Robert

Lower Miocene Nukhul
Formation, Gebel el Zeit,
Egypt: Model for structural
control on early synrift strata
and reservoirs, Gulf of Suez
Robert D. Winn, Jr., Paul D. Crevello,
and William Bosworth

ABSTRACT
The Aquitanian–early Burdigalian (lower Miocene) Nukhul Formation at Gebel el Zeit, Egypt, was deposited during early stages
of Gulf of Suez rifting. The unit dips 8–15⬚ less than underlying
prerift strata, indicating that significant rotation and extension preceded subsidence of the Gebel el Zeit fault block. The Nukhul Formation at Gebel el Zeit is up to 75 m thick in outcrop and consists
of a lower sandstone and an upper carbonate unit. The formation
varies considerably along strike because of syndepositional differential movement of small fault-bounded blocks. The lower clastic
unit at South Gebel el Zeit contains poorly sorted, conglomeratic,
marly sandstone that commonly displays grading and Bouma sequences. Beds were deposited below storm base by sediment gravity
flows. Thicker intervals are inferred to fill small, structurally controlled, submarine gullies that funneled sand and gravel southwestward to a half-graben basin. In contrast, an inferred correlative, thin,
basal conglomeratic unit in North Gebel el Zeit was deposited in a
shallow-marine setting. The presence of basement clasts in Nukhul
strata indicates early syndepositional uplift due to structural tilting.
The upper carbonate unit consists of bioclast, peloid, and intraclast packstone, wackestone, and grainstone with minor floatstone,
rudstone, and coral-algal boundstone. Carbonate strata were deposited variously in deep-marine, low-energy peritidal and subtidal,
and reefal environments. Deeper submerged blocks were the site
of carbonate resedimentation or deeper shelf deposition. Reefs and
shallow-marine bioclast shoals formed on higher submerged blocks.
Nukhul strata show that synrift reservoir prediction in the Gulf of
Suez, the Red Sea, and presumably in other rifts requires mapping
of synrift cross faults and fault block by fault block facies analysis.

Copyright 䉷2001. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received October 22, 1999; revised manuscript received October 23, 2000; final acceptance
January 8, 2001.

AAPG Bulletin, v. 85, no. 10 (October 2001), pp. 1871–1890

1871

AUTHORS
Robert D. Winn, Jr. ⬃ Geology
Department, University of Papua New Guinea,
N.C.D., Papua New Guinea; current address:
1777 Larimer St., #807, Denver, Colorado,
80202-1543; [email protected]
Robert Winn is a consulting geologist. He
received a Ph.D. from the University of
Wisconsin–Madison in 1975 and joined
Marathon Oil Company as a research
geologist in 1977. He left Marathon in 1994
for the University of Papua New Guinea,
where he was senior lecturer and then
associate professor. He was head of the
Geology Department from 1996 to 2000. His
primary interests are in clastic sedimentology
and sequence stratigraphy.
Paul D. Crevello ⬃ Petrex-Petrogeos, P.O.
Box 2905, Bandar, Brunei S.B.;
[email protected]
Paul Crevello is a consulting geologist and
technical director of PetrexAsia, which he
formed in 1997. He received an M.S. degree
from the University of Miami (1978) and a
Ph.D. from Colorado School of Mines (1989).
He was employed by Marathon Oil as a
research geologist from 1978 to 1994 and by
the University of Brunei as senior lecturer
from 1994 to 1997. His specialties are in
sequence stratigraphy and sedimentology of
carbonate and turbidite systems and the
integrated calibration of 3-D reservoir models.
William Bosworth ⬃ Marathon Petroleum
Egypt, Ltd., P. O. Box 52, Maadi, Egypt;
[email protected]
William Bosworth is employed by Marathon
Oil Company. He joined Marathon in 1984
and has worked principally on international
exploration and development projects. From
1980 to 1984 he taught structural geology and
tectonics at Colgate University in Hamilton,
New York. His research interests are
principally in extensional tectonics, continental
stress field evolution, and the
paleogeodynamics of the African plate.

INTRODUCTION
Numerous sedimentologic and structural studies of rift
evolution have involved the Gulf of Suez (Figure 1)
because of the presence of well-exposed, synrift marine
strata, which can be relatively well dated, and because
of the existence of abundant hydrocarbon exploration
data. More than 80 oil fields in the Gulf of Suez produce approximately 600,000 BOPD. Production occurs from basement, prerift Paleozoic and Mesozoic
units, and synrift Miocene clastic and carbonate strata.
The oldest, areally extensive synrift deposit in the Gulf
of Suez is the lower Miocene Nukhul Formation (Figure 2), which is a reservoir in more than 15 fields
(Saoudi and Khalil, 1986). The unit’s presence and reservoir quality vary considerably across the basin. The
Nukhul Formation regionally includes a mixed assemblage of sandstone, conglomerate, limestone, shale,
and evaporite rock. According to most interpretations,
the unit was deposited in fluvial, alluvial fan, lagoonal,
lacustrine, and shallow-marine environments in strucFigure 2. Stratigraphic column of Gebel el Zeit area (modified
from Evans, 1988). Maximum thicknesses of strata are for the
southern Gulf of Suez area. Major unconformities are indicated
by wavy lines. An erosional surface also occurs locally at the
base of the Rudeis Formation. Angular discordance between
prerift and synrift strata is shown schematically. UK FMS ⳱
Upper Cretaceous formations.

Figure 1. Geology of Gebel el Zeit. Parts of South Gebel el
Zeit and North Gebel el Zeit are shown in greater detail, respectively, in Figures 3 and 4. North and South Gebel el Zeit are
separated by a topographically lower area. The location of Gebel
el Zeit is shown in the inset.
1872

Synrift Nukhul Formation, Gulf of Suez, Egypt

turally subdued basins, although minor evidence of
deeper basin deposition has been reported (Beleity,
1984; Sellwood and Netherwood, 1984; Webster and
Ritson, 1984; Scott and Govean, 1985; Evans, 1988;
Richardson and Arthur, 1988; Schu¨tz, 1994; McClay
et al., 1998).
In this article we document the sedimentology of
the Nukhul Formation at Gebel el Zeit at the southern
end of the Gulf of Suez (Figure 1). The unit is exposed
in southwest-dipping outcroppings on the dip slope of
the Gebel el Zeit structural feature. Gebel el Zeit is
part of the central rift that was uplifted in the Pliocene–
Holocene. The Nukhul Formation at Gebel el Zeit consists of a lower siliciclastic and an upper carbonate interval and is reported to have been deposited in a
shallow-marine setting (Evans and Moxon, 1988;
Saoudi and Khalil, 1986; Allam, 1988; Evans, 1990).
Our field work, in contrast, has determined that a major part of the Nukhul Formation at Gebel el Zeit was
deposited by sediment gravity flows below storm base.
Deposition was in a basin with considerable bathym-

etry that was flanked by topographically high areas. We
also found that significant tilting, deformation of prerift strata, and extension occurred before Nukhul sedimentation began (also see Bosworth, 1995; Bosworth
et al., 1998). Evidence of uplift and fault-block rotation during early rifting at Gebel el Zeit includes the
presence of basement pebbles and cobbles in Nukhul
strata.
Both the lower siliciclastic and upper carbonate intervals of the Nukhul Formation at Gebel el Zeit
change considerably along strike. Lithologic and thickness variation and, in places, absence of the unit are
inferred to record differential movement during extension of small blocks within the overall Gebel el Zeit
structural feature. Some of the blocks measure only a
few hundred meters in a strike direction. The blocks
are delineated by intrabasin transfer faults, or cross
faults, that are primarily either orthogonal to the dominant northwest structural trend or strike northnortheast–south-southwest. Cross faults have only minor offsets ranging from tens of meters to possibly 200–
300 m. In this article we show the importance of
recognizing such small blocks and bounding cross faults
in interpreting rift strata in the Gulf of Suez. We infer
that the presence of small transfer faults and slight
changes in elevation and bathymetry from minor differential movement between blocks profoundly influenced deposition in the early Miocene at Gebel el Zeit.
Cross faults commonly separate time-equivalent facies
with significantly different thicknesses and reservoir
characteristics. We deduce that recognizing such cross
faults and the small crustal blocks they bound should
be an important component of exploration and reservoir development in other rifts.

GEOLOGIC SETTING
The Gulf of Suez is the northern segment of a Tertiary
rift system that extends from northern Egypt through
the Red Sea to the Gulf of Aden. The Gebel el Zeit
structural block is located at the southern end on the
southwest, bounding-fault side of the Gulf of Suez
(Figure 1).
Rift Evolution
Most researchers have treated significant rifting in the
Gulf of Suez as beginning in the late early Miocene
(e.g., Angelier, 1985; Steckler, 1985; Moretti and Colletta, 1987; Evans, 1988, 1990; ; Steckler et al., 1988;

Perry and Schamel, 1990; Schu¨tz, 1994). Although
probable rift-related basalt intrusions and flows dated
at 20–25 Ma by K-Ar are present (Steen, 1984; Ott
d’Estevou et al., 1986; Bosworth, 1995), and earliest
Miocene and Oligocene rift-related faulting is recognized (e.g., Garfunkel and Bartov, 1977; Chenet et al.,
1986; Patton et al., 1994), Oligocene–earliest Miocene
tectonism has been considered minor by most
researchers.
In contrast to a late early Miocene age, we believe
that significant rift-related tectonism and extension occurred earlier in the Miocene, if not in the latest Oligocene (also see Plaziat et al., 1998). The latest
Oligocene–earliest Miocene volcanism suggests earlier
rifting. Extensional tectonism also is indicated by the
occurrence of cobbles and boulders of basement rock
in Oligocene conglomerate and breccia along the
northwestern Red Sea coast and by early Miocene tilting of Gebel el Zeit fault blocks (Garfunkel and Bartov,
1977; Allam, 1988; Bosworth, 1995; Bosworth et al.,
1998). Indeed, greater and, at times, faster rotation
likely occurred along bounding faults before and during
Nukhul deposition than during later periods of the
rift’s history (Bosworth et al., 1998). Moreover, fissiontrack ages of apatites in granitic basement indicate that
major uplift of the Red Sea Hills to the west of the
Gulf of Suez, associated with early rifting, occurred at
21–23 Ma in the early Miocene (Omar et al., 1989).
Rift flank uplift from flexural isostacy should be concomitant with extension (McClay et al., 1998).
Continued extension, this time associated with
widespread regional subsidence, occurred in the late
early–middle Miocene, and deep-water Rudeis and Kareem strata were deposited in response (Figure 2) (Sellwood and Netherwood, 1984; Moretti and Colletta,
1987; Evans, 1988; Richardson and Arthur, 1988;
Steckler et al., 1988; Omar et al., 1989; Rodgers et al.,
1989). Later, onset of movement in the middle Miocene along the Gulf of Aqaba transform (Figure 1) apparently lessened extension in the southern Gulf and
restricted active rifting to a central part of the Gulf
(Steckler et al., 1988; Bosworth et al., 1998). Transform motion was approximately correlative with the
onset of evaporite deposition in the Gulf of Suez, although the latter may be related, at least in part, to a
drop in eustatic sea level that occurred at about the
same time (Evans, 1988).
Renewed subsidence occurred in the southern
Gulf of Suez in the late Miocene–Pliocene. Subsequently, deformation and uplift occurred there in the
Pliocene–Holocene because upper Miocene marine
Winn et al.

1873

units and Pliocene terrestrial and marine strata are
now exposed, as at Gebel el Zeit (Figures 1, 3). Latest
Pleistocene–Holocene deformation at Gebel el Zeit
also is indicated because Pleistocene sediment and sedimentary rock are offset by faults and because Pleis-

tocene reefs and ooid lime sand are found at varying
elevations up to 100–150 m above sea level (Garfunkel
and Bartov, 1977; Bosworth and Taviani, 1996). Latest
Pleistocene–Holocene uplift appears to be associated
with continued tilting because very young Pleistocene

Figure 3. Detailed geologic map of a part of South Gebel el Zeit (see Figure 1 for location). Measured sections of the Nukhul
Formation (SGZ-A, SGZ-B, . . . , SGZ-3) are shown schematically in Figures 7 and 14. The Hammam Faraun is the upper member of
the Belayim Formation (see Figure 2). Rose diagrams to the left summarize measurements of long axes of pebbles in three conglomerate beds in the lower Nukhul Formation at section SGZ-3. Pebble measurements are bimodal but are shown as indicating unimodal
southwest transport because structural relationships suggest an approximate southwest-dipping depositional paleoslope. The arrows
on the rose diagrams are vector means. Faults synthetic to major bounding faults strike north-northwest–north. Intrabasin transform
faults (such as cross faults W, X, and Y) strike northeast–east. Recent relative motion on faults is indicated. Note in Figures 3 and 4
that most cross faults do not extend through the Rudeis-Kareem interval.
1874

Synrift Nukhul Formation, Gulf of Suez, Egypt

reefal deposits are present on the east side of Gebel el
Zeit 10–18 m above sea level but are not raised at Zeit
Bay to the southwest (Evans and Moxon, 1988).
Nearby modern earthquakes also indicate active faulting (Jackson et al., 1988). Uplift of the rift margin and
concentration of active rifting to the central Gulf have
restricted the present marine Gulf of Suez to about
one-third the width of the entire rift.
Stratigraphy of Gebel el Zeit
A prerift interval at Gebel el Zeit overlies a basement
of granitic igneous, metasedimentary, and metavolcanic rocks that are cut by mafic and felsic volcanic
dikes. The oldest prerift units are a discontinuous, possible Paleozoic–Triassic red shale and sandstone interval up to several tens of meters thick and a probable
Aptian–Albian, medium-grained to coarse-grained,
quartzose, dominantly cross-bedded sandstone several
hundred meters thick. The cross-bedded sandstone is
correlated to the Malha Formation in the northern
Gulf of Suez (Kerdany and Cherif, 1990). The basal
red shale and sandstone and the cross-bedded sandstone together are referred to informally as either the
Nubia or the Nubian sandstone or formation (Figure
2) (Sellwood and Netherwood, 1984; Evans and
Moxon, 1988). Overlying the Nubia sandstone is an
Upper Cretaceous clastic and carbonate section that
has a maximum exposed thickness of approximately
225 m. The latter interval is overlain in the nearby subsurface by Eocene Thebes Limestone.
In many areas of eastern Egypt, although not at
Gebel el Zeit, and in western coastal areas of Saudi
Arabia, a poorly dated, early rift, primarily continental
interval separates prerift Eocene and older strata from
younger Miocene formations (e.g., Garfunkel and Bartov, 1977; Dullo et al., 1983; Sellwood and Netherwood, 1984; Patton et al., 1994; Abou Ouf and
Gheith, 1998; Bosworth et al., 1998; Montenat et al.,
1998; Plaziat et al., 1998). The early rift sequence in
Egypt includes the Abu Zenima Formation and part of
the Ranga Formation. Deposition of the early rift interval began in the latest Oligocene, but most sedimentation is probably Aquitanian.
Miocene strata in outcrop at Gebel el Zeit consist
of the Nukhul Formation at the base of the synrift interval; overlying marl, shale, sandstone, and minor conglomerate and breccia of the Rudeis-Kareem interval;
and evaporite, marl, shale, and sandstone of middle and
upper Miocene formations (Figure 2). Synrift lower
Miocene strata overlie basement directly, Nubia sand-

stone, or other Cretaceous units, depending in large
part on the amount of Neogene erosion.
An erosional disconformity is present at the lower
contact of the Rudeis locally, but elsewhere the contact
is conformable (Bosworth et al., 1998). The RudeisKareem interval primarily records relatively deepwater sedimentation that apparently began in the late
Aquitanian and extended into the Serravalian. An unconformity also is present at the base of the dominantly
evaporitic Belayim Formation. The latter unit consists
of halite and gypsum and records restriction at the connection between the Mediterranean Sea and the Gulf
of Suez. Belayim and upper Miocene evaporite strata
onlap basement at Gebel el Zeit (Figure 3). Another
unconformity is present at the base of the Pliocene.
Structure of Gebel el Zeit
Gebel el Zeit is an intrarift structural block located on
the western bounding-fault margin in the southern part
of the Gulf of Suez rift (Figure 1). The gebel is separated from the East Zeit Basin under the Gulf of Suez
to the northeast by a normal fault zone that has several
kilometers of offset. The major bounding faults dip approximately 30–40⬚NE (Jackson et al., 1988), although
deep structure is not well documented because of the
absence of wells in the basin center and because of the
poor quality of seismic-reflection images beneath middle and upper Miocene evaporite strata. Strata on dip
slopes of the gebel incline southwestward onto the
Gemsa–Zeit Bay Basin. Approximately 5 and 7 km of
sedimentary strata, respectively, likely fill the East Zeit
Basin and Gemsa–Zeit Bay Basin (Schu¨tz, 1994; Bosworth et al., 1998). Gebel el Zeit and other major
structural blocks and rift-bounding normal faults trend
northwest–north-northwest, parallel with the Gulf of
Suez.
Gebel Zeit consists of two topographically high areas, North Gebel el Zeit and the smaller South Gebel
el Zeit, which are separated by a lower elevation saddle
(Figure 1). Basement is exposed at both North and
South Gebel el Zeit (Figures 3, 4). Synrift lower Miocene beds typically dip 25–30⬚SW and 8–15⬚ less than
the underlying prerift Cretaceous units and the basal
Miocene unconformity (Bosworth et al., 1998). Dip
variation is largely a consequence of rift-related tilting,
although minor structural discordance between preMiocene and Miocene strata may be related to prerift
Cretaceous–Paleogene tectonism (Patton et al., 1994).
South Gebel el Zeit shows more deformation than
North Gebel el Zeit. Structures in South Gebel el Zeit
Winn et al.

1875

NUKHUL FORMATION
The Nukhul Formation is up to approximately 75 m
thick in exposures at North and South Gebel el Zeit
and consists of a lower sandstone and conglomerate
and a concordant upper carbonate interval (Figure 2)
(Saoudi and Khalil, 1986; Evans, 1988). Locally, the
lower clastic interval is absent in outcroppings (e.g.,
section SGZ-B in Figure 3). Nukhul strata are absent
elsewhere, and Rudeis-Kareem marl and shale locally
lie directly on pre-Nukhul units (e.g., section SGZ-A).
Clastic and carbonate units are assumed to be approximately time equivalent between South and North Gebel el Zeit, although biostratigraphic information confirming synchroneity is absent. The Nukhul Formation
is up to several hundred meters thick in the Gemsa–
Zeit Bay Basin and East Zeit Basin (Saoudi and Khalil,
1986; Richardson and Arthur, 1988; Bosworth, 1995),
although maximum thicknesses are unknown because
of sparse data from the basin centers.
Age

Figure 4. Geologic map of a part of North Gebel el Zeit (see
Figure 1 for location). Cross faults, such as the fault at Z, trend
northeast-southwest to east-west and oblique to northnorthwest structural strike of bounding faults.

include a secondary normal fault and syncline that are
parallel with the overall rift trend (Figure 3). In addition, the Nukhul Formation and older units there are
cut by a large number of small normal faults parallel
with the rift trend and by numerous oblique cross
faults. Cross faults strike approximately northeastsouthwest, although they vary in orientation (Figure
3). The major bounding faults, the transfer fault in the
saddle between South and North Gebel el Zeit (Figure
1), and the large normal fault in the center of South
Gebel el Zeit (Figure 3) offset upper Miocene strata.
Most of the small normal faults and cross faults, however, do not extend above the Rudeis-Kareem interval,
indicating that intense internal deformation was limited to the early phase of rift history at Gebel el Zeit.
North Gebel el Zeit has less deformation, but most
cross faults there also do not cut strata younger than
the Rudeis-Kareem interval (Figure 4).
1876

Synrift Nukhul Formation, Gulf of Suez, Egypt

The Nukhul Formation regionally is inferred to be
dominantly Aquitanian (early Miocene), with sedimentation starting in some areas in the Chattian (upper Oligocene) and deposition probably extended to
the early Burdigalian (Figure 2) (Webster and Ritson,
1984; Scott and Govean, 1985; Ott d’Estevou et al.,
1986; Richardson and Arthur, 1988; Montenat et al.,
1998). Age data from Gebel el Zeit include the presence in the Nukhul Formation of the early Aquitanian
benthic foraminifera genus Miogypsinoides (Bosworth
et al., 1998). In addition, the age of the Nukhul at
Gebel el Zeit is constrained by latest Aquitanian and
early Burdigalian foraminifera in the overlying RudeisKareem interval.
Nukhul Conglomerate
Pebbles, cobbles, and boulders in the lower Nukhul
Formation consist of chert, clasts of Nubia sandstone,
other sandstone, partially to completely dolomitized
limestone, marl, and uncommon plutonic and finegrained volcanic clasts (Table 1; Figure 5). Chert clasts
consist of microcrystalline quartz, commonly with
small fossils and ooids. Chert clasts were derived from
the abundant chert nodules and chert beds in Upper
Cretaceous strata and the Eocene Thebes Limestone.
Clasts of Nubia sandstone are less common than chert
pebbles; however, some Nukhul beds are composed

Table 1. Clast Types, Nukhul Formation Conglomerates
South Gebel el Zeit*
Chert
Dolomite/limestone
Dolomitic sandstone
Nubia Sandstone
Other sandstone
Siltstone
Granite/granodiorite
Diorite
Silicic volcanic
Total

16
6
67
1
3
2
3
1
1
100%

South Gebel el Zeit*
100

South Gebel el Zeit**

North Gebel el Zeit†

98

57
43

2

100%

100%

100%

*Clast identifications on bed between sections SGZ-1 and SGZ-3, South Gebel el Zeit.
**Pebble identifications on beds 19 and 24.5 m, respectively, above base of Nukhul Formation, section SGZ-3, South Gebel el Zeit.
†
Clast identifications at section NGZ-2, North Gebel el Zeit.

almost entirely of large, friable sandstone clasts that
display cross-bedding (Figures 6, 7). In addition, the
quartz-rich sand of the lower clastic unit of the Nukhul
probably was derived primarily from the friable Nubia
sandstone. Dolomite and marl pebbles and cobbles, in
turn, were eroded from Upper Cretaceous units and
Thebes Limestone above the Nubia sandstone. Dolomitic sandstone clasts also were derived from Upper
Cretaceous rock types because the Nubia sandstone
lacks dolomite. The uncommon plutonic and volcanic
clasts are identical to rock types of Gebel el Zeit basement, their presumed source.

Lower Clastic Unit Sedimentology
The lower siliciclastic unit of the Nukhul Formation at
South Gebel el Zeit unconformably overlies Nubia
sandstone or younger Upper Cretaceous units and is
more than 30 m thick in places. The siliciclastic unit
varies considerably in thickness along strike. The siliciclastic interval at South Gebel el Zeit consists of
sandstone, conglomeratic sandstone, pebble to cobble
conglomerate, and minor sandy mudstone and marl
(Figure 7). Nukhul sandstone is very fine to very coarse
grained and poorly sorted. Pebbles commonly are

Figure 5. Typical chert-pebble
conglomerate from the lower
siliciclastic unit of the Nukhul
Formation. The bed is 20 m
above the base of section
SGZ-3 (see Figure 7). The bed
is ungraded and contains a
large Nubia sandstone clast immediately above scale. The
scale is 25 cm.
Winn et al.

1877

Figure 6. Nubia sandstone
clasts (at arrows and above the
scale) in the basal bed of the
lower clastic unit of the Nukhul
Formation, section SGZ-3 (see
Figure 7). The scale is 25 cm.

suspended in sandstone beds. Minor glauconite and
shell debris, including bivalve, coral, and echinoid fragments, are present, and beds have a matrix of dolomitized detrital carbonate mud. Quartz sand, in places,
is suspended in carbonate matrix.
Sandstone beds typically are 25 cm–1 m thick, and
most are structureless or normally graded. A few
graded beds consist of structureless sandstone overlain
by horizontal lamination (Figure 8). Sandstone also
commonly occurs as the upper parts of normally
graded conglomerate. Sandstone beds are lenticular for
a few meters to a few tens of meters along outcrop
strike. Burrowing is common.
A complete transition exists between sandstone,
sand-matrix–supported pebble and cobbly sandstone,
and clast-supported conglomerate. Conglomerate beds
range from less than 1 to 8.2 m thick. Thicker, coarser
conglomerate beds tend to be dominated by clasts of
cross-bedded, friable Nubia sandstone. The thickest
and coarsest of these occurs at section SGZ-3 (Figures
6, 7). Nubia sandstone–clast beds are very poorly
sorted, clast-supported, structureless, and ungraded,
and clasts are subangular to subrounded. The largest
Nubia clast found is almost 2 m long. Most conglomeratic beds, however, are composed dominantly of
pebble- to small cobble–size chert clasts (Figure 5) that
have a few dolomitized limestone clasts. The chertclast and limestone-clast beds are poorly to moderately
sorted and are ungraded or, less commonly, normally
1878

Synrift Nukhul Formation, Gulf of Suez, Egypt

or reverse graded. Chert and limestone pebbles and
cobbles are subangular to well rounded. The long axes
of pebbles of some conglomerates are aligned and are
interpreted to indicate south to southwest transport
(Figure 3). Conglomerate beds also are lenticular over
short distances along strike.
The lower clastic interval of the Nukhul Formation
in North Gebel el Zeit is thinner than at South Gebel
el Zeit and typically is less than 2–3 m thick, although
thickness varies (Figure 9). The Nukhul Formation unconformably overlies either Nubia sandstone (to the
south) or Upper Cretaceous clastic and carbonate units
(to the north) at North Gebel el Zeit. Small, angular
Nubia sandstone clasts are present in places where the
Nukhul Formation directly overlies the Nubia sandstone. Some cherty Upper Cretaceous beds at the contact with the Nukhul Formation are contorted, suggesting slumping during the early Miocene (i.e., section
NGZ-5 in Figure 9).
Three separate units are identified in the lower
clastic section of the Nukhul Formation at North Gebel el Zeit. The lowest unit is a crudely stratified,
poorly to moderately sorted conglomerate and sandstone that lacks shell material and detrital carbonate
mud. The conglomerate and sandstone fill and locally
overlie a fissure in the Cretaceous section more than
20 m deep. (The fissure fill is noted on Figure 4.)
Small, narrow fissures less than 30 cm deep in Nubia
sandstone are also present at SGZ-2. The second unit

Figure 7. Schematic diagram of described sections of the Nukhul Formation, South Gebel el Zeit (see Figure 3 for section locations).
Section SGZ-2A is approximately 35 m south of section SGZ-2 (see Figure 10). Thicknesses of the lower clastic interval and upper
carbonate interval are shown separately. Centimeter measurements (cm) refer to the maximum sizes of pebbles and cobbles. Note
the large Nubia sandstone clasts. For carbonate strata in Figures 7 and 9, M ⳱ micrite; G ⳱ grainstone, packstone, or wackestone;
F ⳱ floatstone; R ⳱ rudstone; B ⳱ boundstone.

of the basal Nukhul Formation occurs at section
NGZ-2 and consists of a less than 1 m–thick, moderately sorted, burrowed sandstone and conglomerate
that has some remnant parallel stratification and lowangle cross-stratification. The sandstone and conglomerate contain minor shell debris and only extend ap-

proximately 20 m along strike. Overlying the fissure
fill, the partially low-angle cross-stratified conglomerate and sandstone, and prerift strata in North Gebel
el Zeit is a laterally extensive, approximately tabular,
typically 2 m–thick conglomeratic sandstone to clastsupported sandy conglomerate (Figure 9). The unit
Winn et al.

1879

Figure 8. Graded sandstone from lower clastic unit of the
Nukhul Formation, section SGZ-2A, South Gebel el Zeit. The bed
consists of a thick, structureless lower interval (at a) with floating
pebbles and shells and a thin parallel-laminated upper zone (at
b). Sandstone is interpreted as a Tab turbidite bed. The lower
contact is indicated by arrows. At the top of the photograph,
conglomerate with an erosional lower contact is above the parallel lamination. The scale is 10 cm.
contains rounded boulders up to 80 cm long. The
sandstone and conglomeratic sandstone is burrowed,
poorly sorted, and contains shells and a dolomitized
carbonate matrix.
Lower Clastic Unit Deposition
The lower clastic unit is dominantly marine and records transgression over an unconformity surface. Poor
dating precludes precise correlation of Nukhul deposition to published eustatic sea levels, although onset
of deposition may correlate with a late Chattian–earliest Aquitanian rise in sea level (Haq et al., 1987).
Alternatively, transgression may have been the consequence of tectonic subsidence.
1880

Synrift Nukhul Formation, Gulf of Suez, Egypt

The lower clastic unit at South Gebel el Zeit is
interpreted to have been deposited by sediment gravity
flows. The underlying erosion surface also was likely
eroded by density currents. Marine conditions are indicated by shell debris, detrital carbonate matrix, glauconite, and abundant burrowing. Structureless and
normal- and reverse-graded sandstone, pebbly sandstone, and conglomerate are inferred to have been deposited from high-density sediment gravity flows and
mass flows below depths commonly affected by storm
waves and currents. Strata consisting of structureless
or graded sandstone overlain by parallel lamination are
interpreted as Tab turbidite beds. Water depth is inferred to have been at least many tens of meters deep
and probably 100 m deep or more at deposition; otherwise, some evidence of shallow-water current and
wave-reworked sedimentary structures likely would
have been preserved. Paleowater depths likely deepened basinward to the west into the more structurally
down-dropped parts of the Gemsa–Zeit Bay half graben (Figure 1). Limited paleocurrent data are consistent with south to southwest sediment gravity flow
transport (Figure 3).
Thicker sections of the lower Nukhul interval,
such as at SGZ-2A and SGZ-3, are inferred to represent the fill of small submarine gullies or canyons that
channeled sediment basinward (Figure 10). The submarine gullies were fed from the relatively highstanding South Gebel el Zeit block because pebble
compositions indicate that an interval of Eocene limestone and chert down to local basement was eroded
(Table 1). Uplift, in large part, was due to structural
tilting (Figure 11). Rotation of the South Gebel el Zeit
block concomitantly would have exposed an emergent
area northeast of the modern outcrop and created a
deeper Gemsa–Zeit Bay Basin relative to paleowater
depths at the outcrop site to the southwest.
In contrast, the lower clastic unit at North Gebel
el Zeit appears to record terrestrial deposition followed
by shallow-marine submergence. The fissure fill likely
is a subaerial mudflow and small stream deposit, as indicated by poor sorting, crude stratification, and lack
of marine shells and carbonate mud. The younger, bioturbated, shelly, and partially stratified sandstone and
conglomerate interval represents a shallow-marine unit
that was deposited during transgression. The low-angle
cross-lamination in the strata is interpreted as hummocky cross-stratification. The overlying tabular conglomerate and sandstone, in turn, represent shallowmarine reworking and deposition and more extensive
marine flooding.

Figure 9. Schematic lithologic sections of the Nukhul Formation, North Gebel el Zeit (see Figure 7 for legend and Figure 4 for
locations of sections NGZ-1, NGZ-2, and NGZ-3). The location of the cross fault at Z on Figure 4 is shown below the cross section.
NGZ-4 is located 1.75 km north of NGZ-3. NGZ-5 is 3.4 km north of NGZ-4.

Figure 10. Outcrop view of the Nukhul Formation and an interpreted Miocene submarine gully at South Gebel el Zeit (view is to
the southwest). The locations of sections SGZ-2A and SGZ-2 are indicated (see schematic diagrams in Figure 7). The fault at W on
Figure 3 is in the gully on the left of the photograph. Nukhul strata unconformably overlie Nubia sandstone (white-appearing rocks
exposed in current gullies). Note the lenticularity of lower Nukhul strata. The upper Nukhul carbonate interval forms the small cliff
at the top of the ridge. Beyond the ridge in the upper right is Zeit Bay (see Figure 1), and in the far distance at the arrow is another
uplifted Miocene fault block (Gebel Esh el Mellaha).

Winn et al.

1881

Figure 11. Schematic diagram depicting relative block motion
and differential tilting of South Gebel el Zeit and North Gebel
el Zeit during deposition of the lower siliciclastic unit of the
Nukhul Formation. The figure depicts that uplift can occur from
block rotation. Small-scale block rotation, similar to block motions diagrammed for North and South Gebel el Zeit, was responsible for much of the lithologic and facies variation in the
Nukhul Formation. UK ⳱ Upper Cretaceous.
No sedimentological evidence was identified during the study of sediment gravity flow deposition of the
lower clastics of the Nukhul Formation at North Gebel
el Zeit. The absence of deep-water deposits was a consequence of lesser submergence and probably of only
minor tilting of the North Gebel el Zeit block relative
to South Gebel el Zeit (Figure 11). Islands were subaerially exposed, as indicated by Eocene, Cretaceous,
and basement clasts in Nukhul conglomerate, but
emergent areas probably had low relief. The absence
of significant rotation resulted in the absence of a
northeastern topographically high area and in the presence of a bathymetrically shallower basin immediately
to the southwest.
Upper Carbonate Unit Sedimentology
The upper carbonate unit of the Nukhul Formation
consists of dolomite and calcareous dolomite that un1882

Synrift Nukhul Formation, Gulf of Suez, Egypt

conformably overlies prerift strata or conformably
overlies the lower clastic interval. Thickness and facies
vary considerably along strike, particularly in South
Gebel el Zeit, where changes are abrupt and striking
(Figure 7).
The carbonate unit is slightly less than 20 to almost
60 m thick at South Gebel el Zeit (Figure 7). The transition over most of the area from underlying Nukhul
siliciclastic sediment gravity flow deposits occurs in a
2–5 m transition zone that consists of interbedded
sandstone, conglomerate, and bioclast-intraclast dolomite packstone and wackestone. The sandstone and
conglomerate beds are similar to underlying sediment
gravity flow strata. The packstone consists of very
coarse-grained sand-size to small pebble-size bioclasts
in beds 5–20 cm thick. Erosional contacts are common
at the bases of the transitional carbonate beds, and
small pebbles and floating siliciclastic sand grains are
present. Bioclasts are primarily oriented horizontal to
bedding (i. e., grain oriented), and tops of beds are
burrowed.
Strata overlying the transitional beds at South
Gebel el Zeit dominantly consist of bioclast dolomite
packstone and grainstone and minor floatstone and
rudstone at and near section SGZ-2 (Figures 12a, 7).
Bioclast dolomite packstone, intraclast dolomite floatstone, and dolomite wackestone occur above transitional beds at section SGZ-3. The strata at both localities mostly consist of structureless, poorly sorted,
1–3 m-thick beds with indistinct bed contacts. In
places, however, are beds several centimeters thick
composed of horizontally laminated, contorted, and
graded dolomite grainstone, packstone, and wackestone. The bases of the stratified beds are sharp and
irregular and represent scour surfaces. The stratified
packstone to grainstone contains abundant broken,
poorly to moderately sorted, sand-size to granulesize, thin-walled bivalve debris (Figure 12a). Floatstone contains cobble-size carbonate intraclasts and
bioclasts dispersed in a matrix of bioclast-peloid
wackestone to packstone. Floatstone and rudstone are
more common at SGZ-3 than at SGZ-2. In addition,
a synsedimentary contorted bed is present approximately 10 m above the lower contact of the carbonate in section SGZ-3. Beds typically contain a mixture of fossil types, and burrowing is evident in both
sections.
The upper almost 5 m of the Nukhul carbonate
interval at section SGZ-3 (i.e., the bed 20 m above the
base of the interval) consists of a thick, unbedded coralalgal dolomite floatstone and boundstone and whole

Figure 12. (a) Bioclast packstone with abundant thin-walled
bivalves and moldic porosity from ⬃20 m above the base of
the upper carbonate unit, Nukhul Formation, section SGZ-2 (see
Figure 7). Poor sorting and mixture of bioclasts are interpreted
as due to resedimentation in a relatively deep-marine setting.
(b) In-place coral head (Monostrea) of reefal facies, 18 m above
base of upper carbonate unit, Nukhul Formation, section SGZ-3.

bivalve dolomite floatstone and packstone (Figure 7).
The basal contact of the interval is sharp. The coralalgal floatstone and boundstone contain large, in situ
coral heads of Monastrea sp. (Figure 12b) and Siderastrea sp., encrusting red algae, and a diversity of bivalves. The coral-algal unit extends more than 500 m
to the south, where it varies in thickness from 3 to 7
m and, in places, rests directly on Nubia sandstone.
In contrast, the carbonate unit at SGZ-1 contains
a distinctly different lithofacies from those at SGZ-2
and SGZ-3 (Figure 7). Poorly stratified, bioclastic pebbly sandstone occurs in a 2–4 m transition zone above
Nukhul siliciclastics. Bioclasts in the transition zone

consist of abraded fragments of oysters and platy echinoderms (sand dollars). The lower 30 m of overlying
carbonate beds consist of meter-scale cycles that
coarsen upward. In ascending order, the cycles contain
bioturbated, bioclast packstone and wackestone; bioturbated, whole bivalve and oyster-bivalve packstone
and floatstone; and bioclast-bivalve rudstone. Bioclasts
in the rudstone are moderately sorted and abraded, and
rudstone beds have erosional basal contacts and are
faintly horizontally stratified.
The upper approximately 30 m of the SGZ-1 section lacks cyclical bedding, and beds are structureless
and relatively thick. The interval consists of moderately
sorted, bioclast and peloid dolomite packstone and less
abundant whole oyster-bivalve dolomite packstone
and wackestone. Packstone contains grain-oriented,
comminuted, sand-size to granule-size, thin-walled bivalve and algal plate fragments. Bioturbation is common, and burrows include Ophiomorpha forms.
Carbonate strata in North Gebel el Zeit vary in
thickness from 8 to 45 m because of erosion at the base
of the overlying Rudeis-Kareem interval and northward depositional thinning from NGZ-3 to NGZ-5
(Figure 9). For example, erosion at the base of a submarine channel preceding Rudeis-Kareem deposition
at NGZ-2 left less than 10 m of Nukhul strata there.
At North Gebel el Zeit, the lower contact of the carbonate strata with the underlying Nukhul siliciclastics
is sharp and has tens of centimeters of relief in places.
This contact is interpreted as a marine ravinement.
Sandy to pebbly bioclast dolomite packstone to grainstone occurs immediately above the siliciclastics.
Sedimentology of the upper carbonate interval
varies less along strike at North Gebel el Zeit than it
does at South Gebel el Zeit. The lower almost 10 m
of carbonate strata in NGZ-1, the truncated NGZ-2
interval, and the lower approximately 30 m of NGZ-3
dominantly consist of 0.5–3 m–thick beds of bioclast
and peloid dolomite wackestone and packstone and
micrite and minor grainstone. Bioclast grains are horizontally oriented in some zones. Beds are burrowed,
and complete bioturbation is common. The interval at
17–30 m in NGZ-3 is marly and nodular.
From above approximately 10 m in NGZ-1 to the
upper contact, strata tend to consist of better sorted,
more distinctly bedded, 1–2 m–thick, very coarse
grained bioclast grainstone and packstone, except for
two micritic intervals near the top of the Nukhul Formation. The grainstone and packstone contain abundant whole and broken bivalve debris. Section NGZ-3
at 30–40 m consists of another bioclast grainstone
Winn et al.

1883

zone, which contains whole and broken bivalve and
oyster shells (Figure 13a) and some up to cobble-size
red-algal nodule fragments.
Bioclast floatstone at approximately 5–8 m above
the lower Nukhul contact and coral-algal floatstone,
boundstone, and grainstone (Figure 13b) at 8–13 m
occur in NGZ-4. Coral-algal dolomite boundstone and
floatstone also are present in NGZ-3 at 40–42 m.
Above the floatstone and boundstone in NGZ-4 are
approximately 10 m of burrowed to bioturbated, partially small-scale cross-bedded bioclast grainstone. The
upper few meters of the Nukhul carbonate interval in

NGZ-3 and most strata above 23 m in NGZ-4 consist
of relatively shaly, primarily thinly bedded, bioclast
packstone and wackestone.
The carbonate interval of the Nukhul Formation
at NGZ-5 is less than 20 m thick and consists of 0.1–
2 m–thick, nodular, bioclast and intraclast, dolomite
wackestone and packstone that locally have large coral
and bivalve fossils. Packstone beds are lenticular with
erosional basal contacts and grade upward to wackestone. The Nukhul carbonate interval appears to transitionally grade into overlying Rudeis-Kareem marl and
shale at sections NGZ-3, NGZ-4, and NGZ-5.
Upper Carbonate Unit Deposition

Figure 13. (a) Bioclast packstone with red algae (arrow) in
bed 8–12 m above base of upper carbonate unit, Nukhul Formation, section NGZ-4 (see Figure 9). The bed is interpreted as
having been deposited in a bioclast shoal. (b) Bioclast wackestone to packstone with prominent oysters, other bivalve shell
fragments, and moldic porosity from 30 m above base of upper
carbonate unit, Nukhul Formation, section NGZ-3. The bed is
interpreted as representing relatively deep-shelf sedimentation.
1884

Synrift Nukhul Formation, Gulf of Suez, Egypt

We interpret the change from siliciclastic to carbonate
sedimentation as the response to a rise in sea level. Imprecise age control precludes concluding whether
transgression was due to eustatic sea level rise or tectonic subsidence. Transgression ended the supply of
terrigeneous sand and gravel from uplifted parts of
tilted fault blocks.
The carbonate intervals at sections SGZ-2 and at
SGZ-3 below the uppermost coral-bivalve interval are
interpreted as having been deposited by sediment gravity flows in submarine gullies in water at least several
tens of meters deep and possibly 100–200 m deep (Figures 7, 14). The sharp basal bed contacts in the strata
likely were eroded by density currents. Poorly sorted,
graded bioclast-intraclast packstone and wackestone
(Figure 12a) that contain some parallel lamination are
inferred to have been deposited primarily by turbidity
currents. Thick, structureless bioclast-intraclast floatstone and packstone in SGZ-2 and SGZ-3 are interpreted to be primarily due to mass emplacement. The
mixing of fossil types in beds is consistent with resedimentation; however, the presence of burrow traces indicates that some sediment homogenization was the
result of bioturbation. Uncommon soft-sediment contorted beds represent submarine slumps.
Resedimented carbonate strata at SGZ-3 are overlain by reworked and in situ coral-algal and bivalve
skeletal carbonate beds (Figure 12b). The coral-algal
and coarse-grained bivalve beds were deposited in a
shallow, open-marine reefal setting. Reefs grew directly on outcrops of Nubia sandstone at SGZ-B (Figure 14).
The relatively thick carbonate strata at section
SGZ-1 (Figures 7, 14), in contrast to SGZ-2 and
SGZ-3, are interpreted as recording relatively lowenergy, peritidal and shallow subtidal ramp followed

Figure 14. (a) Stratigraphy
and inferred depositional environments, Nukhul Formation,
South Gebel el Zeit. See Figure
3 for the locations of the described sections and cross faults
and Figure 7 for the section descriptions. Recent relative movement on cross faults is indicated. (b–d) Schematic diagram
of block movement and interpreted structural control on
depositional environments,
South Gebel el Zeit. Sediment
gravity flow deposition of carbonate beds on blocks represented by sections SGZ-2 and
SGZ-3 is inferred to have occurred concurrently with shallower water deposition on the
block represented by section
SGZ-1, although biostratigraphic
evidence for equivalency is
lacking. In addition, deep-water
sedimentation at SGZ-2 is inferred to have occurred simultaneously with the growth of
reefs at SGZ-B and SGZ-3, although fossil age control is
lacking. Note that movement
across some intrabasin transfer
faults changed during the
Miocene.

Winn et al.

1885

by lower energy, deeper shelf deposition. Multiple, cyclic, shallowing-upward carbonate beds in the lower 30
m of SGZ-1 record water-depth fluctuation within individual cycles from shallow, low-energy, subtidal
depths upward to a wave- and current-worked intertidal environment. Rudstone at the top of coarseningupward cycles likely was reworked from muddier,
underlying oyster-bivalve beds during initial resubmergence. The cyclic strata are overlain by 30 m of
bioturbated, more thickly bedded packstone and
wackestone that we interpret as a deeper water, openmarine ramp facies.
Carbonate beds at North Gebel el Zeit directly
overlie a surface likely eroded during transgression.

Figure 15. (a) Stratigraphy of
the Nukhul Formation and
Rudeis-Kareem interval, North
Gebel el Zeit See Figure 4 for
locations of described sections
NGZ-1, NGZ-2, and NGZ-3 and
cross faults. NGZ-4 is located
1.75 km north of NGZ-3. NGZ-5
is 3.4 km north of NGZ-4.
NGZ-A is located 0.5 km south
of NGZ-1. Datum is the RudeisKareem–Belayim contact.
(b) Nukhul Formation basal
conglomerate and sandstone,
North Gebel el Zeit. The cross
fault at Z appears to have been
inactive during conglomerate
and sandstone deposition because the lower unit does not
change in thickness significantly
across the fault. (c) Nukhul carbonate facies, North Gebel el
Zeit. Down-to-the-north movement on the fault at Z and
northward tilting of the northern block is inferred during Nukhul carbonate deposition (see
Figure 9 for Nukhul Formation
section descriptions).
1886

Synrift Nukhul Formation, Gulf of Suez, Egypt

Most of the carbonate beds at North Gebel el Zeit record open-marine, storm-influenced ramp deposition
with local patch reefs and skeletal sand shoals (i.e., sections at NGZ-1, NGZ-2, most of NGZ-3, and the basal
part of NGZ-4 in Figures 9, 15). Boundstone in sections NGZ-3 and NGZ-4 represent patch reefs. Grainstone and some packstone in NGZ-1, NGZ-3, and
NGZ-4 record carbonate sand shoals (Figure 13a). The
basal scour contacts and dispersed pebbles within
grainstone and packstone beds are interpreted as the
products of storm reworking on a carbonate shelf.
More micritic, bioclast packstone and wackestone (Figure 13b) within the dominantly shallow-marine intervals record slightly deeper, open-shelf sedimentation.

Environments deepened to the north and upward
in the Nukhul Formation at North Gebel el Zeit (Figure 15). Nodular, finer grained, micritic, and more
thinly bedded strata at NGZ-5 and the tops of
NGZ-3 and NGZ-4 record deeper shelf and, possibly,
turbidity-current deposition.

INTRABASIN TRANSFER FAULTS, BLOCK
MOVEMENT, AND SEDIMENTATION
Continental rifts consist of structurally linked half grabens. Half grabens originate from movement on large
listric normal faults, called border or bounding faults,
that are located along one margin of the half grabens.
The separate half grabens are linked by major transverse interbasin transfer faults and relay ramps that
connect separate border faults (e.g., Bosworth, 1985;
Morley et al., 1990). Also transverse to oblique to the
major bounding faults are small intrabasin transfer
faults or cross faults within half grabens. Cross faults
connect and accommodate small amounts of varying
motion along more-or-less en echelon border faults and
associated synthetic faults (e.g., Gibbs, 1984; Peacock
and Sanderson, 1991; Gawthorpe and Hurst, 1993).
Cross faults permit minor differential movement of adjacent crustal blocks within half grabens.
Facies and thickness relationships of Nukhul strata
at Gebel el Zeit indicate that differential motion of
small blocks and movement on cross faults were important to controlling sedimentation. For example, the
difference in depositional setting of the lower siliciclastic unit of the Nukhul Formation in South and
North Gebel el Zeit indicates that the two areas behaved differently structurally during early Nukhul deposition. Lower Nukhul strata at South Gebel el Zeit
were deposited in relatively deep water, likely because
of significant tilting of the South Gebel el Zeit block
(Figure 11). Strata in outcroppings of the lower Nukhul at South Gebel el Zeit are inferred to have been
deposited between a relatively high area to the northeast, which includes the area having current exposures
of Precambrian basement, and the deep Gemsa–Zeit
Bay Basin to the southwest.
In contrast, the North Gebel el Zeit area of outcroppings was a shallower depositional site during early
Nukhul deposition as a consequence of less rotation.
In addition, North Gebel el Zeit had a lower and
smaller emergent source area. Bedding attitudes support the structural interpretation from the sedimentologic evidence. The Nubia sandstone in South Gebel

el Zeit dips 45–50⬚SW, but the Nubia sandstone in
North Gebel el Zeit is rotated on average only about
42⬚SW (Bosworth et al., 1998). Structural discordance
indicates that a transfer fault in the topographically low
area between North and South Gebel el Zeit partially
disconnected the two areas in the early Miocene (Figure 1).
In addition to the break between North and South
Gebel el Zeit, numerous, smaller cross faults mapped
from offsets in basement and Miocene strata are present down the entire length of the Gebel el Zeit structure, particularly in South Gebel el Zeit (Figures 3, 4).
The faults bound small structural blocks, some of
which measure only a few hundred meters in length in
a strike direction. Relative movement of blocks profoundly affected Nukhul sedimentation. In particular,
the submarine channels at South Gebel el Zeit appear
to have been located on more deeply subsiding blocks
(Figure 14b). Those blocks became submarine conduits for sediment gravity flows during Nukhul sedimentation. For example, the submarine gully fill preserved at SGZ-2 and SGZ-2A apparently records a low
area during lower Nukhul deposition. The sections are
approximately 300 m south of a cross fault (labeled X
on Figures 3, 14) that we interpret as the northern
boundary of a Miocene intrabasin structural block. The
block immediately to the north, represented by section
SGZ-A, was higher standing, although probably still
submerged. The section at SGZ-A consists of Nubia
sandstone directly overlain by fine-grained RudeisKareem shale, marl, and minor sandstone without intervening Nukhul strata. Only minor erosional relief is
evident at the contact.
Another cross fault is mapped at W on Figure 3.
(The fault zone at W is visible at the left on the photograph in Figure 10.) Nukhul strata southeast of the
fault are poorly exposed but apparently consist of a
resedimented interval significantly thicker than section SGZ-2A. This thickness difference suggests that
the block to the south subsided even more than the
block represented by sections SGZ-2 and SGZ-2A
(Figure 14b).
Section SGZ-3 (Figure 7) records the existence of
another submarine channel and another deeply downdropped block (Figure 14b). Section SGZ-3 has the
coarsest Nukhul debris along the Gebel el Zeit exposure (Figure 6). The northern boundary of the block
represented by SGZ-3 is the fault labeled Y on Figure
3. The southern boundary of the block is less certain.
Miocene stratigraphic relationships suggest a break
between sections SGZ-3 and SGZ-B, although the
Winn et al.

1887

presence or absence of a fault has not been determined
because of poor exposure. The block containing
SGZ-B appears to have been relatively high during
early Nukhul deposition because it lacks the lower clastic interval.
Another submarine gully appears to have been
present on the block represented by section SGZ-1,
although lower Nukhul strata there are not well exposed, making interpretation equivocal. Section
SGZ-1 is just south of a poorly exposed cross fault that
had probable latest Miocene down-to-the-north motion (Figure 3); however, the fault likely had down-tothe-south motion during early Nukhul deposition because deep-water deposits are present south but not
north of the fault (Figure 14a, b). In addition, we infer
the existence of a cross fault between sections SGZ-A
and SGZ-1 from outcrop discordance and Miocene
stratigraphic relationships, but the fault is not evident
at the surface because of poor exposure.
North Gebel el Zeit appears to have acted as a
more coherent structural unit during deposition of the
lower Nukhul siliciclastics because the basal conglomerate is fairly uniform in character and thickness over
the area (Figures 9, 15). The basal lower Nukhul fissure
fill, however, is spatially associated with a major cross
fault (fault Z on Figures 4, 15), suggesting a genetic
relationship.
A rise in sea level likely is responsible for the
change from siliciclastic to carbonate Nukhul deposition. Nukhul carbonate strata were deposited in a range
of environments, with facies and thickness changes also
strongly influenced by movement of small structural
blocks (Figures 14, 15). For example, carbonate strata
at sections SGZ-2 and SGZ-3 in South Gebel el Zeit
were deposited by sediment gravity flows in relatively
deep water on blocks that were low (Figure 14a, c).
Siliciclastic submarine gullies were located at the
SGZ-2 and SGZ-3 sites. Presumably, the blocks continued to subside with the onset of carbonate sedimentation, although carbonate debris may have filled existing low areas.
In contrast, the presumed equivalent basal carbonate beds of section SGZ-1 were deposited on a shallow
peritidal to subtidal, open-marine platform. The beds
were deposited on an uplifted block relative to its position during siliciclastic Nukhul deposition, although
the block likely remained submerged (Figure 14c). The
crests of the blocks immediately to the south and north
represented by sections SGZ-A and SGZ-B, respectively, were in very shallow water or possibly were exposed slightly.
1888

Synrift Nukhul Formation, Gulf of Suez, Egypt

The SGZ-1 section overall records a gradual, but
slight, deepening of open-marine platform conditions,
indicating that subsidence and carbonate sedimentation rates were nearly equal there (Figure 14d). In contrast, the carbonate turbidite strata of SGZ-3 are
capped by shallow-marine reefal and skeletal carbonate
facies, indicating local sea shallowing during latest
Nukhul deposition. The water shallowed probably as a
consequence of uplift of the formerly subsided block
at the SGZ-3 section. The SGZ-B block, in contrast,
subsided, which allowed reefs to grow during latest
Nukhul deposition. The SGZ-2 block, in turn, presumably continued to sink, which permitted continued
accumulation of carbonate sediment gravity-flow
deposits.
Carbonate facies in North Gebel el Zeit show
some evidence of early Miocene faulting and the relative movement of small blocks (Figure 15), although
erosional truncation of the Nukhul make interpretation equivocal. In addition, differential block motion
apparently was much less in North Gebel el Zeit than
to the south. The presence of reefal buildups in sections NGZ-3 and NGZ-4, however, which are absent
in NGZ-1 and NGZ-2, suggests that the area immediately north of the fault at Z (Figures 4, 9) was higher
standing during Nukhul carbonate deposition. In contrast, the dominance of deep shelf to turbidite carbonate at sections NGZ-4 and NGZ-5 indicates that the
northern block might have tilted to the north.
Later, the fault at Z likely controlled the location
of a Rudeis-Kareem submarine channel that cuts
downward into the Nukhul just southeast of NGZ-2
(Figures 4, 15). The close proximity of the RudeisKareem channel to the fault suggests that the fault itself may have controlled location of the submarine
channel, possibly by being a weaker zone easily eroded
by sediment gravity flows. Independent motion of
small blocks within the Gebel el Zeit structure stopped
by the late middle Miocene because the overlying
Belayim Formation generally is not offset by intrabasin
cross faults (Figures 3, 4).
Bathymetric differences and displacement between adjacent structural blocks were likely only tens
of meters to 100–200 m to account for the different
facies and thicknesses of Nukhul strata at Gebel el Zeit
(Figures 14, 15). Facies relationships indicate a complex history of block movement, which included uplift
at times and not just differential subsidence. Offsets on
cross faults in the Miocene also appear to have been
small and limited to tens to possibly 200–300 m of slip.
The consequence of differential block movement,

however, is that strike-slip, vertical, and oblique directions of movement may occur simultaneously or at different times on the same fault.

CONCLUSIONS
Gebel el Zeit at the southern end of the Gulf of Suez
represents an uplifted part of the central Gulf of Suez
rift. Discordance between prerift and rift strata there
indicates that 8–15⬚ of rotation occurred in the latest
Oligocene–early Miocene before subsidence and sedimentation began (Bosworth, 1995; Bosworth et al.,
1998). The oldest synrift strata at the gebel is the lower
Miocene Nukhul Formation, which preserves the early
sedimentary record of rifting. Greater angular discordance between prerift and synrift strata at South Gebel
el Zeit and facies relationships indicate that South Gebel el Zeit rotated more than North Gebel el Zeit during early Nukhul deposition. Clast composition of Nukhul conglomerate indicates unroofing of basement
and also attests to early Miocene block rotation.
We interpret the lower clastic interval of the Nukhul Formation in South Gebel el Zeit as a relatively
deep-marine sediment gravity flow deposit, contrary to
previous interpretations of the unit as entirely shallow
marine. Sediment gravity-flow deposits partially fill
submarine gullies that funneled debris from uplifted
areas to the northeast into a deep Gemsa–Zeit Bay Basin to the southwest. Gully location appears to have
been controlled primarily by the local down-dropping
of small blocks.
The upper carbonate unit of the upper Nukhul
Formation also changes considerably along strike in facies and thickness. Variation is also related to the differential movement of small crustal blocks. Sediment
gravity-flow deposition continued in some areas at
South Gebel el Zeit. Other blocks in South Gebel el
Zeit were higher standing, and strata record evidence
of block uplift as adjacent blocks subsided. North Gebel el Zeit behaved as a more coherent unit, and Nukhul strata record only slight local differential subsidence. Intrabasin block movement during extension
is due to differential rotation and is also the consequence of jostling and space conflicts because of the
various shapes and orientations of structural blocks and
their bounding faults.
Deep basins, high relief, abrupt thickness and facies changes, and differential block movement were
unrecognized and are contrary to the majority of published interpretations of the early Gulf of Suez setting

(e.g., Garfunkel and Bartov, 1977; Angelier, 1985;
Steckler, 1985; Evans, 1988, 1990; Steckler et al.,
1988; Rodgers et al., 1989; Schu¨tz, 1994). We show
Gebel el Zeit to have undergone a depositionally complicated evolution in the early Miocene and that the
site was not the structurally simple basin commonly
envisioned.
The structural and depositional complexity encountered at Gebel el Zeit appears typical of much of
the rest of the Gulf of Suez rift (e.g., Burchette, 1987;
Schu¨tz, 1994), of the coastal geology of the Red Sea
(e.g., Montenat et al., 1988; Gawthorpe et al., 1990;
Purser et al., 1990), and apparently of other continental rifts (e.g., Gawthorpe and Hurst, 1993). In particular, intrabasin transfer faults and small crustal blocks,
important to controlling Nukhul sedimentation, are
common features of extensional basins. Depositional
complexity, as demonstrated by the Nukhul, dictates
that realistic exploration and production models in rifts
require the delineation of small-scale blocks and cross
faults from well, seismic, and potential-field data. Lithologic complexity necessitates fault block by fault
block facies prediction. Last, maturely explored hydrocarbon basins, like the Gulf of Suez in particular, require detailed structural-stratigraphic models of synrift
reservoirs for exploration success as smaller accumulations become targets and as in-field development
occurs.
REFERENCES CITED
Abou Ouf, M. A., and A. M. Gheith, 1998, Sedimentary evolution
of early rift troughs on the central Red Sea margin, Jeddah,
Saudi Arabia, in B. H. Purser and D. W. J. Bosence, eds., Sedimentary and tectonic evolution of rift basins: the Red Sea–Gulf
of Aden: London, Chapman and Hall, p. 135–145.
Allam, A., 1988, A lithostratigraphical and structural study on Gebel
el-Zeit area, Gulf of Suez, Egypt: Journal of African Earth Sciences, v. 7, p. 933–944.
Angelier, J., 1985, Extension and rifting: the Zeit region, Gulf of
Suez: Journal of Structural Geology, v. 7, p. 605–612.
Beleity, A., 1984, The composite standard and definition of paleo
events in the Gulf of Suez: Proceedings of the 6th Exploration
Conference Seminar, Egyptian General Petroleum Corporation, p. 181–198.
Bosworth, W., 1985, Geometry of propagating continental rifts: Nature, v. 316, p. 625–627.
Bosworth, W., 1995, A high-strain rift model for the southern Gulf
of Suez (Egypt), in J. J. Lambiase, ed., Hydrocarbon habitat of
rift basins: Geological Society Special Paper 80, p. 75–112.
Bosworth, W., and M. Taviani, 1996, Late Quaternary reorientation
of stress field and extension direction in the southern Gulf of
Suez, Egypt: evidence from uplifted coral terraces, mesoscopic
fault arrays, and borehole breakouts: Tectonics, v. 15, p. 791–
802.
Bosworth, W., P. Crevello, R. D. Winn Jr., and J. Steinmetz, 1998,

Winn et al.

1889

Structure, sedimentation, and basin dynamics during rifting of
the Gulf of Suez and northwestern Red Sea, in B. H. Purser and
D. J. W. Bosence, eds., Sedimentary and tectonic evolution of
rift basins: the Red Sea–Gulf of Aden: London, Chapman and
Hall, p. 77–96.
Burchette, T. P., 1987, Tectonic control on carbonate platform facies
distribution and sequence stratigraphy: Miocene, Gulf of Suez:
Sedimentary Geology, v. 59, p. 179–204.
Chenet P., J. Letouzey, and E. S. Zaghloul, 1986, Some observations
on the rift tectonics in the eastern part of the Suez rift: Proceedings of the 7th Exploration Conference, Egyptian General
Petroleum Corporation, p. 18–36.
Dullo, W.-C., H. Ho¨tzl, and A. R. Jado, 1983, New stratigraphical
results from the Tertiary sequence of the Midyan area, NW
Saudi Arabia: Newsletter Stratigraphy, v. 12, p. 75–83.
Evans, A. L., 1988, Neogene tectonic and stratigraphic events in the
Gulf of Suez rift area, Egypt: Tectonophysics, v. 153, p. 235–
247.
Evans, A. L., 1990, Miocene sandstone provenance relations in the
Gulf of Suez: insights into synrift unroofing and uplift history:
AAPG Bulletin, v. 74, p. 1386–1400.
Evans, A. L., and I. W. Moxon, 1988, Gebel Zeit chronostratigraphy:
Neogene syn-rift sedimentation atop a long-lived paleohigh:
Proceedings of the 8th Exploration Conference, Egyptian General Petroleum Corporation, p. 251–265.
Garfunkel, Z., and Y. Bartov, 1977, The tectonics of the Suez rift:
Geological Survey of Israel Bulletin 71, 44 p.
Gawthorpe, R. L., and J. M. Hurst, 1993, Transfer zones in extensional basins: their structural style and influence on drainage
development and stratigraphy: Journal of the Geological Society, v. 150, p. 1137–1152.
Gawthorpe, R. L., J. M. Hurst, and C. P. Sladen, 1990, Evolution of
Miocene footwall-derived coarse-grained deltas, Gulf of Suez,
Egypt: implications for exploration: AAPG Bulletin, v. 74,
p. 1077–1086.
Gibbs, A. D., 1984, Structural evolution of extensional basin margins: Journal of the Geological Society, v. 141, p. 609–620.
Haq, B. U., J. Hardenbol, and P. R. Vail, 1987, Chronology of fluctuating sea levels since the Triassic (250 million years to present): Science, v. 235, p. 1157–1165.
Jackson, J. A., N. J. White, Z. Garfunkel, and H. Anderson, 1988,
Relations between normal-fault geometry, tilting and vertical
motions in extensional terrains: an example from the southern
Gulf of Suez: Journal of Structural Geology, v. 10, p. 155–170.
Kerdany, M. T., and O. H. Cherif, 1990, Mesozoic, in R. Said, ed.,
The geology of Egypt: Rotterdam, Balkena, p. 407–438.
McClay, K. R., G. J. Nichols, S. M. Khalil, M. Darwish, and W.
Bosworth, 1998, Extensional tectonics and sedimentation, eastern Gulf of Suez, Egypt, in B. H. Purser and D. J. W. Bosence,
eds., Sedimentary and tectonic evolution of rift basins: the Red
Sea–Gulf of Aden: London, Chapman and Hall, p. 223–238.
Montenat, C., et al., 1988, Tectonic and sedimentary evolution of
the Gulf of Suez and the northwestern Red Sea: Tectonophysics, v. 153, p. 162–177.
Montenat, C., F. Orszag-Sperber, J.-C. Plaziat, and B. H. Purser,
1998, The sedimentary record of the initial stages of OligoMiocene rifting in the Gulf of Suez and the northern Red Sea,
in B. H. Purser and D. W. J. Bosence, eds., Sedimentary and
tectonic evolution of rift basins: the Red Sea–Gulf of Aden:
London, Chapman and Hall, p. 146–161.
Moretti, I., and B. Colletta, 1987, Spatial and temporal evolution of
the Suez rift subsidence: Journal of Geodynamics, v. 7, p. 151–
168.
Morley, C. K., R. A. Nelson, T. L. Patton, and S. G. Munn, 1990,
Transfer zones in the East Africa Rift system and their relevance
to hydrocarbon exploration: AAPG Bulletin, v. 74, p. 1234–
1253.

1890

Synrift Nukhul Formation, Gulf of Suez, Egypt

Omar, G. I., M. S. Steckler, W. R. Buck, and B. P. Kohn, 1989,
Fission-track analysis of basement apatites at the western margin of the Gulf of Suez rift, Egypt: evidence for synchroneity
of uplift and subsidence: Earth and Planetary Science Letters,
v. 94, p. 316–328.
Ott d’Estevou, P., J.-J. Jarrige, J.-C. Icart, C. Montenat, C. Henry,
and J. Cravatte, 1986, Observations structurales sur le section
d’Abu Rudeis marge orientale du Golfe de Suez, in C. Montenat, ed., Etudes geologiques des rives du Golfede Suez et de
la Mer Rouge nord occidentale: Documents et Travaux Institut
Geologique Albert de Lapparent, no. 10, p. 75–92.
Patton, T. L., A. R. Moustafa, R. A. Nelson, and S. A. Abdine, 1994,
Tectonic evolution and structural setting of the Suez rift, in
S. M. Landon, ed., Interior rift basins: AAPG Memoir 59, p. 9–
55.
Peacock, D. C. P., and D. J. Sanderson, 1991, Displacements, segment linkage and relay ramps in normal fault zones: Journal of
Structural Geology, v. 13, p. 721–733.
Perry, S. K., and S. Schamel, 1990, The role of low-angle normal
faulting and isostatic response in the evolution of the Suez rift:
Tectonophysics, v. 174, p. 159–173.
Plaziat, J.-C., C. Montenat, P. Barrier, M.-C. Janin, F. OrszagSperber, and E. Philobbos, 1998, Stratigraphy of the Egyptian
syn-rift deposits: correlations between axial and peripheral sequences of the north-western Red Sea and Gulf of Suez and
their relations with tectonics and eustacy, in B. H. Purser and
D. W. J. Bosence, eds., Sedimentary and tectonic evolution of
rift basins: the Red Sea–Gulf of Aden: London, Chapman and
Hall, p. 211–222.
Purser, B. H., E. R. Philobbos, and M. Soliman, 1990, Sedimentation
and rifting in the NW parts of the Red Sea: a review: Bulletin
de la Socie´te´ Ge´ologique de France, v. 6, p. 371–384.
Richardson, M., and M. A. Arthur, 1988, The Gulf of Suez–northern
Red Sea Neogene rift: a quantitative basin analysis: Marine and
Petroleum Geology, v. 5, p. 247–270.
Rodgers, J. J. W., M. E. Dabbagh, B. M. Whiting, and S. A. Widman,
1989, Subsidence and origin of the northern Red Sea and Gulf
of Suez: Journal of African Earth Sciences, v. 8, p. 617–629.
Saoudi, A., and B. Khalil, 1986, Distribution and hydrocarbon potential of Nukhul sediments in the Gulf of Suez: Proceedings
of the 7th Exploration Conference, Egyptian General Petroleum Corporation, p. 75–96.
Schu¨tz, K. I., 1994, Structure and stratigraphy of the Gulf of Suez,
Egypt, in S. M. Landon, ed., Interior rift basins: AAPG Memoir
59, p. 57–96.
Scott, R. W., and F. M. Govean, 1985, Early depositional history of
a rift basin: Miocene in the western Sinai: Palaeogeography,
Palaeoclimatology, Palaeoecology, v. 52, p. 143–158.
Sellwood, B. W., and R. E. Netherwood, 1984, Facies evolution in
the Gulf of Suez area: sedimentation history as an indicator of
rift initiation and development: Modern Geology, v. 9, p. 43–
69.
Steckler, M. S., 1985, Uplift and extension at the Gulf of Suez: indications of induced mantle convection: Nature, v. 317, p. 135–
139.
Steckler, M. S., F. Berthelot, N. Lyberis, and X. Le Pichon, 1988,
Subsidence in the Gulf of Suez: implications for rifting and
plate kinematics: Tectonophysics, v. 153, p. 249–270.
Steen, G., 1984, Radiometric age dating and tectonic significance of
some Gulf of Suez igneous rocks, in G. Hanter, ed., Proceedings
of the 6th Exploration Seminar, Egyptian General Petroleum
Corporation, p. 199–208.
Webster, D. J., and Ritson, N., 1984, Post-Eocene stratigraphy of the
Suez rift in south-west Sinai, Egypt: Proceedings of the 6th Exploration Seminar, Egyptian General Petroleum Corporation,
p. 276–288.