238 Physics of the Earth andPlanetary Interiors, 18 (1979) 238—246 @ Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands INDICATION OF A GEOMAGNETIC LOW-INCLINATION EXCURSION IN SUPPOSED MIDDLE WEICHSELIAN INTERSTADIAL MARINE CLAY AT RUBJERG, DENMARK NIELS ABRAHAMSEN and KAREN LUISE KNUDSEN Laboratory of Geophysics, University of Arhus, DK-8200 Arhus (Denmark) Department of Micro palaeonto logy, Geological Institute, University of Arhus, DK-8000 Arhus (Denmark) (Received December 6, 1977; revised and accepted March 16, 1978) Abrahamsen, N. and Knudsen, K.L., 1979. Indication of a geomagnetic low-inclination excursion in supposed Middle Weichselian interstadial marine clay at Rubjerg, Denmark. Phys. Earth Planet. Inter., 18: 238—246. Palaeomagnetic measurements have been made on specimens from Late Pleistocene sediments of a piston boring at Rubjerg in Vendsyssel, northern Jutland. The stratigraphy of the deposits is based on content of foraminifera. A total of 70 relatively oriented specimens were investigated palaeomagnetically. Normal steep inclinations close to that of the axial dipole field were found in the Upper Saxicava Sand and in the Younger Yoldia Clay (radiocarbon dated at 14,650 ±190—12,650 ±180 B.P.), and a secular variation with an amplitude of 10—12°in the inclincation and a “period” roughly estimated at about 350—400 years was found in the Younger Yoldia Clay. Seventeen relatively oriented specimens from undisturbed older marine deposits revealed a stable low inclination of 110 with ~ = 3~. The age of this apparent geomagnetic excursion falls somewhere between 23,000 and 40,000 B.P. (Older Yoldia Clay). Among other known geomagnetic excursions and events within this interval are Laschamp in France, Mono Lake in California and Lake Mungo in Australia. Until more definite ages have been obtained, the excursion is provisionally named the “Rubjerg Excursion”. 1. Introduction 2. Methods In connection with a geological investigation of The Rubjerg boring (Fig. 1) was made on top of late-glacial deposits of the N~rre Lyngby area the late-glacial Yoldia plain, described by Jessen (57°25’N, 9°48’E) in north Jutland, Denmark, a (1899, 1936). The upper marine boundary of the series of shallow borings was made in 1974—1976 by Yoldia Sea in the area is about 40 m, and the bore the Department of Micropalaeontology, University of site is 18 m above present-day sea level. Fourteen Arhus. To make use of magnetostratigraphic meth- core sections were obtained in tubes by a 54-mm ods, relatively oriented specimens were collected piston sampler. The tubes, 80 cm long, were pushed from some of the borings, and the purpose of this vertically into the deposits by means of a drop ham- note is to report on the magnetic and stratigraphic mer and pulled up with a hydraulic rod puller. The results from one of the borings situated near Rubjerg Rubjerg Core was 17.7 m in total length and the (Fig. 1). The palaeomagnetic investigation of the pres- boring was stopped because of a boulder. Only virtu- ent material was made by Niels Abrahamsen and the ally undisturbed samples may be used magnetostrati- foraminiferal content and the stratigraphy were graphically, and normally an undisturbed section with studied by Karen Luise Knudsen. a length of 50—65 cm was obtained in the tubes.