Ulla Preeden

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

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.

Åvike Bay — a 10 km Diameter Possible Impact Structure at the Bothnian Sea Coast of Central Sweden.

Avike Bay is a 270° degree wide near-circular, 114 m deep bay on the Swedish coast of the Bothnian Sea, northeast of Sundsvall. The structure has a diameter of about 10 km. It was classified as a probable impact structure because of its extraordinary circular topography in the overwiew of impact structures in Fennoscandia. Recent studies lend further support to this interpretation. The structure has a submarine central mound, which is elevated some 40 m above the adjacent sea floor. It has a very distinct tangential and radial on-shore fracture pattern as seen in the topographic map. Along the southwestern shore of the Bay, an enigmatic quartzite breccia of unknown age occurs as part of a larger outcrop of polymict breccia with clasts of crystalline rocks and quartzite of unknown age. In thin section, planar fractures can be observed in quartz and feldspar grains. A detailed investigation showed that in a few cases the quartz grains contained microdeformation features closely resembling PDFs.

Physical properties of Vilppula drill cores and petrographic analysis of associated breccias in Keurusselkä impact structure, central Finland

The Mesoproterozoic deeply eroded Keurusselkä impact structure in central Finland is situated within the ∼1860–1890 Ma Central Finland Granitoid Complex. An estimate for the original size of the structure is 30 km, yielding a 5 km wide central uplift with insitu shatter cones and shock metamorphic features in quartz. Petrophysical and rock magnetic properties of the three shallow drill cores (V-001, V-002 and V-003) in the vicinity of the central uplift are determined in order to assess the dimensions of the central uplifts magnetic anomalies. The drill core lithologies consist of schists (metagraywackes), metavolcanic rocks, gneisses and breccia. Petrophysical properties of the drill core rocks show average densities (D) of 2644–2752 kg/m3, susceptibilities (κ) of 160–761 × 10−6 SI and natural remanent magnetization (NRM) of 3–306 mA/m and Koenigsberger Q ratios of 0.1–10. Rock magnetic measurements with temperature dependence of susceptibility (κ-T) curves and hysteresis indicated mostly paramagnetic behaviour. However, a fraction of fine-grained ferromagnetic minerals (pyrrhotite and magnetite) was detected from all lithologies. Breccia veins cutting the parautochthonous subcrater floor show lower values of petrophysical properties (D, κ, NRM, Q) and this could be related to the impact event. Amphiboles and micas in the breccia are strongly altered and replaced by secondary chlorite. Chloritization may indicate widespread impact-induced hydrothermal alteration of the target rocks or it may be related to regional tectonic shearing. However, planar deformation features in quartz, found from shatter cones in the central uplift area, are decorated with fluid inclusions indicating that alteration by post-impact processes was present.

The Duobblon Structure - A Small Segment of a Large Precambrian Impact Structure?

The Duobblon Structure is located in northern Sweden, near the Scandinavian mountain range in the county of Vasterbotten. The structure constitutes a 30 km long formation, slightly concave to the NW. The Revsund Granite (1.87 Ga) makes up most of the basement, and grades into a 100 m thick autochthonous basal breccia, overlain by a chert-like, clast-bearing melt rock and a polymict breccia. A fine-grained volcanic-like rock makes up the top of the series. Caledonian nappes of Cambro-Silurian age covers the structure to the W. The possible presence of planar deformation features in quartz and the similarity to the stratigraphy of proven complex impact structures support a possible impact origin. The age of the assumed impact event might correspond to a U-Pb zircon age of about 1.80 Ga, which is the indicated age of a chert-like rock in-between the autochthonous and the polymict breccia. Assuming that the arcuate breccia formations form part of a once circular structure, a crater diameter of approximately 80 km can be inferred, of which 1/7 of the outer margin is preserved.