Ordovician magnetostratigraphy: Llanvirn‐Caradoc limestones of the Baltic platform

Abstract

b Palaeomagnetic sampling has been performed covering 43 stratigraphic levels within the Baltoscandian Ordovician carbonates. After removing a ubiquitous Permo‐Carboniferous (287 ± 14 Ma) remagnetization between 200d̀ and 500 d̀C, a Llanvirn‐Caradoc reversal stratigraphy is delineated by components with maximum unblocking temperatures up to 550d̀‐580 d̀C. Three reversed (SE, down) and three normal (NW. up) antipodal polarity intervals have been recognized. A primary/early diagenetic remanence age is therefore inferred for the stratigraphically linked polarity chrons. Primary magnetizations are resident in detrital/biogenic or early diagenetically formed single‐ and pseudo‐single domain magnetite phases and subordinate early diagenetic pigmentary haematite. The recognition of a primary remanence within these well‐dated Ordovician carbonates has the following important tectonic and magnetostratigraphic consequences. (1) Accurate time‐calibration of the Baltic APW path implies that rapid counterclockwise rotation took place in late Tremadoc and Llandeilo times. The Arenig‐Llanvirn epochs are characterized by a ‘still stand'. Baltica occupied intermediate to high Southerly latitudes during the early Ordovician (Tremadoc‐Llanvirn). Systematic northward drift is recognized in post‐Llanvirn times. (2) A time‐calibrated Ordovician reversal stratigraphy is proposed. Presently available data suggest the geomagnetic field was predominantly reversely polarized during Tremadoc and Arenig times. Two normal polarity zones of short duration are identified within mid‐Llanvirn and mid‐Llandeilo strata. Discontinuities within the succession may mask other short‐period events. Late Llandeilo to mid‐Caradoc times were then characterized by a normal polarity field. Copyright © 1991, Wiley Blackwell. All rights reserved