Burgsvik Beds - Environment of Formation

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.

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.

Subdivisions of the Burgsvik beds
after Manten (1971) -45 — – -40 — – -35 — – -30 — – -25 — – -20 — – -15 — – -10 — – -5 — – 0 — Upper Middle Lower Interbedded oolitic/oncolitic
lime-, sand- and mud-stones Sandstone with minor muddy sand/mudstone Mudstone and muddy sandstone. Rarely exposed. Grain size -→
Vertical scale in Metres