Geological geomorphological features of the Baltic region and adjacent areas: imprint on glacial postglacial development

Geological - geomorphological features of the Baltic region and adjacent areas: imprint on glacial - postglacial development

The aim of the present paper is to describe some geological, geomorphological and tectonic features of the Baltic Sea lowland that could be in one or other case relevant to the history of glacial grows and decays, as well as linked processes of isostatic rebound and possible neotectonic component. We also hope that such extensive overview would be helpful for scientists who deal with different geological problems of the Northern Europe, providing additional information about the development of the region.

• 
  zone of slope of the shield combined with dominant platform depocenters,
  
• 
  central zone of tectonic subsidence, isolated from the slope.
  

Zone of shield's slope runs from the Southern Baltic, Baltic Proper and Northern Baltic with the Gulf of Finland in the direction to the Lakes Ladoga and Onega and then to the White Sea. The saddle of the Åland archipelago demarcates the slope from the inner zone of subsided platform strata, which includes the Bothnian Sea and the Bothnian Bay of the Gulf of Bothnia.

In general, marginal lowlands are typical features of slopes of the crystalline shield’s that undergone intensive multiphase preglacial Tertiary denudation with abundant role of selective Pleistocene glacial– fluvioglacial erosion, like the Baltic, Canadian and (in less degree) Anabar shields. Usually they are more extensive in the bedrock topography, being masked or complicated in the modern topography by the sporadic Pleistocene accumulation. Structural peculiarities and rock’s properties impacted on the topographical factor and erosion variability.

Prominent inner basins occupied by world’s largest lakes and seas like the Baltic and White Seas mark parts of the marginal lowlands, usually pointing its deepest parts in zones of the cropping out of the non-metamorphosed sedimentary that overlies older formations. Simply, the position and shape of all modern great inner basins is linked with the pattern of distribution either of proper sedimentary cover, or of early platform deposits that are closer in properties to the platform strata than to the metamorphic basement (Fig. 2).

Metamorphic and intrusive rocks of the crystalline Archaean-Proterozoic basement of the East-European platform comprise SUV peneplain under the platform cover, emerging from below it in the exhumed zone beneath Quaternary overburden. So far, this exhumed surface forms shield's slope that represents one flange of the major lowland in the Gulf of Finland, Northern Baltic Proper, and offshore along the coast of Sweden. It is widespread onshore as well, where it usually has comparable angle of dipping as below the cover, being somewhere deformed by faults, likely mostly of Caledonian reactivation. Continuation of the SUV peneplain can be reconstructed at the adjacent area of the shield by preserved fragments under sedimentary outliers, areas of distribution of neptunic dykes filled by sediments of basal formations and weathering crusts. At larger distance from the cover the skyline continuation of the peneplain could be easily reconstructed by tracing summit heights of the crystalline bedrock (Fig. ). It normally determines macrorelief of adjacent areas, like of parts of Sweden and Southern Finland up to about 150 m (Tanner, 1938; Heikkinen,1975; Lidmar-Bergstrom, 19..), also existing in a narrow strip along the front of the Caledonian mountains and below the easternmost overthrust sheets (Rudberg,1970).

The heterogeneous basement is usually comprised by reworked Archean or Lower Proterozoic formations of great thickness, with the dominate role of Svekokarelian orogenic event with major folding and metamorphism at 1.9-1.8 Ga ago.

That would not be strong exaggeration to point that the abovementioned belts were in indirect way responsible for the pattern of the Baltic anteclise and later shield, as well as of the modern figure of the shield and the Baltic Sea lowland. At the early-platform tectonothermal anomaly stage the emplacement of hotter material with compositional deviation and contrast density preceded intensive landscape modification and further erosion. Later on after the thermal field slowly normalizing the remaining compositional anomaly could possibly respond in geological time in favorable conditions. However, we are talking about supposed gentle hinge zones hundreds kilometers width, and not linear sutures or megaflexures that don’t exist in reality. Relatively short time gap after the main Svekokarelian event with expected correspondent thermal crust – mantle anomaly should be mentioned, as well as possible involving some Svekokarelian fault zones in partial remelting of the crust. More of all, the transcontinental variations in the mentioned granites are believed to be indicative of broad regional changes in the composition of the lower crust of Laurentia and Baltica (Anderson & Morrison, 2005).

Different types of Riphean negative structures may be determined in this segment of the East-European platform (Amantov, 1989; 1992). They are:

1. 
   Marginal pericratons, like Mezenck-Barentsevomorsky trough that was extended along the north-eastern margin of the craton. The description stays beyound the tasks of this paper.
   
2. 
   Extensive linear aulacogens developed along major sutures or fault zones inside Archean - Proterozoic domains. White Sea Riphean basin is the typical example of such structure, which determines prominent features of the north-eastern flank of the Baltic-White Sea marginal lowland.
   
3. 
   Baltic type of less elongated negative structures that were formed mostly within Svecokarelian domain. Often they are allied with mentioned large rapakivi granite - gabbro-anorthozite intrusions plutons Gothian ( Subiotnian ) complex in space, being related to the formation of circular fault-and-fracture shapes, with complimentary often northwestern trends.
   

We speculate that magma emplacement caused rotational distortion at the margin, with wallrock asymmetric uplift and associated faulting. In addition, relatively slow but variable cooling had been caused by large magma volumes and changing shape of the body. Also, lifting of the country rock and their subsequent erosion together with erosion of the magmatic rocks caused isostatic relaxation in case of huge plutons. Combination of such processes finally shaped Riphean basins of the Baltic type that exhibits negative structures comprised by thick sedimentary sequences from first hundreds meters to almost 2 km (Amantov et. al., 1995, 1996). However, there are indications of their secondary erosional shape for some clay and claystone units that could have broad extent and connection. Their separation likely happened in connection with Svekonorwegian orogeny, and the pattern of overall erosion at that interval seems to be impacted by mentioned gentle hinge zones. Åland Sea, Landsort, Ladoga-Pasha and other basins belong to the Baltic type of negative structures. We specially focus on them because these lowlands have prominent appearance in both the bedrock topography and modern landscape as deepest ones, with the shape approximately corresponding outline of negative structures. That’s it, most contrast lowlands of structural - denudative origin have been formed by selective dominantly Pleistocene glacial exhumation of fragments of negative structures comprised by sedimentary rocks that are relatively soft in comparison with surrounding crystalline frame. Usually Riphean sandstone's were removed with evident deepening of the bedrock surface in comparison with surrounding crystalline frame. Some of these bedrock landforms in their deepest part often have typical profile and some morphometric parameters of glacial cirques including headwall and lip, that gave possibility to introduce separate family of giant glacial cirques of non-mountain areas (Amantov & Amantova, in press). In the deepest proximal part bedrock roof somewhere rests on comparable for all troughs marks at –200 - -350 m and even more, like in the horseshoe marginal ovedeepening of the Landsort trench. "Critical depth" of deepening depends mainly on pliability of the rocks to glacial erosion, fracturing, on the angle of substratum beneath ice massses and in some cases their thickness. Main evidences of the erosional nature of such troughs come to the absence of relevant modern graben-like displacements along SUV peneplain on continuation of such landforms in the area of distribution of platform cover (Amantov, 1992).

The durable platform sedimentation started since the Late Vendian stage after mature planation in Early Vendian time.

Stable deposition at the Fennoscandia region with migration of depocentres to the west continued in Early Paleozoic, while the first abundant erosion with significant reduction of the overburden of the shield has happened in connection with the Main Scandinavian stage of Caledonian orogeny.

At the Middle-Upper Paleozoic cycle wide deposition in the area of the modern shield might be possible since Upper Devonian marine expansions, but Devonian terrigenous sequences could be of importance as well. The widest marine sedimentation in Fennoscandia is expected in Carboniferous, especially during Late Carboniferous when carbonate sedimentation has had maximum distribution at the Russian platform. Some aspects of Carboniferous paleogeography have been discussed by Bergstrцm et.al. (1985).

In the Baltic syneclise Permian - Lower Triassic sedimentation of the Late Paleozoic - Early Mesozoic cycle was more united with younger Mesozoic one in tectonic plan, in comparison with adjacent parts of the Russian platform. During Middle Triassic transformation the Polish - Lithuania syneclise was shaped. Chalk series exhibits maximum (taking into account the eastern region) of Mesozoic transgression, that seems to be widespread, especially in Santonian time.

After the most Late Cretaceous planation Early and Mid-Tertiary phases of uplift and erosion affected the area. Intermediate marine ingressions could cover entire Baltic area. Etchplain or less mature peneplain has been formed to the end of Pliocene, representing pre-erosional surface of the last phase. During the last cycle an influence of ice sheets seems to be dramatic in changing of rates of many processes, but in favorable zones only.

Usual thickness of Quaternary overburden offshore is about 15 - 40 m, increasing in some special forms like a moraine ridges or buried valleyes. As usual, it does not mask bedrock topography, so that general forms of the bottom and buried bedrock relief are comparable.

In the White Sea area comparable bedrock morphology is observed for the axial lowland. In the Onega bay of the White Sea bedrock surface rests in the deepest zone at the heights of - 50 - -80 m, but it occurs under -100 - -150 m in the western and central part of the Gorlo (Headache) of the White Sea.

The most shallow zone occurs around the Lake Onega - Vetryany Poyas paleo-watershed region, where the marginal lowland is also narrowest. It might be continued to the southwest on the Karelic peninsula where average heights of bedrock roof in axial trench below thick Quaternary cover are somewhere only about -20 - -40 m. On the other hand, one of the deepest Lake Ladoga radial - transverse to the axis of the marginal lowland - trough seriously complicates bedrock topography. Such forms of the relief are very noticeable as they exceed usual depthes in 2 - 3 times (under -250/-400 m with exceedings over 300 m), so they need special breef remarks, as follows.

The radial troughs of Ladoga type ( ) take place in the Еland Sea and Landsort dupet of the Baltic, one of the most considerable is the Kandalaksha - Dvina trench of the White Sea that looks like the Norwegian trench. Such forms are also constituent parts of the marginal lowland as subordinate locally-manifested basins. Without exception, they exhibit occurrence of pliable rocks that comprise graben-like structures, and the trenches were created by selective erosion if specific formations, which differ in properties from the crystalline rocks, had been exhumed. They are usually exemplified by mentioned Riphean sequences, but may be represented by younger Paleozoic rocks, as in the case of Norwegian channel precursed by Oslo graben. Two varieties of such local basins can be specified as Onega and Ladoga types (Amantov, 1988).

Overall form of the Bothnian lowland also roughly matches both the shapes of the center of last glaciation at its approximate maximum, and postglacial uplift. Such coincidence of the central depressions with extensive sedimentary basins is attractive geological peculiarity that determines figures of many glaciated areas, like the Hudson bay sedimentary basin of the North America (Amantov & Voronov, 1993).

Earlier we mentioned the additional Bothnian rapakivi belt marking possible broad hinge zone along the Swedish coast of the Bothnian Sea, and also corresponding to the prominent modern bend of the topographic surface. In spite of sparse data, it seems to be important in the determination of the shape of Riphean basin also, at least for the Bothnian Sea. Extensional zone of Gothian age roughly following the shape of the modern coastal area possibly controlled large rapakivi intrusions together with conjugated northwestern zones. Large rapakivi pluton likely follows the Bothnian Sea coastline northeast, at least to Örnsköldsvik accompanied by rapakivi massifs like Strömsbro, Sundsvall and Nordringå (Magnusson et. al., 1958).

General northwestern direction of main structural design step faults (which one can follow in Satakunta, Aranda and Evle grabens) in the posterior Riphean sequence is combined with northeastern one, parallel to the trend of the Caledonian belt and possibly reactivated due to its development. Numerous kimberlite-like dykes of alnoites, silicocarbonates and beforsites dated 1.15 Ga occur in the Kalix and Luleе archipelagos and on the adjacent mainland of the NW Bothnian Bay, as well as alkaline intrusive bodies at 0.55 – 0.575 Ga (Kresten et.al., 1977), being in agreement with the guess on the specific long-lived changes of the deep structure.

Parallel or sub-parallel reflectors with stable seismic signature and rather uniform dip at the reflection seismic profiles across the Riphean sequence of the northern part of the Bothnian Sea (Axberg, 1980) may indicate wider primary sedimentation of some Mezo-Neoproterozoic units, with significant erosion at the end of Late Riphean and in the Early Vendian. Since Late Vendian after the mature planation, several tectonic episodes influenced Bothnian basins during the platform development, but stage related with Caledonian event looks the most important. Probably at that time compressional stress, bending and subordinate faulting deformations forcefully revived old structural element. As a result, chain of foreland depressions occurred, from Västergötland to Bothnian Sea– Bothnian Bay and Imandra. It is elongated in the northeast direction with the axis close to the Lake Vänern shore, being traceable now in the skyline topography of the exhumed sub-Cambrian peneplain (Fig. ), often with preserved outliers of the sedimentary cover (Amantov, 1995). This cover has been more widespread in Cenozoic like around lake Mien circular depression, with indications of a Cambrian cover over Mien at the time of impact in the Early Tertiary (Lidmar-Bergstrom, 1985). Complete separation of the Paleozoic outliers of the central basin in the area of the Åland saddle could be caused by glaciations (Amantov & Voronov, 1993). In addition, the supposed tendency of spatial migration of ice domain centers to pre-glacial depressions with sedimentary bedrock could be mostly controlled by the preglacial landscape, with possible minor input of slightly reducing heat flow in the sedimentary basins, additional consolidation of sediments due to the ice sheet load, water exchange with major sedimemtary aquifers, and yet unclear differences in elastic response.

The preservance of Cambrian and more erosion-resistant Ordovician sediments at the northwestern steepest flank of the major syncline, and wide outcrops of carbonate mounds (Winterhalter, 1972; Axberg, 1980) caused varying erosional pattern and thus changing topography of the bedrock surface and of the bottom that we in high degree refer to glacial erosion (Amantov, 1995; Amantov et. al., 2011). Major overdeepening of the bedrock surface took place offshore along the Swedish coast with intensive placking of the Riphean sediments and culmination at the Härnösand deep. Temporary ice streams seem to produced “blind lowland” at its continuation to the southeast that bear usual features of such forms being less developed than Gotlands one.