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.