Mysteries of the Monterey Formation: Unanswered Questions and Future Research Directions

Richard J. Behl¹ and Shingo Morita^2
1Department of Geological Sciences, California State University, Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840
²Corporate Environmental Affairs Division, Matsushita Electric Industries Co., Ltd., Osaka, Japan

The Monterey Formation is a distinctly siliceous and organic-rich deposit with stratigraphic equivalents that span much of coastal California and the Pacific Rim. It is California’s primary petroleum source rock and records key middle to late Miocene climatic, oceanographic, and tectonic transitions. Outcrops in the Palos Verdes Hills expose distinctive siliceous and tuffaceous sedimentary rocks, including diatomite, siliceous and tuffaceous mudstone, porcelanite, and chert. These lithologies reflect both the sedimentary composition and the extent of burial diagenesis from biogenic opal-A to opal-CT to diagenetic quartz. Opal-A phase rocks are concentrated in the upper parts of the Altamira Shale and the Valmonte Diatomite, whereas opal-CT phase rocks, particularly porcelanites, are well distributed throughout the Peninsula. Diagenetic quartz is not a major rock-forming component in the Palos Verdes Hills. Crystallographic ordering of opal-CT indicates that the siliceous rocks reached up to 79°C. Using the measured geothermal gradient of 55.8^oC/km from the nearby Wilmington oil field, we estimate that these sediments were buried by 323 to 968m of overburden that has been eroded during uplift since ~3.4 Ma. These data suggest an average tectonic uplift rate of ~ 0.29 mm/yr over the past 3.4 m.y., comparable to uplift rates determined by previous studies of marine terraces for only the late Quaternary.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California