The Gulf of Finland is one of the most geologically diverse areas of the Baltic Sea

The Gulf of Finland is the easternmost basin of the Baltic Sea. It is a direct extension of the main basin of the Baltic Sea, with no seabed threshold or sill between them, which allows the unhindered exchange of water.

The Gulf of Finland is a relatively shallow marine area. In addition, it has an elongated and narrow shape. It is 400 kilometres long and its width ranges between 48 and 135 kilometres. The Gulf of Finland covers an area of approximately 29,500 km². The volume of water measures 1,098 km³, which is only about 5% of the total volume of the Baltic Sea.

Continuous land uplift (1-4 mm/year) slowly modifies the area, causing both shoreline and seabed changes due to the deposition, erosion and transportation of material. 

Bedrock

The Gulf of Finland basin is actually a depression of the bedrock. The ancient Precambrian crystalline bedrock is more commonly called the basement bedrock. This crystalline basement is exposed in the northern part of the Gulf of Finland. 

The structure of the bedrock is characterised by tectonic lineaments and fracture zones that divide the bedrock into blocks. These elongated lineaments and fracture zones in the terrain have been created over time, as a result of bedrock movements.

In the southern Gulf of Finland, the basement rock is covered with younger sedimentary rocks, which level out the surface forms and the seabed landscape.

The differences in the landscape are clearly visible when comparing the fragmented archipelago of the Finnish coastline to the straighter coastline of Estonia, with its high limestone cliffs

These differences caused by the bedrock are also reflected by variations in the marine landscape. On the northern coast, bedrock fractures create a maze-like archipelago where the fragmented landscape continues underwater.

In the Eastern Gulf of Finland, the bedrock consists mainly of rapakivi (Finnish: “crumbly rock”) granite. As its name implies, rapakivi granite crumbles easily and has a sharp cubic split. Thus, steep cliffs and large boulders, some as big as a sauna cabin, are typically found in the both coastal and seabed areas of Kotka and Hamina.

Topographical differences and features of the seafloor

The seafloor in the northern Gulf of Finland slopes slightly southwards. The southern slope is even steeper, forming steep cliffs on the northern Estonian coastline. This steep limestone escarpment is an underwater extension of the so-called Baltic Clint, i.e. an erosional limestone escarpment and found on several islands of the Baltic Sea, in Estonia, in the Leningrad Oblast of Russia and in the Gotland and Öland islands of Sweden.

The average depth of the Gulf of Finland is only 37 metres. The deepest point, measuring 123 metres, is found in the Paldiski Deep in the southwestern part of the Gulf. In general, the Gulf of Finland becomes shallower towards its eastern end, where the largest river in the Baltic Sea, i.e. the Neva, flows to the sea.

Localised deep basins also occur in the eastern Gulf of Finland. One such is found in the sea area of Kymenlaakso. That a canyon-like hole, Paskamonttu, to the south of Haapasaari Island, is 90 metres deep, 200 metres wide, and about 800 metres in length. This sea hole is one of the deepest parts of the eastern Gulf of Finland.

The seafloor of the Gulf of Finland is full of alternating elevations and depressions

The current depth and seafloor features of the Gulf of Finland are mainly the result of crystalline and sedimentary bedrock morphology. In addition, both present and glacial deposition and erosion processes have affected the seabed landscape. However, the primary features of the seabed were formed well before the Ice Age.

The seabed of the Gulf of Finland is complex in terms of seafloor features. Several distinct types occur in the area, each covering more than 5% of the area's seabed. The most common seafloor features are basins (33%), elevations (32%), plains (25%), as well as valleys and sea holes (10%).

The seafloor of the northern part of the Gulf of Finland is characterised by exposed bedrock in which valleys, holes and reefs also occur. Reef-like moraines, boulders and bedrock-based elevations are relatively abundant along the Finnish coast. Most of these reef-like elevatoins are found in depths below 30 metres.

The Gulf of Finland has very few sand and gravel banks 

Except for the eastern Gulf, where the proportion of sandy elevations is higher, relatively few banks containing sand and gravel as the dominant sediment type occur in the Gulf of Finland.

In Finnish marine areas, sandbanks and potential sandbank areas occur in the eastern Gulf of Finland, as well as in the Salpausselkä areas in the western Gulf area. Potential areas for sandbanks often correspond to underwater esker extensions.

Seabed composition

In the Gulf of Finland, the regional occurrence of different sediment types on the seabed is very patchy, especially in the northern part of the Gulf. Whether characterised by erosion or by the transportation and deposition of sediments, bottom types vary both locally and temporally.

Large sedimentation basins are primarily found in the central Gulf of Finland. These basins are formed in the bedrock, over which sediment has been deposited over time.

A variety of smaller seabed features occur in coastal areas. Approximately one third of the seafloor in the entire Gulf of Finland can be considered a sedimentation area, i.e. where sediment deposition is currently taking place.

In Finnish marine areas, the seafloor consists of approximately 40% soft sediments, i.e. silt, clay, and mud, 30% bedrock and till, 25% hard clay, and less than 5% of sand and gravel.

The highest occurrence of soft silt and clay bottoms, i.e. greater than 50%, is found in the western sea area south of the Hanko Peninsula 

Compared to the eastern Gulf of Finland, there is more bedrock outcrops in the western Gulf area. For example, the Raseborg-Kirkkonummi area is fragmented with many small, exposed rock outcrops.

The Greater Helsinki Area also has a fragmented seafloor. For example, in the Raseborg district, the seabed is composed of extensive muddy basins, as well as exposed bedrock and till (unsorted glacial sediments). In this area, 27% of the seafloor is bedrock, while 7% is composed of till. By comparison, the eastern Gulf of Finland has 8% bedrock and 25% till.

The eastern Gulf of Finland has extensive moraines, while the Rapakivi granite area in particular, is characterised by boulders. The eastern Gulf also has extensive sand and gravel formations. Otherwise, the seabed of the Gulf of Finland has relatively little gravel and sand, i.e. less than 5%.

Rock, till, as well as sand and gravel bottoms occur or are exposed on the seabed, mainly in areas less than 60 metres in depth. At greater depths, seabed surface deposits are primarily composed of soft sediments.

Geodiversity

Geodiversity refers to the geological diversity of the land or the seabed. In other words, there are many types of substrates and rock that have an impact on the rest of the environment.

Along with the Archipelago Sea, the Gulf of Finland is one of the geologically most diverse areas in the Baltic Sea. Due to its different bedrock, the coastal and seabed features of the northern Gulf of Finland are more fragmented and complex than the southern Gulf area.

The seabed of the Gulf of Finland, especially its northern coast, is both complex and unique when compared across the whole scale of the Baltic Sea.

In general, the landscape and seafloor in areas composed of crystalline bedrock are more fragmented and diverse than those covered by sedimentary rock.

The geological origin of the bedrock plays a significant role in changing living habitats. Indeed, high geodiversity may also lead to a richer biological diversity. The quality of the bedrock beneath the surface also affects the environment. In other words, biodiversity will also differ between areas of basement rock and sedimentary rock, e.g. limestone.

Canyons, sea holes, valleys, and various elevations cut across the Archipelago Sea seafloor

The Archipelago Sea is an extremely fragmented and shallow sea area of more than 40,000 rocky and bedrock islands and islets larger than 0.5 hectares. The area of the Archipelago Sea is approximately 8,890 km².

Continuous land uplift (3-4 mm / year) slowly changes the area, causing changes due to the erosion, transportation, and deposition of sediments.

As a result of land uplift, small submerged reefs emerge to first become skerries, then large islands and finally, they become part of the mainland. As new bays are created, existing shallow sheltered bays shrink, turning into ponds and lakes.

The archipelago acts as a transition zone between the coast and the open sea. In the archipelago, various transition zones or ecotones form separate gradients, based on differences in salinity, coastline and sea bottom openness, as well as water quality. All such zones tend to shrink when moving towards the mainland.

The outer archipelago is rocky or stony and open, while the inner archipelago areas are more sheltered and shallower and often have soft bottoms.

Environmental factors, including water depth, shore exposure, and bottom quality change markedly over short distances. In conjunction with these factors, a long and convoluted shoreline make the archipelago a remarkably diverse natural environment. 

As an extension of the Stockholm archipelago, the Archipelago Sea region, with its thousands of islands, forms an entity which is globally unique.

Bedrock

The bedrock of the Archipelago Sea is primarily composed of ancient Precambrian crystalline bedrock or basement. It is characterised by tectonic lineaments and fracture zones that divide the bedrock into blocks, and which produce complex and fractured topographical differences. This is reflected in the complex, maze-like landscape of the Archipelago Sea.

Seafloor topographical differences and seabed features

While the average depth of the Archipelago Sea is only 19 m, in some areas the water depth exceeds 100 m. The deepest point in the Archipelago Sea is 104 metres.

Bedrock fault lines and fracture zones break up the topography, causing elevation differences. These varying differences manifest as a labyrinthine archipelago, complete with seafloor depressions, deep canyons in fracture zones, as well as a complex spectrum of elevations.

It is estimated that deep basins cover about one quarter of the seafloor of the Archipelago Sea. Several types of mounds or elevations are common here, covering about half of the seabed area.

Most of the seafloor elevations in the Archipelago Sea are either rock or till

Although mounds covered by either hard or soft clay are common throughout the Baltic Sea, they typically occur in the Archipelago Sea. Most elevations here are covered with rock, boulders, or till (moraine material). In all depth zones, 60% of the elevations are mainly rock or till.

However, elevations occur mainly in shallow areas. Some of these hard seafloor elevations form habitats, i.e. reefs, which are typical of the Archipelago Sea.

Only 1% of elevations occur deeper than 50 metres. This is a good indication that sediments deposited on the seabed in deeper areas smooth out its shape.

In the Salpausselkä region, the proportion of rocky and moraine mounds is smaller than in other areas. Here, mounds which are primarily composed of sand and gravel, account for almost half of the surface area of all the occurring elevations.

In the area outside the Salpausselkä formations, the surface area occurrence of sand- and gravel-dominated banks is only 1%. Therefore, sand and gravel are mainly found in the confined Salpausselkä area, where their underwater banks are extensive.

Canyons are important underwater corridors

In general, valleys, sea holes and canyon-like seafloor feeeatures are relatively rare in the Baltic Sea. They are concentrated in the northern parts of the Baltic Sea, such as in the Archipelago Sea, where a few can be found. Here, canyons are often found in bedrock fault lines, as well as in fracture zones.

Several canyons are important undersea passages for both organisms and water currents. In the Archipelago Sea, canyons are an important corridor for water exchange between the Baltic Proper and the Gulf of Bothnia. The Baltic Proper covers the area, from Åland Sea to the Danish sounds, excluding the Åland Sea, the Gulf of Finland, and Gulf of Riga. In terms of seabed topographical differences, the low occurrence of plains (10%) supports the perception of the Archipelago Sea as a complex and fragmented area.

In the Archipelago Sea, the Salpausselkä ice-marginal features and the deep canyons further enrich the benthic environment. Moving outwards from the shallow coastal waters to the deeper and darker outer archipelago, various deposits of mud and clay often cover the seabed bedrock and till, smoothing out their elevation differences.

Typical bottom  features of the Archipelago Sea also include clay mounds which rise from the seafloor. These are composed of bedrock elevations and moraines that have been covered by younger sediment deposits.

The composition of the seabed

When looking at the composition of seabed surface deposits of the Archipelago Sea, just over 50% of the seabed is covered with soft sediments, such as silt, clay, or mud. The remainder of the seabed area experiences either erosion or transportation processes and features coarser surface material, such as sand, gravel, till or rock. On an erosional seabed, the bottom is constantly eroded away, while on a transport seafloor, sediment is deposited on the bottom from time to time.

About one-fifth of the seafloor of the Archipelago Sea is composed of exposed hard clay. This is glacial clay. When such old clay is exposed on the seabed, it has commonly a thin layer of erosional sand on top.

Rock, till, as well as sand and gravel occur or become exposed on the seabed, mainly in areas less than 60 metres in depth. In very shallow areas, i.e. less than 10 meters, these comprise more than 50% of the seabed.

Sand and gravel deposits cover only about 2% of the seabed in the Archipelago Sea. Conversely, both of their percentage occurrences are largest in the Salpausselkä area. In areas with depths exceeding 60 metres, the seabed surface substrates are primarily comprised of softer sediments.

Geodiversity

In general, the northern regions of the Baltic Sea are more diverse than those in the south. The Archipelago Sea is one of the most geologically diverse areas in the Baltic Sea. Archipelagos are distinguished by the occurrence of fragmented and complex environments on their coas and seafloor.

In some Archipelago Sea areas where the continental ice sheet remained for a longer period during a retreat phase or where its retreat was slow, several different seabed features can be identified, which contribute to geodiversity. Such an area can be found in the Salpausselkä zone.

In the Archipelago Sea, as well as throughout the Baltic Sea, the most geologically diverse areas are located mainly in areas of crystalline bedrock. Rocks resistance to erosion and weathering, bedrock fault lines and glacial abrasion processes have influenced the formation of the fragmented topography.

In Baltic Sea archipelago areas, such as the Archipelago Sea, seafloor conditions are very dynamic. Here, both seabed properties and processes change markedly over short distances and the seabed splinters into smaller rocky islands and islets.

The outer archipelago is open and has extensive rocky reefs. By comparison, the inner archipelago is sheltered, and the seabed is often covered with soft sediment deposits.

In the Bothnian Sea, the old basement bedrock is covered by sedimentary rocks

The Bothnian Sea is the name given to the southern Gulf of Bothnia. It is bordered to the south by the Archipelago Sea, the Åland Islands and the Åland Sea and to the north by the Kvarken (Quark). The Bothnian Sea is the largest sea area in Finland. It has a total area of approximately 64,890 km².

The average depth of the Bothnian Sea is 66 m. Its maximum depth, i.e. the Ulvö Deep, measures 293 m, is situated off the Swedish coast. 

The Bothnian Sea is characterised by rapid land uplift (about 5-8 mm/year). Land uplift exposes sediments previously deposited on the seabed to erosion by ice and bottom currents, as well as coastal forces, such as waves and winds.

In the Bothnian Sea, the ancient basement bedrock is covered with sedimentary rocks, which level out differences in seabed elevations. Thus, the relatively low topographical differences of the seabed in open sea areas result from the deposition of sedimentary rocks.

Bedrock

Geologically, the Bothnian Sea is located in the northwestern part of the Fennoscandian or Baltic Shield, i.e. a segment of the Earth's crust representing a large part of Fennoscandia, northwestern Russia and the northern Baltic Sea. Precambrian bedrock is found on both sides of the Bothnian Sea, i.e. on both Swedish and Finnish coasts. This ancient bedrock dates from the oldest time in the earth’s history and is mainly composed of metamorphic and recrystallised crystalline rocks.

It is highly probable that the composition of the crystalline bedrock is also similar underneath the Bothnian Sea. However, over most of the Bothnian Sea, sedimentary rocks of different ages cover this ancient crystalline bedrock.

The sedimentary rocks at the bottom of the Bothnian Sea consist mainly of Proterozoic sand and shale, as well as Cambrian and Ordovician sandstone, shale, and limestone.

Seabed topographical differences and seabed features

The Bothnian Sea basin is a depression situated on the Fennoscandian Shield, which is filled by sedimentary rocks. The topography (depth differences) of the Bothnian Sea is characterised by a flat and gradual deepening of the seabed on the eastern side and a steeply sloping seafloor on the fragmented western side.

On the Finnish side, the seabed surface is composed of Precambrian crystalline bedrock that slopes gradually westwards. This bedrock surface descends from the east and forms the base of the deepest areas of the Bothnian Sea. This basemant is covered by sedimentary rocks, particularly in the southwestern and central areas, which thicken further in a westerly direction. The Swedish part of the seafloor is composed of fragmented fault lines.

In general, the bedrock structure in the Bothnian Sea area can be divided into a topography composed of areas of fragmented basement bedrock and gently sloping sedimentary rocks.

Over a large scale, the seabed features of the Bothnian Sea are dominated by plains (39%), various basins (27%), and elevations (27%). Valleys and sea holes also occur in approximately 7% of the seafloor.

The largest valleys and sea holes are generally located in ancient bedrock fault zones. In some places, such valley-like features represent the submarine extensions of continental rivers.

In Finnish waters, the elevations which occur on the hard substrates of the Bothnian Sea are mostly found in areas less than 30 meters in depth. They are mostly composed of rock or till material and thus appear as reef-like elevations.

By contrast, in soft-bottom areas, mounds or elevations are more evenly distributed throughout all depth zones.

Only a few sand and gravel features are recognised in the Bothnian Sea. On the Finnish side, the potential areas for elevations covered by sand or gravel, i.e. sandbanks, are mainly located off the coast near Pori. They are mainly related to the submari extensions of the land-based Pori esker.

Seafloor composition

Detail marine geological data collected by the Geological Survey of Finland covers only about 5% of the area of the Bothnian Sea. Within this area, seabed surface substrates are mainly composed of about 40 and 26% of moraine material and hard clay, respectively, while approximately 15% of the area is bedrock.

Further, less than 20% of the mapped area’s seabed is composed of mud, clay, and silt, with less than 5% comprising of sand and gravel. Rock and moraine occur in areas below 40 m in depth, while gravel and sand are mainly found at depths ranging from 20 to 60 metres.

The Bothnian Sea is not protected by an extensive archipelago, which is reflected in the prevalence of till (moraine) and rock seabed areas. Soft sea bottoms dominate at depths exceeding 60 metres, consisting mainly of mud, clay and silt, as well as hard clay.

Geodiversity

The seabed is geologically diverse in the coastal areas of the Bothnian Sea, particularly on the seaboard of Sweden. Geodiversity "hotspots" are located in areas of crystalline bedrock. These abrasion-resistant rock types, bedrock faults and glacial erosion processes have contributed to the formation of a fragmented topography.

Shallow coastal areas are often energetic and active environments, where the many different processes shaping the seabed also affect its geodiversity.

The offshore areas of the Bothnian Sea are mainly located in areas of sedimentary rock with seabed topographical differences. With their extensive plains and basins, the open sea areas are generally poorer in seabed geodiversity than the coastal areas. Offshore areas are calm environments where water currents dominate sediment deposition or transport.

The Kvarken Archipelago is a UNESCO World Heritage Site

The Kvarken area is located between Finland and Sweden. It is the narrowest part of the Gulf of Bothnia which forms a threshold or sill at approximately 25 metres that separates the Bothnian Sea to the south and the Bothnian Bay to the north. 

The majority of the Kvarken area on the Finnish side consists of shallow shoals (0-25 m) with approximately 7,000 island and skerries.

Due to its relatively rapid land uplift (8.0-8.5 mm/year), the depth conditions of the area have changed dramatically after the last Ice Age. During the retreat phase of the continental ice sheet around 10,000 years ago, the water depth in the Kvarken was 250 to 280 metres deeper than it is today. 

The ongoing land uplift results in the continued erosion of the seabed. It exposes new seafloor, i.e. by bringing older substrates into the shallower erosion zone and eventually to the shore.

De Geer moraine fields are the most typical features of this area’s seabed and land, making the Kvarken Archipelago unique throughout the world.

The Kvarken Archipelago is Finland's only nature site included in the UNESCO World Heritage List. Together with the Swedish High Coast area on the Swedish coastline, it forms a cross-border geological heritage site.

Bedrock

The bedrock of the Kvarken Archipelago, on both the Finnish and the Swedish coasts, consists mainly of ancient Paleoproterozoic crystalline bedrock. These worn remnants of ancient mountain ranges form the base of the present, flat landscape of the Kvarken Archipelago.

Over millions of years, younger sedimentary rocks in the area have worn away due to erosion, except for the ancient seabed in the Söderfjärden meteorite impact crater area, situated south of Vaasa.

Seafloor topography and seabed features

The Kvarken Archipelago is very shallow and rocky with approximately 7,000 islands and skerries.

The seabed of the Kvarken is characterised by a wide variety of structural features, such as elevations, flat areas (plains), basins, and valleys. However, the seabed contours mainly follow the bedrock surface and the area is relatively flat. Local small-scale variations in structural features and depths are dominated by moraines.

The seabed basins are relatively shallow and small in area. They occur throughout the Kvarken and cover approximately 30 % of the seafloor. The basins are mainly hard-bottomed.

Various mounds or elevations cover 37 % of the Kvarken seabed. These are extensive, particularly in the area between Raippaluoto and Sweden. Hard seafloor mounds are the most common bottom feature of the area (25 %).

The main bottom type of minor mounds is rock or, in most cases, moraine material, i.e. till. These can be considered as reefs in the Kvarken area.

There are only a few sand and gravel areas in the area. Potential sandbank areas only occur in the shallow northern part of the open sea, i.e. in the Helsingkallan area.

Plains cover 23 % of the Kvarken seafloor. Extensive plains, in which surface sediments are coarse in nature, such as till, boulders and bedrock occur particularly in the northern part of the Kvarken and on the Ostrobothnian shoreline, south of Vaasa.

Valleys and canyon-like features are characteristic of the southwestern part of the Kvarken. They also occur in the northern parts, e.g. west of Holmö Island. Valleys cover 11 % of the seabed in this region. It is likely that some were shaped from the courses of ancient rivers.

Moraines make the Kvarken area special

As noted above, the local small-scale topography of the Kvarken is dominated by various moraines. Typical terrestrial formations include De Geer- , hummocky-, as well as Rogen-type moraines and drumlins, which occur both on the seabed and land in the Kvarken Archipelago.

Of all the features in the area, the most typical are the blocky De Geer moraine fields, which make the Kvarken area globally unique. 

De Geer moraines are long and narrow boulder ridges. They often occur as broad fields with parallel ridges. From the air, they resemble a gigantic washboard. In fact, De Geer moraine fields are sometimes called washboard moraines.

De Geer moraines were formed during the last retreat phase of the continental ice sheet. They are deposited at or near the edge of the ice sheet. De Geer moraines can be used to determine the former position of the continental ice sheet edge, as well as the direction of ice retreat.

De Geer moraines are usually 20 to 500 metres long, a few metres high, and from 5 to 25 metres in width. The distance between ridges ranges from 20 to 200 metres. In areas with moraines, especially in De Geer moraine fields, there is a marked small-scale variation in depth.

The combination of seabed moraine ridges and continuous land uplift produces a variety of alternate marine and coastal habitats. For example, globally rare flad lagoons and gloe lakes are formed between the Rogen or De Geer moraine ridges rising from the sea.

Seafloor composition

The seafloor of the Kvarken area is primarily composed of moraine material, which covers over 60% of the charted area. Exposed bedrock occurs rarely, comprising only 5% of the seafloor area. Soft muddy bottoms, i.e. areas in which fine material collects at present, cover over 10% of the seabed. However, the regional distribution of such deposition areas is highly variable.

Soft clay bottoms consisting of sediments deposited during the earlier stages of the Baltic Sea, also cover more than 10% of the area. Such sediments include, for example, sulphide-containing clays deposited during the Littorina Sea phase.

The proportions of mud and clay as seabed surface deposits increases towards the deeper areas. There are very few sand and gravel bottoms in the Kvarken area.

Geodiversity

In terms of geological diversity, the Kvarken is one of the richest marine areas in the Baltic Sea. Although the number of different types of rock features and seabed substrates is less abundant than in either the Gulf of Finland or the Archipelago Sea, the seabed in the Kvarken area is very diverse and dotted with thousands of moraines and boulder fields.

The Kvarken Archipelago is a UNESCO World Heritage Site because of its geological value. Together with the High Coast area of Sweden, it forms a transnational geological heritage site.

Land uplift is fastest in the Bothnian Bay area

The Bothnian Bay is located in the northernmost corner of the Baltic Sea. Its average and maximum depths are 41 and 146 metres, respectively. The surface area covers 36,260 km².

The Botthian Bay is situated in an area dominated by sedimentary rocks, whose erosion has produced an abundance of sand.

In addition, land uplift is the most rapid in the Baltic Sea area, i.e. up to one centimetre per year. Land uplift has contributed to the erosion and redeposition of seabed material.

Sand and gravel are more common in the Bothnian Bay than in other marine areas of Finland. 

Bedrock

Both the Bothnian Bay and its catchment area are located on a portion of the Earth’s crust known as the Fennoscandian Shield. In most areas, the bedrock consists of basement rocks formed during the Precambrian period.

Like the Bothnian Sea, the Bothnian Bay is also located in a depression of the Fennoscandian Shield. The bedrock is similar on the coasts of Sweden and Finland and along the periphery of the Bothnian Bay basin.

The crystalline bedrock of the central part of the Bothnian Bay basin are covered by sedimentary rocks, which were deposited from older eroded bedrock material approximately 1.4 to 1.2 billion years ago.  They consist mainly of sandstone and conglomerates, i.e. coarse-grained sedimentary rocks composed mainly of granules, pebbles, cobbles, and boulders, larger than 2 mm in diameter.

Similar sedimentary rocks are also found from various places, including the district around Oulu, the Muhos formation, the Bothnian Sea seabed, and in the Satakunta region of western Finland. These sediments were deposited in the Ectasian period 1.4 billion years ago and are sometimes covered by younger  Ediacarian sedimentary rocks (circa 635 million years ago). Corresponding Ediacarian sedimentary rocks are found almost everywhere in the southern Gulf of Finland.

Interestingly, throughout the deposition of these Ectasian sediments, Fennoscandia was located near the equator!

Seabed topographical differences and seabed features

The average and maximum depths of the Bothnian Bay are 41 and 146 metres, respectively. In many respects, the depth distribution is similar to that of the Bothnian Sea.

Similarly, the Bothnian Bay is also characterised by a smooth, slightly tilted eastern side and a steeply sloped and broken western area. A similar difference can be observed in the landscapes of the Swedish and Finnish coasts.

On its Finnish side, the seafloor of the Bothnian Bay slopes slightly to the west, representing the ancient eroded surface of the Precambrian crystalline bedrock. In the central part of the Bothnian Bay basin, this crystalline bedrock is covered by sedimentary rocks. Like the Bothnian Sea, the seafloor of the Bothnian Bay on its Swedish side is fragmented by various faults.

As in the Bothnian Sea, the structural features of the bedrock in the Bothnian Bay can be divided into two main types: the fragmented topography of the basement bedrock and the gently sloping topography of the sedimentary rock areas.

The shape of the southern Bothnian Bay seafloor is dominated by basement bedrock of differing topography, covered by till  of varying thickness. In the Bothnian Bay, this crystalline bedrock is generally covered with sedimentary rocks. In turn, sedimentary rocks are sometimes covered with Quaternary period (circa 2.6 million years ago to the present) deposits, measuring up to several tens of metres thick.

Local depth differences in the northern Bothnian Sea are mainly due to Quaternary period sedimentation and erosion. The area has abundant crests and ridges, as well as ancient river channels up to tens of meters deep. Both the ridges and channels run in a northwest to southeast direction. The river beds appear to be extensions of land-based riverbeds, which were apparently formed before the last glaciation.

The northwest-southeast orientation mentioned above has been emphasised due to the direction of the largest fracture zones within the bedrock running parallel to the main flow direction of the continental ice sheet.

The post-glacial deposition which also occurred in the Bothnian Bay contributed to smoothing out structural irregularities on the seafloor. In addition, the  abovementioned land uplift in the Bay of Bothnia area is the fastest in the Baltic Sea. This process has contributed to the erosion and redeposition of seabed material, as well as levelling the seabed topography.

The effect of land uplift is visible, e.g. in the smoothing of eskers by shoreline forces like ice and wave action. Here, eskers are evened out, with both sand and gravel being spread over a wide area near the original formation.

Of all the extensive geomorphologic features occurring in the Bothnian Bay, no single type is dominant. On the seabed, hard- and soft bottom plains cover 19 and 12 %, respectively. In addition, hard- and soft bottom basins cover 14 and 17 %, respectively, while clay and hard clay elevations cover 10 % of the seabed.

Hard bottom elevations (mainly moraines) cover 11 % of the seafloor, some of which are reef-like. Although there are relatively few sandbanks throughout the Baltic Sea, the Bothnian Bay is one of the areas where they occur most abundantly, covering approximately 8 % of the seabed. They are just as common here as in the Kattegat and the Gulf of Riga.

For example, in the Bothnian Bay, large eskers occur as sandbanks, such as the underwater extension of the sandy esker of Vattajanniemi foreland near Lohtaja. Such sand eskers and potential sandbank areas occur widely throughout the Bothnian Bay.

In addition, various valleys and canyon-like seabed features, including sea holes, occur in the Bothnian Bay. As mentioned above, these valley-like features are possible remnants of old river beds, which formed when the water level was significantly lower than today.

Seafloor composition

In the Bothnian Sea, sand and gravel are more common than in other marine areas of Finland. The area has extensive sand fields and eskers. Although the area also has some seabed areas covered by till, there are hardly any exposed bedrock.

In surveyed areas with more accurate marine geological data, the percentage coverages of the dominant substrate types include; hard clay (about 30 %), sand and gravel (each about 20%) and till (20%). However, such surveyed areas represent less than 5 % of the total area. The proportion of hard bottoms in areas deeper than 60 metres is very small.

Geodiversity

Like the seabeds of the Archipelago Sea and the northern coast of the Gulf of Finland, the seabed of the Bothnian Bay is very fragmented. Each of these areas has at least seven different geomorphological feature types, each covering more than 5% of the seabed. These areas are located on the crystalline basement rock and are characterised by the blocky structure typical of the Baltic (or Fennoscandian) Shield.

In the Baltic Sea, geodiversity tends to increase from south to north, from the outer sea towards the archipelago and the coastline, and from sedimentary rock areas to those of crystalline bedrock.

The deepest point in Finland lies in the Åland Sea

The Åland Sea is located in a tectonic depression of the bedrock in a steep-sided basin. This is the deepest sea area of all Finnish marine areas. It has an average and a maximum depth of 75 and 301 metres, respectively. Fun fact! If you sank a tower the height of the Helsinki Olympic Stadium tower there, you could stack it vertically four times end to end, and it would still not be visible on the surface!

The bedrock of the Åland Sea consists of crystalline bedrock. In the deep area formed by a tectonic depression, the crystalline bedrock is covered by younger sedimentary rocks. In the northeast region of the Åland Sea and on Åland Island, the bedrock consists mainly of rapakivi granite.

The seabed topography of the Åland Sea is dominated by a tectonic depression composed of southern and northern basins, which run in east-west and southeast-northwest directions, respectively. These basins are a kind of rift-like formation with deep ravines. The shallow sill areas of the Åland Sea limit water exchange between the Baltic Proper and the Åland Sea to the south, and between the Åland Sea and the Bothnian Sea to the north. Very little of the deep water from the Baltic proper can flow over the ridges on the southern side of the Åland Sea to its basin and from there onwards to the Gulf of Bothnia.

In general, while canyons and valleys are relatively rare in the Baltic Sea, they do occur to some extent in the northern Baltic, such as in the Archipelago- and Åland Seas. Seabed canyons, valleys, and sea holes often correspond to underwater extensions of mainland rivers and river channels or from ancient river beds. Many of these valleys and throughs still function as underwater waterways. As in the Archipelago Sea, the canyons of the Åland Sea are an important channel for water flows between the Baltic Proper and the Gulf of Bothnia. Clay-covered mounds also occur in the Åland Sea, while rock elevations are also typical of this area.

In the Åland Sea basin area, the seabed is in many places erosion bottom. Strong bottom currents have eroded away the seabed sediments over hundreds and thousands of years. However, in certain areas of the seabed, there has also been some sediment deposition. For example, in the western part of the Åland Basin, exceptionally thick sediment deposits occur, which are up to tens of metres in thickness. They have been formed over an extended period as sediment materials transported by currents were deposited on the seabed. Indeed, these types of sediment deposits provide a unique archive for studying environmental changes and their influences over the last centuries and millennia.

The Åland Sea is also one of the Baltic Sea regions with a geologically diverse seafloor.

The Geological Survey of Finland (GTK) has not carried out seabed geological mapping in the Åland Sea area, so accurate seabed geological data are missing from the area.