AGE OF LANDSLIDE-DAMMED LAKE DEPOSITS IN CENTRAL GRAND CANYON, ARIZONA

Until now no lacustrine deposits with characteristics diagnostic of long-lived deep-water lakes have been verified in Grand Canyon, despite evidence that several lava dams impounded the Colorado River in Grand Canyon between 100-630 ka between river miles 179 and 189, and six undated landslides occurred between river miles 134 and 139. Some lava dams were unstable and failed catastrophically, releasing large floods into the Colorado River, but other dams may have been stable. At least one of the landslides, part of the complex known as the Surprise Valley landslides, crossed and dammed the Colorado River, rerouting its channel through bedrock at Granite Narrows. Discovery of a lacustrine deposit at river mile 134.5, 3 km upstream of the landslides and 72-88 km upstream of the lava dams, presents the first credible evidence of long-lived lakes in Grand Canyon. The deposit forms a 52-m thick, well-exposed and laterally extensive section of fine-grained sediments that contain evidence of reduced iron and manganese, suggesting an anoxic environment. The top and bottom of this section are 64 and 12 m above the modern river, and the impounded lake would have extended at least 48 km upstream. At an elevation 55 m above the river, the fine-grained sediment is capped by bedload gravels that contain rounded pebbles transported from sources far upstream. The gravels may be inset into the lacustrine deposits, suggesting downcutting in the lacustrine sediments after the Colorado River had filled the reservoir and breached its dam. Infrared-stimulated luminescence (IRSL) and optically stimulated luminescence (OSL) dates of lacustrine sediments are 18.6±1.6, 20.9±1.3, and 22.5±1.3 ka for the bottom, middle, and top of the section, respectively; given the large 2s error of these dates, we consider these ages to be the same at about 21 ka. The latest Pleistocene ages suggest that the lacustrine deposits are not temporally correlated with lava dams, the youngest of which is 100 ka. The highest preserved lacustrine deposits (665 m) at river mile 134.5 would be consistent with a lake impounded by the Surprise Valley slides, which could have dammed the river to a maximum elevation of 760 m.