MARINE PALEOENVIRONMENTS OF THE PALEOCENE CANNONBALL FORMATION, NORTH DAKOTA

The Cannonball Formation in North Dakota and adjacent areas is the only interior remnant of the last marine incursion of the Western Interior Seaway. Questions remain regarding the timing and extent of this seaway during the early Paleogene and its implications for the Arctic Ocean, the Gulf of Mexico, and biogeographical ranges of marine and nonmarine species. During its last Cretaceous highstand, the Western Interior Seaway maintained a North American marine connection between the Arctic Ocean and the Gulf of Mexico. The precise age and extent of the latter stages of the Western Interior Seaway are in doubt, and the connections with source waters during the deposition of the Cannonball Formation are still unclear. In regards to regional sea levels during the deposition of the Cannonball Formation, the interfingering of the Cannonball Formation with the Boyce Tongue Formation, a coeval nonmarine formation to the west, suggests two transgressive-regressive pulses. The foraminiferal assemblages from the Cannonball Formation offer insight into the environmental conditions during its deposition.

Thirty samples from the Garrison core in southwestern North Dakota were washed, picked for foraminifera, and paleoenvironments were interpreted from the assemblages. Six of the samples are barren. Preliminary data show low diversity in samples with foraminifera and rare occurrences of planktic foraminifera. The bottom portion of the core has a more brackish environment, poorer preservation, and a lower abundance of foraminifera. The middle and upper portions show an increase in salinity up to normal salinity and are from the middle to outer shelf. Further investigation will examine seaway connections based on the paleobiogeography of the Cannonball Formation foraminifera, and contribute to correlating the global planktic foraminiferal zonation with the regional land mammal ages.