Shankhadeep Baul / Earth & Environmental Sciences / Faculty Mentor: Majie Fan
In the western interior of the USA, eolian sedimentary rocks of the middle and late
Cenozoic age are widely distributed. Previously, late Paleogene loess deposits
have been identified in Wyoming and western Nebraska. This study explores a
new site near the Pawnee Buttes (PB) in northeastern Colorado to assist
understanding of the spatiotemporal extent of loess deposition during this period.
Here we present the results of field lithofacies studies and laboratory grain-size
analysis of samples from the White River Group at two nearby sections (PB and
PBE). The two sections were stitched together based on the elevation difference
and lithological markers. Field sedimentology data of the White River Group show
that the sections contain mostly pink to grey, massive siltstone or very fine-grained
sandstone with interbedded brick red, laminated siltstone. End member analysis of
the grain-size data shows three modes at 10 μm (EM1), ~30 μm (EM2), and ~70 μ
m (EM3). The massive siltstone and sandstone all contains EM1 and EM2, with
most of them also have EM3 up to 30%. The laminated siltstone contains 20-30%
EM1, 20-30% EM2, and 40-60% EM3. The grain size distribution of the massive
beds is similar to the loess deposits found on the Chinese Loess Plateau,
suggesting the presence of loess-like deposits in the Pawnee Buttes during the
Oligocene. EM1 reflects long-range transport by higher-altitude winds, while EM2
and EM3 were possibly transported by strong, episodic lower-level winds from
nearby or regional sources as fallout of suspension. The beds with root casts
generally have a higher clay content suggesting more pedogenesis under a warmer
and humid climate during those interval.
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