The combined use of oxygen isotope and K/Ar dating can yield information about the diagenetic history of clay minerals in shales and sandstones. In the Tertiary shale sequences of the United States Gulf Coast, in which the dominant detrital clay mineral is mixed-layer illite/smectite, the progressive conversion of expandable clay layers to illite layers can be monitored by the isotope systematics. Oxygen isotopes of fine-grained clay and quartz approach equilibrium with one another, raising the (as yet unrealized) possibility of an O-isotope geothermometer. Fine-grained quartz becomes isotopically zoned as detrital grains are overgrown by diagenetic quartz that forms, accompanying the smectite-to-illite transformation. The K/Ar clock of the diagenetically formed illite la ers is set to zero age at the time of diagenesis (although that of existing illite layers within the crystals remains unaffected), and it is therefore possible to estimate the mean time of diagenesis in such shale sequences.

Illite is a common cement in sandstones. The time of cementation by illite can be estimated from K/Ar systematics in cases, such as that of the Permian Rotliegendes of the North Sea, where the clay-sized fraction of the original detritus was relatively free of illite or other K-bearing phases. Conditions of cementation can be inferred from oxygen isotope measurements, augmented by knowledge of the geologic/tectonic history of the sandstone. In the case of the Rotliegendes, the timing and conditions of illite cementation were relatively uniform within fault blocks but varied from block to block. Oxygen isotope measurements indicate that meteoric water components were important in illite formation at times when nearby sections of the Rotliegendes were exposed to the surface by uplift an erosion.

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