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The structure and relief of the bedrock sequence in the Gotland-Hiiumaaarea, northern Baltic SeaIgor Tuulinga; Tom Flodénb

a Institute of Geology, Tartu University, Tartu, Estonia b Department of Geology and Geochemistry,Stockholm University, Stockholm, Sweden

To cite this Article Tuuling, Igor and Flodén, Tom(2001) 'The structure and relief of the bedrock sequence in the Gotland-Hiiumaa area, northern Baltic Sea', GFF, 123: 1, 35 — 49To link to this Article: DOI: 10.1080/11035890101231035URL: http://dx.doi.org/10.1080/11035890101231035

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GFF volume 123 (2001), pp. 35–49. Article

The structure and relief of the bedrock sequence in the Gotland–Hiiumaa area,northern Baltic Sea

IGOR TUULING and TOM FLODÉN

Tuuling, I. & Flodén, T., 2001: The structure and relief of the bedrocksequence in the Gotland–Hiiumaa area, northern Baltic Sea. GFF, Vol.123 (Pt. 1, March), pp 35–49. Stockholm. ISSN 1103-5897.

Abstract: Based on high resolution seismic reflection profiling, struc-tural and relief maps of the sedimentary bedrock between Gotland andHiiumaa in the Baltic Sea have been composed and analysed. The gen-eral structure and relief of the submarine Lower Palaeozoic successionreveal a westward extension of the homoclinal structure distinguished inthe Estonian mainland. The main bedrock structures offshore are 1–4km wide, and several tens of kilometres long, linear zones of distur-bances. On the structural map, these disturbances appear as submeridio-nal zones of contour changes, up to several tens of metres in offset. Theseismic profiles usually reveal a faint flexure-like bending of the layersthrough the zone. Locally, this flexure can be intersected by small faults.These bedrock structures are ascribed to fault movements in the crystal-line basement. Two different bedrock relief systems were superposed onthe region during the Cenozoic uplift and the Pleistocene glaciations.The first event resulted in the formation of a subparallel cuesta-like sys-tem of alternating erosional scarps and plains. Glacial erosion createdsubmeridional valleys and troughs. Today three large bedrock forms,namely the Baltic and the Silurian clints and the Ordovician plateau,characterize the area. The outlines of the cuesta relief, and the amount oferoded sediments, advocate a regional increase in erosional activityfrom the St. Petersburg district to the area of the Baltic-Bothnian mobilezone northeast of Gotland. This zone existed as a subsided meridionallower ground during the Cenozoic, accommodating a main river thatcollected water both from the craton margins and the inner platform ar-eas.Keywords: Bedrock structure, bedrock relief, northern Baltic, linearzones of disturbances, Cenozoic uplift, Baltic clint, Silurian clint, Pleis-tocene glaciations, Baltic-Bothnian mobile zone.

I. Tuuling, Institute of Geology, Tartu University, Vanemuise 46, EE-51014 Tartu, Estonia, tuuling@ut.ee. T. Flodén, Department of Geol-ogy and Geochemistry, Stockholm University, SE-106 91 Stockholm,Sweden, tom@geo.su.se. Manuscript received 26 November 1998. Re-vised manuscript accepted 18 January 2001.

In order to study the submarine bedrock geology, the aquatorybetween the Estonian islands of Hiiumaa and Saaremaa in thenortheast and the Swedish islands of Gotland and Gotska Sandönin the southwest was covered by a regular net of north to southdirected high resolution seismic profiles in 1990 to 1992 (Fig. 1).Several auxiliary profiles were shot in 1993–96, either to studyin more detail the geology of some district or to support the inter-pretation of some specific geological structures. The only trans-Baltic, northeast to southwest directed seismic profile (NB9601;Figs. 1, 2) proved to be very useful for the evaluation of the gen-eral bedrock structure, as well as for the style and morphology ofthe prominent zones of disturbances found in the area.

Based mainly on the data just mentioned, a new seismostrati-

graphical scheme for the Ordovician and lowermost Silurian se-quences were worked out for the northern Baltic Sea (Flodén etal. 1994; Tuuling et al. 1995). Two prominent trans-Baltic re-flectors were distinguished in the laterally stable Ordovician se-quence. These are O1, which corresponds to the Cambrian–Ordo-vician boundary, and O4–5 that emerges as an interfering reflectorat two sharp lithological boundaries between the Upper Ordovi-cian Nabala-Vormsi and Vormsi-Pirgu stages. The lowermostSilurian stratal units between the two sides of the Baltic Sea,however, have significantly different lithologies. Hence, thereare no possibilities for direct oversea seismic correlation.

Following the interpretation and digitising of the seismic data,a regular database was created for the mentioned trans-Balticbedrock marker surfaces. The database contains position x/y-values in the UTM-34 coordinate system and elevation z-valuesalong the seismic lines. Structure and relief maps of the bedrockwere contoured by means of grid based mapping programs,Quicksurf inside AutoCAD (release 11) and Surfer for Windows(release 5.02). Based on these maps, the structure of the LowerPalaeozoic bedrock sequence in the northern Baltic Sea was ana-lysed and compared with the sequences in the surrounding on-shore areas. The impact of the internal structure of the bedrocksequence on the formation of its topography was analysed bycomparing geological, structural and bedrock relief maps of theinvestigated area.

The investigated areaIn the seismic profiles, the two trans-Baltic reflectors in the Or-dovician can be followed mainly between the two most con-spicuous morphological features of this region, namely the Bal-tic clint in the north and the Silurian clint in the south (Fig. 1).The O1 reflector always starts from the face of the Baltic clint(see Fig. 12) and it usually becomes obscured somewhat to thenorth of the Silurian clint. The O4–5 reflector appears a little southof the Baltic clint and can still be followed south of the Silurianclint. Hence, the areas covered by the afore-mentioned two re-flectors generally coincide. The isolines of the O1 level are, how-ever, those most representative for the area (Fig. 3). Only a fewzones of disturbances have been followed further southwardsusing the isolines at the O4–5 level as indicators.

In order to extend the area with structural information, the con-tour map of the base of the Cambrian (Tuuling et al. 1997) wasincorporated into the map of the O1 level (Fig. 3). Since the Cam-brian sequence in this region normally rests on the sub-Cambrianpeneplain (Flodén 1980), the former map reflects quite ad-equately the bedrock structure in the area of outcropping Cam-brian rocks. In the east–west direction, the shallow water areasoffshore Hiiumaa in the northeast and Gotska Sandön in thesouthwest limit the seismic information from the bedrock se-quence. To embrace the entire Saaremaa-Gotland clint complex,the bedrock relief map (see Fig. 10) was enlarged a little further

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southwards compared to the structural map (Fig. 3). Further-more, the top of the bedrock relief was distinguishable also in theprofiles, shot in the shallow water area offshore Hiiumaa.

Formation and general structure of the LowerPalaeozoic succession in the Baltic regionThe sedimentary bedrock cover northwest of the East EuropeanCraton started to form extensively in Late Proterozoic, Vendian,time. The main depocentre during the Late Vendian and earliestCambrian was located east-northeast of the presently investi-gated area. Only the Early Cambrian Lontova transgression in-undated for the first time the easternmost part of the northernBaltic Sea (Tuuling et al. 1997). After Lontova time, a new depo-centre started to form south-southwest of the investigated area.During the following Early Palaeozoic the Palaeobaltic Cambro-

Silurian epicontinental basin submerged the northwesternmostpart of the East European Craton. Large areas of the Baltic Shieldbecame buried under a thick sequence of Cambro-Silurian sedi-ments.

Following the main Early Palaeozoic tectonic event, namelythe Caledonian orogeny which in this region culminated in LateSilurian to Early Devonian times, the interior and southeasternslopes of the uplifted Baltic Shield turned into a continental area.During the following more than 350 million years, a hugeamount of Lower Palaeozoic rocks were eroded from this area. Aconsiderable quantity of them was exarated by Pleistocene gla-ciers. As a result, the original extension of the Lower Palaeozoicsedimentary bedrock cover has diminished considerably. Largeareas of Precambrian crystalline bedrock were uncovered fromdifferent Cambrian to lowermost Devonian units (Martinsson1958).

At present, the erosional limit of the Palaeozoic platform cover

36 Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area GFF 123 (2001)

Fig. 1. Area of investigation with the location of the seismic profiles. Numbers 1–8 mark the parts of the seismic profiles presented in Figs. 4–8 and12–14. The boundaries of the different geological-seismostratigraphical units discussed in the text are marked Cm (base Cambrian), O1 (base Ordo-vician), O3 (base Rakvere), O4–5 (Nabala-Vormsi and Vormsi-Pirgu), S1 (base Silurian), S2 (base Raikküla). A–B, B–C, C–D mark the intervals ofthe trans-Baltic geological section presented in Fig. 2.

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extends from the Bornholm area in the southwest, northwardsalong the Swedish coast west of Öland and in a large arc throughthe northern Baltic Sea to the northern coast of Estonia. Onwardsfrom the Gulf of Finland the limit continues through the lakesLadoga and Onega to the White Sea (Amantov et al. 1995, fig.3).The most conspicuous erosional feature of the region, namelythe Baltic clint, is located somewhat platform-wards of the out-cropping siliciclastic Vendian and Cambrian rocks. This escarp-ment, which is up to 60 m high on the Estonian mainland, marksthe northern extension of the calcareous Ordovician rocks. It ismore or less continuous from Öland in the southwest to LakeLadoga in the northeast (Tammekann 1940).

Due to minor differences in the geological history, the com-pleteness of the Lower Palaeozoic sequence, as well as its strikeand dip are slightly different between the different parts of theregion. In general, the area represents a typical homocline. Theslightly (0.1–0.3 degrees) south-southeastwards tilted Cambro-Silurian layers thicken towards the main depocentres - the Balticand the Moscow Syneclises. Depending on the position of thearea with respect to the earlier mentioned two large structuralbasins, the strike of the bedrock layers varies slightly aroundsouth-southeast. The discernible hinge line, separating the twosyneclises, runs along the Lake Peipsi axis. In the Peipsi - Gulf of

Finland isthmus the hinge line is marked by the Zagrivye faultzone (Tuuling 1988). East of it, in the St. Petersburg district, theLower Palaeozoic succession is distinctly tilted in a southeast-erly direction, towards the centre of the Moscow Syneclise. Onthe Estonian mainland, the southwards declivity, towards theBaltic Syneclise prevails (Tuuling 1990).

This homocline structure is complicated by numerous, some1–4 km wide and several tens of kilometres long, locallysmashed and fractured zones known as linear zones of distur-bances (Vaher et al. 1962; Vaher 1983; Puura et al. 1986, 1987;Tuuling 1990). Unveiled in some oil shale and phosphorite quar-ries and mine walls, the linear zones of disturbances appear as aset of small folds and faults and strongly fissured rocks. A geo-logical section across a similar zone, however, reveals a weakflexure-like bending of the bedrock layers. In the structurallyhigher and lower sides of it, the flexural-bending generallyverges into a small anticline and syncline, respectively.

The declivity of the flexural bending, connecting the anticlinaland synclinal parts of structure, does usually not exceed 2–3 de-grees (maximally 7°). The tilt of the anticlinal and synclinallimbs, however, remains in the limit of the first degree (Vaher1983). The total amplitude between the anticline crest andsyncline trough is usually in the limits of 5–10 m, reaching in a

GFF 123 (2001) Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area 37

Fig. 2. Trans-Baltic geological section based on seismic profile NB9601 (for location of the intervals A–B, B–C, C–D, see Fig. 1). F - zones ofdisturbances and faults. O1–O4–5 seismostratigraphical boundaries, see Fig. 1. Numbers 1, 2, 4, 5 mark the parts of the trans-Baltic section illustratedwith seismic lines in Figs. 4–5 and 7–8 (for location see 1, 2, 4, 5 in Fig. 1).

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few cases up to 50 m (Puura & Suuroja 1984). Thus, on thewhole these zones appear in the bedrock sequence as linear,strongly asymmetrical anticlinal belts. Due to their distinctlyasymmetrical shapes, these structures are ascribed mostly tofault movements in the basement (Vaher et al. 1962).

General structure of the bedrock sequence in thenorthern Baltic SeaStructurally, the submarine area of the northern Baltic Sea isvery similar to the Estonian mainland (Tuuling et al. 1995). Thestructural map (Fig. 3) shows the typical homoclinal structurewith 0.1–0.3 degrees south-southeastwards tilted Lower Palaeo-zoic layers. Numerous longer or shorter zones of disturbances,which conditionally divide the entire area into blocks, induce thelocal strike variations. A perceptible regional strike change fromsouth-southeast to southeast occurs some 60–70 km east ofGotska Sandön (Fig. 3). It is marked by several detached sub-meridional zones of disturbances, separating structurally thesouthwestern and northeastern sides of the present area of inves-tigation. The multitude and extensions of the zones of distur-bances on the southwestern side, offshore Fårö and GotskaSandön, exceeds overwhelmingly those offshore Saaremaa andHiiumaa.

38 Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area GFF 123 (2001)

Fig. 3. Structural mapof the sedimentarybedrock sequence.1 - zones of distur-bances,2 - level of the O1

reflector,3 - top level of theacoustic basementcoinciding in mostareas with the top ofcrystalline basement,4 - location of thesupposed structuralhinge line crossingthe northern BalticSea.Numbers 1–5 markthe distinguishedswarms of disturbedzones discussed inthe text.

Fig. 4. Interval of seismic profiles NB9601 (1 in Figs. 1–2) across thestructurally complex area northeast of Fårö. O1(–194.3) - Cambrian–Ordovician boundary reflector with the calculated absolute height inmetres. O4–5 - reflector from the boundary of the Upper OrdovicianNabala-Vormsi and Vormsi-Pirgu stages. F - zones of disturbances andfaults. NB9001–NB9003, positions of the crossing submeridional pro-files. Vertical scale 50 ms equals to c. 36 m at water velocity.

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The slightly southeastwards tilted bedrock sequence of thesouthwestern part is crossed by a series of more or less continu-ous submeridional zones of disturbances. Tentatively, they maybe grouped into five different swarms (numbered 1–5 in Fig. 3),passing meridionally through the entire area of investigation.Each swarm consists of several detached zones of disturbances,which, from south to north, are displaced successively east-wards, i.e., all the swarms have slight southwest to northeastorientations. In the north, the zones 2 and 3 extend into an areathat previously was considered as a zone of increased tectonicactivity (Tuuling et al. 1997). In this area, minor faults and foldswith heavily fractured and epigenetically altered Cambrian rockswere traced.

All zones of disturbances in the limits of the first four swarmshave relatively lowered eastern sides. On the average, the struc-tural height differences are less than 10 m; only rarely they reachmore than 20 m. The zones of disturbances that mark the easternlimit of the southwestern side (swarm 5 in Fig. 3) have mostlytheir western sides structurally lowered. Exceptionally, the fifthswarm has also a slight southeast to northwest orientation andfurther north it merges with the fourth swarm (Fig. 3).

The trans-Baltic Gotska Sandön - Hiiumaa profile (Figs. 1, 2),reveals many additional zones of disturbances which are missingon the level map (Fig. 3). This is either because of their inconsid-erable extension, negligible amplitude or due to the too sparsenet of profiles. The latter factor is particularly acute offshoreFårö and Gotska Sandön in between the profiles NB9001–NB9005, where the distance between profiles is twice as much(10 km) as in the remaining areas (Fig. 1). On account of the in-sufficient density of the profiles, the real positions of thesemainly submeridional zones are often somewhat shifted andtheir complex structure simplified.

This is very impressively demonstrated at one of the mostcomplex sets of tectonic dislocations just northeast of Fårö. This10–12 km wide zone between the profiles NB9001 and NB9003emerges in the level map as a single structurally eastwards low-ering zone (zone 1 in Fig. 3). The trans-Baltic profile, however,reveals two additional closely placed narrow down faultedblocks (Figs. 2, 4). The amplitude of the faults that delimit theblocks (Fig. 4) is relatively high, about 20 m. Due to the limited

set and unfavourable positions of the profiles, these narrowdownfaulted blocks remain unveiled on the level map.

The abundance and extension, as well as the amplitude of thezones of disturbances diminish noticeably east-northeast ofswarm 5 (Fig. 3). Yet, they are somewhat higher in numbers andmore extensive around the 21° longitude. The submeridional ori-entation and the structurally lowered western sides of the zonesof disturbances dominate in this part of the northern Baltic Sea.The estimated amplitude does not exceed 10 m.

One of the most striking structural anomalies in the northeast-ern part of the Baltic Sea seems to be present just outside of thepresented level map. The northeasternmost segment of the trans-Baltic profile (interval C–D in Figs. 1, 2) reveals a complicatedset of dislocations of different styles (Fig. 5). Some 20–25 kmsouthwest of the Kõpu Peninsula the profile crosses a zone ofdisturbances with its northeastern side structurally uplifted some10 m (not shown in Fig. 2). Further northeastwards the profileintersects a 1–2 km wide, 5–10 m downfaulted block (Fig. 5).Northeastwards there follows a zone of disturbances with thenortheastern side structurally lowered some 8–10 m. In betweenthese two structural areas, the profile crosses a few solitary,strongly fractured zones without any significant amplitudechanges.

Neither the extensions nor the orientations of the dislocationsjust mentioned can be determined due to the limited set of seis-mic data. The predominantly northwesterly to southeasterly ori-ented bedrock relief features (see Fig. 10), as expressed by themorphology of the nearby Baltic clint, point towards one largerhighly dislocated and fractured northwest to southeast orientedset of disturbances here. The heavily indented and lobated clintface, incised by deep northwest to southeast oriented valleyssomewhat further towards the northwest (Fig. 6) is to a certainextent inherited from the latter zone.

Style and general spatial pattern of dislocationsThe continuous image of the seismic profiling technique revealsthe style and morphology of every single dislocation, as well asthe entire spatial complexity of their distribution. The trans-Bal-tic profile NB9601 (Figs. 1, 2) supports the results of the Esto-

GFF 123 (2001) Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area 39

Fig. 5. Interval of theseismic profileNB9601 (2 in Figs.1–2) across the struc-turally complex areasouthwest of theKôpu peninsula(Hiiumaa).O1(–116.7) - Cam-brian–Ordovicianboundary reflectorwith the calculatedabsolute height inmetres. F - zones ofdisturbances andfaults. Vertical scale50 ms equals to c. 36m at water velocity.

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nian mainland investigations, namely that considerable struc-tural level changes in the bedrock sequence occur mostly withina complex zone, rather than along one single fault.

Thus, the main structural element of these zones is usually aflexure-like bending of bedrock layers that extends through theentire width of the zone. In places it may be intersected by smallfaults. The distance between the lowermost and highest struc-tural level points across these zones is mostly in the limits of 1–2km, extending only in a few cases up to 3–4 km. The relative im-portance of these two structural elements, versus the total ampli-tude and width of similar zones of disturbances, determines thegeneral style of the zones. In those cases, where the faults aremissing, the zones of disturbances appear as genuine flexures(Fig. 7). On the other hand, increasing numbers and amplitudesof small faults make them resemble step-wise dislocated faultsystems (Fig. 8).

The offsets of the aforedescribed north–south oriented solitaryfaults reach up to 20 m. They usually delimit narrow, about 1 kmwide, downfaulted blocks. Hence, these faults have no consider-able influence on the regional structural background (Figs. 3–5).For that reason, they may easily remain undetected on the struc-tural map. Northeast of Gotland, in places there were solitaryeast–west oriented dislocations (Tuuling et al. 1995, fig. 4).

The dominating north–south orientation of the fault zones inthe northern Baltic Sea has been pointed out earlier (Flodén1980; Winterhalter et al. 1981; Grigelis 1991). Similar orienta-tion of the zones of disturbances in the area of investigation dif-fers distinctly from the prevailing directions of the tectonic line-aments on the adjacent mainland areas. Thus, in northern Estoniathe northeast–southwest direction dominates, whereas offshoreSweden and on the Swedish mainland the northwest–southeastdirection of faulting dominates (Strömberg 1976; Flodén 1980).

Westwards, the longitudinal orientation of the submarinezones of disturbances prevails to a major northwest–southeastdirected regional tectonic hinge line, extending from Landsort onthe Swedish coast to northern Gotland (Flodén 1984). East-wards, the north–south orientation still prevails on the West Es-tonian islands and is terminated only by the Vihterpalu zone ofdisturbances in West Estonia (Vaher 1993). The latter, north-northwest to south-southeast directed zone intersects the Esto-

nian mainland from the Pärnu Bay in the southwest to theVihterpalu area on the Estonian northwest coast and has thehighest known structural amplitude in the region, about 50 m(Puura & Suuroja 1984).

Another specific character, which differentiates the zones ofdisturbances in the northern Baltic Sea and in North Estonia, istheir structurally lowered opposite sides. Except for a few casesin North Estonia, all the zones of disturbances east of theVihterpalu zone have their west-northwestern sides structurallylowered. In the northern Baltic Sea, on the other hand, the easternsides are generally lowered.

The age of the zones of disturbancesThe relief of the sub-Cambrian peneplain (Fig. 3; Tuuling et al.1997) clearly indicates that the zones of disturbances in the sed-imentary bedrock rest on deep basement faults. This is similar tothe zones in the Estonian mainland area. Likewise, the subma-rine zones cut through the entire Lower Palaeozoic bedrock suc-cession, which includes the lowermost Silurian stratal sequencein the presently investigated area.

The analogous mainland zones in the St. Petersburg district cutthrough the Middle Devonian, which is the youngest stratigraph-ic unit that contains similar structures (Tuuling 1988). Thus, thesubmarine zones have certainly been reactivated during post Si-lurian times, too. Presumably, the activity along the zones culmi-nated in the latest Silurian - Early Devonian time interval (Puuraet al. 1987), i.e., at the prime of the Caledonian tectonic move-ments, when the entire region underwent considerable structuraland paleogeographical changes.

Thus, the zones of disturbances have evolved due to recurrentactivation and vertical block movements along the ancient base-ment structures. A few, in detail explored zones of disturbancesin northeast Estonia show some evidences of synsedimentarytectonic movements during the Late Vendian - Early Cambriantime interval (Vaher 1983). Tectonic movements during theEarly Cambrian post-Lontova time were evidenced along theaforenamed Vihterpalu zone of disturbances (Tuuling 1990).Thus, the thickness distribution of the Early Cambrian post-Lontova sequence indicates some subsidence of the area west of

40 Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area GFF 123 (2001)

Fig. 6. Interval of the seismicprofile NB9201 50 km west ofHiiumaa (3 in Fig. 1). The profiledemonstrates the tectonicallydisturbed and indented area of theBaltic clint complex. O1 -Cambrian–Ordovician boundaryreflector. F - zones of distur-bances and faults. Vertical scale50 ms equals to c. 36 m at watervelocity.

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the latter zone during the Sõru-Soela times (Fig. 9). Most of theanalogous mainland structures do not provide any evidence forsynsedimentary movements in the Cambro-Silurian sequence,however. Therefore, the zones of disturbances in this region arecertainly of latest Caledonian age or younger, i.e., in part fromgeological periods which are not present in the covering bedrocksequence.

The present seismic data indicate tectonic activities alongsome submarine zones of disturbances in the Late Ordovician.The most evident mobile zone has an influence on the facies dis-tribution of the trans-Baltic Ordovician–Silurian transitionalbeds (Flodén et al. 1994). This mobile zone coincides with theset of disturbances that mark the gentle change in the regionalstrike of the bedrock layers (zones 5 and 4–5 in Fig. 3).

The zone just mentioned, has played an important role in theformation and distribution of the Late Ordovician carbonatebuildups (Tuuling & Flodén 2000). It is distinctive as some kindof slope-like topographical feature influencing the distributionand extent of erosion during two separate Late Ordovician timeintervals (Tuuling & Flodén 2000). Another zone (zone 2 in Fig.3) delimits eastwards the extension of the Late Ordovician linearreef structure (Tuuling & Flodén 2000). The development of theLate Ordovician barrier-reef-like structure just east of GotskaSandön has occurred along some kind of east–west orientedflexure-like bending of bedrock layers (Tuuling & Flodén 2000).This furthermore indicates an increased Late Ordovician to EarlySilurian tectonic activity.

The general bedrock relief of the northernBaltic SeaBecause of the inconsiderable thickness of Quaternary deposits,the seabottom relief of the northern Baltic Sea also reveals to alarge extent the concealed bedrock surface. Therefore, even thescarce data have permitted rather reliable interpretations andenabled researchers to distinguish the main bedrock relief fea-

GFF 123 (2001) Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area 41

Fig. 7. Interval of the seismic profile NB9601 in the central part of thenorthern Baltic Sea (4 in Figs. 1–2). The interval shows a flexure-likebending of the bedrock layers. O1 (–185.3) - Cambrian–Ordovicianboundary reflector with the calculated absolute height in metres. O4–5 -reflector at the boundary of the Upper Ordovician Nabala-Vormsi andVormsi-Pirgu stages. Vertical scale 50 ms equals to c. 36 m at watervelocity.

Fig. 8. Interval of the seismic profile NB9601 in the central part of thenorthern Baltic Sea (5 in Figs. 1–2) showing a step-wise rising zone ofdisturbances. O1 (–202.8) - Cambrian–Ordovician boundary reflectorwith the calculated absolute height in metres. O4–5 - reflector at theboundary of the Upper Ordovician Nabala-Vormsi and Vormsi-Pirgustages. F - zones of disturbances and faults. Vertical scale 50 ms equalsto c. 36 m at water velocity.

Fig. 9. Thickness distribution of the Vendian - Cambrian Lontova (3)and Cambrian Sôru and Soela (4) sequences across the Vihterpalu zoneof disturbances (1) in West Estonia. Black dots (2) mark the drill coresused for thickness calculation of mentioned sequences.

tures in this submarine area (Martinsson 1958; Svidorov et al.1976; Winterhalter et al. 1981; Grigelis 1991). The bedrock re-lief map of the northern Baltic Sea, presented in this paper (Fig.10), is for the first time based on a large set of regular bedrocklevel data.

The main bedrock relief style in the northern Baltic Sea,namely the cuesta-like alternation of submeridionally slopingterraces and escarpments (Fig. 11), is typical for large areas inthe northwestern part of the East European Platform. Thisdenudational structure is largely induced by the homoclinally

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south-southeast tilted bedrock sequence of rapidly varyinglithologies. Thus, the bedrock consists of alternating layers ofhard erosionally resistant limestone and soft, easily eroded sand-stone, clay and marl.

In the northern Baltic Sea, the main bedrock relief setting has adistinct northeast to southwest orientation (Figs. 10, 11). Theplateaus and escarpments intersect at a slight angle the east towesterly and southeast to northwesterly directed facial zones ofthe Palaeobaltic Ordovician and earliest Silurian basins, respec-tively. Thus, the erosional surface that follows the coeval Ordo-vician–Silurian rocks in a southwesterly direction enters gradu-ally towards the Swedish coast more deep-water facial varieties.This gives the submarine bedrock surface a general declivity inthe same direction (Fig. 11). The southwesterly deepening inci-sion, and the increasing amount of eroded Ordovician–Silurianrocks in this direction, indicates an increase in erosional intensitytowards the Swedish coast.

In summary, three major bedrock relief features can be distin-guished from north to south in the investigated area. These arethe Baltic clint complex, the submarine Ordovician plateau, andthe Saaremaa-Gotland clint complex (Figs. 10, 11). These reliefforms are part of a regional system in the present cuesta land-scape of alternating plains and escarpments. The outlines of thislandscape were established in the wide marginal areas of theFennoscandian block during the Cenozoic uplift (Fromm 1943;Martinsson 1958). Pleistocene glaciers further reshaped the ter-races and escarpments. Together, this has resulted in numerousdeep valleys, larger troughs, heavily indented and lobated sub-marine escarpments and an extremely rugged relief, particularlywithin the narrow area of outcropping friable Cambrian rocks(Figs. 10, 11).

The submarine Baltic clint complexIn the Gulf of Finland, the up to 45 km wide submarine area ofoutcropping Vendian and Cambrian rocks was distinguishedmorphologically as a submarine Cambrian-Vendian plateau(Tavast & Amantov 1992; Tavast 1993). In the case of the north-eastern Baltic Sea it is hardly relevant to talk about a submarineCambrian plateau. Between Hiiumaa and Gotska Sandön, theCambrian outcrop is generally a 3–10 km wide stripe with a rap-idly changing topography of alternating scarps, steep slopes, andnarrow flat areas (Tuuling et al. 1997; Figs. 11, 12). Moreover,the outcrop is intersected by numerous deep submarine valleysand depressions, separating isolated elevations and erosionalremnants (Figs. 6, 10, 11, 12, 13). Thus, from a morphologicalpoint of view, it is reasonable to treat the narrow area of Cam-brian outcrop together with the Baltic clint as one erosional sys-tem of terraces and escarpments, namely the Baltic clint complex(Figs. 11, 12). The latter forms the submarine transition betweenthe outcropping areas of the crystalline and the calcareous rockson the southern slope of the Baltic Shield.

Strictly speaking, the Baltic clint represents an erosional es-carpment at the contact between Lower Palaeozoic siliciclasticand calcareous rocks. The crest of the scarp always consists ofhard erosionally resistant Ordovician limestones, whereas its ba-sal part as a rule terminates in soft Lower Cambrian siliciclasticrocks (Figs. 12, 13). Being more or less distinct the entire dis-tance from Lake Ladoga in the northeast to the island of Öland inthe southwest, it constitutes the most prominent geomorpho-logical feature in the northwestern part of the East EuropeanPlatform (Tammekann 1940).

Unlike the mainland observations, the seismic profiles in thenorthern Baltic Sea embrace the entire system of terraces and

42 Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area GFF 123 (2001)

Fig. 10. Bedrock reliefmap of the northeasternBaltic Sea. The sub-meridional lines markthe zones of distur-bances.

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escarpments between the outcropping calcareous bedrock coverand the crystalline basement. The escarpment, which is strictlyknown as the Baltic clint on the mainland, makes up only one,although usually the major, part of this morphologically complexsystem.

In places, however, the scarp at the contact between the silici-clastic and the calcareous rocks may be negligible compared tosome other escarpment further north and south inside the areas ofoutcropping Cambrian and Ordovician rocks. This is most im-pressively demonstrated along profile NB9110. In the profile,just a few kilometres north of the 10 m high Baltic clint, an es-carpment at the contact between the Cambrian and pre-Cambrianbedrock reaches about 100 m (Fig. 13). Also in areas of intensiveexaration, the transition between the calcareous and siliciclastic

rocks is usually rather smooth and without any considerable es-carpment (Tuuling et al. 1997, fig 6).

Thus, to strictly follow the Baltic clint, as it is defined on themainland, is sometimes rather problematic in the seismic pro-files. This supports furthermore the treatment of the submarineset of escarpments and terraces as one morphological system,enabling a more extensive use of the term Baltic clint complex.Due to the complex pattern created by the glacial exaration, themorphology and height of the different escarpments can varyconsiderably. As on the Estonian mainland, indentations of dif-ferent sizes penetrate deep into the submarine Ordovician pla-teau, forming narrow submarine valleys and larger submarinebays, being separated by protruding submarine capes (Figs. 10,11).

GFF 123 (2001) Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area 43

Fig. 11. Perspective plot showing the main bedrock relief features of the northern Baltic Sea.Downloaded By: [Tartu University Library] At: 18:15 1 March 2011

The indentations are best developed and well pronounced inthe areas of extensive exaration activity like closely northeast ofGotska Sandön and west of Hiiumaa. It seems as two sub-meridionally advancing glaciers formed a pair of large troughsjust northeast of Gotska Sandön (Figs. 10, 11). OffshoreHiiumaa, on the other hand, the clint face is intersected by sev-eral northwest to southeast oriented valleys, reworked and deep-ened by glaciers (Figs. 6, 10, 11). In the south, towards the south-easternmost end of these incisions, the transition from the Ordo-vician carbonate plateau to the outcropping Cambrian rocks, isusually rather smooth, with a negligible escarpment. Due to theirgradual deepening northwards, the scarp becomes more pro-nounced in this direction along the marginal parts of the troughsand valleys, i.e., usually reaching its maximum at the very pointof submarine capes.

In summary, there are two morphologically distinctly differentareas of the Baltic clint complex between Hiiumaa and GotskaSandön. The extensive exarational area northeast of GotskaSandön (between the profiles NB9001 and NB9006) has a largearea of Cambrian outcrop (>30 km) and a smoothly northwardsdescending bedrock relief with only some small escarpments.The scarp at the boundary between the calcareous Ordovicianand the siliciclastic Cambrian rocks is negligible. The area north-east of profile NB9006 has a rather narrow strip of Cambrianoutcrop (3–10 km), usually descending northwards in a well-de-veloped system of alternating escarpments and terraces. The es-carpment at the boundary between siliciclastic and calcareousrocks is, as a rule, considerable.

44 Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area GFF 123 (2001)

The trans-Baltic Ordovician plateauThe step-wise rising Baltic clint complex passes southwardsgradually into the submarine trans-Baltic Ordovician plateau.The width of the plateau between the Baltic clint complex in thenorth and the Saaremaa-Gotland clint complex in the south var-ies between 50 and 70 km. In general, it is narrower and morpho-logically more complex in the area of the most extensive Pleisto-cene exaration, offshore Fårö, enlarging regularly northeast-wards, in the direction of the Estonian islands of Hiiumaa andSaaremaa (Figs. 10, 11).

The plateau surface is sculptured mainly in the hard, erosionresistant, layers of Ordovician limestone. The southernmost partof it is cut into the Lower Silurian rocks (Figs. 1, 10). Within thelarger part of the plateau area, a southern strip, some 7–8 km inwidth, deepens southwards along the layers in between the twostrong Lower Silurian reflectors S2 and S3 (Fig. 14; Flodén et al.1994, figs. 5, 6A, 6B). According to the seismic interpretation(Flodén et al. 1994), the reflectors are limiting the relatively pureand hard limestone sequence, corresponding to the RaikkülaStage in Estonia. In the limits of the Fårö Deep, however, thesoutherly dipping part in large areas follows either the O4–5 re-flector or the Ordovician–Silurian boundary reflector S1 (Tuul-ing & Flodén 2000).

Thus, the strip of the outcropping lowermost Silurian rocksnarrows gradually southwestwards (Fig. 1). This depends on thetrend of the facial changeability of the Lower Silurian rocks. TheEarly Silurian reef and mud-mound deposition in the nearshore

Fig. 12. Interval of the seismicprofile NB9204 in the centralpart of the northern Baltic Sea(6 in Fig. 1) crossing the Balticclint complex. O1 - Cambrian–Ordovician boundary reflector.Vertical scale 50 ms equals to c.36 m at water velocity.

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GFF 123 (2001) Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area 45

facies belt on the Estonian side is progressively replaced by amore offshore mud facies towards Gotland (Bassett et al. 1989).This circumstance has definitely favoured the west-southwest-erly increasing extent of the erosion and the formation of thedeep area just northeast of Fårö.

A more detailed study of the submarine Ordovician plateaushows that it cannot be interpreted as one unique plateau-likemorphological feature. Conditioned by the absolute height, rug-gedness, and prevailing relief declivity of the area, three separateparts can be distinguished.

1) The northeasternmost part offshore Hiiumaa and Saaremaa.This is the highest, slightly southwestwards tilted area with someerosional, northwest-southeasterly oriented depressions. The ab-solute height of the bedrock relief varies from about -20 m justoffshore Hiiumaa, to -100 m in its deepest part some 40-50 kmsouthwest of the island. Towards the southwest it is bordered bya 5-7 km wide, northwest to southeast oriented slope where theabsolute height of the bedrock relief rapidly lowers from -70 to -100 m (Figs. 10, 11).

2) The 50-60 km wide central part of the investigated area ischaracterised by southwest to northeast-directed isolines (Figs.10, 11). Thus, the bedrock relief features in this part are orientedalong the regional setting of alternating terraces and escarpmentsdeveloped during the Cenozoic. Quite possibly, Pleistocene gla-ciers have not considerably reshaped the bedrock relief in thispart of the area. This fact is also supported by the morphology ofthe Baltic clint complex. North of the plateau area, between theprofiles NB9203 and NB9205, the entire Baltic clint complex isexceptionally well developed and has an only slightly indentedmorphology of step-wise rising escarpments. The southernmostpart of the clint complex is here about 10 km wide and sculpturedin hard erosion resistant Ordovician limestone (Fig. 12). Within

Fig. 13. Interval of the seismicprofile NB9110 crossing theBaltic clint complex 40 km westof Hiiumaa (7 in Fig. 1).Vertical scale 50 ms equals to c.36 m at water velocity.

the limits of the plateau, from the Baltic clint complex to theSaaremaa-Gotland clint complex, there can be distinguished: a) a5-10 km large crest area which forms a plateau in between -80and -100 m and b) an up to 40 km wide, slightly southeast tiltedslope of pre Saaremaa-Gotland clint complex, from -100 to -120m.

3) The bedrock relief of the third, some 50-60 km wide areaclosely northeast of Fårö is strongly reworked by Pleistoceneglaciers. It is dominated by two large submeridional glacialtroughs, separated by a saddle-like elevation (Fig. 10, 11). Thus,the subparallel oriented bedrock relief pattern, typical for thecentral part, has been obliterated by submeridionally directed re-lief features of glacial heritage.

Although, different in size, the two troughs are of similar stylefrom north to south in accordance with the resistance of erosionof the outcropping bedrock layers. The deep bay-like incisions inthe Baltic clint complex and the large deeps in the Saaremaa-Gotland clint complex, are formed in areas of less resistant Cam-brian-Lower Ordovician and Upper Ordovician-Lower Silurianrocks respectively. A narrow transitional channel connects thedeep areas in both cases across the crest of the plateau, which ismade up of hard Lower and Middle Ordovician limestone. Thedifference between the highest and the lowest points, in the areaof the saddle-like elevation respectively in the deep some 30 kmnortheast of Fårö (Fårö Deep), is about 150 m.

The Saaremaa-Gotland clint complexTowards the south, the Ordovician plateau is usually terminatedby the next set of submarine escarpments, terraces and slopes,namely the Saaremaa-Gotland clint complex. This, normally 3–10 km, in some cases up to 20 km wide area of rising relief forms

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46 Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area GFF 123 (2001)

the transition to the next submarine plateau, the trans-Baltic Silu-rian plateau. The clint complex is predominantly cut into the softLower Silurian (Llandoverian) sequence of clayey limestone andmarl. Its uppermost part is, however, composed of hard, erosionresistant limestone of Wenlockian age.

Morphologically, the clint complex can be divided into twodistinctly separate parts. A steep stepwise rising set of alternat-ing scarps, steep slopes and narrow plain-like areas in the south,is followed northwards by a slightly rising slope which is usually3–4 km, but occasionally as much as 13–14 km, wide. In a fewcases, within the Fårö Deep and offshore Hiiumaa, the northernregularly ascending part is missing, being replaced by a few ad-ditional, up to 20 m high escarpments. The basal part of thesesteps always rises from the reflectors O4–5 or S1.

One of the foremost and most continuous escarpments of thissystem occurs right at the sharp lithological contact to the Wen-lockian reef-limestone. This feature, also known as the supra-marine Silurian clint (Aaloe 1956), is more or less continuousalong the northwestern coast of the island of Saaremaa and west-northwest coast of Gotland. Most of its face and crown consistsof reefs containing Wenlockian limestone. The basal part of theclint wall, however, is made up of strongly clayey Llandoverian-Wenlockian limestone.

A conodont based micropaleontological investigation (Jepps-son et al. 1994) has proved that the larger part of the trans-Baltic

Silurian clint faces consists of coeval rocks, i.e., the uppermostJaani and lowermost Jaagarahu stages on Saaremaa and LowerVisby, Upper Visby and Högklint Beds on Gotland. The thick-nesses of the corresponding Saaremaa subdivisions are in gen-eral smaller and their lithologies and distribution in the clint faceare somewhat different.

Similarly to the Baltic clint, the submarine Silurian clint, gen-erally forms only one, often a very prominent, part of a largersystem of alternating terraces, slopes and scarps. Likewise, thewidth and the general morphological style of this transitionalarea between the Ordovician and Silurian plateaus can vary nota-bly, depending largely on the paths of the Pleistocene glaciers.

Thus, the glacier lobes that considerably reshaped the mor-phology of the Baltic clint complex also formed distinct indenta-tions along the Saaremaa-Gotland clint complex. Within thesesubmarine glacier incisions, the clint complex is nothing but aregularly rising slope, usually without any major scarps. This isthe case along the aforenamed two glacial troughs northeast ofFårö, where along the profile S9312 (Fig. 1) the largest slope,about 35 km in width, was traced. Similar glacially induced,smoothly rising, slopes were found in a few profiles just offshoreSaaremaa, too.

In the main part of the area, the major ascent of the bedrockrelief between the submarine Ordovician and Silurian plateaus isconfined to about 1–3 km, visualised by 1–4 but usually 2–3

Fig. 14. Interval of the seismicprofile S9311 in the central partof the northern Baltic Sea (8 inFig. 1) crossing the Saaremaa-Gotland clint complex. O4–5, S1,S2, S3 - seismic reflectorsmarking the boundaries of theOrdovician Pirgu and Vormsistages, the Ordovician–Silurianboundary, and lower and upperboundaries of the Adavere Stageequivalent in Estonia, respec-tively. Vertical scale 50 msequals to c. 36 m at watervelocity.

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GFF 123 (2001) Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area 47

steps, the height and face declivity of which can vary consider-ably. The general style of the Saaremaa-Gotland clint complex isdetermined by the multitude, as well as the morphology of theseescarpments. Normally the topmost steps, or in case of a singlestep the uppermost part of it, at the contact between the pure andclayey limestone, are much steeper, often vertical. The lower-most step, which often comprises the largest rise in relief, usu-ally has a talus-like, downwards flattening slope, knittingsmoothly together with the gradually rising slope area of theSaaremaa-Gotland clint complex.

The most impressive interval of the Saaremaa-Gotland clintcomplex was followed along the eastern margin of the glaciertrough some 80 km northeast of Fårö (profiles NB9202, S9311).In this area, the trough bottom is deeply incised into Lower Silu-rian rocks. This way, it extends southwards very close to thesteeply falling clint face (Fig. 14). The relief fall, brought aboutby two upper, very closely spaced (200–300 m) vertical steps, isabout 80 m. A little further north follows a lower third step. Thetotal relief fall within two kilometres is 110 metres. In mostcases, however, the total relief-fall along the Saaremaa-Gotlandclint complex remains in the limits of 60–80 m. The variationsdepend both on the depth of the basal part of clint complex andon the thickness of the preserved uppermost part of hard lime-stone at the clint crest.

The zones of disturbances and the formation ofthe present bedrock reliefThe north–south oriented zones of disturbances in the northernBaltic Sea intersect the cuesta-like bedrock relief setting, whichcontains southwest to northeast-directed escarpments and pla-teaus. Most of the submarine zones of disturbances are not ex-pressed in the present day bedrock topography (Fig. 10). This isalso the case in northern Estonia (Puura et al. 1999). Thus, mostof them seem to have been dormant or at least less active sincethe Cenozoic. Only locally, narrow downfaulted bedrock blocksthat are distinct also in the bedrock relief appear (Figs. 4, 5, 6).This may indicate at least some neotectonic activity in the area.

In places, a clear relation between the heavily ruptured zonesof disturbances and some relief forms of selective linear erosion,namely ancient river valleys and exarational troughs appears. So,in the case of two exarational troughs northeast of Fårö, theCenozoic rivers, and also the advancing Pleistocene glaciers,have seemingly both followed a less resistant path, coincidingwith the crushed and cracked zones of disturbances (Fig. 10).Their promoting influence on the extension of glacier exarationis demonstrated in the Fårö Deep. The unusual depth of thisstructure is partially due to the complex set of zones of distur-bances described northeast of Fårö (Figs. 3, 4, 10). A similarsituation is found in another structurally complex area southwestof Hiiumaa (Figs. 5, 6). On the other hand, the huge continentalice masses may also have caused some movements along thesmall faults, i.e., part of their present amplitudes may haveglaciotectonic heritage.

Discussion and conclusionsThe Lower Palaeozoic stratal sequence in the northernBaltic Sea as a submarine continuation of the EstonianhomoclineStructurally, the submarine sedimentary bedrock sequence in the

Gotland-Hiiumaa area forms a west-southwesterly continuationof the homocline, which covers large slope areas of the BalticShield in Estonia and in the St. Petersburg district. The generalstyle and morphology of the main bedrock structures observedon the mainland, namely linear zones of disturbances can be fol-lowed offshore, too. Their slightly different orientation, as wellas their particularly high multitude northeast of Gotland, pointtowards a tectonic setting which is somewhat diverse from theareas further east.

Thus, the slightly, 0.1–0.3 degrees, towards the Baltic Syn-eclise tilted Lower Palaeozoic sequence undergoes a gentlestrike change from east–west to southwest–northeast about mid-way between Hiiumaa and northeastern Gotland (Fig. 3). Thischange is accompanied by a general increase in the number oflinear zones of disturbances westwards. Towards Gotska Sandönand Fårö, the increasing tectonic activity is best evidenced by thenumerous faults and graben-like structures located just north-west of Gotska Sandön (Flodén 1980).

The northern Baltic Sea - a structural transitional areabetween the East European Craton margin in the westand the inner platform areas in the eastThe investigated area, its western part in particular, has evidentlyserved as some sort of structural transitional area between thecraton margin in the west and the inner platform areas in the east.The prevailing northwest to southeast direction of the tectoniclineaments, as well as the east-southeast tilted surface of thecrystalline basement offshore Sweden (Flodén 1980) and on theSwedish mainland (Strömberg 1976), are evident proofs of dif-ferent tectonic and structural backgrounds between these areas.The structural transitional position of the northern Baltic Sea isfurthermore proved by the prevailing northeast–southwest orien-tation of the linear zones of disturbances on the Estonian main-land (Vaher 1983). In North Estonia, also the domination of thestructurally lowered west-northwestern sides of the linear zonesof disturbances is opposite to that in the submarine area furtherwest.

The tectonically active and more disturbed area northeastof Gotland - a southerly prolongation of a hypotheticalBaltic-Bothnian mobile zoneThe location of the high dynamic area northeast of Gotland coin-cides well with the southerly prolongation of the tectonicallymobile north–south trending zone of graben-like structures in theGulf of Bothnia, Åland Sea, and Landsort-Fårö area, supportingthe idea of a longer hypothetical Baltic-Bothnian regional mo-bile zone (Puura & Flodén 1997). The graben-like structures arelocated along some sort of intra-Svecofennian hinge line, whichever since the Middle Proterozoic has repeatedly reacted on dif-ferent types of regional stresses that have been induced on theplatform margin during the course of its interaction with otherpalaeocontinents. Intensive tectonic movements along this zonehave resulted in opposite and oblique movements of the areaswest and east of it.

The periods of increased tectonic activities along this zone arealso distinctly imprinted into the platform sedimentary cover ofthe Baltic region. The same mobile zone seems to have influ-enced the distribution and facial zonation of the sedimentarybedrock sequence in the northern Baltic Sea during the Late Or-dovician – Early Silurian time interval. In the present Ordovi-

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48 Tuuling & Flodén: Structure and relief of the bedrock sequence in the Gotland–Hiiumaa area GFF 123 (2001)

cian–Silurian transition beds it is expressed in a locally restrictedLate Ordovician reef environment, as well as erosional featuresnortheast of Gotland (Tuuling & Flodén 2000) and resulted inthe entirely different, north–southerly arranged, early Llandov-ery facial zones (Bassett et al. 1989).

The present altitude differences of the coeval Silurian reefstructures, which once formed at about the same depth along theshallow shelf, indicate that the Silurian layers east of Gotlandhave also undergone a considerable structural subsidence. Sohave the two large submarine reef structures in the profileNB9205, a little south of the Silurian clint, located 80–90 m be-low the present sea level. Their morphological and seismostrati-graphical positions, as well as their rather large dimensions (0.5–0.7 km), point towards a correlation with the similar Högklintreef structures on Gotland. Thus, considering their present posi-tion on land, the submarine reefs are located more than 100 mbelow the sea. An even higher difference was found along theLudlovian, Östergarn linear reef on central Gotland, which 50km east of Gotland is located some 170 m below the sea surface(Flodén et al. 1995).

The lack of younger rocks than Silurian prevents a more exactdating of the above discussed structural subsidence. A major pe-riod of activity along the Baltic-Bothnian mobile zone is con-nected with a tension applied on the region from south-southwestin Late Palaeozoic Permian time. Besides a considerable struc-tural rearrangement in the southern Baltic area, this event trig-gered the formation of several Late Palaeozoic graben-like de-pressions, including reactivation and sinking of older similarstructures in the Gulf of Bothnia and Åland Sea. Hence, also thezone of disturbances northeast of Gotland may to a certain extenthave been reactivated in Permian time, contributing to the gen-eral structural lowering of the area east of Gotland.

There is a general lack of direct evidences supporting the ideaof fault tectonic activity in the northern Baltic Sea during theCenozoic uplift and during the course of younger geological timeintervals. Some narrow downfaulted blocks in the present bed-rock relief northeast of Gotland (Fig. 5) and southwest ofHiiumaa (Fig. 6) may, however, partially have been caused bythese forces. But more likely, this coincidence is to a great extentcaused by linear selective erosion, as Cenozoic rivers and theadvancing Pleistocene glaciers used these cracked and crushedzones as a less resistant path to erode. The analysis of the presentbedrock relief, discussed further below, proves that the structurallowering of the mobile zone east-northeast of Gotland played aparamount role in the formation of the entire Cenozoic drainagesystem in the region. Moreover, the present day Baltic Sea basinis probably to a great extent based on the Cenozoic structuraldepression, which formed along the same mobile zone. The sameevent most likely also induced the considerable structural lower-ing of the Silurian reef structures offshore Gotland.

The Cenozoic erosion and Pleistocene glacial exaration -the two main bedrock relief forming agents in the north-ern Baltic SeaTwo different bedrock relief forming periods of different form-ing agents have influenced the northern Baltic Sea. An older sys-tem of east-northeast to west-southwest directed escarpmentsand terraces was superposed on the region as a consequence ofthe Cenozoic uplift. This tectonic event led to an erosional activ-ity upsurge and formation of large east-northeast to west-south-west oriented rivers in the southern slope of the Baltic Shield.

This kind of Cenozoic relief system is best preserved in the cen-tral part of the northern Baltic Sea. In the marginal parts of thearea, in northeast offshore Hiiumaa and particularly in the south-west offshore Gotska Sandön, the primary Cenozoic relief fea-tures are obliterated by younger, northwest to southeast, respec-tively north to south directed exarational relief forms of thePleistocene glaciers. Thus, the advancing glaciers have followedthe less resistant paths of the ancient tectonic structures and al-ready existing Cenozoic riverbeds.

A structural depression east-northeast of Gotland as aprominent bedrock relief forming factor in the Balticregion during the CenozoicCaused by the enlarging streams towards the lower course of theriver, the large Cenozoic rivers on the southern slope of the Bal-tic Shield show a distinct increase in erosional intensity towardsGotland. As a result the east–west oriented Cenozoic cuesta re-lief in the St. Petersburg district and on the Estonian mainlandturns slightly southwestwards down the river, as it approachesGotland. Due to the increase in erosional intensity towards thewest-southwest, the submarine Ordovician plateau surface ex-poses gradually deeper facial varieties in the same direction. Forthe same reason, the outcropping areas of Vendian-Cambrianand lowermost Silurian deposits narrow gradually from east-northeast to west-southwest along the Baltic and the Silurianclints. Thus, the large plateau-like area of the outcropping Cam-brian-Vendian rocks, so typical for the eastern part of the Gulf ofFinland, vanishes further to the west.

The discernible change in orientation of the Baltic clint com-plex towards the southwest occurs in northwestern Estonia.About the same longitudinal area, the first appearances of theSilurian clint emerge further south. The almost horizontal,slightly rugged Lowland and Archipelago in western Estoniaturns to the southwesterly tilted submarine Ordovician plateauthat from the south is terminated by the more or less steady sub-marine Silurian clint. Thus, major changes in the bedrock mor-phology and orientation, as well as a distinct quantity increase ofremoved deposits westwards of West Estonia, point towards asomewhat different structural background further west. Theseappearances may reverberate the Cenozoic reactivation of theBaltic-Bothnian mobile zone, which led to a considerable struc-tural lowering of the area northeast and east of Gotland.

Thus, the main southwards directed Cenozoic riverbed, col-lecting the water from affluents both in east and west, followedthe structural depression east-northeast of Gotland. This Ceno-zoic river valley, which has been considerably reworked byPleistocene glaciers, is in the present bedrock relief preserved asa chain of separate, submeridionally directed deeps, called FåröDeep, Gotland Deep, and Slupsk Deep, respectively (Grigelis1991).

An estimate of the amount of bedrock exarated by Pleisto-cene glaciersSince rivers always progress from higher to lower altitudes, wecan approximately evaluate the minimum thickness of the depos-its exarated by the Pleistocene glaciers from the depressions infront of Baltic and Silurian clints. This is equal to the absoluteheight differences in the lowermost point of the depressions andthe highest point following the depressions down the rivercourse, i.e. 90 and 130 m, respectively. However, whereas the

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exaration intensity inside similar glacier troughs was consider-ably higher than outside of them and the exarated sequencesfrom both depressions to a great extent were composed of fri-able, tectonically crushed Cambrian and lowermost Silurianrocks, the average amount of the exarated sediments in this re-gion has been considerably less than the presented numbersabove.

The Cenozoic northeast to southwest orientation of some mi-nor relief forms is preserved on the elevation between the twoglacier troughs and in the middle part of the northern Baltic Sea.This direction is at right angle to the direction of the advancingglaciers, and this may suggest that the glacier exaration in theseareas has been negligible. On the submarine Ordovician plateau,which was sculptured in the hard Lower and Middle Ordovicianlimestone, the height difference between the areas of preservedprimary Cenozoic northeast to southwest orientation and thelowermost point in the glacier trough is about 60 m. Since theCenozoic river has eroded part of the missing rocks in the glaciertrough, the thickness of exarated deposits in the limits of the sub-marine Ordovician plateau is considerably less than 60 m.

Acknowledgements. – This study was made possible by grants from the Stockholm Univer-sity, the Royal Swedish Academy of Sciences, the Swedish Natural Science Research Coun-cil (NFR), the Estonian Academy of Sciences, and the Helge Axelson Johnsons Stiftelse.Prof. Väino Puura, Institute of Geology, University of Tartu and Dr. Rein Vaher Institute ofGeology, Tallinn Technical University kindly read the manuscript and suggested valuableimprovements. The field works were performed from the R/V Strombus operated by the De-partment of Geology and Geochemistry, Stockholm University, from the Estonian R/VLivonia and from the Russian R/V Professor Multanovskij. We thank the crews of these ves-sels for their cooperation.

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