We have focused our study on the four sites (Sites 998-1001) cored during Leg 165 where sedimentary sections older than Pleistocene age were penetrated. At Site 1001, we also cored through 37 m of Upper Cretaceous basaltic basement. An extensive description of the coring results, including lithologic descriptions, core photos, and preliminary biostratigraphic and paleontological analyses are presented within the Ocean Drilling Program (ODP) Initial Reports Volume 165 (Sigurdsson, Leckie, Acton, et al., 1997). Core photos and detailed core descriptions presented at the end of that volume can be used to determine the position and lithology of our samples, which follow the standard ODP sample naming convention. Below we present a brief summary of some of the relevant aspects of the sections sampled in this study, including their geologic setting, lithologies, ages, and sedimentation rates.
Site 998 (19.49°N, 277.06°E) is located on the Cayman Rise, north of the Cayman Ridge and Cayman Trough (Fig. 1). It thus lies on the southern portion of the North American plate rather than on the Caribbean plate. A sedimentary section spanning the lower Eocene (~52 Ma) to the present was recovered from coring in two holes, the deepest of which penetrated 904.8 meters below seafloor (mbsf). This section consists mainly of carbonates with varying amounts of clay and ash. Discrete clay-rich layers and ash layers are present throughout. The upper 160 m of the section is composed mainly of oozes (nannofossil and foraminiferal) and clayey nannofossil mixed sediments. Below 160 mbsf, the dominant lithologies are nannofossil chalks that grade into limestone with clay at ~700 mbsf. Sedimentation rates vary between 8 and 30 m/m.y., averaging ~17 m/m.y. (1.7 cm/k.y.) over the 52 m.y. interval.
Site 999 (12.74°N, 281.26°E) is located in the Colombian Basin, southeast of the Hess Escarpment. A sedimentary section spanning the upper Maastrichtian (~66 Ma) to the present was recovered from coring in two holes, the deepest of which penetrated 1066.4 mbsf. The upper 347 m of the section consists mainly of clay-rich carbonates classified as nannofossil and foraminiferal clayey mixed sediments. From 347 to 566 mbsf, clayey chalk with foraminifers and nannofossils is the main lithology. Below this the dominant lithology is clayey limestone except for an interval of clayey calcareous mixed sedimentary rock from 887 to 1033 mbsf. Discrete ash layers are found throughout the section. Sedimentation rates vary between 6 and 32 m/m.y., averaging ~16 m/m.y. (1.6 cm/k.y.) over the 66 m.y. interval.
Site 1000 (16.55°N, 280.13°E) is located on the northern Nicaraguan Rise, ~265 km southwest of Jamaica. A sedimentary section spanning the lower Miocene (~19 Ma) to the present was recovered from coring in two holes, the deepest of which penetrated 696 mbsf. The entire section is carbonate rich with the upper 307 m being dominantly nannofossil and micritic oozes, the interval from 370 to 513 mbsf is dominantly micritic nannofossil chalk, and the lower interval is dominantly limestone. Discrete ash layers are present throughout the section. Sedimentation rates vary between 27 and 47 m/m.y., averaging ~37 m/m.y. (3.7 cm/k.y.) over the 19-m.y. interval.
Site 1001 (15.76°N, 285.09°E) is located on the Hess Escarpment, on part of the lower Nicaraguan Rise. Basaltic basement (~81 Ma) and a sedimentary section spanning the Campanian to the present was recovered from coring in two holes, the deepest of which penetrated 522.8 mbsf. The upper 165.7 m, which is dominantly clayey nannofossil ooze, clayey nannofossil mixed sediment, and nannofossil ooze, extends down to the middle Miocene. The middle Miocene nannofossil ooze is separated from the underlying early Eocene-Campanian-age section by 28 cm of Eocene chalk and two unconformities with a total duration of 38 m.y. From 166 to 352 mbsf, the early Eocene-late Paleocene-age section is composed of chalk and mixed sedimentary rock with clay. A 10- to 20-cm-thick K/T boundary interval was recovered from both Holes 1001A and 1001B at 352-353 mbsf. The Upper Cretaceous sediments down to 473 mbsf are limestones and claystones. From 473 to 485 mbsf, in the interval just above igneous basement, there is a significant reduction in carbonate and an increase in volcaniclastic material, including ash layers and several thick ash turbidites. The very base of this interval contains subangular fragments of basaltic lapilli and hyaloclastite breccia. As at the other sites, discrete ash layers are present throughout the sedimentary section. Sedimentation rates vary between ~4 and 30 m/m.y., averaging ~12 m/m.y. (1.2 cm/k.y.) in the Neogene, 14 m/m.y. (1.4 cm/k.y.) in the Paleogene, and 11 m/m.y. (1.1 cm/k.y.) in the Cretaceous.
At Site 1001, we also cored through igneous basement from ~485 mbsf to the bottom of both holes. In Hole 1001A we penetrated 37 m into basement and recovered 20 m of igneous rock (54% recovery), whereas we penetrated only ~3 m into basement with 2.1 m of recovery in Hole 1001B. The basement is probably wholly extrusive in origin and the dominant lithologies are vesicular and massive basalts.
During Leg 165, the basement in Hole 1001B was subdivided into 12 formations, which were thought to be representative of individual lava flows or groups of similar flows and associated hyaloclastite breccias (Sigurdsson, Leckie, Acton, et al., 1997). The formation divisions where based mainly on chilled margins in the form of glassy rinds or on the presence of hyaloclastite breccias or carbonate deposits between formations. The formations were further subdivided into 52 units at coring gaps where changes in texture or composition occurred, some of which were quite subtle (pp. 325-329 and 739-763 in Sigurdsson, Leckie, Acton, et al., 1997). Separating the recovered basalts into independent flows is subjective. We consider several of the units from within a formation as individual lava flows and note that additional subdivision may be necessary between cores, where coring gaps are typically the largest.
Because of the importance of subdividing the extrusive rocks into individual flows that might have sampled the geomagnetic field independently, we present our own "flow unit" picks (Table 1). The flow- unit boundaries, as described in Table 1, are typically at chilled margins, though some of the chilled margins could be the margins of basalt pillows that belong to a single thicker basalt flow. Each of our 27 flow units potentially could represent flows that were extruded far enough apart in time that they could possibly provide independent samples of the geomagnetic field. More likely multiple flow units have been extruded within a short time interval relative to geomagnetic secular variation (SV), which indeed proved to be the case as discussed below. Relative to the subdivision derived during Leg 165 (Sigurdsson, Leckie, Acton, et al., 1997), we have more flow units than formations because we have intentionally subdivided any potentially independent flow from its juxtaposing flows when there was any indication of independence (chilled margins, change of texture across core boundaries, or change in inclination across a coring gap). We also have fewer flow units than the unit subdivision of Sigurdsson, Leckie, Acton, et al. (1997) because their units include breccia intervals and hyaloclastite intervals that were not sampled for paleomagnetic purposes.
We present our paleomagnetic results first as a function of depth, and then convert these to ages using calcareous nannofossil and foraminiferal datums. We use the datums given in Sigurdsson, Leckie, Acton, et al. (1997) except where they have been superseded by publications within this volume. In particular, the Neogene calcareous nannofossil ages for Sites 998, 999, and 1000 come from Kameo and Bralower (Chap. 1, this volume), and the Neogene planktonic foraminifer ages at Site 999 come from Chaisson and D'Hondt (Chap. 2, this volume).
40Ar-39Ar dates of three basalt samples from Hole 1001A give an age of 81 ± 1 Ma for the basement (Sinton et al., Chap.15, this volume). This age is in good agreement with the 76-80 Ma age obtained from nannofossil and foraminiferal datums from the overlying sediments (Sigurdsson, Leckie, Acton, et al., 1997).