Long and Short Period Transitions in the Geologic Record

The seismic stratigraphic record provides a regional framework that illustrates distinct changes in the morphology of the Antarctic margin over the last 60 m.y. Drill cores provide the direct 'ground truth' geologic record of both long-period (m.y.) and short-period (k.y.) transitions (Table D-2). Some of the transitions in drill cores are reflected in the seismic stratigraphic framework and others are not. All transitions, however, are important in deciphering Antarctic paleoenvironments. In this discussion, we focus on lithostratigraphic changes in drill cores, and leave the discussion of isotopic, biostratigraphic and other relevant variations to authors of other chapters of this book. Our intent is to use the proximal lithostratigraphic drilling record from the continental margin to independently evaluate paleoenvironmental history, where possible.

Drill cores from two segments of the continental shelf provide a general lithostratigraphic framework for the long-period systematic transition from non-glacial (Mesozoic) to fluctuating glacial and interglacial (late Cenozoic) paleo-depositional environments. At the mid-shelf of Prydz Bay, cores document the changes from subaerial non-glacial (Cretaceous) to fluvial/ lagoonal early glacial (late Eocene) to shallow marine early glacial (early Oligocene) to subglacial deep shelf (early Pliocene) to interglacial open-marine (Holocene) conditions. A similar transition is documented in the Ross Sea (McMurdo Sound), from subaerial non-glacial (Mesozoic) to shallow marine early glacial (early Oligocene) to fluctuating subglacial and marine glacial (early Miocene to Holocene) to interglacial open-marine (Holocene) environments. Large hiatuses exist in the shelf cores.

Improved resolution of the continuity and timing of changes is seen in drill cores from the continental rise, where geologic continuity and core recovery

Table D2-2: Lithostratigraphic transitions in the geologic records from drill cores from the antarctic margin (listed by duration and decreasing inferred age).

Feature

Regions

Processa

Timing

Long-period changes (m.y.)

On the shelf: up-section lithologic changes from alluvial to fluvial to shallow marine to marine glacial to subglacial On the rise, sedimentation rates of hemipelagic sediments decrease smoothly while on the slope, sedimentation rates increase stepwise (i.e. in distinct stratigraphic units) On the rise, up-section increases in IRD, diatom content; shift kaolinite and glauconite

Short-period changes (k.y.)

On the inner shelf: cyclic changes from diamict (glacial) to glacial marine (interglacial) facies (in early Miocene at Milankovitch frequencies) On the rise: cyclic changes PB, AP from terrigenous (glacial) to biogenic (interglacial) facies at Milankovitch (PB) and similar variable (AP) frequency On the slope: shift in glacial sediment clast type: sandstone to granite

Subsiding graben/ shelf with increasing ice to the shelf

Likely decrease in onshore sediment supply coincident with a shift in deposition from the rise to the slope

Erosion of shelf basins by grounded ice sheets

Glaciers fluctuating onto and off the shelf during glacial and interglacial times

Glaciers fluctuating onto and off the shelf during glacial and interglacial times

Likely change in ice source area: offshore to onshore?

Cretaceous to Pliocene early Miocene to early Pliocene middle Miocene (~ 17-14m.y.)

early Oligocene to middle Miocene early Miocene to early Pliocene

Between 1.1 Ma and 780 k.y.

aThe principal process is listed.

are greater than from the shelf or slope. On the rise, there is a distinct up-section change in seismic character from well layered below to channel-levy development above (all areas) that is widely inferred due to a large influx of sediment when onshore ice sheets initiated in late Eocene to early Oligocene time. Here, the pre-ice to glacial transition has not yet been sampled by drilling. Yet, higher in the stratigraphic section of the rise, drill cores show a clear long-term parabolic decrease in the sedimentation rates within sediment drift deposits from the early Miocene (PB: 10-fold decrease) and the late Miocene (AP: 6-fold decrease) to the present. The large decreases occurred when the PSEs were prograding over distances of several tens of kilometres and aggrading up to several hundred metres, although the detailed timing of the prograding and aggrading is unknown. Regardless, the distinct changes in geometries of seismic sequences beneath the outer paleoshelves in PB and AP are not seen as abrupt changes in sediment deposition rates on the rise. Drilling on the slope (WS) shows large incremental increases in sedimentation rates for this same general period (i.e. early Miocene to early Pliocene), indicating that sediment coming from the shelf may not have reached the rise. The decreases in sedimentation rates on the rise may also reflect a decrease in the amount of sediment being eroded from onshore and shelf areas.

A notable long-term transition occurs in middle Miocene sediments (17-14 Ma) from the rise (PB). Up-section increases in IRD, diatom content, and recycled organic matter, along with changes in the types of clay and the first appearance of glauconite, point to greater ice nearby and initial erosion of shelf sedimentary basins. Evidence for strong erosion on the shelf is also seen in truncated foreset strata beneath the outer PB paleo-shelf. RS shelf drill cores are marked by a long hiatus, from mid- to late-Miocene. The hiatus and truncated shelf reflectors point to an increase in shelf erosion and overdeepening in the RS, similar to the erosion recorded on the rise at PB.

Short-period fluctuations in shelf and rise drill cores provide the strongest evidence that erosion onshore and on the shelf by fluctuating grounded ice sheets was the mechanism for sediment supply and distribution by glacial processes, as inferred from seismic-reflection data. On the RS shelf, cyclic fluctuations in glacial diamict and interglacial glaciomarine lithofacies at Milankovitch frequencies are documented for lower Miocene nearshore facies at the front of the Transantarctic Mountains, and resulted from ice advancing onto and retreating off the shelf at this time (Naish et al., 2001). On the rise (PB and AP), alternating dark- and light-coloured lithofacies with varying amounts of terrigeneous (dark) and biogenic (light) components are described throughout the lower Miocene to lower Pliocene intervals from visual observation of cores, downhole logging and physical properties measurements (PB); similar compositions variations are described from upper Miocene to Pliocene intervals in the AP. The facies are inferred to be of glacial and interglacial origin, respectively, and occur at Milankovitch frequencies in PB and similar order-of-magnitude frequencies in AP. Hence, drilling on the rise in East Antarctica and in West Antarctica has provided similar geologic evidence for fluctuating ice sheets on the shelves during the period of principal shelf progradation and aggradation - from the early Miocene to the early Pliocene.

The geologic transition in the late Miocene to the early Pliocene is the initiation of broad and narrow shelf troughs and widespread banks, lobes and upper-slope fans along the Antarctic margin (all areas); this transition is difficult to see in drill cores, but has been imaged seismically. On the rise (AP, PB), sedimentation rates decrease uniformly during this period and lithologies (e.g. clays, IRD, etc.) do not show systematic long-term changes. However, seismic geometries beneath the adjacent continental shelves show abrupt changes to rapidly prograding sections (S2/S3 beneath AP; PP-12 beneath PB). Elsewhere, drilling information is insufficient to explain why large geomorphic changes on the shelf, probably due to changes in glacial regime, are not reflected in the rates or types of sediment delivered to the rise.

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