The Impact of Depositional Events and Burial Rate On Carbonate–Silica Diagenesis In A Middle Jurassic Stromatactis Carbonate Mud Mound, Sainte-Baume Massif, Se France
Notice bibliographique
Résumé
Abstract: Understanding the processes and conditions of chertification in carbonates is a challenging problem to assess the marine silica cycle. This contribution highlights parameters of internal silica recycling processes from their biogenic source to quartz cementation by describing a unique case of carbonate–silica diagenesis in a Mesozoic mud mound. Stromatactis carbonate mud mounds exposed in the Sainte-Baume Massif (Basse Provence, France) developed in an outer-shelf environment during the late Aalenian concavum ammonite Zone (Middle Jurassic). They form part of a cherty succession punctuated by hardgrounds and stratigraphically condensed intervals, interpreted to record deepening episodes. These mud mounds, rich in siliceous sponge spicules and stromatactis, are similar to their widespread Paleozoic counterparts but are particular in being extensively silicified. Carbonate–silica paragenesis forms a polymud fabric including four microcrystalline carbonates (M1 to M4; all low-Mg calcite, LMC), five spar cements (C1 to C5; C1 with high-Mg calcite, HMC, precursor mineralogy), and four silica phases (S1 to S4; S1 to S3 replacive chalcedony after opal-CT, S4 euhedral quartz cement). M1, assumedly related to the degradative calcification of siliceous sponges, forms a labyrinthine network into which M2 to M4 were infiltrated. Spar cements C1 to C3 include a successive decrease of δ18O at fairly invariable δ13C, interpreted to represent marine to shallow burial conditions, suggesting burial and successive transformation of marine bottom waters. Temperature estimation, based on the average δ18O value of C4 (δ18O ≈ −7.9‰), indicates a temperature of around 50°C at a maximum burial depth of about 1000 meters. Replacive chalcedony (S1 to S3) occurs between calcite cementation C3 and C4 in combination with traces of corrosion on C3. Mg-calcite was preferably replaced, particularly the precursor of cement C1 that surrounds peloids of M2, and preserved some ghost structures. Corrosion and substrate selectivity suggest that acidification, Mg-hydroxyl complexes, and surface area are the triggers for flocculation of a silica gel. Because dissolution of opaline sponge spicules started almost contemporaneously with C1, the dissolved silica was retained in a connate fluid as phases C1 to C3 were precipitated, and stagnant conditions prevailed for around 1 Ma. Bulk δ18O/δD values (S1–S3) below the marine chert line, together with the range of S1–S3 δ18O (δ18O SMOW = 28.3‰ ± 1.0) suggest silica flocculation in a shallow burial environment at temperatures of 25–30°C. The establishment of acidic conditions might have been favored by sulfide-oxidizing micro-organisms. Numerical simulation of early diagenetic silica flux in such marine sediments is consistent with the interpretation that small grain-size changes and fluid barriers (that create diffusion-controlled conditions), such as those induced by stratigraphic condensation, are key parameters to retain pore-water dissolved silica over geologic time. Inversely, Paleozoic stromatactis carbonate mud mounds typically form part of large-scale coarsening- and shallowing-upward successions that correspond to rapid burial. These conditions support convergent pore-water flux and drainage of pore-water dissolved silica in the shallow burial realm. Hence, spicule-rich mounds should tend to silicify if they form part of a condensed section.
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Comment cette classification a été obtenuedéplier
Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,003 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule tête enseignante, pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».