Lithostratigraphy, Paleontology, and Biochronology of the
Chadron, Brule, and Arikaree Formations in North Dakota
John W. and MURPHY, Edward C., North Dakota Geological Survey, Bismarck
Exposed strata of the White River Group (Chadron and Brule formations) and Arikaree Formation in North Dakota occur as sparse and isolated erosional remnants only in southwestern parts of the state, primarily in the Chalky Buttes, Little Badlands, and Killdeer Mountains areas.
The Chadron Formation in North Dakota has been divided into two members. The basal Chalky Buttes Member is lithologically variable and consists of yellow/green to white, gravel-bearing, cross-bedded sandstones and sandy mudstones. This member unconformably overlies strata of the Fort Union Group and conformably underlies the South Heart Member of the Chadron Formation. The South Heart Member consists of green to gray smectitic claystones containing lenticular beds of freshwater limestones and conformably underlies the Brule Formation. Mammalian fossils of Chadronian age have been recovered from both members.
The Brule Formation is composed of pinkish/brown to gray/green complexly interbedded claystones, calcareous mudstones, siltstones, freshwater limestones, tuffaceous beds, and cross-bedded sandstones. The Antelope Creek tuff in the lower part of the formation is a useful marker bed for correlation between isolated outcrops. The Brule Formation is unconformably overlain by the Arikaree Formation, and the Brule is easily distinguishable lithologically and by color, from the Chadron and Arikaree formations. Mammalian remains indicate an Orellan age for the lower part of the formation and a possible Whitneyan age for the upper part.
The Arikaree Formation consists of concretionary, cross-bedded, calcareous sandstones, siltstones, silty claystones, tuffaceous beds, and carbonates, and caps several isolated buttes in southwestern North Dakota. A basal conglomerate commonly marks the disconformity which separates the Arikaree rocks from the White River Group. The Arikaree Formation is overlain by a thin veneer of unconsolidated lag sediments, mostly gravel, of unknown age. Vertebrate fossils are extremely sparse in the Arikaree Formation in North Dakota but tend to suggest a late Whitneyan to late Arikareean age. This is consistent with an age of 25.1+2.2 Ma from fission track dating of volcanic glass from the middle part of the Arikaree Formation in the Killdeer Mountains.
This investigation was initiated to determine the lithostratigraphic and biostratigraphic relationships of the Chadron, Brule, and Arikaree formations in North Dakota, to identify paleofaunas restricted to these formations, to determine the age of the strata using radiometric and biochronologic indicators, and to attempt to identify similarities and differences of these formations in North Dakota compared to those in other areas of the mid-continent. This five-year study began as a Cooperative Geological Mapping Program (COGEOMAP) between the United States Geological Survey and the North Dakota Geological Survey. Results, published in detail as a North Dakota Geological Survey Report of Investigation (Murphy et al., 1993), are summarized and updated here.
Exposures of the Chadron, Brule, and Arikaree formations are sparse and scattered in North Dakota in isolated buttes and butte complexes in the southwestern part of the state. Principal outcrops exist in the areas of Chalky Buttes (Slope County), Little Badlands (Stark County), Rainy Buttes (Slope County), and Killdeer Mountains (Dunn County). These units have been mapped at only 27 butte localities in southwestern North Dakota, a very small area compared to exposures of correlatives in other areas of the mid-continent. Commonly, only the caprock is exposed on these buttes and 11 of the buttes have an areal extent of 10 acres or less. Even though exposures of these units are restricted, they are important because they are the most northeasterly occurrences of these strata in North America.
Cope (1883) was first to introduce White River terminology to North Dakota for outcrops in the Chalky Buttes. Douglass (1909) identified rock exposures in the Little Badlands as White River strata. Outcrops in the Killdeer Mountains were assigned to the "White River formation" by Quirke (1918). The first systematic investigation of White River rocks in southwestern North Dakota was by Leonard (1922). He divided the White River formation into three informal units: "Lower White River beds," "Middle White River (Oreodon) beds," and "Upper White River beds." Powers (1945) also recognized a three-fold division of White River strata in North Dakota but termed these units, in ascending order, the "Titanotherium beds," "Oreodon beds," and "Protoceras beds." Denson et al. (1959D) elevated the White River formation in North Dakota to group status, and Moore et al. (1959M) identified the rocks as the Chadron and Brule formations. Moore et al. (1959Mo) were also first to apply the term Arikaree Formation to the 21 meters of capping sandstone at Chalky Buttes even though Seager et al. (1942) suggested that Miocene rocks may overlie the White River beds in North Dakota.
During the 1940s to 1960s several investigations were carried out in southwestern North Dakota to evaluate the economic potential (uranium, claystone, limestone) of the White River beds. The most comprehensive of these studies was by Denson and Gill (1965) who examined the uranium potential of White River strata. They measured several sections, produced a cross-section that for the first time correlated these units between several buttes, described the lithology and mineralogy of the formations, and noted the biostratigraphy of important vertebrate fossils. In addition, they were first to map the tuffaceous-marlstone cap rocks of the Killdeer Mountains as the Arikaree Formation, and suggested that these strata are Miocene in age.
Two important studies of the White River Group and Arikaree Formation were conducted by doctoral students from the Geology Department at the University of North Dakota. Stone (1973) measured sections in the Chalky Buttes, Little Badlands, and Killdeer Mountains, and correlated the units in a geologic cross-section. He split the Chadron Formation into three, and the Brule Formation into two members. Even though these member names were used by subsequent workers, Hoganson (1986) noted that Stone's members were not formally proposed in accordance with the rules established in the North American Stratigraphic Code, and thus the terms should be considered informal stratigraphic names. Hoganson and Lammers (1992) suggested that Stone's informal Brule members were of questionable utility. Stone (1973) also informally replaced the term Arikaree Formation with the name "Killdeer formation", because he believed that the caprocks on many of the North Dakota buttes were lithologically different than typical Arikaree lithologies in South Dakota and Nebraska. Stone found several vertebrate fossils during his study, and commented on their biochronological value.
Delimata (1975) conducted a detailed stratigraphic study of the Killdeer Mountains for his dissertation. One of Delimata's primary objectives was to determine the age of the caprocks on the Killdeer Mountains by finding diagnostic fossils. He was unsuccessful in that attempt but provided many useful measured sections.
Most previous investigations of White River and Arikaree strata in North Dakota have included reports of isolated fossil occurrences. The vertebrate fossil record of the Brule Formation in North Dakota was outlined by Hoganson and Lammers (1992). The most comprehensive discussion of the paleontology of the North Dakota Chadron, Brule, and Arikaree formations was in a report by us (Murphy et al., 1993). Also, in that report are detailed descriptions of 31 measured sections, and discussions of the petrology, lithostratigraphy, paleontology, biochronology and radiometric ages of the formations. A complete listing of fossils known from the formations was given. Two members of the Chadron Formation, the Chalky Buttes Member and the overlying South Heart Member were formally named in that report.
Field activities included measuring geologic sections, coring in areas of poor or nonexistent outcrops, collection of lithologic samples for petrographic characterization and potential radiometric dating, collection of paleontological specimens for biochronologic and paleobiogeographic analyses, and field reconnaissance of the geologic type-sections of the Chadron, Brule, and Arikaree formations in South Dakota and Nebraska for comparison to the presumed equivalent North Dakota formations.
Laboratory procedures included: hand specimen and thin section petrographic analysis, x-ray analysis of claystones, fission track analysis of glass shards, trace element analysis of glass separates, and fossil preparation and identification.
Chalky Buttes Member As in most places in the upper Midwest, the Chadron Formation in North Dakota is the basal formation in the White River Group. The Chadron Formation is divided into two easily recognizable members in North Dakota, the Chalky Buttes Member and the overlying South Heart Member. The Chalky Buttes Member consists of light colored, generally white to yellow/green, gravel-bearing, cross-bedded sandstone and sandy mudstone. Pebbles and cobbles of volcanic porphyries, apparently unique to this member, and other exotic rocks are oriented along bedding planes in some of the better exposures. At a few localities, a 4.5 meter-thick white to purple mudstone is found at the base of the member. The Chalky Buttes Member is, on the average, 6 meters thick in North Dakota, and reaches a maximum thickness of 24 meters in the Chalky Buttes. Measured sections of this member were given by Murphy et al. (1993).
The Chalky Buttes Member rests unconformably on the Bullion Creek (Paleocene), Sentinel Butte (Paleocene), or Golden Valley formations (Paleocene and Eocene) of the Fort Union Group. The contact is sharp between the overlying brightly colored white to yellow/green gravel-bearing, cross-bedded sandstone or sandy mudstone of the Chalky Buttes Member and usually somber grey to brown beds of alternating sandstone, siltstone, mudstone, claystone, and lignite of the Fort Union Group. In Slope County, North Dakota, at the type locality in the Chalky Buttes, the basal contact of this member is placed above a brightly colored 0.6 to 0.9 meter-thick analcime bed (paleosol) in the Sentinel Butte Formation. The Chalky Buttes Member is conformably overlain by the South Heart Member of the Chadron Formation.
The Chalky Buttes Member is lithologically similar to the Chamberlain Pass Formation in the Big Badlands of South Dakota and Nebraska (Evans and Terry, 1994; Terry et al., 1995), and Lillegraven's (1970) "dazzling white" unit of the Chadron Formation at Slim Buttes, South Dakota.
The South Heart Member, the most distinctive and continuous lithostratigraphic unit in the White River Group in North Dakota, is the upper member of the Chadron Formation. The rocks consist of brown to gray/green smectitic claystone that commonly contains thin (<1.5 meters thick) lenticular beds of freshwater limestone, particularly in the upper part of the member. These limestone beds are often ledge-formers and cap some of the small buttes in the study area. Radially fibrous calcite concretions are often found on South Heart outcrops. South Heart claystones have "popcorn weathering" surfaces and drape over underlying strata, thus making it difficult to locate contacts. The rocks weather to distinctive grayish domed "haystack" hills and butte caps in areas of the Little Badlands and Chalky Buttes. The South Heart Member attains a maximum thickness of 29 meters at White Butte (Slope County), and ranges from about 3 to 18 meters elsewhere in the study area. Measured sections of the South Heart Member in southwestern North Dakota were given by Murphy et al. (1993).
Even though the South Heart Member is conformably underlain by the Chalky Buttes Member of the Chadron Formation, and is conformably overlain by the Brule Formation, it is easily distinguishable from those units lithologically, by color, and by weathering characteristics. The rounded exposures and "popcorn weathered", somber grayish colored claystone of the South Heart Member stands in sharp contrast to the dazzling white of the underlying gravelly sandstones of the Chalky Buttes Member and the pink and brown siltstones and mudstones of the overlying Brule Formation, both of which weather to vertical cliffs. The South Heart Member is an excellent "marker" horizon in southwestern North Dakota.
The South Heart Member is lithologically similar to the Peanut Peak Member of the Chadron Formation in the Big Badlands of South Dakota and Nebraska (Evans and Terry, 1994; Terry et al., 1995) and to Lillegraven's (1970) "typical Chadron" at Slim Buttes, South Dakota.
Vertebrate fossils have been recovered from six localities in the Chadron Formation of North Dakota, and brontothere remains have been found in both members of the Chadron Formation (Murphy et al., 1993). The only known diverse vertebrate fossil assemblage is from the Chadron Formation in the Medicine Pole Hills area of Bowman County. At that locality, erosional remnants of the Chalky Buttes Member are exposed in isolated outcrops and blowouts. Brontothere remains were first recovered from this locality by Leonard (1922), and then by other early workers (Hares, 1928; Benson, 1952; Denson et al., 1959; Kepferle and Culbertson, 1955). Pearson (1993P) determined that a diverse vertebrate fossil assemblage is present at this locality, and called the fauna the Medicine Pole Hills local fauna. Remains of 31 mammalian (within 11 orders and 22 families), 5 fish, 2 amphibian, and 10 reptilian taxa have been identified from this locality; it is anticipated that additional taxa will be found as work continues (Pearson and Hoganson, 1995).
Ostracodes, algal crusts, gastropods, and fish have been found in freshwater limestones in the South Heart Member of the Chadron Formation at several localities (Murphy et al., 1993). No leaf fossils have been reported from the Chadron Formation in North Dakota.
Murphy et al. (1993) suggested that brontothere remains in both the Chalky Buttes Member and South Heart Member of the Chadron Formation imply a Chadronian age for the formation. The Chadronian equid, Miohippus assiniboiensis, reported by Prothero and Shubin (1989) from the Chadron Formation in the Little Badlands, Stark County, also suggests a Chadronian age for the Chadron Formation in North Dakota. The occurrence of Herpetotherium valens, Sinclairella, Centetodon chadronensis, Stibarus montanus, Trigonias, brontotheres and possibly other taxa listed by Pearson and Hoganson (1995) from the Medicine Pole Hills local fauna also imply a Chadronian age. Pearson and Hoganson (1995) suggested that the Medicine Pole Hills local fauna may have affinities with the Calf Creek local fauna of the Cypress Hills Formation in Saskatchewan, believed to be early Chadronian in age (Storer, 1978).
The lower part of the Brule Formation in North Dakota consists of pinkish/brown to gray/green complexly interbedded claystones, calcareous mudstones, siltstones, freshwater limestones, tuffaceous beds, and cross-bedded sandstones. The channel sandstones, such as the "Fitterer bed," are generally less than 3 meters thick, are difficult to trace for long distances, and usually bear mammalian fossils. A useful marker bed in the lower part of the formation was called the Antelope Creek tuff by Murphy et al. (1993). The upper part of the formation is lithologically less variable, and consists primarily of alternating beds of mudstones and thin beds of swelling claystones. In parts of the Little Badlands and at Rainy Buttes, cross-bedded conglomeratic sandstones are in the upper part of the formation. The Brule Formation ranges in thickness from 6 meters in the Chalky Buttes (Slope County) to a maximum of 65 meters at the Fitterer Ranch locality, Little Badlands (Stark County). Measured sections of Brule exposures in southwestern North Dakota were presented by Murphy et al.
The Brule Formation is unconformably overlain by the Arikaree Formation, and is conformably underlain by the South Heart Member of the Chadron Formation. Brule rocks are easily distinguishable by lithology and color from the underlying South Heart Member (i. e., the pinkish/brown siltstone and mudstone of the Brule compared to the gray/green claystone of the South Heart Member). A conglomerate is usually found at the base of the Arikaree Formation where it overlies the Brule. It is at times difficult to distinguish these Arikaree conglomerates from conglomeratic sandstones in upper parts of the Brule Formation. Murphy et al. (1993) placed these channel sandstones in either the Arikaree or Brule Formation, depending upon occurrences of typical Brule pinkish/brown siltstones or mudstones above the conglomeratic unit.
Lithologically, the lower part of the Brule Formation in North Dakota appears similar to the Scenic Member of the Brule Formation in the Big Badlands of South Dakota, and the Orella Member of the Brule Formation in Nebraska. The upper part of the Brule in North Dakota is lithologically similar to the Poleslide Member of the Brule Formation in the Big Badlands of South Dakota and the Whitney Member of the Brule Formation in Nebraska. Layering and color banding, attributed to paleosol development by Retallack (1983), however, is not as obvious in the Brule Formation in North Dakota as in the Big Badlands of South Dakota.
Several important vertebrate fossil sites are known in the Brule Formation in southwestern North Dakota. These include Fitterer Ranch, Obritsch Ranch, and the Little Badlands (Stark County), and the Chalky Buttes, including the White Butte locality, (Slope County). Most of the fossil occurrences, however, are from the lower half of the formation. The most comprehensive paleontological treatments of the Brule Formation in North Dakota are by Hoganson and Lammers (1992) and Murphy et al. (1993). Both reports provide histories of Brule fossil research in North Dakota. Murphy et al. (1993) gave a list of all taxa identified from the North Dakota Brule. At least 90 mammalian (10 orders and 31 families), 3 fish, 2 amphibian, 8 reptilian, and 1 avian taxa have been reported from the Brule Formation in North Dakota. Freshwater and terrestrial gastropods, ostracodes, insect trace fossils, and fish and mammalian coprolites are also found in the North Dakota Brule Formation. No leaf fossils are found in the formation, but algal remains, charophyte oogonia, and hackberry endocarps are present.
Mammalian taxa reported from the lower Brule Formation by Hoganson and Lammers (1992) and Murphy et al. (1993) (including Leptictis haydeni, Ischyromys typus, Eumys elegans, Mesohippus bairdi, Archaeotherium mortoni, Leptomeryx evansi, Hyaenodon horridus, Hyaenodon crucians, Palaeolagus, Merycoidodon, Hesperocyon, Daphoenus, and Dinictis) are generally considered characteristic of the Orellan (Emry et al., 1987), and strongly suggest at least the lower part of the Brule Formation in North Dakota was deposited during the Orellan. Skinner (1951) was first to suggest that most of the Brule Formation in North Dakota is Orellan in age, which was confirmed by Hoganson and Lammers (1992). Magnetostratigraphic interpretation appears to corroborate an Orellan age for strata in the Little Badlands (Prothero et al., 1983).
Skinner (1951) suggested that upper parts of the Brule Formation in North Dakota are Whitneyan in age, without providing much paleontological evidence for his interpretation. Wood et al. (1941) reported a fauna, possibly of Whitneyan age, from presumably the Brule Formation at White Butte, Slope County. They noted, however, that the "White Butte local fauna" differed from other Whitneyan faunas. Moore et al. (1959) suggested that the occurrence of Miohippus, Leptauchenia, and Protoceras? in the Brule Formation at Chalky Buttes indicated a Whitneyan age. Denson and Gill (1965) also interpreted these fossils to indicate a Whitneyan age, but cautioned that the fossils probably were recovered from a slump block. Kihm (1990) cited the occurrence of the rodent Eumys brachyodus in the Little Badlands as evidence for a Whitneyan age.
Hoganson and Lammers (1992) found no evidence of a Whitneyan age for the Brule, and suggested that fossils are too sparse in the upper part of the Brule to confirm the age of that part of the formation. Murphy et al. (1993) noted that upper parts of the Brule Formation at East and West Rainy Butte may be Whitneyan. Magnetostratigraphic results by Prothero et al. (1983P) imply that the upper part of the Brule Formation in the Little Badlands is Whitneyan in age.
The Arikaree Formation in North Dakota is lithologically variable, and consists of concretionary, cross-bedded, calcareous sandstones, siltstones, silty claystones, carbonates, and tuffaceous beds. It unconformably overlies either the Chadron or Brule formations in North Dakota, and is overlain by a thin veneer of unconsolidated sediment, usually lag gravels of unknown provenance. The Arikaree is the butte-capping formation in several areas of southwestern North Dakota. A conglomerate is often found at the base of the formation, which allows easy differentiation except where conglomeratic channel sandstones occur at the top of the Brule Formation. The Arikaree Formation is best exposed in the Killdeer Mountains (Dunn County) where it attains a maximum thickness of 130 meters. Murphy et al. (1993) presented measured sections from most areas of exposure of the Arikaree Formation in southwestern North Dakota.
The Arikaree Formation consists of tuffaceous siltstones, sandstones, and carbonates in the Killdeer Mountains. Carbonates are also found in the Arikaree Fm. at the Chalky Buttes, but in most other places siltstones and sandstones are the dominant Arikaree lithologies. The siltstones, sandstones, and carbonates are ledge-formers in the Killdeer Mountains. One of these ledges, 4.5 to 9 meters thick, consists of interbedded highly tuffaceous sandstone, siltstone, and occasional calcite-indurated beds, and is unique because it contains abundant burrows. Forsman (1986F) termed this the "burrowed marker unit", a terminology also adopted by Murphy et al. (1993Mu). Forsman (1986Fo) also determined that nearly all of the lithologies in the Arikaree Formation in the Killdeer Mountains contain volcanic glass, and could be characterized as slightly to highly tuffaceous. Lithologies and bedding structures in the Arikaree Formation at the Killdeer Mountains suggest deposition in a lacustrine setting (Quirke, 1913; Delimata, 1975; Forsman, 1986; Murphy et al., 1993).
The Arikaree Formation in North Dakota is lithologically similar to the Gering and Harrison formations of the Arikaree Group in northwestern Nebraska. Like the Arikaree Formation in North Dakota, those units are lithologically variable, but are predominantly fine-grained, tuffaceous sandstones and siltstones with interbedded carbonate lenses. The Gering and Harrison formations in Sioux County, Nebraska, also contain small burrows at some localities similar to those in the "burrowed marker unit" in the Killdeer Mountains and in the Arikaree Formation at White Butte and West Rainy Butte, Slope County.
Fossils are extremely sparse in the Arikaree Formation of North Dakota. Remains of two genera of oreodonts (Merychyus and Merycochoerus) were collected from near the top of the Arikaree Formation in the Killdeer Mountains by the Frick Laboratory of the American Museum of Natural History in 1954. We recently collected another skull of an immature Merychyus from near the summit of the Killdeer Mountains. The only other fossils that have been found in the Arikaree Formation in the Killdeer Mountains are burrows and ostracodes. Elsewhere, vertebrate fossils reported from the Arikaree Formation in North Dakota include: Palaeocastor from near the summit of the buttes at Obritsch Ranch in the Little Badlands area of Stark County (Murphy et al., 1993); Miohippus obliquidens from the top of East Rainy Butte, Slope County (Prothero and Shubin, 1989); Elomeryx armatus from the summit of East Rainy Butte; Hypertragulus minor? and Amphicaenopus from White Butte, Slope County (Stone, 1973); and Leptauchenia decora and Nanotragulus, from White Butte, Slope County.
The occurrence of Elomeryx armatus, Miohippus obliquidens, Palaeocastor, Nanotragulus, and Leptauchenia decora in the Arikaree Formation in Slope and Stark counties and Merychyus and Merycochoerus in the Arikaree Formation in the Killdeer Mountains, Dunn County, suggest a late Whitneyan to late Arikareean age for the formation in North Dakota. Tedford (pers. comms., 1989, 1994) wrote that Merychyus and Merycochoerus have overlapping range zones in the late Arikareean. In addition, he noted that this overlap occurs in the uppermost part of the Arikaree Group (Upper Harrison beds, see MacFadden and Hunt, this volume) and base of the Ogallala Group (Runningwater Formation) in Nebraska. Hunt (pers. comm., 1996) also observed that the earliest occurrence of Merycochoerus in the Great Plains is found in the Upper Harrision beds which produced the latest Arikareean fauna of western Nebraska and southeastern Wyoming; this genus continues on into the early Hemingfordian Runningwater deposits in Nebraska, but Hunt reports that the Merychyus of the Killdeer Mountains found with Merycochoerus suggests a late Arikareean age. This interpretation of an Arikareean age for the Arikaree Formation in Dunn County is consistent with an age of 25.1+2.2 Ma from fission track dating of volcanic glass from the middle part of the Arikaree Formation in the Killdeer Mountains (Murphy et al., 1993). The mammalian taxa present in the Arikaree Formation imply that the erosional remnants of the formation exposed in Stark and Slope counties are older than those in Dunn County, although no fossils were found in the lower part of the formation in the Killdeer Mountains to establish a basal age for the formation in Dunn County.
This investigation was initiated as a cooperative project between the North Dakota Geological Survey and the United States Geological Survey through the COGEOMAP program (Project #14-08-0001-A0805). We thank the following people for assistance with various aspects of this project: Richard LeFever, John Kay, Steven McLean, Eric Harvey, James Sorenson, Mitch Reynolds, Glen Izett, Charles Naeser, Nancy Naeser, Roy Knight, James Martin, Phil Bjork, Janet Whitmore, George Lammers, Richard Tedford, Robert Hunt, Dean Pearson, Bob Purdy, Sergiu Mamay, Donald Prothero, Allen Kihm, Johnathan Campbell, and Todd Strand. We thank Jason Lillegraven, James Martin, and Margaret Stevens for providing useful reviews of the manuscript.
We also thank the following landowners and their families for cordially allowing access to their land: Leonard Buzalsky, Ryan Brooks, Ted Pope, Albert Privratsky, Ben Privratsky, William Schmidt, George Schmidt, Edward Schmidt, Ron Obritsch, Bob Obritsch, Bob Fitterer, Jack Murphy, Alec Dvornak, Lorin Dvornak, Ken Urlacher, Ralph Urlacher, Rick Maxiner, Fred Thomas, Nick Biel, Orion Bruvold, Gene Fisher, Sherman Oakland, and Jeff and Laurie Oakland.
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