From western Texas and southeastern New Mexico, core samples of shallow subtidal and peritidal carbonates in the Ellenburger Group (Lower Ordovician) were examined to investigate the effects of deep burial on diagenesis. At present, the burial depth of samples ranges from 6,000 ft. (1.5 km) to 23,000 ft. (7.0 km). Below 10,000 ft. (3.0 km) carbonates are exclusively dolostones, showing evidence of deep-burial dolomitization such as the presence of coarse crystalline dolomite, xenotopic texture, homogeneous cathodoluminescence, high fluid-inclusion homogenization temperatures, and light oxygen-isotope compositions. Locally, neomorphism of older, fine crystalline dolomite occurs; however, the preservation of depositional textures in coarse crystalline, xenotopic dolomite indicates a nonobliterative replacement origin in deep-burial environments. Millimeter-size crystals of euhedral saddle dolomite commonly fill vugs and fractures. The Mg (super +2) ions in the nonferrous dolomites are probably derived from basinal brines via high porosities and fractures. Higher temperatures and longer reaction times coupled with the addition of Mg (super +2) ions from circulating waters are apparently responsible for deep-burial dolomitization.