Dr. Lemons et al., PALEOCLIMATIC IMPLICATIONS OF LATE PLEISTOCENE SEDIMENT YIELD RATES FOR THE BONNEVILLE BASIN, NORTHERN UTAH, Palaeogeography, palaeoclimatology, palaeoecology, 123(1-4), 1996, pp. 147-159
Climate factors such as precipitation and temperature as well as clima
te-driven geomorphic processes such as glaciation affect rates of eros
ion and the amount of sediment supplied from a drainage basin (sedimen
t yield). Thus, sediment yield rates provide paleoclimatic implication
s, although very little work has been published on ancient sediment yi
eld rates. Late Pleistocene Lake Bonneville (Utah, Idaho and Nevada),
with its well-documented lake level history and well-preserved deltaic
sequences, provides a unique opportunity to estimate ancient sediment
yield rates. Two end-member delta systems, coarse-grained (gravel) cl
assic Gilbert-type deltas and fine-grained (sand, silt and clay) delta
s, are exposed along the eastern margin of paleolake Bonneville. One e
xample of each end-member delta is examined in this study (American Fo
rk and Weber River deltas). The established Bonneville hydrograph can
be used to determine time of deposition for a given package of sedimen
ts because these shoreline deltaic sediments were deposited in shallow
water. The geomorphic expressions of these deltas are well-preserved
and their sediment volumes can be determined. River drainage basin are
a can be combined with time constraints and delta volumes to estimate
sediment yield rates. Sediment yield rate estimates for the coarse-gra
ined American Fork and fine-grained Weber River deltas are 478 m(3) km
(-2) yr(-1) +/-34% and 705 m(3) km(-2) yr(-1) +/-15%, respectively. Th
e similarity of these values is somewhat surprising due to the large d
ifference in drainage basin sizes (American Fork approximate to 160 km
(2) and Weber River approximate to 3328 km(2)). However, this similari
ty suggests that precipitation may be the dominant control on sediment
yield. Using sediment yield/precipitation regression equations develo
ped for similar modern basins in the Southern Alps of New Zealand, a f
irst-order approximation of paleoprecipitation is 1.6 m/yr (+/-14% for
American Fork delta) and 1.9 m/yr (+/-7% for Weber River delta). Thes
e paleoprecipitation value are comparable to modern precipitation valu
es of 0.4-1.5 m/yr (range of mean values over both drainage basins). A
maximum late Pleistocene precipitation increase of 33% over the moder
n day value is suggested. This is determined by the difference between
the highest paleoprecipitation rate (1.9 m/yr+7% of the Weber River d
elta) and the modern value (1.5 m/yr). Previous climate models and thi
s 33% paleoprecipitation increase, suggest a maximum decrease in basin
floor paleotemperature of approximately 13 degrees C. This paleotempe
rature decrease is consistent with previous estimates of late Pleistoc
ene temperatures (using amino acid epimerization rates and periglacial
wedges) in the Bonneville Basin and adjacent areas. These estimates s
uggest that although late Pleistocene precipitation rates may have bee
n slightly higher than present (up to 33% higher), late Pleistocene te
mperature and evaporation rates were more important in the development
of Lake Bonneville and associated glaciers.