41DETECTION OF REWORKED NANNOFOSSILS

ASSEMBLAGE-BASED BIOZONATIONS: A KEY TOOL IN THE DETECTIONOF REWORKED CALCAREOUS NANNOFOSSILS

RICHARD A. DENNEApplied Biostratigraphix, 51 Midday Sun Place, The Woodlands, Texas 77382, U.S.A.

Present address: Marathon Oil Company, 5555 San Felipe Rd., Houston, Texas 77056, U.S.A.e-mail: radenne@marathonoil.com

ABSTRACT: Determining the age of oil-well cuttings samples with microfossils is problematic in wells with a significant amount of reworkedfossils. In addition, caving is a common problem in cuttings and a species’ base cannot be used with any confidence. If an age-group plotindicates that some of the fossil markers are older than the overall assemblage would suggest, then a biostratigraphic zonation that utilizesassemblage events (e.g., downhole increases, dominance shifts, and morphologic shifts) is needed to correctly determine the age of thesediment. This is especially true if the age of the reworked fossils is only slightly older than the age of the indigenous assemblage.

A well from the slope of the Gulf of Mexico sampled 1500 m of Pleistocene section with numerous reworked Pliocene specimens. Utilizinga zonation scheme with 18 horizons based on nine last occurrences and 23 assemblage changes, nine events were identified. Twelve specieswere interpreted to be reworked.

In an example from the upper Pliocene of the Gulf of Mexico, a well was noted to contain a large number of reworked nannofossils fromthe lower Pliocene and upper Miocene. Many of these reworked species have their extinction points within nannofossil zone NN16 (basalupper Pliocene) or NN15 (uppermost lower Pliocene). A high-resolution, assemblage-based zonation scheme of the Pliocene was used,which utilizes 26 species to define 23 separate horizons. Five upper Pliocene events were identified using the species assemblage changesand highest occurrences. Eighteen species were interpreted as being reworked.

A third example is a well from the Gulf of Mexico that contained rare but consistent occurrences of upper Eocene calcareous nannofossilsin what was believed to be an Oligocene section. The zonation scheme used to subdivide the section utilizes 28 species to define 27 eventswithin the Oligocene and uppermost upper Eocene. The species were divided into four age groups, which, along with the species’assemblage changes and highest occurrences, were used to identify 11 events within the Oligocene to uppermost Eocene. Fourteen specieswere determined to be reworked in the section.

KEY WORDS: assemblage changes, biostratigraphy, biozonations, calcareous nannofossils, Eocene, Oligocene, Pleistocene, Pliocene,reworking

Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. JonesSEPM Special Publication No. 93, Copyright © 2009SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-137-7, p. 41–55.

INTRODUCTION

As oil exploration has steadily moved into ever deeperwaters, calcareous nannofossil biostratigraphy has become avital tool for stratigraphic correlations and real-time drillingdecisions (e.g., casing points and horizontal drilling). Biostrati-graphic studies of exploration and production wells generallyrely on cuttings samples, which are often contaminated bydownhole caving of rocks from higher up in the well. Cavingmakes it very difficult to accurately determine a species’ truefirst occurrence (FO) (Fig. 1), so most industry zonations arebased on last occurrences (LOs) (Varol, 1989). However, thisreliance on (LOs) in industry zonations makes the problem ofreworking more difficult to solve.

Calcareous nannofossils are defined as calcareous fossils thatare smaller than 30 microns. The most common forms, “cocco-liths” and “nannoliths,” are believed to be the disaggregatedcalcitic plates of haptophyte algae (Bown and Young, 1998). Dueto their size within the clay (< 4 microns) and silt (4–62.5 microns)fractions, calcareous nannofossils are reworked (eroded and thenredeposited into younger sediments) in greater numbers thanmost other fossil groups, and in some cases they are the onlyfossils that are reworked (Bramlette and Riedel, 1954). The stan-dard calcareous nannofossil zonations (e.g., Martini, 1971; Okadaand Bukry, 1980; Sissingh, 1977) rely extensively on species’ FOsbecause reworking often obscures a species’ true LO, althoughthere are still a number of zonal boundaries defined by LOs. ThePliocene, for example, has five of its seven zonal boundariesdefined by LOs as described by Martini (1971).

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FIG. 1.—Plot of the abundances of in situ Cretaceous and downholecaved Tertiary calcareous nannofossils in a Gulf of Mexicowell, demonstrating caving of Tertiary specimens down to atleast 600 m below the top of the Cretaceous.

RICHARD A. DENNE42

Reworking was recognized as a problem in even the earli-est studies on the biostratigraphic use of calcareous nannofos-sils. Two early studies (Hamilton, 1953; Bramlette and Riedel,1954) noted that the calcareous nannofossil flora found within“obviously” reworked foraminifera (individually crushed andsmeared onto a slide) differed greatly from the assemblagesfound within in situ specimens. In many cases, the age differ-ence between the reworked and in situ flora is large enoughthat the reworked specimens are readily detected. Reworkedspecimens are common enough that they have been used forlocal zonations in the Black Sea (Bukry, 1974; Percival, 1978),local correlations in the Gulf of Mexico (Jiang, 1993), determi-nation of sediment provenance in the Arctic (Gard and Crux,1994), and detection of river discharge in Mediterraneansapropels (Negri et al., 1999).

Unfortunately, there are examples where the reworkedspecimens are only slightly older than the in situ nannoflora,making it difficult to establish an accurate zonation. In somecases it is possible to use preservational state to identifyreworked specimens (Stant et al., 2004), although it is notunusual for reworked specimens to be as well preserved as thein situ flora, possibly because they were transported withinsediment clasts (Perch-Nielsen, 1985). In core studies it may bepossible to determine the presence of reworked zones on thebasis of sedimentological identification of slumps (Su et al.,2004) and mass-flow deposits (Robertson, 1998), but this isgenerally not feasible with cuttings samples. If the reworkingis restricted to the calcareous nannofossil assemblage it may bepossible to utilize other fossil groups to recognize the presenceof reworked specimens (Demchuk, 2000). Multivariate analy-ses have been successfully used to determine reworked fora-miniferal assemblages where only nonquantitative data wereavailable (Holcová and Maslowská, 1999), but this successmay be in part due to the different environmental nichesinhabited by various benthic foraminifera.

The most common technique for detecting reworked cal-careous nannofossils in deep-sea sediments assumes that in-digenous occurrences are relatively continuous and of higherabundance than reworked occurrences (e.g., Backman andShackleton, 1983; Rio et al., 1990; Holcová, 2005). This methodrequires the use of quantitative or semiquantitative data. Indi-vidual species’ abundances are plotted either as the totalnumber per area (field of view or square millimeter) or as apercentage of a given group (e.g., discoasters). Discontinuousoccurrences or abundances significantly lower than the “nor-mal” abundance are then identified as possibly being re-worked. Although this technique works well in condensed,high-abundance deep-sea sections, the continental-margindeposits drilled in the search for hydrocarbons are much moreexpanded and usually have considerably lower microfossilabundances. Calcareous nannofossils are also distributed muchmore unevenly in continental-margin deposits than in deep-sea deposits, with most occurrences in discrete floral “pulses”which are a function of both depositional environment (Everett,1982) and the transgressive–regressive cycles of sequencestratigraphy (Shaffer, 1987; Jiang and Watkins, 1992). There-fore this technique is not as useful for hydrocarbon explorationwells.

To successfully address the problem of reworking in hy-drocarbon wells, a two-step technique is proposed. This tech-nique, outlined in the following sections, involves using “fos-sil age group plots” and assemblage-based zonation schemesto detect reworked microfossils and establish accurate bio-stratigraphic zonations.

METHODS

Sample Examination and Fossil Distribution Plots

All of the samples from the examples described here arecuttings samples taken at 30 foot (≈ 10 m) intervals from Gulfof Mexico exploration wells. Representative shale chips fromthe cuttings are wet ground using a mortar and pestle and thensmeared onto a slide. The calcareous nannofossils were countedusing a modified version of Styzen’s (1997) cascading counttechnique; abundances were estimated based on four traversesinstead of one, and the actual numbers are recorded ratherthan using a mnemonic scheme. Rare species identified afterthe fourth traverse were given a value of one. The data wereentered into the biostratigraphic data program “StrataBugs,”which was used to create the plots exhibited here. Fossilabundances are then plotted with the marker fossils dividedinto various age categories (e.g., by zones, series, etc.) (Crux etal., 2005a, 2005b).

Assemblage-Based Zonations

One technique of formulating an accurate biostratigraphiczonation when reworked fossils are present is to concentrateon assemblage changes that have previously been identified asreliable events for correlation. Unless the reworked assem-blage is more abundant than the indigenous assemblage, theseassemblage-based events should be easier to identify and lesssusceptible to inaccuracies. They can be used to detect thepresence of reworked fossils by identifying species occur-rences that do not fit the overall event succession as deter-mined by the assemblage changes, as well as those occurrencesthat are not supported by the total assemblage. They can alsobe used to produce an initial zonation that is independent ofspecies LOs and FOs.

In producing an assemblage-based zonation for industrialuse, focus is placed on downhole increases and acmes (zonesof highest abundance), dominance shifts, and changes in mor-phologic trends (e.g., size, coiling direction in planktic fora-minifera). In an example with four species, four differenthorizons are delineated by five assemblage changes which

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FIG. 2.—Diagram of the abundances of four hypothetical fossilspecies through time, demonstrating how assemblage changescan be used to create a biostratigraphic zonation.

43DETECTION OF REWORKED NANNOFOSSILS

include increases and acmes of species A and B, and a domi-nance shift from species C to species D (Fig. 2). The calcareousnannofossil Discoaster quinqueramus–D. berggrenii–D. bergeniilineage is an excellent example of how morphologic trends canbe used to define biostratigraphic horizons. With sixmorphotypes from this lineage, nine separate horizons can beidentified within the upper Miocene calcareous nannofossilzone NN11 (Fig. 3).

The first step with this technique is to make a biostrati-graphic interpretation relying on the assemblage changes, butalso using the LOs when they fit the general succession ofevents. After an initial zonation is formulated (skipping overportions of the well with significant reworking), then one can goback and identify those specimens that are older than theassemblages can support. As stated previously, caving cansmear younger assemblages downhole into older sediments, sothe succession of events both above and below the problem zonehas to be taken into account. The next step is to reexamine the

portions of the well with reworking and formulate a zonationthat fits reasonably with the rest of the well. The final step is toreconsider those LOs not used in the initial zonation to deter-mine if they are in situ, and if so, to integrate them into a finalzonation.

EXAMPLE 1—GULF OF MEXICO PLEISTOCENE

Setting

The first example is from an exploration well drilled on theslope of the northern Gulf of Mexico (GOM). The Pleistocenesection examined in the well was 1500 m thick and containedtwo zones of reworked nannofossils. These zones were ini-tially identified by the presence of Pliocene species in samplesthat contained a predominantly Pleistocene assemblage. Al-though the Pliocene reworked specimens were readily identi-fied, several Pleistocene marker species (Helicosphaera selliiand Calcidiscus macintyrei) are common in the Pliocene, intro-ducing the possibility that their highest occurrences are actu-ally reworked.

Zonation Scheme

To zone the well, a Pleistocene scheme introduced by Denne(2003) was used. This scheme utilizes a combination of nine LOsand 23 assemblage changes of 14 species (Plate 1) to delineate 18horizons in the Pleistocene of the Gulf of Mexico, 16 of which canbe identified without using LOs (Fig. 4). This compares to Martini’s(1971) three-zone zonation for the Pleistocene, and Okada andBukry’s (1980) four-fold Pleistocene zonation.

Results

Beginning with the highest abundance peak (Fig. 5), Event 6was picked at about 40 m (all depths are given in meters belowthe uppermost sample) based on increases in both smallGephyrocapsa spp. and Pseudoemiliania ovata, and is supported bythe common occurrence of Gephyrocapsa parallela. The highestoccurrence of Gephyrocapsa sp. “A” suggested the top of Event 7at about 300 m, but because it occurred without the usualincrease in Gephyrocapsa margerelii specimens, it was initiallylabeled as a “possible” top. The next abundance peak (1050–1070 m) was problematic, in that it contained both Pleistoceneand Pliocene species. A minor increase of Gephyrocapsa sp. “A”and the highest occurrence of Reticulofenestra asanoi were foundwithin this abundance peak, which mark Events 8 and 8a,respectively. Inasmuch as these fit the general succession, thispeak is initially labeled as Event 8a. The remainder of thePleistocene is relatively condensed. The assemblage changesagree with the succession, with Event 9 (Pseudoemiliania spp.increase) at 1130 m, Event 10 (a weak expression of the smallGephyrocapsa spp. acme; due to the abundance variations pro-duced by sedimentation rates, sampling, and wellbore condi-tions, acme events are picked at the top of a peak of highestrelative abundance rather than on the sample with the highestabsolute abundance) at 1170 m, Event 11 (R. asanoi increase,large Gephyrocapsa sp. top) at 1220 m, Event 12 (large Gephyrocapsasp. increase) at 1440 m, Event 13 (Gephyrocapsa caribbeanicaincrease) at 1460 m, and a possible Event 14 at 1510 m, based ona weak increase of H. sellii. A large turnover in the flora wasfound at 1550 m, which is interpreted as an unconformitycutting into the Pliocene.

With this assemblage-based zonation, the occurrences of markerspecies are then examined to determine if they fit the overall

Time

A

B

C

D

E

FYounger Older

FIG. 3.—Generalized abundances of six morphotypes of theDiscoaster quinqueramus–D. berggrenii–D. bergenii lineage foundwith zone NN11 (Martini, 1971). The morphologic shiftswithin this lineage can be used to define at least ten separatehorizons. Photos shown are all in phase contrast. (A) D.quinqueramus, (B) D. berggrenii, (C) D. berggrenii “A”, (D) D.berggrenii “C”, (E) D. berggrenii extensus, (F) D. bergenii.

RICHARD A. DENNE44

succession of events. In this example, all of the Pliocene species(Cycloperfolithus carlae, Discoaster spp., Reticulofenestra spp., andSphenolithus abies) are interpreted as reworked, as well as thelowermost Pleistocene marker C. macintyrei. Because the LO of H.sellii is associated with Event 12, the occurrences of this markerspecies above 1440 m (40 m; 1050–1060 m) are believed to bereworked. Overall, nine events were identified in the Pleistocenesection of the well, and 12 species were interpreted to be reworked.

EXAMPLE 2—GULF OF MEXICO PLIOCENE

Setting

The second example is also from an exploration well drilledon the slope of the northern GOM. The section examined here isapproximately 400 m of Pliocene sediments. A total of 66 speciesand morphotypes were identified (Fig. 6). Although the floralassemblages are suggestive of the upper Pliocene, lower Plioceneforms are common, and Miocene and older species were alsofound in the some of the samples. Because reworked Cretaceousspecies are endemic throughout the entire Neogene section of theGulf of Mexico, they are not discussed here.

Zonation Scheme

As noted in the introduction, the standard zonation schemes ofthe Pliocene are unusual in that most of the zones are defined byLOs. The zonation used here is a compilation of the standardzonation schemes, previous GOM studies, and observations fromnumerous (hundreds of) GOM wells. It was initially introduced byBreard et al. (2000). Twenty-three horizons were identified in thePliocene of the GOM based on 21 LOs and 22 assemblage changes(Fig. 7) of 26 species (Plates 1, 2), significantly more than Martini’s(1971) eight zones and Okada and Bukry’s (1980) seven zones.

Results

The age-diagnostic species were divided into typical upperPliocene, lower Pliocene, and Miocene to Eocene forms (Fig. 8).Sphenolithus abies and S. neoabies were plotted within the lowerPliocene age group, because they are very rare in the upperPliocene and their increases in abundance occur within the lowerPliocene. Some long-ranging and Pleistocene species were left offthe plot. Within each age group, the species are plotted in orderof highest occurrence. Although species’ occurrences from theolder two age groups are found throughout the section, there aretwo zones where they are concentrated: the upper zone at about 50m, and the lower zone at the bottom of the well from 380 to 400 m.

Although the uppermost samples contained few nannofos-sils, Reticulofenestra minuta was found to be the dominant form,indicating a position below Event 20. A minor increase in abun-dance was found just above 50 m, corresponding to the upperpotential reworked zone. The highest occurrence (HO) of Discoasterpentaradiatus was found at this level, which confirms the sub-Event 20 interpretation. A second, somewhat stronger, abun-dance increase was found from 80 to 120 m, but no older eventswere identified.

The highest strong abundance peak was found from 150 to 190m. The first sample within this peak contained a minor increaseof small Gephyrocapsa spp. and the HO of Umbilicosphaera cricota.The most abundant flora was found in the last sample (185 m) ofthe peak. This sample contained the HOs of Discoaster asymmetricusand Discoaster surculus, and increases in the abundances of D.pentaradiatus and the Discoaster brouweri group. Accordingly thissample is assigned to Event 22, inasmuch as the assemblagechanges match well with the increase in Discoaster spp. associatedwith Event 22. A minor increase at 210 m contains essentially thesame assemblage with the inclusion of an increase in the numberof D. surculus specimens, supporting this interpretation.

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45DETECTION OF REWORKED NANNOFOSSILS

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1B 5 micronsPLATE 1.—A) Pleistocene calcareous nannofossil marker species. All photos taken with a light microscope (CN, crossed nicols; PC,

phase contrast). 1. Calcidiscus macintyrei (a) CN (b) PC. 2. Emiliania huxleyi (CN). 3. Large Gephyrocapsa sp. (CN). 4. SmallGephyrocapsa spp. (CN). 5. Gephyrocapsa sp. “A” (CN). 6. Gephyrocapsa caribbeanica (CN). 7. Gephyrocapsa margerelii (CN). 8.Gephyrocapsa parallela (CN). 9. Helicosphaera inversa (CN). 10. Helicosphaera sellii (CN). 11. Pseudoemiliania lacunosa (CN). 12.Pseudoemiliania ovata (CN). 13. “Reticulofenestra” asanoi (CN). 14. Scyphosphaera pulcherrima (a) CN (b) PC. B) Pliocene calcareousnannofossil marker species. 1. Amaurolithus delicatus (a) CN (b) PC. 2. Amaurolithus tricorniculatus (a) CN (b) PC. 3. Calcidiscusmacintyrei (a) CN (b) PC. 4. Ceratolithus armatus (a) CN (b) PC. 5. Coccolithus pelagicus (a) CN (b) PC. 6. Cycloperfolithus carlae (a) CN(b) PC.

RICHARD A. DENNE46

The next strong increase in abundance was found in thesample from 265 m. Of note in this sample is the HO of C. carlae.Although termed an “increase” of C. carlae in the zonation, thisincrease at Event 22a is not much stronger than a consistentoccurrence, so the C. carlae HO at 265 m may represent a weakdevelopment of Event 22a. A minor abundance peak was foundat about 310 m containing the HO of D. tamalis, which is believedto be lower than its LO (Event 21).

The largest abundance peak in the well occurred from 335 to370 m. The sample at 340 m contained increases of C. pelagicus,D. asymmetricus, and C. carlae, as well as the second downholeoccurrence of D. tamalis. The abundances of C. carlae wererelatively high, so it was initially labeled as its acme point. Allof these assemblage changes either indicate or support an inter-pretation that this horizon is Event 23. The next two samplescontained the same overall assemblage. The last sample in thepeak (365 m) was the most abundant, and contained a very

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FIG. 5.—Plot of the abundances of 41 calcareous nannofossil species from the Example 1 well in the Gulf of Mexico, demonstratinghow an assemblage-based zonation can be used to determine a biostratigraphic zonation (far right) and then identify reworkedspecimens. Depths shown are in meters below the highest sample.

strong increase in the numbers of small Gephyrocapsa spp. aswell as an increase in D. tamalis. Because the former increase isassociated with Event 23a and the latter with Event 23, thissample is believed to represent Event 23a.

All of the specimens in the lower Pliocene age group, withthe exception of S. abies and S. neoabies, have their LOs at orbelow Event 24; thus their occurrences above 370 m are circledas being reworked, and all of the occurrences of the Miocene–Eocene age group are circled. The occurrences of S. abies and S.neoabies above Event 23 (335 m) are also circled, while theirhighest occurrences at or below this sample are marked as theirHO. As noted above, there is a zone of reworking at about 45 m,with scattered occurrences in most of the subsequent samplesdown to the last two samples (385 and 395 m). In these twosamples, especially the last one, strong increases in a number ofthe reticulofenestrids (Reticulofenestra and Dictyococcites) arefound, with the highest occurrences being that of R. haqii. By

47DETECTION OF REWORKED NANNOFOSSILS

Dep

th (

m)

0

50

100

150

200

250

300

350

400

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64 5 2 80 1 1 2 3 96 5 4 1 80 150 2100 24 1 128 160

20 40 2 48 5 2 1 96 6 1 20 120 2400 96 5 40 72

240112 1 1 1 7 1 1 1 2 1 16 52 16 3 28 1 1 3201 240 1 6 1 4 260 900 1 5000 150 5 1 14 4 1 1 2 14 56

1 1 3 1 1 2 240 2 1 1

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10 4 1 1 4 1 3 1 14 7 1 6 24 200 4 1 1 3 1

1 1 4 28 2

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32 14 1 1 3 1 1 1 10 18 1 2 1 1 1 24 6 1 16 1 2 1 48 80 1 2100 128 2 20 128 24

3 6 2 1 3 1 1 12 20 3 120 1 1

4 1 5 2 1 20 4 1 1 600 24 1 56 1

28 28 1 4 1 3 1 3 6 3 2 1 1 72 1 1 20 1 40 180 1 1200 3 2 1 1 5 1 1 48 1 40

1 3 1 2 1 1 1 36 1 2 2 1 2

200 2601 1 45 10 90 48 1 20 112 64 24 1 2 1 1 34040 1 220 1 2 120 600 3600 220 1 1 1 4 24 20 2

220 3202 1 20 2 1 56 120 4 12 112 220 5 8 380 1 150 1 1 5 96 900 4500 200 2 3 3 10 56 3

150 220 1 3 1 56 3 112 32 2 32 180 80 1 1 32 3201 112 1 3 150 1200 6000 320 1 1 1 4 2 4

340 260 1 1 48 4 72 180 16 14 180 1 64 1 16 2 1200 36020 3 2 120 1500 7500 56 1 1 1 1 5 40 1 1

20 3 1 4 16 200 1 1 1 1 24 3 1 4 1 1500 24 96 8 2 2 1 1 40

2 2 1 40 2 48 300 3 4 28 1 3 3 1 28 1 1 900 220 48 32 1 3 1 56 1

FIG. 6.—Abundances of 66 calcareous nannofossil species and morphotypes identified in a 400 m Pliocene section in a well from theGulf of Mexico. Depths shown are in meters below the highest sample.

themselves, the reticulofenestrids indicate a position at or be-low Event 32a (Dictyococcites antarcticus increase) near the Mi-ocene–Pliocene boundary, indicating either a fault or an uncon-formity at 385 m. Sphenoliths did not exhibit an increase in thesesamples, and no lowermost Pliocene (e.g., Ceratolithus acutus) oruppermost Miocene (e.g., Discoaster quinqueramus) markers wereidentified, and a number of Eocene–Miocene forms were foundin them. It is highly likely that this is a reworked zone and notmissing section. There is also no indication of a large fault orunconformity in the nearby control wells, supporting this inter-pretation.

EXAMPLE 3—GULF OF MEXICOOLIGOCENE–UPPER EOCENE

Setting

The last example is from the Oligocene to the uppermostEocene portion of a GOM exploration well, which covers about325 m of section. Sixty-eight species were identified in the samples,ranging in age from upper Oligocene to lower Eocene (Fig. 9).This section is somewhat condensed and generally fossiliferous;

obvious abundance peaks are not present in the data. As with thePliocene, the global zonations for the Oligocene rely heavily onLOs, so it is critical that reworked specimens be identified accu-rately.

Zonation Scheme

The scheme utilized here (Denne, 2007) is a compilation ofLOs from the global schemes (Martini, 1971; Okada and Bukry,1980), LOs from more recent studies (Berggren et al., 1995; deKaenel and Villa, 1996), and LOs from the Gulf Coast (Bybell,1982; Siesser, 1983). It has been recognized for some time, how-ever, that many Paleogene calcareous nannofossils exhibit strongpaleoecologic affinities (see Bukry et al., 1971; Bybell and Gartner,1972), making it difficult to apply a zonation scheme formulatedin the onshore Gulf Coast section to deep-water GOM sediments.The assemblage changes employed here are primarily from ob-servations made from examination of both onshore Gulf Coastwells (Louisiana and Texas) and offshore GOM wells. Twenty-seven horizons were identified using 27 LOs and 20 assemblagechanges (Fig. 10) of 28 different species (Plates 2, 3). This resolu-tion is significantly better than the seven zones and subzones

RICHARD A. DENNE48

produced by a combination of the standard zonation schemes(Martini, 1971; Okada and Bukry, 1980).

Results

The age-group plot of the data for Example 2 was divided intofour groups: upper Oligocene or older, lower Oligocene or older,upper Eocene or older, and middle Eocene or older (Fig. 11).Because Isthmolithus recurvus is very rare in the lower Oligocene,it was plotted in the upper Eocene group. Upper Eocene specieswere restricted to the section below 150 m, but with the exceptionof the last two samples they were not found to be concentrated inany particular sample. The species from the middle Eocene andolder group were limited to single occurrences in various samples,so no reworked zones were obvious.

The uppermost sample contained common occurrences ofDictyococcites bisectus and Sphenolithus ciperoensis and a singlespecimen of Sphenolithus distentus, suggesting a position at orbelow Event Pa5 in the upper Oligocene. The HO of Sphenolithuspredistentus occurs at 15 m, indicative of Event Pa6, and then itsincrease at 40 m, marking Event Pa7 in the basal upper Oligocene.

Single occurrences of Reticulofenestra hillae and the middleEocene species Sphenolithus furcatolithoides were found above 40m, so both were circled as reworked. A single specimen ofHelicosphaera compacta was identified at 57 m, which was labeledinitially as its tentative in situ HO, marking Event Pa8 in the lower

Oligocene. Single specimens of Discoaster tanii ornatus andReticulofenestra laevis were found in the next sample at 67 m,indicating that the section was at least as old as Event Pa7a. Thenext significant occurrences were found at 95 m, where the HOsof Sphenolithus pseudoradians and Ericsonia formosa were identi-fied, as well as three more specimens of H. compacta. Because theoverall assemblage does not support the presence of E. formosa, itis circled as probably being reworked. The HO of D. tanii tanii wasidentified at 110 m, along with two more specimens of H. com-pacta. The next occurrence of D. tanii tanii was not seen until 140m, where it was found along with an increase of H. compacta andthe second downhole occurrence of S. pseudoradians. Below 140 mthe occurrences of D. tanii tanii were relatively consistent. Basedprimarily on the increase of H. compacta, Event Pa8 is placed at 57m and Event Pa9 at 140 m, and the occurrence of D. tanii tanii at110 m is interpreted as being reworked. Alternative interpreta-tions could consider the H. compacta at 57 m as being reworked,put Event Pa7a at 67 m based on the HO of R. laevis, place EventPa8 at 95 m, where the S. pseudoradians HO and the secondoccurrence of H. compacta were found, and move Event Pa9 to theD. tanii tanii HO at 110 m.

A number of older species were found at 150 m, including thesecond occurrence of E. formosa, and the HOs of Reticulofenestraumbilica, Cribrocentrum reticulatum, Helicosphaera heezenii, and thelower Eocene form Zygodiscus adamas. Subsequent samples downto 185 m also contained various Eocene species and a relatively

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OccurrenceMorphologic

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FIG. 7.—Generalized abundances of the 26 Pliocene calcareous nannofossil species illustrating the 23 horizons of the Pliocene of theGulf of Mexico (after Breard et al., 2000).

49DETECTION OF REWORKED NANNOFOSSILS

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Reworked Increase Acme HO Highest In SituOccurrence

INC ACME

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Upper Pliocene & Older

Dep

th (

m)

0

50

100

150

200

250

300

350

400

INC

INC?

HO

INC

HO

INC

INC

HO

HO

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FIG. 8.—Fossil age-group plot of the abundances of 33 age diagnostic calcareous nannofossil species found in the Example 2 well,divided into three age categories. The assemblage changes are noted that were used to produce the zonation at far right as wellas to determine which specimens are in situ and which were reworked.

consistent occurrence of E. formosa, but no significant change inthe overall assemblage. At 185 m an increase of R. laevis was notedalong with the HO of R. hillae, which would not be expected if thesection was actually Eocene or below Event Pa13 (E. formosa LO).Abundances of R. laevis remained frequent for the next fewsamples, so the E. formosa and Eocene species were circled asreworked.

The next potential assemblage change was identified at 230m, where a much larger increase of R. laevis was found alongwith the second occurrence of R. hillae. The present scheme doesnot include an acme of R. laevis, hence this change was not usedin the zonation. The next assemblage changes were noted atabout 267 m, where increases of R. hillae and Lanternithus minutusoccurred. This sample also had four specimens of Sphenolithusakropodus, the first occurrence of E. formosa below 190 m, and asingle specimen of Discoaster barbadiensis. The next sample (277

m) contained a strong increase of E. formosa, two specimens of C.reticulatum, and two specimens of D. barbadiensis. Ericsoniaformosa remained common down to the last sample (322 m),where increases were found of R. umbilica, C. reticulatum, D.barbadiensis, and D. saipanensis, as well as the only specimen ofI. recurvus. The assemblage in this sample is a perfect match forEvent Pa16a in the upper Eocene, so it is used to help interpretthe assemblages higher in the well. Cribrocentrum reticulatumwas consistent up to 277 m; thus Event Pa16 was placed at 277m. The next sample higher (267 m) is interpreted as Event Pa15,based on the increase in R. hillae and the highest consistentoccurrence of D. barbadiensis. Although no assemblage changewas seen at 250 m, this sample contained the HOs of bothHelicosphaera reticulata and L. minutus, so it is interpreted to beEvent Pa12. The last event to be interpreted was the in situ R.hillae HO (Event Pa11). To aid in this, all of the Eocene species

RICHARD A. DENNE50C

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1 2 1 2 1 120 20 1 24 5100 40 2 112 2 1 2 1 5 1 1 2 1 1 28 3 600 1 4 1

2 8 6 2 10 9 280 10 220 6000 40 7 112 4 12 3 4 1 1 3 48 14 750 1 1 2

3 5 3 5 2 2 42096 28 6900 120 3 200 24 3 20 1 2 1 10 1 45 1 48 1000 1 1 1

5 1 3 4 240 150 24 5000 48 3 48 1 5 1 1 32 1 24 2 3 450 32 5

1 7 3 2 16 12 200 48 40 4500 40 28 3 56 4 1 104 1 2 1 24 600 72 1 2 1

12 1 3 10 28 400 150 80 6900 80 112 180 2 1 1 1 6 2 80 4 1 1 48 750 112 2

2 2 2 3 12 320 120 48 6000 96 10 150 1 3 3 1 40 1 1 4 1 1 600 112 2 2

2 3 3 24 10 150 20 12 6000 4 4 128 1 3 3 1 1 32 96 6 1 12 4 20 600 40 1 1 1

6 1 3 14 3 120 1 20 4500 112 1 128 2 3 1 12 112 2 1 10 40 450 112 1

40 1 10 2 220 3 1 2 4500 1 10 128 1 1 1 6 2 1 1 150 3 1 6 2 48 750 56

36 20 12 16 400 48 2 10 9000 1 80 1 90 5 24 2 4 6 2 2 240 1 2 2 64 5 600 150

2 36 9 24 48040 1 48 12000 4 80 180 3 2 1 3 12 1 2 1 3 48 1 1 2 3 24 1000 120 1 1

1 5 16 5 5 150 14 1 9000 80 2 40 1 1 1 3 9 1 2 1 112 1 1 1 2 24 900 80 1 1

1 2 1 5 1 1 320 120 1 10 4500 2 96 2 150 1 2 1 4 1 2 1 32 3 2 10 1 150

1 1 1 2 14 3 3 260 28 7500 1 64 2 1 80 1 2 1 2 1 5 16 1 1 2 1 1 150 3 2 3 4 750 32 2

1 3 10 4 220 5 1 1 4500 150 4 48 3 4 1 1 2 13 2 1 240 4 2 10 750 2 1 1

1 1 1 3 10 40 24 1 2 1 6600 6 18 18 2 1 2 6 4 64 2 2 24 1 600 56 2

6 2 3 6 4 96 2 4 6000 1 5 1 1 3 6 2 8 3 48 2 16 4 600 80

5 5 20 260 112 1 3 4500 180 40 2 1 4 2 6 1 1 6 96 24 2 600 200 2

2 5 12 28 260 120 12000 600260 1 2 150 2 6 3 14 1 1 10 2 1 240 6 1200 160 1 1

28 3 5 180 48 5100 72 3 40 1 12 1 40 1 24 2 450 20

10 56 48 150 96 1 6000 300 112 4 3 4 24 1 1 1 1 20 1 1 10 600 64 6

1 4 2 18 20 240 220 7500 1 150 16 1 2 28 1 1 48 2 6 1400 24 1

2 32 7 6 420128 6000 420 3 1 20 1 1 4 1 3 64 1 1 14 20 4 1 4 96 900 10 1 3

20 72 40 360 40 2 6000 320 3 2 56 1 4 36 3 12 1 2 4 10 2 12 750 1

1 6 4 12 20 260 80 4 7500 64 2 1 3 18 24 16 1 2 1 1 56 28 40 1 3 750 20 1 2

1 1 5 36 320 120 10 4500 200 3 5 28 2 2 48 1 1 1 1 2 1 48 1 36 3 3 300 3

2 1 4 2 48048 80 4500 150 4 40 1 28 3 56 1 3 1 2 1 4 52 1 40 2 450 3 1

Dep

th (

m)

0

50

100

150

200

250

300

FIG. 9.—Abundances of 68 calcareous nannofossil species and morphotypes identified in a 325 m Oligocene to upper Eocene sectionin a well from the Gulf of Mexico. Depths shown are in meters below the highest sample.

and occurrences of E. formosa above 270 m are marked asreworked, showing that the samples above and below 185 m allcontain reworked lower Oligocene to upper Eocene specimens.Therefore, the R. hillae occurrence at 185 m is interpreted asreworked and the sample is labeled as Event Pa10, while thenext R. hillae occurrence at 230 m is believed to be the in situ HO,so this sample is labeled as Event Pa11.

Fourteen species were reworked in the Oligocene section,most of which were concentrated between 140 to 230 m. The topof the lower Oligocene was placed at 67 m at Event Pa8. The basallower Oligocene is believed to be missing, inasmuch as thesection at 267 m appears to jump from Event Pa12 to Event Pa15in the uppermost Eocene.

SUMMARY AND CONCLUSIONS

Although reworking of calcareous nannofossils can be quiteproblematic in determining a biostratigraphic zonation for asection, particularly when working with cuttings samples, as-semblage-based zonations have been used successfully to de-tect the presence of reworked specimens and formulate a bio-stratigraphic succession without relying on potentially mis-leading LOs. Without this or similar techniques, the age of thesection could be determined to be either older than its actual age

or left unzoned due to the reworked fossils present in thesediments. This is especially true in cases where the reworkedfossils are only marginally older than the indigenous assem-blage.

ACKNOWLEDGMENTS

The author would like to acknowledge the mentorship of T.C.Huang, whose ideas on assemblages and sequence biostratig-raphy have led to the zonation schemes used here, the initialguidance of Jason Crux on the paper, and the helpful reviews ofNancy Englehardt-Moore, Peter Thompson, and an anonymousreviewer.

REFERENCES

BACKMAN, J., AND SHACKLETON, N.F., 1983, Quantitative biochronology ofPliocene and Early Pleistocene calcareous nannofossils from theAtlantic, Indian and Pacific Oceans: Marine Micropaleontology, v. 8,p. 141–170.

BERGGREN, W.A., HILGEN, F.J., LANGEREIS, C.G., KENT, D.V., OBRADOVICH, J.D.,RAFFI, I., RAYMO, M.E., AND SHACKLETON, N.J., 1995, Late Neogenechronology: New perspectives in high-resolution stratigraphy: Geo-logical Society of America, Bulletin, v. 107, p. 1272–1287.

51DETECTION OF REWORKED NANNOFOSSILS

BOWN, P.R., AND YOUNG, J.R., 1998, Introduction, in Bown, P.R., ed.,Calcareous Nannofossil Biostratigraphy: British Micro-palaeontological Society Series, Cambridge, U.K., Chapman &Hall, p. 2–15.

BRAMLETTE, M.N., AND RIEDEL, E. 1954, Stratigraphic value of discoastersand some other microfossils related to Recent coccolithophores:Journal of Paleontology, v. 28, p. 385–403.

BREARD, S.Q., CALLENDER, A.D., DENNE, R.A., AND NAULT, M.J., 2000,Gulf of Mexico basin late Tertiary deep-water biostratigraphiczonation: Relationship to standard shelf foraminiferal and cal-careous nannofossil marker terminology, in Weimer, P., ed.,Deep Water Reservoirs of the World: Gulf Coast Section SEPMFoundation, Bob F. Perkins 20th Annual Research Conference, p.116–126.

BUKRY, D., 1974, Coccoliths as paleosalinity indicators—evidence fromBlack Sea, in Degens, E.T., and Ross, D.A., eds., The Black Sea—Geology, Chemistry, and Biology: American Association of Petro-leum Geologists, Memoir 2, p. 353–363.

BUKRY, D., DOUGLAS, R.G., KLING, S.A., AND KRASHENINNIKOV, V., 1971,Planktonic microfossil biostratigraphy of the northwestern PacificOcean, in Fisher, A.G., et al., eds., Initial Reports of the Deep SeaDrilling Project, v. 6, p. 1253–1300.

BYBELL, L.M., 1982, Late Eocene to Early Oligocene calcareous nanno-fossils in Alabama and Mississippi: Gulf Coast Association of Geo-logical Societies, Transactions, v. 32, p. 295–302.

BYBELL, L.M., AND GARTNER, S., 1972, Provincialism among mid-Eocenecalcareous nannofossils: Micropaleontology, v. 18, p. 319–336.

CRUX, J.A., GARD, I.G., GRIGGS, P.H., FARRER, B.J., AND EVANS, N.N., 2005a,Fossil age group plots: A rapid interpretation technique for complexstructural areas (abstract): American Association of Petroleum Ge-ologists, 2005 Annual Convention, Abstracts Volume, p. A31.

CRUX, J.A., GARD, I.G., GRIGGS, P.H., FARRER, B.J., AND EVANS, N.N., 2005b,Fossil age group plots: A rapid interpretation technique for complexstructural areas: Search and Discovery Article #40161, Website,http://www.searchanddiscovery.net/documents/2005/Crux/index.htm.

DE KAENEL, E., AND VILLA, G., 1996, Oligocene–Miocene calcareous nanno-fossil biostratigraphy and paleoecology from the Iberia abyssal plain,in Whitmarsh, R.B., Sawyer, D.S., Klaus, A., and Masson, D.G., eds.,Proceedings of the Ocean Drilling Program, Scientific Results, v. 149,p. 79–145.

DEMCHUK, T.D., 2000, Palynological characterization and multi-disciplinechronostratigraphic interpretation of Cenozoic strata from the Tai-wan Straits Region: Palynology, v. 24, p. 255.

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FIG. 10.—Generalized abundances of the 29 calcareous nannofossil species illustrating the 27 horizons of the Oligocene to uppermostEocene of the Gulf of Mexico (after Denne, 2007).

RICHARD A. DENNE52

7

a b

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12 13

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16 17

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PLATE 2.—A) Pliocene calcareous nannofossil marker species. All photos taken with a light microscope (CN, crossed nicols; PC, phasecontrast). 7. Dictyococcites antarcticus (CN). 8. Dictyococcites productus (CN). 9. Discoaster asymmetricus (a) CN (b) PC. 10. Discoasterbrouweri (a) CN (b) PC. 11. Discoaster pentaradiatus (a) CN (b) PC. 12. Discoaster brouweri “A” (a) CN (b) PC. 13. Discoaster surculus(a) CN (b) PC. 14. Discoaster tamalis (a) CN (b) PC. 15. Discoaster variabilis (a) CN (b) PC. 16. Small Gephyrocapsa spp. (CN). 17.Gephyrocapsa oceanica (CN). 18. Helicosphaera rhomba (CN). 19. Reticulofenestra gelida large form (CN). 20. Reticulofenestra haqii (CN).21. Reticulofenestra minuta (CN). 22. Reticulofenestra pseudoumbilica (CN). 23. Reticulofenestra pseudoumbilica large form (CN). 24.Sphenolithus abies (a) CN – 0° (b) CN – 45°. 25. Sphenolithus verensis (a) CN – 0° (b) CN – 45°. 26. Umbilicosphaera cricota (a) CN (b)PC. B) Oligocene–uppermost Eocene calcareous nannofossil marker species. 1. Blackites creber (a) CN (b) PC. 2. Clausicoccusfenestratus (a) CN (b) PC. 3. Cribrocentrum reticulatum (CN). 4. Dictyococcites bisectus (CN). 5. Dictyococcites stavensis (CN)

53DETECTION OF REWORKED NANNOFOSSILS

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PLATE 3.—Oligocene–uppermost Eocene calcareous nannofossil marker species. All photos taken with a light microscope (CN,crossed nicols; PC, phase contrast). 6. Discoaster barbadiensis (a) CN (b) PC 7. Discoaster saipanensis (a) CN (b) PC 8. Discoaster taniitanii (a) CN (b) PC 9. Ericsonia formosa (a) CN (b) PC 10. Helicosphaera compacta (CN) 11. Helicosphaera reticulata (CN) 12. Isthmolithusrecurvus (a) CN (b) PC 13. Lanternithus minutus (CN) 14. Pemma papillatum (a) CN (b) PC 15. Reticulofenestra hillae (CN) 16.Reticulofenestra umbilica (CN) 17. Reticulofenestra laevis (CN) 18. Sphenolithus akropodus (a) CN – 0° (b) CN – 45° 19. Sphenolithuscapricornutus (CN) 20. Sphenolithus ciperoensis (a) CN – 0° (b) CN – 45° 21. Sphenolithus delphix (a) CN – 0° (b) CN – 45° 22.Sphenolithus distentus (a) CN – 0° (b) CN – 45° 23. Sphenolithus predistentus (a) CN – 0° (b) CN – 45° 24. Sphenolithus pseudoradians(a) CN – 0° (b) CN – 45° 25. Transversopontis sigmoidalis (CN) 26. Transversopontis obliquipons (CN) 27. Triquetrorhabdulus carinatus(a) CN (b) PC 28. Zygrhablithus bijugatus (a) CN – 0° (b) CN – 45°.

RICHARD A. DENNE54

DENNE, R.A., 2003, Sequence biostratigraphy and a new nannofossilzonation of the Pleistocene Gulf of Mexico: Gulf Coast Associationof Geological Societies, Transactions, v. 53, p. 256–265.

DENNE, R.A., 2007, Paleogene calcareous nannofossil bioevents from thefold belts of the central Gulf of Mexico, in Kennan, L., Pindel, J., andRosen, N.C., eds., The Paleogene of the Gulf of Mexico and Carib-bean Basins: Processes, Events, and Petroleum Systems: Gulf CoastSection SEPM Foundation, Bob F. Perkins 27th Annual ResearchConference, p. 211–231.

EVERETT, R.W., 1982, Using nannofossil counts in the interpretation ofsubsurface deltas: Gulf Coast Association of Geological Societies,Transactions, v. 32, p. 579–591.

GARD, G., AND CRUX, J.A., 1994, Reworked Jurassic–Neogene calcareousnannofossils in the central Arctic: Marine Geology, v. 119, p. 287–300.

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Dep

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INC

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FIG. 11.—Fossil age-group plot of the abundances of 41 age diagnostic calcareous nannofossil species found in the Example 3 well,divided into four age categories. The assemblage changes are noted that were used to determine the 11 events listed at the far right.This zonation was used to determine which specimens were in situ and which were reworked.

HAMILTON, E.L., 1953, Upper Cretaceous, Tertiary, and Recent Foramin-ifera from Mid-Pacific flat-topped seamounts: Journal of Paleontol-ogy, v. 27, p. 204–237.

HOLCOVÁ, K., 2005, Quantitative calcareous nannoplankton biostratig-raphy of the Oligocene/Miocene boundary interval in the northernpart of the Buda Basin (Central Paratethys): Geological Quarterly, v.49, p. 263–274.

HOLCOVÁ, K., AND MASLOWSKÁ, H., 1999, The use of multivariate statisticalmethods in analysis of postmortem transport and resedimentationof foraminiferal tests in relation to species composition oforyctocenoses. Miocene of the south Slovak basin (central Paratethys):Revista Española de Micropaleontología, v. 31, p. 1–21.

JIANG, M.M., 1993, Miocene sequence biostratigraphy of the northernGulf of Mexico: Gulf Coast Association of Geological Societies,Transactions, v. 43, p. 137–143.

55DETECTION OF REWORKED NANNOFOSSILS

JIANG, M.M., AND WATKINS, J.S., 1992, Floral pulses, foraminiferal peaksand inferred sequences in the Miocene of the northern Gulf of Mexico:Marine and Petroleum Geology, v. 9, p. 608–622.

MARTINI, E., 1971, Standard Tertiary and Quaternary calcareous nanno-plankton zonation, in Farinacci, A., ed., Proceedings of the SecondPlanktonic Conference Rome 1970: Rome, Edizioni Tecnoscienza, v.2, p. 739–785.

NEGRI, A., CAPOTONDI, L., AND KELLER, J., 1999, Calcareous nannofossils andplanktic foraminifers distibution patterns in Late Quaternary–Ho-locene sapropel of the Ionian Sea: Marine Geology, v. 157, p. 89–103.

OKADA, H., AND BUKRY, D., 1980, Supplementary modification and intro-duction of code numbers to the low-latitude coccolith biostrati-graphic zonation (Bukry, 1973; 1975): Marine Micropaleontology, v.5, p. 321–325.

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