Early mixed-water dolomitization in the Pleistocene reef limestones, west coast of Saudi Arabia

Sedimentary Geology, 53 (1987) 231-245

231

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

EARLY MIXED-WATER DOLOMITIZATION IN THE PLEISTOCENE REEF LIMESTONES, WEST COAST OF SAUDI ARABIA

N.V.N. DURGAPRASADA RAO, O.A.O. AL-IMAM and A.K.A. BEHAIRY

Department of Marine Geology, Faculty of Marine Science, King Abdulaziz University, P.O. Box 1540, Jeddah (Saudi Arabia) (Received April 10, 1986; revised and accepted February 26, 1987)

ABSTRACT Durgaprasada Rao, N.V.N., Al-Imam, O,A.O. and Behairy, A.K.A., 1987. Early mixed-water dolomitization in the Pleistocene reef limestones, west coast of Saudi Arabia. Sediment. Geol., 53: 231-245. Raised Pleistocene reef limestone characterizes the western coastal plain of Jeddah on the eastern margin of the Red Sea. Soon after the Mid-Pleistocene regression, the subaerially exposed limestone was subjected to meteoric processes during which Mg-calcite was converted to stable low Mg-calcite and partial or complete dissolution of aragonite occurred at various depths. Early meteoric diagenesis through dissolution-precipitation processes had produced sparry calcite in the voids formed by the dissolution of aragonite in the limestone. Following the late Pleistocene-Holocene marine transgression, dolomitization was initiated in the reef in a meteoric-marine water mixing zone. Lack of correlation between dolomite and the evaporite minerals and the low Sr concentrations argue against hypersaline solutions as agents of dolomitization. Mineralogical and chemical data suggest that most diagenetic dolomite is formed at the expense of primary aragonite. Vertical and lateral variations in the distribution of dolomite, aragonite and calcite indicate that dolomitization processes have been affected by the fluctuating mixed-water zone associated with sea-level oscillations. Holocene rise in the sea level terminated dolomitization in the lower layers and shifted the dolomitizing front to the upper sections of the limestone. Dolomite is low in the upper horizons of the reef and occurs as scattered perfect rhombs, while in the lower layers it is fine-grained and subhedral.

INTRODUCTION

The coastal plain of Jeddah on the eastern Red Sea is characterized by pediments, terraces, wadi alluvia, eolian sands, sabkha deposits and coral limestone of Quaternary age. The reefal limestone, mostly occupying the western coastal plain, was deposited during a major transgression of the Red Sea in the early Pleistocene. Behairy (1983) reports that the coastal carbonates were formed by a series of transgressive and regressive cycles, in which the early Pleistocene transgression was the most extensive penetrating to a maximum distance inland, while each successive transgression penetrated less deeply inland than the preceding one. As lithostrati0037-0738/87/$03.50

© 1987 Elsevier Science Publishers B.V.

232 graphic studies are lacking, it is difficult to evaluate the exact age and depositional history of the limestones. However, some shells and coral (Porites sp.) from the highest limestone terrace near Sharm Abhur gave an age of about 41,000 and 43,200 years B.P., respectively (Hacker et al., 1984). Though there are indications of large-scale uplift of the coastal plain in the northern Saudi Arabia due to Quaternary tectonic movements (Skipwith, 1973), evidence is lacking for such extensive tectonic activity and associated uplift of the coastal plain in Jeddah region. All along the west coast of Saudi Arabia, the coastal plain was cut through by numerous drowned

Fig. 1. Geologyof the study area and locationof the cores.

233 Core

P s°cenereef limestone E

resent sea ~ter Jeve

Fig. 2. Schematic diagram of the littoral face of the reef limestone near the study area. Numbers 1-6 represent sample locations.

estuaries or Sharms, which extend onto the inner part of the shelf. One such sharm, Sharm Abhur, is located in the northern Jeddah (Fig. 1) and extends for about 10 km inland through the coral limestone. Near the Faculty of Marine Science situated on the banks of the Sharm Abhur, the coral limestone occurs as a raised terrace about 3 m high from the present sea level in the Sharm (Fig. 2). F r o m this raised reefal limestone, two cores were collected and the diagenetic trends in the carbonates were studied. Facies patterns of the Pleistocene limestones were studied by A1-Sayari et al. (1984) in the northwestern Saudi Arabia along the coast of Gulf of Aqaba. The raised coastal plain reefs exhibit the same lateral zonational patterns as the modern reefs in the eastern Red Sea (Mergner and Schuhmacher, 1974). However, vertical facies development differs considerably from place to place. While five characteristic facies occur in the Jordanian coast--(1) micro atoll zone; (2) seagrass zone; (3) coral rock zone; (4) bioturbated layer with Callianassa burrows; and (5) reef crest z o n e - - i n the south only few facies are represented in the raised reefs. The coastal plain reef limestones at Abhur do not show the various types of facies as seen in the coast of Gulf of Aqaba, but are characterized by the well-cemented crest facies. PREVIOUS STUDIES Diagenetic trends in the reef limestone in the Jeddah coastal plain have been described by Behairy (1980) and Durgaprasada Rao and Behairy (1982). In contrast to the modern carbonates in the marginal Red Sea, which are composed of high Mg-calcite and aragonite (Behairy and E1-Sayed, 1984; Durgaprasada Rao and Behairy, 1986), the Pleistocene coastal plain carbonates are composed of abundant

234 low Mg-calcite and minor aragonite. A progressive decrease of aragonite inland from the present coastline due to increasing meteoric diagenesis has also been recognised (Behairy, 1980). While investigating the diagenetic processes in the reef limestone in the coastal plain of northwestern Saudi Arabia, Dullo and Jado (1984) report various stages of aragonite alteration to calcite. The limestone terraces in the coastal plain of Gulf of Aqaba show more or less the same degree of diagenetic alteration in different facies due to aggressive diagenetic environment (A1-Sayari et al., 1984). No dolomite was recorded in any of the above-mentioned studies, which mainly dealt with surface and near-surface limestones. The first report on the occurrence of dolomite in the Quaternary deposits in the west coast of Saudi Arabia was presented by Behalry et al. (1984). In Ras Hatiba lagoon, an evaporite environment in the coastal plain of Jeddah, a thin hard layer covering unconsohdated fine carbonate mud was found to contain dolomite in addition to high Mg-calcite and minor aragonite. This occurrence of dolomite in the hard layers prompted the investigation on the possible dolomitization of the subaerially exposed coastal plain limestones in Jeddah. In this paper we describe heretofore unrecognized dolomites in the Pleistocene reef hmestones on the west coast of Saudi Arabia and interpret them in the context of known diagenetic history. HYDROLOGICAL CONDITIONS The hydrology of the region around Jeddah and its hinterland has been discussed by Hacker and Kollmann (1984). The drainage basins are characterized by numerous narrow and shallow wadis with limited and rather irregular replenishment that results in an ephemeral hydraulic flow system. There is no pubhshed information on the ground-water levels and fluctuations in the coastal plain. However. a recent survey has indicated that the present-day water table is approximately 3 m deep. In the coastal plain, due to proximity to the coast, salt-water intrusion affects the ground water creating an upper zone of mtxing with chlorinity values ranging from fresh to brackish (212-17.800 mg 1-1 C1). The M g / C a ratios of these waters are higher than those in the hinterland attaining values greater than one. Occasionally the Mg concentration reaches as high as 6000 mg 1-1. which is several times higher than normal Red Sea water. SAMPLE COLLECTIONAND METHODS OF STUDY Two bore holes, about 100 m apart, were drilled at Abhur by an engineering firm in order to study some geotechnical properties of the coral limestones in the coastal plain (Fig. 1). Fifty samples from these two cores were obtained by the authors and were used in the present work. Six more samples were collected from the littoral surface of the limestone terrace near the drilling sites (Fig. 2).

235

The carbonate mineralogy of the bulk samples from various depths in the cores was determined by X-ray diffraction analysis using Philips XRD unit equipped with a Ni-filtered Cu radiation source. Low and high Mg-calcite were identified from the shift in the peak value of the (104) calcite reflection (Goldsmith et al,, 1955). Low Mg-calcite is generally defined as containing less than 4 mole% MgCO 3. Relative concentrations of calcite, aragonite and dolomite were estimated using the peak areas of the strongest reflections in the diffraction charts. Carbonate fraction of the samples was extracted with 1M HC1 following the procedure adopted by Robinson (1980) and the solutions were analysed for Mg, Sr and Fe using IL Atomic Absorption Spectrophotometer.

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236

Fresh-broken surfaces of some samples were mounted on SEM stubs, coated with carbon and gold,palladium mixture in an evaporation chamber and were-examined in a JEOL-35 scanning electron microscope. CARBONATE M I N E R A L O G Y

Low Mg-calcite, dolomite and aragonite are the common carbonate minerals in the Abhur reef limestone. Low Mg-calcite commonly occurs both in association with either aragonite a n d / o r dolomite, but monomineralic layers are occasionally encountered. While low Mg-calcite is common in almost all samples; significant downcore distributional variations are observed for dolomite and aragonite. Down-

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237 TABLE 1

Mineralogy, Mg, Sr and Fe concentrations in core II from Abhur reef limestone Depth in core (cm) 10 40 75 115 150 170 195 225 250 270 295 320 370 400 430 455 475 545 575 615 630 645 735 755 780 820 870 900 920 940

Environment

Zone above the dolomitizing front (stage II)

Mineral composition

Low Mg-calcite Aragonite Dolomite

Dolomitizing

front (stage If)

Low Mg-calcite (100%)

Fresh-seawater mixing zone

Low Mg-calcite Dolomite

Zone above the dolomitizing front

Low Mg-calcite Dolomite

(stage I)

Aragonite

Dolomitizing

front (stage I) Fresh-sea-water mixing zone

Low Mg-calcite Dolomite

Mg (%)

Sr (ppm)

Fe (ppm)

0.55 1.20 1.50 0.75 2.90 2.55 0.65 1.40 1.55

1000 850 790 1010 690 760 860 490 750

48 54 52 34 39 92 43 60 72

1.85 0.35 1.35 2.20 1.65 1.50 1.10 0.50

360 180 300 340 250 200 250 240

54 33 30 145 95 113 122 95

1.60 1.50 2.15 2.55

380 430 260 240

61 54 93 74

3.85 3.50 4.30 4.20 4.25 4.35 5.95 7.05 7.20

210 150 170 220 200 240 170 150 160

64 90 100 85 70 67 109 119 189

core variations in the concentrations of low Mg-calcite, aragonite and dolomite in the cores are shown in Figs. 3 and 4. In the top 0.8 m of the core I, dolomite is absent and the sediments are characterized by a calcite-aragonite assemblage. Below 0.8 m, dolomite occurs in association with calcite and aragonite. The same assemblage continues down to 2.8 m, below which aragonite disappears, dolomite is minor to absent and low Mg-calcite is abundant. In core II, aragonite occurs only in the top 2.7 m and in a layer between 5.25 and 6.3 m (Table 1). However, the aragonite content in the lower layers is remarkably smaller than that in the top

238 TABLE 2 Mineralogy of the littoral surface of the raised terrace limestone Sample no.

Depth (cm)

Minerals in order of decreasing abundance

1 (exposed surface) 2 (exposed surface) 3 (sub surface) 4 (exposed surface) 5 (exposed surface) 6 (below seawater)

10 90 100 220 260 280

cal, qtz, fel, hal cal, arag, hal, qtz, fel cal, arag, gyp cal, arag, dol, hal cal, arag, dol, hal Mg-cal, arag, dol

Cal = low Mg-calcite; Mg-cat = high Mg-calcite; arag = aragonite; hal = halite; fel-feldspar; qtz = quartz: gyp = gypsum; dol = dolomite.

section. In the middle of the core between 2.7 and 5 m and from 6.45 m down to the bottom, aragonite is absent and the limestone contains variable proportions of low Mg-calcite and dolomite. The aragonite distribution trend is similar to that recognised in core I, where because of the short core length, only the top cycle is represented. Except in the pure calcitic horizons, dolomite occurs in almost all core II samples but in varying concentrations. Maximum amount of dolomite ( > 25%) occurs in the bottom layers of the core II. In the littoral surface of the raised terrace, halite and traces of gypsum, precipitated from the salt-water spray, occur along with the common carbonate minerals observed in the cores (Table 2). Halite is present in the exposed reef surface, whereas gypsum is identified in the subsurface sample. In the top 1 m low Mg-calcite and aragonite constitute the bulk of the carbonate minerals. In contrast the bottom 2 m of the terrace contain considerable dolomite. In the sample below seawater high Mg-calcite is dominant with minor aragonite and dolomite. The modern shelf sediments and the contemporary cements in the shallow marine environment of Jeddah are typically high Mg-calcite and aragonite, with Mg-calcite commonly the quantitatively dominant phase (Behairy and E1 Sayed, 1984; Durgaprasada Rao and Behairy, 1986). M A G N E S I U M , S T R O N T I U M A N D IRON

In almost all the samples, the dolomite peak position is at 2.90 or 2.92 indicating an excess of about 7.5 mole% CaCO 3 in the dolomite (Goldsmith and Graf, 1958). The concentrations of Mg, Sr and Fe in the bulk samples at various depths in core II are given in Table 1. Mg values vary from a minimum of 0.35 wt.% to a maximum of 7.2 wt.% and show a distinct increasing trend towards the bottom of the core from 4.75 m (Fig. 4). There is a positive correlation between Mg contents and dolomite concentrations. Sr shows an opposite pattern of distribution to that of

239 Mg and exhibits a general upward increasing trend (Fig. 4). Samples with no aragonite have Sr contents less than 250 ppm, whereas those with a high proportion of aragonite contain significantly higher strontium. Such a positive correlation between aragonite and Sr contents is more characteristic in the upper 2.8 m and in a layer between 5.45 and 6.3 m (Fig. 4). Iron concentration is extremely low and shows no discernable vertical variation in the core. SEM OBSERVATIONS Scanning electron micrographs of the limestone samples reveal the diagenetic features produced in the early meteoric process and later dolomitization. In the top layers partially dissolved aragonitic material and the solution channels created by meteoric water are quite common (Fig. 5a and b), While some of the pores developed by the aragonite dissolution are empty, the others are filled with low Mg-calcite, Secondary calcite with pyramidal terminations grown perpendicular to the substrate into the centre of the void is a common feature (Fig. 5c). Besides, some

Fig. 5. (a) Partially dissolved aragonite grain (1.95 m); (b) a solution channel created by the seeping fresh water through an aragonite grain (1.95 m); (c) low Mg-calcite with pyramidal terminations grown perpendicular to the wall into the centre of a solution channel (8.7 m); crystals growingon both sides to fill the cavity; (d) blocky meteoric low Mg-calcite in a void (8.7 m).

240

Fig. 6. (a) Subhedral dolomite rhombs in the upper layers of the limestone(1.95 m); (b) dolomite euhedra m a void (1.95 m); (c) rhombs of dolomite associated with unaltered aragonite needles(6.15 In); (d) finely crystalline dolomite in the lower partsof the limestone(9 m).

of the voids are filled with blocky low Mg-calcite (Fig. 5d). These calcite cements are similar to meteoric low Mg-caleite cement reported from many Quaternary carbonate sequences. In the upper and middle parts of the limestone sequence. dolomite occurs as subhedral and euhedral rhombs (Fig. 6a and b). Within the resubmerged marine interval (8.7 m in core II), dolomite is associated with unaltered needle-shaped aragonite (Fig. 6c). In the lower portions of the limestone, dolomite is fine-grained and anhedral (Fig. 6d). DISCUSSION During the subaerial exposure soon after the eustatic drop in the sea level in the Mid-Pleistocene, the primary Mg-calcite and aragonite in the reef limestone underwent the first phase of diagenesis through early meteoric processes. Such early diagenesis resulted in the leaching of Mg from high Mg-calcite and in the replacement of aragonite in the original sediments by low Mg-calcite through dissolution-precipitation process. Dissolution of aragonite and precipitation of calcite cement has been reported by Friedman (1964), Gavish and Friedman (1969),

241

Friedman and Brenner (1977), Gvirtzman and Friedman (1977) and Buchbinder and Friedman (1980) in the fresh-water diagenesis of coastal carbonates. Partially dissolved aragonitic fragments and the secondary calcite cement filling the voids and solution channels show evidence for such a process in the Abhur limestone. However, monomineralic calcitic layers in between partially altered carbonates and the downcore variations in the abundance of low Mg-calcite suggest differential elimination of aragonite due in part to variations in (1) the availability of meteoric water, (2) lateral and vertical permeability, and (3) the nature of the aragonitic material. Behairy (1980) found that the coastal plain limestones, which are close to the fresh-water recharge areas in the east, were subjected to more vigorous alteration compared to the down flow areas to the west, where aragonite dissolution is slow and incomplete. In the coastal plain of northwestern Saudi Arabia, Dullo and Jado (1984) observed varying degrees of diagenetic alteration of aragonite to calcite in the Pleistocene reef limestones depending on the availability and saturation with meteoric waters and the skeletal composition. Early diagenetic dolomitization in the Abhur reef limestone is indicated by the persistence of aragonite in some layers that are devoid of dolomite and by the aragonite-dolomite association in other layers. Further, dolomitization seems to be confined to unstable aragonite and not to relatively stable low Mg-calcite. Such a process is evidenced by the lack of correlation between low Mg-calcite and dolomite distribution in the cores and by the virtual absence of dolomite in the pure calcitic layer (2.95-3.2 m, Table 1). In the calcite intervals, early meteoric processes have completely altered the unstable minerals and left no aragonite available for dolomitization. Sibley (1980) states that early solution or conversion of aragonite and high Mg-calcite to stable low Mg-calcite could inhibit dolomitization. Experimental results of Gaines (1980) also have shown that under given conditions, aragonite is more readily dolomitized than the low Mg-calcite. However, from the low dolomite concentrations and considering the absence of dolomite in calcitic horizons, a greater diagenetic loss of aragonite appears to have occurred in the early meteoric diagenesis before dolomitization. Two fundamental processes may have led to dolomite formation: (1) an early-stage penecontemporaneous replacement of aragonite by dolomite; and (2) a later-stage diagenetic replacement. Modern environments reveal penecontemporaneous dolomite formation in arid marine marginal areas, sabkhas or supratidal flats (Shinn and Ginsburg, 1964; Gunatilaka et al., 1984). However, these dolomites invariably are associated with evaporites or other sedimentary features characterizing evaporitic environments. Lack of these features in the dolomitized portions of the Abhur reef limestone argues against the penecontemporaneous replacement origin of dolomite. Dolomitization, therefore, is considered to be a post-depositional diagenetic process involving replacement a n d / o r dissolution-precipitation. The concepts and various mechanisms of dolomitization in different environments have been reviewed by Friedman and Sanders (1967) and Zenger and

242 Durtham (1980). Of these models, (1) hypersaline concentration of Mg under evaporative conditions, and (2) mixing of marine and meteoric water, are considered here as possible mechanisms for the formation of dolomite in the Abhur reef limestones. Although hypersaline conditions exist in sabkhas and supratidal environments in the m o d e m coastal plain, the chemical characteristics of the Sharm Abhur waters (Behairy et al., 1983) do not differ greatly from those of the adjacent nearshore Red Sea waters. Furthermore, there are no evaporite minerals associated with dolomite in either of the cores. Even in the samples from the terrace, where evaporation of the salt spray led to the precipitation of halite and gypsum, dolomite is uncommon indicating no correlation between evaporation and the dolomitization process. The excess mole % CaCO3, which is relatively higher than that in the dolomite formed under evaporite conditions (Supko et al., 1974; Patterson and Kinsman, 1982) argue against an evaporitic origin of dolomite. It is inferred therefore, that hypersaline brines were not the dolomitizing fluids in the Abhur limestones. Apparently, the solutions, which led to the dolomite formation in the Abhur reef are interpreted as mixed seawater and meteoric-derived ground waters. Such a mixed-water model explains early diagenetic dolomitization in the Pleistocene limestone without the precxpitation of evaporites. The mixed-water dolomitization model has been discussed by Land (1973), Badiozamani (1973) and Folk and Land (1975). Land (1973) reports diagenesis and dolomitization in the zone of mixing as meteoric water with a high p C O / b e c o m e s mixed with sea water in the Pleistocene limestones. North Jamaica. The same mixing mechanism has been proposed by Badiozamani (1973) to account for the dolomite in the lower Ordovician rocks in Wisconsin. Mixing of fresh and saline waters is an inherent process in the Jeddah coastal plain and the resultant waters, despite their low M g / C a rauos, create a favorable environment for dolomitization due to the continuous supply of Mg derived from seawater. Land (1973) stresses the significance of large supply of Mg ions in the mixing zone rather than a high M g / C a ratio for dolomitization. In the Pleistocene Hope Gate Formation of Jamaica, Land found very finely crystalline micro-sized to sparry dolomite formed by mixed water having M g / C a ratio of about 1, which is comparable to the M g / C a ratio of the ground water-sea water mixture at Abhur. Mineralogical variations with depth suggest that the dolomitization process in the Abhur reef has been affected by the Post-Pleistocene fluctuations in the sea level, which control the location of meteoric-marine water mixing zones. The earlier mixing-zone probably was located below 7 m, where the aragonite was completely altered to dolomite, in contrast to the partial alteration m the section above the mixing zone. Occurrence of minor quantities of aragonite in the section between 5.45 and 6.30 m substantiates such a gradual decrease in the intensity of dolomitization process upward from the mixing zone. With the Holocene rise in the sea level, the mixing zone has been elevated to a higher level terminating the dolomitization in the submerged layers and establishing the dolomitization front in the upper sections

243 of the limestone. The rapid shift of the mixing zone left the unaltered aragonite in the lower layers (5.45-6.30 m) resubmerged in marine environment, where it is less soluble. Possible shifts in the dolomitizing zone due to fluctuations in the fresh-water lens in a tropical climate a n d / o r to the eustatic changes in the sea level were postulated by Wilson (1975). The dolomitization processes in the upper sections of the limestone appear to be uneven, apparently being related to irregularly distributed permeability. While dolomite occurs throughout core II, in core I and in the terrace the top sections are devoid of dolomite. As the chemical and mineralogical characters are almost same, differences in the permeability of the limestone might have caused such localized variation in the dolomitization. McHargue and Price (1982) report that in a marine-meteoric mixing model, dolomitization occurs during either early or late diagenesis depending on the location of the mixing zone and on the permeability controlled fluid flow patterns. CONCLUSIONS With the eustatic drop in sea level, the fringing reefs of the eastern Red Sea were subaerially exposed and subjected to diagenesis. Early fresh-water diagenesis in the Pleistocene coastal plain reefal limestone of Jeddah replaced to a great extent the metastable carbonate minerals by low Mg-calcite. While in-situ conversion of high Mg-calcite is complete, only partial dissolution of aragonite and precipitation of low Mg-calcite occurred in the limestone. With the rise in the sea level, dolomitization was initiated in the meteoric-marine water mixing zone. Lack of correlation between evaporite minerals and dolomite suggests hyposaline dolomitization consistent with the mixing-zone model. The distribution pattern of the carbonate minerals indicates that dolomitization process in the limestone has been affected by fluctuations in the mixed-water zone caused by rising sea level and permeability controlled flow patterns of the dolomitizing fluids. ACKNOWLEDGEMENTS We would like to thank Mr. R.K. Banerjee, Faculty of Science, for his help in the scanning electron microscopic work. Thanks are due to Mr. J.A. Khatwa, Faculty of Science for the chemical analysis, Mr. Sayed Alimuddin, English Language Centre, for the photographic work, Mr. Mir Yakoub Ali Khan, Faculty of Earth Science for draughting the figures and Mr. Shakeel S. Qureshi for typing the manuscript. REFERENCES A1-Sayari, S.S., Dullo, C., Hotzl, H., Jado, A.R. and Zotl, J.G., 1984. The Quaternary along the coast of the Gulf of Aqaba. In: A.R. Jado and J.G. Zotl (Editors), Quaternary Period in Saudi Arabia, Vol. 2. Springer, New York, N.Y., pp. 32-47.

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