SEDIMENT DESCRIPTION AND INTERPRETATION

This section is summarized from Shipboard Scientific Party (1999). Three lithostratigraphic units were identified at Site 1095 (Fig. F2). Unit I (Holocene to late Pliocene, 0-1.77 Ma; 0.0-49.3 meters below seafloor [mbsf]) is composed of clays and silty clays, which are locally biogenic rich, contain scattered ice-rafted debris (IRD), and alternate in color between gray and brown. Unit II (late Pliocene to late Miocene, 1.77-8.93 Ma; 49.3-435.5 mbsf) is characterized by thick and repetitive sequences of greenish gray laminated silt and mud. IRD is scattered throughout Unit II and appears concentrated within bioturbated intervals. Unit III (late Miocene, 8.93-10.1 Ma; 435.5-570.2 mbsf) is mainly composed of dark greenish gray laminated claystone. Unit III will not be discussed further, as the present study only covers Unit I and part of Unit II.

Description

Unit I is mainly composed of fine-grained brown and dark gray diatom-bearing silty clay, silty clay, and clay, with minor siliceous ooze. The sediments are indistinctly laminated and extensively bioturbated. Subunit IA consists of alternating diatom-bearing silty clay (interglacials; 0.1-0.7 m thick) and clay (glacials; 0.9-1.7 m thick), extending from the present back to marine oxygen isotope Stage 11 at ~8 mbsf. Subunit IB consists of alternating massive silty clay with sand grains (interglacials; 0.5-2.1 m thick), and clay with silt laminae (glacials; 1.3-4.0 m thick), down to the top of a coarse-grained unit at 49.3 mbsf.

Interpretation

Unit I records deposition from suspension in a low-energy environment, as indicated by the fine grain size, lack of sorting, and absence of sedimentary structures indicating current winnowing. Slow sedimentation of the biogenic-rich facies in Subunit IA and the massive silty clay in Subunit IB allowed complete reworking by benthic burrowing organisms. In the terrigenous facies of Subunit IA, the diffuse nature of the lamination and the absence of silt laminae or graded-laminated facies suggest an origin as hemipelagites, which have been influenced by weak bottom currents (Pudsey and Camerlenghi, 1998). Dispersed sand grains and granules were transported by ice rafting.

Rare sharp-based, parallel-laminated silt laminae in Unit I are interpreted as distal turbidites.

The contact between Unit I and Unit II occurs over a 10-m-thick transitional zone, including thin layers of massive, matrix-supported diamict (Fig. F2).

Description

Unit II is characterized by sharp-based, graded, variably laminated fine sands and silts and laminated silty clays, interbedded with massive units. Three laminated facies (L1, L2, and L3) are distinguished by the presence and abundance of very fine sand and silt laminae. A massive facies (M) is characterized by the absence of primary sedimentary structures, except for diffuse grading, and results from hemipelagic sedimentation and intense bioturbation. Unit II shows a cyclic alternation of facies at scales from a few meters (L and M alternations) to many tens of meters (predominance of more or less silty/sandy L facies) (Fig. F2).

Facies L1, cross-laminated sand, silt, and silty clay, comprises ~10% of the thickness of Unit II. It consists of repetitive sequences of laminated to very thin-bedded fine sand and silt that grade up into laminated and massive diatom-bearing silty clay. The bases of sequences are conformable or erosional and are composed of cross-laminated very fine sand/silt, with a sharp upper contact with parallel-laminated silt and mud. This passes upward into laminated/graded silty mud, in turn overlain by massive silty clay. In some cases, muds contain sufficient diatoms to be classified as oozes. The massive silty clay shows varying degrees of bioturbation, from intense, where primary structure has been destroyed, to absent. IRD is concentrated in bioturbated bed tops.

Facies L2, parallel-laminated silt and silty clay, accounts for ~70% of the total thickness of Unit II. It consists of repetitive sequences of parallel-laminated silt and mud, passing upward into laminated/graded silty mud, in turn overlain by massive diatom-bearing silty clay. Bioturbation is limited to the upper few centimeters of bed tops.

Facies L3, laminated silty clays, forms ~10% of the thickness of Unit II. It consists of repetitive sequences of the thinly laminated and massive diatom-bearing silty clay that are present in the upper parts of Facies L1 and L2. Color banding is common, and upward transitions from dark to light hues within the depositional sequences suggest subtle size grading. The degree of bioturbation varies from minimal to moderate.

Facies M, massive, bioturbated, diatom-bearing sandy silty clay, lacks any distinct internal structure as a result of intense bioturbation. This facies accounts for ~10% of Unit II. In contrast to the rather thin hemipelagic intervals of the laminated facies, the massive facies forms beds up to 1 m thick and is generally grayish green, in comparison with the dark greenish gray of other Unit II facies. Subtle gradations in texture are present, and beds may show a gradual upward coarsening or upward fining. IRD is common in Facies M. Bed tops are typically sharp; lower contacts are locally blurred by burrowing (typically by Planolites).

Interpretation

Sediments resembling laminated sand, silt, and mud sequences of Facies L1, L2, and L3 are well described in the literature as "parallel silt-laminated mud" (Stow and Piper, 1984), "mud turbidite" (Stow and Townsend, 1990), and "thin-bedded turbidites." These facies are characteristic of deep-sea depositional environments dominated by muddy sediment gravity flows (Pickering et al., 1988; Alonso and Maldonado, 1990). Consequently, L1, L2, and L3 are all interpreted as turbidites.

Facies M probably results from slow hemipelagic settling of fine-grained particles derived from various sources such as low-density turbid flows, sediment plumes following the pycnocline and transported by geostrophic flows, and biogenic productivity. Intense bioturbation indicates low deposition rates and sufficient time to allow infauna to completely mix seafloor sediments. The low carbonate and organic matter content of these sediments indicates well-oxygenated bottom-water conditions and deposition below the carbonate compensation depth. IRD is a more conspicuous component of Facies M than Facies L1 to L3. This may be attributed either to a reduction in the rate of supply of fine-grained sediment relative to the influx of IRD or to an increase in the flux of IRD.

The massive diamict beds at the top of Unit II are interbedded with laminated sediments. They may record episodes of enhanced deposition of debris from floating ice relative to background deposition of mud, or they may have originated as debris flows produced by the local resedimentation and mixing of IRD and silty clay.

The stratigraphic distribution of Facies L1, L2, and L3 shows some long-term trends, both coarsening-upward (L1 L2 L3; e.g., 160-120 m and 300-200 m) (Fig. F2) and fining-upward cycles. Facies L1 indicates higher-energy conditions or a more proximal setting to the sediment source (continental margin or local channel). Those parts of a cycle dominated by more fine-grained T_D-E turbidites record a more distal setting. Provisionally, first-order cycles of several tens of meters can be interpreted as recording long-term (0.5-1.5 m.y.) phases of enhanced sediment deposition, reflecting sediment supply trends and changing position and dimensions of feeder channels or lobes along the margin of the Antarctic Peninsula.

IRD is a ubiquitous component of Units I and II and locally a substantial part of the flux of terrigenous sediment to the site (see also Cowan, Chap. 10, this volume; Hassler and Cowan, Chap. 11, this volume). It occurs as scattered sand grains and granules, as isolated pebbles, and as lenses of granules and sand. IRD lithologies include volcanic (rhyolite and basalt), volcaniclastic, plutonic (granite and granodiorite), and low-grade metavolcanic rocks, which can be matched to Antarctic Peninsula sources (Hassler and Cowan, Chap. 11, this volume).

Scattered sand grains and granules are common in Unit I from 2 mbsf downward. Pebbles up to 5 cm in diameter include a variety of volcanic and acid to intermediate plutonic rocks, with rare, low-grade metasedimentary rocks. Most pebbles are subrounded to subangular, the largest being rounded. In general, the number of ice-rafted pebbles (>0.5 cm diameter) in Unit II fluctuates but remains high until 205 mbsf. Most of these cores contain sand and granules and from one to three pebbles. From 205 to 426 mbsf, cores contain sand, granules, and low numbers of pebbles. Granite and basalt were the only pebble lithologies described from this 221-m-thick interval.