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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Lithologic and thickness data on the "Upper Cretaceous" rocks of the western states were collected in the field and from the literature. The data were arranged into isopach-lithofacies maps showing average lithology of the total interval, and of the Dakota-Colorado and Montana-Laramie groups. Vertical variability maps of the number of sandstones in the units were also prepared.
The isopach-lithofacies maps indicate a general thickening and increasing sand content in all units from east to west. The facies lines are roughly parallel with the isopachs, and suggest a general westerly source for most of the sediments, although local sources were operative at times. The number of sandstone beds is greatest along an intermediate belt, indicating a major intertonguing of sand and shale in the general area of the average shoreline trend.
Detailed study of sand and shale lithology in sections from Arizona, New Mexico, and southwestern Colorado indicates that most of the sands are relatively clean subgraywackes, decreasing in angular components northeastward. The shales are mainly silty, and the association of lithologic types suggests unstable shelf conditions over most of the area.
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