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

Abstract

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. coarsegrained conglomeratic sandstones and locally pebbly conglomerates, with abundant silicified fossil trunks, several large-to mediumscale 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 sandsheet, 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.