The Gulf of Suez—northern Red Sea neogene rift: a quantitive basin analysis

The Gulf of Suez-northern Red Sea Neogene rift: a quantitive basin analysis Mark Richardson and Michael A. Arthur Graduate School of Oceanography, University of Rhode Island, Narragansett, RI. 02882, USA

Received 30 October 1987; accepted 16 March 1988 Subsidence analysis (backstripping) was carried out on a series of wells from the Gulf of Suez and northern Red Sea region of Egypt in order to examine the interplay between tectonic events, basin subsidence, sedimentation and sea level changes in a young, developing ocean basin and continental margin. Using constraints on chronostratigraphy and paleodepth from various sources combined with stratigraphic and structural information from industry wells and other geophysical sources it has been possible to compile the data necessary to perform geohistory analyses throughout the region. Major subsidence due to crustal thinning began ~25 Ma with sedimentation initially occurring in isolated sub-basins. These earliest sediments record the transition from continental to marine depositional environments. Subsequently during early and middle Miocene times subsidence was rapid and uniform along and across the entire rift basin. Open marine sedimentation occurred across all structural regimes. The mid-Clysmic tectonic event (16.5 Ma) resulted in structural rearrangement of the rift basin and uplift of the rift shoulders. Rapid subsidence continued as global sea level fell, producing a series of prograding, siliciclastic fan-deltas at the rift margins. At -15.5 Ma, opening of the Suez rift was terminated, tectonic subsidence decreased dramatically in the southern rift and ceased entirely in the northern rift. Tensional plate motion probably was transferred from the Gulf of Suez to sinistral strike-slip movement on the Dead Sea transform at this time. The quiescence in subsidence combined with a lowered global sea level resulted in the deposition of a thick (up to 4 km) series of evaporites within the central trough of the rift from the middle to latest Miocene. The accumulation of such a thick sequence of sediments during a phase of decreased tectonic subsidence is interpreted as a 'filling-in' of the rift topography which developed during the earlier period of rapid subsidence and rift-shoulder uplift and continued compaction. A rapid global sea level rise concomitant with a subsequent pulse of increased tectonic activity in the latest Miocene-earliest Pliocene returned the rift to dominantly marine conditions. Keywords: Gulf of Suez; basin analysis; rift sedimentation; tectonostratigraphy; rift subsidence; evaporites

Introduction The Red Sea rift and its northern extensions, the Gulfs of Suez and Elat (Aqaba), occupy an essentially north-south elongate depression extending from the. Nile Delta in the north to the Straits of Bab E1-Mandeb in the south where the rift system enters the Gulf of Aden. This series of subsided troughs extending over 2500 km in length and up to 450 km wide is a Cenozoic rift system formed as a result of divergence between the Arabian and African continental plates (Coleman, 1948a). The Red Sea and Gulf of Suez are primarily extensional basins structurally controlled by normal faulting (Garfunkel and Bartov, 1977; Cochran, 1983; Mart and Ross, 1987) whereas the Gulf of Elat and its northern continuation, the Dead Sea Rift, is the result of translational plate motion (Jaffe and Garfunkel, 1987). Various studies (eg. Robson, 1971; Garfunkel and Bartov, 1977) clearly show the Suez section of the rift is fault bounded, whereas in the northern Red Sea (offshore Egypt) interpretation of geophysical data supports the existence of horsts and tilted fault blocks developed on a basinward dipping homoclinal structure

(Tewfik and Ayyad, 1982; Barakat and Miller, 1984). A sketch map showing some of the major structural features in the region is presented in Figure 1. The Gulf of Suez and extreme northern Red Sea rift, on which this paper will focus, separates the Sinai block from the African plate. In this region the rift generally trends N W - S E and is approximately 500 km long and 60-80 km wide. The southern opening of the Gulf of Suez joins the Gulf of E l a t - D e a d Sea rift which is a N E - S W striking transcurrent fault system that separates the Sinai block from the Arabian plate. It is estimated that 100-110 km of sinistral offset has occurred along the E l a t - D e a d Sea rift (Fruend, et al., 1970; Robson, 1971; Garfunkel and Bartov, 1977; Joffe and Garfunkel, 1987). The purpose of this study is to describe the relation between tectonic events, basin subsidence, sedimentation and sea level changes in the Gulf of Suez, a relatively young, continental rift that developed as part of a new oceanic rift system. The northern Red Sea-Gulf of Suez rift is ideally suited for such a study since it is a Neogene feature whose stratigraphy and structure are well exposed and where ample data are

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available from over 25 years of petroleum exploration (Abdine, 1981; Michel, 1986). This, combined with extensive fieldwork in the region, provides a sizeable data base from which to work. Using constraints on chronostratigraphy and paleodepth from various sources combined with stratigraphic and structural

information from industry wells and other geophysical sources it has been possible to compile the data necessary to perform geohistory analyses throughout the region. We outline and synthesize the regional structural and stratigraphic units in the Gulf of Suez-northern Red

248 Marine and Petroleum Geology, 1988, Vol 5, August

Gulf of Suez-northern Red Sea. We then discuss the basic elements of geohistory analysis and the results of 'backstripping' various wells both across and along the axis of the rift.

Geologic background and basin setting The Suez rift of Miocene age is wider than the present day Gulf of Suez (Figure 2A). The rift basin is flanked

Sea Neogene rift: M. Richardson and 114.A. Arthur by uplifted shoulders which, in the southern half of the Gulf, consist of Precambrian crystalline basement from which the sediment cover has been denuded (Figures 1 and 2). Nearly 2 km of relief exists between the Gulf and the high peaks of the Sinai and the Red Sea Hills. There is less uplift flanking the northern Gulf where the rift shoulders are composed of Cretaceous to Eocene

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Marine and Petroleum Geology, 1988, Vol 5, August 249

Gulf of Suez-northern Red Sea Neogene rift: M. Richardson and M. A. Arthur pre-rift sedimentary sequence. The E1 Tih Plateau of pre-Neogene, Red Sea trending depression. west central Sinai, and the Northern and Southern Basalt flows and an extensive system of doleritic Galala Plateaus on the western side of the rift consist of dikes, which strike parallel to the rift trend, are lower Tertiary platform carbonate rocks that dip gently associated with faulting which occurred during the away from the basin margins. initial stage of crustal extension that produced the Structure within the rift consists of a series of horsts S u e z - R e d Sea rift system. The volcanic series are and grabens that trend subparallel to the border faults. radiometrically dated both in the Suez-Sinai and Most of the rotated horsts are buried by upper Miocene southern Arabian regions at 18-29 Ma (Bartov, et al., and Pliocene sedimentary rift-fill but several blocks are 1980; Coleman, 1984a, b). The structure of the initial presently exposed on the rift floor (eg. Gebel Zeit and rift basin was clearly outlined by the latest Oligocene to Esh Mellaha, Figure 2B). Three structural trends are earliest Miocene (Gass, 1977a; Garfunkel and Bartov, evident in the Gulf of Suez because the rift reveals a 1977; Coleman, 1984a: Labrecque and Zitellini, 1985). distinct asymmetry in fault block attitudes. In the Scarcity of upper Eocene sediments in the Gulf of northern and southern regions of the Gulf the fault Suez and Red Sea region is the result of a major blocks are predominantly southwest-dipping, while in regional regression. Deposition of marine carbonates the central area the blocks tend to dip to the northeast. of middle to late Eocene age was limited to an epicontinental seawa~ north of central Egypt. Bohannan et al. (1988) showed that marine sediments Pre-rift stratigraphy of Paleogene age also occur across much of the Arabian The once continuous A r a b i a n - N u b i a n shield is Peninsula with non-marine sedimentation across the constructed of a late Precambrian Pan-African, southern Red Sea, Sudan and Ethiopian regions. Those crystalline basement (Gass, i977b; Stoeser and Camp, upper Eocene sediments that are present in the 1985; Stern and Manton, 1987) and is overlain by a S u e z - n o r t h e r n Red Sea region consist of shallow water Paleozoic to early Cenozoic sedimentary sequence that carbonate and elastic rocks and are perhaps the frst predates the rift system. A thick series of mature, sediments to be deposited within an incipient rift continental sandstones rests upon the basement rocks (Garfunkel and Bartov, 1977: Snavely et al., 1979: and, because of the similarity of lithologies in these Sellwood and Nethcrwood, 1984). This limited sediments over an apparently wide stratigraphic range distribution of uppermost E o c e n e - O l i g o c e n e marine (Cambrian to early Cretaceous) and their sediments in the Gulf of Suez region would appear to unfossiliferous nature, the informal term 'Nubian indicate relative uplift of the area after deposition of sandstones' is assigned to this sequence. Above the the lower-middle Eocene carbonates and thus lend Nubian sandstones is a series of marine, predominantly support to the notion of doming of the continental carbonate-platform sediments of Cenomanian to lithosphere m the region prior to rifting (Lowell and middle or late Eocene age. This sequence was Genik, 1972; Saoudi and Khalil, 1984). It is difficult deposited during a major transgressive cycle which however, to ascertain to what degree this hiatus is a inundated much of the northern A r a b i a n - N u b i a n result of doming, or is the effect of the eustatic sea level platform (Awad and Fawzy, 1956; Hermina and Issawi, drop in the late Eocene documented by Vail et al. 1971; Van Houten, et al., 1984). These pre-rift rocks (1977) and Haq et al. (1987). There is no evidence of a are collectively referred to as the pre-rift sequence in sedimentary sequence of late Eocene to Oligocene age later discussions. of significant thickness or extent adjacent to the region Initiation o f rifting A proto-Gulf of Suez rift as early as the Carboniferous has been suggested by several authors (Said, 1962: Soliman and EI-Fetouh, 1970). Bhattacharyya and Dunn (1986) have argued for recurrent pre-Tertiary block movements parallel to the Miocene S u e z - n o r t h e r n Red Sea rift which suggest control of pre-Miocene structural elements in the development of the present rift. Several authors (eg. Garfunkel and Bartov, 1977: Sellwood and Netherwood, 1984) have shown that claims of a pre-Neogene, Suez depression were based o n erroneous correlation of beds of different ages. Furthermore, thickness and facies variations in the Cretaceous to Eocene sedimentary sequence are very gradual across the Suez region and do not show variations that might reasonably be expected across a proto-rift. Absence of Cretaceous to Eocene sediments from the rift shoulders is probably a result of post-Eocene erosion rather than non-deposition. Where sediments of these ages are present both on the margins and in the centre of the rift, they have similar thicknesses and relatively shallow water depths of deposition (Garfunkel and Bartov, 1977). Based on their studies in the southern and central Red Sea region, Bohannon et al. (1988) similarly concluded that there is no evidence for a 250

which could have resulted from the erosion of a hypothetical dome. Furthermore, the pre-rift sequence penetrated by industry wells in the centre of the Gulf of Suez, does not appear to be more deeply eroded than sites in more marginal locations of the rift overlain by lower Miocene synrift sediments. Although individual faulted blocks have undergone differential rates of erosion during rift development, there appears to be no regional trend of deeper erosion that would be expected over a dome crest situated where the present rift is now. Field relations and lission track studies outlined by Bohannon ,~,Ial. (1988) preclude Oligocene doming in the central and southern Red Sea region as well.

Extension a n d sea.floor ,spreading Various authors (Girdler and Styles, 1974; Styles and Hall, 1980: Labrecquc and Zitellini, 1985; Levi and Riddihough, 1986) cited evidence for oceanic-type crust extending over a wide area of the Red Sea depression, particularly in the southern regions. Bohannon (1986a, b; 1988, pers. comm.) indicated that the crust beneath the shelves and coastal plains in the southern Red Sea also consists of oceanic-type material with fragments of extended, older crust. Others (Lowel and Genik, 1972; Cochran, 1983a; Bonatti et al., 1984;

Marine and Petroleum Geology, 1988, Vol 5, August

Gulf of Suez-northern Red Garfunkel et al., 1987) suggested that much of the rift is floored by attenuated and block-faulted continental crust with intruded oceanic-type material and that seafloor composed primarily of oceanic crust appears to be developed only in the axis of the southern Red Sea. It is suggested that spreading began about 5 Ma in the central axial zone of the southern Red Sea (Phillips, 1970; Roeser, 1975). The main difference between these two 'schools of thought' is the degree of 'oceanization', ie. the relative volumes of each type of crustal material underlying the Red Sea depression, particularly in the southern

Sea N e o g e n e rift: M. Richardson a n d 114. A. Arthur regions. The central axis, with its oceanic-type basalts and linear magnetic anomaly pattern, breaks up into a series of deeps as it progresses northwards and is completely absent in the northern Red Sea. This trend has been interpreted as 'punctiform' propagation of the oceanic-like axial rift northwards and that the Gulf of Suez-northern Red Sea rift is underlain completely by stretched and thinned continental crust with oceanic-type doleritic intrusions decreasing northward (Cochran, 1983a; Bonatti et al., 1984; Bonatti, 1985; Uchupi and Ross, 1986; Crane and Bonatti, 1987; Martinez and Cochran, 1988). We favour this model for

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Marine and Petroleum Geology, 1988, Vol 5, August 251

Gulf of Suez-northern Red Sea Neogene rift: lid. Richardson and M. A. Arthur the northern Red Sea. Indeed, all industry wells structural highs to over 700 m in the depocentres of the available to us which penetrate crystalline basement in rift sub-basins (Figure 13). This distribution is due in the northern Red Sea and Gulf of Suez bottom out in part to erosion or non-deposition during the Precambrian continental crust (see also Tewfik and Aquitanian and partly to post-Aquitanian erosion of Ayyad, 1982; Barakat and Miller, 1984). the Nukhul Formation as tectonic activity continued. The depositional limit of the formation coincides with Syn-rift stratigraphy the rift shoulders to the east and west but the southern Subsidence in the rift basin since the early Miocene has and northern extents of the unit are less well defined. permitted the accumulation of up to 5 km of Lithofacies and thickness distributions are controlled continental and marine sediment. Sedimentation is by rotated fault blocks with relative uplift and adjacent primarily controlled by block structures resulting from subsiding lows. The Nukhut Formation overlies pre-rift fragmentation of continental crust within the rift. basement ranging in age from Precambrian to Generally, a thick series of siliciclastics and evaporites middle-late Eocene, depending upon the degree of are deposited in the grabens while carbonate buildups uplift and erosion prior to Nukhul deposition. The occur on the flanks of the rotated fault blocks. Lateral general trend is for the Nukhul to rest on older, pre-rift changes of facies and in thickness occur from the rift basement in the south where relative uplift and shoulders and margins of horsts into the grabens. subsequent erosion were greater, Lower to mid-Miocene marine rocks are more Depositional environments and facies relationships widespread than marine sedimentation today (Figure have been interpreted from our own studies of over 150 2A). industry wells, cuttings and other geophysical sources Stratigraphy and sedimentation patterns within the as well as in various published reports (Garfunkel and rift basin have been described by Said (1962), Gezeery Bartov, 1977; Sellwood and Netherwood, 1984; Saoudi and Marzouk (1976), Garfunkel and Bartov (1977), and Khalil, 1984; Scott and Govean, 1985). The Hagras and Slocki (1982), Sellwood and Netherwood sediments are of mixed elastic, carbonate and (1984), and Scott and Govean (1985). A summary of evaporitic composition containing such faunal much of the work on lithostratigraphy is illustrated in assemblages as brackish water ostracods and oysters, Figure 3 and presented in a chronostratigraphic coral patch reefs, shallow marine foraminifers such as framework in Figure 4 from which age control used in Miogypsina 37)., Elphidittm crispum and Amphistegina sp., as well as many species of pecten. Reworked the subsidence analysis was derived. The Cenozoic geochronology of Berggren et al. (1985) was adopted Cretaceous and Paleogene foraminifers are also locally abundant (Scott and Govean, 1985). The barren Shoab for the construction of Figures 3 and 4. Ali Member and the October Member contain Faulting and subsidence had started by latest feldspathic sandstones, noncalcareous red shales and Oligocene to earliest Miocene time. The subsequent polymictic conglomerates interpreted to be of Miocene to Recent sedimentary sequence within the alluvial-fluvial origin. These sediments also contain rift has been subdivided into three major oxidized, coal-like (type III) organic carbon. The lithostratigraphic units: the lower to middle Miocene Ghara and Gharamul Members are laterally equivalent continental to open marine Gharandal Group, the to the October Member and their carbonates, marls, middle to upper Miocene, predominantly evaporitic shales and evaporites are representative of both Ras Malaab Group and the mixed siliciclastic, restricted- and shallow-marine environments associated carbonate and evaporitic post-Miocene Group. The with the initial marine transgression into the rift. first stratigraphic analysis of the Gulf of Suez section It is difficult to ascertain the extent of relief due to was Gezeery and Marzouk (1976). Their work has been faulting in the Aquitanian. Chenet et al. (1984) modified using modern facies concepts by Garfunkel proposed normal faults with offsets of up to 1000 m and Bartov (1977); ttagras and Slocki (1982); Fawzy during the early Miocene, and Garfunkel and Bartov and Abdel Aal (1984): Saoudi and Khalil (1984) and (1977) suggested a subdued topography with perhaps Richardson et al. (1986). only a few hundreds of meters relief at most during the Gharandal Group. The Gharandal Group, informally Aquitanian as the rift began to develop. Clearly the known as the 'Globigerina marls', consists of the sedimentary sequence suggests horst relief great Nukhul, Rudeis and Kareem Formations. enough to have their sedimentary cover unroofed down into the Nubian sandstone and perhaps crystalline Nukhul Formation basement at least in the cxtreme southern sections of Apart from the very sparsely represented Oligocene the Suez rift. On the basis of estimates of the average "Abu Zenima' sediments (Garfunkel and Bartov, 1977) thickness of the pre-rift sedimentary sequence in the the lower Miocene Nukhul Formation deposits record southern Gulf of Suez and northern Red Sea the earliest phase of sedimentation in a rift basin (Richardson, 1982; Barakat and Miller, 1984), environment. The lower strata of this formation unroofing of horsts down into the Nubian sandstones document continental sedimentation in the young would suggest initial rifting fault displacements on the incipient rift while the upper portion chronicles the order of 1000 m. It should be pointed out that if initial marine transgression into the basin. This erosional, depositional and fault motion rates are observation is highly significant because it shows that comparable, then subdued relief is not incompatible the floor of the rift basin was at sea level within with fault displacements of 1000 m. 1-3 m.y. of its formation. The Nukhul Formation is Latest Aquitanian-earliest Burdigalian appears to chiefly of Aquitanian age (Gezeery and Marzouk, 1976; be a time of renewed tectonic activity (Post Nukhul Garfunkel and Bartov, 1977; Allen et al., 1984), but its Event; Garfunkel and Bartov, 1977; Beleity, 1982; base is undated due to the lack Of diagnostic fossils. Chenet et al., 1984) which inw)lved the rotation and This formation varies in thickness from 0 m on uplifting of fault blocks as well as renewing subsidence 252

Marine and Petroleum Geology, 1988, Vol 5, August

Gulf of Suez-northern Red Sea Neogene rift: M. Richardson and M. A. Arthur

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Marine and Petroleum Geology, 1988, Vol 5, August 253

Gulf of Suez-northern Red Sea Neogene rift: M. Richardson and M. A. Arthur within the rift. The result was partial erosion of the of the central trough. Our own observations of industry Nukhul Formation at some localities. This tectonic wells and seismic sections and studies by John Smale event and relative high sea level stand (Haq et al., 1987) (Univ. South Carolina pets. comm., 1987) show allowed for greater connection to the open ocean continuous uninterrupted sedimentation of marls and during which the Rudeis and Kareem Formations were shales across the l o w e r - u p p e r Rudeis boundary in deposited. many of the central grabens. This may be due to more marginally positioned, rotated horst blocks acting as sediment dams to the coarser-grained siliciclastics being Rudeis Formation and the Mid-Clysmic Event carried into the basin from the surrounding uplifted margins. The Rudeis Formation consists of up to 1 km of open marine, basinal, globigerina-rich marl and shale with sandstone, conglomerate and carbonate rocks in more Kareem Formation marginal settings. Distribution of the Rudeis extends The Kareem Formation, 350 m thick in places, up to the rift shoulders and at least as far north as the consists of a lower unit of sandstone, carbonate and northern tip of the present gulf. The Rudeis Formation evaporite rocks (Markha or Rhami Member) and an is divided into upper and lower parts whose boundary upper sequence of sandstone, marl and limestone. The corresponds in time to the well known 'mid-Rudeis' or Kareem, like the Rudeis, extends to the rift shoulders 'mid-Clysmic" tectonic event (Garfunkel and Bartov, and at least as far north as the tip of the present gulf. 1977; Beleity, 1982; Chenet et al., 1984; Scott and The age of the Kareem Formation is well constrained, Govean, 1985; Smale et al., 1987). The age of the having been deposited during the Langhian to earliest Rudeis Formation is biostratigraphically well Serravallian (15.5-14 Ma: Figures 3 and 4), (Allen et constrained (Figure 4). The lower Rudeis is Burdigalian al., 1984: Scott and Govcan, 1985; Smale et al.. 1988). (ca. 21-16.5 Ma) in age while the upper Rudeis is The Rhami Member of the formation is distinct partly Langhian (ca. 16.5-15.5 Ma) in age (Garfunkel because of its anhydrite sequence. Core studies reveal a and Bartov, 1977; Andrawis and Abdel Malik, 1981; series of well developed Sabkha-type evaporite cycles Allen et al., 1984; Scott and Govean, 1985; Smale et al., indicating very shallow to emergent conditions in the 1988). These and other authors (eg. Chenet etal., 1984; basin by the end of R u d e i s - o n s e t of Kareem Evans, 1987) reported that the mid-Clysmic tectonic deposition. Some areas contain reworked clasts of the event occurred at approximately 16.5 Ma. The age of anhydritic sequence, either as a result of reworking of this event, distinguishable lithostratigraphically in some the evaporites during deposition of the shallow-marine industry' wells and outcrop due to dip changes and upper unit or erosion from newly rotated, uplifted increase in coarser-grained siliciclastic sedimentation, crests of still active fault blocks within the basin. It is is also biostratigraphically well controlled. difficult in the Kareem to distinguish whether The lower Rudeis is chiefly a marine progradational sedimentation patterns are a response to global sea unit which onlaps structural highs and blankets the level, rift tectonics or a combination of both. relief formed during the post-Nukhul event. Sandstone is abundant in the lower part of the sequence but gives Ras M a l a a b G r o u p . l'he Ras Malaab Group, also way to deeper water marine shale and marl in the known informally as the "evaporite group' consists of central basimd area (Allen et al., 1984). Sedimentation the Belayim, South Gharib and Zeit Formations which in the marginal areas of the rift consists of tire characterized by dominance of anhydrite and coarser-grained siliciclastic lobes interpreted as halite. Microfossils are rare in the group, making fan-deltas (Hagras and Slocki, 1982), occasional determinations of age and duration of deposition conglomeratic wedges and coral-bearing limestones difficult to constrain, The evaporites are generally (Sellwood and Netherwood, 1984). restricted to the central trough of the rift, The mid-Clysmic event is interpreted as a major approximately within the outline of the present tectonic reactivation in the rift which inw)lved the coastline of the gulf, with the exception of some segmenting and rotating of major pre-existing tilt marginally situated rapidly subsiding grabens. The blocks into smaller units. Concomitantly, the shoulders evaporites extend south where they floor much of the of the rift, including the Sinai Massif and Red Sea Hills, Red Sea basin (Whitmarsh et al., 1974). The Ras Malaab Group is very thin and discontinuous in the underwent rapid uplift (Garfunkel and Bartov, 1977). This tectonic event is coincidental with a drop in the northern Suez rift, while up to 4 km of evaporites have accumulated in the central trough of the gulf. eustatic sea level (Haq et al., 1987). The combined effect is the presence of a hiatus over crests of the Surface sections and subsurface data enable precise location and mapping of the various evaporite and structural highs, angular unconformities and a distinct associated facies within the basin. The change in sedimentation patterns. This hiatus is not gypsum-anhydrite facies commonly interfingers with regional as sedimentation was more or less continuous carbonate reefs and alluvial fans or fan deltas along the in the deeper basinal regions of the rift. margins. Thick halite-bittern salt deposits are The upper Rudeis is a marine retrogradational unit in restricted to the basin centre. Distribution of the which conglomeratic alluvial fans and sandy fan-deltas, coarse-grained elastic rocks in discrete lobes up to 1 km that had a source in the progressively uplifting thick indicates point sources for sediments entering the shoulders, prograded out into the rift basin (Hagras basin, probably via major wadi systems (l;Tgures 14 and and Slocki, 1982; Allen et al., 1984; Smale et al., 1987). Smale et al. (1988) have shown that the upper Rudeis Z5). Detailed studies of ltle ewiporites from coastal distal fan deposits downlap onto lower Rudeis outcrops and subsurface cores suggest that they were sediments at the basin margins. The pulse of deposited mainly in the form of bedded selenites (now coarser-grained siliciclastics (conglomerate; sandstone) anhydrite in subsurface) and zoned chevron halites being shed into the basin did not reach into many parts

254

Marine and Petroleum Geology, 1988, Vol 5, August

Gulf of Suez-northern Red (Richardson and Arthur, in prep.). Many of the selenite and halite textures indicate a predominantly shallow subaqueous environment rather than deep-water or sabkha-type deposition of evaporites. Periodic 'freshening' occurred within the basin during deposition of the evaporite series. This freshening is demonstrated by intercalated reef or carbonate-bank deposits. Belayim Formation The mid-upper Serravallian (14-10.4 Ma) Belayim Formation consists of intercalated evaporites (Baba and Feiran Members) and sandstone, shale and carbonate rocks (Sidri and Hamman Faraun Members) indicating greatly fluctuating depositional environments. The more normal marine sediments of these latter members contain faunal assemblages that allow for some constraints to be placed on the age of the unit (Scott and Govean, 1985). The Belayim Formation extends across some of the marginal rift areas, although thicknesses there are substantially lower. South Gharib Formation Coincident with a major, rapid global sea level drop (Haq et al., 1987) the Tortonian (10.4-6.4 Ma) South Gharib Formation consists of a very thick (up to 3 km) sequence of halite. The halite is restricted to the southern two-thirds of the Gulf of Suez rift and the Red Sea basin. With the exception of some of the lower salt strata, individual halite units can be widely correlated across the rift basin. Zeit Formation The Messinian (6.4-5.3 Ma) Zeit Formation is a dominantly sulphate evaporite sequence consisting of thin (cm to m) alternating beds of shale and anhydrite with occasional halite. The top of this formation is the top of the evaporite sequence in the Gulf of Suez and Red Sea rift. It acts as a good seismic marker in the Gulf and is known as 'reflector S' in the Red Sea (Whitmarsh et al., 1974). 'Reflector S' and Gulf of Suez Zeit Formation are tentatively correlated with the top of the Messinian evaporites ('Reflector M': Ryan and Hsti et al., 1973) in the Mediterranean basin. Post Miocene sedimentation With the onset of a rapid sea level rise at the end of the Miocene (Haq et al., 1987), sedimentation patterns indicate a return to more normal marine deposition throughout much of the rift basin, but isolated sub-basins within northern sections of the rift show continued evaporite deposition through the Pliocene (Fawzy and Abdel Aal, 1984). Thickness distribution of Plio-Pleistocene and Recent sediments within the rift is highly variable. Some offshore regions show thick (up to 1500m) accumulations of coarse-grained siliciclastics with subordinate carbonate. Carbonate reef platforms have developed in the offshore areas of the southern Gulf of Suez (Roberts and Murray, 1984), while in more marginal areas alluvial fans, fan deltas and fringing reefs have developed (Hayward, 1985; Purser et aL, 1987). Onshore, Suez rift Plio-Pleistocene deposits of lacustrine origin have been studied by Issar and Eckstein (1969). The coastal plain of the northern Red

Sea Neogene rift: M. Richardson and M. A. Arthur Sea and southern Gulf of Suez contains a series of raised coral reefs and alluvial terraces.

Subsidence analysis Subsidence analysis or 'backstripping' (Steckler and Watts, 1978; Angevine and Turcotte, 1981) was carried out at various localities within the Gulf of Suez rift using lithologic and stratigraphic information from industry exploration wells. By quantitively removing the effects of subsidence caused by sediment compaction, sediment loading and basin water-depth changes, one can estimate the tectonic subsidence. The tectonic subsidence plotted against time can be interpreted in terms of the thermal and mechanical history of the subsiding basin after thinning and post-rift cooling of the lithosphere. The timing, rates and magnitude of subsidence can then be correlated with sedimentation history and known tectonic events within the Suez-northern Red Sea rift. The subsidence history of the Gulf of Suez has been studied by Moretti and Colleta (1987) although they did not account for the effects of evaporite composition on compaction corrections in their backstripping model (see procedure below). Steckler (1985) and Moretti and Chenet (1987) used the results of preliminary backstripping in their estimates of crustal extension and models of secondary convection in the Gulf of Suez region. Tectonic subsidence curves were also used as constraints on the timing of plate boundary developments in the Red S e a - S u e z - D e a d Sea rift system (Steckler and ten Brink, 1986). The procedure used in this study is modified slightly from Steckler and Watts (1978) and Angevine and Turcotte (1981). The 17 sites used (Figure 2C) were carefully selected from over 100 available industry wells. Most petroleum exploration and production wells are sited in areas of prominent structure, such as salt domes, flanks and tops of horst blocks, so caution must be taken in selecting wells for backstripping. Repetition or absence of syn-rift strata due to faulting will lead to erroneous results in subsidence analysis. Using constructed cross sections and seismic lines as a guide, only those wells without significant faulting or possible disruption by salt movement were chosen by us. Seismic section and well log data from the northern Red Sea near Egypt (Safaga and Quseir) showed that faulting and salt flowage was too intense for useful backstripping curves to be derived (Tewfik and Ayyad, 1982; Barakat and Miller, 1984). The distribution of studied wells was selected so that east-west and north-south trends in subsidence could be observed as well as differences due to structural variations such as proximity to rotated horst blocks, marginal rift areas and deep central grabens. The procedure of 'backstripping' involves: 1. Estimation of thickness, lithology, age and approximate water depth of deposition for each of the formations of the Neogene. Well logs were used to obtain thickness and lithology information. Percentages of up to six different lithologies were calculated for each formation. The absolute age of the upper and lower boundaries of each of the formations was determined by comparing the age ranges of syn-rift strata of 18 stratigraphic studies completed for the Gulf of Suez. These estimates

Marine and Petroleum Geology, 1988, Vol 5, August

:)55

Gulf of Suez-northern Red Sea Neogene rift: M. Richardson and M. A. Arthur have been plotted against the Cenozoic estimate the contribution of sediment loading to the geochronology of Berggren et al., (1985; Figure 4). total basin subsidence through time. Choice of age boundaries was weighted more in favour of the more recent biostratigraphic Other recent subsidence analyses for the Suez rift determinations. have not taken into account the special problems of Paleodepth values used in the analyses are evaporite compaction (Moretti and Colletta, 1987). We presented in Figure 5 and are derived from the work assumed zero original porosity for the halite evaporite of Beleity (1982), Scott and Govean (1985) and our components so in that case there is no decompaction own studies of the evaporites (Richardson et al., correction. However, previous work (Richardson et al., 1986). The additional complication of estimated 1986; see also discussion under Ras Malaab Group) has paleodepths during evaporite deposition and the shown that the majority of the sulphate evaporites that effects of evaporite composition on compaction are currently in the form of anhydrite within the rift corrections are discussed in more detail below. sequence were originally deposited as gypsum. At the 2. Each formation in the stratigraphic sequence is range of hydrostatic pressures and geothermal sequentially removed from the top down. Where gradients existing within the Gulf we estimate that the biostratigraphic control permitted, the upper and transition from gypsum to anhydrite takes place over a lower Rudeis were considered as separate steps. At restricted zone 600-700 m deep (McDonald, 1953). each step of removal, the porosity of the remaining Examination of well data supports a transition from lithologic units in the sequence is recalculated to gypsum to anhydrite at about 600 m depth. The correct for the effects of unloading. This calculation mineral transformation from gypsum to anhydrite in effect returns the sedimentary sequence to its results in a 40% reduction in volume (Holser, 1979), original thickness before burial by successive units. providing that the water of dehydration is expelled Since actual porosity versus depth profiles for from the rock. Furthermore, this reduction occurs over individual lithologies was difficult to obtain for the such a short depth range that at the depth scale used it Gulf of Suez strata, we utilized the porosity versus appears step-like and does not decrease exponentially depth profiles of Rieke and Chilingarian (shales; in contrast to the other sediment porosities. 1974), Schmoker and Halley (limestones; 1982) and Consequently when backstripping evaporite strata the Bond and Kominz (sandstones; 1984) summarized increase in volume resulting from re-hydration of in Figure 6a. Figure 6b shows an incomplete shale anhydrite in the sequence above 700 m is calculated as porosity versus depth profile derived from the shale instantaneous rather than with an exponential decay density data available from well E in the Gulf of term (Figure 6a). Depending upon the relative Suez. thickness of the evaporite sediments compared to the 3. At each step the top of the formation is placed with other sediments in the sequence and depending upon its top at a depth below sea level which corresponds the percent of calcium sulphate minerals in the to the best approximate depth of water in which the evaporite suite, the effect of decompacting anhydrite unit was deposited. In the resulting plots this is can mean a difference of up to 5% in the tectonic shown as a shaded band instead of a line in order to subsidence curve (Figure 7). present the range of estimated water depths rather Also in a previous study (Richardson et al., 1986) we than a single value. suggested that the evaporites were deposited in a 4. The density of the entire sedimentary column is also shallow subaqueous environment in a deep basin. The calculated at each of these time steps in order to presence of such a large evaporite deposit in this setting

(M=) I0

20

0

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::i:::

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Figure 5 Estimated basin and water d e p t h s for the Neogene Suez rift sequence. Based on various well information and previous studies (Richardson, Arthur and Katz, 1986; Scott and Govean, 1985; Beleity, 1982)

256

Marine and Petroleum Geology, 1988, Vol 5, August

Gulf of Suez-northern Red Sea Neogene rift: 114.Richardson and M. A. Arthur P O R O S I T Y

(0)

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suggests that the rift basin is isolated from the open ocean and that sea level within the basin is 100-200 m below the global sea level. Since potential evaporite accumulation rates are much greater than basin subsidence rates, it would be difficult to maintain a very deep basin-shallow water environment.

Results The stratigraphic accumulation and tectonic subsidence curves for wells in the Gulf of Suez and northern Red Sea are shown in Figures 9-12. Figure 9 shows example curves from two locations, well E in the marginal, east-central Gulf of Suez and well P from a deep graben in the southern Gulf of Suez. Figure 10 (note scale change) shows two marginal wells from the east-central Gulf of Suez, an area of less total subsidence. With few exceptions, a common, general pattern of subsidence exists throughout the Gulf of Suez and northern Red Sea. After an initial rapid subsidence event within the rift (25 to 15.5 Ma), there was a period of tectonic quiescence (15.5 to 6.4 Ma) before a final pulse of tectonic activity (6.4 Ma to Present). There are also several major regional trends (Figures 11 and 12): 1. the timing of onset and durations of tectonic

subsidence and quiescence are the same for both the northern and southern regions of the rift system; 2. magnitudes of total subsidence, tectonic subsidence and stratigraphic accumulation increase from north to south. During the non-tectonic subsidence period, stratigraphic accumulation (evaporites) in the northern sections of the rift was negligible; 3. magnitudes of total and tectonic subsidence and stratigraphic accumulation are greater in the central

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M a r i n e and P e t r o l e u m G e o l o g y , 1988, Vol 5, A u g u s t

257

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trough than in the onshore marginal rift areas (see also Figure 8); and 4. after tectonic subsidence ceased at 15.5 Ma, stratigraphic accumulation (and thus total subsidence) continued but only in the mid- and southern central trough region. This phenomenon of continued sediment accumulation in the central trough during a tectonically quiescent period is also shown in Figure 8.

southern Suez-northern Red Sea rift. These values correspond to 5-10 km of extension for the 40-60 km wide northern Suez rift, 16-27 km of extension for the 80km wide central Suez rift and 3 0 - 4 0 k m of extension for the extreme southern Gulf of Suez-northern Red Sea rift. Values of 25-35 km of extension in the Gulf of Suez have been suggested by LePichon and Francheteau (1978), Cochran (1981), Steckler (1985) and Steckler and ten Brink (1986). For the extreme northern Suez rift localities and most of the wells in the marginal rift areas, almost 100% of the extension had occurred by - 15 Ma. In the central and southern Suez rift main trough, 65-90% of the extension had occurred by - 1 5 Ma, the remainder (3-10 km of extension) occurring since 6.4 Ma after the middle to late Miocene quiescent period. Caution must be used with these extensional estimates for several reasons. First, the initial subsidence (first 5-10 m.y. following the rifting event) resulting from extension is essentially an isostatic compensation due to the mass change in the vertical sense (thinning). The ensuing thermal subsidence, the result of lithospheric cooling and thickening, may last for over 100 m.y. (Steckler and Watts, 1978). Thermal subsidence is poorly constrained within the first 40 m.y. or so and since the Suez-Red Sea rift has only developed over the past 25 m.y. we are probably not observing the total tectonic subsidence. To further complicate matters, the 'rifting event' appears to have lasted approximately 10 m.y. Cochran (1983b) suggests that a finite rifting event of 10 m.y. with lateral heat conduction taken into account increases synrift subsidence at the expense of post rift (thermal) subsidence by a minimum of 10-15%. Steckler (1985)

O'

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Discussion Tectonic (thermal) subsidence results both from contraction associated with cooling of thinned lithosphere (McKenzie, 1978; Steckler and Watts, 1978) and increased density caused by the crystallization of the upwelled, partially melted asthenosphere. Isostatic readjustment of brittle lithosphere on initial attenuation also adds a 'mechanical' component to the initial tectonic subsidence. The Gulf of Suez-northern Red Sea rift basin most probably resulted from a period of continental lithospheric attenuation, heating and subsequent cooling. The main rifting and subsidence event occurred between latest Oligocene and middle Miocene (latest Chattian- mid-Serravallian; 25-15.5 Ma), a period of approximately 10 m.y. Using field relations and radiometric dates, Bohannon et al. (1988) have been able to constrain the timing of continental rifting and mechanical extension in the southern Red Sea to between 25 to 21 Ma. Using the relationship between subsidence and crustal thinning given by McKenzie (1978) and Steckler (1985) and assuming similar initial thickness values of crust (41 km) and lithosphere (225 km) to those of Steckler (1985) for the Suez-northern Red Sea region, we estimate crustal extension factors (~) of 1.25-1.5 for

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Figure 10 Geohistory for wells F and H, Gulf of Suez. (Note the scale change from Figure 9)

M a r i n e and P e t r o l e u m G e o l o g y , 1988, Vol 5, A u g u s t

259

Gulf of Suez-northern Red Sea Neogene rift." M. Richardson and M. A. Arthur 20

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and Moretti and Chenet (1987) both indicate that uplift of the Suez rift shoulders far exceed the amount of uplift that can be expected by lithospheric heating resulting from 30 km of extension. They invoke secondary, small-scale convection induced by the rifting event to explain the magnitude of shoulder uplift. This convection would produce a thermal perturbation to the normal tectonic subsidence and contribute to variations in subsidence history within the 260

gulf. Finally, most hydrocarbon prospects within the Gulf of Suez are on top or flanking structures such as fault blocks and are rarely in deep grabens. Consequently most calculated subsidence curves do not represent the maximum subsidence in the rift. Tectonic subsidence rates during the first 4 - 5 m.y. are highly variable (0-100 m/m.y.). Total (compaction corrected) subsidence was on the order of 300-350 m/m.y. Absolute subsidence rates are difficult

Marine and Petroleum Geology, 1988, Vol 5, August

Gulf of Suez-northern Red Sea N e o g e n e rift: M. Richardson and M. A. Arthur of the rift, and the overall environment was one of deposition of mixed marl, siliciclastic and evaporite rocks in more rapidly subsiding sub-basins (Figure 13) with carbonate reefs capping or fringing the highs. The rift basin was at sea level soon after its initiation (1-3 m.y.) (Sellwood and Netherwood, 1984) with a marine connection to the Mediterranean in the north. Communication with the open ocean throughout deposition of the lower open marine units (Nukhul to Kareem Formations) and evaporitic Belayim

to obtain for this earliest rift time period since the base of the Nukhul Formation is probably diachronous. However, our own studies and those of Saoudi and Khalil (1984) indicate there is no overall trend of younging in age of the base of that unit from south to north. The variable subsidence rates tend to support Garfunkel and Bartov's (1977) interpretation of subdued relief throughout most of the rift during earliest Miocene times. Locally, horsts were uplifted and stripped, particularly in the extreme southern parts

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Marine and Petroleum Geology. 1988. Vol 5. August 261

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Gulf of Suez-northern Red Sea Neogene rift: M. Richardson and M. A. Arthur Formation was exclusively with the Mediterranean Sea subsidence rates had slowed greatly and in many (Souaya, 1966; Said and El-Heiny, 1967; Garfunkel localities had actually ceased. By early Serravallian and Bartov, 1977; Andrawis and Abdel Malik, 1981; times (end Kareem, ,--14 Ma), tectonic subsidence Scott and Govean, 1985). The post-evaporite Pliocene across the entire Gulf of Suez had ceased. Stratigraphic strata offer the next direct evidence of open marine accumulation and hence total subsidence continued connection, where the affinities are with the Indian only in the central and southern main trough of the Ocean (Whitmarsh et al., 1974; Gaffunkel and Bartov, Suez rift (Figures 8, 11 and 12). 1977). Deposition of evaporite strata in the main trough is Tectonic subsidence rates increased at approximately coincident with the cessation of tectonic subsidence. A 2(1 Ma and, together with a continued rise in global sea major fall in global sea level in the earliest Tortonian level, produced a marine transgression within the rift (10.5 Ma) sent the basin from primarily calcium basin. A thick sequence of marl and shale was sulphate precipitation to the more restricted halite deposited in the subsiding grabens, with carbonate and evaporite phase. Distribution of the main evaporite coarser-grained siliciclastic rocks (Rudeis Formation) facies in the rift basin is shown in Figure 14. Siliciclastic on the flanks of the horsts. Even at this stage, some of sediments were introduced ahmg the margins of the the earlier formed horsts had not subsided sufficiently evaporite basin at point sources in the same general enough to allow sedimentation to occur on them (eg. localities as during N u k h u l - Kareem deposition (Figure well N). Tectonic subsidence rates increased (up to •5). The continued rapid accumulation of evaporites 250 m/m.y.) after the 'Post Nukhul Event' of Beleity within a rift basin having no tectonic subsidence may be (1982) and the total subsidence rates were as high as the result of two related processes. If indeed the main 400 m/m.y, in the deeper, more rapidly subsiding trough was relatively sediment starved, the evaporites grabens. There is no evidence of a south to north would have rapidly begun to fill the subsided rift propagation of rifting in the Gulf of Suez. Tectonic topography and thus contributed to sediment loading events appear to be simultaneous along the entire which, combined with the gypsum compaction history length of the rift basin. Indeed, when one considers the previously outlined, would have perpetuated total, aforementioned 25 to 21 Ma rifting event in the non-tectonic subsidence (i.e. load subsidence). Vertical southern Red Sea (Bohannon et al., 1988), there is little motion continued along major faults bordering the evidence for propagation of initial rifting along the main trough in response to loading observed in the entire length of the Red S e a - S u e z rift system. central rift basin. The Mid-Clysmic Event (16.5 Ma), was a period in The Zeit Formation. with one of the shortest time which the rift basin subsidence, structural and durations (6.4 to 5.3 Ma), is one of the thickest sedimentation patterns were radically changed. Fault sedimentary units within the synrift sequence. Higher blocks were segmented and rotated, producing both global sea level and renewed tectonic activity are new sediment sources and sediment dams. Uplift and responsible for this phase of less restricted evaporite erosion of higher-relief rift shoulders also began about deposition within the rift basin. Since the riffs this time (Garfunkel and Bartov, 1977). Bohannon et connection with the Mediterranean Sea persisted at al., (1988) have suggested similar timing (15 Ma) for least until the end of the Serrevailian (10.5 Ma; top of the onset of shoulder uplift in the southern Red Sea. Belayim Fm.) and the S u e z - R e d Sea basin was In the northern Suez rift, tectonic subsidence ceased connected to the lndian Ocean by the early Pliocene completely. In the central and southern rift subsidence (5.3 Ma), the change-over must have occurred within rates became more diverse. At some localities in the the late Miocene. If the Mediterranean Sea was indeed main trough tectonic subsidence rates increased from a deep basin-shallow water setting during deposition <100 m/m.y, to 400 m/m.y. (eg. wells D and K, Figure of Messinian evaporites there (Hsti et al., 1973: 11) while at other sites tectonic subsidence rates appear Schreiber and Friedman, 1976), then the marine not to have changed. Total subsidence curves show the evaporites accumulating in the S u e z - R e d Sea rift most dramatic increase to over 700 m/m.y, for sites in during the Messinian must have been sourced from the the central trough. Sedimentation (upper Rudeis) at Indian Ocean. We suggest that the change to less this time (16.5-15.5 Ma), switched to large prograding restricted evaporitic conditions in the Suez rift, alluvial fans and fan deltas at the rift margins as rift concomitant with a general rise in eustatic sea level and shoulders uplifted and global sea level fell. There is renewed tectonic activity, was the period in which the little evidence of any absolute uplift within the rift. The Indian Ocean became the primary source of marine tectonic style was characterized by rotated fault blocks waters within the S u e z - R e d Sea rift basin. The subsiding within the rift margins. tentative age of this transition is latest Serravallian It is interesting to note that, at a time of increased (6.4 Ma). A short, sharp fall in sea level during the latest Messinian may be responsible for the persistent subsidence within the main trough, upper Rudeis sedimentation patterns in many central grabens did not 'Zeit Salt' observed in industry wells in the Suez rift. As change from marl and shale to coarser-grained with the Mediterranean basin, the Red Sea rift basin siliciclastic rocks. Although increased elevation may returned to normal marine conditions at the end of the have provided a more abundant local source of Miocene with a rapid rise in global sea level. Evaporite sedimentation, the increased elevation and backtilting deposition in isolated sub-basins within the rift of the rift shoulders would have drastically reduced the continued into the Pliocene. Although the negative area of the potential drainage basin. This change in tectonic subsidence (uplift) observed in some drainage pattern, combined with the earlier suggestion backstripped marginal wells could be an artifact, study of marginal horsts acting as sediment dams, may have of the rift geology shows a plateau consisting of produced sections of the central trough where sediment Belayim Formation uplifted over 450 m above present accumulation rates were drastically reduced. sea level at Ras Malaab (Figure 2b). The uplift By the end of Rudeis deposition (15.5 Ma), tectonic documented along the margins of the present day Gulf

264

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Both the present day structural regime of with Red S e a - G u l f of Elat tectonics en-echelon left-lateral strike-slip faulting in the Gulf of Elat, and the current rift structural trends in the Opening of the northern Red Sea continues today by northern Red Sea (Mart and Hall, 1984: Mart and diffuse extension of continent crust (Cochran, 1983a). Ross, 1987). are interpreted to have been in existence The present opening of the Red Sea rift involves in their present form only for the past 5 m.y. because translation from primarily extensional motion to one of the upper part of the Miocene evaporite group is strike-slip motion in the Gulf of Elat (Ben-Avraham et tectonically deformed in those regions (Uchupi and al., 1979: Mart and Rabinowitz, 1984: Mart and Ross, Ross, 1986; Mart and Ross, 1987). 1987: Courtillot et al., 1987; Joffe and Garfunkel, It seems there is an earlier expression of this change 1987). Our data suggests that the main rifting event in in relative plate motions manifested in the phase of the Gulf of Suez lasted from latest Oligocene to middle renewed tectonic activity in the Gulf of Suez at Miocene (25-15 Ma) and little tectonic activity has ~ 6 . 4 Ma. We suggest that perhaps the 5 Ma date of occurred since. From their studies in the Sinai triple initiation of the tectonic rearrangement in the region junction area, Courtillot et al. (1987) concluded that may be an underestimation. the period 2 0 - 15 Ma is a time of major changes in the Recent minor subsidence in en-echelon lows geodynamics of the northern boundaries of the African immediately to the north of the Gulf of Suez was and Indian plates. Several studies of the Gulf of discussed bv Garfunkel and Bartov (1977). More E l a t - D e a d Sea rift suggest onset of sinistral strike-slip recently, Joffe and Garfunkel (19871 made the motion was at least as early as the Middle Miocene observation that the present-day basins of the central (15 Ma) (Garfunkel et al., 1974; Eyal et al., 1981: trough of the Gulf of Suez and the topographical Courtillot el al., 1987: Kashai and Croker, 1987). expressions of the Bitter Lakes and Lake Timsah to the Ben-Avraham et al. (1979) further divided the Dead north of the Suez Gulf, are spatially' arranged in an Sea motion (105 km) into two distinct phases - - 65 km en-echelo, fashion, which the}' point out, is a geometry in the Miocene and 40 km during the past 4 - 5 m.y. commonly associated with strike-slip motion. More recently, however, it has been suggested that the Courtillot et al. (1987) have suggested a model of overall slip rate has remained constant throughout the present-day, right-lateral slip along the Gulf of Suez history of the Dead Sea plate boundary and that trend on the basis of seismicity, field and L A N D S A T observed change to the current rift structure which studies carried out bv Tapponier and Armijo (19851. occurred at about 5 Ma (Mart and Hall, 1984; Mart and Hence, this final pulse of tectonic activity in the Gulf of Ross, 1987) is more likely to have been caused by an Suez probably' inw)lves more oblique motion than increased component of transverse separation along the normal extension. The structural interpretation of transform fault (Joffe and Garfunkel, 1987). Courtillot et al. (1987) can be further constrained with Based on our subsidence analysis, we suggest that our subsidence data for the Gulf of Suez (Figure 17). extension normal to the trend of the Gulf of Suez rift Onset of more opm; marine conditions throughout was terminated in the Middle Miocene ( ~ 1 5 M a ) and most of the Gulf of S u e z - R e d Sea rift at 5.3 Ma likely that relative plate movements involved in the opening resulted from continuing tectonic activity associated of the Red Sea rift switched to strike slip motion on the with the initiation of seafloor spreading in the southern Dead Sea-Gulf of Elat transform. The mid-Clysmic Red Sea, which led to less restricted marine circulation event of 16.5 Ma may in part signal a change in tectonic with the Indian Ocean through the Straits of Bab stresses thai also led to the initiation of the Dead Sea EI-Mandeb. transform, but onset of significant motion along the strike-slip fault probably did not occur until later ( - 1 5 Ma). It has been proposed by Steckler and ten Summary and conclusions Brink (1986) that this change in relative plate motion resulted from increased lithosphere strength across the Correlation of stratigraphy, sea level and major tectonic events is summarized in Figure 16. The Gulf of Mediterranean continental margin acting as a barrier to S u e z - n o r t h e r n Red Sea rift formed as a result of the opening of the Red Sea rift through the Suez continental attenuation which was initiated - 2 5 Ma. region. The Suez rift was the northernmost extension of the The latest pulse of tectonic subsidence is present only Red Sea rift system. Although total subsidence in the central trough of the rift, approximately outlined increases from north to south, initial rifting and by the present coastline. The onshore marginal rift subsidence (25-15 Ma) were of similar timing and tectonic activity shows up as minor (50-100 m) uplift. magnitude along the entire length of the S u e z - R e d Sea Several observations can be made regarding the latest rift. Initial differentiation of structural blocks took tectonic activity: (1) main trough subsidence decreases place during the Aquitanian with sedimentation towards the extreme northern end of the rift; (2) at (Nukhul Formation) occurring in isolated sub-basins. many localities the activity is confined to an The Nukhul Formation records the initial transition approximately 1 m.y. period of subsidence in which the from continental to marine sediment deposition. rates went from almost 0 m/m.y, up to 500 m/m.y, and Subsequently, during deposition of the Rudeis then back to almost 0 m/m.y. At some localities,

266

Marine and Petroleum Geology, 1988, Vol 5, August

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Marine and Petroleum Geology, 1988, Vol 5, August

267

G u l f o f S u e z - n o r t h e r n R e d Sea N e o g e n e rift: M. R i c h a r d s o n a n d M, A. A r t h u r Formation (21-15.5 Ma), subsidence was rapid and Allen, G., et al. (1984) Subsurface sedimentological study of the Rudeis Formation in Kareem, Ayun, Yusr and Shukheir uniform across the entire rift basin. Open marine Fields, EGPC 7th Exploration Seminar, Cairo sedimentation occurred across all structural regimes. Andrawis, S. F. and Abdel Malik, W. M. (1981) Lower/Middle The 'mid-Clysmic' tectonic event (16.5 Ma) rearranged Miocene boundary in the Gulf of Suez region, Egypt, NewsL fault blocks, uplifted rift shoulders and together with a Stratigr. 10, (3) 156-163 Angevine, C. L. and Turcotte, D. L. (1981) Thermal subsidence eustatic sea level fall resulted in a series of marginal and compaction in sedimentary basins: Application to clastic wedges which prograded across an often tilted Baltimore Canyon Trough, AAPG Bull. 85, (2) 219-225 early Miocene rift sequence. The basin centre may have Awad, G. H. and Fawzy, M. A. (1956) The Cenomanian been somewhat sediment starved at this time. transgression over Egypt, Bull. /'Institute du Desert d'Egypt, Tome VI, No 1 At about 15 Ma opening of the Suez rift was Barakat, H. and Miller, P. M. (1984) Geology and Petroleum terminated with relative plate motions transferred to Exploration, Safaga Concession, Northern Red Sea, Egypt, the east along the newly initiated Dead Sea-Elat EGPC, 7th Exploration Seminar, Cairo, March, 1984 transform. Tectonic restriction of the rift and the Bartov, Y., Steinitz, G., Eyal, M. and Eyal, Y. (1980) Sinistral markedly reduced subsidence rates resulted in a phase movement along the Gulf of Aqaba - its age and relation to the opening of the Red Sea, Nature 285, 220-222 of marine evaporite deposition (Belayim Formation) Beleity, A. (1982) The composite standard and definition of during the Serravallian (14-10.4 Ma). A major sea paleo events in the Gulf of Suez, E.G.P.C. 6th Exploration level lowstand at earliest Tortonian time (10.4 Ma) Seminar, Cairo tipped the basin into a long episode of continuous Ben-Avraham, Z., Almagor, G. and Garfunkel, Z. (1979) evaporite deposition recorded primarily by a thick Sediments and structure of the Gulf of Elat (Aqaba)-northern Red Sea, Sed. GeoL 23, 239-267 halite sequence (South Gharib Formation). Deposition Berggren, W. A., Kent, D. V., Flynn, J. J. and Van Couvering, J. A. of most of these marine evaporites is restricted to the (1985) Cenozoic Geochronology, GeoL Soc. Am. Bull. 96, central trough of the rift (approximately outlined by the 1407-1418 present coastline) where loading subsidence continued Bhattacharyya, D. D. and Dunn, L. G. (1986) Sedimentological evidence for repeated pre-Cenozoic vertical movements due to a filling of pre-evaporite rift topography. As a along the north-east margin of the Nubian Craton, J. African result, maximum subsidence across the rift occurs in the Earth Sci. 5, (2) 147-153 central trough. It is interesting to note that the Bohannon, R. G. (1986a) How much divergence has occurred evaporites occur during a quiescent phase in subsidence between Africa and Arabia as a result of the opening of the Red Sea? Geology 14, 510-513 while more normal marine clastic sediments typify Bohannon, R. G. (1986b) Tectonic configuration of the western episodes of more rapid subsidence. Arabian continental margin, southern Red Sea, Tectonics 5, A subsequent pulse of tectonic subsidence beginning (4) 477-499 at 6.4 Ma may be the result of oblique strike-slip Bohannon, R. G., Naeser, C. W., Schmidt, D. L. and Zimmerman, motion within the Suez rift associated with a change in R. A. (1988) The Timing of Uplift, Volcanism, and Rifting Peripheral to Red Sea: A Case for Passive Rifting, J. the structure of the Dead Sea-Gulf of Elat transform. Geophys. Res. (in press) A rapid sea level rise concomitant with the increased Bonatti, E., Colantoni, P., Vedova, B. D. and Taviani, M. (1984) tectonic activity in the latest Miocene-earliest Pliocene Geology of the Red Sea transitional region (22°N-25°N), returned the rift to dominantly marine conditions. Oceanologica Acta 7, (4) 385-398 Bonatti, E. (1985) Punctiform initiation of seafloor spreading in Minor uplift of the onshore marginal areas and slow, the Red Sea during transition from a continental to an localized subsidence in the offshore basin characterize oceanic rift, Nature 316, 33-37 the present day rift. Bond, G. C. and Kominz, M. A. (1984) Construction of tectonic subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains: Implications for Acknowledgements subsidence mechanisms, age of breakup and crustal thinning, GeoL Soc. Am. Bull. 95, 155-173 This project could not have been accomplished without Bunter, M. A. G. (1980) Gulf of Suez Stratigraphic Summary, CONOCO Internal Report the co-operation of Egyptian General Petroleum Corp. Chenet, Y. -P., Letouzey, J. and Zaghloul, E. S. (1984) Some (Gamal Hantar), GUPCO (A. Shawky Abdine), observations in the rift tectonics in the eastern part of the Mobile Nile (Nabil A. Khalil), General Petroleum Co. Suez rift, E.G.P.C. 7th Exploration Seminar, Cairo (Mahmoud Farid) and Esso Exploration Inc. (Paul M. Cochran, J. R. (1981) The Gulf of Aden: Structure and evolution of a young ocean basin and continental margin, J. Geophys. Miller). Initial fieldwork was funded by an NSF grant to Res. 86, 263-288 the Earth Science and Resource Institute, University of Cochran, J. R. (1983a) A model for the development of Red Sea, South Carolina. Aspects of this research have been AAPG Bull. 67, (1) 41-69 funded by Texaco Inc. and by an ACS-PRF Grant Cochran, J. R. (1983b) Effects of finite rifting times on the (15724-AC2). The authors have benefited from development of sedimentary basins, Earth Planet. ScL Lett. 66, 289-302 discussions of this work with Barry Katz, James Coleman, R. G. (1984a) The Red Sea: A small ocean basin Cochran, Paul Heller, Charles Angevine, John Smale formed by continental extension and sea floor spreading, and Robert Thunell. Thanks to Barbara S. Bakanic for Proc. 27th International Geological Congress 23, 93-121 help with the word processing. The initial manuscript Coleman, R. G. (1984b) The Tihama Asir igneous complex, A was improved by the reviews of Michael Steckler and passive margin ophiolite, Proc. 27th International Geological Robert Bohannon. Congress 9, 221-239 Courtillot, V., Armijo, R. and Tapponnier, P. (1987) The Sinai triple junction revisited: In, Z. Ben-Avraham (ed), Sedimentary basins within the Dead Sea and other rift zones, References Tectonophysics 141, 181-190 AbdeI-Salem, H., and EI-Tablawy, M. (1970) Pliocene diatom Crane, K. and Bonatti, E. (1987)The role of fracture zones during assemblages from East Bakr and East Gharib exploratory early Red Sea rifting: structural analysis using Space Shuttle wells in the Gulf of Suez, 7thArab Petroleum Cong,, Kuwait, radar and LANDSAT imagery, J. Geol. Soc. London 144, Paper No 57, (B-3) 407-420 Abdine, A. S. (1981) The Gulf of Suez has excellent potential, Curray, J. R., Moore, D. G., et al. (1982) Init. Repts. DSDP, v 64, World of Oil July, 147-149 Washington (US Govt. Printing Office)

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