Thickly laminated mud lithofacies is present commonly within the studied interval (Fig. F3). Parallel silt laminae are >2 mm thick and have sharp lower contacts and grade upward into mud. On X-radiographs, they appear as medium-density fuzzy laminations with low-density interlaminae (Fig. F4A, F4B). The MS signature has high-frequency peaks, and thicker silt laminae correspond to peak MS values. Bioturbation is slight, with burrowing parallel to laminae or absent. IRD is present as isolated pebbles or as weak stratification (Fig. F4B). This lithofacies was deposited during glacial periods by distal low-density turbidity currents, probably generated by sediment failure along the continental slope (McGinnis et al., 1997; Barker et al., 1999) or by rapid hemipelagic sedimentation from meltwater plumes originating at glacier margins near the shelf edge (Pudsey, 2000). This lithofacies is punctuated by intervals of intense iceberg rafting.
The thinly laminated mud lithofacies has 1- to 2-mm-thick parallel to wavy laminae that appear color banded or variegated on the split core surface. This lithofacies has a distinctive appearance on X-radiographs: a strong contrast between thin silt and mud laminae (Fig. F4C). This is interpreted to visually indicate sharp contacts between the silt and mud laminae. IRD is low and variable within this lithofacies. The thinly laminated lithofacies appears significantly different from other laminated lithofacies on X-radiographs and on the split core surface. The sharp contacts and wavy laminae may result from reworking and winnowing of fine-grained turbidites by currents. These siliciclastic-dominated laminae have several characteristics in common with glacigenic contourites (Stoker et al., 1998). However, the origin of the thinly laminated lithofacies cannot be definitively determined without further work because of the many similarities that exist between fine-grained turbidites and those that have been reworked by contour currents (Stow et al., 1998). However, these two lithofacies suggest that several processes may have operated on the Antarctic Peninsula continental rise during glacial periods. At present, contour currents are steady but have too low a velocity to erode drift sediment (Camerlenghi et al., 1997). Contour current strength may vary between glacial and interglacial periods, as has been suggested for the Weddell Sea by Grobe and Mackensen (1992).
Homogeneous mud appears as a low-density massive unit on X-radiographs with isolated IRD (Fig. F5A, F5B). Bioturbation may be high, resulting in complete reworking locally. This lithofacies is structureless, either as a result of rapid hemipelagic sedimentation from meltwater plumes or from infaunal activity when sedimentation rates were low. Icebergs were present, but rapid sedimentation could have reduced their overall impact within the record.
Diamicton occurs in 20- to 50-cm-thick beds that are massive or weakly stratified with clasts concentrated in bands and silt laminae (Figs. F3, F5C). The diamicton was deposited by intense iceberg rafting with possible contributions of mud from meltwater plumes. This lithofacies corresponds to the highest IRD MAR and pebble counts within the studied interval.
Foraminifer-bearing mud contains 10%-90% biogenic component and isolated IRD. X-radiographs are low density and show highly burrowed sediment with loss of primary structures (Fig. F5D). Foraminifers are rare near the Antarctic continent (Hodell and Venz, 1992) and at the other sites drilled during Leg 178 (Barker, Camerlenghi, Acton, et al., 1999). Their abundance in the interglacial periods at Site 1101 from 2.2 to 0.76 Ma suggests that conditions favorable for carbonate preservation occurred during these warmer periods (Pudsey, 2000) and not at other times. Slow sedimentation allowed high infaunal activity and bioturbation along with homogenization of IRD.
Features on the IRD MAR curve and >2-mm clast histogram generally correspond (Fig. F3). The highest values coincide with diamicton, which supports its origin by ice rafting. Thinly laminated mud has the lowest IRD MAR, and other lithofacies have variable IRD MAR.