Teaching Slides: Ice-sheet dynamics and the glacial sedimentary record
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10.2: Boulder-claystone (diamictite) with glacially scratched boulders of quartzite (inset) and dolomite in the Moonlight Valley Tillite, East Kimberleys, Western Australia. Sedimentology suggests the former existence of dynamic wet-base ice.

10.3: Prograded till “delta” with sandstone lenses in the Lower Mirbat Formation (Sturtian?), Mirbat inlier, southwestern Oman. Sedimentology suggests subglacial meltwater production.

10.4: Glacially-filled paleovalley from the older Cryogenian (Sturtian) glaciation in the Otavi Group, NW Namibia. Chuos Formation diamictite and sandstone thickens from 65 to 450 m in 2.5 km along strike at the expense of underlying strata (Ombombo Subgroup), which were tectonically rotated 1.5 during the glaciation.

10.5: Mean annual surface temperatures and sublimation rates in an AGCM simulation of a snowball earth with 750 Ma paleogeography (Donnadieu et al., 2003). Note extensive sublimation of sea ice, strongest in the subtropics, and comensurate accumulation of condensate in tropical and subtropical highlands.

10.6: Simulations using an AGCM for 750 Ma with coupled sea-ice and ice-sheet dynamics (Donnadieu et al., 2003): (A) with 900 ppm 2, land-based ice sheets build up but the ocean remains open; (B) with 300 ppm CO2, ocean is completely ice covered but land-based ice sheets continue to grow, covering most global land area by 400 kyr after snowball onset; (C) same as (B) showing basal temperature of land-based ice sheets; (D) same as (B) showing basal sliding velocities of land-based ice sheets. Note the narrow corridors of fast-flowing wet-base ice (ice streams) near the margins of the tropical ice sheets.

10.7: Dynamics of an idealized ice sheet, showing flowage from a central area of accumulation to a peripheral zone of ablation. Note vertical exaggeration.

10.8: Simulated basal sliding velocities for the present Antarctic Ice Sheet, displaying narrow corridors of fast-flowing wet-base ice (ice streams) within the cold-base peripheral zone. Up to 90% of the total ice-sheet drainage is thought to be routed through ice streams.

10.9: Bathymetry of the Barents Sea region of Arctic Europe, displaying transverse troughs with trough-mouth fans or moraines, inferred as having formed through the action of Quaternary ice streams (Ottesen et al., 2005). Troughs range from a few 100’s to a few 10’s of km in width, and 10’s of m in depth. They attest to the erosive potency of ice streams.

10.10: Seismic-reflection profile oriented perpendicular to the Skjoldryggen ice-stream terminal moraine on the western Norwegian margin (Ottesen et al., 2005).

10.11: Cryogenian carbonate platform (Otavi Group) exposed on the Great Western Escarpment of southern Africa in northern Namibia.

10.12: Geological map of the Otavi Group carbonate platform and foreslope of Cryogenian and early Ediacaran age (ca 780-580 Ma), northern Namibia. Red box encloses part of the southern foreslope known as the Fransfontein Ridge.

10.13: Stratigraphic restoration of the Otavi Group carbonate platform and foreslope, Namibia, and representative carbon isotopic records of the platformal succession (Halverson et al., 2005; Hoffman & Halverson, in press). Note the paired glacial and cap-carbonate formations, Chuos-Rasthof and Ghaub-Maieberg respectively.

10.14: Typical thin carbonate-clast diamictite of the Ghaub Formation (635 Ma) on the Otavi Group carbonate platform in Namibia. Diamictite paraconformably overlies shallow-marine carbonate of the Ombaatjie Formation (hammer head), and is conformably overlain by swaley-crossbedded, peloidal, cap dolostone of the Keilberg Member.

10.15: Measured columnar sections of the southern foreslope of the Otavi Group on the Fransfontein Ridge (see inset map and slide 10-12 for location). Line of section is 60 km long. The western 40 km is oriented subparallel to the foreslope-platform break (heavy dashed line on inset map); the eastern 20 km angles obliquely up the foreslope to the platform edge at the easternmost section. Columns are not fixed to a common datum but are adjusted interpretively to suggest sea-floor topography at the end of the younger Cryogenian glaciation (Ghaub Formation). Relief is arbitrarily suppressed at the eastern end of the section to fit the page (from Hoffman, in press, S. Afr. J. Geol.).

10.16: Same as last slide, but with red box indicating glacial marine sections (Ghaub Formation) illustrated in slides 10-16 through 10-23.

10.17: Measured sections 6-12 on the Fransfontein Ridge illustrating the stacking of grounding-line diamictites, separated by stratified proglacial deposits, and the interleaving of packages of lithologically-distinct diamictites. Note continuous upper member, composed of stratified proglacial deposits choked with ice-rafted debris (IRD).

10.18: On the southern foreslope of the Otavi Group, glacial marine deposits of the Ghaub Formation are generally underlain by a low-stand wedge (Franni-aus Member) of dolomitized rhythmite, turbidites and debris flows rich in variably-disaggregated coarse-grained oolite, related to falling base-level accompanying ice-sheet buildup at higher latitudes. Upper photo shows an unusually thick lower member of the Ghaub Formation on Bethanis farm (Camp Xaragu), comprised of terrigenous siltstone drift. IRD occurs in the transition to the massive diamictite.

10.19: Massive grounding-line diamictite composed of unsorted coarse and fine-grained limestone (grey-black) and dolostone (tan colored) debris.

10.20: Laminated “silt-stringer” with fine-grained IRD within a massive diamictite. Silt-stringers (ss) are interpreted to have settled from suspension in quiescent sub-glacial meltwater puddles.

10.21: (Left) Transition from proglacial sediment with IRD (lower member) into massive limestone-clast diamictite. (Right) Stratified proglacial tongue between massive diamictites, the lower of which is clast-size graded at the top. Current ripples in the proglacial unit indicate paleoflow parallel to paleoslope contours.

10.22: Climbing ripples with IRD in fine grainstone tongue between massive diamictites. Paleoflow is directed southward, down the inferred paleoslope.

10.23: Stratified proglacial detrital carbonate with IRD in the lower (below) and upper (above) members of the Ghaub Formation. Note deformation of impacted strata and draping by post-impact deposits. Dropstone in lower photo is an oolitic limestone from the Franni-aus Member; those in upper photo are stromatolitic dolostone (pale tan) from Ombaatjie Formation cycle b7 and limestone (dark grey) from cycle b8.

10.24: "Starved" ripple formed by westerly-directed (contour parallel) traction currents in the proglacial upper member of the Ghaub Formation. Gyre-like contour currents imply that waters south of the Otavi Group platform were not perpetually ice-covered.

10.25: Sub-glacial erosion surface (red line) beneath the Ghaub Formation in sections 1-27 and beneath the Keilberg post-glacial cap dolostone in sections 28-32, where glacials are absent.

10.26: Same as last slide but showing truncation by the sub-glacial erosion surface of underlying units: (blue line) base of low-stand wedge (Franni-aus Member), (purple line) base of terrigenous siltstone (Narachaams Member), and correlative horizons in sections 26-32. Note broad trough cut from underlying units by sub-glacial erosion between sections 13 and 26. Duurwater Trough is ~100 m deep (relative to underlying strata) and ~18 km wide (in the line of section). It is interpreted to be a slope-transverse erosional feature, possibly cut by a paleo-ice stream.

10.27: Same as last slide but indicating the extent of a major prism (outlined in red) of dolomitized grainstone interpreted as submarine channel and levee deposits, situated stratigraphically beneath the postulated ice-stream trough. The prism represents a major submarine drainage system, established well before the Ghaub glaciation apparently in response to a local subsidence anomaly.

10.28: Lateral variation in thickness of the Ghaub Formation glacial marine deposits (outlined in red): 40-135 m west of Duurwater Trough, absent on the upper foreslope (sections 28-32), and relatively thin within Duurwater Trough with the exception of a doubly-crested ridge of massive diamictite rising 600 m above the floor of the trough. The thickest diamictite sections (20 and 22) consist of lithologically-variable diamictites without silt-stringers or proglacial strata. The ridge is 7.5 km wide at its base in the line of section, giving it a minimum aspect ratio of 0.08 (0.6/7.5) assuming transverse orientation. It is interpreted to be a transverse medial moraine (Duurwater Moraine) possibly related to the paleo-ice stream that carved the trough in which it resides.

4.29: Former aragonite crystal fans (sea-floor cement) occur in marly limestone rhythmite directly above the post-glacial cap dolostone (Keilberg Member) exclusively on the crest and flanks of the Duurwater Moraine on Fransfontein Ridge (they occur elsewhere locally on the platform, particularly its outer edge). The crystal fans suggest that bottom waters were highly oversaturated with respect to CaCO3, and may be related to shoaling and vertical mixing associated with the sea-floor topography.

4.30: Terrigenous silt (with rare outsize quartz granules) in the lower member is the only significant terrigenous component in the Ghaub Formation glacial marine deposits. Its logical source is the lithologically identical siltstone (locally with quartz granules) of the Narachaams Member, which was exposed by the erosion of Duurwater Trough. It follows that the trough was eroded before the basal Ghaub Formation was deposited. If the trough was eroded by an ice stream at the glacial maximum (or maxima), then the Ghaub Formation consists entirely of recessional deposits. Conditions recorded by the Ghaub Formation do not represent glacial maximum conditions, and place no constraint on the extent or thickness of ice cover on the adjacent ocean during the glacial maximum.

10.31: Inferences from glacial sedimentology on Fransfontein Ridge. Background photo is from the ablative zone of the Antarctic Ice Sheet (photo courtesy of Steve Warren).

10.32: Thanks to Eugene Domack for glacial sedimentological insights and an open mind. He stands on the contact between Ghaub diamictite and the Keilberg cap dolostone, with his hands near the base of the plumb (tubular) stromatolite biostrome.