Geology Reference
In-Depth Information
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Figure 20.7
Time-Transgressive Cambrian Facies
West
Craton
Wisconsin
Ohio
East
Margin of
Appalachian
mobile belt
Neoproterozoic
basement rock
Upper
Cambrian
Middle
Cambrian
Lower
Cambrian
(a)
Upper
Precambrian
a
Block diagram from the craton interior to the
Appalachian mobile belt margin showing the three
major Cambrian facies and the time-transgressive
nature of the units. Note the progressive
development of a carbonate facies stemming
from submergence of detrital source areas by the
advancing Sauk Sea.
b
Outcrop of cross-bedded Upper Cambrian sandstone in the
Dells area of Wisconsin.
tectonic event taking place in the Appalachian mobile belt,
and which we will discuss later.
Organic reefs
are limestone structures constructed by liv-
ing organisms, some of which contribute skeletal materials
to the reef framework (
The size and shape of a reef are mostly the result of the
interactions among the reef-building organisms, the bottom
topography, wind and wave action, and subsidence of the
seafl oor. Reefs also alter the area around them by forming
barriers to water circulation or wave action.
Reefs have been common features in low latitudes since
the Cambrian and have been built by a variety of organisms.
The fi rst skeletal builders of reef-like structures were
archaeo-
cyathids.
These conical organisms lived during the Cambrian
and had double, perforated, calcareous shell walls. Archaeo-
cyathids built small mounds that have been found on all
continents except South America (see Figure 21.5).
Beginning in the Middle Ordovician, stromatoporoid-
coral reefs became common in the low latitudes, and simi-
lar reefs remained so throughout the rest of the Phanerozoic
Eon. The burst of reef building seen in the Late Ordovician
through Devonian probably occurred in response to evo-
lutionary changes triggered by the appearance of extensive
Figure 20.9) (see Chapter 9 Geo-
inSight on pages 236 and 237). Today corals and calcareous
algae are the most prominent reef builders, but in the geo-
logic past other organisms played a major role.
Regardless of the organisms that dominate reef com-
munities, reefs appear to have occupied the same ecological
niche in the geologic past that they do today. Because of the
ecological requirements of reef-building organisms, present-
day reefs are confi ned to a narrow latitudinal belt between
30 degrees north and south of the equator. Corals, the major
reef-building organisms today, require warm, clear, shallow
water of normal salinity for optimal growth.
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