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Field guide to the Jurassic of the Isles of Raasay and Skye, Inner Hebrides, NW Scotland.
... Rifting is likely to have occurred during the Oxfordian and Kimmeridgian (Late Jurassic) allowing for the development of fully marine, partially anoxic environments, for the deposition of the Staffin Shale Formation (Morton 1992b). Within the SOHB, Jurassic rocks have been dated from the Hettangian (earliest Jurassic) through to the Kimmeridgian (Late Jurassic), whereas within the MB they have only been dated from the Hettangian to Toarcian (Early Jurassic) (Fig. 5) (BP 1990;Morton and Hudson 1995;Hesselbo and Coe 2000;Barron et al. 2012). The difference in preserved strata is predominantly a function of post-Jurassic erosion (Morton 1992b). ...
... Onshore, extensive geological data collected on Skye allow for an assessment of source-rock quality, reservoir quality, depositional patterns and palaeoenvironmental reconstructions (e.g. Hudson 1962;Steel 1974;Harris and Hudson 1980;Morton 1989Morton , 1992bMorton , 1993Harris 1992;Morton and Hudson 1995;Vincent and Tyson 1999;Hesselbo and Coe 2000). Data on porosity and permeability, vitrinite reflectance, and total organic carbon (TOC) (wt%) have been obtained from the wells, which has furthered our understanding of the petroleum potential of the basins (BP 1990;Chevron 1991b;Pentex 1992a;Fyfe et al. 1993). ...
... The potentially oil-prone Portree Shale Formation (early Toarcian) is poorly exposed onshore, with only three accessible outcrops: one on the Trotternish Peninsula, one on the Strathaird Peninsula and one on the east coast of Raasay (Morton and Hudson 1995;Morton 2004). The mudstones of the Portree Shale Formation were deposited during a period when the SOHB was experiencing Morton's (1989Morton's ( , 1992b lithostratigraphy has been used throughout this paper for consistency with wells, boreholes and published geological maps. ...
The Sea of Hebrides Basin and Minch Basin are late Palaeozoic-Mesozoic rift basins located to the northwest of the Scottish mainland. The basins were the target of small-scale petroleum exploration from the late 1960s to the early 1990s, with a total of three wells drilled within the two basins between 1989 and 1991. Although no commercially viable petroleum discoveries were made, numerous petroleum shows were identified within both basins, including a gas show within the Upper Glen 1 well in Lower Jurassic limestones. Organic rich shales have been identified throughout the Jurassic succession within the Sea of Hebrides Basin, with one Middle Jurassic (BajocianBathonian) shale exhibiting a Total Organic Carbon content of up to 15 wt%. The focus of this study is to review the historic petroleum exploration within these basins, and to evaluate whether the conclusions drawn in the early 1990s of a lack of prospectivity remains the case. This was undertaken by analysis of seismic reflection data, gravity and aeromagnetic data and sedimentological data, from both onshore and offshore wells, boreholes and previously published studies. The key findings from our study suggest that there is a low probability of commercially sized petroleum accumulations within either the Sea of Hebrides Basin or the Minch Basin. Ineffective source rocks, likely due to low maturities (due to lack of burial) and the fact that the encountered Jurassic and Permian-Triassic reservoirs are of poor quality (low porosity and permeability) has led to our interpretation of future exploration being high risk, with any potential accumulations being small in size. While petroleum accumulations are unlikely within the basin, applying the knowledge obtained from the study could provide additional datasets and insight into petroleum exploration in other northeast Atlantic margin basins, such as the Rockall Trough and the Faroe-Shetland Basin.
... The Kilmaluag Formation crops out on the Scottish Inner Hebridean islands of Eigg, Skye and Muck, and is approximately 25 m in thickness at the most complete section on the Strathaird Peninsula on Skye (Harris & Hudson 1980;Morton & Hudson 1995) (Figs 1, 2). It was formerly known as the Ostracod Limestone, and the base of the formation is defined by the occurrence of ostracod-bearing calcareous mudstones and marls/fissile mudstones (Anderson & Dunham 1966;Harris & Hudson 1980;Andrews 1985;Barron et al. 2012). ...
... The similarities in vertebrate faunal composition with that from the Kirtlington Cement Quarry (Forest Marble Formation, see below) in England also support a Late Bathonian age. Unlike other formations within the Great Estuarine Group, the Kilmaluag Formation includes predominantly low-salinity and freshwater facies, especially on the Strathaird Peninsula, as demonstrated by the presence of freshwater ostracods (Darwinula and Theriosynoecum : Wakefield 1995), shallow freshwater to oligohaline conchostracans (such as Anthronesteria and Pseudograpta: Chen & Hudson 1991) and freshwater gastropods (Viviparus: Andrews 1985; Morton & Hudson 1995;Barron et al. 2012) (Fig. 3). ...
The Kilmaluag Formation on the Isle of Skye, Scotland, provides one of the richest Mesozoic vertebrate fossil assemblages in the UK, and is among the richest globally for Middle Jurassic tetrapods. Since its discovery in 1971, this assemblage has predominantly yielded small-bodied tetrapods, including salamanders, choristoderes, lepidosaurs, turtles, crocodylomorphs, pterosaurs, dinosaurs, non-mammalian cynodonts and mammals, alongside abundant fish and invertebrates. It is protected as a Site of Special Scientific Interest and by Nature Conservancy Order. Unlike contemporaneous localities from England, this assemblage yields associated partial skeletons, providing unprecedented new data. We present a comprehensive updated overview of the Kilmaluag Formation, including its geology and the fossil collections made to date, with evidence of several species occurrences presented here for the first time. We place the vertebrate faunal assemblage in an international context through comparisons with relevant contemporaneous localities from the UK, Europe, Africa, Asia and the US. This wealth of material reveals the Kilmaluag Formation as a vertebrate fossil assemblage of global significance, both in terms of understanding Middle Jurassic faunal composition and the completeness of specimens, with implications for the early evolutionary histories of mammals, squamates and amphibians.
... Reports exist of abundant graphoceratids far too extensive to comprehensively include here. A brief selection of examples is therefore given, from Britain (Chandler, 1997;Chandler and Callomon, 2009;Morton, 1985Morton, , 1990Morton and Chandler, 1994;Morton and Hudson, 1995), France (Contini, 1969;Contini et al., 1997), Germany (Rieber, 1963;Dietze et al., 2014Dietze et al., , 2022Dietze and Schweigert, 2018), Southern Europe (Ureta, 1985;Pavia et al., 1995;Géczy, 1967), North Africa (Sadki and Mouterde, 1994;Sadki, 1996), Iran (Seyed-Emami et al., 2006, and South America (Hillebrandt and Westermann, 1985;Westermann and Riccardi, 1972). Occasional records of graphoceratids exist in the Far East (Yin pers. ...
... Unlike other formations within the Great Estuarine Group, the Kilmaluag Formation includes predominantly freshwater facies, and there are abundant freshwater ostracods such as Darwinula (Wakefield, 1995), shallow freshwater to oligohaline conchostracans (Chen and Hudson, 1991), and freshwater gastropods (Andrews, 1985;Morton and Hudson, 1995;Barron et al., 2012). Paleoenvironmental reconstructions suggest a low-salinity environment of closed-off lagoons with freshwater input, occasionally drying out to form mudcracks (Andrews, 1985). ...
Docodonta are one of the earliest diverging groups of mammaliaforms, and their morphology provides key information on the transition between non-mammalian cynodonts and Mammalia. We describe the partial skulls of two docodontans Borealestes serendipitus and Borealestes cuillinensis sp. nov. from the Kilmaluag Formation (Middle Jurassic: Bathonian), Isle of Skye, Scotland. We visualize their cranial anatomy using laboratory and synchrotron X-ray micro-CT. The skulls belong to two partial skeletons, currently comprising the most complete Mesozoic mammal fossils reported from the British Isles. The associated upper and lower dentitions show that the lower dentition of Borealestes is not diagnostic to species level. We establish, B. cuillinensis, based on upper molar characters, and re-identify upper molars previously assigned to ‘Borealestes’ mussettae as belonging to B. cuillinensis. ‘Borealestes’ mussettae, based on distinctive lower molars, is found to be morphologically and phylogenetically distinct from Borealestes, necessitating assignment to a new genus, Dobunnodon gen. nov. The skulls of Borealestes retain many plesiomorphic features seen in Morganucodon but absent in more crownward mammaliaforms. Our study highlights that generic and species taxonomy of docodontans are more reliable when based on both upper and lower teeth, while lower molar morphology may underrepresent the true diversity of Mesozoic mammaliaforms.
... An intertidal coastal section in dark-gray claystone at Flodigarry, Staffin Bay, Isle of Skye (northwest Scotland) has yielded detailed Sub-Boreal and Boreal ammonite Successions and a magnetostratigraphy (e.g., Sykes and Callomon, 1979;Wierzbowski et al., 2006Wierzbowski et al., , 2015Wierzbowski et al., , 2016Wierzbowski et al., , 2018Przybylski et al., 2010a) and is now the site proposed for the GSSP, specifically in the upper part of Bed 35 of the Staffin Shale Fm., 1.25 6 0.01 m below the base of Bed 36 (cf. Morton and Hudson, 1995). The proposed GSSP level is the base of the Sub-Boreal ammonite Pict. ...
Ammonites underwent an evolutionary diversification after the mass extinction of the end Triassic induced by the formation of a Large Igneous province (LIP), and this group provides the most useful marine biostratigraphy. Only two levels within the Jurassic are relatively well determined using U–Pb dating from single zircons in ash beds, at the base Hettangian and the Pliensbachian–Toarcian boundary. Otherwise the Lower Jurassic is scaled using astrochronology and the Middle and Upper Jurassic scaled from Pacific seafloor spreading rates correlated to magnetic reversals. LIP activity during the Early Jurassic (Triassic–Jurassic boundary and Toarcian) perturbed global environments to extents not evidenced since the end Permian, and age relationships allow for a strong causal connection between these LIP eruptions and mass extinctions caused by major paleoenvironmental change, including ocean anoxia. Breakup of the supercontinent Pangea dominated paleogeography and paleoceanography and created shallow seaways that form sources and traps for hydrocarbons. Calcareous planktonic algae diversified and migrated from shallow seaways to open oceans to set the stage for the beginning of modern oceanic biogeochemical cycling; calcareous nannofossils provide additional widely used correlation tools.
Arido-eustasy is a model that explains palaeoenvironmental change by linking the covariation of the carbon isotope record and the eustatic sea level to orbitally modulated hydroclimatic intensity. Orbitally forced wet climate modes periodically accelerate the water cycle in the biosphere, modulating the balance between terrestrial fresh-water reservoirs and the eustatic sea level and perturbating the carbon cycle through catastrophic weathering of terrestrial carbon reservoirs, resulting in excessive transfer of carbon into the oceans. These intervals of extreme hydroclimatic intensity and weathering, or Orbito-Hyetal Events (OHEs), are hypothesised to be responsible for major palaeoenvironmental crises such as oceanic anoxic events and biotic extinction events. Here, I consider the available data and show evidence for the occurrence of a strong and short-term OHE at the end of the middle Oxfordian. The event probably lasted ∼40 kyr and was bracketed by characteristic arido-eustatic traits, including a prominent sea level fall (the OX5), a contemporaneous extreme (>2‰) negative excursion in the marine carbonate and terrestrial organic-carbon isotope records, and regional occurrences of anoxic environments with organic-rich deposits. Important prolific source-rock reservoirs, such as the Smackover, Hanifa, and Khodjaipak Formations in the Gulf of Mexico, Arabian Gulf, and Uzbekistan, respectively, seem to have been created during this end-middle Oxfordian OHE.
Late Liassic in the Western Tethys has been the cradle of Middle and Late Jurassic diversity of corals. This is what revised and enhanced taxonomy of corals from Pliensbachian and Toarcian stages reveals. The current new taxonomic study of Pliensbachian corals describes 66 species distributed in 41 genera and 20 identified families. It includes four new genera: Podosmilia n. gen., Tubulosmilia n. gen., Prismastrea n. gen. and Spongiocoenia n. gen.; and 20 new species: Axosmilia amellagouensis n. sp., Apocladophyllia guigouensis n. sp., Coryphyllia bicuneiformis n. sp., Coryphyllia capillaria n. sp., Proleptophyllia calix n. sp., Proleptophyllia magna n. sp., Proleptophyllia subphaceloida n. sp., Fungiaphyllia praecursor n. sp., F. rotunda n. sp., Margarosmilia dividenda n. sp., Paravolzeia calabrensis n. sp., Distichophyllia pauciseptata n. sp., Retiophyllia zizensis n. sp., Epismiliopsis paraeudesi n. sp., Phacelostylophyllum mg. arbustulum n. sp., Podosmilia horologium n. gen., n. sp., Stylophyllopsis bovista n. sp., S. veracolumella n. sp., Tubulosmilia regularis n. gen., n. sp. and Prismastrea organum n. gen., n. sp. So many new species appear surprising at first sight considering the special attention paid in this study to the correction of species diversity overestimations that took place in the literature of the last century as a consequence of a typological approach. Many taxa previously considered extinct at T-J boundary were still living during Pliensbachian times, various genera are known only for Pliensbachian. In addition, a small number of genera namely Isastrea, Montlivaltia and Thamnasteria have their first occurrence during this stage. Despite their low abundance during Pliensbachian, these genera will significantly increase their part in Middle and Upper Jurassic communities. Most collected coral assemblages come from both reefs and level-bottom assemblages found in carbonate platform situation.
The early Toarcian (~183 Ma) was characterized by a prominent volcanism-induced warming event associated with a massive addition of ¹²C-enriched carbon to the ocean-atmosphere system. This warming likely contributed to marked ocean deoxygenation during this time, giving the event its name: the early Toarcian oceanic anoxic event (T-OAE). Although the T-OAE has been recognized globally, clear geographic differences in the character of the event and its environmental effects have been noted. Here we present new carbon isotope, element abundance and organic geochemical data from a lower Toarcian succession on the Isle of Raasay, Scotland (Hebrides Basin, Northwest European Shelf). These data provide the first evidence of the T-OAE in Scotland. The succession is generally enriched in organic matter through the T-OAE interval, though redox-sensitive trace element data indicate that oxic-suboxic bottom water conditions prevailed, potentially interspersed with ephemeral anoxic episodes. Our elemental data contrast with evidence for persistent anoxia/euxinia in nearby basins, and emphasizes how deoxygenation was spatially variable and dependent on water depth and basin hydrography. Similarly, the data emphasize how anoxia was not a prerequisite for the deposition of organic-rich lithologies during the T-OAE. Sedimentological evidence, coupled with inorganic geochemical data, indicates increased coarse-grained detrital flux and enhanced chemical weathering during the T-OAE. Our findings support emerging evidence for a marked strengthening of hydrological cycling and increased storminess at tropical and subtropical latitudes globally in response to global warming during the T-OAE.
Clumped isotope based temperature estimates from exceptionally well-preserved belemnites from Staffin Bay (Isle of Skye, Scotland) reveal that seawater temperatures throughout the Middle-Late Jurassic were significantly warmer than previously reconstructed by conventional oxygen isotope thermometry. We demonstrate here that this underestimation by oxygen isotope thermometry was likely due to a) using the incorrect calcite thermometry equation for belemnite temperature reconstructions and b) by incorrectly estimating the seawater δ18O(δ18Osw) for the Hebrides Basin. Our data suggests that the fractionation factor for oxygen isotopes in belemnites from seawater was closer to that of slow-growing abiogenic calcites than that of other marine calcifying organisms. Our clumped isotope temperatures are used to reconstruct δ18Oswtrends across the Callovian–Kimmeridgian in the Hebrides Basin. The δ18Oswvaried significantly in the Hebrides Basin throughout this interval, possibly as a result of changing currents through the Laurasian seaway. Trends in temperature and δ18Osware compared to published palaeoceanographic studies to shed light on changing palaeoceanography in the Tethyan and Boreal realms throughout the Middle–Late Jurassic.
Fossil shells of benthos and nektobenthos have been shown to be faithful recorders of seawater carbon- and oxygen-isotope geochemistry, and thus also useful to track the relationship between carbon cycle and palaeotemperature. In this study we present an extensive dataset from Lower Jurassic (Hettangian and lower Sinemurian) mollusc and brachiopod hard parts collected from biostratigraphically well-calibrated UK coastal outcrops (Bristol Channel and Hebrides basins). These basins lay palaeogeographically in the southern part of the Laurasian Seaway that connected the Tethys and Boreal oceans. All samples have been subject to screening for diagenesis on the basis of elemental composition, light microscopy, and SEM observations. In the case of some localities within the Hebrides Basin, alteration by hydrothermal systems around Paleogene intrusions has led to re-setting of carbonate oxygen isotopes, but the original carbon isotope values from the shells are largely preserved. Above the prominent and apparently short-lived, ~3 per mil δ¹³Ccarb amplitude positive carbon-isotope excursion (CIE) that occurs immediately above the Triassic-Jurasic (T-J) boundary (in the tilmanni ammonite biozone), a pronounced negative CIE (the so-called Main Negative CIE) spans the entire Hettangian Stage. At the Hettangian-Sinemurian boundary, and through the lower Sinemurian, the carbon-isotope values of the skeletal carbonate again trend towards progressively more positive values, but representing a time of several million years. The heaviest δ¹³Ccarb values of about ~ +4.3 per mil are evident towards the top of the lower Sinemurian, and are comparable with values observed from the tilmanni Zone, and from the lower Toarcian, higher in the Jurassic. This long-term positive hump, which confirms trends derived from bulk organic matter carbon-isotope records, is supporting evidence of prolonged enhanced organic carbon burial that is inferred to have occurred in the extensive system of lacustrine and marine rifts that traversed a fragmenting Pangaea after emplacement of the Central Atlantic Magmatic Province. In parallel, oxygen-isotope values of the skeletal carbonate show a continuous downward trend from the lower part of the Hettangian (~ −1 per mil δ¹⁸Ocarb in the planorbis Zone) to the top of the lower Sinemurian (~ −4 per mil δ¹⁸Ocarb in the higher turneri Zone). Oxygen-isotope values may be interpreted as due to gradually increasing palaeotemperature, and/or addition of a meteoric or cryospheric water component; in the case of the Laurasian Seaway, palaeoceanographic and palaeoecological considerations point towards a dominant palaeotemperature signal. Consequently, any atmospheric carbon-dioxide drawdown effect on global palaeotemperatures, as suggested by progressively increasing δ¹³Ccarb values, and assuming a constant silicate weathering sink, was more than counterbalanced in the seaway by regional processes that led to significantly warmer bottom water temperatures.
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