T21 Terrestrial Impact Breccias

Sunday, 22 October 2006
8:00 AM - 12:00 PM

PALEOMAGNETISM OF THE ALAMO BRECCIA, NEVADA

Paper number: T21-10
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

The Upper Devonian Alamo Breccia (Guilmette Formation, Nevada) is interpreted as a deposit that formed from a wet impact on the continental slope. The lower part of the deposit at some locations is interpreted as ejecta that was hot at deposition whereas as the upper part was reworked by tsunamis. A modified paleomagnetic conglomerate test was conducted to investigate the origin of the breccia. Reworking of clasts by tsunami would presumably produce deposition of cold clasts with random directions as opposed to hot clasts with grouped directions as expected in hot ejecta or melt breccias. Individual clasts from the “clean washed” upper part of the breccia at most locations contain random magnetization directions. This suggests a pre-depositional magnetization which is consistent with an impact-generated tsunami origin. Preliminary results from a breccia clast and matrix in the lower part of the unit indicate a well grouped characteristic remanent magnetization (ChRM) with a pole that falls near the late Devonian part of the apparent polar wander path (APWP). This ChRM may be related cooling of ejecta breccia. Fine grained (coarse sand-pebble size) breccia at the top of the deposit contains a ChRM with southeasterly declinations and shallow inclinations that resides in magnetite. The pole position lies near the mid-Paleozoic segment of the APWP, and the ChRM is tentatively interpreted as a detrital remanent magnetization (DRM) or a post depositional DRM.

In contrast, lapilli clasts at multiple locations contain a ChRM residing in magnetite with south-southeasterly declinations and shallow-moderate negative inclinations that fails the conglomerate test. The pole position suggests remagnetization in the late Paleozoic/early Mesozoic and ChRM is interpreted as a chemical remanent magnetization caused by fluid alteration.