Stromatoporoids from the Silurian of the Oslo Region, Norway KEI MORI Mori, K.: Stromatoporoids from the Silurian of the Oslo Region, Norway.Norsk Geologisk Tidsskrift, Vol. 58, pp. 121-144. Oslo 1978. ISSN 0029-196X. Silurian stromatoporoids from the Oslo Region, southem Norway, have been investigated. Fifteen species belonging to seven genera are described. Among them, one new genus, Os/odictyon, and three new species are proposed: viz. Oslodictyon henningsmoeni, Plectostroma norvegicum, and Stromato­ pora ringerikensis. The stromatoporoid assemblages are characterized by the Clathrodictyids such as Clathlodictyon and Ecc/imadictyon. They are related to those from the Silurian of the Baltic regions such as Gotland and Estonia. K. Mori, Institute ofGeo/ogy and Paleonto/ogy, Tohoku University, Sendai, Japan. The purpose of the present paper is to describe study has revealed that the Silurian stromatopo­ the Silurian stromatoporoids from the Oslo Re­ roids of the Oslo Region are related to those of gion, southem Norway. No detailed systematic Gotland and Estonia. investigation on stromatoporoids from this re­ gion has hitherto been carried out; nevertheless common occurrence of stromatoporoids has Material been noted by many authors. Previous work on the Silurian stromatoporoids in this region has The Silurian stromatoporoids dealt with in this been done only by Broadhurst (l%6). He in­ paper include not only the author's collection vestigated growth forms of stromatoporoids in but also the collections of Paleontologisk Muse­ relation to sedimentation and water currents, um, Oslo. The latter is mainly from the Llando­ although he did not treat them taxonomically. verian collected by Kiær between 1894-1913 and The present author collected stromatoporoids Kjerulf during 1861-1863. The total number of in the Oslo Region in the spring of 1969. The specimens is 85, among which 34 belong to Pale­ material stored in Paleontologisk Museum in ontologisk Museum. Sixty specimens are from Oslo was also investigated. Comparative studies Llandoverian (stages 6c-7c), twelve from Wen­ of stromatoporoids described from England lockian (stages 8c-8d), and thirteen from Ludlo­ (Nicholson 1886-1892), Estonia (Rosen 1867, vian? (Stage 9c). (Conceming the Norwegian Nestor 1962, 1964, 1966), Scania (Mori 1969), stages, see Table 1.) and Gotland (Mori 1968, 1970) were carried out. The number of specimens of each described Fifteen species belonging to seven genera are species is as follows. described in this paper. Among them, one new genus, Oslodictyon, and three new species are Family Labechiidae Pseudostylodictyon sp. ..... ...... .. .. .. .. proposed: viz. Oslodictyon henningsmoeni, Family Clathrodictyidae Plectostroma norvegicum, and Stromatopora Clathrodictyon crickmayi Parks ................... ringerikensis. Clathrodictyon /innarssoni Nicholson . l The Oslo Region is paleogeographically dem­ Clathrodictyon regulare (Rosen) .. 2 C/athrodictyon simplex (Nestor) . 6 onstrated by foreland sediments of the Cale­ Clathrodictyon sulevi Nestor ...................... donian geosyncline. It is important to compare Clathrodictyon variolare (Rosen) . 16 the stromatoporoid assemblages between this Oslodictyon henningsmoeni, n. gen. and n. sp. 2 region and the other Bal tie areas such as Gotland Ecc/imadictyon fastigiatum (Nicholson) . 6 Ecc/imadictyon microvesiculosum (Riabinin) . l and Estonia, where epicontinental sediments are Ecc/imadictyon robustum Nestor . 2 distributed, especially from a biostratigraphical Family Actinostromatidae and paleoecological point of view. The present Plectostroma norvegicum, n. sp. 3 122 K. Mori NORSK GEOLOGISKTIDSSKRIFT 2 (1978) o Fig. l. The distribution of the Cambro-Silurian deposits in the central part of the Oslo Region and the fossil localities shown in Figs. 2-4. Family Stromatoporidae Loe. no. -2. Rail road cutting at Vakås, Asker, Stromatopora discoidea (Lonsdale) . l about 800 m NE of the niilway station of .As­ Stromatopora ringerikensis, n. sp. .. .. .. .. 2 Stromatopora sp. .. 6 ker (stage 6c). Clathrodictyon variolare, Ecc­ U ndescribed poorly preserved specimens . 34 limadictyon fastigiatum. Total number of specimens ......... .. ................ 85 Loe. no. 3. Sea side exposure along the southern coast of Malmøya, S of the Oslo city (stage All the specimens described in this paper are 7a). Clathrodictyon linnarssoni, Clathrodicty­ stored in Paleontologisk Museum, Oslo. on simplex, Clathrodictyon variolare, Eccli­ madictyon fastigiatum. F ossil local it ies Loe. no. 4. Exposure along the road from Has­ lum to Sandvika at Gjettum, 1.8 km NNE of The localities where the stromatoporoid material the railway station of Sandvika, Bærum (stage was collected are shown in Figs. 1-4 and a list of 7a). Clathrodictyon simplex. these localities is given. The species found at Loe. no. 5. Exposure along the road at Dugnad­ each locality are mentioned. The species without veien, Gjettum·, Bærum, 600 mW of the locali­ locality numbers are Paleontologisk Museum ty no. 4 (stage 8c). All the specimens are collections. Unfortunately exact points of the poorly preserved. fossil localities of the museum collections are Museum collections. Clathrodictyon regulare unknown in many cases, because the material (Sandvika, stage 7). Clathrodictyon sulevi often is labelled only 'Ringerike', 'Sandvika', (Sandvika, stage 7). Clathrodictyon variolare etc., although the stratigraphical positions are (Spirodden, stage 6c; Sandvika, stage 7; clearly noted. Malmøya, stage 7a). Ecclimadictyon microve­ siculosum (Sandvika, stage 7). Oslo-Asker district Holmestrand district Loe. no. l. Road side cliff, 250 m SE of the rail road at Ringeriksveien, Sandvika, Bærum Loe. no. 6. Sea side exposure, about 500 m N of (stage 6c). Clathrodictyon regulare. the southernmost end of Langøya, E of Hol- NORSK GEOLOGISKTIDSSKRIFf 2 (1978) Stromatoporoids from Oslo Region 123 • N • l 5 l. •2 5 km Asker Oslofjorden Fig. 2. Collecting localities in the Oslo-Asker district. mestrand (stage 9e ). Ecclimadictyon robus­ tigiatum (Ringerike, stage 7). Stromatopora tum, Stromatopora sp. ringerikensis (Vesleøya, stage 7e). Museum eolleetions. Clathrodictyon variolare (Bjerkøya, stage 7b). Distribution of the stromatoporoids Ringerike district Among 15 species deseribed here, 10 species are Loe. no. 7. Exposure along the lake eoast, 150 m from the Llandoverian (stages 6e,7a, 7b and 7e), NE of the southernmost end of Limåstangen, 3 species from the Wenloekian (stages Se and Ringerike (stage 7a). Ecclimadictyon fastigi­ Sd), and 2 species from the Ludlovian? (stage atum. 9e). Stratigraphie distribution of the speeies is Loe. no. S. Lake side exposure, 200 m NE of the shown in Table l. loe. no. 7 at Rytteraker, Ringerike (stage 7b). Llandoverian stromatoporoids are ehar­ Oslodictyon henningsmoeni, Ecclimadictyon aeterized by Clathrodietyids sueh as Clath­ fastigiatum. rodictyon and Ecclimadictyon. Seven out of ten Loe. no. 9. Exposure beside the main road from Sund vollen to N orderhov, about 200 m N of the eross-road, north of Vik, Ringerike (stage 7b). All the spee imens are poorly preserved. N Loe. no. 10. Exposure along the road from Vik to Kroksund, 900 m NNW of the northwest­ ern margin of the bridge at Kroksund, Ring­ Langøya erike (stage Se). Plectostroma norvegicum. 1 Loe: no. 11. Lake side exposure at the north­ 6 eastern end of Storøya, Ringerike (stage Se). Holmestrand All the specimens are poorly preserved. Museum eolleetions. Pseudostylodictyon sp. (Limåstangen, stage 7a). Clathrodictyon o 2 3km crickmayi (Bragsøya, stage Sd). Clathrodicty­ on simplex (Gjeitøya, stage 7b; Purkøya and Storøya, stage 7e). Oslodictyon henningsmo­ eni (Vesleøya, stage 7b ). Ecclimadictyonfas- Fig. 3. Collecting locality in the Holmestrand district. 124 K. Mori NORSK GEOLOGISKTIDSSKRIFT 2 (1978) Labechiidae. It should be pointed out that the Labechiids are quite insignificant in the Silurian Steins­ of the Oslo Region. In the family Stromatopori­ fjorden dae, only one species, Stromatopora ringeriken­ sis, has been found in the Llandoverian. It is also insignificant in number of species and individu­ als compared with those of the Clathrodictyids. But it is worthy of note that the occurrence of S. ringerikensis is one of the earliest among the Sund- species of the genus Stromatopora. otJJ volden Wenlockian stromatoporoids are represented N by three species: Clathrodictyon crickmayi, 11 Plectostroma norvegicum, and Stromatopora Storøya discoidea. The Wenlockian materials studied are limited; nevertheless the field occurrence of the o Wenlockian stromatoporoids exceeds quantita­ l tively those of Llandoverian ones. Therefore it is probable that the number of species and genera ? will increase with further studies. Present know­ ledge indicates that there is a difference between o 2 3 4 5km the Llandoverian and the Wenlockian assembla­ ges, because no species found in the Llandoveri­ an occur in the W enlockian. In the Actinostro­ matid group, only one species, Plectostroma Fig. 4. Collecting localities in the Ringerike district. norvegicum, is known in the Silurian of the Oslo Region. Stromatoporoids possibly of the Lu.dlovian are species are represented by these two genera. known from the Holmestrand district. Two spe­ The oldest record of the occurrence of the Llan­ eies, Ecclimadictyon robustum and Stroma­ doverian stromatoporoids in the Oslo Region is topora sp., are described. Species in the Llan­ in stage 6c. Three species appear in this stage, doverian and the Wenlockian are