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The AAPG/Datapages Combined Publications Database
Wyoming Geological Association
Abstract
Stratigraphy of the Jurassic Stump Formation
Abstract
It is recommended that the Stump Sandstone be formally called the Stump Formation since siltstones, shales and limestones are part of the unit. Regionally the formation is recognized in the thrust belt area of western Wyoming, southeastern Idaho and northeastern Utah.
The Stump was deposited during the fourth and final marine transgression of Jurassic seas in an inland trough that extended southward from the Arctic Ocean. The Stump Formation has been dated as Oxfordian by fossils. Studies indicate the main source area for the sediments was probably an uplift in northern Utah and southeastern Idaho. A secondary source is postulated to have been the Sweetgrass-Belt Island region in western Montana.
The Stump is thickest in the west where it was deposited in a subsiding trough (Idaho trough) and thins eastward in a pattern similar to the underlying Preuss Sandstone and Twin Creek Formation. The Preuss-Stump contact is unconformable, at least in the western study region. The upper contact with the Ephraim Conglomerate Member of the Cretaceous Gannett Group is unconformable in the thrust belt. At the margins of the Stump the contact with the latest Jurassic continental Morrison Formation is possibly an unconformity in the Uinta Mountain area and is questionable along the northern Wyoming-Montana border.
A prodeltaic model is proposed for the paleoenvironment of the Stump. Sandstones comprise the primary lithology, but interbeds of limestones, siltstones and shale are common and in general the Stump becomes shalier to the east and upward in the section. The occurrence of glauconite in the Stump and equivalent formations is an important characteristic of Oxfordian sedimentation and the top and bottom units of Stump are commonly picked by the recognition of glauconite. A zone of heavy mineral enrichment exists in the northeastern portion of the study area but is too thin to be of economic importance. Hydrocarbon production is documented from the Stump and other formations in the Pineview field in Utah, but production from the Stump Formation has not been established further to the north.
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