Development of microbial carbonates in the Lower Cretaceous Cod o Formation (north-east Brazil): Implications for interpretation of microbialite facies associations and palaeoenvironmental

Development of microbial carbonates in the Lower CretaceousCod�o Formation (north-east Brazil): Implications forinterpretation of microbialite facies associations andpalaeoenvironmental conditions

ANELIZE M. BAHNIUK*, SYLVIA ANJOS† , ALM �ERIO B. FRANC�A† , NILOMATSUDA† , JOHN EILER‡ , JUDITH A. MCKENZIE* and CRISOGONOVASCONCELOS**Geological Institute, ETHZ, 8092 Zurich, Switzerland (E-mail: [email protected])†PETROBRAS/E&P-EXP/GEO/ES, 0031170 Rio de Janerio, Brazil‡California Institute of Technology, Pasadena, CA 9115, USA

Associate Editor – Daniel Ariztegui

ABSTRACT

The study of microbial carbonates has acquired new significance with the rec-

ognition that they retain valuable information related to biomineralization

processes associated with microbial activity throughout geological time.

Additionally, microbialites have a demonstrated economic potential to serve

as excellent hydrocarbon reservoirs. The Lower Cretaceous Cod�o Formation,

located in the Parnaiba Basin of north-east Brazil, comprises a unique strati-

graphic sequence of up to 20 m thick, well-preserved carbonate microbialites.

Deposited in a continental basin during the initial break up and separation of

South America from Africa in the Early Cretaceous, this lacustrine carbonate

sequence provides an excellent example to investigate the palaeoenvironmen-

tal conditions controlling microbialite facies development. Based on macro-

scopic and microscopic observations of outcrop and drill core samples, four

microbialite facies (stromatolite, lamina, massive and spherulite) were

defined and distinguished by textures and microbial fossil content. Changes

in facies type are related to alternating palaeo-water depths, as reflected by87Sr/86Sr cycles resulting from fluctuations in the sources of meteoric water.

Clumped isotope measurements of stromatolitic fabrics yield precipitation

palaeo-temperatures with an average value of 35°C. The d18O values of bulk

carbonate (�6�8 to �1�5& Vienna Pee Dee Belemnite) imply precipitation

from water with calculated d18O values between �1�6& and 1�8& Vienna

Standard Mean Ocean Water, reflecting precipitation from variably modified

meteoric waters. The d13C values of bulk carbonate (�15�5 to �7�2& Vienna

Pee Dee Belemnite) indicate a significant input of carbon derived from aero-

bic or anaerobic respiration of organic matter. Combined, the data indicate

that the evolution of the Cod�o Formation occurred in a closed lacustrine pala-

eoenvironment with alternating episodes of contracting and expanding lake

levels, which led to the development of specific microbialite facies associa-

tions. The results provide new insights into palaeoenvironmental settings,

biogenicity and early diagenetic processes involved in the formation of

ancient carbonate microbialites and, by extension, improve the knowledge of

the reservoir geology of correlative units in deep waters offshore Brazil.

155© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists

Sedimentology (2015) 62, 155–181 doi: 10.1111/sed.12144

Keywords Brazil, carbonate, Cod�o Formation, Lower Cretaceous, micro-bialite facies.

INTRODUCTION

Although microbialites found throughout thegeological record have been well-defined andcategorized through studies of their macroscopicstructures beginning with the early investigationsof Kalkowsky (1908) and later with the definitionof Burne & Moore (1987), the environmental con-text of their formation remains elusive. With theadvent and application of new high-resolutionmicroscopic and geochemical techniques, inves-tigations of microbialites can now yieldadditional information regarding the depositionalconditions at the time of their formation and sub-sequent diagenesis, as well as providing insightsinto the palaeoenvironmental evolution of theEarth’s surface. Furthermore, the palaeoenviron-mental interpretations of ancient microbialite for-mation can be enhanced when combined withinformation from the study of modern microbia-lite depositional environments and well-designedlaboratory experiments. This multi-prongedapproach has been applied in the present studyof the microbialite sequence of the Lower Creta-ceous Cod�o Formation of north-east Brazil.Located in the Parnaiba Basin (Fig. 1), the

Upper Aptian carbonate unit of the Cod�o Forma-tion comprises a unique stratigraphic sequence

of up to 20 m thick, well-preserved carbonatemicrobialites, which has not been investigatedextensively until now. Deposited in a continen-tal basin during the initial break up and separa-tion of South America from Africa in the EarlyCretaceous, this lacustrine carbonate sequenceprovides an excellent example to investigate thepalaeoenvironmental conditions controllingmicrobialite facies development. Furthermore, ithas been correlated with the time-equivalentmicrobialite facies of the major petroleum dis-covery in deep waters offshore Brazil (Vaz et al.,2007; Maizatto et al., 2011). Whereas the on-shore Cod�o Formation and offshore petroleumreserves are laterally distant and probably differin some details of their depositional environ-ment, a detailed understanding of the relativelyaccessible rocks and drill core of the Cod�o For-mation may furnish insights into the propertiesthese two Cretaceous sequences share. For thisreason, constraining the palaeoenvironmentalconditions during deposition and diagenesis ofthe microbial carbonate in the Cod�o Formationhas taken on a new significance.Apart from its economic importance, the Cod�o

Formation has recently been investigated withregard to its sedimentological, stratigraphicand geochemical features in order to reconstruct

Fig. 1. Location map for theParna�ıba Basin (Silurian), with thelimits of the Lower Cretaceousdelineated by the dashed line.Insert map of Brazil shows thelocation of the Maranh~ao State,where samples were collected. Thedistribution of sabkha, carbonateand anhydrite environments of theUpper Aptian Cod�o Formation, aswell as the location of sampledoutcrops and wells in the Cod�o,Graja�u and Imperatriz regions, isdisplayed. (Modified from aninternal report entitled ‘Formac�~aoCod�o’ with permission of PetrobrasMinerac�~ao S.A. PETROMISA.)

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 155–181

156 A. M. Bahniuk et al.

the depositional conditions (Paz, 2000; Paz &Rossetti, 2001; Rossetti et al., 2004; Paz & Ros-setti, 2005). These studies defined the deposi-tional conditions as a closed anoxic hypersalinelacustrine environment, based on lithologicalobservations throughout the basin. Paz & Rossetti(2001) provided the first descriptions of sedi-mentary characteristics and facies, which wereused to reconstruct the palaeo-environmentalconditions during deposition of the Cod�o Forma-tion in the Graja�u region (Fig. 1). The presenceof evaporites, limestones and argillites was takenas evidence for deposition in shallow, low-energy, subaqueous, hypersaline environmentsexposed to meteoric and/or capillary hydrologyconditions. Upward cycles of successive periodsof shallow water followed by expansion of thebasin were defined based on the clay mineralassemblages (Gonc�alves et al., 2006).Some authors place the Cod�o Formation in the

Graja�u Basin, which overlies the Parna�ıba Basin(Paz, 2000; Paz & Rossetti, 2001; Rossetti et al.,2004), However, Vaz et al. (2007) defined theCod�o Formation as a Cretaceous sequencewithin the Parnaiba Basin (Fig. 1). The biostra-tigraphy of the Cod�o Formation, based on non-marine ostracods and palynology, indicates aNeoaptian fauna comprising mainly Candona eHarbinia (Ramos et al., 2006) and the biozone ofSergipea Varriverrucata (Pedr~ao et al., 2002),suggesting a lacustrine environment. Recentstudies of the Cod�o Formation in the Graja�u andImperatriz regions place it at the same age(Maizatto et al., 2011).The main objective of this study of the Cod�o

Formation is to interpret the palaeoenvironmen-tal conditions and bio-geochemical processesrelated to microbialite carbonate facies evolutionduring the initial break up and separation ofSouth America from Africa during the Early Cre-taceous. Different approaches were applied todefine the microbialite facies and the microbialinfluence on carbonate precipitation, as well asthe subsequent diagenesis. These include classi-cal methods, such as description of sedimentaryfacies both from outcrops and drill cores andpetrographic description of thin sections, com-bined with geochemical studies, such as87Sr/86Sr, light stable isotope and ‘clumped’ iso-tope analyses. The combination of all of thesemethods provides new insights into the palaeo-environmental conditions and early diageneticprocesses that occurred during microbial carbo-nate formation in the Lower Cretaceous lacustrinesystem.

Based on field observations, four types ofmicrobialites have been recognized, according totheir internal structures and morphology: (i) stro-matolite; (ii) lamina; (iii) massive; and (iv) sphe-rulite microbialite facies. In order to associatemorphology with environmental conditions, it isnecessary to evaluate both organic and inorganiccomponents in the distinct facies and ultimatelyintegrate the results to reconstruct the palaeo-environment during the microbialite formation.

GEOLOGICAL SETTING

The breakup of the super continent Gondwanabegan in the Early Jurassic, and South Americastarted to drift slowly westward from Africa inthe Early Cretaceous. The incipient initiation ofthis process, during the Aptian, split SouthAmerica away from Africa beginning in thesouth and opening northward (Dietz & Holden,1970). This process established several riftbasins along the equatorial Brazilian margin(Soares Junior et al., 2008). One of these, theParna�ıba Basin, is a cratonic basin located in thenorth-east region of Brazil. It occupies an area ofca 600 000 km² and contains an infill sequenceup to 4000 m thick in the deepest part of thebasin (Fig. 1). The Parnaiba Basin is bounded onthe north-east by the Ferrer-Urbano Santos Arc,which separates the continental basins from theSouth Atlantic Ocean. The Middle San Fran-cisco Arc constrains its southern boundary.Lisboa (1914) and Campbell et al. (1949)

defined the Cod�o Formation as a Lower Creta-ceous succession, up to 450 m thick, composedof sandstones, evaporites, shales and limestones.Paz & Rossetti (2001) recognized three lacustrinefacies associated with the Cod�o Formation con-sisting of: (i) deep-water deposits composed ofbituminous black shale and evaporates; (ii)intermediate-depth deposits composed of argil-lites and limestones; and (iii) marginal shallow-water deposits composed of pelite, limestone,ostracodal wackestone and tufa.Maizatto et al. (2011) have studied the palyno-

logical and non-marine ostracod content of sam-ples from the Cod�o Formation, which date theformation as Aptian, based on the Sergipeavariverrucata palynomorph biozone (Lima, 1982;Batista, 1992; Paz & Rossetti, 2001) and theHarbinia spp. ostracod biozone (Maizatto et al.,2011). According to Pedr~ao et al. (2002), basedon the occurrence of the Sergipea variverrucatapalynomorph biozone, rocks correlative with the


Development of microbial carbonates in the Lower Cretaceous 157

Cod�o Formation can be found in the Braganc�a-Viseu and S~ao Lu�ıs basins, as well as in the IlhaNova Graben.

METHODS

Carbonate rock samples were collected from out-crops and drill cores of the Cod�o Formation alongthe southern border of the Parnaiba Basin duringOctober 2009 and August 2010. Several road out-crops and quarries were described, and 75 sam-ples were obtained from outcrops located in theCod�o, Graja�u and Imperatriz regions. The S~aoBenedito Quarry near the city of Cod�o is consid-ered a ‘type section’ of the microbialite sequencein the Cod�o Formation (Fig. 1). These outcrop-ping sections represent the southern margin ofthe basin. Cores from the following wells weredescribed and sampled: 9-PAF-4R-MA, 9-PAF-7-MA, 1-PA-1-MA, 2-IZST-1-MA and 2-VGST-1-MA from the shale-rich part of the basin; and9-PAF-1-MA, 9-PAF-2-MA from the basin margin,where microbial carbonates predominate (Table 1).The microbial features of the Cod�o Formation

were recognized macroscopically in the threestudy areas of the outcropping southern rim ofthe Parnaiba Basin (Fig. 1). Subsequently, micro-bialite facies were described and defined in 53thin sections using optical microscopy in trans-mitted light (Nikon Optiphot Microscope, ETHZurich; Nikon Instruments Europe BV, Amster-dam, The Netherlands) and cathodolumine-scence microscopes. In addition, minerals wereidentified using a Bruker X-ray diffractometer(XRD) AXS D8 Advance with a Lynxeye detector(Bruker AXS GmbH, Karlsruhe, Germany). Themineralogy was interpreted using the programXRD Wizard at ETH Zurich.Scanning electron microscopy (SEM) studies

coupled with an EDAX detector for elementalanalyses (SEM, Zeiss Supra 50 VP, University ofZurich; Carl Zeiss Microscopy GmbH, Jena, Ger-many) were used to identify microbial featuresin the samples. The samples underwent a slightacid etching before coating with platinum toavoid polishing artefacts; they were alsoobserved without etching. Some samples wereselected for an environmental SEM study basedon specific areas identified by prior petrographicstudies. These samples were observed usingbackscattered electrons allowing the identifica-tion of mineralogical phases.To obtain 87Sr/86Sr ratios, eight samples were

dissolved in a weak acetic acid solution and

measured using a Perkin Elmer, Optima 4300DV inductively coupled plasma optical emissionspectrometer (ICP-OES; Perkin Elmer, Waltham,MA, USA) at the Plasma Analytical Laboratory,Institute of Marine Science, University of Cali-fornia, Santa Cruz. The NBS 987 Sr isotopicstandards were run intermittently to provideinformation on the internal offset. The 87Sr/86Srdata were corrected for the average offset andare given in Table 2.Carbonate samples were also analysed for their

light stable isotope values, including d18O, d13Cand clumped isotope compositions. The latteranalysis (Eiler, 2007) constrains the proportion of13C and 18O that are bonded together to make themultiply substituted carbonate ion group,13C18O16O2

�2. Clumped isotope measurementsare conventionally reported as Δ47 values for CO2

produced by phosphoric acid digestion. Thesemeasurements reflect the enrichment in CO2 ofmass 47 isotopologues (principally 13C18O16O)relative to the amount expected for a random dis-tribution of isotopes among all isotopologues. Ithas been demonstrated that Δ47 values of CO2

extracted from carbonates depend on carbonategrowth temperature and are not directly related tothe oxygen or carbon isotopic composition of thefluid from which the mineral formed (Ghoshet al., 2006a, 2007). Thus, measuring the Δ47 ofcarbonate rocks constrains the temperatures ofcarbonate deposition (in the case of primary car-bonate) or diagenesis (in the case of carbonate thatgrows or recrystallizes during burial). Further-more, combination of a clumped isotope measure-ment with a measurement of the d18O of carbonateallows calculation of the d18O of water withwhich the carbonate was last equilibrated. Thisinformation constrains the source of the waterfrom which the carbonate precipitated in the caseof primary carbonates and formation waters in thecase of diagenetically modified carbonates.Twenty-eight samples containing variable per-

centages of dolomite and calcite were reacted at90°C with 100% phosphoric acid in a commonacid bath. The presence of gypsum in some sam-ples contaminated the isotope analysis produc-ing anomalous results. The high-precisionclumped-isotope measurements were performedusing a modified Thermo Finnigan 253 (ThermoFisher Scientific, Waltham, MA, USA) housedin the Laboratory for Stable Isotope Geochemis-try at Caltech (Eiler & Schauble, 2004; Wanget al., 2004; Affek & Eiler, 2006; Ghosh et al.,2006a,b; Schauble et al., 2006; Ghosh et al.,2007; Huntington et al., 2009).



The oxygen isotopic fractionation factor accom-panying phosphoric acid digestion was assumedto equal 1�0093 and 1�00806 for dolomite andcalcite, respectively (Rosenbaum & Sheppard,1986; Sharma et al., 2002; respectively). The iso-

topic composition of the water in equilibriumwith the dolomite samples was determinedusing the equation 1000 ln adolomite-water = 2�73 9

106T�2 + 0�26 (Vasconcelos et al., 2005) and forcalcite using 1000 ln acalcite-water = 18�03 9

Table 1. Summary of the studied sections: coordinates, thickness and most abundant facies. Locations are givenfor the drill wells, outcrops and quarries described in this study.

Name Region Type

Co-ordinate (SAD 69)

Thickness(m) Main facies

UTM N UTM ES W

9-PAF-1-MA Arari Drill core 9603984 515900 135 60% Shale*3°34058″ 44°51025″ 40% Carbonate

9-PAF-2-MA Arari Drill core 9627245 522260 127 60% Shale*3°22020″ 44°47058″ 40% Carbonate

9-PAF-4R-MA Cajari Drill core 9632964 498577 142 80% Shale*3°19014″ 45°00046″ 20% Carbonate

9-PAF-7-MA Itapecuru Drill core 9634302 564806 98 80% Shale*3°18030″ 44°24059″ 20% Carbonate

1-PA-1-MA Santa Luzia Drill core 9538612 427276 258 80% Shale*4°10026″ 45°39019″ 20% Carbonate

2-IZST-1-MA Imperatriz Drill core 9388503 224077 26 80% Shale*5°31037″ 47°29025″ 20% Carbonate

2-VGST-1-MA Vargem Grande Drill core 9607700 623712 54 80% Shale*3°32055″ 43°53010″ 20% Carbonate

S~ao BeneditoQuarry

Cod�o Outcrop 9495872 615184 10 60% Gypsum4°33037″ 45°57042″ 20% Shale/Sand

20% Unit I, II, III and IV

Gessomar Quarry Cod�o Outcrop 9461836 604757 20 70% Shale4°52005″ 44°03016″ 30% Unit I†

Baixao Quarry Cod�o Outcrop 9497322 614132 10 50% Unit I and II4°32049″ 43°58016″ 30% Shale

20% Sandstone

BarreirinhaQuarry

Graja�u Outcrop 9364856 377011 20 35% Unit II, III and IV5°44″42″ 46°06″39″ 35% Gypsum

30% Shale/Sand

Barreirinha IIQuarry

Graja�u Outcrop 9365536 376346 8 70% Unit II, III and IV5°44″20″ 46°07″00″ 30% Gypsum

Chorado Quarry Graja�u Outcrop 9361786 375481 12 60% Gypsum5°46″22″ 46°07″29″ 40% Unit II

Praia daGaivota/Amor

Imperatriz Outcrop 9382868 224463 4 80% Unit 1‡5°34″41″ 47°29″14″ 20% Sandstone

Microbialite facies associationsUnit I: Flat lamina microbialite facies; wavy lamina microbialite facies, club-shaped stromatolite microbialitefacies – associated with desiccation structures, mud crack and tepee structures on the top.Unit II: Flat and discontinuous lamina microbialite facies.Unit III: Crenulate lamina and massive microbialite facies.Unit IV: Spherulite microbialite facies.*Black bituminous shales, limestones and anydrites intercalations. †Presence of the ostracods nodules associatedwith selenite. ‡Presence of columnar stromatolite microbialite facies.



Table

2.

Summary

table

ofdescriptionsanddata

obtainedfordifferentmicrobialite

faciesin

theCod� oForm

ation.

Sample

IDRegion

Units

Sample

from

Microbialite

facies

Mineralogy(%

)d1

8O

VPDB

d13C

VPDB

Palaeo

T(°C)

d18O

water

VSMOW

87Sr/

86Sr

Stromatolite

Codo7

Codo

Unit

IOutcrop

Club-shaped

Q:25,C:60,

D:15

�5�48

�10�55

40�75

0�94

Codo4

Codo

Unit

IOutcrop

Club-shaped

Q:35,C:55,

D:10

�5�43

�10�64

30�45

�1�51

CodoC2A

Codo

Unit

IOutcrop

Club-shaped

Q:15,C:80,

D:5

�5�19

�9�64

33�55

�0�51

0�70

9438

Codo1

Codo

Unit

IOutcrop

Club-shaped

Q:25,C:60,

D:15

�5�48

�10�51

30�90

�1�80

Codo3

Codo

Unit

IOutcrop

Club-shaped

Q:35,C:50,

D:15

�5�45

�10�58

27�19

�2�67

Codo5

Codo

Unit

IOutcrop

Club-shaped

Q:10,C:70,

D:20

�5�57

�10�51

27�99

�2�60

Codo6

Codo

Unit

IOutcrop

Club-shaped

Q:15,C:85

�5�45

�10�51

27�35

�2�64

Codo8

Codo

Unit

IOutcrop

Club-shaped

Q:35,C:50,

D:15

�5�53

�10�54

28�37

�2�47

CODO

Codo

Unit

IOutcrop

Club-shaped

Q:20,C:20,

D:60

�4�94

�10�01

27�74

�2�03

Codo2

Codo

Unit

IOutcrop

Mudcrack

Q:49,C:51

�4�79

�9�99

42�85

2�12

I-5

Imperatriz

Unit

IOutcrop

Columnar

Q:87,D:13

�6�69

�8�73

51�79

0�46

0�70

9029

I-7

Imperatriz

Unit

IOutcrop

Breccia

Cement

Q:3,C:97

�6�83

�9�31

50�06

0�32

0�70

7958

AFCD1A

Codo

Unit

IOutcrop

Domal

Q:7,C:85,D:8

�5�79

�9�46

Lamina

G8

Grajau

Unit

IIOutcrop

Flat

Q:15,C:25,

D:60

�1�04

�13�40

32�73

0�07

AFCD1B

Codo

Unit

IIOutcrop

Flat

Q:87,D:13

�1�47

�7�18

CD07

9-PAF-4R.M

AUnit

IIW

ell

Flat

Q:60,C:40

�2�25

�8�42

CD11

9-PAF-7.M

AT.04

Unit

IIW

ell

Flat

Q:40,C:40,

D:20

�3�16

�7�35

0�70

8262

C1B

Codo

Unit

IIW

ell

Discontinuous

Q:12,C:88

�7�04

�6�92

45�60

�0�67

C2B

Codo

Unit

IIW

ell

Discontinuous

Q:9,D:91

�7�76

�6�77

C2A

Codo

Unit

IIW

ell

Discontinuous

Q:55,C:45

�6�86

�5�31

51�73

0�57

CD06

9-PAF-2.M

AT.05

Unit

III

Well

Crenulate

Q:20,C:50,

D:15,G:15

�6�68

0�01

CD2B

9-PAF-1.M

AT.06

Unit

III

Well

Crenulate

Q:20,C:50,

D:10,G:20

�7�51

�0�81

0�70

9359



Table

2.(continued)

Sample

IDRegion

Units

Sample

from

Microbialite

facies

Mineralogy(%

)d1

8O

VPDB

d13C

VPDB

Palaeo

T(°C)

d18O

water

VSMOW

87Sr/

86Sr

CD05A

9-PAF-2.M

AT.05

Unit

III

Well

Crenulate

Q:20,C:40,

D:10,G:20;

B:I0

�6�93

2�52

CD05B

9-PAF-2.M

AT.05

Unit

III

Well

Crenulate

Q:20,C:60,

D:10,G:10

�7�52

1�98

Massive

18

Imperatriz

Unit

III

Outcrop

Massive

Q:10,C:65,

D:25

�4�25

�8�03

50�41

1�70

G11

Grajau

Unit

III

Outcrop

Massive

Q:10,D:90

�6�10

�3�20

CD08

9-PAF-7.M

AT.02

Unit

III

Well

Massive

Q:10,D:90

�0�73

�14�60

54�85

2�95

0�70

9759

CD08-A

9-PAF-7.M

AT.02

Unit

III

Well

Massive

Q:10,D:90

�1�49

�14�18

CD01

9-PAF-1.M

AT.06

Unit

III

Well

Massive

Q:20,C:20,

D:60

�4�31

2�5

Spherulite

CD03

9-PAF-2.M

AT.05

Unit

IVW

ell

Spherulite

Q:10,C:80,

D:10

�5�55

�1�54

53�58

1�77

0�70

8698

CD2A

9-PAF-1.M

AT.06

Unit

IVW

ell

Spherulite

Q:10,C:80,

D:10

�5�48

�1�44

C2B

Codo

Unit

IVOutcrop

Cement

Q:9,D:91

Others

Ost

Grajau

Outcrop

Ostracod

Calcite

0�70

9119

G6

Grajau

Outcrop

Evaporite

Gypsu

m:100

G7

Grajau

Outcrop

Evaporite

Gypsu

m:100

CD4A

9-PAF-2.M

AT.05

Well

Evaporite

Anhydrite:100

CD4B

9-PAF-2.M

AT.05

Well

Evaporite

Anhydrite:100

CD09

9-PAF-7.M

AT.01

Well

Sandstone

Quartz:100

CD10

9-PAF-7.M

AT.03

Well

Sandstone

Quartz:100

Mineralogy(usedweightedX-raydiffractiondata

forsemi-quantitativeestim

ationofminerals):Q,Quartz;C,Calcite;D,Dolomite;G,Gypsu

m;B,Barite.



103T�1 – 32�42 (Kim & O’Neil, 1997). The isotopicresults are reported in the conventional per milnotation with respect to Vienna Pee Dee Belem-nite (VPDB) and standard mean ocean water(VSMOW). The data are presented in Table 2.

Microbialite facies in the Cod�o Formation

Based on macroscopic and microscopic analy-ses, four microbialite facies were defined for theCod�o Formation, i.e. stromatolite, lamina, massiveand spherulite. The microbial features found inthese facies are described and compared withdifferent microbialite classifications compiledfrom the literature (Fig. 2). Each facies expressespeculiar characteristics and textures related tomicrostructures and microbial fossil content.The sedimentary structures recorded in the

microbialite facies of the Cod�o Formation have

basically all been previously observed in therock record (Preiss, 1972; Dupraz et al., 2004;Terra et al., 2010), but this study defines anadditional facies, called massive in the catalogue(Fig. 2). The unique microbialite sequence foundin the Cod�o Formation offers the possibility toevaluate the palaeoenvironmental conditions ina well-controlled stratigraphic context.

Stromatolite microbialite faciesThe stromatolite microbialite facies occurs in alloutcrops along the southern border of the Cod�oFormation, but it was not found in any drill coresamples that were recovered more basinwards(Fig. 1). The facies consists of three major stro-matolite types: club-shaped; columnar; anddomal structured (Fig. 2).The club-shaped stromatolites have a singular

characteristic, in that all stromatolites have a flat

Fig. 2. Compilation of different microbialite classifications applied to microbial features described for Cod�oFormation profiles. The various features are modified after Preiss (1972), Dupraz et al. (2004) and Terra et al. (2010).

Fig. 3. Stromatolite microbialite facies (A) and (B) Club-shaped stromatolites from S~ao Benedito Quarry, Cod�oRegion (A = Sample Cod�o 1; B = Sample Cod�o 3) showing a flat laminated substrate at the base, upon which thestructures grow displaying distinct club-like structures at the top. Pencil for scale is 14 cm long and 1 cm wide. (C)Domal stromatolite hand sample from Baix~ao Quarry, Cod�o Region (Sample AF CD 1A) displaying laminae with alow-angle, gently convex structure. (D) Columnar stromatolite hand sample from Praia da Gaivota outcrop, Imperat-riz Region (Sample I-5), exhibiting steep convex upward laminae. (E) Photomicrograph of club-shaped stromatolite(A) where in thin section (crossed polars), peloidal microfabric can be observed. Yellow arrows indicate dark micri-tic peloids composed of dolomite surrounded by lighter microsparitic crystals (white arrows). (F) Photomicrographof columnar stromatolite (D), where in thin section (crossed polars), laminae show megaquartz with drusy texturein transition to micro-laminae of microquartz. (G) SEM photomicrograph of club-shaped stromatolite (A) showingsub-polygonal honeycomb structures in a dolomite matrix. (H) SEM photomicrograph of columnar stromatolite (D)displaying the presence of filament structures associated with remains of EPS in a quartz matrix.



A B

C D

G H

E F



A B

C D

E F

Fig. 4. Sedimentary structures associated with stromatolite microbialite facies. (A) Breccia hand sample from theImperatriz outcrop (Sample I-7), formed by clasts with diverse size and mineralogical composition. (B) Brecciahand sample from S~ao Benedito Quarry, Cod�o Region. (C) Photomicrograph of the thin section (crossed polars)showing that the clasts are sub-angular and faceted, sometimes formed of stromatolite pieces with mosaic calcitecements. (D) SEM photomicrograph showing the presence of fossil filaments in the stromatolite fragments of thebreccia (Fig. 4A). (E) Mudcrack structure from S~ao Benedito Quarry (Codo 2), Cod�o Region, associated with thetop of club-shaped stromatolites. (F) Tepee structure from S~ao Benedito Quarry, Cod�o Region, associated withthe top of club-shaped stromatolites. Pencil for scale is 1 cm wide.



laminated substrate upon which the structuresgrow, displaying a distinct club-like profiletowards the rectangular top (Fig. 3A and B). Incontrast, the domal-shaped stromatolite displayslaminae with a low-angle, gently convex struc-ture and variation in the particle size, whichincreases from silt at the base to sand-size at thetop (Fig. 3C). Columnar stromatolites are charac-terized by the lack of a flat laminated substrate.Figure 3D illustrates columnar morphology withupward steeply convex, sub-regular millimetriclaminae. The thickness of the laminae is irregu-

lar at the base and becomes more uniformtowards the top.There is a remarkable similarity in mineralogical

composition among the stromatolites, which arecomposed of calcite (60 to 80%), dolomite (10 to20%) and quartz (10 to 35%; Table 2). The colum-nar stromatolites are silicified and dominate in theImperatriz Region with lesser amounts in the Cod�oRegion. In contrast, club-shaped and domal stro-matolites are widespread along the southern mar-gin of the basin, with a low degree of silicificationand better preservation of the primary structures.

A

B C

D E

Fig. 5. (A) The Imperatriz outcrop contains bioherms exposed along the Tocantins River. (B) Detail of the sedi-mentary breccia occurring near the contact of bioherm with the palaeo-channel. Part of knife for scale is ca 2 cm.(C) Grey flat laminated carbonate infilling palaeo-channel between bioherm structures. (D) Bioherm structure,which is ca 2 m high. (E) Detail of the bioherm structure composed of columnar stromatolites.



Petrographic analysis of thin sections fromclub-shaped and domal stromatolites displayspeloidal microfabrics or aphanitic peloids, whichare aggregates of dark micrite surrounded bymicrospar (Fig. 3E). Thin sections of columnarstromatolites show an intensive silicification(Fig. 3F). Based on crystal shape and dimensions,two end-members of quartz-forming laminae(megaquartz and microquartz) have been distin-guished. Megaquartz consists of euhedral crystals(30 to 300 lm), showing slightly undulate sur-faces. This type of quartz also forms subsphericalcrystals. The microquartz consists of equal crys-tals of <1 lm with strongly curved surfaces.Scanning electron microscopy observations of

fresh surfaces of stromatolites reveal the pre-sence of filamentous structures associated withremains of exopolymeric substances (EPS). Scan-ning electron microscopy analyses confirm thediffuse presence of traces of fossilized organicmatter associated with the mineral phases, aswell as possible sub-polygonal honeycomb-likenetworks (Fig. 3G). The filaments consist mainlyof empty, sub-spherical tubes of variable lengthwith diameters ranging between 1 lm and 2 lm(Fig. 3H).Three different types of sedimentary structures

associated with the stromatolite facies were rec-ognized, i.e. breccia, mudcracks and tepees. Thebreccia is composed of clasts of diverse size andmineralogical composition (Fig. 4A and B) and isfound in the palaeo-channels between biohermalstructures. Thin sections show that the brecciaclasts are sub-angular and faceted; they are oftenformed by stromatolite pieces with mosaic cal-cite cement (Fig. 4C). The SEM observations offresh surfaces of breccia reveal the presence offossil filaments in the stromatolite pieces(Fig. 4D). The mudcrack and tepee structures areassociated with the top of club-shaped stromato-lites (Fig. 4E and F).In outcrop, the Imperatriz section consists of

large bioherms up to 2 m in height and exhibits

perfectly exposed lateral variations of the micro-bialite facies. The outcrop occurs along theTocantins River covering an area ca 100 m longand 5 m wide (Fig. 5A). In this outcrop, it ispossible to observe the deposition of breccia andgrey flat laminated carbonate infilling a palaeo-channel (Fig. 5B and C) between bioherm struc-tures composed of columnar stromatolites(Fig. 5D and E).

Lamina microbialite faciesThe lamina microbialite facies occurs in allstudied outcrop locations described in thisstudy. It is best exposed in the Graja�u Region,where it is 3 to 6 m thick. Also, it is found indrill core profiles. In this facies, three types oflaminae were distinguished from one another bymorphology and thickness: flat/planar, discon-tinuous and crenulate laminae (Fig. 6A, B andC). All three types have intercalations of darkand light laminae (Fig. 6D) composed of calcite,dolomite and quartz, and crenulate lamina con-tain up to 20% gypsum (Table 2).The laminations consist of an alternation of

micritic layers (0�1 to 0�5 lm) with fine-grainedlayers (0�3 to 1�5 lm) with a mosaic texture. Pet-rographically, this facies generally is intensivelysilicified, but the primary structures are still pre-served. Also, correlation between silicificationand horizontal porosity, as defined by the alter-nation of dark and light lamina, is observed(Fig. 6E). A peloidal microfabric was recognizedin the crenulate lamina microbialite facies(Fig. 6F), but not in the flat/planar or disconti-nuous laminae.Scanning electron microscope studies of fresh

surfaces of flat/planar and crenulate laminamicrobialite facies show the presence of micro-fossils (Fig. 6G and H). Scanning electron micro-scopy analyses confirm the diffuse presenceof remains of fossilized organic matter (EPS),closely associated with the mineral phases. Themajority of these fossil remains consist of planar

Fig. 6. Lamina microbialite facies. (A) Flat/planar lamina hand sample (Sample G8) from Barreirinha Quarry, Gra-jau Region. (B) Discontinuous lamina hand sample from S~ao Benedito Quarry, Cod�o Region. (C) Crenulate laminadrill core sample (Sample CD06) from well 9-PAF-2.MAT T.05. (D) Flat lamina hand sample (Sample AF CD 1B)from S~ao Benedito Quarry, showing the intercalation of dark and light laminations. (E) Photomicrograph of flatlamina (A) in thin section (crossed polars) showing alternation of dark and light lamina. Large cavities form cru-dely laminated layers, whereas light blue areas reflect primary porosity. (F) Photomicrograph of thin section(crossed polars) displaying peloidal microfabric of crenulate lamina microbialite facies (C). (G) SEM photomicro-graph of the flat lamina microbialite facies (A) showing planar structures composed of interconnecting dolomitemicro-crystals, with fragmented and irregular textures associated with EPS remains. (H) SEM photomicrograph ofthe crenulate lamina (C) displaying the presence of filament structures associated with EPS remains.



A B

C

E

G H

F

D



A B

C

E F

D

Fig. 7. Massive microbialite facies. (A) Massive hand sample (G11) from Barreirinha Quarry, Graja�u Region. Notethe absence of any structure. (B) Drill core of massive lithology (CD08) from 9-PAF-7.MA T.02 well displayingobtuse structures. (C) Photomicrograph of sample G11 from Barreirinha Quarry (A) in thin section (crossed polars)showing the massive structures formed by micritic dolomite. (D) Photomicrographs of sample CD08 (B) in thinsection (crossed polars) displaying incipient lamination of the massive microbialite facies formed by micritic dolo-mite. (E) and (F) SEM photomicrographs of sample G11 (A), showing the presence of some fossil EPS structures.



or sheet-like features. The thickness of theseremains ranges from 0�5 to 1 lm; they line thecavities and/or envelop the crystal aggregatesextending in length for several tens of microns.

Massive microbialite faciesThe massive microbialite facies is characterizedby its light cream colour and the absence ofdistinct lamina structures (Fig. 7A). This faciesis present in all studied outcrops and com-monly it is associated with the crenulate lami-nae. It is possible to observe scattered slightlamination structures within the massivemicrobialite (Fig. 7B). The most characteristicfeature of this microbialite facies is the miner-alogy composition, which comprises more than80% micritic dolomite with smaller amountsof calcite and quartz (Table 2). Thin sectionsshow fine-grained crystals of dolomite (lessthan 4 lm in size) forming the massive struc-ture (Fig. 7C and D). The presence of rare fos-sil EPS structures can be observed in SEMphotomicrographs (Fig. 7E and F). The massivemicrobialite facies could be equivalent to theleiolite facies, as described by Braga et al.(1995).

Spherulite microbialite faciesThe spherulite microbialite facies is one of themost distinctive constituents of the Cod�o Forma-tion carbonate successions. Spherulites occur inthe Cod�o and Graja�u outcrops, but were notfound in the Imperatriz section. The spheres arefound in thin laminated beds, 15 to 30 cm thick(Fig. 8A). This microbialite facies is directlyoverlain by evaporites.The spherulites appear to coalesce forming

the discrete laminae (Fig. 8A, B and C), bestobserved in cathodoluminescence analyses(Fig. 8D); their size varies from 2 to 6 mm withradial internal structures (Fig. 8B, E, F and G).These spherulites show diverse configurations,which range from radiating spheres of fibrouscrystals to dense internal structures. The mine-ralogical composition is mostly calcite for thespherulites and dolomite and quartz for thematrix cement (Table 2). No microfossil remainswere observed during SEM analysis of thisfacies. The SEM observations of broken spheru-lite surfaces reveal an association between thecalcite spherulite and dolomite matrix cement(Fig. 8F and G). Occasionally, it is possible toobserve sheet-like features composed of Mg-sili-cates, confirmed by EDAX analysis, envelopingthe spheroidal structures (Fig. 8H).

MICROBIALITE FACIES ASSOCIATIONAND STRATIGRAPHIC CORRELATION

In Fig. 9, stratigraphic columns illustrate themicrobialite facies associations in outcrops fromthe Cod�o, Graja�u and Imperatriz regions; theyare organized into four units, based on the faciessuccessions recognized during field observa-tions. The outcrop located in S~ao BeneditoQuarry, near the city of Cod�o (Fig. 1), containsall four types of microbialite facies, representingthe only complete sequence. Thus, it is consi-dered to be the ‘type section’ of the microbialitesequence of the Cod�o Formation.In the Cod�o Region (S~ao Benedito Quarry), the

stratigraphic sequence shows the typical relationof the four units. The term ‘crenulated’ waschosen over ‘wavy undulatory’ because it morecorrectly represents the low-amplitude andhigh-frequency structures found in the Codomicrobialite. At the base, Unit I gradationallychanges upward from flat planar laminae towavy laminae and ends with club-shaped stro-matolites capped by desiccation, mudcrack andtepee structures. The overlying Unit II is charac-terized by the presence of the lamina microbia-lite facies, with a slight variation from a wavydiscontinuous pattern at the base becomingcrenulated with the transition to the overlyingunit. Unit III is dominated at the base by thecrenulate lamina microbialite facies with somegypsum followed by the dolomitic massive mi-crobialite facies. The uppermost Unit IV com-prises the spherulite microbialite facies at thetop of the sequence and is overlain by evaporites(Paz et al., 2005).The stratigraphic profile described in the Gra-

ja�u Region (Barreirinha Quarry) is divided intothree units, correlating with Units II, III and IVof the Cod�o profile. The lamina microbialitefacies reaches its thickest exposure of ca 6 m atthis location. Also, the best-developed sequenceof overlying evaporites is present in the Graja�uRegion.The stratigraphic sequence in the Imperatriz

Region, located at the Praia da Gaivota/Amoralong of the Tocantins River (Fig. 5), containsonly one unit, which resembles Unit I of theCod�o profile. Two microbialite facies with dis-tinctive structures were described, i.e. wavy anddiscontinuous laminae and club-shaped andcolumnar stromatolites. The individual colum-nar stromatolites, which are unique to the Imper-atriz Region, grow together to form ca 2 m highbioherm structures (Fig. 5E). The evaporites are



A B

C

E

G H

F

D



missing or not exposed in the Imperatriz Region,where the sequence is capped by sandstone ofthe Itapecuru Formation (Vaz et al., 2007).

Results and interpretation of isotope studies

The ‘clumped’ isotope measurements of themicrobialite facies samples from outcrops yieldpalaeo-temperatures with an average estimatedto be 35°C (minimum 27°C; maximum 51°C) and

those from the drill core samples to be 51°C(minimum 45°C; maximum 54°C) (Fig. 10A).Although there is a 16°C difference in the ave-rage palaeo-temperatures of the outcrop versusdrill core samples, with the latter being warmer,palaeo-temperatures for outcropping columnarand massive microbialites are associated withthe highest measured values (Fig. 10B). Samplesfrom the Cod�o Region outcrop indicate a widerange of palaeo-temperatures from 27�2 to

Fig. 9. Illustration showing stratigraphic correlation of the microbialite facies associations for the Cod�o Forma-tion, as defined by outcrop and sample descriptions. Note the absence of horizontal scale. Insert shows location ofthe outcrop sites.

Fig. 8. Spherulite microbialite facies (A) Spherulite hand sample, from S~ao Benedito Quarry, Cod�o Region, withthin lamination formed by the coalescence of different sizes of spherulite. (B) Photomicrograph of spherulite (A)in thin section (crossed polars) showing the radial internal structures and the size variability of the spherulitemicrobialite facies. (C) Photomicrograph of spherulite microbialite facies (A) in thin section (parallel polars) innatural light. (D) Photomicrograph of cathodoluminescence light of sample CD 2A (A), displaying spherulites coa-lescence forming the lamination. (E) Photomicrograph of spherulite microbialite facies in thin section (crossedpolars) showing the internal radial structures. The spherulite core sample (CD 2A) is from well 9-PAF-1.MA T.06.(F) SEM photomicrograph from sample (A) displaying spherulites with radial internal structure. The spherulitesare calcite encased in dolomite matrix. (G) SEM photomicrograph of the spherulite sample (A) reveals the associa-tion between spherulite and crystals of dolomite matrix cement. (H) SEM photomicrograph of spherulite sample(A) enveloped by Mg-silicates consisting of 100 to 200 nm thick gently folded sheets.



40�8°C, with all measurements made on theclub-shaped stromatolites. This temperaturerange is relatively warm for Earth surface tem-peratures, but is still in a range often found inequatorial to subtropical hypersaline environ-ments (Vasconcelos & McKenzie, 1997; Bahniuk,2013). It is conceivable that hydrothermal fluidsmay have influenced the temperature of the for-mation waters. This possibility cannot beexcluded based on the current data set. How-ever, because most of the outcrop samplesrecord palaeo-temperatures below 40°C, it isassumed that any hydrothermal influence wouldhave been either localized or minimal. The largevariability within a single 20 cm thick outcropcontaining club-shaped stromatolites from theCod�o Region suggests preservation of originalprecipitation temperatures. Based on this rea-sonable assumption for relatively good preserva-tion, the measured palaeo-temperatures of all

Cod�o Formation samples were considered usefulto calculate the range of d18OVSMOW values ofthe palaeo-waters, from which the carbonateprecipitated. Because all of the drill core sam-ples record palaeo-temperatures greater than45°C, this suggests a greater degree of burial dia-genesis at temperatures above Earth surface con-ditions rather than a hydrothermal influence.A cross-plot of stable carbon and oxygen iso-

tope values for all of the analysed samples(Fig. 11) shows distinct groupings of the diversemicrobialite facies, that is, stromatolite, lamina,massive and spherulite. Such groupings into iso-topic fields may reflect that facies specific pro-cesses and environmental parameters can bederived from the data to reconstruct the deposi-tional palaeoenvironment.The stable isotope values of the club-shaped

stromatolite microbialite facies are characterizedby low d13C values of �9�6 to �10�6& VPDB

Stromatolite

–16–8 –7 –6 –5 –4

–14

–12

–10

–8

–6

–4

–2

0

2

4

–3 –2

LaminaCrenulate

LaminaFlat

Spherulite

LaminaDiscontinuous

δ18OVPDB of carbonate (‰)

δ13C

VP

DB o

f car

bona

te (‰

)

Legend:OutcropDrill Core

Stromatolite microbialite faciesClub-ShapedColumnarDomal

Lamina microbialite faciesDiscontinuousFlatCrenulate

Massive microbialite faciesSpherulite microbialite facies

–1 0 1

Massive

Breccia cementMud crack

Fig. 11. Cross-plot of stable carbonand oxygen isotope data for samplesof different Cod�o Formationmicrobialite facies, from outcropand drill core. Note that themicrobialite facies tend to groupinto specific fields indicatingsimilar environmental conditionsduring their formation.

A

B Fig. 10. ‘Clumped’ isotope palaeo-temperatures of Cod�o Formationcarbonates where most samples aremixtures of calcite and dolomite.(A) Palaeo-temperature distributionof outcrop versus core samples. (B)Palaeo-temperature distributionamong the different microbialitefacies.



and d18O values of �4�8 to �5�6& VPDB. A sin-gle sample of columnar stromatolite microbialitefacies displays a d13C value of �8�7 VPDB andd18O value of �6�7& VPDB, whereas a singledomal stromatolite sample exhibits a d13C valueof �9�5& VPDB and d18O value of �5�8& VPDB.Thus, the stromatolite microbialite facies has, ingeneral, d13C and d18O values that consistentlycluster in a narrow range (Fig. 11), suggestinggrowth in a similar aqueous environment regard-less of shape. The calcite cement of the sedi-mentary breccia sample deposited in the palaeo-channel of the Imperatriz outcrop (Fig. 5) has ad13C value of �9�3& VPDB and d18O value of�6�8& VPDB, which plot within the stromato-lite field. The mud crack sample with a d13Cvalue of �10�0& VPDB and d18O value of�4�8& VPDB likewise plots within the stromato-lite field.The flat laminae are characterized by the low-

est d13C values (from �7�2 to �13�4& VPDB)measured in the lamina microbialite facies andthe least negative d18O values ranging from �1�5to �3�2& VPDB. The d13C values of the discon-tinuous laminae are slightly less negative, rang-ing from �5�3 to �6�9& VPDB, and the morenegative d18O values are from �6�9 to �7�8&VPDB. A 13C isotopic enrichment was observedin the d13C values for the crenulate laminae from2�5 to �0�8& VPDB along with a narrow rangeof d18O values (�6�7 to �7�5& VPDB). Unlikethe clustering of the isotopic values for the stro-matolite samples, the large isotopic variabilityamong the various lamina microbialite faciessuggests different depositional environments forthe three different types.The massive microbialite facies is distin-

guished from the other facies by a very widescatter of data in the isotopic cross-plot

(Fig. 11). The d13C values vary considerablyfrom those recorded in drill core samples rang-ing from 2�5 to �14�6& VPDB and d18O valuesfrom �0�7 to �4�3& VPDB, whereas the outcropsamples have d13C values of �3�2 to �8�0&VPDB and d18O values of �4�3 to �6�1& VPDB.The spherulite microbialite facies has an averaged13C value of �1�5& VPDB and d18O value of�5�5& VPDB. These values place the spherulitesamples in close proximity to the Unit III crenu-late lamina microbialite facies in the isotopecross-plot (Fig. 11).A plot of the calculated d18OVSMOW values for

the water in isotopic equilibrium with the Cod�ocarbonates versus palaeo-temperatures is illus-trated in Fig. 12. The club-shaped stromatolitemicrobialite facies shows a strong co-varying lin-ear trend with a range of palaeo-temperaturesfrom 27 to 40°C and d18O palaeo-water valuesfrom �2�7 to 0�9& VSMOW. One sample of themud crack structures from the top of club-shaped stromatolite has a palaeo-temperature of43°C, which indicates a d18O value for the pal-aeo-water of 2�1& VSMOW. Because these sam-ples are all from the same stratigraphic Unit I,the co-varying trend implies a synsedimentaryevolution of the d18O value of the water bodywith increasing values reflecting increasing tem-peratures and, hence, more evaporation.Although the d13C and d18O values of all mea-

sured samples of the stromatolite microbialitefacies are generally similar (Fig. 11), the palaeo-temperature of the Unit I columnar stromatolitemicrobialite facies is 51�8°C, yielding a d18O valueof 0�4& VSMOW for the palaeo-water. Also, onesample of calcite cement drilled from a Unit Isedimentary breccia sample has a palaeo-tem-perature of 50°C and a d18O value for thepalaeo-water of 0�3& VSMOW. These latter two

Fig. 12. Cross-plot of ‘clumped’isotope palaeo-temperatures versuscalculated d18O values of palaeo-water for the four microbialite faciesdescribed in the Codo Formation. Astrong linear correlation(R2 = 0�975) exists for Unit I club-shaped stromatolite and mud cracksamples, that is, with higher palaeo-temperature the d18O of the palaeo-water increases proportionally.



Unit I samples do not correspond to the sameco-varying isotopic trend as the Unit I club-shaped stromatolites (Fig. 11), but the higherpalaeo-temperature may indicate precipitationin equilibrium with warmer, possibly shallowerwaters. This interpretation is consistent withthe location of the columnar stromatolite bio-herms on the edge of a palaeo-channel (Fig. 5).Three samples of Unit II lamina microbialite

facies yielded variable palaeo-temperatures with avalue of 32�7°C for the flat lamina and 45�6°C and51�7°C for discontinuous lamina. Unfortunately,the palaeo-temperature for the crenulated laminacould not be measured due the presence of gyp-sum. The calculated d18O value of palaeo-water inequilibrium with the flat lamina sample is 0�1&VSMOW, whereas the discontinuous lamina gavean average d18O value of �0�1& VSMOW. The flatlaminae occur at the base of Unit II and are over-lain by discontinuous lamina in the upper part ofUnit II. The apparent constancy of the d18O valueof the palaeo-water throughout Unit II suggeststhat the lake maintained a rather steady hydrologi-cal inflow versus outflow.Two samples of Unit III massive microbialite

facies yield comparable high palaeo-temperaturesof 50�4°C and 54�8°C, which would be conduciveto dolomite precipitation under evaporative con-ditions. Despite the fact that the d18O values ofthe measured dolomite are quite variable (�6�1&and �0�7& VPDB), the calculated d18O values forpalaeo-waters are at the more positive end of therange of values for the Cod�o microbialites at1�7& and 3�0& VSMOW (Fig. 12). Overall, thepalaeo-waters at the time of dolomite precipita-tion were relatively isotopically enriched, pos-sibly as a result of evaporation from a closedlacustrine basin. Finally, for the Unit IV spheru-lite microbialite facies, only one ‘clumped’ iso-tope palaeo-temperature was calculated for anindividual spherulite, which was sampled bymicro-drilling. The palaeo-temperature obtainedis 53�6°C and the calculated d18O value for thepalaeo-water is 1�8& VSMOW. This positived18O value for the palaeo-water, in which thespherulites grew, is similar to the evaporatedpalaeo-water of the underlying Unit IV massivemicrobialite facies suggesting, along with therelatively high palaeo-temperatures, that thehydrological conditions remained similarly eva-porative within a closed basin.The 87Sr/86Sr results, given in Table 2, are for

eight samples representing different microbialitefacies deposited in the four stratigraphic units.The ratios range between 0�707958 and

0�709759. The highest value was found in thedrill core sample of the massive microbialitefacies (0�709759), and the lowest value was forthe calcite cement drilled in the breccia(0�707958) recovered in the Imperatriz Region.In Unit I, the club-shaped stromatolite has a87Sr/86Sr value of 0�709438, whereas the valuefor the columnar stromatolite is 0�709029. Themeasured 87Sr/86Sr value for the Unit II flat lam-ina microbialite facies is 0�708262. In Unit III,the 87Sr/86Sr value for the crenulate laminamicrobialite facies is 0�709359, similar to thevalues measured in Unit I. The spherulite micro-bialite facies of Unit IV has an 87Sr/86Sr value of0�708698. Finally, an ostracod concretion associ-ated with the selenitic gypsum overlying UnitIV in the Grauja�u Region has an 87Sr/86Sr valueof 0�709119. Thus, in general, an alternating pat-tern is revealed in the 87Sr/86Sr values of thefour units, with Unit I and III microbialite facieshaving a value of ca 0�709 and Unit II and IVmicrobialite facies being ca 0�708.

EVOLUTION OF THE MICROBIOLITEFACIES

The Cod�o Formation comprises a carbonatesequence deposited, in large part, as microbialsediment. The stromatolitic features present inthe Cod�o Formation suggest special environmen-tal conditions required to develop such struc-tures, as is the case for most modern stromatoliteexamples. Rossetti et al. (2004), Gonc�alves et al.(2006) and Maizatto et al. (2011) described theenvironmental setting for the Cod�o Formation asbeing lacustrine, hypersaline and evaporitic witha variable mineralogy. Such conditions are idealfor the development of stromatolite structures.However, these previous studies did not describethe carbonate microbialite structures in the Cod�oFormation, and, thus, the main goal of thepresent study was to define the facies associationof microbialites and interpret their depositionalenvironment.A major controversy concerning the biotic ver-

sus abiotic origin of microbialites exists (Grotzin-ger & Rothman, 1996). The variable associations ofstromatolite and lamina rocks in the Cod�o Forma-tion suggest that these structures may be related tobiological processes. Nevertheless, contemporane-ous abiotic processes may also be involved in theformation of the structures. Indeed, as demon-strated in this study by macroscopic andmicroscopic observations, it is possible to



distinguish organic and inorganic controls usingdifferent approaches to evaluate the biogenicity,as defined by Dupraz et al. (2009).The microbialite facies described in the Cod�o

Formation is herein interpreted to be formedduring an early lithification process leading tothe preservation of primary structures in theseorganosedimentary deposits. An importantaspect of this study is the coupling of laboratoryexperiments with modern environment studies,where microbialites develop, to interpret theancient rock record better. In other words, thefield, petrographic and geochemical results areintegrated with microbial laboratory experimentsand studies of modern environments to supplycomplementary information to evaluate the con-ditions under which ancient microbial structureswere formed (Vasconcelos et al., 2006, 2013).There are two major problems associated with

the interpretation of ancient microbialites. First,modern stromatolites are still relatively poorlyunderstood, but advances have been madebeyond the classical description of microbial matstromatolites to investigate the surface commu-nity of oxygenic cyanobacteria underlain byanoxygenic phototrophic bacteria and anaerobicheterotrophic bacteria (Visscher & Stolz, 2005).Indeed, the microbial mat ecosystem is efficientin element cycling and requires more than justlight as an energy source to function. Thus, min-eral formation with depth in the mat is morecomplex and not simply a response to a lightbased energy input (Petryshyn & Corsetti, 2011).Second, it is difficult to establish a direct con-nection between shape and internal structuresand the depositional palaeoenvironmental condi-tions and, thus, the biochemistry of the microbia-lites, i.e. the building of a biocenosis. Therefore,in this study, the palaeoenvironmental condi-tions were defined using a geomicrobiologyapproach together with traditional stable and‘clumped’ isotope techniques, which were usedto determine palaeo-temperatures and calculatethe palaeo-water isotope composition fromwhich the carbonates precipitated. Furthermore,87Sr/86Sr values of selected samples were used tolink the microbialite facies with cycles of con-tracting and expanding lake levels (Fig. 13), aspreviously outlined by Paz et al. (2005).

Stromatolite and lamina microbialite facies

Lamina structures in microbialite associationsare generally interpreted as evidence of micro-bial metabolisms related to mineral precipita-

tion. There are two models to explain laminaformation: the ‘trapping and binding’ model(Reid et al., 2000); and the ‘amalgamation’model (Vasconcelos et al., 2013). In both mod-els, microbial activity is important for carbonateprecipitation, but they differ in the sense thatthe former involves the cementing of particles toform laminae while, in the latter, in situ precipi-tation and subsequent coalescence of discretelayers form the laminae.This evaluation of the Cod�o Formation stro-

matolite structures, using petrography and SEManalyses, indicates that laminae form via theamalgamation model because minor or no detri-tal grains occur within the laminae. In addition,the presence of fossilized EPS encased in themineral precipitates demonstrates that the bio-genicity associated with the amalgamationprocesses in these structures is more induced(Dupraz et al., 2009). In contrast, the lamina mi-crobialite facies shows a mixed biogenicity, asindicated by the alternation of micritic layerswith fine-grained layers showing a mosaic tex-ture (Fig. 6), which are an indication of inducedand influenced origins, respectively.

Massive microbialite facies

The massive microbialite facies comprises pre-dominantly dolomite and lacks any laminationstructure. Dolomite can be considered a micro-bial biomineral often associated with the meta-bolic activity of sulphate-reducing bacteria,which overcome the kinetic barrier surroundingthe cell and increase the alkalinity (Vasconcelos& McKenzie, 1997). Thus, microbial activity canbe an essential process to promote specific con-ditions for dolomite precipitation.To better define the conditions where the mas-

sive dolomite forms, a comparison with a mod-ern environment, Brejo do Espinho, Rio deJaneiro, Brazil, was applied. Moreira et al.(2004), van Lith et al. (2003), S�anchez-Rom�anet al. (2009) and Delfino et al. (2012) proposethat, during changes in the hydrological parame-ters, the physico-chemical environmental redoxconditions likewise change in this modern waterbody. During periods of extreme dryness, thesurface sediments containing authigenic dolo-mite are exposed and, consequently, mostorganic matter will be degraded by inorganicoxidation. These extreme conditions withoutanoxia lead to minor or no preservation of sedi-mentary structures. However, the predominanceof primary micritic dolomite and the presence of



Fig.13.A

two-dim

ensionaldepositionalmodelillustratingthepalaeoenvironmentalevolutionforthedevelopmentofthevariousmicrobialite

faciesofthe

CodoForm

ation.Seetextfordescriptionofthemicrobialite

faciesform

ingtheunits.

Theprofilesdisplaychemostratigraphic

variationsin

the

34Sr/

86Srand

d18O

palaeo-w

atervaluesofthedifferentunits,

whichare

interpretedto

representchangesin

waterlevel.Themicrobialcarbonate

sequenceis

overlain

by

evaporites.



fossilized EPS in this microbialite facies (Fig. 7)suggest the dominance of an induced process,whereby the metabolism is responsible for theprecipitation (Dupraz et al., 2009).

Spherulite microbialite facies

The spherulitic structures in the samples arecomposed of calcite and could be considered asanother type of precipitation, either co-precipita-tion within a dolomite matrix or at a later stagewith early diagenesis. Many researchers haveassociated the formation of spherulites withdecaying cyanobacteria in recent microbial mats(e.g. Dahanayake et al., 1985). However, theexplanation for their origin is often unclear andusually attributed to possible organic influencewithout further elucidation. Spherulitic or ooli-tic habit has been observed in many other casesinvolving different micro-organisms and diffe-rent mineral species (e.g. Folk, 1993). Braissantet al. (2003) performed experiments to comparecalcium carbonate crystals obtained in bacterialcultures with those obtained during abioticallymediated synthesis. These authors demonstratedthe importance of EPS and amino acids fordetermining the mineralogy and morphology ofcalcium carbonate crystals produced by livingbacteria. In these experiments, the role of EPSand amino acids was tested using increasingconcentrations of organic compounds with time.All of the amino acids tested enhanced sphereformation. Braissant et al. (2003) concluded thatthe presence of EPS and amino acids is impor-tant for the formation of spherulitic calcite andmay indicate the presence of organic com-pounds dissolved in the solution during thetime of formation. These experiments demon-strated that, to form spherulite structures, theorganic compounds play an important rolebeyond the influence of microbial metabolism,which suggests the influence of an organosedi-mentary process (Dupraz et al., 2009). The fine-grained core may be the result of carbonatenucleation in the presence of extracellular poly-meric substances, a process that is known toproduce grainy textures (Bontognali et al.,2008). Thus, although no evidence of fossilizedorganic matter was observed in SEM analysis(Fig. 8), the spherulite microbialite facies mostprobably corresponds to an influenced biogeni-city, whereby only the concentration and type oforganic matter influence mineral formation.In summary, based on a number of criteria

including experimental data, modern environ-

mental observations and petrographic and geo-chemical analyses of the ancient microbialitesamples, the four recognized types of microbia-lite facies in the Cod�o Formation all apparentlyoriginate from biogenic processes. However,abiotic processes cannot be eliminated as animportant component, particularly during earlydiagenesis and cementation.

Possible scenario for deposition of the Cod�oFormation

The application of three model concepts, that isthe lamina formation, microbial dolomite andspherulite formation models, provides informa-tion to help interpret the possible mechanismsinvolved in the development of the four differentmicrobialite facies. Applying these models pro-vides clues to interpret the evolution of the pala-eoenvironment setting for the Cod�o Formation,as illustrated in Fig. 13. The palynological andnon-marine ostracod content of samples from theCod�o Formation indicates that it was a liminicpalaeoenvironment with a high evaporation rateunder warm and arid palaeoclimatic conditions(Antonioli & Arai, 2002; Maizatto et al., 2011).In the microbial context, the minimal ecosys-

tem, microbial mats, is a semi-closed system,where light, water (sea and/or meteoric) andgases can be part of the microcosms. The pro-ducts of microbial metabolisms reflect changesin the environmental conditions during mineralformation. Understanding how these parametersaffect the crystal morphology and mineralcomposition helps to design a better possibledepositional scenario for the Cod�o Formationduring the Aptian. Regardless of different sedi-ment structures described in the profiles, mix-tures of two distinct carbonate mineralogicalend-members were observed: (i) Mg-carbonatemore dominant in the stromatolite, lamina andmassive microbialite facies; and (ii) Ca-carbonatemore dominant in the spherulite microbialitefacies (see Fig. 2; Table 2).The coupling of Sr isotope and ‘clumped’ iso-

tope studies was used to clarify the environmen-tal scenario of the Cod�o Formation samples. Thedifferent microbialite facies have shown distinctoxygen-isotope palaeo-water compositions, cal-culated using the palaeo-temperatures derivedfrom clumped isotope measurements (Fig. 12).For Unit I, the clumped isotope measurementsof club-shaped stromatolitic fabrics drilled fromoutcrop yield estimated average palaeo-tempera-tures of 34�9°C, ranging from 27�2 to 40�8°C.



Assuming that the Unit I outcrop samples of theCod�o Formation preserve original stable isotopesignals, the d18O values of the bulk carbonate(�6�8 to �1�5& VPDB) reflect precipitation fromwater with calculated d18O values between�2�6& and 2�12& (VSMOW). These calculatedd18O (VSMOW) values reflect precipitation fromvariably modified Cretaceous meteoric water,which shows a linearly increasing trend as thepalaeo-temperature increases (Fig. 12). Unit Iappears to be a shallowing-upward sequence ina closed basin terminating with desiccation fea-tures such as tepee structures and mudcracks(Fig. 4). The latter structure registers a palaeo-temperature of 42�8°C and the most positived18O of 2�1& (VSMOW). The palaeo-tempera-tures calculated for samples from Units II, IIIand IV are between 45°C and 55°C, exclusive ofa Unit II flat laminae with a palaeo-temperatureof 32�7°C.The d13C values of the bulk carbonate indicate

a significant input of carbon derived from aero-bic or anaerobic respiration of organic matter,suggesting precipitation in a semi-enclosed orisolated water body. The C and O isotope resultsare consistent with a hydrologically closed basinlacustrine signature for the Cod�o Formationpalaeoenvironment. The interpretation of theresults obtained in the present study is in fullagreement with the palaeohydrological interpre-tation reported in Paz & Rossetti (2006).Based on the Sr isotope composition, the

palaeoenvironment of the overlying evaporitescapping the Cod�o Formation, was most probablyarid continental and had undergone cyclicchanges due to the expanding and contracting ofthe water body (Paz et al., 2005). The Sr isotopevalues for the carbonate microbialite facies rangebetween 0�707958 and 0�709759. Because the87Sr/86Sr value of early Cretaceous sea waterranges between 0�70720 and 0�70735, thepalaeo-water of the Cod�o Formation cannot bemarine and, thus, must be derived instead fromcontinental sources. These results are in accor-dance with 87Sr/86Sr ratios measured in the over-lying Cod�o Formation evaporites, as reported inPaz et al. (2005), and can be divided into twodistinct fields with ratios of ca 0�708 and 0�709(Fig. 13), with their lowest value being 0�707824and highest value being 0�709280. In compari-son, the lowest 87Sr/86Sr value in the microbia-lite facies of the Cod�o Formation was measuredin the sedimentary breccia cement (0�707958)and the highest value was measured in the mas-sive microbialite facies (0�709759).

INTERPRETATION OF THE FACIESASSOCIATION

In Unit I (87Sr/86Sr � 0�709), the facies associa-tion gradationally changes upward from flatlaminae to wavy laminae, and terminates withclub-shaped stromatolites associated with desic-cation structures, such as mud cracks and tepeesat the top. The calcite cement of the sedimentarybreccia deposited near the columnar stromatolitebioherm has an 87Sr/86Sr value of 0�707958,suggesting later precipitation in the presenceof a different palaeo-water. Unit II (87Sr/86Sr� 0�708) is defined by the presence of flat/planarlamina microbialite facies, with a slight variationto a wavy pattern at the base. Towards the top ofUnit II, the laminations become progressively dis-continuous transitioning to the crenulated lami-nae that characterize a gradation into Unit III(87Sr/86Sr � 0�709). This unit comprises the cren-ulated lamina at the base and is overlain by themassive microbialite facies. The sedimentarysequence ends with Unit IV (87Sr/86Sr � 0�708),containing exclusively spherulite microbialitefacies. The ostracod concretion found directlyabove Unit IV within a gypsum bed, representinga change to more evaporative conditions (Pazet al., 2005), has an 87Sr/86Sr value of 0�709119,which is in agreement with the value measuredin the first gypsum bed (Paz et al., 2005).Figure 13 displays a two-dimensional deposi-

tional model illustrating the palaeoenviron-mental interpretation associated with thedevelopment of the Cod�o Formation. In Unit I,the sequence begins with flat laminae changingto club-shaped stromatolite, indicating a shal-lowing upward evolution in a hydrologicallyclosed lacustrine basin, as supported by thegeochemical data showing a direct correlationbetween the d18OVSMOW values of palaeo-waterwith increasing palaeo-temperature (Fig. 12).Subaerial exposure structures associated withstromatolites suggest that the microbialites havegrown in shallow waters within the photic zone.The stromatolites must have grown in a shallowsetting because of the requirement that lightpenetrates the water body to facilitate photosyn-thesis, as well as the presence of subaerial expo-sure structures associated with stromatolites atthe top of the unit.Unit II is related to an isotopically stable,

slightly deeper water body where the inflow isbalanced by outflow, as reflect by the unvaryingd18O values for the palaeo-water. Also, exposurestructures are missing. This unit represents the



maximum water level reached in a possibleopen basin during the deposition of the Cod�oFormation. The microbialite facies associatedwith Unit II is the flat lamina deposited duringthe lake high-stand, which grades into the dis-continuous lamina microbialite facies at the topas the water body begins to contract.Unit III comprises two notable microbialite

facies, crenulate lamina with gypsum and mas-sive dolomite. These microbialite facies areinterpreted to be associated with a high evapora-tion phase, which resulted in hypersaline envi-ronmental conditions with a contracting lakelevel phase (lake low-stand) possibly leading tohydrologically ephemeral state as in the moderndolomite precipitating in Brejo do Espinho,Brazil. Such extreme conditions are ideal toform organominerals with special morphologies,such as produced by microbial mats.Unit IV is associated with an expanding water

body within the closed basin (lake intermediate-stand) and is defined by the presence of thespherulite microbialite facies. The origin of thistype of microbialite facies has been the subjectof many studies and is still a matter of contro-versy. The lower 87Sr/86Sr values (0�708) foundin both Units II and IV with respectively highand intermediate water levels may be controlledby Sr recycling in the catchment area with aninflux of Sr with a different 87Sr/86Sr value thanthe water source for low-stand Units I and III(0�709). The subsequent onset of high evapora-tion in the closed basin is indicated by the pre-sence of evaporites capping the carbonatemicrobialite sequence. The first stage of signifi-cant evaporite deposition may be represented bythe spherulite carbonate.

CONCLUSIONS

Microbialite structures are well-preserved in theCod�o Formation of north-eastern Brazil, record-ing a special episode where micro-organismsand organic content played an important role inthe sedimentation of carbonates during theAptian time. The present research combinesclassical sedimentological/geochemical studieswith a geomicrobiological approach to explainmicrobialite formation in space and time.Detailed characterization of the microbialitefacies yields diverse microbially produced struc-tures, which can be systematically related tospecific depositional environments. A combina-tion of fieldwork observations, petrographic

studies, geochemistry measurements and labora-tory experiments provides a degree of certaintyto reconstruct the palaeoenvironmental evolu-tion of the Lower Cretaceous Parnaiba Basin.The Unit I stromatolite structures are placed in

the shallow-water lacustrine palaeo-environmentbased on the occurrence of desiccation struc-tures, relatively high palaeo-temperatures (ca32°C) and calculated slightly negative d18Owater

values. Similar conditions are also found todayin modern stromatolitic forming hypersalinemilieus (for example, Lagoa Vermelha, Brazil).Following the same methodology as used inmodern studies, Unit II flat/planar and discontin-uous lamina microbialite facies are associatedwith an expanding lake with a stable inflow bal-anced by outflow, within a deeper water pala-eoenvironment (lake high-stand), where theaverage temperature was 40°C and the calculatedd18Owater was consistently ca 0�0& Vienna Stan-dard Mean Ocean Water (VSMOW). For the UnitIII crenulate lamina and massive microbialitefacies, the palaeo-environmental setting becameeven more extremely hypersaline, representingthe contraction of the lake with a minimum waterlevel (lake low-stand) in the closed basin. Themeasured palaeo-temperature for the outcropmassive microbialite facies is 50°C and the calcu-lated d18Owater is 1�7& (VSMOW). Conditions inUnit IV continued to be extreme with high eva-poration, even though deposition occurred in anintermediate water level. The measured palaeo-temperature for the spherulite microbialite faciesis 54°C and calculated d18Owater is 1�8&(VSMOW). To date, it has been possible to inter-pret the evolution of the Cod�o Formation as aclosed basin lacustrine palaeoenvironment withalternating episodes of contracting and expand-ing lake level, as illustrated in Fig. 13.The variation of structures in this environ-

mental context suggests that the microbial com-munities have been responding to changes inenvironmental conditions. The development ofmicrobialite morphogenesis implies that chemi-cal conditions, rather than physical ones, couldbe the dominant control on microbialite mor-phology and type, in shallow and sub-shallowenvironments through time. However, evapora-tion is the physical parameter driving salinityincreases and, as a consequence, the microbialmetabolism could be altered or even inhibitedunder more extreme conditions. Thus, differentbiomineralization processes are reflected in thevariety of microbialite facies found in the fourunits of the Cod�o Formation.



The development of microbialites in the Cod�oFormation is correlated with restriction of theenvironment and climate controls. The geo-chemical studies confirm previous studies,which indicated that the Cod�o Formation wasdeposited under warm and arid palaeoclimateconditions with high evaporation rates in ahydrologically closed lacustrine system. In sum-mary, the combined data presented in this studydemonstrate that new insights can be achievedto evaluate better palaeoenvironmental condi-tions and early diagenetic processes involved inancient microbialite formation. Furthermore, aclear interpretation of the varying environmentalconditions forming the different microbialitefacies provides useful information to evaluatethe porosity and permeability of this potentialreservoir rock.

ACKNOWLEDGEMENTS

We acknowledge the general support of theSwiss National Science Foundation Grant No200020127327. This study is part of the researchcollaboration (PETHROS) between Petrobras andETH Zurich. We thank Adina Payton, Universityof California, Santa Cruz for providing the stron-tium analyses, and for her interpretation of thedata.

REFERENCES

Affek, H.P. and Eiler, J.M. (2006) Abundance of mass 47 CO2

in urban air, car exhaust, and human breath. Geochim.

Cosmochim. Acta, 70, 1–12.Antonioli, L. and Arai, M. (2002) O registro da Ecozona

Subtilisphaera na Formac�~ao Cod�o (Cret�aceo Inferior da

Bacia do Parna�ıba, Nordeste do Brasil): seu significado

paleogeogr�afico. In: 6° Simp�osio Sobre o Cret�aceo do Brasil,pp. 25–30. Boletim UNESP, Rio Claro, S~ao Pedro-SP.

Bahniuk, A.M. (2013) Coupling organic and inorganic

methods to study growth and diagenesis of modern

microbial carbonates, Rio de Janeiro State, Brazil:

implications for interpreting ancient microbialite facies

development. PhD Thesis. ETHZ, Z€urich, 169 pp.

Batista, A.M. (1992) Caracterizac�~ao paleoambiental dos

sedimentos Cod�o-Graja�u, Bacia de S~ao Lu�ıs (MA). PhD

Thesis, Universidade Federal do Par�a, Bel�em, 102 pp.

Bontognali, T.R.R., Vasconcelos, C., Warthmann, R.J.,Dupraz, C., Bernasconi, S. and McKenzie, J.A. (2008)

Microbes produce nanobacteria-like structures, avoiding

cell entombment. Geology, 36, 663–666.Braga, J.C., Mart�ın, J.M. and Riding, R. (1995) Controls on

microbial dome fabric development along a carbonate-

siliciclastic shelf-basin transect, Miocene, S.E.Spain.

Palaios, 10, 347–361.

Braissant, O., Cailleau, G., Dupraz, C. and Verrecchia, E.P.(2003) Bacterially induced mineralization of calcium

carbonate in terrestrial environments: the role of

exopolysaccharides and amino acids. J. Sed. Res., 73, 485–490.Burne, R.V. and Moore, L.S. (1987) Microbialites:

organosedimentary deposits of benthic microbial

communities. Palaios, 2, 241–254.Campbell, D.F., Almeida, L.A. and Silva, S.O. (1949)

Relat�orio preliminar sobre a geologia da Bacia do

Maranh~ao. Conselho Nac. Petr�ol., 1, 160.Dahanayake, K., Gerdes, G. and Krumbein, W. (1985)

Stromatolites, oncolites and oolites biogenically formed in

situ. Naturwissenschaften, 72, 513–518.Delfino, D.O., Wanderley, M.D., Silva e Silva, L.H., Feder,

F., and Lopes, F.A.S. (2012) Sedimentology and temporal

distribution of microbial mats from Brejo do Espinho, Rio

de Janeiro, Brazil. Sed. Geol., 263, 85–95.Dietz, R.S. and Holden, J.C. (1970) Reconstruction of Pangea:

breakup and dispersion of continents, Permian to Present.

J. Geophys. Res., 75, 4939–4956.Dupraz, C., Visscher, P.T., Baumgartner, L.K. and Reid, R.P.

(2004) Microbe–mineral interactions: early carbonate

precipitation in a hypersaline lake (Eleuthera Island,

Bahamas). Sedimentology, 51, 745–765.Dupraz, C., Reid, R.P., Braissant, O., Decho, A.W., Norman,

R.S. and Visscher, P.T. (2009) Processes of carbonate

precipitation in modern microbial mats. Earth Sci. Rev.,

96, 141–162.Eiler, J.M. (2007) “Clumped-isotope” geochemistry – the

study of naturally-occurring, multiply-substituted

isotopologues. Earth Planet. Sci. Lett., 262, 309–327.Eiler, J.M. and Schauble, E. (2004) 18O13C16O in Earth’s

atmosphere. Geochim. Cosmochim. Acta, 68, 4767–4777.Folk, R.L. (1993) SEM imaging of bacteria and nannobacteria

in carbonate systems and rocks. J. Sed. Petrol., 63, 990–999.

Ghosh, P., Adkins, J., Affek, H., Balta, B., Guo, W., Schauble,E.A., Schrag, D. and Eiler, J.M. (2006a) 13C18O bonds in

carbonate minerals: a new kind of paleothermometer.

Geochim. Cosmochim. Acta, 70, 1439–1456.Ghosh, P., Garzione, C.N. and Eiler, J.M. (2006b) Rapid

uplift of the Altiplano revealed through 13C-18O bonds in

paleosol carbonates. Science, 311, 511–515.Ghosh, P., Eiler, J., Campana, S.E. and Feeney, R.F. (2007)

Calibration of the carbonate “clumped isotope”

paleothermometer for otoliths. Geochim. Cosmochim.

Acta, 71, 2736–2744.Gonc�alves, D.F., Rossetti, D.F., Truckenbrodt, W. and

Mendes, A.C. (2006) Argilominerais da Formac�~ao Cod�o

(Aptiano Superior), Bacia de Graja�u, nordeste do Brasil.

Latin Am. J. Sedimentol. Basin Anal., 13, 59–75.Grotzinger, R. and Rothman, D.H. (1996) An abiotic model

for stromatolite morphogenesis. Nature, 383, 423–425.Huntington, K.W., Eiler, J.M., Affek, H., Guo, W., Bonifacie,

M., Yeung, L.Y., Thiagarajan, N., Passey, B., Tripati, A.,Daeron, M. and Came, R.E. (2009) Methods and

limitations of ‘‘clumped’’ CO2 isotope (D47) analysis by

gas-source isotope ratio mass spectrometry. J. MassSpectrom., 44, 1318–1329.

Kalkowsky, E. (1908) Oolith und Stromatolith im

norddeutschen Buntsandstein. Z. Deutschen Geologischen

Ges., 60, 69–125.Kim, S.T. and O’Neil, J.R. (1997) Equilibrium and

nonequilibrium oxygen isotope effects in synthetic

carbonates. Geochim. Cosmochim. Acta, 61, 3461–3475.



Lima, M.R. (1982) Palinologia da Formac�~ao Cod�o na regi~ao de

Cod�o, Maranh~ao. Bol. Inst. Geocienc. – USP, 13, 116–128.Lisboa, M.A.R. (1914) The Permian geology of Northern

Brazil. Am. J. Sci., 37, 425–442.van Lith, Y., Warthmann, R., Vasconcelos, C. and

McKenzie, J.A. (2003) Sulphate-reducing bacteria induce

low-temperature Ca-dolomite and high Mg-calcite

formation. Geobiology, 1, 71–79.Maizatto, J.R., Queiroz Neto, J.V., Ferreira, E.P. and Bahniuk,

A.M. (2011) Palinomorfos e ostracodes nao marinhos de

afloramentos da Formacao Codo, Bacia do Parnaıba. In:

Paleontologia: Cenarios de Vida. (Eds I.S. Carvalho, N.K.

Srivastava, O. Strohschoen and C.C. Lana), Vol. 3, 1st edn,

pp. 367–378. Editora Interciencia, Rio de Janeiro.

Moreira, N.F., Walter, L.M., Vasconcelos, C., McKenzie, J.A.and McCall, P.J. (2004) Role of sulfide oxidation in

dolomitization: sediment and pore-water geochemistry of a

modern hypersaline lagoon system. Geology, 32, 701–704.Paz, J.D.S. (2000) An�alise faciol�ogica da Formac�~ao Cod�o

(Aptiano Superior na regi~ao de Cod�o (MA), Leste da Bacia

do Graja�u. MSc Thesis, Universidade Federal do Par�a,Bel�em, 146 pp.

Paz, J.D.S. and Rossetti, D.F. (2001) Reconstruc�~aopaleoambiental da Formac�~ao Cod�o (Aptiano), borda leste

da Bacia do Graja�u, MA. In: O Cret�aceo na Bacia de S~aoLu�ıs-Graja�u. Colec�~ao Friedrich Katzer (Ed. M.P., Goeldi),

pp. 77–100. Colec�~ao Friedrich Katzer, Bel�em.

Paz, J.D.S. and Rossetti, D.F. (2005) Linking lacustrine

cycles with syn-sedimentary tectonic episodes: an

example from the Cod�o Formation (Late Aptian),

Northeastern Brazil. Geol. Mag., 142, 269–285.Paz, J.D.S. and Rossetti, D.F. (2006) Paleohydrology of an

Upper Aptian lacustrine system from northeastern Brazil:

integration of facies and isotopic geochemistry.

Palaeogeogr. Palaeoclimatol. Palaeoecol., 241, 247–266.Paz, J.D.S., Rossetti, D.F. and Macambira, M.J.B. (2005) An

Upper Aptian saline pan/lake system from the Brazilian

equatorial margin: integration of facies and isotopes.

Sedimentology, 52, 1303–1321.Pedr~ao, E., Lima, H.P., Makino, R.K. and Barrilari, I.M.R.

(2002) Palinoestratigrafia e evoluc�~ao ambiental da sec�~aocret�acea das bacias de Braganc�a-Viseu e S~ao Lu�ıs (margem

equatorial brasileira). Acta Geol. Leopoldensia, 25, 21–39.Petryshyn, V.A. and Corsetti, F.A. (2011) Analysis of growth

directions of columnar stromatolites from Walker Lake,

western Nevada. Geobiology, 9, 425–435.Preiss, W.V. (1972) The systematics of South Australian

Precambrian and Cambrian stromatolites. Trans. Roy. Soc.

S. Australia, 96, 67–100.Ramos, M.I.F., Rossetti, D.F. and Paz, J.D.S. (2006)

Caracterizac�~ao e significado paleoambiental da fauna de

ostracodes da Formac�~ao Cod�o (Neoaptiano), leste da

Bacia de Graja�u, MA, Brasil. Rev. Brasil. Paleontol., 9,339–348.

Reid, R.P., Visscher, P.T., Decho, A.W., Stolz, J.F., Beboutk,B.M., Dupraz, C., Macintyre, I.G., Paerl, H.W., Pinckney,J.L., Prufert-Beboutk, L., Steppe, T.F. and DesMarais, D.J.(2000) The role of microbes in accretion, lamination and

early lithification of modern marine stromatolites. Nature,

406, 989–992.

Rosenbaum, J. and Sheppard, S.M.F. (1986) An isotopic

study of siderites, dolomites and ankerites at high

temperatures. Geochim. Cosmochim. Acta, 50, 1147–1150.Rossetti, D.F., Paz, J.D.S. and G�oes, A.M. (2004) Facies

analysis of the Cod�o Formation (Late Aptian) in the Graja�u

area, Southern S~ao Luis-Graja�u Basin. An. Acad. Brasil.Cienc., 76, 791–806.

S�anchez-Rom�an, M., McKenzie, J.A., Wagener, A.,Rivadeneyra, M.A. and Vasconcelos, C. (2009) Presence of

sulfate does not inhibit low-temperature dolomite

precipitation. Earth Planet. Sci. Lett., 285, 131–139.Schauble, E.A., Ghosh, P. and Eiler, J.M. (2006) Preferential

formation of 13C 18O bonds in carbonate minerals,

estimated using first-principles lattice dynamics. Geochim.Cosmochim. Acta, 70, 2510–2529.

Sharma, S.D., Patil, D.J. and Gopalan, K. (2002)

Temperature dependence of oxygen isotope fractionation

of CO2 from magnesite-phosphoric acid reaction. Geochim.Cosmochim. Acta, 66, 589–593.

Soares Junior, A.V., Costa, J.B.S. and Hasui, Y. (2008)

Evoluc�~ao da Margem Atlantica Equatorial do Brasil: Tres

Fases Distensivas. Geociencias, 27, 427–437.Terra, J.S.G., Spadini, A.R., Franc�a, A.B., Sombra, C.L.,

Zambonato, E.E., Juschaks, L.C.S., Arienti, L.M., Erthal,M.M., Blauth, M., Franco, M.P., Matsuda, N.S., Camarralda Silva, N.G., Moretti, P.A., Davilla, R.S.F., Schiffer, R.,Tonietto, S., Anjos, S.M.C., Campinho, V. and Winter,W.R. (2010) Classificac�~ao de rochas carbon�aticas aplic�avel�as bacias sedimentares brasileiras. Bol. Geocienc.Petrobras, 18, 9–29.

Vasconcelos, C. and McKenzie, J.A. (1997) Microbial

mediation of modern dolomite precipitation and

diagenesis under anoxic conditions (Lagoa Vermelha, Rio

de Janeiro, Brazil). J. Sed. Res., 67, 378–390.Vasconcelos, C., McKenzie, J.A., Warthmann, R. and

Bernasconi, S.M. (2005) Calibration of the d18Opaleothermometer for dolomite precipitated in microbial

cultures and natural environments. Geology, 33, 317–320.Vasconcelos, C., Warthmann, R., McKenzie, J.A., Visscher,

P.T., Bittermann, A.G. and van Lith, Y. (2006) Lithifying

microbial mats in Lagoa Vermelha, Brazil: modern

Precambrian relics? Sed. Geol., 185, 175–183.Vasconcelos, C., Dittrich, M. and McKenzie, J.A., (2013).

Evidence of microbiocoenosis in the formation of laminae

in modern stromatolites. Facies, 60, 3–13.Vaz, P.T., Rezende, N.G.A.M., Wanderley Filho, J.R. and

Travassos, W.A.S. (2007) Bacia do Parna�ıba. Bol.Geocienc. Petrobr�as, 15, 253–263.

Visscher, P.T. and Stolz, J.F. (2005) Microbial mats as

bioreactors: populations, processes, and products.

Palaeogeogr. Palaeoclimatol. Palaeoecol., 219, 87–100.Wang, Z., Schauble, E.A. and Eiler, J.M. (2004) Equilibrium

thermodynamics of multiply substituted isotopologues of

molecular gases. Geochim. Cosmochim. Acta, 68, 4779–4797.

Manuscript received 23 August 2013; revisionaccepted 12 June 2014



Documents

Development of microbial carbonates in the Lower Cretaceous Cod o Formation (north-east Brazil): Implications for interpretation of microbialite facies associations and palaeoenvironmental