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Abstract

Magnetic polarity stratigraphy (magnetostratigraphy) offers a powerful stratigraphic tool of great promise for high-resolution age-dating and correlation in the middle Miocene Monterey Formation of California in which precise age-dating and long-range correlation have not been possible due mainly to the absence or inadequate preservation of age-diagnostic siliceous microfossils (diatoms). For this reason, we have conducted a detailed magnetostratigraphic study of a 290-m thick Monterey section at Shell Beach in Pismo basin, central California. The results of this study, which is the first that provides high-resolution numerical age data for an entire section of the Monterey, clearly illustrate the potential that magnetostratigraphy holds for establishing a much-needed temporal framework for the Monterey. The lack of precise chronologic control in the Monterey has posed a serious obstacle to understanding the true origin of this economically and scientifically important unit of California.

Detailed stepwise demagnetization analysis of some 1,021 closely spaced oriented samples of the Monterey rocks from the Shell Beach section, the age of which is constrained by limited diatom and calcareous nannofossil biochronologic control, shows that the Monterey lithologies, especially its dolomite, have faithfully preserved the geomagnetic reversal record that these rocks acquired at or near the time of deposition during the middle Miocene. Our data, which pass both the fold and reversal tests at greater than 95 percent confidence level, result in the recognition of 17 well-defined, lithologically independent and stratigraphically controlled magnetozones. Nine of these zones are of normal polarity (N1-N9) and eight are of reversed polarity (R1-R8). We correlate these magnetozones with the interval from the lower part of magnetic polarity Chron 5B to the lower part of Chron 5r of the standard geomagnetic polarity time scale of Harland and others (1982). From the ages of polarity zone boundaries and extrapolation of sedimentation rates to the base and top of the section, we conclude that the Shell Beach section of the Monterey was deposited between approximately 15.15 Ma and 11.0 Ma at an average post-compaction sediment-accumulation rate of 94 meters per million years. Our data indicate that the facies boundary between the lower calcareous-phosphatic facies and upper siliceous facies is marked by a hiatus/disconformity that lasted about one million years, from approximately 14.3 Ma to 13.25 Ma. We speculate that this hiatus may have been caused by a eustatic fall in sea level that began around 14.3 Ma due to global climatic cooling. The onset of predominantly siliceous sedimentation may signal the intensification of coastal upwelling caused by the increased pole-to-equator temperature gradients accompanying this same climatic cooling, an event clearly recorded by the Monterey rocks at around 13.25 Ma.

Sandstone of the overlying Pismo Formation truncates the Monterey section at Shell Beach. We suggest that this erosional contact may have been caused by a latest middle Miocene sea-level fall at around 11.0 Ma. We note that a large eustatic fall in sea level is proposed by Haq and others (1987) at around 10.8 to 11.0 Ma.

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