Abstract: Paleomagnetism Applied to the Monterey Formation of California: Results from the Horse Canyon Section, Salinas Basin

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

The Miocene Monterey Formation of California is an organic and silica rich pelagic-hemipelagic unit. It occurs in various Neogene on- and offshore basins in coastal California. It is one of the most extensively studied rock units in California due to its economic importance, complex lithology, uncertain origin, and lack of precise age control. Among the various dating techniques, magnetostratigraphy holds great promise for establishing a high-resolution temporal framework for the Monterey.

In our quest to establish a high-resolution chronologic framework for the Miocene Monterey Formation of California, we have conducted a detailed paleomagnetic study of a 1 km thick section exposed in and along a stream bed in Horse Canyon, Salinas basin. Progressive thermal demagnetization analysis of some 1200 oriented samples of the Monterey rocks reveals the presence of primary remanent magnetization in some 58% of the samples analyzed and results in the recognition of twenty-three stratigraphically controlled magnetozones of both normal and reversed polarity (11 normal, 12 reversed). We correlate these zones with the interval from the upper part of the magnetic polarity Chron C5Dr to the lowermost part of Chron C5r of the global polarity time scale (Cande and Kent, 1995). This correlation shows that the Monterey Formation in Horse Canyon was deposited between approximately 17.9 Ma and 11.9 Ma at an average postcompaction sediment accumulation rate of 207 m/my. Our high-resolution age data show that the facies boundary between the lower calcareous-phosphatic and the upper siliceous facies of the Monterey at Horse Canyon is marked by a hiatus that lasted for about 1.35 my, from 14.30 Ma to 12.95 Ma. We speculate, that this hiatus is closely related to the hiatus we observe in the Shell Beach section (Pismo basin), some 150 km to the south, and may have been caused by a climatically-controlled global eustatic fall in sea level.

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