Magnetostratigraphic (re)calibration of the Paleocene/Eocene boundary interval in Holes 550 and 549, Goban Spur, eastern North Atlantic

Abstract

A magnetostratigraphic reinvestigation of the Paleocene/Eocene boundary intervals in DSDP Holes 550 and 549, Goban Spur, has removed uncertainties associated with the previously published magnetostratigraphic information for these chronostratigraphically important sections. In Hole 550, levels assigned to nannoplankton zones NP4 and 5, previously considered to be of normal polarity, carry a reverse polarity characteristic magnetisation which is correlated with geomagnetic Chron C26r. Unconformably overlying these levels is a thin interval of reversely magnetised sediment which records the latter part of Chron C25r. Chron C25n is redefined and its lower/upper boundaries are placed respectively at 425.08 and 422.12 mbsf in Hole 550. Chron C24r extends upwards ~62.4 m. Respectively, the start and end of Chron C24n.3n are placed at 359.69 and 349.41 mbsf in this hole. Within Core 28, just above C24n.3n, is a record of Chron C24n.2n as well as interval of indetermine polarity within C24n.2r. The problematic issue of assigning numerical ages to the various events associated with the Paleocene/Eocene transition in Hole 550 is then discussed. The remanence in Hole 549 is difficult to interpret because magnetic inclinations in the interval previously assigned to Chrons C25n and C24r are typically <20°compared with the ~60°expected for the Paleogene sediments at this site. Acknowledging the complexities of this record, we have constructed the magnetostratigraphy for this hole, based on results from the present study and previous work [H.A. Townsend, The paleomagnetism of sediments acquired from the Goban Spur on the Deep Sea Drilling Project Leg 80, in: P.C. De Graciansky, C.W. Poag et al. (Eds.), Initial Reports DSDP 80 (1985) 389-410]. Contrary to the previous study, we are not able to fix the base of Chron C25n; it is located either in the lower part of Core 18, where the core pieces are too small to be used reliably for paleomagnetic studies, or in the gap between Cores 18 and 19. The Chron C25n-24r boundary is blurred (~351 to 353 mbsf) because of the shallow inclination problem. Chron C24n.3n, which is characterised by inclinations of ~35°, has its lower/upper boundaries at ~318 and 307.62 mbsf respectively. An apparent record of Chron C24n.2n, based on two samples, is present between 305.53 and 304.65 mbsf.