Magnetostratigraphy of the late Neogene purisima formation, Santa Cruz County, California

Mathematics – Logic

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Scientific paper

The magnetic polarity zonation of a late Neogene sedimentary sequence in Santa Cruz County, California, has provided a chronologic framework for studies of the sedimentologic and tectonic processes involved in an episode of basin formation in the vicinity of the San Andreas fault system in central coastal California. The zonation is based on the analysis of samples from 79 horizons in a 300 m thick section of the Purisima Formation and a portion of the overlying Aromas Formation. Although rock magnetic studies support the hypothesis that the primary carrier of the remanence is magnetite, many samples contain a secondary overprint which cannot be completely removed by alternating field demagnetization. Nevertheless, systematic analysis of the behavior of the samples during demagnetization has led to an unambiguous determination of the polarity of each horizon and to the development of a magnetic polarity zonation containing thirteen magnetozones. These magnetozones can be correlated to the magnetic polarity time scale using biostratigraphic constraints provided by diatoms in the lower portion of the section and invertebrates and vertebrates in the upper portion. The studied section is found to span the interval from the Epoch 5/Epoch 6 boundary (6.07 Mya) to the Matuyama/Gauss boundary (2.47 Mya) with a hiatus corresponding to the upper part of the Gilbert epoch (4.5 Mya to 3.5 Mya). This hiatus does not coincide with major regressions in the global sea-level curve and is interpreted as a period of tectonic uplift. The compression which generated this uplift was probably caused by interplay between the San Andreas fault east of the study area and the San Gregorio-Hosgri fault west of it.

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