|
Distribution of heavy minerals
in Cenozoic sediments of McMurdo Sound, Antarctica: Reconstruction of source
areas and fluctuations of the ice shield
Dipl.-Geol. Kerstin Polozek, Prof. Dr. Werner
Ehrmann
|
Scientific Background:
| During various drilling campaigns, a series
of sediment cores has been recovered along the margin of the Transantarctic
Mountains in the McMurdo Sound area (Ross Sea, Pacific sector of the Southern
Ocean). The sediments are early Oligocene to Quaternary in age. Despite
of intense investigations, little has been known so far on the source area
of the sediments and on the Cenozoic ice dynamics. |
Scientific Objectivs:
| This study encompassed an investigation of the
heavy mineral composition of early Oligocene to Quaternary glacial and
glaciomarine sediment sequences from the Victoria Land Basin and the fjords
of the South Victoria Land, Ross Sea, Antarctica. The main objectives were
to identify possible source rocks and to unravel the provenance regions
of different heavy minerals and minerals groups. Moreover, with the available
data an attempt to reconstruct the general development and configuration
of the Antarctic ice sheets during the Cenozoic was also made. |
Fig. 1: Map of the McMurdo
Sound of the Ross Sea with locations of the investigated drill cores. All
sediment cores except DVDP-12 were drilled from a sea ice platform. Bedrock
geology is simplified after Warren (1969). |
Results:
| Two quite different source regions of the heavy
minerals have been identified. One provenance area, the South Victoria
Land of the Transantarctic Mountains, consisting of rocks from the Skelton
Group, was the main source of the Oligocene-Pliocene sediment sequence.
In most cases, significant heavy mineral abundance fluctuations within
this major association can be related to advances and retreats of glaciers
from the Transantarctic Mountains. A quite different source area is the
McMurdo Volcanic Group in the region of the present-day Ross Ice Shelf.
This volcanic area was the major source of the heavy minerals in the Pliocene-Quaternary
sediment sequence.
The heavy mineral assemblages of the sediment
sequences studied reflect the erosional work done by the glaciers in the
hinterland. The data support that no major change of the glacier catchment
areas has taken place since the Oligocene. The rock debris that were eroded
and transported toward the sea by the Taylor, Ferrar, and Mackay Glacier,
show heavy minerals assemblages that are typical for the catchment areas
of the respective glaciers. Since the Lower Oligocene, the Transantarctic
Mountains glaciers slid in several phases in the direction of the McMurdo
Sound in the Ross Sea. Later, in the Pliocene, grounding ice masses from
the region of the present day Ross Ice Shelf advanced from the sea into
fjords of the Transantarctic Mountains.
The heavy mineral data support that Cenozoic
volcanism in the McMurdo Sound (McMurdo Volcanic Group) began as early
as 27 Ma, which is somewhat older than hitherto supposed. The oldest known
volcanic rocks of the McMurdo Volcanic Group in the Victoria Land are dated
to about 25 Ma. As magnetic anomalies in the McMurdo Sound support the
occurrence of volcanic rocks in that area, a likely candidate for the early
volcanic detritus observed in the cores might be found there.
The sediment cores DVDP-12 (Taylor Valley) and
CIROS-2 (Ferrar Fjord) can be well correlated using their heavy mineral
spectra and abundance patterns. In both cores, the heavy mineral assemblages
in the older Pliocene sediments support a source region in the Transantarctic
Mountains. In the Upper Pliocene to Pleistocene sequences, the assemblages
indicate an additional source from Ross Sea ice masses originating in West
Antarctica. Using the heavy mineral spectra as a correlation tool, it has
been possible to place the hitherto insufficiently dated core DVDP-12 within
a more sharply defined stratigraphic context. |
Fig. 2: Distribution
of the main heavy mineral groups in the drill core CIROS-2 (Ehrmann &
Polozek, 1999). The heavy minerals document different source areas and
therewith major changes in the ice dynamics. During both subunit 1.1 and
1.2 the ice discharged through the Transantarctic Mountains into McMurdo
Sound. Characteristic heavy minerals are apatite, zircon, titanite, garnet,
epidote and green hornblende. Grains of subglacially erupted volcanic rocks
(brown hornblende and palagonit) are present in subunit 1.1, but missing
in subunit 1.2. In the Quaternary, the source area shifted to the south,
to the region of the present-day Ross Ice Shelf. The sediments are characterized
by pyroxene, titanaugite, altered minerals and opaque minerals. |
| Publications:
Ehrmann, W. & Polozek, K. (1999): The heavy
mineral record in the Pliocene to Quaternary sediments of the CIROS-2 drill
core, McMurdo Sound, Antarctica. - Sedimentary Geology, 128: 223-244.
Polozek, K. (2000): Distribution of heavy minerals
in CRP-2/2A, Victoria Land Basin, Antarctica. - Terra Antartica, 7 (4):
567-573.
Polozek, K. (2002): Die Schwermineralverteilung
känozoischer Sedimente des McMurdo-Sunds, Ross-Meer, Antarktis: Rekonstruktion
von Liefergebieten und Eisschildfluktuationen. ? Dissertation, Universität
Leipzig, 120 S.
Polozek, K. & Ehrmann, W. (1998): Distribution
of heavy minerals in CRP-1. - Terra Antartica, 5 (3): 633-638. |
|
