North-Central Section - 39th Annual Meeting (May 19–20, 2005)

Paper No. 6
Presentation Time: 8:00 AM-12:00 PM


FULLER, Theodore and PERG, Lesley, Geology and Geophysics, Univ of Minnesota, 310 Pillsbury Drive, Minneapolis, MN 55455,

River terraces are ubiquitous throughout the world and are used extensively by geologists to infer climate and tectonic events. Identifying a climate or tectonic event from a preserved terrace is a classic example of an inverse problem. The current paradigm for river terrace formation points to external events as the primary driver inferring a one to one correlation between a climate event and a resulting terrace. We suggest that in basin systems characterized by extensive landslides and other autogenic processes, terraces might also be generated by stochastic processes that are internal to the system. The landscape of the South Fork Eel River (SFER) in northern California is characterized by large landslide events making it an ideal fluvial system to investigate the possibility of terrace formation due to internal stochastic processes. We envision three possible scenarios for terrace formation: 1) external processes only; 2) internal processes only and 3) a combination of external and internal processes. We believe it is likely that terrace formation on the SFER is due to a combination of external and internal processes. It is important to note that in both cases 2 and 3 above, the system is no longer a simple inverse problem with a single terrace correlating to a single climate or tectonic event. We hypothesize that terraces with similar elevation found throughout the drainage basin more likely represent an external climatic or tectonic event. Conversely, terraces that are discontinuous and localized represent an internal event. A detailed slope map of the drainage basin has been constructed using airborne LIDAR data. Assuming a stable river profile over the past 103 - 104 years, this slope map will be used to correlate terraces based on elevation. Field study will be used to confirm the initial correlation of terraces. Specifically, our field study will focus on the elevation of the bedrock strath surface and the internal stratigraphy of the deposits overlying the strath surface. This combination of data will allow us to present a qualitative analysis of terraces in the basin and to refine our model of terrace formation. Future work will include dating the terraces using 10Be. The use of cosmogenic dating will allow us to establish a chronology of terrace formation and compare this chronology with recent climate events.