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J
OURNAL OF
S
EDIMENTARY
R
ESEARCH
,V
OL
. 73, N
O
.3,M
AY
, 2003,
P
. 407–420
Copyright
q
2003, SEPM (Society for Sedimentary Geology) 1527-1404/03/073-407/$03.00
SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF EARLY SYN-RIFT TIDAL SEDIMENTS: THE
NUKHUL FORMATION, SUEZ RIFT, EGYPT
IAN D. CARR,
1,
* ROB L. GAWTHORPE,
1
CHRISTOPHER A.L. JACKSON,
1
IAN R. SHARP,
1
†
AND
ALI SADEK
2
1
Basin and Stratigraphic Studies Group, Department of Earth Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.
e-mail: icarr@brookes.ac.uk
2
Department of Geology, Faculty of Science, University of Cairo, Giza, Cairo, Egypt
A
BSTRACT
: Facies and tectono-stratigraphic models for the tidally in-
fluenced Miocene Nukhul Formation are presented, based on outcrop
data from Hammam Faraun fault block, Suez Rift, Egypt. Deposits of
the Nukhul Formation are attributed to two linked depositional set-
tings, offshore to shoreface and estuary settings, and were deposited
during initial stages of rifting in hanging-wall depocenters of early-
formed propagating fault segments. The offshore to shoreface deposits
consist of variably bioturbated mudstones that pass gradationally up-
ward to bioturbated bioclastic sandstones. The more landward estuary
deposits can be separated into a tripartite division of estuary mouth,
estuary funnel with bayhead delta, and upper estuary channel deposits.
Estuarine processes generated a complex intercalation of lithologies,
with both gradational and sharp facies transitions. In the estuary de-
posits, tidal ravinement surfaces are typically characterized by mud-
stones of the estuary-funnel association below, passing abruptly up to
erosionally based estuary mouth sandstones. Maximum flooding sur-
faces are expressed by an abrupt erosional contact separating estuary-
mouth sandstones below and estuary-funnel mudstones above.
Stratigraphic development was strongly influenced by the evolving
early-rift structure. Depocenters were narrow (2–5 km wide) and elon-
gate (
,
10 km long) parallel to the strike of normal-fault segments.
The shoreface shoal prevented wave energy in the estuary and in-
creased the relative influence of tidal currents. The elongate, fault-
controlled geometry of the depocenters confined the bayhead delta and
further enhanced tidal influence. Stratal geometry reflects deformation
associated with low-relief growth folds and surface-breaking faults
that, together, formed part of an evolving fault array. This basin con-
figuration and associated Nukhul stratigraphy is markedly different to
tectono-stratigraphic models for crustal-scale tilted fault blocks that
are applicable from late stages of rifting.
INTRODUCTION
In a sequence stratigraphic framework, tidal facies are commonly de-
scribed from incised valleys and are interpreted to have formed during
relative sea-level rise following fluvial incision (e.g., Allen and Posamentier
1993; Dalrymple et al. 1994; Mellere 1994; Nichol et al. 1996; Zhang and
Li 1996). Previous facies and sequence stratigraphic models have been
developed mainly on the premise that eustasy was the controlling factor
on relative sea level and accommodation, and the driving force for both
fluvial incision and subsequent infill during relative sea-level rise. Other
controlling factors on accommodation, such as local tectonics, have not, as
yet, been considered. Due to fault propagation, fault linkage, and fault death
the structural development of rift basins introduces extra complexity to the
sequence stratigraphy of tidal facies. These structural factors cause varia-
tions in basin physiography and accommodation in four dimensions (see
review in Gawthorpe and Leeder 2000). Furthermore, there are relatively
few studies of tidal deposits in rift basins, especially where tidal facies can
* Present address: Geology (BMS), Oxford Brookes University, Headington, Ox-
ford OX3 0BP, U.K.
† Present address: Norsk Hydro Research Center, Sandsliveien 90, N-5020, Ber-
gen, Norway
be unequivocally related to structural style at the time of deposition (cf.
Surlyk and Clemmensen 1983; Gjelberg et al. 1987; Richards 1991).
This study focuses on the Miocene Nukhul Formation, Suez Rift, which
was deposited in a wave-dominated estuary in tectonically formed, narrow
embayments subject to a mesotidal range. We develop a process-based
facies model for the Nukhul Formation, characterize key stratal surfaces
that subdivide the formation into genetically related packages, and discuss
structural controls on sedimentation and stratigraphic evolution. Thus, we
address controls on the distribution, geometry, facies stacking pattern, and
character of key stratal surfaces of tidal deposits in a rift basin. The ex-
ceptional exposure of un-inverted rift basin structures and wave-dominated,
tidally influenced stratigraphy found in the Hammam Faraun fault block,
Suez Rift, allows: (1) walking out of facies transitions in both depositional
dip and strike orientations, (2) characteristics of key strata surfaces to be
documented, and (3) structural influence on deposition and stratigraphic
evolution to be defined. Development of the sedimentological and tectono-
stratigraphic models for the Nukhul Formation provides insights into the
controls on sequence development during early stages of rifting (so-called
rift initiation), and also leads to better prediction of sandbody location,
geometry and heterogeneity in early syn-rift plays.
GEOLOGICAL SETTING
The Suez Rift is the northern extension of the Red Sea Rift, which
developed during separation of the African plate from the Arabian plate in
the Late Oligocene. The rift trends NW–SE and is up to 300 km long and
80 km wide. Normal faults strike parallel to the elongation of the modern
gulf and are linked by shorter faults, resulting in a classic extensional zig-
zag fault pattern in plan view. In cross section, the rift is characterized by
large tilted fault blocks 10 to 20 km wide, the dips of which subdivide the
rift into three asymmetric dip provinces along its length (Moustafa 1976;
Colleta et al. 1988; Patton et al. 1994). This study concentrates on the
Hammam Faraun fault block of the central dip province, which is exposed
in western Sinai (Fig. 1A).
In the Hammam Faraun fault block, the latest pre-rift deposits are dom-
inantly carbonates (e.g., Garfunkel and Bartov 1977) and these are uncon-
formably overlain by earliest syn-rift continental deposits of the Abu Zen-
ima Formation. The Abu Zenima Formation (23.5–21.1 Ma) ranges from
100 m thick (Fig. 1B) to locally absent, and lies with an erosional contact
above pre-rift lithologies and infills early-formed depocenters in the hang-
ing-wall of short, 1–5 km long faults (Figs. 1A, 2) (Sharp et al. 2000b;
Jackson et al. 2002). The overlying Nukhul Formation (21.1–19.7 Ma), the
subject of this paper (Figs. 1B, 2), was also deposited when the Hammam
Faraun block was characterized by a number of short, intra-fault block
faults with relatively small (
,
1 km) displacements (Sharp et al. 2000b).
The top of the Nukhul Formation is characterized by deposits displaying
evidence of a prominent increase in water depth associated with a change
to basinal mudstones of the Lower Rudeis Formation (Fig. 2).
The distribution, geometry, and lapout relationships of the Nukhul For-
mation are closely related to the structural geometry of fault-controlled
depocenters, and have been interpreted to reflect the growth and linkage of
normal faults and associated folds (Gawthorpe et al. 1997; Gawthorpe et
al. 2000; Gawthorpe and Leeder 2000; Sharp et al. 2000a; Sharp et al.
2000b, Jackson et al. 2002). However, in addition to local fault control on
408 I.D. CARR ET AL.
F
IG
. 1.—A) Location map indicating studied
exposures of the Nukhul Formation in the
Hammam Faraun fault block, Western Sinai,
Gulf of Suez, and locations of other figures in
this paper. Paleocurrents (insets) from the Thal
and Tanka areas show NW–SE oriented tidal
cross-beds and NNE oriented tidal inlets.
Paleocurrents from the Nukhul area show
WNW–ESE directed tidal cross-beds and SSE
directed inlet currents. B) Typical exposure of
the Nukhul Formation at the top of the cliff,
with the continental Abu Zenima Formation at
the base of the cliff and, along the west face of
the Nukhul syncline, 4 km long and 120 m high.
409SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF THE NUKHUL FORMATION, EGYPT
F
IG
. 2.—Idealized chronostratigraphic diagram
illustrating the development of formations across
the Hammam Faraun fault block. The Nukhul
Formation was deposited in the hanging-wall
depocenters of early-formed fault segments.
Fault movement was later concentrated on major
block-bounding faults.
the development of the Nukhul Formation, there are data supporting the
role of regional base-level change. In particular, the prominent transgres-
sive surface and condensed section at the top of the Nukhul Formation
(T10 of Krebs et al. 1997; see our Fig. 2) are developed throughout the
Suez Rift (Krebs et al. 1997). Although this basin-wide change is regionally
extensive, several workers have interpreted it to reflect the tectonic evo-
lution of the rift (e.g., Patton et al. 1994; Gupta et al. 1999; Sharp et al.
2000b).
METHODS
Sedimentological analysis of the Nukhul Formation is based on: sections
logged at 1:25 to 1:50, spaced at intervals between tens to a few hundred
meters; mapping horizontal facies variations; and field-based interpretation
of large-format photo panoramas. The data presented in this paper come
from three main exposures of the Nukhul Formation in the Hammam Far-
aun fault block: the areas around Wadi Thal, Wadi Tanka, and Wadi Nu-
khul (Fig. 1A). The exceptional quality of the exposure enables key stratal
surfaces associated with abrupt facies shifts and facies belts within genet-
ically related stratal packages to be ‘‘walked out,’’ or traced on photo
panoramas where exposures were too steep, over distances of up to 10 km.
This paper develops a generic model for estuary deposits in rift settings,
applicable to all three outcrop study areas. Landward to seaward trends
were identified by walking up depositional dip from shallow marine de-
posits to continental deposits. The estuary depositional system in each sub-
basin was broadly parallel to the fault orientation, in a NW–SE trend.
However, the directions of landward to seaward trends vary within and
between fault segments, as sediment entered the depocenters at one or both
fault tips. In this paper, landward to seaward orientations rather than actual
orientations, are given for the estuaries.
The chronostratigraphic framework (Fig. 2) linking the individual areas
is based on biostratigraphic graphic correlation (Krebs et al. 1997) and
magnetostratigraphy (Bentham et al. 1996). The structural framework for
this study is based on our detailed structural studies (e.g., Gawthorpe et al.
1997; Sharp et al. 2000a; Sharp et al. 2000b; Jackson et al. 2002) and those
of Adel Moustafa (e.g. Moustafa 1993, 1996; Moustafa and Abdeen 1992).
SEDIMENTARY FACIES
This study suggests that the Nukhul Formation was deposited in two
broadly contemporaneous environments: (1) an open-shelf offshore to shor-
eface environment (Facies Association A), and (2) a structurally controlled
estuary. The estuary deposits can be subdivided into estuary-mouth deposits
(Facies Association B), estuary-funnel with bayhead-delta deposits (Facies
Association C) and upper-estuary channel (Facies Association D) deposits.
The facies and facies associations are shown schematically in Figure 3, and
their key characteristics are summarized in Table 1. The facies associations
are described in a seaward to landward order within the estuary.
Facies Association A: Offshore to Shoreface
Units of Facies Association A are up to 30 m thick and can be traced
for over 5 km along both depositional dip and strike. In a depositional dip
orientation, seaward deposits of Facies Association A consist of variably
bioturbated silty mudstones with prominent ferroan carbonate concretions
(Facies A1), and pass landward into bioturbated, fine-grained, bioclastic
sandstones (Facies A2) (Fig. 3, 4A).
Variably Bioturbated Mudstones (A1).—These mudstones form reces-
sive-weathering intervals up to 20 m thick (typically 2–5 m thick), with
ferroan carbonate concretions in the middle of vertical sections and sub-
ordinate decimeter-thick coarsening-upward units towards the top. The
mudstones contain benthic and planktonic foraminifera, and sponge spic-
ules as bioclasts. Bioturbation is highly variable (bioturbation index ranging
between 1 and 6, sensu Taylor and Goldring 1993) and is commonly in-
410 I.D. CARR ET AL.
F
IG
. 3.—A) Genetic facies model for the Nukhul Formation, based on outcrops from the Hammam Faraun block. Offshore to shoreface (Facies Association A) deposits
trace laterally to estuary deposits (Facies Associations B–D). B) Schematic longitudinal cross section, based on outcrops from the Hammam Faraun block, illustrating
autocyclically generated surfaces. Gradational transitions occur from variably bioturbated mudstones (A1) to bioturbated, bioclastic sandstones (A2) and green/red laminated
mudstones (C1) to fine- to medium-grained, cross-bedded sandstones (C3). Sharp transitions occur from bioturbated, bioclastic sandstones (A2) to coarse-grained, trough
cross-bedded sandstones (B1); coarse-grained, low-angle planar cross-bedded sandstones (B2) to green/red laminated mudstones (C1); and green/red laminated mudstones
(C1) to low-angle-laminated sandstones and mudstones (D1). MHWM
5
mean high water mark, MLWM
5
mean low water mark.
distinct, although Planolites and Chondrites can locally be identified. The
decimeter-thick coarsening-upward units grade from mudstone at the base
into fine bioclastic sandstone at the top. Thalassinoides, Ophiomorpha,
Planolites, and Teichichnus are common in the uppermost 50 cm of these
units.
Bioturbated, Bioclastic Sandstones (A2).—This facies consists of 1–
10 m thick, bioturbated, bioclast-rich, fine- to medium-grained silty sand-
stones. They have sharp or gradational contacts with underlying variably
bioturbated mudstones (A1) and typically have sharp contacts with over-
lying facies (Fig. 4A). Very coarse sand–size grains, similar in mineralogy
and grain size to overlying coarse-grained, trough cross-bedded sandstones
(Facies B1), are found evenly distributed towards the top of the facies.
Facies A2 sandstones are generally structureless, although rare beds 10–
20 cm thick containing poorly preserved parallel lamination, ripple cross-
lamination, and hummocky cross-stratification are observed. The sand-
stones have a bioturbation index of 4–6 with Teichichnus, Planolites, and
Chondrites that are locally overprinted by Thalassinoides and subordinate
Ophiomorpha. Bioclasts include abundant benthic and planktonic forami-
nifera, as well as oyster, pectin, and indeterminate bivalve, and echinoderm
fragments.
Interpretation of Facies Association A.—The mudstone-dominated na-
ture of Facies A1 suggests deposition mainly from suspension, with the
presence of marine microfauna suggesting a marine environment. The var-
iable amount of bioturbation, with horizontal burrows dominant, is consis-
tent with a low-energy marine setting, generally below fair-weather and
storm wavebase (e.g., McCave 1984). As such, the mudstones are inter-
preted to have been deposited in an offshore environment (Fig 3A).
The gradational vertical transition from offshore mudstones (A1) to bio-
turbated, bioclastic sandstones (A2) reflects shallowing water depths and
increasing wave energy. This, together with the marine fauna, ichnofabrics,
and rare sedimentary structures, suggests that Facies A2 sandstones were
deposited above storm wavebase, in a shoreface environment (Howard and
411SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF THE NUKHUL FORMATION, EGYPT
T
ABLE
1.—Facies and facies associations of the Nukhul Formation.
Facies
Unit Thickness and
Lateral Extent Grain Size and Structures Fossils Bioturbation Depositional Processes
Facies Association A: Offshore to Shoreface
Variably bioturbated mud-
stones (A1)
Thickness: 2–20 m
Extent: 4 km
Silty mudstone with prominent Fe
concretions, rare laminae, and
decimeter-thick coarsening-up
units
Foraminifera, sponge spicules BI
5
1–6 Planolites, Chondrites,
Thalassinoides, Ophiomorpha,
Teichichnus
Deposition from suspension
Bioturbated, bioclastic sand-
stones (A2)
Thickness: 1–10 m
Extent: 4 km
Fine, fine-medium sandstone, struc-
tureless with subordinate parallel
lamination, cross laminae, and
hummocky cross-stratification
Oyster beds, pectins, bivalves, fora-
minifera, echinoids
BI
5
4–6 Thalassinoides, Ophiom-
orpha, Teichichnus, Planolites
Deposition during rare storm events
and subsequent bioturbation
Facies Association B: Estuary Mouth
Coarse-grained, trough
cross-bedded sandstones
(B1)
Thickness:
,
3m
Extent: 6 km
Pebbly, medium-coarse sandstone,
trough cross-beds, scours, wave
ripples, mudstone drapes, rip-up
clasts
Oyster fragments and abundant frac-
tured thick- and thin-shelled bi-
valves
BI
5
1 Wave and tidal currents passing
through a channel
Coarse-grained, low-angle
planar cross-bedded sand-
stones (B2)
Thickness:
,
1.5 m
Extent:
.
3km
Low-angle, planar cross-beds,
small-scale trough cross-beds on
larger bedforms
Rare fractured oyster debris BI
5
0–2 Flood tidal currents passing into es-
tuary funnel
Facies Association C: Estuary Funnel with Bayhead Delta
Green/red laminated mud-
stones (C1)
Thickness: 2–5 m
Extent: 4 km
Mudstones, laminated BI
5
1–6 Planolites, Skolithos Deposited from suspension during
tidal stillstands
Interbedded mudstones and
rippled sandstones (C2)
Thickness:
,
17 M
Extent: 4 km
Fine-medium sandstone and mud-
stone, cross-laminae, wave rip-
ples, rare planar laminae
BI
5
1–6 Sands—wave reworked current rip-
ples. Muds—deposited from sus-
pension during tidal still-stands
Fine- to medium-grained,
cross-bedded sandstones
(C3)
Thickness:
,
4.5 m
Extent: 2 km
Fine-medium sandstone; 1 m scale,
silt-draped compound cross-bed
sets; oppositely oriented cross-
laminae and cross-beds; and pla-
nar, low-angle laminae
Rare fractured bivalve debris BI
5
1 Flood-dominated tidal currents, with
subordinate ebb tidal currents;
mudstones deposited from sus-
pension during tidal stillstands
Interbedded fine- to medium-
grained sandstones and
mudstones (C4)
Thickness:
.
3m
Extent: 2 km
Fine-medium sandstone and mud-
stone; mud-draped current rip-
ples, planar laminae: microdeltas;
mud-chip breccia
Oysters BI
5
1–6 Rippled sandstones deposited during
flood and ebb tides; mudstone
deposited from suspension at
high-tide stillstand
Facies Association D: Upper Estuary Channel
Low-angle-laminated sand-
stones and mudstones
(D1)
Thickness: 2–3 m
Extent: ?
Fine sandstone with pebbles, heter-
olithic inclined strata; cross-lami-
nae; parallel laminae
Oysters BI
5
1 Lateral accretion
Conglomerates and thin
sandstones (D2)
Thickness: 2 m
Extent: 1 km
Matrix-supported conglomerates
(clasts
.
75 cm) and sandstones,
fining-up, with low-angle planar
cross-beds and trough cross-bed-
ding
BI
5
0 Rooted horizons Fluvial processes
Reineck 1981), with the thorough bioturbation most likely reflecting a low
intensity and frequency of storms. The ichnofabrics and trace fossils present
are reminiscent of the highly bioturbated shoreface sandstones such as the
Fulmar and Ula formations in the Jurassic North Sea (e.g. Taylor and Gaw-
thorpe 1993; Gowland 1996). The coarse-grained clasts, found toward the
top of the deposits, are interpreted to be derived from the overlying coarse-
grained, trough cross-bedded sandstones and redistributed down into the
shoreface units by bioturbation. It is interpreted that there is a genetic
relationship between the bioturbated, bioclastic shoreface sandstones and
the overlying coarse-grained, trough cross-bedded sandstones, because of
the common occurrence of this relationship. The shoreface is thus inter-
preted to be part of a barrier through which the coarse-grained, trough
cross-bedded sandstones eroded.
Facies Association B: Estuary Mouth
This facies association has two facies: a coarse-grained, trough cross-bed-
ded sandstone facies (B1) and a more landward coarse-grained, low-angle
planar cross-bedded sandstone facies (B2). Facies B1 erosionally overlies
Facies Association A (offshore to shoreface) or Facies Association C (estuary
funnel) and is generally abruptly overlain by Facies Association C.
Coarse-Grained, Trough Cross-Bedded Sandstones (B1).—Facies B1
consists of erosionally based, pebbly, medium- to coarse-grained, bioclastic
sandstones with abundant oyster debris (Fig. 4A). Units of Facies B1 are
up to 3 m thick, can be traced in a depositional dip orientation for 6 km,
and erosionally overlie bioturbated, bioclastic sandstones (A2) or Facies
Association C. The basal contact of Facies B1 is typically a pebble-lined
erosional surface that has up to 2–3 m of relief. Internally, the sandstones
are divided into erosionally based, 1–2 m high sets of trough cross-bedding
(Fig. 4C). Set boundaries may be mantled by quartz pebbles or mudstone
rip-up clasts, and draped by oppositely directed current rippled and wave
rippled finer sediment.
Coarse-Grained, Low-Angle Planar Cross-Bedded Sandstones
(B2).—Facies B2 consists of sheet-like, coarse-grained sandstones contain-
ing low-angle, planar cross-sets with rare oyster debris. Units are up to 1.5
m thick and can be traced over distances of
.
3 km. In a seaward direction
they are eroded by Facies B1 sandstones; in a landward-orientation, they
pass into Facies Association C. Paleocurrents indicate a dominant landward
oriented flow, with the low-angle cross-bedding prograding landward over
Facies C1. Facies B2 also contains subordinate 10 cm scale oppositely
directed trough cross-beds superimposed on the larger low-angle, planar
cross-beds.
Interpretation of Facies Association B.—This facies association, being
both coarse-grained and containing wave- and tide-generated sedimentary
structures, is interpreted to have formed in a relatively high-energy estua-
rine environment. Facies B1 sandstones are interpreted as tidal-inlet de-
posits (e.g., Hoyt and Henry 1967) on the basis of their channelized form,
draped trough cross-bedding, and oppositely directed current ripples re-
flecting the tidal currents passing through the channel (Fig. 3). Evidence
for wave activity suggests a relatively seaward location within the estuary.
The tidal-inlet interpretation is also based on the close association of Facies
412 I.D. CARR ET AL.
413SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF THE NUKHUL FORMATION, EGYPT
←
F
IG
. 4.—A) Representative stratigraphic section through the Nukhul Formation from the Thal area (see Fig. 1 for location). Facies association and key surfaces are
indicated, TS is transgressive surface, TRS is tidal ravinement surface, MFS is maximum flooding surface, TST is transgressive systems tract, and HST is highstand systems
tract. B) Current ripples in Facies C4 cut by falling-stage drainage features producing microdeltas. C) Coarse-grained trough cross-bedded sandstone (Facies B1) of the
estuary-mouth facies association from Thal area. D) Fine–medium grained cross-bedded sandstones with mudstone drapes (Facies C3) of the estuary-funnel facies association
in the Thal area.
B1 with shoreface sandstones (Facies A2) and estuary mudstones (Facies
C1). The trough cross-bedding was formed by the migration of sinuous-
crested dunes within inlets; the continuity of the sandstone units in a de-
positional dip orientation reflects the landward migration of tidal inlets due
to their overall transgressive nature (e.g., Dalrymple et al. 1992).
Low-angle, planar-laminated cross-bedded sandstones of Facies B2 dis-
play landward-directed paleocurrents, and interfinger with more landward
facies, for example estuary mudstone (Facies C1), indicating the influence
of flood-directed currents. Facies B2 is interpreted as being deposited in a
flood tidal delta (Fig. 3), with the estuary mouth passing landward into the
estuary funnel via a flood tidal delta (e.g. Dalrymple et al. 1992). The
oppositely directed trough cross-beds are interpreted to have been formed
by the migration of subordinate ebb-tide-directed sinuous crested dunes
over the flood tidal delta.
Facies Association C: Estuary Funnel with Bayhead Delta
Facies Association C consists of four facies: (1) green/red laminated
mudstones (C1), (2) interbedded mudstones and rippled sandstones (C2),
(3) fine- to medium-grained cross-bedded sandstone (C3), and (4) inter-
bedded fine- to medium-grained sandstones and mudstones (C4). There is
a clear seaward to landward order from Facies C1, through C2 and C3, to
C4. The facies typically show gradual landward transitions and may also
interfinger in a seaward direction with estuary-mouth deposits (Facies As-
sociation B). In vertical section, units of Facies Association C generally
have sharp upper and lower contacts with Facies Association B (estuary
mouth) but may be abruptly overlain by upper-estuary channel sandstones
(Facies Association D).
Green/Red Laminated Mudstones (C1).—Facies C1 consists of 2–5 m
thick green to red colored, laminated mudstones that are sheet-like, ex-
tending for distances of 4 km. Units of Facies C1 may have rare Planolites
and Skolithos burrows.
Interbedded Mudstones and Rippled Sandstones (C2).—This facies
forms units up to 17 m thick that can be traced over distances of 4 km.
The facies consists of fine- to medium-grained sandstones with cross-lam-
ination, wave ripples, and rare planar lamination that are thinly interbedded
with yellow to green mudstones (Fig. 4A). Within units of Facies C2, the
interbedded mudstones may change vertically from yellow to green across
a sharp surface, and this color change is associated with an increase in
bioturbation index from 1 to 3. Facies C2 may be completely homogenized
by bioturbation.
Fine- to Medium-Grained, Cross-Bedded Sandstones (C3).—This fa-
cies consists of fine-to medium-grained sandstones, with oyster fragments
and compound cross-sets that reach up to a meter in thickness (Fig. 4D).
Facies C3 may be up to 4.5 m thick and can be traced for 2 km. Individual
sets can be traced for
.
50 m. Cross-bed sets consist of 10 cm-scale cross-
beds, reactivation surfaces, and oppositely directed small-scale current rip-
ples. The toes of foresets and the oppositely directed ripples are commonly
draped by mudstone or mud-chip breccia, and the mudstone drapes may
contain sandstone-filled desiccation cracks. Towards the top of the com-
pound cross-sets are low-angle planar laminae. On their stoss sides, the
compound cross-sets have erosionally-based, oppositely directed, low-angle
surfaces with 10 cm-scale cross-beds above.
Interbedded Fine- to Medium-Grained Sandstones and Mudstones
(C4).—This facies forms units
.
3 m thick that extend for 2 km. The
sandstones constitute
,
50% of the facies and contain mud-draped, small-
scale cross-lamination, interference ripples, planar lamination, and intra-
clast mudstone breccias. Exposure features are common, such as drainage
microdeltas (Fig. 4B). The mudstones are structureless, are present in beds
,
2 cm thick, and appear to drape topography on the tops of the underlying
sandstones. Primary sedimentary structures are commonly disturbed by
soft-sediment deformation, commonly in the form of pseudonodules and
ball-and-pillow structures. Individual beds may be highly bioturbated (bio-
turbation index up to 5), but with a monospecific trace-fossil assemblage
consisting entirely of bivalve resting traces. Erosionally based, decimeter-
thick, 3–5 m wide lenticular units of coarse-grained, oyster-rich, cross-
bedded sandstone are commonly found within this facies.
Interpretation of Facies Association C.—The fine-grained nature of
the deposits compared to the coarse-grained, trough cross-bedded sand-
stones of Facies B1, the increasing landward tidal influence and relatively
diminishing wave influence suggest an estuary-funnel depositional envi-
ronment (Fig. 3, Dalrymple et al. 1992). The fine-grained nature of Facies
C1 mudstones is interpreted to reflect deposition from suspension in the
estuary funnel. By comparison with modern estuaries, the mudstones are
interpreted to have been deposited during tidal slacks from a zone of highly
concentrated suspended sediment, the ‘‘turbidity maximum’’ (Allen 1991).
Facies C2 is transitional between C1 and C3. The thin sandstones are
interpreted to have been deposited as subtidal, wave-reworked current rip-
ples, in front of and between the fine- to medium-grained, cross-bedded
sandstones (Facies C3). The interbedded mudstones were deposited at times
of slack water, between tides. The bioturbated units are interpreted to rep-
resent times when tidal currents were diverted from these areas, allowing
them to become colonized.
The meter-scale, compound, cross-bedded sandstones of Facies C3 are
interpreted as flood-dominated, subtidal–intertidal dunes (cf. Allen and Ho-
mewood 1984; Ashley 1990). This interpretation is based on recognition
of subordinate ebb-tide and emergence sedimentary structures. Planar lam-
ination toward the top of the tidal dunes is interpreted to have formed
during falling tide, under upper-flow-regime conditions. The oppositely di-
rected erosion surfaces and cross-beds found on the stoss sides of the dunes
are interpreted to have formed by drainage as the crest of the dune became
emergent during falling tide, thus allowing ebb-tide direction to be rec-
ognized. Mud drapes on the dune lee side, above ebb-tide cross-laminae,
probably formed during low tide stillstand and suggest that the dunes did
not become fully emergent during low tide. Belderson et al. (1982) sug-
gested that tidal dune height is about one-third of the water depth, thus the
1 m sets suggests a minimum water depth of 3 m. Evidence for emergence
indicates that water depth decreased to below the crest of the dunes, giving
a fall in water depth of at least 2 m and, hence, suggesting a mesotidal (2–
4 m) range.
Facies C4 is interpreted to represent mixed sand/mud flat or mid tidal
flat deposits (e.g., van Straaten 1954). Rippled sandstones were deposited
during flood and ebb tides, with mudstones deposited during slack water
between tides. The mudcracks and drainage features are evidence for emer-
gence, with mud-chip breccias suggesting that subsequent tidal currents
reworked exposed, desiccated tidal flats. Lenticular, oyster-rich sandstones
are interpreted to be small tidal creeks or channels that cut through mud
and mixed flat deposits. Soft-sediment deformation within this facies has
previously been interpreted to have been produced by earthquake-induced
414 I.D. CARR ET AL.
F
IG
. 5.—A) Correlation panel from the Wadi Thal area illustrating horizontal and vertical facies transitions in the lowermost part of the Nukhul Formation and the characteristics of key stratal surfaces (see Fig. 1 for
location). W, X, Y, and Z refer to specific details discussed in the text. See Figure 4 for key to logs. B) Photograph of Wadi Thal section illustrating relationships between the deposits and how they correspond to
systems tracts.
415SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF THE NUKHUL FORMATION, EGYPT
F
IG
. 6.—Fault-parallel strike correlation from
the northern end of Thal Ridge (see Fig. 1 for
location). Note transgressive onlap and overstep
towards the fault tip (NW), but deposits
reflecting more offshore deposition and
becoming thicker towards the center of the fault
segment (SE). Datum for the correlation is the
T10 major transgressive surface/condensed
section (Fig 4A). Y and Z refer to specific
details discussed in the text.
shaking and dewatering due to the orientation of the dewatering features
being parallel to basin-bounding normal faults (e.g., Sharp et al. 2000b).
Facies Association D: Upper Estuary Channels
Facies Association D consists of low-angle-laminated sandstones and
mudstones (Facies D1) and conglomerates and sandstones (Facies D2). The
facies reflect the diminishing tidal influence in a landward direction, pass-
ing in a depositional dip direction landward into continental deposits.
Low-Angle-Laminated Sandstones and Mudstones (D1).—Facies D1
forms units up to 2–3 m thick (Fig. 4A), composed internally of large-
scale, low-angle planar inclined heterolithic strata that downlap onto a sub-
horizontal erosion surface. A pebbly and shelly lag generally overlies the
basal erosion surface. Mudstone-draped parallel-laminated, cross-bedded
and current-rippled sandstones are oriented at a high angle to the dip of
the inclined surfaces. The sandstones contain vertical burrows and soft-
sediment deformation structures. Facies D1 sits with an erosional contact
above Facies Association C estuary-funnel deposits in a seaward direction;
passes gradationally landward into Facies D2; and passes vertically, via a
sharp contact, to Facies A2 and B1.
Conglomerates and Thin Sandstones (D2).—Facies D2 consists of ero-
sionally based, matrix-supported conglomerates and centimeter-thick sand-
stones, forming fining-upward units up to 2 m thick that can be traced over
horizontal distances of
;
1 km. Conglomerate clasts are subrounded frag-
ments of chert and limestone up to 75 cm in diameter, derived from pre-
rift strata. Both the conglomerates and the sandstones are low-angle planar
and trough cross-bedded, and the sandstones have rootletted horizons with-
in them. Conglomerates rest erosionally above pre-rift lithologies and es-
tuary-funnel deposits, commonly pass gradationally up to estuary-funnel
deposits, and pass horizontally seaward into Facies D1.
Interpretation of Facies Association D.—The low-angle-laminated
sandstones and mudstones (D1) are interpreted as estuary point-bar depos-
its, formed by the lateral migration of a tidal channel (Fig. 3, e.g., Reineck
1967). The mudstone drapes indicate the tidal origin, and the large-scale,
low-angle downlapping surfaces are interpreted as lateral accretion surfac-
es. The smaller-scale cross-bedding, oriented approximately perpendicular
to the lateral accretion surfaces, represents bedform migration in the direc-
tion of flow within the channel. Facies D2 conglomerates and sandstones
are interpreted as fluvial channel deposits and are the most landward facies
documented within the Nukhul Formation during this study.
Nukhul Formation Facies Model
Figure 3 illustrates the genetic facies relationships developed in the Nu-
khul Formation that have been identified by ‘‘walking out’’ lateral facies
transitions in the three fault-bounded areas studied (Fig. 1A). The facies
transitions in these three areas suggest that separate depositional systems
developed in each study area, each with its own sediment source(s), hang-
ing-wall depocenter, and connection with the open sea.
The most seaward facies association, A (offshore to shoreface), com-
prises variably bioturbated mudstones (A1) (e.g., W in Fig. 5) that pass
gradationally landward into bioturbated, bioclastic sandstones (A2) (e.g.,
X in Fig. 5). The dominant process that operated in the shoreface environ-
ment was fair-weather wave activity with subordinate storm activity. The
sharp contact between bioturbated, bioclastic sandstones (A2) and coarse-
grained, trough cross-bedded sandstones (B1) is interpreted to have formed
due to tidal currents sweeping through the estuary mouth (e.g., Y in Fig.
5). The presence of scattered, coarse-grained clasts near the top of biotur-
bated, bioclastic sandstone (A2) units suggests a genetic relationship be-
tween the offshore to shoreface and estuary-mouth deposits (cf. de Fa´tima
Rossetti 2000). It is thus interpreted that the estuary-mouth deposits rest
with an erosional surface above the bioturbated, bioclastic sandstones.
Flood-tide-delta sandstones (B2) are generally poorly preserved but,
where found, they interfinger in a landward direction with green/red lam-
inated mudstones (C1) of the estuary funnel facies association (C). The
green/red laminated mudstones (C1) interfinger landward with interbedded
mudstones and rippled sandstones (C2), which, in turn, can be traced in a
landward direction into subtidal–intertidal dune sandstones (C3; Z in Fig.
5). Moving laterally within estuary deposits, Facies C1, C2 and C3 may
pass gradationally into interbedded fine- to medium-grained sandstones and
mudstones (C4) interpreted as tidal-flat deposits dissected by minor tidal
channels. Estuary funnel facies association (C) pass landward into upper
estuary channels facies association (D), characterized by low-angle-lami-
nated sandstones and mudstones (D1) interpreted as lateral accretion units,
which, beyond the tidal limit, pass into fluvial channels (D2). Taken to-
gether, the landward to seaward facies associations D and C are interpreted
as a bayhead delta. Sands derived from a fluvial source were reworked by
tidal currents at the mouth of the upper estuary channel.
In summary, the Nukhul Formation is interpreted to have been deposited
in wave-dominated estuaries (cf. Dalrymple et al. 1992). Most wave energy
was dissipated in the shoreface environment and did not pass through the
416 I.D. CARR ET AL.
F
IG
. 7.—Schematic cross sections through the estuary axis illustrating the formation of key surfaces in the Nukhul Formation. A) Relative sea-level rise, but with
accommodation creation outpacing sediment supply. This produces the landward translation of facies and deposition of the TST. B) Maximum transgression, with the
estuary-mouth deposits at their most landward position. C) Sediment supply is greater than accommodation creation as the rate of sea-level rise decreases. The bayhead
delta progrades forming the HST.
estuary mouth, thus increasing the relative tidal influence within the estuary
itself. Flood tidal deltas built landward into the estuary funnel, and the
deltas interfingered with estuary-funnel mudstones. Clastic sediment en-
tered the confined hanging-wall depocenters from fluvial systems at the
fault tips, were reworked by the tides with a mesotidal range, and were
aligned parallel to the local structures. The relative importance of local
structural versus more regional controls on the evolution of the Nukhul
Formation are discussed later in this paper.
SEQUENCE STRATIGRAPHY
Key Stratal Surfaces and Stacking Patterns
The offshore to shoreface and estuarine Nukhul Formation overlies the
continental Abu Zenima Formation and is, in turn, overlain by the deep
marine to shoreface Lower Rudeis Formation, in an overall transgressive
succession. Internally, the Nukhul Formation consists of repetitions of ge-
netically related transgressive systems tracts (TST) and infilling highstand
systems tracts (HST) estuarine units, bounded by surfaces that can be
walked out within individual hanging-wall depocenters (i.e., up to c. 15
km). Although the TST–HST succession (e.g. TST 1–HST 1 in Fig. 5)
appears similar to shallow marine high-frequency systems tracts, no low-
stand or forced regressive systems tracts have been identified. This section
describes one of these repeating TST–HST successions from a represen-
tative measured section from the Wadi Thal area (Fig. 5). The key char-
acteristics of systems tracts and key stratal surfaces that bound the systems
tract and correlatable surfaces within systems tracts are described, and pro-
cesses that formed them interpreted.
Transgressive Systems Tracts.—The basal part of the repetitive TST–
HST cycle is well illustrated by TST 2 from Wadi Thal (Figs. 4A, 5),
which is bounded beneath by a transgressive surface that separates the
previous highstand shoreface and fluvial deposits beneath from the trans-
gressive central estuary deposits above. The central estuary deposits of TST
2 show a landward-directed transition from estuary-funnel mudstones (C1)
in the NW to backstepping tidal-flat deposits (C4) in the SE (Fig. 5). Near
the top of TST 2, estuary-funnel mudstones are overlain by shoreface sand-
stones in the NW of Wadi Thal (Fig. 5). However, shoreface sandstones
are more commonly overlain above an erosional surface by estuary-mouth
sandstones (B1). The erosional surface beneath the estuary-mouth sand-
stones is sharp and generally planar, but with erosional scours up to 2 m
deep and 5 m wide. The surface can be correlated for up to 10 km before
passing landward into flood tidal delta (B2) and estuary-funnel mudstones
(Z in Fig. 6). Estuary-funnel mudstones beneath the correlatable surface
show a change from rare vertical to abundant horizontal burrows towards
the surface. Vertical burrows descend from the surface, mottling the un-
derlying mudstone and are filled with coarse-grained material from above.
The correlatable surface is interpreted to have been formed by the landward
migration of the estuary mouth during transgression. It is thus a tidal rav-
inement surface (TRS; Allen and Posamentier 1993), which can be traced
from the point where the tidal inlets began to transgress, to the inlets max-
imum landward position. Increased bioturbation toward the surface is in-
417SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF THE NUKHUL FORMATION, EGYPT
F
IG
. 8.—A) Tidal ravinement surface from the Thal area; see Figure 1 for location. Green/red laminated mudstone (C1) at the base changes vertically across a sharp
surface to coarse-grained, trough cross-bedded sandstone (B1), producing an atypical flooding surface character. Lens cap for scale. B) Maximum flooding surface from
Wadi Nukhul, see Figure 1 for location. Photograph shows coarse-grained, trough cross-bedded sandstones (B1) below a sharp surface and green/red-laminated mudstones
(C1). A notebook, 30 cm high, for scale.
F
IG
. 9.—Four sub-basins of the Hammam Farraun fault block. From north to south, Wadi Wasit, Thal Ridge, Wadi Tanka, and Wadi Nukhul, have different sediment
thicknesses, reflecting unique fault movement histories during early syn-rift times. See Figure 1 for locations.
terpreted to indicate an increasing marine influence during the initial stage
of transgression. Although the TRS is a key correlatable surface, it does
not always bound the TST.
TST 2 is bounded above by a maximum flooding surface (MFS), which
is recognized by the change from backstepping transgressive deposits to
progradational infilling deposits, and is typically marked by a shift from
inlet sandstones (B1) beneath the surface to estuary-funnel mudstones (C1)
above (e.g., Y in Fig. 6, Figs. 7C, 8B). Maximum flooding surfaces can
be correlated for
.
10 km above the estuary-mouth sandstones, are gen-
erally planar and non-erosional. However, erosional topography generated
by inlet scour is locally preserved beneath the surface, with estuary mud-
stones filling this topography. Close to sediment input points, the MFS may
be cut by an erosional surface beneath fluvial deposits that built out over
the underlying estuary deposits during HST progradation. The surface be-
neath the fluvial deposits may erode into the previous TST (Figs. 5A, B,
7C).
Highstand Systems Tracts.—Above the MFS, coarsening-upward pro-
gradational facies relationships are recognized in the subsequent HST, e.g.,
HST 2 (Figs. 4A, 5A). Central estuary mudstones at the base of HST 2
pass laterally and vertically into intertidal–subtidal (C3) sandwaves and
lower estuary ripple fields (C2) as the bayhead delta progrades (Figs. 4A,
5A, B). Erosionally based estuary channel sandstones may prograde into
the central estuary area, eroding through the previously deposited central
estuary mudstones, and possibly through the MFS.
418 I.D. CARR ET AL.
F
IG
. 10.—Fault control on Nukhul deposits. A)
View looking north at the northern end of the
Thal Ridge showing convergence of green/red
laminated mudstones (C1) towards the fault
zone. These steeply dipping beds of the Nukhul
Formation formed part of a growth monocline
during Nukhul deposition that is now preserved
in the immediate hanging-wall of the Thal Ridge
fault. See Figure 1 for location. B) Dip-
orientated cross section from the Nukhul area
illustrating the characteristic stratigraphic
geometries in the hanging-walls of normal faults.
The earliest syn-rift Abu Zenima Formation thins
towards the Nukhul Fault, whereas the younger
Nukhul Formation expands towards the fault
zone and onlaps older stratigraphy. These two
geometries reflect surface growth above a blind
fault, and surface-breaking fault stages,
respectively. See Figure 1 for location.
The Nukhul Formation is thus characterized by a repetitive cyclicity of
TST–HST, with TSTs typically being bounded beneath by a TS or TRS
and above by MFS and characterized, in relatively seaward localities, by
tidal-inlet deposits and, landward of the inlets, by backstepping central
estuary deposits (Fig. 7B). Highstand systems tracts are bounded beneath
by MFS, above by a TS or TRS, and are characterized by prograding
bayhead delta deposits. No lowstand systems tracts or major downshift
surfaces have been recognized within the succession. This distinctive TST–
HST cyclicity is interpreted to be an alternation between times of large
ratio of accommodation space to sediment supply (TST) and large ratio of
sediment supply to accommodation space (HST). The role of structural
evolution in controlling cyclicity in the Nukhul Formation is discussed in
the following section.
STRUCTURAL CONTROL ON THE NUKHUL FORMATION
There is a variety of evidence that points to a structural control on the
deposition of the Nukhul Formation. Structural control is demonstrated by
thickness variations between sub-basins, both parallel and perpendicular to
the fault trends within sub-basins. Changes in stratal geometries through
the succession show how the structural influence evolved through time,
with an initial wedge-shaped stratal unit that thins towards the fault fol-
lowed by a wedge-shape stratal unit that thickens into the fault. Onlap
relationships towards fault tips point towards active fault growth during
deposition, and the alignment of paleocurrent parallel to local structures
indicates that the fault confined the depositional systems.
The Nukhul Formation shows major variations in thickness between dif-
ferent fault zones (Fig. 9). For example, in the Nukhul area, this stratal
unit is up to 110 m thick. The same interval in the hanging-wall of the
Tanka fault is
,
40 m thick, and, in the hanging-wall of the Wasit fault
in the north of the study area, the unit is only 30 m thick (Fig. 9). In the
adjacent Baba fault block the Nukhul Formation is locally absent (Sharp
et al. 2000a; Sharp et al. 2000b). These data suggest that the development
of normal fault zones was a primary control on the Nukhul Formation
stratigraphy and the accommodation development was strongly influenced
by fault-controlled subsidence that varied between the fault zones.
Within the hanging-wall of individual fault zones, two distinctive stratal
units can often be observed: a wedge-shaped stratal unit that thins toward
the fault zones, and a wedge-shaped unit that expands toward the fault
zones (Fig. 10 A). In effect these wedge-shaped stratal units are sequence
sets (sensu Mitchum and Van Wagoner 1991), inasmuch as each is com-
posed of several TST–HST cycles. In units that thin toward the fault zones,
bedding dips into the hanging-wall depocenter, away from the fault, and
individual beds converge and are often eroded below intraformational trun-
cation surfaces at the base of younger horizons near to the fault zone (Fig.
10A). Locally, the Nukhul Formation may be extremely thin, or absent, in
the immediate hanging-wall of the faults (cf. Gawthorpe et al. 2000). As
discussed by Gawthorpe et al. (1997) and Gawthorpe et al. (2000), this
wedge-shaped form and stratal geometry are interpreted to reflect deposi-
tion and erosion around growth folds above blind normal faults. In the
second type of stratal unit, strata thicken, diverge, and dip towards the fault
zone (Fig. 10B). This stratal geometry is typical of deposition in the im-
mediate hanging-wall of surface-breaking normal faults, where the rate of
subsidence is greatest in the immediate hanging-wall of the fault (e.g.,
Leeder and Gawthorpe 1987). Along-strike variation in fault displacement
also played an important role in controlling basin morphology. The north-
ern end of the Thal Ridge area, illustrated in Fig. 6, illustrates the along-
419SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF THE NUKHUL FORMATION, EGYPT
F
IG
. 11.—Summary tectono-sedimentary
model for the Nukhul Formation. Deposition
occurred in the hanging-walls of numerous,
small displacement, normal faults, creating
elongate, narrow depocenters. Depocenter (A) is
bounded by the steeply dipping hanging-wall
limb of a fault-propagation monoclinal growth
fold. Depocenter (B) is bounded by a surface-
breaking fault zone. Note how the structural
style influences the geometry and stratal
architecture of the syn-rift deposits. Upper
estuary channel (FAD) and estuary funnel (FAC)
sandstones and mudstones prograde from a
sediment entry point at the fault tip. Tidally
influenced estuary-funnel (FAC) and estuary-
mouth (FAB) sandstones are aligned parallel to
the growth folds and faults. Offshore to
shoreface sandstones (FAA) and mudstones are
aligned parallel to the coast, in an orientation
that may be normal to the orientation of the tidal
sandbodies.
strike (fault-parallel) facies relationships and thickness variations that are
typical of the early syn-rift deposits within the Hammam Faraun area. To-
wards the fault tip, the succession onlaps and thins from 20 m to 6 m over
3 km (e.g., Sharp et al. 2000b). These stratal and facies relationships sug-
gest marked along-strike variations in depositional environments and ac-
commodation development related to decreasing displacement rates to-
wards the tips of fault segments.
Taken together, the distribution of the Nukhul Formation, its thickness
variations and its internal stratal geometry suggest a structural style dom-
inated by depocenters that were typically 4–10 km long, 2–4 km wide and
controlled by a number of isolated normal fault segments and growth folds
(Fig. 11), and were unconnected during the time of Nukhul estuary depo-
sition (Fig. 9). This structural geometry had a strong control on Nukhul
facies geometry and evolution (Fig. 11). In the North Wadi Thal example
(Fig. 6), facies become more proximal towards the fault tip, which are
sediment input points. Lateral transitions from fluvial to tidal to open ma-
rine facies are developed parallel to the fault away from the lateral fault
tip, consistent with an increasing marine influence linked to an increase in
displacement towards the center of the fault segment. Similar stratigraphic
relationships are found parallel to the other fault zones within the Hammam
Faraun fault block, such as in the Tanka and Nukhul areas (e.g., Jackson
et al. 2002).
Paleocurrents (Fig. 1A) from Thal and Tanka, measured from bayhead
delta and flood-tidal delta cross-beds respectively, show a NW–SE orien-
tation for the bayhead delta, and a NNE orientation of tidal inlets. Paleo-
currents from the Nukhul area show WNW–ESE directed tidal cross-beds
and SSE directed inlet currents. The tidal cross-beds are aligned to local
structure that confined the bayhead delta. These relationships indicate that
sediment transport pathways were strongly influenced by the evolving fault-
controlled topography. Sediment, from a fluvial source originating in the
rift shoulders, entered the hanging-wall depocenters predominantly at fault
tips. Sediment derived from the evolving growth folds and footwalls was
only a minor contributor during this early rift stage because of limited
topographic relief of footwall blocks (Fig. 11). Paleocurrents indicate that,
within the depocenters, flow was predominantly parallel to the adjacent
faults (Fig. 1A). The elongate fault-controlled geometry of the depocenters
confined the bayhead delta, and the tidal sandbodies became aligned par-
allel to the fault. The tidal-inlet sandstones (Facies Association B) sat at
the relatively narrow mouth of the estuary, where the elongate depocenter
passed out into a more open marine basin. The inlet facies belt was pref-
erentially located around structural highs, so-called transverse hanging-wall
anticlines (Schlische 1995), formed by low displacement rates at fault tips
(Fig. 11).
The Nukhul Formation displays a characteristic repeated stacking of
TSTs and HSTs (Figs. 4A, 5A, 6, 10A), and lacks incised valleys and
incision surfaces related to relative falls in sea level. This repetition is
attributed to subsidence of hanging-wall depocenters, which caused accom-
modation creation to outpace sediment supply, and hence the retrograda-
tional nature of the TSTs. This is followed by a decrease in the rate of
accommodation creation, and the hanging-wall depocenter was filled as
sediment supply outpaced accommodation creation. Subsequent fault
movement once more increased the ratio of accommodation space to sed-
iment input, and hence another TST was formed (Fig. 7A–C).
Although a wide range of data support the importance of local tectonics
on the development of the Nukhul Formation, data also support more re-
gional base-level controls. In particular, the prominent transgressive surface
and condensed section towards the top of the Nukhul Formation are re-
gionally developed within the Suez Rift (Krebs et al. 1997). Within bio-
stratigraphic resolution, this surface (T10 of Krebs et al. 1997; see our Fig.
2) is contemporaneous across the rift and marks the change to deeper-water
mudstones of the Lower Rudeis Formation. Although regionally extensive,
several workers have interpreted this basin-wide change to reflect the tec-
tonic evolution of the rift (e.g., Patton et al. 1994; Gupta et al. 1999; Sharp
et al. 2000b). In particular, it has been interpreted to reflect the change
from numerous small-displacement faults that were active during Nukhul
deposition, to localized deformation on the major half-graben bounding
faults that dominate the structural style today. However, it is likely that the
faults gradually died out (Sharp 2000b) rather than became inactive all at
once.
CONCLUSIONS
This paper presents facies and tectono-stratigraphic models for the tidally
influenced early synrift sequences of the Miocene Nukhul Formation based
on a detailed outcrop study of the Hammam Faraun fault block, Suez Rift,
Egypt. The Nukhul Formation can be divided into offshore to shoreface
and estuary deposits. The offshore to shoreface deposits consist of two
facies; variably bioturbated mudstones that pass gradationally landward to
420 I.D. CARR ET AL.
bioturbated bioclastic sandstones. In a more landward setting, the estuary
deposits can be separated into a coarse-grained estuary mouth association,
fine-grained mudstones and sandstones of the Estuary Funnel with Bayhead
Delta Association, and interbedded, low-angle, tidally influenced sand-
stones and mudstones of the upper estuary channel association. These es-
tuary deposits pass landward into fluvial and associated continental depos-
its. Estuary transgression and infill generated a complex intercalation of
facies, and facies transitions may be either gradational or sharp.
Stratigraphic development and sandbody geometry was strongly influ-
enced by the evolving early rift structure. Depocenters were narrow (2–5
km wide) and elongate (
,
10 km long) parallel to the strike of normal-
fault segments. The orientation of tidal sandbodies was significantly influ-
enced by the orientation of the early fault segments, with tidally influenced
sandbodies parallel to the fault strike. Stratal geometry also reflects defor-
mation associated with low-relief growth folds and surface-breaking faults
that together formed part of an evolving fault array. This basin configu-
ration and the associated Nukhul stratigraphy are markedly different from
tectono-stratigraphic models for crustal-scale tilted fault blocks that are
more applicable to the late stages of rifting. Although thickness and facies
were strongly influenced by local fault growth, more regional controls on
accommodation development generated key flooding surfaces and influ-
enced the overall sequence stacking pattern within the Nukhul Formation.
These regional controls operated at a spatial scale of individual crustal-
scale fault blocks to the scale of the rift as a whole.
ACKNOWLEDGMENTS
The authors would like to thanks Norsk Hydro, Amerada Hess, BP and NERC
(Grant No. GR3/R9527) for financial support; John Underhill, Sayeed Gooda, Dave
Pivnick are thanked for their encouragement and advice. BP, GUPCO, and EGPC
are thanked for logistical support for field work. The authors would also like to
thank referees for their constructive comments that greatly improved the manuscript.
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Received 15 December 2000; accepted 14 November 2002.
