DAV/DROBERTS NGU-BULL434, 19 9 8 - PA GE77
Ordovician lithistid sponges and gastropods from the
Snasa Limestone, Nord- Trendelaq, Norway
DAVIDROBERT5
Roberts,D.1998:OrdovicianIithistid spo nges andgastropodsfrom theSnasaLimest one,Nord -Trond elag,Norway.
Norges geologiskeundersoke/seBul/etin434,77-82.
Fossilsoccurringin the SnasaLimestone,observedin newroad-cuts alo ng themain E6roadnear Snasavatnet, Nord- Trond elag,areillustrated and brieflydescribed.Altho ug h the effects ofScandiandeformation and crystallisation havemadeidentificati ondifficult,itseemsclearthatthefaunaconsistsmainlyof macluritid gast ropodsand related forms, and lith ist idspongesof the generaArchaeoscyphia,Rhopa/ocoelia,possibly Hudsonospongiaand either Anthaspid el/a or Zittelella.Thereis alsoa possibilitythatst romato poro ids and, lesslikely,arugosecoral maybere- presented.The fauna isofmainly Nort h American aspect and isconsidered to beofMidd le, or possiblyLate Ordovicianage.
DavidRoberts,Norges geologiskeundersoke/se, Post-Box 3006,7002Trondheim,Norway.
Introduction
With in the meta morph ic allo chth on of the Norwegi an Caledonid es,the Ste ren Napp e of the western Trondh eim Region is by farthemost prol if icof the Koli Nappes (Up per Allochthon)in termsof knownfossil occurrencesandfaunal diversity. The low-g rade metasedime ntar y rocks of the Hel onda dist rict,forexample,have yielded a rich variety of brachiopod s,molluscs,trilobites, graptolit es, corals,cono- donts and echinoderms(e.g.,Berry 1968,Neuman&Bruton 1974,1989,Berq strorn1979, 1997,Ryan etal.1980, Bruton&
Bockelie 1982, Neuman et al. 1997) of Early to Middle Ordovician age. Furthernorth,intheSnasavatn district,fos- silsare known only from theSnasaLim estone , oneofseveral formation s of greenschist-facies meta sedimentary and metavolc anic rock swhich have been consideredto repre- sent a north erly extension of the Steren Nappe (Carstens 1960, Wolff 1979), or conceivabl y a separate napp e (Tiet zsch-Tyler 1983).
Carstens (1956,1960)first reporte d the occurrence of fragments ofOrdoviciangastrop od sintheSnasaLimesto ne in a stream sect ion near the south western end of lake Snasavat net but gave no details abo ut the locality.
Spjeldn zes(1985).ina review art icle,note d thatthe fauna consistedmostly of larg egastropod s, andaddedthat hetoo hadfound gastropod s,ceph alop od sand colonialormulti- cellularorganisms(co ralsor strom atoporoids)in road sec- tion s alo ng thenorth w est ern side of Snasavat net.A broad Middl e Ordovician age was inferred.In a doct oral thesis, Tietzsch-Tyler (1983)also not edthe occurrence of spi ral,co- nical andelli pt ical'cellular'markings ofwhite carbonatein several places in this sam e meta limesto ne,and assumed that they repr esented gast ropod s. A few conodontshave alsobeenext racted from theSnasaLimestone from nearthe sout hw este rnend of Snasavatn et(Berqst rorn 1997)butthey are poorl ypreserv ed.
Earlier,in the autumnof1978,thepresent aut hor hadre- corded abundan t defor med gastropods and other shelly
fossilsinnewly blasted road-cutsalong themainE6roadin thissame generalarea.During the following summer,aclo- serinspect ion revealeda diversityof forms,photographsof which formed the basis of a short, internal NGU report (Roberts 1980).ln viewofthe renewedinterest inthe signifi- cance of faunasand their comparative rarity in the meta- morph ic allocht hon,this shortcont rib ut io naim satpresen- ting a series of field photographswhich may proveto be helpfulto futuregeo log istsandpalaeonto logists workingin comparablecarbonateformations.
The fossil locality and host rock
Althoughthere are nowmanynewroad-cut s alo ngthissec- tionof theE6, notallpreserve well definedfossil forms.The road-cut localit y describ ed here is approximately 5 km northeast ofKvam,300-800 m south-south west of thefarm Nesjan, Nod albu kta, Snasavatn et (Fig.1).This is on the 1:50,000 map-sheet 'Snasavat net' 1723 II (M711 series,3- NOR editi on, blue grid). between coordinates (32WPS) 370 192 and369 187.Note thatthenewE6trase isnotshown on the top og raphi c map.Fossilswere also observed and photograph edon ice-poli shedandst riated surfaces within 20 m of the act ual road-cuts. Thesesur faceswere,at that tim e,newly exposed after thesoil-and vegetation-st ripping operatio nsalong the trase of thenewroad. Inthe courseof just 2-3years,how ever,thesesurfacesacquired auniform grey film asaresultof oxidationand air pollution,and over subseq uent yearsmany havebeen concealedby a thi ncar- pet of new soil and vegetat ion.The road-cuts themselves have suffere d over the years,asa result ofgeneralair and traffic pollution,but many ghost-likefossilforms arestillvisi- ble today.
Thelit hologyoftheSnasaLimestoneinthisarea is a med- ium-to thick-b edded,dark blue-grey, medium- to coarse- grained met alim eston e wit h sporadic,darker blue-gr ey to grey,micaceouslam inaea fewmillimetresin thick ness.The
NGU-BULL 434,1998 - PA GE 78 DAVIDROBERTS
3km I
STORENNAPPE BeitstadGroup
Metasandstone.conglomerate
SnasavatnGroup SnAsa limestone
Undifferentiatedmetavolcanic and metasedimentary rocks Thrust
SKJOTINGEN NAPPE Mostly schistsand amphlt>oIite Fossillocahly
Strike-sliptault ExtensionaJfault
Fig.1.Sim plified geological mapof the Kvam-Nodalbuktaarea, Snasavatnet, showing the fossil10- cality.The'locality ' refers to a c.400 m st retchof almost continuous road-cuton the west side of the E6, and a c.100 m-long sectiononthe east side of the road.
formati on has been involved in at least three Caledonian (Scandian)fold phases at a peakmetamorphicgradeof up- per greenschist facies (Robert s 1967,Tietzsch-Tyler 1983), the evidence of which isclearly reflected in the distorted shapes of the fossilremains. The limestoneit selfwas tho- roughly recrystallised duringthepolyphaseScand iandefor- mati on and theoriginalskeletal remains of thefossilsare picked out aswhit e carbonate against the unifo rm blue- grey background.No silicifi ed shell remainshavebeenob- served,such thatit has proved impossibl e to extractanyof the fossils fromthe rock. Some of thefossil forms are trunca- ted, eit her along minor, schistosity-parallel,shear-sur face offsetsorbytheeffects of pressuresolution.
The fossils
The majority of the fossils represented at thislocality are gastropods.On comparing the forms with those described andillustr ated in the Geolog icalSociety of Americamono- graph on Moll usca(Moore1960),most appear to belon gto the genus Macluritesor typesclosely relat ed to this(Fig. 2).
Both 'low-spired' and 'hig h-spired' varieties of gast rop ods arepresent.
Thispreliminaryiden tifi cati onwasconfi rmedbyDr.Ellis Yochelson(pers.comm.1980),who added that it isquite possible that severalother genera may be represente d.A more preciseidenti ficati onisprecluded by thestat eofde- formation ofthe fossilsand also because thephotographs show only profil es thro ug htheunext ractableforms. Some ofthe photos show high-spire d gastropod form s which are not ofmacluri ti d type,but whose genericaffinit y isuncer-
tain.lt is alsopossib lethat a few low- spi redlenti cular gastro- pods maybe present.
According to Ellis Yochelsonthese particu largastropod formsrangein age from Midto Late Ordovician,but inthis casethey are most probably MidOrdovician.Suchtypes of gastropods are know n from three otherareas in Norway - from the Skjolberg Limestone on the island of Smola (cf.
Brut on & Bockelie 1979), from the Otta Conglomerate (Yochelso n 1963)and from Ashgill carbonate units in the Oslodist rict(Brenchley & Newall 1975, Owenet al.1990).
The second mostprominentgroup of forms represented atthis locality appear to belong to the phylum Porifera- thesponges.Althoughthe stateofstrain of the materi alis again ahinde ranceto preciseident ification,the shapes out - linedbythe whitecarbonatein, e.g.,Figs.3 and 4 are remi- niscent of genera in the order Lithistida (de Laubenfels 1955),oneofthemany ordersintheclass ofDemospong ea.
In terms of spo nge morphol ogy, the so-called 'rhaqon struct urecharact erisesc.98% of all sponges,fossil or pre- sent -day, and the architecture of the lithi sti dsis ofa com- pact,generallythick-walled rhagon type,Thisis aninternal, complex organisation of micro-sized flagellate chambers with anintricat e aquiferoussystem ofinhalant and exhalant canals,whichis suppor tedby a solid,internal,skeletalframe- work of discrete, interlocked, opaline spicule eleme nts, Spongeswit h such skeletons will readilyfossilize, with out becoming disintegrated.
Examination of severalof the field photographsby Drs.
Keith Rigbyand Barry Webby hasled to theconclusion that the annulate subcylindr icalto curved,vase-shapedsponge forms shown inFigs.3 and 4 are almostcertainly of thelit his-
DAVIDROBERTS NGU- BU L L 43 4,1998 - PAGE 79
Figs.2.Sectionsthroughdiverse,deformedmacluritid gastropod s.Intheseandother phot ographicfigu resthe coin is 2.5 cm in diameter.
tid genus Archaeoscyphia,even thoughthere is now no sign of the spicules which formed partof the original skeleton.The annulated exteriorof the walls is quite characteristic of this Ordoviciansponge (Fig.5).Although Archaeoscyphia was a siliceous sponge,most specimensare commonly altered to
Fig.3.View ofa longitudinaltolongitud inal-oblique sect ion througha markedly deforme d lithistid sponge,in allprobabilityArchaeoscyphia, showing thetypicallyannulat e exte rnalstr uct ure.
calcareousimpressions (K.Rigby, pers.comm.).Some repre- sentat ives of the family Anthaspidellida e,like Archaeoscyphia, were moderately largesponges,10-20cm in height and near- ly as wide (de Laubenfels 1955).Others, however,are quite small and stick-like,e.g.as in Ounhillia(Rigby&Webby 1988).
Fig. 4.Alongitudinaltolongitudin al-obl ique sectionthrough anot her lithistidsponge,also probablyArchaeoscyphia.
NGU-BULL 434,19 98 - PAGE 80
Fig. 5.Recon structionof the Ordovicianlithistid spongeArchaeoscyp hia mingan ensis (side view), wit h part of the wall removed; from de Laubenfels (1955,p.E53). Reprod uced wit h permi ssion of theUnive rsity ofKansas Press.
Forms which are perhaps less easy to identify are shown in Figs.6 & 7. One suggestion(K.Rigby,pers. comm.) is that they may also belong to the Anthaspidellidae and could possibly be Hudsonospongia or a related genus (e.g.
Anthaspide/la or Zittele/la).These smaller, funnel- or cone- shaped forms lack the conspicuous annulate structure of
DAVIDROBERTS
Archaeoscyphia. Athi rd,mor e cylindricalform may be the anthaspidellidRhopalocoe lia(Fig s.6a & 7).It lacks the annu- lations ofArchaeoscyphi a,is cylind ricalorclub-shaped with a spongocoel reachi ng near to the base,and is more thin- walled than Hudsonospongia.It isinteresti ng to note that Balt ic representativesofArchaeoscyp hi a,Ant haspide/laand Hudsono spong ia,aswell as many otheranthaspide ll idgene- ra, have been recorded from cobble-and pebble-sizeerra- tics in Pleistocene fluvial deposits in the Netherl and s and NW Germany (van Kempen 1978,Rheb ergen &Hacht 1996, Hacht & Rhebergen1997).
Another suggestionforthe forms wit h the radiate struc- ture illustratedin Figs.6b & 6c has been that theycould con- ceivablyrepresent early rugose corals (David Bruton,pers.
comm. 1980).Some of the original photographswere exami- ned(in 1980)by Bjbrn Neumanwho expresseda certain mea- sure of doubt,noting that whilethe radiatestructure could possibly be interpreted as representingthe septa of a solita- ry coral,the irregular nature of the cavit iesbetweenthe'sep- ta'is not typical of rugosans.Howeve r,the shear deformation and recrystallisationof the fossilsmay have partlydestroyed and modified the original internal structure. Acccepting ,for the sake of discussion ,thatwemaybedealing herewit h a rugose coral,then thefeatures would suggest that allthe sections are fort uitous lyhigh up in the calyxand that the forms may be representative of Strepte lasmat idae (B.Neuman, pers. comm.1980).Such corals are known in Scandinavia mainly from Upper Ordovician strata.Since it has not proved possibleto extractthesefossilsfromthe per- vasive lyrecrystallisedmetalimestone and study themin de-
Fig. 6.Theform s with theinternal,radiate structureand darkergrey, central area in all three photos are probably sections through anthaspidellid sponges,possibl yHudsonospo ngiaorlittelel/a.The vertica llyelongate form tothe leftinFig.6a,with the large infilled internal cavity-the spongocoel -isreminiscent of theanthaspide llid spo nge Rhopa locoelia .Section sthroug hgastrop ods anddiverse shell fragments can also be seen in Fig.6b.
DAVID ROBERTS NGU-BULL 434,1998- PAGE 81
Fig.7. Strongly sheared, prolate sectionsthroug hforms which may represent 3 specimens of the anthaspidelliddemosponge Rhopalocoelia.lnthetw o smal- ler specimens,thesectio nexpo- sesthe sedime nt-i nfilledspo ng- ocoelcavity.
tail,it is difficult right now to decide one way or the other - anthaspidellid sponges or rugose corals. However, in the opi- nion of BWebby (pers.comm. 1998) the structures are more characte rist ic of demosponges than of rugosans; a view to which B.Neuman (pers. comm. 1998) also subscribes after having seen the originals to Fig.6.
The possibility that some of the elliptical or rounded form s(e.g.Fig.7 and left part of Fig.6a) present at this locali- ty may be representative ofst romatoporoids has been sug- gestedby both David Bruton and Bjorn Neuman. The possi- ble presence of stromatoporoids in the Snasa Limestone, at another locality, had also been suggested earlier by Spje ldn ees (1985).Examination of mostof the photographs shown here, and inan earlier report (Roberts 1980), or addi- tionalphotos notpublished,did not,however, convince eit- herK.Rigby or BWebb y (pers. comm.)that stromatoporoids werepresent.
Discussion
Although the strong Scand iandeformation and pervasive recrystalli sation preclud es any detailed study of these fos- sils,palaeontolog ist colleagues do agree that gastropo ds andsponges ofOrdov ician age are present. Whether or not solitar y corals and multicellular stromatoporoids are repre- sented is more conj ect ural. Both Ellis Yochelsen and Nils Spjeldnzes(pers.comm.1980 and 1998)have suggested that the age of the fauna is Middle Ordovician, perhaps just a lit- tle younger,based on the gastropods and the various, multi- cellular, frame-building organisms, respectively.
Fossils comparable to some of those present in the Snasa Limestone, though without the sponges, occur further southwest in Tmndelag on the island of Tautra(Spieldnze s 1985).lnterms of provinciality, the faunas on Tautra and tho- se in the Snasa Limestone are considered to be largely of
North American aspect (Spjeldnzes
1985). Southwestof the Trondheim Region proper,on the island of Sm01a in thecounty of Nord-More (inset, Fig. 1),fossils in the Skj el berq Limestone includemacluritid and other gast ropods ,brachi- opods and a trilobite and are ofLate Arenig-Early L1anvirn age (Bruton&Bockelie 1979,Harpe r 1981).A fewconodo nt s have also been retrieved (Bergstr6m 1997),but sponges have not been reported from this form atio n.The fauna in the Skjelberq Limestone is also of Nor th American (Whiterockian) aspect.Despitethesimilarit ies in faunal affi- nity, the ages of the fauna in thesetwo areas, Sm01a and Snasa,are noticeably different and thi sis also ref lected in significant differences in the geology.TheSm01a area has been considered to be part of a separateterrane, characteri- sed by a Mid Ordo vician, Taconic-equivalent defo rm ation and asubsequent, Mid Ordovicianto EarlySilurian, calc-alka- line magmatic arc(Hall&Roberts 1988, Roberts1988).
Considering that the sponges represent an essentially warm-wate r,subt ropi cal totropical fauna,then theirappear- anceincarbonateformat io ns peripheralto the palaeoconti- nent Laurenti a might beexpected at any time during the Ordovician,since Laurenti a was positioned fairly stably at equatorial latitudesthroughoutthis period. Refined palaeo- magnet ic datafor Balt ica, on theotherhand,haveindicated that this paleocontinentmoved, and gradually rotated, from highandinter medi atelati tu desin EarlyOrdovician times to a moreequato rial positio nbytheLateOrdovician(Torsvik et al. 1995,Torsvik 1998).Consequ ently,the sponges would presumably be expected to havebeen more prolific, indi- versity and number, on the Baltican plate in Mid Ordovician to Early Silurian times.The Balticanthaspidellids,for exam- ple, dominated many reefs and bioher msbyMidOrdov ician times,but by the Early Silurian they had been largelyrepla- ced by stromatoporoids and corals (van Kempen 1981).
Lithistid sponges of late Middle to Late Ordovician agealso constitute abundant erratic material in Pleistocene fluvial depositsof parts of mainlandnorth weste rnEurope (e.g.,the Netherlands, NW Germany, N.Poland),and havebeen infer- red to have derived from northernmost Balt ica or Siberia
NGU-BULL 434,1998 - PAGE 82
(Rhebergen&von Hacht 1996).Based on the most recent palaeomagnetic reconstructions (Torsvik 1998), northern Baltica andNW Siberia wereindeed almo st juxt apo sed,and along the palaeo-equato r, during this interval of time.
Moreover,LateOrdovician brachiopod faunas in the cent ral Taim yr Peninsulaof NW Siberiaareof Balt ic affi nityandfa- vo urtheview that the centralblockofTaimyrform edpartof Balt ica at thistim e (Cocks&Modzalevskaya1997). As yet, howeve r,there is norecord ofany occurrenceof sponges in the Ordovician rocks of the Taimyr Peninsula (V.Moralev, pers. comm. 1998). A more likely source for the errati c Ordov icianspongematerialin the Pleistoceneriverdeposits may therefor e have beentheGulfof Bothnia regionand sur- rounding areas(then marine)to the east.
A cknowledgements
Discussions over the years on Norwegian Caledonide geology and es- pecially on the dearth of fossilremainsin many of the allochthons,final- ly persuaded me that it is better to publicise the fewforms that have been encountered during field workrather then'fo rg et'them in phot o - graphic files or unnumbered open-file reports. A specialdebt of gratitu- de is owed to Keith Rigby and BarryWebby for their help with sugge- st ed identifica tionsof the stronglysheared sponges,based so lely on the photographic material. Their helpful commentson the manuscript are alsomuch appreciated.Ellis Yochelson,Bjorn Neuman,DavidBruton and NilsSpjeldneesalso contributedsuggestions when thein it ialNGU Report waswritt en som e 18 years ago,and Professors Bruton and Spjeldnzes kindly reviewed the manuscript to this paper.
Correspondence with Theo van Kempen,Ulrich von Hacht and Freek Rhebergen onthetopicof erratic sponge material in thePleistoceneof west ern Europe has also brought to light a fascinating aspect of Ordovician sponge re-d istr ibutionhitherto unknownto me.
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