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Abrupt westerly deflections of north-south trending facies boundaries, the isopleth of (87sr/86Sr)° = 0.076, and arcuate structural trends of Paleozoic, Mesozoic, and Cenozoic age in the west-central Basin and Range Province, have been explained by (1) post-Early Jurassic oroflexural folding and shear, (2) the existence of a pre-Late Jurassic resistant block along the western continental margin, possibly representing late Precambrian breakup, and (3) regional flexure ('Mina deflection') prior to intrusion of Late Cretaceous plutons which crosscut flexure-related structural trends. Paleomagnetic data gathered from Late Cretaceous plutons and remagnetized metasedimentary rocks along and north of the northern margin of the zone of east-west structural trends are not in support of significant oroflexural folding (and attending major clockwise rotation) since Late Cretaceous time. The data (Water Canyon pluton: D = 1°, I = 57°, α95 = 4°, k = 9, N= 132; Gunmetal pluton: D = 354, I = 60, α95 = 6, k = 21, N = 29; East Garfield Hills pluton: D = 349, I = 74, α95 = 11, k = 14, N = 12; La Panta pluton: D = 22, I = 66, α95 = 10, k = 16, N = 13; Gillis Canyon pluton: D = 354, I = 50, α95 = 4, k = 20, N = 50; and folded units of the Excelsior Mountains: D = 309, I = 79, α95 = 7, k = 12, N = 37) are dispersed from expected Cretaceous directions for the Mina deflection region, but they do not conform to a model of major systematic regional clockwise rotation. The generally small deflections of unit means from expected directions could be explained by irregular components of Late Cenozoic tilting and/or rotation. The paleomagnetic data do not deny the possibility of pre-Late Cretaceous oroclinal bending. Nevertheless, regional structural analysis complimented by the paleomagnetic results indicate that the Mina deflection is in all likelihood a primary feature whose geometry reflects the late Precambrian fragmentation of western North America. Structures related to the accretion of allochthonous assemblages between Middle Devonian and Cretaceous time in this part of the Cordillera also reflect the primary nature of this feature. [ABSTRACT FROM AUTHOR]

The contact between the Daptocephalus to Lystrosaurus declivis (previously Lystrosaurus) Assemblage Zones (AZs) described from continental deposits of the Karoo Basin was commonly interpreted to represent an extinction crisis associated with the end-Permian mass-extinction event at ca. 251.901 ± 0.024 Ma. This terrestrial extinction model is based on several sections in the Eastern Cape and Free State Provinces of South Africa. Here, new stratigraphic and paleontologic data are presented for the Eastern Cape Province, in geochronologic and magnetostratigraphic context, wherein lithologic and biologic changes are assessed over a physically correlated stratigraphy exceeding 4.5 km in distance. Spatial variation in lithofacies demonstrates the gradational nature of lithostratigraphic boundaries and depositional trends. This pattern is mimicked by the distribution of vertebrates assigned to the Daptocephalus and L. declivis AZs where diagnostic taxa of each co-occur as lateral equivalents in landscapes dominated by a Glossopteris flora. High-precision U-Pb zircon (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) age results indicate maximum Changhsingian depositional dates that can be used as approximate tie points in our stratigraphic framework, which is supported by a magnetic polarity stratigraphy. The coeval nature of diagnostic pre- and post-extinction vertebrate taxa demonstrates that the L. declivis AZ did not replace the Daptocephalus AZ stratigraphically, that a biotic crisis and turnover likely is absent, and a reevaluation is required for the utilization of these biozones here and globally. Based on our data set, we propose a multidisciplinary approach to correlate the classic Upper Permian localities of the Eastern Cape Province with the Free State Province localities, which demonstrates their time-transgressive nature. [ABSTRACT FROM AUTHOR]

We report paleomagnetic results from the Jurassic to Lower Cretaceous continental sedimentary succession exposed in the eastern limb of the Los Yariguíes anticlinorium, Eastern Cordillera, Colombia. About 820 m of a strati-graphic section of the upper part of the Girón Group (Angostura del Río Lebrija and Los Santos Formations) was sampled to construct a magnetic polarity stratigraphy. A total of 199 independent samples that yield interpretable and acceptable data have a characteristic remanent magnetization component (ChRM) isolated between 400 °C and 680 °C in progressive thermal demagnetization. Demagnetization behavior and rock magnetic properties are interpreted to indicate that hematite is the principal magnetization carrier with a possible contribution by magnetite in some parts of the section. After tilt correction, 123 samples are of normal polarity (declination [D ] = 44.9°, inclination [/] = +9.7°, R = 110.64, k = 9.87, and α95 = 4.3°), and the other 76 accepted samples are of reverse polarity (D = 216.4°, I = -6.1°, R = 68.29, k = 9.72, and α95 = 5.5°). The statistical reversal test conducted on virtual geomagnetic poles is positive (class B). Based on paleontologic age estimates for the Cumbre and Rosablanca Formations, we assume a Berriasian age for the Los Santos Formation. The magnetostratigraphic data from the Girón Group strata are interpreted to suggest an age for the sampled part of the section between early Kimmeridgian and early Valanginian (ca. 157-139 Ma). The age of the Angostura del Río Lebrija Formation is estimated as between early Kimmeridgian and early Tithonian (ca. 157-146.5 Ma). The age of the Los Santos Formation is estimated between early Tithonian and early Valanginian (146.5-139.3 Ma). With our proposed, but nonunique, correlation with the Geomagnetic Polarity Time Scale, the Jurassic-Cretaceous boundary is interpreted to be located within the Los Santos Formation. The Girón Group is characterized by two periods of high (>8 cm/k.y.) and two periods of low (< 2 cm/k.y.) sedimentation rates. An inferred clockwise rotation of ~44°, based on paleomagnetic declination data from the Girón Group, is similar to rotation estimates reported in some previous studies in the general area, and this facet of deformation could be related to local and regional response to displacement along regional-scale strike-slip faults. [ABSTRACT FROM AUTHOR]

We report U-Pb age determinations of carbonate nodules from an in situ paleosol horizon in the Upper Permian Balfour Formation and from several horizons of pedogenic nodule conglomerate (PNC) in the Triassic Katberg Formation, Karoo Basin, South Africa, using laser ablation-inductively coupled plasma-mass spectrometry (LAICP-MS). The paleosol sample yields an age of 252 ± 3 Ma, which overlaps with a previous high-precision U-Pb zircon date from a volcanic ash deposit 2 m above the paleosol. This relationship demonstrates the reliability of using LA-ICP-MS dating techniques on terrestrial pedogenic calcite. Two PNC samples collected at the base of the Katberg Formation within the same sandstone unit yield ages of 255 ± 3 Ma and 251 ± 3 Ma. The age of 251 ± 3 Ma overlaps with the high-precision U-Pb zircon date below the PNC and is a maximum age estimate of deposition for the base of the Katberg Formation. Our results show that reworked nodules in the same concentrated conglomerate lag can be of different ages, but that similarly aged nodules are spatially associated. In addition, two PNC samples collected higher in the section yield ages of 249 ± 3 Ma and 241 ± 3 Ma, providing maximum depositional ages for the lower to middle Katberg Formation for the first time. We demonstrate that pedogenic carbonate nodules can be dated with meaningful precision, providing another mechanism for constraining the age of sedimentary sequences and studying events associated with the Permian-Triassic transition in the central Karoo Basin, even though the extinction boundary may not be preserved in this area. [ABSTRACT FROM AUTHOR]

The relative roles of tectonics and global climate in forming the hydroclimate for widespread eolian deposition remain controversial. Oligocene loess has been previously documented in the interior of western United States, but its spatiotemporal pattern and causes remain undetermined. Through new stratigraphic record documentation and data compilation, we reveal the time transgressive occurrence of loess beginning in the latest Eocene in the central Rocky Mountains, that expands eastward to the Great Plains across the Eocene- Oligocene transition (EOT). Our climate simulations show that moderate uplift of the southern North America Cordillera initiated drying in the Cordilleran hinterland and immediate foreland, forming a potential dust source and sink, and global cooling at the EOT expanded the drying and eolian deposition eastward by causing retreat of the North American Monsoon. Therefore, the eolian deposition reflects continental aridification induced both by regional tectonism and global climate change during the late Paleogene. [ABSTRACT FROM AUTHOR]

Paleointensity is one of the least determined parameters in geomagnetism, yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the Earth's core and lower mantle. We test two important hypotheses that affect our understanding of how paleointensity has changed in Earth's past: (1) the Mesozoic Dipole Low (MDL) hypothesis and (2) the inverse relationship between geomagnetic polarity reversal rate and paleointensity. We report paleointensity results determined using both the Coe modified Thellier method and the IZZI protocol on the medium-grained hornblende-biotite granite of the Middle Jurassic Shamrock batholith in the southern Singatse Range, western Nevada (USA). Previous zircon U-Pb dating gives an age of batholith emplacement at ca. 165.8 ± 0.4 Ma. This age estimate is coincident with a period of unusually high reversal frequency (~10 reversals/Ma) in the Middle Jurassic. Remanence in the Shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase. The laboratory unblocking temperature range for the well-defined thermal remanent magnetization is narrow, between ~540 and 575 °C, and median destructive fields are between 40 and 50 mT, thus we infer that the age of remanence acquisition is approximated by the U-Pb age estimate for pluton emplacement. Two sets of quality criteria were used to assess the paleointensity results. The first set yielded an 87% success rate while the second set using more stringent criteria resulted in only a 20% success rate. Mean paleointensity values for the Shamrock batholith using the "loose" and "strict" quality criteria are 34.0 ± 6.4 µT (N = 13) and 33.6 ± 9.6 µT (N = 3) respectively, with errors reported at one standard deviation. After correction for both remanence anisotropy and cooling rate, the mean values that pass "loose" and "strict" criteria are respectively 17.9 ± 2.7 µT and 17.5 ± 2.6 µT. The uncorrected and corrected paleointensity values for the "strict" estimates yield respective mean virtual dipole moments (VDMs) of 62.7 ± 18 ZAm² (Z = 1021) and 32.7 ± 4.9 ZAm². The uncorrected value is within error of the VDMs reported for plagioclase grains isolated from ca. 160 Ma basalts from Ocean Drilling Program Site 801, however these values have an absolute difference ranging from ~6 to 20 ZAm², while the corrected VDM value is more consistent with the estimated average Jurassic field strength of ~29 ZAm². Using a recent estimate for the long-term stable field strength of ~42 ZAm², our corrected values add further credence to both the MDL and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses. [ABSTRACT FROM AUTHOR]

The Upper Triassic Chinle Formation is a critical non-marine archive of low-paleolatitude biotic and environmental change in southwestern North America. The well-studied and highly fossiliferous Chinle strata at Petrified Forest National Park (PFNP), Arizona, preserve a biotic turnover event recorded by vertebrate and palynomorph fossils, which has been alternatively hypothesized to coincide with tectonically driven climate change or with the Manicouagan impact event at ca. 215.5 Ma. Previous outcrop-based geochronologic age constraints are difficult to put in an accurate stratigraphic framework because lateral facies changes and discontinuous outcrops allow for multiple interpretations. A major goal of the Colorado Plateau Coring Project (CPCP) was to retrieve a continuous record in unambiguous superposition designed to remedy this situation. We sampled the 520-m-long core 1A of the CPCP to develop an accurate age model in unquestionable superposition by combining U-Pb zircon ages and magnetostratigraphy. From 13 horizons of volcanic detritus-rich siltstone and sandstone, we screened up to ~300 zircon crystals per sample using laser ablation-inductively coupled plasma-mass spectrometry and subsequently analyzed up to 19 crystals of the youngest age population using the chemical abrasion-isotope dilution-thermal ionization mass (CA-ID-TIMS) spectrometry method. These data provide new maximum depositional ages for the top of the Moenkopi Formation (ca. 241 Ma), the lower Blue Mesa Member (ca. 222 Ma), and the lower (ca. 218 to 217 Ma) and upper (ca. 213.5 Ma) Sonsela Member. The maximum depositional ages obtained for the upper Chinle Formation fall well within previously proposed age constraints, whereas the maximum depositional ages for the lower Chinle Formation are relatively younger than previously proposed ages from outcrop; however, core to outcrop stratigraphic correlations remain uncertain. By correlating our new ages with the magnetostratigraphy of the core, two feasible age model solutions can be proposed. Model 1 assumes that the youngest, coherent U-Pb age clusters of each sample are representative of the maximum depositional ages and are close to (<1 Ma difference) the true time of deposition throughout the Sonsela Member. This model suggests a significant decrease in average sediment accumulation rate in the mid-Sonsela Member. Hence, the biotic turnover preserved in the mid-Sonsela Member at PFNP is also middle Norian in age, but may, at least partially, be an artifact of a condensed section. Model 2 following the magnetostratigraphic-based age model for the CPCP core 1A suggests instead that the ages from the lower and middle Sonsela Member are inherited populations of zircon crystals that are 1-3 Ma older than the true depositional age of the strata. This results in a model in which no sudden decrease in sediment accumulation rate is necessary and implies that the base of the Sonsela Member is no older than ca. 216 Ma. Independent of these alternatives, both age models agree that none of the preserved Chinle Formation in PFNP is Carnian (>227 Ma) in age, and hence the biotic turnover event cannot be correlated to the Carnian-Norian boundary but is rather a mid-Norian event. Our age models demonstrate the powers, but also the challenges, of integrating detrital CA-ID-TIMS ages with magnetostratigraphic data to properly interpret complex sedimentary sequences. [ABSTRACT FROM AUTHOR]

Abstract: Incremental heating of basaltic lava groundmass from the Albuquerque Volcanoes, New Mexico, and plagioclase from Ash D at Pringle Falls, Oregon, yield 40Ar/39Ar isochron ages of 218 ± 14 and 211 ± 13 ka, respectively. Sediments in which Ash D was deposited and the eight lava flows of the Albuquerque Volcanoes display excursional paleomagnetic data with virtual geomagnetic poles (VGPs) in the southern hemisphere and together with the statistically indistinguishable 40Ar/39Ar dates, establish that both sites record the Pringle Falls excursion. This excursion is also recorded by the 227 ± 8 ka Mamaku Ignimbrite, New Zealand, and by high deposition rate sediments at ODP site 919 in the North Atlantic Ocean that are dated astrochronologically at 209–207 ka. We propose that the names “Albuquerque” and “Jamaica” excursion be abandoned and that a radioisotopic age of 211 ± 13 ka be adopted for the Pringle Falls excursion, which is one of five globally expressed, well-documented excursions in marine sediment cores or dated by 40Ar/39Ar methods that took place from 220 to 30 ka. Together with at least five other well-dated excursions between 730 and 520 ka, some ten excursions define the Geomagnetic Instability Time Scale (GITS) for the Bruhnes Chron. Eight of these excursions have been dated using 40Ar/39Ar methods. If the temporal clustering of excursions in the GITS is not a sampling artefact, this suggests that the geodynamo is not intrinsically unstable, but rather that instability in the flow pattern of the outer core fluid flow causes the main dipole field to be considerably weakened such that it becomes unstable with a 200 to 300 ka recurrence interval. Excursions are an important part of dynamo behavior over geologic time scales that, in addition to reversals of the main dipole field, need to be fully considered when assessing whether theoretical and numerical simulations of the dynamo produce earth-like results. [Copyright &y& Elsevier]