Overview of relationship between glacio-epochs and times of enhanced crustal extension during the Proterozoic and Phanerozoic.

1. Neoproterozoic glacial epochs – Snowball
Earth, or limited glaciation?
Tek Jung Mahat 7 November 2017
Department of Geography

2. MA = a million years
(Megayear) ago
GA = a billion years
(Gigayear) ago)
The geological
clock: a
projection of
Earth's 4,5 Ga
history on a
clock

3. Glacio-epochs
Schematic representation of glacio-epochs in Earth history and their
relationship to phases of supercontinent assembly and break up
(Tectonic influences on long-term climate change: geotectonic
setting of Archean, Proterozoic and Phanerozoic glaciations)

4. Archean glacio-epochs (c. 4–2.5 Ga)
There are fundamental uncertainties regarding Archean climates because of the dearth of
sedimentary deposits and climate modelling yields very different, opposed perspectives.
Glaciation is recorded at about 2.9 and 2.8 Ga but is restricted to southern Africa. The
geodynamic setting indicates a passive margin setting. A systematic search is needed for new
deposits in other basins.
Paleoproterozoic glacio-epoch (c. 2.4 Ga)
The well-developed relationship between Paleoproterozic rifting and glacial deposits suggests
either a causal relationship between rift-related uplift and climatic cooling or selective
preservation of glaciated rift deposits. Deposits are dominantly submarine debrites associated
with thick turbidites. They are part of very thick marine tectono-stratigraphic successions, often
associated with volcanics, recording the changing interplay of subsidence rates and sediment
supply as rifting progresses. An association between glacials and banded iron formations may
reflect deposition in semi-enclosed basins with incipient spreading centres.
Paleoproterozoic to Mesoproterozoic non-glacial interval (c. 2.3–0.75 Ga)
The absence of any extensive glacial record during the long Paleoproterozoic–Mesoproterozoic
interval between about 2.3 Ga and 750 Ma represents a large gap in Earth's glacial history.
Glaciation should have been a common phenomenon given the known formation of several
large landmasses and the orographic effects of associated high standing orogens, but this does
not appear to be the case other than briefly and locally in Australia at about 1.8 Ga. Possibly the
rock record has not been sufficiently well examined, deposits were extensively reworked and
not preserved along active plate margins, or as yet unknown processes acted to suppress
glaciation.

5. Glacio-epochs of the last billion
years and their relationship to
supercontinent cycle
A. three distinct global pulses of glaciation in
the Neoproterozoic glacio-epoch
B. Variation in 13C over last 1.5 Ga (largest
excursions occur in the Neoproterozoic are
coincident with breakup of Rodinia)
C. Variation in Cosmic Ray Flux (curve) a timing
of Earth's periodic crossings of spiral arms of
the Milky Way
D. Estimated global temperature trends
E. Variation in atmospheric carbon dioxide

6. Schematic glaciated rift basin during Rodinia breakup
after 750 Ma (fault activity and sedimentation in a
marine rift basin)

7. Neoproterozoic glacio-epoch (0.75 Ga to 545 Ma)
• Snowball Earth hypothesis:
severe Neoproterozoic
glaciation occurred at low
latitudes (most controversial
and polarized area of
debate). Researchers claim,
during the Snowball Earth,
some 650 ma ago, earth was
either completely frozen or
was almost completely
frozen. Then, the earth was
covered by a single sheet of
ice extending from pole to
pole. Scientists however
think that Snowball Earth
was not a single incident and
that it happened multiple
times with the duration of
each event varying.
Estimated changes in global mean
surface temperature, based on energy-balance calculations, and ice
extent through one complete snowball event.

8. Neoproterozoic glacio-epoch (0.75 Ga to 545 Ma)
Neoproterozoic glaciations occurred against an overall tectonic backdrop of active
crustal extension as Rodinia broke apart
The Neoproterozoic glacio-epoch and the break up of Rodinia

9. Rodinia
Breakup
stages
Mid-life Rodinia stretching to the high latitude at
above a mantle superplume
continued continental rifting on lower-latitude
Rodinia
onset of Rodinia breakup, and pan-
Rodinian “Sturtian” glaciation
continuing Rodinia breakup and sea-level
rises
Rodinia breakup near completion, and the
global “Marinoan” glaciation
Rodinia breakup completion, early Gondwanaland assembly, and
the “Gaskiers” glaciation.
formation of Gondwanaland, high continental topography, and
the lowering of sea level.

10. Slide Title
Breaking and integrating super-continents
Rodinia was a supercontinent formed about 1.1 ga ago. 750 ma ago, Rodinia
broke into three pieces that drifted apart as a new ocean formed between the
pieces. Then, about 600 ma ago, those pieces came back together with a big
crunch known as the Pan-African orogeny (mountain building event). This
formed a new supercontinent, with the name of Pannotia. By about 550 ma ago,
Pannotia was breaking up into several small fragments, Laurentia (the core of
what is now North America), Baltica (northern Europe), and Siberia, among
others, and one very large piece. This large piece, containing what would
become China, India, Africa, South America, and Antarctica, was called
Gondwana. It is considered a supercontinent in its own right because it is so big,
but it is only part of the earlier supercontinents.
Over the next 200 ma many of the small pieces came together to form another
large continent called Laurasia. Laurasia and Gondwana joined approximately
275 ma ago to form the supercontinent of Pangea. The breakup of Pangea is still
going on today and contributes in the formation of the Atlantic Ocean.
Eventually a new supercontinent will form and then it will break apart and so on.
Source: https://scienceline.ucsb.edu/getkey.php?key=22

11. The northern margin of Gondwana was the locus of active extension after 480 Ma and extension-
related uplift of the Gondwanan Highlands may have triggered polar Saharan glaciation after 440 Ma.
Outlying ice masses lay on the proto Andes and in southern Africa where they reached sea level
(Cancanari Formation and Pakhuis Formation respectively). The Saharan ice sheet was short lived and
disappeared by the Early Silurian but ice remained over the uplifted active margin of South America
into the Devonian of Brazil and Bolivia but not over the pole (see text). When Gondwana collided with
Laurentia to form Pangea beginning in the mid-Carboniferous this remnant ice would expand to form
an extensive Gondwanan ice complex
Palaeogeography of Late Ordovician Saharan glacio-epoch: c. 440 Ma.

12. The Late Cenozoic glacio-epoch after
55 Ma.
The breakup of Pangea moved large
landmasses into higher latitudes,
isolated Antarctica and changed the
configuration and bathymetry of
ocean basins.
Palaeogeography of Late Ordovician Saharan glacio-epoch: c. 440 Ma.
Geometry of continental extension
(after Ebinger et al., 2002) as
occurred during the Paleoproterozic
and Neoproterozoic glacio-epochs
The most extensive uplifts are
created where crust is old, thick and
thus flexurally rigid.

13. Simplified diagram illustrating principal differences between glacio epochs
resulting from uplift resulting from continental collision (A1 and A2) and
resulting from continental extension (B1, B2).
Palaeogeography of Late Ordovician Saharan glacio-epoch: c. 440 Ma.

14. Conclusions:
• There is a close relationship between glacio-epochs and times of
enhanced crustal extension during the Proterozoic and Phanerozoic;
• Most of Earth's glacial record appears to be preserved in extensional
basins. Tectonically generated topography produced by crustal
extension may be an important control on cooling in conjunction with
increased availability of moisture.
• There are times in earth history of rifting with no ice, and ice with no
rifting but the marked association between the two for most ancient
glacio-epochs cannot be simply coincidental.
• Having recognised the importance of tectonic preconditions under
which glacio-epochs develop and glacial deposits are preserved,
detailed consideration of the role of tectonics in influencing climate
and controlling water depths, sediment supply and the age of
sedimentary successions, is essential in future basin investigations
and climate models.

15. Thank You !
Key References:
Eyles, N. (2008): Glacio-epochs and the supercontinental cycle after 3.0 Ga: tectonic
boundary conditions for glaciation. Palaeogeography, Palaeoclimatology,
Palaeoecology, 258, 89–129.
Li, Z.-X., Evans, D.A.D., Halverson, G.P. (2013): Neoproterozoic glaciations in a revised
global palaeogeography from the breakup of Rodinia to the assembly of
Gondwanaland. Sedimentary Geology, 294, 219–232.
Hoffmann, P.F., Schrag, D.P. (2002): The Snowball Earth hypothesis: testing the limits
of global change. Terra Nova, 14, 129–155.
Disclaimer: Most of the Images, tables and charts used in this presentation are either
from the references above or from other sources as cited in respective slides. Some
other images are extracted from unspecified online sources, featured under “Creative
Commons licenses”.

18. QUICK NOTE:
The earliest known glaciation (mid Archean ∼ 2.9 Ga) is recorded in
the marine Mozaan Group of South Africa deposited along the
passive margin of the Kapvaal Craton then part of the early
continent Ur.
It remains unclear whether the passive-margin related Kaapvaal
glaciation represents a glacio-epoch or a short-lived event.
A long Paleo-Mesoproterozoic non-glacial interval (c. 2.3 Ga to
750 Ma?) coincides with continental collisions and high standing
Himalayan-scale orogenic belts marking the suturing of
supercontinents Nena-Columbia and Rodinia. A near absence of glacial deposits other than at 1.8 Ga, may reflect
lack of preservation.
The anomaly of the lack of a glacial record during the Paleo-Mesoproterozoic growth of Nena-Columbia is clearly
evident though Williams (2005) reports evidence of glaciation at 1.8 Ga. The sedimentary record of most glacio-
epochs occurs in the geodynamic context of intracratonic rifting, crustal extension and the formation of passive
margins.
The timing and number of glacial events in the Neoproterozoic (3a, b, c) is uncertain. Paleoproterozoic (c.2.5 Ga)
and Neoproterozoic glacio-epochs (c. 750–580 Ma) occurred during the breakup
of Kenorland and Rodinia respectively. It is also possible that extension along high latitude continental margins
and consequent uplift also played a role in triggering Ordovician glaciation at c. 440 Ma (when terranes rifted off
Gondwana; see text). Most of the Gondwanan glacio-epoch deposits are stored in rift basins even though
glaciation was initiated during the compressional growth phases of Gondwana.
The extensive and prolonged Neoproterozoic glacio-epoch records either diachronous glaciations or discrete
pulses of cooling between ∼ 750 and ∼ 580 Ma, and is overwhelmingly recorded by substantial thicknesses
(1 km+) of glacially influenced marine strata stored in rift basins. These formed on the mid to low latitude (< 30°)
oceanic margins of western (Panthalassa: Australia, China, Western North America) and eastern (Iapetus:
Northwest Europe) margins of a disintegrating Rodinia. The youngest glacially influenced deposits formed about
580 Ma along the compressional Cadomian Belt exterior to Rodinia (Gaskiers Formation) possibly correlative with
the classic passive margin Marinoan deposits of South Australia.
Tectonics played a major role in Cenozoic cooling after 55 Ma culminating in continental scale Northern
hemisphere ice sheets only after 3.5 Ma.
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19. Primary glacial sediment is extensively reworked by mass flow
processes and terrestrial glacial facies are seldom preserved.
Sedimentation is markedly diachronous as a consequence of
propagating faults and the non-synchronous formation and
filling of different sub-basins. The fill of any one sub-basin
comprises a tectonostratigraphic succession recording changing
relationship between subsidence and sediment supply. Marked
intrabasinal variability in the timing of rifting and the
sedimentation response prohibits correlations of like facies
(e.g., diamictites) and also wider extrapolation of age dates on
any one stratigraphic horizon to other basins worldwide.
The well-developed relationship between Paleoproterozic rifting and glacial deposits suggests either a causal
relationship between rift-related uplift and climatic cooling orselective preservation of glaciated rift deposits.
Deposits are dominantly submarine debrites associated with thick turbidites. They are part of very thick marine
tectono-stratigraphic successions, often associated with volcanics, recording the changing interplay of subsidence
rates and sediment supply as rifting progresses. An association between glacials and banded iron formations may
reflect deposition in semi-enclosed basins with incipient spreading centres.
------------------------------------------------------
The bulk of the Neoproterozoic glacial record is stored within thick
marine debrite-turbidite successions that accumulated within rift
basins. Terrestrial ‘tillites’ and associated deposits are poorly
represented. Neoproterozoic glaciers were wet based and
produced abundant meltwater and sediment incompatible with
catastrophically cold conditions of a hard Snowball Earth. The
breakup of Rodinia took place over a 200 million year period and
by analogy with other episodes of rifting there was significant
along-strike diachroneity in the timing of rifting, basin formation
and glacially influenced sedimentation (Kendall et al., 2006).
Large-scale rearrangement of landmasses and oceanic
configurations created by an evolving disintegrating supercontinent may have played a key role in climate change.
There is growing recognition that Neoproterozoic glaciations were initiated as regional ice centres (Halverson et
al., 2005, p. 1198) whose growth was diachronous (op cit., p. 1198) countering the longstanding use of glacial
deposits as precise global time markers. Earlier ideas of ‘instant glaciation’ involving notional albedo-feedback
mechanisms and runaway refrigeration are now underplayed (see Halverson et al., 2005).
In the light of the substantial gaps in knowledge identified above, and the emerging theme of diachroneity of
Neoproterozoic glaciation, it is profitable to revisit the conceptual underpinning of current efforts to subdivide
Proterozoic time using Global Stratotype Sections and Points (GSSP). Knoll et al. (2006, p. 14) believe that ‘the
great ice ages that wracked the later Neoproterozoic world… were global in impact, and because they are
associated with carbon isotopic excursions larger than any recorded in Phanerozoic rocks, the glaciations offer
what are undoubtedly our best opportunities for the sub-division of Neoproterozoic time’. It can be argued in fact
that the geologic consensus is moving away from catastrophic global freeze events and instantaneous
deglaciations. In contrasts to ‘wracking’ the world, the Neoproterozoic rock record informs us that glaciers were
wet-based and may have been part of diachronous events as tectonotopography evolved during the dispersal of
crustal blocks.

20. A superplume occurs when a large mantle upwelling is convected
to the Earth's surface. ... Although similar, a superplume forms at
the mantle-core boundary while a hot-spot occurs at the mantle-
crust layer. Superplumes create cataclysmic events that affect the
whole world when they explode.