Overprinted allocyclic processes by tidal resonance in an epicontinental basin: the Upper Jurassic Curtis Formation, east‐central Utah, USA

Modern, tide‐dominated and tide‐influenced coastlines are characterised by a range of environments, including deltas, estuaries, and lagoons. However, some tide‐dominated basins and related sedimentary units in the rock record, such as the semi‐enclosed, shallow, Utah‐Idaho Trough foreland basin of the Jurassic Curtis sea, do not correspond to any of these modern systems. Persistent aridity caused the characteristic severe starvation of perennial fluvial input throughout this basin, in which the informal lower, middle, and upper Curtis, as well as the underlying Entrada Sandstone, and the overlying Summerville Formation were deposited. Wave energy was efficiently dissipated by the shallow basin's elongated morphology (approximately 800x150 km), as its semi‐enclosed morphology further protected the system from significant wave impact. Consequently, the semi‐enclosed, shallow‐marine system was dominated by amplified tidal forces, resulting in a complex distribution of heterolithic deposits. Allocyclic forcing strongly impacted upon the system's intrinsic autocyclic processes as the lower Curtis was deposited. Short‐lived relative sea‐level variations, along with uplift and deformation episodes, resulted in the accumulation of three parasequences, each separated by traceable flooding and ravinement surfaces. The subsequent transgression, which defines the base of the middle Curtis, allowed for the shallow‐marine part of the system to enter into tidal resonance as a consequence of the flooded basin reaching the optimal configuration of approximately 800 km in length, corresponding to an odd multiple of the quarter of the tidal wavelength given an average minimum water depth of 20 to 25 m. This resonant system overprinted the effects of allocyclic forcing and related traceable stratigraphic surfaces. However, the contemporaneous and neighbouring coastal dune field sedimentary rocks of the Moab Member of the Curtis Formation, characterised by five stacked aeolian sequences, as well as the supratidal deposits of the Summerville Formation, lingered to record allocyclic signals, as the Curtis sea regressed. This study shows that a tide‐dominated basin can enter into tidal resonance as it reaches its optimal morphological configuration, leading to the overprinting of otherwise dominant allocyclic processes by autocyclic behaviour. It is only by considering the sedimentological relationships of neighbouring and contemporaneous depositional systems that a full understanding of the dynamic stratigraphic history of a basin alternatively dominated by autocyclic and allocyclic processes can be achieved

A nutrient control on marine anoxia during the end-Permian mass extinction

Oxygen deprivation and hydrogen sulfide toxicity are considered potent kill mechanisms during the mass extinction just before the Permian–Triassic boundary (~251.9 million years ago). However, the mechanism that drove vast stretches of the ocean to an anoxic state is unclear. Here, we present palaeoredox and phosphorus speciation data for a marine bathymetric transect from Svalbard. This shows that, before the extinction, enhanced weathering driven by Siberian Traps volcanism increased the influx of phosphorus, thus enhancing marine primary productivity and oxygen depletion in proximal shelf settings. However, this non-sulfidic state efficiently sequestered phosphorus in the sediment in association with iron minerals, thus restricting the intensity and spatial extent of oxygen-depleted waters. The collapse of vegetation on land immediately before the marine extinction changed the relative weathering influx of iron and sulfate. The resulting transition to euxinic (sulfidic) conditions led to enhanced remobilization of bioavailable phosphorus, initiating a feedback that caused the spread of anoxic waters across large portions of the shelf. This reconciles a lag of >0.3 million years between the onset of enhanced weathering and the development of widespread, but geographically variable, ocean anoxia, with major implications for extinction selectivity

The sedimentology and sequence stratigraphy of the Curtis Formation along the eastern San Rafael Swell, Utah

Investigation of the marginal marine Curtis Formation of the Middle to Upper Jurassic succession, composed of the Entrada Sandstone, the Curtis Formation and the Summerville Formation, in ascending order. As a whole, the succession represents an alternation between continental and marine strata. The sedimentary succession is well exposed in east-central Utah, USA. Descriptive sedimentological data have been collected, from outcrops along the eastern flank of the San Rafael Swell, to link the Curtis Formation vertical and lateral variability to temporal and spatial distribution of depositional environments, and its relation to stratigraphically neighbouring units. The Curtis Formation was deposited during tectonically induced marine transgression onto a low angle retroarc foreland basin, consistently dominated by tidal mechanisms. At the end of the transgression the Summerville Formation was deposited contemporaneously with the Curtis Formation as proximal and distal equals, respectively, as they record an unconformably bounded, full transgressive-regressive sequence (Peterson, 1994). Utilization of sequence stratigraphic concepts reveal complex interfingering depositional architecture that directly related to the autogenically controlled spatial distribution of tidal sub-environments in a basin of dynamically shifting depositional environments. Accordingly, it is suggested that the Curtis Formation may be used as analogue for other tidally influenced environments in future sequence stratigraphic analyses

A new high-resolution stratigraphic and palaeoenvironmental record spanning the End-Permian Mass Extinction and its aftermath in central Spitsbergen, Svalbard

Research on the Permian-Triassic boundary (PTB) along the northern margins of Pangaea (exposed today in the Arctic region) has been heavily reliant on field observations, where data resolution was consequently determined by outcrop condition and accessibility. Core drilling in central Spitsbergen allowed for a near-complete recovery of two ~90 m cores through the PTB. Analyses of the core and nearby outcrops include stratigraphic logging and sampling, XRF scanning, petrography, biostratigraphy, isotope geochemistry, and geochronology. The First Appearance Datum (FAD) of H. parvus in Svalbard places the base of the Triassic ca. 4 m above the base of the Vikinghøgda Formation, and ca. 2.50 m above the End-Permian Mass Extinction (EPME) and its associated sharp negative δ13C. The PTB therefore falls within the Reduviasporonites chalastus Assemblage Zone in Svalbard. Precise U-Pb TIMS dating of two zircon crystals in a tephra layer just above the first documented Hindeodus parvus in Svalbard gives an age of 252.13 ± 0.62 Ma. High-resolution palaeoenvironmental proxies, including Si/kcps (kilo counts per second), Zr/Rb, K/Ti, Fe/K, and V/Cr, indicate a transition towards a more arid climate in the earliest Triassic, contemporaneous with prolonged bottom-water dysoxic/anoxic conditions, following an increase in volcanic activity in the Late Permian. Statistical analysis of Zr/Rb, K/Ti and V/Cr elemental ratios suggests that the system was impacted by long-eccentricity (400 kyr) cyclicity. The δ13C excursion in organic carbon (δ13Corg) record signals a large negative carbon isotope excursion (CIE) associated with the mass extinction event, but also records a second, smaller negative CIE ca. 22 m above this interval. This younger δ13Corg excursion correlates to similar CIEs in the Dienerian (late Induan) records of other sections, notably in the Tethys Ocean, which have been interpreted as recording a small biotic crisis during the post-extinction recovery. Evidence of this negative CIE in Spitsbergen suggests that the Dienerian crisis may have been global in extent. The negative δ13Corg values are associated with evidence for dysoxia or anoxia in the core, and the occurrence of tephra layers in the same interval suggests a possible connection between the Dienerian crisis and a discrete episode of volcanic activity