The Fröjel formation is a ten metre thick siliciclastic unit lying in the carbonate sequence of Gotland

Gotland

Gotland is a county, province, municipality and diocese of Sweden; it is Sweden's largest island and the largest island in the Baltic Sea. At 3,140 square kilometers in area, the region makes up less than one percent of Sweden's total land area...

, Sweden

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

. Deposited some during the mid Homerian (late Wenlock, mid Silurian), the sediments represent an unusually high supply of terrigenous sediment into the shallow carbonate platform that was to become the island of Gotland. Their deposition is coincident with a excursion of +3‰.

Stratigraphy

The formation, nine to eleven metres thick in total, contains two subdivisions, the Svarvare Mudstone Member, two to three metres thick, which is overlain by the Gannarve Siltstone Member, seven to eight metres in thickness.
(This is also known as the Silte Siltstone, but does not form part of the Slite group.) The water depth was decreasing as the Gannarve member was deposited, so the member shows evidence of shallowing up. The formation is overlain with an oolite, which rests unconformably on an erosion surface - a sequence boundary - created at a time of low sea level. This erosional surface formed a flat - a rocky shore which was innundated by the sea at times, but was also subject to subaerial rainfall. Calcium carbonate penetrated the underlying sediment (the top of the Gannarve member), altering the existing dolomitic siltstone.

Formation

The Svarvare member was deposited during the end of a sea level highstand; the Gannarve member was subsequently deposited when falling sea levels exposed land, increasing the influx of terrigenous material. Deposition occurred under storm conditions.

The sea levels fell as a result of increasing ice buildup in Gondwana, which then occupied the South pole; this caused increased carbonate weathering, and possibly increased organic burial, leading to the positive excursion.

Palaeontology

Graptolites are abundant in parts of the Svarvare member, which also contains Ramphoprionid

Ramphoprionid

The Ramphoprionids are a family of polychaete worms known from the Ordovician and Silurian periods...

 polychaete

Polychaete

The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...

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