Lower-middle Miocene (328-445 mbsf) and mid-Aptian to terminal Oligocene sections (752-1130.37 mbsf) were cored at Hole 1183A (Table T5). By using planktonic foraminifers and calcareous nannofossils, we detected the presence of 11 unconformities (Table T6), of which the longest are intra-Albian (~10.6 m.y.) and Albian/Coniacian (~13.2 m.y.). The upper Coniacian to lower Campanian section overlies the younger of these two unconformities and is extremely condensed (0.3 m/m.y. accumulation rate), devoid of planktonic foraminifers, and contains sparse, atypical nannofossil assemblages. The Eocene/Oligocene and Cretaceous/Paleogene boundaries were recovered in Sections 192-1183A-24R-2 and 39R-4, respectively. The Cretaceous/Paleogene boundary is unconformable, but the Eocene/Oligocene boundary interval is condensed and appears relatively complete. Most significantly, microfossils (Eprolithus floralis and Leupoldina cabri) limit the age of the sediment immediately above basaltic basement to a short stratigraphic interval straddling the lower/upper Aptian boundary.
Foraminifer abundance at Hole 1183A is high overall, except for rare intervals associated with low-oxygen deposition and/or deposition near or below the foraminifer lysocline; foraminifer preservation generally worsens downsection. Calcareous nannofossils are abundant and poorly preserved throughout most of the section; only the basal 73 cm of the Danian section is barren of nannofossils.
We have completed a preliminary, integrated biostratigraphic framework for both the Cenozoic (Table T7) and Cretaceous (Table T8) intervals. The emphasis is on the Cretaceous-Eocene interval.
We applied the standard Cenozoic biozonations for planktonic foraminifers (Berggren et al., 1995) and calcareous nannofossils (Martini, 1971) to Hole 1183A sediments. We examined the Paleogene portion of the sediments (Cores 192-1183A-19R through 39R) in detail for foraminifers, whereas only the Paleocene-Eocene portion (Cores 24R through 39R) was examined in detail for calcareous nannofossils. We examined single samples from individual cores for both microfossil groups.
Our preliminary results demonstrate one major difference in calibration between these two microfossil groups relative to the Berggren et al. (1995) timescale. In Hole 1183A, the occurrences of Sphenolithus heteromorphus and Globorotalia fohsi fohsi overlap for 28 m. However, Berggren et al. (1995) reported that the extinction of S. heteromorphus preceded the appearance of G. fohsi fohsi by 0.82 m.y. We also encountered minor calibration differences around the Eocene/Oligocene boundary in Section 192-1183A-24R-2 and within the middle Eocene (see Table T7).
The Danian (lower Paleocene) interval is ~2.3 m thick in Hole 1183A (Table T5) and is bounded by unconformities (Table T6). The basal Danian planktonic foraminifer zone (P0) is extremely thin (5 cm); it is characterized in Sample 192-1183A-39R-4, 83-87 cm, by dominant, tiny Guembelitria cretacea and also very rare Hedbergella holmsdelensis, Heterohelix globulosa, and Woodringina hornerstownensis. A thin section from the terminal Cretaceous limestone (Sample 192-1183A-39R-4, 87-90 cm) contains abundant planktonic foraminifers and laminae with graded bioclasts disrupted by common horizontal burrowing. This thin section also bisected a vertical burrow that was filled with microfossils of the Zone P0 Danian assemblage. Planktonic foraminifers and calcareous nannofossils indicate that the upper Maastrichtian interval in Hole 1183A is unconformable with the overlying Danian interval.
Calcareous microfossils can be used to subdivide the Cretaceous section in Hole 1183A into three parts:
The mid-Campanian to Maastrichtian limestone is rich in both foraminifers and calcareous nannofossils. The Campanian/Maastrichtian boundary is associated with a possible unconformity (Table T6) located between Cores 192-1183A-44R and 45R (top of nannofossil Zone CC24). This unconformity is marked, in part, by the highest occurrences of the calcareous nannofossils Quadrum trifidum and Quadrum gothicum in Sample 192-1183A-45R-1, 56 cm. The highest occurrences of these two species are 10 m above the base of the Maastrichtian in the new boundary stratotype section near Tercis, France (J. Bergen, unpubl. data). A second unconformity is suggested by the highest occurrences of three nannofossil taxa in Section 192-1183A-46R-2 at 1051.22 mbsf (see Table T8).
Sparse calcareous nannofossil assemblages dominated by nannolith taxa (e.g., Assipetra and Marthasterites) were recovered from samples taken from the condensed interval between Section 192-1183A-50R-1, 60 cm, and 50R-2, 75 cm. This interval is nearly devoid of planktonic foraminifers, indicating deposition below the foraminifer lysocline. Recovered nannofossils indicate that almost the entire upper Coniacian to lower Campanian interval may be present within this 1.65-m section, although a Santonian/Campanian hiatus may be present (see Table T6).
Approximately 40 m of mid-Aptian to upper Albian limestone was recovered below the major unconformity with the upper Coniacian. Within these mid-Cretaceous sediments, calcareous microfossils indicate a significant mid-Albian unconformity between Sample 192-1183A-52R-2, 105 cm, and 52R-3, 107 cm. In contrast, the Albian/Aptian boundary appears conformable. A small unconformity is inferred within the upper Aptian section (between Sample 192-1183A-54R-2, 28-31 cm, and 54R-2, 35 cm) because the calcareous nannofossil Subzone NC7B and foraminifer G. ferreolensis Zone (see Bralower et al., 1995) are missing.
There is ~2 m of limestone between the intra-upper Aptian hiatus and basalt basement (Samples 192-1183A-54R-2, 34 cm, and 54R-3, 119 cm). The calcareous nannofossil Eprolithus floralis is present in the sample taken immediately above basement (Sample 192-1183A-54R-3, 119 cm) and the foraminifer Leupoldina cabri extends only 23 cm higher in the section. The co-occurrence of these two microfossils defines Zone IC25 of Bralower et al. (1995), who placed this short zone at the base of the upper Aptian (~121 Ma) ~3.5 m.y. younger than their estimate for the basal Aptian. In the historical lower Aptian stratotype (see Moullade et al., 1998), the lowest occurrence of Eprolithus floralis is ~5 m below the base of the upper Aptian and immediately below the base of the Dufrenoyia furcata ammonite zone (Zone IC25 straddles the lower/upper Aptian boundary at this locality). Gradstein et al. (1995) estimated the base of the D. furcata ammonite zone to be ~118 Ma, which they placed ~3.3 m.y. above their estimate for the basal Aptian.
The presence of the foraminifer index Praehedbergella sigali within limestones intercalated with lava flows in the upper 6 m of the basement section (Samples 192-1183A-54R-4, 7-9 cm, and 55R-1, 6-9 cm) indicates an early Aptian age.
Paleoenvironmental analysis was often hampered in Hole 1183A by generally poor recovery of benthic foraminifers in pelagic limestone and chalk. Nevertheless, intensive examination of washed residues and thin sections allowed us to make a reasonable paleoenvironmental interpretation for most of the section. Most of the interval is indicative of normal, deep-water, pelagic carbonate mud deposition. However, we also noted several indications of variation in sedimentation rate and bottom-water oxygenation, as well as sporadic occurrences of allochthonous facies distributed through most of the section. Postcruise studies will be necessary to establish the full distribution of these less common facies. Our initial examination of the section, however, allowed an estimation of paleobathymetric trends for Hole 1183A through time (Table T9). These trends proved to be more variable than we originally anticipated.
Relatively shallow-water benthic foraminifers were recovered from Aptian limestone immediately above basaltic basement, dominated by nodosariids such as Lenticulina gaultina, Dentalina nana, and Astacolus incurvata, as well as Gavelinella barremiana and indeterminate species of Polymorphinidae. Relatively shallow water depths, comparable to the middle to outer shelf in continental margin environments, are indicated by these species. The appearance of common small, spherical radiolarians in the upper Aptian section marked the beginning of a rapid deepening that continued into the Late Cretaceous, probably caused by a rapid subsidence of this part of the Ontong Java Plateau. In the lower Albian, the nodosariid assemblage mixed with slope species of agglutinated foraminifers such as Dorothia filiformis and hyaline species such as Gyroidinoides primitiva and Valvulineria loetterlei. Farther upsection within the Albian, the nodosariid species decline and robust agglutinates such as Gaudryina dividens and common Gyroidinoides infracretacea and Osangularia spp. appear, indicating continued deepening to depths comparable to the lower half of the slope in continental margin environments. The condensed Coniacian to basal Campanian interval marks the climax of the initial stage of subsidence at Site 1183. Abyssal benthic foraminifers characterize the section, including Gaudryina pulvina and Eggerellina sp. 1 Moullade (Sikora and Olsson, 1991).
Evidence of shallowing paleobathymetry and uplift of Site 1183 is present in the overlying thick Campanian to Maastrichtian section. The shallowing is first evident in an increase in benthic foraminifer diversity in the middle Campanian section, with a mixture of deep-water agglutinated species, such as Kalamopsis grzybowskii and Verneuilina cretacea, and shallower-water slope species such as Dorothia oxycona, Nuttallides bronnimanni, and Pleurostomella. Deposition at depths corresponding to the lower half of the slope on continental margins is indicated, and the abyssal species of the condensed zone are absent. The upper Campanian through mid-Paleocene section indicates further shallowing as species of Gyroidinoides, Globorotalites, and Osangularia become more common. The peak of this late Cretaceous shallowing occurred in the late Campanian with a reappearance of rare nodosariids such as Lenticulina munsteri and Dentalina communis.
Our analysis of the post-mid-Paleocene section indicates a resumption of rapid deepening in the late Paleocene, with a reoccurrence of lower slope species such as Nuttallides truempyi and Clavulinoides paleocenica. Abyssal taxa, including Cibicidoides grimsdalei and Bulimina callahani, replaced the slope assemblage by the middle Eocene. Abyssal conditions continued through deposition of the younger section, as indicated by facies markers such as Melonis pompilioides forma sphaeroides in the Oligocene and Planulina wuellerstorfi and Laticarinina pauperata in the Miocene.