ABSTRACT One of the widespread Cretaceous units of the Gulf of Mexico is characterized by dark, megapaleonto-logically poor to barren lime wackestones and mudstones, herein referred to as the Tamaulipas. The unit forms the Gulfward component of facies sets which include reef and back-reef facies. These crop out in the Sierra Madre Oriental, have been drilled in the Coastal Plain, and apparently encircle the Gulf of Mexico. Although divisible into rock units (Taraises, Tamaulipas, Cuesta del Cura, lower Agua Nueva), the lithofacies is unified by the overriding physical properties and prevalent bathypelagic paleontological aspect. The temporal span of the facies varies from one locality to another but generally extends from earliest Cretaceous into Turonian. In spite of difficulties resulting from a biologically eccentric biota and limitations resulting from thin-section study necessitated by induration, there has been prolonged investigation of the unit. The biostratigraphic framework is well established in some oil company laboratories and in reports, particularly by Bonet, Trejo, and Longoria, but these are not always readily available, and related literature is widely scattered. Consequently, a current summary is offered here. The most useful taxa of the Tamaulipas are calpionellids, nannoconids, calcisphaerulids, micro-calamoidids, and planktonic foraminifers. For each of these taxa, general properties, illustrative and significant species, and important ranges are given. The distributions of the significant forms delineate detailed zonations for intervals of (1) Berriasian to Hauterivian, based on calpionellids, (2) middle Aptian to early Albian, based on foraminifers, microcalamoidids, and calpionellids, (3) late Albian to middle Cenomanian, based on foraminifers, and (4) early Turonian, based on foraminifers. Calcisphaerulid ranges are somewhat unreliable, but calcisphaerulids provide supporting evidence, particularly for the mid-Albian. Nannoconids are very small and typically obscure in thin sections of conventional thickness, but they often help in dating, particularly for the early Aptian. Their visibility and assistance can be improved by cutting thin sections with a tapered edge or by treating a fragment of sample with calcareous nannofossil techniques. Calcareous nannofossils are present but have not been extensively reported so far, perhaps partly because of the obscuring effect of recrystallization involved in induration.