3Radiolarians, Radiolarites, and MesozoicPaleogeography of theCircum-Mediterranean Alpine BeltsPatrick De Wever

Overview

Radiolarites are of great importance as bathymetricindicators for paleogeographic reconstructions andgeodynamic models. Only recently have we beenable to date radiolarites and many age dates are scat­tered through the specialized geologic literature.The inventory of available data from the Mesozoic ofthe circum-Mediterranean Alpine fold belts revealstwo general periods of 'radiolarite sedimentation,both associated with sedimentary and ophioliticsequences: one period occurred during Triassicand/or Liassic time, the other during Dogger and/orMaIm time. The Triassic-Liassic sections are alloch­tonous. Radiolarites associated with extrusive rocksare considered to be the sedimentary cover of anoceanic crust, either of an open ocean, back-arcbasin or small ocean basin. Radiolarites intercalatedwith other sedimentary rocks belong to nappes withunknown basement rocks, probably of thinned con­tinental crust. Radiolarites deposited during Doggerand MaIm time are of three types and characterizethree environments whose common attribute is abasal diachronism and a synchronous top. Radio­larites intercalated with sedimentary sequencesoccurred in basins that received deposits since Trias­sic time (i.e., Lagonegro, Pindos-Olonos zone) or inregions newly invaded by water masses (i.e., Austro­Alpine zone). Detailed local studies suggest deposi­tion on faulted blocks of a rifting margin. Althoughthe base of radiolarites is not synchronous in differ­ent units, it nevertheless starts around the Doggerand the maximum development was generally dur­ing Oxfordian time. The base of radiolarites asso­ciated with ophiolites is everywhere dated as MaImbut the bases are not exactly synchronous; data arestill too few to analyze the details. The sudden disap-

pearance of all the radiolarites in the UppermostJurassic may be attributed to a drastic change in cir­culation from gyres producing upwelling in theTethyan basin, to latitudinal, east-to-west circula­tion through Central America which broke down theupwelling regime in much of the tethyan area.

Introduction

Radiolarites (rhythmically centimeter-bedded chertsalternating with millimeter-bedded shale) are ofgreat importance for paleogeographic reconstruc­tions and geodynamic models because of their bath­ymetric significance and their frequent associationwith ophiolites. They are a very good marker for thestudy of margin formation, subsidence, and evolu­tion and are important for dating the oceanic crust.

However, direct dating of radiolarite was not pos­sible until recently, since these sedimentary rocks donot bear the fossils commonly used for stratigraphy(e.g., foraminifers, nannofossils). Locally they hadbeen indirectly dated by reworked microfaunaincluded in rare beds of microbreccia (i.e., in thePindos-Olonos zone, Greece - Dercourt et al., 1973;Thiebault et aI., 1981; De Wever and Thiebault,1981). Moreover, the petrographic composition ofthese rocks did not permit use of the usual etchingtechniques for extracting fossils. In the 1970s, thisdifficulty was overcome by the discovery of Dumi­trica (1970). His etching technique has been des­cribed and improved by several authors (Pessagnoand Newport, 1972; De Wever et aI., 1979a; DeWever, 1982). Radiolarites of circum-Alpine oro­genic belts have since been analyzed paleontologi­cally (Baumgartner et al., 1980; Baumgartner, 1984;Kocher, 1981; Thiebault et aI., 1981; De Wever and

32 Patrick De Wever

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FIGURE 3.1. Synthesis of radiolarites sensustricto (level 3b of Radiolarite Formation inFleury, 1980) age dates. From left to right: syn­thetic lithologic column, radiolarite formation.3a: Pelites de Kasteli member; 3b: radiolaritemember (= radiolarites s.s.); 3c: Calcaires aCal­pionelles member. Age assignments: thickhorizontal line represents the well-documentedage range; thin horizontal line represents the lesswell documented age range. The narrow age rangefor each sample is joined to its neighbor (thin line,stippled area); the same is done for wide ageranges (thick line, blank area). a: Age assign­ments of Dercourt et al. (1973) on the basis ofLucasella cayeuxi (Lucas). Time scale is fromOdin and Kennedy (1982). Sedimentation rates(of lithified sediment, equivalent to the sedimen­tation rate of the silica component- Si02

representing 95-99% of the total rock). Lowerpart: 35 m120 Ma = 1.7 m/Ma. Middle part: 10mil 8 Ma = 0.5 m/Ma. Upper part: 18 m/8 Ma =2.2 m/Ma. Average: 65 m/46 Ma = 1.4 m/Ma.

Caby, 1981; De Wever and Thiebault, 1981; DeWever, 1982, 1984; De Wever and Origlia-Devos,1982a,b; Dumitrica and Mello, 1982; Origilia­Devos, 1983; Baumgartner, 1984; EI Kadiri, 1984;De Wever and Miconnet, 1985; De Wever et al.,1985, 1986b; De Wever and Cordey, 1986.) As thislist of references reveals, age information is recentand dispersed throughout the specialized scientificliterature.

Age-Dating Capabilities withRadiolarians and Determinationof Sedimentation Rates of Radiolarite

During the 1980s, our knowledge of Mesozoic radi­olarian stratigraphy increased a great deal, espe­cially with respect to low-latitude belts. Radiolarianstratigraphy now permits precise dating of otherwiseunfossiliferous siliceous rocks.

As an example, in the Pindos-Olonos zone(Greece) (Fig. 3.1), it is possible to record 18 assem-

blages in a sequence about 60 m thick (Fig. 3.1)representing Bajocian to Tithonian. Age determi­nations in different sections allow calculation ofthe average sedimentation rate (for lithified sedi­ment) for the entire formation (1.4 m/Ma). Thelower part of the Radiolarite Formation shows a rela­tively high sedimentation rate of 3 m/Ma; the upperpart reveals a lower rate. Therefore, using radiolar­ians, variation in the sedimentation rate is detect­able in a sequence. Calculation of sedimentationrates assumes that the sedimentary environmentremained closed to the input or release of silica dur­ing diagenetic processes.

The sedimentation rates determined with radio­larians for the Pindos-Olonos zone are compatiblewith rates proposed by other authors for other Mes­ozoic bedded chert sections:

2 m/Ma for Franciscan chert (S. Karl, unpublisheddata in Hein and Karl, 1983)

4 m/Ma as average for different Tethyan radiolaritesaccording to Bernoulli (in Garrision and Fischer,1969)

3. Radiolarites, Circum-Mediterranean Alpine Belts

0.7 to 1.0 m/Ma for East Alpine radiolarite (Garri­son and Fischer, 1969)

1.0 to 5.3 m/Ma (McBride and Thompson, 1970) forpartially lithified chert (novaculites from Texas)

3 to 9 m/Ma for East Alpine radiolarite (Schlagerand Schlager, 1973)

27 to 34 m/Ma for radiolarite in Japan (Iijima et al.,1978)

2.8 m/Ma for bedded cherts from Inuyama region,Japan (Matsuda et al., 1980)

3.3 to 6.2 m/Ma for radiolarite in Northern Italy(Kocher, 1981)

7.5 m/Ma in the Chichibu Chert Formation in Japan(Matsumoto and Iijima, 1983) for the chert bedsand 40 m/Ma for the shale partings

These rates are approximate, because they werenot determined with the same chronostratigraphicscale. Different time length variations used for theJurassic are as follows: Van Hinte (1976), 20 Ma;Odin and Kennedy (1982), 28 Ma; Harland et al.(1982), 25 Ma; Kent and Gradstein (1985), 25 Ma;Westermann (1984), 24 Ma. The maximum differ­ence represents 40 %, and the proposed sedimenta­tion rates have to be considered to be within a 40%range.

More recent (unconsolidated) sediments have highrates of sedimentation; for example, siliceous sedi­ments (carbonate-free) found in Antarctic zonesshow rates of up to 4.8 m/Ma (Brewster, 1980).Some quoted sedimentation rates are up to 40 to 50m/Ma in the east Pacific and even 90 to 235 m/Ma indiatomaceous sediment from the Miocene of Japan(Iijima et al., 1982; Matsumoto and Iijima, 1983),but these correspond to the bulk sedimentation rate(all phases) and not to the sedimentation rate of onlythe silica phase.

According to studies of the variation of porosityand density in diatomaceous rocks (Isaacs et al.,1983), a compaction of 60% was proposed.

Studying a silicified wood within radiolarite of theAdoyama Chert Formation (Honshu, Japan), Iijimaet al. (this volume) also proposed a compaction of60 %. If we apply this factor to the radiolaritesof the Pindos-Olonos zone, the uncompacted ratewould range from 3.5 to 7.5 m/Ma (average 6.2m/Ma). This rate is the same as the average rate pro­posed for the Adoyama Chert Formation (6 m/Ma;Iijima et al., this volume) and to 7.5 m/Ma proposedby Matsumoto and Iijima (1983) for the ChichibuChert Formation.

Because many factors influence the original sed­imentation rate (turbidites, redeposition, input of

33

silica, and dissolution of the silica tests), Barrett(1981) found it more meaningful to report thenumber of events (turbidites) per time unit. If weaccept that a bed represents one turbidite, this calcu­lation reveals an average of 47 events per Ma (thick­ness of the series is 60 m, average bed thickness is 4cm, duration is 32 Ma) for the Pindos-Olonos series(Greece) in contrast to the 100 to 400 events cal­culated by Barrett for Ligurian cherts of northernItaly. One cycle in the Greek Radiolarite Formationrepresents around 21,000 years. This cyclicity cor­responds well to the period of precession in theMilankovitch theory of climate variation (De Wever,1987). Alternatively, it has been proposed that thebedding results from diagenesis. This is suggested bythe observation that diatomites that are massivewhen they are still opal-A acquired a pronouncedrhythmic bedding when they underwent diagenesis(transformed in opal-CT) (Pisciotto and Garrison,1981, pIll). The origin of the bedding is not alwaysclear, and both primary and diagenetic bedding nodoubt occur. The bedding in the rhythmically bed­ded Mesozoic cherts of Austria are clearly primary(Vescei et al., this volume). One would not expectthe bedding in diatomaceous and radiolarian-richdeposits to be the same because of the differenthydrodynamic properties of the two microfossilgroups (Hein and Karl, 1983).

Ages of Tethyan Radiolarite

The inventory of available data from Tethyan radio­larites reveals, in general, two periods of pro­nounced biosiliceous sedimentation, one duringTriassic and/or Liassic time, the other during Dog­ger and/or MaIm time (Fig. 3.2). Both time intervalscontain radiolarites associated with sedimentaryand ophiolite sequences. I will now present a brieflithologic description and ages of the Alpine beltswhere radiolarites have been dated.

Triassic-Liassic Radiolarite

Radiolarite Interbeddedwith Sedimentary Rocks

Pindos-Olonos Zone, Greece

In the Pindos-Olonos zone (one of the externalHellenides nappes, Greece) there exist two series ofradiolarites. The best known is of Jurassic age and isseveral tens of meters thick. The other one, only afew meters thick, belongs to the Drimos Limestone

FIGURE 3.2. Summary table of radiolarite ages based on radiolariandata presented in this paper. (1) Paleontological data for radiolariteassociated only with sedimentary sequences. (2) Paleontologicaldata for chert from interpillow lava spaces. (3) Paleontological data

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3. Radiolarites, Circum-Mediterranean Alpine Belts

Formation (Dercourt et al., 1973; Fleury, 1980). Ithas long been considered to be Triassic but hasrecently been dated as Liassic (De Wever andOriglia-Devos, 1982b)

Budva Zone (Yugoslavia)

This zone in Yugoslavia is equivalent to the Pindos­Olonos zone in Greece and contains the same twolevels of radiolarites. The Budva radiolarite-bearingsequences have recently been dated as Liassic(Obradovic and Gorican, this volume; Gorican,1987).

Maliac Series (Greece)

This sequence (one of the internal Hellenidesnappes, Greece) has an unknown basement (pre­sently subducted under the internal zones). Triassicchert has been dated from different sections. Someare associated with only sedimentary rocks (Logis­tion series, lower Ladinian-Carnian; Ferriere,1982). According to Vrielynck (1982), the Maliacseries may be considered to have been deposited onthe lower part of the Pelagonian margin facingtoward the Neo-Tethyan ocean. These data are cur­rently the only paleontological evidence of anoceanic area existing in Triassic time between Eura­sia and Africa-Arabia.

Antalya Nappe Series (Turkey)

In the Isparta-c;ay series, radiolarite-bearing inter­vals are interbedded with limestone yielding halo­bians, conodonts (Poisson, 1977), and radiolarians(De Wever et al., 1979; De Wever, 1982) of Late Tri­assic (Norian) age.

Lagonegro Series (Italy)

In southern Italy (Lucany, Lagonegro region), radi­olarite overlies a cherty limestone sequence (Trias­sic) which is in turn overlain by the clastic GalestriFormation (Neocomian). The first deposition ofthese cherts is diachronous, from Triassic to thesouth (Lagonegro unit) to Upper Jurassic to thenorth (San Fele unit) (De Wever and Miconnet,1985) (Fig. 3.3).

Meliata Unit (Czechoslovakia)

The Meliata unit (Czechoslovakia) belongs to theSlovac edifice. It is an extension of the Bukk unit(Hungary) and represents the termination of theVardar zone (Dercourt et al., 1985a,b). It has beendated as Triassic using radiolarians (Middle Trias-

35

sic: latest Anisian or early Ladinian; Dumitrica andMello, 1982) and conodonts (Kozur and Mock,1973).

Hawasina Nappes (Oman)

In the Hamrat Dum Group, the Wahra Formationand the Lower Zulla and Upper Sid'r Formations allcontain thick sequences of radiolarian-bearing chertinterbedded with clastic sedimentary rocks. Sam­ples from AI Jil cherts (= lower part of the Zulla for­mation in Glennie et al., 1974) yielded radiolarianfauna denoting late Murghabian and Late Triassic(Carnian-Norian) ages (Bourdillon et al., 1987; DeWever, unpublished data). Additional chert samplesfrom the Matbat Formation (upper part of the Zulla+ lower part of Guwayza in Glennie et al., 1974)indicate an Early Jurassic age (Bourdillon et al.1987; De Wever, unpublished data). Several datesmentioned by Blome et al. (1983) are well in accor­dance with those given here. However, the preciselithostratigraphic position of their samples was notgiven, so it is impossible to ascertain whether theyactually correspond to the formations listed here.

Radiolarites Overlying Ophiolites(Triassic-Liassic)

Darno Series (Hungary)

In the Lower Austro-AIpine unit, chert associatedwith ophiolite pillow basalt has been dated as MiddleTriassic (De Wever, 1984).

Rarau Series (Romania)

In transylvanian nappes (Rarau and Persani Moun­tains, eastern Carpathian Mountains, Romania),radiolarite has been dated as Middle Triassic (Ladin­ian; Dumitrica and Mello, 1982).

Diabase-Chert Unit (Yugoslavia)

In the Diabase-Chert Formation from Serbia (Yu­goslavia), radiolarian chert (near pillow basalts)has been dated as Late Triassic (Carnian-Norian)(Obradovic and Gorican, this volume; Gorican,1986).

Porphyrite-Chert Unit (Yugoslavia)

In the Porphyrite-Chert Unit from Serbia (Yugo­slavia), radiolarian cherts near volcanic rocks havebeen dated as Middle Triassic (Anisian-Carnian)(Obradovic and Gorican, this volume; Gorican,1986).

36 Patrick De Wever

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FIGURE 3.3. Reconstruction of Lagonegro (southern Italy)for Norian time (from De Wever and Miconnet, 1985).(A) Hypothetical rock basement (partly provided detritusto the basin). (B) Basement cover and Scythian platform.(C) Primary dolomite (partly provided detritus to thebasin). (D) Monte Facito Formation (probable thrusting

level). (E-G) Calcaires a Silex Formation (cherty lime­stone). E-San Fele Unit, massive dolomite; F-Pignola­Abriola Unit, bedded dolomite and dolomitic limestone;G-Sasso di Castalda, Armizzone and Lagonegro Units.(H) Radiolarite belonging to Armizzone and Lagonegro(and Sasso di Castalda ?) Units.

Maliac Series (Greece)

Triassic cherts associated with basalts from theMaliac series (internal Hellenides nappes, Greece)have unknown basement rocks (presently sub­ducted under the inner zones). Chert occurs betweenpillow lavas and has been dated as Late Triassic(Tourla series, Norian; De Wever, 1982; Ferriere,1982).

Mid-Upper Jurassic Radiolarites

During this time period, radiolarite is common. Asin the previous examples, it is associated with sedi­mentary and volcanic sequences.

older in basinal deposits (from Bajocian or Batho­nian, Lombardy Basin, Umbria) and younger onplateau deposits (mid-Oxfordian, Trento plateau)(Baumgartner et al., 1980; Kocher, 1981; Baumgart­ner, 1984; Conti et al., 1985; Conti, 1986). Radi­olarites are synchronously capped by Maiolicalimestone (Govi, 1965; Luthi, 1973).

Lagonegro Series (Lucany, Italy)

As noted above (Fig. 3.3), these series have adiachronous base (from Triassic to Oxfordian), buttheir top is synchronous (Before Neocomian time)(De Wever and Miconnet, 1985).

Sciacca Series (Sicily, Italy)

Radiolarites Within Sedimentary Sequences

"Chaines Calcaires" (Rif, Morocco)

The base of this formation is diachronous based onradiolarian dates, from Bajocian or Bathonian toCallovian or Oxfordian within different structuralunits (Fig. 3.4), the oldest being to the south (exter­nal zones) (De Wever et al., 1985). The top of thischert formation is synchronous in all structuralunits, Kimmeridgian or early Tithonian.

Southern Alps (Lombardy, Tuscany,and Umbria, Italy)

Radiolarite sections are well known in northernItaly, where their bases are diachronous. Bases are

The base of this series is unknown, because it is cutby an overthrust fault, but the top has been preciselydated with ammonites, calpionellids, nannofossils,and radiolarians as the exact boundary betweenKimmeridgian and Tithonian (De Wever et al.,1986b).

Pindos-Olonos Series (Greece)

Radiolarite has been studied in several sections fromnorthern and southern Greece (Fig. 3.1). The basesof the sections are dated as Bajocian or Bathonian(Thiebault et al., 1981; De Wever and Thiebault,1981; De Wever and Cordey, 1986), and the top isTithonian (De Wever and Origlia-Devos, 1982a; DeWever and Cordey, 1986) except in southernmost

3. Radiolarites, Circum-Mediterranean Alpine Belts 37

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FIGURE 3.4. Paleogeographic evolution of the Chrafateklippes and the external "Dorsale Calcaire" (Rif,Morocco) zone during Jurassic time (from De Wever etal., 1985). (1) Turbidite; (2) radiolarites; (3) "calcaires a

filaments"; (4) black shale; (5) Ammonitico Rosso; (6)cherty limestone; (7) massive limestone; (8) dolomiticmassive limestone; (9) horizontal grading.

38

Peloponnesus, where a few outer units contain radio­larites as young as the Late Cretaceous. However,during Tithonian time, some thick, coarse clasticintervals are interbedded (Thiebault et aI., 1981; DeWever and Thiebault, 1981).

Budva Series (Yugoslavia)

A series equivalent to the Pindos-Olonos seriesexists in Montenegro (Yugoslavia), where it hasbeen dated as Middle Jurassic to Late Jurassic(Obradovic and Gorican, this volume; Gorican,1987).

Pelagonian Series (Internal Hellenides,Strimbes, Greece)

Radiolarites from Pelagonian series (Othrys, con­tinental Greece) have been dated as Kimmeridgian­Tithonian with radiolarians (det. Origlia-Devos inFerriere, 1982, p 930)

Pelagonian Series (Asklepeion Nappe,Angelokastron Cherts, InternalHellenides, Greece)

Radiolarites have been dated in the Peloponnesus.Their first occurrence is Bathonian (AsklepeionNappe-Baumgartner, 1984) or late Oxfordian (An­gelokastron Chert and Ayios Nikolaos Formation­Baumgartner, 1984; they correspond to the Epidaurezone and Trapezona zone ofVrielynck, 1978, 1980,1982 at this time period).

Pelagonian Series (Evvoia, Greece)

Radiolarite from northern Evvoia has been datedwith radiolarians as Oxfordian or Kimmeridgian,except the latest Kimmeridgian (Baumgartner,1984; Ferriere et aI., 1986). This radiolarite wasdated as MaIm or Neocomian (Baumgartner andBernoulli, 1976), but more recent information per­mits us to restrict the age as mentioned above.

Upper Austro-Alpine Series in Hungary(Bakony and Gerecze Nappes)

In the upper Austro-Alpine nappes of Hungary(Bakony and Gerecze Hills), the base of the radi­olarite section is late Bajocian or Bathonian (inKozoskut, Lokut, Bakonycsernye sections - FUlop,1969, 1976; De Wever, unpublished data) but else­where is Bathonian (Gyenespuszta-Galacz, 1970,1976, 1980; De Wever, unpublished data). Theradiolarite-bearing formation is overlain in someplaces by a coarse clastic section of Oxfordian age

Patrick De Wever

(Lokut and Tata Hills), but in other places radiolaritepersists to the Kimmeridgian (Cszernye Hills) oreven to the Kimmeridgian-Tithonian boundary(Margit Hill-De Wever, unpublished data).

Upper Austro-Alpine SeriesCzechoslovakia (Silica Nappe)

Radiolarite (equivalent to the series of the SlovakKarst) is dated as Callovian or Oxfordian (Dumitricaand Mello, 1982).

Hawasina Nappes (Oman)

In the Hamrat Duru Group, the Wahra Formation,the Lower Zulla and Upper Sid'r Formations all con­tain thick sequences of radiolarian-bearing chertinterbedded with clastic sedimentary rocks (turbi­dites). Radiolarian fauna obtained from the Sid'rchert indicate a Late Jurassic to Neocomian age(Bourdillon et aI., 1987; De Wever, unpublisheddata). Several dates have been mentioned by Blomeet aL (1983) and by Baumgartner (1984,) for the AlAridh Formation. Their dates are in accordancewith those given here. However, they did not pro­vide precise lithostratigraphic positions for theirsamples, so it is impossible to ascertain whether theycorrespond to the stratigraphic levels analyzed here.

Autochtonous Unit (Oman)

Overlying the Sahtan Group (shallow-water Jurassiclimestone) is the Kahmah Group, which yieldedradiolarians in chert at the base of the sectioninterbedded with limestone (with Calpionellids).The chert was dated as late Tithonian or early Berri­asian (Bourdillon et al., 1987; De Wever, unpub­lished data).

Radiolarite Overlying Oceanic Crust

Good outcrops of ophiolite-hosted radiolarite arerare because of metamorphism. Nevertheless, somesignificant data already exist.

Series of the Ligurian Ophiolitic Nappe (Queyrasand Corsica, France; Liguria and Elba, Italy)

The metaradiolarite, which overlies the ophioliteand ophiolitic breccia of the Chabriere series (baseof the "Schistes Lustres"), has been dated in sectionsfrom France as late Oxfordian or early Kimmer­idgian (De Wever and Caby, 1981) and on rocksfrom Italy (Traversiera) as middle-upper Callovian(De Wever et aI., 1987a).

3. Radiolarites, Circum-Mediterranean Alpine Belts

Nonmetamorphosed radiolarite from Elba overly­ing pillow lava (Monte Campannelo-Nisporta, Vol­terraio) or ophicalcite (San Felo Namia) has beendated as middle Oxfordian or early Kimmeridgian(Baumgartner, 1984; Conti and Marcucci, 1986).This age is the same as that determined in othersections (Murlo, Southern Toscany-Conti andMarcucci, 1986; San Colombano and kIn 59,Corsica - De Wever et aI., 1987b). The shalebetween ophicalcite and radiolarite has been datedat Rocchetta di Vara (eastern Ligurides) (Baum­gartner, 1984) as middle Callovian or basal Oxford­ian, and the basal part of the chert sequences is datedas late Oxfordian or early Kimmeridgian (Contiet aI., 1985; Conti and Marcucci, 1986). In thesection at II Conventino (Monti Rignosi, easternTuscany), the radiolarian assemblage gives a lateCallovian or early Kimmeridgian age (Conti andMarcucci, 1986). The onset of deposition of theMonte Alpe Chert gives an upper age limit for basaltextrusion in each section. The age of sedimentationoverlying Ligurian ocean crust was therefore notsynchronous.

Subpelagonian Nappe Series (MigdhalitsaUnit, Argolis, Greece)

Radiolarite from the Migdhalitsa unit (Baumgartner,1984; Fanary Wildflysch ofVrielynck, 1982) overly­ing pillow basalt of the Ophiolite Suite (Akros Hill)is dated near its contact with basalt as middle Callo­vian or early Oxfordian (Baumgartner, 1984)

Subpelagonian Nappe Series (Evvoia, Greece)

Chert samples from interpillow space of the ophiolitein northern Evvoia are dated as Oxfordian (Baum­gartner, 1984; Ferriere et aI., 1986).

Subpelagonian Nappe Series (Thessaly, Greece)

Oxfordian and/or Tithonian age was obtained for ra­diolarite overlying ophiolite at Theopetra (De Weverin Ardaens, 1978; Ferriere, 1982), but the rocksdated were not from the base of the section.

Diabase-Chert Formation, Serbian Units(Internal Dinarides, Yugoslavia)

In the Diabase-Chert Formation from western Ser­bia (Yugoslavia), Upper Jurassic chert (Callovianand/or early Kimmeridgian) containing radiolarianshas been identified (Obradovic and Gorican, thisvolume; Gorican, 1986, p 58).

39

Bukk Ophiolitic Nappe (Bukk Massif, Hungary)

The Bukk may be considered as a Neo-Tethyan sphe­nochasm between Apulia and Europe (Dercourt etaI., 1984a,b, 1985a,b). A Bajocian and/or Batho­nian age was obtained (Kozur, 1984) for the upperpart of the radiolarite occurring in the wildflyschoverlying ophiolites in the Bukk Massif (northeastHungary) (Geyssant and Lepvrier, 1984).

Bucovinian Nappe (Romania)

Pojorim radiolarite (crystalline Mesozoic zone ofsoutheastern Carpathian Mountains) has long beenconsidered to be of Middle Triassic age (Bancila andPapiu, 1953) but is now known to be of Callovianand/or Oxfordian age (Mutihac, 1968; Cioflica etaI., 1981; Dumitrica, unpublished data), as are mostradiolarites of this zone.

Metalliferous Mountains (Southern ApuseniMountains, Romania)

The Dro<;ea-Cris Unit (of the Cris Nappe) containsradiolarite dated as Oxfordian (Dumitrica in Bleahuet aI., 1981) overlying basalt. The Curechiu-Smnijaunit has radiolarite dated as Oxfordian-Tithonian(Dumitrica in Bleahu et aI., 1981) overlying ophio­lite and tuffs.

The Capllnas-Techereu Nappe (well developed inMetalliferous Mountains) has radiolarite dated aslate Callovian to late MaIm (Dumitrica, unpublisheddata) interlayered with volcanic rocks, both of whichoverlie ophiolite with a radiometric age of 180 Ma.This ophiolite volcanism was thus probably activebefore the Jurassic until the Callovian.

Marmaros Unit (Klippen Zone, USSR)

Radiolarite from the Marmaros unit associated withophiolites of the northeast Eastern Carpathians hasbeen dated as Oxfordian and Kimmeridgian (Tik­homirova, 1983, 1984).

Summary

All Triassic and Liassic radiolarite sections associ­ated with sedimentary rocks are parts of allochto­nous sequences. These radiolarites belong to nappeswith unknown basement rocks, probably thinnedcontinental crust (Dercourt et aI., 1985a,b; Ricou etaI., 1985)(Fig. 3.5).

Radiolarite interbedded with sedimentary se­quences exists in basins that have received depositssince Triassic time (Lagonegro, Pindos-Olonos

olarite; (3) limestone; (4) flysch; (5) breccia; (6) emergent land(on any type of crust); (7) oceanic crust; (8) thin continentalcrust (basin); (9) thick continental crust (platform); (10) activespreading ridge; (11) spreading ridge when dying out.

FIGURE 3.5. Location ofradiolarite outcrops that were dated byradiolarian assemblages. Paleogeographic map from Dercourtet al. (1985a) map No.1 (Pliensbachian) is used for this report.(A) Radiolarite associated with sedimentary sequences; (B)radiolarite associated with ophiolites. (1) Volcanism; (2) radi-

Liassic

'90"---<---...

2-><,/ j

:' i\i"

"

_.-;:.//",\~HaWaSinri

~(~H\"

.~ A

• B

= 2

'52' 3

..... 4

'.' 5

U 6

~7

DB0 9

::::::: 10

:7-.:;':';-'; 11

~

~'"1

n':><;"

u(l>

~<:(l>'"1

3. Radiolarites, Circum-Mediterranean Alpine Belts

zone, ... ) or in regions newly invaded by majortransgressive water masses (Le., Austro-Alpinezone, Inner Maghrebides). Detailed local studies(Rif, Morocco; Lagonegro, southern Italy; southernAlps, northern Italy) suggest deposition on faultedblocks of a rifting margin. Although the base of theradiolarite is not synchronous in different units, itnevertheless generally starts around the Dogger. Themaximum development of biosiliceous sedimenta­tion occurred during Oxfordian time. Their disap­pearance occurs synchronously near the Tithonian(Fig. 3.6).

All allochtonous radiolarites associated with vol­canic rocks have been presumed to be the sedimen­tary cover of oceanic crust, either of an open-oceanbasin crust or of a smaller ocean basin.

The base of radiolarite-bearing sequences (exclud­ing mudstone locally known as umbers in Cyprusand Oman) associated with ophiolite is everywheredated as MaIm, but it is not exactly synchronous;data are still too few for a precise analysis.

The above descriptions consider only true radio­larites, as defined in the Introduction. Other radio­larian dates have recently been obtained from chertnodules in circum-Mediterranean Alpine fold beltsbut are not considered in this paper because radio­larians were extracted from rock types other thanradiolarite sensu stricto. Two sets of results are nev­ertheless mentioned because of their geodynamicimplications. They are as follows: radiolarian­bearing rocks associated with the Samail ophiolite,Oman, and the Troodos ophiolite, Cyprus, yielded aradiolarian fauna dated as Cretaceous: Turonian inCyprus (Blome and Irwin, 1985): Campanian inOman (Schaaf and Thomas, 1986; De Wever et al.,1988); and Albian-Cenomanian, Cenomanian­Turonian, and Santonian-Campanian in Oman(Beurrier et al., 1987; Bourdillon et al., 1987).Dated rocks are red mudstone or umbers (not radio­larite).

Work is in progress on the radiolarian stratigra­phy of other regions: (1) to the east in the Sam­khet Karabagh (Lesser Caucasus, Georgia, USSR),where the Jurassic Lesser Caucasus radiolaritesequences have been dated as Callovian (to Bar­remian?) with the Kimmeridgian missing (Vishnev­skaya, 1984), and (2) in the Klippen Belt of thePienninnic sequences of the Csorstyn and Kysucaregions (Birkenmayer, 1977). With these new datait will be possible to date the distension of thismargin.

41

Diachronism/synchronismof Radiolarites

Diachronism characterized the earliest radiolaritesedimentation, whereas synchronism characterizesthe latest radiolarite sedimentation. In early studiesof radiolarites, it appeared that the main radiolaritesequences belonged to the Jurassic, confirming theclassic hypothesis on the age of these deposits. Suc­ceeding work dated the base and the top of radio­larite sections more precisely and revealed that thebases were not always of the same age. Detailedstudies of the Dogger and MaIm radiolarites indifferent regions documented a diachronism for theearliest radiolarite sedimentation and a synchronismfor the top of the sections.

The age variations correspond to different tiltedblocks of a continental margin. In some places, agevariations correspond to a stretching from the oceantoward the craton (Rif, Morocco; Lagonegro, Italy).Such a margin may have corresponded to thedevelopment of a transform fault zone (Lemoine,1985; Dercourt et al., 1984a, b) between the open­ing Atlantic Ocean and the subducting Neo-Tethys tothe south of the Alboran-Kabyly-Calabria block (DeWever et al., 1985). Radiolarite basins in northernMorocco migrated northward during Middle to LateJurassic time, suggesting sedimentation on tiltedfault blocks (Fig. 3.4) with progressive northwardformation of listric faults (Lemoine, 1982, 1985;Lemoine et al., 1981; De Wever et al., 1985).

The diachronism of the first radiolarite deposits inLagonegro (De Wever and Miconnet, 1985) andArgolis (Baumgartner, 1985) represent the samekind of phenomenon-deposition on tilted blocks ofan actively forming margin.

In other places, age variations correspond to thedifference noted between basins and plateaus or sea­mounts such as the Lombardian basin and Trentoplateau, northern Italy (Baumgartner, 1984) andBakony Hills, Hungary (Galacz, 1984; Galacz et al.,1985; De Wever, unpublished data). The first radio­larite deposits are not synchronous, but their maxi­mum extent of development is generally during theOxfordian.

Disappearance of Radiolarites

It is now accepted that most radiolarites were notdeposited in wide-open basins or on continental

FIGURE 3.6. Location of radiolarite outcrops that were dated bytheir radiolarian assemblage as MaIm. Paleogeographic mapfrom Dercourt et al. (l985a) map No.2 (Callovian) is used for

'90,",,0'~""""'"

"'<""';;"

-+:>.N

~""1(S.;;>:;"

o~

~<:~""1

".~

:::::: 10

';-:7:';-: 11

D6~7

D8D94

• B

<e> A

'.' 5

~ 3

this report. (A) Radiolarite associated with sedimentarysequences; (B) radiolarite associated with ophiolites. Legend issame as for Figure 3.5.

? 40_

----!.~

i 't- I

~"""""'2~ (' ('

)~)"",/' /

\ i\~i i

i // i

Maim

'0.

l<l~:

\\~~\

\\

3. Radiolarites, Circum-Mediterranean Alpine Belts

slopes but in elongate basins (gutters) or small basinswith restricted oceanic circulation (Steinberg et aI.,1977; Marcoux and Ricou, 1979; Jenkyns and Win­terer, 1982; Iijima and Utada, 1983). These rela­tively confined basins represent gulfs rich in organicmatter such as in the modern Gulf of California (Cal­vert, 1966; Nelson and Goering, 1978; Schrader etaI., 1980; De Wever and Thiebault, 1981), Red Sea(Goll, 1969), and marginal seas such as the Sea ofJapan (Steinberg et aI., 1977), Sea of Okhotsk, andBering Sea (Jenkyns and Winterer, 1982).

Areas of intense upwelling provide three condi­tions which enhance the deposition and preservationof radiolarian-rich sediments: (1) high nutrient levelsin upwelling regimes promote higher phytoplanktonproductivity, in turn causing higher rates of radiolar­ian productivity and deposition of radiolarian-richdeposits. Increased rates of radiolarian depositiondecrease the rate of dissolution of siliceous skeletonson burial: (2) increased rates of production oforganic matter beneath upwelling zones elevate theCCD, which removes calcareous organisms thatdilute the radiolarian-rich sediment: and (3) highorganic content prevents the dissolution of silica bycoating the tests (Kastner, Scripps Institution ofOceanography, personal communication, 1986).

The general geometry of the Tethys during Juras­sic time (an eastwardly open triangle) promotedupwelling on the western side and led to the develop­ment of a clockwise gyre (De Wever and Thiebault,1981; Thiebault et aI., 1986). This triangular basinwas present from the creation of the neo-Tethys inTriassic time. During the Jurassic, the basin openedand enlarged or spread to the west. This basin evolu­tion fits well with the progressive onset of radiolaritesedimentation from east to west from Triassic toJurassic time. In addition to the cause cited byJenkyns and Winterer (1982), the absence of radio­larites in the Atlantic can be explained by the lack ofupwelling in this basin and the large distance of thisbasin from areas of upwelling.

In Tithonian times, radiolarian sedimentationsuddenly stopped and was replaced by carbonatesedimentation (e.g., Oberalm in Northern Alps,Maiolica in Southern Alps (Lombardian zone),Biancona in Venetian Alps, Calpionellids Limestonein Elba, Calcare Rupestre in Appennines (Umbria,Marches), Vigla limestone in Greece (Ionian zone),"Calcaires aCalpionelles" in Greece (Pindos-Olonoszone), Lattimusa in Sicily).

Although this change was widespread, its causeneed not have been a major one, because only slight

43

biophysical changes in the seas can produce strongmodifications in the planktonic life (see EI Ninoeffects; 5- to 20-fold reduction of primary produc­tivity-McGowan, 1984; Barber and Chavez, 1983;Pisias et al., 1986), and because slight changes inproduction of silica have an exaggerated result in sil­ica sedimentation (Fig. 3.7) (Renz, 1976; De Wever,1982).

This abrupt change in sedimentation may haveoccurred worldwide, but outside Tethys (Japan, Cal­ifornia, Mexico), radiolarite sedimentation gradu­ally gave way to terrigenous clastic sedimentation.

Modifications in oceanic circulation could haveresulted from tectonism associated with the InnerHellenide zones (De Wever and Thiebault, 1981),but this mountain range did not totally close theTethyan triangular basin. Moreover, such a closurewould have increased the quantity of anoxic sedi­ment, which is not found; rather, anoxic depositsaccumulated in isolated basins (Valaisan, Caucasus)since the MaIm. In any case, there were shallowseas in this region from Triassic time. This tec­tonism was probably not extensive enough to mod­ify the oceanographic conditions as far away asMorocco.

Several characteristics we know about the Tethyshelp to explain the disappearance of radiolarite sed­imentation.

The Tethys opened in a westward direction since Tri­assic time (Aubouin and Tardy, 1980).

East-west exchanges of waters occurred betweenNorth and South America from Callovian time, asis attested to by the migration of ammonites (Rein­eckeidae from Pacific to Tethys-Cariou, 1984;Cariou et aI., 1985; or Parasenia from Tethysto Pacific-Enay and Mangold, 1982). Theseexchanges involved surface water masses, neverdeep water masses.

In the Latest Jurassic, a new current formed in theAtlantic, as is evidenced by the arrival of Pygopein Greenland (Enay, 1980).

Oceanic conditions existed from Pamir to Mexico(Dercourt et aI., 1984a, 1985a).

The exchange of deep water masses began duringTithonian or Berriasian time somewhere betweenNorth and South America (Berggren and Hol­lister, 1974; Thiede, 1979; Kennett, 1982). Anoceanic communication has been illustrated byPindell (1985) between Tethys and Pacific(between the Yucatan block and Mexico) whichsignificantly affected circulation.

44 Patrick De Wever

....~

r<l

E"­.".:

QlUc::o."c::::J

.Qex

1000000

800000

600000

400000

200000

19 21 23 23 10

66

33 3327

Number of species

8.11.30

FIGURE 3.7. Comparison between biocenosis and than­tocenosis. (A) Radiolarian fauna caught on both sides ofthe equator in the Pacific Ocean. (B) Radiolarian fauna

The sudden disappearance of all the radiolarites inthe Latest Jurassic could be explained by a drasticchange in circulation due to a latitudinal flowthrough Central America. Such a new communica­tion drastically changed the current pattern from agyre in the Tethyan triangular basin (and its asso­ciated upwelling) to a latitudinal current and cantherefore explain the disappearance of radiolaritesedimentation (De Wever et al., 1986a). This latitu­dinal current could have, at least locally, flowed fromeast to west, as suggested by Thiebault et al. (1986).

This hypothesis accords well with the long dura­tion of radiolarite sedimentation of the east Arabianmargin of the Tethys Sea (Oman). In Oman, radio­larite sedimentation continued until the Late Creta­ceous, because this area remained under the influ­ence of the upwelling conditions (De Wever andBourdillon, 1987; De Wever et al., 1988; De Wever,unpublished data). When the current patternchanged to latitudinal circulation rather than a gyre,there was a sudden deepening of the basin recordedin the sedimentary rocks of the Autochtonous Unitin Oman. Sedimentation became progressively shal­lower again during Neocomian time. This initial

dredged on the sea floor in the same localities. The shift ofthe maximum abundance is a result of equatorial gyres.

deepening could have resulted from a "flush" effectat the beginning of the latitudinal current. In Japan,being farther east, where radiolarites were notdiluted by clastic sedimentation, this modificationin currents had no effect, and radiolarite sedimenta­tion persisted until Neocomian time.

The new circulation pattern affected most of thezones: the oldest rifted platforms (Austro-Alpine,southern Alps) as well as the troughs (Lagonegro,Pindos-Olonos zone, Budva zone). We have there­fore to interpret the paleogeography and/or paleo­bathymetry ofthe areas where radiolarite depositionpersisted after the installation of latitudinal currents(i.e., southernmost Pindos zone, Greece) as con­fined basins. The regions where radiolarites existedthrough the Cretaceous were regions sheltered fromthe new latitudinal current (Oman, Japan).

Acknowledgments. This work has been supported bythe CNRS (UA 319, ATP GGO No. 98 1039) andUNESCO lUGS (project IGCP 187). I am indebtedto A. Kemp (University of Southampton, U.K.) and1. Tauxe for their help with the English and to R.Garrison (University of California at Santa Cruz), 1.

3. Radiolarites, Circum-Mediterranean Alpine Belts

Dercourt (University of Paris), J. Obradovic (Uni­versity of Belgrade and J.R. Hein (USGS, MenloPark, CA) for reading the manuscript.

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