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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
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.
... 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). ...
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.
... 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]. ...
Intermediate composition titanohematite, Fe2-yTiyO3 with 0.5 ≤ y ≤ 0.7, is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature. While largely overlooked in magnetostratigraphic studies, intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of Late Mesozoic and Early Cenozoic Laramide clastic deposits throughout the central United States. Intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite, except its unique self-reversing property. Due to these similarities, and with detrital remanent magnetizations masking its self-reversing nature, intermediate titanohematite is often misidentified in sedimentary deposits. Past studies relied upon non-magnetic techniques including X-Ray diffraction and electron microprobe analysis. While these techniques can identify the presence of intermediate titanohematite, they fail to test whether the mineral is the primary recorder. To facilitate the identification of intermediate titanohematite in sedimentary deposits, we characterize this mineral using low-temperature magnetometry and high-temperature susceptibility experiments, and present a new identification technique based on titanohematite’s self-reversing property, for sediments that span the Cretaceous-Paleogene boundary (Hell Creek region, Montana). Results from the self-reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field, but that intermediate, self-reversing titanohematite (y = 0.53–0.63) is an important ancillary carrier of remanence. While earlier literature suggests that intermediate titanohematite is rare in nature, reanalysis using specialized rock magnetic techniques may reveal that it’s more abundant in the rock record, particularly within depositional basins adjacent to calc-alkaline volcanics, than previously thought.
... 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. ...
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.
... The Tongue River Member is 425 to 485 m thick in the Moorehead coal field, which includes this site (Bryson and Bass, 1973). The age of the Tongue River Member has been estimated based on paleomagnetic correlations to range from 62.8 to 61.5 Ma in sediments exposed at Signal Butte near Miles City ~115 km to the north (Peppe et al., 2011). The Newels Nook vertebrate fauna and associated fruit and seed collection from the level of the lowest coal of the Tongue River Member on the western edge of the Powder River Basin 85 km west-northwest of Horse Creek was judged to be lower Tiffanian, or about 60 Ma (Robinson and Honey, 1987;Huegele and Manchester, 2022). ...
We investigated leaf, cone and fruit impressions preserved in a siltstone deposit between coal seams in the late Paleocene Tongue River Member near Otter, southeast Montana, to assess the floristic diversity and composition. We document the presence of Taxodium based on seed cone scales as well as foliage branches and associated pollen cones, and a low-diversity angiosperm assemblage of about 15 genera dominated by Platanaceae (Platanus, Macginitiea, Macginistemon) and Cornales (Cornus, Davidia, Amersinia, Browniea), cf. Trochodendraceae (Zizyphoides, Nordenskioeldia), augmented by Aesculus, Trochodendroides, Ulmites and Porosia. Most of these genera were widespread in the Paleocene of the Northern Hemisphere. The low diversity of this florule is consistent with that of Tiffanian assemblages elsewhere in Montana and Wyoming. The leaf known as Phyllites demoresii Brown is proposed as a likely candidate to correspond with the extinct fruits called Porosia verrucosa (Lesquereux) Hickey emend. Manchester et Kodrul.
... Occurrence.-Highway Blowout Site, Fallon County, Montana (Tongue River Member, Fort Union Formation, Tiffanian- Clarkfordian, ∼62-60 Ma; Peppe et al., 2011). Precise coordinates are available to qualified researchers at YPM. ...
Ray-finned fishes comprise nearly half of extant vertebrate species and include several ancient lineages with fossil records that stretch over 200 Myr in time. One of these old clades, the sturgeons and paddlefishes, is distributed across the Northern Hemisphere and includes some of the largest known freshwater fishes. Yet, the fossil record of this lineage (Acipenseriformes) is poor compared to similarly ancient ray-finned fish clades. Here, I describe sturgeon fossils from two geological units in North America < 10 Myr younger than the Cretaceous-Paleogene boundary. Both come from individuals of ~1.5 m in length. These Paleogene forms establish the long history of large body size in Acipenseriforms and reveal sturgeons were some of the largest inhabitants of freshwater ecosystems that were still recovering from the Cretaceous-Paleogene extinction.
A 400 m-thick sequence characterized by prominent tabular sandstone beds and a significant amount of marl and limestone occurs in Paleocene strata of the northern part of the Bighorn Basin (Clarks Fork Basin) of Wyoming and Montana. These strata, currently designated as the Belfry Member of the Fort Union Formation, actually consist of two separate but related lithogenetic units. The lower unit, which includes the Belfry Member stratotype, shows a gradual upward increase in tabular sandstone and marl or limestone and is inferred to have been deposited on a drowning flood plain under paralacustrine conditions. The upper unit, here proposed as the Chance Member, is characterized by the presence of six asymmetrical, basin-wide cyclothems. Each cycle begins abruptly with a transgressive surface overlain by laterally extensive tabular sandstone, followed by a micrite-dominated interval that together represent the lacustrine phase of the cycle. These are succeeded by lenticular interbeds of mudstone and sandstone inferred to have been deposited as a prograding fluvio-deltaic and flood plain sequence. The cyclothems are of variable thickness, ranging from ∼30m in the lowest cycle to ∼10 m in the uppermost cycles. Detailed stratigraphic mapping and correlation with the paleomagnetic and vertebrate biostratigraphic framework for the Bighorn Basin places the entire Chance Member within a portion of one vertebrate zone, Ti4, of the middle Tiffanian Provincial Age (59.2 to 58.5 Ma). The variable thickness of the cycles points toward deposition during unequal time intervals and suggests a tectonic origin most likely related to episodic movement of faults bounding the Bighorn Basin.
Six fossil leaf species are described from impression fossils collected from the Paleocene Fort Union Formation in the Williston Basin in southwestern North Dakota, USA. They are Meliosma vandaelium sp. nov., Meliosma thriviensis sp. nov., Ternstromites paucimissouriensis sp. nov., Macginitiea nascens sp. nov., Dicotylophyllum horsecreekium sp. nov. and Dicotylophyllum han-sonium sp. nov. These species represent some of the elements of the Fort Union Formation that are biostratigraphically important megafloral zone taxa or are species that demonstrate an evolutionary relationship to floras from the Eocene Golden Valley Formation. Some of the species described here suggest that new species endemic to the Williston Basin evolved through the Paleocene and into the Eocene. If the pattern of the origination of endemic daughter species seen in the Williston Basin is consistent across the Western Interior basins of North America, it might have driven up gamma diversity through the Paleogene. This provides a possible explanation for the relatively steep vegetation diversity gradients seen in mid-and high-latitude pollen floras in North America during the late Paleocene and early Eocene.
Comparisons of Tertiary floras of North America with those of Europe and Asia document a long history of floristic interchange. The stratigraphic and geographic ranges of selected conifer and angiosperm genera that are easily recognized in the fossil record provide a basis for discerning patterns in the routes and timings of intercontinental dispersals through the Tertiary.
The warmest global temperatures of the Cenozoic Era occurred in early Eocene time, following a warming trend that started in late Paleocene time. The greater Green River Basin of southwestern Wyoming is one of the best areas in the Rocky Mountains for paleobotanical investigation of the Paleocene-Eocene climatic transition. Intensive sampling has resulted in the recovery of an estimated 189 species of plant macrofossils from the Tiffanian, Clarkforkian, Wasatchian, and Bridgerian land mammal "ages." The leaf morphologies and taxonomic affinities of these fossils were used in combination with other indicators to evaluate Paleocene-Eocene climates. Following cool humid conditions in the Tiffanian, the Clarkforkian was humid and subtropical, and several plant families with modern tropical affinities appeared. However, as in the Tiffanian, Clarkforkian floras had low diversity and were dominated by a single species in the birch family. Mean annual temperature (MAT) rose from an estimated 12 °C in the Tiffanian to 19 °C in the Clarkforkian, while mean annual precipitation (MAP) for the Tiffanian and Clarkforkian is estimated to have been 130-150 cm. Little fossil plant material is preserved from the latest Clarkforkian or the earliest Wasatchian, which is thought to have contained an interval of cooling and drying followed by renewed warming. By the middle Wasatchian, the time of the Cenozoic thermal maximum, the inferred MAT was about 21 °C, and the MAP was near 140 cm. A second influx of plant families with tropical affinities appeared in the area, and diversity increased significantly, but most plant families known from the Clarkforkian persisted. Species turnover from the Clarkforkian to the Wasatchian was greater than 80%. A second turnover of more than 80% of species (but not families) from the Wasatchian to the early Bridgerian accompanied drying and increased seasonality of precipitation. The early Bridgerian MAT is inferred to have been near 20 °C and the MAP to have been about 80 cm. Except for the Tiffanian and possibly portions of the early Wasatchian, paleoclimates during the study interval were predominantly frost free. Although the moderating influence of the Green River lake system has been suggested as a possible explanation for mild Eocene winters in Wyoming, this study shows that virtually frost-free climates existed in the area prior to and independent of significant lake development.
An unconformable relationship is observed within the Paleocene Fort Union Formation in the western Williston Basin at the
contact between the Tongue River Member and the underlying Lebo and Ludlow Members. Isotopic dates and pollen biozone data
reported here are integrated with previously published data. A new correlation of these facies results in a revised history
of localized depositional and tectonic events. One unconformity occurs at this lithological contact in the Pine Hills (PH),
Terry Badlands (TB), and Ekalaka (E) areas west of the Cedar Creek anticline (CCA), and another unconformity occurs at the
same lithological contact in the Little Missouri River (LMR) area east of the CCA. The two unconformities differ in age by
about two million years. The older is the U2 and the younger is the U3, which initially were recognized in the Ekalaka area of southeastern Montana (Belt et al., 2002). The U2 crops out in the TB, PH, and E areas, where at least 85 m of Tongue River strata bearing palynomorphs characteristic of biozone
P-3 are found above the unconformity. Radiometric dates from strata (bearing palynomorphs characteristic of biozone P-2) below
the U2 range in age from 64.0 to 64.73 Ma. The U2 unconformity west of the CCA thus occurs in strata near the base of the lower P-3 biozone.
The U3 crops out in the LMR area (east of the CCA), where only 13 m of strata characterized by the P-3 pollen biozone occur above
it. Radiometric dates from an ash <1 m above the U3 in that area range in age from 61.03 to 61.23 Ma, and the P-3/P-4 pollen biozone boundary is located 13 m above the ashes.
The U3 thus occurs in strata characterized by upper parts of the P-3 pollen biozone east of the CCA. The U3 is also identifiable in the middle of the ca. 200 m-thick Tongue River Member west of the CCA, where mammal sites 40 to 80
m above it are Tiffanian-3 in age. The strata below this unconformity are tilted gently to the northwest; strata above the
unconformity are flat lying. This mid Tongue River unconformity probably correlates with the unconformity at the base of the
Tongue River Member in the LMR area east of the CCA, where a Ti-2 mammal site (the “X–X” locality) occurs <10 m above it.
Depositional and tectonic events can be summarized using North American Mammal Age nomenclature as a relative time scale.
From latest Cretaceous through Puercan time, paleodrainage was toward the east or southeast, in the direction of the Cannonball
Sea. The Black Hills did not serve as an obstruction at that time. During early Torrejonian time, the Miles City arch (MCA)
and Black Hills were uplifted and partially eroded, leading to the U2 unconformity. When deposition resumed, paleodrainages shifted to a northeasterly course. During middle and late Torrejonian
time, facies of the lower Tongue River (“Dominy”) sequence and the Ekalaka Member of the Fort Union Formation were deposited
in the middle of a subbasin between the MCA and the CCA. Simultaneously, smectite-rich components of the Ludlow Member were
being deposited east of the CCA. During latest Torrejonian time, uplift of the Black Hills tilted the “Dominy” sequence toward
the northwest and local erosion led to the U3 unconformity. Following this tilting, during Tiffanian time, deposition of the upper Tongue River (“Knobloch”) sequence shows
continuity from western North Dakota across eastern Montana and into the northern Powder River Basin.
We present a quantitative analysis of megafloral turnover across the Cretaceous/Paleo- gene boundary (K/T) based on the most complete record, which comes from the Williston Basin in southwestern North Dakota. More than 22,000 specimens of 353 species have been recovered from 161 localities in a stratigraphic section that is continuous across and temporally calibrated to the K/T and two paleomagnetic reversals. Floral composition changes dynamically during the Cre- taceous, shifts sharply at the K/T, and is virtually static during the Paleocene. The K/T is associated with the loss of nearly all dominant species, a significant drop in species richness, and no subse- quent recovery. Only 29 of 130 Cretaceous species that appear in more than one stratigraphic level (non-singletons) cross the K/T. Only 11 non-singletons appear first during the Paleocene. The sur- vivors, most of which were minor elements of Cretaceous floras, dominate the impoverished Pa- leocene floras. Confidence intervals show that the range terminations of most Cretaceous plant taxa are well sampled. We infer that nearly all species with last appearances more than abou t5mb elow (approximately 70 Kyr before) the K/T truly disappeared before the boundary because of normal turnover dynamics and climate changes; these species should not be counted as K/T victims. Max- ima of last appearances occur from 5 t o3mb elow the K/T.Interpretation of these last appearances at a fine stratigraphic scale is problematic because of local facies changes, and megafloral data alone, even with confidence intervals, are not sufficient for precise location of an extinction horizon. For this purpose, we rely on high-resolution palynological data previously recovered from continuous facies in the same sections; these place a major plant extinction event precisely at the K/T impact horizon. Accordingly, we interpret the significant cluster of last appearances less tha n5mb elow the K/T as the signal of a real extinction at the K/T that is recorded slightly down section. A max- imum estimate of plant extinction, based on species lost that were present in the uppermost 5 m of Cretaceous strata, is 57%. Palynological data, with higher stratigraphic but lower taxonomic res- olution than the megafloral results, provide a minimum estimate of a 30% extinction. The 57% estimate is significantly lower than previous megafloral observations, but these were based on a larger thickness of latest Cretaceous strata, including most of a globally warm interval, and were less sensitive to turnover before the K/T. The loss of one-third to three-fifths of plant species sup- ports a scenario of sudden ecosystem collapse, presumably caused by the Chicxulub impact.
Field surveys of the Upper Cretaceous Hell Creek Formation in southwestern North Dakota since 1986 have produced a total of 10 124 specimens from 42 vertebrate microsites and an additional 41 dinosaur skulls or partial skeletons or skulls from separate sites representing 61 taxa of vertebrates dominated by fish, dinosaurs, turtles, and crocodilians. Common elements of this diverse fauna occur to within 2.37 m of the Cretaceous-Tertiary (K-T) boundary. The stratigraphically highest fossil in the study is a partial ceratopsian skeleton 1.77 m below the K-T boundary in the basal Fort Union Formation. All dinosaurs that occur at more than two sites also occur at the highest level that yielded more than 500 specimens (8.40 m below the K-T boundary). The fine-grained uppermost 2 m of the Hell Creek is nearly devoid of all fossils, including taxa known to occur in the overlying formation. The absence of channel deposits in this part of the formation may be the reason for the absence of fossil localities. The presence of marine-tolerant taxa suggests that the study area may have been adjacent to a previously unidentified seaway of latest Cretaceous age. Rarefac-tion analysis indicates no evidence for a decline in vertebrate diversity through the formation or dinosaurian diversity in the 3 m below the K-T boundary. Our results are not compatible with gradual vertebrate extinction at the end of the Cretaceous.
An unconformable relationship is observed within the Paleocene Fort Union Formation in the western Williston Basin at the contact between the Tongue River Member and the underlying Lebo and Ludlow Members. Isotopic dates and pollen biozone data reported here are integrated with previously published data. A new correlation of these facies results in a revised history of localized depositional and tectonic events. One unconformity occurs at this lithological contact in the Pine Hills (PH), Terry Badlands (TB), and Ekalaka (E) areas west of the Cedar Creek anticline (CCA), and another unconformity occurs at the same lithological contact in the Little Missouri River (LMR) area east of the CCA. The two unconformities differ in age by about two million years. The older is the U 2 and the younger is the U 3 , which initially were recognized in the Ekalaka area of southeastern Montana (Belt et al., 2002). The U 2 crops out in the TB, PH, and E areas, where at least 85 m of Tongue River strata bearing palynomorphs characteristic of biozone P-3 are found above the unconformity. Radiometric dates from strata (bearing palynomorphs characteristic of biozone P-2) below the U 2 range in age from 64.0 to 64.73 Ma. The U 2 unconformity west of the CCA thus occurs in strata near the base of the lower P-3 biozone. The U 3 crops out in the LMR area (east of the CCA), where only 13 m of strata characterized by the P-3 pollen biozone occur above it. Radiometric dates from an ash <1 m above the U 3 in that area range in age from 61.03 to 61.23 Ma, and the P-3/P-4 pollen biozone boundary is located 13 m above the ashes. The U 3 thus occurs in strata character-ized by upper parts of the P-3 pollen biozone east of the CCA. The U 3 is also identifiable in the middle of the ca. 200 m-thick Tongue River Member west of the CCA, where mammal sites 40 to 80 m above it are Tiffanian-3 in age. The strata below this unconformity are tilted gently to the northwest; strata above the unconformity are flat lying. This mid Tongue River unconformity probably correlates with the unconformity at the base of the Tongue River Member in the LMR area east of the CCA, where a Ti-2 mammal site (the "X-X" locality) occurs <10 m above it. Depositional and tectonic events can be summarized using North American Mammal Age nomenclature as a rela-tive time scale. From latest Cretaceous through Puercan time, paleodrainage was toward the east or southeast, in the direction of the Cannonball Sea. The Black Hills did not serve as an obstruction at that time. During early Torrejonian time, the Miles City arch (MCA) and Black Hills were uplifted and partially eroded, leading to the U 2 unconformity. When deposition resumed, paleodrainages shifted to a northeasterly course. During middle and late Torrejonian time, facies of the lower Tongue River ("Dominy") sequence and the Ekalaka Member of the Fort Union Formation were The present report is concerned mainly with Paleocene strata that crop out: (1) between the two Cretaceous-cored uplifts, the CCA and the MCA (SB, TB, PH, E, and 7-UP, Fig. 1); and (2) east of the CCA in the area of the Little Missouri River (LMR, Fig. 1). Evidence is presented first for an unconformity at the base of the Tongue River Member; this boundary also coincides with a major facies change (Fig. 2). Later, when bio-and chronostratigraphic evidence for dating is presented, it becomes apparent that the facies change and its associated deposited in the middle of a subbasin between the MCA and the CCA. Simultaneously, smectite-rich components of the Ludlow Member were being deposited east of the CCA. During latest Torrejonian time, uplift of the Black Hills tilted the "Dominy" sequence toward the northwest and local erosion led to the U 3 unconformity. Following this tilting, during Tiffanian time, deposition of the upper Tongue River ("Knobloch") sequence shows continuity from western North Dakota across eastern Montana and into the northern Powder River Basin.
Abundant plant and vertebrate fossils have been recovered from fluvial sediments deposited in the Bighorn Basin, Wyoming, during the first 13 m.y. of the Tertiary. Here we outline and discuss changes in the composition and diversity of floras and faunas during this period, which includes the recovery of terrestrial ecosystems from the K/T boundary extinctions, and later, during the Paleocene-Eocene transition, the greatest global warming of the Cenozoic.Floral diversity has been studied at three levels of spatial resolution: sub-local (at individual collecting sites), local (along a single bed or stratigraphic horizon), and basin-wide (regional). Sub-local diversity shows a moderate increase from the early to late Paleocene, followed by a decrease across the Paleocene/Eocene boundary, then an increase into the later early Eocene. Local heterogeneity was lower in Paleocene backswamp floras, although distinct groups of species dominated in different local fluvial settings such as backswamps and alluvial ridges. Heterogeneity of backswamp forests increased by about 65% from the early to late Wasatchian (early Eocene). The number of plant species inferred from the Bighorn Basin dataset rose gradually from the Puercan to an early Clarkforkian peak of about 40 species, declined sharply to about 25 species by the Clarkforkian/Wasatchian boundary, then rose through the Wasatchian to about 50 species.A regional analysis of mammalian genera shows high turnover and a rapidly increasing number of genera within a million years of the K/T boundary (10–50 genera), a slight decline to 40 genera by the early Clarkforkian, then an increase from 40 to 75 genera by the late Wasatchian.Our analyses found no major extinctions in mammals during the Paleocene and early Eocene in the Bighorn Basin, but a one-third decrease in the number of plant species at about the Paleocene/Eocene boundary. Rates of taxonomic turnover were much higher for mammals than plants. The diversity trends for plants and mammals show little congruence, implying that the two groups responded in a very different manner to post K/T extinction opportunities. There is also little congruence between plant diversity levels and change in mean annual temperature (MAT) as inferred from foliar physiognomy.
Empirical estimates of climate parameters such as mean annual temperature (MAT) are essential to describe both ancient climate and to ground truth climate model simulations of past climates. In terrestrial settings, no single proxy record is able to provide detailed temporal ...
In the Rocky Mountain region between central Montana and central New Mexico, sedimentologically isolated nonmarine basins were produced by basement deformation during the Laramide orogeny within the area formerly occupied by a broad Late Cretaceous foreland basin in which laterally continuous marine facies had accumulated previously. Laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere. Laramide basins include perimeter basins along the cratonic periphery of the arcuate Laramide province, axial basins along a north-south intramontane trend, and ponded basins located farther west closer to the overthrust belt. Twelve specific stratigraphic and sedimentologic criteria for the onset, duration, and termination of Laramide deformation allow the chronology of basin development to be inferred independently for each basin. Maastrichtian initiation of Laramide deformation was approximately synchronous throughout the Laramide province, but termination of Laramide deformation was systematically diachronous from north to south between early and late Eocene time. Widespread Eocene erosion surfaces truncate syntectonic Laramide sequences and are overlain by largely volcanic and volcaniclastic post-Laramide strata of Eocene age in the north and Oligocene age in the south. Fluvial depositional systems draining toward the Great Plains were dominant in perimeter and axial basins, but ponded basins were occupied at times by large lakes that served as regional sediment traps. Paleocene drainages from ponded basins also led eastward toward the continental interior, but partly interconnected lakes that developed within several ponded basins by mid-Eocene time were either closed hydrologically, or else they drained westward into the "Tyee" paleoriver of the Pacific Northwest.
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.
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.
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.
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.
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
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
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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
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
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).
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.
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.
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.
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.
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.
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.
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.
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.
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
- J F Lerbekmo
- K C Coulter
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- S L Wing
- J Alroy
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- K H Freeman
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Astronomical calibration of the Paleocene time: Palaeogeography Palaeoclimatology Palaeoecology
- T Westerhold
- U Rohl
- I Raffi
- E Fornaciari
- S Monechi
- V Reale
- J Bowles
- H F Evans
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