ABSTRACT: Paleomagnetism Applied to the Miocene Monterey Formation of California: Paleomagnetic Properties of the Monterey Rocks
OMARZAI, SHERAZ KHAN, and ROBERT COE, University of California at Santa Cruz, Santa Cruz, CA, and JOHN BARRON, U.S. Geological Survey, Menlo Park, CA
An inherent problem in investigating the Miocene Monterey Formation of California is determining its precise age. The lack of a precise chronologic framework within the Monterey is due mainly to the paucity of age-diagnostic siliceous microfossils, most of which were destroyed by diagenesis during burial. In the past, geologists relied on benthic foram biochronology for relative age determination in some of the Monterey sections. Although this chronologic scheme has been useful, its resolution is too coarse to address many of the current problems.
A high-resolution temporal framework must be developed for the Monterey before we can improve our basic understanding of this economically important sedimentary unit. A promising candidate to provide such a framework is paleomagnetism/magnetostratigraphy, and, for this reason, we conducted a detailed paleomagnetic study of the Monterey sections in coastal California. The results of our study of the Monterey sections at Ano Nuevo (outer Santa Cruz basin), Shell Beach (Pismo basin), and in Horse Canyon (Salinas basin) demonstrate how paleomagnetism, in concert with diatom biochronology, can furnish precise sedimentation rates, unconformity ages, and high-resolution absolute age data with which to probe the origin of the Monterey and to perceive possible links between its lithologies and major middle Miocene climatic-oceanographic events.
Apart from highlighting the role of paleomagnetism as a numerical age-dating tool, this presentation also focuses on paleomagnetic properties of the Monterey lithofacies and basin-to-basin differences of these properties. Fresh, well-indurated, impermeable, hard, dense dolomites, which have not experienced intense fracturing and groundwater percolation, generally give excellent results, with an unambiguous direction of stable remanence over a broad range of unblocking temperature after removal of a less-stable viscous component of magnetization. Fine-grained and unweathered siliceous mudstones, especially from the more distal basins, also provide the ancient polarity reversal record, which can be confidently used for age assignment and tectonic interpretation. About 30-50% of the samp es from fresh and unweathered dolomitic calcareous-phosphatic shales and dolomitic-porcelaneous rocks also yield reliable data that can be used for determining polarity, especially when the data can be independently checked by data from the more robust Monterey siliceous mudstones and dolomites.
AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-EMD Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)