Geology Reference
In-Depth Information
Chapter Summary
Tables 20.1 (page 557) and 20.2 (page 558) provide sum-
maries of the geologic history of the North American cra-
ton and mobile belts, as well as global events and sea-level
changes during the Paleozoic Era.
Most continents consist of two major components: a rela-
tively stable craton over which epeiric seas transgressed
and regressed, surrounded by mobile belts in which
mountain building took place.
Six major continents and numerous microcontinents and
island arcs existed at the beginning of the Paleozoic Era;
four of the major continents were located near the
paleoequator.
During the Early Paleozoic (Cambrian-Silurian), Lau-
rentia was moving northward and Gondwana moved to a
south polar location, as indicated by tillite deposits.
During the Late Paleozoic, Baltica and Laurentia col-
lided, forming Laurasia. Siberia and Kazakhstania col-
lided and fi nally were sutured to Laurasia. Gondwana
moved over the South Pole and experienced several
glacial-interglacial periods, resulting in global sea-level
changes and transgressions and regressions along the
low-lying craton margins.
Laurasia and Gondwana underwent a series of collisions
beginning in the Carboniferous. During the Permian,
the formation of Pangaea was completed. Surrounding
the supercontinent was a global ocean, Panthalassa.
Geologists divide the geologic history of North America
into cratonic sequences that refl ect cratonwide transgres-
sions and regressions.
The fi rst major marine transgression onto the craton
resulted in deposition of the Sauk Sequence. At its maxi-
mum, the Sauk Sea covered the craton except for parts
of the Canadian shield and the Transcontinental Arch, a
series of large, northeast-southwest trending islands.
The Tippecanoe Sequence began with deposition of an
extensive sandstone over the exposed and eroded Sauk
landscape. During Tippecanoe time, extensive carbon-
ate deposition took place. In addition, large barrier reefs
enclosed basins, resulting in evaporite deposition within
these basins.
The basal beds of the Kaskaskia Sequence that were
deposited on the exposed Tippecanoe surface consisted
either of sandstones derived from the eroding Taconic
Highlands, or of carbonate rocks.
Most of the Kaskaskia Sequence is dominated by
carbonates and associated evaporites. The Devonian
Period was a time of major reef building in western
Canada, southern England, Belgium, Australia, and
Russia.
Widespread black shales were deposited over large areas
of the craton during the Late Devonian and Early
Mississippian.
The Mississippian Period was dominated for the most
part by carbonate deposition.
Transgressions and regressions, probably caused by advanc-
ing and retreating Gondwanan ice sheets over the low-lying
North American craton, resulted in cyclothems and the
formation of coals during the Pennsylvanian Period.
Cratonic mountain building, specifi cally the Ancestral
Rockies, occurred during the Pennsylvanian Period and
resulted in thick nonmarine detrital sediments and evap-
orites being deposited in the intervening basins.
By the Early Permian, the Absaroka Sea occupied a nar-
row zone of the south-central craton. Here, several large
reefs and associated evaporites developed. By the end of
the Permian Period, this epeiric sea had retreated from
the craton.
The eastern edge of North America was a stable car-
bonate platform during Sauk time. During Tippecanoe
time, an oceanic-continental convergent plate bound-
ary formed, resulting in the Taconic orogeny, the fi rst
of three major orogenies to affect the Appalachian
mobile belt.
The newly formed Taconic Highlands shed sediments
into the western epeiric sea, producing a clastic wedge
that geologists call the Queenston Delta.
The Caledonian, Acadian, Hercynian, and Alleghenian
orogenies were all part of the global tectonic activity that
resulted from the assembly of Pangaea.
The Cordilleran mobile belt was the site of the Antler
orogeny, a minor Devonian orogeny during which
deep-water sediments were thrust eastward over
shallow-water sediments.
During the Pennsylvanian and Early Permian, moun-
tain building occurred in the Ouachita mobile belt. This
tectonic activity was partly responsible for the cratonic
uplift in the southwest, resulting in the Ancestral Rockies.
During the Paleozoic Era, numerous terranes, such as
Avalonia, existed and played an important role in form-
ing Pangaea.
Paleozoic-age rocks contain a variety of mineral
resources, including petroleum, coal, evaporites, silica
sand, lead, zinc, and other metallic deposits.
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