Geology Reference
In-Depth Information
A
1350
1625
peak: ~13 ka
peak: ~16 ka
1575
1300
Bonneville
& Lahontan
Lake Levels
1525
1250
1475
1425
1200
1375
Lahontan
1150
1325
Bonneville
1275
1100
1225
1175
1050
0
5000
15,000
25,000
35,000
Time (yr B.P.)
B
Number of basins with data
33 12 12 14 13 13 14 15 15 17 18 15 13 17 12 12 11 11 12 17 11 10 10 11 10
9
7
5
6
5
100
intermediate
level
80
low
level
high
level
60
40
Great Basin
Lake Levels
20
0
0
5
10
15
20
25
30
Thousands of years before present (
14
C)
Fig. 2.8
Lake-level changes.
A. Lake-level history for Lake Bonneville and Lake Lahontan. Despite the proximity of these two large lakes in the
Great Basin, they have significantly different histories due both to contrasts in discharge and to changing topographic
thresholds within each basin. Modified after Benson
et al.
(1990). B. Late Quaternary lake-level records in southwestern
United States. These data indicate that the majority of lakes fluctuate synchronously at the time scale of a few
thousand years, but that lake-level variations in any individual lake may differ markedly from the mean. Modified
from Smith and Street-Perrott (1983).
Gilbert (1890) described the internal bedding
geometries of lacustrine deltas long ago, and
showed that the contact of the topset and fore-
set beds closely approximates the level of the
body of water into which the delta is prograding
(Fig. 2.9). If a lake only existed during one par-
ticular interval, mapping of the foreset-topset
contact for many deltas formed along its margin
would define a paleo-horizontal surface that
may have been subsequently displaced. As with
lake shorelines, the magnitude of isostatic
rebound or fault displacements can be deduced
from crustal warping of the paleo-horizontal
surface defined by deltas (Thorson, 1989). If,
however, multiple high lake levels occurred
within the basin, then not only must the height
of a topset-foreset contact be determined, but
the age of the delta must also be ascertained to
