extended 4 to more than 9 m, are vertically and laterally overlapped. The bounding surfaces of the sets are erosive, 0 – 15° dipping, and are parallel to the laminae of the overhead set (Fig. 3A). Locally, asymmetrical undulating laminations may be observed in gradual transition with the planar laminations (Fig. 3C). The laminated sandstone sets constitute 84% of the thickness of the measured succession and form interval more than 50 m thick. Horizontal or low-angle planar and parallel laminae are interpreted as subcritical climbing translatent strata (Hunter 1977, Hunter 1981) (Figs. 3A and 3B). This sedimentary structure is produced by migration of climbing wind ripples. The similarity with the “pin stripe lamination” of Fryberger and Schenk (1988), the grain-size segregation (Eriksson & Simpson 1998) and the presence of high rounded sandstone clasts, typical of wind transport (Mahaney 2002), suggest this interpretation. Also the architectural framework, in which the laminated sandstone sets are separated by inclined (< 15°) bounding surfaces (truncation surfaces), supports this interpretation (Kocurek & Nielson 1986). The asymmetrical undulating laminations are considered supercritically climbing translatent strata related to the increase of deposition rate (Hunter 1977) similar to interpreted for Basilici and Dal Bó (2014) in Tulum Valley (Argentina). 85

Figura 3. Laminated sandstone. (A) Low-angle, parallel laminated sandstone, corresponding to subcritical translatent climbing strata, organized in sets separated by erosive surfaces (red lines indicate truncation surfaces). (B) Observe the grain-size segregation emphasised the planar and parallel laminations (arrows point the fine-grained laminae). (C) Asymmetrical undulating laminations (dotted rectangle) in gradual transition with planar and parallel laminations. Pencil: 14 cm.

Cross-stratified sandstone

Well- to very well-sorted, very fine- to fine-grained sandstone characterizes this facies. Cross-stratified sets, 0.12 – 0.3 m thick, 3 – 7 m laterally extended, constitute the sedimentary structure (Fig. 4). The foreset laminae are 2 – 10 mm thick, and dip 21 – 26º. The foresets display alternation of fine- and very fine-grained laminae. Reactivation surfaces (sensu Brookfield 1977) are also observed. This facies forms lenticular and isolated beds, which are embedded in laminated sandstone sets with erosive contact. Cross-stratified sandstone corresponds to 2% in thickness of the measured sandstone succession. 86

Figura 4. Cross-stratified sandstone facies (arrows) interbedded with laminated sandstone facies.

Cross-stratified sandstone indicates that this deposit was formed by avalanching processes, allowing its interpretation as lee side of small dunes. The fine- and very fine- grained foreset laminae, the interbedding with wind-rippled strata, the presence of reactivation surfaces and the absence of sedimentary structures associated to subaqueous flows suggest that these structures are eolian dunes (Hunter 1977; Kocurek 1996; Mountney 2006). Real height of the dunes is unknown because the erosive boundary surfaces, but the restricted lateral extension and rare occurrence of this structure suggests small dimension of the original bedforms.

Sandy conglomerate

Sandy conglomerate facies has an erosive and slightly concave-up bottom surface and planar top surface. Two types of sandy conglomerate subfacies are present: structureless sandy conglomerate and stratified sandy conglomerate (Fig. 5A). The structureless sandy conglomerate represents 12% of the thickness of the sedimentary succession; it is constituted of poorly sorted, clast-supported, pebble- to boulder-grained conglomerate within medium- to coarse-grained sandstone, poorly sorted matrix. The thickness of this lithofacies is 7–18 m, its bounding surfaces are erosive and concave-up, and its upper surface, when it is in contact with translatent climbing ripples strata, has bounding surface planar and probably non- erosive. The conglomerate clasts are constituted of laminated sandstone facies (Fig. 5B), 87

whose abundance is 87 – 100%, and the others clasts (vein-quartz, quartzite, banded iron formation and schist), whose abundance is 13 – 0%. Laminated sandstone clasts are angular to rounded, with prevalence of subangular (Fig. 5B); the other clasts are rounded or subrounded. The dimension of the clasts is extremely variable. Maximum particle size of laminated sandstone clasts is 0.35 m. The other clasts have maximum particle size of 0.09 m. This subfacies does not display any internal organization. Stratified conglomerate is less than 1.5 m thick, and it constitutes 2% of the thickness of the measured sedimentary succession. Maximum particle size of the clasts is 0.14 m; the matrix is a medium- and coarse-grained, poorly-sorted sandstone, and it is more abundant than structureless sandy conglomerate. Roundness and composition of the clasts are similar to the other lithofacies. The beds of this lithofacies are commonly interbedded with fine- and medium-grained, well sorted, inverse grading laminated sandstone, 0.04 – 0.15 m thick, which form lenticular beds laterally extended up to 3 m (Fig. 5A and 5C). These sandstone beds have gradual transition at the base to sandy conglomerate, but the top surface is sharp and smooth (Fig. 5C). The sandstone beds display planar parallel laminations, constituted of laminae 3 – 10 mm thick, at times with inverse grading. The concave-up bottom and the planar top surface of the sandy conglomerate are evidences that this facies represents the filling of channelized morphological structures, generated by subaqueous flows. The grain-size heterogeneity and the absence of internal structures of the conglomerate subfacies suggest highly concentrated subaqueous flows as depositional mechanisms (Tooth 2000, Rodriguéz-Lopez et al. 2012). More diluted subaqueous flows deposited the stratified sandy conglomerate subfacies. Most of the clasts that constitute these deposits were originated by the erosion of the substratum, constituted of laminated sandstone. This demonstrates that laminated sandstone had certain degree of competence before the transport (Trewin 1993, Mountney & Howell 2000). Angular or subangular roundness of these clasts means that the laminated sandstone clasts (intraclasts) did not undergo a long transport. The rounded exotic clasts indicate that the catchment area was external to the depositional basin. 88

Two hypotheses are possible to explain the laminated sandstone beds associated with stratified sandy conglomerate. These deposits may correspond to subaqueous or eolian deposits. If interpreted as subaqueous deposits they represent parallel laminations formed in upper flow regime (Fielding 2006). However, we do not observed parting laminations and

Figura 5. Sandy conglomerate facies. (A) The dashed line indicates the contact between stratified sandy conglomerate subfacies and structureless sandy conglomerate subfacies. The lenticular sandstone beds are indicated by arrows. (B) Sandstone clasts (rounded to angular) - Pencil: 14 cm. (C) The detailed scale to observe the sandstone beds - Jacob stick: 1.5 m. 89

laminae with normal grading, which are characteristics of this structure. Thus, we propose an eolian origin for these laminae due to clasts dimensions, sorting and inverse grading, consequently these laminae are interpreted as wind ripple laminations. Indeed, sand reworking by wind inside ephemeral channels in dry periods is very common (Cowan 1993, Tooth 2000, Jain et al. 2005, Krapf et al. 2005). 90

DISCUSSION

Paleoenvironment analysis

The Bandeirinha Formation is constituted by a monotonous sedimentary succession, composed by laminated sandstone with small lenticular cross-stratified sandstone sets, and sandy conglomerate (Fig. 2). Laminated sandstone deposits are interpreted as climbing wind ripples, cross-stratified sandstone as small eolian dunes, while sandy conglomerate were produced by ephemeral stream flows. The depositional environment is interpreted as dry eolian sand sheet, which was episodically occupied by ephemeral river systems that cut through the depositional surface. Laminated sandstone facies were probably part of zibar-like dunes (Fig. 6) and cross-stratified sets corresponded to small and isolated slipfaced dunes (Kocurek & Nielson 1986, Nielson & Kocurek 1986, Mountney & Russell 2004). Zibars bedforms may be considered like dunes, but that do not have time to fully development (proto-dunes), their internal architecture is composed by low-angle or tabular sets separated by truncation surface (Nielson & Kocurek 1986, Bristow et al. 2010). This eolian sand sheet was characterized by abundance of zibar dunes, as well as many present-day eolian systems (Nielson & Kocurek 1986, Wang et al. 2009). Nielson and Kocurek (1986) analyzed zibars deposits in a sand sheet area from Entrada Sandstone Formation and observed that slipfaced dunes deposits had to fine- to medium-grained sand (0.1 – 0.3 mm) and the zibars fine- to coarse -grained sand (0.1 – 1.0 mm) and claimed that this grain-size may be an important factor to preventing the slipfaced dunes formation. Laminated sandstones deposits of Bandeirinha Formation have grain-size analogous to the zibars described by Nielson & Kocurek (1986) suggesting an analogous interpretation of this facies. In present day, zibar dunes are common in sand sheet areas that are near to dune fields where the clastic input is high, but the availability is low, to form slipfaced dunes (Nielson & Kocurek 1986). The availability do not depends only of the input but of other factors that limit the quantity of loose sand due to the creation of protection surfaces, as well as the effect of water table rise and capillarity fringe action, which aggregate sand, coarse to pebble grained materials concentration, that shield the surface, early cementation, freezing and seasonal flooding (Kocurek & Nielson 1986, Basilici & Dal Bó 2014). The Bandeirinha Formation is a sandstone succession deposited in eolian- dominated paleoenvironment interbedded with three ephemeral fluvial episodes (Fig. 2). Eolian processes predominated for long time and they were interrupted by fluvial ephemeral 91

systems (Fig. 6). During the fluvial episodes, the increase of precipitations inhibited the eolian processes of transport and sedimentation, and formed fluvial channels that cannibalized previous eolian deposits, bringing about the filling of the channels with sandstone intraclasts. Probably the alternation between eolian and fluvial episodes has climatic causes, as reported by Trewin (1993) and Mountney & Howell (2000) to other units.

Unusual high thickness of the eolian sand sheet succession

Eolian sand sheet sedimentary successions are commonly less than 20 m thick (Mountney 2006), because these depositional systems are transitional to dune fields or because geographic position makes difficult their accumulation (e.g. proximity to river systems). Chakraborty & Chakraborty (2001) described the Shikaoda Formation (Hosangabad, India), 40 m thick, as the thickest eolian sand sheet deposit that has been recognized. These authors noted that the eolian system developed in supratidal setting, in which the sand sheet area was affected, directly or indirectly, by the tides, as suggested by subaqueous structures, adhesion strata, and evaporites deposits. Chakraborty & Chakraborty (2001) concluded that frequent tidal flooding moist the sand, supplied by the coastal setting, thus inhibiting the availability of sand and the consequent construction of slipfaced dunes. Differently from the Shikaoda Formation, the eolian sand sheet succession of the Bandeirinha Formation does not have subaqueous deposits, mud cracks or adhesion structures that may indicate subaqueous processes, flooding and high humidity in the surface (Nielson & Kocurek 1986, Clemensen & Dam 1993, Rodriguéz-Lopez et al. 2010, Basilici & Dal Bó 2014). However, alike to the Shikaoda Formation, the eolian sand sheet sedimentary succession of the Bandeirinha Formation is characterized by a high thickness, which suggests high sandy supply. In general, when an eolian system receives large input of sediment slipfaced dunes are formed (Kocurek & Nielson 1986). Nevertheless, Bandeirinha Formation shows rare dunes with slipface, testifying low availability of sand. To explain the apparent inconsistency between high supply and low availability of sediment, two other factors may be invoked according to recorded features: early cementation of the sand and coarse grain-size. Early cementation of the sand, soon after the deposition, was probably the main control factor that allowed the accumulation of these sand materials and its poor availability. This is suggested by the sandy conglomerate facies, which is constituted by intraclasts of laminated sandstone. Stream flow cannibalized the previously cemented eolian sand sheet deposits, which furnished the main clastic source of the fluvial conglomerates, indicating the sand 92

materials were been subjected to early cementation processes. In dry and hot continental environments, where the evaporation exceeds the meteoric influx, the groundwater flux is upward directed, facilitating the mineral precipitation (Worden & Burley 2003). Similar interpretation was described by Rodriguéz-Lopez et al. 2012 to explain the origin of sandstone intraclast in eolian Teruel Basin (Spain). Early cementation may be due to evaporitic mineral precipitation, as observed by Basilici & Dal Bó (2014) in present-day sand sheet deposits, but the later metamorphism and diagenesis masked or modified previous evaporite cements of Bandeirinha Formation.

Figura 6. The interbedding between laminated sandstone and sandy conglomerate facies is interpreted as due to alternating climate phases, where laminated sandstone, which represents an eolian sand sheet with zibars bedforms, formed in dry phase and sandy conglomerate, that represent a river channel filling, is related to more humid periods. 94

In conclusion, early cementation was considered one of the most important factor of control that determine the accumulation of the sandstone. The preservation of Bandeirinha Formation was possible by accommodation space that placed eolian accumulation below the erosional base level (Kocurek & Havholm 1993). Accommodation space was related to high subsidence rate attributed to the early stages of rift evolution proposed for Espinhaço Basin (Dussin & Dussin 1995, Silva 1998, Martins-Neto 2000, Chemale et al. 2012).

CONCLUSIONS

The Bandeirinha Formation is constituted of a sandstone succession interbedded with three sandy conglomerate episodes. Sandstone beds were deposited in an eolian sand sheet paleoenvironment, whereas the sandy conglomerate beds were formed in ephemeral river channels for highly concentrated subaqueous flows. Probably, alternating climate phases, dry and more humid, were responsible for eolian sand sheet and fluvial depositional palaeoenvironments, respectively. The anomalous thickness of the eolian sand sheet deposits was due to high supply of sand material combined with early cementation of the sand, which decreased the sand availability and caused the accumulation of low and wide dunes (zibar). High rate of subsidence associated with an extensional tectonic phase enabled the eolian sand sheet preservation. 96

ACKNOWLEDGEMENT

The authors gratefully acknowledge the Capes for financial support. We would like to thank also Mrs. Carmem and Mr. Rommel, wonderful employees of the Casa da Gloria in Diamantina.

REFERENCES

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APÊNDICE III

TEMPORAL EVOLUTION OF AN EARLY CRETACEOUS PLAYA LAKE: THE SEDIMENTARY RECORD OF QUIRICÓ AND TRÊS BARRAS FORMATION (SANFRANCISCANA BASIN, SOUTH-EASTERN BRAZIL) 102

ABSTRACT

Playa lakes are depositional environments developed in endorheic continental basins in which the hydric balance is normally negative. The sedimentary record of the formations Quiricó and Três Barras (Sanfranciscana Basin) suggest the transition from a playa lake to aeolian dune field. Detailed sedimentological analyses of 28.3 m thick sedimentary succession allowed distinguishing four stages of local depositional evolution. In the first stage the sedimentary environment was dominated by an inner saline mudflat, a flat area with high relief efflorescent salt crusts, which suggest a shallow groundwater table. The second stage consists in the record of expansion of a temporally perennial shallow lake, indicative of a wetter period. The record of the third stage consists in an outer saline mudflat, characterized by thin efflorescent salt crusts, which indicates the relatively deeper groundwater table, and a drier period. The fourth stage resembles an extremely dry environment, the playa lake disappeared and the aeolian dune field was constructed. The analysis suggest the progradation of an aeolian dune field over the playa lake environment, indicating a transitional period controlled by decrease of the water influx through the groundwater table, which produced a progressive increase in the availability of sand to aeolian dunes field construction.

KEYWORDS: Playa lake, Saline mudflat, Efflorescent salt crust, Aeolian dune field, Sanfranciscana Basin.

103

INTRODUCTION

Playa lakes may be defined as end members of closed drainage systems, located in lowland areas of continental basins (Briere 2000, Teller and Last 1990, Tunbridge 1984). These environments are developed under influence of semi-arid or arid climates and are strongly influenced by environmental moisture content, which varies according to the balance between water influx and outflux, induced respectively by surface and/or subsurface water flows and evaporative processes (Benison and Goldstein 2001, Bobst et al. 2001, Smoot 1983). Considering that the playa lakes are depositional environments composed of genetically linked subenvironments, Hardie et al. (1978) defined that in the shallow and wide sedimentary basins, setting in arid sub-tropical high pressure belts, the environments are composed of a salt pan surrounded by a saline mudflat, which, in turn, is bordered by aeolian dune field and ephemeral fluvial streams. Although this depositional setting is common and easy to recognize in several present- day playa lake environments (Bobst et al. 2001, Hardie et al. 1978, Lokier 2012, Smoot and Castens-Seidell 1994), their sedimentary records are not so easy to identify, especially when the evaporite minerals are not preserved. Indeed, the saline mudflats are characterized by the wide diversity of efflorescent salt crusts, which may be partially or completely filled by clastic sediments (Smoot and Castens-Seidell 1994), producing a variety of sedimentary records (Goodall et al. 2000, Smoot and Castens-Seidell 1994). In this article, we examined the transition from a playa lake to an aeolian dune field. The idea is to discuss the factors of control that were responsible for the paleoenvironmental transformation in this part of the Sanfranciscana Basin. This study may contribute to the recognition and interpretation of other ancient playa lakes. 104

GEOLOGICAL SETTING

The studied area occurs in the city of Presidente Olegário, in Minas Gerais State, southeast of Brazil. The sedimentary succession is part of the formations Quiricó and Três Barras, and it is composed of poorly cemented coarse-grained sandstone to mudstone; rare beds show calcite pseudomorphs after gypsum. These two units constitute part of the Sanfranciscana Basin, which is interpreted as a sag type basin (Campo and Dardenne 1997). The Sanfranciscana Basin is divided in two sub-basins (Abaeté and Urucuia sub-basins) (Fig. 1A). The tectonic evolution of this basin is related to the uplift of the Alto Paranaíba where isostatic adjustment creates the accommodation space on the Abaeté sub-basin (Hasui and Haralyi 1991). The Alto Paranaíba Uplift also constitutes the main source of clastic sediment to the Abaeté sub-basin (Sgarbi et al. 2001). The analysed sedimentary succession is part of the Abaeté sub-basin (Fig. 1A), in which the sedimentary filling (~300 m thick) is constituted by deposits of Areado and Mata da Corda Groups; an angular unconformity separates the bottom of Areado Group from the Proterozoic Bambuí Group (Fig. 1B) (Fragoso et al. 2011, Sgarbi et al. 2001). The Areado Group is divided in three formations: from the bottom to the top, Abaeté, Quiricó and Três Barras Formation (Fig. 1C) (Campos and Dardenne 1997, Fragoso et al. 2011, Sgarbi et al. 2001). The Abaeté Formation occurs locally and it is composed of conglomerates, breccias and sandstones and it is interpreted as alluvial fan deposits. The Quiricó Formation is constituted of mudstones and subordinately sandstones, which are formed in playa lake systems (Sgarbi et al. 2001). The Três Barras Formation is formed of well-sorted sandstones deposited in an aeolian environment. This sedimentary succession is overlain by an Upper Cretaceous mafic/ultramafic volcano-clastic succession (Mata da Corda Group) (Fig. 2A). 105

Figure 1. Geological setting and location of the study area. A) In the left side, look at the extension of Sanfranciscana Basin, which is divided in two sub-basin (Abaeté and Urucuia), and in the right side the geographical location of the study area. Stratigraphy of the study area and C) the geological map, that the study area is belong. Modified after Campos and Dardenne (1997), Sgarbi et al. (2001) and Fragoso et al. (2013). 106

METHODS

The study area is illustrative of the change from playa lake to aeolian dune field paleoenvironment. A sedimentary section, 28.3 m thick, was measured and analyzed in detail (Fig. 2B). The sedimentary deposits were subdivided in facies associations, which are considered a group of facies genetically related (Walker 2006). The description of facies associations demanded the observation of lithology, sedimentary texture and structure (Reading and Levell 1996). Geometry of the beds and their bounding surfaces were described during the field activity. The deposits subdivided as facies association were interpreted comparing them with examples of the present-day depositional environments and using analogous from ancient and well-studied sedimentary succession. 107

Figure 2. The sedimentary succession analysed and the four depositional stages described in this study. A) Regional correlation of the depositional interval analised in this study, which correspond to the transition of the Quiricó to Três Barras Formation, modified after Fragoso (2011). B) The drying upward sequence, which is representative stratigraphic section of the progradation of aeolian dune field on the playa lake palaeoenvironment.

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RESULTS: THE FACIES ASSOCIATIONS

Six facies associations compose the studied stratigraphic interval (Fig. 2B). They are named as shallow lake, inner saline mudflat, outer saline mudflat, flooding deposits, small channel and aeolian dunes facies associations.

Shallow lake facies association

This facies association consists of claystone laminae alternated with muddy sandstone laminae (Fig. 3). The claystone laminae are red or yellowish-grey, 1 to 6 mm thick. The contacts in the base of the laminae are normal graded and are composed of fine-grained sand to clay, are pale-yellow, 3-8 mm thick (Fig. 3A). Ripple bedforms are common (Fig. 3B) and they form beds, 3-22 mm thick; in cross-section they show cross-laminations; in plan-view, they show rectilinear, flatted and bifurcate crests, normally covered by thin film of claystone (Fig. 3B). Sometimes interference ripples are observed in plain view. This facies association constitutes intervals up to 2.5 m thick, and are laterally extend more than 40 m. The bottom boundary is sharp, not erosive, planar to slightly undulating; they are interbedded with inner saline mudflat facies associations. Interpretation The general characteristics described in this facies association suggest deposition in shallow-lake with periods of stagnant conditions alternated with water flows. The subaqueous flows are probably associated with flood events, which introduced sediment to the lake system by underflows and decrease of flow velocity; depositing sand to clay by the mechanism of down flow (Abdul-Aziz et al. 2003, Armenteros et al. 1995, Paik and Kim 2006). Sandstone lenses observed in cross-section were formed by oscillatory movement, as well as the symmetrical to weakly asymmetrical ripples bedforms observed in plain-view. The cross-laminations were produced by combination of oscillatory and unidirectional flows. The clay films suggest period of stagnant water. The relative large extension combined with the small thickness of this facies association indicates large water bodies with long periods of stagnancy. The absence of evaporite deposits or pseudomorphs of evaporite minerals may suggest freshwater chemistry. 109

Figure 3. Shallow lake facies association. A) Planar-parallel laminae of interbedded muddy sandstone (yellow) and claystone (red). The laminae are slightly undulated. B) Sandstone layer with asymmetrical ripples in plain view showing rectilinear and bifurcate crests covered by clay. The arrow indicates the flow direction.

Inner saline mudflat facies association

This facies association is constituted of red or yellowish green, sandy mudstone beds with small sandstone lenses (Fig 4). These deposits consist of tabular beds, 0.20-3.4 m thick and laterally extended to more than 100 m. The sandy mudstones are poorly sorted deposits composed mostly of clay to fine-grained sand and in some beds medium-grained sand; the sand grains are angular to sub-rounded. Sedimentary structures are rarely observed in these beds; planar- or cross-laminations, and some rippled structures (Fig. 4A) are weakly preserved. Lenses composed of sandstones or claystones are common. The sandstone lenses are white or yellowish red and are composed of very fine- to medium-grained sand. The shapes of lenses are slightly spherical or elliptical. Spherical shape is 4-30 mm in diameter and elongated shape is 2-3.5 cm thick and 90-190 mm wide. In addition, plane to concave-up, 4 to 13 cm thick and 5-20 cm wide, medium-grained sandstone lenses with planar- and/or cross-laminations, were observed embedded in some beds (Fig. 4B). Although rare, ripple bedforms are preserved in some beds. Common feature observed in structureless sandy mudstone deposits are the intrasediment calcite pseudomorphs after gypsum (Fig. 4C). The crystals are preserved in two different ways: as displacive lenticular crystals of 2-4 cm high (Fig. 4C and D) or aggregates of millimeter scale (less than 2 mm). This facies association is dominant in the lower part of the sedimentary succession (Fig. 2B) and this occurs interstratified with shallow lake and outer saline mudflat facies association. 110

Interpretation The poorly sorted and almost structureless deposits reflect a peculiar process of deposition, in which silt- to fine-grained sand was transported by wind and trapped by capillary adhesion on the high relief efflorescent salt crusts in the inner parts of the saline mudflat (Goodall et al. 2000, Smoot and Castens-Seidell 1994). The process of trapping creates a film of sediments on the crusts, which isolated it and inhibit the insolation, and by consequence, making the underlying saline crust be dissolved by the contact with the capillarity fringe (i.e., without evaporation the subsurface water may be unsaturated). New saline crusts grew above the film of sediment and the process continues until the next period of insolation loss (Smoot and Castens-Seidell 1994). The repetition of these processes produces a structureless sandy mudstone. Efflorescent salt crusts are rarely preserved in the sedimentary record by itself (Smoot and Castens-Seidell 1994, Lokier 2012). The small spherical or elongated lenses, were produced by deposition of sand on the irregular tops of high relieve efflorescent salt crusts. In the inner saline mudflats, efflorescent crusts are thick, with steep and narrow depressions. The distance to source-area is likely the main reason of the scant presence of sand deposits on the efflorescent salt crusts. The sandstone lenses with preserved planar- and cross-laminations embedded in sandy mudstone beds consists in the filling of hollows, which were produced by differential dissolution of the efflorescent salt crusts surface after flooding periods. The hollows were filled by sand sediments transported by ephemeral water flows (Goodall et al. 2000). Lenticular calcite pseudomorphs after gypsum and crystals aggregates were formed after deposition (Benison and Goldstein 2001, Eugster and Hardie 1975). Smoot and Castens-Seidell (1994) argument that displacive evaporites associated with deposits of poorly sorted and structureless sandy mudstone correspond to an important feature to recognition of deposits from inner saline mudflats. 111

Figure 4. Inner saline mudflat deposits. A) Observe in this image the structureless feature of the inner saline mudflat facies association. Small lenses (white) in sandy mudstone bed (red). Observe the slightly preserved ripple bedform in the lower and right corner. B) The image shows sandstone lens with cross or low-angle laminations embedded in sandy mudstone, “lobate projections” from Goodall et al. (2000). C) Calcite pseudomorph after desert-rose gypsum (black arrow). D) Crystal of calcite pseudomorphs after gypsum. 112

Outer saline mudflat facies association

This facies association consists of red or purple muddy sandstones with white sandstone lenses, which is widespread (Fig. 5A). The muddy sandstones are poorly sorted and composed of clay to medium-grained sands. The sand grains are sub-rounded to well- rounded. The white lenses are well-sorted, fine- to medium-grained sandstones. They show different types, which may be divided into three different groups. (1) The most common type consists of small lenses (2-4 cm thick) that display cuspate and jagged edges with muddy sandstones (Fig. 5B), which occur as aggregates and shows a large variety of forms, similar to mottling aspect (Fig. 5A). (2) Another type is constituted of bowl-shaped lens and upturned edges (Fig. 5B and C), which are composed of well- to very well-rounded and well-sorted sandstones. The bowl-shaped lenses variates in dimensions and may be 1-3 cm thick and 6-46 cm wide, although the most are less than 10 cm wide and 2 cm thick. (3) The third type is composed of ripple lenses in vaguely layered and deformed aspect; sometimes they have hump-shaped crests (Fig. 5D). Other structures occur in this facies association; tepee-like structures are observed in sandstone beds. The crests are 0.4-0.5 m distant and 3-4 cm high (Fig. 5E). This facies association has sharp, but non-erosive contact with inner saline mudflat at the base of the succession. In the upper portion of the succession this facies association is alternated with aeolian dune facies association.

Figure 5. Outer saline mudflat deposits. A) Muddy sandstone (red to violet) with widespread sandstone lenses (white). B) Observe the bowl-shaped sandstone lenses with upturned edges (left side) and the lenses with jagged edges (right side). C) Detail to thin sandstone lens with jagged boundaries, feature imitative of “popcorn surface”. D) Deformed ripple are also observed in some lenses. E) Observe the tepee-like structures. The arrows indicate the crests. (pencil: 140 mm) 113

Interpretation The muddy sandstones were formed by sediment trapping due to capillary adhesion on thin efflorescent salt crusts, followed by salt dissolution after loss of insolation and evaporation, and consecutive re-growth of efflorescent salt crusts. Thin efflorescent salt crusts are typical from outer parts of saline mudflats (Goodall et al. 2000). The small sandstone lenses with cuspate and jagged edges reflect the morphological features of thin efflorescent salt crusts that sometimes are characterized by irregularities with depressions and humps, which were nicknamed “popcorn surface” by Smoot and Castens-Seidell (1994). These surfaces, due to the proximity with the edges of saline mudflat, have their depressions completely filled by sand grains. Similarly, the bowl-shaped lenses reflect the shape of the polygons structures that are bordered by salt ridges, other common morphology from outer saline mud flat (Bobst et al. 2001, Goodall et al. 2000, Lokier et al. 2012). Some lenses show upturned edges, which suggest that the depression keeps growing in the same time that are filled by sand grains (Smoot and Castens-Seidell 1994). Probably, the greater volume of sandstones in this facies association results of the proximity with fluvial and aeolian systems. Rippled structures with hump-shaped crests correspond to ripples deformed by the growth of efflorescent salt crusts on the poorly sorted sandy mud (Goodall et al. 2000). The tepee-like structures are deformations, which reflect the development of large polygons on the depositional surface, typical from outer edges of saline mudflats (Smoot and Castens-Seidell 1994). The common interbedding with aeolian dunes facies association and the upward decrease of thickness of this facies association suggest the proximity and progressive progradation of the aeolian dune field.

Flooding deposit facies associations

This facies association is constituted of tabular beds with horizontal and erosive planar to slightly concave-up bottom, and planar top. The bed thickness varies from 11 to 30 cm and they are laterally continuous up to 20 m. Cross-stratifications (Fig. 6A) and planar laminations (Fig. 6B) are the main sedimentary structures that compose these beds. The cross- stratifications occur in single sets and are composed of fine- to medium-grained, angular to sub-rounded and moderately sorted sandstones. The foresets are planar in the basal contact and 17° dip (Fig. 6A). The beds with planar laminations are composed by very fine to fine- grained, angular to sub-rounded, well-sorted sandstones. The laminations are 1-3 mm thick, and characterised by the differences of grain-size and colour variations (Fig. 6B). Red to 114

brown mudstone clasts, 15 mm large, are observed in the some beds. This facies association occasionally occur interbedded with inner saline mudflat and outer saline mudflat facies associations; its lower boundary is sharp and erosive, the top is sharp. Interpretation The features described in this facies association indicate deposition in flood event, where turbulent flows were responsible to generation of erosive scouring observed in the bottom contacts. The absence of channelized shape of the beds and the large lateral extension of the beds indicate unconfined or poorly confined flows. The cross-stratifications are interpreted as depositional record of bi-dimensional subaqueous dunes. Planar laminations suggest deposition in upper regime flow (Alexander et al. 2001, Bridge 2006, Fielding 2006).

Small-channels facies associations

This facies association consist of lenticular beds of sandstones, up to 2.5 m thick and 1.5 m wide in section perpendicular to the cross-stratification (Fig. 6C). The bottom is erosive and concave-up and the top is planar. Well-sorted, fine- to medium-grained sandstones filled this channelized form. Internally, various sets of low angle cross-stratifications were observed. This facies association is rare; it occurs embedded within outer saline mudflat facies association. Interpretation 115

Figure 6. Subaqueous deposits. A) Sandstone bed with cross-stratification in angular basal contact. (Pencil: 14 cm). B) Sandstones bed with planar lamination. Observe the small mudstone clast. (Coin: 2 cm). Each lamination exhibits a lateral continuity of 0.1 to 0.6 m. (Pencil: 0.14 m). C) Channel structure isolated in sandy mudstone bed of outer saline mudflat facies association. The structure shows erosive concave-up base (arrows) and top planar filled of low angle cross-bedded sandstone.

Concave-up with erosive bottom and planar top suggest ribbon-shaped channelized flows. Grain-size features and sedimentary structures indicate that these channels were filled by subaqueous and small dunes (Ramos et al. 1986). Isolated small channels are common to present-day saline mudflats, as described by Smoot and Castens-Seidell (1994) in the Saline Valley from California.

Aeolian dune facies association

This facies association consists of red to pale-yellow, very fine- to medium-grained sandstone, with well-sorting or bimodal sorting. Sets of planar cross-bedding (9-15° dip) may overlap one another in angular or tangential contact. Sometimes, sets of tangential cross- bedding overlap planar and parallel to low angle cross-lamination (Fig. 7). The sets are 0.1- 1.2 m thick and its lateral extension may be more than 15 m. The cross-stratifications are composed of foresets of very fine-grained sandstone alternated with wedge-shaped medium- 116

grained sandstone (i.e. bimodal sorting). In section perpendicular to the dip direction, the foresets are large trough sets with cross-bedding, 5-9 m wide and 0.6-1.2 m high, and <7° dip; in this case the boundaries are tangential. The contact with planar and parallel to low angle cross-laminations, which are 0.8-1 cm thick and more than 15 m large, are gently inclined (<6°). This facies association may occur intercalated with outer saline mudflats, and in this case they may occur intercalated with deformed laminations.

Figure 7. Aeolian deposits. Sandstone beds with cross-strata (CR) separated by erosive surfaces (dotted line), sometimes intercalated with deformed ripples (DR) or planar laminations (PL). Deformed lamination are related with efflorescent salt crusts.

117

Interpretation This facies association is interpreted as aeolian dune deposits. The very fine sandstone layers are interpreted as grain-fall strata whilst the medium-grained with wedge-shape as grain-flow strata (Hunter 1977). The alternation of grain-flow and sometimes grain-fall strata correspond to a typical product of migration of aeolian dunes (Kocurek and Dott 1981, Hunter 1977, Mountney 2006). The sandstones with trough cross-bedding correspond to sedimentary record of sinuous crested aeolian dunes. The planar and parallel to low angle cross- laminations correspond to subcritical climbing translatent strata produced by climbing wind ripple migration (Hunter 1977). The sets composed of sandstone with cross-bedding in gently inclined and tangential contact with planar and parallel to cross-laminated beds are interpreted as plinth dune deposits (Pye and Tsoar 2008). The cross-bedded sets intercalated with deformed ripples indicate alternation of periods of efflorescent crust formation (rise of groundwater table) and aeolian dunes. The association of this facies association with outer saline mudflat indicate decrease of local moisture. 118

DESCRIPTION OF THE SEDIMENTARY SEQUENCE

The analyzed sedimentary succession in the study area is stratigraphically positioned in the transition interval between Quiricó and Três Barras Formation and results from the superposition of aeolian dune field deposits over playa lake deposits (Fig. 2). The interval analyzed is divided in four stages that are characterized by peculiar facies associations (Fig. 8). Stage I (21.3% of succession by thickness) - The first stage is mostly composed of inner saline mudflat facies association, which represent 95% of the stage. Within this stage, flooding deposits (5%) overlap inner saline mudflat facies association, in erosive and slightly planar bottom contact. The transition from stage I to stage II occurs when inner saline mudflat passes to shallow lake facies association, where the contact is abrupt, but not erosive. Stage II (12.2% of succession by thickness) - This second stage is mainly constituted of shallow lake facies association, which represent 76.6% of the stage. The deposits of shallow lake consist of a 2 m thick tabular body that is observed for more than 1 km. An erosive and slightly horizontal surface separates this facies association of a thin bed of flooding deposits (5.5%). This facies association is overlapped by inner saline mudflat deposits, in sharp and non-erosive boundary. The limit of this stage consists in a non-erosive, irregular to slightly horizontal surface which separates the inner saline mudflat facies association of the outer saline mudflat facies association. 119

Figure 8. This picture represents a sequential development during the transition from Quiricó to Três Barras Formation, where occurred the progradation of the aeolian dune field on the playa lake. Observe the four stages of deposition and the relationship between subenvironments controlled by the water balance (wet to dry). From the lower to upper part, the environment is dominated by: (1) the inner saline mudflat, (2) shallow lake, (3) outer saline mudflat and (4) eolian dune field. 120

Stage III (38.7% of succession by thickness) - This third stage is mainly composed of outer saline mudflat facies association, which constitutes 65.9% of the stage. The contact that separates this stage of the lower (stage II) is slightly horizontal. In this stage, aeolian deposits (29.7%) are interbedded with outer saline mudflat facies association (Fig. 7); the aeolian deposits increase their thickness upward in the succession. The contacts are sharp, planar and erosive in the base of aeolian deposits. At the top of this stage, inner saline mudflat facies association (2.7%) is interbedded with outer saline mudflat. Locally, small channel facies association (1.7%) occur into outer saline mudflat facies association. This stage ends when the outer saline mudflat facies association is substituted by aeolian dune field facies association. Stage IV (27.8% of succession by thickness) – This stage is totally composed of aeolian dune facies association. The boundary corresponds to an erosive and horizontal surface that may be observed for kilometers (Fig. 9). This stage is composed by aeolian dunes cross-strata without interdune deposits, at least locally.

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DEPOSITIONAL ENVIRONMENT

The sedimentation in the Sanfranciscana Basin took place under influence of arid climate (Sgarbi et al. 2001), in which minor variations in water balance can induced severe changes in the depositional environment. Temporal changes were demonstrated by the analysis of the local sedimentary succession, which were observed in the transition of a playa lake environment (sensu Eugster and Hardie 1975) to an aeolian dune field (Fig. 8). At least in the study area, this interval may be divided and interpreted as stages, which probably underwent climate changes that resulted in variations in the water balance, with associated decreasing in the salinity of the playa lake, and rising in the clastic availability to aeolian reworking. The deposits analyzed in the first stage reveal that initially, at least in the study area, the sedimentation occurred within an inner saline mudflat. During this stage, the main control parameter was the height of the water table; the surface flows were rare and did not exerted great influence in the system. The main evidence in favor of the influence that groundwater flows were relevant consists in the fact that the inner saline mudflat subenvironment was characterized by the presence of thick efflorescent salt crusts (Goodall et al. 2000, Lokier 2012, Smoot and Castens-Seidell 1994). The relative thickness of the efflorescent salt crusts resulted of enrichment of the brine solutes from the edge to the center of the playa lake which combined with high evaporation rates, produced the ideal conditions to saturation of solutes and efflorescence of evaporite minerals. Other evidence of influence of the groundwater movement to the deposition consists in the presence of displacive or aggregate minerals, as calcite pseudomorphs after gypsum, preserved within structureless sandy mudstones (Benison and Goldstein 2001, Eugster and Hardie 1975, Rosen 1994). The rare presence and small thickness of flooding deposits demonstrate that the inner saline mudflat subenvironment was rarely crossed by sheet floods, attesting the episodic nature of subaqueous flows in the surface. The second stage is mainly constituted of shallow lake facies association, thus recording an increase of moisture produced by the increase of water input. Differently from what is observed in the first stage, in this case there was more surface runoff due to probable increase of the number of water springs, resulting in greater water supply, as evidenced by presence of ripples cross-laminations and planar laminations and non-observation of evidences related to the surface exposure. Another difference consists of the non-observation of evaporite minerals, which indicates an environment with water too fresh for the development of brines, which would be necessary for the precipitation of evaporite (Rosen 122

1994). This wet period finished when the playa lake changed into an inner saline mudflat again. The onset of the third stage occurred in the contact of inner saline mudflat and outer saline mudflat facies association, which was geographically close to aeolian dune field and ephemeral fluvial systems. Because of this, the outer saline mudflat was susceptible to a bigger sediment input from the neighbor subenvironments. Outer saline mudflat has low relief efflorescent salt crusts, which allows distinguishing it from inner saline mudflat (Goodall et al. 2000, Lokier 2012, Smoot and Castens-Seidell 1994). The low relief of the saline crusts reveal groundwater table relatively deep and poor solute concentration (Smoot and Castens- Seidell 1994). The stage III constitutes evidence that the groundwater table rarely was deeper, given that, only few and thin beds of inner saline mudflat facies association were recognized. The upward increasing of the thickness of aeolian deposits, which are interbedded with outer saline mudflat, suggests progressive deepening of groundwater table and consequent increase of sand availability (Kocurek and Havhlom 1993). The subenvironment rarely was disturbed by surface runoff, since only small channels entered in the system. The fourth stage recorded the definitive establishment of an aeolian dune field over the playa lake (Fig. 8). The progradation of aeolian dune field on the playa lake was possible because the water input decreased and created the necessary condition to aeolian construction. The increase of availability of sand was the main parameter responsible to aeolian dune construction (Kocurek and Havholm 1993). The preserved deposit of the stage IV corresponds to aeolian dunes facies association where interdunes are absent, which may be resulted from the probable combination of the extremely dry surface and a limited storage of sand materials.

Figure 9. Observe the boundary surface (dotted line) in the contact of deposits of the stage III and stage IV.

123

CONCLUSIONS

The deposits of Quiricó and Três Barras Formation were sedimented during the Early Cretaceous in the Sanfranciscana Basin, where occurred the transition from a playa lake to aeolian dune field. The process of progradation of an aeolian dune field over a playa lake was the result of the progressive decrease of water influx, as demonstrated by the analyses of the 28.3 m thick sedimentary succession. The deposits analysed were divided in four stages, according to the dominant facies association. The stages of paleoenvironmental evolution record possible climate changes. (1) In the first stage the inner saline mudflat, characterized by high relief efflorescent salt crust, was the dominant subenvironment, where the groundwater table was shallow and the brine concentration greater. (2) The second stage records a temporal increase in the water influx and consequent expansion of the shallow lake. (3) The third stage was dominated by an outer saline mudflat subenvironment characterised by thin efflorescent salt crusts, which indicate relatively deeper groundwater table and poor solute concentration. This subenvironment was sensitive to minor variations in water balance and more it a more susceptible subenvironment to the incursions of the aeolian dunes. (4) The fourth stage recorded a period in which the groundwater table stopped to feed the playa lake, leading to the disappearance of the playa lake and follow construction of aeolian dune field. 124

ACKNOWLEDGEMENT

The authors would like to thank the Geosciences Program from the State University of Campinas for the financial support and scholarship grant through to the Program of Academic Excellence (PROEX) conceded for the first author. In addition, we would like to thank the National Council for Scientific and Technological Development (CNPq) for part of the financial support, obtained from the research project n.474227/2013-8. Also, we would like to thank the anonymous reviewer who improved the quality of this manuscript.

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Fragoso D.G.C. 2011. Geologia da região de Presidente Olegário e evolução tectono- sedimentar do Grupo Areado, Eocretáceo da Bacia Sanfranciscana, Minas Gerais. Belo Horizonte: Inst. Geoc., Univ. Fed. Minas Gerais / UFMG. 133p. (MSc. Dissertation). Grossi-Sad J.H., Cardoso R.N., Costa M.T. 1971. Formações cretáceas em Minas Gerais: uma revisão. Rev. Bras. Geoc., 1: 2-13. Goodall T.M., North C.P., Glennie K.W. 2000. Surface and subsurface sedimentary structures produced by salt crusts. Sedimentology, 47: 99-118. Hardie L.A., Smoot J.P., Eugster H.P. 1978. Saline lakes and their deposits: a sedimentological approach. In: Matter A, Tucker M.E. (eds.). Modern and Ancient Lake Sediments. IAS Special Publication 2. p. 7-41. Hasui Y., Haralyi N.L.E. 1991. Aspectos lito-estruturais e geofísicos do Soerguimento do Alto Paranaíba. Geociências, 10: 57-77. Hunter R.E. 1977. Basic types of stratification in small eolian dunes. Sedimentology, 24: 361-387. Kattah S.S. 1991. Análise faciológica e estratigráfica do Jurassico Superior/Cretáceo Inferior na porção meridional da Bacia Sanfranciscana, Oeste de Minas Gerais. Ouro Preto: Dep. Geol., Univ. Fed. Ouro Preto / UFOP. 227 p. (MSc Dissertation). Kocurek G., Dott Jr R.H. 1981. Distinctions and uses of stratification types in the interpretation of eolian sand. Jour. Sed. Res., 51: 579-595. Kocurek G., Havholm K.G. 1993. Eolian sequence stratigraphy - a conceptual framework. In: Weimer P., Posamentier H.W. (eds.). Recent Developments in Siciliclastic Sequence Stratigraphy. Am. Assoc. Pet. Geol. Mem, 58, p. 393-409. Lokier S.W. 2012. Development and evolution of subaerial halite crust morphologies in a coastal sabkha setting. Jour. Arid Env., 79: 32-47. Mendonça K.R.N. 2003. Estratigrafia de Sequencias da Formação Areado na porção sul da Bacia Sanfranciscana. Rio Claro: Inst. Geoc. Ciênc. Ex., Univ. Est. J. Mesq. Fil. / Unesp. 124p. (PhD Thesis) Mountney N.P. 2006. Periodic accumulation and destruction of aeolian erg sequences in the Permian Cedar Mesa Sandstone, White Canyon, southern Utah, USA. Sedimentology, 53: 789-823. Paik I.S., Kim H.J. 2006. Playa lake and sheetflood deposits of the Upper Cretaceous Jindong Formation, Korea: occurrences and palaeoenvironments. Sed. Geol., 187: 83-103. Pye K., Tsoar H. 2008. Aeolian Sand and Sand Dunes. Berlin: Springer, 458p. 127

Ramos A., Sopenã A., Perez-Arlucea M. 1986. Evolution of Buntsandstein fluvial sedimentation in the northwest Iberian Ranges (Central Spain). Jour. Sed. Pet., 56: 862-875. Reading H.G., Levell B.K. 1996. Controls on the sedimentary rock record. In: Reading H.G. (ed.). Sedimentary Environment and Facies. Oxford: Blackwell Publishing. p. 5-36. Rosen M.R. 1994. The importance of groundwater in playas: a review of playa classifications and the sedimentology and hydrology of playas. Geological Society of America Special Papers, 289: 1-18. Sgarbi G.N. 1991. Arenitos eólicos da Formação Areado (Bacia Cretácea do São Francisco): caracterização, diagênese e aspectos químicos. Rev. Bras. Geoc., 21: 342-354. Sgarbi G.N.C., Sgarbi P.D.A., Campos J.E.G., Dardenne M.A., Penha U.C. 2001. Bacia Sanfranciscana: o registro fanerozoico da Bacia do São Francisco. In: Pinto C.P., Martins- Neto M.A. (eds.). Bacia do São Francisco: Geologia e Recursos Naturais. Belo Horizonte: Sociedade Brasileira de Geologia, p. 93-138. Smoot J.P. 1983. Depositional subenvironments in an arid closed basin; the Wilkins Peak Member of the Green River Formation (Eocene), Wyoming, USA. Sedimentology, 30: 801-827. Smoot J.P., Castens-Seidell B. 1994. Sedimentary features produced by efflorescent salt crusts, Saline Valley and Death and Valley, California. In: Renaut R.W., Last W.M. (eds.). Sedimentology and Geochemistry of Modern and Ancient Saline Lakes. Soc. Econ. Paleont. Min., Special Publication, 50, p. 73-90. Teller J.T., Last W.M. 1990. Paleohydrological indicators in playas and salt lakes, with examples from Canada, Australia, and Africa. Palaeog., Palaeoc., Palaeoe., 76: 215–240. Tunbridge I. P. 1984. Facies model for a sandy ephemeral stream and clay playa complex; the Middle Devonian Trentishoe Formation of North Devon, UK. Sedimentology, 31: 697-715. Walker R.G. 2006. Facies models revisited. In: Posamentier H.W., Walker R.G. (eds.). Facies model revised. Society for Sedimentary Geology, Special Publication, 84. p. 1-17. 128

APÊNDICE IV

THE ROLE OF GROUNDWATER VARIATION ON A DRYING UPWARD SEDIMENTARY SEQUENCE (SANFRANCISCANA BASIN, EARLY CREATACEOUS)

129

ABSTRACT

Playa lakes may be assumed as a depositional environment where there are alternations of periods with subaerial or subaqueous exposure, in which the equilibrium is mostly controlled by the water balance. The analyses of the sedimentary features in this study have been used to understand the sequential evolution and demonstrate the relevance of the groundwater to the environmental transformations. The Early Cretaceous of the Sanfranciscana Basin experimented a period of aridification in which was recorded the transition from a permanent lake to a playa lake environment that changed to an eolian dune field. The study indicates that most of water input resulted of groundwater flows and only a minor part resulted from stream flow. Evidences of subsurface water flows were recognized and include, displacive and nodular evaporites and features related to thick and thin efflorescent salt crusts. In conclusion, this study suggests that the paleoenvironmental changes reflect the variations in the depth of groundwater table by consequence of changes in the volume of water recharge.

Keywords: Sanfranciscana Basin, Aridification, Playa Lake, Eolian Dune Field, Groundwater. 130

INTRODUCTION

The deposits analysed in the Sanfranciscana Basin consist in an important model to understand the sedimentation during the Early Cretaceous, period in which sedimentary records of desert environments were preserved in the most of the continental basins in the Brazil (Carvalho & Kattah 1998, Sgarbi & Dardenne 1996). The present study is devoted to analyse deposits produced in a prevalent arid climate (Sgarbi et al. 2001), where the records of three depositional environments were preserved in specific intervals of time. Lacustrine was the first environment developed, and represent a period of relatively humid climate. The lacustrine environment changed to a playa lake, which consists in a drier environment characterized by alternation of humid and dry seasons. In the end of the evolutionary process, the environment shifted to an eolian dune field in the driest interval. Most of analysed deposits were produced in a playa lake setting. Present-day playa lake environments show a great variety of morphological features, in which the subenvironments are extremely diversified (Briere 2000, Hardie et al. 1978). For example, the end member of shallow and wide sedimentary basins, developed under arid zones (i.e., sub-tropical high pressure belts) is characterized as flat lands from a closed drainage system in which a central and ephemeral lake is surrounded by wide mudflats (saline or not), fluvial distributary and eolian systems (i.e., eolian sand sheet or eolian dune field) (Hardie et al. 1978). Playa lakes in which the main features consist in areas of extensive mudflats have been analysed in recent and ancient sedimentary records (Benison & Goldstein 2001; Benison et al. 2005, Bobst et al. 2001, Eugster & Hardie 1975, Goodall et al. 2002, Lokier 2012, Smoot 1983, Smoot & Castens-Seidell 1994, Tunbridge 1984, Teller & Last 1990). Because they are constituted in locations of hot and dry climate, in which the rainfall and floods events are rare, these sedimentary environments are strongly sensitive to alterations in hydric balance. In this regard, variations in the subsurface water flows, produced by decrease in the rate of recharge, may be the main factor of changes in the playa lake environments. The aim of this article is to reconstruct a sequence of deposition related to the transition from a lacustrine to an eolian dune field environment, which represents the possible process of climate change, recorded in part of the Early Cretaceous Sanfranciscana Basin. In this context, the present work intends to demonstrate how the deposition was affected by hydric balance.

131

GEOLOGICAL SETTING

The Early Cretaceous Sanfranciscana Basin is divided in two sub-basins, Abaeté and Urucuia (Fig. 1A). The Quiricó and Três Barras Formation are the main units that occur in the study area (Fig. B), which form significant part of the Abaeté sub-basin in the south-western of Brazil, in the Minas Gerais State. The study area is localized in the north of the city of Presidente Olegário. The tectonics responsible to the establishment of accommodation space in the Abaeté sub-basin is correlated with extensional forces in the continental crust, which were responsible to promote the subsidence in the basin, by the reactivation of Proterozoic faults (Fragoso et al. 2011). The event is related to the break-up of Gondwana supercontinent. Linked to the Abaeté sub-basin, the Alto Paranaíba Uplift is a tectonic structure positioned in their western border (Sgarbi 2000), which is considered the main source of clastic sediments that filled the basin (Sgarbi et al. 2001). Composed of metasedimentary and intrusive igneous rocks, the Alto Paranaíba Uplift, that is incorporated in the Brasília Fold and Thrust Belt in the southern boundary of São Francisco Craton (Almeida 1967), consists in a structural arch of NW-SE direction that was reactivated during the Mesozoic Era. The stratigraphy of Abaeté sub-basin is composed by Areado and Mata da Corda groups (Campos & Dardenne 1997). The Areado Group is the main lithologic unit of the study area and overlap the Proterozoic Bambuí Group, being separated by an angular unconformity (Sgarbi et al. 2001); the Bambuí Group is composed of phyllites and meta- carbonates. The Areado Group, ~130 m thick, is divided in the Abaeté, Quiricó and Três Barras formations (Campos & Dardenne 1997, Fragoso et al. 2011, Sgarbi et al. 2001). The Abaeté Formation is composed of conglomerates, breccias and sandstones, the Quiricó Formation constituted of mudstones and subordinately sandstones and the Três Barras Formation composed of sandstones. The sedimentary records that are observed in the Areado Group were previously analysed in their sedimentological aspects (Campos & Dardenne 1997, Fragoso et al. 2011, Kattah 1991, Sgarbi 1991, Sgarbi 2001, Sgarbi & Dardenne 1996, Mendonça 2003). In general terms, the authors agree that most of the sedimentation occurred under influence of arid climate, in which evidences of alluvial and fluvial environment are recorded in sandstones and conglomerates, the lacustrine environments in mudstones and locally black shales with fossil of fishes (Coelacanth Mawzonia) (Carvalho & Mayse 2008, Fragoso et al. 2011, Sgarbi 2000), the playa lake in sandy mudstones and muddy sandstones, with evidence 132

of evaporites and dissection cracks, and the eolian dune field environment in sandstones with large cross-beds.

Figure 10. Geological map of the study area and relative localization in the Sanfranciscana Basin. A) The study area in the Sanfranciscana Basin is divided in Abaeté and Urucuia sub-basin. B) Geological map of the study area, in which is highlighted the paleoenvironmental interpretation with upward transition from lacustrine to playa lake and aeolian dune field deposits. Observe distribution of paleocurrent data.

133

METHODS

The field works were interested to construct a map of facies and sedimentary logs to understand the paleoenvironmental issues. A series of thirteen sedimentary logs (i.e., less than a meter to tens of meters) were performed in the study area and helped to understand minor variations in the physical processes, as well as the distribution of subenvironments along the sequential deposition. Five sedimentary logs were chosen to the stratigraphic correlation of the facies association, covering a thickness of up to 100 m and a lateral extension of 9 km (Fig. 2). The facies associations are understood as a group of facies genetically related, which have palaeoenvironmental significance (Walker 2006). The description of facies associations demanded the observation of lithology, textures and sedimentary structures (Reading & Levell 1996, Walker 2006), which were made during the fieldwork and the laboratory analyses. Geometry of the beds and their bounding surfaces were described during the field activity and 43 slabs were analysed in laboratory. The facies association were interpreted comparing present-day depositional features and using analogous from ancient and well- studied sedimentary succession. 134

Figure 211. The stratigraphic correlation of the five sedimentary logs. Observe the transition of different environments and subenvironments associated. 135

FACIES ASSOCIATION

For the purposes of facies analysis, the deposits has been divided in eight facies associations to the studied area, which are characterized and interpreted in terms of subenvironment.

Permanent lake facies association

This facies association is constituted of mudstones and sandstones beds (Fig. 3A). The mudstones consist of laminae to beds, 0.27 to 2.3 cm thick and laterally extensive up to 6.9 m. Most of mudstone layers are clayey, but sometimes sand materials can occur; the sand sediments are very fine-grained. In the last situation, the beds with different grain size materials (clay to sand) are amalgamated and poorly defined (Fig. 3B). The basal contacts of mudstone layers are sharp and erosive; they are highlighted by thin layers or lenses of very fine-grained sand. The lenses are up to 0.4 cm thick in the thickest part and 3 to 11 cm laterally extended (Fig. 3A). The mudstone beds forms bodies up to 7.2 cm thick and with more than 26 m in lateral extension. The mudstones are interbedded with sandstone bodies. The sandstone deposits occur in beds that are 0.6 to 1.13 m thick and laterally extended to more than 26 m, in its maximum lateral extension. The beds are yellowish brown and they are constituted of very fine- to fine-grained, angular to sub-rounded, well- to very well-sorted sand. Most of sandstone beds are structureless, although sometimes some of beds show gradual transition from structureless to undulating laminations, from the base to their tops. The laminae are highlighted by subtle grain-size variation, and are composed of very fine- grained sand. The undulations are 1 to 4 mm high and has wavelength up to 10 cm. In some situations, ripple cross-laminations may be observed between largest undulations (Fig. 3C). Locally, horizontal layers of claystone, less than 1 mm thick, may occur isolated into the sandstone beds and, in many cases, these layers may be fragmented and laterally discontinuous, sometimes showing flame or dish-like structures. This facies association is recognized in the lower portion of the sedimentary sequence, showing vertical transition to shallow lake facies association (Fig. 2). 136

Figure 3. A) Sandstone beds without sedimentary structures intercalated with claystone beds (side bar). B) Claystone intercalated with sandstone lenses, sometimes with amalgamated aspect. Coin=2 cm. C) Sandstone beds showing the upward change from massive aspect to undulating, in which some lenses shows cross-laminations. 137

Interpretation The characteristics described in this facies association indicates deposition by currents that carrying clastic materials to a steady body of water (Talbot & Allen 1996, Nichols 2005). The massive laminae or beds of mudstones suggest deposition by the mechanism of settling (Zavala et al. 2006), and the erosive basal contact, combined with presence of very fine- grained sand, in very thin layers on the base of mudstone, suggest a final stage of a turbulent flow. The sandstone beds, characterized by the absence of sedimentary structures and lack of grain-size variation suggests high rates of sediment fallout by rapid deceleration of the flows (Zavala et al. 2006, Fisher et al. 2007, Basilici et al. 2012). Another factor that contributes to idea of rapid decelerating flows consists in the occurrence of water-escape structures, as dish- and flame-like structures. The dominant aspect to both mudstone and sandstones is almost absence of sedimentary structures, which may be considered an evidence of deposition below the fair weather wave base. However, in the other side, the undulated structures and the associated ripples cross-laminations, showed in some beds with gradual transition, consists in features that indicate combined flows as mechanism of formation, which is characterized by combination of unidirectional and oscillatory component (Basilici et al. 2012). Furthermore, the transition from sandstone deposits with absence of sedimentary structures to undulating lamination may indicate oscillation of water depth. The characteristics of this facies association suggest a system dominated by turbidity currents in a permanent lake.

Shallow lake facies association

This facies association consists of laminated mudstone that is basically composed of clay to sandy mudstone layers that are alternated with sandstone layers or small lenses of sandstone (Fig. 4A). The mudstones layers are green or red to yellowish red, composed of clay- to very fine-grained sand, normally 0.2 to 1.5 cm thick, although sometimes some beds are up to 12 cm thick (Fig. 4B). Internally, they exhibit planar laminations, which are <1 mm thick and are up to 3 m of lateral extension. Normally, the thickest mudstone beds show lenticular structures. The contacts are sharp (not erosive) above sandstone layers, although sometimes shows gradual transition of silt to clay grain-size. Some of mudstone beds may be followed laterally to more than 40 m, in its maximum outcrop extension. The sandstone beds are composed of very fine to fine-grained and well sorted sand. They occur in beds 0.2 to 4 cm thick, with lateral extension similar to mudstone beds. The contacts are sharp and in many cases erosive. The sandstone deposits may exhibit absence of sedimentary structures or rare 138

planar to wave laminations (Fig. 4B). Sometimes, in cross-section, the sandstone beds exhibit upward changes of thin planar laminations (<1 mm) to lenticular forms composed of sandstone with cross-laminations; mud drapes are common in the foresests of cross- laminations. The lenses are separated by truncations surfaces of undulating layers with concave-up forms (Fig. 4B). The lenses are up to 1.2 cm thick and 9 cm wide. Ripple bedforms commonly occur preserved; they exhibit rectilinear, flatted and bifurcate crests, normally covered by thin film of claystone (Fig. 4C). Interference ripples were observed in some of the bed surfaces. Sometimes, in some beds there are pillar structures in the contact of thin mudstone with sandstones layers. Beside these features, in some parts of sedimentary succession, this facies association may be characterized by widespread presence of cracks filled by sand materials (Fig. 4D). In plain-view, the cracks show polygonal geometry, which sometimes shows sandstone ridges. The sides of the thickest ridges exhibit moulds of planar laminations (Fig. 4E). In cross-section, most of the cracks consist in V-shaped or tabular forms that cross the mudstone layers below sandstone beds. Mud curls are preserved in some beds (Fig. 4D). Normally, the depth of cracks is consistent with the thickness of the mudstone layers, but in some cases the fractures crosses several layers of both mudstones and sandstones. Other aspect interesting to this facies association is that some of the beds consists in pseudomorphs of calcite after anhydrite or gypsum alternated with mudstone layers (Fig. 4F), in which in detail shows nodular structures (Fig. 4G). Although rare, another important evidence of evaporite consist of moulds of cubic geometry (< 1 cm wide) that occur randomly distributed on the surface (Fig. 3H). This facies association was recognized as intervals of 2.5 to 12 m thick and laterally extend to more than 40 m. The bottom and top boundaries are sharp, planar to slightly undulating, although not erosive, and are interbedded with inner saline mudflat facies associations. 139

Figure 4. A) Outcrop of tabular beds composed of laminated mudstone. Handbook = 20 cm. B) Alternation of claystone (red) and silt to sand sized layers (yellow) and lenses with cross-lamination. Coin = 2 cm. C) Wave ripple with sinuous and bifurcated crests in plain-view. D) Mud curls composed of claystone in sandstone layer, and sand ridges in claystone layer (arrow). E) Sand ridge of polygonal forms. Coin = 2.5 cm. F) Layer of calcite pseudomorphs after gypsum intercalated with mudstone. Scale = 13 cm. G) Detail to the nodular structure of the calcite pseudomorphs after gypsum. Bar = 1 cm. H) Halite moulds in mudstone layer. 140

Interpretation The characteristics of this facies association suggest deposition in a shallow water body, in which, during some periods, the subaqueous deposition alternated with subaerial exposure. The subaqueous deposition is indicated to sandstone and mudstone layers. The sandstones probably were deposited rapidly, as indicated by the absences of sedimentary structures in some layers, and for waning flow mechanism, because of gradual transition of sand to clay size materials in some beds (Armenteros et al. 1995, Abdul Aziz et al. 2003, Paik & Kim 2006). The mudstone layers were deposited by mechanism of settling in stagnant water body. The sandstone lenses, sometimes associated with mud drapes, were formed after deposition in a shallow water (above level of fairweather wave base), yielded by oscillatory water flows generated by energy from wind on a stagnant water surface. Periods of changes in the wind direction resulted in the formation of interference ripples. Slightly asymmetrical ripple were produced by the combination between unidirectional flows combined with oscillation movement (combined flows) (Basilici et al. 2012). Clay films on the wave ripple surfaces indicate conditions of stagnant water following the wavy, which permitted settling of clay above the ripple bedforms. The sand filled polygonal cracks in plain view and “V” or tabular shape in cross-section, sometimes associated with mud curls, corresponds to desiccation cracks (Plummer & Gostin 1981, Paik & Kim 2006). After deposition, the water body dried-up and the floor became exposed for periods of time, in which the variations of exposures were short or long-time, as evidenced by widespread mud cracks (Paik & Kim 2006, Hadlari et al. 2006). The shorter periods of subaerial exposure are suggested because of the smaller cracks filled by sand that cross single layers of mudstones while the longer periods are emphasized by bigger cracks that cross several layers of both mudstones and sandstones. The features observed in the vertical transition indicate enrichment of solutes and precipitation of evaporite minerals in water body and accumulation in lake floor. After deposition these minerals probably were modified in subsurface, where the nodules grow-up due to intrasediment precipitation and replacement by capillary evaporation of the pore fluid (Hussain & Warren 1989). In addition to pseudomorphs, the cubic moulds are interpreted as indirect evidence of syndepositional halite, similar to analysed by Paik & Kim (2006) in the Upper Cretaceous Playa Lake deposits from Jindong Formation. The features described in this facies association indicate periods of lake expansion followed by lake contraction, which represent a cycle typical of playa lake environments during wet periods (Bobst et al. 2001, Paik & Kim 2006, Hadlari et al. 2006).

141

Inner saline mudflat facies association

This facies association is constituted of red or yellowish green sandy mudstone beds that are combined with small sandstone lenses (Fig 5). These deposits consist of tabular beds, 0.20-3.4 m thick (Fig. 5A) and laterally extended to more than 100 m. The sandy mudstones are poorly sorted deposits composed of clay- to fine-grained sand. Although most deposits are structureless (Fig. 5B), sometimes weakly preserved planar laminations or cross-laminations may be observed in some beds (Fig. 5A and B). In some outcrops there are symmetrical or slightly asymmetrical ripple bedforms preserved in plain-view, some layers (Fig. C). Lenses of sandstones are common and they are white or yellowish red, composed of very fine- to medium-grained sand (Fig. 5B). The shape of lens is slightly spherical or elongated. Spherical shape is 4-30 mm in diameter and elongated shape is 20-35 mm thick and 90-190 mm wide. In addition to these types, another kind of lenses consist of bodies with plane to concave-up lower boundary, 40-130 mm thick and 50-200 mm wide, that are embedded within of the mudstone beds, internally composed of medium to fine grained sandstone with horizontal to low-angle or cross laminations as typical sedimentary structures (Fig. 5D). This kind of lens occurs horizontally aligned at further 40 m in the same stratigraphic levels. Calcite pseudomorphs after gypsum are common in this facies association (Fig. 5E). The crystals are preserved in two different ways, as displacive lenticular crystals of 20-40 mm or aggregates of millimetre scale (less than 2 mm). This facies association is interbedded with shallow lake, outer saline mudflat and fluvial distributary facies association, in sharp contact. 142

Figure 5. A) Tabular beds of massive sandy mudstone that are superposed, separated by sharp contact (black arrow). Observe the bed with planar to low angle cross-lamination (red arrow) and contact with aeolian dunes deposits (yellow arrow). Jacob stick = 1.5 m. B) Structureless sandy mudstone with slightly preserved lenses of cross-lamination (arrow). Pencil = 14 cm. C) Wave ripple intercalated with massive sandy mudstone (arrow). Hammer = 25 cm. D) Inclined lens filled by sandstone with cross lamination embedded in sandy mudstone. Apparently, the base is not erosive. E) Pseudomorphs of calcite after gypsum in centimetre scale crystals with lenticular shape (circle). Observe slightly preserved planar lamination in the sand lens (arrow).

143

Interpretation The poorly sorted and structureless deposits reflect a peculiar process of deposition, by means silt- to fine-grained sand was transported by wind and trapped by capillary adhesion on the high relief efflorescent salt crusts from inner parts of the saline mudflat (Goodall et al. 2000, Smoot & Castens-Seidell 1994). This process trapped a film of sediments on the crusts, which isolated it of the insolation and consequent evaporation, making the underlying saline crust been dissolved by the raising of the capillarity fringe. A residue of clastic materials, previous trapped on the saline crust, rest as a proof of previous saline crust. The newly formed saline crusts grew above the film of sediment and the process continues (Smoot & Castens-Seidell 1994). The repetition of these processes (salt crust precipitation, sediment trapping and partial dissolution) creates a structureless sandy mudstone, which is similar to an “aggradation fabric” (Goodall et al. 2001, Smoot & Castens-Seidell 1994). Efflorescent salt crusts are rarely preserved in the sedimentary record (Smoot & Castens-Seidell 1994, Lokier 2012). Spherical and elongated lenses were produced by deposition on the irregular tops of high relieve efflorescent salt crusts. In inner saline mudflats, efflorescent crusts are thick, with steep and narrow depressions. Long distance to source-area is likely the main reason of the scant presence of sand deposits on the efflorescent salt crusts. Cross-laminated sandstone lens were formed within small hollows, formed by differential dissolution of the efflorescent salt crusts surface after flooding (Goodall et al. 2001). The symmetrical or slightly asymmetrical ripple bedforms testify periods in which the water table rise, creating pools of water, which dissolved the underlying salt crusts and making the sediments loose to be reworked by waves. The slightly asymmetrical aspects were formed also during rare floods where the wind drags on the water surface with wave. Lenticular calcite pseudomorphs after gypsum and crystals aggregates were formed after deposition, due to evaporative process in subsurface (Eugster & Hardie 1975, Benison & Goldstein 2001). Smoot & Castens-Seidell (1994) argument that displacive evaporites associated with deposits of poorly sorted and structureless sandy mudstone correspond to an important feature to recognition of “aggradational deposits” from inner saline mudflats.

Outer saline mudflat facies association

This facies association consists of red or purple muddy sandstones with widespread white sandstone lenses (Fig. 6A). The muddy sandstones are poorly sorted, composed of clay- to medium-grained sand materials that are sub-rounded to well-rounded. The lenses are 144

composed of well-sorted, fine- to medium-grained sand, exhibit different shapes, which may be divided into three different types. (1) The most common type consists of small lenses (20- 40 mm thick) that display cuspate and jagged edges, composed of well-rounded and well- sorted sandstone, that composed aggregates of a large variety of forms, similar to mottling aspect. (2) The second type is constituted of bowl-shaped lens with upturned edges (Fig. 6B), which are composed of well- to very well-rounded and well-sorted sandstones. The bowl- shaped lenses vary in dimensions and may be 10-30 mm thick and 60-460 mm wide, although the most are less than 100 mm wide and 20 mm thick. (3) The third type is composed of ripple lenses layered and deformed aspect; sometimes they have hump-shaped crests. Many lenses exhibit wrinkle forms in the lower boundary (Fig. 6C). Tepee-like structures are observed in sandstone beds. The crests are 0.4-0.5 m distant and 30-40 mm high (Fig. 6A). Concave-up structures, 70-90 mm deep and 0.5 m wide, occur in beds of the small lenses. The deposits that underlying the bottom of these concave-up structures are deformed. These structures have geometry emphasized by claystone beds, 20 mm thick, which occurs concentrated in their bottoms and shows jagged edge. The contacts of this facies association are irregular and abrupt, but not erosive, with inner saline mudflat (Fig. 6A). Differently, the contacts with eolian dune facies association that sometimes overlap these deposits are sharp and erosive.

145

Interpretation The features described in the muddy sandstones with sandstone lenses are similar to described to marginal areas of saline mudflats (Bobst et al. 2001, Goodall et al. 2000, Bobst

Figure 6. A) Muddy sandstone of red colour and widespread white sandstone lenses with bowl-shape and the tepee-like evidences (black arrow show the crests). Observe the contact with inner saline mudflat facies association and the partial preservation of paleo-relief of efflorescent salt crusts (orange line), and also the sharp contact with aeolian dunes facies association (yellow line). Jacob stick bar = 10 cm. B) Bowl-shape lens with upturned edges (black arrow). C) Imitative “popcorn surface”, typical from thin efflorescent salt crusts. et al. 2001, Lowenstein et al. 2003, Lokier et al. 2012, Smoot & Castens-Seidell 1994), in which the massive aspect of muddy sandstone results from sediment accumulation for trapping by capillary forces on the hygroscopic surface of thin efflorescent salt crusts. The adhesion is followed by salt dissolution, resultant of the loss of insolation promoted by dust cover. Subsequently, on the surface in which the dust is accumulated, the evaporation may promote the new growth of efflorescent salt crusts. The thin efflorescent salt crusts are typical 146

of the outer parts of saline mudflats (Goodall et al. 2000). The small sandstone lenses reflect the morphological features of thin efflorescent salt crusts that sometimes are characterized by irregularities with depressions and humps (wrinkles), which were nicknamed “popcorn surface” by Smoot and Castens-Seidell (1994). These surfaces, due to the proximity with the edges of saline mudflat, have their depressions completely filled by sand grains. Similarly, the bowl-shaped lenses reflect the shape of the polygonal structures bordered by salt ridges, which consists in other common morphology from outer saline mud flat (Bobst et al. 2001, Lokier et al. 2012, Goodall et al. 2000). Some lenses show upturned edges, which suggest that the depression keeps sinking in the same time that are filled by sand grains (Smoot & Castens-Seidell 1994). The greater volume of sandstones in this facies association result of the probable proximity with eolian systems. Ripple structures with hump-shaped crests correspond to ripples deformed by the growth of efflorescent salt crusts. The tepee-like structures are deformations, which reflect the development of large polygons on the depositional surface, typical of the outer parts of saline mudflats (Smoot & Castens-Seidell 1994). The concave-up structures, overlapped by claystone beds in non-erosive boundary correspond to hollows generated after periods of floods. The flooding produces dissolution and subsequent settling of fine grained materials, like silt and clay. Subsequently, the efflorescent crusts grew-up again inside the hollow, forming other efflorescent crust fabrics. The deformation of the underlying beds is consequence of dissolution processes. The association with inner saline mudflat and eolian dune deposits suggest proximity in the edges of playa lake, near adjacent eolian systems.

Ephemeral pools facies association

This facies association consist of sandstone beds intercalated with claystone laminae (Fig. 7A and B). The sandstone beds are white to yellowish red, composed of fine- to medium-grained, well-sorted sand. Most of sandstone beds are laterally discontinuous in terms of thickness, sometimes showing lenticular forms. The lenses have up to 2.2 cm in thickness and 7 to 22 cm in lateral extension, and normally exhibit cross-laminations (Fig. 7A). The sandstone beds are 0.6 to 4.1 cm thick and laterally continuous at least to 9 m, in its maximum outcrop extension. Differently of sandstone lenses, in the beds there are a great variety of features: (1) massive aspect that are combined or not with cylindrical structures (Fig. 7C), (2) tepee-like structures, and in many cases (3) crinkled laminae associated with very thin claystone laminae (Fig. 7A). The claystone laminae are red or reddish brown, 1 to 3 147

mm thick and laterally extended up to 3 m. Most of laminae are crinkle in cross-section (Fig. 7A). Sometimes they exhibit dome-like structures that are 2 to 7 cm wide and up to 1.3 cm high (Fig. 7B). The cylindrical forms are recognized in cross-section, starting from claystone and cross part of sandstone layers (Fig. 7C). In plain-view, above sandstone layers, the cylindrical forms occur like imprints emphasized by claystone (Fig. 7D). Sand filled cracks are common to this facies association, and occur in the interface claystone and sandstone (Fig. 7E). The depths are limited by the thickness of the claystone layers; in thicker layers are the cracks are large and deep and in the thinnest layers they are narrow and shallow. In plain view, most of these structures occur in form of counter moulds, in which the ridges area sometimes associated with ripples (Fig. 7F). This facies association occur in flat to slightly irregular contact, in both base and top, with inner saline mudflat facies association.

Interpretation The features described in this facies association suggest clastic deposition in stagnant and shallow water body combined with biological activity. Evidences of water flows were not observed in the sandstone layers, which may have been destroyed by post-depositional processes. Only the sandstone lenses indicate occurrence of physical process, such as oscillatory forces that reworked the sediment in the water floor. These oscillatory forces sometimes were combined with unidirectional water flows, which produced cross- laminations, observed in some lenses. Tepee-like structures were generated by intergranular brine precipitation and growing of efflorescent salt crusts (Smoot & Castens-Seidell 1994) and the cylindrical forms are interpreted as burrows left by bioturbation of invertebrate organisms (Gringas et al. 2014). The claystone with crinkled lamination and dome-like structure, both observed in cross-section, suggest biological activities. The crinkled laminae are similar to microbial mat laminae, as described by Schieber (1998). Sometimes, crinkled laminae may be intimately related to efflorescent salt crust (Goodall et al. 2000), which could mimic the morphology of the biogenic mat when the ephemeral pools, where they were formed, dried out and the mat started to decompose. Simultaneously to this process of decomposition, the surface may have been encrusted with efflorescence crust, which preserves their morphology. Both, organic matter and salt crust normally have low potential of preservation, usually leaving clay-silt materials as the indirect evidence of their formation, which makes it difficult to know what kind of processes they are made (Goodall et al. 2000). The dome-like structures are formed by growth of biological mat due to competition for light, necessary to photosynthesis. So, these structures consist of successive crinkled laminae 148

(Schieber 1998). The sand filled cracks are interpreted to be the mudcracks formed by desiccation of mud layers that contract and creates cracks, which are after that filled by sand (Plummer & Gostin 1981). The features interpreted suggest the formation of a shallow water pool with stagnant water, which permitted the development of microbial mats responsible to the mud formation on the floor. Subsequently, the water body dried-up resulting mud cracking.

Figure 7. A) Sandstone lens (sl) with cross-lamination in claystone layers. Observe the crinkled laminae (cr). B) Claystone layer with dome-like structure (arrow) and bioturbation associated (bt). C) Cylindrical forms interpreted as burrow structures. D) Imprints enhanced by clay film. E) and F) Sand ridges observed in plain view. Coin = 2 cm.

149

Distributary channel facies association

This facies association consists of tabular sandstone bed-sets with flat to slightly concave-up base and flat tops (Fig. 8). Most of the sets are single and show thickness of 0.11 to 0.6 m and more than 20 m in lateral extension. These single sets occur in the centre to the west of study area and are composed of cross-stratifications and planar laminations (Fig. 8A). The cross-stratifications are composed of fine- to medium-grained, angular to sub-rounded and moderately sorted sandstones. The foresets show mean dip of 17° and are planar and show angular and erosive basal contact. The beds with planar laminations are composed of very fine to fine-grained, angular to sub-rounded, well-sorted sandstones. The laminations are 1-3 mm thick and they are distinguished by grain-size and colour variation (Fig. 8A). In some beds the planar laminations are associated with small scours (centimetre scale) of concave up base and planar top, which are filled by massive sandstone (Fig. 8B). Mud-chips are common in both planar laminations and in the fill of the scours; these chips are rectangular, red to brown and 15 mm large (Fig. 8C). Although most of deposits consist in single beds, sometimes in some outcrops in the east to northeast part of the study area, compound sandstone bed-sets may occur as bodies up to 1.9 m thick and with more than 17 m in lateral extension (Fig. 8D). These bodies are composed of bed-sets of sandstone that consists in combinations of sets of trough cross-beds, 15.3 to 36.6 cm thick and 85.6 to 141 cm wide, or association of different kinds of structures, as planar cross-beds and tabular cross-beds (Fig. 8D). Tabular and planar laminations sets eroded and superimposed by trough cross-beds (Fig. 8D and E). Moreover, some beds show the transition of trough cross-beds to sandstone beds with ripple cross-laminations that are symmetrical to asymmetrical (Fig. 8E). The paleocurrent data show direction to south-southwest with a main vector to 205° (Fig. 8D). These beds are tabular, up to 25 cm thick and with more than 26 m in the maximum lateral extension. Commonly these deposits exhibit mud drapes (Fig. 8F). This facies association is occasionally interbedded with inner saline mudflat and outer saline mudflat facies associations and its lower boundary is sharp and erosive and their top is sharp.

Interpretation The features described in this facies association, which consists of single or composed bed-sets, suggest deposition associated to rare flood events, where turbulent flows were responsible to the generation of erosive scouring observed in the basal contacts. Planar laminations suggest deposition in upper regime flow (Alexander et al. 2001, Bridge 2006, 150

Fielding 2006). The tabular cross-beds are interpreted as depositional record of bi- dimensional subaqueous dunes generated in lower regime flows (Allen 1982, Miall 1977, Miall 1985). The absence of channelized structures and the large lateral extension of most of the single bed-sets indicate unconfined or poorly confined flows. The single bed-sets with tabular cross-bedding and also the planar cross-bedding represent features common to areas where the subaqueous flows spread-out (Nichols 2005, Nichols & Fisher 2007). The deposits with trough cross-beds, which occur only in compound bed-sets, indicate transport and deposition by tri-dimensional subaqueous dunes. The thickest compound bed-sets with erosive and slightly concave up basal contacts are evidence of transport and deposition in slightly channelized erosional structures. The paleocurrent data indicates sediment transport direction to south-southwest. The composition of compound sets of trough cross-beds superposed in erosive contact suggests deposition by high energy turbulent flow, condition which was constant during specific periods. In other side, the transition of planar cross-beds to trough or tabular cross-beds indicates variation in water flow conditions, which may change to stagnant water, where the ripple cross lamination may be formed by oscillatory flows. Additional evidence of stagnant water consists in the presence of mud-drapes associated with ripple cross-lamination. All these features indicate strong variation in discharge downstream, by the change of poorly confined to unconfined flows, which may interpret this subenvironment as probable distributary channel (?) system from marginal area of playa lake (Kelly & Olsen 1993, Nichols 2005, Nichols & Fisher 2007). 151

Figure 8. A) Sandstone with planar laminations and rare mudstone clasts. Coin = 2.5 cm. B) Small channel structure filled with structureless sandstone and rare mudstone clasts. C) Detail to the mudstone clast. D) Sandstone with trough cross-beds with an erosive surface at the base of each set. Observe the erosive contact with inner saline mudflat facies association in the lower part (dotted line) and the convolute lamination in the upper part. The paleocurrent data show direction to south-southwest with a main vector to 205°. E) Sandstone with trough cross-beds on the lower part and in planar contact the bed with wave ripple lamination associated with mud drapes. F) Sandstone with wave lamination and mud drapes. 152

Small-channels facies association

This facies association consist of lenticular beds of sandstones, up to 2.5 m thick and 1.5 m wide in section perpendicular to the palaeocurrents direction (Fig. 9). The bottom is erosive and concave-up and the top is planar. Well-sorted, fine- to medium-grained sandstones fill this channelized form. Internally, various sets of low angle cross-stratifications were observed. This facies association is rare; it occurs embedded within outer saline mudflat facies association.

Interpretation Concave-up erosive bottom and width/thickness ratio <15 indicate ribbon-shaped channelized deposits. Grain-size features and sedimentary structures testify that these channels were filled by subaqueous small dunes (Ramos et al. 1986). Isolated small channels are common to present-day saline mudflats, as described by Smoot and Castens-Seidell (1994) in the Saline Valley from California.

Figure 9. Channelized structure filled by low angle cross-lamination embedded with muddy sandstone (red) with widespread sand lenses (white). 153

Eolian dune facies association

This facies association is dominant in the uppermost part of the sedimentary sequence (Fig. 10A) and consists of red to pale-yellow, very fine- to medium-grained sandstone, which may have well- or bimodal-sorting. Sets of planar cross-bedding (9-15° dip) may occur as single bed or be superimposed by other sets in angular or tangential contact (Fig. 10B and C). Sometimes, sets of tangential cross-bedding overlaps planar and parallel to low angle cross- lamination. The sets are 0.4-1.2 m thick and its lateral extension is more than 15 m (Fig. 10A). The cross-stratifications foresets are composed of very fine-grained sand alternated with wedge-shaped medium-grained sand (Fig. 10D) (i.e., bimodal sorting). In section perpendicular to the dip direction of the foresets are observed large trough sets with cross- bedding, 5-9 m wide and 0.6-1.2 m high, and <7° dip; in this case the boundaries are tangential. The paleocurrent data shows dip direction to the south-west with a main vector to 231.7° (Fig. 10A). The contact with planar and parallel to low angle cross-laminations, which are 8-10 mm thick and more than 15 m large, are gently inclined (<6°). Sometimes, lenses of sandstone with cross-lamination are preserved in some beds (Fig. 10C). Aeolian dunes deposits are normally associated with outer saline mudflat facies association.

Interpretation This facies association is interpreted as eolian dune deposits. The very fine-grained sandstone layers are interpreted as grain-fall strata whilst the medium-grained with wedge- shape as grain-flow strata (Hunter 1977). The alternation between grain-flow and sometimes grain-fall strata corresponds to typical product of eolian dunes migration (Hunter 1977, Kocurek & Dott 1981, Mountney 2006). The sandstones with trough cross-bedding correspond to sedimentary record of sinuous crested eolian dunes. The planar and parallel to low angle cross-laminations correspond to subcritical climbing translatent strata produced by climbing wind ripple migration (Hunter 1977). Lenses of cross-lamination are interpreted as deposits of wind ripple. The sets composed of sandstone with cross-bedding in gently inclined and tangential contact with planar and parallel to cross-laminated beds are interpreted as plinth dunes deposits (Pye & Tsoar 2008). The association of this facies association with outer saline mudflat indicate decrease of local moisture. 154

Figure 10. A) Aeolian dunes facies association (yellow bar) in erosive contact (dotted line) with outer saline mudflat facies association (orange bar). The paleocurrent data shows dip direction to the south-west with a main vector to 231.7°. B) Sandstone with low angle cross-beds. C) Sandstone with planar cross bed. Observe e the lens of wind ripple cross-lamination in the middle part of the image. D) Grain flow strata associated with grain fall strata and wind ripple cross-lamination.

155

STRATIGRAPHIC CORRELATION

The locations of the five sedimentary logs, measured in the study area, allow to an assessment to the vertical and lateral continuity of sedimentary records relatives to different kinds of environments and subenvironments, which comprise at least 9 km of extension in the N-S direction (Fig. 2). Essentially, the correlations illustrate the change of environments, where a permanent lake is shifted to a subaerially exposed eolian dune field. The interval of transition is enclosed by the development of a hybrid playa lake environment, where subaqueous and subaerial subenvironments coexisted. In this regard, the primary element of stratigraphic correlation was the interval of transition of playa lake to eolian dune field, which are bounded by an horizontal erosive surface (Fig. 10A). In addition to the correlation of sedimentary logs, in this study was made a mapping of deposits generated in different paleoenvironment (i.e., lacustrine, playa lake and aeolian dune field), in which it could be observed that the limits of transition between the environments is in agreement with the topography of the study area (Fig. 1B). The permanent lake facies association was measured only in one location because of the poor exposure, representing only 2% of the measured stratigraphic sequence. However, although the correlation is poor in relation to this facies association, the lateral continuity could be safely estimated, with the aid of geological mapping performed in the study area (Fig. 1B). in more than 9 km. Overlapping to this facies association, the playa lake records consist of a range of subenvironments that coeval a wide plain. The dominant facies associations analysed in this study were deposited in a playa lake setting, which was characterized by extensive saline mudflats (playa mudflat). These deposits represent almost 58% of sedimentary sequence and occur in a lateral extension estimated at over 9 km. The saline mudflat was divided in two facies associations, inner saline mudflat and outer saline mudflat. Shallow lakes represent 25% of the sequence and their records are concentrated in the lower part of playa lake interval (Fig. 11A). The shallow lake overlaps the permanent lake facies association, being thickest and wide in the central region of study area, where the estimated width is 3.6 to 4.7 km (Fig. 2). However, although important in the lower part of the sedimentary sequence, this kind of deposit decrease in thickness toward the top, where alternate with deposits of inner saline mudflats. The upper part of the sequence is characterized by a relative increase of the thickness and frequency of the saline mudflat deposits that change upward, from inner saline mudflat to outer saline mudflat. The upper part is also followed by an increase of other subenvironments records, including deposits of eolian 156

dunes, ephemeral channels and pools; in this part is emphasized the contemporary absence of shallow lake facies association (Fig. 11B).

Figure 11. Hypothetic playa lake depositional environment, in which is illustrated the differences of (A) wet and (B) dry intervals.

There are eolian dunes facies association in almost all sedimentary sequence, most in the south part of the study area (Fig. 11), where is increased upward the frequency and thickness of this kind of deposits (Fig. 2). Although frequent, eolian deposits account only to 9% of analysed deposits that vary from 0.8 to more than 9 km of width. Distributary channel facies association occur mostly in the upper and central part of the sequence and represent 4% of the intervals. The estimated width varies of 1 to 3.6 km in random vertical and lateral distribution. These deposits are alternated with saline mudflat facies association. The ephemeral pools facies association occur locally in the upper part of sedimentary sequence, on the north of sedimentary sequence, corresponding to 3% in an estimated greater than 4 km, alternated with inner saline mudflat facies association. 157

Bounded by an erosive surface, the records from eolian dune field environment, at least in the study area, consist in bed set with planar and trough cross-beds, typical dunes deposits. Thin planar laminations are observed only in the base of the bed-sets. Bed-sets of planar laminations overlapped and separated by truncation surfaces, common to interdune or sand sheet deposits were not observed in this study area. 158

DISCUSSION PALEOENVIRONMENT

The stratigraphic contacts and the estimated lateral distribution of the various facies associations, interpreted as subenvironments in this study, allowed to the reconstruction of part of the Sanfranciscana Basin. Probably related to arid climate (Sgarbi et al. 2001, Sgarbi & Dardenne 1996), the sedimentary sequence analysed in this study suggests a climate change, recorded in a drying upwards sequence, in which minor variations in water balance induced proportional variations in the depositional processes. The lacustrine environment, interpreted to the lower part of the sedimentary sequence, records a temporal interval in which the water input greatly exceeded the water output. Although not observed in the study area, in other studies (Carvalho 1982, Carvalho & Mayse 2008, Sgarbi 2000, Sgarbi & Dardenne 1996, Fragoso et al. 2011) was reported the occurrence of fresh water coelacanth fishes at bituminous shale in the base of Quiricó Formation. This evidence reinforces the idea of a permanent freshwater lake as dominant environment. Probably, most of water inflow have been provided from alluvial fan system, which the intercalation of alluvial fan with lacustrine deposits have been documented in other works on the margin of the Abaeté sub-basin (Sgarbi et al. 2001, Fragoso et al. 2011). The sedimentary records of the playa lake environment were preserved in the interval above the lacustrine environment (Fig. 2), which suggest a gradual environmental change. As part of playa lake, the shallow lake is the dominant subenvironment in the lower part of sedimentary sequence, which indicates that the transition was gradual, where the permanent lake dried upward and transformed to playa lake with a central shallow lake (Fig. 11). Expansions and retractions of the shallow lake are suggested by deposits of laminated mudstones, which initially show features of permanent water body and progressively change upward, starting periods of alternation with subaerial exposure (e.g., mudcracks). Sometimes, the shallow lake subenvironment became enrichment in brine solutes, as evidenced by nodular gypsum and halite casts. In the middle of the sequence, shallow lake facies association start to alternate with inner saline mudflat facies association. Unlike the shallow lake, the inner saline mudflat consists in a subenvironment characterized by long periods in which the surface is subaerially exposed with consequent formation of efflorescent salt crusts (Smoot & Castens- Seidell 1994). The saline mudflat were controlled by groundwater table, where in the inner part of the playa lake tend to be shallow (<1 m), which facility the formation of thickest efflorescent crusts (Smoot & Castens-Seidell 1994). Also, in this condition the saline water 159

table induced to the growing of displacive evaporites such as gypsum or anhydrite (Benison et al. 2015). Deposits of eolian dunes are rare and occur as thin lenses in the south, suggesting the existence of a dune field margin playa lake (Fig. 11). The upper part of the sequence still exhibit evidences of dominant subaerial exposure, where the inner saline mud flat progressively shifted to outer saline mudflat facies association (Fig. 11), which differently, is characterized by a deeper groundwater table that results in thin efflorescent salt crusts (Goodall et al. 2000). The marginal part of the playa lake is more affected by progradation of subenvironments from the border, such as fluvial systems, which were ephemeral and recorded paleoflow from the north-northeast to the south-southeast. The eolian dunes facies association was raised at frequency and thickness upward in the upper part of the sedimentary sequence (Fig. 2), probably because the deepening of groundwater table and consequent rise of sediment availability. The eolian dunes migrated from the northwest to the southeast with initial progradation in the south margin of the playa lake (Fig. 2C). In this same interval, in the north of the study area was developed a kind of lake, an ephemeral pool that records alternation of flooding and subaqueous exposure. During flooding episodes occurred intense microbiological activity, with development of algal mats and in some periods the growing of invertebrate associated. The uppermost part of sedimentary sequence recorded the complete progradation of eolian dune field, that initially eroded the depositional surface of playa lake and after accumulated eolian sediments. 160

THE ROLE OF GROUNDWATER IN THE PALEOENVIRONMENTAL CHANGES

Playa lakes characterized as saline mudflat bordering ephemeral and/or perennial shallow lakes, and associated with eolian dune fields and fluvial environments, similar to described by Hardie et al. (1978), were recognized in the Sanfranciscana Basin. The paleoenvironmental dynamic in this part of the basin and their resultant sedimentary deposits, were controlled by water balance (positive or negative), where the environments changed from permanent lake to playa lake and aeolian dune field, indicating the change from initially positive to negative hydric balance. There are no clues of rainfall (i.e. rain drops in mudstone) on the playa lake, which may be explained by the rarity of rain, since this kind of evidences is common to desert lakes (Paik & Kim 2006). The stream flows were ephemeral and locally affected the paleoenvironment, and most of their preserved deposits occur only in the east of study area, where the deposits are thicker (Fig. 2). Thin and single bed-set deposits are sporadic in the sedimentary sequence. The absence of well-developed channelized structures associated with equally well developed fluvial bars, combined with ephemerality of channels, preserved in the sedimentary sequence and the absence of rainfall evidences suggest small participation of surface runoff for the filling of lacustrine basin. Evidences indicate that most of water input resulted of groundwater flows and only a minor part resulted in stream flow. The evidences of subsurface water flows in shallow lake subenvironment consist in nodular gypsum, which is generated by solute concentration in water brine that comes in contact with previously deposited anhydrite/gypsum layers. The saline mudflat shows many evidences of water brine concentration. Firstly, the structureless sandy mudstones with small sandy lenses are features produced by the deposition on high relieve efflorescent salt crusts (Goodall et al. 2000, Lokier 2012, Smoot & Castens-Seidell 1994), which are produced only if the groundwater table is shallow (<1 m). The inner part of saline mudflats, where the thicker efflorescent salt crusts were formed, had the conditions necessary for growing of displacive calcite pseudomorphs after gypsum, that are groundwater table shallower than 1 m and high solute concentration (Benison et al. 2015, Benison & Goldstein 2001, Eugster & Hardie 1975, Rosen 1994). The increase in groundwater depth produced several changes in the playa lake environment. The efflorescent salt crusts become thin and the environment sandier, typical features of outer saline mudflats, in which the deposits are characterized as larger sandstone lenses in muddy sandstone beds (Goodall et al. 2000, Lokier 2012, Smoot & Castens-Seidell 161

1994). The clues of displacive evaporites disappeared and the environment began to experience greater influence of adjacent environment. The continuous increase of depth creates the necessary conditions to the progradation of eolian dunes, which resulted in the paleoenvironmental change. The sand availability (Kocurek & Havhlom 1993) increased and associated with aerodynamic features made possible the accumulation of eolian dunes deposits that was partially preserved in the sedimentary record. The present research does not explain the reasons which produced variations in the groundwater table. In fact, is probable that these variations resulted of changes in the catchment area whereas evaporation rate remained constant. Sgarbi & Dardenne (1996) argue that the Alto Paranaíba Uplift operated as an Orographic Barrier during the period of deposition in the Abaeté Sub-basin, being also the source-area of clastic sediments and brine solution to the sedimentary basin. In this context, there is possible that the Alto Paranaíba Uplift may have acted as a zone of high relief around the basin, which trapped the rain- shadow that catchment the low topographic area of the depositional environment analysed in this study, where the basin floor remained arid. Playa lake setting formed in areas surrounding mountains are interpreted in Eugster & Hardie (1975) and Rosen (1994). 162

CONCLUSIONS

The sedimentary records of the Early Cretaceous Sanfranciscana Basin indicate deposition in an arid climate. In this climatic context, the present study recognized, through analysis of the sedimentary features, an interval with local aridification processes, in which a permanent lake gradually shifted to a playa lake and after to an eolian dune field. The stratigraphic correlation includes vertical transition and lateral distribution of the facies association, which produced an assessment of the paleoenvironmental transformation. Thus, the lower part of the sequence was interpreted as a record of a more wet environment, dominated by a permanent lake facies association. The environment changed to a playa lake initially dominated by shallow lakes that, up to the middle part of the sequence, started to alternate with inner saline mudflat that culminated with the disappearance of the shallow lake record, which demonstrate that the surface started to remain subaerially exposed for longer periods. The drying-upward process became more evident when, from the middle to the upper part of the sequence, the inner saline mudflat began the transition to outer saline mudflat. This interval is outstanding to development of the eolian dune field, because the sand availability started to increase, which lead to the increase of eolian dunes that occupied the playa lake for longer periods and in larger areas. Also, this interval record most of the distributary channel facies association, which represent ephemeral flows that affected local and temporally the playa lake. The completeness of aridification occurred when the eolian dune field become the dominant environment. The variations in the water balance induced paleoenvironmental changes. The water input was controlled by subsurface water flows, where the changes in the depth of groundwater table resulted in changes, from dominance of subaqueous to subaerial subenvironments. Evidences of importance of subsurface flows includes: deposits relative of thick and thin efflorescent salt crusts, nodular and displacive evaporite and liesingang rings.

163

ACKNOWLEDGEMENT

The authors would like to thank the Geosciences Post Graduation Program from State University of Campinas by the financial support and scholarship grant through to the Program of Academic Excellence (PROEX) for the first author. In addition, we would like to thank the National Council for Scientific and Technological Development (CNPq) for part of the financial support, obtained from the research project n.474227/2013-8. 164

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