Site 1129, the shallowest (202.4 m water depth) site of an upper slope/shelf edge transect that includes Site 1131 (333.6 m water depth) and Site 1127 (479.3 m water depth), is situated at the present-day shelf edge. This site includes two biostratigraphic units: (1) an expanded Quaternary section more than 540 m thick that is underlain by a thin (16 m thick) and conformable Pliocene section (see "Paleomagnetism"); and (2) a middle-lower Miocene section (Fig. F8). These units are separated at 556 mbsf by an unconformity of ~12 m.y. The unconformity is marked by a bryozoan turbidite overlying indurated sediments and chert layers (see "Lithostratigraphy"). Calcareous nannofossils are generally abundant and moderately well preserved in the upper parts of the succession and poorly preserved below 371 mbsf. Preservation and abundance of planktonic foraminifers become increasingly degraded in the upper part of the section. Below ~68 mbsf, most characteristic features of foraminifers are obscured by carbonate cement and recrystallization.
Benthic foraminifers are generally abundant and well preserved in the upper part of Hole 1129C, and both preservation and abundance deteriorate markedly at ~140 mbsf. Three main assemblages are recognized at Site 1129: a distinctive well-preserved Pleistocene assemblage found in bryozoan-rich accumulations (down to ~140 mbsf); a Pleistocene-Pliocene assemblage (140 mbsf-565 mbsf), which includes a variable redeposited neritic component; and a sparse Miocene assemblage in Core 182-1129D-24R. The two Pleistocene assemblages indicate upper bathyal paleodepths, whereas the Miocene assemblage indicates an upper to middle bathyal paleodepth.
Sedimentation rates for most of the Pleistocene were relatively high, reaching a maximum of 542 m/m.y. (Fig. F9). However, in the upper Pliocene and lower Pleistocene to the bottom of the Jaramillo, the sedimentation rate was considerably lower (142 m/m.y.). A sedimentation rate was not calculated for the sediments below the unconformity because of the lack of datum levels.
Site 1129 contains an expanded Pleistocene sequence (~554.70 m), overlying a short middle-lower Miocene sequence (~40 m), a succession similar to those at Sites 1127 and 1131. Hole 1129C is rich in nannofossils that are moderately well preserved in the upper 360 mbsf, but degraded to poorly preserved in the bottom 80 m of the hole. Although sediments from Hole 1129D are also rich in nannofossils, they are generally poorly preserved.
The upper 160 mbsf at Site 1129 is characterized by common to abundant Gephyrocapsa caribbeanica, common small Gephyrocapsa spp., and common occurrences of Calcidiscus leptoporus. The lack of Pseudoemiliania lacunosa indicates that this section should be assigned to the combined Zones NN21-NN20. Also present in this section are Coccolithus pelagicus, Dictyococcites antarcticus, Dictyococcites productus, Helicosphaera carteri, Oolithotus fragilis, Rhabdosphaera clavigera, and Umbilicosphaera sibogae.
Sample 182-1129C-18H-CC, 8-11 cm (168.85 mbsf), contains the highest stratigraphic occurrence of P. lacunosa, indicating that sediments below this horizon in Hole 1129C (to 444.88 mbsf) and from Samples 182-1129D-1R-CC, 6-9 cm (282.33 mbsf), to 21R-CC, 31-34 cm (554.70 mbsf), belong to Zone NN19. Proceeding downhole, abundances of small Gephyrocapsa spp. increase, G. caribbeanica decrease, and C. leptoporus varies from rare to common. The highest stratigraphic occurrence of Calcidiscus macintyrei is found in Sample 182-1129C-37X-CC, 24-27 cm (347.47 mbsf). This occurrence may be caused by reworking because it does not occur again until Sample 182-1129C-45X-CC, 20-23 cm (414.00 mbsf).
Samples 182-1129D-22R-CC, 16-19 cm (557.68 mbsf), to 26R-CC, 30-33 cm (594.90 mbsf), contain Sphenolithus heteromorphus, together with Cyclicargolithus floridanus, Sphenolithus moriformis, and Braarudosphaera bigelowii. This association indicates a zonal assignment to the combined Zones NN5-NN4. This section lies directly below a bryozoan turbidite marking an unconformity that spans 12 m.y. (see "Lithostratigraphy").
The section at Site 1129 is comprised of two biostratigraphic units: (1) a Pleistocene interval that exceeds 540 m in thickness, underlain conformably by an ~17-m-thick upper Pliocene unit; and (2) a middle Miocene unit. The biostratigraphic units are separated by an unconformity that coincides with the boundary between lithostratigraphic Units II and III at 556.7 mbsf (see "Lithostratigraphy"), and appears to correspond to the base of seismic Sequence 2 (see "Seismic Stratigraphy" in the "Site 1127" chapter). The succession of biostratigraphic units is similar to those at Sites 1127 and 1131.
Planktonic foraminifers are comparatively rare, although beautifully preserved, among abundant bryozoans, gastropods, ostracodes, and benthic foraminifers in the sand-sized fraction above 36.28 mbsf. However, preservation and abundance decline rapidly, from moderate below 36.28 mbsf to poor below ~68 mbsf, and carbonate cement and recrystallized overgrowths obscure test features throughout the remainder of the section. Some intervals are so degraded that they contain no recognizable taxa. In general, the inferior condition of the tests makes us uncertain whether we accurately recognized the first and last occurrences of species used to mark zonal boundaries. This is borne out by magnetostratigraphic results and is discussed in "Paleomagnetism".
The two most abundant species of the well-preserved, temperate assemblage above ~68 mbsf are Globigerinoides ruber and Globorotalia inflata. Eleven other species are rare to common and include Globigerina bulloides, Globigerina falconensis, Globigerina quinqueloba, Globigerinita glutinata, Globorotalia hirsuta, Globorotalia truncatulinoides, Orbulina universa, Neogloboquadrina pachyderma (dextral), and Zeaglobigerina rubescens. Only a few robust and distinctive species, however, are consistently recognized in the poorly preserved section below ~68 mbsf. They are Globorotalia inflata, G. ruber, N. pachyderma, and G. truncatulinoides.
Samples above ~414 mbsf are placed in Pleistocene Zones Pt1 (Berggren et al., 1995) and SN14 of Jenkins (1985, 1993), based on the presence of G. truncatulinoides. The last occurrence of Globorotalia tosaensis between 335.83 and 347.47 mbsf (Sample 182-1129C-37X-CC, 24-27 cm) is used to divide the zone into Subzones Pt1b and Pt1a. Poor preservation, however, may have deepened the last appearance of G. tosaensis, which occurs between 335.83 and 347.47 mbsf. This is only slightly above the Brunhes/Matuyama boundary, which lies between 338.8 and 347.2 mbsf in Hole 1129C (see "Paleomagnetism"). The last occurrence of G. tosaensis occurred at 0.65 Ma in subtropical latitudes and locally at 0.71 Ma. Based on data from Site 1127, this datum should lie 71 to 38 m above the Brunhes/Matuyama boundary, based on estimates from the sedimentation rate for this interval (Fig. F9).
Placement of the base of Zone Pt 1 also may be affected by poor preservation. The poor preservation may have caused the first appearance of G. truncatulinoides to be recognized at a depth that is too shallow. This horizon occurs 42 m below the onset of the Jaramillo Subchron, and its age is estimated to be 1.04 Ma on the basis of the sedimentation rate. The estimate is much younger than the age of the first appearance of G. truncatulinoides in subtropical regions (excluding the southwest Pacific Ocean according to Berggren et al., 1995). The estimate is also younger than the age estimated at 1.23 Ma at nearby Site 1127 in which preservation is superior.
Samples from 413.86-432.93 mbsf to 554.7-554.70 mbsf contain most of the species found in the Pleistocene Zone Pt1 described above, although without G. truncatulinoides and with more common G. crassaformis. Therefore, the unit is placed in the G. crassaformis interval, which at this site includes a lengthy portion of the lower Pleistocene and a portion of the upper Pliocene (Fig. F8) on the basis of magnetostratigraphy (see "Paleomagnetism") (Berggren et al., 1995).
The middle Miocene section below the disconformity at 556.7 mbsf (see "Lithostratigraphy") contains two samples (Samples 182-1129D-22R-CC, 16-19 cm, and 24R-1, 20-25 cm, from 557.68 and 575.60 mbsf, respectively) containing sparse and poorly preserved planktonic foraminifers. Both samples contain Globigerinoides trilobus and Globoconella conoidea, an association consistent with a late-middle Miocene age. The latter sample also contains Globigerinella obesa, Zeaglobigerina woodi, Globigerinita glutinata, O. universa, Globoconella miozea, Globoquadrina dehiscens, Globigerinita juvenilis, and Neogloboquadrina continuosa. The association of the last five species is consistent with a middle Miocene age.
Benthic foraminifers were studied from the only two core-catcher samples recovered at Hole 1129B and from every fourth core-catcher sample from Holes 1129C and 1129D. Additional samples were examined from intervals in which marked lithologic change occurred. Benthic foraminifers are generally abundant and well preserved at Hole 1129B and in the upper part of Hole 1129C (Cores 1H-15H). Abundance decreases significantly and preservation deteriorates markedly below Core 182-1129C-15H. Between 100 and 300 benthic foraminifers were picked from the >63-µm fraction, except in samples in which abundance was low. The benthic foraminiferal assemblages at Site 1129 contain mainly cosmopolitan taxa, although they also include species with a more geographically restricted distribution. Postcruise studies will be necessary to fully document benthic foraminifer distribution at Site 1129 during the Pleistocene and to investigate faunal changes in relation to climate, sea-level, and/or circulation fluctuations within a sequence stratigraphic framework. A shallowing-upward trend from middle bathyal paleodepths in the Miocene to upper bathyal paleodepths in the Pleistocene is exhibited by the following assemblages at Site 1129.
This relatively diverse, well-preserved assemblage is found in core-catcher samples together with abundant, well-preserved bryozoan fragments. It includes some large specimens (>1 mm) of Sigmoilina obesa, Heterolepa dutemplei, Textularia spp., Spirillina spp., and miliolids. Also present as rare to few constituents of the assemblage are Sphaeroidina bulloides, Martinottiella communis, Sphaeroidina variabilis, Hoeglundia elegans, Bigenerina nodosaria, Uvigerina hispidocostata, Bulimina marginata, Bolivina spp., Loxostomum spp., Loxostomoides spp., Rosalina spp., Cibicidoides spp., Anomalinoides spp., and various nodosariids. The presence of S. bulloides, Hoeglundia elegans, B. nodosaria, and Bulimina marginata indicates upper bathyal paleodepths. Similar benthic foraminiferal assemblages were found at Sites 1131 and 1132. They appear to have been associated with the establishment of diversified bryozoan communities at the seafloor at various times during the Pleistocene. Variations in the composition of the assemblage may reflect changes in circulation or sea level in the Great Australian Bight, which may have also influenced the development of bryozoan buildups during the Pleistocene. Further sedimentological and micropaleontological work will clarify the evolution of these bryozoan buildups and the paleoceanographic setting of the Great Australian Bight.
This impoverished assemblage is characterized by fluctuating numbers of Hoeglundia elegans, M. communis, H. dutemplei, S. bulloides, Planulina wuellerstorfi, U. hispidocostata, Loxostomum spp., Loxostomoides spp., Elphidium spp., Rosalina spp, Cibicidoides spp., Anomalinoides spp., Palliolatella spp., miliolids, and various nodosariids. Upper bathyal paleodepths are suggested by the presence of the depth-indicative species Hoeglundia elegans, S. bulloides, M. communis, H. dutemplei, and U. hispidocostata. Variable abundance of small miliolids and Elphidium spp. suggests that part of the assemblage was redeposited from shallower depths. However, poor preservation in many of the samples and generally low abundance overall prevent detailed analysis of faunal changes within this interval.
A sparse assemblage was found in Sample 182-1129D-24R-1, 20-25 cm, within an interval of poor recovery in which mostly chert fragments were recovered. This assemblage is characterized by Vulvulina spinosa, Plectofrondicularia vaughni, Rectuvigerina striata, Siphonina tenuicarinata, Bolivina spp., and various nodosariids. Middle bathyal paleodepths are suggested by the presence of V. spinosa, P. vaughni, and R. striata.
Sediment accumulation rates were calculated from preliminary biostratigraphic and paleomagnetic data from Site 1129. The results are presented in Figure F9. The biostratigraphic datum levels, together with the paleomagnetic data used, are listed in Table T2.
An exceptionally high sedimentation rate, averaging 364 m/m.y., is calculated for the Pleistocene section. During the Jaramillo polarity reversal, the sedimentation rate reached its peak of 542 m/m.y. In contrast, the underlying lower part of the Pleistocene registered 142 m/m.y., the slowest rate for the entire Pliocene section. A major disconformity below the Pliocene section is indicated by the simultaneous downhole disappearance of two planktonic foraminifers, G. crassaformis and G. puncticulata, that originate in the lower Pliocene, with the downhole appearance of the middle-lower Miocene nannofossil S. heteromorphus. Biostratigraphic data suggest that the entire lower Pliocene and the greater part of the Miocene (nannofossil Zones NN18-NN6) are missing. The duration of this hiatus is >11 m.y.