Shipboard analyses of calcareous nannofossils and planktonic foraminifers reveal a late Pleistocene to early Miocene age for the 523-m-thick sequence cored at Site 1198 (Table T4). See "Age Model" for age vs. depth and sedimentation rate plots. Microscopic analysis of sand-sized (>63 µm) biogenic sediment constituents provided data for paleoenvironmental interpretations (Table T5). Core catcher samples were the basis for the biostratigraphic analyses, and additional samples were taken where necessary.
Calcareous nannofossils are abundant and well preserved in all the samples examined from this hole except in the last two core catcher samples, which yielded poorly preserved early Pliocene nannofossils. Sample 194-1198A-1H-2, 80 cm, contains few Emiliania huxleyi and abundant Gephyrocapsa >4 µm among other Pleistocene taxa. This suggests that oxygen isotope Stages 1-4, if present, lie above this sample. The first occurrence (FO) of E. huxleyi, which is known to correlate with oxygen isotope Stage 8 (at ~0.26 Ma), is located between Samples 194-1198A-1H-CC and 2H-2, 80 cm. The last occurrence (LO) of Pseudoemiliania lacunosa, which correlates with oxygen isotope Stage 12 at 0.46 Ma, is placed between Samples 194-1198A-2H-CC and 3H-CC. Calcidiscus macintyrei was first encountered in Sample 1194-1198A-1H-CC, indicating that the sample is older than 1.7 Ma. The Pliocene/Pleistocene boundary is generally drawn just below the LO of C. macintyrei.
Few Discoaster brouweri were found in Sample 194-1198A-14H-CC. The LO of D. brouweri (2.0 Ma) is placed between this and Sample 194-1198A-13H-CC. Discoaster pentaradiatus and Discoaster surculus were first found in Sample 194-1198A-18H-CC and Discoaster tamalis in Sample 194-1198A-19H-CC, indicating an age older than 2.6 and 2.8 Ma, respectively, for the two samples. The LO of Reticulofenestra pseudoumbilicus (3.7 Ma) is placed between Samples 194-1198A-21H-CC and 22H-3, 85-87 cm. This datum can be used to approximate the early/late Pliocene boundary.
Samples 194-1198B-2R-CC through 16R-CC contained generally rare and poorly preserved nannofossils with several samples being barren of nannofossils, leading to relatively poor biostratigraphic resolution for this interval. The LO of Reticulofenestra umbilicus (>7 µm) (9.0 Ma) is placed between Samples 194-1198B-8R-CC and 10R-CC based on absence of the species in the former sample and its presence in the latter sample. Similarly, the LO of Cyclicargolithus floridanus is placed between Samples 194-1198B-13R-CC and 16R-CC based on absence of the species in the former sample and its presence in the latter sample. Because Sample 194-1198B-14R-CC is barren of nannofossils and recovery for Core 194-1198B-15R was 0%, there is a relatively large error bar in the current placement of the datum. The LO of Sphenolithus heteromorphus (13.6 Ma) is located between Samples 194-1198B-18R-CC and 19R-CC. The FO of this index fossil (18.2 Ma) is tentatively placed between Samples 194-1198B-31R-CC and 33R-2, 10 cm, based on the presence of the species in the former sample and its absence in the latter sample. Sphenolithus belemnos, which has a range of 18.5-19.3 Ma, was not found in Core 194-1198B-33R. Thus, an age of 18.2-18.5 Ma is tentatively assigned to Core 194-1198B-33R.
Samples 194-1198A-1H-CC through 20H-CC revealed a late Pleistocene (Zone N23) to late Pliocene (Zones N20-N21) sequence reaching to a depth of 185.8 mbsf. Samples 194-1198A-21H-CC and 22H-CC were determined to be early Pliocene in age.
Samples 194-1198A-1H-CC through 5H-CC comprise foraminifers of the late middle Pleistocene Zone Pt 1b, based on the presence of Globigerionoides ruber (pink) and the absence of Globorotalia tosaensis. Sample 194-1198A-1H-CC contains the "pink" morphotype of G. ruber, whose LO datum is ~0.12 Ma in the Pacific (although it does vary), thus defining the youngest possible age for the sample. The LO datum of G. tosaensis, which marks the base of Zone Pt 1b was placed between Samples 194-1198A-5H-CC and 6H-CC. The LO datum of Globigerinoides fistulosus is placed between Samples 194-1198A-11H-CC and 12H-CC. The LO and FO datums of both Globigerinoides extremus and Globorotalia truncatulinoides are positioned between Samples 194-1198A-15H-3, 85-87 cm, and 15H-CC. Samples 194-1198A-6H-CC through 15H-CC therefore represent the early Pleistocene Zone Pt 1a, probably with some overlap into the Pliocene PL6 Zone (synonymous with the Zone N21/N22 boundary) (Fig. F8 in the "Explanatory Notes" chapter). The precise placement of this boundary in Hole 1198A is marred by downhole contamination and appears to be smeared out.
Foraminifers in Samples 194-1198A-21H-CC through 23H-CC are only moderately preserved and are difficult to precisely identify. Sample 194-1198A-21H-CC contains an assemblage that includes G. extremus, Dentoglobigerina altispira, and Globorotalia pseudomiocenica, whereas Sphaeroidinellopsis seminulina and Globorotalia margaritae are absent. This sample is assigned to the time interval defined by Zones N21 and N20 (PL4-PL5). Sample 194-1198A-22H-CC may be placed in Zone N19 based on the absence of Globorotalia plesiotumida and Globigerina nepenthes. G. plesiotumida and Sphaeroidinella dehiscens are present in Samples 194-1198A-23X-CC and 23X-1, 0-25 cm, which can therefore be constrained to an age at the Zone N18/N19 boundary.
Samples 194-1198B-1R-CC through 16R-CC indicate a late to possibly middle Miocene sequence in Hole 1198B. The preservation quality of foraminifers is moderate to poor, with many samples containing recrystallized specimens and/or few age-diagnostic taxa.
The FO of G. extremus is placed between Samples 194-1198B-3R-CC (214.8 mbsf) and 6R-CC (243.7 mbsf), indicating that the Zone N17/N16 boundary occurs between these two samples. The G. plesiotumida FO datum is observed between Samples 194-1198B-11R-CC and 13R-CC; thus, the Zone N15/N16 boundary can be placed here. This boundary approximates the middle/late Miocene boundary. The last reliable datum recorded was the FO of Orbulina spp. between Samples 194-1198B-19R-CC and 20R-CC. This datum defines an early middle Miocene age for these samples. Core 2194-1198B-6R through Sample 194-1198B-32R-CC contains no age-diagnostic planktonic foraminifers and thus could not be assigned to any zone.
All samples from Hole 1198A, (194-1198A-1H-CC [4.95 mbsf] through 23H-CC [195.02 mbsf]) contain planktonic foraminiferal tests and test debris, as well as a generally common diverse assemblage of benthic foraminifers characteristic of upper bathyal water depths. Most samples also contain at least a small amount of reworked material.
Samples 194-1198A-1H-CC (4.95 mbsf) through 6H-CC (52.24 mbsf), which span lithologic Subunit IA (see "Lithostratigraphy and Sedimentology"), include a minor coarse component of outer neritic/upper bathyal hard-substrate macrofauna. Pteropods are also common to abundant in this interval. Samples 194-1198A-17H-CC (63.37 mbsf) through 21H-CC (195.02 mbsf) from lithologic Subunit IB (see "Lithostratigraphy and Sedimentology") differ from those of Subunit IA, as they lack any significant coarse component.
Samples 194-1198A-22X-CC (202.96 mbsf) and 23X-CC (205.79 mbsf) from the top of lithologic Subunit IIA (see "Lithostratigraphy and Sedimentology") are bimodal in size distribution, representing two very different sources. Medium to fine sand fractions as well as the samples overall are dominated by planktonic foraminiferal tests. Coarse sand fractions are characterized by a diverse assemblage of larger benthic foraminifers, with debris from bryozoan and other neritic macrofauna. Reworked clasts are also common.
Samples 194-1198B-1R-CC (199.43 mbsf) and 2R-CC (205.6 mbsf) from the top of lithologic Subunit IIA (see "Lithostratigraphy and Sedimentology"), are dominated by medium to fine sand-sized planktonic foraminiferal tests with a significant coarse sand-sized fraction containing a diverse assemblage of larger benthic foraminifers, with debris from bryozoan and other neritic macrofauna as well as reworked clasts. Samples 194-1198B-3R-CC (214.8 mbsf) and 4R-CC (224.93 mbsf), which are also from lithologic Subunit IIA, are bimodal in grain size but with much less planktonic influence. Instead, the fine fraction is dominated by silt and fine sand of probable neritic origin.
Samples 194-1198B-5R-CC (234.27 mbsf) through 15R-1, 24-26 cm (330.5 mbsf), from lithologic Subunit IIB (see "Lithostratigraphy and Sedimentology"), are dominated by neritic material. A diverse assemblage of larger benthic foraminifers, particularly Lepidocyclina spp., are abundant in all samples and often are the dominant constituent. Rhodoliths, red algal fragments, coral, and bryozoans are also important components. Planktonic foraminifers and typical upper bathyal benthic foraminiferal taxa such as Cibicidoides and nodosarids are also found in most samples in this subunit (see Table T5). Samples from lithologic Subunit IIC (i.e., 194-1198B-16R-CC [340.3 mbsf] through 21R-CC [391-44 mbsf]) contain sediments from both planktonic and neritic sources, although compared to Subunit IIB, the planktonic foraminifers are more abundant and the neritic debris is generally much finer grained.
Fine neritic debris dominates the sediments of lithologic Unit III (Samples 194-1198B-22R-CC [407.48 mbsf] through 33R-CC [501.23 mbsf]) (see "Lithostratigraphy and Sedimentology"). Planktonic foraminifers and outer neritic/upper bathyal benthic foraminifers become increasingly difficult to recognize with depth through this interval.
A short interval of skeletal floatstone to rudstone, topped by a phosphatic hardground (lithologic Unit IV; see "Lithostratigraphy and Sedimentology"), is represented only by Sample 194-1198B-33R-2, 114-116 cm (505.97 mbsf). This small unit is characterized by a foraminiferal boundstone consisting of large, very thin larger benthic foraminifers, Lepidocyclina (Eulepidina) sp., bound together by thin red algal crusts (Fig. F11). This unit overlies lithologic Unit V, which is olivine basalt (see "Lithostratigraphy and Sedimentology").
Major biogenic constituents of Unit IV include thin red algal crusts and very large, flat larger benthic foraminifers, both of which indicate very deep euphotic environments, possibly as deep as 100-150 m (Tsuji, 1993). The flooding of the basaltic basement rocks is possibly recorded in a thin interval of sediments that immediately overlie the basalts. These sediments are characterized by more robust rhodoliths, as well as smaller sizes and more robust morphologies among the larger benthic foraminifers. These factors are indicative of shallower paleowater at the base of Unit IV than on top.
Planktonic foraminifers, as well as benthic foraminifers characteristic of outer neritic to upper bathyal paleowater depths, occur through most of lithologic Units I through III (Table T5). Although sea level fluctuations undoubtedly affected both the source and supply of sediments, paleowater depth may have had less influence on sedimentation through this interval than the source and rate of supply of hemipelagic clays and periplatform carbonates as a result of the proximity of the site to the Australian continental shelf, upcurrent platforms, and the nearby SMP. Unit III exhibits significant amounts of very fine neritic carbonate sediment and is interpreted to represent a distal periplatform depositional setting. Input of medium to coarse periplatform carbonates increases upsection through Subunit IIC, becoming the dominant component in Subunit IIB. A hardground on top of Subunit IIA marks an abrupt end to the deposition of neritic carbonates and probably nearby shedding and production.
Unit II provides enigmatic biostratigraphic data of potentially profound significance relative to the age ranges of the larger benthic foraminifers. Seismic data indicate that sediments at least in the upper part of this unit were shed from the SMP (see "Seismic Stratigraphy"). Planktonic foraminifers and nannofossils indicate a late Miocene age for this unit (Table T4). Yet the dominant component of the coarser neritic sediments in this unit is a diverse assemblage of larger benthic foraminifers, including, and often dominated by Lepidocyclina spp. These foraminifers were long considered to be predominantly of early Miocene age in northern Australia (e.g., Chaproniere 1981, 1984). Betzler and Chaproniere (1993) and Chaproniere and Betzler (1993) considered that all Lepidocyclina found in northeastern Australia and Queensland Plateau sediments younger than Zone N12 (12.1 Ma) were reworked. Betzler (1997) later showed that on the Queensland Plateau, Lepidocyclina has an extended age range into the late Miocene. Allan et al. (2000) also found strontium isotopic evidence that the Lepidocyclina lineage continued into Zone N16 (8.3-10.9 Ma) in the late Miocene in the western Pacific, at least in New Guinea. The predominance, sorting, and excellent preservation of Lepidocyclina in periplatform sediments in Subunit IIA, which both planktonic foraminifers and nannofossils indicate to be no older than Zone N17 (6.0-8.3 Ma), provide strong evidence that these foraminifers did indeed survive well into the late Miocene.
Neritic sediments abruptly diminish above the phosphatic nodules that mark the boundary between lithologic Units II and I. In Subunit IB, sand-sized sediments are overwhelmingly planktonic in origin, even in the small reworked component. Substantial variations in the predominant sizes of the planktonic foraminiferal tests are apparent in this interval, which may be indicative of changes in current velocities and therefore sediment winnowing. Subunit IA is also of predominantly pelagic origin, but it contains a minor but conspicuous macrofaunal component, apparently transported either downslope from the nearby drowned SMP or from a more distant source by strong currents. The benthic foraminifers, some of which are likely from the same sources as the macrofaunal debris, are particularly common and diverse throughout Unit I.