Planktonic Foraminiferal Biostratigraphy and Paleo-Ecologyof the Brasso Formation (Middle Miocene) at St. Fabien Quarry,

Trinidad, West Indies

BRENT WILSON

Petroleum Geoscience Programme, Department of Chemical Engineering, Faculty of Engineering, The University of theWest Indies, St. Augustine, Trinidad, West Indies, bwilson@eng.uwi.tt

ABSTRACT.—Planktonic foraminifera >0.125 mm are examined in six samples from the calcareous clays ofthe Brasso Formation at St. Fabien Quarry, Trinidad, for which they indicate an early Middle Miocene age(Globorotalia fohsi fohsi Zone, latest N10). The percentage of the foraminiferal fauna comprising planktonics(%P) is suggestive of outer neritic to upper bathyal paleo-depths, with a trend towards deeper water in theupper part of the section. Planktonic foraminiferal diversity, measured using the Information Function (H�),is significantly correlated with %P. However, there is no correlation between %P and species dominance,measured using the Equitability Index (E). This implies that, while planktonic foraminiferal diversity of the>0.125 mm fraction increased with paleo-depth or distance from shore, dominance was not affected. Globi-gerina praebulloides, rare elsewhere in the tropics after the earliest Early Miocene, is abundant in the lowerMiddle Miocene at St. Fabien Quarry. This may indicate a tropical refuge provided by local upwelling ofcool, nutrient-rich water during mid Miocene times.

KEYWORDS.—Globigerina praebulloides, upwelling, Miocene, Trinidad, Brasso Formation

INTRODUCTION

The Order Foraminiferida comprises mi-croscopic protists that abound in marineand marginal-marine environments. Indi-vidual foraminifera are mostly <1 mm insize, and most species are benthonic, al-though those in the Suborder Globigerinina(the subject of this note) are planktonic.Their size and rapid evolution make fos-sil foraminifera invaluable in paleo-oceanographic work: a small sedimentsample can yield a statistically significantnumber of specimens. Micropaleontolo-gists use fossil benthonic foraminifera pri-marily to determine paleo-environmentalconditions prevailing on the seabed at thetime of deposition of marine sedimentaryrocks, and the planktonic Globigerinina,because of their relative facies indepen-dence, to correlate rock sequences biostrati-graphically. The proportion of the entireforaminiferal assemblage comprising

planktonics may be used to discern paleo-bathymetric trends (Berger and Diester-Haas 1988).

Micropaleontological work associatedwith hydrocarbon exploration on Trinidadplayed a major role in the development ofplanktonic foraminiferal biostratigraphy(Bolli 1957; Saunders and Bolli 1979). How-ever, perhaps because the 1200 m-thick, cal-careous clays of the Miocene Brasso Forma-tion do not yield hydrocarbons, theplanktonic foraminifera therein have beenthe subject of few, mostly qualitative, stud-ies. Renz (1948) recorded the followingfrom the Brasso Formation: Globigerinoidessacculifera (Brady), Globorotalia canariensis(d’Orbigny), Globorotalia fohsi Cushman andEllisor, Globorotalia menardii (d’Orbigny), andOrbulina universa d’Orbigny. However, hegave few details of their distribution. Ku-gler (1996, map 20) indicated that theBrasso Formation at Brasso Village rangesbetween the Globigerinatella insueta and Glo-borotalia fohsi robusta Zones (Zones N8 toN12 of Blow 1969), and elsewhere (Kugler2001, p. 8) listed 24 species recovered from

ms. received December 6, 2004; accepted September4, 2005

Caribbean Journal of Science, Vol. 41, No. 4, 797-803, 2005Copyright 2005 College of Arts and SciencesUniversity of Puerto Rico, Mayaguez

797

the formation. Wilson (2003) examined thedistribution of planktonics in a ∼150 m sec-tion along the Guaico-Tamana Road. He re-corded 28 species and subspecies in the sec-tion, which ranges from the Globigerinatellainsueta Zone to the Globorotalia fohsi fohsiZone (Zones N8-N10). He noted only spe-cies’ presence/absence, but recorded thepercentage of the entire (planktonic + ben-thonic) fauna comprising planktonics. Thepercentage of planktonics (%P) was used toinfer paleo-depths (although it might alsoindicate distance from shore [see Smart2002b, p. 19]).

Renz (1948) selected the Upper CaparoValley as the type section for the BrassoFormation: however, because exposurethere is poor, and there are few others per-manent exposures, Kugler (2001) movedthe type section to the rocks drilled by thewell Montserrat-1. Material from this is notnow readily available. Wilson (2003, 2004)noted that manmade, frequently temporaryexposures can provide valuable informa-tion: this note reports a quantitative studyof the planktonic foraminiferal assemblagein a ∼7 m, manmade exposure of the BrassoFormation in St. Fabien Quarry, Trinidad,West Indies (UTM Zone 20, grid reference672300, 1143000).

MATERIAL AND METHODS

Six samples (JBW-84 to JBW-89, lowest touppermost) were collected at 1 m verticalintervals from the man-made cliff exposingthe Brasso Formation in the St. FabienQuarry. The rocks, exposed in the northface of the quarry, comprise uniform, inky-blue calcareous claystones. No sedimentarystructures, either primary or biogenic, areevident. The samples were soaked in waterfor two days, boiled for ten minutes, washedover a 0.063 mm sieve, and air-dried.

For each sample the residue was furthersieved over a 0.125 mm mesh because the0.063-0.125 mm fraction, while it may con-tain environmentally useful benthonic fora-minifera (Schroder et al. 1987), yields pri-marily juvenile planktonics that aredifficult to identify to species level (cf. Kel-logg 1984; Kandiano and Bauch 2002; Smart2002a) and thus provide little biostrati-

graphic information. The >0.125 mm frac-tion was split into two aliquots that, to con-found bias by the micropaleontologist,were sorted as follows:

Aliquot 1: this comprises ∼300 specimensof planktonic foraminifera. These weresorted into species that were used to deter-mine biostratigraphic ages according to theranges given by Bolli and Saunders (1985).Ages are stated using both the namedzones of Bolli and Saunders (1985) and theN zones of Blow (1969). The number offoraminifera picked (N) and the speciesrichness (S) was recorded, the diversity ofthe total planktonic assemblage calculatedusing the Information Function H�, anddominance determined using the Equitabil-ity Index E (see Wilson 2004).

Aliquot 2: this comprises all foraminiferapicked to a total of 200 benthonics. Thenumber of planktonics was counted andthe percentage of the total assemblage com-prising planktonics (%P) calculated.

Correlations between N, S, H�, E (Aliquot1) and %P (Aliquot 2) were examined usingthe Pearson’s product moment correlationcoefficient.

RESULTS

Biostratigraphy

Almost all planktonics in the Aliquot 1>0.125 mm fraction were readily assignedto species (Table 1). Selected species are il-lustrated on Figure 1. The entire planktonicassemblage (1972 specimens, mean 329specimens) is dominated by Globigerinoidestrilobus immaturus (24.0%) with lesser Glo-bigerina praebulloides (17.9%) and Globorota-lia continuosa (12.4%).

The majority of the 31 species identifiedhave long biostratigraphic ranges. Whenthe ranges given by Bolli and Saunders(1985) are used, the presence of Globorotaliafohsi praefohsi and G. praemenardii in sampleJBW-84, and of G. fohs i per iphero-acuta in JBW-89, indicates an early MiddleMiocene age (Globorotalia fohsi fohsi Zone,latest N10). However, Kennett and Sriniva-san (1983) apparently disagree regardingthe concordance of Bolli and Saunders’(1985) named zones and Blow’s (1969) Nzones: ranges given in their species descrip-

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Globigerinapraebulloides

Globigerinasp.

Globigerinavenezuelana

Globigerinawoodi

Globigerinellapraesiphonifera

Globigerinitaglutinata

Globigerinoidesbulloideus

Globigerinoidesobliquus

Globigerinoidesruber

Globigerinoidestrilobusimmaturus

Globigerinoidestrilobussacculiferus

Globigerinoidestrilobustrilobus

Globoquadrinaaltispira

Globoquadrinacf.altispira

Globoquadrinadehiscens

Globorotaliacontinuosa

Globorotaliafohsiperipheroacuta

Globorotaliafohsipraefohsi

Globorotaliamayeri

Globorotaliaobesa

Globorotaliapraemenardii

Globorotaliascitula

Globorotaloidesvariabilis

Orbulinabilobata

Orbulinasuturalis

Orbulinauniversa

Sphaeroidinellopsisseminulina

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MIOCENE PLANKTONIC FORAMINIFERA 799

tions suggest that G. fohsi periphero-acuta and Globorotalia fohsi praefohsi overlapin Zone N11, but elsewhere (Kennett andSrinivasan 1983, fig. 2) they equate thiswith a Globorotalia praefohsi zone not recog-nized by Bolli and Saunders (1985). For thisreason a Globorotalia fohsi fohsi Zone, but lat-est N10, age is accepted here, albeit withcaution.

Diversity

The Aliquot 1 values of H� and E aregiven in Table 2, together with the numberof specimens (N) and species richness (S).Despite the small ranges in N and S, there isa significant positive correlation betweenthese measures (r = 0.87, p = 0.025). There isno significant correlation between either Nor S and H�, but H� and E are significantlycorrelated (r = 0.834, p = 0.039).

Variations in %P

The Aliquot 2 values for %P are given inTable 2. The mean %P is 42.6% and the only

significant correlation between Aliquot 2%P and any other measure is with Aliquot 1H� (r = 0.852, p = 0.031).

DISCUSSION

%P and Paleo-Depth Interpretations

In the modern oceans %P, the percentageof the foraminiferal assemblage comprisingplanktonics, is generally correlated withwater depth (e.g., de Rijk et al. 1999). How-ever, due to factors such as freshwater in-flow, differential preservation of plankton-ics and benthonics, and upwelling, therelationship between %P and depth mayvary from one area or time to another (see,for example, Krijgsman et al. 2002). Never-theless %P has been used as a proxy forpaleo-depths (Grimsdale and van Mork-hoven 1955; Wilson 2003), and %P curveshave potential as correlation tools.

Wilson (2003), when estimating paleo-depths in the Brasso Formation along the

TABLE 2. Faunal characteristics and comparison of %P.

JBW-84 JBW-85 JBW-86 JBW-87 JBW-88 JBW-89

Aliquot 1 faunal characteristicsNumber of planktonic foraminifera (N ) 306 322 321 338 341 344Number of planktonic species (S) 20 21 19 23 26 25Information Function (H�) 2.301 2.503 2.083 2.457 2.426 2.370Equitability Index (E) 0.499 0.582 0.321 0.475 0.453 0.428

Aliquot 2 %P 31.0 48.5 31.7 50.2 50.0 44.4

FIG. 1. A. Globigerina bulloides d’Orbigny, 1826, ×68. B. Globigerina druryi Akers, 1955, ×85. C. Globigerinapseudociperoensis Blow, 1969, ×60. D. Globorotalia continuosa Blow, 1959, ×68. E. Globorotalia mayeri Cushman andEllisor, 1939, ×44. F. Globorotalia fohsi peripheroacuta Blow and Banner, 1966, ×60. G. Globorotalia praemenardiiCushman and Stainforth, 1945, ×58. H. Globorotalia scitula (Brady, 1882), ×55.

BRENT WILSON800

Guaico-Tamana Road, Trinidad, employedthe expression

depth = e(81.9 + %P)/24

that was obtained by de Rijk et al. (1999)from a study of relationships in the easternMediterranean Sea, including around theNile delta, Egypt. Admittedly this may notbe the best choice of area for comparison:the Mediterranean Sea is an evapora-tive silled basin, whereas Trinidad bor-ders the open Atlantic. However, de Rijket al.’s (1999) study has the advantage thatused only the >0.125 mm fraction (i.e., thesame as this study): it is not yet clearhow inclusion of smaller fractions might af-fect %P.

In further support of using de Rijk et al.’s(1999) expression: their study includedtransects off the Nile, and is the only one toexamine the impact of a major river on %P.The Recent fauna around Trinidad isstrongly influenced by freshwater flowingfrom the Amazon, Essequibo and OrinocoRivers (van der Zwaan and Jorissen 1991),and the SE Caribbean has long been influ-enced by freshwater runoff from SouthAmerica: A paleo-Amazon was in existenceas early as the Middle Miocene (Hoorn1994) and, while it is unclear how large thiswas compared to the modern AmazonRiver, which delivers ∼20% of the world’sfreshwater to the oceans (Dagg et al. 2004),the Guyana Current carries much Amazonwater to the SE Caribbean (Agard andGobin 2000), and may have done sinceMiddle Miocene times. Furthermore, a pa-leo-Orinoco has been delivering freshwaterto the Caribbean at least since Eocenetimes, albeit prior to mid Middle Miocenetimes through mouths in western Venezu-ela (Kugler 1953; Diaz de Gamero 1996).

When de Rijk et al.’s (1999) expression isapplied to the St. Fabien Quarry section, itindicates paleo-depths ranging betweenshallow outer-neritic (∼110 m: JBW-84) andshallow upper-bathyal (∼250 m: JBW-87).Values of %P are generally greater in theupper part of the section (JBW-87 to JBW-89), perhaps indicating deposition during atransgression.

Some workers studying benthonic fora-minifera have noted a general positive cor-relation relationship between S and paleo-depth (e.g., Li and McGowran 2000). Such arelationship might also be expected amongnearshore planktonic foraminifera. The de-crease in turbidity and freshwater influencewith increasing distance from shore mightencourage the development of a more spe-cies rich community in offshore areas.However, Buzas et al. (1977) noted that spe-cies richness (S) is positively correlatedwith the number of specimens examined(N), and this is the case in the planktonicfauna in St. Fabien Quarry (r = 0.8762, p =0.025). Because N was here determined bythe author’s picking whim, S is not usedhere for environmental interpretation.

The Information Function H� is here usedinstead of S to test relationships betweendiversity and %P. In the St. Fabien QuarryH� is not significantly correlated with N (r =0.391, p = 0.442), as is to be expected (seeBuzas and Hayek 2005), but is positivelycorrelated with %P (r = 0.852, p = 0.031).However, despite the positive correlationbetween H� and E, there is no significantcorrelation between E and %P (r = 0.462,p = 0.356). This implies that the diversity ofthe planktonic fauna increased with paleo-depth/distance from shore, but that thesefactors had no effect on overall dominance.

Globigerina praebulloides

The high relative abundance of Globige-rina praebulloides (Figure 2) in the St. FabienQuarry section is noteworthy. According toKennett et al. (1985) this species was duringmost of the Early Miocene common in sub-tropical regions, where it formed up to∼35% of the planktonic fauna. From the lat-est Early Miocene onward it retreated tohigh latitudes to form >50% of assemblagessouth of ∼40°S, but became rare in the trop-ics. However, G. praebulloides at St. FabienQuarry forms ∼35% of the total planktonicfauna in the lower Middle Miocene sampleJBW-86.

Kennett et al. (1985) state that the pole-ward retreat by G. praebulloides may reflecta switch in environmental tolerance. Itshigh abundance in the early Middle

MIOCENE PLANKTONIC FORAMINIFERA 801

Miocene at St. Fabien Quarry indicates,however, persistence in a tropical refuge.The cause of this is unclear, but it may in-dicate either seasonal fluctuation in salinityand turbidity associated with runoff fromSouth America, or upwelling off northernSouth America during Middle Miocenetimes. In the modern oceans Globigerina bul-loides, which is descended from G. praebul-loides, shows a preference for areas influ-enced by run-off or upwelling (Bé 1977;Hilbrecht 1996). There is other evidence forupwelling in the Trinidad region duringMiocene times: Wilson (2004) recorded inthe Brasso Formation a slightly older fauna(Globigerinatella insueta to Globorotalia fohsiperipheroronda Zones, 18-N9) dominated byBrizalina alazanensis and Plectofrondiculariafloridana that he suggested was typical of anoxygen minimum zone. Such dysoxic zonesdevelop beneath areas of oceanic upwelling(e.g., Sen Gupta et al. 1981).

Conclusions

The section of the Brasso Formation ex-posed in the St. Fabien Quarry is of earlyMiddle Miocene age. A Globorotalia fohsifohsi Zone age (latest N10) is proposed here.

The percentage of planktonic foramini-fera (%P), which may be used as a proxy forpaleo-depth or distance from shore, is in

the St. Fabien Quarry section of the BrassoFormation (>0.125 mm fraction) suggestiveof paleo-depths ranging between outerneritic and upper bathyal. There is a trendtowards higher values of %P in the upperpart of the section, which is suggested toindicate a deepening upward trend.

In the >0.125 mm fraction the values of%P are significantly correlated with the di-versity of the planktonic foraminiferalfauna, as measured using InformationFunction H�. This reflects an increase in di-versity with either paleo-depth or distancefrom shore.

Globigerina praebulloides during MidMiocene time largely retreated to high lati-tudes, but persisted in a tropical refuge inTrinidad. This may be related to either run-off from northern South America, or up-welling in the region, during mid Miocenetimes.

Acknowledgments.—Partial support forthis work came from the Research and Pub-lication Fund of The University of the WestIndies. Thanks are due to John Frampton(BioStratigraphic Associates, Trinidad),Robert Fleisher and an anonymous re-viewer for their constructive criticism.

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