Shales and mudstones are fine-grained sedimentary rocks of global extent and considerable economic importance. Dating them is not straightforward, because clay minerals such as illite can reset isotopic systems during diagenesis. K-Ar, Ar-Ar and Rb-Sr dates of shales therefore often record diagenetic events rather than deposition. Sm-Nd and detrital-zircon U-Pb data mainly constrain provenance, whereas Re-Os data from organic-rich mudrocks can give depositional ages, and Re-Os data from oils constrain hydrocarbon generation. U-Pb dating of zircon from interbedded volcanic ash provides the most direct depositional age. Carbon-isotope excursions, elemental ratios, redox-sensitive trace metals,... More >