Catastrophic Events Conference 3075.pdf

THE EARLY ORDOVICIAN CATASTROPHIC EVENT ON OSMUSSAAR ISLAND(ESTONIA) - RESULT OF AN IMPACT OR EARTHQUAKE? Kalle Suuroja and Sten Suuroja, Geological Survey of Estonia, Kadaka tee 80/82, 12618 Tallinn, Estonia, e-mail: [email protected]

The Osmussaar (Odensholm in Swedish) Island (Estonia) is situated in the southern part of the entrance to the Gulf of Finland (59018´ N; 230 28´ E ). It is a relict island of the ca 1200 km long Baltic Klint, extending from the Öland Island (Sweden) to Lake Ladoga (Russia). The up to 8 m high island emerged from the Baltic Sea ca 3000 years ago. In the coastal cliff of the island Ordovician limestones crop out within ca 5 km . The most intriguing part of the Osmussaar bedrock section is related to the 1.0-1.5 m thick layer of rocks of the Billigen, Volkhov and Kunda stages at the foot of the coastal escarpment, where the limestones are split into blocks and penetrated by veins and bodies of strongly cemented breccia-like limy sandstones. The rocks above (limestones of the Lasnamägi Stage) and below this level (glauconitic sandstones of the Hunneberg Stage) are undisturbed. Since the middle of the 19th century, several hypotheses concerning the origin of the described disturbed level have been proposed. Several terms have been used in geological literature to designate this formation: earthquake cracks (Öpik 1927), sedimetary veins (Orviku 1960), clastic dykes (Puura and Tuuling 1988) and sediment-intrusions (Suuroja et al 1999). By opinion of the majority of authors the catastrophic earthquake is proposed to be the main cause of this phenomenon. In connection with discovering of the submarine Neugrund impact structure a short way off (some miles to north- east) from the island in 1996, the authors (Suuroja et al 1996) proposed that the traces of destruction observed on Osmussaar Island and to a lesser degree on Pakri isalands are connected with the Neugrund impact. However, later investigations (1997–1999) showed that these two appearances have not nothing in common. The study of the crater filling rocks in the Neugrund impact structure and target rocks in the surrounding area has shown that the Neugrund impact took place in the Early Cambrian time (ca 535 Ma ago), while the Osmussaar event (splitting sea floor and penetrating sediment-intrusions) happened considerably later (ca 475 Ma ago). Likewise it was proved that distribution of the split sea floor is not restricted only to coastal outcrops of Osmussaar Island but can be followed in Northwestern Estonia on several thousands square kilometers. Inside the Neugrund impact crater the complex of rocks with sediment- intrusions (limestones of the Pakri and Toila Formations) is undisturbed and forms an unusually thick layer (ca 8 m in contrast to ca 1-2 m in the surrounding area). The differences in lithological composition are also noticeable. The chemical, lithological and mineralogical composition of the breccia-like sediment intrusions has been studied as well. In this strongly cemented rock two types of limy sandstone (light and dark) both bearing evident signs of mudflow can be distinguished,. Alongside with these traces, angular pieces of one rock type (usually dark) occur in the other (light) one. The sharp-edged pieces and blocks of pre-event limestones (the Pakri and Toila Formations) make up 0–50% (average 15%) of this three-component breccia-like limy sandstone. The dark (brownish) component differs from the light one mainly by its limy substance, cementing the clastic material only. In the light component the cementing material is fine- grained calcite, in the dark one the latter contains also a slight admixture of kerogene and diffused pyrite. In the thin (1-20 cm) veins injected into the surrounding rocks the dark modification prevails. At some distance from the Osmussaar Island (on the Suur-and Väike-Pakri islands ) the veins occur rarely, are thinner (1-10 cm) and grey in colour. On Osmussaar Island the veins and bodies of sediment-intrusions make up ca 10 % of the host layer , but on the Suur-Pakri Island the content of the intrusions is less than 1% (one vein to every ten meters). In the aforesaid veins is found shocked quartz (with planar features)! By the authors opinion it does not refer directly to the impact origin of these veins. The grains of shocked quartz are derived from the pre-impact (target) sandstones which had been subjected to the Neugrund impact in the Early Cambrian time (ca 535 Ma ago). To the sediment-intrusions they have been carried from the slopes of the Neugrund impact crater by the mudflow accompanying the catastrophic earthquake ca 60 Ma latter during the Middle Ordovician erosion in Upper Kunda time (ca 475 Ma ago). Analogous processes that caused the formation of the sediment-intrusions on Osmussaar Island and its surroundings embraced a great deal of the shallow part of the basin at this time. The about 50 km wide and several hundred kilometers long zone extended from Pakri Peninsula via Hiiumaa Island (Estonia) to the Öland Island (Sweden). In different parts of this area the results of the described processes connected with global tectonic movements (forming the Kaledonites) and of uplifting the aforesaid zone, have also been different.