A new tectono-sedimentary model for Cretaceous mixed nonmarine–marine oil-prone Komombo Rift, South Egypt
The Komombo Basin is a recently discovered mixed nonmarine–marine, petroliferous basin of Cretaceous age in South Egypt. It is an asymmetrical half graben, synchronous with the Neothys opening and filled with up to 4 km of continental to open marine strata ranging from Early to Late Cretaceous. Despite its great relevance, no detailed sedimentological study concerning this basin has been carried out to date. Here, we present an integrated approach to the borehole and core data, as well as unique outcrop sections to construct a new detailed sedimentological interpretation on depositional systems, controls on basin evolution, basin configuration and regional tectonic setting. Seven depositional systems were recognized: (I) a fluvial fan system, (II) a braidplain system, (III) a siliciclastic lacustrine system, (IV) a lacustrine/lagoonal system, (V) a fluvial-estuarine system, (VI) a tidally affected delta, and (VII) an open marine system. The Komombo Basin evolution can be compartmentalized into three main rifting phases: the Berriasian–Early Barremian, Late Barremian, and Aptian–Albian. The first and third rifting phases are comparable with the rifting phases reported for several basins in North and Central Africa. The second rifting phase represents a transitional event between the other two phases. The first three depositional systems consist mainly of continental siliciclastics and are dominant in the Berriasian–Early Barremian and Late Barremian rifting phases. The lacustrine/lagoon and fluvial-estuarine systems correspond to the Aptian–Albian rifting phase, while the Campanian–Maastrichtian open-shelf deposits represents the post-rift stage.
KeywordsCretaceous Nonmarine Rift Komombo Egypt
Introduction and exploration history
The study area is located on the central part of the Komombo Basin (Fig. 1b). It was part of a larger former Repsol concession which was relinquished in 2001 after the drilling of three wells in this area. These wells are located in a NW–SE trending half graben. After Centurion obtained exploration rights on Komombo Block in 2004, additional 2D seismic and 3D seismic acquisition and processing were done. Despite this, the basin represents unique petroleum activity in South Egypt, the geologic evolution, geometry, and petroleum system of the basin remain poorly understood.
The Komombo Basin was studied by Fathy et al. (2010), Wood et al. (2012), and Dolson et al. (2014). The aim of this study is integrated approach which describes and interprets the borehole image logs, dipmeter, wire-line logs and the available cores and combines them to establish depositional environments, paleosediment transport directions within these environments, and the tectono-stratigraphic development of the Cretaceous succession within a palyno-stratigraphically calibrated framework. The results were used as input for basin modeling and development purposes in this basin and related basins in North Africa; however, this aspect is not discussed here.
Data set and methodology
This study is based on results from the description and interpretation of core material from 6 wells, wire-line logs from 24 wells, and borehole images from 10 wells. The wire-line logs, cores, borehole images, and seismic data used in this study are courtesy of SeaDragon-Egypt. The detailed palynological description and biostratigraphic zonation were carried out for the studied wells by Abu El Ella (2006, 2011). The core and the borehole images were acquired from the same wells and are directly correlated. These data sets were first described and interpreted separately and subsequently integrated into one final interpretation. This interpretation led to the subdivision of the studied Cretaceous succession into facies based on: lithology, grain size, internal structures, color, and bioturbation. These facies are grouped into facies associations and depositional systems. Unique surface outcrops were used as analogs to the facies associations and depositional systems described from the subsurface data. These outcrops are located at the area between Faris Village and Kobet El Hawa (south of the study area, Fig. 1a). Paleogeographic maps were built using the combined primary suite of maps and incorporating interpreted depositional systems and facies associations. Each paleogeographic map reflects the distribution of depositional systems and facies at the time of maximum progradation. Maps also show the locations and nature of sedimentary elements, structural features and depocenters, axes of sediment input, and dominant transport. One seismic line from the 3D seismic survey has been used to show the basin architectures and lateral thickness variations of the sedimentary strata.
Lithofacies and depositional environments
The interpretations presented here for the Cretaceous Komombo Basin filling succession are based on detailed sedimentological analysis, detailed stratigraphic correlation, lithofacies, and paleocurrent analyses. The results suggest that the whole succession was deposited in seven depositional systems (Fig. 2b), some of them contemporaneous and some succeeding each other during basin evolution. These are: (I) a fluvial fan system, (II) a braidplain system, (III) a siliciclastic lacustrine system, (IV) a lacustrine/lagoonal system, (V) a fluvial-estuarine system, (VI) a tidally affected delta, and (VII) an open marine system. The fluvial fan system comprises two facies associations: (Ia) stream-dominated fan and (Ib) floodout-dominated fan. The braidplain system can be described in terms of three facies associations: (IIa) distal, (IIb) mid-, and (IIc) proximal braidplain. The lake/lagoonal system, siliciclastic lacustrine, and open marine systems, each one encompasses one facies association. The fluvial-estuarine system includes five facies associations: (Va) mixed-load fluvial channels, (Vb) tidally affected fluvial channels, (Vc) interfluve, (Vd) tidal channels, and (Ve) tidal-flat deposits. The tidally affected delta system includes two main associations: (VIa) a deltaic mouthbar and (VIb) deltaplain deposits.
Description and interpretation of the lithofacies
F1—Massive conglomerates and conglomeratic sandstone
Amalgamated and stacked discontinuous ribbons and continuous sheets, 10–50 cm thick, internally organized as fining-upward pattern with many internal reactivation surfaces (Fig. 6a)
Product of destabilization of upper-flow regime antidunes during sheet flood on alluvial fans
F2—Massive pebbly muddy sandstone to muddy pebbly sandstone
Decimeter to meter thick bodies of pebble to mud grade, poorly sorted, structureless sandstones
Rapid deposition and/or destruction of structure due to bioturbation (c.f. Cain and Mountney 2009)
It is made of decimeter to meter scale thick of medium- to coarse-grained sandstone to pebbly sandstone. It rests on basal scour
Medium- to coarse-grained, sandstone to pebbly sandstone locally conglomerates with plane-parallel and low-angle cross-beds composing apparently tabular meter-scale successions often associated with F13
Migration of straight-crested dunes
F5—Cross-stratified sandstone to pebbly sandstone
Medium- to coarse-grained, small to medium-scale trough cross-stratified sandstones with sparse granules and pebbles disposed in erosive-based 10–200-cm-thick lenses, internally organized as 5- to 25-cm-thick sets
Channelized lower-flow regime deposits formed during recessional stages of sheet-flood events migration of sinuous-crested dunes
Inverse-graded tabular beds of very coarse sandstone,
locally conglomerate, to fine-grained sandstones with a few plane-parallel laminations (Fig. 8c). Beds compose centimeter to meter scale
Rapid dumping from flood-dominated river discharge
Decimeter to meters-scale thick, light gray to brownish gray medium- to fine-grained cross-stratified sandstone with mud drapes along the cosets and reactivation surfaces. It ranges from flaser to lenticular bedding in nature
F8—Muddy sand heterolithics
Repetitive alternations of fine sand and shale laminae The fine sand and silt laminae range from millimeter-thick lenses (‘linsens’ of Reineck and Wunderlich 1968), to thicker rippled beds (less than a centimeter thick) showing planar and trough cross-bedding
Tidal rhythmites (Smith et al. 1991)
F9—Inclined muddy sand (HIS)
Sand layers are composed of very fine sand, varying in thickness from few millimeters to up to 30 cm and show upper-flow regime parallel lamination or current ripple lamination with opposite directions of migration
The IHS is particularly abundant in tide-influenced settings (Smith 1987, 1988; Thomas et al. 1987; Eberth 1996; Dalrymple et al. 2003; Choi et al. 2004) as a result of lateral meander migration. Point bars may develop IHS in the inner bends of meandering channels
F10—Contorted mudstone and sandstone
Mudstone and fine- to medium-grained sandstone beds with convolute folds (Fig. 9a)
Product of slumping
F11—Massive and laminated red mudstones
Decimeter to meter scale thick, of red to reddish brown mudstone that can be either massive or laminated (Fig. 8b). The laminated variety usually presents sparse sand grains and some sandstone interbeds
Deposition from suspension fallout under quite oxidized conditions
F12—Variegated sandy mudstone
Gray to light gray sandy mudstone to muddy sandstone with remains of plant debris
Suspension fallout in quite conditions as channel abandonment or lake
F13—Gray organic-rich mudstones
Decimeter to meter scale thick, of gray to dark gray, organic-rich mudstone that can be either massive or laminated (Fig. 8b). The laminated variety usually presents sparse sand grains and some sandstone interbeds
Deposition from suspension fallout in quite reducing water column
F14—Thinly laminated mudstone and siltstone
Dark gray to gray laminated to bioturbated mudstone and siltstone with some traces as planolites, teichichnus, and skoliths
Suspension fallout from oscillating tidal currents in flooded bays or tidal flats
It is made of thinly laminated shales and siltstone with dominance of marine dinocycts
Suspension fallout in open marine conditions.
F16—Coal to coaly mudstone
Coals and carbonaceous shales are easily identified on the open-hole logs because they show distinctly different log signatures from the sandstones and mudstones
Accumulation and preservation of plant material over a prolonged period in a swamp on a poorly drained floodplain
Fluvial fan system I
Fluvial fan deposits occur in the central part of the Komombo Basin. They comprise two main facies associations: (Ia) stream-dominated FA and (Ib) floodout-dominated FA. The facies association (Ia) dominates the axial depocentral occurrences, while the facies association (Ib) dominates the transverse hanging wall ones. These deposits are characterized by coarse-grained facies, mainly conglomerates and conglomeratic sandstones, and are characteristic of the rift initiation phases.
Stream-dominated mid-fluvial fan FA (Ia)
The common occurrence of scour-based, meter-scale fining-upward cycles of facies F1, F3, F4, and F12 reveals that the main process responsible for the deposition of this facies association was characterized by large variations in discharge. These cycles are interpreted as deposits of high-energy floods, which started with the erosion of previously deposited sediment at peak discharge and generated different sedimentary facies at different stages of the waning flow. The basal, channel-form scours are interpreted as the product of lower-flow-regime currents developed in the deeper portions of the stream. Above the scour fill, deposition was controlled by the migration of three-dimensional dunes (F5) and local gravel bars (F1), both in the lower-flow regime, suggesting that deposition took place inside confined streams. This was followed by waning of the floods and deposition of fine-grained facies (F12). The ephemeral nature of the streams is suggested by the recurrence of these cycles, marked by episodic peak discharge followed by waning flow, with no evidence of long-lasting active channels. This association is interpreted as fluvial channel deposits intercalated with overbank muds or playa muds probably of fluvial fan setting. Similar association was described by Ito et al. (2006) and Almeida et al. (2009).
Floodouts-dominated distal fluvial fan FA (Ib)
The dominance of massive to weakly laminated fabric and poorly sorting fabric of F2 indicates rapid deposition or destruction of structure due to bioturbation (c.f. Cain and Mountney 2009). This facies is attributed to the diffuse flow of unchannellized flood waters across the alluvial plain that probably emanated either from breaches of the main channel bank or from points where fluvial channels terminated on the alluvial plain (cf. Tooth 2000a, b, 2005). This facies is probably interpreted as frontal lobe or floodout deposits fed by the terminal parts of channels or alluvial fan as the flow expanded out onto the floodbasin. The flood deposition was culminated by dominance of floodbasin to lacustrine deposition from suspension fallout as seen from dominance of the muds. The general successive coarsening and thickening upward indicates general progradation of the floodout probably at the distal part of alluvial fan toward the depocenter of the basin with the rotation of the hanging wall (c.f. Leeder and Gawthorpe 1987).
Braidplain depositional system II
The braidplain depositional system comprises three main facies associations: (IIa) distal braidplain, (IIb) mid-braidplain, and (IIc) proximal braidplain. These three facies associations are defined, with contrasting sand/shale ratios and similar sedimentary structures.
Lacustrine depositional system III
Lacustrine/lagoonal system IV
Fluvial-estuarine depositional system (V)
The fluvial-estuarine depositional system is widely distributed in the Komombo Basin through the Upper Cretaceous, and it is penetrated in all the studied wells. It includes a wide spectrum of the mixed-load fluvial channels (Va), tidally affected fluvial channels (Vb), interfluve (Vc), tidal channels (Vd), and tidal-flat (Ve) facies associations.
Mixed-load fluvial channels FA (Va)
Tidally affected fluvial channels FA (Vb)
This facies association consists of fining-up, light gray to yellowish gray, fine- to medium-grained, calcite-cemented, cross-stratified and wavy-bedded, partly massive sandstones (F7, F3, and F5) and some gray mudstone interbeds (F14) at the top (Fig. 11a, b). Slumped or contorted cross-stratified sandstone (F10) is described at the base. Some foresets are delineated by small, flattened coal intraclasts. Stylolites and rare, very thin mudstone drapes occur toward the top (Fig. 11d). The scale of the cross-bed sets decreases toward the upper part, and the bi-directional cross-stratification can be observed (Fig. 11b). The presence of planar cross-stratification indicates migration of unidirectional, subaqueous dunes. Fining-up trends and heterolithic facies reflect gradual decrease in current speed from deeper parts of the channel to the channel bank (Mossop and Flach 1983). Abundance of mud drapes suggests slack-water periods during high- or low-tide stillstands. This together with opposingly directed ripples and herringbone cross-stratification reflects a strong degree of tidal influence, typical of the fluvial-tidal transition (Dalrymple and Choi 2007; Van den Berg et al. 2007). The diagnostic features of this facies are diverse soft-sediment deformation structures (slumps, ball-and-pillow, pseudonodules, and fluid-escape features).
Interfluve areas FA (Vc)
Tidal channels FA (Vd)
The constituents of this association are mainly cross-stratified, fine- to medium-grained and wavy-bedded sandstones with mudstone intercalations (F3, F7, F8, and F14, Fig. 13b–d). Sandstones up to 15.2 m thick are cross-stratified, contain single and double carbonaceous drapes on foresets, and are slightly bioturbated (Fig. 13). Palynoflora from mudstone interbeds in the thick sandstone is dominated by land derived species as Stellatopollis barghoornii and Stellatopollis spp., and rare dinoflagellates (Abu El Ella 2011). One of the most common facies recorded in this association is the inclined heterolithics (F8). It consists of light gray, very fine- to fine-grained sandstones interbedded with dark gray to black siltstones (Fig. 13b). Wavy bedding and flaser bedding (single, double, and bifurcated) are dominant, in addition to lenticular bedding (with connected or single, thick lenses) being very common (Fig. 13b–d).
The fining-upward nature and the presence of the basal scour reflect the channel regime. Planar cross-stratification probably indicates ripple or dune migration across the channel floor. The occurrence of the heterolithics and mud drapes indicates a tidal influence on the depositional environment. Mud drapes record countercurrent flows and slack-water conditions, or weak tidal inundation (Fischbein et al. 2009). Double mud drapes and bundle thickness variations in cross-bed sets evidence the in-shore tidal environment of deposition (Visser 1980; Nio and Yang 1991). The inclined-heterolithic stratification is interpreted as having been produced by point-bar accretion, a dominant structure in upper-intertidal channels (e.g., Reineck 1958; Bridges and Leeder 1976; De Mowbray 1983; Thomas et al. 1987).
Tidal-flat FA (Ve)
Tidally affected deltas system (VI)
Tidally affected deltas are widely distributed in the Komombo Basin in the subsurface and the outcrop sections. It is well recorded at the top of the Coniacian–Santonian Quseir Formation. It includes two main facies associations: prograding mouthbar and deltaplain deposits.
Prograding mouth bar FA (VIa)
Delta plain FA (VIb)
The deltaic channel subassociation is composed of cross-stratified, fine- to medium-grained sandstone with general fining-upward pattern (Fig. 16a). It rests on erosional base (Fig. 16b), and it may erode the underlying mouth bar deposits.
Open marine system (VII)
The seven depositional systems described above represent the preserved Cretaceous sediments in the Komombo Basin. The basin evolution is characterized by successive tectonic rifting. The first phase begins in the Barrasian-Hautervian, the second phase begins during Late Barremian, and the third phase begins in the Aptian time. The first and third rifting phases are correlated with Early Cretaceous rifting phases which were recorded in North and Central Africa (Guiraud and Maurin 1992). The second Late Barremian rifting phase represents a transitional event between the two rifting phases.
The first phase of basin evolution is characterized by fluvial fan, lacustrine, and braidplain depositional systems in successive cycles or a general upward coarsening sequence (A–E members of the Six Hills Formation, Fig. 2b). The second phase, distal floodbasin and paleosols of F member of the Six Hills Formation are well developed and were followed by mid-/proximal braidplain of G member of the Six Hills Formation, generally in an upward coarsening sequence (Fig. 2b). The boundary between the first and second rifting phases is considered as unconformity due to reversing of the sediment dispersal trends (Figs. 8, 10). On the other hand, the boundary between the second and third rifting phases was characterized by marine ingression. A similar upward coarsening succession was described in the Muglad Basin (Central Sudan) but with a wider time span extending from Berriasian to Early Albian (McHargue et al. 1992). Numerous rift basins along the NE African margin were showing strong subsidence during Neocomian and Barremian times, notably the Alamein, Abu Gharadig, and Shushan in the northern Western Desert of Egypt (Bayoumi and Lotfy 1989; Taha 1992; Moustafa et al. 1998; Rodriguez 2015) and the Hameimat and Sarir in the southeastern Sirt (Exploration Staff of the Arabian Gulf Oil Company 1980).
During the third phase, lacustrine/lagoonal deposits (Abu Ballas Formation) are dominated by the tectonic subsidence and marine ingression. They are followed by the existence of fluvio-estuarine depositional system (Taref-Sabaya and Quseir formations). The tidal-flat and associated deposits were the main elements of the basin fill characterizing the third stage. The Aptian–Albian rifting phase is well documented in Abu Gabra, Anaza, Kaisur Basin of Sudan (Bosworth 1992), and Alamein and Abu Gharadig basins of North Western Desert (El Emam et al. 1990; Rodriguez 2015) in addition to Beni Suef Basin (Zahran et al. 2011). These rifting phases are recorded as well-defined distinct successions, here interpreted in terms of the external controls on the basin architecture (Fig. 18a–o). The successive rifting phases will be discussed in detail below.
Berriasian–Early Barremian rifting
This rifting phase is almost synchronous with Late Berriasian continental rifting in NE Africa–Arabia (see Maurin and Guiraud 1990; Guiraud and Maurin 1991, 1992). The initial rifting stage is characterized by increasing rate of fault-related subsidence. The onset of deposition in the Komombo Basin is recorded in the A member of the Six Hills Formation being penetrated in the depocenter of the basin (Figs. 5, 6, 18) and is characterized by conglomerates, conglomeratic sandstones, and mudstones of the fluvial fan depositional system. This sedimentary package is characterized by wedge shape, which attains 75 m thickness and limited areal extent. Thus, I inclined to consider the deposition of this sedimentary package occurred in small fault-bounded basin (Fig. 18a). The small isolated depocenters, developed prior to the linkage of the early faults due to fault propagation (e.g., Gawthorpe and Leeder 2000), are diagnostic features of the rift initiation according to Prosser (1993). A similar succession was deposited at Sudan troughs such as Muglad Basin (McHargue et al. 1992).
This rift initiation is characterized by the establishment of locally sourced transverse and axial sedimentary lobes (Ravnås and Steel 1998). These are dominated by coarse-grained clastics of the stream-dominated fluvial fan in the axial domain and floodout-dominated fluvial fan in the lateral hanging wall. The hanging wall fluvial fan shows general progradation due to successive increase in the accommodation with the rotation of the hanging wall block (See Leeder and Gawthorpe 1987). This was shown from the general upward coarsening and thickening of the coarse-grained facies. The axial stream-dominated alluvial plain shows general aggradation to retrogradation due to the relative higher tectonic subsidence rate in contrary to the sediment supply rate (Fig. 4a).
Spatial distribution and provenance and paleocurrent patterns of the stream-dominated fluvial fan (Fig. 4a) point to an axial river system with transport directions from north to south and southwest with a northern source area (Fig. 18a). This phase of basin evolution is interpreted as the rift initiation with maximum preserved thickness of around 61 m.
The approximately 137-m-thick B member of the Six Hills Formation was probably deposited during this stage in a starved sedimentary environment where high rate of fault-related subsidence produced relatively deep anoxic water with relatively low clastic input. This resulted in deposition of offshore lacustrine mudstones (Fig. 9a, b). The limitation of the B member of the Six Hills Formation to the basin depocenter reflects the relatively high tectonic subsidence in the basinal area with more reduced sediment supply and limited drainage of the basin (Fig. 18b). Similar organic-rich lacustrine mudstones were deposited in Muglad Basin (McHargue et al. 1992) and Salamat trough of Central African Republic-Chad (Genik 1992). The transition from the fluvial fan system of A member of the Six Hills Formation to the lacustrine system of the B member delineates a transgressive surface.
This late stage of Berriasian–Barremian rifting is characterized by gradual increase in the clastic supply relative to the tectonic subsidence with general successive progradation of the axial and lateral lobe/channel systems. The deposits of this stage show the braidplain progradation or basinward shift of the facies toward the floodbasin or shallow water lake. This will be encountered from the general upward coarsening pattern of the C–E members of the Six Hills Formation. This progradation is very clear from the vertical facies transition from distal to proximal braidplain (Fig. 18c–e). The proximal braidplain deposits are widely distributed through the rift basin especially nearby the master bounding fault of the Komombo Basin as seen from the drilled wells and the surface outcrops (Figs. 8a, 18e).
Spatial distribution and provenance and paleocurrent patterns of the mid- to proximal braidplain (Fig. 8a) refer to the complex channels system with transport directions from north/northwest to south and southeast with a northern source area (Fig. 18d, e).
Late Barremian rifting
The Late Barremian rifting phase represents the second phase of rifting at Komombo Basin and includes two stages as follow:
The synrift stage characterized by the dominance of the distal floodbasin/distal braidplain to shallow lacustrine mudstone (F member of the Six Hills Formation) reflects the increase in the rate of the accommodation outpaced the sediment supply (high-accommodation-system tract, Fig. 18f). The distal braidplain deposits characterize the underfilled phase in the evolution of the basin.
This stage is characterized by tectonic sag as the distal floodbasin/distal alluvial plain succession passes upwards to mid- and proximal braidplain system with overbank deposits of G member of the Six Hills Formation (Figs. 10a, 18g) of the filling phase. The general change from underfilled to filled conditions is attributed to a shift in the balance between accommodation and the ability of sedimentary systems to fill the available space. The Late Barremian rifting phase shows reversing in the sediment transport direction from south/southeast to north/northwest with change in the source area to the south (Fig. 10a).
This synrift stage is characterized by marine flooding through the Aptian with dominance of the Abu Ballas mudstones that reflect the high rate of tectonic subsidence with low sediment supply rate. This event represents the first marine ingression in the Komombo Basin.
The dominance of the lacustrine/lagoonal deposits (Fig. 10a, b) reflects the creation of accommodation which outpaced the sediments supply from the hinterland source area. The lack of dinoflagellates and sparse pollens and spores reflects the deposition of lacustrine to lagoonal setting (Fig. 18h).
During the Sabaya stage, no new accommodation is generated and sediment supply gradually consumes the available space leading to a change from an underfilled to overfilled basin. Typically, sediment supply increases after a tectonic pulse, when the newly created differential relief between the source area and the basin allows for a more efficient delivery of sediment to the basin. The tectonic subsidence that was associated with deposition of Abu Ballas shales was followed by increase in the siliciclastics with dominance mixed-load fluvial channels during Sabaya-Taref Formation (Fig. 18j, k). The progradational pattern of siliciclastics through Abu Ballas to Sabaya-Taref stages probably reflects the waning of tectonic activity.
Post-rifting phase (epicontinental sag)
The rifting episode described above was followed during the Mid-Cretaceous by a tectonically quiet period (Guiraud 1998). This phase is characterized by the tectonic quiescence and degradation of source rock with dominance of fine-grained facies deposition with the development of the epicontinental sea. This stage began with major marine flooding of the Albian–Cenomanian systems with the dominance of the tidally affected fluvial channels, interfluve areas, and development of tidal estuaries (Figs. 11, 12, 13, 18k, l). This reflects the growth of the sea level role on the tectonic role in the deposition and sediment architectures and distribution. The gradual decrease in the clastic supply from the hinterland shows the dominance of the fine-grained deposition in mudflats and paleosol formation. The rate of relative sea level rise increased through Turonian–Santonian with development of the tidal flats and tidal channels in the inner shelf setting (Figs. 14, 18m). During the late Quseir (Santonian), the marine facies regression occurred with gradual decrease in the marine influences. This was followed by increase in the fluvial action and formation of tidally affected deltas (Figs. 14, 15, 16, 18n). This marine regression is confirmed by vertical facies transition from mudflats to prograding deltaic mouth bar deposits. This was eroded or capped by the fluvial or deltaic channels that intercalated with coaly and mudstone interbeds of the deltaplain setting. This event is probably simultaneous with Late Cretaceous Syrian Arc inversion pulses. Finally, the whole basin and surrounding areas were covered by Campanian–Maastrichtian (Dakhla) sea with deposition of open marine muds (Figs. 17a, b, 18o). The open marine depositional system was confirmed by the abundance of marine dinocysts and the limited land-derived miospores.
The Komombo Basin is an approximately 70-km-long and 30-km-wide, WNW-ESE-oriented Cretaceous rift basin formed during opening of the Neotethys. The sedimentary fill of the Komombo Basin can be ascribed to seven depositional systems: fluvial fan, braidplain, siliciclastic lakes, fluvio-estuarine, lake/lagoon, tidally affected deltas, and open marine settings. These depositional systems are in part coeval and in part succeed each other during the basin evolution.
In agreement with the main literature, the study reveals two main rifting phases commonly identified in the Central and North African Mesozoic basins through the Early Cretaceous. These two main phases of rifting are recognized in the history of Komombo Basin: Berriasian–Early Barremian and Aptian–Albian. A third additional Late Barremian rifting stage has been identified in the transition between those two main rifting phases. The Berriasian–Early Barremian rifting includes three stages: The rift initiation stage (A member of Six Hills Formation) is represented by conglomeratic and coarse-grained sandstones of fluvial fan-dominated successions. The rift climax stage (B member of the Six Hills Formation) of basin evolution began with a major reactivation tectonic subsidence with deposition in an anoxic lacustrine setting. This was followed by gradual decrease in tectonic subsidence and growth of the sediment supply to outpace the tectonic accommodation resulting in the development of the prograding distal to proximal braidplain deposits with late rift phase. The spatial distribution of the depositional systems and associated paleocurrent patterns indicate southerly sediment dispersal during Berriasian–Early Barremian. This rifting phase was described in many basins along the Northeastern African margin such as Hameimat and Sarir basins of Libya and Shushan and Abu Gharadig basins of Egypt that were dominated by fluvio-lacustrine deposits (Guiraud and Bosworth 1999) as well as Muglad Basin of Central Sudan (Mchargue et al. 1992).
Aptian–Albian rifting was associated with the creation of new accommodation space and accompanied marine ingression. This led to the formation of mixed-load fluvial channel and tidally affected fluvial channel with successive development of tidal estuaries (indicating the first ingression of the sea into the graben). This was followed by further transgression with the development of extensive tidal-flat domain with some tidal channels. This succession is interpreted as the record of the initial and continued thermal subsidence prior to post-rift thermal cooling, recorded in the Turonian–Santonian (Taref-Maghrabi stage). By the end of this stage, delta deposits are formed as consequence of marine regression and probably Santonian tectonic inversion. The post-rift deltaic deposits and the following open marine muds of Dakhla stage were deposited in a broader area than the synrift successions, recording deposition in a shallow sea-oriented parallel to the former rift. The Aptian–Albian rifting was described in Central African basins such as Abu Gabra, Anza, and Kaisur basins (Bosworth 1992). During the Albian–Aptian, most of Cretaceous rifts of southern Sudan represent sites for lacustrine sedimentation in the, resulting in widespread deposition of source rocks (Schull 1988). In the north Western Desert (Abu Gharadig Basin), the Dahab shales represent the deposition under tectonic subsidence (El Emam et al. 1990) as well as lower Kharita shales of Beni Suef Basin (Zahran et al. 2011).
An additional rifting stage has been identified in the transition between the two main phases: Berriasian–Early Barremian and Aptian–Albian. This stage (Late Barremian) is associated with creating accommodation with the abundance of distal floodbasin/distal braidplain deposits and paleosol formation. This was followed by a relative decrease in the tectonic subsidence and increase in the sediment supply from the hinterland with the development of mid- and proximal braidplain deposits. The spatial distribution of the depositional systems and associated paleocurrent patterns indicate reversing of sediment dispersal direction, formerly southerly during Berriasian–Early Barremian to Northerly during Late Barremian.
The author is grateful to SeaDragon-Egypt for the permission to use their data and to publish this paper. Prof. M. Darwish and Prof. A. El Manawi (Cairo University) and Eng. Alaa Ghoneimy (General Manager of south asset, SeaDragon-Egypt) are acknowledged for their critical and thorough reviews and suggestions. Dr. Ray Smith (CSIRO, Australia) is thanked for reviewing and editing the manuscript. The author is also grateful to the two reviewers: Prof. Thomas Voigt and Prof. Jochen Kuss, for their critical reading that improved the paper. The author also thanks Editor Prof. Wolf-Christian Dullo for handling the article for International Journal of Earth Sciences.
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