late quaternary evolution of gravel deposits in tromper ...journal of coastal research si 51 173-186...

Journal of Coastal Research SI 51 173-186 West Palm Beach, Florida Summer 2010

Late Quaternary Evolution o f Gravel Deposits in Tromper Wiek, South-western Baltic SeaValérie Bellec1*, Markus Diesing2**, and Klaus Schwarzer2

1 G h en t U niversity 2 C hristian-A lbrechts U niversitä t Kiel,R e n a rd C en tre o f M arine G eology Institu te o f G eosciences, Coastal an dKrijgslaan 281, s. 8 C o n tin en ta l S helf R esearchB-9000 G ent, B elgium O lshausenstrasse 40, 24118K iel. G erm any

* P resen t address: ** P resen t address:G eological Survey o f N orw ay C e n tre fo r E nv ironm en t, Fisheries a n dLeiv Eirikssons vei 39 A q u acu ltu re Science7040 T ro ndheim , N orw ay R em em b ran ce Avenue

B urnham -on-C rouch , Essex CMO 8BQ, U n ite d K ingdom

ABSTRACT

T h e Late Q u a te rn a ry h isto ry o f th e Baltic Sea is m ark ed by a com plex seq u en ce o f glacial, lacustrine a n d m arin e phases (late P leistocene , Baltic lee Lake, Yoldia Lake, Ancylus Lake, L itto rin a S ea ) . B oom er elata, acq u ired in O cto b er 2004, p e rm itte d to im prove th e know ledge o f th e late Q u a te rn a ry geological evo lu tion o f T ro m p er W iek, a semi- enclosecl bay, lo ca ted in th e n o rth -eas te rn p a r t o f R ügen Island. T h e sed im en ta ry deposits can be subdiv ided in 6 seism ic un its (U l to U 6). T h e u p p e r p a rt o f th e lowest u n it (U l) co rre sp o n d s to P leistocene till. C hannels incise th e to p o f th is till (su rface S2), p robab ly c rea ted d u rin g th e first d ra in ag e o f th e Baltic Sea d u r in g th e L ate Glacial. S u b seq u en t ch an n e l filling (U 2) o ccu rred in two phases b e g in n in g with chao tic deposits, p robab ly fluviatile o f origin, follow ed by g rad ed deposits. This filling was s to p p e d by an erosive p e r io d w ith th e fo rm a tio n of su rface S3, show ing ch annels a t th e sam e location as S2. T h e facies o f th e ch an n e l filling (U 3 a n d U 4), d u rin g a seco n d phase, is sim ilar to th e first one, b u t resem bles a p rogracling se d im e n t body, in te rca la ted betw een th e two un its in th e shallow er part. U 3 shows a bar-shaped deposit a t its top . T h e facies o f U 4 is very sim ilar to a b a rr ie r /b a c k -b a rr ie r facies sim ilar to th e facies o f un it U5, partly com posed o f gravel. T h e deposits o f U 6 co rre sp o n d to th e post-L itto rina Sea deposits.T h e p resen c e o f gravel is lin k ed to coastal cliffs, in w hich chalk layers, p u sh e d u p by glaciers, a lte rn a te w ith sections o f till a n d m eltw ater deposits a n d w ith sub m arin e ou tcrops o f till. Gravel deposits are p re se n t in u n it U5. T hey are strongly linked to th e p resen c e o f a barrier. F our o f th e six un its show a b a rrie r facies (U2, U3, U 4 a n d U 5); gravel deposits cou ld be p re se n t inside all o f these un its a n d w ould re p re se n t a la rger d ep o sit th a n estim ated previously.

ADDITIONAL INDEX WORDS: Baltic Sea, coastal evolution, barrier des’elopment, marine resources.

INTRODUCTION AND AIM OF THE STUDY

G ravel-dom inated coastal deposits occur in several places w here sed im ent supply an d wave energy favour th e accum ula­tion of coarse debris in th e litto ra l zone. The p resence of rocky cliffs, subm arine outcrops an d tectonic se ttin g (e.g. ra ised gravel beaches, associated w ith co-seism ic uplift, such as in New Z ealand (B erryman et al., 1992; W ellm a n , 1967), favour th ese deposits (D a v ies , 1972; O r fo r d , F o r b e s , and J e n n in g s , 2002). Moreover, th e re is a la titu d in a l control (>40° N and S) on th e common occurrence of g ravel deposits in con tinen ta l shelves and shore zones (D a v ies , 1972; H a yes, 1967), w hich correspond to perig lacial deposits (C h u rch and R y d er , 1972). On storm w ave-dom inated coasts gravel o rig inates m ainly from glacigenic deposits (C arter et al., 1987; F orbes an d T ay­lo r , 1987; F orbes and S yv itsk i, 1994).

The coastline of th e S ou thw estern B altic Sea (from D en­m ark v ia G erm any to Poland) consists of an a lte rn a tio n of

DOI: 10.2112 / SI51-016.1 received 14 January 2007; accepted inrevision 15 January 2008.

Pleistocene cliffs an d low lands, w here th e cliffs a re composed m ain ly of till, p a rtly of m e ltw a te r deposits or older m ateria l, pushed-up by advancing ice during th e la s t glaciations. Most of th ese cliffs a re u n d e r erosion w ith an average re tre a t of ap ­prox im ately 30 cm /year (S chw arzer , 2003).

E xploration an d exploitation of offshore m inera l resources have been carried ou t in th e form er G erm an D em ocratic R e­public since th e seventies (H a r ff et al., 2004; J ü r g e n s , 1999; L em k e et al., 1998; L e m k e , S chw arzer, and D ie s in g , 2002). E x trac tion h a s been carried out by m eans of anchor hopper d redging in dep ths of up to -9 m m ean sea level (msl). As a resu lt, th e sea bottom is covered w ith furrow s and p its w ith d iam eters be tw een 20 to 50 m and dep th s of up to 6 m below th e sea bed (D ie sin g , 2003; D iesin g et al., 2004; K l e in , 2003; K u b ic k i, Ma n so , an d D ie sin g , 2007; Ma n so et al, th is volume). O ur study a rea is s itu a te d in th e n o rth w es te rn p a r t of T rom p­e r W iek (F igure 1) w here th e seafloor is dom inated by gravel. The aim of th is p ap e r is to im prove th e know ledge of th e geo­logical se tting of th e study a rea and especially to u n d e rs tan d th e geological developm ent of gravel resources.

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174 Belle c, et al.

Norway

S k ag errakB ilhngc

mSw eden

NORTH

Denmark},o.

BALTIC SEAA rkona Basin ^

P om eranian

F ault(F r a n k e , 1993) Tromper - , P o lan d

Wiek

public

seismic profiles

(in Sc hw arzer al., 2000)

TROMPER WIEK

Lag Deposits

u . . A ' \

10G ravel

Fine S and

S I C o a rse S and S trip s

S B each Ridge

13.45° Longitude E (dec.)

Figure 1. Localisation of Tromper Wiek. A- General map; B- Localisation of the seismic profiles and geological interpretation of the sea bottom (modified from Schwarzer et al., 2000).

14C ky BP 14

30—1 Lm msl

20 -

10-

0-

- 10 -

- 2 0 -

-30_

- 4 0 -

- 5 0 -

13 12 11 10_l______I______ I______I_

M U iperm anent connection w ith the ocean

? /• /

_ _

Water-level used in this paper 9

9 - '

. relative w ater-level evolution in West Pom erania (Lam pe, 2005), dashed line is less w ell established

_ relative w ater-table evolution in the A rkona B asin according to Jensen (1995) and B ennike and Jensen (1996)

■ shoreline displacem ent curve o f Rügen 1 Island (Schum acher, 2002), the dottest line is

less w ell established

possib le connection w ith the ocean (Björck, 1995; Lam pe, 2005)

PLENI- LATE GLACIALM ecklenb- O L D E ST BO LLIN G b L LLEROD|Y O U N G E Rurg stage DRYAS | | d | | DRYAS

ICESHEET

ICEMARGINAL

LAKE

BALTIC ICE LAKE

PRE-BO REA L

HOLOCENEATLAN TIC I SU B BO REA L SU BATLA N TIQ U E

YOLDIASEA

ANCYLUSLAKE

MASTO­GLOIA

SEA

LITTORINA SEA AND POSTLITTORINA SEA

- 0ms tw t

--10

--20

--30

.-4 0

Figure 2. Water-level changes close to the study area (modified from Lampe, 2005). In grey: water-level, as used in this paper.

Jou rnal o f Coastal Research, Special Issue No. 51, 2010

European Marine Sand and Gravei Resources 175

Table 1. Baltic Sea stages (adapted from Lampe, 2005). The transgressive stages noted Rügen 1 to Rügen 7 are derived from Schumacher and Bayeri (1999). WL: water-level.

Baltic Sea stages D ate 14C (ky BP) Possible evolution of th e w ater-level EventsPost-Littorina 0

1.5 Rügen 73-2 Rügen 6 4-0 ky: only tectonic movements of minor importance

Rügen 5 (Uscinowicz, 2002)5.8 Abrupt regression 6-5 ky: regression phase? Neotectonic movements?6 Much slower rise

Littorina Sea Rügen 3 7-6 ky : temporary increase of the uplift7.3-7.2 Rügen 4 then WL fall of ~ lm Depth: -2 m (Janke and Lampe, 2000)

WL fall (from —6 to —7.5 m) Flooding of the Danish Strait — Depth: -15 m

7.8 Rapid rise (Lemke, 1998)

Ancylus Lake 7.8 Connection with the ocean8-7.3 WL rise (Rügen 2) 8-7 ky: decrease of the rate of uplift9.2-8.8 Sudden WL fall after 8.8 ky Regression (32 m below wl, Lemke et al., 1998)9.5 Rapid rise (Rügen 1)

Yoldia Lake 9.59.9 Temporary link to the ocean10.3

Baltic lee Lake 10.3 WL drop of 25 m Connection with open ocean-drain age. Start of the earlyWL rise Holocene incision phase on the mainland (Janke, 1978).

Late-glacial Lake transformed into a delta or river plain.Melt-water pulse (Fairbanks, 1989) - Subglacial drainage,

11.5 WL drop of 5-10 m channel incisions.12.5 14-11 ky: main uplift (Uscinowicz, 2002)13.5-13 WL rise Connection with the ocean

GEOLOGICAL HISTORY OF THE BALTIC SEA AND STUDY AREA

The B altic Sea is an alm ost non-tida l w a te rb o d y w ith only one narrow connection (Skagerrak) to th e N orth A tlan tic via th e N orth Sea. I ts h is to ry is controlled by isostacy, eu stasy and re su ltin g connections to th e N orth Sea, for d is tinc t p e ri­ods during th e L a te P leistocene and early Holocene (B jô e k , 1995). T herefore its evolution is m ark ed by lacu strin e and m a­rine phases, resu ltin g in four stages: B altic lee Lake, Yoldia Sea, Ancylus L ake and th e L itto rin a Sea (F igure 2, T able 1) (B.jô r c k , 1995; D u ph o r n et al., 1995; L a m pe , 2005). Below, a descrip tion of th e evolution of th e B altic Sea is given, w ith an indication of conventional radiocarbon years.

D uring th e pleniglacial (Figure 2), the w ater-level w as high in Pom erania, betw een 3 and 25 m above th e m ean sea level (Ja n k e , 2002b). Tiere, th e la te Pleistocene h isto ry of th e B al­tic Sea s ta rted w ith th e re tre a t of th e active ice from Rügen Is land and th e Pom eranian B ight around 14 ky BP (G ö e sd o e f and K a ise r , 2001; K ra m a rsk a , 1998; L a g e r lu n d et al., 1995; LTs- CINOW ICZ, 1999). W ith th e opening of several, probably subgla­cial drainage channels a t Mt. B illingen app. 11.2 ky BP, the w ater-level dropped to a t least —25 m m sl and extensive river erosion occurred. D uring the Y ounger Dryas, th e w ater level rose again from -40 to -20 m m sl (Lampe, 2005) leading to the full developm ent of th e B altic lee L ake (Table 1). The reopening of a drainage pathw ay a t Mt. Billingen, due to th e re tre a t of the Scandinavian ice sheet around 10.3 ky BP, caused a drop of the w ater tab le to about -40 m (B .jô rck , 1995). The early Holocene m arine incision phase Yoldia Sea s ta rted (Ja n k e , 1978), b u t due to rap id uplift of Scandinavia th e closure of th e connection to th e open ocean followed 9.5 ky BP (Table 1). The Ancylus Lake period began w ith a w ater level rise reaching a m axim um high-

stan d of —18 m m sl (La m pe , 2005; L em k e , 1998; L em k e et al., 1999), sim ilar to th e level of th e Baltic lee Lake. This h ighstand w as followed by a w ater level fall during th e second h a lf of the A ncylus L ake period. The first phase of th e following L itto rina Sea is m arked by a rap id w ater level rise betw een 7.8-6 ky BP. Since then , th e w ater level h ad fluctuated w ith in a range of a few m eters betw een —5 m m sl and th e p resen t w ater level (S chum acher and B ayerl, 1999). A fter 5 ky BP, th e w ater level alm ost reached its m odem position (Figure 2). W ater level low- stands occurred a t th e end of D ryas 1, a t th e Yoldia Sea stage and th e regression of th e Ancylus L ake (Table 1).

Rügen Is land w as reached by th e L itto rina transgression about 7.2 ky BP ( J a n k e , 2002; L ampe et al., 2002) and shows a strongly un d u la ting shoreline displacem ent curve w ith up to 17 regression and transg ression phases (S chum acher , 2002). This island, a form er archipelago com prising m ore th a n a dozen la rg ­e r and sm aller Pleistocene islands, connected by b a rr ie r and spit developm ent during the younger Holocene (D u ph o r n et al., 1995, J a n k e , 2002), is an uplifted a rea w ith ra te s of 0.24 nnn/yr for th e n o rth -easte rn p a r t (S chum acher , 2002). K o lp (1979) and D ietrich and L iebsch (2000) have shown th e presence of a h inge line of zero isostatic uplift, w hich stre tches from th e southern Zingst pen insu la to LTsedom Island, separa ting an uplifted (Rü­gen Island) from a subsiding a rea (southw estern Baltic Sea, e.g. bays of W ismar, Lübeck and Kiel). The strong regression b e ­tw een 6-5 ky BP h as been re la ted to an uplift of 6 m betw een 7-5 ky BP (S chum acher and B ayerl, 1999; S chwarzer, D iesin g , and T rieschm ann , 2000) and as a land upheaval on Rügen Is land of 8 m (S chum acher , 2002). The age of th is uplift fits to th e age of th e uplift of th e P om eran ian B ight shoreline around 5.8-5 ky BP ( J anke and L a m pe , 2000).

T rom per W iek is a sem i-enclosed bay located in th e n o rth ­ea s te rn p a r t of R ügen Is lan d betw een th e cliffs of W ittow and

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176 Belle c, et al.

Ja sm u n d (F igure 1). The cliffs are connected by a 12 km long Holocene b a rr ie r nam ed Schaabe, w hich developed a fte r th e L itto rin a T ransgression (D u ph o r n et al., 1995; S ch um a ch ­er and B ayerl, 1999). The cliffs, w ith a m axim um he igh t of 118 m a t Jasm und , are characterized by a com plicated p a t ­te rn of glacio-tectonically up lifted L a te C retaceous chalk and Pleistocene deposits subdividing th e chalk u n its (H er r ig and S ch n ic k , 1994). The chalk is soft and w eakly cem ented, in h e r­iting b lack flin t concretions ( J a n k e , 2002; S c h n ick , 2002). The w aters off Ja sm u n d and W ittow are characterized by a steep b a th y m etric g rad ien t w hich continues in th e no rth -w este rn p a r t of T rom per W iek w here th e w ate r dep th increases rap id ly from -12 to -18 m m sl (S teph a n et al., 1989).

The la te s t re su lt of sed im ent d is tribu tion p a tte rn s in th is bay (F igure IB) can be found in S chw arzer , D ie s in g , and T r i- esch m a n n (2000). L ag deposits occur in front of W ittow and Ja sm u n d cliffs. They s itu a te th e gravel deposit, w hich is lo­ca ted in front of W ittow cliff and Schaabe b a rr ie r betw een —8 and —14 m msl. This deposit shows p rom inen t morphological ridges composed of w ell-rounded pebbles and cobbles up to 25 cm in d iam eter. Shallow er th a n -10 m some till crops out. F ine san d is located in front of Schaabe sp it betw een —10 and —14 m msl. M uddy fine san d and sandy m ud occurs in deeper p a rts of T rom per Wiek.

J e n s e n (1992); J e n sen et al. (1997); L em ke et al. (1998); L em k e , S chwarzer, and D iesin g (2002) have identified five seis- m ostratig raphic u n its (E l to E5) in th e w estern B altic Sea and th e a rea around Rügen. An upperm ost till (E l) w as incised by la te glacial channels, probably filled w ith glacio-lacustrine sedi­m ents (E2) of the early Baltic lee L ake stage. A th ick sedim en­ta ry complex (E3) covered these deposits during th e final phase of th e B altic lee Lake. The boundary separa ting E2 and E3 cor­responds to a m ajor discontinuity. A t least in T rom per W iek E3 is subdivided into E 3a and E3b. E 3a corresponds to an associ­a ted beach ridge - lagoon system and E3b is in te rp re ted to be e ither of fluvial or coastal origin, deposited during th e low stand of th e Yoldia Sea. E4 w as deposited in th e deeper cen tra l p a rt of th e bay during th e final phase of th e Yoldia stage and in th e beginning phase of th e Ancylus Lake. The m axim um h ighstand of th e Ancylus L ake w as around -18 m msl. I t w as followed by a regression to about 30 m msl. LTnit E5 is a b rack ish m arine mud, w hich reflects recent sedim entation.

In th e shallow er p a r t of T rom per Wiek, an in n e r b asin is charac te rised by lagoonal deposits of E 3a w hich are covered by gravelly beach ridges. F u r th e r offshore, tow ards th e cen tra l p a r t of T rom per Wiek, th e till surface dips steeply, reach ing a level of m ore th a n —40 m m sl and delim iting an o u te r basin crea ted by form er ice (L e m k e , S chw arzer , an d D ie sin g , 2002).

METHODS

The Uniboom is an electro-acoustic sound converter p roduc­ing a b road frequency b an d of acoustic pu lses (0.5 to 15 kHz) em itted vertically into th e w a te r colum n (A tzler , 1995). The boom er acoustic source is m oun ted on a ca ta m aran an d tow ed beh in d th e ship. The sound signal is reflected from boundaries betw een d ifferent lay e rs /s tru c tu res w ith in th e sed im en tary sequence, consisting of d ifferent im pedances. Reflected sig­n a ls a re received by a stream er, add itionally tow ed beh in d th e ship, close b en ea th th e sea surface. T his signal is tu n e d in a receiver and tra n sfe rre d to analogue an d dig ita l acquisition un its . Processing of th e d ig ita l d a ta consists of b an d p ass filte r­

ing, stack ing and th e ad ju s tm en t of a tim e varied gain (TVG). V ery-high reso lu tion seism ic profiles are in te rp re ted accord­ing to se ism o-stra tig raphy princip les (P o sam entier et al., 1992; P o sa m en tier , J erv ey , and V a il , 1988; V ail et al., 1977; V an W agoner et al., 1988). O riginally developed for low resolution seismics, it can also be applied for h igh to very-high resolution seism ic (e. g. B ro w n e , 1994; C h io c c i, O rlando , and T ortora , 1991; C ir Ac et al., 1997; L er ic o la is , B e r n é , and F e n ie s , 2 0 0 1 ).

L im ita tio n s in th e q u a lity of th e seism ic profiles, due to b a d w ea th e r conditions d u ring th e surveys, com plicate p a r t ­ly th e in te rp re ta tio n of th e d a ta in to d ifferen t se ism o stra ti- g raph ic un its .

RESULTS

Several seismic u n its (U 1 to LT6) are p resen t on th e seismic profiles (Figures 3 to 7). They are bounded by h igh am plitude and often strongly erosive surfaces S2-S6. These u n its essen­tia lly correspond to th e filling of two basins. The first one, in th e shallow er p a rt of th e bay, would correspond to a lagoonal facies (L em k e , S chwarzer, and D iesin g , 2002) and is located be­tw een —13 to —20 m s tw t (app. —10 to —15 m msl) beh ind the gravel b a rr ie r (S chwarzer, D iesin g , and T rieschm ann , 2000) on our seismic profiles. Its m axim um depth is about 20 m s (app. -15 m) tw t (two-way trave l tim e) in our study area. The second b asin is situ a ted offshore deeper th a n —24 m s tw t (app. -18 in msl) (Figure 3) and is m arked by a steeply dipping surface. The th ickness of th e sedim ent fill is more or less 25 m s tw t (app. 20 in). C orrelation betw een th e two basin s w as achieved by com­paring th e seism ic facies and th e num ber of th e seismic un its above u n it U l, occurring in th e whole study area w ithout in te r­ruption. The u n it U l dips steeply offshore w here it delim its the offshore basin . The base of U l is not accessible. Its u pper p a rt constitu tes of indented reflections which form channels.

T he b a se of th e onshore (lagoonal) b a s in co rresponds to an u n ev en h ig h am p litu d e an d to low to good c o n tin u ity s u r ­face S2 w hich can be follow ed th ro u g h o u t th e w hole stu d y a re a (F igu res 3, 4 an d 5). S2 show s ch an n e ls r ig h t to th e off­shore b o u n d a ry of th e in n e r b a s in w here it a lm ost reach es th e sea bo ttom . In th is a rea , th re e ch an n e ls show a g en e ra l N W -SE s tr ik e a n d inc ise th e s u b s tra tu m dow n to 36 m s tw t (app. -27 in) (F igu re 5). T hey a re se p a ra te d by in te rflu v es sh a llo w er th a n 28 m s tw t (app. 21-22 in h). T he channels, w hich a lm ost d isa p p e a r a t th e b o u n d a ry be tw een th e two b asin s , a re filled by tw o d iffe ren t facies: th e f irs t one (LT2a) is chao tic a n d evolves u p w ard s in to a second facies (LT2b), w hich show s w avy p a ra l le l reflec tions (F igu re 4). T here is no c lea r reflection horizon v is ib le b e tw een th e se tw o seism ic facies.

Surface S3 show s sim ilar ch arac te ris tics as S2. I t is an uneven h igh am plitude and good continu ity surface showing channels. N evertheless, th e channels a re generally sm aller th a n th e previous ones. The firs t deposits filling th e channels (LT3a) a re composed of a chaotic facies w ith few para lle l reflec­tions on th e in terfluves. A nother type of deposits (U3b) is only located in th e w esternm ost area. I ts base is qu ite tab u la r. The seism ic facies corresponds to p rograd ing reflections. LT3b ra p ­idly p inches out offshore. LT3 shows a b a r-sh ap ed body in th is u p p er p a r t (F igure 6).

The am plitude of th e surface S4 is variable, b u t its conti­n u ity is good (Figure 4). I t erodes th e top of u n it LT3. LTnit LT4 corresponds to the las t filling of th e channels form ed by S2 and

Journal o f Coastal Research, Special Issue No. 51, 2010

European Marine Sand and Gravei Resources 177

54.66F igure

ïshore basin.

54.64-

: S tI)ffkhore basin

54.62- B oundar$ \betw een th e 2 b a sin s (up lift)

O n sh o re p in c K p u t o f th e o f fsh o m ^ a s in

13.42 13.44 13.46 13.48 13.5

s w ONSHORE BASIN OFFSHORE BASIN NE500 mU5 (barrie r)

Figure 3. Boomer profile showing the different seismic units. See details A and B on Figure 4.

SW (onshore) NE (offshore)channel

£§p*U 4i

«¿Ms*.

2 0 0 m

B SW (onshore) Berms NE (offshore)Barrier

30 • ' ¡ : î f e a w r l ^ Ç

Figure 4. Details of the seismic profile displayed in Figure 3.

Jou rnal o f Coastal Research, Special Issue No. 51, 2010

178 Belle c, et al.

b a r r ie r (U S )'

channelc h a n n e l ,

. 3 ° i

....

channelsource .

o n s h o r e 11 o f fs h o r e b a s in basfyi

c h a n n e l

LegendI I <18 m s tw t

I I 18-20

I 120-22 í . :;i 22-24

24-26

26-28

28-30

30-32

32-34

34-36

H >38 m stw t

fe*“*.1*,c h a n n e l

SCHAABE

Figure 5. Isochrons of surface S2 and details of the seismic profiles.

onlaps on S4. F u rth e r south th is u n it shows a complex p a tte rn of retrograding, prograding reflections and sm all channels w ith only a few milliseconds (twt) deep. An offshore bar-shaped body is p resen t betw een th e two basins, a t th e sam e place th a n th e U3 bar-shaped body. I ts facies is chaotic (Figure 6).

The u p p er surface S5 exhib its h igh am plitude and good continu ity an d can be followed alm ost th ro u g h o u t th e in n er b asin (F igures 3, 4 an d 7). I t show s NW -SE orien ted isochrons betw een 16 and 24 m s tw t (app. -12 and -18 m) and a sm all E-W orien ted channel of only a few m illiseconds deep. I t is cov­ered by th e h igh am plitude facies U5 w hich is composed of two un its: a b asin filling show ing chaotic facies (U5a), prograd ing and re trog rad ing reflections as well as channels of 3-4 m s tw t w hich a re very sim ilar to th e facies of U4, and a b a rrie r-sh ap ed body (U5b) w hich ends up th e facies offshore. The steep slope of th e ridges on th e b a rr ie r (F igure 4) is d irected tow ards th e coast and th e gentle slope tow ards th e sea. T his b a rr ie r facies

is located exclusively in th e shallow p a r t of th e bay w here it shows a th ickness of up to 6 m s tw t (app. 4,5 m). The th icker p a r ts a re located on th e b a rr ie r and in th e shallow est area. The u n it U6 is a th in layer (less th a n 1 m) of deposits, w hich is difficult to follow because it is m ixed w ith th e seafloor signal.

The seism ic u n its U2, U3 an d U4 reach th e position of th e ou ter ridges and U3 an d U4 p inch out a t th e end of th e ridge deposit. The th ick n ess of each u n it does no t exceed 8 m s tw t (app. 6 m), w ith a m ean a round 3-4_ms tw t (app. 2-3 m).

B etw een th e tw o basins, th e till deposit U l is bounded by th e surface S2, covered by U2 in th e n o rthw est (F igures 3, 4 and 6). The facies of th is u n it shows channel filling in th e n o rth (U2a, F igure 4). T ow ards th e sou theast, p rograd ing and re trog rad ing reflections form a dom e-shaped deposit (U2c) on S2 and som etim es cover th e channels form ed by th is surface. Sm all channels are also p re sen t in th e dom e-shaped deposit. U2c w as gently eroded by th e form ation of S3, except on its top

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European Marine Sand and Gravei Resources 179

B arrier (U 4 and U 5)

,B arrier (U 3)

m ultip le

m s tw t

Figure 6. Boomer profile showing the interfluve between the two basins.

500 m

Dome-shaped body (U2c)

Isochrons S5

channel Isopachs U5

barrierdeposits

prograding reflections retrograding'reflections

b a r r ie r

SC H A A B E

Figure 7. Isochrons and isopachs of S5 andU5 and seismic profiles details.

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180 Belle c, et al.

w here th e erosion w as stronger and probably younger th a n th e form ation of S3, as U3 deposits a re only p resen t on each side of th e dom e-shaped deposit (F igure 6). U2 an d U3 are cov­ered by a th in layer composed of th e younger u n it U6, w hich th ickens ju s t a t th e foot of th e b a rr ie r (app. 2-3 m thick), show ­ing p a ra lle l reflections a t th is location.

In to th e offshore b asin (F igure 3), th e uneven surface S2 also corresponds to th e base of th e basin , w hich dips offshore a round 30 m s tw t (app. 22 m). S2 shows channels of less th a n 5 m s tw t (app. 3,5 m). S2 is covered by U2, w hich p resen t sim i­la r seism ic facies th a n in th e onshore basin : th e bottom deposit is chaotic and evolves upw ards from w avy to m ore or less p a r ­allel reflections. S3 does no t correspond to an uneven surface in th e offshore basin . I t corresponds to a p la n a r surface w ith a m edium am plitude and continuity , locally d is tu rb ed by gas presence. U3 p resen ts h igh frequency p a ra lle l reflections. The u p p er surface S4 is quite ho rizon ta l w ith a h igh am plitude and continuity . I t shows sm all channels of 2-3 m s tw t deep (app. 1.5-2 m). The seism ic facies of u n it U4 also corresponds to h igh frequency para lle l reflections. I t is p a rtly difficult to d ifferen tia te U5 from U4 in th e offshore basin , as U5 could correspond to a p a r t of facies U4. U6 is composed of p ara lle l reflections and becom es th ick e r offshore, increasingly.

The th ickness of th e u n its is reg u la r an d corresponds to 8 m s tw t (app. 6,5 in) for U2, about 5-7 m s tw t (app. 4-5,5 in) for U3, 6-8 m s tw t (app. 5-6,5 in) for U4 (plus U5?) and 1 to 3 ms tw t (app. 1-2,5 ni) for U6.

DISCUSSION AND INTERPRETATION

History of the Western Part of Tromper WiekC o m p a r in g t h e r e s u l t s w i th p r e v io u s in v e s t ig a t io n s o n

la n d , (H o ffm a n n , L a m pe , andBAKNASCH, 2 0 0 5 ; S chum acher a n d

B a y e r l , 1999) and inside T rom per W iek (L em ke et al., 1998; L em ke, S c h w a rz e r , and D ie s in g , 2002; S c h w a rz e r , D ies in g , and T rie sc h m a n n , 2000), th e u p p er p a r t of th e u n it U l corre­sponds to Pleistocene till (un it E 1 in L em k e et al., 1998; L em ke, S c h w a rz e r , an d D ie s in g , 2002; Table 2). In fact, th ese au th o rs ind ica te th a t th e surface of th e upperm ost till is charac te rised by a h igh re lief and channel-like depressions w ith th e surface dipping steeply tow ards th e cen tra l p a r t of T rom per Wiek, as is th e case for surface S2.

A fter th e la s t glacial m axim um about 18.5 ky BP (L am b eck et al., 2000), du ring th e lee Sheet and th e lee M arg inal L ake periods, th e m elting of th e ice led to an opening of th e B altic Sea to rw ard s th e N orth Sea (Lam pe, 2005). The w ater-level dropped m ore th a n 40 m, w hich probably in itia ted th e for­m ation of th e channels bounded by S2 (F igures 8A and 8B). C hannels generally form during w ater-level fall, b u t can also be form ed due to m elt-w ate r pu lses or subglacial draining. T here w ere tw o im p o rtan t w ater-level falls due to th e d ra in ­age of th e B altic lee Lake. The first one (d rainage a t 13 ky BP, F igure 2) occurring in th e B altic Sea w as com bined w ith a m elt-w ate r pulse, as th e ice re tre a t on R ügen Is lan d is s i tu ­a ted about 14 ky BP ( G ö r s d o r f an d K a is e r , 2001; K ra m a r- ska , 1998; L a g e r lu n d et al., 1995; U sc in o w icz , 1999), and so could lead to th e form ation of th e m ore im p o rtan t channels, bounded by surface S2. B etw een about 13 and 10 ky BP, the w ater-level h a d been m ore or less stable, a round -20 to -25 m m sl (S c h u m a c h e r , 2002) or rose up to a few m eters on Rügen Is lan d considering th e w ater-level evolution in th e A rkona b a ­sin or in w est P om eran ia (B e n n ik e and J e n s e n , 1996; J e n s e n , 1995; L am pe, 2005). The th re e channels, p re sen t in F igure 4, incise from 26-28 m s tw t (19-21 m msl) down to m ore th a n 36 m s tw t (27 m msl). Therefore a w ater-level a round 20-25 m m sl probably allowed th e filling of th e channels (U2, F igure 8C). U n it U2 w as deposited in two steps du ring th e w ater-

T ab le 2. C o m p a riso n be tw een th e s e ism ic u n i ts o f L em k e e t al. (1998) a n d th o se in th is p a p er .

Lemke et a l 1998 This paper

Seismic units Sedim ent type Age Seismic un its Supposed AgeEl: Tik hyp: grey, partly clayey,

chalk fragmentsPleistocene U l Pleistocene

Channel surface Late glacial S2 (channel) ~13 ky (drainage)

E2: channel filling, glacio- lacustrine sequence

hyp : Silty to sandy material Early Baltic lee Lake stage U2 Baltic lee Lake

Major unconformity S3 (channel) ~11.5 ky (drainage)

E3 : E3a: Thick silt then olive grey,

fine laminated silt

Baltic lee Lake

Upper part: ~10.1-10.5 kyU3?U4?U5?

Baltic lee Lake to Yoldia Sea

E3b (below 35 m depth): fluvial or coastal

Silty fine sand From ~10.3 ky

U5?E4: fresh water lake deposit Below -34 m : grey silts Base: ~9.6 ky

Ancylus LakeU5 ? U6? Ancylus Lake

E5 Olive grey sandy mud Post-Littorina U6 Post-Littorina Sea

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European Marine Sand and Gravei Resources 181

c ky bp 14 12 10 8 6 4 2

m e te rm m sl

A- lee Marginal Lake- High WL and WL fall (> 40 nr) due to the 1 st drainage. End of the till deposits ( U l ) .

B- lee Marginal Lake- Low WL. Till erosion and fonnation of the 1st channels (S2). Beginning of channel filling and formation of the barrier beach (U2).

\

c- r

o15

30

m e te r

C-Baltic lee Lake- WL stagnation. .End of channel filling (U2). Formation of the barrier beach (U2).

D-Baltic lee Lake- 2nd drainage of the Baltic Sea (maximum fall ~20 m?). Formation of narrow channels onshore (S3). Channel filling during the WL rise (U3) then formation of barrier (back- barrier) deposits.

E- Yoldia Sea and Ancylus Lake- 3rd and last drainage. Formation of S4. Then rapid WL rise and formation of barrier and back­hander deposits (U4).

F- Ancylus Lake- WL stagnation. Erosion of U4. Formation of a ravinement surface (S5) and barrier and back-barrier deposits (U5).

G-Littorina Sea- Rapid WL rise. Uplift of 6-7 m. Preservation of U5. Deposition of U6.

TILL

TILLm eters

m m sl

TILL

TILL

TILLm m sl

m etersm m sl m s tw t

Figure 8. Deposition model of the 6 seismic units, interpreted from the boomer profiles . WL: waterdevel.

level stagnation a fte r 13 ky BP (Figure 8C). The low est u n it (LT2a) show s a chaotic facies, w hich m ight rep resen t th e final m elt w a te r deposits, composed of heterogeneous and/or coarse m ateria l. The u n it above (LT2b) shows a lte rn a tin g wavy b ed ­ded reflections, following th e underly ing relief. T his generally gives evidence of m ore hom ogeneous and/or finer deposits. T his wavy facies is very sim ilar to th e E2 facies m entioned in L e m k e , S chw ärzer , and D ie sin g (2002) w here th e au tho rs

also ind ica te th a t th e seism ic facies rep resen ts silty to sandy m ate ria l. They in te rp re t E2 as a g lacio-lacustrine sequence form ed im m ediately a fte r th e final deglaciation of th e area. In front of th e offshore basin , a dom e-shaped body (LT2c, F igure 6) seem s to correspond to a b a rr ie r beach, form ed during or af­te r th e filling of th e channels since th e ir deposition. I t should ind ica te a stab ilisa tion of th e w ater-level a round 20-25 m m sl for a du ra tio n w hich is sufficient to crea te th ese deposits. This

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Back-barrier Barrier

Direction of progradation A-WL stagnation -20-25 m depth. Formation of the ravinement surface S5 then deposition of the gravel marsh and the barrier (U5).

Back-barrier Barrier

ky bp 14 12 I Q S ^ 4 2 0

Direction of progradation B- Rapid WL rise. Preservation of a part of the old barrier which shows berms (landwards steep slope). Onshore formation of a new barrier and back-barrier system.

o ,0

-20 15

■ -4 0 30

r meters

C- WL rise before uplift (about 6000 years BP). WL ~2 m. Deposit of gravels and formation of barriers goes on. Gravels located between -14 m and -22 m depth.

E rosion ......................................BerTill outcrop

p 14 12 io a 6

S Uplift

D- Uplift of 6 m after 6000 years BP. Regression. Erosion of the sediment, till outcrops. Onshore gravel boundary: 8 m depth, offshore gravel boundary: 16 m depth.

- - . ■ - - ~ - - - ̂ -

■ 1 ■

E- Actual. Preservation of the gravel deposits.

J Post-gravel deposits ] Gravels

J Post-till deposits I Till

Figure 9. Model of gravel barrier deposits in Tromper Wiek. WL: waterdevel.

m eets th e w a te r level curves for R ügen Island, p resen ted by S c h um acher (2 0 0 2 ) and th e position a round 2 0 -2 5 m depth of th e channels (S2). In th e offshore basin , th e in itia l filling of th e channels consists of sim ilar facies.

The form ation of th e second set of channels, located in th e onshore basin (surface S3, F igure 8D), should be due to th e second im p o rtan t d ra inage of th e B altic lee L ake a round 11 ky

BP p re sen t on th e w ater-level curves of th e A rkona b asin or th e w est P om eran ia (B e n n ik e a n d J e n s e n , 1996; J e n s e n , 1995; L a m pe , 2005) (F igure 2). W hen form ing th is surface S3, a p a r t of th e beach b a rrie r, located offshore, w as eroded. S3 seem s to correspond to th e boundary betw een th e u n its E2 and E3 w hich is m en tioned in L e m k e , S chw arzer , and D iesin g (2002); S chw arzer , D ie sin g , and T r iesch m a n n (2000). Moreover, these

European Marine Sand and Gravei Resources 183

au th o rs ind icate th a t E3, corresponding to th e seism ic u n it of U3, w as deposited p rio r to 10.3 ky BP.

M ore in detail, th e low erm ost deposits of U3 in th e ch an ­n e ls look sim ilar to th e facies of U2, i.e. heterogeneous and/ or coarse deposits (U3a). The facies above is d ifferent and cor­responds to a sm all p rograd ing deposit (U3b). N orm ally, th is ind icates sed im ent in p u t du ring a constan t w ater-level. As it is only located in th e w est p a r t of th e profile (F igures 3 and 4), it could be form ed due to local conditions, e.g. p rog radation of th e channel wall. U3 deposits do no t show a large beach b a rr ie r facies as U2c. N evertheless, a deposit sim ilar to a b a rr ie r is p re sen t dow nstream of U2c (F igure 6). L e m k e , S chw ärzer , and D iesin g (2002); S chw arzer , D ie sin g , an d T riesch m a n n (2000) also ind ica te th a t th is u n it is co rre la ted w ith a barrier-lagoon system in th e cen tra l p a r t of T rom per Wiek, w here th e b a r ­rie rs are composed of gravel. A b a rr ie r is also p re sen t in our study area. So th e barrier-lagoon system probably extended tow ards th e no rth . N evertheless, in our study a rea th e gravel deposits do no t correspond to LT3, b u t to th e younger u n it (LT5). T herefore th e re should have been several periods of gravel deposition w ith a sh ift of th e cen tre of deposition tow ards th e no rth .

A th ird erosive surface is ind ica ted by S4. T his surface could have been form ed during th e la s t d ra inage a fte r 10.3 ky BP. O nlaps, w hich are ch a rac te ris tic for a tran sg ressiv e facies, a re p resen t on S4 (F igures 4 and 8E). As such, LT4 is a t r a n s ­gressive facies w hich should have been form ed during th e first p a r t of th e w ater-level rise about 10-9.5 ky BP. The channel- filling con tinued as well, as also th e construction of th e b a rr ie r w hich is a lready p resen t in LT3. A back-barrier/lagoon facies is p re sen t also (F igure 6).

A bout 9.5 ky BP, th e speed of th e w ater-level rise slowed down and rem ained stab le a t a level of 23-25 m msl, w hich is about 10 m below th e back -barrie r system . N evertheless, if we consider an up lift of 6 m afte r 7 ky BP, th e n th e gravel deposits w ould have been located a round 16-20 m msl, w hich w as th e dep th of th e shoreface 9.5 ky BP ago (Figure 9A). The shape of th e gravel confirm s th is fact, as observations by scuba divers revealed th a t th ese ridges are composed of w ell-rounded peb ­b les an d cobbles of up to 25 cm in d iam eter (S chw arzer , D ie s ­in g , an d T riesch m a n n , 2000).

The w ater-level s tagna tion m ay have favoured th e fo rm a­tion of a wave erosion surface (S5) a t th e top of LT4 (F igures 8F and 9A). D ue to stab le w ater-level conditions du ring sev­era l centuries, a b arrie r, la rg e r th a n th e form er ones, h ad been developed. B ehind th is b a rrie r, a facies sim ilar to th e LT4 facies h a s been deposited. The LT5 deposit is o rien ted p a ra l­lel to th e b a rr ie r (F igure 7). LT5 w ould correspond to a b a rr ie r an d a back -barrie r facies w ith channels a lte rn a tin g in terfluves show ed by re trog rad ing or p rograd ing reflections.

The nex t w ater-level rise, a fte r 9 ky BP, w as likely re la ­tively fast. D ue to th e very coarse m ateria l, th e gravel deposits w ere p reserved partly . N evertheless, th e b a rr ie r w as probably in p a r t eroded due to its shallow w a te r location, an d its m or­phology evolved in a berm system (steep slope sh ifted la n d ­w ards; F igure 9B). The deposition of gravel probably decreased quickly w ith increasing w ater depth. No gravel is p re sen t on th e ac tua l coast, below -15 m msl. New system s of b a rr ie rs an d berm s m ight have form ed on th e gravel deposits (Figure 9C). P rio r to th e up lift approxim ately 5-7 ky BP, th e gravel deposits w ere probably located in 22 m w ater-dep th . The onshore boundary is m ore difficult to estab lish , but, by com­parison w ith th e ac tu a l depth, it w as likely in approxim ately

15 m w ater-dep th . LTplift ra ised R ügen Is lan d w ith about 6 m (S c h um acher and B ayerl , 1999; S chw arzer , D ie sin g , and T r ie ­sch m ann , 2000). T hen th e gravel deposits w ere located betw een 8 and 16 m msl, w hich is th e ac tua l dep th (F igure 9D an d E). D ue to th e very coarse g ranu lom etry of th e b a rr ie r sedim ents, it w as m ostly preserved.

The la s t un it, LT6, is a th in cover of fine sand, w hich rep re ­sen ts th e ac tu a l sed im en ta tion onshore. Offshore, it can be sub­divided in several sub-units, w hich have probably recorded th e oscillations of th e w ater-level since 9000 years BP (F igure 8G). T his u n it probably corresponds to th e u n it E5 of S chw arzer, D ie sin g , and T riesch m a n n (2000), w hich shows typ ical deposits of th e post-L itto rina b rack ish m arine B altic Sea.

Gravel deposits: Barrier and back-barrier faciesWe found a b a rr ie r facies in four of th e six u n its (LT2, LT3, LT4

and LT5), generally in th e ir u p p er part/top . In our study area, a back-barrier/lagoon facies is p re sen t in two u n its (LT4 and U5) and o ther investiga tions (L e m k e , S chw arzer , and D ie sin g , 2002; S chw arzer , D ie sin g , and T riesch m a n n , 2000) show ed th e presence of a lagoon in E3/LT3 in th e cen tra l p a r t of T rom per Wiek. Two of th ese u n its crop ou t on th e sea floor (LT3 in th e cen tra l p a r t of T rom per W iek and LT5 in our study area) show ­ing b a rr ie rs composed of gravely sed im ents w ith a no rthw est- so u th east o rien ta tion for LT3 and a n o rth east-so u th w est o rien ­ta tio n for LT5.

The w ith gravel bu ilt-up u n it LT5 is in tensively dredged, show ing p its of up to a few m deep (D ie sin g , 2003) (F igure 7). The th ickness of th e gravel u n it (LT5) reaches 6 m s tw t on our seism ic profiles (about 5 m thick) (F igure 7). LT5 sp reads over m ore th a n 3500 m in a NE-SW direction an d from about 300 m (in th e north ) to m ore th a n 1000 m (in th e south) in a NW -SE direction.

I t is possible th a t each of th ese u n its (LT2, LT3, LT4 and LT5) shows gravel deposits on th e ir u p p e r part, especially in th e b a rr ie r facies. T h a t m eans th a t th e to ta l g ravel deposit is p rob­ably m uch m ore sp read th a n th e gravel deposits on th e sea bottom shows.

G enerally, th e re a re two sources of gravel: th e seafloor itse lf and th e erosion of th e cliffs (An th o n y , 2002; C aviola , 1997; J o h n so n , 1919; O rfo r d , F o r bes, and J e n n in g s , 2002; R eg n a u ld , M auz, an d M o rzadec-K e r fo u r n , 2003; S c h ro ttk e , 2001). The gravel deposits form ed w hen th e w ater-level w as about 15-20 m msl, considering th e up lift of about 15-10 m m odern msl. Moreover, th e b a rr ie r deposit b u ilt du ring quite h igh and s ta ­ble w ater-levels. If th e seafloor w as th e only source of th e grav ­el, we should find gravel deposits in th e ou ter basin ; th is is not th e case. So, th e m ost probable source of th e gravel deposits is th e erosion of W ittow cliff, composed of chalk, m eltw a te r sedi­m ents, bou lder an d clay, p re sen t close to our study area. This erosion is only possible w hen th e w ater-level w as about 15-20 m msl. Following, th e cliff w as eroded by wave and cu rren t action and supplied th e gravel needed for th e form ation of th e gravel deposits.

CONCLUSIONS

Six u n its (LU to LT6) have been identified in th e w estern p a r t of T rom per W iek and are bounded by 5 surfaces (S2 to S6). LU is a ttr ib u te d to th e presence of Pleistocene till; its u p ­p e r p a r t w as eroded by th e form ation of channels (S2), p roba­bly re la ted to th e w ater-level drop during th e L a te G lacial. The

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184 Belle c, et al.

filling of th e se channels (U2) began by fluviatile sedim ents, w hich w ere la te r rep laced w ith finer a n d hom ogeneous sedi­m ents. U2 show s a beach b a rr ie r facies deposited during or a fte r th e filling of th e channel. The location of th is b a r r ie r is sligh tly offshore of th e m odern b a rr ie r . The firs t reac tiva tion of th e channels (S3) occurred probably during th e B altic lee L ake about 11 ky BP. T he ir filling (U3) is very s im ila r to U2. A t th e top of U3, a b a rr ie r is found a t m ore or less th e sam e position as th e ac tu a l ba rr ie r . O utside of th e study area, investiga tions have show n th a t U3 is p a rt ly com posed of g ravel b a rr ie rs . S4 corresponds to th e la s t reac tiva tion of th e channels form ed by S2 an d could h ave been form ed about 10 ky BP. The filling of th e channels (U4) occurred during th e Yoldia Sea an d th e A ncylus L ake. I t is sligh tly d ifferen t as it show s tr a n s gres sive deposits a n d a b a r r ie r an d b ack -b a rrie r facies. S 5 w ould be an erosional surface caused by w aves as th e w ater-level rise speed decreased. U5 also show s a b a rr ie r a n d a b ack -b a rrie r facies s im ila r to th e U 4 facies. T his u n it is now adays dredged in te n ­sively, because of its h igh gravel content. D ue to th e up lift of 6 m about 5-7 ky BP, th e gravel deposited orig inally a round 15- 20 m m sl depth, w hich a re th e m ean dep th of th e h igh w ater- level betw een 9 an d 13 ky BP, a re now a t about 10-15 m msl. U6 w ould correspond to th e la s t L itto rin a deposits.

F o u r of th e six u n its (U2, U3, U 4 an d U5) show b a rr ie r deposits. The tw o u n its (U3 a n d U5), outcropping on th e sea bottom , an d show ing a b a rr ie r , a re com posed of gravelly sedi­m ents. I t is possible th a t th e tw o o th e rs u n its (U2 an d U4), show ing a b a rrie r, a re also composed, a t le a s t p a rt ly of gravel. T he gravel cam e from th e erosion of th e W ittow cliff du ring periods of h igh w ater-level. The volum e of g ravel resources can be m ore im p o rtan t th a n e s tim a ted before.

ACKNOW LEDGEM ENTS

T his study fram es in to th e research objectives of th e project EUM ARSAND (“E uropean M arine S an d a n d G ravel R esourc­es: E va lua tion an d E n v ironm en ta l Im pact of E x trac tion”, con­tra c t HPRN-CT-2002-00222). The cap ta in an d th e crew of th e R.V. A lko r a re th a n k e d for th e ir flexibility a n d ass is tance d u r­ing th e cam paigns.

LITERATURE CITED

A nthony, E. J., 2002. Long-term marine bedload segregation, and sandy versus gravelly Holocene shorelines in the eastern English Channel. Marine Geology, 187(3-4), 221-234.

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