Midyan Peninsula, Northern Red Sea, Saudi Arabia: Seismic Imaging and Regional Interpretation

GeoArabia, 2014, v. 19, no. 3, p. 165-184 Gulf PetroLink, Bahrain

Midyan Peninsula, northern Red Sea, Saudi Arabia: Seismic imaging and regional interpretation

Robert E. Tubbs Jr., Hussein G. Aly Fouda, Abdulkader M. Afifi, Nickolas S. Raterman, Geraint W. Hughes and Yousuf K. Fadolalkarem

ABSTRACT

The Midyan Peninsula of northwest Saudi Arabia offers an exceptional opportunity to observe a complex interplay of rifting, salt tectonics, and strike- slip faulting. Recently onshore 3-D, transition zone 2-D, and offshore 2-D seismic data have been acquired in the area. In addition, ongoing fieldwork and an active drilling program have provided new insights into the geologic history of the region. The initial stages of continental rifting began during the Early Oligocene (ca. 33 Ma) and often utilized pre-existing basement fault trends. The early syn-rift sedimentary record is typified by formation of deep half-grabens filled with thick wedges of primarily continental sediments, with lesser amounts of evaporitic and marine deposits.

Seismic data show a distinct break in deposition occurred ca. 21 Ma characterized by a persistent angular unconformity near the basin-bounding fault, before a shift to marine and offshore deposits of the Lower Miocene Burqan Formation. Post-Burqan a second angular unconformity termed the mid-clysmic event is evident away from the basin edge. This surface exhibits significant relief created by re-activation of older EW-trending faults and lower Maqna Group sediments display substantial thickening across these faults. Overall, the Maqna section transitions from normal marine sedimentation to more restricted basin conditions before being succeeded by the thick-layered evaporite sequence of the Mansiyah Formation.

Approximately 15–12 Ma active strike-slip faults appeared in the Red Sea and shifted the extension from rift normal to highly oblique directed at N15°–20°E, parallel to the Gulf of Aqaba. During this transition the composition of the rift- fill changed as well from basin-wide precipitates to thick siliciclastic wedges of the Ghawwas Formation. Seismic images of the Ghawwas show abrupt thickness changes and stratal geometries that date deposition as coincident with both the growth of Mansiyah Formation diapirs and the movement of a large detachment at the base of the Mansiyah.

Roughly five million years ago, organized seafloor spreading began in the southern Red Sea and strike-slip motion intensified as deformation began to focus along the Dead Sea/Aqaba strike-slip fault system. Adjacent to Midyan, a pull-apart basin in the Gulf of Aqaba has opened over 26 km perpendicular to the strike- slip system resulting in significant footwall uplift. The positive interference of the Aqaba/Dead Sea and Red Sea footwall uplifts has uniquely exposed the full syn- rift stratigraphic section from basement to Upper Miocene at Midyan, making the area an ideal locality for field studies. Presence of the complete Miocene section on the Aqaba shoulder uplift clearly indicates the uplift occurred after the Miocene. Salt-filled pull-apart basins in the same orientation as the Gulf of Aqaba are also observed on 3-D seismic data in the Ifal Basin.

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INTRODUCTION

Exploration efforts were first carried out in the Saudi Arabian side of the Red Sea Basin in the late 1960s by the Saudi Arabian Ministry of Petroleum, together with Sun Oil - an American company, and Auxerap, a French company that later became ELF and then TOTAL. Their efforts were focused in the onshore and shallow offshore regions and resulted in the discovery of the offshore Burqan gas field in 1969, ca. 30 km south of the Midyan Peninsula (Figure 1). With little interest in gas efforts eventually ceased in the mid-1970s. The next exploration phase was by Saudi Aramco in the 1990s and focused onshore. Extensive 2-D seismic was acquired and numerous wells drilled along the west

34°30’0”E 35°0’0”

N 0 20

km Gulf of Aqaba Older basement trends

28°30’0”N 28°30’0”

Pull-apart Basin Red Sea trend

Ifal Basin

Figure 12

BAC Najd trend graben Midyan Field

3-D boundary Older trends 28°0’0” 28°0’0” Red Sea

Burqan Field

34°30’0” 35°0’0” Figure 1: False-color landsat image of the Ifal Basin, Midyan area, northwest Saudi Arabia. The basin is located between the Gulf of Aqaba and Red Sea footwall uplifts, both of which bring basement to the surface (shown in transparent red). West of the Midyan area a pull-apart basin in the Gulf of Aqaba has opened 26 km perpendicular to the Aqaba/Dead Sea strike-slip fault system due to a slight rotation of the Arabian Plate as it migrated northward. Major faults are shown in white. Midyan Field wells are labeled A, B and C. The Midyan 3-D seismic survey is shown in orange.

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coast of the Kingdom throughout the 1990s decade. Three of these wells were drilled in the Midyan area of northwest Saudi Arabia resulting in the discovery and delineation of the Midyan Field in 1992. The early 2000s saw a period of inactivity until Saudi Aramco resumed exploration activities in 2006. In this third phase, which continues to this day, 3-D seismic was acquired across the Ifal Basin (Figure 1). In this paper we integrate the new seismic data, drilling results, and field observations to present a tectonic-stratigraphic framework for Midyan Peninsula and northern Red Sea. Our study also links Miocene tectonics of the Arabian Plate to the structural and sedimentary response in the basin.

DATA ACQUISITION

Saudi Aramco has conducted geologic field programs for outcrop study in the Midyan area every year since the winter of 2006/2007. Two to three weeks have been spent in the field each year by members of the Red Sea exploration team. The primary objective of these outings has been outcrop observation and description, sample collection for age dating, rudimentary fault mapping, and reservoir description. The fieldwork has proven invaluable in understanding the facies relationships of the syn-rift section, the deformation style in the basin, and reservoir characterization.

Extensive 3-D seismic data was acquired over the Ifal Basin in two phases. Both phases were acquired by an Argas vibroseis crew with a 4,000 trace capacity. Bin size is 25 m x 25 m with cross-line and in-line far off-sets of 4,975 m. Record length is 6 seconds. The first phase acquired about 357 sq km of data from December 2009 to February 2010, and covers the southwestern portion of the basin. The second phase, acquired from March to June of 2011, is ca. 745 sq km and includes all remaining onshore areas of the Ifal Basin south of the village of Al Badaa and not covered by phase one (Figure 1). The nominal fold of phase one is 500. This was increased to 1,000 fold for phase two to improve shallow imaging.

The data acquired in phase one was processed in the time domain and then the gathers were extracted and carried through pre-stack depth migration using an interpreter-guided velocity model. After completion of phase two all the traces were merged and the larger volume was again time-processed prior to performing pre-stack depth migration.

Saudi Aramco moved a drilling rig into the Ifal Basin in July of 2009 and it has been operating continuously since that time. The company considers all well information from ongoing drilling operations confidential and specific well results, or locations, are not discussed in this paper. Insights, correlations, and understanding gained from the well data are.

STRUCTURAL SETTING

The Ifal Basin in the Midyan Peninsula of northwest Saudi Arabia (Figure 1) occupies a clear indentation in the eastern flank of the Red Sea Basin, and it is likely that this situation has persisted since the first stages of rifting (Figures 2 and 3). Influence by pre-existing basement fabrics on fault patterns and geometry of the northern Red Sea rift was recognized by Polis et al. (2005) just south of the Midyan Peninsula. Albeit some of the structural grains exploited at Midyan are of a different orientation than those described by Polis et al. (2005), the propensity of rift deformation to temporarily align with older trends is evident. The strike of the master rift-bounding fault at Midyan aligns on multiple older basement trends before stepping back to the Red Sea trend and continuing up the Gulf of Suez (the trend was later offset by the Aqaba/Dead Sea strike-slip system).

The Ifal Basin shares a similar geologic history with the entire Red Sea Basin and Gulf of Suez throughout the Late Oligocene to Middle Miocene. Syn-rift basalts penetrated by onshore exploration drilling near Jeddah, Saudi Arabia, are dated at 33–34 Ma (Hughes and Johnson, 2005), and document rifting was underway by Early Oligocene. Likewise recent offshore exploration drilling in the northern Red Sea near Midyan encountered Upper Oligocene nannofossils in the lower syn-rift section (Hughes, 2012).

The complex path of the rift-bounding fault from south to north is as follows (Figure 1): (1) a few kilometers south of the Midyan area it first aligns along an older trend striking N44ºW; (2) steps north across a Najd trend (Al-Husseini, 2000) horst and graben; (3) picks up a short segment of the Red Sea

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40°E 50°60° TURKMENISTAN NAF ANATOLIA EAF Caspian Sea 40°N TURKEY 40°

CYPRUS IRAN EURASIA SYRIA Zagros Mediterranian Sea IRAQ

A/DSF JORDAN

30° Area of Figure 1 30° BAHRAIN

QATAR

EGYPT UNITED ARAB EMIRATES

SAUDI ARABIA Red Sea OMAN

20° 20°

AFRICA

ERITREA SUDAN

ETHIOPIA Gulf of Aden 30°40° 50°60°

Figure 2: Tectonic framework of the Arabian Plate (modiﬁed from Sharland et al., 2001). Abbreviations: NAF = North Anatolean Fault; EAF = East Anatolean Fault; and A/DSF = Aqaba/Dead Sea Fault. Area of Figure 1 shown with red rectangle. Dashed mid-ocean ridge in the northern Red Sea indicates embryonic sea ﬂoor spreading; an organized through going ridge is not present.

trend; (4) deflects 12–15 degrees eastward to another basement trend striking N12ºW; and (5) finally jumps back to the Red Sea trend at the northern edge of the Ifal Basin along an older fault striking E20ºS. All of these older fault trends are expressed in adjacent basement exposures (Clark, 1987). Two of these older fabrics (ca. N12°W and ca. E20°S) are distinct in basement outcrops in the Midyan area but do not appear south of Midyan (Davies, 1985; Davies and Grainger, 1985; Grainger and Hanif, 1989).

The penchant of the rift-bounding fault to align on older fabrics in the Midyan area may have also controlled the eventual location of the Arabian/Sinai plate boundary. It is hypothesized that the curve in the rift-bounding fault favored motion on the Aqaba strike-slip system over other strike-slip faults, eventually helping to fix the plate boundary. The deflection of the rift master fault must have occurred at the onset of deformation suggesting Midyan has been a persistent embayment in the basin edge since the opening of the Red Sea rift (Figure 3).

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15 Ma 10 Ma JORDAN JORDAN Incipient Aqaba / Dead Sea strike-slip fault Gulf of Suez Gulf of Suez (eventual plate boundary)

SAUDI ARABIA SAUDI ARABIA EGYPT EGYPT Area of Figure 1

Red Sea 0 100

km Red Sea

5 Ma 32°E 33° 34° 0 Ma 35° 36° 37°

JORDAN JORDAN

Gulf of Suez Gulf of Suez Gulf of Aqaba 29°N 29°

SAUDI ARABIA SAUDI ARABIA 28° 28° EGYPT EGYPT

Red Sea 27° 27°

Red Sea 26° 33° 34° 36° Figure 3: Map restorations of the Midyan area for 15 Ma, 10 Ma, 5 Ma and 0 Ma. The original Red Sea rift opened using older basement faults in the Midyan area resulting in a persistent indentation in the rift shoulder since Oligocene time. Notice the Aqaba shoulder uplift did not appear until the last 5 Myr. Magnitude and location of the footwall uplifts are represented by Λs. Area of Figure 1 shown with red rectangle.

Active rifting continued along the Red Sea trend until ca. 11–12 Ma (Reilinger and McClusky, 2011), accumulating thick sequences of continental, shallow-marine, and restricted-basin deposits (Hughes and Johnson, 2005; Figure 4).

LITHOSTRATIGRAPHY AND SEISMIC STRATIGRAPHY

Basement and Pre-Rift Section

The pre-rift section in the Midyan Peninsula is comprised of Neoproterozoic basement and occasional pockets of pre-rift Cretaceous Adaffa Sandstone (Clark, 1987) preserved in grabens bounded by pre-existing faults, which were re-activated in the earliest stages of Oligocene rifting (Aly Fouda et al., 2008). The basement composition is commonly granitic but also consists of metasediments, metavolcanics, and gabbro (Clark, 1987). The contact is rarely sharp, and is usually loosely located in a thick sequence of highly weathered and fractured granitic rocks that grade into very arkosic sediments.

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Rift Phase Tectonic Events Generalized Lithology Extension Orientation Era Period Epoch Age Time (Ma) Planktonic Foraminiferal Zone Group Formation Member

Pleistocene N22 Reefs and Clastics Quat.

Pleist. 1.8 Holocene 1.95 N21 Badr

Late Drift/hyper- 3.1 N19- N N20 extension SA 15°N - 20°E N19 LI Ifal Pliocene 4.8 N18 5.0 5.33 N17 Messinian Unconformity ca. 5 Ma Organized sea floor spreading 7.7 begins in the southern Red Sea. N16 Rift Oblique 10 Extension Embryonic sea floor spreading N15 (Aqaba Trend) and oblique rifting present in the Late Miocene11 Early northern Red Sea 15°N - 20°E N14 12.0 ca. 11-12 Ma N13 12.4 Sea floor spreading begins in the Gulf of Aden;initiating strike-slip N12 faulting in the Red Sea rift. 13.9 N11 The Gulf of Suez is structurally

14.7 Mansiyah Ghawwas isolated from the Red Sea. N10 15.3 Sidr Conglomerate Middle Miocene Nakhlah Sandstone

N 9 Kial Yuba Shale Salt/halite Neogene Rayaman Anhydrite Dolomitic limestone MAQNA

Miocene 16.0 Dhaylan Limestone

Cenozoic N 8 Crystalline basement

17.2 Luj Wadi Jabal Kibrit Umm Waqb N 7 Rift Normal Mid-clysmic Unconformity Extension ca. 17.5 Ma 18.0 (Red Sea Trend) Widespread up-lift and erosion 55°N - 65°E with re-activation of (older) N 6 basement trend faults. 18.6

N 5 Burqan Early Miocene

20.5 Musayr Yanbu ca. 21 Ma N 4 Localized erosion prior to the "Burqan" marine incursion. 23.03

Late 25.0

28.4 P 22 Al Wajh to Oligocene Paleogene

Early P18 ca.33 ca.33-28 Ma Pre-rift Unconformity ca. The Red Sea rift first opens as 65.5 series of deep half grabens. Adaffa Suqah Mesozoic

Cretaceous Figure 4: Generalized lithostratigraphy of the

Campanian- ca. Maastrichtian 83.5 Midyan Peninsula (based on Hughes and ca. 541 Johnson, 2005) showing major tectonic events Neoproterozoic Basement (this study).

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Al Wajh Formation

The Al Wajh Formation represents the oldest syn-rift deposits in the Midyan Peninsula (Figure 4). It is typified by thick wedges of primarily conglomerates and red mudstones that attain a thickness of up to 2,200 m. The deposits accumulated in half-grabens, which formed on top of rotated basement blocks sliding towards the basin center on listric normal faults (Figure 5). The clastics are mainly of lacustrine origin as evident by the presence of cyprid ostracods, freshwater charophyte oogonia, as well as arkosic, non-marine, alluvial plain deposits with interspersed sandstone channels (Hughes and Johnson, 2005). Some of the Al Wajh mudstones contain the benthonic foraminifera Ammonia beccarii, which is indicative of brackish to shallow-marine conditions.

Yanbu and Musayr Formations

A thin, often less than 10 m, section of Yanbu evaporite intermittently overlies the Al Wajh Formation (Figure 4). The Yanbu Formation represents the first appearance of restricted marine conditions in the Ifal Basin. Localized outcrops of marine carbonates, named the Musayr Formation (Figure 4), are also seen to overlie the Al Wajh but their age relationship to Yanbu evaporites is unclear (Hughes and Johnson, 2005). These were likely reefs situated on the high corners of rotated fault blocks, which shed debris down-dip in either direction off the crest of the blocks. The occurrence of both of these formations is sporadic but they appear to be a precursor to, and have a closer affinity to, the marine incursion characterized by the overlying marly clastics of the deep-marine Burqan Formation (Figure 4).

Burqan Formation

On seismic images the Al Wajh/Burqan contact typically displays a localized angular unconformity (Figure 6). Often the high corners of rotated fault blocks are eroded to basement prior to Burqan deposition (Figure 7). Field and subsurface observations indicate this is a widespread unconformity (Winn et al., 2001; Hughes and Johnson, 2005), which is best-expressed seismically near the basin edge (Figure 6). Hughes and Johnson (2005) assigned the Musayr and Yanbu formations to the Tayran Group, which would necessarily place them below this sub-Burqan unconformity. The overlying Burqan Formation unquestionably denotes marine deposition (Hughes and Johnson, 2005) and the Yanbu/Musayr appear to be related to this transgression. Altogether the Yanbu-Musayr-Burgan sequence (Figure 4) embodies conformable deposition in a deepening seaway as it transitions from restricted to open-marine conditions. This facies shift has also been interpreted to represent a more energetic phase of rifting (Polis et al., 2005). We propose that the Tayran Group nomenclature be dropped and the three component formations: Al Wajh, Yanbu, and Musayr be maintained as stand- alone units with no Group assignment (Figure 4).

Mid-Clysmic Event

A hiatus in deposition, referred to as the mid-clysmic event in the Gulf of Suez (Garfunkel and Bartov, 1977), is also recognized in the Midyan area. The equivalent of the Burqan Formation is known as the Rudeis Formation in the Gulf of Suez and there the mid-clysmic event is an intra-Rudeis unconformity (Hughes et al., 1992). Nannofossil evidence indicates that the gap endured longer at Midyan, continuing past Burqan time into the beginning of Maqna Group sedimentation in the very latest Early Miocene (Hughes and Filatoff, 1995; Figure 4).

The origin of the mid-clysmic event has been attributed to a change in Arabian Plate motion and introduction of active strike-slip faulting in the basin associated with the onset of active spreading in the Gulf of Aden by Bosworth et al. (2005).

Wadi Waqb Member of the Jabal Kibrit Formation

The pre-Maqna depositional surface in the Midyan area exhibits significant relief that was filled-in by early Maqna sediments, primarily the Wadi Waqb Member of the Jabal Kibrit Formation (Figure 4). Abrupt thickness changes in the Wadi Waqb Member can be observed in both outcrop (Hughes and

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A A’

Ghawwas

Burqan / Al Wajh contact Al Wajh Al Wajh Al Wajh

Thickening of Al Wajh sediments into the fault

Rift bounding fault

0 4

Figure 5: Un-interpreted and interpreted pre-stack depth-migrated 3-D seismic traverse (location shown on inset map) displaying Gulf of thick half-graben-ﬁll of early syn-rift sediments. Notice the onlap Aqaba A’ of Al Wajh sediments towards the crest of the rotated fault block SAUDI and thickening back towards the fault scarp. Vertical exaggeration ARABIA is ca. 3:1. Interpreted section: black lines = faults; green = top Mansiyah Formation, purple = top Maqna Group, cyan = top Al A Wajh Formation, and red = top basement, Mansiyah Formation highlighted in pink.

Red Sea

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B B’

Ghawwas Maqna Burqan

Area of figure7 Al Wajh

Basement

Gulf of Aqaba B’

B SAUDI ARABIA Rift Bounding fault 0 2 0 2 km Red Sea km Figure 6: Un-interpreted and interpreted pre-stack depth-migrated 3-D seismic traverse (location shown on inset map) exhibiting an angular unconformity between the older, early syn-rift ﬁll of the Al Wajh Formation and overlying Burqan Formation. A close-up of the erosional and depositional geometries at the crest of the rotated fault block is shown in Figure 7. Vertical exaggeration is ca. 2:1. Interpreted section: black lines = faults; green = top Mansiyah Formation, purple = top Maqna Group, blue = top Burqan Formation, cyan = top Al Wajh Formation, and red = top basement, the Mansiyah Formation is highlighted in pink.

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Evaporite

Al Wajh-Burqan Maqna- contact Burqan Angular contact unconformity at the Burqan - Al Wajh Basement contact

1.00

Figure 7: Close-up of pre-stack depth-migrated 3-D seismic traverse shown in Figure 6 at the crest of a rotated hanging wall fault block. Notice the angular unconformity between the older Al Wajh Formation and overlying Burqan Formation. Over the basement high, the Burqan Formation lies directly on basement. The high corner of the basement block also experienced erosion and beveling prior to Maqna deposition where the Burqan section is thinned. Vertical exaggeration is approximately 2 to 1. Interpreted section: black lines = faults; green = top Mansiyah Formation, purple = top Maqna Group, and red = top basement.

Johnson, 2005) and subsurface data (Figure 8). Elongate depositional troughs formed by fault scarps on one side and the rotated top of the hanging wall blocks on the other (Figure 8), were infilled by subaqueous debris flows of reef material on the sea floor (Hughes and Johnson, 2005). Interestingly, most of these faults are oriented east-west, following an older fabric rather than the Red Sea or Aqaba trends. The carbonate material is believed to have been derived from a predominantly rhodophytic fringing reef complex situated at the basin edge and/or at the crests of rotated hanging-wall fault blocks (Hughes, 2007). The presence of unkeeled planktonic foraminifera mixed with the transported reef debris suggests a middle shelf environment denoting water depths of 50 to 75 m (Hughes, 2007).

Kial Formation and Mansiyah Formation

Based on diatom evidence, marine influence faded and restricted conditions prevailed as the mixed clastics and evaporites of the Kial Formation slowly graded into massive evaporites and minor clastics of the Mansiyah Formation (Figure 4) in the late Middle Miocene. The age of the Mansiyah is poorly constrained, but necessarily represents a time of hydrologic isolation and evaporitic drawdown across the basin to accumulate precipitates of such widespread and thick character (Warren, 2006). The presence of basin-wide evaporites also indicates a time of slow subsidence, possibly due to transition of the basin from rifting into hyper-extension/embryonic sea-floor spreading.

Ghawwas Formation

Following the deposition of the Mansiyah evaporites, immense accumulations of Ghawwas Formation siliciclastics, which often make up 50% or more of the complete Miocene sedimentary package, began to infill newly created accommodation space (Figure 9). Earliest Ghawwas sedimentation appears to be marine, but the bulk of the section exhibits only freshwater fauna, denoting lacustrine deposition. Thickness changes due to salt mobilization and withdrawal from beneath mini-basin depo-centers is quite evident in the Ghawwas section (Figures 8 and 9). Loading from mini-basin siliciclastics displaces the salt and concentrates it into diapirs and salt walls situated between the mini-basins (Figure 9).

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C C’

C CA B C’

Welded Ghawwas Ghawwas Formation

Wadi Waqb Top basement section

Gulf of 0 2 Aqaba km SAUDI ARABIA Figure 8: Un-interpreted and interpreted pre-stack depth-migrated 3-D seismic traverse (location shown on inset C C’ map) showing thickening of the pre-evaporite section across faults re-activated during the mid-clysmic event. Location is shown for Midyan Field wells A, B and C. Top Maqna Group to Red Sea basement isopachs are as follows: well A = 410 m, well B = 445 m and well C = 191 m. Much of the Mansiyah Formation salt has been evacuated from this area and overlying Ghawwas Formation has welded to the Maqna Group. Vertical exaggeration is ca. 3:1. Interpreted section: black lines = faults; green = top Mansiyah Formation, purple = top Maqna Group and red = top basement, Mansiyah Formation is highlighted in pink, and Wadi Waqb Member of the Jabal Kirbrit Formation is highlighted in cyan.

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Tubbs et al.

D D’

Slight collapse and disruption of sediments due to late salt withdrawal

Ghawwas

Salt-filled Diapir pull-apart Basement Pre-evaporite x x sediments Pre- evaporite sediments

Gulf of Aqaba 0 4 SAUDI ARABIA km D’ Figure 9: Pre-stack depth-migrated 3-D seismic traverse (location D shown on inset map) showing a cross section through a pull-apart rhombochasm which is bounded by strike-slip and normal faults. Red The pull-apart is filled with Mansiyah Formation salt which Sea “flowed” into the feature primarily from the upper right as faulting opened accommodation space in the rhombochasm. Disruption of the overlying Ghawwas Formation next to the salt wall on the right of the section is interpreted to be caused by flowage into the pull-apart by salt after Ghawwas deposition. Vertical exaggeration is ca. 4.5:1. Interpretation: black lines = normal faults, cyan lines = strike-slip faults; green = top Mansiyah Formation, purple = top Maqna Group, and red = top basement, Mansiyah Formation is highlighted in pink.

South of the Midyan Peninsula, where the Arabian coast trends at N55ºE, a large detachment exists in the Mansiyah Formation, with 15 to 20 km of displacement down-dip where the evaporite has been completely evacuated and mobilized towards the basin center (Figure 10). Massive growth of the Ghawwas section on the hanging wall of this detachment clearly indicates active faulting throughout Ghawwas time (Figure 10). Active diapirism is also coincident with Ghawwas deposition, as is shown by abrupt thickness changes and stratal geometries adjacent to Mansiyah diapirs (Figure 9). Formation of the detachment is interpreted to be primarily due to massive sediment loading, but regional subsidence focused in the center of the rift due to necking of continental crust from the rift edge (Péron-Pinvidic and Manatschal, 2009) likely also played an important role. Curiously, virtually all of the Mansiyah evaporites are evacuated along the detachment south of Midyan (Figure 10) but this is not the case in the Ifal Basin (Figure 9) where diapirism is observed.

By Ghawwas time the active faulting had stepped basin-ward, as only older syn-rift sediments are present adjacent to the rift-bounding master fault (Figures 5 and 6). A thick growth section of the Ghawwas is evident on the hanging wall of subsequent down-to-the-basin normal faults in the Midyan area (Figure 5).

Ongoing strike-slip deformation related to the nearby Aqaba Plate boundary manifests itself in the Ifal Basin by the late development of through-going oblique-slip faults parallel to the Aqaba trend (Figures 9 and 11). These faults characteristically scissor, tip-out updip, and exhibit a component of left-lateral displacement of up to four kilometers.

Distinctive rhomboid or triangle-shaped pull-apart basins (Figure 12) that have been filled with mobilized evaporites (Figures 9 and 11) are associated with the oblique-slip faults. It is likely that these

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faults became active contemporaneous with the formation of the Aqaba strike-slip plate boundary. Since the evaporite flows into and fills the voids created by the rhombochasms, the maximum possible age of these faults is syn-Mansiyah. The overlying Ghawwas sediments show little sag over the pull-apart basins (Figures 9 and 11). If the local salt budget was sufficient to completely fill the accommodation space with mobilized evaporite as it opened, the overlying stratigraphy would have been for the most part unaffected. A signature of late salt movement is seen in the Ghawwas siliciclastic sediments by a slight disruption of the reflectors next to the Mansiyah salt diapir above the pull-apart (Figures 9 and 11). This minor collapse on the flank of the salt diapir records late salt movement after Ghawwas deposition, dating the pull-apart basins as post-Ghawwas. The ductile evaporite section has absorbed the late strike-slip deformation by flowing into the accommodation space created as the pull-apart basin opened. Similar pull-apart features crop out on the western side of the Gulf of Aqaba (Ben-Avraham et al., 1979).

Post-Ghawwas Section

The post-Ghawwas stratigraphy is thin in the Midyan area and characterized by carbonate and fine- to medium-grained siliciclastic deposition (Figure 4). This part of the section generally falls in the shallow mute zone of the 3-D seismic volumes and little can be deduced about its structure and stratigraphy from seismic data.

E E’

Detachment Ghawwas

surface Growth section

Basement

0 4

Figure 10: Pre-stack depth-migrated 2-D seismic line (location shown on inset map) showing major detachment in the Gulf of Aqaba Mansiyah Formation (black arrows). The Mansiyah evaporite SAUDI has been largely evacuated and mobilized down-dip. Growth ARABIA section of Ghawwas Formation siliciclastics have back-ﬁlled over the detachment, coincident with movement on the detachment. Vertical exaggeration is ca. 4:1. Interpretation: black E’ lines = faults; green = top Mansiyah Formation, purple = top E Maqna Group, and red = top basement, Mansiyah Formation is Red highlighted in pink. Sea

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F F’

Ghawwas Disturbed sediments due to late salt withdrawal Salt flowage to fill void

Salt-filled pull-apart Pre-evaporite Basement Basement sediments x x Gulf of Aqaba SAUDI ARABIA

F’ F 0 10

km Red Sea Figure 11: Un-interpreted and interpreted pre-stack depth-migrated 3-D seismic traverse (location shown on inset map and Figure 12) showing a cross section through a pull-apart rhombochasm which is bounded by strike-slip and normal faults. The pull-apart basin is filled with Mansiyah Formation salt which “flowed” into the feature primarily from the right as faulting opened accommodation space in the rhombochasm. Disruption of the overlying Ghawwas Formation next to the salt wall on the right of the section and over the pull-apart basin is interpreted to be caused by flowage of salt into the rhombochasms as it opened after Ghawwas deposition. Vertical exaggeration is ca. 3:1. Interpreted section: black lines = normal faults, cyan lines = strike-slip faults; green = top Mansiyah Formation, purple = top Maqna Group, and red = top basement, Mansiyah Formation is highlighted in pink.

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Midyan Peninsula, northern Red Sea, Saudi Arabia

N 0 4 Older basement trend fault Aqaba trend km extension

Salt-filled pull-apart

Red Sea trend fault Location of seismic line shown in Figure11

Figure 12: Horizon slice at basement level from the pre-stack depth-migrated 3-D seismic volume. Notice the salt-ﬁlled pull-apart basin formed by late strike-slip deformation (see Figure 11). The strike-slip faulting in this area aligned onto older basement and Red Sea trend faults. Blue lines are faults.

REGIONAL INTERPRETATION

The Midyan Peninsula exhibits a history of rifting that was perpendicular to the direction of extension between Africa and Arabia from ca. 33 Ma until about 12 Ma. The rift opened in a series of deep half- grabens, which were completely filled by coarse wedges of Al Wajh Formation clastics up to 2,200 m thick. Thermochronologic (Omar and Steckler, 1995) and geodetic evidence (ArRajehi et al., 2010) indicate that the entire length of the 2,200-km-long Red Sea rift opened nearly simultaneously as the Arabian Plate began to move away from the African Plate (Figure 3) along an azimuth of N55°E (Polis et al., 2005). This supports the argument by McQuarrie et al. (2003) that the rift is driven by far-field slab-pull stress due to the Arabian Plate subducting beneath the Eurasia Plate (Figure 2). The Midyan area was near, or below, sea level and the area experienced little, if any, thermal uplift prior to rifting (Garfunkel and Bartov, 1977; Steckler, 1985; Coleman and McGuire, 1988; Bosworth et al., 1998).

This early phase was followed by a more tectonically quiescent time during deposition of the restricted marine Yanbu and Musayr formations, followed by the fine-grained deposits of the open- marine Burqan Formation. The Burqan seaway was terminated about 18 Ma by the mid-clysmic event. The event is interpreted as a re-activation of older EW-orientated faults that propagated from the basement into the syn-rift section at Midyan.

At the end of the mid-clysmic event, basal Maqna Group carbonates and silts filled-in considerable depositional relief as apparent throughout the Midyan Peninsula. The Maqna Group and succeeding Mansiyah Formation stratigraphy show increasingly restricted conditions developed as the rocks grade from mixed siliciclastics and carbonates, to mixed siliciclastics and evaporites, and finally into massive evaporites. This conformable sequence denotes another period of relative tectonic quiescence and measured subsidence until a major shift in basin tectonics occurred about 12 Ma.

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The shift at ca. 12 Ma involved a change in the direction of movement between Africa and Arabia from rift-normal to highly oblique. The shift caused strike-slip faults oriented parallel to the new extension direction of N15°–20°E to appear throughout the basin. The shift has been generally dated at ca. 14 Ma (Bosworth et al., 2005); however it may have occurred as late as 11–12 Ma based on geodetic and plate configuration evidence (Reilinger and McClusky, 2011). We favor the later occurrence based on correlation of tectonic events with the notable change from evaporitic to siliciclastic deposition (Figures 4 and 9) in the late Middle Miocene, which has been dated using diatom assemblages.

The shift from normal to highly oblique rifting was probably caused by the commencement of seafloor spreading in the Gulf of Aden south of Arabia (Reilinger and McClusky, 2011), and/or collision of the Arabian and Eurasian plates to the north and northeast (Bosworth et al., 2005) (Figure 2). In either case, the Arabian Plate began to rotate counterclockwise around an Euler pole located in North Africa (Sultan et al., 1992), and the direction of extension with respect to the African Plate became highly oblique at N15°–20°E.

Although the rate of strike-slip motion is controversial (Bosworth et al., 2005; Reilinger and McClusky, 2011), its appearance had a profound influence upon subsequent Late Miocene deposition. It is not clear if the rate of extension changed at this time as well. Most likely some of the strike-slip faults created by this shift matured into transform faults of the Red Sea spreading ridge. The Dead Sea/Aqaba strike-slip fault system, in particular, evolved into the Sinai microplate/Arabian Plate boundary (Figure 2) and tectonically isolated the Gulf of Suez from the rest of the Red Sea Basin (Bayer et al., 1988).

Tectonism was re-energized after 12 Ma across the Ifal Basin and numerous normal faults re-activated due to ridge push from the opening of the Gulf of Aden, which was caused by a slight rotation in the motion of the Arabian Plate. This intensified strike-slip deformation in the rift as faults acted to accommodate differential extension from south to north. Motion centered on strike-slip faults continued to open the rift in a highly oblique manner.

The Mansiyah evaporite was mobilized and began to slide down-dip towards the basin center creating vast amounts of accommodation space that was filled by thick clastic wedges of the Late Miocene Ghawwas Formation. This was also a period of active diapirism at Midyan.

Griffin (1999, 2002) discounts any tectonic influence on the change from layered evaporite to dominantly siliciclastic deposition at the Mansiyah/Ghawwas contact, and attributes the shift solely to climate change ascribed to a shift in the monsoonal rainfall patterns across North Africa and Asia. Increased precipitation may certainly have affected later Ghawwas sedimentation, but diatom evidence dates the basal Ghawwas in the Midyan area to be late Middle Miocene coincident with the shift to oblique rifting, and far older than the Messinian-aged deposits described by Griffin (1999).

The structural evolution of the Gulf of Suez and the rest of the Red Sea Basin diverges significantly once the Gulf of Suez became isolated from the rest of the rift (Polis et al., 2005). Deformation continued in the Red Sea as it evolved into a plate boundary and the Gulf of Suez became quiescent by comparison.

Around the beginning of the Pliocene at ca. 5 Ma, strike-slip motion significantly intensified along the Aqaba/Dead Sea strike-slip system (Ehrhardt et al., 2005), coincident with initiation of active seafloor spreading in the southern Red Sea (Roeser, 1975). We correlate the Messinian Unconformity, which we position at the Ghawwas Formation/Lisan Group contact (Figure 4) in the Midyan area, with this event. In the absence of an organized, through-going spreading ridge, the northern Red Sea is thought to be in an embryonic spreading stage characterized by an axial depression punctuated by localized elongate deeps of volcanic activity and broadly distributed seismicity (Martinez and Cochran, 1988; Guennoc et al., 1990; Cochran et al., 1991; Cochran, 2005; Al-Ahmadi et al., 2013). Nevertheless the northern Red Sea is widening and has essentially entered a hyper-extension phase with deformation concentrated in the zones of active volcanism and strike-slip faulting (Al-Amri, 1995; Aldamegh et al., 2009).

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Although ca. 107 km of left-lateral, strike-slip movement has been documented along the Aqaba/ Dead Sea strike-slip fault system (Quennell, 1959), the southern end of the system, along the Gulf of Aqaba, has also opened over 26 km perpendicular to the transform due to rotation of the Arabian Plate (Sultan et al., 1992; Smit et al., 2010). Associated with this opening are substantial footwall uplifts that expose basement rocks on both sides of the Gulf of Aqaba (Figure 1). West of Midyan, bathymetry almost immediately dives to depths that exceed 1,800 m on what are assumed to be steep fault scarps (Ben-Avraham et al., 1979). Polis et al. (2005) describe the uplift on the Midyan side of the Gulf of Aqaba as transpressional, but we believe the area is clearly adjacent to the largest pull-apart basin in the Aqaba/Dead Sea strike-slip system (Ben-Avraham et al., 1979), and thus transtensional.

The basin moved into a drift/hyper-extension phase as deformation was concentrated at the basin center in the weaker embryonic spreading zones and associated strike-slip faults. The Aqaba transform evolved into an ongoing strike-slip plate boundary and strike-slip faulting has been the dominant deformation style throughout the Pliocene and Pleistocene.

Approximately 5 Ma sea-floor spreading commenced in the southern Red Sea, transferring most of the deformation to the weaker ridge. In the northern Red Sea a combination of focused strike- slip deformation and embryonic spreading began operating. Each of these tectonic events directly influenced the sedimentary record in the basin.

At present time, any remnant of continental rifting has ceased as regional stresses are absorbed at the weaker spreading zones and/or large strike-slip faults located offshore (Cochran, 2005). The constructive interference of the Aqaba and Red Sea “rift” shoulders has uplifted the Midyan area and uniquely exposed the entire Oligocene–Miocene section of the Ifal and greater Red Sea basins. Preservation of the complete Miocene section at Midyan implies the Aqaba shoulder uplift did not commence until the Pliocene and Smit et al. (2010) argue for similar timing of footwall uplifts northeast of Midyan along the same Aqaba-Dead Sea strike-slip system. Briem and Blumel (1984) also present a strong geomorphological case for recent uplift of the Gulf of Aqaba footwall (Jabal Hamdah) along the Arabian coast and adjacent to the Ifal Basin.

CONCLUSIONS

The decision by Saudi Aramco to resume their Red Sea exploration effort in 2006 has allowed us to update the conventional tectonic framework for the northern Red Sea. The rekindled program made available new 2-D and 3-D seismic, well data from the current drilling program, and instigated a fresh episode of fieldwork in the area. These data and studies have been used to synthesize a more refined geologic history for Midyan, which has assisted Saudi Aramco’s greater Red Sea exploration effort.

ACKNOWLEDGEMENTS

We thank the management of Saudi Aramco and the Ministry of Petroleum and Minerals, Kingdom of Saudi Arabia, for their support and permission to publish this paper. Discussions with Mark Rowan, Michael Hudec, and Gianreto Manatschal greatly increased our understanding of salt tectonics and rift evolution. Their insights were instrumental in helping us decipher what the rocks were telling us. We particularly appreciate the unremitting efforts of Saudi Aramco’s depth migration team to improve the seismic imaging of the Midyan area. Finally, input from two anonymous reviewers significantly improved the manuscript. The authors thank GeoArabia’s Production Co-manager, Nestor “Nino” Buhay IV, for designing the paper for press.

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ABOUT THE AUTHORS

Robert E. Tubbs Jr. is a Geophysical Consultant who has been working in Saudi Aramco’s Red Sea Exploration Division since 2006. He received a Bachelor of Science degree from Brigham Young University (USA) in 1982 and Master of Science degree from Texas A&M University (USA) in 1984. He worked exploration projects in the Rockies, Arkoma Basin, and Alaska with both Pennzoil and ARCO before joining Saudi Aramco in 2000. His primary interests are margin evolution and hydrocarbon habitat of the Red Sea, tectonic control of basin fill architecture, and salt tectonics. He is a member of the Dhahran Geoscience Society and AAPG. [email protected]

Hussein G. Aly Fouda is a Geological Specialist who has been working in Saudi Aramco’s Unconventional Gas Exploration since 2013 and worked for Red Sea Exploration Department from 2006 to 2013. He received a Bachelor of Geology Degree and Master of Structural Geology from Ain Shams University (Egypt) in 1980 and 2003 respectively. He worked as an Exploration Geologist in the Gulf of Suez rift basin, Sinai inverted basins, and Paleozoic- Mesozoic basins in Western Desert with both APACHE and GUPCO Companies before joining Saudi Aramco in 2004. His primary interests are the analysis of complex structural terrains, such as rift basins with salt tectonics, inverted basins and the application of sequence stratigraphy. He is a member of the Dhahran Geoscience Society (DGS) and AAPG. [email protected]

Abdulkader M. Afifi has worked for Saudi Aramco since 1991, and has held various technical and leadership positions in its Exploration Organization. He also worked, from 1980–1986, for the US Geological Survey in Saudi Arabia on geologic mapping and mineral evaluation of the Mahd Adh Dhahab gold district. He obtained his degrees in Geology, a bachelors from the King Fahd University of Petroleum and Minerals (Saudi Arabia) in 1977, a masters from the Colorado School of Mines (USA) in 1980 and a PhD from the University of Michigan (USA) in 1990. Abdulkader is interested in most aspects of Earth Science, particularly the geology of the Middle East. He has authored several papers on topics ranging from the

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phase equilibria of telluride minerals to the Paleozoic stratigraphy and hydrocarbon habitat of the Arabian Plate. He is a member of AAPG, DGS, and SPE. He served the AAPG as Councilor (2002–2004), Distinguished Lecturer (2004), and President of the Middle East Region (2006– 2009). He served the DGS as speaker (1991, 2004), field trip leader (1992, 2005), and Vice President (1995). He also served on several organizing committees for the GEO conferences and is the Saudi Aramco Editor for GeoArabia. [email protected]

Nickolas Raterman received a BSc (2003) in Geology from the University of Idaho (USA) and an MSc (2006) in Geology from the University of California, Davis (USA). He is the Lead Geoscientist for deep-water exploration in the Red Sea Exploration Department at Saudi Aramco. He began his career with Schlumberger and has contributed to several international deep-water exploration projects. [email protected]

Geraint Wyn Hughes is currently a Micropalaeontology Consultant and Director of Applied Microfacies Limited, based in North Wales, having recently left his position after 21 years as Senior Geological Consultant in the Biostratigraphy Group of Saudi Aramco’s Geological Technical Services Division. He gained considerable experience in the integration of micropaleontology and microfacies with sedimentology to support exploration activities and assist reservoir characterization, and has an extensive publication record. He gained BSc, MSc, PhD and DSc degrees from Prifysgol Cymru (University of Wales) Aberystwyth, UK, and in 2000 he received the Saudi Aramco Exploration Professional Contribution award, in 2004 the best paper award, and in 2006 the GEO 2006 best poster award. His biostratigraphic experience, prior to joining Saudi Aramco in 1991, includes 10 years with the Solomon Islands Geological Survey, and 10 years as Unit Head of the North Africa-Middle East- India region for Robertson Research International. He maintains links with academic research as an Adjunct Professor of the King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia and visiting lecturer at various United Kingdom universities and the University of Urbino micropalaeontology summer school. He is an Associate Editor for the AAPG, Saudi Journal of Earth Sciences, GeoArabia reviewer, and a member of the British Micropaleontological Society, the Grzybowski Agglutinated Foraminiferal Society, SEPM Society for Sedimentary Geology, the Cushman Foundation for Foraminiferal Research and Geoscience Wales. He has recently been accepted as a Scientific Associate of the Natural History Museum in London. [email protected]

Yousuf K. Fadolalkarem is a Master’s Degree candidate at the University of Kansas (USA). He started his job with Saudi Aramco in 2007. He was a geophysicist working in Saudi Aramco’s Red Sea Exploration Division from 2010 to 2012. He received a Bachelor of Science degree in Geophysics from University of Leeds (UK) in 2007. His primary interests are geophysical modeling of complex structure, geophysical attributes, and salt tectonics. He is a member of the Dhahran Geoscience Society, SEG and AAPG. [email protected]

Manuscript submitted May 13, 2013 Revised December 23, 2013 Accepted March 8, 2014

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