A PRIMARY MAGNETIZATION FINGERPRINT FROM THE CRETACEOUS LAYTONVILLE LIMESTONE - FURTHER EVIDENCE FOR RAPID OCEANIC PLATE VELOCITIES

Paleomagnetic data from the Cretaceous (101-88 Ma) Laytonville Limesto ne of northern California are of geodynamic interest because they sugg est a rate of absolute plate motion (> 15 cm/yr) that exceeds those of all present-day plates. Such findings assume that a primary magnetiza tion is preserved in these accreted pelagic limestone outcrops. This a ssumption is supported by a trend in the paleomagnetic inclination dat a that matches, in magnitude and sign, the expected sense of motion pr edicted by comparing the mean paleolatitude value with the North Ameri can apparent polar wander path. The pervasiveness of remagnetizations seen in shallow water carbonates of North America and elsewhere, howev er, raises questions as to whether the Laytonville Limestone is remagn etized, regardless of tests indicating a primary magnetization. The re magnetized carbonates have a distinctive ''wasp-waisted'' magnetic hys teresis signature, thought to reflect a bimodal distribution of magnet ite grain sizes. This signature can be quantified by the relationship between saturation remanence (M(r)), saturation magnetization (M(s)), coercivity (H-c), and the coercivity of remanence H-cr: M(r)/M(s) = 0. 89(H-cr/H-c)(-0.6). This power law is compared with reference data fro m Pacific deep-sea carbonates (Deep Sea Drilling Project Sites 167, 31 7, and 463; Ocean Drilling Program Site 806) and Tethyan pelagic carbo nates (Valle del Mis section, Southern Alps) which together define a d ifferent curve: M(r)/M(s) = 0.53(H-cr/H-c)(-1.0) The Laytonville Limes tone falls in two categories. On the basis of unblocking temperature s pectra, resistance to alternating field demagnetization and behavior d uring the acquisition of isothermal remanent magnetization, red Layton ville Limestone is thought to contain pigmentary hematite in addition to magnetite. Hysteresis curves from red Laytonville Limestone are was p waisted, but because such curves also typify magnetite-hematite mixt ures, these data do not provide an unambiguous test for remagnetizatio n. Some white Laytonville Limestone, however, appears to contain negli gible amounts of hematite and can be directly compared with the remagn etized carbonates. Hysteresis parameters derived from white samples ar e indistinguishable from those of the reference pelagic limestones. Su ch samples have a stratigraphic significance, occurring where paleolat itude data indicate an equatorial crossing. These samples may have dep leted pigmentary hematite due to increased magnetic dissolution as the limestone entered the equatorial belt of high productivity. The lack of a remagnetization fingerprint in these white samples, together with paleomagnetic, lithologic, and paleontologic data, form a consistent data set supporting the primary nature of the Laytonville Limestone ma gnetization and rapid oceanic plate velocities. A small plate size cou pled with the unusually vigorous mantle plume volcanism of the Cretace ous Pacific basin could have combined to reduce the effectiveness of a sthenospheric drag, accounting for the rapid motion.