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doi:10.2204/iodp.sp.324.2009

Proposed drill sites

Tamu Massif sites

The program envisions coring at three sites on Tamu Massif, the massive volcano that formed from initial Shatsky Rise eruptions. One site is located at the summit, whereas two are situated on the lower reaches, one on the southwest (oldest) flank, and one on the northeast (youngest) flank. Primary sites were chosen where sediments are thin and the reflector thought to represent igneous basement is clear. For each primary site, at least one nearby alternate site was chosen.

Primary sites

Site SRSH-8

Proposed Site SRSH-8 is located on the southwest flank of Tamu Massif at 4670 m water depth, ~50 km downslope from Site 1213 where igneous rocks were cored on Leg 198. Like Site 1213, the interpreted upper igneous section on the Cruise TN037 seismic profile has a curious layered signature that is seen all along the seismic profile traversing the southwest flank. This site was chosen at a location where erosion has partly stripped away the sediment cover, leaving only a thin layer of sediments overlying igneous basement. From the seismic profile a total sediment thickness of 155 m is anticipated, with 116 m of ooze overlying 39 m of probable chalk with chert. The purpose of drilling at this site is to recover igneous rocks from the southwestern flank of Tamu Massif, which is thought to be the oldest part of Shatsky Rise. At proposed Site SRSH-8, the plan is to core igneous rock with a single bit, ending with bit destruction or when allotted time is exhausted. This should achieve ~100 m penetration of igneous basement.

Site SRSH-3B

Proposed Site SRSH-3B is situated on the upper slope of the east side of the summit of Tamu Massif at 3477 m water depth. It is located on Cruise TN037 seismic Profile 14C at 1625 on 23 August 1994. The objective of proposed Site SRSH-3B is to core a long section of igneous rocks from the Tamu Massif summit for comparison of geochronology, geochemistry, and isotopes with other locations on Tamu Massif and elsewhere on Shatsky Rise. A deep hole is planned at this site, using two drill bits to achieve the planned penetration of ~300 m into the igneous basement (assuming each RCB bit penetrates ~150 m). A FFF will be emplaced at the top of the hole to allow reentry after bit changes.

Originally chosen as an alternate to proposed Site SRSH-3, which is located at the intersection of two seismic lines, proposed Site SRSH-3B was elevated to primary status for two reasons. First, sediments are thinner at proposed Site SRSH-3B. The total sediment thickness at this site is estimated to be 169 m, with 78 m of ooze and 91 m of chalk with chert, in contrast to the 311 m sediment thickness estimated for proposed Site SRSH-3. Second, at proposed Site SRSH-3B, the character of the upper igneous basement is different than at most other Tamu Massif sites. In most locations on the summit and south flank of Tamu Massif, the upper igneous basement appears layered, much like sediment, and this layered section has a thickness of ~0.25 s TWT. At Site 1213, igneous material with this signature was cored, recovering apparent sills. At proposed Site SRSH-3B, this layered basement is thin and more normal-appearing basement (lower frequency, less coherent reflectors) is closer to the surface. Assuming a seismic velocity of 4000 m/s, this surface layer is estimated to be 90 m thick. Since deep penetration is planned at this site, it should be possible to penetrate through the upper basement layer into the lower layer.

Site SRSH-6

This site is located on the north flank of Tamu Massif at 3285 m water depth. Appearing on Cruise TN037 seismic line 13 at 0225 on 22 August 1994, the site is situated on a basement high where the thick pelagic sediment cover atop the high plateau is thinned, allowing drilling of igneous basement with minimal cover. The purpose of drilling at this site is to recover igneous rocks from the northern extent of Tamu Massif for comparison with the summit and southwest flank sites. Proposed Site SRSH-6 has been chosen where the overlying sediments are ~177 m thick, with ~58 m of ooze atop ~119 m of cherty chalk and limestone. Coring of basement will be done with a single bit, giving an estimated igneous section penetration of ~100 m.

Alternate sites

Site SRSH-2A

Proposed Site SRSH-2A is an alternate for primary proposed Site SRSH-8. It is located in deep water (4743 m) at the foot of the Tamu Massif southwest flank, just to the east of the M21 magnetic lineation. The objective for this site is to sample igneous rocks from the oldest part of Shatsky Rise. The proposed site is located on Cruise TN037 seismic Profile 17D at 0115 on 29 August 1994. At this location, the drape of pelagic sediments covering the Tamu Massif flank becomes thinner as it goes over a basement high, located to the southwest of the proposed site. Based on the seismic layers, there is a covering of 297 m of sediment of which the upper ~41 m is predicted to be ooze and the rest is likely to be cherty chalk. Basement will be cored to refusal, bit destruction, or allotted time running out. This is estimated to provide ~100 m penetration of the igneous section.

Site SRSH-2B

Proposed Site SRSH-2B is located in deep water (4783 m) on the lower southwest flank of Tamu Massif and is an alternate for primary proposed site SRSH-8. It is situated on Cruise TN037 seismic Line 17C at 2301 on 28 August 1994. Unlike the other southwest Tamu Massif flank sites, this site has a thick sediment cover, estimated at ~511 m. The overall objective is the same as at proposed Site SRSH-8 and other nearby prospective sites: to sample ~100 m of basalt from the southwest flank for geochemical, isotopic, and geochronologic study. However, the sediment column at proposed Site SRSH-2B contains an unusual feature. At ~0.075 s TWT above the igneous basement, the sediments show a thin, strong reflector. This reflector is limited in areal extent and appears to connect to a zone of chaotic reflections that appear to be side-swipe from a buried volcanic cone. The strong, mid-sediment reflector is interpreted as a lava flow from this cone that must have occurred long after shield-building volcanism stopped forming the south flank. Thus, a second objective for this site is to study this postshield volcanism. Drilling with a single bit is planned at this site, which should give ~100 m penetration of the igneous basement.

Site SRSH-3

Proposed Site SRSH-3 is an alternate for proposed primary Site SRSH-3B. This site is situated on the east side of the summit of Tamu Massif at 2987 m water depth. The objective of this hole is to core a long section of igneous rocks from the Tamu Massif summit. The proposed site is located on Cruise TN037 seismic Profile 14B at 1207 on 23 August 1994. At this site, sediments are predicted to be 311 m thick with ~50 m of ooze overlying ~261 m of cherty/shaley chalk and limestone. Coring at this site is planned for two bits and two round trips of the drill string with reentry into a FFF; however, depending on penetration, we might need three bits to reach target depth. If each bit gives ~150 m of penetration into the igneous section, the total igneous section drilled will be ~300 m.

Site SRSH-5

Proposed Site SRSH-5 is located near the summit of Tamu Massif in a channel on the west side of a summit ridge. The water depth is 3499 m. The site was chosen at the axis of a channel, at 2246 on 25 August 1994 on Cruise TN037 seismic Line 15C. The objective at this site is to core igneous rocks from the summit of Tamu Massif but at a location on the other side of the summit from proposed Site SRSH-3. Seismic data indicate a vanishing cover of ooze and ~210 m of Cretaceous cherty chalks and limestone. The total igneous penetration is planned for one bit, which should give ~100 m of igneous section. This site was moved to alternate status because of time constraints and because another hole is planned for the Tamu Massif summit.

Site SRSH-7

Proposed Site SRSH-7 is an alternate for proposed primary Site SRSH-6, which is located ~35 km upslope on the northeast flank of Tamu Massif. It is located on Cruise TN037 seismic Line 13 at 2350 on 21 August 1994. This site was chosen because it is a spot on the seismic line where acoustic basement is clear and appears to show the normal flank slope and the sediments are thin. In contrast, basement at proposed Site SRSH-6 is a high of undetermined origin, so proposed Site SRSH-7 would allow drilling of apparently normal flank slopes. The coring objectives and drilling plan at this site are the same as for proposed Site SRSH-6.

Ori Massif

Primary site

Site SRCH-5

Proposed Site SRCH-5 was chosen as primary over other Ori Massif sites because it is located atop a volcanic ridge at the summit of the volcanic edifice where sediments are thin. Proposed Site SRCH-3 was originally proposed as the primary site and it was chosen because it has a basement high at the intersection of two seismic lines. However, it displays poorly defined basement structure on seismic lines, perhaps caused by rough basement topography. In contrast, proposed Site SRCH-5 shows a strong, coherent acoustic basement reflector that apparently represents the almost-flat top of a summit ridge. Furthermore, pelagic sediments on the flat ridge top have apparently been sculpted by currents, giving rise to a tapering sediment cover that allows us to choose a location with as much covering sediment as desired.

Proposed Site SRSH-5 is located on Cruise TN037 seismic Line 8 at 0005 on 15 August 1994 where sediments have a thickness of 184 m with 54 m of ooze and 130 m of chalks and chert. The objective of coring at this site is to recover igneous rocks from Ori Massif, specifically at the summit of this large volcanic structure, for the purpose of geochronologic, geochemical, and isotopic studies. Proposed Site SRSH-5 is one of two sites where drilling deeper than ~100 m is the plan (the other is proposed Site SRSH-3B). To achieve this extra depth, two RCB bits might be needed, which should allow ~200 m penetration into igneous material. In this case, a FFF will be used to facilitate reentry of the hole after the bit change.

Alternate sites

Site SRCH-3

Proposed Site SRCH-3 is an alternate site for proposed Site SRCH-5 that is located at the summit of the Ori Massif at 3243 m water depth. Proposed Site SRCH-3 was chosen at a location where two seismic lines intersect over a basement volcanic high. The site is located on Cruise TN037 seismic Line 11 at 1741 and seismic Line 9 at 0801Z. The seismic line indicates ~133 m of sedimentary cover with the upper ~25 m consisting of ooze and the remainder of cherty chalk and limestone. The scientific goals and operational plan for proposed Site SRCH-3 are the same as for proposed Site SRCH-5.

Site SRCH-4

Proposed Site SRCH-4 is also an alternate site for proposed Site SRCH-5. It was chosen to provide a site located on the lower flank of Ori Massif. Proposed Site SRCH-4 is situated on the lower east flank of Ori Massif at 4079 m water depth. It is found on cruise Cruise TN037 seismic Line 8 at 1537 on 14 August 1994. At this location, the seismic profile shows a sediment cover that thins downslope. The scientific goals and operational plan for this site are the same as for proposed Site SRCH-5.

Shirshov Massif

Primary site

Site SRNH-2

Proposed Site SRNH-2 is located on Cruise TN037 seismic Line 5A at 2130 on 09 August 1994 at 3654 m water depth. Originally this site was proposed as an alternate to proposed Site SRNH-1, which is located near the intersection of two seismic lines. It was elevated to primary status because the sediment is thinner than at proposed Site SRNH-1 and the acoustic basement is simpler. At proposed Site SRNH-2, the acoustic basement is relatively flat near the edge of the summit platform of Shirshov Massif, whereas there is a basement high of uncertain origin near the location chosen for proposed Site SRNH-1. The seismic profile indicates that at proposed Site SRNH-2, the sediment cover is 194 m thick with the uppermost ~54 m being Cenozoic ooze and the remainder being cherty chalk and limestone. As at other Expedition 324 sites, the objective for proposed Site SRNH-2 is to core igneous rock for geochronologic, geochemical, and isotopic studies in comparison to other Shatsky Rise sites. Drilling with a single bit is planned at this site, which should give ~100 m penetration of the igneous basement.

Alternate sites

Site SRNH-1

Proposed Site SRNH-1 is located on the summit of Shirshov Massif at 3339 m water depth. The site location has been chosen where currents have eroded the uppermost sedimentary layers and therefore the sedimentary cover is thinner than elsewhere. The site is situated on Cruise TN037 seismic Line 5C at 0030 on 11 August 1994. The sedimentary cover appears to be 285 m thick, with the upper ~83 m being ooze and the remainder being cherty chalk and limestone. The scientific objectives and drilling plan are the same as at proposed Site SRNH-2.

SRNH-2A

Proposed Site SRNH-2A is an alternate for proposed primary Site SRNH-2, situated where sediment thins on the north rim of the Shirshov Massif summit. This site is found on Cruise TN037 seismic Line 5A at 2144 on 09 August 1994. It is situated ~2 km south of proposed Site SRNH-2 at a location where the basement is better defined than at the primary site. Because the sediment dome on top of Shirshov Massif thickens toward the center of the edifice, sediments at this site are significantly thicker than at the primary site. The estimated sediment thickness for proposed Site SRNH-2A is 328 m, with 41 m of Cenozoic–Late Cretaceous ooze overlying 287 m of Cretaceous chalk, limestone, and chert. Because of the better basement definition at this site, it could be moved to primary status if the expedition is allowed to drill through the uppermost sediments without coring. The objectives for proposed Site SRNH-2A are the same as those for the primary site: to core igneous rock for geochronologic, geochemical, and isotopic studies in comparison to other Shatsky Rise sites. Drilling with a single bit is planned at this site, which should give ~100 m penetration of the igneous basement.

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