Date of Award:

2014

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Joel L. Pederson

Abstract

Arcing eastward from the deep gorge of Cataract Canyon on the Colorado River is a series of aligned valleys (graben) and ridges (horst). This unusual landscape has formed as subsurface salt deforms toward the river and dissolves away, causing the overlying rocks to fault, slide, and subside. Geologists have long been interested in this actively evolving area they call the Needles fault zone, because understanding its mechanics and origin may shed light on how faults work in general and similar, yet inaccessible places like offshore rift zones or even the surface of the Moon. Despite this interest, the timing and long-term patterns of deformation here and are poorly constrained.

This study uses analysis of digital landscape models to better delineate these patterns and provide better age constraints on the development of the Needles fault zone. We find that the Colorado River incision that led to deformation here began as recently as 1 million years ago, and that faulting due to subsurface salt movement initiated between 700 and 200 thousand years ago.

The first part of this study takes advantage of how the development of graben valleys has changed the path of many of the streams in the study area, resulting in numerous captured streams terminating into a type of sinkhole, called a swallow hole, that develops above opening faults. These fissures are so named because, by ongoingopening, they are “swallowing” material that is flushed into them by local drainages. By recording and numerically dating the exposed upper 6-14 m of basin-fill strata, we determined that sediment was deposited to an alluvial fan and to ponded water. We also compared calculated sediment yields over time to paleoclimate records for the region to test extant hypotheses about how drylands respond to changing climate of the same scale as modern climate change. Against expectations, our results suggest that the greatest sediment yield and storage in these upper basins occurred during the relatively warm and dry time from 9 to 5 thousand years ago, when overland flow to transport sediment was weak. This implies that we are actually measuring sediment storage, as the faults that form swallow holes were relatively less active, allowing sediment to accumulate, rather than be flushed out of the basins.

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Geology Commons

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