Abstract

A limited suite of trace fossils dominates the fossil record of the marginal marine deposits of the Fox Hills Formation (Late Cretaceous, Maastrichtian) in outcrops at the south end of the Cedar Creek anticline in Bowman County, North Dakota. The 30-meter thick section contains feeding traces (Phycosiphon, Rhizocorallium) and two species of the burrow trace Ophiomorpha (O. borneensis, O. nodosa). Feeding traces dominate the silty sands of the lower shoreface deposits (Trail City and lower Timber Lake members). The species of Ophiomorpha dominate the very fine and fine hummocky bedded sands of the middle and upper Timber Lake Member. The delicate O. nodosa marks the relatively low-energy conditions of the lower shoreface, while upsection, the robust O. borneensis marks the relatively high-energy deposits of the middle and the upper shoreface. Ophiomorpha are rare in the medium-grained crossbedded channel facies of the upper Fox Hills Formation (Colgate Member). A similar suite of trace fossils occurs in other outcrops of the Fox Hills Formation on the southwest rim of the Williston basin. The younger Fox Hills strata of the southeastern basin flank, including the type area of the formation, however, contain a varied assemblage of bivalves, oysters, and ammonites. Ophiomorpha, the only widely recognized trace fossil on the southeast flank, occurs with the bivalve Tancredia in sands of relatively high-energy barrier bar deposits.

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