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During the Cryogenian (720 to 635 Ma ago) Snowball Earth glaciations, ice extended to sea level near the equator. The cause of this catastrophic failure of Earth's thermostat has been unclear, but previous geochronology has suggested a rough coincidence of glacial onset with one of the largest magmatic episodes in the geological record, the Franklin large igneous province. U-Pb geochronology on zircon and baddeleyite from sills associated with the paleo-equatorial Franklin large igneous province in Arctic Canada record rapid emplacement between 719.86 ± 0.21 and 718.61 ± 0.30 Ma ago, 0.9 to 1.6 Ma before the onset of widespread glaciation. Geologic observations and (U-Th)/He dates on Franklin sills are compatible with major post-Franklin exhumation, possibly due to development of mafic volcanic highlands on windward equatorial Laurentia and increased global weatherability. After a transient magmatic CO2 flux, long-term carbon sequestration associated with increased weatherability could have nudged Earth over the threshold for runaway ice-albedo feedback.

Despite being a prominent continental-scale feature, the late Mesoproterozoic North American Midcontinent Rift did not result in the break-up of Laurentia, and subsequently underwent structural inversion. The timing of inversion is critical for constraining far-field effects of orogenesis and processes associated with the rift’s failure. The Keweenaw fault in northern Michigan (USA) is a major thrust structure associated with rift inversion; it places ca. 1093 Ma rift volcanic rocks atop the post-rift Jacobsville Formation, which is folded in its footwall. Previous detrital zircon (DZ) U-Pb geochronology conducted by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) assigned a ca. 950 Ma maximum age to the Jacobsville Formation and led researchers to interpret its deposition and deformation as postdating the ca. 1090–980 Ma Grenvillian Orogeny. In this study, we reproduced similar DZ dates using LA-ICP-MS and then dated 19 of the youngest DZ grains using high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS). The youngest DZ dated by CA-ID-TIMS at 992.51 ± 0.64 Ma (2σ) redefines the maximum depositional age of the Jacobsville Formation and overlaps with a U-Pb LA-ICP-MS date of 985.5 ± 35.8 Ma (2σ) for late-kinematic calcite veins within the brecciated Keweenaw fault zone. Collectively, these data are interpreted to constrain deposition of the Jacobsville Formation and final rift inversion to have occurred during the 1010–980 Ma Rigolet Phase of the Grenvillian Orogeny, following an earlier phase of Ottawan inversion. Far-field deformation propagated >500 km into the continental interior during the Ottawan and Rigolet phases of the Grenvillian Orogeny.

New geochronologic and paleomagnetic data from the North American Midcontinent Rift (MCR) reveal the synchronous emplacement of the Beaver River diabase, the anorthosite xenoliths within it, and the Greenstone Flow—one of the largest lava flows on Earth. A U-Pb zircon date of 1091.83 (Formula presented.) 0.21 Ma (2 (Formula presented.)) from one of the anorthosite xenoliths is consistent with the anorthosite cumulate forming as part of the MCR and provides a maximum age constraint for the Beaver River diabase. Paired with the minimum age constraint of a cross-cutting Silver Bay intrusion (1091.61 (Formula presented.) 0.14 Ma; 2 (Formula presented.)), these data tightly bracket the age of the Beaver River diabase to be 1091.7 (Formula presented.) 0.2 Ma (95% CI), coeval with the eruption of the Greenstone Flow (1091.59 (Formula presented.) 0.27 Ma; 2 (Formula presented.))—which is further supported by indistinguishable tilt-corrected paleomagnetic pole positions. Geochronological, paleomagnetic, mineralogical and geochemical data are consistent with a hypothesis that the Beaver River diabase was the feeder system for the Greenstone Flow. The large areal extent of the intrusives and large estimated volume of the volcanics suggest that they represent a rapid and voluminous ca. 1,092 Ma magmatic pulse near the end of the main stage of MCR magmatism.

A goal of undergraduate geoscience programs is to prepare students for the geoscience workforce. In order to understand the extent to which programs succeed at this goal, we identified desired workforce skills, compiled undergraduate geoscience program requirements, and mapped workforce skills onto faculty responses to the 2016 National Geoscience Faculty Survey categorized by respondent-provided course names for majors-level courses (n = 1037). We used a Bayesian statistical model to determine the posterior probability of students practicing desired workforce skills in undergraduate geoscience programs, using program profiles as the model prior, conditioned by the likelihood of a student practicing workforce skills in each course category based upon survey responses. Probabilistic Monte Carlo simulations of students in each example program were used to calculate the likely number of courses in which a student practiced each skill. Activities to help students develop skills related to geologic reasoning, working as part of a team, quantitative skills (algebra), applying skills in new scenarios, evaluation of scientific literature, temporal thinking, spatial thinking, written communication, and managing uncertainty are frequently encountered by students; on average, students encounter these activities at least seven times during an undergraduate program. In contrast, students encounter activities to help them develop an understanding of societal relevance and systems thinking, on average, fewer than three times during an undergraduate program. Our results provide a snapshot, via instructors' reporting, of the state of workforce skill development across undergraduate degree programs, and both the approach and data set can be built upon for future studies.

The Mascall fauna is a well-known middle Miocene (Barstovian) mammalian assemblage in the Pacific Northwest. It has been collected for over 100 years and collecting intensity has increased since the establishment in 1975 of a national monument enclosing the type area of the formation. Despite its importance to biostratigraphy, biogeography of Barstovian taxa, and paleoecological studies, the fauna at the type locality has not been taxonomically examined in more than 50 years. Evaluation and classification of the stratigraphy of the Mascall Formation (Bestland et al. 2008) has prompted a faunal revision in order to place taxa within the new stratigraphic framework. Here we report on the fauna from the type area of the Mascall Formation in central Oregon, and conclude that 20 taxa are new to the fauna, and several taxa previously assigned to distinct species are synonymized. We also place specimens and taxa within a robust stratigraphic framework, calibrated with new U-Pb radioisotopic ages for the Mascall Tuff (15.122±0.017 Ma), the most fossiliferous layer in the formation, and the Kangaroo Tuff (13.564±0.018 Ma), the upper most tuff in the formation.

Sedimentary deposits north of the western Snake River Plain host Idaho's first and only producing oil and gas field. They consist of the lower to middle Miocene Payette Formation, the middle to upper Miocene Poison Creek and Chalk Hills Formations, and the Pliocene to lower Pleistocene Glenns Ferry Formation. Using new geochronology, palynomorph biostratigraphy, and geologic mapping, we connect updip surface features to subsurface petroleum play elements. The Payette Formation is a likely main source of the hydrocarbons, and acts as one of the reservoirs in the unnamed basin. Here, we redefine the Payette Formation as 0 to ~3,500 ft (0 to ~1,000 m) of mudstone, with lesser amounts of sandstone overlying and interbedded with the Columbia River Basalt Group and Weiser volcanic field. Index palynomorphs, including Liquidambar and Pterocarya, present in Idaho during and immediately after the middle Miocene climatic optimum, and new U--Pb ages of 16.39 and 15.88 Ma, help establish the thickness and extent of the formation. For the first time, these biostratigraphic markers have been defined for the oil and gas wells. The Poison Creek Formation is sandstone interbedded with mudstone that is ~800-1,800 ft (250-550 m) thick. The Chalk Hills Formation is a tuffaceous siltstone, claystone, and sandstone that is as much as ~4,200 ft (1,280 m) thick. New U--Pb ages are 10.1, 9.04, and 9.00 for the Poison Creek Formation, along with maximum depositional ages of 10.7 to 9.9 Ma for four samples from the Poison Creek Formation. A single U--Pb age of 7.78 Ma was determined from pumice low in the Chalk Hills Formation. Like the Payette Formation, the Poison Creek Formation can be a reservoir, whereas the Chalk Hills Formation acts as a sealing mudstone facies. The overlying sandstone, siltstone, and conglomerate of the Glenns Ferry Formation act as the overburden to the petroleum system in the subsurface, and were important for burial and hydrocarbon maturation. The Glenns Ferry Formation is up to 500 ft (150 m) thick in the study area, as much has been eroded. Whereas the Payette and Poison Creek Formations were deposited during the mid-Miocene climatic optimum amongst and above volcanic flows, the Chalk Hills and Glenns Ferry Formations were deposited within ancient Lake Idaho during an overall increase in aridity and cooling after the mid-Miocene climatic optimum. [ABSTRACT FROM AUTHOR]

Relating stratigraphic position to numerical time using age-depth models plays an important role in determining the rate and timing of geologic and environmental change throughout Earth history. Astrochronology uses the geologic record of astronomically derived oscillations in the rock record to measure the passage of time and has proven a valuable technique for developing age-depth models with high spatial-temporal resolution. However, in the absence of anchoring dates, many astrochronologies float in numerical time. Anchoring these chronologies relies on radioisotope geochronology (e.g., U-Pb, 40Ar/39Ar), which produce high-precision, randomly distributed point estimates of age. In this study, we present a new R package, astroBayes for a Bayesian inversion of astrochronology and radioisotopic geochronology into age-depth models. Integrating both data types allows reduction in uncertainties related to interpolation between dated horizons, and the resolution of subtle changes in sedimentation rate, especially when compared to existing Bayesian models that use a stochastic random walk to approximate sedimentation variability. The astroBayes inversion also incorporates prior information about sedimentation rate, superposition, and the presence/ absence of major hiatuses. The resulting age-depth models preserve both the spatial resolution of floating astrochronologies, and the accuracy and precision of modern radioisotopic geochronology. We test the astroBayes method using two synthetic data sets designed to mimic real-world stratigraphic sections. Model uncertainties are relatively constant with depth, and predominantly controlled by the precision of the radioisotopic dates and are significantly reduced relative to "dates-only" random walk models. Since the resulting age-depth models leverage both astrochronology and radioisotopic geochronology in a single statistical framework they can resolve ambiguities between the two chronometers. Finally, we present a case study of the Bridge Creek Limestone Member of the Greenhorn Formation where we refine the age of the Cenomanian-Turonian Boundary. [ABSTRACT FROM AUTHOR]

The geologically rapid appearance of fossils of modern animal phyla within Cambrian strata is a defining characteristic of the history of life on Earth. However, temporal calibration of the base of the Cambrian Period remains uncertain within millions of years, which has resulted in mounting challenges to the concept of a discrete Cambrian explosion. We present precise zircon U–Pb dates for the lower Wood Canyon Formation, Nevada. These data demonstrate the base of the Cambrian Period, as defined by both ichnofossil biostratigraphy and carbon isotope chemostratigraphy, was younger than 533 Mya, at least 6 My later than currently recognized. This new geochronology condenses previous age models for the Nemakit–Daldynian (early Cambrian) and, integrated with global records, demonstrates an explosive tempo to the early radiation of modern animal phyla. [ABSTRACT FROM AUTHOR]

Abstract: Alkaline rocks and carbonatites, including nepheline syenites, are well established as mantle-derived magmatic products, but the nature and location of their mantle sources is debated. Some workers have used [...]