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Late versus early leachings of ooid cortices: 
Facts, findings and conclusions
Bruno R.C. Granier

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-06 02:30 PM – 02:50 PM
Last modified: 2018-12-06

Abstract


According to most authors (e.g., see references in Granier et al., 2014), fossil calcitic radial and radial-concentric ooids have retained their fibrous fabric. They also state that fossil aragonitic ooids have lost their original fabric, either via the replacement of aragonite by a calcitic mosaic, or via the leaching of aragonite. Thus, occurrence of a drusy calcitic cement in lieu of some cortical layers or of the whole cortex commonly interpreted as "two-phase"/"bimineralic" or aragonitic ooids.

Several authors (e.g., see references in Granier et al., 2014) reported the find of ooids with partly leached cortices from the lower Cretaceous strata of the French and Swiss Jura Mountains. They commonly interpreted "these features" (...) "as evidence for subaerial exposure".

Ooids with partly leached cortices from both the Grand Essert Fm (ex "Pierre jaune de Neuchâtel", early Hauterivian: Charollais et al., 2008; Comité Suisse de Stratigraphie, 2014) and the Gorges de l'Orbe Fm (ex "Urgonien jaune", late Hauterivian in age: Charollais et al., 2008; Comité Suisse de Stratigraphie, 2014) have been studied by Granier et al. (2014). According to these authors, it is not the mark of an early leaching of "two-phase"/"bimineralic" ooids but the mark of a late leaching of calcitic ooids "probably related to tectonics and/or karstification that favoured the seepage of acidic waters".

The same phenomena affected calcitic ooids from the lowermost Cretaceous Vuache Fm (ex "Calcaires roux", early Valanginian in age: Charollais et al., 2008; Comité Suisse de Stratigraphie, 2014) at Crozet, Ain, France (Granier, 2019). There besides the late leaching there are also locally few evidences of early leaching of some aragonitic allochems, hence rare evidences for genuine "subaerial exposure".