Ages for the Navajo Sandstone
Boulder, Colo., USA: The real Jurassic Park was as an ancient landscape
home to a vast desert covered mostly in sand dunes as far as the eye could
see, where dinosaurs and small mammals roamed southern Utah. The Navajo
Sandstone is known for its beautiful red and tan crossbedded sandstones
that grace many of the national parks and monuments in the southwest
USA––for example Arches, Canyonlands, Capitol Reef, and Zion national
parks.
The sands were deposited in dunes within the largest known sand sea (erg)
in Earth’s history during the Early Jurassic. These deposits show a record
of desertification—the process by which fertile lands become desert. How
did this landscape lose its water bodies, vegetation, and animals? How long
did desertification take to happen? How long did it last? What amount of
time is actually represented by these deposits? Understanding the timing,
scale, and duration of this significant period in Earth’s history is
challenging, and many questions are unanswered due to the lack of age
constraints in these deposits.
A new study by Parrish et al., published in Geology, has
determined numerical ages from several calcium carbonate (i.e., carbonates,
CaCO3) rock layers that represent lake deposits that once
occupied interdune areas, which served as watering holes for a variety of
dinosaurs and small theraspids (relatives of mammals). These carbonates
were age dated using the radiometric method of uranium-lead (U-Pb),
providing ages of 200.5 ± 1.5 million years (Ma) and 195.0 ± 7.7 Ma.
These age dates show that in eastern Utah parts of the Navajo desert are
much older than previously thought, and together with age dates from
Arizona show that the giant sand sea became younger to the south. The lake
and associated spring deposits also show that this vast desert, at times,
had a wetter climate and more active hydrologic cycle than had been
previously assumed.
This work demonstrates that the desertification process is complex, and
that age dates from carbonates and correlation of rock layers will help
answer major questions of how desertification takes place in continental
interiors.
This study has societal relevance because the history of hydroclimate
(i.e., groundwater and climate) change recorded in the Navajo desert
deposits can serve as a model for modern marginal environments that may be
impacted by desertification from a warming climate. With the projected rise
in global temperature, regions in marginal zones are anticipated to become
even more vulnerable to desertification. That is, these zones will become
part of the growing desert regions. Particularly vulnerable areas are found
in Africa and Asia, areas with large population densities that are already
exceeding the capacity to supply food and water. By studying how the Navajo
erg evolved, we can provide important insights into rates of modern
desertification.
FEATURED ARTICLE
Earliest Jurassic U-Pb ages from carbonate deposits in the Navajo
Sandstone, southeastern Utah, USA
Judith Totman Parrish, Univ. of Idaho, and colleagues. Corresponding
author: E. Troy Rasbury, Stony Brook Univ., troy.rasbury@stonybrook.edu.
URL:
https://pubs.geoscienceworld.org/gsa/geology/article/573442/Earliest-Jurassic-UPb-ages-from-carbonate-deposits
.
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