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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