Chert from at least 9 different stratigraphic zones, ranging in age from Cenomanian (Late Cretaceous) through late Oligocene, was recovered at 4 sites in the southeastern Gulf of Mexico during Leg 10 of the Deep Sea Drilling Project. The sites are on the northern and eastern edges of the Campeche Scarp, in the Catoche Tongue area of the Yucatan Channel, and on the western approach to the Straits of Florida.

Chert formation in all locations took place by late postdepositional replacement of deep-water, calcareous, foraminiferal nannoplankton oozes. At each site the degree of silicification of the carbonates increases with increasing age. Depth of burial and/or thickness of the overlying water mass appears to be independent of the degree of silicification.

Preliminary petrographic analyses of the cherts reveal the presence of amorphous (opaline) silica; fibrous quartz (variety chalcedony); fibrous cristobalite; and anhedral, subequant, microcrystalline quartz. The type of silica present is dependent upon the stage of silicification and on the composition and texture of the components being replaced. Finely comminuted plant material and other forms of organic detritus are locally common but are unaltered by silicification.

The exact source of silica for these cherts is not presently established. Radiolaria and other siliceous organisms are generally common in overlying sediments and may be important silica sources. Although drill-core data in the Gulf of Mexico are incomplete and far from conclusive, there is a suggested increase in the amount of silica both in the form of siliceous organisms and in chert--in the southeastern Gulf of Mexico.

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