The Paleocene Cannonball Formation, cropping out primarily in southwest-central North Dakota, is a marine deposit with variable lithologic characteristics ranging from medium to dark-gray-weathering mudstone to fine-grained, well-sorted, brownish-yellow-weathering sandstone. Also, two distinct tongues of the formation, exposed in southwestern North Dakota, are comprised of organic-rich siltstones and claystones.

There are 30 known species of bivalves in the Cannonball. Because bivalves are abundant and well known, and because their morphology and life habits are highly reflective of environmental demands, they are used to more accurately define depositional environments of the Cannonball sea. Based on Q-mode and R-mode cluster analysis, five bivalve associations are defined: Ostrea-Corbicula, Crassostrea-Corbula, Isognomon, Crassatella-Nucula, and Glycymeris-Arctica associations.

The Ostrea-Corbicula association, in the lower Cannonball tongue, and the Crassostrea-Corbula association, in the upper tongue, suggest that the Cannonball sediments in southwestern North Dakota were deposited in lagoonal or estuarine environments.

Where present, Isognomon occurs in abundance. However, it is found at only a few known localities in southwest-central North Dakota, and it has not been found in association with any other macrofossils. Isognomon, found in organic-rich sands, appears to have lived attached to vegetation in shallow-water environments.

The Crassatella-Nucula and Glycymeris-Arctica associations, common throughout southwest-central North Dakota, are most characteristic of the Cannonball. The Crassatella-Nucula association occurs in silty, clayey sand with moderately high organic content. It is dominated by both deposit and suspension-feeding bivalves, and has a high species diversity. It appears to have been deposited in a low energy environment with moderately high turbidity. In contrast, the Glycymeris-Arctica association is found in fine-grained, well-sorted sandstone with low organic content. It is dominated by infaunal suspension-feeding bivalves that indicate both a higher energy environment and low turbidity. Crabs and Ophiomorpha commonly occur stratigraphically above this association. These two associations m st likely represent foreshore, shoreface, and/or shelflike environments.

The distribution of these bivalve associations, along with lithologic characteristics, suggest that the Cannonball Formation was primarily deposited in a barrier island complex and included lagoonal, beach, and offshore environments.

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