21. DOLOMITIC BASAL SEDIMENTS FROM NORTHERN END OF NINETYEAST RIDGE

Christopher C. von der Borch, Flinders Institute for Atmospheric and Marine Sciences, Flinders University, South Australiaand

Norman A. Trueman, Mines Exploration Pty. Ltd., Pasadena, South Australia

ABSTRACTA variety of dolomitization textures occurs within the lower

64 meters of Deep Sea Drilling Project Site 217, situated on thecrest of the northern end of Ninety east Ridge. Immediately abovethe dolomitized zones near the bottom of the column and belowpelagic sediments are strata characterized by layers ofundolomitized gravel-sized bioclastic debris interbedded withbiomicrite beds. This sediment type has shallow off-reef affinities.Below this level, saccharoidal dolomite layers occur with rhombstypified by one dolomite overgrowth around corroded sparrycalcite nuclei. These are interbedded with silicified biomicrites.At the deepest cored levels similar layers of dolomite arecomposed of rhombs showing one to two calcite zones alternatingwith one to two dolomite zones around corroded dolomitenuclei. The sequential variation in dolomitization texture withdepth, along with independent evidence for occasional shallowwater to subaerial conditions, provides good evidence for originof the dolomite by dolomitization of back-reef sediments. Themechanism which best fits the facts is one of seepage refluxion ofhigh Mg/Ca solutions from a periodically desiccating lagoon.

INTRODUCTION

Dolomitized bioclastic limestones of coral reef, fore reef,and lagoonal environments are relatively widespread in theiroccurrence. Ancient examples from continental areas aretypified by the Permian reef complex of the GuadalupeMountains in the United States (Newell et al., 1953).Comparable examples in an oceanic environment have beendescribed from Guam (Schlanger, 1964) and from drillholes in mid-Pacific atolls such as Eniwetok, Funafuti,Kita-daitojima, Bikini, and Midway (Schlanger, 1963; Laddet al., 1970), where dolomites are of lower Tertiary age andappear to replace coarse bioclastic reef debris. Somewhatsimilar dolomitized limestones, in this case lime sands(calcarenites) of Pleistocene age, have been described fromthe Island of Bonaire in the Netherlands Antilles (Deffeyeset al., 1964).

The origin of dolomitizing fluids in the above instancesis of geological significance. Weight of opinion appears tofavor dolomitization of pre-existing calcareous material bysome form of seepage refluxion through the sediments ofhypersaline brines. The Mg/Ca values of the percolatingsolutions are postulated to have been enhanced byevaporation and consequent gypsum or aragonite precipita-tion in a lagoonal environment (Newell et al., 1953; Adamsand Rhodes, 1960). This interpretation is strengthened bythe observation that oxygen isotopes occurring withinTertiary dolomites from mid-Pacific atolls indicate an originfrom evaporated, isotopically heavy seawater (Berner,1965) rather than from extended contact with pore watersof normal seawater composition.

Drilling on the crest of the northern end of theNinetyeast Ridge during Leg 22 of the Deep Sea DrillingProject sampled a shallow-water sequence of interbeddedcherts, silicified biomicrites, and saccharoidal dolomites.These sediments, of Campanian age, were drilled at Site 217(Figure 1). They occur near the base of the sedimentcolumn and are overlain by approximately 600 meters ofmostly oceanic pelagic nannofossil ooze which ranges in agefrom Campanian to Quaternary. The sedimentary succes-sion and biostratigraphy indicate that the basement ofNinetyeast Ridge stood at or near sea level in the LateCretaceous and subsequently subsided to its present depthof 3010 meters at Site 217 (The Scientific's Staff, Leg 22,1972). The replacement dolomites from the basal section ofthis site, which will be described in detail below, closelyresemble the /dolomitized sequences from Guam and thePacific atolls. In addition they contain textural evidencewhich is in accord with an origin by seepage refluxiondolomitization of bioclastic reef debris.

STRATIGRAPHY AND SEDIMENTOLOGY

The sediment column in Holes 217 and 217A waspenetrated to a depth of 664 meters, with the deepestsampled sediments being at least as old as Campanian.Increasing proportions of chert down the hole causedcurtailment of drilling before igneous rocks were reached;however, seismic evidence suggests that the deepestrecovered sediments lie in close proximity to basalt.

The stratigraphy and biostratigraphy of the site aredescribed in detail in the site report (Chapter 8, thisvolume). Briefly, upper sections of the column comprise

477

C. C. VON DER BORCH, N. A. TRUEMAN

90° 100°

100c

locr

Figure 1. Location of Deep Sea Drilling Project Site 217 oncrest on Ninetyeast Ridge at its northern extremity.Water depth is 3010 meters.

pelagic nannofossil oozes and chalks ranging in age fromQuaternary to mid Eocene. Below this, at a sediment depthof about 370 meters; thin chert stringers of Paleocene ageappear, interbedded with micrites and shelly micrites. Amolluscan fauna is associated with these lower sediments,including Inoceramus sp. and oysters. This, along withassociated glauconite, suggests a relatively shallow-waterenvironment. Similar carbonate sediments without inter-bedded cherts occur below this zone to a depth of about550 meters, where chert stringers reappear in earlyMaastrichtian to late Campanian sediments. Below about600 meters and extending down to the bottom of the holeat 664 meters, an interbedded sequence of saccharoidaldolomites, cherts, and silicified biomicrites appears,associated with a possible mud-cracked layer near the basewhich suggests subaerial exposure. Core recovery in thelower 110 meters is shown in Figure 2.

A typical 150-cm core section from the dolomite-chert-silicified micrite zone is shown in detail in Figure 3 toillustrate the interbedded nature of these lithotypes.Statistics compiled from the entire dolomitic zone indicatethat dolomite beds vary from 2 to 50 cm in thickness andaverage 12 cm, while cherts vary from whisps to 10 cm,averaging about 5 cm.

As indicated in Figure 3, contacts between dolomitezones and silicified micrites vary from sharp to diffuse andgradational. The dolomites, which appear to have replacedcoarse-grained layers of gravel-sized bioclastic debris,occasionally preserve relict sedimentary structures whichare clearly predolomite in origin and which includecross-stratification and bioturbation. Evidence of bioturba-tion is also widespread in the interspersed fine-grainednondolomitic layers.

A plot of percent dolomite versus chert for each 150-cmcore section is shown in Figure 4. There are two zones

METERS

550-1

570^

590-

610-

630-

650-

Y/////Z

y///////.

o o<->Z

32

33

34

37

T. D.

664 m

STAGE 3

Figure 2. Summary stratigraphic column of the basalsediments of Holes 217 and 217A. Shaded zones on thecolumn represent recovered material, the remainder be-ing voids. Line drawings (not to scale) of repres-entative textures are shown, based on thin-sectionpetrography. These are grouped as: Stage 1, bioclasticsparry calcite debris with micritic matrix from abovedolomitized zone. Stage 2, dolomite rhombs ofuppermost dolomitized zone showing an outer dolomiteovergrowth over a rounded calcite core of preexistingbioclastic material. Dolomite rhombs with no calcitenuclei are also present at this level. Stage 3, dolomiterhombs of deepest sampled interval showing one to twocalcite zones and one to two dolomite zonesovergrowing corroded dolomite nuclei.

478

DOLOMITIC BASAL SEDIMENTS

627m

50-

100-

DoiareniteFine-grained saccharoidal dolomite rhombs,0.1 to 0.2 trm across, dominate these zones.Contacts with silicified biomicrite zonesmay be sharp or gradational. Some layersshow suggestions of current cross-lamination.

ChertOlive-black. Composition quartz. Appears to bereplacing s i l i c i f i e d biomicrite.

Si l ic i f ied biomicriteDominantly Cristobalite-tridymite. Containsghosts of microfossils, some of which s t i l lretain some original CaC03.

628.5 m

Figure 3. Lithology of a typical core section from theCampanian dolomitized basal zone of Hole 217A (14-2).The alternation of beds rich in sugary dolomite rhombswith fine-grained silicified biomicrites and olive blackchert layers is apparent.

which are particularly rich in dolomite layers, with theamount of chert appearing to covary with the dolomite.

MICROSCOPIC TEXTURES

A total of 20 microscopic thin sections of thedolomitized basal sediments was prepared. Before cement-ing cover slips onto slides, the sections were stained withalizarin red solution in order to distinguish between calciteand dolomite. Selected photomicrographs are illustrated inFigures 5 and 6.

Studies of mineralogy and texture of thin sections ofsediments immediately overlying the dolomites and thedolomite-rich layers themselves reveal a vertical sequence indegree and texture of dolomitization, summarized as Stages1,2, and 3 in Figure 2.

Stage 1, exemplified by Figures 5a and b, is typified byundolomitized coarse bioclastic debris. Recrystallizationmakes it difficult to identify the original material, although

METERS

0 0 0

610

620 -

630 -

640 -

650 -

600

DOLOMITE

50 100 °/β

Figure 4. Graph of percent dolomite and chert versusdepth in hole. The percentages were calculated bysumming up for each 150-cm core section the lengthsrepresenting dolomite and chert and recalculating this topercentage of length of a particular core section. Thegraph shows a covariance between dolomitization andchertification, suggesting a genetic relationship. The twopeaks in dolomite content may represent an abundanceof suitable host beds for dolomitization at these twolevels.

fragments of coral (Figure 5a and 5b) and calcareous algaecan be recognized with confidence. In all cases thebioclastic fragments are composed of sparry calcite and areset in a calcite-micrite matrix. This lime mud also infillsinterseptal spaces in the corals.

Stage 2, illustrated in Figures 5c and 5d, occurs at agreater depth in the sediment than Stage 1. Textures consisteither of an interlocking mosaic of euhedral to subhedraldolomite rhombs with rounded sparry calcite cores (Figure5c) or in some layers of euhedral rhombs composed entirelyof dolomite (Figure 5d). The poly crystalline calcite cores inFigure 5c may be relict bioclastic material and mayoriginally have been aragonitic. In both of these cases thematrix between the rhombs is isotropic silica.

Stage 3, which occurs below Stages 1 and 2 in thesediment column, is shown in Figures 6a, b, and c. At thislevel dolomite rhombs are notably zoned, with somewhatcorroded dolomite cores surrounded by one (Figure 6a) ortwo (Figures 6b and 6c) zones of calcite alternating withone or two zones of dolomite. Matrix between the rhombsis generally isotropic silica as in Stage 2. At this lowest levelin the column any evidence of former bioclastic debris hasbeen obliterated by the growth of the dolomite rhombs.

479

J00

opp

OzσmjoCO

OJO

n

Figure 5. ^y .Samp/e 217A-12-1, 78 cm, coarse biocalstic debris (sparry calcite) of Stage 1 (Figure 2) associated with micritic calcite matrix. Material similar to this ispresumably host to dolomitization at depth, (b) Sample 217A-12-1, 78 cm. Fragment of possible coral, composed of calcite. Equivalent to Stage 1 (Figure 2).(c) Sample 217-13-1, ? cm. Close-packed mosaic of dolomite rhombs of Stage 2 (Figure 2). Sparry calcite cores, representing relict bioclastic calcite, show asrounded dark centers of rhombs in this stained slide. Rare matrix probably cristobalite. Fragment of Inoceramus to right center, (d) Sample 217A-12-1,140 cm.Close-packed mosaic of interpenetrating dolomite rhombs of Stage 2 (Figure 2) without nuclei. Brown isotropic matrix probably cristobalite.

DOLOMITIC BASAL SEDIMENTS

(c)Figure 6. (a) Sample 217A-17-1, 83 cm. Euhedral dolomite rhombs of Stage 3 (Figure 2) showing dolomite

cores followed by a calcite (stained) and dolomite zone. Matrix possibly cristobalite. Innermost calcitezone appears to be corroding dolomite core. Calcite zone retouched in black, (b) Sample 217A-17-1, 65cm. Euhedral dolomite rhombs of Stage 3 (Figure 2) showing corroded dolomite nuclei, followed bycalcite-dolomite-calcite-dolomite zones. Slide stained, calcite zones retouched in black, (c) Sample217A-17-1, 65 cm. Similar to (b).

481

C. C. VON DER BORCH, N. A. TRUEMAN

X-RAY MINERALOGY

Table 1 illustrates the mineralogical composition ofsamples from the dolomite-chert-silicified micrite sequence.

It is apparent that the dolomites are either dominantlycomposed of the mineral dolomite or of subequal portionsof stoichiometric dolomite and calcite. Accessory minerals,possibly representing the groundmass between the rhombs,are generally cristobalite and tridymite. The interbeddedfine-grained sediments, the silicified biomicrites, aredominantly cristobalite and tridymite and may represent anearly stage in the process of chertification. The olive blackcherts are dominantly quartz in composition, associatedwith traces of calcite which may represent relict calcareousmicrofossils.

INTERPRETATION

The presence of coarse, occasionally current-bedded,bioclastic debris and interbedded biomicrites of Stage 1(Figure 2) suggests that the basal portion of the sedimentcolumn was originally a reef talus, possibly formed in ashallow off-reef environment. The occurrence of a possiblemud crack near the base of the sediment column indicatesperiodic subaerial exposure as would be expected duringperiodic desiccation in a lagoon.

Stage 2 textures (Figure 2), which underlie Stage 1, aretypified by almost complete obliteration of original formsby dolomitization. Textures at this level consist in somelayers of rhombs made up entirely of dolomite, and inothers, of single euhedral overgrowths of dolomite overrelict corroded calcite nuclei. A single episode ofdolomitization is suggested at this depth by these textures,with sparry calcite nuclei representing the remnants ofbioclastic debris similar to Stage 1. The dolomite rhombswithout calcite cores may represent complete dolomitiza-tion with consequent obliteration of preexisting nuclei, orthey could represent rhombs which nucleated originally asdolomite.

Stage 3 textures (Figure 2), which occur at the deepestlevels, consist of corroded dolomite nuclei, overgrown byone or two zones of calcite alternating with one or twozones of dolomite. This suggests two to three episodes ofdolomite growth separated by periods conducive to calciteformation.

If the sampling is representative of the lithologies, thenthe zoned textures of the dolomite rhombs indicate thatdeepest sediments have been subjected to more episodes ofdolomitization than overlying sediments. This is in accordwith an origin of the dolomite by periodic seepagerefluxion of high Mg/Ca solutions during or soon aftersedimentation. Climatic variations or oscillations in relativesea level could cause periodic hypersalinity of the lagoon atwhich times dolomitizing fluids would be produced. Thefact that deepest sediments record two or even threeperiods of dolomitization while shallower sediments onlyrecord one argues for dolomitization during building up ofthe sediment column. The earliest phase of dolomitization,perhaps due to an initial phase of lagoonal hypersalinity,produced the dolomite nuclei of Stage 3 textures beforemuch of the overlying sediments were deposited. This wasfollowed by further sedimentation of reef debris andinterbedded biomicrites during which time a calciteovergrowth formed over the early-formed dolomite rhombsunder the influence of normal seawater pore solutions. Asubsequent episode of lagoonal hypersalinity producedanother influx of dolomitizing pore solutions, causingcalcite fragments from younger sediments to be overgrownby dolomite and adding a second dolomite overgrowth tothe deeper, older dolomite rhombs. This process may havebeen repeated three times before the crest of NinetyeastRidge finally sank below sea level with consequentdeposition of oceanic nannofossil ooze which has persistedto the present. The possibility should also be stated that thecalcite zones of the rhombs could have formed duringperiods of emergence of the island, with subaerial solution

TABLE 1Semi-quantitative X-ray Diffraction Analyses of Selected Samples of Dolomites, Fine-grained

Biomicrites, and Silicified Biomicrites

Description Quartz Calcite Dolomite Montomorillonite Clinoptilolite Cristobalite Tridymite

217A-12,CC

217A-12-l,88cm

217A-14-2, 12 cm

217A-14-2, 33 cm

217A-14-2, 90 cm

217A-17-1,83 cm

217A-17-l,83cm

217A-17-1, 101 cm

217A-17-1, 128 cm

Dolomitized Zone

Biomicrite

Silicified Biomicrite

Dolomitized zone

Silicified Biomicrite

Dolomitized zone

Silicified Biomicrite

Silicified biomicrite

Silicified biomicrite

FTr

Tr

Tr

Tr

Tr

Ac

Tr

Tr

Tr

CD

D

Tr

-

Tr

CD

Tr

SD_

CD

-

-

D

-

CD

-

Tr

Ac

D

Tr

AcCD

Ac

CD

Ac

D

Tr

D

AcCD

-

CD

Tr

SD

Tr

SD

Legend: D = Dominant (> 50%)CD = Codominant (subequal abundance of major components)SD = Subdominant (20%-50%)Ac = Accessory (5%-20%)Tr = Trace « 5%)FTr = Faint trace « 1 % )

482

DOLOMITIC BASAL SEDIMENTS

of limestones providing downward-percolating CaC03-richsolutions.

Zoned, rhombic carbonates were reported by Schlanger(1964) in lagoonal sediments from the Island of Guam.Episodic crystallization of dolomite and calcite zonesindicated stages of postdepositional uplift and submergencebefore final emergence and cementation. On ChristmasIsland, one of the few oceanic islands in the eastern IndianOcean (10°30'S, 105°40'E), zoned dolomitic carbonaterocks occur in the depressed central area of the upperplateau (Trueman, 1965). The fauna and textures of theChristmas Island carbonate rocks allow reasonable faciesreconstruction in terms of reef limestone complexes andthe dolomitic sparry carbonates represent original lagoonalenvironments in which dolomitization occurred duringperiodic emergency and submergence.

There are, therefore, close similarities between the formof the dolomitic carbonates from Site 217 and those ofback-reef carbonates reported from islands in the Indianand Pacific oceans.

ACKNOWLEDGMENTS

This study benefitted greatly from the meticulousattention to detail in the shipboard core logs by Drs. R. W.Thompson and A. C. Pimm. Ian Dyson, of FlindersUniversity, prepared the excellent set of microscope thinsections. Use of the X-ray diffraction equipment of

Australian Mineral Development Laboratories is gratefullyacknowledged.

REFERENCES

Adams, J. E. and Rhodes, M. L., 1960. Dolomitization byseepage refluxion: Am. Assoc. Petrol. Geol. Bull., v. 44,p. 1912-1920.

Berner, R. A., 1965. Dolomitization of the Mid-PacificAtolls: Science, v. 147, p. 1297-1299.

Deffeyes, K. S., Lucia, F. J., and Weyl, P. K., 1964.Dolomitization: Observations on the Island of Bonaire,Netherlands Antilles: Science, v. 143, p. 678-679.

Ladd, H. S., Tracy, J. I., Jr., and Grant Gross, M., 1970.Deep drilling on Midway Atoll: U. S. Geol. Surv. Prof.Paper 680-A.

Newell, N. D., Rigby, J. K., Fischer, A. G., Whitemen, A. J.,Hickox, J. E., and Bradley, J. S., 1953. The Permian reefcomplex of the Guadalupe Mountains region, Texas andNew Mexico—A study in paleoecology: San Francisco(Freeman).

Schlanger, S. O., 1963. Subsurface geology of EniwetokAtoll. U.S. Geol. Surv. Prof. Paper 260-BB, p.991-1066.

., 1964. Petrology of the limestones of Guam: U. S.Geol. Surv. Prof. Paper 403-D.

The Scientific Staff, 1972. Deep Sea Drilling Project, Leg22: Geotimes, v. 17, p. 15-17.

Trueman, N. A., 1965. The phosphate, volcanic andcarbonate rocks of Christmas Island, Indian Ocean: J.Geol. Soc. Aust., v. 12, p. 261-283.

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