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A regional thinning of the marine Niobrara Formation and equivalent strata, trending northeast across Colorado and adjacent states, reflects movement on the Transcontinental arch during the Late Cretaceous. The isopach pattern reflects low rates of deposition on the arch, a regional unconformity at the base of the Niobrara with onlap onto the broad structural high, and other unconformities within or at the top of the formation.
Isopach maps of the Lower Cretaceous and of four time-stratigraphic intervals in the lower part of the Upper Cretaceous (approximating the Graneros, Greenhorn, Carlile, and Niobrara Formations) were prepared from surface and subsurface data. These maps suggest that tectonic movement of the Transcontinental arch affected the seafloor during times of worldwide changes in sea level. The major movement occurred during deposition of the upper Carlile and Niobrara Formations (late Turonian to early Santonian).
This structural movement on the Transcontinental arch may have been significant in early stratigraphic entrapment of petroleum in high structural positions in the Dakota sandstones at two of the largest gas fields in the area: Wattenberg field in the Denver basin and the Blanco field in the San Juan basin. The downwarping of the traps into their present low structural positions occurred during the Laramide orogeny in latest Cretaceous and early Tertiary time.
The Transcontinental arch is but one of several northeast-trending cross-basin arches that influenced sedimentation and petroleum occurrences in Cretaceous of the Western Interior of North America.
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