The Antarctic Ice Sheet: Neogene Glacial Record from the Sirius Group Sonia Tikoo Ge112...

The Antarctic Ice Sheet:Neogene Glacial Record from the

Sirius Group

Sonia Tikoo

Ge112 Geomorphology and Stratigraphy

November 27, 2006

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Tectonic

- Geomorphologic

- Fossilliferous

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Tectonic

- Geomorphologic

- Fossilliferous

Modern Antarctica

• continental glaciation• cold-based (friction of

ice-ground contact greater than shear strength ice)

• polar glacier• area 12.5x10^6 m^2

Radsat image of Antarctica: http://earthobservatory.nasa.gov/Study/RampingUp/

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Tectonic

- Geomorphologic

- Fossilliferous

Hambrey, et. al.

Hambrey, et. al.

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Tectonic

- Geomorphologic

- Fossilliferous

Background:• Sirius Group: collection of Neogene

(23-1.5 Ma) glaciogenic strata

Two theories:• (1) Deposits fairly old, landscape/glacial

setting remained stable for past 14 m.y.• (2) Glacier formed 35 Ma, but has

undergone major fluctuations until 2.5 Ma– Regime more temperate during fluctuations

Which is correct? Want to infer paleoclimate.

• Mahaney (1995) studied quartz grains that showed intensive corrosion and weathering features indicative of continental glaciation as opposed to mountain glacier modification (< 500m thickness)

• Passchier (2001) studied provenance of sirious group – showed deposition took place due to glacial denudation of Transantarctic Mountains over a long time period, not quickly.

• Ages disputed, but mainly show Pliocene/Miocene ages.

What we know:

Shackleton Glacier

• North-flowing

• Moves from East Antarctic Ice Sheet, through Transantarctic Mountains into Ross Ice Shelf

• 130 km long x (7-11) km wide, descents from 2400-200 masl.

Roberts Massif

• Nunatak (mountaintop not covered by land ice, protrudes from glacier)

• 10 km x 10 km

• Relief 2723-2000m

• Low area of northern part covered by Sirius Group diamict

Bennett Platform

• Similar nunatak to Roberts Massif, crowned by Mount Black

• Contains near-vertical cliff of diamict. Glacier surface at 1820 masl, top of cliff at 140 m above that.

• Reference for Sirius Stratigraphy from this area.

Sedimentological Approach:Hambrey, M.J., Webb, P.-N., Harwood, D.M.,

Krissek, L.A., 2003. Neogene glacial record from the Sirius Group of the Shackleton Glacier region, central Transantarctic Mountains, Antarctica. Geol. Soc. Amer. Bull. 115, 994–1015.

• Hambrey, et. al. conducted extensive sedimentological investigation.

• Sirius Group esp. well exposed near Shackleton Glacier.

• Intent to shed more light on provenance, when correlated with tectonic and geomorphologic studies.

Hambrey, et. al.

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Stratigraphic columns

- Facies associations

- Clast orientations (glacial motion)

- Tectonic

- Geomorphologic

- Fossilliferous

Hambrey, et. al.

Example of measured facies from Bennett Platform.Hambrey, et. al.

Example of measured facies from Bennett Platform.Hambrey, et. al.

Lithofacies (context):

Summary of 11 primary lithofacies in Shackleton Glacier and Bennett Platform Formations.

Hambrey, et. al.

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Stratigraphic columns

- Facies associations

- Clast orientations (glacial motion)

- Tectonic

- Geomorphologic

- Fossilliferous

Facies Associations

Three main facies can be inferred from the stratigraphic sections measured:

• Terrestrial glacial facies association

• Ice-proximal grounding-line fan association

• Ice-distal-bottomset association

Terrestrial Glacial Facies Association• Most of BP and SG

formations comprise massive and weakly stratified diamicts.

• Massive diamicts of BP interpreted as lodgement till, angular gravels as supraglacial melt-out.

• Upper BP implies deposition of glaciogenic sediment in terrestrial env.

Hambrey, et. al.

Grounding-Line Fan Association• Massive/weakly strat diamic

(ice-prox glaciolacustrine sed.)

• Laminite from background sedimentation from suspension, occasional dropstones

• Member 6 sits on top of northward prograding wedge of well-sorted sand and gravel: ice contact delta.

• Fluvial activity => “wet” glacial conditions.

• Member 6 of Shackleton Fm due to presence of:

Hambrey, et. al.

Ice-Distal Lake-Bottomset Association• Unit of laminite and diamict

underlying grounding-line fan association.

• Sand-mud couplets – lake varve succession.

• Accumulation on bottom of lake basin, dropstones this indicate ice-contact with lake.

• M5-6 bounded on both sides by lodgement till implies phase of glacial recession.

• Member 5 of Shackleton Fm due to presence of:

Hambrey, et. al.

Clast Orientation Fabrics

• Clast orientations coupled with lithofacies and grooves show SW-NE glacial sweep near Polar Plateau and SSE-NNW trend in Robert Massif Lowlands. Hambrey, et. al.

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Tectonic

- Geomorphologic

- Fossilliferous

Tectonics

• Both field and aerial photographic observations reveal a series of post-depositional normal faulting through the Sirius Group

Normal faulting

Hambrey, et. al.

Selected Evidence of Faulting:

• Uniform facies distributions irrespective of present topography, at wide range of topographic levels/elevations

• Faulting of Shackleton erosion surface and overlying Sirius Group

• Block-like terrain (steps in topography, etc.)• Larger fault scarps show unweathered rock

faces and fallen blocks, imply less weathered blocks emplaced more recently

Tectonic Reconstruction

• Normal faulting shows uplift ~500m (Hambrey et.al. state deposits uplifted >800m)

• Implies sediment was deposited at landscape much lower than that of today.

Normal faulting

Hambrey, et. al.

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Tectonic

- Geomorphologic

- Fossilliferous

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Tectonic

- Geomorphologic

- Environment determination

- Geomorphology reconstruction

- Fossilliferous

Correlations with Environment

Hambrey, et. al.

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Tectonic

- Geomorphologic

- Environment determination

- Geomorphology reconstruction

- Fossilliferous

Hambrey, et. al.

Hambrey, et. al.

Hambrey, et. al.

Hambrey, et. al.

Outline

Introduction to Antarctica

Background region of study

Approaches:

- Sedimentological

- Tectonic

- Geomorphologic

- Fossilliferous

Fossiliferous Data

In addition to sedimentological evidence of fluvial Sirius Group deposits discussed above (wet glacial conditions, warmer paleoenvironment):

Studies of recycled microfossils (diatoms) in the upper Pliocene-lower Pleistocene Sirius Group sediments along the Transantarctic Mountains also imply deglacial (warm) conditions during the Cenozoic.

(Webb 1996,1998, 1990) Stephanodiscus (diatom found in Quartz Hills Fm., Sirius Group). Image from microbes.limonology.wisc.edu

Fossiliferous Data (2)

• Fossilized leaves of Nothofagus contain fossilized root traces indicating they supported wood flora.

• Associated paleosols (Meyer Desert Formation, Sirius Group) date to 3.1-4.1 Ma, and also show evidence of two warming conditions ~3.5 Ma and 5 Ma, consistent with hypothesis of paleoclimatic fluctuations (Retallack, et. al 2001).

Nothofagus:

Southern Beech

(Wikipedia)

Conclusions

• From the sedimentological data, we see signs of two major glaciations corresponding to the Shackleton Glacier and Bennett Platform Formations.

• These two formations are separated by a formidable unconformity, and can divide the Sirius Group into two main successions here.

Conclusions

• Presence of diatoms and Nothofagus (silicified wood) in Sirius Group strata imply a significantly warmer regime than that of today.

• Post-depositional faulting of the Sirius Group and resulting uplift implies lower depositional environment.

Conclusions

• Transantarctic Mountains were lower, also during phases of more temperate climatic regimes. – Ice is wet-based, shear strength of ice stronger than

friction of frozen glacier-bed contact. – Makes for faster ice motion, allowing less time for

buildup of thickness.

• Resultingly, the ice sheet was thinner during the Neogene. Transantarctic mountains were not as much of ice flow barrier. Ice sheet does not have to be this thick today in order to have deposited these sediments -> Mtn. uplift and switch to cold-based glaciation led to that.

References:Hambrey, M.J., et. al. 2003. Neogene glacial record from the Sirius

Group of the Shackleton Glacier region, central Transantarctic Mountains, Antarctica. Geol. Soc. Amer. Bull. 115, 994–1015.

Passchier, S., 2001. Provenance of the Sirius Group and related Upper Cenozoic glacial deposits from the Transantarctic Mountains, Antarctica: Relation to landscape evolution and ice-sheet drainage. Sedimentary Geology. 65, 191-194.

Retallack, et. al., 2001. New grounds for reassessing palaeoclimate of the Sirius Group, Antarctica. Journal of the Geological Society, London. 158, 925-935.

Webb, P.N., 1990. The Cenozoic history of Antarctica and its global impact. Antarctic Science. 2(1), 3-21.

Wilson G.S., et. al., 1998. Late Neogene Sirius Group strata in Reedy Valley, Antarctica: a multiple-resolution record of climate, ice-sheet and sea-level events.