Communication Dans Un Congrès Année : 2022

Do evaporites record tides ? Observations in the Lower Cretaceous of Southern Tunisia

Résumé

Classic evaporite facies models consider the formation of gypsum in peritidal settings to be mostly due to an increase in brine concentration. Such a concentration increase can be mediated by per-ascensum evaporation within the sediment or dessiccation of the sediment-water interface. However, little attention has been paid to the possibility that tidal pumping could favor the formation of such gypsum deposits, which in turn could record tidal cyclicities. The Hauterivian-Barremian Bouhedma Formation in the Chott Basin (Southern Atlas of Tunisia) exhibits a 450 mthick, overall transgressive succession of facies intersecting the shoreline of a major evaporitic basin of the Southern Margin of the Tethys Ocean. The facies comprise 1) cross-bedded or structureless sandstones; 2) laminated green clays and silts; 3) dolomitic stromatolites or microbial mats; 4) bioclastic and oolithic shoals; 5) gypsum beds. The gypsum beds are ubiquitous in the succession, being interstratified with the green clays, or occurring as separate units displaying massive or chickenwire-type beds. A lagoonal marine to peritidal setting with intermittent marine and fresh-water influences is indicated by the coexistence of both marine and brackishwater fauna of ostracods preserved both in the carbonate and clay beds. Crossbeddings in the oolithic shoals and in the sandstones were likely formed by tidal currents. The sandstones at the bottom of the section are part of inclined heterolithic stratification infilling meandering channels. These channel fills grade upwards to the green clays, dolomitic and gypsum beds. Much of the gypsum in those beds has been recrystallized, as supported by (i) the light isotopic signature of gypsum mother waters (d18OH2O and dDH2O) and their alignment along a typical slope (x) reflecting the fractionation of oxygen and hydrogen isotopes during gypsum crystallization, and (ii) by syntaxial cements overprinting the primary texture, which is basically laminated. The laminae are commonly grouped in cm-thick layers grading from pure gypsum laminae with a recrystallized fibrous to palissadic texture, to clay- or organicrich laminae where the gypsum is predominantly composed of micronodules (chickenwire texture). The laminae are deformed by syneresis and wrinkle structures, evidencing their origin as intertidal bacterial mats. Similar features are observed in the dolomitic facies. The gypsum laminae are composed of bundles of microlaminae down to <10 micrometers in thickness. We hypothesize that these bundles are tidal cycles, the frequency of which may fall in the lunar to daily periods of common tidalites. Further work, involving the kinetics of gypsum precipitation, and, where possible, the counting of laminae/microlaminae, is necessary to test and consolidate the hypothesis of tidal controls in laminated marginal-marine gypsum formation.

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Dates et versions

hal-04274855 , version 1 (08-11-2023)

Identifiants

  • HAL Id : hal-04274855 , version 1

Citer

Jean-Yves Reynaud, Mohamed Soussi, Chafia Akrout, Kamel Boukhalfa, Marwa Chnayna, et al.. Do evaporites record tides ? Observations in the Lower Cretaceous of Southern Tunisia. Tidalites 2022 International Congress, May 2022, Matera, Italy. ⟨hal-04274855⟩
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