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Palaeomagnetism of Middle Ordovician Carbonate Sequence, Vaivara Sinimäed Area, Northeast Estonia, Baltica

Plado J., Preeden U., Joeleht A., Pesonen L. J., Mertanen S.

Acta Geophysica

2016

Vol. 64, no. 5

1391--1411

The hill range of Vaivara Sinimäed in northeast Estonia consists of several narrow east- to northeast-trending glaciotectonic fold structures. The folds include tilted (dips 4-75°) Middle Ordovician (early Darriwilian) layered carbonate strata that were studied by mineralogical, palaeomagnetic, and rock magnetic methods in order to specify the postsedimentational history of the area and to obtain a better control over the palaeogeographic position of Baltica during the Ordovician. Mineralogical studies revealed that (titano)magnetite, hematite, and goethite are carriers of magnetization. Based on data from 5 sites that positively passed a DC tilt test, a south-easterly downward directed component A (Dref = 154.6°± 15.3°, Iref = 60.9°± 9.7°) was identified. The component is carried by (titano)magnetite, dates to the Middle Ordovician (Plat = 17.9°, Plon = 47.3°, K = 46.7, A95 = 11.3°), and places Baltica at mid-southerly latitudes. Observations suggest that in sites that do not pass the tilt test, the glaciotectonic event has caused some rotation of blocks around their vertical axis.

Palaeomagnetic age of remagnetizations in Silurian dolomites, Rástla quarry (Central Estonia)

Plado J., Preeden U., Puura V., Pesonen L. J., Kirsimäe K., Pani T., Elbra T.

Geological Quarterly

2008

Vol. 52, No 3

213-213

Alternating field and thermal demagnetization of dolomite samples from the Silurian (Llandovery) horizontally-bedded sequence of central Estonia reveal two secondary magnetization components (A and B) both of chemical origin. A low-coercivity (demagnetized at -50 mT) component A (D = 60.7°, I = 7.7°, alfa95 = 16.6°) with high dispersion (k = 14.2), yields a palaeopole at 18.2°N and 139.5°E that points towards the Late Devonian — Mississipian segment of the Baltica APWP (Apparent Polar WanderPath). A high-coercivity component B (D = 13.5°, I = 60.7°, k = 67.0, alfa 95 = 4.7°) carries both normal and reversed polarities. Comparing the palaeopole (71.1°N and 173.3°E) with the European APWP reveals a Cretaceous age. These two remagnetizations are linked to mineral assemblages of magnetite and maghemite (A), and hematite (B) determined from mineralogical (X-ray, SEM and optical microscopy) and rock magnetic (acquisition and thermal demagnetization of a 3-component IRM; Lowrie-test) studies. The results suggest that the first (A) Palaeozoic remagnetization was caused by low-temperature hydrothermal circulation due to the influence of the Caledonian (more likely) or Hercynian Orogeny after the diagenetic dolomitization of carbon ates. Hematite, carrying the component B, and goethite, are the latest ferromagnetic minerals that have precipitated into the existing pore space (hematite) and walls of microscopic fractures (goethite) that opened to allow ac cess for oxygen-rich fluids during the Late Mesozoic.