The geology of the Baltic Sea is characterized by having areas located both at the Baltic Shield of the

East European Craton The East European Craton (EEC) is the core of the Baltica proto- plate and consists of three crustal regions/segments: Fennoscandia to the northwest, Volgo-Uralia to the east, and Sarmatia to the south. Fennoscandia includes the Baltic Shield ( ...

and in the Danish-North German-Polish Caledonides. Historical geologists make a distinction between the current

Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and ...

depression, formed in the Cenozoic

era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the geological eras defined for the history of Earth. Comp ...

, and the much older

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subside ...

s whose sediments are preserved in the zone. Although

glacial erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sediment transport, tra ...

has contributed to shape the present depression, the Baltic trough is largely a depression of

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

origin that existed long before the

Quaternary glaciation The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma (million years ago) and is ongoing. Although geologists describ ...

Ancient sedimentary basin

Tectonic evolution

The Baltic Sea sedimentary basin was formed on top of the

millions of years after it consolidated. This occurred during the Late Ediacaran and the

Early Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

when the weakest part of the craton was reactivated. Since then the basin has been deepened mainly through subsidence caused by extensional tectonics. In the northern part of the Baltic Sea basin, including the Gulf of Bothnia and the

Bothnian Sea The Bothnian Sea ( sv, Bottenhavet; fi, Selkämeri) links the Bothnian Bay (also called the Bay of Bothnia) with the Baltic proper. Kvarken is situated between the two. Together, the Bothnian Sea and Bay make up a larger geographical entity, t ...

, the depression originated as a series of interconnected

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

s that were not active enough to split the East European Craton. Subsidence in the southern parts was most pronounced during a period starting in the

Late Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

and lasting to

Middle Silurian Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek ( ...

. At this time the Baltic sea basin and the East European Craton were part of a continent and tectonic plate known as

Baltica Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains. The thick core of Baltica, the East European Craton, ...

. Baltica collided with the continent

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...

in the Late Silurian and

Early Devonian The Early Devonian is the first of three epochs comprising the Devonian period, corresponding to the Lower Devonian series. It lasted from and began with the Lochkovian Stage , which was followed by the Pragian from and then by the Emsian, ...

. This caused compression in the basin in a north-west to south-east direction. This collision further led to the formation of a

fold and thrust belt A fold and thrust belt (FTB) is a series of mountainous foothills adjacent to an orogenic belt, which forms due to contractional tectonics. Fold and thrust belts commonly form in the forelands adjacent to major orogens as deformation propagates o ...

and faults north of the Danish–North German–Polish Caledonides. During the Carboniferous and

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...

the basin areas around

Rügen Rügen (; la, Rugia, ) is Germany's largest island. It is located off the Pomeranian coast in the Baltic Sea and belongs to the state of Mecklenburg-Western Pomerania. The "gateway" to Rügen island is the Hanseatic city of Stralsund, where ...

and northern

Poland Poland, officially the Republic of Poland, is a country in Central Europe. It is divided into 16 administrative provinces called voivodeships, covering an area of . Poland has a population of over 38 million and is the fifth-most populou ...

were subject to the intrusion of magma. After the

there was little tectonic activity in the Baltic for most of the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

until the Cretaceous. During the Cretaceous a

tectonic inversion In structural geology inversion or basin inversion relates to the relative uplift of a sedimentary basin or similar structure as a result of crustal shortening. This normally excludes uplift developed in the footwalls of later extensional fau ...

occurred in the southwestern part of the basin.

Sedimentary fill

The oldest sedimentary rocks in the Baltic Sea area that are not

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

are

Jotnian In north European geology, Jotnian sediments are a group of Precambrian rocks more specifically assigned to the Mesoproterozoic Era ( Riphean), albeit some might be younger. Jotnian sediments include the oldest known sediments in the Baltic area ...

orthoquartzite, siltstones and conglomerates. Jotnian sediments are scattered. In the south and east of the Baltic crystalline shield the rocks are overlain by an extensive sedimentary cover that constitutes part of the East European platform. In the Baltic area of the platform there are sedimentary units of all geological periods from the Ediacaran (

Vendian The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and the ...

) to the Cenozoic. Paleogene and Neogene marine sediments are however absent in the whole Baltic area except for the southern fringes.

Origin and shape of modern depression

StadtmuseumBerlin GeologischeSammlung SM-2012-2855-1-2

Scientists do not agree on how the present depression the Baltic Sea occupies formed. Scholars like Voipio (1981) and Šlaiupa (1995) consider the depression to have been formed in the Cenozoic, before the Quaternary, by tectonic processes. Some others like Marks (2004) stress erosion was key to the formation of the depression. In the Cenozoic, long before the

s the Baltic was the site of a large river called Eridanos. This river drained west in direction towards the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea, epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the ...

. The Neogene uplift of the South Swedish Dome deflected Eridanos river from its original path across south-central Sweden into a course south of Sweden in the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58scoured an elongated area of the Baltic spanning from northern Småland, via Stockholm and

Åland Åland ( fi, Ahvenanmaa: ; ; ) is an autonomous and demilitarised region of Finland since 1920 by a decision of the League of Nations. It is the smallest region of Finland by area and population, with a size of 1,580 km2, and a populat ...

to the coast at the Finnish-Russian border. The scoured area and other zones made up of crystalline

Fennoscandian Shield The Baltic Shield (or Fennoscandian Shield) is a segment of the Earth's crust belonging to the East European Craton, representing a large part of Fennoscandia, northwestern Russia and the northern Baltic Sea. It is composed mostly of Archean an ...

rocks (northern and western Baltic Sea) have experienced overall very limited glacial erosion during the Quaternary.

Bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water ...

of linear depressions in the Baltic seabed shows that the depression has been subject to glacial

overdeepening Overdeepening is a characteristic of basins and valleys eroded by glaciers. An overdeepened valley profile is often eroded to depths which are hundreds of metres below the deepest continuous line (the thalweg) along a valley or watercourse. This p ...

. The date during which glacier ice carved these troughs is not known but it could have happened during the first glaciations that affected the Baltic region. Alternatively overdeepening could have built-up gradually during the successive glaciations that affected the area.

Quaternary seas and lakes

The hydrology, extent and nature of the Baltic Sea has varied considerably through the Quaternary. Its great overturns are linked to its geographic position, in particular its high latitude, that has made it prone to

glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate be ...

s. During many

s the whole depression was repeatedly covered by

ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...

s. These glaciation have influenced the Baltic Sea depression by bringing eroded sediments from adjacent areas to the depression, downwarping the lithosphere during glaciations and causing

isostatic rebound Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound ...

after thinning and retreat. Important to the understanding of the Baltic basin during the Quaternary was its connection to the open ocean in the west through the area of

Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of Denmark , establish ...

and

Scania Scania, also known by its native name of Skåne (, ), is the southernmost of the historical provinces (''landskap'') of Sweden. Located in the south tip of the geographical region of Götaland, the province is roughly conterminous with Skåne ...

. The depth of the connection in particular has been critical to shape oceanographic conditions such as salinity.

Eemian and Weichselian

When the Eemian interglacial began about 130 thousand years ago, the formerly ice-filled Baltic depression turned to a freshwater lake for a "brief" time of 300 years before becoming a more saline sea. This sea was during its first 2500 to 2000 years of existence connected to the

White Sea The White Sea (russian: Белое море, ''Béloye móre''; Karelian and fi, Vienanmeri, lit. Dvina Sea; yrk, Сэрако ямʼ, ''Serako yam'') is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is s ...

through Karelia due to the isostatic depression caused by the ice sheet of the earlier Saalian ice age. Little is known about the Baltic Sea during the oscillations of the Weichselian ice age, the last glaciation. The first

Weichselian glaciation The Weichselian glaciation was the last glacial period and its associated glaciation in northern parts of Europe. In the Alpine region it corresponds to the Würm glaciation. It was characterized by a large ice sheet (the Fenno-Scandian ice sheet) ...

s, corresponding to

MIS MIS or mis may refer to: Science and technology * Management information system * Marine isotope stage, stages of the Earth's climate * Maximal independent set, in graph theory * Metal-insulator-semiconductor, e.g., in MIS capacitor * Minimally ...

5b and 5d, penetrated parts of the Baltic Sea depression from north to south but did not got south beyond

. The Weichsel ice sheet reached northern Denmark about 65–60 thousand years ago.

Holocene

When the last ice sheet began to retreat north from the Baltic Sea depression in the early Holocene (ca. 10,000 years ago) glacier meltwater accumulated between the front of the ice sheet and the southern shores that were free of glacier ice. This accumulation of freshwater constituted a lake known as the

Baltic Ice Lake The Baltic Ice Lake is a name given by geologists to a freshwater lake that evolved in the Baltic Sea basin as glaciers retreated from that region at the end of the last ice age. The lake existed between 12,600 and 10,300 years Before Present ...

. This lake did not mix with seawater from the west because the ground on the whole depression rose faster than sea level did. Eventually mixing and connection to the ocean did occur when the glacier front retreated to the north beyond Billingen about 11,500 years ago. The Baltic Ice Lake drained then rapidly reaching sea level. From this point on the Baltic water body became the

Yoldia Sea Yoldia Sea is a name given by geologists to a variable brackish water stage in the Baltic Sea basin that prevailed after the Baltic Ice Lake was drained to sea level during the Weichselian glaciation. Dates for the Yoldia sea are obtained mainly b ...

. About 10,700 years ago the continued land rise separated the Baltic water body from the ocean again and Yoldia Sea turned into the Ancylus Lake. This lake lasted until 10,000 years before present when a new connection to the ocean was established, this time in the

Danish straits The Danish straits are the straits connecting the Baltic Sea to the North Sea through the Kattegat and Skagerrak. Historically, the Danish straits were internal waterways of Denmark; however, following territorial losses, Øresund and Fehmarn B ...

forming the

Littorina Sea Littorina Sea (also Litorina Sea) is a geological brackish water stage of the Baltic Sea, which existed around 7500–4000 BP and followed the Mastogloia Sea, a transitional stage of the Ancylus Lake. This stage and form of the body of wate ...

. Land rise did not close off Littorina sea but it made its connection the ocean over time more shallow allowing less salt water to enter it, thus gradually it evolved into the current Baltic Sea with somewhat brackish water.Eliason ''et al''. 2010, pp. 25–26 File:LateBalticIceLake.jpg, Approximate reconstruction of the Baltic Ice Lake File:Baltic_History_7500-BC.svg, Ancylus Lake about 9500–8000 years ago File:Baltic_History_5000-BC.svg, Littorina Sea about 7000 years ago File:BalticSea March2000 NASA-S2000084115409.png, Baltic Sea within its present conditions

Economic geology

Baltic amber

Limestone

Hydrocarbons

There are two kinds of

oil shale in Estonia There are two kinds of oil shale in Estonia, both of which are sedimentary rocks laid down during the Ordovician geologic period. Graptolitic argillite is the larger oil shale resource, but, because its organic matter content is relative ...

, both of which are sedimentary rocks laid down during the

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

geologic period The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochr ...

. Graptolitic argillite is the larger resource, but, because its organic matter content is relatively low, it is not used industrially. The other one is kukersite, which has been mined for almost a hundred years and is expected to last for another 25–30 years. By the end of 2012, the total kukersite resource was 4.8 billion tonnes, of which up to 650 million tonnes was recoverable. Kukersite deposits in Estonia account for 1.1% of global oil shale deposits. Exploratory wells have revealed the existence of

petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...

of Lower Paleozoic age beneath Gotland.

References

Bibliography * * * {{Geology of Europe

Baltic Sea