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IODP Expedition 318:
Wilkes Land Glacial History
Site U1360 Summary
PDF file is available for download.
4 March 2010
(WLSHE-09B) is located in the continental shelf off the Adélie Coast at 495 m
water depth. The main objective at Site U1360 was to core across the regional
unconformity WL-U3 to determine the timing and nature of the first arrival of
the ice sheet to the Wilkes Land continental margin. Site U1360 lies at the
eastern edge of the Adélie Bank and receives drainage from the East Antarctic
Ice Sheet (EAIS) through the Wilkes Subglacial Basin. Glacier ice and the
entrained debris draining through the basin and extending to the continental
shelf would give the evidence for a large-scale ice sheet on Antarctica.
unconformity WL-U3 is interpreted to record the first expansion of the EAIS
across the shelf in this sector of the East Antarctic margin and to, therefore,
separate pre-glacial strata below from glacial strata above (Eittreim et al.,
1995; Escutia et al., 1997; De Santis et al., 2003). Drilling in Prydz Bay
during ODP Leg 188 (O'Brien, Cooper, Richter et al., 2001), and results from
DSDP 269 (Hayes and Frakes, 1975), ODP Leg 189 in the Tasman Gateway (Exon,
Kennett, Malone, et al., 2001), and ODP Leg 182 Site 1128 in the Great
Australian Bight (e.g., Mallinson et al., 2003), among others, led Escutia et
al (2005) to postulate an Early Oligocene age (i.e., 33.5-30 Ma) for the
development of the WL-U3 unconformity.
U1360, unconformity WL-U3 is predicted to occur at ~165 mbsf (0.81s TWTT).
Multichannel seismic reflection profiles crossing Site U1360 show gently
dipping strata on the shelf truncated near the seafloor. This provided a unique opportunity to sample across the unconformity with very shallow penetration. The
proximal record from Site U1360 was to complement the distal record of the
first arrival of the EAIS to the Wilkes Land margin obtained at Site U1356.
Hole U1360A was drilled to a total depth
of 70.8 meters below seafloor (mbsf) using the Rotary Core Barrel (RCB) system.
Only 60 cm were recovered in the upper 14.3 mbsf and sediments are
unconsolidated and moderately-to-strongly disturbed by drilling (Core
U1360A-1R). Below 14.3 mbsf (Cores U1360A-2R to -6R), the sediments are
consolidated and most recovered intervals are only slightly disturbed by
drilling. Based on visual core descriptions and smear-slide analyses, Hole
U1360A constitutes diamictons, mudstones, sandstones, and diamictites, which
are placed into two lithostratigraphic units. Unit I consists of unconsolidated
clast-rich sandy diamicton. The diamicton is slightly compacted, but soft,
crudely stratified, and includes one lamina of clay-rich diatom ooze with a
yellowish color at Interval U1360A-1R-1, 18-20 cm. A trace of diatoms is
present in the matrix of the diamictite. Rare shell fragments are also present
in this unit. Clast percentages are up to 25% and clasts are primarily composed
of angular, indurated, olive green to olive brown mudstone fragments, 2-8 mm in
size. Crystalline rock clasts, including basalt and gneiss, up to 7 cm in size,
are also present and have sub-rounded and facetted shapes. The unconsolidated
diamictons were probably deposited from floating ice, and most likely represent
deposition from a floating glacier tongue or icebergs releasing debris over the
site. The lamina of diatom ooze is indicative of a brief period of open marine
conditions with high productivity and low terrigenous sedimentation rates.
top of Unit II marks a sharp change in lithology and induration of the cores,
from unconsolidated diamicton above, to carbonate-cemented claystone below. An
Early Oligocene age is assigned to the interval below U1360A-3R-1, 8 cm,
whereas no age assignment is possible for Core U1360A-2R. Five different
lithofacies are recognized in a sequence from top to bottom in this unit: (1)
olive green claystone with moderate bioturbation, (2) dark green claystone with
dispersed clasts, (3) dark greenish gray sandy mudstone with dispersed clasts,
(4) olive brown sandstone with dispersed clasts, and (5) gray clast-rich sandy
diamictite. Overall, Unit II can be characterized as a fining upward sequence
from diamictite at the base to claystone at the top. Bivalve shell fragments,
some of which are pyritized, are common in the lower portion of Unit II. The
lithofacies distribution is consistent with an ice-proximal to ice-distal
glaciomarine depositional environment, similar to that described from the
Oligocene and Miocene strata of the Victoria Land Basin, Ross Sea, Antarctica
(Naish et al., 2001; Powell and Cooper, 2002). Five samples from Hole U1360A
were prepared for XRD analysis of the clay fraction. A mixture of all the
major clay mineral groups characterizes the clay mineral assemblages in these
samples. The dominant clay-mineral components are smectite, illite, and chlorite,
with a lesser contribution of kaolinite and pyrophyllite-talc. The cores
assigned to the Early Oligocene have clay mineral assemblages similar to those
reported from Lower Oligocene shelf strata around the Antarctic margin (e.g.,
Hambrey et al., 1991; Ehrmann et al., 2005). The abundance of illite and
chlorite are consistent with a glacial-marine depositional setting for the
claystone, mudstone, sandstone, and diamictite facies described within
Lithostratigraphic Unit II. The relatively large contribution of talc,
however, is not typical of Paleogene sediments on the Antarctic shelf and may
reflect the weathering of a low-grade metamorphic facies, derived from a basic
or ultrabasic igneous protolith, locally on the Wilkes Land margin or within the
Wilkes subglacial basin.
Dinoflagellate cysts (dinocysts) and diatoms
provide age-control for Hole U1360A. They suggest that Core U1360A-1R (0 to
0.54 mbsf) comprises an uppermost Pleistocene matrix, with intraclasts of Upper
Eocene to Lower Oligocene material. An age could not be assigned to the strata
between Samples U1360A-1R-CC and -3R-1, 8 cm (0.54 to 23.38 mbsf) due to poor
recovery. Samples U1360A-3R-1, 8 cm to -6R-CC (23.38 to 53.78 mbsf) are of
Early Oligocene (< 33.6 Ma) age.
Sediments within Core U1360A-1R (0 to
0.54 mbsf) comprise an uppermost Pleistocene matrix with intraclasts of Late
Eocene to Early Oligocene age. Diatoms indicate that during the latest
Pleistocene, the shelf at Hole U1360A was not subjected to year-round ice cover
but was influenced by seasonal sea ice either directly, or indirectly. Combined
microfossil analyses suggest the sedimentary interval below U1360A-3R-1, 8 cm (23.38 mbsf) is Early Oligocene (< 33.6 Ma) in age. Early Oligocene microfossils indicate
a shallow-water shelf environment with low salinities and high nutrient levels,
likely driven by seasonal sea ice. Sporomorphs may represent reworking from
older strata and/or contemporaneous vegetation in the hinterland.
Two samples from Hole U1360A (Samples
U1360A-4R-1, 15 cm and -4R-2, 72 cm) were reversely magnetized, consistent with
an age of Chron C12r as predicted by the biostratigraphy.
The fining upward sequence from
diamictites at the base (Core U1360A-6R) to claystones at the top (Cores
U1360A-2R to -3R) is also documented in the general decrease in MS values from
the bottom to the top of the hole. Variations in GRA density nicely reflect
variations in lithology between clast-rich diamictite, sandy mudstone with
dispersed clasts and claystone. Calculated porosity ranges from 49% to 17% and
generally decreases with depth. Grain densities range from 2.62 to 3.13 g/cm3.
The slightly higher grain densities in Core U1360A-4R are most likely related
to the occurrence of pyrite in the sediment (up to 8%).
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
Eittreim, S.L., Cooper, A.K., Wannesson,
J., 1995. Seismic stratigraphic evidence of ice-sheet advances on the Wilkes
Land margin of Antarctica. Sedimentary Geology 96, 131–156.
Escutia, C., Eittreim,
S.L., Cooper, A.K., 1997. Cenozoic glacio- marine sequences on the Wilkes Land
continental rise, Antarctica. Proceedings Volume-VII International Symposium on
Antarctic Earth Sciences, pp. 791–795.
Escutia, C., De Santis, L., Donda, F.,
Dunbar, R. B., Brancolini, G., Eittreim, S. L., & Cooper, A. K. (2005).
Cenozoic Ice Sheet history from east Antarctic Wilkes Land continental margin
sediments. Global Planet. Change, 45, 1–3.
Exon, N.F., Kennett, J.P., Malone, M.J., et al., 2001. Proceedings of the Ocean Drilling Program. Initial Reports, 189, 1-170. ODP, College Station, TX, CD-ROM.
Hayes, D.E., Frakes, L.A., et al., 1975. Initial Reports of the Deep Sea Drilling Project, 28. US Government Printing Office, Washington. 1017 pp.
Mallinson, D.J., Flower, B., Hine, A.,
Brooks, G., Molina Garza, R., 2003. Paleoclimate implications of high latitude
precession- scale mineralogic fluctuations during early Oligocene Antarctic
glaciation. The Great Australian Bight record. Global and Planetary Change 39,
Naish, T.R., Woolfe, K.J., Barrett, P.J.,
Wilson, G.S., Atkins, C., Bohaty, S.M., Bucker, C., Claps, M., Davey, F.,
Dunbar, G., Dunn, A., Fielding, C.R., Florindo, F., Hannah M., Harwood, D.M.,
Watkins, D., Henrys, S., Krisseck, L., Lavelle, M., van der Meer, J. J.P.,
McIntosh, M.C., Niessen, F., Passchier, S., Powell, R., Roberts, A.P.,
Sagnotti, L., Scherer, R.P., Strong, C.P., Talarico, F., Verosub, K. L., Villa,
G., Webb, P-N., Wonik, T., 2001. Orbitally induced oscillations in the East
Antarctic ice sheet at the Oligocene/Miocene boundary. Nature 413:
O'Brien, P.E., Cooper, A.K., Richter, C., et al., 2001. Proceedings of the Ocean Drilling Program. Initial Reports, 188. ODP, College Station, TX, CD-ROM.
Powell, R.D. and Cooper, J.M., 2002. A sequence stratigraphic model for temperate, glaciated continental shelves. In Dowdeswell, J.A. and Cofaigh, C. (eds) Glacier-influenced sedimentation on high latitude continental margins: Ancient and modern. Geol. Soc. London, Special Pub., 203: 215-244.