MAGNETIC STRATIGRAPHY OF THE UPPER PALEOCENE SILVERADO FORMATION, ORANGE COUNTY, CALIFORNIA

The southernmost Paleocene rocks in California are the Silverado Formation of northern Orange County. This unit consists of about 500 m of terrestrial fluvial sandstones which grade upward into shallow marine sandstones, exposed in numerous isolated fault blocks in the Santa Ana Mountains and San Joaquin Hills. Paleomagnetic samples were taken at the type section east of Irvine Park, and at a section in Silverado Canyon. After AF and thermal demagnetization, a stable remanence was isolated held mainly in hematite, which passes a reversal test. The entire formation is of reversed polarity. Based on the presence of molluscs of the Turritella infragranulata pachecoensis zone, and the occurrence of these molluscs below planktonic foraminifera of Zone P4 in the Santa Susana Formation, we correlate the Silverado Formation with Chron C26r (58.0-60.9 Ma). The Silverado Formation gives a paleolatitude about 3 +/- 17 degrees south of its present latitude, consistent with slight northward transport (although the error bars are large). It shows a clockwise tectonic rotation of 69 +/- 15 degrees, consistent with previous data that suggest that the Transverse Ranges and Peninsular Ranges were on the same tectonic block that has undergone 70-90 degrees clockwise rotation since the early Miocene.