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Magnetostratigraphy of the Lebo and Tongue River Members of the Fort Union Formation (Paleocene) in the northeastern Powder River Basin, Montana

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Abstract

We analyzed paleomagnetic samples and documented the stratigraphy from two sections near Miles City, Montana to determine the geomagnetic polarity stratigraphy and to constrain the age and duration of the Lebo and Tongue River Members of the Fort Union Formation in the northeastern Powder River Basin. The resulting polarity sequence can be correlated to subchrons C29n–C26r of the geomag-netic polarity time scale. By interpolating measured sediment accumulation rates from the base of C28r to the top of C27n, and then extrapolating to the top of the Tongue River Member and the bottom of the Lebo Member, we developed two age models to estimate the durations of the Lebo and Tongue River Members. Based on the first model, which uses different sedimentation rates for the Lebo and Tongue River Members, we estimate the duration of deposition of the Lebo to be between 1.30 and 1.74 million years and of the Tongue River to be between 1.42 and 1.61 million years. Using the second model, which uses the same sedimentation rate for the Lebo and Tongue River Members, we estimate the duration of deposition of the Lebo to be between 1.33 and 1.76 million years and of the Tongue River to be between 1.00 and 1.25 million years. Our results indicate a decrease in sediment accumulation rates in C27r, which is likely the result of a 0.26 to 0.62 million-year long depositional hiatus in the middle of C27r, represented by the Lebo–Tongue River contact. This unconfor-mity occurs 2 million years earlier than previously suggested and is likely contempo-raneous with unconformities in the Williston Basin and in southwestern Alberta, suggesting that it may be regionally significant.

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... The Mexican Hat fossil site is located approximately 15 m above the contact between the Tullock and Lebo Members (ES Belt, personal communication 2012). At Signal Butte, Montana, located 40 km west of Mexican Hat, the magnetic polarity reversal between chrons C29n and C28r occurs approximately 10 m above the Tullock-Lebo contact [30]. Because of potential differences in sedimentation rates and topography, the sediments at Mexican Hat could have been deposited either during C29n or C28r, whose reversal boundary is currently calibrated to 64.95 Ma [31]. ...
... These specimens are housed at DMNH and YPM. Collections made by DJP from early to middle Paleocene strata in the Williston Basin in North Dakota (2916 specimens at YPM [14,39] [31] and sedimentation rates from [30]) were examined for insect damage for the first time in this study. For broader regional and temporal context, the leaf fossils from early late Paleocene localities in Wyoming (59.0-57.5 Ma) collected by PW and CCL and scored for insect damage in [19] were also reexamined. ...
... In modern rainforests, plant genera with wide ranges can support insect faunas with low beta diversity [94], so low diversity Paleocene floras dominated by widespread species might be expected to support similar insect faunas across fossil localities. We did not observe any Paleocene sites with insect damage diversity comparable to Mexican Hat, even at Lebo Member localities deposited nearby and at an approximately similar time (Signal Butte, MT section [14,30,39], ca. 32 km from Mexican Hat). ...
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Plant and associated insect-damage diversity in the western U.S.A. decreased significantly at the Cretaceous-Paleogene (K-Pg) boundary and remained low until the late Paleocene. However, the Mexican Hat locality (ca. 65 Ma) in southeastern Montana, with a typical, low-diversity flora, uniquely exhibits high damage diversity on nearly all its host plants, when compared to all known local and regional early Paleocene sites. The same plant species show minimal damage elsewhere during the early Paleocene. We asked whether the high insect damage diversity at Mexican Hat was more likely related to the survival of Cretaceous insects from refugia or to an influx of novel Paleocene taxa. We compared damage on 1073 leaf fossils from Mexican Hat to over 9000 terminal Cretaceous leaf fossils from the Hell Creek Formation of nearby southwestern North Dakota and to over 9000 Paleocene leaf fossils from the Fort Union Formation in North Dakota, Montana, and Wyoming. We described the entire insect-feeding ichnofauna at Mexican Hat and focused our analysis on leaf mines because they are typically host-specialized and preserve a number of diagnostic morphological characters. Nine mine damage types attributable to three of the four orders of leaf-mining insects are found at Mexican Hat, six of them so far unique to the site. We found no evidence linking any of the diverse Hell Creek mines with those found at Mexican Hat, nor for the survival of any Cretaceous leaf miners over the K-Pg boundary regionally, even on well-sampled, surviving plant families. Overall, our results strongly relate the high damage diversity on the depauperate Mexican Hat flora to an influx of novel insect herbivores during the early Paleocene, possibly caused by a transient warming event and range expansion, and indicate drastic extinction rather than survivorship of Cretaceous insect taxa from refugia.
... Ma; Fig. 3; Gradstein et al., 2012). Up to 60 m of paleotopography is recognized on this surface (Kupsch, 1957), and it is overlain by a late Maastrichtian to Danian succession (Lerbekmo and Sweet, 2008;Peppe et al., 2011) characterized by an upward transition from coarser, more amalgamated fluvial channel sandstones to finer-grained, better preserved floodplain clastics and coal (Fig. 3;Eberth and O'Connell, 1995;Murphy et al., 2002). This transition is characteristic of the late lowstand systems tracts (LST) to transgressive systems tracts (TST) of fluvial sedimentary sequences (Shanley and McCabe, 1994;Ethridge et al., 1998). ...
... This transition is characteristic of the late lowstand systems tracts (LST) to transgressive systems tracts (TST) of fluvial sedimentary sequences (Shanley and McCabe, 1994;Ethridge et al., 1998). This succession is unconformably overlain by the latest Danian to Thanetian fluvial Paskapoo Formation and time-equivalent strata (Lerbekmo and Sweet, 2008;Peppe et al., 2011). Thus, in the study area, the Cretaceous-Paleogene boundary occurs within the late LST to TST of an ~3-6-m.y.-duration (third order; cf. ...
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Petrological and delta C-13 analyses were undertaken on contiguous specimens of coal and intercalated minor organic-rich clastic sediments collected from coal seams spanning the Cretaceous-Paleogene boundary in the Alberta-and Saskatchewan portions of the Western Interior Basin. The generally high smectite content of the coal suggests that the original mires were largely small, disconnected, and rheotrophic, readily receiving abundant waterborne detrital clastic material of largely volcanic origin. Nevertheless, using the distinctive claystone that marks the Cretaceous-Paleogene boundary as a regional datum, it is possible to correlate cycles in the vitrinite and inertinite composition of the coals over >500 km. Estimates of peat accumulation rates suggest that the cycles in vitrinite and inertinite composition represent regional, cyclic fluctuations in wildfire and oxidation of the peatlands and overlying canopy at a frequency of hundreds to thousands of years. The likely causes of these fluctuations were cyclic, regional-scale changes in temperature. The Cretaceous-Paleogene boundary event occurred early during a phase of gradually increasing temperature and/or decreasing rainfall, but peak wildfire and desiccation of peat occurred up to 14,000 yr later than the Cretaceous-Paleogene boundary, and the mires did not experience significant water stress in the immediate aftermath of the extinction event. A persistent, 1.5%-3.0% negative delta C-13 excursion occurs across the Cretaceous-Paleogene boundary, but it cannot be readily separated from four, further negative excursions later in the earliest Danian. The negative carbon isotope excursion linked to the Cretaceous-Paleogene boundary began a few hundred years before the event itself, and recovery occurred within 21 k.y., and possibly in as little as just a few thousand years, consistent with recently calibrated shallow-marine delta C-13 records. Hence, the atmospheric and surface ocean carbon pools were coupled at this time. The absence of evidence for catastrophic change in the climatic regime at the time of the Cretaceous-Paleogene extinction in these mires supports the notion that the negative shift in atmospheric delta C-13 was brought about by changes in the delta C-13 composition of the surface ocean. This is consistent with the greater magnitude of extinction experienced by marine fauna relative to the terrestrial realm.
... Since Butler and Lindsay [1985], intermediate titanohematite has been identified using similar techniques in KPg age Laramide clastic deposits across central North America, including the San Juan Basin (northwest New Mexico) [Butler and Lindsay, 1985;Force et al., 2001], the Bighorn basin (south-central Montana, north-central Wyoming) [Butler et al., 1987;Force et al., 2001], and much of the Williston basin (Western and central North Dakota and Eastern Montana, USA and Southern Alberta, Saskatchewan, and Manitoba, Canada) [Lund et al., 2002;Swisher et al., 1993;Lerbekmo, 1999;Force et al., 2001] (see Figure 1). We speculate that intermediate titanohematite is present in additional KPg age Laramide deposits, but that it has been misidentified as intermediate titanomagnetite or titanomaghemite due to inadequate rock magnetic analysis [e.g., Peppe et al., 2009Peppe et al., , 2011. identification of this mineral in the future; moreover, by more accurately understanding its properties, it will be possible to better analyze sources of error and complexity in paleomagnetic analysis when this mineral is an important carrier of remanence. ...
... In all Hell Creek paleomagnetic studies, coercivity spectra reveal that the characteristic remanent direction is constrained between 20 and 80 mT [Archibald et al., 1982;Swisher et al., 1993;LeCain et al., 2014] (Table 1). These results are similar to those observed for other KPg Laramide continental deposits in the San Juan Basin, NM, Big Horn Basin, WY and MT, Powder River Basin, WY and MT, and other parts of the Williston Basin (southeastern MT, southern Canada, and eastern and central North Dakota) Lindsay et al., 1981;Butler et al., 1987;Peppe et al., 2009Peppe et al., , 2011Lerbekmo andCoulter, 1984, 1985;Lerbekmo, 1999;Lund et al., 2002]. No further rock magnetic analyses have been conducted on rocks from the Hell Creek region since Swisher et al. [1993]. ...
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The effect of the Cretaceous-Paleogene (K-Pg) (formerly Cretaceous-Tertiary, K-T) mass extinction on avian evolution is debated, primarily because of the poor fossil record of Late Cretaceous birds. In particular, it remains unclear whether archaic birds became extinct gradually over the course of the Cretaceous or whether they remained diverse up to the end of the Cretaceous and perished in the K-Pg mass extinction. Here, we describe a diverse avifauna from the latest Maastrichtian of western North America, which provides definitive evidence for the persistence of a range of archaic birds to within 300,000 y of the K-Pg boundary. A total of 17 species are identified, including 7 species of archaic bird, representing Enantiornithes, Ichthyornithes, Hesperornithes, and an Apsaravis-like bird. None of these groups are known to survive into the Paleogene, and their persistence into the latest Maastrichtian therefore provides strong evidence for a mass extinction of archaic birds coinciding with the Chicxulub asteroid impact. Most of the birds described here represent advanced ornithurines, showing that a major radiation of Ornithurae preceded the end of the Cretaceous, but none can be definitively referred to the Neornithes. This avifauna is the most diverse known from the Late Cretaceous, and although size disparity is lower than in modern birds, the assemblage includes both smaller forms and some of the largest volant birds known from the Mesozoic, emphasizing the degree to which avian diversification had proceeded by the end of the age of dinosaurs.
Article
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Rapid global warming of 5 degrees to 10 degrees C during the Paleocene-Eocene Thermal Maximum (PETM) coincided with major turnover in vertebrate faunas, but previous studies have found little floral change. Plant fossils discovered in Wyoming, United States, show that PETM floras were a mixture of native and migrant lineages and that plant range shifts were large and rapid (occurring within 10,000 years). Floral composition and leaf shape and size suggest that climate warmed by approximately 5 degrees C during the PETM and that precipitation was low early in the event and increased later. Floral response to warming and/or increased atmospheric CO2 during the PETM was comparable in rate and magnitude to that seen in postglacial floras and to the predicted effects of anthropogenic carbon release and climate change on future vegetation.
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Food web recovery from mass extinction is poorly understood. We analyzed insect-feeding damage on 14,999 angiosperm leaves from 14 latest Cretaceous, Paleocene, and early Eocene sites in the western interior United States. Most Paleocene floras have low richness of plants and of insect damage. However, a low-diversity 64.4-million-year-old flora from southeastern Montana shows extremely high insect damage richness, especially of leaf mining, whereas an anomalously diverse 63.8-million-year-old flora from the Denver Basin shows little damage and virtually no specialized feeding. These findings reveal severely unbalanced food webs 1 to 2 million years after the end-Cretaceous extinction 65.5 million years ago.
Article
The Power River basin is a Laramide foreland basin that was filled by a combination of fluvial, deltaic, paludal, and lacustrine sediments. The depositional history of the Fort Union Formation was unraveled in a regional subsurface study using data from approximately 1,400 geophysical well logs. The depositional model developed from the subsurface study was tested by selective fieldwork. This paper discusses the methodology used in the study, describes the regional distribution of lithofacies and sand-body geometry, and presents a regional depositional model based on subsurface mapping with supporting fieldwork. The lacustrine fluvial-deltaic model provides a viewpoint for evaluating energy resources of the Fort Union Formation, including coal, coal-bed methane and uranium. Refs.
Article
A 60km (37 mile) section of the E flank of the Bighorn Mountains, Wyoming, between the Tensleep and Shell lineaments is broken into fault bounded segments. The principal fault trends are: N40o-45o E; N60oE; and N20o-25oW. A major high angle reverse fault closely parallels the axis of the Powder River basin from Sheridan southward to the Casper arch. The fault location is based on seismic profile and drill records, and the displacement is in the order of 1250m (4000ft). Structural relief from the axis of the Powder River basin to the crest of the range is approximately 7800m (25 000ft). Cross sections based on surface geology and well geology through Ts. 45N-53N demonstrate that movement on W dipping thrust faults accounts for much of the deformation. Tectonic transport is up and to the E. The pattern of deformation of the mountain flank is that of differential eastward movement. The Bighorn Mountains are essentially a double plunging anticline with Precambrian basement exposed in the core. The segmented flank lies opposite the region of maximum differential elevation of the Precambrian basement, and is the response of an upward and eastward bulging crest under compression. The reverse fault pattern is pervasive in the sedimentary rocks, and the faults are believed to be rooted in the basement. The geometry of the deformed rocks supports the concept that the mountain flank deformation took place under compression rather than as simple vertical block uplift.-from Author
Article
The Tullock Member was deposited in a continental fluvial environment. Channel deposits (comprising about one-third of the sequence) contain mainly trough and tabular planar crossbedded and climbing-ripple laminated sandstone; reactivation surfaces, liquefaction fronts, and structures resulting from soft-sediment deformation are also common, suggesting episodic rapid deposition, saturation of sediments, and a high watertable. Fine-grained overbank deposits (making up about two thirds of the sequence) show color mottling, contain plant, wood, and coal fragments, have obscure bedding or no apparent internal structure, and contain thin coal beds. -from Author
Chapter
We have carried out a magnetostratigraphic study of uppermost Cretaceous and lower Paleocene strata of theWilliston Basin that are exposed in central North Dakota and easternmost Montana. Five overlapping sections were sampled that extend from the marine Fox Hills Formation through the predominantly continental Hell Creek Formation and Ludlow Member of the Fort Union Formation into the overlying marine Cannonball Member of the Fort Union Formation. Rock magnetic study indicated that the detrital magnetic mineral carrying the sediment's natural remanent magnetization is an ilmeno-hematite with low Curie temperature (<225 °C) and that secondary viscous and chemical overprints are routinely present. The base of magnetic polarity Chron 29r, which contains the K-T boundary, is located at the Hell Creek-Fort Union formational contact in central North Dakota but lies within the uppermost Hell Creek Formation in eastern Montana. This is consistent with palynological placement of the K-T boundary within the Ludlow Member in central North Dakota and at the Hell Creek-Fort Union formational contact in eastern Montana. The paleomagnetic data suggest that the Hell Creek Formation in central North Dakota terminated at least 90000 yr before the palynological K-T boundary, while the Hell Creek Formation in eastern Montana terminated synchronously with the palynological K-T boundary. This resulted from a time-transgressive change in paleoenvironmental conditions related to westward transgression of the Cannonball Sea, which was present in central North Dakota about 160000 yr after the K-T boundary. The earlier Upper Cretaceous marine Fox Hills regression and overlying marine Breien Member (Hell Creek Formation) incursion occurred during normal magnetic polarity Chron 30n.
Chapter
The patterns of marine magnetic anomalies for the Late Cretaceous through Neogene (C-sequence) and Late Jurassic through Early Cretaceous (M-sequence) have been calibrated by magnetostratigraphic studies to biostratigraphy, cyclostratigraphy, and a few radiometrically dated levels. The geomagnetic polarity time scale for the past 160 myr has been constructed by fitting these constraints and a selected model for spreading rates. The status of the geomagnetic polarity time scale for each geological period is summarized in Chapters 11–22 as appropriate. PRINCIPLES Magnetic field reversals and magnetostratigraphy The principal goal of magnetostratigraphy is to document and calibrate the global geomagnetic polarity sequence in stratified rocks and to apply this geomagnetic polarity time scale for high-resolution correlation of marine magnetic anomalies and of polarity zones in other sections. The basis of magnetostratigraphy is the retention by rocks of a magnetic imprint acquired in the geomagnetic field that existed when the sedimentary rock was deposited or the igneous rock underwent cooling. The imprint most useful for paleomagnetic directions and magnetostratigraphy is recorded by particles of iron oxide minerals. Most of the material in this chapter is updated from summaries in Harland et al. (1990) and Ogg (1995). Excellent reviews are given in Opdyke and Channell (1996) for magnetostratigraphy and McElhinny and McFadden (2000) for general paleomagnetism.
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The composition, species abundances, and spatial and temporal distributions of mollusc assemblages were controlled by the environments in which they lived and the depositional processes that affected the molluscs after death and before final burial. Post-mortem transport, reworking and concentration of shells, and mixing of faunal elements from discrete habitats produced a taphonomic 'overprint' on assemblage characteristics that directly reflects the processes of alluvial plain and floodbasin lacustrine sedimentation. The 'overprint' can be interpreted from outcrop analysis of molluscan biofabric, which consists of: 1) orientation, fragmentation, size-sorting, abrasion, density, and dispersion of shells, 2) the nature and extent of shell-infilling, and 3) ratio of articulated to disarticulated bivalves. Taphonomic characteristics were used with sedimentological properties to differentiate in-place, reworked, transported, and ecologically mixed mollusc assemblages. This study also defines the paleoecology of habitat preferences of mollusc species as a basis for recognition of the environments in which these assemblages were deposited: 1) large floodbasin lakes, 2) small floodbasin lakes, and 3) crevasse deltas and splays. Integration of sedimentology and paleoecology provides an interdisciplinary approach to the interpretation of alluvial environments through time in the Tongue River Member. -Authors
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The U.S. Geological Survey, in cooperation with the Bureau of Land Management, Wyoming State Engineer's Office, and coalbed methane operators, is conducting multidisciplinary studies in the Powder River basin, Wyoming and Montana. These studies are investigating regional geology and hydrology, coal composition, gas composition, methane desorption and water composition. This report presents data from ongoing studies to determine the chemical and isotopic composition of water coproduced with methane throughout the basin. Groundwater samples in the Lower Tertiary Wasatch and Fort Union Formations are being collected, primarily from producing coalbed methane wells, for analysis of major, minor and trace elements and selected isotopes. Results of these studies have implications for future and present coalbed methane development. Total dissolved-solids concentrations in water produced from the Paleocene Fort Union Formation coal beds increase from south of the Belle Fourche River to the north and west over the basin. The trend in values for sodium adsorption ratios parallels the trend of dissolved solids. Water produced from Fort Union Formation coal beds is exclusively sodium bicarbonate-type water, whereas water produced from Eocene Wasatch Formation coal beds and sandstones varies from mixed cation-mixed anion types to a sodium-bicarbonate type. The ionic composition of water in the Wasatch Formation apparently is related to depth and may be the product of geochemical reactions along horizontal and vertical flow paths. Trace-metal concen-trations in water produced from Fort Union coal beds are uniformly low and below the U.S. Environmental Protection Agency's maximum contaminant and secondary maximum contaminant levels for drinking water, except for iron and manganese. The stable-isotopic composition of water from the Wasatch and Fort Union Formations is consistent with a meteoric water origin and has not evolved away from the global meteoric water line. Tritium dating of selected groundwater and spring samples indicates that water in the deeper Wasatch Formation and in the Fort Union Formation is older than about 1952, the age of application of the method.
Article
THEORIES that explain the extinctions characterizing the Cretaceous/Tertiary (K/T) boundary1-3 need to be tested by analyses of thoroughly sampled biotas. Palynological studies are the primary means for stratigraphic placement of the terrestrial boundary and for estimates of plant extinction4-12, but have not been combined with quantitative analyses of fossil leaves (megaflora). Megafloral studies complement palynology by representing local floras with assemblages capable of high taxonomic resolution13, but have previously lacked the sample size and stratigraphic spacing needed to resolve latest Cretaceous floral history5,14-18. We have now combined megafloral data from a 100-m-thick composite K/T boundary section in North Dakota with detailed palynological analysis. Here the boundary is marked by a 30% palynofloral extinction coincident with iridium and shocked-mineral anomalies and lies ~2 m above the highest dinosaur remains. The megaflora undergoes a 79% turnover across the boundary, and smaller changes 17- and 25-m below it. This pattern is consistent with latest Cretaceous climatic warming preceding a bolide impact.
Article
Sequential morphologic changes in pollen of the Momipites‐Caryapollenites lineage from Paleocene strata in the Wind River Basin of Wyoming appear to reflect evolution within the family Juglandaceae. The stratigraphic occurrence of the species within this complex permits the establishment of six biostratigraphic zones that can be correlated between outcropping and subsurface sequences within the basin and to surrounding areas.Trends involving changes in shape and size of the pollen and in structure of exine at the poles proceed from a basic form of Momipites to six other distinguishable species. The genus Caryapollenites appears to have been derived from the basic form of Momipites by changes in size and development of heteropolarity in pore position.Species of Momipites described as new are M. wyomingensis, M. waltmanensis, M. ventifluminis, M. actinus, M. anellus, and M, leffingwellii. The genus Caryapollenites is redescribed and C. prodromus, C. imparalis, C. inelegans, and C. wodehousei are described as new.
Article
An integrated magnetostratigraphic and palynostratigraphic study of the continental, mostly lower Paleocene upper Coalspur and middle to upper Paleocene Paskapoo Formation was carried out in the Coal Valley to Hinton area of west-central Alberta. Magnetostratigraphic samples were taken from 2 cores and one outcrop in the Coal Valley Mine, a roadcut at Coalspur, the Bryan Creek Mine pit near Robb, 13 outcrop locations, and 4 cores in the Hinton-Obed Mountain area (totaling 313 samples). About 300 palynostratigraphic samples were taken from cores and outcrops within Coal Valley Mine, Coalspur roadcut, Bryan Creek Mine, the Hinton area and Obed Mountain and vicinity. Sample localities were structurally fitted into a composite Paleocene section from the Cretaceous-Tertiary boundary to the top of the No. 1 coal in the Obed Mountain Mine, totaling about 300 m of upper Coalspur and about 770 m of Paskapoo Formation. The section encompasses parts of magnetozones 29r to 24r and palynozones Wodehouseia spinata Zone, Aquilapollenites reticulatus Subzone through the Pistillipollenites mcgregorii Zone. Preserved sediment accumulation rates were up to about 250 m per m.y., but averaged much less due to hiatuses. These hiatuses involve the omission of parts of magnetochrons 28r, 28n and 27r in the Arbour-Val d’Or coal zone, 27n at the base of the Paskapoo, and 26n in the upper Paskapoo, totaling at least 2.2 m.y.
Article
A chronology for Paleocene strata of the Calgary region is developed from the integration of magnetostratigraphy with palynological and mammal zones. Structural reference to the underlying Battle Formation adds stratigraphic control. These strata range in age from magnetochron 29r to the lower half of 26r and palynomorph zones Wodehouseia fimbriata through Aquilapollenites spinulosus. Four mammal sites have been placed in the magnetostratigraphic framework. The youngest is a Tiffanian (Ti1) site in 26r. Sites judged to be Torrejonian occur in 27n, 28r and 29n. The presence of thin coals indicates correlation of lower Paleocene outcrops south and southeast of Calgary to the upper Scollard Formation. Younger strata in Calgary City have characteristics of a seasonally dry facies association most comparable to that of the Porcupine Hills Formation of southwestern Alberta. To the west and north of Calgary, the occurrence of thin coals in strata of late Paleocene age justifies assigning these rocks to the Paskapoo Formation. However, the placement of all formational boundaries is somewhat arbitrary in this transitional region between the nomenclatural systems of southwestern versus central Alberta. The type Porcupine Hills Formation is shown to be correlative to the upper part of the upper Scollard Formation and the lower part of the Paskapoo Formation.
Article
Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geo-qtagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene anq Pliocene and with a new timescale for the Mesozoic.
Article
Any topological framework requires the development of a theory of errors of characteristic and appropriate mathematical form. The paper develops a form of theory which appears to be appropriate to measurements of position on a sphere. The primary problems of estimation as applied to the true direction, and the precision of observations, are discussed in the subcases which arise. The simultaneous distribution of the amplitude and direction of the vector sum of a number of random unit vectors of given precision, is demonstrated. From this is derived the test of significance appropriate to a worker whose knowledge of precision lies entirely in the internal evidence of the sample. This is the analogue of 'Student's' test in the Gaussian theory of errors. The general formulae obtained are illustrated using measurements of the direction of remanent magnetization in the directly and inversely magnetized lava flows obtained in Iceland by Mr J. Hospers.
Article
Fossils from the Hell Creek and Tullock Formations in northeastern Montana provide detailed documentation of terrestrial faunal and floral evolution during latest Cretaceous (Lancian) and early Paleocene (Puercan) time. Here the replacement of Lancian faunas by those of Puercan age, most obviously signaled by the extinction of dinosaurs, and the changes in pollen floras sometimes used to mark the Cretaceous-Tertiary boundary occurred during a period of reversed magnetic polarity. Paleontological correlations suggest that dinosaur extinction and the change in pollen floras took place in the Red Deer Valley area, Alberta, during the same period of reversed polarity. Furthermore, also on the basis of paleontological correlations, the extinction of dinosaurs in the San Juan Basin, New Mexico, appears to have occurred either during the same period of reversed polarity or, possibly, during the preceding period of normal polarity.
Article
Depositional controls on peat-forming environments which produce thick (>10 m) coal beds can be inferred from relationships between coal bed geometry, maceral composition and associated lithologies. The Wyodak-Anderson peat is interpreted to have formed in restricted parts of the floodplain that were separated by deposits of contemporaneous, anastomosed channels. The Felix coal bed is interpreted to have formed as a raised but widespread peat on an abandoned platform of meander-belt sands. These models may be useful as predictive tools for coal exploration and production. -from Authors
Article
The recovery and subsequent prolific radiation of mammals in the northern Western Interior of North America following the Cretaceous-Paleogene (K-P) boundary is well documented in rocks attributed to the Puercan Land Mammal Age. The most complete Puercan record is that of the Tullock Formation, which overlies the dinosaur-bearing Hell Creek Formation and consists of a stratigraphic series of channel and overbank deposits from which well-preserved Puercan faunas have been collected. These channel deposits are typically bracketed by widespread coal beds. The IrZ- and Z-coals mark the base of the Puercan at the K-P boundary. The IrZ-Coal is overlain sequentially by the Z-, HFZ-, Y-, W-, and U-coals. 40Ar/39Ar dating of single crystals of sanidine separated from bentonites in the IrZ-, Z-, HFZ-, W-, and U-coals has yielded high-precision ages with standard errors of 0.1%. Replicate analyses of single crystals of sanidine yield weighted mean ages for bentonites in the following coals: IrZ, 65.16 ± 0.04 Ma; Z, 65.01 ± 0.03 Ma; HFZ, 64.77 ± 0.06 Ma; W, 64.11 ± 0.02 Ma; and U, 63.90 ± 0.04 Ma. These ages provide detailed calibration of Puercan through earliest Torrejonian land mammal ages and provide crucial calibration for the nonmarine K-P boundary and the Early Paleogene portion of the geomagnetic polarity time scale. -from Authors
Conference Paper
Coal beds, as much as 250 ft thick, and adjacent sandstones in the Paleocene Tongue River Member of the Fort Union Formation are reservoirs for coal-derived natural gas in the Powder River basin. The discontinuous coal beds were deposited in raised, ombrotrophic peat bogs about 3 mi{sup 2} in size, adjoining networks of fluvial channels infilled by sand. Coal-bed thickness was controlled by basin subsidence and depositional environments. The average maceral composition of the coals is 88% huminite (vitrinite), 5% liptinite, and 7% inertinite. The coals vary in rank from subbituminous C to A (R{sub o} values of 0.4 to 0.5%). Although the coals are relatively low rank, they display fracture systems. Natural gas desorbed and produced from the coal beds and adjacent sandstones is composed mainly of methane with lesser amount of Co{sub 2} ({lt}10%). The methane is isotopically light and enriched in deuterium. The gases are interpreted to be generated by bacterial processes and the fermentation pathway, prior to the main phase of thermogenic methane generation by devolatilization. Large amounts of bicarbonate water generated during early stages of coalification will have to be removed from the fracture porosity in the coal beds before desorption and commercial gas production can take place. Desorbed amounts of methane-rich, bacterial gas in the Powder River basin are relatively low ({lt}60 Scf/ton) compared to amounts of thermogenic coal-bed gases (hundreds of Scf/ton) from other Rocky Mountain basins. However, the total coal-bed gas resource in both the coal beds and the adjacent sandstones is considered to be large (as much as 40 Tcf) because of the vast coal resources (as much as 1.3 trillion tons).
Article
Paleomagnetic results have been obtained from the Late Cretaceous-early Tertiary igneous complexes of the north-central Montana alkalic province. Data from 94 sites in Eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82.0°N, 170.2°E (A95 = 3.5° k = 18.6), while 36 sites in Paleocene intrusions yield a paleomagnetic pole at 81.8°N, 181.4°E (A95 = 5.4° k = 20.2). These poles differ by only 1.6° and are not significantly different statistically. The 130-site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander (APW) during the period of magnetization of the igneous centers. Postmagnetization structural complications in these rocks are minimal. The presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration. The northcentral Montana data together with other early Tertiary, Cretaceous, and mid-Tertiary paleomagnetic results require modification of our earlier APW chronology [Diehl et al., 1980]. It now appears that APW relative to North America since the Early Cretaceous consists of a polar still-stand during much of the Cretaceous (120-75 m.y. B.P.), a period of rapid movement in latest Cretaceous time (75-65 m.y. B.P.), and a period of slow polar movement thereafter. The onset of this period of rapid APW correlates well with a major change in plate motions at ˜80 m.y. and the beginning of the Laramide Orogeny.
Article
Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geomagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene and Pliocene and with a new timescale for the Mesozoic.
Article
It is standard practice that a positive reversal test is claimed on the basis of inability to reject the hypothesis that two distributions share a common mean direction, and thus the claim of a positive reversal test is in fact often based on a lack of information. This is unsatisfactory. Therefore it is suggested that positive reversal tests should be classified according to the amount of information that was available for the test. This amount of information is readily indicated by the critical angle (e.g., at the 95 per cent confidence level) between the two sample mean directions at which the hypothesis of common mean direction for the distributions would be rejected. It is recommended that 5°, 10° and 20° be used as the breakpoints in the classification.
Article
USA. We analyzed paleo-magnetic samples from nine of the logged sections. The principal magnetic carrier in the Ludlow Member sediments is likely titanomaghemite, as indicated by predomi-nantly irreversible thermomagnetic curves measured from sandstone, siltstone, and carbonaceous shale samples. The analyzed paleomagnetic samples document a series of polarity zones that can be correlated from C29n to C27r on the geomagnetic polarity time scale (GPTS). We infer that the mag-netization of the samples is primary because the characteristic directions are consistent with those of the Paleocene of North Amer-ica, and the reversal stratigraphy from this section corresponds to the GPTS with rea-sonable sediment accumulation rates. By extrapolating the measured sediment accu-mulation rate from the Cretaceous-Tertiary (K-T) boundary to the top of C28n and then to the top of the Ludlow Member, we esti-mate the duration of the member to range from 2.31 to 2.61 m.y. This is the fi rst esti-mate for the duration and age of the Ludlow Member, and it can be used as an important tool for interpreting rates of biotic recovery after the K-T extinction.
Article
The classic, multivariate technique of principal component analysis can be used to find and estimate the directions of lines and planes of best least-squares fit along the demagnetization path of a palaeomagnetic specimen, thereby replacing vector subtraction, remagnetization circles and difference vector paths with one procedure. Eigenvalues from the analysis are the variance of the data along each principal axis, and provide a relative measure of collinearity or coplanarity which may be used to define a general palaeomagnetic precision index. Demagnetization planes found with principal component analysis may be used in place of difference vector paths for locating Hoffman—Day directions, avoiding unnecessary vector subtraction and intensity truncation steps. Two methods are discussed for jointly estimating an average remanence direction from demagnetization lines and planes.
Article
Cleaning is a term used in palaeomagnetism to refer to laboratory methods designed to demagnetize preferentially the less stable components of magnetization. Although the diversity of methods suggests that there is a wide choice available, this is illusory. Except in simple cases, the wrong cleaning strategy may yield misleading results. Cleaning methods include thermal, magnetic (alternating field or a.f.), low-temperature, chemical, shock and microwave cleaning. A cleaning strategy involves one or more cleaning methods applied in a specific sequence. The design of a strategy is made easier with the a priori knowledge of magnetic mineralogy and granulometry, or in other words, rock magnetic properties. Although not a substitute for other rock magnetic experiments, the variation of low-field susceptibility with temperature (k−T) not only provides information on magnetic mineralogy and granulometry, but it also draws attention to chemical alteration that may occur during thermal cleaning.
Article
A newly discovered Cretaceous-Tertiary (K-T) boundary locality in the western Powder River basin, Wyoming, is characterized by a palynologically defined extinction horizon, a fern-spore abundance anomaly, a strong iridium anomaly, and shock-metamorphosed quartz grains. Detailed microstratigraphic analyses show that about one third of the palynoflora (mostly angiosperm pollen) disappeared abruptly, placing the K-T boundary within a distinctive, 1- to 2-cm-thick claystone layer. Shocked quartz grains are concentrated at the top of this layer, and although fern-spore and iridium concentrations are high in this layer, they reach their maximum concentrations in a 2-cm-thick carbonaceous claystone that overlies the boundary claystone layer. The evidence supports the theory that the K-T boundary event was associated with the impact of an extraterrestrial body or bodies. Palynological analyses of samples from the K-T boundary interval document extensive changes in the flora that resulted from the boundary event. The palynologically and geochemically defined K-T boundary provides a unique time-line of use in regional basin analysis.
Article
The first complete cyclic sedimentary successions for the early Paleogene from drilling multiple holes have been retrieved during two ODP expeditions: Leg 198 (Shatsky Rise, NW Pacific Ocean) and Leg 208 (Walvis Ridge, SE Atlantic Ocean). These new records allow us to construct a comprehensive astronomically calibrated stratigraphic framework with an unprecedented accuracy for both the Atlantic and the Pacific Oceans covering the entire Paleocene epoch based on the identification of the stable long-eccentricity cycle (405-kyr). High resolution X-ray fluorescence (XRF) core scanner and non-destructive core logging data from Sites 1209 through 1211 (Leg 198) and Sites 1262, 1267 (Leg 208) are the basis for such a robust chronostratigraphy. Former investigated marine (ODP Sites 1001 and 1051) and land-based (e.g., Zumaia) sections have been integrated as well. The high-fidelity chronology is the prerequisite for deciphering mechanisms in relation to prominent transient climatic events as well as completely new insights into Greenhouse climate variability in the early Paleogene. We demonstrate that the Paleocene epoch covers 24 long eccentricity cycles. We also show that no definite absolute age datums for the K/Pg boundary or the Paleocene–Eocene Thermal Maximum (PETM) can be provided by now, because of still existing uncertainties in orbital solutions and radiometric dating. However, we provide two options for tuning of the Paleocene which are only offset by 405-kyr. Our orbitally calibrated integrated Leg 208 magnetostratigraphy is used to revise the Geomagnetic Polarity Time Scale (GPTS) for Chron C29 to C25. We established a high-resolution calcareous nannofossil biostratigraphy for the South Atlantic which allows a much more detailed relative scaling of stages with biozones. The re-evaluation of the South Atlantic spreading rate model features higher frequent oscillations in spreading rates for magnetochron C28r, C27n, and C26n.
Article
The palynologically defined Cretaceous-Tertiary boundary in the western interior of North America occurs at the top of an iridium-rich clay layer. The boundary is characterized by the abrupt disappearance of certain pollen species, immediately followed by a pronounced, geologically brief change in the ratio of fern spores to angiosperm pollen. The occurrence of these changes at two widely separated sites implies continentwide disruption of the terrestrial ecosystem, probably caused by a major catastrophic event at the end of the period.
Magnetostratigraphy and Biostratigraphy Correlations of Late Cretaceous to early Paleocene Strata between Alberta and North Dakota
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Astronomical calibration of the Paleocene time: Palaeogeography Palaeoclimatology Palaeoecology
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Westerhold, T., Rohl, U., Raffi, I., Fornaciari, E., Monechi, S., Reale, V., Bowles, J., and Evans, H. F., 2008, Astronomical calibration of the Paleocene time: Palaeogeography Palaeoclimatology Palaeoecology, v. 257, n. 4, p. 377– 403, http://dx.doi.org/10.1016/j.palaeo.2007.09.016