DRILLING STRATEGYSecular Variation in Previous Ocean Drilling Studies
We propose to drill five to six basement sites along the Emperor seamounts chain. We group the drilling sites and order as follows: Group 1 will be at the oldest (northern) end of the chain, Group 2 at Detroit Seamount, and Group 3 at seamounts south of Detroit, along the youngest portion of the chain. All sites will be drilled and cored using the rotary core barrel (RCB). We propose basement penetrations to moderate depth (150-250 m). At the northern sites with thicker sediment cover, our strategy will be to employ minicones for reentry after a single bit change.
Site survey data used in the approval of these DSDP/ODP locations have been used to guide our proposed coring. For each site proposed, previous nearby DSDP or ODP coring has touched basement or penetrated the sediment cover, providing information on the nature of the sediments and basement depth as well as drilling times. We propose to drill without coring through the sediments to a few cores above the basement because (1) these sediments have been cored previously, (2) there is problem with bias in paleomagnetic inclinations derived from sediments, and (3) time saved can be used toward coring more of the lava flow sequences. An exception to this plan is the Meiji Seamount site (see below). We have estimated depths for the basement penetration based on drilling of other Pacific seamount and plateau sites (e.g., Legs 143, 144, and 192). Whereas these estimates are needed for the planning process, we envision an interactive process based on the recovery. We hope to recover at least 15 flow units at each site for detailed paleomagnetic and radiometric age (40Ar/39Ar) analysis. If this is achieved in a given hole, we would prefer to drill additional sites on a seamount (or additional seamounts) to improve the accuracy of paleolatitude determinations and assist in the overall test. Below we include a brief description and rationale for each of the drilling sites.
Group 1: Meiji Seamount (~86 Ma)
A precisely determined paleolatitude from a well-dated site in the northernmost Emperor Seamount is of the highest priority. Given the new age data from Leg 145, the age of Meiji Seamount is presumably older than 81 Ma but how much older is uncertain. There is also a bend in the northern Emperor trend that, if better dated, could be used to examine independently some of the issues of plate and hotspot motion discussed here. We identified two sites on Meiji Seamount (Fig. 1, Fig. 9) near DSDP Site 192. If the current trend of the Emperor paleomagnetic data reflects continuous hotspot motion, we expect to find a paleolatitude of ~40° for Meiji. Site HE-1A, at the location of DSDP Site 192, is our alternate site on Meiji Guyot. Prior drilling at DSDP Site 192 indicates a sediment cover of 1044 m, composed of ooze, chalk, and clays above subaerial basalt. Our primary site (HE-1B) is located 6 km southwest of DSDP Site 192. We expect this site to have a thinner sediment cover than that penetrated at Site 192. However, the sediment cover should include a relatively thick (200-300 m?) section of Paleogene to Cretaceous sediments of paleoceanographic importance that we plan to core before penetrating basement. We note that failure to obtain clearance for drilling on Meiji Guyot (which is in Russian territorial waters) will force us to adopt an alternate drilling strategy. If clearance is denied, we will drill two primary sites on Detroit Seamount (see below) or, should a seismic data package be prepared (and approved by the appropriate JOIDES panels) prior to the leg, a site on the ridge between Meiji Guyot and Detroit Seamount in international waters.
Group 2: Detroit Seamount (81 Ma)
We identified five potential drill sites on Detroit Seamount. With paleolatitude data from one or more of these sites, we expect to improve the existing time-averaged results available from just a single site (Site 884) (Tarduno and Cottrell, 1997). Primary Site HE-3B is located on the summit region of Detroit Seamount between ODP Sites 882 and 883 and has a relatively thin (<500 m) sediment cover. Site HE-3A (an alternate site if clearance is obtained for drilling on Meiji Guyot or a primary site if clearance is denied) is in a similar region and has a similar sedimentary thickness above basement. Alternate Site HE-2 is located 7 km northwest of ODP Site 882 and has a sediment column composed of ~800 m of oozes, chalks, and clays. Alternate Site HE-3 is ODP Site 883, where the sediment column above basaltic basement is 840 m thick.
Group 3: Nintoku (>56 Ma), Ojin (56 Ma), and Koko Seamounts (>48 Ma)
Present paleomagnetic data from basalt cores are insufficient to determine how the 8° paleolatitude discrepancy between Suiko Seamount and present-day Hawaii accumulated and the potential relative contributions of true polar wander and hotspot motion in causing the discrepancy. We propose drilling Nintoku and Ojin Seamounts and Koko Guyot (Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9) to examine this question. Paleolatitude results can also be compared with global data to test for true polar wander. We note that at these sites the difference in measured paleolatititude with present Hawaii may be quite small, so the number of independent cooling units (flows) needed for a significant statistical test will be larger and require penetration greater than that at the more northerly sites (or additional sites on each seamount or guyot). We are helped somewhat in that the decreased angular dispersion of paleomagnetic directions at the lower latitudes of these sites, however, acts to allow a more precise paleolatitude estimate for a given number of independent flow units.
If the Emperor trend represents southward hotspot motion of the Hawaiian hotspot, we should obtain a paleolatitude of 25°-27° for Nintoku Seamount. Two sites have been identified on Nintoku Seamount. Proposed alternate Site HE-4A is positioned at DSDP Site 432A, near the northwest edge of the seamount on flat-lying stratified sediments. Previous drilling indicates the sediments are 42 m thick above the lava flows. The uppermost flows are separated by soil horizons, indicating significant time between cooling units. Primary Site HE-4B is offset by ~28 km to the northwest on the volcano summit. Geochemical and radiometric age data from Nintoku Seamount, however, indicate that prior drilling at DSDP Site 432 penetrated late-stage alkalic lavas (Dalrymple et al., 1980). Although such alkalic rocks are suitable for paleomagnetic tests, it is desirable to obtain as wide an age range as available. Accordingly, we will use short seismic surveys during the leg to evaluate whether a suitable flank site free of tectonic complications can be identified (i.e., whether early tholeiitic shield lavas might be sampled).
Four sites have been identified on Ojin Seamount. Alternate Site HE-5A is positioned on Ojin Seamount at DSDP Site 430 (Fig. 1, Fig. 9). Approximately 60 m of sediments (ooze, sand, and volcanic ash) overlie lava flows at this site. Alternate Site HE-5B is located to the northeast of DSDP Site 430 on the summit flank. Sites HE-5C and HE-5D are on the seamount summit to the east of DSDP Site 430. Site HE-5C is designated as the primary site.
Primary Site HE-6A is positioned at DSDP Site 308 (Fig. 1, Fig. 9) on Koko Guyot. Previous drilling penetrated ~70 m of clays and volcaniclastic sandstone. Our alternate Site HE-6B is located at DSDP Site 309. Although previous DSDP drilling at those sites was also terminated prior to penetrating basement, they are located in sedimented areas where drilling can be easily started.
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