NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 101

New data on the Bruflat Formation and the Llandovery/ Wenlock transition in the Oslo Region, Norway.

David Worsley, B. Gudveig Baarli, Michael P.A. Howe, Frodi Hjaltason and Dagfinn­ Alm

Worsley, D., Baarli, B. G., Howe, M. P. A., Hjaltason, F. & Alm, D.: New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region, Norway Norsk Geologisk Tidsskrift, Vol. 91, pp. 101-120. Oslo 2011. ISSN 029-196X.

The Bruflat Formation of the Oslo Region is a siliciclastic unit of Early Silurian (Telychian) age consisting of fine, turbiditic sand- and siltstones alternating with shales. Five different benthic assemblages are recognized in the formation, revealing a wide depth range from outer shelf BA 5 to nearshore BA 1. Analyses of benthic assemblages from the Bruflat and under- and overlying formations have been combined with new bio­ stratigraphical data to produce a series of palaeobathymetric maps. These show an advancing foreland basin that apparently followed the shape of the present Caledonian front. Parts of the Bruflat Formation and possibly also the Ekeberg Greywacke Formation of Jämtland in Sweden were deposited in the outer to the middle sections of several small sandy submarine fans on the slopes of the foreland basin. Before the first effects of the Scandian phase of the Caledonian orogeny recorded in the strata of the Oslo Region came to a temporary halt, progradation and a eustatic drop in sea-level brought siliciclastic deposition from deep water up to the shoreface, where sediments were reworked. Further eustatic shallowing led to the development of a hiatus between the Bruflat Formation and the overlying Braksøya Formation while a carbonate shoal developed to the southeast.

D. Worsley, Fergestadveien 11, 3475 Sætre, Norway; B. G. Baarli, Department of Geosciences, Williams College, 947 Main Street, Williamstown, MA 01267, USA; M.P.A. Howe, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK; F. Hjaltason, Statoil ASA, Oslo, Norway, D. Alm, Sta­ toil ASA, Harstad, Norway.

Introduction of the basin migrated eastwards during deposition of the formation. In the far central and eastern parts of the region calcareous shales and nodular limestones of The Silurian of the Oslo Region has been studied for the upper Vik Formation developed. Baarli et al. (2003), over 100 years since the epic description of Kiær (1908), who investigated bathymetric conditions throughout the but there are such rapid and complex facies changes entire Silurian succession in the Oslo Region, explained over short distances that many of the discontinuous and this as reflecting the development of a foredeep basin in tecton­ised sections remain to be studied and correlated front of the Caledonian orogenic front. Exactly how the in detail. A general overview by Worsley et al. (1983a) Bruflat Formation fitted into this foreland basin was not presented the present lithostratigraphical scheme for the considered at that stage. marine Silurian succession and corrected several mis­ understandings regarding ages of the various formations. This paper explores the origin of the Bruflat Formation.­ A later brief survey of the Late Llandovery (Telychian) New exposures of the formation from the central Modum to lowermost Wenlock successions from the entire Oslo District that occur between the northern and central dis­ Region by Cocks & Worsley (1993) made it clear that the tricts (along Route 285 in Sylling, NM7340) have proved Bruflat Formation in the northern and western districts most helpful, since they show a distal transitional­ devel­ merited closer attention. opment between the other areas. Our aim is to investigate the bathymetric relationships by Benthic Assemblage The formation is a relatively coarse siliciclastic unit analyses throughout the formation in order to test the occurring within the carbonate and shale- succession hypothesis of a foreland basin. As the form­ation has few that otherwise dominates the region. The formation was age-diagnostic fossils, we have also tried to attain better earlier thought to have been deposited in deep water, stratigraphical resolution of these deposits by including but Cocks & Worsley (1993) showed that fossil com­ new material and updating older stratigraphical ­data to a munities suggested deposition over a wide depth range. modern standard. We also apply available sedimentologi­ There were deep conditions with deposition of graptolite cal data to propose a better deposition­al model for the shale to the north prior to the deposition of the upwards-­ Bruflat Formation. shallowing Bruflat Formation, while the deepest parts 102 D. Worsley et al NORWEGIAN JOURNAL OF GEOLOGY

Geological setting the shallowest facies to the north and west and deeper facies to the southeast in the Oslo Region. During the The Silurian successions of the Oslo Region (Norway) early Llandovery, before the Scandian phase onset, sedi­ and Jämtland (Sweden) were deposited in an epiconti­ ment provenance areas were mainly to the west. In Had­ nental sea on the western periphery of the Baltic craton eland and Jämtland, however, there was sediment influx along the developing Caledonian front. The Scandian from the east, indicating that the northern parts of the phase of the Caledonian orogeny to the NW impacted the depositional basin were fairly narrow and elongate at that northern districts the most, while the southernmost dis­ time (Heath & Owen 1991; Baarli et al. 2003; Dahlquist tricts were left relatively undisturbed, although they are & Calner 2004). now regarded as parautochthonous (Fig. 1). The entire Oslo Region might thus have been tectonically shortened Baarli (1990) postulated that prior to the deposition of by a third and Figure 1 shows the assumed original posi­ the Bruflat Formation, a peripheral bulge connected with tion of localities during Llandovery time. This resulted in a foreland basin affected the Oslo Region. Garfunkel & the development of the present arcuate facies belts, with Greiling (1998) discussed the relatively modest sedi­ ment wedge deposited to the east of the Caledonian front both in Jämtland and the Oslo Region. They claimed Jämtland that there was little evidence of a foreland basin and 100 km explained the lack of sediment by modeling the develop­ ment of a thin upwards concave Caledonian orogenic wedge above a foreland lithospheric plate that was too thick to accommodate much depression. The presence and size of a foreland has been much discussed in recent NORWAY years, some workers (e.g. Karlson et al. 1999) arguing Ringsaker that fission track studies indicate a wide foreland basin

Toten filled with >2.5 km of sediment in southern and western Hadeland Sweden, while Hendriks & Redfield (2005) used similar evidence to suggest only a minor foreland basin with thin Ringerike sediment fill. The latter view agrees with that of Baarli et Modum Malmøya Asker al. (2003), who suggested a narrow, long and relatively Holmestrand shallow foreland basin developed in a nearly north­south direction during Telychian time, and in the western part Skien Porsgrunn of which the Bruflat Formation was deposited. Figure 1: Map of study area. Silurian outcrops are shown in black. The key areas are shown in their present day The Bruflat Formation is developed in the northern position with open circles and in projectedFig. 1 W originalorsley position et al. and western Oslo Region districts of Ringsaker, Toten, assuming a subsequent tectonic shortening by one third. The Hade land, Ringerike and Modum (Fig. 2). It overlies present­day Caledonian front is marked.

A B C D Jämtland Ringsaker Toten Hadeland Ringerike Modum Sandvika Malmøya Holmestrand Skien Porsgrunn W e n 11 Braksøya l Monograptus Braksøya riccartonensis >75m o 3 E. angelini Braksøya E. angelini 80m E. angelini

A11 c 10 6 2 C Malmøya k Cyrtograptus murchisoni Ps. bicornis P o Röde Braksøya ? 9 e s E. sulcata A10 Skinnerbukta n t >65m A6-10 Cyrtograptus centrifugus 5 Skinnerbukta 4 m A9 E. sulcata t r 30m A8- ~190m 21 m a i 182m 8 m D3 A6-10 55-60 m 8 s Wenloch A6-9 A8-9? PF120 m D3, B5 Cyrtograptus insectus P.a.amorpho- m 51 m P P e t F PF 85m B5 P 80m FP gnathoides Ekeberg Porsgrunn F r r Bruflat 76 m D3 41m PF Porsgrunn 7 o i Greywacke Bruflat F 50-60 m 76m D 3 49-53m B5 E. sulcata Porsgrunn Cyrtograptus lapworthi i c T F Bruflat Bruflat 69m B5 e d k Bruflat A7 e l ? A6 A6? l 4P.a.lithuanicus 20-40m ? 46-49m B4 A6 6 s a ? >35m 95m P 60-70m y Oktavites spiralis P. a.lennarti n 80m L 3 A6 -4 m A6 80m A6 40-45 B3 Vik c d -26 m A6? D3 Vik l 5 2 i Vik h a A5? a i Monoclimacis crenulata- P. a. angulatus Ek/Vik 39m D3 44 m C2 35-40m C2 C2 M. griestoniensis Ek Ek

n a 2 37m B2-4 Bangsåsen Vik Vik 26.7-37.9 B2 10 m C2/D2 d n 1 S 32 m 20 m C2 D2 4 1 Shale C2/C1 Streptograptus satorius . A4 20m A4 o A4 30m C1 Vik Fm Vik C1 80m P. eopennatus P l A3 v 3 e a A3 A3 Monograptus crispus n A3 ? 24 m C1 e e 50m t v 4m C1- D2 r 2 a i 18 m D2 Spirograptus turriculatus A2 y m s 8m 2m D2 Rytteråker Rytteråker e Rytteråker Rytteråker 61m 0m D1 Rytteråker r A1 1 Rytteråker Fm. 89m 99m Spirograptus guerichi u 30m Rytteråker Rytteråker Rytteråker s Rytteråker Figure 2: Biostratigraphic chart for the Telychian and early Wenlock formations in Jämtland and the Oslo Region. A to D give the biostratigraphic range of A: graptolite biozones, B: conodont biozones, C: the Pentamerus to Pentameroides brachiopod lineage and D: parts of the Stricklandia to Costistricklandia lineage. Stars mark the positions of biostratigraphically importantFig. 2 W orslfinds:ey etE= al.Eocoelia, F=Erinostrophia and P=Paleocyclus. The graptolite biozones follow Zalasiewicz et al. (2009), while the conodont biozones are used according to Männik (2007). NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 103 the graptolite-rich shales of the Ek Formation in the Ek Formation at Toten, 4 m below the base of the Bruflat Ringsaker and Toten districts (Fig. 2). The Ek Forma­ Formation, new data from the Modum District, as well as tion grades laterally into the more lime-rich Vik Forma­ an updated detailed correlation scheme of conodonts in tion to the west, with a transitional Ek/Vik formational the Vik Formation from Malmøya and Bærum by Män­ development in Hadeland. The top of the Bruflat For­ nik (pers. comm. 2009) as well as an update of the grap­ mation has not yet been found in the northern districts, tolite data to fit a modern biozonal framework. Map ref­ although a red shaly facies largely devoid of identifiable erences are included herein for localities not previously fossils, and called the Reinsvoll Formation, is found in described in earlier published works, viz. new localities the Toten District; this has rare microcoquinas indicat­ in this paper and localities only previously described in ing a Silurian age. The shallow-water reefoid limestones the thesis of Howe (1982). For more extensive locality of the Braksøya Formation overlie the Bruflat Forma­ references the interested reader is referred to Worsley et tion in the western Ringerike District and are them­ al. (1983) and Cocks & Worsley (1993). selves overlain by carbonates and lagoonal shales of the Steinsfjorden Formation (Worsley et al 1983a). Moving Graptolites southwards, the Modum District exhibits a distal devel­ opment of the Bruflat Formation, followed by a thin suc­ Graptolites are restricted to a very few horizons within cession of shales which we assign to the Skinnerbukta the Bruflat Formation and these faunas generally consist Formation. This occurs below exposures showing typical of long-ranging taxa of little stratigraphical significance. shallow marine carbonates of the Braksøya Formation. In strong contrast, some of the underlying and adjacent Parts of the Vik Formation form an older and synchro­ formations contain datable faunas of well-preserved and nous facies to the Bruflat Formation in the central Oslo stratigraphically well-constrained graptolites, providing Region, where they are overlain by the graptolite-rich significant evidence regarding the age of the Bruflat For­ Skinnerbukta Formation. Lateral facies developments in mation. the southernmost districts of Skien and Porsgrunn were assigned to the Vik Formation and to the overlying non- Kiær (1908) was the first to define graptolite zones sys­ graptolitic shales of the Porsgrunn Formation by Cocks tematically within the underlying Ek Formation of the & Worsley (1993). Mjøsa District. Based primarily on exposures on Eksber­ get, Helgøya, he recognised: Similar lithofacies to those in the Bruflat Formation are found in the Ekeberg Greywacke Formation of the Jämt­ • Zone with Monograptus turriculatus and Petalograptus land area, about 500 km to the north of the Oslo Region. palmeus. 3 – 4 m. (7cα of his notation) Little work has been done in this area, partly because of the strong degree of deformation and scarcity of out­ • Zone with Monograptus discus 16 – 17 m. (7cβ of his crops. However, the Telychian section there shows a notation) somewhat similar shallowing-upwards development to the northern districts of the Oslo Region in that the • Zone with Monograptus spiralis subconicus and Retio­ underlying Bångsåsen shales contain a rich graptolite lites geinitzianus. ca. 35 m. (7cγ of his notation) fauna, while the overlying Röde Formation displays a lithofacies suggesting a fluvial origin (Bassett et al. 1982; • Highest beds of calacarous shale with carbonate Dahlqvist & Bergström 2005). nodules­, approximately 30 m.? (7cδ of his notation)

Kiær (1908) was unaware of the presence of similar Biostratigraphy grapto­lite faunas in Hadeland, presumably due to the lack of suitable outcrops before the railway was complet­ed. The Bruflat Formation is not especially fossiliferous, The first record was that of Hagemann (1966). although brachiopods are locally abundant. There are scattered occurrences of biostratigraphically important Howe (1982) re-collected from all available Silurian taxa such as graptolites and conodonts and only sporadic grapto­lite localities in the Mjøsa, Hadeland, Ringe­ representatives of key brachiopod evolutionary lineages rike and central Oslo districts, and re-examined all are also found. Thus, correlation relies heavily on more the reference ­material in the Paleontologisk Museum, precise dating of the underlying, overlying and laterally University­ of Oslo. The results were then interpreted correlative formations. using the graptolite zonation system of Rickards (1976), although a number of difficulties were noted. Subsequent­ All available biostratigraphic information on the Bru­ refinement of the graptolite zonal scheme (e.g. Loy­ flat Formation and its contiguous formations is plot­ dell & Cave 1993; Loydell & Cave 1996 and Zalasiewicz ted in Figure 2. This information is gleaned from Wors­ et al. 2009) has provided a more workable system for ley et al. (1983a), Howe (1982), Baarli & Johnson (1988), these horizons. The graptolite zones recognised by Howe Cocks & Worsley (1993) and Baarli et al. (2003). Data (1982) are herein revised in the light of Zalasiewicz et al. added in this paper include a new graptolite find in the (2009). No attempt will be made to interpret the results 104 D. Worsley et al NORWEGIAN JOURNAL OF GEOLOGY beyond individual graptolite zones; the possible recogni­ and Oktavites spiralis. The fauna indicates the spiralis tion of subzones will be deferred, pending the results of Biozone, with the abundance of M. parapriodon suggest­ an ongoing revision of the taxonomy of certain key spe­ ing its lower part. Similar faunas have been recorded cies. elsewhere in Toten (e.g. railway cuttings north of Eina & Bruflat), on Helgøya, and in Brumunddalen (cliff north In the cliff south of Torsæter Bridge, Brumunddalen, Spi­ of Torsæter Bridge and Grini roadcut, near Byflaten). rograptus turriculatus, Cochlograptus veles (= Monograp­ The Grini road cut has the most diverse fauna (some 21 tus discus) and ‘Monograptus’ crispus (plus four other graptolite taxa), including, in addition to the above: Reti­ taxa) collected 1.0 m above the base of the Ek Formation olites angustidens, Barrandeograptus bornholmensis and indicate the crispus Biozone. Streptograptus sartorius Monograptus cultellus. The latter species is particularly was collected 4.6 m above the base of the Ek Formation, characteristic of the spiralis Biozone. marking the base of the sartorius Biozone. There are two localities at Toten where graptolites On Helgøya, the lowest graptolite fauna in the Ek have been collected from the Ek Formation at a known Formation­ includes Streptograptus sartorius, indicating distance­ below sandstone beds of the basal Bruflat Form­ the sartorius Biozone and suggesting that the base of the ation. In the railway cutting north of Eina, a spiralis Ek Formation is slightly younger than in Brumunddalen. Biozone fauna has been collected from ca. 4 m below the basal Bruflat Formation. In the railway cutting north of Identifiable graptolite faunas have not yet been collect­ed Bruflat, there are several graptolitic horizons with a simi­ close to or from known heights above the base of the lar fauna in the top 10 m of the Ek Formation. However, Ek Formation in Toten. However, a temporary sec­ there is a single record of ?Cyrtograptus lapworthi from tion exposed during the construction of a barn at Stor ca. 4.5 m below the presumed base of the Bruflat Forma­ Kyset during 1977, exposed some 8 m of Ek Formation. tion, so it is possible that the base of the formation might ‘Monograptus’ crispus and Cochlograptus veles (plus 7 be of lapworthi Zone age in some places. other taxa) were recorded from 0.6 m above the base of the exposure, indicating the crispus Biozone. The incom­ Records of graptolites from the Bruflat Formation itself ing of Streptograptus sartorius 4.4 m above the base of the are rare. The railway cutting north of Eina includes a exposure, indicates the sartorius Biozone. typical spiralis Biozone fauna in a graptolitic horizon within approximately 4 m of shale between sandstone The base of the Ek Formation in Hadeland is signifi­ units. Bjørlykke (1903) recorded Monograptus priodon, cantly older than in the Mjøsa district. The formation Monograptus crenulatus [= Monoclimacis vomerina] and exposed in the railway cutting known as “Storskjæringa” Retiolites angustidens from exposures now assigned to [= large cutting], near Netberg, Hadeland (Hagemann, the Bruflat Formation near Mariendal (Brumunddalen). 1966) includes four graptolitic bands, numbered I – IV This association is not particularly diagnostic and could in ascending order. Band IV includes Cochlograptus veles range from the spiralis to at least the centrifugus Biozone. and Streptograptus sartorius (plus 6 other taxa), indicat­ ing the sartorius Biozone. Band III includes Cochlograp­ An exception to this general rarity is found in the Bru­ tus veles and ‘Monograptus’ crispus (plus 8 other taxa), flat Formation of Ringerike, where abundant graptolites but no Streptograptus Sartorius, which is otherwise typ­ were recorded by Bassett and Rickards (1971) from occa­ ical of the underlying crispus Biozone. Band II con­ sional bedding planes, particularly in the lower part of tains Spirograptus guerichi, Streptograptus plumosus and the formation by the lakeshore between Vik & Kroksund: Monograptus marri, indicating the guerichi Biozone. M. priodon, Mcl. sp., Mcl. vomerina vikensis, Retiolites Band I includes Streptograptus plumosus, Monograptus sp. (?geinitziatius), Pristiograptus sp., “robust cyrtograp­ marri and Pristiograptus sp. which is also likely to be the tid” (C. ex gr. centrifugus). This fauna was assigned to the guerichi Biozone. pre-riccartonensis Biozone, Wenlock, largely on the basis of the cyrtograptid specimen, which was collected by The top of the Ek Formation is generally poorly exposed, Whitaker (1966) from a different locality to most of the but there are a number of outcrops that must be close to material, but apparently within the same part of the unit. the base of the Bruflat Formation. Band VI, which is , Unfortunately, the specimen has been mislaid, but Dr. found in the railway cutting at Gulla (NM7884) in Hade­ R.B. Rickards’ original comments were quoted by Whita­ land, has a spiralis Biozone fauna, including Monograptus ker (1966): “Cyrtograptus aff. insectus Bouček. The nature priodon, M. parapriodon, Oktavites spiralis, Monoclima­ of the rhabdosome and proximal spiral coiling place it in cis vomerina, Pristiograptus sp. and Retiolites angustidens. the murchisoni—centrifugus— insectus complex, and the considerable length without cladia, and one possible cla­ The new locality in Toten, a temporary road cut north of dium distally, suggest insectus. The degree of proximal Reinsvoll (NM877304), has yielded the following grapto­ coiling in insectus is however, supposed to be less than lite taxa from 4.5 m below the base of the Bruflat For­ this. The proximal coiling is most impressive.” A speci­ mation: Monograptus priodon, Monograptus parapri­ men of C. insectus without cladia, and with very strong odon, Monoclimacis vomerina, Monoclimacis cf. geinitzi, proximal coiling could be almost indistinguishable from NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 105

M. spiralis, in which case, the fauna would be of spiralis Bassett and Rickards (1971) commented on the age of the Zone age. If the specimen is of C. insectus, then an insec­ beds (8a – 8b of Kiær, 1908) now assigned to the Skinner­ tus Zone age would be appropriate. bukta Formation on Malmøya. They identified the follow­ing taxa from newly collected material from the The Porsgrunn Formation has yielded graptolites from lowest beds (“8aα”): Monoclimacis vomerina vomerina, two localities (Cocks & Worsley, 1993). They reported Mcl. vomerina vikensis, Cyrtograptus ex gr. centrifugus, Monograptus priodon/?M. parapriodon from their ‘Local­ Monograptus flexuosus and M. priodon. They suggested ity 2’(shore exposure, Kommersøya) and suggested it these were of centrifugus Zone age, and that the over­ “indicated a crenulata Zone age”. Whilst abundant M. lying (“8aβ”) beds were of murchisoni Zone age, based on parapriodon is characteristic of the spiralis Biozone Kiær’s collections in the Paleontologisk Museum, Oslo. (which would be included in the crenulata Biozone They also suggested that the overlying “8b” beds, tradi­ (sensu Cocks & Worsley, 1993), M. priodon is much lon­ tionally considered as of the riccartonensis Biozone age ger ranging. A better interpretation of the find would be (Kiær, 1908), extended up into the middle Wenlock. This tosuggest a possible spiralis Biozone age for it. They also was based on the identification of Monoclimacis flumen­ reported “Monoclimacis (long) & cyrtograptid arm” from dosae and Monograptus flemingii on a single museum their locality 3 (1.2 m above ‘Locality 2’). This suggests a slab. However, recent work (Zalasiewicz et al. 2009) sug­ possible lapworthi Biozone age. gests this association could be of upper riccartonensis or dubius Biozone age. Graptolites from the Skinnerbukta Formation demon­ strate marked diachroneity between the Bærum District ­ Conodonts in the west and Malmøya (Oslo District) in the east. Bassett­ and Rickards (1971) identified the following Conodonts are common in the Vik Formation of the cen­ species ­in Kiær’s collections from Bærum, held in the tral Oslo Region, but rare in the Ringerike and Modum Paleontologisk Museum, Oslo: Monoclimacis vomerina districts both in the Vik and Bruflat formations. Nakrem s.l., Mcl. vomerina vikensis, Mcl. griestoniensis s.l., Retio­ (1986) studied the conodont fauna in the Vik Formation lites geinitzi­anus geinitzianus, Monograptus priodon and of the central Oslo Region. Männik (pers. comm. 2009) Pristio­graptus aff. watneyae. They noted that this assem­ re-examined these collections and subdivided and rede­ blage could be highest Llandovery or basal Wenlock. fined the Pterospathodus celloni and overlying P. amor­ They also recorded the discovery of a single specimen of phognathoides conodont zones according to Männik Monoclimacis cf. linnarssoni from a higher horizon in the (2007). Skinnerbukta Formation, and noted that Mcl. linnarssoni was a high Llandovery to lowest Wenlock species, not The only occurrence of conodonts in the Bruflat known with certainty above the centrifugus Zone. Cocks Formation­ is a fauna representative of the Pterospatho­ & Worsley (1993) reported additional graptolites from dus amorphognathoides zone in the uppermost 30 m of three temporary sections in the Skinnerbukta Formation the formation ­at Ringerike, where the base of the zone is of the Bærum District: missing (Worsley et al. 1983b). Kiipli et al. (2001) stud­ ied volcanic ash beds in the middle Garntangen Member • “Locality 26” – Monoclimacis vomerina vomerina, Mcl. of the Vik Formation and used the overlying conodont v. aff. basilica, Monograptus cf. priodon occurrence in the Bruflat Formation to infer that the tuffs could be correlated with the Pterospathodos amor­ • “Locality 27” – Retiolites genitzianus angustidens, Pris­ phognathoides lithuanicus conodont zone because the tiograptus cf. watneyae oldest P. amorphognathoides identified by Nakrem (1986) at Malmøya in the Oslo District was identified by Kiipli • “Locality 33” – Monograptus priodon; Monoclimacis (2001) as Pterospathodos amorphognathoides lithuanicus. vomerina basilica However, the ash beds occur well below these conodont occurrences and close to the Pentamerus/Pentameroides transition in Ringerike. That same brachiopod lineage They interpreted the faunas from localities 26 and 33 transition occurs within or slightly below the P. a. angu­ as “indicating centrifugus or low murchisoni Zone age” latus conodont zone in Malmøya (Fig. 2), making the and from 27 as “probably indicating centrifugus Zone Garntangen Member at Ringerike older than assumed by a g e”. Monoclimacis basilica is recorded in Britain from Kiipli et al. (2001). the middle of the lapworthi Biozone to the riccartonensis­ Biozone, Retiolites angustidens from the griestoniensis­ Brachiopods Biozone to the lower murchisoni Biozone and Mono­ climacis linnarssoni from the spiralis to the lower insectus ­ In Telychian successions in Norway and also in the type Biozone (Zalasiewicz et al. 2009). In the absence of more section in the Welsh Llandovery (Cherns et al. 2006), diagnostic taxa, all that can be suggested is that the brachiopods belonging to the Pentamerus/Pentamero­ Skinner­bukta Formation in Bærum is likely to fall within ides, Stricklandia/Costistricklandia and Eocoelia lineages the range of spiralis to low murchisoni biozones. have taken on prime importance for correlation because 106 D. Worsley et al NORWEGIAN JOURNAL OF GEOLOGY of the relative scarcity of conodont and graptolite data. the Costistricklandia find of Baarli & Johnson (1982, Pl. An overview of these lineages was presented by Bassett 1, figs. 12-14) close to the Pentamerus/Pentameroides and Rong (2002, p. 126), but new information from the transition in the middle Vik Formation in their Figure present study validates an adjustment of some of these 3, while Stricklandia as shown in Figure 5 of Cocks and lineage boundaries following earlier work by Baarli & Worsley (1993) would require a co-occurrence of S. lae­ Johnson (1988), Cocks & Worsley (1993) and Jin (2002). vis and Costistricklandi lirata. Further, the only known occurrence of stricklandiids in the Vik Formation of the Pentamerus/Pentameroides. Baerum section is found 4 m below the top of the forma­ tion and designated by Baarli (1986) as Costistricklandia The transition between Pentamerus and Pentamero­ides lirata, 3 m above the first occurrence of the Pterospath­ was set in the Spirograptus turriculatus graptolite Biozone odus amorphognathoides conodont Biozone or the lower by Bassett & Rong (2002). Baarli & Johnson (1988) set Cyrtograptus lapworthi graptolite Biozone. The record of the same transition a little up in the Monograptus grie­ S. laevis in the Malmøya area by Cocks & Worsley (1993) stoniensis graptolite Biozone, while Cocks & Worsley is correct, although it occurs in the lower part of the (1993) set it at the base of the same zone and this was fol­ Vik Formation, not in the upper part. It is found there lowed and partially corroborated by Jin (2002). The last together with Pentamerus oblongus and below the first occurrence of Pentamerus at Malmøya in the Oslo Dis­ occurrence of Pentameroides cf. subrectus. trict occurs at 24 m and the first Pentameroides is found between 35 m and 40 m above the base of the Vik For­ The only other known occurrence of Costistricklandia mation (Fig. 2 and Baarli & Johnson, 1988). Reinterpre­ lirata in the Bruflat Formation at Ringerike was men­ tation of conodont data by Männik (pers. comm. 2009) tioned in Baarli & Johnson (1988), where it was located shows the conodont Biozone Pterospathodus amorphog­ near the very top of the formation. Our present study nathoides angulatus from 27.7 m to 37.9 m above the also recorded Costistricklandia 8 m above the base of the base of the formation, i.e. 2-3 m above the last appear­ Skinnerbukta Formation in Modum (NM7340, Fig. 2) ance of Pentamerus. This conodont zone correlates and it is found near the top of the Porsgrunn Formation with the Monoclimacis crenulata graptolite Biozone and in Skien. Costistricklandia is known to range slightly into slightly into the Oktavites spiralis Biozone (Männik, the Wenlock Cyrtograptus murchisoni graptolite Biozone 2007). This is immediately above the Monoclimacis grie­ in Great Britain (Cherns et al. 2006). stonienis Biozone. There are no signs of any considerable gap in the section, making a M. griestoniensis or a tran­ The Eocoelia lineage. sition to a higher zone likely. For now, we follow Cocks & Worsley (1993) and Jin (2002) and place the transition Eocoelia is not common in the Oslo Region and has not as corresponding with the base of the M. griestoniensis yet been found in the Bruflat Formation. However rep­ graptolite Biozone. resentatives of the lineage, transitional forms between E. sulcata and E.angelini, are present in overlying and late­ Stricklandia/Costistricklandia. rally equivalent formations (Fig. 2). E. sulcata has been shown to occur together with Costistricklandia slightly The transitions from Stricklandia laevis to Costistrick­ above the Llandovery/Wenlock boundary in the C. mur­ landia lirata and from Pentamerus to Pentameroides are chisoni graptolite Biozone in Great Britain (Cherns et al. important stratigraphic markers in the Oslo Region and 2006), which is corroborated by the record from Norway. elsewhere. As noted by Jin (2002), the co-occurrence of Costistricklandia and Pentameroides makes this a use­ Erinostrophia ful concurrent biozone for middle to upper Telychian rocks. However, Cocks & Worsley (1993) redesignated Cocks & Worsley (1993) erected the new genus Erinos­ the stricklandiids found together with Pentameroides in trophia and this is frequently found together with the the Skien District from Costistricklandia (as previously coral Paleocyclus. Both of these were earlier only known determined by Baarli & Johnson, 1988) to S. laevis, thus from the Lower Visby Formation in Gotland and thought making this the first co-occurrence of Pentameroides and to be confined to that formation. However, Hoel (2005) Stricklandia anywhere. Therefore, we have moved the also reported Erinostrophia from the Upper Visby For­ Stricklandia laevis to Costistricklandia lirata transition mation. The base of the Upper Visby Formation is corre­ slightly above the Pentamerus to Pentameroides transi­ lated as being close to the boundary between the C. cen­ tion in the M. griestoniensis zone. trifugus and C. murchisoni graptolite biozones (Jeppson et al. 2006). Erinostrophia alone may therefore not According to Cocks & Worsley (1993, Fig. 5), Strick­ exclude an early Wenlock age. landia appears in the upper parts of the Vik Formation in the Bærum, Malmøya and Ringerike districts, high Corals above the first occurrence of Pentameroides and is cor­ related with the upper M. crenulata zone. This we assume The button coral Paleocyclus together with the brachio­ is a drafting mistake, because the same authors register pod Erinostrophia occur in all districts uppermost in NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 107 the Bruflat Formation and were also found 41 m above The Bruflat Formation is largely argillaceous, where the the formational base at Sylling (NM7340). This possi­ namesakes of traditional Llandovery Clorinda, Strick­ bly indicates a fairly synchronous top to the formation. landia, Pentamerus, Eocoelia and Lingula communi­ Paleocyclus porpita occurs exclusively in the Monograptus ties are not well represented. Cocks and Worsley (1993) crenulatus graptolite Biozone globally (Scrutton & Deng, described several of these more specialized assemblages 2002) except for one record from the Upper Visby Form­ from the Telychian of the Oslo Region, but they pro­ ation possibly M. murchisoni graptolite Biozone, of vided only raw data and a qualitative description of each Gotland (Stel 1978). P. porpita is otherwise very com­ community. Here we incorporate some of their informa­ mon in the Lower Visby Formation, and its last appear­ tion from the Bruflat Formation with the data collected ance is used to define the boundary to the Upper Visby for this paper to obtain a more quantitative descrip­ Formation. tion. Twenty­one 10­kilo bulk samples were collected throughout the Bruflat Formation in the Ringerike Dis­ trict. These samples were augmented by 6 bulk sam­ Faunal assemblages ples from the Modum District and compared with sam­ ples from the Hadeland and Toten districts, as described The Benthic Assemblage zones 0­6 of Boucot (1975) are in Cocks & Worsley (1993). Different authors recorded recognized and widely used in Silurian sections world­ the fossil content from the samples and some of the data wide. In this system zone 0 represents land and BA 1 cor­ are therefore updated. Atrypids are clearly problematic. responds to assemblages once living under nearshore, The samples from Ringerike register only Atrypa reticu­ intertidal conditions. Subsequent assemblage zones are laris. In Modum, Atrypina sp. occurs alone in the lower distributed along a depth gradient stretching to BA6, the parts of the section. In the middle parts, Atrypina gradu­ latter being equated with the deeper parts of the outer ally gives way to Atrypa reticularis. We believe that the shelf. Brett et al. (1993) placed the absolute depth of pattern at Ringerike is the same as at Modum and note Silurian BA 1 to BA 4 between 0 and 40 to 60 m. This that Worsley & Broadhurst (1975) discussed the ecologi­ system was used and further defined by Baarli (1990), cal significance of atrypid communities in Ringerike, Cocks & Worsley (1993) and Baarli et al. (2003) for the but without distinguishing between the different taxa. Telychian of the Oslo Region. Additional new data are A mix of Atrypa and Atrypina sp. was also recorded by added here from Ringerike and from Modum, where Tel­ Cocks & Worsley (1993) in the middle of the Bruflat For­ ychian strata were not previously well known (Fig. 3). mation at Hadeland. There may also be some confusion Baarli (1988) used proximality data to verify the bathy­ around a smooth athyridacean brachiopod that is quite metric record and Baarli et al. (2003) applied this to aug­ common. In Modum and at Ringerike, this is identified ment the record where few fossils are available. Normal as Hindella , with the characteristic long median septum (fair­weather) wave base was assigned to the boundary in the brachial valve and long dental plates. Cocks & between BA2 and BA3, while maximum storm wave base Worsley (1993) registered Hyattidina at Hadeland. was set in BA 4 for the Lower Silurian succession in the Because the exteriors of Hyattidina and Hindella are the Oslo Region. same, we suspect that all should be referred to Hindella.

Jämtland Ringsaker Toten Hadeland Ringerike Modum Sandvika Malmøya Holmestrand Skien Porsgrunn W e n Malmøya Braksøya Braksøya l M. riccartonensis o Röde Braksøya c Malmøya 3 C. murchisoni Fluvial k Skinnerbukta Braksøya Braksøya ? 2 C. centrifugus 6 6? 5 Skinnerbukta 3 4? Skinnerbkt. 6 1 2/3 2 4/5 3 C. insectus 3 4 4 3 4 ? 2 3/2 4 Ekeberg Bruflat Bruflat 3 4 Porsgrunn Bruflat ? 4 Bruflat 3 Porsgrunn Porsgrunn T Greywacke Bruflat 4 5 5 C. lapworthi 3 5 4/5 e ? ? 4 5 l 4/5 ? ? 4 5 5 4/5 5 L y O. spiralis 5 6 6/5 5 c 4 4 Vik l h Vik M. crenulatus 5 Vik Vik Vik 3 Vik a i Ek/Vik 3 ? M. griestoniensis Ek Ek 5 n a 6 4 3 d n Vik 3/4 4 6 6 4 o S. satorius Bangsåsen 6 5 v 4 Shale ? e M. crispus ? 4 4 r 5 Rytteråker y 6 Rytteråker S. turriculatus Rytteråker 5 4/5 Rytteråker Rytteråker Rytteråker 6 Rytteråker Rytteråker Rytteråker S. guerichi Rytteråker Figure 3: Stratigraphical chart for the Telychian to Lower Wenlock of the Oslo Region with Benthic Assemblages drawn in. Fig. 2 Worsley et al. The three grey­shaded lines in the Oktavites spiralis, Cyrtograptus Lapworthi and C. insectus graptolite biozones represent the time zones for the bathymetric maps of figures 4, 5 and 6, respectively. 108 D. Worsley et al NORWEGIAN JOURNAL OF GEOLOGY

There may also have been some lumping of enteletacean The trilobite Phacops sp. and bryozoan colonies are also brachiopods. At Modum Resserella is the most common common, as they are in the overlying assemblages. taxon, but Isorthis sp. is also recorded in the upper parts of the section - as Cocks & Worsley (op. cit.) also found The Atrypa-Hallopora assemblage in their samples from Hadeland and Ringerike. Correct­ ing for these factors, we find four clear associations, as This assemblage was named and recognized by Cocks shown in tables 1-4. & Worsley (1993) as a common mid-shelf assemblage in the Oslo Region. They collected mainly in the south­ The Resserella-Atrypina assemblage ern Skien, Porsgrunn and Holmestrand districts, where there are fewer silt- and sandstones. However, what is This assemblage was called the Eoplectodonta-Atrypa described below from Ringerike and Modum (Table 2) Community by Cocks & Worsley (1993). We prefer to clearly belongs to the same assemblage. In these areas rename it, because both community name-bearers are there is a gradual transition from the Resserella-Atrypina very eurotropic and dominant under variable condi­ assemblage to the Atrypa-Hallopora assemblage. The tions. Eoplectodonta seems to be dependent mainly on a main difference between these assemblages is the occur­ soft-bottom substrate, while Atrypa is an all-round gen­ rence and eventual dominance of Atrypa and an increase eralist. We prefer to use the names of taxa more spe­ in the number and sizes of taxa. Large strophomenids are cific to their environment. The assemblage belongs to especially characteristic, with Leptaena, Erinostrophia the Dicoelosia­-Skenidioides community group of Boucot and Cyphomenoidea as common components. The two (1999) and Watkins et al. (2000) and both Skenidioides­ former may dominate in some samples. Likewise Cyrtia­ and Dicoelosia­ are nearly always present, although as is sometimes very abundant in the lower parts of this minor components (Table 1). Eoplectodonta is very com­ assemblage. Cyrtia dominated communities are found in mon, as it also is in all the other assemblages. Atrypina the upper parts of BA4 into BA3 in North America (Bou­ is more common in the Ringerike and Modum districts, cot, 1999) but Cyrtia also occurs as a minor component with their higher silt content than in the southern dis­ in BA5, both in the Oslo Region and other places in the tricts. This fits with the occurrence of Atrypina as a com­ world. Cyrtia seems to generally occur in environments mon component in some North American assemblages, with high argillaceous input and consequently a soft bot­ although it is also known in Europe. It most often occurs tom, such as the High Lake Member of the Laketown in the shallower and more argillaceous parts of the Dolomite in Utah (Sheehan 1980) or the Cock Rig For­ Skenidioides-Dicoelosia, BA5 assemblage (Boucot 1999). mation in the Pentland Hills, Scotland (Robertson 1989).

Table 1. The Resserella-Atrypina association. The number of specimens over rank is given for each sample taken at x m above base of the Bruflat Foramtion. The total gives total number of specimens in all samples over mean rank abundance. Name Storøya Storøya Storøya Storøya Storøya Storøya Sylling Sylling Syllig Total: 6m 15m 25m 33m 44 m 54m at base 5 m 84 m Resserella cf. 24/1 52/1 18/1 34/1 17/1 25/1 8/1 1/3 6/2 185/1.33 springfieldensis Atrypina sp. 16/2 2/5 4/3 15/3 1/6 13/2 6/2 1/3 8/1 66/3 Eoplectodonta penkillensis 14/3 1/8 5/2 26/2 11/2 7/4 4/3 4/1 2/4 74/3.22 Phacops sp. 5/4 9/2 1/6 6/5 3/3 5/6 -/8 2/2 -/8 31/4.89 Bryozoan colonies -/11 2/5 3/4 9/4 2/4 9/3 1/6 -/5 3/3 29/5 Skenidioides leweesi 5/4 1/8 1/6 1/7 1/6 3/7 1/6 -/5 1/5 14/6 Rugose corall 1/8 1/8 -/9 1/7 2/4 7/4 -/9 -/5 -/8 12/6.89 Dicoelosia alticavata 1/8 1/8 -/9 2/6 1/6 1/9 1/6 -/5 -/8 7/7.22 Encrinurus sp. 2/6 2/5 -/9 1/7 -/9 1/9 -/9 -/5 -/8 6/7.44 Cyrtia exporrecta -/11 -/12 3/4 1/7 -/9 -/13 3/4 -/5 1/5 8/7.78 Hindella?furcata 2/6 5/3 -/9 -/9 -/9 -/13 -/9 -/5 -/8 7/7.89 Gastropods -/11 3/4 -/9 1/7 -/9 -/13 -/9 -/5 -/8 4/8.33 Chypomenoidea -/11 -/12 -/9 -/9 -/9 1/9 -/9 -/5 1/5 2/8.67 wisgoriensis­ Coolinia applanata -/11 -/12 -/9 -/9 -/9 3/7 -/9 -/5 -/8 3/8.78 Katastrophomena -/11 -/12 -/9 -/9 -/9 -/13 3/4 -/5 -/8 3/8.89 penkillensi­s Leptaena purpurea -/11 -/12 -/9 -/9 -/9 1/9 -/9 -/5 -/8 1/9 Pentlandina tartan -/11 -/12 1/6 -/9 -/9 -/13 -/9 -/5 -/8 1/9.11 Calymene sp. 1/8 -/12 -/9 -/9 -/9 -/13 -/9 -/5 -/8 1/9.11 Total 71 79 36 97 38 76 27 8 22 454 NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 109

Table 2. The Atrypa-Hallopora association. The number of specimens over rank is given for each sample taken at x m above base of the Bruflat Formation. The total gives total number of speci- mens in all samples over mean rank abundance. The samples from Gjesvalåsen, Storøya l.,=lower, middle and Storøya, m., =middle, is taken from the unpublished thesis of Aarhus (1978). Sample B39, Garntangen er fra Cocks & Worsley (1993). Unspecified trilobites are marked by a star. Name Storøya Storøya Storøya Gjesval­ Storøya Strorøya Gesval- Garn- Sylling Sylling Sylling Rank 59m 64m 93m åsen l. l. m. åsen u. tangen 41 m 30 m 15.5 m Abun­ middle middle middle middle B39 dance Atrypa reticularis 27/1 86/1 9/1 27/1 28/1 80/1 26/1 22/2 23+2/3 10/2 5/1 357/1.34 Atrypina sp. Eoplectodona 22/3 30/5 -/12 7/2 8/2 15/3 9/3 19+4/1 51/1 14/1 5/1 212/3.09 penkillensis Resserella cf. spring­ 26/2 46/3 2/3 9/2 1/7 12/5 15/2 3/7 5/6 2/4 3/3 116/4.00 fieldensis Isorthis sp. Leptaena purpurea 2/11 14/7 2/3 5/4 5/3 20/2 -/5 7/3 2/7 2/4 -/8 60/5.18 Bryozoan colonies 16/4 39/3 -/12 -/7 -/8 -/10 -/5 6/4 15/4 4/3 1/6 82/6.00 Phacops stokesi 5/6 -/19 1/8 4/5 3/5 14/4 -/5 5/5* 2/7 2/4 1/6 39/6.73 Cyrtia exporrecta 2/11 62/2 -/12 3/6 4/4 3/8 3/4 4/6 2/7 -/10 -/8 85/7.09 Rugose corall 3/9 18/6 1/8 -/7 -/8 -/10 -/5 1/10 34/2 1/7 -/8 61/7.27 Hindella? furcata 2/11 2/13 2/3 -/7 -/8 -/10 -/5 -/15 2/7 -/10 -/8 15/8.82 Katastrophomena -/17 6/8 -/12 -/7 -/8 3/8 -/5 1/10 2/7 -/10 -/8 13/9.09 penkillensis­ Coolinia applanata 3/9 4/10 -/12 -/7 -/8 -/10 -/5 3/7 -/19 -/10 -/8 11/9.55 Skenidioides lewisii 6/5 1/17 -/12 -/7 -/8 -/10 -/5 -/15 1/13 -/10 3/3 11/9.55 Meifodia sp. -/17 -/19 -/12 -/7 -/8 -/10 -/5 -/15 2/7 1/7 3/3 6/10.00 Pterinea sp -/17 4/10 -/12 -/7 -/8 -/10 -/5 -/15 1/13 -/10 -/8 5/10.45 Cyphomenoidea 1/15 2/13 -/12 -/7 2/6 5/5 -/5 1/10 -/19 1/7 -/8 12/9.73 wisgoriensis­

Encrinurus sp. 2/11 5/9 -/12 -/7 -/8 -/10 -/5 -/15 -/19 -/10 -/8 7/10.36 Paleocyclus porpita -/17 -/19 7/2 -/7 -/8 -/10 -/5 -/15 1/13 -/10 -/8 8/10.36 Dinobolus sp. -/17 -/19 -/12 -/7 -/8 -/10 -/5 -/15 6/5 -/10 -/8 6/10.55 Favosites sp 5/6 -/19 -/12 -/7 -/8 -/10 -/5 -/15 -/19 -/10 -/8 6/10.81 Eospirifer sp. -/17 2/13 -/12 -/7 -/8 -/10 -/5 -/15 1/13 -/10 -/8 3/10.73 Dicoelosia alticavata 4/8 -/19 -/12 -/7 -/8 -/10 -/5 -/15 -/19 -/10 -/8 5/11.00 Hesperorthis sp. -/17 1/17 -/12 -/7 -/8 -/10 -/5 -/15 1/13 -/10 -/8 4/11.09 Gastropods 1/15 -/19 -/12 -/7 -/8 -/10 -/5 2/9 -/19 -/10 -/8 3/11.09 Erinostrophia undata -/17 -/19 1/8 -/7 -/8 4/7 -/5 -/15 -/19 -/10 -/8 5/11.18 Pentlandina tartana -/17 3/12 -/12 -/7 -/8 -/10 -/5 -/15 -/19 -/10 -/8 4/11.18 Ostracoda -/17 -/19 -/12 -/7 -/8 -/10 -/5 -/15 1/13 -/10 -/8 11.27 Calymene sp. -/17 2/13 -/12 -/7 -/8 -/10 -/5 -/15 -/19 -/10 -/8 3/11.27 Glassia sp. -/17 -/19 -/12 -/7 -/8 -/10 -/5 1/10 -/19 -/10 -/8 1/11.36 Craniops sp -/17 -/19 -/12 -/7 -/8 -/10 -/5 1/10 -/19 -/10 -/8 1/11.36 Howellella sp. -/17 -/19 1/8 -/7 -/8 -/10 -/5 -/15 -/19 -/10 -/8 2/11.45

Atrypa dominated communities may occur from upper transitional Atrypa-Hallopora assemblage with dominant BA5 to BA 3 but because of their position between a typ­ Hindella and the presence of Protochonetes was noted by ical BA 5 assemblage and the overlying BA3 Hindella Cocks & Worsley (1993) at Hadeland. This community community, we assign them to BA 4. type was earlier known as the Cryptothyrella community. However, the taxon name Cryptothyrella is now invalid The Hindella assemblage and incorporated under Hindella so herein it is called the Hindella community. Ziegler and Boucot (1970) first This is a very low diversity and often low density assem­ described this low diversity community. It is widely dis­ blage, where Hindella may be the sole component (Table tributed, especially in North and South America (Boucot et 3). So far it has been found only in the Ringerike Dis­ al. 1999), where it occupies the same shallow, high-energy trict. Like the other assemblages, there is a gradual transi­ niche as Pentamerus. Hindella is a eurytopic pioneer taxon tion from the Atrypa-Hallopora assemblage and these two that was able to rapidly colonize physically disturbed areas name-bearers are the two other common components. A (Sheehan, 1975). This fits very well with the conditions 110 D. Worsley et al NORWEGIAN JOURNAL OF GEOLOGY

Table 3. The Hindella association. The number of specimens over rank is given for each sample taken at x m above base of the Bruflat Foration. The total gives total number of specimens in all samples over mean rank abundance. Name Storøya Storøya Storøya Storøya Storøya Sylling R. A. 74m 76m 85m 93m 100m 20 m Hindella furcata 14/1 12/1 -/6 1/3 56/1 3/2 86/2.33 Atrypareticularis Atrypina sp. 1/3 -/4 15/1 2/2 -3 9/1 27/2.33 Bryozoan colonies -/4 1/3 8/2 3/1 -/3 -/6 12/3.17 Eoplectodontapenkillensis 2/2 4/2 -/6 1/3 -/3 -/6 7/3.67 Leptaena purpurea -/4 -/4 2/3 -/6 -/3 2/3 4/3.83 Resserella cf. -/4 -/4 1/4 1/3 -/3 -/6 2/4 springfieldensi Isorthis sp. Pentlandinia sp -/4 -/4 -/6 -/6 -/3 1/4 1/4.5 Gastropod -/4 -/4 -/6 -/6 -/3 1/4 1/4.5 Paleocyclus porpita -/4 -/4 -/6 -/6 7/2 -/6 7/4.67 Phacops stokesi -/4 -/4 1/5 -/6 -/3 -/6 1/4.67 Howellella sp. -/4 -/4 -/6 -/6 -/3 1/6 1/4.83 Protochonetes sp. -/4 -/4 -/6 -/6 -/3 1/6 1/4.83 Total 17 17 26 8 63 18

predicted in the shallower parts of the Bruflat Formation and Salopina sp. commonly occur together in sequences above storm wave-base, where an already unstable bottom displaying interference ripples, indicating deposition above was frequently reworked by storms. A similar community normal wave base. Paleocyclus has a short stratigraphic is also found in the Aeronian upper and shallower parts of range and may occur in several depth zones from BA4 to the Solvik Formation in the central Asker area with silici­ BA2, where it is mainly adapted to a soft bottom. Protocho­ clastic deposition in the transition zone (Baarli 1988). The netes and/or Salopina dominated assemblages are common association occupies a BA 3 position. both in Wales (Lawson 1999) and North America (Bou­ cot 1999) in a BA 2 position, although Salopina also may The Protochonetes-Paleocyclus assemblage be common in a BA 4 environment. These assemblages are most prevalent in Upper Llandovery rocks. Howellella- Although there are few collections of this community so dominated communities are cosmopolitan and belong that a definitive description is not possible, there is clearly a most frequently to BA 2 or 3. A BA 2 assignment is the separate assemblage present both at Toten and in Ringerike most likely position for this assemblage. that can be compared with similar shallow-water assem­ blages from South Wales and North America. The three Mollusc-dominated assemblage collections from Ringerike (Table 4) show very common Paleocyclus together with ostracodes. Howellella sp. and There are no new collections for this BA1 assemblage Protochonetes sp. are also found. At Toten Protochonetes sp. other than those described by Cocks & Worsley (1993) from the Toten District. Molluscs make up 83 percent of their collections, and of these the gastropods Oriostoma Table 4. The Protochonetes-Paleocyclus cf. acuta, Loxoema sp., Gyronema? sp. and Raphistomina association. The number of specimens over sp. are the most common. Of the bivalves Pterionella rank is given for each sample taken at x m sp. dominates, while other groups are rare. See Cocks & Worsley (1993, p. 36) for further details. above base of the Bruflat Foramtion. The total gives total number of specimens in all samples Assemblages in the Skinnerbukta Formation of Modum over mean rank abundance. Name Storøya Storøya Rank In the Modum District, a thin shale development which 107m 116m Abundance­ we assign to the Skinnerbukta Formation overlies the Paleocyclus porpita 67/1 36/1 103/1 Bruflat Formation. This section is poor in fossils, but Ostracods 20/2 27/2 47/2 large individuals of Costistricklandia lirata and Eospirifer Atrypareticularis Atrypina sp. 2/3 1/5 3/4 sp. have been found. They are followed by a very sparse Howellella sp. -/6 3/3 3/4.5 fauna of small brachiopods of the Resserella-­Atrypina Protochonetes sp. -/6 2/4 2/5 assemblage near the base of the Braksøya Formation. Hindella furcata 1/4 -/6 1/5 This indicates a slight deepening from the deeper parts Hindella furcata 1/4 -/6 1/5 of the Hindella assemblage to a BA 4 position and may Total 101 69 extend further into BA5. NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 111

Palaeobathymetric maps correspond to a slight peripheral bulge in front of a fore­ deep as described earlier by Baarli (1990). However, the The Benthic Assemblage zones defined above have been resulting biostratigraphic maps as plotted here show a plotted in terms of the stratigraphic framework shown on better fit for a basin congruent with the Caledonian front Figure 3. Three horizons were chosen for the construction as seen today with its westward turn near Hardanger­ of bathymetric maps: 1) the lower Oktavites spiralis grap­ vidda, rather than the rounded Caledonian front por­ tolite Biozone, a horizon just below the base of the Bruflat trayed by Baarli et al. (2003). Formation; 2) the middle Cyrtograptus lapworthi grapto­ lite Biozone in the middle of the formation, and 3) the Cyr­ Figure 5 shows the distribution of Benthic Assemblage tograptus insectus Biozone near the top of the Bruflat For­ zones during deposition of the Bruflat Formation in the mation. Due to considerable uncertainty in the age desig­ Cyrtograptus lapworthi graptolite Biozone. Eustatic sea­ nation of different stratigraphic sections and their tectonic level was still high, although approaching the start of the displacement, there is some leeway in the interpretation. sea­level drop in the upper C. lapworthi through the C. These maps mark the development of the formation from centrifugus Biozone (Johnson 2010). The deepest part of approximately 431 to 428 Ma, both ± ca. 2 Ma. the trough was now situated further east, near the Ring­ erike District. At this point the trough was less pro­ Figure 4 shows the bathymetric configuration for the O. nounced ­ possibly because it was being partly filled in spiralis graptolite zone. During this time interval there by sediments of the Bruflat Formation. was a eustatic deepening according to Johnson (2010). However, there was a clearly developed trough between Eustatic sea level started to fall during Cyrtograptus the northern Ringsaker District and the central Oslo insectus graptolite Biozone time as the Scandian phase Region. The Jämtland area accrued deep­water deposits of the Caledonian orogeny came to a halt and shallower from the beginning of Telychian time and now registered conditions were evident all over the region. The foreland a slightly shallower position. trough was no longer clearly defined (Fig. 6) and a car­ bonate shoal developed in the Asker area and further The same is true of the Ringsaker District, although the south. deepest conditions there developed a little later than in Jämtland. The central Oslo Region and the southern Benthic Assemblage zones cannot be identified in the Skien District were in relatively shallow­water condi­ Jämtland sections, because fossils are very scarce. The tions, while further east at Malmøya and in Porsgrunn, few fossils found in the uppermost beds include a coral, conditions were slightly deeper. This pattern would graptolites, nautiloids and hyolithids.

Figure 4: Bathymetric 100 km 5 map using Benthic Assemblage zones coeval with the Oktavites spi­ ralis interval graptolite NORWAY Biozone.

56 Ringsaker Hamar 6 Toten6 5

Hadeland 6/5

Ringerike5 Modum 5 4/5 Asker? 4/5Malmøya SWEDEN

Holmestrand ?

Skien 5 4/5 5Porsgrunn O. spiralis

Worsley et al 112 D. Worsley et al NORWEGIAN JOURNAL OF GEOLOGY

? 100 km J5ämtland Figure 5: Bathymetric map using Benthic Assemblage zones coeval with the Cyrtograptus NORWAY lapworthi interval grap­ tolite Biozone.

6 ? Ringsaker

Hamar ? Toten 3 3/4 Hadeland 3/4

Ringerike 4/5 4 Modum 4 Malmøya Asker 3 SWEDEN

3 Holmestrand 4 4/5 C. lapworthi Skien 5Porsgrunn

100 km 5? Figure 6: Bathymetric map using Benthic Assemb lage zones coeval with the Cyrtograptus Worsley et al. NORWAY insectus interval grapto­ lite Bio zone.

4?Ringsaker Hamar Toten 1 3 Hadeland 2/3

Ringerike 3 4 Modum 2/3 4 Asker Malmøya SWEDEN

Holmestrand 3 Skien 3/4 3 Porsgrunn C. insectus BA1 BA2 BA3 BA4 BA3 BA4 BA3 BA4 BA3

Sedimentological characteristics thickest sandstone interbeds up to 2 m thick. There is a decrease in coarse interbeds from Jämtland southwards The Bruflat Formation and the Ekeberg Greywacke con­ through the Toten, Hadeland and Ringerike districts to sist of silt­ and sandstones intercalated with shale. The the Modum District, where the Bruflat Formation has its Jämtland area shows the coarsest sediments, with the most southerly outcrop. Likewise, there is a decrease in NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 113 sandstone thickness across the same outcrops. based on exposures at the railway cutting south of the drive to Vollenga between Bruflat and Reinsvoll sta­ The Ekeberg Greywacke Formation in Jämtland tions, consists of massive, thick and fine sandstones and sandy limestones showing few internal structures. The outcrop in Jämtland is strongly tectonized and poorly These beds are often amalgamated. Interbeds con­ studied. However, the entire Silurian section and its fauna sist of very thin siltstone or silty shale partings. A few are quite similar to that seen in the Toten Distric.­ Karis shallow channels are observed. The bases of the thick (1998) noted that the Ekeberg Greywacke Formation var­ beds often contain coarse, shelly material. The tops ies lithologically over short distances. Near the base at of the thick beds may show interference ripples. This Järntangen, there is a 4-6 m thick section with massive fine facies was deposited in a proximal setting and above sand- and siltstones (up to 2 m thick.) interbedded with normal wave-base, in the shoreface to intertidal zones greywacke. This is followed by 40 m of thin, alternating as suggested by Benthic Assemblages 1 and 2. siltstone and shale layers. Bassett et al. (1982) described these sandstones in the lower Ekeberg Formation as mas­ The Toten District sive to graded turbidites with intercalated mudflake conglomerates. Towards the top of the formation they The Bruflat Formation in Toten shows its thickest recorded thinly bedded fine-grained sandstones and silt­ develop­ment of at least 190 m here, although the top of stones interbedded with shale and interpreted these as the formation is not known. The interbeds are mostly distal turbidites. Fluvial red beds of the Röde Formation fine-grained sandstones, but this is the district with overlie the Ekeberg Formation (Bassett et al. op.cit.). interbeds that show the coarsest grain sizes. All three facies are present in order from Facies A to Facies C, with The Bruflat Formation in the Oslo Region a thick development of Facies C. In Figure 7, the per­ centage of silt- and sandstones is shown to be very high Detailed sedimentological analyses are only available ­from through most of the section except for in Facies B. The the Toten, Hadeland, Ringerike and Modum districts­. The direction of toolmarks at the base of thick interbeds and Ringsaker outcrops are still poorly studied­. According to current ripples in the Bouma sequences is approximately Kiær (1908) the development here is more or less as seen towards the ESE/SE (110°-130°, Fig. 8), while hummocky in Hadeland, although reddish shales with Cyrtia expor­ cross-stratification indicates a more north-south orienta­ recta are found near the top of the formation (Worsley et tion for wave axes (180°-190°/0°-15°). al. 1983a). In the main areas that we have examined, we recognize three different facies developments: The Hadeland District

• Facies A, interbedded sandstones/siltstones and The Hadeland District shows a similar development to shales, shows silt- to fine sandstones and shale inter­ Toten, with a 182 m thick sequence. Again, the top of the beds that are approximately equally thick (5-100 cm Bruflat Formation is not known. The proportion of silt- for the sandstones and 2-300 cm for the shale). There and sandstone interbeds is consistently lower than found is no clear pattern of upwards thinning or thicken­ in Toten (Fig. 7). The order of facies at Hadeland is possi­ ing in the silt- and sandstone interbeds. The coarser bly A, B, C followed by a repetition of B and C. The lower interbeds often have erosive bases with abundant tool occurrence of Facies C lacks bioclastic stringers and cal­ marks and gradational tops. These beds generally ­ careous sandstones. The typical development of Facies C, appear to be uniform in thickness. Where it is pos­ as seen near the top, is thinner than that found at Toten sible to discern structures, graded planar laminat­ and with thicker shale interbeds. ion followed­ by ripple cross lamination and low- energy planar lamination is common. Hummocky The Ringerike District cross-stratification may also be present. This facies is interpreted as representing turbidity flow and storm- This is the district that has received the most study. The reworked deposits. complete section is 120 m thick, i.e. much thinner than at Toten and Hadeland. The interbeds here are continu­ • Facies B, interbedded shale and sandstones, showing ous siltstones with coarser bioclastic stringers common abundant fine sandstone beds in a thinning and thick­ towards the top of the profile (Fig. 7). Only Facies A is ening up pattern. The sandstones are mainly thin with present. However, there are significant changes through a few thicker beds in a shale-dominated sequence. The this facies. The lower part consists of intervals with thick siliciclastic interbeds have erosive bases and trace fos­ siltstone and thin shale interbeds alternating with thick sils are common. This facies is interpreted as repre­ shale and thin planar laminated siltstone interbeds. senting distal channel lobes deposited on a steep shelf Hummocky cross-stratification is common at the base during times with less sediment accumulation. of many siltstone beds in the middle parts of the forma­ tion. This is often followed by a normal-graded inter­ • Facies C, massive planar bedded sandstones, inform­ val of planar lamination and ripple lamination typical ally designated the Vollenga member by Howe (1982) of storm beds deposited above a storm wave base. The 114 D. Worsley et al NORWEGIAN JOURNAL OF GEOLOGY

FORMATION LITHOLOGY GRAIN SIZE FORMATION LITHOLOGY GRAIN SIZE m and and TEXTURE m TEXTURE BRAKSØYA 190 FORMATION LITHOLOGY GRAIN SIZE m and 120 TEXTURE

180 180

110 170 170

160 160 100

150 150

90 140 140

T

h

i 130 130 n n

i n 80 g FORMATION LITHOLOGY u GRAIN SIZE p m and 120 120 TEXTURE BRAKSØYA SKINNERBUKTA 110 T 110 70 70 h

i n

n

i

n T

g

h u i

100 100 c p

k

e

n i

n 60 60

g BRUFLAT

T

u

BRUFLAT h 90 90 p i

c BRUFLAT

k

e

n

i

n

80 g 80

B u p 50 50

70 70

40 40 60 60 B

T BRUFLAT h i 50 50 n n i n 30 30 g

u 40 40 p B

30 30 20 20

B 20 20 10 10

10 10

0 Sh Si Vfs Fs 0 Sh Si Vfs Fs 0% 50 100 0% 50 100 0 Sh Si Vfs Fs 0 Sh Si Vfs Fs M W P G M W P G 0% 50 100 0% 50 100 M W P G M W P G TOTEN HADELAND RINGERIKE SYLLING, MODUM

Figure 7: Lithostratigraphic columns for the Bruflat Formation from the Toten, Hadeland, Ringerike and Modum districts. The Modum District also includes the transition from the Vik Formation and the overlying Skinnerbukta Formation because those parts have not been published previously. The column to the left on each section gives the percentage of silt or sandstones, shales and limestone averaged for each metre. The next column gives grain sizes for Sh=shale, Si=siltstone, Vfs=veryFig. fine 7 sandstone Worsley oret Fs=fineal. sandstone. In the case of limestones there is a scale for M=mudstone, W=wackestone, P=packstone and G=grainstone. B=bentonites. NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 115

upper 50 m of the succession exhibits a high siliciclas­ m above base 120 tic and carbonate content similar to that seen in Facies C, although the siltstones are not very thick or massive. Some of the silt beds are amalgamated with shell hash at the base. The reefoid carbonates of the Braksøya Forma­

tion overlie the Bruflat Formation with a marked erosive 100 contact. The upper parts of the Bruflat Formation were probably deposited under very proximal conditions in the shoreface, with frequent reworking and winnowing by storms. Tool marks are very common throughout the Bruflat Formation here and the general direction points eastwards (average 97°, Fig. 8). Only five silt beds dis­ play current ripples, suggesting a general southeasterly directed current (average 137°/317°).

Petrological investigations have also been undertaken in

the Ringerike District. XRD analyses for semi-quantita­ Shale x = 0.873 + 0.00793Y

Siltstone x = 2.108 + 0.01920Y tive variation of the seven most common minerals 50 and modal analyses for 10 different minerals (Table 5) were made for 46 and 37 samples throughout the Bru­ flat Formation­. The two types of analyses gave closely comparable­ results. They show that the mineral content of both shales and sandstones is fairly steady throughout the section, indicating provenance from a single source area. There is an average alkali- and plagioclase feldspar content close to 15 % as found by the modal- and 17 % by the XRD analysis. The XRD analysis indicates a slight increase of the proportion of quartz to feldspar towards = shale samples = siltstone samples the top of the section (Fig. 9), possibly suggesting greater 0 maturity and reworking of the sediments. 0 1 2 3 4 5 Quartz + Potash feldspar + Plagioclase Illite + Chlorite 100 km Figure 9: Semiquantitative changes measured by XRD- Jämtland Fig.analyses 9. Worsley in the etrelationship al. between quartz + potash feldspar + plagioclase/illite + chlorite throughout the Bruflat Formation in the Ringerike District based on the results NORWAY from Table 5. A best-fit line is drawn for shale and siltstones respectively.

The Modum District The Bruflat Formation is exposed in one continuous 51 m thick section along the eastern side of Route 285 in

n=12 Sylling (NM7340). It is underlain by the Vik Formation Toten and followed by a 21 m succession of dark grey shales assigned to the Skinnerbukta Formation, which termi­ nate at an erosive contact with shallow marine carbon­ n=351 ates of the overlying Braksøya Formation. This is the Ringerike thinnest known development of the Bruflat Formation, Modum with alternations of shale and minor, thin to medium n=9 thick siltstone beds (Fig. 7). Only Facies A is present. The proportion of siltstones is well under 50 % through most SWEDEN of the formation. A section with very thin to medium thick siltstone beds interbedded with very silty shale occurs between 37 and 42 m above formational base and Figure 8: Measured current directions from the Toten, this may represent a lateral development of Facies C. A Ringerike and Modum districts shown on a map coeval with few horizons with limestone nodules occur in the lower the C. lapworthi graptolite Biozone. part of the formation and there is a gradual upwards Fig. 8. Worsley et al. 116 D. Worsley et al NORWEGIAN JOURNAL OF GEOLOGY

Table 5. XRD-analyses from Ringerike, showing semi-quantitative mineral content (%) from siltstone and shale samples. *) Relative height in measured profile from base Bruflat Formation Sample Height (m)* Lithology Calcite Dolomite Quartz P-feldspar Plagioclase Illite Chlorite LÅ 005 119.2 siltstone 28.0 15.0 24.3 3.1 19.6 3.1 6.8 SV 403 118.6 siltstone 14.2 29.6 30.2 6.1 14.0 2.6 3.3 GÅ 004 117.9 siltstone 31.3 28.5 16.2 1.3 10.8 4.9 7.0 SN 015 116.2 siltstone 27.8 24.8 10.8 3.7 10.1 9.8 12.9 SN 012 104.4 siltstone 36.1 13.9 24.7 7.8 13.7 0.2 3.6 SV 215 99.0 siltstone 36.4 16.8 20.1 4.8 9.8 4.7 7.2 SN 009 95.2 siltstone 37.1 20.9 14.9 1.5 12.2 6.5 6.9 SN 004 84.2 siltstone 25.0 13.8 22.6 10.5 12.0 4.8 11.3 SV 206 81.2 siltstone 34.8 14.2 20.0 4.0 16.0 3.3 7.8 SN 002 75.4 siltstone 44.3 10.3 16.4 5.6 11.5 5.1 6.7 SV 202 66.0 siltstone 41.7 12.1 15.0 2.3 13.5 6.0 9.3 SV 120 56.9 siltstone 33.0 11.6 17.1 1.9 15.3 6.9 14.1 GV 003 54.3 siltstone 35.4 16.8 24.5 2.7 13.1 1.2 6.3 SV 118 42.9 siltstone 62.6 7.0 10.1 0.2 11.2 2.7 6.1 SV 116 37.9 siltstone 42.5 8.3 22.9 4.4 14.7 0.8 6.3 SV 113 31.8 siltstone 38.1 10.5 11.0 4.7 11.8 10.0 14.0 SV 111 31.0 siltstone 61.7 5.2 8.8 3.0 10.7 4.1 6.5 SV 109 27.5 siltstone 29.8 11.9 16.9 8.5 12.0 8.7 12.2 SV 107 18.9 siltstone 27.0 15.0 19.9 7.0 16.6 6.7 7.8 PU 003 2.6 siltstone 43.2 12.5 14.0 2.1 13.8 6.2 8.3 SV 102 0.1 siltstone 36.6 13.5 14.9 4.3 17.0 5.4 8.3

GÅ 005 119.1 shale 18.4 19.8 18.8 3.4 11.6 12.8 15.3 SN 016 119.1 shale 27.6 14.1 14.7 1.0 11.6 11.3 19.7 SV 401 117.2 shale 6.7 19.3 12.7 4.2 12.3 23.4 21.4 LÅ 004 116.3 shale 17.7 4.6 17.2 4.8 22.6 11.3 21.7 SN 013 111.3 shale 32.7 15.5 12.1 2.8 13.2 8.5 15.1 SN 010 97.9 shale 18.7 17.7 28.4 4.7 14.6 5.7 10.1 SV 213 92.1 shale 32.7 15.2 13.1 2.5 13.7 9.1 13.7 SV 210 87.1 shale 30.1 14.4 13.2 3.5 11.3 10.1 17.4 SN 006 85.6 shale 23.4 10.2 18.9 4.2 19.0 11.4 13.0 SV 207 83.5 shale 21.1 11.5 17.6 4.4 15.1 12.8 17.6 SN 003 78.4 shale 38.9 5.9 21.8 4.5 18.0 3.2 7.8 SV 205 74.2 shale 25.4 13.4 14.4 2.2 12.2 13.4 19.0 SN 001 72.9 shale 32.9 11.4 21.5 1.2 15.0 7.0 11.1 SV 201 59.0 shale 22.5 13.7 11.7 2.3 10.0 18.4 21.4 GV 004 58.0 shale 26.8 9.4 20.2 6.9 11.3 10.6 14.8 SV 119 51.9 shale 18.0 13.2 14.6 4.5 13.2 15.3 21.2 SV 117 42.6 shale 24.5 11.2 15.7 1.7 11.3 14.8 20.8 SV 115 33.7 shale 20.5 11.4 10.6 0.8 10.4 21.2 25.0 SV 114 32.2 shale 29.7 14.1 14.9 0.7 12.9 11.6 16.1 SV 112 31.2 shale 19.6 15.9 11.1 3.0 12.9 15.2 22.3 SV 110 28.4 shale 23.2 14.1 11.4 2.7 13.3 15.3 20.0 SV 108 23.4 shale 41.0 12.5 18.1 1.4 15.6 3.6 7.8 PU 004 4.5 shale 21.5 11.6 12.1 1.8 13.0 19.8 20.3 SV 103 1.4 shale 14.0 7.0 19.6 4.3 14.0 17.4 23.8 SV 101 -0.1 shale 12.7 9.7 9.9 2.0 13.0 23.4 29.2 NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 117 transition into the shales of the Skinnerbukta Forma­ very deep, as seen from maps of the Benthic Assemblage tion. This is the only district where both the Bruflat and zones. Directions measured in Toten and in the Ring­ the Skinnerbukta formations are developed above the erike and Modum districts are somewhat different. This Vik Formation. Further north and west the Skinner­ fits with a multiple sandy fan model, where the fans typi­ bukta Formation is missing, while in the central Oslo cally are small (on average 25 km across) and curve radi­ Region the Bruflat Formation is missing. A few silt beds ally (Mattern 2005). The distance between the Toten Dis­ show lineations and toolmarks at their base with a gener­ trict and the Ringerike District is 70 to 80 kilometres, ally east-west lineation (70°-100°/250-280°), the tool and thus making it likely that the siliciclastics were derived flute marks pointing towards the east (Fig. 8). There is from different fans, as were also the siliciclastics of the no indication of deposition as shallow as at the shoreface. Ekeberg Greywacke Formation 500 km further north along the Caledonian front. The average current direc­ tion in the Modum District is slightly more northerly Interpretations and discussion than in the Ringerike District and fits with a curved fan model, as Modum was situated further out in the basin. The foreland basin XRD and modal analyses show that sediments of the Our palaeogeographic reconstructions (Figs. 3-6) indi­ Bruflat Formation in Ringerike are immature, with fairly cate a foreland basin along the advancing Caledonian high feldspar and clay contents and have the same com­ front during late Telychian time. Typically, such basins position throughout the formation, indicating a sin­ are filled with thick, coarse-grained turbidite systems gle provenance area. Bjørlykke (1983) proposed that the deposited in a rapidly subsiding foredeep (Mutti et al. source area might be the Osen-Røa nappe. This nappe 2003). The Telychian sections in the Oslo Region are has an extensive distribution, consists mainly of sedi­ around 300 m thick or thinner and the Bruflat Forma­ mentary rocks and is the most external tectonic unit in tion is the only formation with silt- and fine-grained the southern Norwegian Caledonides (Morley 1989). sandstones. There is no coarser siliciclastic material. This might explain the relatively fine clastic grain size in Garfunkel & Greiling (1998) discussed the lack of thick the formation. siliciclastic wedges along the Caledonian front in Bal­ tica and they also claimed that the foreland basin is miss­ Lower and mid-fan deposits ing. These authors provided a model which postulated that the continental crust below the advancing Caledo­ Mattern (2005) advocated a model whereby sandy fans are nian front was quite old and, therefore, presumably, very subdivided into three facies, representing upper fan, mid- thick. They argued that because the thrust sheets were so fan and lower fan sectors. Facies A of the Bruflat Forma­ thin, only a minor basin could be expected to develop. tion would correspond to a lower fan sector with its evenly This agrees with our suggestion of a shallow, minor fore­ thick and laterally continuous turbiditic silts and sand­ land basin. We believe that the Ek and the Bruflat forma­ stones. The most distal districts, Ringerike and Modum, tions, together with the corresponding Bangsåsen Shale display only outer fan deposits and also have the thin­ and the Ekeberg Greywacke formations in Jämtland, rep­ nest sections. Midfan with channel deposits Facies B is resent the first clastic infill from the Caledonian front only encountered in the more proximal Toten and Hade­ into a minor foreland basin. land districts. Typically, the midfan and outer fans are no more than 200 to 300 m thick (Mattern 2005) and this is Sand-rich submarine fan deposits the range represented by the Bruflat Formation.

Mattern (2005) discussed the recognition of ancient Tectonic quiescence sand-rich submarine fans. Such fans typically develop on continental crust with tectonic activity in the hinter­ The Bruflat Formation was deposited in the upper Tely­ land. This is the situation in the Oslo Region, where the chian and this was followed by a long period with shal­ approaching Caledonian front finally terminated marine low carbonate deposition through early to mid-Wenlock deposition in an epicontinental sea in late Wenlock time time. Tectonically induced clastic deposition resumed (Davies et al. 2005; Garfunkel & Greiling 1998). Typi­ during the late Wenlock with the accumulation of thick cal environments for such fans are small, active marginal continental red-bed deposits represented by the Ring­ basins with steep gradients (Mattern 2005) as found to erike Group. Therefore, there were clearly several phases have been the case for the foreland basin that developed in the advance of the Caledonian front and the Bruflat during the Telychian in the Oslo Region. Formation represents only the first such phase. The facies development at the top of the Bruflat Formation shows The current directions for the Oslo Region in all the dis­ how the coarser clastic input was gradually phased out. tricts measured with sole marks in turbidite beds point This is especially well seen in the Modum District and from west towards east (Fig. 8). Traction comes from to a lesser degree in Ringerike. Facies C was deposited the direction of the approaching front and into a fore­ towards the end of the tectonic phase and indicates sta­ land basin with a steep slope, although the basin was not bilization and establishment of a shallow shelf (Worsley 118 D. Worsley et al NORWEGIAN JOURNAL OF GEOLOGY et al. 1983a) in the west, north and south. It is interesting formation, viz. the Resserella-Atrypina assemblage, the that the Modum District is close to the barrier suggested Atrypa-Hallopora assemblage, the Hindella assemblage, to reflect a Caledonide blind thrust fault by Davies et al. the Protochonetes-Paleocyclus assemblage and a mollusk- (2005) in order to explain depositional patterns in the dominated assemblage. These are arranged from deep younger Ringerike Group sediments. We may speculate to shallow marine life environments. All are adapted to that a similar tectonic hinge-line may also have been a soft substrates, and they suggest that the Bruflat Forma­ controlling factor on sedimentation in the late Telychian tion was deposited over a range of depths from BA 5 to to early Wenlock tectonic phase. BA 1. Existing biostratigraphic data, revised and refined by new collections, have been used as a basis for a new Eustatic sea-level variations palaeo­bathymetric reconstruction of the Oslo Region and the Jämtland area in Sweden. This suggests the The Bruflat Formation was deposited in water depths presence of a foreland basin following the present- day inhabited by faunas of Benthic Assemblage zones 5 to 1. Caledonian­ front. The foreland basin advanced from Eustatic sea-level was high during the O. spiralis and C. the northwest ahead of the Caledonian front, and this is lapworthi graptolite Biozone times, followed by a strong reflected by fairly deep-water conditions in the north­ drop in sea-level beginning in the C. insectus graptolite ern districts during the early Telychian, before proceed­ Biozone (Johnson 2010) and culminating during the C. ing into the western Ringerike and Modum districts murchisoni-C. centrifugus graptolite Biozone. This drop during­ mid- to late Telychian time. Due to a thick crust corresponds to the Ireviken Excursion, an important and possi­bly due to thin, advancing nappes, this foreland extinction level found with a cosmopolitan development basin showed only moderate subsidence. (Jeppson 1997, Cramer & Saltzman 2007). XRD analyses suggest that the siliciclastic rocks of the The transition to overlying formations Bruflat Formation in the Ringerike District are relatively immature and all are derived from the same source. The In the Modum District, the Bruflat Formation is over­ lower and middle parts of the formation were deposited lain by shales of the Skinnerbukta Formation. Ben­ as outer to midfan sandy deposits on a steep basin slope thic Assemblage analysis shows that there was a grad­ in a moderately deep basin where parts were above storm ual deepening as the siltstones of the Bruflat Formation base. There were several active fans along the exposed became phased out and shales were being deposited. parts of the foreland basin. As the first phase of tec­ Further east, in the central Oslo Region, there is a much tonic activity in the hinterland terminated, the shelf sta­ thicker development of the Skinnerbukta Formation in a bilized and eustatic shallowing exposed the sandy fan deeper setting. The gap below the Braksøya Formation deposits in a shoreface environment where the silici­ in Ringerike and Modum is not well dated but it prob­ clastics were reworked by normal wave activity. In cen­ ably corresponds to the Ireviken extinction event and tral Asker and in the southern districts a carbonate shoal the protracted sea-level drop associated with that event, developed simultaneously. This eustatic shallowing cor­ of which extended for about 0.2 Ma around the Lland­ responds to the Ireviken Excursion that had a maximum overy/Wenlock transition. Gaps around the Llandovery/ sea-level drop near the Llandovery/Wenlock bound­ Wenlock boundary are found many places in the world ary. This sea-level change eventually led to the hiatus and may relate to this event: e.g. throughout Laurentia recorded between the Bruflat and Braksøya formations (Cramer & Saltzman 2007) and in much of eastern Bal­ in the Ringerike District. The carbonate-rich and reefoid tica (Loydell et al. 2003). Breaks are also widespread in Braks­øya Formation reflects renewed deposition, but China in the Yangtze Region (Xu et al. 2002) and com­ with little ­clastic influx. mon in the British Isles (Aldridge et al. 2002). It is likely that parts of the upper Bruflat Formation in the Ring­ erike and Modum Districts have been eroded away dur­ Acknowledgements – This paper is based on several unpublished theses ing this event. The Braksøya Formation, however, reflects supervised by David Worsley and augmented by new data. Different authors are responsible for specific data. The Toten - and to a lesser renewed deposition on a shallow carbonate platform. degree the Hadeland District was the focus of a thesis by Michael Howe. The palaeoecology of the Ringerike District was investigated by Dagfinn Alm, while Frodi Hjaltason described the sedimentology of Summary the same district. The Modum District and new outcrops in the Toten District were investigated by David Worsley and B. Gudveig Baarli. We thank Markes E. Johnson for his assistance in the field, and for A temporary section in the Toten District and investiga­ subsequent discussions and inspiration. Magne Høgberget was very tions of hitherto unknown exposures of the Bruflat For­ helpful in sharing information on his collections of graptolites from mation in the Modum District, together with older pub­ the new, temporary road section in the Toten District. We are also lished and unpublished data from the other districts, have grateful for updated information on conodonts from the Vik Formation provided new and/or revised litho- and biostratigraphical provided by Peep Männik and for comments and discussions regarding the Jämtland section from Peter Dahlgren. Mike Howe publishes data for the Bruflat and closely contiguous formations. by permission of the Director, British Geological Survey (NERC). We are grateful for the positive and helpful reviews of D.L. Bruton, Five distinct benthic assemblages are recognized in the T.R. Redfield and L.R.M. Cocks. David Worsley expresses his deep NORWEGIAN JOURNAL OF GEOLOGY New data on the Bruflat Formation and the Llandovery/Wenlock transition in the Oslo Region 119 gratitude to the late Fred Broadhurst, whose inspiring supervision first the Natural History Museum (Geology) 49, 31-46. introduced him to the fascinating succession of the Oslo Region. 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