Abstract: Control of Global Climate-Eustacy on the Stratigraphic Architecture of the Miocene Monterey Formation of California

COE, ROBERT S., SHERAZ K. OMARZAI, and JOHN A. BARRON

The Miocene Monterey Formation is one of the most intensively studied rock units of California.. However, despite intensive geologic scrutiny of the Monterey, many questions remain. One question that prempts all others is: what is the precise age of the Monterey (in years, not relative geologic units) and what specific factors influenced its stratigraphic architecture. It is with respect to this question that we have been conducting a long-range paleomagnetic study of the Monterey in coastal California

We have successfully completed a detailed study of the Monterey sections at Shell Beach (Pismo basin) and Horse Canyon (Salinas basin). Our results show that the Monterey section at Shell beach was deposited between #15.15 and 11.535 Ma at an average sediment-accumulation of 107 m/my and that the facies boundary between the lower calcareous-phosphatic facies and the upper siliceous facies of the Monterey at Shell Beach is marked by a hiatus that lasted about one million years, from 14.3 Ma to 13A Ma.

Our data from the Horse Canyon section show that the Monterey Formation at Horse canyon was deposited between #17.9 Ma and 11.9 Ma at an average sedimentation rate of 207 m/my. Again, our high-resolution age data show that the facies boundary between the lower and upper facies at Horse Canyon is also marked by a hiatus, that lasted for about 1.35 my, from 14.30 Ma to 12.95 Ma. We speculate that the hiatuses observed in the Shell Beach and Horse Canyon sections are closely linked and may have been caused by a global eustatic fall in sea level that began around 14.3 Ma due to global cooling related to the rapid growth of continental ice cap in East Antarctica The onset of predominantly siliceous sedimentation in the Monterey may signal the intensification of the wind-driven coastal upwelling caused by the global cooling, an event clearly recorded by the Monterey rocks around 13.0 Ma. Our results from the Shell Beach and Horse Canyon sections show that the changes in global climate-eustacy during the middle Miocene appear to have markedly influenced the development of the two-fold facies division of the Monterey Formation.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California