MAGNETIZATION AND REMAGNETIZATION PROCESSES IN SOME EARLY PALEOZOIC LIMESTONES FROM THE GREAT BASIN

Paleomagnetic data from Early Paleozoic rocks in North America are scattered, and the causes of this scatter are controversial. Several related studies of Cambrian and Ordovician strata, in which the focus was on determining the origin and nature of the magnetization by emphasizing geologic controls, are reported herein. The rock types sampled are mostly gray limestones, which have been little used in paleomagnetism because of their very weak magnetization, but which can now be routinely measured on cryogenic magnetometers. As such rock types are extremely common in the geologic record, the potential data base for paleomagnetic study is greatly broadened if they can be employed; additionally, much is known about the diagenetic history of such rocks, so that geologic constraints are easier to establish on the magnetization.;In western Utah, a contact aureole in late Cambrian limestone surrounds a granitic rock at Notch Peak. Samples farthest from the pluton yield a "Paleozoic" magnetization. In contrast, the only consistent magnetization inward from this is a low-blocking temperature magnetization that is rather scattered, has steep inclination with both polarities present, and that appears to reside in very fine-grained hematite. This magnetization is inferred to reflect a very late oxidation related to the intrusion, possibly from the mobilization of groundwater. Directions of magnetization from the pluton itself are similar in that they are scattered, two-polarity, and steep, but they appear to reflect the original thermoremanent magnetization (TRM) in magnetite.;Late Cambrian limestones from eastern Tennessee, sampled across "biomere boundary" defined by contrasting trilobite faunas, yield a consistent, "Paleozoic" magnetization that appears to reside in detrital magnetite. The average directions derived from sites in different thrust sheets differ, however, suggesting that relative tectonic rotation has occurred between the two sites. Further, comparison of other paleomagnetic data from North America from at or near this biomere boundary reveals gross discrepancies between the paleomagnetic poles from these nearly coeval rocks.;Cratonic Cambrian strata from tectonically complicated areas in southern Nevada appear to have been completely remagnetized. Red sandstone in the Bright Angel Shale yields a consistent magnetization that resides in diagenetic hematite; magnetization residing in magnetite in the overlying Jangle and Muav Limestones appears to be a VPTRM imposed during the late Tertiary.;Cambrian and Ordovician miogeoclinal carbonate rocks from the Desert Range of southern Nevada yield a well-grouped characteristic magnetization that appears to reside in detrital magnetite; its paleomagnetic pole is near late Paleozoic poles and it appears to reflect remagnetization at that time, probably from a viscous partial thermoremanence (VPTRM) that was imposed during regional uplift. In the Ordovician strata, the magnetization is younger than bedding disruption by major, late, through-going stylolites. Remagnetization is also grossly consistent with the degree of heating inferred from conodont color alteration index. A secondary component of magnetization, having lower blocking temperature and also residing in magnetite, is also present in the Desert Range Strata and probably reflects a VPTRM imposed during Cretaceous thrusting and uplift.;The studies carried out herein yielded ancient magnetizations that could in most cases be shown to result from remagnetization long after deposition. Upon reviewing the published literature on early to mid-Paleozoic paleomagnetism in North America, it appears that the scatter of published paleomagnetic poles from the early Paleozoic results from pervasive remagnetizations and tectonic rotations. I conclude that careful paleomagnetic study must be integrated with geologic data if reliable paleomagnetic data are ever to be derived from rocks as old as Early Palezoic.