Current chemostratigraphical studies of the Jurassic System primarily involve the use of one sedimentary component (marine organic carbon), one divalent transition metal substituted in carbonate (manganese), and two isotopic tracers: strontium-isotope ratios ( 87 Sr/ 86 Sr) and carbon-isotope ratios (δ 13 C carb and δ 13 C org ) in carbonate and in organic matter. Other parameters such as Mg/Ca and Sr/Ca ratios in calcite, oxygen-isotope ratios (δ 18 O) in carbonate, sulphur-isotope ratios (δ 34 S) in carbonate-hosted sulphate, nitrogen-isotope ratios (δ 15 N org ) in organic matter, osmium-isotope ratios ( 187 Os/ 188 Os) in black shales and neodymium-isotope ratios ( 143 Nd/ 144 Nd) in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors. Stratigraphical patterns in total organic carbon (TOC) can be of either local or regional significance, depending on the lateral extent of the former nutrient-rich and productive water mass. Divalent manganese follows a similar pattern, being concentrated, most probably as a very early diagenetic phase, only in oxygen-depleted waters that typically underlie zones of elevated organic productivity. Shifts in Mg/Ca and Sr/Ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes. Strontium-isotope ratios from pristine skeletal calcite provide a global signal; δ 13 C values from carbonates with minimal diagenetic overprint potentially do the same, although small spatial differences in palaeo-water-mass composition may have been locally significant. Oxygen-isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful, because they reflect palaeotemperature, the evaporation–precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase. Nitrogen-isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification, and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis. Neodymium-isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems. Preliminary data on sulphur-isotope ratios in carbonates and osmium-isotope ratios in organic-rich shales, both potentially offering global correlation, indicate that these tracers may be valuable, although the records at present are not sufficiently well established to allow high-resolution regional correlation. In all cases, biostratigraphically well-dated reference sections, against which the relevant geochemical data have been calibrated, are required in the first instance. To date, studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern (Boreal) and southern (Tethyan) Europe; carbon-isotope stratigraphy has been undertaken primarily, but not exclusively, on bulk pelagic sediments from the Alpine–Mediterranean or Tethyan domain; and strontium-isotope stratigraphy has been undertaken largely on calcitic skeletal material (belemnites and oysters) from northern and southern Europe. In many sections, including those containing ammonites, multi-parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means. Strontium-isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record.

/pdf/chemostratigraphy-of-the-jurassic-system-applications-fuep9zfrjr.pdf

Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography

For the purposes of a high-resolution multi-disciplinary study of the Upper Jurassic Kimmeridge Clay Formation, two boreholes were drilled at Swanworth Quarry and one at Metherhills, south Dorset, UK. Together, the cores represent the first complete section through the entire formation close to the type section. We present graphic logs that record the stratigraphy of the cores, and outline the complementary geophysical and analytical data sets (gamma ray, magnetic sus- ceptibility, total organic carbon, carbonate, δ 13 C org ). Of particular note are the new borehole data from the lowermost part of the formation which does not crop out in the type area. Detailed logs are available for download from the Kimmeridge Drilling Project web-site at http://kimmeridge. earth.ox.ac.uk/. Of further interest is a mid-eudoxus Zone positive shift in the δ13C org record, a feature that is also registered in Tethyan carbonate successions, suggesting that it is a regional event and may therefore be useful for correlation. The lithostratigraphy of the cores has been precisely correlated with the nearby cliff section, which has also been examined and re-described. Magnetic-susceptibility and spectral gamma-ray measurements were made at a regular spacing through the succession, and facili- tate core-to-exposure correlation. The strata of the exposure and core have been subdivided into four main mudrock lithological types: (a) medium-dark-dark-grey marl; (b) medium-dark-dark grey-greenish black shale; (c) dark-grey-olive-black laminated shale; (d) greyish-black-brownish- black mudstone. The sections also contain subordinate amounts of siltstone, limestone and dolostone. Comparison of the type section with the cores reveals slight lithological variation and notable thick- ness differences between the coeval strata. The proximity of the boreholes and different parts of the type section to the Purbeck-Isle of Wight Disturbance is proposed as a likely control on the thickness changes.

/pdf/integrated-stratigraphy-of-the-kimmeridge-clay-formation-24xh671soo.pdf

Integrated stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) based on exposures and boreholes in south Dorset, UK

Electron microscopy and pyrolysis of kerogens from the Kimmeridge Clay Formation, UK: Source organisms, preservation processes, and origin of microcycles

Controls on the molecular and carbon isotopic composition of organic matter deposited in a Kimmeridgian euxinic shelf sea: Evidence for preservation of carbohydrates through sulfurisation

Source- and age-related biomarker and isotopic data were measured for more than 1000 crude oil samples from wells and seeps collected above approximately 55N latitude. A unique, multitiered chemometric (multivariate statistical) decision tree was created that allowed automated classification of 31 genetically distinct circum-Arctic oil families based on a training set of 622 oil samples. The method, which we call decision-tree chemometrics, uses principal components analysis and multiple tiers of K-nearest neighbor and SIMCA (soft independent modeling of class analogy) models to classify and assign confidence limits for newly acquired oil samples and source rock extracts. Geochemical data for each oil sample were also used to infer the age, lithology, organic matter input, depositional environment, and identity of its source rock. These results demonstrate the value of large petroleum databases where all samples were analyzed using the same procedures and instrumentation.

Circum-Arctic petroleum systems identified using decision-tree chemometrics

The Kimmeridge Clay of northern England is part of a highstand system tract, rich in organic material, whose offshore correlatives sourced much of the North Sea oil province. Spatial and temporal variation over a 35-km-long transect (extended to 100 km by correlation offshore), some 200 m thick ranging from the Cymodoce to the Pallassioides zones, and representing some 6.5 m.y. of marine sedimentation, was evaluated by study of four continuously cored boreholes sited to sample both basin and shelf facies. Thin-bed stratigraphy, established by geophysical log signatures calibrated by the ammonite succession, enables organic-rich beds to be traced throughout the transect and into other English basins. These results show that total organic carbon may be computed reliably from a combination of resistivity, density, and sonic logs and increases by over 50% as each level is traced from shelf to basin where deeper waters are thought to have favored its entrapment and preservation in an environment more depleted in dissolved oxygen and with a more rapid burial. Associated, coccolith-rich, marker bands of the shelf pass into dolomites in the basin owing to the precipitation of early dolomite through bacterial decay of the organic-rich material. Vertical distribution of the organic content of each section identifies a hierarchy of sedimentary cycles with periodicities of about 25,000 and 280,000 End_Page 67------------------------- years. The short period cycles, less than 1 m thick, comprise alternations of more or less organic-rich beds. The analysis of the extracts suggests unity of origin of the organic matter of type II origin (zoo- and phytoplankton) throughout the period of sedimentation. The differences observed in the kerogens can be interpreted in terms of a fluctuation of oxygen deficiency whose variations (dysaerobic, anaerobic, or anoxic) are recorded in time and in space. Some of the variation, near coccolith-rich beds, can be related to a very high organic productivity (up to 40 wt.% total organic carbon). The second order cycles show maximum kerogen enrichment in the middle of the transgressive tract intervals, or at the base of high level, or platform edge prisms. These relationships allow the d stribution of the organic matter to be deduced from sequence stratigraphic studies.

Jean Paul Herbin

Papers

Variation of the Distribution of Organic Matter Within a Transgressive System Tract: Kimmeridge Clay (Jurassic), England: Chapter 6