Middle Eocene to mid-Aptian sediments (697.4-968.6 mbsf) were cored at Site 1186 (Table T5). Poor sediment recovery (18%) limited our ability to detect potential unconformities, especially within the Cenozoic (average recovery, 8.6%). Within the Cretaceous, two long hiatuses in deposition identical to those recognized at Site 1183 are present: intra-Albian (~10.6 m.y.) and upper Albian to upper Coniacian (~13.2 m.y.). Also, at both sites unconformities are present within the upper Aptian and at the Campanian/Maastrichtian boundary. As at Site 1183, the presence of the calcareous nannofossil Eprolithus floralis and the planktonic foraminifer Leupoldina cabri limit the age of the sediment immediately above basaltic basement to a short stratigraphic interval straddling the lower/upper Aptian boundary.
An integrated biostratigraphic framework is compiled for the Cenozoic in Table T6 and for the Cretaceous in Table T7.
We applied the standard Cenozoic biozonations for planktonic foraminifers (Berggren et al., 1995) and calcareous nannofossils (Martini, 1971) to the lower Paleocene to middle Eocene sediments recovered from Hole 1186A. The extremely low recovery (8.6%) in Cores 192-1186A-2R through 13R makes it impossible to precisely delimit any unconformities or establish precise correlations to other Leg 192 sites. However, the following unconformities may be present:
The Cretaceous/Paleocene boundary was not recovered because only 0.22 m of sediment was recovered in Core 192-1186A-14R. Sample 192-1186A-13R-CC contains Danian calcareous nannofossils (Zone NP2) and planktonic foraminifers (mid-Zone P1b). Common reworked upper Maastrichtian planktonic foraminifers and rare calcareous nannofossils also characterize the sample. The presence of the calcareous nannofossil Micula praemura and the planktonic foraminifer Rugotruncana subpennyi in the highest Cretaceous sample examined (Sample 192-1186A-14R-CC) may indicate that the terminal Maastrichtian is absent in Hole 1186A (Table T7).
Calcareous microfossils can be used to subdivide the Cretaceous section from Hole 1186A into three parts: (1) microfossil-rich mid-Campanian to Maastrichtian chalk in interval 192-1186A-14R-CC to 26R-2; (2) condensed upper Coniacian to lower Campanian claystone in interval 192-1186A-26R-2 to 26R-3, 32 cm; and (3) mid-Aptian to upper Albian limestone in interval 192-1186A-26R-3, 33 cm, to 30R-1, 43 cm.
The mid-Campanian to Maastrichtian chalk is rich in calcareous nannofossils. Planktonic foraminifers are abundant and well preserved in the Maastrichtian section (Cores 192-1186A-14R through 19R) but are nearly absent in the Campanian, indicating deposition below the foraminifer lysocline. The one exception is Sample 192-1186A-23R-CC, 10-15 cm, in which moderately well preserved, upper middle Campanian planktonic foraminifers are common, including the stratigraphically short-ranging index, Radotruncana calcarata. The Campanian/Maastrichtian boundary is associated with a potential unconformity (Tables T5 and T7) located between Cores 192-1186A-19R and 20R (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-20R-CC. The highest occurrences of these two species are 10 m above the basal Maastrichtian in the new boundary stratotype section near Tercis, France (J. Bergen, unpubl. data).
Sparse calcareous nannofossil assemblages dominated by nannolith taxa (e.g., Assipetra and Marthasterites) were recovered from samples taken from the condensed interval in Section 192-1186A-26R-3, between 4 and 32 cm. This interval is nearly devoid of planktonic foraminifers, indicating deposition below the foraminifer lysocline. Recovered nannofossils indicate upper Coniacian and lower Campanian material within this 0.28-m interval. The Santonian is either absent at this locality or was not recovered (a chert nodule was recovered between 7 and 10 cm in Section 192-1186A-26R-3).
Thirty-seven meters of mid-Aptian to upper Albian limestone was recovered below the major unconformity at the base of the upper Coniacian. Within these mid-Cretaceous sediments, calcareous microfossils indicate a significant mid-Albian unconformity between Samples 192-1186A-28R-2, 3 cm, and 28R-3, 22 cm. Planktonic foraminifers place the Albian/Aptian boundary between Section 192-1186A-28R-CC and 29R-1, 68 cm. A small unconformity is inferred within the upper Aptian section (between Section 192-1186A-29R-CC and 30R-1, 11 cm) because of the absence of the Globigerinelloides ferreolensis foraminifer zone (see Bralower et al., 1995).
The calcareous nannofossil Eprolithus floralis is present in the sample taken immediately above basement (Sample 192-1186A-30R-1, 43 cm) and the foraminifer Leupoldina cabri occurs only 33 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.
Brecciated limestone and fracture fill within the upper 14 m of the basalt basement have an early Aptian age based upon the presence of the planktonic foraminifer indices Praehedbergella sigali and Blefuscuiana occulta (Samples 192-1186A-30R-1, 73-75 cm, and 32R-4, 89-92 cm).
Other than in sediment deposited below the foraminifer lysocline, benthic foraminifers are abundant, diverse, and well preserved in the Aptian to Eocene assemblage cored at Site 1186. Paleo-water depths range from middle slope for the upper Aptian to abyssal for the post-Albian section. There are indications of shallowing in the upper Campanian to Maastrichtian section similar to that observed at Site 1183. However, at present, detailed paleoenvironmental analyses have been completed only for the upper Aptian to lower Albian section.
The upper Aptian to upper Albian section (192-1186A-30R-1, 11-15 cm, through 27R-1, 37-39 cm) is characterized by slope species of benthic foraminifers diluted by more common shallow-water components, such as diverse nodosaroid benthic foraminifers, ostracodes, and often coarse-grained mollusk and echinoid debris. Volcanic ash is also common to abundant. The section indicates persistent downslope movement of sediment at or near Site 1186 during late Aptian to late Albian time. Upper Aptian to lower Albian in situ benthic species indicate a slope assemblage near the lower-upper slope transition and include Gyroidinoides crassa, G. subglobosa, Gavelinella schloenbachi, and Clavulina gabonica. Upper Aptian to lower Albian planktonic foraminifers exhibit a complementary pattern. A deep-water, open-marine assemblage dominates most of the section (i.e., abundant, large Globigerinelloides, such as G. algerianus and G. barri, with, in the upper Aptian, a very diverse assemblage of Blowiella species). However, several discrete intervals (e.g., Sample 192-1186A-29R-2, 131-134 cm) are strongly dominated by an assemblage of small Blefuscuiana species, similar to that in "shelf" limestone at Site 1183. These Blefuscuiana-dominated intervals likely represent deposits of fine-grained calcareous debris flows.
The upper Albian section marks the beginning of a long period of deposition below the foraminifer lysocline (i.e., spanning upper Albian through upper Campanian, Cores 192-1186A-28R through 20R). The lower part of the upper Albian section (Sample 192-1186A-28R-2, 136-139 cm) contains a lower-slope agglutinated benthic assemblage similar to that noted by Scheibnerová (1974) in the Albian of the western Australian slope; e.g., Verneuilina howchini, Haplophragmoides chapmani, and Textularia wilgunyaensis. Higher in the upper Albian (Sample 192-1186A-27R-1, 37-39 cm) is an interval containing only calcareous nannofossils and diverse radiolaria. Calcareous nannofossils precisely correlate between this radiolarian interval and the radiolarian influx noted in the upper Albian at Site 1183 (Samples 192-1183A-52R-1, 139-141 cm, to 51R-3, 13-15 cm).
The remainder of the interval deposited below the lysocline is composed of limestone that contains only 2%-5% bioclasts by volume, mainly very rare deep-water benthic foraminifers. Rare volcanic ash grains are consistently present. The interval containing late middle Campanian planktonic foraminifers in Sample 192-1186A-23R-CC, 17-22 cm, probably indicates a debris flow that transported bioclasts from above the lysocline to Site 1186, where rapid burial prevented dissolution.
The Paleogene section cored at Site 1186 (Cores 192-1186A-2R through 13R) exhibits alternation between a very indurated, silicified, and/or recrystallized limestone and a very soft, planktonic-foraminifer limestone transitional to ooze. The alternation can occur on a very fine scale; for example, in Core 192-1186A-6R, a silicified limestone with extensive secondary dissolution (Sample 192-1186A-6R-CC, 2-5 cm) lies <30 cm above a very soft, ductile ooze (Sample 192-1186A-6R-CC, 28-30 cm) with abundant, well-preserved planktonic foraminifers. Bedded chert separates the two intervals. This alternation in limestone lithology contrasts sharply with the Paleogene interval at Site 1183 in which highly indurated, silicified, and recrystallized limestone occurs from the uppermost Maastrichtian to middle Eocene. The contrast between the two areas likely is the result of differences in both paleoceanographic conditions and diagenetic pathways. An integrated paleontologic, sedimentologic, and geochemical study will be needed to resolve these differences.