Teaching Slides: Timing and extent of Proterozoic glaciation

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1.2: Ice extent at Present and at the Last Glacial Maximum, and the secular variation in oxygen isotopes (δ18O) of bottom-dwelling foraminifera (proxy for the global volume of non-floating ice) over the last 450 kyr (modified from Broecker, 1985).

1.3: Paleogeographic extent of continental ice sheets and permanent sea ice over the last 800 million years. Red lines indicate major mass extinctions.

1.4: Snowball Earth episodes correlate broadly with major turning points in the chemical history of seawater and the evolution of life.

1.5: Rise in atmospheric oxygen over geological time and its relationship to ice ages (blue bars) and biological innovations.

1.6: Histogram of papers relating to Precambrian glacials and glaciation by year of publication (445 papers) from 1930 to 2005.

1.7: Stratigraphic setting of the three glacial and glacial-marine formations in the Paleoproterozoic (2450-2220 Ma) Huronian Supergroup, north shore of Lake Huron, Ontario, Canada.

1.8: Present global distribution of older (“Sturtian”) Cryogenian (ca 720-700 Ma) glacial and glacial-marine deposits. Red dots indicate deposits that include banded iron or iron-manganese formations (BIF).

1.9: Present global distribution of younger (“Marinoan”) Cryogenian (ca 660-635 Ma) glacial and glacial-marine deposits. Red dots indicate deposits that include banded iron or iron-manganese formations (BIF).

1.10: Present global distribution of Ediacaran (635-542 Ma) glacial and glacial-marine deposits.

1.11: Present global distribution of Sturtian, Marinoan and Ediacaran glacial and glacial-marine deposits (left), and secular variation in the carbon isotopic composition (δ13C) of marine carbonates (right) from 860 to 490 Ma (modified after Halverson et al., 2005) and their relation to the three glacial epochs and early faunal diversification (after Knoll and Carroll, 1999). Note the generally elevated δ13C values before the Marinoan glaciation and major negative δ13C anomalies informally named after formations in which they were originally documented: B (Bitter Springs, Australia), M (Maieberg, Namibia), R (Rasthof, Namibia), S (Shuram, Oman), and T (Trezona, Australia). The R, T and M anomalies suggest that the Sturtian and Marinoan glaciations were globally synchronous.

1.12: The Australian Antarctic explorer and geologist Sir Douglas Mawson (1882-1958) was an early advocate of global Neoproterozoic glaciation, based on the mistaken belief that continents were fixed in location, and therefore that glacial deposits now near the equator originated at low latitude.

1.13: The English geologist and stratigrapher Brian W. Harland (1917-2004) marshaled evidence for a “great infra-Cambrian glaciation” in the context of continental drift. He argued for the existence of low-latitude ice sheets on the basis of climate-dependent sedimentological indicators and primitive paleomagnetic data.

1.14: This outcrop near Biggenjarga on the Varanger Peninsula of northern Norway was interpreted as a sub-glacial pavement, grooved by ice movement (red arrow), overlain by boulder-claystone (tillite) of glacial origin by Hans Reusch of the Geological Survey of Norway in 1891. The tillite belongs to the Smalfjord Formation, which is tentatively correlated with the younger Cryogenian (Marinoan) glaciation, and was apparently deposited at a middle latitude. The hammer handle (circled) is 33 cm long.

1.15: A useful criterion for glacial action is the presence in tillites (diamictites) of pebbles that were faceted and multiply-scratched by entrainment at the base of a moving glacier or ice sheet. This example is from the Jbéliat Formation in the Taoudeni Basin in Mauritania (West Africa), which is tentatively correlated with the younger Cryogenian (Marinoan) glaciation.

1.16: A Neoproterozoic carbonate platform (Otavi Group) containing both Cryogenian glacial horizons is spectacularly exposed on the Great Western Escarpment of southern Africa in Namibia. The escarpment rises from the hyper-arid coastal sand sea (Namib Desert).

1.17: Bedrock tectonic elements of central and northwestern Namibia. The Damara orogen separates the Kalahari paleocontinent to the south and the Congo paleocontinent to the north, and their respective continental-margin successions exposed in the Witvlei and Otavi fold belts.

1.18: Geological map of the Otavi fold belt showing the Neoproterozoic carbonate-dominated succession (Otavi Group) and the foreslope-platform facies change.

1.19: Composite stratigraphically-restored cross-section of the western Otavi fold belt showing relations between the platform and southern foreslope. Representative carbon isotope (δ13C) records are shown for Otavi Group platformal carbonates.

1.20: Stratigraphic architecture, generalized sedimentary facies, U-Pb ages of volcanic zircons and composite carbon isotope profile for the Otavi Group, projected onto a north-south cross-section of the carbonate platform and its southern foreslope. Note rift-drift transition at the base of the Ombaatjie Formation. The 635-Ma Ghaub glaciation occurred after the rift-drift transition and the older Chuos glaciation before it.

1.21: Marine tillite (diamictite) from the younger Cryogenian glaciation (Ghaub Formation) in the Otavi Group, Namibia. Both the large clasts and fine-grained matrix are composed entirely of detrital carbonate, dolostone (tan color) or limestone (grey-black). The tillite-bearing Ghaub Formation has been mapped for >600 km along the outer arc of the Otavi fold belt.

1.22: Stratified proglacial carbonate with ice-rafted debris (dolostone and limestone) near the top of the Ghaub Formation on the southern foreslope. Of the Otavi Group, Namibia.

1.23: Ice-rafted “dropstone” within stratified proglacial carbonate near the top of the Ghaub Formation on the southern foreslope of the Otavi Group, Namibia. Diameter of all coins is 2 cm.

1.23: Ice-rafted dropstone (oolitic limestone) within stratified proglacial carbonate near the base of the Ghaub Formation on the southern foreslope of the Otavi Group, Namibia.

1.25: Representative glacial marine sediments on the southern foreslope of the Otavi Group, Namibia.

1.26: Nature of host strata (carbonate, mixed, or terrigenous) for Neoproterozoic glacial deposits (inverted triangles) in different regions.

1.27: Theoretical distribution of carbonates and glacial deposits on earth-like planets with high and low orbital obliquities. Obliquities >54 degrees result in a reversed meridional temperature gradient (i.e., poles warmer than the equator).