The Quaternary geology of the Half Moon Bay region, directly south of the Dammam Peninsula, Saudi Arabia, has been mapped and is outlined in the present study. The bay is surrounded by sabkha plains and low sand dunes in a complex morphology which results from the interplay of global sea-level changes, local uplift of the Dammam Peninsula, dune migration of the Jafurah Sand Sea, coastal sabkha formation, and influx of the Al-Hasa perennial river. Half Moon Bay was emergent in the Late Pleistocene for at least 50,000 years, due to eustatic sea-level changes, but was inundated again during the Late Pleistocene/Holocene “Flandrian” transgression that started about 12,500 years before the present (BP) and reached its culmination 4,000 years BP. The regional climate has fluctuated considerably in the geologic past, with colder, pluvial episodes occurring, amongst others, during the Early Pleistocene (1.6 to 0.7 million years BP), Middle Pleistocene (560,000 to 325,000 years BP), Wurm glacial (36,000 to 17,000 years BP) and Neolithic wet phase (10,000 to 6,000 years BP). Karstification and sinkhole hazards, present in all major carbonate units of the Eastern Province of Saudi Arabia, originated from dissolution during these past pluvial episodes. During the present-day hot and arid climate, torrential rains episodically cause overflow of the irrigation network of the Al-Hasa Oasis. The excess water fills large evaporation ponds which, in turn, feed a perennial river that transects the dune landscape for some 80 kilometers flowing towards Half Moon Bay.
Dawhat Zulum, informally known as “Half Moon Bay” or “Khaleej Nisf Al-Qamar”, is located directly south of the Dammam Peninsula. Its shores consist of sand sheets, sabkhas and eolian dunes (Figure 1). Three major morphological elements of the coastline of Half Moon Bay can be distinguished (Figures 1 and 2): the east shore is formed by the Dammam Peninsula with Ra’s Abu Urayqat at the southern most tip; the southern shore is formed by the Ra’s Al-Qurayyah Peninsula; and the western shore comprises low dunes of the Jafurah Sand Sea. The complex morphology of Half Moon Bay is controlled by Quaternary global sea-level changes, local uplift of the Dammam Peninsula, dune migration of the Jafurah Sand Sea, coastal sabkha formation, and influx of the Al-Hasa perennial river.
A systematic description of the geology of Half Moon Bay region was undertaken for several reasons. Firstly, the interleaving and mixing of marine and continental silici-clastics within a sabkha environment provides a valuable analog for understanding other sabkha deposits in the Arabian Platform stratigraphy (e.g., Rus Formation). Secondly, its involved Holocene history at the interface of clastic and carbonate facies in an eolian setting may provide a template for understanding similar depositional environments elsewhere. Thirdly, the Half Moon Bay region is well-suited for geological field-trips by the professional and academic communities of Saudi Aramco, King Fahd University of Petroleum and Minerals (KFUPM), and others. Although some sections and subfacies of this environment have been discussed by Fryberger et al. (1983), a concise geologic description of the Half Moon Bay region has never before been attempted.
DAMMAM DOME AND RA’S ABU URAYQAT
The bedrock of the Dammam Peninsula are Tertiary formations, which have been gently uplifted by a subsurface salt dome (Tleel, 1973). The slow uplift of the Dammam Dome (Weijermars, 1999) has resulted in progressive marine regression and the development of extensive coastal sabkhas, now partly covered by eolian dunes. The Quaternary deposits, which form the southern tail of the Dammam Peninsula, had several provenances. The eolian silici-clastics were blown from the north-northwest, while marine sediments were transported by longshore, southbound currents and cemented by chemical precipitation and biogenic processes.
Much of the Dammam area, occupied by the Late Tertiary Hadrukh Formation (Figure 1), could be classified as dikaka. This term is sometimes used for sediments characterized by a dense, networklike arrangement of fossil roots stuffed with fine sand or coarse gypsum. Dikaka formed from vegetation growing on the silty floor of a wadi (valley) that retains humidity for some time (Glennie and Evamy, 1968). It covers relatively flat terrains, underlain by poorly-exposed bedrock, generally blanketed by scattered rocks, patches of eolian sand and some vegetation.
The Half Moon Bay region itself is devoid of any Tertiary outcrops and is dominated by surficial deposits of silici-clastic eolian sands of the Jafurah Sand Sea (Figures 1 and 2), which shift as dune systems across the coastal sabkha plains, principally toward the south-southeast under the action of the dominating Shamal winds (Figure 3). The eolian sand is commonly fine- to medium-grained, clean and well-sorted (mesh is 0.26 to 0.31 and sorting 1.38 to 1.53) (Roger, 1985). Many of the dunes have been excavated for use in local construction as aggregate and filler. The silica content of the eolian sands ranges between 80% to 90% and the remainder is calcite. The ferric iron content is less than 0.46% and impurities generally do not exceed 2.6%.
A coastal and marine Quaternary deposit (Qcd), occurs as narrow low ridges at only a few meters above present sea-level (Figure 1). These ridges comprise oolitic sands, coquinas with a sandy matrix, calcarenites and beach rocks. These are interpreted here to mark the high shoreline at the end of the Flandrian transgression about 4,000 years ago (4 ka). Shells of gastropods, bivalves, molluscs and barnacles from this Holocene shoreline deposit have been sampled and yielded radiocarbon dates of 3,700 to 6,000 years BP (McClure and Vita-Finzi, 1982; Vita-Finzi and McClure, 1991). The beaches and spits which surround Half Moon Bay consist of a mixture of marine sand, eolian sand and reworked eolian sand, with minor amounts of coquina, beach rock and muds. The bay itself is probably floored by similar deposits, but no core samples are available. The true depth of the Tertiary bedrock is unknown, but can be estimated from the surface slope of the nearby Dammam Dome to be no more than several tens of meters.
EASTERN PROVINCE SABKHAS
The area surrounding Half Moon Bay is underlain by sabkhas, which develop where the regional water table is close to the ground surface. The surface of the sabkhas is commonly covered by muds composed of calcite, aragonite, gypsum, anhydrite and halite, mixed with various amounts of (commonly eolian) quartz sand (Figure 4). Below the sabkha floor may occur alternating layers of horizontally laminated eolian sand sheets, cross-bedded eolian dune deposits, marine beachrock and coquina. The sabkhas of the Eastern Province are hostile for construction projects, because of excessive settlement under normal loading, corrosive ground water and flooding risk (Shehata et al., 1990).
Sabkhas are flat, often water-saturated land surfaces deflated by wind down to the level of the capillary fringe of the water. These normally occur in hot climates and evaporation of the ground water through the surface results in the precipitation of evaporite minerals (Kinsman, 1969; Johnson et al., 1978). Isolated barchanoid dunes migrate over the otherwise flat sabkhas. Simultaneously, deflation occurs in the sabkha around these dunes down to the level where the sediment grains are held together by adhesive forces of water in the capillary zone. Fryberger et al. (1983) concluded that all sabkhas in the Dammam area are underlain by silici-clastics in contrast to the classical carbonate sabkhas of the Abu Dhabi coast. The quartz sand fraction is commonly much larger than the evaporitic fraction (Roger, 1985). Holm (1960) and Glennie (1970) suggested that even the inland sabkhas in the eastern Province, are remnants of coastal sabkhas of Pleistocene and Late Pliocene sea-levels.
Two major sabkha ponds occur on the Ra’s Al-Qurayyah Peninsula (Figure 1). Parts of these sabkhas remain submerged in shallow brine for most of the year. The size of the submerged area fluctuates in accordance with the seasonal precipitation pattern. Heavy rainfall in December 1992, resulted in a 40 centimeters (cm) rise in the water-level, which flooded most of the sabkha. The water receded to its previous level by April 1993 (Al-Guwaizani, 1994). The southernmost of the two ponds is stripped of its surface salt along its western margin (Figure 5a). The salt concentration of the brines is enriched by evaporation and the building of small dams. The precipitation of salts from evaporating brines like sea-water occurs in the order of reciprocal solubility: calcium carbonate, gypsum, halite (see references in Weijermars, 1991). In the area of salt mining, the brine concentrations are kept such that sodium chloride is the principal evaporite deposit. Gypsum precipitates when the brine concentration is three to four times denser than that of standard sea-water. Halite is the principal salt precipitating when the brine concentration is ten times or more than that of standard sea-water.
Along the southeastern margin of the Ra’s Al-Qurayyah Sabkha, a coarse crystalline mush predominantly composed of gypsum and more than one meter thick, is the principal deposit. The gypsum forms 30% to 100% of the deposit and occurs together with halite and carbonates (calcite, dolomite and aragonite) mixed with minor amounts of both marine and eolian quartz sand. Anhydrite (CaSO4) is also found and forms when the gypsum (CaSO4H2O) dehydrates at temperatures above 45° to 60° centigrade. Polygonal patterns of cracks are present after the seasonal recession of the flood-water (Figure 5b). The fresh-water gastropod Melanoides tuberculata is widespread on these sabkha plains. This is a very adaptable form and may also be found in brackish and hyper-saline waters at present.
STRUCTURE OF JAFURAH BARCHAN DUNES
Some extraordinary clear internal stratification patterns were observed inside modern Jafurah dunes when deflated after exceptionally heavy rainfalls in April 1995. Annual rainfall of 202.95 millimeters (mm) was measured in Abqaiq (located 40 kilometers (km) west of the dune studied) for the whole of 1995, which was the largest recorded since the start of meteorological measurements in 1939. On 4 April, 1995, an entire barchan (located at coordinates 25° 54’ 43”N and 49° 53’ 97”E) was found completely soaked and saturated by rain-water following heavy rainfall overnight (Figure 6a). Sand grains were held together by the intergranular capillary water. A strong Shamal wind had deflated all of the disorderly, surficial, dry sand which normally covers the dune crest and thereby obscures any internal stratification (compare Figures 6b and 6c). However, the deflation surface revealed a well-structured pattern of internal cross-stratification in a tilted transversal section (Figures 7a and 7b). The observed cross-stratification includes truncated sets similar to those seen in ancient eolian deposits.
Although the barchan migration mechanism is well-studied (Ahlbrandt and Fryberger, 1981), the present observations allow the proposition of a simple model of barchan anatomy development (Figures 8a to 8c). Barchan dunes are crescent-shaped in plan view and each has two horns facing down-wind (Figure 8a). Eolian sand is continually sliding down the slip-face at the down-wind or lee-side of the dune, after cascading over the dune crest. Sand grains reach the crest by saltation and rolling and tend to become airborne in the relatively high wind speed at the crest of the dune. However, the aerodynamics of barchans is such that low-speed air currents at the lee-side of the barchans cause airborne particles to fall onto the slip-face. Consequently, sand continually shifts from the wind-ward slope to the lee-ward slope to be temporarily buried in new slip-faces until re-emerging at the windward slope for further surface transport. Theoretically, there should be a vertical plane bisecting barchan dunes into two similar, symmetrical halves (Figure 8a). However, many barchans show internal stratification more complex than that of Figure 8a. Figures 8b and 8c attempt to visualize the way in which cross-stratification may develop inside migrating barchans by sideways shift during migration down-wind. The cross-stratification observed in the transversal sections, through the rear end of the barchans in Figures 7a and 7b, can be explained by the simple model of Figures 8b and 8c.
RA’S AL-QURAYYAH PENINSULA AND QUATERNARY SEA-LEVEL CHANGES
The Ra’s Al-Qurayyah Peninsula is dominated by two low hills, North Hill and South Hill (Figures 1 and 2). Both are principally made up of sands, emplaced as eolian dunes, which must have migrated southward to their present position over the floor of what is now Half Moon Bay. The floor of Half Moon Bay itself is here interpreted to have been a supratidal sabkha for some 50,000 years, until about 6,000 years BP, when it was flooded (see below). Directly north of the North Hill, a north-northwest-striking landspit extends into Half Moon Bay (Figures 1 and 9a). This prominent barrier is made up of quartz sand and covered by beach rock (Figure 9b). Three kilometers southwest of the North Hill, a shallow topographic ridge composed of Holocene marine sands and coquina beds, occurs to form an ancient coastal barrier extending to the south-southeast as far as the Ra’s Al-Qurayyah desalination plant. This Holocene beach deposit (approximately 4,000 years BP) is now exposed (Figure 9c), probably due to eustatic regression around the entire Arabian Gulf possibly accompanied by minor uplift of the Ra’s Al-Qurayyah Peninsula.
The Arabian Gulf is a marginal sea of the Indian Ocean with a maximum depth of 120 to 130 meters (m) near the Strait of Hormuz, and has an average depth of only 35 m (Figure 10). Its shallow depth has made it extremely susceptible to the effect of sea-level changes. Figure 11 summarizes the reconstruction of Holocene and Pleistocene eustatic sea-level changes. Before the onset of a major regression about 110,000 years BP, the Early Pleistocene was marked by a high sea-level of + 120 to +150 m (Figure 11), which covered much of the Eastern Province and inundated large parts of the Rub’ Al-Khali (Figure 12). During the Late Pleistocene regression about 105,000 years BP, sea-level dropped 70 m below the present level (Kassler, 1973), and exposed large areas of the Arabian Gulf floor (Figure 13a). A brief transgression occurred between 105,000 and 80,000 years BP (Figure 13b), followed by the major Late Pleistocene regression with sea-level again steadily dropping toward a Pleistocene record low-stand of 120 m below the present level at 17,000 years BP (Figure 13c).
For several millennia, the Gulf was a vast alluvial plain (Seibold et al., 1973), through which the Tigris and Euphrate Rivers flowed southeastward to a coastline in the Gulf of Oman (Sarnthein, 1972). Much of the Gulf remained essentially dry between 17,000 and 14,000 years BP, and was subjected to subaerial erosional conditions (Lambeck, 1996). Fresh, pluvial water dissolved aragonitic shells in skeletal grainstone and provided cement to underlying quartz sands (Darwish and Conley, 1990). Eolian dunes migrated southward over the Gulf floor, mostly under arid conditions (Sarnthein, 1972; Al-Hinai et al., 1987). During the tens of thousands of years it took the Arabian Gulf to attain its former and regain its present level, large areas of the now submerged Gulf plains have supplied reworked sandy alluvial sediments to Qatar and the Rub’ Al-Khali by means of northwesterly Shamal winds (Shinn, 1973). Wind even may have carried airborne foraminiferids from the Gulf’s floor to the central Rub’ Al-Khali when the Gulf was dry (McClure, 1984).
The Late Pleistocene regression was followed by the Holocene or Flandrian transgression when the sea slowly rose to its present level. By about 12,500 years BP, the marine incursion was well underway (Lambeck, 1996). The sea-level rose to -20 m at about 10,000 years BP (Figure 13d). Further transgression led to marine water entering the Gulf of Salwah to pass the -7.2 m level around 6,000 years BP, isolating Bahrain as an island for the first time in almost 70,000 years (Doornkamp et al., 1980). The Flandrian transgression attained its highest point about 4,000 years ago, when it reached 2 m above the present level, before falling to the present-day level (Felber et al., 1978). This latest regression is unlikely to reflect a purely local tectonic uplift rather than eustatic sea-level changes (Vita-Finzi and McClure, 1991). Raised beach deposits of the Middle Holocene reaching upto approximately 2 m above present sea-level also occur on the coast of the United Arab Emirates (Evans et al., 1969; Kapel, 1967; Kirkham, 1997). About 3,750 years BP the sea-level first fell rapidly by one meter and then more gradually regressed to the present level about 1,000 years BP. In contrast, the Iranian shore, subjected to the Zagros folding of the Fars Platform, has locally undergone as much as 25 m tectonic uplift during the last 6,000 years (Vita-Finzi, 1980).
AL-HASA PERENNIAL RIVER
Another, surprising factor in the development of Half Moon Bay is the episodic appearance of a perennial river originating from the Al-Hasa Oasis, about 80 km to the southwest. The oasis drains large amounts of fresh-water from subsurface aquifers in the Umm Er Radhuma and Wasia formations. A major portion of this water is used for irrigation. Occasionally, torrential rains during December to March (Figure 14) inundate the drainage and irrigation networks of Al-Hasa. The excess water is discharged into two large artificial lakes or evaporation ponds. One such lake is located northeast of the oasis, another occurs at the southeast margin. The northern lake appears with dark albedo in the southwest corner of the satellite image of Figure 15.
The water volume of the northern lake may increase considerably during the “rainy season” and sometimes feeds the perennial river flowing towards Half Moon Bay. The general flow path is visible on satellite images, and appears most clearly in false-color satellite images recorded during February and March (not shown here). This river is here referred to as the Al-Hasa River. When the Al-Hasa water first leaves the northern lake for Half Moon Bay it takes one to two weeks to re-establish its actual flow-path. Migrating transverse and barchan dunes locally block the drainage course of previous years, but as water accumulates behind the sand obstructions, overflow is followed by breaching. Subsequently, a narrow drainage channel is eroded into the sand bed and water continues to seek its way through the desert in this fashion with many sandwalls temporarily delaying its advance (Figures 16a to 16d).
Between March and April, 1995, the Al-Hasa River meandered through the desert sands for about 80 km. Water flowed continuously for over one month, bringing with it fresh-water fish, upto 30 cm long, from the Al-Hasa irrigation ponds (Figure 17a). The regional gradient between Al-Hasa and Half Moon Bay is about 2.5 m per km. In places, the water course is only several meters wide and up to one meter deep. In spring 1995, flow rates of about 1 km per hour were observed in such narrow conduits. In other locations, the water course widens and extends onto wide sabkha plains. Sabkha Hammam, south of Abqaiq, is one of several areas where the river locally disappears into a lake of several square kilometers, which in turn feeds a continuation of the river which leaves the lake and heads to the northeast. With the cessation of rains, the Al-Hasa River slowly retreats and dries up, leaving thousands of fish to die in numerous evaporation ponds along the river bed (Figure 17b). The annual rainfall of 202.95 mm measured in Abqaiq for 1995 was the largest recorded since the onset of measurements in 1939 (The Arabian Sun, 1996). Abqaiq’s previous record stood at 194.31 mm for 1982. Abqaiq’s 24-hour record stands at 79.5 mm for 12 December, 1955. For comparison, the annual rainfall averaged over a 27-year period is only 84 mm, according to the monthly distribution summarized in Figure 14.
QUATERNARY CLIMATIC CONDITIONS
The Al-Hasa River reached a few kilometers short of Half Moon Bay during the 1995 flow, to dissipate in the sabkha area at its southwest margin. However, it seems certain that occasionally, perhaps only once per century, the river reaches Half Moon Bay to discharge continental detritus into the marine (oolite) environment. The Al-Hasa River may have been much more prominent in the geologic past, particularly during the Quaternary pluvial episodes. Four such pluvials have been recognized, i.e., from 1.6 to 0.7 Ma; 560,000 to 325,000 years BP; 36,000 to 17,000 years BP; and 9,000 to 6,000 years BP (Table 1). A chronology of Quaternary climatic conditions and some associated geological observations are summarized in Table 1.
In Saudi Arabia, the cold intervals of the Quaternary coincided with pluvial phases, and warm intervals corresponded to arid conditions, as at present (Chapman, 1971). Oxygen isotope ratios indicate warm pluvial episodes existed in the Gulf region during the Early Pleistocene (1.6 to 0.7 Ma) and Middle Pleistocene (560,000 to 325,000 years BP) (Whybrow and McClure, 1981; Whybrow et al., 1987). Lakes were formed in interdune depressions of the Rub’ Al-Khali during the Mid-Pleistocene pluvial. Stalactites and sinter drapers of karst caves in the Umm Er Radhuma Formation of the Summan Plateau of northeastern Saudi Arabia have been radiometrically dated and show a range of ages (see references in Edgell, 1990a) coinciding with the Early Pleistocene and Middle Pleistocene, and the later “Wurm” and “Neolithic” Pluvials. The Empty Quarter or Rub’ Al-Khali has been occupied by sand sheets for at least the last 700,000 years, but was covered by lakes and grassy vegetation during the pluvial episodes (see references in Edgell, 1990a, b).
Although the current arid conditions began to prevail in Saudi Arabia near the end of the Middle Pleistocene, two important minor pluvial episodes can be distinguished: (1) between 36,000 and 17,000 years BP (Wurm Glacial); and (2) during the “Neolithic wet phase” from 9,000 to 6,000 years BP (McClure, 1974, 1976 and 1984). Lakes in the Rub’ Al-Khali also show high levels for at least two different episodes: from 36,000 to 17,000 years BP and from 9,000 to 6,000 years BP (McClure, 1978). These correspond to lacustrine highs reported from other Afro-Arabian areas. The prehistoric Mudafan Lake, some 300 km east of Abha, is now dry and obscured by dunes, but contains lacustrine deposits 24 m thick which accumulated between 30,000 and 21,000 years BP (McClure, 1976). Also, worthy of note is that most of the karstification of the Arabian limestone formations has taken place during the various pluvial episodes of the Quaternary. Any new cave systems are unlikely to have formed since the last “Neolithic” pluvial (Edgell, 1990a).
Another perennial river seems to occasionally find its way to the coastal village of Al-Uqayr (approximately 20 km south of Ra’s Al-Qurayyah), originating from the southern evaporation pond of Al-Hasa. This river course, mapped as Darb Al-Uqayr by Steineke et al. (1958), is earlier referred to as the Al-Asfar River (yellow river), or Al-Aftan River, and was first mentioned in writings by Greek historical travelers (Golding, 1984). It may well be that both the Al-Hasa and Al-Aftan Rivers were much more prominent and permanent during the pluvial episodes mentioned above.
Half Moon Bay was dry for at least 50,000 years in the Late Pleistocene, until the Flandrian transgression inundated the bay, starting about 12,500 years BP and culminating 4,000 years BP. The high shoreline at the end of the Flandrian transgression, 4,000 years BP, is thought to be represented by coastal deposits, including coquinoid silici-clastics (Figures 1 and 9c). The modern shore region of the bay is surrounded by sabkha plains and low sand dunes. The North and South Hills at Ra’s Al-Qurayyah Peninsula are remnants of eolian sand dunes, which migrated southward over the dry floor of the bay before its inundation by the Flandrian transgression. The bay is episodically fed with fresh-water when the perennial Al-Hasa River crosses the Jafurah Sand Sea. The interleaving and mixing of marine and continental clastics within the sabkha environment of the Half Moon Bay shores provides a template for understanding similar deposits elsewhere.
Dr. Moujahed Al-Husseini provided valuable advise and encouragement to go ahead with this work inspite of my withdrawal from the Gulf region. The author acknowledges the Research Institute of King Fahd University of Petroleum and Minerals, for use of the Remote Sensing Laboratory. This paper further benefited significantly from comments and suggestions by three anonymous expert reviewers of GeoArabia. All graphics were professionally prepared by GeoArabia’s technical staff.
ABOUT THE AUTHOR
Ruud Weijermars is currently working as a consultant and Board Member at the Alboran Media Group in Amsterdam. He served as an Associate Professor in the Department of Earth Sciences, King Fahd University of Petroleum and Minerals in Dhahran from 1992 until mid-1998. The geology of Half Moon Bay attracted his attention not only on land, but also during frequent sailing trips at the bay. Ruud previously worked as a visiting research scientist at the University of Uppsala (Sweden), University of Texas at Austin (USA), and the Technical High School of Zurich (Switzerland). He holds a PhD in Geodynamics from the University of Uppsala, and BS and MS degrees in Geology and Structural Geology from the University of Amsterdam. Ruud has studied the basement and cover rocks of Saudi Arabia in numerous field localities. He has authored over sixty research articles and published two textbooks: “Structural Geology and Map Interpretation” and “Principles of Rock Mechanics,” which have been reviewed in GeoArabia (1997, vol. 2, no. 3, p. 340). Ruud is a member of the Editorial Board of GeoArabia.