Sedimentology and the facies architecture of the Ghaggar‐Hakra Formation, Barmer Basin, India: Implications for early Cretaceous deposition on the north‐western Indian Plate margin

Fluvial strata of the Lower Cretaceous Ghaggar‐Hakra Formation are exposed in fault blocks on the central‐eastern margin of the Barmer Basin, Rajasthan. The sedimentology of these outcrops are described from 114 logs (thicknesses up to 100 m) and 53 two‐dimensional correlation panels. The formation comprises three distinct channel belt sandstone packages defined as the Darjaniyon‐ki Dhani, Sarnoo and Nosar sandstones separated by thick siltstone‐dominated floodplain successions. The sediments were deposited in a sub‐tropical, low sinuosity fluvial system that matures into a highly sinuous fluvial system. The Nosar Sandstone, the youngest of the three packages, exhibits a significant increase in energy and erosive power compared to those underlying it. This distinct change in fluvial style is interpreted as being rejuvenation due to an actively developing rift network forming accommodation space, rather than climatic controls acting on part of the depositional system. Consequently, the Ghaggar‐Hakra Formation at outcrop represents Lower Cretaceous syn‐rift deposition within the Barmer Basin with active localized fault movement from Nosar Sandstone times onward. These findings provide sedimentological evidence in support of pre‐Palaeogene northwest–southeast extension in the Barmer Basin. Moreover, they imply Cretaceous extension took place widely along the northern extremity of the West Indian Rift System consistent with plate tectonic models of the break‐up of Gondwana and evolution of the Indian Ocean. Outcrops of Lower Cretaceous strata are patchy across India and Pakistan. This study provides valuable material which, when combined with the available published data, facilitates a re‐evaluation of Lower Cretaceous palaeogeography for the north‐west Indian Plate. The reconstruction demonstrates a complex fluvial system, where the sediments are preserved sporadically as early syn‐rift strata. The findings imply a high preservation potential for early Cretaceous fluvial successions within rifted fault blocks near Saraswati and Aishwarya of the Barmer Basin beneath the Palaeogene fill that likely have significant potential for further hydrocarbon exploration.


| INTRODUCTION
The Barmer Basin is the northern most basin of a series of rifts that comprise the West Indian Rift System (WIRS; Figure 1a), and hosts large hydrocarbon reserves within continental sediments of the basin fill . However, the early geological history of the basin, and the nature of the Mesozoic sedimentary fill, remains poorly understood. This lack of knowledge stems mostly from the limited surface exposure which restricts outcrop studies and thereby constrains interpretations of subsurface well and seismic data.
The Barmer Basin was considered to have developed in response to passage of the Indian Plate over the Reunion hotspot, giving rise to a syn-and post-rift Palaeogene to Eocene sedimentary fill, overlying Precambrian rocks of the Malani Igneous Suite (Figure 2a, Crawford & Compton, 1969) and pre-rift Mesozoic sediments (Compton, 2009;Sisodia & Singh, 2000). Recently, an important earlier northwest-southeast extensional event has been recognized, preserved in structural geometries exposed on the basin margins and overprinted by the perceived main Palaeogene rifting Dasgupta & Mukherjee, 2017). Fluvial sediments preserved on the rift margin are ascribed to the Lower Cretaceous Ghaggar-Hakra Formation and rift margin geometries suggest that they were deposited contemporaneously with the pre-Palaeogene normal faulting (Bladon, Burley, Clarke, & Beaumont, 2015;. Although the Ghaggar-Hakra Formation occurs in relatively small outcrops around Sarnoo, they are regionally significant as they provide excellent exposures of the early Cretaceous sediments which document intraplate continental deposition during the early break-up of India from Gondwana. It follows that the Ghaggar-Hakra Formation is likely to be the time and depositional-equivalent of the Lower Cretaceous fluvial Himatnagar Sandstone of the Cambay Basin (Bhatt, Solanki, Prakash, & Das, 2016;Mohan, 1995;Mukherjee, 1983), the fluvial to marine Nimar Sandstone of the Narmada Basin (Ahmad, 1988), the fluvial to coastal and Principal extensional faults that define the geometry and limits of the Barmer Basin (adapted from Dolson et al., 2015), displaying the field locations for this study on the eastern, central margin. The settlement of Sarnoo is shown deltaic Bhuj Formation of the Kachchh Basin (Akhtar & Ahmad, 1991;Biswas, 1987;Desai & Desai, 1989), the predominantly fluvial Dhrangadhra Group of the Than Basin in Saurasthra (Casshyap & Aslam, 1992) and the fluvial Lumshiwal and Goru and marine-coastal Sembar successions of the Central and Southern Indus basins of Pakistan (Ahmad, Fink, Sturrock, Mahmood, & Ibrahim, 2012;Ahmad & Khan, 2012;Khalid, Qayyum, & Yasin, 2014;Zaigham & Mallick, 2000). These basin margin early Cretaceous sediments are likely to be contemporary with rifting between Madagascar and India (Bastia, Reeves, Pundarika, D'Silva, & Radhakrishna, 2010;Reeves, 2014;Torsvik et al., 2000) and separation of the Seychelles microcontinent from Greater India (Eagles & Hoang, 2014). Therefore, the Ghaggar-Hakra Formation and these isolated occurrences of fluvial sediments are important in reconstructing the Lower Cretaceous depositional palaeogeography of the north-western Indian Plate. Identification and detailed characterization of a well-preserved fluvial succession of Lower Cretaceous age in the Barmer Basin not only has significant implications for the evolution of the basin, but also for potential reservoir distribution and hydrocarbon potential of early Cretaceous sediments in the subsurface across the northern Indian Plate.  (Compton, 2009;Dolson et al., 2015Sisodia & Singh, 2000Tabaei & Singh, 2002;Tripathi et al., 2009); (b) generalized vertical section of the Ghaggar-Hakra Formation, displaying the separate lithostratigraphically informal sandstone units recognized by previous authors (adapted from  Here, a comprehensive description and interpretation of the Lower Cretaceous Ghaggar-Hakra Formation exposed in the Sarnoo and adjacent hills close to the settlements of Sarnoo (also known as Sarnu or Saranu), Karentia and Nosar, Rajasthan (Figure 1b) is presented as a basis for reevaluating the early Cretaceous palaeogeography of the northern Indian Plate margin. Detailed sedimentary logging, combined with geometrical analysis of large-scale, twodimensional inclined exposures, is used to characterize and interpret the evolution of the fluvial succession. Variations in fluvial style and the controls upon them are discussed, along with the implications for the evolution of the Barmer Basin and its palaeogeographical setting in the early Cretaceous Period during the early separation of Indian Plate from Gondwana.

| GEOLOGICAL SETTING
The WIRS (Figure 1a) includes, from south to north, the Narmada, Cambay, Kachchh (Kutch), and Barmer basins (Akhtar & Ahmad, 1991;Biswas, 1987;Pandey, Fursich, & Sha, 2009;Rai, Singh, & Pandey, 2013). The formation of the WIRS was initiated in the mid to late Jurassic Period by the break-up of Gondwana as the eastern part, including the terranes of Greater India, Madagascar and Antarctica, separated from West Gondwana (Africa) in response to the development of the Mozambique and Somali proto-oceans west and north of Madagascar and India (Reeves, 2014;Reeves & De-Wit, 2000). Strike-slip reactivation of pre-existing Proterozoic structures (from tectonic inheritance: Misra & Mukherjee, 2015) in the Jurassic produced the Kachchh Basin (Biswas, 1982(Biswas, , 1999. By early Cretaceous times, development of the Mozambique and Somali proto-oceans gave way to sea floor spreading between the Greater Indian and Antarctic continents (Eagles & Hoang, 2014; fig. 3 of . Consequently, the northward movement and anticlockwise rotation of the Greater Indian continent occurred as break-up progressed (Chatterjee, Goswami, & Scotese, 2013;Storey et al., 1995;Torsvik et al., 2000) forming a series of interlinked failed rifts that constitute the WIRS. Although the Jaisalmer and Indus basins are well-known to have a late Gondwanan origin, the relationship between the Cretaceous sediments of the WIRS, the Jaisalmer and Indus basins remains obscure as it is patchily preserved across the northern margin of the Indian Plate below the Deccan Traps (Ahmad & Amad, 1991;Akhtar & Ahmad, 1991;Chowdhary, 1975;Jaitly & Ajane, 2013;Khosla et al., 2003;Misra & Mukherjee, 2015;Raju, 1968;Sharma, 2007;Sheth, 2007) and the geometry and extent of the associated rifts are poorly documented. That said, preservation of isolated fluvial Lower Cretaceous sediments clearly suggests the presence of an established rift system through the north-western Indian Plate prior to the onset of the main Deccan eruptions around 64 Ma in the Danian (Mukherjee, Misra, Calvès, & Nemčok, 2017). Regional palaeogeographical reconstructions (Biswas, 1999;Chatterjee et al., 2013) also include continental rift basins between India, Madagascar, and the Seychelles microcontinent, although the geometry of these rifts and their sedimentary fills is very poorly described (see Plummer & Belle, 1995).
Subsidence and sedimentation along the WIRS presumably continued contemporaneously with rifting of the Seychelles microcontinent and eruption of the Deccan volcanics at the Cretaceous-Palaeogene boundary~65 Ma (Collier et al., 2008;Eagles & Hoang, 2014;Ganerød et al., 2011;Plummer, Jospeph, & Samson, 1998). The exact causes of this extension remain equivocal, with many authors citing the migration of the western margin of India over the Reunion Plume (Morgan, 1971;Plummer & Belle, 1995;Simonetti, Bell, & Viladkar, 1995), even though the present-day Reunion Plume impinges on a remnant of continental crust (Torsvik et al., 2013).

| The Barmer Basin
Rifting in the Barmer Basin resulted from two distinct noncoaxial extensional events Dasgupta & Mukherjee, 2017). An earlier north-west to south-east extensional phase, oblique to the present orientation of the basin, was followed by dominantly Palaeogene north-east to south-west extension . Together, these two rift events produced an asymmetrical half-graben in the north and a more symmetrical graben in the south. Seismic data from the Barmer Basin indicate that pre-Deccan rifting is present along the full extent of the basin with stratigraphical thickening being pronounced on the present-day eastern margin .
Synrift strata are ascribed to the Palaeogene Mallinath Group and the early part of the Eocene Jagmal Group (Figure 2a). Earliest syn-rift strata of the Mallinath Group comprise acidic pyroclastics and basaltic lavas of the Raageshwari Volcanic Formation erupted predominantly in the centre of the basin (Compton, 2009), whilst contemporary alluvial fan sediments (the Dhandlawas Formation) were deposited along the western margin, adjacent to major fault scarps . From Maastrichtian to Thanetian times, braid-plain fluvial and alluvial systems of the Fatehgarh and Barmer Hill formations dominated with alluvial fan deposition (Jogmaya Formation) along the western faulted margin and flanking horsts within. Regional uplift removed the youngest Barmer Hill strata to produce a widespread unconformity across the basin before deposition resumed in Ypesian times (Jagmal Group). Lacustrine systems (Dharvi Dungar Formation) dominated during the early Jagmal Group giving way to swamp clastics of the Thumbli and Akli formations by Lutetian times exhibiting minor marine incursions.
Post-rift subsidence and in-fill began in Lutetian times with lacustrine sediments of the Nagarka Formation . A period of uplift coincided with collision of the Indian and Eurasian plates developed the regional base Miocene unconformity, before deposition of the fluvial and alluvial Neogene Jagadia and Uttarlia formations of the Rawal-Umed Group (Najman et al., 2018).

| The Ghaggar-Hakra Formation
In the Sarnoo Hills, a succession of fluvial sediments unconformably overlay volcanic rocks of the Karentia Volcanic Formation and, in places, the Malani Igneous Suite (Figure 2b; Baksi & Naskar, 1981;Compton, 2009;Mishra et al., 1993;Sisodia & Singh, 2000). The formation is~100 m thick at outcrop (Mishra et al., 1993) and comprises three distinct, quartz-rich, fluvial channel belt sandstone packages separated by~30 m of variegated white and red coloured, horizontally laminated and crosslaminated sands, with rhizoliths and soft-sediment deformation structures , attributed to deposition on a fluvial floodplain . The sandstone packages have been assigned informal lithostratigraphical status and named (in ascending order): the Darjaniyon-ki Dhani, Sarnoo and Nosar sandstones (Figure 2b;. The Darjaniyon-ki Dhani Sandstone comprises poorly sorted and clast-supported lithic arenites along with coarse sand to pebble-grade conglomerates attributed to deposition in an immature, braided system with a high sediment load . The Sarnoo Sandstone comprises cyclic, well-sorted, fine to mediumgrained, quartz arenites with regular cross-bedded sets (Sisodia & Singh, 2000) and deposited in a mobile meandering system . The Nosar Sandstone caps the formation at outcrop and comprises medium to coarse-grained sands and granule-grade conglomerates with cross-bedded sets and channel geometries with erosive bases, typical of deposition in an actively migrating braided fluvial system .

FIELD AREA
The Ghaggar-Hakra Formation is exposed in a series of three low-lying, fault-bounded hills separated by a peneplained surface covered in modern desert deposits near the village of Sarnoo. Progressive down-stepping of the faulted blocks towards the west and north provides excellent exposures that can be correlated providing continuous, composite vertical sections. Extensive quarrying provides continuous lateral exposure of the sediments in the scarp slopes of these hills. The dataset consists of 114 detailed sedimentary logs ( Figure 3) and 53 detailed two-dimensional panels providing correlation and geometries between the logs.

Formation
The details of lithofacies are given in Table 1 and Figure 3. Eight architectural elements (Table 2 and Figures 4-6) are identified and summarized below with the bounding surface terminology of Miall (1985Miall ( , 1988

| Channel element-Ch Description
These "U"-shaped elements (Table 2)-laterally up tõ 25 m and~7 m thick-are bound by sharp, sometimes erosional, convex-down fifth-order surfaces at their bases, and generally planar and concordant fourth-order surfaces at their tops. The basal fifth order bounding surface erodes other elements of this type (Ch), overbank (Ob), or channel margin (Cm) elements but the fourth order surface, where preserved, is gradational over approximately 2 m into overlying point bar (Pb) or overbank (Ob) elements. Typically, the full geometry is not preserved, eroded by younger channels (Ch), channel margins (Cm), gravel bars (Gb), point bars (Pb), or sheet flows (Sf; Figures 4a, 5a and 6a).
The fifth order surface is immediately overlain by the pebble-grade quartz clasts of grain-supported conglomerate (G) characterized by indistinct cross-bedding. The  Table 1, the arrows represent the palaeoflow orientation and logs are in metres conglomerate is typically overlain by cross-bedded sandstone (Stx, Sx) in sets (30 cm thick) and cosets (50-95 cm thick) bound by first-to third-order surfaces. The contact between the basal conglomerate and sandstone is sharp, generally with pebble-grade lenses within the toe-sets of the lowermost cross-bedded sets. Sporadically, convolute bedding, tree stumps and rip-up clasts are preserved, and packages of structureless sand (Sm) occur between sets of cross-bedded sandstone (Stx, Sx). Where preserved, the upper half of the element comprises horizontally laminated sandstone (Sh) and cross-laminated sandstone (Scl) in sets climbing at 5°. These sediments are overlain by mottled, bioturbated, very fine-grained pedogenic sands (Sp; Figure 6a).

Interpretation
These elements are interpreted as representing small-scale, erosively based, fluvial channels. The initial conglomeratic deposits overlying the channel base indicate high-energy flow with significant bedload transport. Pebble lenses are concentrated in areas marking the thalweg, indicating unsteady flow with localized periods of scour and fill (Froude, Alexander, Barclay, & Cole, 2017). Cross-bedded sandstone (Stx, Sx) sets record the development and migration of lower flow regime bedform trains and barforms, generally migrating towards the south-west. The irregular nature of the sets and the range of bounding surfaces (along with convolute bedding, tree stumps, and rip-up clasts where preserved) suggests flow variability and reactivation of bedforms and barforms.
Above this, the sediments (Scl, Sr, Sp) represent a decrease in energy as the flow wanes and ripple-scale bedforms develop in trains and migrate. Bioturbation and structureless sands below the fourth order surface most likely represents the final stages of sedimentation within shallow, largely stagnant waters, followed by pedogenesis.

| Channel margin element-Cm Description
These tabular to wedge-shaped elements-up to 5 m high and 10 m long-are bound by planar fifth-order surfaces at their bases and planar fourth-order surfaces at their tops (Table 2). Fifth-order surfaces are sharp and typically concordant with the underlying and overlying sediments. Typically, they merge laterally with fifth-order surfaces of

Interpretation
These elements are interpreted as representing the deposition of sediments on the channel margin as flooding occurs. Sets of cross-bedded sandstone (Sx) and cross-laminated sandstone (Scl) represent the development and migration of dune-and ripple-scale bedforms as the flow overtops the channel on to the floodplain and wanes (O'Conner, Jones, & Haluska, 2003;Wakefield, Hough, & Peatfield, 2015). The presence of horizontally laminated sandstone suggests periodic episodes of upper flow regime conditions as water moves from the restricted channel to an unrestricted floodplain (Brierley, Fergusin, & Woolfe, 1997). Mottled sands and silts with poorly preserved bedding possibly indicate destruction of primary depositional fabrics by vegetation.  Table 2). Fifth-order surfaces are laterally extensive and can transition laterally into fifthorder surfaces at the base of channel elements (Ch). Matrix-supported conglomerates (M) and/or clast-supported conglomerates (G) immediately overly the bases. Matrix-supported conglomerates (M) comprise rounded quartz clasts and extraformational lithics, supported in a matrix of very fine to fine-grained sandstone. Sporadically, indistinct foresets of cross-bedding are present. Clast-supported conglomerates (G) comprise poorly sorted but rounded pebbles of quartz and extraformational lithic clasts with sporadic and indistinct cross-beds.
The upper third comprises either massive sandstone (Sm) or horizontally laminated sandstone (Sh) facies (Figure 6c), exhibiting sharp contacts with the underlying sediments. The massive sandstone is moderately to poorly sorted with numerous quartz clasts. The clasts are typically distributed randomly, but sporadically form indistinct horizontal layers. The horizontally laminated sands are up to 9 mm thick but thin upwards. The full succession is rarely preserved and conglomerates (M, G) dominate.

Interpretation
Highly erosional fifth-order surfaces typically contiguous with those of channels and conglomeratic sediments indicate gravel bar deposition primarily by bedload transport under high-energy conditions (Froude et al., 2017). This deposition is likely to have occurred along the bases of developing channels with long axes of bars parallel to the channel. Grainsize is not conducive to producing bedforms, and a high sediment load promotes rapid deposition that further suppresses their development (Bridge & Best, 1988). Where sediment load is reduced, deposits grade from matrix to clast-supported with sporadic and poorly developed cross-bedding indicating some migrating bedforms (Blair & McPherson, 1994) and implying transient gravel bar movement (Figures 4c, 5c and 6c; cf Bridge, 1993). Massive and horizontally laminated sandstone represents deposition in shallow water on top of the developing barforms. Horizontally laminated sandstone was deposited under upper flow regime conditions (Ghazi & Mountney, 2009) when the gravel bars were fully submerged. Massive sandstone represents deposition when the top of the bar was at surface or slightly emergent, and stationary waters allow suspension fall out (Banham & Mountney, 2013;Jones & Rust, 1982).

| Chute channel element-Cc Description
These small, symmetrical "U"-shaped elements are up to 70 cm in height and no more than 2 m wide (Table 2). Their lower fifth-order bounding surfaces are convex-down and erosional into either point bar (Pb) or overbank (Ob) elements. Upper fourth-order surfaces are generally gradational into the overbank elements (Ob) over a thickness of 50 cm (Figures 4d, 5d and 6d).
Internally, the succession fines upwards from grain-supported conglomerates (G) composed of quartz clasts, through massive sands and silts (Sm, Im) containing quartz clasts up to 5 mm. This succession culminates in parallel-bedded sands and silts forming~50 cm thick packages with individual beds ranging between 10 and 15 cm thick and the horizontally laminated sandstone (Sh) with packages up to 80 mm with laminations up to 9 mm thick. Capping this element are bioturbated, rooted, pedogenically modified fine-grained sands and silts (Sp, Ip). Typically, the succession is fully preserved, but sporadically, granule-grade conglomerates (G) pass straight into pedogenically modified sands and silts (Sp/lp) producing a strongly bimodal grain-size profile.

Interpretation
The "U"-shaped geometries and their erosional lower bounding surfaces are indicative of small-scale channels. Conglomerates (G) consisting of locally reworked material immediately overlying fifth-order surfaces suggest initial deposition of sediments in a high-energy flow cutting channels (Bordy, Hancox, & Rubidge, 2004). Subsequently, flow waned rapidly to stagnant conditions with a high sediment load promoting deposition of massive sands and silts (Sm, Im; Martin & Turner, 1998). Where present, parallel-bedded and horizontally laminated sandstone (Sb, Sh) may suggest periods of extended and fluctuating upper flow regime conditions prior to rapid waning (Wakefield et al., 2015). In other examples, flow waned rapidly to stagnant water, depositing a strongly bimodal grainsize (Martin & Turner, 1998).
The strong spatial association between these elements and point bar elements (Pb), coupled with palaeocurrents that are generally perpendicular to the main channel system, suggests small-scale, high-energy chute channels formed during flood conditions (Ghinassi, 2010;Wakefield et al., 2015). During flooding, sediment load is high, increased discharge promotes "short-cutting" of the system across meanders, and energy levels wane rapidly to promote rapid and structureless deposition (Wakefield et al., 2015). Further evidence for this interpretation is provided by the presence of locally reworked quartz clasts most likely derived directly from the main channel system. Pedogenic sands and silts (Sp, Ip) represent bioturbation and reworking of later h F I G U R E 5 Continued stage sedimentation. The geometries of these elements suggest "low sinuosity" chute channels comparable to the "type 1" chute channel of Ghinassi (2010).

| Point bar element-Pb Description
These wedge-shaped elements (Table 2), up to 4 m thick and a maximum of 15 m wide, are bound by lower fifth-order surfaces that are sporadically slightly erosional but are generally concordant with underlying strata and typically extend laterally into the base of a channel (Ch). Upper fourth-order surfaces, where preserved, are concordant with sediments of overbank (Ob) and/or channel margin (Cm) elements. Typically, the upper parts of these elements are not preserved because of erosional down-cutting by chute channel (Cc), channel (Ch) or sheet flow (Sf) elements (Figures 4e, 5e and 6e).

Interpretation
Elements of this type are interpreted as representing the deposits of laterally accreting point bars. Multiple sets and cosets of cross-strata decreasing in size from dune to ripple-  (Ghazi & Mountney, 2009;Jackson, 1976), this, combined with the reduction of grainsize indicates a waning flow. The complex and varied geometry of sets and cosets and numerous bounding surfaces of varying scales suggest dominantly lateral but slightly downstream migrating barforms. Third-order surfaces suggest frequent reactivation and meniscate trace fossils towards the top suggest calm, possibly emergent, conditions with ripplelamination representing wash-over of the bar (Bridge, Alexander, Collier, Gawthorpe, & Jarvis, 1995). Fifth-order bounding surfaces grading laterally into the bases of channel elements (Ch) and the spatial association of these two elements demonstrate an evolutionary relationship between them suggesting attachment of the bar to the channel.

| Sheet flow element-Sf Description
These sheet-like tabular elements (Table 2), with a thickness of~0.5 m, are bound by lower fifth-order surfaces that are convex-down and erosive. Where the elements are fully preserved, fourth-order surfaces bound their tops and separate them from the overlying overbank (Ob) elements. However, the tops of the elements are typically marked by erosive surfaces at the bases of successive elements of sheet flow (Sf), or at the bases of channel (Ch), chute channel (Cc) or point bar (Pb) elements (Figures 4f, 5f and 6f).
Facies of these elements form an ordered succession. Beds up to 30 cm thick of parallel-bedded sandstone (Sb) fine and thin upward into beds of horizontally laminated sandstone (Sh). Overlying these are sets (10 cm) of rippled sandstone (Srha), climbing super-critically up to 30°, and stacked 30 cm cosets of cross-laminated sandstone (Scl) climbing sub-critically at 5°. Pedogenic sandstone and siltstone (Sp, Ip) display mottled textures, and contain fossil leaf imprints along with meniscate trace fossils typical of Taenidium or Beaconites (Gowland et al., 2018).

Interpretation
Deposition of subaqueous facies above fifth-order surfaces with sheet-like geometries indicates a largely unconfined flow (Blair, 2000). Upper flow regime conditions initially prevailed, depositing parallel-bedded sandstone and horizontally g h F I G U R E 6 Continued laminated sandstone (Sb, Sh). Waning occurred rapidly to lower flow regime conditions depositing rippled sandstones (Srha, Scl;Hjellbakk, 1997) indicating bedform development and migration. Variations in climbing angle suggest variations in sediment load, flow competency, and capacity (Blair, 2000;Hampton & Horton, 2007;Hunter, 1977aHunter, , 1977b. Pedogenic facies with high levels of bioturbation and abundant plant remains implies times of depositional quiescence (Bromley & Asgaard, 1979Buatois & Mangano, 2002).
The relationships between sheet flows, channels, and bars (Sf, Ch, Pb, and Gb) suggest that, whilst largely "unconfined" (Banham & Mountney, 2013), the sheet flows may be restricted to the lateral extent of active channel belts and most probably represent times of high discharge when channels were filled to capacity.

| Overbank element-Ob Description
These elements are dominantly tabular (Table 2), up to 1.5 m thick and 2 km in lateral extent and bound at their tops and bottoms by planar fourth-order surfaces except where tops are eroded by overlying channel (Ch) or gravel bar (Gb) elements. Lower boundaries can be sharp where overbank elements overlay gravel bars (Gb), or gradational where they overlay point bar or sheet flood (Pb, Sh) elements (Figures 4g, 5g and 6g).
The facies occur in any order, but when massive sandstone and siltstone (Sm, Im) and/or horizontally laminated sandstone (Sh) are present they typically overlay the fourth order surface, interbedded in an apparently random manner by individual sets of cross-laminated sandstone (Scl). When present, together these facies account for no more than 15% of the element with the remainder comprising pedogenic facies (Sp, Ip, Ihe). Pedogenic facies are dominantly red, and contain abundant granular peds (Retallack, 1988) between 2 and 3 cm long, along with grey to white patches of generally coarser grain. Abundant orange goethite-rich rhizoliths reach lengths of 15 cm (Figure 7), and carbonate-cemented rhizoliths reach lengths of 1 m (Figure 4g) along one particular horizon. All facies are remarkably fissile, and rare root structures and soils slickensides can be found throughout.

Interpretation
The physical dimensions and particularly the lateral extents indicate sediments of unconfined overbank floods. The facies indicate sub-aqueous upper (Sh Sm) and lower flow regime conditions with limited bedform development or migration (Scl) and occasional suspension settlement (Sm, lm). Facies displaying granular peds and extensive colour mottling indicate significant bioturbation (Sp, Ip, Ihe;Retallack, 1988;Tennvassås, 2018), subaerial exposure, and palaeosol formation (Retallack, 1988(Retallack, , 1990. Elements of this type typically overlie one another, with little to no erosion, and likely represent cumulative soil growth on a floodplain supplied regularly with sediment from flooding (Kraus, 1999;Kraus & Aslan, 1999). The rhizoliths indicate floodplain areas were imperfectly to poorly drained soils (Kraus & Hasiotis, 2006) suggesting they were seasonally wet (Retallack, 1990).

| Pond element-Po Description
These elements form lenticular bodies-up to 2 m thick and 50 m wide ( Table 2). The lower and upper boundaries are both fourth-order surfaces and this element grades into the overbank (Ob) element over~30 cm.

F I G U R E 7 Rhizoliths within the floodplain (Ob) sediments (a)
and (b) orange rhizoliths within the formation; (c) white reduction zones due to the organic material within the roots and rhizoliths, and; (d) the orange rhizoliths from Kraus and Hasiotis (2006), to allow for an easy comparison Facies do not form regular definable successions, but where present cross and horizontally laminated sandstones (Scl, Sh) are typically preserved near element bases, followed by massive fine-grained sandstone and siltstone (Sm, lm), with pedogenic (Ip) and haematitic siltstone (lhe) dominating.

Interpretation
Elements of this type are interpreted as small ponds of limited lateral extent developed on the overbank areas. Horizontally laminated sandstone, massive sandstone, and massive siltstone (Sh, Sm, lm) indicate that sediment deposition was dominated by suspension settlement. Sporadic cross-lamination (Scl) indicates development and migration of small ripple-scale bedforms when pond levels were recharged by overbank flooding. Wind shear on standing water combined with localized current turbulence formed symmetrical, asymmetrical, and interference ripple patterns (Sr; Wilson, 1993). The development of soils (Sp, Ip, lhe) as ponds dried-out obliterated many primary bedding structures.

| DEPOSITIONAL MODEL FOR THE GHAGGAR-HAKRA FORMATION
All the field observations support the presence of a fluvial system where the transport and deposition of sediment takes place within erosive channels by the development of in-channel barforms (Ch), accompanied by bedload transport (Gb). The fluvial channels are accompanied by laterally accreting bars (Pb) with chute channels (Cc), channel margin sediments (Cm), and sheet flows (Sf). The elements are arranged to form distinct "channel belt" depositional elements (terminology of Grotzinger et al., 2005;Posmentier & Kolla, 2003). Each belt is separated by "floodplain" depositional elements dominated by fine-grained sediments deposited in bodies of standing water (Po) or by unconfined flooding (Ob). Three channel belt depositional elements are recognized that correspond to the informal lithostratigraphical subdivisions of Bladon, Burley, et al.

F I G U R E 8
Facies model of the gravel bedload dominant low sinuosity fluvial system, the Darjaniyon-ki Dhani Sandstone, which contains the channel (Ch), gravel bar (Gb) and the overbank (Ob) architectural elements. There are 4th to 6th order bounding surfaces within. The sets and cosets within are inconsistent suggesting the gravel bars are transient, suggesting fluvial immaturity. The proportion of channels to floodplain is 90% to 10%, respectively (2015); : the Darjaniyon-ki Dhani, Sarnoo and Nosar sandstones. Conceptual facies models for these depositional elements are presented in Figures 8-10 along with a description of the key features and element relationships that define them. An interpretation of each is given below.
The sediments of the Sarnoo Sandstone are dominated by stacked and amalgamated transient gravel bars in the initial deposits indicating a significant degree of channel avulsion. Sinuosity and stability increase up-section to develop channels of stable flow with associated point bars and chute channels (Ielpi & Ghinassi, 2014; fig. 13 of Miall, 1985). However, discharge is still sufficiently irregular to cause sheet flows (Sf) and support ponding on the floodplain; overall this system represents a mixed load, high sinuosity fluvial system ( fig. 11 of Miall, 1985).
The Nosar Sandstone displays channels with some degree of stability but separated by transient bars. The system was influenced by avulsion and flooding (Best et al., 2003;Bridge, 1993). Sparse preservation of the overbank may indicate limited and patchy development, or poor preservation because of frequent fluvial avulsion. All features indicate a bedload-dominant, low sinuosity fluvial system ( fig. 9 of Miall, 1985).
Channel belt elements are separated by significant sections of floodplain sediments (Figure 2b), that formed through cumulative soil growth and were likely imperfectly to poorly drained due to being seasonally wet. Cyclicity suggests regular flooding which supplied the floodplains with new sediment and recharged ponds, probably a F I G U R E 9 Facies model of the mixed load high sinuosity fluvial system, the Sarnoo Sandstone, as evidenced by the channels (Ch), channel margin (Cm), gravel bars (Gb), chute channels (Cc) sheet flows (Sf), and overbank (Ob) elements. The consistency of sets and cosets representing the migration of in-channel bedforms suggests discharge stability. The proportion of sand to mud increases from 80% sand and 20% mud to 60% sand and 40% mud vertically throughout the facies model consequence of channel instability caused by variations in discharge and sediment load. The thickness of preserved floodplain suggests that the channel belts themselves were reasonably stable for significant periods.
The change in fluvial style from the Darjaniyon-ki Dhani to the Sarnoo sandstones, particularly the increase in sinuosity and the decrease in the dominance of bedload transport, indicates progressive maturing of the Ghaggar-Hakra fluvial system through time (Schumm, 1981). This interpretation is supported by evidence for less flooding and fewer floodplain ponds up-section. However, a decrease in sinuosity from the Sarnoo to Nosar sandstones, coupled with an increase in the proportion of bedload, an increase in bedform and barform migration, and stacking at all scales is atypical of increasing fluvial maturity ( Figure 11) and indicates rejuvenation of the whole system.

| The Ghaggar-Hakra succession
The Ghaggar-Hakra Formation records the maturing evolution of an early Cretaceous fluvial system, followed by a sudden rejuvenation, preserved on an atypical relay ramp on the margin of the Barmer Basin. Dating indicates the succession is of Aptian-Albian age. Deposition of fluvial sediments always represents the complex interplay between the intrinsic processes of sediment transport and deposition and allogenic-controls acting at a variety of spatial and temporal scales (Leeder, 1993). Notwithstanding the constraints of the limited spatial extent of the outcrop available in this study, the relative dominance of broad-scale allocyclic-controls of climate and tectonics (Gawthorpe & Leeder, 2000;Robinson & McCabe, 2012) and the implications for the evolution of the Barmer Basin, warrant some discussion as presented below.
During deposition of the Ghaggar-Hakra Formation, initial rifting between India and Madagascar had started but India and Madagascar remained a single island continent (Biswas, 1987;. The WIRS was located~40°south of the equator with the Indo-Tethyan Ocean to the north, and the Aravalli Mountain Range and continental India to the south, in a position between the subtropical arid and temperate climatic belts (Acharyya & Lahiri, 1991;Chatterjee et al., 2013). The abundance of plant remains in the Ghaggar-Hakra Formation indicates that the Overbank (Ob) composed of mudto fine-sands. Little vegetation.
Channel fill (Ch) bound with erosive 5th order surfaces interbedded with coarse sands and granule-grade conglomerates; containing quartz clasts and rip-up clasts with irregular sets and cosets. The channels approx. 10 m across and stacked and amalgamated with gravel bars (Gb) forming tabular geometries.
Gravel bar (Gb) granule-to pebble-grade conglomerates, deposited by fluid flow Sheet flood (Sf) medium-to very fined -grained sands which is laterally extensive throughout the field area.

Modern channel with sinuous crested duneforms
Rarely preserved overbank (Ob) accounting for 5% of the model Studies and models from other fluvial systems evolving under well-vegetated sub-tropical regimes (Fielding, Allen, Alexander, & Gibling, 2009) display a range of characteristics that are like many of those observed in the Ghaggar-Hakra, including stacked and amalgamated channels with convolute bedding, tree stumps, imperfectly to poorly drained palaeosols (Kraus & Hasiotis, 2006) and variable but significant amounts of floodplain deposition.
However, preserved floodplain thicknesses predicted from fluvial models for subtropical conditions are notably thinner than those observed in the Ghaggar-Hakra (Fielding et al., 2009), and the thick cyclic floodplain successions formed are attributed to periodic flooding, the result of channel instability caused by temporal variations in discharge and sediment load.
Changes in discharge and load may relate to local and/ or regional controls of either tectonics or climate. Given that the floodplain is very uniform, with a lack of internal variation in its pedogenic nature, and that the Indian Plate was within the sub-tropical arid and temperate climate belt (Chatterjee et al., 2013;Scotese, 2011;Scotese, Illich, Zumberge, & Brown, 2007) throughout the time of deposition; it is argued that climatic variation is unlikely to have had a significant influence upon the fluvial rejuvenation. However, the climate did influence overbank conditions and soil growth. Based on the palaeosol profile exhibited by overbank elements, the floodplain likely formed oxiosols that imply it received at least 100 mm per month of rain over 7 months of the year (Cecil & Dulong, 2013).
In the absence of climate control upon the rejuvenation of the fluvial system during Nosar times, local or regional tectonics is the most likely influence on accommodation space and sedimentation. Recent studies indicate a Mesozoic section is preserved in the subsurface up to 6 km deep, in the centre of the Barmer Basin Kothari et al., 2015) and recognize a west-striking, pre-Palaeogene tectonic grain on the eastern margin (termed the Saraswati Terrace  that is overprinted by the younger Palaeogene extensional event.  conclude that early northwest-southeast rifting is a consequence of the transtensional structural regime that existed between Greater India and Madagascar prior to their separation and the main phase of Deccan volcanic eruption. The sedimentological work on the Ghaggar-Hakra presented herein indicates that this early rift event did indeed influence depositional style and architecture as many of the sedimentary characteristics of the Nosar Sandstone, and rejuvenation of the system, can be explained by deposition on a tectonically subsiding continental alluvial plain. However, there is no direct evidence from outcrop for stratal growth patterns in the Ghaggar-Hakra, or for a strong relationship between fluvial drainage patterns and fault geometry in the Darjaniyon-ki Dhani to Sarnoo sandstones, which is atypical as these features are generally common in fluvial systems strongly controlled by contemporaneous rifting (Gawthorpe & Leeder, 2000). Apart from the high-energy system recorded in the initial deposits of the Darjaniyon-ki Dhani Sandstone, the succession through to the base of the Nosar Sandstone exhibits progressive fluvial maturation suggesting stability and quiescence. It is only during Nosar times that significant rejuvenation of the fluvial system occurs and that can be attributed to tectonically induced changes in fluvial gradient (Bridge & Leeder, 1979;Leeder, 1993;Schumm, 1993). Consequently, it is tempting to conclude that the base of the Nosar Sandstone, rather than the base of the Ghaggar-Hakra, represents the onset of active rifting in the Barmer Basin during the early northwest-southeast phase of extension recognized by . Alternatively, rift flank uplift accelerated significantly at the base of the Nosar Sandstone, indicating syn-rift deposition during Lower Cretaceous times. This in turn implies high preservation potential for thick early Cretaceous fluvial successions within rifted fault blocks beneath the Palaeogene fill that likely have significant potential for further hydrocarbon exploration.

| Implications for Palaeogeography of the north-west Indian Plate in the Lower Cretaceous Epoch
During Early Cretaceous times, clastic deposition across the north-west Indian Plate was dominated by fluvial systems carrying sediment to coastal plains and deltas forming along the edge of the Indo-Tethyan Ocean. In the Kachchh and the Middle and Lower Indus basins, the Lower Cretaceous Bhuj and Lower Goru formations are established reservoirs for hydrocarbons Biswas, 1999;Mukherjee, 1983). However, Cretaceous sediments are rarely exposed at outcrop across the Indian Plate, being preserved only within rift basins or at basin margins, where they have been downfaulted and protected from the effects of Palaeogene and Neogene uplift and erosion. Therefore, reconstructing even local Cretaceous palaeogeography is difficult because of limited outcrop, and collation of descriptions of the Lower Cretaceous sediments is required.
The depth to top basement map for north-west India of Kothari et al. (2015) is used to establish a detailed structural framework to depict the early Cretaceous rift systems which in turn define the depositional systems and their distribution ( Figure 12). The extent, type, and distribution of the depositional systems are initially established from the Sarnoo Hills work, together with published accounts of early Cretaceous outcrop and subsurface sediments, before speculatively extrapolating facies following Walther's Law.  ( During Cretaceous times, the Indian Plate was rotated 90°clockwise with respect to its present orientation (Figure 12;Chatterjee et al., 2013), so its leading edge comprised much of present-day Pakistan where the Lower Cretaceous sediments were deposited in coastal embayments, deltas, and shorefaces Smewing, Warburton, Daley, Copestake, & Ul-haq, 2002). As India was within the subtropical arid and temperate climate belt, the overall precipitation across India was low to medium (1.5-12 cm/month; Chatterjee et al., 2013). The FOAM palaeoclimate simulations (Goswami, 2011;Scotese, 2011;Scotese et al., 2007) indicate that the WIRS and surrounding areas were comparatively dry and warm (17°C) when compared to the east of the India which is separated from them by the 600 km long Aravalli Mountain Range.
Along the northern leading edge of the Indian Plate, the coastal Sembar Formation siliciclastics were deposited on top of an extensive carbonate platform (Khalid et al., 2014). This resulted from a gradual and long-term baselevel rise leading to high-stand shedding with the formation of a westerly prograding delta (the "Goru Fan Delta"). An active longshore drift and tidal influence restricted these sands to the east of the shelf where they formed a ramp 200 km wide (Ahmad et al., 2004) that gradually deepened to the west and north ( fig. 9 of Khalid et al., 2014). The Goru Fan Delta fed by fluvial systems draining the north-west Indian Plate built out into the marine embayment, across the Jaisalmer Basin and the Punjab Platform and is imaged on regional two-dimensional seismic data ( Figure 12; Khalid et al., 2014). Consequently, the Lower Cretaceous shoreline, shelf edge positions, and lateral shifts are reasonably well-known across the north-west Indian Plate in south-central Pakistan (Khalid et al., 2014). Fluvial palaeocurrent directions across the Lower Goru Fan Delta are from the east and south-east consistent with sources on the Indian Plate (Ahmad et al., 2004). The Ghaggar-Hakra Formation palaeocurrent directions, present day, are to the south-west (Figures 7-9; Sisodia & Singh, 2000) into the Barmer Basin reflecting their location on a feeder relay ramp (Saraswati Terrace) into the rift and directed towards the Cambay Basin. Therefore, it is possible to speculate that these sediments actually fed the southerly part of the WIRS and potentially drained into a poorly known early Cretaceous rift basin beneath the present-day Gulf of Khambhat. Early Cretaceous rivers draining the Devikot High and the northern Aravalli Range likely supplied the Lower Goru and Sembar formations across the Punjab Platform ( Figure 12).
This palaeogeographical reconstruction suggests a much more complex drainage system than has previously been envisaged for the northern leading edge of the Indian Plate. Upland deposits had a very low preservation potential which, compounded with later uplift and widespread erosion preceding the Deccan volcanism, resulted in the disparate preservation of fluvial early Cretaceous sediments. Along with Biswas (1999), it is proposed here that most of the early Cretaceous sediments preserved in the WIRS are indeed remnants of a syn-rift continental succession. By contrast, the coastal plain and deltaic deposits of the Sembar-Lower Goru succession were much more regionally extensive at deposition and accumulated along the Indo-Tethyan leading edge in coastal plain, deltaic, and shallow marine shoreface settings.

| CONCLUSIONS
The outcrops at Sarnoo, Karentia, and Nosar provide continuous sections and significant insight through Lower Cretaceous fluvial strata of the Ghaggar-Hakra Formation. The sedimentary succession includes channels, bars, sheet floods, and overbank deposits in varied proportions that typify a fluvial system deposited under sub-tropical climate conditions. The immature, low sinuosity system of the Darjaniyon-ki Dhani Sandstone, dominated by gravel bars and isolated channels, along with its frequently flooded and poorly drained floodplain, matures upward into the laterally migrating channel system of the highly sinuous Sarnoo Sandstone. The Nosar Sandstone completes the formation at outcrop and comprises stacked and amalgamated channels and gravel bars indicating fluvial rejuvenation.
Fluvial rejuvenation is most likely a response to faulting and it is concluded tentatively that the Nosar Sandstone may represent the onset or acceleration of rifting and development of the eastern margin of the Barmer Basin. Consequently, the Nosar Sandstone, along with contemporaneous and later sediments of the Ghaggar-Hakra Formation, is syn-rift in nature. This conclusion supports that of  derived from structural analysis and provides further evidence that extension in the Barmer area of north-west India was probably established prior to the start of the Palaeogene Period. Given this interpretation, well-developed successions of Cretaceous fluvial strata may be preserved in Mesozoic sections of the subsurface of the WIRS, offering hydrocarbon potential below the presently explored Palaeogene succession of the Barmer and related basins.
Early Cretaceous sediments are rarely exposed at outcrop across the north-west Indian Plate. Comparisons between the Ghaggar-Hakra Formation and other successions of comparable age, between their relative positions in a plate tectonic framework, and with regional structural data, allow for a reconstruction of a detailed palaeogeographical map for this part of the north-west Indian Plate for the early Cretaceous. The reconstruction suggests a more complex fluvial drainage system than previously envisaged, with continental early Cretaceous sediments preserved only within contemporaneous rifts.