Maverick Science mag 2013-14

Photo credits:
Above and right,
Brandon Wade; top
right, Majie Fan.
At top left, master’s student Sara Ayyash drills
holes in a freshwater carbonate sample collected
near Douglas, Wyoming, in order to collect
the powder for stable isotope and element
concentration analysis. Above, doctoral student
Min Gao conducts primary field work last
summer at Firehole Canyon in southwestern
Wyoming, where the upper stream of the Colorado
River cut the rocks formed in a lake environment
about 52 million years ago. At left,
Fan in her lab with master’s student Ohood Al
salem, who is examining a thin section of sandstone
collected in Wyoming. The thin section
of sandstone, which dates from the Miocene
epoch, can be seen on the computer monitor.
ern soil and water samples in order to understand
the stable isotope signatures of modern climate
and environment, “because the interpretation of
geologic data derived from the rock record relies
on the understanding of such data in modern geologic
context,” Fan said.
The project has three goals, Fan said. The first
is to investigate the occurrence and timing of a regional
transition of post-Laramide basin fill from
fluvial to eolian deposition, by applying field observations,
sandstone petrography and detrital
zircon geochronology study, and grain-size analysis.
The second is to constrain the timing of surface
elevation changes by reconstructing the
stable isotope compositions of middle and late
Cenozoic surface water and surface temperature
from the combined analysis of hydrogen isotope
ratios of volcanic glass, oxygen and clumped isotope
ratios of pedogenic and lacustrine carbonates.
The third is to evaluate and refine proposed
competing mechanisms of formation of the high
central Rockies.
Gao spent three weeks last summer with Fan
in Wyoming, locating areas with good outcrops
from which to collect samples. Gao is focusing on
sediment constraint and basin subsidence modeling
in order to understand the influence of mantle
upwelling on surface uplift when the subducting
flat oceanic slab was detached underneath the
continental plate.
“In the next field trip, I will be working on
measuring stratigraphic sections and collecting
rock samples,” said Gao, who wants to teach and
do research at a university in her native China
after earning her Ph.D. “Specifically, after we find
a good outcrop to work on, we will measure the
thickness of every stratigraphic layer and record
as many characteristics of the rock as possible,
such as the lithology and the sedimentary structure.”
The field work involves a great deal of hiking.
Due to the remote location of the outcrops and in
order to keep costs down, the team normally
camps in tents.
Fan and her students are in the final stages of
lab data collection and have recently published
one paper and submitted two others based on
their research. They also have three papers in different
stages of preparation. Their research has
yielded the discovery that modern soil carbonate
in the central Rocky Mountains is formed during
the short periods of soil dewatering, which happen
irregularly during hot summer weather.
“This particularly challenges the previous assumption
that soil carbonate is formed continuously
during the growth season,” Fan said.
They also found that the central Rockies and
adjacent Great Plains in western Nebraska experienced
another stage of uplift after the end of the
Laramide orogeny, which caused concomitant
drying in both areas. Lastly, they found that there
was a regional transition from water-lain sedimentation
to wind-lain sedimentation which occurred
32-35 million years ago, and the transition
became younging eastward.
“The transition represented a regional drying
event caused by the initiation of a rain shadow
due to the uplift of the central Rocky Mountains.
However, global cooling due to Antarctica glaciation
at around 34 million years ago may be another
factor causing the drying events. Such
findings bring new research questions,” Fan said.
In a separate project, Fan is teaming with Barbara
Carrapa of the University of Arizona to research
the temporal and spatial patterns of
Laramide uplift to evaluate and refine the proposed
competing geodynamic models of the tectonic
processes during the Laramide orogeny.
22 Maverick Science 2013-14