Mesozoic Earth and Life History - The Changing Earth: Exploring Geology and Evolution - page 606

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(a)

Figure 22.10 The Franciscan Complex

Land

Trench

Sea
level

Continental
crust

Ophiolite

Great Valley
Group

Franciscan
Complex —
Low-temperature,
high-pressure zone
where blueschist
facies develops

Lithosphere

Upper
mantle

Sediment

Oceanic crust

a Location and reconstruction of the depositional
environment of the Franciscan Complex during the
Late Jurassic and Cretaceous.

( b Exposures of the Franciscan Complex along the
central California coast.

So far, most terranes have been identifi ed in mountains
of the North American Pacifi c Coast region, but such plates
are suspected to be present in other mountain systems as
well. They are more difficult to recognize in older moun-
tain systems, such as the Appalachians, however, because of
greater deformation and erosion. Thus, terranes provide an-
other way of viewing Earth and gaining a better understand-
ing of the geologic history of the continents.

Particularly widespread in western North America are coals
of Cretaceous age. Mesozoic coals are also known from Aus-
tralia, Russia, and China.
Large concentrations of petroleum occur in many areas
of the world, but more than 50% of all proven reserves are
in the Persian Gulf region (see Chapter 2 Geo-Focus on page
55). During the Mesozoic Era, what is now the Gulf region
was a broad passive continental margin conducive for the
formation of petroleum. Similar conditions existed in what
is now the Gulf Coast region of the United States and Cen-
tral America. Here, petroleum and natural gas also formed
on a broad shelf over which transgressions and regressions
occurred. In this region, the hydrocarbons are largely in
reservoir rocks that were deposited as distributary channels
on deltas and as barrier-island and beach sands. Some of

MESOZOIC MINERAL RESOURCES
Although much of the coal in North America is Pennsylvanian
or Paleogene in age, important Mesozoic coals occur in the
Rocky Mountain states. These are mostly lignite and bitu-
minous coals, but some local anthracites are present as well.