Trajectory analysis and inferences on geometric relationships of an Early Triassic prograding clinoform succession on the northern Barents Shelf

Marine and Petroleum Geology 54 (2014) 167e179

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Research paper

Trajectory analysis and inferences on geometric relationships of an Early Triassic prograding clinoform succession on the northern Barents Shelf I. Anell a, *, I. Midtkandal b, A. Braathen b, a a b

UNIS, The University Centre in Svalbard, Postboks 156, 9171 Longyearbyen, Norway Department of Geosciences, The University of Oslo, Sem Sælands Vei 1, 0371 Oslo, Norway

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 September 2013 Received in revised form 3 March 2014 Accepted 6 March 2014 Available online 16 March 2014

A sedimentary succession studied along three parallel seismic lines details a platform-edge progradation of 21e36 km in a northwesterly direction across the northwestern Barents Shelf. The intra-shelf clinoform succession is bounded at bottom and top by Base Olenekian and Early Ladinian seismic reflectors. The ca 800 m thick succession can be resolved into seven distinct clinothems. The system is characterized by an early sub-horizontal platform-edge trajectory with extensive progradation, limited relative sea level rise and restricted accommodation. Thereafter the system outlines a largely ascending trajectory, marking a major rise in relative sea level and creation of significant accommodation. The platform-edge appears to back-step along one line suggesting that relative sea level rise out-paced sediment influx and preserved a clinothem with a trajectory characterized by accretionary transgression. Thereafter the trajectory is overall ascending regressive, with some variation of the trajectory angle, culminating in a flat and finally descending trajectory with oblique clinoforms outlining extensive progradation and another period of limited accommodation. The clinoforms downlap onto a succession of basin-floor deposits which appear to comprise at least two separate periods of deposition, forming two separate units. The first five clinothems downlap onto the first basin-floor unit. The shift to downlap onto the second unit occurs around the second period of extensive platform-edge advance, suggesting limited accommodation promoted bypass of significant amounts of sediment to the basin floor. The Gardarbanken High has been considered an obstacle to Early Triassic sediment progradation in this part of the basin. This inference can be corroborated based on the seismic attributes, which show sediment infill and onlap near the High. The influence is also noticeable in the reduced slope relief near the High, indicating that the basin floor was topographically higher. However, other geometric attributes cannot provide any definitive measures of structural influence. The thickness of preserved topsets and the distance from the platform-edge to the toe pinch-out point of each clinothem is found to be inversely proportional. This relationship is most marked in the fully developed sigmoidal clinoforms, whereas the link appears weaker in the oblique clinoforms. A nearperfect correlation between clinothem average vertical thickness (the average sedimentary rock accumulation within the clinothem) and advance of the toe is found, with only a relatively close relationship between clinothem average vertical thickness and advance of the platform-edge. In the studied system it therefore appears the advance of the toe is governed solely by sediment influx while the advance of the platform-edge is also influenced by relative sea level. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Barents Shelf Triassic Clinoforms 2D seismic Platform-edge Trajectory

1. Introduction

* Corresponding author. E-mail addresses: [email protected], [email protected] (I. Anell). http://dx.doi.org/10.1016/j.marpetgeo.2014.03.005 0264-8172/Ó 2014 Elsevier Ltd. All rights reserved.

The Triassic succession on the northwestern Barents Shelf (Fig. 1) is characterized by progressive infill from prograding deltas and coastlines from the south and southeast, migrating across the underlying Permian carbonate platform (Glørstad-Clark et al., 2011,

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stacking pattern provide insight into changes in the rates of sediment influx and sea level, as well as paleogeography. These Triassic prograding delta-systems have been studied previously in a regional context by Glørstad-Clark et al. (2010), Høy and Lundschien (2011) and Riis et al. (2008). The present study offers a different approach in that it considers a more localized scale with study focused on detailed comparison of geometric features and trajectory analysis along three parallel seismic lines separated by w10 km (Fig. 1). The Early Triassic deposits stack up against the Gardarbanken High and the platform-edge is curved around it. Early Triassic deposits are thin or absent across the High and the seismic data appears to indicate a structural hindrance to Early Triassic progradation (Anell et al., 2014). The three lines are located on the northeastern edge of the Gardarbanken High, with Line 1 located closest to it. Geometric and volumetric changes along the lines are studied here, in order to see if it is possible to detail structural influence of the High on accommodation and sediment influx. An improved understanding of variations and geometric relationships within a single delta-system across depositional strike could provide further insight into Early Triassic sedimentary development in the northwestern Barents Sea, and to prograding shelf-scale systems in general.

2. Geological background

Figure 1. Structural elements of the western Barents Sea adapted from Faleide et al. (2008). The dotted line of the Caledonian thrust front is based on Gernignon and Brönner (2012). The three lines used for the detailed study are marked.

2010). The Triassic basin was successively filled with clastics originating from the newly formed Uralian mountain chain and the Baltic Shield (Høy and Lundschien, 2011; Mørk et al., 1999; Riis et al., 2008). The delta-systems advanced across the shelf from the Early Triassic and are believed to have reached the eastern parts of presently exposed Svalbard by the Late Triassic (Høy and Lundschien, 2011). The prograding depositional-system is preserved in the form of shelf-edge scale clinoforms (Fig. 2), visible in the seismic data (Glørstad-Clark et al., 2011; Skjold et al., 1998). Clinoforms are sloping sub-aqueous depositional forms comprising topsets, foresets and bottomsets. A clinothem is a rock-body, bounded by clinoforms (Rich, 1951). In the present study the clinoforms did not develop on the shelf-edge but rather within the platform area that encompassed the entire Barents Shelf. Therefore, the point that demarcates the transition to slope, the rollover-point, is herein referred to as the platform-edge (Fig. 2). The platform-edge trajectory (between successive rollover points) and the depositional

The assembly of Pangea culminated in the Late CarboniferousPermian with the collision and growth of the Uralide Mountains (Berzin et al., 1996; Cocks and Torsvik, 2007; Nikishin et al., 2002; Otto and Bailey, 1995; Puchkov, 2002). The Uralide foreland deposition was centered on the North and South Barents Basins (Doré, 1992; Johansen et al., 1992; Riis et al., 2008), which subsided rapidly (Gudlaugsson et al., 1998). As the foreland filled with sediment, the delta-systems advanced west and northwestward toward the northern Barents Shelf (Høy and Lundschien, 2011). Sediment was largely sourced from the Uralides, but the TimanPechora and Baltic Shield were also likely sources of sediment (Mørk et al., 1989). The source signal from the Uralide/Baltic region is apparent in detrital zircon analysis, with a dominant new source traceable early in the Triassic on Bjørnøya and later in the Triassic onshore Svalbard and Franz Josef Land (Bue, 2012; Bue et al., 2010; Worsley et al., 1986). The transition to Triassic sedimentation in the Barents Sea is marked by a regional reflector (the near Top Permian) (Gérard and Buhrig, 1990; Rønnevik et al., 1982), which identifies the transition from silicified carbonates below, to dominantly mudstones and sandstones above. The Early-Middle Triassic Sassendalen Group forms three large progradational units: the Havert, Klappmyss and Kobbe formations (Mørk et al., 1999) (Fig. 3). The studied succession is bounded by two regionally traceable surfaces, the Base Olenekian and the Early Ladinian reflectors (Anell et al., 2014; Glørstad-Clark et al., 2011, 2010; Riis et al., 2008). The lower boundary is a high impedance reflector overlying a progradational system that also advances across the shelf, marking the boundary between the Havert and Klappmyss formations (Mørk et al., 1999) (Fig. 3). The upper boundary has a lower impedance contrast but is frequently defined by downlapping reflectors. It marks the offshore transition between the Sassendalen and Kapp Toscana groups (between the Kobbe and Snadd formations) (Høy and Lundschien, 2011). The time-equivalent transition onshore is marked by a maximum concentration of organic material associated with a maximum flooding (Krajewski and Weitschat, submitted for publication). This lithostratigraphic transition occurs at the end of the Ladinian onshore (between the Botneheia/Bravaisberget and

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Figure 2. To the right, a 3D schematic figure of a single platform-edge clinothem with the various geometric parameters used in this study marked, redrawn from Helland-Hansen and Hampson (2009). On the left three schematic figures show the manner in which the CAVT (Clinothem Average Vertical Thickness) was obtained for a single clinothem along the three separate lines, along with the parameters which constrained the calculations.

Tschermakfjellet formations), which is associated with a hiatus (Krajewski and Weitschat, submitted for publication; Mørk et al., 1999). 3. Data Three WNWeESE oriented seismic lines located southeast of Hopen on the northeastern flank of the Gardarbanken High (Fig. 1) form the basis for this study. They reveal a set of Early Triassic clinoforms with a depositional dip-direction parallel to the seismic lines, from ESE to WNW. The first clinoforms downlap onto the high-impedance Base Olenekian reflector, while the successive clinoforms aggrade/prograde upward and basinward, with thick successions accumulating at the basin floor. Seven distinct

clinoforms can be identified along each of the three seismic lines, forming seven clinothems, termed C1 e C7 (Fig. 4). Above C7, bounded stratigraphically higher by the Early Ladinian reflector and hence included in the same sedimentary succession as C1eC7, there are deposits which cannot be resolved into distinct clinothems, and are hence not discussed further here (Fig. 4). Throughout the results and discussion, geometric parameters, including trajectory and trajectory angle, refer to the value obtained between two upper bounding surfaces, except in the case of volumetric calculations. Variations in clinoform geometry and attributes occur between the seismic lines, however, the platform-edge trajectory remains distinct and the separate clinothems correlate across all three lines. The clinoform succession in total comprises ca 300e400 ms of

Figure 3. Lithostratigraphy of the Sassendalen Group and lowermost Kapp Toscana Group from the Barents Sea and Svalbard, redrawn from Mørk et al. (1999). The Early Triassic clinothems from this study (C1eC7) are compared to the Early Triassic clinothems studied in the southern Barents Sea by Glørstad-Clark et al. (2010). Also shown on the right hand side are two eustatic sea level curves covering the Triassic (Haq et al., 1987; Vail et al., 1977).

170 I. Anell et al. / Marine and Petroleum Geology 54 (2014) 167e179 Figure 4. The three seismic lines (courtesy of the Norwegian Petroleum Directorate, NPD) used in this study detailing the platform-edge clinoform succession. The gray box in the corner of the upper-most images shows the main seismic reflectors and their probable age based on previous studies (Anell et al., 2014; Glørstad-Clark et al., 2011; Glørstad-Clark et al., 2010; Høy and Lundschien, 2011; Riis et al., 2008). The map shows the location of the lines and in red the section of the seismic line displayed. The middle images trace the reflectors and outline their continuity, with the colored reflectors showing the probable ages as shown by the key. The images below this show close-ups of the seven clinothems. The interpreted trajectory is stippled between successive platform-edge breaks. The yellow unit at the base of the clinoforms shows the basin-floor deposits, note the separation into two units along lines 2 and 3. The seismic section in the lower left corner shows Line 1 flattened on the Early Ladinian reflector showing the advance of the clinoforms towards the flank of the Gardarbanken High. The two close-up images show the sigmoidal clinoforms near the high (upper image) to progressively flatter and finally horizontal reflectors filling available accommodation (lower). The lower right seismic section shows Line 3 flattened on the Early Ladinian reflector showing how further from the flank of the Gardarbanken High the structural influence is less apparent and in the close-up progradation continues albeit at a very low angle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

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deposits. By assuming a velocity of 4000 m/s, which is typical of Triassic sediments in this part of the Barents Sea (Czuba et al., 2008; Faleide et al., 1991; Grønlie and Elverhøi, 1979; Renard and Malod, 1974), the cumulative clinoform succession is w700e800 m thick. At the basinward termination point of the clinoform reflectors, there is a series of slightly convex-up, sub-parallel strata. They form a topographically higher area on the basin floor which the clinoform bottomsets onlap in the base of clinothems C3 and C4, whereas higher numbered clinoforms downlap this architecture. This seismic appearance is consistent with basin floor fan deposits described in other literature (e.g. Shanmugam et al., 1996; Posamentier and Kolla, 2003), and they are henceforth referred to as basin floor deposits in this study. 4. Methods 4.1. Trajectory analysis Shelf-edge trajectory analysis is a sequence stratigraphic method which provides a dynamic approach to sedimentary successions and insight into ancient paleogeography, sediment-type distribution, relative sea level change and sediment influx rates (Helland-Hansen and Hampson, 2009). By tracing the trajectory of the shelf-edge (or platform-edge) through time (i.e. through a sedimentary succession, typically a seismic line), a proxy for the relative relationship between rate of sedimentation and rate of sea level change can be established for the relevant time period. The trajectory is determined by plotting successive positions of the platform-edge (Fig. 2) along a succession of clinoforms (Fig. 5) (Helland-Hansen and Gjelberg, 1994). Trajectories can be categorized as ascending, flat or descending (Helland-Hansen and Hampson, 2009). Flat and descending trajectories are characterized by predominantly oblique clinoforms, meaning that the strata chiefly build up on the slope basinward of the shelf-edge break, while ascending trajectories promote development of full

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sigmoidal forms, where the entire bathymetric profile is represented (Mougenot et al., 1983). Thus, during periods when the shelf-edge trajectory is ascending, accommodation is being generated across the entire basin, expanding in all directions. In contrast, the accommodation is being shifted basinward during periods of flat and descending trajectories, and expansion of the accommodation is limited to the basinward direction. In turn, this promotes the aforementioned oblique clinoform geometries. The trajectory angle is the angle determined by connecting successive shelf-edge breaks, in this case platform-edge breaks, relative to a datum that approximates palaeo-horizontal (HellandHansen and Hampson, 2009) (Fig. 2). While an exact horizontal datum for calculating the angle of trajectory can be difficult to define in seismic data, flooding surfaces are commonly used as a reference. Flooding surfaces are generally widespread, and represent relatively short periods of time when the basin floor topographic relief was at a minimum, and the continental shelf would typically have a largely constant gradient in the range of 0.1 e0.3 (Johnson and Baldwin, 1996; Suter, 2006). The Base Olenekian flooding surface has been selected as a horizontal datum for this study. The lowest trajectory angles are near 0 , and given the difficulties in correcting exactly to datum, slightly descending, flat or slightly ascending trajectories cannot be distinguished with confidence. Due to the limitations in finding exact trajectory angles, values between 0.5 and 0.5 are considered to be flat regressive (Table 1). It is possible that platform-edge breaks have been eroded and incised and, given the lack of well control and limitations due to data quality, there is potential for some erroneous inferences of platform-edge breaks, basin floor deposits and overall clinoform geometry. 4.2. Geometric relationships A suite of properties for each clinothem has been measured including the platform-edge trajectory and trajectory angle, the

Figure 5. A. The main study area in which a clear prograding succession can be recognized is outlined in relation to surrounding structural elements. The Gardarbanken High is redrawn from Mørk (1999). The three seismic lines parallel to the advancing system that have been used for detailed study are marked in bold, with other seismic lines used to map the platform-edge also displayed. The advance of the platform edge in the early Triassic is shaded in gray showing the apparent curved path around the Gardarbanken High. B. The study area is shown in a simplified 3D representation. The location of the platform-edge for each of the seven clinothems is shown. C. Simplified cartoon of an advancing shoreline and development of a platform-edge (schematic figure, not to scale).

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Table 1 The third column shows the trajectory angle between clinothems along each of the three lines (for details on trajectory angle see Fig. 2). Based on these values we define if the trajectory is ascending, descending or flat (trajectory angles are considered flat if they are  0.5 ) and if the platform-edge is regressive or transgressive (column 2). The table also shows the values for platform-edge advance, the advance of the toe and the preserved topsets, which are displayed in graph form in Figure 8. The summarizing total or average of the values for each line is presented at the bottom (italics indicate average values) and the average of these findings for all three lines is presented at the bottom of the table. Trajectory Line 1 C1eC2 C2eC3 C3eC4 C4eC5 C5eC6 C6eC7 Line 2 C1eC2 C2eC3 C3eC4 C4eC5 C5eC6 C6eC7 Line 3 C1eC2 C2eC3 C3eC4 C4eC5 C5eC6 C6eC7

Trajectory angle (degrees)

Platform edge advance (m)

Toe-advance (m)

Preserved topsets (m)

Flat - ascending (regressive) Ascending (aggrading) Ascending (regressive) Ascending (regressive) Ascending (regressive) Flat - descending (regressive) Total/average

0.31 63 1.83 3.57 2.01 0.67 1.10

7226 100 3126 1900 2850 5980 21,182

3437 3241 12,751 1446 4627 5726 31,228

72 202 114 68 70 0 526

Flat - ascending (regressive) Ascending (transgressive) Ascending (regressive) Ascending (regressive) Flat - descending (regressive) Flat - descending (regressive) Total/average

0.08 172 1.19 2.53 0.1 0.18 0.66

7447 1250 7199 2265 5800 13,110 34,330

5431 2975 18,648 3267 4302 5211 39,834

70 196 152 84 44 0 546

Ascending (regressive) Ascending (aggrading) Ascending (regressive) Ascending (regressive) Flat - ascending (regressive) Flat - descending (regressive) Total/average 3 Lines average

1.03 114 0.74 1.74 0.06 0.04 0.78 0.85

4100 100 6900 2500 8962 13,334 35,696 30,403

2978 3667 12,550 3434 12,334 3953 38,916 36,659

92 200 172 78 34 0 576 549

platform-edge and clinothem toe advance (or retreat), thickness of preserved topsets, slope relief and clinothem average vertical thickness (CAVT, described below) (Fig. 2). Comparisons between the various values to assess changes across the three lines were performed to ascertain any correlations between different indicators. Vertical values in meters assume a velocity of 4000 m/s, which is a typical value for Triassic sedimentary rocks on the northern Barents Shelf (Czuba et al., 2008; Faleide et al., 1991; Grønlie and Elverhøi, 1979; Renard and Malod, 1974). Rate of sedimentation is a one-dimensional value given by decompacting the volume of sediment/area divided by time. For the studied clinothems there are no age-constraints, hence, a true rate of sedimentation is not possible to calculate. Instead, in this study, we calculate the Clinothem Average Vertical Thickness (CAVT). The CAVT is essentially the rate of sedimentation without time, and hence represents a one-dimensional value of sedimentary rock contained within each clinothem along each line. For adequately comparable values between the seismic lines, the distance between the platform-edge and the southeastern end (proximal direction) of Line 1 (which covers the least of the platform-edge system), was used for each calculation (Fig. 2). As such, the values reflect equal horizontal distance relative to the platform-edge for individual clinothems (for example the C1’s on each line), but not equal distance for separate clinothems. The CAVT of a single clinothem is measured from volumes contained subsurface within an area 250 m wide on either side of the seismic line, lengthwise encompassing the whole clinothem to the proximal cut-off measured on Line 1. The volume is then divided by the areal extent of the 3D clinothem body for which volume has been calculated. The results are not decompacted, which may skew the results slightly towards lower values for the presumably more compacted deeper clinothems. The seismic reflectors bounding the clinothems can be correlated across the three lines and are assumed to represent practically isochronous surfaces. It follows that the amount of time it has taken to deposit the volume of

sediment within one clinothem, for example C3, is the same across all three lines. Therefore, calculating the CAVT of each clinothem serves as a proxy for a comparison of the “relative depositional rate” between clinothems of the same age, along different lines. The CAVT can also be compared to various clinothem parameters, to determine if sediment influx influences any geometric parameters. Slope relief is a vertical value measured from the platform-edge on the upper boundary of the clinothem to the flattening of the toe of the same upper boundary (Fig. 2). This value represents the minimum vertical space available basinward from the platformedge at the time when deposition of a single clinothem culminated. As with the aforementioned clinothem average vertical thickness, the slope relief is only compared between coeval clinothems across the different seismic lines. The trajectory of the platform-edge results from interplay between eustatic sea level, subsidence/uplift of the basin and sediment supply. The slope relief can be used as a proxy for relative sea level changes expressed by the platform-edge trajectory, thus strengthening the validity of values applied in calculations.

5. Results Clinothems C1 to C7 average a basinward progradation distance of 30 km along the three seismic lines, (Figs. 5 and 6a, Table 1), with an average trajectory angle of 0.85 . There is variation between the lines, however, with a steeper average trajectory (w1.1 ) along Line 1, with a shortened progradation distance of 21 km, whereas Line 2 shows a more extensive progradation of 34 km with a trajectory angle of 0.66 . The various trajectory angles, essentially providing a value of the relationship between progradation and aggradation between clinothems, is similar across all three lines. The angle is relatively low between C1 and C2, higher between C2 and C5 and thereafter generally very low and negative for the final two clinothems C6 and C7 (Table 1).

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Figure 6. A. Graph showing the cumulative platform-edge advance and the general trend from Line 1 to Line 3. B. Graph showing the cumulative clinothem average vertical thickness across lines 1 to 3, with the general trend marked with a line. C. Graph showing the cumulative platform-edge to toe distance across the three lines with the general trend, which is relatively even, marked with a line. The contribution of individual clinothems to the cumulative value is delineated by shades of gray (see key).

The clinoforms downlap a relatively thick succession of basin floor deposits. The basin-floor deposits are possible to distinguish from the clinothems as parallel to sub-parallel reflectors forming a convex-up body on the basin floor onto which the sigmoidal reflectors terminate. The basin-floor deposits form a wedge-shaped body that thickens toward the northwest. The basin floor deposits are difficult to resolve with precision in the seismic data, however, two separate units can be identified on lines 2 and 3 (Fig. 4). The location of the platform-edge during the Triassic has been mapped out in detail in the northern part of the Barents Shelf (Høy and Lundschien, 2011) as well as for the whole western region (Glørstad-Clark et al., 2010). The two do not link together perfectly as seen in Figure 7a, but with the detailed study presented here, a tentative link between the two is suggested (Fig. 7b). 5.1. Clinothem and platform-edge trajectory description On each of the profiles discussed here, the clinothem platformedge breaks follow a similar trajectory (Fig. 4). C1e C2 forms a flat to ascending (Line 3) trajectory. The platform-edge advance from C1 to C2 is around 4e7 km. Thin topsets are preserved on C2 along each line, indicating that accommodation was limited and relative sea level was rising slowly. C1 and C2 are both characterized by generally low amplitude internal reflectors and medium amplitude bounding reflectors. Early C2 deposits downlap onto the C1 bottomsets, eventually prograding past C1 and in places partly onlapping onto the aforementioned basin-floor fan deposits. The trajectory between clinothems C2 and C3 is near vertical, slightly back-stepping on line 2, marking an aggradational to retrogradational development (Fig. 4). The platform-edge appears to have moved forward 100 m on Line 1 and back some hundred meters on Line 3. However, the back-step could be interpreted to be more than 1 km on Line 2 (Fig. 4). The highly aggradational to potentially retrogradational deposits in C3 preserves the, comparatively, thickest topsets. Relative sea level rose considerably and created significant accommodation. C3’s upper boundary is marked by a high amplitude reflector but generally low to medium amplitude reflectors are recorded within. There are indications of onlap onto the top of C2, consistent with C3’s retrogradational character. While the platform-edge aggrades or retrogrades, the toe of the clinothem advances slightly and downlaps onto the thin basin-floor deposits on lines 2 and 3. The trajectory from C3 to C5 marks an ascending regression. The C3 to C4 transition is marked by renewed progradation following the generally aggrading C3 deposits. The platform-edge advance is 7 km on lines 2 and 3, and w3 km on Line 1. C4 displays thick

foresets, with comparatively thinner topsets, followed by a steeper trajectory and more aggrading deposits in the succeeding clinothem, C5. C4 contains the highest sediment volume and the greatest CAVT. It has readily identifiable upper and lower boundaries, and includes some medium amplitude reflectors within. The clinoforms downlap onto basin-floor deposits on lines 2 and 3 but tangentially onlap onto the mound-shaped basin floor deposits on Line 1. The transition to C5 marks the second steepest trajectory angle (after the aggrading to retrograding angle between C2 and C3) in the system, between ca 2e4 . The increase in trajectory gradient indicates a reduced progradation rate, in favor of increased sediment storage on the platform. The CAVT is lower than in C4 and both toe and platform-edge advance is less, although the termination point is difficult to determine in some places. C5 is outlined by generally high amplitude reflectors, however, the top reflector loses its signature around the break-point and within the topsets. The bottomsets of the clinothem downlap onto the basin-floor succession. C6 and C7 generally display flat to descending trajectories and extensive progradation distances. The trajectory angle is generally very low between C5 and C6 and negative between C6 and C7. An exception occurs along Line 1 where the C5eC6 transition has a comparatively high ascending trajectory angle. Along lines 2 and 3 the platform-edge advances significantly, in total ca 19e22 km between C5eC7 in comparison to 9 km on Line 1. Relative sea level was stable or falling during deposition of C6 and C7. Platform-edge regression was extensive and accommodation limited. C6 is generally characterized by sigmoidal to tangential oblique clinoforms, with thin topsets preserved, while C7 transitions into oblique clinoforms with no topsets preserved. Low to medium amplitude, in part mildly chaotic reflectors characterize both clinothems. It appears that the basin floor deposits outline two distinguishable successions along Line 2 and 3. The break occurs between C5 and C6 where it is possible to delineate basin-floor deposits overlying the C5 bottomsets, with C6 bottomsets clearly downlapping onto this unit. 5.2. Geometric relationships The cumulative CAVT (Fig. 6b) shows an increasing trend from Line 1 to Line 3. The advance of the platform-edge, by contrast, is almost identical along lines 2 and 3 but significantly less on Line 1 (Fig. 6a). Line 1 has, on the whole, higher trajectory angles (Table 1), indicating a comparatively more stacked and aggradational profile than the other two lines. This is of particular note for C5eC6 and C6eC7, where the platform-edge advances significantly along lines 2 and 3 and much less along Line 1 (Table 1). Of note is also that on

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Figure 7. A. Previously inferred location of the platform-edge from the studies of Glørstad-Clark et al. (2010, 2011) in the south and Riis et al. (2008) in the north. B. Links between the two more regional studies using the detailed analysis done in this study in the outlined quadrangle.

Line 1 the clinoforms onlap the basin-floor deposits at a relatively high angle; while lines 2 and 3 they tangentially downlap onto the older substrate. The sigmoidal form of the reflectors near the flank transitions to flat and chaotic closer to the High along Line 1, while retaining a more continuous downlapping geometry along lines 2 and 3 (Fig. 4). While both platform-edge advance and sediment influx is lower on Line 1, the increase in the distance between the platform-edge and toe is relatively similar across all three lines, although it too increases slightly from Line 1 to Line 3 (Fig. 6c). Comparing thickness of preserved topsets and distance between platform-edge and toe, an inverse relationship is expected. Clinoforms with steep ascending trajectories, during more dominantly aggradational development, will preserve a thicker succession of topsets. A more dominantly progradational system, with sedimentation dominantly occurring beyond the platform-edge, is expected to promote development of greater distances between the platform-edge and the toe of the bottomset. The relationship is found to be generally inversely proportional, although in the oblique clinoforms (C2 and C7), the trend weakens (Fig. 8a-1). By comparing the CAVT between the three lines, it is apparent that although the values differ, the trend is very similar (Fig. 8b-1). Comparing the CAVT to two averaged values; the average advance of the platform-edge and average advance of the toe, it is noted that while the platform-edge advance shows a similarity in trend, the advance of the toe reveals a near-perfect correlation between the lines (Fig. 8b-1). Detailed comparisons of on each line shows that this relationship holds true independently on each line (Fig. 8b-2e 8b-4). The slope relief, which is a proxy for accommodation, shows a near complete sea level curve, with depth increasing away from the Gardarbanken High (Fig. 8c). 6. Discussion 6.1. Relative sea level change and sediment influx The prograding system is characterized by a relatively flat trajectory (C1eC2), followed by an ascending stationary to landward

migrating (transgressive) trajectory (C2eC3), then by ascending regressive successions (C3eC5), and finally by a flat to descending trajectory (C5eC7) (Fig. 9). Accommodation, while a concept with some definition challenges (Muto and Steel, 2000), is governed by creation/removal of space (subsidence/uplift, eustatic sea level rise/ fall) and sediment influx (Catuneanu et al., 2009). Stacking patterns develop as a result of the relative rates of change in accommodation and sediment input (A/S ratio). Individual clinothems typically represent a small segment of the relative sea level curve, with the bounding reflectors produced by a combination of regression and ensuing flooding. However, the overall trajectory of the system studied here appears to delineate a near-full cycle of relative sea level change (Fig. 9). C1eC2 is generally characterized by a very low angle, basinward ascending trajectory. The C2 clinothem is strongly progradational, with a platform-edge positioned slightly above the platform-edge of C1 (Fig. 4). The preservation of topsets generally indicates a rise in relative sea level. The thin topsets and low trajectory angle of C2 (Table 1) is inferred to represent the earliest stages of rising relative sea level, with a low to moderate rate of rise (Fig. 9). A landward migrating trajectory, with preserved clinoforms, as potentially observed between C2 and C3 on Line 2, is termed accretionary transgression according to Helland-Hansen and Gjelberg (1994). The C2eC3 succession outlines the highest rates of rising relative-sea level (Fig. 9), which potentially outpaced sediment supply causing the platform-edge to back-step on Line 2. On lines 1 and 3 the trajectory is near-vertical, reflecting an aggradational architecture during which deposition and rate of accommodation were matched (Catuneanu et al., 2009). The succeeding high-angle basinward climbing trajectories (characterized by building upward and basinward, C3eC5) indicate long-term rise in relative sea level with sediment supply higher than the rate of creation of accommodation (Bullimore et al., 2005; HellandHansen and Hampson, 2009; Henriksen and Vorren, 1996; Mougenot et al., 1983). The trajectory indicates normal regressive development, with preserved deposits in the topset area. The highest trajectory angles, C4 to C5, indicate higher rate of rise in relative sea level compared to sediment influx rate (Bullimore et al.,

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Figure 8. Graphs depicting various geometric relationships in the studied platform-edge system. Graph A-1 shows the inverse relationship between thickness of preserved topsets (y-axis) and distance between platform-edge and toe (x-axis). The main deviations from the trend, the largely oblique C2 and C7 clinothems, are circled in orange. Graph A-2 shows, in bold lines, the thickness of preserved topsets (left axis) compared to the length between the platform-edge and toe, in stippled lines (right axis), of each individual clinothem (xaxis). Graph B-1 shows the clinothem average vertical thickness along all three lines (left axis) compared to the average value (of the three lines) of the advance of the toe and advance of the platform-edge (right axis), highlighting the strong relationship between clinothem average vertical thickness and advance of the toe. B-2 to B-4 show the same comparisons but made along individual lines, the scale bars on the right and left y-axis are the same as for B-1. Graph C shows changes in the slope relief along the three lines between the seven clinothems. The vertical distance was measured from the platform-edge to the toe of the bounding clinoform. The relative sea level curve, inferred from the trajectory analysis, is stippled in black. All vertical values in meters were depth converted using a velocity of 4000 m/s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

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Figure 9. A summary of the main features of each individual clinothem and associated trajectory (as seen on Line 3) is presented. The upper boundary of a set of clinothems is traced and tilted back to a near-horizontal position based on the underlying base Olenekian flooding surface, used for datum correction. An arrow indicates the direction of the platform-edge trajectory. A short description (based on the observations made in this study) is presented next to each clinothem. A relative sea level curve based on the platformedge trajectory is shown in the upper right corner together with the inferred stage of development for each of the clinothems.

2005), in turn suggesting that at this time a reduction of sediment influx rate and/or an increased rate of relative sea level rise occurred. Flat/low angle to descending platform-edge trajectories, without or with very little preserved topset material, generally characterize C5eC7. These trajectories represent a stable to early stages of falling relative sea level and indicate less accommodation available on the existing platform areas (Helland-Hansen and Hampson, 2009; Kolla et al., 2000; Mougenot et al., 1983). Sediment bypass is promoted, and progradation is accelerated as a result. Additionally, greater amounts of sediment reach the basin floor (Carvajal and Steel, 2006; Johannessen and Steel, 2005). Smaller basin floor fans can still be created in association with rising trajectories, however, flatter trajectories are linked to thicker and more extensive fans (Carvajal and Steel, 2006). These deposits mark the end of rising relative-sea levels and the start of a new falling stage (Fig. 9). Negative trajectory angles are typically ascribed to forced regression (Catuneanu et al., 2009; HellandHansen and Gjelberg, 1994; Hunt and Tucker, 1992; Van Wagoner et al., 1988). Overall C1eC7 appears to reflect the response of a high sediment input system to the onset of relative sea level rise and early fall during deposition of C7. The depositional cycle is then broken, followed by a rate of relative sea level fall high enough to make it impossible to distinguish a clear clinothem after the deposition of C7. The deposits succeeding C7 (fully exposed only on Line 1, Fig. 4), outline a clear falling trajectory trend. 6.2. Structural influence The slope relief (Fig. 2) is controlled by relative sea level changes and basin floor sediment accumulation. From the slope relief curves (Fig. 8c) the inferences from the trajectory analysis can largely be confirmed. The height curves are similar to the inferred changes in

relative sea level (Figs. 8c and 9). The discrepancy between the two curves occurs because when relative sea level rise/fall slows and stabilizes, sediment accumulation on the basin-floor continues, thus reducing the water depth further. This means that the slope relief minimum will occur after the relative sea level curve has bottomed out, and the slope relief maximum high will occur before relative sea level peaks. The early clinothems, C1 and C2, reveal a decreasing to minimum slope relief. The C1eC2 trajectory is inferred to indicate stable to rising relative sea level. The decreasing slope relief may indicate significant basin-floor accumulation. C2 to C5 show increasing to stabilizing slope relief in line with the ascending platform-edge trajectory and major transgression around C3. The slope relief stabilizes prior to the inferred relative sea level high. This is, again, an indication of sediment accumulation on the basin floor. During deposition of C3 to C5 basin floor accumulation appears to balance relative sea level rise (Fig. 8c). C6 and C7 reveal decreasing accommodation space as relative sealevel began to fall. The three lines are located on the northeastern edge of the Gardarbanken High, with Line 1 closest to the High (Fig. 5). Along Line 1 the northeastern edge of the High can be seen in the anticlinal form of the deep reflectors (Fig. 4), which is not apparent on the other two lines. There are some notable differences in the seismic attributes near the High between the three lines, most marked if the data is flattened on the Early Ladinian flooding surface (Fig. 4). Along Line 1 the advancing clinoforms become progressively less sigmoidal near the flank of the High, eventually transitioning to vaguely curved reflectors, truncated at the top and filling the space up to the Early Ladinian reflector. Further east along Line 3 the advancing clinoforms also flatten out, but appear to continue advancing northwestward (Fig. 4). Seismic data further west shows Early Triassic progradation piling up against the High, and very thin deposits across it (Anell et al., 2014). The advance of the platform-edge is curved around the High in the Olenekian and

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Anisian (Fig, 5). The High has therefore been suggested to have acted as a structural hindrance to Early Triassic progradation (Anell et al., 2014). If the Gardarbanken High influenced clinoform development during deposition of C1eC7 we might expect limited accommodation reflected in lower slope relief values, less preserved topsets and a more rapid advance of the platform-edge until the space was filled, and the advance would stop. CAVT values, which reflect sedimentation rate, would probably not be influenced by the structure unless accommodation was completely filled, in which case CAVT values would be lower. While Line 1 has lower CAVT values this is mainly due to a low C4 value (Fig. 6b). C6 and C7, located right on the inclined flank (Fig. 4), show similar CAVT values along all three lines (Fig. 6). It therefore appears that the sediment influx remained relatively constant along all 3 lines up to this point. The lower slope relief along Line 1 (Fig. 8c) indicates that the basin floor (the flank of the High) was topographically higher than surrounding sea-floor. There is also a propensity towards onlap (as opposed to tangential downlap) of the clinoforms onto the basin floor deposits along Line 1. No distinct differences in preserved topsets are appreciable, nor does the platform-edge advance faster along Line, rather it is significantly less (Fig. 6a). This is strange given that the greater advance along lines 2 and 3 is mainly a result of C6 and C7, for which the CAVT is similar for all three lines. This in turn might be the result of rapid advance promoting slope collapse along Line 1, with removal of the original platform edge. One can speculate that if the platform-edge was hindered completely further west that the limited advance along Line 1 is the result of the deflection of the platform edge causing it to curve around the High. This would imply that the measured values along Line 1 are at an angle to main direction of propagation (which would be more northward). It appears that while the seismic data corroborates the structural influence of the Gardarbanken High on progradation, the measured clinoform geometric features, barring slope relief, do not appear to provide insight into structural control on sedimentary architecture. More detailed work on higher resolution data might provide further insight. 6.3. Regional correlation The base for the clinoform succession is a large flooding surface inferred to have displaced the shoreline hundreds of kilometers landwards (Glørstad-Clark et al., 2011), which is in accordance with the sharp base of the studied succession. The top of C3 is inferred to mark a major flooding, and is considered the best surface for regional correlation. It can be linked to the base of the Anisian Kobbe Formation which is marked by a transgressive pulse (Gabrielsen et al., 1990) and correlates with a major 2nd order transgression in the Sverdrup Basin, Svalbard and Eastern Siberia (Mørk and Smelror, 2001). This transgression is the base of the Bravaisberget Formation of West Spitsbergen, and the base of the Botneheia Formation of East Svalbard, both marked by fine-grained mudstone on top of coarser sediments (Mørk et al., 1999). In the southwestern Barents Sea this event has been mapped in the prograding successions (the T1-T2 transition of Skjold et al., 1998, and O3-A1 transition of Glørstad-Clark et al., 2010, 2011). Internal characteristics can be correlated between several clinoform packages in the work of Glørstad-Clark et al. (2010, 2011) further linking the successions (Fig. 3). The onshore Sassendalen Group exposures on Svalbard are believed to be sourced chiefly from a western or southwestern source (Harland and Geddes, 1997; Krajewski and Weitschat, submitted for publication; Nagy et al., 2011; Riis et al., 2008; Worsley, 2008; Worsley and Aga, 1985), with the coarsest

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deposits attributed to local tectonic activity (Krajewski and Weitschat, submitted for publication). The overall coarsening upward of the Bravaisberget Formation and subdivision into several coarsening upward members (Mørk et al., 1999), is much like expected in the time-equivalent C4eC7. However, the deposits on western Svalbard are influenced by local tectonism and extensive sediment reworking (Krajewski and Weitschat, submitted for publication). Nonetheless, with further study it may be possible to link onshore and offshore development and determine the degree of regional influence on sedimentation. The Botneheia Formation is characterized by a coarsening upward trend from mudstone into siltstone forming the distal deposits of advancing delta systems. Eustatic sea level is difficult to ascertain and there are large discrepancies in global sea level curves (Haq and Al-Qahtani, 2005; Haq et al., 1987; Vail and Mitchum, 1977; Vail et al., 1977). However, when comparing two eustatic curves covering the Triassic, they both detail an overall transgressive trend with a significant fall near the top of the Anisian (Haq et al., 1987; Vail et al., 1977). This fall is appreciable in the platform-edge trajectory and is most likely what generated the regional upper bounding reflector of the succession. 7. Conclusions This study details a succession of clinoforms which prograded northwestward across the northern Barents Shelf during the Early Triassic. The platform-edge followed a curved path around the northeastern edge of the Gardarbanken High. The sedimentary succession displays a large-scale fluctuation in relative sea level and marks infill between the end of falling relative sea level through rising relative sea level, to the early stages of a renewed fall. This is indicated by early limited accommodation and significant sediment bypass, driving lengthy basinward progradation. The early sediment influx can probably be linked to large-scale basin-floor sediment accumulations, onto which several successive clinoforms downlap. Eustatic sea level rose and matched or possibly outpaced sediment supply during the regional late Olenekian-early Anisian transgression and generated an accretional to slightly transgressive development. The transgression created significant accommodation for the succeeding clinoforms, which show ascending trajectories owing to high sediment influx. The youngest clinoforms in this study are oblique and prograde far into the basin, onlapping a secondary basin floor unit. Relative-sea level was stable to falling at this time, and lowered relative sea level accounts for the significant influx of sediment into the deeper parts of the basin. The secondary basin floor unit is presumably associated with the limited accommodation and advancing platformedge. Rapid sea level fall is probably responsible for the last poorly outlined deposits succeeding deposition of C7. The thickness of preserved topsets and the length between the platform-edge and the toe of each clinothem is inversely related. The relationship is less evident in oblique clinoforms, whereas a clear trend is evident within the fully developed sigmoidal geometries. It is also noted that in this platform-edge system, the clinothem average vertical thickness is directly proportional to the general trend of the advance of the toe of clinothem, while the advance of the platform-edge shows a relatively similar trend but does not match as closely. Hence, amount of sediment appears to solely control toe-advance, whereas platform-edge advance is also influenced by accommodation. The close inspection of a suite of clinothem properties along with careful trajectory analysis has revealed quantifiable relationships, which in turn are useful tools for evaluating large-scale sedimentary architecture and how it is affected by allogenic factors such as tectonic forcing.

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