Burgsvik beds

The Burgsvik beds are a sequence of shallow marine limestones and sandstones found near the town of Burgsvik in the southern part of Gotland, Sweden. The beds were deposited in the Upper Silurian period, around Ma|420, in warm, equatorial waters frequently ravaged by storms, in front of an advancing shoreline.

Appearance

The beds consists of thin to very thick layers of a light grey, fine grained argillaceous sandstone, containing a small calcareous element. The sandstones are occasionally intercalated with very thin-bedded blue-grey claystone. In places, the sandstone is overlain by the upper Burgsvik beds, which comprise thin-bedded, light-to-bluish grey, oolitic limestone with, here and there, alternating sandy beds containing problematic structures described by Manten (1966).

Environment of formation

Manten (1966) deduces that the Burgsvik beds were formed fairly close to the shoreline on a beach "faintly sloping towards the open sea", and that they were extensively reworked by the action of tides and storms. Evidence from cross-bedding and ripple marks is taken to imply a subaquaeous origin; rounded oolite pebbles and slightly rounded, size-sorted fossils are evidence of a high-energy environment. The presence of certain species of lamellibranch molluscs suggest a marine setting, and the thick shells present are also indicative of that type of environment. Rare burrows, sometimes found in clay lenses, may have formed in quieter waters that were protected by low sand or reef barriers from wave action. Features that only form on sub-aerial ground, including erosion channels, pothole-like excavations, mud cracks and dendritic rill marks are all present, and provide firm evidence that parts of the environment consisted of beaches or unvegetated ground that occasionally ran dry. Detailed petrographic and paleæoecological analysis of the upper and top few metres of the middle Burgsvik beds by Stel and de Coo (1977) confirm that this section of the sequence was deposited between the beach and the lower foreshore; oolites and oncolites in the upper strata form in an "agitated shallow marine setting", implying a minor tidal influence. The paleoshoreline was located to the north-east, and facies become progressively more marine in character progressing to the south-west (Jeppsson 2005).

Recent studies suggest that the sandstone might in fact represent delta deposits.citation
author = Eriksson, M. J.; Calner, M.
year = 2008
journal = Facies
volume = 54
issue = 2
pages = 253–276
url = http://www.springerlink.com/index/T423145276717242.pdf
publisher = Springer]

Palaeogeographic reconstructions allow the position of Gotland at the time of deposition to be deduced, and it appears that the Burgsvik beds were deposited near the equator (Torsvik "et al". 1993). Combined with the high temperatures of the Silurian, this may have led to very hot, hypersaline waters.Fact|date=November 2007

edimentology

Long (1993) recognises three lithofacies in the Burgsvik beds; a poorly exposed silty/sandy mudstone facies dominant in the lower beds, appearing as interbeds in the middle beds; a fine to very fine sandstone; and a "biofacies" consisting of ooids, oncolites and bioclasts. He challenges three interpretations of the sub-aerial sandstone facies. Contrary to Gray "et al."'s (1974) tidal mud flat interpretation, Long surmises that it may represent locally emergent offshore bars, near-shore sands or beach deposits. Tides cannot be a dominant factor, as cross-stratification is abundant; storm beds, recognised by hummocky cross-stratification, are also common, suggesting that storms were important in shaping the landscape. Irregularly aligned prod and scour marks on the sea floor shows that waves also played a role. The most favourable conclusion appears to be that the facies represents a shoaling sequence — the migration of sand wave complexes, detached offshore bars (Swift & Field 1981, Brenner "et al". 1985) or isolated mid-shelf bars (La Fon 1981).

Correlation

Mainly due to the inhomogeneous nature of shoreline deposits, lateral variation is intense throughout the Burgsvik beds, making correlation difficult (Laufeld 1974). However, using freshly available borehole data, Manten (1971) was able to further sub-divide the Burgsvik beds into 3 members, illustrated above. The upper bed can be recognised across the entire outcrop belt, varying slightly along strike, and has a distinctive lower contact. The lower bed, however, is easily eroded and rarely exposed. To further complicate the matter, the depositional area was being continually provided with sediment — and thus filling up — from the north west. As bioherm detritus and terrigenous infill accumulated, the coast prograded, and the reef zone advanced in front of it to the south west. This pattern is complicated further by sea level changes, making precise interpretation troublesome, to say the least (Laufeld 1974).

Correlation to units elsewhere in the world is aided by the high-resolution conodont data available; the beds are in the "Ozarkodina snajdri" conodont subdivision of the "Pseudomonoclimacis latilobus" graptolite zone, which is also well displayed in, for example, Estonia (Jeppsson "et al." 1994, Jeppsson & Männik 1993).

Palæontological interest

As well as reef-building organisms and the thick-shelled Lamellibranchia mentioned above, the Burgsvik beds are also of interest to micropalæontologists. Their quiet tectonic history — with the depth of burial never exceeding 200 metres, and "no thermal maturation" occurring (Jeppsson 1983) — means that organic material is preserved relatively unscathed, to a degree of quality barely rivalled anywhere else on earth for rocks of this age - indeed, the preservation is equivalent to that expected from the Tertiary (Sherwood-Pike and Gray 1985). Dissolution of the rocks in hydrofluoric acid leaves the organic matter unscathed, and putative fungi ("Ornatifilum") and fæcal pellets have been unearthed (Sherwood-Pike and Gray 1985), as well as supposed euglenids (Gray and Boucot 1989): the latter being of particular interest as not one other fossil euglenid is known! A lack of marine macrofossils in plant-rich beds suggests that large grazers or predators may have been absent, perhaps because water depths were so shallow - this may have aided fossil preservation (Gray "et al." 1974).

The beds are the first location where it was recognized that "elephant skin" wrinkles in marine sediments are trace fossils of microbial mats, cite journal
author = Manten, A.A.
year = 1966
title = Some problematic shallow-marine structures
journal = Marine Geol
volume = 4
pages = 227–232
doi = 10.1016/0025-3227(66)90023-5
url = http://igitur-archive.library.uu.nl/geo/2006-1213-210452/UUindex.html
accessdate = 2007-06-18 ] which were Earth's most sophisticated form of life for nearly 2 billion years and are still the major factors maintaining life on Earth. citation | contribution=Biofilm, Biodictyon, Biomat Microbialites, Oolites, Stromatolites, Geophysiology, Global Mechanism, Parahistology | author=Krumbein, W.E., Brehm, U., Gerdes, G., Gorbushina, A.A., Levit, G. and Palinska, K.A. | pages=1-28 | title=Fossil and Recent Biofilms: A Natural History of Life on Earth | editor=Krumbein, W.E., Paterson, D.M., and Zavarzin, G.A. | date=2003 | publisher=Kluwer Academic| isbn=1402015976 | url=http://134.106.242.33/krumbein/htdocs/Archive/397/Krumbein_397.pdf | accessdate=2008-07-09 ]

Association with mass extinction

Martma "et al". (2005) assign a Mid-Ludfordian (Upper Ludlow) age to the Burgsvik, which places the beds in close temporal proximity to the Lau event, a late Silurian mass extinction. They also note a positive delta|13|C_org excursion in the Burgsvik and underlying Eke beds. Such excursions are normally associated with the decrease in oceanic productivity caused by mass extinctions. However, it could also be interpreted as climate, mainly precipitation, controlling the distribution of facies; high delta|13|C is often observed in deposits formed in arid conditions.

Calner (2005) notes that anachronistic facies are observed in other strata spanning the Lau event, places the Burgsvik beds immediately after this mass extinction (Calner 2005b), noting the occurrence of flat-pebble conglomerates in the underlying Eke beds.

Correlation with the P-S episodes postulated by Jeppsson (1990) suggests that the beds were deposited during a wet period - a P episode.

ee also

*Stratigraphy of Gotland
*Fröjel formation

References

* Agterberg, F.P. (1958). "An undulation of the rate of sedimentation in southern Gotland". "Geologie en Mijnbouw", New Series 20: 253-260.

*cite journal
author = Brenner, R.L.
coauthors = Swift, D.J.P.; Gaynor, G.C.
year = 1985
title = Re-evaluation of coquinoid sandstone depositional model, Upper Jurassic of central Wyoming and south-central Montana
journal = Sedimentology
volume = 32
issue = 3
pages = 363–372
doi = 10.1111/j.1365-3091.1985.tb00517.x

* cite journal
author = Calner, M.
year = 2005
date = 2005-04-01
title = A Late Silurian extinction event and anachronistic period
journal = Geology
volume = 33
issue = 4
pages = 305–308
issn =
doi = 10.1130/G21185.1
url = http://geology.geoscienceworld.org/cgi/content/abstract/33/4/305

*cite journal
author = Comments by R. Riding and M. Calner
year = 2005
title = Reply to 'A Late Silurian extinction event and anachronistic period'
journal = Geology
volume = 33
pages = e92-3
url = http://www.gsajournals.org/pdf/online_forum/i0091-7613-31-6-e92.pdf

* cite journal
author = Gray, J.
coauthors = Boucot, A. J.
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* cite journal
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title = Silurian Trilete Spores and Spore Tetrads from Gotland: Their Implications for Land Plant Evolution
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issue = 4147
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pmid = 17812053

*

* cite journal
author = Jeppsson, L.
date = December 2005
title = Conodont-based revisions of the Late Ludfordian on Gotland, Sweden
journal = GFF
volume = 127
issue = 4
pages = 273–282
url = http://www.geol.lu.se/PERSONAL/ltj/Jeppsson%202006,GFF.pdf
accessdate = 2007-08-10
issn = 1103-5897

* Jeppsson, L. (November 30, 2000) Journal of the Geological Society (ProQuest). " [http://findarticles.com/p/articles/mi_qa3721/is_200011/ai_n8917153 Ludlow (late Silurian) oceanic episodes and events.] " Page 1137.

* cite journal
author = Jeppsson, L.
coauthors = Männik, P.
year = 1993
title = High-resolution correlations between Gotland and Estonia near the base of the Wenlock
journal = Terra nova(Print)
volume = 5
issue = 4
pages = 348–358
url = http://cat.inist.fr/?aModele=afficheN&cpsidt=4933561
accessdate = 2007-08-10
doi = 10.1111/j.1365-3121.1993.tb00268.x

* cite journal
author = Jeppsson, L.
coauthors = Viira, V.; Männik, P.
year = 1994
title = Silurian conodont-based correlations between Gotland (Sweden) and Saaremaa (Estonia)
journal = Geological Magazine
volume = 131
pages = 201–218

* cite journal
author = Jeppsson, L.
year = 1990
title = An oceanic model for lithological and faunal changes tested on the Silurian record
journal = Journal of the Geological Society
volume = 147
issue = 4
pages = 663–674
doi = 10.1144/gsjgs.147.4.0663

* cite book
author = Laufeld, S.
year = 1974
title = Silurian Chitinozoa from Gotland
publisher = Universitetsforlaget
series = Fossils and Strata
number=5

* cite journal
author = Long, D.G.F.
year = 1993
title = The Burgsvik beds, an Upper Silurian storm generated sand ridge complex in southern Gotland
volume = 115
number=4
journal = Geologiska Föreningens i Stockholms Förhandlingar (now GFF)
pages = 299–309
issn = 0016-786X

* cite journal
author = Manten, A.A.
year = 1966
title = Some problematic shallow-marine structures
issn =
journal = Marine Geol
volume = 4
pages=227–232
doi = 10.1016/0025-3227(66)90023-5
url = http://igitur-archive.library.uu.nl/geo/2006-1213-210452/UUindex.html
accessdate = 2007-06-18

*cite journal
author = Martma "et al".
title = The Wenlock-Ludlow carbon isotope trend in the Vidukle core, Lithuania, and its relations with oceanic events
journal = Geological Quarterly
year = 2005
volume = 49
issue = 2
pages = 223–234
url = http://www.pgi.gov.pl/pdf/gq_49_2_p223.pdf

* cite journal
author = Sherwood-Pike, M.A.
coauthors = Gray, J.
year = 1985
title = Silurian fungal remains: probable records of the class Ascomycetes
journal = Lethaia
volume = 18
pages = 1–20
doi = 10.1111/j.1502-3931.1985.tb00680.x

* cite journal
author = Stel, J.H.
coauthors = de Coo, J.C.M.
year = 1977
title = The Silurian Upper Burgsvik and Lower Hamra--Sundre Beds, Gotland
journal = Scripta Geologica
volume = 44
pages = 1–43
publisher = Rijksmuseum van Geologie en Mineralogie
oclc = 4000723

*citation
author = Swift, D. J. P.; Field, M. E.
year = 1981
journal = Sedimentology
volume = 28
issue = 4
pages = 461–482
url = http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-3091.1981.tb01695.x
publisher = Blackwell Synergy

*cite journal
author = Torsvik, T.H.
coauthors = Trench, A.; Svensson, I.; Walderhaug, H.J.
year = 1993
title = Palaeogeographic significance of mid-Silurian palaeomagnetic results from southern Britain-major revision of the apparent polar wander path for eastern Avalonia
journal = Geophysical Journal International
volume = 113
issue = 3
pages = 651–668
doi = 10.1111/j.1365-246X.1993.tb04658.x