Geology Reference
In-Depth Information
Distance upstream from the basin mouth (km)
200x vertical
exaggeration
10
20
50
A
25
position of present stream and inferred pre-terrace gradient
0
5
Diachronous
Terrace Tread
+
B
+
10
+
Fig. 2.14
Terrace formation.
A. Height of the top surface of terrace
T6 at Cajon Creek, California,
compared with the modern river.
The gradient of the modern river is
removed, in order to emphasize the
apparent upward bowing of the
surface. The highest part of the terrace
occurs where it crosses the San
Andreas Fault, but there is no apparent
offset here. B. Variations in the timing
of initiation of aggradation and incision
along Cajon Creek, both of which are
highly diachronous. Radiocarbon dates
(crosses) on the fill are used to
calibrate the aggradation-degradation
cycle. Incision is assumed to begin
shortly after the maximum thickness of
fill is attained. C. Interpretation of the
evolving topographic profile of the
terrace due to migration of the locus of
deposition and erosion. Modified after
Weldon (1986).
+
+
+
15
radiocarbon
dates
0
10
20
distance upstream from basin mouth (km)
Distance upstream from the basin mouth (km)
200x vertical
exaggeration
10
20
50
10ka
9ka
8ka
7ka
6ka
C
25
12ka
15ka
14ka
13ka
11ka
0
coeval (Pan
et al.
, 2003; Pazzaglia and Brandon,
2001; Thompson
et al.
, 2002). Thus, correlation
of undated terrace surfaces with ones dated at
another locality or with known climatic changes
has often been used to estimate the age of the
unknown surface (Avouac and Peltzer, 1993).
Even where the physical downstream continu-
ity of a terrace is clear, it is not always safe to
assume that the upper surface represents an
essentially isochronous horizon. The potentially
complex nature of terrace formation is strikingly
illustrated by a terrace that crosses the San
Andreas Fault in southern California near Cajon
Pass (Weldon, 1986). When the terrace surface
is compared with the modern stream gradient,
the terrace appears to have been warped
upwards more than 30 m (Fig. 2.14A), although
it is not physically disrupted across the fault.
