Geology Reference
In-Depth Information
Michigan
basin
Craton
Barrier reefs
Carbonate bottom
Land
Mountains
Barrier reefs
Evaporites
Epeiric sea
Deep ocean
◗
Figure 20.10
Silurian Paleogeography of North America Note the development of reefs in the
Michigan, Ohio, and Indiana-Illinois-Kentucky areas.
the Middle and early Late Devonian epochs were times of
major reef building.
The Middle and Late Devonian reefs of western Canada con-
tain large reserves of petroleum and have been widely stud-
ied from outcrops and in the subsurface (
precipitated in much the same way as in the Michigan Basin
during the Silurian (Figure 20.11). More than half of the
world's potash, which is used in fertilizers, comes from these
Devonian evaporites. By the middle of the Late Devonian,
reef growth had stopped in western Canada, although non-
reef carbonate deposition continued.
◗
Figure 20.14).
These reefs began forming as the Kaskaskia Sea transgressed
southward into western Canada. By the end of the Middle
Devonian, they had coalesced into a large barrier-reef system
that restricted the fl ow of oceanic water into the back-reef
platform, creating conditions for evaporite precipitation (Fig-
ure 20.14). In the back-reef area, up to 300 m of evaporites
In North America, many areas of carbonate-evaporite
deposition gave way to a greater proportion of shales and
coarser detrital rocks beginning in the Middle Devonian
and continuing into the Late Devonian. This change to
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