Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

A.M. Cvancara


Middle Cenozoic (Oligocene and Miocene) continental deposits of North America have been most studied in southwestern South Dakota and western Nebraska where the White River Group constitutes a fairly complete Oligocene record, and the Arikaree Group of early Miocene age is commonly the sole remaining record of that epoch. Isolated outcrops in adjacent states also offer opportunities for studying the middle Cenozoic history of the Midcontinent but disagreement as to the physical stratigraphy of such deposits has hindered further work in these areas. The main purpose of this study is to present a viable stratigraphic framework for the middle Cenozoic deposits of one of these isolated outcrop regions, southwestern North Dakota. A secondary purpose is to report preliminary findings concerning the petrography and sedimentary history of these rocks.

Middle Cenozoic strata in North Dakota are largely restricted to isolated buttes in Adams, Bowman, Dunn, Golden Valley, Grant, Hettinger, Slope, and Stark Counties. These deposits consist of 125 to 560 feet of cross-bedded sandstone, clay, claystone, silty claystone, siltstone, and minor limestone and dolostone, spanning Chadronian (early Oligocene) through Arikareean (early Miocene) times.

The Chadron and Brule Formations of the White River Group are recognized in North Dakota. Proposed subdivision of these formations includes six new rock-stratigraphic units of bed and member rank. The name "Arikaree Formation," previously applied to the strata overlying the White River in North Dakota, is rejected and a new formation name proposed for deposits because they are lithologically distinct from type Arikaree in Nebraska and Arikaree of even closer outcrops in the Big Badlands of South Dakota.

Three new members are proposed for the Chadron Formation in North Dakota: the Amidon, Chalky Buttes, and South Heart Members in ascending order. The Amidon includes 7 to 16 feet of clay and silty claystone that weather very pale orange or light brown but are very pale orange to grayish orange where fresh. The Chalky Buttes is made up of 8 to 75 feet of medium-grained to very coarse-grained, cross-bedded sandstone that weathers white, very pale orange, dark yellowish orange, or light brown and is very pale orange to pinkish gray where fresh. The South Heart consists mainly of 8 to 55 feet of clay that weathers pale greenish yellow and is yellowish gray to pale olive where fresh.

Two new members and one new bed are proposed for the Brule Formation in North Dakota: the Dickinson Member below, including the Fitterer Bed, and the Schefield Member above. The Dickinson consists of 60 to 130 feet of clay and pitted-weathering, silty claystone. The clay is yellowish gray where weathered but pale olive to light olive gray where fresh; the claystone weathers very pale orange and is yellowish gray where fresh. The Fitterer Bed is a 5-foot to 9-foot thick, cross-bedded, fine-grained to medium-grained sandstone, occurring at various horizons within the Dickinson. The Fitterer sandstone weathers pale greenish yellow but is yellowish gray where fresh. The Schefield includes 16 to 85 feet of alternating beds of siltstone and clay. The siltstone weathers very pale orange but is grayish orange pink to yellowish gray where fresh; the clay weathers moderate reddish orange but is pale reddish brown where fresh. Chadronian, Orellan, and Whitneyan fossils have been previously reported from the Chadron and Brule of North Dakota.

The name "Killdeer Formation" is proposed for the strata overlying the White River Group in North Dakota but below unconsolidated deposits whose source and age are unknown. The Killdeer consists of 25 to 200 feet of green-colored, concretionary, calcareous sandstone, siltstone, silty claystone, and dolostone. Meager fossil evidence (Paleocastor sp., Hypertragulus minor, and Amphicaenopus(?)] suggests an Arikareean (early Miocene) age for the Killdeer.

From petrographic analyses it has been determined that the sandstones of the Chadron, Brule, and Killdeer Formations are arkose. X-ray-diffraction analyses showed that montmorillonite is the predominant clay mineral in these deposits and that the zeolites clinoptilolite, erionite, and heulandite are present in both the mudstone and sandstone units. No distinct mineralogic criterion for distinguishing the new units was found. Although previous workers have mentioned the tuffaceous aspect of these deposits and several lines of evidence were found in the present study to suggest the presence of ash in some of the beds, no attempt is made to apply a pyroclastic nomenclature to them until further work has been done.

The source of the detritus in the middle Cenozoic deposits was probably the Tertiary igneous intrusions of the northern Black Hills, based on (1) an examination of available radiometric dates of Tertiary igneous rocks there, (2) a cursory comparison of igneous rock types in the gravel of the Chalky Buttes Member of the Chadron with igneous rock types in the northern Black Hills, (3) an approximation of the pre-Oligocene slope between the Black Hills and the study area, and (4) a review of what is known of the Cenozoic geomorphic history of the Black Hills.

The middle Cenozoic strata of North Dakota represent deposition in various fluvial subenvironments: channel, floodplain, and floodbasin. All of the deposits may be accounted for by a fluvial model in which divides are low such that flood waters of adjacent streams comingle to form large but shallow, temporary lakes. Possible modern analogs of such conditions include the Punjab region of Pakistan, the Amazon River Basin of Brazil, and the Pampa of Argentina.

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