Sedimentary Geology 335 (2016) 166–179

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Sedimentary Geology

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Variability of tidal signals in the Brent Delta Front: New observations onthe 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, Chinab Jackson School of Geosciences, University of Texas at Austin, 1 University Station, C1100, Austin, TX 78712, USAc A/S Norsk Shell, Tankvegen 1, Tananger 4056, Norwayd Department of Geological Sciences, Indiana University Bloomington, 1001 East 10th Street, Bloomington, Indiana, 47405, USA

⁎ Corresponding author at: School of Energy Resources,(Beijing), 29 Xueyuan Rd, Haidian, Beijing 100083, China.

E-mail address: vivi-stefanie2008@hotmail.com (X. W

http://dx.doi.org/10.1016/j.sedgeo.2016.02.0120037-0738/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 November 2015Received in revised form 10 February 2016Accepted 12 February 2016Available online 19 February 2016

Editor: Dr. B. Jones

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 drapesare prominent but discretely distributed, II): a few tidal bundles within bottomsets and foresets of up to10 cm-thick sets cross-strata, and III): dm-thick heterolithic lamination showing multiple, well-organizedsand–mud couplets.During progradation of the Brent Delta, the Rannoch shoreline system passed upward from 1) a successiondominated by clean-water, storm-event sets and cosets frequently and preferentially interbedded with type Itidal beds, and occasional types II and III tidal deposits, toward 2) very clean storm-event beds less frequentlyseparated by types II and III tidal beds, and then into 3) a thin interval showingmuddier storm-event bedsmainlyalternating with type II tidal beds.It is likely that those variations in preservation bias of storm and tidal beds in each facies succession result fromcombined effects of 1) the frequency and duration of storms; 2) river discharge; and 3) the absolute and relativestrength of tides. Tidal deposits are interpreted as inter-storm, fair-weather deposits, occurred preferentially inlonger intermittent fair-weather condition and periods of lower river discharge, and well-pronounced in thedistal-reach of delta-front. The formation and preservation of tidal signals between storm beds, indicate thatthe studied Rannoch Formation was most likely a storm-dominated, tide-influenced delta front 1) near themouth of a large Brent river, where a significant tidal prism and high tidal range might be expected, and 2) ina setting where there were relatively high sedimentation rates associated with high local subsidence rates, sothat the storm waves did not completely rework the inter-storm deposits. The documentation of theunconventional Rannoch Formation contributes to our understanding of mixed-energy coastal systems.

© 2016 Elsevier B.V. All rights reserved.

Keywords:Brent Delta frontTidal signalsStorm-event bedsStorm–tide coupletsMixed-energy coastal system

1. Introduction

The Rannoch Formation in the northern North Sea, is widely consid-ered to be formed during an early pre-rift tectonic stage of basin devel-opment (Johannessen et al., 1995; Fjellanger et al., 1996) as a storm-dominated delta-front, with shoreface segments in an open marineenvironment (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 Seapresent quite a different and unconventional Rannoch Formationdelta-front, as seen by the presence of well-pronounced, double mud

China University of Geosciences

ei).

drape tidal signals, intermittently interbeddedwith the storm-wave de-posits, thus presenting a unit showing alternating storm–tide interac-tion, not previously documented on the Brent Delta front.

Theoretically, though tides can effectively operate and be expressedin the wave-dominated system (Short, 1991), preservation of discern-able tidal signals, such as double mud drapes, in the open-coast settingis not common (e.g., Short, 1991; Dashtgard et al., 2009; Dalrymple,2010). This is particularly so in the ancient record, because doublemud drapes are typically generated in a constricted, wave-protectedsetting, where slack water periods and asymmetry in tidal reversalscan be expected (Dalrymple et al., 2003; Dalrymple and Choi, 2007).Mud drapes are simply easier to destroy by episodic storms and persis-tent shoaling waves, breaker and surf, or swash and backwash(Vakarelov et al., 2012), and are much less likely to be depositedbecause the wave-generated near-bed turbulence should inhibitsettling 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 lowtide. If the site is far enough offshore, there is even the possibility of arotary component to the tides, which also inhibits the formation oftrue slack water periods (Dalrymple and Choi, 2007; Dalrymple,2010). The co-existence of prominent storm-wave and tide-generatedstructures within the Rannoch Formation is of interest, leading to theconclusion that the Rannoch Formation in this study area is not solelya wave- or a tide-dominated end member. On the contrary, the storm-waves and tides co-existed and interacted, and were not spatiallyseparated.

A spectrum of tidal shoreline deposits has been documented recent-ly, as tidally influenced shoreface (TIS) (Dashtgard et al., 2012;Ainsworth et al., 2008), tidally modulated shoreface (TMS) (Dashtgardet 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 wave-dominated end, TIS and TMS are expected in micro- to meso- and inmacro- to mega-tidal settings, respectively (Dashtgard et al., 2012). Inthese two types of mixed-energy, wave and tidal environment, tidesexert influence by shifting the wave zone during the rising and fallingtide periods (Dashtgard et al., 2009, 2012), which in turn affects themode and timing of wave regime in a given part of the shoreface profile(Short, 1991; Masselink and Hegge, 1995). With an increase of tidalrange, the wave-zone shift in TMS is more significant than it is in TIS.Thus, tidal current processes in TMS become more evident. This isshown by a ‘disturbance’ of the predictable trend of the shoreface pro-file, specifically leading to increased thickness of foreshore deposits,commonly interbedded sedimentary structures across the shorefaceprofile, and redistribution of grain size and the associated colonizingburrows (Dashtgard et al., 2009, 2012). As tidal influence progressivelyincreases 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 anddepositional environment of the “problematic” Rannoch Formation,and 2) present an ancient case where mixed storm-tidal processesoccur, aiming to contribute to a facies model capable of expressing thefull variability of such systems.

2. Geological setting

The Middle Jurassic Rannoch Formation was deposited as the openmarine front of the Brent Delta system in the northern North Sea, andit persisted as the Brent Delta prograded northward for at least130 km (Graue et al., 1987; Helland-Hansen et al., 1992; Steel, 1993;Johannessen et al., 1995; Fjellanger et al., 1996). Two major episodesof rifting (Fig. 1B) affected the Mesozoic development of the northernNorth 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, therewere also additional phasesof extension during the Middle Jurassic (Figs. 1B and 2). The first ofthese led to some uplift of the western and eastern basin shoulders,and the development of the Broom (UK section) and Oseberg (Norwe-gian sector) formation clastic wedges as fan-delta systems that deliv-ered 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. Im-mediately after the generation of the Broom and Oseberg formations,there was an important late Aalenian transgression that generated anopenmarine basin, intowhich theBrentDelta prograded (Fig. 3). Drivenby the uplift and erosion of the Central North Sea thermal dome, theBrent Delta system prograded northward and was always classed as awave-dominated delta system (Graue et al., 1987; Helland-Hansenet al., 1992) that accumulated the Rannoch, Etive and part of the Nessformation deposits. The wave-process dominance on the front of the

system was defined by the presence of hummocky and swaley crossstrata in the Rannoch Formation. Increased fault-related subsidenceand eustatic sea-level rise prior to the main late Jurassic rifting, led togradual and widespread transgression of the Brent Delta from the LateBajocian~Early Bathonian (Helland-Hansen et al., 1992; Ravnås andBondevik, 1997). The Brent Delta had now become an axial estuarinesystem that retreated southward by punctuated transgressions. Duringthe earlier northward growth of the delta-front, the Rannoch Formationis also now known to have been affected by lesser extension andminorfault-related tilt-block subsidence, which undoubtedly hastened the re-treat of the delta system (Folkestad et al., 2014). The observed wells ofthis study are mainly from Kvitebjørn–Valemon, Nøkken and HuldraFields, the studied Rannoch succession of which were formed in a highlatitude setting of N60°~62° where reasonable magnitude and episodicstorms would be expected (Duke, 1985).

3. Database and methodology

This study is based on core observations from four Viking Grabenwells, 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) andHuldra (well 30/2-1S) field areas (Fig. 1A), two wells 34/10-23 and34/11-2S show unconventional Rannoch Formation successions andthe other two are reference wells recording conventional Rannoch For-mation. To present the range of Rannoch depositional processes, corephotos are shown from both key and reference wells. Detailed descrip-tions and interpretations are from key well 34/10-23 (Figs. 5, 6, 7). Wemeasured grain-size, internal sedimentary structures, facies stackingpatterns, as well as thicknesses of beds that are showing storm-wave,and tidal current generated signals, with the aim to quantitatively eval-uate 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 beeninterpreted as wave-dominated delta-front or shoreface in an open-marine setting (Richards and Brown, 1986; Graue et al., 1987; Scott,1992; Fjellanger et al., 1996). It is typically characterized by amalgamat-ed storm-event beds (in a succession up to 100 m thick in places), eachcapped by a bioturbated and muddy interval produced in fair-weatherconditions as shown in well 34/11-1 (Fig. 4A). However, the coredRannoch Formation in two of the studied wells shows a remarkably dif-ferent succession (Figs. 4B, C, and 5), notably 1) a moderately commonpresence ofmud drapes in a variety of forms (ca. 15% of the entire thick-ness), creating various types ofmud-draped, tidal intervals; and 2) fairlyuniform grained, unburrowed, sandy storm-event beds that lack a cap-ping bioturbated and muddy interval. These two facies are frequentlyinterbedded, forming amalgamated “storm–tide couplets” (see detaileddescription and interpretation below), with the storm-event beds volu-metrically exceeding the tidal beds. Nevertheless, storms and tideswere the dominant processes, and they reflect significant interaction,often expressed as paired processes, operating on the Rannoch shore-line system.

4.2. Criteria and examples for recognizing storm and tidal processes

4.2.1. Storm-wave signaturesWave 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 wavesand storms is termed fair-weather wave base and storm-wave base, re-spectively. In the storm-dominated shelf setting, below the fair-weatherwave base, oscillatory combined-flow, typically associated with storms,

Fig. 1.A. Locationmap of studiedwells andmain structure elements of thenorthernNorth Sea,modified fromFolkestad et al. (2014); B. Stratigraphic columnof the Jurassic northernNorthSea, modified from Davies et al. (2000).

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is themost common hydrodynamicmechanism, operating in the transi-tion zone between shoreface and offshore, leading to the preferentialpreservation of storm-event beds (Reading and Collinson, 1996; Petersand Loss, 2012). An ideal single storm-event bed is typically initiatedfrom a scoured surface, then overlain by planar laminated sandstone,HCS or SCS stratification (Dott and Bourgeois, 1982; Duke, 1985), follow-ed by flat laminated, or wave ripple cross-laminated sandstone andcapped by muddier bioturbated intervals (Duke, 1985), representingpost-storm fair-weather conditions. However, the idealized successionis not commonly complete in the geological record, as each storm succes-sion is readily eroded by the following storm. Typical storm-wave gener-ated bed examples in this study are shown in Fig. 6, and they arerepresented by 3 sorts of units as discussed below: (1) HCS stratification

(Fig.6A–C), is most likely to form above storm wave base under com-bined flows that have a strong oscillatory component (Dumas andArnott, 2006). It is commonly of large wavelength (a few meters), anddifficult to differentiate from planar lamination especially in cores(e.g., Fig. 6A, B, D, E), as those two are commonly closely associated andsimilar in appearance, and mainly expressed as amalgamated sets ofmicro-scale (mm), delicate laminations. HCS is generally bounded bytruncation or undulating surfaces, internally sub-parallel or low-anglelaminated (b10° in dip) but apparently randomly oriented. In addition,HCSwith smallerwavelength (several decimeters) (Fig. 6C) is also recog-nized in the core, and its formation may reflect a different paleogeo-graphic setting from HCS of large wavelength; (2) SCS (Fig. 6D, F, G), isgenetically-related with HCS, but more common or better preserved in

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 intectonically low and high positions, respectively.

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shallower water depth where the aggradation rate is not sufficient toform hummocks and scouring is more frequent (Dumas and Arnott,2006). SCS resembles HCS but lacks hummocks and is typically initiatedby 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 associatedhomogenous sandstone (which may be a major component of SCS),may have been produced by storms of lower magnitude than HCS strat-ified sandstone requires (Brenchley and Newall, 1982; Richards, 1992);(3) Planar lamination (Fig. 6A, B, D, E), is considered as a type of upperflow regime lamination formed by intense wave action such as thatwhich occurs during storms under strong oscillatory and unidirectionalflows (Cheel, 1991; Arnott and Southard, 1990). It can be recognized bythe absence of any undulating surface and by its well-organized tabularbedsets.

4.2.2. Tidal signaturesDirect evidence of tidal current processes in the “unconventional”

Rannoch Formation is expressed as cyclic deposits, ranging from several

Fig. 3. Paleogeography of Brent Delta progradation (after Fjellanger et al., 1996). (A)Delta positiowas very subtle. Multiple large river systems were shown from southern, eastern and westpronounced synsedimentary faulting. The major sediment supplies were from eastern and wblack stars represent the locations of wells 34/10-23, and 34/11-1, respectively.

millimeters to several decimeters in scale, likely reflecting a hierarchy oftidal cycles (Davis, 2012).

(1) The first sort of tidal deposits is an alternation of thick cross-laminated sandstone and thin mud-draped sandstone (type 1 tidaldeposits) (Fig. 7A–C), expressed as very low abundance of thin(1~2 mm) mud drapes or double mud drapes that are discretelydistributed within well-sorted, unidirectional cross-laminae. Themost readily recognizable tidal signature is the repetition ofdouble mud drape, which is diagnostic of tidal processes, recordingthe diurnal or semi-diurnal tidal cycle (Visser, 1980; Nio and Yang,1991; Dalrymple, 1992) and occurs as sand–mud couplets of fewmillimeters thick. There are two sand layers, a thicker onerepresenting the dominant tide and a much thinner one from thesubordinate tide, as well as a slack-water mud drape cappingeach. The twomud drapes on either side of the very thin subordi-nate sand layer is the eye-catcher, and gives rise to the term doublemud drape. The capping muddy or carbonaceous layers highlightrhythmic changes in the thickness of successive sand laminae orbeds, reflecting changes in current velocity and flow reversals

n after themain progradation (on SB169Ma). During this period, synsedimentary faultingern directions. (B) The flooding during Brent aggradation (MFS 167 Ma), generated byestern directions. (C) The maximum delta extension (on SB 166 Ma). Note: The red and

Fig. 4. Comparison between the “conventional” and “unconventional” Rannoch Formation. The conventional Rannoch Formation example (A) is from well 34/11-1, showing HCS/SCSdominated 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; andMD—mud-draped intervals.

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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 tidaldeposits and overall lowabundance ofmud drapes can be ascribedto longer strong tidal energy and shorter weak tidal energy pe-riods, as a result of neap–spring tidal asymmetry. As cross-laminae within type I deposits are somewhat similar to HCS incore, care should be taken to observe the orientation of cross-laminae and their stacking pattern to identify the boundary be-tween storm and tidal beds.

(2) The second tidal criterion (tidal signal type 2) is tidal bundling ofstrata, of a few tidal cycle duration. It is a style of repetitive muddrapes and sand layer thickening and thinning within cross-stratal foresets and bottomsets (Fig. 7D–F), reflecting the tidal fre-quency of slack-water periods (Steel et al., 2012) and recording aseries of strengthening or weakening tidal cycles; whereby the re-petitive mud drapes reflect the regular daily or twice-daily slack-water period as the tide turns (Archer et al., 1995). The alternationof thicker and thinner layers within foresets suggests fluctuatingflow 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 andwell-organized sand–mudcouplets (Fig. 7G–I), reflecting the repetition of tidal cycles duringneap–spring cycles. It is vertically accreted, horizontally or sub-

Fig. 5. Core description of key well 34/10-23: facies proportion and facies succession. The profile(FS3), each showing coupled storm-tidal deltaic depositional components. The thickness proporthe figure.

horizontally laminated sandstone layers capped by mud drapes,internally showing either evenly or slight increasing and decreas-ing spacing, as well as slightly increasing and decreasing abun-dance of mud drapes. It is interpreted as deposited fromsandstone generated by flood–ebb tidal currents alternating withmudstone from intervening slack water suspension fallout (Nioand Yang, 1991) during neap–spring tidal cycles.

Someof the above signatures in the study coreswould be consideredweaker tidal criteria, as other process, e.g., variation in wave-groupfrequency and fluctuating fluvial currents, cannot be unequivocally ex-cluded from generating such features, but the occurrence of all threecriteria together and their internal regularity and cyclicitymake a strongcase (Nio and Yang, 1991; Dalrymple, 2010; Davis, 2012) for the tidalinfluence 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 anaverage 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 thecores but the top boundary can be recognized both on well logs andon cores by an abrupt grain-size increase to the overlying Etive Forma-tion. The entire Rannoch succession is composed by relatively uniform

shows two coarsening-upward successions (FS1 and FS2) and one finning-upward intervaltions of storm- and tide-dominated facies of each succession are shown in the pie-chart of

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grained and consistently unburrowed, well-sorted storm beds periodi-cally alternating with tidal beds.

During the progradation of the Brent Delta, the Rannoch shorelinesystem developed three successions, two broadly upward-coarseningand thickening, and the uppermost one showing upward finingand thinning (Fig. 5). These are: 1) a succession dominated by clean-water, storm-event beds frequently and preferentially interbeddedwith 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 fre-quently separated by some types II and III tidal beds (Figs. 5 and 8B),and then upward into 3) a thin interval showing muddier storm-eventbeds mainly alternating with few type II tidal beds (Figs. 5 and 8C).

4.3.1. Descriptions of the Rannoch facies successions4.3.1.1. 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 tolight-gray colored storm-event sandstone beds, which are characterizedbyplanar laminated sandstone (45%), HCS (27%) and SCS (12%) stratifica-tion (Fig. 5).

The most striking feature of FS1 is the frequent interbedding of tidaldeposits, 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 are1~5 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 slightlycoarsening-upward successions (e.g., Figs. 5 and 8B). The sandstonesare notably clean and homogenous in nature (i.e., very little mud is pres-ent in the system). There are significant differences between successionsFS1 and FS2, particularly the dominance ofHCS in FS1 in contrast tomain-ly 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. Thethickness of each mud drape is greater (1–7 mm) and mud drapes arewell-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 mud-dier and thinner (ca. 2 m) unit (e.g., Figs. 5 and 8C), composed by someerosionally, amalgamated, dark-colored storm-event beds (91%), withHCS of smaller wavelength (decimeters), which are mainly separatedby type II tidal beds (Figs. 5 and 8C). The entire interval exhibits a verti-cal fining-upward trend. The thickness of mud drapes ranges from 2 to8 mm, which is generally thicker than the mud drapes in FS1 and 2.

4.3.2. Interpretations of the Rannoch facies successionsThe 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 (N80%) of storm-wave generated sedimenta-ry structures, such as planar lamination, HCS, and SCS and lowproportion(b20%) of tide-generated structures within the succession, suggest thatthe primary process is storm-wave and tide influence is secondary, inthe overall shallowing-upward succession. The system is therefore cate-gorized as storm-dominated, and tide-influenced. The complete absenceof bioturbation within the storm-event beds, may be related to the high

Fig. 6. Examples of storm-generated structures. (A, B) HCS and planar lamination; and (C) Mudshowing faint laminae. (E) Planar lamination. (F) Homogenous sandstone bounded by high-angP—planar lamination, S—SCS, and the black arrows point to erosional surface(s).

frequency and high magnitude of storms, and/or high sedimentationrates (MacEachern et al., 2005; Li andBhattacharya, 2015). The preservedrecord of stacked, erosionally based, HCS, SCS andplanar laminated sand-stone reflecting constant high-energy oscillatory currents, is likely to rep-resent the storm-dominated middle-lower shoreface deposits betweenfair-weather and storm wave base (Reading and Collinson, 1996; Petersand Loss, 2012). The non-negligible amount (ca. 10%), and moderatelyfrequent occurrence of mud drapes in storm-dominated setting, indi-cate that there were 1) intermittent fair-weather (inter-storm) periodswith minimal wave influence during a train of storms; 2) availablesuspended 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 pairedprocesses and the preser-vation bias of frequency and style of the storm-wave and tidal currentgenerated structures, in three distinct units may reflect temporal andspatial, absolute and relative changes of storms and tidal currents oper-ating on the Rannoch shoreline.

4.3.2.1. Interpretation of facies succession 1. Thepreferential occurrence ofcharacteristically asymmetric type I tidal bed, and the thinness of muddrapes might reflect a low fluvial input and relatively strong tidal cur-rents in the distal delta-front reaches. The small amount of thicker andamalgamated well-organized double mud drapes in the foresets andbottomsets of cross-strata (type II) or heterolithic lamination (type III)may be produced in prolonged fair-weather conditions, representingmoderate tidal currents via sufficient suspended sediment settling.

4.3.2.2. Interpretation of facies succession 2. In FS2, the large-scale struc-tureless, to planar or low-angle laminated sandstones bounded byscour surfaces, may represent SCS dominated storm-event beds. Thepreferential occurrence of SCS sandstone may be related to storms ap-proaching shallow water depths (Duke, 1985; Dumas and Arnott,2006), where the aggradation rate is not sufficient and the waterdepth is not enough to form and preserve hummocks. The good sortingand relatively homogenous nature of sandstone beds may also indicatea dominance of high-energy and high-frequency storm-wave action,which winnows out mud (Vakarelov et al., 2012), thus making theSCS sandstone appear structureless. The increased thickness of muddrapesmay be due to increasing suspended sediment supply. The betterpreservation of tidal signal types II and III may reflect better preserva-tion of tidal deposits or less frequent and intense storms, thus allowinglong-term fair-weather conditions to create and preserve tidal deposits.FS2 is therefore interpreted as a middle delta-front succession, equiva-lent to middle shoreface setting.

4.3.2.3. Interpretation of facies succession 3. FS3 differs from FS1 and FS2by its entirely muddier character, and its abundance and thickness ofmud drapes, perhaps reflecting proximity to the river-mouth wherethere would have been sufficient suspended sediment supply. Thefining-upward trend and the modest thickness of storm-event bedsmay be the combined effects of a temporal increase of fine-grainedsediment and a decrease of storm-wave energy that is effective inwinnowing out fine-grainedmaterial. Compared with FS2, the slight in-crease in proportion of tidal beds and occurrence of smaller HCS maysuggest that FS3 represents a transgressive interval and that theRannoch shoreline away from the active deltaic output was beingreworked by small waves.

4.3.3. Processes summaryIt is evident that the studied Rannoch succession was controlled by

coupled tidal hydrodynamics and pronounced storm process; tidal cur-rentswere only able to redistribute sediment at times. Stormdominance

dier HCS, with smaller wave-length (several decimeters). (D) SCS and planar lamination,le scoured surfaces. (G) Homogenous sandstone bounded by sharp surfaces. Note: H—HCS,

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is readily recognizable as well-sorted HCS and SCS stratifications. Tidalinfluence is shown by the three types of tidal beds, and the good sortingof fine-grained sands within tidal beds. Fluvial influence was importantin transporting sediment onto the shoreline and amplifying or dampingtidal influence.

Variation in the preservation bias of storm and tidal beds mightbe the combined effects of 1) the frequency and duration of storms, orthe 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 directimpact, as no tidal beds could be deposited in long-lasting storms. Thelonger periods between storm events promoted the accumulation oftidal beds, particularly type III tidal beds; the short duration of fair-weather (inter-storm) periods allowed the formation of thin, type IItidal beds.

Secondly, in the prograding deltaic lobes, tidal process likely occurredpreferentially in the seaward reaches of the system, i.e., in FS1, wherefluvial deposition rates were relatively low, compared to the sedimentredistribution capacity of tidal currents; in this context, tides were re-sponsible for redistributing both sand and mud (Plink-Bjorklund,2012). In addition, the abundance and thickness of mud drapes wouldalso have varied with river discharge, as thicker drapes would haveformed in landward reaches, i.e., FS2 and FS3, or during high dischargeperiods, from high suspended sediment loads (Dalrymple and Choi,2007).

Thirdly, as suggested by the overall sand-rich nature of the succes-sion, deposition occurred near the mouth of a large river, where sandbedload was abundant, and relatively strong tidal currents would bepresent because of the significant tidal prism associated with flow inand out of the river mouth area (Dalrymple et al., 2003; Dalrympleand Choi, 2007). Besides, tidal process is significant especially in areaswhere wave energy is dissipated, therefore, the presence of mud drapesin the delta-front suggests that theremight have been local topographicrestrictions e.g., sheltering from a wave-approach direction. This couldhave resulted in an enhanced tidal range or tidal-current velocity whenthere was lateral restriction (e.g., bifurcation of distributary channel) orwhere the tidal wave entered into tidal resonance (Dalrymple andZaitlin, 1994; Dalrymple et al., 2003). Variations in current velocities aswell as in tidal rangesmight be twomechanisms to explain the formationof tidal beds. As tidal current velocities fluctuate between values close tozero and velocities capable of transporting bedload material, associationof large tide range and strong tidal current velocities or low tide rangeand weak tidal current velocities could have resulted in association ofsandstone and mud drapes of different types (Liu et al., 2002; Hemeret al., 2004; Plink-Bjorklund, 2012).

5. Discussion

5.1. Formation and preservation conditions for tidal signals

5.1.1. Conceptual model for the formation of storm–tide coupletsAs suggested, the formation andpreservation of tidal signals between

storm-beds, require a relatively protected shoreline setting, and thearrival of a sufficient river supply. Both of these conditions were likelymet: 1) when the Brent Delta prograded northward across theKvitebjørn–Valemon area, and a slight extension and a locally accelerat-ed subsidence across the depositional region may have produced someirregularity of the Rannoch shorelines. The shorelines became veryslightly embayed in places insteadof the normal deltaic protrusive coasts(as suggested inwell 34/10-17 aboveMFS 167Ma, Fig. 3B). 2) Therewasat least one large river driving the overall progradation of Brent Deltaregardless of the shoreline configuration (Fig. 3A–C). We thereforepropose that the following two stages were responsible for the forma-tion of tidal signals between storm beds (e.g., Fig. 9A, B).

Stage 1: Initial stage of deposition under storm conditions.

During storm periods, storm beds (S) were generated and tendedto be preserved between fair-weather and storm wave base, andlaterally flank the river mouths and shoreface. The sandy stormbeds in FS1 and FS2 were most likely deposited above the nearshoremudline (Fig. 9A).

Stage 2: Secondary stage of deposition under fair-weather conditions.During this period, wave energy was minimal and the fair-weather

wave basemoved upward and became higher than that in storm condi-tions. Wave influence was minimal and sediment supply through riverswas distributed and reworked by river and tidal currents. With thedecrease of river currents and increase of tidal currents in the seawarddirection, at the point where the velocity of tidal currents exceedsriver currents, tidal beds (T) accumulate in river mouths and embay-ment in the intertidal and subtidal zone (Fig. 9B).

5.1.2. Suspended sediment sourceSuspended sediment in tectonically active, mixed-energy deltas

can be derived from a range of sources; in addition to the river cur-rents, tidal currents and storms that have potential to transportand suspend/resuspend fine-grained sediment, the uplifted flanksand internal high blocks of the North Viking Graben are also likelyto have contributed to the deltaic sediments (Richards, 1992;Helland-Hansen et al., 1992).

The studied succession is part of a large delta complex, and mudwould have been provided by distributary channels as shown inthe Aalenian paleogeographic map (Fig. 3). The best signal of BrentRiver mud is in FS3, because FS3 is relatively mud rich and occupiedthe most proximal (albeit a more sheltered lateral development)Rannoch site. Besides, the landward-directed, residual tidal flow, inthe seaward part of Rannoch also has potential to resuspend mudfrom the prodelta areas with landward transport onto the delta-front(Dalrymple and Choi, 2007). Though storm-erosion can generate a con-siderable amount of mud, any mud resuspended during a storm wouldbe deposited relatively quickly after the end of the storm, therefore it isless likely to have been a source of the fair-weather (inter-storm)muddy tidal deposits.

In addition, the uplifted flanks on themargins of the northern VikingGraben were likely an important sediment source for the Brent Delta(Ravnås and Bondevik, 1997). In some areas including parts of the pres-ent study area, tectonic activity and fault-block rotationmay have accel-erated or fault-related subsidence started earlier, as has been suggestedby Folkestad et al. (2014). Limited by seismic and thin section data, wecannot test the hypothesis and evaluate this process precisely. However,the abrupt change from light-colored sandstone (likely enriched inquartz and feldspar) of successions FS1 and FS2, to dark-colored sand-stone (with a higher matrix content or lithic grains) characterizing thethin interval FS3, may suggest influx of immature detritus from nearbysource areas in addition to the conventional southerly provenance(Fjellanger et al., 1996).

5.1.3. Preservation potentialThe tidal signals described herein would normally have been se-

verely masked by fair-weather waves and storms, so their occur-rence here indicates that the setting must also have been onewhere there were relatively high sedimentation rates associatedwith the mouth of one of the Brent distributary rivers here, so thatthe storm waves did not completely rework the inter-storm deposits(Fig. 9C). The high sedimentation rates at the delta mouth would alsohave been enhanced by progradation of Rannoch, Etive and thelower part of the Ness Formations during a period of slow sea-levelrise where accommodation was outpaced by high sedimentationrates (Eschard et al., 1993). Both the absolute and relatively highsedimentation rates would have contributed to the preservation oftidal signals.

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 ofthick 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 bottomsetsand foresets of cross-strata, showingmoderate abundance ofmud drapes that are well-organized. (3) type III (G~I): heterolithic lamination, showingmultiple well-organized sand–mudcouplets, 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; theyellow 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.

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5.2. Comparison with modern and other ancient systems

Although there is a widespread spectrum of deposits expectedto be deposited between the storm and tidal end members of coasts,there are only some modern and rare ancient examples recordingmixed wave and tide signals (Yang et al., 2005, 2006, 2008; Choi et al.,2004; Dashtgard et al., 2009, 2012; Vakarelov et al., 2012; Cummingset al., 2015).

One relatively poor analog to the Rannoch deposits is the westKorean coast described by Yang et al. (2005, 2006, 2008), which is bor-dering 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 Rannochconditions. In the Korean open-coast tidal-flat case the sediment supplywas relatively limited and the suspended sediment concentration

was higher, resulting in a more complete winter storm reworking ofthe inter-storm deposits and retention of the tidal signal only as a mod-ulation of the stormdeposits themselves rather than asmud drapes. An-other analog is the muddy open-coast tidal flats along the east coast ofChina described by Fan (2012). However, they are fringing a verylarge river and receiving gigantic volumes of fluvial muddy sediment.Besides, due to theirmarginal sheltered location and their low sedimen-tation rates as well as modification by high magnitude storms duringtidal cycles, they are preferentially preserving low-energy sediments,i.e., wave ripples, combined flow ripples, rather than high-energystorm deposits as in Rannoch Formation.

Other cases showing mixed storm-wave and tidal processes are thetidal-modulated shorefaces described by Dashtgard et al. (2009, 2012)and Vakarelov et al. (2012). However, in these cases the tidal signalsare also only indirectly expressed by tidal modulation of the storm

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 shorelineconfiguration; 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 stormwave base. Notably, the sandy storm beds are likely deposited above the nearshore mudline. (B) Secondary stage of deposition under fair-weather conditions, indicates theaccumulation of tidal beds, when wave influence is minimal. In the seaward direction, particularly at low river discharge periods, tidal beds likely occurred when tidal currentsoutpaced 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, theeffective 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 isredistributed 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 largelydependent on high accommodation rates. When the accommodation rates outpaced the sedimentation rates, tidal deposits are likely preserved between storm deposits.

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effects (i.e., shifting of the tidal zones during tidal cycles) in a meso- ormega-tidal setting.

Another possible analog, perhaps especially for FS3, is the opencoast tidal-flat area near the mouth of the Han River, west coast ofSouth Korea described by Choi et al. (2004) and Cummings et al.(2015). However, these open coast tidal flats are subject to lesswave energy than the Rannoch case, due to the dampening of incom-ing waves by the broad subaqueous delta platform. Another weak-ness of this analog is its close association with tidal bars and pointbars.

The ancient Lajas Formation (Argentina) example (Rossi and Steel,2016), is another possible analog in the formation and preservation oftidal signals in a mixed-energy delta-front. However in this case thesouthern Neuquen Basin was moderately protected from high-energystormwaves and the mixed energy components were mainly the inter-action of periodic river flood input with tidal currents that increased intheir intensity basinward, either because of impinging against a low-

gradient axial tidal system or because of approach to the shelf edgewhere there was an increased tidal prism (Rossi and Steel, 2016).

It can be seen that the Brent delta front cannot be classified intoexistingmixedwave-tide system schemes but would otherwise classifyas 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 studyarea are important for two reasons. Firstly, we encourage the re-examination of other ancient storm-wave dominated successions fortidal signals. Secondly, the study segment of the Rannoch delta-front,thoughwave-dominated, was significantly tide-influenced in a configu-ration that is poorly known amongmodern or ancient deltaic examples.As such it serves to strengthen our current classification of mixed-energy coastlines.

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6. Conclusions

(1) In the deep-graben Kvitebjørn–Valemon fields, the shorelinesystem of the prograding Brent Delta in the northern North Seaexhibits an “unconventional” delta-front succession, generallydominated by storms but significantly influenced by tides.The storm-wave signals are stacked, erosionally amalgamatedstorm-event beds characterized by unburrrowed flat-laminated,HCS and SCS beds; the tidal signals are of three types: 1) thinmud drapes and double mud drapes in an alternating associationwith cross-laminated sandstones; 2) a few tidal bundles in theforeset or bottomsets of cross-strata; and 3) heterolithic lamina-tion 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 faciessuccessions. From bottom to top, this is expressed as 1) a faciessuccession dominated by clean-water, storm-event beds fre-quently interbedded with type I tidal intervals and occasionaltypes II and III tidal beds, passed toward 2) a facies succession ofclean storm-event beds less frequently separated by some typesII and III tidal beds, and then into 3) a thin interval showing mud-dier storm-event beds alternating with few type II tidal beds. Thevariations in the preserved bias of storm and tidal beds, reflectthe combined effects of 1) the frequency and duration of storms,or the duration of non-storm, fair-weather periods; 2) riverdischarge; and 3) the absolute and relative strength of tides.Tidal deposits are interpreted as inter-storm, fair-weatherdeposits and tidal processes occurred in longer intermittent fair-weather condition and lower river discharge periods, and prefer-entially occurred in the distal-reach of the delta-front.

(3) The facies and facies stacking of the studied Rannoch Formationare consistentwith a highly episodic storm setting periodically in-fluenced by tides. The studied Rannoch Formation can thereforebe interpreted as a storm-dominated, tide-influenced delta-frontnear the mouth of large river, where relatively strong tidal cur-rents would be expected. The better preservation of tidal beds, isattributed to the high delta mouth accumulation rates and risingnormal regression of the delta front.

Acknowledgments

We thank A/S Norske Shell for financial support, the Jackson Schoolof Geosciences at the University of Texas at Austin for administrativesupport, and the China Scholarship Council (CSC) for financial supportto cover some associated international travel. We also appreciateFernando De Miguel Esteban, Bente Reinertsen, and Ben Hull-Baileyfor assistance in core arrangements, and Atle Folkestad, Valentina M.Rossi, Shunli Li, and Yifan Li for valuable discussions. We particularlythank Editor Dr. Brian Jones, and reviewers Piret Plink-Björklund andRobert W. Dalrymple for their constructive reviews and suggestions,which helped improve the manuscript greatly.

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