Magnetic history of Early and Middle Ordovician sedimentary sequence, northern Estonia

Abstract

SUMMARY Alternating field and thermal demagnetization of lime- and dolostones from the Lower and Middle Ordovician (Floian to Darriwilian stages) subhorizontally bedded sequences in NW and NE Estonia reveal two characteristic magnetization components (named P and S). The intermediate-coercivity (demagnetized at 30–60 mT, up to 300–350 °C) reversed polarity component P (mean of Floian Stage: Dref= 147.8 ± 10.8°, Iref= 65.8 ± 5.4°; combined mean of Dapingian and Darriwilian stages: Dref= 166.0 ± 8.4°, Iref= 56.1 ± 6.5°) is regarded as the primary remanence of early diagenetic (chemical) origin. On the Baltica's apparent polar wander path (APWP), the palaeopoles (Floian: Plat= 25.0 °N, Plon= 50.8 °E, K= 52.7, A95= 7.2°; Dapingian and Darriwilian: Plat= 11.4 °N, Plon= 39.1 °E, K= 33.8, A95= 6.7°) are placed on the Lower and Middle Ordovician segment. The poles indicate that Estonia was located at southerly latitudes, decreasing with time (Floian: ∼48 °S; Dapingian and Darriwilian: ∼37 °S), when the remanence was acquired. A high-coercivity and high-unblocking-temperature component S (mean of samples: Dref= 33.7 ± 6.3°, Iref= 51.9 ± 5.7°) that is regarded as a secondary remanence has both normal and reversed polarities. On the European APWP, its palaeopole (Plat= 52.5 °N, Plon= 157.9 °E, K= 38.9, A95= 5.3°) gives middle to late Permian age. According to mineralogical (SEM and optical microscopy) and rock magnetic (three-component induced remnant magnetization) studies, component P is carried by magnetite (coexisting with glauconite) and component S by haematite. Magnetite is of chemical origin, formed in the course of early diagenesis and/or dolomitization. During the Permian continental period haematite, the carrier of component S, was likely precipitated from oxidizing meteoric fluids in the already existing or simultaneously formed pore space between the dolomite crystals.