User:Smith609/Gotland
- Succession of Limestones and Shales
- Shallow shelf above thin (75-125m) Ord. succession
- 500-200m thick, thickest in south. Oldest @ NW.
- Shallow epicontinental sea
- Near equator (Creer 1973)
- Water depth never > 175-200m (Gray, Laufield & boucot, 1974)
- Dominated by reefs, starting to grow in Llandovery (sea then 50-100m deep.
- NW shelf becomes shallower, from bioherm detritus and terriginous infilling. Migration of reef zone to SW, but complicated by sea level change.
- Burgsvik group:
- Thin → v thick beds, light grey, fine grained, slightly calcareous, argillaceous sst - intercalated w/ v. thin bedded blue-grey claystone.
- The sst in places overlain by the "Upper Burgsvik Beds": thin-bedded, light-to-bluish grey, oolitic lst with, in places, alternating sandy beds. These are the ones containing problematic structures (Manten 1966).
- Strong lateral variation - correltation difficult.
Re. the Upper Burgsvic Beds. Age is probably lowermost Devonian (Downtonian)
- Formed close to the shoreline on a beach "faintly sloping towards the open sea" affected by tides.
- Evidence
- Cross-bedding
- Ripple marks
- Offshore bars
- oolites
- rounded oolite pebbles in ssts and oolites
- slightly rounded fossils, sorted by size, between sst and uppermost oolite
- lamellibranch (molluscs?) with thick shells
- Rare burrows
- clay lenses (deposited behind low barriers?)
- erosion channels
- Pothole-like escavations
- mud cracks
- "dendritic patterns of rill marks" - suggest form on beach/unvegetated ground which periodically ran dry.
Manten 1971
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Divides Burgsvic beds into 3 members, based on bore-hole. Upper bed varies a little along strike but can be recognised across whole outcrop belt, with distinctive lower contact. Lower beds are exposed at Kullunde 1 & Glasskär 1-3
Thorough sedimentological analysis. Surmises and challenges sedimentary descriptions:
- Near-shore sands: Munthe 1910, Manten 1966
- Locally emergent, offshore bars: Agterberg 1958, Manten 1971
- beach deposits: Stel & de Coo 1977, Laufield & Bassett
- Intertidal exposure: Grey et al 1974, Laufield & Martinsson 1981
- tidal influence: Manten 1966, Stel & de Coo 1977
Contains logs of cores
Microbial origin of unusual wrinkle structures described by Manten 1966.
Lithofacies
[edit]- → zst-s/mst
- Poor exposure. Interbeds in middle, thick seq. in lower.
- → f-vf. ssf
- → biofacies - oo, onco, bioclastic
Problematic structures
[edit]- wrinkles are kinneyia ripples (Martinsson 1965) or Runzel-marken (Reineck 1969, or wrinkle marks (Reineck & Sing 1973, Allen 1984).
- Wind stress on freshly deposited seds? Not nec.. soft sed failure of sand beds over mud.
Interpretation
[edit]Shoaling sequence - migration of storm dominated sandbanks, or sand-wave complexes.
- Formation: detatched offshore bars (Swift & Field 1981, Brenner et al 1985)? Isolated bars in mid-shelf (La Fon 1981)?
- Cross stratification abundant. so can't be tide-dominated.
- Storm beds abundant. So Storms probably created bars.
- Prod & scour marks etc irregularly aligned - waves influenced sea floor.
Detailed Petrographic and paleæoecological analysis of top few metres. sst at top of middle member, and in upper member, deposited @ beach to lower foreshore. Oolites & oncolites in upper strate form, in "agitated shallow marine setting" - minor tidal influence
Calner 2005
[edit]- Anachronistic facies found near Lau event in Late Silurian.
- Basal Eke Fm. contiains flat-pebble conglomerates
Cherns 1983
[edit]In a SGU we don't have.
Jux & Stauber
[edit]Assign a Mid-Ludfordian (Upper Ludlow) age to the Burgsvik. Positive δ13Corg excursion during Eke and Burgsvik beds. Could be interpreted as climate (arid vs. wet) controling facies distribution, sea level shifts less important; high δ13C observed in arid deposits.
Key text.
See
[edit]- Agterberg 1958:254-256 ==1958, Agterberg, F.P.: An undulation of the rate of sedimentation in southern Gotland: Geologie en Mijnbouw, New Series Vol. 20, pp. 253-260.
Reflist
[edit]- ^ Laufeld, S. (1974). Silurian Chitinozoa from Gotland. Fossils and Strata. Universitetsforlaget.
- ^ Manten, A.A. (1966). "Some problematic shallow-marine structures". Marine Geol. 4 (3): 227–232. doi:10.1016/0025-3227(66)90023-5. Retrieved 2007-06-18.
- ^ Long, D.G.F. (1993). "The Burgsvik beds, an Upper Silurian storm generated sand ridge complex in southern Gotland". Geologiska Föreningens i Stockholms Förhandlingar (Now GFF). 115 (4): 299–309. doi:10.1080/11035899309453917. ISSN 0016-786X.
- ^ Stel, J.H. (1977). The Silurian Upper Burgsvik and Lower Hamra-Sundre Beds, Gotland. Rijksmuseum van Geologie en Mineralogie.
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(help)CS1 maint: date and year (link) - ^ Bassett; et al. (1989). "The Wenlock-Ludlow carbon isotope trend in the Vidukle core, Lithuania, and its relations with oceanic events" (PDF).
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(help) - ^ Sherwood-pike, M.A. (1985). "Silurian fungal remains: probable records of the class Ascomycetes". Lethaia. 18: 1–20. doi:10.1111/j.1502-3931.1985.tb00680.x.
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