Geology Reference
In-Depth Information
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unit of sequence stratigraphy is the
sequence,
which is a suc-
cession of rocks bounded by unconformities and their equiv-
alent conformable strata. Sequence boundaries result from a
relative drop in sea level. Sequence stratigraphy is an impor-
tant tool in geology because it allows geologists to subdivide
sedimentary rocks into related units that are bounded by
time-stratigraphically signifi cant boundaries. Geologists use
sequence stratigraphy for high-resolution correlation and
mapping, as well as interpreting and predicting depositional
environments.
What Would You Do
You work for a travel agency and are putting together a raft trip
down the Colorado River through the Grand Canyon. In addition
to the usual information about such a trip, what kind of geo-
logic information would you include in your brochure to make
the trip appealing from an educational standpoint as well?
above sea level and experiencing extensive weathering and
erosion. Because North America was located in a tropical cli-
mate at this time and there is no evidence of any terrestrial
vegetation, weathering and erosion of the exposed Precam-
brian basement rocks must have proceeded rapidly.
During the Middle Cambrian, the transgressive phase of
the Sauk began with epeiric seas encroaching over the craton
(see Geo-Focus on pages 532 and 533). By the Late Cambrian,
Rocks of the
Sauk Sequence
(Neoproterozoic-Early Ordo-
vician) record the fi rst major transgression onto the North
American craton (Figure 20.5). During the Neoproterozoic
and Early Cambrian, deposition of marine sediments was
limited to the passive shelf areas of the Appalachian and
Cordilleran borders of the craton. The craton itself was
Cordilleran
mountain-building
episodes
Appalachian-Ouachita
mountain-building
episodes
Period
Craton
Coast ranges
Tejas
C
Laramide
Sevier
K
Zuni
Sequence
Nevadan
J
R
T
R
P
Alleghenian
Absaroka
Sequence
P
Ouachitan
M
Acadian
Kaskaskia
Sequence
Antler
D
S
Tippecanoe
Sequence
Taconic
O
Sauk
Sequence
C
◗
Figure 20.5
Cratonic Sequences of North America A cratonic sequence is a large-scale
lithostratigraphic unit representing a major transgressive-regressive cycle and bounded
by cratonwide unconformities. The white areas represent sequences of rocks separated by
large-scale unconformities (brown areas). The major Cordilleran orogenies are shown on
the left side, and the major Appalachian orogenies are on the right.
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