The thermal, mechanical, and chemical evolution of a sedimentary basin exerts important controls on porosity and permeability of reservoir rocks. Oxygen isotope ratios of individual diagenetic cements record evidence of this history, but cannot be analyzed accurately by conventional techniques. Recent improvements for in situ analysis by ion microprobe provide high precision and accuracy at a scale of 5–10 μm. In combination with cathodoluminescence imaging, in situ analysis of δ18O (quartz) from the Cambrian Mount Simon Sandstone in the Illinois Basin (USA) reveals gradients within single overgrowths of as much as 7.7‰/50 μm. While the inner portions of overgrowths remain approximately constant in δ18O across the basin, the δ18O of the rim becomes lower with depth. These data suggest that overgrowths formed during burial and heating, possibly with minimal changes in δ18O of pore fluids. If δ18O(H2O) = −3‰, the highest temperature calculated for the rim of an overgrowth is 107 °C at a paleodepth of 3.5 km. The variability both in average δ18O of overgrowths and patterns from individual overgrowths corresponds with a geotherm of 30 °C/km, and there is no evidence of quartz precipitation from higher temperature hydrothermal fluids.