V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 75CONTRIBUTION OF QUANTITATIVE ECOLOGICALMETHODS TO THE INTERPRETATION OFSTRATIGRAPHICALLY HOMOGENEOUSPRE-QUATERNARY SEDIMENTS: A PALYNOLOGICALEXAMPLE FROM THE OLIGOCENE OF VENEZUELA

VALENTÍ RULLUniversitat Autònoma de BarcelonaDept. Biologia Animal, Vegetal i EcologiaU. Botànica (Paleopalinologia)08193 Bellaterra, BarcelonaSpaine-mail: valenti.rull@uab.es

Palynology, 27 (2003): 75–98© 2003 by AASP Foundation ISSN 0191-6122

Abstract

This paper deals with an Oligocene section that is stratigraphicallyhomogeneous from both a lithological and palynological point ofview. It has been impossible to subdivide it into discrete units,using either taxon-range analysis or assemblage-zone approachbased on the relative abundance of palynomorphs. Furthermore,the common multivariate numerical methods used so far withsuccess in the region (Cluster Analysis, Principal ComponentsAnalysis, etc.) gave no useful results in this case. The search forcyclicity using palynocycles and ecologs has been also unsuccess-ful. Instead of considering the section of low interest, an alterna-tive, high-resolution ecological approach was attempted to ex-tract the information contained in these sediments. Paleoecologi-cal trends were deduced from statistical methods commonly usedin modern and Quaternary ecology, mainly TWINSPAN andgradient analysis, combined with diversity analysis. As a result,the fine-scale stratigraphic variability of the data could be suc-cessfully explained in terms of paleoecological succession takingplace in upper delta environments, characterised by a complexmosaic vegetation including morichales, herbaceous fern swamps,and gallery forests. The succession could be reconstructed indetail, and would be of indirect stratigraphic value for high-resolution correlation. This is an example of how the search fornarrow or biased objectives can hidden significant information. Itis more fruitful to have a wider perspective, and to be open to anyinformation that sediments can provide us, without a priorilimitations.

INTRODUCTION

Stratigraphic analysis of pre-Quaternary sediments isconcerned mainly with the recognition and differentiationof discrete rock bodies with distinctive lithological, physico–chemical and biological characteristics. These units are

easily recognisable when stratigraphic breaks (e.g. uncon-formities) separating sedimentary bodies deposited in con-trasting environments are present. In this case, the presenceor absence of indicator properties or key fossils would beenough to differentiate between stratigraphic units. Thedifferentiation becomes more difficult if environmentalchanges are small and gradual, in which case quantitativemethods are commonly needed. In biostratigraphy, thebasic stratigraphic unit which considers quantitative data isthe ‘assemblage zone’, that can be defined using only visualexamination of fossil diagrams, or through the use ofstatistical methods like cluster analysis and other relatedones. Methods developed by Quaternary palynologistshave notably contributed to these aspects (for example:Birks and Birks, 1980; Birks and Gordon, 1985; Berglund,1986). In addition, the recent development of sequencestratigraphy, based on the existence of synchronous world-wide eustatic cycles, has contributed to the tremendousincrease in the search for cycles as stratigraphic units(Wilgus et al., 1988). In their biostratigraphic expression,these cycles are mainly quantitatively-defined through therelationship between the abundance of fossils typical ofeither continental or marine paleoenvironments. In thisway, the almost forgotten approach of the ecostratigraphy(Martinsson, 1973; Rull, 1997a) has been revived, and anumber of high-resolution quantitative methods have beendeveloped (Howell and Aitken, 1996), includingpalynocycle and ecolog analysis (Poumot, 1989; Reyment,1980; Rull and Poumot, 1987; Rull, 1992a, 1997b, 2000,2001a, 2002a). Using these procedures, some sedimentarysections which had shown resistance to be subdivided into

76 PALYNOLOGY, VOLUME 27 — 2003

assemblage zones, were successfuly interpreted as cyclicsuccessions involving cycles of several orders (for ex-ample: Rull, 1999, 2000).

However, in spite of all these new approaches, somesections keep showing a homogeneity which makes theirstratigraphic subdivision difficult. This suggests that thosegeological procedures used are not adequate to extract theinformation contained in the sediments. These types ofsections are commonly neglected for stratigraphic pur-poses, but a more scientific attitude would be to use otherapproaches, in order to discover what is hidden in suchstratigraphic uniformity. A possible alternative is theecological approach, that is, the search for ecologicalprocesses in time rather than fragmentation into hypo-thetical units. Ecological changes might be more subtlethan strong environmental changes, but they are detectablewith appropriate tools, and can provide useful paleocologicalreconstruction. All these procedures are not exclussive butcomplementary, and their combined use produces a farmore clear picture than if only one or few of them areattempted.

The purpose of the present study is to illustrate the use ofseveral quantitative methods developed mainly in modernand Quaternary ecology, to adequately understand bothstratigraphic and ecological characteristics of homoge-neous sections. For this purpose, a very uniform Oligocenesection from southwestern Venezuela has been chosen.First, numerical methods including numerical zonation, aswell as palynocycle and ecolog analysis were attempted tosubdivide the section into assemblage zones and/or cycles.Since the units defined were not especially meaningful, aset of ecological methods such as TWINSPAN and gradi-ent analysis were employed, in order to understand thepaleoecological processes involved. Other methods can beused depending on the particular nature of each subject.

GEOLOGICAL ANDPALYNOSTRATIGRAPHIC FRAMEWORK

The section studied lies on the Barinas–Apure Basin ofwestern Venezuela (Text-Figure 1). In this area, the UpperEocene–Oligocene sedimentation patterns reflect a majorstratigraphic outbreak, represented in extensive areas of theMaracaibo Basin by an outstanding Middle Eocene/LowerMiocene uncomformity (Lorente et al., 1997). As a conse-quence, Oligocene sediments in western Venezuela show adiscontinuous distribution, ranging from depths around1000–1500 m in the Falcón Basin to their total absence ina wide area of the Maracaibo Basin (Osuna and Arnstein,1997). The present work is concerned with the OligoceneGuafita Formation (Text-Figure 1) originally described by

Ortega et al. (1987), and subsequently studied by Monroyand Van Erve (1988) and Kiser (1989). It consists of analternation of quartzitic and arcosic sandstones, sands,shales, claystones, siltstones and thin layers of ligniteswhich were deposited in a prograding deltaic system (Ortegaet al., 1987), most probably within the proto-Orinoco deltasystem (Díaz de Gamero, 1996).

In northern South America, the Oligocene has beencharacterised palynologically by the Magnastriatites–Cicatricosisporites dorogensis zone, a concurrent range-zone defined by the top occurrence (LAD) ofCicatricosisporites dorogensis and the basal occurrence(FAD) of Magnastriatites grandiosus (Lorente, 1986;Muller et al., 1987). The Oligocene pollen assemblagespublished for western Venezuela are typical of continentalenvironments, mainly from alluvial and coastal plains(Colmenares and Terán, 1993; Kiser, 1989; Lorente, 1986;Monroy and Van Erve, 1988; Rojas et al., 1997; Rull,1997c). Notable is the abundance of palynomorphs fromfresh- and brackish-water swamps, together with relativelylow numbers of pollen from the most important post-Eocene mangrove taxa (Graham, 1995; Rull, 1998b), andmarine indicators.

MATERIALS AND METHODS

This study was carried out on samples from well COT-1X (Text-Figure 1). The present work is concerned with theinterval 4580–4970 m, which consists mainly of shalesdeposited in a coastal paleoenvironment. The faunal con-tent is scarce, and is represented by isolated occurrences ofthe foraminifers Trochamina sp., Halophragmoides sp.,Miliammina sp., Eggerella sp., Ammobaculites sp., etc.together with some gasteropods, pelecipods, ostracods, andreworked Cretaceous representatives (Bejarano et al., 1992).According to the environmental models of Barbeito et al.(1985) and MARAVEN (1983), this assemblage is typicalof brackish-water swamps and marshes in coastal plains.

Of this interval, 60 cuttings were processed for palynol-ogy through HCl and HF digestion and density separationby centrifugation in a Zinc bromide solution. Slides weremounted with glycerine-jelly. The identification ofpalynomorphs followed the classical literature for northernSouth America (Germeraad et al., 1968; Regali et al., 1974;Muller et al., 1987), but emphasis was placed on the post-Eocene taxa depicted and described by Lorente (1986) andHoorn (1994). A selection of these taxa, including the moreimportant types, are depicted in plates I to IV. Fungalspores, dinoflagellate cysts and foraminiferal linings werenot identified taxonomically but treated as three summarycategories. Counts were conducted until both a minimum

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 77

pollen sum (all the sporomorphs, except the unknown ones)of 200 and the saturation of diversity were reached (Rull,1987).

Rare taxa were excluded from statistical analysis, in orderto avoid potential noise effects. The cut point previously usedto exclude rare taxa ranged from 1 to 2% (Rull, 1998a, 1999,2000). In the present work, the relative ‘superabundance’ oftwo types (Mauritiidites franciscoi and Laevigatosporitesvulgaris, which are commonly over 50% taken together)produce an artificial lowering of the percentages of theremaining taxa. Therefore, the threshold value for rare taxahas been reduced to 0.3%, which is approximately equiva-

lent to a 1% cut point if Mauritiidites franciscoi andLaevigatosporites vulgaris are not considered.

Diversity analysis was done using the Shannon–Weaverindex (H), which considers both taxon richness (R, num-ber of taxa) and equitability (Eq, the evenness with whichthe grains are distributed among these taxa) (Pielou, 1966,1975). The equitability is conceptually opposite to domi-nance, and is expressed here as the ratio between thediversity measured in a given assemblage and the maxi-mum H possible with the same richness; that is, if all thetaxa have the same relative abundance. By definition, itranges from 0 to 1. Dominance (D) is expressed as 1-Eq.

Text-Figure 1. Upper Eocene–Oligocene paleogeography of western Venezuela. The arrow indicates the main direction of thesediment supply, which coincides with that of the proto-Orinoco fluvial system. Summarised from Díaz de Gamero (1996),Pestman et al. (1998), Osuna and Arnstein (1997), Parnaud et al. (1995) and Zambrano et al. (1970).

78 PALYNOLOGY, VOLUME 27 — 2003

The program PSIMPOLL 3.00 (Bennett, 1994, 1998)was used to plot the diagrams, as well as to performnumerical zonations, randomisation tests and zonationmodelling. Weighted average (WA) regression was doneusing the program TRAN 1.41 (S. Juggins, University ofNewcastle). Locally weighted regression was carried outwith SYSTAT 9 for Windows. Two way indicator speciesanalysis was done with TWINSPAN 2.2a and TWINDEND0.4 (Hill, 1979; with modifications by C. J. F. ter Braak andH. J. B. Birks). Diversity, Cluster Analysis, PrincipalComponents Analysis (PCA) and Dentrended Correspon-dence Analysis (DCA) were carried out using the programMVSP 3.1 for Windows (Kovach, 1989, 1999). Univariateprocedures were carried out with ΣSimstat 1.22 for Win-dows.

Formation-water chlorinity was estimated from resistiv-ity measures of the induction log ILD. Salinity, expressedas chlorinity or g of Cl- per kg of water (‰), was estimatedusing empirical tables made from NaCl solutions (Asquithand Gibson, 1982).

RESULTS AND INTERPRETATION

Abundance Patterns and Zonation

In well COT-1X, the Oligocene, represented by theMagnastriatites– Cicatricosisporites dorogensis zone, liesbetween 4663 and 4873 m depth (Text-Figure 2). The topis defined by the last occurrence (LAD) ofCicatricosisporites dorogensis, and its base is placed at theLAD of the Eocene marker Echitriporites trianguliformis.Both LADs have been checked upwards until the top of thewell. In this case, the first appearance (FAD) ofMagnastriatites grandiosus (below the base of the Oli-gocene zone) has not been considered a suitable criterion,because downward contamination is expected in cuttings,and tops are more reliable indicators of in situ fossiloccurrence. Below the Oligocene, the Upper EoceneEchiperiporites estelae zone is recorded, due to the co-occurrence of Echiperiporites estelae, Echitriporitestrianguliformis, Cicatricosisporites dorogensis,Laevigatosporites catanejensis and Wilsonipitesmargocolpatus (Muller et al., 1987). The Lower MioceneVerrutricolporites rotundiporus–Echidiporitesbarbeitoensis zone is identified above the Oligocene, basedon the absence of Cicatricosisporites dorogensis and thepresence of Verrutricolporites rotundiporus, Retitricolpitesamapaensis, Jandufouria seamrogiformis andBombacacidites brevis (Lorente, 1986; Muller et al., 1987).Some taxa with LADs known to have occurred before theOligocene (Muller et al., 1987) are present. These re-

worked pollen and spores correspond to two main classes:a Maastrichtian/Lower Paleocene group (Concavisporitessp, Proteacidites dehaani, Annutriporites iversenii andEphedripites sp), and an Eocene group (Retibrevitricolpitestriangulatus and Wilsonipites margocolpatus). Both groupsare present in the intervals 4800–4855 m and 4725–4750 m,whereas the first group alone occurs between 4700 and4715 m. This reworking of Maastrichtian/Lower Paleoceneand Eocene rocks on Oligocene strata is indirect evidencefor the occurrence of orogenic events during the Oligocene.A similar situation was found in the Oligocene sediments ofthe Mérida Range foothills (Rull, 1997c).

There is no palynological indication of a hiatus in theEocene/Oligocene boundary, because of the apparent con-tinuity of the flora, and the lack of simultaneous appear-ances or disappearances of groups of taxa. Therefore, it canbe reasonably assumed that this boundary is comformable,and its age estimated as 33.7 Ma (Berggren et al., 1995). Atthe Oligocene/Miocene boundary, the simultaneous topoccurrence of several taxa could be indicative of a minorextinction event or a sedimentary break (either non-depo-sitional or erosive). Hence, continuity is not guaranteed,and the possible absence of some part of the upper Oli-gocene section can not be disregarded.

From this point, only the Oligocene part of the section (46samples) will be considered. The counts for the maincategories of palynomorphs are shown in Table 1, and thestratigraphic distribution of their relative abundances isdepicted in Text-Figure 3. Visually, two different parts canbe distinguished in the diagram: the lower half, characterisedby pollen values mostly around 20%, together with highpercentages of freshwater algae remains (30–40%); and theupper half, where pollen is more abundant (around 40%),and algae diminish to values of 10% and lower. Otherpalynomorphs do not show significant changes. This ap-parent inverse relationship between pollen and algae can beexplained by assuming that the lower half represents avegetation with more reduced plant cover in an environ-ment inundated by freshwater, while the upper half ac-counts for a more dense vegetation type in a less (orseasonally) flooded environment.

The site was probably not far from the coast, becausesome marine influence is revealed by the occurrence of

→Text-Figure 2 (opposite page). Stratigraphic range chart of

all the sporomorphs found in well COT-1X. Key-markertaxa for the stratigraphy are noted with an asterisx, andtheir tops marked with arrows. The reworked grainswithin the Oligocene interval are noted with the abbre-viation ‘rw’ followed by their age range (K/T = UpperCretaceous/Paleocene, E = Eocene), and their actualrange is emphasised by grey areas.

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 79

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dinoflagellate cysts and the sporadic presence of benthicforaminifers from the lower coastal plain (Bejarano et al.,1992). However, their low numbers suggest that tidalinfluence was only occasional and limited (high tides?), asis expected in upper coastal plain/alluvial plain transitionalenvironments from northern South America (Rull, 1998c).

This is supported by the faunal content (Text-Figure 3), i.e.lower coastal plain foraminifers are almost absent in thelower half of the diagram, indicating mostly freshwaterenvironments, and scarce in the upper half, suggestingsporadic tidal influence. In support of this, the averageresistivity of the formation water is high (Text-Figure 3)

Palynomorph category Total counts

Average

(per sample)

1- Pollen 5977 130

2- Fern and allied spores 5734 125

3- Fungal spores 2415 255

4- Freshwater algae 2886 63

5- Dinoflagellate cysts 41 1

Sporomorphs (1+2) 11711 255

Others (3+4+5) 5342 319

Total (1+2+3+4+5) 17053 574

TABLE 1. Total and average number of palynomorphs counted in the Oligocene samples of well COT-1X, subdivided into generalcategories.

Text-Figure 3. Percentage diagram of the main groups of sporomorphs found in the Oligocene interval from well COT-1X.Freshwater algae include Botryococcus (the major component), Pediastrum and Pseudoschizaea. Marine palynomorphs areDinoflagellate cysts and foraminiferal linings. Percentages are computed with respect to the total palynomorphs. Solid lines in the‘Marine’ curve represents x10 exaggeration. Foraminifers (presence/absence) after Bejarano et al. (1992).

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 81

and corresponds to fresh or slightly oligohaline waters(estimated chlorinity between < 0.06‰ and 0.29‰, aver-age 0.13‰). This water is assumed to be very similar incomposition to the connate water, that is the water origi-nally deposited with the sediment. A number of formationwaters are saline at the beginning and are then diluted afterthe penetration of fresh meteoric waters. In this case, this isimprobable, because the involved sediments are fine-grainedshale where water penetration is difficult. On the contrary,the most probable post-sedimentary alteration of thesewaters is some increase in ion concentration, due to physico–chemical diagenetic processes such as membrane filtration(Drever, 1997). However, this concentration is not ex-pected to be significant, due to the absence of evaporites inthe region (Kiser, 1989). On the other hand, there is no

evidence for the presence of hydrocarbons that could in-crease the resistivity measures.

The relative abundance of the pollen and spore taxaconsidered statistically significant are shown in Text-Fig-ure 4. Rare taxa altogether represent a minor percentage ofthe total assemblage. Despite the ordination of taxa byweighted averaging (WA) regression with depth as thepredictor variable (ter Braak and Looman, 1995), no clearzones can be distinguished visually. Few taxa show varia-tions with any stratigraphic significance. Indeed, several ofthem are more abundant at the lower half and fall in the leftside of the diagram, while others lie in the right side,because they are more abundant upwards.

Several methods of numerical zonation on non-trans-formed percentages were attempted and compared (Text-

Text-Figure 4. Percentage diagram of the taxa selected for statistical analyses (>0.3%) in the Oligocene of well COT-1X.Percentages were computed on total sporomorphs. Rare taxa not considered are shown as a single summary category. Solid linesindicate 10x exaggeration. Taxa are ordered by WA regression using depth as the predictor (ter Braak and Looman, 1995).

82 PALYNOLOGY, VOLUME 27 — 2003

Figure 5): binary splitting by sum of squares (BSSQ),binary splitting by information content (BSIC), optimalsplitting by sum of squares (OSSQ), optimal splitting byinformation content (OSIC), and constrained incrementalsum-of-squares cluster analysis (CONISS)(Birks and Gor-don, 1985; Gordon and Birks, 1985; Grimm, 1987). Thedivisive methods (optimal and binary splitting) producealmost the same zonation, with a repetitive pattern of twosmall upper zones (from the top to 4683 or 4686 m), and twolarger zones below, from 4683 (or 4686) to 4774 and from4774 to the base. CONISS, which is an agglomerativemethod, coincides with others only in the uppermost zone.

Given the similarity among divisive methods, the OSIC isselected here, as recommended by Bennett (1998). Aftercomparison of the zonation on the actual data with thezonation on a randomised (by samples) data set (Bennett,1996), the maximum number of zones that can be reliablydistinguished with the COT-1X data set resulted to be 17.

In order to test the repeatability of the obtained zonationpattern, which is dependent on counting features such ascounting size, potential errors, etc. a set of 100 randomsimulations (with their corresponding zonations) wasmade, maintaining the counting size for each sample.Results clearly indicate that the more consistent pattern isof three zones, with boundaries at 4674 and 4774 m.Therefore, this zonation was considered the best one forthe studied data set (Text-Figure 5). There are no elementsrestricted to a particular zone, only changes in relativeabundance can be observed. In OLG-1 Retitricolporitesguianensis, Cicatricosisporites dorogensis andDeltoidospora adriennis are comparatively higher, whileMauritiidites franciscoi shows its minimal values. Thissituation is reversed at OLG-2, and emphasised in OLG-3, where a strong decrease in Laevigatosporites vulgaris,together with an increase of Jandufouria seamrogiformisare evident.

However, despite these general features, there is stillsome remaining variability within these zones, expressedas highly oscillating trends. This may be a combination ofnoise and intra-zonal patterns that have to be resolved withother methods.

Cyclicity: Palynocycles and Ecologs

The first approach to palynocyclicity considered in thepresent work uses the methodology of Poumot (1989),which is based on the definition of five basic palynologicalphases, connected to eustatic variations in sea level andclimate, and intimately related to the depositional systemsdefined in the sequence stratigraphy framework. Thesephases are represented in the percentage pollen diagramsby the sequential occurrence of abundance peaks of fernspores, palm pollen, mangrove pollen, open forest pollenand herbs. In the present work all the elements belong to thefirst phases of the section, especially fern and forest groups,

Text-Figure 5. Results of the different methods of nu-merical zonation attempted in the Oligocene of wellCOT-1X. The optimal number of zones was determinedby the broken-stick method (Bennett, 1996). The columnat the right side (ZONES) is the subdivision chosen afterrandomisation tests and modelling (see text).

1–3 Laevigatosporites vulgaris4–5 Laevigatosporites catanejensis6–7 Verrucatosporites usmensis8 Verrucatosporites sp.9–10 Deltoidospora adriennis

PLATE 1

11 Magnastriatites grandiosus12–13 Cicatricosisporites dorogensis14 Echitriletes muelleri15–16 Verrutriletes spp.

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 83

1

5

9

11 12

13

14 15 16

10

6

78

10 mm

20 mm (11,12)

2

3

4

Plate 1

84 PALYNOLOGY, VOLUME 27 — 2003

and no representatives were found for mangroves andherbs. Therefore, the palynocycle method, although it hasbeen successfully applied to the Neotropics formerly (Rulland Poumot, 1997) is not applicable in this case.

Ecologs were originally described as logs in whichenvironmental variables are related to frequencies of spe-cies over time (Reyment, 1980). Ecologs based on quanti-tative pollen assemblages such as diversity log (DIVLOG)and Principal Components Analysis log (PCALOG) havebeen useful to find cyclic patterns in the stratigraphic record(Rull, 2000). Ecological diversity is related with syntheticecosystem properties such as taxonomical structure, com-plexity, successional maturity, energy utilisation, etc.(Margalef, 1968; Pielou, 1975), and is therefore a suitablesynecological descriptor, useful also in paleoecology (Rull,1990). In the present work, diversity is primarily controlledby equitability (r = 0.881, p < 0.001), while richness has alower influence (r = 0.315, p < 0.05). Therefore, internalquantitative re-organisations within the involved ecosys-tems are more important than appearances or disappear-ances of taxa, in determining their taxonomic structure. Forquantitative treatment, the curve was linearly detrended(Legendre and Legendre, 1998).

The PCALOG was derived from a PCA analysis on thenon-transformed percentages, using the centred covari-ance matrix. Rull (1997b) recommended to buildPCALOGs through some ratio between the more coastaland the more inland component. In the present study, thefirst three components explained more variance thanexpected under a random model (broken stick method),and were considered statistically significant. Component1 (54.31% of total variance) separates two types of veg-etation: morichal (palm stands dominated by Mauritia)and fern-dominated herbaceous swamps. The second com-ponent (21.16%) most probably reflects the general abun-dance pattern and is not useful for the analysis attemptedhere. Component 3 (11.96%) represents gallery forests.Component 1 was selected as the PCA log itself becauseferns can grow more close to the mangrove belt, whilemorichales do not tolerate brackish waters (Muller, 1959;Rull, 1998d, 2001b), and live in more inland environ-ments. As in DIVLOG, there is a significant linear trend,and the PCALOG was detrended linearly.

RPCALOG (the detrended PCALOG) suggests the ex-istence of two morichal-fern swamp-morichal cycles in theupper half of the plot (4750 upwards) and a possible, butless evident, cycle in the lower half (Text-Figure 6). Thedetrended diversity log (RDIVLOG) also shows the cyclesrecorded in RPCALOG, but in an inverse sense. Althoughthe boundaries do not coincide, the cyclicity pattern ob-tained is very similar to the quantitative zonation foundpreviously, so there is no significant gain in the strati-graphic interpretation. Autocorrelation analysis was thenapplied to both RDIVLOG and RPCALOG, to evaluate theoccurrence of periodic cycles. No significant autocorrelation

Text-Figure 6. Diversity and PCA logs smoothed using aloess regression, and cycles inferred.

17–18 Podocarpidites sp.19–21 Psilamonocolpites spp.22 Proxapertites operculatus23 Mauritiidites franciscoi24 Mauritiidites sp.25 Gemmamonocolpites sp.

PLATE 2

26 Psilatricolporites sp.27–28 Retitriporites sp. 129–32 Retistephanoporites angelicus33 Psilaperiporites robustus34 Clavatricolpites sp.

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 85

17 1819

20 21 22

23

2425

26

27 2829 30

31 3233 34

10 mm

Plate 2

86 PALYNOLOGY, VOLUME 27 — 2003

was found at any lag, hence the existence of periodicity isnot supported. Other time series analysis, like periodogramsand spectral analysis could not be used, because samplepoints are too few and not equispaced (Legendre andLegendre, 1998).

In summary, the methods commonly used in the regionto found cyclicity in the stratigraphic pollen records havenot succeed in this case.

Paleoecological Assemblages and Succession

A first trial to extract sporomorph assemblages from thepercentage data matrix was done using the same agglom-erative clustering methods previously applied with suc-cess to the Venezuelan Paleogene, that is, Gower’s (1971)similarity coefficients on log-ratio transformed data, andthe centroid clustering algorithm (Rull, 1998a, 1999). Inthis case, however, the elements that dominate throughoutthe diagram, embracing around 78.5% of the total counts,appear together in the same cluster, and the groups ob-tained seem to respond more to the general abundancepatterns of the taxa involved, than to their stratigraphicvariations. The use of other clustering algorithms such asunweighted and weighted-pair group (UPGMA,WPGMA), median and minimum variance gave similarresults.

For this reason, a divisive classification method wasemployed, i.e. the two way indicator species analysis(TWINSPAN), developed by Hill (1979) and adapted toquantitative data sets by Hill et al. (1975). The method hasbeen widely used in community ecology, and its basicidea is that each group of samples can be characterized bya group of differential taxa (the indicator taxa). Thedivision into groups is hierarchical and iterative, using asthe division point the centroid of the first axis from aCorrespondence Analysis on species abundances trans-formed to pseudo-species which are defined by theirminimum abundance or cut level (van Tongeren, 1995).In this case, nine pseudospecies cut levels were definedfor the sporomorph percentages (Hjelle, 1999): 0, 0.5, 1,2, 5, 10, 20, 50, and 70; the other defaults of TWINSPANwere accepted. Subdivision was stopped automatically byTWINSPAN for groups of 4 or less elements, or by theanalyst when the mean dispersion of a given group, ascalculated by TWINDEND, was less than 50% of the totaldispersion of the data set (Birks and Kernan, 2000). Text-Figure 7 shows the resulting hierarchical dichotomy, thatallows differentiating 14 groups. However, most of themare too small to be interpretable and their major assem-blages were considered (for example, group 10 was con-sidered instead of groups 20, 42 and 43). In this way six

main groups were obtained: 4, 6, 10, 11, 14 and 15. Group4 is anomalous as it is represented by one single samplewith the lowest palynomorph count (127), and will not beconsidered in the interpretation.

A rough characterisation of these groups in terms ofsporomorph assemblages is possible, after classificationby an ordination of taxa according to their fidelity, i. e. thedegree to which they are confined to particular groups(van Tongeren, 1995). The results are shown in Text-Figure 8, where a clear disagreement between theTWINSPAN classification of samples and stratigraphy isobserved, suggesting that subdivision into assemblagezones based on taxonomic composition is not, in this case,the more adequate description of stratigraphical varia-tion. On the contrary, the TWINSPAN results suggest thatsporomorph assemblages do not occur sequentially, but ina recurrent way. In order to interpret the groups recognised,the abundances of their main components are analysed(Text-Figure 9). The first subdivision separates samplesdominated by Mauritiidites franciscoi from others domi-nated by fern spores (mainly Laevigatosporites vulgaris,Verrucatosporites usmensis, Cicatricosisporitesdorogensis and Deltoidospora adriennis). Like in thePCA analysis, these two sporomorph assemblages repre-sent, respectively, morichal vegetation and fern-domi-nated herbaceous swamps from the upper coastal plain/alluvial plain transition. This interpretation is based onthe assumption that sporomorph assemblages have beensedimented in situ and represent local vegetation types.Studies on modern sedimentation support this view. In-deed, Mauritia has only local dispersal potential, and itspresence in sediments is almost restricted to where thispalm is growing, even in environments with high trans-port potential such as larger deltas (Muller, 1959). Fur-thermore, its abundance is highly related to the abundanceof the parent plants in the community, and percentagesbetween 10–30% are indicative of in situ morichal veg-etation (Rull, 1992b). On the other hand, other importantcomponents, especially some fern spores, are relativelyheavy due to their large size and thick sculptural elements(for example, Magnastriatites grandiosus, Cicatricosi-sporites dorogensis and Verrucatosporites species, seeplates), and are most probably locally sedimented. Fur-thermore, modern analogue studies have shown that palmpollen and fern spores of the type found are major compo-nents of the assemblages from upper coastal plain andalluvial plain sediments (Rull, 1998c).

None of the groups seem to represent a ‘pure’ vegetationtype, since both have elements of the other association;therefore, a close spatial relationship between morichalesand fern swamps is assumed (mosaic pattern?). Group 1(the fern group) is subdivided into three other groups.

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 87

Group 10 and group 11 differ only in inter-mediate components, notably Cicatricosi-sporites dorogensis and Jandufouriaseamrogiformis. The highest frequency offerns is found in group 10, and representsthe more ‘pure’ fern vegetation found. Themorichal group (2) is also subdivided intothree minor groups (6, 14 and 15) based onsmaller differences, but it is noteworthythat only group 15 can be considered torepresent the morichal vegetation proper,the others having almost the same abun-dance of Mauritiidites franciscoi andLaevigatosporites vulgaris and can be con-sidered intermediate between morichalesand fern swamps, probably successionalstages between these two major types ofvegetation. The diversity is higher in thefern swamp group than in the morichalgroup (Table 2). Individually, the diver-sity is higher in groups 4 and 10 (thepurest fern swamp) than in groups 6 and15 (the purest morichal). In all cases, thediversity is more influenced by theequitability than by richness. Indeed, themore diverse groups are not those withmore taxa, but the more equitable ones. Incontrast, the less diverse groups are thosewith one or two clearly dominant taxa(Table 2, Text-Figure 9).

Detailed paleosuccessional trends areresolved by analysing the stratigraphicalarrangement of the TWINSPAN groups,and by detrended correspondence analysis(DCA). Text-Figure 10 shows some rela-tionship between the sequencing groupsand the assemblage zones obtained previ-ously (see zonation). Indeed, in zone OLG-1 fern associations predominate, whereasin zone OLG-2, a clear oscillating trendbetween the morichal group and the ferngroup is observed, and in zone OLG-3 allthe associations present belong to themorichal group. Overall, a major trendfrom fern swamp to the morichal is ob-served. Ordination by DCA was performedon the non-transformed percentage matrix(rare species downweighted). Detrendingwas done by segments, using 26 segmentsand 4 detrending cycles (Hill and Gauch,1980). The first two axes were significantaccording to the broken-stick method ap-T

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88 PALYNOLOGY, VOLUME 27 — 2003

Stratigraphic order of samples38 41 42 43 44 21 33 35 18 27 39 13 23 28 37 46 08 45 30 05 06 07 22 10 17 31 36

1 Bomb. spp. 4 2 – 2 3 2 3 2 4 3 – – 1 – – – – 3 4 – – – – – – – –6 Malv. spin. – – 1 4 3 2 2 1 1 1 4 3 1 1 3 4 – – 1 – – – 1 – – 1 –

12 Reti. simp. 4 2 2 2 1 – 2 3 1 3 3 1 2 1 3 3 – – – – 1 – 1 2 – 1 –28 Psit. spp. 4 – – 3 – – 2 1 2 1 3 3 4 2 3 – 4 4 1 – 2 – 1 2 3 2 28 Prox oper. – 3 2 3 2 – – – 2 1 2 – 1 – – 2 1 3 2 2 – 1 1 1 1 1 –

13 Reti. guia. 4 2 – 4 3 – 2 2 – – – – – – 4 3 – 3 1 – 1 – 1 1 – 2 –19 Delt. adri. 4 5 5 6 6 5 4 6 4 5 5 5 5 5 4 6 6 6 4 3 4 3 3 3 4 4 325 Magn. gran. 4 4 1 4 4 4 3 2 1 3 3 4 4 4 – 4 4 3 1 1 1 1 3 1 5 2 43 Bomb. brec. – 2 1 – – – – – 4 3 2 – – – – 2 1 3 1 2 2 – – – 1 – –

23 Peri. poko. 4 2 4 4 3 – 3 1 3 3 3 3 3 1 – 2 1 3 2 1 1 2 1 2 1 1 25 Jand. seam. 4 5 4 4 4 4 4 5 6 5 5 5 4 5 5 7 6 5 5 6 6 5 5 4 4 6 59 Pmon. spp. – 4 1 5 4 3 4 4 4 4 2 4 4 4 3 3 – 3 5 – – 1 1 3 3 4 3

10 Psil. macu – 4 4 4 4 2 3 3 3 2 3 3 2 3 3 4 – 4 2 3 3 2 3 2 2 3 318 Cica. doro. 4 6 6 6 6 5 5 4 5 5 5 4 4 4 6 4 4 3 4 4 2 4 4 3 3 5 520 Laev. cata. 4 4 4 4 3 4 3 3 4 3 4 4 3 4 4 3 5 5 4 3 4 4 3 4 3 3 321 Laev. vulg. 6 6 7 7 7 7 6 7 7 7 7 7 8 7 7 6 7 7 7 7 8 8 7 7 7 7 727 Verr. usme. 7 5 5 5 5 5 6 5 5 6 6 6 6 6 5 4 2 4 5 4 5 4 4 4 4 5 67 Maur. fran. 6 5 5 5 5 7 7 6 7 7 6 6 6 6 6 6 7 6 7 7 6 7 7 8 7 6 7

26 Verr. spp. – 3 4 2 3 2 3 3 2 3 2 – 3 4 5 3 2 3 4 2 – 1 2 4 3 3 414 Reti. hisp. 5 2 4 3 4 3 4 3 1 2 4 – 2 1 – – – – 3 – – 1 1 1 2 3 317 Stri. cata. 4 2 2 2 2 – 3 1 1 1 2 1 1 1 – – – – – – – 1 1 3 1 2 222 Reti. cata. 4 3 – 3 2 2 2 1 1 1 – 3 – 1 – – – 4 – – 1 – 1 – – – 32 Bomb. baum. – – 3 – 1 2 – – – 1 – 1 – 1 – 3 – – – – 1 – 1 1 – 1 –

11 Reti. ange. 4 – 1 3 2 – – 3 2 1 2 2 – – – 2 – – – – 1 – 1 1 2 2 324 Psil. tria. 4 3 1 – 1 2 – 2 – – – – – 1 – – – 4 1 – 3 – 2 1 1 1 24 Ilex ilia. 4 3 1 – 1 – – – – – – – – – – – – – – – 1 – 1 1 1 1 –

15 Reti. irre. – 4 2 – 1 2 3 1 – 1 – 2 1 – – – – – 2 – 2 2 2 2 1 2 216 Reti. sp. 1 – – – – – – – – 1 – 2 – – – 3 – – – – – – – – 2 1 2 –

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 10 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 00 0 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 1 1 1

0 0 1 1 1 0 0 0 0 0 0 1 1 0 1 1 1 10 0 0 1 1 1

Stratigraphic order of samples01 09 12 14 15 19 32 40 16 20 11 24 25 26 29 02 03 04 34

1 Bomb. spp. – – – – – – – 2 – – – 1 – 1 – – – – – 006 Malv. spin. – 1 1 2 2 1 1 2 1 1 2 1 1 1 – – 1 4 – 00

12 Reti. simp. 1 – – – – 1 1 – 3 – – – – – 1 – 2 – 2 0028 Psit. spp. 1 1 – – 1 1 1 – 1 1 – 2 3 1 1 – 1 3 – 01008 Prox oper. – 1 – – 2 4 – 2 2 – 3 – – – 1 – – – – 0101

13 Reti. guia. – 3 1 1 – – 1 3 – – 3 1 2 – – – 1 – 3 010119 Delt. adri. 2 4 4 4 4 2 5 4 4 4 4 4 4 4 4 3 1 4 4 010125 Magn. gran. – 4 4 5 4 3 3 3 4 1 1 1 1 2 2 2 2 2 – 01013 Bomb. brec. – 1 – 1 1 1 1 2 – 2 2 2 – 2 – 2 2 – 2 011000

23 Peri. poko. 4 3 3 3 4 2 3 – 2 1 3 3 3 4 2 2 1 2 2 0110005 Jand. seam. 6 5 6 5 6 5 4 5 5 5 5 3 4 4 4 7 6 7 5 0110019 Pmon. spp. 1 4 2 4 3 4 4 4 5 5 4 4 4 4 4 4 4 – – 011001

10 Psil. macu – 2 3 3 2 3 1 3 4 4 – 3 3 4 4 2 3 3 3 01100118 Cica. doro. 3 4 4 4 5 5 5 5 4 4 5 4 4 4 5 3 3 3 5 01100120 Laev. cata. 2 2 3 3 3 4 4 3 3 4 2 2 3 3 1 2 4 3 4 01100121 Laev. vulg. 5 7 7 7 7 7 7 7 7 7 6 6 6 6 6 5 5 6 6 01100127 Verr. usme. 5 5 4 5 5 5 5 5 5 4 5 5 5 6 5 5 5 5 5 0110017 Maur. fran. 6 7 7 7 7 7 7 7 7 7 8 8 8 7 8 7 8 7 7 01101

26 Verr. spp. 5 2 3 4 3 – 2 3 3 5 4 4 4 5 4 4 4 4 4 0110114 Reti. hisp. – – – – – 3 3 3 4 2 2 2 3 – 2 – 1 4 4 011117 Stri. cata. – 1 1 – – 1 3 4 3 4 – – 1 1 1 1 1 2 – 011122 Reti. cata. – 1 – 1 1 2 4 – 4 5 – 2 3 3 – 2 1 3 3 01112 Bomb. baum. – – – – 1 3 – 2 3 1 – – 1 – 1 2 2 3 2 10

11 Reti. ange. 1 2 – – 3 1 1 – 3 4 – 3 1 – 2 3 1 3 4 1024 Psil. tria. – – 1 – – 2 1 – 2 – 1 1 3 3 1 3 3 3 – 104 Ilex ilia. 1 2 2 – 3 – – 2 1 1 3 2 – – – – 1 2 2 11

15 Reti. irre. 4 3 2 2 3 – 2 4 6 – 4 3 3 4 4 4 4 4 3 1116 Reti. sp. 1 – 1 – – 3 1 1 3 3 4 3 3 3 4 1 – 3 – 2 11

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 10 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 1 0 0 0 0 0 1 1 1 1 1 1 1 1 10 1 1 1 1 0 0 0 1 1 0 0 0 0 0 1 1 1 1

0 1 1 1 1

Text-Figure 8. Taxa-by-samples TWINSPAN table ordered according to the sample classification, and the fidelity of taxa (see text).Taxa are at the left side (abbreviations as in Text-Figure 9), and stratigraphic levels (1 to 46) at the top of the table. Numbers (0and 1) at the right side and at the bottom classify both taxa and samples in a dicotomical way (either 0 or 1) along 5 iterations..

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 89

plied to ordination analysis (Rull and Vegas-Vilarrúbia,1993), and account for 37.14% of the total variance. Axis1 is methodologically similar to that obtained in the firstsubdivision of TWINSPAN (see above), and reflects thesame vegetational gradient with Mauritiidites franciscoi(representing the morichal vegetation) at one side andDeltoidospora adriennis and other ferns (representing fernswamps) at the other (Text-Figure 11). However, there issome dispersion along axis 2 which is significant, indicat-ing that other factors should be considered.

In axis 2, the higher scores correspond to Striatricolpitescatatumbus, Retitriporites sp. and Bombacacidites sp., whilethe lower ones are those of Jandufouria seamrogiformis andBombacacidites baumfalki. Since all of them except theunknown Retitriporites sp. are trees typically growing onriver banks (Rull, 2002b), it is possible that the type of galleryforest accompanying water currents is the second moreimportant criterion for paleovegetation classification. How-ever, the lack of known modern analogues for these forestsprevents a more accurate reconstruction. Both Mauritiidites

franciscoi and ferns fall around the middle of axis 2, whichindicates that the occurrence of gallery forests of one type oranother may occur despite the regional vegetation is beingdominated by either morichales or fern swamps.

The diversity gradient is oblique with respect to the spacedefined by these two axes (Text-Figure 12). The highestdiversity corresponds to fern swamp vegetation accompa-nied by gallery forests of the type A (characterised byStriatricolpites catatumbus), while the lowest diversityoccurs in the region of morichal vegetation with galleryforests of the type B (characterised by Jandufouriaseamrogiformis). Paleosuccessional trends are also de-picted in Text-Figure 12, in order to compare them withdiversity gradient (simultaneous plotting would be confus-ing). The starting point, within group 11, is a mixedvegetation type of intermediate diversity (3.11), with gal-lery forests of the type B (characterised by Jandufouriaseamrogiformis). A rapid change to fern swamps of highdiversity (3.4–3.6), with intermediate gallery forests is thenobserved (group 10, the purest morichal assemblage),

Text-Figure 9. Average percentages of the major components (typically > 2%) of the first two subdivisions of TWINSPAN.Abbreviations as in Text-Figure 9.

1st division

group 1 group 2

gr. 11

gr. 10gr. 4

MAURFRAN

90 PALYNOLOGY, VOLUME 27 — 2003

followed by a shift towards intermediate vegetation again,but this time associated to the type-A gallery forest,characterised by Striatricolpites catatumbus (group 4). Inthis phase, the maximum observed diversity is recorded(> 3.6). After that, the trend crosses the transition zonegoing into the morichal, and starting an oscillation betweengroups 10 and 11 on the one side, and 14 and 15 (the purestfern swamp vegetation) on the other, coinciding with theassemblage zone OLG-2 (see also Text-Figure 10). Thegeneral aspect of this phase is not horizontal but slightlyoblique, so that in the morichal area, the gallery forests tendto be closer to type A, and in the fern swamp zone, theyapproach the type B. The differences among gallery forests,however, seem to be small, as inferred by the low disper-sion of the trend along axis 2. This oscillating trend isparallel to the diversity band of H = 3.0–3.4. Finally, analmost linear trend within the lowest diversity values(2.0–2.4), involving groups 6 and 15, goes from thetransition zone to the most extreme part of the morichalzone, where the observed paleosuccession ends.

CONCLUSIONS AND DISCUSSION

The Oligocene section of the well studied is notablyhomogeneous from a palynostratigraphical point of view. Aclear stratigraphic subdivision was not possible using both

Text-Figure 10. Stratigraphic arrangement of the groupsobtained by TWINSPAN, as compared with assemblagezones.

Group H R Eq

1 (fern swamp) 3.30 20.39 0.76

4 3.65 19 0.86

10 3.42 21.75 0.77

11 3.15 19.33 0.74

2 (morichal) 2.92 21.04 0.67

6 2.71 19.79 0.64

14 3.39 23.20 0.75

15 2.99 21.78 0.68

TABLE 2. Average of the diversity parameters, accord-ing to the TWINSPAN classification. H = Shannon–Weaver index, R = Richness (number of taxa), Eq =Equitability. See materials and methods for details.

35 Crototricolpites annemariae36–37 Bombacacidites baumfalkii38–39 Retitricolporites amapaensis40–43 Jandufouria seamrogiformis44 Malvacipollis spinulosa45 Psilatricolporites triangularis

PLATE 3

46 Psilatricolporites pachydermatus47 Psilatricolporites crassiexinatus48 Psilatricolporites operculatus49 Psilatricolporites maculosus50–51 Retibrevitricolpites catatumbus

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 91

3536 37

38 39 40

41 42 43

44

48 49 50 51

4546 47

10 mm

Plate 3

92 PALYNOLOGY, VOLUME 27 — 2003

marker-species analysis, and visual assemblage-zone ap-proach. Several methods of numerical zonation were at-tempted, the most successful being the Optimal Splitting byInformation Content (OSIC), which produced three statisti-cally-significant zones. However, a high degree of intra-zone variability remained, suggesting the existence ofcyclicity. This possibility was checked using palynocycleand ecolog analysis. It was not possible to establish completepalynocycles, due to the lack of highstand phases, repre-sented by mangorve pollen assemblages, which were absentalong the section. Two types of ecologs were used, theDetrended Residual Diversity Log (RDIVLOG), and theDetrended Principal Components Analysis Log (RPCALOG).Autocorrelation analysis of these logs did not supported theexistence of periodic cycles in the section. Three non-periodic cycles were found but, as in the case of numerical

zonation, a significant amount of intra-cycle variabilityremained unexplained. Cluster analysis (Gower similarityand centroid clustering algorithm) has been commonly suc-cessful in paleoecological studies of western VenezuelanTertiary. In this case, however, the groups defined have noeasily interpreted paleoecological meaning.

A method widely used in community ecology, the TwoWay INdicator SPecies ANalysis (TWINSPAN), was thenattempted. As a result, palynological assemblages wereclassified into two major groups: the morichal vegetation,and the fern-dominated herbaceous swamps, with severalintermediate subgroups interpreted as stages of an ecologi-cal succession. These stages alternate through the strati-graphic section, explaining the intra-zone and intra-cyclevariability previously observed. The paleoecological trendswere analysed in more detail through gradient analysis,

Text-Figure 11. Scatter plot of the the first two axes from a DCA (Hill’s scaling by samples: ter Braak, 1995), showing the arragementof taxa, abbreviated as in Text-Figure 9. Ferns are indicated with empty triangles, and the percentage of the total variance explainedby each axis is indicated in the axes legends.

52–53 Striatricolpites catatumbus54–57 Retitricolporites irregularis58–59 Retitricolporites hispidus60 Retitricolporites guianensis

PLATE 4

61–62 Perisyncolporites pokornyi63–66 Botryococcus spp.67 Pediastrum68 Foraminiferal lining

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 93

52

56

53

57

54

58

55

59

6061

62

6364 65

66 67 68

10 mm

20 mm (66)

Plate 4

94 PALYNOLOGY, VOLUME 27 — 2003

Text-Figure 12. Scatter plot of samples in the space defined by the first two DCA axes, showing A) the diversity gradient, representedthrough ‘isodiversity’ lines considering the individual diversity value for each sample, and B) the successional trends (obtainedby connecting DCA sample scores in the direction of time) and the groups obtained with TWINSPAN. The shaded area representsthe zone in which morichal and fern swamp vegetation types overlap, also called transition zone.

B

A

V. Rull: Quantitative ecological methods used in the interpretation of pre-Quaternary sediments in Venezuela 95

using Detrended Correspondence Analysis (DCA). Resultsindicate that paleosuccession is a gradual process involvingfour main vegetation types: the morichales, the fern swamps,and two types of gallery forests (A and B). The mostprobable picture is an upper delta environment, supportinga complex mosaic vegetation pattern combining thesecommunities, and changing through time in composition,diversity and spatial arrangement.

Summarizing, the information contained in the sectionseems to have a priori little stratigraphic interest, and ismostly of paleoecological nature. If the study were focusedonly to the search for a stratigraphic subdivision (eitherqualitative or quantitative) or cyclicity, the section wouldbe considered useless or of very little utility. In this way,important and detailed information about the sedimentaryenvironment and the ecological succession would be lost.Paradoxically, this paleoecological information could beuseful for stratigraphic purposes. Indeed, the stratigraphicarrangement of paleoecological assemblages (for exampleText-Figure 10) can be compared with other sequencesfrom similar environments for stratigraphic correlation.From a methodogical point of view, It is also noteworthythat the most successful methods (TWINSPAN and gradi-ent analysis) are not stratigraphically- constrained, that is,they are independent of the stratigraphic arrangement ossamples. This could be an avantage in detecting a part of thevariance that is not detectable by stratigraphically-con-strained methods, such as zonation (Bennett, pers. comm.).

The main lesson of this study is that the a priori establish-ment of limited or partial objectives (a common practice invery specialised surveys) usually prevents finding of valu-able information, that could be crucial to understand the past.More than ask simple and concrete (and sometimes subjec-tive and biased) questions to the sediments, and then discardthem if they can not give the answers we expect; the moreuseful approach is to remain completely open to what thesediments can tell us, from the widest perspective possible.Of course many more approaches and methods than thoseused in this work are potentially useful for this purpose. Thepresent study is only an example. Another lesson is that theuse of statistical methods can not substitute reasoning.Under inadequate premises, numerical techniques willprovide unsatisfactory results. In the example presented,the biased search for the subdivision into stratigraphic unitswas unsuccessful, even after the use of sophisticated meth-ods. The explanation is simple: such units do not exist.

ACKNOWLEDGMENTS

The author is grateful to H. John B. Birks for kindlyproviding some of the computer programs used in the

present work and for critically reading the manuscript.Two referees (Keith D. Bennett and another anonymous)contributed to the imptovement of the manuscript. Thanksto M. Carmen Gómez for petrophysiscal data and help intheir interpretation, Irene Truskowski and Armando Fasolafor literature and advice on foraminifers and dinoflagel-lates, and Rosina Pittelli for geological input and discus-sions.

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