Neil SheaFollow

Date of Completion

7-15-2013

Embargo Period

7-15-2013

Advisors

Robert Thorson, Glenn Warner

Field of Study

Geological Sciences

Degree

Master of Science

Open Access

Open Access

Abstract

The Boulder Creek Critical Zone Observatory (BcCZO) aims to understand the history, architecture, and evolution of hillslopes found within the diverse topography and climate regimes of the Colorado Front Range. Here, I present the results of a systematic study that documents spatial patterns of mobile regolith thickness and uses concentrations of meteoric 10Be to assess the mechanisms and time scales of sediment transport and storage in Gordon Gulch, a mid-elevation watershed representative of topography throughout the region. Gordon Gulch lies within the unglaciated portion of the Front Range and is hypothesized to be in long-term steady state evolution. This 3.7 km2 watershed is characterized by soil mantled hillslopes with a high degree of surficial variability due to prominent bedrock outcrops (tors) on ~10% of slopes and intermittent gullying. Varying fracture spacing, rock strength of the local bedrock, and topographic slope, curvature, and hillslope aspect provide distinctive lenses through which to interpret the recent and long-term evolution of hillslopes in this basin. Our analysis of over 200 soil pits reveals high variability in mobile regolith thickness, which we define as the depth to immobile weathered bedrock and/or saprolite. We combine our analysis of regolith thicknesses on hillslopes with mapping of toe slopes, alluvial fans, stream terraces, and colluvial fills to understand sediment storage in the watershed. In general, the mobile regolith cover is similar on both slopes, and a thickening of mobile regolith occurs on the toes of steep north-facing slopes. 10Be derived residence times indicate a redistribution of mobile material on north-facing slopes, and widespread transport efficiency on south-facing slopes. These data also suggest a decoupling of the hillslopes from the drainage channel due to the lack of thin/bedrock mantled slopes expected to evolve in response to a migrating knickpoint in the central stream and relatively young (~21 ky) residence times. In addition to meteoric 10Be analysis, carbon-14 dating provides further constraints of the age, rates, and timing of sediment transport, storage, and removal from the watershed. The possibility that the current state of Gordon Gulch hillslopes is governed by a combination of transient response and steady-state condition is supported by mapping within the lower Gordon Gulch stream valley, which indicates a larger volume of sediment is stored within Holocene alluvial fans and toe-slopes at the base of north-facing slopes than within similar features at the base of south-facing slopes.

Major Advisor

William Ouimet

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