3 Radiolarians, Radiolarites, and Mesozoic Paleogeography of the Circum-Mediterranean Alpine Belts
Patrick De Wever
Overview pearance of all the radiolarites in the Uppermost Jurassic may be attributed to a drastic change in cir Radiolarites are of great importance as bathymetric culation from gyres producing upwelling in the indicators for paleogeographic reconstructions and Tethyan basin, to latitudinal, east-to-west circula geodynamic models. Only recently have we been tion through Central America which broke down the able to date radiolarites and many age dates are scat upwelling regime in much of the tethyan area. tered through the specialized geologic literature. The inventory ofavailable data from the Mesozoic of the circum-Mediterranean Alpine fold belts reveals Introduction two general periods of 'radiolarite sedimentation, both associated with sedimentary and ophiolitic Radiolarites (rhythmically centimeter-bedded cherts sequences: one period occurred during Triassic alternating with millimeter-bedded shale) are of and/or Liassic time, the other during Dogger and/or great importance for paleogeographic reconstruc MaIm time. The Triassic-Liassic sections are alloch tions and geodynamic models because of their bath tonous. Radiolarites associated with extrusive rocks ymetric significance and their frequent association are considered to be the sedimentary cover of an with ophiolites. They are a very good marker for the oceanic crust, either of an open ocean, back-arc study of margin formation, subsidence, and evolu basin or small ocean basin. Radiolarites intercalated tion and are important for dating the oceanic crust. with other sedimentary rocks belong to nappes with However, direct dating of radiolarite was not pos unknown basement rocks, probably of thinned con sible until recently, since these sedimentary rocks do tinental crust. Radiolarites deposited during Dogger not bear the fossils commonly used for stratigraphy and MaIm time are of three types and characterize (e.g., foraminifers, nannofossils). Locally they had three environments whose common attribute is a been indirectly dated by reworked microfauna basal diachronism and a synchronous top. Radio included in rare beds of microbreccia (i.e., in the larites intercalated with sedimentary sequences Pindos-Olonos zone, Greece- Dercourt et al., 1973; occurred in basins that received deposits since Trias Thiebault et aI., 1981; De Wever and Thiebault, sic time (i.e., Lagonegro, Pindos-Olonos zone) or in 1981). Moreover, the petrographic composition of regions newly invaded by water masses (i.e., Austro these rocks did not permit use of the usual etching Alpine zone). Detailed local studies suggest deposi techniques for extracting fossils. In the 1970s, this tion on faulted blocks of a rifting margin. Although difficulty was overcome by the discovery of Dumi the base of radiolarites is not synchronous in differ trica (1970). His etching technique has been des ent units, it nevertheless starts around the Dogger cribed and improved by several authors (Pessagno and the maximum development was generally dur and Newport, 1972; De Wever et aI., 1979a; De ing Oxfordian time. The base of radiolarites asso Wever, 1982). Radiolarites of circum-Alpine oro ciated with ophiolites is everywhere dated as MaIm genic belts have since been analyzed paleontologi but the bases are not exactly synchronous; data are cally (Baumgartner et al., 1980; Baumgartner, 1984; still too few to analyze the details. The sudden disap- Kocher, 1981; Thiebault et aI., 1981; De Wever and 32 Patrick De Wever
6 6 ~ FIGURE 3.1. Synthesis of radiolarites sensu !:: .- c stricto (level 3b of Radiolarite Formation in .5! c c 0' Fleury, 1980) age dates. From left to right: syn N C C C c C CD o "~ ~ 0 0 o a ~ "~ c QI thetic lithologic column, radiolarite formation. V "; 'E C ~ c o 0 QI 0 .r; o o E 0 .... .r; 3a: Pelites de Kasteli member; 3b: radiolarite )( "~ 0 0 0 '0 o ~ ..... ~ CD CD u o ~ member (= radiolarites s.s.); 3c: Calcaires aCal pionelles member. Age assignments: thick Ir----.-.----\ 3 c horizontal line represents the well-documented age range; thin horizontal line represents the less well documented age range. The narrow age range for each sample is joined to its neighbor (thin line, stippled area); the same is done for wide age ranges (thick line, blank area). a: Age assign ments of Dercourt et al. (1973) on the basis of Lucasella cayeuxi (Lucas). Time scale is from Odin and Kennedy (1982). Sedimentation rates (of lithified sediment, equivalent to the sedimen
tation rate of the silica component-Si02 representing 95-99% of the total rock). Lower part: 35 m120 Ma = 1.7 m/Ma. Middle part: 10 mil8 Ma = 0.5 m/Ma. Upper part: 18 m/8 Ma = 2.2 m/Ma. Average: 65 m/46 Ma = 1.4 m/Ma.
30
Caby, 1981; De Wever and Thiebault, 1981; De blages in a sequence about 60 m thick (Fig. 3.1) Wever, 1982, 1984; De Wever and Origlia-Devos, representing Bajocian to Tithonian. Age determi 1982a,b; Dumitrica and Mello, 1982; Origilia nations in different sections allow calculation of Devos, 1983; Baumgartner, 1984; EI Kadiri, 1984; the average sedimentation rate (for lithified sedi De Wever and Miconnet, 1985; De Wever et al., ment) for the entire formation (1.4 m/Ma). The 1985, 1986b; De Wever and Cordey, 1986.) As this lower part ofthe Radiolarite Formation shows a rela list of references reveals, age information is recent tively high sedimentation rate of 3 m/Ma; the upper and dispersed throughout the specialized scientific part reveals a lower rate. Therefore, using radiolar literature. ians, variation in the sedimentation rate is detect able in a sequence. Calculation of sedimentation rates assumes that the sedimentary environment Age-Dating Capabilities with remained closed to the input or release of silica dur Radiolarians and Determination ing diagenetic processes. The sedimentation rates determined with radio ofSedimentation Rates ofRadiolarite larians for the Pindos-Olonos zone are compatible with rates proposed by other authors for other Mes During the 1980s, our knowledge of Mesozoic radi ozoic bedded chert sections: olarian stratigraphy increased a great deal, espe cially with respect to low-latitude belts. Radiolarian 2 m/Ma for Franciscan chert (S. Karl, unpublished stratigraphy now permits precise dating ofotherwise data in Hein and Karl, 1983) unfossiliferous siliceous rocks. 4 m/Ma as average for different Tethyan radiolarites As an example, in the Pindos-Olonos zone according to Bernoulli (in Garrision and Fischer, (Greece) (Fig. 3.1), it is possible to record 18 assem- 1969) 3. Radiolarites, Circum-Mediterranean Alpine Belts 33
0.7 to 1.0 m/Ma for East Alpine radiolarite (Garri silica, and dissolution of the silica tests), Barrett son and Fischer, 1969) (1981) found it more meaningful to report the 1.0 to 5.3 m/Ma (McBride and Thompson, 1970) for number of events (turbidites) per time unit. If we partially lithified chert (novaculites from Texas) accept that a bed represents one turbidite, this calcu 3 to 9 m/Ma for East Alpine radiolarite (Schlager lation reveals an average of47 events per Ma (thick and Schlager, 1973) ness ofthe series is 60 m, average bed thickness is 4 27 to 34 m/Ma for radiolarite in Japan (Iijima et al., cm, duration is 32 Ma) for the Pindos-Olonos series 1978) (Greece) in contrast to the 100 to 400 events cal 2.8 m/Ma for bedded cherts from Inuyama region, culated by Barrett for Ligurian cherts of northern Japan (Matsuda et al., 1980) Italy. One cycle in the Greek Radiolarite Formation 3.3 to 6.2 m/Ma for radiolarite in Northern Italy represents around 21,000 years. This cyclicity cor (Kocher, 1981) responds well to the period of precession in the 7.5 m/Ma in the Chichibu Chert Formation in Japan Milankovitch theory ofclimate variation (De Wever, (Matsumoto and Iijima, 1983) for the chert beds 1987). Alternatively, it has been proposed that the and 40 m/Ma for the shale partings bedding results from diagenesis. This is suggested by the observation that diatomites that are massive These rates are approximate, because they were when they are still opal-A acquired a pronounced not determined with the same chronostratigraphic rhythmic bedding when they underwent diagenesis scale. Different time length variations used for the (transformed in opal-CT) (Pisciotto and Garrison, Jurassic are as follows: Van Hinte (1976), 20 Ma; 1981, pIll). The origin ofthe bedding is not always Odin and Kennedy (1982), 28 Ma; Harland et al. clear, and both primary and diagenetic bedding no (1982), 25 Ma; Kent and Gradstein (1985), 25 Ma; doubt occur. The bedding in the rhythmically bed Westermann (1984), 24 Ma. The maximum differ ded Mesozoic cherts of Austria are clearly primary ence represents 40 %, and the proposed sedimenta (Vescei et al., this volume). One would not expect tion rates have to be considered to be within a 40% the bedding in diatomaceous and radiolarian-rich range. deposits to be the same because of the different More recent (unconsolidated) sediments have high hydrodynamic properties of the two microfossil rates of sedimentation; for example, siliceous sedi groups (Hein and Karl, 1983). ments (carbonate-free) found in Antarctic zones show rates of up to 4.8 m/Ma (Brewster, 1980). Some quoted sedimentation rates are up to 40 to 50 Ages of Tethyan Radiolarite m/Ma in the east Pacific and even 90 to 235 m/Ma in diatomaceous sediment from the Miocene of Japan The inventory of available data from Tethyan radio (Iijima et al., 1982; Matsumoto and Iijima, 1983), larites reveals, in general, two periods of pro but these correspond to the bulk sedimentation rate nounced biosiliceous sedimentation, one during (all phases) and not to the sedimentation rate ofonly Triassic and/or Liassic time, the other during Dog the silica phase. ger and/or MaIm time (Fig. 3.2). Both time intervals According to studies of the variation of porosity contain radiolarites associated with sedimentary and density in diatomaceous rocks (Isaacs et al., and ophiolite sequences. I will now present a brief 1983), a compaction of 60% was proposed. lithologic description and ages of the Alpine belts Studying a silicified wood within radiolarite ofthe where radiolarites have been dated. Adoyama Chert Formation (Honshu, Japan), Iijima et al. (this volume) also proposed a compaction of Triassic-Liassic Radiolarite 60 %. If we apply this factor to the radiolarites of the Pindos-Olonos zone, the uncompacted rate Radiolarite Interbedded would range from 3.5 to 7.5 m/Ma (average 6.2 with Sedimentary Rocks m/Ma). This rate is the same as the average rate pro Pindos-Olonos Zone, Greece posed for the Adoyama Chert Formation (6 m/Ma; Iijima et al., this volume) and to 7.5 m/Ma proposed In the Pindos-Olonos zone (one of the external by Matsumoto and Iijima (1983) for the Chichibu Hellenides nappes, Greece) there exist two series of Chert Formation. radiolarites. The best known is ofJurassic age and is Because many factors influence the original sed several tens of meters thick. The other one, only a imentation rate (turbidites, redeposition, input of few meters thick, belongs to the Drimos Limestone St>l NltIH.lA::>c; 6Cl NltlSINIt w +::- NltlNIOltl -f 6ll :IJ
NltINl:lltJ > (J) Oll (J) ()
Nltll:lON
Nltl.l3ltHl:l y' 1901-om: I t>Ol ( Wl ) _~!,.!..~_N'y!.:!:~.H__
Nltll:lnV NltiJorlt8 c... c ------t-IOH :IJ > (J) NltlNOH.llt8 (J) BS~ o NVl/\OllltJ S~-lv~ os~ NVIOl:lO::lXO ot>~ Nltl~OIl:l3WWI>l (S£~) ------NltlNOH.ll.l ~-Ll~ I oc~ NltISltll:ll:l38 9U 00 ~ mr~ -n~lf)- l/l :;~b n ;0 ~~~~ ~~S:~=i~~S'~ > o Ie$:;>; l/l::: ""0 CD lJl >::l 0 C ;0 S' cu' C s: 01 l/lOl=:OCUnn>I;o0 C C ~. ., c 3 Ie - >->IS: i;f 01 CDIC'> CDII ~ ~.g Formation (Dercourt et al., 1973; Fleury, 1980). It sic: latest Anisian or early Ladinian; Dumitrica and has long been considered to be Triassic but has Mello, 1982) and conodonts (Kozur and Mock, recently been dated as Liassic (De Wever and 1973). Origlia-Devos, 1982b) Hawasina Nappes (Oman) Budva Zone (Yugoslavia) In the Hamrat Dum Group, the Wahra Formation This zone in Yugoslavia is equivalent to the Pindos and the Lower Zulla and Upper Sid'r Formations all Olonos zone in Greece and contains the same two contain thick sequences of radiolarian-bearing chert levels of radiolarites. The Budva radiolarite-bearing interbedded with clastic sedimentary rocks. Sam sequences have recently been dated as Liassic ples from AI Jil cherts (= lower part of the Zulla for (Obradovic and Gorican, this volume; Gorican, mation in Glennie et al., 1974) yielded radiolarian 1987). fauna denoting late Murghabian and Late Triassic (Carnian-Norian) ages (Bourdillon et al., 1987; De Maliac Series (Greece) Wever, unpublished data). Additional chert samples This sequence (one of the internal Hellenides from the Matbat Formation (upper part of the Zulla nappes, Greece) has an unknown basement (pre + lower part of Guwayza in Glennie et al., 1974) sently subducted under the internal zones). Triassic indicate an Early Jurassic age (Bourdillon et al. chert has been dated from different sections. Some 1987; De Wever, unpublished data). Several dates are associated with only sedimentary rocks (Logis mentioned by Blome et al. (1983) are well in accor tion series, lower Ladinian-Carnian; Ferriere, dance with those given here. However, the precise 1982). According to Vrielynck (1982), the Maliac lithostratigraphic position of their samples was not series may be considered to have been deposited on given, so it is impossible to ascertain whether they the lower part of the Pelagonian margin facing actually correspond to the formations listed here. toward the Neo-Tethyan ocean. These data are cur rently the only paleontological evidence of an Radiolarites Overlying Ophiolites oceanic area existing in Triassic time between Eura (Triassic-Liassic) sia and Africa-Arabia. Darno Series (Hungary) Antalya Nappe Series (Turkey) In the Lower Austro-AIpine unit, chert associated In the Isparta-c;ay series, radiolarite-bearing inter with ophiolite pillow basalt has been dated as Middle vals are interbedded with limestone yielding halo Triassic (De Wever, 1984). bians, conodonts (Poisson, 1977), and radiolarians Rarau Series (Romania) (De Wever et al., 1979; De Wever, 1982) ofLate Tri assic (Norian) age. In transylvanian nappes (Rarau and Persani Moun tains, eastern Carpathian Mountains, Romania), Lagonegro Series (Italy) radiolarite has been dated as Middle Triassic (Ladin In southern Italy (Lucany, Lagonegro region), radi ian; Dumitrica and Mello, 1982). olarite overlies a cherty limestone sequence (Trias Diabase-Chert Unit (Yugoslavia) sic) which is in turn overlain by the clastic Galestri Formation (Neocomian). The first deposition of In the Diabase-Chert Formation from Serbia (Yu these cherts is diachronous, from Triassic to the goslavia), radiolarian chert (near pillow basalts) south (Lagonegro unit) to Upper Jurassic to the has been dated as Late Triassic (Carnian-Norian) north (San Fele unit) (De Wever and Miconnet, (Obradovic and Gorican, this volume; Gorican, 1985) (Fig. 3.3). 1986). Meliata Unit (Czechoslovakia) Porphyrite-Chert Unit (Yugoslavia) The Meliata unit (Czechoslovakia) belongs to the In the Porphyrite-Chert Unit from Serbia (Yugo Slovac edifice. It is an extension of the Bukk unit slavia), radiolarian cherts near volcanic rocks have (Hungary) and represents the termination of the been dated as Middle Triassic (Anisian-Carnian) Vardar zone (Dercourt et al., 1985a,b). It has been (Obradovic and Gorican, this volume; Gorican, dated as Triassic using radiolarians (Middle Trias- 1986). 36 Patrick De Wever N San Fele U. s ~~~n:::~~~~===------pjQr~Abric;i~~------'-iii=--IOII. Pignola - Abriola Unit Sasso di Castaldo Unit - -lOOOm Armizzone Unit Lagonegro Unit --2000m o, 30km FIGURE 3.3. Reconstruction ofLagonegro (southern Italy) level). (E-G) Calcaires a Silex Formation (cherty lime for Norian time (from De Wever and Miconnet, 1985). stone). E-San Fele Unit, massive dolomite; F-Pignola (A) Hypothetical rock basement (partly provided detritus Abriola Unit, bedded dolomite and dolomitic limestone; to the basin). (B) Basement cover and Scythian platform. G-Sasso di Castalda, Armizzone and Lagonegro Units. (C) Primary dolomite (partly provided detritus to the (H) Radiolarite belonging to Armizzone and Lagonegro basin). (D) Monte Facito Formation (probable thrusting (and Sasso di Castalda ?) Units. Maliac Series (Greece) older in basinal deposits (from Bajocian or Batho nian, Lombardy Basin, Umbria) and younger on Triassic cherts associated with basalts from the plateau deposits (mid-Oxfordian, Trento plateau) Maliac series (internal Hellenides nappes, Greece) (Baumgartner et al., 1980; Kocher, 1981; Baumgart have unknown basement rocks (presently sub ner, 1984; Conti et al., 1985; Conti, 1986). Radi ducted under the inner zones). Chert occurs between olarites are synchronously capped by Maiolica pillow lavas and has been dated as Late Triassic limestone (Govi, 1965; Luthi, 1973). (Tourla series, Norian; De Wever, 1982; Ferriere, 1982). Lagonegro Series (Lucany, Italy) As noted above (Fig. 3.3), these series have a Mid-Upper Jurassic Radiolarites diachronous base (from Triassic to Oxfordian), but During this time period, radiolarite is common. As their top is synchronous (Before Neocomian time) in the previous examples, it is associated with sedi (De Wever and Miconnet, 1985). mentary and volcanic sequences. Sciacca Series (Sicily, Italy) Radiolarites Within Sedimentary Sequences The base of this series is unknown, because it is cut by an overthrust fault, but the top has been precisely "Chaines Calcaires" (Rif, Morocco) dated with ammonites, calpionellids, nannofossils, The base of this formation is diachronous based on and radiolarians as the exact boundary between radiolarian dates, from Bajocian or Bathonian to Kimmeridgian and Tithonian (De Wever et al., Callovian or Oxfordian within different structural 1986b). units (Fig. 3.4), the oldest being to the south (exter Pindos-Olonos Series (Greece) nal zones) (De Wever et al., 1985). The top of this chert formation is synchronous in all structural Radiolarite has been studied in several sections from units, Kimmeridgian or early Tithonian. northern and southern Greece (Fig. 3.1). The bases of the sections are dated as Bajocian or Bathonian Southern Alps (Lombardy, Tuscany, (Thiebault et al., 1981; De Wever and Thiebault, and Umbria, Italy) 1981; De Wever and Cordey, 1986), and the top is Radiolarite sections are well known in northern Tithonian (De Wever and Origlia-Devos, 1982a; De Italy, where their bases are diachronous. Bases are Wever and Cordey, 1986) except in southernmost 3. Radiolarites, Circum-Mediterranean Alpine Belts 37 Outer s " Dorsole Colcoire" N INNER PART OUTER PART ==== unit 1 ======~ unit 2 ~~======unit 3 ======MALM ® .Ie 0 m 000 ~ 6 .2 n~iJ 3 l---~~I 4 6 5 !f±J 6 @) DOGGER 7 e Om-- 0 ~8 000 000 9 i'o \ I D_O_M_E_R_IA_N_-_T_O_A_R_C_IA_N;-- \"---'=""'e 0 m __ --::=:::=x:~~~\,3! \ \ '4 000 SINEMURIAN-CARIXIAN ® Ie, Om-- HETTANGIAN CD * Om-- FIGURE 3.4. Paleogeographic evolution of the Chrafate filaments"; (4) black shale; (5) Ammonitico Rosso; (6) klippes and the external "Dorsale Calcaire" (Rif, cherty limestone; (7) massive limestone; (8) dolomitic Morocco) zone during Jurassic time (from De Wever et massive limestone; (9) horizontal grading. al., 1985). (1) Turbidite; (2) radiolarites; (3) "calcaires a 38 Patrick De Wever Peloponnesus, where a few outer units contain radio (Lokut and Tata Hills), but in other places radiolarite larites as young as the Late Cretaceous. However, persists to the Kimmeridgian (Cszernye Hills) or during Tithonian time, some thick, coarse clastic even to the Kimmeridgian-Tithonian boundary intervals are interbedded (Thiebault et aI., 1981; De (Margit Hill-De Wever, unpublished data). Wever and Thiebault, 1981). Upper Austro-Alpine Series Budva Series (Yugoslavia) Czechoslovakia (Silica Nappe) A series equivalent to the Pindos-Olonos series Radiolarite (equivalent to the series of the Slovak exists in Montenegro (Yugoslavia), where it has Karst) is dated as Callovian or Oxfordian (Dumitrica been dated as Middle Jurassic to Late Jurassic and Mello, 1982). (Obradovic and Gorican, this volume; Gorican, 1987). Hawasina Nappes (Oman) In the Hamrat Duru Group, the Wahra Formation, Pelagonian Series (Internal Hellenides, the Lower Zulla and Upper Sid'r Formations all con Strimbes, Greece) tain thick sequences of radiolarian-bearing chert Radiolarites from Pelagonian series (Othrys, con interbedded with clastic sedimentary rocks (turbi tinental Greece) have been dated as Kimmeridgian dites). Radiolarian fauna obtained from the Sid'r Tithonian with radiolarians (det. Origlia-Devos in chert indicate a Late Jurassic to Neocomian age Ferriere, 1982, p 930) (Bourdillon et aI., 1987; De Wever, unpublished data). Several dates have been mentioned by Blome Pelagonian Series (Asklepeion Nappe, et aL (1983) and by Baumgartner (1984,) for the Al Angelokastron Cherts, Internal Aridh Formation. Their dates are in accordance Hellenides, Greece) with those given here. However, they did not pro Radiolarites have been dated in the Peloponnesus. vide precise lithostratigraphic positions for their Their first occurrence is Bathonian (Asklepeion samples, so it is impossible to ascertain whether they Nappe-Baumgartner, 1984) or late Oxfordian (An correspond to the stratigraphic levels analyzed here. gelokastron Chert and Ayios Nikolaos Formation Baumgartner, 1984; they correspond to the Epidaure Autochtonous Unit (Oman) zone and Trapezona zone ofVrielynck, 1978, 1980, Overlying the Sahtan Group (shallow-water Jurassic 1982 at this time period). limestone) is the Kahmah Group, which yielded radiolarians in chert at the base of the section Pelagonian Series (Evvoia, Greece) interbedded with limestone (with Calpionellids). Radiolarite from northern Evvoia has been dated The chert was dated as late Tithonian or early Berri with radiolarians as Oxfordian or Kimmeridgian, asian (Bourdillon et al., 1987; De Wever, unpub except the latest Kimmeridgian (Baumgartner, lished data). 1984; Ferriere et aI., 1986). This radiolarite was dated as MaIm or Neocomian (Baumgartner and Radiolarite Overlying Oceanic Crust Bernoulli, 1976), but more recent information per mits us to restrict the age as mentioned above. Good outcrops of ophiolite-hosted radiolarite are rare because of metamorphism. Nevertheless, some Upper Austro-Alpine Series in Hungary significant data already exist. (Bakony and Gerecze Nappes) Series of the Ligurian Ophiolitic Nappe (Queyras In the upper Austro-Alpine nappes of Hungary and Corsica, France; Liguria and Elba, Italy) (Bakony and Gerecze Hills), the base of the radi olarite section is late Bajocian or Bathonian (in The metaradiolarite, which overlies the ophiolite Kozoskut, Lokut, Bakonycsernye sections - FUlop, and ophiolitic breccia of the Chabriere series (base 1969, 1976; De Wever, unpublished data) but else ofthe "Schistes Lustres"), has been dated in sections where is Bathonian (Gyenespuszta-Galacz, 1970, from France as late Oxfordian or early Kimmer 1976, 1980; De Wever, unpublished data). The idgian (De Wever and Caby, 1981) and on rocks radiolarite-bearing formation is overlain in some from Italy (Traversiera) as middle-upper Callovian places by a coarse clastic section of Oxfordian age (De Wever et aI., 1987a). 3. Radiolarites, Circum-Mediterranean Alpine Belts 39 Nonmetamorphosed radiolarite from Elba overly Bukk Ophiolitic Nappe (Bukk Massif, Hungary) ing pillow lava (Monte Campannelo-Nisporta, Vol The Bukk may be considered as a Neo-Tethyan sphe terraio) or ophicalcite (San Felo Namia) has been nochasm between Apulia and Europe (Dercourt et dated as middle Oxfordian or early Kimmeridgian aI., 1984a,b, 1985a,b). A Bajocian and/or Batho (Baumgartner, 1984; Conti and Marcucci, 1986). nian age was obtained (Kozur, 1984) for the upper This age is the same as that determined in other part of the radiolarite occurring in the wildflysch sections (Murlo, Southern Toscany-Conti and overlying ophiolites in the Bukk Massif (northeast Marcucci, 1986; San Colombano and kIn 59, Hungary) (Geyssant and Lepvrier, 1984). Corsica- De Wever et aI., 1987b). The shale between ophicalcite and radiolarite has been dated Bucovinian Nappe (Romania) at Rocchetta di Vara (eastern Ligurides) (Baum gartner, 1984) as middle Callovian or basal Oxford Pojorim radiolarite (crystalline Mesozoic zone of ian, and the basal part ofthe chert sequences is dated southeastern Carpathian Mountains) has long been as late Oxfordian or early Kimmeridgian (Conti considered to be ofMiddle Triassic age (Bancila and et aI., 1985; Conti and Marcucci, 1986). In the Papiu, 1953) but is now known to be of Callovian section at II Conventino (Monti Rignosi, eastern and/or Oxfordian age (Mutihac, 1968; Cioflica et Tuscany), the radiolarian assemblage gives a late aI., 1981; Dumitrica, unpublished data), as are most Callovian or early Kimmeridgian age (Conti and radiolarites of this zone. Marcucci, 1986). The onset of deposition of the Metalliferous Mountains (Southern Apuseni Monte Alpe Chert gives an upper age limit for basalt Mountains, Romania) extrusion in each section. The age of sedimentation overlying Ligurian ocean crust was therefore not The Dro<;ea-Cris Unit (of the Cris Nappe) contains synchronous. radiolarite dated as Oxfordian (Dumitrica in Bleahu et aI., 1981) overlying basalt. The Curechiu-Smnija Subpelagonian Nappe Series (Migdhalitsa unit has radiolarite dated as Oxfordian-Tithonian Unit, Argolis, Greece) (Dumitrica in Bleahu et aI., 1981) overlying ophio lite and tuffs. Radiolarite from the Migdhalitsa unit (Baumgartner, The Capllnas-Techereu Nappe (well developed in 1984; Fanary Wildflysch ofVrielynck, 1982) overly Metalliferous Mountains) has radiolarite dated as ing pillow basalt of the Ophiolite Suite (Akros Hill) late Callovian to late MaIm (Dumitrica, unpublished is dated near its contact with basalt as middle Callo data) interlayered with volcanic rocks, both ofwhich vian or early Oxfordian (Baumgartner, 1984) overlie ophiolite with a radiometric age of 180 Ma. This ophiolite volcanism was thus probably active Subpelagonian Nappe Series (Evvoia, Greece) before the Jurassic until the Callovian. Chert samples from interpillow space ofthe ophiolite Marmaros Unit (Klippen Zone, USSR) in northern Evvoia are dated as Oxfordian (Baum gartner, 1984; Ferriere et aI., 1986). Radiolarite from the Marmaros unit associated with ophiolites of the northeast Eastern Carpathians has Subpelagonian Nappe Series (Thessaly, Greece) been dated as Oxfordian and Kimmeridgian (Tik homirova, 1983, 1984). Oxfordian and/or Tithonian age was obtained for ra diolarite overlying ophiolite at Theopetra (De Wever in Ardaens, 1978; Ferriere, 1982), but the rocks Summary dated were not from the base of the section. All Triassic and Liassic radiolarite sections associ Diabase-Chert Formation, Serbian Units ated with sedimentary rocks are parts of allochto (Internal Dinarides, Yugoslavia) nous sequences. These radiolarites belong to nappes with unknown basement rocks, probably thinned In the Diabase-Chert Formation from western Ser continental crust (Dercourt et aI., 1985a,b; Ricou et bia (Yugoslavia), Upper Jurassic chert (Callovian aI., 1985)(Fig. 3.5). and/or early Kimmeridgian) containing radiolarians Radiolarite interbedded with sedimentary se has been identified (Obradovic and Gorican, this quences exists in basins that have received deposits volume; Gorican, 1986, p 58). since Triassic time (Lagonegro, Pindos-Olonos ~ .~ A '90" ---<--- ... •B U 6 ~7 = 2 DB 2->< '52' 3 ,/ j ..... 4 0 9 :' i ::::::: 10 \ '.' 5 i :7-.:;':';-'; 11 " " _.-;:.//",\ ~HaWaSinri Liassic ~(~H\" ~ '"1 FIGURE 3.5. Location ofradiolarite outcrops that were dated by olarite; (3) limestone; (4) flysch; (5) breccia; (6) emergent land n' :><;" radiolarian assemblages. Paleogeographic map from Dercourt (on any type of crust); (7) oceanic crust; (8) thin continental u et al. (1985a) map No.1 (Pliensbachian) is used for this report. crust (basin); (9) thick continental crust (platform); (10) active (l> (A) Radiolarite associated with sedimentary sequences; (B) spreading ridge; (11) spreading ridge when dying out. ~ <: (l> radiolarite associated with ophiolites. (1) Volcanism; (2) radi- '"1 3. Radiolarites, Circum-Mediterranean Alpine Belts 41 zone, ...) or in regions newly invaded by major Diachronism/synchronism transgressive water masses (Le., Austro-Alpine of Radiolarites zone, Inner Maghrebides). Detailed local studies (Rif, Morocco; Lagonegro, southern Italy; southern Diachronism characterized the earliest radiolarite Alps, northern Italy) suggest deposition on faulted sedimentation, whereas synchronism characterizes blocks of a rifting margin. Although the base of the the latest radiolarite sedimentation. In early studies radiolarite is not synchronous in different units, it of radiolarites, it appeared that the main radiolarite nevertheless generally starts around the Dogger. The sequences belonged to the Jurassic, confirming the maximum development of biosiliceous sedimenta classic hypothesis on the age of these deposits. Suc tion occurred during Oxfordian time. Their disap ceeding work dated the base and the top of radio pearance occurs synchronously near the Tithonian larite sections more precisely and revealed that the (Fig. 3.6). bases were not always of the same age. Detailed All allochtonous radiolarites associated with vol studies of the Dogger and MaIm radiolarites in canic rocks have been presumed to be the sedimen different regions documented a diachronism for the tary cover of oceanic crust, either of an open-ocean earliest radiolarite sedimentation and a synchronism basin crust or of a smaller ocean basin. for the top of the sections. The base ofradiolarite-bearing sequences (exclud The age variations correspond to different tilted ing mudstone locally known as umbers in Cyprus blocks of a continental margin. In some places, age and Oman) associated with ophiolite is everywhere variations correspond to a stretching from the ocean dated as MaIm, but it is not exactly synchronous; toward the craton (Rif, Morocco; Lagonegro, Italy). data are still too few for a precise analysis. Such a margin may have corresponded to the The above descriptions consider only true radio development of a transform fault zone (Lemoine, larites, as defined in the Introduction. Other radio 1985; Dercourt et al., 1984a, b) between the open larian dates have recently been obtained from chert ing Atlantic Ocean and the subducting Neo-Tethys to nodules in circum-Mediterranean Alpine fold belts the south of the Alboran-Kabyly-Calabria block (De but are not considered in this paper because radio Wever et al., 1985). Radiolarite basins in northern larians were extracted from rock types other than Morocco migrated northward during Middle to Late radiolarite sensu stricto. Two sets of results are nev Jurassic time, suggesting sedimentation on tilted ertheless mentioned because of their geodynamic fault blocks (Fig. 3.4) with progressive northward implications. They are as follows: radiolarian formation of listric faults (Lemoine, 1982, 1985; bearing rocks associated with the Samail ophiolite, Lemoine et al., 1981; De Wever et al., 1985). Oman, and the Troodos ophiolite, Cyprus, yielded a The diachronism ofthe first radiolarite deposits in radiolarian fauna dated as Cretaceous: Turonian in Lagonegro (De Wever and Miconnet, 1985) and Cyprus (Blome and Irwin, 1985): Campanian in Argolis (Baumgartner, 1985) represent the same Oman (Schaaf and Thomas, 1986; De Wever et al., kind ofphenomenon-deposition on tilted blocks of 1988); and Albian-Cenomanian, Cenomanian an actively forming margin. Turonian, and Santonian-Campanian in Oman In other places, age variations correspond to the (Beurrier et al., 1987; Bourdillon et al., 1987). difference noted between basins and plateaus or sea Dated rocks are red mudstone or umbers (not radio mounts such as the Lombardian basin and Trento larite). plateau, northern Italy (Baumgartner, 1984) and Work is in progress on the radiolarian stratigra Bakony Hills, Hungary (Galacz, 1984; Galacz et al., phy of other regions: (1) to the east in the Sam 1985; De Wever, unpublished data). The first radio khet Karabagh (Lesser Caucasus, Georgia, USSR), larite deposits are not synchronous, but their maxi where the Jurassic Lesser Caucasus radiolarite mum extent of development is generally during the sequences have been dated as Callovian (to Bar Oxfordian. remian?) with the Kimmeridgian missing (Vishnev skaya, 1984), and (2) in the Klippen Belt of the Pienninnic sequences of the Csorstyn and Kysuca Disappearance of Radiolarites regions (Birkenmayer, 1977). With these new data it will be possible to date the distension of this It is now accepted that most radiolarites were not margin. deposited in wide-open basins or on continental -+:>. ? 40_ N ".~ l \ \ '90,",,0' ~ """"'" "'<""';;" .... ~ r 1000000 800000 600000 Ql U 400000 c:: o 66 ." c:: ::J 200000 .Q ex 19 21 23 23 10 33 3327 8.11.30 Number of species FIGURE 3.7. Comparison between biocenosis and than dredged on the sea floor in the same localities. The shift of tocenosis. (A) Radiolarian fauna caught on both sides of the maximum abundance is a result of equatorial gyres. the equator in the Pacific Ocean. (B) Radiolarian fauna The sudden disappearance of all the radiolarites in deepening could have resulted from a "flush" effect the Latest Jurassic could be explained by a drastic at the beginning of the latitudinal current. In Japan, change in circulation due to a latitudinal flow being farther east, where radiolarites were not through Central America. Such a new communica diluted by clastic sedimentation, this modification tion drastically changed the current pattern from a in currents had no effect, and radiolarite sedimenta gyre in the Tethyan triangular basin (and its asso tion persisted until Neocomian time. ciated upwelling) to a latitudinal current and can The new circulation pattern affected most of the therefore explain the disappearance of radiolarite zones: the oldest rifted platforms (Austro-Alpine, sedimentation (De Wever et al., 1986a). This latitu southern Alps) as well as the troughs (Lagonegro, dinal current could have, at least locally, flowed from Pindos-Olonos zone, Budva zone). We have there east to west, as suggested by Thiebault et al. (1986). fore to interpret the paleogeography and/or paleo This hypothesis accords well with the long dura bathymetry ofthe areas where radiolarite deposition tion of radiolarite sedimentation of the east Arabian persisted after the installation of latitudinal currents margin of the Tethys Sea (Oman). In Oman, radio (i.e., southernmost Pindos zone, Greece) as con larite sedimentation continued until the Late Creta fined basins. The regions where radiolarites existed ceous, because this area remained under the influ through the Cretaceous were regions sheltered from ence of the upwelling conditions (De Wever and the new latitudinal current (Oman, Japan). Bourdillon, 1987; De Wever et al., 1988; De Wever, unpublished data). When the current pattern Acknowledgments. This work has been supported by changed to latitudinal circulation rather than a gyre, the CNRS (UA 319, ATP GGO No. 98 1039) and there was a sudden deepening of the basin recorded UNESCO lUGS (project IGCP 187). I am indebted in the sedimentary rocks of the Autochtonous Unit to A. Kemp (University of Southampton, U.K.) and in Oman. Sedimentation became progressively shal 1. Tauxe for their help with the English and to R. lower again during Neocomian time. This initial Garrison (University of California at Santa Cruz), 1. 3. Radiolarites, Circum-Mediterranean Alpine Belts 45 Dercourt (University of Paris), J. Obradovic (Uni Samail (Sultanat d'Oman): Consequences tectogene versity of Belgrade and J.R. Hein (USGS, Menlo tiques. Comptes Rendus de 11\.cademie des Sciences, Park, CA) for reading the manuscript. Paris 304:907-910. Birkenmayer K (1977) Jurassic and Cretaceous lithostrati graphic units of the Pieniny Klippen belt Carpathians, References Poland. 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