Supradetachment to rift basin transition recorded in continental to marine deposition; Paleogene Bandar Jissah Basin, NE Oman

A transition from supradetachment to rift basin signature is recorded in the ~1,500 m thick succession of continental to shallow marine conglomerates, mixed carbonate‐siliciclastic shallow marine sediments and carbonate ramp deposits preserved in the Bandar Jissah Basin, located southeast of Muscat in the Sultanate of Oman. During deposition, isostatically‐driven uplift rotated the underlying Banurama Detachment and basin fill ~45° before both were cut by the steep Wadi Kabir Fault as the basin progressed to a rift‐style bathymetry that controlled sedimentary facies belts and growth packages. The upper Paleocene to lower Eocene Jafnayn Formation was deposited in a supradetachment basin controlled by the Banurama Detachment. Alluvial fan conglomerates sourced from the Semail Ophiolite and the Saih Hatat window overlie the ophiolitic substrate and display sedimentary transport directions parallel to tectonic transport in the Banurama Detachment. The continental strata grade into braidplain, mouth bar, shoreface and carbonate ramp deposits. Subsequent detachment‐related folding of the basin during deposition of the Eocene Rusayl and lower Seeb formations marks the early transition towards a rift‐style basin setting. The folding, which caused drainage diversion and is affiliated with sedimentary growth packages, coincided with uplift‐isostasy as the Banurama Detachment was abandoned and the steeper Marina, Yiti Beach and Wadi Kabir faults were activated. The upper Seeb Formation records the late transition to rift‐style basin phase, with fault‐controlled sedimentary growth packages and facies distributions. A predominance of carbonates over siliciclastic sediments resulted from increasing near‐fault accommodation, complemented by reduced sedimentary input from upland catchments. Hence, facies distributions in the Bandar Jissah Basin reflect the progression from detachment to rift‐style tectonics, adding to the understanding of post‐orogenic extensional basin systems.

On a different note, broad isostatic uplift from detachment movements produce large sediment source areas and basin fill dominated by alluvial fan deposits resulting from extension-parallel (detachment-transverse) transport of sediments derived from within the basin system (Friedmann & Burbank, 1995;Oner & Dilek, 2011). Basin fill in many cases record a transgressive development from alluvial fans via braided streams to fan deltas and carbonate ramps (Massari & Neri, 1997), reflecting a setting of mixed shallow marine carbonate-siliciclastic depositional systems that may prevail in low-latitude areas with arid climatic conditions and elevated drainage catchments (e.g. rift shoulders or pre-rift orogens). The arid conditions favour ephemeral runoff from hinterland catchments, leading to deposition of continental to marginal marine coarse clastic sediments. Down depositional dip, the coarse clastic sediments grade into marine carbonates produced under favourable conditions, as highlighted in this study. Examples include the Miocene deposits of the Lorca Basin, Spain (Thrana & Talbot, 2006), the Miocene Suez Rift strata with recent analogues (Cross & Bosence, 2008;Cross, Purser, & Bosence, 1998;Friedman, 1988;Roberts & Murray, 1988), Upper Jurassic sediments in the Neuquén Basin (Spalletti, Franzese, Matheos, & Schwarz, 2000), the Carboniferous succession in the Billefjorden Trough on Svalbard (Braathen, Baelum, Maher, & Buckley, 2011;Smyrak-Sikora, Johannessen, Olaussen, Sandal, & Braathen, 2019) and Devonian deposits in the Canning Basin, Western Australia (Holmes & Christie-Blick, 1993).
This article is devoted to basin characteristics during the transition between different extensional basin styles. We demonstrate how sedimentation in the Paleogene Bandar Jissah Basin changed as the controlling mode of deformation evolved from detachment to high-angle extensional faulting (Figures 2 and 3). Our investigation shows that the early detachment-style basin fill was dominantly transgressive, with depositional environments spanning from alluvial fans to carbonate ramps. The transition to a rift-style basin system was recorded by mixed carbonate-siliclastic shallow marine deposits that occasionally experienced subaerial exposure. Eventually, the basin became truly carbonate-dominated as sediment sources in the footwall were cut-off or became exhausted.

| GEOLOGICAL SETTING
The Oman Mountains (Al-Hajar Mountains) features the world's most well-studied ophiolite complex (e.g. Rollinson, Searle, Abbasi, Al-Lazki, & Al Kindi, 2014). Inside the range, the Semail Ophiolite forms part of a nappe stack of Permian to Upper Cretaceous shelf-to deep-water rocks that was obducted onto the Arabian Neo-Tethys margin during the Late Cretaceous (e.g Cooper, Ali, & Searle, 2014;Glennie et al., 1973;Glennie et al., 1974;Lippard, Shelton, & Gass, 1986;Searle, 2007;Searle, Warren, Waters, & Parrish, 2004). Subsequent extensional collapse of this orogen is evidenced by the Jebel Akhdar and Saih Hatat tectonic windows. Eclogite facies rocks that were exhumed from depths exceeding 30 km in the Late Cretaceous currently outcrop in the Saih Hatat window/metamorphic core complex (Figure 2a; e.g. Lippard, 1983). Sediments were shed to surrounding areas and alluvial fan conglomerates of the Al Khawd and Qahlah formations developed directly on the Semail Ophiolite northeast of the orogen in the Late Campanian-Maastrichtian (Mann, Hanna, & Nolan, 1990;Nolan, Skelton, Clissold, & Smewing, 1990). Exhumation of Saih Hatat is recorded by the reverse stratigraphy of Al Khawd and Qahlah Formation conglomerates, where clasts derived from structurally highest nappes were deposited lowest in the post-obduction stratigraphy (Abbasi, Salad Hersi, & Al-Harthy, 2014;Nolan et al., 1990). Several periods of extension have been suggested to have followed ophiolite obduction based on field data from Upper Cretaceous to lower Eocene sedimentary growth packages and interpretation of steep faults in post-obduction slope sediments offshore northern Oman (Fournier, Lepvrier, Razin, & Jolivet, 2006;Mann et al., 1990;Mattern & Scharf, 2018;Ricateau & Riche, 1980;White & Ross, 1979). Extensional faulting controlled post-obduction deposition and led to rapid lateral thickness and facies variations in Upper Cretaceous to Eocene strata Mann et al., 1990;Nolan et al., 1990). Fournier et al. (2006) suggested that extensional faulting persisted until the early Eocene, when deposition of the Jafnayn Formation was affected by syn-sedimentary normal faults. A major regional unconformity separates Upper Cretaceous from Paleocene strata. Paleocene to lower Eocene strata (Jafnayn and Rusayl formations) thin and onlap towards Saih Hatat, indicating its topographic prominence and role as a sediment source area (Searle, 2007 Métour et al. (1992). Stereoplots for the Wadi Kabir, Marina and Yiti Beach faults display fault planes and slickenlines. Banurama Detachment stereoplot shows fault planes with slickenlines. Ruwi-Yiti-Yenkit shear zone stereoplot displays foliations and lineations (contoured poles to lines). Stereoplot for the Qantab subbasin monocline displays contoured poles to bedding planes. Satellite photo courtesy of Bing/Microsoft. (c) Geological map of the study area by Le Métour et al. (1992) Nolan et al., 1990). Hence, uplift of the Al Hajar Mountains to their current elevation (highest peak is Jebel Shams, 3,000 m.a.s.l.) took place during or after the late Eocene. The timing and cause for the uplift is debated; Miocene (Saddiqi, Michard, Goffe, Poupeau, & Oberhänsli, 2006), Oligocene (Gray, Kohn, Gregory, & Raza, 2006;Mount, Crawford, & Bergman, 1998;Würsten et al., 1991) or late Eocene to middle Miocene (Hansman et al., 2017) uplift have been suggested. Suggested causes for the uplift include far-field stresses from the Zagros collision (Ali & Watts, 2009;Fournier et al., 2006;Glennie et al., 1974;Nolan et al., 1990;Searle & Ali, 2009) or crustal thickening following a retardation of Makran subduction causing north Oman to accommodate Arabia-Eurasia convergence (Hansman et al., 2017). A complementary view is that postobduction extension of the Semail Ophiolite lasted throughout the Eocene (Braathen & Osmundsen, 2020). Another phase of extension that started in the Oligocene has been suggested by Fournier et al. (2006). The following brief review based in literature is complicated by the tectonic picture; growth basins around the Saih Hatat culmination may differ significantly in terms of sedimentary facies distributions although they are the results of the same tectonic event(s). This hampers regional stratigraphic correlations.

Stübner
The Paleocene to Eocene sedimentary succession in northeastern Oman consists of the Jafnayn, Rusayl and Seeb formations. They are all dominated by carbonates formed in a shallow marine environment, but they vary in terms of depositional subenvironments, fossil fauna and siliciclastic content (Figure 3). The characteristics of the late Paleocene Jafnayn Formation vary between localities in terms of thickness, amount of terrigenous debris and substrate. In the Bandar Jissah Basin, Jafnayn Formation conglomerates are deposited directly onto the Semail Ophiolite  (Fournier et al., 2006;Le Métour, Béchennec, Roger, & Wyns, 1992;Mann et al., 1990;Nolan et al., 1990;Özcan et al., 2016;Racey, 1995). The Jafnayn Formation records a regional transgression event during the late Paleocene and consists primarily of shallow-shelf wackestones to grainstones. Larger benthic foraminifera such as Orbitolites, miliolids and Alveolina, together with coral fragments, mixed carbonate-siliciclastic sandstones and conglomerate interbeds, reflect variable energy and water depth on the shelf (Haynes, Racey, & Whittaker, 2010;Nolan et al., 1990;Özcan et al., 2016;Racey, 1995).
The Seeb Formation consists of nodular foraminiferal wackestones to grainstones with a varied fossil assemblage that indicate energy variations in a carbonate ramp setting with an overall transgressive trend (Beavington-Penney et al., 2006;Nolan et al., 1990;Racey, 1995). In the lower part of the Seeb Formation the microfauna is dominated by Alveolina and miliolids, while the upper part display a predominance of Nummulites and Assilina at the type locality . Bio-retexturing is generally complete although occasional storm beds and preserved hummocky cross-stratification suggest the carbonate ramp was wave-affected (Beavington-Penney et al., 2006). Some karstification and paleosol development in the Seeb Formation reflect intermittent subaerial exposure (Dill et al., 2007).
The study area is located between Yiti Beach and Wadi Al Kabir, SE of Muscat in the Sultanate of Oman ( Figure 2). The area was mapped by Le Métour et al. (1992) (Figure 2c) and included in studies by Racey (1995), Searle et al. (2004), Fournier et al. (2006) and Haynes et al. (2010).
In its current configuration, the southern margin of the Bandar Jissah Basin is bounded by three faults: The NW striking Wadi Kabir Fault, the Marina Fault striking WSW, and the Yiti Beach Fault striking approximately W (Figures 2b  and 3a). Paleocene to Eocene strata are preserved in the hanging walls of these faults. Towards the northwest, the footwall of the Wadi Kabir Fault contains an outlier klippe of moderately SW-dipping Paleogene strata ( Figure 2). This klippe is bound underneath by the sub-horizontal Banurama Detachment, which separates it from underlying north-dipping Triassic low-grade metamorphic carbonates, henceforth termed marbles for simplicity ( Figure 4a; Braathen & Osmundsen, 2020). In the northernmost part of the outlier, Eocene strata rests unconformably on the ophiolite over the Banurama Detachment. The Wadi Kabir Fault truncates and offsets this detachment down to the NE (Figure 4d). Hence, the Bandar Jissah Basin with its depositional substrate sits in an allochthonous position, which is cut and offset by the younger and steeper Wadi Kabir Fault. Together with the Marina and Yiti Beach faults, the Wadi Kabir Fault represents faulting that post-date the Banurama Detachment. The basin fill consequently records two different settings: An initial basin setting controlled by the detachment and a later setting controlled by the steeper faults.
We subdivide the Bandar Jissah Basin into the informally named Qantab and Yiti Beach subbasins. The former is located NW of the Marina Fault towards Muscat and the latter occupies a position between the Marina and Yiti Beach faults (Figures 2 and 3). Qantab subbasin strata onlap ophiolitic rocks above a 5-10 m high paleo-relief. Footwall rocks to the south consists of Triassic to Jurassic low-grade carbonates (marbles).
Contractional inversion has been proposed for the basin (Fournier et al., 2006). However, we observe mostly extensional structures and see no evidence for syn-contractional deposition. Accordingly, we will not discuss contraction or inversion structures in this work.

| METHODS
Conventional fieldwork was carried out over a period of four weeks in January and December 2017, measuring sedimentological sections and collecting structural data (Figures 2, 3 and 5). The dataset includes a large collection of photographs including high-resolution photomosaics suitable for analysis of depositional architecture of km-scale outcrops. Structural measurements were plotted using OpenStereo software (Grohmann & Campanha, 2010). The basin stratigraphy is divided into facies based on depositional processes (Table 1). Carbonate-dominated facies are classified according to Dunham (1962) and Embry and Klovan (1971). Facies associations define depositional environments (Figures 6  and 7). 25 thin sections were made from collected rock samples to determine their ages and depositional sub-environments ( Figure 8). 4.2, 7; Table 1; Figures 6 and 7a). Conglomerate beds display normal, inverse, normal-to-inverse or no grading. Bases of conglomerate beds vary from strongly erosive (typical for normal graded beds) to non-erosive (typical for inverse graded or ungraded beds). Clast sorting and internal organization/clast fabric of individual conglomerate beds range from unsorted and disorganized to better sorting with well-developed clast fabrics, the latter more typical for normal-graded conglomerate beds and with a higher occurrence frequency towards northeast. Similarly, silty sandstone (Facies 7) deposits that drape conglomerate beds and fill scour/channel features are increasingly preserved towards the northeastern part of the study area.

| RESULTS AND
Interpretation FA A represents a spectrum of alluvial fan to braidplain deposits characterized by high relief and significant discharge events. Flow types range from cohesive mass flows, seen as coarse, disorganized, ungraded or inverse graded beds suggesting steep gradients and high discharge, to fully turbulent streamflow as indicated by strongly erosive, normal graded beds with welldeveloped internal structure (e.g. Talling, Masson, Sumner, & Malgesini, 2012;Zavala, Arcuri, Di Meglio, Diaz, & Contreras, 2011). These unconfined mass flow conglomerates and scouring braided stream conglomerates represent a proximal alluvial fan depositional setting. Furthermore, very coarse deposits and immature flow types in the sedimentary record suggest proximity to a high-relief source area. The limited thickness and lateral persistence of paleosols (Facies 7) in the proximal alluvial fan relate to (a) frequent blanketing by unconfined debris flows that inhibits soil development and (b) rapid avulsions in braided river systems, eroding into paleosols (Facies 7). The preservation potential of paleosols (Facies 7) increase from proximal alluvial fan to distal alluvial fan and braidplain, where the depositional gradient was lower and flows were more turbulent. Turbulent flows scoured into the substrate and kept channel belts entrenched with F I G U R E 4 The Banurama Detachment and relationship with the Bandar Jissah Basin. The approximate extents of the photos are shown in Figure 2. (a) The 10-30 m thick Banurama Detachment separates NE dipping Triassic marbles from SW dipping Paleogene sediments (Jafnayn Formation). Tectonic transport in the Banurama Detachment is top-to-NNE. (b) The boundary between the Banurama Detachment and the Paleogene basin in its hanging wall. See (a) for location of photo. (c) Details of fault rocks at the boundary between the Banurama Detachment and the Paleogene hanging wall basin. The boundary is constituted by several distinct rock units: 1) Sheared out marls and disintegrated limestone beds in the highly sheared basal part of the sedimentary succession in the proximal hanging wall of the detachment, 2) mixed layer of disintegrated hanging wall sediments and clasts of underlying fault breccias, 3) tectonic breccias of serpentine cataclasites cemented by white magnesite, 4) cataclasite to phyllonite in semi-brittle shear zone partly comprising talcserpentine fabric, 5) carbonate and serpentine breccias superimposed on serpentine cataclasites with remnant clasts of ultramafic rocks from the ophiolite in the footwall of the detachment. Units 1) and 2) in particular display low-angle down-to-NE shear zones. (d) The Wadi Kabir Fault offsets the Banurama Detachment with its Paleogene hanging wall basin down-to-the-NE approximately 500 m (Braathen & Osmundsen, 2020). Note location of photo (a)   less possibilities for significant avulsions as compared to more proximal parts of the alluvial fan. This allowed for development of thicker and more laterally extensive paleosols.

| FA B
Description FA B consists of Facies 4.4 and 6, differing in grain size but both displaying a high textural maturity and well-developed parallel laminations and low-angle cross-stratification (Table 1; Figures 6 and 7b). Facies 4.4 consists of parallel bedded to low-angle cross-stratified well-rounded and sorted fine quartz gravel. It overlies grainstones to wackestones (Facies 8) and is overlain by bioturbated sandstones (Facies 6). Facies 6 consists of very fine to coarse sand displaying sedimentary structures such as parallel lamination and ripple-to dune-scale cross-stratification that reflect a variety of oscillatory, bidirectional and unidirectional current regimes. Facies 6 sandstones have a variable content of skeletal fragments and Ophiomorpha trace fossils.

Interpretation
We interpret FA B as beach deposits with significant differences in grain size and identifiable structures; Facies 4.4 represent a gravel beach deposit on the basis of its sedimentary structures, unusually high textural maturity and stratigraphic context. Parallel lamination in Facies 6 sandstones is indicative of upper flow regime typical for the foreshore/ swash zone (Clifton, Hunter, & Phillips, 1971). We are not able to support this interpretation with observations of marine fauna. However, we note that the preservation potential for body fossils in such a depositional environment is inherently low.

Interpretation
FA C represents conglomeratic mouth bar/delta front deposits in a carbonate-producing marine basin. Shallow water is indicated by modest foreset heights (e.g. Patruno, Hampson, & Jackson, 2015), but a lack of well-preserved topsets prevents further quantification of paleowater depth. Coarse siliciclastic conglomerates deposited from high-density turbidity currents and subaqueous mass flows establish FA C as a marine equivalent of FA A; the marine affiliation is suggested by the grainstones to wackestones (Facies 8) matrix of conglomerates, Ophiomorpha trace fossils and a bimodal current regime reflected in the sedimentary structures. Interbedded conglomeratic and sandy clinothems indicate flow separation as different flows (streamflows, debris flows) met standing water (e.g. Bhattacharya, 2006). The subsequent decrease of viscosity and sediment concentration led to increased runout and deposition of tangential foresets. Preservation potential for FA C relates to fluvial discharge and the nearshore energy regime. During periods of low fluvial discharge, carbonate production was active and fine-grained siliciclastic deposits were reworked by wave-and tidal currents.

| FA D
Description FA D consists of relatively fine-grained sediments (Facies 6 and 8) with the exceptions of the rudstones and boundstones of Facies 10 and 11, respectively (

Interpretation
FA D represents a shoreface environment in which the sediment was affected by waves and tidal currents. Further differentiation can be inferred from the specific sedimentary structures observed; hummocky cross-stratification is representative for offshore transition/lower shoreface whereas low-angle-and trough cross-stratification and asymmetrical ripples suggest an upper shoreface setting (e.g. Clifton, 2006). The rate of carbonate to siliciclastic grains relate to fluvial discharge and position with regards to shoreline. The increased karstification towards the southwest suggests a position near the shoreline, which suggests a strong influence of relative sea level fluctuations.

| FA E
Description FA E consists of graded conglomerate -sandstone (Table 1; Facies 5), grainstones to wackestones rich in benthic foraminifera and calcareous algae (Facies 8), rudstone (Facies 10), boundstone (Facies 11) and silty sandstone (Facies 7; Figures 6 and 7e). Bioturbation has destroyed most primary sedimentary structures to the point where even bedding planes are vague and difficult, if not impossible, to correlate through the basin (Figure 7e). Isolated pockets of less bioturbated FA E rocks display parallel lamination, lowangle cross-stratification and trough cross-stratification. Facies 8 deposits display foresets with heights up to 2 m at one locality (Log 9). Rudstone beds consist of gastropods, coral fragments, larger foraminifera and thick, broken oyster tests. Rudstone and coral boundstone beds are generally thin (<1 m) and localized. An exception of this is a tens of meters thick coral aggregate near the Yiti Beach Fault (Figure 9c). Foraminiferal content varies but individual beds or bedsets tend to be dominated by similar species. FA E is frequently karstified (dm-scale) in the southwest, where karstified surfaces are often observed together with paleosols (Facies 7). Paleosols (Facies 7) are also observed occasionally towards the northeast (log 20).

Interpretation
We interpret FA E as the shallow part of a carbonate ramp on the basis of the complete bioturbation, proximal position with regards to the shoreline suggested by siliciclastic content and karstification, scattered nature of coral reefs and reef mounds, few abrupt lateral facies changes and migrating, possibly wave-breaking bars. Reefs and mounds indicate periods of high carbonate productivity, little siliciclastic sedimentary input and/or an elevated bathymetric position (e.g. uplifted footwall high) that favour carbonate production while inhibiting siliciclastic input (e.g. Cross & Bosence, 2008;Dorobek, 2008). The carbonate ramp was storm-affected, as suggested by grainstones (Facies 8) interbedded with tempestites (Facies 5 normal graded conglomerate -sandstone/ grainstone). Bathymetric variations on the carbonate ramp can also result from shoreward sediment transport during storms, resulting in deposition of barrier complexes. These positive bathymetric features favoured carbonate production/ reef development. The shallow water associated with these features brought about a sensitivity to sea-level variations; paleosols (Facies 7) developed on the barrier complexes during periods of low relative sea level. Rudstones with thicktested oysters indicate an agitated depositional environment related to wave-action in the barrier complex (Racey, 1995). Occurrence of coral fragments in grainstones indicates proximity to reef mounds. The limited continuity and thickness of rudstones (Facies 10) and boundstones (Facies 11) favour the interpretation of a sediment-driven barrier complex over a tectonically induced high bathymetric position. Foraminiferal content indicates a variety of water depths (shallower -Alveolina, deeper -Nummulites), energy regimes (miliolids -low energy), environmental stress (low vs. high-diversity fauna) and vegetation on the seabed (Orbitolites indicate vegetation; Figure 8). The nearly complete bio-retexturing of the sediment has previously been ascribed to seagrass roots and rhizomes, annelid seaworms and other burrowing organisms such as echinoids (Beavington-Penney et al., 2006). Karstification in the southwestern area might relate to tectonically driven accommodation adjustments (slip events in basin-controlling faults).

| FA F
Description FA F consists of foraminiferal grainstones to wackestones (Facies 8), marl (Facies 9), rudstone (Facies 10) and boundstone (Facies 11; Table 1; Figures 6 and 7f). Generally, the sediment lacks primary sedimentary structures due to extensive bioturbation, however plane-parallel lamination, lowangle-and trough cross-stratification is preserved in places. The grainstones are dominated by larger foraminifera, with guest appearances by gastropods, echinoids, oysters and coral fragments and contain a variable proportion of siliciclastic grains. The body fossils described above are either encased in a foraminiferal grainstone/packstone/wackestone matrix or concentrated in rudstones. Beige to dark red marl (Facies 9) is littered with gastropods and undifferentiated shell fragments and displays distinct Ophiomorpha burrowing.

Interpretation
We interpret FA F as lagoonal deposits, particularly because of the combination between rudstones and lagoonal 558 | marls, which has previously been described for the Rusayl Formation by Racey (1995). Calm conditions prevailed with stormy interruptions resulting in trough cross-stratification of wackestones to grainstones. Upon first sight, Facies 9 marls are strikingly similar to Facies 7 paleosols, however, the fossil assemblage and Ophiomorpha trace fossils strongly indicate a marine origin. Occasional occurrences of algal mats indicate elevated salinity levels (e.g. Paerl, Pinckney, & Steppe, 2000).

| Basin structures
The Banurama Detachment and Wadi Kabir, Marina and Yiti Beach faults are fundamental structures in the Bandar Jissah Basin. The Wadi Kabir Fault is considered part of a regional range-front fault complex (Braathen & Osmundsen, 2020;Mattern & Scharf, 2018). Also of significance for this study is the Qantab subbasin monocline, which is refolded in a rollover fold in the hanging wall of the Marina Fault (Figures 2b  and 3d).

| Banurama Detachment
The Banurama Detachment is exposed near the southeastern end of the Wadi Al Kabir urban area, as a klippe in the footwall of the Wadi Kabir Fault and in a down-faulted block/ lens within this fault. It is inferred below the Paleogene basin fill in the hanging wall of the Wadi Kabir Fault (Figure 2). In the Wadi Kabir Fault footwall, the sub-horizontal Banurama Detachment separates steeply NE-dipping Triassic marbles in the footwall from 40-60° SW-dipping Paleogene strata (Jafnayn Fm) in the hanging wall ( Figure 4). These dips are comparable to dips of Paleogene strata in the juxtaposed Wadi Kabir Fault hanging wall basin (Figure 4d; Wessels, 2012). Furthermore, the Paleogene strata display a depositional contact with mildly sheared ophiolites on both sides of the Wadi Kabir Fault. The Banurama Detachment constitutes a 20-30 m thick section of fault rocks, which include serpentine-talc phyllonites and cataclasites hosting clasts of magnesite, all with ophiolite affinity, overlying mainly marble breccias of footwall affinity. Within this zone, which has a semi-brittle to brittle style, most primary rock characteristics are erased, with strain diminishing towards the margins. The fundamental shear boundary between rocks of ophiolite affinity (footwall) and deformed sediments (hanging wall), where exposed, is approximately 10 cm thick, overlain by a few meters of highly strained sediments. The shear zone shows extensive folding of partly intact rock coupled with cm to dm-wide shear zones hosting typical brittle fault rocks, many with signs of plastic deformation elements (particularly associated with serpentine-talc formation; Figure 4c). Many shear zones host slip surfaces (slickensides; Figure 4b).
There is consistent top-NNE (ca. 030°) tectonic transport in the Banurama Detachment and shear zones within the core complex as indicated by slickenlines and stretching lineations and reported by previous workers and in our data (see Ruwi-Yiti-Yenkit shear zone in Figure 2b; e.g. Braathen & Osmundsen, 2020;Jolivet, Goffé, Bousquet, Oberhänsli, & Michard, 1998;Warren & Miller, 2007). A minimum displacement of ~1,500 m on the Banurama Detachment is estimated by measuring the length of the outcrop in a direction parallel to tectonic transport (~750 m) and considering that the detachment cuts the bedding at 45°. Judging from the rotation of strata and thickness of the detachment, however, the displacement is likely significantly larger.

| Wadi Kabir Fault
The

| Marina fault
The footwall affinity. Strata in the immediate hanging wall dip toward NW and consist of footwall-derived clasts floating in a matrix of fossiliferous wackestones. The Marina Fault is associated with sedimentary growth packages in the upper Seeb Formation (Figure 9).

| Qantab subbasin monocline
A basin-scale ENE-WSW trending monocline is observed in the hanging wall of the Marina and Wadi Kabir faults. The informally named Qantab subbasin monocline displays a nonsystematic distribution of bedding orientations suggesting non-cylindrical folding (Figure 2b). The most prominent trend fits a moderately ESE-plunging axis, another trend a sub-horizontal ENE axis. Notably, the first trend is perpendicular to the kinematic axes of shear zones in the footwall core complex and the Banurama Detachment (Braathen & Osmundsen, 2020), conforming to an interpretation of the basin as a supradetachment half-graben. The distinct plunge of the ESE trending fold-axis can be compared to moderate bedding dips in the proximal hanging wall of the Marina Fault, consistent with rotation of the hanging wall block including the Qantab subbasin monocline during movements on this fault. A WSW-ENE Marina Fault trend parallels the less prominent ENE fold axis suggested by bedding data. The Qantab subbasin monocline is associated with sedimentary growth packages expanding towards the southeast, suggesting syn-depositional (Rusayl and Seeb formations) growth of the monocline (Figure 9).

| Yiti Beach fault
The

| TECTONOSTRATIGRAPHIC DEVELOPMENT
The Bandar Jissah Basin is characterized by a two-fold tectonic evolution reflected in lateral and vertical facies architecture variations of the basin fill. The Banurama Detachment controlled the initial basin development before it was rotated, deactivated and cut by the Wadi Kabir Fault as steep faults became active and generated rift-style subbasins. The timing of activity on the Banurama Detachment is given by the ages of strata that were rotated before being cut by the Wadi Kabir Fault. The upper Paleocene to lower Eocene Jafnayn Formation was deposited in a supradetachment basin controlled by the Banurama Detachment. Broad uplift-subsidence patterns following displacement in the Banurama Detachment gave a gradual proximal-to-distal facies transition from south to north. Facies distributions and stratal geometries in the Rusayl and Seeb formations, however, are more complex, reflecting the transition from a supradetachment basin to a rift-style basin system. Below, we subdivide the tectonic evolution of the Bandar Jissah Basin into three phases based on the structural chronology and spatio-temporal stratigraphic trends.

Formation -late Paleocene to early Eocene
The lowermost basin fill is characterized by a series of prograding coarse clastic wedges of the Jafnayn Formation deposited onto the ophiolitic substrate. Deposition of alluvial fan to braidplain deposits (FA A) was punctuated by rapid transgressions with deposition of beach (FA B), mouth bar (FA C), shoreface (FA D) and/or carbonate ramp (FA E) onto alluvial fans. The prograding wedges display a strong south-to-north proximaldistal trend, reflecting depositional gradient, shoreline proximity and depositional processes ( Figure 10). The proximal deposits are continental and consist of alluvial fan conglomerates grading downdip into coarse braidplain deposits (FA A; Figures 10 and 3, logs 1 and 10). A gradual transition from alluvial fan to braidplain deposits is reflected by (a) increasingly organized bedding, suggesting gradually more turbulent flow types, (b) increasing prevalence of fining-upwards conglomerate beds, typical for fluvial deposits and (c) thicker and more spatially extensive paleosols, reflecting a run-off pattern confined to channel belts, allowing paleosols to establish on the floodplains (Figure 3, logs 1 and 10). Braidplain deposits (FA A) grade into shallow marine deposits; conglomeratic mouth bars (FA C), beach sands (FA B), shoreface deposits (FA D) and ultimately carbonate ramp deposits with variable siliciclastic content (FA E; Figure 10). The shallow marine deposits vary according to shoreline morphology and position with respect to the shoreline and river mouths, demonstrating a dominance of fluvial, wave or tidal processes at different stratigraphic levels and positions both laterally along the paleo-shoreline and along the proximal-distal axis. Shoreline transgressions led to deposition of shallow-marine deposits (FA B, C, D, E) onto continental deposits. The lack of convincing evidence for extensive transgressive reworking during flooding suggests shoreline transgressions were rapid, reflecting high accommodation rates, possibly | 561 EAGE SERCK Et al.
related to slip events in faults controlling accommodation, likely the Banurama Detachment. The transgressive development from alluvial fans to fan delta, shoreface and carbonate ramp is similar to supradetachment basin characteristics in the Dolomites as documented by Massari and Neri (1997).
The only preserved Paleogene sediments in the footwall of the Wadi Kabir Fault are located NW of the Bandar Jissah Basin, and thus are not directly comparable. Nevertheless, we argue that observations from this Paleogene outlier are fundamental for understanding stratigraphic development in the Bandar Jissah Basin (Figure 2). The Banurama Detachment is described both at the base of the ophiolite in the footwall of the Wadi Kabir Fault and in a rider block along the Wadi Kabir Fault, suggesting the Wadi Kabir Fault post-dates the Banurama Detachment (Figure 2d). Moreover, similar rotation of Paleogene strata in both the hanging wall and footwall of the Wadi Kabir Fault indicate that the Banurama Detachment controlled basin morphology during deposition of the Jafnayn Formation (Figure 4d). Continued control of the Banurama Detachment on basin evolution is evidenced by sedimentary growth packages at higher stratigraphic levels (lower Seeb Formation) in the Qantab Subbasin monocline. We classify the Bandar Jissah Basin as a supradetachment basin during deposition of the Jafnayn Formation, which displays strong similarities with generalized supradetachment basin successions (Figure 1; Friedmann & Burbank, 1995): Firstly, sedimentary transport directions (NNE-NNW) in alluvial fan to braidplain conglomerates compare with tectonic transport directions in the Banurama Detachment (top-NNE) (Figures 2  and 3). Secondly, the spectrum of coarse subaerial debris flow to high-energy streamflood deposits in the Qantab subbasin indicates a high-relief source area in the south. This is consistent with large-magnitude footwall uplift (isostatic compensation) following displacement in the Banurama Detachment, and perhaps deeper detachments within the Saih Hatat window (Braathen & Osmundsen, 2020;Warren & Miller, 2007). A southerly sediment source also conforms to a Maastrichtian to Paleocene uplift of the Saih Hatat window, as documented by Hansman et al. (2017). Finally, continental deposits in extensional basins indicate limited accommodation near the controlling fault. This is typical for supradetachment basins where deposition of footwall-derived strata takes place in distal positions with respect to the fault (Friedmann & Burbank, 1995).
Steep dips (~40-60°) of originally (sub)horizontal Paleogene carbonates above the Banurama Detachment in the Wadi Kabir Fault footwall suggests that the detachment initiated as a steep fault before being rotated together with its hanging wall basin, likely as an isostatic response to faulting. Considerations around whether the studied supradetachment basin system can be classified as a breakaway basin or a ramp basin (sensu Vetti & Fossen, 2012) are difficult for several reasons; (a) limited exposure of the Banurama Detachment, constrained to a rider block and the footwall of the Wadi Kabir Fault, (b) no outcrops of the supradetachment basin further inland and (c) a lack of suitable sub-surface data offshore the study area. The tectonic contact between the Paleogene carbonates and Banurama Detachment, which initiated as a steep fault, might suggest that the sediments were deposited in a breakaway basin. However, we recognize that other mechanisms can have placed the Paleogene sediments in contact with the detachment (see e.g. Asti et al., 2019). Hence, a classification of the studied supradetachment basin system will be highly uncertain. Beach sands (FA B) between the two lowermost alluvial fan packages (FA A) in the Qantab subbasin ( Figure 5 -log 1; Supplementary Material) contain the calcareous algae Distichoplax biserialis, which constrains the age of the lowermost basin fill and onset of accommodation generation in the Bandar Jissah Basin to the late Paleocene to Eocene (Figure 8a; Denizot & Massieux, 1965;Dietrich, 1927;Pia, 1934). Distichoplax biserialis been have also been recorded in Jafnayn Formation deposits of the nearby Sunub Basin (Mattern & Bernecker, 2018). Accumulation of sediments in extensional basins around the Saih Hatat metamorphic core complex have been reported to commence in the Late Cretaceous (e.g. Abbasi et al., 2014;Braathen and Osmundsen, 2020;Mann et al., 1990;Nolan et al., 1990). Accordingly, we speculate that the lack of Upper Cretaceous sediments in the Bandar Jissah Basin is related to the position with regards to the Saih Hatat metamorphic core complex. Here, isostatic uplift following large-magnitude detachment faulting limited accommodation space near the detachment and progressively cannibalized earlier supradetachment basin sediments with each uplift episode.
Flooding, possibly driven by basin subsidence following slip events in the controlling detachment, led to carbonate ramp (FA E) deposition over the coarse siliciclastic wedges ( Figure 10). Wackestones to grainstones with fluctuating amounts of quartz grains, pebble horizons and conglomerate interbeds suggest that carbonate production persisted even with significant terrigenous input to the basin. We attribute this to the coarse nature of the siliciclastic input, which has previously been suggested to have limited detrimental effects on carbonate production (Cross & Bosence, 2008;Friedman, 1988). FA E limestones in the Jafnayn Formation are completely bio-retextured with a nodular appearance, weak to indiscernible bedding surfaces and only occasional and locally preserved primary sedimentary structure ( Figure 6). Bathymetric variations are evident from shallower-water facies (barrier system) in distal positions; wave-breaking gravel bars, oyster banks, and coral reefs (facies 5, 10, 11). Karst surfaces (facies 8) in log 9 constrain the extent of the Qantab subbasin on the carbonate ramp (Figure 5 -log 9, 3; Supplementary Material). Fault-driven local facies variations on the carbonate ramp present an alternative explanation for shallow-water facies and subaerial exposure of more distal parts of the ramp (e.g. Massari & Neri, 1997). Without evidence of such faults, however, we recognize that these shallow marine to continental facies likely result from shoreward transport of sediment during storms that affect the carbonate ramp. Sediments accumulate to form barrier complexes on which reefs may develop, but which are also sensitive to sea level falls. The modest lateral extent and thickness of bars, mounds and reefs (log 9) provide additional support for this interpretation.

Formation -early to middle Eocene
The boundary between the Jafnayn and Rusayl formations in the Qantab subbasin represents unconformable deposition of continental conglomerate (FA A) over the carbonate ramp (FA E), corresponding with other observations in the region (Figures 3 and 5 -log 21;Supplementary Material;Nolan et al., 1990;Özcan et al., 2016). The Rusayl Formation's basal alluvial fan grade upward into fan delta (FA C) and shoreface deposits (FA D) affected by tidal currents (Figure 7). The shoreface assemblage consists of alternating siliciclastic conglomerate and mixed carbonate-silicliclastic tidally influenced sandstone (Figure 7). Paleocurrent data from gravelly foresets indicate an overall sedimentary transport towards east, with some sandstone cross-sets indicating bidirectional ~N-S currents (Figure 3). The dominantly easterly sedimentary transport is perpendicular to the Qantab subbasin monocline, contrasting the northerly transport recorded in the Jafnayn Formation and emphasizing influence by initial monocline growth. The monocline was active during deposition of the Rusayl Formation and lower Seeb Formation. Together with associated growth packages, the monocline closely resembles structures and sedimentary architectures from the Suez rift, where syn-sedimentary fault-propagation folding was documented by Sharp, Gawthorpe, Underhill, and Gupta (2000). Here, however, we lack observations of blind faults within the monocline. Additionally, the most prominent trend obtained from bedding orientations in the monocline indicates an ESE-trending fold axis, which is perpendicular to tectonic fabrics on the Banurama Detachment ( Figure 2b). Hence, we suggest that monocline growth during deposition of the Rusayl and lower Seeb formations represents rollover folding related to the geometry of, and movement on, the underlying Banurama Detachment rather than being affiliated with the Marina Fault at this stage. We relate backstepping of the Rusayl Formation alluvial fan (FA A), as indicated by the upward grading into fan-delta (FA C) and shoreface deposits (FA D), to increasing accommodation rates east of the monocline (

| Late transition to rift-style basin phase: Seeb Formation -middle Eocene
The base of the Seeb Formation records a major flooding event that resulted in deposition of a thick ramp-type carbonate succession with scattered gravel beds (FA E) onto the more proximal Rusayl Formation 8,13,14,19;Supplementary Material). The south-to-north proximal-distal trend recorded in the Jafnayn Formation is readily identifiable also in the Seeb Formation; distal sections consist of relatively clean carbonates dominated by well-preserved larger benthic foraminifera (Alveolina, Nummulites) and lack evidence for Modifications to the overall northerly accommodation increase resulted from displacement, first on the Banurama Detachment with associated inception of the Qantab subbasin monocline and subsequently on the Marina Fault, fitting the suggested chronology of the monocline. Growth packages in the Qantab subbasin monocline in the lower Seeb Formation, display thickness increase and progradation of nummulitic limestone foresets towards the east (Figure 9a). This growth package documents the syn-sedimentary relevance of the Qantab subbasin monocline, which became active during deposition of the Rusayl Formation. Another growth package succession is recorded in the upper Seeb Formation, located in the immediate hanging wall of the Marina Fault, which display stratal expansion towards the inferred maximum displacement on this fault (Figure 9b). Displacement in the Marina Fault tilted hanging wall strata, resulting in the gentle east to south stratal dips observed throughout the Qantab subbasin (Figure 3). This agrees with the interpretation of both the ESE-plunge of the detachment-related fold axis and the sub-horizontal ENE trend that is parallel to the Marina Fault (Figure 2b). The reduction of siliciclastic material in the upper Seeb Formation, particularly in distal (northerly) positions, relate to the transition from supradetachment to rift-style setting, with (a) less footwall rebound and thus smaller and lower-relief footwall catchment areas and (b) increasing near-fault accommodation (Figure 1). The Marina Fault likely merged with or was cut by the Wadi Kabir Fault in the southern corner of the Qantab subbasin. No Paleogene strata are preserved in the Wadi Kabir Fault footwall in this position. Coarse shallow marine siliciclastic conglomerates (FA D) in the upper Seeb Formation, deposited in the proximal hanging wall near the junction between the two faults, suggest a relay zone between the two faults existed during deposition, feeding sediments into the basin. This relay zone promoted sediment transport from the footwall to the hanging wall basin and a structurally high position in the hanging wall conforms to deposition of hinterland-derived conglomerates in a shallow marine setting. Accordingly, we propose that both the Wadi Kabir Fault and Marina Fault were active at this time. This is supported by the component of sinistral oblique-slip documented for the Wadi Kabir Fault (Figure 2b).
We suggest that the deposits in the Yiti Beach subbasin belong to the Seeb Formation, contradicting Le Métour et al. (1992), who interpreted these sediments as part of the Jafnayn Formation (Figure 2c). We observe plentiful nummulitic limestones and coral reefs in the Yiti Beach subbasin that match well with previous descriptions of the Seeb Formation  and facies of the Seeb Formation in the Qantab subbasin. Displacement on the Yiti Beach Fault established the Yiti Beach subbasin as a half-graben basin where it controlled accommodation development during deposition of the upper Seeb Formation. Coral reefs grew on the Yiti Beach Fault surface to form large, vertically stacked reef complexes, suggesting increasing water depth in the hanging wall of the fault, likely driven by fault slip events (Figure 9c). This fault-generated bathymetry led to deposition in distinct facies belts in the Yiti Beach subbasin; in-situ reefs and reef debris grade basinward into skeletal wackestones. With insitu reefs and no observations of other exotic clasts in the hanging wall basin, we suggest the Yiti Beach Fault footwall was subaerially exposed without significant drainages delivering sediment across the fault scarp during deposition of the Seeb Formation in the Yiti Beach subbasin.
The inception and growth of the Wadi Kabir, Marina and Yiti Beach faults could reflect a changing stress-regime that triggered new faults unrelated to detachment tectonics, as proposed by Fournier et al. (2006). More likely, however, complying to crustal scale extension tectonic models as reviewed in Platt, Behr, and Cooper (2015) and Brun et al. (2018), the steep faulting in the upper-plate formed after abandonment of the rotated Banurama Detachment when a new detachment nucleated at deeper crustal levels ( Figure 1). This is similar to descriptions from the Dolomites (Massari & Neri, 1997) and the Sacramento Basin (Fedo & Miller, 1992), and conforms to the observation of the Banurama Detachment below the Jafnayn Formation resting on ophiolite in the footwall of the Wadi Kabir Fault (Figure 4c). Nevertheless, the general northerly accommodation increase linked with the Banurama Detachment became modified by displacement on the Marina and Yiti Beach faults, giving a rift-style topography/bathymetry filled by carbonate growth packages organized in well-defined facies belts (Figure 11). At this stage, the basin was barren of siliciclastic input, suggesting that slopes in continental uplands were tilted away from the basin. This would be expected by footwall rebound behind normal faults (e.g. Gawthorpe & Leeder, 2000). Oman from the Maastrichtian onward following collapse of the orogen that constituted the paleo-Oman Mountains (e.g. Abbasi et al., 2014;Braathen & Osmundsen, 2020). Significant siliciclastic input to these basins, indicating the topographic prominence of Saih Hatat, has been documented in the Late Cretaceous Al Khawd Formation, late Paleocene to early Eocene Jafnayn Formation and early to middle Eocene Rusayl Formation (Dill et al., 2007;Fournier et al., 2006;Mann et al., 1990;Nolan et al., 1990;Searle, 2007). It has been suggested that the Saih Hatat was submerged by middle Eocene times on the basis of lacking siliciclastic input and few evidence for subaerial exposure during deposition of the Seeb Formation (Hansman et al., 2017;Nolan et al., 1990). Stratigraphy in the Bandar Jissah Basin, however, records both a significant siliciclastic input and prolonged periods of subaerial exposure during deposition of the Seeb Formation (Figures 3 and 5 -logs 15 and 19; Supplementary Material). These observations suggest both active tectonics and presence of a sediment-producing hinterland during the middle Eocene. Our subdivision of the basin evolution into three phases (supradetachment basin, early transition and late transition to rift-style basin phase) reflect changes in basin configurations that impact sedimentary systems in the basin. Furthermore, the changing basin configuration alters the significance of uplands south of the study area as sediment sources. The early wave of coarse clastic sediments discharged into a shallow but broad marine basin became obstructed as the basin starts to roll over in a growth monocline, coinciding with regressive events that facilitate development of karst and paleosols. This potentially reflects the arrival of isostasy-driven uplift and deactivation of the Banurama Detachment. When significant accommodation again developed, clean carbonate growth sections were deposited proximal to steep normal faults in a configuration of fault blocks.

CURRENT OUTCROP PATTERN
The distribution of outcrops in ancient extensional basin systems may reflect characteristics of their controlling faults. In the hanging wall of the Wadi Kabir Fault, Paleogene sediments and the Semail Ophiolite outcrop, respectively, in synclines (Qantab and Al Bustan) and anticlines (area between Qantab and Al Bustan, Wadi al Kabir urban area) that are perpendicular to the fault. Such transverse structures are common in extensional basin systems (see e.g. Friedmann & Burbank, 1995;Schlische, 1995;Gawthorpe & Leeder, 2000;Kapp et al., 2008;Serck & Braathen, 2019). The transverse folds in the Bandar Jissah Basin and adjacent areas may be explained by both detachment and rift tectonics; either reflecting corrugations in the underlying Banurama Detachment or resulting from fault displacement variations along an initially segmented Wadi Kabir Fault. The transverse fold axes, however, are both roughly parallel with the kinematic axis of the Banurama Detachment, and perpendicular to the strike of the Wadi Kabir Fault (Figure 2b). Moreover, because of limited along-strike exposure of the Banurama Detachment and few indications of initial segmentation of the Wadi Kabir Fault, such as breached relay structures, the origin of the transverse folds remain elusive. We note, however, that transverse folds contribute to controlling the presentday outcrop pattern, and that the Bandar Jissah Basin was likely much wider than in its current configuration as it evolved as a supradetachment basin in the hanging wall of the Banurama Detachment from the late Paleocene to early Eocene.

| CONCLUSIONS
This study documents the tectonostratigraphic development of the Paleogene Bandar Jissah Basin, which occupied a position between the Late Cretaceous obduction orogeny and the Tethys Ocean. The Bandar Jissah Basin resulted from different modes of extensional tectonics, and the basin fill records both substantial siliciclastic input from external catchment areas as well as extensive carbonate production within the basin. We establish how faulting and fault-related folding controlled accommodation development and facies distribution during basin history, as follows: 1. The stratigraphy and structures mapped in the Bandar Jissah Basin document a transition from continental to marine depositional environments influenced by active faulting. Changes in sedimentary style and distribution reflect a transition from a supradetachment basin setting to a rift-style basin setting through three phases; 2. Supradetachment phase: The Bandar Jissah Basin initiated as a supradetachment basin in the late Paleocene, when a siginificant pulse of continental conglomerates mixed with shallow marine carbonate ramp deposits of the late Paleocene to early Eocene Jafnayn Formation were deposited onto ophiolitic rocks above the Banurama Detachment. 3. Early transition phase: During deposition of the early to middle Eocene Rusayl Formation and lower part of the middle Eocene Seeb Formation, the overall northerly increase in accommodation was modified by growth of the Qantab subbasin monocline, which was caused by rollover into the Banurama Detachment. Decreasing siliciclastic input during this time, together with regressive events that result in karstification and paleosol development, signal a re-configuration of hinterland drainages that likely relates to isostasy-driven uplift and abandonment of the Banurama Detachment. 4. Late transition to rift-style basin phase: Fault-related sedimentary growth packages and alignment of facies belts to fault strike suggest the steep Marina, Yiti Beach and Wadi Kabir faults controlled accommodation during deposition of the upper section of the Eocene Seeb Formation. The Wadi Kabir Fault cut and offset the Banurama Detachment and supradetachment basin down towards the northeast. 5. The evolution of basin style suggested herein agrees with models deduced from other supradetachment basins, where isostatic uplift following large-magnitude faulting caused abandonment of detachment faults. Subsequent extension manifests as steep faults that dissect the upper plate, altering drainage patterns and accommodation distribution. 6. Paleogene basins around Muscat were likely much larger than reflected in the current outcrop pattern. Transverse folds that either represent corrugations on the Banurama Detachment or result from displacement variations along an initially segmented Wadi Kabir Fault have modified the hanging wall geometry and ultimately affected which parts of the Bandar Jissah Basin were preserved and eroded.