Burgsvik beds
Encyclopedia
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 , 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.
s consists of thin to very thick layers of a light grey, fine grained argillaceous sandstone, containing a small calc
areous 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, oolit
ic limestone with, here and there, alternating sandy beds containing problematic structures described by Manten (1966).
s and storms. Evidence from cross-bedding and ripple marks is taken to imply a subaquaeous origin; rounded oolite pebbles and slightly rounded, size-sorted fossil
s 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.
dominant in the lower beds, appearing as interbeds in the middle beds; a fine to very fine sandstone; and a "biofacies" consisting of ooids, oncolite
s and bioclast
s. 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).
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).
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 euglenid
s (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 mat
s, which were Earth's most sophisticated form of life for nearly 2 billion years and are still the major factors maintaining life on Earth.
, a late Silurian mass extinction. They also note a positive 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 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.
Burgsvik
Burgsvik ['buschvick] is a locality situated in Gotland Municipality, Gotland County, Sweden with 347 inhabitants in 2005. Burgsvik lies in the southern part of the island of Gotland. The Burgsvik beds are a geological sequence found there.-External links:*...
in the southern part of Gotland
Gotland
Gotland is a county, province, municipality and diocese of Sweden; it is Sweden's largest island and the largest island in the Baltic Sea. At 3,140 square kilometers in area, the region makes up less than one percent of Sweden's total land area...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
. The beds were deposited in the Upper Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
period, around , 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.
Appearance
The bedBed (geology)
In geology a bed is the smallest division of a geologic formation or stratigraphic rock series marked by well-defined divisional planes separating it from layers above and below. A bed is the smallest lithostratigraphic unit, usually ranging in thickness from a centimeter to several meters and...
s consists of thin to very thick layers of a light grey, fine grained argillaceous sandstone, containing a small calc
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
areous 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, oolit
Oolite
Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites...
ic 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 tideTide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
s and storms. Evidence from cross-bedding and ripple marks is taken to imply a subaquaeous origin; rounded oolite pebbles and slightly rounded, size-sorted fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s are evidence of a high-energy environment. The presence of certain species of lamellibranch molluscs suggest a marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
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
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
barriers from wave action. Features that only form on sub-aerial ground, including erosion channels, pothole-like excavations, mud cracks and dendritic rill
Rill
A Rill can be a:*1.) natural fluvial topographic feature;*2.) functional constructed channel to carry a water supply from a water source some distance away;*3.) aesthetic garden water feature.-Natural:...
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
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
(Torsvik et al.. 1993). Combined with the high temperatures of the Silurian, this may have led to very hot, hypersaline waters.
Sedimentology
Long (1993) recognises three lithofacies in the Burgsvik beds; a poorly exposed silty/sandy mudstone faciesFacies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
dominant in the lower beds, appearing as interbeds in the middle beds; a fine to very fine sandstone; and a "biofacies" consisting of ooids, oncolite
Oncolite
Oncolites are sedimentary structures formed out of oncoids, which are layered spherical growth structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but instead of forming columns they form approximately spherical structures...
s and bioclast
Bioclast
Bioclasts are skeletal fragments of marine or land organisms that are found in sedimentary rocks laid down in a marine environment—especially limestone varieties, some of which take on distinct textures and coloration from their predominate bioclasts—that geologists, archaeologists and...
s. 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
Hummocky cross-stratification
Hummocky cross-stratification is a type of sedimentary structure found in sandstones. It is a form of cross-bedding usually formed by the action of large storms, such as hurricanes. It takes the form of a series of "smile"-like shapes, crosscutting each other. It is only formed at a depth of...
, 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 boreholeBorehole
A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...
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
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
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
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
and terrigenous
Terrigenous
In oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, that are derived from terrestrial environments. Consisting of sand, mud, and silt carried to sea by rivers, their composition is usually related to their source rocks; deposition of these sediments...
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
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
data available; the beds are in the Ozarkodina snajdri conodont subdivision of the Pseudomonoclimacis latilobus graptolite
Graptolite
Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...
zone, which is also well displayed in, for example, Estonia (Jeppsson et al. 1994, Jeppsson & Männik 1993).
Palæontological interest
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
(Sherwood-Pike and Gray 1985). Dissolution of the rocks in hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
leaves the organic matter unscathed, and putative fungi (Ornatifilum
Ornatifilum
Ornatifilum is an artificial form genus, which is used to categorise any small, branched filaments with external ornamentation. It has been applied to microfossils of Devonian age with possible fungal affinities; two "species" have been described, and further Silurian fossils closely resemble it...
) and fæcal pellets have been unearthed (Sherwood-Pike and Gray 1985), as well as supposed euglenid
Euglenid
Euglenoids are one of the best-known groups of flagellates, commonly found in freshwater especially when it is rich in organic materials, with a few marine and endosymbiotic members. Most euglenids are unicellular. Many euglenids have chloroplasts and produce energy through photosynthesis, but...
s (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 mat
Microbial mat
A microbial mat is a multi-layered sheet of micro-organisms, mainly bacteria and archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces but a few survive in deserts. They colonize environments ranging in temperature from –40°C to +120°C...
s, which were Earth's most sophisticated form of life for nearly 2 billion years and are still the major factors maintaining life on Earth.
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 eventLau event
The Lau event was the last of three relatively minor mass extinctions during the Silurian period, having a major effect on the conodont fauna...
, a late Silurian mass extinction. They also note a positive 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 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.