In both sandstones and mudrocks, dissolution and replacement can cause substantial modification of detrital feldspar assemblages. Over burial depths from < 1 km to 5.5 km in the Frio Formation (Oligocene) of South Texas, mudrocks lose most K-feldspar and Ca-plagioclase. Both of these minerals, however, survive to greater depths in mudrocks than in associated sandstones and are not completely removed even in the deepest samples examined. Albite is the most abundant replacement phase; calcite and Al-rich sphene are common. Sphalerite, pyrite, and TiO 2 also occur as replacements of detrital feldspar. Replacement phases are introduced subsequent to dissolution of the detrital grain and require transport of material over minimum distances of tens of microns from sources external to the detrital feldspar.