A primary magnetization fingerprint from the Cretaceous Laytonville Limestone: Further evidence for rapid oceanic plate velocities
Authors: Tarduno John A. , Myers Michael 
Published: February 2004

Paleomagnetic data from the Cretaceous (101–88 Ma) Laytonville Limestone of northern California are of geodynamic interest because they suggest a rate of absolute plate motion (> 15 cm/yr) that exceeds those of all present‐day plates. Such findings assume that a primary magnetization is preserved in these accreted pelagic limestone outcrops. This assumption is supported by a trend in the paleomagnetic inclination data that matches, in magnitude and sign, the expected sense of motion predicted by comparing the mean paleolatitude value with the North American apparent polar wander path. The pervasiveness of remagnetizations seen in shallow water carbonates of North America and elsewhere, however, raises questions as to whether the Laytonville Limestone is remagnetized, regardless of tests indicating a primary magnetization. The remagnetized carbonates have a distinctive “wasp‐waisted” magnetic hysteresis signature, thought to reflect a bimodal distribution of magnetite grain sizes. This signature can be quantified by the relationship between saturation remanence (Mr), saturation magnetization (Ms), coercivity (Hc), and the coercivity of remanence Hcr: Mr/Ms = 0.89(Hcr/Hc)−06. This power law is compared with reference data from Pacific deep‐sea carbonates (Deep Sea Drilling Project Sites 167, 317, and 463; Ocean Drilling Program Site 806) and Tethyan pelagic carbonates (Valle del Mis section, Southern Alps) which together define a different curve: Mr/Ms = 0.53(Hcr/Hc)−1.0. The Laytonville Limestone falls in two categories. On the basis of unblocking temperature spectra, resistance to alternating field demagnetization and behavior during the acquisition of isothermal remanent magnetization, red Laytonville Limestone is thought to contain pigmentary hematite in addition to magnetite. Hysteresis curves from red Laytonville Limestone are wasp waisted, but because such curves also typify magnetite‐hematite mixtures, these data do not provide an unambiguous test for remagnetization. Some white Laytonville Limestone, however, appears to contain negligible amounts of hematite and can be directly compared with the remagnetized carbonates. Hysteresis parameters derived from white samples are indistinguishable from those of the reference pelagic limestones. Such samples have a stratigraphic significance, occurring where paleolatitude data indicate an equatorial crossing. These samples may have depleted 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 magnetization and rapid oceanic plate velocities. A small plate size coupled with the unusually vigorous mantle plume volcanism of the Cretaceous Pacific basin could have combined to reduce the effectiveness of asthenospheric drag, accounting for the rapid motion.