The 260-million-year-old Emeishan volcanic province of southwest China overlies and is interbedded with Middle Permian carbonates that contain a record of the Guadalupian mass extinction. Sections in the region thus provide an opportunity to directly monitor the relative timing of extinction and volcanism within the same locations. These show that the onset of volcanism was marked by both large phreatomagmatic eruptions and extinctions amongst fusulinacean foraminifers and calcareous algae. The temporal coincidence of these two phenomena supports the idea of a cause-and-effect relationship. The crisis predates the onset of a major negative carbon isotope excursion that points to subsequent severe disturbance of the ocean-atmosphere carbon cycle.
Abundant lagoonal oncoids occur in the Late Oxfordian Hauptmumienbank Member of the Swiss Jura Mountains. Four oncoid types are observed in the studied sections and classiWed according to the oncoid surface morphology, the structure and composition of the cortex, and the texture and fauna of the encasing sediment. Micritedominated oncoids (types 1 and 2) have a smooth surface. Type 1 has a rather homogeneous cortex and occurs in moderate-energy environments. Type 2 presents continuous or discontinuous micritic laminae. It is associated with a low-diversity fauna and occurs in high-energy facies. Bacinella and Lithocodium oncoids (types 3 and 4) display a lobate surface. They are dominated by microencrusters (Bacinella irregularis and Lithocodium aggregatum) and are found in low-energy facies. The stratigraphic and spatial distribution of these oncoid types shows a correlation with the sequence-stratigraphic evolution of the studied interval, and thus with relative sea-level Xuctuations. It can be shown that these sea-level Xuctuations were controlled by orbital cycles with 100-and 20-kyr periodicities. At the scale of 100-and 20-kyr sequences, types 1 and 2 oncoids are preferentially found around sequence boundaries and in transgressive deposits, while types 3 and 4 oncoids are preferentially found around maximum Xoodings and in highstand deposits. This implies that changes of water energy and water depth were direct controlling factors.Discrepancies in oncoid distribution point to additional controlling factors. Platform morphology deWnes the distribution and type of the lagoon where the oncoids Xourished. A low accumulation rate is required for oncoid growth. Additionally, humidity changes in the hinterland act on the terrigenous inXux, which modiWes water transparency and trophic level and thus plays a role in the biotic composition and diversity in the oncoid cortex.
Stéphanie, Dating the onset and nature of the Middle Permian Emeishan large igneous province eruptions in SW China using conodont biostratigraphy and its bearing on mantle plume uplift models, LITHOS (2010), doi: 10.1016/j.lithos.2010.05.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.A C C E P T E D M A N U S C R I P T supported by these data; rather a more complex interaction between plume and lithosphere with minor localized uplift and subsidence is inferred.
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Recent global change occurs within decades and leaves a significant imprint on shallow carbonate platforms. To what extent can rate and synchronicity of environmental changes in the past be evaluated in order to allow comparisons with the processes and products of today? Sections of a carbonate-dominated platform of Late Oxfordian age have been logged in detail in the Swiss Jura Mountains. The time frame, based on ammonite biostratigraphy and the hierarchical stacking of the sedimentary sequences, suggests that these formed by low-amplitude, high-frequency sea-level changes in tune with the orbital cycles. The smallest unit, the elementary sequence, can thus be attributed to the 20 kyr precession cycle. By identifying sequence-stratigraphic elements such as maximum-flooding surfaces within an elementary sequence, an even higher time resolution can be obtained and reconstructions of changes in water depth and sea-level are attempted. Sedimentation rates were highly irregular and discontinuity surfaces are common. Correlation of such surfaces from one section to the other is tentative because they could also have formed locally and are not necessarily synchronous. On the scale of elementary sequences, the vertical facies evolution permits interpretation of changes in water depth, which were produced by eustatic sea-level changes (allocyclic) and/or by autocyclic processes. The distribution of clay minerals indicates changes of sea-level, as well as of rainfall in the hinterland. Biotic changes were controlled by water depth and water quality, and also by nutrient and clay input. Carbon and oxygen isotopes do not show significant shifts within the elementary sequences. The original ecological, mineralogical and geochemical signals have partly been homogenized because of time-averaging through bioturbation and through reworking by storms, and because of diagenetic modifications. Consequently, there is potential to interpret environmental changes on the studied carbonate platform but the time resolution is not better than a few thousand years. The rates and the synchronicity of the processes leading to the observed environmental changes can thus be estimated only within this time frame.
Modern shallow-water carbonate systems commonly display a complex pattern of juxtaposed depositional environments with a patchy facies distribution (facies mo saics). On ancient carbonate platforms, the reconstruction of lateral facies distribution is often hampered not only by discontinuous outcrop but also by lack of suffi ciently high time resolution. This case study from the Oxfordian (Late Jurassic) of the Swiss Jura Mountains demonstrates a way to improve the temporal and spatial resolution for the interpretation of carbonate rocks. Sequence-stratigraphic and cyclostratigraphic analyses have been performed that provide a basis for defi ning depositional sequences, which formed through sea-level changes that were induced by the 400-, 100-and 20-kyr orbital cycles. On the 100-kyr scale, sequence boundaries are well developed and can be correlated between sections. However, identifi cation and correlation of sequences related to the 20-kyr cycle may be diffi cult if local processes overprinted the record of orbitally controlled sea-level changes. The reconstruction of facies distribution along selected time lines gives a dynamic picture of platform evolution with time steps of a few ten thousand years and helps to interpret the controlling factors such as differential subsidence, low-amplitude eustatic sea-level fl uctuations, climate and ecology of the carbonate-producing organisms. Reefs and ooid shoals developed preferentially on topographic highs and thus accentuated platform morphology. Siliciclastics were shed onto the platform during sea-level falls and increased rainfall in the hinterland; their distribution was controlled by platform morphology. Siliciclastics and associated nutrients hindered carbonate production and thus indirectly infl uenced platform morphology. In addition to these controls, random processes acted on the smaller-scale facies relationships. Sedimentation rates can be estimated for each facies type over time spans of 10-20 kyr. They can thus be compared more easily with Holocene rates than if averaged over millions of years. This study shows that the Oxfordian platform in the Swiss Jura was as complex and dynamic as Holocene carbonate systems.
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