Sedimentary Geology 271-272 (2012) 28–43

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Sedimentary Geology

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Shallow-water marl–limestone alternations in the Late Jurassic of western France:Cycles, storm event deposits or both?

Claude Colombié a,b,⁎, Johann Schnyder c, Damien Carcel b

a CNRS UMR 5276 LGLTPE, Université Lyon 1, La Doua, bâtiment Géode, 69622 Villeurbanne cedex, Franceb CNRS UMR 5125 PEPS, Université Lyon 1, La Doua, bâtiment Géode, 69622 Villeurbanne cedex, Francec Université Pierre et Marie Curie-Paris 6, case 117, 4 place Jussieu, 75252 Paris cedex 05, France

⁎ Corresponding author at: CNRS UMR 5276 LGLTPEbâtiment Géode, 69622 Villeurbanne cedex, France.

E-mail address: claude.colombie@univ-lyon1.fr (C. C

0037-0738/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.sedgeo.2012.05.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 January 2012Received in revised form 27 April 2012Accepted 23 May 2012Available online 29 May 2012

Editor: G.J. Weltje

Keywords:Late JurassicWestern FranceInner mixed siliciclastic-carbonate rampMarl–limestone alternationMilankovitch cycleStorm event deposit

The contribution of event deposits to various basin fills can be very significant, higher than 90% in some cases.Events may lead to the formation of marl–limestone alternations, which can also result from cyclic changes insea level or climate, for example. The marl–limestone alternations of the Late Jurassic of western Francecontain abundant coarse-grained accumulations that resemble storm deposits described in other westernEuropean successions. The detailed analysis of facies evolution and hierarchical, high-frequency stacking pat-tern of depositional sequences of the Phare de Chassiron section (Ile d'Oléron, western France) allows thecontrols on marl–limestone formation to be defined. This section contains nearshore and shallow-marinemud deposits that were exposed to high-energy events. Elementary, small-, and medium-scale depositionalsequences are defined. The stacking-pattern and the duration of these sequences suggest an orbital controlon sedimentation. Precession (20 ka) cycles notably controlled the formation of elementary sequences thatcorrespond to marl–limestone alternations. The deposition of marly or carbonate mud occurred in thisstorm-dominated system because of muddy sea beds, the gentle slope of the shelf, and the great amount ofparticles in suspension, which reduced water energy resulting from storms. Sediment supply was also suffi-cient to limit bioturbation and favour the preservation of numerous storm deposits. The production of car-bonate mud was localised on positive structures and partly controlled by Milankovitch-scale sea-levelcycles. Transport by storms of carbonate mud to the adjacent marly depressions during high carbonateproduction periods led to the formation of calcareous beds. Marl–limestone alternations in the Late Jurassicof western France therefore result from the combined effects of cyclic changes in carbonate production andhigh-energy, episodic events.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The origin of marl–limestone alternations is still in debate (see,e.g., Westphal et al., 2008a; and references therein). Marl–limestonealternations can reflect sedimentary rhythms (i.e., resulting from en-vironmental cyclicity), diagenetic overprint or both (see, e.g., Einsele,1982; Einsele and Ricken, 1991; Westphal et al., 2010). They can be adirect response to periodic fluctuations in carbonate productivityduring steady deposition of clay, in the supply of terrigenous materialduring steady carbonate productivity, in the input of carbonatemud from adjacent shallow-water carbonate factories, in carbonatedissolution, and in the redox conditions at the sea floor, which allresult from cyclic changes in environmental conditions (see, e.g.,Westphal, 2006; Westphal et al., 2010; and references therein).There is, however, abundant evidence for post-depositional alter-ations that augment these primary differences, modify them or even

, Université Lyon 1, La Doua,

olombié).

rights reserved.

create new diagenetic rhythms (see, e.g., Hallam, 1986; Munneckeand Samtleben, 1996; Munnecke, 1997; Westphal et al., 2004,2008b, 2010; Biernacka et al., 2005; Westphal, 2006; Bádenas et al.,2009). During early diagenesis, dissolution and cementation of calcar-eous sediments lead to redistribution of calcium carbonate that gen-erates systematic differences between limestone beds and interbeds(see, e.g., Bathurst, 1971; Ricken, 1986; Munnecke and Samtleben,1996; Westphal et al., 2000, 2010). Marl–limestone alternationsform mostly in pelagic to hemipelagic marine environments but alsooccur in shallow-marine environments (see, e.g., Einsele and Ricken,1991; Munnecke and Samtleben, 1996; Munnecke et al., 1997).They can reflect cyclic variations in allocyclic (i.e., changes in eustaticsea level, climate, etc.) but also in autocyclic processes, which play aprominent role in shallow waters (Westphal et al., 2010). In particu-lar, the role of storms in the formation of marl–limestone alternationsis discussed in various publications (see, e.g., Biernacka et al., 2005;Bádenas et al., 2009). These autocyclic sequences form within abasin or part of it and usually show only limited lateral continuity(Einsele et al., 1991, 1996). Stratigraphic correlation over long dis-tances or from one basin to another allows these autocyclic sequences

N

la Brée-les-BainsD 273

D 734

Pnte de Chassiron

Pnte des Trois Pierres

Pnte des Boulassiers

1 km

Main road

Phare de Chassiron

Studied section

Secondary road

Main village

Pnte de Prouard

St-Denis--d'Oléron

FRANCE

Bordeaux

La Rochelle

20 km

Atlantic Ocean

Ile d'Oléron

Ile de Ré

45°

0°

Fig. 1. Geographical location of the studied section (from IGN topographic map 1 : 100 000 “La Rochelle-Royan” no. 39). The base of the section is located at 46°02′48″N 1°24′46″W;the top, at 46°02′47″N 1°24′10″W.

29C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

to be distinguished from allocyclic sequences, which are mainlycaused by external variations to the basin. Lateral continuity isalso useful for differentiating between primary and diagenetic calcar-eous rhythmites (Westphal, 2006; Westphal et al., 2010). In mostcases, diagenetic, allocyclic, and autocyclic processes combine and re-sult in a sedimentary record that is difficult to decipher (Einsele,1982; Einsele et al., 1991; Strasser, 1991; Einsele et al., 1996;Westphal, 2006; Westphal et al., 2010).

The Late Jurassic (Late Kimmeridgian to Early Tithonian) of thePhare de Chassiron outcrop section at Ile d'Oléron (off the coast ofwestern France, Fig. 1) contains four distinct lithological parts (frompart 1 to part 4), which are several tens metres in thickness andconsist of marl–limestone alternations (Schnyder, 2003) (Figs. 2; 3).Parts 1 and 3 are more calcareous than parts 2 and 4 where marlydeposits dominate. These changes in lithology are associated withlong-term (i.e., third- or second-order) changes in eustatic sea leveland may be climatically-driven (Schnyder et al., accepted). ThePhare de Chassiron section also includes many coarse-grained accu-mulations, which are randomly superimposed on marl–limestone al-ternations (Figs. 2; 3), and interpreted as storm deposits (Schnyderet al., accepted). Chassiron is therefore a prime locality to studythe combined effects of episodic, storm-related processes and cycliccontrols (i.e., eustatic sea level and climate) on marl–limestonealternation formation. Nevertheless, this requires the determinationof 1) the origin of marl–limestone alternations (i.e., diagenetic or pri-mary), 2) depositional conditions of alternations in case of primaryorigin, and 3) controls on alternation formation. On the basis of verydetailed facies and microfacies analyses and sedimentological inter-pretation, an accurate facies model is established. This then allowsthe exact recognition of cyclic patterns in the sedimentary recordwhich, in turn, are interpreted in terms of a high-resolution sequenceand cyclostratigraphical framework. Conclusively, a conceptionalmodel is proposed that describes the role of episodic versus cyclic de-position in the formation of marl–limestone alternations.

2. Geological setting

The Phare de Chassiron section is located north of Oléron Island inwestern France (Fig. 1). The outcrop is a cliff that is a fewmetres high,which faces the sea and was recently partially covered by seawalls.Ammonites (Hantzpergue, 1989; Hantzpergue et al., 2004),

brachiopods (Hantzpergue et al., 2004), ostracods (Donze, 1960;Malz, 1966), foraminifera (Bousquet, 1967), calcareous nannofossils,and dinoflagellate cysts (Schnyder et al., accepted) indicate affinitiesto the sub-boreal faunal realm and a Late Kimmeridgian to EarlyTithonian age (from the Autissiodorensis Zone to the Albani Zone).Hantzpergue et al. (2004) indicated that the boundary between theAutissiodorensis and the Gigas Zones (=Kimmeridgian–Tithonianboundary) is located ca. 9.5 m below the conglomerate bed (bed141), which corresponds to the base of lithological part 2 (Fig. 2). Os-tracods, discovered in beds 155, 162–163, 174–175 base, 176–179base, 176–179 top, and 180, are characteristic of the HudlestoniZone (Wilkinson, 1983; Wilkinson et al., 1997; Schnyder, 2003;Hantzpergue et al., 2004) (Fig. 2). Calcareous nannofossil Tubirhabduspatulus (beds 182–184) does not occur above the top of thePectinatus Zone or the base of the Pallasioides Zone (Bown, 1998;Schnyder et al., accepted) (Fig. 2). Dinoflagellate cysts, discovered be-tween beds 182–184 and 192, are not younger than the PectinatusZone (Schnyder et al., accepted) (Fig. 2). The co-occurrence of dino-flagellate cysts Dichadogonyaulax culmula (Early Tithonian, AlbaniZone, to Berriasian) and Rhynchodiniopsis cladophora (Kimmeridgianto Early Tithonian, Fittoni Zone) in bed 204 indicates that thetop of the studied interval is not younger than the Albani Zone(Schnyder et al., accepted) (Fig. 2). Bousquet (1967) and Schnyderet al. (accepted) gave a detailed sedimentological description of thePhare de Chassiron section. Schnyder (2003) defined four distinctlithological parts (from part 1 to part 4), which are several tens of me-tres in thickness and contain marl–limestone alternations. Parts 1 and3 are rather composed of bed-dominated alternations while interbed-dominated alternations dominate in parts 2 and 4. Sedimentaryfeatures such as dinosaur footprints described by de Lapparent andOulmi (1964), microbial mats, bird's eyes, desiccation cracks, pholadeborings, and serpulid bioherms indicate nearshore to shallow-marineenvironments (Bousquet, 1967; Schnyder, 2003; Schnyder et al.,accepted). These environments formed the La Rochelle shelf thatwas located north of the Aquitaine basin (Fig. 4). This basin wasopen to the west (to the Atlantic seaway), and closed to the northand to the east by the Armorican massif and Massif Central, respec-tively (Fig. 4a). On the La Rochelle shelf, NW–SE-trending faultsbounded tilted blocks, the uplifted part of which formed local topo-graphic highs where coral reefs developed (Hantzpergue, 1985;Olivier et al., 2008) (Fig. 4b). During the Late Jurassic, the La Rochelle

LATE KIMMERIDGIAN(Autissiodorensis Zone, Hantzpergue et al. 2004)

EARLY TITHONIAN(Gigas Zone, Hantzpergue et al. 2004)

?

Schnyder (2003) Schnyder (2003) Schnyder (2003)

AB

CD

EF

GH

IJ

Coarse-grainedaccumulations

Grain abundances

grey filling indicates incom

pleteobservation

Bed numbers1

mM

WP

G

0 5 10 15 20 25 30

Stages

OystersGastropods

BivalvesSerpulidsRhaxesForaminiferaEchinodermsIntraclastsPeloids

DolomiteGypsumOoids

QuartzMuscovite

PhosphateGlauconite

Ostracods

Metres

Textures andgrains

Quartz sizes

1 from B

ousquet (1967)2 from

Schnyder (2003)

12

34

56

78

910

11

Limestone microfaciesand environments

More proximal

More distal

vff

mc

vc

HS

DMFD? MFD?

MF

D

MF

DTD?LSD?HSD?

TD

TD

LSD

HS

D

LSD

TS

TS

Elementary sequencesSmall-scale sequencesMedium-scale sequences

Third-order sub-boreal sequence boundaries(Hardenbol et al. 1998, Ogg et al. 2008)

Ti 1

Ti 2

?

?

SB

Z

SB

Z SB

Z

450

350

250

150

500

0

1

0.5

450

350

250

150

50

0

0

1

0.5

Alternation

thickness (cm)

Ratioa/b

a: the most calcareous bed thickness

b: alternation thickness

Fieldobservations2

microbial m

ats, fenestrae,desiccation cracks

grey-blue marl

abundant pyrite

abundant pyrite

pyrite

pyrite

abundant pyrite

wood debris

wood debris

wood debris

fenestrae

fenestrae

fenestrae

wood debris

conglomerate

dinosaur footprints(Lapparent and O

ulmi, 1964)

13-22a

2523

26-29

30-33

34

35-37

38-46

120?

121?122?

123-126?

127-128

129-134

135

136-137

138

140

141

142a

142b-144

139?

47-54

55 74

75-83

84-88

92-119

108-112?

56-62

63-67

68-73

PART 1 (Schnyder 2003, Fig. II-15)more marine environments and more humid climate

PART 2 (Schnyder 2003, Fig. II-15)more continental environments and more arid climate

Fig.2.ThePhare

deChassiron

section.Grey

fillingscorrespond

tothe

coarse-grainedaccum

ulationsfor

which

theattribution

toaspecific

geometry

istricky

dueto

incomplete

observations.Ratioa/b

reflects

thealternation

lithology.ais

thethickness

ofthe

most

calcareousbed

inthe

alternation,andb,the

thicknessof

thealternation.

30C.Colom

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al./Sedim

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271-272(2012)

28–43

EARLY TITHONIANHudlestoni Zone? (Schnyder 2003) ante-Pectinatus Zone (Schnyder 2003)

Schnyder (2003)Schnyder (2003) Schnyder (2003)

145-146

147-148

149

150

151-152

155

156157158

159

160-161

162-163

164-165

167

168

166

169

170

171172-173174-175

176-179

180

181

182-184185

186-189

190

191

144

153

154

AB

CD

EF

GH

IJ

Coarse-grainedaccumulations

Bed numbers1

Fieldobservations2

mM

WP

G

Quartz sizes

Metres

Textures andgrains

1 from B

ousquet 19672 from

Schnyder 2003

OystersGastropods

BivalvesSerpulidsRhaxesForaminiferaEchinodermsIntraclastsPeloidsOoidsGypsumDolomiteMuscoviteQuartzPhosphateGlauconite

Ostracods

Stages

1vf

fm

cvc

23

45

67

89

1011

Limestone microfaciesand environments

More proximal

More distal

30 35 40 45 50 55 60

brown and lam

inated marl

black-blue marl

greenish marl

greenish marl

blue marl

black-blue marl

blue marl

grey marl

grey marl

pebbles, brachiopods,oysters

charophyte oogonia (Bousquet 1967)

wood debris

greenish marl

MF

D

MF

D

MF

D

MF

D

HS

D

HS

D

HS

D

TD

TD

TD

TD

HS

D

HS

D

LSD

LSD

LSD

LSD

LSD

TS

TS

TS

TS

ElementarysequencesSmall-scale sequencesMedium-scale sequences

450

350

250

150

500

0

1

0.5

450

350

250

150

500

0

1

0.5

Grain abundances

Alternation

thickness (cm)

Ratioa/b

a: the most calcareous bed thickness

b: alternation thickness

Third-order sub-boreal sequence boundaries(Hardenbol et al. 1998, Ogg et al. 2008)

Ti 3

Ti 4

PART 2 (Schnyder 2003, Fig. II-15)more continental environments and more arid climate

PART 3 (Schnyder 2003, Fig. II-15)more marine environments and more humid climate

grey filling indicates incom

plete observation

Fig.2(continued).

31C.Colom

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EA

RLY

TIT

HO

NIA

N

192

?

193

194

195

196197

198199200

201

191

202

203a-203c

204

A B C DE FGH I J

Coa

rse-

grai

ned

accu

mul

atio

ns

Bed

num

bers

1

Fie

ldob

serv

atio

ns2

Grain abundances

mMWP G

Qua

rtz

size

s

Met

res

Tex

ture

s an

dgr

ains

1 from Bousquet (1967)2 from Schnyder (2003)

Oys

ters

Gas

trop

ods

Biv

alve

sS

erpu

lids

Rha

xes

For

amin

ifera

Ech

inod

erm

sIn

trac

last

sP

eloi

dsO

oids

Gyp

sum

Dol

omite

Mus

covi

teQ

uart

zP

hosp

hate

Gla

ucon

ite

Ost

raco

ds

Sta

ges

Am

mon

ite z

ones

(Sch

nyde

r 20

03)

1vf vcf m c 2 3 4 5 6 7 8 9 10 11

Lim

esto

ne m

icro

faci

esan

d en

viro

nmen

ts

Mor

e pr

oxim

al

Mor

e di

stal

60

65

70

SCS andserpulid bioherms

black marl

black marl

pyrite

grey-greenish marl

gypsumpseudomorphs

grey with coal debris

wood debris

MFD

MFD

TD

TD

HSD

HSD

LSD

LSD

Ratioa/b

Alternationthickness (cm)

a: the most calcareous bed thicknessb: alternation thickness

450

350

250

150

500 0 1

0.5

450

350

250

150

500 0 10.5

Ele

men

tary

seq

uenc

esS

mal

l-sca

le s

eque

nces

Med

ium

-sca

le s

eque

nces

Thi

rd-o

rder

seq

uenc

e bo

unda

ries

(Har

denb

olet

al.

1998

, Ogg

et a

l. 20

08)

Ti 5TS

TS

PA

RT

4 (S

chny

der

2003

, Fig

. II.1

5)m

ore

cont

inen

tal e

nviro

nmen

ts a

nd a

rid c

limat

e

incomplete grey filling indicates

observation

ante

-Pec

tinat

usF

itton

i-Alb

ani?

PA

RT

3 (S

chny

der

2003

, Fig

. II-

15)

mor

e m

arin

e en

viro

nmen

ts a

nd m

ore

hum

id c

limat

e

Fig. 2 (continued).

32 C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

shelf was exposed to cyclones during summer (i.e., June July August)(Moore et al., 1992) (Fig. 4a). Cyclones formed in the eastern Tethys.Then, they move westward either parallel to the coasts or inland(Moore et al., 1992). The northwestward flowing surface winds,which concerned the Aquitaine basin during summer (Fig. 4a),might have locally determined cyclone tracks. In winter (i.e., Decem-ber January February), dominant surface winds conversely blew to-wards the southeast and storms probably did not develop in thestudy area (Moore et al., 1992) (Fig. 4a). Schnyder (2003) also per-formed palynofacies, clay mineralogy, and pollen analyses. On thebasis of these results he associated the four lithological parts definedin the Phare de Chassiron section with changes in allocyclic processessuch as eustatic sea level and climate (Fig. 2). Sedimentological inter-pretation and changes in the ratio between marine- and continental-derived organic particle contents (MAR/CONT ratio) indicate thatparts 1 and 3 correspond to more open-marine environments thanparts 2 and 4 where restricted shallow-water environments dominate(Schnyder, 2003). The boundaries between these four lithologicalparts easily correlate with long-term (i.e., third- or second-order)sequence boundaries and maximum-flooding surfaces defined in thesame age succession of the Paris basin, and coincide with changes ineustatic sea level (Schnyder, 2003). Clay mineral and palynologicalanalyses indicate that these lithological changes also correspond toclimate changes (Schnyder, 2003). Clay mineral associations mainlycontain chlorite, illite, illite/smectite mixed-layers (I/S, smectitess.l.) of R0 type, and kaolinite. Relative proportions of kaolinite de-crease within parts 2 and 4, and match with relatively higher propor-tions of Classopollis sp. pollen. This probably indicates drier conditions

in parts 2 and 4 than in parts 1 and 3 (Schnyder, 2003). Clay mineralproportions can result neither from burial diagenesis, which ratherleads to increase in chlorite and illite and occurrence of subregularand regular mixed-layer minerals (Chamley, 1989), nor from diagen-esis of porous rocks such as sandstones, which are absent here. Lastly,parts 2 to 4 correspond to the Purbeck facies that globally indicatemore continental environments and drier climate than part 1(Schnyder et al., accepted). In the Tithonian, the mid and centralEurope were similarly affected by regressive tendencies probablycoupled with uplifting activities that progressively enlarged the drylands (Rhenish and Armorican massifs and Massif Central) and dras-tically reduced the marine areas (Thierry, 2000). The Purbeckian fa-cies were formed during the transgressive–regressive second-ordercycle that began in the Early Tithonian and ended in the EarlyBerriasian (Ogg et al., 2008). The boundary between parts 1 and 2corresponds to a major erosive surface capped by a conglomeratebed (bed 141) that coincides with the base of this sequence(Schnyder et al., accepted) (Fig. 2). Jacquin et al. (1998) andRusciadelli (1999) also defined this second-order sequence boundaryin the adjacent Paris basin (Schnyder et al., accepted).

3. Methodology

Limestone beds and marl interbeds that form diagenetic alter-nations show the same sedimentary features (see, e.g., Westphal etal., 2010; and references therein). The study of the origin of marl–limestone alternations requires to describe marl interbeds and lime-stone beds. A bed-by-bed analysis of Chassiron was performed.

COARSE ACCUMULATIONA cemented patchB coarse patchC continuous coarse layerD discontinuous coarse layerE fine patchF continuous fine layerG discontinuous fine layerH hummocky cross-beddingI continuous laminaJ discontinuous lamina

SEDIMENTARY STRUCTUREbioturbationagglutinated oystersboringswaley cross-bedding

BIOCLASTIC GRAINshellserpulid

gastropodurchin

DEPOSITIONAL SEQUENCE

sequence boundary

lowstand depositsequence boundary zone

transgressive surfacetransgressive deposit

highstand depositmaximum-flooding depositMFD

TD

LSDTS

HSD

SBZ

GRAIN ABUNDANCE

common

very abundant

sparse

abundant

QUARTZ SIZEvf

m

f

c

vc

very fine (< 50 µm)finemediumcoarsevery coarse (> 200 µm)

LITHOLOGY

limestone marl

gypsum

SCS

silty marlsilty limestone

Fig. 3. Key relative to the Phare de Chassiron section.

33C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

Fig. 2 includes the measured outcrop section and field observations,bed numbers according to Bousquet (1967), thicknesses in metres,textures after Dunham (1962), and grains. The key is given in Fig. 3.Seawalls recently covered some parts of the outcrop, which arereplaced in this study by parts of the log of Schnyder (2003). Somefield observations come from de Lapparent and Oulmi (1964),Bousquet (1967), and Schnyder (2003). Facies and microfacies analy-ses of limestone beds were realised from field observations and thestudy of polished rockslabs and 111 thin sections. The preparationof rockslabs and thin sections being difficult from marls, facies analy-ses of marl interbeds were realised from field observations only. Thediversity of bioclasts, the relative abundance of grains, and the sizeof quartz grains were determined in thin sections. The bioclast diver-sity is low when thin sections contain less than four different types ofbioclasts, and high when four different types of bioclasts (includingechinoderms in most cases) or more are present in thin sections.The relative abundance of grains ranges between sparse and veryabundant. Grains are sparse when they occur at least once in thin sec-tions; common when they occur at least once in each field of view;abundant when they occur at least twice in each field of view, andvery abundant when they occur more than twice in each field ofview. The size of quartz grains ranges from very fine (b50 μm) tovery coarse (>200 μm). It results from mean estimation and compar-ison between thin sections.

Primary marl–limestone alternations reflect cyclic changes inenvironmental conditions. The determination of these conditions re-quires a facies model that results from the interpretation of faciesand microfacies in depositional environments. Key features such as li-thology, texture, grains, and sedimentary structures allow facies andmicrofacies types to be defined. These features as well as the size ofquartz grains were used to assign facies and microfacies types todepositional environments. This sedimentological interpretationleads to a facies model that reflects the lateral distribution of theseenvironments.

The definition of controls on marl–limestone formation requires tostudy changes in depositional conditions through time. The vertical var-iations in depositional environments, alternation thickness and litholo-gy allow the identification of elementary, small-, and medium-scaledepositional sequences, which are hierarchically stacked (Colombié

and Strasser, 2003). Ratio a/b (i.e., the proportion of limestones inmarl–limestone alternations) reflects the lithology of marl–limestonealternation. Alternation thickness and lithology form a continuous sig-nal, while depositional environments correspond to parcel data that de-pend on the number of observations and samples. Elementary, small-,medium-, and large-scale sequences differ by their thickness (Strasseret al., 1999). Elementary sequences are the thinnest sequences. Small-scale sequences include several elementary sequences, and stack intomedium-scale sequences. Medium-scale sequences are the thickest se-quences. Whatever their sizes these sequences contain diagnostic sur-faces that define deposits. Deposits are used instead of systems tractsbecause the classical geometries of systems tracts cannot be distin-guished in metre-scale sequences (Strasser et al., 1999). Lowstand de-posit (LSD) occurs between the sequence boundary (SB) and thetransgressive surface (TS). SB forms in response to relative fall in sealevel (e.g., Sarg, 1988; Vail et al., 1991; Handford and Loucks, 1993).SBZ is used instead of SB when SB is not developed (Strasser et al.,1999). TS is the first major flooding surface, and marks the end of re-gression. Transgressive deposit (TD) is comprised between the TS andthe maximum-flooding deposit (MFD). In shallow-marine carbonatedeposits,maximum-flooding surfaces can be difficult to identify, and in-tervals of maximum flooding are defined instead (Strasser et al., 1999;Colombié and Strasser, 2005).MFD corresponds to the end of transgres-sion. Highstand deposit (HSD) forms between theMFD and the next SB.The criteria used to identify these diagnostic surfaces and depositschange from one system to another depending on the facies evolution,and have to be defined for Chassiron. The studied interval was corre-lated with the sequence-chronostratigraphic charts of Hardenbolet al. (1998) and Ogg et al. (2008) by means of ammonite biostratig-raphy. This stratigraphic correlation lends further support to thesequence-stratigraphic interpretation by filtering out autocyclicallyformed depositional sequences. It also allows sequence duration tobe defined by counting the identified sequences between datedlevels. The resulting durations combined with the hierarchical,high-frequency stacking of depositional sequences may refer toorbital control on sedimentation. Frequence analysis can infersequence duration as well but it is not the most appropriateapproach to study such a storm-dominated shallow-marine succes-sion (Strasser et al., 1999).

4. Lithofacies description

4.1. Coarse-grained accumulations

The studied section contains abundant coarse-grained accumu-lations, which are randomly superimposed on the studied marl–limestone alternations (Fig. 2). They occur in marly interbeds aswell as in calcareous beds. The texture, composition, grain size, andsedimentary structures of coarse-grained accumulations are differentfrom the host sediment (marl or limestone) (Figs. 5; 6). Texturesrange from wackestones to packstones (Fig. 5). Grains correspondto bioclasts, quartz or peloids, and are sand-sized and smaller. Struc-tures are planar or wavy laminations, cross-laminations, normal grad-ed beddings, or hummocky or swaley cross-beddings. Ten coarse-grained accumulation types (from A to J) were defined in the fieldaccording to the size of the grains they contain and their geometry(Fig. 6). Accumulation type A corresponds to cemented patches.Coarse bioclastic accumulation types B, C, and D are patches and con-tinuous and discontinuous layers, respectively. Fine bioclasticaccumulation types E, F, and G are patches and continuous and dis-continuous layers, respectively. Grains in accumulation types H, Iand J are too small to be determined in the field. Accumulationtypes H, I and J only differ by their geometry. Accumulation type His characterised by hummocky cross-bedding. Accumulation types Iand J display continuous and discontinuous laminae, respectively.

25°

30°

35°

40°

45°

25°

30°

35°

40°

45°

500 km

AM

MC

PB

AB

TO

CA

Exposed lands

Epicontinental deposits

Oceanic basins

High-energy platforms

Low-energy platforms

Basins

Winter storms

Cyclones

Summer winds

Mid-oceanic ridges

Studied section

Main faults

Coral reefs

N

La Rochelle

Island ofOleron

Niort

Exposed lands

N

AQUITAINEBASIN

ARMORICANMASSIF

PARISBASIN

CENTRALMASSIF

La Rochelleshelf

20 km

a b

AM Armorican massif

AB Aquitaine basin

CA Central Atlantic

MC Massif Central

PB Paris basin

TO Tethys Ocean

Studied section

Winter winds

Fig. 4. Late Jurassic palaeogeography: a) Simplified map of the northwestern margin of the Tethys ocean during the Early Tithonian (modified from Thierry, 2000) with the pre-ferred region of development, maturity, and decay of winter storms and cyclones (modified from PSUCLIM, 1999), and winter (i.e., December/January/February) and summer(i.e., June/July/August) surface wind vectors (modified from Moore et al., 1992). The studied section was exposed to cyclones that came from the East; b) Zoom on the northernmargin of the Aquitaine basin during the Late Oxfordian and the Early Kimmeridgian (modified from Hantzpergue andMaire, 1981; Olivier et al., 2008). The studied section includesthe Late Kimmeridgian to the Early Tithonian deposits. These deposits correspond to nearshore to shallow-marine environments that develop because of regressive tendencies thatcharacterised the mid and central Europe during the Early Tithonian.

34 C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

4.2. Marl–limestone alternations

Marls have different colours and can contain coal debris, quartzgrains (i.e., be silty, Figs. 2; 3), planar laminations or coarse-grainedaccumulation types B, C, E, F, G, H, I, and J (Fig. 2).

Eleven limestone microfacies types (from facies 1 to 11) weredefined (Table 1, Fig. 5). Only the texture, the abundance and diver-sity of bioclasts, and the amount and size of quartz grains are pres-ented below because they are key features that allow microfacies tobe distinguished. Sedimentary structures, which can be the samefrom one microfacies to another, are given in Table 1. Textures aregenerally mudstones except in microfacies types 1, 2, and 3. Micro-facies type 1 corresponds to alternating layers of mudstones orwackestones–packstones–grainstones with bioclasts or quartzgrains and layers with gypsum pseudomorphs only. Microfaciestypes 2 and 3 correspond to packstones in most cases. Microfaciestype 2 contains sparse muscovite, very abundant quartz, and lowbioclast diversity. Microfacies type 3 is chiefly composed of peloids,but also contains very abundant quartz and low bioclast diversity.The abundance and diversity of bioclasts and the amount and sizeof quartz grains generally decrease from microfacies types 4 to 11(Table 1, Figs. 5; 7). Microfacies types 4 to 7 are characterised byhigh diversity of bioclasts. Microfacies types 4 and 5 differ frommicrofacies types 6 and 7, respectively, because they include

Fig. 5. Thin-sections of themost common coarse-grained accumulations andmicrofacies typesvery abundant bivalves (Bi) (a), bioclasts (ostracods (Os) in this case) and quartz (Qu) (b), quamon to very abundant bivalveswhilemicrofacies types 6 (f) do not contain asmany bivalves. Boostracods, bivalves, foraminifera (Fo), and echinoderms (Ec)) and common to very abundantquartz is typical of microfacies type 9 (h). Both include low bioclast diversity (less than 4 diffderms). Lower left corners of labels indicate grains.

common to very abundant bivalves. Microfacies types 5 and 7 con-tain few grains of quartz compared to microfacies types 4 and 6, re-spectively. Microfacies types 8 and 9 are characterised by low diversityof bioclasts. Microfacies type 8 differs from microfacies type 9 by itshigher quartz content. Microfacies type 10 contains sparse quartzonly. Only low diversity of bioclasts indicates microfacies type 11.

5. Sedimentological interpretation

5.1. Coarse-grained accumulations

The sedimentary features of coarse-grained accumulations A to Jresemble those of storm deposits described by Pedersen (1985)from the Lower Jurassic of the eastern part of the Danish Basin, byFürsich and Oschmann (1986) from the Upper Jurassic of northernFrance, and by Carcel (2009) and Carcel et al. (2010) from theOxfordian–Kimmeridgian of La Rochelle (Figs. 5; 6). Schnyder et al.(accepted) interpreted these coarse-grained accumulations as stormdeposits but did not dealwith the processes responsible for their forma-tion and preservation. Most appear as patches or discontinuous layersor laminae because of the early lithification that characterises calcare-ous tempestites and the strong bioturbation that affects the upperpart of storm beds (Aigner, 1985; Seilacher and Aigner, 1991; Einseleet al., 1996). Storm deposits are rarely preserved in muddy shelf

fromChassiron. Coarse-grained accumulations arewackestones or packstones that containrtz (c) or peloids (Pe) (d). Microfacies types 4 (e) are mudstones in most cases with com-th contain high bioclast diversity (4 ormore different types of bioclasts among gastropods,quartz. Microfacies type 8 (g) includes common to very abundant quartz, whereas sparseerent types of bioclasts among gastropods, ostracods, bivalves, foraminifera, and echino-

e f

a b

c d

hg

Bi

Qu

Bi

Ec

Qu

Fo

Bi

Qu

Qu

Qu

Qu Qu

Pe

Bi

Bi

Os

35C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

36 C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

deposits because they are often overprinted and more or less obliterat-ed by post-event bioturbation (Pedersen, 1985; Einsele et al., 1996) orreworked by subsequent storms (Einsele and Seilacher, 1991).Schnyder (2003) showed that the Late Jurassic deposits of Chassironprobably formed in or seaward of a shelf embayment,maybe an estuary.In such a setting, the accumulation of fine-grained sediments was prob-ably fast enough to limit bioturbation. This togetherwith the highmarlyand carbonate mud cohesiveness, which makes them difficult to erode,favoured the preservation of numerous storm deposits.

Effects of storms vary with bathymetric, sedimentological, and cli-matic parameters as well as with their timing (Seilacher and Aigner,1991). Storms lead to exchange of sediments between beach anddeeper-water environments according to processes that are wind-generated currents, waves, wave–current interactions, tidal currents,rip currents, and turbidity currents (Aigner, 1985; Nummedal, 1991;Immenhauser, 2009; and references therein). Most studied materialsin thin sections are calcareous mudstones, which can contain coarse-grained accumulations including quartz, bioclasts, and peloids. Sincethese grains are sand-sized or smaller, most sediment was transportedin suspension across the shelf (Aigner, 1985, p. 45; and references there-in). Recent field observations from several shelf environments showthat gravity-driven transport within negatively buoyant layers is animportant mode of fine sediment transport across storm-dominatedshelves (Wright and Friedrichs, 2006). Resulting tempestites (or stormbeds) are generally well graded and contain Bouma-like sequences(Myrow et al., 2002; Lamb et al., 2008). Here, graded bedding is scarceand the orientation of bioclasts often emphasises planar and low-angle laminations, which are together with bioturbation themost com-mon sedimentary structures (Table 1). Planar lamination can reflect de-position under high-energy flows while low-angle cross laminationprobably develop under storm-generated waves (Myrow et al., 2002).These structures are typical of shoreface and foreshore deposits. Thelack of additional evidence does not allow going farther in the interpre-tation of the processes responsible for sediment transport. Features ofstorm deposits depend on the characteristics of the sediment source,sediment uptake underway, and the mode of final deposition (Einseleet al., 1996). Therefore, it is often problematic to determine the mecha-nisms responsible for sediment transport and the character of the flow(Einsele et al., 1996; Myrow and Southard, 1996).

5.2. Marl–limestone alternations

Schnyder (2003) performed palynofacies analyses from marls andlimestones of Chassiron. Marls exhibit lower MAR/CONT ratios thanlimestones, suggesting that marls settled in environments that weremore proximal (i.e., closer to land) than the environments wherelimestones formed.

Differences in texture and sedimentary structure between lime-stone microfacies types are small (Table 1, Figs. 2; 5). Most of themindicate quiet-water sedimentation in shallow-marine environmentsthat were subject to high-energy events. The type, relative abun-dance, diversity, and size of grains help to clarify the sedimentologicalinterpretation of microfacies types 1 to 11. Alternating layers of lime-stones and gypsum pseudomorphs, which correspond to microfaciestype 1, commonly indicate supratidal sabkha environments (Flügel,2004, p. 622). Facies containing muscovite and quartz, which comefrom weathering of rocks on land, generally indicate more proximalenvironments than similar facies including neither muscovite norquartz. According to Mohs hardness scale, muscovite is less resistantto physical and chemical destructive processes during sedimenttransport than quartz. Consequently, microfacies type 2, which con-tains sparse muscovite and very abundant quartz, indicates moreproximal environments than microfacies types 3 to 10, which donot include muscovite.

The size and the abundance of quartz grains generally decreasefrom proximal environments to distal (i.e. far away from land towards

the open sea) environments. In most ancient siliciclastic shelves,transport of terrigenous sediments was downslope (Pettijohn et al.,1987, p. 332), along a concave upward, exponential equilibriumprofile the steepest portion of which is at the shore (Swift, 1970;Pettijohn et al., 1987, p. 301). Seaward both slope and grain sizebecome less. In this study, coarse-grained accumulations indicatethat the Late Jurassic shelf of La Rochelle was storm-dominated. Theabundance of quartz grains decreases with distance from land, whilethe proportion of mud increases. This pattern is due to the decreasingcapacity of storm-induced flows to transport sands towards the opensea (Aigner, 1985, p. 37). Microfacies types 2 and 3, which exhibitmuch higher coarse-grained quartz contents than microfacies types4 to 11, therefore indicate more proximal environments than micro-facies types 4 to 11.

Most microfacies types include gastropods, ostracods, bivalves,foraminifera or echinoderms. These nonphotosynthetic organisms (ornon symbiotic animals) reflect meso- to eutrophic conditions that arecharacterised by high and oscillating levels of nutrient supply (Brasier,1995; Mutti and Hallock, 2003). Olivier et al. (2008) studied the reefcommunities of the Kimmeridgian of La Rochelle. The identified corals'genera and the periodic development of microbialites reflect low- tohigh-mesotrophic conditions. These conditions favour suspensionfeeders (Hallock and Schlager, 1986; Brasier, 1995; Hallock, 2005;Lokier et al., 2009) such as suspension-feeding bivalves the dominanceof which points to food particles largely suspended in thewater column(Flügel, 2004, p. 624). Meso- to eutrophic conditions usually occur innearshore environments where nutrient availability may be higherthan in shallow-marine environments because of continental run-off(Brasier, 1995; Mutti and Hallock, 2003). In these meso- to eutrophicecosystems, decreasing amount of terrestrially derived nutrients fromnearshore to shallow-marine environments may result in a parallelproximal–distal trend in biotic diversity (Lukasik et al., 2000). Conse-quently, microfacies types 4 and 5, which contain common to veryabundant bivalves, indicate more proximal environments than micro-facies types 6 and 7, which contain as many bivalves as other bioclasts.Microfacies types 4, 5, 6, and 7, which exhibit high bioclast diversity(i.e., when four or more different types of bioclasts are present in thinsections), correspond tomore proximal environments than microfaciestypes 8, 9, and 11, which are characterised by low bioclast diversity (i.e.,when thin sections contain less than four different types of bioclasts).Thus, the decrease in nutrient availability from nearshore to shallow-marine environments would explain that bivalves dominate in micro-facies types 4 and 5, and that biotic diversity decreases between micro-facies types 4 and 11. This change in biotic diversity correlates with thedecrease in abundance and size of quartz grains that reflect a decreasein siliciclastic sediments from nearshore to shallow-marine environ-ments. This decrease in terrigenous inputmay be responsible for the de-crease in nutrient availability (Larcombe et al., 2001; Lokier et al., 2009)and changes in faunal communities (Hallock, 2001, 2005; Lokier et al.,2009).

Some of the microfacies criteria used in this study can also be cau-sed by other environmental constraints. However, the above interpre-tation does not result from a single source of information but includesinformation provided by each type of grains that form microfaciestypes. Moreover, the evolution through time of all of the encounteredmicrofacies types is gradual and lends further support to the proposedfacies model (Fig. 7). Lastly, Carcel (2009) and Carcel et al. (2010) de-veloped a similar model for the Oxfordian–Kimmeridgian deposits ofLa Rochelle, which are close to the studied Late Kimmeridgian–EarlyTithonian deposits of Chassiron.

6. Sequence- and cyclostratigraphical interpretations

Changes in depositional environment, thickness alternation, andratio a/b allow several orders of depositional sequences to be defined(Figs. 2; 8). Elementary sequences correspond to marl–limestone

d

E

G

a b

D

B

C

c

e f

H

J

g

I

A

hmarl burrow

limestone

F

G

A B

G

I

Fig. 6. Coarse-grained accumulations from Chassiron, classified according to the size of the grains they contain and their geometry. Accumulations A (photo a, g) correspond to cementedpatches. Coarse-bioclastic accumulations B (photo b, c), C (photo c), and D (photo c) are patches and continuous and discontinuous layers, respectively. Fine-bioclastic accumulations E(photo d), F (photo e), and G (photo d, e, f) are patches and continuous and discontinuous layers, respectively. Accumulation H (photo f) includes hummocky cross-bedding. AccumulationsI (photo g, h) and J (photo g) contain continuous anddiscontinuous laminae, respectively. These coarse-grained accumulations are superimposed onmarl–limestone alternations that have aprimary origin as testified by burrows filled with calcareous sediments in the upper part of marls for example (photo h).

37C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

Table 1Limestone microfacies types.

Microfacies Textures Skeletal and non-skeletal components Sedimentary structures

1 M or W/P/G Very abundant cilia of soft corals (Alcyonium digitatum according to Bousquet, 1967)or very abundant quartz in the case of W/P/G

Alternating layers of M or W/P/G and gypsumpseudomorphs

2 P in most cases Sparse muscovite, very abundant quartz (coarse to very coarse in most cases), andlow bioclast diversity (ostracods, bivalves or echinoderms)

3 P in most cases Very abundant peloids, very abundant quartz (fine to coarse in most cases), and lowbioclast diversity (gastropods, ostracods, bivalves or echinoderms)

Cross-lamination, bioturbation

4 M in most cases Common to very abundant bivalves, high bioclast diversity (gastropods, ostracods,bivalves, foraminifera or echinoderms), and common to very abundant quartz (fineto coarse in most cases)

Fenestrae, planar lamination, cross-lamination,bioturbation

5 M/W in most cases Common to very abundant bivalves, high bioclast diversity (gastropods, ostracods,bivalves, foraminifera or echinoderms), and sparse quartz (fine in most cases)

Bioturbation

6 M in most cases High bioclast diversity (gastropods, ostracods, bivalves, foraminifera or echinoderms)and common to very abundant quartz (fine to coarse in most cases)

Planar lamination, cross-lamination, bioturbation

7 M in most cases High bioclast diversity (gastropods, ostracods, bivalves, foraminifera or echinoderms)and sparse quartz (fine to coarse in most cases)

Fenestrae, normal graded bedding, planarlamination, cross-lamination, bioturbation

8 M in most cases Low bioclast diversity (gastropods, ostracods, bivalves, foraminifera or echinoderms)and common to very abundant quartz (fine to medium in most cases)

Planar lamination, cross-lamination, bioturbation

9 M Low bioclast diversity (gastropods, ostracods, bivalves, foraminifera or echinoderms)and sparse quartz (very fine to medium in most cases)

Normal graded bedding, planar lamination,cross-lamination, bioturbation

10 M Sparse quartz (very fine to fine in most cases) Fenestrae, planar lamination, cross-lamination,wavy lamination, bioturbation

11 M Low bioclast diversity (ostracods and bivalves)

38 C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

alternations (Fig. 2). Small-scale sequences are about 2 m thick andcontain five alternations on average (Figs. 2; 8). Sequence boundaries(SB) of small-scale sequences are located at the base of the thickestand the most marly alternations (Fig. 2). Small-scale sequences stackinto medium-scale sequences, which are about 7 m thick and includethree to four small-scale sequences on average (Figs. 2; 8). SBsof medium-scale sequences show the same characteristics thanthose of small-scale sequences. Most of the medium-scale SBs alsocorrespond to the most proximal environments (Fig. 2). Decreases inalternation thicknesses and increases in ratios a/b indicate transgres-sive surfaces of medium-scale sequences, which mostly coincidewith sudden shifts to more open environments as well (Fig. 2).Maximum-flooding deposits of medium-scale sequences include thethinnest and the most calcareous alternations and generally corre-spond to the most distal environments (Fig. 2). All these sequencesare very similar to those described by Carcel (2009) and Carcel et al.(2010) in the Oxfordian–Kimmeridgian of La Rochelle.

Biostratigraphy of ammonites, ostracods, dinoflagellate cysts, andcalcareous nannofossils gives evidence for correlating the studiedsection with the third-order sub-boreal sequences (Hardenbol et al.,1998; Ogg et al., 2008) (Fig. 2). According to Hardenbol et al. (1998)and Ogg et al. (2008), third-order sequence boundary Ti 1 (SB Ti 1)is located between the Kimmeridgian–Tithonian boundary and SB Ti2. It probably corresponds to the base of bed 120, which is the baseof the thickest and the most marly alternation in this interval(Fig. 2). SB Ti 4 is located at the top of the Pectinatus Zone(Hardenbol et al., 1998; Ogg et al., 2008), and probably coincideswith the base of the thickest and the most marly alternation in bed190 (Fig. 2). SB Ti 3 is located between SB Ti 2 and SB Ti 4, and probablycorresponds to the base of beds 162–163. It is the base of a thick and

Shallow-marine(or distal)

11 10 9 8

Decreasing bioclast diversDecreasing quartz abun

LIMES

DEPOSIT

Sea levelFair weather wave base

Ramp morphology

Seaward

Fig. 7. Interpretation of limestone microfacies types in depositional environments, which

marly alternation that formed just before a shift to more proximal en-vironments (Fig. 2). SB Ti 5 is probably located at the base of bed 197,which is the base of the thickest and the most marly alternation aboveSB Ti 4 (Fig. 2). Specific sedimentary features such as fenestrae, wooddebris, and desiccation cracks, which generally indicate proximal de-positional environments, help establish T–R trends that confirm thecorrelation between the sequence boundaries defined in this studyand the third-order sub-boreal sequence boundaries of Hardenbolet al. (1998) and Ogg et al. (2008) (Fig. 2). Fenestrae and wood debrischaracterise the interval above SB Ti 1 (Fig. 2). Fenestrae and desicca-tion cracks occur below SB Ti 2 (Fig. 2). Limestone microfacies typicalof proximal environments indicate SB Ti 3 and SB Ti 4 (Fig. 2). Theinterval above SB Ti 5 includes wood debris, serpulid bioherms, andswaley cross-bedding (Fig. 2).

The third-order sub-boreal sequences defined by Hardenbol et al.(1998) and Ogg et al. (2008) contain one or several medium-scalesequences (Fig. 8). Together with the further subdivision of the sedi-mentary record into small-scale and elementary sequences thisallows for the definition of a high-resolution chronostratigraphicframework (Fig. 8). Due to a major phase of subaerial platform expo-sure at SB Ti2, highly marly and thinly bedded deposits between SB Ti2 and SB Ti 3, andmostly covered outcrop between SB Ti 3 and SB Ti 4,the interval between SB Ti 4 and SB Ti 5 is the most appropriate forcyclostratigraphic interpretation (Fig. 8). The stacking pattern ofsequences in this interval is five elementary sequences on averagein one small-scale sequence, and four small-scale sequences in onemedium-scale sequence. This stacking pattern suggests an orbitalcontrol on sedimentation. Moreover, Hardenbol et al. (1998) andOgg et al. (2008) give time spans that allow duration of sequencesdefined in this work to be calculated (Fig. 8). Even if Hardenbol

Nearshore(or proximal)

17 6 5 4 3 2

ity and abundancedance and size

TONE MICROFACIES

IONAL ENVIRONMENTS Landward

range from the proximal to the distal part of a nearshore to shallow-marine ramp.

39C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

et al. (1998) and Ogg et al. (2008) are in agreement on the position ofthe third-order sub-boreal sequences according to the ammonitezones, time spans differ from one time scale to the other (Fig. 8).The division of the time spans of Hardenbol et al. (1998) by the num-ber of elementary, small-, and medium-scale sequences defined inthese intervals gives average durations of less than 57, 181, and692 ka for elementary, small-, and medium-scale sequences, respec-tively. Applying the time scale of Ogg et al. (2008), however, the av-erage duration of elementary sequences defined in this work is lessthan 30 ka. Small-scale sequences last less than 108 ka on average,and medium-scale sequences, less than 421 ka on average. These lat-ter values, combined with the stacking ratios 1:5 and 1:4 describedabove from the interval between SB Ti 4 and SB Ti 5, imply that sedi-mentation in the Late Kimmeridgian–Early Tithonian of Oléronchanged in tune with orbital parameters. Precession (20 ka), first(100 ka), and second (400 ka) eccentricity cycles controlled the for-mation of elementary, small-, and medium-scale sequences, respec-tively. This study therefore indicates that sedimentary cycles in theLate Kimmeridgian–Early Tithonian of Chassiron partly formed inresponse to high-resolution changes in climate.

These cycles show the same sedimentary features than those de-scribed by Carcel (2009) and Carcel et al. (2010) in the Oxfordian–Kimmeridgian of La Rochelle. However, their durations are not thesame. The elementary sequences of Chassiron correspond to thesmall-scale sequences of La Rochelle, the small-scale sequences ofChassiron to the medium-scale sequences of La Rochelle, and themedium-scale sequences of Chassiron to the large-scale sequences ofLa Rochelle. This apparent contradiction is possible when applyingthe approach of Strasser et al. (1999). Elementary, small-, andmedium-scale sequences are only used descriptively and do not

Platform exposure

Most of the outcrop is covered

Marly and thinly bedded deposits

Thi

rd-o

rder

sub

-bor

eal

sequ

ence

bou

ndar

ies

Tim

e sp

ans

(Ma)

from

Har

denb

ol e

t al.

(199

8)

Tim

e sp

ans

(Ma)

fr

om O

gg e

t al.

(200

8)

Ti 1

Ti 2

Ti 3

Ti 4

Ti 5

0.50

0.40

0.70

0.70

1.25

1.14

1.70

0.29

Ave

Ear

ly T

ithon

ian

Fig. 8. Durations of the Chassiron elementary, small-, and medium-scale sequences calculateand Ogg et al. (2008) for the Early Tithonian. The stacking pattern of sequences in the intesequences calculated from time spans given by Ogg et al. (2008) suggest an orbital controlcycles probably controlled the formation of elementary, small-, and medium-scale sequenc

involve durations. The Late Kimmeridgian–Early Tithonian depositsof Chassiron correspond to more proximal environments than thoseof the Oxfordian–Kimmeridgian of La Rochelle because of thesecond-order regression that began in the Late Kimmeridgian andended in the Early Tithonian or even in the Early Berriasian and ledto the formation of Purbeckian facies. Therefore, the lack of accommo-dation space in these particularly proximal environments probablydid not favour the preservation of the smallest cycles. Rameil (2005)observed quite the same in contemporary successions in the JuraMountains.

7. Discussion

7.1. Origin of marl–limestone alternations

Field observations made in this study as well as the results in Carcel(2009), Carcel et al. (2010), and Schnyder (2003) clearly indicate pri-mary differences between marls and limestones. Marls as well as lime-stones include coarse-grained accumulations. The geometry and thecontent of these accumulations differ between marls and limestones(Fig. 2), suggesting changing environmental conditions between thetime of marl and limestone deposition. Moreover, marls can containabundant burrows filledwith carbonate sediments and be sharply over-lain by less bioturbated limestones (Fig. 6). This succession indicates adrastic change in deposition conditions between marls and limestones.According to these field observations, marl–limestone alternationsclearly reflect cyclic changes in environmental conditions and do not re-sult from diagenetic alterations. Carcel (2009) and Carcel et al. (2010)did calcimetry and palynofacies and calcareous nannofossil analyses inall of the lithologies encountered in the Oxfordian–Kimmeridgian

Num

ber

of e

lem

enta

ryse

quen

ces

Seq

uenc

e du

ratio

ns (

ka)

acco

rdin

g to

Har

denb

ol e

t al.

(199

8)

Seq

uen

ce d

ura

tio

ns

(ka)

acco

rdin

g t

o O

gg

et

al. (

2008

)

Num

ber

of s

mal

l-sca

lese

quen

ces

Num

ber

of m

ediu

m-

-sca

le s

eque

nces

THIS WORK

>4

9

8

4

>1

3

2

1

>9

54

32

20

<21 - <127 - <380

<53 - <212 - <850

14 - 72 - 290

<139 - <312 - <1 250 <55 - <125 - <500

<22 - <87 - <350

35 - 175 - 700

<7 - <44 - <133

<30 - <108 - <421<57 - <181 - <692rage durations

d from the third-order sub-boreal sequence durations given by Hardenbol et al. (1998)rval between third-order sequence boundaries Ti 4 and Ti 5 and the mean duration ofon sedimentation. Precession (20 ka), first (100 ka), and second (400 ka) eccentricityes, respectively.

Sea level

Ramp morphologyLOW and LOCALISED carbonate

production

HIGH but LOCALISED carbonateproduction

LOW carbonate inputdepending on storms

HIGH carbonate inputdepending on storms

HIGH siliciclastic input(nearby clastic source)

LOW siliciclastic input(pounding of siliciclastics

on flooded area shelf)

THICKMARLY

BED

THINCALCAREOUS

BED

Low relative sea level

High relative sea level

Fig. 9. Formation of the sequences from the Late Jurassic of western France. Carbonate input mainly depended on carbonate production and storms. It was high during high sea level,when carbonate production was high, and siliciclastic input, low, and led to the deposition of the thin calcareous beds that are typical of the maximum-flooding deposits of the LateJurassic sequences of western France.

40 C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

deposits of La Rochelle. There, the sampling resolution for these analy-ses was too low for getting results on marl–limestone alternations.However, calcimetry and palynofacies and calcareous nannofossil anal-yses clearly show differences between marls and limestones. The calci-um carbonate percentages range from 35 to 65% in marls; from 65 to78% in calcareous marls; and from 78 to 90%, in marly limestones,and are higher than 90% in limestones. The positive correlation be-tween the calcium carbonate and the amorphous organic matter(which results from the degradation of marine or continental or-ganic matter) contents, the negative correlation between the calci-um carbonate contents and the absolute abundances of ascidianspicules, coccoliths, and schizopheres, and the decrease in theMAR/CONT ratio and in the inertinite PM4T over PM4E ratio frommore calcareous to more marly deposits indicate changing environ-mental conditions between the time of marl and limestone deposi-tion. Schnyder (2003) performed clay mineralogical analyses frommarls and limestones of Chassiron. Kaolinite over illite ratios are higherin calcareous intervals (parts 1 and 4) than inmarly ones (part 2 and 4),reflecting more humid climatic conditions during the deposition ofcalcareous intervals than during the deposition of marly intervals.Whatever the performed analyses and their resolutions marls andlimestones differ quite considerably, proving the primary origin of thestudied alternations.

7.2. Depositional conditions of alternations

The facies analysis performed in this work shows that muddeposits dominated the La Rochelle shelf during the Late Jurassic.These deposits indicate shallow-marine and low-energy environ-ments. However, coarse-grained accumulations, which are super-imposed on marl–limestone alternations, clearly result from storms.The sedimentary features of coarse-grained accumulations andmarl–limestone alternations of Chassiron are close to those of theOxfordian–Kimmeridgian of La Rochelle (Carcel, 2009; Carcel et al.,2010). Consequently, the following discussion includes evidencefrom both the Late Kimmeridgian–Tithonian of Chassiron and theOxfordian–Kimmeridgian of La Rochelle.

The first possibility to explain the co-existence of mud and stormdeposits is that the La Rochelle shelf was protected from fair-weather and storm waves and currents except during the strongestevents. Mud deposition occurred during fair-weather periods orweak storms whereas storm deposits only formed during the most vi-olent events. Olivier et al. (2008) proposed that the Late Jurassic fine-grained sediments of the La Rochelle shelf accumulated in waters thatwere most often quiet. Sheltering by e.g., reefal barriers, shoals, or

structural highs could have significantly damped storm-inducedwaves and currents. The La Rochelle shelf was tectonically structuredin horsts and grabens (Hantzpergue, 1985; Olivier et al., 2008).Emerged land appeared seaward of Ile d'Oléron at the end of theLate Jurassic and beginning of the Early Cretaceous because of thebasement uplift and the second-order sea-level fall that characterisedthis period (Schnyder, 2003). However, the great amount of stormdeposits in the Late Kimmeridgian–Early Tithonian of Chassiron andin the Oxfordian–Kimmeridgian of La Rochelle indicates that thesesheltering mechanisms were not efficient enough to keep all storm-impact away from the La Rochelle shelf. In the parts of the Phare deChassiron section that are not covered and can still be observed,marl–limestone alternations can include several coarse-grained accumu-lations (Fig. 2).Moreover, the Rock-Eval pyrolysis analyses performed bySchnyder (2003) and Schnyder et al. (accepted) indicate that the studieddeposits formed in well-oxygenated waters. Lastly, Chassiron does notinclude sufficiently coarse grains or developed reefs that could form acontinuous barrier (Bousquet, 1967; Hantzpergue, 1985; Schnyder,2003; Olivier et al., 2008; Schnyder et al., accepted). All these criteriarule out the existence of an efficient barrier that sheltered the La Rochelleshelf from storm-inducedwaves and currents during the studied period.

The second possibility is that muddy sediments settled in environ-ments open to the influence of waves and currents. During the Late Ju-rassic, the northern margin of the Aquitaine basin was subjected tocyclones that went northwestward following large-scale circulation(Barron, 1989; Moore et al., 1992). Even if these results may be diffi-cult to apply at a regional scale because of local parameters that prob-ably disturb the track of cyclones, the La Rochelle shelf was exposed towinds. The west Florida continental shelf is a distally steepened car-bonate ramp (Read, 1985; Brooks et al., 2003) that extends westwardinto the Gulf of Mexico. This shelf is regularly influenced by tropicalstorms including hurricanes (Davis et al., 1989). The west-centralFlorida inner shelf represents a transition between the quartz-dominated barrier-island system and the carbonate-dominated mid-outer shelf (Brooks et al., 2003). Such a contour-parallel facies patternalso characterises the Late Jurassic deposits of Chassiron and LaRochelle and is typical of open shelves (Reading, 1993, p. 304). Inthe west-central Florida inner shelf, carbonate sediments are of bio-genic origin, consisting dominantly of bivalve shell fragments withsubordinate amounts of gastropods, benthic foraminifera, bryozoa,and coralline algae (Brooks and Doyle, 1991; Brooks et al., 2003).The Late Jurassic of Chassiron and La Rochelle contains most of thesebioclastic components. However, they constitute sands in the west-central Florida inner shelf while muddy sediments dominate in theLate Jurassic of Chassiron and La Rochelle.

41C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

Bottom friction can slow down wave propagation, especially inshallow waters and across gentle slopes (Friedrichs and Madsen,1992; Le Hir et al., 2000). Moreover, if soft mud forms the seabed, asignificant proportion of the wave energy may be absorbed as it pro-gresses across the bed (Le Hir et al., 2000). The Late Kimmeridgian–Early Tithonian of Chassiron and the Late Oxfordian–Kimmeridgianof La Rochelle mainly consist of fine-grained deposits (i.e., alternatingmarly and calcareous mudstones or wackestones). The Oxfordian–Kimmeridgian depositional sequences of La Rochelle often showmost facies types that the entire studied section includes (Carcel,2009; Carcel et al., 2010). This means that sea level systematicallyswept across all the environments that formed the La Rochelle shelfduring a cycle of sea-level change, and suggests that the slope of theshelf was smooth (Carcel, 2009). When exposed coasts are muddy(because of large input of sediment nearby), mud liquefaction is likelyto occur and the waves experience strong damping (Kaihatu et al.,2007). Extreme dissipation rates (from 80 to 95%) have been reportedin laboratory experiments and off the coasts of Surinam and India(Kaihatu et al., 2007). The study of processes responsible for loss ofwave energy over muddy sea beds is still in progress (Kineke et al.,2006). The studied depositional environments probably formed near-by an estuary (Schnyder, 2003). Moreover, the sedimentation ratewas sufficiently high to preserve many storm deposits. Lastly,Olivier et al. (2008) studied the Late Jurassic reefs of the La Rochelleshelf and showed that microbialites and corals developed in turbidwaters. Waves and currents therefore maintained large volumes offine-grained sediment in suspension. This high concentration of par-ticles in suspension probably subsided waves and currents resultingfrom storms and greatly helped mud deposition in these open en-vironments. A recent analogue might be the Guiana Coast (SouthAmerica), which is the world's longest continuous mud coastline(Wells and Coleman, 1981; Rine and Ginsburg, 1985; Orton andReading, 1993). Although the sediment accumulation is likely muchhigher than during the Late Jurassic of western France, the factors in-volved seem to be close. Along the Guiana Coast, immense volumes ofmarly mud are being deposited in environments normally associatedwith sand deposits during periods of relatively high wave and currentactivity. Shoreface and foreshore are subdivided into bank and inter-bank zones. Banks consist of mud that is so fluid that it interacts withsurface waves causing them to be altered and damped.

7.3. Controls on alternation formation

Many reefs and carbonate platforms grew preferably during sea-level rise and early highstand (e.g., Sarg, 1988; Tucker and Wright,1990; Leinfelder, 1993, 1994, 1997; Schlager, 2005). This is particular-ly true for mixed platforms where coastal sediment trapping systemssuch as estuaries will particularly develop during sea-level rise(Leinfelder, 1997). Therefore, thick beds generally developed onshallow-carbonate platform tops during maximum floodings. Howev-er, the Late Jurassic sequences of Chassiron and La Rochelle show thinand calcareous maximum-flooding deposits. Moreover, they probablyformed in a storm-dominated setting where sea level changed in tunewith Milankovitch cycles and partly controlled sedimentation. Thesouth Florida margin shows a complex framework of modern andrelict carbonate geomorphic features (Mallinson et al., 2003). Marlybasins develop around reefs that grow on positive structures(Mallinson et al., 2003). The growth of these reefs is related to sea-level variations and is additionally modified by changes in climate.These reefs represent localised carbonate factories and frameworkproducers that exert a fundamental control on the long-term evolu-tion of this margin. During the Late Jurassic on the La Rochelle shelf,Hantzpergue (1985, 1988) showed that reefs developed on basementuplifts while siliciclastic sediments were stored in morphological de-pressions. Moreover, in the Oxfordian–Kimmeridgian deposits of LaRochelle, the dilution of calcareous nannofossils by carbonate mud

indicates that carbonates came from another source than calcareousnannofossils (Carcel, 2009; Carcel et al., 2010). Consequently, the car-bonate production was the highest during maximum floodings, as it isin other shallow-carbonate platforms, but it was probably localisedon reefs and transported to the surrounding marly depressions bystorms. The carbonate supply was particularly important whensea level and carbonate production were high but it was not enough toform calcareous maximum-flooding deposits as thick as marly deposits.

The following conceptional model describes how a combinationof storms and high-resolution changes in sea level and concomitantfluctuations in carbonate production generates the Late Jurassicsequences of western France, and notably the marl–limestone alter-nations described here (Fig. 9). During low sea level, siliciclasticinput was high. Carbonate production was low and localised bythe reefs that developed on positive structures. Carbonate input,which depended on storm events (see above), was low. All theseconditions resulted in the formation of the thick marly beds thatcharacterise the lowstand deposits of the Late Jurassic sequencesof western France. Conversely, high sea level led to decreasingsiliciclastic input, trapped in the proximal parts of the platform,and increasing reef growth (see above). The carbonate productionwas higher than before but stayed localised. The transport of car-bonate mud to the surrounding depressions by storms resulted inthe deposition of the thin calcareous beds that are typical of thestudied maximum-flooding deposits. Consequently, the formationof marl–limestone alternations in the Late Jurassic of westernFrance results from the combination of 1) cyclic changes in carbon-ate production due to Milankovitch-scale sea-level variations, and2) transport of the produced carbonate mud to the surroundingmarly depressions by high-energy events. In the absence of storms,the produced carbonate mud would stay by the reefs and onlymarly sediments would fill the surrounding depressions.

8. Conclusions

Field observations and microfacies analyses of the Late Jurassicmarl–limestone alternations of western France lead to sedimentolog-ical, sequence- and cyclostratigraphical interpretations that yield aconceptional model describing the role of storms in the formation ofmarl–limestone alternations.

1. The calcareous parts of the Phare de Chassiron section chiefly in-clude marl–limestone alternations that indicate quiet sedimenta-tion in nearshore and shallow-marine environments. However,marly interbeds as well as calcareous beds contain coarse-grainedaccumulations that are interpreted as storm deposits.

2. Storm andmuddeposits probably form in open environmentswherei)muddy sea beds, ii) gentle slope of the shelf, and iii) a large volumeof fine-grained sediments in suspension damped storm-relatedwaves and currents. Sedimentation rate was sufficient to preservenumerous storm deposits from bioturbation and reworking.

3. Durations of the hierarchically stacked elementary, small-, andmedium-scale sequences indicate high-resolution climate changesin the Milankovitch frequency band. Elementary sequences corre-spond to marl–limestone alternations that formed in tune withthe precession cycle (20 ka).

4. Climatically induced changes in sea level controlled carbonate pro-duction that was localised on reefs. Transport by frequent stormsof carbonate mud to the adjacent marly basins during periods ofhigh sea level and carbonate production results in the formationof relatively thin calcareous beds compared to marly interbeds.

5. Marl–limestone alternations perfectly reflect the combination of cyclicchanges in carbonate production and episodic high-energy events. Themethodology applied in this study allows one to decipher the sedi-mentary record that depends on both allo- and autocyclic processes.

42 C. Colombié et al. / Sedimentary Geology 271-272 (2012) 28–43

Acknowledgements

The authors thank the UMR CNRS 5125 PEPS of the UniversityLyon 1 and the Foundation MAIF for their financial support. Theyare grateful to Dr. Fabienne Giraud from Université de Grenoble inFrance and Dr. Niels Rameil fromMaersk Oil, Copenhagen (Denmark)for their constructive comments that greatly help to improve the firstversion of this manuscript. Five anonymous reviewers are also greatlythanked for their constructive and helpful comments.

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