Silurian Cliffs on Saaremaa Island
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SILURIAN CLIFFS ON SAAREMAA ISLAND Institute of Geology at Tallinn University of Technology University of Turku, Department of Geology SILURIAN CLIFFS ON SAAREMAA ISLAND Tallinn 2007 Silurian cliffs on Saaremaa island. MTÜ GEOGuide Baltoscandia. Tallinn, 2007 ISBN 978-9985-9834-3-0 Compiled: Tiiu Märss Alvar Soesoo Heldur Nestor Acknowledgements: Olle Hints Helle Perens Kaidi Tilk Igor Tuuling Edited: MTÜ GEOGuide Baltoscandia Layout: Andres Abe Figures: Heikki Bauert Photos: Heikki & Tarmo Bauert, Tiiu Märss, Oive Tinn, Igor Tuuling Front cover: Püssina cliff Back cover: Panga cliff (photo by T. Märss) © MTÜ GEOGuide Baltoscandia, 2007 Release of this booklet in English was co- financed by Environmental Investment Centre, Estonia. It was previously published in Estonian and Finnish languages under the INTERREG IIIA Southern Finland and Estonia programme. 2 Cliffs are coastal escarpments abraded in resistance. Wave-cut notches occur when bedrock by waves. In the West Estonian the rocks forming a cliff’s lower part are Archipelago, cliffs occur on the northern less resistant to abrasion than those in the coast of Kesselaid and Kõinastu islets, cliff’s upper part. The appearance of cliffs Muhu and Vilsandi islands, and the changes constantly. Severe storms, during eastern, northern, and western coast of which the sea level rises and the intensity Saaremaa Island. The appearance of cliffs of wave activity increases considerably, may differ: some rise as vertical walls, play an important role. some are dissected by wave-cut notches, For geologists, cliffs are extensive out- and others retreat landward as several crops allowing the study of rocks and steps. Active cliffs rise directly from the fossils in them along extensive shore sea and are washed by waves, while the sections. The characteristics of rocks and extinct ones are located away from the their vertical and lateral changes allow modern shoreline. The latter are coastal one to restore the conditions that pre- escarpments of earlier stages of the Baltic vailed in the ancient sea. The cliffs on Sea. The height of cliffs is different as Saaremaa and other cliffs in western well; the highest cliff in the West Estonian Estonia display the development of the Archipelago is Mustjala Cliff (21.3 m Palaeobaltic Sea and its biota during the high) on Saaremaa. The appearance of Silurian period (416 million to 443 million a cliff depends on the composition of years ago). the rocks forming the cliff, their bed- ding, the presence of fissures, and also Position of the continents in the mid- Silurian, ca 425 Ma ago (after C.R. Sco- the bedding conditions of rock beds and tese “Plate tec tonic maps and Conti- bedrock topography. Vertical cliffs are nental drift animations”, PALEOMAP formed when the rocks are of a similar Project, www.scotese.com) PANTHALASSIC North OCEAN China Siberia Kazakstania Barentsia Equator Greenland PALEO - TETHYS North OCEAN Australia America Baltica South Mexico Avalonia China RHEIC Antarctica Arabia IAPETUS OCEAN OCEAN GONDWANA Florida Africa SILURIANPosition CLIFFS of ON continents SAAREMAA in ISLAND mid-Silurian, ca 425 Ma (after C.R. Scotese "Plate tectonic maps 3 and Continental drift animations”, PALEOMAP Project, www.scotese.com) Spitsbergen. As a result of uplift, the seas SILURIAN PERIOD gradually retreated toward the margins of the continents. However, the retreat The Silurian period started 443 million and shallowing of the sea was not a years ago and ended 416 million years continuous and unidirectional process. It ago, lasting 27 million years. The Silurian was interrupted by repeated transgres- is an important time in the Earth’s history. sions and the deepening of the sea, the The rifting and dispersal of the Rodinia interruptions being at least partly related supercontinent, which had started in to changes in the sea level of the world the Proterozoic eon, was substituted ocean. As always, in the shallow areas with the reassembling and reunion of near shore, carbonate sediments (lime- the pieces of the continents. At that stones and dolostones) were deposited, time, the territory of present-day Estonia while in the deeper parts there accumu- was incorporated into the Baltica craton, lated clayey sediments (marlstones, clay- which included what is now northern stones, and shaley claystones). During the and eastern Europe. During the Silurian whole Silurian period, Baltica remained in period, the Baltica and North America the tropics. For this reason, its climate (Laurentia) drifted rapidly toward each was not influenced by the ice ages that other. They merged at the end of the at the beginning of the Silurian took period. The collision of these superconti- place in South America and Africa and nents started in the north and continued that served as a continuation of the ice toward the south. As a result of the ages at the end of the Ordovician. mutual stress of huge crust blocks, the northwestern edge of the Baltica craton Silurian marine life consisted mainly of started to uplift above the present-day seafloor invertebrates. However, the share Scandinavian mountains. Simultaneously, of freely swimming vertebrates increased above what are now the Baltic Sea and considerably during the Silurian. In the the Baltic countries, there appeared a seas, the diversity of corals and stromato- depression oriented northeast to south- poroids grew rapidly and their contribu- west. The depression formed the deepest tion to reef forming increased. The first axial part of the ancient Palaeobaltic Sea. coral-stromatoporoid barrier reefs were At the site of the collision of the conti- formed; they were comparable to those nental plates, the Caledonian Mountains at present and were hundreds of kilo- formed. The Caledonian Mountains meters long. Among the echinoderms, extended over the British Isles, western primitive stalkless cystoids gave up their Scandinavia, and eastern Greenland to leading position to sea lilies (crinoids). The 4 latter formed dense thickets and gener- the Silurian low terrestrial vegetation, ated an abundance of skeletal fragments primitive and sparse, started to develop. for the formation of thick limestone It was made up of primitive cryptogams: beds. Among arthropods, the diversity psilophytes. Thus, the Silurian can be and frequency of trilobites decreased considered an epoch-making and revolu- steadily; at the same time, entirely new tionary period also because the first firm evolutionary branches developed (e.g., evidence of terrestrial plants and animals sea scorpions – eurypterids) that were comes from that time. capable of living in freshwater lagoons. They reached imposing dimensions. The SILURIAN ROCKS IN ESTONIA fossils of the above life-forms can be found on Saaremaa in the cliffs and on In Estonia, the Silurian is divided into 10 stony beaches. regional stages grouped into the four Some terrestrial plant fossils originate series of the global chronostratigraphical from approximately the same period. standard: Llandovery, Wenlock, Ludlow, This suggests that in the second half of and Přidoli. Silurian stratigraphy in Estonia, thicknesses & prevailing rocks AGE Thickness Prevailing rocks in Prevailing rocks in Ma SERIES STAGE m Central Estonia South Estonia 416.0 ± 2.8 N Ohesaare 29+ missing dolomitic marl with limestone interbeds PŘIDOLI Kaugatuma 41-86 missing marl with lime- stone interbeds Kuressaare 19-27 missing marl, clayey ja bioclastic limestone LUDLOW biomicritic & reef Paadla 11-28 missing 422.9 UPPER SILURIA limestone, dolomite ± 2.5 clayey dolomite, Rootsiküla 19-40 missing limestone Jaagarahu 32-140 biomicritic & reef marl (upper part), dolo- WENLOCK limestone, dolostone mitized limestone (lower) Jaani 24-70 marl, domerite, marlstone, domerite clayey limestone Adavere 10-52 marl, domerite, marlstone, clay, domerite biomicritic limestone LLANDO- Raikküla 16-176 micritic-, biomicritic & micritic limestone, VERY reef limestone, dolomite marl, black shale LOWER SILURIAN Juuru 20-64 clayey, biomicritic, coqu- marl with limestone 443.7 noid & reef limestone nodules ± 1.5 SILURIAN CLIFFS ON SAAREMAA ISLAND 5 The System was earlier divided into the ited. In the second half of the Juuru Lower and Upper Silurian. The Lower Age, the sea became shallower. A huge Silurian included the Llandovery and shelly limestone deposit was formed of Wenlock; the Upper Silurian consisted the shells of the brachiopod Borealis. It of Ludlow and Přidoli. Unfortunately, the extended from the island of Hiiumaa as subdivision into Lower and Upper Silurian far as Alutaguse. This extremely pure is no longer internationally accepted. limestone, popularly known as “ring” limestone, was used for lime burning. At The Silurian strata overlie the Ordovician the end of the Juuru, on Hiiumaa Island and are covered with the Devonian beds. and the Ridala Peninsula, pure, granular They crop out in central and western crinoidal limestones consisting of the Estonia to the south of the Haapsalu- clasts of sea lilies and with coral and Risti-Tamsalu-Mustvee line. In mainland stromatoporoid reefs began to form. The Estonia they are discordantly overlain process continued in the Raikküla time. by the Devonian beds. The southern Those platy limestones were used in the boundary of the Silurian outcrop area building of Haapsalu and Ungru castles spreads over Tõstamaa-Pärnu-Suurejõe and the churches at Ridala and Käina. and Kolga-Jaani in the direction of Mustvee. The Devonian is missing on The Raikküla Stage is characterized by Saaremaa. In eastern Estonia, the Silurian the alternation of muddy (micritic) and rocks (except the lowermost, Juuru and granular (bioclastic) limestones in cen- Raikküla stages) had been eroded before tral Estonia, and micritic limestones and the Devonian. Southwestward, the marls in southern Estonia. At the end sections gradually become more com- of the Raikküla time, western Estonia plete. The Upper Silurian stages (Paadla, was subject to an extremely extensive Kuressaare, Kaugatuma, Ohesaare) have lowering of the sealevel, in the course of been preserved only on Saaremaa and which most of the earlier sediments were at the western margin of the Tõstamaa eroded. This is evidenced by the highly Peninsula.