Folkeslunda Limestone

Ordovician sedimentary rocks are exposed in patches across the southern half of Sweden. The northernmost exposure is surrounding the Siljan Ring, a Devonian impact crater in Dalarna. Other outcrops are found west and south of Örebro, northeast of Livköping, west and southwest of Skövda and surrounding Falköping. Underlying the Granby crater, west of Linköping, the Ordovician has a larger exposure. The southernmost Ordovician outcrops are situated in Skåne, due east of Lund and at the southeastern tip of Sweden.[1]

Most of the island of Öland consists of Ordovician sediments, where Folkeslunda is located, the namesake for the formation.[2] Previous names for the formation were Upper Grey Orthoceratite Limestone, Chiron Limestone, Centaurus Limestone and Schroeteri Limestone (lower part). The type section is located in the southeastern part of the parish of Långlöt and the northeastern part of the parish of Runsten on Öland. The best section of the unit on Öland is in a quarry close to the road from Vedby to Bäcklunda, east of Hornsjön.[3]

The formation is the uppermost unit in the traditional Orthoceras Limestone of Sweden, overlying the Seby Limestone and the Folkesunda Limestone is overlain by the Furudal Limestone.[4] The Folkeslunda Limestone dates to the Lasnamägi stage, part of the Purtse, belonging to the Virunian in the regional stratigraphy of Sweden, corresponding to a late Darriwilian age of the Middle Ordovician.[5]

The unit is 2 metres (6.6 ft) thick in the Vikarby section and 2.6 metres (8.5 ft) thick in the Kårgärde section of the western Siljan Ring.[6] In the Lunne section of Jämtland the unit is between 0.99 and 3.51 metres (3.2 and 11.5 ft) thick.[3][7]

Eustatic sea level of the Ordovician

The base of the formation is formed by a 15 centimetres (5.9 in) thick grey calcarenitic limestone bed, which is overlain by thin-bedded calcarenitic and calcilutitic limestones. The upper half of the formation consists of thick-bedded calcarenitic limestones. In Kårgärde, the formation comprises medium-bedded, coarse-grained, grey to dark grey limestones with chamositic grains in some beds.[8] Other parts of the formation contain finely nodular lime mudstones and mudstones,[9] and thick-bedded, coarse grained, in part coquinoid grey to dark grey limestones.

The depositional environment of the limestone has been interpreted as reef platform facies.[10] The eustatic sea level change globally during deposition of the Folkeslunda Limestone was a transgression,[11] and based on the low implosion rate of fossil shells during fossilization, the water depth at time of deposition of the Folkeslunda Limestone has been estimated at 150 to 200 metres (490 to 660 ft).[12]

Erratic boulders belonging to the formation have been found in Mecklenburg-Vorpommern,[13] Germany and Lubusz and Kuyavia-Pomerania, Poland.[14][15] The German boulder was described by Schlotheim (1820) and Boll (1857). Boll's collection is fortunately preserved at the regional Müritz Museum of Waren, Mecklenburg. Dzik (1984) described and illustrated Ordovician nautiloids of the erratic boulders in Poland.[16]

With respect to both fauna and lithology, the unit in the Siljan District resembles its development on Öland. The fauna shares many common species with the Seby Limestone, but is also distinct with respect to the cephalopods and hyolithids. Faunal differences with respect to the underlying formation were notes in the phosphatic brachiopod fauna, although the dominant species is the same as in the Seby Limestone. Trilobites from the Folkeslunda Limestone were described by Holm (1883) and Törnquist (1884), while Janusson (1957) described ostracods. Species of nautiloid cephalopods were reported by Holm in 1896, Troedsson (1931, 1932), and Kröger (2004). Biostratigraphically, the Folkeslunda Limestone spans the Pygodus serra conodont zone and the Eoplacoghnathus reclinatus subzone.[3]

Fossil content

Interpretation of Orthoceras

Paleogeography of the late Middle Ordovician. The Folkeslunda Limestone was deposited around 30°S

The following fossils have been found in the rocks belonging to the formation:

Group	Genus	Species	Location	Status	Notes
Cephalopods	Archigeisonoceras	A. folkeslundense	Öland	In situ	[18]
	Arionoceras	A. lotskirkense	Löts Kyrka, Öland	In situ	[19]
			Mecklenburg-Vorpommern	Erratic block	[20]
	Bactroceras	B. angustisiphonatum	Mecklenburg-Vorpommern	Erratic block	[13]
	Cyrtendoceras	C. hircus	Öland	In situ	[21]
	Nilssonoceras	N. latisiphonatum	Öland	In situ	[22]
	Orthoceras	O. bifoveatum	Öland	In situ	[23]
		O. regulare	Öland	In situ	[24]
		O. scabridum	Södra Bäck, Öland	In situ	[25][26][27]
	Oonoceras	O. priscum	Vistula River, Bydgoszcz	Erratic block	[15]
	Phthanoncoceras	P. ellipticum	Lerkaka, Kalmar	In situ	[28]
			Vistula River, Bydgoszcz	Erratic block	[15]
			Żary, Lower Silesia		[14]
	Plagiostomoceras	P. fragile	Böda, Stora Mossen, Öland	In situ	[29]
	Polygrammoceras	P. oelandicum	Folkeslunda, Öland	In situ	[30][31]
	Richardsonoceras	R. gerhardi	Lerkaka, Kalmar		[28]
	Troedssonella	T. endoceroides	Öland	In situ	[32]
	Clinoceras	sp.	Vistula River, Bydgoszcz	Erratic block	[15]
	Cochlioceras	sp.	Sjöstorp, Öland	In situ	[33][34]
			Vistula River, Bydgoszcz	Erratic block	[15]
			Żary, Lower Silesia		[14]
	Lituites	sp.	Kårgärde, Dalarna	In situ	[35]
Trilobites	Illaenus	I. chiron	Kårgärde, Dalarna	In situ	[36]
	Pseudoasaphus	P. aciculatus
	Pseudobasilicus	P. ?brachyrachis
	Plectasaphus	P. plicicostis			[35]
	Pseudomegalaspis	P. patagiata			[37]
	Ampyx (Cnemidopyge)	sp.
	Asaphus	sp.	Żary, Lower Silesia	Erratic block	[14]
	?Lonchodomas	sp.	Żary, Lower Silesia	Erratic block	[14]
	Nileus	sp.	Kårgärde, Dalarna	In situ	[37]
	Remopleurides	sp.	Östergötland	In situ	[38]
			Kårgärde, Dalarna		[35]
Gastropods	Eccyliopterus	E. alatus	Lerkaka, Kalmar	In situ	[28]
		E. declivis
	Pararaphistoma	P. vaginati
	Sinuites	S. vetustus
Ostracods	Euprimites	E. bursa	Kårgärde, Dalarna	In situ	[37]
		E. effusus
	Steusloffia	S. linnarssoni
	Laccochilina	sp.	Östergötland		[38]
			Kårgärde, Dalarna		[37]
	Sigmobolbina		Östergötland		[38]
Lingulata	Biernatia	B. holmi	Kårgärde, Dalarna	In situ	[35][36][37]
	Conotreta	C. mica
		C. siljanensis
	Eoconulus	E. cf. clivosus
		E. robustus
	Hisingerella	H. unguicula
	Myotreta	M. dalecarlica
	Rowellella	R. cf. lamellosa
	Scaphelasma	S. mica
	Spinilingula	S. radiolamellosa
	Talasotreta	T. deformis
	Cyrtonotreta	sp.
	Paterula	sp.
	Spondylotreta	sp.
	Lingulellinae	indet.
	Orbiculoideinae	indet.
Rhynchonellata	Nicolella	sp.	Östergötland	In situ	[38]
Strophomenata	Christiania	sp.	Kårgärde, Dalarna	In situ	[35]

• B. Kröger, Y. Zhang, and M. Isakar. 2009. Discosorids and Oncocerids (Cephalopoda) of the Middle Ordovician Kunda and Aseri Regional Stages of Baltoscandia and the early evolution of these groups. Geobios 42:273-293
• D. H. Evans. 2005. The Lower and Middle Ordovician cephalopod faunas of England and Wales. Monograph of the Palaeontographical Society 623:1-81
• J. Dzik and G. N. Kiselev. 1995. The Baltic nautiloids Cyrtoceras ellipticum Lossen 1860, C. priscum Eichwald 1861, and Orthoceras damesi Krause 1877. Palaeontologische Zeitschrift 69(1/2):61-71
• V. Jaanusson. 1963. Lower and Middle Viruan (Middle Ordovician) of the Siljan district. Bulletin of Geological Institutions of the University of Uppsala 42(3):1-40
• V. Jaanusson. 1962. The Lower and Middle Viruan sequence in two borings in Östergotland, central Sweden. Bulletin of Geological Institutions of the University of Uppsala 38(9):1-30
• V. Jaanusson. 1960. The Viruan (Middle Ordovician) of Öland. Bulletin of the Geological Institutions of the University of Uppsala 38:207-287