Reduced plate motion controlled timing of Early Jurassic Karoo-Ferrar large igneous province volcanism

Past Large igneous province (LIP) emplacement is commonly associated with mantle plume upwelling and led to major carbon emissions. One of Earth’s largest past environmental perturbations, the Toarcian oceanic anoxic event (T-OAE; ~183 Ma), has been linked to Karoo-Ferrar LIP emplacement. However, the role of mantle plumes in controlling the onset and timing of LIP magmatism is poorly understood. Utilizing global plate reconstruction models and Lower Toarcian sedimentary mercury (Hg) concentrations, we demonstrate (1) that the T-OAE occurred coevally with Karoo-Ferrar emplacement, and (2) that timing and duration of LIP emplacement was governed by reduced Pangean plate motion, associated with a reversal in plate-movement direction. This new model mechanistically links Earth’s interior and surficial processes, and the mechanism is consistent with the timing of several of the largest LIP volcanic events throughout Earth history, and thus the timing of many of Earth’s past global climate change and mass extinction events

Glacioeustasy during the middle Eocene? Insights from the stratigraphy of the Hampshire Basin, UK

The sedimentary strata of the Hampshire Basin constitute some of the best-preserved Palaeogene sequences worldwide, and include the traditional [`]unit stratotype' for the Bartonian (~ 36.9-41.4 Ma). The Barton Clay Formation at Alum Bay on the Isle of Wight (IOW) was studied to assess the evidence for middle Eocene sea-level variation in records of grain size, sediment properties, faunal assemblage, foraminiferal diversity indices and foraminiferal stable isotopes. Sedimentary cycles of 1-10 Myr (third order) and 0.2-0.5 Myr (fourth order) duration are reported and interpreted to reflect ~ 20-60 metre variations in water depth. Additionally, an integrated magneto-bio-chemostratigraphical age model for the succession at Alum Bay is presented and new and published litho- and bio-stratigraphical markers are used to correlate additional successions. Based on this age model, it appears that during the late Lutetian and early Bartonian (~ 42-38 Ma), water depth variation identified within the basin was synchronous. Sedimentary and fossil evidence supports episodic uplift in the eastern part of the Hampshire Basin during the Bartonian, which at present precludes the calculation of eustatic sea-level. However, the amplitude and frequency of water-depth variations identified in the Barton Clay Formation, and correlations to published sea-level curves, are consistent with a component of these changes being glacioeustatic during the middle Eocene. There is also evidence for a large excursion ([delta]18O > 1[per mille sign]) in the mono-specific benthic foraminiferal oxygen-isotope record (Alum Bay) ~ 39.9 Ma, which is correlated to the isotope excursion at the [`]middle Eocene climatic optimum' previously reported in the Southern Ocean, and other localities. A contemporaneous water-depth increase of ~ 40 m at Alum Bay may indicate that a component of this [`]global' oxygen-isotope excursion results from a reduction in continental ice storage

Initial results of coring at Prees, Cheshire Basin, UK (ICDP JET project): towards an integrated stratigraphy, timescale, and Earth system understanding for the Early Jurassic.

International audienceDrilling for the International Continental Scientific Drilling Program (ICDP) Early Jurassic Earth System and Timescale project (JET) was undertaken between October 2020 and January 2021. The drill site is situated in a small-scale synformal basin of the latest Triassic to Early Jurassic age that formed above the major Permian–Triassic half-graben system of the Cheshire Basin. The borehole is located to recover an expanded and complete succession to complement the legacy core from the Llanbedr (Mochras Farm) borehole drilled through 1967–1969 on the edge of the Cardigan Bay Basin, North Wales. The overall aim of the project is to construct an astronomically calibrated integrated timescale for the Early Jurassic and to provide insights into the operation of the Early Jurassic Earth system. Core of Quaternary age cover and Early Jurassic mudstone was obtained from two shallow partially cored geotechnical holes (Prees 2A to 32.2 m below surface (m b.s.) and Prees 2B to 37.0 m b.s.) together with Early Jurassic and Late Triassic mudstone from the principal hole, Prees 2C, which was cored from 32.92 to 651.32 m (corrected core depth scale). Core recovery was 99.7 % for Prees 2C. The ages of the recovered stratigraphy range from the Late Triassic (probably Rhaetian) to the Early Jurassic, Early Pliensbachian (Ibex Ammonoid Chronozone). All ammonoid chronozones have been identified for the drilled Early Jurassic strata. The full lithological succession comprises the Branscombe Mudstone and Blue Anchor formations of the Mercia Mudstone Group, the Westbury and Lilstock formations of the Penarth Group, and the Redcar Mudstone Formation of the Lias Group. A distinct interval of siltstone is recognized within the Late Sinemurian of the Redcar Mudstone Formation, and the name “Prees Siltstone Member” is proposed. Depositional environments range from playa lake in the Late Triassic to distal offshore marine in the Early Jurassic. Initial datasets compiled from the core include radiography, natural gamma ray, density, magnetic susceptibility, and X-ray fluorescence (XRF). A full suite of downhole logs was also run. Intervals of organic carbon enrichment occur in the Rhaetian (Late Triassic) Westbury Formation and in the earliest Hettangian and earliest Pliensbachian strata of the Redcar Mudstone Formation, where up to 4 % total organic carbon (TOC) is recorded. Other parts of the succession are generally organic-lean, containing less than 1 % TOC. Carbon-isotope values from bulk organic matter have also been determined, initially at a resolution of ∼ 1 m, and these provide the basis for detailed correlation between the Prees 2 succession and adjacent boreholes and Global Stratotype Section and Point (GSSP) outcrops. Multiple complementary studies are currently underway and preliminary results promise an astronomically calibrated biostratigraphy, magnetostratigraphy, and chemostratigraphy for the combined Prees and Mochras successions as well as insights into the dynamics of background processes and major palaeo-environmental changes