Kr6lodvorlkosov Boundory Qnd the Jote Ordovicion Environmentol
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@L Sbor. Geologie Pages 4 z ls Praha 1989 vdd 44 Eeol. 777 -753 figs tabs. I nls. IS'SN 0581-9172 Kr6lodvorlKosov boundory qnd the Jote Ordovicion environmentol chonges in the Progue Bosin (Borrondion oreo, Bohemio) Hrqnice kr6lodvorfkosov o zm6ny prostiedi ve svrchnrm ordoviku v proisk6 p6nvi Petr Storchl - Mlchal MergIl Received March 19, 1986 storch p. - Mergl M. [1989J: xrdl'odvor/Kosov boundary and the late Ordovician environmental changes in the Prague Basin IBarrandian area, Bo- hemial. - Sbor. geol. VEd,..Geot,., 44, 11.7-L53. praha. Abstract: strong changes in facies and faunal assemblages at the Kra- lodvor/K,osov boundary are described from the Prague Basin. This boundary as well as the whole sequence of the Kosov Series are influenced by targe late Ordovician Slaciation of G,ondwanaland. We erplain the changes in lltho- logy and f,ossil biota by glacio-eustatic changes in sea-Ievel and we try to use it for chronostratiSraphic correlation. Sudden extinction of high-diverstfied, relatively deep-water proboscisambon community, succeeding Mucnonaspls community and pronounced change in lithology permit to correlate Kr6lo- dvor/Kos'ov b'oundary interval with that of Anglo-Scandic Rawtheyan/Hirn- antian boundary. This interval coresponds to the start of glacio-eustatic drop in sea-level and associated environmental changes. Overlying flysch-like sequence of the Kosov Formation dep'osited during further glacio-eustatic l,owering of the sea-level. Regression culminated by deposition of thick banks ,of shall,ow-water clastics with monot'onous bivalve fauna. The upper- miost part of the Kosov Formation means starting transgression which also brought the rich Hirnantia fauna to the Prague Basin. r 0stVedni fr.staD geologickg, Matosttanskd ndm. 19, 178 21 Ptaha 7 lntrod uction Global changes in lithOlogy and fossil biota on the Ordovician/Silurian boundary recently have attracted attention of many geologists and paleontologists. The first symptom of such revolutional changes could be identified in the upper Ashgill and corresponds to the Kr6lodvor/Ko- L77 sov boundary in the Prague Basin. It is connected with strong changes in sedirnentation and faunal assemblages. The first sections through the boundary between the Kr6hiv Dvtr and Kosov Formations rvere described by chlup6d [1951a, 1951b, 1953J and Marek [1952]. Major attention has been paid to stratigraphy and faunal associations. The biostratigraphy and fauna were described. by H avli d e k - vanEk t19661 in detail. Due to difficulties in strati- graphic correlatron of the Mediterranean region tsp jelcinaes 1g61J with the British standard scale, new chronostratigraphic units IKrdhiv Dviir series and Kosov Seriesl were introduced by Havlf dek - Ma- r e k [1973). Sedimentology of the uppermost ordovician strata was studied by Boudek - PiibyI t1958) and Kukal (1961, 1963J. A new conception of the prague Basin as a linear sedimentary depres- sion was given by Havlidek t19B1J. According to him, gradual syn- sedimentary deformations of the basin during the ordovician time control both the lithofacies and the distribution. of benthic communities I Havlidek 1980, 1981, 1sB2). pronouirced'change in sedimentation at the Kr6hiv Dvrir/Kosov boufldary was explained in the same sense IHavlidek 1981]. Both the change in sedimentation and extinction of shelly fauna and trilobites during this event was thought to be caused mostly by synsedimentary tectonic activity IHavIidek 19g2]. AIso substantial shallowing of the basin during Kosov age has been explained by tectono-eustatic mechanism although the glacial influence had formerly been assumed I i{avIid ek 1gT4). A new investigation in the uppermost ordovician of the prague Basin has been initiated by the worldwide discussion about the ordovician/si- lurian boundary and late Ordovician glaciation. The present authors accepted Haviidek,s [1gg0, 1gg1] model of linear sedimentary depression of the prague Basin but they assumed the Iate ordovician facies and faunal changes to have chiefly been controlled by glacio-eustatic causes. The authors are aware of the possibility of somewhat different explanation of these changes. However, the gtacio- eustatic conception of the late ordovician environmental changes pre- sented by Berry - Boucot [1973], Brenchiey t1984l and Brenchley - NewaIl [1984] best fits with the datafrom the upper Ordovician of the Prague Basin. Glaclo-eustatic environmental changes can be recognized on separate continents and plates I Berry - Boucot 1gZ3]. The resuitingfacies and faunal changes have often been precisely identified in the strati- graphical record I Berry - Boucot 7973,8renchIey - NewalI 1980, Cooper 1980, Brenchley - Cocks 7982, Brenchley - 118 CuIl en 1984J and could weII be correlated with those recorded from the Prague Basin. Sudden extinction of relatively deep watel trilobite-ostracode assem- blage with common brachiopods, cystoids and gastropods Itopmost Kr6- lodvor Proboscisambon Community in the sense of Havlidek 1,982 and Havlidek - MergI 1982J is correlated.in this paper with the extinction of trilobite-cystoid-gastropod faunas in the outer shelf con- ditions which is described by B re n chI e y [1984]. The drastic change in sedimentation at the Kr6lodvor/Kosov boundary ancl regressive sequence of the Kosov Formation could well be compared with a sequence controlled by gtacio-eustatic phenomena summarized by Brenchley - Newall [1984]. Recent correlation of the Kr6lodvor and Kosov with the British standard Stages proposes a synchronous natule of environmental changes caused by extensive late Ordovician glaciation IBeuf - tsiju-DuvaI Stevaux - Kulbicki 1-966, Hambrey - Harland 19811 and related glacio-eustatic changes in sea-Ievel I B er.ry - B ouc ot 1973]. The accurate biostlatigraphical correlation of the Kr6lodvor and Kosov with the British standard scale is complicated by both scarce occulrence of graptolites and different composition of benthic assemblages. in the Prague Basin, glacio-eustatic effect seems to coincide with the synsedimentary tectonic activity noted by Havlidek [1980, 1981, 1982J and K u k a I [1961, 1903). The similar situation was described alsoby Brenchle y - C ocks [1982] fromthe Osloregion. According to Brenchley - C ocks [1982), soon after beginning of the Hirn- antian, there were radical changes in the palaeogeography, which were probably initiated by differential block movement within the region, but emphasized by the significant late Ordovician glacio-eustatic regression. Thirteen sections were studied during the recent investigation. They are nearly regulariy distributed around the whole Prague Basin [text- -fig. 1i. They cover the Kr6hiv Dvrir/Kosov boundary interval. The Kraltv Dvrir/Kosov boundary sequence is subdivided here into several informal Iithostratigraphic units Ilevels Al, A2, A3, B1-a, B1b,82,C, D, E, F, G, HJ. AII the ievels are characterized by single type of sediment and corres- ponding faunal assemblage. AIl metric data in the description of the sections refer to distances above the first layer of coarse-grained sub- graywacke at the base of the Kosov Formation Ilevel D]. This layer has been found at aII the secti.ons in nearly the sarne thickness and indicates a consid.erable change in sedimentation. It is considered to be syn- chronous and referring to the base of the Kosov Series. Because of strongly similar character of all the sections, one ideal sectlon summarizing all the measurements is being described in detail. 119 6 I CN .: (D (D trq0 @6 @> oui bob 6iDLo O{ cd o0) !(J-dk I iq oll rde \ oA D} A E o- iq QC l!o .p'6rtr <o lJo -r\G^ os g6do idNo ^6 od E+ ooatD H EI@ HN o AP 6: E6a E3dt' gHo 'di o N dE d O 6q0 6# @fr(, g l:.fi-I ..t Eo.Ex t....t I l:.:j.:.1 3E t-....t t*h I : ..i q oO N o f,a9 @ Ho= OFE m at 0) .9o6 H ollEIII llHvI t20 Individual levels are discussed in this ideal section. All the Iocalities of the sections measured are only briefly commented. This work forms a contribution to the International deological corre- lation Programme IIGCP), project no. 216 - BIO-EVtrNTS. V"re wou]d like to thank M. Sna jdr who kindly gave us unpublished ljst of tri- lobites from the Kr6lodvor/Kosov Boundary interval which completed our findings. We would like also to thank I. Chlup6d who kindly Ient us upper Krdlodvor graptolites from Litef,, zadni riebai and velk6 Chuchle localities. Locolities This chapter attempts to give a brief characterization of the localities- More complete data are presented in pl. 1, locations of the sections are depicted by text-fig. 1. MaI6 Ohrada Deep excavations about 300 rn northeast of the MaI6 ohrada exposed a large section through the uppermost ordovician including Krahhv Dvrir/Kosov boundary interval. Due to tectonic disturbance it was im- possible to study certain parts of the section tpl 1J. The topmost Krd- Itiv Dvfir Fm. [evel 82] yields many fossils. some graptolites were collected in the level 41. ZIi d,in similar excavations 200 m southeast of the Zlidin railway station exposed complete Krdhlv Dvrir/Kosov boundary interval. The last Krdto- dvor fossils (Mucronaspis grandls and some undetermined gastropodsl have been found in nod.ular limestone bed near the top of the formation flevel C]. The main subgraywacke sequence Ilevel FJ yields numerous well rounded pebbles of sandstones and subangular pebbles of green shales. VrdZ A highway cut on the northern slope of the Herinky hill between vrdi and Beroun shows another outcrop with Krdhiv Dvrir/Kosov boundary interval. There are the two formations developed in a relatively srnall thickness. Richly fossiliferous level 81 is formed by brown-grey silty- clayey shale, probably decalcified Ithe typical B1b developmentJ. Level 82 is poorly developed. LzL Table 1 Percentage presence of brachiop,ods in samples JE-1 and VR-1 IJezerka and Yrdl, ,localities] " Lingula" incongtua. Barrande Aeanthambonia sp. Conotteta sp. Orbiculoidea squamosa I BarrandeJ lezercia ostiaria Havlidek & Mergl RaDozetina opima Havlidek & Mergl Salopina sleholeri Havl{dek & Mergl EpitomAonia dorsicaua Havlidek & Mergl Cliltonia sp.