Paper Abstracts

Micas in cross-bedded sandstones

K. Borsch, J.H. Whitmore, R. Strom, G. Hartree

Discipline: Geology

Abstract: In our study of thin sections of the cross-bedded Coconino Sandstone, we discovered muscovite in almost every thin section and biotite in a lesser number of thin sections. This is surprising given that micas have not previously been reported in this deposit. The mica found is detrital in character thus is part of the depositional fabric. This led us to investigating other cross-bedded sandstones from around the world, all of which have been conventionally interpreted as wholly or partly eolian. Subsequent laboratory experiments provided some framework for understanding this discovery. Based on those experiments it was found that mica cannot survive more than a few days during continuous transport in a simulated eolian environment but can last more than a year with continuous transport in a simulated subaqueous environment. Modern ergs contain none to virtually no micas except in cases where mica sources (like granite outcrops, beach sand or fluvial sand) are located in the immediate vicinity (~<10 km) of the erg.

This paper catalogs and illustrates a large number of cross-bedded sandstones that contain mica (mostly muscovite) as an accessory mineral. The dominant conventional view is that these sandstones are eolian, but experimental data and field observations suggest otherwise. The presence of muscovite in cross-bedded sandstones can be used as one of many criteria to argue for subaqueous deposition of cross-bedded sandstones.