The <0.05-µ-size fractions of a suite of upper Oligocene Frio Formation shale samples taken from below 10,500 ft in a well on the Texas Gulf Coast fall on a Rb-Sr isochron corresponding to an age of 23.6 ± 0.8 m.y. The isochron shows an initial 87Sr/86Sr ratio of 0.7088 ± 0.0004. This suggests that regularly interstratified illite/smectite simultaneously equilibrated soon after burial over a range of burial depths at 7,000–8,000 ft shallower than at present. Oxygen-isotope values for these clays are also consistent with equilibration under shallower burial conditions. Such a pattern conflicts with the prevailing idea of a gradual illitization of smectite with progressive burial. A new model of episodic or “punctuated” diagenesis is proposed, whereby sudden diagenesis is initiated by a change in pore-water chemistry. Clay reaction occurs over a range of temperature at this time, such that the deeper clays are transformed completely to 80% illite, and the shallow clays are transformed less completely. Subsequent pore-water chemical changes inhibit further clay reaction, and the mineralogic gradient and the isotopic signature formed at 23.6 m.y. B.P. are “frozen in.” Seven to eight thousand feet of additional post-Oligocene burial has not resulted in the creation of younger illite/smectite. Consequently, there is no evidence that illite/smectite diagenesis is currently taking place in the Frio Formation.