GeoArabia, Vol. 4, No. 1, 1999 Gulf PetroLink, Bahrain Quaternary Evolution of Dawhat Zulum (Half Moon Bay) Region Eastern Province, Saudi Arabia Ruud Weijermars Alboran Media Group (Previously at King Fahd University of Petroleum and Minerals) ABSTRACT 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. INTRODUCTION 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. 71 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/4/1/71/4552952/weijerm.pdf by guest on 24 September 2021 Weijermars 50°00' 50°10' D Dammam a m (Tdm) m a Qes m Hadrukh Th Bahr Old Abqaiq Road ain (Th) y a Causeway w h ig H f fu o l H A Qes Qsb Aziziyah 26°10' 26°10' Qsb Railway Coastal Deposits (Qcd) (4,000 years BP) Dominant directions of long shore currents HALF MOON BAY Qsb (Maximum depth 10 meters) Figure 2 Qcd (Recent) Ra's Abu Urayqat North Hill 26°00' 26°00' South Hill ARABIAN GULF JAFURAH Qsb SAND SEA Silt Deposits RA'S AL-QURAYYAH PENINSULA (Qsb) Qsb Eolian Sand (Qes) Eolian Sand Silt Deposits Coastal Deposits (Qcd) Calcareous Deposits (4,000 years BP) Ra's Al-Qurayyah Hadrukh Formation 010N Dammam Formation Late Flandrian Shoreline (4,000 yr BP) Km 50°00' 50°10' Figure 1: Geological map of the Half Moon Bay area. The core of the Dammam Peninsula is occupied by the Tertiary Dammam Formation (Tdm), and is overlain by the Late Tertiary Hadrukh Formation (Th). Quaternary deposits distinguished around the bay are: silt deposits on sabkha plains (Qsb), eolian sands (Qes), and calcareous deposits at the Flandrian high shore (Qcd). Major access roads are indicated in grey with black outline. High 4,000 year BP shoreline marking the end of the Flandrian transgression is dashed in blue. The geology was mapped in the field using Landsat images, basemaps of Steineke et al. (1958) and Roger (1985), and descriptions of Powers et al. (1966). 72 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/4/1/71/4552952/weijerm.pdf by guest on 24 September 2021 Half Moon Bay, Saudi Arabia 50°00' 50°10' 26°10' HALF MOON BAY Service Road North Hill 26°00' South Hill 26°00' Delta Region Al-Hasa River Dune of Figures N 6 and 7 Road to Ra’Ra’ss Al-Qurayyah 02 Bridge over Al-Hasa River Km 50°00' Figure 2: False color, enhanced Landsat Thematic Mapper image of the Half Moon Bay area. Shorelines are flanked by sabkha plains across which transverse and barchan dunes migrate south- southwest with the prevailing Shamal winds. The Ra’s Al-Qurayyah Peninsula, south of Half Moon Bay, hosts two major evaporite basins, separated from the bay by the eolian dunes of North Hill and South Hill. Image recorded on 28 May, 1990. Image width as cropped here is 26 km. 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, network- like 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 73 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/4/1/71/4552952/weijerm.pdf by guest on 24 September 2021 Weijermars 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 North 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 Rose Diagram local construction as aggregate and filler. The silica content of Annual Wind of the eolian sands ranges between 80% to 90% and the Directions 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, West East 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 Annual Average deposit have been sampled and yielded radiocarbon dates Wind Direction of 3,700 to 6,000 years BP (McClure and Vita-Finzi, 1982; Vita-Finzi and McClure, 1991). The beaches and spits South 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 Figure 3: Annual average wind core samples are available. The true depth of the Tertiary direction for the Eastern Province bedrock is unknown, but can be estimated from the (after Fryberger et al., 1983). 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.