Geology Reference
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Sandstone-Carbonate-Shale Assemblages Fully 60%
of all Proterozoic rocks are sandstone-carbonate-shale as-
semblages that were deposited along rifted continental
margins and in basins within cratons. Their widespread
occurrence indicates that large, stable cratons were pres-
ent with depositional environments much like those of the
present. Paleoproterozoic assemblages of these rocks are
common in the Great Lakes region of the United States and
Canada (Figure 19.11), and in the western part of the con-
tinent they are found in three Meso- and Neoproterozoic
basins (Figure 19.13).
What Would You Do
Suppose you visit a planet that, like Earth, has continents
and vast oceans. What evidence would indicate that this
hypothetical planet's continents formed and evolved like
those on Earth?
distribution of these glaciers, and even though we do not
know the exact geographic extent of these glaciers, it seems
that they existed even in near-equatorial areas.
Based on the fact that Neoproterozoic glacial deposits
are so widespread, some geologists think that glaciers covered
all land and the seas were frozen—a snowball Earth , as it has
come to be known. The snowball Earth hypothesis is con-
troversial, but proponents claim that the onset of this glacial
episode may have been triggered by the near-equatorial
location of all continents, and as a result, accelerated weath-
ering would absorb huge quantities of carbon dioxide from
the atmosphere. With little CO 2 in the atmosphere, glaciers
would form and refl ect solar radiation back into space and
more glacial ice would form.
So if there actually was a snowball Earth, why wouldn't
it stay frozen? Of course, volcanoes would continue to erupt
spewing volcanic gases, which includes the greenhouse
gases carbon dioxide and methane, which would warm the
Proterozoic Glacial Deposits What appear to be glacial
deposits, as well as striated and polished bedrock, indicate
that two major episodes of Proterozoic glaciation took
place. North America probably had an extensive ice sheet
centered southwest of Hudson Bay during the Paleopro-
terozoic (
Figure 19.15a). Similar deposits of about the
same age are present in the United States, Australia, and
South Africa, but their ages are not known precisely enough
to determine whether there was a single widespread period
of glaciation or a number of glacial events in different areas
at different times.
Another time of widespread glaciation is indicated by
Neoproterozoic-age deposits on all of the continents except
Antarctica (Table 19.1) Figure 19.15b shows the approximate
Figure 19.12 Proterozoic Igneous Rocks and Rocks of the Grenville Orogeny
1.6
1.7 BY
Ilmenite-series granite
Magnetite-series granite
Two-mica granite
Anorthosite
Age in billions of years ( BY )
of granite plutonism
1.5
1.3
1.4
Anorthosite and
ilmenite-series granite
1.1
1.5 BY
1.4
1.4 BY
1.1
1.3
Two-mica
granite
Magnetite-series
biotite-hornblende
granite
1.1
0
400
800 km
a Some of these igneous rocks are 1.7 billion years old, making
them Paleoproterozoic, but most of them are of Mesoporterozoic
age.
b The steeply dipping rocks in the lower part of this image
were metamorphosed during the Grenville orogeny. They are
now overlain by nearly horizontal layers of Late Cambrian-
age sedimentary rocks.
 
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