Variability of tidal signals in the Brent Delta Front: New observations on the Rannoch Formation, northern North Sea

Sedimentary Geology 335 (2016) 166–179

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Variability of tidal signals in the Brent Delta Front: New observations on the Rannoch Formation, northern North Sea Xiaojie Wei a,b,⁎, Ronald J. Steel b, Rodmar Ravnås c, Zaixing Jiang a, Cornel Olariu b, Zhiyang Li d a

School of Energy Resources, China University of Geosciences (Beijing), 29 Xueyuan Rd, Haidian, Beijing 100083, China Jackson School of Geosciences, University of Texas at Austin, 1 University Station, C1100, Austin, TX 78712, USA A/S Norsk Shell, Tankvegen 1, Tananger 4056, Norway d Department of Geological Sciences, Indiana University Bloomington, 1001 East 10th Street, Bloomington, Indiana, 47405, USA b c

a r t i c l e

i n f o

Article history: Received 23 November 2015 Received in revised form 10 February 2016 Accepted 12 February 2016 Available online 19 February 2016 Editor: Dr. B. Jones Keywords: Brent Delta front Tidal signals Storm-event beds Storm–tide couplets Mixed-energy coastal system

a b s t r a c t Detailed observations on the Rannoch Formation in several deep Viking Graben wells indicate that the ‘classical’ wave-dominated Brent delta-front shows coupled storm–tide processes. The tidal signals are of three types: I): alternations of thick cross-laminated sandstone and thin mud-draped sandstone, whereby double mud drapes are prominent but discretely distributed, II): a few tidal bundles within bottomsets and foresets of up to 10 cm-thick sets cross-strata, and III): dm-thick heterolithic lamination showing multiple, well-organized sand–mud couplets. During progradation of the Brent Delta, the Rannoch shoreline system passed upward from 1) a succession dominated by clean-water, storm-event sets and cosets frequently and preferentially interbedded with type I tidal beds, and occasional types II and III tidal deposits, toward 2) very clean storm-event beds less frequently separated by types II and III tidal beds, and then into 3) a thin interval showing muddier storm-event beds mainly alternating with type II tidal beds. It is likely that those variations in preservation bias of storm and tidal beds in each facies succession result from combined effects of 1) the frequency and duration of storms; 2) river discharge; and 3) the absolute and relative strength of tides. Tidal deposits are interpreted as inter-storm, fair-weather deposits, occurred preferentially in longer intermittent fair-weather condition and periods of lower river discharge, and well-pronounced in the distal-reach of delta-front. The formation and preservation of tidal signals between storm beds, indicate that the studied Rannoch Formation was most likely a storm-dominated, tide-influenced delta front 1) near the mouth of a large Brent river, where a significant tidal prism and high tidal range might be expected, and 2) in a setting where there were relatively high sedimentation rates associated with high local subsidence rates, so that the storm waves did not completely rework the inter-storm deposits. The documentation of the unconventional Rannoch Formation contributes to our understanding of mixed-energy coastal systems. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Rannoch Formation in the northern North Sea, is widely considered to be formed during an early pre-rift tectonic stage of basin development (Johannessen et al., 1995; Fjellanger et al., 1996) as a stormdominated delta-front, with shoreface segments in an open marine environment (Richards and Brown, 1986; Graue et al., 1987; Scott, 1992), because of its dominance of amalgamated storm-generated, hummocky (HCS) and swaley (SCS) cross-stratified intervals. However, a particular series of wells in the deep parts of the northern North Sea present quite a different and unconventional Rannoch Formation delta-front, as seen by the presence of well-pronounced, double mud ⁎ Corresponding author at: School of Energy Resources, China University of Geosciences (Beijing), 29 Xueyuan Rd, Haidian, Beijing 100083, China. E-mail address: [email protected] (X. Wei).

http://dx.doi.org/10.1016/j.sedgeo.2016.02.012 0037-0738/© 2016 Elsevier B.V. All rights reserved.

drape tidal signals, intermittently interbedded with the storm-wave deposits, thus presenting a unit showing alternating storm–tide interaction, not previously documented on the Brent Delta front. Theoretically, though tides can effectively operate and be expressed in the wave-dominated system (Short, 1991), preservation of discernable tidal signals, such as double mud drapes, in the open-coast setting is not common (e.g., Short, 1991; Dashtgard et al., 2009; Dalrymple, 2010). This is particularly so in the ancient record, because double mud drapes are typically generated in a constricted, wave-protected setting, where slack water periods and asymmetry in tidal reversals can be expected (Dalrymple et al., 2003; Dalrymple and Choi, 2007). Mud drapes are simply easier to destroy by episodic storms and persistent shoaling waves, breaker and surf, or swash and backwash (Vakarelov et al., 2012), and are much less likely to be deposited because the wave-generated near-bed turbulence should inhibit settling and drape formation. If waves are larger at high tide, because

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there is less frictional attenuation of incoming waves in deeper water, mud drapes might be less common at high “slack water” than at low tide. If the site is far enough offshore, there is even the possibility of a rotary component to the tides, which also inhibits the formation of true slack water periods (Dalrymple and Choi, 2007; Dalrymple, 2010). The co-existence of prominent storm-wave and tide-generated structures within the Rannoch Formation is of interest, leading to the conclusion that the Rannoch Formation in this study area is not solely a wave- or a tide-dominated end member. On the contrary, the stormwaves and tides co-existed and interacted, and were not spatially separated. A spectrum of tidal shoreline deposits has been documented recently, as tidally influenced shoreface (TIS) (Dashtgard et al., 2012; Ainsworth et al., 2008), tidally modulated shoreface (TMS) (Dashtgard et al., 2009, 2012; Frey and Dashtgard, 2012; Vakarelov et al., 2012), and open-coast, wave-influenced tidal flat (Yang et al., 2005, 2006, 2008). The existing shoreface model, based on a few modern cases, given by Dashtgard et al. (2012), illustrates that at the wavedominated end, TIS and TMS are expected in micro- to meso- and in macro- to mega-tidal settings, respectively (Dashtgard et al., 2012). In these two types of mixed-energy, wave and tidal environment, tides exert influence by shifting the wave zone during the rising and falling tide periods (Dashtgard et al., 2009, 2012), which in turn affects the mode and timing of wave regime in a given part of the shoreface profile (Short, 1991; Masselink and Hegge, 1995). With an increase of tidal range, the wave-zone shift in TMS is more significant than it is in TIS. Thus, tidal current processes in TMS become more evident. This is shown by a ‘disturbance’ of the predictable trend of the shoreface profile, specifically leading to increased thickness of foreshore deposits, commonly interbedded sedimentary structures across the shoreface profile, and redistribution of grain size and the associated colonizing burrows (Dashtgard et al., 2009, 2012). As tidal influence progressively increases to take a dominant role, an open-coast tidal flat is expected (Vakarelov et al., 2012; Yang et al., 2005, 2006, 2008; Fan, 2012). The objectives of this paper are to 1) interpret the genesis and depositional environment of the “problematic” Rannoch Formation, and 2) present an ancient case where mixed storm-tidal processes occur, aiming to contribute to a facies model capable of expressing the full variability of such systems. 2. Geological setting The Middle Jurassic Rannoch Formation was deposited as the open marine front of the Brent Delta system in the northern North Sea, and it persisted as the Brent Delta prograded northward for at least 130 km (Graue et al., 1987; Helland-Hansen et al., 1992; Steel, 1993; Johannessen et al., 1995; Fjellanger et al., 1996). Two major episodes of rifting (Fig. 1B) affected the Mesozoic development of the northern North Sea basin, Permian–Triassic rifting (Steel and Ryseth, 1990; Færseth et al., 1995a) and Late Jurassic rifting (Færseth and Ravnås, 1998; Ravnås et al., 2000). In addition, there were also additional phases of extension during the Middle Jurassic (Figs. 1B and 2). The first of these led to some uplift of the western and eastern basin shoulders, and the development of the Broom (UK section) and Oseberg (Norwegian sector) formation clastic wedges as fan-delta systems that delivered sediment into the basin from west and from east, respectively (Richards, 1992), eventually producing a platform (Olsen and Steel, 1995) for the broad, subsequent advance of the Brent Delta system. Immediately after the generation of the Broom and Oseberg formations, there was an important late Aalenian transgression that generated an open marine basin, into which the Brent Delta prograded (Fig. 3). Driven by the uplift and erosion of the Central North Sea thermal dome, the Brent Delta system prograded northward and was always classed as a wave-dominated delta system (Graue et al., 1987; Helland-Hansen et al., 1992) that accumulated the Rannoch, Etive and part of the Ness formation deposits. The wave-process dominance on the front of the

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system was defined by the presence of hummocky and swaley cross strata in the Rannoch Formation. Increased fault-related subsidence and eustatic sea-level rise prior to the main late Jurassic rifting, led to gradual and widespread transgression of the Brent Delta from the Late Bajocian ~ Early Bathonian (Helland-Hansen et al., 1992; Ravnås and Bondevik, 1997). The Brent Delta had now become an axial estuarine system that retreated southward by punctuated transgressions. During the earlier northward growth of the delta-front, the Rannoch Formation is also now known to have been affected by lesser extension and minor fault-related tilt-block subsidence, which undoubtedly hastened the retreat of the delta system (Folkestad et al., 2014). The observed wells of this study are mainly from Kvitebjørn–Valemon, Nøkken and Huldra Fields, the studied Rannoch succession of which were formed in a high latitude setting of N60°~62° where reasonable magnitude and episodic storms would be expected (Duke, 1985). 3. Database and methodology This study is based on core observations from four Viking Graben wells, distributed in the Kvitebjørn (well 34/11-1) (Figs. 1A, 2, 3), Valemon (well 34/10-23) (Figs. 1A, 2, 3), Nøkken (34/11-2S) and Huldra (well 30/2-1S) field areas (Fig. 1A), two wells 34/10-23 and 34/11-2S show unconventional Rannoch Formation successions and the other two are reference wells recording conventional Rannoch Formation. To present the range of Rannoch depositional processes, core photos are shown from both key and reference wells. Detailed descriptions and interpretations are from key well 34/10-23 (Figs. 5, 6, 7). We measured grain-size, internal sedimentary structures, facies stacking patterns, as well as thicknesses of beds that are showing storm-wave, and tidal current generated signals, with the aim to quantitatively evaluate the primary depositional processes at short-time scales. 4. Results 4.1. Comparison of conventional and unconventional Rannoch Formation The Rannoch Formation, showing a strong storm-wave dominance, has been widely documented in the northern North Sea, and has been interpreted as wave-dominated delta-front or shoreface in an openmarine setting (Richards and Brown, 1986; Graue et al., 1987; Scott, 1992; Fjellanger et al., 1996). It is typically characterized by amalgamated storm-event beds (in a succession up to 100 m thick in places), each capped by a bioturbated and muddy interval produced in fair-weather conditions as shown in well 34/11-1 (Fig. 4A). However, the cored Rannoch Formation in two of the studied wells shows a remarkably different succession (Figs. 4B, C, and 5), notably 1) a moderately common presence of mud drapes in a variety of forms (ca. 15% of the entire thickness), creating various types of mud-draped, tidal intervals; and 2) fairly uniform grained, unburrowed, sandy storm-event beds that lack a capping bioturbated and muddy interval. These two facies are frequently interbedded, forming amalgamated “storm–tide couplets” (see detailed description and interpretation below), with the storm-event beds volumetrically exceeding the tidal beds. Nevertheless, storms and tides were the dominant processes, and they reflect significant interaction, often expressed as paired processes, operating on the Rannoch shoreline system. 4.2. Criteria and examples for recognizing storm and tidal processes 4.2.1. Storm-wave signatures Wave signals can be generated by either fair-weather (normal) waves or storm waves (Dumas and Arnott, 2006; Peters and Loss, 2012); the maximum water depth that can be penetrated by waves and storms is termed fair-weather wave base and storm-wave base, respectively. In the storm-dominated shelf setting, below the fair-weather wave base, oscillatory combined-flow, typically associated with storms,

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Fig. 1. A. Location map of studied wells and main structure elements of the northern North Sea, modified from Folkestad et al. (2014); B. Stratigraphic column of the Jurassic northern North Sea, modified from Davies et al. (2000).

is the most common hydrodynamic mechanism, operating in the transition zone between shoreface and offshore, leading to the preferential preservation of storm-event beds (Reading and Collinson, 1996; Peters and Loss, 2012). An ideal single storm-event bed is typically initiated from a scoured surface, then overlain by planar laminated sandstone, HCS or SCS stratification (Dott and Bourgeois, 1982; Duke, 1985), followed by flat laminated, or wave ripple cross-laminated sandstone and capped by muddier bioturbated intervals (Duke, 1985), representing post-storm fair-weather conditions. However, the idealized succession is not commonly complete in the geological record, as each storm succession is readily eroded by the following storm. Typical storm-wave generated bed examples in this study are shown in Fig. 6, and they are represented by 3 sorts of units as discussed below: (1) HCS stratification

(Fig.6A–C), is most likely to form above storm wave base under combined flows that have a strong oscillatory component (Dumas and Arnott, 2006). It is commonly of large wavelength (a few meters), and difficult to differentiate from planar lamination especially in cores (e.g., Fig. 6A, B, D, E), as those two are commonly closely associated and similar in appearance, and mainly expressed as amalgamated sets of micro-scale (mm), delicate laminations. HCS is generally bounded by truncation or undulating surfaces, internally sub-parallel or low-angle laminated (b 10° in dip) but apparently randomly oriented. In addition, HCS with smaller wavelength (several decimeters) (Fig. 6C) is also recognized in the core, and its formation may reflect a different paleogeographic setting from HCS of large wavelength; (2) SCS (Fig. 6D, F, G), is genetically-related with HCS, but more common or better preserved in

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Fig. 2. Fault populations and fault linkage through Jurassic time (from Folkestad et al., 2014). It illustrates that the typical key well 34/10-23 and reference well 34/11-1 were located in tectonically low and high positions, respectively.

shallower water depth where the aggradation rate is not sufficient to form hummocks and scouring is more frequent (Dumas and Arnott, 2006). SCS resembles HCS but lacks hummocks and is typically initiated by a prominent scour surface and capped by flat lamination (Fig. 6D), thus forming a concave-down geometry. Some scoured surfaces (Fig. 6F) or sharp bounding surfaces (Fig.6G), detected in the associated homogenous sandstone (which may be a major component of SCS), may have been produced by storms of lower magnitude than HCS stratified sandstone requires (Brenchley and Newall, 1982; Richards, 1992); (3) Planar lamination (Fig. 6A, B, D, E), is considered as a type of upper flow regime lamination formed by intense wave action such as that which occurs during storms under strong oscillatory and unidirectional flows (Cheel, 1991; Arnott and Southard, 1990). It can be recognized by the absence of any undulating surface and by its well-organized tabular bedsets. 4.2.2. Tidal signatures Direct evidence of tidal current processes in the “unconventional” Rannoch Formation is expressed as cyclic deposits, ranging from several

millimeters to several decimeters in scale, likely reflecting a hierarchy of tidal cycles (Davis, 2012). (1) The first sort of tidal deposits is an alternation of thick crosslaminated sandstone and thin mud-draped sandstone (type 1 tidal deposits) (Fig. 7A–C), expressed as very low abundance of thin (1~2 mm) mud drapes or double mud drapes that are discretely distributed within well-sorted, unidirectional cross-laminae. The most readily recognizable tidal signature is the repetition of double mud drape, which is diagnostic of tidal processes, recording the diurnal or semi-diurnal tidal cycle (Visser, 1980; Nio and Yang, 1991; Dalrymple, 1992) and occurs as sand–mud couplets of few millimeters thick. There are two sand layers, a thicker one representing the dominant tide and a much thinner one from the subordinate tide, as well as a slack-water mud drape capping each. The two mud drapes on either side of the very thin subordinate sand layer is the eye-catcher, and gives rise to the term double mud drape. The capping muddy or carbonaceous layers highlight rhythmic changes in the thickness of successive sand laminae or beds, reflecting changes in current velocity and flow reversals

Fig. 3. Paleogeography of Brent Delta progradation (after Fjellanger et al., 1996). (A) Delta position after the main progradation (on SB 169 Ma). During this period, synsedimentary faulting was very subtle. Multiple large river systems were shown from southern, eastern and western directions. (B) The flooding during Brent aggradation (MFS 167 Ma), generated by pronounced synsedimentary faulting. The major sediment supplies were from eastern and western directions. (C) The maximum delta extension (on SB 166 Ma). Note: The red and black stars represent the locations of wells 34/10-23, and 34/11-1, respectively.

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Fig. 4. Comparison between the “conventional” and “unconventional” Rannoch Formation. The conventional Rannoch Formation example (A) is from well 34/11-1, showing HCS/SCS dominated storm-wave deposits, capped by bioturbated and muddy intervals that were produced under fair-weather conditions. The unconventional Rannoch Formation examples (B) and (C) are from well 34/10-23, illustrating mud drapes that occurred between storm-event deposits. SW—storm-wave deposits; BM—bioturbated and muddy deposits; and MD—mud-draped intervals.

during both daily and neap–spring tidal cycles (Nio and Yang, 1991; Archer et al., 1995; Dalrymple and Choi, 2007). Therefore, the characteristic asymmetry (thick–thin) feature of type I tidal deposits and overall low abundance of mud drapes can be ascribed to longer strong tidal energy and shorter weak tidal energy periods, as a result of neap–spring tidal asymmetry. As crosslaminae within type I deposits are somewhat similar to HCS in core, care should be taken to observe the orientation of crosslaminae and their stacking pattern to identify the boundary between storm and tidal beds. (2) The second tidal criterion (tidal signal type 2) is tidal bundling of strata, of a few tidal cycle duration. It is a style of repetitive mud drapes and sand layer thickening and thinning within crossstratal foresets and bottomsets (Fig. 7D–F), reflecting the tidal frequency of slack-water periods (Steel et al., 2012) and recording a series of strengthening or weakening tidal cycles; whereby the repetitive mud drapes reflect the regular daily or twice-daily slackwater period as the tide turns (Archer et al., 1995). The alternation of thicker and thinner layers within foresets suggests fluctuating flow velocities over neap–spring tidal cycles (e.g., Williams, 1991; Dalrymple, 1992). (3) The third type of recognized tidal deposits is heterolithic lamination (dm-thick), showing amalgamated and well-organized sand–mud couplets (Fig. 7G–I), reflecting the repetition of tidal cycles during neap–spring cycles. It is vertically accreted, horizontally or sub-

horizontally laminated sandstone layers capped by mud drapes, internally showing either evenly or slight increasing and decreasing spacing, as well as slightly increasing and decreasing abundance of mud drapes. It is interpreted as deposited from sandstone generated by flood–ebb tidal currents alternating with mudstone from intervening slack water suspension fallout (Nio and Yang, 1991) during neap–spring tidal cycles. Some of the above signatures in the study cores would be considered weaker tidal criteria, as other process, e.g., variation in wave-group frequency and fluctuating fluvial currents, cannot be unequivocally excluded from generating such features, but the occurrence of all three criteria together and their internal regularity and cyclicity make a strong case (Nio and Yang, 1991; Dalrymple, 2010; Davis, 2012) for the tidal influence on the Brent Delta front. 4.3. Facies successions and variability within “unconventional” Rannoch Formation The examined cores of Rannoch Formation in the study area have an average thickness of 45 m, and 24 m of it is cored in key well 34/10-23 (Fig. 5). The lower boundary of Rannoch Formation is absent in the cores but the top boundary can be recognized both on well logs and on cores by an abrupt grain-size increase to the overlying Etive Formation. The entire Rannoch succession is composed by relatively uniform

Fig. 5. Core description of key well 34/10-23: facies proportion and facies succession. The profile shows two coarsening-upward successions (FS1 and FS2) and one finning-upward interval (FS3), each showing coupled storm-tidal deltaic depositional components. The thickness proportions of storm- and tide-dominated facies of each succession are shown in the pie-chart of the figure.

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grained and consistently unburrowed, well-sorted storm beds periodically alternating with tidal beds. During the progradation of the Brent Delta, the Rannoch shoreline system developed three successions, two broadly upward-coarsening and thickening, and the uppermost one showing upward fining and thinning (Fig. 5). These are: 1) a succession dominated by cleanwater, storm-event beds frequently and preferentially interbedded with type I tidal intervals and occasional types II and III tidal beds (Figs. 5 and 8A), toward 2) one of cleanest storm-event beds less frequently separated by some types II and III tidal beds (Figs. 5 and 8B), and then upward into 3) a thin interval showing muddier storm-event beds mainly alternating with few type II tidal beds (Figs. 5 and 8C). 4.3.1. Descriptions of the Rannoch facies successions 4.3.1.1. Storm-dominated (84%), tide-influenced (16%) facies succession 1. The lowermost part of the cored Rannoch Formation is a ca. 12 m thick, clean-water (i.e., little mud is present), facies succession (FS1) (e.g., Fig. 5), containing mostly upward-coarsening motifs (e.g., Fig. 5,). In general, FS1 consists mainly of very fine to fine-grained, gray to light-gray colored storm-event sandstone beds, which are characterized by planar laminated sandstone (45%), HCS (27%) and SCS (12%) stratification (Fig. 5). The most striking feature of FS1 is the frequent interbedding of tidal deposits, particularly type I tidal beds, with storm-event beds (e.g., Figs. 5 and 8A); a second feature is that there is a lesser volume of tidal beds, accumulating amalgamated and well-organized sand–mud couplets, i.e., tidal bundling within cross-strata (type II) and heterolithic lamination (type III) (Fig. 8A) where individual mud drapes or mud laminae are 1~5 mm thick. 4.3.1.2. Storm-dominated (94%), tide-influenced (6%) facies succession 2. Facies succession 2 (FS2) is fine to upper fine grained sandstone, ca. 10 m thick, light-gray colored and forming several blocky to slightly coarsening-upward successions (e.g., Figs. 5 and 8B). The sandstones are notably clean and homogenous in nature (i.e., very little mud is present in the system). There are significant differences between successions FS1 and FS2, particularly the dominance of HCS in FS1 in contrast to mainly SCS in FS2 and the overall decreased volume of tidal beds (6%) in FS2, which are expressed as types II and III, and lack of type I tidal beds. The thickness of each mud drape is greater (1–7 mm) and mud drapes are well-organized in FS2 compared to FS1. 4.3.1.3. Storm-dominated (91%), tide-influenced (9%) facies succession 3. The uppermost facies succession (FS3) of Rannoch Formation is a muddier and thinner (ca. 2 m) unit (e.g., Figs. 5 and 8C), composed by some erosionally, amalgamated, dark-colored storm-event beds (91%), with HCS of smaller wavelength (decimeters), which are mainly separated by type II tidal beds (Figs. 5 and 8C). The entire interval exhibits a vertical fining-upward trend. The thickness of mud drapes ranges from 2 to 8 mm, which is generally thicker than the mud drapes in FS1 and 2. 4.3.2. Interpretations of the Rannoch facies successions The three Rannoch facies successions share common features in the presence of well-developed storm-wave and tide-generated signatures, which indicate the interaction of storm-wave and tidal current processes. The overall high proportion (N 80%) of storm-wave generated sedimentary structures, such as planar lamination, HCS, and SCS and low proportion (b20%) of tide-generated structures within the succession, suggest that the primary process is storm-wave and tide influence is secondary, in the overall shallowing-upward succession. The system is therefore categorized as storm-dominated, and tide-influenced. The complete absence of bioturbation within the storm-event beds, may be related to the high

frequency and high magnitude of storms, and/or high sedimentation rates (MacEachern et al., 2005; Li and Bhattacharya, 2015). The preserved record of stacked, erosionally based, HCS, SCS and planar laminated sandstone reflecting constant high-energy oscillatory currents, is likely to represent the storm-dominated middle-lower shoreface deposits between fair-weather and storm wave base (Reading and Collinson, 1996; Peters and Loss, 2012). The non-negligible amount (ca. 10%), and moderately frequent occurrence of mud drapes in storm-dominated setting, indicate that there were 1) intermittent fair-weather (inter-storm) periods with minimal wave influence during a train of storms; 2) available suspended sediment supply; and 3) well-defined slack-water periods (i.e., rectilinear tidal current activity) (Dalrymple and Choi, 2007; Dalrymple, 2010). The variations in the paired processes and the preservation bias of frequency and style of the storm-wave and tidal current generated structures, in three distinct units may reflect temporal and spatial, absolute and relative changes of storms and tidal currents operating on the Rannoch shoreline. 4.3.2.1. Interpretation of facies succession 1. The preferential occurrence of characteristically asymmetric type I tidal bed, and the thinness of mud drapes might reflect a low fluvial input and relatively strong tidal currents in the distal delta-front reaches. The small amount of thicker and amalgamated well-organized double mud drapes in the foresets and bottomsets of cross-strata (type II) or heterolithic lamination (type III) may be produced in prolonged fair-weather conditions, representing moderate tidal currents via sufficient suspended sediment settling. 4.3.2.2. Interpretation of facies succession 2. In FS2, the large-scale structureless, to planar or low-angle laminated sandstones bounded by scour surfaces, may represent SCS dominated storm-event beds. The preferential occurrence of SCS sandstone may be related to storms approaching shallow water depths (Duke, 1985; Dumas and Arnott, 2006), where the aggradation rate is not sufficient and the water depth is not enough to form and preserve hummocks. The good sorting and relatively homogenous nature of sandstone beds may also indicate a dominance of high-energy and high-frequency storm-wave action, which winnows out mud (Vakarelov et al., 2012), thus making the SCS sandstone appear structureless. The increased thickness of mud drapes may be due to increasing suspended sediment supply. The better preservation of tidal signal types II and III may reflect better preservation of tidal deposits or less frequent and intense storms, thus allowing long-term fair-weather conditions to create and preserve tidal deposits. FS2 is therefore interpreted as a middle delta-front succession, equivalent to middle shoreface setting. 4.3.2.3. Interpretation of facies succession 3. FS3 differs from FS1 and FS2 by its entirely muddier character, and its abundance and thickness of mud drapes, perhaps reflecting proximity to the river-mouth where there would have been sufficient suspended sediment supply. The fining-upward trend and the modest thickness of storm-event beds may be the combined effects of a temporal increase of fine-grained sediment and a decrease of storm-wave energy that is effective in winnowing out fine-grained material. Compared with FS2, the slight increase in proportion of tidal beds and occurrence of smaller HCS may suggest that FS3 represents a transgressive interval and that the Rannoch shoreline away from the active deltaic output was being reworked by small waves. 4.3.3. Processes summary It is evident that the studied Rannoch succession was controlled by coupled tidal hydrodynamics and pronounced storm process; tidal currents were only able to redistribute sediment at times. Storm dominance

Fig. 6. Examples of storm-generated structures. (A, B) HCS and planar lamination; and (C) Muddier HCS, with smaller wave-length (several decimeters). (D) SCS and planar lamination, showing faint laminae. (E) Planar lamination. (F) Homogenous sandstone bounded by high-angle scoured surfaces. (G) Homogenous sandstone bounded by sharp surfaces. Note: H—HCS, P—planar lamination, S—SCS, and the black arrows point to erosional surface(s).

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is readily recognizable as well-sorted HCS and SCS stratifications. Tidal influence is shown by the three types of tidal beds, and the good sorting of fine-grained sands within tidal beds. Fluvial influence was important in transporting sediment onto the shoreline and amplifying or damping tidal influence. Variation in the preservation bias of storm and tidal beds might be the combined effects of 1) the frequency and duration of storms, or the duration of non-storm, fair-weather periods; 2) river discharge; and 3) the absolute and relative strength of tides (Dalrymple, 2010; Plink-Bjorklund, 2012; Rossi and Steel, 2016). Firstly, the frequency and duration of storms have the most direct impact, as no tidal beds could be deposited in long-lasting storms. The longer periods between storm events promoted the accumulation of tidal beds, particularly type III tidal beds; the short duration of fairweather (inter-storm) periods allowed the formation of thin, type II tidal beds. Secondly, in the prograding deltaic lobes, tidal process likely occurred preferentially in the seaward reaches of the system, i.e., in FS1, where fluvial deposition rates were relatively low, compared to the sediment redistribution capacity of tidal currents; in this context, tides were responsible for redistributing both sand and mud (Plink-Bjorklund, 2012). In addition, the abundance and thickness of mud drapes would also have varied with river discharge, as thicker drapes would have formed in landward reaches, i.e., FS2 and FS3, or during high discharge periods, from high suspended sediment loads (Dalrymple and Choi, 2007). Thirdly, as suggested by the overall sand-rich nature of the succession, deposition occurred near the mouth of a large river, where sand bedload was abundant, and relatively strong tidal currents would be present because of the significant tidal prism associated with flow in and out of the river mouth area (Dalrymple et al., 2003; Dalrymple and Choi, 2007). Besides, tidal process is significant especially in areas where wave energy is dissipated, therefore, the presence of mud drapes in the delta-front suggests that there might have been local topographic restrictions e.g., sheltering from a wave-approach direction. This could have resulted in an enhanced tidal range or tidal-current velocity when there was lateral restriction (e.g., bifurcation of distributary channel) or where the tidal wave entered into tidal resonance (Dalrymple and Zaitlin, 1994; Dalrymple et al., 2003). Variations in current velocities as well as in tidal ranges might be two mechanisms to explain the formation of tidal beds. As tidal current velocities fluctuate between values close to zero and velocities capable of transporting bedload material, association of large tide range and strong tidal current velocities or low tide range and weak tidal current velocities could have resulted in association of sandstone and mud drapes of different types (Liu et al., 2002; Hemer et al., 2004; Plink-Bjorklund, 2012).

During storm periods, storm beds (S) were generated and tended to be preserved between fair-weather and storm wave base, and laterally flank the river mouths and shoreface. The sandy storm beds in FS1 and FS2 were most likely deposited above the nearshore mudline (Fig. 9A). Stage 2: Secondary stage of deposition under fair-weather conditions. During this period, wave energy was minimal and the fair-weather wave base moved upward and became higher than that in storm conditions. Wave influence was minimal and sediment supply through rivers was distributed and reworked by river and tidal currents. With the decrease of river currents and increase of tidal currents in the seaward direction, at the point where the velocity of tidal currents exceeds river currents, tidal beds (T) accumulate in river mouths and embayment in the intertidal and subtidal zone (Fig. 9B).

5.1. Formation and preservation conditions for tidal signals

5.1.2. Suspended sediment source Suspended sediment in tectonically active, mixed-energy deltas can be derived from a range of sources; in addition to the river currents, tidal currents and storms that have potential to transport and suspend/resuspend fine-grained sediment, the uplifted flanks and internal high blocks of the North Viking Graben are also likely to have contributed to the deltaic sediments (Richards, 1992; Helland-Hansen et al., 1992). The studied succession is part of a large delta complex, and mud would have been provided by distributary channels as shown in the Aalenian paleogeographic map (Fig. 3). The best signal of Brent River mud is in FS3, because FS3 is relatively mud rich and occupied the most proximal (albeit a more sheltered lateral development) Rannoch site. Besides, the landward-directed, residual tidal flow, in the seaward part of Rannoch also has potential to resuspend mud from the prodelta areas with landward transport onto the delta-front (Dalrymple and Choi, 2007). Though storm-erosion can generate a considerable amount of mud, any mud resuspended during a storm would be deposited relatively quickly after the end of the storm, therefore it is less likely to have been a source of the fair-weather (inter-storm) muddy tidal deposits. In addition, the uplifted flanks on the margins of the northern Viking Graben were likely an important sediment source for the Brent Delta (Ravnås and Bondevik, 1997). In some areas including parts of the present study area, tectonic activity and fault-block rotation may have accelerated or fault-related subsidence started earlier, as has been suggested by Folkestad et al. (2014). Limited by seismic and thin section data, we cannot test the hypothesis and evaluate this process precisely. However, the abrupt change from light-colored sandstone (likely enriched in quartz and feldspar) of successions FS1 and FS2, to dark-colored sandstone (with a higher matrix content or lithic grains) characterizing the thin interval FS3, may suggest influx of immature detritus from nearby source areas in addition to the conventional southerly provenance (Fjellanger et al., 1996).

5.1.1. Conceptual model for the formation of storm–tide couplets As suggested, the formation and preservation of tidal signals between storm-beds, require a relatively protected shoreline setting, and the arrival of a sufficient river supply. Both of these conditions were likely met: 1) when the Brent Delta prograded northward across the Kvitebjørn–Valemon area, and a slight extension and a locally accelerated subsidence across the depositional region may have produced some irregularity of the Rannoch shorelines. The shorelines became very slightly embayed in places instead of the normal deltaic protrusive coasts (as suggested in well 34/10-17 above MFS 167 Ma, Fig. 3B). 2) There was at least one large river driving the overall progradation of Brent Delta regardless of the shoreline configuration (Fig. 3A–C). We therefore propose that the following two stages were responsible for the formation of tidal signals between storm beds (e.g., Fig. 9A, B). Stage 1: Initial stage of deposition under storm conditions.

5.1.3. Preservation potential The tidal signals described herein would normally have been severely masked by fair-weather waves and storms, so their occurrence here indicates that the setting must also have been one where there were relatively high sedimentation rates associated with the mouth of one of the Brent distributary rivers here, so that the storm waves did not completely rework the inter-storm deposits (Fig. 9C). The high sedimentation rates at the delta mouth would also have been enhanced by progradation of Rannoch, Etive and the lower part of the Ness Formations during a period of slow sea-level rise where accommodation was outpaced by high sedimentation rates (Eschard et al., 1993). Both the absolute and relatively high sedimentation rates would have contributed to the preservation of tidal signals.

5. Discussion

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Fig. 7. Examples of tidal signatures. They are classified into three types, according to the abundance, thickness, and organization of mud drapes, namely: (1) type I (A~C): alternations of thick cross-laminated sandstone and thin mud-draped sandstone. There is a low abundance of thin (mm-thick) mud drapes, whereby mud drapes are commonly discretely distributed, showing occasional regularity. The black arrows illustrate individual mud drapes; and the white arrows point to the erosional surfaces. (2) type II (D~F): tidal bundles in the bottomsets and foresets of cross-strata, showing moderate abundance of mud drapes that are well-organized. (3) type III (G~I): heterolithic lamination, showing multiple well-organized sand–mud couplets, of even or slightly increasing and decreasing spacing. Note: the dashed lines refer to the bounding surfaces. The green bars point to intervals dominated by cross-lamination; the yellow bars indicate storm-event bed; and the black lines and bars refer to mud drapes and mud-draped intervals, respectively.

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Fig. 8. Summary of the key features and variabilities of “storm–tidal couplets” in the three facies successions.

5.2. Comparison with modern and other ancient systems Although there is a widespread spectrum of deposits expected to be deposited between the storm and tidal end members of coasts, there are only some modern and rare ancient examples recording mixed wave and tide signals (Yang et al., 2005, 2006, 2008; Choi et al., 2004; Dashtgard et al., 2009, 2012; Vakarelov et al., 2012; Cummings et al., 2015). One relatively poor analog to the Rannoch deposits is the west Korean coast described by Yang et al. (2005, 2006, 2008), which is bordering the outer part of an open-mouth estuary and classified as sandy, open-coast tidal flat, displaying storm and tidal processes' dominance. However, their depositional conditions contrast with the Rannoch conditions. In the Korean open-coast tidal-flat case the sediment supply was relatively limited and the suspended sediment concentration

was higher, resulting in a more complete winter storm reworking of the inter-storm deposits and retention of the tidal signal only as a modulation of the storm deposits themselves rather than as mud drapes. Another analog is the muddy open-coast tidal flats along the east coast of China described by Fan (2012). However, they are fringing a very large river and receiving gigantic volumes of fluvial muddy sediment. Besides, due to their marginal sheltered location and their low sedimentation rates as well as modification by high magnitude storms during tidal cycles, they are preferentially preserving low-energy sediments, i.e., wave ripples, combined flow ripples, rather than high-energy storm deposits as in Rannoch Formation. Other cases showing mixed storm-wave and tidal processes are the tidal-modulated shorefaces described by Dashtgard et al. (2009, 2012) and Vakarelov et al. (2012). However, in these cases the tidal signals are also only indirectly expressed by tidal modulation of the storm

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Fig. 9. Schematic model showing the genesis and preservation of tidal beds between storm beds (“storm–tide couplets”) in an assumed setting 1) of relatively protected shoreline configuration; and 2) with sufficient river supply. (A) Initial stage of deposition under storm conditions, illustrates the accumulation of storm beds (S) between fair-weather and storm wave base. Notably, the sandy storm beds are likely deposited above the nearshore mudline. (B) Secondary stage of deposition under fair-weather conditions, indicates the accumulation of tidal beds, when wave influence is minimal. In the seaward direction, particularly at low river discharge periods, tidal beds likely occurred when tidal currents outpaced river currents. Note: the fair-weather wave base is lower than that during storm conditions. (C) Erosional and depositional stage during storm conditions. At this period, the effective fair-weather wave base is lowered again as indicated and approached to the storm-wave base. A large volume of sediment transports through rivers to delta-front and is redistributed by storm-waves forming storm beds (S), which largely erode the existing deposits (tidal beds). In this context, the preservation potential of tidal beds is largely dependent on high accommodation rates. When the accommodation rates outpaced the sedimentation rates, tidal deposits are likely preserved between storm deposits.

effects (i.e., shifting of the tidal zones during tidal cycles) in a meso- or mega-tidal setting. Another possible analog, perhaps especially for FS3, is the open coast tidal-flat area near the mouth of the Han River, west coast of South Korea described by Choi et al. (2004) and Cummings et al. (2015). However, these open coast tidal flats are subject to less wave energy than the Rannoch case, due to the dampening of incoming waves by the broad subaqueous delta platform. Another weakness of this analog is its close association with tidal bars and point bars. The ancient Lajas Formation (Argentina) example (Rossi and Steel, 2016), is another possible analog in the formation and preservation of tidal signals in a mixed-energy delta-front. However in this case the southern Neuquen Basin was moderately protected from high-energy storm waves and the mixed energy components were mainly the interaction of periodic river flood input with tidal currents that increased in their intensity basinward, either because of impinging against a low-

gradient axial tidal system or because of approach to the shelf edge where there was an increased tidal prism (Rossi and Steel, 2016). It can be seen that the Brent delta front cannot be classified into existing mixed wave-tide system schemes but would otherwise classify as tide-influenced, wave-dominated.

5.3. Implications of tidal signatures in the Brent Delta front The tidal signals captured in the Rannoch delta front of the study area are important for two reasons. Firstly, we encourage the reexamination of other ancient storm-wave dominated successions for tidal signals. Secondly, the study segment of the Rannoch delta-front, though wave-dominated, was significantly tide-influenced in a configuration that is poorly known among modern or ancient deltaic examples. As such it serves to strengthen our current classification of mixedenergy coastlines.

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6. Conclusions (1) In the deep-graben Kvitebjørn–Valemon fields, the shoreline system of the prograding Brent Delta in the northern North Sea exhibits an “unconventional” delta-front succession, generally dominated by storms but significantly influenced by tides. The storm-wave signals are stacked, erosionally amalgamated storm-event beds characterized by unburrrowed flat-laminated, HCS and SCS beds; the tidal signals are of three types: 1) thin mud drapes and double mud drapes in an alternating association with cross-laminated sandstones; 2) a few tidal bundles in the foreset or bottomsets of cross-strata; and 3) heterolithic lamination and higher abundance of well-organized mud drapes. (2) The variations in the frequency and style of storm and tidal beds, allow the Rannoch Formation to be subdivided into three facies successions. From bottom to top, this is expressed as 1) a facies succession dominated by clean-water, storm-event beds frequently interbedded with type I tidal intervals and occasional types II and III tidal beds, passed toward 2) a facies succession of clean storm-event beds less frequently separated by some types II and III tidal beds, and then into 3) a thin interval showing muddier storm-event beds alternating with few type II tidal beds. The variations in the preserved bias of storm and tidal beds, reflect the combined effects of 1) the frequency and duration of storms, or the duration of non-storm, fair-weather periods; 2) river discharge; and 3) the absolute and relative strength of tides. Tidal deposits are interpreted as inter-storm, fair-weather deposits and tidal processes occurred in longer intermittent fairweather condition and lower river discharge periods, and preferentially occurred in the distal-reach of the delta-front. (3) The facies and facies stacking of the studied Rannoch Formation are consistent with a highly episodic storm setting periodically influenced by tides. The studied Rannoch Formation can therefore be interpreted as a storm-dominated, tide-influenced delta-front near the mouth of large river, where relatively strong tidal currents would be expected. The better preservation of tidal beds, is attributed to the high delta mouth accumulation rates and rising normal regression of the delta front.

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