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Evolution of the Dinocyst assemblages, organofacies and pal ynofacies: a paleoenvironmental interpretation

Dinocyst assemblages from the studied 959D samples are characterized by a relative high abundance of Spiniferites spp. and Achomospharea spp. (Fig. 1.7), but successive rises and falls mark the history of the two genera. In the lower part of the studied interval (1045 – 1033.7 mbsf), Coniacian in age, Palaeohystrichopora spp. dominate and Dinogymnium spp. are also common. From the lower part of the Campanian (1011.91 mbsf) to the base of the Danian (893.8 mbsf) the taxa showing important peaks are; Circulodinium spp., Trichodinium castaneum, Spinidinium spp., Trithyrodinium spp., Kallosphaeridium spp. and Areoligera spp. In the upper part of the studied interval (from 886.8 mbsf to the top) dinocyst assemblages do not show clear peaks of dominance by a single taxon. The frequency distribution of taxa is more equitable in the assemblages.
Previous studies on the sedimentological, structural and paleoenvironmental evolution of the Côte d’Ivoire – Ghana transform margin suggest a first marked phase of deepening in the 959D site from the upper Coniacian to the lower Campanian (ca. 1035 to 1015 mbsf, Fig. 1.3) (Saint-Marc and N’Da, 1997; Basile et al., 1998; Wagner and Pletsch, 1999; Pletsch et al., 2001). The assemblages of benthic foraminifera from this interval suggest an extended oxygen minimum zone in the water column (Pletsch et al., 2001; Friedrich and Erbacher, 2006). Subsidence rates of the CIGMR crest and, northern and southern flanks are different, and reworking processes are considered almost constantly active (Wagner and Pletsch, 1999). After the high subsidence interval and low oxygen availability, the deepening rate was lower and almost constant during most of the Campanian-Maastrichtian interval. From the early Campanian upwards, the initial reliefs of the sea bottom fall deep through the subsidence (Fig. 1.3), which allowed the deep-water circulation (Saint-Marc and N’Da, 1997; Pletsch et al., 2001; Friedrich and Erbacher, 2006). Depth remained stable for the entire Paleogene (from ca. 890 mbsf to the top of the studied interval) (Pletsch et al., 2001).

Density of sampling and biostratigraphic resolution

Some interesting observations can be made form the comparison of sampling density, the stage boundaries position, the extension of intervals not assigned to any stage and the distance from the lower limit of stages proposed in each of the three studies herein considered to the lower limit of the stages proposed in the consensual biostratigraphy (Table 1.2, Fig. 1.6).
At a first sight it seems that the lower stratigraphic density of our samples prevents the identification of some stages, as the Santonian. Based on the co-occurrence of Isabellidium acuminatum and Unipontidinium grande (Pl. 1, Fig. 1-2) at 1025.37 mbsf (Fig. 1.6), Masure et al. (1998a) suggested a Santonian age for this level. Oboh-Ikuenobe et al. (1998) placed the lower Santonian boundary at 1031.7 mbsf because of the identification of FO of Dingodinium undulosum. The distance between the position of the lower Santonian boundary suggested from dinocyst biostratigraphy by Masure et al. (1998a) and Oboh-Ikuenobe et al. (1998) and that suggested from the combination of all micropaleontological data by Moullade et al. (1998d) result in 7.8 m and 2.0 m of error in the boundary placement, respectively. In the present study the dinocyst assemblage at 1025.37 mbsf was not studied, I. acuminatum was not observed in any of the studied slides and the FO of U. grande was identified at 1015.62 mbsf, within the interval dated as Campanian. The non-observation of these taxa implies that we cannot define the Santonian stage from our dinocyst dataset and that the error associated to the lower Santonian boundary rises to 40.9m in the present study (Table 1.2).
However, sometimes it is not the sampling resolution that determines the precision of the stage boundaries placement but the sample position. Oboh-Ikuenobe et al. (1998) placed the upper Coniacian boundary at 1043.24 mbsf (Fig. 1.6), because of the observation of the only occurrence of Cyclonephelium vannophorum. In the present study the presence of C. vannophorum at 1037.10 mbsf lead us to place the upper Coniacian boundary at 1037.1 mbsf (Table 1.2). Although the sample resolution of the study by Oboh-Ikuenobe et al. (1998) was slightly higher (1 sample each 8.4 m) than in the present study (1 sample each 9.8 m) the error in the determination of the lower Coniacian boundary is higher from Oboh-Ikuenobe et al. (1998) results, 9.7 meters, than from our results, 3.4 m. 5.2 . Rarity effect and the recognition of age markers.

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Rarity effect and the recognition of age markers

All dinocyst events identified by Masure et al. (1998a) and by the present study do not coincide exactly. The dinocyst events herein identified but not identified by Masure et al. (1998a), are the FO of Areoligera spp. and Cerodinium spp. at 996.16 mbsf; the FO of Palaeocystodinium lidiae at 981.18 mbsf; the FO of Carpatella cornuta at 901.9 mbsf; the FO of Adnatosphaeridium multispinosum at 842.48 mbsf and; the occurrence of Areoligera gippingensis at 828.7 mbsf. Obviously, the analysis by Masure et al. (1998a) also identified of some dinocyst events that have not been identified in the present study, like the FO of Glaphyrocysta group, Alterbidinium varium and Cerodinium leptodermum at 952.74 mbsf and the FO of Damassadinium californicum at 901.9 mbsf. When considering individually both studies these differences in the identification and placement of the dinoflagellate events would result in a difference of about 15 meters in the placing of the lower boundary of the Campanian and of about 14 meters for the lower boundary of the Thanetian.

Influence of paleoenvironmental factors on the dinocyst stratigraphic record

Paleoenvironmental control on the dinocyst taxa relative frequency can impact the reconstruction of their stratigraphic distribution. The comparison of dinocyst distribution (Fig. 1.7) with the paleoenvironmental reconstructions proposed by previous studies shows a good fit see Wagner and Pletsch, 1999; Pletsch et al., 2001; Friedrich and Erbacher, 2006. Modifications of environmental conditions have already been proposed for explaining variations in the distribution of dinocyst taxa along sedimentary sequences. For example, see surface temperature variations have been proposed as triggers of dinocyst frequency variations along the K/T boundaries from different regions (e.g. Brinkhuis et al., 1998; Vellekoop et al., 2015). They found that some of the chronobiostratigraphic dinocyst markers (as Palynodinium grallator, Pierceites pentagonus, Phelodinium maginficum or Senegalinium bicavatum) are influenced by SST variations. Improving our knowledge about how paleoenvironmental variation control dinocyst distribution is fundamental to realize the full potential of dinocyst as biostratigraphic markers.

Biostratigraphical framework

Dinoflagellate cysts distribution was studied in the five outcrops of the Lusitanian Basin from a biostratigraphical perspective by Hasenboehler (1981) and Berthou and Hasenboehler (1982). Dinoflagellate cysts biostratigraphy in São Julião, Magoito Aguda and Guincho outcrops was also studied by Horikx et al. (2014) and by Heimhofer et al. (2007), respectively. The precise age of the base of the Galé Formation and of the boundary between the two members vary slightly among authors, the age differences emerging mainly as result of differences in the location of the first occurrences of Xiphophoridium alatum and Chichaouadinium vestitum, two species considered as indicators of the transition from the Early to the Middle Albian (Heimhofer et al., 2007, 2012; Horikx et al., 2014). In the present work X. alatum is found in the lowest sample in the five sections so we consider that the base of the studied outcrops is not older than Middle Albian.
Dinoflagellate cysts distribution of the 398D samples was studied by Masure (1984). The results from dinoflagellate cyst biostratigraphy are consistent with those from foraminifera study (Sigal, 1979) regarding the position of the Early Albian – Middle Albian boundary. Middle Albian – Late Albian boundary and the lower boundary of the Vraconian (uppermost Late Albian) are placed lower with dinoflagellate data (Masure, 1984) than with foraminifera (Sigal, 1979) according the first occurrence of Leberidocysta chlamydata and Cribroperidinium? intricatum.
Pending a clearer resolution of age assignment, we retained a dating and correlation scheme based on dinoflagellate age markers. Xiphophoridium alatum and Litosphaeridium arundum are selected as markers for the Middle Albian interval, Cribroperidinium? intricatum and Leberidocysta chlamydata for the lower part of the Late Albian, and Endoceratium dettmanniae, Litosphaeridium siphoniphorum, Ovoidinium verrucosum and Palaeohystrichophora cf. infusorioides for the Vraconian. All species considered are among the most widespread across the Lusitanian margin and are recognized elsewhere as valuable for stratigraphic purposes (Davey and Verdier, 1971, 1973; Verdier, 1975; Davey, 1979; Fauconnier, 1979; Foucher, 1979; Monteil and Foucher, 1998; Oboh-Ikuenobe et al., 2007; Gradstein et al., 2012).

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