BIOSTRATIGRAPHY AND SEDIMENTATION OF THE EOCENE AND OLIGOCENE KREYENHAGEN FORMATION, CENTRAL CALIFORNIA

The middle Eocene to lower Oligocene Kreyenhagen Formation of the Great Valley of California records the changing character of slope and basinal sedimentation on the evolving California continental margin from 52 to 36Ma. Nonmarine to shallow-marine sandstones occur stratigraphically above and below the Kreyenhagen Formation which is dominated by bathyal fine-grained sediments. The Kreyenhagen Formation represents a record of widespread slope and basinal deposition.;Diagenetic changes in the biogenic silica-rich portions of the Kreyenhagen Formation are associated with increased depth of burial. Opal A biogenic silica is found in the Kreyenhagen Formation in the Sacramento Basin and in portions of the northern San Joaquin Basin. Opal CT and biogenic quartz are found in the Kreyenhagen Formation on the west-central border of the San Joaquin Basin and in the southern San Joaquin Basin.;Planktonic foraminifera from the Domengine Sandstone and Kreyenhagen Formation recovered from core material and pipeline trench samples are studied and correlated with coccoliths. Three middle Eocene faunas are recognized, and these are impoverished in tropical index taxa. Undescribed taxa often occur in abundances of greater than 30%, and these are illustrated, described, and compared to similar taxa. Consequently, the various existing tropical planktonic foraminiferal zonation schemes cannot be systematically applied in California because the zonal marker taxa are absent.;Faunas present suggest that subtropical to temperate surface water was present on the California continental margin during the time interval studied. An eastern boundary current operating off the west coast of North America during the middle Eocene would bring cool water from much further north down into the mid latitudes, in a manner analogous to the modern California Current.;The character of the fine-grained bathyal sedimentation varies greatly throughout the Kreyenhagen Formation. Sedimentation is in the form of complex mixtures of clay and silt-sized calcareous and siliceous planktic microfossils, clay minerals, and silt-sized detrital quartzofeldspathic grains. Fine-grained sediments are often rich in organic matter (up to 5% total organic carbon) and may contain authigenic minerals associated with reducing conditions, including glauconite, pyrite, and phosphate, with laminated portions of the formation indicative of a low-oxygen setting and impingement of an oxygen minimum layer.