Arthur R. Sweet

The relationship between the iridium anomaly and palynological floral events at three Cretaceous-Tertiary boundary localities in western Canada

Profiles illustrating the abundance of iridium and the changes in the relative abundances of major groups within the palynoflora are given for three localities from western Canada. The peak abundance of iridium ranges from 1.35 to 5.60 ppb and corresponds to the base of a coal and to a floral-extinction event at the three localities. Immediately above the iridium-peak anomaly, angiosperm pollen is conspicuous in the assemblage, in contrast to the fern-spore spike described for sections in the mid-continental United States and at the Morgan Creek locality in south-central Saskatchewan.

Fireball passes and nothing burns¿The role of thermal radiation in the Cretaceous-Tertiary event: Evidence from the charcoal record of North America

High soot contents have been reported in Cretaceous-Tertiary (K-T) sedimentary rocks, leading to the suggestion that the amount of thermal power delivered from the Chicxulub impact was sufficient to have ignited wildfires. Soot cannot be used to indicate fire location, however, as soot from one large fire could spread globally. Sources other than biomass burning could also yield soot. Charcoal in nonmarine sedimentary rocks (here quantified in situ in polished blocks) provides a unique tool to record the distribution of wildfires and therefore assess the extent of any thermal radiation associated with the impact at Chicxulub. The K-T and lowermost Tertiary sedimentary rocks of six nonmarine sequences (Colorado to Saskatchewan) contain no charcoal or below-background levels of charcoal and a significant quantity of noncharred organic materials, revealing that there was no distinctive wildfire across the North American continent related to the K-T event. This finding indicates that the K-T impact cannot have delivered a peak irradiance of >95 kW.m - 2 of thermal power to the atmosphere and <19 kW.m - 2 to the ground. Therefore, the thermal power delivered from the impact to North America did not have the destructive potential previously predicted. High amounts of thermal radiation were not responsible for the environmental perturbations or extinctions associated with the K-T event.

Sedimentological and palynological evidence of regional climatic changes in the Campanian to Paleocene sediments of the Rocky Mountain Foothills, Canada

Abstract Evidence of paleoclimatic variations in the upper Campanian to lower Paleocene post-Wapiabi sequence of strata is visible both laterally between the central and southern Foothills, and vertically in the stratigraphic record. These differences are expressed by the distribution of climatically sensitive sediments, i.e. coal and caliche, and associated palynological assemblages within floodplain facies, as well as by changes in the style of fluvial channels. The interpretation of a semi-arid environment for mature caliche paleosols is supported by the impoverished character of the associated palynological assemblages, both in terms of diversity and the number of specimens recovered, and by the conspicuous presence of Classopollis . The lateral extent of the semiarid floodplain facies and associated broad and mobile channel deposits is limited to the southern part of the basin. No signs of a caliche facies have been found in the post-Wapiabi strata of the central Foothills. Instead, some of the floodplain deposits associated with meandering streams in this part of the basin contain numerous, thick, coal-bearing intervals. The relative climatic differences between the more humid central part and the drier southern part of the basin prevailed throughout the entire post-Wapiabi interval. As this cannot be satisfactorily explained by the position of the sea shore or by orographic influences alone, there was probably another external factor. This could have been the pattern of atmospheric circulation, such as that responsible for the present-day climatic differences existing between the southern and central Foothills. Semiarid floodplain facies occur at two levels in the stratigraphic column of the southern Foothills (in the late Campanian, Belly River Formation and in the upper Maastrichtian to lower Paleocene, Willow Creek Formation). They correspond to regressive episodes of the epicontinental seaway. Intervening between these semiarid floodplain facies are the marine shales of the Bearpaw Formation and, both overlying and underlying them, thin coal beds. These represent recurrent periods of swamp growth and peat accumulation in back-barrier environments (lagoon-fill and supratidal marsh respectively). Because of the proximity of these swamps to the sea shore it is difficult to assess the influence of climate on the relative humidity of these settings on strictly sedimentological grounds. However, the terrestrial components of the palynological assemblages recovered from these marginal-marine settings and correlative lacustrine sediments (containing fresh-water stromatolites but no coal) in the central Foothills are in sharp contrast with the impoverished assemblages known from the caliche facies. The former are prolific, diverse, and contain a number of triprojectate pollen species, which indicate a relatively humid climate during the latest Campanian time.

Maastrichtian-Paleocene foreland-basin stratigraphies, western Canada: a reciprocal sequence architecture

Palynological and magnetostratigraphic chronostratigraphic correlations of lower Maastrichtian to Paleocene strata along an east–west Western Canada Basin transect allow for the recognition of a reciprocal sequence architecture in nonmarine strata. Reference sections include three Canadian Continental Drilling Program Cretaceous–Tertiary Boundary Project core holes and outcrops in Alberta, southern Saskatchewan, and north-central Montana. The spatial and temporal position of the third-order sequences provides evidence for the correlation of proximal sector regional disconformities and sedimentary wedges with distal sector sedimentary wedges and regional disconformities, respectively. The boundary between the two sectors is represented by a hingeline, which separates the foreland-basin “syncline” from the “peripheral bulge.” The stratigraphies defined by reciprocal third-order sequences are complicated by fourth-order boundaries, developed within proximal sedimentary wedges and with no correlative distal strata. These results support tectonic control on foreland-basin sedimentation. A model for interpreting the various types of sequences in terms of foreland-basin evolution, vertical tectonics, and orogenic cycles is provided. It is argued that nonmarine sequence boundaries (times of maximum uplift in the foreland region) may be expressed as disconformities, incised valleys, top of mature paleosol levels, or base of fluvial channels, whereas nonmarine equivalents of marine maximum flooding surfaces (times of maximum basinal subsidence) may be indicated by extensive coal seams and (or) lacustrine sediments. Résumé: Les corrélations palynologiques, magnétostratigraphiques et chronostratigraphiques des strates échelonnées du Maastrichtien inférieur au Paléocène, le long du transect est-ouest du bassin de la Cordillère occidentale au Canada, permettent la reconnaissance d’une ordonnance architecturale de séquences réciproques dans les strates continentales. Les coupes de référence incluent trois trous de forage carottés du «Canadian Continental Drilling Program Cretaceous– Tertiary Boundary Project», et des affleurements localisés en Alberta, dans le sud de la Saskatchewan et dans le centrenord du Montana. La position spatiale et temporelle des séquences de troisième ordre plaide pour la mise en corrélation des discordances régionales et des dépôts sédimentaires en coin dans le secteur proximal avec les dépôts sédimentaires en coin et les discordances régionales dans le secteur distal, respectivement. Une ligne de jonction représente la limite entre les deux secteurs, qui sépare le bassin d’avant pays «en forme de cuvette» du «bombement périphérique». Les stratigraphies définies par les séquences de troisième ordre réciproques sont embrouillées par les limites de quatrième ordre, développées au sein des dépôts sédimentaires en coin proximaux sans la présence de strates distales corrélatives. Ces résultats appuient la thèse d’un contrôle tectonique sur le bassin sédimentaire d’avant-pays. On présente ici un modèle interprétant les divers types de séquences en termes d’évolution d’un bassin d’avant-pays, de tectonique verticale et de cycles orogéniques. Nous tentons de démontrer que les discordances, les vallées entaillées, les horizons sommitaux de paléosols matures ou la base des chenaux fluviaux peuvent aider à définir les limites des séquences continentales (périodes de soulèvement maximum dans la région de l’avant-pays), tandis que les couches de charbon étendues et (ou) les sédiments lacustres représentent plutôt les équivalents continentaux des aires d’inondation maximale (périodes de subsidence maximum du bassin). [Traduit par la Rédaction] Catuneanu and Sweet 703

Life in a temperate Polar sea: a unique taphonomic window on the structure of a Late Cretaceous Arctic marine ecosystem

As the earth faces a warming climate, the rock record reminds us that comparable climatic scenarios have occurred before. In the Late Cretaceous, Arctic marine organisms were not subject to frigid temperatures but still contended with seasonal extremes in photoperiod. Here, we describe an unusual fossil assemblage from Devon Island, Arctic Canada, that offers a snapshot of a ca 75 Myr ago marine palaeoecosystem adapted to such conditions. Thick siliceous biogenic sediments and glaucony sands reveal remarkably persistent high primary productivity along a high-latitude Late Cretaceous coastline. Abundant fossil faeces demonstrate that this planktonic bounty supported benthic invertebrates and large, possibly seasonal, vertebrates in short food chains. These ancient organisms filled trophic roles comparable to those of extant Arctic species, but there were fundamental differences in resource dynamics. Whereas most of the modern Arctic is oligotrophic and structured by resources from melting sea ice, we suggest that forested terrestrial landscapes helped support the ancient marine community through high levels of terrigenous organic input.

Thrust-top basin formation along a suture zone, Cantwell basin, Alaska Range: Implications for development of the Denali fault system

The Cantwell Formation consists of a lower sedimentary sequence as much as 4000 m thick and an upper volcanic sequence with a maximum thickness of 3750 m that was deposited in the Cantwell basin, south-central Alaska. Previous to this study, the Cantwell basin was interpreted as a Paleogene, nonmarine (mainly fluvial), pull-apart basin that formed in response to dextral, strike-slip displacement on the Denali fault system. This study proposes that the Cantwell basin formed as part of the Mesozoic accretionary phase of deformation, prior to the development of the Cenozoic postaccretionary Denali fault system. Our reinterpretation is based on several new lines of data. (1) Age . New data based on palynologic analyses of 135 fine-grained samples indicate that the lower Cantwell Formation was deposited during the late Campanian and early Maastrichtian. On the basis of previous regional tectonic studies and this new age constraint, the formation of the Cantwell basin was coeval with regional Late Cretaceous shortening associated with accretionary tectonics in southern Alaska. (2) Depositional systems . Our analysis of the Cantwell Formation demonstrates that sedimentation occurred mainly in stream-dominated alluvial fan, axial braided stream, and lacustrine settings. These depositional systems were strongly influenced by a southward dipping, asymmetric basin floor. The presence of abundant terrestrially derived organic material, together with palynological assemblages that include marine dinoflagellates and the associated presence of oncolites, may be suggestive of a time of marginal marine influence during the deposition of the upper part of the lower Cantwell Formation. The late Campanian to early Maastrichtian timing of this possible marine influence is within the range of the Bearpaw transgressive event of the Cordilleran foreland basin and allows for regional stratigraphic correlation of the Cantwell basin with other sedimentary basins in northwestern North America. (3) Structural controls on basin formation . Mapping of intraformational angular unconformities and progressively tilted strata along the southern margin of the Cantwell basin provides direct evidence that thrust fault deformation and lower Cantwell Formation sedimentation were synchronous. Distinctive Cantwell Formation conglomerate clasts derived from the uplifted hanging walls of nearby thrust sheets adjacent to the southern basin margin also support a syndepositional thrusting interpretation. Provenance data and the concentration of proximal alluvial fan deposits along the northwestern basin margin adjacent to the Hines Creek fault indicate that it, too, was active during deposition of the Cantwell Formation. On the basis of the new data, the Cantwell basin is interpreted to have formed as a thrust-top basin (i.e., piggyback basin) along the Late Cretaceous suture zone between the accreting Wrangellia composite terrane and the North American continental margin. In contrast to previous studies, this reinterpretation of the formation of the Cantwell basin implies that the lower Cantwell Formation is not a synorogenic deposit directly associated with strike-slip displacement along the Denali fault system. Therefore, the Cantwell basin cannot be used to constrain the timing for the early development of the Denali fault system.

Regional characterization of the Paskapoo bedrock aquifer system, southern AlbertaGeological Survey of Canada Contribution 2008-0479.

The Paskapoo Formation of southern Alberta supports more groundwater wells than any other aquifer system in the Canadian Prairies. Located in a region of rapid population growth and straddling watersheds where no new surface water licenses are available, this aquifer system is under increasing pressure to provide water supply. The Paskapoo Formation represents a foreland deposit of a siltstone- and mudstone-dominated fluvial system. The system is highly heterogeneous with broad ranges in physical properties that impact groundwater production. High-porosity coarse-grained channel sandstone can provide productive wells, whereas thin and fractured sands and siltstones are low producers. The basal Haynes Member and western portion of the Paskpaoo Formation have higher sandstone volumes than other portions of the system. Fracture density shows a strong inverse relationship to bed thickness, such that fracture flow becomes more important for thinner sandstone beds. There is no regional-scale flow system associated with the Paskapoo Formation; rather it is dominated by local-scale recharge processes. The geochemistry of Paskapoo Formation groundwater is largely controlled by the variable composition of immediately overlying glacial deposits.

A First Account of Organelle Preservation in Eocene Nonmarine Diatoms: Observations and Paleobiological Implications

Abstract Post-eruptive lacustrine sediments that infill a Middle Eocene kimberlitic diatreme near Lac de Gras (Northwest Territories, Canada) have spectacular preservation of diatom microfossils. A single undiagnosed species belonging to the genus Aulacoseira is abundant in this material and reveals, in addition to the full complement of siliceous structures comprising the frustule, an array of soft parts that hitherto have been unavailable for study in diatoms of this age. These features include the velar complex, which lines the interior of valves, layered internal membranes, lamellate plastid fragments with pyrenoid-like inclusions, and extracellular mucilagenous bodies. Both ultrastructural and cytoplasmic characters have pronounced affinities with extant congeneric taxa, with an especially close resemblance to the resting cells observed in living Aulacoseira. These observations suggest that modern cellular organization and associated ecological adaptations were present early in the history of nonmarine aulacoseiroid diatoms, implying that prolonged evolutionary stasis has characterized the ecologically important genus Aulacoseira.

Climatically induced floristic changes across the Eocene–Oligocene transition in the northern high latitudes, Yukon Territory, Canada

Global temperature decline associated with the Eocene–Oligocene transition resulted in extinctions of plants and animals in both marine and nonmarine environments. The extensive stratigraphic exposures, well-preserved palynological assemblages, and interbedded coal seams of the nonmarine Amphitheatre Formation, Burwash Basin, Yukon Territory, provide a comprehensive record of this transition. The formation spans a paleoclimatically significant interval otherwise poorly represented in high-latitude deposits of the northwestern Cordillera. Palynological data constrained by the chronologic and stratigraphic framework established for the Amphitheatre Formation indicate that the global temperature decline resulted in a shift from warm temperate, angiosperm-dominated to cooler temperate, gymnosperm-dominated (mainly coniferous) forest types. This turnover is seen in the increase in the percentage of gymnosperm compared to angiosperm pollen upsection. The effect of climate change is also recorded in the systematic composition of the angiosperm pollen spectra. The highly diverse palynoflora of the lower Amphitheatre Formation is dominated by angiosperm pollen characteristic of plants favoring warm temperate climates, and consistently includes pollen of broad-leaved evergreen taxa. In contrast, the angiosperms most conspicuous in the low-diversity palynoflora of the upper Amphitheatre Formation are characteristic of plants favoring a cooler temperate climatic setting. Petrographic compositional changes in the coals document the same plant community changes. The floristic data also indicate that, at high latitudes, there may have been a change to a wetter and less seasonal climate during the overall global cooling trend.

Palynofacies, coal petrographic facies and depositional environments: Amphitheatre formation (Eocene to Oligocene) and Ravenscrag Formation (Maastrichtian to Paleocene), Canada☆

Abstract Low rank coals from two disparate geological settings have been subjected to petrographic and palynological analysis. The stratigraphic units studied and their locations are the Amphitheatre Formation, St Elias Mountains, Yukon Territory, of Eocene to Oligocene age; and the base of the Ravenscrag Formation, south-central Saskatchewan, of latest Cretaceous to earliest Paleocene age. The depositional setting of the Amphitheatre Formation ranges from distal sand-dominated braided stream and lacustrine environments to proximal gravel-dominated fluvial environments. The coals are low in inertinite ( 85%). In localities with a dominance of angiosperm pollen the relative abundance of eu-ulminite B and densinite is greatest whereas in the ones with a dominantly coniferous pollen assemblage eu-ulminite A is the most prominent maceral. These observations suggest as an immediate cause/effect relationship, an at least partial dependance of present maceral content on floral precursors. This in turn probably reflects a certain combination of depositional environment conditions (pH, Eh, temperature, etc.), that likely controlled the plant community and the preservation of vegetal matter. The depositional environment of coals from the basal part of the Ravenscrag Formation contrasts sharply with that of the Amphitheatre. The Ravenscrag coals formed within a low energy, stable, floodplain environment. Palynological and coal maceral profiles for the basal Ravenscrag Formation coal, which spans the Cretaceous-Tertiary boundary, indicates that the environment of deposition progressed from an open canopied swamp forest with areas of open water, through a phase of low-lying to open water swamps with herbaceous, pterophytic vegetation to, at least locally, raised bogs. The change in coal petrography across the Cretaceous-Tertiary boundary is shown to be relatively minor in comparison to subsequent changes in coal petrography in the earliest Paleocene and to changes that occur in the palynological assemblages across the boundary interval.