Clast data from CRP cores (Victoria Land Basin): Implications on the uplift history of the Transantarctic Mountains

The international Cape Roberts Project drilled three holes (CRP-1, 2/2A, 3), distributed along a SE-NW trending traverse at 16-8 km from Cape Roberts in the McMurdo Sound (southern Ross Sea, Antarctica). The 3 holes comprehensively recovered an almost continuous c. 1600 m thick section of Cenozoic glacio-marine sediments at the western margin of the Victoria Land Basin (Fig. 1). At the deepest (939 metre below the sea floor, mbsf) borehole (CRP-3), the Cenozoic sediments were found to rest unconformably on Devonian age arenites of the Beacon Supergroup, indicating a total post-Jurassic offset of the CRP block of c. 3000 m with respect to the adjacent Transantarctic Mountains block south of Mackay Glacier. Clasts (granule- to boulder-grade) are a significant component throughout most of the recovered sediments, which range in lithology from diamictite/sandstone/argillite alternated sequences to conglomerate-bearing sandstone units (confined to the section below 200 mbsf in CRP-3). During all three field drilling seasons, petrological investigations on the coarser-grade clast fractions were focused on the preliminary petrographical characterization of the main litologies and their distribution and abundance throughout the core (Cape Roberts Science Team, 1998, 1999, 2000). Subsequent laboratory analysis were then carried out in order to refine the initial report dataset and highligt some preliminary interpretations about provenance and meaning of detected distribution patterns (Talarico & Sandroni, 1998; Smellie et al., 1999; Talarico et al., 2000; Brink et al., 2000; Sandroni & Talarico, subm.). Similarly to previous drillholes (MSSTS-1, CIROS-1) in the McMurdo Sound, CRP clast lithologies were found to closely reflect the lithological composition of the major onshore geological units (granitoid and metamorphic basement, Beacon Sandstone, Ferrar dolerite, Kirkpatrick basalt, McMurdo Volcanics), thus providing a sound indication of a local provenance. A number of evidence lines based on modal and petrological investigations on granitoid basement clasts (mainly biotite±hornblende, foliated or undeformed monzogranites and ubiquitously distributed) consistently support a supply from areas of the Transantartic Mountain block located to the west and south-west of the drillsites. Nevertheless the scattered occurrence of rare phyllites, apparently confined to the lower part of the CRP-3 section (below 200 mbsf), might provide some hints for a distant (>200 km) provenance from the south (Skelton Glacier-Koettlitz Glacier region). The overall distribution patterns for coarse clasts in the three CRP drillholes provide a clear evidence of an evolving provenance which can be at least partly related to the uplift/erosion history of the on shore TAM blocks west and south of the drillsites. Several erosion phases can be distinguished and a major compositional discontinuity identified at c. 307 mbsf in CRP-2/2A, with granitoid-dominated clasts (plus McMurdo Volcanic Group clasts) above and mainly Ferrar Group lithologies (dolerite, basalt) below. The tectonic meaning of this and other petrological discontinuities detected in CRP cores will be discussed in order to assess their bearing in providing further constrains on the timing and style (gradual, short-lived rapid?) uplift history of the Transantarctic Mountains in Southern Victoria Land.