HR: 17:30h
AN: V22G-07 INVITED     [PDF]
TI: Cosmogenic $^{26}$Al and $^{10}$Be Depth Profiles in High-level Terrace Gravels Demonstrate Early Pleistocene Entrenchment of the San Juan River in the Canyonlands Region of Utah
AU: * Granger, D E
EM: dgranger@purdue.edu
AF: Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AU: Wolkowinsky, A J
EM: arapacz@purdue.edu
AF: Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AU: Caffee, M W
EM: mcaffee@physics.purdue.edu
AF: Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AB: Cosmogenic $^{26}$Al and $^{10}$Be measured in a vertical profile can be used to date sedimentary deposits such as alluvial terraces, provided that the profile is deep enough to shield the lowest portion from secondary cosmic-ray nucleons and muons. Radioactive decay at depth lowers the $^{26}$Al/$^{10}$Be ratio with time, allowing sediments to be dated up to 5 million years old. Accurate dating requires correction for post-depositional production of $^{26}$Al and $^{10}$Be, which can be important at depths up to 10-20 meters. Production at depth can be extrapolated from the upper portion of the profile. Here we present data from two terraces ~150 meters above the San Juan River, Utah, near the towns of Bluff and Mexican Hat. The terraces are on the brink of the canyon upstream from the entrenched Goosenecks for which this river is known. These terraces and others grade evenly to the rim of Glen Canyon on the Colorado River, and thus constrain the age of canyon incision east of Grand Canyon. The terrace at Bluff is mantled with over 11 meters of gravel, while at Mexican Hat the gravel is 6 meters deep. These gravels have previously been correlated with glacial deposits in the San Juan mountains. The alluvium is capped by Stage IV-V pedogenic carbonate. We collected sediment from a profile 11.7 meters deep at Bluff, and 5.5 meters deep at Mexican Hat. We also collected a sample for cosmogenic surface exposure dating to compare with the profile dating method at Bluff. Our cosmogenic profiles yield an age of 1.36 (+0.20/-0.15) My for the deposit. The average incision rate of the San Juan River at this site is thus 113 +/- 16 m/My, a value that is similar to many others in the Colorado River system, but nearly 5 times slower than incision rates that have been inferred from nearby cosmogenic nuclide exposure dates on terraces and pediments. To test a possible source for this discrepancy, we measured $^{26}$Al and $^{10}$Be in a sample collected from the undisturbed terrace surface. We determined that the effective surface exposure age of this deposit is only 625 thousand years, only half the true age of the terrace. This discrepancy underscores the caution that must be taken when interpreting very old surface exposure ages, which are known to be sensitive to erosion at the surface. These data suggest that Glen Canyon on the Colorado River is at least 1.3 million years old, and that incision rates in the Canyonlands area are similar to those downstream in Grand Canyon, Marble Canyon, and on the Little Colorado River.
DE: 1035 Geochronology
DE: 1094 Instruments and techniques
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting