Architecture and sequence stratigraphy of a late Neogene incised valley at the shelf margin,...

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JOURNAL OF SEDIMENTARY RESEARCH, V OL. 69, NO. 2, MARCH, 1999, P. 351–364Copyright 1999, SEPM (Society for Sedimentary Geology) 1073-130X/99/069-351/$03.00

ARCHITECTURE AND SEQUENCE STRATIGRAPHY OF A LATE NEOGENE INCISED VALLEY AT THESHELF MARGIN, SOUTHERN CELTIC SEA

JEAN-YVES REYNAUD1, BERNADETTE TESSIER1, JEAN-NOEL PROUST2, ROBERT DALRYMPLE3, JEAN-FRANCOIS BOURILLET4,MARC DE BATIST5, GILLES LERICOLAIS4, SERGE BERNE4, AND TANIA MARSSET4

1 Laboratoire de Sedimentologie et Geodynamique, Universite des Sciences et Techniques de Lille, 59655 Villeneuve d’Ascq Cedex, France2 Geosciences Rennes, UPR 4661 du CNRS, Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France

3

Department of Geological Sciences, Queen’s University, Kingston, Ontario K7L 3N6, Canada4 Institut Francais de Recherche pour l’Exploitation de la Mer (IFREMER), Departement Geosciences Marines, BP 70, 29280 Plouzane Cedex, France

5 Renard Centre of Marine Geology, Seismostratigraphy Unit, University of Gent, Krijgslaan 281 S8 B 9000 Gent, Belgium

ABSTRACT: Valleys on the outer Celtic Sea shelf were cut and filledduring the late Pliocene/early Pleistocene. The Kaiser valley is one of several valleys forming an anastomosed network. The main valley, di-rected 20 (Azimuth true N), is 50 m deep and 10 km wide. It isconnected to the parallel Dompaire and Parsons valleys by several 120–140 directed incisions of lesser width and depth. Analyzed by meanof very high-resolution seismic data, the Kaiser valley is interpreted ascontaining a compound fill consisting of eight erosionally based depo-sitional sequences. A typical sequence comprises two facies: (1) fluvialchannels at the base, which represent lowstand to early transgressive

deposits; and (2) onlapping transgressive bay-fill deposits that are lo-cally interbedded at the top with isolated small channels attributed toflood tidal deltas. The erosional bases of the fluvial facies correspondto sequence boundaries. These are interpreted to result from relativesea-level falls. Successive fluvial and bay-fill facies are separated by flaterosional surfaces of high acoustic amplitude, which extend laterallyacross the entire composite valley, locally beveling sequence boundariesand creating terraces on the valley walls. These flat facies contacts areinterpreted as bay ravinement surfaces produced by waves in an es-tuarine setting. The larger-scale stacking pattern of the depositionalsequences defines a progradational–retrogradational trend, in whichthe lowest sequence is mainly constituted by fluvial channel deposits,whereas upper sequences display mostly bay-fill facies. The sequencesare related to fifth-order glacioeustatic fluctuations, whereas their pro-gradational–retrogradational trend reflects fourth-order eustatic vari-

ations and/or rapid tectonic tilting of the area, as indicated by thepresence of two incision orientations. The preservation of the systemtook place during a third-order sea-level rise, and was favored by sub-sidence of the margin, leading to its present occurrence down to 240m below present sea level.

INTRODUCTION

Incised valleys are elongate erosional features that are several kilometerswide and several meters to tens of meters deep. In coastal settings they arecut during relative sea-level falls and filled during relative sea-level rises(Dalrymple et al. 1994). They are of great scientific interest because theyare the best sinks for lowstand and transgressive sediments on continental

shelves and they are key elements for unraveling continental-margin se-quence stratigraphy (Posamentier et al. 1988; Van Wagoner et al. 1990;Thorne 1994). The importance of estuarine deposits in incised valleys wasnoted by early workers (e.g., Wilson 1948). The link between estuaries andincised valleys gave rise recently to an incised-valley depositional model(Zaitlin et al. 1994). This model integrates results from modern estuaries(Dalrymple et al. 1990; Allen and Posamentier 1993) with information fromtheir ancient counterparts (e.g., Weimer 1984; Rahmani 1988; Nichol 1991;Pattison and Walker 1994; Martinsen 1994; Thomas and Anderson 1994;Kindinger et al. 1994). Incised valleys are filled by one (simple incisedvalley) or several (compound incised valley) transgressive–regressive se-quence(s). The sequences comprise deposits ranging from fluvial and es-

tuarine settings (e.g., Shanley and McCabe 1992; Lesueur et al. 1990) tofully marine settings (e.g., Reinson et al. 1988; Harris 1994).

The recent developments of very high-resolution seismics (Lericolais etal. 1990; Lericolais et al. 1994) give an opportunity for new insights onthe large-scale, 3D geometry of incised-valley deposits. In this paper wepresent a seismic study of a compound incised-valley fill on the deepSouthern Celtic Sea shelf (Fig. 1). Little is known about valleys of thiskind at the shelf margin, which are incised in response to relative sea-levelfalls below the shelf edge. Because of this situation near the knickpoint onthe depositional profile (i.e., at the break in slope between the flat conti-

nental shelf and the steep continental slope), such valleys are particularlysensitive to incision and aggradation under the control of high-frequencyrelative sea-level changes. Because of their location near the lowstandshoreline, they are likely to be filled during the early stages of transgres-sion(s). By contrast, the more studied valleys incised at the landward edgeof the continental shelf are mainly subjected to late transgressive to high-stand sedimentation.

SETTING

The southern Celtic Sea, located seaward from the English Channel, isthe deepest segment of the western European outer shelf, with a shelfbreakbetween 180 and 240 m (Fig. 1; Bourillet and Loubrieu 1995). TheCeltic Sea valleys are incised into the Neogene strata of the Western Chan-nel Approaches syncline, which are composed of three stacked prograding

wedges of Miocene to Lower Pliocene age (Figs. 1, 2; Andreieff et al.1972; Pinot 1974; Evans and Hughes 1984; see synthesis by Evans 1990).These wedges are crossed by the Alderney–Ouessant fault, which may havebeen active up to the time of valley filling (Evans 1990; Lericolais 1997).

The incised valleys form a network that covers an area 300 km wide,pinching out landward at a depth of 150 m (in the west) to 130 m (inthe east). At their seaward (southward) ends, the valleys terminate in thecanyon heads at the edge of the continental shelf (Kenyon et al. 1978), ata depth of about 240 m (Fig. 3). In the west, the valley-fill depositsconstitute a continuous sheet (the Little Sole Formation; Fig. 1) that com-pletely buries the underlying valley network (Pantin and Evans 1984). Thevalley paths are, however, much more distinct in the studied area in theeastern part of the Western Channel Entrances, where they were initiallymapped by Bouysse et al. (1976) on the basis of sparker reflection seismics

(Figs. 1, 3). There, individual valleys are 7–10 km wide and 50–70 m deep,and form a network of slightly sinuous anastomosed incisions flowing tothe SSW (Bouysse et al. 1976). In this paper we focus on the Kaiser valley(named for the sand bank that overlies it; Fig. 3). The valleys are com-pletely filled by sediments and are overlain by Holocene to late Pleistocenemarine deposits 0–40 m thick (Bouysse et al. 1976; Bouysse et al. 1979;Pantin and Evans 1984). Because of their depth beneath the surface, littleis known about the nature of the valley-fill deposits. Pantin and Evans(1984) interpreted the locally well stratified and channel cut-and-fill seismicpatterns of the deposits as interbedded clays and gravelly sands, but thesehave never been cored deeper than a few meters.

Bouysse et al. (1976) attributed valley incision to the lowest Quaternary



352 J.-Y. REYNAUD ET AL.

FIG. 1.—Location map of the study areashowing the geological framework of theNeogene succession of the southern Celtic Sea.Formation limits are after Evans and Hughes(1984). Incised valleys are after Bouysse et al.(1976) eastward from 730’ and after Evans andHughes (1984) to the west. Crosses indicatelocations of samples from the top of valley-filldeposits (Evans and Hughes 1984) used toconstrain their age. Bathymetry is after Evans(1990). Isobaths are drawn for every 20 m above200 m and for every 500 m below. A) Seismic

line A in Figure 2. B) Seismic line B in Figure 2.

FIG. 2.—Margin-normal cross sections of Neogene and Quaternary formations, outer CelticSea. A) In the Kaiser bank study area (afterVanhauwaert 1993). B) In the southwesternCeltic Sea (after Evans and Hughes 1984). SeeFigure 1 for locations.

glacioeustatic sea-level fall. However, the great depth of erosion (240 m)is well below the maximum sea-level fall in late Quaternary time (Shack-leton 1987), even taking into account any possible uplift associated withthe glacial forebulge (Lambeck 1995). Pantin and Evans (1984) suggestdeep-seated crustal movements as the cause of the uplift responsible forvalley incision. Planktonic foraminifera recovered from sediments at thetop of the valley fill are of latest Pliocene to earliest Pleistocene age (Evansand Hughes 1984). Therefore, the timing of the valley incision is relatedto the maximum relative sea-level fall during the Pliocene (Evans andHughes 1984; Pantin and Evans 1984). If so, it most likely occurred during

mid-Reuverian or Tiglian times (2.8–2.3 Ma), according to stratigraphicdata from Brittany (Morzadec-Kerfourn 1990) and the eustatic sea-levelcurves based on isotopic studies (Shackleton et al. 1991) or the seismicstratigraphy of passive margins around the world (Haq et al. 1988). Wesuggest that the maximum lowstand of the mid-Reuverian times may beresponsible for valley incision, because it corresponds to the deposition of the fluvial ‘‘red sands’’ in southern Brittany as far as the present-day loweroffshore area (Bourcart 1947).

METHODS AND DATA

The target area was explored first using 20 watergun profiles recoveredduring a first campaign at sea by Renard Centre of Marine Geology(RCMG) and IFREMER in 1992 in order to delineate the Kaiser and Dom-paire valley boundaries (Fig. 3; Vanhauwaert 1993). Subsequently, morethan 1500 km of very high-resolution seismics were shot on a 400 m squaregrid during three cruises conducted by IFREMER in 1992 (Sedimanche 1)and 1993 (Sedimanche 2, Fig. 4), and by RCMG in 1994 (Belgica 94, Fig.4). These surveys were originally designed to study the Celtic Sand Banks(Fig. 3), but they also provided superb images of the fill of the valleys

below, so that they were used to complete the valley mapping (Bourillet1997; Peyre 1997). The seismic device was a SIG 1580A sparker, shootingat 1 s time intervals with an energy of 700 J. The receiver was a single-channel streamer. Recorded numeric data were processed using an ELICSDELPH2 system (gain, pass-band filter, swell filter) and enhanced in somecases using the IFREMER–SITHERE software (multiple filter). The two-way travel time below the sea floor is converted into sediment thicknessassuming a homogeneous velocity of 1600 m/s. Time profiles display a



353 ARCHITECTURE OF AN INCISED VALLEY AT THE SHELF MARGIN

FIG. 3.—Incised valleys in the southern CelticSea. A) Shaded areas show valley outlines asmapped by Bouysse et al. (1976) and

Vanhauwaert (1993). Bathymetric map is afterBouysse et al. (1976). The valleys are namedafter sand banks resting above. B) Contour mapof base of Kaiser and Dompaire incised valleys(after Vanhauwaert 1993).

maximum vertical resolution about 3 ms TWT (i.e., about 0.5 m, cf. Fig.5). Geographic positions were calculated and recorded for each shot takinginto account the distance between the streamer and the DGPS ship antenna,so that the intersections between profiles are precisely located. The overallunit or valley surface dips that are reported correspond to apparent slopesrelative to a horizontal surface, as calculated from one profile parallel tothe valley axis. The slopes of internal surfaces are true, local maximumdips as calculated at the intersection of two profiles.

SHAPE OF THE KAISER VALLEY

The Kaiser valley is incised into the Miocene calcarenitic wedge of theCockburn Formation (Evans and Hughes 1984). The valley is directed 20(Azimuth True N) and is connected at its northern end to the Dompairevalley by a 120-trending incision (Fig. 3; Vanhauwaert 1993). In thesouthern part of the studied area, the Kaiser valley is divided into twosubparallel valley branches (Fig. 4). In two places the eastern branch itself splits into several smaller branches that merge seawards. The western valleyis connected to several NW–SE-trending tributaries that probably connectlandwards to the Parsons valley (Fig. 3). These tributaries have the same120 direction as the valley branch connecting the Kaiser and Dompairevalleys (Fig. 4). The width of the valleys is of the same order (5–10 km)

as the northward-trending valleys mapped by Bouysse et al. (1976), exceptat local constrictions where they narrow to 700 m (Fig. 3). The tributariesof the western valley are 200–1000 m wide.

The depth of the Kaiser valley incision below the surrounding sea floorranges from 20 m in the north of the studied area to more than 60 m inthe south, with an overall seaward dip of 0.045. The base of the westerntributaries is about 10 m higher than that of the main valley (Fig. 6F). TheKaiser valley has a flat or gently concave-upward floor. It is bounded bywalls with a maximum slope of about 2–3. In the north, the valley wallsare slightly concave-up, regular surfaces (Fig. 6A). In the south, the valleywalls display several terraces that enlarge the valley upward (Fig. 6D–F).The base of the tributaries corresponds to the level of the most pronounced

of these terraces (Fig. 6E). This terrace also forms the flat interfluve be-tween the western and eastern branches of the main valley (Fig. 6C, D).

STRUCTURE OF THE INFILL

General Organization and Geometry of the Valley Fill

The Kaiser valley fill is composed of eight distinct seismic units (U0 atthe base; U7 at the top) that are defined objectively on the basis of the

most prominent seismic reflectors. Their thickness ranges from 3 to 45 m.Each unit can be traced across the entire width of the valley and over tensof kilometers along the length of the valley (Fig. 6B–F). Each unit isbounded by planar, locally slightly concave-upward surfaces marked byhigh-amplitude and laterally continuous seismic reflectors (Fig. 5A, B).These surfaces truncate the underlying valley-fill deposits and may locallycrosscut underlying units (Figs. 5A, B, 6F). The unit-bounding surfacescommonly merge with the valley walls at the level of the terraces (Fig.5A, B). The units dip toward the south with an overall slope angle of 0.03(locally 0.06). This slope angle is higher than that of the surrounding shelf (0.015) but smaller than that of the mean valley base (0.045). The unitspinch out northwards because of truncation by the shelf planation surfaceand therefore display an overall offlapping stacking pattern (Fig. 8).

The shape of the units in plan view typically has a branching pattern

(Fig. 7B–H), which may be dissected locally by erosion at the base of overlying units (see particularly the southern part of U1 and U2; Fig. 7B–C). The nature of the branching pattern changes upward from systems witha smaller number of thick, 2–3-km-wide branches (U1 to U3, Fig. 7B–D)to more complexly anastomosed systems with narrower (0.5–1 km wide),sinuous branches that merge in the valley axis to form sheet-like deposits(U6 and U7, Fig. 7G–H). This pattern evolution is correlated with an up-ward decrease of unit thicknesses, from 30–40 m in U1 to a few meters inU7. The thalweg of the main valley (Fig. 7A) is almost totally filled byU1. As a result, the branches of subsequent units (U2 to U4, Fig. 7C–E)are not strongly constrained by the initial geometry of the valley. By con-trast, the main branches of U5 and U7 are generally located along the valley




FIG. 4.—Seismic lines used for the reconstruction of the Kaiser valley fill (Sedi-manche 2 and Belgica 94 campaigns). Bold straight lines correspond to seismicprofiles given as examples in Figure 5 and interpreted cross sections in Figure 6.Bold arrowed lines underline main thalwegs at the base of the valley. Shaded areashows the valley outlines.

axis (Fig. 7F–H). Units U2 to U7 also have branches that link with thetributaries that join the main valley from the northwest (Figs. 3, 7C–H).

There are systematic changes between units U1 to U7 with regard totheir maximum landward extent or pinchout (Fig. 8) and the slope of theirbases. From U1 to U4, the landward limit of each unit is farther south thanthe unit beneath (Figs. 7B–E, 8). This seaward shift is accompanied by anupward increase in the average southward dip of the unit base, which variesfrom 0.045 at the base of U1, to 0.15 at the base of U2, and to 0.3 atthe base of U3. Because it is observed on a too short length, the slope of U4 is not reliable. From U5 to U7, the landward limit of the units migratestoward the north (Figs. 7F–H, 8). This landward shift is correlated with anupward decrease in the average southward dip of the unit bases, whichvaries from 0.1 at the base of U5, to 0.08 at the base of U6, and to 0.075at the base of U7.

Facies Architecture of the Valley Fill

Each seismic unit except U0 contains two seismic facies (A and B), withfacies A at the base and facies B at the top of each unit. Unit U0 consistsentirely of facies B. Facies A exhibits parallel, planar reflectors of lowamplitude and high continuity, each of which extends for many kilometers.On profiles parallel to the main valley axis, these reflectors display low-angle landward onlap against either the basal bounding surface of the unit

or the valley floor and walls (Figs. 5A–B, 8). Facies A contains sparse flat-topped, channel-form features that are several tens of meters in lateral ex-tent and few meters in depth, and increase upward in size and abundance.The top of facies A is truncated by either the next unit-bounding surfaceor the base of facies B. Facies B exhibits numerous crosscutting, sinuous,channel-form reflectors of high amplitude and variable continuity. Theyhave a lateral extension of 200–500 m, a vertical radius of curvature thatis locally as low as 30 m, and a maximum slope of 3. Because the densityof reflectors is locally very high, facies B has an amalgamated, somewhatchaotic appearance. In places, these channel forms are filled with down-lapping high-angle parallel reflectors. Locally, facies B incises the depositsof the underlying unit. At the top, facies B is truncated by the boundingsurface at the base of the overlying unit.

Facies A and B are not equally abundant along the length of most of the seismic units. Overall, facies B is more abundant in the northern part

of units U1 and U2 (Figs. 6A, 8), whereas facies A becomes progressivelymore prominent in southern areas (Figs. 6F, 8). This may be due to thestrong erosion of these units by overlying units in the south. The situationis reversed in U3, U4, and U5, where facies B is most prominent to thesouth (Figs. 6F, 8). The relative proportion of facies A and B remainsunchanged along the length of U6 and U7 (Fig. 6A–F).

Facies A and B are also not equally abundant in every unit. Facies A isoverall predominant in U6 and U7, which are thinner and more sheetlikethan the other units (Fig. 6C–F). Facies B comprises almost all of U0 andis much more abundant than facies B in U1 to U5, which have a thicker,channelized geometry. The character of facies B in U0 also differs fromits expression in the upper units. In U0, facies B is composed of severalpackages that are many kilometers in lateral extent and bounded by gentlyundulating erosional surfaces (Fig. 6B, east end). The thickest of these

packages forms a 25-m-thick lateral-accretion structure, characterized bysteeply dipping (up to 4–5), parallel reflectors (Fig. 6C, west) flanking theeastern side of the western valley. In the upper units, occurrences of faciesB are more discontinuous. Individual channels commonly amalgamate toform wider and deeper channel forms that are incised into facies A. Themean width:depth ratio of these channel complexes is 30:1. Their depthincreases from U2 to U4, reaching a maximum of 20 m (Fig. 6F). Themost developed of these channels commonly scours down into the under-lying unit (Fig. 6F). From U5 to U7 the amalgamation of individual channelforms in facies B decreases, until, by the top of the valley fill, they occuronly as rare, individual channel forms of a few meters depth (Fig. 6D).

There is also a relationship between the thickness of a unit, the ratio of facies A and B in that unit, and the presence of terraces on the valley walls:the thinner the unit, the higher the facies A:B ratio, and the wider thevalley-wall terraces on which the unit rests. The lower units U0 to U2,

which are locally thick and contain a high proportion of (or only) facies Bin the north, do not display terraces in that area (Fig. 6A–B). The terracesappear first in the central and southern part of U3, where facies A predom-inates (Figs. 6C–D, 7D) and are best developed in U5 and U6, whichcontain relatively small amounts of facies B and significant quantities of facies A (Fig. 7F, Fig. 6E).

INCISION AND DEPOSITION INSIDE THE VALLEY

Interpretation of Valley-Fill Facies

On both transverse and longitudinal profiles facies B exhibits curvedreflectors of high amplitude and low continuity. These reflectors are inter-




preted as lag deposits at the bases of sandy channel fills. The similarity of reflector geometry in both directions suggests that these channels werehighly sinuous, with a curvature radius significantly less than the 400 mprofile spacing. In all of the units except U0, the high sinuosity of thechannels, their small width/depth ratio (mean: 30), the numerous vertical-amalgamation features and few lateral-accretion features on their flanks,and the nested geometry in plan view of the units in which they occurargue for an anastomosed channel-belt origin for facies B. The low-ampli-tude, horizontal reflectors of facies A, by contrast, suggest that these sed-iments are of low lithological contrast (perhaps muddy) and were depositedin a quiet, low-energy, continuously aggrading environment. The higher-amplitude, channel-form reflectors within this facies indicate that it wasepisodically traversed by sinuous channels.

Given the incised-valley setting of these facies, facies A is believed tocorrespond to estuarine muds deposited in the low-energy central basin(lagoon) of a wave-dominated estuary (e.g., Dalrymple et al. 1992; Thomasand Anderson 1994; Kindinger et al. 1994; and others). The landward onlapof facies A reflectors onto the unit-bounding surfaces is consistent with thetransgressive, estuarine character of this facies. The sparse channel formsin facies A are thought to correspond to distal cross sections of either floodtidal channels or distributaries of the bayhead delta. Because these channelforms are very few and of small size, and do not exhibit an abundance of

lateral-migration features, facies A is unlikely to fit into the tide-dominatedestuarine facies model. In this model, there is a gradual transition fromwell developed tidal–fluvial channels to tidal sand bars near the estuarinemouth (Dalrymple et al. 1992). No barform that could be related to sucha system has been observed in the Kaiser valley fill, so that the wave-dominated model seems more likely. The channel deposits of facies B maycorrespond to the amalgamation of estuarine channels of the same originas in facies A truncated by deeper and wider fluvial channels. The anas-tomosing, highly sinuous pattern of the channel network in U1–U7 suggeststhat it is incised into relatively fine-grained and cohesive sediments(Tornqvist 1993), as expected for the facies A deposits that these channelstruncate. By contrast, the thicker facies B packages in U0 with their large-scale lateral accretion structures (Fig. 6C, west end) are interpreted to haveaccumulated in a coarser-grained, meandering channel system (Mjos andPrestholm 1993).

Interpretation of Unit-Bounding surfaces

The unit-bounding surfaces at the base of facies A truncate the under-lying fluvial deposits as well as the preceding units in some areas. Thehigh amplitude of the reflectors associated with these surfaces could reflectthe presence of a lag deposit and/or the lithological contrast of muddy baysediments over sandy channel deposits. Because these surfaces are very flatand underlie inferred estuarine, central-basin deposits, they are interpretedas wave-formed, estuarine bay-ravinement surfaces. A strong wave climateis argued by the proximity of the study area to the edge of the continentalshelf. In such a setting the tidal range was probably equal to that in theopen Atlantic Ocean (i.e., small) and wave energy would have been veryhigh.

Therefore, the cliffed terraces shaping the valley walls are interpreted aslateral extensions of the unit-bounding, wave ravinement surfaces (Fig. 5A,terrace 1). They could form in response to successive floodings by the seaduring valley infilling. Alternatively, the terraces could be inherited fromsuccessive base-level falls that could have stepped the fluvial valley inci-sion. In the latter case, the transition from facies B to facies A could havebeen triggered partly by abrupt increases of the valley width as terraceswere flooded. However, because some unit boundaries do not merge lat-erally with terraces, especially near the base of the valley fill (Fig. 6, C–F), wave control seems to have been more important in the terrace for-mation than the inherited valley morphology. In the wave-control hypoth-esis, the absence of terraces correlated to the lower units may be the result

of limited internal fetches during the early phases of valley filling, becauseof the presence of the axial high (an island?) that separates the eastern andwestern branches of the valley (Figs. 4, 6). The upward increase in theprominence of the terraces is consistent with eventual drowning of thisisland and the resulting increase in fetch within the estuary. At the time of deposition of U5–U7, the estuary may have been more than 10–15 kmwide and several tens of kilometers long.

Because the erosion associated with the unit-bounding, bay ravinementsurfaces is enough to produce unit truncations (e.g., Fig. 6E, U3 on U2)or cliffed terraces on the valley walls, it is likely to be due to the propa-gation of the oceanic swell into a relatively open estuary (e.g., the SevernRiver estuary; Allen 1990). In this hypothesis, the bay ravinements oc-curred rapidly as the valley was flooded by the sea. Then, facies A (estu-arine fine-grained, low-energy deposit) would have been deposited in re-sponse to the growth of a barrier, which would have protected the estuarinecentral basin from open-sea energy. Therefore, the sediment supply duringthe bay ravinement could not keep pace with the rate of creation of ac-commodation space, so that no barrier could form. Alternatively, the bayravinement surfaces could have been produced within the central basin of a barred estuary, provided that the central basin was sufficiently large (e.g.,Chesapeake Bay; Biggs 1967). In this hypothesis, there could have been apermanent barrier seaward of the study area during the transgression. What-

ever the case, in the bay ravinement interpretation, the barrier would havebeen seaward from the place where unit boundaries formed. This wouldexplain why no preserved barrier deposits have been recognized in thevalley fill.

Sequence Stratigraphic Interpretation

The presence of eight repeated units within the valley fill suggests thatthere were short-term variations of space available for sedimentation. Themain facies A deposition corresponds to periods during which the rate of relative sea-level rise was greater than the rate of sedimentation, whereasfacies B, with its amalgamated channels, represents deposition during timeswhen the rate of relative sea-level rise was smaller than the rate of sedi-mentation. As defined above, the units are separated by bay ravinement(flooding) surfaces and show an overall regressive evolution from estuarine

sediments (facies A) to fluvial sediments (facies B). Thus, each depositionalunit could be interpreted as a parasequence (Van Wagoner et al. 1990).However, the depth of channel incision observed at the base of the fluvialchannels (as much as 20 m) suggests instead that a relative sea-level falloccurred during deposition of facies B.

Therefore, we interpret the valley-fill units (U) as related to depositionalsequences (S) sensu Posamentier et al. (1988). The sequence boundariesare not very prominent on the seismic records, because they mostly occuras sand-on-sand contacts within facies B (Fig. 9). The more prominent bayravinement surface would instead be the transgressive surface, as proposedby Allen and Posamentier (1993), separating the fluvial deposits (deepestchannels in facies B) of the lowstand systems tract (LST) from the estuarinecentral-basin sediments (lower part of facies A) of the transgressive (TST)and highstand (HST) systems tract. If the isolated channels in the upper

part of facies A and/or the shallowest channels in the lower part of faciesB are bayhead-delta distributaries, then they form part of the progradationalhighstand systems tract and the maximum flooding surface (MFS) lies be-low them, probably in the horizontally bedded part of facies A (Fig. 9,hypothesis 1). Alternatively, if these isolated channels are tidal channelsassociated with the flood-tidal delta, then all of facies A is transgressiveand the MFS has probably been erosionally removed by incision at the nextsequence boundary (Fig. 9, hypothesis 2).

It has been suggested that depositional sequences such as these may becreated by autocyclic variations in sediment flux, without any change inrelative sea level (Galloway 1989; Homewood et al. 1992; Wescott 1993).However, it is unlikely that the sequences within the Kaiser valley fill are







F I G .

5 . —

S e i s m i c p r o fi l e s a n d t h e i r i n t e r p r e t a t i o n . A ) s e c t i o n t r a n s v e r s e t o t h e v a l l e y . B

) s e c t i o n p a r a l l e l t o t h e v a l l e y ( s e e F i g u r e 4 f o r l o c a t i o n s ) . N u m b e r s r e f e r t o u n i t s .

S e i s m i c f a c i e s i n t h e v a l l e y fi l l : f a c i e s A ,

d a r k

g r a y ; f a c i e s B ,

l i g h t g r a y .




FIG. 6.—Interpreted seismic cross sections of the Kaiser valley (see Figure 4 for locations). Numbers refer to units. Note the terraces on the valley walls (lines C to F),the most prominent of which corresponds to the elevation of the interfluve between the western and eastern valley branches. Note on line E the main terrace on the westernvalley wall, which connects westward with the base of the southernmost 120-trending tributary valley (Fig. 4). Note on line F the scouring of U3–U6 into underlyingunits.

of purely autocyclic origin, because (1) their bases are deeply incised intothe underlying units; and (2) each of the sequences contains an amount of sediment that seems too large to be related to autocyclic geomorphic cycles(Wescott 1993). Thus, these units are more likely related to sea-level var-iations.

Because of the presumed Pliocene age of the Kaiser valley system (Pan-tin and Evans 1984), sea-level oscillations responsible for the valley-fill

sequences are believed to be of glacioeustatic origin (Auffret 1983; DeGraciansky and Poag 1985). Indeed, two features of the succession suggestsuch a regularly cyclic forcing of sedimentation in the Kaiser valley, per-haps coupled with glacial/interglacial sediment-flux variations: (1) theamount of sediment involved in the sequences changes little from one toanother, the upward thinning of the units being compensated by wideningof the valley; and (2) the architecture and facies content of each sequencealso remains constant. The glacioeustatic variations in the Pliocene are fifth-order cycles (Williams 1988), associated with sea-level changes of 40 kyperiod and 30–40 m amplitude (Fig. 11; Shackleton and Opdyke 1977;Shackleton et al. 1991). Despite the smaller amplitude of the Pliocene gla-cioeustatic cycles than those of the late Pleistocene (Shackleton 1987), the

former were probably enough to have a strong effect on fluvial base levelsin the Kaiser valley area, which is near the shelf break.

Sequence Stacking Pattern

Relative to the other units, U0 contains an anomalously thick, amalgam-ated, and coarse-grained stack of fluvial-channel deposits (facies B) and

lacks any deposits of facies A. These attributes suggest that this unit maycontain several amalgamated sequences that together form one or morelowstand systems tracts (LST) (Fig. 10). From U1 to U4 the maximumlandward extension of the units decreases (Fig. 7B–E), indicating a seawardshift in the extent of coastal onlap. At the same time, the relative proportionof facies B increases (Fig. 6F). Thus, sequences S1–S4 represent a progra-dational sequence set (sensu Mitchum and Van Wagoner 1991). Becausesequence boundaries rise from S1 to S4, producing a vertical stacking of estuarine deposits (Fig. 6F), this progradational sequence set formed duringan overall relative sea-level rise. As such, these deposits could correspondeither to a highstand systems tract or to the late lowstand systems tract of a composite sequence (Mitchum and Van Wagoner 1991). The latter in-




FIG. 7.—Isopach maps of valley-fill units.Isopachs every 5 m. Bold lines delineate valleymargins. Shaded areas show areas where the unitbase corresponds to the valley floor.

terpretation is the simplest, given that S0 is interpreted as an amalgamatedlowstand sequence set, but it does not account for the presence of a rapidlandward shift in coastal onlap (i.e., a significant transgression) at the baseof U1 (from U0 to U1, Fig. 8). Therefore, we suggest that the S1–S4progradational sequence set accumulated under conditions of a decreasingrate of relative sea-level rise and hence represents the highstand systemstract (HST) of a composite sequence (Fig. 10). The lowstand systems tractof this composite sequence would be represented by either all of S0 oronly its upper part if S0 comprises more than one lowstand sequence set.From U5 to U7, the landward extension of the deposits increases (Fig. 7E–H) and the relative amount of facies A increases (Fig. 6F). This suggestsan increased rate of relative sea-level rise and an overall transgression. The

retrogradational stacking pattern of sequences S5–S7 is thus interpreted asthe transgressive systems tract (TST) of another composite sequence.

The regressive–transgressive evolution of valley-fill sedimentation be-tween U1 and U7 is corroborated by the changing slope of the erosionalunit boundaries (bay ravinement surfaces). The slope of these surfaces isprobably not inherited from the transgressed fluvial profile, given that thefluvial systems (facies B) are all interpreted as low-gradient anastomosingrivers. More likely their slope is controlled by the speed of relative sea-level rise, relative to the sedimentation rate in the fluvial–estuarine transi-tion zone, which together determine the rate of bayshore transgression. If it is assumed that (1) sedimentation rate did not change significantly, assuggested by the constant volume of the sequences, and (2) the measured




FIG. 8.—Synthetic longitudinal section of theKaiser valley fill. At the landward end, note theinterpretation of U0 as consisting of severalfacies B complexes, the lowest of whichcomprises point-bar structures on the edge of thevalley (see Figure 6C).

FIG. 9.—Sequence-stratigraphic interpretationof units constituting the Kaiser valley fill. Notethat sequences do not correspond to theseismically defined units, which are boundedinstead by transgression surfaces. SB, sequenceboundary; LST, lowstand systems tract; TS,transgression surface; TST, transgressive systemstract; MFS, maximum flooding surface; HST,highstand systems tract.

slope of the bay ravinement surface is reliable, then this slope should beinversely proportional to the rate of sea-level rise. Therefore, the increasingslope of the bay ravinement surface from U1 to U4 indicates a slowing of the rate of sea-level rise. This is consistent with highstand sedimentationat the composite sequence scale (Fig. 10). Conversely, the upward decreasein the slope of the bay ravinement surface from U5 to U7 would be cor-related to an overall sea-level rise at the composite-sequence scale (Fig.10).

The progradational to retrogradational turnaround at the base of S5 is acomposite sequence boundary. Because this surface does not appear muchmore erosional than those at the bases of other sequences, it is inferred thatit is a type 2 sequence boundary (Posamentier and Vail 1988). This explainswhy there is no LST at the base of the upper composite sequence. Instead,S5 can be considered as the shelf-margin systems tract (SMST) of this

sequence (Posamentier and Vail 1988). In this interpretation, there is noneed for a significant sea-level fall at this time and the valley is believedto have filled during a continuous overall relative sea-level rise, but witha decrease in the rate of rise during the deposition of S2–S5 (Fig. 10).

Eustatic and Tectonic Influences on Sedimentation

Given the late Pliocene age of the valley-fill deposits, it is logical toassume that the stacking pattern of the sequences in the Kaiser valley fillis mainly controlled by third-order eustatic sea-level changes (Haq et al.1988; Williams 1988), which would have had a magnitude of about 100m and a period of 800 ky (Fig. 11). If so, then incision of the main valleymay correspond to the mid-Reuverian third-order eustatic fall (ca. 2.9 Ma),whereas the amalgamated fluvial deposits in S0 could have formed duringthe following third-order LST/TST, which is correlated with the late Reu-

verian transgression at 2.7 Ma in mainland Brittany (Morzadec-Kerfourn1990). The rest of the valley fill must have been deposited during the lastPliocene third-order sea-level rise flooding the outer shelf. This is relatedto the Tiglian ages, following a major relative sea-level fall on land at 2.35Ma (Morzadec-Kerfourn 1990). The third-order sea-level fall between bothsedimentation stages would have excavated most of the deposits inside thevalley, including those from the Reuverian HST, so that there are probablyonly few remnants of these deposits at the base of S0. In this context, theHST of the lower composite sequence (S1–S4) is due either to a breakdown

of the regional subsidence or to the interplay of the falling limb of a fourth-order eustatic cycle superimposed to the third-order Tiglian sea-level rise.In turn, both TSTs of the lower and upper composite sequences (part of S0 to S1 and S6–S7, respectively) were deposited when sea level rose atthe same time at third and fourth order (Fig. 11).

Despite the apparent predominant eustatic control on the stacking pat-terns of the valley-fill sequences, valley-fill preservation is due to thedrowning of the area. If it is assumed that there are no significant breaksduring the deposition of S4–S7 and that each sequence represents a 40 ky,fifth-order sea-level cycle, then the rate of relative sea-level rise calculatedfrom their thickness (mean 12 m) is of the order of 500 m/My between S4and S7 (given that all of the deposits are interpreted to have accumulatedclose to sea level). This rate is ten times higher than the long-term subsi-dence of the margin in the Pleistocene (Pantin and Evans 1984), and ex-

ceeds significantly the rate expected from the addition of the rate of third-order eustatic rise (200 m/My) to the long-term subsidence since the upperPliocene (55 m/My, Pantin and Evans 1984). This suggests that acceleratedtectonic subsidence was an important additional control on drowning of the valley (Fig. 11).

During the Pleistocene, the Celtic Sea area experienced a general warp-ing toward the southwest, perhaps with slightly different rates on eitherside of the Alderney–Ouessant fault (Pantin and Evans 1984; Morzadec-Kerfourn 1990) (Fig. 1). This oceanward tilting of the shelf surface resultedto some extent in the present-day depth of the valleys and caused theirlandward truncation (Fig. 1). In the southern part of the Celtic Sea, thistilting is depicted by the 30 angle between the SSW orientation of themain valleys and the present-day SW dip of the shelf (Bouysse et al. 1976).This pattern may have been controlled by faulting along the Alderney–

Ouessant axis, as suggested by the deepest incisions of the valleys and thewider development of the canyon reaches west of this fault zone (Bourilletand Lericolais 1996). Consistent with Ruffell (1995), the subsidence cal-culations above suggest that the movements were discontinuous and veryrapid. Differential movement is also indicated by the fact that the orien-tation of the rivers responsible for the branching valley network changedfrom NNE–SSW during the incision of the main valley and deposition of S0 and S1 (Fig. 7B) to NW–SE during the cutting of the western tributariesand the deposition of S2 and subsequent units (Fig. 7C). Assuming that




FIG. 10.—Stacking pattern of sequencesinfilling the Kaiser incised valley and inferredrelative sea-level variations. The amplitude of sea-level variation is assumed to be 40 m, asdeduced from the Pliocene isotope record(Williams 1988; see discussion). SB, sequenceboundary; LST, lowstand systems tract; SMST,shelf margin systems tract; TST, transgressivesystems tract; HST, highstand systems tract.

FIG. 11.—Chronology of the Kaiser valleyincision and infilling, inferred from matching thestacking pattern of sequences in the valley fillagainst the relative sea-level history of thesouthern Celtic Sea. Thin curve: eustatic curvederived from 18O stratigraphy, with theaddition of the third-order eustatic cyclesinferred from the Haq et al. (1988) curve (boldblack envelope). The 18O signal is fromShackleton et al. (1991) from 2.6 Ma to thepresent, and from Shackleton and Opdyke (1977)from 2.6 to 3.2 Ma. Thick curve: relative sea-level curve, obtained by constraining the eustaticcurve using: (1) the incision of the Celtic valleysto 260 m (Bouysse et al. 1976) during theReuverian third-order lowstand at 2.8 Ma; (2)earliest Pliocene valley-sealing marine sedimentsrecovered at 165 m (round spot), which weredeposited in more than 50 m water depth (Evansand Hughes 1984); and (3) paleobeaches aroundBrittany, suggesting that late Pleistocenehighstand levels varied little from the present-day sea level (Morzadec-Kerfourn 1990).

the rivers flowed directly down the depositional dip, this change suggestsa tilting of the shelf toward the southeast or uplift of the area to the north-west of the Kaiser valley between deposition of S1 and S2. If there is nosignificant hiatus between S1 and S2 (i.e., less than half of a 40 ky, fifth-order cycle), this would indicate the very rapid response of the incisedvalley to pulses of tectonic tilting.

DISCUSSION

Because the fill of the Kaiser valley is composed of several sequences,it is a compound valley fill in the terminology of Zaitlin et al. (1994). Thisis consistent with their ideas, because the river feeding into the Kaiservalley (the ‘‘English Channel River’’; Gibbard 1988) is a piedmont system

with an extensive catchment area, and because the initial valley is large,thereby taking a considerable time to fill. The complexity of the fill is,however, greater than that of most previously described compound fills,given that it is believed to contain at least eight fifth-order depositionalsequences, which are in turn grouped into two fourth-order composite se-quences, overlying a basal unit that may contain remnants of the precedingthird-order lowstand (Fig. 11).

In our hypothesis, this long filling history was also accompanied by alengthy erosional history, such that the surface underlying the valley net-work is also a compound surface, formed during several episodes of erosionthat alternated with periods of infilling. Thus, the northwest-oriented trib-utaries and the terraces along the valley walls (Fig. 7) represent modifi-cations to the initial, simpler valley form. Indeed, the existence of such




FIG. 12.—Hypothetical relationship betweenthe position of an incised valley on the shelf and

the valley segmentation scheme in the model of Zaitlin et al. (1994). Although other controls canbe of importance (the slope of the incised shelf,the amplitude and duration of sea-level cycles,etc.), the major control, the only one taken intoaccount here, is the rate of sea-level rise at thetime of valley flooding. The facies sketchesrepresent the hypothesized facies record in theseaward part of the valley fill during a singlesequence. For valleys incised on the lower shelf,which are flooded early, the most importantfilling stages are fluvial to estuarine LST/earlyTST. This contrasts with valleys incised fartherlandward on the shelf (modern estuaries), inwhich the marine HST may be preserved.

complex valley shapes may be an indication that they are multigenerationalfeatures. If present in the subsurface, the fill of such systems would havea very complex internal architecture with many potential reservoir-seal con-figurations.

Our interpretation of the valley-fill deposits does not recognize the ex-istence of significant amounts of estuary-mouth or open-marine deposits.Instead, the valley fill is interpreted to consist almost entirely of fluvial,fluvial–estuarine, and estuarine central-basin sediments. This situation issurprising, given the location of the study area at the outer edge of thecontinental shelf, very near the presumed lowstand shoreline, in an areaexpected to lie within segment 1 of the valley as defined by Zaitlin et al.(1994). Even in the absence of core evidence, it seems worth discussingthe apparent absence of coastal and marine facies with respect to the in-cised-valley model, because examples of deep-shelf incised valley are poor-

ly documented:(1) If the sea-level falls responsible for the simple sequences inside the

valley fill were significantly lower than the shelfbreak (ca. 260 m), thevalley in the study area may not have been transgressed by the open-coastshoreline during the fifth-order highstands. However, the lack of deep in-cision at the base of each sequence and the resulting preservation of olderfifth-order sequences does not support this idea.

(2) The shoreline may indeed have transgressed the study area numeroustimes, but almost no estuary-mouth deposits remain because of wave rav-inement at the main shoreline. Such truncation of the barrier complex dur-ing transgression is common (e.g., Demarest and Kraft 1987; Willis 1997).In addition, fluvial erosion during the succeeding lowstand would havepartially to entirely removed the marine deposits and any remnants of theestuary-mouth barrier.

The second hypothesis is more likely. Even if marine deposits are pre-served on top of the bay ravinement surface, they are too thin to be dis-tinguished seismically. Because of the location of the valley on the outerpart of the continental shelf, the part of the valley under study probablywould have been far from the shoreline at the time of maximum trans-gression, leading to sediment starvation and limited accumulation of marinesediments (Fig. 12A–B). This situation contrasts with that of those partsof incised valleys located near the present-day coast (i.e., on the inner partof the shelf) that are close to the highstand shoreline and are more likelyto be buried by late transgressive and early highstand marine deposits (Fig.12C).

In this perspective, the interpretation that the lower part of facies B

consists of amalgamated progradational bayhead-delta channels of the HST(Fig. 9, hypothesis 1) is less likely to be correct than the alternative hy-pothesis that these channels belong to the estuary-mouth complex (floodtidal delta, tidal inlets) (Fig. 9, hypothesis 2). More data are still neededto confirm or invalidate these hypotheses.

CONCLUSIONS

(1) The Kaiser valley contains a compound valley fill composed of eightor more fifth-order sequences generated by glacioeustatic sea-level fluctu-ations. Each sequence consists of lowstand, amalgamated fluvial-channeldeposits at the base, overlain by transgressive bay-fill (central-basin) sed-iments that accumulated in a barred, wave-dominated estuary. There are

several points of difference between the Kaiser valley system and the ide-alized incised-valley model of Zaitlin et al. (1994) that relate to the natureof the sea-level oscillations governing valley filling and the position of thestudy area near the lowstand shoreline:

(i) The most obvious erosion surface, seismically, within each sequenceis not the sequence boundary but an interpreted bay ravinement surfacethat underlies the bay-fill deposits and is considered to have formed bywave erosion within the valley. Wave ravinement is interpreted as beingresponsible for truncating the underlying fluvial deposits and eroding thevalley walls, thus enlarging the valley and creating terraces along the valleymargins. The terraces are overlain by estuarine deposits (estuarine beach-es?) instead of fluvial sediments as cited in the model of Zaitlin et al.(1994).

(ii) Because of proximity to the lowstand shoreline, the study area isbelieved to have been transgressed by the shoreline during each sea-levelcycle and thus lies within segment 1 of Zaitlin et al.’s (1994) model. How-ever, no transgressive barrier or marine deposits are recognized seismically,despite the fact that the model predicts their presence. Wave ravinementduring transgression and fluvial erosion during lowstands appear to haveeffectively removed evidence of these transgressions. Prograding highstanddeposits are also absent, presumably because the moderately high-ampli-tude fifth-order sea-level oscillations were too short to allow progradationof the highstand shoreline this far seaward.

(iii) The closeness of the area to the shelf edge and lowstand shorelinemay account for the limited degree of dissection of older sequences duringeach succeeding sea-level fall. Individual sequences typically have a sheet-




like geometry and can be traced for long distances. Farther inland, greaterflushing of the valley might be expected during each lowstand.

(2) The valley incision and infilling occurred during the late Pliocene.The stacking pattern of the sequences produces a progradational–retrogra-dational succession that is believed to contain parts of two composite se-quences mainly produced during one third-order eustatic cycle, modulatedby a fourth-order superimposed signal and/or a rapid tectonic movement.Such rapid subsidence events are inferred from the thickness of the pre-served fifth-order sequences. The first of them may be related to a south-eastward tilting of the shelf, which is recorded by the oblique orientationof tributary valleys that were eroded during filling of the main valley,relative to the orientation of the main valley itself. Long-term subsidenceduring the Pleistocene controlled the preservation of the valley fill.

(3) The sequence boundary that underlies all of the valley-fill depositsis a composite surface that reflects local amalgamation of high-frequencysequence boundaries and several bay ravinement surfaces.

ACKNOWLEDGMENTS

This work, initiated in the French SEDIMANCHE project, was financially sup-ported by the EEC-funded MAST2-Starfish European program, through the part-nership of RCMG–Gent and IFREMER–Brest. We extend our thanks to the scientificstaff and crew who assisted us during the campaigns at sea: people from IFREMER

and GENAVIR during Sedimanche 1 and 2; and people from the Belgian Navy andUniversity of Gent during Belgica 92 and 94. We especially thank Pieter Vanhau-waert, who drew the first map of the Kaiser valley. Herve Chamley is also acknowl-edged for reviewing an early draft of the manuscript. The final version benefitedgreatly from reviews by Andy Pulham, Daniel Belknap, and Corresponding EditorJohn Southard.

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Received 12 January 1998; accepted 1 August 1998.

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Architecture and sequence stratigraphy of a late Neogene incised valley at the shelf margin,...

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