Geology Reference
In-Depth Information
Mountain-building activity took place primarily along
the eastern and southern margins (known as mobile
belts) of the North American craton during the Paleo-
zoic Era.
In addition to the large-scale plate interactions dur-
ing the Paleozoic, microplate and terrane activity also
played an important role in forming Pangaea.
Paleozoic-age rocks contain a variety of important
mineral resources.
The Paleozoic history of most continents involves major
mountain-building activity along their margins and numer-
ous shallow-water marine transgressions and regressions
over their interiors. These transgressions and regressions
were caused by global changes in sea level that most probably
were related to plate activity and glaciation.
In this chapter, we provide an overview of the geologic
history of the world during the Paleozoic Era in order to
place in context the geologic events taking place in North
America during this time. We then focus our attention on
the geologic history of North America—not in a period-by-
period chronology, but in terms of the major transgressions
and regressions taking place on the continents, as well as the
mountain-building activity occurring during this time. Such
an approach allows us to place North American geologic
events within a global context.
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August 1, 1815, is an important date in the history of geology.
On that date William Smith, a canal builder, published the
world's fi rst true geologic map. Measuring more than eight
feet high and six feet wide, Smith's handpainted geologic
map of England represented more than 20 years of detailed
study of the rocks and fossils of England.
England is a country rich in geologic history. Five of
the six Paleozoic geologic systems (Cambrian, Ordovician,
Silurian, Devonian, and Carboniferous) were described
and named for rocks exposed in England. The Carbonifer-
ous coal beds of England helped fuel the Industrial Revo-
lution, and the need to transport coal cheaply from where
it was mined to where it was needed set off a fl urry of canal
building during the late 1700s and early 1800s. During
this time, William Smith, who was mapping various coal
mines, fi rst began to notice how rocks and fossils repeated
themselves in a predictable manner. During the ensuing
years, Smith surveyed the English countryside for the most
effi cient canal routes to bring the coal to market. Much of
his success was based on his ability to predict what rocks
canal diggers would encounter. Realizing that his obser-
vations allowed him to unravel the geologic history of
an area and correlate rocks from one region to another,
William Smith set out to make the fi rst geologic map of an
entire country!
The story of how William Smith came to publish the
world's first geologic map is a fascinating tale of determi-
nation and perseverance. However, instead of fi nding fame
and success, Smith found himself, slightly less than four
years later, in debtors' prison and—upon his release after
more than two months—homeless. If such a story can have
a happy ending, William Smith at least lived long enough to
fi nally be recognized and honored for the seminal contribu-
tion he made to the then fl edgling science of geology.
Just as William Smith applied basic geologic principles
in deciphering the geology of England, we use these same
principles to interpret the geology of the Paleozoic Era. In
this chapter, we use the geologic principles and concepts
discussed in earlier chapters to help explain how Earth's
systems and associated geologic processes interacted during
the Paleozoic to lay the groundwork for the distribution of
continental landmasses, ocean basins, and the topography we
have today.
CRATONS AND MOBILE BELTS
During the Precambrian, continental accretion and orogenic
activity led to the formation of sizable continents. Movement
of these continents during the Late Proterozoic resulted in
the formation of a single Pangaea-like supercontinent that
geologists refer to as Pannotia. This supercontinent began
breaking apart sometime during the latest Proterozoic, and
by the beginning of the Paleozoic Era, six major continents
were present. Each continent can be divided into two major
components: a craton and one or more mobile belts.
Cratons
are the relatively stable and immobile parts of
continents and form the foundation on which Phanerozoic
sediments were deposited (
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Figure 20.1). Cratons typically
consist of two parts: a shield and a platform.
Shields
are the exposed portions of the crystalline base-
ment rocks of a continent and are composed of Precambrian
metamorphic and igneous rocks (see Figures 7.1 and 19.5)
that reveal a history of extensive orogenic activity during
the Precambrian. During the Phanerozoic, however, shields
were extremely stable and formed the foundation of the
continents.
Extending outward from the shields are buried Precam-
brian rocks that constitute a
platform,
another part of the cra-
ton. Overlying the platform are fl at-lying or gently dipping
Phanerozoic detrital and chemical sedimentary rocks that
were deposited in widespread shallow seas that transgressed
and regressed over the craton. These seas, called
epeiric seas,
were a common feature of most Paleozoic cratonic histories.
Changes in sea level, caused primarily by continental glacia-
tion, as well as by plate movement, were responsible for the
advance and retreat of these epeiric seas.
Whereas most of the Paleozoic platform rocks are still
essentially fl at lying, in some places they were gently folded
into regional arches, domes, and basins (Figure 20.1). In many
cases, some of these structures stood out as low islands during
the Paleozoic Era and supplied sediments to the surrounding
epeiric seas.
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