Sedimentary vs. diagenetic control on rhythmic calcareous successions (Pliensbachian of Asturias,...

Sedimentary vs. diagenetic control on rhythmic calcareoussuccessions (Pliensbachian of Asturias, Spain)

Beatriz Badenas,1 Marc Aurell,1 Jose Carlos Garcıa-Ramos,2 Beatriz Gonzalez3 and Laura Pinuela2

1Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain; 2Departamento de Geologıa, Universidad de Oviedo,

33005 Oviedo, Spain; 3Departamento de Explotacion y Prospeccion de Minas, Universidad de Oviedo, 33004 Oviedo, Spain

Introduction

The significance of rhythmic calcare-ous successions commonly recorded inancient hemipelagic environments hasbeen debated. In particular, theknowledge of the diagenetic overprintis an important requirement forunderstanding initial depositionalprocesses and the origin of the bed-ding rhythms (Ricken and Eder,1991). Many authors propose thatthe limestone–marl rhythms record adirect primary environmental signal(e.g. Weedon and Jenkyns, 1999; Pit-tet and Mattioli, 2002; Wendler et al.,2002). Early or late diagenetic redis-tribution of calcium carbonate in thesediments is believed by severalresearchers to enhance the pre-dia-genetic lithological differences (e.g.Ricken, 1986; Frank and Bernet,2000; Reuning et al., 2006). However,other authors demonstrate that fornumerous successions, no proof forprimary sedimentary variations isfound, leaving open whether primaryvariations have been present at all(e.g. Hallam, 1986; Westphal et al.,2000; Munnecke and Westphal, 2005).

The debate about the significance ofrhythmic calcareous successions re-mains open because of the difficultyto distinguish between primary anddiagenetic features in most of the fieldexamples (Westphal, 2006). The hemi-pelagic rhythmic calcareous Pliensba-chian successions superbly exposed inthe Asturias coastline (Spain, Fig. 1)offer a unique opportunity to addressthis issue. The outcrop quality allowsan unequivocal identification of aprimary feature, which consists of analternation of bioclastic levels fol-lowed by laminated ⁄burrowed inter-vals. These primary facies alternationscan be compared with the lithologicalrhythms formed by cemented lime-stone beds alternating with clay-richintervals.The purpose of this paper was to

first explain the sedimentary pro-cesses controlling the developmentof the different facies recorded inthe Pliensbachian hemipelagicsuccessions of Asturias. Second, wediscuss on the effects of diagenesisredistributing the CaCO3, resultingin the eventual formation of thelithological rhythms. The obtainedresults show a major diagenetic dis-tortion of the sedimentary patternrather than a mere enhancement ofthe primary signal and provide someclues for the correct interpretationof the origin (sedimentary vs.diagenetic) of the limestones–marlsalternations.

Geological setting and methods

The Lower Jurassic succession ofAsturias was deposited in an epicon-tinental platform developed west-wards of the Basque-CantabrianBasin. This succession shows along-term transgressive–regressiveevolution (Fig. 1), from carbonate-evaporite coastal sediments to openplatform lime mudstones and marls(with successive development of blackshale intervals), up to the major dis-continuity at the end of the Toarcian(Aurell et al., 2003). The studied Pli-ensbachian successions representdeposition in the outer area of astorm-dominated carbonate ramp,which was open to the north. Theselected interval covers most of thedavoei Zone (Suarez-Vega, 1974; Val-enzuela et al., 1989) and was devel-oped during the regressive hemicycleR3 defined by Quesada et al. (2005).Two sections, 18 km apart, were

analysed: Punta La Llastra (PL), 10 mthick, and Playa de Vega (PV), 6.3 mthick. These two sections have lateralfacies changes, reflecting the sedimen-tation from the relatively shallow (PV)to the deeper (PL) outer ramp do-mains. The excellent exposure of thetwo analysed outcrops allows detailedcm-scale determination of facies andlithological variations (Fig. 2). A totalof 152 fresh rock samples were alsocollected for the analysis of CaCO3

content across the carbonate-rich to

ABSTRACT

The diagenetic overprint in rhythmic hemipelagic successionscan either enhance or change the original distribution of CaCO3,but it is difficult to evaluate its effect because in most fieldexamples, it is not possible to distinguish between sedimentaryand diagenetic features. The rhythmic succession of thePliensbachian of Asturias (Spain) shows alternation of bioclasticand laminated ⁄ burrowed intervals. The original content ofcarbonate brought by storms from shallow areas was larger inthe bioclastic horizons. However, there is a widespreadmismatch between facies alternations and the observed litho-

logical rhythms. The diagenetic redistribution of CaCO3 resultedin successive limestone–marl ⁄ clay couplets that do not match(either in number or in the location of the boundaries) thesedimentary cycles defined by facies alternations. We concludethat interpreting the limestone–marl rhythms as a directresponse to primary changes is highly questionable, unlessthere is unequivocal proof of a sedimentary origin of thealternation of the two lithologies.

Terra Nova, 21, 162–170, 2009

Correspondence: Beatriz Badenas, Area de

Estratigrafıa, Departamento Ciencias de la

Tierra, Universidad de Zaragoza, 50009

Zaragoza, Spain. Tel.: +34 976 762247;

fax: +34 976 761106; e-mail: bbadenas@

unizar.es

162 � 2009 Blackwell Publishing Ltd

doi: 10.1111/j.1365-3121.2009.00869.x

clay-rich intervals. Comparison ofsedimentary cycles and lithologicalprofiles (Figs 3 and 4) contributesseveral evidences to illustrate thecombined effect of the sedimentaryprocesses and diagenetic overprintoriginating rhythmic calcareoussuccessions.

Facies and sedimentary cycles

Description

The two studied logs are characterizedby the alternation of three facies (A, Band C), which form sedimentarycycles (sensu Einsele, 1992) easily dis-tinguishable from field observation.

The definition of facies was based oncriteria that refer only to depositionalfeatures of sediments (i.e. fossil con-tent, lamination, burrowing). Thelithology was not used for the faciesdefinition, because carbonate contentin each facies is variable and is actu-ally importantly controlled by diagen-esis (see next section).Bioclastic facies A is characterized

by cm- to dm-thick pale grey-colouredlevels, including skeletal grains withfining-upward trends, forming mud-to grain-supported textures (Figs 5a,band 6). Skeletal grains can be para-autochthonous and allochthonous.Allochthonous fossils are millimetricfragments mainly of echinoids, bi-

valves and gastropods, derived fromshallower ramp areas. Para-autoch-thonous are disarticulated or wholefossils (inner moulds or preservedshells) mainly of bivalves, brachio-pods, belemnites and ammonitesslightly reworked in the outer ramp.The proportion of CaCO3 is variable:from 70–90% in most of the cases, to20–30% in some particular cases(cycles 2, 19 and 20 of PL log). TheCaCO3 content may also have signi-ficant variation in a discrete bioclastichorizon, from 60–80% in discontinu-ous limestone beds to 30–50% in theclay-rich equivalent levels (Fig. 6a).The more common bioclastic facies

A1 forms single levels that have basal

Fig. 1 Location of the two studied logs (Punta La Llastra and Playa de Vega) in the Asturias coastline (North Spain) and summaryof the facies distribution and long-term transgressive–regressive (T–R) cycles of the Lower Jurassic of Asturias (compiled fromAurell et al., 2003 and Quesada et al., 2005). The studied Pliensbachian interval is enclosed in a dashed box.

Terra Nova, Vol 21, No. 3, 162–170 B. Badenas et al. • Sedimentary vs. diagenetic control on rhythmic carbonates

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� 2009 Blackwell Publishing Ltd 163

erosive surfaces (locally with dm-scalescours filled by coarse bioclasts) ornon-erosive irregular surfaces withbioclastic debris filling the empty bur-rows of previous intervals. Amalgam-ated bioclastic facies A2 consists ofsuccessive bioclastic levels with fining-upward trend, separated by internalerosive surfaces with local scours.They are more frequent in the rela-tively proximal log of PV. Intenselyburrowed bioclastic facies A3 is char-acterized by different generations ofPhymatoderma, Chondrites and Thal-assinoides traces (�complex tiering�,after Taylor et al., 2003).Laminated facies B is formed by dark

laminated clays ⁄marls (Fig. 6) withvariable amount of CaCO3 (from 8%to 48%) including scarce mm-thickaccumulations of para-autochthonousfossils (brachiopods, belemnites,bivalves and ammonites) and scatteredallochthonous bioclasts (crinoids, echi-noids and bivalves).Burrowed facies C also covers a

wide lithological range, from clayey-marls to marly-limestones (calciumcarbonate content up to 80%). Thefacies displays abundant burrow mot-tling mainly with Phymatoderma,Chondrites and Thalassinoides. Lo-cally, these traces may penetrate inter-nal moulds of ammonites. The tracesshow pale greyish mud- to grain-supported passive fillings (Fig. 5c).Burrowed facies C1 shows well-devel-oped vertical zonation of trace fossils(simple tiering) with greater depth ofpenetration of Phymatoderma andminor depth of large and small Chon-drites (Fig. 6b,c). Mixed burrowedfacies C2 shows intense bioturbation,

Punta lallastra Number of

limestone bed

Number of primary cycle

52

1314

15

16

17

18

19

20

21

22

23

24

25

51

56

61

59

58

57

555453

50

26

27

12

11

30

29

28

Playa de vega

Number of limestone bed

Number of primary cycle

62

61

13

14

15

16

17

18

19

20

21

22

23

24

25

64

63

60

59

58

57

55

54

53

51

26

2728

29

Fig. 2 Field view of the middle andupper part of Punta La Llastra andPlaya de Vega logs (see Figs 3 and 4,respectively) showing the distribution ofsedimentary cycles (numbers in circles)and lithological beds (numbers insquared boxes). Light coloured intervals(bioclastic facies a) and dark intervals(laminated and bioturbated facies B andC) define the sedimentary cycles. Somemarly intervals may include several sed-imentary cycles. Limestone beds arecontinuous and discontinuous at theoutcrop scale and may contain bound-aries between facies.

Sedimentary vs. diagenetic control on rhythmic carbonates • B. Badenas et al. Terra Nova, Vol 21, No. 3, 162–170

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with different generations of traces(complex tiering).The three facies described above

form sedimentary cycles: 39 cycleswith 25 cm of average thickness inthe relatively distal log of PL; and 34cycles with about 19 cm of average inPV log (Figs 2–4). Three types ofcycles have been distinguished: com-plete, incomplete and amalgamated. Acomplete cycle is formed by a lowerbioclastic interval (facies A) and anupper laminated interval (facies B),which has been eventually burrowed(facies C). The number of the com-plete cycles is larger in the more distalPL log: 16 cycles compared with thefour recorded only in PV log. Theincomplete cycles characterized byfacies A–C form most of the succes-sion exposed in PV (24 of the 34defined cycles) and one-third of thecycles found in the more distal PL log.A more particular type of incompletecycle formed by facies A–B is onlylocally found in PL log (i.e. cycles 19,29, 38). The lower interval of the so-called amalgamated cycles consists ofsuccessive bioclastic levels separatedby erosive surfaces (i.e. the facies A2),followed either by the burrowed faciesC (five and six cycles in PL and PVlogs, respectively) or forming �com-plete cycles� with facies A, B and Cpresent (cycles number 6 and 16 in PLlog).

Interpretation

The described sedimentary cycles werepreviously interpreted as the result ofrapid deposition during storms (bio-clastic facies) followed by long periodsof slow sedimentation (laminated andburrowed facies; Valenzuela et al.,1989). Facies data reflect that anindividual sedimentary cycle repre-sents several storm episodes (Fig. 7).

Fig. 3 Punta La Llastra log, showinglithological data and distribution offacies and sedimentary cycles. The dotslabelled with A in the %CaCO3 curveare analysis in bioclastic facies A. Thedashed line left to bed 52 indicates twodifferent analyses in the same bioclasticlevel (either in limestones or in equiva-lent lateral marly interval). Notice thewidespread mismatch between the num-ber and boundaries of sedimentarycycles and lithological rhythms.


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Fining-upward trend and amalgama-tion in bioclastic facies A indicatedeposition by pulsating flows of asingle storm-density current or of

several storm events. Facies B repre-sents hemipelagic sedimentation com-bined with dilute suspended cloudslofted by successive storm-density

currents (e.g. Damholt and Surlyk,2004). This laminated sediment can beeventually partially to completely bur-rowed depending on the sedimentaryrates and ⁄or the amount of oxygenavailable.The relative proximal–distal loca-

tion of the two studied sectionsexplains some lateral variations inthe recorded sedimentary cycles. Thesmaller number and thickness ofcycles in the shallower PV log areexplained by a more frequent amal-gamation and storm-current winnow-ing in the shallower areas of the outerramp. There is a larger proportion ofcomplete cycles (with thicker lami-nated facies B) in the deeper PL log,whereas incomplete cycles A–C aremore frequent in the proximal log ofPV. Larger preservation of the lami-nated facies B in the deeper PL log isfavoured by higher sedimentary ratesand more limited storm winnowingcompared with the shallower areas.The local absence of episodes of bur-rowing on top of the cycles could bedue to the high rates of sedimentationand ⁄or to deposition below theboundary of the oxygen-deficientwaters in relatively deep areas.The sedimentary cycles show

decreasing CaCO3 content from faciesA to facies B–C, which is interpreted toreflect the decrease of imported car-bonate from the shallowareas.Accord-ingly, the original content of carbonatebrought by storm-related currentsfrom shallow productivity areas waslarger in the bioclastic levels than in thelaminated ⁄burrowed intervals. Thefluctuations in platform carbonate pro-duction and offshore resedimentationby storm driven currents have beenproposed as amajor primary control toexplain other Jurassic hemipelagic cal-careous successions (e.g. Pittet et al.,2000; Colombie and Strasser, 2003;Munnecke and Westphal, 2004; Bade-nas et al., 2005).

The effects of the diagenesis on theredistribution of the carbonate

The studied successions show thealternation of carbonate-rich bedsand clay-rich intervals that result in avery evident field expression of thelithological profile on weathered expo-sures: from continuous to discontinu-ous decimetre-thick cemented beds(limestone to marly-limestone), toFig. 3 Continued.


.............................................................................................................................................................


recessive dark clayey-marls to marlyintervals (Fig. 2). The number of theserhythms increases from PL log (21cemented beds, eight of those discon-tinuous: Fig. 3) to the more proximalPV log (25 cemented beds, including11 discontinuous beds: Fig. 4). Theboundaries and the number of theselithological rhythms rarely match thesedimentary cycles; the only excep-tions are cycles 17, 24 and 29 in PVlog. This is surprising because, asstated above, the storm events resultedin alternations of carbonate-richerlevels (i.e. the bioclastic facies A)followed by clay-richer levels (thelaminated ⁄burrowed facies B ⁄C). Thismismatch can be explained by theredistribution of the calcium carbon-ate during diagenesis.Evidence for redistribution of the

calcium carbonate during the diagen-esis is given by the vertical trend of theCaCO3 content curve. The CaCO3

content in the carbonate-rich cemen-ted beds usually ranges from 70% to85%, a much larger value than thoseof the lateral and intercalated marlybeds (<30% in most of the cases).The sharp difference of the CaCO3

content from cemented to non-cemen-ted beds fits well the �diagenetic bed-ding� of Ricken (1986); the originalcarbonate became augmented in lime-stones during diagenesis, but wasreduced in the marly beds.Differential compaction between

carbonate-rich beds and clay-richintervals is also indicative of a diage-netic redistribution of the carbonate(e.g. Ricken and Eder, 1991). Whenthe burrowed intervals of facies C areincluded in a �marl-limestone� contact,trace fossils are strongly deformed inthe marly beds, whereas they areonly slightly compacted in limestones(see Fig. 6b). This indicates that

Fig. 4 Playa de Vega log, showing lith-ological data and distribution of faciesand sedimentary cycles (legend of sym-bols in Fig. 3). The dashed line left tobeds 47 and 56 indicates two differentanalyses in the same bioclastic level(either in limestones or in equivalentlateral marly interval). Notice the wide-spread mismatch between the numberand boundaries of sedimentary cyclesand lithological rhythms, except forsedimentary cycles 17, 24 y 29 (indicatedin grey).


.............................................................................................................................................................


cementation of the carbonate-rich lev-els took place before the main com-paction phase, in very shallow burialdepths (e.g. Bathurst, 1970). Earlydiagenetic aragonite dissolution couldprovide the cement for the limestonebeds (e.g. Munnecke and Samtleben,1996). In the studied successions, thereis evidence of very shallow burialdissolution of aragonite, with Chon-drites traces affecting internal mouldsof ammonites.Comparison between primary cy-

cles and the lithological profile sug-gests that only in some cases,diagenesis enhanced the originallycarbonate-rich sediments. Cementedlimestone beds do not exactly coincidewith a facies type, although they tendto be equivalent with some of thebioclastic intervals of facies A. Clay-rich intervals may generally includeseveral sedimentary cycles, which arealso reflected by decreasing CaCO3

content from 20–50% in bioclasticfacies A to 10–25% in facies B-C(e.g. cycles 12–16 in PL log, Fig. 3).

1 mm

(a)

1 mm

(c)

1 mm

(b)

Fig. 5 Key aspects of the facies under microscope. (a) Bioclastic facies A (upper part of bed 59, PL log: see Fig. 3), dominated byallochthonous fossils (gastropods, echinoids, bivalves, brachiopods). (b) Bioclastic facies A (bed 59, PV log: see Fig. 4) with small-size allochthonous bioclasts. (c) Burrowed facies C with very scarce skeletal grains (lower part of bed 67, PV log: see Fig. 4),showing burrow mottling (Chondrites) with pale grey mud-supported fillings.

Fig. 6 (a) The upper part of the PL log(between beds 59–63, see Fig. 3) is usedto illustrate the lateral lithological var-iability of the bioclastic facies A (seepale grey coloured intervals), includedor lateral to discontinuous cementedbeds 59, 60 and 61. (b) Field view ofdifferent facies; note the different defor-mation of the traces (Facies C1) in theclayey marl interval and in the limestonebed. (c) Field view of different faciesfound in a clayey marl interval; notelarge para-autochthonous fossils dis-persed and relatively well preserved inthe dark laminated interval (facies B).

A1

B

C1

Limestone

Clayey marl

(b)

(a)

60

5962

63

61

50 cm

A1

B

C1

(c)

5 cm


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The increasing number of lithologicalrhythms towards the more proximallocalities (i.e. 21 vs. 25 rhythms fromPL to PV logs) can be related to thelarger original carbonate content inthe areas located closer to the carbon-ate factory, resulting in a larger num-ber of the limestone beds produced bythe diagenetic redistribution of thecarbonate.The relationship between the

changes in the CaCO3 curve and thefacies alternations supports the ideathat �a sedimentary succession withouta primary cyclic change in composi-tion is unlikely to produce a clearlyrhythmic diagenetic pattern� (Reuninget al., 2006). However, in the studiedcase, the diagenetic enhancement andchanges of the primary differencesresulted in the formation of aprominent lithological signature (i.e.21 and 25 limestone–marls ⁄clays

couplets in PL and PV, respectively),independent of the sedimentary cycledefined by the facies alternations (i.e.39 and 34 cycles in PL and PV,respectively).

Conclusions

The calcareous hemipelagic Pliensba-chian successions of Asturias (North-ern Spain) record successivesedimentary cycles formed after sev-eral storm events, alternating bioclas-tic levels and darker laminated and ⁄orburrowed intervals. The number ofpreserved cycles and the thickness ofthe laminated intervals are signifi-cantly reduced in the relatively prox-imal domain of the outer ramp. Thesefacts are related to more frequentprocesses of erosion and amalgama-tion and ⁄or deposition above theboundary of the oxygen-deficient

waters compared with the more distallocalities.The storm-processes controlling the

sediment accumulation in the outerramp involved episodic changes in theoriginal content of the calcium car-bonate derived from shallow platformareas; the amount of carbonate waslarger in the bioclastic basal faciescompared with the laminated ⁄bur-rowed interval. In spite of this, thereis a widespread mismatch between thesedimentary cycles and the lithologicalprofile, formed by successive rhythmsof carbonate-rich beds and clay-richrecessive intervals.In the studied succession, the dia-

genetic redistribution of calciumcarbonate created a lithological signa-ture (i.e. the limestone–marls ⁄claysbedding rhythm), which does notmatch the number and the boundariesof the sedimentary cycles (i.e. the

Fig. 7 Summary of the main sedimentary processes giving rise to the different facies and sedimentary cycles.


.............................................................................................................................................................


alternation between bioclastic andlaminated ⁄burrowed intervals). Thisobservation questions the interpreta-tion of limestone–marls coupletsfound in similar hemipelagic succes-sions as a direct response to the cyclicvariation of palaeoenvironmentalparameters. The assumption of thelink between the changes in the cal-cium carbonate content and the var-iation of any primary parameter needsto be based on the existence ofunequivocal sedimentological proofs.

Acknowledgements

The authors are indebted to the editorialwork carried out by M. Coleman, L.Martire, H. Westphal and anonymousreviewer, which helped to improve theoriginal versions of the manuscript. Finan-cial support was provided by the Spanishand Aragon Governments (ProjectsCGL2007-62469 ⁄BTE, CGL2008-01237 ⁄BTE and Grupos Consolidado ‘‘Recon-strucciones Paleoambientales’’). The workof J.C.G-R. and L. P. was supported bygrants of the Principado de Asturias andUniversity of Oviedo (CN-04-226).

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Received 6 November 2008; revised versionaccepted 4 February 2009


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