Sedimentology and palynology of the Lower Cretaceous succession of central Spitsbergen: integration of subsurface and outcrop data

The Lower Cretaceous succession in Svalbard is commonly considered as an important analogue to age-equivalent strata on the Barents Shelf which are sporadically targeted by exploration wells. In this study, the stratigraphic and genetic relationship between the Rurikfjellet (open marine), Helvetiafjellet (paralic) and Carolinefjellet (open marine) formations of the Lower Cretaceous succession in Svalbard is evaluated by combining sedimentological outcrop studies with well log and core data across Nordenskiöld Land, central Spitsbergen. Sedimentological characteristics and stratigraphic units are mapped within a refined dinocyst biostratigraphic framework, enabling relatively well-constrained palaeogeographic reconstructions. The Valanginian–lowermost Barremian Rurikfjellet Formation consists of a lower shale-dominated unit of offshore origin which grades upwards into storm-reworked lower shoreface sandstones displaying hummocky cross-stratification. Local occurrences of prodeltaic successions and thick successions of gravity flow deposits containing coal-bearing slump blocks of delta plain origin in some wells, reveal a late Hauterivian progradational pulse which has previously not been recorded in Svalbard. The lower Barremian– lower Aptian Helvetiafjellet Formation consists of fluvial braidplain and paralic deposits which rest unconformably on the Rurikfjellet Formation across the entire study area, reflecting regional uplift and widespread subaerial exposure prior to the onset of paralic deposition. The Helvetiafjellet Formation exhibits increased marine influence upwards, and in the investigated cores the uppermost part of the unit consists of wave-reworked mouth-bar deposits which are truncated by a transgressive conglomerate lag dominated by extrabasinal lithic clasts and intraformational siderite clasts. An up to 10 m-thick, regionally extensive, organic-rich (TOC up to 2.1 wt.%) shale unit of early Aptian age marks the base of the overlying Carolinefjellet Formation. The shale accumulated during a regional flooding event which drowned and eventually transformed the Helvetiafjellet Formation coastal plain into a shallow shelf. The organic facies of the shale unit (Type II–III kerogen) and a high Pr/Ph ratio (>2), in combination with abundant long-chained n-alkanes, suggest that the unit was deposited in a suboxic paralic marine environment strongly influenced by input of terrestrial organic matter. The investigated succession exhibits stratigraphic and petrographic resemblance to age-equivalent strata in NE Greenland, suggesting that these successions may have formed part of the same drainage system located on the northwestern margin of the Barents Shelf. Thus, by highlighting the Early Cretaceous palaeogeographic evolution in Svalbard, this study contributes to the regional stratigraphic understanding of the Lower Cretaceous succession on the wider northern Barents Shelf.


Introduction
The Lower Cretaceous succession in Svalbard ( Fig.  1) consists of the Rurikfjellet (open marine), the Helvetiafjellet (paralic) and the Carolinefjellet (open marine) formations (e.g., Dypvik et al., 1991a;Gjelberg & Steel, 1995;Fig. 2). Early work on the succession mostly focused on lithostratigraphic aspects (Parker, 1967;Nagy, 1970), and particularly the Barremian Helvetiafjellet Formation (Fig. 2) received considerable attention due to its coal content, possible local reservoir potential, and for representing a possible analogue to time-equivalent reservoir units for oil and gas in the Barents Sea (Edwards, 1976(Edwards, , 1979Smith & Pickton, 1976;Steel, 1977;Nemec et al., 1988;Nemec, 1992;Gjelberg & Steel, 1995). Coal seams in the Helvetiafjellet Formation were briefly mined at Advent City and Hiorthamn in Adventfjorden and at Bohemanneset at the northern shore of Isfjorden ( Fig. 1) during the early 1900s. Despite these early efforts, only limited volumes of coal have been extracted from the unit, and there is presently no commercial interest for the unit in regard to coal exploration. The Lower Cretaceous succession was further subject to intense mapping during the onshore oil and gas exploration campaigns in central Spitsbergen between 1963 and1994. Following these campaigns, some hydrocarbon shows and technical gas discoveries were reported in some drilled wells (e.g., the 7714/3-1 Ishøgda I, 7815/10-1 Colesbukta and 7816/12-1 Reindalspasset I wells; Nøttvedt et al., 1993). In addition, a coal exploration well drilled in the inner part of Adventdalen in the early 1980s incidentally struck dry gas in Lower Cretaceous strata. However, the gas was of unknown origin and in non-commercial volumes (Mackenzie et al., 1983). Exploration for moveable hydrocarbons onshore Svalbard was eventually abandoned in the early 1990s following the overall  , Ul -Ullaberget. (C) Map of the study area showing the location of the CO 2 well sites and nearby outcrop sections. The map is modified from the Norwegian Polar Institute (http://toposvalbard.npolar.no).
(e.g., Nøttvedt et al., 1993). More recently, parts of the Lower Cretaceous succession in the subsurface of central Spitsbergen have been penetrated by seven fully cored wells drilled in relation to a CO 2 sequestration project disappointing results. Most of the subsurface data generated during the exploration campaigns are largely unavailable for the public and still remain unpublished, only being sporadically mentioned in previous literature  (BCU) in the offshore basins. The cross-section integrates data compiled from Parker (1967), Nagy (1970), Steel & Worsley (1984), Dypvik et al. (1991a), Gjelberg & Steel (1995), Midtkandal et al. (2007Midtkandal et al. ( , 2008, Hurum et al. (2016b), Koevoets et al. (2016;2018), , and this study. FS -lower Aptian flooding surface; SU -lower Barremian subaerial unconformity. in the vicinity of Longyearbyen ( Fig. 1; Braathen et al., 2012). Although the Lower Cretaceous succession was initially regarded as overburden to the targeted Triassic to Middle Jurassic CO 2 reservoir sandstones, aquifer potential (largely due to fractures) was proven in the Festningen Member of the Helvetiafjellet Formation (e.g., Braathen et al., 2012;Magnabosco et al., 2014). However, the unit was disregarded as a possible CO 2 reservoir due to its direct connection to the land surface along many valley and fjord sides. The investigations also demonstrated that the shale-dominated Rurikfjellet Formation forms part of a viable top seal unit (Braathen et al., 2012;Bohloli et al., 2014). To date, there have been few attempts to link outcrop and subsurface observations of the Lower Cretaceous succession in Svalbard (e.g., , despite its relevance for understanding Lower Cretaceous play models and related petroleum system elements on the northern Barents Shelf (e.g., the presence of inferred sandstone-dominated Barremian to Aptian delta-scale clinoforms and the distribution of oil-prone source rocks demonstrated in the Fingerdjupet Subbasin by well 7321/9-1S).
This contribution reports on the sedimentology and biostratigraphy of the Lower Cretaceous succession in Nordenskiöld Land, central Spitsbergen, by combining well and outcrop data. The genetic relationship between the Rurikfjellet, Helvetiafjellet and Carolinefjellet formations is thoroughly documented, with special attention given to new subsurface observations. In addition, a refined biostratigraphic age model based on dinoflagellates (dinocysts) is presented. The age data aid correlation of wells and selected outcrop sections across the study area, including the nearly continuous outcrop section at Festningen in the westernmost part of Isfjorden (Fig. 1). Thus, the findings of the present study contribute to the regional stratigraphic and palaeogeographic understanding of the Lower Cretaceous succession on the northern Barents Shelf.

Tectonic setting
The study area is confined to Nordenskiöld Land on Spitsbergen, the largest island of the Svalbard archipelago, Norway (Fig. 1). The archipelago represents the uplifted and exposed northwestern corner of the Barents Shelf. The bulk of the Mesozoic strata on Spitsbergen crop out along the margins of the NNW-SSE-trending Central Tertiary Basin (Fig. 1B). Due to Paleogene transpressional tectonics (Steel et al., 1985), the Mesozoic strata are steeply dipping or vertically tilted in western Spitsbergen, and dip towards the southwest at a low angle (<3°) in central and eastern Spitsbergen.
Svalbard has been subjected to recurring uplift since the Late Cretaceous through Cenozoic, as well as glaciations from the Pliocene and onwards. As a result, Upper Cretaceous strata are not present in Svalbard (Fig. 2). However, reworked Late Cretaceous terrestrial and marine microfloras reported in the Paleocene strata of southern Spitsbergen suggest that Upper Cretaceous strata may once have been present (Smelror & Larssen, 2016).
The Helvetiafjellet Formation (Barremian-lower Aptian; Grøsfjeld, 1992; This study) is 40-155 m thick and consists of the lower sandstone-dominated Festningen Member and the upper heterolithic and coal-bearing Glitrefjellet Member (Parker, 1967;Midtkandal et al., 2008). The formation represents deposition in a fluvial to paralic setting (Steel, 1977;Steel et al., 1978;Nemec Steel & Worsley (1984), Dypvik et al. (2002), Torsvik et al. (2002), Tullius et al. (2014), Marin et al. (2016) and . BFZ -Billefjorden Fault Zone; LFZ -Lomfjorden Fault Zone; KKL -Kong Karls Land; MIC -Mjølnir Impact Crater.   The non-weathered nature of the cores makes them ideal for geochemical studies and screening of potential source rocks. A total of five source-rock geochemical samples were collected in the basal shale unit of the Carolinefjellet Formation in well DH5R (Table 2). In addition, the dataset includes nine samples from the same stratigraphic unit in DH1 previously collected and analysed by Midtkandal et al. (2016) ( Table 2). The TOC and Rock-Eval analyses of the DH5R samples were conducted by ENI, Norway, and samples from DH1 were analysed at the Institute for Energy Technology (IFE), Norway. The TOC content was determined by hightemperature combustion in a Leco SC-632 instrument. Prior to combustion, the rock samples were crushed and treated with diluted HCl to remove carbonate minerals. A Rock-Eval 6 instrument was used for Rock-Eval pyrolysis, and the Jet-Rock 1 standard was run and checked against the acceptable range given in NIGOGA (4 th edition) for every tenth sample. The temperature program was 300°C for 3 minutes with the temperature subsequently increased at a gradient of 25°C/min to a final temperature of 650°C. Data measured during pyrolysis include maximum temperature (T max ) measured

Dataset and methods
The dataset used in this study includes seven onshore sediment cores with a cumulative thickness of c. 1800 m, as well as wireline gamma-ray logs from four of the wells (Fig. 4). Wells DH1 and DH2 are located 3 km to the northwest of Longyearbyen, and were drilled with c. 20 m of spacing ( Fig. 1). Wells DH3 to DH7A are located in Adventdalen ( Fig. 1), approximately 7 km to the southeast of the wells DH1 and DH2, and are referred to as the UNIS CO 2 LAB well park, Longyearbyen (Braathen et al., 2012). Wells in this area are clustered together within a radius of c. 80 m. Well DH5R is also included in two regional correlation panels (Figs. 5 & 6). The cores were measured bed-by-bed, and include descriptions of rock type, grain size, sorting, sedimentary structures, body and trace fossils, and degree of bioturbation (expressed following the bioturbation index (BI) of Taylor & Goldring (1993). Data collected from nearby outcrops aided in the interpretation of facies and facies associations (Table 1) recorded in the cores and provided palaeocurrent flow directions.
A total of 38 dinoflagellate cyst (dinocyst) bio stratigraphic samples have been collected and analysed. Biostratigraphic microscope slides were prepared at the Geological Survey of Denmark and Greenland (GEUS) by conventional techniques as described by Nøhr-Hansen (2012), and subsequently scanned for key species of the Lower Cretaceous. When possible, the assemblage is referred to the dinocyst zonation of Nøhr-Hansen (1993). Out of the 38 samples, the age of the Rurikfjellet at the peak generation of hydrocarbons, S 1 (the amount of free liquid hydrocarbons), S 2 (potential amount of hydrocarbons to be generated), S 3 (the amount of CO 2 generated during pyrolysis), oxygen index (OI; data only available for well DH1) and hydrogen index (HI; Table 2). Calculated data include the production index (PI; S 1 / (S 1 + S 2 )). Collectively, these parameters are used to assess the thermal maturity of the organic matter present in the samples, source-rock potential and kerogen type. Solvents from one sample from DH5R were further extracted, de-asphalted and fractionated before the saturated hydrocarbons were analysed by gas chromatography-mass spectrometry (GC-MS). The GC-MS data provide detailed information on the

Facies associations
A total of 12 facies associations are recognised (FA 1-12; Table 1). Their characteristics and stratigraphic distribution within the three Lower Cretaceous formations are briefly summarised below. Deposition primarily took place below storm-wave base in a relatively shallow shelf environment below stormwave base (Dypvik et al., 1991b). FA 2 ( Fig. 7E-H) is more heterolithic and rich in hummocky cross-stratified (HCS) sandstones, suggesting deposition in open marine, shallow shelf to nearshore environments above the storm-wave base (Dypvik et al., 1991b). Together, these associations form an overall regressive succession recording a long-term shoaling of the basin (Dypvik et al., 1991a;Midtkandal et al., 2008). The orientation of waveripple crests in FA 2 are widely scattered (Fig. 8A), but a WSW-ENE trend is evident in the western and northern parts of the study area, and a SSW-NNE to N-S trend dominates in the eastern part ( Fig. 8A). Dip-azimuths of anisotropic HCS foresets are also widely scattered, but locally display strong modes towards the south and southeast (Fig. 8A). FA 3 consists of sandy mudstones that grade upwards into sandstones with abundant softsediment deformation structures ( Fig. 9A-D). A prodelta slope to delta-front depositional environment is inferred for this facies association. FA 4 consists of thick-bedded, poorly sorted diamicts and sandstones with various gradings and chaotic fabrics ( Fig. 9E-I). This facies association represents deposition from a wide range of mass flows in a lower slope to basin-floor setting. FAs 3 and 4 only occur in wells DH1 and DH2, and have not previously been documented in detail (only briefly by Braathen et al., 2012). Thus, thorough descriptions of these facies associations are provided later in the text.

Facies associations of the Helvetiafjellet Formation
FAs 5-9 constitute the Helvetiafjellet Formation ( Fig.  4; Table 1) and broadly represent deposition in fluvial Both the Festningen section and well DH5R record the stratigraphic development typical for the Lower Cretaceous succession elsewhere in Svalbard (e.g., Gjelberg & Steel, 1995). However, note the thickness variation of the Glitrefjellet Member between the two sites. The correlation of the lower Aptian flooding surface is supported using δ 13 C org curves from Festningen (Vickers et al., 2016) and from well DH1 .   and this study.

Facies associations of the Carolinefjellet Formation
The Carolinefjellet Formation resembles the Rurikfjellet Formation in terms of the abundance of HCS sandstones and the inferred storm-influenced, open-marine shelf, depositional environment (Table 1). However, due to the high content of volcanically derived sediments (Maher et al., 2004), the generally higher content of sandstones, increased bed thickness and degree of amalgamation, as well as the difference in trace fossil assemblage (Table  1), the deposits of the Carolinefjellet Formation are given separate FA codes (FAs 10-12; Fig. 4; Table 1). FA 10 consists of an up to 10 m-thick, dark grey to black, organic-rich (sapropelic) shale-unit of regional extent (Figs. 4,5,6 & 11). The association was deposited in a restricted shelf setting during a major flooding . Because this unit has not previously been described in detail, a detailed description is provided later in the text. FA 11 consists of up to 15 m-thick, tabular-shaped, sandstone units dominated by amalgamated HCS, which make up the bulk of the strata of the Dalkjegla Member ( Fig. 10I-M; Nagy, 1970). Deposition took place in a storm-dominated and tidally influenced (cf., Birkenmajer, 1966) offshore transition to lower shoreface setting. Coarse-grained sandstones exhibiting trough cross-bedding occur at Festningen and at Ramfjellet in the western and northwestern parts of the study area, respectively (Figs. 5 & 6). Thus, these sections are attributed to deposition in more shoreproximal settings (e.g., middle to upper shoreface). Wave ripple crests in FA 11 display N-S to NNE-SSW orientations (Fig. 8C), and foresets of anisotropic HCS are broadly oriented perpendicular to the wave ripple crests (Fig. 8C). Throughout the study area, FA 11 grades upwards into lenticular to wavy-bedded sandstones and interbedded mudstones of FA 12 to form a 150 m thick shale-dominated succession of the Innkjegla Member ( Fig. 4; Nagy, 1970;Maher et al., 2004). FA 12 was deposited in storm-dominated, open-marine shelf environments (Table 1).  Gjelberg & Steel (1995). (C) The blue rose diagrams indicate wave-ripple crest orientations, whereas the black represent the migration direction of anisotropic hummocky cross-stratification. The red rose diagram represents the migration direction of anisotropic hummocky cross-stratification obtained from Nøttvedt & Kreisa (1987).

New subsurface observations
Sedimentary characteristics of FA 3 (prodelta slope to delta front) and FA 4 (base-of-slope mass-transport complex) in the Rurikfjellet Formation, the sideritecontaining conglomerate locally occurring in FA 8 (delta front) of the Helvetiafjellet Formation, as well as FA 10 (restricted shelf) of the Carolinefjellet Formation (Table  1) have not previously been described in detail. Detailed descriptions and interpretations of these units are thus given here.   4). The mudstones are sand-rich, poorly sorted, and normally graded with inclined and distorted bedding (Fig. 9A). Interbeds of erosionally-based, normally graded, fine-grained sandstones to siltstones occur (Fig.  9B). The sandstone content increases upwards in concert with a gamma-ray decrease (Fig. 4). Soft-sediment deformation structures (Fig. 9C), current ripple cross- lamination, and massive to bioturbated sandstone beds are variably present (Fig. 9D). The bioturbation index ranges from 0-1 in the mudstones and increases up to 2 in the sandstones. Trace fossils include Phycosiphon incertum, Planolites, Palaeophycus and Thalassinoides.

Interpretation
Based on the frequent occurrence of inclined and distorted mudstones, as well as their sandy texture, this facies association is attributed to deposition in a prodelta slope environment dominated by gravity-flow processes. The coarsening-upwards trend indicates progradation and a transition into a lower deltafront environment dominated by traction and marine flood-related processes. The poorly sorted mudstones indicate gravity-flow deposition by various processes including suspension settling, rapid deposition from river-fed hyperpycnal flows (Mulder & Syvitski, 1995;Bhattacharya & MacEachern, 2009), turbidity currents initiated by rapid collapse of hypopycnal mud plumes  (Parsons et al., 2001), and by offshore flushing of fluid mud during river floods (Neil & Allison, 2005). The sharp-based normally graded beds suggest deposition from flood-generated turbidity and hyperpycnal flows (Bhattacharya & MacEachern, 2009 Micro-faults with a few centimetre offsets (Fig. 9G), sand-filled fractures resembling small-scale injectites, distorted bedding, and folded heterolithic units occur frequently. The lower part of the succession is dominated by lithic conglomerates and interbedded bioturbated siltstones. The better sorted sandstones display planeparallel lamination and current ripple cross-lamination (Fig. 9H). The upper part of this association gradually fines and grades upwards into the overlying prodelta slope deposits (FA 3;Figs. 4 & 12). In borehole DH2, a 1.6 m-thick, strongly inclined, sharp-based coarseningupwards unit is observed (Figs. 9I & 12C). It consists of very fine-to medium-grained sandstones with rootlets and a coal cap (Figs. 9I & 12C). Bioturbation is absent in the sandstones, but the bioturbation index ranges from 3 to 4 in the interbedded fines. Typical trace fossils include Planolites, Phycosiphon incertum and Chondrites. Rare Palaeophycus and Schaubcylindrichnus freyi also occur.

Interpretation
The chaotic fabric, abundance of soft-sediment deformation structures, the poorly sorted character of the sandstones, and the presence of inversely graded beds rich in mudstone clasts and lithic conglomerates, suggest deposition from slumps and debris flows. Normally graded and current-ripple cross-laminated sandstone beds record deposition from surge-type, decelerating turbidity currents (e.g., Bouma, 1962). Thicker, ungraded, massive beds are interpreted to represent rapid deposition from collapsing high-density turbidity flows (Lowe, 1982), continuous aggradation from sustained, depletive turbidity flows (Kneller & Branney, 1995), or alternatively as deposition from sandy debris flows (Shanmugam, 1996). The trace fossil assemblage resembles the Zoophycos Ichnofacies, largely representing shelf to slope-transitional environments (e.g., MacEachern et al., 2007). The 1.6 m-thick, inclined, sharp-based, coarsening-upwards coal-bearing unit in DH2, is interpreted to be a slumped block of delta-plain origin. Based on its stratigraphic position below prodelta slope deposits (FA 3), and the dominance of gravity-flow deposits, FA 4 is interpreted to represent a mass-transport complex which accumulated in a lower slope to basin floor setting prior to delta progradation into the study area.

FA 8. Delta-front deposits Description
In order to provide stratigraphic context for the sideritebearing lithic conglomerate below the basal shale unit of the Carolinefjellet Formation (FA 10), the subsurface expression of the delta-front deposits immediately below the conglomerate is described here (Fig. 11). In outcrops, this facies association forms tabular to wedgeshaped sandstone bodies up to 12 m thick, internally displaying coarsening-upwards trends and clinoform geometries at the scale of a few to ten metres (Nemec, 1992;Gjelberg & Steel, 1995;Midtkandal et al., 2007).
In the wells, this facies association is up to 6 m thick and consists of lenticular-bedded, very fine-to fine-grained sandstones alternating with thin mudstones (Fig. 11). Medium-to coarse-grained sandstone beds up to 10 cm thick occur sporadically. This facies association typically overlies mudstones of FA 6 with the sandstone content and bed thickness gradually increasing upwards to form a vague coarsening-upwards unit in the uppermost part of the Helvetiafjellet Formation in the wells (Figs. 4 & 11). Current-ripple cross-lamination, wave-ripple lamination, plane-parallel lamination, low-angle crossstratification, and soft-sediment deformation structures occur (Fig. 11). Siderite concretions are present locally. Trace fossils are variably present (bioturbation index of 0-5) and include dominant Schaubcylindrichnus freyi and subordinate Chondrites and Planolites. The facies association is capped by a 10-40 cm-thick, sharp-based conglomerate bed (Fig. 11A-C). The conglomerate is poorly sorted and consists of a medium-to coarsegrained quartz-rich sandy matrix with variable contents of larger extrabasinal lithic clasts and intraformational siderite clasts (Fig. 11C). Thin section analysis of the conglomerate in well DH1 reveals the presence of rounded spiculite clasts originating from the Upper Permian Tempelfjorden Group (Fig. 11E). Low-angle stratification occurs in places.

Interpretation
Based on the overall coarsening-and thickeningupwards trend, its stratigraphic position above mudstones of FA 6 (floodplain to interdistributary bay deposits), and the occurrence of traction-generated structures, this facies association is attributed to deposition by prograding delta lobes (e.g., Nemec, 1992;Gjelberg & Steel, 1995). The occurrence of wave-ripple cross-lamination and low-angle cross-stratification (possibly representing hummocky cross-stratification) indicates wave and storm-wave reworking of the delta front. Siderite and intensely bioturbated beds reflect periods of non-deposition. The conglomerate bed capping the delta front deposits is interpreted to be a lag deposit formed by transgressive ravinement during the final flooding of the Helvetiafjellet Formation coastal plain. The presence of the overlying regional extensive shale unit of FA 10 (restricted shelf deposits; Table 1) support a transgressive origin for the conglomerate.  Table 2.

FA 10. Restricted shelf deposits Description
This facies association is poorly exposed in most outcrops. Thus, the following description is primarily based on core observations. FA 10 occurs across the entire study area as an up to 10 m-thick, dark grey to black, finely-laminated, organic-rich (TOC up to 2.1 wt.%; Table 2) shale unit in the lowermost part of the Carolinefjellet Formation (Table 1; Figs. 4, 5, 6 &11). In all the CO 2 -wells, the shale caps a sharp-based lithic conglomerate unit (FA 8;Figs. 4A & 11;Tennevassås, 2018). A vague coarsening-upwards trend accompanied by decreasing gamma-ray values, increasing velocities, and a change to light grey-coloured shales are observed (Fig. 11). The amount of interbedded, very fine-grained, lenticular, wave-ripple cross-laminated sandstones also increases upwards, marking a gradual transition into FA 11 (Figs. 4 & 11). Pyrite and siderite nodules are also present. Bioturbation is sparse (BI 0-1), but tends to increase upwards (BI 1-3). The ichnodiversity is low and limited to Arenicolites, Palaeophycus, Gyrophyllites and Schaubcylindrichnus freyi in the sandstones.

Interpretation
Based on its fine-grained, laminated nature, dark colour and TOC content, and low-diversity trace fossil assemblage, this facies association is interpreted to have been deposited in a restricted shelf setting below storm wave base. The presence of pyrite and siderite indicates reduced oxygen levels and variable salinity levels on the shallow shelf, possibly due to dilution of brackish water and stratification of the water column. The widespread occurrence of the shale unit suggests deposition during regional flooding of the Helvetiafjellet Formation coastal plain . The association has been assigned an early Aptian age  This study), and a negative followed by a positive δ 13 C org excursion demonstrated in well DH1 and in the Festningen outcrop section ( Fig. 5; Midtkandal et al., 2016;Vickers et al., 2016) suggests a regional event that may correlate globally with the early Aptian Oceanic Anoxic Event 1a (OAE1a). The coarsening-upward trend and the presence of waveripple cross-laminated sandstones indicate shallowing of the shelf and marks the arrival of the Dalkjegla Member inner shelf to shoreface sandstone sheet (FA 11). The trace fossil assemblage is attributable to the distal Cruziana Ichnofacies (e.g., MacEachern et al., 2007).

Source-rock characteristics
The TOC and Rock-Eval data from the shale unit in the lower part of the Carolinefjellet Formation (FA 10) are given in Table 2, and selected parameters are shown in Fig. 13. The results of the GC-MS analysis are shown as a total ion chromatogram in Fig. 14. Within the short sample interval (<10 m; Fig. 11) and with the limited number of samples analysed (14 in total;  Fig. 13). The GC-MS analysis shows a smooth distribution of n-alkanes extending beyond n-C 35 with a relatively high concentration of long-chained n-alkanes (Fig. 14). There is, however, a prominent lack of frontend n-alkanes (<n-C 15 ). The measured pristane/n-C 17 and the phytane/n-C 18 ratios are around 0.76 and 0.33, respectively, with a relatively high Pr/Ph ratio of 2.32 (Fig. 14).

Dinocyst biostratigraphy
Dinocysts are rather rare and poorly preserved in the analysed samples, and the organic matter is generally dominated by saccate pollen and wood fragments. The dinocyst biostratigraphy of the Helvetiafjellet and Carolinefjellet formations is referred to the zonation of NE Greenland established by Nøhr-Hansen (1993). Only the most important Lower Cretaceous dinocyst markers recognised in the studied material are discussed here (Fig. 15).

Age of the Rurikfjellet Formation
Numerous samples, especially in the basal part of the formation, are barren of dinocysts. The most common dinocyst species within the Rurikfjellet Formation is Oligosphaeridium complex, whereas the stratigraphically most important dinocysts include: Endoscrinium hauterivianum, Oligosphaeridium abaculum, less abundant Nelchinopsis kostromiensis and Stanfordella fastigiata (Fig. 15). The stratigraphic range of E. hauterivianum is restricted to the Hauterivian, with its last occurrence in the lowermost upper Hauterivian (Duxbury, 2001). O. abaculum is regarded as an upper Hauterivian to lowermost Barremian marker (Costa & Davey, 1992), and N. kostromiensis was previously recorded from the upper Hauterivian to lower Barremian in NE Greenland (Nøhr-Hansen, 1993), from the upper Valanginian to lower Barremian in the North Sea (Duxbury, 2001), and from the Hauterivian to lower Barremian in the Barents Sea (Århus et al., 1990). S. fastigiata ranges from lower Hauterivian to lowermost upper Barremian (Bailey, 2017). Thus, the co-occurrence of E. hauterivianum, N. kostromiensis and O. abaculum suggests an early late Hauterivian age for the upper part of the Rurikfjellet Formation (232-288 m in well DH5R; Fig. 4). The occurrence of O. abaculum and N. kostromiensis indicates a latest Valanginian to early Hauterivian age for the lower part of the formation. Our results confirm the previous age assessment of Århus (1992), and constrains the age of the Adventpynten member to the earliest late Hauterivian (Fig. 4).
Samples from the Bohemanflya outcrop section and the uppermost sample from the DH5R well yield few specimens of questionable Pseudoceratium anaphrissum (Fig. 15K, P). These specimens differ from the holotype by their densely distributed spines and more rounded outline (cf., Sarjeant, 1966). In addition, the uppermost samples from the Bohemanflya outcrop section yield common to abundant specimens previously recorded as Nyktericysta? pannosa by Grøsfjeld (1992) (Fig. 15M-O). These specimens of N.? pannosa differ slightly from the holotype by their less pronounced lateral horns (cf., Duxbury, 1980). However, the presence of questionable P. anaphrissum and N?. pannosa indicate a Hauterivian to early Barremian age for the uppermost few metres of the Rurikfjellet Formation at Bohemanflya, confirming the age suggestion by Grøsfjeld (1992).

Age of the Helvetiafjellet Formation
Samples of the Helvetiafjellet Formation (Fig. 15) yield dinocysts with relatively long age ranges, e.g., O. complex (first occurrence in the lower Valanginian; Duxbury, 2001) and Sirmiodinium grossii (last occurrence in the lower Aptian; Costa & Davey, 1992;Nøhr-Hansen, 1993). Sporadic occurrences of Subtilisphaera perlucida, P. anaphrissum and Odontochitina nuda are recorded within the Helvetiafjellet Formation (Fig. 15). In NE Greenland, O. nuda is observed in the Barremian to lower Aptian, S. perlucida is recorded in the lower Barremian to middle Albian (Nøhr-Hansen, 1993), and P. anaphrissum is restricted to a short time range in the lower Barremian (Costa & Davey, 1992;Bailey 2017). The dinocyst assemblage is similar to the Pseudoceratium anaphrissum Subzone (I.2 of Nøhr-Hansen, 1993), suggesting that the lower half of the formation is of early Barremian age as previously suggested by Grøsfjeld (1992). The upper half of the Helvetiafjellet Formation yields O. nuda which is characteristic of the lower Aptian in NE Greenland (Nøhr-Hansen, 1993). Based on these age determinations, the uppermost part of the Helvetiafjellet Formation is referred to the early Aptian. This is in agreement with an absolute Ar/Ar age of 123.3 ± 0.2 Ma obtained from zircons in a bentonite layer in the middle of the formation in the DH3 well (Corfu et al., 2013;Midtkandal et al., 2016).

Age of the Carolinefjellet Formation
Two samples were studied from the base of the Carolinefjellet Formation. The samples yield amongst others O. nuda, Pseudoceratium cf. retusum, S. perlucida and Muderongia pariata (Fig. 15). In NE Greenland, M. pariata and P. cf. retusum are present in lower Aptian strata (Nøhr-Hansen, 1993). Thus, this assemblage can be referred to the earliest Aptian (Pseudoceratium nudum Zone II of Nøhr-Hansen, 1993). A similar age for the basal part of the Carolinefjellet Formation has also recently been suggested by Midtkandal et al. (2016) and Vickers et al. (2016). Previous ammonite studies by Nagy (1970) assigned an Aptian age for the Dalkjegla Member and late Aptian to early Albian age for the transitionally overlying Innkjegla Member.

Subsurface and outcrop correlations
The well correlation confirms the threefold lithostratigraphic subdivision typical of the Lower Cretaceous succession elsewhere in Svalbard (Fig. 4;Parker, 1967;Midtkandal et al., 2008). The seven wells are lithostratigraphically correlated by using two datum surfaces (Fig. 4). Datum 1 is the top of the Festningen Member, and Datum 2 is the flooding surface on top of the Helvetiafjellet Formation (FA 10; Table 1). Datum 1 is marked by an abrupt transition from cross-bedded sandstones (FA 5) into a generally <1 m-thick coaly shale unit. In some outcrops, particularly in eastern Spitsbergen, the shale thickens by several metres and delta-front deposits (FA 8) are observed to downlap onto the Datum 1 surface (e.g., Nemec et al., 1988;Steel et al., 2000;Midtkandal et al., 2007;Onderdonk & Midtkandal, 2010). In the Festningen outcrop section (Figs. 1C & 5), the coaly shale separates two fluvial sandstone bodies, where the lower one accumulated in a braidplain setting and the upper one in a distributary channel in a lower delta-plain setting (Fig. 5;Steel et al., 1978). Thus, the coaly shale layer may represent a good candidate for constituting the proximal part of a marine flooding surface. Datum 2 separates paralic deposits of the Helvetiafjellet Formation from open-marine shelf deposits of the overlying Carolinefjellet Formation (Fig. 11). The surface is overlain by an up to 10 m-thick early Aptian shale unit (FA 10; Table 1 ;Figs. 4,5,6 & 11) which accumulated during a regional flooding event ( Fig. 6; Midtkandal et al., 2016;. At the Festningen and Forkastningsfjellet outcrop sections, mouth-bar units (FA 8) >10 m thick characterise the uppermost part of the Helvetiafjellet Formation (Figs. 5,6 & 11). In these locations, the marine shale unit rests conformably on top of the mouth-bar units. In the wells, the laterally equivalent delta-front deposits are thinner (<5 m thick), heavily wave-reworked, and truncated by a matrix-supported conglomerate containing extrabasinal lithic clasts and intraformational siderite clasts (Fig. 11). This suggests that wave winnowing and ravinement took place prior to deposition of the marine shale unit. Thickness variations at the order of some few tens of metres occur within the Helvetiafjellet Formation (Figs. 4-6). This does not only imply that the amount of erosion associated with the transgressive ravinement varied significantly across the study area, but also testifies to the erosive base of the unit (e.g., Midtkandal & Nystuen, 2009) and the overall lobate character of the deltas which deposited the Helvetiafjellet Formation (e.g., Steel et al., 2000).
Another prominent surface of regional extent is the subaerial unconformity which separates the Helvetiafjellet Formation from the underlying Rurikfjellet Formation (Figs. 4-6). The unconformity is of early Barremian age (e.g., Grøsfjeld, 1992; This study), and is recognised by an abrupt facies transition from open-marine shelf and delta-front/shoreface deposits in the Rurikfjellet Formation (FAs 1 and 2) below to fluvial braidplain (FA 5) deposits of the Festningen Member (Fig. 4) above the unconformity. In the subsurface, a prominent drop in gamma-ray values defines the unconformity (Figs. 4-6). For correlation purposes, the highly undulating character of the unconformity ( Fig. 4; Gjelberg & Steel, 1995;Midtkandal et al., 2007Midtkandal et al., , 2008 Fig. 4). Formally, the base of the Kikutodden Member is defined by the first occurring sandstones above the shales of the Wimanfjellet Member (Midtkandal et al., 2008). In the wells, however, a high gamma-ray peak (e.g., 242 m in well DH4; Fig. 4) immediately followed by a prominent decrease and a coarsening-upwards trend occur approximately 20 m below the first occurring sandstone. In the cores, the coarsening-upwards trend seen in the gamma-ray logs is consistent with an increase in the amount of thin-bedded lenticular sandstones and more silty to sandy shales (e.g., 242-227 m in well DH4; Fig. 4). Collectively, this suggests that the regression represented by the Kikutodden Member was initiated earlier than what is captured by the lithostratigraphic correlation. The uppermost heterolithic, thin-bedded, hummocky cross-stratified sandstone units of the Kikutodden Member are in many outcrops capped by a 2-30 m-thick marine shale package (Figs. 4-6). The thickness variation of this shale package probably reflects the erosive character of the overlying Helvetiafjellet Formation, and its presence points to a semi-regional flooding event prior to uplift and formation of the Barremian subaerial unconformity.
In the subsurface, the most prominent deviation from the typical stratigraphic expression of the Rurikfjellet Formation is recorded in wells DH1 and DH2, where the 150 m-thick succession of mass-flow (FA 4) and prodelta slope (FA 3) deposits of the Adventpynten member occur (Figs. 2, 4 & 12). No similar deposits have been observed in outcrop elsewhere within the study area.

Discussion
The ensuing discussion primarily focuses on the palaeogeographic development of the LCS in central Spitsbergen based on facies distributions, palaeocurrent flow directions, as well as the subsurface and outcrop correlations (Figs. 4-6). The timing of events is based on the biostratigraphic framework established in this study (Figs. 2 & 15).

Palaeogeographic development of the Rurikfjellet Formation
The lower shale-dominated (FA 1) Wimanfjellet Member ( Fig. 2; Dypvik et al., 1991a;This study) represents the most distal part of the depositional system during Valanginian-early Hauterivian times and formed on an open-marine shelf well below storm wave base (Fig. 16A). The unit exhibits a slight upwards increase in the amount of siltstone and the degree of bioturbation, indicating shoaling from outer-to innershelf environments as nearby shorelines successively approached the study area from the west or northwest ( Fig. 16B; Dypvik et al., 1991b). This interpretation is consistent with previous studies based on microfossils (Løfalidi & Thusu, 1976) and geochemistry (Dypvik, 1985;Hvoslef et al., 1986). Due to its shale-dominated character, extensive regional distribution (present in all investigated sections and wells; Fig. 6) and laterally continuous thickness (c. 200 m in average), the offshore deposits of the Wimanfjellet Member constitute an important part of the top seal for the underlying Triassic to Middle Jurassic CO 2 reservoir unit (Braathen et al., 2012).
The transitionally overlying Hauterivian to lowermost Barremian Kikutodden Member comprises vertically stacked coarsening-upwards parasequences (Figs. 4-6). The parasequences consist of offshore transition to lower shoreface deposits (FA 2). The lack of shallower facies suggests either that such deposits were eroded during intervening transgressions (due to wave ravinement) or that they never developed due to rapid progradation into shallow water; or due to the low-gradient ramp setting of the basin (e.g., Midtkandal & Nystuen, 2009) giving rise to widely distributed facies belts. No transgressive lags typical of transgressive wave ravinement or evidence of forced regression (e.g., bypass surfaces, sharpbased shoreface deposits or foreshortened stratigraphy) have been identified in the parasequences. Thus, it seems more likely that the parasequences developed as a fully subaqueous system (within the study area) on a low gradient ramp. Although upper shoreface and shallower facies are not present in the Kikutodden Member in the study area, such deposits do occur within the member in southernmost Spitsbergen . Although these southern deposits lithostratigraphically belong to the Kikutodden Member, they probably represent a clastic wedge that developed separately from the parasequences documented in this study .
The lowermost parasequences of the Kikutodden Member represent successive basinward progradation of distal shoreline tongues (Fig. 16B-D). The uppermost parasequences contain less sandstone and exhibit retrogradational stacking with units becoming successively thinner upwards (Figs. 4 & 6). This indicates that the Kikutodden Member shoreline system retreated (Fig. 16E) prior to the formation of the subaerial unconformity at the base of the overlying Helvetiafjellet Formation (Fig. 16F). The open-marine shale package, which in many outcrops (locally eroded) separates the upper parasequences in the Kikutodden Member from the overlying Helvetiafjellet Formation, may thus represent a latest Hauterivian to earliest Barremian semi-regional marine flooding. Collectively, the Rurikfjellet Formation forms a large-scale, secondor third-order, regressive-transgressive sequence ( Fig.  16A-E) which developed in the Valanginian to earliest Barremian as an early sedimentary response to ongoing uplift north and west of Svalbard and igneous activity related to the HALIP in the east-northeast.

The origin of mass-flow deposits in the Rurikfjellet Formation
The most remarkable observation from this study is the 130 m-thick succession of mass-flow deposits of the Adventpynten member in the lower part of the Rurikfjellet Formation in wells DH1 and DH2 (FA 4;Figs. 2, 4A & 12;Mork-Jansson et al., 2010;Braathen et al., 2012). The mass-flow deposits are dated to be of an earliest late Hauterivian age, where the equivalent stratigraphic interval in the other wells is characterised by Valanginian to earliest Hauterivian shales of the Wimanfjellet Member (Fig. 4A). Thus, the age of the Adventpynten member corresponds directly to the hummocky cross-stratified sandstone-rich parasequences of the Kikutodden Member in the other wells (Fig. 4). It is noteworthy, however, that these sandstone units occur stratigraphically above the mass-flow deposits of the Adventpynten member and laterally correlate to prodelta slope deposits in wells DH1 and DH2 (FA 3; Fig. 4A). The origin of the mass-flow deposits is thus enigmatic in the sense that the Rurikfjellet Formation is traditionally interpreted to have been accumulated on a flat to gently sloping shelf (Dypvik et al., 1991b;Midtkandal & Nystuen, 2009).
Mass-flow deposits with a remarkably similar character have been described as syn-sedimentary infill deposits of collapse scars involving the Rurikfjellet and Helvetiafjellet formations in Kvalvågen, eastern Spitsbergen ( Fig. 1; Nemec et al., 1988). The mass-flow deposits were derived from nearby slopes by retrogressive slumping of the slump scar margins, and by later delta-front progradation into and eventually across the collapse scar.
We suggest two alternative interpretations for the origin of the Adventpynten member. The first interpretation is that it represents a base-of-slope mass transport complex that accumulated in front of the advancing delta system of the Kikutodden Member. Its localised distribution and the occurrence of a slump block of delta-plain origin within the mass-flow deposits (in well DH2;Figs. 4,9 & 12) indicate sourcing from an unstable delta front of early late Hauterivian age, thus predating the Barremian Helvetiafjellet Formation fluvio-deltaic system. Prodelta slope to delta-front deposits (FA 3) conformably overlying the mass-flow deposits (FA 4) suggest that this delta front prograded in concert with and later across its associated base-of-slope mass transport complex. Recently, it has been suggested that the Lower Cretaceous succession in Svalbard may represent the proximal part of a prograding shelfprism-type clinoform system . Such clinoform successions are reported widely on the Barents Shelf (Marin et al., 2016). Although clinoform geometries at these scales are yet to be demonstrated in Svalbard due to outcrop limitations, their presence cannot be ruled out with certainty.
The second interpretation is that the Adventpynten member represents a mass transport complex similar to the slump-scar deposits described from Kvalvågen in eastern Spitsbergen (Nemec et al., 1988;Onderdonk & Midtkandal, 2010). Seismic activity along the N-Strending Lomfjorden Fault Zone has generally been invoked as the main triggering mechanism for these collapse scars (Nemec et al., 1988;Onderdonk & Midtkandal, 2010). It is noteworthy that wells DH1 and DH2 are located c. 20 km west of the long-lived Billefjorden Fault Zone. Although it has been disputed whether this lineament was active during the Mesozoic, similar-trending Late Palaeozoic lineaments on the shelf immediately east of Svalbard were reactivated both in the Late Jurassic and in the Hauterivian to early Barremian (Kairanov et al., 2018). Thus, it cannot be excluded that short-lived tectonic pulses led to small, localised collapses in the studied succession. A diamictite representing a debris-flow deposit also occurs in the Upper Jurassic Agardhfjellet Formation in well DH5R (Koevoets et al., 2018). This may support the notion of proximity to an active fault scarp that periodically provided a local source for sediments and enabled sufficient relief to initiate gravity flows.

Palaeogeographic development of the Helvetiafjellet Formation
The unconformity at the base of the Helvetiafjellet Formation (Fig. 16F) is of early Barremian age (Grøsfjeld, 1992;This study). A similar age is also inferred for the uppermost part of the Rurikfjellet Formation, suggesting that the unconformity represents a hiatus of <2 myr (Grøsfjeld, 1992; This study). The Festningen Member was deposited in the early Barremian during relative sea-level rise immediately following the uplift which created the subaerial unconformity ( Fig. 16F-H; e.g., Midtkandal et al., 2007).
Based on the lateral thickness variations of the Festningen Member and the variable degree of preservation of the underlying shale package in the uppermost Rurikfjellet Formation, it is suggested that the Festningen Member was deposited in wide, and partially coalescing, low-relief river valleys (Fig. 16G).
The palaeo-topography is demonstrated from the well correlation (Fig. 4A), where the braidplain deposits of the Festningen Member display a thickness decrease in well DH5R. These braidplain deposits were also deposited within a river valley, but were probably situated on the valley margin or a topographic saddle between two valleys. Similar observations were made in Louisdalen (Fig. 1C) where the braidplain deposits of the Festningen Member thin from more than 8 m to 0.5 m thickness over a distance of c. 300 m. Upon filling the valleys, the fluvial system developed into an extensive braidplain depositing a fluvial sandstone sheet across the study area (Fig. 16H). The lateral extent of the fluvial sandstone sheet is attributed to the rapid southeastward shoreline progradation in the early Barremian due to uplift of the northern shelf. This resulted in extension of the shelf equilibrium profile, reduction of the shelf gradient, and thereby limited vertical accommodation space in the continental areas immediately following the regressive-transgressive turn-around. The limited accommodation space promoted frequent channel avulsion and cannibalism resulting in the channels building laterally instead of aggrading vertically (Midtkandal & Nystuen, 2009). Ultimately, this resulted in the development of an extensive fluvial sandstone sheet (Fig. 16H).
The transgressive coaly shale unit which caps the Festningen Member most likely formed due to a rising groundwater table during an early stage of the flooding (Fig. 16I;Nemec, 1992;Midtkandal et al., 2007). Delta-front deposits (FA 8) downlapping onto the top of the Festningen Member at several localities (e.g., Bohemanflya;Figs. 1C & 6), indicate that at one stage, the relative sea-level rise must have exceeded the rate of fluvial sediment supply and resulted in a regional flooding of the fluvial braidplain (Fig. 16I). When the rate of relative sea-level rise eventually outpaced the rate of sediment supply, enough accommodation was created for a deltaic system to subsequently develop and prograde across the shallow shelf which had formed by drowning of the braidplain (Fig. 16J).
At the Festningen locality, the Festningen Member originally includes two sandstone channel bodies (Fig. 6, level 180-200 m;Steel et al., 1978). The two channel bodies are fundamentally different in their architectures. The lower one has been interpreted to be a braidplain deposit, and the upper one to be a meandering channel deposit showing strong indications of lateral migration and point-bar development (Steel et al., 1978). A thin (few metres thick) shale deposited in an interdistributary bay environment separates the two channel bodies (Fig. 6). Thus, it is suggested here that only the lowermost channel body formally belongs to the braidplain deposits of the Festningen Member (Fig.  6), and that the interdistributary bay unit above formed during the suggested marine flooding of the braidplain. The upper channel body may represent a distributary channel feeding the delta-front deposits (FA 8) which locally occur on top of the Festningen Member (e.g., at Bohemanflya or in Kvalvågen;Figs. 1C & 6;Nemec et al., 1988). The change from an incised valley and braidplain setting to an unconfined lower delta plain setting may explain the architectural differences between the two channel bodies.
No obvious stacking trends are observed in the Glitrefjellet Member, indicating that the coastal plain was aggrading and that autogenic processes had a major influence on the facies development (e.g., Midtkandal et al., 2007). However, accommodation was continuously being created through subsidence and relative sealevel rise, resulting in marine incursions occurring more frequently as is evident from the presence of marine trace fossils. The mouth-bar unit (FA 8) which occurs in the uppermost Glitrefjellet Member at the Forkastningsfjellet and Festningen outcrop sections (Fig.  1C) reflects the overall transgressive development and retrogradational retreat of the Helvetiafjellet Formation shoreline (Figs. 6 & 16L). Due to localised transgressive ravinement associated with the final drowning of the Helvetiafjellet Formation shoreline, the mouth-bar unit is thinner and more wave-reworked in the wells (Fig.  4A). Notice also that Nemec (1992) documented barrier bar deposits in the uppermost part of the formation, both in Kvalvågen east of the study area and in Hiorthamn some few km north of Longyearbyen (Fig. 1C).

Palaeogeographic development of the Carolinefjellet Formation
The transgressive retreat of the Helvetiafjellet Formation shoreline culminated in a marine flooding in the early Aptian, depositing the regionally extensive sapropelic shale unit which marks the boundary between the Helvetiafjellet and Carolinefjellet formations (FA 10). The Helvetiafjellet Formation coastal plain drowned and the flooding thus created a shallow, gently-sloping shelf (Fig. 16M). The lower sandstone-dominated part of the Dalkjegla Member exhibits a shoaling-upward facies trend recording renewed progradation of the shoreline onto the shelf (Figs. 11 & 16N). The Dalkjegla Member is characterised by interbedded shales and sandstones with wave-and storm wave-generated structures suggesting deposition in an offshore transition to lower shoreface setting (FAs 11-12; Table 1). Coarse-grained sandstones of upper shoreface origin occur in the west and northwest, suggesting the presence of a shoreline in those directions beyond the present-day outcrop belt. The sheet-like architecture of the unit directly relates to the low-angle, distally-deepening (from the northwest to the southeast) ramp setting. The relatively shallow-water depths and the high energy of depositional processes inhibited accumulation of fine sand and mud on the inner shelf. Reworking by storm waves, potentially in combination with tidal currents (e.g., Birkenmajer, 1966), thus resulted in a net basinward transport of the sediments and shaped nearshore sands into amalgamated, laterally persistent sandstone sheets.
Similar to the Rurikfjellet Formation, no foreshore, backshore or paralic deposits have been recorded at any locality in the Carolinefjellet Formation in Svalbard . This points to predominantly subaqueous deposition or low-angle and extensive facies belts with the shoreline itself located somewhere in the west to northwest beyond the present-day outcrop belt (Fig. 16N).
The upper part of the Dalkjegla Member transitionally grades into the overlying mudstone-dominated Innkjegla Member (FA 12; Fig. 4). This facies trend is recorded in the wells and investigated outcrops, indicating a longterm, regional-scale retreat of the shoreline and a return to outer-shelf environments across the entire study area (Fig. 16O). This demonstrates that the Dalkjegla Member represents a second-or third-order regressivetransgressive sequence (Fig. 16M-O) similar to the trend of the Rurikfjellet Formation.

Source-rock potential of the lower Aptian shale
The TOC analyses of the samples of the shale unit in the lowermost part of the Carolinefjellet Formation (Fig.  11) all show values greater than 1 wt.%, indicating a fair to good source rock (Fig. 13A, B; Table 2). However, most of the samples have relatively low HI values, particularly the well DH1 samples (generally <200; Table 2). Vitrinite reflectance analyses of coal seams in the Paleocene Firkanten Formation yield R o % values up to 0.7 (Marshall et al., 2015), and coals from the Helvetiafjellet Formation in central Spitsbergen yield R o % values up to 1.1 (Nøttvedt et al., 1993). The average T max value of 446°C, in combination with the published vitrinite reflectance data, suggests peak oil window maturities (Fig. 13C). The organic facies in the sourcerock unit appear to be mostly of Type III (gas-prone) kerogen for the well DH1 samples and mixed Type II-III (oil-and gas-prone) kerogen for the DH5R samples ( Fig.  13B-D). The HI and OI values plotted in a modified van Krevelen diagram (Fig. 13D) also confirm a mostly gas-prone source rock in well DH1. However, the well DH5R samples in general have higher S 2 and HI values than the well DH1 samples (Fig. 13B, C; Table 2), possibly indicating that the source-rock unit represents an increased oil potential eastwards in the basin. The two well sites are located c. 7 km apart, with well DH1 representing a more proximal position relative to the palaeoshoreline (to the NW) and the West Spitsbergen Fold-and-Thrust Belt compared to well DH5R (Fig.  1). Thus, the observed differences may reflect lateral variations in primary depositional conditions (e.g., proximity to a fluvial point source) or regional thermal maturity trends. Alternatively (but less likely), the differences may reflect the various analytical approaches used by the two labs analysing the samples.
Based on the Rock-Eval and GC-MS analyses, as well as the kerogen type, a wet gas condensate-prone source rock is suggested for the unit. The analysed extracted samples show a lack of front-end n-alkanes (<n-C 15 ), possibly reflecting evaporation of lighter components (or slight biodegradation) which may commonly result in the loss of n-alkanes at around C 8 -C 15 . A higher Pr/ Ph ratio (2.32; Fig. 14) than most anoxic marine-derived oils (Pr/Ph ratios <1) is also evident, and the abundance of n-alkanes above n-C 21 thus suggests a significant contribution from terrigenous organic matter. This indicates that the source-rock unit was not deposited in an anoxic marine setting, but rather in a suboxic environment (Fig. 13D) strongly influenced by input of terrestrial organic matter. Consequently, a more paralic marine setting for the shale unit is suggested. Rising sea level in shallow marginal seas typically result in increased organic production, salinity-or thermallyinduced water-column stratification, as well as nearshore trapping of terrestrial material, collectively leading to condensation and deposition of organic-rich sediments (Arthur & Sageman, 2004). This is consistent with the interpretation that the investigated shale unit (FA 10; Table 1) was deposited during the early Aptian flooding of the Helvetiafjellet Formation coastal plain (e.g., Midtkandal et al., 2016). Although the present source rock is interpreted to be condensate-to gas-prone in the study area due to its proximal position, a potentially oilprone source rock may be present in distal positions on the shelf south of Svalbard where fully marine conditions prevailed during deposition. Barremian to Aptian oilprone source rocks are known to be locally present in, for example, the Fingerdjupet Subbasin (proven by well 7321/9-1S, see www.npd.no).

Regional correlations and provenance perspectives
Our study of the Lower Cretaceous succession in Nordenskiöld Land is consistent with previous regional palaeogeographic reconstructions indicating a northwestern provenance area, in particular for the Rurikfjellet and Helvetiafjellet formations ( Fig. 16; Birkenmajer, 1984;Steel & Worsley, 1984;Dypvik et al., 1991bDypvik et al., , 2002Gjelberg & Steel, 1995;Midtkandal & Nystuen, 2009). Across the study area, the Festningen Member is dominated by quartzose arenites and shows a consistent southeastward palaeocurrent direction (except in tidally influenced sections with bidirectional cross-strata; Fig. 8). Previous detrital zircon and petrographic studies indicate that the Helvetiafjellet Formation in general was sourced from a mixed metamorphic basement and sedimentary terrane to the west, with the interior of Northeast Greenland being the most obvious candidate (Røhr et al., 2007). A similar terrane also occurs in northern Svalbard, and apatite fission track studies demonstrate that northern Svalbard has been subjected to uplift and exposure since the Early Jurassic, particularly during Jurassic-Cretaceous times (Dörr et al., 2012). Thus, detrital inputs from northern Svalbard seem likely (e.g., Edwards, 1979). A northwestern provenance area has also been suggested for the underlying Bathonian to Ryazanian Agardhfjellet Formation (Dypvik et al., 1991(Dypvik et al., , 2002Koevoets et al., 2018). There is, however, a pronounced petrographic change from quartzose arenites in the Helvetiafjellet Formation to volcanic arenites in the overlying Carolinefjellet Formation (Edwards, 1979;Maher, 2001;Maher et al., 2004). In eastern Nordenskiöld Land (e.g., the Agardhbukta area; Fig.  1), this change is also observed in the upper part of the Helvetiafjellet Formation (Edwards, 1979;Birkenmajer, 1984). A 90° switch in palaeocurrent directions from southeast-to northwest-directed is also evident in these areas (Birkenmajer, 1984). In addition, basaltic lava flows of Barremian-Aptian age are intercalated with the Helvetiafjellet Formation on Kong Karls Land (Worsely, 1986). Collectively, these findings point to influx of volcanic sediments attributed to erosion of the large igneous terrane (i.e., the HALIP) that formed east to northeast of Svalbard in the Barremain to Aptian (e.g., Maher et al., 2004;Corfu et al., 2013;Senger et al., 2014;Polteau et al., 2015). Although this source area became increasingly important from the early Aptian and onwards, it did not influence sediment routings in western Svalbard in pre-Aptian times (Edwards, 1979). In a circum-Arctic perspective, the Lower Cretaceous succession in Nordenskiöld Land bears similarities to age-equivalent strata at Kilen in the Wandel Sea Basin, NE Greenland ( Fig. 3; Dypvik et al., 2002;Hovikoski et al., 2018), and partly to the Isachsen Formation of the Sverdrup Basin in the Canadian Arctic ( Fig. 3; e.g., Tullius et al., 2014). In these successions, the sediments are very rich in quartz suggesting craton-interior and recycled orogenic provenances (Røhr et al., 2008(Røhr et al., , 2010. Provenance studies demonstrate that the interior parts of the Greenland Shield was the main source area for both successions (Røhr et al., 2008(Røhr et al., , 2010. Of particular interest is also the stratigraphic development reported from Kilen (Dypvik et al., 2002;Hovikoski et al., 2018), which during the Early Cretaceous was located south of Svalbard adjacent to the western margin of the Barents Shelf (Fig. 3). At Kilen, a Barremian-lower Aptian paralic unit (the Lichenryg Formation) truncates a Valanginian-Hauterivian offshore shale-dominated unit (the Dromledome Formation; Hovikoski et al., 2018), thus resembling the stratigraphic relationship observed between the Helvetiafjellet and Rurikfjellet formations of the Lower Cretaceous succession in Svalbard. The paralic unit at Kilen is further capped by a late Aptianmiddle Albian heterolithic unit (the Pil Member of the Galadriel Fjeld Formation) representing offshore to shoreface environments similar to the Carolinefjellet Formation in Svalbard. The stratigraphic similarities may thus indicate that the Lower Cretaceous successions in western Svalbard and Northeast Greenland were part of the same drainage system, at least during Valanginianearly Aptian times. Thus, it is concluded that western Svalbard, and probably the western rim of the Barents Shelf, had its main source in the west to northwest in Bathonian-early Aptian times. In Svalbard, a regional switch to a volcanic source area associated to the HALIP terrane to the northeast took place from the early Aptian and onwards, eventually becoming the dominant source area for most of the basins on the northern Barents Shelf (e.g., Kairanov et al., 2018).

Conclusions
The Lower Cretaceous succession in central Spitsbergen, Svalbard, consists of the Rurikfjellet, Helvetiafjellet and Carolinefjellet formations, and includes sediments deposited in environments ranging from fluvial to outer shelf. In this study, core-and outcrop-based sedimentological and biostratigraphic analyses have provided new insight into the depositional development of the Lower Cretaceous succession. The main conclusions are as follows: • The Rurikfjellet Formation is of Valanginian to Hauterivian and earliest Barremian age, and consists predominantly of mudstone-dominated outer-shelf deposits (the Wimanfjellet Member) grading upwards into a sandstone-rich offshore transition to lower shoreface deposits in the upper part of the formation (the Kikutodden Member). The offshore transition to lower shoreface deposits stack into a series of shallowing-upwards parasequences which contain abundant hummocky cross-stratified sandstones. These units record successive shoreline progradations onto a storm-dominated shelf and reflect basinwide regression prior to the onset of shelf uplift and exposure in the Barremian.
• Base-of-slope mass-transport deposits (here informally referred to as the Adventpynten member) overlain by prodelta slope and delta-front deposits occur in wells DH1 and DH2, and record an earliest late Hauterivian regressive pulse which has previously not been recorded in Svalbard. The character of the mass-flow deposits, which include turbidites, debrites and slump deposits with rafted blocks of deltaplain origin, suggests that the flows originated from collapse of an unstable delta front.
• The Helvetiafjellet Formation is of early Barremian to early Aptian age and consists of fluvial braidplain, floodplain and interdistributary bay, crevasse splay, mouth-bar, and fluvial distributary channel deposits. The base of the formation is demarcated by a lower Barremian subaerial unconformity. Because the uppermost few metres of the Rurikfjellet Formation are also dated to be of early Barremian age, the subaerial unconformity records a hiatus of a limited time span (<2 myr).
• The sandstones of the Festningen Member were deposited in a laterally and regionally extensive fluvial braidplain environment which persisted through the early Barremian. Lateral thickness variations occur within the unit, suggesting that erosional topography formed by fluvial incision existed locally.
• The upper part of the Helvetiafjellet Formation (the Glitrefjellet Member) shows an overall transgressive development. Aggradationally stacked fluvial overbank deposits are overlain by wavereworked mouth-bar deposits which are capped by a transgressive conglomerate containing extrabasinal lithic clasts and intraformational siderite clasts. Thus, an upwards increase of marine influence prior to the regional marine flooding of the Helvetiafjellet Formation coastal plain in the early Aptian is evident.
• A lower Aptian shale unit up to 10 m thick, which caps the transgressive conglomerate unit, marks the boundary between the Helvetiafjellet and Carolinefjellet formations. The shale accumulated during a regional flooding event which drowned the low-angle coastal plain of the Helvetiafjellet Formation and resulted in widespread (regional) deposition of organic-rich mud. Rock-Eval and GC-MS analyses indicate a fair to good source rock that may be capable of generating gas or condensate. The predominance of mixed Type II-III kerogen and the relative abundance of long-chained n-alkanes suggest a significant contribution from terrigenous organic matter, thus pointing to a suboxic paralic marine depositional environment.
• The lower half of the Dalkjegla Member is characterised by abundant hummocky cross-stratified sandstones and represents renewed storm-dominated shoreline progradation onto the shallow shelf of the drowned Helvetiafjellet Formation. The sandstones of the Dalkjegla Member gradually pass upwards into the shale-dominated Innkjegla Member, recording a long-term shoreline retreat.
• The investigated succession displays clear stratigraphic similarities to age-equivalent strata in NE Greenland, suggesting that these successions may have formed part of the same drainage system.