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Along its contact with the Baltic Shield, the margin of the East European Platform reveals a well-developed, flooded terraced relief. The most striking and consistent set of escarpments at the contact of the Lower Palaeozoic calcareous and terrigenous rocks, known as the Baltic Klint (BK), extends from northwest of Russia to the Swedish island of Oland. Marine seismic reflection profiling in 1990–2004 revealed the central Baltic Sea Klint (BSK) section in detail and enabled comparison of its geology/geomorphology with a classical klint-section onshore, namely the North Estonian Klint (NEK). The conception of the BK onshore, which is based on the land-sea separating terraced relief in northern Estonia, is not fully applicable beneath the sea. Therefore, we consider that the BSK includes the entire terraced Cambrian outcrop. We suggest the term “Baltic Klint Complex” to include the well-terraced margin of the Ordovician limestone outcrop, which is weakly developed in Estonia. Because of a steady lithological framework of the bedrock layers across the southern slope of the Fennoscandian Shield, the central BSK in the western and the NEK in the eastern part of the Baltic Homocline have largely identical morphologies. The North Estonian Ordovician limestone plateau with the calcareous crest of the BK extends across the central Baltic Sea, whereas morphological changes/variations along the Klint base occur due to the east-westerly lithostratigraphic/thickness changes in the siliciclastic Cambrian sequence. The verge of the NEK, located some 30–50 m above sea level, starts to drop in altitude as its east-westerly course turns to northeast-southwest in western Estonia. Further westwards, the BK shifts gradually into southerly deepening (0.1–0.2°) layers as its crest drops to c. 150 m below sea level (b.s.l.) near Gotska Sandon. This course change is accompanied by a considerable decrease in thickness of the platform sedimentary cover, as below the central Baltic Sea the stratal sequence is 150–250 m thinner than in northern Estonia. This has facilitated a deviation of the terraced relief-forming Cenozoic rivers traversing east-westerly across the southern slope of the Fennoscandian Shield and forming different morphostructures in its eastern and western parts. Thus, a low-lying central Baltic Sea depression with well-developed asymmetrical cuesta valleys and terraces occur in the western half of the Baltic Homocline. In its eastern part, however, the only explicitly shaped cuesta valley along the shield-platform boundary forms a narrow east-westerly branch/gulf of the Baltic Sea (Gulf of Finland). The well-terraced southern margin of this gulf runs along the contact of the outcropping terrigenous and calcareous rocks, rises above the sea and forms a complex landform known as the NEK.

Based on structural maps, the bedrock structure on the southern slope of the Baltic Shield is analysed. Gently southerly dipping (0.1–0.2°) Paleozoic layers from Estonia to the Swedish east coast form the Baltic Homocline (BH). Numerous monoclinal folds, forced by basement faulting, induce slight variations in bedrock attitude across the BH. Studies on faults exposed in Southern Finland suggest that many monoclinal folds inducing basement faults within the BH are of Precambrian origin and have been active in pulses. The present monoclines were shaped by the Caledonian Orogeny. Submeridional tectonic hinge-lines induce minor regional-scale changes in the structural setting and control trends/styles of the forced folds. From the west, the BH bounds with the Baltic–Bothnian mobile zone that has been intermittently active since Mesoproterozoic times. The evolving Baltic Syneclise in the Silurian rearranged the structural setting typical for the Ordovician Baltic Basin. The Cenozoic uplift of Scandinavia created a SE-to-E-dipping bedrock sequence around the Gotland–Oland area. As the latter area had a different attitude than the BH, two Cenozoic cuesta-sets started to evolve around the northern Baltic Proper. They both had an outlet to the N–S-flowing Eridanos River around the Gotska Sandon area, creating thus two independent sections of the Baltic Klint.

The Silurian reefs off Saaremaa in the Baltic Sea were studied by means of high-resolution seismic reflection profiling. The abundance and diversity of the reefs increases off Saaremaa concomitantly with the deepening of the Baltic Silurian Basin towards Gotland. The peak of the reefs around Saaremaa occurred during the Middle Wenlock. The reef facies retreated further off Saaremaa during the Late Wenlock, became episodically restored around the island during the early Ludlow and finally ceased during the late Ludlow. A similar SW–NE–SW migration of reefs reflects an alternating transgressive–regressive pattern in the nearshore shallow shelf environment, where already minor sea-level fluctuations strongly influenced the conditions for reef growth. During the early Wenlock, a bathymetric break with a large barrier-reef-like structure divided the shallow shelf offshore Saaremaa facially into back-reef and southerly sloping fore-reef areas. The width of this SW-migrating barrier (c.8 km) and the extent of reef bodies within it (c.4 km) are the largest known in the Baltic region. During the Silurian, the reefs between Saaremaa and Gotland were flourishing to a much larger extent than is visible today. The primary Silurian reef pattern was increasingly destroyed towards Gotland by later erosion. This has resulted in a large area void of Wenlock reefs off northern Gotland. Off southern Gotland, the narrow conical low-energy deeper-water reefs prevail, as the wave-agitated shallow-water facies with larger and lenticular Wenlock and Ludlow reefs are largely eroded around the Gotland Deep.

The Lateglacial and postglacial sequence in the northern Gulf of Riga is sedimentologically subdivided into nine distinctive layers. In the seismo-acoustic sequence these layers are correlated with seven seismic/acoustic units, which largely reflect different stages in the development of the Baltic Sea. A uniform layer of the Late Weichselian till, a layer of waterlain glacial diamicton (WGD), a varved succession of the Baltic Ice Lake, a brackish-water/freshwater sandy/silty clay of Yoldia Sea, a FeS-rich layer of Ancylus Lake and discordantly bedded sand of the Litorina Sea and present-day gyttja are revealed both in sediment cores and in acoustic recordings. In general, the lateral extent of the distinguished sediment layers is gradually shrinking upwards in the Quaternary sequence towards the deepest, central depression of the gulf. Two distinguished regional discontinuities divide the Lateglacial and postglacial sediment sequence into three allounits: glacial diamicton deposits in the lower part; ice-proximal WGD, glaciolacustrine and postglacial lake/marine deposits in the middle; and brackish-water marine deposits in the uppermost part of the sequence. The presented detailed seismostratigraphic subdivision of the Quaternary sediment sequence of the Gulf of Riga permits a correlation/comparison with similar sequences across the Baltic Sea and in other former glaciated basins.

The Wenlock–Přidoli sequence has been subdivided and mapped offshore Saaremaa in the Baltic Sea by means of high resolution seismic reflection profiling. The most striking and consistent seismic reflectors correlate with contrasting lithological boundaries of the Silurian regional stages, the Jaani/Jaagarahu (S8), the Jaagarahu/Rootsikula (S10), the Rootsikula/Paadla (S11), the Paadla/Kuressaare (S12), the Kuressaare/Kaugatuma (S13) and the Kaugatuma/Ohesaare (S14) stages, which often coincide with erosional disconformities on Saaremaa. The shallow-shelf Silurian sequence that crops out on Saaremaa is also exposed in the closest offshore area of the island. Due to the westerly to south-westerly increasing erosional cut below the Baltic, the deeper facies varieties that are concealed in southern Saaremaa, are revealed and exposed at the seafloor towards Gotland. This is particularly well expressed in the Jaagarahu Stage, which undergoes a facies change with significant increase in thickness on the basinal slope of the Baltic Silurian Basin, both onshore and offshore Saaremaa. The Silurian sequence overlying the Jaagarahu Stage does not show any considerable thickness or facies changes neither below nor offshore Saaremaa. Offshore Saaremaa, seismic reflectors limiting new deeper facies subunits in the Jaagarahu Stage become either successively or simultaneously exposed inside the Vilsandi (S8–S9) respectively the Maasi–Tagavere (S9–S10) seismic units towards Gotland. These outcrop patterns were largely formed by two different erosional agents that during different time periods have played a considerable role in the formation of the present day Baltic Sea depression. Hence, the gradually south-westerly deepening cuesta landscape formed by the Cenozoic erosion, namely the Silurian Klint and the plateaus north and south of it, discloses progressively deeper part of the Baltic Silurian Basin. The deeper facies subunits are exposed rapidly by the submeridional erosional incisions of the Pleistocene glaciers. Except for the erosional component, there appears also a distinct tectonic factor in the genesis of the present Baltic Sea depression. Today, the coeval Silurian rocks that once were formed in separate facies belts, i.e. in different depth intervals of the same sedimentary basin on Saaremaa and on Gotland, are presently located at about equal altitude.

After a short historical review of the correlation of Palaeozoic rocks between Estonia and Sweden, this paper focuses on the results of marine seismic studies, achieved during the cooperative Swedish-Estonian project since 1990. The most recent seismic correlation scheme of the Cambrian, Ordovician, and Silurian strata and their distribution at the seafloor across the northern Baltic Proper are presented. Thickness changes and trends, as well as the sedimentary structures, reef bodies, and erosional features of different seismic units, are treated in connection with structural and facies changes in the Palaeobaltic Basin. The immediate background of this project is outlined and the locations of the seismic lines shot during the joint expeditions to the Baltic Sea are shown.

The Llandovery-lowermost Wenlock sequence in the Baltic Sea between Saaremaa and Gotland has been studied by means of high resolution seismic reflection profiling. The number and the positions of the seismic reflectors within this sequence change considerably even within short distances, thus showing a high lateral inconsistency both in lithology and thickness. Stratigraphically two unambiguous (S-1 and S-2) and two roughly fixed trans-Baltic seismic reflectors (S-5 and S-7) enabled us to distinguish the pre-Adavere, the Adavere (S-1-S-2 and S-2-S-5) and the Mustjala seismic units (S-5-S-7) and to follow their east-westerly changes across the Baltic. The seismic signatures and thicknesses of these units undergo a sharp transformation midway Baltic, dividing between the Saaremaa type and the Gotland type of Llandovery-lowermost Wenlock sections. This is due to a short and steep northeast-southwest facies transect that was induced by a well differentiated bathymetry in the intracratonic Palaeobaltic basin, thus clearly distinguishing the shallow-shelf (basinal), the slope (transitional) and the deep-shelf (basinal) areas. The slope is furthermore accentuated by the intensive submarine erosion and channelling during the Late Ordovician and Early Silurian times. Thus, the Saaremaa type of section includes the inner to outer shallow-shelf transect, where the pre-Adavere and Adavere units are decreasing in thickness, and the Mustjala unit is increasing in thickness towards Gotland. In the slope area offshore Gotland, all the Llandovery-lowermost Wenlock units thin out rapidly towards the deep-shelf area in south to southeast and the thickness of the pre-Adavere and Adavere units approach zero. Similar features of submarine erosion on Gotland and around the Estonian Latvian border reveal that an unstable basinal slope, from the point of sediment accumulation, extended over large areas along the shallow-shelf edge of the Palaeobaltic basin during the latest Ordovician and earliest Silurian times. Seismic data suggest that the uppermost unidentified subsurface of the Silurian succession in northern Gotland includes a considerable amount of marlstone layers that are coeval with the Mustjala Member in Estonia.

Seismic reflection profiling east of Gotland has given information on a succession of four reefal units in the lower Ludlow stratal sequence ranging from the Klinteberg Formation to the Hemse Group in the Gotland sequence. In this paper they are named the Klinte Reef (Klinteberg Formation unit f), the Hammarudden Reef (Hemse Group ‘younger’ units a-c), the Ostergarn Reef (Hemse Group unit d) and the Millklint Reef (Hemse Group unit e). The reef units form a mainly regressive succession with younger reefs resting on the seaward slopes of older reefs. The reef succession is exposed on Gotland and in the Baltic Sea east of Gotland, whereas towards the East Baltic coast it is overlain by Pridoli sedimentary bedrock. Each reef unit corresponds to a well-defined seismic unit with distinct facies zonality ranging from lagoon via reef barrier and biohermal slope to basin facies. The ‘fore-reef’ facies is in this paper termed biohermal slope to express its characteristic reefal features. Reconstruction of...

Based on high resolution seismic reflection profiling, structural and relief maps of the sedimentary bedrock between Gotland and Hiiumaa in the Baltic Sea have been composed and analysed. The general structure and relief of the submarine Lower Palaeozoic succession reveal a westward extension of the homoclinal structure distinguished in the Estonian mainland. The main bedrock structures offshore are 1-4 km wide, and several tens of kilometres long, linear zones of disturbances. On the structural map, these disturbances appear as submeridional zones of contour changes, up to several tens of metres in offset. The seismic profiles usually reveal a faint flexure-like bending of the layers through the zone. Locally, this flexure can be intersected by small faults. These bedrock structures are ascribed to fault movements in the crystalline basement. Two different bedrock relief systems were superposed on the region during the Cenozoic uplift and the Pleistocene glaciations. The first event resulted in ...

The dimensions and form, structure and distribution of the Upper Ordovician carbonate buildups in the Baltic Sea, northeast of the Island of Gotland, have been determined by means of shallow-marine seismic reflection profiling. They are shown to decrease in size and number downslope, towards the deeper part of the Palaeobaltic Basin. The seismic signature variations, reflecting the structural and compositional changes of the carbonate buildups point to an increase in reef skeleton forming organisms and enhanced rigidity and stability of the buildups upslope. Supported by the tectonic setting, a local shallow-water break was formed on the slightly sloping carbonate ramp of the Late Ordovician sea, closely east of the Isle of Gotska Sandon. This depth development promoted the formation of a major linear reef of fringing or barrier character, separating a patch reef complex and reefs in the offshore belt adjacent to deep shelf muds from a beach or tidal flat. In the offshore area northeast of northe...

The erosional relief of Ordovician and Silurian deposits in Estonia was developed during the continental period in late Silurian and early middle Devonian times. The uplift of the area and marine regression were induced by compressional tectonics in the continental interior related to the closure of the Iapetus and Tornqvist Oceans. In the northern part of the Baltic sedimentary basin (Estonia), on the gentle southerly dipping slope between the Fennoscandian Shield (Finland) and Baltic Syneclise (Latvia), a pre-Devonian, slightly rugged erosional relief with few cuesta was developed. The preDevonian erosional landforms hills, depressions and escarpments reaching 150m in height were probably buried under the Devonian deposits and then partly re-exposed by pre-Quaternary erosion. These landforms are described in detail using data from several thousands of cores drilled in the course of oil-shale and phosphorite exploration and mining. Erosional relief on Ordovician and Silurian deposits in the northwest of the East European Craton was developed during the continental period of exposure in the late Silurian and early-middle Devonian. Uplift of the territory and marine regression were induced by compressional tectonics in the continental interior due to the closure of the external Iapetus Ocean to the northwest (Phillips et al. 1976), and the Tornqvist Ocean to the southwest (Ziegler 1990; Schwan 1997). At the beginning of pre-Devonian erosion, the land surface was of generally low relief due to Ordovician and Silurian near-shore marine sedimentation. Structurally induced local landforms were almost non-existent before the late Caledonian phase of compression. Before the late Silurian-early Devonian phase of deformation, the whole East Baltic area was a shallow, intracontinental seabed or near-shore continental flat with few structural complications in the early Palaeozoic sedimentary cover. The compressional tectonics along the craton margins caused uplift of the region and deformation of the crystalline basement and sedimentary cover in the western corner of the Russian Platform. East of the present Baltic Sea, at least two areas of different types of structure were formed. In the central part of the Baltic sedimentary basin, i.e. in Latvia, tectonic deformation was most extensive, and fault throws reached 700m (Suveizdis 1979). After early Devonian erosion, the faultand dome-related residual escarpments and hills had altitudes of up to 150m (Polivko & Ulst 1969). In the northern part of the sedimentary basin (Estonia), within the gentle southerly dipping slope between the Fennoscandian Shield (Finland) and Baltic Syneclise (Latvia), a flatter hilly relief developed. The influence of faults with throws of up to 50 m on the formation of the pre-Devonian topography was negligible. These oldest erosional landforms are now partly buried under Devonian deposits and partly re-exposed during late-Phanerozoic erosion. Following the previous preliminary interpretations published in Estonian and Russian (Puura 1974; Puura et al. 1987; Tuuling 1988), this paper discusses sedimentary bedrock structure-topography relationships and reconstruction of the pre-Devonian landscape. Geological and geomorphological setting The study area is situated in the northwestern part of the Russian Platform of the East European Craton south of the Fennoscandian Shield (Fig. 1). Oil-shale deposits of the Baltic Oil-Shale Basin occur in northeastern Estonia and adjacent areas of northwestern Russia extending from 25~ 59~ to 29~ 59~ (Fig. 2). It is the best known portion of the basin because of extensive exploration for oil shale and phosphorite (over 15000 boreholes in 5000 km2). The oil-shale basin is located in the sub-surface of a slightly hilly plain south of the coastal cliffs of the Gulf of Finland. The highest point of the plain is 166 m above sea level on the Pandivere Heights (PH; Fig. 2). The Devonian is distributed around the periphery of the bedrock hills south of the Pandivere Heights and between the Ahtme Elevation (AE) and Izhora Heights (IH). From: SMITH, B. J., WHALLEY, W. B. & WARKE, P. A. (eds) 1999. Uplift, Erosion and Stability: Perspectives on Longterm Landscape Development. Geological Society, London, Special Publications, 162, 75-83. 1-86239-030-4/99/ $15.00 9 The Geological Society of London 1999. by guest on January 6, 2014 http://sp.lyellcollection.org/ Downloaded from

Seismic reflection profiling in the Baltic Sea between Gotland and Hiiumaa reveals that the lower boundary of the Cambrian mainly coincides with the crystalline sub‐Cambrian peneplain here. In the Gotska Sandon area, the Cambrian sequence rests partially on Proterozoic sedimentary bedrock, whereas Vendian rocks may underlie the Cambrian at the Estonian coast. The sandy‐argillaceous Cambrian sequence is covered by Lower Ordovician limestones in the western part, and by an up to 10 m thick layer of Lower Ordovician siliciclastic rocks in the eastern part. At the end of Lontova time, the early Cambrian depocenter shifted westwards from the Moscow Basin to the Baltic Basin, prohibiting any detailed Trans‐Baltic seismic correlation. Two Cambrian reflectors extend westwards from Hiiumaa, and seven reflectors eastwards from Gotska Sandon. Only one reflector extends across the area. It is located near the base of the Cambrian sequence within the Nar Member shale unit in the Gotska Sandon drill core, and ...