The Kaali Crater Field and Other Geosites of Saaremaa Island (Estonia): the Perspectives for a Geopark
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Geologos, 2010, 16 (1): 59–68 doi: 10.2478/v10118-010-0004-z The Kaali crater field and other geosites of Saaremaa Island (Estonia): the perspectives for a geopark Anto Raukas 1 & Wojciech Stankowski 2 1 Institute of Ecology, Tallinn University, Uus-Sadama 5, 10120 Tallinn, Estonia;e-mail: [email protected] 2 Institute of Geology, Adam Mickiewicz University, Maków Polnych 16, 61-606 Poznan, Poland; e-mail: [email protected] Abstract The Island of Saaremaa in Estonia offers highly spectacular geological features that belong to the most interesting in the Baltic Sea area. A unique geological monument on the island is the Kaali meteorite–crater field, formed by nine meteori- te impacts. There are also attractive coastal cliffs, huge erratics, alvars (limestone areas covered by a very thin soil) and well-developed glacial and marine landforms. Limestone cliffs and shingle beaches abound with Silurian fossils and offer great opportunities to fossil collectors. The island is a prospective geopark. During the past few years, the geology of the island has become an intensely studied object of Estonian and Polish geologists. Keywords: geopark, geosites, meteorite–crater field, Saaremaa Island, Estonia Introduction opment. Worldwide importance has the Kaali crater field, the first one in Europe of which The Island of Saaremaa (Ösel in German this origin was scientifically proven by finds and Swedish) is one of the largest islands in of meteoritic iron. The geological/morphologi- the Baltic Sea, measuring 2,673 km2. It belongs cal characteristics of the island, in combina- to the West Estonian Archipelago. The island tion with the peculiar flora and fauna, together has a low population density, with only about with the archaeological, historical and cultural 40,000 people. heritage deserve to be preserved in the form of Like the whole Estonian territory, Saaremaa a geopark. is located on the southern slope of the Fenno- scandian Shield and the peripheral part of the East European Platform, built of Lower Pa- Geological setting laeozoic strata. The geological structure is, in combination with morphogenesis, responsible Saaremaa is located on the southern slope of for an interesting landscape. Spectacular are the Fennoscandian Shield, which is composed tectonics-related seashore cliffs. Glacial topog- of crystalline rocks that actually form a south- raphy formed mainly during the glaciation, ward extension of the Precambrian complexes whereas young abrasion forms were created that crop out in southern Finland. The sur- during various phases of the Baltic Sea devel- face of the Fennoscandian Shield dips slightly Anto Raukas & Wojciech Stankowski 60 Anto Raukas & Wojciech Stankowski Fig. 1. Bedrock map of the Saaremaa and Muhumaa islands (from material of the Estonian Geological Survey) with the most important geosites (compiled by Heikki Bauert). Ice-marginal zones date from 12,400 years BP (Pandivere Stadial) and 11,700 years BP (Palivere Stadial). Af- ter Raukas & Stankowski (2005). southwards, with about 3.2 m/km. Based on Most of Saaremaa is located within the fold- borehole data and geophysical evidence, the ed Latvian-Estonian basement complex, which crystalline rocks occur in Saaremaa at a depth is 1.9–1.8 Ga old. The northern edge of this of several hundreds of metres; the 500-metre complex is marked by the somewhat younger isobath runs through the central part of Saare- (1.6–1.55 Ga) Riga rapakivi pluton, which ex- maa in a roughly W-E direction. The Proterozo- tends to the south-western corner of the island. ic formations are represented by several types Eastern Saaremaa consists predominately of of magmatic and metamorphic rocks (Perens, metamorphic rocks, while acid igneous rocks 2002). (mostly different types of granites) dominate in the western part of the island. Cambrian rocks are not exposed anywhere but are known from drilling cores. The exposed limestones and dolostones are of Silurian age. The boundary between Lower and Upper Silu- rian carbonate rocks (between the Rootsiküla and Paadla regional stages) runs through central Saaremaa (Fig. 1). The youngest Silu- rian rocks – limestones of the Ohesaare Stage, which formed approx. 417 Ma ago – crop out in the south-western cliffs of the Sõrve peninsula, southernmost Saaremaa. The total thickness of the Silurian succession is impressive, reaching 445.7 m according to core data from the Ohe- saare drill site. It consists of 10 chronostrati- graphical units (stages). The Juuru, Raikküla, Adavere, Jaani, Jaagarahu and Rootsiküla stages belong to the Lower Silurian succession, whereas the Paadla, Kuressaare, Kaugatuma Fig. 2. Silurian carbonate rocks of the Ninase cliff at the and Ohesaare stages constitute the Upper Silu- northern shore of Saaremaa (photo by H. Bauert). rian succession (Perens, 2002). The Kaali crater field and other geosites of Saaremaa Island (Estonia): the perspectives for a geopark 61 The Silurian carbonate rocks of Saaremaa sula (the limestone bedrock lies 92.5 m below are rich in fossils. Several Silurian limestone sea level at Mõntu). and dolostone varieties are not only good The glacial topography of the island stems building stones, but are also highly valued mainly from the last glacial. Large, massive as excellent source material for the manifac- continental glaciers derived from the Scandi- turing of natural finished stone products. For navian mountains transported a large number instance, the Upper Silurian Selgase mottled, of erratics to Saaremaa; some of them are truly easily handled limestone is widely used not huge (Raukas, 1995). The glacial and glacioflu- only for limestone carvings (cups, plates, can- vial sediments are fairly thin and reworked to dlesticks) but also for the production of fin- a large extent by the waters of the Baltic Sea. ished natural stone objects such as fireplace Waves strongly eroded the emerging land and, parts, wall blocks and wall veneers. For centu- in some places, changed the originally ice- ries, impurity-free, chemically pure limestone sculptured landscapes beyond recognition. The has been used for burning high-quality lime. most magnificent glacial landform is without This kind of limestone was mined at the Jaa- any doubt the West-Saaremaa Upland, a huge garahu limestone quarry North of Kihelkonna end moraine composed mainly of till and ris- village in NW Saaremaa. ing 20–35 m above its surroundings. The West- Saaremaa Upland owes its characteristics to its position between two ice flows, which moved Landscape in different directions and which were not si- multaneous (Karukäpp et al., 2002). The most Spectacular are the numerous seashore cliffs, recent glacier moved from north-west to south- which actually are coastal escarpments eroded east and carried erratic boulders to western into carbonate bedrock by waves (Märss et al., Saaremaa; these boulders differ in composition 2007). The Saaremaa cliffs display a variety of from those found in eastern Saaremaa (Rau- shapes: some of them stand as almost verti- kas, 1995). The southern extension of the West- cal walls (Panga cliff) but other cliffs look as Saaremaa Upland is the Sõrve Upland. scarps with wave-cut notches in their lower The other glacial landforms of Saaremaa are part (Üügu cliff). There are also cliffs rising in of a much smaller scale. Some glacial accumu- the form of terraces towards the land. The cliffs lations in Saaremaa are reflected by the hilly should be considered as active phenomena, de- landscape near Pöide and Valjala. The largest scending straight into the sea and subjected to glaciofluvial landforms are represented on the direct wave action (Fig. 2), but farther inland West-Saaremaa Upland near Viidumäe (for in- also �fossil� cliffs exist. These are coastal scarps stance at Kodaramägi) and on the Sõrve penin- formed during earlier stages of the Baltic Sea sula (for instance at Viieristi). development. The height of the cliffs varies considerably, ranging from only a few to 21.3 m for the Panga Post-glacial history (Mustjala) cliff. Vertically uprising steep walls have developed in carbonate rocks of uniform The deglaciation history of Estonia (see resistance to erosion and weathering. Wave-cut Fig.1) has been dated using radiocarbon, varve- notches are characteristic for the lower parts of chronology, palynology, electron spin reso- cliffs that consist of more argillaceous carbon- nance, thermoluminescence, optically stimu- ate rocks. The latter are less resistant to marine lated luminescence and cosmogenic-beryllium erosion than the more pure limestones. methods. The results are highly heterogeneous Some ten million years ago, the whole Bal- (Raukas, 2009). During the past few years, the tic area was emerged, reaching considerable present authors studied the suitability of lumi- absolute heights, and dissected by giant rivers nescence (TL and OSL) dating techniques for cutting deep into the rocks. Erosion caused the establishing a precise Late Pleistocene chronol- formation of deep valleys in the Sõrve penin- ogy for the Baltic area (Raukas & Stankowski, 62 Anto Raukas & Wojciech Stankowski 2005). It turned out that, in addition to reliable and at a height of 15 m on the Sõrve peninsula. dates, many entirely unreliable dates were The transgressive coastal forms are impressive, thus obtained, but the Late Pleistocene history because the water-level rise kept more or less could nevertheless be reconstructed. When the up with the uplift of the land, so that accumu- land-ice cap had retreated from the area, the lation of shoreline deposits in a narrow zone surface started to rise rapidly as a result of gla- could proceed for a long time. cioisostatic adjustment of the Earth�s crust: the About 4000 14C years ago, the sea level land was elevated in the order of 40–50 m. The started to drop slowly. This was caused by ice-freed areas became almost completely inun- a climate cooling that also induced ice-sheet dated by waters of local ice-dammed lakes. The growth in the polar regions.