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 paleolatitudinal offset originally observed (Champion et al., 1984) for these rocks was all, or in part, due to compaction shallowing of their paleomagnetic inclination. The study consisted of two parts: (1) A standard paleomagnetic study, including detailed thermal and alternating field demagnetization, was conducted on oriented cores collected at Pigeon Point, approximately 50 km south of San Francisco, California The results of this study were combined with the alternating field demagnetized results for samples provided by D. Champion from the initial Pigeon Point paleomagnetic study. The combined data set has a mean direction for Pigeon Point (I = 41.6 degrees, D = 346.9 degrees) similar to that originally obtained by Champion et al. (1984). (2) Material from the Pigeon Point Formation was disaggregated, given a laboratory analog of a postdepositional remanence, and compacted to pressures as high as 0.13 MPa which caused volume losses up to 53%. The laboratory-compacted samples were alternating field demagnetized, and their magnetic inclination and anisotropy of anhysteretic remanence were both measured. These data were used to derive correction curves, following Jackson et al. (1991), which describe the specific relationship between remanence anisotropy and inclination shallowing for the Pigeon Point Formation. Two correction curves were determined, one assuming that the magnetic particle orientation distribution experienced a prolate deformation after remanence acquisition and one assuming an oblate deformation. These two different corrections were necessary because the anisotropy of anhysteretic remanence indicates a composite fabric due to both a prelithification tectonically caused lineation and a burial compaction foliation. The anisotropy of anhysteretic remanence measured for each paleomagnetic sample and the correction curves determined from the laboratory compaction experiments indicate that the inclination of the Pigeon Point Formation has been shallowed by burial compaction. The compaction-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 al., 1984). The resulting paleolatitude for the Pigeon Point Formation could indicate that Salinia served as a link between the cratonic Sierra Nevada are to the north and the Peninsula Ranges/Baja-Borderlands allochthon to the south. Alternatively, our results suggesting a 10 degrees compaction inclination shallowing for the Pigeon Point turbidites may indicate that many of the paleomagnetic studies placing the Peninsula 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.