J. Helenes, D. Cabrera: Oligocene–Miocene palynomorph assemblages from eastern Venezuela 5OLIGOCENE–MIOCENE PALYNOMORPHASSEMBLAGES FROM EASTERN VENEZUELA

JAVIER HELENESDepartamento de Geología, C.I.C.E.S.E.,Km 107 Carretera Tijuana-EnsenadaEnsenada, Baja California, 22860, MéxicoandCICESE, Geology Department,P.O. Box 434843,San Diego, CA 92173-4843.e-mail: jhelenes@cicese.mx

DIANA CABRERAPDVSA INTEVEPApartado Postal 76343Caracas, 1070A, Venezuela

Palynology, 27 (2003): 5–25© 2003 by AASP Foundation ISSN 0191-6122

Abstract

Palynological analyses of several subsurface sections in easternVenezuela, show varied and abundant Oligocene–Miocenepalynomorphs overlying a Cretaceous–Oligocene unconformity.Two palynomorph assemblages are recognized in the Cretaceousinterval, and two more in the Tertiary strata.The Cretaceous strata contain the spores Foveotriletes margaritae,Buttinia andreevi and elateres of Ariadnasporites sp., togetherwith the dinoflagellates Dinogymnium, Andalusiella, Cerodinium,Senegalinium, and locally Isabelidinium and Odontichitina. Theterrestrial assemblage represents the undifferentiated Auriculiiditesreticularis (Campanian to lower Maastrichtian) to Crassitricol-porites subprolatus (Maastrichtian) zones, and the Proteaciditesdehanii (Maastrichtian) zone of Müller et al. (1987), while thedinoflagellates indicate a Campanian to Maastrichtian age, de-pending on the location within the area.The lowermost Tertiary rocks are late Oligocene in age andcontain the terrestrial palynomorphs Cicatricosisporitesdorogensis, Jandufouria seamrogiformis, Polypodiisporitesusmensis and Mauritiidites franciscoi, together with the dinoflagel-lates Cordosphaeridium spp., Polysphaeridium congregatum,Homotryblium spp. and Tuberculodinium vancampoe. The terres-trial assemblage represents the Magnastriatites–Cicatricosisporitesdorogensis zone of Müller et al. (1987), while the dinoflagellatesindicate a late Oligocene age.Higher in the section, the early Miocene strata contain the pollenPsilatricolporites pachydermatus, P. maculosus, Bombacaciditesbrevis and Bombacacidites noremii together with the dinoflagel-lates Cribroperidinium tenuitabulatum and Diphyes latiusculum.The terrestrial assemblage represents the Verrutricolporitesrotundiporis–Echidiporites barbeitoensis zone of Müller et al.(1987), and the concurrent presence of the dinoflagellates indicatean early Miocene age.

INTRODUCTION

In eastern Venezuela (Text-Figure 1), marine sedimen-tary rocks represent all of the Late Cretaceous and most ofthe Tertiary Systems. In the northern part of this area, in theSerranía del Interior (Text-Figures 1, 2), a complete Paleo-gene section has been reported (González de Juana, et al.,1980) and studied in some detail (Crespo-de-Cabrera, S.and Pizon, J. 1988). To the south, exploratory wells inVenezuela’s Eastern Basin allow to recognize a Tertiarysuccession, which is very similar to the one known fromsurface localities, even when the paleoenvironments tendto be shallower.

In the western Guarico sub-basin and the southern Orinocobelt (Text-Figure 2), a hiatus including most of the Paleo-gene has been observed (González de Juana, et al., 1980;Muñoz, 1985). A more complete Tertiary section has beenreported from the eastern and northern portions of theMaturin sub-basin, but a hiatus has been identified (Fasolaand Paredes de Ramos, 1991) at the bottom of this Tertiarysection. The area studied is included in the Maturin sub-basin (Text-Figures 1, 2), where the paleoenvironmentsrepresented are paralic to shallow marine. At the base of theTertiary section, the content of planktonic calcareous mi-crofossils is poor and so the palynological analyses are veryuseful in providing age assignments. This paper reports onthe stratigraphy and the palynological content of strataabove and below a Cretaceous–Oligocene unconformablesurface drilled in the northern part of the Marturin sub-

6 PALYNOLOGY, VOLUME 27 — 2003

basin. The objectives are to improve intrabasinal correla-tion, and increase the knowledge on the stratigraphic distri-bution of dinoflagellate assemblages in tropical, shallowmarine environments.

Materials and Methods

The material used includes core and cuttings samplesfrom several exploratory wells drilled by Petroleos deVenezuela (PDVSA) in the Maturin sub-basin ofVenezuela’s Eastern basin. A total of 287 core samples (96from Well 1, 59 from Well 2 and 132 from Well 3) and 34cuttings samples (18 Well 1 and 16 from Well 2) wereprocessed and analyzed (Appendices A, B in: Helenes andCabrera, 2003). The samples were prepared following thenormal palynological treatment with HCl, HF and ZnBr2(Wood, Gabriel and Lawson, 1996). The residue was thensieved through 100 µm and 20 µm mesh sieves, and strew

mounts were prepared with the residue retained in the 20µm. Slides are kept at INTEVEP-PDVSA, Caracas, Ven-ezuela. One slide per sample was analyzed under themicroscope, and absolute abundances of marine and terres-trial palynomorphs were recorded (Appendix A in: Helenesand Cabrera, 2003). As a basis for our stratigraphic inter-pretations, we used the lithology as indicated by the GammaRay well logs and the samples. Age assignments of thesections studied are based mainly on the stratigraphicranges of the palynomorphs contained in core samples.However, some age assignments of the uppermost shalesare also based on planktonic foraminifera and calcareousnannofossils reported in internal reports of PDVSA. Thegamma ray logs were used to determine grain size grada-tion, together with the lithology observed in core andcuttings samples. The lithology and the values of diversity,or number of species per sample, of marine and terrestrialpalynomorphs obtained from the palynological analyses(Appendix B in: Helenes and Cabrera, 2003), allow the

Caracas

Caribbean Plate

Orinoco River

SOUTHAMERICA

Arch

El Pilar F.Z.

Serranía del Interior

El Baul Eastern Basin Bocono F. Z.

Barinas-Apure Basin

Colombia

500 km Brazil

Venezuela

Guyana Shield

Andes

MaracaiboBasin

N

Anzoategui State Mo

na

ga

s

Sta

te

5 Km

Well 1

Well 3

Well 2

Pirital Fault

Studyarea

72o

68o 64

o 60o

12o

8o

4o

0o

Trinidad

Text-Figure 1. Main Tectonic and geologic elements in Venezuela and location of wells studied.

J. Helenes, D. Cabrera: Oligocene–Miocene palynomorph assemblages from eastern Venezuela 7

recognition of changes in depositional environments andstratigraphic units.

The assigned ages are based on the presence of thedinoflagellates, spores and pollen grains observed in thesamples analyzed. The stratigraphic ranges used here (Text-Figure 3) for the terrestrial palynomorphs are reported inGermeraad et al. (1968); Pares-Regali et al. (1974), Mülleret al. (1987), and Lorente (1986).

Ranges used for the dinoflagellates are also shown onText-Figure 3. They were compiled from Drugg and Stover(1975), Williams and Bujak (1985), Haq et al. (1988),Williams et al. (1993), Wrenn and Kokinos (1986); Helbyet al. (1987); Matsuoka and Bujak (1988); Powell (1992);Paredes et al. (1994); Helenes et al. (1998), Helenes andSomoza (1999) and in the computer data base TAXON(R.L. Ravn, personal communication, 1993).

Geologic Setting

The northern edge of the South American continent iscurrently deformed by right-lateral, strike-slip faulting,while an oblique subduction area is present to the north ofit (Molnar and Sykes, 1969; Burke et al., 1984; Pindell andBarrett, 1990). This configuration is the result of the evo-lution of the region during the late Tertiary, which isdominated by compression of the Caribbean plate againstthe South American plate. In eastern Venezuela, the maingeological features (Text-Figure 1) are the stable Guyanashield to the south, a Mesozoic–Cenozoic sedimentaryfilling (Eastern basin) in the central part, a mountain chain(Serranía del Interior range) formed by folded Mesozoic

and Cenozoic rocks to the north of the basin, and a Mioceneor younger right-lateral, strike-slip fault zone (El Pilarfault) along the northern portion (González de Juana, et al.,1980; Schubert, 1984).

In the subsurface of the Eastern basin, a complex struc-tural regime is dominated by a late Miocene thrust faultdipping to the north, known as the Pirital fault and severalright lateral strike-slip faults. This paper presents data onthe sedimentary sequences drilled to the southeast of thePirital fault (Text-Figure 1).

Stratigraphy. The Tertiary strata of this oil-producingbasin have been drilled and studied for a long time(González de Juana, et al., 1980; Fasola and Paredes deRamos, 1991). These strata overlie a Cretaceous sequenceranging in age from Barremian to Maastrichtian (Paredeset al., 1994). The uppermost Cretaceous unit (San JuanFormation) is conformably overlain in places by alternat-ing siltstones and dark-gray shales (Vidoño Formation)Paleocene in age, and/or by Eocene glauconitic sand-stones and siltstones (Caratas Formation). Overlying theseunits (Text-Figure 4), are the medium to fine clastics of

Caracas

Orinoco RiverArch

El Pilar F.Z.Serranía del Interior

El BaulGuarico Subasin

Maturin Subasin

500 km

Orinoco Belt

Text-Figure 2. Location of the subdivision of Venezuela’sEastern Basin, into the Guarico (to the west) and Maturin(to the east) Subasins, and the Orinoco belt to the south.

Location of Core Samples

Location of important taxa in core samples

Location of Cuttings Samples

Location of important taxa in cuttings samples

Unconformity

Location and extent of Core

Sandstone

Conglomerate

Shale

Terrestrial Diversity

Marine Diversity

Stratigraphic range extends before Campanian.

Complete stratigraphic range of the taxon.

Interval where taxon occurs more abundantly.

Guideline to appropriate range.

Legend to Text-Figures 3–8.

8 PALYNOLOGY, VOLUME 27 — 2003A

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Text-Figure 3. Stratigraphic ranges of the main palynomorph taxa found in the Campanian to Miocene interval. See text forreferences of the sources.

J. Helenes, D. Cabrera: Oligocene–Miocene palynomorph assemblages from eastern Venezuela 9

the “Merecure” Group. This unit is locally assigned to the“Naricual“ Formation due to its sandy composition and tothe presence of occasional coal fragments. Conformablyoverlying this unit, the shales of the Carapita Formationcontain abundant planktonic and benthonic foraminiferaand calcareous nannoplankton. These Tertiary units havebeen shown to include the Oligocene–Miocene boundary(Di Gianni-Canudas et al., 1984; Crespo-de-Cabrera andDi Gianni-Canudas, 1984; Mitacchione and Odehnal,1984). At the top of the sequence are found the La Pica(sandstones and siltstones) and Morichito Formations(conglomerates). In the area studied, the “Naricual” For-mation unconformably overlies the sandy San Juan For-mation, or locally the sandy–shaly San Antonio Forma-tion. The samples analyzed come from the interval includ-ing from the late Cretaceous San Antonio Formation tothe early Miocene Carapita Formation.

PALYNOLOGICAL ASSEMBLAGES

Cretaceous Assemblages

The assemblages observed in the Cretaceous strata showan abrupt change from predominantly marine in the basalpart, to mostly terrestrial towards the top. The marine

dominated interval corresponds to the shaly–sandy SanAntonio Formation. The change of dominance from marineto terrestrial is easily observable and a simple way todistinguish the shallower sandstones of the San Juan For-mation.

The richest marine assemblages are generally found inthe Campanian interval and include gonyaulacoid,peridinoid, ceratioid and gymnodinioid taxa (Plate 1).These assemblages indicate deeper marine environmentsthan those of the overlying San Juan Formation. The SanAntonio strata were probably deposited in a middle neriticenvironment, with minor terrestrial components likeCyathidites australis and Ariadnaesporites sp.

The richer terrestrial assemblage of the San Juan Forma-tion usually contain the species Foveotriletes margaritae,Ariadnaesporites sp. and Buttinia andreevi (Plate 4). To-gether with these taxa, occur sparse Dinogymnium,Senegalinium, Cerodinium and Andalusiella (Plate 1). Theseassemblages are related to a shallowing event in the basin,towards inner neritic to transitional environments.

Oligocene–Miocene Assemblages

The assemblages observed in the Tertiary strata, gener-ally contain more terrestrial than marine elements (Text-Figures 5, 6 and 7), related to the shallow marine environ-ments represented in the sections studied. The Oligocene–Miocene terrestrial assemblages (Plate 4) are diverse andprobably correspond to a deltaic system with large fluvialinflux and sparse mangrove areas. A nearby wet tropicalforest environment is indicated by the common presence ofseveral pollen taxa such as Bombacacidites spp. 7 speciesof Bombacacidites were, found in this study, they are: B.aff. B. baculatus Müller et al., 1978 (differs from the typicalB. baculatus by the lack of baculae, instead has coarselyreticulate polar areas); B. sp. A in Müller et al., 1987; B.baumfalki Lorente 1986; B. brevis Muller, de Di Giacomo& van Erve, 1985; B. noremi Salard 1974, and B. zuatensisLorente 1986. Together with these species we found largenumbers of fern spores like Polypodiisporites usmensis,Cicatricosisporites dorogensis, Kuylisporites waterbolkiand Laevigatosporites spp., common to rare mangroveelements like Verrutricolporites rotundiporus,Spinizonocolpites echinatus and Laganiopollis crassa, andsparse Mauritiidites franciscoi and Magnastriatitesgrandiosus. The dinoflagellate assemblages (Plates 2, 3)are dominated by shallow, mostly tropical gonyaulacoidtaxa such as Homotryblium spp., Polysphaeridium spp. andOperculodinium spp., with less common Batiacasphaeraspp., Spiniferites spp. and Cribroperidinium tenuitabulatum.Also rare are specimens of Tuberculodinium vancampoe

Oligocene

early Paleoceneto Maastrichtian

middle to earlyMiocene

Carapita

"Naricual"

San Juan

San Antonio

La PicaPliocene to

late Miocene

Morichito

FORMATIONAGESLITH.

Pleistocene?

Maastrichtianto Campanian

Text-Figure 4. Upper Cretaceous to Tertiary stratigraphicunits recognized in the wells studied.

10 PALYNOLOGY, VOLUME 27 — 2003

and Systematophora placacantha. Among the fewperidinioid forms found, the more common taxa areSelenopemphix spp., Lejeunecysta spp., and Quadrinapallida.

Previous Palynological Studies

Cretaceous Interval. The Cretaceous–Tertiary terres-trial palynoflora from South America has been described inseveral papers (Germeraad et al., 1968; Pares-Regali et al.,1974a, b; Müller et al., 1987; Lorente, 1986; Jaramillo andDilcher, 2001). Some of these works have proposed zona-tions based on terrestrial palynomorphs. Several palyno-logical studies including Cretaceous material from this areahave been published. Some of them focus on the terrestrialpalynofloras (Müller et al., 1987), while others have dealtwith the Cretaceous dinoflagellates (Fasola and Paredes deRamos, 1991; Paredes et al., 1994; Helenes and Somoza,1999; Yepes, 2001). These works present more generalresults and they do not document the palynological contentof the Tertiary strata involved. The present paper docu-ments the uppermost Cretaceous palynofloras, particularlythe dinoflagellates, together with the assemblages from theunconformably overlying Oligocene–Miocene strata.

High Latitude Oligocene–Miocene Interval. A largenumber of studies dealing with dinoflagellates of the Oli-gocene to Miocene interval have been carried out byvarious authors since at least the 1960s, most of themdealing with high latitude locations of the northern hemi-sphere (e.g. Gerlach, 1961; Benedek, 1972; Habib, 1972;Manum, 1976; Stover, 1977; Edwards, 1984, 1986; Bujakand Matsuoka, 1986). All this information has been re-viewed and/or used to establish zonations for that region

(Williams, 1975; Harland, 1978; Costa and Downie, 1979;Williams and Bujak, 1985; Costa and Manum, 1988; Manumet al., 1989; Powell, 1992).

Mid Latitude Oligocene–Miocene Interval. A smaller,but rapidly growing number of papers deal with mid-latitude locations, mainly around the Mediterranean region(Jan du Chene, 1977; Corradini, 1978; Powell, 1986 a, b;Biffi and Manum, 1988; Brinkhuis et al., 1992; Brinkhuisand Biffi, 1993; Wilpshaar et al. 1996; Torricelli and Biffi,2001). Most of these works present detailed accounts of thedinoflagellate assemblages found in well dated sectionsrepresenting deep water facies. Based on these studies, theproposed zonations for this part of the world are increas-ingly refined and well calibrated. Some of the most com-mon forms in Oligo-Miocene strata from mid- to high-latitude locations are the peridinioid Deflandrea andWetzeliella, as well as the gonyaulacoid Chiropteridium,Distatodinium and Pentadinium. The zonations proposedfor the mid- to high latitude regions rely on representativesof these genera, they include: Deflandrea, 5 species;Wetzeliella, 2 species; Chiropteridium, 4 species;Distatodinium, 6 species, and Pentadinium, 2 species.These genera are not present in the tropical material studiedfor this paper.

Low Latitude Oligocene–Miocene Interval. There isa large amount of published information concerning theOligocene–Miocene continental palynology from north-ern South America (Van Der Hammen, 1957; Van DerHammen and Wijmstra, 1964; Germeraad et al., 1968;Regali et al., 1974a, b; Dueñas, 1980; Müller et al., 1987;Lorente, 1986; Colmenares and Teran, 1993; Hoorn,1993; Rull, 1997). However, published information con-cerning Oligocene–Miocene dinoflagellate assemblagesfrom lower latitude regions is sparse (Jarvis & Tocher,

1 Odontochitina costata Alberti 1961 emend. Clarke &Verdier 1967. Low focus, dorsal view.

2 Palaeocystodinium australinum (Cookson) Lentin &Williams 1976. Medium focus, lateral view.

3 Andalusiella gabonensis (Stover & Evitt) Lentin &Williams 1989. High focus, dorsal view.

4 Cerodinium boloniensis (Riegel) Lentin & Williams1989. Low focus, dorsal view.

5 Andalusiella polymorpha (Malloy) Lentin & Williams1977. Low focus, lateral view.

6 Senegalinium bicavatum Jain & Millepied 1973. Highfocus, lateral view.

PLATE 1

Maastrichtian–Campanian Dinoflagellates. Photomicrographs taken with Nomarski Interference Contrast. Length in µm represented byscales bars is indicated in the corresponding photographs.

7 Dinogymnium digitus (Deflandre) Evitt, Clarke &Verdier 1967. Low focus, outline.

8 Cerodinium boloniensis (Riegel) Lentin & Williams1989. High focus, lateral-dorsal view.

9 Kallosphaeridium? helbyi Lentin & Williams 1989.High focus, lateral-dorsal view.

10 Senegalinium obscurum (Drugg) Stover & Evitt 1978.Low focus, dorsal view.

11 Senegalinium microgranulatum (Cookson & Eisenack)Lentin & Williams 1977. Low focus, lateral-dorsalview.

12 Isabelidinium glabrum (Cookson & Eisenack) Lentin& Williams 1977. Low focus, lateral-dorsal view.

J. Helenes, D. Cabrera: Oligocene–Miocene palynomorph assemblages from eastern Venezuela 11

50 m�(1)m 50 m��(2–5)m

50 m�(6–12)m

1

2

34

6 7 8

9

10 11

12

5

Plate 1

12 PALYNOLOGY, VOLUME 27 — 2003

1 Tuberculodinium vancampoe (Rossignol) Wall 1967.High focus, dorsal view.

2 Operculodinium centrocarpum (Deflandere & Cookson)Wall 1967. High focus, dorsal view.

3 Diphyes latiusculum Matsuoka 1974. Medium focus,dorso-ventral view.

4 Polysphaeridium congregatum (Stover) Bujak et al.1980. High focus, dorsal view.

5 Polysphaeridium zoharyi Rossignol 1962. High focus,ventral view.

6 P. zoharyi var ktana Rossignol 1962. High focus,antapical view.

7 Lingulodinium polyedrum (Deflandre & Cookson) Wall1967. High focus, dorsal view.

PLATE 2

Tertiary Dinoflagellates. Photomicrographs taken with Nomarski Interference Contrast. Scale bar represents 20 µm.

CO

RE

0 150

(13000’)

(13500’)

(14000’)

Depthm(ft)

3810

(14500’)

(15000’)

(15500’)

MainBioevents

R.sommeri*

T. vancampoe,A. coronata, B. brevis*

P. pachydermatus*C.dorogensis*

T.vancampoe, H.vallum

D.digitus,F.margaritae*

S.bicavatum,C? tocheri

P.australinum

K? helbyi,I.glabrum

O.porifera,O.costata

P.magnificum

Fm

s.

Ca

rap

ita

Na

ricu

al

Sa

n A

nto

nio

Sa

nJu

an

Maast.

Paly. Ages

(14190’) 4325.1

(13783’2”)

Undetermined

earlyMiocene

lateOligocene

Campan.

(15480’)

(12760’)

(13711’)

4718.30

4201.5

4179.1

(4101.7)

(4087.5)

3889.2

(12500’)

3962.4

4267.2

4419.6

4572

4724.4

GammaRay(Well 1)

Ma

r. D

iv.

Te

rr. D

iv.

20 2000

4114.8

Sa

mp

les

(13428’) 4094

8 Kallosphaeridium capulatum Stover 1977. High focus,dorsal view.

9 Batiacasphaera hirsuta Stover 1977. High focus, ven-tral view.

10 Homotryblium tenuispinosum Davey & Williams 1966,High focus, ventral view.

11 Operculodinium microtriainum (Klumpp) Islam 1983.High focus, lateral-right view.

12 O. microtriainum (Klumpp) Islam 1983. Medium fo-cus, apical view of the granular surface of the cyst.

13 Hystrichokolpoma rigaudiae Deflandre & Cookson1955. High focus, lateral view.

Text-Figure 5. Well 1, stratigraphy, samples studied, palynological diversities (Mar. Div. = Marine Palynomorph Diversity; Terr.Div. = Terrestrial Palynomorph Diversity). Palynological ages and main palynological events. Core (indicated in the column ofdepth) contains the Maastrichtian/Oligocene unconformity and the Oligocene/Miocene limit. Asterisk (*) indicates terrestrialpalynomorphs.

J. Helenes, D. Cabrera: Oligocene–Miocene palynomorph assemblages from eastern Venezuela 13

1

50 mm

2 3

4

7

5

9

6

108

11 12 13

Plate 2

14 PALYNOLOGY, VOLUME 27 — 2003

50

(14500)

(15000)

(15500)

(16000)

Depthm(ft)

GammaRay

MainBioevents

CO

RE

4419.6

4572

4724.4

(15016’)

(15049’)

(14095’)

4577

4296

P. cretaceum,O. operculata,

Subtilisphaera sp.

K?. helbyi

S. obscurum

C.tenuitabulatum, P.pachydermatus*

P.pachydermatus*,B.sp.A Muller et al.*

P.pachydermatus*

B.noremi*,P.transversalis*

P. usmensis*,M. spinulosa*

H.salacium,C.dorogensis*,O. centrocarpum

F. margaritae*,Senegalinium sp.

Ariadnaesporites sp.*,C. australis*

B. andreevi*

4587

Fm

s.

Sa

mp

les

Cara-pita

Na

ricu

al

Sa

n A

nto

nio

Sa

n J

ua

n

Paly. Ages

earlyMiocene

lateOligocene

early Mioc.-

late Olig.

Maas.- Paleocene

Maas-trichtian

Campanian

(15620’)

(15171’)

(14838’)

4761

4624

4523

(14300’) 4360

X. ceratioides

(Well 2)

20 2000

Mar. D

iv.

Terr

. D

iv.

Text-Figure 6. Well 2, stratigraphy, samples studied, palynological diversities (Mar. Div. = Marine Palynomorph Diversity; Terr.Div. = Terrestrial Palynomorph Diversity). Palynological ages and main palynological events. Core (indicated in the column ofdepth) contains the Maastrichtian/Oligocene unconformity. Asterisk (*) indicates terrestrial palynomorphs.

1 Cribroperidinium tenuitabulatum (Gerlach) Helenes1984. High focus, ventral view.

2 C. tenuitabulatum (Gerlach) Helenes 1984. Same speci-men as in fig. 1. Low focus, ventral view on dorsalsurface.

3 C. tenuitabulatum (Gerlach) Helenes 1984. High focus,lateral-dorsal view.

4 Sumatradinium hispidum (Drugg) Lentin & Williams1976. Low focus, dorsal view.

5 Selenopemphix nephroides Benedek 1972. Low focus,apical surface.

6 Selenopemphix brevispinosa Head et al. 1989. Highfocus, right lateral-dorsal view.

7 Lejeunecysta fallax (Morgenroth) Artzner & Dörhöfer1978. High focus, right-lateral dorsal view.

PLATE 3

Tertiary Dinoflagellates. Photomicrographs taken with Nomarski Interference Contrast. Scale bar represents 50 µm.

8 L. fallax (Morgenroth) Artzner & Dörhöfer 1978. Highfocus, right-lateral ventral view.

9 Spiniferites mirabilis (Rossignol) Sarjeant 1970. Lowfocus, ventral view.

10 Spiniferites mirabilis (Rossignol) Sarjeant 1970. Highfocus, dorsal view.

11 Quadrina pallida Bujak in Bujak et al. 1980. Highfocus, uncertain view.

12 Systematophora placacantha (Deflandre & Cookson)Davy et al. 1969. High focus, dorsal view.

13 S. placacantha (Deflandre & Cookson) Davy et al.1969. Medium focus, ventral view.

14 Spiniferites pseudofurcatus (Klumpp) Sarjeant 1970.High focus, uncertain view.

J. Helenes, D. Cabrera: Oligocene–Miocene palynomorph assemblages from eastern Venezuela 15

A

2

1u

6

1i

IV

B

4

50 mm8

7

32

65

1

9 10 11

141312

Plate 3

16 PALYNOLOGY, VOLUME 27 — 2003

50 20 0 20

(14700)4481

4511

4541

(14800)

(14900)

(15000)

GammaRay

Depthm(ft)

Bioevents

C.tenuitabulatumL.hyalina

C.tenuitabulatum,P.pachydermatus*

B. brevis*

P.triangularis*

P.divisus*

D.latiusculum

C.columbianus*

P.pachydermatus*

H.plectilum

K.waterbolki*

CO

RE

P.triangularis*

Bombacid. *sp. A

Fm

s.

Sa

mp

les

Mar.

Div

.

Terr

. D

iv.

4572

0

Cara-pita

Naricual

(Well 3)

Paly.Ages

earlyMiocene

4485(14715’ 4”)

4558(14952’ 10”)

Text-Figure 7. Well 3, stratigraphy, samples studied, palynological diversities (Mar. Div. = Marine Palynomorph Diversity; Terr.Div. = Terrestrial Palynomorph Diversity). Palynological ages and main palynological events. Core (indicated in the column ofdepth) includes exclusively the lower Miocene unit. Asterisk (*) indicates terrestrial palynomorphs.

1 Psilatricolporites maculosus Regali, Uesugui & Santos1974.

2 P. pachydermatus Lorente 1986.3 P. pachydermatus Lorente 1986. Same specimen as in

figure 2.4 Momipites africanus Van Hoeken-Klinkenberg 1966.5 Bombacacidites noremi Salard 1974.6 B. brevis (Dueñas) Jan Muller et al. 19877 Jandufouria seamrogiformis Germeraad, Hopping &

Muller 1968.8 Mauritiidites franciscoi (van der Hammen) Van Hoeken-

Klinkenberg 1964.

PLATE 4

Tertiary Terrestrial Palynomorphs. Photomicrographs taken with Nomarski Interference Contrast. Scale bar represents 50 µm.

9 Cicatricosisporites dorogensis Potonié & Gelletich1933.

10 Bombacacidites baumfalki Lorente 1986.11 Psilatricolporites triangularis van der Hammen &

Wymstra 1964.12 Cyathidites mesozoicus (Thiergart) Potonié‚ 1956.13 Polypodiisporites usmensis (van der Hammen) Hekel

1972.14 Foveotriletes margaritae Germeraad, Hopping et Muller

1968.15 Ariadnaesporites sp. (Potonié) Tschudy 1966. Only

elateres of this taxon were found.16 Buttinia andreevi Boltenhagen 1967.

J. Helenes, D. Cabrera: Oligocene–Miocene palynomorph assemblages from eastern Venezuela 17

56

1

2 3

4

7

14

13

50 mm

11

16

910

15

12

8

Plate 4

18 PALYNOLOGY, VOLUME 27 — 2003

1985; McMinn, 1992, 1993b). Some publications (Duffieldand Stein, 1986; Lenoir and Hart, 1986; Wrenn andKokinos, 1986) describe dinoflagellates from Miocene toPleistocene strata in the northern Gulf of Mexico. How-ever, no zonation has been proposed for lower latitudes.Based in the reports just mentioned, the results reportedhere and data from other studies based on cuttings samplesfrom several other wells studied in the region (Helenes etal., 2001; 2002), and in the southern Gulf of Mexico (DelValle-Reyes, 2001; Carrillo Berumen, 2003) the follow-ing trends are evident:

First, the dinoflagellate cyst diversity in equatorial as-semblages seems to be lower than the diversity in assem-blages reported from higher latitudes. Recent sedimentsseem to present low diversity both in tropical and polarenvironments (Jarvis and Tocher, 1985; Edwards andAndrle, 1992; McMinn, 1993a; Dale, 1996). Conversely,diversity of dinoflagellate cysts is normally higher in tem-perate regions where a combination of representatives ofboth provinces are found.

Second, there is a dominance of gonyaulacoid taxa in ourmaterial. The dinoflagellate genera mentioned above andwhich are the basis for higher latitude zonations, are almostalways absent from all the samples studied. In the studyreported here, only one specimen of a cavate peridinioidcyst questionably assigned to the genus Deflandrea wasobserved in the early Oligocene interval of well 2 (sampleat 14920 ft).

A possible explanation of this gonayaulacoid dominancein tropical material could be the following. In Recentsediments, peridinioid taxa are more common in coldenvironments, while gonyaulacoid taxa are more commonin tropical environments (Matsuoka, 1992, 1994; Mehrotraand Rawat, 1992; Edwards and Andrle, 1992). This relationof gonyaulacoid taxa with warmer environments is alsorecognizable in interglacial stages of the Quaternary (DeVernal, et al., 1992; Harland, 1994).

This difference of dominance can be explained in rela-tion to the trophic strategies of both orders. On the onehand, taxa from the order Peridiniales are mostly het-erotrophic and diatoms are their main food source. Diatomsare abundant in regions with high levels of nutrients,particularly silicates. These conditions are more commonlyfound in cooler mid to high latitudes, where peridinioidtaxa are also more common.

On the other hand, taxa from the order Gonyaulacales aremostly photosynthetic and rely mainly on nitrogenatedcompounds. Blooms of these taxa seem to be related totemperatures above 17°C and concentrations of more than0.5 µm for nitrites and nitrates (Anderson, 1998; Peña-Manjarrez, et al., 2001). So, gonyaulacoid taxa are morelikely to proliferate in warmer waters.

Provincialism is common among planktonic organisms.For example, foraminifera have shown a distribution inbelts which roughly parallels latitude since at least Jurassicto Recent times (Gordon, 1970; Sliter, 1972; Boersma,1978; Bé, 1977). Also, the geographic distribution ofcalcareous nannoplankton has been related to a latitudinalclimatic gradient at least from Tertiary (Haq and Lohmann,1976) to Recent times (McIntyre, et al., 1972; Okada andHonjo, 1973). Among dinoflagellates, Cretaceous provin-cialism has also been noted (Lentin & Williams, 1980;Powell, 1992; Yepes, 2001; Helenes & Téllez-Duarte,2002) particularly for the Campanian–Maastrichtian inter-val.

Third, since the peridinioid taxa (Deflandrea andWetzeliella) commonly found in higher latitudes, are nor-mally considered indicators of shallow marine to brackishenvironments, we believe that the differences are not en-tirely attributed to the shallow marine environments repre-sented in the sections studied.

The information presented in this paper indicates that thezonations proposed for the Oligocene–Miocene intervalfrom higher latitudes, including the Mediterranean region,are not applicable in strata from more equatorial locations,specifically the Caribbean region. However, many di-noflagellate species are commonly found in both areas andsome of the age assignments are also supported by plank-tonic foraminifera and calcareous nannoplankton. Addi-tionally, our data contains information on the simultaneouspresence of terrestrial and the marine palynofloras, and thiscombination will help to compare the stratigraphic rangesof both types of palynomorphs.

Palynological Biostratigraphy

Cretaceous Interval. Strata assigned here to a Campanianto Maastrichtian age were observed in wells 1 (Text-Figure5) and 2 (Text-Figure 6). In general, the dinoflagellateassemblage reported here are similar to those found in strataof the same ages in Colombia (Yepes, 2001) and Venezuela(Fasola and Paredes de Ramos, 1991).

The lower San Antonio Formation contains the dinoflagel-lates (Plate 1) Andalusiella spp., Cerodinium spp.,Dinogymnium spp., Isabelidinium spp., Odontochitina spp.,Subtilisphaera sp., Cassiculosphaeridia? tocheri,Kallosphaeridium? helbyi, Palaeoperidium cretaceum,Phelodinium magnificum, Senegalinium obscurum andXenascus ceratioides.

This interval contains few terrestrial palynomorphs, butthe presence of Ariadnaesporites sp. and absence ofDroseridites senonicus suggest correlation to the undiffer-entiated Auriculiidites reticularis (Campanian to lower

J. Helenes, D. Cabrera: Oligocene–Miocene palynomorph assemblages from eastern Venezuela 19

Maastrichtian) to Crassitricolporites subprolatus (Maas-trichtian) zones of Müller et al. (1987).

The sandy strata of the San Juan Formation include thedinoflagellates (Plate 1) Andalusiella spp., Cerodiniumspp., Dinogymnium, Senegalinium spp., together with theterrestrial palynomorphs Ariadnaesporites spp., Buttiniaandreevi, Cyathidites mesozoicus and Foveotriletesmargaritae (Plate 4). This terrestrial assemblage iscorrelatable with the Proteacidites dehanii zone of Mülleret al. (1987), unfortunately the nominate species was notobserved in our study.

On well 2 (Text-Figure 5), the interval 4624–4577 mcontains the spore Foveotriletes margaritae and the di-noflagellate Senegalinium sp., therefore can only be corre-lated to an undifferentiated Maastrichtian to Paleocene age.This interval can only be assigned to the interval fromProteacidites dehanii to Foveotricolpites perforatus zonesof Müller et al. (1987).

Late Oligocene Interval. Late Oligocene palynologicalassemblages were observed in wells 1 (Text-Figure 4) and2 (Text-Figure 5). This interval commonly contain thedinoflagellates Areoligera coronata, Batiacasphaerahirsuta, Batiacasphaera sphaerica, Cordosphaeridiummicrotriaina, Cribroperidinium tenuitabulatum,Homotryblium plectilum, Homotryblium vallum,Hystrichokolpoma salacium, Kallosphaeridium capulatum,Lejeunecysta hyalina, Operculodinium centrocarpum,Polysphaeridum congregatum and Tuberculodiniumvancampoe (Plates 2, 3). These strata also contains theterrestrial palynomorphs Bombacacidites brevis,Bombacacidites noremi, Cicatricosisportites dorogensis,Malvacipollis spinulosa, Polypodiisporites usmensis andPsilabrevitricolporites triangularis (Plate 4).

The lowermost Tertiary rocks studied in wells 1 and 2,are late Oligocene in age and contain the terrestrialpalynomorphs Cicatricosisporites dorogensis, Jandufouriaseamrogiformis, Polypodiisporites usmensis andMauritiidites franciscoi, together with the dinoflagellatesCordosphaeridium spp., Polysphaeridium congregatum,Homotryblium spp. and Tuberculodinium vancampoe.According to Müller et al. (1978), the stratigraphic rangesof the concurrent species Bombacacidites brevis (Zones25–30, Oligocene to Pliocene) and Cicatricosisporitesdorogensis (Zones 22–25, middle Eocene to Oligocene),indicates that the terrestrial assemblage can be assigned tothe Magnastriatites–Cicatricosisporites dorogensis zone(Zone 25, Oligocene) of Müller et al. (1987). While thedinoflagellates Polysphaeridium congregatum andTuberculodinium vancampoe indicate a late Oligocene age(Text-Figure 3).

Early Miocene Interval. The palynological content ofthis interval indicates an early Miocene age, and was ob-

served in the 3 wells. These strata commonly contain someof the same dinoflagellates from the underlying interval,excluding Homotryblium plectilum, Hystrichokolpomasalacium and Cordosphaeridium microtriaina. These strataalso contains the terrestrial palynomorphs Bombacaciditessp. A in Müller et al., 1987, Bombacacidites zuatensis,Kuylisporites waterbolki, Psilatricolporites divisus,Psilatricolporites pachydermatus, Retitriletes sommeri andVerrutricolporites rotundiporus.

The limit Oligocene–Miocene was clearly observed incore samples from well 1, just above the uppermost occur-rence of the spore Cicatricosisporites dorogensis (4101.7m), and below the lowermost occurrence of the pollenPsilatricolporites pachydermatus (4087.5 m). This lastspecies has been reported from the Miocene (Text-Figure3) of Venezuela (Lorente, 1986), and the results presentedhere support this age assignment. In well 2, the uppermostuppermost occurrence of C. dorogensis (4523 m), is alsolocated below the lowermost occurrence of P.pachydermatus (4360 m).

The samples from well 3 represent deposition during theearly Miocene, based on the concurrent presence of thepollen taxa P. pachydermatus, Kuylisporites waterbolki,Bombacacidtes sp. A in Müller et al., 1987, Psilatricolporitestriangularis and Psilatricolporites divisus, together withthe dinoflagellates Diphyes latiusculum, Homotrybliumplectilum, Lejeunecysta hyalina and Cribroperidiniumtenuitabulatum (Text-Figure 3).

The terrestrial assemblage represents the undifferenti-ated Verrutricolporites rotundiporis–Echidiporitesbarbeitoensis to Echitricolporites spinosus zones (Zones26–29, Miocene) of Müller et al. (1987), while the dino-flagellates Diphyes latiusculum, and Cribroperidiniumtenuitabulatum (Text-Figure 3) indicate an early Mioceneage.

Correlation. The studied intervals from wells 1 and 2include from Campanian to early Miocene strata, while thecore from well 3 includes only early Miocene strata. How-ever, based on the ages assigned here and considering thecharacteristics of the Gamma Ray logs, the 3 wells can becorrelated as shown on Text-Figure 8. Unfortunately, inthis study we did not have the opportunity to analyzesamples from levels lower than the core (Text-Figure 8),but it is advisable to confirm the presence of the Maastrich-tian–Oligocene unconformity and the Oligocene–Miocenelimit on well 3.

CONCLUSIONS

The combination of pollen, spores and dinoflagellatebiostratigraphy are the basis to date the section studied, and

20 PALYNOLOGY, VOLUME 27 — 2003

to define that in the area there is an unconformity betweenCampanian–Maastrichtian and late Oligocene sedimentaryrocks. This unconformity has been recognized before in thearea, but there was uncertainty about the interval encom-passed. We estimate that the hiatus encompasses approxi-mately 40 Ma, ranging from late Maastrichtian to earlyOligocene.

Additionally, the strata overlying the unconformity con-tain the Oligocene–Miocene boundary. Age diagnosticpalynomorphs used to identify the Oligocene–Mioceneboundary are the terrestrial: Cicatricosisporites dorogensis,P. pachydermatus, Kuylisporites waterbolki, Bombacaci-dites sp. A in Müller et al., 1987 and Psilatricolporitesdivisus; together with the dinoflagellates: Polysphaeridiumcongregatum, Kallopshaeridium capulatum, Cribroperi-dinium tenuitabulatum, Homotryblium plectilum andDiphyes latiusculum

A semi-quantitative analysis of the diversities of marineand terrestrial palynomorphs was used, together with thelog analyses, to recognize changes in the sedimentaryenvironments. These are useful to correlate the sections inthe area.

Significantly, the Oligo-Miocene dinoflagellate assem-blages do not contain taxa generally reported in sectionsfrom higher latitudes, suggesting that provincialism shouldbe carefully considered when utilizing any zonation. How-ever, the stratigraphic ranges of the species found in bothareas seem to be the same.

ACKNOWLEDGMENTS

Many thanks to PDVSA for permission to publish thispaper and to our friends from INTEVEP for all their help.

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Text-Figure 8. Correlation of the 3 wells studied. The Maastrichtian/Oligocene unconformity, and the late Oligocene interval arebased only on the log analyses, therefore they are tentative.

J. Helenes, D. Cabrera: Oligocene–Miocene palynomorph assemblages from eastern Venezuela 21

The study was conducted partially at the LaboratorioGeológico from CORPOVEN (now part of PDVSA) atPuerto La Cruz, Venezuela, and partially at CICESE,Ensenada, Mexico. Thanks to Oscar Yepes and CarlosJaramillo for their thorough and useful reviews, whichhelped improve the original manuscript.

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