Geol Rundsch (1996) 85 :310–326 Q Springer-Verlag 1996

ORIGINAL PAPER

P. De Wever 7 F. Baudin

Palaeogeography of radiolarite and organic-rich deposits

in Mesozoic Tethys

Received: 13 February 1995 / Accepted: 9 October 1995

P. De Wever1 7 F. Baudin (Y)CNRS-URA 1761, Univ. PM Curie, Dept. GéologieSédimentaire, T15-16 E4, 4 Place Jussieu,F-75252 Paris Cedex 05, FranceFax: c33-1-44 27 38 31

Present address:1 Laboratorie de GéologieMuseum National d’Histoire Naturelle43 rue Buffon, F-75005 Paris, FranceFax: c33-1-40 79 37 39

Abstract Siliceous and marine organic-rich depositsare sometimes associated, sometimes separate in spaceand time; however, both are generally accepted to bethe result of high planktonic productivity. Among thesiliceous marine deposits, the phtanite family facies isdistinguished from the radiolarite family facies by sev-eral characteristics: They contain organic material andas a result are blackish (vs red/green for radiolarite fa-cies), their time of deposition corresponds with strongfaunal modifications and they are deposited generallyin shallower environments. A palaeogeographic analy-sis of locations of Tethyan biosiliceous and marine or-ganic-rich rocks, both resulting from a high planktonicpalaeoproductivity, for three Mesozoic high sea-levelintervals, Toarcian, Kimmeridgian and Cenomanian,show: (a) during Jurassic times these Tethyan depositswere dissociated, the siliceous deposits being closer toopen ocean waters than the organic-rich ones. This is acommon disposition in modern upwelling systems andsuggests a common process; (b) during Cretaceoustimes these Tethyan deposits were often associated, i.e.both occur at the same site, and are probably the resultof a different process from that in the Jurassic.

Key words Palaeogeography 7 Radiolarite 7 Organicmatter 7 Mesozoic 7 Tethys

Introduction

Distal basinal or oceanic environments are often opti-mal sites for the preservation of siliceous and lipid-richorganic material derived from plankton. Radiolariansare associated with rich marine source rocks through-out the Phanerozoic (Ormiston 1993) and are commonconstituents of source rocks since at least Silurian time.This is connected with the elevated planktonic produc-tivity of such environments in the geological record.Other silica-rich deposits, however, are not associatedwith organic-rich deposits.

A recent multidisciplinary work (Dercourt et al.1993) provides a set of palaeoenvironmental maps ofthe Tethyan realm, from Indonesia and Australia in theeast to the Caribbean in the west. These maps providean opportunity to integrate information on radiolaritesand organic-carbon-rich facies in order to obtain amore coherent picture of the distribution of these facieswith respect to tectonic, climatic and circulationchanges.

The purpose of the present paper is to discuss thedepositional controls of radiolarites and marine or-ganic-carbon-rich facies and to describe briefly the pal-aeogeography and palaeoenvironments of the Tethyanrealm for three Mesozoic high sea-level intervals: Toar-cian, Kimmeridgian and Cenomanian. The selectedtime intervals correspond to three different steps in theevolution of the Tethyan realm. The Toarcian marksthe paroxysm of the opening of the Neotethys, theKimmeridgian is an early stage of opening of the NorthAtlantic and the Cenomanian illustrates the beginningof closure in the Neotethys and opening of communica-tion between the North and South Atlantic.

Radiolarian accumulation in sediments

Radiolarians have existed since the Cambrian (Nazarovand Ormiston 1993) and several thousand species haveevolved since then. They are useful tools in correlating

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Fig. 1a, b Relationships between planktonic silica and silica con-tents in sea-floor sediments. a Distribution of dissolved silica inthe uppermost 100 m of the sea (northern winter) in mg at Si l–1.b Amorphous silica (chiefly diatom, frustule and radiolarianskeletons) in surface sediments in percent of dry sediment (incarbonate-free sediment). (Adapted from Lisitzin 1985)

basinal deposits, including source rocks. Radiolariansare exclusively marine protozoans with a siliceous skel-eton and a cytoplasm divided into two parts by a mem-brane. They are present in all the oceans and open seasfrom pole to equator. The main biochemical compo-nents of radiolarians are carbohydrates, proteins andlipids, the last being an important component of or-ganic molecules deposited in the sediment along withmaterial from other organisms. Certain colonial radio-larians are particularly rich in endoplasmic oil droplets(Anderson 1983). The organic carbon content of thesecolonial forms is, naturally, high. When this materialaccumulates in substantial amounts in oceanic sedimentit may be a significant contributor to the organic frac-tion of those sediments.

Ecology, abundance and accumulation in sediments

Most living radiolarians are symbiont-bearing (usuallyzooxanthell). They are most abundant within the photiczone (50–200 m depth interval), but some species arepanbathyal. Radiolarian abundances in the differentoceans and water masses are influenced by climate, lati-tude and hydrological conditions (temperature, salinity,etc.), but are related primarily to the abundance of nu-trients. It is the amount of available nutrient, not thedissolved silica content, that is the primary factor in-fluencing the abundance of radiolarians (Figs. 1 and 2;

Fig. 2 Oceanic distribution of organic carbon. Top: Primary pro-ductivity in the world ocean (mg C/m2!day); bottom: Organiccarbon in superficial sediments (%TOC). (After Pelet 1985)

Lisitzin 1985; Anderson 1983). The long-held assump-tion of a relationship between the abundance of radio-larians and volcanic processes is largely erroneous. It iswell known today that radiolarian abundance in sedi-ments increases particularly beneath high productivityareas such as water-mass fronts (equatorial zones, polarbelts) and more generally under all areas of active up-welling.

In Miocene-Recent sediments the radiolarian abun-dance at a site can be related either to high or low sea-level stands (e.g. South Atlantic coast off Africa, Wal-vis Ridge; Diester-Haass et al. 1992; Hay and Brock1992). The height of the sea level itself is apparentlyrelatively unimportant. Variations in the height of sealevel are, however, important. These induce modifica-tions in currents and other factors (silica content, nu-trients, etc.) are influenced by the operating conditionsof upwelling. The intensity of upwelling is of prime im-portance in influencing the abundance of radiolariansat a site.

A latitudinal distribution does exist for modern ra-diolarian assemblages. It is also possible to differentiatesurface from subsurface assemblages. In sediments be-low upwelling sites a systematic mixture of cold andwarm waters species occurs: as in North Africa Ceno-manian deposits where tropical species are mixed with

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temperate forms (Kuhnt et al. 1990), or as off Peruwhere Recent cold water forms are mixed with tropicalforms (De Wever et al. 1994). Surface and subsurfacemodern species are also mixed off Somalia (Caulet etal. 1992). Hence, the possibility of identifying tropical(pTethyan) vs boreal assemblages of radiolarians inradiolarite is almost null, because radiolarite faciesresult from upwelling with faunal mixing.

Preservation in sediments

Four major processes control the evolution of a sili-ceous biogenic ooze from its initial stage to its presentstate: supply of biogenic material, dissolution of thismaterial during settling, dilution within the sediment byother components (biogenic or other), and diagenetictransformation of the initial product.

Not all individuals and species of plankton are rep-resented in the sediment, and fewer still are repre-sented in the rock. The ratio of individuals in sedimentover those in plankton decreases rapidly on both sidesof the equator. The loss is 25% at the equator, whereplankton is abundant, but it represents 98% near 257Nlatitude where the plankton is slightly less abundant(Renz 1976).

After death, individual tests are at least partially dis-solved, during settling through the water column, whilethey lie on the bottom and finally within the sediment.Chemical and physical characteristics of tests vary ac-cording to taxa (King 1977), and so does the dissolutionthat affects them. A selective dissolution effect can berecognized for species (Riedel 1958; Swanberg andBjørklund 1992). Species with a “positive preservationpotential” are normally found in sediments in quantitythat generally average background variations such asseasonal variations. Robust forms and bloom-gener-ated specimens are over-represented in sediments whencompared with the common plankton (Swanberg andBjørklund 1992).

The rate of silica dissolution is highest in surface wa-ters, specifically the upper 500–1000 m, and does not in-crease with depth (Leinen 1979; Hurd and Takahashi1981), in contrast to the rate of dissolution of carbon-ates. Abundant radiolarians and foraminifers oftenshare the same geographical water domain, but the un-derlying sediments frequently do not reflect this: Sili-ceous fossils may be preserved when calcareous fossilsare not, and vice versa.

Caulet (1977, 1978) was the first author to show thatoccurrence or disappearance of biogenic silica in sedi-ments was more a result of bioproductivity variationthan of dissolution. Skeletons are better preserved un-der high productivity zones and differences in abun-dance are amplified by the sedimentary record (Renz1976). At the water–sediment interface silica dissolu-tion is nearly three times greater than in sediments.Less than 1% of the silica fixed by planktonic organ-isms is preserved within the geological record. The

chance that an isolated test will be deposited at the bot-tom is small, but it does not change greatly with waterdepth.

In sedimentary rocks radiolarians are usually abun-dant or absent. This results from a double enhancement(a) during sedimentation and (b) during diagenesis. Se-diments that are almost undifferentiated when deposi-ted become more and more differentiated during diag-enesis. Silica moves to layers already silica-rich. Origi-nal differences are considerably magnified. Each mod-ification of the initial signal (during sedimentation anddiagenesis) accentuates the original fluctuation.

A scenario where transgressions are associated witha significant input of organic matter and a radiolarianbloom has been proposed by Steinberg (1981) for themain epochs of silica deposition. In sedimentary rocks,however, a high concentration of silica is now consid-ered to be an indicator of very fertile surface watersand is almost always connected with the most active ar-eas of upwelling (Thiede and Junger 1992; Garrison1992).

Sites of siliceous deposits

Among the necessary conditions for deposition of bio-siliceous rocks the most important is high productivityof siliceous organisms (Maliva et al. 1990). The input ofother components, clays, detrital, carbonates, organicmatter, etc. may vary or not.

Fossil radiolarians occur in facies representing anextremely wide range of depositional depth. Indeed,these organisms, as with other pelagic fauna, may occurin shallow water deposits. It is unjustified to automati-cally assume a deep water origin for all deposits bear-ing radiolarians. Siliceous rocks are usually red-greenor blackish colored. Generally, dark cherts occur in Pal-aeozoic strata and are known as “phtanite”, whereaslight ones occur in Mesozoic strata and are known as“radiolarite”. Isolated chert beds and/or nodules of dif-ferent colours occur in many geological sequences.They are siliceous “accidents” and therefore are notconsidered herein.

Depositional sites of radiolarite type rocks

“Light” cherts are red and/or green. An alternation ofchert-shale layers is known as radiolarite facies and oc-curs in most of the Mesozoic periods. There are a num-ber of theories on the site of deposition of radiolaritesin Tethyan regions. For a long time it was assumed thatsiliceous sediments accumulated in large deep basins(`3000 m) which were enriched in silica by volcanismalong spreading ridges. We now know that depositionsites of siliceous sediments are not necessarily deep (i.e.off Peru; De Wever et al. 1990a).

Mesozoic radiolarite basins were elongate, narrow,relatively small and locally restricted, with depositionoccurring at specific periods. Some present localities

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Fig. 3 Comparison betweenthe modern Owen and Somalybasins, and the Pindos-Olonos,Pichakun (Pic), and Hawasina(Hw) basins during Jurassictime, at the same scale. Notethe common points: size andlatitude. Both are or werezones of intense upwelling.(From De Wever et al. 1994)

that are geographically comparable with ancient sites ofradiolarite deposition are: Gulf of Baja California (DeWever et al. 1994), Owen basin (NW Indian Ocean, offArabia; Fig. 3; De Wever et al. 1994) and Somalia ba-sin.

All these basins, present and fossil, are partiallyopen to the ocean from which they are separated by aplatform: the Owen ridge for the modern, and the Pela-gonian ridge for the Mesozoic. Comparing these basinsis of interest for several reasons. They are: (a) subjectto bottom water currents, (b) located at similar latitude(Fig. 3) and (c) are located on the northwestern edge ofan ocean. Their position on the eastern side of a conti-nent is unusual for upwelling; nevertheless, monsoonactivity allows these regions to experience seasonal up-welling. Explanation of the Tethyan upwelling as beingdue to monsoons (De Wever et al. 1994) integrates sev-eral propositions previously used to explain the pres-ence or absence of radiolarites: (a) connection to sealevel (Steinberg 1981), (b) water currents (Jenkyns andWinterer 1982), (c) hot vs cold climates (Hallam 1984,1986) and (d) climatic cycles of Milankovitch type (DeWever 1987; Hori et al. 1993).

Monsoons are a result of the relative position in lati-tude of seas vs lands. During Jurassic times the landsand seas involved were far larger thanat present. Theintensity of the moonson was therefore probably veryhigh.

Depositional sites of phtanite-type rocks

According to their location, setting and age, dark sili-ceous facies are named Phtanites (in western countries)or Domanik-type cherts (in the former Soviet Union).

Dark siliceous facies are known from the Cenomanianin Central Tethys: they are named “phtanites” in Mor-occo, “Livello Bonarelli” and “Livello Selli” in theApennines from the Early Aptian (base of the Marne afucoidi; Coccioni et al. 1987). These facies are all some-what peculiar (one is especially rich in organic matter,another is clay-rich and the third is powdery). Theironly common point with true radiolarite is their highsilica content.

Several dark siliceous facies indicate shallow waterdeposition:1. Visean phtanites in the Ardenne massif as indicated

by ostracods from the over- and underlying limes-tone levels (Crasquin 1983)

2. Palaeozoic silica deposits known as Domanik inRussia, as indicated by their chemical composition,lithofacies and faunal content (Vishnevskaya 1993a,b). These basins were separated partly from theopen Iapetus Ocean by an island arc and a barrierreef (Ormiston 1993; Vishnevskaya 1993a).Devonian Domanik-type rocks are major oil-source

rocks of the Russian plate (i.e. Pripjat, Timan-Pechora,Volga-Urals regions). In the United States the Chatta-nooga formation is of similar age and character to theDomanik shale of the Volga-Urals province. The pri-mary source rock of hydrocarbons is Domanik organicshale, bituminous limestones and phtanite.

Colour and the depth of deposition aside, there areseveral other differences between radiolarite-type fa-cies and other dark siliceous facies. The deposition ofblack cherts s.l. (phtanites, Bonarelli, etc.) often marksa drastic faunal change, a characteristic pointed out byseveral authors for several periods and regions: theLower Cretaceous of Italy and Spain (Coccioni et al.

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1987; O’Dogherty 1994; De Wever and Lambert 1995),and the Toarcian of Japan (Hori 1990; Matsuoka et al.1994). Such faunal changes have not been mentioned asoccurring during periods of radiolarite deposition.

Organic matter accumulation in marine environments

The accumulation of marine organic matter in sedi-ments is influenced by both biological and physico-chemical factors. Biological factors include primaryproductivity of the surface waters and biochemical de-gradation of organic matter after the death of primaryproducers. Physical factors include the mode of settlingof organic matter, the sedimentation rate and the sizesof particles. All these factors interact to determine thequantitative and qualitative preservation of organicmatter in a given sediment.

Biological and biochemical factors

The primary production of organic matter in the oceanis from unicellular planktonic organisms whose activi-ties are governed by nutrient supply and solar illumina-tion. Nutrients are present generally in very low con-centrations in the euphotic zone, because they are in-tensively consumed by living organisms. Nutrients arereturned into solution by bacterial degradation duringthe settling of organic debris. Sustained planktonic pro-ductivity takes place in regions where nutrients are con-tinuously replenished. Such regions exist along coastswhere rivers can supply large amounts of nutrients byrun-off, and at sites where upwelling is the source ofreplenishment.

Another major source of organic carbon in the ma-rine environment is terrestrial input. Land-plant pro-ductivity is dependent largely on the amount of rainfallon emerged land. Modern tropical rain forests sustainextremely high organic productivity, but relatively lowaccumulation of organic matter occurs because of thehigh rate of decomposition. Because of the high level ofdegradation prior to transport, only a small proportion(0.5%) of the terrestrial organic matter escapes thecontinental cycle to make its way into the marine envi-ronment (Huc 1980). Significant amounts of terrestrialorganic matter may, however, accumulate in coastal ordeltaic environments. This is clearly shown in Fig. 2where coastal regions are areas of preferential organicmatter accumulation.

In this paper we focus on marine organic matteronly. Because there is no map showing phytoplankton-derived organic carbon concentrations in the surfacesediment, we are unable to determine precisely thezones of accumulation of such organic matter. Nev-ertheless, the comparison of the maps in Figs. 1 and 2indicates that zones of high phytoplankton productivitycoincide generally with organic-carbon-rich sediments.In detail, however, some areas of high phytoplankton

productivity, such as the Grand Banks off Newfound-land or the northeastern Brazilian shelf, do not corre-spond to organic-enriched sediments. Factors otherthan surface productivity interact clearly to determinesuch a situation.

Physical factors

Dead zoo- and phytoplankton, fecal pellets and animalcarcasses originate in the euphotic zone and contin-uously sink to the bottom. Part of this material is oxid-ized during settling, part is used as food by other zoo-plankton and benthic organisms, part is further de-graded in the sediment by bacterial activity and the re-mainder is buried. The relative importance of theseprocesses varies greatly from place to place dependingon the amount of production, the water depth, the rateof sedimentation and the availability of oxidants.

The particles settle at rates from 0.1 to 5 m per day,according to their size and buoyancy (Spencer et al.1976), which is comparable to what is known for radio-larians. The smallest particles are the slowest to reachthe sea floor. Fecal pellets are much faster. If organicparticles fall through a well-oxygenated water column,their chance of reaching the bottom depends mainly onthe water depth. Suess (1980) estimates that the settlingflux of organic carbon decreases by a factor of ten forevery tenfold increase in water depth. The flux is ap-proximately 10% of the production at 400 m depth, butonly 1% at 4000 m depth.

In addition to being affected by scavengers duringsettling and bioturbation near the sediment–water in-terface, organic matter accumulation is also controlledby the size of the mineral particles and the sedimenta-tion rate. Fine-grained sediments, where diffusion ofoxygen is limited, have lower bacterial activity thancoarse-grained sediments. As a result of these effects,preservation of organic matter varies greatly and directinterpretation of productivity based on the organicrichness of sediments must be undertaken with care.

Association of siliceous rocks and marine

organic-carbon-rich sediments

Radiolarians are one of the primary producers of or-ganic matter in marine environments, and were proba-bly a much more important producer during Palaeozoicand early Mesozoic times because foraminifera, coccol-iths and diatoms had not yet emerged (Ormiston 1993).Radiolarians have been common constituents of petro-leum source rocks since Silurian time. This is clearlyconnected with the elevated plankton productivity.

The most productive areas (300 mg Corgm–2a–1) areupwelling sites such as certain margins of continents oratop monsoonal upwelling (Takahashi 1986; Diester-Haass et al. 1992; Caulet et al. 1992; Sarnthein et al.1992). Production of biogenic silica and marine organic

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matter both result from a high plankton activity. Theydo not, however, always remain associated in sedi-ment.

In the geological record organic matter and radiolar-ian-rich deposits are associated in a number of cases:

1. The Miocene diatomites of the Monterey forma-tion, California (Garrison et al. 1981; Isaacs etal.1983), and of the Pisco formation, Peru (Ochoa1980)

2. The Miocene to Recent sediments off Peru (Suessand Thiede 1983; Suess et al. 1986)

3. The Cretaceous and Palaeogene siliceous depositsin Costa Rica (Hein et al. 1983; Von Rad andRösch 1974)

4. The Cenomanian-Turonian Bonarelli horizon inItaly (Marcucci et al. 1991)

5. The Cenomanian “phtanites” in Morocco, Spainand Italy (Thurow and Kuhnt 1986; Kuhnt et al.1990)

6. The Aptian-Albian beds of the “Marne a fucoidi”in Italy (Coccioni et al.1987)

7. Several horizons of the Jurassic “Schistes à Posi-donies”, Ionian zone in the Hellenides (Baudin etal. 1988; Baudin and Lachkar 1990; Danelian andBaudin 1990)

8. The extended Palaeozoic and Mesozoic silica de-posits known as Domanik- or Domanikoid-typecherts in Russia (Ormiston 1993)

9. The Visean phtanites in Ardennes massif, Belgiumand France (Demanet 1938)

10. The Dinantian “lydiennes” and Devonian blackcherts in Montagne Noire, France and Spain(Boyer et al. 1974).

As shown by Lisitzin (1971) those areas in modernoceans which have a high organic productivity are in-variably rich in diatoms, radiolarians or both, and theirskeletons are deposited in the bottom sediment. Of theworld’s known oil reserves, 60% are concentrated inthe Arabian basin where the rich source horizons in theJurassic (i.e. Hanifa formation) and Cretaceous are allof marine origin. Many of these source rocks are well-laminated sediments from environments in which theoxygen content was sufficiently low to prohibit the de-velopment of a benthic infauna which could bioturbatethe sediment. The pre-Cretaceous source rocks are alsolaminated and, lacking diatoms, radiolarians can be thedominant biosilicate fraction. In source rocks, such asthe Kimmeridgian shales of the North Sea, the light-coloured fraction of the laminae consists of predomi-nantly calcareous nannofossils, but radiolarians are alsopresent. Thus, in many post-Palaeozoic source rocks ra-diolarians are present, often in abundance, but may besecondary to other planktonic groups.

The abundance of radiolarians relative to otherplanktonic or supposed planktonic organisms is propor-tionately much greater in older rocks (Palaeozoic) thanin most younger source rocks (Ormiston 1993). Thenumber of Mesozoic source rocks known to include sig-nificant abundances of radiolarians is considerable. Ex-

amples are the Bazhenov suite of the Siberian platform,Turonian of Morocco, Glenn shale of Alaska and manyothers (Ormiston 1993).

Kerogen content of some source rocks is structure-less, amorphous material not attributable to any specif-ic organism. It is not uncommon, however, to note ref-erences to such structureless amorphous material as be-ing of “algal” origin, despite a lack of rigourous proof.It has been suggested by Ormiston (1993) that radiolar-ians, which are a common biotic element in Palaeozoicsource rocks, could have contributed significantly totheir organic richness because radiolarians are knownto contain significant concentrations of lipids. Radiolar-ians are predators and feed on a wide variety of organ-isms (Swanberg and Anderson 1985), which allowsthem to contain a wide variety of biochemical mole-cules. We also know that most radiolarians lead a sym-biotic life with zooxanthells. It has been demonstrated(Anderson 1983) that there exists a direct pathwayfrom the symbiotic algal cells to the lipid droplet in-cluded in their central capsule. Therefore, these drop-lets might have molecules with an “algal signature”analogous to the production of a distinctive sterol bysymbionts in sponges (Imhoff and Truper 1976). Thesteady contribution of radiolarian lipids to ocean sedi-ments could have included the minute fecal pellets(minipellets 10–30 mm in size) which these organismsproduce (Gowing and Silver 1985).

Certain living radiolarians are known to containhigh levels of lipid material (Ormiston 1993) and,therefore, in addition to being associated with an envi-ronment favourable to the accumulation and preserva-tion of organic-rich sediments, radiolarians may havebeen significant contributors to the ultimately pre-served petroleum precursors. Thus, radiolarians havehad both an environmental association with, and mayhave contributed significant organic material to, the ac-cumulation of organic-rich rocks which represent po-tential source rocks for hydrocarbons throughout mostof the Phanerozoic.

Interpretation of a strong relationship between up-welling systems and Palaeozoic source rocks is under-lain by a very strong belief in the bioproductivity mod-el, implying that source rocks are fundamentally gener-ated under conditions of extremely high bioproductivi-ty when other factors are relatively unimportant. Forexample, Parrish et al. (1983) used the term “biopro-ductite” for organic- and radiolarian-rich rocks, imply-ing a belief in upwelling-induced bioproductivity as themain factor controlling their occurrence.

Palaeogeography of Mesozoic Tethyan radiolarites and

marine organic-carbon-rich facies

Toarcian

The Toarcian (Fig. 4; Table 1) was marked by an activephase in the breakup of Pangea. The general palaeo-

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Fig. 4 Reconstructed Toarcian palaeogeography (after Bassoul-let et al. 1993) and distribution of radiolarites and marine organic-carbon-rich facies of the Tethys

geographic picture shows the continental masses pene-trated by a V-shaped oceanic wedge: the Neotethys.This ocean was relatively narrow in its western part(500 km in the Mediterranean Tethys) and becamebroader and more well developed eastwards (5000–6000 km along the Australian meridian). Toarciandeep-sea deposits are unknown because all the oceaniccrust of the Neotethys and its sedimentary cover haseither been subducted or strongly metamorphosed inorogenic belts.

On the southern border of this ocean the Gondwanashield, composed of the Australian and Indian blockslinked together, was separated from the Arabo-Afri-can-South American megablock by a southern embay-ment between Ethiopia-Somalia and Madagascar. Anarrow proto-Atlantic seaway was forming betweenwestern Africa and North America. This western cul-de-sac of the Neotethys, corresponding to Iberia andnorthwestern Africa, was a narrow and complex zoneof intense deepening. There is no evidence of marinecommunication between the western Neotethys and thePacific Ocean during the Toarcian times (Bassoullet etal. 1993). The northern boundary of the Neotethys wasmarked by the subduction of the oceanic floor underthe Eurasian plate and the continental Cimmerianblock composed of central Afghanistan, southern Pam-irs (Pakistan), southern Tibet and western Thailand.

This northern margin of the Neotethys was marked byintense arc-type volcanism.

The southern North Sea and northwestern Europeformed a wide epicontinental terrigenous platformwhere Toarcian deposits were well developed. Theseinclude the Jet rock in Great Britain, the Posidoniashales in the southern North Sea, the Schistes cartonsin Paris basin and the Posidonienschiefer in Germanyand Switzerland (Table 1).

In the Mediterranean Tethys the Toarcian was also aperiod of intense tectonic activity and strong subsi-dence. This was especially true in the Alpine and Apu-lian domains where numerous gravity deposits testify tointense block faulting. The overall situation created acomplex palaeogeography with small carbonate plat-forms surrounded by basins. Marine organic-carbon-rich facies occurred in the newly created basins espe-cially in northern and central Italy, western Greece,Hungary and northern Tunisia (Table 1).

During the Early Jurassic the Mediterranean Seuil(as defined by Dercourt et al. 1993; Vrielynck et al.1994) was a domain with a complex topography of plat-forms and basins, connected by slopes where calcareousdeposits, nodular limestones (ammonitico-rosso, parti-cularly during the Toarcian) and pelagic limestones ac-cumulated in association with clastic deposits. In thissetting of a relatively shallow bathymetry, radiolariteswere deposited in some rare basins. In the Mediterra-nean Seuil the complex topography promoted the onsetof water stratification, sometimes reinforced by the in-flux of saline water from surrounding shallow-waterevaporitic platforms (Farrimond et al. 1990). These fac-

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Table 1

Code Basin or province (country) Name of formation, member orgroup

Selected references

Ar Ardèche (France) Schistes cartons Dromart et al. (1989)Bel Beluno (Italy) Igne Bitterli (1963), Farrimond et al.

(1988), Baudin et al. (1990)Bk Bakony (Hungary) Polgari et al. (1989), Jenkyns (1991)BuT Budva Trench (Montenegro) Lastva Obradovic and Gorican (1989),

Gorican (1993)GeB Westphalia, Saxe, Franconia

(Germany)Posidonienschiefer Küspert (1983), Mann et al. (1986),

Rüllkotter et al. (1987)Hw Hawasina (Oman) Matbat, Sayfam, Aqil De Wever et al. (1990)I Ionian (Greece) Schistes à Posidonies Inférieurs Baudin et al. (1988), Jenkyns (1988),

Baudin and Lachkar (1990)Jur Jura (France and Switzerland) Schistes cartons, Posidonia shales Broquet and Thomas (1979), Met-

traux et al. (1986), Gorin and Feist(1990)

KoB Koçali basin (Turkey) Fourcade et al. (1991)L Lombardia (Italy) Sogno Jenkyns (1988), Farrimond et al.

(1988), Baudin et al. (1990)LCB Lesser Caucasus Basin (Russia,

Georgia, Armenia, Azerbaidjan)Lordkipanidze et al. (1984)

LnT Lagonegro (Italy) Lagonegro, Armizzone Units De Wever and Miconnet (1985)MB Mamonia basin (Cyprus) “Formation sédimentaire de

Mamonia”Lapierre (1972)

MC Causses (France) Schistes cartons Trümpy (1983)NL Rijswijk (Netherland) Posidonia shales Bodenhausen and Ott (1981)NSB North Sea Posidonia shales Barnard and Cooper (1981, 1983)ParB Paris Basin, Lorraine (France) Schistes cartons Huc (1976, 1977), Espitalié and

Madec (1981), Espitalié et al.(1987)

Str Strandja (Bulgaria) Dolno-Lukovo Tikhomirova et al. (1988)T North-South Axis (Tunisia) Soussi et al. (1988, 1989)TrK Trejklano (Bulgaria) Dobridol Tikhomirova et al. (1988)UMB Umbria-Marches (Italy) Marne de Monte Serrone Jenkyns (1988), Farrimond et al.

(1988), Baudin et al. (1990)Yor Yorkshire (Great-Britain) Jet-Rock Morris (1979), Myers and Wignall

(1987)

tors, in conjunction with an adequate phytoplanktonproductivity, contributed to the development of oxy-gen-depleted deep waters and hence to the preserva-tion and accumulation of marine organic matter (Bau-din et al. 1990).

Kimmeridgian

Compared to the map of Toarcian, the reconstructionfor the Kimmeridgian (Fig. 5; Table 2) is characterizedby a new kinematic regime which resulted from theopening of the North Atlantic Ocean (Cecca et al.1993). On the southern border of the Neotethys the In-dian and Australian blocks were still linked together.Rifting on the northern margin of Australia, however,caused the formation of small and isolated basins suita-ble for local deposition and accumulation of marine or-ganic matter. They were especially well developed onthe northwestern shelf of Australia and southern Ti-mor. On the northern margin of India organic-rich de-posits are reported from the Oxfordian to TithonianSpiti shales and Nupra formations. Rifting withinGondwana resulted in the opening of an oceanic basinbetween eastern Africa and the India–Madagascar

block. This new corridor promoted the deposition ofblack shales facies at the Horn of Africa. The openingof the North Atlantic Ocean was paralleled by thesoutheastward drift of the Africa–South America me-gablock. The Arabian Peninsula moved from the equa-torial belt to the tropical arid zone. Such a location wassuitable for deposition of marine organic-carbon-richfacies. Another consequence of the southward move-ment of Africa relative to North America was the open-ing of the western arm of the Neotethys (the Ligurianand Alboran-Penninic basins). A continuous oceaniccorridor extended from the Gulf of Mexico to Indone-sia and Australia.

In the young and narrow North Atlantic Ocean, noKimmeridgian organic-rich deposits are known. In theGulf of Mexico province, Kimmeridgian-Tithonian or-ganic-rich shales containing predominantly marine or-ganic-carbon-rich facies were deposited in the Tampi-co-Tuxpan and Sabinas basins in Mexico. These facieswere deposited on the newly created continental mar-gin in deep isolated troughs where circulation was re-stricted.

At the northern margin of the Neotethys, the Cim-merian block was approaching collision with Asia. Ac-

318

Fig. 5 Reconstructed Kimmeridgian palaeogeography and distri-bution of radiolarites and marine organic-carbon-rich facies ofthe Tethys. (After Cecca et al. 1993)

tive arc-type volcanism was still present from northernTurkey to the northern Himalayas. Eurasia had shiftedfrom the temperate humid belt to the north tropicalone since the Toarcian. The Kimmeridgian deposits inthe Channel and the Paris basin are dominated by ma-rine organic-carbon-rich facies. Marine organic-carbon-rich facies are known in the Porcupine trough, Jeanned’Arc basin and along the Scotian shelf (Table 2).

In the Mediterranean Tethys the general organiza-tion of troughs and platforms had not changed. Duringthe Kimmeridgian, the troughs deepened and radiolar-ite deposits were widespread. Few basins had condi-tions favourable for organic-carbon-rich deposition andthese were mainly silled basins (southern Turkey) orisolated medium-deep troughs (western Greece).

The Late Jurassic, Kimmeridgian and Tithonian wasa period of continuous development of the Atlantic Te-thys and of the Caribbean domain. The wide radiolarit-ic domain which had existed during the Callovian (DeWever et al. 1994) was fragmented. In the Mediterra-nean Seuil during the Kimmeridgian, radiolaritictrenches, gutters and basins formed a complex network.Radiolarites were present roughly in the same places asbefore, but they were generally more restricted in area.Distribution of radiolarite facies was similar to that ofthe preserved marine organic matter (Baudin et al1992), but the deposition of organic-rich facies was

closer to the continental areas. This is true for examplefor radiolarites of the Pindos-Olonos basin. These aredevoid of organic matter, whereas organic material isknown in the Schistes à Posidonies in the Ionian basin(Fig. 4). In the western part of the Mediterranean Seuil,the radiolaritic facies is absent, but organic matter ispreserved. This is probably the result of areas with up-welling which was not strong enough to produce biog-enic silica in sufficient quantity to be over the thresholdabove which the silica was preserved. The organic mat-ter would be preserved because of lack of oxygena-tion.

Cenomanian

The palaeogeography of the Cenomanian (Fig. 6; Ta-ble 3) was characterized by a wide North AtlanticOcean and by opening of new seaways. At the southernmargin of the Neotethys the Indian and Australianblocks were separated. The change in the direction ofspreading between India and Australia also coincidedwith the separation of Madagascar from India. The lat-ter started its rapid northward motion. Some thin or-ganic-carbon-rich levels with radiolarians are known onthe Australian and Indian plates during the Cenoman-ian (Table 3).

Extensive carbonate platforms occupied the easternand northern shelves of Africa and Arabia. In some lo-cations corresponding to protected environments(Turkey, Israel and Lebanon) black shales were pre-served. In the Arabian peninsula (Iraq, United Arab

319

Table 2

Code Basin or province (country) Name of formation, member ongroup

Selected references

ADB Al-Mado Daror (Yemen) Madhbi Beydoun (1986), Haitham and Nani(1990)

Af (Afghanistan-Tadzikistan) Ulmishek and Klemme (1990)BaB Baër Bassit (Syria) Delaune-Mayère et al. (1977)BarB Barrow-Dampier (Australia) Dingo Claystones Osborne and Howell (1987)Bel Belluno basin (Italy) Galacz (1980)Bk Bakony (Hungary) Galacz (1980)BrB Browse (Australia) Dingo Claystones Thomas (1982), Volkman et al.

(1983), Master and Scott (1986)BuT Budva trench (Montenegro) Lastva Gorican (1993)CaB Carson basin (Canada) Mic Mac Powell (1985)CarB Carnarvon (Australia) Thomas (1982)GC Great Caucasus (Armenia,

Azerbaidjan)Ulmishek and Klemme (1990)

DA Dras Arc (India, Pakistan) Frank et al. (1977)Dor Dorset (Great Britain) Kimmeridge Clay Cox and Gallois (1981)ET Taurus (Turkey) Akkuyu Baudin et al. (1994)Hw Hawasina (Oman) Buwaydah, Al Aridh De Wever et al. (1990)I Ionian (Greece) Paliambela Danelian and Baudin (1990)JAB Jeanne d’Arc (Canada) Mic Mac Powell (1985), Grant et al. (1988),

von der Dick (1989)KoB Koçali basin (Turkey) Konak Fourcade et al. (1991)KRD Kabylo-Riffian (Morocco–Algeria) Raoult (1974), De Wever et al.

(1985), Maaté et al. (1993)LCB Lesser Caucasus basin (Russia, Ar-

menia, Georgia, Azerbaidjan)Lordkipanidze et al. (1984), Vish-nevskays (1993)

LMB Lugh Mandera (Somalia) Uarandab, Gehodleh Beydoun (1989)LnT Lagonegro (Italy) Armizzone, Lagonegro, Saso di

CastaldaDe Wever and Miconnet (1985)

MB Mamonia basin (Cyprus) “Formation sédimentaire deMamonia”

Lapierre (1972)

Mek Mekele (Ethiopia) Agula Shale Beydoun (1989), Savoyat et al.(1989)

N Normandie (France) Argiles d’Octeville Baudin (1992)NSB North Sea basin Kimmeridge ClayOB Ogaden (Ethiopia) Uarandab Savoyat et al. (1989)PB Poiana-Botisei (Romania) Sandulescu (1984)Pic Pichakun (Iran)PO Pindos-Olonos (Greece) Radiolarites De Wever and Dercourt (1985)PoB Porcupine (Ireland) Croker and Shannon (1987)Qa Qatar Dukhan Murris (1980), Alsharshan and Naim

(1990)SaB Sabinas (Mexico) La Casita Longoria (1984)ScS Scotian Shelf (Canada) Mic Mac Purcell et al. (1979, 1980), Mukho-

padhyay and Wade (1990)SP Serbo-Pelagonian (Greece) Mavrolakhos unit Stais et al. (1990)SeB Seram basin (Maluku, Indonesia) Nief Beds Audley-Charles et al. (1979)SuB Subbetic basin (Spain) Azéma et al. (1979)TamB Tampico-Tuxpan (Mexico) Taman Guzman-Vega (1991)Th Thakkhola (India) Spiti Shales, Nupra Gradstein et al. (1989, 1991), Bau-

din, unpublishedTiB Timor basin (Timor) Ofu, Wai Bua, Wai Luli Audley-Charles et al. (1979)Tts Southern Tibet (China) Wu (1993)TuB Tuscan basin (Italy) Monte Alpe Cherts Marcucci and Marri (1990)Tur Amu Darya (Turkestan) Ulmishek and Klemme (1990)UAE (United Arab Emirates) Hanifa Alsharshan (1985), Beydoun (1986)UB Umar basin (Oman) Aqil De Wever et al. (1990)VB Vienna (Austria) Ladwein (1988)W Waigeo (Irian Jaya, Indonesia) Pigram and Davies (1987)Yem (Yemen) Sabatayn, Madhbi Aboul Ela (1987)Yor Yorkshire (Great Britain) Kimmeridge Clay Herbin et al. (1991)Zh Zhob (Pakistan) Muslin Bagh Japanese–Pakistan Research

Group (1989)

320

Fig. 6 Reconstructed Cenomanian palaeogeography and distri-bution of radiolarites and marine organic-carbon-rich facies ofthe Tethys. (After Philip et al. 1993)

Emirates and Oman) shelf areas were dominated bythe marine organic-rich Misrif formation.

Cenomanian Atlantic deposits, well known from nu-merous DSDP and ODP sites, show important varia-tions in lithology and sedimentation rate. Organic-car-bon-rich layers appeared at all bathymetric levels, butthe highest accumulations occurred in outer-shelf envi-ronments and in low-latitude areas. During Cretaceoustimes radiolarites s.s. were absent in the Atlantic Te-thys and they declined in abundance in the Central andEastern Tethys.

The Caribbean domain was characterized by a com-plicated palaeogeography, with numerous isolated car-bonate platforms and with subduction beneath the ad-vancing Greater Antilles island arc. Organic-rich de-posits are rare,except in DSDP sites from the FloridaStrait. The South American plate was independent ofboth Africa and North America. In western Venezuelathe basal part of the La Luna formation, and its coevalequivalent in eastern Venezuela, the more pelagicQuerecual formation consisted of organic-rich blackand cherty fine-grained limestone.

Along the northern margin of the Neotethys a nar-row furrow, infilled by flysch facies, separated westernEurope from the Apulian promontory. Eastwards, fromthe Rhodope massif to Borneo, subduction of the Te-thyan ocean crust gave birth to volcanic arcs and back-arc basins.

In the Mediterranean Tethys, Cenomanian organic-rich facies were distributed mainly in deep environ-ments where redeposition was common. They consistedfrequently of thin black chert or shaly limestones withradiolaria, such as the famous “Livello Bonarelli” inItaly and coeval beds in the Rif and Gibraltar arch do-mains and in the Vocontian trough in the Alpine do-main.

Discussion

Palaeogeographic evolution of the Tethys during Meso-zoic times shows several trends for sites of radiolaritedeposition. Radiolarite is (De Wever et al. 1994): (a)absent in the Atlantic Tethys (western Tethys); (b)present from Norian to Cenomanian, more or less con-tinuously, off the Arabian Platform on the southernedge of the Tethys to the Mediterranean Seuil; (c) de-veloped extensively on the Mediterranean Seuil duringCallovian and Oxfordian times; (d) well represented inthe eastern Tethys, mainly on the southern margin (theside of India and Australia), during the Kimmeridgianand Tithonian.

Tethyan radiolarite has a latitudinal extent from 307

north to 207 south in the Mediterranean Seuil and inCentral Tethys. The Kimmeridgian to Tithonian radio-larite facies of the southern margin of the Eastern Te-thys extend to 507S latitude (De Wever et al. 1994).

The abundance of radiolarite of Callovian and Ox-fordian age on the Mediterranean Seuil allows recon-struction of narrow furrows separated by shallow basinsor insular platforms. These N–S troughs represent a

321

Table 3

Code Basin or province (country) Name of formation, member orgroup

Selected references

5 DSDP Site 5 (western N. Atlantic) Top of Hatteras97 DSDP Site 97 (Gulf of Mexico) Boyce (1973)105 DSDP Site 105 (western

N. Atlantic)Top of Hatteras Herbin et al. (1986)

137 DSDP Site 137 (central N. Atlantic) Top of Hatteras Herbin and Deroo (1982), Deroo etal. (1984)

367 DSDP Site 367 (offshore Senegal)386 DSDP Site 386 (central N. Atlantic) Top of Hatteras Herbin and Deroo (1982)387 DSDP Site 387 (central N. Atlantic) Top of Hatteras Herbin and Deroo (1982)398 DSDP Site 398 (Galicia bank) Deroo et al. (1979), Herbin et al.

(1986)540 DSDP Site 540 (Gulf of Mexico) Patton et al. (1984)545 DSDP Site 545 (offshore Morocco) Deroo et al. (1984)547 DSDP Site 547 (offshore Morocco) Deroo et al. (1984)549 DSDP Site 549 (Goban Spur) Waples and Cunningham (1985)551 DSDP Site 551 (Goban Spur) Waples and Cunningham (1985)603 DSDP Site 603 (western

N. Atlantic)Top of Hatteras Herbin et al. (1987)

635 ODP Site 635 (Bahamas) Katz (1988)641 ODP Site 641 (Galicia Bank)A (Algeria) Thurow and Kuhnt (1986), Herbin

et al. (1986)Ao Alboran (Morocco) Herbin et al. (1986), Bachaoui et al.

(1992)Ben Benue (Nigeria) Kuhnt et al. (1990)Ca Calabria (Italy) Thurow and Kuhnt (1986), Herbin

et al. (1986)CbT Celtiberic (Spain) Herbin et al. (1986)E Ethia (Creta) Serie d’Ethia Bonneau (1991)ET Eastern Taurus (Turkey) Karababa A Baudin, unpublishedEu Euganean Hills (Italy) Herbin et al. (1986)Hw Hawasina (Oman) Musallah, Al Aridh De Wever et al. (1990)Kr Kermanshah (Iran) Braud (1989)LeB Israël and Lebanon Daliyya Lipson-Benitah et al. (1990)MrB Venezuela La Luna-Querecual Talukdar et al. (1985), Tribovillard

et al. (1991)Pic Pichakun (Iran) Ricou and Marcoux (1980)PnB Penibetic (Spain) Thurow and Kuhnt (1986)PO Pindos-Olonos (Greece) Radiolarites De Wever and Dercourt (1985), De

Wever and Thiébault (1981)RiB Rif (Morocco) Kuhnt et al. (1990)SB Casamance (Senegal) Herbin et al. (1986)SiCr Sierra de Santa Cruz Rosenfield (1981)Sq Siquisique (Venezuela) Rio Tocuyo unit Stephan (1982)SuB Subbetic (Spain) Herbin et al. (1986)T (Tunisia) Balhoul Herbin et al. (1986)TarB Tarfaya (Morocco) Herbin et al. (1986)UAE Abu Dhabi/Saudi Arabia Misrif Ulmishek and Klemme (1990)UB Uman Basin (Oman) De Wever et al. (1990)UMB Umbria-Marches (Italy) Bonarelli level Herbin et al. (1986)Va Carpathian (Roumania) Herbin, unpublishedVoB Vocontian (France) Thomel level Crumière et al. (1990)

transition between the opening western Tethys (Atlan-tic) and the old rejuvenated central and eastern Tethys.For the first time the Mediterranean Seuil was sub-jected to intense oceanic circulation. The location ofthis Seuil, the relative latitudinal position of land vs seaand the increase of circulation caused powerful upwell-ing systems (De Wever et al. 1994; Cottereau and Lau-tenschlager 1994).

In Arabia radiolarite facies occurred troughout theperiod from Norian to Cenomanian. During this time

Arabia was divided into several basins separated fromthe oceanic domain by platforms. Such sites are subjectto strong oceanic currents and upwellings. They benefitfrom the huge oceanic stock, but are protected from theopen ocean.

The presence of radiolarite on the southern marginof the eastern Tethys (northern margins of India andAustralia), during Kimmeridgian and Tithonian time, isa peculiar case. The latest Jurassic was a period of pro-found modification in the ocean current circulation.

322

These changes had influences on geographically distantzones and probably also on climates.

The absence of radiolarite s.s. in the Atlantic Tethysand its scarcity in Caribbean regions may be explainedby the absence of strong monsoonal-driven upwelling.

Marine organic-rich sediments and radiolarians areassociated in many cases, but are not associated in oth-er cases. Such a contradiction may be explained by sev-eral factors:1. Silica may act as a dilutant of the organic matter.

Bogdanov et al. (1980) have shown that in modernsediments the TOC/SiO2 ratio in suspended par-ticles (at the surface) decreases when productivityincreases. Examples are known in the Central Pa-cific, the Gulf of California and the Monterey for-mation, California (Donegan and Schrader 1982;Aplin et al. 1992).

2. Some radiolarite may also be depleted in organicmatter due to its high water-depth deposition (VonBreymann et al. 1992) and its low rate of sedimen-tation. Note that this does not mean that the sedi-ments were not organic rich at the time of theirdeposition. Organic matter is recycled during set-tling or at the water–sediment interface. (Note:This is also true for red clays; Leeder 1982).

3. Porosity has an important role in whether or notthe organic matter is preserved. It permits ex-changes, oxygenation or sulphate action, all ofwhich destroy organic matter (Aplin et al. 1992).Organic matter is very sensitive to the oxygen con-tent of the interstitial waters.

In conclusion, there is a relationship between or-ganic matter and silica in sedimentary rocks, but morethan one parameter controls their abundance. Theseparameters and their importance have not yet beenwell identified. Organic matter and silica both resultfrom high plankton productivity, but the requirementsfor their preservation are not the same. As a result,they are not systematically associated in sedimentaryrocks. From Norian to Late Jurassic times radiolariteand organic-rich facies are generally not associated andtheir relative position is quite constant: organic-rich fa-cies are situated behind siliceous deposits with respectto the open ocean. Such a relationship is a common fea-ture of modern upwelling systems. During the Creta-ceous period, mainly during Cenomanian times, sili-ceous facies and marine organic-rich facies were notdistributed with the same duality as in previous periodsof time (and at present). Siliceous facies and marine or-ganic-rich facies of this age occupy the same sites. Theirorigin is probably of a different type than those ofTriassic-Jurassic age.

Acknowledgements This study was financed by the PeriTethysprograms, GDR 88 and URA 1761. We are indebted to MoniqueTroy (CNRS-UPMC, Paris) for the Reference List. We are alsograteful to several colleagues for their help for various reasons:Jean Dercourt (UPMC, Paris), who carried out the Tethys pro-gramme, Jean-Pierre Caulet (CNRS-MNHN, Paris) helped to im-prove the manuscript, Robert J. Langridge (Queens University,

Kingston, Ontario, Canada) and Dr. E. Urquhart (University Col-lege of London, UK) painstakingly helped us to improve the Eng-lish.

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