Date of Award
Doctor of Philosophy (PhD)
F.D. Holland, Jr
The Upper Cretaceous Niobrara Formation in eastern North Dakota consists of calcareous shales, chalks, and shaly chalks. The formation was studied from surface exposures, cores, and well logs. The Niobrara is 62 feet (19m) to 272 feet (83 m) and is thickest to the northeast, in a central linear area, and to the southwest. In a northern area, it was divided into a lower, calcareous shale member, and an upper, chalky member. In a.southern area, five stratigraphic units were recognized, in ascending order: a lower shaly chalk, a shale, a middle shaly chalk, a chalk, and an upper shaly chalk. The formation is bounded by unconformities that exhibit considerable relief. In a central area, thick Niobrara was deposited in a northwest-trending channel eroded into the underlying Carlile Shale. Numerous diastems were recognized within the formation on the basis of detailed comparisons of logs of closely spaced wells. Within the formation, intervals of non-bioturbated strata with "white specks" and bioturbated strata were recognized.
Thirty-nine species of calcareous nannoplankton were identified, and four stratigraphically delineated nannoplankton assemblages were recognized. Occurrences of species were related to paleobiogeography, the age of the strata, and early diagenesis. Macrofossils include Lingula, ammonites, and four bivalve taxa, including the abundant oyster, Pseudoperna congesta, and fragments and prisms of inoceramids. Trace fossils include non-pyritized burrows (Chondrites) and pyritized burrows (Planolites montanus, Trichichnus, and pellet-filled burrows). Pyritization of burrows is explained as the result of post-depositional, in-sediment, reducing conditions.
The Niobrara in northeastern North Dakota is Santonian, based on the occurrence of Clioscaphites cf. C. saxitonianus septentrionalis at the base, and a Late santonian nannoflora, including Arkhangelskiella specillata, at the top of the formation.
Eastern North Dakota was the last area of inundation by the Western Interior sea during a Coniacian-santonian transgression. The bounding unconformities and resultant topography were the result of exposure during low-stands of sea level. The Niobrara was deposited in shallow to deep sublittoral depths. Shifts from anaerobic to aerobic bottom water conditions, and vice versa, were the result of changes in water level that, in turn, controlled the position of the pycnocline (aerobic anaerobic boundary) in a stratified water body.
Reiskind, Jeremy, "Paleontology and stratigraphy of the Niobrara Formation (Upper Cretaceous) of eastern North Dakota with emphasis on the calcareous nannoplankton" (1986). Theses and Dissertations. 242.