PRINCIPAL RESULTS


Site 1139
Site 1139 lies on Skiff Bank (Leclaire Rise), a bathymetric and gravimetric high ~350 km west-southwest of the Kerguelen Archipelago (Figs. 3, 4). Flanked to the south and west by Cretaceous oceanic crust of the Enderby Basin, Skiff Bank appears to be structurally related to, and bathymetrically continuous with, the NKP. At least two major faults, however, offset interpreted igneous basement between Skiff Bank and the large massif containing the Kerguelen Archipelago. Skiff Bank has been proposed to be the current site of the Kerguelen hot spot (Fig. 2), but hundreds of meters of sediment on parts of the elevated feature argue against Skiff Bank originating entirely by recent volcanism. Both Skiff Bank and Elan Bank trend east-west, approximately perpendicular to the trends of fracture zones in the Enderby Basin and thus parallel to the axis of breakup between Antarctica and India. The free-air gravity signatures of the two features are also similar; pronounced negative anomalies flank their southern margins, but not their northern margins (Fig. 4). Many rock types, including both aphyric basalt and plutonic rocks such as alkali granite, were recovered in a single dredge haul from Skiff Bank, quite close to Site 1139. The plutonic rocks were interpreted as ice-rafted debris. Hence, the age and composition of Skiff Bank's igneous crust and its relationship to the contiguous northern Kerguelen Plateau are not established. The NKP is commonly believed to have formed since ~40 Ma, when the SEIR separated Broken Ridge and the CKP (Fig. 2), but submarine igneous basement of the NKP has never been drilled.
Site 1139 lies at a depth of 1427 m on Skiff Bank's southwestern terrace, which is >1000 m lower than the crest, located <50 km to the northeast (Fig. 3). We chose this location as representative of the entire Skiff Bank on the basis of its relatively simple structural setting and thin sedimentary section. The top of acoustic basement is flat lying, and the overlying basement is a sediment sequence ~500 m thick (Fig. 32). The fault scarp marking the boundary between Skiff Bank and the Enderby Basin lies ~20 km southwest of Site 1139 and offsets the basement by more than 2700 m.
The major objectives at Site 1139 were to obtain igneous basement to characterize the ages, petrography, and compositions of the lavas, the physical characteristics of the lava flows, and the environments of eruption (subaerial or submarine). We were especially interested in testing the hypothesis that the age of the uppermost igneous basement at Skiff Bank is <40 Ma. The sedimentary objectives at Site 1139 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 this high latitude site. As discussed below, we largely achieved our goals at Site 1139. We drilled 233 m into igneous basement that is overlain by early Oligocene shallow marine sediments.
Sediments were recovered from the upper 461 mbsf of Hole 1139A, whereas extensively altered felsic volcaniclastic rocks and mafic to intermediate composition lava flows were recovered from the lower 233 m of the hole (Fig. 33). We recognize six lithologic units. Units I-V are sediment and sedimentary rock resting on volcanic basement (Unit VI). Unit I (0 to 47.5 mbsf) consists of foraminifer-bearing diatom-bearing nannofossil ooze (Subunit IA) of Quaternary age and foraminifer-bearing nannofossil ooze (Subunit IB). Scattered basaltic sand grains and rare pebbles as well as traces of pumice are present in Subunit IA. Unit II (47.5 to 380.7 mbsf) consists of nannofossil-bearing clay and claystone with interbedded nannofossil-bearing ooze and chalk of early late Miocene to mid-Oligocene age. Trace fossils are very common. Unit II records a substantial influx of terrigenous clay from an adjacent volcanic landmass. In Subunit IB and the upper portion of Unit II (to ~107 mbsf) P-wave velocity averages 1822 m/s, bulk density ranges from 1.5 g/cm3 to 1.7 g/ cm3, grain density ranges between 2.6 and 2.8 g/cm3, and porosity changes from 60% to 74%. Sediments become semilithified by 100-110 mbsf. An unusual nannofossil (Braarudosphaera) bloom in late Oligocene time, reported previously on the SKP, may have been synchronous with other occurrences in the Atlantic and Indian Oceans. Minimum sedimentation rates were ~16 m/m.y. in the Miocene and ~20 m/m.y. in the Oligocene. We observed very rare tephra layers and disseminated volcanic ash, locally concentrated in burrows. Chert nodules appear only at the base of Unit II. We correlate normal and reverse magnetic polarities between ~100 and ~380 mbsf to early Miocene to early Oligocene geomagnetic Chrons C5D to C12 (or C13). From 108.9 mbsf to the base of Unit II at 380.7 mbsf, velocities increase linearly with depth, from 1785 to 4331 m/s. Within this depth interval, three volcanic ash layers have high P-wave velocities. In the same interval, bulk density increases from 1.3 g/cm3 to 2.1 g/cm3 with a mean of 1.7g/cm3, and porosity decreases from a maximum of 75% to 42%. grain density maintains a nearly constant value of ~2.8 g/cm3. Unit III (380.7 to 383.5 mbsf) is foraminifer nannofossil chalk of anomalous brownish to reddish yellow color. The P-wave velocity averages 3616 m/s. Units I-III represent deep-marine pelagic sedimentation. The base of the pelagic section is earliest Oligocene in age.
Unit IV (383.5 to 384.9 mbsf) consists of dusky red to greenish pink sandy packstone with rare planktonic foraminifers and bivalve shell fragments (Fig. 33). Index properties change significantly near the boundary between Units III and IV, from 381.4 to 384.4 mbsf. The bulk density in this zone averages 2.0 g/cm3, grain density averages 2.8 g/cm3, and porosity ranges between ~50% and ~31%. Grains are predominantly highly altered volcanic lithic fragments. P-wave velocity averages 3616 m/s. Unit V (384.9 to 461.7 mbsf) consists of interlaminated grainstone and sandstone with some thin interbeds of rudstone and cross-bedded intervals. Bryozoans, bivalves, and echinoids are the major biogenic components. Units IV and V were deposited in a shallow-marine neritic environment in low-energy and very high-energy (near shore) settings, respectively. Well-rounded cobbles at the top of the basement suggest a beach deposit at the base of the sedimentary succession.
At Site 1139, we drilled 232.5 m into igneous basement with a 37.4% recovery rate (Fig. 34). We identify 19 basement units; an upper succession of variably welded trachytic to rhyolitic volcanic and volcaniclastic rocks (Units 1-5) underlain by 14 lava flows (Units 6-19). All basement units are highly altered and fractured. The high degree of alteration and poor core recovery in Units 1-5 make it difficult to identify physical volcanic features and interpret modes of emplacement. However, these units can be distinguished in the natural gamma ray logs. Rocks in Units 1-5 have P-wave velocities varying from 2577 to 4770 m/s, with a mean value of 3616 m/s. Their bulk densities average 2.3 g/cm3, grain densities range from 2.6 to 2.9 g/cm3, and porosities average ~25%. The underlying 14 subaerial lava flow units (Units 6-19) have P-wave velocities that are typically >3000 m/s, with a mean of 4416 m/s. Bulk densities vary widely, with a mean of 2.4 g/cm3; grain density approaches a mean of 2.8 g/cm3 and decreases slightly with depth; and porosity varies widely, from 65% to 3%. All basement units have positive magnetic inclinations, corresponding to reversed polarity.
Unit 1, which had poor recovery (57 m thick; 5.3 m recovered), contains a variety of felsic volcanic and volcaniclastic rocks (Fig. 34). Unit 1A consists of rounded, massive to flow banded rhyolite cobbles. Unit 1B is a lens of bioclastic sandstone that resembles the grainstone at the base of the sedimentary section. Beneath this, a thin felsic pumice breccia (Unit 1C) overlies a zone of altered, perlitic felsic glass that contains lithic fragments (Unit 1D). We interpret the glassy zone to be the densely welded core of a pyroclastic flow deposit. The base of Unit 1D is a silicified basal breccia with lithic fragments and pumice. Beneath this are highly sheared and altered, clay-rich volcaniclastic sediments (Unit 1E) that we interpret as a fault zone. Within Units 1C through 1E, both clasts and the matrix commonly display cataclastic fabrics, and slickensides are ubiquitous on broken clay-rich surfaces. Unit 2 (10.5 m thick; 1.35 m recovered) consists of dark red (oxidized) rhyolite with ~10% sanidine and minor quartz phenocrysts. Flattening and agglutination textures suggest that this is a welded pyroclastic flow deposit. Unit 3 (9.7 m thick; 4.6 m recovered) is a green, highly altered, crystal-vitric tuff. It contains abundant sanidine phenocrysts, minor quartz, and lithic clasts, in a perlitic glassy matrix that is locally banded. As with Unit 1C, we interpret Unit 3 to be the densely welded core of a pyroclastic flow deposit. Unit 4 (30.1 m thick; 5.9 m recovered) contains massive to brecciated, dark red (oxidized) rhyolite that is similar to Unit 2. Unit 5 (17.4 m thick; 4.2 m recovered) is highly altered, sanidine-phyric trachyte that consists of a massive central zone bounded by a brecciated top and base; this unit is probably a lava flow.
Basement Units 6-17 (65.7 m drilled; 41.4 m recovered) consist of aphyric to sparsely plagioclase-phyric volcanic rocks ranging in composition from trachybasalt to trachyandesite (Fig. 35). We subdivide this sequence into individual lava flow units on the basis of brecciated flow tops and increased vesicularity toward the margins. The flow units vary from 1.8 to 19.8 m in thickness, but most are <6 m thick. Unit 10 consists of small pahoehoe lobes, Unit 11 is an aa flow, and the other flow units have brecciated margins of indeterminate character. The breccias are highly altered and sheared, with both matrix and clasts nearly completely altered to clay minerals. Breccia clasts are oxidized and cemented by calcite and siderite(?) as well as clay minerals. The relatively thin massive portions of the flows have many moderately to steeply dipping fractures and pronounced streaks of mesostasis, now altered to green clay minerals. Thin veins, commonly containing carbonate, pervade the basalt units. Rarely, the rock has a pale gray hue, and the groundmass is bleached because of the replacement of igneous minerals by secondary calcite.
Basement Units 18 and 19 are highly to completely altered highly sanidine-phyric trachyandesite and trachyte, respectively. Alteration consists of either intense hematitic red staining or white/pink bleaching. The minerals in the bleached rocks include quartz, sanidine, and siderite. The latter mineral is a common phase that cements groundmass, replaces primary phases, and fills veins and vesicles. The veins within the bleached intervals have prominent red alteration halos (hematite) and are filled with hematite, quartz, siderite, and calcite. The alteration of these rocks is probably the result of interaction between felsic igneous rocks with large volumes of hydrous fluids in some form of geothermal (subaerial?) system.
Although their compositions were affected by posteruption alteration, the major element compositions of the volcanic and volcaniclastic rocks comprising the basement at Site 1137 clearly form a series from trachybasalt to trachyte and quartz-bearing rhyolite (Fig. 35). These lavas are significantly more alkaline than the dominantly transitional to tholeiitic basement lavas recovered from all other Kerguelen Plateau drill sites. However, with the exception of the rhyolites, the alkaline Skiff Bank lavas are quite similar to alkaline lava series erupted in the Southeast Province of the Kerguelen Archipelago in the early Miocene and again in the Pliocene and Pleistocene.
In summary, significant results bearing on the origin and evolution of Skiff Bank (Site 1139) are
1. Subsidence of the NKP is recorded by the paleoenvironments of volcanic rocks and overlying sediments; since Eocene time, environments have changed from subaerial (volcanic and volcaniclastic rocks) to intertidal (beach deposits) to very high-energy, near-shore (grainstone and sandstone) to low-energy, offshore (packstone) to bathyal pelagic (ooze).
2. The oldest sediments overlying igneous basement are earliest Oligocene; this minimum age for basement is consistent with a <40 Ma age for the uppermost igneous basement of Skiff Bank.
3. The 233 m of igneous basement consists of an uppermost 124-m succession of variably welded trachytic to rhyolitic volcanic and volcaniclastic rocks; in addition, the two lowermost lava flow units comprise 33 m of sanidine-phyric trachyandesite and trachyte. As at Elan Bank (Site 1137), highly evolved magmas erupted, in some cases explosively, during the final stages of volcanism that formed Skiff Bank.
4. The volcanic basement at Skiff Bank, with a minimum age of earliest Oligocene, includes an alkaline lava series ranging from trachybasalt to trachyte. Similar alkaline lavas have erupted in the Kerguelen Archipelago in early Miocene and Pliocene/Pleistocene time; therefore Skiff Bank, which crests <500 m below sea level, may have been a somewhat older island analogous to the Kerguelen Archipelago 350 km to the east-northeast.

Leg 183 Principal Results - Site 1140
Leg 183 Table of Contents