TIMING OF FORMATION OF GRAND CANYON FROM U-PB DATES ON GROUNDWATER-TABLE SPELEOTHEMS

Cave mammillaries – groundwater-table-indicator speleothems collected from 10 sites in Grand Canyon – yield U/Pb-ages that support an old western Grand Canyon, ca. 17 Ma old, connecting to a much younger eastern Grand Canyon, ca. 4 Ma, by headward erosion. Five western Grand Canyon samples yielded apparent groundwater-table descent rates ranging from 50 to 128 m/Ma over the last 17 Ma. In contrast, five eastern Grand Canyon/Marble Canyon samples produced groundwater-table descent rates ranging from 186 to 410 m/Ma over the last 4 Ma. The oldest date using a 3-D concordia age was derived from mammillary calcite in Grand Canyon Caverns (16.8 ± 1 Ma, 62 km south of river-mile 190) ~1160 m above river-level today, suggesting an overall groundwater table descent of 69 ± 4 m/Ma for the western Grand Canyon. Another 3-D concordia date was derived from mammillary calcite near Bat Cave (3.79 ± 0.39 Ma, 0.5 km north of river-mile 266) ~260 m above river-level, which also supports a groundwater-table descent of 69 ± 7 m/Ma for the western Grand Canyon. A U-Pb date derived from mammillary calcite near Coronado Butte situated in the eastern Grand Canyon (3.46 ± 0.43 Ma, 4.6 km south of river-mile 80) 580 m above the river yielded a faster groundwater-table descent rate of 168 ± 20 m/Ma. The youngest U/Pb date derived from mammillary calcite in Bedrock Canyon (0.755 ± 0.44 Ma, 1.6 km north of river-mile 32) 310 m above the river gave the fastest groundwater table descent rate of 410 +25 -100 m/Ma. Overall, the data demonstrate dramatic differences in the ages and incision rates between the western and eastern Grand Canyon. Accordingly, Grand Canyon incision started in the west at least 17 Ma ago with a slow near-uniform incision rate throughout its history, integrated with a much faster incising eastern portion of the canyon through headward erosion. The accelerated incision rates in the east probably began prior to 4 Ma and after the through-flowing Colorado River was established at ~5.5-6 Ma. An older western Grand Canyon history compliments an interpretation that the source waters for the Hualapai Limestone issued from a smaller, western Grand Canyon river that started truncating the carbonate-rich Redwall-Muav aquifer as early as 16 to 9 Ma ago.