Sites 1141 and 1142
Sites 1141 and 1142 are situated near the crest of Broken Ridge ~350 km east of DSDP/ODP Sites 255, 752, 753, 754, and 755 (Figs. 5, 6). Flanked to the south by Eocene and younger oceanic crust of the Australia-Antarctic Basin and to the north by Cretaceous oceanic crust of the Wharton Basin, Broken Ridge appears to have formed during Late Cretaceous time as a result of Kerguelen hot spot magmatism (Duncan, 1991; Duncan and Storey, 1992; M.K. Coffin et al., unpubl. data). Subsequently, Broken Ridge and the Kerguelen Plateau began to separate along the nascent SEIR at ~40 Ma. Igneous basement of Broken Ridge had not been sampled previously by drilling; dredge samples from three locations along the feature's southern, faulted boundary yield dates of ~62, ~83, and 88-89 Ma (Duncan, 1991). Because of the scatter in ages of the dredged rocks and the absence of in situ basement samples from Broken Ridge, knowledge of Broken Ridge's age and composition remains extremely limited. We located Sites 1141 and 1142 on the JOIDES Resolution single channel seismic Line JR183-101. Sites 1141 and 1142 lie at depths of 1197 m and 1201 m, respectively, ~3-4 km north of the crest of Broken Ridge. We chose this location primarily on the basis of its thin sedimentary section (Fig. 41). The top of acoustic basement has an apparent dip to the north-northeast of 0° at Site 1141 and 2.5° at Site 1142. An ~100-m-thick sediment sequence overlies igneous basement. Since basement of Broken Ridge had never been drilled, our major objective at Sites 1141 and 1142 was to determine its age and composition. Additional basement objectives were to determine the physical characteristics of the lava flows and the environment of eruption (subaerial or submarine). The sedimentary objectives at Site 1141 were to determine sequence facies, to define the ages of seismic sequence boundaries, to estimate the duration of possible subaerial and shallow marine environments, to obtain minimum estimates for basement age, and to determine the paleoceanographic history of Broken Ridge. At Sites 1141 and 1142, these objectives were achieved by coring 72 and 51 m of volcanic basement, respectively, and ~113 m of overlying sediment at Site 1141 (Figs. 42, 43). The abrupt termination of Hole 1141A led to an unanticpated experiment whereby we compared two basement sections separated by only 800 m.
At Site 1141, sediments were recovered from 0 to 103.8 mbsf (Fig. 42). We recognize only one sedimentary unit, lithologic Unit I. The basement volcanic rocks are designated lithologic Unit II. Unit I (0-113.5 mbsf) consists of white foraminifer nannofossil ooze of Pleistocene to early Miocene age. Core 183-1141A-1R consists of nannofossil-bearing foraminifer ooze that is predominantly composed of sand-sized foraminifers and displays slight normal size-grading. Traces of aragonite are present in Core 183-1141A-1R. Temperate calcareous microfaunas and floras characterize the current-winnowed Neogene calcareous ooze recovered at Site 1141. They are joined by subtropical index taxa in the Pliocene-Pleistocene section, a result of northward movement of Broken Ridge into warmer, lower latitude waters. The average sedimentation rate of 6 m/m.y. for the entire carbonate ooze section is the lowest Neogene rate for Leg 183. In Cores 183-1141A-8R and 9R, we obtained reliable remanent magnetization and correlated normal and reversed polarities with middle Miocene Chrons C5 to C5AD. Bulk densities in Unit I vary from 1.6 to 1.8 g/cm3, and porosity ranges from 54% to 65%, with a mean of 62%. P-wave velocities in Unit I show very little scatter, with a mean value of ~1860 m/s. The base of Unit I consists of a layer of sandy foraminifer limestone with abundant sand- to pebble-sized rock fragments and mineral grains. The limestone, late-middle to late Eocene in age (35-38 Ma), postdates rifting and separation of Broken Ridge and the CKP. The pebbles include basalt with ferromanganese crusts. The thickness of this basal layer is uncertain as only two small fragments were recovered. The pelagic sedimentary succession at Site 1141 indicates that Broken Ridge has been at bathyal water depths since at least early Miocene time. Neritic fossils in the basal limestone indicate redeposition from shallow-water areas to a bathyal environment during the Eocene or later. Unit II consists of basalts, which are highly altered in the upper portion of the section. It is subdivided into six basement units. Below the boundary between Units I and II (~114 mbsf), index properties change abruptly. From 115.8 to 116.7 mbsf in the upper part of basement Unit 2, bulk densities increase to a mean value ~2.0 g/cm3, grain densities increase to 2.9 g/cm3, and porosities decrease to 48%.
At Site 1142 (Fig. 43) no sediments were recovered from the drilled interval (0-91 mbsf; Core 183-1142A-1W, except for some small fragments of sandy pebbly foraminifer limestone with Oligocene or Eocene nannofossils.
The six basement units at Site 1141 represent 71.2 m of basement penetration and consist of five mafic lava flows overlain by a coarse-grained sedimentary deposit of three small fragments of moderately altered, medium-grained, plagioclase-clinopyroxene-olivine gabbro, which could be from a dike, sill, or gravel bed. These lava flows appear to have been erupted subaerially; no evidence suggests interaction with water during emplacement. The top of most volcanic units in Hole 1141A have been highly to completely altered to clay; in some cases this intense alteration affects entire units. Red flow tops and green to gray flow interiors suggest decreasing oxidation with depth. In many intervals, traces of native copper are in the groundmass, and abundant native copper line some fracture surfaces. Vesicles are filled with dark green clay, calcite, zeolite, amorphous silica, and quartz and have well-developed colloform textures. Slickensides are numerous along some fractured surfaces. Perhaps the most noteworthy alteration within Hole 1141A is the spectacular alteration halos associated with quartz veins. In some instances, a single quartz vein extends for >120 cm with multiple, symmetrical alteration halos progressively altering the surrounding wall rock. Common calcite veins are generally <0.5 mm wide and crosscut the quartz-filled veins.
Basement Unit 2 (20.0 m thick) is fine-grained, aphyric basalt. The least altered portion of Basement Unit 3 (8.3 m thick) is fine- to medium-grained, sparsely plagioclase-phyric basalt. Basement Unit 4 (19.6 m thick) is aphyric to moderately plagioclase- or plagioclase-olivine-phyric basalt. Most of the massive interior of this unit is moderately altered, but a fresher, denser, and finer grained zone might represent a dike intruded into the basement Unit 4 flow; however, no contacts were recovered. Basement Unit 5 (8.0 m thick) is sparsely olivine-phyric. Basement Unit 6 (15.3 m thick) ranges from aphyric to moderately olivine-and-plagioclase-phyric basalt. In addition to olivine phenocrysts, groundmass olivine and minor apatite in the lower part of basement Unit 4, and throughout basement Units 5 and 6, suggest that these basalts are alkalic. Thin sections show that carbonate, clay, and iron oxides completely replace the mafic phases and groundmass glass from the top of Unit 1 to the upper part of Unit 6; the bottom part of Unit 6 retains a large proportion of relatively fresh phenocryst and groundmass olivine. All index properties change markedly near the boundary between basement Units 2 and 3 and reach extremes in basement Unit 6, where bulk densities vary from 2.6 to 2.9 g/cm3 with a mean of 2.7 g/cm3, grain density approaches a mean of 2.8 g/cm3, and porosity varies from 16% to 3%. P-wave velocities in basement Unit 6 increase gradually with depth, from 4276 to 6902 m/s.
At Site 1142, 50.9 m of basement penetration recovered six basement Units (Fig. 43). They include a diverse range of lithologies, including olivine-phyric basalt lava flows, possible pillow basalts, subaerial deeply weathered (felsic?) lavas, and volcanic sediments. Alteration and weathering of these basement rocks suggest subaerial exposure. The lithologies and alteration intensity within Hole 1142A are heterogenous. Some units are relatively massive and only slightly altered basalt, whereas other volcanic units are variably brecciated by both volcanic and tectonic processes and have been completely altered to clay. Primary igneous textures are still visible in most units and are typically accentuated by the replacement of feldspar by light green clay and mafic minerals by red brown clay.
Basement Unit 1 (1.9 m thick) is a slightly to moderately altered, massive, fine-grained, aphyric to sparsely plagioclase-and-olivine-phyric basalt; the upper portion has prominent oxidation halos, suggesting a period of exposure and weathering, possibly related to Eocene rifting and breakup between Broken Ridge and the CKP. Basement Unit 2 (1.2 m thick) consists of a single section containing 20 moderately to completely altered cobble-sized pieces of genetically unrelated rock types, including volcanic breccia, clinopyroxene-phyric, plagioclase-phyric, olivine-phyric, and aphyric basalt, and feldspar- and feldspar-quartz-phyric felsic volcanic rocks. Some of the pieces have abraded, slightly weathered surfaces, suggesting the unit may be a near-source debris flow or a talus pile. Basement Unit 3 (19.3 m thick) is a completely altered, aphanitic, aphyric to moderately olivine plagioclase-phyric basaltic breccia with three subunits defined on the basis of textural characteristics. Basement Unit 4 is a 4.3-m-thick, well-indurated, normally graded claystone or mudstone with very highly to completely altered, very coarse sand-sized to small granule-sized lithic clasts and crystals of quartz and altered feldspar in a red clay matrix. We interpret this unit to be a mudflow deposit. Basement Unit 5 (8.8 m thick) is a very highly to completely altered, aphanitic, aphyric volcanic breccia. The only hint of the original rock type is provided by very rare quartz crystals, which suggest an evolved composition. Basement Unit 6 is composed of >=9.6 m of nonvesicular, massive, and possibly pillowed basalt. It contains several fine-grained clay margins that strongly resemble highly altered glass. However, dark green to black alteration halos adjacent to calcite veins within massive basalt appear similar to features interpreted as highly altered glassy pillow margins. Thus, evidence is equivocal as to whether these are actually pillow basalts. Basalt in Unit 6 is aphyric and fine-grained to aphanitic. Spectacular sinuous and semicircular red-brown oxidation fronts and halos are common and are often crosscut and offset by calcite veins. These oxidation bands, similar to those in weathered pillow basalts, may represent subaerial weathering after the uplift of Broken Ridge that accompanied its breakup with the CKP.
All basement rocks in Holes 1141A and 1142A are normally magnetized. Despite a lateral separation of only 800 m, we cannot correlate the basement units at Sites 1141 and 1142. Seismic reflection data collected over the sites during Leg 183, as well as seismic reflection data and drilling results from Sites 752-755 lying ~350 km to the west, indicate that to the north of the bathymetric crest of Broken Ridge, prebreakup sediment and presumably igneous basement dip consistently to the north. This may account for the differences in basement units between Sites 1141 and 1142, in that Site 1142 penetrated a deeper stratigraphic section than Site 1141 did. Alternatively, lateral variability of igneous basement in Broken Ridge could be much greater than that in continental flood basalts.
The major results of drilling Sites 1141 and 1142 include
1. Six basement Units at Site 1141 include five mafic flows that appear to have been erupted subaerially.
2. Several of these lava flows, 8 to 20 m thick, contain phenocryst and groundmass plagioclase and olivine.
3. Six basement Units at Site 1142 range from olivine-phyric basaltic flows to subaerial deeply weathered (felsic?) lavas and volcanic sediments. A surprising result is that basement Unit 6 may be a pillow basalt that was subsequently oxidized in a subaerial environment, perhaps after the ~40-Ma uplift of Broken Ridge that accompanied its breakup with the CKP.
4. Differences between igneous basement sections at Sites 1141 and 1142 may result from penetration of different stratigraphic levels or from considerable, unanticipated lateral variability in volcanism.

Leg 183 Summary
Leg 183 Table of Contents