Latest Cenozoic foraminifera from the 154-0 m interval of the DVDP 8, 9, and 10 drillholes, Taylor Valley, Antarctica
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Date
2003-08
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The Ohio State University
Abstract
Benthic foraminifera have been examined from the upper 154 meters of a 186 m
thick Pliocene-Holocene glacial succession recovered in the DVDP 8, 9, and 10 drillholes
of eastern Taylor Valley, Transantarctic Mountains, Antarctica. The principal objectives
of the investigation center on resolving the stratigraphic position of the Pliocene-Pleistocene
boundary, establishing the full extent of the Pleistocene succession, and
evaluating the potential value of these sediments for refining latest Cenozoic
chronostratigraphy. The upper 154 meters of the succession were subdivided into
twenty-four lithostratigraphic subunits by McKelvey (1981). Subunits are expressions of
discrete lithofacies packages such as diamictite, conglomerate, breccia, sands, and
mudstone. A well-defined disconfonnity at a drillhole depth of ~154 m (disconformity
H3) separates the mid-Pliocene sediments below from the younger succession above.
The latter is investigated here. Numerous disconformities of unknown duration, probably
produced during repeated phases of glacier and ice sheet advance and grounding, occur
within the younger succession. They also appear to be associated with repeated phases of
erosion and re-transportation of sediments and their in situ fossil material. This upper
sedimentary succession provides, therefore, only a partial and reprinted geological
record.
In situ benthic foraminiferal assemblages have been documented within the
eastern Taylor Paleofjord at its juncture with the western margins of McMurdo Sound and the southwestern Ross Sea at ~140 m, ~125m, ~99-90 m, and 25-0 m. The
assemblages are interpreted as being deposited during marine deglacial events, such as
receding tidewater glaciers, ice shelves, and possibly open marine waters with icebergs.
Assemblage content in the lower three of the four intervals cited above is identical to
assemblages in the Holocene (4-5 kyr bp) sediments between 25-0 min DVDP 8, 9, and
10, mid Pleistocene (~1 Ma) assemblages in the CRP-1 drillhole on submarine Roberts
Ridge (western McMurdo Sound), and living assemblages in New Harbor and other
coastal regions of McMurdo Sound. All four DVDP drillhole foraminiferal assemblages
identified above are significantly different from the Pliocene and late Miocene
assemblages that occur below the H3 disconformity in both the DVDP 10 and 11
drillholes of Taylor Valley (Taylor Paleofjord). This implies that the succession above
~140 m, and perhaps that above H3 at ~154 m, are Pleistocene-Holocene in age. Published magnetostratigraphic data has been interpreted so as to date the upper
154 m of DVDP 10 as part of the Gauss (<3 Ma, late Pliocene), as well as the Matuyama
(late Pliocene-Pleistocene), and Brunhes (Pleistocene-Holocene) polarity zones; with the
Pliocene-Pleistocene boundary placed at a drillhole depth of ~71 m. Pliocene diatoms
and Pliocene- recent silicoflagellates and radiolarians have been reported from the DVDP
154-0 m interval and would appear to support age determinations derived from
magnetostratigraphical data. Benthic foraminifera data presented in this study do not
support this chronostratigraphical interpretation. Pliocene benthic foraminifera also occur
in the DVDP 154-0 m interval and in some instances co-occur with apparent PleistoceneHolocene
foraminiferal assemblages discussed herein. This is ample evidence to support mixing among fossil assemblages above 154 m. It follows that Pliocene and possibly
upper Miocene sediments and their fossil contents were eroded from older successions
during the cutting of the H3 disconformity. This process is most adequately explained by
grounded glacier ice or ice shelves transporting the sediments and microfossils into the
Pleistocene-Holocene sediments during their re-deposition. This hypothesis gains support
from the fact that Holocene and Pleistocene sediments across the floor of the Ross Sea
also contain abundant and identical recycled Pliocene and Miocene fossil material.
Because recycled volcanic material of the McMurdo Group (upper Miocene to
modem) is distributed throughout the 154-0 m interval, 40 Ar/39 Ar radiometric dating of
clasts might provide an independent method by which to test the validity of the
magnetostratigraphical data. Radiometric dating would also assist in identifying those
parts of the upper succession that are younger than the Pliocene-Pleistocene boundary
(i.e., <~1.8 Ma).