Punch bowl filled with phosphate nodules from various units at I-170 exposure
Punch bowl filled with phosphate nodules from the I-170
Most nodules are roughly spherical or ovoid in shape, but a few are
rather elongate. For some, the shape correlates with the kind of
phosphatic fossil nucleus around which the phosphate precipitated.
The color reflects the color of adhering shaly matrix.
Pennsylvanian phosphate nodules apparently precipitated within soft
(nonlithified) shaly sediment deposited under conditions of poor
oxygenation. Surface waters were highly productive, and much of the
phosphate was derived from skeletal and fecal material that rained onto
the sea floor. Today, phosphate precipitation commonly occurs on the
outer reaches of continental shelves, associated with upwelling of
deep oceanic water. Similar conditions have been inferred for the
Pennsylvanian phosphatic shales, except these units were deposited
in a broad interior seaway. The Midcontinent Pennsylvanian sea was
probably never much deeper than about 100 meters, but this may
have been deep enough for upwelling. However, some Pennsylvanian
phosphate occurrences, such as in the Mulberry Member, suggest
deposition in near shore settings (see text).
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