HEAVY MINERALS FROM THE ST. PETER SANDSTONE NEAR STARVED ROCK STATE PARK, ILLINOIS

The Ordovician St. Peter Sandstone is an important formation throughout much of the Midwest. The proportions of minerals in the St. Peter sandstone commonly consist of 99% quartz, with much of the remainder being heavy minerals. Zircon has been reported as the most common heavy mineral in the St. Peter from a variety of localities. Although zircon has been used successfully in provenance studies (Winters et al. 1999), other heavy minerals may also provide provenance information.

Samples were collected from the Starved Rock member of the St. Peter sandstone, near the entrance to Starved Rock State Park. The samples were examined using SEM, XRD and light microscopy to determine if minerals other than zircon occurred and if present, extract provenance information from these minerals.

Garnet and zircon were the most common heavy minerals found. The zircon and garnet specimens were very well rounded. The garnets have chemical compositions dominated by Mg, Fe, and Al with lesser amounts of Ca and Ti. TEM investigation has revealed authigenic microcrystals of apatite in some samples, which commonly occur as 1 to 10 mm clusters. A high pressure silica phase (HPSP) was also identified in the heavy mineral separate. The high pressure phase was identified by density, SEM, and EDS analysis operated at 25kv. The high pressure silica phase grains are subangular and are commonly 0.25mm in diameter. Coesite has been recognized in the St. Peter Sandstone in the Kentland Impact structure of Indiana. The HPSP observed in the starved rock member may be ejecta from the Kentland impact structure and may provide an event stratigraphic horizon within the Starved rock member of the St. Peter sandstone.