GeoLog

Colorado Plateau

Imaggeo on Mondays: Of ancient winds and sands

Imaggeo on Mondays: Of ancient winds and sands

Snippets of our planet’s ancient past are frozen in rocks around the world. By studying the information locked in formations across the globe, geoscientist unpick the history of Earth. Though the layers in today’s featured image may seem abstract to the untrained eye, Elizaveta Kovaleva (a researcher at the University of the Free State in South Africa) describes how they reveal the secrets of ancient winds and past deserts.

In summer 2016 we toured the Western US in a minivan. We visited many of the gems of Utah, Arizona, and New Mexico, such as Monument Valley, Antelope Canyon, Grand Canyon, The Arches, Bryce Canyon, White Sands Monument… But the most precious and memorable for me was Zion National Park in Utah. This canyon is a unique and special place. First, because you access it from the bottom, unlike most of the other canyons, which you observe from cliff tops, such as the Grand Canyon. Thus, as you drive along the road, leading into Zion National Park, you look upward into the magnificent cliffs and rock temples. Small hiking trails lead up to waterfalls, arches and breathtaking views.

The cliffs of Zion National Park are built of Navajo Sandstone and display aeolian deposits, which have been shaped by winds, on a massive scale. They are the remnants of an ancient fossil-bearing sand desert, one of the greatest and largest wind-shaped environments that has ever existed on Earth.

In the Early Jurassic, up to 200 million years ago, the Navajo desert covered most of the Colorado Plateau (which today includes the states of Utah, Colorado, New Mexico and Arizona). Fossils, found in these sand deposits, include ancient trees, dinosaur footprints and rare dinosaur bones.

In Zion National Park, the thickness of sand deposits reaches 762 m. Beautiful cross-beds are cross-sections through fossilized towering sand dunes. They indicate the direction of the ancient winds, which were mainly responsible for moving and accumulating the sand in the Navajo desert. On the top, the Navajo sandstone is abruptly truncated by a regional unconformity, which indicates the erosion of the overlying sediments, and is covered by Middle Jurassic sediments. In remains unknown how much of the Navajo sandstone was eroded from the top of the formation during this weathering episode. It might be that the thickness and height of the Navajo sand dunes used to be even more impressive than it is now.

The cliffs of Zion National Park. Pictured is Checkerboard Mesa (South-Eastern entrance to the Zion National Park. Credit: Credit: Elizaveta Kovaleva.

By Elizaveta Kovaleva, post-doctoral researcher at University of the Free State, in South Africa

Movement of ancient sand is one of the winners of the 2017 Imaggeo Photo Contest.

References

Ron Blakey and Wayne Ranney, Ancient Landscapes of the Colorado Plateau, Grand Canyon Association, 2008, p.156.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

 

Imaggeo on Mondays: Natural Bridges Monument, Utah, USA

Imaggeo on Mondays: Natural Bridges Monument, Utah, USA

Slowly but surely, the force of water has carved out a beautiful landscape in the sandstones of the Colorado Plateau. Suspended over canyons, naturally formed bridges and arches are the starts of Utah’s first national monument. The geological and modern  history of the region is rich as Kimberly Galvez, a student of the University of Miami, describes below.

This image shows an overview of a portion of the Natural Bridges Monument in Utah, taken from the Colorado Plateau. In the lower center of the picture is the Sipapu Bridge, one of the 3 bridges within the national park.

The dominating sandstone comes from the Permian Cedar Mesa Formation that is part of the Colorado Plateau, a geologic province that extends through southern Utah and northern Arizona, northwest New Mexico, and western Colorado; with geologic units including the Lower Cutler Beds from the Pennsylvanian, Permian Organ Rock and Cedar Mesa Formation, Triassic Chinle and Moenkopi Formations, Jurassic Wingate Sandstone and the Quaternary alluvium.

Due to the regional uplift the Colorado Plateau experienced, meandering rivers progressively cut through the crossbedded sandstone – cross-bedding reflects the transport of sand by a flow of water over a river channel – of the Cedar Mesa Formation forming the bridges, leaving behind the exposed erosional surfaces carved out by the river.

Today, these bridges face many challenges: The region is still seismically active. Earthquake activity could lead to the collapse of (some of) the bridges and other small formations. Due to the low stability of the sandstone from the lack of anchoring from root structures, slope failures and slumps can be quite common and especially in the event of flash floods. Continuous wind patterns increase erosion and alter the exposed surfaces and the structure of the channel is constantly changing due to seasonal rainstorms.  Sediment transport and deposition, caused by streamflow, is a major factor in channel morphology and, therefore, the ecosystem of the Natural Bridges Monument.  The National Park Service constantly monitors the changes and issues that arise.

A final remark: Thanks to the entire group of the 2015 Annual AAPG Student Field Trip for making this photo possible. Members of the CSL – Center for Carbonate Research and the UM Student Chapter of AAPG for funds, and field trip leaders: Gregor Eberli and Donald McNeill.

By Kimberly C. Galvez, University of Miami – RSMAS

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.