CHEMICAL AND SR-ISOTOPIC VARIATIONS DURING DIAGENESIS OF MIOCENE SILICEOUS SEDIMENTS OF THE MONTEREY FORMATION, CALIFORNIA

ASSTRACT: The Monterey Formation of California preserves Miocene siliceous sediments at progressive (generally with depth) diagenetic stages: diatomite (opal-A) ~ CT-chert (opaI-CT) ~ quartz chert for initiallyclay-poor sedimentsand diatomaceous shale CT-porcelanite (porous opal-CT) ~ quartz porcelanite for sediments that were initially clay-rich. The chemistry of most diatomite is controlled by three primary phases (biogenic silica, carbonate, and detrital clay) and several subsidiary phases (organic material, apafite, pyrite, and detrital minerals including quartz and plagioclase). The elemental abundances of the clay-poor diagenetic sequence decrease sharply with advancing diagenetic state for those elements controlled by clay (AI, Ti, K, Rb, Cs, Fe, some Group B metals), carbonate (Ca, Mg, St, Ba), apafite (P, Ca, St, Ba, V), and organics (V, Ni), while silica shows a congruent increase. These first-order variations reflect the initial compositions of the siliceous sediments modified by the addition of silica, the removal of carbonate and apatite, and the migration of petroleum during silica diagenesis. Anomalous abundances of Mn, Ni, Cr, Mo, and some other Group B metals are associated with trapped refiactory organic material and suggest that petroleum catagenesis and migration occurred during silica diagenesis: migration may have been facilitated by the release of bound water during silica-dehydration reactions. Second-order variations (in K/Rb, K/Cs, Rb/Sr, sTSr/S~Sr)with advancing diagenetie state suggest ion-exchange and ion-expulsion by reaction of detrital phases with pore fluids. Diaganetic processes partially equilibrated, but did not homogenize, depositional STRb/~Sr-gTSr/SrSrmixing lines generated by detfital clays and authigenic carbonate and silica in siliceous sediments from the Santa Maria region. However, similar mixing lines in diatomites from the Santa Lucia area are reset to nearly horizontal slopes in associated CT-porcelanites. The degree of equilibration may be a function of the rate of formation of relatively impermeable quartz chert which isolated the siliceous system from further interaction with pore fluids. The rapid formation ofqnartz chert in the Santa Maria region is believed to have prevented isotopic homogenization processes from reaching completion. Initial chemistry of the siliceous systems controlled the rates and pathways of silica diagenesis. Silica diagencsis, in turn, altered the chemistry of the siliceous sediment.