application of seismic and sequence stratigraphic concepts to a lava-fed delta system in uk.pdf

Application of seismic and sequence stratigraphicconcepts to a lava-fed delta system in theFaroe-Shetland Basin,UKand FaroesK. A. Wrightn, R. J. Daviesn, D. A. Jerramz, J. Morrisw and R. FletcherwnCentre for Research into Earth Energy Systems (CeREES), Department of Earth Sciences, DurhamUniversity,Durham, UKwStatoil UKLimited, London, UKzDougalearthLtd,12 Nevilles CrossVillas, DurhamDH14JR, UK

ABSTRACT

Detailed seismic stratigraphic analysis of 2D seismic data over the Faroe-Shetland Escarpment hasidenti¢ed13 seismic re£ection units that record lava-fed delta deposition during discrete periods ofvolcanism.Depositionwas dominated by progradation, during which the time shoreline migrated amaximumdistance of �44km in anESEdirection.Localised collapse of the delta front followed the endof progradation, as a decrease in volcanic activity left the delta unstable. Comparisonwith modern lava-fed delta systems onHawaii suggests that syn-volcanic subsidence is a potential mechanism for apparentrelative sea level rise and creation of new accommodation space during lava-fed delta deposition. Afterthe main phase of progradation, retrogradation of the delta occurred during a basinwide syn-volcanicrelative sea level rise where the shoreline migrated a maximum distance of �75km in aNNWdirection.This rise in relative sea level was of the order of175^200m, andwas followed by the progradation ofsmaller, perched lava-feddeltas into the newly created accommodation space.Active delta deposition andthe emplacement of lava £ows feeding the delta front lasted �2600 years, although the total duration ofthe lava-fed delta system, including pauses between eruptions, may have been much longer.

INTRODUCTION

Seismic re£ection imagery of sedimentary basins hasresulted in the recognition of speci¢c re£ection con¢gura-tions and re£ection discordances that have informed thereconstruction of relative sea level changes and an under-standing of basin ¢ll histories (e.g. Payton, 1977; Wilguset al., 1988). The seismic re£ection method was initiallyapplied to siliciclastic (e.g.Vail et al., 1977; Posamentier &Vail, 1988) and then carbonate successions (e.g. Bubb &Hatlelid, 1977; Sarg, 1988), and more recently to volcanicrifted margins (e.g. Spitzer et al., 2008; Jerram et al., 2009;Ellefsen et al., 2010). Growing interest in exploration andproduction of hydrocarbons from o¡shore successionswith a volcanic component has resulted in seismic databeing acquired over such areas, including the M�re andV�ring Basins (o¡shore Norway) and the Faroe-ShetlandBasin (UK and Faroes).

Signi¢cant volumes of £ood basalts were erupted insubaerial to submarine settings in the North AtlanticRegion during the Late Palaeocene (e.g. Ellis et al., 2002;

Jerram et al., 2009). The volcanic succession displays avariety of re£ection con¢gurations that are indicative ofdepositional environment and subsequent mass transport.These include parallel bedded re£ections that areinterpreted to be subaerially erupted plateau lava £ows(Boldreel & Andersen, 1994). In contrast, seaward dippingre£ections exhibit inclined, smooth to hummocky geome-tries and are interpreted to be subaerial to shallow submar-ine lava £ows erupted during the early stages of sea £oorspreading.They erupted close to, or on the axis of spread-ing and were later a¡ected by post-rift subsidence, withthe greatest inclination seen in the oldest lava £ows(Andersen, 1988; Planke et al., 2000; Parkin et al., 2007).Prograding re£ections with a steeper inclination (4201)are interpreted to be subaerially erupted lava £ows enter-ing the sea, forming steep delta escarpments of hyaloclas-tic breccias (Smythe et al., 1983; Ki�rboe, 1999; Spitzeret al., 2008).

Lava-fed deltas preserve the transition from subaerialto submarine strata, and are a record of the palaeo-shore-line. They often display similarities to siliciclastic deltasystems, by ¢lling available accommodation space, react-ing to changes in relative sea level and variations in thesupply of material (Fig. 1) (Jones & Nelson, 1970; Mooreet al., 1973; Jerram et al., 2009).This has led to comparisonsof lava-fed deltas with Gilbert-type siliciclastic deltas and

Correspondence: K. A.Wright, Centre for Research into EarthEnergy Systems (CeREES),Department of Earth Sciences, Dur-ham University, Durham DH1 3LE, UK. E-mail: [email protected]

© 2011 The AuthorsBasin Research © 2011 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 91

Basin Research (201 ) 24, 91–106, doi: 10.1111/j.1365-2117.2011.00513.x2

EAGE

the identi¢cation of comparable facies components (Full-er,1931; Jones&Nelson,1970; Porebski &Gradzinski,1990;Naylor et al., 1999). However, lava-fed delta systems, parti-cularly those formed during £ood basalt eruptions, recordvariations in the supply of volcanic material, which can bemuch greater than in siliciclastic systems. Huge volumesof lava erupt over geologically short timescales, resultingin the very e⁄cient ¢lling of accommodation space and ra-pid progradation of the shoreline.

Modern examples of lava £owing into the sea (such asseen on Hawaii), undergo quenching and fragmentationinto hyaloclastic breccias which are then rapidly depositeddown slope under gravitational processes to form inclinedforesets (Kokelarr, 1986; Fisher & Schmincke, 1994). Thegrowth of the delta is through deposition of new lava £owsand hyaloclastic breccias, with successive phases of vol-canism producing a stacking pattern that is directly relatedto the interaction of relative sea level, lava supply andavailable accommodation space. The geometry of thestacking pattern depends on the dominant factor atthe time of deposition, making it possible to reconstructthe depositional environment and interpreted the lava-fed deltawithin a sequence stratigraphic framework (Jones&Nelson, 1970; Gatli¡ et al., 1984; Ki�rboe, 1999).

This paper investigates in detail the re£ection geome-tries of the Faroe-Shetland Escarpment and the applic-ability of seismic and sequence stratigraphy to de¢ne aseries of volcanic units that can be interpreted in terms ofrelative sea level, lava supply and available accommodationspace. Understanding how £ood basalts develop from asubaerial to submarine environment and the identi¢cationof key horizons within the volcanic succession can be usedto investigate the onset, development and closing stages of£ood basalt volcanism (e.g. Jerram&Widdowson, 2005). Itcan constrain the spatial and temporal distribution of keyvolcanic facies (Nelson et al., 2009) and be a valuable re-source for exploration in volcanic rifted settings.This al-lows us to reconstruct in detail the development andevolution of Faroe-Shetland Escarpment and how the pa-laeo-shore line evolved due to £ood volcanism during thebreak-up of Europe fromNorth America.

GEOLOGICAL SETTING

The Faroe-Shetland Basin is a product of North Atlanticrifting between Greenland and Eurasia during the Meso-zoic to early Cenozoic (England et al., 2005; Passey & Bell,

2007). Continental break-up and the onset of sea£oorspreading were accompanied by extensive £ood basalt vol-canism.The main phase of volcanism occurred during thePalaeocene, at 62^54Ma (e.g. Ritchie &Hitchen, 1996; Han-sen etal., 2009;S�ager&Holm, 2009) and is characterised bythe extrusion of subaerial basaltic lavas (e.g. Passey & Bell,2007), the intrusion of sills (e.g. Thomson & Scho¢eld,2008; Hansen et al., 2011) and the formation of individualigneous centres, such as the Erlend Complex and BrendansDome (e.g. Gatli¡ etal., 1984; Ritchie &Hitchen,1996).

To the east of the Faroe Islands, the Faroe-Shetland Es-carpment has been identi¢ed as the subaerial extension ofthe £ood basalts, which £owed to the southeast in¢llingpre-existing topography before reaching the palaeo-shore-line (Smythe et al., 1983; Ki�rboe, 1999; Ritchie et al., 1999).At the shoreline, a number of the £ood basalt £ows enteredthewater and formed a prograding body of hyaloclastic brec-cias pushing the shoreline basinward. Initialwork has shownthat the distribution of these systems or deltas can be exten-sive, recording a signi¢cant syn-volcanic migration of thepalaeo-shoreline in this region (e.g. Ki�rboe, 1999; Spitzeret al., 2008; Jerram et al., 2009).Volcanism within the basinceased when sea £oor spreading became established to thenorth of the basin, with post-rift subsidence and late Ceno-zoic compression creating the tilted and folded structuresidenti¢ed today (Ritchie et al., 2003; S�rensen, 2003; Davieset al., 2004; Praeg et al., 2005).

DATA ANDMETHODOLOGY

This studyhas used avariety of 2D seismic re£ection surveysgatheredwithin the Faroe-Shetland Basin between1983 and2005 with large areas of geographical overlap. The greatestconcentration of survey lines is located over Faroe-ShetlandEscarpment (Fig. 2) and images the £ood basalt successionand contemporaneous deep water strata at an average verti-cal resolution of 20^30m. Analysis included detailed map-ping of460 lines that have an average line spacing between1 and 3km.The top surface of the £ood basalts throughoutthe basin is identi¢ed by a prominent, high amplitude andstrongly continuous re£ection (Fig. 3).The strong re£ectiv-ity of the top surface and the internal heterogeneity withinthe volcanic succession presents a challenge for imaging,particularly near the base of the succession (e.g.White et al.,2003; Roberts et al., 2005).

Despite these challenges, seismic amplitudes variationsand various re£ection geometries have been clearly imaged,

Fig.1. Schematic cross-section througha developing lava-fed delta. Based on thisstudy, Fuller (1931) and Jones &Nelson(1970).

© 2011 The AuthorsBasin Research © 2011 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists92

K. A. Wright et al.

andwith the application of seismic stratigraphy it is possibleto de¢ne the gross stratigraphic architecture. The recogni-tion of units composed of relatively conformable re£ectionsand bounded by unconformities is through identi¢cation ofsystematic discordances or re£ection terminations againstthe bounding re£ection (Mitchum et al., 1977a;VanWagoneretal.,1988).Additional use of seismic facies analysiswith char-acterisation in terms of amplitude, continuity and con¢gura-tion can be used to interpret the depositional processes,lithologies and environmental conditions (Mitchum et al.,1977b; Sangree &Widmier,1977; Cross &Lessenger,1988).

The majority of wells are located in the south east of thebasin, beyond the extent of the Faroe-Shetland Escarpment(Fig. 2), so well control is limited. These wells include205/9-1and 213/23-1, which encountered inter-bedded suc-cessions of hyaloclastites, lavas and siliciclastic successionsof varying thickness (Larsen et al., 1999; Jolley & Morton,2007).Wells 214/4-1 and 214/9-1 penetrated the most distal£ood basalts, with 214/4-1encountering lava £ows overlyinga �1000m thickness of hyaloclastic breccias, which havebeen identi¢ed to extend some distance beneath the Faroe-Shetland Escarpment (Fig. 3) (Davies et al., 2002).

On the Faroe Islands, the £ood basalts have a strati-graphic thickness of at least 6.6 km, which have been sub-

divided into seven formations on the basis of lithology,geochemistry and £ow structure (Fig. 4) (Ellis et al., 2002;Passey&Bell, 2007; Jerram etal., 2009).TheLopra Forma-tion is composed ofvolcaniclastic material thought to havebeen deposited in an estuarine environment, proximal tothe eruption (Ellis et al., 2002; Boldreel, 2006; Passey &Bell, 2007).The Beinisv�rj andMalinstindur Formationsconsist of signi¢cant thicknesses of lava with an average£ow thickness of 25 and 2m, respectively.TheEnniForma-tion has thinner, less extensive lava £ows with an average£ow thickness of 15m (Ellis et al., 2002; Passey & Bell,2007; Passey & Jolley, 2009). The Prestfjall, Hvannhagiand Sneis Formations consist of siliciclastic and volcani-clastic interbeds which may record periods of local volca-nic quiescence (Passey & Bell, 2007; Jerram et al., 2009).

The onshore volcanic succession is penetrated by threewells, including theGlyvursnes-1borehole, which reacheda depth of 700m, encountering 450m of theMalinstindurFormation and 250m of the Enni Formation (Fig. 4)(Japsen et al., 2004). The Vestmanna-1 borehole, whichreached a depth of 660m, encountered 550m of the lowerpart of the Malinstindur Formation and 110m of theuppermost part of the Beinisv�rj Formation (Fig. 4)(Japsen etal., 2004).The oldest £ood basaltswere penetrated

Fig. 2. Map of the Faroe-Shetland Basin showing the study area and location of Figs 3, 5 and 6. Extent of £ood basalts and Faroe-Shetland Escarpment modi¢ed fromRitchie &Hitchen (1996), Ritchie et al. (1999), Ellis et al. (2002) and S�rensen (2003).


Stratigraphy of a lava fed delta system

Fig. 3. (a) Regional correlation of seismic re£ection con¢gurations and interpreted lithologies identi¢ed inwell 214/4-1 (see Fig. 2 forlocation). (b) Schematic correlation of onshore and o¡shore stratigraphy,modi¢ed after Smythe et al. (1983) andRitchie et al. (1999). (c)Wireline log responses and interpreted lithologies for the volcanic succession in 214/4-1 (MD,measured depth;TVD, total vertical depth).

Fig.4. Distribution of the Faroe Island Basalt Group on the Faroe Islands and stratigraphy compiled from both onshore and boreholedata.Modi¢ed from Passey & Bell (2007), Jerram et al. (2009) andNelson et al. (2009).


K. A. Wright et al.

by theLopra-1/1Aborehole,which reached a depth of 3.6kmwithout encountering the base of the volcanic succession.The borehole encountered42500m of lava £ows from theBeinisv�rj Formation and41000m of volcaniclastic mate-rial from the Lopra Formation (Fig. 4).The volcanic succes-sion exhibits avariety ofvelocitieswhich are indicative of thevolcanic facies.Tabular lava £ows, as identi¢ed in theBeinis-v�rj Formation have high velocities, varying from 4 to7km s�1 with an average of 5.5 km s�1. Compound lava£ows, as identi¢ed in theMalinstindur Formation have lowvelocities, varying between3and6km s�1with an average of4.5km s�1. Hyaloclastite breccias as identi¢ed in the LopraFormation have the lowest velocities, varying between 3 and5km s�1 with an average of 3.5km s�1 (Planke, 1994; Bol-dreel, 2006; Nelson et al., 2009).

OBSERVATIONS

Reflection configuration analysis

The top of the £ood basalts is identi¢ed by a prominent,high amplitude and strongly continuous re£ection that

de¢nes the upper limit of a succession of high amplitude,subhorizontal and continuous re£ections that decrease inamplitude and continuity with depth (Fig. 5).This succes-sion extends from theFaroes shelf into theFaroe-ShetlandBasin, where they rapidly change to inclined, moderateamplitude re£ectionswith the transition marked by a clearo¥ap break. Basinward of the o¥ap break, the re£ectionsde¢ne units composed of moderate to low amplitude,continuous re£ectionswith prograding, sigmoidal geome-tries.The re£ections often onlap the underlying boundingre£ection and vary from downlapping to being terminatedby the overlying re£ections. The base of each seismic re-£ection unit is identi¢ed by downlap on to deeper re£ec-tions (Fig. 5). In total, 13 seismic re£ection units with are£ection con¢guration of high amplitude topsets andlow amplitude foresets have been identi¢ed (Fig. 6). Wehave numbered these in stratigraphic order, with 1 beingthe oldest and13 being the youngest.

Seismic re£ection units 1^11 have a sheet to wedge-likemorphology, with heights varying from 700 to1050m.Thestacking pattern of the units is largely progradational withan aggradational component that becomes increasinglyapparent in units 6^11 (Fig. 6). Unit 11 is the most distal

Fig. 5. Seismic stratigraphic methodology used to identify seismic re£ection units afterVail et al. (1977), Posamentier & Vail (1988) andKi�rboe (1999) (see Fig. 2 for location).



unit, and displays shallower internal re£ections whichvaryfrom continuous to semi-continuous, with a locally dis-rupted and curved upper bounding re£ection. Seismic re-£ection units 12 and 13 have a similar wedge-shapedmorphology as units 1^11, with height varying from175 to200m.These units are directly above units1^11and displaya retrogradational stacking pattern, with the extent of eachunit located progressively to thewest, towards theFaroe Is-lands (Fig. 6). Identi¢cation of seismic re£ection units, inparticular the bounding re£ections, becomes increasinglydi⁄cult deeper within the succession of re£ections due toheterogeneity of the system causing scattering and ab-sorption of the seismic energy.

The position of the o¥ap break for the most easterly ly-ing clinoform identi¢es the limit of the individual seismicre£ection unit (Fig. 7). The distal limits of the units areinferred, as the thickness of the units thins below seismicresolution and prohibits reliable identi¢cation of unitterminations. Seismic re£ection units1^11display progra-dation to the east.The extent of units 1^5 is irregular andhighly sinuous, with an NE^SW orientation. Units 6^11have a less irregular extent with a smoother, curvi-lineargeometry, with unit11displaying localised areas of disrup-tion (Fig. 7). To the north, the o¥ap break continues tobe orientated NNE^SSW, whereas in the south, theo¥ap break gradually rotated anticlockwise, becomingorientated N^S. Seismic re£ection units 12 and 13 recordretrogradation to the west (Fig. 7). Unit 12 has a similar

extent and o¥ap break orientation to unit 11, with onlyminor westerly movement. Unit 13 is located signi¢cantlyfurther west towards the Faroe Islands, with a irregular,sinuous extent and NE^SW orientated o¥ap break asdisplayed by seismic re£ection units 1^5.

Seismic facies analysis

Detailed analysis of the seismic re£ection con¢gurationshas identi¢ed ¢ve seismic facies, using key observationalcriteria such as amplitude and continuity (Table 1), witheach facies named according to their distinctive re£ectioncharacteristics, as suggested byWest etal. (2002).The iden-ti¢ed facies have distinct distributions and spatial rela-tionships, often with indistinct facies boundaries (Fig. 6).The ¢rst and uppermost facies identi¢ed is composed ofhigh amplitude, continuous re£ections (HAC facies) thatextend from theFaroes shelf into the basin.The second fa-cies is composed of moderate amplitude, continuous re-£ections (MAC facies) that are located basinward of theo¥ap break.The re£ections are inclined and prograde ina south easterly direction.The third seismic facies is com-posed of low amplitude, semi-continuous re£ections(LASC facies) that are located further basinward of theMAC facies. The fourth facies is composed of moderateamplitude, semi-continuous re£ections (MASC facies)that extend from the east and terminated half way beneaththe body of the MAC facies. The ¢nal and deepest facies

Fig. 6. (a) Uninterpreted seismic sections through the Faroe-Shetland Escarpment. (b) Interpreted seismic section through the Faroe-ShetlandEscarpment, displaying bounding re£ections of the seismic re£ection units anddistribution of seismic facies (seeFig. 2 for location).


K. A. Wright et al.

identi¢ed is composed of high amplitude, semi-continu-ous re£ections (HASC facies) that are located beneath allof the previously described facies, extending across the ba-sin and towards the Faroe Islands.

INTERPRETATIONS

Seismic reflection units

The idea that the Faroe-Shetland Escarpment was formedthrough the deposition of subaerial lava £ows into marinehyaloclastic breccias is well established (e.g. Smythe et al.,1983; Ritchie et al., 1999; Spitzer et al., 2008).We interpretthat the seismic re£ection units identi¢ed in this study re-cord continuous volcanic deposition during discrete peri-ods of active volcanism. The seismic re£ection unitsappear to have been deposited sequentially, with the grossstacking pattern revealing variations in the available accom-modation space, relative sea level rise and the supply ofvol-canic material. As in conventional delta systems, the heightof the delta-front clinoforms (MAC and LASC facies) maybe a proxy for water depth at the time of delta deposition(Schmincke et al., 1997; Ki�rboe, 1999; Spitzer et al., 2008).The initial stacking pattern is progradational, with seismicre£ection units 1^11 extending progressively further intothe basin (Fig. 8). Deposition was likely controlled by thevolumes of erupted lava entering the basin and in¢lling theavailable accommodation space. Units 1^5 also display aminor aggradational component, which is interpreted to bethe product of a gradual rise in the position of relative sealevel.The degree of aggradation increases with the deposi-tion of units 6^11, as the delta front intersects with, andclimbs over theMASC facies (Fig. 8).

Seismic re£ection unit11extends furthest into the basinand de¢nes the extent of progradation of the lava-feddelta.The unit displays a variation of inclinations, includ-ing shallowly dipping re£ections (LASC facies) which havea limited lateral distribution (Figs 7 and 8).This delta frontmorphology is interpreted to be controlled by collapsescarps, which developed along the delta front. The col-lapse of unit 11 resulted from a prolonged hiatus ordecrease in the supply of new material, which left thedelta front prone to erosion and reworking by tides, wavesand storms (cf. Skilling, 2002; Sansone & Smith, 2006).Large scale section collapse scarps (up to10km across) mayresult from the subsidence of the delta, as seen in modernHawaiian lava-fed deltas identi¢ed by Kauahikaua et al.(2003) and Mattox & Mangan (1997). During active deltadeposition, the unconsolidated delta front subsides, caus-ing fractures to propagate up through the delta front.Thearea located basinward of these fractures is known as a ‘lavabench’and can be inherently unstable due to the unconsoli-dated material. During full or partial bench collapses, ex-plosive interactions between lava and ocean water canoccur, resulting in the catastrophic collapse of the deltafront (Mattox &Mangan,1997; Heliker &Mattox, 2003).

The stacking pattern changes to one of retrogradationduring the deposition of seismic re£ection units 12 and13, with each unit located progressively towards the FaroeIslands.The units are located directly above, and downlaponto seismic re£ection units1^11.We interpret that duringthe hiatus between units 11 and 12, there was a continuedsyn-volcanic rise in relative sea level, creating a newvolume of accommodation space above the previouslydeposited units. Recommencement of lava supply in¢lledthe newly created accommodation space above seismic re-£ection units 1^11. Lava supply is inferred to be limitedand short lived with deposition of unit 12 rarely reaching

Fig.7. Map of the extent of seismicre£ection units, with the position of theo¥ap break for the most easterly lyingclinoformwithin each unit identi¢ed.Distal limits of individual units areinferredwith a dotted line, as thethickness of the units thins below seismicresolution and prohibits reliableidenti¢cation of unit terminations.



Tab

le1.

Description

ofseismicfacies,including

observationalcriteria,externalgeom

etry

andtypicalre£ection

con¢

guration

s

Seismicfacies

Seismicre£ections

Geometry

Topre£ections

Internalre£ections

Basere£ections

Low

Amplitud

eSe

mi-

Con

tinu

ous(LASC

)Wedge

Mod

erateam

plitu

deinclined

andsubp

arallelre£ection

sLow

amplitud

e,inclined,

subp

arallel,varies

between

semi-continuo

usand

disrup

tedre£ections

Low

amplitud

e,subh

orizon

-tal,subp

arallelre£ection

s

HighAmplitud

eCon

tinu

ous(HAC)

Sheet-like

Higham

plitud

e,ho

rizontalto

subh

orizon

tal,parallellargely

continuo

uswithairregu

laror

undu

lating

re£ections

Highto

mod

erateam

plitu

de,

horizontal

tosubh

orizon

tal,

paralleltosubp

arallel,

continuo

usre£ections

Mod

erateam

plitud

e,subh

ori-

zontal,parallel,

varies

from

continuo

usto

disrup

ted

re-

£ections

Mod

erateAmplitud

eCon

tinu

ous(MAC)

Wedge

Mod

erateam

plitud

e,subh

orizon

tal,parallelto

subp

arallelre£ection

s

Mod

erateto

low

amplitu

de,

inclined,p

arallelto

subp

aral-

lel,continuo

usto

semi-

continuo

usre£ections

Low

amplitud

e,parallel

tosubp

arallel,

semi-continuo

usre£ections

Mod

erateAmplitud

eSe

mi-Con

tinu

ous

(MASC

)

Sheet-like

towedge

Higham

plitud

e,ho

rizontalto

subh

orizon

tal,parallelto

subp

arallel,occasion

ally

rugose

re£ections

Highto

mod

erateam

plitu

de,

horizontal

tosubh

orizon

tal,

paralleltosubp

arallel,

semi-continuo

us,hu

mmocky

re£ections

Mod

erateto

low

amplitud

e,subh

orizon

tal,parallel,varies

from

semi-continuo

usto

dis-

rupted

re£ections

HighAmplitud

eSe

mi-Con

tinu

ous

(HASC

)

Sheet-like

Higham

plitud

e,ho

rizontal

tosubh

orizon

tal,largely

continuo

usre£ections

Highto

mod

erateam

plitu

de,

paralleltosubp

arallel,varying

continuitywithstrong

layered

re£ections

Mod

erateto

low

amplitud

e,parallelto

subp

arallel,semi-

continuo

usre£ections,di⁄-

cultto

identify


K. A. Wright et al.

the extent of unit 11, therefore recording the retrograda-tion of the delta front (Fig. 8).The syn-volcanic rise in re-lative sea level continued after the deposition of unit 12,and once again created a new volume of accommodationspace above the previously deposited units. When lavasupply resumed, deposition was greatly limited, with unit13 never reaching the extent of unit 12, recording furtherretrogradation of the delta front (Fig. 8).

The delta front is identi¢ed by the position of the o¥apbreak,where re£ectiongeometries change from subhorizon-tal to inclined.The o¥ap break is also interpreted to identifythe location of the palaeo-shoreline and the position of rela-tive sea level during delta deposition (Ki�rboe,1999;Spitzeret al., 2008; Ellefsen etal., 2010).Mapping of the o¥ap breakis widely used in siliciclastic seismic stratigraphy to de¢neshoreline trajectory and identify changes in the position ofthe palaeo-shoreline (e.g. Helland-Hansen & Martinsen,1996; Helland-Hansen &Hampson, 2009).

The lava-fed delta front prograded SE, with a gradualanticlockwise rotation from NE^SW to N^S during de-position of seismic re£ection units 1^11 (Fig. 7).The rota-tion of the delta front was caused by variations in thevolume of material ¢lling the available accommodationspace.The height of the individual seismic re£ection unitsdisplays little variation across the delta front, while thewidth of the units varies from 1^2 km in the north to 3^5 km in the south.This increase in unit width caused thedelta front to migrate further in the south than the north,and rotate anticlockwise. Such variation in the ¢lledaccommodation space may indicate the location anddistribution ofvolcanic sources,with more ¢ssures locatedto the south of the Faroe Islands and the Faroese shelf.Following deposition of unit11, the delta underwent retro-gradation during deposition of unit12, migrating between1and 6 km to theNE andwith a similar distribution as unit11. The greatest retrogradation occurred during the ¢nalstage of delta construction. During deposition of seismicre£ection unit 13 the delta front migrated �31km in thenorth and �75 km in the south, causing a sharp clockwiserotation of the delta front from N-S to NE^SW (Fig. 7).Deposition of unit 12 and 13 suggests volcanism was wan-ing and becoming more sporadic.

Seismic facies

Interpretation of the facies identi¢ed within this study isbased on the re£ection characteristics and distinct spatialdistributions, with comparison to lithologies known to existwithin lava-fed delta systems.The uppermost facies withinthe lava-fed delta system is theHAC facies, which is locatedat the top of each of the seismic re£ection units.The contin-uous nature and lateral extent suggest that the facies is com-posed of tabular lava £ows that fed the delta (Fig.8) (Planke etal., 1999; Spitzer et al., 2008; Jerram et al., 2009).The MACfacies is located belowandbasinward of theHACfacies,withthe re£ections displaying progradation into the basin andthe transition from the HAC to the MAC facies identi¢edby the o¥ap break.The facies is interpreted to be composedof hyaloclastic breccias, which record the £ow of lava directlyinto the o¡shore basin (Fig. 8) (Spitzer et al., 2008; Jerram etal., 2009).TheLASC facies has a limited lateral distributionalong the delta front in unit 11 and is interpreted to be theproduct of remobilisation of theMAC facies (Fig.8).The re-£ections of the LASC facies display a semi-continuous nat-ure that indicates that the delta front may have been semi-consolidatedduring the period of collapse,which is re£ectedin the limited distance that the remobilised material tra-velled downslope and the shallower dip of the delta front(Porebski &Gradzinski, 1990; Planke et al., 2000).

The lava-fed delta system is underlain by two di¡erentfacies.The ¢rst is theMASC facies which has been identi-¢ed beneath seismic re£ection units 6^11, appearing towedge out beneath unit 5. The boundary between theMAC and MASC facies varies from distinct to more am-biguous downlap of the MAC on to the MASC facies. Attimes the MAC appears to interdigitise with the MASCfacies, with previous studies suggesting the re£ections oftheMASC are extensive toesets (e.g.Ki�rboe,1999; Plankeetal., 1999, 2000). However this interpretation does not ac-count for why theMASC facies is found beneath the laterseismic re£ection units, rather that all of the units, aswould be expected for toesets.The MASC facies extendseast out into the basin where it is intersected by well 214/4-1, which identi¢ed hyaloclastic breccias capped by tabu-lar lava £ows (see Fig. 3).

Fig. 8. Schematic cross- section through the lava-fed delta based on Fig. 6, including seismic re£ection units and distribution ofseismic facies (not to scale).



We have interpreted that the MASC facies was depos-ited from an easterly volcanic sourcewithin the basin, suchas the Erlend Complex and Brendans Dome (Fig. 2) that£owed west towards the Faroe Islands. This was followedby the subsequent deposition and progradation of thelava-fed delta system east into the basin.Where the deltasystem became spatial coincident with the MASC facies,the seismic re£ection units traversed over anddownlappedon to theMASC facies (Fig. 8).We believe that the cause ofthe ambiguous downlap of the MAC facies on to theMASC facies is due to the both facies containing hyalo-clastic lithologies. The second facies that underlies thelava-fed delta system is the HASC facies which has beenidenti¢ed to extend beneath the entire delta and theMASC facies, and east into the Faroe-Shetland Basin.This facies is interpreted to be part of the basin ¢ll beforethe onset of lava-fed delta deposition and thereforemay contain subaerially eroded volcanic material or minorvolcanic intrusions.

Correlation to onshore stratigraphy

The eruption of the Faroe Island Basalt Group was brokeninto distinct episodes by small pauses or migration of thevolcanic centres with the identi¢cation of seven distinctformations (Passey&Bell, 2007; Jerram etal., 2009).Correla-tion of the lava-fed delta system to known onshore volcanicsuccessions is based on the nature of the formations, theirkey volcanic facies and their stratigraphic position (e.g.Jerram et al., 2009; Nelson et al., 2009). Interpretation fromseismic facies analysis and onshore stratigraphy suggeststhat the lava £ows that fed the delta system were tabular innature and are likely to be the o¡shore equivalent of theBeinisv�rj Formation. The Beinisv�rj Formation is com-posed of tabular lava £ows thatwere emplaced though in£a-tion and lobe coalescing, during ¢ssure eruptions withrelatively continuous supply of lava during each eruption.The structure of these lava £ows may account for the dis-tance that the lava would have had to travel before reachingthe palaeo-shoreline and forming hyaloclastic breccias (Selfet al., 1997; Jerram&Widdowson, 2005; Passey &Bell, 2007).

Lava-fed delta duration

Flood basalt volcanism is characterised by repetitive, long-lived eruptions (weeks to10s years) that are capable of produ-cing large volumes (41km3) of lava,with the overall durationof volcanism lasting over a few (1^5)million years (e.g. Co⁄n& Eldholm, 1994; Eldholm & Grue, 1994; Bryan et al., 2010).The onset of £ood basalt volcanism is characterised by rela-

tively lowvolume eruptions, controlled by pre-existing topo-graphy or stress regime. The main phase of £ood basaltactivity is typi¢edbyan increase in eruptionvolumewithhighintensity volcanic £ux (e.g.1011kg s�1) eruptions.The end of£oodbasaltvolcanism is signi¢edbya rapiddecrease in erup-tion volume and the development ofwidely distributed loca-lised volcanic centres (Jerram &Widdowson, 2005; Bryan etal., 2010). Relative and absolute dating between eruptionscanbe di⁄cult and relies on the preservation of erosional sur-faces, deposition of non volcanic units, palynology and geo-chemical ¢ngerprinting of di¡erent eruptive units. Ino¡shore settings, it can be extremely di⁄cult to obtain thisinformation, especially if the volcanic succession is undrilled.

Hon et al. (1994) calculated the length of time a lava £owhas taken to in£ate and cool based on the thickness of the£ow crust, by the empirical equation:

t ¼ 164:8C2

where t is time in hours,164.8 is an empirically determinedconstant andC is the thickness of the £ow crust in metres.This information has been constrained by observing thedevelopment of p�ahoehoe sheets through time (Hon et al.,1994). Passey & Bell (2007) used this equation to estimatethe duration of individual £ow lobes on the Faroe Islands,with results varying from10.3 h for small, isolated lobes to22.2 days for the better developed lobes that display in£a-tion structures such as de¢ned vesicle zones. Onshore ex-posures of the Beinisv�rj Formation suggest that theaverage £ow lobe thickness is 25m (Ellis etal., 2002; Passey& Bell, 2007; Passey & Jolley, 2009) and will be composedof 40% crust (Nelson et al., 2009). Use of the empiricalequation (Hon et al., 1994) estimates it took 1.88 years for aindividual lava £ow lobe to in£ate to 25m.

Further to this, we have estimated the average total £owthickness for the seismic re£ection units 1^13 (Table 2)using two-way travel time from the seismic data and velo-cities of 5.5 km s�1 for tabular lava £ows gathered fromboreholes on the Faroe Islands (Boldreel, 2006; Nelson etal., 2009). The total thickness of lava £ows for each unitwill be composed of a number of individual £ows, mostlikely with a similar thickness as the onshore exposures ofthe Beinisv�rj Formation but are below seismic resolu-tion. In order to calculate the thickness of crustC, we haveused core to crust ratios from Nelson et al. (2009), whoplotted the core proportions of onshore Faroes lava £owsidenti¢ed within the Vestmanna-1, Glyvursnes-1 andLopra-1/1A boreholes. By using data based on lava £owsfrom equivalent onshore stratigraphy, we have an accurate

Table 2. Average thickness for lava £ows feeding the seismic re£ections units and the calculated time taken to in£ate to the total £owthickness (values to two decimal places).

Seismicre£ection unit

Average total £owthickness (m) C (m) t (h) t/245 days t/24/3655 years

1^11 275 110 1994 080 83 086.67 227.6312^13 137.5 55 498520 20 771.67 56.91


K. A. Wright et al.

assessment of lava thicknesseswhere the core to crust ratiohas beenwell constrained statistically (Nelson etal., 2009).

Seismic re£ection units 1^11 have an average totalthickness of 275m, with 40% crust equating to 110m(Table 2). The average duration (t) for each unit is 227.63years, culminating in the active progradation of units 1^11occurring over 2503.93 years. In contrast, seismic re£ectionunits 12 and 13 have a much smaller average thickness of137.5m, with 40% curst equating to 55m (Table 2). Theaverage duration (t) for each unit is 56.91years, culminatingin the active retrogradation of units 12 and 13 over 113.82years. The sum of the duration (t) for all the units (1^13)gives a value of 2617.75 years of active delta deposition andlava £ow emplacement.

Geochemical and isotopic dating of the Beinisv�rjFormation suggests that volcanism occurred between60.1 � 0.6 and 56.8 � 0.6Ma (Waagstein etal., 2002; Storeyet al., 2007), while palynological and sequence strati-graphic analysis suggest volcanism occurred between 56.8and 54.9Ma (Ellis et al., 2002; Jolley & Bell, 2002; Jolley,2009). The calculated total duration of active depositiondoes not include any periods of volcanic quiescence whichcould have varied from 10 to 104 years (Co⁄n & Eldholm,1994; Jerram &Widdowson, 2005). By including extendedpauses between volcanic pulses, the duration of deltaconstruction would be in keeping with the timing of theeruption of the Beinisv�rj Formation. However, it is di⁄ -cult to constrain the e¡ects of erosion, which would havereduced the total thickness of the lava £ows and thereforegive an underestimate of the time taken to in£ate and cool(Eldholm&Grue, 1994).

DISCUSSION

Seismic reflection units

Interpretation of the seismic re£ection units has beenbased on the seismic facies associations, stratigraphicposition and the juxtaposition of one unit against another.We suggest that each unit represents an individualvolcanicsuccession created by a discrete period of active volcanism,with the internal re£ections recording the continuousdeposition of hyaloclastic breccias (Schmincke et al., 1997;Ki�rboe, 1999).The inference that each seismic re£ectionunit represents a period of active volcanism also suggeststhat each period of activitywas followed by a period of littleor no volcanic activity. During these hiatal periods no newlava £ows or hyaloclastic breccias were deposited over theprevious unit, leaving them prone to erosion, remobilisa-tion and resedimentation. We propose the boundingre£ections are surfaces produced during such hiatuses.

In a subaerial environment, weathering and erosion ofsubaerial lava £ows forms volcanogenic soils. Genesis of asoil from basaltic lava parent material is slower than thatfor scoria or ash of the same composition, and is muchslower than for unconsolidated sedimentary deposits suchas sand or glacial deposits (Dan & Singer, 1973; Pillans,1997). Rates of soil genesis are di⁄cult to estimate due to

a wide range of factors that in£uence soil formation, suchas climate, temperature and mechanisms of erosionincluding weathering and leaching. However, it has beenestimated that genesis of avolcanogenic soil can take as lit-tle as 45^70 years in a tropical climate and up to 500 yearsin a cool climate (Corbett, 1968; Buol et al., 1989). In anumber of onshore outcrops, siliciclastic deposits, oftenwith associated plant material were deposited after theprevious phase of lava-fed delta deposition and indicatethe re-emergence of a pre-existing sedimentary regimeduring periods of volcanic inactivity (Porebski & Grad-zinski, 1990; Yamagishi, 1991; Trodeson & Smellie, 2002;Jolley et al., 2009).

In a submarine environment, erosion can result fromreworking by tides, waves and/or storms and are theequivalent of the subaerial palaeosols and erosional sur-faces, with coastal sandstones and deeper marine mud-stones accumulating during periods of volcanic inactivity(Furnes & Fridleifsson, 1974; Bergh & Sigvaldason, 1991).Submarine erosional surfaces may also occur due to theavulsion of the actively depositing lava lobe during periodsof volcanism. Avulsion occurs when the feeder systemsshifts location, causing construction of the active deltalobe to cease and the build-out of a new lobe to occur atanother location which is usually in close proximity alongshore (Coleman, 1988; Correggiari et al., 2005). In theFaroe-ShetlandEscarpment area, we interpret the bound-ing re£ectors of the seismic stratigraphic units to repre-sent submarine erosional surfaces at the top of eachvolcanic succession.

Lava-fed delta development

The stacking pattern of the seismic re£ection units is afunction of the interaction between lava supply, the posi-tion of relative sea level and available accommodationspace, and it records how these parameters a¡ected thelava-fed delta system. It is clear that lava supply varied,with deposition occurring during periods of active volcan-ism and no deposition during volcanic hiatuses.Volcanicsystems are known to display a pulsed or cyclic nature,withvariations in distribution, volume and geochemistry oferupted products (Paterne &Guichard, 1993; Knight et al.,2004; Jerram & Widdowson, 2005). Variations in deposi-tional extent can also occur during a waning of the erup-tion rate, migration of the vent or location switching ofthe depositing lava tube or in£ation lobe (Self et al., 1997;Heliker et al., 1998; Passey & Bell, 2007).

The subaerially erupted lava £ows of the delta systemare suggested to be extensive p�ahoehoe £ows that coa-lesced and formed on large in£ating sheet £ows (e.g. Selfet al., 1997). Evidence from lavas in onshore exposures inthe Faroes and in the British Palaeogene indeed point tothe p�ahoehoe nature of the subaerial £ows (Single &Jerram, 2004; Passey & Bell, 2007), while ‘a’�a lava £ows,comprised largely of autoclastic breccias are rare in most£ood basalt provinces (Brown et al., in press). It is unlikelythat the lava £ows of the delta entered the basin simulta-



neously and fed the entire delta front. The delta systemwas more likely fed from point sources along the palaeo-shoreline, with each location building a delta thateventually merged into one continuous delta body, as seenwhere modern lava £ows enter the ocean (Moore et al.,1973; Mattox et al., 1993; Kauahikaua et al., 2003). Hiatusesoccurring in this system would only record local varia-tions, representing a waning of volcanism closer to thesource or sites of lobe switching, with deposition removedto another location. Importantly, any signi¢cant hiatusesin volcanism are likely to be recorded by degradation andcollapse of parts of the delta front as the sea starts to erodethe shoreline. Therefore, periods of signi¢cant lava £uxwould be seen as a prolonged, probably pulsed, period ofdelta progradation, as seenwith the early to middle phasesof delta development in this study.

Variations in both the position of relative sea level andthe volume of accommodation space are also evident. Ag-gradation of the seismic re£ection units is seen to increasethrough the stratigraphic succession and is de¢ned by themigration of the o¥ap break in units 1^11 (Fig. 8). Thisapparent rise in relative sea level is interpreted to be aproduct of the loading and subsidence of the growing deltasystem. Studies of modern lava-fed deltas on Hawaii

have identi¢ed that deltas subside as they form, with thegreatest subsidence during active deposition. Geodeticmonitoring of active lava-fed deltas on Hawaii hasrecorded subsidence of up to 7 cm a month (Mattox &Mangan, 1997; Kauahikaua et al., 2003). Such syn-volcanicsubsidence would have been localised, with the greatestsubsidence occurring in areas of active deposition. Themore regional subsidence seen within the basin (e.g. Deanet al., 1999; Lamers & Carmichael, 1999) is a product of theunderlying rift architecture at the time of extension (e.g.Davies et al., 2004). Deposition of the retrogradationalseismic re£ection units (Fig. 8) occurred after periods ofvolcanic inactivity when the delta system was no longeractively depositing and subsiding. This retrogradation ofthe delta front towards the Faroe Islands in the latterstages of delta development records a far more signi¢cantrise in relative sea level and the creation of new accom-modation space.

Comparisonwithmodern lava-fed deltas

Volcaniclastic units in £ood basalt and volcanic marginsettings are not as well studied as the more distinct lava£ow units, but recent work has shown that they can occur

Fig.9. Distribution and ages of lava £ows originating from the Pu’uO’o volcano on the southeast side ofHawaii.Modi¢ed fromMattox&Mangan (1997), Heliker et al. (1998), Smith et al. (1999), Kauahikaua et al. (2003), Heliker &Mattox (2003), Sansone & Smith (2006).


K. A. Wright et al.

in a number of settings and are particularly importantat and near the onset of £ood volcanism (e.g. Jerram &Stollhofen, 2002; Ukstins Peate et al., 2003; Ross et al.,2005). Indeed, Iceland, Greenland and Antarctica containdocumented outcrops of lava fed delta systems similar inthickness and geometry to the one presented in this study(e.g. Furnes & Fridleifsson, 1974; Porebski & Gradzinski,1990; Pedersen et al., 1997; Smellie et al., 2008). The mostwell known and studied example of modern lava-fed deltasystems is that of the eruptions on the Island of Hawaii.Multiple lava £ows enter the sea from a number of discretevents and ¢ssures, forming extensive hyaloclastite depos-its in an o¡shore apron along the eastern coastline of theMain Island of Hawaii (Fig. 9) (Moore et al., 1973; Mattoxet al., 1993; Heliker et al., 1998). Hawaiian lava £ows arepredominantly emplaced as p�ahoehoe £ows, which cantravel signi¢cant distances from source to emplacements(Heliker et al., 1998; Heliker & Mattox, 2003), and areconsidered as analogues to the way in which continental£ood basalts are emplaced with similar mechanisms ofemplacement and character of eruptions (Self et al., 1997;Kauahikaua et al., 1998).

The Pu’u ‘O’o vent on Hawaii has been erupting almostcontinuously since1983 in a series of distinct, eruptive epi-sodes that on average continue for 3^4 years (Heliker &Mattox, 2003; Kauahikaua et al., 2003). Lava-fed deltashave been identi¢ed at Kalapana and Kamonamoabays (Fig. 9), with rates of build out of �38500 and�18500m2/day. Although delta construction was over2 years, active deposition lasted only 11 months in total(Mattox et al., 1993; Mattox & Mangan, 1997). Intermit-tently shifting lava streams have also been identi¢ed alongthe delta front,where the lava tubes feeding the £owofma-terial become blocked and the £ow only resumes when anew tube has formed. These shifting £ows behave in asimilar manner to distributaries as seen in river deltas,where the delta builds out as a lobe that is sourced fromthe delta mouth, and then shifts lateral position (Mooreet al., 1973;Mattox et al., 1993).

In between, and occasionally during the eruptive epi-sodes, periods of little or no volcanic activity occur with alack of any new eruptive products (Mattox etal., 1993;Heli-ker et al., 1998). These periods are likely a function of theplugging of the eruptive vent and/or the injection of newmaterial into the magma chamber, causing a new vent or¢ssure to open up which is often in close proximity to theprevious one.With no new lava £ows, erosion of the pre-viously deposited £ows commences through both chemi-cal and mechanical mechanisms with a current rate of11.9 t km� 2 yr�1 (Dessert et al., 2003; Navarre-Sitchler &Brantley, 2007). Onshore, the product of weathering anderosion is often volcanogenic soils or boles that form onthe top surface of the lava £ow. O¡shore, mass wasting ofthe hyaloclastic delta front can occur, forming a debris¢eld consisting of ¢ne sand to large boulder fragments(Sansone & Smith, 2006). The rapid re-establishment ofcoral communities that have been submerged by lava £owshas also beenwidely documented (Grigg&Maragos,1974).

CONCLUSIONS

This study demonstrates the utility in using seismic andsequence stratigraphic concepts to reconstruct the volca-nic sediment basin- ¢ll history of rifted margins. Detailedanalysis of re£ection geometries has identi¢ed a series ofseismic re£ection units that record the evolution of theFaroe-Shetland Escarpment during discrete periods ofvolcanism. Overall, the resulting lava-fed delta systemshows a major period of progradation due to high volcaniclava £uxes during which the shoreline migrated a maxi-mum distance of �44 km in an ESE direction (away fromtheFaroes).The later stages of delta depositionwere domi-nated by reduced volcanic input coupled with basinwiderelative sea level rise, which caused retrogradation of thedelta during which the shoreline migrated a maximumdistance of �75 km in a NNW direction (towards the Far-oes).We conclude that the encroachment of £ood basaltsinto the basin and the resulting palaeo-shoreline in thecentral Faroe-Shetland Basin has recorded the depositionof a lava-fed delta system over several thousand years. Im-portantly, this studyhighlights how the preservation of an-cient volcanic systems in o¡shore settings has thepotential to record key aspects of basin development, in-cluding the histories of relative sea level, volcanic sedi-ment supply and available accommodation space, whenmore conventional depositional systems were absent.

ACKNOWLEDGEMENTS

Suggestions by BobWhite, Gary Hampson, Peter van derBeek and an anonymous reviewer resulted in a substantialimprovement to this manuscript. Seismic data is shownwith the permission of Fugro Multi Client Services andCGGVeritas.We gratefully acknowledge use of the Land-mark Universities software grant program.We thank DaveStevenson and Gary Wilkinson for data loading, softwareand hardware support. K. A.Wright is grateful to Statoilfor funding this research as part of a PhD project withinthe Volcanic Margin Research Consortia, and to RichardBrown, Dorthe M�ller Hansen, David Ellis and AdamPugh for useful discussions.

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Manuscript received 16 September 2010; Manuscript accepted 6May 2011.


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