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The Fisher Valley basin (FVB), located adjacent to the Onion Creek salt diapir, Paradox basin, Utah, USA, constitutes one of the thickest collections of Quaternary sediments within the Colorado Plateau. These sediments are important for constraining regional paleoclimate environments as well as recent tectonic movement of the Onion Creek salt diapir. Here, we combine magnetic susceptibility data with previously published age constraints (Bishop Tuff and Lava Creek B ash) into a cyclostratigraphic analysis of these sediments. We present a refined and astronomically tuned age model that demonstrates that deposition of the upper basin fill was between ca. 765 ka and 212 ± 8 ka. Correlating this chronologic model to environmental magnetic proxies, we show that from ca. 765 ka to ca. 535 ka, magnetic mineral assemblages deposited during glacials were characterized by generally finer grain sizes (elevated χARM/χlow) than during interglacials. These data are consistent with glacial periods being characterized by either wetter conditions amenable to pedogenesis, or drier conditions associated with increased concentrations of windblown dust. Interglacials are characterized by generally coarser magnetic grain sizes (lower χARM/χlow), consistent with periods of episodic alluvial and colluvial deposition in the FVB. At ca. 535 ka, χARM/χlow reach their lowest value (coarsest magnetic grain size) and then begin a progressive transition to higher values, consistent with a generally fining upward stratigraphic sequence throughout the rest of the section. This transition at ca. 535 ka coincides with a peak in sediment accumulation rate of ∼19 cm/k.y. and is most plausibly linked to halokinetic activity of the nearby Onion Creek salt diapir. Thus, although sediments in the FVB appear to be sensitive to global climate patterns between 765 ka and ca. 535 ka, local tectonic processes appear to episodically obscure this sensitivity. 

Abstract Nepheline syenites from the ∼1.2 Ga Ilímaussaq Complex of southern Greenland are examined to assess the utility of anisotropy of magnetic susceptibility (AMS) fabrics as proxies for silicate petrofabrics. Mineral lamination is a relatively common structural feature in cumulate rocks, including in the Ilímaussaq intrusion, but there is little consensus on the process (or processes) responsible for its formation. The Ilímaussaq AMS data are combined with rock magnetic experiments and electron backscatter diffraction (EBSD) measurements to characterize the magnetic mineralogy and compare the magnetic fabrics obtained to the silicate petrofabric. The data show that Na-amphibole (arfvedsonite) is most likely the dominant control on the AMS fabrics in the coarse-grained nepheline syenites (referred to as kakortokites), and that the AMS fabric is inverse relative to the observed silicate fabric. The EBSD data for a kakortokite sample suggests that the petrofabric is defined by arfvedsonite and is wholly planar, with evidence of only weak cross-lineation of c axes. The fine-grained nepheline syenites (lujavrites), two of which have a well-developed lamination carried by Na-pyroxene (aegirine), appear to have composite AMS fabrics that are considered to be a consequence of a mixed aegirine (normal) and arfvedsonite (inverse) response. The combined datasets shed light on the mechanisms of fabric acquisition in both lithologies. In the kakortokites, the AMS fabrics and silicate crystallographic preferred orientations, as well as the lack of observed microstructural evidence for subsolidus intra-crystal deformation, support models invoking gravitationally controlled crystal mats in the development of the macro-rhythmic layering of these rocks. In the lujavrites, the strong planar fabrics revealed by both the AMS and EBSD datasets, with some evidence of subsolidus deformation, point to fabric formation and perhaps even aegirine crystallization at the postcumulus stage. The combination of EBSD and AMS fabric datasets is a powerful means of deciphering the processes responsible for mineral alignment in igneous cumulates. 

Ancient magnetization(s), often recorded by hematite (Fe₂O₃), provide key paleomagnetic constraints on plate interactions through time. Primary remanent magnetizations may be modified or overprinted by secondary processes that complicate interpretations of paleomagnetic data. Hematite (U‐Th)/He (hematite He) dating has the potential to resolve when secondary magnetizations were acquired. Here, we compare hematite He data and paleomagnetic results in Paleoproterozoic crystalline rocks, meters below a major nonconformity in the Colorado Front Range, USA. Prior work and new rock magnetic data indicate that pervasive hematite alteration records a secondary chemical remanent magnetization (CRM) during the Permo‐Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountain orogeny. We target minor hematite‐coated faults cutting basement for (U‐Th)/He analyses because they are of sufficient hematite purity to yield geologically meaningful dates. Two samples yield overlapping and scattered individual hematite He dates ranging from ∼138 to 27 Ma (n = 33), significantly younger than the age of the late Paleozoic CRM. Scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy indicate that aliquots have variable grain size distributions and fluorocarbonate impurities. Thermal history models support hematite on fault surfaces mineralized coeval with CRM acquisition during the late Paleozoic, and hematite He data scatter reflects variable He loss during Mesozoic burial owing to differences in grain size distribution from fault slip comminution and in chemistry among aliquots. Our results underscore the differences in temperature sensitivity and sampling requirements between paleomagnetic and hematite He investigations and illustrate that hematite He dates will usually be younger than preserved remanent magnetizations. 

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 (LA-ICP-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. 

The Whitehorse Group and Quartermaster Formation are extensive red-bed terrestrial sequences representing the final episode of sedimentation in the Palo Duro Basin in north-central Texas, U.S.A. Regionally, these strata record the culmination of a long-term regression sequence beginning in the middle to late Permian. The Whitehorse Group includes beds of abundant laminated to massive red quartz siltstone to fine sandstone and rare dolomite, laminated to massive gypsum, and claystones, as well as diagenetic gypsum. The Quartermaster Formation exhibits a change from nearly equal amounts of thin planar and lenticular fine sandstone and laminated to massive mudstone in its lower half to overlying strata with coarser-grained, cross-bedded sandstones indicative of meandering channels up to 7 m deep and rare overbank mudstones. Paleosols are absent in the Upper Whitehorse Group and only poorly developed in the Quartermaster Formation. Volcanic ash-fall deposits (tuffs) present in uppermost Whitehorse Group and lower Quartermaster Formation strata permit correlation among five stratigraphic sections distributed over ∼150 km and provide geochronologic age information for these rocks. Both the Whitehorse Group and Quartermaster Formation have traditionally been assigned to the late Permian Ochoan (Changhsingian) stage, and workers assumed that the Permian-Triassic boundary is characterized by a regionally significant unconformity. Chemostratigraphic or biostratigraphic evidence for this age assignment, however, have been lacking to date. Single zircon U-Pb CA-TIMS analyses from at least two distinct volcanic ash fall layers in the lower Quartermaster Formation, which were identified and collected from five different localities across the Palo Duro Basin, yield interpreted depositional ages ranging from 252.19 ± 0.30 to 251.74 ± 0.28 Ma. Single zircon U-Pb CA-TIMS analyses of detrital zircons from sandstones located only a few meters beneath the top of the Quartermaster Formation yield a range of dates from Mesoproterozoic (1418 Ma) to Middle Triassic (244.5 Ma; Anisian), the latter of which is interpreted as a maximum depositional age, which is no older than Anisian, thus indicating the Permian-Triassic boundary to lie somewhere within the lower Quartermaster Formation/upper Whitehorse Group succession. Stable carbon isotope data from 180 samples of early-burial dolomicrite cements preserve a chemostratigraphic signal that is similar among sections, with a large ∼−8‰ negative isotope excursion ∼20 m beneath the Whitehorse Group-Quartermaster Formation boundary. This large negative carbon isotope excursion is interpreted to be the same excursion associated with the end-Permian extinction and this is in concert with the new high precision radioisotopic age data presented and the fact that the excursion lies within a normal polarity stratigraphic magnetozone. Dolomite cement δ 13 C values remain less negative (between about −5 and −8 permil) into the lower part of the Quartermaster Formation before becoming more positive toward the top of the section. This long interval of negative δ 13 C values in the Quartermaster Formation is interpreted to represent the earliest Triassic (Induan) inception of biotic and ecosystem “recovery.” Oxygen isotope values of dolomicrite cements show a progressive trend toward more positive values through the boundary interval, suggesting substantially warmer conditions around the end-Permian extinction event and a trend toward cooler conditions after the earliest Triassic. Our observations on these strata show that the paleoenvironment and paleoclimate across the Permian-Triassic boundary in western, sub-equatorial Pangea was characterized by depositional systems that were not conducive to plant preservation.