A key question concerning paleomagnetic tests such as those conducted during Leg 197 is the penetration needed to adequately average secular variation. Analysis of cores from Site 884 on the eastern flank of Detroit Seamount indicates that at some sites as little as 85 m of basement penetration is sufficient to obtain an average of secular variation. Where basement penetration was >120 m at Cretaceous plateaus, seamounts, and guyots in the Pacific, enough independent time units were recovered to average secular variation (Tarduno and Sager, 1995; Tarduno and Gee, 1995). This depth does not differ greatly from the that over which secular variation is averaged (100–200 m) in analyses (Holt et al., 1996) of cores from lava flows obtained by drilling on Hawaii.
However, it is not possible to determine the time sequence represented by the volcanic section at a given site prior to drilling. It is necessary to evaluate the angular dispersion of independent lava flows (inclination units) and compare this with global paleomagnetic data to confirm whether secular variation has been adequately sampled at a given site. We planned to collect on-site paleomagnetic data and to make angular dispersion calculations to evaluate whether the resulting record provides an adequate average.
We planned basement penetration to moderate depth (150–250 m) at the Emperor Seamount sites (Fig. F1). Our preferred strategy was to employ minicones for reentry. The nominal depths for basement penetration were based on drilling of other Pacific seamounts and plateaus (Tarduno and Gee, 1995). Whereas these estimates were needed for the planning process, we envisioned an interactive process based on recovery. Ideally, we hoped to recover at least 15 flow units (distinct lava flows rather than lobes of compound flows) from each hole for detailed paleomagnetic and radiometric age (40Ar-39Ar incremental heating) analysis. Below, we include a brief description and rationale for each of the drilling sites. This approach, with age information provided by micropaleontology and relative time information provided by physical volcanology, guided our decisions at sea.
Originally, the northernmost site to be drilled during Leg 197 was on Meiji Guyot. Unfortunately, clearance for drilling was denied by the Russian government in May 2001. Consequently, drilling on Detroit Seamount (˜81 Ma) became the highest priority for Leg 197. We planned single holes at two sites; two summit sites (proposed Sites HE-3A and HE-3B) were given high priority and were included in a seismic survey by the JOIDES Resolution en route. Proposed Site HE-3A became our first site drilled (Site 1203).
The relatively thick sequences of volcaniclastic rocks recovered at Site 1203 indicated proximity of a Campanian volcanic source; we felt that nearby proposed Site HE-3B might largely repeat the section already cored. Therefore, we elected to return to Site 883 (proposed Site HE-3) to drill deeper and obtain more flow units than previously acquired (during Leg 145) to increase the precision of the oldest paleomagnetic data to be used in our paleolatitude test. The final selected location near Site 883 became Site 1204, where two holes were drilled.
In addition to confirming the already recognized discrepancy between the paleolatitudes of Suiko and Detroit Seamounts and the present latitude of Hawaii, we saw an opportunity to investigate how this difference accumulated with time. We planned sites on Nintoku, Ojin, and Koko Seamounts (Fig. F1) to obtain this record. Because of time devoted to deepening Site 1203 below volcaniclastic intervals (to obtain additional time-independent lava flows) and operational delays related to a clogged drill bit at Site 1204 (forcing the drilling of a second hole), drilling at Ojin Seamount was canceled. We allotted the time saved in roughly equal proportions to deeper drilling at Nintoku Seamount and Koko Guyot (which became Sites 1205 and 1206, respectively).
If the Emperor trend represents southward motion of the Hawaiian hotspot, we expected to obtain a paleolatitude of 25°–27° for Nintoku Seamount. Our planned Site HE-4A was positioned at DSDP Hole 432A, near the northwest edge of the seamount on flat-lying, stratified sediment that could be used to stabilize the bottom hole assembly (BHA). Previous drilling indicated that the sediment was 42 m thick, above basement, and that the uppermost lava flows were separated by a soil horizon. Soils indicate significant time intervals between lava flows and ideal conditions for deeper drilling.
Our planned site on Koko Guyot (proposed Site HE-6A) was positioned at DSDP Site 308. Previous drilling penetrated ˜70 m of clay and volcaniclastic sandstone. Biostratigraphic data indicated an Eocene age for the base of the section, suggesting the hole was terminated (due to sea conditions) near basement.