High-resolution Chronostratigraphy of Holocene Terraces in Eastern Grand Canyon: A Record of Paleofloods or Aggradation-incision Cycles?

Changes in climate, affecting sediment supply and hydrology, have influenced deposition and erosion along the Colorado River corridor in Grand Canyon. Studying the nature of alluvial deposits in the canyon will help reveal how this large river responds to changes in its Colorado Plateau tributaries and Rocky Mountain headwaters. This study focuses on Holocene alluvial terraces at a site near Ninemile Draw in Glen Canyon and two sites at Tanner Bar downstream in Grand Canyon, areas with well preserved and exposed deposits. The goal is to combine stratigraphic relations with dating methods to reconstruct the Holocene alluvial history and relate it to broader climate. The hypothesis being tested is that the Holocene alluvial deposits in Grand Canyon are a series of inset aggradational packages that correlate to the valley fills and arroyo cycles identified in tributaries on the Colorado Plateau. This hypothesis was proposed by Hereford and others (1996), and it implies deposits should be laterally continuous and correlatable throughout the river corridor. An alternate hypothesis, following work by O’Connor et al. (1994), is that the alluvial packages instead record floods along a mainstem river with constant grade over Holocene time. Data from each site include stratigraphic panels, detailed sedimentologic descriptions, and facies interpretations of each exposure. Results can be related to archaeological evidence and radiocarbon dates, and deposits are being dated by optically stimulated luminescence (OSL). This project will improve our ability to decipher alluvial records of large rivers, and help determine if cycles of erosion and deposition identified in small streams are preserved by a larger river that inherits signals of many tributaries