Site 1128 is located on the upper continental rise, close to the toe of slope in 3874.4 m of water. Coring penetrated sediment in a small perched depression (see "Seismic Stratigraphy") within a seaward-thickening wedge of Cenozoic carbonate and terrigenous clastic sediment. Seismic data indicate that the lower part of the continental slope directly north of the site is an ~2000-m-high relatively steep incline with an irregular topography of exposed Mesozoic (see "Seismic Stratigraphy"), mainly terrigenous, clastic sedimentary rocks. The upper part of the slope is a relatively smooth seaward-dipping incline that represents the outer part of the cool-water carbonate ramp. The continental slope is cut by numerous canyons.
The succession intersected at Site 1128 (Fig. F3) consists of 70 m of mainly Neogene pelagic carbonate sediment overlying a 380-m succession of Paleogene deep-water siliciclastic deposits. These two sediment types are separated by a zone of sediment gravity-flow deposits and a major hiatus.
Unit I is a succession of deep-water pelagic carbonate sediment, mostly nannofossil ooze, punctuated by centimeter- to meter-scale planktonic foraminifer glauconite packstones and wackestones and several coarse- to very coarse grained boulder conglomerates. The base of the unit is defined by the transition from brown calcareous clay to gray and green clay. The grainy carbonates and conglomerates have all the attributes of sediment gravity-flow deposits and are interpreted as calciturbidites and debris-flow deposits, respectively. The sediments are pink to brown throughout and characterized by extensive bioturbation.
Subunit IA is pelagic nannofossil ooze punctuated by numerous thin calciturbidites. The sediment is pink to brown in color, interrupted by discrete white intervals. These colors, which become somewhat muted with increased depth, are most probably caused by small amounts of oxidized clay. The noncarbonate component of these sediments is generally ~10% (see "Organic Geochemistry"). Darker hues commonly pass gradationally into lighter hues, defining repetitive decimeter-scale units. Alternatively, color changes abruptly across obvious omission surfaces. Contacts between pink/brown units and white units are generally sharp.
Portions of the section, as thick as 3 m at ~20.0-20.4, 21.2-22.0, and 27.5-28.5 mbsf, are characterized by steeply inclined (as much as 50°) and/or contorted bedding and are interpreted to be the result of synsedimentary deformation. It is not clear whether these are syndepositional or were formed later within the sediment pile.
Pelagic Sediment. The nannofossil ooze has a mudstone to wackestone texture and is conspicuously spiculitic. Well-preserved planktonic foraminifers, generally small and in the fine to very fine sand-size range, form most of the coarser particles. Other rare grains include bioclasts, clay particles, and tunicate sclerites (see "Site 1128 Smear Slides" in PDF format). White sediment is typically totally homogenized through bioturbation, whereas pink/brown sediment, although extensively burrowed, displays multi-tiered trace fossils with discrete Zoophycos, Chondrites, and Planolites.
Calciturbidites. These white to gray grainy sediments have a packstone or grainstone texture and are generally fine- to medium-grained sand. Most are graded turbidites with sharp to eroded bases that gradually pass upward from grainstone to packstone into nannofossil ooze (Fig. F4). Less common grain flows are composed of grainstone throughout and have sharp bases and tops. Units range in thickness from <1 cm to >2 m, with thicker beds usually composed of stacked turbidites. Virtually all the calciturbidites can be correlated between Holes 1128B and 1128C, and they show only minor thickness changes.
The sediment is typically dominated by the tests of planktonic foraminifers and lesser but conspicuous glaucony grains. Glaucony ranges from dark green/black to light pale green and typically has a well-developed botryoidal texture. Some sediment contains numerous limonite/goethite grains, many of which are clearly altered glaucony, whereas others have an indeterminate origin. Other important particles, although usually present in trace amounts, are angular to subangular quartz grains (clear and iron stained) and sedimentary rock fragments, particularly fine- to very fine grained green sandstone. These carbonate sands also occur in isolated burrows, suggesting either infiltration down from overlying turbidites or burrow filling by otherwise bypassing sediment. Other sand-sized grains include benthic foraminifers, sponge spicules, and occasional infaunal echinoid spines.
Coarse-Grained Sediment Gravity Flows. One interval 19 m below the top of the succession is an ~1.2-m-thick sandy conglomerate (Fig. F5). This deposit is mostly a planktonic foraminifer sand with granule-sized pieces of bryozoans (mostly delicate branching growth forms; cf. Nevianopora and Hornera), serpulid worm tubes, benthic foraminifers (mostly nodosariids), and fine-grained green sandstone. The middle portion contains cobble-sized clasts of compacted nannofossil ooze with a wackestone texture, together with planktonic foraminifers. This unit appears to represent several stacked beds.
This interval consists of large clasts, fine sediment, and deformed strata (Figs. F6, F7). Although there are differences between the two holes, there is a general similarity in stratigraphy, and the interval is divisible into two zones.
The upper zone is a series of three meter-scale polymict conglomerates and pebbly mudstones separated by black clay and nannofossil ooze. The lower zone from Hole 1128C is deformed nannofossil ooze and black clay with basal conglomerates. The conglomerates are mainly matrix supported, with clasts in the coarse pebble to cobble grade and a calcareous ooze matrix. Clasts represent a large variety of lithologies from different stratigraphic levels. They consist mainly of claystone and mudstone lithologies, but siltstone and sandstone are also present.
Clasts are well segregated. Some are slightly compacted but not severely deformed, although clast boundaries locally show shearing. A succession of clasts in Sections 182-1128C-7H-1 and 7H-2 are considerably wider than the core diameter (6.6 cm), extending over 20-30 cm in thickness and showing an inclination of 35°-45°. They seem to represent elongated slabs. Consolidation ranges from very soft to moderately consolidated to lithified. This last type is mainly seen in a few specimens of relatively hard green sandstones, with most clasts being poorly to moderately consolidated. Most fragments are randomly oriented, although some beds show a crude subhorizontal clast fabric. Steeply inclined to vertical clast orientations are common.
The matrix consists of massive, nonbioturbated, light gray to white nannofossil and foraminiferal ooze. The clast/matrix ratio varies considerably from very matrix-rich beds, which may be described as pebbly mudstones, to almost clast-supported beds. The well-consolidated greenish black clay occurs in Sections 182-1128C-7H-2 through 7H-CC. It contains scattered vertical or inclined pebble- to cobble-sized clasts of nannofossil ooze and green sandstone. A 55-cm-thick clast? of nannofossil ooze occurs in the clay at the base of Section 182-1128C-7H-3. The lower 340 cm of the greenish black clay unit shows burrow mottling and a poorly developed banding or layering.
The clasts occur in what seem to be fairly well-defined beds, some of which are amalgamated, as indicated by abrupt changes in overall clast size or composition. Bed thickness is on the order of 0.3-1.5 m, and bed boundaries are mainly well defined. Some very matrix rich beds have lower and upper portions dominated by whitish ooze-like matrix that is difficult to distinguish from the enclosing nannofossil and foraminiferal oozes. A few beds show very sharp boundaries with signs of syndepositional shearing. Vertical trends in clast size or clast/matrix ratios of individual beds are difficult to evaluate because of the large clast size compared to the width of the core (6.6 cm). Basal inverse grading and top coarse-tail grading seem, however, to be present in some beds.
The ages of the clasts are known only in a reconnaissance fashion, and further study is needed. Nannofossil ooze clasts analyzed from Subunit IB are middle and late Miocene in age (see "Biostratigraphy"). Lithostratigraphic similarities between the sandstone clasts and Cretaceous sediment encountered at Site 1126 suggest that the terrigenous clastic materials may be of Cretaceous age.
This fine-grained unit is composed of clay and ooze punctuated by several small sediment gravity-flow deposits. The sediment is transitional in character, varying from clay rich in nannofossils to nannofossil ooze with a high clay content. The unit is usually a dull brown to gray color with a yellow to red hue, punctuated by some white units of nannofossil ooze. There are few obvious sediment gravity-flow deposits. Overall, the sediment grades from red at the top to gray toward the base. The interval is characterized by striking alternations of dark and light units, 0.5-1.0 m thick, that pass gradually into one another or are separated by sharp omission surfaces.
Pelagic muds are usually subequal in carbonate and clay content, with the calcareous portion dominated by nannofossils, whereas the biosiliceous component is mostly abundant sponge spicules. Although many sediments are totally mud size, those with a "wackestone" texture (silty muds) also contain benthic and planktonic foraminifers, lesser sponge spicules, bioclasts, tunicate sclerites, and trace amounts of glaucony and quartz. Calcareous nannofossils in most of this interval are poorly preserved and show some evidence of corrosion (see "Biostratigraphy").
Sediment gravity-flow deposits make up a very small proportion of these sediments (Fig. F3), consisting of centimeter-thick calciturbidites similar to those in the overlying Unit I.
This part of the succession is a uniform green, slightly calcareous clay that is locally interrupted, particularly in the upper parts, by a few beds of redeposited planktonic foraminiferal and nannofossil ooze (Fig. F8). The sediment is divided on the basis of composition into three subunits: an upper clay, an intermediate clayey chalk, and a lower claystone. The unit is light to dark green in color. Bright green millimeter-scale laminations are present throughout.
This unconsolidated slightly calcareous clay is thoroughly bioturbated, with burrows manifest as mottles and a few discrete trace fossils. The sediment is clay with trace amounts of angular quartz and glaucony silt and a variable carbonate component, generally less than 20 wt% (see "Organic Geochemistry"). The carbonate component is dominated by coccoliths with accessory small planktonic foraminifers. Many such skeletons are corroded by dissolution. Accessory biosiliceous components are volumetrically dominated by sponge spicules with lesser discoasters, radiolarians, diatoms, and silicoflagellates. Sponge spicules are commonly concentrated in small subcentimeter burrows.
The carbonate intervals are white and either nannofossil ooze with a mudstone to wackestone texture, planktonic foraminiferal ooze with a packstone texture, or layers that grade upward from planktonic foraminiferal ooze to nannofossil ooze. Most are less than 0.5 m thick, except for one 6.0-m-thick layer at ~135 mbsf. The grading suggests that most of these intervals are turbidites.
This clayey chalk is partially lithified and contains chert layers and numerous discrete multigeneration ichnofossils. The most prominent of these trace fossils are Planolites, Chondrites, Zoophycos, Terebelina, and Thalassinoides. Pyrite locally fills some small burrows.
This poorly recovered sediment is a moderately consolidated clay in the upper part of the interval and a lithified claystone below 260 mbsf. This unit is an olive-green clay rich in sponge spicules with ~30% carbonate (see "Organic Geochemistry"), mainly in the form of nannofossils. The section contains four centimeter-thick carbonate beds, mudstone or wackestone in texture, composed of nannofossils and pelagic foraminifers and interpreted as calciturbidites. Numerous centimeter-thick chert layers are present. Ichnofossils are superbly preserved and dominated by Zoophycos.
This 2-m-thick unit stands out from the enclosing sediment because of its light-colored, coarse-grained character. Although thin, the unit is distinguished because it is so different from sediment above and below and because it marks a major change in overall sediment composition. The unit is composed of a lower glauconitic sandstone, an intermediate cross-laminated sandstone grading upward to a carbonate nannofossil wackestone (Fig. F9), and an upper glauconitic sandstone with water-escape structures grading upward to a nannofossil carbonate mudstone (Fig. F10). The sandstones, interpreted as turbidites, are separated by green burrowed claystone.
This unit is composed of green, highly burrowed, silty clay to sandy siltstone. Two subunits are distinguished. Subunit IVA is silty clay and claystone; Subunit IVB, a clayey siltstone to sandy siltstone. Although extensively burrowed, there are short sections with millimeter-scale laminations.
The green silty clay of this unit is mostly partially lithified but grades into a lithified claystone below 355 mbsf. Although trace fossils are characteristically flattened and compacted throughout, Zoophycos, Chondrites, Paleophycus, and Planolites are still distinguishable. The claystones contain silt-sized quartz and glauconite with biotite, opaque minerals, and clay flakes present in trace amounts. Opaline diatoms are common. Some of the sediment contains rare sponge spicules, radiolarians, and planktonic foraminifers. There are a few centimeter-scale chert layers between 140 and 160 mbsf.
The section is punctuated by occasional centimeter-scale sandstone Ta-Tc turbidites at ~335 mbsf. These sandstones are fine sand-sized lithified arkose with angular quartz grains, glauconite grains, intraclasts, lithoclasts, and planktonic foraminifers (see "Site 1128 Smear Slides" in PDF format). Sediment is normally graded with the coarser lithoclast and glauconite grains at the base, planktonic foraminifers in the middle, and silt-sized quartz grains at the top. The matrix is locally partially silicified with alteration rims around glauconite and phosphatic grains. There are also traces of bryozoans, sponge spicules, and benthic foraminifers. Other terrigenous grains consist of traces of opaque minerals, biotite, microcline, and plagioclase.
The sediment is mostly clayey siltstone with silt- to occasional fine sand-sized grains of glaucony, quartz, mica, unidentified opaque minerals, and trace amounts of sponge spicules. Burrowing is extensive throughout. The basal 5 m is coarser grained, grading downward from sandy siltstone to silty sandstone. The sandy sediment near the base has centimeter-thick brown-colored layers and contains abundant quartz and feldspar grains, mica, glaucony, and minor silicified benthic foraminifers.
These Eocene and lowermost Oligocene clays, siltstones, and basal sandstones, with their almost total lack of calcareous material except for a few layers with corroded microfossils, suggest deep-water deposition below the CCD. The green color and extensive bioturbation of the sediment throughout most of this section, except for the base of Subunit IVB, indicate dysoxic depositional conditions.
Overall, Unit IV fines upward from basal sandy silts to pelagic clays at the top, indicating gradual fining of terrigenous clastic sediment delivery. This may be due in large part to climate. The middle Eocene in much of southern Australia was a time of abundant rainfall, coastal rainforests, and much runoff (Kemp, 1978; Macphail et al., 1994). Fluvial siliciclastic and paralic lignite deposits are common onshore. Sedimentation became more marine and carbonate rich toward the end of the late Eocene as climate became more semiarid. The fining-upward trends observed in this interval may reflect this change as a result of fluvial systems delivering less coarse sediment to the ocean.
The succession of sandstone-carbonate turbidites of Unit III, resulting from either seismicity or sea-level fall, signals a change in depositional style. The uppermost Eocene and lowermost Oligocene green sediments of Unit II are variably calcareous and similar to shallower Neogene pelagic sediments. The thick section of clays and clayey chalks most probably accumulated near the lysocline, as indicated by the corroded and partially dissolved calcareous nannofossils and overall paucity of planktonic foraminifers (see "Biostratigraphy"). The green color and diverse trace fossil assemblage indicate a continuing dysoxic seafloor environment.
The upper Eocene-lowermost Oligocene record contains numerous chert bands, in most cases representing silicified clay. The upper Eocene in shallow-water facies across southern Australia was characterized by biosiliceous deposition, particularly spiculites, spongiolites, and silicified chalks. This similarity implies some sort of oceanographic control on silica deposition during this time.
These fine-grained sediments are transitional in color in the lower part, from gray upward to brown, reflecting a gradual oxygenation of the seafloor and presumably more vigorous mixing and ventilation of the entire water column. The pink nannofossil oozes are classic pelagic sediments composed mostly of fine coccolith plates derived from the postmortem disintegration of calcareous phytoplankton (cf. Scholle, 1983). The accessory planktonic foraminifers are well preserved, indicating accumulation above the calcite lysocline. Numerous siliceous sponge spicules were probably derived from upslope because there are no sponge body fossils or root mats. One possible source is the zone of prolific sponge growth near the shelf edge and upper slope (James et al., 1992).
The presence of bryozoans in coarser conglomerates also suggests a link to a relatively shallow-water sediment source. Delicate branching bryozoans, like those in the resedimented material, accumulate today on the upper slope and characterize sediments between water depths of 200 and 350 m (Bone and James, 1993). Likewise, the lack of material characteristic of slightly downslope environments (~350-600 m water depth) like those encountered at Site 1127 implies sediment bypassing, possibly by travel down submarine canyons and gullies. Nevertheless, the presence of green terrigenous sand grains and lumps of compacted nannofossil ooze suggests erosion and acquisition of some deeper material as the flows moved downslope. Local synsedimentary deformation of the pelagic sediment points to periodic slumping or mass movement, presumably caused by oversteepening or seismicity.
This assemblage of matrix-supported, soft to hard sediment clasts with exotic lithologies such as black clay and green sandstone, together with abundant soft-sediment deformation and evidence of local shear with a fine-grained locally derived matrix of pelagic carbonate, indicates a complex accumulation process involving downslope movement. The three main lithologies are (1) graded foraminiferal packstone, (2) nonbioturbated, unlithified foraminiferal wackestone, and (3) matrix-supported, unlithified pebble and cobble conglomerates with clasts composed of a variety of strongly colored consolidated clays and oozes. The graded packstone beds indicate deposition by turbidity currents, and the interbedded nannofossil and foraminiferal oozes indicate deposition at deep bathyal to abyssal water depths. The nonbioturbated wackestones were probably deposited from mudflows or short-traveled turbidity currents. The conglomerates were deposited from viscous debris flows, and benthic foraminifers in the clasts and matrix suggest a middle to upper bathyal source. The size of clasts and other features in such deposits is best seen and interpreted at the large outcrop scale; two cores can only hint at the precise nature of these sediments. Nevertheless, the relatively intact nature of the materials, even though commonly poorly lithified, implies that their origin is local and that they were derived from immediately upslope environments. On balance, this interval is interpreted as a series of debrites whose origin is relatively local, but which has sampled rocks possibly as old as Paleogene or Cretaceous.
The timing of the events is somewhat constrained by upper Miocene sediment directly overlying these deposits. The late middle Miocene-late Miocene was a period of prolonged low eustatic sea level (Haq et al., 1987). In southern Australia, however, it is also a time when the southern continental margin underwent tilting and uplift, exposing Paleogene and lower-middle Miocene shallow-water sediments (Lowry, 1970). The coincidence of the mass flows with this uplift episode suggests that the two events may be linked and that the triggering mechanism for movement was seismic.
The event that presumably removed upper Oligocene-lower Miocene sediment might be due to either submarine erosion/corrosion before the resedimentation event or removal of the underlying section by erosion during mass flow deposition.