ca palynological and sequence-stratigraphic study...
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A palynological and sequence-stratigraphic studyof Santonian–Maastrichtian strata from the UpperMagdalena Valley basin in central ColombiaSandra Garzon a b c , Sophie Warny a b & Philip J. Bart aa LSU Department of Geology and Geophysics, Baton Rouge, Louisiana, 70803, USAb LSU Museum of Natural Science, Baton Rouge, Louisiana, 70803, USAc Smithsonian Tropical Research Institute, PO Box 0843-03092, Balboa, Ancón, Panama City,Panama
Available online: 20 Mar 2012
To cite this article: Sandra Garzon, Sophie Warny & Philip J. Bart (2012): A palynological and sequence-stratigraphic study ofSantonian–Maastrichtian strata from the Upper Magdalena Valley basin in central Colombia, Palynology, 36:sup1, 112-133
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A palynological and sequence-stratigraphic study of Santonian–Maastrichtian strata from the
Upper Magdalena Valley basin in central Colombia
Sandra Garzona,b,c*, Sophie Warnya,b* and Philip J. Barta
aLSU Department of Geology and Geophysics, Baton Rouge, Louisiana 70803, USA; bLSU Museum of Natural Science, BatonRouge, Louisiana 70803, USA; cSmithsonian Tropical Research Institute, PO Box 0843-03092, Balboa, Ancon, Panama City,
Panama
This paper presents a sequence-stratigraphic interpretation from the palynological analysis and lithologic data oftwo outcrop sections on the NE flank of the Upper Magdalena Valley (UMV) basin primarily comprising theSantonian–Lower Maastrichtian interval. Important stratal horizons are identified in the northeastern part of theUMV basin and ages assigned to them. A cyclic pattern of palynomorph distribution was recognized in both sectionsand tied to the different stages of the stratigraphic chart. Spikes in abundance of spores accompanied by pollencharacterize the lowstand systems tracts and are replaced by the occurrence of euryhaline dinoflagellate cysts(ceratioids and/or gymnodinioids) during the subsequent transgressive phase. Maximum flooding surfaces (MFS)are recognized by a sudden increase in open marine palynomorphs (peridinioids and/or gonyaulacoids) and thescarcity of terrestrial representatives. As sea level starts to fall, the gradual decrease in open marine dinoflagellatesalong with the occurrence of euryhaline dinoflagellate cysts and terrestrial representatives corresponds to highstandsystems tracts. The sequence-stratigraphic interpretation from palynological analysis was correlated to the globalsea-level curve allowing the identification of the Santonian–Campanian and Campanian–Maastrichtian boundaries.System tracts from supercycles ZC-3, ZC-4 and TA-1 were recognized from the palynological data.
Keywords: Campanian; dinoflagellate cysts; Maastrichtian; pollen; Santonian; sequence stratigraphy; spores
1. Introduction
Late Cretaceous global sea-level fluctuations generatedgeological surfaces that constitute important correla-tion horizons in Cretaceous oil-producing basins fromnorthwest South America. Various authors havediscussed the synchronous character of these horizons(e.g. Pocknall et al. 1997; Helenes and Somoza 1999;Guerrero et al. 2000; Vallejo 2002; Jaillard et al. 2005).In Colombia, a number of studies (e.g. Vergara andRodriguez 1997; Villamil 1998; Guerrero 2002) haveprovided a sequence-stratigraphic framework forCretaceous sequences of the Eastern Cordillera,Catatumbo, Llanos Foothills and Upper MagdalenaValley (UMV) basins. The synchronicity of sea-levelfluctuations in the UMV basin has been extensivelyanalyzed (e.g. Guerrero and Sarmiento 1996; Villamil1998) and important boundaries including the Santo-nian–Campanian and the Campanian–Maastrichtianhave been identified from sequence-stratigraphic ana-lysis and correlated with eustatic changes (Guerreroet al. 2000).
Sequence-stratigraphic interpretations are com-monly based on stacking patterns and lithologicalchanges, together with marine fossil abundances.
However, identifying maximum flooding surfaces andsequence boundaries can be quite difficult in certainsettings; for example, different system tracts of thesequence may be represented by monotonous fine-grained deposits. This is potentially the case with theUpper Cretaceous units from the northeastern part ofthe UMV basin, where the fine-grained lithology andthe scarcity of well-preserved calcareous microfossils incertain intervals limit traditional sequence-strati-graphic interpretations. If present, organic-walledmicrofossils (dinoflagellate cysts, spores and pollen)may constitute an important tool to overcome thisproblem as the combined lithological/palynologicalapproach can provide a way to refine the sequence-stratigraphic interpretation. Jaramillo and Yepes(1994), Guerrero and Sarmiento (1996) and Yepes(2001) have described palynomorph-rich intervals fromUpper Cretaceous UMV strata; however, despite thewell-known occurrence of diverse palynomorph assem-blages in the area, their application as a tool forsequence-stratigraphic interpretation has rarely beenfully explored.
Sequence-stratigraphic interpretations based onpalynological analysis have proven to be effective in
*Corresponding authors. Email: [email protected]; [email protected]
Palynology
Vol. 36, Supplement 1, 2012, 112–133
ISSN 0191-6122 print/ISSN 1558-9188 online
� 2012 AASP – The Palynological Society
http://dx.doi.org/10.1080/01916122.2012.675147
http://www.tandfonline.com
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different geographic regions and time intervals. Severalstudies have established the direct relationship betweenthe palynological record and sea-level fluctuations (e.g.Poumot 1989; Gregory and Hart 1992; Li and Habib1996; Holz and Dias 1998; Rull 2000, 2002). Poumot(1989) was the first to tie cyclic patterns of terrestrialpalynomorph distribution on tropical coastal settingsto a sequence-stratigraphic framework. He correlatedpalynological assemblages dominated by fern sporesand forest pollen to the lowstand, higher abundancesof palm pollen to early sea-level rise and an increase ofmangrove pollen to the later transgressive phase.Poumot (1989) explained these fluctuations of pollenand spore assemblage composition on grounds of theenvironmental preferences of the parent plants andpalynomorph transport. A similar pattern was de-scribed by Gregory and Hart (1992) who noticed thatduring lowstand deposition, palynological assemblageswere dominated by terrestrial palynomorphs producedby both hydrophilic and xerophilic plants; highstanddeposition was however characterized by an increase ofpalynomorphs derived from hydrophilic plants only.In another study, Holz and Dias (1998) reported highspore content in lowstand system tracts and agradually decreasing trend in spore abundance intransgressive systems tracts. The environmental pre-ferences of certain groups of marine palynomorphsalso provide valuable information on sea-level changes(e.g. Li and Habib 1996; Schioler et al. 1997; Warnyand Wrenn 2002; Warny et al. 2003).
In the present study, new palynological data fromtwo Upper Cretaceous outcrop sections in centralColombia are analyzed and interpreted within asequence-stratigraphic framework with a view towardunderstanding the important stratal horizons anddetermining their respective ages.
2. Geological setting
The UMV is an intermontane basin located in thecentral part of the Colombian Andes, between theEastern and Central cordilleras. The Upper Cretaceousunits from the northeastern side of the UMV basinwere deposited in a passive margin setting on inner toouter shelf environments and are dominated by fine-grained sediments (Dengo and Covey 1993; figure 1).
The lithostratigraphic succession of the UMV basincomprises, from base to top, the lower Lidita Forma-tion, El Cobre Formation, upper Lidita Formationand Buscavidas Formation (Figure 2), following theterminology proposed by Guerrero et al. (2000). In ourstudy area, the lower Lidita Formation is characterizedby a succession of siliceous mudstones with inclusionsof thin phosphatic laminae, cherts and black shales; theEl Cobre Formation is composed of monotonous
calcareous black shales; the upper Lidita Formationconsists of siliceous mudstones and black cherts; andthe Buscavidas Formation is characterized by calcar-eous gray mudstones interbedded with limestones andcalcareous shales.
3. Methods
Two outcrop sections were measured. The Aico Creeksection, located at 38 440 N and 758 230 W, comprisesfrom base to top the lower Lidita Formation, El CobreFormation, upper Lidita Formation and BuscavidasFormation (Figure 2). The Buitrera Creek section,located at 48 300 N and 748 370 W, is composed of theuppermost part of the El Cobre Formation, the upperLidita Formation and the Buscavidas Formation(Figure 2). Both sections were previously studied byJaramillo and Yepes (1994) and Yepes (2001). Accord-ing to these previous studies, the Aico Creek sectioncomprises sediments of Coniacian–Early Maastrich-tian age and the Buitrera Creek section includessediments of Campanian–Early Maastrichtian age.
Ninety palynological samples were analyzed. Allsamples were treated with hydrochloric and hydro-fluoric acids. Following these steps, centrifuging inheavy liquid (ZnBr2) was used to further separate theorganic fraction of the samples (Traverse 1988; Brown2008). Up to 300 palynomorphs were counted persample although, when recovery was extremely low,tabulation stopped when the full residue was analyzed.Identification and description of the palynomorphswas carried out using an Olympus BX 41 transmittedlight microscope at the Center for Excellence inPalynology (CENEX), Louisiana State University,Baton Rouge, Louisiana. The results were tabulatedin Excel spreadsheets and plotted on biostratigraphicdistribution charts.
3.1. Sequence-stratigraphic analysis using palynologyand lithology
The sequence-stratigraphic interpretations from theAico Creek and Buitrera Creek sections are based onthe Haq et al. (1987) global sea-level curve plottedagainst the Geologic Time Scale of Gradstein et al.(2004). The chert unit is interpreted to representdeposition in a more basinal paleoenvironmentwhereas sandstone is interpreted to represent deposi-tion in a more proximal paleoenvironment. Within thiscontext, the upsection lithologic changes were utilizedto infer relative sea-level change.
Furthermore, distinct palynological assemblageswere identified in order to infer sea-level fluctuationsand environments of deposition. The assemblages weregrouped by environmental affinities indicative of
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Figure 1. Paleogeographic map illustrating the Late Cretaceous depositional setting of the Upper Magdalena Valley Basin(modified from Dengo and Covey 1993) and the location of the Aico and Buitrera Creek stratigraphic sections. The insert showsthe modern geographical location of Colombia.
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coastal to open marine depositional conditions andpartly supported by previously published studies (e.g.Germeraad et al. 1968; May 1977; Poumot 1989;Gregory and Hart 1992; Schrank 1994; Li and Habib1996; Holz and Dias 1998; Rull 2000, 2002). Figure 3illustrates the distribution of the palynological assem-blages on a hypothetical onshore–offshore transect andtheir relation to the sequence-stratigraphic cycle. Thecomposition of the palynological assemblages ispresented in Table 1. Several age and environmentallysignificant palynomorphs are illustrated in Plates 1 and2. Three broad palynological groups are describedbelow.
3.1.1. Marine group
In this study, both peridinioid and gonyaulacoiddinoflagellate cysts are interpreted as indicators ofopen marine environments, although their distributioncan be constrained by paleoecological preferencesother than distance from shoreline. For instance,peridinioid dinoflagellate cysts possess heterotrophicpreferences (Jacobson and Anderson 1986; Powellet al. 1990) and higher abundances are often associatedwith upwelling events or nearshore environments withhigh terrigenous input (Bujak 1984; Pearce et al. 2003).Conversely, most gonyaulacoid dinoflagellate cysts areautotrophic (Harland 1988; Powell et al. 1990) and aretherefore not constrained by nutrient availability,although certain species of gonyaulacoid dinoflagellatecysts (e.g. some species of Spiniferites) have beenrecorded in great abundance in nearshore settings dueto salinity preferences (Davey 1970; Downie et al.1971; Li and Habib 1996).
3.1.2. Estuarine group
Ceratioid and gymnodinioid dinoflagellate cysts aregrouped because of their nearshore paleoenvironmen-tal preference as discussed by May (1977), Wilpshaarand Leerveld (1994) and Leerveld (1995). According tothese authors, ceratioid cysts indicate restricted marineor low-salinity environments suggesting nearshore,lagoonal or back-barrier environments. Based onmorphological studies, May (1977) concluded thatgymnodinioid dinoflagellate cysts are indicative ofestuarine environments. He proposed that the uniqueaccordion shape and wall canals of gymnodinioidspecies are highly effective mechanisms that allow themto tolerate rapid changes in salinity.
3.1.3. Terrestrial group
Pollen and spores dominate this group andindicate terrestrial input. If the parent plant can be
Figure 2. Stratigraphic columns of the Aico and BuitreraCreek sections showing the main lithostratigraphic unitsfrom the Santonian–Maastrichtian succession in the UpperMagdalena Valley basin.
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identified, then we are able to make some inferencesabout the local paleoenvironment prevailing at thetime of deposition and the coastal and hinterlandvegetation. Consequently, we can also determine ifhydrophilic or xerophilic conditions (whether or nota plant can grow and reproduce in conditions withhigh/low-water availability, respectively) existed.Plants such as ferns that reproduce via spores areconsidered hydrophilic, as the spores require soilwith high-moisture content to produce gametophytesthat will generate the next generation of plants.Their predominance in a section can reflect swampenvironments. Xerophylic pollen-producing plants do
not require moist soil to reproduce and spread, sotheir occurrence in coastal settings is considered toindicate transport via river input from neighboringdry lands. At times, reconstructing the vegetationcan be difficult because many of the Late Cretaceouspollen and spore types have no known livingcounterpart.
4. Results
4.1. Palynostratigraphy
The stratigraphic distributions of terrestrial, estuarineand marine palynomorphs for both sections are shown
Figure 3. Schematic model showing the distribution of palynological assemblages from onshore–offshore and their relationshipto an idealized sequence-stratigraphic section.
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in Figures 4–7. Based on the distribution of speciesrecovered, four stratigraphically distinct palynologicalassemblage zones (1–4) are defined at Aico Creek(Figure 8). Only Zones 3 and 4 are identified atBuitrera Creek (Figure 9).
From oldest to youngest, Zone 1 is characterizedby the co-occurrence of Araucariacites sp. andScabratriletes sp. Zone 2, the Odontochitina zone, ischaracterized by the occurrence of Odontochitinacostata, O. operculata and O. porifera. Zone 3 isdivided into two subzones: the Hamulatisporis zonedefined by the occurrence of Hamulatisporis caperatus,Muerrigerisporis ardilensis, Araucariacites australis andNeorastrickia densus; and the overlying Andalusiellazone characterized by the common occurrence ofAndalusiella mauthei and A. polymorpha. Finally,Zone 4, the Cerodinium zone, is defined by the firstoccurrence of Cerodinium diebelii, and the last occur-rences of A. australis. Buttinia andreevi, Foveotriletesmargaritae and Echimonocolpites protofranciscoi occursparsely within this zone. The zonation helps correlatethe two sections in the absence of stratigraphicallysignificant marine microfossils (e.g. foraminifera,nannofossils).
4.2. Distribution of key palynological assemblages
Within the broad palynomorph groups, six distinctiveassemblages were recognized: those dominated byceratioid dinoflagellate cysts, gonyaulacoid dinoflagel-late cysts, gymnodinioid dinoflagellate cysts, peridi-nioid dinoflagellate cysts, spores and pollen.
Changes in these assemblages are visible in thevertical distribution of species (Figures 4–7). Therelative abundances of species in both sections (Figures10 and 11) reveal some interesting trends. Thesevertical changes in palynological assemblages aredescribed below.
4.2.1. Aico Creek section
Figure 10 illustrates the vertical distribution of the sixassemblages recovered at the Aico Creek section. Paly-nomorph recovery varies from fair to abundant withsome barren intervals occurring throughout the section.
A spike in ceratioid cysts occurs in the lower LiditaFormation at 43.5 m although numbers quicklydecline. At 46.4 m, gonyaulacoid cysts dominate withminor abundances of spores and pollen. From 79 to89 m in the El Cobre Formation, gymnodinioid cystsdominate the assemblage with gonyaulacoid cysts,pollen and spores as minor components. The upper-most El Cobre Formation (114.5–127 m) contains apredominantly marine assemblage dominated by go-nyaulacoid cysts with decreasing gymnodinioid cystabundances, and with ceratioid cysts and sporescommon at 125 m. At the base of the Upper LiditaFormation (134 m), gonyaulacoid cysts are recordedfollowed by the occurrence of gymnodinioid cysts,spores and pollen at 143.5 m. The interval from 145 to
Table 1. List of palynomorphs used to define the palyno-logical groups for sequence-stratigraphic interpretation.
Marine
Peridinioid GonyaulacoidAndalusiella gabonensisAndalusiella mautheiAndalusiella polymorphaAndalusiella/Palaeocystodiniumcomplex
Cerodinium spp.Cerodinium diebeliiIsabelidinium sp.Palaeocystodinium spp.Palaeohystrichophorainfusorioides
Senegalinium spp.Trithyrodinium fragile
Areoligera spp.Circulodinium distinctumCoronifera sp.Cyclonephelium spp.Diphyes sp.Exochosphaeridium sp.Exochosphaeridium bifidumHeterosphaeridium difficileHystrichodinium pulchrumOligosphaeridium complexSpiniferites spp.
Nearshore/estuarineCeratioid GymnodinioidOdontochitina costataOdontochitina operculataOdontochitina porifera
Alisogymnium spp.Alisogymnium euclaenseDinogymnium spp.Dinogymnium acuminatumDinogymnium digitusDinogymnium ‘‘lamarense’’Dinogymnium undulosum
TerrestrialSpores PollenAppendicisporites‘‘protoerdtmanii’’
Cicatricosisporites sp.Cicatricosisporites‘‘bifurcates’’
Distaverrusporites spp.Echitriletes spp.Echitriletes intercolensisFoveotriletes‘‘protomargaritae’’
Foveotriletes margaritaeGabonisporis vigourouxiiHamulatisporis caperatusLaevigatosporites spp.Laevigatosporites tibuensisMuerrigerisporis sp.Neoraistrickia sp.Polypodiisporites sp.Psilatriletes spp.Rotverrusporites spp.Rugotriletes sp.Rugulatisporis‘‘vermiculatus’’
Scabratriletes sp.Scabratriletes granularisStriatriletes spp.Trilites ‘‘psilatus’’Verrucatotriletes spp.Zlivisporis blanensis
Araucariacites spp.Araucariacites australisBombacacidites ‘‘inciertus’’Buttinia andreeviEchimonocolpites protofranciscoiEphedripites sp.Gnetaceaepollenites‘‘pseudoboltenhagenii’’
Psilatricolpites sp.Retistephanocolporites sp.Arecipites spp.Monocolpopollenites spp.Monocolpopollenitesspheroidites
Psilamonocolpites sp.Psilamonocolpites mediusProxapertites ‘‘foveoreticulatus’’Proxapertitesminutihumbertoides
Proxapertites maracaiboensisRacemonocolpites sp.Retimonocolpites sp.Spinizonocolpites sutae
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Plate 1. Light photomicrographs of key species of marine and terrestrial palynomorphs present in the studied sections. Figure 1.Buttinia andreevi, Buitrera Creek 609.8 m, EF X36. Figure 2. Echimonocolpites protofranciscoi, Aico Creek 203 m, EF S14. Figure 3.Cerodinium diebelii, Aico Creek 229.9 m, EF O44. Figure 4. Foveotriletes margaritae, Aico Creek 229.9 m, S36/3. Figure 5.Spinizonocolpites sutae, Aico Creek 203 m, EF V27/3. Figure 6. Oligosphaeridium complex, Aico Creek 79 m, EF R27/2. Figure 7.Alisogymnium euclaense, Aico Creek 229.9 m, EF K9. Figure 8. Odontochitina operculata, Aico Creek 125 m, EF E18/4.
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Plate 2. Light photomicrographs of species of marine and terrestrial palynomorphs present in the studied sections. Figure 1.Bacumorphomonocolpites tausae, Buitrera Creek 619.4 m, EF T42/1. Figure 2. Bombacacidites ‘‘inciertus’’, Aico Creek 207.7 m,EF P6/4. Figure 3. Gnetaceapollenites ‘‘pseudoboltehagenii’’, Aico Creek 205 m, EF P25/2. Figure 4. Dinogymnium undulosum,Aico Creek 89 m, EF M25. Figure 5. Alisogymnium ‘‘glotonicus’’, Aico Creek 221.4 m, W39. Figure 6. Psilamonocolpites medius,Aico Creek 221.4 m, EF P12/3. Figure 7. Hamulatisporis caperatus, Aico Creek 216.7 m, EF H12/3. Figure 8. Gabonisporisvigourouxii, Aico Creek 143.5 m, EF Q18/2. Figure 9. Proxapertites maracaiboensis, Buitrera Creek 609.8 m, EF V38/1.
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185 m is barren of palynomorphs. At the base of theBuscavidas Formation (188.5–191.5 m), peridinioidand gonyaulacoid cysts dominate the assemblage.Gymnodinioid cysts, spores and pollen increase inabundance at 200.7 m followed by a spike in sporeabundance at 203 m with minor pollen numbers.Terrestrial palynomorphs dominate the assemblage at205.7 m with gymnodinioid and gonyaulacoid cysts asminor components. From 207.7 to 229.5 m gonyaula-coid, gymnodinioid and peridinioid cyst abundancesincrease. Pollen and spores dominate the palynologicalassemblage at the top of the Buscavidas Formationfrom 275.7 to 276.7 m.
The first occurrence of the important Campanian–Maastrichtian palynological markers, Echimonocol-pites protofranciscoi and Buttinia andreevi (Mulleret al. 1987; Jaramillo and Rueda 2004), are at200.7 m and 202 m, respectively, in Zone 4 (seeFigures 5 and 8). We place the Campanian–Maas-trichtian boundary above the first occurrence withinthe Buscavidas Formation at 240 m in the Aico Creeksection (Figure 10). This choice of placement for theCampanian–Maastrichtian boundary is consistent withour sequence-stratigraphic interpretation for the posi-tioning of the 71 Ma sequence boundary (Figure 10,see discussion in Section 5).
Figure 4. Distribution chart of terrestrial palynomorphs from the Aico Creek section.
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Figure 5. Distribution chart of marine palynomorphs from the Aico Creek section.
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4.2.2. Buitrera Creek section
Figure 11 illustrates the vertical distribution of the sixassemblages recovered at the Buitrera Creek section.Palynomorph recovery ranges from fair to abundantthroughout, with the exception of barren intervals atthe base and middle part of the upper LiditaFormation.
At 19.2 m, at the base of the section in the El CobreFormation, the palynological assemblage is dominatedby terrestrial palynomorphs associated with gymnodi-nioid cysts. Peaks of peridinioid cysts are observed at28.8, 56 and 78.4 m and dominate the upper part of theEl Cobre Formation. Near the top of the Upper LiditaFormation (198.6 m) and base of the BuscavidasFormation (219.6 m), only peridinioid cysts arerecorded. Gonyaulacoid and gymnodinioid cysts re-appear in the section at 238.3 m but peridinioid cysts
remain the dominant component up to the sample at359.5 m. Pollen and spores are minor components ofthese predominantly marine assemblages. Recovery inthe upper part of the Buscavidas Formation is poorwith the exception of common spores at 409 m andabundant pollen at 609.8 m. In general, at the BuitreraCreek section, peridinioid cysts dominate the marineassemblages; this is in contrast to their rare presence atthe Aico Creek section where gonyaulacoid cystsdominate the dinoflagellate assemblages.
The first observed occurrence of the Campanian–Maastrichtian palynological markers, Echimonocol-pites protofranciscoi and Buttinia andreevi (Mulleret al. 1987; Jaramillo and Rueda 2004), is in theuppermost part of the Buscavidas Formation atBuitrera Creek at the top of Zone 4 (i.e. the twosamples above 600 m on Figures 7 and 11). No
Figure 6. Distribution chart of terrestrial palynomorphs from the Buitrera Creek section.
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samples could be obtained between 469.8 m and609.8 m where the section is not accessible. Thesespecies could therefore be present as low as 470 m, i.e.above 469.8 m. On this basis, and on the basis of oursequence-stratigraphic interpretation (see Figure 11and Section 5 for details), we placed the Campanian–Maastrichtian boundary at 475 m.
5. Sequence-stratigraphic interpretation and discussion
To analyze the distribution of palynomorph assem-blages within a sequence-stratigraphic framework, it isassumed that a regular pattern of upsection changes inpalynological assemblages correspond to environmen-tal changes associated with sea-level rise and fall(eustacy). Spearman correlation tests and DetrendedCorrespondence Analysis (DCA) (Hill and Gauch1980) were performed on the palynomorph percentage
data to assess the significance of the observed patterns.DCA is an ordination method used to summarize datavariation in a small number of dimensions. Theordination was performed on 5 of the 6 palynologicalassemblage (i.e. peridinioid cysts, gonyaulacoid cysts,gymnodinioid cysts, pollen and spores) percentagedata from both of the studied stratigraphic sections;ceratioid cysts were not included due to their sparseoccurrence at Aico Creek and their absence at BuitreraCreek.
Marine affinity palynomorphs (i.e. sum of peridi-nioid and gonyaulacoid cyst percentages) were com-pared with estuarine affinity palynomorphs (i.e. sum ofgymnodinioid cyst percentages) from Aico Creek andyielded a Spearman’s rank correlation coefficient of –0.39, p-value 0.1075, which indicates a moderatenegative correlation between the two groups. Estuarineaffinity palynomorphs were then compared with
Figure 7. Distribution chart of marine palynomorphs from the Buitrera Creek section.
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Figure
8.
Palynologicalrangechart
oftheAicoCreek
section.(Terrestrialtaxain
black,marinetaxain
gray.)
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terrestrial affinity palynomorphs (i.e. spores andpollen) from Aico Creek and a Spearman’s rankcorrelation coefficient of –0.58, p-value 0.01341, wasdetermined. Comparison between marine affinitypalynomorphs and terrestrial affinity palynomorphsfrom Aico Creek yielded a Spearman’s rank correla-tion coefficient of –0.56, p-value 0.009114, indicatingmoderate negative correlation. An onshore–offshoregradient is apparent on the DCA1 axis for the AicoCreek DCA biplot (Figure 12); pollen plots closely tospores, followed by gymnodinioid cysts, and thengonyaulacoid cysts and peridinioid cysts. A similarpattern for palynological assemblages ordered along agradient on the DCA1 axis is seen for the BuitreraCreek DCA biplot (Figure 12).
The upsection changes of the palynomorph assem-blages described above (Figures 10 and 11) werecombined with a conventional lithological interpreta-tion to establish a sequence-stratigraphic framework.In general, the Lowstand System Tract (LST) ischaracterized by abundance spikes of spores accom-panied by pollen whereas the Transgressive SystemsTract (TST) is indicated by the occurrence of estuarinedinoflagellates (ceratioid cysts and/or gymnodinioidcysts). Maximum Flooding Surfaces (MFS) are usuallyrepresented by abundant open marine dinoflagellates
(gonyaulacoid cysts and/or peridinioid cysts) and thescarcity of terrestrial palynomorphs. A gradual de-crease in open marine dinoflagellates, the occurrence ofestuarine dinoflagellates and a consistent presence ofterrestrial palynomorphs represent the onset of theHighstand Systems Tract (HST).
In terms of sedimentary facies, limestone and chertare considered to represent basinal paleoenvironmentswhereas mudstone and sandstone are considered torepresent more proximal paleoenvironments. Sincethese basinal and proximal paleoenvironments arelateral facies equivalents, the sequence-stratigraphicinterpretation of lithologic data relied heavily on theobserved patterns of upsection lithologic changesrather than lithology at any particular depth. Theobserved upsection lithology change (and/or asso-ciated palynomorph-assemblage change) thereforedepended upon the paleoenvironment existing at thetime prior to the onset of relative sea-level changes.
5.1. Aico Creek section
At the base of the section, up to 43.5 m, the poorrecovery of palynomorphs prevented sequence-strati-graphic interpretation as only a few specimens ofestuarine dinoflagellates (i.e. gymnodinioid cysts),
Figure 9. Palynological range chart of the Buitrera Creek section. (Terrestrial taxa in black, marine taxa in gray.)
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peridinioid cysts and spores were found in this part ofthe section. In this zone, system tracts are interpretedon the basis of lithological changes as well as
correlation to the global cycle chart (Haq et al.1987). At 43.5 m a MFS is placed near the top of thelower Lidita Formation based on the occurrence of a
Figure 10. Stratigraphic distribution chart showing the relative abundance of the six palynological assemblages (percentage) atthe Aico Creek section. Also shown are the lithostratigraphy and the sequence-stratigraphic interpretation correlated with theHaq et al. (1987) eustatic curve plotted against the Geologic Time Scale of Gradstein et al. (2004). Percentage values for eachassemblage is calculated using the total number of specimens of each assemblage against the sum of specimens of all sixpalynological assemblages.
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Figure 11. Stratigraphic distribution chart showing the relative abundance of the six palynological groups (percentage) at theBuitrera Creek section. Also shown are the lithostratigraphy and the sequence-stratigraphic interpretation correlated with theHaq et al. (1987) eustatic curve plotted against the Geologic Time Scale of Gradstein et al. (2004). Percentage values for eachassemblage is calculated using the total number of specimens of each assemblage against the sum of specimens of all sixpalynological assemblages.
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Figure 12. DCA biplots of palynological assemblage distribution at the Aico and Buitrera Creek sections.
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spike in abundance of ceratioid cysts coincident withan overall lithologic change from limestone in theunderlying LST to chert/limestone in the TST, whichsuggests an overall deepening of water depth at thesite. Within the top of the overlying HST, i.e. from 79to 89 m, the dominance of gymnodinioid cysts coupledwith the occurrence of terrestrial palynomorphs isinterpreted to correspond to relative sea-level fall at theend of the HST. Sandstone deposition at 99.7 m isinterpreted to represent the onset of the Shelf MarginSystems Tract (SMST¼ lowstand deposition), i.e. asequence boundary (at 83 Ma) is placed below thesandstone. At 114.5 m, the occurrence of estuarinedinoflagellates coupled with high abundances ofgonyaulacoid cysts is associated with the transgressivesystem tract (TST). A subsequent decrease of gonyau-lacoid cysts, together with the occurrence of spores andestuarine dinoflagellates at 125 m, is interpreted torepresent the end of the HST, i.e. a sequence boundary(at 80 Ma). The sequence boundary is at the base of thesandstone at 125 m.
Starting at c. 125 m, the upsection vertical changefrom sandstone to mudstone to limestone is interpretedas representing the progressive deepening from LST toTST within the overlying sequence. The subsequentshift from chert to limestone at 135 m is taken torepresent slow relative sea-level fall during the lateHST. The sequence boundary (at 79 Ma) is placedabove the finer-grained limestone and below the baseof the overlying coarser-grained limestone. The over-lying LST contains gymnodinioid cysts, pollen andspores which represents the major sea-level fall(associated with the 79 Ma sequence boundary). Asequence boundary (at 77.5 Ma) is placed at c. 155 mat the base of the coarse limestone.
Above the sequence boundary, the lithologicchange from limestone to chert is interpreted torepresent an overall relative sea-level rise during theLST and TST. The shift from chert to limestone tomudstone is interpreted to represent progressiveshoaling during relative sea-level fall associated withthe HST. Palynomorphs are absent from the LST, TSTand base of the HST; however, marine and estuarinepalynomorphs co-occur in the top of the HST. Withinthe context of the sequence-stratigraphic interpreta-tion, the late HST represents the lowest water depthpaleoenvironments for this sequence at the Aico Creeksection. The sequence boundary (at 75 Ma) is placed atthe base of the coarse limestones. The basal-most LSThas an increase in spore and pollen, which is taken torepresent the relative sea-level fall associated with the75 Ma sequence boundary. In the overlying part of theLST and overlying TST, the occurrence of gonyaula-coid and peridinioid cysts is interpreted to representprogressive relative sea-level rise. If correct, the
continued presence of spores and pollen representsbasinward transport of palynomorphs from terrestrialsettings. The inferred decrease of marine and estuarinepalynomorphs above 230 m and continued presence ofterrestrial palynomorphs are interpreted to representshoaling in the HST. The sequence boundary (at 71Ma) is placed at c. 232 m. The overlying sequence andsequence boundary (at 68 Ma) is inferred primarily onthe basis of the coarse sandstone beds above the 68 Masequence boundary at 252 m, i.e. the base of thesandstone beds. Lowstand deposition is interpretedfrom the entire interval above the sequence boundarybased on the abundances of terrestrial palynomorphsat 275.7 and 276.7 m.
5.2. Buitrera Creek section
At the base of the Buitrera Creek section (9.6–28.8 m),the occurrence of gymnodinioid cysts along withspores and pollen are taken to represent the LST. Anincrease in peridinioid cysts at 56 m indicates the onsetof the TST and a spike of peridinioid cysts at 78.4 m atthe top of the TST is interpreted as the MFS. Asequence boundary (80 Ma) is placed at 81 m. In thesequence above the 80 Ma sequence boundary, theoverlying lithologic change from mudstone to lime-stone to chert is taken to represent an overalldeepening associated with relative sea-level rise.
A sequence boundary corresponding to relativesea-level fall at 79 Ma is inferred to be within the chertsequence. This is consistent with the stratigraphicframework interpreted from the Aico Creek section(Figure 10) although the samples are barren and thereare no lithologic changes. The absence of lithologicchange may simply mean that water depth remaineddeep despite a relative sea-level fall. Indeed, the sea-level fall at 79 Ma was relatively minor.
A sequence boundary is placed at the base of thelimestone bed at c. 160 m which is interpreted tocorrespond to an abrupt relative sea-level fall at 77.5Ma. The increase in peridinioid cysts at 198.6 m istaken to represent the top of the TST, i.e. the MFS.The overlying lithologic change from chert to lime-stone to mudstone is taken to represent progressiveshoaling and relative sea-level fall in the HST. At thetop of the HST estuarine and terrestrial palynomorphsbecome successively abundant, which likewise isconsistent with upward shoaling. The sequence bound-ary (at 75 Ma) is placed at c. 260 m, just below thedecrease in marine palynomorphs at 262.2 m. In theoverlying LST, the upsection transition from mudstoneto limestone is taken to indicate relative sea-level rise.The increased abundance of peridinioid cysts at294.9 m is taken to correspond to the top of theTST. The increased abundance of pollen/spore at
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302.9 m and concomitant decrease in peridinioidcysts is taken to indicate shoaling and relative sea-level fall in the HST. The 71 Ma sequence boundaryis placed at c. 400 m based on the abrupt increase inspores and pollen at 409 m. The sandstone at c.465 m is considered to be within the LST. Thecontinued presence of terrestrial and estuarinepalynomorphs together with the absence of peridi-nioid cysts from the overlying TST and HST is takento indicate progressive shoaling. The 68 Ma sequenceboundary is placed at the base of the sandstone at c.490 m.
Based on (1) the absence of Echimonocolpitesprotofranciscoi and Buttinia andreevi up to the sampleat 469.8 m; (2) their presence at 609.8 m; and (3) thesequence-stratigraphic interpretation which places the71 Ma sequence boundary at 400 m and the 68 Masequence boundary at 490 m, we place the Campa-nian–Maastrichtian boundary at 475 m (Figure 11).This interpretation also minimizes the difference inthickness of the Buscavidas Formation between theAico and Buitrera Creek sections.
The section just below the unsampled portion ofthe Buscavidas Formation is placed within the over-lying LST. The sandstone above the inaccessibleportion of the Buscavidas Formation is placed withinthe LST of the overlying sequence. In other words, the63 Ma sequence boundary is placed at c. 600 m.
6. Age and correlation
Age dating the Coniacian–Santonian and Santonian–Campanian boundaries at the Aico Creek section wasaccomplished using ammonite and inoceramid datafrom previous studies (Etayo-Serna 1994; Villamilet al. 1998) as well as some key palynological markersobserved in this study and our sequence-stratigraphicinterpretation. On these bases, the Santonian–Campa-nian boundary is identified at 89 m in Aico Creekwithin the middle part of the El Cobre Formation. TheSantonian–Campanian boundary is below the sectionat Buitrera Creek.
For the Campanian–Maastrichtian boundary, pa-lynologic markers such as Foveotriletes margaritae,Buttinia andreevi and Echimonocolpites protofranciscoi(Muller et al. 1987; Jaramillo and Rueda 2004) wereidentified in both sections and a framework zonation(Zones 1–4) was established. Poor and erratic recoveryin some parts of both sections and covered intervalspresented a challenge to age assignment based solelyon palynology. The Campanian–Maastrichtian bound-ary is picked at 240 m in the Aico Creek section withinthe Buscavidas Formation. At the Buitrera Creeksection, the Campanian–Maastrichtian boundary isplaced at 475 m within the Buscavidas Formation. In
both outcrop sections, the boundary is between thesequence boundaries assigned to sea-level falls at 71and 68 Ma.
Figure 13. Graphic comparison showing correlationbetween the Aico Creek and Buitrera Creek sections usingpalynological zones, first and last appearance datum,maximum flooding surface and sequence boundary asstratigraphic markers. The table provides the depths of keystratigraphic events used for the graphic comparison. Thenumbers in red are inferred elevations for the top of Zone 1and Zone 2 at the Buitrera Creek section based on thiscomparison.
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Correlation between Aico and Buitrera Creek wasachieved using the palynostratigraphic Zones 1–4 andthe sequence-stratigraphic approach described in theprevious section. Figure 13 presents the correlation ofthe two sections based on the biozones and sequenceboundaries.
7. Conclusions
Palynological data recovered from the Aico andBuitrera Creek sections in the UMV has provideduseful biostratigraphic and paleoenvironmental infor-mation. Four palynological zones are defined that helpto differentiate the lithologically monotonous LowerLidita, El Cobre, Upper Lidita and Buscavidasformations.
Cyclic patterns in palynomorph abundances andlithological changes were used to construct asequence-stratigraphic framework. In both sections,the absolute abundances of the various palynomorphassemblages enabled recognition of lowstand systemstracts, transgressive systems tracts, maximum flood-ing surfaces (MFSs) and highstand systems tracts.The sequence-stratigraphic interpretation was basedon calibration to the Haq et al. (1987) eustatic sea-level curve for the Santonian–Early Maastrichtianinterval. Additionally, previous conventional se-quence-stratigraphic interpretations for the samelithostratigraphic units in other parts of the UMVbasin (e.g. Guerrero 2002) have been incorporatedwithin our study.
The results of this study support earlier observa-tions by Poumot (1989), Gregory and Hart (1992)and Holz and Dias (1998) who proposed thatpalynological data is a valuable tool for sequence-stratigraphic interpretation. In diverse geologicalsettings such as the structurally complex UMV,where conventional sequence-stratigraphic tools (i.e.seismic, stacking patterns, lithological changesalone or calcareous microfossil content) cannot beapplied, palynological data is essential. Future workon additional outcrop sections is needed to sub-stantiate and build on the framework presented inthis paper.
Acknowledgements
The authors are grateful to Dr David Pocknall andtwo anonymous reviewers for their in-depth critique ofthe manuscript, which resulted in a much-improvedpaper. S. Garzon would like to thank the Museum ofNatural Science and the Department of Geology andGeophysics CENEX Laboratory at Louisiana StateUniversity (Baton Rouge, LA), the Smithsonian TropicalResearch Institute in Panama and the ColombianPetroleum Institute for their financial and logistical
support during her masters thesis research project.The authors also thank Dr Carlos Jaramillo, CarlosSantos and Manuel Paez for their collaboration with thisproject.
Author biographies
SANDRA GARZON is from Bucaramanga,Colombia. She obtained her bachelor’s degreein geology from the Universidad Industrialthe Santander in 2007. Sandra worked at theColombian oil company ECOPETROL fortwo years as a junior biostratigrapher beforecoming to the United States in the summer of
2009 to obtain her Master of Science degree from theDepartment of Geology and Geophysics at Louisiana StateUniversity. Once at Louisiana State University she worked aseducational curatorial assistant for the LSU Museum ofNatural Science. Sandra worked as an intern with BP in thesummer of 2011 and joined BP full-time in February 2012based in Houston, Texas.
SOPHIE WARNY is an Assistant Professorof Palynology in the department of Geologyand Geophysics & at the Museum of NaturalScience at Louisiana State University inBaton Rouge. She has a long history withAASP as she won the AASP Student Awardin 1996. She received her Ph.D. from the
Universite Catholique de Louvain, in Belgium working withDr Jean-Pierre Suc. Her doctoral dissertation focused on theMessinian Salinity Crisis. Since graduating, she has beenworking on Antarctic sediments that were acquired via theANDRILL SMS and the SHALDRIL programs. In 2011,she received the prestigious NSF CAREER award to supporther palynological research in Antarctica. Now that she livesin Louisiana, her research focus has expanded to the Gulf ofMexico region and to some other sequences thanks to herstudents’ research interests. In addition to her research, sheteaches Historical Geology, Paleobotany, and Micropaleon-tology. She also manages the education and outreachprograms for the Museum. She currently has a researchgroup at the remodeled CENEX facility at LSU (BatonRouge, Louisiana) that is composed of four master’s studentsand three Ph.D. students.
PHILIP BART is an Associate Professor inthe Department of Geology and Geophysics atLouisiana State University in Baton Rouge.He earned his Ph.D. degree from RiceUniversity in Houston, working with DrJohn Anderson. He received the PresidentialEarly Career Award for Scientists and En-
gineers (PECASE) in 2001 for his seismic and sequencestratigraphic research in Antarctica. His other research interestsconsists in evaluating the sequence stratigraphy of the lastglacial cycle and the sea-level elevation at the peak of the LGMbased on seismic data from the northeastern Gulf of Mexico.
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