Age and Paleo-oceanographic Significance of Silica-Carbonate Cycles in Miocene Monterey Formation, California
Lisa D. White
Middle Miocene Monterey Formation cherts are often cyclically interbedded with laminated dolomites. Their co-occurrence is paradoxical because cherts are produced in relatively cold water, whereas dolomite is associated with warm water. Chert and dolomite intervals generally have poor age control, which has hindered further paleo-oceanographic interpretations.
Examination of diatom assemblages that are well preserved in the dolomites has resulted in high-resolution age dating and estimation of cycle duration in 200 m of section exposed at Pt. Ano Nuevo, northern California. The age of the section ranges from late Luisian to early Mohnian, at approximately 15-12 Ma. Cycle duration estimates for the Pt. Ano Nuevo section imply that the chert-dolomite cycles may be on the order of 100,000 and 50,000 years. Such durations are consistent with, and were perhaps influenced by, changes in the earth's orbital parameters (Milankovich cycles), namely eccentricity and tilt.
Warm and cold cycles have been identified by previous investigators in nondiagenetically altered middle Miocene sections of the Monterey Formation and in northeastern Pacific DSDP cores. These cycles are recognized and dated by the use of abundance fluctuations of planktonic foraminifera and diatoms. The chert-dolomite cycles are likely time equivalent to the microfossil-based cold-warm cycles and further strengthen the paleo-oceanographic and paleoclimatic interpretation.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.