Architectural and facies characterization of the Aptian fluvial Barbalha Formation, Araripe Basin, NE Brazil

Abstract

The Aptian Barbalha Formation records the beginning of the post-rift stage of the Araripe Basin. It consists predominantly of sandstones and mudstones interbedded with thin layers of bituminous black shales and conglomerates. The depositional and architectural features of the alluvial succession of the Barbalha Formation were characterized by detailed study and descriptions of the selected outcrops and analysis of well core data. In this study, two main depositional sequences were identified. The lower depositional sequence is more than 100 m thick and comprises a vertical facies succession composed of amalgamated, multistorey, braided fluvial channel sandstone bodies overlain by a widespread lacustrine black shale up to 10 m thick. The lacustrine black shales–carbonate mixed interval is known as the Batateira Beds and constitutes a regionally important stratigraphic correlation marker in the basin. This interval records the establishment of a large lake that experienced severe water-level fluctuations and anoxic events. The upper depositional sequence is 60–95 m thick, and mainly consists of thin, yellowish, medium- to fine-grained sandstones and variegated shales. The upper sequence rests unconformably on the lacustrine black shales of the Batateira Beds. Thin and discontinuous conglomerate beds at the base of the upper sequence laterally grade into coarse-grained sandstones. These coarse-grained sandstones are overlain by interbedded sandstones and mudstones organized in fluvial cycles. The upper and lower sequences of the Barbalha Formation are separated by an erosive unconformity, traceable throughout the study area, formed during a period of stratigraphic base-level lowering. This surface marks a change in the lower sequence from a dominantly fluvial depositional style, with amalgamated multistorey braided fluvial channel sand bodies, to a lacustrine system in the top to an eminently fluvial sedimentation, which in the basal section comprises amalgamated, multistorey, braided fluvial channel sand bodies, and in the superior section the amalgamated fluvial channels are overlain by floodplain and overbank sandstone bodies with fixed fluvial channel deposits, interpreted as a suspended-load-dominated fluvial system in the upper sequence. This change in the depositional style is accompanied by a reduction in grain size and a change in the fluvial regime, suggesting that the drainage system was restructured due to tectonic movements in the basin and climatic variations. In addition to the restructuring of the drainage basin, the characteristics of the discharge of the river system have changed, probably because of the more humid climatic conditions that dominated during the deposition of the upper sequence. The fluvial deposition in the lower sequence is associated with more ephemeral river systems, while the facies architecture of the upper sequence is associated with perennial systems and is suggestive of a suspended-load-dominated fluvial system. This fluvial system is capped by lacustrine deposits of the Crato Formation. The upper sequence grades upwards into the Crato Formation. The boundary between these two units is delineated by the presence of greenish calciferous shales that are covered by lacustrine laminated limestones and shales of Neoaptian age. Palaeocurrent readings from the fluvial deposits of both sequences display a consistent palaeoflow to the SE. Sedimentological and palaeontological evidence indicates a tectonic control on sedimentation and humid to subhumid climate conditions.

Análise tectonossedimentar das fases início de rifte e clímax de rifte da Bacia do Araripe, Nordeste do Brasil

“…Um exemplo dessas transformações importantes pode ser encontrado na Bacia do Araripe, no Nordeste do Brasil. Esses eventos envolvem a presença de quatro principais sequências tectônicas (Figura 3), recentemente redefinidas (Fambrini et al, 2010a(Fambrini et al, , 2010b(Fambrini et al, , 2010c(Fambrini et al, , 2011b(Fambrini et al, , 2017(Fambrini et al, , 2019 com base na proposta de Prosser (1993): 1. sequência Paleozoica, formada apenas pela Formação Cariri; 2. sequência Início de Rifte (Rift Initiation), composta da Formação Brejo Santo e da porção basal da Formação Missão Velha (Neojurássico? ); 3. sequência Clímax de Rifte (Rift Climax), formada pela parte superior da Formação Missão Velha e pela totalidade da Formação Abaiara (Neocomiano); 4. sequência Pós-Rifte (Post Rift), separada em duas sequências pós-rifte, de acordo com Assine (2007) e Araujo et al (2008).…”

“…A sequência pós-rifte I (Aptiano-Albiano) é constituída das formações Barbalha, Crato, Ipubi, Romualdo e Araripina (Fambrini et al, 2015(Fambrini et al, , 2017(Fambrini et al, , 2019, enquanto a sequência pós-rifte II (Cenomaniano) caracteriza-se exclusivamente pela Formação Exu. Uma das características mais significativas desses eventos é o desenvolvimento de longa e estreita bacia denominada de Depressão Afro-Brasileira, iniciada durante o Neojurássico (Cesero e Ponte, 1997;Küchle et al, 2011;Fambrini et al, 2017).…”

“…A coluna estratigráfica da bacia engloba desde depósitos do Mesopaleozoico até o Mesozoico (Neocretáceo) (Assine, 1992;Neumann et al, 2000). A evolução mesozoica da Bacia do Araripe relaciona-se com os eventos tectônicos que resultaram na ruptura e na fragmentação do Supercontinente Gondwana e na abertura do Oceano Atlântico Sul (Cesero e Ponte, 1997;Asmus e Ponte, 1973;Ponte e Asmus, 1978;Garcia, 1991;Matos, 1992Matos, , 1999Ponte e Ponte-Filho, 1996a, 1996bAssine, 2007;Fambrini et al, 2007Fambrini et al, , 2008Fambrini et al, , 2009aFambrini et al, , 2009bFambrini et al, , 2011aFambrini et al, , 2011bFambrini et al, , 2013Fambrini et al, , 2017Fambrini et al, , 2019, e seu preenchimento posterior durante o Mesozoico (Petri, 1987). As tectonossequências pré-rifte e rifte (e.g., Ponte e Asmus, 1978), aqui redefinidas como início de rifte e clímax de rifte, respectivamente, constituem-se em formidável registro tectonossedimentar da tectônica rifte que afetou essas bacias no Mesozoico, particularmente a Bacia do Araripe.…”

The geoengineering approach to the study of rivers and reservoirs

“…In this section a number of studies on modern systems (Blum 2019;Sinclair et al 2016;Nicholson et al 2019) are compared with ancient counterparts, from the Cenozoic (Stow et al 2016) and Mesozoic (Dal' Bó et al 2018;Yeste et al 2018;Fambrini et al 2019) through to the Paleozoic (Reesink 2016;Ellen et al 2018). In reconciling these two timescales, it is necessary to find common ground between studies which focus on the plan-view (topdown) evolution of fluvial systems (often the focus of modern studies) and the cross-sectional view often employed by studies of ancient relict landforms in the geological record.…”

“…In the previous section a number of authors generated 2D panels (Stow et al 2016;Fambrini et al 2019) and developed 3D reconstructions of outcrop architecture (Ellen et al 2018;Yeste et al 2018). These models and the data collected in the field can be used for training images and statistical datasets for the building of 3D reservoir models (Martinius et al 2014;Beaumont et al 2016).…”