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«Paleontological Studies on the Neogene (Miocene and Quaternary) Carbonate Rocks of Rabigh-Ubhur Areas, Red Sea Coast, Saudi Arabia Mohammed H. ...»

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JAKU: Earth Sci., Vol. 21, No. 2, pp: 105-143 (2010 A.D. / 1431 A.H.)

DOI: 10.4197 / Ear. 21-2.5

Paleontological Studies on the Neogene (Miocene and

Quaternary) Carbonate Rocks of Rabigh-Ubhur Areas,

Red Sea Coast, Saudi Arabia

Mohammed H. Mandurah

Department of Petroleum Geology and Sedimentology,

Faculty of Earth Sciences, King Abdulaziz University,

Jeddah, Saudi Arabia

mhmandurah@yahoo.com

Received: 21/4/2009 Accepted: 22/12/2009

Abstract. The syn-rift Miocene carbonate rocks in Rabigh and Ubhur areas belong to the Jerba carbonate member of Dafin Formation and Ubhur Formation, respectively. The carbonate rocks of Rabigh overlie Wadi Al-Hakkak clastic section and are considered as a lateral facies equivalent to the Jehfa evaporite section. The Ubhur carbonate rocks underlie the Middle Miocene shale and evaporite rocks. Raised Quaternary coral terraces are dominant in Rabigh area.

Field, paleontological and palynofacies studies of the Neogene rocks (carbonate and shale) of Ubhur and Rabigh areas, Red Sea coast of Saudi Arabia indicate the existence of corals, gastropods, bivalves, echinoderms, foraminifers and amorphous organic matter. Taxonomic and paleoecological interpretations of the recorded fossils are the purpose of this work.

Keywords: Neogene, paleontology, Rabigh, Ubhur, Red Sea, Saudi Arabia.

Introduction Most paleontological, palynological and stratigraphical studies on the eastern shore of the Red Sea coast of Saudi Arabia were done on the Midyan Peninsula, Gulf of Aqaba, where profitable hydrocarbon occurrences are discovered in the sixties of the last century. Based on 105 Mohammed H. Mandurah 106 these studies, various informal lithostratigraphic schemes have been applied to the Saudi Arabian Red Sea succession by numerous authors (e.g. Beydoun, 1991; Hughes & Beydoun, 1992; Filatoff & Hughes, 1996; Hughes et al., 1999; and Hughes & Johnson, 2005). Whereas in Makkah and Rabigh Quadrangles, most sedimentological and stratigraphical studies deal with the pre-rift sequences (Shumaysi and Usfan formations); (e.g. Brown et al., 1963; Moltzer & Binda, 1984;

Spencer & Vincent, 1984; Srivatsava & Binda, 1991; Gheith & Abou Ouf, 1994; Abou Ouf,1998; and Hughes & Johnson, 2005), and the coastal sediments of the Red Sea region (e.g. Basaham, 1998; Basaham et al., 2006; Bantan, 2006; and Al-Sofyani & Niaz, 2007). The clay deposits of the syn-rift Miocene sediments are the interest of Taj et al. (2002), and Basyoni et al. (2002). However, no paleontological studies are dealt with for the Neogene sediments at the central part of the eastern shore of the Red Sea coast of Saudi Arabia. So, the purpose of this work is to study the taxonomy and paleoecology of the Neogene rocks in Ubhur-Rabigh areas, north of Jeddah city (Fig. 1).

Location and General Geology Ubhur and Rabigh areas are located north of Jeddah city by ~ 20 km and 175 km respectively (Fig. 1). The two areas are located in Makkah and Rabigh Quadrangles where the Precambrian basement rocks are exposed to the east that are unconformably overlain by Tertiary sedimentary rocks in the west, and by Miocene to Pliocene lavas in the north (Moore & Al- Rehaili, 1989). Extensive areas of Quaternary surficial deposits (sand and gravel) are extended on the coastal plain and in the major wadis. The stratigraphic succession of the Tertiary formations, as well as, the paleogeography and structures of these sediments are studied by Spencer & Vincent (1984).

The Tertiary layered rocks of Makkah Quadrangle are composed of tilted and faulted strata, which are exposed sporadically, and usually poorly, beneath a cover of flat-lying lavas and Quaternary deposits in the western part of the Quadrangle. They were assigned by Brown et al. (1963) to the Shumaysi and Usfan formations. Spencer & Vincent (1984) replaced the Shumaysi Formation with several formations; they are Haddat Ash-Sham, Shumaysi, Khulays and Buraykah formations. It is noticed that the Haddat Paleontological Studies on the Neogene (Miocene and Quaternary) … 107 Ash-Sham, Usfan and Shumaysi formations are facies variants and thus chronostratigraphic near-equivalents (Spencer & Vincent, 1984).

–  –  –

The Tertiary sedimentary rocks in the western part of Rabigh Quadrangle are best preserved adjacent to where they have been covered by Tertiary basalt flows; elsewhere, they occur in low hills rising above the coastal plain, largely concealed by sand and gravel (Ramsay, 1986). The rocks are generally horizontal but have been faulted Mohammed H. Mandurah 108 and consequently attain flexure dips, in places, as much as 40°. They lie unconformably beneath Tertiary lava flows and unconformably overlie, or are faulted against, the Precambrian rocks. They have been assigned to the Usfan, Shumaysi (?), and Daffin formations (Ramsay, 1986).

Lithologically, the Tertiary formations in Makkah and Rabigh Quadrangles that were studied by Taj et al. (2002 & 2004) have a limited extent that consist mainly of siliciclastic material, derived from the surrounding Precambrian basement and pre-existing sedimentary rocks. However, the overwhelming clastic sedimentation is temporarily interrupted by limited but persistent marine and lacustrine carbonate units in Usfan Formation and Dafin Formation (Taj et al., 2002).

Despite the similarity of the lithology, age and the close geographic locations of Ubhur and Rabigh areas, the exposed Miocene rocks in these areas are assigned with different formation names. In Ubhur area, the Ubhur Formation as described by Moore & Al-Rehaili (1989) consists of green sandy clay, siltstone and soft, white bioclastic limestone with gypsum bed interbedded with the clay. Based on the microfossils and bivalves present in the limestone, Andreieff (1983) assigned an Early Miocene age to the Ubhur Formation.





In Rabigh area, Taj & Hegab (2005) studied the lithostratigraphy, sediment characteristics and depositional environments of the Miocene rocks of Dafin Formation. They classified Dafin Formation into three sedimentary facies; they are siliciclastic, carbonate and evaporite rocks.

The siliciclastic rocks are deposited in a meandering system, the carbonates are formed in shallow marine to protected lagoon, whereas the evaporites are formed in saline pond and in sabkha (Taj & Hegab, 2005).

Mandurah & Aref (2009) interpreted the deposition of the evaporite facies of Dafin Formation in a very shallow salina dominated with microbial activity.

Lithostratigraphy Two composite lithostratigraphic sections have been measured and sampled in detail for the Miocene carbonate rocks in Rabigh and Ubhur areas, and one section for the Quaternary terraces in Rabigh area (Fig. 2).

In addition, samples from the marls, claystone, shale and mudstone beds are checked for their microfaunal and palynological contents. However, Paleontological Studies on the Neogene (Miocene and Quaternary) … 109 Fig. 2. Lithostratigraphic sections of the Neogene carbonates in Rabigh and Ubhur areas, Red Sea, Saudi Arabia.

sampling and interpretation of the evaporite sections at Rabigh and Ubhur areas (Mandurah & Aref, 2009) and the clastic section at Rabigh area (Taj & Hegab, 2005), and Ubhur area (Taj et al., 2001) are out of the scope of this work. It is important to mention that Taj & Heqab (2005) subdivided the Miocene Dafin Formation of Ramsey (1986) in Rabigh Mohammed H. Mandurah 110 area into three members, these are from the base to the top; Al-Haqqaq clastic member, Wadi Jehfa gypsum member and Wadi Jerba carbonate member. The following is a description of the measured carbonate sections in Rabigh and Ubhur areas.

1. Wadi Jerba Miocene Carbonate Rocks The Miocene carbonate rocks are exposed only at Wadi Jerba, which conformably overly the clastic member of Rabigh Formation. At Wadi Jehfa, the evaporite rocks are also conformably overlying the clastic member of Dafin Formation. So, the carbonate and evaporite rocks are considered as lateral facies equivalent. The carbonate rocks are recorded as residual hills and elongated ridges that were covered with black boulders from Al-Harrat basalt.

The following is a description of the carbonate section at Wadi Jerba

from base to the top:

Bed Symbol thickness Description (cm) Jb-1 100 Massive, hard, olive to yellow medium sized sandstone.

Massive, hard, dirty white dolomitic limestone, with few gastropod and Jb-2 70 bivalve shells.

Very hard, vuggy, pale yellow to white dolomitic limestone that forms prominent, resistant ridge for the whole carbonate exposures. The dolomitic limestone consists of scattered shells of foraminifers, gastropods (small and large), (Fig. 3) and bivalves embedded in fine bioclastic matrix (Fig. 4). The distribution of the fossils is sporadic, where bivalve shell concentrations are observed locally (Fig. 4).

Jb-3 120 Thalassinoids burrows are also recorded, that may enclose the mold or shell of gastropods. Sand sized quartz grains and pebble and cobble-sized volcanic rock fragments are recorded also in the dolomitic limestone layer.

Millimetric moldic vugs of bivalve and gastropod shells are common, where the internal and external molds of the shells are filled with bioclasts and fine sand sediments.

Medium hard, dolomitic bioclastic limestone with gastropod and bivalve shells and molds. The upper surface of this layer shows high concentration of shells and shell fragments from Clypeaster and Echinolampas sp. that Jb-5 40 are dispersed at the weathered surface of the dolomitic limestone. No pebbles are recorded from this layer. Also, a fewer shell concentration is observed than the underlying carbonate layer.

Massive, medium hard, varicolored yellow to pink to white carbonate Jb-6 120 mud, without any observed macrofossils or ghost of them.

Massive, vuggy dolomitic limestone that consists of foraminifers, gastropods and bivalves embeded in a matrix of bioclasts. The gastropod and bivalve shells are mostly dissolved leaving moldic cavity, whereas the Jb-7 150 internal molds of the shells are composed of fine bioclastic material. Coral debris are recorded overlying this layer.

Paleontological Studies on the Neogene (Miocene and Quaternary) … 111 Fig. 3. Internal mold of large sized gastropod steinkern that is filled with bioclastic materials, Miocene, Jerba Member, Dafin Formation,Wadi Jerba.

–  –  –

2. Ubhur Miocene Carbonate Rocks The Miocene carbonate rocks have limited exposure near the piedmont of the hill (~200 m) of Ubhur area, and they are exposed only as a result of excavation by dredger as they are covered with aeolian sand, basalt and other volcanic boulders. The sequence at a newly

excavated face is as follows from the base to the top:

Mohammed H. Mandurah 112

–  –  –

3. Quaternary Coral Reefs Sandy beach sediment forms the coast of Ubhur area, whereas carbonate rocky beach forms the coast of Rabigh area. This is due to the existence of raised coral terraces facing the shore line in Rabigh area, whereas coral debris are recorded far from the beach (~ 1500 m) below sabkha sediments and aeolian sands in Ubhur area.

At Rabigh city, raised coral reefs form two prominent terraces throughout all the coastal area from the refinery station of Saudi Aramco to the north of the city. Each terrace starts with dissolution notches at the base and a hard ledge or overhang at the top (Fig. 5). The lower notch is encrusted with thin black organic crust. The sequence in the coral reefs

from the base to the top is as follows:

Fig. 5. Two reefal terraces start with dissolution notches and capped by a hard ledge or overhang, Quaternary, Rabigh.

Paleontological Studies on the Neogene (Miocene and Quaternary) … 113

–  –  –

Fig. 10. Calcareous rhizocretion at the base of the upper terrace, Quaternary, Rabigh.

Systematic Paleontology The Miocene and Quaternary carbonate rocks of Rabigh and Ubhur areas are highly dominated with faunal content (both macroinvertebrates and microinvertebrates). The following is a systematic description of the

recorded fauna:

1. Macroinvertebrates The studied specimens have been cleaned from the attached rocks and washed in the laboratory, fully described, photographed and generically classified. Specific identification was carried out by comparing the studied fauna with previous works. The studied macrofossils include corals, bivalves, gastropods and sea urchins.

Paleontological Studies on the Neogene (Miocene and Quaternary) … 115

–  –  –

Description Colony shape massive; attachment of skeleton attached; wall development between buds always; symmetry of bud geometry multidirectional; calice or valley width medium (5-10mm); calice relief low (2mm); calicular platform shape u-shaped; coenosteum narrow ( valley width); exothecal dissepiments present; number of septal cycles 3; septal spacing (per 5mm) 6-12; relative septa thickness unequal;

relative costae thickness equal; continuity of costae continuous; septal lobes absent; paliform lobes absent; columella continuity continuous;

columella width 1/2; endothecal dissepiments abundant; wall structure parathecal; double or single wall single.

Distribution Red Sea coast, Arabian Gulf, Atlantic and Pacific Oceans.

Range Pleistocene-Recent.

Family Fungiidae Dana, 1846 Genus Fungia Lamarck, 1801 Fungia sp.

(PI. 1, Fig. 2 & 3) Mohammed H. Mandurah 116

–  –  –

Description The corallite is discoid, solitary with rounded sharp margins and convex and elevated upper surface and without a calicular depression.

The lower surface is flat to slightly concave and without epitheca. The septa are thick, compact and long. The lateral faces of septa are strongly pennulat. Septa are arranged in five cycles and 6 systems. The first two cycles extend to the center of the corallite and joined by their inner margins to form a trabecular columella.

Distribution Red Sea coast, and Arabian Gulf.

Range Pleistocene-Recent.

1. B. Bivalves Abbreviations used (All linear measurements are given in millimeter) H - Shell height. RV - Right valve L - Shell length. LV- Left valve T - Shell thickness. EL- Estimated length.



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