Geology Reference
In-Depth Information
and when particularly large and turbulent rivers
transport heavy sediment loads that enable them
to bevel laterally into bedrock.
The controls on the formation of bedrock
straths are still debated. The traditional model for
the formation of strath terraces involves gradual
fluvial widening of the valley floor to create a
broad, flat, bedrock surface into which the river
later incises, leaving behind a strath (Fig. 2.12).
This process was inferred to occur during intervals
of tectonic quiescence, and sequences of straths
were interpreted as records of episodic tectonic
uplift. More recently, straths have been suggested
to form when valleys have aggraded due to
increased sediment loads (Hancock and Anderson,
2002). Not only does the aggraded sediment shield
the bedrock beneath the river from further
erosion, it also provides
tools
that collide with
the valley walls and can bevel them laterally
(Hartshorn
et al.
, 2002; Sklar and Dietrich, 2004;
Turowski
et al.
, 2008). Subsequent changes in sed-
iment or water discharge cause the river to incise
down through its alluvial fill (which is readily
removed), leaving behind strath terraces that are
elevated above the bedrock floor (Fig. 2.12). In
this scenario, climate changes are also likely to
modulate strath formation and abandonment.
In order to use deformed terraces to calculate
tectonic rates, their ages must be known. Although
obtaining reliable ages is commonly difficult, we
know that many paired aggradational terraces
form in response to climatic cycles. In many
areas of alpine glaciation, for example, fluvial
terraces can be demonstrated to correlate with
moraines associated with glacial advances or
stillstands (Penck and Brückner, 1909). Changes
in sediment and water fluxes during these cli-
matic intervals can lead to river aggradation that
is followed by incision. The ages of such climati-
cally controlled surfaces are often similar across
a region that has experienced similar climatic
conditions. Notably, when straths are beveled
across weak bedrock (commonly Cenozoic
sedimentary strata), their formation appears to
be modulated primarily by climate, and hence
their ages (Fig. 2.13) are likely to be regionally
Fluvial Terraces in the Kyrgyz Tien Shan
14
C age (
1
σ
)
Probability distribution
Terrace locations
T
i
e
n
11,800±40
Kyrgyz Range
11,860±50
11,700±50
11,930±50
Common age:
14,000 ± 250 yr
11,770±50
S
h
a
n
12,340±40
12,160±50
0
100
12,190±80
13 14
Calibrated date (x 10
3
cal yr BP)
km
15
16
Fig. 2.13
Late Pleistocene terrace ages in the Tien Shan, Kyrgyzstan.
Radiocarbon ages from eight fluvial terraces are each depicted as probability density plots of the calendar ages
corresponding to the radiocarbon age of each site and its uncertainty. These dates were collected from terraces in
three tectonically distinct basins as much as 200 km apart. Each terrace surface is underlain by a strath that was cut
on Tertiary sedimentary rocks and covered by 2-10 m of fluvial gravels. The terrace ages derive from organic matter
preserved in the gravels. Their consistency argues that their formation was climatically controlled. Note that the
probability distribution for each age is a function of the radiocarbon date, its uncertainty, and variations in
atmospheric
14
C through time, as explained in Chapter 3. Modified after Thompson
et al.
(2002).
