Provenance interpretation

Introduction Sedimentologists have, for a great many decades, displayed a persistent interest in unraveling the provenance of siliciclastic sedimentary rocks. This interest has extended to interpretation of both the lithology of parent source rocks and the tectonic setting of source areas. Many properties of siliciclastic sedimentary rocks such as texture, sedimentary structures, chemical composition, and facies relationships may play some role in provenance analysis (e.g., Boggs, 1992, ch. 8); however, provenance analysis has focused on identification and interpretation of the particulate components (minerals and rock fragments) of conglomerates, sandstones, and, to a lesser extent, shales. Large clasts in conglomerates can be easily and reliably identified; therefore, interpretation of source-rock lithology from study of conglomerates is relatively straightforward. That is, the coarse clasts in conglomerates can commonly be readily traced to the specific kinds of plutonic igneous, volcanic, metamorphic, or sedimentary source rocks from which they were derived. On the other hand, conglomerates probably make up less than about one percent of all sedimentary rocks; so they may not be readily available for provenance analysis in many cases. The provenance of shales, which make up roughly 50 percent of all sedimentary rocks in the geological record, is unfortunately much more difficult to assess reliably because the fine grain size of shales makes them difficult to analyze petrographically. Sandstones, which are sufficiently coarse grained to be readily studied by petrographic methods and abundant enough (about 25 percent of all sedimentary rocks) to be important, have consequently received the bulk of attention from geologists interested in provenance analysis.