Chronostratigraphy in Chemical Sediments and Ashfalls

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Chronostratigraphy in Chemical Sediments and
Ashfalls J. Wood Michigan Technological
University
Environmental Clues in Ancient Rocks
The sediments in the Michigan Basin contain a
rich record of the early history of North America
in the Early Paleozoic, about 542 Mya (million
years ago) to about 400 mya. This was a time when
Michigan was located about 10-20 degrees south of
the equator and a time when vast quantities of
salt, reefs and volcanic ash accumulated. The
excursions on oil well logs, such as the one
shown on the right, can be correlated over most
of the Michigan subsurface and thus provides not
only a record of the geologic environments of the
time, but also how long those environments
lasted. We have used data from well logs like
this to determine how long certain periods
persisted as either arid or humid, and also to
constrain a detailed model of the
paleoenvironments present at the time. This is
especially pertinent to helping understand the
present period of global warming.
Section of the Salina Fm.(4500-7000 ft) in the
Michigan Basin illustrating the prominent log
excursions that define relative time markers in
salts (red). The gamma ray trace is shown on the
left, the density log (RHOB) on the right.