The Cannonball Formation (Paleocene, Danian) is best exposed in southwest-central North Dakota. Here, two major lithotypes, mudstone and sandstone, occur. In this area, the Cannonball can be informally subdivided into four lithologic units: lower (primarily thinly-bedded sandstone and mudstone), lower-middle (sandstone), upper-middle (mudstone), and upper (sandstone). The Cannonball is also exposed in southwestern North Dakota where it occurs as two mudstone tongues (upper and lower).
During July and August, 1982, thirteen Cannonball outcrops were studied. Bivalves, along with other macrofossils, were collected quantitatively in an attempt to determine fossil species dominance. Rock samples were also collected and analyzed for textural characteristics and organic carbon content. In addition, R-mode and Q-mode cluster analyses were done using the reported occurrences of 20 bivalve species, along with other macrofossils, from 97 localities in North Dakota.
Five bivalve associations are defined: Ostrea-Corbula-Corbicula, Crassostrea-Corbicula-Corbula, Isognomon, Glycymeris-Arctica, and Crassatella-Nucula associations. One sub-association, the modified Crassatella-Nucula association, and a biotic association of Ophiomoroha Crab-Driftwood-Shark Teeth, are also defined. These associations aid in the interpretation of the relationships of Cannonball lithologic units and depositional environments.
The Ostrea-Corbula-Corbicula association (lower tongue) and the Crassostrea-Corbicula-Corbula association (upper tongue) contain low-diversity faunas that are characteristic of brackish-water environments associated with· lagoons.
The Isognomon association, found in the lower Cannonball unit, occurs in muddy sandstone rich in organic material. The abundance of Isognomon, a large, thick-shelled, suspension-feeding bivalve, along with the lack of other macrofossils, suggests an environment where conditions were prohibitive to most organisms. A shallow-water environment associated with a tidal flat is proposed.
The Glycyrneris-Arctica association, found in well-sorted sandstone in the lower-middle unit, is dominated by suspension-feeding bivalves. The presence of Glycymeris, a bivalve well adapted to high-energy conditions, indicates a nearshore, wave-agitated beach environment. The Ouhiomorpha-Crab-Driftwood-Shark Teeth association also occurs in well-sorted sandstone but is not restricted to any Cannonball unit. The association of these nearshore inhabitants suggests a beach environment where deposition took place in the vicinity of the strandline.
The Grassatella-Nucula association occurs in the lower-middle unit in muddy sandstone. This association has high diversity and both deposit-feeding and suspension-feeding bivalves are present suggesting a stable, subtidal environment. The modified Crassatella-Nucula associa tion contains bivalves similar to the veritable Crassatella-Nucula association but lacks the high diversity. The modified association occurs in variable lithology and is not restricted to any Carmonball lithologic unit. It also represents a subtidal environment; however, conditions were probably not conducive to high-diversity communities.
Two major transgressions and regressions occurred during Cannonball time. Based on the proposed associations, it seems likely that deposition of the brackish-water tongues in southwestern North Dakota is correlative with two major transgressive events in southwest-central North Dakota
