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.
Portable gamma-ray spectrometry provides an objective and quantitative method of studying sedimentary cyclicity in otherwise-cryptic mudrock successions. Spectral gamma-ray data are presented from the Lower Jurassic sections of the Yorkshire and Dorset Coasts (England), from Peniche (Portugal) and from inland exposures in southern Germany, including a recent road-cut. In Yorkshire, where proximal-distal relationships are readily demonstrated from sedimentological evidence, there is good correspondence between more proximal facies and elevated Th/K ratios. This relationship may be extended to the Dorset succession where the sequence-stratigraphical interpretation of Lower Lias mudrocks has been a source of some controversy. Th/K ratio data here suggest a distal, starved, Hettangian-earliest Sinemurian (Blue Lias) and a prograding Sinemurian interval (Black Ven Marls and Shales-with-‘Beef’). Flooding and further progradation in the Early Pliensbachian (Belemnite Marls) is also suggested. These results provide support for common ‘second order’ stratigraphical forcing mechanisms between the Dorset and Yorkshire successions. Data from mainland Europe suggest that there are systematic regional variations in Th/K ratio, upon which local temporal variations are superimposed. These may reflect climatic or regional sediment transport effects.
We describe a method of thin-bed petrophysical analysis that exploits the availability of both acoustic and resistivity images of the same rock. In developing our method we have gained a number of insights into the contrasting response of these images to rock properties, which may be of general interest to the imaging community. The case-study we present is from a thin-bedded, deep-water siliciclastic depositional environment. The resistivity image was used to increase the resolution of a conventional resistivity device. Comparison of bedding dips between the two images suggests a resistivity depth of investigation of around three inches. However, the best fit between image and conventional resistivity was obtained using a somewhat deep-reading resistivity measurement. The acoustic image was used to increase the resolution of our porosity measurement. The best fit to acoustic image amplitude was with the neutron porosity log, rather than with density, slowness, or with any combination of two porosity logs. Given high-resolution resistivity and porosity measurements from the images, and the precise information on bed boundaries that images provide, we proceeded with petrophysical analysis at a much higher resolution than obtainable from the conventional logging suite.
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