Ice Sheet Development Oligocene Miocene

The older glacial section of the shelf comprises tabular units that pinch out shoreward due to inner-shelf erosion, and that extend seaward into prograding slope deposits (Cooper et al., 1991a). Palaeo-shelf edges for these units are better defined up-section as foreset strata steepen seaward (Fig. PB-2, Foldout PB-1). Shelf drilling (ODP Sites 739, 740, 741, 742 and 1166) recovered probable subglacial and glacimarine diamicts, with thin interbedded diatomaceous mudstones deposited during warm episodes (Hambrey et al., 1991; Erohina et al., 2004). The drilling and seismic evidence indicates glacial advance well across the Prydz Bay shelf during cold episodes, probably reaching the shelf edge. Over-compacted horizons indicate periods of glacial erosion and ice loading during the early Oligocene, Miocene and Plio-Pleistocene (Solheim et al., 1991; Shipboard Scientific Party, 2001a). Before the late Miocene, the Prydz Bay shelf prograded uniformly across its width, with the bulk of the ice and entrained sediment coming from the southern end of the bay (i.e. from the Lambert Graben). The Prydz Bay continental slope became progressively steeper from the early phase of glaciation in early Oligocene time, to reach angles of as much as 8° on the present slope (Foldouts PB-1 and PB-2).

On the continental rise, a pre-ice-sheet unit is overlain by one exhibiting channel-levee geometries. The nature of the change in geometry and the tracing of reflectors to the shelf drilling suggest that this change originated from the glacial expansion and increased sediment supply in the early Oligocene (Kuvaas and Leitchenkov, 1992). Overlying the channel-levee complexes are sequences that include thick mounds and sediment waves suggestive of contourite deposition, in addition to turbidite channels and associated levees formed by intensified down-slope and along-slope currents in the early Miocene (Fig. PB-2, Foldout PB-2).

ODP Site 1165 (Leg 188) drilled 999 m with 69% recovery into a thick mound of lower Miocene and younger contourite sediments with turbidites only in the upper 5 m (Cooper and O'Brien, 2004). The hole penetrated the base of the mounded sequences, which was still of early Miocene age (Handwerger et al., 2004). The drilling confirmed the seismic interpretation that deposition of the thick contourite mounds had commenced by at least early Miocene time, but sediments above and below the surface were typical contourites - fissile claystones with abundant silt laminae (Handwerger et al., 2004). Therefore, there was no obvious lithological change in the hole to suggest a reason for the change from low relief submarine fans to highly mounded deposits, previously inferred to be mixed turbidite-contourites.

ODP Site 1165 intersected a surface that can be mapped along the rise, and that marks a middle Miocene (14-16 Ma) change in sedimentation from laminated contourites to hemipelagic and pelagic facies (Cooper and O'Brien, 2004). Also, minerals and fossils recycled from shelf deposits first appear, suggesting the start of intense erosion by ice and overdeepening of the shelf. At this time, sedimentation rates slow more rapidly at the drill site, falling from 100m/m.y. in the early Miocene to 37m/m.y. in the mid-Miocene to 10m/m.y. during the Plio-Pleistocene (Shipboard Scientific Party, 2001c; Florindo et al., 2003; Fig. PB-4).

On shorter time scales, Griitzner et al. (2003) examine the proportions of terrigenous sediment and biogenic opal in ODP Site 1165 between 3.4 and 7.6 Ma. They find high opal content from 5.8 to 5.2 Ma, which they relate to reduced sea ice and increased productivity. They also identify terrigenous intervals with high sedimentation rates from 7.2-6.6 Ma and 5.2-4.8 Ma, which they interpret as indicating high erosion rates and a fluctuating ice sheet under the influence of obliquity forcing. Grutzner et al. (2003) also report cyclic variations in sediment composition and physical properties that have

Figure PB-4: Age-depth model for ODP Site 1165 from Shipboard Scientific Party (2001c) showing rapid sedimentation during the early Miocene, reducing rapidly through the late Miocene to Pliocene.

spectral peaks at ~94, 41, 31, 21, and 18 k.y. cycles. Williams and Handwerger (2005) report that geophysical log parameters detect cycles of biogenic and terrigenous input at periods of ~15-23 and ~ 135 k.y., probably representing Milankovich-scale forcing of paleoenvironmental processes. Uncertainty in the age model of the hole prevents them from exact matching of peaks.

Was this article helpful?

0 0

Post a comment