THE EARLY MIOCENE OF WESTERN VENEZUELA,...

V. Rull: A quantitative palynological record from the Early Miocene of western Venezuela, with emphasis on mangroves 109A QUANTITATIVE PALYNOLOGICAL RECORD FROMTHE EARLY MIOCENE OF WESTERN VENEZUELA,WITH EMPHASIS ON MANGROVES

Palynology, 25 (2001): 109–126© 2001 by AASP Foundation ISSN 0191-6122

VALENTÍ RULLPDVSA Exploration and ProductionCaracasVenezuelaPostal address: PA1394PO Box 02-5304Miami, FL 33102-5304U.S.A.e-mail: [email protected]

Abstract

The quantitative reconstruction presented in this paper documentspaleoecological trends of the northern Maracaibo basin (westernVenezuela), during the Early Miocene, through pollen analysis ofdrill core samples from the La Rosa (shallow marine) and Lagunillas(coastal plain) formations. Palynological assemblages weregrouped statistically into three assemblages representing man-groves, herbaceous back-mangrove swamps, and inland palm/fern swamps. Mangroves were the local vegetation during thedeposition of La Rosa Formation and, after a regressive event,were replaced by herbaceous back-mangroves during the sedi-mentation of Lagunillas Formation. The palynological assem-blage representing palm/fern swamps dominates the entire se-quence and is interpreted as a large background signal reflectingtransport by rivers. Mangrove communities had few species, andrepresented a transitional phase in the mangrove communityevolution, after the terminal Eocene biotic crisis.

INTRODUCTION

This paper presents a palynological study of Early Mio-cene La Rosa and Lagunillas formations from the NE of theMaracaibo basin in Venezuela, in order to reconstruct thepaleosuccession of coastal plant communities, with empha-sis on mangroves. It is a part of a broader project which isaimed at reconstructing coastal and mangrove communitiesand their paleosuccession in the Tertiary of the Maracaibobasin using quantitative pollen analysis and statistical paleo-ecological methods to increase the degree of objectivity inboth the composition and the interpretation of palynologicaltaphocenoses. Paleocommunities should be reconstructedfrom suitable taphocenoses, and not with qualitative or semi-quantitative data. In palynology, this is achieved throughrepresentative counts including all the palynomorph typespresent in samples (Rull, 1987), and comparison with mod-ern analogs (Rull, 1998a). Furthermore, empirical associa-

tions based solely in the botanical affinities of the fossil taxaand the taxonomic composition of present communities (acommon procedure) implicitly assume that communitieshave not changed through time, which is totally untrue,especially for mangroves (Graham, 1995; Rull, 1998b). Toproperly record past associations, more objective methods,such as statistical associations, should be applied, beforelooking at botanical affinities (Rull, 1998c). So far, severalPaleocene and Eocene mangroves communities have beenreconstructed with these methods (Rull, 1992; 1997b; 1998b;1998c; 1999; 2000). However, post-Eocene mangroves,although well-known taxonomically (Lorente, 1986; Mulleret al., 1987), have not yet been studied with these methods.

From a palynostratigraphic point of view, the Early Mio-cene in Venezuela is characterized by two zones (Lorente,1986): Verrutricolporites and Psiladiporites, which are equi-valent to the northern South American pollen zonesVerrutricolporites rotundiporus–Echidiporites barbeitoensis,and Echitricolporites maristellae–Psiladiporites minimus, re-spectively (Muller et al., 1987). The zone of Verrutricolporitesis an interval zone, having its base in the first occurrence ofVerrutricolporites rotundiporus and its top below the firstoccurrence of Psiladiporites minimus or Echitricolporitesmaristellae. In addition, two other taxa: Bombacaciditeszuatensis and Psilatricolporites pachydermatus have theirfirst occurrence at the base of this zone. According to Lorente(1986), pollen assemblages for this zone are dominated in theentire basin by Zonocostites ramonae, the other importantcomponents being Psilatricolporites crassus, Bombacaciditesbaumfalki, Retitricolporites irregularis, Retitricolporiteshispidus, Retitricolporites simplex, Jandufouriaseamrogiformis, Verrucatosporites usmensis, Proxapertitesoperculatus, Monoporites annulatus, Mauritiidites franciscoi,Striatricolpites catatumbus, Psilatricolporites maculosus, and

110 PALYNOLOGY, VOLUME 25 — 2001

Podocarpidites sp. The inferred sedimentary environment iscoastal to shallow marine. The interval zone of Psiladiporitesis defined by the first occurrence of Psiladiporites minimus, atthe base. In western Venezuela, the first occurrence ofEchitricolporites maristellae is also characteristic, and thezone is called Psiladiporites–Echitricolporites (Lorente, 1986).The top is defined by the first occurrence of Crassoretitriletesvanraadshooveni. In the Maracaibo basin, this zone can besubdivided into two well-differentiated intervals. The lowerinterval is dominated by Monoporites annulatus,Echitricolporites maristellae, Malvacipollis spinulosa,Psilatricolporites operculatus, Crototricolpites annemariae,Bombacacidites baculatus, Retimonocolpites longicolpatus,Verrucatosporites usmensis, Magnastriatites grandiosus,Psilamonocolpites medius, Zonocostites ramonae,Mauritiidites franciscoi, and Retitricolporites guianensis, andwas deposited under alluvial to lower coastal plain condi-tions, with some geographical variations. The upper intervalis similar but in it Zonocostites ramonae decreases in abun-dance, and disappears in some areas. The sedimentary envi-ronments were alluvial to coastal plains, with more marineconditions in the south (Lorente, 1986).

In the mangrove communities, Zonocostites ramonaewas abundant and geographically widespread in westernVenezuela, during the Early Miocene. This form-speciescorresponds to the fossil pollen of Rhizophora, the domi-nant genus in present-day Neotropical mangrove commu-nities (Chapman, 1976; Tomlinson, 1986). The fossil recordshows that this pollen appeared in the Late Eocene in lowquantities, and has been dominant in lower coastal man-grove communities from the Miocene to the present. Othermangrove-forming trees progressively appeared in theMiocene (Avicennia), Pliocene (Laguncularia) and Qua-ternary (Conocarpus) (Graham, 1995). The Eocene man-grove communities were radically different in composi-tion. Their main elements are nowadays extinct in theNeotropics (Nypa, Brevitricolpites variabilis) or reducedto a relict area (Pelliciera). The only extant component isthe fern Acrostichum aureum, represented in the fossilrecord by Deltoidospora adriennis. The evolutionary changefrom Eocene to Miocene–Recent mangroves is assumed tohave been not gradual, and related to the terminal Eocenebiotic crisis, probably due to climatic causes (Rull, 1998b).

MATERIAL AND METHODS

Study Site and Lithology

This study was carried out on core samples from well TJlocated at the NE part of the Maracaibo basin, in westernVenezuela (Text-Figure 1). The geological formations

involved are La Rosa and lower Lagunillas. The La RosaFormation unconformably overlies the Eocene Misoa For-mation. It is approximately 200 m thick in the type locality,but can be as thick as 1000 m in the eastern part of the basin.It consists of greenish fossiliferous clayey shales withintercalated sandstone layers, and has been subdivided intofour units: the basal Santa Bárbara Member (unconsoli-dated clayey sandstones), the La Rosa shales (with thetypical lithology described for the formation), the Interme-diate Sand (with a mixture of the main La Rosa lithologies)and the La Rosa sands (fine-grained sandstones, and shaleswith mollusks and foraminifers). The fossil content indi-cates an Early Miocene age, and initial deposition in aneritic environment. The Santa Bárbara Member repre-sents the initial transgression on the eroded Eocene, laterculminating in a shallow sea covering most of the Maracaibobasin (represented by La Rosa shales). During the subse-quent regression, the ‘intermediate sand’ and the ‘La Rosasand’ (informal names) were deposited in estuarine andbeach, and ultimately deltaic environments, transitional tothe overlying Lagunillas Formation (González de Juana etal., 1980).

The Lagunillas Formation is 300 to ~1000 m thick,consisting of unconsolidated sandstones, with intermingledclays, shales, and lignites. In the eastern part of the basin,it has been subdivided into three members: Lower Lagunillas(fine-grained sandstones with intercalated shales), Laguna(gray fossiliferous shales and gray sandstones), andBachaquero (thick clayey sandstones with clays and shales).The Lagunillas Formation ranges from Lower to MiddleMiocene in age, and represents the continuation of theregressive sequence initiated in the La Rosa Formation,although an intermediate increase of marine conditions arerepresented by the Laguna Member. The Lower LagunillasMember was deposited in a deltaic environment, prograd-ing from the S–SE, and the Bachaquero Member wasdeposited in fluvial and deltaic environments (González deJuana et al., 1980). In the present study, only the LowerLagunillas member has been examined.

Laboratory and Analytical Methods

Palynological analyses were carried out on all the shalyintervals of the selected cores (31 samples). Samples weredigested in HF and HCl, and centrifuged with ZnBr2, beforemounting in glycerin-jelly. In order to have statistically-representative proportions, counts were done on randomtransects until the saturation of diversity (Rull, 1987). Finalaverage counts were 424 for sporomorphs (pollen and fernspores) and 497 for the total, which include fungal spores,freshwater algal remains (Pediastrum, Botryococcus,

V. Rull: A quantitative palynological record from the Early Miocene of western Venezuela, with emphasis on mangroves 111

Pseudoschizaea) and marine microplankton (dinoflagellatecysts, foraminiferal linings, acritarchs). The most importanttaxa identified are shown in Plates 1 to 4. Range analysis wasbased on the chronostratigraphically-meaning taxa, accord-ing to the Lorente (1986) and Muller et al. (1987) zonations.Percentages were computed with respect to the ‘pollen sum’,which included pollen and fern spores. Diagrams wereplotted with PSIMPOLL (Bennett, 1994). Palynologicalassemblages were obtained from cluster analysis using theGower (1971) similarity coefficient on log-e transformedpercentages and the unweighted centroid agglomerativemethod, using Multivariate Statistical Package (MVSP)software (Kovach, 1989). Only taxa over 1% of the total wereselected for cluster analyses, in order to avoid random noise

in paleoecological interpretation (Rull, 1997a; 1998c). Bo-tanical affinities, modern analogs, and vegetational patternsused in the paleoecological and paleoenvironmental inter-pretation are from Lindeman (1953), Muller (1959), van derHammen (1963), Germeraad et al. (1968), Wijmstra (1968),Caratini et al. (1973), Murillo and Bless (1974), Andersonand Muller (1975), Gastony and Tryon (1976), Muller(1981), Sheihing and Pfefferkorn (1984), Thanikaimoni etal. (1984), Frederiksen (1985), Lorente (1986), Tomlinson(1986), Muller et al. (1987), Tissot et al. (1988), Thanikaimoni(1987), Westgate and Gee (1990), Roubik and Moreno(1991), Srivastava and Binda (1991), Tryon and Lugardon(1991), Hoorn (1994), Velásquez (1994), Rull (1997b, 1998a,d), Rull and Vegas-Vilarrúbia (1999), and Graham (1999).

Text-Figure 1. Location map showing the approximate distribution of the formations involved, according to the updated version(on-line) of the Venezuelan Stratigraphic Code (www.intevep.pdv.com:80/~ibc03/).


RESULTS AND INTERPRETATION

Palynostratigraphy

The section studied was subdivided into the two EarlyMiocene palynostratigraphic zones described by Lorente(1986) and Muller et al. (1987) for northern South America(Text-Figure 2). The Verrutricolporites zone was recog-

nized between the base of the section and the first occurrenceof Echitricolporites maristellae, which defines thePsiladiporites–Echitricolporites zone, until the top of thesequence. The Verrutricolporites zone coincides with the LaRosa Formation, while the Psiladiporites–Echitricolporiteszone corresponds to the part of the Lower Lagunillas Mem-ber considered in this study. Few rare species are restrictedto each zone. It is also worth mentioning the occurrence of

Text-Figure 2. Presence/absence distribution chart and palynostratigraphical interpretation, for the La Rosa and Lagunillasformations.

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several Paleogene taxa, such as Spinizonocolpites echinatus,Spinizonocolpites baculatus, Echitriporites trianguliformis,Proxapertites cursus, Foveotriporites hammenii, andRetidiporites magdalenensis. They usually show single oc-currences in isolated samples, and can be assumed to bereworked from older upstream paleo-outcrops.

Paleoecology and Paleoenvironments

Fern and allied spores are generally dominant throughoutthe whole section (Text-Figure 3). Pollen and fungal sporesshow lower values, and aquatic elements (freshwater andmarine together) are only important in the La Rosa Forma-

Text-Figure 3. Percentage diagram for the major groups of palynomorphs.

418418

countscountsPollen

Pollen

Spo

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Fungi

Fungi

Aqu

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Aqu

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0 20 400 20 40 0 20 40 600 20 40 60 0 10 0 10 0 20 %0 20 %

413413514514

449449540540411411

370370

547547

527527

521521529529

431431

444444

372372

565565

409409

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396396

458458

512512

542542

623623

503503

468468

484484

648648

502502

429429

607607

452452

661661

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EA

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CE

NE

10701070

Depth(m)

Depth(m)

10801080

10901090

11001100

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11201120

11301130

11401140

SANDSAND

SHALESHALE

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sa

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1%).

Verrutricolporites VerrutricolporitesPsiladiporites-Echitricolporites Psiladiporites-Echitricolporites

EARLY MIOCENE EARLY MIOCENE

10

70

10

70

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pth

(m)

De

pth

(m)

10

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SAMPLES SAMPLES

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tion (Verrutricolporites zone). In the percentage diagramfor selected taxa (>1%), a clearer differentiation can beseen than using only presence/absence data (Text-Figure4). Dominant and less abundant elements show little varia-tions across the section, although Laevigatosporites vul-garis is slightly more abundant in the Lagunillas Formation(Psiladiporites–Echitricolporites zone). Other elementswith intermediate abundance, however, are clearly distinc-tive. Characteristic of the La Rosa Formation are themangrove elements, Psilatricolporites crassus andZonocostites ramonae, the marine representatives (foram-iniferal linings and acritarchs) and the freshwater algaBotryococcus, indicating sedimentation in coastal and shal-low marine environments, close to mangrove vegetation.Botryococcus, although more typical of freshwater envi-ronments, can also occur in slightly brackish water, due toits tolerance to salinity (Rull, 1997b). The main elementsthat characterize the Lower Lagunillas Member of theLagunillas Formation are fern spores (Magnastriatitesgrandiosus, Laevigatosporites catanejensis, Perino-monoletes sp.), together with Retitricolporites guianensis(of unknown affinity), and the freshwater alga Pediastrum.This is consistent with the presence of alluvial and coastalplain environments flooded by freshwater.

Cluster analysis provided 3 groups, EM1–3, which wereinterpreted according to the known botanical affinities oftheir components and the available modern analog studies(see Table 1 for a summary). Group EM1 is also recognizedin the percentage diagram, and is typical of the La Rosa

Formation (Text-Figures 5 and 6). From a vegetationalpoint of view, it is considered to be a mangrove assemblagedeposited near the mangrove belt or in adjacent shallowbrackish water. The lower Lagunillas Member is character-ized by group EM2 (Text-Figure 6), which is composed ofelements typical of vegetation flooded by fresh or low-salinity water (Magnastriatites grandiosus, Deltoidosporaadriennis, Laevigatosporites catanejensis, Pediastrum). Italso includes other elements with a wide range of environ-mental conditions, which also present in coastal swampsand marshes (Monoporites annulatus, Perisyncolporitespokornyi), and a form-species with unknown botanicalaffinity and environmental requirements (Retitricolporitesguianensis). The majority of these components are fernsor herbs, hence, they probably represent herbaceousswamps and marshes behind the mangrove belt. GroupEM3 is composed of palms (Mauritiidites franciscoi,Psilamonocolpites sp.), ferns (Verrucatosporites usmensis,Laevigatosporites vulgaris), and fungal spores, represent-ing another type of coastal swamps (palm/fern swamps)common in the Neotropics. The palm Mauritiiditesfranciscoi typically develops almost monotypical stands(‘morichales’) in the freshwater swamps of upper coastalplains and savannas, beyond the limit of tidal influence.Therefore, this assemblage is thought to represent the moreinland vegetation recorded in this study.

The succession of these assemblages through timeshows a dominant ‘background’ pollen signal from palm/fern–back-mangrove swamps (EM3) with little changes,

Text-Figure 5. Cluster analysis on the percentages of selected palynomorphs (>1%).

-0.2 0 0.2 0.4 0.6 0.8 1

Gower General Similarity Coefficient

Mauritiidites franciscoiPsilamonocolpites sp.Laevigatosporites vulgarisVerrucatosporites usmensisFungal sporesMonoporites annulatusPerisyncolporites pokornyiDeltoidospora adriennisPerinomonoletes sp.Retitricolporites guianensisLaevigatosporites catanejensisMagnastriatites grandiosusPediastrumPsilatricolporites crassusForam liningsZonocostites ramonaeAcritarchsBotryococcusDinoflagellate cysts

EM1

EM2

EM3


and the replacement of mangroves (EM1) by herbaceousback-mangrove swamps (EM2), in the La Rosa–Lagunillas transition. Indeed, during deposition of theLa Rosa Formation (Verructricolporites zone), pollensedimentation was almost exclusively composed of palm/

fern swamp sporomorphs and pollen locally depositedunder or in front of the mangrove fringe, with a negli-gible contribution from herbaceous back-mangroveswamps. During deposition of the Lagunillas Formation(Psiladiporites–Echitricolporites zone), although sporo-

1, 2 Deltoidospora adriennis (Potonié & Gelletich 1933)Frederiksen 1983. 3572' (106.4–17.6), 3673' (76.3–25.0).

3, 4 Foveotriletes ornatus Regali et al., 1974. 3481' (84.1–1.5).

5, 6 Magnastriatites grandiosus (Kedves & Solé de Porta)Dueñas 1980. 1963. 3551' (85.0–22.3); 3686' (108.8–2.6); bar = 16 µm for these two specimens.

PLATE 1

7, 8 Laevigatosporites vulgaris Potonié 1934. 3727' (89.9–3.6), 3623' (81.3–17.3).

9, 10 Laevigatosporites catanejensis Muller et al., 1987.3713' (108.3–8.2), 3673' (94.3–18.8).

11, 12 Verrucatosporites usmensis (van der Hammen 1956)Germeraad et al., 1968. 3767' (75.7–5.0).

13, 14 Perinomonoletes sp. Krutzsch 1967. 3673' (110.5–7.5),3760' (112.2–19.9).

Text-Figure 6. Percentage diagram showing the quantitative assemblages established, and the sporomorphs under 1%, withsignificant correlation.

EM

1EM

2EM

3Oth

ers

Ech

itrile

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uelle

ri

Ech

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uelle

ri

Ret

itricolpo

rites

hispidu

s

Ret

itricolpo

rites

hispidu

s

Ret

itricolpo

rites

irre

gularis

Ret

itricolpo

rites

irre

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Psilatri

colpor

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us

Psilatri

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us

Pod

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Pod

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Ret

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Ret

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V. Rull: A quantitative palynological record from the Early Miocene of western Venezuela, with emphasis on mangroves 117Plate 1

8 µm


1 Polyadopollenites sp. Pflug & Thompson 1953. 3681'(109.5–24.0)

2, 3 Podocarpidites sp. Cookson 1947 ex Couper 1953.3492' (104.2–5.4), ; 3760' (102.1–1.4),

4, 5 Psilamonocolpites spp. van der Hammen & García deMutis 1965. 3673' (79.0–14.9), 3755' (104.6–8.4).

6 Mauritiidites franciscoi (van der Hammen 1956) vanHoeken-Klinkenberg 1964. 3481' (99.7–7.5).

7, 8 Echidiporites barbeitoensis Muller et al., 1987. 3673'(11.2–11.2).

9 Verrutricolporites rotundiporus van der Hammen &Wijmstra 1964. 3745' (91.5–3.0).

PLATE 2

10, 11 Psilatricolporites maculosus Regali et al 1974. 3551'(72.8–11.5), 3673' (93.5–20.2).

12, 13, Zonocostites ramonae Germeraad et al., 1968. 3486'14 (91.6–23.0), 3755' (98.1–3.7), 3589' (92.5–18.0).15 Psilatricolporites venezuelanus Lorente 1986. 3551'

(98.7–21.0)16, 17 Psilatricolporites operculatus van der Hammen &

Wijmstra 1964. 3767' (89.7–6.3), 3673' (94.3–3.6).18 Psilatricolporites crassus van der Hammen & Wijmstra

1964. 3700' (105.1–22.5).19 Perfotricolpites digitatus González 1967. 3572' (98.5–

3.9).

TABLE 1. Pearson product-moment coefficients between the palynomorph assemblages and the taxa below 1%. Significance level(α): * 0.05, ** 0.01, *** 0.001.

EM1 EM2 EM3Bombacacidites baumfalki 0.1096 -0.1441 -0.1388Bombacacidites brevis 0.1923 -0.5312** 0.0998Clavamonocolpites lorentei 0.1286 0.0905 -0.2168Corsinipollenites oculus-noctis 0.3186 -0.0051 -0.3785*Echitricolporites maristellae -0.4762** 0.7853*** -0.0028Foveotriletes ornatus 0.0665 -0.0098 -0.1311Ilexpollenites sp. 0.1859 -0.1786 -0.1932Malvacipollis spinulosa -0.0652 0.2543 -0.1381Perfotricolpites digitatus -0.1354 0.3478 -0.1322Podocarpidites sp. 0.4691** -0.3523 -0.4291*Proteacidites triangulatus 0.1287 -0.0779 -0.1857Psilaperiporites minimus 0.1206 0.0718 -0.1970Proxapertites operculatus 0.0059 0.0012 -0.1475Psilatricolporites maculosus 0.6771*** -0.2457 -0.7173***Psilatricolporites operculatus -0.3077 0.6683*** -0.2226Psilatricolporites venezuelanus -0.1764 0.1041 0.0977Psilatricolporites triangularis 0.2878 -0.3527 -0.2090Retistephanoporites crassiannulatus 0.3406 -0.3669* -0.1147Retitricolpites simplex 0.3580* -0.0305 -0.3996*Retitricolporites hispidus 0.3617* -0.3138 -0.2963Retitricolporites irregularis 0.5083** -0.3890* -0.4308*Scabratricolporites planetensis 0.1273 0.0576 -0.2704Striatricolpites catatumbus 0.1373 -0.1027 -0.2323Echitriletes muelleri 0.5745*** -0.1980 -0.5522**Polypodiaceoisporites sp. 0.1249 -0.0403 -0.2356

morphs from palm/fern swamps continued to be domi-nant, sporomorphs from herbaceous swamps increased,and the pollen from mangroves was substantially re-duced to very few grains, probably transported landwardby wind. Therefore, the replacement of mangrove as-

semblage EM1 (coastal) by assemblage EM2 (continen-tal) across the boundary between the La Rosa and theLagunillas formations (Text-Figure 6) can be inter-preted as a regressive trend, from shallow shelf to back-mangrove environments of deposition.


8 µm


Minor taxa

Elements under 1% (called here minor taxa) were alsoinvestigated for paleoenvironmental and paleoecologicalpurposes by computing their correlations with the threeassemblages found (Rull, 1991). For this analysis, thereworked sporomorphs and the very sparse taxa withonly single occurrences in a few samples, such asBombacacidites baculatus, Bombacacidites bellus,Crototricolpites annemariae, Echidiporites barbeitoensis,Jandufouria seamrogiformis, Kuylisporites waterbolki,Polyadopollenites sp., Proxapertites tertiaria, Psila-tricolporites pachydermatus, Retibrevitricolpitescatatumbus, Retimonocolpites longicolpatus, Retitri-colpites amapaensis were excluded. Four minor taxa havehighly significant correlations (α < 0.01) with the man-grove assemblage (Table 1, Text-Figure 6): Podocarpidites,Psilatricolporites maculosus, Retitricolporites irregularis,and Echitriletes muelleri. The first taxon is the pollen of thegymnosperm tree genus Podocarpus, living in highlandrain forests of northern South America; its pollen is widelydispersed by wind. The form-species Retitricolporitesirregularis corresponds to Amanoa (Euphorbiaceae), a treefrom upper delta environments (Germeraad et al., 1968;Lorente, 1986). Little ecological information is availablefor Psilatricolporites maculosus and Echitriletes muelleri,that have been related to the families Sapotaceae andSelaginellaceae respectively, with wide ranges of occur-rence (Van der Hammen, 1963; Lorente, 1986; Tissot et al.,1988). Two other taxa (Retitricolpites simplex andRetitricolporites hispidus) show lower but still significantcorrelations (α < 0.05). They are similar to the pollen ofinland forest trees of the families Anacardiaceae andFlacourtiaceae, respectively (Lorente, 1986; and observa-tions of the author from herbarium material). Rull (1998c)included Retitricolpites simplex in an Eocene assemblagerepresenting inland forest communities.

Two taxa show highly significant correlations with theback-mangrove herb swamps component: Echitricolporitesmaristellae and Psilatricolporites operculatus. The firstone is a marker species having its first occurrence at the

base of Psiladiporites–Echitricolporites zone; therefore,the significant correlation could be due in part to evolution-ary and not environmental factors. However, when only therange of occurrence of the species is considered, the corre-lation is still significant (r = 0.6772, α < 0.001), indicatinga real statistical association. The botanical affinity ofEchitricolporites maristellae is not well established, al-though it has been related to the Malvaceae or Bombacaceae(Lorente, 1986). The most probable botanical affinity ofProxapertites operculatus is with the palm Astrocaryum,from inland and back-mangrove communities (van derHammen, 1963; Lorente, 1986). None of the taxa consid-ered have a positive significant relationship with the assem-blage EM3. However, the negative correlations of someelements are worth mentioning. Almost all the taxa withsignificant positive correlations with the mangrove assem-blage are negatively associated to the assemblage EM3,except for Retitricolporites hispidus (Table 1).

DISCUSSION

The ‘background’ palynological assemblage from palm/fern swamps is considered to be transported from inlandcommunities, probably upper delta environments, consid-ering that 1) it originated in these environments, 2) themajor transport agents for palynomorphs in coastal envi-ronments are water currents flowing towards the sea, and 3)the La Rosa and Lagunillas Formations were deposited inmore distal environments (mangroves and back-mangroves,respectively). The large amounts of this background signal,however, are surprising. In modern analog studies fromnorthern South America, such quantities are never attainedin shelf environments (Rull, 1998a). This can be explainedby an increased water-transport capacity, which involves avery humid climate, the existence of a major fluvial system,or both. Current paleogeographic reconstructions supportthese points. According to Díaz de Gamero (1996), duringthe Early Miocene, the proto-Orinoco fluvial system drainedmost of the western part of northern South America and itsdelta was situated in the northeast Maracaibo basin.

1, 2 Perfotricolpites digitatus González 1967. 3572' (98.5–3.9), 3690' (79.0–23.8).

3, 4, 5 Bombacacidites brevis Dueñas 1979. 3767' (93.3–10),3713 (106.7–8.9), 3594' (111.2–25.1).

6, 7, 8 Bombacacidites baumfalki Lorente 1986. 3755' (102.8–2.5), 3745' (82.9–21.8).

PLATE 3

9, 10, 11 Retitricolpites simplex González 1967. 3636' (110.1–13.2; 85.6–13.6), 3745' (79.4–14.4).

12, 13, Retitricolporites hispidus van der Hammen & Wijmstra14 1964. 3727' (84.3–3.4; 72.8–13.4).15, 16, Retitricolporites guianensis van der Hammen &17 Wijmstra 1964. 3636' (97.6–9.6; 110.3–6.8), 3673',

(77.8–25.0).


8 µm


The Early Miocene mangrove communities are repre-sented in this study by the assemblage EM1 of theVerrutricolporites zone (La Rosa Formation), and prob-ably associated minor taxa. The only known mangrove-forming trees present were Rhizophora (Zonocostitesramonae) and Pelliciera (Psilatricolporites crassus), form-ing an intermediate association between typical Eoceneand post-Eocene mangroves, and representing a transi-tional phase after the terminal Eocene impoverishment ofthese communities and their progressive Neogene enrich-ment (Rull, 1998b; Graham, 1995). None of the minor taxastatistically associated with the mangrove assemblage inthe present study has been previously considered a man-grove or mangrove-related element (Graham, 1995). In thecase of Podocarpus, the correlation can be explained bywind transport to coastal environments (Muller, 1959). Apotential source in the southeast could have been theincipient northern Andes, where uplift was initiated shortlybefore, during the late Oligocene (Rull, 1997b). Some ofthe other minor elements also correspond to present mon-tane taxa (Sapotaceae, Alchornea and Anacardiaceae) andhave been found in modern mangrove sediments (Rull andVegas-Vilarrúbia, 1999). Therefore, in the study area,Early Miocene mangroves seem to have been of lowdiversity. However, more work is needed to establish thebotanical affinities of many minor taxa in order to infer abetter paleovegetational history.

Compared with modern analog studies, in which themangrove pollen commonly ranges from 20% to more than70% (Muller, 1963; van der Hammen, 1963; Tissot et al.,1988), the mangrove signal found in this study is not strong(Text-Figure 4). The same is true for several sites of theregion (Root et al., 1998). This probably indicates that thesediments were deposited some distance away from anydense mangrove vegetation, although both marinepalynomorphs (Text-Figure 4) and micropaleontologicalanalysis clearly indicated a shallow-water (0–20 m depth)coastal environment of deposition (Echeverría and Ruiz,1999). Therefore, mangroves were probably reduced and/

or were not dense in the region, contrasting with other areasof the same basin and eastern Venezuela, where mangrovepollen is abundant in the Early Miocene (Lorente, 1986). A‘dilution’ effect due to the background EM3 assemblage isundoubtedly present, but insufficient to explain such lowvalues. A possible explanation for Pelliciera emerges afterthe consideration of its biogeography. Aside from doubtfulrecords in Europe and Africa (Muller, 1981), Pelliciera isknown to be common in Eocene to Miocene records ofCentral America, northern South America and the Carib-bean–Gulf of Mexico region (Text-Figure 7). A dramaticreduction began in the Miocene (Wijmstra, 1968), leadingto its present relict distribution. It disappeared from thenorthern limits of its range beginning in the Miocene(Graham, 1995), but remained as a scarce element innorthern South America until the Pliocene. Therefore, theEarly Miocene mangrove communities of the Maracaibobasin were probably affected by the initial reduction.

The relative scarcity of Rhizophora, however, remains tobe explained. In the available modern analog studies for theregion that include mangroves, similar numbers of thispollen occur only inland, in freshwater environments, mostprobably due to wind transport (Muller, 1959; ten Broekand Nijssen, 1971). However, inland fluvial paleoenviron-ments for the La Rosa Formation are unlikely because ofthe abundance of marine palynomorphs (Text-Figure 4),and the micropaleontological evidence (see above). Lowpercentages of Rhizophora pollen have been also foundwithin a disturbed mangrove stand surrounded by montaneforests (Rull and Vegas-Vilarrúbia, 1999; Rull et al., 1999).These modern pollen assemblages, however, are very dif-ferent from those found in the present work, because theyare dominated by herbaceous pollen (Gramineae,Cyepraceae, Chenopodiaceae) with negligible percentagesof marine elements. On the other hand, It is known thatRhizophora can survive in low-saline, even in freshwater,where is not as abundant as in truly brackish and marineones (Lindeman, 1953; Van der Hammen, 1963). There-fore, the possibility of shallow marine environments of low

1 Retitricolporites guianensis van der Hammen &Wijmstra 1964. 3623' (86.7–1.9).

2, 3, 4, 5 Retitricolporites irregularis van der Hammen &Wijmstra 1964. 3693' (99.3–24.1), 3686' (78.0–10.8).

6, 7, 8 Striatricolpites catatumbus González 1967. 3600'(110.2–14.5), 3481' (104.1–17.4).

9 Ilexpollenites sp. Thiegart 1937 ex Potonié 1960. 3481'(75.0–19.6).

10, 11, Echitricolporites maristellae Muller et al., 1987. 3628'

PLATE 4

12 (94.7–16.7), 3486' (95.5–21.0).13, 14 Psilaperiporites minimus Regali et al., 1974. 3636'

(105.1–20.8).15 Perisyncolporites sp. Germeraad et al., 1968. 3727'

(93.3–10.0).16 Perisyncolporites pokornyi Germeraad et al., 1968.

3628' (90.4–17.4).17 Malvacipollis spinulosa Frederiksen 1983. 3673' (77.8–

8.4).


8 µm


salinity should not be neglected. This could be supported inpart by the fact that Pelliciera can also live in low-salinitywater (Fuchs, 1970; Jiménez, 1984). According to theavailable paleogeographical reconstructions, shallow brack-ish-water environments could have existed in the northernMaracaibo basin during the Early Miocene. Saline waterwould have been provided by the marine connection situ-ated at the north of the basin (Hoorn, 1994), whereasfreshwater was probably carried by the large proto-Orinocofluvial system (Díaz de Gamero, 1996).

ACKNOWLEDGMENTS

The author wishes to thank Drs. M. Antonieta Lorente,Francisca Oboh-Ikuenobe, and David Pocknall for theircomments and suggestions that improved the clarity andquality of the manuscript. Special thanks to the formereditor of Palynology, David Goodman, for his excellent jobduring these years.

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