Eocene Willwood Formation Research Articles

Long-eccentricity pacing of alluvial stratigraphic architecture in the Eocene Bighorn Basin, Wyoming, USA

Abstract Alluvial stratigraphy builds up over geologic time under the complex interplay of external climatic and tectonic forces and internal stochastic processes. This complexity makes it challenging to attribute alluvial stratigraphic changes to specific factors. Geological records indicate pronounced and persistent climatic changes during the Phanerozoic, while the effects of these changes on alluvial stratigraphy remain insufficiently documented. We provide evidence for 405 k.y. long-eccentricity climate forcing of alluvial stratigraphy in the lower Eocene Willwood Formation of the Bighorn Basin, Wyoming (USA). Two ~90-m-thick intervals, characterized by a relative paucity of sand, dominance of sinuous-river channels, and floodplain sediments with better-developed paleosols, coincide with eccentricity maxima as determined through integrated stratigraphic methods. These intervals are interspersed with three contrasting intervals, marked by relatively high sand content, prevalent braided-river channels, and less-developed paleosols, corresponding to eccentricity minima. A comprehensive genetic model that integrates climate, source-to-sink system, and alluvial dynamics to explain these findings remains to be elucidated. Given the consistent presence of the 405 k.y. eccentricity cycle throughout Earth’s history, it is plausible to infer that its influence may be discernible across a wide array of alluvial stratigraphic records.

Lateral and vertical characteristics of floodplain aggradation cycles in the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

Sedimentation on river floodplains is a complex process that involves overbank flooding, crevasse splaying, and river avulsion. The resulting floodplain stratigraphy often exhibits floodplain aggradation cycles with alternating fine-grained overbank flooding deposits that underwent significant petrogenesis, and coarser-grained, avulsion-belt deposits largely devoid of pedogenic impact. These cycles are linked to lateral migration and avulsion of channels driven by internal dynamics, external factors, or a combination of both. To better understand the spatial and vertical variability of such floodplain aggradation cycles, we map these in three dimensions using a photogrammetric model of the lower Eocene Willwood Formation in the northern Bighorn Basin, Wyoming, USA. This allows identifying 44 floodplain aggradation cycles in ∼300 m of strata with an average thickness of 6.8 m and a standard deviation of 2.0 m. All the cycles are traceable over the entire model, pointing to their spatial consistency over the 10 km2 study area. At the same time, rapid lateral thickness changes of the floodplain aggradation cycles occur with changes up to 4 m over a lateral distance of 400 m. Variogram analyses of both field and numerical-model results reveal stronger consistency of floodplain aggradation cycle thicknesses along the paleoflow direction compared to perpendicular to paleoflow. Strong compensational stacking occurs at the vertical scale of 2−3 floodplain aggradation cycles (14−20 m), while full compensational stacking occurs at larger scales of more than six floodplain aggradation cycles (>41 m). The lateral and vertical thickness variability of the floodplain aggradation cycles, as well as their compensational stacking behavior, are interpreted to be dominantly driven by autogenic processes such as crevasse splaying and avulsing that preferentially fill topographic lows. External climate forcing may have interacted with these autogenic processes, producing the laterally persistent and vertically repetitive floodplain aggradation cycles. The spatial variability of floodplain aggradation cycles demonstrated in this study highlights again the need for three-dimensional data collection in alluvial floodplain settings rather than depending on one-dimensional records.

Biostratigraphically significant palynofloras from the Paleocene–Eocene boundary of the USA

Pollen and spores were recovered from the Paleocene Fort Union Formation and Paleocene–Eocene Willwood Formation of the Bighorn Basin (BHB), northwestern Wyoming, USA. In many local stratigraphic sections in the BHB, the base of the Eocene has been identified by the characteristic negative carbon isotope excursion (CIE) that marks the beginning of the Paleocene–Eocene Thermal Maximum (PETM). The palynotaxa from outcrop samples were examined using light microscopy (LM) and scanning electron microscopy (SEM). Seven new species are formally described (Tricolpites vegrandis, Rousea spatium, Striatricolporites astutus, Striatopollis calidarius, Friedrichipollis geminus, Retistephanocolporites modicrassus and Retistephanocolporites pergrandis). The temporal and geographic distributions of many of these palynotaxa suggest that hotter and more seasonally dry climates facilitated their northward range shifts during the PETM from the tropics or subtropics of the USA. For the temperate palynotaxa, the hotter and seasonally dry conditions resulted in local extirpation. A re-evaluation of the palynostratigraphic schemes established for the Paleocene–Eocene boundary confirms that the first appearance of Platycarya platycaryoides denotes the Paleocene–Eocene boundary in the Rocky Mountains region. A new Striatopollis calidarius Subzone, associated with early Wasatchian (Wa) Wa-0 and Wa-R faunas, is also recognized for CIE body localities in the BHB.

Sandstone body character and river planform styles of the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

AbstractThe lower Eocene Willwood Formation of the Bighorn Basin, Wyoming, USA, is an alluvial succession with a sand content varying around 25%. It has been studied intensively for palaeontology, palaeoenvironments and palaeoclimates, as well as sedimentological and stratigraphic analysis. Channel dynamics were studied at a relatively low resolution throughout the basin over the geological time from late Palaeocene to early Eocene. Here, a high‐resolution study is reported to complement previous research at the basin scale. Efforts are made to document the characteristics and river planform styles of most sandstone bodies encountered through ca 300 m of alluvial stratigraphy in a 10 km2 area of the Deer Creek part of the McCullough Peaks area situated in the basin axis of northern Bighorn Basin. Four channel facies associations are recognized and ascribed to four river planform styles: crevasse channel, trunk channel, braided‐like channel and sinuous‐like channel, with the latter two types dominant. Braided‐like and sinuous‐like channel sandstone bodies differ significantly in thicknesses, being on average 6.1 m versus 9.0 m, but they have similar palaeoflow–perpendicular widths of on average 231 m and palaeoflow directions of on average N 003°. Braided‐like and sinuous‐like river planform styles show no spatial dependency in the 10 km2 study area. Results of this study are in line with existing basin‐scale depositional models that are composed of a single axial system fed by several transverse systems dominantly from the west. The feeding of these systems could be influenced by palaeoclimate changes possibly controlling their contribution over time, thereby impacting river planform styles. At the same time, changing water discharge hydrograph, sediment load and overbank cohesiveness may have equally driven the observed river planform style changes within the basin without a major role of catchments.

A dense sample of fossil primates (Adapiformes, Notharctidae, Notharctinae) from the Early Eocene Willwood Formation, Wyoming: Documentation of gradual change in tooth area and shape through time.

The Willwood Formation of the southern Bighorn Basin, Wyoming is a fluvial rock sequence that spans approximately 3 million years of early Eocene time. It has yielded one the largest collections of fossil mammals in the world including thousands of dentitions of extinct lemur-like primates known as notharctines. In the southern Bighorn Basin, specimens of these primates have been collected on numerous paleontological expeditions and the stratigraphic levels yielding the dentitions have been carefully recorded. Notharctine dentitions represent a rare opportunity to study morphological variation in a single anatomical system through time among closely related individuals. Prior studies of Bighorn Basin notharctines through time produced measurements of hundreds of specimens but I report here results from measurement and comparison of the dentitions and dentaries of more than 3,000 specimens, all stratigraphically mapped. Variation in premolar and molar area and variation in dentary depth are apparent throughout the section. Specimens with relatively small teeth and dentaries are known from the older part of the section. In younger rocks, variation in tooth area among specimens increases. Variation in tooth area is continuous and overlaps extensively both within and between stratigraphic levels. Other dental variables examined by inspection change in a mosaic and continuous fashion through the section. These features include variation in the presence and number of paraconids on the lower fourth premolar (p4), the size and shape of the entoconid notch on the lower first and second molars, and the relative development of the pseudohypocone, mesostyle, and cingula on the upper molars. These broad patterns can be identified despite notharctine alpha taxonomy being in need of extensive revision and, importantly, simplification. Such revision is beyond the scope of this article but is essential if we are to develop a taxonomy that is both free of stratigraphic influence and useful for rapid, repeatable species assignment. Boundaries among the patterns of tokogenesis, anagenesis, and cladogenesis are blurred in this dense sample of extinct primates. While pattern of evolution, a population-level phenomenon, may be difficult to falsify in the fossil record, this notharctine sample suggests that in the rare instance such as this, when the fossil record is densely sampled, change through time is continuous and more consistent with gradual evolution.

Stratigraphic relationships along the monoclinal eastern base of Bald Ridge and northwestern edge of Wyoming’s Bighorn Basin, U.S.A.

ABSTRACT This geologic study is focused on a less than 5 square-mile (ca. 13 km2) tract of public land in northwestern Wyoming, 8 miles (12.9 km) south-southwest of the small town of Clark in Park County. The study area is south of Clarks Fork of Yellowstone River along the eastern base of the topographic feature called Bald Ridge, also known structurally as Dead Indian monocline. Since the Middle Eocene, the study area has been along the northwestern margin of the Bighorn Basin. Prior to that time, the study area existed near the west–east center of the basin. Bald Ridge became elevated late in the Laramide orogeny (no older than the Early Eocene) through east-directed faulting of basement rocks via the extensive Line Creek–Oregon Basin thrust system. As that active faulting occurred, the overlying Phanerozoic strata (Lower Cambrian through Lower Eocene) responded with numerous west-directed, out-of-the-basin thrusts as a new western-basin margin developed along the eastern realm of the newly born Absaroka volcanic field. Most of that deformation occurred after deposition of uppermost levels of the Lower Eocene Willwood Formation. The key purpose of the present paper was to improve the accuracy of mapping of the Jurassic into Eocene stratigraphy along the newly restricted, northwestern edge of Wyoming’s Bighorn Basin. The stratigraphic column in a north–south band along the eastern flank of the Beartooth Mountains and continuing southward into the present study area was markedly deformed and deeply eroded late during the Laramide orogeny. The present small, more southerly study area is structurally and erosionally simpler than its more northerly equivalent. Thus, its study adds important geological information to the history of the northern Cody Arch, a convex-westward string of related basement-involved uplifts extending southward to southwest of the city of Cody. Progressively steepening eastward dips of strata characterize a west-to-east transect from the summit of Bald Ridge (capped by the shallowly dipping, Mississippian Madison Limestone) to the western edge of strongly overturned outcrops of the Eocene Willwood Formation. The Upper Cretaceous Meeteetse Formation is the stratigraphic horizon at which the dips attain vertical or slightly overturned orientations. All consequential faults within the newly mapped area are thrusts, and they show generally westward (out-of-the-basin) displacements. Despite those west-directed displacements, their primary cause was tectonic shortening at depth below Bald Ridge that was directed to the northeast or east-northeast. During the Laramide orogeny, certain thrust planes within the east-dipping Phanerozoic rock column cut down-section stratigraphically (but uphill relative to Earth’s surface) and thereby placed younger strata upon older. The cumulative result, as recognized at several levels within the present area of study, was marked thinning of the total section. For example, surface exposures of the mostly Paleocene Fort Union Formation, 4,000 feet (1,219 m) thick only 7 miles (11.3 km) to the east, was completely eliminated from the local surface stratigraphy by that means. The northern end of Bald Ridge is formed by the highly asymmetric Canyon Mouth anticline. That structure differs strongly in the attitude of its hinge line from the general east-northeast dip of strata cloaking Bald Ridge. The Canyon Mouth anticline’s hinge line plunges steeply to the southeast, and dips on its northeastern flanks are vertical to partly overturned. Surprisingly, hinge lines and flanks of all other anticlinal/synclinal structures recognized within the present map area share those same orientations with Canyon Mouth anticline. These consistent but unexpected differences in orientation from unfolded strata may represent very late events in the history of Laramide strain vectors across the study area. Working in northern parts of this study area, an independent group determining radiometric ages of detrital-zircon grains reported close agreements in age with their host localities in the Early Cretaceous Mowry Shale and Frontier Formation. However, under the present paper’s interpretation of the local stratigraphy, the other workers misidentified formational hosts for all three samplings. That resulted in age-determination errors of depositional history within the Upper Cretaceous section of as much as 28.8 million years.

Detrital Zircon U-Pb geochronology and provenance of the Eocene Willwood Formation, Northern Absaroka Basin, Wyoming

We report the results of U-Pb ages from detrital zircon populations in the lower Eocene synorogenic Willwood Formation in the northern Absaroka Basin, Wyoming. Zircons (n=229) were extracted from three sandstone beds and one ash layer in the Willwood Formation at the base of Jim Mountain in the North Fork Shoshone River Valley. K-S statistical analysis indicates that the three sandstones, which were sampled from the base, middle, and top of the formation, have identical age spectra, indicating that the sandstone provenance remained the same during the duration of Willwood deposition. The zircon age spectra are dominated by Archean zircons (61%), with peak ages at 3270 and 2770 Ma. These sandstones also have very early Paleoproterozoic zircons (∼2450 Ma), which likely were derived from the Tobacco Root Mountains. The final significant age peak is ∼70 Ma, which is likely associated with the Cretaceous Tobacco Root batholith. The Jim Mountain ash, which occurs at the top of the succession, just beneath the allocthonous volcanic rocks of the Heart Mountain slide, has a maximum depositional age of ∼50 Ma. Between 49–50 Ma, as Eocene volcanism in the northern Absaroka Range became more prominent, stratovolcanoes grew and disrupted sediment transport into the Absaroka basin. Lower Wapiti sandstones to the southwest show a mix of Eocene, recycled Proterozoic and Archean grains. The coeval Crandall Conglomerate, which was dismembered by the emplacement of the Heart Mountain slide in the northern Absaroka Range, has a distinct detrital zircon age spectrum. Thus these stream systems that deposited the Crandall did not share the headwaters with the streams that supplied sediment to the Absaroka basin.

Deciduous premolars of Eocene Equidae and their phylogenetic significance

The deciduous premolars of early Eocene Equidae from North America, conventionally attributed to ‘Hyracotherium’, are described and compared, based primarily on a sample of >60 specimens from the early Eocene Willwood Formation of the Bighorn Basin, Wyoming. The sample represents six to nine species assigned to multiple genera by some other authors, but in the absence of diagnostic characters of the deciduous dentition, species assignments remain ambiguous for most specimens. Consequently it is not clear which generic names should be applied, and we employ the widespread name ‘Hyracotherium’ (acknowledging that the type species may not be an equid). We observed considerable variation in our sample, but most differences are minor and are largely inconsistent with respect to time or taxa, hindering attempts to characterise deciduous premolar anatomy of particular species. Comparisons were also made to Bridgerian Orohippus and Uintan Epihippus, to other early perissodactyls, and to non-perissodactyls that have been considered close to the origin of Perissodactyla (phenacodontid condylarths and Cambaytherium). Based on these comparisons, we confirm Butler’s observation that the deciduous premolars of equids show increasing molarization and lophodonty through the Eocene. However, our evidence suggests that there was little directed change through most of the Wasatchian until Wa-7.

On the taxonomy and osteology of the Early Eocene North American Geranoididae (Aves, Gruoidea)

Whereas our knowledge of small arboreal Paleogene birds has greatly increased in the past years, that of the larger birds coexisting with them is far less detailed. Particularly poorly known and, hence, widely ignored are the North American Geranoididae, of which six species were described from the Early Eocene Willwood Formation. The published material of all of these consists only of incomplete leg elements, and previous descriptions and comparisons mainly focused on selected bones. Here, a revision of some geranoidids from the Willwood Formation is undertaken, and it is concluded that the taxon Geranoides Wetmore, 1933, the type genus of the Geranoididae, is a junior synonym of Palaeophasianus Shufeldt, 1913. Eogeranoides campivagus Cracraft, 1969 is poorly differentiated from Paragrus prentici (Loomis, 1906), and a synonymy of both species also seems likely. Complete leg bones of a large Eogeranoides/Paragrus-like species are reported, which constitute the most substantial record of a geranoidid from the Willwood Formation. It is detailed that geranoidids are likely to be stem group representatives of the Gruoidea, the clade including trumpeters, cranes, and allies, and the potential occurrence of geranoidids in the Eocene of Europe is discussed.

Structure, Timing, and Kinematics of the Early Eocene South Fork Slide, Northwest Wyoming, USA

AbstractThe South Fork slide (SFS) is exposed over an area of 800 km2 in northwest Wyoming, and its evolution is connected in space and time to the adjacent and overlying 48.87 Ma Heart Mountain slide (HMS). The youngest rocks deformed as part of the SFS are the Eocene Willwood Formation sands and mudstones, which have a U/Pb detrital zircon youngest depositional age (TuffZirc calculation) of 51.80 +1.04/−1.08 Ma as well as a spectrum of older zircons that were eroded from the Sevier Highlands to the west and the Archean Beartooth uplift to the north (n = 379 U-Pb zircon ages). The SFS is overlain by allochthonous Paleozoic carbonate and Eocene volcanic rocks of the HMS. The SFS deformation involves shallowly plunging thin-skinned folds that trend northeast-southwest, where the slip surface could have been the top of the Jurassic Gypsum Springs Formation. Detailed mapping reveals the absence of any cleavage or joints and a plethora of minor structures unreported in any thin-skinned belt as well as the fac...

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.

Detrital zircon geochronology from the Bighorn Basin, Wyoming, USA: Implications for tectonostratigraphic evolution and paleogeography

Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, subsidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, influence paleogeography and sediment-dispersal patterns, they should also yield similar patterns, or facies, of detrital zircon age spectra. Such age-distribution patterns should be documented on the craton in order to make meaningful comparisons to sedimentary rocks from suspect terranes along continental margins. The Rocky Mountains of western North America provide excellent outcrops of sedimentary rocks that record >500 m.y. of tectonostratigraphic evolution. One such Phanerozoic section is exposed along the margins of the Bighorn Basin in northwest Wyoming, from which we report over 4000 U/Th/Pb detrital zircon ages from 48 samples that span a stratigraphic interval from the Middle Cambrian Flathead Sandstone through the Eocene Willwood Formation. These data provide one of the most complete records of detrital zircon age patterns from this part of cratonic North America. The stratigraphic record of the Bighorn Basin is subdivided into four tectonostratigraphic assemblages (TSA1–TSA4). These assemblages record an initial passive margin, followed by a transition to a convergent margin, followed by a marine-dominated retroarc foreland basin, followed by a retroarc foreland segmented by local basement uplifts. This tectonostratigraphic architecture is expressed as four, first-order patterns within the detrital zircon age distributions. TSA1 represents a Paleozoic–Triassic proximal continental margin assemblage dominated by Proterozoic zircons with abundant grains in the 1600–1950 Ma range, a Grenville population at ca. 1100 Ma, and a Phanerozoic population at ca. 420 Ma. TSA2 is a transitional assemblage associated with the Jurassic–Early Cretaceous organization of a west-facing convergent margin and Cordilleran orogen. The TSA2 detrital zircon age distribution is characterized by the appearance of Mesozoic grains, age peaks at ca. 420 and 600 Ma, and a dominant population of Grenville (1.0–1.1 Ga) grains with a suite of Proterozoic grains diminishing in abundance as age increases to 1.9 Ga. TSA3 sedimentary rocks were deposited in the Cretaceous Interior Seaway in a retroarc foreland basin and are dominated by zircons for which ages are close to the depositional age of the strata, reflecting input from the active Idaho Batholith and Sierran segments of the Cordilleran magmatic arc. The older zircon fractions from TSA3 sedimentary rocks are characterized by a dominant detrital zircon age peak at 1.7–1.8 Ga, which probably reflects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 reflects the phase of basin fill associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this assemblage record erosion and redeposition of all previous sedimentary rocks from surrounding basement uplifts. Patterns of detrital zircon ages reflect fundamental changes in paleogeography and sediment dispersal at the 10–100 m.y. time scale and are clearly related to major tectonic events or phases. Detrital zircon ages also provide evidence for linkages between convergent margin processes such as arc magmatism and sedimentation in the retroarc foreland. During these times of strong arc-retroarc linkage, detrital zircon geochronology provides a potentially useful tool for high-resolution chronostratigraphy.

Precession‐scale cyclicity in the fluvial lower Eocene Willwood Formation of the Bighorn Basin, Wyoming (USA)

AbstractLittle is known about controls on river avulsion at geological time scales longer than 104 years, primarily because it is difficult to link observed changes in alluvial architecture to well‐defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch‐sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super‐elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7·1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21·6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large‐scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change.

Astronomical climate control on paleosol stacking patterns in the upper Paleocene–lower Eocene Willwood Formation, Bighorn Basin, Wyoming

Research Article| July 01, 2008 Astronomical climate control on paleosol stacking patterns in the upper Paleocene–lower Eocene Willwood Formation, Bighorn Basin, Wyoming Hayfaa Abdul Aziz; Hayfaa Abdul Aziz 1Faculty of Geosciences, Utrecht University, Budapestlaan 4, P.O. Box 80021, 3508 TA Utrecht, Netherlands * Current address: Deltares/TNO, Geological Survey of the Netherlands, Princetonlaan 6, 3508 AL Utrecht, Netherlands; e-mail: haziz@geo.uu.nl. Search for other works by this author on: GSW Google Scholar Frits J. Hilgen; Frits J. Hilgen 1Faculty of Geosciences, Utrecht University, Budapestlaan 4, P.O. Box 80021, 3508 TA Utrecht, Netherlands Search for other works by this author on: GSW Google Scholar Gerson M. van Luijk; Gerson M. van Luijk 1Faculty of Geosciences, Utrecht University, Budapestlaan 4, P.O. Box 80021, 3508 TA Utrecht, Netherlands Search for other works by this author on: GSW Google Scholar Appy Sluijs; Appy Sluijs 2Palaeoecology, Institute of Environmental Biology, Faculty of Science, Utrecht University, Laboratory of Palaeobotany and Palynology, Budapestlaan 4, 3584 CD Utrecht, Netherlands Search for other works by this author on: GSW Google Scholar Mary J. Kraus; Mary J. Kraus 3Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA Search for other works by this author on: GSW Google Scholar Josep M. Pares; Josep M. Pares 4Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar Philip D. Gingerich Philip D. Gingerich 4Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar Geology (2008) 36 (7): 531–534. https://doi.org/10.1130/G24734A.1 Article history received: 27 Dec 2007 rev-recd: 05 Mar 2008 accepted: 07 Mar 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Hayfaa Abdul Aziz, Frits J. Hilgen, Gerson M. van Luijk, Appy Sluijs, Mary J. Kraus, Josep M. Pares, Philip D. Gingerich; Astronomical climate control on paleosol stacking patterns in the upper Paleocene–lower Eocene Willwood Formation, Bighorn Basin, Wyoming. Geology 2008;; 36 (7): 531–534. doi: https://doi.org/10.1130/G24734A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Willwood Formation of the Bighorn Basin (Wyoming, USA) is a thick succession of upper Paleocene and lower Eocene fluvial-floodplain sandstones and mudstones. Reddish paleosols, formed on the floodplain mudstones, alternate rhythmically on various scales with heterolithic intervals of small-channel sandstones and mudstones showing weak pedogenesis. Spectral analysis of redness in the Willwood successions at Polecat Bench and Red Butte reveals significant spectral peaks corresponding to cycle thicknesses of ~8 and ~3 m. The ~8 m cycle reflects distinct clusters of 3–5 paleosols. Age constraints show that the period of this cycle closely matches the ~21 k.y. climatic precession cycle. The ~3 m cycle corresponds to individual paleosols, with a period of 7–8 k.y. This period is similar to millennial-scale sub-Milankovitch cycles found in marine and lacustrine successions of Pliocene–Pleistocene age. Precession and millennial-scale climate variations probably affected paleosol development through cyclic changes from predominantly overbank to predominantly channel-avulsion deposition, with the latter periodically halting soil formation because of high sediment accumulation. A new age model was developed for the Paleocene-Eocene carbon isotope excursion (CIE) at Polecat Bench, based on the precessional origin of paleosol clusters. The main body of the CIE spans ~5.5 precession cycles, or ~115 k.y., and the recovery tail of the CIE spans 2 precession cycles, or ~42 k.y. This outcome is consistent with, and independently confirms, recent estimates of CIE duration based on deep-sea cores. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A relationship between alluvial backswamps and avulsion cycles: an example from the Willwood Formation of the Bighorn Basin, Wyoming

Stratigraphically limited intervals from the Lower Eocene Willwood Formation contain laterally extensive carbonaceous shales and ribbon sandstone networks associated with channel avulsion. We present data from one such interval that documents the avulsion sequence. Vertical sections measured along the outcrop of this interval are similar and comprise a basal carbonaceous shale overlain by fine-grained deposits on which weakly developed, hydromorphic paleosols formed. The paleosols enclose and are locally incised by ribbon sandstones, some of which cut down to and partly through the carbonaceous shale. The ribbons have width/thickness ratios between 3 and 13. Some ribbons cluster at a particular stratigraphic level, which, together with paleocurrent trends, suggests that they formed channel networks. Sections are capped by yellow-brown paleosols showing moderate pedogenic development. We suggest that the carbonaceous shales developed in low-lying topogeneous swamps in distal portions of the floodplain far from the trunk channel. Such a location set limits on the sediment that they received. The mudrocks with weakly developed paleosols and associated ribbon sandstones are interpreted as crevasse-splay complexes resulting from avulsion of the trunk river. The ribbon sandstones represent ancient feeder channels of the avulsion complex. The rapid influx of avulsion deposits appears to have been crucial to preserving the organic material, and this study reveals an important and as yet uncharacterized link between trunk channel processes and the accumulation of organic-rich deposits in distal alluvial swamps. Similar deposits are found in other stratigraphic units in the Rocky Mountain region, and the development of these and other organic-rich deposits should be reassessed in terms of channel avulsion.

ABSTRACT: Alluvial Architecture and Subsidence Rates: Variability in the Early Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

ABSTRACT: Alluvial Architecture and Subsidence Rates: Variability in the Early Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

Skull of Early Eocene Cantius abditus (Primates:Adapiformes) and its phylogenetic implications, with a reevaluation of "Hesperolemur" actius.

A substantially complete skull and mandible of the primitive adapiform Cantius is reported from the Early Eocene Willwood Formation of Wyoming. The mandible contains an almost complete lower dentition in which the lower incisors are strongly inclined and have spatulate crowns, I(2) is larger than I(1), and the canine is large and projecting. The cranium shares many features with those of Notharctus and Smilodectes but differs in having nasals that broaden proximally. Presence of a prominent canine and strong sagittal crest may indicate that it represents a male. The basicranium preserves auditory structures almost identical to those in extant noncheirogaleid lemurs, including a large bony tube for the stapedial artery and a small, open sulcus for the distal portion of the promontorial artery. The dentition is sufficiently primitive to be compatible with a relationship to either strepsirrhines or anthropoids, but the anatomy of the auditory region is more consistent with either specific relationship to lemurs or, more likely, a basal position that approximates the euprimate morphotype. Certain features of the basicranium of "Hesperolemur" actius, described by Gunnell ([1995] Am. J. Phys. Anthropol. 98:447-470) as being unlike that of any other adapiform, were either misinterpreted or are apparently no longer present in the holotype. Reassessment of these and other features indicates that in fact "H." actius differs little from Cantius and should not be separated from the latter at the genus level, although on dental grounds the species appears to be distinct (as C. actius).

Postcranial Skeleton of the Early Eocene MesonychidPachyaena(Mammalia: Mesonychia)

ABSTRACT Analysis of postcranial skeletons of three species of the wolf- to bear-sized mesonychid Pachyaena from the early Eocene Willwood Formation, Bighorn Basin, Wyoming, indicates that it was a derived cursor combining specializations typical of modern Artiodactyla, Perissodactyla, and Carnivora. Skeletal anatomy supports the hypothesis, based on the dentition, that mesonychids evolved from Arctocyonidae. Numerous characters in the limb skeleton of Pachyaena are derived relative to Arctocyon, however, and resulted in restricted rotatory and mediolateral mobility at most joints, promoting predominantly sagittal movement. These characters, present in ungulates and cursorial carnivores, include a broad, caudally displaced humeral head; high greater tuberosity; proximal deltoid tuberosity; narrow, proximodistally elongate distal humeral articulation with moderately constricted capitulum; reduced epicondyles; deep olecranon fossa; anteriorly shifted radioulnar joint; posteriorly concave ulnar shaft; high greater trochanter; deep distal femur; elongate distal tibiofibular syndesmosis; moderately grooved astragalar head; and mediolaterally compressed manus and pes with vestigial pollex and hallux. Some of these characters in Pachyaena are more carnivore-like in grade of cursorial modification. Distinctly ungulate-like specializations of Pachyaena include a wide and unevenly excavated radial head, a double-faceted radiocarpal articulation, an astragalar head with distinct cuboid and navicular facets, and hoofs. Pachyaena had relatively robust limbs and somewhat elongate distal limb segments like modern tapirs, but was functionally paraxonic like cursorial Carnivora and Artiodactyla. These features indicate that Pachyaena was a cursor adapted more for endurance than speed. Cursorial specializations in mesonychids probably arose independently from those in modern ungulates and cursorial carnivores.

Floodplains and paleosols of Pakistan Neogene and Wyoming Paleogene deposits: a comparative study

Abstract Comparative study of fossil-bearing fluvi' deposits in the Eocene Willwood Formation of northern Wyoming and the Miocene Chinji Formation of northern Pakistan indicate how tectonic and climatic processes operating at different scales controlled physical and chemical features of floodplain environments and affected preservation of the paleontological record. The architecture of Willwood Fm. floodplain deposits represents a combination of avulsion-belt sediment packages and overbank sediments that formed alluvial ridges. The architecture of the Chinji Fm. floodplain deposits was controlled by widely distributed crevasse-splay deposition and floodplain topography. Similarities in individual paleosol-bounded overbank sequences from the two formations indicates that the internal structure of such deposits can be independent of channel belt proximity to areas of aggradation. Chinji Fm. paleosols have little vertical zonation and show no consistent pattern of lateral change in relation to major channels, while overbank paleosols in the Willwood Fm, exhibit considerable soil horizon development and a pattern of increasing maturity from alluvial ridge to distal floodplain. The “pedofacies model” of Bown and Kraus (1987) based on such lateral trends in the Willwood paleosols is not applicable to the Chinji Fm. Plant and animal fossils are abundant in the Willwood overbank deposits, with vertebrate remains concentrated in paleosol A horizons. Plant remains are rare in the Chinji Fm. and vertebrate fossils occur primarily in channel fills rather than in paleosols. These differences relate to contrasting patterns of floodplain deposition and to levels of oxidation that controlled penecontemporaneous recycling of organic material, particularly in paleosols. Different large-scale climatic and tectonic controls on temperature and rainfall, water table fluctuations, and soil biota are proposed to account for the differences in organic preservation. Large and small-scale environmental processes also affected spatial and temporal resolution of the organic record, resulting in important differences in the paleoecological and evolutionary information that can be reconstructed from the two sequences.

Partial Skeleton of the Primitive Carnivoran Miacis petilus from the Early Eocene of Wyoming

A partial skeleton of Miacis petilus , a primitive carnivoran from the early Eocene (Was-atchian) Willwood Formation of the Bighorn Basin, Wyoming, is described. This is the oldest skeleton reported for Miacis , a genus that lies at or near the base of the family Miacidae, from which all caniform Carnivora (and possibly all living Carnivora) are derived. Anatomical comparisons to modern carnivorans suggest that M. petilus was as well adapted for exploiting arboreal habitats as almost any extant member of the order. A weight estimate of 1.3 kg is calculated for this individual based on regressions of body mass against humeral and femoral cross-sectional properties derived for small-bodied arboreal and scan-sorial carnivorans. Differences in estimates of body mass for two individuals of the middle Eocene Miacis parvivorus imply that this species was dimorphic in size.