Task 50 - deposition of lignites in the Fort Union Group and related strata of the northern Great Plains Page: 7 of 40
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Formations of the Fort Union Group are ovate-elongate in shape and are simply sculptured with
concentric (nonchevron) umbonal corrugations (Plesielliptio) or dorsal postumbonal dorsally
directed curvilinear ridges (Rhabdotophorus) or are without sculpture ("Unio"). This dramatic
faunal change and the reduction in general abundance were long lasting. Freshwater bivalve
localities remained relatively uncommon for at least four million years (late middle Paleocene, late
Torrejonian). Simple sculpture and shell form persisted throughout the Paleocene and into the
Eocene, and unionid diversity remained low. What molluscan record exists just below the
boundary suggests a Paleocene faunal composition, but these occurrences are rare and possibly
environmentally influenced. This history of unionids is presently not ascribable to a K/T event.
Williston Basin (North Dakota)-Latest Cretaceous-Earliest Eocene. The uppermost
Cretaceous and Lower Tertiary (Paleocene-Eocene) strata of the North Dakota portion of the
Williston Basin contain significant geologic sections including 1) the type area of the Fort Union
Group; 2) the most complete record of the Cannonball Formation; and 3) the youngest Cretaceous
(Fox Hills and Breien) and oldest Tertiary (Cannonball) marine strata in the interior of North
America. Present studies have correlated these strata to the North America land mammal age
biochronology and a current radiometric and polarity chronology. On the basis of palynomorphs,
the Hell Creek-Ludlow formational contact is effectively isochronous and a near match to the K/T
boundary across western North Dakota. Planktic forams from the main body of the Cannonball
Formation indicate the unit as lower but not basal Paleocene. The Cannonball Formation is
correlated to a radiometric timescale on the basis of current planktonic foram zone age dates.
Three tongues of the Cannonball Formation have been recognized. Radiometric dates and
mammalian fossils suggest a Puercan age for the lowermost tongue, while the Boyce and Three V
Tongues are probably of Torrejonian (To) age. The diachronous nature of the Slope Formation
appears to be confirmed by a radiometric date near the base of the Bullion Creek Formation and the
middle Tiffanian (Ti3) Brisbane mammalian local fauna. The Bullion Creek-Sentinel Butte
formational contact is temporally well controlled on the basis of a number of low-diversity
mammalian local faunas. Fort Union strata, as examined by F.V. Hayden in the middle 1800s, can
now be restricted to the Ti3 and Ti4 ages. The age of the Sentinel Butte-Golden Valley
formational contact is not well controlled. Floral similarities between the Bear Den Member and
the Fort Union southeast of Red Lodge, Montana, suggest an early Clarkforkian age for the lower
part of the Golden Valley Formation in North Dakota. The mammals, mollusks, and flora of the
Camels Butte Member indicate a Wasatchian (Wa) age. The mammals from the White Butte
Locality further suggest the Wa3 biochron.
Bighorn Basin (Wyoming-Montana)-Late Paleocene-Early Paleocene. The nonmarine
molluscan fauna in the Western Interior of the United States underwent large-scale changes during
the time represented by the Laramide Orogeny. This period began with the loss of many bivalve
and gastropod taxa near the Cretaceous-Paleogene boundary, followed by an increase in
mesogastropod diversity in the late Torrejonian and Tiffanian, then a reduction in such diversity
into the Clarkforkian that is associated with a dramatic increase in the representation of terrestrial
prosobranch and pulmonate families prior to the beginning of the Wasatchian age. The latter
portion of this record is well documented in the northern portion of the Bighorn Basin in Wyoming
and Montana. The Clarkforkian-Wasatchian boundary, equated by some with the
Paleocene-Eocene boundary, on the basis of the pattern of mammalian evolution and other
evidence in this area, is delimited by aquatic and terrestrial mollusks, but the faunal change is less
dramatic in the Bighorn Basin than changes associated with the increase in terrestrial taxa during
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Hartman, J.H.; Roth, B. & Kihm, A.J. Task 50 - deposition of lignites in the Fort Union Group and related strata of the northern Great Plains, report, August 11, 1997; Grand Forks, North Dakota. (https://digital.library.unt.edu/ark:/67531/metadc693470/m1/7/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.