ABSTRACT: Application of Paleomagnetism for Absolute Age-Dating and Correlating in the Miocene Monterey Formation of California

Sheraz M. Khan, R. S. Coe, J. A. Barron

Our understanding of the genesis of the Monterey Formation of California has been hampered by the poor preservation of age-diagnostic siliceous microfossils, making accurate age dating and precise correlation difficult. Paleomagnetism offers a powerful stratigraphic tool of great promise for absolute age determination and long-range correlation within the Monterey. For this reason, we have conducted a detailed magnetic polarity stratigraphic study of a 290-m-thick section at Shell Beach, Pismo basin, central California.

Constrained by limited diatom control, our Shell beach data result in the recognition of nine normal and eight reverse polarity zones, which we correlate with the interval from the lower part of magnetic polarity Chron 5B (15.23-15.09 Ma) to the lower part of Chron 5r (11.47-10.99 Ma) of the standard magnetic polarity time scale. Extrapolation of postcompaction sedimentation rates, which range from 77-112 m/m.y., show that the Shell Beach section of the Monterey was deposited between 15.15 and 11.10 Ma at an average sedimentation rate of 94 m/m.y.

Our data indicate 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 for about 1 m.y. from approximately 14.3 to 13.25 Ma. This hiatus may have been caused by a global eustatic fall in sea level that probably began at approximately 14.3 Ma; we note that other workers have proposed a sea level fall at around 14.20 Ma. The onset of predominantly siliceous sedimentation marking the facies boundary may also have been caused by global climatic cooling and intense coastal upwelling, an event that is seen in the Shell Beach section at approximately 13.25 Ma. With more data from other sections of the Monterey we are currently studying, we should be able to confirm or reject the interpretation and correlations presented here.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990