ASSOCIATION OF PHOSPHORITFS, ORGANIC-RICH SHALES,CHERT AND CARBONATE ROCKS

RICHARD P.SHELDON3816 T Street NW

Washington, D.C 20007

AllsTRAcr: The single-basin association of sequences of marine phosphorite, organic-rich shale, chert, dolomite and biogenic limestone hasbeen known for a number of well studied deposits for many years. It has long been known that organic matter, silica and phosphorus aresupplied to shallow shelf areas by upwelling ocean currents. Recent marinesedimentologic researchhas shownthat phosphorite and dolomiteare formed by earlydiagenesis of organic matterand, in the case of dolomite, by earlydiagenesis of carbonate sediments.

A literature survey of 135 world-wide phosphatic sequences from LowerProterozoic to Quaternary age showsthat the association of organic­rich sediment, phosphorite, chert and carbonate has been constant over time, but that the type of carbonate rock has shown a temporalvariation. Dolomite is the only significant carbonate in Proterozoic and Cambrian sequences, whereas both limestone and dolomite occurin younger sequences. Some of the organic-rich phosphatic sediments are oil shales or petroleum source rocks. It can be concluded thata significant percentage of worldpetroleum resources werederivedfromsuchsourcerocks.

These phosphatic sequences tend to be cyclic in nature and, in some instances, the regressive sediments include restricted basin evaporites,carbonates and shales. However, thesesediments havenot been countedin the upwelling marineassociations.

INTRODUCTION

Marine phosphorite is a biogenic sediment deposited inshallow-water areas, open to the ocean, where deeper oceanicwaterwells uptothesurface. Otherbiogenic chemical sedimentsare closely associated with phosphorites, including biogeniccarbonate, siliceous and carbonaceous sediments. Because thesource of phosphorus is thedeep-ocean geochemical sink,otherelements trapped in the sink tend to be associated withphosphorites, including silica, about 25 trace elements and, inrare instances, nitrates. Most phosphorites are formed inreducing environments, at least at one stageof theirformation,and tend to be associated with reduced elements includingcarbon, iron, manganese, sulfur, and uranium. Phosphorites canonly be deposited in alkaline environments and are therebyassociated with calcium, magnesium andmanganese carbonatesto varying extents. Phosphorus is derived from marine waters;thus, the conservative marine elements - most importantlycalcium, fluorine and magnesium - are available in theenvironment ofphosphorite deposition andcan makeupapatiteand other associated minerals in the sediments. Finally, thetemperature of upwelling water is normally low, so thatphosphorites are associated with sediments containing cool orcoldwaterbiota.

All of these associations are not synchronous; that is, theyare not necessarily a mixture with or a facies of phosphorite.Because phosphorites are generally found in cyclic sequencesthat are the result of changing paleooceanographic conditions,paleoclimates, andsealevels, manyof thesediment associationsarealsotheresult ofthese changing conditions within thegeneralupwelling environment

The continental shelfupwelling environment on its landwardside commonly adjoins platform seas, which are indirectlyaffected by the upwelling environment Modified deeper-oceanwaterfeeds suchplatform seas. The areasof coastal upwellingform meteorological high-pressure areas because of the coldwater at the surface, while adjacent landward areas formmeteorological low-pressure areas because they are warmerthan the coldocean areas. Thesepressure areas are stable andgenerate a surface equatorward wind system that enhancesoceanic upwelling in the trade wind belt along west coastalareas (Parrish, 1982). Adjacent to continental shelves wherephosphorite is being deposited, the normal sediments areevaporites in cratonic basins and eolian sand deposits interrestrial environments. Trade winds are primarily responsibleCARBONATES ANDEVAPORITES, Vol. 2,No.l,1987,p. 7-14Copyright 1987by the Northeastern Science Foundation, Inc.

for this genetic link, because they cause trade-wind deserts,high evaporation in cratonic seas and strong continental shelfupwelling. Desert conditions also result in only minor clasticcontributions to the basins of deposition, unless adjacent landareashavehighrelief.

Thus,a complex butgenetically interrelated association existsamong chemical and biochemical phosphorites, organic-richand metal-rich shales, siliceous sediments, sediments rich inmagnesium, manganese and ironcarbonates, cold-water biota,evaporites, and eolian sands. At the same time, the chemicalsediments normally contain only minor clastic components.These associations, which have been reviewed by Cook andMcElhinny (1979)and Riggs (1986), are discussed below.

PHOSPHORITES

The sedimentation of phosphorite has been in dispute formanyyears, andnotallquestions haveyetbeensettled. However,a consensus among most present workers is that phosphorusis chemically weathered from rocks on land and transportedby streams to the ocean, where it is used by phytoplanktonas a nutrient Thisstarts a food chainthat results inphosphorus­rich organic mattersinking intodeep waterwhere it is trappedin the deeper stratified ocean - the deep ocean sink. Wherethis deep water is brought back to the surface by upwellingoceancurrents, the waters become highly productive of organicmatter. Ifthisoccurs overdeepwater, thesinking organic matter- primarily diatoms in the present ocean - is decomposed bybacterial activity and oxidation, and the phosphorus is restoredto solution in deep ocean water. Where the upwelling occursover shallow water, the organic matter accumulates in thebottom sediment. During early diagenesis, sulfate-reducingbacterial decayreleases thephosphorus intotheinterstitial water,where it reacts in an alkaline environment to form carbonatefluorapatite. The fluorine, carbonate and calcium come fromthe interstitial water; or, in some cases, the carbonate andcalcium comefromcarbonate shell material.

Ithasbeenarguedthatreworking of thesediment isnecessaryto form apatite pellet concentrate (Cook and Shergold, 1986,p. 369). However, it has been shown that, in Quaternarysediments on the Peru-Chile shelf, apatite pellet concentrateis formed without reworking in water ranging from 100 to450 meters deep (Froelich and others, 1983; Burnett andFroelich, in press). This gives strong evidence that reworkingis not necessary to form the pellet concentrate, although there

8 RICHARD P. SHF.l.DON

is no argument that, in some cases, reworking does occur. Inmy opinion, most of the major phoshorites are unreworked,earlydiagenetic sediments. Exceptforrecentpapers,this subjectis reviewed by Sheldon (1981).

ORGANICMAnER

The genetic link between phosphorite and organic matteris shown by a close association between organic-rich blackshale and phosphorites in rocks of Proterozoic to Quaternaryage. In basin sequences, the two rock types are commonlyinterstratified. Phosphorites normally grade basinward intoblack shale. This relationship is well shown in Quaternaryphosphorites on thecontinental shelfof Peru and Chile(Burnett,1977),and in older phosphorites that have been well studied.as, for example, the Cretaceous Mishash Formation in Israel(Benter, 1953; Wurzburger, 1968), the Lower CaIboniferousDeseret and related formations of the western United States(Roberts, 1979; Sandberg and Gutschick, in press), and thePhosphoria Formationof the western UnitedStates (McKelveyet aL, 1959;Sheldon, 1984).

Some of the black shales associated with phosphorites areoil shales of potential economic value. In Morocco, in theTimahdit region, a Maastrichtian sequence of interbeddeduraniferous phosphorites, phosphatic marls, and bituminousmarls occurs in a sequence 150-300 m thick, showing fourdistinct cycles. The bituminous marls contain 36-120 litersofoilpermetrictonand arebeingevaluatedforpossible productionby the Office National de Recherches et d'ExploitationsPetrolieres (ONAREP, 1983; Rahhali, 1983). In Egypt, theMaastrichtian Dahkla Shale, which overlies the upper Cam­panian to early Maastrichtian phosphorites of the DuwiFormation (Germann et aL, 1984), contains oil shale with upto 40 gallons of oil per ton (167 liters per metric ton). Thisdeposit is being studied for its possible economic value, asreported by Darling (1983). These shales may be the sourcerocks for nearby recent oil discoveries (W.E. Harrison, inDarling, 1983). The black shales of the Permian PhosphoriaFormation in the western United States contain abundantcarbonaceous matter (up to 9 percent organic carbon) and,in someareas of Montana, up to 24 gallonsper ton (100 litersper metric ton) of retortable oil (Swanson, in McKelvey etal., 1959). However, these beds are not being considered forexploitation at the present time. In Estonia, USSR, Lower andMiddle Ordovician rocksin a sequence 150 metersthickincludecommercial phosphorite (Lower Ordovician) and commericaloil shale (Middle Ordovician). The Estonian oil shale depositcontains about 8 million tons of oil shale resource, which isof major importance to the economy of northwestern USSR(Academy of Sciences, Estonian SSR, 1984).

Some phosphatic sequences contain black shales that arepetroleum source beds. The organic-rich shale and diatomiteof the Miocene Monterey Formation, which are the sourceof much of the petroleumof southern California(Graham andWilliams, 1985), are interbedded with thin phosphorite beds(Roberts, in press; Garrison, in press). The Upper Cretaceousand Lower Tertiary rocks of northeast Africa and the MiddleEastcontainphosphorite andblackshales, and in Libya, Tunisiaand Egypt, theseblack shalesare considered to be sourcebedsof petroleum in each country(peterson, 1983).The petroleumresources of Iran and of a portion of Iraq are derived fromUpper Cretaceous to Lower Tertiary source beds (Masters etaL, 1982) that also contain phosphorite beds (Samimi and

Ghasemipour, 1977). Similarly, in Venezuela, the UpperCretaceous La Luna Formation, which contains economicphosphate deposits, is regarded as the major source ofVenezuelan oil (Bockmeulen et aL, 1983).On the North Slopeof Alaska, the Triassic Shublik Formation contains beds ofboth phosphorite and carbonaceous shale. A study of thePrudhoe Bay Field (Magoon and Claypool, 1981) suggestedthe likelihood that the the crude oil was derived from aphosphatic, calcareous shale, carbonate,or other iron-deficientsource rock; and Seifert et aL (1979) interpreted, on the basisof biologic marker compounds in the oils and shale extracts,that the major source of Prudhoe Bay Field crude was theShublik Formation. In the western United States, the basinalblack phosphatic shales of the Phosphoria Formation are thesource beds for oil fields to the east that contain about 1'hbillion barrelsof oil (Sheldon, 1967;Maughan, 1980;Peterson,1984). Other examples of associated petroleum source rocksand phosphorites exist, but are less well established. It can beconcluded that a significant percentage of world petroleumresources were derived from black shale associated withphosphorite.

The association oforganicmatterandphosphorite isobscuredby weathering and oxidation of the organic matter. This isparticularly evidentin semi-aridand arid regions whereerosionis slow and exposure long. The phosphate rock at the surfaceis characteristically light-colored and soft due to weatheringand leaching. This is economically important, as the grade andquality of the ore are thereby improved. Fresh samplesof thesamerock arenormally dark-coloreddueto theirorganicmattercontent,a featurethatiswellshownbycomparison ofphosphaterock mined underground in Morocco with rock mined at thesurface.

Anotherfeaturethatobscures the organicmatter-phosphoriteassociation is the facies relationship between the two.In basinalsequences the association is clear, but in shallow watersequences near the pinchout of phosphorite, organic matteris commonly not present The phosphorite has either had itsoriginal organic matter removed by reworking or it wasoriginally poor in organic matter, as, for example, a coquinaof apatite shells of inarticulate brachiopods. Most examplesof unweathered phosphorite not associated withorganicmattercome from suchoccurrences, and examination of a wider areausually shows that phosphorite and organic matter areassociated in deeperwater facies.

SlllCEOUS SEDIMENTSt

Phosphorus andsilica, alongwithorganiccarbonas discussedabove,are commonly associated in sedimentary rock sequencesas organic-rich bedsof phosphorite and chert or its equivalentsin young rocks, porcellanite or diatomite. However, not allphosphorites are associated with chert, nor are all chertsassociated with phosphorites. Both phosphatic and siliceoussediments occur in minor amounts in many different types ofrocksin manyareas, andingeneralarenotassociated. However,when major phosphorite deposits are considered, a distinctassociation appears. Of 37 major phosphorite deposits, 27 orthree quarters showa chert association, and of all phosphoritedeposits, both major and minor, half are associated with chert(tables 1 and 2).

McKelvey etaL (1953,p.61) pointedout the closeassociationbetween chertand phosphorite in the Phosphoria Formationof Permian age, which OCClD'S in the western United States,t The association of pbosphatic IDd siliceous mariDc sedimentaIy deposits

wastreatr.dinddailbySbcldon(1987),1Ddd1e<XJDclusionsare summarizedin tbissectioa.

PHOSPHORITES ANDCARBONATE ROCKS 9

andextended theassociation to othergeosynclinal phosphorites.apparently for the first time. They pointed out that althoughboth chert and phosphorite occur together in some layers, themost important association is a geographic one. Both chertand phosphorite are most prominent near the margin of thegeosynclinal belt Later workhas confinned the generalizationthat chert and phosphorite are associated but lack precisecoincidence. Petrographically, rocks made up of nearly equalmixtures of apatite and chalcedony or opal are rare. Rather.the association is characterized by interbedding of stratigraphicunits of phosphorite with units of chert. The fact that theseunits do not show facies relationships, that is, time-rockequivalence, agrees with what might be predicted from thelack of mixed rocks.

The differingdistributions ofchertandphosphoritebedsshowup on a broad paleogeographic view. Beotor (1979) pointedout that many ancient phosporites were deposited in epicon­tinental seas adjacent to continental shelves. In North Africaand the Middle East, the epicontinental phosphatic sequencescontain little, if any, chert, whereas the adjacent continentalshelf deposits contain prominent chert. A strong chert­phosphorite association occurs in the deposits of Turkey, Syria,Israel, and eastern Sahara, all of whichweredeposited on theouter continental shelf adjacent to the Tethys seaway. Thephosphorites of Iraq, Jordan, and Saudi Arabia, which weredeposited on the inner continental shelf, contain fewer chertbeds, and the Egyptian deposits, which are farthest away fromthe Tethys seaway, contain the least amount of chert. Thephosphorites of Morocco and Tunisia-Algeria were depositedin narrowinlets that openednorthward intothe Tethys seawayand southward intoa broadepicontinental sea in NorthAfrica.Phosphorite extends farther into the open epicontinental seathan does chert, which is concentrated at the·mouths of theinlets near theTethys Sea. Comparable innershelfphosphoriteswithout a chert association exist in the Permian PhosphoriaFormation in the United States, in the Late Cretaceous LaLuna Formation in Venezuela and Colombia, and in theMiocene-Pliocene phosphorites of the southeastern UnitedStates.

The marine geochemical cycle of silica is similar to thatof phosphorus. The supply of both phosphorus and silica totheoceanispredominantly fromsubaerial chemical weatheringof rocks and transport to the ocean by rivers, and is in minoramounts fromvolcanic activity (WollastandMacKenzie, 1983;Froelich and others, 1982). In a process similar to that ofphosphorus, silica from the frustules and tests of plankton istransferred intact to deeper levels, primarily as diatom-filledfecal pellets, and isreleased back to theseawater bydissolution.These processes lead to a buildupof silica in the deep oceanbelow 500-1000 m; there, the concentration of silica reaches3000-5000 #lgtl, whereas at the surface its concentrationapproaches zero,except at high latitudes in the winter months.The dissolved silica follows phosphorus in its return to theocean surface by upwelling currents, and supports blooms ofphytoplankton, Settling siliceous frustules and tests do notdissolve completely and tend to accumulate on the seafloor.

Silica andphosphate mustbe separated duringsedimentationto giverelatively pure siliceous and phosphatic sediments. Forthe Phanerozoic. before diatoms evolved, the process seemsstraightforward Siliceous sediments were made up primarilyof sponge spicules. When organic matter accumulation wassufficient for phosphorite formation, the bottom water wasprobably too oxygen-deficient to allow sponge growth, and

when sufficient oxygen was available to allow sponge growth,insufficient organicmatter was available to allow phosphositeformation. However, thishypothesis may be too simple, as thepossible role of pre-Cretaceous silica-bearing microfauna hasnotbeenthoroughly studiedForthetimeafterdiatoms evolved,in the Early Cretaceous or, possibly, in the Late Jurassic, theprocess ofseparation ofsilica fromphosphate ismore uncertain.Of the several hypotheses, variation of diagenetic pH appearsmost likely to cause separation, with silica fluxed out of thesediment at high pH and phosphorus fluxed out at low pH(Sheldon, 1987); however, the separation of silica andphosphorus clearly needs additional research to be understood.

CARBONATES

Phosphorites from Proterozoic to Quaternary age aredominantly (77%) associated with carbonate rocks (tables 1and 2). This association occurs as interbedding of phosphoriteandcarbonate rock in sequences of organic-rich rocks in singlebasins, andisdue to variations oftheconditions ofsedimentationin time and not to facies. Thus, a continuous petrologic seriesfrom phosphorite to carbonate rock does not normally OCCW',and phosphorite beds do not nonnally grade into carbonaterock beds. The kind of carbonate rock associated withphosphorite differs with age. All Proterozoic and Cambrianphosphorites are associated with dolomite, with only rare,subordinate limestone; in contrast, most younger phosphoritesare associated withlimestone or, to a lesser extent, withmixedlimestone and dolomite.

Limestone associated with Phanerozoic phosphorite isbiogenic in origin. In the Paleozoic and Lower Mesozoic, thelimestones were made up mainly of benthonic and nektooicshell debris. In Early Cretaceous time,theevolution of pelagicplanktonic globigerinids caused limestone formed fromforaminiferal oozeto be addedto the phosphorite suite of rocks.The upwelling origin of the phosphorite suite of rocks wasaccompanied bylowtemperatures, which inhibited thepresenceof warm temperature biota. For example, coralline limestoneis not directly associated withphosphorites.

Dolomite in the phosphorite suite of rocks of Phanerozoicage is diagenetic in origin. It is forming at present overa largepartoftheoceansoncontinental shelves, during earlydiagenesisoforganic-rich sediments (Baker and Burns, 1985). Dissolutionofcalcium carbonate inthesediments gives calcium andcarbondioxide; oxidation of organic matter gives carbon dioxide; andmagnesium is addedto porewaterby migration from overlyingsea water. Dolomite has formed in a comparable manner inLate Tertiary continental margin sediments (various papers inGarrison etaL, 1984). Kastner etaL (1984)report that dolomiteis the stable carbonate in seawater because of its high MgtCa ratio - about 5:1, and that calcite is dolomitized exper­imentally in a solution that has a MgtCa ratio of 1:3. Becausedolomitization of CaC03 is inhibited by high concentrationsof sulfate in seawater, they suggest that the low sulfateconcentration in the sulfate reduction zone of organic-richsediments enhances the formation of dolomite. Thus, organicmatter may be the genetic link between phosphorite anddolomite. Both dolomite and phosphorite require an alkalineenvironment of sedimention. Phosphorites require organicmatterasa sourceofphosphorus, anddolomite requires organicmatter to reduce the sulfate of the pore water. Dolomiteformation also requires CaCO shell debris to raise the pHto sufficiently high levels. In Pn;terozoic time, however, no

10 RICHARD P. SHElDON

Table 1. Chert and carbonate rock associations with phosphorites. D=dolomite, DL=dolomite and limestone, L=limestone,N=no carbonate.

Carbonate CarbonateNumber Deposit Country Chert Rock SIze Number Deposit Country Chert Rock Sbe

Lower Proterozoic Upper Carboniferous (Pennsylvanian)LP-I Baraga Group Michigan, US + D minor UC-l cyclothems Mid-continent, N minorLP-2 Bijawar Series Central India + D major USLP-3 Aravalli Series Western India D major UC-2 ClareShales Fm. Ireland + L minor

UpperProterozoic and Cambrian UC-3 Alton marine bands UK ? L minor

PC-I Khubsugul Series Mongolia + D major PermianPC-2 Tamdy Series Kazakhstan, + D major P-l Phosphoria Fm. western US + DL major

USSR P-2 UralMtns. USSR + DL minorPC-3 YuhucunFm. China + D major P-3 Sonnebit unit Timor, + L minorPC-4 Kodjari Fm. Benin, Niger, + D major Indonesia

Burkina Faso 'fria!Ilic:PC-5 Beetle CreekFm. Australia + D major T-l ShublikFm. Alaska, US + DL m~orPC-6 TalFm. UttarPradesh + D minor 1-2 Arctic.Islands USSR ? ? minorPC-7 Sturtian Series Australia + D minorPC-8 Lao Cai Vietnam D major JIII'lRicPC-9 Abbottabad Fm. Pakistan + D minor J-l Aramachay Fm. Peru + L majorPC-tO Bambui Group Brazil D major J-2 FernieFm. Canada + L minorPC-II Fontanarejo Deposit Spain N minor J-3 LaCaja Fm. Mexico + L minor

Ordovician Jurassic-Cretace0-1 Kallavere Fm. Estonia, USSR N major JK-l Vyatka Basin USSR N minor0-2 Maquoketa Fm. Iowa, US + DL minor JK-2 Volga Basin USSR N minor0-3 Stairway Sandstone Australia L minor Lower Cretaceous0-4 Baltic Shield Sweden L minor LK-l VacaMuerta Fm. Argentina + DL minor0-5 Trenton Group Tennessee, US L major LK-2 Pabellon Fm. Chile + L minor0-6 Capinota deposit Bolivia N minor LK-3 Muertoand Peru + DL minor0-7 Labradoand Argentina N minor Pariatambo Fms.

Centinela Fms.Upper Cretaceous0-8 Swan Peak and Idaho, Utah, N minor

US UK-I Karababa, Kasrik, Turkey + L major

Hardin Fms. Wyoming, US Karabogaz andGermavFms.

Silurian UK-2 Campanian Syria + L majorS-l LosEspejos Fm. Argentina N minor UK-3 DigmaFm. Iraq + L minorS-2 RedMountain Fm. Alabama, US N minor UK-4 Belqa Sereis Jordan + L majorDevonian UK-5 MishashFm. Israel + L m~or

D-I GeirudFm. Iran L minor UK-6 TurayfFm. Saudi Arabia + DL ~or

D-2 Balong Guangxi, ? L minor UK-7 Duwiphosphate Egypt + DL majorChina UK-8 La LunaFm. Venezuela, + L major

D-3 Onondaga and Pennsylvania, L minor ColombiaRidgely Fms. US UK-9 ParisBasin France + L minor

D-4 Indiana, US L minor UK-lO Ciply chalk Belgium + L minorD-5 MoscowFm. NewYork, US L minor UK-ll Aktyubinsk Basin USSR N minorD-6 Sekondi Sandstone southern N minor UK-12 Gramame Fm. Brazil L minor

Ghana UK-13 NapoFm. Ecuador + L minorD-7 Beacon Subgroup Antarctica N minor UK-14 LasHayas Fm. Argentina L minor

Lower Carboniferous <Mississippian) Lower TeI1iaryLC-l Chattanooga Shale Tennessee, + L minor LT-l Metlaoui Tunisia- + L major

and MamyFm. Alabama, AlgeriaGeorgia, US LT-2 UrnEr Radhuma Iraq + L m~or

LC-2 Deseret and Utah,US + L minor and DammamFms.BrazerFms. LT-3 phosphatic series Morocco + DL m~or

LC-3 Lisburne Group Alaska, US + DL minor LT-4 BuCraa deposit Western + L majorLC-4 Tournasian SWFrance + DL minor SaharaLC-5 Fayetteville and Arkansas, US L minor LT-5 Eocene Senegal- + DL major

PitkinFms. MauritaniaLC-6 Exshawand Canada; US + L minor LT-6 Eocene Syria + L majorLC-7 St Ann'sdeposit Australia ? ? minor LT-7 Eocene Togo L majorLC-8 TalakFm. Niger N minor LT-8 Tilemsi Mali L minorLC-9 fore-flysch Dem.Rep. ? ? minor LT-9 KisilKum USSR L major

Germany LT-lO PabdehPm. Iran + L minorLC-lO Poland ? ? minor

PHOSPHORITES ANDCARBONATE ROCKS 11

Table 1 continued Chert and carbonate rock associations with phosphorites. D-dolomite, DL-dolomite and limestone,L=limestone, N=no carbonate,

CarbmBIe CarIJoaIIeNumber Deposit Coontry Chert Rock Size Number DepoIit CouaIry Chert Rack Size

Oligocene Tm-22 Clarenden Hill, NewZealand L minorTo-I CooperFm. S.Carolina, N minor Ontago

US Tm-23 ChathamRise and NewZealand L major

OIigocme-Mioceoe Campbell Platformoffshore

Tom-l Monterrey Fm. Mexico + N major Tm-24 Clifton Fm.Victoria Australia L minorMiocene PlioceneTm-l PungoRiver Fm. North + DL major Tp-l Monterey andSanta California, US + N minor

Carolina, US BaJbaraCountiesTm-2 Hawthorne and Florida,US DL major Tp-2 Yorktown Fm. North DL minor

Bone Valley Fms. Carolina, USTm-3 Blake Plateau USAtlantic DL major Tp-3 Varswater Fm. South Africa N minorTm-4 Monterey Fm. California, US + DL minor Tp-4 Salentina Peninsula Italy L minorTm-5 offshore California, US + DL minor (maybe sameas Tm-15)Tm-6 Cuba + DL minor Tp-5 eastemJava Indonesia L minorTm-7 Sechura Peru + N majorTm-8 Pisco Peru + N minor QuaternaryTm-9 CaletaHerradura Chile L minor Q-I offshore N.Carolina, ? minor

Fm. USTm-tO Tongoi Chile N minor Q-2 offshore BajaCalif. Mexico + DL majorTm-ll offshore Morocco + N minor Q-3 offshore Chile-Peru + D majorTm-12 offshore Gabon-Congo N minor Q-4 offshore France ? ? minorTm-13 Varswater Fm. South Africa N minor MediterraneanTm-14 offshore South Africa N minor Q-5 offshore Namibia + ? minorTm-15 PietraLeccese Fm., Italy L minor Q-6 offshore Arabian ? minor

Salentino Pen. Penin.Tm-16 Sicily Italy L minor Q-7 offshore Andaman 1 India ? minorTm-17 offshore Spain ? ? minor Q-8 offshore, western India + ? minorTm-18 offshore Portugal DL minor continental shelfTm-19 Seaof Okhotsk USSR ? ? minor Q-9 offshore, eastern Australia L minorTm-20 Negros Philippines L minor continental shelfTm-21 Java Indonesia L minor

12 RICHARD P. SHElDON

no carbonatedolomitechert

Table 2. Chert and carbonate association with phosphorite over geologic time. Percentage of phosphorites having indicatedassociation. Percentages in parentheses are associations of majorphosphorites alone.

dolomite +limestoneageinterva1

7150100(100)100(100)100 (100)

10079 (100)70 (75)

100 (100)32 (40)2050 (100) 33

Lower ProterozoicUpper Proterozoic + CambrianOrdovicianSilurianDevonianLower CarboniferousUpper CarboniferousPermianTriassicJurassicJurassic-CretaceousLower CretaceousUpper CretaceousPaleocene-EoceneOligoceneOligocene-MioceneMiocenePlioceneQuaternary

67 (50)73 (71)12

100(100)91 (100) 9

50 (50)100281433

100

7

100100(100)32 (20)40

CaCO~ shells existed, so a different mechanism of dolomiteformation was required. Tucker(1982) suggests that dolomitemay have precipitated directly from seawater in Precambriantime, as a result of seawater chemistry that was different fromthatof the Phanerozoic.

Kastner et at. (1984) explain the dolomite-phosphoriterelationship in the Monterey Formation of California by adecrease in the Mg concentration of pore water as a resultof dolomite formation, which resulted in enhanced apatiteformation. This cause and effect relationship is plausible, butdoes not appear to be operating in the modem formation ofphosphorite without dolomite on the Peru-Chile continentalshelf(Burnett, 1977). Also, many ancient phosphorites are notassociated withdolomite (table 2).

In a broader paleogeographic view, the phosphorite suiteof rocks, including the carbonate rock discussed above, isdepositedoncontinental shelves andonepicontinental seafloors.Semi-restricted and restricted basins commonly occuradjacenttophosphorite basins onthecraton, andarethesites ofcarbonaterock and evaporite sedimentation not associated with phos­phorites. Such carbonate rocks consist of bioclastic, algal orpelletal limestones, which in many cases have been replacedbydolomite. Sabkha-type dolomites associated withevaporitesare also found in the restricted evaporite basins. In a broadsense, these carbonates are associated regionally with thephosphorite suite of rocks and are time equivalents. However,such cases are not counted as associated in table 2, whichdeals with sediments deposited in the same basin. Thisdistinction is sometimes difficult to make. The phosphorite suiteof rocks isnormally found in cyclic sequences, caused bymajoreustatic changes of sea level. The sequences are composedof interstratified continental shelf and cratonic sediments, sothat transgressive marine sediments can overlie regressiverestricted basin sediments. Such relationships are shown in thePermian rocks of the western United States (Sheldon, 1984)andin Tunisia (Sassi. 1980).

The carbonate rock of the phosphorite suite of rocks showscompositional changes through geologic time. Essentiallydolomite alone occurs in Proterozoic to Cambrian phosphoritesuites. Limestone, on the otherhand, is thedominant carbonaterock in younger Phanerozoic phosphorite suites. In phosphoritesuites of the Ordovician through Carboniferous and of theJurassic through Paleogene, 63% contain limestone alone, 15%contain mixed limestone and dolomite, and 22% contain nocarbonate rock. In Permian, Triassic, andNeogene phosphoritesuites, 35%contain limestone alone, 35%mixed limestone anddolomite, and 29% no carbonate.

The causes of these temporal variations in the type ofcarbonate rockpresent inphosphorite rocksuites arenotknown,but speculations can be made. As pointed out above, Tucker(1982) speculated that Precambrian dolomite sedimentationwas caused by a seawater composition that was different fromthat of the Phanerozoic. This may also have been true of theLowerand Middle Cambriandolomites. Two major phospho­genic episodes occurred in the Phanerozoic during times ofpolar glaciation, and include the Permian and the Neogene.Both were also times of mixed dolomite and limestonesedimentation inthephosphorite suite. OftheotherPhanerozoicphosphorite suites, only the Upper Cretaceous and LowerTertiary include majorphosphorite deposition. Thesesuites aredominantly limestone-only suites, and were not deposited attimes of polar glaciation. It can be speculated that watertemperature wasone control on carbonate sedimentation, withlimestonepreferentially deposited inwarmer wateranddolomitein coolerwater.

CONCLUSIONS

Phosphorite, chert and dolomite are commonly associatedin early diagenetic, organic-rich environments in areas ofupwelling ocean currents over shallow water. Biogeniclimestone, derived from planktonic and benthonic organisms,

PHOSPHORITES ANDCARBONATE ROCKS 13

andblackshalearealsofound in therocksuite. Theassociation,except for that of black shale with all other rock types andof chert with limestone, is generally one of interbedding andnot one of lateral facies gradation. The interbedded sequencesgenerally form lithologic cycles that are probably the resultof changes in sea level and upwelling activity.

The association of black shale, chert and phosphorite isconstant overProterozoic to Holocene time, but the associatedcarbonates show a temporal variation. Dolomite is the onlycarbonate in Proterozoic and Cambrian sequences, and bothlimestone and dolomite occur in younger sequences.

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14 RICHARD P. SHEWON

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Manuscript received June20, 1987;Manuscript accepted September 15,1987