Fröjel Formation Explained

57.3611°N 18.1753°WThe Fröjel Formation is a ten-metre thick siliciclastic unit lying in the carbonate sequence of Gotland, Sweden. 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.^[1] 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.^[2] (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^[3] – created at a time of low sea level.^[3] 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.^[4]

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.^[5]

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.^[6]

Exposure

The formation is no longer exposed at the surface; it is only known from drill cores.^[7]

Palaeontology

Graptolites are abundant in parts of the Svarvare member,^[8] which also contains Ramphoprionid polychaetes.^[9]

See also

• Geology of Gotland

Notes and References

1. 10.1007/s10347-005-0050-0. Silurian carbonate platforms and extinction events—ecosystem changes exemplified from Gotland, Sweden. 2005. Calner, Mikael. Facies. 51. 1–4. 584–591.
2. Calner, M. . 1999 . Stratigraphy, facies development, and depositional dynamics of the Late Wenlock Fröjel Formation, Gotland, Sweden . GFF . 121 . 1 . 13–24 . 10.1080/11035899901211013.
3. Calner. Mikael. Säll. Ewa. Transgressive oolites onlapping a Silurian rocky shoreline unconformity, Gotland, Sweden. GFF. 121. 2. 1999. 91–100. 1103-5897. 10.1080/11035899901212091.
4. Calner, Mikael . 2002 . A lowstand epikarstic intertidal flat from the middle Silurian of Gotland, Sweden . Sedimentary Geology . 148 . 3–4 . 10.1016/S0037-0738(01)00144-0 . 3–4. 2002SedG..148..389C .
5. 2005 . C. Karlsson . Provenance studies of the Middle Jurassic strata of the Tornquist Zone in southern Sweden and of storm generated siliciclastic strata of Upper Silurian Burgsvik Formation of southern Gotland, Sweden . Geophysical Research Abstracts . 7.
6. 10.1002/gj.1090 . Early Silurian positive δ13C excursions and their relationship to glaciations, sea-level changes and extinction events . 2007 . Loydell, David K. . Geological Journal . 42 . 531–546 . 5 . free .
7. Detailed geological map differentiating members is available on second page of:

   2002 . The Silurian Mulde Event and a scenario for secundo–secundo events . Transactions of the Royal Society of Edinburgh: Earth Sciences . 93 . 135–154 . 10.1017/S0263593302000093 . 2.

8. Samtleben, C. . Munnecke, A. . Bickert, T. . 2000 . Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment . Facies . 43 . 1 . 1–38 . 10.1007/BF02536983.
9. Eriksson, Mats . September 2001 . Journal of Paleontology . Silurian Ramphoprionid Polychaetes From Gotland, Sweden . 1 . 75 . 993–1015 . 10.1666/0022-3360(2001)075<0993:SRPFGS>2.0.CO;2 .