The Middle Ordovician of the Oslo Region, Norway, 29. Stratigraphy of the MjØsa Limestone in the Toten and Nes-Hamar areas
PAUL R. OPALINSKI & TOM L. HARLAND
Opalinski, P.R. & Harland, T. L.: The Middle Ordovician of the Oslo Region, Norway, 29. Stratigraphy of the MjØsa Limestone in the Toten and Nes-Hamar areas. Norsk Geologisk Tidsskrift Vol. 6/, pp. 59-78. Oslo 1981. JSSN 0029-/96X.
The upperrnost Ordovician strata in the Toten and Nes-Hamar ares of the Oslo Region, southem Norway, are represented by a ca. l 00 m thick, predominantly carbonate sequence, named the Mjøsa Limestone. Five subdivisions of the forrnation are recognizable: the lower Fumesfjorden Member; the rniddle, laterally equivalent Eina, Gålås, and Bergevika Members; and the upper Holtjern Member. Further subdivision into formal and informal units illustrates the lateral and tempora! facies diversity,
· which is -characteristic of the formation. Three shallowing-upward sequences are distinguished and !hese perrnit a chronostratigraphic correlation to be effected.
P. R. Opa/inski, Texaco Canada Resources Ltd., 605 Fifth Avenue S. W., P. O. Box 3333, Station M, Calgary, Alberta T2P 2P8, Canada. T. L. Har/and, Department ofGeology, University of Kee/e, Keele, Stajfordshire ST5 5BG, England.
The Ordovician of the Oslo Region consists of a of the Upper Ordovician elsewhere in the Oslo 450 m thick succession of marine sediments, Region (StØrmer 1953, Harland 1980). Instead it mainly of a restricted facies range, varying be is overlain, often with marked unconformity, by tween shales and shales with nodular limestones. a coarse sandstone of Silurian age. Initially, the Important departures from this pattern occur at MjØsa Limestone was considered equivalent to the top of the Upper Ordovician where coarse the Gastropod Limestone (substage 5a. Zone of clastics and oolites form parts of the sequence Dicellograptus anceps), which is present in sev (Brenchley & Newall 1975) and at the top of the eral districts of the Oslo Region (Kiær 1897, Middle Ordovician where massive limestones of Holtedahl 1909), but, after inspection of other widely varying facies were deposited. It is the Ordovician sequences in the Baltic regions, stratigraphy of the massive limestone present in Raymond (1916) and subsequently Kiær (1920·, the Toten and Nes-Hamar areas of the Mjøsa 1927) formed the opinion that the MjØsa District (Fig. l) which is described here. Limestone was in fact Middle Ordovician in age Kiær (1897, 1908) made the first extensive and penecontemporaneous with the Up per studies in the MjØsa District and introduced the Chasmops Limestone (substage 4b8. Upper part term 'MjØskalk' or 'Mjøsa Limestone' for the of Zone of Dicranograptus clingani) of the Oslo massive limestone unit forming the uppermost Asker district. This view has been supported by Ordovician beds there. Further important con the majority of later workers and is held here, tributions to the geology of the Mjøsa District although unpublished work on conodont as are those of Holtedahl (1909, 1934), StØrmer semblages by Hamar (see BjØrlykke 1974) sug (1953), and Skjeseth (1963), while numerous gests that the up per beds of the MjØsa Limestone mentions of the Mjøsa Limestone occur in the may extend in to the Up per Ordovician. The many palaeontological publications dealing with history of stratigraphical terminology applied to the Oslo Region as a whole. The age of the Mjøsa the Mjøsa Limestone is summarized in Table l Limestone has been subject to considerable con together with the divisions proposed herein. troversy as it is not succeeded by the The stratigraphical procedure adopted within characteristic dark Lower Tretaspis Shale' this account follows I.S.S.C. recommendations or a correlative (substage 4ca. Zone of (Hedberg 1972, 1976). Detailed descriptions of Pleurograptus linearis), which occur at the base sections through the MjØsa Limestone, including 60 P.R. Opalinski & T. L. Har/and NORSK GEOLOGISK TIDSSKR!Ff l (1981)
thological variation presents difficulty in the selection of a representative formational stratotype and may explain why Kiær (1908, 1927) did not suggest a particular section. Strand & StØrmer (1956) stated the type locality to be the Nes-Hamar district without specifying further, and Skjeseth (1963) indicated that ex posures on HelgØya and sections at Furuberget, both in the Nes-Hamar district, were type localities. Recent studies by the present authors (Opalinski 1977, Harland 1978) have revealed a Jack of complete sections through the Mjøsa Limestone in either the Nes-Hamar or Toten areas. By far the most representative and com plete section available is that present at Eina (Fig. 2) and, despite this locality being in the Toten area rather than the Nes-Hamar area, we designate this the formational stratotype. How ever, we do stress that the full character of the MjØsa Limestone can on! y be described by refer ence to each of the following five members: the lower Furnesfjorden Member, the middle, later ally equivalent Eina, Gålås, and Bergevika Members, and the upper Holtjern Member (Fig. 3). For this reason we have selected a stratotype for each member and describe each in detail. Three of the members have their stratotype in O km 45 the Nes-Hamar area; viz. the Furnesfjorden and Gålås Members at Snippsand and the Bergevika Figure l. Ordovician outcrop in the Oslo Region showing the geographical subdivisions used by Størmer (1953). The Mjøsa Member at Bergevika on HelgØya (Fig. 2), Dis tri et as u sed in this account consists of StØrmer's Toten and whereas the remaining two have their Nes-Hamar districts. l= Hamar, 2 =Gjøvik, 3 = Eina. stratotypes in the Toten area; viz. the Eina Member as part of the formational stratotype at Eina and the Holtjern Member at Holtjern (Fig. 2). We believe this satisfies both the historical all selected stratotypes, are in Opalinski (1977). aspect and the modem concepts of stratigraphic The majority of locality names referred to in the definition and description (Hedberg 1976). text are taken from Norge 1:50,000 Blad 1816 I (Gjøvik), Blad 1816 Il (Eina), Blad /916 Ill (Østre Toten) and Blad 1916 IV (Hamar) and the The Mjøsa Limestone remainder are taken from l:5,000 map sheets Derivation of name. -The name is taken from supplied by local works departments (Fig. 2). the geographical area known as the MjØsa Dis Localities are listed in the appendix. trict (StØrmer 1953) in which the unit forms the uppermost Ordovician strata, and translates Kiær's original name.
Stratigraphy General character. - The MjØsa Limestone is The MjØsa Limestone has been accepted as a distinctive as a limestone-rich lithological unit, distinct formation since Kiær's (1908) studies approximately l00 m thick, occurring between and has been treated as such subsequently the quartz-siltstone/ shale sequence of the under (StØrmer 1953, Strand & StØrmer 1956, Skjeseth lying Furu berg Formation (Skjeseth 1963:65) and 1963), though a detailed definition has never the equally distinctive coarse quartzites of the been proposed. The high degree of internal li- overlying HelgØya Quartzite (Skjeseth 1963:78). NORSK GEOLOGISK TIDSSKRIFT l (1981) - Stratigraphy of Mjøsa Limestone 61
Ta ble l. History of terminology of the Mjøsa Limestone and related strata in the Toten and Nes-Hamar areas.
KlÆR STØRMER SKJESETH HAMAR unpub. KlÆR 1927 HARLAND 1897 HØEG OPALINSKI • -in 1953 1963 conodonts 1981
HELGØYA HELGØYA QUARTZITE sandstone quartzite QUARTZITE
M 5 Upper limes tone Assernblage C J Hedstroemia norveqica w. ø HOLTJERN MC'i·1BF.R 10-30 m s A Sandy crystalline M 4 u. argillaceous 5b reef limestQne J schists 11-12 m MJØSA MJØSA L shale ...•.. (Zone Ill) "' LIMESTONE LIMESTONE I S 3 Middle limestone ?4b6+ 4b6 M K 22 m (c.lOO m) EINA BERGEVIKA GAL AS (30-100 m) Assemblage B E A MEMBER MEMBER MEMBER delicata s L 2 L. argillaceous T K schists 7 m o
Dark crystalline l Lower limestone FURNESFJORDEN MEMBER sandy limestone Solenopora •••••••••••• (Zone II) 18 m F U Assemblage A R ramosa u Cyclocrinus B Cyclocrinus Sa zone... ?4by E beds R G untreated FURUBERG FORMATION Shales with lime F �;.tone beds ... (Zone I) o R
Coelo- � Coelo- sphaeridium T sphaeridium zone •..?4b a 1 beds o N
Formational Stratotype. - The section expo nearby localities, notably at Kyset (Fig. 5a), Rud sed at Eina (Map reference: 60°38'22"N, (Fig. 5b), DØlbakken, and Bøverbru Kalkverk 10"36' lO" E) on the east side of Route 4 approxi (Fig. 2). This boundary is placed at the uncon mately one kilometre north of Eina Railway formable contact with the overlying coarse sand Station is selected as being the most complete stones of the HelgØya Quartzite. Over much of and representative of the availab1e exposures. the Toten area this contact takes the form of a The stratotype is illustrated in Fig. 4. In addi spectacular palaeokarstic surface. tion, almost complete sections through the for mation occur at Snippsand, Bergevika and Description Furuberget in the Nes-Hamar area and at Sivesinhagen and Ånnerud in the Toten area. As previously stated, the full character of the MjØsa Limestone can on1y be described with
Lower boundary. - At Eina the base of the reference to its five component members, details formation is placed immediately below a thick of which are given below. (40 cm), fine bioclastic limestone bed, be1ow which fissile mudstones and calcareous silt The Furnesj}orden Member stones of the Furuberg Formation are discon tinuously exposed. In the Toten area this bound Name. - This member is named after Furnes ary is clearly exposed at Heiset but is rarely seen fjorden (Fig. 2) around which the most elsewhere. In the Nes-Hamar area the lower characteristic exposures are present. boundary is well exposed at Snippsand,
Bergevika, and Furuberget, where a distinct Geographical extent. - Throughout the MjØsa change from fissile mudstones with thin calcare District. ous siltstones and sandstones to thick bioclastic limestones is seen. General character. - The characteristic sedi ments of the member are massive or cross Upper boundary. -The upper boundary is not stratified bioclastic limestones together with exposed at Eina but can be inspected at several more thinly bedded calcareous sandstones and 62 P.R. Opalinski & T. L. Har/and NORSK GEOLOGISK TIDSSKRIFT l (1981)
Figure 2. Map of the Toten and Nes-Hamar areas showing all localities and place names mentioned in the text. Locality names as follows: 1 Eina, 2 Gamme, 3 Sivesinhagen, 4 Hemstadmoen, 5 Dølbakken, 6 Holtjern, 7 Hole Kalkverk, 8 Bøverbru Kalkverk, 9 Bøverbru, 10 Rud (2), 11 Rud (1), 12 Kallerud, 13 Kyset, 14 Sellanrå, 15 Ånnerud, 16 EngØydegård, 17 Granvang, 18 Ile, 19 Heiset, 20 Eriksrud, 21 LØken, 22 Bergevika south, 23 Bergevika north, 24 Bergevika quarry, 25 HelgØya skole, 26 Kjelsrud, 27 Furuberget south, 28 Furuberget north, 29 Snippsand, 30 Gålås quarry, 31 Børsvollen.
siltstones and argillaceous mudstones. The Lower boundary. -This coincides with the Iower calcareous alga Solenopora is abundant through boundary of the formation. At Snippsand it is out and locally forms large bioherms. In many placed immediately below a thick 47 cm irregu parts of the Toten area the upper parts of the larly cross-stratified bioclastic limestone bed member are typified by peloidal limestones and which overlies thinly interbedded calcareous some red or green siltstones; where present sandstones and argillaceous mudstones of the these are termed the Sivesinhagen Beds. Furuberg Formation.
Unit stratotype. -The unit stratotype is selected Upper boundary. -At Snippsand this is placed as the section at Snippsand in the Nes-Hamar immediately above the massive fine bioclastic area (Fig. 2) where it occurs in the southern part limestones containing peloidal horizons aitd re of the extensive cliff exposures (Map reference: worked Solenopora debris and below the base of 60°48'02"N, ll000'40" E). The stratotype sec the lowermost stromatoporoid-rich horizon of tion is illustrated in Fig. 6. the overlying Gålås Member. A similar contact is seen at Furuberget. Elsewhere some variation Thickness. -At Snippsand the member is 30.70 occurs due to lateral facies changes at the top of m thick and is of comparable thickness through the Furnesfjorden Member and particularly at out, although thins to 23.30 mat Bergevika. the bases of the overlying Eina, Gålås, and NORSK GEOLOGISK TIDSSKRIFT l (1981) Stratigraphy of Mjøsa Limestone 63
TOTEN NES-HAMAR AREA AREA SOUTH NORTH
Figure 3. Areal and stratigraphic distribution of the five members of the Mjøsa Limestone.
Bergevika Members. At Bergevika, Solenopora regular trough cross-stratification accentuated rich bioclastic limestones of the Furnesfjorden by discontinuous silty drapes and argillaceous Member are overlain by the basal flaggy calcare laminae. Internal scours are quite common in ous sandstones of the Bergevika Member, these beds. Coarse bioclastic material increases whereas in the Toten area the upper beds of the in abundance vertically through the unit, Furnesfjorden Member are often highly peloidal culminating with the higher beds being very and can be overlain by thinly interbedded densely and coarsely bioclastic and often includ calcareous siltstones and bioclastic limestones, ing unsorted shell layers up to l m thick. Fauna) e.g. at Eina (Fig. 4), grey calcisiltites, e.g. at assemblages in the thick limestones occur solely Holtjern, or red calcareous siltstones, e.g. at as dense, unsorted lenses, layers and horizons, Ånnerud, of the Eina Member. typically including abundant trilobite debris (mostly indeterminate asaphids and Toxo chasmops extensus (Boeck), orthocone Description nautiloids, Rafinesquina sp., bryozoans, and The Furnesfjorden Member is best described by Tallinnella kiaeri Henningsmoen. Sowerbyella reference to three subdivisions, these being an sericea (Sowerby), Tallinnella mjoesensis Hen interbedded bioclastic limestone, calcareous ningsmoen, and a diverse gastropod assemblage sandstone and argillaceous mudstone unit, a unit are locally abundant in the lower limestones of consisting of Solenopora bioherms with their the unit whereas assemblages in the higher, more related coarse bioclastic limestones, and the bioclastic limestones are dominated by Sivesinhagen Beds. 'Rhynchotrema borealis', Orthambonites sp., and abundant Solenopora debris. As stated, Interbedded bioclastic limes tone, calcareous these thick limestones are interbedded with fis
sandstone and argillaceous mudstone unit. - sile argillaceous mudstones with thin calcareous The lowermost beds of the Furnesfjorden sandstones. The mudstones are highly Member are typified by thickly bedded (0.25- bioturbated, while npple-drift lamination is com 2.60 m), pale grey weathering trough cross mon in the sandstone beds, which are parallel stratified bioclastic limestones which are in sided or markedly lenticular. Wash-outs and terbedded with variable thicknesses of brown gutter casts are also present. Bioclastic material weathering, fissile argillaceous mudstones with occurs as layers or basal veneers in these sand thin (2-10 cm) calcareous sandstones. Despite stones, which include fauna! assemblages domi considerable thickness variation, this unit can be nated by Rafinesquina sp., Sowerbyella sericea recognized throughout the MjØsa District. At (Sowerby), and occasionally Ctenodont(l nuda Snippsand the unit is 17.40 m thick (Fig. 6) and is Soot-Ryen & Soot-Ryen. dominated by thick beds of bioclastic limestone The unit is similarly developed at the other which characteristically exhibit large-scale, ir- localities in the Nes-Hamar area. At Furuberget 64 P.R. Opalinski & T. L. Har/and NORSK GEOLOGISK TIDSSKRIFT l (1981)
48 --
47 -=-=
46 -- -
-- 57 -- -- 56 -- ....l 51 -- 50 -- -- 49 - � Bioturbated limestone [2] Rubbly bioclastics - Hemstadmoen Beds lnterlaminated dolomite & micrite Rud Beds Peloidal limestone Skeletal limestone Coral biostrome Bioclastic limestone Dolomitic siltstone Grey calcisiltite lnterbedded mudstone & bioclastic limestone Dark cross· stratified limestone ) Sivesinhagen Beds Peloidal limestone Bioclastic limestone Solenopora bioherm Bioclastic limestone & calcareous sandstone NORSK GEOLOGISK TIDSSKR!Fr l (1981) Stratigraphy of Mjøsa Limestone 65 a. Kyset b. Rud Fig ure 5. Detailed stratigraphic sections of the upper parts of the Mjøsa Limestone at Kyset (a) and Rud (b) showing the [:-.::.:::.:.='::] Coarse sandstones (Helg6ya Quartzite) palaeokarstic upper surface of the formation and its unconformable lil il Bioturbated limestones -mainly grainstones contact with the overlyiog � Bioturbated limestones -mainly micrites HelgØya Quartzite, logged in metres. � Peloidal, intraclastic limestones (north and south) the unit is thicker (ca. 26 m) dolomitic limestones. Indistinctly and massively and is richer in argillaceous mudstones, whereas bedded coarse pelmatozoan bioclastics occur at Bergevika it is considerably thinner (ca. 4 m). immediately adjacent to the bioherms, thinning In the Toten area the unit is generally poorly rapidly away from them. The bioherm at Eina exposed, but where seen, e.g. at Eina, Sivesin consists largely of packed clusters and individuals hagen and Løken, is more highly bioclastic of Solenopora and large colonies are less common throughout arid contains lower proportions of than at Snippsand and Bergevika. Underlying argillaceous mudstone. the bioherm at Eina are 14 m of irregularly bedded bioclastic and arenaceous limestones Solenopora bioherms and associated lithologies. with numerous small, separated in situ clusters - Large bioherms consisting almost exclusively of Solenopora and horizons of reworked debris. of the calcareous red alga Solenopora form an Similar beds are widespread in the Toten area important part of the Furnesfjorden Member but exposures are generally poor. Discontinu and, together with their associated lithologies, ously exposed sequences through these beds dominate the member at several localities. Large occur at Ånnerud, EngØydegård, Kallerud, and bioherms are exposed in the cliffs at Snippsand LØken (Fig. 2), where Eojletcheria irregularis and Bergevika and similar features are seen at Hill is occasionally present with the algal Eina and BØrsvollen. At Snippsand and clusters. Bergevika exceptionally large nodular colonies of Solenopora up to 1.5 m in diameter are The Sivesinhagen Be ds. - This unit consists of common, but laterally extensive, closely packed the uppermost beds of the Furnesfjorden lenses and clusters are most common and in Member and is well developed in the Toten area terdigitate with layers of coarse bioclastic mat but poorly so in the Nes-Hamar area. The unit erial, finer arenaceous limestones, and some consists of a sequence of peloidal limestones interbedded with red and/or green siltstones and has been formally named due to its important Figure 4. Stratigraphic section showing the sequence through the MjØsa Limestone at the stratotype locality of Eina in the palaeoenvironmental implications (Opalinski Toten area, logged in metres. 1977 and in prep.). The most complete and 5-No,-,;k Geologisk Tidsskr. 1181 66 P.R. Opalinski & T. L. Har/and NORSK GEOLOGISK TIDSSKRIFT l (1981) 30.7m 11 10 0'------J � Bioclastic � Solenopora bioherm 1::::::::c::1 l i mestones � Figure 6. Detailed stratigraphic Calcareous section through the Furnesfjorden D sandstones Peloidal limestones Member at the stratotype locality of Snippsand, Nes-Hamar area, - Argillaceous mudstones logged in metres. characteristic sequence through the unit is seen similar sequence is seen at the top of the at Sivesinhagen (Fig. 2) where it is 10 m thick Furnesfjorden Member, e.g. at Hole Kalkverk and consists of very coarse peloidal limestones where exposed bedding planes in the red silt with grainstone textures interbedded with red stones reveal large-scale desiccation cracks and siltstones. Several of the limestones contain Sol small-scale symmetrical ripple marks. The enopora de bris but its abundance is lower than in Sivesinhagen Beds are thinner at Eina (8 m) the underlying beds. Ancistrorhyncha sp. also where siltstones are lacking,the abundance of occurs sporadically but otherwise the unit is peloids is reduced and the upper 3.20 m consist unfossiliferous. Elsewhere in the Toten area a of several (20-70 cm) sets of low angle accretion NORSK GEOLOGISK TIDSSKRIFT l (1981) Stratigraphy of Mjøsa Limestone 61 cross-stratified dark bioclastic limestone (Fig. underlying dark bioclastic limestones of the 4). The cross-sets are concave upwards and each Furnesfjorden Member to interbedded argilla set is bounded by an erosional base. Sets are ceous mudstones and thin bioclastic limestones. graded, with coarse basal lags rich in Solenopora Elsewhere the base of the member is rarely seen debris. In general, the erosion surfaces are es due to poor exposure. sentially planar with irregularities of the order of only 1-2 cm. However, the lowermost erosion Upper boundary. -The top of the Eina Member surface is highly irregular, cutting a minimum of is defined by a well marked erosion surface 35 cm into the underlying peloidal limestone and which cuts into the Rud Beds. also showing undercutting. Lateral equivalents of the Sivesinhagen Beds Description are poorly developed in the Nes-Hamar area. At Snippsand the upper 2.30 m of the Furnesfjorden The Eina Member is best described by reference Member consist of thick bedded, sparsely to two units, one informally termed the lower bioclastic limestone with discontinuous layers beds and one formally termed the Rud Beds. and horizons of peloids (Fig. 6). A similar, though thinner (1.90 m) sequence is seen at Lower beds. - At Eina (Fig. 7) the lowermost Furuberget, whereas at Bergevika no equivalent 8.50 m of the member are dominated by slightly of the Sivesinhagen Be ds is seen. fissile, brown weathering silty argillaceous mud stones interbedded with thin (I-S cm), often discontinuous beds of bioclastic limestone. The Eina Member Small-scale colour mottling is typical in the Name. -The member is named after the expo mudstones and arises from bioturbation by or sure at Eina in the Toten area. ganisms producing Chondrites burrows. Sow erbyella sericea (Sowerby), Rafinesquina sp., Geographical extent. -Only present in the Toten Zygospira sp., bryozoans, Tallinnella kiaeri area. Henningsmoen and trilobite fragments, notably Toxochasmops extensus (Boeck) and General character. - Grey calcisiltites predomi Paraharpes ottawaensis similis Nikolaisen, oc nate in the lower parts of the member while the cur quite commonly in these beds, particularly in upper parts are characterized by a sequence of lenses and layers at the bases of the bioclastic peloidal limestones and interlaminated dolomites limestones. Similar lithologies are also present at and micritic limestones, which are termed the Eriksrud (only upper parts exposed) and at Rud Beds. In addition to these main lithologies, LØken (only lower parts are exposed). At Ån thinly interbedded argillaceous mudstones and nerud, EngØydegård, and Sellanrå (Fig. 2) the bioclastic limestones and red calcareous silt lowermost parts of the Eina Member consist of stones are present locally at the base of the red cakareous siltstones and mudstones. At member; small reefs, coral-stromatoporoid Ånnerud approximate1y 9 m of these red, mas layers, and oncolitic bioclastic limestones are sive to slightly fissile muddy calcareous silt locally present in the middle parts. stones occur and a similar sequence is seen at EngØydegård where the siltstones are 6.5 m Unit stratotype. - The unit stratotype of the thick and include 1.25 m of greenish-brown member is selected as the section exposed at weathering fissile mudstone. Both the interbed Eina, where the member is well exposed in the ded silty mudstones and bioclastic limestones disused quarry and adjacent pavement surfaces and the red siltstones are overlain by grey (Map reference: 60°38'22 "N, 10°36'10 "E). The calcisiltites, which are characteristic of the lower stratotype section is illustrated in Fig. 7. half of the Eina Member throughout the Toten area. At Eina (Fig. 7) the calcisiltites are 12.90 m Thickness. - At the stratotype locality the thick and include two zones where small (2-4 cm member is 38.40 m thick and is of comparable diameter) spherical or ellipsoidal m•cnt•c thickness wherever present. nodules occur. Part of a similar sequence is seen at Rud (Fig. 2). In both cases the nodules include Lower boundary. - At Eina the base of the discrete globular or tubular features composed member is marked by a rapid transition from of coarsely crystalline calcite, which resemble 68 P. R. Opalinski & T. L. Har/and NORSK GEOLOGISK TIDSSKRIFT l (1981) 8 16 - ---- 7 23 6 14 -=-=- 22 5 12 ==�= 3 11 2 o Bioclastic � Peloidal limestones l:!::x::::l limestones F:$�� �Dolomitic � siltstones f�®Coral biostrome � - lnterlaminated dolo- Figure 7. Detailed stratigraphic a · t1tes· � c 1 CISI·1 mites and micrites section through the Eina Member at the stratotype locality of Eina, 6 lnterbedded bioclastic limestones & mudstones Toten area, logged in metres. Phytopsis borings (see Raymond 1931, Textoris and receptaculitids. An additional widespread 1%8). Sedimentary structures are absent from faunal assemblage in these calcisiltites is as the calcisiltites, which are only sparsely fos sociated with horizons of widely spaced low siliferous although large isolated individuals of domal and irregular sheet stromatoporoids, Strophomena norvegica Spjeldnaes are particu which includes Ancistrorhyncha sp., Zygospira larly characteristic. The higher beds at Eina sp., Vellamo sp., and, occasionally, Solenopora. include a denser and more diverse assemblage of Such horizons are well exposed at Rud and Ancistrorhyncha sp., bryozoans, Dalmanella Ånnerud. At Eina the grey calcisiltites are tran sp., Hedstroemina sp., orthocone nautiloids, sitionally overlain by yellow-weathering lami- NORSK GEOLOGISK TIDSSKRIFT l (1981) Stratigraphy of Mjøsa Limestone 69 nated dolomitic siltstones, which are often indis commonly massive. Stylolites are abundant and tinctly colour-mottled. This mottling is due to where particularly well-developed give the differential dolomitization of the siltstones and is limestones the appearance of being thinly bed closely related to moderately intense bioturba ded. Lenses and layers of unmicritized bioclasts tion. The dolomitic siltstones are 5.50 m thick at are sometimes present at the bases of beds, and Eina, while similar siltstones elsewhere, e.g. at horizons of composite grains (grapestones) are Eriksrud and Ånnerud, are thinner (ca. l m) and also common, together with intraclasts up to 7 have a slightly fissile aspect. Small biohermal mm in diameter. At several horizons the peloidal patch reefs, approximately 5 m in vertical extent or bioclastic grains have thin oolitic coats, and and 15 m in lateral extent, are exposed at Kal poorly developed oncolitic coatings have been lerud and Bøverbru (Fig. 2). The reefs overlie noted at Eina but not elsewhere. Thin (20-210 thin Solenopora-rich bioclastics and are built up cm) laminated units consisting of orange and rust of Eofletcheria irregularis Hill, Lyopora favosa coloured fine-grained dolomite and interlami Hill, stromatoporoids and Solenopora together nated pale grey micrites and orange dolomites with other minor associates. They are im are common and are characteristic of the Rud mediately flanked by coarse bioclastic Beds. Individual laminae are laterally continu limestones which rapidly grade laterally into ous up to tens of metres and are typically planar, massive "grey calcisiltites. Well-developed although low amplitude undulations and irregu Girvanella-oncolites are present in both the Iar crinkles also occur. Though present at Rud (3 flanking and sub-reef beds. At Eina the beds m below the top of the Rud Beds) these lith equivalent to these biohermal reefs overlie the ologies are best seen at Kyset, Holtjern, dolomitic siltstones and consist of a thin (65 cm) Dølbakken, and Ånnerud. Bedding surfaces are laterally continuous biostromal deposit with rarely exposed but where seen they often reveal large (up to l m diameter) colonies of regular or irregular systems of desiccation Eofletcheria irregularis Hill and small, irregu cracks (e.g. at Bøverbru Kalkverk). Laterally larly domal stromatoporoids. This overlies dark discontinuous horizontal veins of crystalline bioclastic limestones (70 cm thick) which include calcite (?sheet cracks) are occasionally present Girvanella-oncolites, and is overlain by a thick interlayered with the dolomites and micrites. (1.95 cm) bed of high angle, large-scale av Further desiccation features are present at sev alanche and trough cross-stratified skeletal eral localities where irregular clasts of pale limestone (Fig. 7). A comparable sequence to micrite surrounded by a dolomitic matrix occur that at Eina is seen at Eriksrud (Fig. 2) where in layers within the Iaminated units, e.g. at exceptionally well-developed oncolites occur in Kyset. association with small domal stromatoporoids and are overlain by a 1.40 m thick bed of The Gålås Member cross-stratified skeletal limestone. Name. - The member is named after its occur Rud Beds. - This unit is uniform throughout the rence in the vicinity of Gålås in the Nes-Hamar Toten area and is easily distinguished by its area (Fig. 2). coarse peloidal limestones and interlaminated dolomites and micrites. The lower boundary of Geographical extent. - The member is of re the unit is usually a clear lithological change stricted extent, occurring only in the northern from grey calcisiltites (e.g. at Ånnerud, parts of the Nes-Hamar area. Eriksrud, Rud and Holtjern) or biohermal- or biostromal-associated bioclastics (e.g. at Eina General character. - Pale grey mtcntes and and Kallerud), while the upper boundary peloidal limestones predominate throughout and coincides with that of the Eina Member and is usually exhibit an irregularly 'reticulate' aspect marked by a well-developed erosion surface (see resulting from the selective dolomitization of p. 67). The character of the Rud Beds is best dense burrow systems. Horizons with widely seen at Rud, after which the unit is named, spaced stromatoporoids recur through the where coarse peloidal limestones are dominant member. in the upper 18 m of the Eina Member. Infre quently these peloidal limestones exhibit low Unit stratotype. - The extensive cliffexposure at angle accretion cross-stratification, but are more Snippsand on the western shore of Furnes- 70 P. R. Opalinski & T. L. Harland NORSK GEOLOGISK TIDSSKRIFT l (1981) fjorden is selected as stratotype (Map reference: the member at Furuberget (north and south) 60°48' 02 "N, Il ooo• 40 "E). The stratotype section show little variation from the stratotype although is illustrated in Fig. 8. fewer stromatoporoid horizons are present. In Gålås quarry (Fig. 2) the micrites and peloidal Thickness. - The precise upper limit of the Jimestones characteristic of the member are in member is not exposed at Snippsand (see below) terbedded with thin purple or green siltstones, but an approximate thickness of 36.50 m is and much purple staining has been Ieached into estimated. A similar thkkness is estimated at the underlying limestones. Short vertical burrows quarry at Furuberget. are occasionally present in the green siltstones and are infilled with purple coloured sediment. Lower boundary. - The stratotype of the Gålås Member overlies that of the Furnesfjorden The Bergevika Member Member and this is defined above (p. 62). Name. -The member is named after the classi Upper boundary.- The top of the member is not cal lakeshore exposures around Bergevika on exposed at Snippsand where a covered zone HelgØya (Fig. 2). corresponding to a thickness of 8.55 m occurs between beds of the Gålås Member and overly Geographical extent. -The member is confined ing interbedded calcareous sandstones, bioclas to HelgØya in the Nes-Hamar area. tic limestones, and argillaceous mudstones. Although the stratigraphical position of the lat General character. -The member is dominated ter beds is uncertain due to the presence of an by large stromatoporoid reefs of the Bergevika unusual coral fauna, the upper limit of the Gålås Reef Complex, which overlies thinly interbed Member is here regarded as being close to the ded calcareous sandstones and argillaceous base of the covered zone. mudstones and is, in turn, overlain by dark micrites. Description Unit stratotype. -The stratotype of the member The Gålås Member consists of massiv ely bedded is selected at Bergevika south (Fig. 2) where it pale grey micrites with subordinate peloidal forms part of the extensive cliff exposures (Map limestones. Many beds have a reticulate aspect, reference: 60°44'45"N, ll00l'IO"E). The which results from the preferential dolomitiza stratotype sequence is illustrated in Fig. 9. tion of dense, irregular burrow systems. Con Thickness. -The member is 35.45 m thick at the centrations of insoluble materials along stylo stratotype locality, which is the only complete lites, which are abundant throughout the section. member, also contribute to the reticulate pat tern. Fossils are scarce in these limestones, Lower boundary. - The base of the Bergevika though Solenopora, bryozoans and orthocone Member is sharply transitional between thickly nautiloids are occasionally present, and primary bedded Solenopora-rich bioclastic limestones of sedimentary structures are absent. Numerous the underlying Furnesfjorden Member and the laterally extensive horizons with stroma interbedded flaggy calcareous sandstones and toporoids are interlayered with the micrites and argillaceous mudstones at the base of the peloidal Jimestones (Fig. 8). Within these hori member. zons stromatoporoid coenostea vary greatly in morphology, with domal, Iaminate and irregular Upper boundary. - The upper boundary is sheet-Iike forms all common, and are usually placed at the slightly erosional contact between widely spaced along the horizons. The horizons the massively bedded micrites at the top of the vary between 15 and 55 cm in thickness and have member and the overlying fissile mudstones with a quartz-silt-rich, often dolomitic matrix. In bioclastic layers of the Holtjern Member. addition to the stromatoporoids, these horizons are moderately fossiliferous, usually containing Description an assemblage of Ancistrorhyncha sp., large bivalves, occasional bryozoans, pelmatozoan For the purposes of description the Bergevika debris and S -""" _.., ,..., '' <.J '' 7 '• " r' f r 1 J 15 �:-T�::0--:1 ·..L: . - 6 14 22 30 r�+ ·��;:�. 5 13 21 l '7) �:,�. ··<� 29 """� ·.�·�c�· .. ':;;.\ t""'J r .. . {' 36.5m � �··r}':J �r.' t;;J::, 0' ·' r.:. '•'r. r r: ..... f.' cJ. /J r rJ '' •... ' r, •� ,..•j 4 12 20 28 36 - ��:r�-r,�, ��[lut:),...... r_ , �.,.o' (l r,�c.� ,. 3 11 19 27 35 �.. .. ,�) n , 'rl:; [',• ' ·.. ) . � ' _;;1f-l.C .. -;, rJ l "'•· - � •l \. .- ...... w.. ··�· ... -\1. '(� •) :;'J 2tJ��� ... d �!' 10 18 - - 26 34 E-: l • r"J ' ...r, '�' ," t:' (•' '..' .. 'r:':10l r: l 9 25 - - 17 33 l (�J C> '::/. C�J�' •• (�:�� ':.ri' F-�--�if .-;· ,-._j o-���� 8 16 24 32 Bioturbated limestones Figure 8. Detailed stratigraphic � section through the Gålås Mem Stromatoporoid·rich horizons ber at the stratotype locality of Snippsand, Nes-Hamar area, � Shaley calcareous siltstones logged in metres. �-:-:-::::-:-� interbedded calcareous sandstone and argilla 8.60 m ofthe member consist ofthinly interbed ceous mudstone unit, the Bergevika Reef Com ded flaggy calcareous sandstones and biotur plex, and an upper dark micrite unit (Fig. 9). bated argillaceous mudstones. The sandstones are generally parallel-sided and laterally continu lnterbedded calcareous sandstones and argil ous, though several have asymmetrical ripple laceous mudstones. - At Bergevika the lower marked surfaces. Bioclastic horizons and lenses 72 P.R. Opa/inski & T. L. Har/and NORSK GEOLOGISK TIDSSKRIFT l (1981) 16 15 l .. 30 . · •.:.:.· ... .l . ···.:�.:.·:: ; ... · .. • · ·; .• � :.::..:l· -."·:..·-::. . . .· , . � .- ��'<'. ·,. l� l 'l .... ,!•'.,..· ; ..... , .. ·. .·. . ··:.--. 35.45m '1:·' ·.-; .• �: 11 lO 9 33 8, Ill 32 p::;:::::J Bioclastic � Dark micrites � limestones Figure 9. Detailed .tigraphic r---1 Calcareous . Arenaceous section through the B\��Jevika L.__j sandstones limestones - Member at the stratoty� Argillaceous locality of Bergevika sou� Nes t<{:']Reefs , mudstones Hamar area, logged in metre�. are common in the lower sandstone beds and unit and occur with less common Zygospira sp., bioclastic material is also common in the upper Cyclospira sp., Diplotrypa sp., abundant metre of the unit where it sometimes forms bryozoans and crinoidal debris. discrete beds. Sowerbyella sericea (Sowerby), Strophomena keilhaui Holtedahl, and Bergevika Reef Complex. - Several reefs are Da/mane/la sp. are characteristic fossils in this exposed in the unit stratotype (Fig. 9) and addi- NORSK GEOLOGISK TIDSSKRIFT l (1981) Stratigraphy of Mjøsa Limestone 73 tional examples are present in the cliff-top particularly well-developed and conspicuous. At quarry at Bergevika south, the coastal exposure Bergevika south, this algal-rich bed grades up at Bergevika north, Kj elsrud and HelgØya skole into a sequence of thinly bedded arenaceous (Fig. 2). The larger reefs (up to 15 m vertical limestones (Fig. 9), which thins towards the extent, 50 m lateral extent) are dominated by larger reefs. Small-scale cross-stratification, stromatoporoids, especially of the genus Pac scours, small wash-outs and ripple-drift lamina hystylostroma, while the smaller reefs of the tion are common in the lower of these beds complex are more diverse and resemble those which also contain lenses and horizons of present in the Eina Member (see p. 69). bioclastic material, especially gastropods. Oetailed descriptions of the reefs are given in Higher in the sequence the abundance of Opalinski (1977, chapter 4). bioclastic admixture is reduced and irregular, Sediments included in the reef complex are thin (1-5 mm) horizons of fissile calcareous diverse but can be divided into four broad mudstone sometimes separate the limestone groups: beds. The higher limestone beds become prog l. lrregularly bedded coarse bioclastic ressively muddier and are transitional to overly limestones occur stratigraphically below the ing dark micrites. leve! of reef development and lateral to the southern limits of reef growth at Bergevika Dark micrites.- The upper 5.60 m of the member south. Encrinites are particularly characteris at Bergevika south are composed of thickly tic, especially immediately underlying the reefs. bedded dark micrites (Fig. 9). These micrites Below the lev el of reef development, the bioclas characteristically weather to a pale bluish-grey tic limestones are generally regularly bedded and colour and are similar to those of the Gålås often exhibit accretion cross-stratification and a Member (p. 70) but lack the latter's reticulate grading from coarse bioclastic bases to finer aspect. The lower beds of the unit contain scat tops. In contrast, beds lateral to the reefs are tered bioclastic material and sparse fossils. most commonly thickly and irregularly bedded Large indeterminate bivalves are particularly and, although large-scale accretion and trough typical, together with Mjoesina mjoesensis cross-stratification can be detected in several of Spjeldnaes and Oepikina sp. these beds, the majority are massive and appear poorly sorted due to the presence of large, The Ho/tjern Member scattered Solenopora fragments. 2. Interbedded dolomitic limestones and Name. -The member is named after the series of bioclastic Iimestones predominate between the quarries adjacent to the south shore of Holtjern reefs, where beds are most usually 5-20 cm thick (Fig. 2). though the bioclastic Iimestones thicken towards Geographical extent. - Throughout the Mj Øsa reefs. In situ colonies of Eojletcheria irregularis Oistrict. Hill, Lyopora favosa Hill, and Palaeophyllum sp. are common in the dolomitic limestones, General character. - The member is typified by together with small stromatoporoids and intensely bioturbated limestones in which prefe orthocone nautiloids. rential dolomitization has affected the burrows 3. Thinly bedded (4-10 cm) dark micrites are and produced a reticulate aspect. Coarse con also present between the reefs and especially glomeratic bioclastic limestones, herein termed adjacent to them to the north. These are identical the Hemstadmoen Beds, occur at the base of the with the more thickly bedded micrites in the member in the Toten area and fissile siltstones upper part of the Bergevika Member which are form the basal beds in the Nes-Hamar area. described below. Large, poorly preserved bivalves and the alga Vermiporella inconstans Unit stratotype. - Several quarries adjacent to HØeg are present in the inter-reef micrites. the southern shore ofHoltjern form a continuous 4. At Bergevika south a characteristic thin (ca. section through the member and provide the 60 cm) bed with abundant Girvanella oncolites stratotype (Map reference: 60°39'35 "N, and Dimorphosiphon rectangularis HØeg frag 10°39'40 " E), which is illustrated in Fig. 10. ments overlies the reefs. A similar unit is seen overlying reefs at other localities, e. g. at Thickness. -At Holtjern the member is 25.90 m HelgØya skole (Fig. 2) where the oncolites are thick and is of comparable thickness elsewhere, 74 P.R. Opalinski & T. L. Harland NORSK GEOLOGISK TIDSSKRIFT l (1981) 5 10 15 20 25 -- s - - - - s 4 9 14 19 24 s 3 8 13 18 23 2 7 12 17 22 -- 6 16e:E:E 25.9m 5 10im 20� 25 l s � Bioturbated limestones - mainly micrites � Bioturbated limestones - mainly grainstones [i}}] Rubbly bioclastic limestones with Solenopora S Stromatoporoids Figure /0. Detailed stratigraphic section through the Holtjern Member at the stratotype locality of Holtjern, Toten area, logged in metres. with the exception of Kyset (Fig. 5a) where it is moen Beds, a fissile siltstone unit, a bioturbated considerably thinner. limestone unit, and a cross-stratified peloidal and/or bioclastic Iimestone unit. Lower boundary. -At the base of the member at Holtjern a well-marked erosion surface cuts into Hemstadmoen Beds. - Though exposed in the the upper beds of the underlying Eina Member Holtjern quarries as part of the unit stratotype and is overlain by the Hemstadmoen Beds. This (Fig. 10) and throughout the Toten area, the erosional contact is characteristic throughout the Hemstadmoen Beds are most accessible and Toten area and a similar, though less weU most characteristically developed at the locality marked erosional contact is seen at several of Hemstadmoen (Fig. 2), after which the unit is localities on HelgØya in the Nes-Hamar area. named. The Hemstadmoen Beds infill and over The lower contact is not exposed elsewhere. lie the irregular erosion surface which marks the base of the Holtjern Member and are typically Upper boundary. -The upper boundary of the poorly sorted, coarse, rubbly bioclastic Holtjern Member coincides with that of the limestones which sometimes exhibit vague Mjøsa Limestone and has been described above accretion cross-stratification. Orange weathering (p. 61). detrital dolomite is evident in the matrix, and dolomite and quartz-silt laminae occur in the occasionally distinguishable cross-sets. The Description Hemstadmoen Beds are always less than 1.50 m Four units are referred to in the following de thick (0.80 m at Hemstadmoen) and include scription of the Holtjern Member: the Hemstad- abraided Solenopora fragments with NORSK GEOLOGISK TIDSSKRIFT l (1981) Stratigraphy of Mjøsa Limes tone 75 Streptelasma sp., Hesperorthis sp., Orthambo and despite considerable variation in grain type nites sp., Vellamo sp. and Platystrophia dentata across the Mjøsa District they are instantly rec (Pander). A diverse fauna of bryozoans and ognizable in the field due to their irregularly and gastropods is also present locally, e.g. at Bøver intense! y bioturbated nature, which, in combina bru Kalkverk (Fig. 2). At Ånnerud several scour tion with preferential dolomitization of the bur surfaces are present and sediments of the unit row systems, has resulted in an irregular reticu are much finer than at other localities, with thin Jate aspect. Bioturbated limestones overlie the veneers of fine debris infilling irregularities of Hemstadmoen Beds and the fissile siltstones and the scours, overlain by highly irregular, high to form the uppermost beds of the Holtjern moderate angle cross-stratified fine bioclastic Member at the majority of localities. At these limestones. localities the palaeokarstic upper surface of the MjØsa Limestone forms the upper limit of the Fissile siltstones. -At Bergevika south the low unit. At several other localities in the Toten area ermost 10.70 m of the Holtjern Member consist the bioturbated Jimestones are overlain by predominantly of brown weathering fissile silt thickly bedded, unbioturbated peloidal or stones with thin, often discontinuous bioclastic bioclastic Iimestones. At Holtjern the layers. The base of this unit is marked by bioturbated limestones are 24.70 m thick (Fig. bryozoan-rich fine bioclastic limestone which 10) and similar thicknesses are present at Rud infills smooth irregularities in the erosional base and Bøverbru Kalkverk (Fig. 2). Elsewhere, of the member. Several flaggy calcareous sand however, they are often much thinner. Typi stones (7-10 cm thick) occur in the lower part of cally, the bioturbated sediments are bluish-grey the unit and occasionally exhibit ripple-drift weathering micrites with scattered bioclasts, lamination although they are more commonly which include lenses and thin beds of fine parallel-laminated or massive. The siltstones are bioclastic Jimestone. However, in the higher intensely bioturbated and Chondrites burrows beds at Eina, Holtjern, Gamme, Rud, and can be identified. The unit at Bergevika south is throughout the unit at Ånnerud and Kyset, highly fossiliferous, typical forms including bioclastic limestones and some peloidal layers Mjoesina mjoesensis Holtedahl, Strophomena are predominant and sometimes exhibit small keilhaui Holtedahl, Zygospira sp., Cyclospira scale accretion cross-stratification. The reticu sp., Oepikina sp., Dalmanella sp., Vellamo sp., late aspect of these beds is less marked due to Tallinnella kiaeri Henningsmoen, Diplotrypa the fact that short, vertical and steeply inclined sp., stick bryozoans, Subulites sp., Conularia Tigillites burrows are the main biogenic sp. and trilobite fragments. At Snippsand, beds structures rather than the dense sub-horizontal which we regard as belonging to the Holtjern networks which are typical of the finer sedi Member contain a higher proportion of bioclastic ments. In the Nes-Hamar area the sequences material which occurs in irregular lenses or beds through the bioturbated limestones are domi within bioturbated fissile siltstones. In addition nated by micrites, although beds of bioclastic to the faunas present at Bergevika south, and peloidal material occur in the uppermost 2 m Catenipora? sp., Favosites? sp., Streptelasma at Eik on HelgØya (Fig. 2). The bioturbated sp. and Rafinesquina sp. are common. At limestones are generally poorly fossiliferous. Eriksrud in the Toten area (Fig. 2), 11.20 m of Scattered individuals of Vella mo sp., Platystrop thinly interbedded dark green dolomitic silt hia sp., Leptaena sp., planar spiral gastropods stones, mudstones, and pale bioclastic and orthocone nautiloids are present in the finer limestones overlie the Hemstadmoen Beds and sediments. Horizons with sparsely or densely are lateral equivalents of the siltstones seen at distributed stromatoporoids are also present and Bergevika south and Snippsand. The proportion Vermiporella inconstans HØeg occurs occasion of clastic material decreases sharply towards the ally in the Toten area. upper part of the unit at Eriksrud and is minimal in the higher 2.50 m where in situ Vermiporella Cross-stratified limestones. - These are thickly inconstans Høeg, Solenopora sp., small do mal bedded, pale grey or white bioclastic, peloidal stromatoporoids and Heliolites? sp. are present. and occasionally intraclastic limestones which exhibit well-developed avalanche and herring Bioturbated limestones. - Bioturbated lime bone cross-stratification. Where present, these stones are characteristic of the Hol tjern Member beds form the uppermost part of the Holtjern 76 P.R. Opalinski & T. L. Har/and NORSK GEOLOGISK TIDSSKRIFf l (1981) Toten Nes-Hamar South Central South North f..:,�;·;:'::'.::-::;:::_:1 h::�;:,:;;:·,:,:/1 Quartzite ----- _. '!�>':'-:·:·. _... "' E "' Holtjern Member J ø T s ��-tr-E rwr A L T��� Berge vika Gålås l Member Member M E E ------Furnesfjorden Member Format ion [.:::::=.=.:;.:'.:Yi Carbonates � Very shallow subtidal Formation boundary � & intertidal -+-- � Subtidal � Member boundary Clastics E Erosional contacts f!i!fii\fii\] Very s ����rd:ubtidal & i , S Sharp contacts bi/).J Subtidal T Trans itional contacts NORSK GEOLOGISK TIDSSKRIFT l (1981) Stratigraphy of Mjøsa Limestone 77 Member. The cross-stratification is particularly so regarded here. The rhythms are particularly weU-marked at Rud where the lower contact of well-developed in the Toten area where clear the unit is smoothly erosional and cuts at least 2 changes from transgressive subtidal clasties m into the underlying bioturbated limestones through subtidal carbonates to intertidal and (Fig. 5b). Elsewhere, e.g. at Kyset (Fig. 5a), this possibly supratidal carbonates are seen in each lower contact is sharp but exposure is insuffi of the Furnesfjorden, Eina and Holtjern cient to detect downcutting. Members (Fig. Il). Following Walther (1893) and the principles of correlation outlined above, these can be related with the less well-developed subtidal clastic to subtidal carbonate rhythms of Summary and conclusions the Nes-Hamar area. Thus, the Furnesfjorden The Mj øsa Limestone composes the uppermost and Holtjern Members are chronostratigraphic Ordovician strata in the Toten and Nes-Hamar units recognizable across both areas white the areas and is considered upper Middle Ordovician Eina, Gålås and Bergevika Members are time in age. It consists of predominantly carbonate equivalents representing the development of a lithologies which are distinct from underlying large reef complex (Bergevika Member) and and overlying clastic units (Furuberg Formation related back-reef and Jagoonal sediments (Gålås and HelgØya Quartzite respectively). The Mj Øsa and Eina Members). The final regressive phase Limestone is approximately l 00 m thick and has (represented by the Holtjern Member) culmi been subdivided into five distinct members: the nated with emergence and a prolonged period of lower Furnesfjorden Member; the middle Eina, subaerial solution weathering of the limestone Gålås, and Bergevika Members; and the upper before a Silurian transgression deposited the Holtjern Member. Although a suitably refined coarse clastics of the HelgØy a Quartzite. biostratigraphical correlation of the various sec tions through the Mj øsa Limestone is not possi ble due to the scarcity of biostratigraphically Appendix "useful" faunas, a satisfactory internat chronostratigraphy can be established using sedimentary criteria. The majority of sediments l. List of localities mentioned in the text making up the formation were deposited at or very dose to mean sea level on a wide, essen Toten area tially flat epicontinental sea floor where even 60°42'05"N, 10°38' 55 "E minor fluctuations in sea levet bad profound Ånnerud BØrsvollen 60°30'20"N, to•39'40"E effects on bathymetrical and hydrological condi Bøverbru tions, which in turn control sediment ( = Åsjordet) 60°40' 05 "N, !0°4l'IO"E characteristics. In such situations it is possible to Bøverbru Kalkverk 60°40'00"N, 10°39' JO "E 60"39'32 "N, 10°38'55 match sedimentological changes in several sequ DØlbakken "E Eina 60°38'22 "N, 10°36' JO "E ences and to recognise interrelated sedimentary EngØydegård 60.41'47 "N, 10"40'55 "E facies (Krumbein & Sloss 1963). In the case of Eriksrud 60°42'17 "N, 10°43'30"E the Mj Øsa Limestone this method of correlation Gamme 60°38' 55 "N, 10°37'20 "E 60042' 20 "N, 10"42' 15 "E is enhanced by the presence of shallowing up Heiset Hemstadmoen 60°39'05 "N, 10°39'10 "E ward rhythms and particularly by the presence Holtjern 60°39'35 "N, I0°39'40"E of clastic influxes which form the bases of the se Kallerud (=Holte) 60°40' 50"N, 10.42'40"E rhythms (Fig. Il). Such clastic influxes into Kyset 60.41'17"N, to•39'IO"E 60°41 '50 "N. 10°44'35"E predominantly carbonate shelf areas have been LØken Rud l 60°40'35"N, 10°40'40"E utilized as time markers elsewhere (Krumbein & Rud2 60°40'35 "N, 10°40'40 "E Sloss 1963, Coogan 1972, Wilson 1975) and, in Sellanrå 60°41'55 "N, 10°38' 10 "E view of the sharp or erosional bases of the clastic Sivesinhagen 60°39'05 "N, 10°39'00"E units within the MjØsa Limestone (Fig. Il), are Nes-Hamar area Figure 11. Generalized stratigraphic sequences from various Bergevika south 60°44'45"N, ll001'10"E parts of the Toten and Nes-Hamar areas illustrating the rhyth Bergevika north 60°45'00 "N, 11°01' 10 "E mic nature of the Mjøsa Limestone and the lithostratigraphic Bergevika quarry 60°44'45"N, ll001'00"E units described in the text. Eik 60°44'45"N, ll000'05"E 78 P.R. Opalinski & T. L. Har/and NORSK GEOLOGISK TIDSSKRIFT l (1981) Furuberget south 60°48'52 "N, 11°00'40"E Harland, T. L. 1978: Geology of the Steinvika and MjØsa Furuberget north 60°49'00"N, 11°00'30"E Limestones (Middle Ordovician) of southern Norway with G ålås quarry 60°47'42"N, 10°55'50 "E particular reference to depositional environments and their HelgØya skole 60°44'32"N, J0058'50"E related benthos. Unpubl. Ph.D. thesis, Univ. of Liverpool, Kjelsrud 60°44'25 "N, 11°00' 55 "E 571 pp. Snippsand 60°48'02"N, 11°00'40"E Harland, T. L. 1980: The Middle Ordovician of the Oslo Region, Norway, 28. Lithostratigraphy of the Steinvika Limestone Formation, Langesund-Skien district. Nor. Geo/. 2. Additional localities n�t mentioned in the text Tidsskr. 60, 269-278. Hedberg, H. D. 1972: Summary of an International Guide to Stratigraphic Classification, Terminology and Usage, Inter Toten area national Subcommission on Stratigraphic Classification Re port No. 76. Lethaia 5, 297-323. Bru flat 60°39'20 "N, 10°36'30"E Hedberg, H. D. 1976: International Stratigraphic Guide. xviii Finnstad 60°41'30 "N, 10°41'38"E & 200 pp. New York. Fredeng 60°45'55 "N, 10°41'25"E Holtedahl, O. 1909: Studien iiber die Etage 4 des norwegischen Granvang 60°41'45"N, 10°43'00"E Silursystems beim MjØsen. Skr. Nor. Vidensk. Akad. i Oslo, Hole Kalkverk 60°39'47 "N, 10°39' 15 "E Mat.-Naturvidensk. Kl., 7, 76 pp. Ile 60°42'08"N, 10°42'55 "E Holtedahl, O. 1934: The geology of parts of Southem Nor lnndal 60°41'26"N, 10"40'07"E way. Geo/. Soc. Lond. Proc. 45, 307-377. Lyngås 60°41'55 "N, 10°42'30"E Kiær, J. 1897: Faunistische Uebersicht der Etage 5 des nor Sillongen 60°41'55"N, 10°44'00"E wegischen Silursystems. Skr. Nor. Vidensk. Akad. i Oslo, Sive sin 60"38' 55 "N, 10°39' 10 "E Mat.-Naturvidensk. Kl., l, 76 pp. Slatsveen 60°39'25"N, 10°37'42"E Kiær, J. 1908:Das Obersilur im Kristiania. Skr. Nor. Vidensk Steffensrud 60°41'00 "N, 10°42'30"E Akad. i Oslo, Mat.-Naturvidensk. Kl., 2, 595 pp. Steffensrudtjern 60°40'50 "N, 10°42' 15"E Kiær, J. 1920: Oversigt over Kalkalgelloraene i Norges Stepperud 60°42'05 "N, I0°44'50"E Ordovicium og Silur. Nor. Geo/. Tidsskr. 6, 113. Stor-Kyset 60°41'27"N, 10°39'37"E Kiær, J. 1927: In Høeg; O. A. Dimorphosiphon rectangu/aris. Velt-Malterud 60°42'58"N I0°41'20"E A preliminary note on a new Codiacean from the Ordovician of Norway. Årh. Nor. Vidensk. Akad. i Oslo, Mat. Nes-Hamar area Naturvidensk. Kl., 4, 1-15. Krumbein, W. & Sloss, L. L. 1%3: Stratigraphy and Sedimentation. 660 pp. W. H. Freeman & Co., San Bergevika cross-roads 60°44'35 "N, 11°00'45"E Francisco-London. Fjell 60°44'30"N, 10°57'55"E Opalinski, P. R. 1977: The stratigraphy and sedimentology of Knatterud 60°48'15 "N, 10"55'40"E the MjØsaLimestone (Middle Ordovician) Norway. Unpubl. Kvam 60°47'35"N, 10°56'25"E Ph.D. thesis, Univ. of Liverpool, 412 pp. Raymond, P. E. 1916: Expedition to the Baltic Provinces of Acknowledgements. - We sincerely thank Dr. P.J. Brenchley, Russia and Scandinavia: the correlation of Ordovician strata Dr.G. Newall, and Dr. D.L. Bruton for considerable help and of the Baltic Basin with those of eastern North America. JO, discussion involving Mjøsa sediments before, during and after Bull. Mus. Comp. Zool. Harvard 179-286. preparation of an earl y manuscript, and Dr. B. Webby and Dr. Raymond, P. E. 1931: On the nature of Phytopsis tubu/osum P. D.Lane for much appreciated suggestions for its improve Hall. Bull. Mus. Comp. Zool. Harvard Geo/. Ser. 9, 194- ments. Barbara Hunt prepared the figures and Joan Clifftyped 198. the final manuscript. Financial support was given by the Skjeseth, S. 1%3: Contributions to the geology of the MjØsa Natura! Environment Research Council ofGreat Britain. district and the classical sparagmite area in southern Nor way. Nor. Geo/. Unders. 220, 1-126. Størmer, L. 1953: The Middle Ordovician of the Oslo Region, Norway, l. Introduction to stratigraphy.Nor. Geo/. Tidsskr. 3/, 37-141. References Strand, T. & Størmer, L. 1956: Lexique stratigraphique inter national, Vol. l, Fase. 2a Norvege, 44. BjØrlykke, K. 1974: Depositional history and geochemical Textoris, D. A. 1%8: Petrology of supratidal, intertidal and composition of Lower Palaeozoic sediments from the Oslo shallow subtidal carbonates, Black River Group, Middle Region. Nor. Geo/. Unders. 305, 1411. Ordovician, New York, U.S.A. XXllllnt. Geo/. Congr. 8, Brenchley, P. J. & Newall, G. 1975: The stratigraphy of the 227-248. Upper Ordovician Stage 5 in the Oslo-Asker district, Nor Walther, J. 1893: Einleitung in die Geo/ogie als historische way. Nor. Geo/. Tidsskr. 55, 243-275. Wissenschaft - Beobachtungen iiber die Bildung der Coogan, A. H. 1972: Recent and ancient carbonate cyclic Gest eine und ihre organischen Einsch/usse, Vol. I. Fischer, sequences, 5-16. In: Eland, C. & Chuber, S. (eds), Cyclic Jena. Sedimentation in the Permian Basin. Midland, Texas, West Wilson, J. L. 1975: Carbonate facies in geologic history. 471 Texas Geo!.Soc. pp.Springer-Verlag, Berlin-Heidelberg-New York.