The Aalter Formation (, abbreviation: Aa; named after the town of

Aalter Aalter () is a municipality located between Bruges and Ghent in the Belgian province of East Flanders. The municipality comprises the towns of Aalter, Bellem, , Poeke, Knesselare and . It is bordered on the north by Maldegem, on the east by Zom ...

East Flanders East Flanders ( ; ; ; ) is a Provinces of Belgium, province of Belgium. It borders (clockwise from the North) the Netherlands, Dutch province of Zeeland and the Belgian provinces of Antwerp (province), Antwerp, Flemish Brabant, Hainaut (provinc ...

) is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

in the

subsurface In geology, bedrock is solid rock that lies under loose material (regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bedro ...

of northwest

Belgium Belgium, officially the Kingdom of Belgium, is a country in Northwestern Europe. Situated in a coastal lowland region known as the Low Countries, it is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeas ...

. The formation consists of marine

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

and

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

, deposited in the shallow sea that covered northern and central Belgium in the

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

. The Aalter Formation crops out in the provinces of East Flanders and

West Flanders West Flanders is the westernmost province of the Flemish Region, in Belgium. It is the only coastal Belgian province, facing the North Sea to the northwest. It has land borders with the Dutch province of Zeeland to the northeast, the Flemis ...

and forms a maximally 30 meters thick layer in the subsurface. To the east, the formation wedges out and becomes thinner. Due to this wedging out, the formation does not occur anymore in the

province of Antwerp Antwerp Province (; ; ; ), between 1815 and 1830 known as Central Brabant ( , , ), is the northernmost province both of the Flemish Region, also called Flanders, and of Belgium. It borders on the North Brabant province of the Netherlands to the ...

. The base of the formation consists of glauconiferous clayey

alternating with organic rich (

humus In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...

and

peat Peat is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. ''Sphagnum'' moss, also called peat moss, is one of the most ...

) layers (the Aalterbrugge Member). On top of this is a sequence of clay, sand and

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

layers, rich in

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s (the Beernem Member). The top of the formation consists of fossil rich, glauconiferous fine sand (the Oedelem Member). The Aalter Formation was formed during the late

Ypresian In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...

to early

Lutetian The Lutetian is, in the geologic timescale, a stage (stratigraphy), stage or age (geology), age in the Eocene. It spans the time between . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it ...

ages, giving it an age of around 48 million years. It is part of the Zenne Group and is covered by the Brussels Formation. If the Brussels Formation is absent, the Lede Formation can be found on top of the Aalter Formation. Both are like the Aalter Formation part of the Zenne Group. Stratigraphically below the Aalter Formation are deposits of the Gentbrugge Formation (late Ypresian marine clays and sands).

Fossil content

Chondrichthyes

References

Bibliography

* (eds.); 2001: ''Paleogene and Neogene lithostratigraphic units (Belgium)'', Geologica Belgica 4(1-2), p. 135-152 Geologic formations of Belgium Eocene Series of Europe Paleogene Belgium Lutetian Stage Ypresian Stage Aalter {{Europe-geologic-formation-stub

Fossil content

Chondrichthyes

See also

References

Bibliography