A COMPACTION CORRECTION FOR THE PALEOMAGNETISM OF THE CRETACEOUS PIGEON-POINT FORMATION OF CALIFORNIA

The paleomagnetism of the Cretaceous Pigeon Point Formation turbidites was reexamined to determine whether the 25 degrees of southerly paleo latitudinal offset originally observed (Champion et al., 1984) for the se rocks was all, or in part, due to compaction shallowing of their pa leomagnetic inclination. The study consisted of two parts: (1) A stand ard paleomagnetic study, including detailed thermal and alternating fi eld demagnetization, was conducted on oriented cores collected at Pige on Point, approximately 50 km south of San Francisco, California The r esults of this study were combined with the alternating field demagnet ized results for samples provided by D. Champion from the initial Pige on Point paleomagnetic study. The combined data set has a mean directi on for Pigeon Point (I = 41.6 degrees, D = 346.9 degrees) similar to t hat originally obtained by Champion et al. (1984). (2) Material from t he Pigeon Point Formation was disaggregated, given a laboratory analog of a postdepositional remanence, and compacted to pressures as high a s 0.13 MPa which caused volume losses up to 53%. The laboratory-compac ted samples were alternating field demagnetized, and their magnetic in clination and anisotropy of anhysteretic remanence were both measured. These data were used to derive correction curves, following Jackson e t al. (1991), which describe the specific relationship between remanen ce anisotropy and inclination shallowing for the Pigeon Point Formatio n. Two correction curves were determined, one assuming that the magnet ic particle orientation distribution experienced a prolate deformation after remanence acquisition and one assuming an oblate deformation. T hese two different corrections were necessary because the anisotropy o f anhysteretic remanence indicates a composite fabric due to both a pr elithification tectonically caused lineation and a burial compaction f oliation. The anisotropy of anhysteretic remanence measured for each p aleomagnetic sample and the correction curves determined from the labo ratory compaction experiments indicate that the inclination of the Pig eon Point Formation has been shallowed by burial compaction. The compa ction-corrected Pigeon Point mean directions assuming either a prolate (I = 53.1 degrees, D = 347.2 degrees) or an oblate (I = 49.8 degrees, D = 346.8 degrees) deformation suggest only 13 degrees to 16 degrees of southerly paleolatitudinal offset for the Pigeon Point Formation in the Cretaceous, not the 25 degrees originally observed (Champion et a l., 1984). The resulting paleolatitude for the Pigeon Point Formation could indicate that Salinia served as a link between the cratonic Sier ra Nevada are to the north and the Peninsula Ranges/Baja-Borderlands a llochthon to the south. Alternatively, our results suggesting a 10 deg rees compaction inclination shallowing for the Pigeon Point turbidites may indicate that many of the paleomagnetic studies placing the Penin sula Ranges/Baja-Borderlands 15 degrees to the south of North America in the Cretaceous and Tertiary may have suffered from a similar effect and that the allochthon has been nearly in place since the Cretaceous .