Iron oxide layer at base of Mulberry Member
A conspicuous layer of claystone stained by red iron
oxide marks the
base of the Mulberry Member along much of the I-170 road cut.
The iron-rich layer has a sharp base, and it grades upward into shale
or claystone that contains limestone lenses and clasts, phosphate
nodules, and thin layers of impure coal. The iron oxide probably
became concentrated here due to the lesser permeability of claystone
below the contact (in the varicolored claystone unit) making a barrier
to infiltration of mineral-saturated ground water. Locally the iron-rich
layer is absent, possibly because in places the composition of
sediment resting on top of the varicolored claystone inhibited iron
oxide precipitation. Only after moving laterally to a more "hospitable"
host rock did iron oxide precipitate. The base of the Worland is at the
top of the photo, above a thin zone of greenish gray shale with
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