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IODP Expedition 318:
Wilkes Land Glacial History
Site U1355 Summary
PDF file is available for download.
19 February 2010
objective at Site U1355 (WLRIS-06A) was to core across the WL-U3 unconformity
to obtain the timing and nature of the first arrival of the ice sheet to the
Wilkes Land continental margin in a distal setting. Site U1355 is located at the transition between the
continental rise and the abyssal plain in a water depth of 3729 m.
seismic reflection profiles crossing Site U1355 image three of the Wilkes Land
margin regional unconformities WL-U3, -U4, and -U5. Unconformity WL-U3 is
observed at ~782 m (5.95ms TWTT) and was interpreted to separate pre-glacial
strata below from glacial strata above. Thus, coring across WL-U3 unconformity
was intended to document the first arrival of the ice sheet to the Wilkes Land
continental margin. This "onset" of glaciation is presently inferred to have
occurred during the earliest Oligocene.
was also designed to provide a distal record of Oligocene to the Pliocene(?)
glacial/interglacial (i.e., colder vs. warmer) and ice sheet variability.
Regional unconformity WL-U5 is imaged in the seismic data at ~709 m (5.6 s
TWTT). WL-U5 represents a major shift in continental rise sedimentation with
the onset of thick levee deposits above the unconformity. Coring across WL-U5
aimed to document the timing, nature, and cause of this shift in sedimentation.
the regional seismic interpretations, Site U1355 is located where the uppermost
sedimentary section is relatively thin, or has been eroded, so that WL-U3 could
be reached at a shallower depth in contrast to other locations offshore the
Wilkes Land margin.
the seismic facies at Site U1355, the lithologies expected were fine-grained
distal turbidites, contourites and hemipelagites (Escutia et al., 1997, 2000,
2002; De Santis et al., 2003; Donda et al., 2003). This interpretation was
supported by the sediments recovered from DSDP Site 269 (Hayes and Frakes,
1975) located on the abyssal plain ~280 km seaward from U1355.
from one hole were obtained at Site U1355. Cores U1355A-1R to 4R penetrated
from 0 to 31.7 mbsf and recovered 14.95 m (47%). The stratigraphic integrity of
most of the core was highly compromised by drilling disturbance. The sediments
are composed of angular igneous and metamorphic fragments. These are
unconsolidated clast-supported, moderately-to-well sorted, sandy granule-pebble
conglomerates grading upwards into well-sorted fine, crudely stratified sands.
One 3-cm thick interbed of dark greenish gray, diatom-bearing, silty clay was
preserved between two upward fining units. The mechanism for the formation of
the upward fining beds is through gravity flow, most likely a high-density
Hole U1355A were analyzed for siliceous microfossils, foraminifers and
palynomorphs. Core catcher samples from Cores U1355A-1R to 4R and additional
samples from clay-rich clasts within the cores were analyzed for diatoms. The
core material yielded an abundant Antarctic flora dominated by Fragilariopsis
kerguelensis and Thalassiosira lentiginosa. The
association of these typical Pleistocene-Holocene Antarctic diatoms along with
common Actinocyclus ingens and A. ingens var. ovalis indicates an age no older than late
Pleistocene. Reworking from Miocene and Eocene material was recorded. A sample
from the top of the hole yielded a rich and diverse modern (Holocene) Antarctic
diatom assemblage. Radiolarians typical of late Pleistocene-Holocene Antarctic
waters were also found in the core-catchers and seafloor samples with an
overall low abundance. The seafloor sample yielded a low diversity planktonic
foraminifer assemblage dominated by Pachyderma neogloboquadrina indicating an age <9.2 Ma.
Palynomorphs were recorded in the seafloor sample and Samples U1355A-1R-CC and
-4R-CC. Notable finds included Holocene organic walled dinoflagellate cysts
(dinocysts), foraminifer linings, copepod eggs, besides reworked late Eocene
dinocysts, and reworked Paleogene and/or Cretaceous spores and pollen.
properties program for Hole U1355A cores included non-destructive measurements
of gamma-ray attenuation densitometer (GRA) bulk density, magnetic
susceptibility, natural gamma-ray emission (NGR) and P-wave velocity on
whole-round core sections. Whole-round and section-half core logging
measurements are significantly affected by poor core quality and the data are
therefore compromised. Magnetic susceptibility values are relatively high
reflecting the lithologic composition of the individual clasts in the gravels
and sands. The silty, diatom-bearing clay clasts are characterized by
pronounced lower magnetic susceptibility, bulk density and sonic velocity
values, but higher natural gamma ray counts. The P-wave velocities increase
from 1800 m/s at the seafloor to more than 1920 m/s at the base of Core
operations at Site U1355 revealed that the nature of the seafloor was not the
expected fine-grained sediments. After failing to core into the seafloor with
the APC, the RCB system was tried, which yielded 31.7 m of Pleistocene to
Holocene unconsolidated coarse gravels and sands. The nature of the sediment
prevented any further advance. We decided to abandon Hole U1355A and move to
the alternate Site U1356 (WLRIS - 07A) where we could achieve the same
De Santis, L., Brancolini,
G., Donda, F., 2003. Seismic-stratigraphic analysis of the Wilkes Land
continental margin (East Antarctica). Influence of glacially-driven processes
on the Cenozoic deposition. Deep-Sea Research. Part 2. Topical Studies in
Oceanography 50 (8–9), 1563–1594.
Donda, F., Brancolini, G., De
Santis, L., Trincardi, F., 2003. Seismic facies and sedimentary processes on
the continental rise off Wilkes Land (East Antarctica). Evidence of bottom
current activity. Deep-Sea Research. Part 2. Topical Studies in Oceanography 50
Escutia, C., Eittreim, S., and Cooper., A.K., (1997). Cenozoic
sedimentation on the Wilkes Land continental rise, Antarctica. In: C.A. Ricci
(Ed.) The Antarctic Region: Geologic Evolution and Processes. Terra
Escutia, C., Eittreim, S.L., Cooper, A.K., and Nelson, C.H., (2000).
Morphology and acoustic character of the Antarctic Wilkes Land turbidite
systems: ice-sheet sourced versus river-sourced fans. Journal of Sedimentary
Research, Vol. 70, No. 1, p. 84-93.
Escutia, C., Nelson, C.H., Acton, G.D., Cooper, A.K., Eittreim, S.L.,
Warnke, D.A., and Jaramillo, J. (2002) Current controlled deposition on the Wilkes Land continental rise. In D. Stow et al. (eds.): Deep-Water
Contourite Systems: modern drifts and ancient series, seismic and sedimentary
characteristics. The Geological Society of London, Memoirs, 22,
Hayes, D.E., Frakes, L.A., et al., 1975, Initial Reports of the Deep Sea
Drilling Project, v. 28, Washington (US Government Printing Office), 1017 pp.