BIOSTRATIGRAPHY AND PALEOENVIRONMENTS

Biostratigraphic analyses at Site 1193 reveal a Pleistocene to lower Pliocene sequence overlying a lower to middle Miocene carbonate platform and slope succession. Table T4 lists the nannofossil and planktonic foraminifer datums used for age assignments (see "Age Model" for age vs. depth and sedimentation rate plots). In addition, larger benthic foraminiferal assemblages and morphologies were evaluated as environmental indicators. Core catcher samples were the basis for the primary analysis, but additional samples from within selected intervals were studied to refine the biostratigraphy. Thin sections were examined to verify selected larger foraminiferal taxa and morphologies. These analyses reveal lower to lower middle Miocene assemblages.

All available core catcher samples from the three holes (1193A through 1193C) plus ~30 samples, mostly from the top three cores in Hole 1193A, were examined for calcareous nannofossils. Pleistocene through lower Miocene nannofossil assemblages were recovered from these samples. Nannofossils are abundant in the top five cores in Hole 1193, virtually absent from Cores 194-1193A-6H through 38X, and generally common to rare from Core 39X to the bottom of the hole (Core 83X). Correspondingly, nannofossil biostratigraphic resolution is relatively good for the top five cores, not available for Cores 194-1193A-6H through 38X, and relatively low for Core 39X to the bottom of the hole. Nannofossils were rare to absent in the core catcher samples from Holes 1193B and 1193C; thus, no useful age information could be produced from these latter samples.

A series of nannofossil datums (last occurrences [LOs] of Calcidiscus macintyeri, Discoaster brouweri, Discoaster pentaradiatus, Discoaster surculus, and Discoaster tamalis) are apparently truncated between interval 194-1193A-1H-3, 30 cm, and 1H-5, 30 cm, indicating a hiatus at least from 1.7 to 2.8 Ma or at most from 0.9 to 3.7 Ma, as constrained by the presence of Reticulofenestra asanoi in samples above and the absence of Reticulofenestra pseudoumbilica in samples immediately below.

The nannofossiliferous samples bracketing the carbonate-platform sequence (Cores 194-1193A-6H through 38X) are Samples 194-1193A-5H-CC and 39X-CC. The former contains abundant nannofossils, including Discoaster quinqueramus and D. surculus, suggesting an age range of 5.6-7.5 Ma. The latter sample contains rare nannofossils, including Cyclicargolithus floridanus, which indicates an age older than 11.9 Ma. More precise age information for the carbonate-platform sequence was not possible because the few core catcher samples available from this interval were barren of nannofossils.

Sample 194-1193A-82X-CC contains common and well-preserved nannofossils, including few Zygrhablithus bijugatus and Cylcicargolithus abisectus and rare specimens that resemble those of Reticulofenestra bisecta (except smaller in size than the latter species). As the LO of Z. bijugatus, located above Sample 194-1193A-82X-CC, generally occurs only slightly above the last occurrence of R. bisecta, which approximates the Oligocene/Miocene boundary, Sample 82X-CC is thus only slightly above the Oligocene/Miocene boundary.

The shipboard planktonic foraminifer biostratigraphy of this site is based on Hole 1193A. Holes 1193B and 1193C were cored to improve NMP recovery in lithified material not suitable for shipboard planktonic foraminiferal analysis.

The sequence overlying the NMP (Samples 194-1193A-1H-CC through 4H-CC) contains lower Pleistocene to lower Miocene assemblages. Within the platform interval (35-229 mbsf), sample lithification and low planktonic foraminiferal abundance hindered analysis. Below the platform interval, limited planktonic biostratigraphy can be provided because the sediments were either barren or showed very low abundance of planktonic foraminifers.

The uppermost Sample 194-1193A-1H-1, 20 cm, contains Pulleniatina finalis, placing the sample between the middle of Zone N22 to the Holocene. This was further confirmed by the absence of Globorotalia tosaensis and the presence of Globorotalia truncatulinoides, indicating an age of middle to late Pleistocene (Zone N23). In Sample 194-1193A-1H-3, 20 cm, the first occurrence (FO) datum of G. truncatulinoides coupled with the absence of Globigerinoides fistulosus suggests an age of Zone N22 or younger. The presence of G. tosaensis with its LO datum at the Pt 1b/Pt 1a boundary together with the absence of P. finalis (FO datum in middle Zone N22) implies a lower to middle Zone N22 age (Pt 1a) for this sample. Sample 194-1193A-1H-5, 18 cm, reveals the absence of G. truncatulinoides, whose FO datum defines the base of Zone N22. The first recorded presence of G. fistulosus occurs in this sample, and its range defines lower Zone N21 to lower Zone N22; therefore, the overlap with the absence of G. truncatulinoides implies an upper Zone N21 age for this sample. Sphaeroidinellopsis seminulina is absent in this sample but present in Sample 194-1193A-2H-2, 10 cm, and thus the LO datum (approximating the middle of Zone N21) occurs between the two intervals. This helps to constrain the age of Sample 194-1193A-1H-5, 18 cm, as upper Zone N21 and the age of 194-1193A-2H-2, 10 cm, as Zone N20. An upper age limit in Sample 194-1193A-2H-2, 10 cm, was provided by the absence of G. fistulosus as the top of Zone N20. The lower age limit of the sample was defined by the first appearance of Dentoglobigerina altispira and its FO datum at the base of Zone N20. This is supported by the absence of Globorotalia crassaformis, whose FO datum lies in the lower Zone N20. Sample 194-1193A-2H-5, 20 cm, contains the same assemblage as above but includes the Neogloboquadrina acostaensis FO datum, which occurs at the base of Zone N20.

Samples 194-1193A-3H-2, 10 cm, through 4H-5, 100 cm, were dated within Zone N19 (early Pliocene) based on the LO datum of Globorotalia margaritae and the presence of Globorotalia puncticulata. The LO datum of Globigerina nepenthes is close to the top of Zone N19 and occurs between Samples 194-1193A-4H-1, 80 cm, and 4H-2, 10 cm. Based on G. margaritae, this datum seems to be lower than the top of Zone N19. Samples 194-1193A-1H-CC through 2H-CC contain no index species for the late Pliocene (i.e., G. truncatulinoides, G. tosaensis, Globorotalia miocenica, or G. fistulosus). Abundant specimens of Sphaeroidinella dehiscens, are just at the base of Zone N18, and their first occurrence juxtaposed with the absence of the listed index species indicates a Zone N18 age for these samples. This is further supported by the presence of Pulleniatina obliquiloculata whose FO occurs from Zone N19 to Zone N20 as well as Globorotalia plesiotumida, whose LO is at the base to middle of Zone N19. Samples 194-1193A-3H-CC and 4H-CC lack S. dehiscens and contain Globorotalia cibaoensis, which has its LO in the lower Zone N19 and, therefore, can be placed in Zone N18. The FO of Globorotalia tumida is close to the top of Zone N17. This taxon is present in Sample 194-1193A-4H-CC but not in 5H-CC, which implies a N18 zonation for Sample 4H-CC.

Planktonic foraminifers were not of use in establishing the middle to late Miocene biostratigraphy. Samples 194-1193A-45X-CC through 60X-CC usually contained <1% (by volume) planktonic foraminifers and showed high levels of test alteration and overgrowth. In addition, the specimens found were all globigerines (i.e., shallow dwellers common in the photic zone, which broadly equates to the surface mixed layer). This evidence could be loosely construed as a paleodepth indicator. Although the surface mixed layer varies in depth, it averages ~100 m water depth. No deep/subthermocline dwelling Globorotaliid forms are present, which would normally be present with a shallow mixed layer.

Samples 194-1193A-45X-CC through 60X-CC contain many nonindex fossils of extended biostratigraphic range characteristics (e.g., Globigerina woodi [Zones P22-N21], Globigerinoides triloba [Zones N4b-N22], Globigerina praebulloides [Oligocene-Zone N17], and Globoquadrina dehiscens [Zones N4b-N18]). Some species such as Globigerinoides parawoodi (Zones N4b-N7) and Globigerina connecta (Zones N4b-N7) were used in conjunction with the zone fossil Globigerinoides primordius (Zones N4a-N5) to tentatively assign a Zone N4-N7 age range to Samples 194-1193A-45X-CC through 60X-CC; however, many of the key defining surface and apertural characters were abraded or overgrown with calcite. Samples 194-1193A-60X-CC through 84X-CC were barren of planktonic foraminifers.

Microscopic analysis of biogenic constituents, particularly the diverse and often abundant benthic foraminifers, provide data for paleoenvironmental interpretation of the sediments at Site 1193 (Table T5). Distinctive larger benthic foraminiferal assemblages also provide limited biostratigraphic resolution within intervals lacking diagnostic planktonic foraminifers or nannofossils. Core catcher samples were the basis for primary analysis, supplemented by direct observations of selected cores. Thin sections were examined to verify larger foraminiferal taxa and morphologies.

Biostratigraphy

With the exception of the hemipelagic sediments on top of the NMP (lithologic Unit I), larger benthic foraminiferal associations characteristic of the early to middle Miocene were found in Samples 194-1193A-5H-CC (37 mbsf; lithologic Unit III) through 194-1193C-6X-CC (522 mbsf; lithologic Unit V) just above basement (for lithologic unit descriptions, see "Lithostratigraphy and Sedimentology").

The larger benthic foraminiferal assemblage in lithologic Units II-V appears to be consistent with Chaproniere's (1981, 1984) larger foraminiferal associations Zones LF5-LF7, with abundant Amphistegina spp. (Fig. F30), Lepidocyclina (Nephrolepidina) howchini (Figs. F30, F31, F32), Cycloclypeus sp. (Fig. F32), Operculina complanata (Fig. F33), and, less commonly, Miogypsina spp. (Fig. F34). Chaproniere (1981, 1984) correlated Zones LF5-LF7 biostratigraphically to Neogene planktonic foraminiferal Zones N6 to N9, which are latest early and early middle Miocene in age (~18.8-14.8 Ma). Betzler and Chaproniere (1993) report similar assemblages as ranging from early to early middle Miocene (~24-12 Ma). This age span, which represents all or part of the time of platform buildup, is consistent with the gap in age control from the nannofossil and planktonic foraminiferal data set (Table T4). Detailed biometric analyses of the embryons of specimens of Lepidocylina and Miogypsina will be necessary to determine the species in lithologic Units II-V to refine the biostratigraphy (e.g., Chaproniere 1981, 1984).

Paleoenvironmental Analysis

Sediments recovered in Samples 194-1193A-1H-CC (6.6 mbsf) through 5H-CC (37 mbsf) are overwhelmingly dominated by well-preserved tests of planktonic foraminifers. Rare benthic foraminifers found in Samples 194-1193A-1H-CC through 3H-CC are characteristic of upper bathyal depths (Table T5). Laticarinina, Cibicidoides, and agglutinated foraminifers are conspicuous in this interval. Reworked benthic foraminifers occur in Sample 194-1193A-4H-CC and are abundant in Sample 5H-CC, which is very near the top of the NMP platform (see "Lithostratigraphy and Sedimentology"). Most notably, specimens of the larger benthic foraminifers, Amphistegina and Lepidocyclina, range from white and very well preserved to brown and rounded, indicating both reworked material and in situ deposition. These samples are part of lithologic Unit II (see "Lithostratigraphy and Sedimentology") and represent hemipelagic sedimentation. Unit I occurs near the top of Core 194-1193A-1H, above the core catcher, and was not sampled for biostratigraphic analysis.

Larger benthic foraminifers are common to abundant throughout lithologic Subunit IIIA, including Samples 194-1193A-5X-CC (37 mbsf) through 30X-CC (171 mbsf), 194-1193B-1X-CC (37.4 mbsf) through 17Z-CC (115 mbsf), and 194-1193C-1X-CC (115 mbsf) through 4X-CC (163 mbsf). Bryozoans are the dominant sediment constituent in these grainstones and packstones; larger benthic foraminifers are locally very abundant. The modern analogs of these foraminifers host algal endosymbionts and live in habitats within the euphotic zone, which is typically <100 m water depth (e.g., Hallock, 1987a, 1999; Hohenegger, 1999). Thus, the larger foraminifer-rich limestones and dolomites of lithologic Subunit IIIA are interpreted to have been deposited at depths of <100 m (inner to middle neritic) in a platform depositional environment. Within this lithologic unit, intervals of very robust and abraded larger foraminifers are found that are indicative of inner neritic (30 m, sometimes <10 m) habitat and deposition (Table T5). These foraminifers often occur abundantly immediately below exposure surfaces. Elsewhere in the unit, very large (>2 cm), very flat, well-preserved Cycloclypeus and Lepidocyclina are observed, indicating untransported assemblages with habitats at paleodepths near the limits of the euphotic zone (likely 60-100 m). More detailed reconstruction of depositional histories and paleodepth estimates for lithologic Subunit IIIA will require detailed analysis of thin sections from this interval.

Sediments in Samples 194-1193A-31X-CC (171 mbsf) through 35X-CC (196 mbsf), from the uppermost part of lithologic Subunit IIIB, have three distinct components that result in a bimodal sediment size distribution. The matrix includes terrigenous clays and silt-sized carbonate debris (see "Lithostratigraphy and Sedimentology"). Each sample also has a coarse component of relatively well sorted bioclastic material that is rich in larger benthic foraminifers. For example, the coarse fraction of Sample 194-1193A-33X-CC consists of sorted, rounded Lepidocyclina tests and strongly resembles the larger foraminiferal beach sands found today at Lizard Island, Australia, Bali, and Okinawa (Lee, 1998). Subsequent samples from lithologic Subunit IIIB (Samples 194-1193A-34X-CC through 42X-CC) generally have a greater coarse bioclast-to-matrix ratio but are similar in their three-component composition that includes terrigenous muds, very fine carbonate debris, and coarse bioclasts. Bryozoan fragments are the dominant constituent, but larger benthic foraminifers are often abundant. Large, flat Cycloclypeus and Lepidocyclina morphologies are common in these samples, indicating middle neritic water depths in an outer platform depositional setting (Table T5).

Samples 194-1193A-43X-CC (234 mbsf) through 46X-CC (253 mbsf) (lithologic Unit IV; see "Lithostratigraphy and Sedimentology") contain benthic foraminiferal assemblages (Cibicidoides, Lenticulina, Uvigerina, etc.) characteristic of outer neritic to upper bathyal depths (>120 m). The muddy sediments are predominantly silt-sized carbonate debris with a significant terrigenous clay fraction (see "Lithostratigraphy and Sedimentology" and "Geochemistry"). These sediments lack coarser shallow-water constituents, including the tests of larger benthic foraminifers, and are interpreted to represent a distal periplatform depositional environment.

Sediments in Samples 194-1193A-47X-CC (263 mbsf) through 61X-CC (378 mbsf) (lithologic Unit V; see "Lithostratigraphy and Sedimentology") are intermediate in composition between those of lithologic Subunit IIIB and Unit IV. The dominant constituent of most of these samples is silt and very fine sand-sized carbonate debris, including tiny fragments of bryozoans, red algae, larger benthic foraminifers, echinoids, and worm tubes. Tiny benthic and planktonic foraminiferal tests are also abundant in the fine sediments. Whole juvenile specimens of Amphistegina, Lepidocyclina, and Operculina, as well as comparable-sized fragments of bryozoans, red algae, and echinoids, are relatively common in thin sections from this unit. However, the presence of outer neritic benthic foraminifers as well as common planktonic foraminifers indicates that debris from shallower-dwelling photosynthetic organisms, particularly red algae and larger foraminifers, were transported into deeper water. These sediments are interpreted as having been deposited in a proximal periplatform depositional environment.

Samples 194-1193A-62X-CC (388 mbsf) through 83X-CC (513 mbsf), which correspond to lithologic Unit VI (see "Lithostratigraphy and Sedimentology"), include both coarse bioclastic and terrigenous clastic sediments. Packstones dominated by Lepidocyclina and other larger benthic foraminifers (Figs. F30, F31, F32, F34) are found at the top of Unit VI. Terrigenous constituents generally increase downsection. The presence of larger benthic foraminifers indicates inner to middle neritic water depths for most of this section. The presence of large, flat, well-preserved Cycloclypeus in Sample 194-1193A-70X-CC (431 mbsf) in very coarse terrigenous and bioclastic sediments indicates that the sediments were neither transported during the lives of these delicate foraminifers, nor abraded prior to their burial. The flat shape of these foraminifers indicates low light conditions characteristic of middle neritic depths (Table T5).

Interpretation

Most of the neritic carbonate sediments and rocks encountered at Site 1193 represent sedimentation by a diverse bryozoan community in which larger benthic foraminifers were an important component. Bryozoans are commonly associated with cool-water carbonates, often at subeuphotic depths (e.g., James, 1997). However, the abundant larger foraminifers found in lithologic Units III and VI indicate at least a cool subtropical (e.g., 17°-23°C) climate and paleodepths within the euphotic zone (<120 m), which is the depositional environment of modern bryozoan-coralline algal-larger foraminiferal sediments found on the southwest Australian shelf (James et al., 1999).

The larger foraminifers found in lithologic Units III, V, and VI between 37 and 520 mbsf represent a reduced Australian lower to middle Miocene assemblage (e.g., Chaproniere, 1981, 1984; Betzler and Chaproniere, 1993; Chaproniere and Betzler, 1993). The Site 1193 assemblages are characterized by having only rotaline, low-Mg calcite taxa including at least two species each of Lepidocyclina, Amphistegina, Miogypsina, and Cycloclypeus, as well as Operculina complanata. Lepidocyclina howchini is the most consistently abundant species and is the dominant constituent of packstones in the upper portion of Unit VI (Figs. F30, F31, F32, F34). Porcellaneous, high-Mg calcite larger foraminifers such as Marginopora and Flosculinella are strikingly rare in these units, as are zooxanthellate coral and Halimeda, which may be a geochemical indication of carbonate saturation state below the threshold for coral-reef constructions (e.g., Hallock, 1987b; Kleypas et al., 1999).

The sedimentology and larger benthic foraminifers both indicate that lithologic Unit VI was deposited at inner to middle neritic water depths (Table T5). This unit likely represents initial flooding of terrigenous basement at this location (see "Lithostratigraphy and Sedimentology"). Paleoenvironmental data indicate deepening to upper bathyal depths in lithologic Unit V, as indicated by the presence of planktonic and benthic foraminiferal assemblages. This unit is dominantly composed of very fine bioclastic sediments, possibly deposited by contour currents. Pulses of coarser bioclastic sediments from inner and middle neritic settings, which are common in this interval, were likely transported directly downslope to this proximal periplatform setting. Lithologic Unit IV has the same outer neritic/upper bathyal benthic foraminiferal assemblage in fine bioclastic sediments of neritic origin as are found in Unit V, but lacks pulses of coarser, distinctively shallow-water bioclastics, possibly indicative of a distal periplatform setting. However, Unit IV also contains significantly more fine terrigenous clays and organic matter, suggesting an environmental mechanism that increased terrigenous input to the coastal system. Terrigenous siliciclastic sediments, accompanied by dissolved nutrients and organic matter, may have reduced carbonate production by inner and middle neritic benthic communities, resulting in reduced downslope transport of coarser bioclastic sediments.

Subunit IIIA sediments, which are bryozoan-dominated boundstones and coarse bioclastics with common to abundant larger foraminifers, appear to represent platform sedimentation at inner to middle neritic depths (Table T5). The depositional environment of sediments deposited in lithologic Subunit IIIB appears to be middle neritic and intermediate between the inner to middle neritic platform of lithologic Subunit IIIA and the outer neritic to upper bathyal setting of lithologic Unit V. The sediments of lithologic Unit IV, although occurring between Subunit IIIB and Unit V, indicate the least shallow neritic influence.