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Lower Cretaceous stratigraphy of the Rocky Mountain region is complicated by problems of nomenclature, lateral extent, correlation, and age variation of the Dakota group. Lower members of the Dakota group have three different relationships to the underlying Morrison formation: (1) in some areas the basal units were deposited in channels and are unconformable on the Morrison; (2) in many areas the basal units have an intertonguing facies relationship with the upper Morrison, and, (3) a regional unconformity at the base of the Dakota group is developed only in the Great Plains region.
Regional time lines are necessary for an understanding of facies relations. Marine Jurassic formations define an approximate time line at the base of the Morrison in the northern part of the Rocky Mountain region. The Todilto limestone forms an approximate time line in the San Juan basin, but it can not be related precisely to the marine formations farther north. In the northern part of the region the boundary between the Upper Cretaceous and the Lower Cretaceous is placed at the top of the Mowry shale which is marked by the Clay Spur bentonite bed, an excellent time horizon. The marker bentonite of the Denver basin ("X" bentonite), 100-300 feet above the Mowry shale, can be traced throughout the Powder River basin of Wyoming where it marks the base of the Greenhorn faunal zone (Upper Cretaceous). The "X" bentonite bed, and in its absence, the base of the Greenhorn limestone form an excellent time line.
Sediments of the Dakota group were deposited in a sea that advanced from the north and thus these basal sediments become younger in easterly, southerly, and westerly directions. Marine shales of the Thermopolis and overlying Mowry and Graneros formations of Wyoming and Montana were deposited contemporaneously with sandstones of the Dakota group farther south. The lower part of the Dakota group of Wyoming and Montana was deposited contemporaneously with upper Morrison sediments on the south. Over most of the Rocky Mountain region the Dakota group is Lower Cretaceous, but in deposits just south of the San Juan basin the upper part is as young as the Greenhorn limestone, a relation similar to that found in eastern Nebraska, at the type locality, where the Dakota sandstone is largely, if ot entirely, Upper Cretaceous.
In Wyoming and Montana, sandstones of the lower Dakota group (Lakota, Fall River, Dakota, Cloverly, Cat Creek, and upper Morrison) produce oil in areas where distinctive, linear, thick sandstones are developed and are notably unproductive in areas of patchy, heterogeneous sand development. In eastern Colorado, the lower part of the Dakota group (Dakota, Lytle, "M", "O", "R", and "T" sandstones) is essentially non-productive. Significant oil production in the Dakota sandstone of western Colorado and the San Juan basin is found in areas characterized by linear sandstone development.
"D" and "J" sandstones of the Denver basin produce petroleum from stratigraphic traps and the southeast limit of significant production coincides with the zero isopach of the Mowry shale. Oil production in the Muddy and Newcastle sandstones of Wyoming (equivalents of lower "J") is stratigraphically trapped in or near linear sandstone trends. Structural entrapments are present where linear trends cross structural highs.
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