Bounty Fan is the terminus for a sediment-transport system that begins in the rapidly rising Southern Alps of New Zealand. During glacial periods of low sea level, large quantities of terrigenous sediment were delivered by rivers to an emergent continental shelf. These rivers continued across the shelf to discharge their loads at the paleoshoreline which, in major lowstands such as those that occurred during isotope Stage 2, was near to the shelf edge. There, sediment was captured by a suite of submarine canyons as well as being deposited on the upper continental slope. Mobilization of the sediment periodically generated turbidity currents that swept down canyons and into the Bounty Channel system (Carter and Carter, 1987, 1993). These currents traveled 900 km along the channel to eventually disgorge onto Bounty Fan at the mouth of Bounty Trough. En route, the overspilling turbidity currents deposited an extensive levee complex, characterized by a higher left bank (facing downchannel) that formed in response to deflection by the Southern Hemispheric Coriolis force. Instability within the overspilling currents ultimately was responsible for the formation of an extensive field of sediment waves (Carter et al., 1990).
In sharp contrast to this glacial regime of active transport and deposition, interglacial periods were previously thought to have been times of relative quiescence in Bounty Trough, judging by piston cores described by Griggs et al. (1983), Neil (1991), and Carter and Carter (1988). Under the Holocene highstand of sea level, the cores suggest that terrigenous sediment was diverted from the trough head, thereby allowing the deposition of mainly calcareous pelagite. The exception is near the head of Bounty Trough, where small turbidites accumulated in the upper reach of the channel system.
The abyssal Bounty Fan has developed across the path of the DWBC near the zone where the modern Antarctic Circumpolar Current (ACC) turns east from the margin of Campbell Plateau. Acoustical facies derived from 3.5-kHz profiles suggest that the outer Bounty Fan has been eroded by the abyssal currents (Carter and Carter, 1996). Ostensibly, the dominant flow was the DWBC, but there is an argument that this flow may have been reinforced by the ACC in glacial times when the Subantarctic Front, marking the northern limit of the ACC, probably migrated northward (Nelson et al., 1993). Such a change would presumably have been a response to a northward expansion of subantarctic waters and equatorward migration of strong winds (e.g., Nelson et al., 1993).
Site 1122 was positioned on the crest of a prominent left bank levee containing a 350-m thick sequence of Pleistocene sediment waves, resting on an additional 240 m of normally layered sediment interpreted as extending back to the Miocene (Carter et al., 1994). Coring at Site 1122 obtained a detailed record of Quaternary climatically controlled turbidite sedimentation on the levee, underlain by Pliocene and Miocene contourite drift deposits recording earlier DWBC flow across most of Bounty Trough.
Unit I consists predominantly of a thick sequence of turbidites, which is subdivided into four subunits (Fig. F4). The upper three subunits are distinguished mainly by the thickness and grain size of the turbidites. The lowest subunit, Subunit ID, shows the transition from a turbidity current-dominant to bottom current-influenced depositional environment in its lower half. Unit I extends from 0 to 386.9 mbsf.
The upper 20 cm of Subunit IA is composed of light brownish gray (2.5Y 6/2) silty clay, which may represent Holocene sediments. Because of an offset between Holes 1122A and 1122C, the Holocene sediments and the upper ~8 m are absent from Hole 1122A (see "Operations"). Underneath, Subunit IA is composed of greenish gray (5GY 6/1) silty clay with intercalations of gray (5Y 5/1) very fine sand turbidites. These turbidites are generally <15 cm thick (Table T3), sharp based and normally graded, fining upward into greenish gray (5GY 5/1) silty clay (Fig. F5).
At 4.2 mbsf, an interval appears where the bases of the turbidites are pyritized. A similar pyritization zone occurs between 15 and 18 mbsf. These zones of pyritization are typical features throughout the upper three subunits of Unit I.
A tephra layer occurs at 11.93 mbsf, which probably is the Kawakawa Tephra, dated at 22,590 radiocarbon years (26,000 calibrated years; I.N. McCave, pers. comm., 1998). The base of Subunit IA is probably close to the isotope Stage 3/Stage 4 boundary at 59 ka.
Subunit IB is composed of greenish gray (5G 4/1) silty clay that is interbedded with dark greenish gray and greenish gray (5BG 4/1, 5G 5/1) sand and fine sand turbidites. The thickness of the turbidites is greater than in Subunit IA, varying from 10 to 100 cm (Fig. F6). This subunit also contains three intervals with significant pyrite in the basal sands of the turbidites (23.5-28 mbsf, 47.2-62 mbsf, and 91.7-109.5 mbsf). Subunit IB is moderately bioturbated throughout; only the top of the Section 181-1122C-14X (103.7-104.2 mbsf) represents heavy bioturbation and may reflect an interglacial stage. The lower boundary of Subunit IB is marked by a color change at 109.35 mbsf from greenish gray (5GY 5/1) to grayish green (5BG 5/1). The base of this subunit is estimated to be close to the isotope Stage 8/9 boundary at 285 ka.
Subunit IC is composed of sequences of dark greenish gray (5GY 4/1) silt and very fine sand turbidites with intercalations of grayish green (5GB 5/1) and greenish gray (5GY 5/1) silty clay. The thickness of the turbiditic layers is <20 cm. It is probable that the apparent decrease in turbidite thickness, compared to Subunit IB, is an artifact of the change from APC to XCB drilling. Low recovery rates (averaging ~40%) indicate the likelihood of washout of the thicker sand beds. Three intervals of higher pyrite content are present in Subunit IC (at 146.5-150.43 mbsf, 157.2-166.4 mbsf, and 242.9-245 mbsf). Below 245 mbsf, zones of pyritization are not observed. On average, bioturbation is moderate throughout the entire subunit. In Core 181-1122C-20X there occurs a heavily bioturbated, light greenish gray and light gray (5BG 6/1, 2.5Y 8/1) nannofossil-rich, silty clay. A tephra layer occurs in Section 181-1122C-18X-1 at 138.24 mbsf.
Subunit ID includes greenish gray and gray (5GY 5/1, 5Y 5/1) silty clay interbedded with dark greenish gray (5GY 4/1) very fine sand and silt turbidites of <10-cm thickness. The top of Subunit ID, Core 181-1122C-31X (261.7-271.36 mbsf), marks the first appearance of turbidites with eroded upper contacts and planar lamination. A lighter colored interval with color change from light gray to olive-yellow (5Y 7/1 to 5Y 6/6) coincides with heavier bioturbation. In Section 181-1122C-34X-2, another gradational color change to olive (5Y 5/3) is observed, which is followed by an interval of higher bioturbation in Core 181-1122C-35X (300-308.01 mbsf). There are two tephra layers in Subunit ID, in Section 181-1122C-37X-1 (320.3 mbsf) and in 37X-6 (327.92 mbsf) (Fig. F7).
Subunit ID shows a gradational transition upward from a current-influenced depositional environment to a more turbidite-dominated facies. The higher intensity of bioturbation in the hemipelagic sediment separating turbidite units may indicate a slower rate of deposition or a lower frequency of turbidity currents compared to Subunits IA-IC.
Unit II represents bioturbated, pelagic/hemipelagic sediments interspersed with current-laminated deposits. This unit extends from 386.9 to 550.4 mbsf and is divided into the three Subunits IIA, IIB, and IIC, according to compositional changes.
Poor core recovery in Core 181-1122C-43X suggests that a suitable upper boundary for Subunit IIA lies at the top of Core 44X. The improved core recovery downhole, starting at Core 44X, produced sediments quite distinct from Unit I. The mottled, greenish gray (5G 5/1 to 5GY 5/1) olive-gray (5Y 5/2) or light greenish gray (5BG 7/1) pelagic/hemipelagic silty clay beds are commonly bioturbated with Zoophycos, Planolites, Terebellina, Chondrites, Anconichnus, and Gyrolithes. Pyritization is very sporadic being typically limited to pyritized Zoophycos traces. Interbedded with the silty clay is dark gray (N 4) or dark greenish gray (5GY 4/1) fine sand and silt beds that commonly exhibit scoured basal contacts and conspicuous planar and cross laminations. Such structures suggest a stronger, episodic benthic flow regime, in contrast to that of the decelerating turbidity currents. Laminae are accentuated by concentrations of foraminifers and carbonate debris. Sporadic ripples were observed in the thicker beds (>10 cm) (Figs. F8, F9). Top and bottom contacts of the fine sand and silt beds are generally sharp, with few gradational boundaries. These sand and silt beds are tentatively interpreted as contourite deposits (see "Interpretation and Discussion"). Interspersed among the contourites are sporadic fine sand beds with sharp basal contacts and normal grading, which suggests a turbidity current origin. These beds may have been deposited during periods of weak contour-current activity.
Pinkish gray (5YR 6/2) tephra occurs in Sections 181-1122C-44X-2 (388.96-388.99 mbsf), 50X-5 (450.82-450.86 mbsf), and 51X-1 (454.48-454.52 mbsf). Drilling disturbance increases toward the base of Subunit IIA with moderate to heavy biscuiting starting in Core 181-1122C-47X.
Subunit IIB is heralded by a sudden downcore increase in detrital chlorite. The contact between Subunits IIA and IIB is marked by the occurrence of a pale olive (5Y 6/3) silty clay. Dark greenish gray (5GY 4/1 to 5G 4/1) to greenish gray (5G 5/1 to 5GY 6/1) pelagic/hemipelagic silty clay beds are interspersed with light brownish gray (2.5Y 6/2) to gray (5Y 6/1), nannofossil-rich layers. Bioturbation is common throughout the silty clay beds and includes an ichnofauna of Zoophycos, Chondrites, Planolites, Terebellina, Thalassinoides, and Teichichnus. The interbedded dark greenish gray (5G 4/1) to greenish gray (5GY 5/1) fine sand and silt beds contain sharp, typically scoured basal contacts, sharp upper contacts, and planar laminations; gradational boundaries are not common. We suggest that the sand and silt beds are contourites (see "Interpretation and Discussion") Intense disturbance has caused brecciation in Core 181-1122C-53X, and moderate biscuiting is observed in Core 54X.
The top of Subunit IIC occurs in Core 181-1122C-55X at a distinct color change from the normal greenish gray (5G 4/1) silty clay of Subunit IIB. The color changes gradationally to a yellowish brown to light reddish brown (10YR 6/4) in Section 181-1122C-55X-2, to (10YR 6/3) in Section 55X-3, to a 20-cm white (10YR 8/1) bed to pale yellow (5Y 7/3) in Section 55X-4, and finally to pale olive (5Y 6/3) in Section 55X-5. The suggested location of the boundary between Subunit IIB and Subunit IIC is placed at the beginning of the gradational color change in Section 181-1122C-55X-2 (494.5 mbsf). The extensive bioturbation in Section 55X-2 by Thalassinoides suggests the possibility of a firmground or discontinuity in deposition. The interval of distinct color variability (494.5-499.15 mbsf) is rich in nannofossils and foraminifers and coincides with the unconformity separating the early Pliocene and the middle Miocene.
The dominant lithology of Subunit IIC is dark greenish gray (5GY 4/1 to 5G 4/1) to greenish gray (5G 5/1 to 5GY 6/1) pelagic/hemipelagic silty clay beds that are interspersed with white (5Y 8/1) nannofossil ooze layers. Detrital chlorite is observed in smear slides (see the "Core Descriptions" contents list). Bioturbation is common throughout the silty clay beds and includes an ichnofauna of Zoophycos, Chondrites, and Planolites. The Thalassinoides at the upper boundary of the subunit are interpreted to be ichnofaunal (Pliocene?) reworking of the Miocene-age sediments. The interbedded dark greenish gray (5G 4/1) to greenish gray (5GY 5/1) fine sand and silt beds contain sharp, typically scoured basal contacts, sharp upper contacts, and planar laminations; gradational boundaries are not common. We suggest that the sand and silt beds are contourites (see "Interpretation and Discussion"). Moderate biscuiting is observed throughout Subunit IIC.
Unit III represents bioturbated, pelagic/hemipelagic sediments interspersed with laminated current deposits common to drift deposits. Toward the bottom of the hole, two probable debris-flow deposits have been identified. This unit extends from 550.4 to 617.85 mbsf and is divided into Subunits IIIA and IIIB, according to a change in lithification and the appearance of debris-flow deposits. Recovery was poor, averaging <25%.
Because of the poor recovery in Core 181-1122C-60X the upper contact of Subunit IIIA is located at the top of Core 181-1122C-61X. The green (5G 5/1, Munsell plant color chart) to greenish gray (5G 5/1, Munsell soil color chart) clayey silt to silty clay, composing these pelagic/hemipelagic sediments, tend to be coarser grained than those of Unit II. Sediments are commonly bioturbated with observed traces of Zoophycos, Chondrites, Planolites, Terebellina, and Gyrolithes. Interbedded throughout are distinctive fine sand and silt beds of a greenish hue (5G 5/1, Munsell plant color chart) and greenish gray (5G 5/1-5G 6/1) to dark greenish gray (5G 4/1) as well as interspersed white (10Y 8/1) or gray (10YR 5/1) to dark gray (10YR 4/1) nannofossil-bearing foraminifer sand (Fig. F10). Sand and silt beds have pronounced planar laminae. Top and bottom contacts of the sand and silt beds are sharp with normal grading only sporadically in sand beds. The sand and silt interbeds are interpreted as contourite deposits with intermittent, unreworked turbidites (see "Interpretation and Discussion").
The upper contact of Subunit IIIB is located in Section 181-1122C-64X-CC at the top of a poorly sorted, greenish gray (5G 5/1) fine sand that is reverse graded to gravelly coarse sand. This bed has greenish gray (5GY 6/1) intraclasts of silty clay up to 1.2 cm long, and the beds also contain abundant wood fragments that are typically <1 cm. These features suggest the deposits were emplaced as debris flows. Another debris-flow deposit with similar composition is observed in Section 181-1122C-67X-1, 608.8 mbsf (Fig. F11). Between the two debris flows is a series of heavily bioturbated, interbedded greenish gray (5GY 5/1) to dark greenish gray (5GY 4/1) fine sand-bearing siltstones and light greenish gray (5BG 7/1) foraminifer-bearing nannofossil chalks, which appear to be the lithified equivalents of Subunit IIIA. The siltstone beds in Core 181-1122C-65X contain abundant wood fragments. Observed trace fossils in the chalk and siltstone beds include Chondrites and Planolites. Interspersed among the siltstone and chalk beds are greenish gray (5GY 5/1) laminated, fine sand beds. The fine sand beds have sharp top and bottom contacts and are usually not graded. Drilling disturbance is pronounced throughout the subunit, with moderate to heavy biscuiting and very poor recovery (averaging <20%).
The overall pattern of deposition at Site 1122 exhibits the interplay between the DWBC, possibly reinforced by the ACC, and the inflowing turbidity currents carrying their high sediment load from the Bounty Channel.
Unit III contains pelagic/hemipelagic sediments interbedded with contourites and a few turbidites. The debris-flow deposits in Subunit IIIB indicate a closer source than the South Island. Unit II continues to be affected strongly by boundary currents, with most of the sand and silt beds containing fine planar laminae and cross laminae, which imply steady flow conditions. The presence of chlorite-bearing sediments in Subunit IIB suggests either (1) a source off the Campbell Plateau that has been eroded by the DWBC and redeposited as contourites at Site 1122, or (2) a South Island source in the Bounty Trough catchment. Isolated turbidites do occur throughout Units II and III, but are a minor component of the coarser grained deposits.
Unit I shows the stronger influence and greater sediment load brought by the Bounty Channel. Subunit ID contains mainly thin turbidites that have been eroded and slightly reworked, with typically sharp top contacts. Subunits IC to IA contain sequences of turbidites that represent the emplacement of the prograding channel/levee complex of the Bounty Fan.
Units II and III have distinctive nongraded, laminated fine sand and silt beds indicating the presence of a persistent but variable current. This flow caused basal scouring, yet allowed alternating deposition of cross-laminae and planar laminae. It is likely that sporadic influxes from the Bounty Channel interacted with the DWBC, which was (1) reinforced at times by benthic storms such as those recorded at the High Energy Benthic Boundary Layer Experiment (HEBBLE) site off Nova Scotia (Hollister and McCave, 1984), (2) affected by glacial/interglacial cyclicity, or (3) periodically reinforced by the ACC.
The reverse-graded, coarse-grained sediments containing clay intraclasts and wood fragments are interpreted as debris-flow deposits. Several lines of evidence suggest the debris flows carried sediment from proximal sources into deep water: (1) the deposits are interbedded with pelagic sediments, (2) the composition is different from the distal turbidites of the Bounty Fan, and (3) the benthic foraminiferal fauna is shallow water (see "Biostratigraphy"). The most likely sources of the debris flows are the flanks of the Chatham Rise and the Campbell Plateau. Several of the fine sands and white foraminifer sands appear to be turbidites, which may also represent shallower material originally deposited on Chatham Rise or Campbell Plateau.
The observed changes in the depositional environments also document the changes in the character of DWBC and its effect on the area. The DWBC was a dominant factor in the depositional history of Units II and III. The chlorite-bearing sands and muds within an abyssal flow sequence may reflect erosion upstream by the combined DWBC/ACC. These sediments may represent the unroofing of chlorite-bearing sediments at the base of the Campbell Plateau (see "Lithostratigraphy" in the "Site 1121" chapter). The presence of reworked Eocene diatoms also suggests erosion of earlier deposits (see "Biostratigraphy").
The ACC can be argued to have reinforced the DWBC at least during the time of deposition (and erosion) in Subunit IIC where the foraminifers in the silty clay located in Section 181-1122C-55X-4 comprise almost entirely well-sorted, cold-water Antarctic taxa.
Subunits IA-IC consist of a 262-m-thick Pleistocene turbidite sequence in which the sediment was delivered to the shelf and slope by river input during glacial sea-level lowstands and deposited by turbidity flows that may coincide with eustacy, seismicity, or other triggering mechanisms. The base of Unit I approximates seismic reflector R2a, which is the contact between large-amplitude sediment waves above and a planar bedded sediment wedge below. It appears that the turbidite-dominant Unit I marked the main period of levee growth by the Bounty Channel.
One of the major concerns in separating out the lithostratigraphic units and subunits is the generally poor core recovery. This is particularly related to the observed changes in thickness and grain size of the turbidite intervals throughout Unit I (Fig. F12). Four variables control fan sediments, namely the degree of glacial sea-level lowstands, seismicity affecting the slope, sediment supply, and the influence of the passing DWBC. However, any evaluation of these variables must be made with due consideration of the drilling operations. Of significance is the apparent change in both turbidite thickness and coarsest grain size from Subunit IB to Subunit IC. With the beginning of XCB coring at Core 181-1122C-14X, the amount of recovered core dropped significantly (<50% recovery). It is likely that the use of XCB coring washes out the less cohesive and thicker sand layers, causing only the fine-grained, thinner sands and muds to be recovered in the core. Because of the lack of supporting data such as downhole logs, it is imprudent to interpret the lithology of the missing intervals. When defining the units and subunits, changes that were apparent in lithology, deposition, or composition were considered primarily; core recovery was a necessary secondary consideration, as the definition of the units could not be based entirely on what was observed. For example, it is uncertain if a lithologic change exists between Subunit IB to IC, or whether the change rather represents an artifact of the drilling method.
The transition from a current-influenced (Unit II) to a more turbiditic-influenced environment (upper part of Subunit ID) is marked by an abrupt increase in coarser sand and silt beds. There, the very fine sand and silt turbidite intervals are relatively thin (<10 cm) and sometimes are truncated and reworked. These coarser beds probably represent an early stage of the Bounty Fan with a less distal sedimentation of the terrigenous material. The overlying Subunit IC contains slightly thicker silt and very fine sand turbidites (~10 to 20 cm thick). An erosional influence from the DWBC is not apparent because the turbidites are normally graded. Compared to the overlying turbidites of Subunit IB, Subunit IC beds are relatively thin and could indicate a slow progradation of the fan. Subunit IB has thick (<100 cm) sandy turbidites, implying more rapid fan progradation. Subunit IA is composed of thinner, fine-grained turbidites and could represent either a marked restriction in sediment supply through the Bounty Channel or that the height of the levees allowed only the fine fraction of the turbidity currents to overspill. Another possibility is that the alternation in turbidite thickness is tied to deposition on a migrating sediment wave field. The migration of these antidunes would cause the deposition of thicker turbidites on the upcurrent side and thinner turbidites on the lee side of an antidune. As the sediment waves migrated relative to the location of the drill site, a periodic pattern of thicker and thinner turbidites might be expected.
Pyritized bases of the turbidites appear in distinct intervals of several decimeters to several meters thickness throughout Unit I (Fig. F13). These intervals probably depend on the content or concentration of organic matter being deposited and its degradation under reducing conditions. There appears to be a connection at ~260 mbsf between the last interval of pyritized bases and the elevated methane content (see "Inorganic Geochemistry"), which would confirm a higher organic content in Unit I. Total organic carbon, however, does not show a clear trend of higher values, but the reason for this could be the low-resolution sampling.
Pyritized zones develop best in the bases of sand turbidites but also occur in the very fine sand and silt turbidites. This leads to the possibility that organic detritus carried within the turbidity currents is percolating through the sand bed and is reduced at the contact with the underlying fine-grained deposit. This slows down the interstitial-water circulation. However, the mechanism is not supported by the occurrence of pyritized bases in fine-grained turbidites. Therefore, a more likely explanation is the rapid burial of organic-rich surface sediment (silty clay hemipelagic sediment below turbidites), thereby causing a sudden change from oxidizing to reducing conditions during degradation of organic matter. Still another explanation for the pyrite zones is that the higher permeability of the basal turbidite sands promotes the concentration of reducing pore waters expelled from the muds below. Only in distinct intervals is the supply of organic matter higher; this may be correlated with glacial periods in which productivity and organic carbon content of hemipelagic sediments is higher (Lean and McCave, 1998).
In Unit III, the observed trace fossils in the lithified pelagic sediments of Subunit IIIB and the unlithified pelagic sediments of Subunit IIIA include Zoophycos, Planolites, Chondrites, Terebellina, and Gyrolithes. The low diversity and the high abundance imply a Zoophycos ichnofacies, which indicates a fairly low depositional rate in a slightly oxygen-poor, deep-water environment (Pemberton and MacEachern, 1995).
Subunit IIC contains an ichnofauna abundant with Zoophycos, Planolites, and Chondrites, continuing the Zoophycos ichnofacies from Unit III. The top of the subunit in Core 181-1122C-55X is likely to be directly related to the unconformity separating early Pliocene age sediment in Core 181-1122C-54X from the middle Miocene age sediments in Core 181-1122C-56X. The unconformity is marked by the start of a gradational color change in Section 181-1122C-55X-2. The presence of Thalassinoides at the top of Subunit IIC is interpreted as later (possibly Pliocene) burrowing into the Miocene sediment.
Subunit IIB possesses a more variable depositional history. The trace fossil assemblage in the subunit includes Zoophycos, Chondrites, Planolites, Terebellina, Thalassinoides, and Teichichnus. The presence of Thalassinoides and Teichichnus suggests a transition from Zoophycos ichnofacies to a Cruziana ichnofacies, signifying a more energetic and oxygenated environment (Fig. F14). The carbonate sediments of Core 181-1122C-55X mark the first (last downcore) occurrence of Thalassinoides.
In Subunit IIA, the presence of Chondrites, Terebellina, Anconichnus, and deep-spiraling Zoophycos in relatively high abundance, but low diversity, again implies a Zoophycos ichnofacies. Nannofossil-bearing sediments increase in this unit and may represent glacial-interglacial periods of higher carbonate levels relative to terrigenous input into the ocean.
Unit I contains few identifiable trace fossils; those observed traces are generally robust burrows of opportunistic colonization (primarily Skolithos, with singular occurrences of Thalassinoides in Core 181-1122C-16X and Gyrolithes in Core 181-1122C-17X). The lack of identifiable trace fossils in the hemipelagic silty clays suggests the depositional rates were high, thereby preventing a well-developed ichnofabric from being established. Starting at 280 mbsf and occurring more consistently from 300 mbsf to 389.9 mbsf is an ichnofauna limited to Chondrites, which suggests a lower depositional rate accompanied by a restriction in oxygen (Pemberton et al., 1992) (Fig. F15).