Depositional architecture and sequence stratigraphic framework of the fluvio‐lacustrine Ash Shumaysi Formation, Jeddah‐Makkah Region, Saudi Arabia: Implications for climatic and tectonic changes in a local‐scale sub‐basin

This study aims to interpret and document the depositional architecture styles and sequence stratigraphic framework of the Ash Shumaysi Formation in the Jeddah‐Makkah region, the west‐central part of Saudi Arabia, and presents an example of rarely discussed, local‐scale sub‐basins (half grabens). It also shows the relationships between synchronous sedimentary processes and pre, syn and post‐rift conditions. The described lithofacies and their facies associations indicate the presence of seven architectural depositional styles: proximal‐distal braided fluvial, meandering fluvial (point bar), crevasse splay, floodplain, estuarine and lacustrine. A proposed depositional model for the Ash Shumaysi Formation is drawn. The Ash Shumaysi Formation forms a second order depositional sequence, which is organised into two third order depositional sequences (sequences I and II) bounded by three sequence boundaries. Each third‐order sequence encloses the low accommodation systems tract and high accommodation systems tract. The low accommodation systems tract represents the coarse‐grained, braided‐distal fluvial facies developed during low accommodation space associated with high sediment supply (high discharge). The high accommodation systems tract encloses the fine‐grained deposits of point bar, estuarine and lacustrine facies that reflect the creation of significant accommodation space and low sediment supply (low discharge). Vertical and lateral variations of the inferred depositional architectural styles, sequences and systems tracts reflect that tectonic forces and climate are the main controlling factors during deposition of the Ash Shumaysi Formation, although base‐level changes in response to sea‐level changes cannot be ruled out.


| GEOLOGICAL SETTING AND GENERAL STRATIGRAPHY
The study area is located in the west-central part of the Arabian Shield between the Red Sea to the west and the Hejaz Mountains to the east (Moore & Ar-Rehaili, 1989; Figure 1).It is situated between latitudes 21°15′37.12″and 21°35′59.06″N, and longitudes 39°29′48.00″and 39°42′01.77″E, and extends along the western banks of Wadi Fatima, Wadi As Sayl and Wadi Ash Shumaysi (Figure 1).Tectonically, the Jeddah-Makkah region is part of the Jeddah Terrane that was affected by the Precambrian Orogenesis (HiJaz Orogeny) and the Oligo-Miocene-Quaternary Red Sea rifting (Al-Shanti, 1966).The Precambrian Orogenesis led to the development of different grades of metamorphism and intrusion of plutons within the basement complex, while the Red Sea rifting was associated with volcanic eruptions (Harrat Basalt), and normal faulting developed a series of half grabens (trending NNW-SSE; Spencer & Vincent, 1984).Palaeogeographically, the entire Red Sea area (including the study area) was in a pre-rift status (Ziegler, 2001) and influenced by sea incursion during Maastrichtian to Palaeocene times (Basahel et al., 1982).During the Late Palaeocene to early Oligocene, the Red Sea area was characterised by a fall in sea level and a narrowing of the Neo-Tethys Ocean (Haq et al., 1988).This interval is dominated by fluvio-lacustrine and coastal clastic sediments (Ziegler, 2001).In contrast, during the Late Oligocene-Miocene times, the Red Sea area was in a rifting stage, with the advent of sea floor spreading and splitting of the African-Arabian Plate (Ziegler, 2001).At that time volcanic activity was widespread in western Arabia, a process that continues today (Camp & Roobol, 1991).Also, the Wadi Shumaysi area (area of the present study) was believed to be a half-graben (longitudinal basin) formed along a north-western regional fault parallel to the Red Sea (Abou-Auf & Gheith, 1997).This half-graben was infilled with syn-rift continental siliciclastic sediments of the uppermost portion of the upper member of the Oligo-Miocene Ash Shumaysi Formation (Al-Shanti, 1966;Abou-Auf & Gheith, 1997.The sediments of the lower and middle members of the Ash Shumaysi Formation, however, were laid down in a pre-rift basin (Abou-Auf & Gheith, 1997).Both syn-rift and pre-rift sediments were probably derived from the Precambrian Arabian Shield (Abou-Auf & Gheith, 1997;Spencer & Vincent, 1984).
Lithologically, the study area is characterised by Precambrian basement rocks of the Arabian Shield that are non-conformably overlain by Tertiary rocks and Quaternary deposits (aeolian sand and alluvial wadi infills; Al-Shanti, 1966;Karpoff, 1957).As reported by Al-Shanti (1966) and Johnson (2006), the Precambrian basement rocks are represented by three main rock units: (1) intrusive rocks (Makkah suite, Hashfate complex, Kamil suite and Rumayda granite; (2) metamorphic rock of the Jeddah/Zibara group; and (3) extrusive igneous rocks, mainly basalt, andesite, dacite and rhyolite.The Tertiary Ash Shumaysi Formation (rock unit of the present study) and the Bathan Formation (Brown & Jackson, 1983) are overlain by flat-lying basalts, Late Miocene to Pliocene in age (Moufti et al., 2013).The Tertiary rocks were deposited in half grabens and are currently tilted to the north-west (Brown & Jackson, 1983).Raised reefs of limestone that underlie the coastal plain are assigned Pleistocene to Holocene ages.Recent poorly-cemented sands and gravels fill the wadis of the region.The Ash Shumaysi Formation was first named for the Tertiary clastic sedimentary sequence (conglomerates, sandstones and mudrocks) in the western banks of Wadi Fatima and Wadi Ash Shumaysi (Figure 1) that occur above the Precambrian basement rocks, and below the Late Miocene-Pliocene Harrat Basalt.Outcrops of the Ash Shumaysi Formation are found in the form of discontinuous dark brown to black, small isolated hills (ridges) bounding the western banks of Wadi Ash Shumaysi and Wadi Fatima (Al-Shanti, 1966).Beds within these isolated hills are tilted in a NNE direction (dip angle is 15°-25°), and are mainly offset by north-west oriented faults flanking Wadi Ash Shumaysi (Al-Shanti, 1966;Moore & Ar-Rehaili, 1989).The tilting is probably related to the reactivation of the north-westerly regional block faulting in the area (Al-Wash & Zakir, 1992).As described by Al-Shanti (1966) at Wadi Ash Shumaysi (type locality), the Ash Shumaysi Formation consists of three members (lower-middle and upper members) reaching up to 141 m F I G U R E 1 Geological map of the study area (after Al-Shanti, 1966;Moore & Ar-Rehaili, 1989).
in thickness.The lower member is 64 to 70 m thick and is mainly made up of coarse-grained sandstone with subordinate siltstone and claystone.The middle member is 13 to 18 m thick and consists of alternating sandstones and siltstones bounded by two beds of oolitic ironstone (1.5 m and 2.5 m thick, respectively).The upper member is 44 to 53 m thick and consists of shale, siltstone and a few tuffaceous beds enclosing one bed of a gastropod-bearing siliceous limestone.Chert bands and nodules are recorded in the upper member.

| METHODOLOGY
This work has been achieved through fieldwork augmented by laboratory investigation.Satellite images and geological maps, together with GPS, were used to locate and trace outcrops of the Ash Shumaysi Formation in the studied area.Three composite stratigraphic sections in outcrops of the Ash Shumaysi Formation (Figure 1) were measured and described in terms of lithology, primary sedimentary structures, palaeo-current trends, stacking patterns and geometry.Palaeo-current directions were mainly measured in cross-stratified beds to reconstruct palaeo-flow pathways.The lithofacies are identified and grouped into facies associations.Lithofacies types were classified using the facies codes of Miall (1977), andBridge (2006).Facies associations were subdivided and interpreted according to Walker (1984) with the resulting facies analysis of the outcrops used to draw a depositional model.The terminology of sequence stratigraphic elements (key surfaces, systems tracts, sequences) was adapted from that of Shanley and Mccabe (1994), Currie (1997) and Martinsen et al. (1999) for continental strata.In view of the scarcity of fossils, the strata of the measured sections were correlated mainly using the oolitic ironstones as isochronous deposits and marker beds.

Shumaysi Formation
Field descriptions and measurements (Figures 1 and 2) clarified that the Ash Shumaysi Formation is a clastic sedimentary sequence that non-conformably overlies the Precambrian basement rocks (granites and granodiorites) and underlies the Late Miocene-Pliocene Harrat Basalt (Figure 3A).At the lower contact (below the Ash Shumaysi Formation), the Precambrian igneous rocks (granite, granodiorite) are heavily weathered (Figure 3B,C).The Ash Shumaysi Formation is subdivided into lower, middle and upper members (Figures 2 and 3A).Both the lower and upper members are subdivided into three informal rock units: lower, middle and upper parts (Figures 2 and 3A).
The lower part of the lower member (50-70 m thick, Figure 3A) directly overlies the basement rocks (granites and granodiorite; Figure 3A-C).Its maximum thickness (50 m) is recorded at Wadi Ash Shymaysi, and is reduced to 30 m at Wadi Fatma (Figures 1 and 2).It consists of repeated fining-upwards cycles (Figures 2 and 3B), in which each cycle begins with cross-bedded conglomerate and conglomeratic to pebbly sandstone (80-90%), followed upward by laminated siltstone that terminates in greyishwhite massive claystone (20-10%; Figures 2 and 3C).The middle part of the lower member (35-50 m thick, Figure 3C) is made up of fining-upward cycles.Each cycle consists of fine-grained to medium-grained sandstone 1 to 1.5 m thick, followed upward by siltstone and/or claystone 0.5 m thick and capped by iron crusts 10 to 20 cm thick (Figures 2 and 3C).At the upper part (30-40 m thick) of the lower member, abundant-root traces are observed in sandstone beds capped by iron crusts (Figure 2).The iron crusts, which occur as massive layers of iron-cemented sandstones/siltstones, show an upward increase in thickness from 5 to 20 cm.Apart from the presence of some root traces and desiccation cracks, the iron crusts lack a vertical pedogenic profile (A and B horizons), and/or a mottled horizon underlying them.
The middle member conformably overlies the lower member and is made up of a succession (10-18 m thick) of yellow to reddish yellow, root-traced fine-grained sandstone, mudstone and siltstone (Figure 3D) bracketed by two reddish-brown beds.These two red-brown beds are oolitic to pisolitic ironstone, each of which attains a thickness of 1.5 to 2.5 m (Figures 2 and 3D).The upper member ranges from 30 to 53 m thick, and conformably overlies the middle member and underlies the Miocene-Pliocene Harrat Basalt, the Sita Formation, (Figures 2 and 3A,E).The lowermost portion of the upper member (15-20 m thick) is represented by alternating clastic beds consisting of massive coarse-grained to medium-grained sandstone (1-3 m thick) interbedded with red siltstone (0.2 m thick).These beds directly overlie the oolitic ironstone of the middle member (Figure 2).The middle part of the upper member (10-15 m thick) is mainly made up of heavily-rooted sandstone (1-1.5 m thick), intercalated with siltstone and claystone (0.2-0.3 m thick; Figures 2 and 3E).The uppermost part of the upper member (20-25 m thick) comprises tuffaceous mudrocks, enclosing nodular to banded chert and a siliceous limestone bed containing siliceous gastropod shells (Figure 2) which resemble the Oligo-Miocene aged Lanistes carinatus of Pickford et al. (2010).
The Ash Shumaysi Formation has been variously dated to the Early Eocene or the Oligo-Miocene.Based on macrofossil evidence, Al-Shanti (1966) favoured an Oligocene age for the whole of the Shumaysi Formation, whereas Moltzer and Binda (1981) and Srivastav and Binda (1991) assigned an early Eocene age to the middle member based on the palynomorph assemblages.An Oligo-Miocene age was attributed to the upper member of the Ash Shumaysi Formation (Abou-Auf & Gheith, 1997).The present study agrees with the Oligo-Miocene age assignment for the upper member given by Abou-Auf and Gheith (1997) because of the overlying Miocene-Pliocene aged Harrat Basalt (Moufti et al., 2013), and the presence of Lanistes carinatus of Oligo-Miocene age (Pickford et al., 2010) (Figure 2).The age assignment of the middle member followed the age assignments of previous authors (Abou-Auf & Gheith, 1997;Moltzer & Binda, 1981;Srivastav & Binda, 1991) who assigned an Early Eocene age.The lower member of the Ash Shumaysi Formation was not previously dated because of the poor fossil record.This study assumes the pre-Eocene age (probably Late Cretaceous?) for the lower member of the Ash Shumaysi Formation.This is because of its close similarity with the lithofacies of the lower unit of the Upper Cretaceous Nubia Sandstones that directly overlie the basement complex in nearby eastern Egypt, on the western side of the Red Sea (Van Houten et al., 1984).Despite this, the lower member of the Ash Shumaysi Formation needs further study to determine its precise age.

| Sedimentary facies and related depositional architectural styles
Seventeen sedimentary lithofacies types are identified.These lithofacies are grouped into eight genetic facies associations (FA 1-8).Description and interpretation of the lithofacies types and their related facies associations are provided below and summarised in Table 1.

| Lithofacies types
The recognised lithofacies include the following:

Massive conglomerate (Gm)
This lithofacies is made up of pebbles and granules (of quartz grains, granitic fragments and rare mud clasts)

Shows low angle and unidirectional planar cross-stratification
The occurrence of unidirectional, low-angle, planar cross-bedded sandstones indicates deposition by migration of straight-crested subaqueous 2D dunes under the lower flow regime of a proximal braided stream (Allen, 1963;Miall, 1996) Trough cross-stratified sandstone (St) Displays trough cross-stratification, in which individual sets range in thickness from 0.2 to 0.3 m (Figure 4D) Trough cross-stratification in sandstones suggests migration of large-sized 3D dunes with sinuous crests in a deep proximal channel with high-energy discharge (Allen, 1963) Bioturbated (rooted) massive sandstones (Smb) A massive, medium-grained sandstone with heavy bioturbation by roots (Figure 4F) Roots in the medium-grained sandstone probably developed on the surfaces of elevated mid-channel bars under low rates of flow and sedimentation (Lelpi et al., 2022) Rippled sandstone (Sr) A fine-grained sandstone, moderately sorted and rippled (Figure 4G).This lithofacies dominates throughout the upper part of the lower members Attributable to the down-current migration of ripples among bars under relatively slow sedimentation in a low-flow regime (Cain & Mountney, 2009) Inclined heterolithic, sandstones (IHS) Interbedded siltstone and fine-grained sandstones recorded at the base of a horizontally-oriented sandstone bed (Figure 4H) Heterolithic inclined beds of fine-grained sandstone and siltstone may be a result of lateral accretion in sinuous channels (Shanely et al., 1992;Thomas et al., 1987).Also recorded in fluvial-point bars (Dalrymple & Choi, 2007;Thomas et al., 1987) Horizontally stratified sandstone (Sh) Fine to medium sandstones; moderately sorted; horizontally stratified (0.5-1 m thick).Laminated to thin-bedded (Figure 5A) Recorded in more distal fluvial systems (Miall, 1996) and estuarine point bars (Dalrymple & Choi, 2007) Laminated sandstone (Sl) Fine-grained sandstone, laminated to thinbedded (Figure 5B) May be deposited as bedload transport through weak traction currents in more distal fluvial systems (Collinson, 1996;Hjellbakk, 1997) Laminated siltstone (Sll) Siltstone, laminated (Figure 5C).Occurs in the middle and upper members, and is characterised by sheet-like geometry Reflects deposition from suspension in weak currents in an overbank/lake setting (Allen, 1963;Collinson, 1996) Variegated massive mudstone (Fmv) Mottled (grey to yellow) in colour, and blocky in structure (Figure 5D) Indicates deposition from suspension in an overbank/ lake setting from weak currents or standing water (Abdul Aziz et al., 2003;Kraus & Aslan, 1999) Tuffaceous massive mudstone (Fmt) Greyish brown in colour, and blocky in structure with root traces, rhizoliths (Figure 5E) Non-marine fine-grained siliciclastic rocks, with plant debris and rhizoliths, mostly recorded in a lowgradient, shallow fresh-water lake (Abdel- Aziz et al., 2003) embedded in a sandy to silty matrix.It ranges in thickness from 10 to 20 cm between the cross-bedded coarsegrained and/or massive sandstones while also filling the scoured surfaces (Figure 4A,B).It is recorded in the lowermost part of the lower member (Figure 2).Such lithofacies can be related to deposition in low-sinuosity fluvial channels under high-velocity flows (Miall, 1996).The massive, poorly-sorted gravels that fill scoured surfaces (Figure 4B) could belong to channel base-lag deposits (Miall, 1996).

Planar cross-stratified conglomeratic to pebbly sandstone (GSp)
This lithofacies consists of granule to pebble-sized clasts of quartz embedded in very coarse-grained sands.It is characterised by planar cross-stratification (with each foreset 10 to 20 cm thick; Figure 4A-C) It occurs in the lower part of the lower member.This lithofacies is a result of 2D migration of subaqueous sandy dunes in proximal braided fluvial channels (Allen, 1963;Collinson, 1996).

Massive conglomeratic to pebbly sandstone (GSm)
This lithofacies consists of granules and pebbles of quartz and other rock fragments embedded in a medium-grained to fine-grained sandy matrix (Figure 4B,C).This lithology reflects bed load deposition within a high-energy, steepgradient fluvial channel that is dominated by a proximal braided stream (Miall, 1996).

Trough cross-stratified sandstone (St)
This lithofacies is a medium-grained to coarse-grained, poorly to moderately-sorted sandstone, with sporadically distributed quartz pebbles.It shows trough cross-stratification, in which individual sets range in thickness from 0.2 to 0.3 m (Figure 4E).Cross-sets contain coarse-grained clasts at their bases.This lithofacies dominates in the lower and upper members.Trough cross-stratification in sandstones is related to the migration of 3D dunes and ripples with sinuous crests under low-flow conditions (Cain & Mountney, 2009).The thick trough cross-stratification suggests the migration of large 3D dunes, with sinuous crests in a deep proximal channel with high-energy discharge (Allen, 1963).

Bioturbated (rooted) massive sandstone (Smb)
This lithofacies is a massive, moderately-sorted, coarsegrained to medium-grained sandstone, penetrated by long roots (Figure 4F).This lithofacies dominates the upper part of the lower member.The presence of long root traces in the medium-grained sandstones probably suggests development on the surfaces of elevated midchannel bars under low rates of flow and sedimentation

Lithofacies types Description Interpretation
Oolitic to pisolitic ironstone (Oi) Ooids/pisoids varying in shape from subspherical to irregular (Figure 5F).Also show red to reddish brown and sometimes reddish yellow colour Occurrence within continental (lacustrine) deposits may reflect formation by reworking and re-deposition of nearby local lateritic sediments derived via a river mouth into a marginal lacustrine setting (Taylor, 1992) Iron crust (IC) Caps the fine-grained sandstones and sandy clays.Ranges in thickness from 2 to 10 cm (Figure 2) Iron crusts could be diagenetic in origin developing in areas of iron-rich groundwater discharge, particularly swamps, estuaries, and lake beds (Widdowson, 2007) Siliceous sandy limestones (Silg) Yellow to greyish-yellow coloured limestone with gastropod shells (Figure 5G) Carbonate beds (with small gastropods), in the absence of marine fossils, most likely occur in a low-gradient, shallow fresh-water lake (Abdel- Aziz et al., 2003) Chert nodules and bands (Ch) Chert occurs as laterally discontinuous layers (Figure 5H), and lenses that locally alternate with beds of laminated siltstone and tuffaceous mudstone Discontinuous chert bands and tuffaceous materials may be a result of a synchronous volcanic eruption (Krainer & Spotl, 1998) (Lelpi et al., 2022), and connection of a deep water table with the atmosphere (Knutsson, 1988).

Rippled sandstone (Sr)
This lithofacies, which dominates the upper part of the lower members, is made up of rippled fine-grained sandstone (Figure 4G).Such lithofacies possibly reflects a deposition during the down-current migration of ripples among bars under relatively slow sedimentation rates in a low-flow regime (Cain & Mountney, 2009).

Inclined heterolithic stratified sandstone and siltstone (IHS)
This lithofacies is made up of interbedded fine-grained sandstones and siltstones.These inclined heterolithic interbeds are bounded by horizontally-oriented sandstone beds (Figure 4H).This lithofacies dominates the upper part and lower part of the lower member and upper member, respectively (Figure 2).Heterolithic inclined strata of fine-grained sandstone and siltstone may be a result of lateral accretion in sinuous channels (Shanley et al., 1992;Thomas et al., 1987).Similar sediments have been also recorded in fluvial-point bars (Dalrymple & Choi, 2007;Thomas et al., 1987).

Horizontally-stratified sandstone (Sh)
This lithofacies is a fine-grained to medium-grained sandstone, moderately sorted and horizontallystratified (0.5-1 m thick).Its beds are laminated to thinbedded and show sheet-like geometry (Figure 5A).This lithofacies is recorded in the uppermost and lowermost parts of the lower and upper members, respectively.Such lithofacies is interpreted as bar-top sand sheets that develop during fluvial sheet flood in the upper flow regime (Cain & Mountney, 2009;Hjellbakk, 1997;Miall, 1996).It is recorded in more distal fluvial systems (Miall, 1996), and estuarine-point bars (Dalrymple & Choi, 2007).

Laminated sandstone (Sl)
This lithofacies is represented by fine-grained, laminated to thin-bedded sandstone (Figure 5B).It is recorded in the middle member, and the uppermost part of the lower member.It may be deposited as bedload transport through weak traction currents in the more distal fluvial systems and lake settings (Collinson, 1996;Hjellbakk, 1997).

Laminated siltstone (Sll)
This lithofacies (Figure 5C) dominates the middle and upper members.It reflects deposition from suspension and weak currents in an overbank/lake setting (Allen, 1963;Collinson, 1996).

Variegated massive mudstone (Fmv)
This lithofacies shows a mottled grey to yellow colour and blocky structure (Figure 5D).It ranges in thickness from 20 cm to 5 m.It is recorded in the middle and upper members, but it dominates in the upper member.It indicates deposition from suspension in an overbank/lake setting from weak currents or standing water (Kraus & Aslan, 1999).The red-violet colour-mottling in a mudstone reflects a well-drained floodplain in an oxidised area, while grey colour mottling in mudrocks reflects a poorly drained, waterlogged, distal floodplain setting (Bown & Kraus, 1987;Kraus & Gwinn, 1997;Retallack, 2008;Wanas & Abu El-Hassan, 2006).

Tuffaceous massive mudstone (Fmt)
This lithofacies is greyish-brown in colour, exhibiting a blocky structure with root traces and rhizoliths (Figure 5E).It is recorded in the upper member.The nonmarine, fine-grained siliciclastic rocks, with plant debris and rhizoliths, are mostly recorded in a low-gradient, shallow, fresh-water lake (Abdul Aziz et al., 2003).The greyish-brown and blocky mudstone is closely similar to the tephra deposits associated with a synchronous volcanic eruption in lacustrine successions (Krainer & Spotl, 1998).
Consequently, this tuffaceous massive mudstone (Fmt) could have been developed in a volcanic-influenced lacustrine environment.

Oolitic to pisolitic ironstone (Oi)
This lithofacies occurs as two beds (1.5-2 m thick) within the middle member.It is made up of ooids (up to 2 mm in size) and pisoids (up to 0.5 cm in size) of iron minerals.These ooids/pisoids vary in shape from sub-spherical to irregular (Figure 5F).The dominance of red and reddish brown colouration seen in these ooids/pisoids may reflect their haematitic and/or goethitic composition.
Regarding their origin, oolitic and pisolitic ironstones are well-documented developing in a shallow marine environment (Van Houten et al., 1984;Van Houten & Bhattacharyya, 1980).However, iron ooids/pisoids in a non-marine siliciclastic succession could have formed in situ during diagenetic crystallisation, dehydration or oxidation of the amorphous iron oxyhydroxide of green chamositic clays (Galmed et al., 2021).In the present study, and compared with those described and interpreted by Umeorah (1987), Taylor (1992) and Taylor et al. (2002) in a non-marine siliciclastic succession, the oolitic/pisolitic iron could have developed via the reworking of nearby lateritic clay and subsequent settlement in the well-oxygenated bottom-water of a marginal freshwater lacustrine environment.However, further petrographic, mineralogical and geochemical studies would

Iron crusts (IC)
Two to 10 cm thick iron crusts cap the fine-grained sandstones and sandy clays.They dominate in the uppermost part of the lower member.In some places, they occur as massive layers, while others contain traces of plant roots (Figure 2).Most of the massive layers are iron-cemented sandstones/siltstones.The presence of root traces in iron crusts and desiccation cracks could reflect pedogenic processes during subaerial exposure and lateritic weathering (Tardy, 1992).Alternatively, Tanner and Khalifa (2010) and Afify et al. (2015) declared that root traces and desiccation cracks in massive iron crusts (decimetre in thickness) repetitively capping individual beds within a stratigraphic section does not reflect pedogenesis, but could be diagenetically formed by iron-rich groundwater discharge.In this study, apart from some root traces and desiccation cracks in the iron crusts, an absence of vertical pedogenic profiles (A and B horizons), and/or a mottled horizon underlying the iron crust suggests that the iron crusts could be diagenetic in origin and developed in areas of iron-rich groundwater discharge, particularly swamps, estuaries and lake beds (Widdowson, 2007).The iron for groundwater enrichment was probably derived from the underlying basement rocks.

Siliceous sandy limestone (Silg)
This lithofacies is yellow to greyish-yellow in colour and contains small gastropod shells (Figure 5G) which are very similar to the L. carinatus of Pickford et al. (2010).This limestone attains a thickness of 0.5 to 1 m and is very hard.
It is siliceous and displays sheet-like geometry.It occurs in an association of fine-grained siliciclastic rocks (with plant debris and/or rhizoliths).In the absence of marine fossils, this carbonate bed was most probably deposited in a low-gradient, shallow freshwater lake (Krainer & Spotl, 1998;Pickford et al., 2010).

Chert nodules and bands (Ch)
Chert occurs as laterally discontinuous layers (Figure 5H), and lenses that locally alternate with beds of laminated siltstone and tuffaceous mudstone.This lithofacies forms the uppermost part of the upper member.An occurrence of discontinuous chert bands and tuffaceous materials may be a result of a synchronous volcanic eruption and/ or intense alteration of volcanic glass and related tephra from nearby volcanic sources in the freshwater lacustrine facies (Krainer & Spotl, 1998).

| Facies associations
Seven depositional architectural styles are reflected in the eight facies associations (FA 1-8) identified: proximal-braided fluvial channel (FA-1), distal-braided fluvial channel (FA-2), fluvial point bar (FA-3), upper estuarine channel (FA-4), crevasse splay channel (FA-5), floodplain (FA-6), lacustrine (FA-7) and volcanicinfluenced lacustrine (FA-8; Figure 6).The lower member shows proximal-braided channel deposits grading upwards into fluvial point bar through distal braided fluvial deposits.Freshwater lake sediments are mainly recognised in the middle member and the uppermost part of the upper member.The upper member begins with fluvial point bar deposits followed upward by upper estuarine ones that are capped by volcanicinfluenced freshwater lake sediments.Palaeocurrent measurements of cross-stratification in the sandstones indicate a dominant current flow towards the NNE.Description and interpretation of the inferred depositional architecture styles are given below.
Interpretation: This facies association is interpreted as representative of proximal braided (high-energy, lowsinuosity) fluvial channels.This assessment is supported by: (1) Absence of marine fossils and features indicative of tidal and/or wave action, which indicates deposition by fluvial processes only (Shiers et al., 2014).( 2) The coarse pebbly sandstone lacking bioturbation suggests rapid deposition by decelerating high energy, heavily sedimentladen currents (Miall, 1996).( 3) Multistory, sheet-like geometry of stacked massive conglomerates (Gm), pebbly trough cross-stratified (St) to massive sandstones (Sm) with scarce fine-grained sediments (siltstone and claystone) at their tops (Figure 2; Bridge, 2006;Miall, 1996;Rust, 1978).(4) Limited internal geometry in the Gm to Sm sheet-like strata, which is indicative of deposits of a longitudinal bar in a steep-gradient channel by traction current flows (Allen & Gibling, 1989;Bridge, 2006;Miall, 1978Miall, , 1985)).( 5) Abundance of large-scale (up to 2 m in thickness) trough bedded cross-strata (Figure 7A,B) when compared to the limited distribution of small-scale planar cross-stratification (Figures 4D and 7C,F), which suggests migration of large 3D dunes with sinuous crests in a deep proximal channel with high-energy discharge (Allen, 1963(Allen, , 1983;;Cain & Mountney, 2009).( 6) Relatively low occurrence of overbank mudstones, with a dominance of conglomerate-pebbly sandstone beds (Figure 2; Bridge, 2006;Miall, 1996).( 7) Uni-directional palaeocurrent trend (NNE) of the individual sandstone bodies cross-stratification, which reflects poorly developed lateral accretion and transportation by high-energy currents characteristic of low sinuosity channels (Allen et al., 2014;Miall, 1996).( 8) Abundance of erosional (scoured) surfaces filled with lag deposits, mud clasts and rip-up pebbles from the top of the underlying white mudstone (Figure 7A) that may represent basal channel boundaries and reactivation surfaces during the initiation of proximal braided channels (Allen et al., 2014;Bridge, 2003;Rust, 1972).( 9) Occurrence of planar cross-stratified conglomeritic pebbly sandstone (GSp) on the massive conglomerate, Gm (Figure 7D), which is common during the vertical aggradation of channel bars in proximal-braided streams (Miall, 1996).( 10) Gravels grading within foresets of gravelly pebbly sandstones (Figures 4A,B and 7C), which records the fluctuating discharge of a migrating proximal braided river (Miall, 1977(Miall, , 1978)).The gravel-tosand deposits described here seem more like the Donjek or South Saskatchewan-type stream model of Miall (1977).This high energy and low sinuosity river reflects high discharge and, in turn, a semiarid to dry climate (Manna et al., 2021).This type of fluvial facies could be associated with tectonic uplift (Catuneanu & Elango, 2001).Therefore, it is possible to conclude that FA-1 was related to a NNE sheet flooding of alluvial deposits from the adjacent western high-relief Precambrian Arabian Shield.Deposits of this sheet flooding event were settled in a lowland area.The high relief of the Precambrian Arabian Shield (from which FA-1 deposits were derived), accompanied by lowland areas (where the deposits were settled) could belong to the regional tectonics of the Precambrian Arabo-Nubian Shield (Abdelsalam & Stern, 1997).

Distal-braided to meandering fluvial channel: FA-2
Description: This facies association usually overlies FA-1.It is present in the middle part of the lower member and the lowermost part of the upper member of the Ash Shumaysi Formation (Figure 2).The facies association dominates towards the west of the study area.Eastward, FA-2 decreases in thickness.FA-2 directly overlies the conglomerates and pebbly sandstones of FA-1 (Figure 2).It is mainly composed of planar cross-stratified (Sp) to massive (Sm), and pebbly coarse to medium-grained sandstones; Gsm, interbedded with laminated siltstone (Sll; Figure 7E,G) forming multiple fining-upward cycles (Figure 7E,G).The deposits of FA-2 exhibit sheet-like geometries with marked lateral extent (Figure 7E).Planar cross-stratifications are of low-angle (<15°), small-scale type with individual sets ranging in thickness from 20 to 30 cm (Figure 7D).They exhibit uni-directional (NNE) palaeocurrent patterns.Small-scale planar cross-bedding is abundant while trough cross-bedding is uncommon (Figure 7F).Flat to rippled sandstone is observed in this facies association (Table 1; Figures 4G and 5A,B).Eastward of the study area, the laminated siltstones increase in thickness (Figure 7G) relative to those of the western localities but, in general, sandstone beds are dominant (70-80%) relative to the siltstone and mudstone beds (30-20%; Figures 2 and 7E,G).
Interpretation: The lack of marine fossils and features indicative of tidal and/or wave action, the sheet-like geometries, the smaller-scaled planar cross-bedded, pebbly coarse to medium-grained sandstones with some laminated siltstone inter-beds, all arranged in a fining-upward pattern (Figure 7E,G), may reflect deposition in more distal parts of a braided fluvial channel, in which the pebbly coarse-grained to medium-grained sandstone units were probably deposited during periods of relatively high-river discharge, whereas the intermittent intervals of siltstone and mudrock were probably deposited during inter-flood periods (low-river discharge; Allen, 1983;Miall, 1996;Scherer et al., 2015;Taj & Mesaed, 2012).The abundance of small-scale planar cross-stratification (Figures 4E and  7D,F), together with the absence or rarity of trough crossstratification in the sandstone beds, suggests downstream migrating bars (Ghazi & Mountney, 2009;Walker & Cant, 1984).This may also represent typical features of bar deposits in a more distal, lower-energy braided to meandering channel (Miall, 1977;Walker & Cant, 1984).In FA-2, the scarcity of Gm and Gps facies and an increase in Sp facies are indicative of more distal-braided (Miall, 2006;Walker & Cant, 1984) to meandered (Ghazi & Mountney, 2009) fluvial channels.

Fluvial point bar: FA-3
Description: This facies association occurs in the uppermost part and lowermost part of the lower member and upper member, respectively (Figures 2 and 8A,B,C,G,H).It dominates towards the north-eastern part of the study area (Figure 2).It is mainly built up of superimposed cycles.Each cycle is made up of intensively-rooted, mediumgrained to fine-grained sandstones (Smb, 1-2 m thick) interbedded with laminated siltstones (Sll, 0.2-0.5 m thick) to silty claystone that is capped by an iron crust (Figure 8C through G).Inclined heterolithic sandstonemudstone strata (IHS) are dominantly observed in this facies association (Figures 2 and 9A,B).Meanwhile, the laminated siltstones (Sll) and silty claystone increase in thickness in the extreme north-east.The root traces are characterised by downward tapering and/or branching into smaller branches (Figure 8H).In FA-3, the root traces in the sandstone beds (Smb) become denser towards the NNE.
Interpretation: The sequence of fine-grained sandstone and siltstone beds, in which the fine-grained sandstones are dominant (60-70%) relative to siltstone and mudstone (30-40%), suggests deposition in a meandering channel during relative base-level rise (Miall, 1996).Sandstones accumulated in a high-energy flow regime, unlike mudstones which should settle down from suspension (Miall, 1996).Although root traces are more abundant in river overbank (Kraus & Hasiotis, 2006), their occurrence in the medium to fine-grained sandstones (Figure 8C through H) probably represent growth on the surfaces of elevated point bars under low rates of flow and sedimentation (Lelpi et al., 2022;Miall, 1996).Inclined heterolithic stratification of sand-mud interbeds is known to form during lateral accretion on the point bar of distributary/high sinuosity channels (Johnson & Dashtgard, 2014;Miall, 1996).The deposits of FA-3 are also similar to those interpreted as fluvial point bars in other geological settings (Choi et al., 2004;Dalrymple & Choi, 2007;Johnson & Dashtgard, 2014;Nouidar & Chellaı, 2001;Thomas et al., 1987).

Estuarine channel: FA-4
Description: This facies association occurs in the middle part of the upper member of the Ash Shumaysi Formation where it overlies FA-3 (Figure 2).FA-4 is made up of three beds of green to yellowish green shale (each bed is 1-1.5 m thick) interbedded with three beds (each 0.2-0.3m thick) of reddish-yellow siltstone and fine-grained sandstone (Figure 9C).The beds are organised vertically in coarsening-upward cyclothems.Agglutinated (Ammobaculites) and planktonic (Globigerinoides) foraminifera were recorded in the shales of this facies association by Moltzer and Binda (1981) and Abou-Auf & Gheith (1997), respectively.Plant root traces have been also observed in these deposits.
Interpretation: The estuarine channel is dedicated to a high-sinuosity fluvial channel opening into a shallow sea (Dalrymple et al., 1992;Dalrymple, 2010).In this facies association, although features indicative of tidal activity (i.e.sigmoidal bedding, mud drapes, flaser bedding, lenticular bedding, herringbone cross-bedding and/or trace fossils including Teredolites, Arenicolites and Skolithos) are not recognised, the occurrence of agglutinated foraminifera (Ammobaculites) can reflect deposition in shallow marine conditions and/or near an upper estuarine area that opened into a shallow sea (Moltzer & Binda, 1981).The presence of planktonic (Globigerinoides) foraminifera in the shale beds reflects deposition in an open marine environment (Abou-Auf & Gheith, 1997).Thus, these deposits are closely similar to deposits that reflect episodes of marine transgression during periods of reduced sediment supply in a drowned river, upper estuarine situation (Dalrymple et al., 1992;Thomas et al., 1987).The marine influence could be related to the small embayment (narrow sea) that invaded the study area during Red Sea rifting (Haq et al., 1988).

Overbank facies
This facies association includes crevasse splay channel (FA-5) and floodplain (FA-5, FA-6) as will be discussed below: Crevasse splay channel: FA-5.Description: The deposits of FA-5 are common in the upper member of the Ash Shumaysi Formation.They are dominant towards the

Oolitic iron stone
Heavily-rooted SS Heavily-rooted SS.

A B C D E F G H
part of the study area (Figure 3).This facies association is characterised by sheets (1-0.5 m thick) of massive (Sm) and/or horizontally stratified (Sh) fine to medium-grained sandstones with very thin mudstone/ siltstone layers embedded in grey mudstones (Fmv; Figure 9D).The sandstone package displays a slightly undulatory base contact (Figure 8E).Individual sandstone beds show flat geometries (Figure 9D).In the extreme north-east of the study area, the horizontally-bedded massive sandstone (Sh) shows a relative decrease in thickness (Figure 9E).
Floodplain: FA-6.This facies association occurs as proximal and distal floodplains.It is dominant in the eastern part of the study area.Proximal floodplain.Description: Proximal floodplain deposits are made up of sheet-like thin beds (3-6 cm thick) of fine-grained massive sandstone encased in red massive mudstone (Fmv; Figure 9F).The Increasing thickness of mudrock relative to sandstone is usually seen upwards throughout the facies association.The mudrocks show pedogenic features such as colour mottling.
Distal floodplain.Description: Distal floodplain deposits are dominant in the north-eastern part of the study area but are absent in the western part.The deposits consist of a 5 to 15 m thick succession of red to violet massive mudrocks (Fmv) with subordinate thin layers (0.5-1 cm thick) of parallel laminated siltstone (Sll) and finegrained sandstone (Sll) interbeds (Figure 9F).This facies association is traceable laterally several tens of metres, below sandstones of other channels (Figure 9F).Contacts with overlying channels are irregular (Figure 9F).Eastward of the study area, this facies association consists mainly of mudrocks enclosing isolated lenses (ribbons) of fine-grained sandstone (10-15 cm in width, 30-40 cm in length; Figure 9G) and/or fully red mudrocks.The massive mudstone (Fmv) facies show colour mottling (grey, red, violet; Figure 6D).Root traces are observed in the mottled massive mudrocks.

Lacustrine: FA-7
Description: This facies association is widely developed in the middle member of the Ash Shumaysi Formation.It reaches up to 12 to 15 m in thickness.To the west of the study area, the deposits of FA-7 consist mainly of greenish-white mudstone (0.3-0.5 m thick) alternating with beds of laminated siltstone (Sll) and fine-grained sandstone, Sl, (2-3 cm thick).All are bracketed by two beds of oolitic/pisolitic ironstone (Oi; Figure 10A).These two ironstone beds range in thickness from 1.5 to 2 m, and are laterally continuous.They lie at the contacts of the middle unit with the underlying (lower member) and overlying rocks (upper member; Figure 2).Iron occurs as sub-spherical to irregular pisoids (3-5 mm in diameter) and ooids (≤2 mm in diameter; Figure 5F).The red and dark brown colours reflect the haematitic and/or goethite composition.Towards the south-west of the study area, the mudrock beds decrease in thickness concomitant with an increase in siltstone and fine-grained sandstone (Figure 10B).No marine fossils or evaporites are found in the deposits of FA-7.
Interpretation: In this facies association, the widespread extent of greenish-yellow laminated mudstone interbedded with a few thin beds of siltstone and finegrained sandstones without marine fauna and evaporites can be interpreted as fine-grained materials settling in a quiet freshwater lake (Sáez et al., 2007;Scherer et al., 2007;Wanas et al., 2015).In this unit, the increasing thickness of mudstones and siltstones towards the eastern part of the study area (Figure 2) could reflect the lake centre (Abdul Aziz et al., 2003).
F I G U R E 9 Field photographs showing: (A, B) inclined heterolithic strata, IHS, Section-2, (C) repeated coarsening-upward cycles, each cycle starts with yellowish green shale followed upward by reddish yellow siltstone to fine-grained sandstone, Section-1, (D, E) sheets of massive horizontally stratified sandstones with thin layers of mudstone, Section-3, (F) sheet-like thin beds (2-3 cm thick) of fine-grained massive sandstone in reddish massive mudstones (Fmv), Section-3.(G) Ribbons of fine-grained sandstone (dotted white circles) in variegated mudrock beds (Fmv), Section-3.Description: This facies association is present in the uppermost part of the upper member of the Ash Shumaysi Formation.The deposits consist mainly of greyish violet tuffaceous siltstones and mudrocks (Fmt) enclosing nodular to banded chert (Figure 10C).The strata generally ).The recorded gastropod shells are similar to the Oligocene L. carinatus described by Pickford et al. (2010) and Abbass (1971).Ichnofossils are also observed on finegrained sandstone surfaces (Figure 10E).These ichnofossils are similar to Beaconites sp. of Bromley (1996) and Buatois and Mángano (2011).Likewise, Charophytes are recorded by Abou-Auf and Gheith (1997).Eastward of the study area, FA-8 contains more tuffaceous mudrocks (Fmt) when compared to those in the south-west (Figure 10F).Root traces are observed in the mottled massive tuffaceous mudrocks.
Interpretation: The dominance of massive mudrocks and siltstones suggests deposition in quiet water under reducing conditions (Abdul Aziz et al., 2003).The lack of marine fossils and the absence of evaporites are indicative of the fresh to brackish water of a marginal lake environment (Abdul Aziz et al., 2003;Wanas et al., 2015).An occurrence of L. carinatus suggests deposition in a freshwater lake (Pickford et al., 2010).Bioturbation by Beaconites sp in fine-grained sandstones and siltstones reflects deposition in the marginal lake/and or overbank areas of a delta plain (Buatois & Mángano, 2011).The fine-grained siliciclastic rocks, with plant debris and rhizoliths in an absence of marine fauna (Figure 5E) is mostly recorded in a low-gradient, shallow fresh-water lake (Abdul Aziz et al., 2003;Wanas et al., 2015).The greyish-brown colour, blocky mudstone enclosing abiogenic chert nodules/ bands (Figure 5H) and ferruginous plant debris and rhizoliths (Figure 5E) are closely similar to tephra deposits associated with a synchronous volcanic eruption in a lacustrine succession (Krainer & Spotl, 1998).Therefore, FA-8 can be interpreted as freshwater-lacustrine deposits influenced by synchronous volcanic activity.

| CONTROLS ON DEPOSITION
Variations in the deposition styles of fluvial-lacustrine successions in areas near coastal marine settings can be controlled by both allogenic (base-level change, climate and tectonics) and autogenic factors (switching, abandonment and/or avulsion of channels; Batezelli, 2017;Catuneanu, 2006;Shanley & Mccabe, 1994).Relative sealevel change is also included as a controlling factor in the deposition of fluvial-lacustrine successions in areas near coastal marine settings (Currie, 1997;Schumm, 1993).Blum and Törnqvist (2000) and Catuneanu (2006) reported that base-level fall (concomitant with low accommodation space during sea-level fall) is associated with the deposition of braided gravel deposits.Conversely, the base-level rise (concomitant with high accommodation space during sea-level rise) is associated with deposition of fine-grained floodplain and lacustrine deposits.A relative increase in the base-level rise is also associated with the deposition of coastally-induced point bars and estuarine deposits (Blum & Törnqvist, 2000;Catuneanu, 2006).Such vertical change in depositional styles also developed during climate fluctuations (Alqahtani et al., 2017;Blum & Törnqvist, 2000).In the present work, the sedimentary sequences of the Ash Shumaysi Formation could provide a good example to clarify the factors controlling deposition during pre and syn-rift sediments.
Deposits of the lower members are sand-rich (fully braided facies, FA-1, FA-2) reflecting a high sediment supply rate relative to a low accommodation creation rate (S > A), and mechanical weathering of elevated source rocks, which favours the production of sand over clay (Nystuen et al., 2014).This could be developed through tectonic pulses that led to an uplift of the Arabo-Nubian Shield (Abdelsalam & Stern, 1997) producing the source sediments for the fully braided facies (FA-1 and FA-2) of the lower member (Figures 11 and 12).The palaeocurrent measurements indicate a NNE flow of water, which documents the derivation of sediments from the southwest where the elevated Arabian-Nubian Shield occurs (Figures 11 and 12).Such conditions could be accompanied by a fall in sea level (base-level fall, Schumm, 1993) in association with arid to semi-arid climates (Alqahtani et al., 2017;Manna et al., 2021).The subsequent remarkable upward change in the fluvial architecture style from braided rivers (FA-1, FA-2) to point bar deposits (FA-3; Figure 12) could be associated with the relative rise of sea level (base-level rise, Schumm, 1993) and tectonic subsidence in the basin that was in response to tectonic pulses of the Arabo-Nubian Shield (Abdelsalam & Stern, 1997).In addition, these point bar deposits (FA-3) could have developed during humid to semi-humid conditions (Alqahtani et al., 2017;Manna et al., 2021), and low discharge that facilitates a change from coarse-grained, low-sinuosity braided to fine-grained, high-sinuosity streams (Alqahtani et al., 2017;Blum & Törnqvist, 2000;Schumm, 1993).The fine-grained lacustrine deposits of the middle member (FA-7) could have accumulated during aggradation of the base level after its retrogradation during deposition of its underlying point bar deposits (FA-3; Figure 12).This reflects a slow base-level fall relative to those of the underlying fluvial point bar.
In the upper member, the dominance of fine-grained lacustrine, overbank and upper estuarine facies (FA-4, FA-5, FA-6, FA-7 and FA-8) reflects an increase in the accommodation space/sediment supply (A > S) that could be developed when tectonic pulses lead to a change in the fluvial channel style as a small lake and estuary developed on the flood plain.In such a setting, the studied fine-grained lacustrine and floodplain facies (FA-4, FA-5, FA-6, FA-7 and FA-8) could have developed during semi-humid to semi-arid conditions (Alqahtani et al., 2017;Blum & Törnqvist, 2000).These deposits accumulated in a local-scale basin (half graben) of the Oligo-Miocene rift phase (Al-Shanti, 1966;Moore & Ar-Rehaili, 1989).Limited and local occurrence of marine-influenced (upper estuarine) deposits (FA-4) in the upper member of the Ash Shumaysi Formation could be a result of a small incursion of a narrow sea (Neo-Tethys) that prevailed during the Late Palaeocene to Oligo-Miocene in western Arabia (Haq et al., 1988).Therefore, it is possible to conclude that the interplay between tectonic forces and climate, in association with base-level changes, played an important in the deposition of the Ash Shumaysi Formation.

| PALAEOGEOGRAPHY AND DEPOSITIONAL MODEL
The vertical and lateral distribution of the identified facies combined with their depositional environments shows that the Shumaysi Formation preserves pre-rift and synrift sediments (Figure 11).The pre-rift sediments constitute the lower and middle members of the Ash Shumaysi Formation, whereas the sediments of the upper member point to the syn-rift phase.The pre-rift sediments are fully-fluvial deposits represented by coarse-grained sandstone channel bodies fed by proximal-distal braided fluvial channel deposits (FA-1, FA-2, FA-3) that dominate the south-west portion of the study area (Figure 11).These sediments were deposited in topographic lows adjacent to the high-relief Precambrian Arabian Shield and were probably formed after late Precambrian tectonic movements F I G U R E 1 1 A proposed depositional model for the Ash Shumaysi Formation in the study area.For symbols, see Figure 2. (Abdelsalam & Stern, 1997).The syn-rift sediments include fine-grained overbank and lacustrine deposits with 4, FA-5, FA-6, FA-7, FA-8), that characterise the upper member of the Ash Shumaysi Formation.In this manner, the Pre-Eocene-early Eocene (pre-rift) sediments (lower and middle members of the studied formation) accumulated during flowing NNE braided to meandering streams that discharged at some distance into a small lake (Figure 11).Subsequently, including during the Oligo-Miocene, the study area was influenced by Red Sea rifting, and localscale basins (half grabens) were developed (Figure 11).These local-scale basins (half grabens) were a result of the development of north-west regional faulting parallel to the Red Sea (Abou-Auf & Gheith, 1997).These local-scale basins (half grabens) were infilled with syn-rift continental siliciclastic, lacustrine and upper estuarine sediments (the upper member of the Ash Shumaysi Formation).This was also associated with increased volcanism, as evidenced by the dominance of volcanic tuffaceous sediments (FA-8) in the lacustrine deposits (Figure 2).Both pre-rift and syn-rift sediments were mainly derived from the south-western part of the study area, as their primary structures (crossbedding) reflect unidirectional (NNE) palaeocurrent.

| SEQUENCE STRATIGRAPHY
After successfully applying sequence stratigraphy to siliciclastic successions formed in marginal to shallow marine settings (Catuneanu, 2006), similar techniques were applied to siliciclastic successions in continental settings (Aitken & Flint, 1995;Batezelli, 2017;Currie, 1997;Martinsen et al., 1999;Miall, 2006;Scherer et al., 2015;Shanley & McCabe, 1994).In non-marine strata, parasequences, parasequence sets and key surfaces are not easy to identify.Nevertheless, sequences and systems tracts in non-marine strata can be identified according to changes in facies associations, architectural styles and the geometries of depositional channels that are in response to variations in the rate of accommodation creation and fluvial discharge (Allen et al., 2013;Currie, 1997;Martinsen et al., 1999).Non-marine depositional sequences can be subdivided internally into three systems tracts (lowstand, transgressive and highstand; Catuneanu, 2006;Shanley & McCabe, 1994), which are analogous to those of marine successions (Catuneanu, 2006).However, only a few authors have introduced other terms for non-marine (in particular fluvio-lacustrine) systems tracts (Currie, 1997;Martinsen et al., 1999).Thus, Martinsen et al. (1999) F I G U R E 1 2 Generalised stratigraphic column of the Ash Shumaysi Formation showing the changes of systems tracts, climate and base level during pre-rift and syn-rift phases in the study area.
suggested that in fully fluvio-lacustrine settings lying 65 km away from the coast (shoreline), the sequence is made up of only systems tracts (low-accommodation and high-accommodation systems tracts) depending on variations in available accommodation space/sediment supply (A/S) ratio.Currie (1997), however, pointed out that in fluvio-lacustrine settings lying up to 80 km from the coast, the sequence can be subdivided into three systems tracts (degradational, transitional and aggradational), which are analogous to the lowstand, transgressive and high stand systems tracts of marine depositional sequences, respectively.The systems tracts nomenclature of Martinsen et al. (1999) is the most appropriate for the Ash Shumaysi Formation because the studied fluvio-lacustrine system was developed near the coast as indicated by the occurrence of estuarine FA-4.
The estimated time (<150 m over about 20 Myr) to deposit the Ash Shumaysi Formation supports that it corresponds to a second order depositional sequence.This second order sequence comprises two third order (about 10 Myr) sequences (Sequences-I and -II) bounded by three sequence boundaries (SB).The sequence boundaries are placed where regional subaerial unconformities (major SB) and/or an abrupt change of facies architecture (minor SB) can be determined.Regarding the nomenclature of systems tracts (low-accommodation or LAST, and highaccommodation systems tracts or HAST) proposed by Martinsen et al. (1999), each sequence encloses HAST and LAST.Sequence-I includes the lower and middle members of the Ash Shumaysi Formation whereas Sequence-II is equivalent to the upper member of the Ash Shumaysi Formation (Figure 2).Discussion about how and why these sequences were developed is illustrated below.

| Sequence-I
Sequence-I includes the lower and middle members of the Ash Shumaysi Formation ranging from 80 to 120 m in thickness (Figure 2).It is bounded by two sequence boundaries.The lower sequence boundary (SB-1) is defined by an erosional surface (non-conformity surface) between the lower member of the Ash Shumaysi Formation and the underlying Precambrian rocks (Figure 1).The upper sequence boundary (SB-2) is distinguished by an abrupt change from lacustrine facies (FA-7) below and the overlying point bar facies (FA-3) of Sequence-II (Figure 2).Sequence-I encloses LAST and HAST.The LAST includes the lower and middle members of the Ash Shumaysi Formation whereas HAST is equivalent to the Upper member of the Ash Shumaysi Formation (Figure 2).The LAST represents the braided-distal fluvial facies (FA-1, FA-2) developed during the low accommodation space associated with high sediment supply (high discharge).This is followed upward by relatively fine-grained successions of the fluvial point bar (FA-3), and the overlying lacustrine facies (FA-7; Figure 2) included in the HAST.Such change from proximal-distal fluvial facies to point bar facies indicates a relative increase in accommodation creation associated with low sediment supply (low discharge) that in turn reflects a relative base-level rise.The boundary that occurs at the transition from the coarse-grained proximal-distal fluvial facies (FA-1, FA-2) to fine-grained point bar facies (FA-3) represents a change in style from a relatively regressive facies (proximal-distal fluvial facies) to a relatively transgressive facies (point bar facies).Therefore, it can be referred to a transgressive surface (TS).

| Sequence-II
Sequence-II includes the whole of the upper member of the Ash Shumaysi Formation, ranging in thickness from 50 to 80 m.It rests over Sequence 1, is bounded by two sequence boundaries and encloses LAST and HAST.The lower (SB-2) is amalgamated with the upper SB of sequence 1.This lower sequence boundary (SB-2) is marked by an abrupt facies change from lacustrine facies (FA-7) below, and fluvial (point bar) facies above (Figure 2).The upper sequence boundary (SB-3) lies between the volcanic-influenced lacustrine facies (FA-8) and the Harrat Basalt (Figure 2).In Sequence-II, the LAST is characterised by coarse-grained deposits of the fluvial point bar facies (FA-3) developed during a relative decrease in accommodation creation accompanied by a relatively high sediment supply.The HAST encloses fine-grained estuarine deposits (FA-4) and subsequent volcanic-influenced lacustrine facies .This change from fluvial point bar facies to estuarine facies indicates a relative increase in the rate of accommodation creation and low sediment supply that in turn reflects a relative base-level rise.The boundary that occurs at the transition from the point bar facies to fine-grained estuarine facies represents a change from regressive facies (point bar facies) to transgressive facies (estuarine facies).Therefore, it can be identified as a transgressive surface (TS).The topmost Sequence-II is marked by a sequence boundary (SB-3) that represents a base-level fall.This is followed by basaltic lava.

THIS STUDY
Recently, several studies have been conducted to discuss the influence of pre-rifting to syn-rifting on sedimentation at a regional scale (Ashley & Renaut, 2002;Bosence, 1998 and references therein).Good examples of these regionalscale basins include the Central Graben in the United Kingdom and Norway et al., 1992), the Gulf of Mexico in the United States (Seni, 1992), Whale-Horseshoe in Canada (Tankard et al., 1989), the Gulf of Suez in Egypt (Carr et al., 2003) and the Lower Congo in Angola (Valle et al., 2001).However, at the local-scale sub-basin, such influence could be more complex (Ashley & Renaut, 2002;Bosence, 1998 and references therein).The data presented in this study provide a valuable example with which to interpret the relationship between synchronous sedimentary processes with pre-rifting to syn-rifting in local-scale subbasins (half grabens).The present study shows a marked vertical facies change from fluvial facies (Pre-Eocene lower member) to lacustrine (Eocene middle member) and a mix of volcanic-influenced lacustrine -estuarine facies (Oligo-Miocene upper member).The study area during the prerift phase (pre-Eocene) was stable and characterised by low-accommodation space (relative sea-level fall) and high-sediment influx (lower member of the Ash Shumaysi Formation) from adjacent high-relief mountains (uplifted basement complex) via river systems.However, the siliciclastic sediments of lacustrine, estuarine and volcanic influenced lacustrine (upper members of the Ash Shumaysi Formation) were synchronous with syn-rift and post-rift phases that were accompanied by high accommodation space and a relative rise of sea level.This marked a vertical facies change from fluvial facies (lower member) to lacustrine (upper member) and could be closely related to a change in tectonics from pre-rift to syn-rift and post-rift phases.These results indicate that the pre-rift sediments in the local-scale sub-basins (half grabens) comprise continental sediments (fully fluvial), whereas in the syn-rift phase within the local-scale sub-basins, the volcanic-influenced lacustrine and estuarine are the dominant facies.Consequently, this study clarifies the influence of pre-rifting to synrifting on sedimentation at little studied local-scale basins that could be extrapolated to other situations worldwide.

| CONCLUSIONS
The Ash Shumaysi Formation in the Jeddah-Makkah district, west-central Saudi Arabia represents a good case study to clarify how depositional architectural styles and the sequence-stratigraphic framework of fluvio-lacustrine successions can be developed in areas affected by tectonism and climate.This is documented by changes in rates of accommodation space and sediment supply.Facies analysis of the Ash Shumaysi Formation allows seven depositional architectural styles to be defined: proximalbraid channel, distal-braided channel, point bar, estuarine channel, crevasse splay channel, floodplain and lacustrine.The proximal-distal braided fluvial channels are abundant in the south-west part of the study area and enclose coarse-grained sandstone bodies.In contrast, meandering fluvial channels (point bars), crevasse splays, as well as floodplain/lacustrine and upper estuarine deposits, are dominant in the north-east part of the study area and include relatively fine-grained deposits.The depositional style framework of the Ash Shumaysi Formation represents two depositional sequences separated by three sequence boundaries.Sequence-I includes the lower and middle members of the Ash Shumaysi Formation whereas Sequence-II is equivalent to the upper member of the Ash Shumaysi Formation.Internally, each depositional sequence encloses a HAST and LAST.The LAST represents the braided-distal fluvial facies, which refer to the low accommodation space associated with a high-sediment supply (high discharge).The HAST includes the relatively fine-grained facies (the fluvial point bar, estuarine and lacustrine facies).The HAST develops when there is relatively high accommodation creation associated with a low-sediment supply (low discharge).In a comparison with other nomenclatures of systems tracts (degradational, LST, transitional, TST and aggradational, AST systems tracts) provided by other authors in the fluvio-lacustrine successions, the LAST can be correlated with the LST.The HAST is equivalent to the TST and AST.
In terms of a broader implication, this study showed how the types of sediments change with tectonics (prerift to syn-rift and post-rifting) in local-scale sub-basins (half grabens), an area that has received little attention previously.In general, this study declared that fully fluvial sediments dominate during pre-rifting, whereas lacustrine and estuarine sediments, together with intermittent volcaniclastics are mainly developed during the syn-rift to post-rift phase.

ACKNO WLE DGE MENTS
We would like to thank my colleagues (Profs.El-Sawy K. El-Sawy; Amara Masrouhi and Abdelhamid El-Fakharani) at the Faculty of Earth Sciences (King Abdul-Aziz University, Saudi Arabia) for their kind help during the first field trip.Reviewers and editors are also thanked for their valuable comments and editing that improved the manuscript.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.

F
Correlation chart of the studied stratigraphic sections and their sequence stratigraphic framework.F I G U R E 3 Field photographs showing: (A) the three members of the Ash Shumaysi Formation with their informal parts.(B) Nonconformable contact (see arrow) between the Ash Shumaysi Formation and its underlying weathered basement rocks.(C) The subdivision of the lower member into three informal parts.(D) The middle member that is made up of alternating beds of fine-grained sandstone, siltstone and claystone bracketed by oolitic ironstone beds.(E) The subdivision of the upper member into informal three parts.Description and interpretation of the lithofacies of the Ash Shumaysi Formation.
document the origin of these oolitic/pisolitic ironstones.

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I G U R E 7 Field photographs showing: (A, B) large-scale trough cross-stratified conglomeratic to pebbly sandstones, St, Section-1, (C, D) small-scale planar cross-stratified pebbly coarse-grained sandstones, Sp, Section-1, Section-2, respectively, (E) fining-upward depositional cycle starts by Sp, through Sm, followed upward by Sll, Section-2, (F) small-scale planar cross-bedded medium-grained sandstone, Sp, Section-2 and (G) a depositional cycle at Section-2 formed of a thick base of Sll, compared with that of depositional cycle in Figure7E.

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I G U R E 8 Field photographs of point bar facies associations (FA-3) showing: (A, B) massive medium to fine-grained sandstone interbedded with laminated siltstone in the basal part of the upper member that directly overlies oolitic ironstone, Section-1, (C, D) heavily rooted sandstones of the upper part of the lower Mb.Section-1, (E, F) heavily rooted sandstones in the middle part of the upper member, Section-1 and (G, H) heavily rooted sandstones in the middle part of the upper member, Section-3. lacustrine: