Magnetic reversal history of Pleistocene sediments at Old Crow, northwestern Yukon Territory

Abstract

The banks of the Porcupine River near Old Crow in the unglaciated part of the northern Yukon consist of about 55 m of interbedded fluvial and lacustrine gravels, sands, silts, and clays. This sequence was sampled closely for palaeomagnetic analysis and the sediments were found to possess a natural remanent magnetization (NRM) that is stable in direction with mean destructive fields of 200–300 Oe (16–24 kA/m). Therefore, good palaeomagnetic data can be obtained from sediments that have been subjected to permafrost conditions, although some samples show disturbance of NRM directions by cryoturbation.Magnetic polarity transitions are centred at 17 and 26 m above river level and the 15 samples between them define an interval of reversed polarity of considerable duration. The behaviour of NRM during the transition periods is complex with declination switching abruptly by 180° but with the inclination displaying very erratic variation over a considerable time. This may be caused by mixing in single samples of components of NRM with ages that straddle the transition—a situation that would be possible if NRM were post-depositional in origin and so could be locked in at times long after sedimentation.Sedimentological, palaeobotanical, and palaeomagnetic data all support the presence of a major unconformity in the middle portion of the sedimentary sequence. This unconformity lies at the top of the transition that closes the reversed episode so that the sediments below it are likely older than 700 000 years.These results demonstrate that palaeomagnetic studies on the sedimentary sequences exposed in the many isolated sites within the Old Crow region should greatly facilitate their correlation and indirect dating, effecting thereby an improved understanding of the recent geological history of this anthropologically important area.