A new approach to determine the precursor grains of detrital and diagenetic clay in sandstones requires a reiterative use of optical microscopy and electron beam microanalysis. Excluding muddy sandstone successions from analysis of the siliciclastic fill of a basin may lead to incomplete if not erroneous provenance interpretation because of biased samping of the sediments. To minimize problems inherent in muddy sandstones, we have combined a two-tier optical modal analysis with energy dispersive X-ray analysis (EDXA) to identify precursors of clay. This is a new method of point counting and estimating the mineral compositions of muddy sandstones. Because we apply the principle of identifying the mineral under the cross hair, albeit on a micron scale, we call this the new or modified Gazzi-Dickinson method. Calculations of mass-balance type using the results of this new method may be performed to reconstruct the original composition of the parental detrital material. We have used muddy siliciclastic rocks (outcrop and core samples of siltstones and sandstones) from the Oligocene, intra-arc Medicine Lodge Beds of southwestern Montana, USA for this study. Eocene volcanic rocks of the Challis region and thrusts that have transported pre-Cenozoic sedimentary and metasedimentary rocks in this region were the sources of the siliciclastic sediments. Results of modal analysis by the standard Gazzi-Dickinson method and by the new/modified Gazzi-Dickinson method, using both optical and EDXA on a micron scale, plot in different clusters in a QFL diagram. Our results show that the standard Gazzi-Dickinson method is not truly independent of grain size even in the context of provenance analysis. On the other hand, the modified version has the promise of achieving truly mineralogical modal petrologic analysis of detrital rocks of any grain size.

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