Mesozoic Era Formations
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Pierre
formation
(Tour
Map symbol 'P')
The Pierre shale is the surface formation in the midsection of western South
Dakota from the Missouri River to the Wyoming border (geology
map). It consists of light to dark
gray shales, mudstones, and siltstones that were deposited in a marine environment
during the Cretaceous period. Occasionally it is banded with lighter-colored
bentonite derived from volcanic
ash. Well-developed drainage channels dissect the landscapes
near the Missouri River and surrounding uplands. These landscapes eventually
grade into more gently sloping hills away from other major west to east drainage
basins.
Fox
Hills formation (Tour
Map symbol 'Fh')
Overlying the Pierre shale (stratiography
map) are the late Cretaceous sandstones and shales exposed
in the north central part of the state. The Fox Hills formation is a light-colored
sandstone that forms the resistant caps
of buttes over the less resistant Pierre shale.
Hell
Creek formation (Tour
Map symbol 'Hc')
Overlying the Fox Hills formation is the multicolored and banded Hell
Creek formation that is famous for dinosaurs fossils. The Hell Creek formation
consists of bands of mudstones and thin seams of volcanic ash and lignite and
was deposited in terrestrial environment as erosional and accretion events.
At the top of the Hell Creek formation is the
KT boundary between the Cretaceous and Tertiary strata. In locations around
the world, a greater Iridium concentration is measured in this zone than in
strata above or below it. The content of Iridium and Platinum family elements
is thought to have originated from the fallout of an asteroid impact that led
to the extinction of dinosaurs. This is a controversial subject of much
investigation and discussion.
