Trenching the Cheraw Fault in Southeastern Colorado

2020-03-20 | CGS Admin

One of the missions of the CGS is to better understand the various geological risks that face the residents of the state. While not seen as a seismically active region by most people, Colorado does have its share of geologic faults and historical earthquake activity. The state has over 90 mapped faults that are considered potentially hazardous.

One of the best techniques for understanding the historical activity on a particular fault is called “trenching.” Trenching is an excellent tool for charting disturbances in near-surface sediments that then help establish the historical activity of a fault. The process begins with the determination of where exactly on the fault is of greatest interest, and what orientation of trenching will, hopefully, yield the most information. Once that is decided, the actual removal of a significant amount of surface soil begins. Ideally the trench will get to bedrock and deeper to expose disturbed sediments caused by historical fault movement. Following the careful mapping of those disturbances, the trench is ultimately back-filled.

One fault of interest in Colorado is the Cheraw fault, named for the nearby town of Cheraw in rural southeastern Colorado. The fault was initially recognized and mapped in the 1970s by the USGS, and was first trenched by the USGS in the mid-1990s. It was cursorily re-examined by the CGS in the early 2000s. Later, in 2016 the CGS funded an investigation of a poorly studied segment of the fault led by paleoseismologists Dean Ostenaa and Mark Zellman, the authors of the recent CGS publication that documents this research: MI-97 Paleoseismic Investigation of the Cheraw Fault at Haswell, Colorado. They have been performing active research on the Cheraw fault since 2011.

A second CGS-funded project aimed at augmenting knowledge of the paleoseismic history of the Cheraw commenced during the week of 29 April – 3 May, 2019. A team, consisting of CGS geologists, along with Ostenaa and Zellman, and a cadre of scientists from the USGS, NRCS, USBR, and private consultants re-excavated a former USGS trench across the fault near the town of Arlington in southeastern Colorado.

Data gathered from the prior 1995 trenching effort by the USGS was limited by the geologic dating technologies of the time as well as groundwater conditions. Utilizing contemporary optically stimulated luminescence (OSL) dating technology and additional carbon-14 samples—to accurately date sediments and faulting events—this current research effort hopes to resolve the complex paleoseismic issues on the fault. The new excavation allowed geologists to re-map the sediments exposed along the trench walls and to collect and analyze of previously dated and undated sediments that were exposed in the prior USGS trench. In addition, new samples from adjacent areas were gathered along the fault scarp, and the trench was also deepened to search for evidence of additional ruptures and to better determine any offsets of the stratigraphic surface of the bedrock. Knowing the ages of the faulted sediments, amount of fault displacement, and number of faulting events directly feeds into models that attempt to understand the possible recurrence of fault activity and ultimately drive building codes.

Stay tuned for the results of this project that we will be publishing in the future.