171B Scientific Prospectus


Primary Sites (by order of drilling)

Proposed Site BN-1
Proposed Site BN-1 is a reoccupation of Site 390. The principle objectives are to recover a sequence of Eocene though Campanian strata from deep water that can be compared with age-equivalent, but more expanded, sections at shallower water depths. In particular, the middle Eocene and lower Paleocene-Maastrichtian are thick enough to permit high-resolution studies of Eocene paleoceanography and the nature of the K/P boundary event.

About 200 m of sediment was recovered at Site 390, including 160 m of pelagic sediment that ranges in age from middle Eocene(?) to Barremian and 40 m of Barremian oolitic limestone. The pelagic sediments were entirely stiff to soupy nannofossil ooze with the exception of seven 5-cm-thick limestone and chert stringers encountered in the 30-m-thick lower Eocene and upper Paleocene sections. Eocene sediments comprise about 75 m of the cored section whereas the Paleocene (35 m), Maastrichtian (28 m), Campanian (2 m), and Barremian to lower Albian pelagic sediments (20 m) make up the remainder. During rotary core barrel (RCB) drilling of Hole 390A, coring penetrated about 50 m/hr except during drilling through the cherts, when the drilling rate dropped to about 20 m/hr.

Proposed Site BN-1 is included mainly to increase the total depth range of the transect (to about 1371 m total water-depth range) and recover sediments deposited well within the Paleogene equivalent of modern upper deep water. Further, because Site 390 was rotary cored, the existing core is useless for high-resolution paleoceanography. This is a good site for recovering a moderately thick and well-preserved Eocene-Paleocene section and for estimating the chemistry of upper deep water. However, the Cretaceous sequence is too condensed to be of much use for paleoceanography. The Mesozoic section should be of interest to sedimentologists interested in the control of water depth on sedimentation at the K/P boundary and processes of sedimentation (e.g., tying expanded sections at proposed Site BN-3 into the more condensed intervals at proposed Site BN-1).

Proposed Site BN-2Alt
Proposed Site BN-2Alt is located in 2257 m water depth. The site is located to take advantage of an unusually thick sequence of upper Paleocene(?) strata. At this location, the depth to Reflector Purple is about 400 m, primarily because of an unusually expanded section above Reflector Green (lower/upper Paleocene) and below Reflector Red (Paleocene/Eocene Boundary). The upper Paleocene is probably about 200 m thick at this site and the lower Paleocene-Campanian section (between Reflectors Green and Purple) is about 75 m thick. The sediment packages between the other reflectors have similar thicknesses to their equivalents at Site 390.

This is a high-priority site because it represents an expanded section of Paleocene-Cretaceous sediments that are ideal as a deep-water end-member of the depth transect. The expanded section should help considerably in refining Paleocene and Eocene stratigraphy and paleoceanography because the best available low-latitude sites (such as DSDP Sites 577 and 384 and ODP Site 758) are comparatively condensed. The Aptian and older Cretaceous section is also a good end-member for a thermocline-intermediate water depth transect in the Early Cretaceous. In addition, recovery at the the site will most likely include a complete K/P boundary with well-preserved microfossils. Drilling is proposed to about 600 mbsf to recover 400 m of middle Cretaceous-Eocene sediments and 200 m of Lower Cretaceous sediments.

Proposed Site BN-3
Proposed Site BN-3 is located at shotpoint 1821 on MCS Line TD-5. It is similar to proposed Site BN-2Alt in terms of the thickness of the sediment packages between all the reflectors. As at Site BN-2Alt, the middle Eocene is about 75 m thick and overlies a 50-m section of lower Eocene to uppermost Paleocene (Reflector Orange to Reflector Red). The middle Paleocene is nearly 200 m thick (above Reflector Green) and overlies a 100-m-thick drape of lower Paleocene to Campanian strata (above Reflector Purple). The site is located at the intersection of the Glomar Challenger SCS line and MCS Line TD-5 at about 1972-m water depth.

The section below Reflector Purple consists of 370 m of Lower Cretaceous(?) deposits. The middle Cretaceous represents the distal upper package of clinoforms that overlaps the pre-Barremian(?) reef about 40 km to the southwest and can be traced updip to deposits below proposed Site BN-4. Coring is proposed to a depth of about 600 mbsf to recover the Paleogene and part of the Lower Cretaceous sections.

Proposed Site BN-3 is about 300 m shallower than proposed Site BN-2Alt so both sites add to the resolution of the depth transect. Further, both proposed Sites BN-2Alt and BN-3 penetrate a thick section of Paleocene sediments, which should provide a fairly high-resolution, well preserved, sedimentary sequence that is singularly lacking in existing ODP/DSDP holes that penetrate this paleoclimate. Recovery at proposed Site BN-3 should include a well-preserved K/P boundary interval and expanded Aptian to older Cretaceous sequence. The site could be used as an alternate for proposed Site BN-2Alt, but it would also serve as a site at intermediate depth between proposed Sites BN-4 and BN-2Alt, where it improves the likelihood of correlating accurately between shallower and deeper sites and increases the probability of recovering a complete section of the Lower Cretaceous strata.

Proposed Site BN-4Alt
Proposed Site BN-4Alt is an alternate to proposed Site BN-4. The proposed site is located on shotpoint 1461 at a water depth of 1652 m. The objectives are similar to those for proposed Site BN-3, but the site would enable recovery of a slightly thicker section (~600 m) at a deeper water depth than at proposed Site BN-4. Recovery of a more expanded section above Reflector Purple than at proposed Site BN-4 would be at the expense of about 100 m of section below this reflector.

Proposed Site BN-5Alt
Proposed Site BN-5Alt is located on shotpoint 1241 on MCS Line TD-5 at a water depth of 1293 m. Proposed Site BN-5Alt would core about 100 m of Oligocene and younger strata, 200 m of Eocene to Upper Cretaceous(?) deposits, and 150 m of Lower Cretaceous deposits for a total penetration of about 450 m.

Proposed Site BN-5Alt is included because it is the shallowest site that is likely to have a good Cenozoic section. At 1293 m depth, proposed Site BN-5Alt is well within modern intermediate water and hence is a good end-member for paleoceanographic studies. The site probably lacks a K/P boundary section, or it is very condensed relative to proposed Sites BN-2Alt, BN-3, and BN-4. However, recovery should include an Eocene to upper Paleocene section similar to the deeper sites as well as a shallow-water, end-member Lower Cretaceous section. Hence, this is a desirable site primarily because it expands the total depth range of the transect and increases the probability of reconstructing the vertical hydrographic structure from the late Paleocene to middle Eocene and in the Early Cretaceous. Correlation to proposed Site BN-2Alt will permit a depth transect of about 900 m between proposed Sites BN-5Alt and BN-2Alt, where the sections are expanded compared to that at proposed Site BN-1.

Alternate Sites

Proposed Site BN-1Alt
Proposed Site BN-1Alt would allow recovery of a thicker section than at proposed Site BN-1 (400 m compared to ~170 m) at a slightly shallower depth (2480 mbsf). Nearly all the increased thickness would be in a more expanded Paleocene-Eocene section. The site is located at shotpoint 2241 on MCS Line TD-5. This site is proposed as an alternate to proposed Site BN-1.

Proposed Site BN-4
Proposed Site BN-4 lies at a water depth of 1410 m where about 200 m of upper Paleocene to Eocene strata over about 100 m of Campanian to lower Paleocene deposits are apparently preserved. Reflector Purple is underlain by about 250 m of deposits that appear to correlate with the seismic package that lies between Reflectors Blue and Purple at proposed Site BN-3. These are most likely Barremian to pre-lower Albian nannofossil ooze and clay similar to sediments recovered from the equivalent interval at Site 390. Coring will penetrate about 250 m below Reflector Purple for a total recovery of about 550 m of section.

Proposed Site BN-4 is ~560 m shallower than proposed Site BN-3 and more than 800 m shallower than proposed Site BN-2Alt and should contain a record of the Paleogene equivalent of modern lower intermediate water. This is a critical site primarily because it provides the best shallow-water end-member for the depth transect through Paleocene and Lower Cretaceous strata. In addition, recovery at the site should include a K/P boundary section.

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