Chadron Formation Research Articles

Provenance of Paleogene Strata at Slim Buttes, South Dakota: Implications for post-Laramide Evolution of Western Laurentia

Slim Buttes is a 30 km long by 10 km wide set of buttes containing Paleogene strata in northwest South Dakota. At Reva Gap in northern Slim Buttes, Eocene-Oligocene terrestrial strata of Chadron and Brule Formations of the White River Group unconformably overlie the Paleocene Fort Union Formation. An angular unconformity separates the White River Group from overlying Oligocene and Miocene strata of the Arikaree Group. Using detrital zircon U-Pb ages, we determine the provenance of these rocks as part of a broader synthesis of post-Laramide sedimentation in the Rocky Mountains and western Great Plains. The Chadron Formation age spectrum is dominated by Cretaceous and Proterozoic grains that are interpreted to be locally recycled from the underlying Cretaceous and Paleocene strata. The Brule Formation has a maximum depositional age of ~34 Ma; Paleogene zircons dominate the age spectrum, and a wide variety of older zircons are also present. The Oligocene zircons are interpreted to have been sourced from volcanic systems in the Great Basin to the southwest, while the subsequent proportions of the zircons were derived from a variety of source areas in the Nevadaplano and Rocky Mountain areas to the southwest. Sparse amounts of Archean zircons are thought to represent the burial of Laramide uplifts throughout Wyoming at the time of Brule deposition, making for a regional paleotopography with little relief across the western interior of the United States. The Miocene-age Arikaree Group sand has a maximum depositional age of ~26 Ma and a multimodal detrital zircon age spectrum. The Arikaree Group provenance likely represents continued sourcing in the Great Basin volcanic systems and Nevadaplano, the beginnings of the re-exhumation of Laramide basement uplifts, and subsequent sediment evacuation out of the western interior and into the Gulf of Mexico to the southeast. Our findings indicate that the transport process and detrital zircon provenance signatures of these strata are decoupled, and each have their own independent evolution. The volcanic signature is primarily transported via aeolian processes (i.e. volcanic ash), and the recycled detrital zircon signature is primarily transported via fluvial processes.

Controls on Soft Tissue and Cellular Preservation in Late Eocene and Oligocene Vertebrate Fossils from the White River and Arikaree Groups of Nebraska, South Dakota, and Wyoming

Previous studies on microtaphonomy have identified multiple types of organic microstructures in fossil vertebrates from a variety of time periods and past environmental settings. This study investigates potential taphonomic, paleoenvironmental, and paleoclimatic controls on soft tissue and cellular preservation in fossil bone. To this end, fifteen vertebrate fossils were studied: eight fossils collected from the Oligocene Sharps Formation of the Arikaree Group in Badlands National Park, South Dakota, and seven fossils from formations in the underlying White River Group, including the Oligocene Brule Formation of Badlands National Park, and the Eocene Chadron Formation of Flagstaff Rim, Wyoming; Toadstool Geologic Park, Nebraska; and Badlands National Park, South Dakota. A portion of each fossil was demineralized to identify any organic microstructures preserved within the fossils. We investigated several factors which may have influenced cellular/soft tissue decay and/or preservation pathways, including taxonomic identity, paleoclimatic conditions, depositional environment, and general diagenetic history (as interpreted through thin section analysis). Soft tissue microstructures were preserved in all fossil samples, and cellular structures morphologically consistent with osteocytes were recovered from 11 of the 15 fossil specimens. Preservation of these microstructures was found to be independent of taxonomy, paleoclimate regime, apatite crystallinity, depositional environment, and general diagenetic history, indicating that biogeochemical reactions operating within microenvironments within skeletal tissues, such as within individual osteocyte lacunae or Haversian canals, may exert stronger controls on soft tissue and biomolecular decay or stabilization than external environmental (or climatic) conditions.

First FossilAlligatorfrom the Late Eocene of Nebraska and the Late Paleogene Record of Alligators in the Great Plains

Abstract Fragmentary fossils have been tentatively attributed to Alligator from the Eocene of Wyoming, North Dakota, Saskatchewan, and Utah. The earliest definitive temporal and geographic range of Alligator has been limited to the late Eocene and early Oligocene Chadron and Brule Formations of the White River Badlands in South Dakota, represented by fossils of Alligator prenasalis. New fossils from the Chadron Formation in northwestern Nebraska expand the definitive geographic range of Alligator during the late Eocene. Results from a cladistic analysis of 28 alligatoroid species using 181 morphological characters support the identification of the new specimens as representing the most basally divergent taxon within Alligator, suggesting that they likely represent A. prenasalis; however, a lack of diagnostic features in the Nebraska specimens prevents a definitive assignment to this species. Limited occurrence data may indicate that Alligator experienced a reduction in biogeographic range following global...

Heavy minerals in the late Eocene sandstone of Medicine Pole Hills, southwestern North Dakota

Late Eocene sandstone capping hills and small buttes in the Medicine Pole Hills in southwestern North Dakota has been correlated with the Chalky Buttes Member of the Chadron Formation. Heavy-mineral analysis was used to assess this correlation. Optical microscopy and chemical analysis using a scanning electron microscope/energy dispersive spectrometer (SEM/EDS) system were used to determine abundances and compositions of heavy minerals separated from 0.25–0.30 mm fractions of nine samples from seven depositional units (353–602 grains per sample). The 0.063–0.125 mm fraction was also studied for one of these samples. Samples ranged from coarse- to medium- to fine-grained poorly consolidated sandstone. Total heavy mineral recovery (0.4–14.1%) generally corresponded with the grain-size distribution of samples, with higher recoveries in coarser samples. Opaque grains (ferruginous aggregates, Fe-Ti oxides, and leucoxene) comprised 0.8–15.6 percent of heavy mineral grains. Non-opaque grains were dominated by amphibole (24.4–52.2%) and epidote-clinozoisite (10.6–38.1%), with variable diopside (0–51.1%). Generally less abundant minerals included garnet, apatite (bone fragments), biotite, and muscovite. One sample was biotite-rich (36.5%). Chemical compositions of heavy minerals showed no significant stratigraphic variations, suggesting the nature of the sources did not vary. Diopside, biotite, and approximately 1/3 of the amphibole grains were interpreted as derived from Tertiary volcanic sources. The majority of the amphibole grains (tremolite and blue-green hornblende), epidote, and garnet were derived from Precambrian metamorphic sources. The relative contribution of volcanic sources was variable (19–64%), being highest in those samples with more diopside and (with the exception of the biotite-rich sample) highest in those samples with higher overall heavy-mineral abundances. Individual heavy-mineral abundances also correlated fairly well with grain size. Overall, variations in heavy minerals were explained by variable volcanic source contributions and variations in grain-size distributions of the samples. Despite the variability found, cluster analysis showed that the samples from Medicine Pole Hills were consistently different than the generally much more stable heavy-mineral assemblages found in the Chalky Buttes Member. They were also consistently different when volcanic heavy minerals were ignored. The differences could not be explained by variable volcanic contributions, grain-size effects, and/or differences in heavy mineral stabilities alone. They require differences in provenance, suggesting that the sandstone of Medicine Pole Hills should not be correlated with the Chalky Buttes Member, or, at the very least, it should be considered a distinct facies of the Chalky Buttes Member.

Avian eggs from the Eocene Willwood and Chadron formations of Wyoming and Nebraska

ABSTRACT Partial eggs from the Eocene Willwood Formation of Wyoming contain unidentifiable embryonic remains; the eggs are referable to a theropod on the basis of their structural layering of calcite and to an avian theropod because of their Eocene age. We assign the specimens to the oofamily Medioolithidae as Microolithus wilsoni, oogen. et oosp. nov., on the basis of the following unique combination of characters: 600-μm-thick eggshell composed of three structural layers; abrupt and undulating contact between the mammillary and continuous layers; smooth and glossy outer egg surface; faint or obscure prisms of the continuous layer; non-branching pores; and mammillary-to-total shell thickness ratio of 1:4. The eggshell microstructure resembles that of some extant neognath bird eggs. We refer a single egg from Chadron Formation of eastern Nebraska to incertae sedis as Metoolithus nebraskensis, oogen. et oosp. nov. A combination of characters distinguishes the egg from other fossil and modern avian eggs, namely, thicker eggshell, prominent ornamentation, flared upper portion of the prisms, variable mammillary thickness, and irregular squamatic texture in the continuous layer. These characteristics more closely resemble Mesozoic non-avian theropod eggs and likely reflect mosaic evolution in the non-avian to avian-theropod transition. Finally, an external layer occurs in a wide range of Paleogene and Cretaceous eggs of variable size and taxonomic affinities, indicating that this feature may not represent an apomorphic character for the avian crown group with respect to non-avian theropod eggshell. Therefore, an external layer cannot be used to identify neognath birds in the Mesozoic.

Macroevolutionary and morphofunctional patterns in theropod skulls: a morphometric approach

The synonymization of the cimolestan taxa Cymaprimadon and Chadronia from the Late Eocene Chadron Formation is consistently upheld, despite a lack of supporting evidence. Here we show that the synonymization is unjustified, owing to distinct differences between these taxa in the mandibular tooth count (1-1-3-3 vs. ?-1-4-3), the identity of the enlarged anterior mandibular tooth (incisor versus canine), and the morphology of the crown of m3 (e.g., paraconid on m3 in Cymaprimadon). We also refer a specimen recently collected from the Early Oligocene Brule Formation within the Badlands National Park (BADL 16917) to Chadronia sp., thus making it the youngest occurrence of a pantolestan from North America. Examination of an additional specimen (FMNH UC 349) revealed the presence of a further cimolestan taxon in the White River Group of South Dakota, although the poor quality of the locality and stratigraphic data associated with this specimen precludes erecting a formal name. In total, this study doubles the n...

A New Method for Finding Small Vertebrate Fossils: Ultraviolet Light At Night

Vertebrate fossils from many different formations fluoresce when exposed to ultraviolet (UV) light. In this paper field observations and controlled experiments in the Chadron Formation (White River Group, late Eocene) of Wyoming are used to assess the utility of searching for fossils at night using ultraviolet light. The results indicate that, especially for very small teeth and egg‐shell fragments, searching with ultraviolet light at night can result in significantly more specimens than searching during daylight hours. This method has the potential to increase sample sizes for small vertebrate specimens that are often overlooked when using standard collecting techniques.

Using phytolith assemblages to reconstruct the origin and spread of grass-dominated habitats in the great plains of North America during the late Eocene to early Miocene

This paper outlines a general approach for analyzing siliceous microfossils from plants (phytoliths) in geologic sediments and uses this method to reconstruct the evolution of grasslands in the central Great Plains during the late Tertiary. Phytolith assemblage analysis is widely employed for reconstructing vegetation types in Pleistocene–Holocene paleoecology and archaeology, but no standardized methodology is currently available. Published phytolith studies are often system-specific and tailored after specific modern analogues. As a result, they vary in extraction technique, classification scheme, and method of inference. For ancient phytolith assemblages whose modern analogues are not well understood, a more general analytic approach is necessary. This analytic approach includes studying all size fractions (2–250 mm), using a classification scheme based on the available literature and a comprehensive modern phytolith reference collection, and comparing relative frequencies through time of all diagnostic phytoliths to interpret changes in habitat structure. This general method was applied to late Eocene to early Miocene sediment samples from northwestern Nebraska to deduce vegetation history and the results were compared to those obtained using a more specific technique [Strömberg, Paleogeogr. Paleoclimatol. Paleoecol. 177 (2002) 59]. In both studies, phytoliths were extracted by means of heavy liquid separation from the following lithostratigraphic units: (1) the late Eocene Chadron Formation and the early Oligocene Brule Formation (White River Group); (2) the late Oligocene–early Miocene Monroe Creek Formation, the early Miocene Harrison Formation and Anderson Ranch Formation (Arikaree Group); and (3) the early Miocene Runningwater Formation and Box Butte Formation (Ogallala Group). The two techniques, general vs. specific, produced results that differ markedly. Data from the general approach indicate the presence of relatively closed habitats in late Eocene to early Oligocene, typified by the presence of phytoliths from bambusoid grasses, woody and herbaceous dicotyledons, and palms. The introduction and spread of modern, open-habitat grasses, such as pooids, arundinoids, and panicoids, occurred between the late Oligocene and early Miocene. This resulted in a savanna or woodland type habitat by the early Miocene. In contrast, analysis using the specific method suggested the presence of open habitats in northwestern Nebraska from the late Eocene onward. The difference in interpretation based on the two approaches is principally due to inclusion of phytoliths >50 mm and the use of an expanded reference collection in the current study (general analytic approach). These results point to the importance of methodological choices in phytolith analysis and emphasize the usefulness of phytoliths as paleoecological indicators in the fossil record.

Redescription of Eopelobates grandis, a Late Eocene anuran from the Chadron Formation of South Dakota

Redescription of Eopelobates grandis, a Late Eocene anuran from the Chadron Formation of South Dakota

Paleopedology of the Chadron Formation of Northwestern Nebraska: implications for paleoclimatic change in the North American midcontinent across the Eocene–Oligocene boundary

The Big Cottonwood Creek Member is a newly recognized lithostratigraphic unit of the Chadron Formation of northwestern Nebraska. I measured 23 superimposed paleosol profiles within 29 m of strata exposed 0.4 km (0.25 mile) east of the type section of the Big Cottonwood Creek Member at Toadstool Park in northwestern Nebraska and recognized four main types of paleosols. The paleosols are all well drained, moderately alkaline to alkaline, oxidized, and range from weakly developed azonal Ustifluvents, to moderately and well-developed Paleustalfs and Argiustolls with argillic and petrocalcic horizons. Based on changes in soil structure, horizons, root traces, and geochemical trends, these paleosols record a gradual change from humid, forested conditions of the late Eocene to more seasonal, semi arid, open range conditions leading up to the global climatic deterioration event of the early Oligocene at 33.2 Ma. This interpretation is supported by associated changes in sedimentology, fluvial architecture, floras, and faunas over this period of time. The section at Toadstool Park is significant in that it establishes a link between the Big Badlands of South Dakota and correlative deposits of Wyoming. The contact of the Chadron and Brule Formations in the Big Badlands of South Dakota is marked by an unconformity that is in part correlative to the earliest Oligocene sediments that elsewhere record the global climatic deterioration event at 33.2 Ma. The paleosols of the Big Cottonwood Creek Member of northwestern Nebraska fill in a significant part of this gap and record paleoclimatic conditions of the late Eocene leading up to the global climatic deterioration event of the early Oligocene. Using the latest lithostratigraphic, biostratigraphic, magnetostratigraphic, and tephrostratigraphic data for the White River Group, this change to more arid conditions in the midcontinent appears to have progressed from west to east over time.

Recognition and implications of Eocene tufas and travertines in the Chadron Formation, White River Group, Badlands of South Dakota

There are few descriptions of pre‐Quaternary tufa or travertine, yet these deposits have important climatic, hydrological and tectonic implications. In the Badlands of South Dakota, the Late Eocene Chadron Formation contains a variety of tufa (fluvial, lacustrine, paludal and perched springline types), travertine and calcrete (pedogenic and non‐pedogenic types). In fluvial channels, tufas and tufa barrages (oncolites, tufa lithoclast conglomerates and phytoherms) are transitional to travertines (pseudostromatolitic boundstone gravels and small pinnacle resurgences with rimstone edges and pools). Lacustrine tufas formed in small ponds adjacent to other types of tufas and are similar to the ‘low‐energy, bench‐margin facies’ types observed in modern, groundwater‐fed, temperate‐zone hardwater lakes. Paludal tufas are laterally discontinuous sheets of pisoid‐intraclast wackestone that fill palaeotopography. Distal perched springline tufas are mounds of numerous sloping tufa sheets. Concretions and rhizoliths associated with palaeosols are recognized as pedogenic calcretes, whereas non‐pedogenic calcretes are variably cemented, vertically oriented conduits for palaeogroundwater discharge that are located in fault zones. These deposits resulted from palaeogroundwater discharge along faults during the Late Eocene. Coeval unroofing of the Black Hills uplift created a regional groundwater flow system through karsted Palaeozoic limestones exposed in the Black Hills. The Chadron Formation is an avulsion‐dominated, siliciclastic unit, yet important bodies of carbonate accumulated in it as a result of groundwater processes.

Fluoride in Nebraska's Ground Water

AbstractFluoride concentrations in ground water are generally low but play an important role in dental health. This study evaluates the vertical and spatial distribution of fluoride in Nebraska's ground water and examines the geological and geochemical processes that control its concentration. Data from 1794 domestic wells sampled by the Nebraska Department of Health and Human Services. Regulation, and Licensure (NDOH) had a range of fluoride concentrations from <0.1 to 2.6 mg/L. and a median concentration of 0.3 mg/L. The median fluoride concentrations for Nebraska's 13 ground water regions varied from 0.2 to 0.7 mg/L. In each of these regions, individual wells may have either insufficient or overabundant F concentrations; we recommend that individual private water systems be tested for fluoride. Based on these data, system‐specific recommendations can be made regarding the necessity for fluoridation.Geochemical data indicated that the majority of fluoride occurs as F. Dissolution of F‐bearing minerals controls fluoride occurrence. Apatite plus minor amounts of fluorite along with significant ground water residence times are the primary factors controlling F in the water from the Dakota Formation in Knox County, as well as in other parts of northeastern Nebraska. In western and southwestern Nebraska, dissolution of volcanic glass is the most probable source of F Long residence times plus fluorite also may contribute to the F concentrations in the Chadron Formation.

Additional comments on neutron activation analysis of stonefrom the great plains: Reply to church

Criticisms levelled by Tim Church on a previously published paper (Hoard et al., Neutron activation analysis of stone from the Chadron Formation and a Clovis Site on the Great Plains, Journal of Archaeological Science 19, 655–665, 1992) are addressed. We maintain that the methods and conclusions of our previous paper are sound. Our ongoing research is refining our methodology and continuing to produce significant archaeological interpretations.

The significance of incision and fluvial sedimentation in the Basal White River Group (Eocene-Oligocene), Badlands of South Dakota, U.S.A.

A newly defined lithostratigraphic unit, the Chamberlain Pass Formation (CPF), records the initial episode of incision, fluvial sedimentation, and pedogenesis in SW South Dakota following the retreat of the Cretaceous Interior Seaway. The CPF is Middle(?) to Late Eocene in age, and consists of fluvial sandstone and mudstone. Pedogenic modification of the unit has created a distinctive pedostratigraphic unit, the Interior Paleosol Series. The CPF thickens from west to east, achieving a maximum channel-belt thickness of ⩾ 11 m. Paleoflow data indicate that deposition of the CPF was restricted to a fault-controlled basin southeast of the Black Hills uplift. Sandstones in the CPF were derived from a recycled sedimentary rock source area to the west. In contrast, sandstones in the overlying Chadron Formation (Late Eocene) had a variety of sources including the Precambrian core rocks of the Black Hills uplift. Deposition of the CPF brackets four significant Paleogene changes in baselevel that occurred in this region. These events were: (1) Late Cretaceous to Middle(?) Eocene relative baselevel fall, weathering and erosion of the Cretaceous Pierre Shale to form the Yellow Mounds Paleosol, and fluvial incision; (2) Middle(?) to Late Eocene relative baselevel rise and deposition of the CPF; (3) Late Eocene relative baselevel fall, weathering and erosion of the CPF to form the Interior Paleosol, and fluvial incision; and (4) Late Eocene to Oligocene relative baselevel rise and deposition of the Chadron Formation. The first event was probably eustatic, the second was controlled primarily by local subsidence in a fault-bounded basin, the third records the tectonic uplift and unroofing of the Black Hills, and the fourth event was probably primarily controlled by eustasy, but other factors may have been important.

A new fossilized tree fern stem, Nishidacaulis burgii gen. et sp. nov., from Nebraska-South Dakota, USA

A permineralized tree fern stem assigned to the new genus and species Nishidacaulis burgii was collected from northwestern Nebraska. Although collected in the basal conglomerate of the Upper Eocene-Lower Oligocene Chadron Formation, it was most likely reworked from sediments of Early Cretaceous age in the Black Hills, South Dakota. The stem appears to have had radial symmetry and was surrounded by persistent leaf bases and adventitious roots. Uniseriate, multicellular epidermal trichomes are also persistent on and around the stem. A sclerenchymatous sheath around the solenostelic stele is absent. Leaf and petiolar traces are undivided throughout and are modified reverse omega- or U-shaped with incurved ends. Mucilage cells or cavities occur in the outer phloem of the petiolar vascular strand, but are lacking in the fundamental tissue of the petiole. This new genus may represent a transitional form in a possible lineage from Upper Jurassic-Lower Cretaceous “ Cibotium” tasmanense to Upper Cretaceous C. iwatense and extant members of Cibotium. The basal pinna trace being derived from the adaxial corner only of the petiolar vascular strand in N. burgii indicates the original plant had a small or reduced basal pinna and a short petiole. Its entire frond was more or less rhombic, suggesting a habit with an erect stem with radiating fronds forming a crown.

Neutron activation analysis of stone from the Chadron Formation and a Clovis Site on the Great Plains

Abstract Cryptocrystalline silicates from the Chadron Formation were widely used by prehistoric human groups in the Great Plains of North America. There are two documented quarry areas: Flattop Butte in Colorado and the White River Badlands of South Dakota. Cryptocrystalline silicates from these sources are visually indistinguishable, making it difficult to determine the sources of artefacts found at archaeological sites. In this investigation, neutron activation analysis is used to distinguish the two sources. This technique also identifies lithic material from the Clovis-age Eckles site in Kansas as being from Flattop Butte.

EROSIONAL PROCESSES AND LANDFORMS IN BADLANDS NATIONAL MONUMENT, SOUTH DAKOTA

Small-scale erosional features in the Big Badlands of South Dakota include miniature mountain ranges and pediments, with morphology resembling that of mature mountainous desert regions of the southwestern United States. Miniature badland pediments have steeper gradients than large-scale pediments of the arid southwest, but angles of mountain fronts average about the same for both—approximately 35°. There is an abrupt reduction of slope where the miniature mountain front meets its bordering pediment. The miniature mountain drainage is poorly integrated; it consists of closely spaced rill channels having the same gradient as the mountain slope. At the expense of the retreating miniature mountain front the pediment is extended headward by erosion at the margins of spreading sheetwash. The escarpment retreats by disaggregation through absorption and desiccation and by rill erosion. Topographic texture varies with underlying bedrock. Topography developed on the Chadron formation is fine-textured; that on the Brule formation is ultra-fine-textured. Badlands drainage is typically dendritic with regularly bifurcating finger-tip tributaries. The mean maximum angle of slope in typical Chadron topography is 26.2°, with estimated standard deviation of 4.2°. For the Brule formation mean maximum slope angles of 56.0°, 49.5°, and 51.0° were obtained in three areas. A t test revealed that the differences between means of Brule and Chadron are significant. Each slope mean represents an equilibrium condition in which the slope has been adjusted to achieve a steady state between the resistivity of the material to erosion and the intensity of erosional processes.

A New Bird (Family Cracidae) from the Early Oligocene of South Dakota

THE FAMILY CRACIDAE, including the curassows, guans, and chachalacas, ranges in the present day from the lower Rio Grande Valley in Texas south to Argentina. The modern distribution of the family gives no clue to the considerable radiation once enjoyed by the group in what is now temperate North America. Fossil species are known from Tertiary deposits in Florida, Nebraska, and South Dakota. This paper describes a new fossil cracid, older than any previously discovered, from the top of the Chadron formation, lower Oligocene of South Dakota.

A Remarkable New Mammal from the Lower Chadron of Nebraska

While we were examining an unusually fossiliferous exposure in the lower member of the White River beds near Whitney, Dawes County, Nebraska, my wife, Mrs. Margaret C. Cook, discovered tiny, scattered fragments of a lower jaw on the surface of the gray-green member of the 'Chadron formation. While screening the dry surface, using binocular glasses and working into the matrix, she found an articulated skull and cervical vertebrae under where the jaws had weathered out. Near the skull were several disarticulated vertebrae and ribs. These remains were removed in one block. Preparation has been difficult, both because of the nature and condition of the fossils and matrix, and because slippage in seams of the clay-silt matrix has caused many fractures and dislocations. Though the skull is virtually complete, the facial region from the orbits forward has been compressed laterally and twisted to one side. The maxillae and teeth are intact, but much remains to be done before the characters of the skull can be fully determined. The vertebrae also are to be prepared. They appear to be very large and heavy in proportion to the skull. Meanwhile, many of the tiny fragments of the lower jaws and teeth have been fitted together, revealing dentition that is complete except for the lower incisors. The specimen indicates a mammal that is surprisingly primitive for its horizon in the Tertiary, with characters reminiscent of Paleocene and early Eocene mammals. Several of these characters are either homologous with or analogous to those of early creodonts, insectivores and artiodactyls. For the presenit I am referring the creature to the creodonts, though it does not seem to be closely related to any creodont of later than early Eocene age with which I have compared it. A partial summary of these comparisons is presented in the following description.

Geographic designation of the members of the Chadron formation in South Dakota

Geographic designation of the members of the Chadron formation in South Dakota