Quaternary faulting in the offshore Flaxbourne and Wairarapa Basins, southern Cook Strait, New Zealand

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This article was downloaded by: [UOV University of Oviedo]On: 10 November 2014, At: 00:26Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UKNew Zealand Journal of Geology and GeophysicsPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tnzg20Quaternary faulting in the offshore Flaxbourne andWairarapa Basins, southern Cook Strait, New ZealandPhilip M. Barnes a & JeanChristophe Audru b ca National Institute of Water and Atmospheric Research , P.O. Box 14901, Kilbirnie,Wellington, New Zealand E-mail:b UMR Gosciences Azur , Universit de Nice Sophia Antipolis , Valbonne, 06560, Francec Bureau de Recherches Geologiques et Minieres , BP6009, Orleans, 45060, FrancePublished online: 23 Mar 2010.To cite this article: Philip M. Barnes & JeanChristophe Audru (1999) Quaternary faulting in the offshore Flaxbourne andWairarapa Basins, southern Cook Strait, New Zealand, New Zealand Journal of Geology and Geophysics, 42:3, 349-367, DOI:10.1080/00288306.1999.9514851To link to this article: http://dx.doi.org/10.1080/00288306.1999.9514851PLEASE SCROLL DOWN FOR ARTICLETaylor & Francis makes every effort to ensure the accuracy of all the information (the Content) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources of information. 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Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditionshttp://www.tandfonline.com/loi/tnzg20http://www.tandfonline.com/action/showCitFormats?doi=10.1080/00288306.1999.9514851http://dx.doi.org/10.1080/00288306.1999.9514851http://www.tandfonline.com/page/terms-and-conditionshttp://www.tandfonline.com/page/terms-and-conditionsNew Zealand Journal of Geology & Geophysics, 1999, Vol. 42: 349-3670028-8306/99/4203-0349 $7.00/0 The Royal Society of New Zealand 1999349Quaternary faulting in the offshore Flaxbourne and Wairarapa Basins, southernCook Strait, New ZealandPHILIP M. BARNESNational Institute of Water and Atmospheric ResearchP.O.Box 14901, KilbirnieWellington, New Zealandemail: p.barnes@niwa.cri.nzJEAN-CHRISTOPHE AUDRUUMR Gosciences AzurUniversit de Nice - Sophia Antipolis06560 Valbonne, FrancePresent address: Bureau de Recherches Geologiques etMinieres, BP6009, 45060 Orleans, France.Abstract Marine seismic reflection profiles, bathymetricdata, and seabed samples reveal the stratigraphy andQuaternary structure of the southern Wairarapa andFlaxbourne Basins in southeastern Cook Strait and easternMarlborough. These SW-NE-trending basins began formingbefore the late Miocene (>10 Ma), but their developmenthas been mainly during and since that time and continuestoday within the Pacific-Australia plate boundary zone.Recently active structures deforming and bounding thebasins are recognised by growth strata and deformation ofQuaternary sediments. Observed structural geometriesreflect Pliocene-Recent changes in the kinematics of faultingin central New Zealand. The 12-22 km wide southernWairarapa Basin contains up to c. 2.9 km of strata and isdeforming between offshore segments of the dextral strike-slip Wairarapa Fault and associated Wharekauhau Thrust onthe western margin, and offshore extensions of the AorangiMountains range-front reverse faults on the eastern margin.To the southwest, in the eastern Marlborough Fault System,the 15-20 km wide, 80 km long Flaxbourne Basin contains>4.5 km of strata and is deforming by strike-slip and oblique-slip faults including offshore sections of the Hope andKekerengu Faults. A new set of strike-slip faults, probablyyounger than 1 Ma, strike parallel (c. 080 10) to thecurrent Pacific-Australian plate motion vector and obliquelyto inherited structural trends. Three of these faults arepossibly separating the Flaxbourne and Wairarapa Basinsin central, southern Cook Strait. Curved traces of the Needlesand Wairarapa Faults on the western margins of the basinsare aligned, and may cut across disrupted Miocene structuresto link part of the eastern Marlborough Fault System withthe North Island Dextral Fault Belt.G98033Received 9 September 1998; accepted 3 May 1999Keywords Cook Strait; Marlborough; Wairarapa; plateboundary; tectonic; faulting; structure; Quaternary; seismicreflection; bathymetry; stratigraphy; sedimentary basinsINTRODUCTIONSouthern Cook Strait forms the transition between NorthIsland's Hikurangi subduction margin and South Island'scontinental transpression zone, and is an actively deformingregion between the mountains of both islands (Fig. 1) (Lewiset al. 1994). Reconstructions of the New Zealand plateboundary zone show that the strait has been situatedapproximately within this transition area for at least 20 m.y.(Fig. 1) (e.g., Walcott 1978, 1987; Little & Roberts 1997).Throughout most of the Neogene, the transition has beenessentially fixed to the northern end of South Island andwestern end of the Chatham Rise, despite the shape of theplate boundary, the azimuth and velocity of relative platemotion, and the paleogeography changing significantly.Three major sedimentary basins exist beneath southernCook Strait (Fig. 2B) (Carter et al. 1988; Uruski 1992; Lewiset al. 1994). The Wairau, Flaxbourne (Clarence Basin ofUruski 1992), and southern Wairarapa Basins contain stratawith seismic reflectivity of >2.5 s two-way travel (TWT)(>3 km) thick, and the first two are associated with prominentnegative gravity anomalies (Fig. 2A) (Fenaughty 1987; Rose1991). The structural evolution of the basins should reflectthe tectonic complexities associated with changes in theconfiguration of the plate boundary. The basins are beingactively faulted and folded above the subducted Pacific plate.The subducted Pacific plate lies 15-25 km beneathsouthern Cook Strait (Fig. 2B) and has been thrust beneathnorthwestern South Island to a depth of at least 200 km(Robinson 1986; Eberhart-Phillips & Reyners 1997). Thesubduction decollement reaches the seabed at the thrust-faulted deformation front southeast of the strait on the innerflank of the 2800 m deep Hikurangi Trough (Lewis &Pettinga 1993; Collot et al. 1996; Barnes et al. 1998).Onshore southwest of Cook Strait are major, strike-slip faultsof the Marlborough Fault System (MFS) trending 055-075(Fig. 2). These faults accommodate >80% of the predicted38 mm/yr Pacific-Australian plate motion in northern SouthIsland (Freund 1971;Bibby 1981;Cowan 1990; Van Dissen& Yeats 1991 ; Holt & Haines 1995) and have controlled thesedimentation of Neogene basins (Little & Roberts 1997;Audru & Delteil 1998). Onshore northeast of the strait andwest of the Wairarapa forearc basin are the strike-slip faultsof the North Island Dextral Fault Belt (NIDFB) trending035-045, which uplift the axial ranges of southern NorthIsland and accommodate c. 21 mm/yr strike-slip dis-placement (Beanland 1995; Van Dissen & Berryman 1996).Most of the major active strike-slip faults in the NIDFBand the MFS project offshore beneath Cook Strait and thenortheastern Marlborough continental shelf. The crustalDownloaded by [UOV University of Oviedo] at 00:26 10 November 2014 350 New Zealand Journal of Geology and Geophysics, 1999, Vol. 4243 mm/yrFig. 1 Present tectonic setting of the plate boundary through New Zealand and simplified reconstructions at 10 Ma and 20 Ma (afterBeanland 1995; Little & Roberts 1997). Abbreviations include: SA, Southern Alps; HM, Hikurangi margin; EHS, Esk Head Subterranc;DM0, Dun Mountain Ophiolite. Crosses are active volcanoes. Shaded area offshore is the approximate region of continental crust asdepicted by the 2000 m isobath. The area in the inset box is the approximate area in Fig. 2.Fig. 2 A, Contoured gravity anomalies in the southern North Island and Cook Strait region (Rose 1991 ). Anomalies (in uN/kg, when;10 uN/kg = 1 mgal) are Free Air measured at sea level and Bouguer on land. Abbreviations include: WB, Wairau Basin; FB, Flaxbounu-Basin; CB, Campbell Bank; RR, Rimutaka Range; AM, Aorangi Mountains; ECGH, East Coast Gravity High. B, Simplified basementgeology and late Quaternary faults in central New Zealand, and distribution of major sedimentary basins (stippled) in southern CookStrait. Abbreviations not in A include: CC, Cape Campbell; CSC, Cook Strait Canyon; CF, Carterton Fault; Jordan F., Jordan Thrust;MFS, Marlborough Fault System; MKF, Mokonui Fault; MF; Masterton Fault; NIDFB, North Island Dextral Fault Belt; NC, NicholsoiCanyon; PB, Palliser Bay. Bold contours labelled 15 km, 25 km, and 50 km are depths to the subducted Pacific plate (Ansell &Bannister 1996; Eberhart-Phillips & Reyners 1997). MesozoicTorlesse subterranes (Begg & Mazengarb 1996) include: Rakaia Subtern.n(South Island) and Wellington Belt (North Island) light shading; Esk Head Subterrane (South Island) and Rimutaka Belt (North Islanddashed pattern; Pahau Subterrane (South Island) and Wairarapa Belt (North Island), dark shading. Contours offshore are isobaths inmetres. Broken lines are the axis of Cook Strait Canyon and the Hikurangi Channel.structure and continuity of the faults beneath the strait,however, have been debated (e.g., Carter et al. 1988). Carteret al. and Lewis et al. (1994) interpreted seismic reflectiondata and regional structural trends bounding the sedimentarybasins, and concluded that none of the faults, with thepossible exception of the Wairau Fault, link directly acrossthe strait (Fig. 2B).Carter et al. (1988) considered that the offshore ends ofthe strike-slip faults have been rotated clockwise relative totheir onshore segments and terminate within the sedimentaryDownloaded by [UOV University of Oviedo] at 00:26 10 November 2014 Barnes & AudruMarine seismic study of basin faults, southern Cook Strait 351basins against a NW-SE-trending crustal-scale structurebetween the Flaxbourne and Wairarapa Basins. This structurewas thought to have developed originally as the northernsection of the ancestral Wairau Fault, which connected theAlpine Fault transform system to the Hikurangi subductionfront during the early - middle Miocene (Fig. 1, 2B) (Walcott1978). It was inferred the structure has since been rotatedclockwise and fragmented as the geometry of the plateboundary changed and as new, more favourably alignedstrike-slip faults developed southeast of the fault incontinental crust of the Pacific plate (Fig. 2) (Walcott 1978;Lamb 1988; Little & Roberts 1997). That the ancestralWairau Fault once cut through what is now Cook Strait isevinced by an apparent c. 140 km dextral offset of Mesozoicmarkers exposed in North and South Islands (Fig. 2B)(Mazengarb et al. 1993). Carter et al. (1988) considered thisstructure to be now masked by deep erosion of thesedimentary cover by three arms of the Cook Strait Canyonsystem.Late Neogene (42 S 174 41 30' 174 30'EMultichannel seismic Single channel air gun or sparker -* 3 5 kH?oOeto*CuODJOODownloaded by [UOV University of Oviedo] at 00:26 10 November 2014 Barnes & AudruMarine seismic study of basin faults, southern Cook Strait 353SOUTHERN WAIRARAPA BASINMorphology, gravity anomalies, and stratigraphyBouguer and free-air gravity anomalies across the southernWairarapa region are dominated by positive anomaliesassociated with the Aorangi Mountains (part of the EastCoast Gravity High) and the southern Rimutaka Range, anda negative anomaly associated with the offshore Cook Straitcanyons (Fig. 2A). Modelled residual gravity anomaliesderived from removal of the regional gravity field show that:( I) the 1222 km wide southern Wairarapa Basin onshore ischaracterised by a prominent negative gravity anomaly andcontains up to 3.2 km of probable Miocene-Recentsedimentary strata adjacent to the NW-dipping reverse strike-slip Wairarapa Fault (Hicks & Woodward 1978); and (2) aseries of SW-NE striking, SE-dipping reverse faultsincluding the Waihora Fault, southern Dry River Fault, andseveral unnamed subsurface structures with cumulativevertical separation of c. 1 km, occur beneath the northwesternedge of the Aorangi Mountains (Fig. 4) (Dunkin 1995).Marine seismic reflection data and a negative residualgravity anomaly show that the Wairarapa Basin continuesoffshore as a 15 km wide subsurface feature beneath PalliserBay between the southern Rimutaka Range and AorangiMountains (Fig. 2B, 4) (Carter et al. 1988; Uruski 1992).The basin, however, is not evident in the bathymetry (Fig.3A), which shows Palliser Bay to be a gently slopingcontinental shelf deeply incised by Wairarapa and Nicholsoncanyons.An apparently westward-dipping sedimentary section upto 2.5 s TWT thick (c. 2.9 km assuming an average seismic\elocity of 2.3 km/s; Rollo 1992; Dunkin 1995) occursbeneath Palliser Bay (see Fig. 6, 7) and is exposed on thewestern flank of the Aorangi Mountains, where it consistspredominantly of late Miocene-Pliocene mudstone of thePalliser and Onoke Groups (Dunkin 1995). Approximatecorrelation from onshore exposures into seismic profiles(Fig. 6, profile A) and reflection ties to the late Miocene -early Pliocene seabed core U630 (Fig. 5, see also Fig. 12,profile R) imply that much of the section beneath the bay isTongaporutuan age (c. 10-6 Ma). We infer that the packetof strong reflectors typically occurring at 1-2 s TWT beneaththe bay, including reflector eTt (Fig. 6, 7), represent earlyTongaporutuan conglomerate of the Putangirua Formation(Bates 1967). Discontinuous reflectors between eTt and theacoustic basement may be equivalent to the CretaceousWhatarangi Formation that is exposed in a faulted block onthe western flank of the Aorangi Mountains. Marine 3.5 kHzprofiles show that much of the bay is blanketed by post-lastglacial sediment up to 40 ms TWT (c. 30 m) thick..< Fig. 3 A, Contoured bathymetry (in metres) and selected majorstructures with bathymetric expression in southeastern Cook Straitand the eastern Marlborough continental margin. Data from theshelf and Cook Strait Canyon system are derived from narrow-beam echo-soundings (Mitchell 1988,1996), and are merged withSimrad EM 12 Dual multibeam data from the continental slope(Collot et al. 1996). Abbreviations not on Fig. 2 include: WC,Wairarapa Canyon. B, Bathymetric details of an apparent dextraloffset of Cook Strait Canyon (CSC) by the Needles Fault. Lightlyweighted contours are 10 m isobaths in the canyon axis. C,Distribution and type of seismic reflection profiles used in thisstudy.StructuresWairarapa Fault and Wharekauhau Thrust: basin'swestern marginThe southwestern side of the basin is bounded by the oblique-slip Wharekauhau Thrust segment of the Wairarapa Fault,which extends along the southeastern edge of the RimutakaRange and offshore beneath the western flank of WairarapaCanyon (Fig. 3, 4) (Grapes & Wellman 1993). Onshore theWairarapa Fault has a late Quaternary dextral-slip rate of c.12 4 mm/yr (Beanland 1995) and it last ruptured duringan Ms ~8 earthquake in 1855 with lateral displacement of11 + 2 m (Grapes & Wellman 1993) and maximum coseismicuplift of c. 6 m at Turakirae Head (McSaveney & Hull 1995).In seismic profiles, acoustic basement that can confidentlybe interpreted as Mesozoic Torlesse Terrane on the hangingwall is thrust over the late Miocene-Recent section (Fig. 6,profiles A and B). The vertical separation of acousticbasement across the fault is of the order of 1.7-2.0 s TWT(c. 2.0-2.3 km), which is consistent with the displacementpredicted from gravity modelling on land (Hicks &Woodward 1978). An absence of fanning stratal geometryinto the fault on profile A suggests that the verticaldisplacement postdates the deposition of the Tongaporutuansediments. Considering the late Quaternary uplift rate of c.3 mm/yr at Turakirae Head (McSaveney & Hull 1995), theobserved minimum vertical separation of basementimmediately offshore could have been achieved in 0.60.8 m.y.East of Turakirae Head the Wharekauhau Thrust appearsto diverge into two main active splays of the Wairarapa Fault(Fig. 4). Between Nicholson and Cook Strait Canyons theactive splays are evident as discontinuous SW-NE-trendingbathymetric lineaments up to 10 km in length crossing thesoutheastern end of Nicholson Bank (Fig. 3 A, 8). Seismicprofiles show that the southeastern lineament correspondswith a subsurface fault with normal-slip separation (e.g., Fig.6, profile B). Unpublished 10 m interval isobaths on the wallsand in the axis of Nicholson Canyon between the lineamentsand the Wharekauhau Thrust show constriction of the canyonbut no evidence of lateral offset despite the probability ofdextral motion on the fault splays. The absence of a clearoffset may result from the combination of Holocene activityin canyon sedimentation and erosion, a reduction in dextral-slip rate south of the Wharekauhau Thrust, and/ordistribution of oblique slip onto more than one splay.Central Palliser BayThe central part of the Wairarapa Basin beneath PalliserBay is dominated by two SW-NE-striking thrust faultsthat each displace reflector eTt with vertical separationof c. 0.2 s TWT (c. 230 m) and fold the upper part of thelate Miocene section (Fig. 6, profile A; Fig. 7, profile C).Stratal geometry across the faults indicates negligiblethrust fault and fold growth during deposition of most ofthe late Miocene strata, although extensional growthfaulting may have occurred on one of the faults prior toreflector eTt (centre of profile A). The thrusts thereforedeveloped predominantly since the late Miocene, anobservation consistent with many other contractionalstructures within the Wairarapa Basin (Cape et al. 1990;Beanland 1995; Lamarche et al. 1995; Nicol et al. 1996).The faults, however, have no bathymetric expression (Fig.3A), and in 3.5 kHz profiles there is no evidence of activeDownloaded by [UOV University of Oviedo] at 00:26 10 November 2014 42 S\42 41 30' 17430'E\ y174yQuaternary StructuresStrike-slip fault J Thrust faultNormal faultAnticlineSynclineShelf edgeSubmarine canyonInactive Structures-R^EER -J42 S\174 41 30' 174 30' EPost last-glacial+ HolocenePleistoceneI _ . PlioceneI. Miocene - e. PlioceneOffshoree-m. MioceneSubmarineCanyonShelf edgeExposed PleistoceneunconformitiesSediment core/rock dredgeOffshoreQuaternaryCretaceous & TertiaryMesozoic Torlessebasement3o'OflHa.erSSOooKX/i3Fig. 5 Sedimentary units exposed within and on the margins of the Flaxbourne and southern Wairarapa Basins, eastern Marlborough and Palliser Bay continental shelves, constrained byseabed cores and seismic stratigraphy. Offshore structure is simplified from Fig. 4. Abbreviations not in Fig. 4 include: CR, Clarence River mouth.Downloaded by [UOV University of Oviedo] at 00:26 10 November 2014 356 New Zealand Journal of Geology and Geophysics, 1999, Vol. 42 '. ' i . " : ' . . ' . i.Fig. 6 Interpreted six-folcstacked seismic profiles A and Ifrom the western and centraWairarapa Basin, Palliser BI>Abbreviations include: Bst?basement surface; and Mzprobable Mesozoic basemenrocks. Reflector eTt is of probaliearly Tongaporutuan age (c. 1 CMMa). A shallow unconformity a'0.2-0.4 s TWT depth is oiprobable Pleistocene age. Prof1.locations are shown on Fig. 4. 5folding of the post-last glacial transgressive erosionsurface (c. 18-12 ka).Waihora Fault: basin 's eastern marginThe inner eastern margin of the Wairarapa Basin in PalliserBay is bounded by a SW-NE striking, SE-dipping reversefault that we infer to be the offshore extension of the WaihoraFault (Fig. 4; Fig. 7, profile D). Vertical separation ofreflector eTt is c. 1 s TWT (c. 1.2 km), comparable to thevertical separation on reverse faults along the western edgeof the Aorangi Mountains (Dunkin 1995). A structure evidentto the east of the Waihora Fault in profile D may be theoffshore extension of the Dry River Fault (Fig. 7).FLAXBOURNE BASINMorphology and stratigraphyThe SW-NE-trending, 1520 km wide Flaxbourne Basilextends beneath the eastern Marlborough continental shellfor c. 80 km from the Clarence River mouth to Cook StraitCanyon (Fig. 2B). The central part of the basin coincide-*with an elongate negative gravity anomaly (Fig. 2A) andbathymetric depression lying between the coast and an oute:shelf structural high which includes Campbell Bank at thenortheastern margin of the basin (Fig. 3A, 4,5). On a regionalscale the basin plunges to the northeast with seismicreflectivity from basin strata imaged to at least 1.8 s TW1Downloaded by [UOV University of Oviedo] at 00:26 10 November 2014 Barnes & AudruMarine seismic study of basin faults, southern Cook StraitFig. 7 Interpreted six-foldstacked seismic profiles C and Dfrom the central and easternWairarapa Basin, Palliser Bay.Abbreviations as in Fig. 6. Dashedreflector at 0.3-0.8 s TWT is ofprobable late Miocene-earlyPliocene age. Profile locations areshown on Fig. 4, 5.o -,DP?M"D0)CO2 -NliSffli! i l l "' : ' iBBi l lmmm ISSSSTXmmMMH^Ai1A1&$!$aihora Fault11ijlmmmi*s? sammSfHEsmmC _ilm1I1Wm!mmmsm1I1m11113ry RivEr1Fa ult?liillrHai1111pr ?m|mmmmI 15wmm1i1unoilLsSFia8ismm1Vit91$mHinMA L11iii ? 3hVE =SS5S2.6 @ 2.3 km/smmmaJ(> 2 km) in the southwest deepening to at least 4 s TWT(>4.5 km based on velocity analyses) in the depocentrebetween Cape Campbell and Campbell Bank (e.g., Fig. 9,10, profiles G, H, and I) (Uraski 1992).Whereas much of the basin is covered by post-last glacialmud up to c. 45 m thick, the uplifted margins of the basinexpose strata of Miocene-Pleistocene age (Fig. 5). At leastseven deformed Pleistocene unconformities, includingreflectors B-G in profiles I-M (Fig. 10), exist within theupper 0.25 s TWT (c. 200-230 m) of sedimentary section.The exposed Miocene strata represent the Motunau andAwatere Groups outcropping nearby in coastal hills (Audru1996). With the existing seismic data reflector eTt in theWairarapa Basin cannot be tied directly into the FlaxbourneBasin. We infer the reflector to correlate approximately witha strongly reflective unconformity at 1.7-2.5 s TWT onprofiles G and H (Fig. 9). By analogy with the stratigraphyof coastal hills onshore the unexposed 2 s TWT thick (c. 3km) sedimentary section beneath this unconformity,ncluding the high amplitude reflectors between 3 and 4 sTWT in profiles G and H, probably correlates with lateCretaceous- Miocene strata of the Coverham, Muzzle, andlower Motunau Groups (Uruski 1992).StructuresThere are numerous faults that displace pre-Quaternarybasin strata but are now buried by Quaternary sediments,appear to be no longer active (e.g., Fig. 9, 10; profiles G,H, and I), and cannot be mapped in plan view with theavailable data. Numerous active faults and folds, however,that are easily mapped from their deformation of theQuaternary sediments, occur within and along the marginsof the basin, and the pattern of relative uplift is partiallyreflected in the distribution of sedimentary units exposedat the seabed (Fig. 4, 5). The Quaternary structures in thesouthwestern and central part of the basin have beendescribed in detail elsewhere (Barnes & Audru 1999) andare briefly summarised here. The southeastern marginconsists of a complex, transpressive structural high thatextends along the outer continental shelf and includes theoffshore extension of the Hope Fault and the entirelymarine Kaikoura Fault in the south (Fig. 2B), a 30 kmDownloaded by [UOV University of Oviedo] at 00:26 10 November 2014 358 New Zealand Journal of Geology and Geophysics, 1999, Vol. 42E1 0.2-Seconds(o toN?VE =N i C10@h o l s o n B a n kBathymtrielineament,3 km1.6 km/sSFig. 8 3.5 kHz profiles E and Iof bathymetric lineaments crossing the eastern end of NicholsotBank. Profile locations are shov/ion Fig. 4, 5.N i c h o l s o n B a n kNWFig. 9 Interpreted six-foldstacked seismic profiles G and Ffrom the northeastern end of theFlaxbourne Basin. Reflector a0.4-1.5 s TWT is of probable lat


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