Abstract
Calcareous nannofossil biostratigraphy and carbon-isotope stratigraphy on bulk organics of the middle-upper Eocene Souar Formation (Tunisia) allow for a refined stratigraphy of this interval in the southwestern Neo-Tethys margin. The Souar Formation represents the deepest sedimentary deposits of the central northern Tunisian Basin and was only dated previously by coarse data on planktonic foraminifera. Together with our new data, a correlation is proposed to previous Tunisian records of the Halk El Menzal carbonate platform to the NE and shallow-water deposits of central Tunisia to the SW, which leads us to the erection of a synthetic stratigraphic chart of the middle-upper Eocene in central and northeastern Tunisia. Our results allow for a review of the accurate position of the Lutetian / Bartonian and the Bartonian / Priabonian stage boundaries in Tunisia relative to calcareous nannofossil biohorizons. We apply Bayesian statistics to build our age-depth model which points to a hiatus in the Lutetian (CNE12 Zone) that we relate to the middle Lutetian regressive sequence leading to the Lu-4 M-cycle. Radiolarian-rich sediments and cherts of the late Lutetian and early Bartonian are interpreted as a response to an increase in productivity, tied to the seasonal upwelling of nutrient-rich waters. Correlation to other sections in Tunisia highlights the strong contrast in facies between the carbonate-rich sequences of the edges of the dorsal and the silica-rich deposition in the Souar Formation. This contrast is particularly pronounced in the interval before and across the Middle Eocene Climatic Optimum (MECO) characterized by a maximum of nummulitic carbonate production on the platform and a minimum of carbonate production in the deep basinal parts of the Tunisian dorsal, rather dominated by silica-rich sediments. • Middle-Late Eocene calcareous nannofossil biostratigraphy in the SW Neo-Tethys. • The MECO corresponds to the Reneiche member in the Tunisian stratigraphic chart. • The MECO is characterized by a δ 13Corg excursion and biosiliceous productivity. • Increase in terrigenous flux to the Tunisian dorsale during the MECO. • Major sedimentation rate variations due to changes in the on-land weathering.
Highlights
Following the high temperatures of the Early Eocene Climatic Opti mum (EECO, ~52–50 Myr), the ~17 Ma cooling trend that culminated in the Eocene-Oligocene Transition (EOT, 33.88 Ma, Westerhold et al, 2014; Galeotti et al, 2016; Messaoud et al, 2020).was interrupted by short-lived hyperthermal events (Miller et al, 1987; Zachos et al, 1996, 2008; Zachos, 2001; Coxall et al, 2005; Edgar et al, 2007)
A coinciding B and Bc of R. erbae is recorded at 447 m (Fig. 5), to what we have described above for R. umbilicus and R. reticulata, and we consider that this level likely correlates with the Bc of the species that defines the base of CNE17 in the Agnini et al (2014) zonal scheme
A combined calcareous nannofossil biostratigraphic and chemo stratigraphic (δ13corg) framework has been used as input for constructing a Bayesian age-depth model and to provide the first high-resolution stratigraphic record of the middle-Late Eocene in the south-western Neo-Tethys (Tunisian dorsal)
Summary
Following the high temperatures of the Early Eocene Climatic Opti mum (EECO, ~52–50 Myr), the ~17 Ma cooling trend that culminated in the Eocene-Oligocene Transition (EOT, 33.88 Ma, Westerhold et al, 2014; Galeotti et al, 2016; Messaoud et al, 2020).was interrupted by short-lived hyperthermal events (Miller et al, 1987; Zachos et al, 1996, 2008; Zachos, 2001; Coxall et al, 2005; Edgar et al, 2007). One major Middle Eocene hyperthermal is the Middle Eocene Climatic Optimum (MECO, Bohaty and Zachos, 2003; Bohaty et al, 2009). It was identified as a transient negative oxygen isotopic excursion of about − 1.0‰ (in benthic foraminifera, Bohaty and Zachos, 2003) with noticeable peak warming conditions at the C18r/C18n.2n magnetochron boundary (Bohaty et al, 2009). The opening of the Tasmanian-Antarctic Gateway and the opening of the Drake Passage yielded to the onset of a circumAntarctic pathway eventually leading later to the Antarctic glaciation at the Eocene-Oligocene transition (e.g., Barker and Burrell, 1977; Bohaty and Zachos, 2003; Exon et al, 2004; Wei, 2004; Livermore et al, 2005; Sijp et al, 2014)
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