3D architecture of lowstand and transgressive Quaternary sand bodies on the outer shelf of the Gulf of Lion, France

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A restricted area (700 km 2 ) of the outer continental shelf in the Gulf of Lions (Western Mediterranean Sea) has been surveyed in detail, in order to reconstruct the 3D architecture of large Quaternary sand bodies that are exceptionally well
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  Marine and Petroleum Geology 04 "0887# 328Ð341S9153Ð7061:87:,ÐSee front matter Þ 0887 Elsevier Science Ltd[ All rights reservedPII] S9153Ð7061"87#99904Ð3 2D architecture of lowstand and transgressive Quaternary sandbodies on the outer shelf of the Gulf of Lion\ France M[ Rabineau\ S[ Berne\ Eł[ Ledrezen\ G[ Lericolais\ T[ Marsset\ M[ Rotunno IFREMER "Institut Francžais de Recherche pour l|Exploitation de la Mer#\ DRO:GM\ Laboratoire Environnements Se dimentaires\ BP 69\ 18179Plouzane  \  France Received 8 September 0886^ revised 12 January 0887^ accepted 0 February 0887 Abstract A restricted area "699 km 1 # of the outer continental shelf in the Gulf of Lions "Western Mediterranean Sea# has been surveyed indetail\ in order to reconstruct the 2D architecture of large Quaternary sand bodies that are exceptionally well preserved in the westernpart of the Rho¼ne delta[ Data sources include digital {very high resolution| seismic\ swath bathymetry and some shallow cores[In two dimensions\ the seismic data display a complex superposition of alternating seismic units\ consisting of high and low angleprograding clinoforms\ corresponding to {high| and {low| energy environments[ The combination of faunal and grain size analysisfrom shallow cores\ geometry of sedimentary bodies\ showed that these two types of seismic facies can be attributed\ respectively\ toupper shoreface and prodelta(c settings[Despite a similar geometry in 1D\ it appears that the prograding sand units have very di}erent 2D geometries "lateral extent\direction of progradation with respect to the paleoshorelines[[[#\ implying di}erent sedimentary processes[ At least two major sandunits have shore parallel orientations and can be mapped along the entire study area[ The mechanism responsible for the formationof these sand bodies seems similar to that described for many {sharp!based| sand bodies from the stratigraphic record\ especially inthe Western Interior Seaway^ their isolated position on the shelf being related to "forced# regressions[ In contrast\ some other sandunits have lobate shapes\ with divergent directions of progradation and limited lateral extent[ They are often overlying "~uvial#incisions and they pinch out landward^ it is still not clear whether they are related to some allocyclic or autocyclic processes\ likeshifting of delta(c lobes as observed on the Texas coast[The very high resolution\ and high quality of the seismic data have enabled us to characterise the three!dimensional architectureof small!scale sedimentary bodies[ The study has pointed out the variability of depositional processes and has given an insight onreservoir characteristics[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Keywords]  Western Mediterranean^ Shelf sedimentation^ Lowstand shoreface^ Quaternary 0[ Background The srcin of many ancient shelf sandstones\ generallyencased in shales\ has been a matter for speculation formany years[ Based on comparison with modern tide! andwave!dominated shelves "Swift\ 0864^ Swift and Field\0870^ Stride et al[\ 0871#\ many shelf sandstones weresrcinally interpreted as {o}shore ridges| or {o}shorebanks| resulting from the accumulation of sand by physi!cal processes under steady conditions of relative sea level[On the other hand\ detailed analysis of the 2D archi!tecture of sandstone in the Western Interior Seaway of the U[S[A[ and Canada led several authors to re!interpretthem as older lowstand or transgressive shorelines result! Corresponding author[ Tel] 9922 1 87 11 31 33^ fax] 9922 1 87 1134 69^ e!mail] rabineau Ý ifremer[fr ing from an allocyclic process^ relative sea level changes"Plint\ 0877^ Walker and Wiseman\ 0884#[ To date\ mostofthedetailedinterpretationsofQuaternarydepositionalsequences within a sequence stratigraphic frameworkhave focused on the Gulf of Mexico\ because of theavailability of numerous boreholes and a dense grid of high and very high resolution seismic re~ection surveys"Suter and Berryhill\ 0874^ Coleman and Roberts\ 0877^Sydow and Roberts\ 0883^ Anderson et al[\ 0885#[ InEurope\ despite the lack of abundant borehole data\ itappears\ from several geophysical investigations\ that theMediterranean Sea is a favourable environment for thepreservation of Quaternary depositional units\ probablybecause of a relatively moderate hydrodynamic regime inmost areas "Alo(si\ 0875^ Chiocci\ 0883^ Ercilla et al[\0884^Hernandez!Molinaetal[\0883^Tessonetal[\0883^Trincardi and Field\ 0880#[  M[ Rabineau et al[:Marine and Petroleum Geolo`y 04 "0887# 328Ð341 339 The Gulf of Lions\ in the French part of the WesternMediterranean Sea "Fig[ 0#\ and more precisely the Quat!ernary shelf deposits in front of the Rho¼ne delta\ havebeen used to exemplify the concept of stranded lowstandshorelines associated with {forced regressions| "Posa!mentier et al[\ 0881#[ Our study documents the 2D Fig[ 0[ Simpli_ed geographic and bathymetric map of the Gulf of Lions coast and shelf showing location of the study area and the grid of seismiclines[ Water depth are in meters[ AC] Aude Canyon "also called Bourcart Canyon#^ HC] Herault Canyon^ LDC] Lacaze!Duthiers Canyon^ PvC]Pruvot Canyon[ geometry of Quaternary prograding sand bodies locatedin an area slightly to the west of the area described byPosamentier etal[ "0881#[Thestudy area displaysthe mostdetailed record of depositional units in the Gulf of Lions\probably because of a good balance between sedimentsupply and accommodation variations within the basin[  M[ Rabineau et al[:Marine and Petroleum Geolo`y 04 "0887# 328Ð341  330Fig[ 1[ Location of the seismic re~ection pro_les used in this study\ theheavy lines refer to the lines illustrated in the _gures[ 1[ General setting The Gulf of Lions\ in the Western Mediterranean Sea\is a modern wave!dominated shelf where the tidal rangeis only a few cm\ and waves have moderate energy[ Thelargest swells are from the south!east\ with maximumwave heights of 4 m and periods of about 7 s\ occurring9[0) of the time[ A geostrophic current\ named the Lig!uro!Provencal current\ ~ows south!westward along thecontinental slope[ Its average current velocity near thesea ~oor on the outer shelf is only about 4 cm s −0 \ butthe orientation and magnitude of this current may bemodi_ed by seasonal strati_cation and wind e}ects "Mil!lot\ 0870#[ In such situations\ current velocities near thebed reach 19Ð29 cm s −0 on the outer shelf "Millot andCrepon\ 0870#[The study area corresponds to the zone where thecontinental shelf is of maximum extent\ about 49 km"Fig[0#[Theshelfbreakisdissectedbynumerouscanyonsthat have experienced several phases of incision andin_lling during the Quaternary[ As a result\ the shelf break ranges from 019Ð049 m deep\ depending on theoccurrence of recent slope failures within the canyons[Previous regional geophysical investigations have indi!cated that Quaternary sediments of the region can bedivided into] ,  An upper postglacial "07\999 to present# depositionalcomplex comprised of a transgressive unit capped byregressive prograding deposits of the highstand systemtract[ this complex is restricted to the subaerial deltaplain and inner shelf "L|Homer et al[\ 0870^ Alo(si\0875^ Gensous et al[\ 0882#[ ,  Alowercomplexonthemiddletooutershelfthatformsa seaward!thickening wedge pinching out landwardbeneath the 79 m isobath "Alo(si\ 0875#[ This wedge isinterpreted as a shelf perched lowstand wedge complex"Tesson et al[\ 0889#[ It consists of several "at least 5#superimposed prograding wedges that extend acrossthe entire shelf[ Each wedge is composed of low angleprograding clinoforms\ subdivided into lozenge!shapedsubunits separated by internal surfaces of erosion[ Thewedges are interpreted as having been deposited during{forced regression|\ in response to successive sea levelfalls[The dense seismic grid used in this study is located on theouter shelf between the 89 m isobath and the shelf break[Postglacial deposits are very condensed in this area\ somost of the sediments belong to the lowstand progradingcomplex previously described[ 2[ Methods Most of the geophysical data were acquired in 0883and 0884 with the research vessel {L|Europe|[ Duringthese cruises\ a di}erential GPS system was employed foraccurate "about 4 m# positioning[ Some data come fromprevious "0880 and 0881# surveys with the research vessel{Georges Petit|\ using a Syledis positioning system[ Inorder to obtain su.ciently dense coverage of the studyarea\ a pseudo!2D seismic grid was dimensioned\ con!sisting of 36 dip!sections and 15 strike sections\ with aspacing ranging from 399Ð0499 m[ The grid covered atotal area of 14×17 km\ or 699 km 1 [ The two mainseismicsourcesusedforthesurveyswerea01Ð0499JoulesSIG sparker and an EDO 404A sub!bottom pro_lerworkingat1[4kHz[ADELPH1digitisingandprocessingsystem was used\ allowing simultaneous use of the spar!ker and sub!bottom pro_ler[ The shooting interval was0Ð0[4 s for the sparker and 9[249 s for the sub!bottompro_ler[ A total of 0599 km of high resolution sparkerseismic pro_les and of 249 km of 1[4 kHz sub!bottompro_les were recorded\ both in analogue and digitalmodes[ The resolution of sparker seismic pro_le is about0mwithapenetrationaround499ms\butthepresenceof the sub!bottom multiple decreases the amount of usabledata[ The sub!bottom pro_les display a resolution up to49 cm with a shallow penetration "few tens of ms#[ Thedata were post!processed with SITHERE "Lericolais etal[\ 0889#[ This processing includes swell removal\ band!pass _lter\ deconvolution\ multiple attenuation\ and timevariable gain[Analysis of re~ection terminations "erosional trunc!ation\ onlap\ downlap# and of re~ection con_gurations"i[e[ acoustic facies#\ allowed identi_cation of small!scaleseismic units and their boundaries[ Discontinuity depthswere checked at the numerous tie!points[ The main inter!preted seismic units were numbered and mapped in orderto produce isopach "in ms Twtt# maps[ The general con!cepts of seismic stratigraphy "Payton\ 0866# may beapplied to our high resolution data set\ however\ it mustbe emphasised that inferences in terms of processes can!not always be applied in the same way as for low res!  M[ Rabineau et al[:Marine and Petroleum Geolo`y 04 "0887# 328Ð341 331 olution seismic data because scales of investigations arevery di}erent[ For instance\ autocyclic processes such aslobe switching or bedform migration will have greatere}ect at our high!resolution scale[ 3[ Results Within the upper 019 m of sediment\ at least 04 seismicunits are identi_ed in the study area\ amongst which atleast 5 major sand bodies are well preserved[ 3[0[ Surfaces Most of the seismic discontinuities are both surfacesof erosional truncation "upper boundary of underlyingdepositionalunit#and downlapsurfaces"lowerboundaryof overlying unit#[ It is possible to distinguish on thepro_les planar very low angle dipping surfaces "9[1Ð9[2>#\higher angle and often variable dipping surfaces "0Ð3>#that mould bodies with high angle clinoforms\ and undu!lating surfaces "suggesting a subaerial or very shallowmarine srcin#[ The undulating surfaces are particularlypronounced at deeper stratigraphic levels "D39\ D29\D19 in Fig[ 2#[ Toward the shelf break\ these surfaces aregenerallytruncatedbydeepincisions"seebelow#[Insomelocations\ however\ especially along the inter~uvesbetween canyons\ they become correlative conformitiesas water depth increases[Not all the surfaces encountered in the area have thesame level ofimportance[Many surfaces mergewith eachother[ Some are major erosional surfaces "i[e[ D39# whileothers may represent secondary erosional surfaces[ How!evertheywerecorrelated throughouttheareaanddelimitdistinct sedimentary bodies[ 3[1[ Seismic facies and their relationship Withinthedepositionalunits\mostoftheseismicfaciesconsist of progradational re~ections[ In addition\ di}er!ent types of chaotic facies may be distinguished\ that _lldeep erosional structures "{_lling| facies#[ Except near the Table 0Seismic characteristics\ geometry and depositional environments of the main unitsUnit Re~ection Geometry Lateral extend km Thickness Max Clinoforms dip Depositionalcon_guration width length ms m direction environment039 chaotic:h[a[c lobate 8 × 19  ½ 59  ½ 37 N29 to N049 Incised valley029s h[a[c[ lobate 6 × 05  ½ 24  ½ 17 N034 to N259 Deltaic lobe009 h[a[c[ elongated  × 07 ×  × 17  ½ 39  ½ 21 N019 and N064 Lowstand shoreface79 h[a[c[ elongated 05 ×  × 27  ½ 34  ½ 25 N019 Lowstand shoreface049s h[a[c[ elongated 17 ×  × 42  ½ 29  ½ 13 N029 to N069 Lowstand shoreface049m l[a[c[ not mapped * * * Prodelta(c deposits shelf break "Fig[ 2#\ very few aggradational patterns areobserved[Prograding clinoforms are either low true angle "lessthan 0># continuous re~ections "l[a[c[# in Fig[ 2\ Table 0and U049m in Fig[ 3#[ or high angle "more than 2>#oblique to sigmoidal clinoforms with less lateral con!tinuity "h[a[c[ in Fig[ 2\ Table 0\ U049s in Fig[ 3 and U79in Fig[ 4#[The lithologic composition of the upper part of the lowangle and high angle clinoforms may be assessed usingshallow cores "1Ð4 m long# sampled from the upper seis!mic unit[ The upper part of the high angle clinoformsin Unit 049s corresponds to medium sands "grain sizebetween 049 and 249  m m\ median at 179  m m# with muchshell debris and a carbonate content of about 14)#[ Alayer of coarse sand\ with large shell debris and pebbles\is present at a depth of 9[49Ð9[79 m "Fig[ 4#[ This coarseunit overlies sands similar to the upper deposits[ The lowangle clinoforms consist\ at least in the upper sampledpart "up to 4 m#\ of clayey silt "grain size between 2Ð39 m m\ median at 00  m m#\ with numerous\ thin\ inter!strati_ed _ne sand layers "Fig[ 5#[ Burrows and organicdebris are abundant[ Compilation of lithological\ faunal\andgeometrical characteristicsofthesetwo seismicfaciesled us to interpret the low angle prograding units ashaving been deposited in a medium to low energyenvironment*from lower shoreface to prodelta(csettings\ whereas the high angle prograding units rep!resent a high energy environment*upper to mid shore!face deposits[The transition from low angle clinoforms to high angleclinoforms in the seaward direction is everywhere abruptand underlain by an erosional discontinuity "e[g[ fromUnit 67 to Unit 79 in Fig[ 2 and from Unit 018 to 029sin Fig[ 5#\ the high angle clinoforms downlap on thissurface\ with angular or tangential contacts "Fig[ 4#[The transition from high angle to low angle clinoformsin the seaward direction can also be an erosional dis!continuity"e[g[unit049inFig[2#[However\aprogressivechange from high to low angle along one single re~ectionis sometimes observed\ without any discontinuitybetween the two types of clinoforms "Fig[ 5\ Unit 029#[  M[ Rabineau et al[:Marine and Petroleum Geolo`y 04 "0887# 328Ð341  332Fig[ 3[ "A# Uninterpreted and "B# interpreted portion of the very high resolution seismic pro_le P!910 showing the low angle "Unit 049m# and thehigh angle "Unit 049s# clinoforms and their lithological nature "respectively silt and sand# assessed thanks to some shallow cores[ A distinct seismicfacies is observed in Unit 099m\ which is possibly related to sediment waves[ Also note the fossil canyon head below Unit 59[ "Thickness in meter isapproximate and calculated considering a velocity in the sediments of 0599 m s −0 [# In this case the low angle clinoforms clearly represent thebottomset part of the prograding body[Wavy re~ections are very distinct within some of thelow angle clinoforms\ especially within Unit 099 "Figs 2\3#\ and to a lesser extent in Units 029 and 049 "Figs 2\ 3#[Their spacing is from 0199Ð1999 m[ The two possibleinterpretations of these facies are "a# sliding structuresrelated to retrogradational slumps or "b# sediment waves[This second interpretation is more plausible because of the regularity of these features and because they have noconnection with slides in a seaward direction[ On thecontrary\ the data show an attenuation of these featuresfrom the outer shelf toward the shelf break\ possiblybecauseofhighercurrentvelocitiesintheshallowestarea[
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