ABSTRACT: Paleomagnetism Applied to the Petroleum Bearing Monterey Formation of California
Sheraz M. Khan, Robert S. Coe
Numerical age-dating and long-range correlations have not been possible in the petroleum-bearing Miocene Monterey formation of California that has experienced wholesale burial diagenesis during which the majority of the age-diagnostic siliceous microfossils were totally destroyed. As part of our ongoing project aimed at providing a high-resolution temporal framework for the Monterey, we have conducted a detailed magnetic polarity stratigraphic study of a 290 m thick coastal section of the Monterey at Shell Beach in Pismo Basin, CA. The results of the study are presented here.
Step-wise thermal demagnetization analysis of some 1000 closely spaced oriented samples, taken from dolomites, dolomitic calcareous and siliceous shales, and siliceous mudstones, result in the recognition of nine "normal" and eight "reversed" magnetozones, which we correlate with the interval from the lower part of magnetic polarity Chron 5B (15.23-15.01 Ma) to the lower part of Chron 5r (11.47-10.99 Ma) of the standard magnetic polarity time scale of Harland et al. Constrained by limited diatom and calcareous nannofossil biochronologic control, our data pass both fold and reversal tests, thereby implying that the high-temperature stable remanent magnetization so faithfully recorded and preserved by the Monterey rocks at Shell Beach is "primary," and its age is, therefore, equal to th age of the rock formation. We conclude that the Shell Beach section of the Monterey was deposited between 15.15 Ma and 11.0 Ma at an average post-compaction sedimentation rate of approximately 94 m/m.y.
Our data demonstrate that the facies boundary between the 75 m thick lower calcareous-phosphatic facies and the 215 m thick upper siliceous facies is marked by a disconformity/hiatus that lasted for about l m.y. from 14.3 to 13.25 Ma. We speculate that this disconformity may have been caused by a global glacio-eustatic sea-level fall that began around 14.3 Ma; we note that other investigators propose a large eustatic fall in sea level at approximately 13.8 to 14.2 Ma. Our data show rather convincingly that the lithologic anatomy of the Monterey at Shell Beach was markedly influenced by eustatic sea level changes operating along western California margin during the middle Miocene time.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990