The objective at Sites 1141 and 1142 on eastern Broken Ridge was to sample basaltic basement to discern its relationship to the CKP. Broken Ridge and the CKP formed as one entity, and then separated by rifting and seafloor spreading during the middle Eocene. During its subsequent journey north as part of the Australian Plate, Broken Ridge passed from the high latitudes to its present mid-latitude setting as the Earth's global climate cooled. This change in latitude has been noted in concomitant changes in the microfaunas and microfloras (Pierce, Weissel, et al., 1989). The fossil assemblages changed progressively from those with high-latitude, Southern Ocean affinities in the Eocene to more temperate ones in the Miocene. Pliocene-Pleistocene assemblages contain some subtropical elements. As a consequence, we adopted primarily temperate zonations for the Miocene-Quaternary section cored at Site 1141 but used a number of low-latitude index fossils for the Neogene section (see Fig. F5 in the "Explanatory Notes" chapter). Nevertheless, some of the more tropical marker taxa are missing.
Sediments were cored only from Hole 1141A because Hole 1142A was washed to basement. Ten of the first eleven rotary cores recovered nannofossil ooze in variable amounts. We only examined core-catcher samples on board ship, and these suggest an average sedimentation rate of 6 m/m.y. for the entire carbonate ooze section (Fig. F6), although several hiatuses are probably present in this section. Coarse-grained unconsolidated to poorly consolidated foraminifer-nannofossil ooze (lithologic Unit I) suggests an unusually high percentage of planktonic foraminifers, perhaps concentrated because of winnowing of nannofossils by currents passing along or across the ridge crest. If so, some reworking in the assemblages might be expected.
Sample 183-1141A-1R-CC yielded a characteristic Pleistocene assemblage with Ceratolithus cristatus, Ceratolithus simplex, Rhabdosphaera stylifer/claviger, Scapholithus fossilis, various gephyrocapsids, and small (2.5 to 3 µm) forms that may be Emiliania huxleyi. If scanning electron-microscope study shows that the latter is present, the sample's age would be <0.248 Ma (Zone CN15). Preservation is good, although some specimens show etching.
The subjacent core catcher (Sample 183-1141A-2R-CC) contains a Pliocene assemblage including common Discoaster pentaradiatus, Discoaster brouweri, few Discoaster triradiatus, and a few possible Discoaster surculus (strongly overgrown by secondary calcite), plus Oolithotus fragilis, Ceratolithus rugosus, and a single specimen of Sphenolithus abies. Assuming that the latter is reworked, the assemblage probably belongs to Zone CN12.
Sample 183-1141A-3R-CC yielded a variety of Amaurolithus species, including Amaurolithus primus, Amaurolithus delicatus, and some forms with small apical horns characteristic of Amaurolithus tricorniculatus. Also present are Reticulofenestra pseudoumbilicus, S. abies, D. surculus, D. pentaradiatus, and D. brouweri. Ceratolithus spp., Discoaster quinqueramus/berggrenii, or Triquetrorhabdulus rugosus/farnsworthii were not observed. Ordinarily we would readily assign such an assemblage to the lowest Pliocene Zone CN10b because of the absence of D. quinqueramus and T. rugusus. Some caution should be exercised in making this age interpretation, however, because none of the D. quinqueramus-D. berggrenii lineage has been found on Broken Ridge or in this region south of ODP Site 757 (Leg 121) on Ninetyeast Ridge (now at 17°S; see Shipboard Scientific Party, 1989b).
Abundant T. farnsworthii is present in Sample 183-1141A-4R-CC, however, along with abundant D. pentaradiatus, few D. veriabilis (including large, three-rayed forms and overgrown specimens with stellate bosses but diminished ray tips); however, amauroliths are absent. As Amaurolithus species are present in the uppermost Miocene at ODP Site 754 (Leg 121; Shipboard Scientific Party, 1989a), we assumed that their absence here is not a result of ecological exclusion, although the absence of D. quinqueramus-D. berggrenii probably is (see discussion above). Based on this reasoning and the presence of abundant D. pentaradiatus, we assigned this sample to the uppermost Miocene Zone CN9, probably the lower part (CN9a).
Preservation diminishes in Sample 183-1141A-5R-CC in that many nannoliths are fragmented and some discoasters are heavily overgrown. Some massive discoasters that resemble Discoaster kugleri might have been reworked. Discoaster pentaradiatus continues to be abundant along with abundant Calcidiscus leptoporus/macintyrei; the assemblage is joined by abundant representatives of the cool-water Reticulofenestra perplexa/gelida plexus. We also assign this sample to the lower part of Zone CN9.
We attributed a few large (up to 15 µm), smoothly overgrown five-rayed discoasters in Sample 183-1141A-6R-CC to Discoaster hamatus, although we could not distinguish their characteristic ray-tip spurs. A few six-rayed discoasters with indications of such features were assigned to Discoaster neohamatus. The sample was, therefore, tentatively assigned to Zone CN7 (D. hamatus Zone). These two index taxa were not noted by Leg 121 shipboard scientists at their Broken Ridge sites; however, they were detected at Site 757 on Ninetyeast Ridge to the northwest (Shipboard Scientific Party, 1989b). Their occurrence at Site 1141, therefore, probably represents the southernmost limits of their range, and some of their more distinctive features may not be well expressed here. Alternatively, the smallish D. hamatus may be in the lowest part of their biostratigraphic range. Parker et al. (1985, p. 568) noted that this taxon starts out small and becomes larger with time, as recorded in DSDP Hole 558 (Leg 82). This is a common trend among nannofossils. We observed no catinasters in Sample 183-1141A-6R-CC, in keeping with observations by Leg 121 shipboard scientists, who reported them absent as far north as Site 757 (Shipboard Scientific Party, 1989b). The barrel for Core 183-1141A-7R returned no sediment.
We noted the first downhole occurrence of Cyclicargolithus floridanus/abisectus in Sample 183-1141A-8R-CC, along with large, long-rayed discoasters attributed to Discoaster exilis. Also present are large (12 µm) elliptical C. macintyrei, abundant S. moriformis, R. pseudoumbilicus (up to 10 µm), and pointed six-rayed discoasters that may be D. intercalaris. This and the subjacent Sample 183-1141A-9R-CC belong to Subzone CN5a, based on the presence of the Cyclicargolithus plexus in the absence of Sphenolithus heteromorphus. Also present in the latter sample are Coronocyclus nitescens, Geminilithella rotula, Helicosphaera granulata, and common heavily overgrown discoasters that resemble illustrations of Discoaster woodringii.
Sphenolithus heteromorphus is present in Samples 183-1141A-10R-CC and 11R-CC, indicating that we reached the combined Zones CN4/CN3 in this hole. As this index taxon and its probable ancestor, Sphenolithus belemnos, have both been identified during previous drilling at Broken Ridge Sites 255 (Leg 26) and 754 (Leg 121) (Thierstein, 1974, and Shipboard Scientific Party, 1989a, respectively), we assumed that the stratigraphic range of neither taxa was restricted for ecological reasons at this site. Discoasters are even more heavily overgrown in Sample 183-1141A-11R-CC than in overlying cores; however, those present appear to be predominantly relatively short-rayed Discoaster deflandrei, rather than the longer-rayed D. exilis. If this is correct, then the last carbonate-ooze core belongs to Zone CN3, which spans the early/middle Miocene time boundary. Unfortunately, Core 183-1141A-11R only recovered 0.06 m of sediment, and the next core attempt (12R) recovered none, so little is known about this part of the section.
Besides a small amount of carbonate ooze, however, Sample 183-1141A-11R-CC also contained a well-cemented, manganese-coated, orange-colored conglomeratic foraminiferal limestone with various kinds of pebbles and clasts (see "Lithostratigraphy"). Nannofossils are rare in this rock and most are poorly preserved; however, Reticulofenestra bisecta, Zygrhablithus bijugatus, Coccolithus eopelagicus (up to 20 µm), large (up to 12 µm) Coccolithus formosus are rare to common, and a single specimen of Discoaster saipanensis (seven rayed) was also observed. If in situ, this assemblage ranges from late middle to late Eocene in age. A similar-looking limestone from wash core Section 183-1142A-1W-1 also contained small R. bisecta, which ranges in age from the late middle Eocene to Oligocene; this assemblage is probably of a similar age to that of Sample 183-1141A-11R-CC.
A white streak of nannofossil chalk within Sample 183-1141A-11R-CC contained a nannofossil assemblage with C. floridanus, C. abisectus, Coccolithus pelagicus, and Discoaster sp. The assemblage looks similar to those in the lower part of the Miocene carbonate ooze section above.
Planktonic foraminifers recovered from the Neogene carbonate ooze at Site 1141 are abundant and very well preserved. Compared to those from the Kerguelen Plateau, these assemblages show high species diversity and are virtually identical to assemblages recovered from Broken Ridge Site 754 (Leg 121). Pliocene and Pleistocene faunas reflect temperate conditions with some affinities to subtropical assemblages. Upper Miocene assemblages are rich in species associated with temperate waters with forms characteristic of the high latitudes becoming increasingly important in lower and middle Miocene sediment. This gradual change in faunal composition, also observed by Leg 121 shipboard scientists during drilling on Broken Ridge (Pierce, Weissel, et al., 1989), reflects warming of surface waters associated with the post-mid-Eocene northward motion of the Broken Ridge drill sites from higher latitudes to lower latitudes. Thus, for this site we were able to employ the more detailed temperate planktonic foraminifer zonation of Srinivasan and Kennett (1981). This zonal scheme is largely based on species of Globorotalia, reflecting the importance of these evolutionary lineages in temperate areas (Kennett and Srinivasan, 1983).
The Quaternary is represented by one core of Pleistocene, winnowed, nannofossil-bearing foraminifer ooze. Sample 183-1141A-1R-CC contains a rich assemblage of foraminifers with pristine preservation of tests and wall textures. The assemblage is dominated by Globorotalia inflata and Globorotalia truncatulinoides, which are species typical of temperate water masses. However, the assemblage also contains the tropical to subtropical forms such as Orbulina spp., Globigerinoides ruber, and Globorotalia menardii. More cosmopolitan species, including Globigerina bulloides and Globigerina falconensis are also present. We assign this sample to the upper Pleistocene G. truncatulinoides Zone owing to the common occurrence of the nominate taxon and absence of Globorotalia tosaensis, from which the latter evolved. This Broken Ridge Pleistocene fauna contrasts markedly to Kerguelen Plateau and other high-latitude assemblages that show extremely low diversity, as well as dominance by Neogloboquadrina pachyderma.
Cores 183-1141A-2R to 11R contain well-preserved planktonic-foraminifer assemblages that contain a mixture of Southern Ocean and subtropical elements. Sample 183-1141A-2R-CC is below the last appearance datum (LAD) of G. truncatulinoides. Globorotalia tosaensis is present in this sample, and we therefore assign it to the uppermost Pliocene zone bearing that name. Other elements of this fauna include cool-water Globigerina woodi, G. falconensis, and G. bulloides plus the warmer water-loving taxa Orbulina, Globigerinella siphonifera, G. inflata, G. menardii, and G. ruber. As noted at Site 754 by the Leg 121 Shipboard Scientific Party (1989a), fully tropical species such as Globorotalia tumida, Sphaeroidinella dihiscens, and Pulleniatina obliquiloculata are absent.
The next three samples contain conical forms belonging to the Globorotalia miozea-conoidea plexus. In Sample 183-1141A-3R-CC, we found the short-ranging species Globorotalia conomiozea and the first downhole occurrence of Globoquadrina altispira, indicating an early Pliocene or late Miocene age. At Site 754, the first appearance datum (FAD) of Globorotalia puncticulata and Globorotalia margaritae was used to approximate the Miocene/Pliocene boundary (Leg 121 Shipboard Scientific Party, 1989a). We did not find these species during preliminary examination of core-catcher samples and were thus unable to delineate the G. puncticulata-G. conomiozea Zones. More detailed sampling during shore-based studies should allow us to locate this boundary more precisely.
Upper Miocene assemblages are characterized by G. conoidea, G. miozea, Globorotalia panda, G. woodi, Globigerina nepenthes, G. falconensis, Globigerina bulloides, Globquadrina altispira, Dentiglobigerina sp., Globigerinella aequilateralis, Orbulina sp., Globigerina trilobus, and G. menardii. The downhole FAD of G. conomiozea occurs in Sample 183-1141A-4R-CC. Neogloboquadrina continuosa, the last appearance datum (LAD) of which marks the lower boundary of the late Miocene G. nepenthis Zone, is rare or absent in Hole 1141A. Therefore, we assign Samples 183-1141A-4R-CC to 6R to the combined G. nepenthes-N. continuosa Zones. Core 183-1141A-7R was empty, so no paleontological sample was available.
In addition to abundant G. miozea, we noted Globorotalia mayeri in Sample 183-1141A-8R-CC and accordingly assigned it to the mid-Miocene zone bearing that name. In Sample 183-1141A-9R-CC, we recognized a small, inflated, six-chambered form, identified as Globorotalia peripheroacuta, that allowed us to place this sample in the Globorotalia peripheronda-G. peripheroacuta Zone. We did not find this form in the subjacent sample and therefore placed it in the lower mid-Miocene Orbulina suturalis Zone.
Globoquadrina sp., Dentigloboquadrina sp., and G. miozea dominate planktonic foraminifer assemblages from the last core catcher, Sample 183-1141A-11R-CC. This sample occurs below the FAD of O. suturalis and above the LAD of Catapsydrax dissimilis. We did not distinguish Preaorbulina glomerosa curva, indicative of uppermost lower Miocene strata, and therefore assign this sample to the G. miozea Zone, suggesting a minimum age of 16-18 Ma for the last carbonate ooze sediments above basement.
In addition to the unconsolidated Miocene carbonate ooze described above, the core catcher of Core 183-1141A-11R contained pebbles of a sandy foraminiferal limestone. We could not isolate individual planktonic foraminifers from these pebbles but were able to recognize foraminifers in petrographic thin section. Postcruise examination of thin sections (I. Premoli Silva, pers. comm., 1999) confirmed that limestone Sample 183-1141A-11R-CC contained abundant planktonic and benthic foraminifers. Among the planktonic forms we recognize bisereal Chiloguembelina spp. and possible Globigerinatheka index. Acarininids appear to be absent, indicating a late Eocene age (~34.3-38 Ma). Chips of similar sandy foraminifer limestone were recovered from between basalts at Site 1142. In postcruise studies of thin sections (Sample 183-1142A-3R-1, 1-6 cm), we also found planktonic foraminifers indicative of a middle-late Eocene in age.
The age date for the oldest sediment in these sites (late middle to late Eocene, 34.3-38 Ma) probably does not provide a date close to the age of basaltic basement. Sites 1141 and 1142 were chosen to avoid Cretaceous limestones interpreted from seismic lines that lie to the north; thus, the holes penetrated only the youngest overlying sediment. This strategy was to avoid drilling problems that prevented sampling of Broken Ridge's igneous basement during Legs 26 and 121. Indeed, this strategy worked to the point that basement was cored. Eventually, however, Hole 1141A collapsed, and Broken Ridge claimed yet another bottom-hole assembly to add to its collection (see "Operations").
The basal sediments in Hole 1141A are late middle to late Eocene in age, which dates from the time that Broken Ridge rifted from the Kerguelen Plateau. Conglomeratic in nature with pebbles that include pieces of altered basalt (see "Igneous Petrology"), these sediments apparently record processes of erosion and redeposition that accompanied that tectonic episode (see Pierce, Weissel, et al., 1989, for further details of this event).