Late versus early leachings of ooid cortices: Facts, findings and conclusions (regular talk on the occasion of JK2018)

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

2. Calcitic radial and radial-concentric ooids
retain/have retained their fibrous fabric

3. Aragonitic ooids can loose their original
fabric via the leaching of aragonite …

4. Aragonitic ooids can also loose their
original fabric ... via replacement by
calcite
?

5. The organic material present in the ooid cortices is
probably a key factor to explain features such as the
incomplete cortical leaching and 'suspended' nuclei

16. 1. Some ooids are partly leached.
2. More specifically, the ooid cortices were partly leached and some
ooid nuclei appear suspended in the middle of cortical moldic
cavities created by leaching, rather than having fallen to the bottom
of these cavities before the final cementation take place.

17. 1. Some ooids are partly leached.
2. More specifically, the ooid cortices were partly leached and some
ooid nuclei appear suspended in the middle of cortical moldic
cavities created by leaching, rather than having fallen to the bottom
of these cavities before the final cementation take place.
3. These ooids were originally calcitic, not aragonitic, not "two-phase"
nor "bimineral".
4. This leaching is not an early diagenetic feature related to subaerial
exposure,
but a late diagenetic feature, possibly related to the migration of
acidic pore waters, brought about by Alpine tectonics and/or
karstification.

18. HG

21. 5. In this last example, leaching of a formerly aragonitic Coscinoconus
test is an early diagenetic feature related to subaerial exposure.