Geology Reference
In-Depth Information
OF NORTH AMERICA
The beginning of the Mesozoic Era was essentially the same
in terms of tectonism and sedimentation as the preceding
Permian Period in North America. Terrestrial sedimentation
continued over much of the craton, and block faulting and
igneous activity began in the Appalachian region as North
America and Africa began separating (
encompassed approximately 300 degrees of longitude. Such
a confi guration exerted tremendous infl uence on the world's
climate and resulted in generally arid conditions over large
parts of Pangaea's interior.
The world's climates result from the complex interac-
tion between wind and ocean currents and the location and
topography of the continents. In general, dry climates occur
on large landmasses in areas remote from sources of mois-
ture and where barriers to moist air exist, such as mountain
ranges. Wet climates occur near large bodies of water or
where winds can carry moist air over land.
Past climatic conditions can be inferred from the distri-
bution of climate-sensitive deposits. Evaporite deposits result
when evaporation exceeds precipitation. Although sand dunes
and red beds may form locally in humid regions, they are
characteristic of arid regions. Coal forms in both warm and
cool humid climates. Vegetation that is eventually converted
into coal requires at least a good seasonal water supply; thus,
coal deposits are indicative of humid conditions.
Widespread Triassic evaporites, red beds, and desert
dunes in the low and middle latitudes of North and South
America, Europe, and Africa indicate dry climates in those
regions, whereas coal deposits are found mainly in the high
latitudes, indicating humid conditions. These high-latitude
coals are analogous to today's Scottish peat bogs or Canadian
muskeg. The lands bordering the Tethys Sea were probably
dominated by seasonal monsoon rains resulting from the
warm, moist winds and warm oceanic currents impinging
against the east-facing coast of Pangaea.
The temperature gradient between the tropics and the
poles also affects oceanic and atmospheric circulation. The
greater the temperature difference between the tropics and
the poles, the steeper the temperature gradient, and the faster
the circulation of the oceans and atmosphere. Oceans absorb
about 90% of the solar radiation they receive, whereas con-
tinents absorb only about 50%, even less if they are snow
covered. The rest of the solar radiation is refl ected back into
space. Areas dominated by seas are warmer than those domi-
nated by continents. By knowing the distribution of conti-
nents and ocean basins, geologists can generally estimate the
average annual temperature for any region on Earth, as well
as determine a temperature gradient.
The breakup of Pangaea during the Late Triassic caused
the global temperature gradient to increase because the
Northern Hemisphere continents moved farther north-
ward, displacing higher-latitude ocean waters. Because of
the steeper global temperature gradient produced by a de-
crease in temperature in the high latitudes and the chang-
ing positions of the continents, oceanic and atmospheric
circulation patterns greatly accelerated during the Meso-
zoic (
Figure 22.4). The
newly forming Gulf of Mexico experienced extensive evapo-
rite deposition during the Late Triassic and Jurassic as North
America separated from South America (Figure 22.2 and
◗
◗
Figure 22.5).
A global rise in sea level during the Cretaceous resulted
in worldwide transgressions onto the continents such that
marine deposition was continuous over much of the North
America Cordilleran (
Figure 22.6).
A volcanic island arc system that formed off the western
edge of the craton during the Permian was sutured to North
America sometime later during the Permian or Triassic. This
event is referred to as the
Sonoma orogeny
and will be dis-
cussed later in the chapter. During the Jurassic, the entire
Cordilleran area was involved in a series of major mountain-
building episodes resulting in the formation of the Sierra
Nevada, the Rocky Mountains, and other lesser mountain
ranges. Although each orogenic episode has its own name,
the entire mountain-building event is simply called the
Cor-
dilleran orogeny
(also discussed later in this chapter). With
this simplified overview of the Mesozoic history of North
America in mind, we will now examine the specifi c regions
of the continent.
Recall that the history of the North American craton is divided
into unconformity-bound sequences reflecting advances
and retreats of epeiric seas over the craton (see Figure 20.5).
Although these transgressions and regressions played a major
role in the Paleozoic geologic history of the continent, they
were not as important during the Mesozoic. Most of the con-
tinental interior during the Mesozoic was well above sea level
and was not inundated by epeiric seas. Consequently, the two
Mesozoic cratonic sequences, the
Absaroka Sequence
(Late
Mississippian to Early Jurassic) and the
Zuni Sequence
(Early
Jurassic to Early Paleocene) (see Figure 20.5), are not treated
separately here; instead, we will examine the Mesozoic history
of the three continental margin regions of North America.
During the Early and Middle Triassic, coarse detrital sedi-
ments derived from erosion of the recently uplifted Appala-
chians (Alleghenian orogeny) fi lled the various intermontane
basins and spread over the surrounding areas. As weathering
and erosion continued during the Mesozoic, this once lofty
mountain system was reduced to a low-lying plain.
During the Late Triassic, the fi rst stage in the breakup of
Pangaea began with North America separating from Africa.
Fault-block basins developed in response to upwelling
◗
Figure 22.3). Although the temperature gradient
and seasonality on land were increasing during the Juras-
sic and Cretaceous, the middle- and higher-latitude oceans
were still warm because warm waters from the Tethys Sea
were circulating to the higher latitudes. The result was a
relatively equable worldwide climate through the end of the
Cretaceous.
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