ALBERT

Beschreibung: In this paper, we present paleomagnetic, geochemical, mineralogical, and geochronologic evidence for correlation of the mid-Miocene Cougar Point Tuff (CPT) in southwest Snake River Plain (SRP) of Idaho. The new stratigraphy presented here significantly reduces the frequency and increases the scale of known SRP ignimbrite eruptions. The CPT section exposed at the Black Rock Escarpment along the Bruneau river has been correlated eastward to the Brown's Bench escarpment (6 common eruption-units) and Cassia Mountains (3 common eruption-units) regions of southern Idaho. The CPT record an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations. Paleomagnetic characterization of eruption-units based on geomagnetic field variation has a resolution on the order of a few centuries, providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events. To obtain reliable paleomagnetic directions, the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy, and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization.

Beschreibung: Individual ignimbrite cooling-units in southern Idaho display significant variation of magnetic remanence directions and other magnetic properties. This complicates paleomagnetic correlation. The ignimbrites are intensely welded and exhibit mylonite-like flow-banding produced by rheomorphic ductile shear during emplacement, prior to cooling below magnetic blocking temperatures. Glassy vitrophyric lithologies commonly have discrepantly shallow remanence directions rotated closer to the orientation of the sub-horizontal shear fabric when compared to the microcrystalline center of the same cooling-unit. To investigate this problem, we conducted a detailed paleomagnetic and rock magnetic study of a vertical profile through a single ignimbrite cooling-unit and its underlying baked soil. The results demonstrate that large anisotropy of thermal remanent magnetization (ATRM) correlates with large (up to 38°) deflections of the stable remanence direction. AMS revealed no strong anisotropy. A strong lineation and deflection of the remanence declination suggest that rheomorphic shear above magnetic blocking temperatures is the dominant mechanism controlling formation of the magnetic fabric, with compaction contributing to a lesser extent. Nucleation and growth of anisotropic fine-grained magnetite in volcanic glass at high temperatures after, and perhaps also during, emplacement is indicated by systematic variation of magnetic properties from the quickly chilled ignimbrite base to the interior. These properties include remanence directions, anisotropy, coercivity, susceptibility, strength of natural remanent magnetization, and dominant unblocking temperature. The microcrystalline ignimbrite center has a magnetic direction that is the same as the underlying baked soil and, therefore, is a more reliable recorder of the paleofield direction than the glassy margins of highly welded ignimbrites.

Beschreibung: Static three-axis alternating field (AF) demagnetization is the most common method regularly implemented for removing magnetic components of rock samples. This method is so widely used that one of its main limitations, the acquisition of gyroremanence (GRM), is often not accounted for or even discussed. The presence of GRM likely interferes more than is recognized in accurate determination of the most stable remanence. The accepted method proposed by Dankers and Zjiderveld (1981) for excluding GRM affected measurements requires nearly triple the amount of lab work, and by consequence, is almost never regularly implemented on large batches of samples. Here, we present a laboratory procedure and subsequent analysis (SI method) that removes the effects of GRM in static AF demagnetization without requiring extra laboratory work. This paper, therefore, describes a new standard protocol for efficient static AF demagnetization of rocks. This article is protected by copyright. All rights reserved.