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Cretaceous Rocks

COMMENTARY
Cretaceous rocks provide reservoirs, source beds, and seals for most shallow gas accumulations in the Northern Great Plains and adjacent Rocky Mountains. Reservoirs range from conventional sandstones to unconventional interbedded sandstones and shales and include fractured chalk and shale.

The Cretaceous System is spread as a relatively continuous blanket over most of the Northern Great Plains. I was raised and educated in this area, so most of my early experiences with geology related directly to Cretaceous rocks. And, because I continue to live and work in the Northern Plains, that focus has remained throughout my career.

There is an interesting asymmetry to Cretaceous geology. In the Rocky Mountains, the rocks outcrop extensively, have important economic resources, and have been extensively described and studied. In contrast, on the eastern side of the Great Plains, there are limited outcrops, undeveloped resources, and comparatively few geologists studying the rock record.

As a consequence of this fundamental asymmetry, interpretations of history and paleogeography on the western side of the Cretaceous Seaway are much more detailed than those for the eastern coast Ideas: Figure 4). A group of us working on Cretaceous rocks in the eastern plains attempted to raise consciousness about the eastern side of the seaway. In the early 1990’s, we compiled a volume (GSA Special Paper 287) that summarized the more recent work. The volume has had only a limited impact.

Extensive publications devoted to the Cretaceous during the past decade emphasize global themes: paleoclimatic reconstructions, sequence stratigraphy, and geochemical event cycles. The eastern margin of the Western Interior Seaway, however, generally continues to be treated in a superficial manner. Perhaps it is time to again revitalize interest in the eastern margin.

CHRONOLOGY
This chronology provides an overview of the topical bibliography on Cretaceous Rocks included in the Publications portion of this website.

Work during the 1970’s:
••Shallow gas potential (1978)
••Paleotectonics (1978 and 1979)
••Pierre Shale (1977 and 1980)
••Eastern side of the Seaway (1980 and 1981)

Work during the 1980’s:
••Niobrara Formation (1980 and 1984)
••Eagle Sandstone (1982, 1983, and 1986)
••Shannon Sandstone marine ridges (1984 and 1988)
••Judith River (1989)
••Regional paleotectonism (1986 and 1989)

Work during the 1990’s:
••Eastern margin, Western Interior Seaway (1994)

FEEDBACK STARTER QUESTIONS
1. Are the chalks in the Pierre, Niobrara, and Greenhorn on the eastern seaway margin really “deep water” facies or simply bioclastic deposition beyond the dispersal of fine-grained clastics from the west?
2. Are the sandstone bodies in the Shannon and Judith River, large shelf bedforms “frozen” as hydrodynamic features or are they palimpsest pieces of coastal sands preserved on down-dropped blocks?
3. Has sequence stratigraphy really been applied in an objective way in the Northern Great Plains or has it mainly been an explicit search for global sequences with little regard for local variations?
4. Is there anybody currently doing work on Cretaceous rocks on the eastern seaway margin, other than the on-going studies by people at the Iowa Geological Survey?



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