Title

An Evaluation of the Depositional Setting of the Virgelle Sandstone in Northwestern Montana

Presenter Information

Sarah P. WashkoFollow

Presentation Type

Poster

Abstract

The Virgelle Sandstone is a grey to buff colored sandstone, deposited during the late Cretaceous along the western margin of the Cretaceous Interior Seaway. As the basal member of the Eagle Formation, the Virgelle sandstone overlies the poorly exposed mudstones of the Telegraph Creek Formation. Overlying the Virgelle is the regressive Two Medicine Formation in the Choteau Valley of northwestern Montana. Outcrops of the Virgelle Sandstone are capped by magnetite and ilmenite rich sands (up to 1 m thick) that thin to the east. To understand the change in depositional setting from west to east three measured sections were obtained in the Choteau Valley, which contains both ironstone rich and ironstone poor sandstone outcrops. Sedimentary features noted included grain size, bedforms, paleoflow indicators, inchnofacies, and heavy mineral content.

All measured sections indicate a coarsening upward succession, suggestive of a deltaic and/or shoreface succession with one section capped by a coal. Heavy mineral content and grain sorting also increase up section. Sandstones are dominated by planar and trough cross bedding. Paleocurrent data suggests an easterly flow direction. Trace fossil assemblages are indicative of Skolithos and Cruziana ichnofacies or sandy shore and sublittoral environments respectively. Petrographic analysis reveals a mix of microcrystalline and polycrystalline quartz, feldspars, volcanic and metamorphic lithics, glauconite as well as the heavy mineral magnetite and ilmenite. The presence of these minerals and lithics as well as paleocurrent indicators suggest that the source terrain was an igneous and metamorphic provenance to the west. The increase in heavy mineral content up-section, particularly in the upper most meter or so, is thought to be the result of selective winnowing in a shoreline setting.

Category

Physical Sciences

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Apr 11th, 11:00 AM Apr 11th, 12:00 PM

An Evaluation of the Depositional Setting of the Virgelle Sandstone in Northwestern Montana

The Virgelle Sandstone is a grey to buff colored sandstone, deposited during the late Cretaceous along the western margin of the Cretaceous Interior Seaway. As the basal member of the Eagle Formation, the Virgelle sandstone overlies the poorly exposed mudstones of the Telegraph Creek Formation. Overlying the Virgelle is the regressive Two Medicine Formation in the Choteau Valley of northwestern Montana. Outcrops of the Virgelle Sandstone are capped by magnetite and ilmenite rich sands (up to 1 m thick) that thin to the east. To understand the change in depositional setting from west to east three measured sections were obtained in the Choteau Valley, which contains both ironstone rich and ironstone poor sandstone outcrops. Sedimentary features noted included grain size, bedforms, paleoflow indicators, inchnofacies, and heavy mineral content.

All measured sections indicate a coarsening upward succession, suggestive of a deltaic and/or shoreface succession with one section capped by a coal. Heavy mineral content and grain sorting also increase up section. Sandstones are dominated by planar and trough cross bedding. Paleocurrent data suggests an easterly flow direction. Trace fossil assemblages are indicative of Skolithos and Cruziana ichnofacies or sandy shore and sublittoral environments respectively. Petrographic analysis reveals a mix of microcrystalline and polycrystalline quartz, feldspars, volcanic and metamorphic lithics, glauconite as well as the heavy mineral magnetite and ilmenite. The presence of these minerals and lithics as well as paleocurrent indicators suggest that the source terrain was an igneous and metamorphic provenance to the west. The increase in heavy mineral content up-section, particularly in the upper most meter or so, is thought to be the result of selective winnowing in a shoreline setting.