Pitfalls of Illite Geothermometry: Probable Smectite Precursor Minerals in the Mississippian Chainman Shale of Nevada

Donna M. Herring and Regina N. Tempel

Shaly basins with few or no discrete bentonites may still have received significant volcanic ash input. If unrecognized, this could lead to erroneous interpretations of basin subsidence history. The primary effect of dissemination of volcanic ash in a detrital mud is to reduce the average illite/smectite (I/S) crystallinity, thereby misrepresenting the time of deepest burial. If the original chemistry of the glass can be discerned, a qualitative estimate of this "dilute ash" effect is possible, and might be used to correct burial histories.

The reduction in average I/S crystallinity with addition of volcanic glass to detrital mud is related to the fact that there are more kinetic steps in the transformation (glass to diagenetic smectite to illite) than in the transformation (detrital smectite to illite). The glass-to-smectite transformation can be rapid under certain conditions, but glass may also transform to palygorskite or other smectite-precursor minerals, depending on its original chemistry.

In the Mississippian Chainman Shale of eastern Nevada, recent stratigraphic work has identified an overabundance of smectite, when the age and burial history (as inferred from independent data) of the unit are considered. Bulk chemistry, trace element analysis and X-ray diffraction studies through a 3000 ft interval confirm concentrations of immobile elements, most notably Cr and V, which are highly anomalous for average North American Mississippian shales but typical of tholeiitic basalts. We are working to tie the Chainman smectites chemically to their probable volcanic glass origins, and to discern the rates of the glass-to-smectite-to-illite transformation in the case of basaltic ash disseminated in the mudrock column.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California