Abstract

Deeply eroded Cretaceous rocks of the eastern margin of the Western Interior seaway are nearly 520 m (1700 ft) thick and record discontinuous open marine, marginal marine, and nonmarine deposition along a west-sloping continental surface. Structural and paleotopographical features, including the Transcontinental arch, Sioux ridge, and Precambrian uplands in central Minnesota influenced the distribution of clastic sediment from eastern and western sources. East and west migration of broadly diachronous marine and nonmarine facies occurred during five major transgressive-regressive cycles: Kiowa-Skull Creek (Albian), Greenhorn (Cenomanian-Turonian), Niobrara (Coniacian-Santonian), Claggett (Campanian), and Bearpaw (Campanian-Maestrichtian).

As base level rose during the Kiowa-Skull Creek cycle, eastward migration of marine shale facies into central South Dakota, Nebraska, and Kansas likely produced aggradation of coarse-grained fluvial facies in northwest Iowa and northeast Nebraska (lower part of the Nishnabotna Member of the Dakota Formation). The regressive phase is recorded by westward progradation of nonmarine mudstone and sandstone facies (Dakota Formation) into the central part of the seaway. Eastward migration of marine shale facies (Graneros Shale) during the transgressive phase of the Greenhorn depositional cycle displaced fine-grained fluvial-deltaic facies (Woodbury Member of the Dakota Formation) into south-central Minnesota. Continued transgression led to migration of marginal marine facies into south-central and northern Minnesota (Coleraine Formation) with probable contemporaneous fluvial deposition in southeast Minnesota (Windrow Formation). Stranding of terrigenous sediment derived from eastern sources led to widespread pelagic carbonate sedimentation (Greenhorn Formation). Clastic-rich facies of the Greenhorn Formation are present adjacent to the Sioux ridge and Precambrian uplands in western and central Minnesota, and the Greenhorn probably grades eastward into marine shale units preserved in southeastern Minnesota. As transgression slowed, deposition of terrigenous sediment derived from eastern sources migrated west, producing shaly chalk (Fairport Member of the Carlile Shale); overlap of the Sioux ridge and Precambrian uplands of Minnesota continued. Regression is recorded by the westward progradation of marine shale facies (Blue Hill Member of the Carlile Shale). Deposition of coarse-grained shallow marine sandstone (Codell Member of Carlile Shale) was initiated along the Sioux Ridge and probably prograded southwest to develop a fine-grained shallow marine sandstone facies in Kansas. That facies represents the final regressive phase of the Greenhorn depositional cycle.

Regional uplift and erosional truncation of Carlile strata along the Transcontinental arch preceded sedimentation during the Niobrara cycle. Pelagic carbonate sediment (Niobrara Formation) accumulated southeast of the arch, and calcareous silty mud, derived from western sources, accumulated on the northwest flank of the arch. Open marine deposits lapped onto the crest of the arch. Terrigenous sediment derived form eastern sources diluted carbonate sedimentation to produce calcareous siltstone and sandstone in western Minnesota which grade west into silty sandy chalk in southeastern South Dakota and northeastern Nebraska. Local intertonguing of Niobrara chalk and terrigenous clastic deposits marginal to the Sioux ridge suggests that the ridge served as a local source area. Continued transgression stranded eastern clastic sources, and the inundation of most of the rugged paleotopography of the Sioux ridge led to deposition of unusual biogenic siliceous sequences where marine waters impinged on the irregular coast line. The Pierre Shale, deposited during the Claggett and Bearpaw cycles, consists primarily of marlstone and clay shale derived from western sources. Generally Pierre strata have been extensively eroded from the eastern margin area, but all preserved strata belong to an open marine facies. Minor amounts of terrigenous clastic sediment derived from eastern sources are present in the Pierre around the Sioux ridge.