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Local Variations in the Distribution of Silurian Conodonts of the Distomodus Kentuckyensis Zone of the Oslo Region, Norway

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Local Variations in the Distribution of Silurian Conodonts of the Distomodus Kentuckyensis Zone of the Oslo Region, Norway Local variations in the distribution of Silurian conodonts of the Distomodus kentuckyensis Zone of the Oslo Region, Norway M. B. IDRIS Idris, M. B.: Local variations in the distribution of Silurian conodonts of the Distomodus kentuckyensis Zone of the Oslo Region, Norway. Norsk Geologisk Tidsskrift, Vol. 64, pp. 181-191. Oslo 1984. ISSN 0029-196X. The distribution of conodonts in the early Silurian successions of the Oslo Region is assessed with respect to water depth and energy level changes. Although there is no strong lateral segregation nor increase in diversity in deeper waters, species seem to be selective in their preference for environments. A few exhibited anomalies in the near-shore, high energy environments, others in the off-shore, lower energy environments. The distributional trends of the main zonal indices Distomodus kentuckyensis and lcrio­ della discreta - lcriodel/a deflecta suggest that there are some interactions between these species. Generally, the distribution patte ms of the coniform apparatuses indicate that they are least tolerant of the shallow water, high-energy environments. M. B. /dris, Department of Geology, University Malaya, Kuala Lumpur, Malaysia. The uncertainty over the nature of the conodont kentuckyensis Zone in the Oslo Region by ex­ animal and the function of its element place a amination of distributional patterns and the de­ severe constraint on the interpretation of the termination of environmental preferences. The paleoecology of conodonts, but within the frame­ relative percentages of each conodont species in work of existing evidence, ecological models have successive samples were plotted stratigraphically. been duly proposed based mainly on empirical No coherent pattern emerged, even after the examination of distribution patterns and at­ application of similarity index (Murra y & Wright tempts to correlate these with identifiable pa­ 1974, p. 3), to combine similar samples. The leoenvironmental factors. Seddon & Sweet similarity index involves the comparison of per­ (1971) proposed a depth stratified,pelagic model centage data of two succeeding samples and, on the basis of geographical distributional pat­ where a species is common to both, the smaller terns of Ordovician and Devonian conodonts. In abundance is noted. The negative results ob­ contrast, Barnes, Rexroad & Miller (1973) and tained are probably due to the mixing of cono­ Barnes & Fåhraeus (1975) considered that a nek­ dont faunas in the samples as some strata in tobenthic model is more appropriate for Ordovi­ Norway have been interpreted to be storm-gen­ cian conodont faunas. Several authors (including erated deposits (Worsley et al. 1982, Thomsen Bergstrom & Carnes 1976, Aldridge 1976, Chat­ 1982). However, an assessment of the environ­ terton 1976, Sandberg 1976) have concluded that mental preferences of conodont species using these simple models are insufficient to account brachiopod communities as a guide to water for the distributions that they have encountered. depth (Johnson & Worsley 1982) yielded inter­ Klapper & Barrick (1978) argued that distribu­ esting results. Several species show comparable tional patterns alone could not definitely show a restrictions to those found in younger Silurian pelagic or a benthic mode of life. Alternatively, strata of the Welsh area by Aldridge & Mabillard Aldridge & Mabillard (1981) suggested that the (1981). determination of environmental preferences of This study formed part of my Ph. D. project at particular conodont species and the compilation the University of Nottingham, England, on early of these inferences may throw light on their Silurian conodonts. Some 13,700 conodont sped­ modes of life. mens (representing 14 multi-element species) In this study, attempts were made to assess from 103 samples collected at eight localities of conodont palaeoecology during the relatively the Solvik and Sælabonn Formations in the Ask­ brief interval represented by the Distomodus er and Ringerike districts, respectively, of the 182 M. B. Idris NORSK GEOLOGISK TIDSSKRIFT 3 (1984) A • o OSLO FJORD Fig. l. Map of Asker (A) and Ringerike (B) districts showing locations of early Silurian exposures (stippled). Oslo Region (Fig. 1), were assessed. At present districts, recognised mainly on relative water the specimens are deposited at the Department depth as indicated by brac,hiopod communities of Geology, University of Nottingham and have documented by Johnson & Worsley (1982: text been given Nottingham numbers. They will final­ fig. 2, reproduced here as Fig. 2) and on the ly be housed at the Paleontologisk Museum, general lithological characteristics. The term University of Oslo, Sars Gate l, Norway. 'Point' used here is defined as a set of strata which may not be at the same locality, but were deposited under similar water depth conditions. Deposition of the Solvik Formation at Asker Localities (localities Nl-NS) was under relatively low ener­ Altogether 9 'Points' are established in the early gy, quiet water environments (Spjeldnæs 1982). Silurian successions of the Asker and Ringerike In contrast, sediments of the Sælabonn Forma- NORSK GEOLOGISK TIDSSKRIIT 3 (1984) Local Silurian conodont distribution 183 tion at localities N6, N7 and N8 in Ringerike ASKER LEPSCG RINGERIKE LEPSCG RytterOker \ Bru flat have been interpreted by Thomsen (1982) as Formot•on Formot1on c having been deposited in shallow water environ­ PIF o -- ·oc r ments under stormy conditions. Spjeldnæs (1982) E � Pl 7c r- � has also noted the turbulent nature of the deposi­ f3 -- � f3 D Pl :> r- tional environments at these localities. Se -- a i' r- t ' E l; l �� 7b c a el- �� " 2 Pt.C a: · o o r- r-- The points at Asker are: r- c �r- o�- o u. -- A. Locality N1 and the first 20 m of the profileat f3 o 6b ra- �· Sel the nearby locality N2. The bathymetric curve � �Vl -----r- � Pt.B c r-r-- (Fig. 2) indicates that this Point represents '! c o O Pt.l 6o f3 D 6b --- - the deeper water Clorinda community (see -- - o - - - - � � Ziegler et al. 1968). Further evidence of a PtA Vl 1-- Pt.H - rs.- PtG ;l deeper water environments is provided by the 5b 5b greater dominance of shale in the strata. l B. Encompasses the 20-90 m marks of the sec­ Fig. 2. Bathymetric curves for the Asker and Ringerike dis­ tion at locality N2. This Point Iies within the tricts based on the brachiopod communities (L = Lingula, E = Eocoelia, P = Pentamerus, S = Stricklandia, C = Clorinda) shallower water Stricklandia community and and the graptolite shales (G) of Ziegler et al. (after Johnson & the bathymetric curve indicates gradual shal­ Worsley 1982). The approximate positions of Points A-1 are lowing upwards. Limestone and siltstone lay­ indicated in the figure above. ers occur more frequently here and most of them are dark grey in colour, suggesting that the environment was also muddy. C. Includes the remaining (upper) part of the exposure at locality N2. In contrast to Point light grey in colour. Sandstone horizons occur B, this Point (C) represents the deepest water more frequently than at Point E. of the Stricklandia community (Fig. 2) and the limestone beds here are mostly light grey The Points in the Ringerike district are: in colour. The colour of the limestone, to­ G. Locality N6. The bathymetric curve (Fig. 2) gether with the quantity of undigested residue indicates that this Point represents the from the samples which is mostly less than shallowest water of the Stricklandia commu­ 25% per sample, suggest 'deaner' environ­ nity, comparable to Point F, but the energy ments. leve! here is higher (Thomsen 1981, D. Locality N3. This Point still Iies within the Spjeldnæs 1982). However, the rarity of rip­ Stricklandia community, but with a gradual ple marks, pebbly biosparite and sandy layers shallowing upwards as a continuation of Point suggest that this Point represented the !east C. The bathymetric curve suggests that the shallow and !east high energy environments in water depth here is comparable to Point B. the district. The strata here (Point D) are dominated by H. Locality N7. This Point represents the shal­ dark grey siltstone horizons which have been low water Pentamerus community and the fre­ interpreted by Worsley et al. (1982) as being quent occurrences of ripple marks, pebbly storm generated and deposited below normal biosparite and sandy layers also suggest in­ wave base. creasingly shallow water and higher energy E. Locality �4. The bathymetric curve (Fig. 2) environments. indicates gradual shallowing within the Strick­ l. Locality N8. This Point Iies in the shallowest landia community as a continuation of Point water Lingula community (Fig. 2). The more D. The more frequent occurrences of light frequent occurrences of ripple marks, pebbly grey limestone, siltstone and sandstone layers biosparite, and sandy layers and horizons also suggest shallowing and 'deaner' environ­ bearing current orientated corals are indica­ ments. tive of shallower and very high energy envi­ F. Locality NS. Represents the shallowest water ronments. of the Stricklandia community, and limestone strata at this Point are coarser grained and 184 M. B. Idris NORSK GEOLOGISK TIDSSKRIFf 3 (1984) Fig. 3. Relative abundances of the commoner conodont species at Points A-1, Oslo Region. Points A-F are in the Solvik Formation, Asker and Points G-1 are in the Sælabonn Forma­ tion, Ringerike. 80 l 2 3 4 5 6 7 8 9 10 11 12 13 14 80 l 2 3 4 5 6 7 8 9 10 11 12 13 14 60 16 Samples 60 25 Somples 60 26Samples 1907 Specimens 44 79 Specimens 1328 Specimens % % % 40 40 40 20 20 20 A B c 80 2 3 4 5 6 7 8 9 10 11 12 13 14 80 l 2 3 4 5 6 7 8 9 10 11 12 13 14 80 l 2 3 4 5 6 7 8 9 10 11 12 13 14 5 Samples 14 Somples 6 Samples 60 60 60 246 Specimens 2877 Specimens 47 3 Specimens % % % 40 40 40 20 20 20 D E F 80 2 3 4 5 6 7 8 9 IQ 11 12 1314 80 12 13 14 8 12 13 14 6 Somples 16 Somples 10 Samples 60 60 365 Specimens 1506 Specimens 597 Specimens % % 40 40 20 20 G H NORSK GEOLOGISK TIDSSKRIFT 3 (1984) Loca/ Silurian conodont distribution 185 Conodontfaunasof the From Fig.
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