Jökull

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Jökull - 01.01.2004, Qupperneq 57

Jökull - 01.01.2004, Qupperneq 57
Reviewed research article A reconnaissance study of paleomagnetic directions in the Tjörnes beds, Northern Iceland Leó Kristjánsson Institute of Earth Sciences, University of Iceland, Askja, Sturlugötu 7, 107 Reykjavík, email: leo@raunvis.hi.is Abstract — Paleomagnetic direction measurements were carried out on core samples collected at 30 sites in sediments and one lava site, on the west coast of the Tjörnes peninsula, Northern Iceland. The sediments are mostly sandstones, more coarse-grained than younger sediments in the peninsula sampled by Eiríksson et al. (1990). The remanence in the present collection also has a lower intensity and stability during alternating field demagnetization. Several sites yield reasonably consistent directions, and both polarities are found. More extensive collections and improved experimental techniques are needed before the reversals can be used in local stratigraphic work. HISTORICAL INTRODUCTION General geology The fossil-bearing sediments in the Tjörnes peninsula of Northern Iceland are the country’s most extensive and best known sediment sequence. These sediments which are largely of shallow marine origin, have been the focus of research by many geologists, paleontolo- gists and other scientists since the mid-eighteenth cen- tury. A bibliographic review of this research was pub- lished by Eiríksson (1981a), and more recent develop- ments have been summarized e.g. by Eiríksson et al. (1990) and Buchardt and Símonarson (2003). The pioneer geologists Þorvaldur Thoroddsen and Helgi Pjeturss around 1900, Guðmundur G. Bárðar- son (1925) and subsequent authors have considered the age of the entire Tjörnes sequence to be Pliocene to Pleistocene. Eiríksson (1981b) divides the sedi- ment sequence into two main parts: the younger one is the Breiðavík Group which contains a number of lava flows and is underlain by a series of lavas named after the small Höskuldsvík inlet. The lower part of the se- quence is the Tjörnes beds, which only contains one occurrence of lava flows. It is generally divided into three zones: from top to bottom the Serripes, Mactra and Tapes zones according to characteristic mollusc fossils. Various methods have been employed in attempts to obtain definite ages, either absolute or relative, of parts of the sequence. These methods have in- volved e.g. climate variations, K-Ar radiometric dat- ing of lavas, and faunal changes. An approach used at Tjörnes by Th. Einarsson et al. (1967) as well as by several later authors (see Eiríksson, 1981, Buchardt and Símonarson, 2003) was based on correlation of paleomagnetic polarities with a global time scale of geomagnetic reversals. Unfortunately, the polarity time scale of the 1960’s and its later versions (de- rived partially from dated volcanics worldwide and partially from ocean-floor anomaly lineations) have turned out to underestimate the actual number of re- versals. Therefore it is becoming increasingly clear that paleomagnetic polarities in sections where only a few reversals occur, are not to be relied on by them- selves for long-distance (> 20 km) correlation or age estimates. However, extended polarity patterns and particular smaller-timescale features such as rever- sal paths can be used for short-distance correlations. Here, new paleomagnetic direction results from sites JÖKULL No. 54 57
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