ABSTRACTCombined petrographic and geochemical data document several kinds of detrital carbonate in subsurface Miocene‐Oligocene mudstones of the Texas Gulf Coast. In the extreme south of Texas, in muddy sediments deposited by ancient precursors to the modern Rio Grande, mudstone carbonate is dominated by extrabasinal detritus derived from Cretaceous limestones of the Edwards plateau. Further north, Oligocene mudstones contain carbonate that is mostly syndepositional skeletal material. Minor amounts of authigenic carbonate are found as replacements of silt sized feldspars in all the mudstones. Depth‐related shifts in δ18Ocalciteand δ13Ocalcitesuggest that small amounts of authigenic carbonate also form by replacement of detrital carbonate (‘recrystallization’), although this cannot be demonstrated petrographically. Pervasive carbonate cementation in primary pores in mudstones is not generally observed. Textural evidence of carbonate loss through pressure dissolution is widespread, providing a mechanism for the depth‐related decline in carbonate content observed in many Gulf Coast wells. Carbonate dissolution in mudstones, and export of the CaCO3, implies a massive acid source, probably resulting from silicate reactions within the mudstones during burial