Electron Microprobe Model Analysis of Muddy Sediments for Provenance Determination: ABSTRACT

Abstract

The authors are experimenting with a method of energy dispersive electron microprobe (EMP) modal analysis and testing the method on Tertiary continental back-arc muddy sediments; relative abundances of quartz, feldspars, and clay minerals constitute the primary data. Resolution of a focused electron beam <5 m in diameter, achieves the ultimate in Gazzi-Dickinson modal analysis scheme. Proportions of detrital and authigenic materials, preferred diagenetic alteration of framework grains into clay minerals, and grain replacement characteristics of associated cogenetic coarser sediments are determined by quantitative optical methods. Assuming that clay minerals are alteration products of only feldspars and lithic fragments, modal abundances of clay minerals and grain replacements are redistributed into framework grains, within constraints of mass balance and paragenetic relations obtained from optical studies. Modal analysis of 27 samples by EMP and of 22 samples by optical methods (300 points/sample) from volcaniclastic Medicine Lodge beds in southwestern Montana show that recalculated framework grain proportions of muddy siliciclastic sediments cannot be assigned to proper tectonic provenance fields using Dickinson's boundaries set up for sand-size sediments. Errors increase with increasing grain size of sediments because quartz and feldspar grains are not redistributed as lithic fragments or pseudomatrix. The dispersion of modal data correspondingmore » to suspected subtle variations in source rocks suggests that many EMP modal analyses of a variety of muddy siliciclastic sediments may yield clusters characteristic of tectonic provenance.« less