Sites 1135 and 1136
Sites 1135 and 1136 (water depths of 1567 and 1931 m, respectively) are on the southern Kerguelen Plateau, approximately midway between two ODP Sites (738 and 750) where basaltic basement has previously been recovered. Sites 1135 and 1136 are ~350 km north of Leg 119 Site 738 and 300 km south of Leg 120 Site 750 (Figs. 3, 4). Major objectives of drilling on the southern Kerguelen Plateau were to obtain 150 m of igneous basement to characterize the age, petrography, and compositions of the lavas, the physical characteristics of the lava flows, and the environment of the eruption (subaerial or submarine). A specific goal was to evaluate the areal extent of the continental lithosphere component that has been recognized in the Site 738 lavas using trace element and isotope geochemistry; such a component is not present in the more northerly Site 750 lavas (Figs. 9, 10, 11, 12, 13). Sedimentary objectives at Sites 1135 and 1136 were to determine sequence facies, to define the ages of seismic sequence boundaries, to estimate the duration of possible subaerial and shallow marine environments, and to obtain minimum estimates for basement age. Hole instability forced us to abandon Site 1135 after drilling to ~70 m above acoustic basement (Fig. 15), but we were able to accomplish some of our basement-oriented objectives at Site 1136 (Fig. 16), located ~30 km east of Site 1135. In particular, we penetrated 33.3 m of basaltic basement that included three inflated pahoehoe flows, two of which are characterized by massive, relatively unaltered interiors. These rocks provide excellent samples for radiometric dating and geochemical analyses.
The 526-m-thick late Pliocene to Late Cretaceous sedimentary sequence recovered at Site 1135 is almost entirely pelagic calcareous ooze and chalk (Fig. 17). Chert nodules are common from ~140 m below seafloor (mbsf) to the bottom of the hole. We recovered an expanded (238 m thick) middle Eocene to latest Paleocene nannofossil ooze section, an interval not well represented during previous coring on the Kerguelen Plateau or elsewhere in the Southern Ocean at these high latitudes (~60°S). The study of this section will improve high-latitude biostratigraphic correlations. Furthermore, a Cretaceous/Tertiary boundary section at ~260 mbsf is possibly marked by a bed of light greenish gray calcareous clay with an irregular upper contact and scattered well-rounded clasts of white nannofossil ooze. These features suggest an erosional or mass-wasting event. We have tentatively identified Chron C29n above, and Chron C29r below the boundary, respectively. Near the boundary, velocity, magnetic susceptibility, and natural gamma ray intensity change significantly; in addition, water content, porosity, and carbonate content decrease below the boundary. Sedimentation rates were high in the Paleogene ooze (up to 15 m/m.y.) and Cretaceous chalks (8-10 m/m.y.); the Paleocene section, however, is abbreviated by hiatuses.
The 128-m-thick sedimentary sequence recovered at Site 1136 (Fig. 18) includes an expanded upper lower Eocene to lower middle Eocene section of pelagic calcareous ooze and chalk (Unit II) that is not well represented in other drill holes on the Kerguelen Plateau or in any other southern high latitude sites. Study of these sediments will refine high-latitude middle Eocene biostratigraphic zonations. Underlying these pelagic sediments is calcareous zeolitic volcanic clayey sand (Unit IV), probably deposited in a high-energy neritic (shelf) environment, and a carbonate-bearing zeolitic silty clay (Unit V). Fossil debris in Unit V is common and suggests deposition at shallow paleodepths (upper bathyal to outer neritic) in more tranquil conditions than prevailed during deposition of the overlying clayey sand. The sands and clays overlying basement basalt contain diluted but relatively well-preserved micro- and nannofossil faunas of middle Albian age, thereby providing a minimum age for the section and underlying basalts of ~104.5-106.5 Ma. Lower Albian marine sediments have not been recovered during previous drilling on the Kerguelen Plateau, but sands and clays recovered in Hole 1136A resemble lower Albian sediment drilled on the Falkland Plateau. The sands and clays may correspond to nonmarine, palynomorph-bearing Albian sediment found in silt and claystone cored at Site 750 on the SKP. Albian and Late Cretaceous nannoplankton, foraminifers, and pollen assemblages will provide information on regional paleoceanographic conditions during those times. The epiclastic succession (Units IV and V) and overlying calcareous sediments reflect increasing water depths with time concomitant with a decreasing volcaniclastic component in the sediments. Basaltic volcanic components in the epiclastic sediments at this site are probably derived from erosion of the basaltic plateau.
At Site 1136, from 128.1 to 161.4 mbsf, we cored three normally magnetized tholeiitic basalt flows (55% recovery; Fig. 18). We infer that the vesicular tops of the two upper flows were not recovered. Basalt from the uppermost flow (6.2 m recovered from an ~10-m-thick flow) varies downward from moderately altered to a massive interior to a fine-grained, vesicle-rich (~10% clay filled vesicles) and oxidized base. Horizontal vesicle sheets and the general vertical distribution of vesicles within the massive interior and lower crust of the upper flow imply that it formed as an inflated pahoehoe flow. Basalt from the middle flow (13.3 m recovered from an ~20-m-thick flow) varies downward from a massive interior to a fine-grained, vesicle-rich (10%-15%) base. This flow is also probably an inflated, large-volume pahoehoe flow. We only recovered 53 cm of a vesicular basalt breccia that forms the rubbly flow top of the lower flow. Although we cannot unambiguously determine the eruption environment of these flows, the inference that they are inflated pahoehoe flows and the absence of features indicating submarine volcanism (e.g., pillows and quenched glassy margins) suggest subaerial eruption.
All lavas are sparsely to moderately phyric basalts containing phenocrysts of plagioclase with lesser amounts of clinopyroxene olivine. Phenocrysts are found as either isolated grains or as two texturally distinct types of glomerocrysts. Corroded plagioclase cores in one glomerocryst type resemble those in small (~1 cm) microgabbro xenoliths. Vesicle-rich segregations (1-2 cm wide) contain 10%-30% vesicles in a nonporphyritic, fine- to medium-grained matrix rich in glass and titanomagnetite. The basaltic rocks are slightly to completely altered to low temperature secondary phases that partially replace primary minerals, completely replace mesostasis, fill veins, and partially to completely fill vesicles. The most common secondary minerals are clays (Mg-saponite and celadonite), calcite, and zeolites. In general, clay minerals abound at all depths, whereas the abundances of calcite and zeolites exhibit more pronounced downhole variations. The wide variation of K and Rb contents in the lavas analyzed by XRF reflects formation of these secondary phases.
The four least altered samples (loss on ignition [LOI] = 0.9% to 2.1%) from the upper and middle flows have 50.0-51.0 wt% SiO2, 6.4-6.7 wt% MgO, and 1.60-1.76 wt% TiO2. Both flows are quartz-normative tholeiitic basalts (Fig. 19) with relatively low MgO and Ni contents and low Mg numbers; they are similar to basement rocks from other parts of the Kerguelen Plateau. In detail, the upper flow has marginally lower Ti, Nb, Zr, Y, and Ce, distinctly lower V, and higher Cr abundances than the middle flow. Primitive-mantle-normalized abundances of highly incompatible trace elements (Ba, Nb, and Ce) are only slightly greater than those of less incompatible elements (Ti and Y). Site 1136 lavas do not have the anomalously low Nb/Ce and Ti/Zr ratios that have been used in conjunction with isotope data to infer a continental lithospheric component in basalt from Site 738 on the southern plateau (Fig. 20). In many geochemical characteristics, Site 1136 lavas are similar to the low Al2O3 group at Site 749. No evidence indicates that these lavas contain a component derived from continental lithosphere.
Major results of drilling at Sites 1135 and 1136 on the SKP include the following:
1. Expanded middle Eocene to uppermost Paleocene (Site 1135) and upper lower Eocene to lower middle Eocene (Site 1136) pelagic calcareous sediment sections not previously recovered at any Southern Ocean sites will improve our understanding of high-latitude paleoceanography at critical times of Paleogene cooling as well as aid high-latitude biostratigraphic correlations.
2. Albian and Late Cretaceous nannoplankton, foraminifers, and pollen assemblages will provide information on regional paleoceanographic conditions during those times of high relative sea level and high global temperatures.
3. Paleoenvironments of volcanic rock and overlying sediment range from subaerial (basalt) to neritic (clay and sand) to pelagic (chalk and ooze), documenting subsidence of the Kerguelen Plateau since Early Cretaceous time.
4. The >105-Ma basement basalts at Site 1136 are 10- to 20-m-thick inflated pahoehoe flows similar to continental flood basalts, such as the Columbia River Basalt; there is no geochemical evidence for the continental lithosphere component present in Site 738 basalts.
Leg 183 Principal Results - Site 1137
Leg 183 Table of Contents