Cretaceous rocks in Central America and Mexico crop out over great areas, comprise many lithologic types, and range in thickness from a few thousand feet to more than 25,000 feet. Sources of sediments were mainly north, west, and south of the Mexican sea.

During the Neocomian, the sea was confined to the central part of the Mexican geosyncline, was indented by peninsulas in Coahuila and Oaxaca, and overlapped islands in eastern Mexico. Clastic deposits were formed near shore mainly during the Berriasian, lower Valanginian, and upper Hauterivian. Thin-bedded limestone and marl of Berriasian to Hauterivian ages characterize most of the Mexican geosyncline. The Barremian is represented offshore mainly by thick- to medium-bedded limestone, but nearshore contains some lenses of shale and sandstone and locally some gypsum.

During the lower Aptian the Mexican sea was apparently almost as restricted as during the Neocomian, but during the upper Aptian and Albian it spread widely and may have connected with the Pacific Ocean in both northern and southern Mexico. An angular unconformity may have been developed in southern Mexico during the lower and middle Albian. In northern Mexico a major change in sedimentation occurred in the lower Cenomanian and unconformities developed in marginal parts of the Mexican geosyncline. The lower Aptian consists mainly of thick- to medium-bedded limestone, but nearshore contains some shaly beds, arkosic sandstones, and in some places considerable gypsum. The upper Aptian and basal Albian are represented widely by shaly to thin-bedded limestone, but in southeastern Puebla an in Sonora include much coarse, clastic material.

The middle Albian and the upper part of the lower Albian are represented by a number of facies. Rudistid- or Orbitolina-bearing limestone, generally of considerable thickness, occurs over areas that were landmasses in Upper Jurassic or Neocomian time, or that were adjacent to landmasses. Throughout the central parts of the Mexican geosyncline occurs a thinner facies of thin-bedded limestone interbedded with much black chert that passes rather abruptly into the rudistid reefs. Along the present front of the Sierra Madre Oriental from Lampazos, Nuevo Leon, to Victoria, Tamaulipas, the rudistid reefs change eastward into a dense, thick-bedded limestone containing highly ornamented ammonites but no rudistids. Over the site of the Coahuila Peninsula gypsiferous beds are characteristic. In ast-central Sonora are thousands of feet of interbedded limestone, shale, agglomerate, and lava. In the late middle Albian a thin unit of marl and thin-bedded limestone, comparable stratigraphically with the Kiamichi shale, was deposited over the northern parts of the Mexican states adjoining Texas.

The upper Albian and lower Cenomanian are represented throughout much of Mexico by wavy-bedded, thin-bedded limestone and many lenses of black chert that apparently were deposited in the offshore, deeper-water portion of the Mexican sea. This facies grades shoreward into marl and limestone similar to the Georgetown limestone or into a rudistid-miliolid-bearing limestone.

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The Mexican sea during the upper Cenomanian and Turonian probably did not extend as far west as during the Albian, and its deposits are remarkably uniform over large areas. The greatest known downwarping occurred in southern Coahuila. In southern Mexico rudistid limestone predominates as far north as central Hidalgo and Queretaro. In north-central and eastern Mexico occurs a flag facies consisting of alternating beds of thinly laminated, platy limestone, shaly limestone, and shale that are poorly fossiliferous. In eastern Chihuahua and in Jeff Davis County, Texas, occurs a thick shale and marl facies that is highly fossiliferous. Over the site of the Coahuila Peninsula occurs a thin unit of highly fossiliferous shale and platy limestone. Sandstone interbedded with shale has been found in marginal areas of the Mexican geosyncline in Chihuahua, Zacatecas, and Guerrero.

The Mexican sea of the Coniacian and Santonian was considerably more restricted than during the Turonian and was being crowded eastward by rapidly rising landmasses in western Mexico as shown by thick sections of shale and sandstone in eastern Chihuahua. Thousands of feet of tuffaceous beds were deposited during the Coniacian in northern Zacatecas. Deposition of at least 5,000 feet of shale and some sandstone occurred in a trough extending eastward across southern Coahuila into west-central Nuevo Leon. In northeastern Mexico this shale passes northward into the much thinner Austin chalk and southward into the much thinner San Felipe limestone. A thick, rudistid-bearing limestone is fairly common in Central America and southern Mexico as far north as southeastern San Luis Potosi. The l ndmass south of the Mexican sea does not appear to have furnished much coarse sediment.

The sea during the Campanian and Maestrichtian became more and more restricted as shown by the occurrence of coarse conglomerate and coal beds in eastern Chihuahua and northern Coahuila, of fine conglomerate in southwestern Coahuila, of large quantities of sandstone as far east as western Nuevo Leon and eastern San Luis Potosi, and by the occurrence of more coarse materials in the Maestrichtian than in the Campanian beds. About 20,000 feet of coarse, clastic sediment was deposited in a trough extending eastward across southern Coahuila. Thousands of feet of continental beds were deposited in the Cabullona area of northeastern Sonora. A much thinner shale facies was deposited in Tamaulipas, northern Veracruz, and eastern Nuevo Leon. Thousands of feet of shale and sandstone occur in sou hern Nicaragua. Rudistid-bearing limestone associated with shale and sandstone extends from southeastern San Luis Potosi southward into Central America. At the end of the Maestrichtian the sea withdrew completely from the interior of Mexico.