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The Cenozoic history of the Armorican Massif: New insights from the deep CDB1 borehole (Rennes Basin,
France)
Hugues Bauer, Paul Bessin, Pierre Saint-Marc, Jean-Jacques Châteauneuf, Chantal Bourdillon, Robert Wyns, François Guillocheau
To cite this version:
Hugues Bauer, Paul Bessin, Pierre Saint-Marc, Jean-Jacques Châteauneuf, Chantal Bourdillon, et al.. The Cenozoic history of the Armorican Massif: New insights from the deep CDB1 bore- hole (Rennes Basin, France). Comptes Rendus Géoscience, Elsevier, 2016, 348 (5), pp.387-397.
�10.1016/j.crte.2016.02.002�. �insu-01296796�
Stratigraphy, Sedimentology
The Cenozoic history of the Armorican Massif: New insights from the deep CDB1 borehole (Rennes Basin, France)
Hugues Bauer
a,*, Paul Bessin
b, Pierre Saint-Marc
c, Jean-Jacques Chaˆteauneuf
d, Chantal Bourdillon
e, Robert Wyns
a, Franc ¸ois Guillocheau
faBureauderecherchesge´ologiquesetminie`res(BRGM),3,avenueClaude-Guillemin,45060Orle´anscedex2,France
bUniversite´ duMaine,Ge´osciencesLeMans,avenueOlivier-Messiaen,72085LeManscedex09,France
cSEMMLogging,LesMaufras,18360Vesdun,France
dBRGM(retired),8,quaiduChaˆtelet,45000Orle´ans,France
eERADATA,170,avenueFe´lix-Geneslay,72100LeMans,France
fUniversite´ deRennes-1,UMR6118Ge´osciencesRennes,campusdeBeaulieu,35042Rennescedex,France
1. Introduction
TheArmoricanMassif’spost-Variscangeologicalevolu- tionisknownonlyfromscatteredsmallCenozoicbasins, anditiscommonlyadmittedthat,fromtheCarboniferousto the Eocene, the Massif merely underwent erosion then weathering (e.g., Sissingh, 2006). Divergent, but notable, studieshaveindicatedthepossibilityofMesozoicdeposits ontheMassifduringperiodswhenitwouldhavebeenpartly orlargelyunderwater(AndreieffandLefort,1972;Guillo- cheauetal.,2003;Wyns,1991;Wynsetal.,2003).Evidence forCretaceousreworkingisplentiful(seeBessinetal.,2015
for a review), with the nearest in situ deposits being CenomaniansandsintheLavalBasin(Durandetal.,1973).
The RennesBasin, as the deepest Cenozoicbasin of the ArmoricanMassif(Wyns, 1991),is a good candidate for trackingpossiblepreservedpre-Eocene(Mesozoic)series.A 5-kmseismiclineacrossthebasinfromthe2000GeoFrance 3D programme (Wyns etal., 2002; Fig.1)reveals three seismicunitsinterpretedaspreservedMiddleJurassicand UpperCretaceousseriesunderthe outcroppingCenozoic cover(Wynsetal.,2002). Withoutanydrillingdata,this interpretationhasremainedhypothetical.
Finally, along with water-resource and geothermal- potentialprospecting, theCINERGYProject drillingpro- grammewasfundedtoresolvethisquestion(Baueretal., 2010).BoreholeCDB1wascompletedin2010atadepthof 675.05m.Thefirstresultsandimplicationsoftheborehole ARTICLE INFO
Articlehistory:
Received30November2015
Acceptedafterrevision25February2016 Availableonline31March2016 HandledbySylvieBourquin Keywords:
Eocene Oligocene EOT
Weatheringprofile Continentaldrilling
ABSTRACT
Borehole CDB1 (675.05m) crosses the deepest Cenozoic sedimentary basin of the ArmoricanMassif,theRennesBasin, toreach theunderlyingbasementat adepth of 404.92m, madeup of the Late Neoproterozoicto Early Cambrian Brioverian Group, weathereddownto520mdepth.Thebasin’sCenozoicdepositsaredividedintoseven formations, ranging from Early–Middle Bartonian to Late Pliocene in age. Coastal sedimentsattheverybase,alongwithathickPriabonianlacustrineepisode,implyamajor revisionoftheregionalpalaeogeography,whilstaverysteadyandlow-energylacustrine- palustrineenvironment throughout the Priabonian and Early Rupelian argue for an aggradationalsystemassociated with uniformsubsidence. Palynologicalassemblages attesttoenvironmentalandclimaticchangesthroughtheEoceneandEarlyOligocene,in accordancewithregionalandglobaltrends(Eocene–OligoceneTransition).
ß2016Acade´miedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/
4.0/).
* Correspondingauthor.
E-mailaddress:h.bauer@brgm.fr(H.Bauer).
ContentslistsavailableatScienceDirect
Comptes Rendus Geoscience
w ww . sc i e nce d i re ct . co m
http://dx.doi.org/10.1016/j.crte.2016.02.002
1631-0713/ß2016Acade´miedessciences.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://
creativecommons.org/licenses/by-nc-nd/4.0/).
stratigraphy,includingboththebasin’ssedimentaryinfill anditsbasement,arepresentedbelow.
2. GeologicalsettingoftheCenozoicRennesBasin Thesmallnorth–southRennesBasin(15kmlong,2km wide)isboundedtothewestbythedextralN1608Pont-Pe´an
FaultandtotheeastbyhiddenN1358toN1608enechelon faultsrelayedbysubsequentN208faults(Fig.1).Itsoverall shapeisknownfromagravimetricsurveythatrevealsan elongatebasinpartitionedbynumerousfaultsintoblocks with verydifferent sedimentary thicknesses (Trautmann etal.,1994;Wynsetal.,2002)andwithitsdeepestpart centredontheChartres-de-Bretagnearea(JaegerandCorpel, 1967).ThebasinislargelysurroundedbyaBrioverianGroup (Gp) basement of folded Late Neoproterozoic to Early Cambrian azoic siltstone and sandstone, and rests on Palaeozoic synclines to the south. Six lithostratigraphic formations, ranging in age from Late Eocene (Lower Sapropels Fm.) to Pliocene (Azoic Sands Fm.), were previouslydefined throughintegratingoutcropandshal- low-borehole data (e.g., Durand, 1960; Este´oule-Choux, 1967;Ollivier-Pierre,1980;Thomas,1999;Trautmannetal., 1994, 2000;Fig. 1).Two majorunconformitieswere also identified:
a first angular and erosional one between the Upper Sapropels Fm. (Late Rupelian) and the Faluns and Lithothamniids Limestone Fm. (Langhian to Serraval- lian);
asecondpurelyerosionalone,whichunderliestheAzoic SandsFm.
Attemptstoestimatethethicknessofeachformation havebeeninvainduetotheirhighvariabilityinoutcrop (Trautmannetal.,1994).
Finally,theChartres-de-Bretagneareawasselectedas thebestlocationforadeepboreholetointersecttheentire basininfill.
3. Materialandmethods
3.1. BoreholeCDB1
Thefirst66moftheBoreholeCDB1(hereinafterCDB1) wererotarydrilledwithcuttingsbeingcollectedevery3m then,from66to675.05m,thedrillingwasfullycoredin HQ diameter (cores of 85mm diameter). Core analysis comprisedadetailedsedimentologicaldescriptionatthe 1:20 scale, colour description (MunsellCharts) and full 3608core scanningona DMTCorescan1.Samplingwas performed on half cores split into two and placed in nitrogen-filledplasticbags.
3.2. Biostratigraphy
Theentireboreholeserieswassampledat105levels formicropalaeontological(0–80and645–675mindepth) and palynological (0–675m in depth) analysis (Fig. 2).
Although pollen grains provide the first clues for palaeoenvironmental and climatic reconstruction, some palynomorph assemblages or species are fairly good stratigraphic biomarkers where dinocysts are absent.
Samplepreparationfollowedstandardprotocolforpaly- nology in terms of acid attack, washing, sieving, and mounting, and for micropalaeontology in terms of soft drying,watersoaking,and50to200
m
msieving.Fig.1.GeologicalmapoftheRennesBasinshowingthelocationofBorehole CDB1andtheGeoFrance3Dseismicprofile.P-PF.:Pont-Pe´anFault.
Fig.2.Litho-andchronostratigraphyofBoreholeCDB1.Depositionalenvironments:1:fluvial;2:palustrinetolacustrine;3:lagoontorestrictedmarine;4:
coastalmarinetoopenmarine.
4. Results
4.1. Lithostratigraphy,faciesandpalaeoenvironments 4.1.1. TheBrioverianGp.basementanditsweatheringprofile
TheBrioverianbasementunderlyingtheRennesBasin consistsmostlyoffolded(dipvariesfrom508to908)shale andsiltstoneofdistalturbidites(probablydepositedina prodeltaenvironment;Trautmannetal.,2000)sedimen- taryfacies(Fig.2).
Thebasement isalso marked by adeep weathering profile (Fig. 2),from whichthe topmost ferricrust and mostoftheupper(non-isovolumetric)saprolite(allote- rite) were eroded by the overlying Cenozoic deposits;
remnantsofthe saprolite,i.e.massive softbeigeclays, occurfrom 404.92to 405.90m.Below 405.90m, base- ment structures (stratification and cleavage) are pre- servedwithinanisovolumetricsaproliteandbeigeclays change to grey towards 406.60m. Between 420 and 440m,saprolitegraduallyturnstoanunderlyingfissured layer(indurated butfracturedrock;Wynset al., 2004) withsmallquartz-,carbonates-orgoethite/pyrite-filled fractureswithrareshortargillizedintervals.Thefractured horizonprogressivelypassestothefreshrockatca.520m indepth.
4.1.2. TheChartres-de-BretagneFm.
The sedimentary succession of the Rennes Basin beginswitha30-m-thickformation (404.92–375.40m) termed the Chartres-de-Bretagne Fm. (Fig. 2). It starts withathinconglomeraticsoleoverlainbyshalydeposits, includingsomeorganic-richlevels.Theshaleismassive (claystone) from 400.00 to 391.80m, and it becomes thinlylaminateduptoca.386.00m.Theshaleprogres- sivelyreturnstomassiveupto380.80m,withasandier fractionappearing at 382.00m.The uppermost partof thisformationis madeupof loosetopoorlycemented quartziticsandstone.
Thedeposits of the Chartres-de-Bretagne Fm.reflect variousenvironments.The lowermosttwometres area transgressivelag,followedby coastalsandwithbenthic fauna (C. parisiensis, C. columnatortilis, A. kerfornei, V. globularis) attesting to temperate to warm marine waters.Thefirstorganiclevel,followedbyaclayeyinterval up to382m, contains pollen assemblages prevailingin lacustrine and palustrine environments surrounded by subtropicalforestwithdominantMyricaceaeandrareor absentherbaceousplants.Theuppersandyinterval,upto 375m,is typicalofacoastalenvironmentwithbivalves andbenthicforaminifera.Nearbymangrovesareindicated by Avicennia pollen grains and some brackish water dinocystsoccurrences.
4.1.3. TheLowerSapropelsFm.
The Lower Sapropels Fm., whose contact with the underlying Chartres-de-Bretagne Fm. is marked by the disappearanceofthesandfractionat375.40m(Fig.2),is thethickestsuccessioncrossedbytheborehole(375.40–
85.15m) and includes the Mohria Shales Fm. formerly describedbyDurand(1960).Theentiresuccessionexhibits pluri-todecametricalternationsoflaminatedshaleand
massiveorbrecciatedclaystone,similartothatdescribed in the Chartres-de-BretagneFm. Theorganic contentof thesedepositsvariesfromzero(whitishclaystone)tomore than 10% (carbonaceous shale), but independently of laminatedormassivefacies.Thelaminatedshaleispale ochre to dark brown-blackish in colour (depending on the organic content) with laminae from less than a millimetre to a few millimetres thick. Microfaults and soft sediment deformation,suchasmicro-slumping,are observedinplaces.Themassiveandbrecciatedclaystone has various colours (whitish, ochrous, greenish, and brownishtoblackish)dependingontheorganiccontent and the redox conditions. Where purely massive, the claystone is blocky textured and commonly associated with roottraces. The brecciated facies is mostly insitu breccia resulting from pedoturbation/desiccation, but rounded muddy intraclasts attesting to episodic flows arealsoreported.
Both the claystone and shale are sulphide-rich with disseminated silt-sized pyrite grains or pyritized roots, plusbioturbationsorfracturefillings.Theonlycarbonate occurrenceswithinthetwofaciesaremadeupof:
rare centimetric mudclasts found basically in the laminatedfacies;
rarecalcitic/ankeritic(epigenized) ostracodvalvesand gastropods(associatedwithorganiclayers);
asinglepedogenicnodulefoundat308.80m.
The only limestone bed in this shaly succession is an unexpected massive 70-cm-thick calcrete (158.80–
159.50m).
The entire Lower Sapropels Fm. deposited in a lacustrine/palustrine environment, with the laminated shalecorrespondingtohypoxiclacustrinedeposits(highly organic, no bioturbation) and the massive, blocky and pedoturbatedclaycorrespondingtohydromorphicpalus- trinedeposits.
The massive clay contains abundant plant tissue ranging from sub-aquatic algae to monocotyledon or dicotyledonfenplantssuchasHydrocharitaceae,Alisma- taceae, Potamogetonaceae, Lemnaceae, Typhaceae-Spar- ganiaceae,Restionaceae,Cyperaceae,andNelumbonaceae (Lotus).Suchassemblagesprobablyformedlargefloating rafts,similartothoseknown todayinNorth Americaor easternAfrica(Chapmanetal.,2001),thatblockedoutthe light and anyotheraeolian input,as attested tobythe absence of any pollen grains from the surrounding terrestrial vegetation. Conversely, the laminated shale showshighextra-palustrinepollencontentandisrichin Botryococcusbraunii,attestingtomoreopenconditionsand a better clarity in the lacustrine surface waters. Rare dinocyst occurrences (Homotryblium, Spiniferites) in the uppermost part of the formation suggest short marine incursionsintothelake,asoriginallyreportedbyOllivier- Pierreetal.(1993).
4.1.4. TheNaticacrassatinaMarlsFm.
TheNaticacrassatinaMarlsFm.(85.15–57m),marked atitsbasebya20-cm-thickcalcarenitebedwithanerosive base (Fig. 2), is characterized by a metre-scale cyclic
succession starting with greyish marl with numerous molluscs,followedbygreenishclaystonewithoccasional root traces topped by a thin black shale layer. The sequences become more carbonate-rich upward and a bioclasticlimestoneispresentasof69.00m.
The episodic marine incursions into the lacustrine environment towards the top of the underlying Lower Sapropels Fm.announced environmentalchanges atthe transition withtheN.crassatinaMarls Fm.The malaco- faunaatteststoarestricted(L.munieri,Hydrobiidaea,and Potamididae),but marine(P.trochleare)environment,as confirmed by the numerous dinocystoccurrences, with episodicemergenceindicatedbythemassivegreenishclay bedswithroottraces,interpretedassaltmarshdeposits.A marine transgression is perceptible upwards, with in- creasedcarbonatecontentandtheappearanceofmarine benthic microfauna, such as the Peneroplis genus. The basin’senvironmentevolvedfromarestrictedbaytoan openmarineenvironment(shoreface).
4.1.5. TheArchiacinaLimestone,UpperSapropels,Falunsand LithothamniidsLimestone,andAzoicSandsformations
The upperseriesof theRennes Basin(0–66m)were rotarydrilled,providingonlyapartialviewofthefacies andstratigraphicboundariesthroughcuttings(Fig.2).The ArchiacinaLimestoneFm.,from57to33m,comprisesgrey fossiliferouslimestonewithabundantP.armorica,agrazer foraminifer found in clear, oxygenated, marine coastal waters where seagrass develops. The absence of any Nummulitidaeisprobablyduetoashallowseawithina changingenvironment(WrightandMurray,1972)whilst early cementing and recrystallization around bioclasts, together withthe foraminiferal assemblages, indicate a highlyfluctuatingsealevel,possiblyendinginemersion.
Restrictedmarineepisodesarealsomarkedbylevelswith low microfaunaldiversity and dwarfforms.Overall,the ArchiacinaLimestoneFm.marksthemaximumtransgres- sionoftheseries.
TheUpperSapropelsFm.from33.0to25.5m,ismade upofinterbeddedclaystone,marlstoneandlimestone.The dwarf benthic foraminiferal assemblages indicate a restrictedmarineenvironment.Thepalynologicalcontent is devoid of dinocysts, but rich in onshore herbaceous plantsandlacustrinehydrophilouselements.
The Faluns and Lithothamniids Limestone Fm. from 25.5to4.0m,isacoarserecrystallizedbioclasticlimestone depositedinahigh-energy,upper-shorefacetoforeshore environment. The microfauna came from the offshore/
lowershorefaceenvironmentandweredepositedonthe beachbywavesandcoastalcurrents.
Finally,from4mtothesurface,wehavethequartzitic AzoicSandsFm.,madeupofmilkyangularquartzgrains, translucentroundedquartzgrainsandsubroundedlitho- clasts. No fossils or glauconitic grains were found, suggestingfluvialdeposits.
4.2. BiostratigraphyofCDB1
The biostratigraphical results are summarized in Table1andFig.2,withthemostimportantstratigraphic taxaillustratedassupplementarymaterial(SM1,SM2).
4.2.1. TheBrioverianbasement(LateNeoproterozoic–Early Cambrian)
Nopalynomorphorothermicrofossilwasobservedin the10samplestakenfromthewell-knownazoicsequence oftheArmoricanBrioverianGp.,whichhasbeenassigneda Latest Neoproterozoic to Early Cambrian age (Guerrot etal.,1992;Trautmannetal.,2000).
4.2.2. TheChartres-de-BretagneFm.(Bartonian)
The biostratigraphy of the Chartres-de-Bretagne Fm.
relies on 24 levels sampled for both palynology and micropalaeontology(Table1,Fig.2).Thefirstfossiliferous levels(401.70to 400.60m) compriseargillaceous sand- stone providing rare agglutinated benthic foraminifera.
Theassemblages,althoughincomplete,stronglyresemble theclassicBartonianones(e.g.,Dugue´ etal.,2005),with C. parisiensis (d’Orbigny), C. columnatortilis (d’Orbigny), A.kerfornei(Allix)andV.globularis(d’Orbigny).Associated withtheseEocene foraminiferaarerarereworkedspeci- mensofLateCretaceousbenthicforaminifera(Arenobuli- Arenobulimina,Ataxophragmium,andTritaxia)suggesting nearby(ormoredistal)Cretaceousdepositsbroughtinto theRennesBasinbyfluvialprocesses.Theuppersandstone towardsthetopoftheformationyieldedamorediversified assemblageofbenthicforaminifera,togetherwithbival- ves, rare echinoderms, ostracods, and gastropods. This coastalassemblageisalsoattributedtotheBartonian.
Thefirstorganic-richlayerat399.90m,andsubsequent ones up to ca. 392m, yielded pollen assemblages composedmainlyofJuglandaceae,Pinaceae,andSchiza- ceae. Diporites and otherspecies suchas R. loburgensis, knownin theCampbonBasinandtheBois-Goue¨tSands Fm.oftheSaffre´ Basin(Ollivier-Pierre,1980),werealso found.Thesearetypicaloftheinformal‘Biarritzian’French stage(Ollivier-Pierre,1980), whosebiomarkersareassi- gned to the Early–Middle Bartonian (Serra-Kiel et al., 1998). From 391.50 up to 316.50m in the overlying formation,thepollenassemblagecontainsthelastformsof P. subhercynicus (Myricaceae) along with typical forms suchasR.gibbosus(Araliaceae),S.cf.huloti(Rhamnaceae), T.clavatus(Leeaceae),P.crassus(Theaceae),andP.cognitus (Apocynaceae?), occurring elsewhere during the Late Bartonian.Theuppersandstoneyieldedraredinocystsof no biostratigraphic value(S. ramosusand Homotryblium sp.).TheentireChartres-de-BretagneFm.thusdeposited duringtheBartonian(Table1,Fig.2).
4.2.3. TheLowerSapropelsFm.(LateBartonian–Early Rupelian)
TheLowerSapropelsFm.beingdevoidofdinocystsof stratigraphicvalue,itsbiostratigraphyisdeterminedsolely onthe palynologicalcontent ofthe49 analysedsamples (Table 1, Fig. 2). Pollen assemblages provide sufficient changestodifferentiateLateBartonian–EarlyPriabonian, LatePriabonian,andEarlyRupelianintervals(Table1,Fig.2).
From375.40to278.00m, anintervalshowinga first developmentofconifers,thepollenassemblageissimilar tothoseofLatestBartoniantoEarlyPriabonianagefound inotherwesternEuropeanbasins(e.g.,theLateMarinesian and Early Ludian from the Paris Basin). The top of this biozone is marked by the Last Occurrence (LO) of
Burmaniaceae,T.duvigneaudi(Rubiaceae),S.tenuis(Elea- gnaceae),andofmanyformsofRetitricolpitestogetherwith theacmeofA.cyclops.
From 278.00 up to 204.98m, most of identified megathermicformsareknownfromtheLatePriabonian (LateLudian)oftheParisBasin,particularlyT.raguhnensis (whichisstrictlylimitedtothisinterval)associatedwith T. ludensis and T. rhombus. The interval also shows an increaseinconifers,Nyssaceae,Umbelliferaeand Hama- melidaceae,alongwithadiversificationofherbaceousand shrubvegetation.
ThebaseofOligoceneinEuropeancontinentalstratais classicallymarkedbytheFirstOccurrence(FO)ofB.hohli (Eleagnaceae; Sittler et al., 1975). In CDB1, this pollen appearsbetween195.45and195.08m(Table1,Fig.2).The boundary is also marked by a strong development of conifers,theFOofC.simplex(Juglandaceae),Cistaceaeand P. persica (Hamamelidaceae), and the development of EphedraceaeandUmbelliferae.TheLOoftwoimportant markersoftheLatestPriabonian(P.cognitusandP.crassus) at 204.83m, together with the very thin (196.24–
195.08m)M.vanwijkeibiozone(LatestLudianandEarliest SannoisianoftheParisBasin;Chaˆteauneuf,1980),confirm a ca. 10minterval for theEocene–Oligocene Transition (EOT).
4.2.4. TheN.crassatinaMarlsFm.(EarlyRupelian) The biostratigraphy ofthe N.crassatina Marls Fm. is basedonmalacofauna,micropalaeontology,andpalynol- ogy.Althoughthefirstblackshalelayeryieldedonlytwo speciesofmolluscs,themarlyielded24speciesdominated byL.munieri(bivalve),F.dollfusi(gastropod)andotherwell represented taxa such as the emblematic Rupelian gastropod A. crassatina (formerly associated with the Natica genus). The benthic foraminiferal assemblages (Quinqueloculina,Spiroloculina,Miliola,Discorbidae,Rosa- linidae, etc.) belong to the Early-Middle Rupelian SBZ 21 biozone (Cahuzac and Poignant, 1997). The pollen assemblagesaresimilartothosefromtheupperpartofthe Lower Sapropel Fm., with B. hohli as a marker of the Rupelian. Dinocysts are found within three samples between 83.25 and66.60m;theassemblagesbelongto theD14nabiozoneasdefinedinGermanyandthesouthern North Sea (Ko¨the, 2012), which includes G. conopea, G. paupercula, H. palmatum/recurvatum subsp. parvum, I.multispinosum,C.giuseppei,etc.Thisbiozonecorrelates withtheplanktonicforaminiferabiozonesO1p.p.andO2, and the nannofossilzone NP23, which cover a ‘Middle’
Rupelian interval and correspond to the Fontainebleau Sands Fm.oftheParisBasinand theBoomClaysFm.in Belgium(Lozouet,2012).
Table1
Litho-andchronostratigraphysynthesisofBoreholeCDB1.
Depth(m) Thick.
(m)
Formation Age
Informalage
Indexfossils Biozones
Drilling material
Lithology
Top Base
0.00 4.00 4.00 AzoicSands Pliocene – Cuttings Sand
4.00 25.50 21.50 Falunsand
Lithothamniids Limestone
LateLanghian toSerravallian
Elphidiumspp.&
P.serrata assemblage
Limestone
25.50 33.00 7.50 UpperSapropels Rupelian/Late
Stampian
SBZ22 SBZ21
Marlandclay
33.00 57.00 24.00 ArchiacinaLimestone SBZ21 Limestone
57.00 85.15 28.15 Naticacrassatina Marls
SBZ21&D14na Cuttings/
cores
Marl,clay, limestone
85.15 195.26 110.11 LowerSapropels FOofB.hohli Cores Clay
195.26 278.00 82.74 LatePriabonian/
LateLudian
T.raguhnensis T.ludensis T.rhombus
Clay
278.00 375.40 97.40 LateBartonian
toEarly Priabonian/Ludian
LOofBurmaniaceae, T.duvigneaudi, S.tenuisand A.cyclopsacme
Clay
375.40 404.92 29.52 Chartres-de-Bretagne Bartonian C.parisiensis, C.columnatortilis, A.kerforneiand V.globularis P.subhercynicus (pollen)
Clay,sand, conglomerate
404.92 405.90 0.98 Weath.Brioverian–
Alloterites
LateNeoproterozoic toEarlyCambrian
– Clay
405.90 445.00 39.10 Weath.Brioverian–
Isalterites
– Clay,quartz
445.00 520.00 75.00 Weath.Brioverian–
fissuredlayer
– Shale,sandstone,
conglomerate 520.00 675.05 155.05 FreshBrioverian
bedrock
–
Depthsinitalicsremainapproximate.
4.2.5. TheArchiacinaLimestone(EarlyRupelian),Upper Sapropels(LateRupelian),FalunsandLithothamniids Limestone(LanghiantoSerravallian)andAzoicSands(Plio- Pleistocene)formations
The biostratigraphy of the Archiacina Limestone Fm.
relies mainly on micropalaeontology (Table 1, Fig. 2).
Among themany foraminifera, A. globula (Bourdon and Lys),A.kerfornei(Allix)andP.armorica(d’Orbigny)belong totheSBZ21biozone.Otherfossilsincludeostracodsand various molluscsofno stratigraphicvalue.Rarepalyno- morphswereretrievedfromthelimestone,includingan Early Rupelian pollen assemblage similartothat in the upper partofthe LowerSapropels Fm. and animpove- risheddinocystassemblage.
ThelowerpartoftheUpperSapropelsFm.stillcontains SBZ21biozonemarkers,butthesearelackingintheupper part(from30mupto27m),despiteotherspeciesshowing astablecoastalenvironment.Wethusproposeassociating thisintervalwiththeLateRupelianSBZ22biozone,evenif SBZ 22 markers are missing. The pollen and dinocyst assemblagescontainRupelianmarkers.
ThebiostratigraphyoftheFalunsandLithothamniids Limestone Fm. is based only on benthic foraminifera assemblages (e.g., Elphidium sp., Elphidiella, P. serrata, Rosalina)consistentwithaLateLanghian–Serravallianage, in accordance with previous studies (Trautmann et al., 2000).
No biostratigraphical data is available for the Azoic SandsFm.intersectedbyCDB1.Yet,aLatePlioceneagewas measuredbyESRintheequivalentestuarinedepositafew kilometrestothenorth(VanVliet-Lanoe¨ etal.,1998).
4.3. Palaeoclimatereconstruction
The palynology study also enabled the climate changestobereconstructed(Fig.3).TheEarlyBartonian assemblagesattesttoawarmhumidclimatecomparable tothatoftheLutetian(Gruas-Cavagnetto,1977).TheLate Bartonianis markedbytheonsetofashortdryseason associatedwitha first coolingevent attested tobythe disappearanceofasetoftropicalwarmtaxasuchasNypa and Bombax in the La Trinite´-Porho¨et and Campbon basins(Ollivier-Pierre,1980).ThesameeventinCDB1is markedbythecoevalLOofBurmaniaceaeandAvicennia togetherwiththeFOofxericEphedraandHohenakeria genera.
The pollen assemblage reveals marked changes throughouttheLowerSapropelsFm.,relatedtotheglobal climate changes spanning the Late Eocene to Early Oligocene.Thebaseoftheformationischaracterizedby abundantmegathermicflora,butConiferalesandHerba- ceaedevelopedfrom265.90mupwardsandmesothermic forms became increasingly abundant, implying a new temperaturedrop.TheshortEOT(196to204minCDB1)is markedbyamajorimpoverishmentofwarmdicotsfloraso that M.vanwijhei(Caesalpiniagrowing inxericzonesof SouthAmericaandNorthAfrica)wascommonlyisolated before the Rupelian marker B. hohli appeared. This is interpretedasacooler,drierepisode(Chaˆteauneuf,1980;
Schuler,1983,1988)beforetheRupelianclimatereturned tomorehumidconditions.
Finally,thecoolerRupelianclimateoftheN.crassatina Marls,ArchiacinaLimestoneandUpperSapropelsforma- tions is marked by mesothermic flora replacing the megathermicfloraoftheEocene,asisalsoknowninthe Paris Basin from the Lowermost Oligocene (Sannoisian facies)uptotheFontainebleauSands Fm.(Chaˆteauneuf, 1980;Gruas-Cavagnetto,1978).Therareremainingwarm forms, such as Mussaenda or Alchornea, disappeared in responsetoatemperaturedropduringthecoldestmonths (Mosbrugger et al., 2005), whilst an increase in the herbaceousfractionintheUpperSapropelsFm.indicates amarkeddryerseason.
5. Discussion
5.1. Implicationsonthelocalandregionalpalaeogeography ThefirstresultsfromCDB1providenewinsightsabout the Armorican palaeogeography. At least one period of intenseweathering(see§4.1)isrecordedbeforetheonset of the Rennes Basin, bearing out the pre-Bartonian weatheringofthemassifalreadyhighlightedinprevious studies (e.g., Wyns et al., 2003). Moreover, reworked Late Cretaceous foraminifera possibly argue for earlier marinefloodingoftheArmoricanMassifduringthisperiod (seeBessinetal.,2015fordiscussion).
FortheCenozoictimes,fourofthesixwell-recorded marinefloodingeventsthataffectedtheArmoricanMassif (Guillocheauetal.,2003)areidentifiedinCDB1.Newdata combinedwithpublishedstudies(seeGuillocheauetal., 2003 for references) call for some palaeogeographical revision(Fig.4):
theinnermostextensionoftheEarlytoMiddleBartonian (ParisBasin:Biarritzian)transgressionisseenin CDB1.
Thistransgressionwasdocumentedattheedgesofthe massif[e.g.,atSaffre´ (Borneetal.,1991),Fye´ (Chaˆteau- neuf,1980)andTre´guier(Guennocetal.,2015)]andin surroundingbasinswiththeLimestonesandSandswith NummulitesBrongniartiFm.(AndreieffandLefort,1972;
Paquetetal.,2010;Thomas,1999).Thebiostratigraphic assemblagesfromthesedeposits,however,arespatially homogeneous,sono privileged(northward,southward orboth)marineconnectioncouldbeproposedforthe RennesBasin;
thePriabonianstageisformallyidentifiedinCDB1(Paris Basin:Ludian).Togetherwithbiostratigraphicrevisions ofthescatteredCenozoicbasinsonthemassif(e.g.,the Saffre´, Laval and Lande´an basins), it clarifies the Priabonianpalaeogeography.Aregressivephaseaffected the overall massif, which only records lacustrine environments(180mthickintheRennesBasin)with somelagoonalareaslocatedonitsedges;
theLower Rupelian transgression,already reported in the Rennes Basin (e.g., Ollivier-Pierre et al., 1993), probably came from the south, as argued by the distributionof preservedmarine occurrences close to theRennesBasin(Fig.4);
fortheLanghian–Serravalliantransgression,thelocations of marine occurrences and their palaeobathymetries favourtheestablishmentofanorth–southmarinecorridor
linking the Channel basin and the South Armorican Margin, as already proposed by several authors (e.g., Thomas,1999).
These reviews suggest repeated flooding of the Armorican Massif during Cenozoic and even Mesozoic times.
5.2. GeodynamicsoftheRennesBasin
Thesedimentationratesforthedifferentstratigraphic intervalsoftheRennesBasinvarygreatlyfromca.9m/Myr duringtheBartoniantoca.44m/MyrforthePriabonian interval.ThethickPriabonian–Rupeliansuccessionassoci- atedwithasteadyenvironmentarguesforanaggrading systemwithsedimentationcompensatingtheaccommo- dationrate.Thesedimentationratehereismuchhigher thanintheSaffre´ Basinfor thesameinterval (ca.18m/
Myr;Borneetal., 1991)withsimilar shalydeposits.By comparison(Fig.3),thecoevaldepositsintheEbroBasin andtheRhineGrabenaremuchthickerwithcompacted
sedimentation rates estimated from 90 to 200m/Myr (Costa etal., 2011; Rousse´,2006). Conversely,theParis Basin,BelgiumBasin,andHampshireBasinshowverylow sedimentationrates(10to22m/Myr;Briais,2015;Hooker, 2010;Vandenbergheetal.,1998,2012).Thesedimentary recordofthesebasinsalsoshowdiscontinuitiesrelatedto the overall low accommodation rates (intracratonic basins).TheRennesBasinsedimentationvaluesaveraging 42m/Myr clearly lie between the high and low ‘end- members’.
Considering thegeologicalsettingsandvarying sedi- mentationrates,weconsiderthattheRennesBasinwas initiatedduetofloodingofapreexistingtopographiclow ofpossiblygreaterextent,and thenunderwentahigher subsidencerate,maybeassociatedwithstrike-slipmove- mentsalongtheN1408faultsduringPriaboniantoEarly Rupeliantimes.Thissecondphaseisfurthersupportedby evidence of synsedimentary deformation (seismic line pinchout;Wynsetal.,2002)andcouldbeduetorelative movements betweentheAfricanand Eurasianplates,as wellasEnglishChannelinversions(e.g.,Ziegler,1990).
Fig.3.PalaeoclimaticevolutionandpalaeoenvironmentalchangesoftheRennesBasinwithregardstootherwesternEuropeanbasins(adaptedfrom Ollivier-Pierreetal.,1987).Atthebasin’sscale,theEocene–OligoceneTransition(EOT)iscommonlymarkedbychangesindepositionalenvironmentfrom marineorlagoonaltolacustrineorbyoscillationsaroundcoastalsettings.Nevertheless,theEOTintheEbroBasinisfullyterrestrial,whilstintheRhine Basinitappearstobecrossedcontinuouslybysaltdeposits.DatafortheEbroBasinfromCavagnettoandAnado´n(1996),Alonso-Zarzaetal.(2002),Costa etal.(2010,2013)andBarro´netal.(2010);fortheHampshireBasinfromCollinson(1992),Hookeretal.(2004,2009)andCostaetal.(2011);fortheParis BasinfromChaˆteauneuf(1980)andBriais(2015);forBelgiumandtheNorthSeafromVandenbergheetal.(1998)andRoche(1982);andfortheRhine GrabenfromSittler(1965),Schuler(1983),andRousse´ (2006).(HaqandSchutter,2008).
5.3. PalaeoclimatechangerecordoftheRennesBasin Supportingthestratigraphicframework,thepalynologi- calstudybringsnewdataonthemajorEocene–Oligocene climaticchange(Fig.3).TheBartoniantoPriaboniansawa progressive change from a warm humid climate to a stronger seasonality, as previously reported by Ollivier- Pierre (1980) and Chaˆteauneuf (1980) on a western European scale. The stronger seasonality was relatedto thedevelopmentofafresherdrierseasonasindicatedby Mosbruggeretal.(2005)andattestedtobytheprogressive developmentofConiferalesandmesothermicflora.TheEOT intheRennesBasinismarkedbyanabruptcoolingduring theLatestPriabonianbeforetheFOofB.hohli;anabrupt climaticchangealsoreportedintheParisBasin(Chaˆteau- neuf, 1980), the Rhine Graben (Schuler, 1988), the HampshireBasin,andtheBelgiumBasin(seeOllivier-Pierre etal.,1987,forareview).Thiscanalsoberelatedtothe globalcoolingeventsrecordedworldwide,i.e.theEOTand subsequent‘‘Oi’’glacialevents(e.g.,Katzetal.,2008),andis connectedtotheinitiationoftheAntarcticicesheet(e.g., Zachosetal.,2001).Althoughtheassociatedbase-leveldrop isnotwell-recordedintheRennesBasin,amoderatesea leveldropof15mormoreisestimatedinthelesssubsiding
HampshireBasin(Hooker,2010;Hookeretal.,2007).Itis thereforelikelythatthehighersubsidenceintheRennes Basincompensatedthesealeveldropwithoutanymajor environmentalchange.
The climate record of the Rennes Basin appears to reflectastepwisechangefromawarm,humidBartonian climatetoacooler,drierclimatearoundtheEOTbeforea returntomorehumidconditions.Thisrecordisconsistent withthoseofotherwesternEuropeanbasinsandnotably that of the Ebro Basin (Fig. 3). Whether the slight discrepancies are due to real local climatic differences, ortohigh-frequencychangescoupledwithlow-resolution samplingofvariousstudies,isstillanopenquestion.The ParisBasin recordappears morechaotic, but this could reflectahigher-resolutionfluctuationortheinfluenceof variabledepositionalenvironments.Suchenvironmental changes across theEOT are widespread in thewestern Europeanterrestrialrecords,soclimatesignalswouldbe morecomplextodecipher(Fig.3).Inthisrespect,theCDB1 recordlookslikepromising materialin thatthedeposi- tional environment remainedlacustrine/palustrine from EarlyPriaboniantoEarlyRupelian,embracingtheentire EOT.
6. Conclusionsandoutlook
TheCDB1coreisthefirstfullrecordofthesedimentary infilloftheRennesBasinanditsweatheredsubstrate,and themostcomprehensiverecordofCenozoicdepositsinthe Armorican Massif. It brings new findings towards the understanding oftheCenozoicgeological, palaeogeogra- phical,andclimaticevolutionofthisregion:
acontinuousCenozoiclithostratigraphicandbiostrati- graphicframeworkisdescribed,withsevenformations defined from CDB1 and previous studies. The basin deposits begin with the Chartres-de-Bretagne Fm.
(Early–MiddleBartonian)followedbythe290-m-thick organic and shaly Lower Sapropels Fm. (Priabonian–
EarlyRupelian)overlainbytheN.crassatinaMarlsand ArchiacinaLimestone formations(Early Rupelian),and then by theUpper Sapropels (Late Rupelian) and the FalunsandLithothamniidsLimestoneformations(Mid- dleMiocene);
a hinterland marineincursionof the Bartoniantrans- gression,largelydocumentedfromthesouthernArmor- ican coast, is evidenced. The Priabonian is formally described and the Eocene–Oligocene boundary recog- nizedwithinthelacustrinedeposits;
comparisonwithotherwesternEuropeanbasinsshows that the sedimentation rates of the Rennes Basin are intermediatebetweenanintracratonicbasinandatrue graben.Italsoremainedacontinuouslacustrineenviron- mentfromtheLateEocenetoEarlyOligocene,whereas most of the other basins underwent environmental changesattheEOT;
the Rennes Basin is a unique record of the EOT and illustrates a gradual climate change from the Early PriaboniantotheveryLatePriabonianbeforearapidcool arideventtookplacejustpriortotheOligocene.
Fig.4.RegionalpalaeogeographicalevolutionfromtheBartoniantothe Langhian–SerravallianmodifiedfromThomas(1999)andarguedfrom publishedsedimentologicalandbiostratigraphicaldata(seeGuillocheau etal.,2003,forreferences).
These first results from CDB1 are really promising for further studies concerning the Armorican Cenozoic historyandalsoforabetterunderstandingoftheEOTin continentalsettings.
Acknowledgements
TheauthorswouldliketothanktheBRGMandtheAELB and ADEME agencies for their financial support to the CINERGYproject.BoreholeCDB1wasfundedbytheCG35, Re´gionBretagne,Me´tropoleRennes,Chartres-de-Bretagne citycouncil,the‘‘Pre´fecturedeBretagne’’,IAV,SMPBR,and SMG35,andwasdrilledbyCOFOR.Wealsothankallthe CDB1scientificparty,aswellasPierreLozouet(MNHN)for the malacostratigraphy. The initial manuscript greatly benefitedfrom comments and suggestionsfrom Miguel Garce´sandTorstenUtescher.Weshouldalsoliketothank SirPatrickSkipwith,formerHeadofTranslationatBRGM, foreditingtheEnglish.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbe found,intheonlineversion,athttp://dx.doi.org/10.1016/j.
crte.2016.02.002.
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