Chronostratigraphic constraints and paleoenvironmental interpretation of the Romualdo Formation (Santana Group, Araripe Basin, Northeastern Brazil) based on palynology
Introduction
The Aptian-Albian post-rift succession of the Araripe Basin (Assine, 2007, Fig. 1) is a major paleontological site in Brazil, known globally and studied since the end of the 19th century. Initially referred to as Santana Limestones (Small, 1913), it was formalized as Santana Formation by Beurlen (1963). Its fossil content is incredibly rich and diverse (Silva Santos and Valença, 1968; Mabesoone and Tinoco, 1973; Maisey, 1991), and the presence of several species of fish, pterosaurs and reptiles is especially noteworthy.
Numerous attempts have been made to subdivide the Santana Formation with lithostratigraphic nomenclatural changes, including various type sections, as summarized by Assine (1992) and Martill (2007). Beurlen's (1971) proposal warrants special mention. He subdivided the formation into three members, in ascending order: Crato (laminated limestones, with interbedded sandstones and shales), Ipubi (evaporites), and Romualdo (shales and sandstones, with thin limestone interbeds). The Santana formational designation and its tripartite subdivision have been utilized with some variations in most subsequent works (e.g. Mabesoone and Tinoco, 1973; Ponte and Appi, 1990; Assine, 1992; Fara et al., 2005; Arai, 2006; Assine, 2007; Scherer et al., 2015). On the other hand, some authors (e.g. Martill and Wilby, 1993; Martill, 1993, 2007; Maisey, 2000) have restricted the Santana Formation to the interval corresponding to the Romualdo Member.
The stratigraphy of the Aptian–Albian interval in the Araripe Basin has been considerably improved since the recognition of a psammitic basal section (previously considered part of the Missão Velha Formation), that however contains a lacustrine shale interval with Aptian palynomorphs (Lima and Perinotto, 1984; Hashimoto et al., 1987). This psammitic section has been termed the Barbalha Formation (Assine, 1990, 1992) and the Rio da Batateira Formation (Ponte and Appi, 1990), but, together with the Santana Formation, is now considered part of the post-rift I sequence of Assine (2007).
Adopting the original proposal by Neumann (1999) and Neumann and Cabrera (1999), Assine et al. (2014) and Neumann and Assine (2015) elevated to formational status the four members defined by Beurlen (1971), including them in what they termed the Santana Group. In ascending order, these are the Barbalha, Crato, Ipubi and Romualdo formations (Fig. 2). This nomenclature is utilized in this work.
The Barbalha Formation is Aptian and its temporal range has been extended into the Albian based on ages attributed to the Romualdo Formation (Fig. 3).
Considering the important marine ingression into the Araripe Basin manifested by the Romualdo Formation (Custódio et al., 2017), its definitive dating is highly relevant to establishing regional correlations among Brazilian marginal basins and for paleogeographic reconstructions vis-à-vis the Gondwana breakup and the opening of the South Atlantic Ocean.
In this paleotectonic and paleogeographic context, this work aims to establish the vertical distribution of the palynological associations in the most complete section of the Romualdo Formation, seeking to define the range of species present and characterize paleoenvironmental variations in geological time. The main objective is to obtain a precise chronostratigraphic definition of this unit, provided by the species Sergipea variverrucata (Fig. 4), guide fossil of the Aptian in Brazil.
Section snippets
Records of Sergipea variverrucata in the Araripe Basin
The proposal of guide palynomorphs by Regali et al. (1974a, b) was an important innovation for the biostratigraphy of the Cretaceous and Cenozoic of Brazilian basins. Sergipea variverrucata (Fig. 4), as described by these authors, has been highlighted as the only species that does not cross the Aptian–Albian boundary, and according to Arai et al. (1989) is to be recognized as the foremost guide palynomorph to determine the upper limit of the Aptian.
The genus Sergipea Regali et al. (1974b)
Methods and dataset
Because the Sobradinho section has been described in detail, in terms of both facies (Custódio et al., 2017) and paleontology (Fürsich et al., 2019), it provides a valuable opportunity to record the vertical ranges of constituent palynomorphs notably that of Sergipea variverrucata. Among 55 samples collected from the Sobradinho section (Fig. 5), 40 fine-grained samples from the 30–80 m interval were palynologically analyzed in the present study (Appendix A).
The samples were prepared by an
Occurrence of Sergipea variverrucata
Sergipea variverrucata is generally well preserved in the Sobradinho section (Fig. 4), thus ensuring confidence in its identification. Some specimens show sculptural damage (Fig. 4H, J), but the presence of associated warts (verrucae) confirmed their specific identity.
Sergipea variverrucata was recorded in 12 of the 40 samples studied here (see Appendix A). The highest occurrence of the species was in sample 49: 22 m below the top of Romualdo Formation and 36 m above the top of the well-known
Chronostratigraphic significance
The Aptian is a crucial time interval in the evolution of Brazilian marginal basins, because it records a tectonic style change in the region. This is manifested by conspicuous angular unconformity between the rift and post-rift sequences (pre-breakup unconformity) and deposition of a thick evaporite section. It reveals a distinct shift of rotation poles within Gondwana, and thus the beginning of the rift phase in the equatorial Brazilian marginal basins. Furthermore, this time interval also
Palynological data and paleoenvironmental interpretation
The palynological record shows important environmental changes that occurred through time during deposition of the Romualdo Formation. Quantitative representation of palynomorph groups in the samples studied is depicted in Fig. 5 (based on data in Appendix A).
The presence of dinoflagellate cysts and foraminiferal linings in 15 samples confirms the marine deposition of the Romualdo Formation's shale intervals. Subtilisphaera (Fig. 6, Fig. 7D) – the most abundant dinoflagellate taxon – is
Conclusions
From this detailed palynological analysis of the most complete surface section measured in the Araripe Basin we conclude that the 100-m thick section of the Romualdo Formation is attributable to the upper Aptian (P-270 palynozone). This is confirmed by the presence of Sergipea variverrucata, a guide palynomorph for determining the upper part of the Aptian in Brazilian basins. Because this species was also found in the underlying Barbalha, Crato and Ipubi formations, the entire Santana Group is
CRediT authorship contribution statement
Mitsuru Arai: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft, approved the final version. Mario Luis Assine: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft, approved the final version.
Acknowledgements
The authors are thankful to the São Paulo Research Foundation (FAPESP grant 2004/15786-0) and Petrobras (grant 2014/00519-9) for financial support of the research; to Michele A. Custódio, Marcello G. Simões, Filipe G. Varejão, Mariza G. Rodrigues and and Suzana Ap. Matos for fieldwork assistance; to Hermes Dias Brito for scanning electron microscopy (SEM); to Professor Geoffrey Playford (The University of Queensland, Brisbane, Australia) for reviewing the manuscript and making linguistic and
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