Print this pageFracture Formation in Relation to Burial Diagenesis, Miocene Monterey Formation, Coastal California
EICHHUBL, PETER, and JAMES R. BOLES
Repeated fracturing and cementation is observed throughout the burial history of the Monterey Formation prior to folding, resulting in a characteristic sequence of dolomicrite, quartz, baroque dolomite, and calcite-cemented veins. The heavy oxygen and carbon isotopic composition of the earliest dolomicrite veins (delta{18}O[PDB] approx. +1 o/oo, delta{13}C[PDB] approx. +15 o/oo) suggests precipitation in the zone of bacterial methanogenesis. Repeated events of fluid flow deduced from vertical sedimentary structures of laminated dolomicrite may relate to melting of clathrate or some other mechanisms of episodic fluid escape. Formation and quartz cementation of the next generation of fractures and breccias is accompanied by opal CT dissolution and quartz precipitation in hosting porcellanite, opal CT chert, and siliceous dolostone. Renewed fracturing and brecciation, restricted to organic rich dolomitic host rocks, is accompanied by dissolution and recrystallization of host rock dolomite and by precipitation of baroque dolomite and minor amounts of dolomicrite in fracture space. Silica, if present in the host rock, is not found re-precipitated in adjacent fractures but may instead get transported upsection and precipitated in 'earlier' quartz veins. The youngest observed generation of burial related veins is cemented by calcite, reactivating all earlier fracture sets but also forming new ones in organic-rich porcellanite. Based on the isotopic composition of calcite (delta{18}O[PDB] approx. -6 o/oo, delta{13}C[PDB] approx. +1 o/oo), on the present-day formation water delta{18}O[SMOW] composition of +3 to +5 o/oo, and on the silica diagenetic grade of surrounding opal CT pomellanite, calcite cemented veins are interpreted to form at maximum burial depth at about 60 to 70 degrees C. A lower {87}Sr/{86}Sr ratio of vein calcite as compared to the stratigraphic age of the host rock suggests an upward flow during calcite vein formation on a scale of a few hundred meters. The characteristic, although frequently incomplete, sequence of vein fill - dolomicrite, quartz, baroque dolomite, calcite - suggests that vein cementation reflects distinct stages of the burial diagenetic evolution of the sequence: degradation of organic matter by bacterial methanogenesis (dolomicrite), opal CT instability (quartz), and dissolution of earlier formed dolomite and of calcite shell tests, presumably during thermal degradation of organic matter (baroque dolomite, calcite).