Petrographic and geochemical screening of clay genesis for paleoweathering and geochronology applications

Fine-grained sedimentary rocks form in response to a range of tectonic, climatic, biospheric and basinal conditions, preserving characteristic signatures which make them important archives of Earth System and biospheric evolution. However, they are commonly comprised of reactive constituents prone to post-depositional modification. It can be very difficult, therefore, to distinguish meaningful paleoenvironment signatures from those due to post-depositional alteration, particularly in Precambrian or early Paleozoic successions. A further difficulty in sediments of this age is the determination of robust depositional ages required for precise stratigraphic correlation. This thesis develops novel approaches to address these long-standing problems, taking advantage of newly available microbeam and mass spectrometry tools. A new generation of high-resolution electron imaging and mineral mapping tools are utilized to define robust petrographic criteria to distinguish between depositional and diagenetically altered sedimentary constituents. The main targets here are clay minerals, for which potential secular changes in abundance and composition have been proposed, linked to an increase in chemical weathering associated with rise of biologically active soils (the ‘Precambrian greening of the continents’). Careful petrographic and mineralogical analysis of a set of well-preserved shale and mudstone samples spanning a time interval from Mesoproterozoic (the Velkerri Formation, McArthur Basin, North Australia) to late Neoproterozoic (the Enorama Shale, Brachina and Bunyeroo formations, Adelaide Geosyncline, South Australia) permits the assessment of this hypothesis. It is shown that the high content of illite and kaolinite in the Velkerri Formation, previously interpreted as a signal of intense chemical weathering of adjacent landmasses, is in fact due to the burial diagenetic replacement of primary igneous minerals such as mica or feldspar. Similar mineral compositions and post-depositional processes are identified in the late Cryogenian Enorama Shale and the early Ediacaran Brachina Formation. These sediments record physical erosion with limited pedogenic clay mineral formation in the absence of biologically active soils (i.e. abiotic weathering). The mid Ediacaran Bunyeroo Formation, by contrast, is characterized by a substantially higher content of pedogenic 1M and 1Md illite coupled with trace amounts of readily weathered phases such as feldspar, indicating enhanced chemical weathering. This is consistent with previous studies arguing for a growing influence of biologically-mediated pedogenic weathering in the Ediacaran. A similar approach is developed for the screening of glauconite grains with the aim of testing whether reliable in situ Rb-Sr depositional ages (from laser-ablation reaction cell mass spectrometry) can be obtained from partially altered sequences, focusing on paleontologically significant lower Cambrian glauconite bearing sequences from the Arrowie Basin (South Australia) and the Amadeus Basin (Central Australia). It is shown that illitised glauconites are petrographically distinct from pristine grains, but geochemical screening is complicated by the significant compositional overlap of both classes, attributed to the fine, μm to sub-μm intergrowth of illitised and unaltered glauconite domains. In addition, geochemical screening of petrographically ‘pristine’ grains identifies the widespread presence of cryptic apatite and/or carbonate inclusions, with truly unaltered glauconite being rare. Consistent with this, the majority of grains show 87Sr loss so that apparent Rb-Sr ages are mostly younger than the known stratigraphic age. A subset of apatite and calcite-inclusion bearing grains show significant 87Sr enrichment, likely sourced from decomposition of adjacent high Rb-Sr detrital phases during diagenesis, resulting in apparent ages older than the stratigraphic age of the samples. While the combined petrographic –geochemical screening approach developed here shows potential for identifying the best preserved glauconite grains in partially altered samples, further work is required to confirm the suitability of these grains for geochronology, with only one sample containing ‘pristine’ glauconites with apatite inclusions returning an age within error of the expected stratigraphic age.</p