Wednesday, January 25, 2017

Zion National Park - Land of Landslides

Rockfall from the January 13, 2017 event on Zion National Park's Scenic Road
Earthly- Musings is back from vacation and I will be posting photos and stories from a fantastic trip to the Yucatan, Belize and Guatemala soon.

While the blog was on vacation, Zion National Park in southwest Utah experienced some amazing rock failures. These events were due to over saturation of soils from recent rains and freeze/thaw conditions. However, the bedrock geology inside the Park and surrounding areas lends itself as well to these types of catastrophic events. The main trigger is the presence of hard-over-soft strata, whereby cliff-forming rock units such as the Shinarump Conglomerate or the Navajo Sandstone, lie above very soft, easily eroded rock units like the Petrified Forest Member of the Chinle Formation and the Kayenta Formation (respectively).

Hard-over-soft couplets that can lead to undercutting and rock failure (from Kansas Landscapes, Frank Wilson, 1978)
When soft units erode, they undercut harder units above them and rock failures occur. (Recall a massive rockfall that happened just outside the Zion Park boundary in Springdale Utah in December 2013. In this event, two people were crushed to death in their home. The event was triggered when huge blocks of Shinarump Conglomerate were undercut and fell on the home. See the E-M's post for this event here and another newspaper article about the couple killed here).

This year, Zion closed two trails on January 10 when rockfalls and river erosion affected them. They were the Emerald Pools Trail and the Sand Bench Trail. See the official Park announcement here. Then three days later on January 13, 200 tons of rock in the Kayenta Formation fell on the Scenic Road north of the Zion Canyon Lodge. See the official announcement here. The Scenic Road was reopened a few days later with the official announcement visible here. Note from the photo at the top of the page that interbedded sandstone and mudstone in the Kayenta Fm. were involved in the slippage.

Just last year, researchers announced new data on the massive Sentinel landslide that occurred in the canyon some 4,800 years ago. An article in Earth magazine appeared in October, 2016 and can be read here. GSA Today published an extensive article on the work in their June, 2016 issue and can be read here.

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