The upper succession at N orderhov and on FrognØya in Ringerike,

ALAN W. OWEN

Owen, W. : The upper Ordovician succession at and on Frognøya in Ringerike, Norway. A. Norsk Geologisk Tidsskrift, Vol. 58, pp. 245-258. 1978. ISSN 0029-196X. Lithostratigraphical units in the Ordovician succession in Ringerike are named formally for the first time. The distribution of species of Tretaspis indicates that the highest beds of the oldest unit studied, the Solvang Formation, are younger on Frognøy a (lowest PusgiUian) than at Norderhov (highest Onnian). The overlying shale unit, the Frognøy a Shale, contains weU preserved graptolites of the Pleurograptus linearis Zone (mid-Onnian to ?mid-Pusgillian) . The succeeding units described are dominantly calcareous and are named the Sørbakken Limestone (late Pusgillian to ?low Rawtheyan) and the Bønsnes Formation (probably whoUy Rawtheyan) .

Owen, Department ofGeo/ogy, The University, Dundee, DD14HN, Scotland. A. W.

Historically, the description of Cambro- succession referred to herein are interpreted successions within the Oslo Region has been purely lithostratigraphically. undertaken largely with direct reference to the named units of the Oslo- sequence (e. g. Strand 1933, Størmer 1953, Strand Hennings­ & Setting moen 1960). This has led to imprecision and misconceptions in correlation, examples of The Ringerike district is situated to the north­ which are detailed below for the Ringerike dis­ west of Oslo (StØrmer 1953, fig. 1). The local trict. The primary aim of this paper is to Lower Palaeozoic succession has a general establish a named lithostratigraphical framework north-east to south-west strike and youngs for the upper Ordovician succession in Ringerike south-eastwards. The succession rests on Pre­ prior to the description of the faunas. gneiss and is overlain in the east by It must be stressed from the outset that the lavas. In the north-east, Ordovician writer advocates the discontinuation of the etas­ rocks of Ringerike abut Silurian sediments of the je nomenclature originally applied to the Oslo­ district against the Randsfjorden fault Asker succession but subsequently imposed on (Holtedahl 1960, pl. 13). A brief general account other sequences in the Region. This terminology of the geology of the district and an extensive list was devised by Kjerulf (1857) and subsequently of papers published since 1950 are given by developed by, amongst others, BrØgger (1882, Whitaker (1977). 1887), Kiær (1897, 1902, 1908), StØrmer (1953), The sections described here are the most and Henningsmoen (1957). Originally no more complete upper Ordovician exposures in the than a shorthand method of expressing litho- and district and are at Norderhov, 3 km south of biostratigraphical units, this nomenclature de­ Hønefoss and, 11 km along the regional strike, veloped a chronostratigraphical connotation and on the island of Frognøya (Fig. 1). At Norderhov Henningsmoen (1955) outlined a scheme for its 70 m of strata crop out by the roadside, dipping use purely in chronostratigraphy. While this may gently southwards, but only the lowest 15 m is be feasible for the Cambrian Alum Shale and readily accessable as much of the exposure is Ceratopyge beds (Henningsmoen 1957:34-61), high and overhanging. The succession on evidence is accumulating to show that the Frognøya (Fig. 2) is well exposed on the north Silurian etasjer may lack even local and west coasts of the istand and there is in­ chronostratigraphical significance (W orsley termittent outcrop on the east coast: exposure 1971, Bassett Rickards 1971) and work on inland is very limited. & upper Ordovician successions in Ringerike and Few accounts of the Ordovician rocks of Hadeland shows that many of the se units have a Ringerike have appeared in the literature very limited application. Units of the Oslo-Asker although Kiær carried out extensive fieldwork.

16 - GeologiskTidsskr. 4/78 246 A. W. Owen NORSK GEOLOGISK TIDSSKRiFf 4 (1978)

"+ + + + + + + + + + "· + + + + + + + J . rvvvl Permian + + + + + + + + � + + + + + + + + 1111 .l VY + + + + + + vvvvvv + + + + + + + + '1111Jf Ringerike Group + + + + + +.�1 vvvvvvvv + /. 1 vvvvvvvvvv + + + + + 1 vvvvvvvvvvv Marine Silurian + + + + + + + + + + +.� vvvvvvvvvvvv + 1 vvvvvvvvvvvvv Ordovician + + + + .111 1 vvvvvvvvvvvvv + 1 ,'L vvvvvvvvvvvvvv + + +,ni\IY /VVVVVVVVVVVVVV Cambrian +: +: : :_ /,�1 � vvvvvvvvvvvvvvv vvvvvvvvvvvvvvv + + +,ni iJU' �vvvvvvvvvvvvvvv Hønefoss V V V V V V V V V V V V V V V Precambrian r-'---r-..__,--'---r-"'-r-'-r"--Atvvvvvvvvvvvvvvvv + vvvvvvvvvvvvvvvv l7 �� vvvvvvvvvvvvvvvv 5km vvvvvvvvvvvvvvvv :Mil V V V V V V V V V V V V V V V _.·:·:·.'o/

+ + N + + + + + + + + + + + + + + + + + + + + + + + + + + + \ + + + ++ + + + + + + l + + + + ++ + + NORWAY ) + + +.+ ( + + Ringerike + + + 'j ++ + ++ + + + + o o�1o + + + \) + + ++ + + + + + ++ ++ + + + + + + ++

Fig. Simplified geological map of the Ringerike district (based on Holtedahl 1%0, pl. I. 3).

His collections and fieldnote hooks pertaining to Nerby Member) and as interbedded limestones the Ordovician succession are housed in Pale­ and shales (the Lieker Member) in the south-east ontologisk museum, Oslo. More recently, David . of the district. The concept of the Solvang For­ L. Bruton of Paleontologisk museum, Oslo has mation is here broadened to include a coeval examined the Frognøya succession and has limestone unit in Ringerike, and the outcrop at kindly given the writer free access to his col­ Norderhov (NM71256585) is designated an lections. A summary of the lithostratigraphical hypostratotype. Although the base of the forma­ units defined and newly named herein and their tion is not seen at Norderhov, the upper 12 m interpretation by previous workers is given in (Fig. 4) is exposed and small outcrops to the east Fig. 3. suggest that this is close to the total thickness of the unit here. 80 m to the north of the Norderhov section, shales containing Broeggerolithus dis­ cors (Angelin, 1854) and Lonchodomas The Solvang Formation at aff. Norderhov rostratus (Sars, 1835) crop out by the roadside ( = Upper Chasmops Shale of Størmer 1953:86) Owen (1978:8-10) defined the Solvang Forma­ and are interpreted as belonging to the unit tion in Hadeland where it is developed as a below the Solvang Formation. nodular limestone over much of the area (the The Solvang Formation at Norderhov is com- NORSK GEOLOGISK TIDSSKRIFT 4 (1978) Upper Ordovician succession in Ringerike 247

N o 200m

Limeston e

32/Dip in degrees -+- Vertical strata --"' Fault

� / _....,..4' ,../ / 100m /

s

Fig. Geological map (omitting Permian dykes) of Frognøya and section along the west coast. Horizontal line on section 2. representsfjord leve!- approximately 60 m a.s.l. Inset map shows the position of the major dislocations on Frognøy a and the mainland and is taken from an unpublished study of aerial photographsby F. Bockelie ( 1970). J. 248 A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 4 (1978)

Størmer Strand Kiær This paper 1953 1933 1921

����!,'-,>:�·\:���.�;;1··�)���}��� '.· .\1. \ •.• & \ . �e,",.��r.�o1Æ��',1;{�\�1� GASTROPOD M BØNSNES FORMATION >.� 'T LIMES TONE ��i{' K\' ·' ·:(li.\"''�\J�}p'}�� %: ��1· ... . f,, . r\ ...�)���-· 4 • ���\:�@·h�i·.. ; ..�§�1·... ��it��; ISOTELUS LIMESTONE

4d r-disconformity- SØR BA KKEN LIMESTONE TRINUCLEUS l l l l TRINUCLEUS LIMESTONE

4cØ LIMESTONE

· .·········...... ······················. ....···...... ······································· ·································••••••••••••••••••• •••••••••••• ••••••••···· ·· ·o · ·····••••••••····••••••••••·········· ••••••••••••······· ·······••••••••······ o ...... LOWER ··················· ....·········...... ······ · ····-·····-···..····...·····.... . TRINUCLEUS TRINUCLEUS ········· ········· · ····...... ······ ········· · ···...·······...... TRETA SPIS · ········· . · ····· · · · ØYA ·········· SHALE ··········...... ··· ······ · . . ·········· ·········· ·········· SHALE · ········ ·········· ·········.· ········· · 4ca SHA LE ········ SHALE ········· · · ········· · ········ · · ········ · ········· 4ccr ··· ······· · · ······ ·· ········· · ········· · ····· ···· · ···· ···· · .·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·:.·.·.·.·.·.·.·.·.·:.·.·.·.·. · · · · · · · · · · · · · · ··· ······ · · ··· · · · · · · · · · · · : LOWER :::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::� 4c

. TRINUCLEUS : -:·: -:·: -: -: -: -: -:·:-: -:-:·: -:·:-: -: -: -:-::: " . . . 4b02 .·::.·::.·:.·.·.·::.·.�gberg Mbr LIMESTONE · · ···· · · · · · ··· · . ····· ······· ·· UPP ER UP PER ·········· · ------:-:·: -::::_.:..:.r ·------CHASMOPS UPPER � - -1-1-1 - l. �.J ------CHASMOPS - - - -- � SOLVANG - - -- LIMESTONE CHASMOPS � - - - - - 4b61 - - FORMATJO - - � - - N - -- LI MESTONE - - - - 4bo LIMESTONE � ------

Fig. The historical development of terms applied to the upper Ordovician successions at Norderhov and on Frognøy a (not to 3. scale). The Solvang Formation was originaUy recognised in Hadeland (Owen 1978), all other terms are new. The Høgberg Member of the Sol vang Formation on Frognøy a is the lateral equivalent of the lowest part of the Frognøya Shale at Norderho" NORSK GEOLOGISK TIDSSKRIFT 4 (1978) Upper Ordovician succession in Ringerike 249

ally impersistent and nodular horizons predomi­ nate in the lowest 2 m of the section. Between 2.07 and 2.27 m below the top of the unit are 10 closely-packed hands of limestone nodules with very Iittle intervening shale. The junction of the Solvang Formation with the overlying shale unit is abrupt and lithologically distinctive. Although most horizons in the section are ex­ tremely poorly fossiliferous, some contain a large, diverse fauna of , brachiopods, cystoids, crinoids, ostracods, and gastropods. A detailed log of the trilobite fauna is given by Bruton Owen (1979, fig. 5) and it is sufficient & to note here that the fauna is very similar to that of the Upper Chasmops Limestone in Asker.

The Solvang Formation on FrognØya

The upper 7.5 m of the Solvang Formation is exposed on the north-west promontory of Frognøya (NM65205820) where its development bedded limes tone is very different from the exposure at nodular Norderhov. The lower 4.42 m is composed limestone almost entirely of thin horizons of nodular limestone with very thin shale partings. This shale nodular development may be represented as a thin tongue in the upper part of the section at Norderhov and strongly resembles the Nerby Member in Hadeland. 0.92 m below the top of the nodular beds on Frognøya there is a 14 cm thick zone of nodular limestone striking east­ west and dipping northwards at 45°. This con­ trasts markedly with the gentle eastward dip of the succession above and below. It is not possi­ ble to assess the degree of dislocation at this level. The only fossiliferous horizon in these nodular beds is at the base of the section where the cystoid Echinosphaerites is abundant. The upper 2.88 m of the Solvang Formation on FrognØya is composed of altemating limestone and shale beds and is here termed the HØgberg Member. lts base marks the abrupt change from · Fig. Measured section through the Solvang Formation at nodular limestone and its top is the local base of 4. Norderhov. Metre intervals are indicated. the FrognØya Shale. It may prove desirable to regard the beds above the thin nodular develop­ ment at Norderhov as belonging to the HØgberg posed of altemating limestone and shale hori­ Member also but this must await the documenta­ zons and thus most closely resembles the Lieker tion of the spatia! development of the nodular Member in Hadeland. Most of the limestones horizons as must the use of member names form continuous beds, although some are later- currently used for the unit in Hadeland. 250 A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 4 (1978)

Lingula?, also occur, as do cystoids and 'Y·� TY crinoids. Strand (1933) described a new nautiloid species, Ephippiorthoceras frognoense, and L Sweet (1958) an indeterminate specimen of Diestoceras. Hamar (1966) detailed a very large I. l.. conodont fauna from the unit. rJ; The trilobite fauna of the HØgberg Member is ?'t. � listed in Fig. 6. No complete specimens are known and thoracic segments are rare. Many of the forms are known from a fossiliferous lens �� X"'L'"'� 2.35-2.52 m below the top of the unit and from AJ( 1.5-1.6 5 -:z: .Jl.. beds m below the top. In addition there Q)... are many specimens in Paleontologisk museum, X .c Oslo, whose precise level is not known. Størmer ..l.. EQ) A. (1930) described a specimen clearly belonging to � Tretaspis seticornis seticornis, but its precise

Q)� horizon is unclear, since it was recorded by him .c as being from 1.5 m (1930:57), m (1930:76), 1.3 Ol l (;) m (1945:76) and m (1953:87) above the l.J I trilobite-rich lens. Størmer's comment (1953:87) ="�\. ..1:� .:X: that the boundary between the 'Chasmops � Series' and the 'Tretaspis Series' (marked by the 6 ;r'[ --c:!!: ;;_!!; incoming of T. seticornis) Iies in the middle of 'Y -;'()C ..A..'lt. Y� .X what is now regarded as the HØgberg Member indicates that the specimen of T. seticornis seticornis must have been from the upper half of the unit. A second specimen of this form is LJ..I known from the unit but its precise level is also uncertain. �� Stratigraphical relationships of the 7 .l( }(_ HØgberg Member The trilobite-rich lens, probably a remnant of a ::J; A..J:;X shell bank, first attracted the attention of Kiær (1921), who correlated what he termed the

Lower Trinucleus Limes tone = HØgberg ..{_ � ( Member) with a disconformity near the base of nodular limestone the Lower Tretaspis Shale in Oslo postulated by bedded limestone Raymond (1916:244). Størmer (1953:68, 87) con­ shale curred with this, pointing out that T. kiaeri, which occurs abundantly in the HØgberg Member, is unknown in Oslo-Asker where, he argued, its Fig. Measured section through the Solvang Formation on 5. range is equivalent to a hiatus indicated by the Frognøya showing the development of the Høgberg Member. phosphorite band near the base of the Lower Metre intervals from the base of the overlying Frognøy a Shale are indicated. Tretaspis Shale. The resultant changes in the stratigraphical nomenclature exemplify the con­ fusion associated with the etasje terminology. A diverse fauna, dominated by trilobites, is StØrmer (1953:68, 128) correctly implied that the k,nown from the HØgberg Member. Spjeldnæs lowest part of the Lower Tretaspis Shale on (1957) described the brachiopods Strophomena Nakholmen in Oslo contains Tretaspis Cf!riodes, sp. and Gunnarella cf. rigida (Barrande, 1879). a species which occurs also in the upper part of Orthoids and inarticulate brachiopods, including the Upper Chasmops Limestone in Asker to the NORSK GEOLOGISK TIDSSKRIFT 4 (1978) Upper Ordovician succession in Ringerike 251

Abun- 1 1.5-1.6m TRILOBITE SPECIES ens below dance top

Bronteopsis sp. VR + Carrickia sp. VR Deacybele gracilis (Nikolaisen, 1961) VR + Decoroproetus sp. VR + Flexicalymene sp. VR + + Frognaspis stoePmeri Nikolaisen, 1965 c + harpid gen. et sp. indet. c + Harpidella (s.l.) sp. VR + Illaenus (Parillaenus) aff. fallax Holm, 1882 c + Lonchodomas aff. pennatus (LaTouche, 1884) A + + Miraspis sp. VR + Phillipsinella preclara Bruton, 1976 c + Prionocheilus aff. obtusus (McCoy, 1846) VR + Pseudosphaerexochus densig�nulatus Nikolaisen, 1965 R + Raymondella sp. VR + Remopleurella s . nov. p VR Remopleurides sp. nov. VR + Staurocephalus sp. nov. · VR + Stygina minor Skjeseth, 1955 c + + Teleph1:na s . p VR Tretaspis kiaeri størmer, 1930 A + + Tretaspis seticornis seticornis (Hisinger, 1840) VR

Fig. Trilobite species occurring in the Høgberg Member of the Solvang Formation on FrognØya. YR - very rare (less than 5 6. specimens known), R- rare (6- 10 specimens), C- common (11-20 specimens), A- abundant (many hundreds of specimens).

west. He advocated therefore that the term tool. The highest beds of the Solvang Formation '4b8', which previously was no more than at Norderhov contain populations ofT. ceriodes, shorthand for the Upper Chasmops Limestone in including specimens of a morphological type a purely lithostratigraphical sense (Fig. 7A), (with a short extra are of pits) which is known should be redefined biostratigraphically and di­ also from the youngest samples ofT. ceriodes in vided into two subzones - a lower T. ceriodes the Upper Chasmops Limestone of Asker. Simi­ zone ('4b81') extending into the lowest part of larly, Deacybele gra ei lis, Phillipsinella preclara, the Lower Tretaspis Shale in Oslo and aT. kiaeri and Raymondella sp. are all restricted to the zone ('4b�') represented by the T. kiaeri-bearing upper parts of the Upper Chasmops Limestone beds on Frognøya but absent in Oslo-Asker (Fig. in Asker and the .Solvang Formation at · 1m. Norderhov, but they occur also in the Høgberg T. ceriodes is succeeded stratigraphically by Member on Frognøya. Staurocephalus sp. nov. members of the T. seticornis group (of lngham occurs in the upper part of the Upper Chasmops 1970, see also Hughes, lngham Addison 1975) Limestone in Asker and in the HØgberg & without overlap, irrespective of lithology and Member. Despite this broad fauna! similarity, commonly over a few tens of centimetres or less the HØgberg Member also contains T. seticornis of strata in Oslo-Asker, Ringerike, Hadeland, and therefore is probably younger than either the , and northern England. This stratigraph­ Upper Chasmops Limestone or the Solvang For­ ical relationship is a close approximation to a mation at Norderhov. This indicates that time line and has great potential as a correlation limes tone deposition and the incoming of new, 252 A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 4 (1978)

A BRØGGER 1887 lithostratigraphical Oslo Asker

l Tr inucleus Shale, 4ca ffilimestone and shale, 4b5

8 STØRMER 1953

LITHOSTRATIGRAPH Y CHRONOSTRATIGRAPHY BIOSTRATIGRAPHY

Oslo Asker Frognøya

4ca Tretaspis Lower I seticornis liJTretaspis Shale T. kiaeri 4bO, I tcedode•

C THIS PAPER

LITHOSTRATIGRAPHY CHRONOSTRATIGRAPHY LITHOSTRATIGRAPHY

Oslo Asker Frognøya ' ·.·.·.·.·.·.·:.·.·.·.·.·.·.·:.·.·.·.·.·: l �:::::::::::::::;::::}::::�:::::::::::: ! Frognøya Shale ···.······:···.·:.·.·:: : ·-: _ _ , __ _ j _ _ _ Lower � Tr etaspis Høgberg: fil Member: Shale ---· l : Solvang -T--:':-T'-=-'T'-=-'�'T- l Format ion l l

l

Fig. Schematic illustration of the historical development of stratigraphical terms applied to units at the Caradoc-Ashgill 7A-C. boundary in Oslo-Asker and Ringerike.

limestone-restricted species evident in the upper Miraspis sp. T. seticornis seticornis also made parts of these units persisted longer around its first appearance at this time but was much FrognØya with, amongst others, the appearance more facies-tolerant and thus occurs in the of T. kiaeri, Pseudosphaerexochus den­ HØgberg Member limestones as well as the sigranulatus, Frognaspis stoermeri, and coeval shales of the Lower Tretaspis Shale of NORSK GEOLOGISK TIDSSKRIFT 4 (1978) Upper Ordovician succession in Ringerike 253

Oslo-Asker (Fig. 7C). There is, therefore, no Orbiculoidea sp., a large strophomenid, and an palaeontological evidence for a significant hiatus ortocone. The horizon also contains the graptol­ in the Oslo-Asker succession. ites Dicellograptus cf. johnstrupi (Hadding, 1915) and Orthograptus sp. preserved in full relief. The FrognØya Shale Toni (1975) described the bivalves Nuculites sp. Tancrediopsis plandorsata (Ulrich, 1892), Much of this unit at Norderhov is tal us covered, and an indeterminate pterinid from the Frognøya although the lowest 0.25 m is exposed. The Shale. Specimens assigned by Toni to stratotype is taken to be on the north-west Cyrtodontula ventricosa (Hall, 1847) may be promontory of Frognøya (NM65205820) where from this unit also. the formation is fully exposed and is 23.85 m thick (Owen 1977, fig. 23). Both the base and the top are sharp lithological boundaries with dom­ The Sørbakken Limestone inantly calcareous units above and below the formation. As is demonstrated in the preceding There is no complete section through this unit. discussion, the base of the FrognØya Shale is 37 m crop out at Norderhov but the base and the diachronous. The lowest 4.25 m on Frognøya is lowest few metres are talus covered here. The composed entirely of shale and is succeeded by lowest 34 m crop out on the north-west prom­ 4.85 m of shale with some limestone beds and ontory of Frognøya (NM65205820) and this con­ isolated limestone nodules. The remaining 14.75 stitutes the boundary stratotype for the base of m of the unit contains some limestone nodules the formation. The upper part of the unit crops and a large number of Iimestone beds, many of out on the west, east, and north-east coasts of which are argillaceous; some are strongly Frognøya, and the stratotype for the top of the bioturbated. formation is taken on the west coast of the island Fossils in the shale are commonly fragmentary south of the major north-easterly striking dis­ and crushed, but the hetter preserved material location where the upper 40 m is seen shows that F. trinucleina? occurs throughout the (NM65305775). The greatest measurable thick­ formation. On FrognØya, Tretaspis sp. nov. ness of the formation is north of this dislocation ( = T. sp ?nov. of lngham 1970) is found at many where 50 m of strata crop out, but neither the levels in the unit but the lowest few centrimetres base nor the top of the unit is exposed here. The contain very poorly preserved specimens re­ trilobite faunas of the lower 34 m and upper 40 m sembling T. hadelandica StØrmer, 1945. In addi­ of the Sørbakken Limestone suggest that there is tion, the upper half of the formation on FrognØya no overlap between these two profiles and that contains T. seticornis anderssoni Størmer, 1945. the thickness of the formation based on outcrop Few complete trilobite specimens are known pattem is of the order of 100m. from the Frognøya Shale, and thoracic segments There is no structural, palaeontological, or are rare. Some of the limestone beds contain a sedimentological indication of any hiatus in the well preserved fauna and the collections of SØrbakken Limestone. Strand's statement Paleontologisk museum, Oslo contain sped­ (1933: 102) that the re is a disconformity between mens of the trilobites Cybeloides (?Paracy­ the 'Trinucleus' and 'Isotelus' limestones was beloides) girvanensis Reed, 1906), Ca/y­ probably based on a reluctance to accept lateral aff. mene (sensu lata) aff. holtedahli Størmer, facies variation and therefore the belief that a 1945), Stenopareia sp. and Calyptaulax nor­ shale unit, the Upper Tretaspis Shale of Oslo­ vegicus Størmer, 1945. A Iimestone bed 7.5 m Asker, was missing on Frognøya and conse­ below the top of the unit on Frognøya has quently must be represented by a break in yielded a particularly rich fauna including the sedimentation or by deposition followed by ero­ trilobites Chasmops cf. extensa (Boeck, 1838), sion. A more acceptable explanation is that part Flexicalymene trinucleina (Tullberg, 1882)?, of the Sørbakken Limes tone is the lateral equiv­ Primaspis evoluta (Tomquist, 1884) subsp. nov., alent of the Upper Tretaspis Shale. Tretaspis seticornis anderssoni Størmer, 1945, The base of the Sørbakken Limestone marks a and T. sp. nov. Lenses within this bed are made distinctive change from shale to limestone de­ up almost entirely of small gastropods and the position. The top of the unit is marked by the brachiopod Chonetoidea; also known are first appearance of the calcareous alga 254 A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 4 (1978)

Base To p

Tretaapia sp. nov.

FZexicaZymene trinucZeina (Tu11berg, 1882)? Primaspia evoZuta (Tornquist) subsp. nov.

Tretaspis seticornis (Hisinger) anderssoni

l ----·X stønner 1945

CaZyptau�x norvegicus stønner, 1945 IZZaenus (PariZZaenus)cf roemeri Vo1borth, . 1864

Stenopareia ff. gZaber ( K ru1f , a je 1865) RemopZeurides emarginatua Tornquist, aff. 1884 -X CaZymene (s. Z.) cf. marginata Shir1ey, 1936 X illaenid indet. X CybeZoides (?ParacybeZoides)sp.

Chasmops sp.

IsoteZus sp.

Stygina aff. minor Skjeseth, 1955 harpid indet.

- Known range in unit c:::::=J Range wi thin the se 1imi ts

Range extends into under1ying/succeeding unit

Fig. The distribution of trilobite species in the Sørbakken Limestone on FrognØJo:a. Specimens from m above the lowest 8. 35-37 exposed part of the unit at Norderhov are indicated by crosses, but the precise relationship of this leve! to the Frognøya section is not known.

Palaeoporella in the basal beds of the overlying SØrbakken Limestone has a very diverse fauna BØnsnes Formation. The lowest 9 m of the dominated by brachiopods and, to a slightly Sørbakken Limestone on FrognØya consists of lesser extent, trilobites. Most of the brachiopods grey, argillaceous, nodular limestone beds and have both valves preserved but few complete or silty shales up to cm thick. In most of the almost complete trilobites are known and lO remainder of the unit the limestones are less thoracic segments are otherwise rare. silty, and whilst most horizons are less than The known distribution of trilobite species in l O cm thick, beds up to 30 cm thick are known. the SØrbakken Limestone is illustrated in Fig. 8. Most limestone horizons are to some extent Many forms have a long range in the unit and nodular. The intervening shales are calcareous, thus reflect a long period of sedimentary sta­ and although most are only a few centimetres bility. Only one specimen of Isotelus sp. has thick, some up to 20 cm thick occur, commonly been found by the writer, but there is a large containing isolated limestone nodules. Calcare­ amount of material of this form in Paleontologisk ous siltstones and fine sandstones are developed museum from unspecified horizons in the upper at some horizons. Towards the top of the unit the part of the unit. The following trilobites, also limestones are rather platy and dark grey, almost from the upper part of the unit, are known only black, in colour. from museum specimens: Atractopyge sp., With the exception of the lowest 9 m, the Chasmops eichwaldi Schmidt, 1881, en- aff. NORSK GEOLOGISK TIDSSKRIFT 4 (1978) Upper Ordovician succession in Ringerike 255 crinurid gen. et sp. indet., Panderia insulana mentioned outcrop is taken as the stratotype for Bruton, 1968, and Sphaerexochus sp. A the base of the formation (NM65305775), but the cranidium ascribed to Holotrachelus cf. punctil­ greatest exposed thickness is on the east coast of losus (Tomquist, 1884) is known frorri an un­ the island where the lowest 34 m of the unit specified horizon probably in the upper part of contains many Palaeoporella-rich limestones. the formation, and Stygina sp. occurs in the The se beds are platy, have irregular tops and lower part of the unit. bases, and many contain pyrite grains. On fresh The brachiopod fauna of the Sørbakken surfaces the limestones are black, whilst on Limestone is dominated by orthoids and weathered faces the matrix is light grey and the plectambonitaceans with most forms occurring rod-like algae are much darker in colour. Weath­ at levels throughout the formation. These long­ ered surfaces also reveal pockets of ochreous ranging forms include Anisopleurella? sp., fine sandy material. The Palaeoporella beds are Cyclospira cf. pentagonalis (Reed, 1897), overlain by lighter coloured bedded limestones, Dalmanella sp., /sorthis sp., Octoplecia sp., and many of which contain silicified colonies of the plectambonitacean sp. A few forms are more corals Catenipora sp. and Sarcinula sp. up to restricted in their distribution. A large stroph­ 0.25 min diameter. These beds are exposed only omenid similar to the form found in the intermittently on the west coast of Frognøya and FrognØya Shale occurs between 9 and 17 m higher beds are not seen. above the base of the Sørbakken Limestone on The Palaeoporella beds contain a varied fauna Frognøya and the up per few tens of metres of the including the trilobite Stenopareia sp., leptaenid unit contain Platystrophia sp., leptaenids, and plectambonitacean brachiopods, and also Orbiculoidea sp., and Lingula? sp. chain corals. No shelly fossils have been found The Sørbakken Limestone has a large mol­ in the overlying coral-rich beds. luscan fauna, and gastropods are common ThePalaeoporella beds form a convenient and throughout the unit. Toni (1975) described the mappable base to a unit here termed the Bønsnes bivalve Cleonychia from an unspecified horizon Formation. It would be premature to suggest in the formation and Strand (1933) described 15 that the formation corresponds precisely to the nautiloid species, man y new, from various hori­ 'Gastropod Limestone' until the more complete zons. Stick bryozoans and diplotrypid colonies section at Stamnestangen and the top of Kiær's occur throughout the formation and a fenestel­ unit on Vestre SvartØya have been documented. late form encrusting a nautiloid was found in the This is being done by N. M. Hanken (pers. lower part of the unit. Cystoids ar.e known from comm. 1978). Strand (1933) noted that the nodu­ the lower part of the formation, crinoid ossicles lar limestones and shales which constitute much occur at many horizons, and a pyritized ostracod of Kiær's Gastropod Limestone pass gradually from the top 0.5 m of the unit on Frognøya into the calcareous sandstones of the succeeding strongly resembles specimens figured by Hen­ unit (Kiær's '5b' or 'Die obersten Chasmops ningsmoen as cf. Schichten' =the 'Calcareous Sandstone Forma­ (1954) Macronotella praelonga (Steusloff1894). tion' of Strand Henningsmoen 1960). & Although the incoming of sandstone horizons is .gradual, this lithology actually becomes domi­ nant quite abruptly and it should be possible to The Bønsnes Formation definethe base of the overlying unit at this lev el. Only the lower part of this unit is exposed on FrognØya, where its base is mar�ed by the first appearance of the alga Palaeoporella. These Lithofacies distribution algal limestones were described by Kiær (1922: 122), who considered them to underlie the When viewed in its regional context the succes­ 'Gastropod Limestone' (of Kiær 1897). Strand sion described here lends support to the concept (1933) however, considered them to be the basal of concentric facies beits during the Ordovician beds of the Gastropod Lirri.estone. within the Oslo Region, such beits being concave The lower part of the Bønsnes Formation is eastwards and suggesting a westward shallowing exposed on the west coast of Frognøya, just to (e.g. Skjeseth 1952, Størmer 1967). Thus, the the south of the north-east promontory and in Solvang Formation in Ringerike and Hadeland the central part of the east coast. The first and the Upper Chasmops Limestone in Oslo- A. Owen NORSK GEOLOGISK TIDSSKRIFf (1978) 256 W. 4

Asker represent deeper-water limestone de­ (Brenchley Newall 1975) in Asker, and the & posits than the locally biohermal Encrinite and KalvsjØ Formation in Hadeland. The overlying MjØsa Limestones to the south and north respec­ arenaceous unit has broad equivalents in Hade­ tively (see StØrmer 1953 for stratigraphy). land (the Skøyen Sandstone Formation), Oslo Mon!over, the dominantly nodular limestone (the LangØyene Sandstone Formation), and in characterizing the Solvang Formation over much Skien-Langesund. of Hadeland and on FrognØya may be a shal­ lower-water facies than the interbedded limestone and shale development at Norderhov Correlation and the upper part of the formation in south-east Hadeland. The Upper Chasmops Limestone in All the trilobites known from the Solvang For­ Oslo-Asker is of the latter type also. mation at Norderhov occur also in the Upper The FrognØya Shale is one of a number of Chasmops Limestone of Oslo-Asker, and the shale units reflecting a transgression over much restriction of some forms to the youngest parts of the Oslo Region. There is no evidence for this of both units is very similar. As Dean (1%0, transgression in the north of the Region, where 1%3) noted, the trilobite faunas of the Upper the age of the upper part of the Mjøsa Limestone Chasmops Limestone and the Actonian and On­ is still unclear although it is known to contain nian stages in England are very similar with Ashgill conodonts (Hamar in StØrmer 1%7:204, species or closely related forms of Ampyxella, Hamar in BjØrlykke 1974: 18). The Mjøsa Chasmops, Lonchodomas, Platylichas, and Tret­ Limestone has a karst top locally (Størmer 1953, aspis common to both. In addition, Triarthrus Skjeseth 1963) and is overlain by Llandovery linnarssoni Thorslurid, 1940 occurs in the highest quartzite. In Oslo-Asker and Ringerike this part of the Onnian Stage in Salop and the lowest transgression is demonstrably from the east, but part of the Lower Tretaspis Shale on Nak­ in Hadeland it is from the north, or even north holmen, Oslo, where it is associated with Tret­ west (Owen 1978: 14-15), and here may be a aspis ceriodes (s. l.). Similarly there is a strong result of fault control south of the Mjøsa dis­ faunal similarity between the Upper Chasmops tricts. Limestone and its correlatives in the Oslo Re­ The SØrbakken Limestone is very similar to gion and the Macrourus Limestone and its cor­ the Gagnum Limestone Formation in Hadeland relatives in Sweden. (see Owen 1978: 17) but differs in ha ving thicker The stratigraphical importance of members of sha1e horizons. The Herøy Limestone (Dahl in the Tretaspis seticornis group succeeding T. Kjerulf 1S57, Brøgger 1884) in the Skien-Lange­ ceriodes is discussed above. In northem Eng­ sund district is in need of modem description land, the type region for the Ashgill Series, this but is of the same broad type as the Gagnum change-over occurs at the Onnian-Pusgillian (i.e. Limestone and SØrbakken Limestone. The three Caradoc-Ashgill) boundary with T. convergens may eventually be best regarded as local rep­ Dean, 1962 succeeding T. ceriodes alyta Ingham, resentatives of one formation. This broad unit 1970. The HØgberg Member of the Solvang For­ interdigitates with a more easterly shale succes­ mation on FrognØya contains T. seticornis sion. Such an interfingering is relatively simple seticornis and thus is probab1y lowest Pusgillian in Hadeland, where the Gagnum Limestone di­ in age. This species and T. sp. nov. are not vides the shales of the Formation into known from Britain, but their relative distribu­ two tongue-like members in all but the south­ tion in Ringerike and the Lower Tretaspis Shale east of the district (Owen 1978). In Oslo-Asker, is the same as in the Fjiicka Shale and its it is probably represented by the altemation of correlatives in Sweden. limestone and shale units - the so-called Tret­ The upper part of the Lower Tretaspis Shale aspis and Isotelus series (BrØgger 1887, Strand around Oslo contains specimens of Dicel­ Henningsmoen 1960). lograptus which, like those of the upper part of & Facies pattems in the uppermost part of the the FrognØya Shale, closely resemble D. john­ Ordovician succession of the Oslo Region are strupi, a species which occurs in the much more complex, but the Bønsnes Formation Pleurograptus linearis Zone (late Onnian to is probably a lateral equivalent of the upper part ?mid-Pusgillian) in Sweden. Thus the upper part of the HerØy Limestone in Skien-Langesund, of the FrognØya Shale is no younger than mid­ the lower part of the Langåra Formation Pusgillian in age. NORSK GEOLOGISK TIDSSKRIFT 4 (1978) Upper Ordovician succession in Ringerike 257

The Sørbakken Limestone contains many Acknowledgements. This study was undertaken during the long-ranging trilobite species which are of lim­ te nu res of aN.E.R.C. studentship at Glasgow University and a ited value in correlation. A cranidium from N.A.T.O. postdoctoral fellowship at Paleontologisk museum, Oslo. The writer is grateful to Professors T. N. George, B. E. 35-37 m above the lowest exposed part of the Leake, and G. Henningsmoen for facilities at these institu­ unit at Norderhov is here compared with tions. I thank Dr. J. K. Ingham, who supervised the project, Calymene (s. /.) marginata Shirley, 1936 from Dr. D. L. Bruton for much discussion and for his criticisms of an earlier draft of this paper, Professor A. D. Wrjght for the lower Drummuck Group (Cautleyan) of identifying some of the brachiopods, the Engebretsen family of south-west Scotland. lngham (1977) has also Ringerike for their hospitality, and my wife Gill for her help at compared material with this species from the various stages of this work. highest part of the PusgillianStage and the lower half of the CautleyanStage in northem England. /sote/us sp. and Chasmops aff. eichwaldi oc­ References

cur in the upper part of the sørbakken Limestone Angelin, N. P. 1854: Palaento/ogia Scandinavica I, Crustacea and are known also from Hadeland, where they formationis transitionis. Fase. 2, 21-92. Lund. occur with a demonstrably Rawtheyan fauna Barrande, J. 1879: Systeme Silurien du centre de la Boheme. (Owen 1978: 19). Stygina aff. minor occurs both Iere partie. Recherches pa/eontologiques 5. Classe des Mollusques. Ordre des Brachiopodes, 226 pp. Prague. in the upper part of the SØrbakken Limestone Bassett, M. G. & Rickards, R. B. 1971: Notes on Silurian and in the Husbergøya Shale Formation stratigraphy and correlation in the Oslo district. Nor. Geo/. (Brenchley & Newall 1975) in Oslo-Asker, Tidsskr. 51, 247-260. where it is associated with Mucronaspis kiaeri BjØrlykke, K. 1974: Depositional History and Geochemical Composition of Lower Palaeozoic Epicontinental Sediments (Troedsson, 1918). Troedsson's species is closely from the Oslo Region. Nor. Geo/. Unders. 305, l-Sl. allied toM. mucronata (Brongniart, 1822), afo�m Boeck, C. 1838: Obersicht der bischer in Norvegen gefunde­ which first appears in northem England in the nen Formen der Trilobiten Familie (138-145). In Keilhau, B. Cystoid Limestone (late Rawtheyan) and which M. (ed.) Gæa Norvegica. Christiania. Brenchley, P. J. & Newall, G. 1975: The Stratigraphy of the is known from the upper part of the Ashgill Upper Ordovician Stage 5 in the Oslo-Asker District, Nor­ Series in Britain, Sweden, Bohemia, and way. Nor. Geo/. Tidsskr. 55, 243-275. . Brongniart, A. In Brongniart, A. & Desmarcest, A.-G. 1822: The lowest part of the Bønsnes Formation Histoire naturelle des Crustaces fossiles: les trilobites, 65 pp. Paris. exposed on FrognØya contains no faunal ele­ Bruton, D. L. 1%8: The trilobite genus Panderia from the ments of detailed correlative value but there is a Ordovician of Scandinavia and the Baltic areas. Nor. Geo/. very diverse trilobite fauna in collections in Tidsskr. 48, 1-53. Paleontologisk museum, Oslo from the Bruton, D. L. 1976: The trilobite Phillipsinella from the Ordovician of Scandinavia and Great Britain. Palaeontology 'Gastropod Limestone' in Ringerike. This fauna /9, 699-718. contains many elements in common with the Bruton, D. L. & Owen, A. W. 1979: Late Caradoc-early Husbergøya and lower part of the Langåra for­ Ashgill trilobite distribution in the central Oslo Region, Norway. Nor. Geo/. Tidsskr. 59. mations in Oslo-Asker ( = '5a' of Brenchley & Brøgger, W. C. 1882: Die silurischen Etagen 2 und 3 im Newall 1975). The upper few metres of the Kristianiagebiet und auf Eker, ihr Gliederung, Fossilien, HusbergØya Formation contain Tretaspis sortita Schichtenstorungen, und Kontaktmetamorfosen. Uni­ (Reed, 1935) broeggeri Størmer, 1945, which is versitats-programm (Kristiania), 376 pp. no more than a local population of the nominate BrØgger, W. C. 1887: Geologisk kart over øerne ved Kristiania. Nyt. Mag. Naturvidensk. 31, 1-36. subspecies from the upper Drummuck Group Dean, W. T. 1%0: The Ordovician trilobite faunas of south (highest Rawtheyan) of south-west Scotland. Shropshire, I. Bull. Brit. Mus. Nat. Hist. (Geo/.)4, 73-143. The fauna of the 'Gastropod Limestone' of Dean, W. T. l %2: The trilobites of the Caradoc Series in the Ringerike contains forms known also from the Cross Fell Inlier of northern England. Bull. Brit. Mus. Nal. Hist. (Geo/.) 7, 65-134. Boda Limes tone of theSiljan district of Dalarna, Dean, W. T. 1%3: The Ordovician trilobite faunas of south Sweden. The field relationships of the Boda Shropshire, IV. Bull. Brit. Mus. Nat. Hist. (Geo/.) 9, 1-18. Limestone indicate that it has a more extended Hadding, A. 1915: Der mittlere Dicellograptus-Schiefer auf Bornholm.Lunds Univ. Årsskr. N. F. Afd. 2, Il, 1-40. stratigraphical range than the literature suggests Hall, J. 1847: Palaeontology of New York, I. Geo/. Surv. N. Y., (J. K. lngham pers. comm. 1975), but until the 1-338. distribution of species through the Boda Hamar, G. 1966: The Middle Ordovician of the Oslo Region, Limestone has been documented, detail ed corre­ Norway, 22. Preliminary Report on Conodonts from the lations are not possible. Oslo-Asker and Ringerike Districts. Nor. Geo/. Tidsskr. 46, 27-S3. Henningsmoen, G. 1954: Upper Ordovician Ostracods from the Oslo Region, Norway. Nor. Geo/. Tidsskr. 33, 69-108. 258 NORSK GEOLOGISK TIDSSKRIFT 4 ( 1978)

Henningsmoen, G. 1955: Om navn på stratigrafiske enheter. Schmidt, F. 1881: Revision der ostbaltischen silurischen Nor. Geo/. Unders. 191, 5-17. Trilobiten nedst geognotischer Obersicht des ostbaltischen Henningsmoen, G. 1957: The trilobite family Olenidae. Skr. Silurgebiets. Abt l, Phacopiden, Cheiruriden und Encrinuri­ den. Mem. Acad. Imp. Sei. St. Pt!tersbourg (=Zap. lmp. Nor. Vidensk. Akad. Oslo, Mat.-naturvidensk. Kl., 1957, · No. l, 1-303. Akad. Nauk.),(7),30, 1-237. Hisinger, W. 1840: Lethaea Sveeica seu Petrijicata Sveeiae, Shirley, J. 1936: Some British trilobites of the family iconibus et characteribus illustrata. Suppl. secundum. 6 pp. Calymenidae. Q. J. Geo/. Soc. Lond. 92, 384-422. Norstedt: Holmiae. Skjeseth, S. 1952: On the Lower Didymograptus Zone (3b) at Holm, G. 1882: De Svenske Artene of TrilobitsUigtet Illaenus Ringsaker, and contemporaneous deposits in Scandinavia. (Dalman). K. Sven. Vetensk.-Akad. Hand/. 7, 1-148. Nor. Geo/. Tidsskr. 30, 138-182. Holtedahl, O. (ed.) 1960: Geology of Norway. Nor. Geo/. Skjeseth, S. 1955: The Middle Ordovician of the Oslo Region, Unders. 208, 540 pp. Norway, 5. The trilobite family Styginidae. Nor. Geo/. Hughes, C. P., lngham, J. K. & Addison, R. 1975: The Tidsskr. 35, 9-28. morphology, classific