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14. Neogene and Quaternary Calcareous Nannofossil Biostratigraphy of the Walvis Ridge1

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14. Neogene and Quaternary Calcareous Nannofossil Biostratigraphy of the Walvis Ridge1 14. NEOGENE AND QUATERNARY CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY OF THE WALVIS RIDGE1 Ming-Jung Jiang, Robertson Research (U.S.) Inc., Houston, Texas and Stefan Gartner, Department of Oceanography, Texas A&M University, College Station, Texas ABSTRACT During DSDP Leg 74 in the South Atlantic, 11 holes were drilled at 5 sites (525-529) on the eastern end of Walvis Ridge. The sediments recovered range in age from late Campanian or early Maestrichtian to Holocene. This chapter deals only with the Neogene to Holocene interval. The calcareous nannofossils over this interval have been corroded as well as overgrown, but a precise biostratigraphy is possible at all sites because most of the key index species are present. INTRODUCTION CALCAREOUS NANNOFOSSIL ZONATION During DSDP Leg 74, 11 holes were drilled at 5 sites Martini's standard Tertiary and Quaternary zonation (525-529) on Walvis Ridge in the South Atlantic near (1971), which is widely used elsewhere, was the starting South Africa (Fig. 1). Sediments ranging in age from basis for the zonation described here. Because of the late Campanian or early Maestrichtian to Holocene scarcity or apparent lack of some of Martini's index fos- were recovered. This chapter deals with the calcareous sils, possibly owing to environmental exclusion of the nannofossils found in the Neogene to Holocene sedi- parent organism or to poor preservation, other species ment from these five sites. The calcareous nannofossils were selected to determine zones. Deviations from Mar- from the Paleogene and the Upper Cretaceous are docu- tinis zonation, necessary for the studied area, are dis- mented by Hélène Manivit in this same volume. cussed in the following. The age and zone assignments Calcareous nannofossils from the Neogene through of the Neogene through Holocene cores based on this Holocene recovered during DSDP Leg 74 are abundant modified zonation are then given in Table 1. at all sites, except for the middle Miocene red clay inter- val cored at Site 527—the deepest site. Their state of Triquetrorhabdulus carinatus Zone (NN1) preservation, however, is ranked as "moderate" in gen- Definition. Interval from the last occurrence of Helicosphaera rec- eral. Dissolution and recrystallization have progressed ta to the first occurrence of Discoaster druggii above it. Remarks. Because of environmental exclusion, Helicosphaera rec- to such an extent that only solution-resistant or robust ta together with H. perch-nielseniae is present only at Site 526, the forms can be recognized for most of the middle and shallowest site. The last occurrence of Sphenolithus ciperoensis is ten- lower Miocene sections. Fortunately, most of the zonal tatively used to define the base of Zone NN1, i.e., the Oligocene/ markers are present and can be identified with certainty. Miocene boundary, in this study. Other important and solution-resis- tant species, which become extinct slightly above the last occurrence The main purpose of this chapter is, therefore, to docu- of Sphenolithus ciperoensis, are Dictyococcites bisectus (Dictyococci- ment the biostratigraphic distribution of the calcareous tes abisectus included), aff. D. dictyodus, S. capricornutus, S. del- nannofossils rather than to do a complete taxonomic phix, and Zygrhablithus bijugatus. S. capricornutus and S. delphix study. have very short ranges. Both forms first occur above the last occur- rence of S. ciperoensis and become extinct at about the same level as Smear slides of the original unprocessed samples Dictyococcites bisectus, aff. D. dictyodus, and Z. bijugatus. were examined with a light microscope for this study. In Because of the strong overgrowth of discoasters within this inter- order to obtain a precise position of the zonal bounda- val, typical specimens of Discoaster druggii cannot be identified with ries, one sample was taken for each section (150 cm) of a certainty. The first occurrence of an unusual form of discoaster is at core and investigated for its calcareous nannofossil con- the top of Zone NN1. This species is consistently much larger and more robust than the common D. deflandrei specimens in this inter- tent. For some critical intervals, more closely spaced val. It has straight and slightly tapered rays and is documented as Dis- samples were analyzed. Therefore the stratigraphic first coaster sp. aff. D. druggii in this study. and last appearance of the selected species was deter- mined with an accuracy of 150 cm or less for this report. Discoaster druggii Zone (NN2) Checklists of a selected set of species were prepared for Definition. Interval from the first occurrence of Discoaster drug- each site. gii to the last occurrence of Triquetrorhabdulus carinatus. Remarks. The first occurrence of Discoaster sp. aff. D. druggii is used to mark the base of Zone NN2, whereas the first occurrence of Sphenolithus belemnos marks the top of this zone in this study. This Moore, T. C, Jr., Rabinowitz, P. D., et al., Init. Repts. DSDP, 74: Washington (U.S. zone is therefore equivalent to Zone CN1C of Okada and Bukry Govt. Printing Office). (1980). 561 M.-J. JIANG, S. GARTNER W Angola Basin 20° S -2 25< -3 30c 35C i 1 1 1 1 r 20 Time (hr.) Ship Course Track 315° Figure 1. Site locations on the Walvis Ridge in the South Atlantic Ocean for DSDP Leg 74. The last occurrence of T. cαrinαtus is uncertain, because in some boundary has been identified variously as older, equal to, and younger cases it is difficult to distinguish T. cαrinαtus from an elongate calcite than this datum.) lath in an assemblage with moderate or poor preservation and recrys- tallization. Reticulofenestrα pseudoumbilicα sample and Cαlcidiscus Sphenolithus heteromorphus Zone (NN5) leptoporus have their first occurrence in this zone. Definition. Interval from the last occurrence of Helicosphαerα αm- pliαpertα to the last occurrence of Sphenolithus heteromorphus. Sphenolithus belemnos Zone (NN3) Remarks. At the boundary of Zones NN4/NN5, i.e., the lower/ Definition. Interval from the last occurrence of Triquetrorhαb- middle Miocene boundary, assemblages of discoaster change signifi- dulus cαrinαtus to the last occurrence of Sphenolithus belemnos. cantly. Discoαster deflαndrei, which is a dominant species in the lower Remarks. The total range of S. belemnos marks Zone NN3 in this Miocene, is replaced abruptly by (overgrown) D. vαriαbilis, although study. Sphenolithus sp. aff. S. belemnos documented in the tables the former persists upward. The first common occurrence of D. vαri- could be small specimens of S. dissimilis. αbilis s. ampl. is therefore used to make the base of Zone NN5 because this datum is easy to recognize (see Bukry, 1973). Helicosphαerα αmpliαpertα Zone (NN4) The top of this zone is also easy to recognize in the studied sites. Definition. Interval from the last occurrence of Sphenolithus be- lemnos to the last occurrence of Helicosphαerα αmpliαpertα. Remarks. S. heteromorphus first occurs immediately above the ex- Discoαster exilis Zone (NN6) tinction of S. belemnos. Neither gap nor overlap of the ranges of the Definition. Interval from the last occurrence of Sphenolithus two species is observed at the studied sites. The first occurrence of S. heteromorphus to the first occurrence of Discoαster kugleri. heteromorphus may be a good marker for the boundary of Zones Remarks. Because of strong overgrowth of discoasters, typical D. NN3 and NN4 (Bukry, 1972, 1973). kugleri cannot be identified with certainty. The top of Zone NN6 is The index fossil, H. αmpliαpertα, is absent, probably because of therefore placed at the last occurrence of Cyclicαrgolithus floridαnus, ecological exclusion. The first common occurrence of Discoαster vαri- a species which is common within and below this zone (see, e.g., αbilis s. ampl. is tentatively used to mark the top of Zone NN4, i.e., Bukry, 1975). the top of the lower Miocene. (The lower Miocene/middle Miocene Coronocyclus nitescens seems to become extinct within this zone, boundary is for the sake of convenience somewhat arbitrarily placed whereas Triquetrorhαbdulus rugosus has its first occurrence in this at the top of the H. αmpliαpertα Zone, recognizing, however, that the zone. 562 Table 1. Zonal and geologic age assignments of Leg 74 cores, based on calcareous nannofossils in samples from holes at Sites 525-529. Calcareous Nannofossil Age Zonation (Martini, 1971) 525 525A 525B 526 526A 526B 526C 527 528 528A 529 NN21 Emiliania huxleyi 1-1, 30-32 1-1, top 1-1, 80-82 1-1, 40-41 late 1-2, 30-32 1-1, 101-102 Pleistocene NN20 Gephyrocapsa oceanica 1-1, 20-21 1-1, 28-29 1-1, 150 2-1, 106-107 early NN19 Pseudoemiliania lacunosa 1,CC l.CC 1-2, 80-82 2,CC 1-1, 30-32 1-1, 29-31 1-2, 20-21 1-1, 120-122 1-2, 40-41 3-1, 76-78 3-1,35-36 2-1, 50-51 1,CC 5-3, 19-20 2-6, 40-41 NN18 Discoaster brouweri 3-2, 53-54 3-2, 35-36 2,CC 5-4, 19-20 2,CC 4-1, 75-77 3,CC 6-3, 19-20 late NN17 Discoaster pentaradiatus 4-2, 65-66 3-1, 35-37 NN16 Discoaster surculus 4-3, 65-66 3-1, 50-51 6-3, 111 3-1, 30-31 5-1, 81-83 3-4, 35-37 7-2, 100 5-2, 81-83 4-2, 20-21 Pliocene NN15 Reticulofenestra 1-1, 75-76 1-1, 100 3-4, 50-51 7-3, 40-41 3-2, 30-31 pseudoumbilica 8-2, 86-87 5,CC l.CC 5,CC 11-3, top 4-6, 40-41 NN14 Discoaster asymmetricus 8-3, 3-4 1-2, 75-76 6-1,40-42 11,CC 4,CC 13-2, 63-64 early NN13 Ceratolithus rugosus 3-1, 29-30 1-3, 75-76 6-2, 40-42 13-3, 63-64 12-1.
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