The onset of the Early Toarcian flooding of the Pliensbachian carbonate platform of central Tunisia (north–south axis) as inferred from trace fossils and geochemistry

Abstract

The flooding of the Lower Jurassic shelf in the North Gondwana Palaeomargin during the early Toarcian occurred on a fragmented and irregular topography affected by differential subsidence—due to the activity of listric faults along the North-South Axis of Tunisia—that favoured lateral changes in facies and thickness at a kilometric scale. The onset of Toarcian sedimentation (Polymorphum ammonite Zone, NJT5c nannofossil Subzone) in two adjacent sections was characterised by the deposition of limestones under high-energy conditions. The Châabet El Attaris section was located in a depressed sub-basin, and recorded restricted environmental conditions owing to water stagnation and an oxygen-depleted sea-bottom. Therefore, dark mudstones developed, with increased TOC contents and enhanced accumulation of redox-sensitive elements. The sedimentation of limestones bearing gutter cast structures is related to gravity flows probably linked to storm activities. These processes favoured the remobilization of sediments at the sea floor, as well as oxygen input to bottom waters, as shown by the record of trace fossils including Zoophycos, Ophiomorpha, and secondarily, Chondrites and Diplocraterion. The thinly interbedded dark mudstones are locally rich in thin-shelled bivalves that re-colonised the sea bottom after the sedimentation of these high-energy deposits, and subsequently underwent mass mortality related to the return of oxygen-depleted conditions. The Kef El Hassine section is located in the upper part of a tilted, less subsident block, as indicated by its reduced thickness compared with the Châabet El Attaris section; the absence of dark mudstones implies oxic conditions. The Polymorphum Zone consists of limestones showing evidence of sedimentation under high-energy conditions, along with hardgrounds. The occurrence of Zoophycos (deep-tiers) in the upper part of some limestone beds of the Polymorphum Zone is linked to minor erosive processes. The top of the high-energy sequence—below the deposits of a marly interval corresponding to the Levisoni Zone—is interpreted as a hardground given the high content of belemnites and Arenicolites, some of them boring on the eroded Zoophycos and Thalassinoides. This study shows that the sedimentary expression of the Jenkyns Event is not uniform across Tunisia, supporting the importance of local conditions in determining the development of anoxic conditions.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5510162

Differentiate allogenic and autogenic controls on the formation of parasequences and depositional sequences of a tide‐ and wave‐dominated carbonate platform (Lower Cretaceous, Kopet‐Dagh Basin, Iran)

“…The thick package of the green–grey shale–marl of the FE association indicates a long duration of the relatively calm conditions and low sedimentation rate in the deepest part of the sedimentary basin. In this context, storm‐induced waves favoured water circulation at bottom as well as the input of nutrients (Brandano, Lipparini, Campagnoni, & Tomassetti, 2012; Pomar et al, 2012; Reolid et al, 2021; Sharafi et al, 2021).…”

“…The greater concentration of predominantly horizontal feeding traces is indicative of the Cruziana ichnofacies, from sea-bottoms having low to moderate energy and a substrate consistency varying from softground to firmground (e.g., Ekdale, Bromley, & Pemberton, 1984;Fürsich, Oschmann, Singh, & Jaitly, 1992;McEachern, Pemberton, Gingras, & Bann, 2007;Reolid, Marok, & Lasgaa, 2014;Reolid, Nieto, & Rey, 2010). The poorly sorted, coarse grains of the dominant allochems-including mixed shallow open-marine fauna and inner ramp-generated carbonate grains (e.g., ooids, intraclasts, peloids), as well as highly fragmented bioclasts-indicate high hydrodynamic energy and generalized permanent reworking of the seafloor during storms (Fürsich, Werner, & Schneider, 2009;Fürsich et al, 2018;Kietzmann et al, 2014;Peng, Steel, & Olariu, 2019;Sharafi et al, 2016Sharafi et al, , 2021 favoured water circulation at bottom as well as the input of nutrients (Brandano, Lipparini, Campagnoni, & Tomassetti, 2012;Pomar et al, 2012;Reolid et al, 2021;Sharafi et al, 2021).…”

Environmental Conditions in the Pre-jenkyns Event Times (Late Pliensbachian – Early Toarcian) in the Southiberian Palaeomargin (Betic External Zones, Southern Spain)

“…The lower part of the section, including the top of the Gavilán Formation and the lower part of the Zegrí Formation, records high values of Zr/Rb and Sr/Cu ratios that are interpreted as related to warm conditions. Sr/Cu is a weathering proxy that increases under warmer conditions(Bai et al, 2015;Cao et al, 2015;Moradi et al, 2016;Reolid et al, 2023). The Zr/Rb ratio is used as a proxy of the silt/clay ratio(Kylander et al, 2011) with Zr content related to eolian input(Pye, 1987; Hamroush and Stanley, 1990) and Rb related to uvial transport to the basin(Chester et al, 1977;Reolid et al, 2012).…”