Stratigraphy and paleontology of the upper Cretaceous Morden Member (Vermilion River Formation) in the outcrop area, northeastern North Dakota

The Late Cretaceous Morden Member is the oldest exposed bedrock unit in North Dakota, cropping out in eastern Cavalier County in the northeastern part of the state. The Morden overlies the Greenhorn Formation and underlies the Niobrara Formation. The Canadian Morden Member of the Vermilion River Formation is used in lieu of the Carlile Formation because of the lithologic similarity and proximity to the Morden type area, and the distance from the Carlile Formation type section in southeastern Colorado. Three principal sections were measured and sampled at 2-meter intervals for microfaunal and lithologic analysis. A single well was used for microfaunal analysis of the cuttings. The Morden Member is a dark gray to black, noncalcareous, organic-rich shale of uniform lithology. Three calcareous, septarian concretion zones are the basis for surface to subsurface lithologic correlation. A zone of ferruginous concretions is associated with the lowest concretion zone. Structure contours decrease in elevation to the southwest, at approximately eight feet per mile, indicating a depocenter toward the center of the Williston Basin. Isopach trends indicate a more localized depocenter to the southeast, the result of local structural influences. The Morden is more than 225 feet thick in the study area with thinning in northwestern Manitoba and thickening to the southwest, toward the center of the Williston Basin. Illite is the main clay mineral present followed by kaolinite. Other minerals include quartz, muscovite, biotite, and feldspars. Secondary minerals include gypsum, jarosite, pyrite, calcite, and minor carbonates and sulfides. X-ray analysis of six samples from a single measured section suggests the mineralogy to be essentially uniform throughout the vertical extent of the Morden Member. Thirty-six species of benthonic foraminiferids and three species of planktonic foraminiferids were collected from 48 outcrop and 17 well cutting samples. The dominant species are rotaliines whereas the daminant individuals are textulariines. The foraminiferid fauna occurs throughout the Morden with the well cuttings dominated by calcareous foraminiferids. The upper half of the outcrops is dominated by arenaceous, benthonic foraminiferids with the lower half dominated by calcareous, benthonic and planktonic foraminiferids. Haplophragmoides carlilensis is the dominant species in the outcrop samples. Hedbergella delrioensis is the dominant species in the Greenhorn and Niobrara well cuttings and its increase in abundance is the basis for determining the Greenhorn-Morden and Niobrara-Morden subsurface contacts. Macrofossils, identified from the lowest calcareous concretion zone, include Scaphites carlilensis, Inoceramus aff. I. cuverii, and Euspira ednae. The Morden Member is interpreted as a regressive shale unit, deposited under normal salinities during deposition of the lower half and normal to brackish salinities during deposition of the upper half of the member. Depths were sublittoral to littoral, less than 100 to 200 meters. The Morden Member is homotaxially equivalent to the Blue Hill Shale and Sage Breaks Members of the Carlile Formation of the Great Plains. Faunal analysis indicates the Morden is Middle Turonian in Age