Hanna Basin Research Articles

Interactive Effects of Temperature, Aridity, and Plant Stoichiometry on Insect Herbivory: Past and Present.

AbstractThe influence of climate on deep-time plant-insect interactions is becoming increasingly well known, with temperature, CO2 increases (and associated stoichiometric changes in plants), and aridity likely playing a critical role. In our modern climate, all three factors are shifting at an unprecedented rate, with uncertain consequences for biodiversity. To investigate effects of temperature, stoichiometry (specifically that of nitrogen), and aridity on insect herbivory, we explored insect herbivory in three modern floral assemblages and in 39 fossil floras, especially focusing on eight floras around a past hyperthermal event (the Paleocene-Eocene Thermal Maximum) from Bighorn Basin (BB). We find that higher temperatures were associated with increased herbivory in the past, especially among BB sites. In these BB sites, non-N2-fixing plants experienced a lower richness but higher frequency of herbivory damage than N2-fixing plants. Herbivory frequency but not richness was greater in BB sites compared with contemporaneous, nearby, but less arid sites from Hanna Basin. Compared with deep-time environments, herbivory frequency and richness are higher in modern sites, suggesting that current accelerated warming uniquely impacts plant-insect interactions. Overall, our work addresses multiple aspects of climate change using fossil data while also contextualizing the impact of modern anthropogenic change on Earth's most diverse interactions.

Paleocene (65–63 and 58.5 ma) marine flooding and 62–60 ma sediment bypass in southern Wyoming, U.S.A.: Implications for Laramide sediment flux to the Gulf of Mexico

ABSTRACTSedimentary structures unique to tidally influenced environments and unambiguously salinity-stressed marine ichnofossil assemblages in the lower Paleocene Ferris and upper Paleocene Hanna formations of Wyoming's Hanna Basin (HB) necessitate major revision of local and regional reconstructions of the Paleocene Western Interior Seaway (WIS). Preserved in sandy estuarine bars, sandy tidal flats, heterolithic distributary channels, bayhead delta, and flood-tide-delta deposits similar those in the modern Trinity River, its bayhead delta, and the San Luis Pass flood-tidal delta on the Texas coast, these these assemblages include Arenicolites, Bergaueria, Fuersichnus, Gyrochorte, Ophiomorpha, Palaeophycus, Planolites, Psilonichnus, Rhizocorallium, Rosselia, Siphonichnus, Skolithos, Spongeliomorpha, Taenidium, Thalassinoides, and tetrapod tracks. Mapping an ∼ 325-m-thick succession of lower Paleocene strata (65 to 63 Ma) around the western HB reveals a series of marine flooding events, each followed by coal accumulation. A similar, 170-m-thick succession of interfingering coastal-plain and restricted-marine strata occurs in the upper Paleocene (58.5 Ma) Hanna Formation, following accumulation of lacustrine and floodplain deposits and an episode of major gravel and cobble progradation from 62 to 60 Ma. These younger ichnofossils record the final major transgression of the WIS and have major implications for the depositional environment of the time-equivalent Waltman Shale in the Wind River Basin to the north and for sediment routing to the Gulf Coast Wilcox sands. Ichnofossils are an underutilized source of physicochemical proxy data that are lifting the veil from the cryptic Paleocene transgressions of the WIS that have for so long remained undetected because of the absence of open-marine body fossils.

Local differences in paleohydrology have stronger influence on plant biomarkers than regional climate change across two Paleogene Laramide Basins, Wyoming, USA

Plant biomarkers in the geologic record are commonly used to reconstruct paleovegetation and paleoclimate. Yet the representativeness of these biomarker-based reconstructions is not entirely straightforward because both local and regional climatic influences are convolved in the recovered biomarker composition. Furthermore, geomorphic processes including sedimentation, deposition, burial rates, and surface and groundwater levels can also influence biomarker preservation and taphonomy. The combination of leaf compression fossils with the biomarker record provides a robust framework in which ecological reconstructions can be made. Herein we present a case study examining the biomarker responses to a major regional climatic signal, the Paleocene Eocene Thermal Maximum (PETM), from local effects in two basins with different paleohydrology: the Hanna Basin (HB) and Bighorn Basin (BHB), in Wyoming (USA). Data collected include (1) leaf wax n-alkanes and plant-derived non-steroidal di- and triterpenoids, (2) pristane/phytane vs. terrestrial to aquatic ratio (Pr/Ph:TAR), plant material source proxy (Paq), hopanes, and bacteria which provide constraints on organic matter sourcing, redox conditions, and bacterial inputs, and (3) carbon (δ13Cn-alk) and hydrogen (δ2Hn-alk) isotopic compositions of plant wax n-alkanes. We find the two basins differed significantly in weight % total organic carbon (TOC), alkane and terpenoid concentrations, and δ13Cn-alk and δ2Hn-alk values. We examined the effects of δ13Catm on δ13Cn-alk values by assuming comparable regional climate in the two basins. Differences in paleohydrology and paleovegetation were assessed using the above proxies and previously published paleobotanical data. Local paleohydrologic conditions, an abiotic factor, likely drove variability in plant biomarker preservation and paleovegetation structure. Notably, an abundance of water in the HB during the PETM created a conifer refugium while drying within the BHB caused a local extirpation. This study highlights the importance of considering the influence of local environment, specifically paleohydrology, on the local fossil record and the need to compare multiple sites to disentangle regional climate effects on terrestrial plant communities. Distinguishing local from regional differences is imperative if we are to understand how ecosystems respond to future climate change.

Magnetostratigraphy of lower Paleocene strata in the Ferris Formation, Hanna Basin, Wyoming, with refined resolution of the Pu1–Pu2 interval-zone boundary of the Puercan North American Land Mammal Age

ABSTRACT Recent studies of early Paleocene stratigraphic sections across the U.S. Western Interior are refining our understanding of the biotic recovery in the aftermath of the Cretaceous–Paleogene (K–Pg) mass extinction event. Herein we present magnetostratigraphic data from an approximately 600-m-thick section of strata of the Ferris Formation in the Hanna Basin, south-central Wyoming, that spans the K–Pg boundary and includes in conformable superposition the three subdivisions of the earliest Paleocene Puercan North American Land Mammal Age: interval zones Pu1, Pu2, and Pu3. Prior studies on early Paleocene stratigraphic sections in Montana to the north of Hanna Basin and in Colorado and New Mexico to the south have been only marginally successful in defining the temporal boundaries of these divisions. This earlier work typically restricted Pu1 entirely within reversed magnetochron C29r, Pu2 entirely within normal magnetochron C29n, and at least the early part of Pu3 in C29n, as well. Results of the present study confirm what has been only tentatively suggested previously: that interval zone Pu2 begins in the youngest part of C29r, with later Pu2 fauna extending into C29n. Although Pu3 is known to begin in C29n, its younger limit remains unknown in our Hanna Basin section, because the top of the local Puercan is well above the stratigraphic level of our sampling for this project. We estimate a date for the Pu1–Pu2 boundary in the Hanna Basin section of approximately 65.82 Ma—nearly 70 k.y. earlier than prior estimates. This boundary marks the transition from the Pu1 survival fauna to later phases of the mammalian recovery characterized by a significant increase in diversity, appearance of considerably larger-bodied mammals, and greater dental and dietary specializations.

Alternative viewpoints on the nature and importance of a prominent syncline at the northeastern edge of Wyoming’s Hanna Basin

ABSTRACTThe geologic history of Wyoming’s Hanna Basin is still being written. Surprisingly, here appeared an opportunity to share insights from previously accomplished work with that conducted anew by other scholars. The area of study was in the southeastern quadrant of Wyoming, which exhibits the state’s most complex history with respect to the Laramide orogeny. Especially important for present purposes were the tectonic conditions of the late Paleocene and earliest Eocene, recorded within the Hanna Formation. Of central focus is the 2020 publication by Dechesne and her six co-authors. Geographically, the landscape they covered was a thin, synclinal slice of the northeastern margin of the Hanna Basin. Key goals for the present publication have been to illustrate positive linkages and to highlight discrepancies between Dechesne et al. (2020) and relevant prior geological work. A concern that permeates all facets of this approach is the ability to verify viability of brand-new geologic descriptions, data, and resulting conclusions. Essential graphical elements were introduced first into this present publication. Once that package of background information was available, more focused analyses were rigorously pursued on diverse issues within the Dechesne et al. (2020) publication. Dechesne’s team presented a significantly modified but adequately defended approximation of the Paleocene–Eocene boundary. Data from fossil plants (macro- and palynofloras), continental mollusks, and bulk organic-carbon isotopes all agree within one measured section (of five sections studied) with an approximated Paleocene–Eocene boundary along with a ‘carbon isotope excursion’ (CIE). Strength of available evidence seems questionable, however, in that the inordinately high variability in bulk organic carbon (characteristic of a CIE) has been demonstrated only in the Hanna Draw Section. Although fluvial, paludal, and lacustrine facies are considered in several contexts, in no sense does the publication’s organizational form provide a ‘detailed stratigraphic framework.’ One zircon-based U–Pb depositional date (54.42 ± 0.27 Ma) came from this study that matched early Wasatchian time. Participants in the Dechesne et al. (2020) project are to be commended in that their resulting paper ranged broadly across the geologic setting, stratigraphy, paleocurrents, paleobotany, continental mollusks, zircon geochronology, associated lithofacies, and paleogeography. Despite that breadth, there exists a plethora of unexpected and wholly avoidable inconsistencies, strong contradictions within what should be homogeneous datasets, and seemingly inexplicable omissions of obviously necessary and sometimes clearly existing but unutilized data, one must question the reliability of much of the information presented in their paper.

A new stratigraphic framework and constraints for the position of the Paleocene–Eocene boundary in the rapidly subsiding Hanna Basin, Wyoming

AbstractThe Paleocene–Eocene strata of the rapidly subsiding Hanna Basin give insights in sedimentation patterns and regional paleogeography during the Laramide orogeny and across the climatic event at the Paleocene–Eocene Thermal Maximum (PETM). Abundant coalbeds and carbonaceous shales of the fluvial, paludal, and lacustrine strata of the Hanna Formation offer a different depositional setting than PETM sections described in the nearby Piceance and Bighorn Basins, and the uniquely high sediment accumulation rates give an expanded and near-complete record across this interval. Stratigraphic sections were measured for an ∼1250 m interval spanning the Paleocene–Eocene boundary across the northeastern syncline of the basin, documenting depositional changes between axial fluvial sandstones, basin margin, paludal, floodplain, and lacustrine deposits. Leaf macrofossils, palynology, mollusks, δ13C isotopes of bulk organic matter, and zircon sample locations were integrated within the stratigraphic framework and refined the position of the PETM. As observed in other basins of the same age, an interval of coarse, amalgamated sandstones occurs as a response to the PETM. Although this pulse of relatively coarser sediment appears related to climate change at the PETM, it must be noted that several very similar sandstone bodies occur with the Hanna Formation. These sandstones occur in regular intervals and have an apparent cyclic pattern; however, age control is not sufficient yet to address the origin of the cyclicity. Signs of increased ponding and lake expansion upward in the section appear to be a response to basin isolation by emerging Laramide uplifts.

Plant and insect herbivore community variation across the Paleocene-Eocene boundary in the Hanna Basin, southeastern Wyoming.

Ecosystem function and stability are highly affected by internal and external stressors. Utilizing paleobotanical data gives insight into the evolutionary processes an ecosystem undergoes across long periods of time, allowing for a more complete understanding of how plant and insect herbivore communities are affected by ecosystem imbalance. To study how plant and insect herbivore communities change during times of disturbance, we quantified community turnover across the Paleocene­–Eocene boundary in the Hanna Basin, southeastern Wyoming. This particular location is unlike other nearby Laramide basins because it has an abundance of late Paleocene and Eocene coal and carbonaceous shales and paucity of well-developed paleosols, suggesting perpetually high water availability. We sampled approximately 800 semi-intact dicot leaves from five stratigraphic levels, one of which occurs late in the Paleocene–Eocene thermal maximum (PETM). Field collections were supplemented with specimens at the Denver Museum of Nature & Science. Fossil leaves were classified into morphospecies and herbivore damage was documented for each leaf. We tested for changes in plant and insect herbivore damage diversity using rarefaction and community composition using non-metric multidimensional scaling ordinations. We also documented changes in depositional environment at each stratigraphic level to better contextualize the environment of the basin. Plant diversity was highest during the mid-late Paleocene and decreased into the Eocene, whereas damage diversity was highest at the sites with low plant diversity. Plant communities significantly changed during the late PETM and do not return to pre-PETM composition. Insect herbivore communities also changed during the PETM, but, unlike plant communities, rebound to their pre-PETM structure. These results suggest that insect herbivore communities responded more strongly to plant community composition than to the diversity of species present.

Hanna of Wyoming—The Rockies’ Deepest Basin

The Hanna Basin is one of the world’s deeper intracratonic depressions. It contains exceptionally thick sequences of mature, hydrocarbon-rich Paleozoic through Eocene rocks and has the requisite structural and depositional history to be a significant petroleum province. The Tertiary Hanna and Ferris formations consist of up to 20,000 ft of organic-rich lacustrine shale, shaly mudstone, coal, and fluvial sandstone. The Upper Cretaceous Medicine Bow, Lewis, and Mesaverde formations consist of up to 10,000 ft of marine and nonmarine organic-rich shale enclosing multiple stacked beds of hydrocarbon-bearing sandstone. Significant shows of oil and gas in Upper Cretaceous and Paleocene rocks occur in the basin. Structural prospecting should be most fruitful around the edges where Laramide flank structures were created by out-of-the-basin thrust faults resulting from deformation of the basin’s unique 50-mile wide by 9-mile deep sediment package. Strata along the northern margin of the basin were compressed into conventional anticlinal folds by southward forces emanating from Emigrant Trail-Granite Mountains overthrusting. Oil and gas from Pennsylvanian to Upper Cretaceous aged rocks have been found in such structures near the Hanna Basin. Only seven wells have successfully probed the deeper part of the Hanna Basin (not including Anadarko’s #172 Durante lost hole, Sec. 17, T22N, R82W, lost in 2004, hopelessly stuck at 19,700 ft, unlogged and untested). Two of these wells tested gas at commercial rates from Upper Cretaceous rocks at depths of 10,000 to 12,000 ft. Sparse drilling along the Hanna Basin’s flanks has also revealed structures from 3,000 to 7,000 feet deep which yielded significant shows of oil and gas.

Interactions of A Paleocene River, A Rising Fold, and Early-Diagenetic Concretions

Abstract: The relative rates of sediment accumulation, erosion, and structural uplift determine whether a growing fold develops positive topographic relief, is beveled by antecedent streams, or is buried under thick growth strata. When folds rise in subsiding basins, upward, convergent flow of groundwater through the permeable growth strata that underlie antecedent streams enhances the flux of ions required for concretion growth. Early diagenetic concretions that grow in such alluvial strata may constitute the only clasts larger than sand size available for transport when antecedent streams become erosive. The first reworked concretions deposited by these streams should accurately mark the transition from aggradation to erosion as folds rise into the paths of streams. In this situation, the ability to differentiate reworked from in situ concretions is crucial. The west-vergent Simpson Ridge anticline, a N–S-trending, thick-skinned Laramide structure in east-central Wyoming, separates the larger Hanna basin from the Carbon basin. Near the north nose of this anticline, in situ iron-oxide-rich concretions are abundant in folded Paleocene strata (Ferris Formation) and, just to the east, large, reworked , iron-rich concretions are abundant in younger, more gently dipping conglomerates in the basal Hanna Formation of the backlimb. Smaller reworked concretions are also present near the base of the Hanna Formation at least 7 km south of the anticlinal nose and just east of the fold9s axis. At the anticlinal nose, in situ (non-reworked) concretions up to 3 m × 1 m × 1 m are abundant at the top of an ∼ 7-km-thick sequence of sandstones and siltstones that constitute the Late Cretaceous–early Paleocene Ferris Formation. Reworked concretions are absent in the strata hosting these in situ concretions, but reworked concretionary clasts up to 2 m in diameter are present in exposures of conglomerates in the lowermost Hanna Formation that lie just above the in situ Ferris concretions and southeast of the anticlinal nose. These early-diagenetic concretions were originally cemented by siderite (FeCO 3 ). Oxidation of some small, rinded siderite-cemented clasts took place after their fluvial transport into the Hanna Formation, but abundant angular, un-rinded, iron-oxide-cemented clasts indicate that many large, in situ siderite concretions had resided in the vadose zone before they were entrained. The distribution of reworked concretions and the orientations of crossbeds show that antecedent Hanna streams eroded a swath at least 5 km wide across the rising structure. These streams transported Ferris Formation concretions southeastward into the Carbon basin, and deposited them in a conglomeratic sandstone body in the Hanna Formation. Large calcite-cemented concretions, many with a pipe-like morphology, then grew within Hanna crossbeds. In many cases, these in situ concretions enclose transported, iron-rich concretions, but there is no evidence any calcite-cemented concretions were reworked. The NW–SE alignment of the pipes record southeastward flow of groundwater and thus also provide evidence (together with the orientation of the crossbeds) of the original continuity of the Hanna Formation across the anticline. Reworked concretions reveal the interplay of deposition, diagenesis, and erosion. Due to convergent groundwater flow over growing anticlines, early diagenetic concretions, both in situ and as reworked clasts, are especially likely to be found in growth strata.

Early Cenozoic drainage reorganization of the United States Western Interior–Gulf of Mexico sediment routing system

Abstract Continental-scale drainages host the world’s largest rivers and offshore sediment accumulations, many of which contain significant petroleum reserves. Rate of sediment supply in these settings may be a signal of external controls (e.g., tectonics) on landscape evolution, yet deciphering these controls remains a major challenge in interpreting the ancient stratigraphic record. Integration of new and published detrital zircon U-Pb ages from the United States Rocky Mountain region and Gulf of Mexico (GOM) sedimentary basin demonstrates profound changes in the U.S. continental drainage divide that controlled the rate of sediment delivery to the northern GOM during Paleocene–Eocene time. Sedimentation rate increased dramatically during deposition of the lower Wilcox Group, reaching approximately three times the Cenozoic average, accompanied by pronounced shoreline regression and delivery of a large volume of sand to the basin floor. We hypothesize that this increase in sediment delivery to the GOM resulted from drainage capture of a significant portion of the Sevier-Laramide structural province (∼900,000 km2) that included the headwaters of the California and Idaho Rivers. Capture of the California River drainage may have occurred in the vicinity of the Hanna Basin of eastern Wyoming that previously emptied northward into a shallow seaway, but was subsequently diverted southward to the Rockdale delta, which accumulated within the Houston embayment during the time of deposition of the lower Wilcox Group. Detrital zircon U-Pb ages from Wilcox samples within the Rockdale delta show a remarkable similarity with contemporaneous Laramide synorogenic units, including enrichment in detritus derived from the Cordilleran arc and basement terranes of western North America relative to older and younger units in the Houston embayment. A subsequent order of magnitude decline in sedimentation rate to the GOM can be partly attributed to well-documented drainage closure (∼800,000 km2) that accompanied lake formation in interior Laramide basins (ca. 53–51.8 Ma). Our results demonstrate that tectonically induced drainage migration in the high-relief segments of continental-scale drainages can have a pronounced effect on the rate of sediment transfer to continental margins.

Architecture and recognition criteria of ancient shelf ridges; an example from Campanian Almond Formation in Hanna Basin, USA

AbstractShelf ridges are sedimentary bodies formed on the continental shelf due to transgressive reworking (tidal or storm) of lowstand deposits. Common on modern shelves, they are under‐represented in the geological record due to a lack of recognition criteria and facies model. This article proposes a new facies and architectural model for shelf ridges, linked to their inception–evolution–abandonment cycle and the process regime of the basin. The model is mainly based on new outcrop data and interpretations from three sandstone bodies of the Almond Formation, an overall transgressive interval during the infill of the Campanian Western Interior Seaway. Building from the case study, and ancient and modern examples, six characteristics are proposed for the recognition of ancient shelf ridges. Shelf ridges: (i) are encased between thick marine mudstone intervals; (ii) have a basal unconformity that erodes into marine muds or into the remnants of a previous shoreline; (iii) have a non‐erosional upper boundary that transitions into marine muds; (iv) are characterized by clean and well‐sorted sandstones, often cross‐bedded; (v) contain fully marine ichnofauna; and (vi) present compound architectures with large accretion surfaces and lower order structures. Although shelf ridges have been described in previous studies as generated exclusively by either tidal or storm currents, it is clear, from modern examples and the case study, that these two processes can be recorded and preserved in a single shelf ridge. The stratigraphy of these sandstone bodies is therefore much more complex than previously recognized, bearing the signature of changing tidal and storm intensity through time. Because they are developed during transgressions, shelf ridges are commonly subject to strong changes in process regime as sea‐level changes can easily affect the oceanographic conditions and the morphology of the basin. For this reason, shelf ridges can provide the best record of shelf process variability during transgressions.

Late Laramide tectonic fragmentation of the eastern greater Green River Basin, Wyoming

Today9s greater Green River Basin is limited to the southwestern quarter of Wyoming. From late in the Cretaceous into late Paleocene time, however, sedimentary accumulations within that basin continued uninterruptedly much farther to the east, connecting areas now occupied by the isolated Hanna, Carbon, Pass Creek, and Laramie basins. Field-based research resulted in three contiguous geologic maps that focus on modern basin margins and boundaries among those eastern elements. Analyses of derived cross sections and restored stratigraphic columns suggest that active subsidence and rapid sedimentary accumulation persisted with only minor interruptions until very late in the Laramide Orogeny. That led to a generally symmetrical north–south cross-sectional configuration of the original Hanna Basin, with its true depositional axis set well south of its apparent position of today. The Hanna Basin9s present strong asymmetry developed only secondarily. That basin9s modern configuration reflects Paleogene influences of: (1) late Laramide (early Eocene and probably younger) basement-involved contractional tectonics and associated uplifts; (2) out-of-the-basin thrusting passively responding to stratigraphic crowding; (3) prodigious syntectonic erosion; and (4) resulting basin fragmentation. North–south dimension of the late Paleocene (i.e., pre-fragmentation) greater Hanna Basin sedimentary sequence was roughly twice that of today, and near-sea-level topographic conditions persisted until late Eocene time. As expected, remnants of basin margins universally show major thinning of stratigraphic sequences. Principal thinning was from tectonic causes, however, exhibiting erosional angular unconformities only rarely. Out-of-the-basin, younger-on-older faulting (in which fault planes cut down-section) accompanied by massive erosion was the rule at all basin margins. Uplift of Simpson Ridge Anticline postdated deposition of upper Paleocene strata in direct continuity between what is now the separated Hanna and Carbon basins. The basement-involved fault system responsible for westward relative tectonic transport (and ca. 8 km of elevation) by Simpson Ridge also led to raising the attached Carbon Basin. Original Hanna Formation of the Carbon Basin was beveled away by erosion and soon thereafter became replaced by shallow-slope sliding of a long-runout allochthon, the Carbon Basin Klippe. The klippe9s original site of deposition probably was to the northeast, above what later became Flat Top Anticline. Uplift of Flat Top and Simpson Ridge anticlines was essentially synchronous (latest Paleocene or, more probably, early Eocene), establishing a lengthy, faulted-synclinal separation of the Hanna/Carbon Basin from the Laramie Basin. That syncline also bifurcated Simpson Ridge Anticline into western and eastern segments. A second allochthon, the Dana Klippe, rests upon southern parts of the Hanna Syncline (of Hanna Basin). That klippe9s site of deposition probably was above the area that later elevated as Elk Mountain, thus causing origin of Pass Creek Basin. Elk Mountain9s ca. 12 km uplift could not have occurred prior to the early Eocene, and that event contributed to tight folding of the Hanna Syncline. Coal Bank Basin is a giant footwall syncline ahead of the out-of-the-basin, thrust-faulted (with relative tectonic transport to the southwest) Dana Ridge Anticline. That fault–fold complex represents a common structural style seen at all scales across the Hanna Basin.

Interpreting stratigraphic relationships and Laramide structural history of the northeastern margin of the Hanna Basin (Wyoming): Meniscoessus (Mammalia, Multituberculata) exposes its faults

Linkage of paleontological and geological discoveries provides new opportunities to strengthen interpretations of paleogeographic evolution of the Rocky Mountains9 deepest structural basin. We report discovery in the northeastern Hanna Basin (south-central Wyoming) of a lower first molar of Meniscoessus cf. M. robustus , an advanced form of multituberculate mammal known only from the North American Western Interior in Upper Cretaceous local faunas of the Lancian North American Land Mammal Age. It aids in dating patchy outcrops of the Ferris Formation, overlain and covered laterally by significantly younger, thrust-emplaced Hanna Formation. The specimen documents a member of the M. robustus species group, also recovered from more southwesterly strata of the Ferris Formation in the Hanna Basin. The fossil-bearing strata were deposited close to ancient sea level but are tectonically overturned and bounded above and below by what originally were north-vergent thrust faults. We present a new geologic map (scale 1:24,000) including two representative cross sections. Using an interpretive cross-sectional evolutionary model, we propose that the Hanna Basin, until late in Laramide orogenesis, had a markedly more extensive northern existence in the upland areas now occupied by the Freezeout Hills and southern Shirley Mountains. Local Laramide orogenic history in the mapped area is dominated to the north by development of at least 10 kilometers of Cretaceous–early Eocene structural relief across Archean granitic rocks. Those ancient rocks today form the NNE–SSW-oriented, axial core of the asymmetrical Shirley Mountains Anticline. Completion of the Shirley Mountains9 uplift postdated deposition of almost the entire stratigraphic sequence now exposed along the northern Hanna Basin. North-vergent, out-of-the-basin thrust faults developed in response to crowding initiated by the much larger, south-vergent, basement-involved thrust complex known as the Shirley Fault. Those out-of-the-basin thrust faults had mostly bedding-parallel planes of displacement. But they commonly cut stratigraphically down -section during basin-margin deformation, thus placing younger strata of the hanging walls onto older strata of the footwalls. These thin-skinned, younger-on-older fault relationships today exhibit steeply basinward-dipping to overturned strata. The faulting led to greatly thinned stratigraphic sections when juxtaposed against basin-margin, mountainous uplifts expressing oppositely vergent, basement-involved thrust-fault systems. These kinds of down-section thrust faults probably will become recognized as common expressions of basin subdivision along steeply dipping, basin-margin strata throughout the Rocky Mountain province. Furthermore, several occurrences of this phenomenon appear to have been long-misinterpreted as depositional/erosional angular unconformities. Such recognition demands re-thinking of the areas9 geologic histories. Complexities of erosional history constitute central parts of our evolutionary scenario. Locally derived clastic deposits within uppermost Cretaceous and Paleogene sequences of the northern Hanna Basin originated principally from north of the Shirley Mountains and other upland areas that today closely border the basin. Multiple source areas existed across deeply eroding, mountainous landscapes (the Granite Mountains) that existed during latest Cretaceous through Eocene time. Beginning in the latest Eocene and continuing late into the Miocene, dominantly airfall volcaniclastic materials from distant sources covered all but the high peaks of Wyoming. Late in the Miocene, however, the heavily eroded core of the Granite Mountains collapsed via extensional tectonics, allowing preservation of remnant volcaniclastic strata atop the Granite Mountains Graben.

Field test of autogenic control on alluvial stratigraphy (Ferris Formation, Upper Cretaceous-Paleogene, Wyoming)

Internally generated (autogenic) sedimentary processes can obscure signals of tectonic movements, climate conditions, or sea-level fl uctuations in alluvial basins. The stratigraphic effects of autogenic dynamics have been considered negligible on time scales of 10 4 –10 6 yr. However, recent physical and numerical models have shown that over basin-fi lling time scales, self-organization in sedimentary systems can produce stratigraphic patterns similar to those resulting from changing basin boundary conditions. Here, we present the fi rst fi eld-based test for autogenic control on stratigraphy in an ancient alluvial basin. The Ferris Formation (Upper Cretaceous–Paleogene, Hanna Basin, Wyoming) is composed of clusters of closely spaced channel deposits separated by intervals dominated by overbank material—a pattern that has been proposed as an example of a potentially autogenic stratigraphy. In order to evaluate controls on Ferris Formation stratigraphy, spatial patterns of key channel properties were analyzed. These variables (including sand-body dimensions, paleofl ow depth, maximum clast size, paleocurrent direction, and sediment provenance) were chosen because they can change in response to climate, tectonic, or sea-level forcing and are commonly measurable in ancient alluvial successions. No clear statistical trends were detected in the measured variables, which suggests that external forces did not likely control stratigraphic organization in the unit. This study demonstrates that autogenic dynamics in natural, fi eld-scale systems can produce organized stratigraphic patterns over much larger spatial and temporal scales than is typically presumed. This fi nding emphasizes the need for further understanding of autogenic stratigraphy and greater care interpreting climate, tectonic, and eustatic changes from alluvial basins.

Scale-dependent compensational stacking: An estimate of autogenic time scales in channelized sedimentary deposits

Recent studies show that paleoenvironmental (allogenic) signals preserved in the stratigraphic record may be contaminated or overprinted by internally generated (autogenic) sedimentation. This is problematic, but it is unclear over what temporal and spatial scales autogenic patterns are most prevalent. We propose that scale breaks in basin-fi lling trends can be used to identify the transition between allogenic and autogenic stratigraphy. Using data from numerical and physical experiments and an ancient outcrop, we explore how compensation, the tendency for sediment transport systems to preferentially fi ll topographic lows, varies with stratigraphic scale. Object-based models demonstrate the temporal scales at which stratigraphy changes from being partially infl uenced by autogenic processes to being completely determined by allo- genic forcings and suggest that this transition occurs at a time scale set by the maximum scale of surface roughness in a transport system divided by the long-term aggradation rate. This hypothesis is validated in a physical experiment where delta topography was monitored along fl ow-perpendicular transects at a high temporal resolution relative to channel kine matics. The strength of compensation in the experiment changes at the predicted time scale, where the maxi mum surface roughness is equal to the depth of the experimental channels. Above this compensation time scale deposits stack purely compensationally, but below this time scale deposits stack somewhere between randomly and deterministically. Similar scale-dependent stacking is also observed in the Ferris Formation (Cretaceous-Paleogene, Hanna Basin, Wyoming, United States). This study demonstrates that scale-dependent compensational stack- ing may be useful for isolating allogenic and autogenic stratigraphy in sedimentary basins.

Persistence of the Western Interior Seaway: Historical background and significance of ichnogenus Rhizocorallium in Paleocene strata, south-central Wyoming

We report and describe abundant, well-preserved, parallel-sided, U-shaped spreite burrows ( Rhizocorallium ) in the upper part of the latest Cretaceous into early Paleocene Ferris Formation of south-central Wyoming's Hanna Basin. Rhizocorallium typically is a component of the widely represented ‘ Cruziana ichnofacies,’ principally involving benthic marine environments seaward of the intertidal zone in shallow to offshore settings. Traditionally, this Ferris section has been interpreted as coal-bearing, continental deposits formed after full withdrawal of the Western Interior Seaway from eastern Wyoming and adjacent areas. The burrowed strata overlie fossiliferous rocks diagnostic of parts of the Puercan Land Mammal Age, early Paleocene. At time of burrow formation, and for several million years thereafter, the vicinity of the future Hanna Basin remained as an undivided, eastern component of an enormous, greater Green River Basin that encompassed almost all of Wyoming's southern half. The Rhizocorallium -bearing marine strata represent westward expansion of a previously more restricted Western Interior Seaway that persisted through latest Cretaceous time in what is now the western Great Plains. Even though tidal influences may have affected rapidly aggrading fluvial systems far upstream to the west in Montana and Wyoming, we regard actual Paleocene marine inundations to have been uncommon and geologically ephemeral events as far west as the Hanna Basin. Diverse fossil assemblages from strata of the Williston Basin, representing the first five million years of Paleocene time, have led to documentation of fully open, marine conditions as the Cannonball Formation was deposited. Stratigraphic distribution of fossils within the Cannonball shows persistence of the Western Interior Seaway in the northern Western Interior through the Cretaceous, followed by expansion (renamed the ‘Cannonball Sea’) during early Paleocene time. Connections of that seaway to the south, however, remain poorly understood because of later Cenozoic massive erosion of any Paleocene rock record that had existed south of the borderland between the Dakotas. No Paleocene localities in southern Wyoming or Colorado have yet yielded assemblages of marine invertebrate body fossils or microfossils as known from the Williston Basin. No verifiable means, therefore, have been recognized to characterize the Paleocene marine record of the Hanna Basin in terms of species uniquely shared with the Cannonball biota. Short-lived, Paleocene seaway excursions into the Hanna Basin may have been: (1) direct and exclusively from the Gulf Coast; (2) solely from the Cannonball Sea, with seaway contiguity east of the emerging Black Hills; or (3) initiated from a more extensive, midcontinental seaway connecting the Arctic Ocean to the Gulf of Mexico. Substantial structural uplift of the Laramie Mountains prior to mid-Paleocene time would have precluded even brief westward pulses of marine inundation into the vicinity of the presumptive Hanna Basin.

FOSSILS TO FERTILIZER: TAPHONOMIC IMPLICATIONS OF URANIUM ROLL FRONTS

Wyoming is famous for its deep structural basins containing both abundant vertebrate fossils and uranium-ore deposits in the form of roll fronts. Research conducted in The Breaks, an area of badlands in the northeastern corner of the Hanna Basin, south-central Wyoming, shows that these two disparate phenomena have an important relationship. Vertebrate bones and teeth, composed of bioapatite, are chemically stable under most diagenetic conditions. The chemical changes that occur at the leading edge of a propagating roll front, however, are capable of eliminating these fossils. This happens as pyrite is oxidized, which releases sulfuric acid to the ground water. Sulfuric acid reacts vigorously with apatite, releasing soluble phosphate. Phosphate liberated from the dissolved bones and teeth provides a valuable, often limiting, nutrient to plants or lichens once the roll front is exposed on the ground surface. In The Breaks, bedrock permeability is the most important factor controlling the distribution of roll fronts and, therefore, also of vertebrate fossils. Thus, patterns of roll-front distribution, observed from differing lithologic characteristics or floral assemblages growing on the rock, might provide a useful prospecting tool for paleontologists.

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In the late 1980s, two coal gasification concepts were tested at a 15- acre site in the Hanna Basin of south-central Wyoming. Both tests were conducted in the Hanna No. 1 Coal Seam, which is about 30 feet thick and 365 feet below surface at this site. Approximately 4500 tons and 11,300 tons of coal were burned in the two tests, respectively. The Land Quality Division (LQD) of the Wyoming Department of Environmental Quality (WDEQ) issued Research and Development License No. 15 (RD15) to Stearns Catalytic Corporation (now Washington Group International, Inc.) for the tests. Per the license requirements, ground water restoration and surface reclamation were necessary after the tests. Because of the efficiency of the burns and the effectiveness of venting and flushing the burn cavities after each test, ground water restoration was completed in 1989. Once ground water restoration was deemed complete, the 30+ wells were abandoned and surface reclamation began. The reported seed mix included Wheatgrass (western, thickspike, and slender), Indian Ricegrass, Fourwing Saltbush, and Big Sagebrush. In addition, Big Sagebrush seedlings were also planted. Surface reclamation was completed in 1993, with limited repair work (primarily weed control) in subsequent years. The LQD released the reclamation bond and terminated the license in late 2005. Even though this site was tiny in comparison with most in situ noncoal mines and traditional coal mines, it provides a microcosm of reclamation issues, including: drought impacts; selection of areas for evaluation of reclaimed vegetation; and comparison of species planted with species expressed. It also provides information specific to reclamation issues at in situ mining projects, such as extent of ground water contamination, measurement of revegetation success at sites with multiple, relatively small areas of disturbance, and soil sampling for water treatment impacts. The site was one of the first evaluated for bond release under Category 5 of the LQD Coal Rules.

Structural analysis of a Laramide, basement-involved, foreland fault zone, Rawlins uplift, south-central Wyoming

The western border of the Hanna Basin is defined by the Rawlins uplift, a Laramide, basement-involved, faulted arch. This north-northwest–south-southeast-trending structure separates the Hanna Basin on the east from the Great Divide Basin (part of the greater Green River depositional basin) on the west. The Rawlins uplift is a west–southwest-vergent, macroscale, fault-related fold. Detailed geologic mapping, construction of serial cross sections, and the incorporation of data from a seismic-reflection profile indicate that displacement along the fault zone flanking the uplift's western margin cannot account for the net structural relief between the Hanna and Great Divide Basins (∼37,000 vertical ft [∼11,285 vertical m] and ∼27,000 vertical ft [∼8,235 vertical m], respectively). The exposed frontal fault traces are interpreted as high-angle (∼70°) splays off a shallowly dipping (∼25°), master fault zone developed within Archean granitic rocks of the Wyoming province. A low-dipping, braided, plastic-to-brittle thrust-fault zone in the Precambrian basement is inferred to accommodate much of the fault displacement and thus account for the structural relief between the core of the uplift and the adjacent basins. Within the study area, displacement along the exposed frontal fault zone decreases from south to north. Within the map area, bedding attitudes along the southwest limb (forelimb) of the uplift range from ∼30–90° with only local areas of overturned beds. However, southwest of the map area, Upper Cretaceous strata are sub-vertical to overturned. On the homoclinal backlimb, dips are ∼10–15° east-northeast into the Hanna Basin. Eastward structural bends at the southern and northern margins of the uplift suggest a component of left-lateral, oblique-slip displacement along the southern margin and right-lateral, oblique-slip displacement along the northern margin. Pre-existing basement anisotropies or discontinuities are likely responsible for these important changes in structural grain.

Tectonic and paleogeographic implications of late Laramide geologic history in the northeastern corner of Wyoming's Hanna Basin

The Hanna Basin of south-central Wyoming has been considered anomalous among other Laramide depocenters of the Rocky Mountain region because of its combination of small size and great thickness of synorogenic strata. Most prior interpretations of the Hanna Basin have assumed a history of subsidence and sedimentary infilling discrete from surrounding basins. In contrast, we summarize new geological and paleontological information from the northeastern corner of the modern Hanna Basin suggesting that, prior to late Paleocene time, the Hanna Basin and nearby Carbon, Pass Creek, Laramie, and Shirley Basins were unified and depositionally continuous with the much larger Green River Basin to the west. Only late in the local expression of the Laramide orogeny (late Paleocene and early Eocene) did this “greater Green River Basin” become subdivided through completion of development of intrabasinal, basement-involved thrust faulting and associated anticlines. We view the present Hanna Basin as only a small, structurally defined remnant of an enormous, ponded basin that extended eastward during most of Paleocene time from the Wyoming–Idaho–Utah thrust belt to the newly uplifted Laramie Mountains. That basin was bounded on the south by the Uinta Mountains and combined Sierra Madre–Medicine Bow Mountains and on the north by the Gros Ventre Range, Wind River Mountains, and Sweetwater arch. When the original configuration of the unified greater Green River Basin is taken into account and combined with palinspastic removal of late Laramide faulting that defines its various margins, this basin at least rivaled dimensions of the Powder River Basin of Wyoming and Montana; the greater Green River Basin greatly exceeded the volume of sedimentary accumulation within the Powder River Basin. Rates of Paleogene basinal erosion associated with structural subdivision of eastern components of the greater Green River Basin were prodigious. Within context of a measured section, we describe and interpret the lithologic nature, depositional settings, source areas, previous geographic extent, and deformational history of the Hanna Formation as seen today in the eastern Hanna Basin. Temporal control is linked to summaries of included mammalian fossils, freshwater molluscs, leaf macrofloras, and palynomorphic assemblages. Unexpectedly early occurrences of certain palynomorphic assemblages suggest that paleoecological controls on distributions of pollen-forming plants during Paleocene time were particularly important. Deformation of strata in the eastern Hanna Basin chiefly reflects a complex interaction between a major, south-directed, low-angle, basement-involved thrust system and the thick and generally incompetent, mudstonerich Hanna Formation. We argue that out-of-the-basin faults, the result of spatial crowding in the basin, are kinematically linked to the basement-involved thrust system through a triangle-zone geometry. This trishear-style of deformation involved north-directed, out-of-the-basin, decollement-type thrusts and a synchronous south-directed, basement-involved, blind-thrust system. Map relationships along the northern margin of the basin indicate that many of the out-of-the-basin faults emplaced younger rocks onto older rocks, seemingly an anomalous structural relationship for contractional deformation. One explanation of these relationships involves synchrony of the: (1) cutting down section of north-directed, out-of-the-basin fault planes; with (2) upturning of basinal strata related to south-directed, basement-involved faulting along the northern margin of the Hanna Basin. Finally, the inherited Precambrian structural grain of the Archean Wyoming province was instrumental in controlling the orientation of many of the Laramide-age, basement-involved structural features. Tectonic heredity, rather than changing stress orientations, probably accounts for the variation in orientations of structural features associated with the northern and eastern margins of the Hanna Basin.