Stephen L. Gillett

Paleomagnetism of the late CambrianCrepicephalus-Aphelaspis trilobite zone boundary in North America—divergent poles from isochronous strata

Paleomagnetic samples from the Nolichucky Formation (Late Cambrian), sampled at two sites in the Valley and Ridge Province of east Tennessee, yield a possibly penecontemporaneous characteristic magnetization that appears to reside in detrital magnetite. The paleomagnetic pole positions are “Paleozoic”, but differ: site I, lat. 41°N, long. 109°E,dp = 1°, dm = 2°; site II, lat. 39°N, long. 131°E,dp = 4°, dm = 7°. The difference in poles reflects a significant difference in declination between the site-mean directions, and this declination difference probably reflects relative tectonic rotation as the sites are in different thrust sheets. The paleontologic age of both sections is exceptionally well-constrained as they are sampled across an abrupt “biomere boundary” between contrasting trilobite faunas. Comparison of these results with paleomagnetic data from coeval strata elsewhere in North America reveals gross discrepancies, so that at least some of the published data must reflect remagnetization and/or tectonic rotation.

Major, through-going stylolites in the Lower Ordovician Goodwin Limestone, southern Nevada; petrography with dating from paleomagnetism

ABSTRACT The Early Ordovician Goodwin Limestone, a shallow-water, miogeoclinal calcarenite that occurs in the southern Great Basin, has been studied paleomagnetically and petrographically at one site in the Desert Range, southern Nevada. Major, through-going stylolites that occur in the Goodwin are relatively late diagenetic features because they truncate or deform all other fabrics. Locally, development of these stylolites has caused major disruption of primary bedding owing to differential compaction around chert nodules. The Goodwin yields a well-grouped characteristic magnetization that can be no younger than Early Triassic; because this magnetization is not affected by the stylolitic compaction, however, the magnetization could not have been acquired penecontemporaneously with deposition ut rather must reflect remagnetization. In turn, the paleomagnetic data show that the stylolites had formed by Early Triassic time at the latest and were not affected by Mesozoic thrusting or Cenozoic Basin-and-Range deformation.

Paleomagnetic Applications in Hydrocarbon Exploration and Drilling Operations: ABSTRACT

A new generation of high-sensitivity cryogenic magnetometers permits paleomagnetic applications in weakly magnetized sedimentary rocks. One of the most useful paleomagnetic applications is drill-core orientation, which is important for determining fracture orientations, for stress analysis, and for determining sediment transport directions. A 2-year study involving approximately 600 core plug samples from five wells in three Rocky Mountain basins yielded paleomagnetic orientations that agree with those obtained using the conventional photographic multishot technique. The strongest paleomagnetic signal in these rocks points toward the late Cenozoic paleomagnetic pole and probably represents a secondary magnetization imposed by thermal effects associated with the late Cenozoic uplift and tectonism in this region. Weaker paleomagnetic signals, reflecting earlier thermal, diagenetic, or depositional magnetizations are also commonly preserved in sedimentary rocks and can also be used to orient core.