New Data on the Bruflat Formation and the Llandovery/Wenlock Transition in the Oslo Region 101
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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. 101120. Oslo 2011. ISSN 029196X. 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 sealevel 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 tecton ised 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 form ation has few that otherwise dominates the region. The formation was agediagnostic 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 deposition al 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 present-day 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.