Biostratigraphic levels with age significance at Site 1122 were derived from LO, FO, acme, LCO, and FCO events, using diatoms, radiolarians, nannofossils, and foraminifers. The 29 levels are shown in Table T8. To show the principal trends, the assembled age-depth data are plotted together with magnetic polarity datums in Figure F17. FO events may have been estimated to be too shallow, based on limited sampling. The position of arrows in Figure F17 reflects the possibility that further work may extend these datums downhole. Last occurrence events may be too deep, again as a result of the limited sampling interval. The position of arrows in Figure F17 reflects the possibility that further work may extend these datums uphole. The solid line in Figure F17 shows the preferred age depth model using magnetic polarity constrained by the biostratigraphic datums (see "Paleomagnetism"). Given the fact that the sequence is dominated by turbidites down to ~400 mbsf, the scatter of event data is surprisingly low. Hence, average sedimentation rates may be expressed with relatively long linear segments. This observation suggests that any reworking of microfossil marker events was relatively instantaneous. An ~5-m.y. hiatus at 490 mbsf separates middle Miocene strata below from early Pliocene strata above. Below the hiatus, middle Miocene (compacted) sedimentation rates of lithostratigraphic Unit III and Subunit IIB were low, varying between 50 and 5 m/m.y. The late Miocene pelagic mud and laminated sand (contourite deposit), classified as the lower part of Unit IIA, has a much lower sedimentation rate of 2 m/m.y. Early Pliocene net sedimentation is also of low rate but poorly constrained. The sedimentation rate accelerated to ~100 m/m.y. from the latest Pliocene to early Pleistocene and became very fast (400 m/m.y.) in the middle and late Pleistocene when most of the Bounty Fan levee was deposited at this site.
Sediment decompaction and accumulation rates were assessed using 124 porosity measurements from Hole 1122C (see "Physical Properties"). These were analyzed for trends, using the programs DEPOR and BURSUB (Stam et al., 1987; Gradstein et al., 1989). Coarser grained lithostratigraphic Subunits IA, IB, IIIA, and IIIB were decompacted to a greater extent, relative to the middle, finer grained Subunits IC, ID, IIA, and IIB. Next, compacted and restored rates of sedimentation were derived, with the age intervals slightly smoothed; these are shown in Table T12. In lithostratigraphic Unit III, decompaction resulted in an 80% increase of rates, from 50 to 80 m/m.y. Decompaction had negligible effect on sedimentation rates of younger strata. This is in accord with the measured porosities in the basal siltstones, showing accumulation rate values ~60% lower than stratigraphically higher units. It must be remembered that this decomposition was performed on data from the preferentially preserved fine material; the coarse (and less compactible) sand turbidites were washed away during drilling or were not subjected to compression testing. The results in Table T12 are thus biased toward high values.
The character of basement on multichannel seismic lines across Site 1122 and gravity and magnetic survey data (Davey, 1993; Carter et al., 1994) demonstrate that the distal part of the Bounty Trough is situated on oceanic crust in over 4 km of water. The abyssal ocean floor of the Bounty Fan is part of the most distal southwest Pacific Ocean, which started opening before marine magnetic Anomaly 32 (late Campanian) (Stock and Molnar, 1987). Anomaly 32 is mapped adjacent to Campbell Plateau, Bounty Trough, and Chatham Rise. The Bounty Fan Site 1122 itself is situated in the geomagnetic quiet zone, with an estimated age of ocean basement around 85 Ma (Lawver and Gahagan, 1994).
According to recent interpretations (Carter et al., 1994), the Bounty Trough is a failed and sediment-starved rift, at right angles to the Southwest Pacific Ocean spreading basin. The Bounty Trough lies between the continental blocks of Campbell Plateau to the south and Chatham Rise to the north. Drilling on the southern block has recovered a sequence of middle to upper Cretaceous coal measures and shallow marine terrigenous clastics, unconformably overlying metamorphic basement. Similar and coeval strata are inferred for the northern block (Raine et al., 1993). The Cretaceous sediments may be interpreted as synrift strata, formed during regional subsidence on a passive margin, adjacent to the rifted and opening Southwest Pacific Ocean.
The thermal history of the distal Southwest Pacific oceanic crust constrains the subsidence history and sediment accommodation space at Site 1122. Figure F29 sketches the backtrack history, based on a ridge crest age at the location of Site 1122 at 85 Ma, which is slightly older than Anomaly 32, and on the observed present-day water depth at the site of nearly 4500 m. Initial ocean sediments at the site are postulated to be Late Cretaceous through Paleocene siliceous pelagic clays (see "Lithostratigraphy" in the "Site 1121" chapter). The presence of rare, reworked Oligocene nannofossils, diatoms, and planktonic foraminifers in middle Miocene strata at Site 1122 indicate that deep marine Oligocene strata also formed in the vicinity of the site. Regional sedimentation rate curves, considered representative for the Chatham Rise and part of the Campbell Plateau, show sustained sedimentation through the Late Cretaceous, reduced Paleocene-Eocene sedimentation, and a hiatus in the Oligocene through the early Miocene (fig. 3.2 in Wood et al., 1989). Hence, Oligocene fossils may have been transported from the New Zealand paleo-shelf.
From the seismic profile across Site 1122, the pre-middle Miocene ocean sediments are ~1 km thick. Figure F29 also shows the >130-m-thick middle Miocene marine silt and nannofossil ooze resting unconformably on older strata and the >490 m of uppermost Pliocene-Pleistocene fine sands and nannofossil ooze unconformably overlying the middle Miocene sediments. The rapid, late-stage sedimentation, beginning in the Miocene and steadily increasing thereafter, reflects the uplift of the Southern Alps in New Zealand. This shed an enormous volume of terrigenous material on the nascent shelf, slope, and rise to the east (Wood et al., 1989).
The combined evidence supports an oceanic history at the site that traces to ~85 Ma, pre-Anomaly 32 time. The abundance of shallow marine microfossils observed at some levels in the Pleistocene turbidites and middle Miocene drifts (see "Biostratigraphy") may be explained by geologic reworking, as part of the slow and inexorable fill of Bounty Trough, estimated to have taken at least 200 m.y. at the present rate of sedimentation (Carter et al., 1994).