Fröjel Formation
Coordinates: 57°21′40″N 18°10′31″E / 57.36111°N 18.17528°E
The Fröjel Formation is a ten-metre thick siliciclastic unit lying in the carbonate sequence of Gotland, Sweden. Deposited some 424 million years ago 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 δ13C excursion of +3‰.
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 inundated 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.
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