We report Rb-Sr, Sm-Nd and mineralogical data for leached whole rock, < 2 btm and < 0.1 #m clay size fractions of lower-to mid-Tertiary shales spanning the smectite/illite transition, from a single deep well in the Texas Gulf Coast. The abrupt transition at 2400 m, from smectite-to illite-dominated assemblages is accompanied by a marked increase in 87Sr/S6Sr from 0.708 to 0.711 in exchangeable sites. Leached authigenic illite from the < 0.1 /~m fraction, sampled from below the smectite/illite transition, defines a Rb-Sr isochron age of 34.8 _+ 2.0 Ma. These data are consistent with a single episode of authigenic illite precipitation over a depth range of > 1200 m, independent of the duration of burial. The exchangeable Sr in the clay has remained an open system and has undergone isotopic exchange since that time. Minimum fluid/rock ratios of 0.2 to 2 are modeled from Sr isotopic data, consistent with rock-dominated, open system diagenesis. Leachates and residues of the < 0.1/zm fraction, below the smectite/illite transition appear to have achieved Nd isotopic equilibrium during clay diagenesis, and show no signs of subsequent exchange. The clay residues have low 147Sm/144Nd (< 0.085), indicating that authigenic illite has a lower Sm/Nd ratio than its bulk rock protolith. Conversely the coexisting exchangeable REE inventory has elevated Sm/Nd, as reflected by leachate 14VSm/144Nd as high as 0.188. This Sm/Nd fractionation, in conjunction with the preserved isotopic equilibrium may be useful for dating diagenesis and associated fluid-rock interaction in older rocks.