Burgsvik beds

Burgsvik beds Stratigraphic range: Ludfordian PreЄ Є O S D C P T J K Pg N ↓
The distribution of the Burgsvik Beds (blue)
Underlies	Burgsvik Oolite
Overlies	Eke formation
Lithology
Primary	Limestone, sand and mudstone
Location
Coordinates	57°02′13″N 18°17′03″E / 57.036990°N 18.284230°E / 57.036990; 18.284230
Region	South Gotland, Sweden

The Burgsvik beds are a sequence of shallow marine limestones and sandstones found near the locality of Burgsvik in the southern part of Gotland, Sweden. The beds were deposited in the Upper Silurian period, around 420 million years ago, in warm, equatorial waters frequently ravaged by storms, in front of an advancing shoreline. The Burgsvik Formation comprises two members, the Burgsvik Sandstone (on which this article focusses) and Burgsvik Oolite.

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).

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).