QUANTIFICATION OF THE EFFECTS OF SECONDARY MATRIX ON THE ANALYSIS OF SANDSTONE COMPOSITION, AND A PETROGRAPHIC-CHEMICAL TECHNIQUE FOR RETRIEVING ORIGINAL FRAMEWORK GRAIN MODES OF ALTERED SANDSTONES

Most studies Of sandstone provenance involve modal analysis of framewo rk grains using techniques that exclude the fine-grained breakdown pro ducts of labile mineral grains and rock fragments, usually termed seco ndary matrix or pseudomatrix. However, the data presented here demonst rate that, when the proportion of pseudomatrix in a sandstone exceeds 10%, standard petrographic analysis can lead to incorrect provenance i nterpretation, Petrographic schemes for provenance analysis such as QF L and QFR should not therefore be applied to sandstones containing mor e than 10% secondary matrix, Pseudomatrix is commonly abundant in sand stones, and this is therefore a problem for provenance analysis, The d ifficulty can be alleviated by the use of whole-rock chemistry in addi tion to petrographic analysis, Combination of chemical and point count data permits the construction of normative compositions that approxim ate original framework grain compositions, Provenance analysis is also complicated in many cases by fundamental compositional alteration dur ing weathering and transport, Many sandstones, particularly shallow ma rine deposits, have undergone vigorous reworking, which may destroy un stable mineral grains and rock fragments, In such cases it may hot be possible to retrieve provenance information by either petrographic or chemical means, Because of this, pseudomatrix-rich sandstones should b e routinely included in chemical-petrological provenance analysis, Bec ause of the many factors, both pre- and post-depositional, that operat e to increase the compositional maturity of sandstones, petrologic stu dies must include a complete inventory of matrix proportions, grain si ze and sorting parameters, and an assessment of depositional setting.