Recognition of Diagenetic Dolomite and Chemical Surface Features of the Quartz Grains in Coastal Sabkha
Sediments of the Hypersaline Shuaiba Lagoon, Eastern Red Sea Coast, Saudi Arabia
H.A. AL-WASHMI and A.M. GHEITH
Faculty of Marine Science, King Abdulaziz University, P.O. Box 80207, Jeddah 21589, Saudi Arabia
ABSTRACT. X-ray diffraction analysis and scanning electron micro- scope have been used to study the mineralogy and quartz grain sur- face features of the supratidal and intertidal sabkha sediments of Shuaiba lagoon.
Results of X-ray diffraction analysis of the supratidal sabkha sedi- ments below 60 cm at a location farther inland indicate that diagenetic dolomite is developed at 30 cm depth. Percentage of aragonite varies inversely with that of dolomite indicating that aragonite is consumed in the production of dolomite.
Sabkha quartz grains are characterized by the predominance of chemical surface features which are generally developed by solution and precipitation of recent silica in the near surface diagenetic envi- ronment.
Introduction
There are extensive areas along the eastern Red Sea coast intruded by coastal lagoons and most of them are surrounded by largely developed sabkha. Shuaiba lagoon 80 km south of Jeddah suffered hard ecological conditions and is charac- terized by intensive dry climate. The western and eastern borders of the lagoon are bounded by extensive intertidal and supratidal sabkhas. The lagoon water bodies may continue to move landward by the winds until they are depleted by evaporation and infiltration.
101
Very little has been published on the coastal sabkha deposits on the eastern coastal plain of the Red Sea (El-Sayed, 1987; Behairy et al., 1991; Basyoni, 1997 and Gheith, 1999).
Coastal sabkhas are supratidal surfaces produced by depositional offlap of marine sediments with the addition of aeolian sediments. The associated brines are derived primarly from sea water and partly from the groundwater. Evapora- tion through the surface causes the formation of brines and the precipitation of evaporated minerals.
Progressive evaporation of marine and continental waters drives sabkha pore- fluid to gypsum saturation and leads to the precipitation of aragonite and gyp- sum within lagoon-derived sabkha muds (Mckenzie et al., 1980). This precipita- tion causes a dramatic increase in the molar Mg/Ca ratio of the pore fluids.
These are favorable for the formation of dolomites, either by the replacement of aragonitic sabkha muds (Mckenzie, 1981) or as a direct interstitial precipitate (Hardie, 1987).
The greater meteoric water limits the production and preservation of evapor- ites in a supratidal environmental setting. Therefore, reduced evaporite produc- tion may diminish the dolomitzation potential. The dolomite forming today under surface evaporative marine conditions are generally poorly ordered, con- tain excess calcium and have been termed proto dolomites or calcian dolomites (Gavish, 1980; Land, 1985 and Reeder, 1983). While Hardie, 1987 pointed out that ancient dolomites are generally more ordered and less soluble than their modern, surface formed disordered counterparts.
Protodolomite (diagenentic dolomite) is recorded in several areas; the Arabian Gulf, Gulf of Elat and the Bahamas and lately in the supratidal sabkha sediments of the present study (Deffeyes et al., 1965; Patterson and Kinsman, 1982 and Mcken- zie et al., 1980). The essential aim of the present paper is to shed light on the devel- opment of diagenetic dolomite in the supratidal sabkha sediments of Shuaiba la- goon and to deduce the characteristic surface features in the quartz sand grains.
Study Area
The study area is located on the east and west shores of Shuaiba lagoon and include the intertidal and supratidal sediment (Fig. 1). This area was exposed subaerially for a long period of time and covered by spring or storm tide. The recent carbonate sediments that were deposited in this area appear to be pack- stones with gastropod (cerithides) shells. Shuaiba lagoon, a representative hy- persaline lagoon, is situated between longitude 20º42' to 20º51'N and latitude 39º26' to 39º32'E, and is 80 km south of Jeddah. It consists of two lagoons of about 20 km long and reaches to the maximum of 5 km width. It is connected
FIG. 1. Location map showing the studied samples and sediment mineralogy, Shuaiba lagoon, western coast of Saudi Arabia.
with the Red Sea by small narrow canal probably formed on the continental shelf during the Late Pleistocene after sea regression. The lagoon is separated from the Red Sea by coral reefs sometimes covered with small sand dunes which provide protection from the crashing waves and support the development of mangroves.
Physiography and Hydrography
Shuaiba lagoon experiences hard ecological conditions and is characterized by intensive dry climate. The weather is very hot, averaging to 35ºC tempera- ture during summer months. Daily maximum temperature generally exceeds 40ºC. Rainfall is very erratic and seasonal rains usually occur during the winter months. Tide affected winds drag the lagoon water towards the land until it be- comes depleted by evaporation and infiltration or until the wind diminishes. The western and eastern borders of the lagoon are bounded by extensive intertidal and supratidal sabkhas. Flood water may move as far as 2 km from the intertidal zone. Around the lagoon and farther toward the land the sabkhas are developed.
Toward the sea, there is a common reef terraces, sometimes covered with an as- semblage of small seif dune sands that extend parallel to the coast indicating northly wind direction.
Sample Collection and Analysis
A total of ten sediment samples were collected from the upper 60 cm of the supratidal flat and from 30 cm of the intertidal sabkha sediments around Shuai- ba lagoon through small trenches, in addition to two samples from the lagoon bottom. A small portion of each sample was dried and ground in agate mortar and then passed through 0.063 mm sieve then mounted in aluminium holders.
Total mineral content in each sample was determined by X-ray diffraction method. The height of the principal peak for each mineral present has been measured and the relative percentages of each mineral have been calculated ac- cording to Carver (1971) and Milliman (1974).
The coarse quartz grains were separated also from the studied samples and grain surface features using techniques of Krinsely and Doornkamp (1973). The characteristic features present were identified and photographed with Jeol 50A scanning electron microscope at King Abdulaziz University.
Results and Discussion Bulk Mineralogy of Sabkha Sediments
Trench sites in the supratidal and intertidal flats where sediment mineralogy has been determined are shown in Fig. (1). Mineralogy of the lagoonal surfacial
sediments have been determined to characterize the primary mineralogy of sedi- ments before they become buried and react with the concentrated sabkha brines.
TABLE 1. Relative percentages of bulk minerals in the supratidal, intertidal and bottom sediments of Shuaiba lagoon determined by X-ray diffraction.
S. no. Depth
Aragonite Calcite High
Dolomite Quartz Potash Plagioclas
Gypsum Halite Py.
cm Mg-calcite feldspar feldspar
1 Surface – 5.8 2.2 1.6 15.3 18.1 43.2 10.1 3.6 –
2 30 – – – 77.6 5.9 2.6 10.6 – 3.3 5.9
3 50 9.6 – 27 37 5.6 – 11.8 – 9.3 –
4 60 22.9 40 8.3 6.3 3.3 13.7 – – 5.4 –
5 30 21.3 34.2 36.1 – – – 8.4 – – –
6 30 20.4 37.3 9 – – – 33.3 – – –
7 30 8.1 56.9 – – 12.2 – 22.8 – – –
8 30 4.8 32.2 – – 12.8 31.9 18.3 – – –
9 Bottom 24 21.4 23.5 – 6.4 5.8 8.7 – 9.3 –
10 Bottom 23.3 – 21.2 – 11.5 12.7 26.1 – 5.2 –
Data of the gross mineralogy are determined and given in Fig. (1). Their x- ray diffractograms are shown in Figs. (2 & 3). In general binocular examination of the studied sediments proved that they are mainly composed of biogenous skeletal components mixed with little amount of detrital constituents. The inter- tidal sediments analyzed from Shuaiba lagoon are mainly composed of carbo- nate minerals (aragonite, calcite and high Mg-calcite) and terrigenous constitu- ents (quartz, plagioclase and potash feldspars). It is clearly observed that calcite is a very common mineral in the intertidal zone and nearshore zone (seaward).
This is probably related to the disintegration of emergent coral reef terraces which supported the shore zone of Shuaiba lagoon.
Calcite is the main constituent of the coastal plain sediments (Behairy, 1980) and is the only carbonate mineral in the eolian dust in the coastal area of Jeddah (Behairy, et.al., 1985). High Mg-calcite with a peak at 2.94 A and correspond- ing to 32 mol% MgCO3 (Goldsmith and Graf, 1958) is common only in two samples of the intertidal sediments and is usually attributed to the differences in composition of sediments-producing organic communities. Calcareous red al- gae, benthic forams, echinoids and bryozoans contribute to high Mg-calcite, whereas green algae, mollusks and coral fragments are responsible for arago- nite. In general, there is a marked concentration in the detrital constituents that controlled significant difference in the mineralogy. Quartz and plagioclase feld-
FIG. 2. Typical X-ray diffraction patterns of supratidal and intertidal sediments, Shuaiba lagoon.
FIG. 3. Typical X-ray diffraction patterns of bottom sediments from Shuaiba lagoon.
spar proved that they were derived from the land either by aeolian transporta- tion or brought to the marine environment through ancient wadies. The domi- nance of plagioclase feldspar over the k-feldspar envisages basic volcanic source rocks over felsic rocks.
Results of X-ray diffraction analyses of the supratidal sabkha sediments below 60 cm at location farther inland which is shown in Fig. (1) indicate the development of diagenetic dolomite at 30 cm depth. Its distribution appears to support the idea of Mckenzie et al. (1980) that dolomitization occurs farther in- land in the intermediate sabkha. It is interesting to mention here that the per- centage of aragonite varies inversely with the dolomite indicating that aragonite is consumed in the production of dolomite. Partly dissolved aragonite cerithid shells and molds of cerithids provide direct evidence of aragonite loss. The maximum percentage of diagenetic dolomite in the supratidal flat sediments at this location (Fig. 1) reaches 71% .
Potentially dolomitizing fluids in the sabkha are formed within a narrow zone farther inland where strong wind moves shallow bodies of sea water that are periodically propelled over the lower portions of the supratidal sabkha. As the water bodies move inland, evaporation causes loss of calcium through gypsum precipitation and increase in the mol Mg/mol Ca ratio. The shallow water mass- es continue to move landward until the water volume is depleted by evaporation and infiltration or until the wind diminishes.
Quartz-Grain Surface Features
Scanning electron microscopic investigation of quartz grains surface texture (Fig. 4) have shown that Shuaiba sabkha is characterized by an abundance chemical surface features due to chemical action. They are characterized by oriented triangular etching pits (Fig. 4A & B), solution of silica and formation of deep haloes (Fig. 4C), deep etching pits and silica precipitation (Fig. 4D), sil- ica precipitation on rugged surface and cavity filling (Fig. 4E), precipitation of silica on triangular etching pits (Fig. 4F), precipitation of silica on upturned plates (Fig. 4G) and etching and precipitation of silica on upturned plates (Fig.
4H). The most diagenetic features of this environment are the triangular etch de- pressions where chemical etching along planes of weakness occur extensively.
This texture usually indicates a high energy chemical environment. Solution (etching) and precipitation are mostly developed in the recent nearsurface diage- netic environment. The increasing rate of evaporation and the abundance of salts in the pore water generally increase the pH which affects the quartz grains and influence the development of chemical features. Al-Saleh and Khalaf (1982) found the dominance of chemical features in the Gulf sabkha deposits while Gheith (2000) differentiated between various modern environments in the
FIG. 4. Surface features of the quartz grains from Shuaiba supratidal and intertidal sediments, A&B – Oriented triangular etching pits. C – Solution of silica and formation of deep ha- loes. D – Deep etching and silica precipitation. E – Silica precipitation on rugged surface and cavity filling. F – Precipitation of silica on triangular etching pits. G – Precipitation of silica on upturned plates. H – Etching of the silica covered the upturned plates.
eastern coastal plain of the Red Sea using the natural surface features observed in the quartz grains.
Conclusions
Study of the intertidal and supratidal flat deposits of the Shuaiba lagoon by X-ray diffraction and scanning electron microscope proved considerable verti- cal and lateral variations in the proportion and composition of the carbonate minerals. Etching and silica precipitation on the quartz grain surfaces are the causes of dominant chemical features especially in a high energy chemical envi- ronment. Aragonite and calcite are the most common carbonate minerals in the intertidal zone while dolomite and high Mg-calcite predominate in the suprati- dal sediments. Diagenetic dolomite was recorded at a depth farther inland where dolomitization and lithification take place at a depth in the upper limit.
References
Alsaleh, S. and Khalaf, F.I. (1982) Surface textures of quartz grains from various recent environ- ments in Kuwait. Jour. Sed. Petrology, V. 52, No. 1, pp. 215-225.
Basyoni, M.H. (1997) Sedimentological and hydrochemical characteristics of Al Lith sabkha, Saudi Arabia. JKAU: Earth Sci., V. 9, pp. 75-86.
Behairy, A.K.A. (1980) Clay and carbonate mineralogy of the reef sediments north of Jeddah, west coast of Saudi Arabia. JKAU: Mar. Sci., V. 4, pp. 265-275.
Behairy, A.K.A., El-Sayed, M.Kh. and Durgaprasada Rao, N.V.N. (1985) Eolian dust in the coastal area north of Jeddah, Saudi Arabia. Jour. of Arid Environment.V. 8, pp. 89-98.
Behairy, A.K.A., Durgaprasada Rao, N.V.N. and El-Shater, A. (1991) A siliciclastic coastal sabkha, Red Sea coast. Saudi Arabia. J.K.A.U. Mar. Sci., V. 2, pp. 65-77.
Carver, R.E. (1971) Procedures in sedimentary petrology. John Wiley, New York, 653 p.
Deffeyes, K.S., Lucia, F.J. and Weyl, P.K. (1965) Dolomitization of recent and plio-pleistocene sediments by marine evaporate waters on Bonaire, Netherlands Antilles, in dolomitization and limestone diagenesis: SEPM Spec. Pub. V. 13, pp. 71-80.
El-Sayed, M.Kh. (1987) Chemistry of modern sediments in a hypersaline lagoon, North of Jed- dah, Red Sea. Estuarine, Coastal and Shelf Science. V. 25, pp. 467-480.
Gavish, E. (1980) Recent sabkhas marginal to the southern coasts of Sinai, Red Sea. In : Hyper- saline Brines and Evaporitic Environments. Developments in Sedimentology. V. 29, Elsevi- er, Amsterdam, pp. 233-251.
Gheith, A.M. (1999) Mineralogy and diagenesis of coastal sabkha sediments of the hypersaline lagoons on the eastern coast of the Red Sea, Saudi Arabia. Arab Gulf J. Scientific Res. V.
17, No. 17, pp. 199-219.
Gheith, A.M. (2000) Sedimentary reflections of coastal processes on the shore zone sediments along the eastern Red Sea coast, Saudi Arabia. Z. Geomorph. N.F.V. 44 No. 4, pp. 449-468.
Goldsmith, J.R. and Graf, D.L. (1958) Relation between lattice constants and composition of the Ca-Mg carbonates, Am. Mineral., V. 43, pp. 84- 101.
Hardie, L.A. (1987) Dolomitization: a critical view of some current views. J. Sediment. Petrol., V. 57, pp. 166-183.
Land, L.S. (1985) The origin of massive dolomite. J. Geol. Educ.,V. 33, pp. 112-125
Krinsely, D.H. and Doornkamp, J.C. (1973) Atlas of quartz sand surface textures, London, Cambridge Univ. Press. 91 p.
Mckenzie, J.A. (1981) Holocene dolomitization of calcium carbonate sediments from the coastal sabkha of Abu Dahbi. U.A.E. a stable isotope study. J. Geol. V. 89, pp. 185-198.
Mckenzie, J.A., Hsu, K.J. and Schneider, J.F. (1980) Movement of subsurface water under the sabkha, Abu Dhabi, U.A.E. and its relation to evaporative dolomite genesis. Concepts and models of dolomitization SEPM. Spec. Publ. No. 28, pp. 11-30.
Milliman, J.D. (1974) Marine carbonates. Springer, Verlag, Berlin. 375 p.
Patterson, R.J. and Kinsman, D.J.J. (1982) Formation of diagenetic dolomite in coastal sabkha along Arabia (Persian) Gulf. The Amer. Assoc. Petrol. Geol. V. 66, No. 1, pp. 28-43.
Reeder, R.J. (1983) Crystal chemistry of the rhombohedral carbonates. In: R.J. Reeder (Ed.) Car- bonates: Mineralogy and Chemistry. Mineralogical Society of America, Washington D.C.
Reviews in Mineralogy, 1-47 pp.
