Nature and genesis of silt-size carbonate sediments; Northern Red Sea, Egypt

NATURE AND GENESIS OF SILT-SIZE CARBONATE SEDIMENTS; NORTHERN RED SEA, EGYPT

Amr AEI-SammakOceanographyDepartment, FacultyofScience, Alexandria University, Egypt.; Present address: UnitedArab EmiratesUniversity, Faculty ofScience,geologyDept. AI-Ain P.O. Box 17551, UnitedArab Emirates.; Fax: +97137671291;

[email protected]

ABSTRACT: Thenatureandgenesisof silt-sizecarbonatefractions intheNorthernRedSeais investigated via detailedSEM,XRDandICPanalysesof the sediments. Resultsshowthat inorganicaragoniterepresents only25% of the fine silt and clay fractions,butup to 60% of thetotal coarsesilt. In generalthere is a compositional trend of increasing proportionof'Mg-calcitewithdecreasinggrain sizeof the sediments.Petrographic examination demonstrates thatthe coarsesilt sizecarbonates aremainlycomposedof comminutedpiecesof unaltered carbonateor micritizedmarine skeletal debris. On the other hand, the fine and very fine silt fractionsare mainlycomposed ofauthigenic carbonate,formedinsitu,andreplacingthe comminuted piecesof carbonatedebris. Theargumentforthe inorganic originof the finecarbonate fractionsisbasedon the detectableincreasein theSrcontentof aragonite,andthe increasing proportionofMg-calcite with decreasing grainsizeof thesediments. It is concludedthat the Mg-calcite componentprovidesa more importantclue to the originof the silt fraction. Micritization ofskeletal grains is an important source of carbonate mud. Thus Mg-calcite from the Red Sea reefal sediments, either as skeletal orcryptocrystalline lumps, is the principalsourceof this componentin the carbonatemud, especiallyin the clay size fractions. The Carbonatemud in the study area is mineralogically similar to the Belize carbonate mud, and differs from the Bahamas and Arabian Gulf carbonatesediments.

INTRODUCTION

Carbonate sediments belong to a group of deposits that arepredominantly composed ofchemical or skeletal precipitates.The most important carbonate elements are: (1) whole orfragmented organic skeletons that become part of thesedimentary record as grains upon death ofthe organisms; (2).grains and compound grains that are the result of directprecipitation or oforganic binding; (3) crystals that form in theinter- and intragranular pore space and occlude this to variousdegrees; and (4) microcrystalline carbonate mud i.e. micrite(Reid et al. 1990).

The early detailed studies of the ongm and genesis ofcarbonate sand and mud have focused mostly on the Bahamasand similar environments. The silt-size carbonates have beenlooked at in Belize, the Caribbean Sea (Matthews 1966),Bimini (Stieglitz 1972) and in detail during the work ofFreileand Milliman (1991) on Bahamas mud. According to theseauthors, silt-size carbonate sediments in the western Bahamasrepresents a combination of both destructional andconstructional processes, and the relative contribution ofthesetwo components appears sensitive to the size partitioning ofthe silt fraction. However, the study of silt-size carbonatesediment has not been adequately investigated. The origin ofsuch size particles still remains inconclusive (Friedman 1985).This question usually arises: are the silt size carbonatefractions the product of a destructive process of skeletal andnon-skeletal grains; constructive mechanism (aggregating offme carbonate mud), and/or authigenic precipitation?

To answer this question, the nature and genesis of silt-sizefractions in the sediments from the northern Red Sea will belooked at. The predictable results will be compared with thoseofthe Bahamas, Belize and elsewhere.

Carbonatesand Evaporites, v. 16, no. 1,2001, p. 37-45.

METHODS

The area ofstudy is located in front ofthe National Institute ofOceanography and Fisheries, at Hurghada near Abu Shaa'rBay, Northern Red Sea, Egypt. Sediment samples (26samples) were collected, using a grab sampler. It has beennoted that only those samples from depths ranging between 25m or deeper have significant amount of fine fractions.

In the laboratory, samples were washed with distilled waterand treated with HP2 to remove salt and organic matter, thenoven dried. Part of the samples were subjected to grain-sizeanalysis. The separation ofsilt and clay fractions was done bydecantation using the settling tube according to Prokopovich(1958). Because of the interference of dispersant with thechemical analysis (Milliman et al. 1993), and the effect ofgrinding on the peak height and peak area (Gavish andFriedman 1973), the samples were only ultrasonicallydispersed. The carbonate mineralogy of the various siltfractions was analyzed by X-ray diffraction, scanning theinterval from 250 to 330 2e employing Kn, Cu target, at 1.00 perminute. The relative amounts of aragonite, calcite andmagnesium calcite were quantified using the peak area methodof Milliman and Bornhold (1973) with a reproducibility of2.3% and accuracy within 5%. Samples were analyzed for Srand Mg on an Yvon-Jobin Induced Coupled Plasma-EmissionSpectrometer and calibrated against standards. The analyticalprecision oftriplicate analyses was 1.1% for Sr and 1.96% forMg. Thin sections were made and examined under apetrographic microscope. The mud fractions were probedwith SEM equipped with EDX analysis.

RESULTS

Among the collected sediment samples only few samples have


50 r-----.-------.----.....------.------""T'"-----,

-..--

~ 40CJ)CJ)ro()

Q) 30.NCJ)....c~ 20~"'0

'0 10?F-

0'-------'------'--------'-----"'--------'------'>4 phi 5phi 6phi 7phi 8phi

Average Grain Size (Mz) in <p<8phi

Figure J. Averagegrain size distribution (in cp) at different size classes.

\IT] Aragonite II Calcite mMg-calcite60

50

40

30

20

10

0,phi4 phi5 phi6 phi7

Grain Size

phi8

Figure2. Average mineralogical contentsat differentsize classes.

a considerable amount of fme fractions; accordingly thesesamples were subjected to the full investigations. Sampleswith dominant fine fractions exhibited a log-normal sizedistribution (Fig. 1). The most dominant size fraction is themedium silt (15.6 urn) i.e. 28% to 46% ofthe total sediments(average of37.4% ± 6.2). The sand fractions range between

38

4% and 17% (average of10.9% ±4.9) ofthe total weightofthesilty sample, while clay-size fractions representapproximately5% ± 0.88 of the total weight. The average mineralogicalcontent (± standard deviation) for various grain-size fractions(Fig. 2) shows that very fine sand (125-63 urn), has ± 50%aragonite, the smaller the size the lower the aragonite content.

EL-SAMMAK

Figure3.SEMmicrographs (a)veryfine sandandsiltfractions,(b)skeletalgrainshowingtheeffectofboring organisms, wherea board,pitted skeletalgrain is shown, (c) clayparticles aggregatearounda non-carbonate grain.

On the other hand, Mg-calcite increases in amount withdecreasing grain size. Mg-calcite attains its maximwn value inclay size, while aragonite is minimum. Calcite is nearlyinvariable.

SEM analysis shows that silt fractions consist predominantlyof cryptocrystalline grains mixed with skeletal grains (i.e.Echinoid fragments, coralline algae, forams, and others); somehave anhedral shape (Fig. 3a). Grain surfaces are in placesrough while others are smooth and rounded. Microboring, pitsand skeletal cavities are filled with Mg-calcite together withAl-Silicate minerals (Fig. 3b). Clay-sized sediments consistpredominantly of clwnps of fme anhedral crystal (Fig. 3c)aggregates around non-carbonate particles.

39

DISCUSSION

According to Milliman et al. (1993), the Sr" within thearagonite phase of a carbonate sediment (Sr .) could be

aragorute

estimated, asswning that the carbonate sediments consistentirely of aragonite and calcite (including Mg-calcite); usingthe following equation:

Sr" % (bulk) = Sraragonite (% Aragonite) +0.2(% Calcite) ..... (1)

Employing this equation; it is obvious that the clay-sizedcarbonate fractions have a Sr content almost above 1.0% (Fig.4). A slight increase in Sr content ofthe medium silt fractions(0.65 % Sr .) is also noticed. The range ofSr . within

aragontte aragonite


1.4

1.2

1.0

..- 0.8'eft-10- 0.6en

0.4 ..

0.2 .,

0.0Phi4 PhiS Phi 6

Grain Size

Phi 7 Phi 8

Figure4. Sr . contentsat different size classes.a,..agonue

the clay fractions «4 urn) varies from 0.9% to 1.25%, whilewithinthe silt fractions (64 - 4 11m) the variationof Sr . isr- aragorute

limited.

Strontium contents vary within the different aragoniticphases. Modemaragonitecement may containas much as1-2%sreo3 (MorseandMackenzie 1990). Ontheotherside,aragonite produced by codiacean algae has Sr contentsranging from 0.8% to 0.9%, whereas inorganicallyprecipitated aragonite (i.e. ooids, grapestone, cementedpelltoids) contains 0.95 - 1.0% Sr (Milliman et al. 1993).Exceptformollusks, whichincorporate diminutive amountofstrontium, (mollusks contains between 0.08 and 0.4 % ofstrontium averaging 0.24%),skeletal aragonite has strontiumconcentrations which correspond to those predicted fromcalculation i.e.0.95- 1.0% Sr (Milliman 1974; Loreau1982).Coralscontain0.911% Srwithintheiraragonite phase(Tuckerand Wright 1990).

Coralsand mollusks are the only skeletal fragments that arecomposed entirelyof aragonite(Morseand Mackenzie 1990;Tucker and Wright 1990). Average Sr values for aragoniticcoral and aragonitic mollusks from the Gulf of Aqaba are0.82%and 0.13%,respectively (Friedman 1968).

Providing thataragonite in theRedSeasediments isdepositedas skeletal (corals and mollusks) and non-skeletal (inorganic

40

precipitated), theratioof skeletal to non-skeletal aragonite canbe calculated using a simple two-part mixing equation asfollows:-

(100 - X)% of Skeletal x (averageSr content in skeletal) + X(Sr in inorganic aragonite) = Sraragonite % (2)

whereX is the percentage of inorganicaragonite.

According to El-Sammak et al. (l997) the average coral andmollusc fragment contents of the same samples fromHl~:JEgh1:B17D± 8% for corals and 8.0 ± 2% formolluscs. Considering also that the averageSr value for theinorganic aragonite phase (Bathurst 1975) is 1.0%;accordingly equation number2 will be modified as follows,

(lOO-X)% x (coral%x 0.82+ Mollusks% x 0.13)+ 1.0(X)%= SrangOnile (3)

whereCoral%= Coral/ (Coral+ Mollusks)and Mollusks%= Mollusks/ (Coral+ Mollusks)

«lOO-X)% x (0.68x 0.82+ 0.32 x 0.13» + 1.0(X) %= Srlll1lg0nite (4)

(100 -X)% x 0.5992+ 1.0(X)%= Srlll1lgonite .......(5)

EL-SAMMAK

Figure5. (a)Foraminiferalgrainsshowingthearagoniteneedlesinsidethegrain cavities. (b) In somegrains, the intraskeletalcementation completelyblocked thegrain cavities; the alteration oforiginalskeletalgrain is also noted.

Apparently, from equation (5) the calculatedaverage valuesfor skeletal aragonite will be sometimes higher than thedetermined Sr .t values calculated from the equation (1).

aragorure

Using the minimum value of corals (9%) and the maximumvalue for mollusks (10%) "lower estimate", the strontium inthe skeletal aragonite will account for 0.3854 + 0.0689 =

0.4543 % Sraragonite

For the higher estimates (higher value for corals, 25% andlower value for mollusks, 6%), the strontiumcontent will be0.6642 + 0.0247 = 0.6889 % Sraragonite

The interpretation for the inorganic contribution using the

41

lower,average, and higher estimatedat differentgrain sizesislisted in Table 1.

Petrographicevidence for internalmicrite is shown in Fig. 5a,where aragonite needles can be seen inside the grain cavities.The medium silt fraction contains skeletal and non-skeletalfragments. Ascidianspiculesare found in themediumsilt-sizefraction. According to Milliman (1974), these spicules arearagonite containing high strontium (0.82%) and low Mg(0.15%).

Mineralogically, the aragonitedecreaseswith decreasingsizeof the sediments, while the 64 urn have 57.4% ± 10.7


Table 1. Estimated inorganic aragonite contribution at different silt size carbonate fractions.

Grain Size Lowerestimate (%) Average estimate(%) Higherestimate(%)(fIIl1) (Sr....... : 0.4543%) (Sr.......: 0.5992%) (Sr""....,~:Q.6889 %)

63 13 UE UE

31 36 8 13

16 22 UE UE

8 14 UE UE

<4 100 100 100

UE = UnderEstimated

55

50 0

045 0

0 00

- 40 0 0~0

00-- 35Q)-'0

CO 30oI

~ 25Q

200 0

15 0 0

10-0.5 0.5 1.5 2.5 3.5 4.5 5.5 6.5

Mean Size (phi)

Figure6. Relationship betweenmeangrainsize (<p) and the Mg-calcite contentsat different stations.

aragonite, the clay fraction « 4 urn) has an aragonite contentof 24% ± 1.4. In fact the amount of precipitated aragonitewithin the different sizes is similar, but the amount ofskeletalaragonite increases in the medium silt and coarse silt, andcompletely disappears in the fme silt and mud, therefore, therelative percentage of precipitated aragonite increases in thefme silt.

Evidence for the inorganic origin ofclay-size aragonite mud «4 um) comes from the Sr data. Corals have the highest Srcontent (0.82 %) among the skeletal carbonate observed

42

during the present study, whereas carbonate mud contains1.038% ± 0.176 Sr, suggesting that inorganic precipitatedaragonite is the prime contributor to the Red Sea aragonitemud. The medium silt fraction having low Sr . value is

aragonite

mainly due the intermixing of both breakdown of micritizedskeletal grains and the division of internal micritization.Alternatively coarse silt fractions composed primarily fromskeletal aragonite with minor inorganic aragonite asintraskeletal precipitates.

The inorganically precipitated aragonite mud in the Red Sea is

EL-SAMMAK

not unique. High Sr aragonite mud was also found along theArabian Sea(EllisandMilliman 1985)andBahamas(Shinnetal. 1989). A high Sr content within the banktop muds in theGreat Bahama Bank, i.e. 0.91%, implies the inorganicprecipitationof silt-sizesediments(Millimanet al. 1993). In asubsequent detailed study on the silt fractions (Freile andMilliman 1991), only very coarse and very fine silt matchedthe value ofthe silt size sediments. The mid-sizesilt fractionhave considerablylower Sr value. A similartrend oflower Srcontent in silt compared to clay was observed during thepresent study,where the 15.611m and 7.8 11m fractions have aSrlll1lg0nite values of 0.573% ± 0.04 and 0.53% ± 0.08,respectively, compared with the mud value i.e. 1.038% ±0.176 and the coarse silt (0.653%±0.042). The present studyshowedthatcoarsesiltsizecarbonatesaremostlycomposedofcomminuted pieces of unaltered carbonate or micritizedmarine skeletal debris. On the other hand, the fine and veryfinesilt fractionsaremostlycomposedofauthigeniccarbonateformed in situ and replacing the comminuted pieces ofcarbonate debris. Many of the recognizableskeletalparticlesshow the effect of dissolution and/or replacement byauthigenic carbonate. Authigenic carbonate here is of twotypes: carbonate precipitatedwithin the primary intraskeletaland replacement of skeletal carbonate to constitutecryptocrystalline grains (Fig. 5b). Stieglitz(1972) observedamarked increase in the number of aggregatesin samples< 1611m. Reid et al. (1992)studiedthe origin of the carbonatemudin the northern Belize lagoon, they observed that the silt-sizefraction, consists predominantly of cryptocrystalline grainswith less abundant skeletal fragments at all stages ofalterations. They also foundthat clay size« 4 11m) sedimentsconsist predominantly of clumps (1-4 11m) of fme anhedralequatecrystals. Theseclumps,are moreor lesssimilarto thoseobserved for the studied samples from the Red Sea.

In general aragonitein the Red Sea samplesconsists less than60% of the total coarse silt and almost 25% of the carbonatemud. Accordingly Mg-calcite should answer the questionabout the origin of silt size carbonate sedimentsin the studiedsamples. According to Milliman et al. (1969), in the coarsegrained (> 63 11m) Red Sea sediments, aragonite is commonand calcite is relativelyminor;whereas in sedimentsfiner than63 11m, aragoniteis rarelypresentand calcitecontributesaboutone-half of the carbonate. The unusual characteristic of theRed Sea lutite,however is the dominanceofMg-calcite (12 ±1.4 mol% MgC0

3) .

The relationship between total Mg-calcite and the mean size(rp) ofthe reefalsedimentsfromthe studyareais shownin (Fig.6). Themean sizeofthe sedimentsarerelatedto theMg-calcitecontents in the Red Sea samples by the followingequation:

Mz (tp)= -2.485 + 0.141 Mg-Calcite (r= -0.7669)

Ingeneral thereis a trendtowardincreasingtheamountofMgcalcitewith decreasingthe sizeofthe sediments. AccordingtoReid et al. (1992), the distributionof'Mg-calcitein the various

size fractions of samples from northern Belize shows anincreaseofMg-calcite in clay fractionrelativeto sandfraction.The increase of Mg-calcite content in clay sized fractions isalso perceived during the present study, where Mg-calcitecontent reaches almost 75% of the total carbonate minerals.The general trend of increasing Mg-calcite toward the finesediments may reflect the alteration of all mineral species(aragonite or Mg-calcite) to Mg-calcite during themicritization process. EDX analysis shows that boring andskeletalcavitiesare filled with micriticMg-calcite ratherthanaragonite. Alterationof skeletalgrains to Mg-calciteprovidesa counterpart to the Bahamas, where grains are altering toaragonite(FrielandMilliman 1990). This isagreeablewiththework of Reid et al. (1992) on the northern Belize shelf. Theyrecognizeddifferentstages in thetexturalalterationof alltypesof skeletalgrains, regardless of the original mineralogy. Theinitial stage is recrystallization of laths and needles inforaminifera, where microboring by endolithic organismsenhancedtheprocessof recrystallization. A secondstageis thesize reduction of micritized skeletons in some of the openneedle and lath structures of foraminifera. During this stagethe finer secondary crystals coalesce and recrystallize. BothBathurst (1966, 1975) and Alexanderssson (1972), noticedsimilarmicritizationprocesses.

However, it is uncertain that all Mg-calcite fragments in thefinesedimentswere formedby aragoniteor mineralinversion.SomeMg-calcitemay havebeen precipitateddirectlyfromtheseawater. Undoubtedly, high salinityand high temperatureinthe Red Sea permit inorganic precipitation of Mg-calcite.Normal salinity (40 0/00) and normal temperature (> 21°C) aresufficiently high to permit inorganic precipitation of Mgcalcite (Millimanet al. 1969).

Mg ion concentration determined using the ICP "Mg(ICP)"arerelatedto Mg ion concentrationscalculatedfromtheX-raydeterminative curve "Mg(XRD)" by the following equation(Fig. 7):

Mg (ICP) = -0.0402 + 1.182 Mg (XRD) (r = 0.9768)

This highly significant direct relation between Mg(ICP) andMg(XRD) may indicate that Mg-calcite exhibits chemicalhomogeneity i.e. composed entirely of either inorganic orbiogenicMg-calcite. This homogeneitymay be explainedbythe inorganic origin of Mg-calcite rather that the biogenicorigin. This argument is based mainly on the fact that manyalgae are known to contain significantmagnesium (Bischoffet al. 1983). Most of coralline algae contains considerablemagnesiumin their calciticskeletonsthan can beexplainedbytheshift in theirX-ray diffractionpeaks (Millimanetal. 1971).According to Bischoff et al. (1983), errors over 5 mol%MgC0

3can be made in using the determinative curve on

biogenicspecimens. Taking an averageMg-calcitecontentofabout 40% and an error up to 5 mol% MgC03 (in case ofbiogenicorigin),an error up to±0.5% Mg couldbe estimated.Accordingly, an inorganic origin for the Mg-calcite is

43


3.42.82.21.6

Mg % (rep)

1.0

· . ......... " ~ '" ';' "i ..

· .· .· .· .· .· .· .O . .· .......p ~ j ; .

o . :

o ¢.................................... :, ! " _o ~ ;.

: : :· .· .· .· .· . . .· . .............................J ) .;. C?-.& l .

1 ~ CD a l·····························1························) ···············r··?·······················~··········· .

. .. .

2.4

2.0

1.6

1.2

0.8 .

0.4 ~-~--~~~~~----.i. ~_...l....-~~~~--'-_~_~----t

0.4

-C0::><-?f!.0)

:i

Figure 7.Relationship betweenthe calculatedMg contentusingXRD charts and the determinedMg concentrations using thechemicalanalysis(ICP).

assumed. This inorganic origin could indicate either directprecipitation or mineral inversion.

In this study micritization ofskeletal grains is considered to bean important source for clay-size fraction. Consequently Mgcalcite in the present samples is either skeletal, inorganicallyprecipitated, or cryptocrystalline. These grains will be thefundamental source of mud especially the clay-size fraction.Reid et al. (1992) indicated the importance ofcryptocrystallineand micritization to Mg-calcite as a source of lime mud inBelize and they related the contrast from the Bahamas and theArabian Gulf, to the chemical alteration ofcarbonate grains inthese areas. Inorganically precipitated carbonates mayaccount for more than half of the deep-sea carbonates in thesouthern Red Sea (Milliman et al. 1969).

incorporates mostly of skeletal aragonite. The amount ofinorganic aragonite in different silt fractions (i.e. coarse,medium and fine silt) is almost similar. The distinction inSraragonitepercentage is mainly due to the difference in the ratioof skeletal to nonskeletal ingredients.

Mg-calcite is more important in the explanation ofthe origin ofthe silt fraction. Micritization ofskeletal grains is an importantsource for carbonate mud. Itfollows that Mg-calcite in the RedSea reefal sediments as either skeletal or cryptocrystallinelumps will be the principal source ofcarbonate mud, especiallythe clay size fractions. Carbonate mud from the mineralogicpoint of view is consistent with Belize carbonate mud andadverse to Bahamas and Arabian Gulf carbonate sediments.

ACKNOWLEDGMENTSCONCLUSIONS

The present study on silt size carbonate sediments in thenorthern Red Sea suggests that the clay-size carbonate mud «4 urn) in this area is mostly authigenic. Aragonitic mud in thenorthern Red Sea represents only 25% of the total carbonate.The medium silt fractions with lower aragonitic values couldbe attributed to the result of a breakdown ofskeletal grains aswell as the micritized skeletal grains, in addition to theovergrowth of carbonate mud and the internal micritizationwhich occur inside the cavities of the grains. The coarse silt

The XRD and SEM analysis were both done at USGS atWoods Hole Oceanographic Institution. The chemicalanalysis was done at Boston University. Acknowledgmentsare due to Prof. Mahmoud Kh. El-Sayed, Prof. Mohammed A.El-Sabrouti and Prof. John D. Milliman for their valuablediscussion and comments.

REFERENCES

ALEXANDERSSON, T., 1972,Micritizationofcarbonateparticles:

44

EL-SAMMAK

processes ofprecipitation and dissolution in modem shallowmarine sediments: Bull. GeoL Instn. Univ. Upsal. N.S. v. 3, p.201-236.

BATHURST, R.G.C., 1966, Boring Algae, micrite envelopes andlithification ofmolluscan biospharites: GeologicalJournal, v.5, p. 15-32.

BATHURST, RG.C., 1975, Carbonate sediments and theirdiagenesis. Developments in Sedimentology, v. 12. (2ndedition), Elsevier Publishing Co., Amsterdam, 658 p.

BISCOFF, W.D., BISHOP, F.e., and MACKENIZIE, F.T., 1983,Biogenically produced magnesian calcite; inhomogeneties inchemical and physical properties; comparison with syntheticphase: American Mineralogist, v. 68, p. 1183-1188.

ELLIS, J.P. and MILLIMAN, J.D., 1985, Calcium carbonatesuspended in Arabian Gulfand Red Sea waters: Biogenic andDetrital, Not "Chemogenic": Journal of SedimentaryPetrology, v. 55, p. 805-808.

EL-SAMMAK, A.A., EL-SAYEO, M.KH., and EL-SABROUTI,M.A., 1997, Analysis of reef sedimentation in Hurghada(Egypt), Northern Red Sea: Carbonates and Evaporites, v. 12,p.254-263.

FRIEDMAN, G.M., 1968, Geology and geochemistry of reefs,carbonate sediments, and waters, Gulf of Aqaba, Red Sea:Journal ofSedimentary Petrology, v. 38, p. 895-919.

FRIEDMAN, G.M., 1985, The term micrite or micritic cement is acontradiction - discussion ofmicrite cement inmicroborings isnot necessarily a shallow-water indicator: Journal ofSedimentary Petrology, v. 55, p. 777.

FREILE, D. and MILLIMAN, J.D., 1991, Nature and Genesis ofsiltsize carbonate, Great Bahama Bank. GSA Meeting,Baltimore.

GAVISH, E. and FRIEDMAN, G.M., 1973.Quantitative analysis ofcalcite and Mg-calcite by X-ray diffraction: effect ofgrindingon peak height and peak area: Sedimentology, v. 20, p. 437444.

LOREAU, J.P., 1982, Sediments aragonitiques et leur genese: Paris,Museumd'Histoire Naturelle, Memoires,SerieC, v. 47, 300p.

MATTHEWS, RK., 1966, Genesis ofRecent lime mud in southernBritish Hondruras: Journal ofSedimentary Petrology, v. 37, p.428-454.

MILLIMAN, J.D., 1974, Marine carbonates. Springer-Verlag, 375 p.

45

MILLIMAN, J.D. and BORNHOLD, B.D., 1973, Peak height versuspeak intensity analysis ofX-ray diffraction data: Sedimentology, v. 20, p. 445-448.

MILLIMAN, J.D., ROSS, D.A., and KU, T.H., 1969, Precipitationand lithification of Deep Sea carbonate in the Red Sea:.Journal ofSedimentary Petrology, v. 39, p. 724-736.

MILLIMAN, lD., GASTNER, M., and MULLER, J., 1971,Utilization of magnesium in coralline algae: GeologicalSociety ofAmerica Bulletin, v. 82, p. 573-580.

MILLIMAN, J.D., FREILE, D., STEINEN, RP., and WILBER,J.R, 1993, Great Bahama Bank aragonitic muds: mostlyinorganically precipitated, mostly exported: Journal ofSedimentary Petrology, v. 63, p. 589-595.

MORSE, J.W. and MACKENZIE, F.T., 1990, Geochemistry ofsedimentary carbonates. Developments in sedimentology. v.48. Elsevier Publishing, Co., Amsterdam-Oxford-New YorkTokyo, 707 p.

PROKOPOVICH,N.,1958,SettlingtubeforMechanicalanalysisbydecantation: Journal ofSedimentary Petrology, v. 28, p. 508.

REID, R.P., MACINTYRE, I.G., and JAMES, N.P., 1990, Internalprecipitation of microcrystalline carbonate: a fundamentalproblem for sedimentologists: Sedimentary Geology, Y. 68, p.163-170.

REID, RP., MACINTYRE, I.G., and POST, J.E., 1992, Micritizedskeletal grains in northern Belize lagoon: A major source ofMg-calcite mud: Journal ofSedimentary Petrology, v. 62, p.145-156.

SHINN, EA, STEINEN, RP., LIDZ, B.H., and SWART, P.K.,1989, Whitings, A sedimentological Dilemma: Journal ofSedimentary Petrology, v. 59, p. 147-161.

STIEGLITZ, R.D., 1972, Scanning electron microscopy of the finefraction ofrecent carbonate sediments from Bimini, Bahamas:Journal ofSedimentary Petrology, v. 42, p. 211-226.

TUCKER, M.E. and WRIGHT, V.P., 1990, Carbonate sedimentology. Blackwell Scientific Publications, Oxford-LondonBoston, 482 p.

Received: June 2, 2000Accepted: October29,2000

Documents

Nature and genesis of silt-size carbonate sediments; Northern Red Sea, Egypt