Early Glacial Late Oligocene to Early Miocene

This period includes seismic sequences RSS-2 and -3 (Fig. RS-3 and Foldouts RS-1 and RS-2). Sedimentary rocks from this period were recovered at CIROS-1, and CRP-1, -2 and -3, and MSSTS-1 drilling sites in the McMurdo Sound area. Such rocks include compacted diamicton indicative of deposition by/under grounded ice, as well as mud and ice-rafted debris (IRD) indicative of open-water environments (Barrett, 1986; Barrett, 1989; Hannah, 1994; Cape Roberts Science Team, 2001), in lower Miocene sediments at CRP 2/2A sites. Compacted diamicton and mud layers at site CRP-1, vary with uniform cyclicity, and document systematic oscillation of the EAIS size (Naish et al.. 2001). The oscillations are at orbital periodicities similar to those recorded by isotope studies in distal deep-ocean sediments.

Seismic facies along the border of the Victoria Land basin suggest that tidewater glaciers all along the Transantarctic Mountains intermittently extended onto the continental shelf and carried abundant glacial sediment to the sea (Brancolini et al., 1995; Bartek et al., 1996; Henrys et al., 2001).

In the eastern Ross Sea, at DSDP Site 270, Nothofagus-dominated flora in lower Miocene sediments (Kemp and Barrett, 1975) are similar to those recovered in McMurdo Sound drillcores (Hill, 1989; Mildenhall, 1989; Askin and Raine, 2000), and indicate cool-temperate climates during interglacial periods. DSDP Site 270 also recovered lower Miocene ice-proximal glaciomarine sediments from the early Miocene section, but the size and character of the ice sheet that deposited these sediments is debated. Anderson and Bartek (1992) suggest, based on high-resolution singlechannel seismic data and drill cores, that by late Oligocene to early Miocene time, the continental shelf was deeply scoured and foredeepened (i.e. landward dipping) by a massive ice sheet. In contrast, Brancolini et al. (1995) and De Santis et al. (1995), utilize regional stratigraphic maps (Cooper et al., 1995) and their seismic facies analyses to postulate that, during the same period the Central high was partly exposed and partly covered by small subpolar ice caps (i.e. subpolar as defined by Anderson and Ashley, 1991). A semi-quantitative evaluation of the water depth of the Eastern basin during the early Miocene, based on the backstripping of the seismic section in Foldout RS-1, indicates that the foredeepeened profile of the Eastern basin was only attained after middle Miocene time (De Santis et al., 1999).

The end of this early glacial period is marked by a change in reflection geometries beneath the outer continental shelf (Foldouts RS-1 and RS-2) from principally aggrading (RSS-2 and -3) to principally prograding, (RSS-4). Cooper et al. (1991b) postulate that this change marks the start of grounded-glacier advances to the shelf edge and erosion of a normal-water-depth shelf by episodic grounded ice.

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