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New Results of Studies on the Fossils in the Lower Cambrian Sediment Deposit of the Söderfjärden Basin

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New Results of Studies on the Fossils in the Lower Cambrian Sediment Deposit of the Söderfjärden Basin NEW RESULTS OF STUDIES ON THE FOSSILS IN THE LOWER CAMBRIAN SEDIMENT DEPOSIT OF THE SÖDERFJÄRDEN BASIN RISTO TYNNI TYNNI, RISTO 1982: New results of studies on the fossils in the Lower Cambrian sediment deposit of the Söderfjärden. Bull. Geol. Soc. Fin- land 54, 1 -2, 57-68. Fossils Mickwitzia monilifera and olenellid trilobite fragments are found in the greenish grey shale in the lower sediment formation of Söderfjärden at a depth of 237.8 and 278.05 m. Microfossils (acri- tarchs) Archaeodiscina umbonulata, Baltisphaeridium cerinium, Granomarginata squamacea, Lophosphaeridium tentativum, Tasma- nites bobrowskii, T. piritaensis were also observed in the lower part of the Söderfjärden sedimentary formation. In this lower intersection the microfossil genera resemble those in the upper sediment forma- tion. Only one new species, Dictyotidium bottnicum, is described in the lower portion of the formation. The occurrence of the fossils and microfossils in the Söderfjärden sediment formation suggest Lower Cambrian sedimentation. Risto Tynni, Geological Survey of Finland, Kivimiehentie 1, SF-02150 Espoo 15, Finland. Introduction Drilling started again in 1979 under the su- pervision of Jyrki Lehtovaara. The whole sed- The subject of the description of the sedi- imentary sequence close to drill hole No. 3 ment deposit of Söderfjärden (Lauren et al. was intersected at a depth of 74.35 to 318.08. 1978) were the fossils and microfossils found Three sites showing fossil imprints or shell at a depth of 73.6 to 139.55 m in hole No. 3 fragments were revealed. drilled by Paraisten Kalkki Oy. On the basis of its fossils this portion was interpreted as Lower Cambrian. The most important guide Fossils fossils and microfossils on which the dating was based were Volborthella tenuis and Tas- The drill core showed the imprint of a manites. Since seismic sounding suggested worm at a depth of 178 m. At 237.8 m a few that the sedimentary deposit might be mark- shell fragments of Mickwitzia (monilifera edly thicker than the portion intersected by Lns) and at 278.05 two fragments, probably the drill hole, the fossils and dates of the of trilobite, were observed. Both fossils, the underlying deposit have still to be estab- brachiopoda Mickwitzia and the trilobite, lished. have an important bearing on the dating and New results of studies on the fossils in the Lower Cambrian sediment... 59 will be discussed in detail in the present con- bed. Volborthella also occurs in Finngrundet, text. at a depth of 82.22 m (Thorslund & Axberg Mickwitzia monilifera Linnarsson has a 1979). A similar sequence, Volborthella at characteristic surface structure that had al- 119 m and Mickwitzia at 237.8 m, has been lowed Wiman (1903) to identify the species noted at Söderfjärden. in the fragmentary occurrences collected by In Estonia Mickwitzia monilifera is asso- G. von Schmalenséer in the Aland islands. ciated with Schmidtiellus mickwitzi trilobite The structure typical of Mickwitzia monili- and Volborthella tenuis cephalopod in the fera was a fairly dense nodular ruling. The Lükati formation (Mens and Pirrus 1977). Ac- surface of the fragments is occasionally fold- cording to Öpik (1956), the occurrence of ed perpendicular to the nodular ruling (Wi- Mickwitzia is confined to the Lükati forma- man op.cit.). The dense nodular ruling shows tion and the upper portion of the Lontova up clearly in the Söderfjärden occurrence formation. In southern Lithuania, Mickwitzia (Fig. 1). Moreover the surface of the fragment cf. monilifera has been encountered in some exhibits undulatory folding, which is, how- cores drilled into the Pirita formations, i.e. ever, oriented diagonally to the nodular rul- in rocks of the final stage of the Lower Cam- ing. The sedimentary rock of the occurrence brian (Korkutis, Lapinskas, Lashkov 1972). is a greenish grey shale. The trilobite fragments found at Söder- Mickwitzia fossils or Mickwitzia sand- fjärden in the greenish grey shale at a depth stones are encountered in many places in the of 278.05 m (Fig. 2 and 3) probably correspond Baltic area. They have been reported by Wi- to the olenellid trilobite, i.e. the oldest types man (op.cit.) on the basis of boulders from the in the Baltic area. Fig. 2 bears a marked re- following sites in Aland: South Lumparn; semblance to the Olenellus sp. fragment, Slemmern, east of Marianhamn; Ytternäset, Schalenskulptur von gewöhnlichem Aus- in Marianhamn; and Granboda, in Lemland. sehen, reported by Wiman (1903, PL 1, Fig. 6). In Sweden they have been found in the Närke The material described by Wiman derives area (Westergård 1940), Kinnekulla (Holm mainly from the Gävle Bay, although he also 1901, Högbom and Ahlström 1924), Biludden has an observation on Olenellus sp. from (Wiman 1903), and in the drilling at File Hai- Rosenberg in Aland (Wiman op.cit.). This dar in Gotland below the Mobergella and Vol- may, however, be another genus resembling borthella occurrences (5 m) close to the up- Olenellus. The surface structure of the Ole- per portion of the Lower Cambrian deposit nellus? curvicornis described by Poulsen (Thorslund and Westergård 1938, Martins- (1932) from the Lower Cambrian formation in son 1974). In Östergotland Mickwitzia Ella Island, Greenland, is similar to that en- monilifera has been identified in local errat- countered in the Söderfjärden occurrence. ics but not in situ (Martinsson op.cit.). In The trilobite Schmidtiellus mickwitzi (Mo- Dalsland it has been reported from a clastic berg 1892) and the brachiopod Mickwitzia dyke west of Lake Vaner (Gavelin 1909). The monilifera occur in Västergötland, Närke and most important site of comparison for the Estonia in conglomerates that formed during Söderfjärden Mickwitzia occurrence may be the Lower Cambrian transgression (Martins- Finngrundet, where Mickwitzia was encoun- son 1974). In the Mjosa area, Schmidtiellus tered in a drill core from Lower Cambrian cf. mickwitzi is encountered in the Brenn- Fig. 1. Mickwitzia monilifera, shell fragments from a depth of 237.8 m in shale. Photo Kalevi Hokkanen (A-C) and Erkki Halme (D). 60 Risto Tynni Fig. 2. Fragmentary cephalon of olenellid trilobite from a depth of 278.05 m in shale. A lateral border furrow visible at the right-hand margin. Photo K. Hokkanen. saeter limestone above the Precambrian by Thorslund and Axberg (1979) from the (Skjeseth 1963,' Martinsson op.cit.). In the Lower Cambrian Västra Banken formation at Mjosa area, however, the olenellid trilobite a depth of 133.26 to 133.35 m. At Söderfjärden Callavia sp. occurs together with Vol- the conglomeratic sandstone contains small borthella tenuis in the Bråstad shale, which is amounts of glauconite as well. A larger indi- younger than the Brennsaeter limestone. vidual phosphorite nodule was detected in With its stripes (Fig. 3), the fossil fragment the Söderfjärden formation above the ole- from Söderfjärden resembles the surface nellid trilobite at a depth of 277 m. These structure of the rear of the head of the tri- phosphorites are free from fossils. Lehto- lobite Strenuaeva sp. (Ahlberg 1979) and also vaara had earlier found what may be the rem- the structures of the pleural areas of some nant of a fossil in a phosphorite-bearing poly- other trilobites (cf. Dean and Martin 1978, mictic conglomerate in the upper part of the p. 33). Söderfjärden sedimentary formation (Laurén Since fossils are sometimes associated with et al. 1978, p. 25). phosphorite nodules, attention was focused on the sedimentary rock of Söderfjärden at Microfossils (acritarchs) a depth of 314.55 m, where there is a layer of a conglomeratic sandstone, less than 10 cm The main objects of our attention are the thick, with phosphorite nodules (Figs 4 and microfossils in the lower sediment formation 5). The layer is very similar to that described of Söderfjärden at a depth of 140.0 to 317.75 m. New results of studies on the fossils in the Lower Cambrian sediment... 61 Fig. 3. A shell fragment of trilo- bite from a depth of 278.05 m. Probably a remnant of the same individual as that shown in Fig. 2. Fig. 4. Conglomeratic sand- stone from a depth of 314.55 m. Fig. 5. A detail of conglomer- atic sandstone. Small rounded phosphorite nodules in thin section. 62 Risto Tynni 0 250|j- o o O 200p- o ° OD so O °o°0 o OO O O o o O c® oo g o g o 1* ms o 150 p O OO O O o n Oo o o o o ° cP so o 100 80p- Fig. 6. Diameter and thickness of the shell of the Tasmanites from the depth 100 150 Söderfjärdén sediment thickness O 5p Ol|i O o 2>j formation. New microfossil analyses were performed on the diameter and the thickness of the shell in 35 sites at an average interval of 5 m. In this graphic presentation. The third important lower intersection the microfossil genera re- quantity, the spacing between the pores, is 3 semble those in the upper sediment forma- to 5 [j. for the majority of the species. As tion. The number of genera in the lower for- shown by the figure, most of the occurrences mation is, however, smaller than in the upper of Tasmanites bobrowskii fall within the size formation. Thus, the Cymatiosphaera genus, range of 90 to 160 |i (the upper limit 130 whose proportion is almost 40 % at a depth of which I reported previously, should read 129 m, is practically lacking from the sedi- 160 n, Tynni 1978). mentary formation at a depth of 140.0 to Table 1 lists the acritarch genera in the mid- 317.75 m. dle and lower portions of the Söderfjärden The genus Granomarginata is very rare in sediment formation and whose existence has the lower portion.
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