Late Barremian / Early Aptian Re–Os age of the Ipubi Formation black shales: Stratigraphic and paleoenvironmental implications for Araripe Basin, northeastern Brazil

Abstract

The Ipubi Formation of the Santana Group, Araripe Basin, Brazil, is characterized by black shales and overlying evaporite deposits and is suggested to record the transition from lacustrine to marine depositional environments. To date, the age of the black shales, constrained only by microfossils, is poorly determined, with ages spanning ~25 myrs from 125 to 100.5 Ma (Aptian-Albian). Here we present new Re-Os elemental and isotopic data to provide the first absolute age for those rocks of the Ipubi Formation and an improved understanding of the depositional paleoenvironment of the Araripe Basin.The Re-Os isotope data for Ipubi Formation black shales yields a depositional age of 123 ± 3.5 Ma, with a highly radiogenic initial 187 Os/ 188 Os composition (Os i ) of 1.97 ± 0.02. The Re-Os age indicates that the deposition of the Ipubi Formation black shales occurred during the Late Barremian / Early Aptian, prior to the onset of OAE 1a, in a highly restricted marine / lacustrine setting.

Age of the Post-rift Sequence I from the Araripe Basin, Lower Cretaceous, NE Brazil: implications for spatio-temporal correlation

“…However, neither Coimbra et al (2002) nor Tomé et al (2014) indicated such a wide age interval for the Ipubi Member. Tomé et al (2014), mainly based on the works of Arai et al (2001Arai et al ( , 2004, both not registered in the references by Lúcio et al (2020), indicated an Aptian/early Albian age for the entire Santana Formation. Heimhofer & Hochuli (2010), another work not mentioned by Lúcio et al (2020), proposed a late Aptian age for the Rio da Batateira Formation and the Crato Member, an early Albian age for the Romualdo Member and probably a late Aptian/early Albian age for the Ipubi Member, based on agediagnostic palynomorphs and dinoflagellates recorded below and above the Ipubi evaporites.…”

“…Considering that the works of Arai & Assine (2020) and Melo et al (2020) were published after that of Lúcio et al (2020), the present discussion focuses only on the works on microfossils and biostratigraphy available to the last authors.…”

“…Teixeira et al (2017) detected specimens of Sergipea variverrucata at the base and top of 'Sítio Sobradinho', constraining the Romualdo Member age to the late Aptian. Unfortunately, Lúcio et al (2020) were not aware of that work. Associated with this, there is the absence of the ostracod biozones referring to the local Brazilian stages Aratu, Buracica and Jiquiá, which revealed a large hiatus between the Rio da Serra local Brazilian Stage and the Alagoas local Brazilian Stage (Arai et al, 2001;Coimbra et al, 2002).…”

“…Despite the robustness of the Lower Cretaceous biostratigraphic framework in the NE Brazilian basins, Lúcio et al (2020) diminished the potential of the current zonal scheme in correlating the Alagoas local Brazilian Stage with the International Chronostratigraphic Chart. They performed a geochemical analysis in nine samples of black shales of the Ipubi Member based on Rhenium-Osmium isotope data.…”

“…More recently, the debate on the age of the Post-rift Sequence I has raised up once again, with Lúcio et al (2020) introducing a Barremian age for part of this section, centered on Re-Os isotopes. Subsequently, two new biostratigraphic studies were Figure 1.…”