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.