Chapter 15 Facies and Diagenesis of the Miocene Monterey Formation, California: A Summary

Abstract

The Miocene Monterey Formation records the deep marine phase of a major late Tertiary cycle of basin formation and filling associated with wrench-fault tectonism along the California margin. Monterey sedimentation occurred in basin-floor, slope, shelf, and ridge-top environments which were variously affected by the oxygen-minimum zone. in many places the Monterey Formation consists of a lower calcareous facies, a middle transitional phosphatic facies, and a thick upper siliceous facies composed of diatomaceous rocks and their diagenetic equivalents (chert, porcelanite, etc.). Aquagene tuffs and volcanic rocks underlie the Monterey locally. the foraminifer-coccolith mudstones of the calcareous facies represent an early Miocene phase of equable climate and relatively low-nutrient water masses. the widespread siliceous facies represents rapidly deposited diatom ooze, recording high plankton productivity caused by climatic cooling and intensified upwelling in the late Miocene. the middle phosphatic facies formed during a middle Miocene transition from low-fertility calcareous deposition to high-fertility diatomaceous deposition. Diagenesis of the Monterey Formation typically involved the widely recognized sequence of silica phases, biogenic opal-A → diagenetic opal-CT → diagenetic quartz. Accompanying this sequence was a typical diagenetic succession of rocks: diatomites and diatomaceous shales → opal-CT cherts, porcelanites, and mudrocks →- quartz cherts, porcelanites, and mudrocks. the two phase transformations occurred by solution and in-situ precipitation of silica accompanied by abrupt reductions of 15 to 30 porosity percent resulting from compaction. Estimates indicate that opal-CT rocks became abundant at about 40° to 50° C or at burial depths of 750 to 1100 m, whereas quartz rocks became abundant at about 80° C or at burial depths of 1500 to 2400 m. the temperatures at which the silica phases transformed varied somewhat with the abundance of detrital clay, but the presence of calcite had little effect on the kinetics of the transformations. Past geothermal gradients can be inferred from the thickness of the opal-CT zone and from the rate of change of opal-CT d-spacings with depth in porcelaneous rocks. Thermal diagenesis of the abundant organic matter, largely kerogen, produced a highly asphaltic hydrocarbon fraction–and probably generated oil–at temperatures lower than 100° C. Secondary carbonate minerals, mainly dolomite, exhibit complex patterns of carbon- and oxygen-isotope ratios and probably formed at all stages of diagenesis.