Middle from allochthonous limestones at Høyberget, southeastern Norwegian Caledonides

JANAUDUN RASMUSSEN & SVEND STOUGE

Rasmussen, J. A. & Stouge, S.: Middle Ordovician conodonts from allochthonous limestones at Høyberget, southeastern Norwegian Caledonides. Norsk Geologisk Tidsskrift, Vol. 69, pp. 103--110. Oslo 1989. ISSN 0029-196X.

Middle Ordovician (Liandeilo-Early Caradoc) conodonts are recorded from the limestone at Høyberget, southernNorwegian Caledonides. The con odont fauna, including Pygodusanserinus Lamont & Lindstrom and Ba/toniodus variabilis (Bergstrom), corresponds to the upper part of the Pygodus anserinus and the lower part of the Amorphognatus tvaerensis zones. On the basis of the stratigraphic position the overlying black shale unit is correlated with the upper Nemagraptus gracilis and the Diplograptus multidens graptolite zones.

Jan Audun Rasmussen, Institute of Historica/ Geology and Palaeontology, University of Copenhagen, Øster Voldgade JO, DK-1350 Copenhagen K, Denmark; Svend Stouge, Geological Survey of Denmark, Thoravej 8, DK-2400 Copenhagen NV, Denmark.

Non-fossiliferous and fossiliferous limestones out­ stone at Høyberget is overlain by a fossiliferous crop sporadically within the Norwegian Cale­ black shale which was considered contempor­ donides (Spjeldnæs 1985; Bruton & Harper 1988). aneous with the 'Ogygiocaris Series' in the Oslo Some of the fossiliferous limestone units have Region, i.e. Llanvirn-EarlyLlandeilo (e.g. Bjør­ been correlated with the Arenig-Early Llanvim lykke 1905; Holtedahl 1920, 1921). 'Orthoceras Limestone' in the Oslo area and the Recently the nautiloid genus Ormoceras Stokes lateral equivalent Stein Limestone in the Rings­ has been recognized in material collected from aker area. This correlation has usually been the limestone at Høyberget, and a Late Arenig based on lithological and broad stratigraphical to Late Llanvim age was suggested (Spjeldnæs similarities and/or weak fossil evidence. 1985). As a consequence, the overlying unnamed During our work on Arenig-Llanvirn conodont black shale unit would correspond to the Hus­ faunas in the Oslo Region (Rasmussen & Stouge tedograptus teretiusculus graptolite Zone 1988; Rasmussen in prep.) the limestone at Høy­ (Llandeilo) (Spjeldnæs 1985). berget (formerly Høgberget) east of Snerta (Fig. The conodont data presented here show the age l) was also sampled. The conodonts however, of the limestone to be Llandeilo-Early Caradoc were much younger (Middle Ordovician rather (Middle Ordovician). The Høyberget conodont than Early Ordovician) than was expected from fauna is clearly younger than the Lower Ordo­ earlier interpretations. Here we present the data vician conodont successions known from the obtained from Høyberget and demonstrate for 'Orthoceras Limestone' and the Stein Limestone the first time the occurrence of Llandeilo-Early (Kohut 1972; Rasmussen & Stouge 1988; Ras­ Caradoc conodonts within the Norwegian Cale­ mussen in prep.). Consequently, it is likely that donides. the black shale overlying the limestone at Høy­ Actinoceroid cephalopods have been known berget covers the upper part of the Nemagraptus from the limestone at Høyberget since 1863 gracilis graptolite Zone and may extend into the (Kjerulf 1863), and on this evidence it was cor­ Diplograptus multidens graptolite Zone - that is, related with the 'Orthoceras Limestone' and the an early Caradoc rather than Llandeilo (Hus­ Stein Limestone (Schiøtz 1883; Bjørlykke 1905; tedograptus teretiusculus Zone) age . Holtedahl 1920, 1921; Skjeseth 196:2). The lime- 104 /. A. Rasmussen & S. Stouge NORSK GEOLOGISK TIDSSKRIFT 69 (1989)

Locality -l l l- ' / ./ : \ Allochthonou ' \ strata � / l / l 1 \ ll-:= //\ \-

Caledonian fl ' \'-­ \/ Front l l\\ ' l - ., '\ l\ l Ordovician autocht.- 1- parautocht. strata

Precambrian basement /\

/ " \ - '\/ / .// --/ '' '...-\ \\ --1 /{{/ - / ...... ' 'l --- - / " / 1/ ,�, /-- \ \ \ \ / /' \ l ,-- 50km Skm

Fig. l. Location map. NORSK GEOLOGISK TIDSSKRIFr 69 (1989) Conodontsfrom Høyberget, Norwegian Caledonides 105

Location and geological setting emplacement has brought the Vendian Enger­ dalen Group into structural superposition on the The locality is situated at the western side of Ordovician sediments (Nystuen 1980, 1983). The Høyberget, approximately l km east of Snerta thrust plane between the Osen-Røa Nappe Com­ village within the map sheet Elvdal 2018 Ill (UTM plex and the Kvitvola Nappe is marked by a coordinates PP452441) (Fig. 1). conspicuous 6 m thick mylonite zone (Bjørlykke The limestone at Høyberget is about 5-10 m 1905). thick (Holtedahl 1921) and is overlain by a black shale unit. It is fossiliferous, impure, and has intercalations of argillaceous material. The bed­ Previous investigations ding planes of the limestone show a characteristic The first recorded fossils from the limestone at weathered, reticulate surface. Høyberget are those mentioned by Kjerulf These two Ordovician units overlie the Upper (1863), who documented the nautiloid cephalo­ Proterozoic sandstones and shales of the Hed­ pods which are most abundant in the lower mark Group (Vendian) (e.g. Schiøtz 1883; Bjør­ part of the unit. One species was identified as lykke 1905; Holtedahl 1920; Nystuen 1982). At Orthoceras crebriseptum Hall, or a closely related Høyberget, the Hedmark Group and the Ordo­ species. Recently, Spjeldnæs (1985) noted that vician limestone and the black shale units are some of the Høyberget nautiloids have annulate all incorporated in the Lower Allochthon of the siphuncles, and belong to the genus Ormoceras Osen-Røa Nappe Complex of the Norwegian Stokes. The Høyberget cephalopods were com­ Caledonides (Nystuen 1981; Bockelie & Nystuen pared with those known from the top of the 1985). The limestone and the black shale unit are 'Orthoceras Limestone' in the Oslo Region structurally overlain by the Middle Allochthon (Spjeldnæs 1985), and therefore the limestone at Kvitvola Nappe, which was thrust discordantly Høyberget was interpreted to span the interval upon the Osen-Røa Nappe Complex (Nystuen from the upper part of the 'Orthoceras Limestone' 1980, 1983; Bockelie & Nystuen 1985). The to well in to the 'Ogygiocaris Series', at or above

BRITISH GRAPTOLITE CONODONT RANGES OF OSLO HØY BERGET SERIES ZONES ZONES AND SUBZONES SELECTED SPECIES ----1 r Boltoniodus Black Diplogropfus Lower gerdoe i Amorpho- shale Caradoc _ f!!.uJJ. f/!!Q..S_ Chasmops gnofhus -r- -r- unit Shale tvoerensis r------Nemogropfus variabilis - - gro ei/is - -- - Limestone Llandeilo Amorpho- at Pygodus gnothus Ampyx 14eyberget ------anserinus inoequalis Limes tone ------.... ------Husfedo- Amor�thus .IQ lUn- grop fus � kielcensis classitiedl teretius- .$;! ------y, �:o. eu/us .� � � - - Ogygiocaris IMissingl - -- � Pygodus serra t:i ·� Shale Didymo- � Llanvirn gropfus � murchisoni � Cl.. y J1 r Fig. 2. Correlation table. (Data from Bergstrllm 1971; Bergstrllm et al. 1987; Berry 1964; Hamar 1964, 1966.) 106 J. A. Rasmussen & S. Stouge NORSK GEOLOGISK TIDSSKRIFf 69 (1989)

Table l. Distribution of conodonts from the limestone at Høyberget.

Species Sample no. 1 2 Total

?A nse/la sp...... 1 1 Baltoniodus variabi/is (Bergstroml ...... 6 6 12 Drepanoistodus sp...... 1 1 ."Oistodus" venus fus Stautter ...... 1 5 6 aculeatus Hadding ...... 2 31 33 Protopanderodus robustus (Haddingl ...... 6 1 7 Protopanderodus varicostatus (Sweet & Bergstroml ...... 1 1 Pygodus anserinus Lamont & Lindstrom ...... 11 11 Scabbardel/a a/tipes (Henningsmoen) ...... 12 15 27 Strachanognathus parvus Rhodes ...... 1 1 Plattorm conodonts (tragmentaryl ...... 2 1 3 Unidentitiable elements ...... 7 9 16

Total 38 81 119

the top of the D. murchisoni Zone. Other fossil strom 1964). The limestone at Høyberget is partly groups known from the limestone include gastro­ covered in the road section l km east of Snerta pods (Kjerulf 1863; Schiøtz 1874; Holtedahl 1921) village (Fig. 1), but loose blocks are well dis­ trilobites (Schiøtz 1874), brachiopods, ostracodes tributed in the area. Sample l (0.7 kg) was col­ and echinoderms (Henningsmoen 1979). None of lected from a large erratic with cephalopods, these records, however, have biostratigraphical whereas sample 2 (1.4 kg) is from beds in situ. importance. One hundred and three reasonably well-pre­ The only fossils reported from the overlying served conodont elements have been identified black shale unit are biostratigraphically undiag­ (Table l; Fig. 3) and all have a CAI of 5-{i. This nostic. The poorly preserved fauna comprises shows that the rocks have been heated from a cephalopod fragments (Holtedahll921), echino­ minimum of 300°C to about 550°C (Epstein et al. derms (Schiøtz 1874; Bjørlykke 1905), asaphid 1977; Rejebian et al. 1987). trilobites (Holtedahl 1921), gastropods (Kjerulf The fauna is characterized by Pygodus anser­ 1863; Schiøtz 1874; Bjørlykke 1905; Holtedahl inus Lamont & Lindstrom, Baltoniodus variabilis 1921), and brachiopods (Henningsmoen 1979). (Bergstrom) and Periodon aculeatus Hadding, and the coniform genera Protopanderodus, Scab­ bardella, Strachanognathus and 'Oistodus' venustus Stauffer are present in varying numbers Conodont collection (Table 1). Two samples were collected and processed for conodonts using standard techniques (e.g. Lind-

Fig. 3. O A-C. Pygodu.sanserinu.s Lamont & Lindstrom. A. Pastinate P element. MGUH 18900. Sample 2: 70x. B. Stelliscaphate P element. MGUH 18901. Sample 2: 50x. C. Stelliscaphate P element. MGUH 18902. Sample 2: 85x . O D-F. Ba/toniodu.s variabilis (Bergstrom). D. Geniculate coniform M element. MGUH 18903. Sample 1: 65x. E. Quadriramate ramiform S element. MGUH 18904. Sample 2: 100x. F. Pastinate pectiniform P element. MGUH 18905. Sample 1: 60x. O G-K. Periodon acu/eatu.s Hadding. G. Geniculate coniform M element. MGUH 18906. Sample 2: 140x. H. Dolabrate ramiform S element. MGUH 18907. Sample 2: 85X. I. Digyrate P element. MGUH 18908. Sample 2: 90x. J. Geniculate coniform M element. MGUH 18909. Sample 2: 60x. K. Dolabrate ramiform S element. MGUH 18910. Sample 2: 95x. O L, O. 'Oistodu.s' venu.stu.s Stauffer. L. Geniculate coniform element. MGUH 18911. Sample 2. 90x. O. Geniculate coniform element. MGUH 18912. Sample 2: 80x. OM, P, Q. Scabbardella a/tipes (Henningsmoen). M. Non-geniculate coniform element. MGUH 18913. Sample 2: 70x. P. Non-geniculate coniform element. MGUH 18914. Sample 2: 90x. Q. Non-geniculate coniform element. MGUH 18915. Sample 1: 95x. ON. Protopanderodu.s robu.stu.s (Hadding). N. Symrnetrical non-geniculate coniform element. MGUH 18916. Sample 1: 105x. (All illustrated specimens are deposited at the Geological Museum, Copenhagen, Denmark (MGUH).) NORSK GEOLOGISK TIDSSKRIFT69 (1989) Conodontsfrom Høyberget, Norwegian Caledonides 107 108 J. A. Rasmussen & S. Stouge NORSK GEOLOGISK TIDSSKRIFf 69 (1989)

(several localities) (Bergstram 1962, 1971, 1973a, Age of the fauna 1986) and Estonia (Viira 1967, 1974). Outside Pygodus anserinus (Fig. 3A-C) is the index Baltoscandia the Pygodus anserinus conodont species of the P. anserinus conodont Zone (Berg­ Zone is widely distributed. In the Caledonian­ stram 1971), but the species ranges up to the Appalachian fold belt it has been recorded from top of the Baltonioduspariabilis Subzone of the Scotland, Ireland, Newfoundland and eastern succeeding Amorphognathus tvaerensis Zone United States (Lamont & Lindstram 1957; Berg­ (Bergstram 1971; Bergstram et al. 1987) (Fig. 2). stram 1971). It has also been documented from The presence of Baltoniodus variabilis (Fig. 3D­ small limestone units associated with volcanic arcs P) suggests that the limestone at Høyberget cor­ (Bergstram et al. 1974; Stouge 1980a, c; Fåhræus relates with the interval from the Amorphog­ & Hunter 1981) and other sites along the eastern nathus inaequalis Subzone of the Pygodus and western margins of the Appalachians (Berg­ anserinusZone to the Baltoniodus variabilis Sub­ stram 1971, 1973b; Stouge 1980a, b). zone of the Amorphognathus tvaerensis conodont Zone (Fig. 2). This interval mainly covers the Nemagraptus gracilis graptolite Zone (Bergs tram Significance of the fauna 1986; Finney & Bergstram 1986; Bergstram et al. 1987). Other fragmentary platform conodonts Eustatic changes during the Middle Ordovician have been found but their poor state of pres­ are well established and unconformities at or near ervation allows for neither a specific nor a generic the base of the Llandeilo in shelf areas on both identification. The other conodonts recognized sides of the Iapetus Ocean are known (Leggett (Table l) are longer ranging species with lower 1978; Leggett et al. 1981; Stouge 1981a, c; Fortey biostratigraphical resolution. 1984; Thickpenny & Leggett 1987; Fortey & Cocks 1988), while a major transgression is evi­ dent during the late Llandeilo and early Caradoc. The Llandeilo regressivejtransgressive turn­ Correlation over is marked by a transition from radiolarian The conodont fauna from the limestone at Høy­ cherts to Nemagraptus gracilis-bearing black berget is similar to that of the Middle Ordovician shales in the Caledonides of Ireland and southern (Llandeilo) Ampyx Limestone in the autoch­ Scotland (Leggett 1978). Shallow water lime­ thonous and parautochthonous strata of the stones were deposited in volcanic regions of cen­ Oslo Region (Hamar 1964, 1966). Pygodusanser­ tral Newfoundland during the lowstand (Stouge inus, Baltoniodus variabilis, Protopanderodus 1980a; Fåhræus & Hunter 1981). These lime­ robustus (Hadding), Periodon aculeatus, Scab­ stones were succeeded by Nemagraptus gracilis­ bardella altipes (Henningsmoen) and 'Oistodus' Diplograptus multidens-bearing black shales venustus occur in both units. It should be noted (Bergstram et al. 1974). In view of this the lime­ that the older species Pygodus serra (Hadding) stone at Høyberget was probably deposited dur­ and Baltoniodus prevariabilis (Fåhræus) are ing the initial transgression that reached Norway present in the lower part of the Ampyx Limestone in late Llandeilo-early Caradocian and the suc­ (Hamar 1964) while they have not been found in ceeding black shale unit is probably equivalent to the limestone samples from Høyberget (Table the widespread N. gracilis-D. multidens-bearing 1). The possible appearance of Amorphognathus black shales of the Caledonian-Appalachian tvaerensis Bergstram ( = Ambalodus triangularis Orogen. Branson & Mehl sensu Hamar 1966) within the Lower Chasmops Shale (Bergstram 1971, p. 104) indicates that the limestone at Høyberget is Summary younger than the lower part of the Ampyx Lime­ stone, but older than the Lower Chasmops lime­ The Late Llandeilo-Early Caradoc age for the stone (Caradoc) in the Oslo Region (Fig. 2). limestone at Høyberget is based on the occurrence In Baltoscandia, Llandeilo-Caradoc conodont of the diagnostic conodont species Pygodus anser­ faunas are well known from Scania (Hadding inus and Baltoniodus variabilis. As a conse­ 1913; Lindstram 1955; Bergstram 1971, 1973a, quence, the overlying black shale unit is 1986; Bergstram & Nilsson 1974), central Sweden correlated with the upper N. gracilis-D. multidens NORSK GEOLOGISK TIDSSKRIFT 69 (1989) Conodontsfrom Høyberget, Norwegian Caledonides 109

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