Middle to upper Eocene calcareous ooze and limestone are present immediately above basement at Site 1185. Extremely rare calcareous nannofossils recovered from limestone interbeds within the upper 15 m of basaltic basement in both holes indicate an Albian to Cenomanian age. Recrystallized planktonic foraminifers found within thermally metamorphosed limestone 126 m below the sediment/basement contact in Hole 1185B tentatively indicate a late Aptian age.
The calcareous microfossil biozonations used here are discussed in "Biostratigraphy" in the "Explanatory Notes" chapter. A summary of the main biostratigraphic events identified in the sediments recovered from Site 1185 is presented in Table T5.
Fifty-eight meters of middle to upper Eocene calcareous ooze and limestone (Cores 192-1185A-2R to 8R) were cored from Hole 1185A. Planktonic foraminifers indicate a possible hiatus spanning the middle to upper Eocene boundary (encompassing Zone P15; Berggren et al., 1995) between Sections 192-1185A-5R-CC and 6R-CC. Calcareous nannofossils recovered from limestone interbeds in the uppermost basalts (Sections 192-1185A-9R-4 and 10R-1) indicate an Albian to early Turonian age but may be no older than late Albian.
Calcareous nannofossils are abundant and moderately to well preserved in Cores 192-1185A-2R through 7R; the few middle Eocene nannofossils recovered from the uppermost portion of Core 8R (5-14 cm) are poorly preserved. The presence of Cribrocentrum reticulatum and Chiasmolithus solitus in these samples from Core 8R, which are immediately above basement, indicate Zone NP16 of Martini (1971) and an age no older than 42 Ma (see Berggren et al., 1995). Above this level, the sedimentary sequence is well dated by a succession of nannofossil events (see Table T5), although the zonal markers for the bases of Zones NP18 and NP19 were not recognized in recovered assemblages. The presence of Chiasmolithus consuetus in the topmost sample examined (Sample 192-1185A-2R-1, 0-1 cm) indicates Zone NP19 (see Perch-Nielsen, 1985) and that we did not recover rocks of terminal Eocene age.
Within basement, extremely rare and poorly preserved calcareous nannofossils were recovered from five samples taken from limestone interbeds in Sections 192-1185A-9R-4 and 10R-1. Section 9R-4, 17 cm, contains Rhagodiscus asper, which indicates an age no younger than early Turonian. Tranolithus exiguus (two specimens) and Microstaurus chiastius (one specimen) were recovered from Section 192-1185A-10R-1, 49 cm; their co-occurrence indicates an early Albian to latest Cenomanian age (see Table T5). A questionable specimen of Eiffellithus turriseiffelii observed in Section 192-1185A-9R-4, 17 cm, may indicate an age no older than late Albian for this sample.
Planktonic foraminifers are common and moderately well preserved in Cores 192-1185A-2R through 3R but are nearly absent in Cores 192-1185A-4R and 5R. They become common again in Cores 192-1185A-6R through 8R but indicate a dissolution-affected assemblage. Cores 192-1185A-2R and 3R contain large globigerinids and subbotinids with first stratigraphic appearances in late Eocene Zone P16 (Tourmakine and Luterbacher, 1985). The most common taxa include Globigerina euapertura, G. venezuelana, G. pseudovenezuelana, and Subbotina gortanii. The interval is restricted to the latest Eocene by the co-occurrence of Tuborotalia cerroazulensis, which has a last stratigraphic occurrence within Zone P17 (Berggren et al., 1995). The interval from Cores 192-1185A-4R through 5R is nearly barren of planktonic foraminifers, indicating deposition below the foraminifer lysocline. Only two corroded specimens of large, thick-walled species from the late Eocene assemblage were recovered (e.g., G. euapertura). Cores 192-1185A-6R to 8R, however, are marked by a moderate increase in planktonic foraminifer recovery, but the assemblage is strongly dominated by robust, very thick walled species of Globigerinatheka, especially G. senni and G. subconglobata. An assemblage affected by dissolution is indicated, with deposition near the foraminifer lysocline but under less corrosive conditions than indicated by the interval represented by Cores 192-1185A-4R and 5R. With a middle Eocene last stratigraphic occurrence (Zone P14; Tourmarkine and Luterbacher, 1985), G. senni indicates a stratigraphic hiatus spanning the middle to upper Eocene boundary (Table T5). However, given that the planktonic assemblage is depleted because of dissolution, the duration of the hiatus cannot be defined precisely. In Cores 192-1185A-7R and 8R, the co-occurrence of Tuborotalia cerroazulensis and Globigerinatheka index, species with middle Eocene first stratigraphic appearances, and Globigerinatheka micra, with a middle Eocene last stratigraphic occurrence, indicates Zone P12 (Toumarkine and Luterbacher, 1985). In addition, the first appearance of G. index has been calibrated by Berggren et al. (1995) at 42.9 Ma (Table T5).
Thin section study of limestone recovered from the upper basalt section (Core 192-1185A-10R) reveals common radiolarians and rare, very small (<80 µm) planktonic foraminifers. Preservation is very poor: the radiolarians are calcified to coarsely crystalline spar, and the foraminifers are recrystallized. The limestone is baked by basalt and shows concentric zones of lessening thermal alteration away from the limestone/basalt contact. From their gross morphology, the planktonic foraminifers appear to be species of either the genus Hedbergella or Blefuscuiana. Very similar biofacies composed of large radiolarians and small planktonic foraminifers are present in two discrete intervals from Hole 1183A: the upper Albian limestone (Samples 192-1183A-51R-3, 13-15 cm, to 52R-1, 139-141 cm) and the middle Aptian limestone immediately overlying basaltic basement (Samples 192-1183A-54R-3, 59-61 cm, to 54R-3, 96-99 cm). However, poor preservation prevents any definite biostratigraphic determination from Hole 1185A.
Benthic foraminifers are common and well preserved in all samples examined from the Eocene section of Hole 1185A (Cores 192-1185A-2R to 8R) but are absent from limestone interbeds of Core 10R. The benthic foraminifers, including most of the agglutinated taxa present, are composed almost entirely of calcareous specimens. However, two upper Eocene samples (Samples 192-1185A-3R-CC, 13-15 cm, and 6R-1, 51-53 cm) contain rare, noncalcareous agglutinated species composed of silt-size, yellow quartz grains with rare black (heavy?) mineral grains and very rare volcanic glass shards. Some agglutinated species are restricted to these samples (e.g., Rhabdammina abyssorum and Hormosinella ovicula), whereas other taxa, noncalcareous in these samples, are also present in other parts of the upper Eocene section, where they are composed of nannofossil ooze, a consequence of a lack of volcaniclastic grains (e.g., Adercotryma glomeratum). These agglutinated species are useful in marking the presence of very rare volcaniclastic material that otherwise might escape detection in macroscopic core or sedimentologic analysis.
The Eocene assemblage shows a relatively high species diversity and indicates a uniform, deep abyssal paleobathymetry >3000 m (Table T6) (van Morkhoven et al., 1986). However, there is a distinct faunal turnover between the middle and upper Eocene assemblages, first described from the abyssal Eocene of the Atlantic Ocean (Tjalsma and Lohmann, 1983). The abyssal benthic foraminifer assemblage of the early Eocene, dominated by Nuttallides truempyi and associated taxa, is gradually replaced in the middle Eocene by an assemblage that dominates upper Eocene and Oligocene abyssal facies. Below the upper Eocene, N. truempyi is a dominant component of abyssal and bathyal benthic foraminifer assemblages but in the upper Eocene is restricted to abyssal paleodepths before becoming extinct near the end of the Eocene (Tjalsma and Lohmann, 1983). Although termed the "late Eocene" assemblage, this group of benthic foraminifer species is actually well established in middle Eocene limestone from Hole 1185A (Table T6). The assemblage is composed of Oridosalis umbonatus, Stilostomella species, Cibicidoides praemundulus (= C. ungerianus of Tjalsma and Lohmann, 1983), and planoconvex taxa of Cibicidoides such as C. grimsdalei and C. eocaenus. The turnover in Hole 1185A is illustrated in Table T6. Components of the "late Eocene" benthic assemblage are already well established by the middle Eocene, but the overall assemblage is still dominated by Nuttallides truempyi and associated species. The lowest stratigraphic level marked by a decline in N. truempyi occurs immediately above the middle to upper Eocene unconformity between Samples 192-1185A-6R-1, 108-110 cm, and 6R-2, 6-8 cm. The lower portion of the upper Eocene section (Samples 192-1185A-4R-CC, 17-19 cm, to 6R-1, 6-8 cm) marks a transitional interval in which components of both assemblages are rare. However, by the time of deposition of the overlying upper Eocene section in Cores 192-1185A-2R and 3R, the "late Eocene" abyssal assemblage is firmly established and components of the Nuttalides truemypi assemblage are rare, including the nominate taxon. The cause of this benthic foraminifer turnover has not been generally determined but, in Hole 1185A, is clearly associated with a rise in the foraminifer lysocline and, hence, presumably the CCD as well. The middle to upper Eocene hiatus may indicate a rise in the CCD above Site 1185, and the overlying transition zone in the lower part of the upper Eocene (Samples 192-1185A-4R-CC, 17-19 cm, to 6R-1, 6-8 cm) was deposited below the foraminifer lysocline. By the time deposition resumed above the lysocline (Cores 192-1185A-2R through 3R), the turnover in abyssal benthic foraminifer assemblages was complete.
One full core section (Section 192-1185B-2R-1) of middle Eocene limestone was recovered above basalt basement (encountered in Section 2R-2) from Hole 1185B; this section is correlative with the basal limestone interval from Hole 1185A (Core 8R). Calcareous nannofossils recovered from limestone interbeds in the uppermost basalts (Core 192-1185B-4R) indicate an age of Albian to Cenomanian but may be late Albian. Recrystallized planktonic foraminifers recovered from thermally metamorphosed limestone much deeper in the basalt sequence (Core 192-1185B-17R) tentatively indicate a late Aptian age.
Middle Eocene nannofossil assemblages recovered from six samples taken in Section 192-1185B-2R-1 are all poorly preserved and have a low to moderate abundance. The presence of Cribrocentrum reticulatum and Chiasmolithus solitus in these samples immediately above basement indicates Zone NP16 of Martini (1971) and an age no older than 42 Ma (see Berggren et al., 1995).
Within basement, extremely rare and poorly preserved calcareous nannofossils were recovered from five samples taken in limestone interbeds in Sections 192-1185B-4R-2 through 6R-1. Section 192-1185B-4R-2, 42 cm, contains Microstaurus chiastius, which indicates an age no younger than the end of the Cenomanian; the presence of Prediscosphaera columnata (one specimen) in the subjacent Section 192-1185B-4R-3, 58 cm, indicates an age no older than earliest Albian (see Table T5). A questionable specimen of Eiffellithus monechiae found in Section 192-1185B-4R-4, 43 cm, may indicate a late Albian age for this sample. The ages of calcareous nannofossils recovered from two samples in Section 192-1185B-6R-1 are indeterminate.
The middle Eocene section in Hole 1185B, as in Hole 1185A, was deposited below the foraminifer lysocline. Sample 192-1185B-2R-1, 35-37 cm, is nearly barren of planktonic species, containing only two poorly preserved specimens of the dissolution-resistant middle Eocene species, Globigerinatheka senni. Sample 192-1185B-2R-1, 146-148 cm, contains more common and diverse planktonic foraminifers and common volcaniclastic grains and displays wispy laminae and diffuse grading of bioclasts. The sample represents an allochthonous deposit, transitional between a mudflow and turbidite, in which rapid burial of planktonic species transported from above the lysocline prevented extensive dissolution. The co-occurrence of the planktonic foraminifer species Globigerinatheka micra and Tuborotalia cerroazulensis in this sample indicates an upper P12 zonal assignment, as in Core 8R from Hole 1185A.
Thin section examination of limestone interbeds in the basalt of Core 192-1185B-4R reveals a recrystallized carbonate with common radiolarians and very small planktonic foraminifers (<60 µm), a biofacies very similar to that noted from Core 192-1185A-10R. However, preservation is slightly better in Core 192-1185B-4R, allowing identification of one species, Guembelitriella graysonensis. A long-ranging species, G. graysonensis, has been documented previously from the upper Aptian to lower Cenomanian (Leckie, 1984). The only other limestone in the basalt section to contain foraminifers was sampled in Sample 192-1185B-17R-1, 80-82 cm. It is thermally altered limestone composed of spar and iron oxides. Common, recrystallized planktonic foraminifers varying widely in size are evident; radiolarians are very rare, indicating a biofacies quite different from that of the limestones in the upper basalt section from Holes 1185A and 1185B (Cores 192-1185A-10R and 192-1185B-4R). A large species of Globigerinelloides is present, which closely resembles the upper Aptian index G. ferreolensis (i.e., it is suboval in spiral view, with eight to nine chambers in the final whorl, increasing in size at a moderate rate). The poor preservation, however, prevents a definite identification.
The benthic foraminifer assemblage of the Eocene limestone in Core 192-1185B-2R is very similar to the deep abyssal assemblage recovered from limestone immediately overlying the basalt in Core 192-1185A-8R. The assemblage is diverse and dominated by Nuttallides truempyi, Cibicidoides grimsdalei, and Alabamina dissonata. No benthic foraminifers are present in the limestone interbeds analyzed in Cores 192-1185B-4R and 17R.