4 Reading Landscapes and Dirt: Understanding Past Environmental Change

Wednesday, 18 May 2011
8:00 AM - 12:00 PM
Elm

INVESTIGATING ARROYO CUT-FILL CYCLES AND THEIR LINK TO HOLOCENE CLIMATE CHANGE ALONG KANAB CREEK, SOUTHERN UTAH

Paper number: 4-5
Program: Theme Sessions
Time: 9:10 AM – 9:30 AM

Authors

Abstract

In the late 1800's and early 1900's, many river channels in the southwestern US incised and formed deep arroyos, leaving many historical settlements perched above their water source. Kanab Creek is an excellent example of a mature arroyo, which has incised 30- 40 meters into its alluvium exposing fine-grained sediment packages along the arroyo walls and leaving behind fluvial terraces. Research objectives were to determine the timing of past valley-filling and arroyo-cutting episodes along a 20 km-long reach of Kanab Creek in southern Utah.

In order to reconstruct the Holocene alluvial history of Kanab Creek, fluvial deposits were mapped at the 1:12,000 scale and sediments were described and dated using Optically Stimulated Luminescence (OSL) and radiocarbon dating methods. Sixteen OSL samples were collected and analyzed using the Single Aliquot Regenerative (SAR) technique on small aliquots and single grains of quartz sand. The majority of OSL samples collected along Kanab Creek exhibit some degree of partial resetting of the previous luminescence signal, causing an age overestimation. With the aid of the Minimum Age Model of Galbraith et al. (1999) and fifteen radiocarbon samples for comparison, accurate ages could be estimated for the terrace and basin-fill deposits along Kanab Creek. Results suggest at least four periods of aggradation over the middle to late Holocene: ~6-3.5ka, ~2.5->1ka, ~0.7-0.12ka, and post-1880 AD. Each of these aggradational intervals is separated by 5-40 meters of incision. Linkages to climate-induced changes in discharge regime and relationships between geomorphic thresholds and arroyo cutting will be discussed.