Estratigrafia, arquitetura deposicional e faciologia da formação Missão Velha (Neojurássico-Eocretáceo) na área-tipo, bacia do Araripe, nordeste do Brasil: exemplo de sedimentação de estágio de início de rifte a clímax de rifte

The aim of this study was to carry out a stratigraphic revision of the Missão Velha Formation (Araripe Basin, northeastern Brazil) based on detailed facies analysis, architectural elements, depositional systems and palaeoflow data. The main facies are: 1. coarse-grained conglomeratic sandstones and locally pebbly conglomerates, with abundant silicified fossil trunks, several large-to medium-scale trough cross-stratifications and predominantly lenticular geometry; 2. medium-to coarse-grained lenticular sandstones with granules, abundant silicified fossil wood and trunks, and large-to medium-scale trough cross-stratifications, cut-and-fill features and mud drapes on the foreset cross-strata; 3. conglomerates and poorly sorted medium-grained sandstones with sparse pebbles and horizontal stratification; 4. fine-to very fine-laminated silty sandstone interlayered with 5. decimetric pelitic layers with parallel stratification and climbing-ripple cross-lamination. Ten architectural elements were identified: CH: Channels, GB: Gravel bars and bedforms, SG: sediment gravity flows, SB: Sand bars and bedforms, SB(p): sand bedform with planar cross-stratification, OFch: Overbank flow of channel (levee, crevasse and channel splay), DA: Downstream-accretion macroforms, LS: Laminated sand-sheet, LA: Lateral-accretion macroforms and FF: Floodplain fines. Not all of these features were observed at each outcrop. These elements, which are defined by their geometry and bounding surfaces, form the basis for interpreting depositional environments. The Missão Velha Formation is interpreted as: a. high energy braided fluvial systems with fining-upward cycles, channelized features, truncated cross-strata (diastema), lenticular geometry, channeled sandstones, sparse pebbles and b. meandering river systems due to the presence of flood plain deposits, crevasse splays and point-bars deposits. Aeolian deposits can also occur.Este trabalho enfoca revisão da Formação Missão Velha na área-tipo (Bacia do Araripe, Nordeste do Brasil), através de análises de fácies, elementos arquitetônicos, sistemas deposicionais e paleocorrentes. As principais fácies reconhecidas foram: 1. conglomerados de seixos e grânulos, com abundantes troncos fósseis silicificados, formando ciclos granodecrescentes, estratificações cruzadas acanaladas e tabulares, de pequeno e médio portes e geometria predominantemente lenticular; 2. arenitos grossos a médios, conglomeráticos, com grânulos, quartzosos, com troncos fósseis silicificados, com estratificações cruzadas acanaladas de médio e grande porte e geometria lenticular; 3. conglomerados e arenitos médios com seixos esparsos, mal selecionados, com estratificação plano-paralela; 4. arenitos finos a muito finos, sílticos, tabulares, laminados, intercalados com 5. camadas decimétricas de horizontes pelíticos com estratificação plano-paralela e laminações cruzadas cavalgantes. Dez elementos arquitetônicos foram reconhecidos: CH: canais, GB: formas de leito cascalhosas, SG: barras, SB: formas de leitos arenosos, SB(p): formas de leitos arenosos com estratificação cruzada tabular (p), OFch: depósitos de transbordamento (overbank) como diques, crevasse e channel splays, DA: macroformas de acréscimo para jusante, LS: lençóis de areia laminada, LA: depósito de acréscimo lateral e FF: depósitos finos de planície de inundação. Nem todos esses elementos foram observados em cada afloramento. Estes elementos, definidos pela geometria e superfícies delimitantes, formam a base para interpretação dos ambientes deposicionais. A Formação Missão Velha foi interpretada como: a. sistemas fluviais entrelaçados de alta energia com as seguintes características: ciclos granodecrescentes, estratificações cruzadas tabulares e acanaladas, truncamentos entre estratos cruzados, geometria lenticular, feições canalizadas, seixos esparsos e b. sistemas fluviais meandrantes pela presença de depósitos de planície de inundação, crevasse splays e barras em pontal. Ocorrem também depósitos eólicos

Stratigraphy, Depositional Architecture and Facies Analysis of the Missão Velha Formation (Neojurassic-Eocretaceous) in the Type-area, Araripe Basin, Northeastern Brazil: an Example of Sedimentation of Rift Initiation to Rift Climax Transition

The aim of this study was to carry out a stratigraphic revision of the Missão Velha Formation (Araripe Basin, northeastern Brazil)based on detailed facies analysis, architectural elements, depositional systems and palaeofl ow data. The main facies are: 1. coarsegrained conglomeratic sandstones and locally pebbly conglomerates, with abundant silicified fossil trunks, several large-to mediumscale trough cross-stratifi cations and predominantly lenticular geometry; 2. medium-to coarse-grained lenticular sandstones with granules, abundant silicified fossil wood and trunks, and large-to medium-scale trough cross-stratifications, cut-and-fill features and mud drapes on the foreset cross-strata; 3. conglomerates and poorly sorted medium-grained sandstones with sparse pebblesand horizontal stratifi cation; 4. fi ne-to very fine-laminated silty sandstone interlayered with 5. decimetric pelitic layers with parallel stratification and climbing-ripple cross-lamination. Ten architectural elements were identifi ed: CH: Channels, GB: Gravel barsand bedforms, SG: sediment gravity fl ows, SB: Sand bars and bedforms, SB(p): sand bedform with planar cross-stratifi cation,OFch: Overbank fl ow of channel (levee, crevasse and channel splay), DA: Downstream-accretion macroforms, LS: Laminated sandsheet,LA: Lateral-accretion macroforms and FF: Floodplain fines. Not all of these features were observed at each outcrop. Theseelements, which are defi ned by their geometry and bounding surfaces, form the basis for interpreting depositional environments. TheMissão Velha Formation is interpreted as: a. high energy braided fluvial systems with fining-upward cycles, channelized features,truncated cross-strata (diastema), lenticular geometry, channeled sandstones, sparse pebbles and b. meandering river systems due tothe presence of flood plain deposits, crevasse splays and point-bars deposits. Aeolian deposits can also occur