244 SiqueirosBeltronesD.A.et al.

Hidrobiológica

Hidrobiológica2012,22(3):244-257

Further remarks on the role of pro-thrombolites in the origin of coastal lagoons for Northwestern Mexico

Nuevas observaciones sobre el papel de protrombolitos en el origen de lagunas costeras del NW de México

DavidAlfaroSiqueirosBeltrones,1ÓscarUbishaHernándezAlmeida2andJanetteMagalliMurilloJiménez3

1Dpto. Plancton y Ecología Marina, CICIMAR, Instituto Politécnico Nacional. Av. Instituto Politécnico Nacional S/N, Col. Playa Palo de Santa Rita, La Paz, BCS. 23096. México

2Laboratorio de Producción Primaria, CINVESTAV-IPN, Unidad Mérida, Carretera Antigua a Progreso Km 6, Mérida, Yucatán, 97310. México

3Laboratorio de Geología, CICIMAR-IPN e-mail: dsiquei@ipn.mx

SiqueirosBeltrones,D.A.,O.U.HernándezAlmeidaandJ.M.MurilloJiménez.2012.Furtherremarksontheroleofpro-thrombolitesintheoriginofcoastallagoonsforNorthwesternMexico.Hidrobiológica22(3):244-257.

ABSTRACTAlongthemarginsofcoastallagoonsinBajaCaliforniaSur,Mexico,cyanobacteria-dominatedcommunitiesformthickmats thateventuallygenerateconsolidatedmudplatformsorpro-thrombolites (unlithified thrombolites).Pro-throm-boliteplatformsgrowseawardand formspits thatalterwater flowandpromotesanddeposition thatdevelop intobarriers, thus influencing coastal geomorphology, promoting lagoon formation. In this paper we provide evidenceson the role of pro-thrombolites in the generation of coastal lagoons that includes different regions of NW Mexicowherepro-thromboliteand/orthrombolithicplatformsarecommon.InBahíaMagdalena,desiccatingmicrobialmatsaredominatedbyMicrocoleus chthonoplastesThuretexGomontandtheirremainingemptysheatsoccurtogetherwithlithifiedstructures(thrombolites)cementedwithmicrite,consideredofbioticorigin.Previouslyproposedasalacunarprocess,itisnowsuggestedthatpro-thrombolithicdevelopmentleadingtocoastallagoonformationactuallyoccursinnon-lacunarenvironmentswithsimilarhidrologicalconditions.Likewise,thisprocessisfurtherenhancedoncethecoastallagoonhasbeenformed.Wealsosuggestthattheestablishmentofmangroveforestsisindeedprecededbytheformationofprothrombolithicplatformsthatprovideastabilizedsubstratefortheseplantsintropicalandsubtropi-calcoastallagoons.

Key words:Coastallagoons,Cyanophytemats,geomorphology,pro-thrombolites.

RESUMENAlolargodelosmárgenesdelagunascosterasdeBajaCaliforniaSur,México,comunidadesdominantesdecianofitasformantapetesconglomerados,gruesosqueeventualmentegeneranplataformaslodosasconsolidadasoprotrombo-litos,quepermanecensinlitificar.Lasplataformasprotrombolíticascrecenmaradentroformandopuntasquealteranelflujodelascorrientesyporconsiguienteeltransporteyacumulacióndesedimentos,creandobarreraseinfluyendoen la geomorfología costera, lo que resulta en la formación de lagunas costeras. En este trabajo se proporcionanevidenciassobreelpapeldelosprotrombolitosenlageneracióndelagunascosterasdediferentesregionesdelNWdeMéxico,endondelasplataformasprotrombolíticasy/otrombolíticassonunrasgocomún.EnBahíaMagdalena,se

Pro-thrombolitesandoriginofcoastallagoons 245

Vol.22No.3•2012

INTRODUCTION

Microbial mats and pro-thrombolites. Coastalmicrobialmatsex-tendhundredsofsquaremetersoverlagoonbeachesintropicalandsubtropicalenvironments(Golubicet al.,1999)formingdense,organic-richsedimentarystructuresthatfrequentlyaccreteabovehightide(Stal,2000).Thesemicrobialmatsareformedmainlybycyanobacteria that trap and bind sediments, and resist erosiontoagreatextent, forming thusconspicuous livingsubstrates intropicalcoastallagoons.Consequentlytheirsedimentstabilizingeffecthasbeenconsideredofgreatimportanceforcoastalmor-phogenesis(Stal,2000).

Extensivebenthicmatsformedbyfilamentouscyanobacte-riaoccurinmostoftheintertidalandsubtidalmarginsoftheLaPazlagoon,B.C.S.,Mexico(Siqueiros-Beltrones,2006).Thesearemostly associated to soft, conglomerated (vertically accreted)matsandtomoreorlessconsolidatedsedimentplatforms,whichduetotheirrockyappearancewithagreenishcovering(Siqueiros-Beltrones,2008)hadbeenhithertounnoticed.Thesemaybeplat-formsand/or fragmentsofassortedsizesresemblingmudstone,butinallcasestheyoccurasseawardhardgroundextensions.Inthecaseoflithifiedstructures,theirclottednatureindicatedthatthese were thrombolithic in nature (Siqueiros-Beltrones, 2008).Thromboliteshavebeendefinedasbenthicbiosedimentarystruc-turesgeneratedbytheentrapment,binding,andconsolidationofsediments as result of the metabolic activity and growth of mi-croorganisms,particularlycyanobacteria;unlikestromatolitesdonotshowinternallamination(Charpyet al.,1999;Stal,2000;Shap-iro,2000).Thismostcloselydepictstheprocesswearecurrentlyinvestigating, inasmuch the platforms initially recorded from LaPazlagoonweredescribedaspro-thrombolites,i.e.,nonlithifiedsedimentary structures generated by the growth of cyanophytemats, i.e. clotted sediments with inclusions of shell fragments,irregularly packed, surrounded by a sheet of active cyanobac-teria(Siqueiros-Beltrones,2008).Althoughsomelackitbecauseofprolongedexposureor forbeingcoveredbyothersediments(Siqueiros-Beltroneset al.,2009)manystillcharacteristicallyshowaconspicuoussheetoffilamentouscyanobacteria,mainlyMicro-coleus chthonoplastesThuretexGomont andLyngbya aestuarii LiebmanexGomont.Althoughotherspeciesarecommon,suchasSpirulinasp.,Oscillatoriaspp.,Calothrix,andunicellularforms:Chroococcus sp.andAphanotece sp.(Siqueiros-Beltrones,2008),

muchlikeinthereportedcyanobacterialmats(García-Maldona-do,2005).Also,inspiteoftheperiodicaldesiccation,asmanyas150diatomtaxathriveonthepro-thrombolitesurfaces,mainlyofmangroveaffinity(Siqueiros-Beltrones,2006).Someofthemoreconsolidated platforms lacking a cyanobacterial cover stronglyresemble the microbialithic structures from the Bahamas, as inWhittonandPotts(2000).

Living microbialites, including those in the Baja Californiapeninsula,areimportantfortheunderstandingofrecentenviron-mentalchanges,andbecausethrombolitesandothermicrobial-ites are also represented in the ancient geological record, thisinformationiscriticalfortheunderstandingofpastenvironmentsaswell.Inparticular,pro-thrombolitesformingtodayaroundtheLa Paz lagoon margins exert important changes in the overallgeomorphologicaldevelopmentofthearea,includinglandscapechanges and alteration of water flows and upper tidal limits(Siqueiros-Beltrones,2008).

Geomorphology of coastal lagoons. Most coastal lagoonsare closely related to the presence of a sand barrier or barrierislands.Nonetheless,thegeomorphologicalmechanismsfortheformation of barriers are not fully understood and their originhas been the subject of debates for more than a century. Sev-eralexplanationsormodelshavebeenproposedonsandbarrierformationfromwhichthreetheoriesaresaidtoprevail(Kjerfve,1994). One of them is the existence of low lying areas or spits,i.e., extensions of beaches that protrude into a bay as a resultofdepositionofsedimentscarriedby long-shorecurrents.Like-wise, theformationofcoastal lagoonbarriersdependsonsandsupply, wave energy, and tidal fluctuations (Phleger, 1969). Yet,deposition alone does not imply permanence of the sediments,unlessacohesiveagent isatplay.Conspicuouscyanobacterialmats growing in coastal lagoon environments may play such arole.Hence,wepropose the inclusionofa long-termbiologicalfactor into these theoretical models, as a mechanism of ensur-ingthestabilizationofloosesediment(bytrappingandbinding)inspitsandbarriers.

LaPazlagooninBahíadeLaPaz,Mexico(Fig.1)beganform-ingaround6000yearsagobycoastaltransportationanddeposi-tionofsediments,eventuallyformingasandbar,andtransformingaprimitivecoveintothepresentcoastal lagoon(Nava-Sánchez

encontrarontapetesdecianofitasbajodesecación,conformadosporMicrocoleus chthonoplastesThuretexGomontysusvainasvacíasremanentesjuntoconestructuraslitificadas(trombolitos)cementadosconmicrita,consideradadeorigenbiótico.Previamentepropuestocomounprocesolagunar,ahorasesugierequeeldesarrolloprotrombolíticoqueconducehacialaformacióndelagunascosteras,ocurreenambientesnolagunaresconcondicioneshidrológicassimilares.Noobstante,talprocesoseexacerbadentrodelalagunacosteraformada.Sesugieretambiénqueelesta-blecimientodebosquesdemanglaresprecedidoporlaformacióndeplataformasprotrombolíticasqueproporcionanunsustratoestableparamanglesenlagunascosterastropicalesysubtropicales.

Palabras clave:Geomorfología,lagunascosteras,protrombolitos,tapetesdecianofitas.

246 SiqueirosBeltronesD.A.et al.

Hidrobiológica

&Cruz-Orozco,1989).Notwithstanding,thehypotheticalspitpro-posed by the former authors needed to trigger-off and mantainthe growth of the sandbar lacked a feasible origin. It was thusproposed that pro-thrombolithic growth might have defined thehypothetical spit where the armpit of the La Paz lagoon is de-fined.Throughcontinuousgrowth,thisprovidedabasementforthedevelopmentofthesandbarElMogotethattransformedtheprimitive cove into the (Ensenada de La Paz lagoon (Siqueiros-Beltrones,2008).

TheverysimilargeomorphologyofmostcoastallagoonsintheGulfofCalifornia,alsoleadustohypothesizethattheprocessofformationinvolvingpro-thrombolithicdevelopmentinallcasescouldbethesame.Itwaslatersuspectedthatthepro-thrombo-lithic platforms might have played and could still be playing anessential role in the geomorphological development of coastallagoonsinalltheNWregionofMexicothroughseawardaccre-tionofpro-thrombolitesandbarformation.Additionally,recentlygatheredstratigraphicevidencesuggestedthatpro-thrombolithicprocesses are responsible for the formation of the substratesaroundtheLaPazlagoon(andprobablyothers)supportingman-

groveforestsandhumansettlements(Siqueiros-Beltroneset al.,2009).

Accordingtotheabovehypothesis,inthemarginsofcoastallagooncyanobacterialmatscontinuetoevolveintoconglomeratedmats that generate unconsolidated pro-thrombolites (Siqueiros-Beltrones,2008).Althoughpreviousmineralogicalanalysisofpro-thrombolitesshowedthepresenceofcementingmicrocrystallinecalcite(micrite)thesestructuresareunlithifiedandrepresentanintermediate stage between microbial mats and thrombolites.Thissubstratecanbecolonizedbymangroves,whicharecom-monintropicalcoastallagoons.However,answerstothefollow-ingquestionsarecompelling,isthisthrombolithicaccretionpro-cessfoundexclusivelyincoastallagoonenvironments?Canweconfirmthetransitionfromgrowingmicrobialmatstopro-throm-bolitestothrombolites?Arethrombolithicstructuresthecommonsubstrateformangroves?

First, we predicted that pro-thrombolites will not be foundoutsidelacunarsystemsgiventheirrestrictiontoshallowwatersandtherequirementofcoarsesedimentfluxfortheirformation,whichismoreactivealongtheshore.However,comparisonsof

Figure1.Siteswiththrombolitesandpro-thrombolitesoutcropsinthecoastsoftheGulfofCaliforniaandthewestcoastofB.C.S.,Mexico.1)CloseupofsitesinBahíadeLaPaz,includingCaleritaandIslaEspírituSanto;2)BahíaMagdalena(SanCarlos);3)IslaSanJosé;4)BahíaConcepción;5)LagunaSanIgnacio;6)EsteroElSoldado,Guaymas,Sonora.

Pro-thrombolitesandoriginofcoastallagoons 247

Vol.22No.3•2012

lacunar and non-lacunar environments in the Gulf of Californiaare so far inexistent. Second, that the transition between con-glomeratedmatsandthrombolitesshouldbefoundsomewhereintheintertidalmargins,becausecoastallagoonmarginscontinueto accrete while microbial mats evolve into more consolidatedpro-thrombolitesandeventually intothrombolites.Third,theob-servationofmangrovesgrowingonpro-thrombolitesandthrom-bolites,suggeststhatthesemicrobialitesserveasthepreferredanchoringsubstrateformangroveseedsintropicalandsubtropi-calcoastallagoons.

Theaimofthisstudywastoprovidesupportontheproposedpro-thrombolithic nature of coastal ground and their relation tocoastallagoonformationandontherelationshipbetweencyano-bacteriamats,pro-thrombolitesandthrombolites.Also, toshowthedependenceofmangroverecruitsoncyanobacteriamatsandpro-thrombolitesasananchoringsubstrate.

MATERIAL AND METHODS

Study area. Our observations comprise coastal lagoons in theNWregionofMexico.WeparticularlyfocusedontheLaPazla-goonandBahíaMagdalenabecausetheformerhasbeenmorecloselystudied,andthelatterbecauseitiswherethehypothesisthatthrombolitesandpro-thrombolitesweretobefoundinothercoastallagoonswasfirstcontrasted.However,wealsosearchedforsaidstructuresinBahíaConcepción(B.C.S),andEsteroElSol-dado(Guaymas,Sonora)(Fig.1).AgoodaccountofthegeologicalnatureoftheBajaCaliforniaPeninsulacoastflankingtheGulfofCaliforniaisgiveninJohnsonandLedesma-Vázquez(1997).

Themarginsoftheexploredcoastallagoonsarepopulatedbymarshvegetation,mainlySalicorniaspp.andthreespeciesofmangrove (Avicennia germinans (Linnaeus) Stearn, Rhizophora mangle Linnaeus and Laguncularia racemosa (Linnaeus) Gaert-ner)closelyassociated to thepro-thrombolithicgrounds.Thesegroundsarecoveredbysheathsofcyanobacteriaandareassoci-atedwithmicrobialmatslocatedseaward,orlandwardexposedtodesiccationamongthemangroveandmarshvegetation.

IntheLaPazlagoon,locatedinsoutheasterncoastoftheBa-jaCaliforniaPeninsula,theblackmangrove Avicennia germinansis the most common mangrove recorded although Rhizophora mangle (redmangrove) isalsoconspicuous,whileLaguncularia racemosa (whitemangrove)isscarce.Description(andotherref-erences)of this lagoonandthefirstrecordofpro-thrombolithicstructuresassociatedtomangrovesarefound inSiqueiros-Bel-trones(2006,2008).Likewise,inBahíaMagdalenalagoonlocatedon thesouthwesterncoastof theBajaCaliforniapeninsula, thelagoonmarginsaredenselypopulatedbymangroveforestsofthesamethreespecies,althoughmuchbigger,alsocloselyassoci-atedtopro-thrombolites.But,unlikeintheLaPazlagoon,Lagun-cularia racemosa is very common. Several sites were exploredspecificallyaroundPuertoSanCarloswherethrombolithicplat-

formswereexposedandstructuresassumedtorepresenttran-sitional stages between conglomerated mats and thromboliteswererecordedlandwardamongthemangrovetrees.

Also,structuresfromnon-lacunarsitessuchas inLasBri-sasandCaleritaalongthecoastlineofsouthernBahíadeLaPaz(Fig. 1) were recorded along vast extensions of exposed sandybeaches.Therethevegetationconsistsmostlyofmacroalgaeinboththeintertidal,andinthesubtidalalongwithmanyotherfoul-ing species that use the thrombolithic platforms or boulders assubstrate. Besides, because lacunar-like environments are evi-dentintheislandcovesinthesouthernGulfofCalifornia,explora-tionsweredoneat IslaEspírituSantoandIslaSanJoséwheremangroveforestsarewellestablished.

Observation of pro-thrombolithic formations. Observationsofpro-thrombolites,thrombolites,andconglomeratedmatstruc-tures associated with mangroves carried out in the southernpartoftheGulfofCaliforniacamemainlyfromBahíadeLaPaz,particularlyLaPazlagoon.Weaddedobservationsofstructureslocated in thesandbarand fromMarinaSurapreviouslyunex-ploredsiteinsidethelagoon(Fig.1).Thesestructureswerefoundonthebasisofthehypothesisthatthepro-thrombolihticprocessshouldbecommonthroughout the insidemarginsof the lagoon(Siqueiros-Beltrones, 2008). Closer observations, plus sampling,weremade inBahíaMagdalenaon thewesterncoastofB.C.S.Allthiswascomplementedwithimagesfrom2008inothercoastallagoonsinnortheastB.C.S:BahíaConcepción,andfromEsteroElSoldado(Guaymas,Sonora)in2010(plusarecentslidefromEs-teroSantaRosa,Hermosillo,Son.),inthecentralGulfofCalifornia.However,closeranalysesfortheselocalitiesarestillpending.

Becauselagoon-likeenvironmentsareevidentintheislandcovesinthesouthernGulfofCalifornia,explorationsweredoneatIslaEspírituSantoandIslaSanJoséwheremangroveforestsarewellestablished.Additionally,observationsonthrombolithic-likeplatformsandblocksoutsidelacunarenvironmentsintheGulfofCaliforniaaregraphicallydocumentedtothenorth(Calerita)andnorthwest(LasBrisas)of theLaPaz lagoon, insideBahíadeLaPaz in thesouthernGulf. In thiscasethenullhypothesisstatedthatsuchstructureswouldnotbefoundinasmuchastheexploredsiteswerenotlacunarenvironments.

InMarch2009inSanCarlos,BahíaMagdalena,wesearchedbothforthrombolithicstructuresandtransitionalstagesbetweencyanobacterialmatsandthrombolites,i.e.,consolidatedsedimen-tarystructureswithevidenceofcyanobacteriaremains,suchasfilamentsheaths.Wemadedirectobservationsof thrombolites,cyanobacterial mats and pro-thromboliths that we documentedphotographically.Also,inordertomorecloselycomparethislo-calitywiththeLaPazlagoon,samplesofallstructureswerecol-lected(atSanCarlos)withinanareaofapproximately25m2formicroscopic analysis to identify the filamentous cyanobacteriacovering the structures. And likewise for comparing sediment

248 SiqueirosBeltronesD.A.et al.

Hidrobiológica

texture and mineralogy of the consolidated and soft structureswiththosedescribedinSiqueiros-Beltrones(2008).

Microbiological observations. Wet mounts were preparedwith samples from the surface of pro-thrombolites sampled inSan Carlos. Portions of the samples were shaken with distilledwaterinessaytubesandsubsamplesrecoveredforobservation(Siqueiros-Beltrones, 2008). Wet mounts were observed underphase contrast microscopy and photographed using Kodak 400ASAfilm.

Analysis of thrombolithic and pro-thrombolithic structures. As previously described for the La Paz lagoon (Siqueiros-Bel-trones, 2008), samples of consolidated and soft structures fromSanCarloswereexaminedtodeterminetheirdegreeofconsoli-dationandfortexturalandmineralogicalanalysis;theseincluded:a)fragmentofsemi-consolidatedpro-thrombolite;b)isolatedrootlikesedimentarystructures;c)root-likestructuresassociatedtopro-thrombolite;d)softdesiccatingmatassociatedtopro-throm-bolite; e) soft isolated microbial mat. In order to examine the

samples,texturalanalysesweremadeinsixdifferentways:1)Apartwasbrokendownbyhand,andtheanalysiswasconductedusingalaserparticleanalyzer(LPA);2)15%HClwasaddedtoasubsampletoeliminatecarbonatedmatter,andatexturalanaly-siswasdonewiththeLPA;3)Thissubsamplewasdisintegratedby hand, and acetone and hexametaphosphate were added toremove organic matter and to preclude the cohesion betweengrains, respectively; textureanalysiswasdonewith theLPA;4)Thispartwasimpregnatedwithepoxytomakeathinsection,andthe components were described under a petrographic micro-scope;5)Thissubsamplewasdisintegratedbyhand,anditscom-ponentsweredescribedunderastereoscope;6)Thesubsamplewastreatedwith15%HCltoeliminatecarbonates;thinsectionswereexaminedunderapetrographicmicroscope(Table1).

RESULTS

Sediment and petrographic analyses of samples from San Car-los,BahíaMagdalena,showthatbothconglomeratedmatsand

Table1. Processingandmineralogicalcompositionofpro-thrombolithicsamplescollected inSanCarlos,BahíaMagdalena,B.C.S.A)abundant,P)present,S)scarce,St)strong,W)weak,VW)veryweak,MA)moderatelyabundant.

Sample A A1 A2 B B1 B2 C C1 C2 D D1 D2 E E1 E2

Process Crumbledbyhand x x x x x

Impregnated x x x x x

TreatedwithHCl x x x x x

Equipment Petrographicmicroscope

x x x x x x x x x x

Magnifyingglass x x x x x

Hardness Semi-consolidated x x x x x x x x x

Non-consolidated x x x x x

Organicmater P A A A A

Minerals Quartz A A A A A A A A A A A A A A A

Phelderspate-plagioclase

P P P P P P P

Amphibolites P P MA MA P MA P

Hornblend A P P P P p

Olivine P P P P

Phosphateoolites P P P P P P P P P P

Opaqueminerals P P p

Biotite P P P

Matrix CaCO3microcrystals

A A S

Microsparite A S

Micrite P

ReactiontoHCl St W VW VW

Fiber P A

Pro-thrombolitesandoriginofcoastallagoons 249

Vol.22No.3•2012

semi-consolidatedpro-thrombolitesareverysimilarintheirmin-eralogicalcomposition.Allsamplescarryabundantquartz,moreorlessabundanthornblende,andtracesoffeldspars,phosphoriteoolites,olivine,andshellfragments.Thesemi-consolidatedsam-plesshowedgrainsofmicrocrystallinetexture,mostlikelymicrite(carbonateorphosphate)surroundedbydecayingorganicmat-ter.Thecementingmicrite,however,detachedwhenthesamplewascrumbledbyhand(Table1).Inmostsamples thedegreeofgrain selection is poor indicating an ample range in grain size,whichaccordingtotheirfrequencydistributionleanstowardthefinesizegrains;between40and60%isfineandveryfinesand,and30to45%ismediumandcoarsesand(Table2),whichreflectthelowenergyconditionsofthesamplingareainSanCarlos.

Description of thrombolithic and pro-thrombolithic forma-tions in lacunar and non-lacunar environments. Consolidatedpro-thrombolithicblocksof50cmhigh(approx.)werediscoveredatMarinaSur,intheLaPazlagoon,associatedtoAvicennia ger-minans.Theseblockswerebroken-offfromaplatformstructure,andthosemoreexposedtowaveenergyappearederoded,par-ticularlyaroundthelowerpart(Figs.2-6)roughlyresemblingthebunshapedstromatolitesofSharkBay,Australia,albeitnotwiththe consolidation (lithification) and separation, probably due toerosion,of the latterwhicharemillionsofyearsoldversus thefew hundreds of years of the former. These showed the sameclotted matrix and sheet cover as other pro-thrombolithic plat-formsdescribedearlierforthelagoon(Fig.6).

In non-lacunar environments we observed similar thoughlithifiedsedimentarystructuresatLasBrisasandCalerita.Intheformer,extensivethrombolithicplatformsoccurredalongthein-tertidalareawithbrokenoffblocksthatwerelocatedinthelowerintertidalarea(Figs.7-9)andshowedheavyfouling.InCalerita,asinLasBrisas,platformswerealsofoundandblockswerebrokenoffduetowaveerosionundertheplatforms.Inthiscaseseparateepisodes could be identified based on the two observed stratainthislocation(Fig.10);theplatformsexhibitedperforationsthatcould be interpreted as remains of mangrove roots being an-choredintheoncepro-thrombolite(Figs.11-12).

Theaboveobservationsobligeustorejectourhypothesisthatthrombolithicstructureswouldnotbefoundina(modern)non-la-cunarenvironment.Thisraisedthequestiontowhetherthesear-easpresentedcoastallagoonenvironmentsinthegeologicalpastornot.However,theassumedassociationofpro-thrombolitestomangroves may be supported by fossil-like structures from ElMogoteintheLaPazlagoon.Thesecomplexformationsderivedfrom the interaction between dead mangrove root systems andpro-thrombolites(Figs.13-15),aremostlikelyrhizoliths.

The exploration for thrombolithic and pro-thrombolithicstructures in the southern gulf islands, Espíritu Santo and SanJosé,showedcontrastingstagesofdevelopment.Intheformer,thrombolithicplatformsofdifferentappearanceswereconspicu-

Table2.Percentagesofgrainmeansizeandstandarddeviationofsedimentsinthepro-thrombolithicsamplesA-B(crumbledbyhandand(1)Impregnated)fromSanCarlos,BahíaMagdalena,B.C.S.

Phi(f) A A1

Granules 1-2 3.59 0.00

Verycoarsesand -0.75-0 18.72 0.62

Coarsesand 0.25-1 16.95 7.30

Mediumsand 1.25-2 28.35 36.90

Finesand 2.25-3 23.15 41.55

Veryfinesand 3.25-4 3.56 7.07

Silt-clay 4.25-14.5 5.69 6.56

f B B1

Granules 1-2 4.92 0.00

Verycoarsesand -0.75-0 25.76 6.97

Coarsesand 0.25-1 15.60 9.28

Mediumsand 1.25-2 20.82 30.73

Finesand 2.25-3 18.96 34.19

Veryfinesand 3.25-4 4.56 6.16

Silt-clay 4.25-14.5 9.40 12.78

f C C1

Granules 1-2 0.25 0.21

Verycoarsesand -0.75-0 3.43 5.32

Coarsesand 0.25-1 4.84 1.65

Mediumsand 1.25-2 25.83 28.99

Finesand 2.25-3 44.10 50.05

Veryfinesand 3.25-4 7.03 6.31

Silt-clay 4.25-14.5 14.59 7.45

f D D1

Granules 1-2 0.00 5.06

Verycoarsesand -0.75-0 0.00 1.30

Coarsesand 0.25-1 26.54 29.01

Mediumsand 1.25-2 56.11 51.12

Finesand 2.25-3 9.64 7.72

Veryfinesand 3.25-4 7.68 5.69

Silt-clay 4.25-14.5

f E E1

Granules 1-2 0.00 0.00

Verycoarsesand -0.75-0 1.25 1.29

Coarsesand 0.25-1 4.28 8.71

Mediumsand 1.25-2 31.82 34.92

Finesand 2.25-3 35.58 38.04

Veryfinesand 3.25-4 6.40 8.52

Silt-clay 4.25-14.5 20.71 8.48

250 SiqueirosBeltronesD.A.et al.

Hidrobiológica

ousinvarioussites,eitherasexposed(Fig.16)orprotected(lacu-nar)platforms(Fig.17),andlithifiedbridge-likestructures(Fig.18).InIslaSanJosé,however,pro-thrombolithicformationsseemedincipient,associatedtothickmicrobialmats(Figs.19-20).Furtherexploration is required at Isla San José inasmuch formation ofsandbars,verysimilartoElMogoteinLaPazlagoon,suggestpastpro-thrombolithicactivityinseveralsites.

InthenorthernpartofB.C.Sincidentalobservationsweredone in Bahía Concepción specifically at El Requesón. There,thrombolithicplatformsshowingdifferentstagesofdevelopmentwere associated to other geological processes besides thoseof sedimentary nature (embedded boulders). We recorded aplatform of a muddy appearance located landward and a moredefined(cemented)one,albeitassociatedtoaLaguncularia rac-emosaspecimen(Figs.21-22).

In order to gather information on the proposal that pro-thrombolithicgroundformationinfluencingthegeomorphologyofcoastal lagoonsoccursall throughouttheGulfofCaliforniaandthe rest of the Mexican NW, we explored the coast of Sonora.Theclosestandmorerecentobservationstotheselagoonsweredone at Estero El Soldado in Guaymas, where the same con-glomerated formationswerenotedalongwith thrombolithic-likeplatforms,alsoassociated tomangroves (Fig.23).This initiativehowevercamefromviewingaslidefromEsteroSantaRosa,So-norathatshowesSeriIndiansseatingonprothrombolithicblocks(Fig. 24). Albeit samples from these sites are not yet availableandsedimentologicalandotherdirectexaminationsarepending,theimageshowsSerinativesseatingonwhatwerecognizedasthrombolithicblocksinamangroveenvironment.

In San Carlos (Bahía Magdalena) uncovered thrombolithic

Figures2-7.MicrobialstructureslocatedintheBahiadeLaPaz,B.C.S.,Mexico,mainlyinsidethelagoon:2)Extensivecyano-bacterialmats(farview)andpro-thrombolithicplatform(closeview)inCentenarioSWmarginofLaPazlagoon;3)Fragmentedpro-thromboliteplatformsupportingmangroves(Avicennia germinans)inMarinaSur;4)ErodedplatformshowingchannelsinMarinaSur;5)DetachedblockswitherodedbasesinMarinaSurthatremindofthebunshapedstromatolitesofSharkBay,Australia;6)Structuresshowingthesameclottedmatrixandsheetcoverasotherpro-thrombolithicplatformsinMarinaSur;7)Non-lacunarthrombolithicblocksandplatformsshowingsmoothsurfacescoveredwithcyanobacteriaatLasBrisasnorthoutsideofthelagoon.

Pro-thrombolitesandoriginofcoastallagoons 251

Vol.22No.3•2012

structures (Fig. 25) may be observed in the beach areas freefrommangrovecover,appearingasplatformslimitingthesandyareas.Theirfragmentationtodifferentdegreesisevident.Inthemangroveareasdensemicrobialmatsareconspicuous,closelyassociatedtotheLaguncularia racemosarootsystemsand,justbehindthemangroveline,evidencesofthetransitionfrommattopro-thrombolithcanbefound.Extensivesoftdiscoloredmats(Fig.26)arefoundfusedwiththeconsolidatedthrombolithicstructure(Fig.27).

Microscopic examination of the above samples howeverrevealed mostly sediments together with empty sheaths and

drying filaments mainly of Microcoleus chthonoplastes that arealso common. However, green (live) filaments of Microcoleus chthonoplastes (Figs.28-30)andLyngbya aestuarii are less fre-quent.Thescaffoldfunctionofthemicrobialmatisevidencedbythe presence of soft bridge-like structures (Fig. 31) as the (lith-ified)oneinIslaEspírituSanto(Fig.18),inwhichemptysheathsare common along with scarce live filaments. The presence ofbothemptysheathsandgreenfilamentssuggeststhateithernewcolonizationisoccurringorthat,asithasbeenobservedwithliv-ingstromatolites,alivepartremainsfromtheoriginalconglomer-ate mats that originated the thrombolite. This would apparently

Figures8-13.ThrombolithicformationsindifferentlocalitiesofBahíadeLaPaz,B.C.S.,Mexico:8)BlocksintheintertidalcoveredbygreenmacroalgaeathightideatLasBrisas;9)ThrombolithicformationintheintertidalcoveredbygreenmacroalgaeduringlowtideatLasBrisas;10)Non-lacunarthrombolithicplatformswheretwoseparateepisodescanbeidentifiedbasedonthetwolayersofbeach-rockplatformsatCalerita;11)ThrombolithicplatformsshowingperforationsatCalerita;12)Closeupofperfora-tionsinthrombolithicblocksatCalerita;13)Fragilerhizolith-likestructuresinthelowerintertidalofElMogote,LaPazlagoon.

252 SiqueirosBeltronesD.A.et al.

Hidrobiológica

supporttheproposedhypothesisthatatransitionalstructureex-isted.However, thishas tobemoreaccuratelystated,becausethe transitional structures between mats and thrombolites arepro-thrombolites, while in the observed structures they werepastthepro-thrombolithicstage(moreconsolidated)andmatsinwhichcyanobacterialsheathsarestillpresentandthusrepresentseparategeologicalevents.

Finally,concerningourquestion:arethrombolithicstructuresthecommonsubstrateformangroves?Ourobservationsshowthatbothmangroverecruitsandtreeswereonlyanchoredeitherincon-glomeratedmatsorinthrombolithic/pro-thrombolithicoutgrowths

butnotintheborderingsand(Fig.32-36).Ourhypothesisisthusconsistentwithourobservationsinall thesites, i.e.,mangrovesusethepreformedstructuresbycyanobacterialmats(conglom-erate mats, pro-thrombolites, thrombolites) as fixing substrate.Nonetheless,mangrovetreeswereseenanchoredonothernon-thrombolithicrockysubstrata,asinIslaSanJoséwhereincipientpro-thrombolithicformationswererecorded(Figs.19-20).

DISCUSSION

Theabovedescription for theSanCarlossamplesdoesnotde-partfromtheonegivenearlierforthrombolithicstructuresofthe

Figures14-19.Pro-thrombolithicstructuresindifferentlocalitiesofB.C.S.:14)ExtensiveplatformwiththickrootlikeformationsintheupperintertidalofElMogote;15)LithifiedplatformwithfragilerhizolitesystemintheupperintertidalofElMogote;16)ThrombolithicplatformontheexposedeastcoastintertidalofIslaEspírituSanto;17)ThrombolithicplatformsinacoveonthewestcoastofIslaEspírituSanto;18)Bridge-likethrombolithicstructures(ca.25cm)adjacenttomangrovesonthewestcoast(SanCarlos,B.C.S.);19)Incipientpro-thrombolithatIslaSanJosé.

Pro-thrombolitesandoriginofcoastallagoons 253

Vol.22No.3•2012

La Paz lagoon (Siqueiros-Beltrones, 2008) where environmentalconditionsaremuchalike.Mineralogicalanalyses indicate thatgrainsfromSanCarlosareprobablyformedundermarinecondi-tionsandtheacidtreatedsamplesshowasimilardegreeofse-lection indicating that they come from sites under similar (low)energyconditions.

Thecommondenominatoristhepresenceofmicritewhichweexpectedtobepresentorabundantinconsolidatedtolithifiedstructures,whereasmostofoursampleswerenon-lithifiedpro-

thrombolithicstructures.Althoughthemeansbywhichmicrobialcommunitiesmediatetheprecipitationprocessesofmicro-crys-tallineCaCO3remainunclearandaresubjecttodebateandspec-ulation (Kazmierczak et al., 1996; Paerl et al., 2001), it has beendetermined that huge amounts of micrite may be produced byprecipitationmainlybycyanobacteriainmarginalmarinemarsh-es(Tucker&Wright,1990)anditspresenceisconsistentwiththeshallow(intertidal)marineenvironment.Thuswehaveassumedabiogenicoriginfortheidentifiedmicritethatmaybeassociatedtothemicrobialmatphotosynthesis.

Figures20-25.Pro-thrombolithicstructuresinotherlocalitiesoftheGulfofCalifornia,Mexico:20)ThickcyanobacterialmatfromtheintertidalofIslaSanJosé;21)Pro-thrombolithicmuddyplatformatElRequesón,BahíaConcepción;22)LithifiedthrombolithicplatformandLaguncularia racemosaspecimen;23)ThrombolithicplatformatEsteroElSoldado,Sonora;24)SerinativesseatingonathrombolithicblockinEsteroSantaRosa,Sonora;25)ThromboliteplatformandfragmentsintheupperintertidalareaofSanCarlosbeach,BahíaMagdalena,westcoastofB.C.S.

254 SiqueirosBeltronesD.A.et al.

Hidrobiológica

In the above scenario, activity would continue as long asa cyanobacterial cover remained on the conglomerated sedi-ment platforms (prothrombolite) that is subject to tidal flooding,inwhichcasetheprecipitatedmicritewouldbe forcedthroughthecyanobacterialmeshduringebbtide,depositingbetweenthesandgrainsandgraduallycementingthem.

A report by Holser et al. (1981) at Laguna Ojo de Liebre(GuerreroNegro,B.C.S.)supportsourobservationsthatthemaincyanobacteriaspeciesMicrocoleus chthonoplastes andLyngbya aestuarii involved in pro-thrombolithic processes are the same;thesetaxaareimportantcomponentsofmicrobialmatsinseveralpartsoftheLaPazlagoon(García-Maldonado,2005)andparticu-

larly those associated to pro-thrombolites (Siqueiros-Beltrones,2008).Moreover,theformerspeciesisknowntobethedominanttaxon in marine intertidal microbial mats worldwide (Stal et al.,1985).AlthoughpresentatSanCarlos,diatomswerenotexam-ined,andfurtherfloristicanalyseswerenotdoneforthecyano-bacteria inasmuchoursole interestwas focusedonconfirmingthedominanttaxainvolvedinthepro-thrombolithicprocess.

MicrobialstructuresarecommonalongtheBajaCaliforniaPeninsula, including stromatolites (Miranda-Avilés et al., 2005)andhypersalinemats (Holseret al., 1981;Horodysky&VonderHaar,1975;Horodysky,1977),whichareamongthemoststudiedintheworld(Stal,2000).Loganet al.(1964)andAltermannet al.

Figures26-31.Earlystagesofpro-thrombolithicdevelopmentinthewestcoastofB.C.S.,Mexico:26)Softconglomeratingcyano-bacterialmatsdryingintheintertidalareaatSanCarlosbeach,B.C.S,alongsidethe“transitional”pro-thrombolithicformation;27)Transitionfromsoftconglomeratedmats(embeddedcoin)fusedtoconsolidatedpro-thrombolithicformation(laidcoin)inSanCarlos;28)Photomicrographofempty Microcoleus chthonoplastessheathsactingasscaffoldinthetransitionalpro-throm-bolitematfromSanCarlosinBahíaMagdalena(200x);29)FilamentofMicrocoleus chthonoplastesfromSanCarlosshowingthicksheaths(400x);30)TrichomefromMicrocoleus chthonoplastesfilamentat630xfromSanCarlos;31)Softbridge-likepro-thrombolithicmat(ca.20cm)structure(scaffold)atSanCarlos.

Pro-thrombolitesandoriginofcoastallagoons 255

Vol.22No.3•2012

(2006)havesuggestedthatduringthePre-Cambrian,stromatolitesandhencemicrobialmatshadcoveredlargeareas.Thesemayberepresentedbyrockygroundfar fromthecoastand livingstro-matolitesintheintertidal,butalsobyextensive(nonconsolidatedsediment)pro-thrombolithicplatformsandtrombolitesincoasts.Therefore,extensivemicrobialmatssuchasthoseshowninStal(2000)andhereinfigure2mayhavedevelopedintoconglomer-ated mats and pro-thrombolithic ground that eventually was to

beoccupiedbyhumansettlementsormangroves(Siqueiros-Bel-trones et al., 2009). Thus, the seaward development of spit-likepro-thrombolites would lead eventually to costal lagoon forma-tion,buttheirhypotheticroleinthegeomorphologyofcoastalla-goonshadnotbeenproposeduntilrecently(Siqueiros-Beltrones,2008).

The above observations show that the described processoccurs throughout the coasts of the Gulf of California and NW

Figures32-36.AssociationsbetweenmangrovesanddifferentstagesofthromboliteformationinB.C.S.,Mexico:32)Blackman-grove(Avicennia germinans)anchoredonthrombolihticplatform(LaPazlagoon;33)Whitemangroveshrub(Laguncularia rac-emosa)anchoredonpro-thrombolithicplatform(ElMogote,LaPazlagoon);34)Recruitsofredmangrove(Rhizophora mangle)anchoredinsoftconglomeratedmatatSanCarlos;35)Recruitsofredmangrove(Rhizophora mangle)anchoredinthrombolithoutcropatSanCarlos;36)Rhizophora manglerecruitsanchoredinthrombolithicplatformsurroundedbysandwithnorecruitsatSanCarlos.

256 SiqueirosBeltronesD.A.et al.

Hidrobiológica

Mexicobothinlacunarandnon-lacunarenvironments.Thenon-lacunarthrombolithicplatformsobservedindicatethat,althoughthrombolithicformation is favoredbytheconditionsestablishedbythe lacunarenvironment, italsoshowsthatverysimilar(lowenergyandshallow)non-lacunarconditionswouldhaveinitiallypromoted pro-thrombolite formation. Said process is furtherenhanced within the formed coastal lagoon. We have recentlyobserved that similar non-lacunar thrombolithic platforms andpro-thrombolithic grounds are common off the coast of Yuca-tán, Mexico. Most likely, this process occurs in other parts ofthe world, contributing to the generation and eventual filling ofcostallagoonsandtheformationofcoastalterrainsasdepictedinFigure37.Evidenceforthisprocesswasgeneratedbyexplor-ingexcavationsandanalyzingsedimentcoresfromLaPazlagooncoast(Siqueiros-Beltroneset al.,2009).

Previousobservationshavesuggestedthattheroleofman-grovesassedimenttrapsthateventuallycausegroundformation(Dawes,1981;Kathiresam,2003) isapreexistingprocess that isacceleratedbymangrovesaftercolonizingsuitableareas(Wood-roffe, 1992). Our study, provides observations supporting thatground formation is initially carried out by cyanobacterial matsthrough binding, stabilizing, and clotting sediments, leading tothe development of pro-thrombolithic platforms, and that thesesubstratesareopportunisticallycolonizedbymangroverecruits,asdepictedinfigures32-36thatshownomangroverecruitsoffthe microbial mats or off the microbialites. Plus, although man-grove recruiting has been observed to occur on other type ofrockwheremangrovetreesthrive,inthecostallagoonswehavesurveyed, both mangrove recruits and trees are only observedanchored either in conglomerated mats or in thrombolithic/pro-thrombolithic outgrowths but not in the sand. Furthermore, ac-cretingmats (conglomerated)developextensively in the lagoonbeachesweremangroveshavenotyetcolonized(Fig.2)thoughwe have observed ephemeral recruiting on them in La Paz la-goon.Theabovesupportsourhypothesisthatgroundformationleading to island formation and shoreline extension hitherto at-tributedtomangroveforests(Dawes,1981)that“claimlandfromthesea”(Woodroffe,1992)isactuallyinitiatedbymicrobialmats,andcontinuesthroughpro-thrombolithicgrowth,andisonlylatercolonizedbymangroves.Thepastoccurrenceofthisprocessis,according to our hypothesis, recorded here geologically by therhizoliths(Figs.13-15)andinSiqueiros-Beltrones(2008);muchliketheonesdescribedbyJohnsonet al.(1997)intheareaofBahíaConcepciónclosetoElRequesón,B.C.S.

Basedontheabovealongwithreportsofpro-thrombolitesfromLagunaSanIgnaciointhewesterncoastoftheBajaCalifor-niapeninsula(Siqueiros-Beltroneset al.,2008),andtheobserva-tionsinBahíaMagdalenawefurthersupportthehypothesisthatmicrobialmats throughpro-thrombolithicground formationmayhavepromotedthegenerationofcoastallagoonsthroughoutNWMexico.Thus,asmoreinformationbecomesavailable,Lovelock´s

(1979) assumption “let us now make the bold speculation that(coastal) lagoons formed as a consequence of the presence oflifeinthesea”comesintofocuswithpropheticconnotation.

ACKNOWLEDGEMENTS

This work was supported by grants from projects IPN-SIP:20080010, 20090154 and 20100036, Influencia de protrombolitosenlageomorfologíadelagunascosterasdelGolfodeCalifornia(I,II,III).UriArgumedohelpedwithsampleprocessingandfieldphotograph;RubénGarcíaaidedinfieldworkandwithimageed-iting. Diana Luque “Ecoturismo Seri, CIADand CtamCoyai, A.C.,Sonora”providedthethrombolitesimagefromEsteroSantaRosa,andAlejandraÁlvarezeditedfigures1and37.EnglisheditionwasdonebyDianaDorantesSalasinanearlierMS.WethankHugoBeraldi,threeanonymousreviewers,andRosaluzTaverafortheirobservations.ThefirstandthirdauthorsareCOFAAandEDIfel-lowsatIPN.

REFERENCES

AltermAnn, W., J. KAzmierczAK, A. Oren & D. t. Wright.2006.Cyanobacte-rialcalcificationand their rock-buildingpotentialduring3.5billionyearsofEarthhistory.Geobiology4:147-166.

chArpy, l. & A. W. D. lArKum (eDs.).1999.Marine Cyanobacteria.Bulletinde l’Institut Océanographique, Musée Océanographique, Monaco.Numérospécial19.624p.

DAWes, c. J.1981.Marine Botany.JohnWiley&SonsLtd.628p.

Figure37.LaPazcoastallagooninthesouthernBajaCalifor-niapeninsula.Largearrowindicatesthegrowthofthesand-bar along approximately 6000 years. Small arrows indicatepro-thrombolithicaccretionalongtheinternalmarginsofthelagoon, based on recorded platforms and extensive cyano-bacterialmats.

Pro-thrombolitesandoriginofcoastallagoons 257

Vol.22No.3•2012

gArcíA-mAlDOnADO, J. Q.2005.DiversidadydistribucióndecianobacteriasenlaEnsenadayBahíadeLaPaz,B.C.S.,indicadoresdeactividadhumana.TesisdeLicenciatura.Dpto.BiologíaMarina,UniversidadAutónomadeBajaCaliforniaSur.LaPaz,México.69p.

gOlubic, s., t. le cAmpiOn-Al sumArD & s. e. cAmpbell.1999.Diversityofmarinecyanobacteria.In:Charpy,L.&A.W.D.Larkum(Eds.). Ma-rine Cyanobacteria.Bulletinde´l InstitutOcéanographique.MuséeOcéanographique.Mónaco.pp.53-76.

hOlser, W. t., b. J. JAvOr, c. pierre & l. Ortlieb.1981.GeochemistryandecologyofsaltpansatGuerreroNegro,B.C.S.In:Ortlieb,L.&Q.J.Roldán (Eds.).Geology of Northwestern Mexico and Southern Ari-zona; Field Guides and Papers.EstaciónRegionaldelNoroeste,In-stitutodeGeología,U.N.A.M.,Hermosillo,Sonora,México.pp.2-56.

hOrODysKy, r. J. & s. J. vOn Der hAAr.1975.Recentcalcareousstromato-lites from Laguna Mormona (Baja California) México. Journal of Sedimentology and Petrology45:894-906.

hOrODysKy, r. J.1977.LyngbyamatsatLagunaMormona,BajaCalifornia,México:comparisonwithProterozoicstromatolites.Journal of Sedi-mentology and Petrology47:1305-1320.

JOhnsOn, m. e. & J. leDesmA-vázQuez.1997.Pliocene carbonates and re-lated facies flanking the Gulf of California, Baja California, México.Specialpaper318,GeologicalSocietyofAmerica.171p.

JOhnsOn, m. e., J. leDesmA-vázQuez , m. A. mAyAll & J. minch.1997.UpperPliocenestratigraphyanddepositionalsystems:ThePeninsulaCon-cepciónbasinsinBajaCaliforniaSur,México.In:Johnson,M.E.&J.Ledesma-Vázquez(Eds.).Pliocene carbonates and related facies flanking the Gulf of California, Baja California, México.Specialpaper318,GeologicalSocietyofAmerica.pp.57-72.

KAzmierczAK, J., m. l. cOlemAn, m. gruszczynsKi & s. Kempe.1996.Cyano-bacterial key to the genesis of micritic and peloidal limestones inancientseas.Acta Palaeontologica Polonica41(4):19-338.

KAthiresAm, K. 2003. How do mangrove forests induce sedimentation?Revista de Biología Tropical 51(2):355-360.

KJerfve, b.1994.(Ed.).Coastallagoonprocesses.ElsevierOceanographicSeries.Amsterdam.577p.

lOgAn, b. W., r. rezAK & r. n. ginsburg.1964.Classificationandenviron-mental significanceofalgal stromatolites.TheJournal of Geology72:68-83.

lOvelOcK, J. e.1979.Gaia: AnewlookatlifeonEarth.OxfordUniversityPress.154p.

mirAnDA-Aviléz, r., h. berAlDi-cAmpesi, m. J. puy-AlQuizA & A. l. cArreñO.2005. Estromatolitos, tufas y travertinos de la sección El Morro:depósitosrelacionadosconlaprimeraincursiónmarinaenlaCuen-cadeSantaRosalía,BajaCaliforniaSur.Revista Mexicana de Cien-cias Geológicas22(2):148-158.

nAvA-sánchez, e. h. & r. cruz-OrOzcO.1989.Origenyevolucióngeomor-fológicadelaLagunadeLaPaz,BajaCaliforniaSur,México.Inves-tigaciones Marinas CICIMAR 4(1):49–58.

pAerl, h. W., t. f. steppe & r. pAmelA reiD.2001.Bacteriallymediatedpre-cipitationinmarinestromatolites.Environmental Microbiology3(2):123-130.

phleger, f. b.1969.Somegeneralfeaturesofcoastallagoons.In:Ayala-Castañares,A.&F.B.Phleger(Eds.).Coastal Lagoons: A symposium.ProceedingsoftheInternationalCoastalLagoonSymposium(origin,dynamicsandproductivity),Nov.28-30,1967,UniversidadNacionalAutónomadeMéxico-UNESCO.México,D.F.pp.5-26.

shApirO, r.2000.Acommentonthesystematicconfusionofthrombolites.Palaios15(2):166-169.

siQueirOs-beltrOnes, D. A.2006.Diatomeasbentónicasasociadastrombo-litosrecientesenlaEnsenadadeLaPaz,B.C.S.,México.CICIMAR-Oceánides21(1-2):113-143.

siQueirOs-beltrOnes, D. A.2008.Roleofpro-thrombolitesinthegeomor-phologyofacoastallagoon.Pacific Science62(2):257-269.

siQueirOs-beltrOnes, D. A., O. u. hernánDez-AlmeiDA, s. gOnzález-cArrillO & u. ArgumeDO-hernánDez.2008.ProtrombolitosenLagunaSanIgna-cio,B.C.S,México.CICIMAR-Oceánides23(1-2):83-86.

siQueirOs-beltrOnes, D. A., e. f. félix-picO & O. u. hernánDez AlmeiDA.2009.Stratigraphicevidenceofpro-thrombolithicgroundformationaroundtheLaPazlagoon(México).CICIMAR-Oceánides24(1):59-63.

stAl, l. J. 2000. Cyanobacterial mats and stromatolites. In: Whitton, B.A.&M.Potts (Eds.).The ecology of cyanobacteria; their diversity in time and space. Kluwer Academic Publishers. Netherlands. pp.61-120

stAl, l. J., h. vAn gemerDen & W. e. Krumbein.1985.Structureanddevel-opmentofabenthicmarinemicrobialmat.FEMS Microbial Ecology31:111-125.

tucKer, m. e. & v. p. Wright.1990.Carbonate sedimentology.BackwellScience,Malden,MA,USA.482p.

WhittOn, b. A. & m. pOtts (eDs.).2000.The ecology of cyanobacteria: their diversity in time and space.KluwerAcademicPublishers.Nether-lands.669p.

WOODrOffe, c.1992.Mangrovesedimentsandgeomorphology.In:Robert-sonA.I.&D.M.Alongi(Eds.).Tropical mangrove ecosystem.Ameri-canGeophysicalUnion.WashingtonDC.pp.7-41.

Recibido:23defebrerode2012.

Aceptado:27deagostode2012.