The paleontology and biostratigraphy of Tertiary, Cretaceous, and Paleozoic rocks in the upper Mississippi embayment are incompletely known because marine fossils are only locally present in these rocks. This study concerns material from two U.S. Geological Survey test wells drilled in New Madrid County, southeastern Missouri, as part of earthquake hazard studies in the northern Mississippi Embayment. Test well1 sampled lower Tertiary strata to a depth of 146ft; these strata were found to be late Eocene in age on the basis of sporomorphs. Test well1-X, 29ft northwest of well1, provided cuttings and cores from lower Tertiary, Upper Cretaceous, and lower Paleozoic rocks to a total depth of 2,316 ft below the Kelly bushing (at an altitude of 288 ft). Lithologic evidence suggests that the base of the Jackson Formation may be at 270 ft, but sporomorphs indicate that the base of the Jacksonian Stage (upper Eocene) is possibly at a depth of about 350ft in test well1-X. Lithologic units of the Claibornian Stage (middle Eocene) here consist of the Cockfield(?) and Cook Mountain(?) Formations and Memphis Sand, in descending order. The Claibornian could not be subdivided using sporomorphs from cuttings, but sporomorphs show that the top of the Sabinian Stage (top of the lower Eocene) is at about 1,055 ft; lithologically, the top of the Wilcox Group (top of the Flour Island Formation) is at 1,048 ft. Sporomorphs suggest that the top of the lower Sabinian (top of the Paleocene) is within the Flour Island Formation at about 1,105 ft. The next major lithologic break, the top of the Fort Pillow Sand at 1,186 ft, does not coincide with any detectable biostratigraphic boundary. Lithologically, the interval from 1,339 to 1,377 ft may belong to the Old Breastworks Formation; dinoflagellates from cuttings indicate that this interval is likely to be late Midwayan in age and, therefore, correlative with the Naheola Formation of the eastern Gulf Coast. The Porters Creek Clay extends from a probable top at 1,377 ft to 1,691 ft, and the base of the underlying Clayton Formation (Tertiary-Cretaceous contact) is at 1, 703 ft. Calcareous nannofossils, dinoflagellates, foraminifers, mollusks, ostracodes, and sporomorphs from continuous cores of the Porters Creek Clay and Clayton Formation indicate that the upper half of the Porters Creek correlates with the upper part of the same formation or perhaps partly with slightly younger rocks in the eastern Gulf Coast, the lowermost Porters Creek appears to correlate with the upper part of the Clayton Formation of the eastern Gulf Coast and with the Kincaid Formation of Texas, and the thin Clayton of the test well probably correlates with only the lower part of the thicker Clayton of the eastern Gulf Coast. Sporomorphs from McNairy Sand (Upper Cretaceous) cores indicate Maestrichtian and perhaps latest Campanian Age. Lower Paleozoic dolostone from 2,023 ft to total depth at 2,316 ft is barren of identifiable fossils except for a probable fragment of the fish Anatolepis; the dolostone is probably Late Cambrian in age. The McNairy Sand of New Madrid well 1-X seems to have been deposited in nonmarine to nearshore marine environments; the Clayton Formation and lower part of the Porters Creek Clay were deposited under marine conditions during an early Paleocene marine transgression. A major regression followed that continued through the end of Porters Creek time and perhaps into Naheola (late Midwayan) time; the Sabinian, Claibornian, and Jacksonian strata in southeastern Missouri were deposited primarily or entirely in nonmarine environments.