LATE PLEISTOCENE FAN ACTIVITY IN JAKES VALLEY, CENTRAL GREAT BASIN, USA

Decades of research have yielded comprehensive understanding of late Pleistocene climate change and geomorphic activity at the periphery of the Great Basin. However, little has been published detailing climate change and geomorphic activity during the late Pleistocene in the interior of the Great Basin. Primary data is presented regarding timing of late-Pleistocene time lake-level fluctuation and fan deposition in a high altitude, relatively small basin in the central part of the Great Basin. The timing of “Jakes Lake” highstand and recession is established, and the late-Pleistocene time histories of two fans in Jakes Valley are documented. Data were collected through 1:12,000 scale field mapping, soil profile characterization, topographic surveys using a digital theodolite, and radiocarbon dating of mollusk shells collected from beach ridges. “Jakes Lake” highstand (1943m) occurred at 13,870 +/- 50 radiocarbon years before present (rcybp) followed by recession to 1936m by 12,440 +/- 50 rcybp, and rapid recession thereafter. The numerically dated beach ridges provide a temporal and spatial context for evaluating fan activity during the late Pleistocene. At least two fans prograded during Jakes-Lake time. The Cottonwood fan Qf3a fan-lobe surface projects down fan to beach ridges that are convex toward the Jakes Valley playa. The downfan “deflection” of the beach ridges coupled with the relationship of the Qf3a surface to the beach ridges indicates that Qf3a was deposited during Jakes Lake time. As Jakes Lake receded, the Cottonwood fan incised across beach ridges and into Qf3a. Fan building resumed after 12,440 +/- 50 rcybp with deposition of Qf3. The relationship of late Pleistocene/Holocene fan lobes (Qf3a and Qf3) to beach ridges on the Yamaha fan is slightly different than on the Cottonwood fan. Yamaha fan beach ridges are convex toward the playa, but are partly buried by Qf3 sediments. This suggests that Yamaha fan Qf3 was deposited during Jakes Lake time, but sediment deposition by the Yamaha fan was uninterrupted by Jakes Lake recession. Future work will characterize geomorphic activity on other fans during Jakes Lake time with the aim of developing a model for landscape response to the Pleistocene-Holocene transition in Jakes Valley.