Abstract The Oi2b isotope event is a significant Late Oligocene global cooling period for which physical evidence and knowledge are scarce. It is represented by the glaciomarine Polonez Cove Formation (PCF) on King George Island (South Shetland Islands), the oldest and most intensively studied Cenozoic glacial deposit exposed on Antarctica. The PCF is overlain by the much less well-studied, slightly younger Boy Point Formation (BPF), which also preserves vital environmental information. The lithofacies are dominated by phases of coeval volcanism. Autogenic and allogenic processes are identified for the first time and used to determine the history of both formations. The PCF documents the ice terminus, its retreat and sub-polar thermal regime. The recession was followed by isostatic uplift, then by continuous sea level rise due to eustasy. The South Shetland Islands supported an ice dome probably detached from another on the Antarctic Peninsula and separated by at least seasonally open water (proto-Bransfield Strait). The existence of open water in the Weddell Sea explains the presence of erratics dumped by icebergs sourced from East Antarctica and the Antarctic Peninsula, and the study has enabled the configuration of Oi2b ice across Antarctica to be reconstructed for the first time. Rapid uplift terminated marine deposition, helping terrestrial ice to accumulate during a hitherto-unrecognized advance on King George Island. Wet-based ice of a further, slightly younger glacial was probably responsible for deep erosion that truncated the BPF and PCF, cutting >200 m down to present sea level.
Abstract Deccan volcanism in India represents one of the major continental flood basalt provinces on Earth. The drill holes of Continental Scientific Deep Drilling Project at Koyna in southern Deccan Province are unique in sampling a continuous subsurface stratigraphic record of the basalt down to basement. Element geochemistry of the 1251 m thick KBH-7 drill hole sequence shows that geochemistry alone cannot reliably demarcate discrete eruptive phases. However, geochemical logs of incompatible trace-element patterns and ratios, combined with multivariate statistical analysis, establish a subsurface succession comprising the Mahabaleshwar, Ambenali and Poladpur formations of the Wai Subgroup and the Bushe and Khandala formations of the Lonavala Subgroup directly overlying Archean basement. The older Kalsubai Subgroup is absent in the drill hole and this provides subsurface evidence for southward overstepping of older formations by younger formations in the Deccan Province. Palaeomagnetic inclination data from KBH-7 and nearby KBH-8 drill hole identify eight directional groups; each directional group may have erupted in short span of time (decades to centuries) relative to speed of secular variation. Considering the inferred lava-production rates, relative thicknesses of directional-group and non-directional lava sequences and the estimates of time lapsed for intermittent red boles formation, the main phase of Deccan eruption was likely to have occurred within a few thousand years (<4000–5000 years), in discrete phases, over a cumulative timescale of ∼200–300 ka.
Abstract The uppermost Triassic Cotham Member of the Lilstock Formation preserves two regionally extensive microbialite horizons – the well-known stromatolitic Cotham Marble and the older, largely overlooked Estheria Bed – offering new insights into environmental change during the Rhaetian. Integrated field, facies and literature review data demonstrate that both horizons are far more geographically widespread than previously recognized, extending across multiple depocentres in southwest England and the English Midlands. The Estheria Bed comprises thrombolitic–stromatolitic bioherms with variable mesostructures, including vugs, grainstone infill and locally preserved lamination, indicating fluctuating hydrodynamic and early diagenetic conditions. The Cotham Marble exhibits a consistent architecture of laminated stromatolites and columnar branching forms, initiated by shallow burrows or borings and variably accompanied by Microtubus microfossils. Combined evidence from microbial fabrics, associated fauna, sedimentary structures, geochemistry and biomarker studies indicates deposition within a laterally extensive, shallow freshwater–brackish lagoon at least 9500 km 2 in area. This lagoon experienced two mudstone-dominated packages culminating in microbialite formation. Together, the Cotham Marble and Estheria Bed provide useful horizons for regional correlation, revealing a dynamic lagoonal system sensitive to tectonic and hydrological change.
Abstract A sample of an elongated, hollow, tube-like fulgurite (labelled NC-CH) was collected in the shorelines’ sand of Outer Banks, NC (USA). The goal of this study is to shed light on the origin of this material and phase transformations induced by lightning discharge. X-ray powder diffraction (XRPD) coupled with Rietveld quantitative phase analysis of the surrounding sand (NC-sand) identified quartz, plagioclase, biotite, cordierite and calcite. XRPD analysis of NC-CH revealed stishovite, absent in NC-sand, unequivocally confirmed through high-resolution XRPD and Rietveld quantitative phase analysis. The fulgurite exhibits a high X-ray amorphous abundance (85.3(4) wt%) compared to the sand (49.3(2) wt%), indicating melting and partial melting of primary minerals such as quartz, plagioclase, phyllosilicates and cordierite. Scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) revealed well-crystallized pyrite clusters (<1 µm) in NC-CH. Pyrite precipitation likely resulted from reducing conditions during sulfidation of ferromagnesian silicates in the presence of H 2 S fluids derived from hydrocarbons, water vapour and organosulfur compounds from burned organic matter. By investigating shock-impacted minerals at the nanometric scale, this study provides new insights into physicochemical constraints influencing mineral formation and phase transitions during extreme events such as lightning strikes or meteor impacts.
Abstract Eo-Alpine metamorphism in the Western Carpathians is mostly recorded by low-to-medium grade rocks of the Veporic Superunit. However, the multimethod geochronological data from this key unit to understand Alpine evolution of the Western Carpathians are scarce. To better understand the timing of burial (subduction), peak conditions and post-peak exhumation, we applied in situ garnet Lu/Hf and mica Rb/Sr geochronology. Garnet yielded a somewhat imprecise, yet valid and geologically meaningful, age of ∼116 Ma interpreted as a record of subduction. In contrast, biotite Rb/Sr ages of ∼91 Ma and ∼75 Ma record resetting in the near-peak conditions upon the earliest stages of exhumation and fluid-aided re-equilibration, respectively. A white mica Rb/Sr dating also reflects Alpine disturbance of the isotopic system. The geochronologic data presented herein, together with the other already published geochronologic results, allow us to formulate a coherent subduction-exhumation model for the Veporic Superunit in an intracontinental setting. The upper plate for this subduction system would be a continental block belonging to Adria. We postulate that the rocks in question were subducted due to prior thinning of the continental crust, much as in the Eastern Alps, where the Lower Central Austroalpine units were subducted beneath the Adria plate. Altogether, these temporal and tectonic similarities demonstrate that the major allochthonous units in the Western Carpathians could indeed be equivalents to those defined in the Eastern Alps.
