Mid-Jurassic through mid-Cretaceous extension in the Central Graben of the North Sea - part 1: estimates from subsidence

Abstract The post early Carboniferous subsidence history of the Central North Sea basin can be separated into three major periods: Permian, Triassic and post Mid-Jurassic. Prior efforts to account for this subsidence within an extensional framework have concentrated on the post Mid-Jurassic. These efforts have assumed that the effects of the previous periods of extension necessary to create the Permian and Triassic subsidence are negligible. We consider the 80-km value for the Mid-Jurassic-mid-Cretaceous extension from these efforts a reasonable upper estimate of the likely amount of extension. This value has received considerable criticism as it is almost four times as great as that determined by summing the horizontal displacement (heave) on faults observed on industry seismic lines in the area.
We treat the two earlier phases of extension as one phase and develop a method to estimate the maximum value of this extension. We use this value, with estimates of the total extension from the early Carboniferous to Present, to determine a likely minimum value for the mid-Mid-Jurassic through mid-Cretaceous extension. After justifying the use of Airy isostasy for the loading response of the lithosphere we show that the observed unloaded basement subsidence history is compatible with the parameters we derive for the pre and post Mid-Jurassic extension. Our minimum estimate of 38 km is still significantly higher than that: made by summing the heave on the faults active throughout the Upper Jurassic and lower Cretaceous.     

Faults and other shears in bedded Pleistocene deposits on the Wirral, United Kingdom

Normal and reverse faults, thrusts and shears with traceable lengths rarely over 0.35 m and throws of up to 0.014 m are locally developed within a series of bedded glacial meltwater deposits. They are believed to have originated during undrained loading of those deposits by an overlying flow till of late Devensian age. Local disturbance of beds by injection of fluid fine sands indicates past generation of high porcwater pressures.     

Norwegian corals: radiocarbon and stable isotopes in Lophelia pertusa

The ahermatypic coral Lophelia pertusa which produces aragonitic skeletons is widely distributed along the Norwegian coast. Specimens from a number of localities have been analyzed for oxygen and stable carbon isotope composition and ¹⁴C age. Stable isotope ratios of recent corals provide information on growth rate and seasonality of oceanographic conditions. Lophelia can be useful in paleoenvironmental reconstructions. ¹⁴C dates of fossil Lophelia from Drøak in the Oslofjord, collected from 20 m ahove and 40 m below present day sea-level, indicate regional extinction between 8700 and 7800 years ago. We suggest that the extinction resulted from the cut-off of deep waters by a rising sill in connection with the postglacial shoreline displacement. Radiocarbon dating of coral bushes suggests a fairly rapid growth rate as older and younger parts of recent corals do not reveal any difference in activity despite the short time scale of the history of bomb-produced ¹⁴C in the oceans.     

A New Interpretation of Centimetre-scale Variations in the Progress of Infiltration-driven Metamorphic Reactions: Case Study of Carbonated Metaperidotite, Val d'Efra, Central Alps, Switzerland

Progress () of the infiltration-driven reaction, 4olivine +5CO₂ + H₂O = talc + 5magnesite, that occurred during Barrovianregional metamorphism, varies at the cm-scale by a factor of3·5 within an 3 m³ volume of rock. Mineral and stableisotope compositions record that X_CO2, ¹⁸O_fluid, and ¹³C_fluidwere uniform within error of measurement in the same rock volume.The conventional interpretation of small-scale variations in in terms of channelized fluid flow cannot explain the uniformityin fluid composition. Small-scale variations in resulted insteadbecause (a) reactant olivine was a solid solution, (b) initiallythere were small-scale variations in the amount and compositionof olivine, and (c) fluid composition was completely homogenizedover the same scale by diffusion–dispersion during infiltrationand subsequent reaction. Assuming isochemical reaction, spatialvariations in image variations in the (Mg + Fe)/Si of the parentrock rather than the geometry of metamorphic fluid flow. Ifinfiltration-driven reactions involve minerals fixed in composition,on the other hand, spatial variations in do directly imagefluid flow paths. The geometry of fluid flow can never be determinedfrom geochemical tracers over a distance smaller than the oneover which fluid composition is completely homogenized by diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress     

Diagenetic signatures of stratal surfaces in the Upper Jurassic Fulmar Formation, Central North Sea, UKCS

Sedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried Upper Jurassic Fulmar Formation, South Central Graben, UKCS. As a result of the bioturbated nature of the shallow marine strata, variations in ichnofabrics and cross-cutting relationships between trace fossils are important in identifying and correlating stratal surfaces. Two transgressive and two regressive surfaces/sequence boundaries were studied, using petrographical, mineralogical and geochemical techniques. Cements, including early diagenetic pyrite framboids, grain-rimming apatite, microcrystalline dolomite and ankerite, occur at, above and immediately below both transgressive and forced regressive surfaces/sequence boundaries. Breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time, enhancing diagenetic reactions at and below the surfaces. The distribution of ankerite, despite being a deep-burial cement, was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces. The diagenetic character of the forced regressive surfaces/sequence boundaries is more complex than that of the transgressive surfaces, both in terms of mineralogy and paragenesis. This is attributed to the superimposition of diagenetic assemblages as a result of erosion, re-sedimentation and sediment by-passing. Although the diagenetic signature of the surfaces has been modified by dissolution/replacement reactions during deep burial, early diagenetic signatures can still be distinguished.     

Within-valley asymmetry and related problems of Neoglacial lateral moraine development at certain Jotunheimen glaciers, southern Norway

A difference in the size of Neoglacial lateral moraines on either side of a valley axis (within-valley asymmetry of lateral moraine development) is described. Analysis of clast roundness has revealed subangular material in latero-terminal and terminal moraines; lateral moraines, however, exhibit a compositional gradient of increasing angularity with distance from the former glacier snout. Comparisons with clasts of known origin suggest that this 'roundness gradient' may be explained with reference to either or both of two hypotheses: (1) a variable proportion of supraglacial (or englacial) to subglacial transported material; and (2) the variable composition of regolith incorporated by a push mechanism from the valley sides. Within-valley asymmetry is inferred to result where the supply of debris to lateral moraines from these sources is unequal either side of a valley axis. Both interpretations are also consistent with the relatively large size of latero-terminal sections of end moraines. In order to account for the discrepancy between moraine size and apparent debris supply rates, it is suggested that the largest lateral moraines may have been formed over a longer time scale than the 'Little Ice Age', and that reworking of deposits may have occurred. The supply of debris to the north-facing lateral moraine at Nordre Illåbreen has been so great that it has developed into a rock glacier; this suggests the possibility that subglacial material and valley-side regolith, as well as supraglacial material, contributes to the formation of ice-cored rock glaciers.     

Traces produced by chironomid larvae in sediments of an ice-contact proglacial lake

Scdimentological studies of a small ice-contact proglacial lake at Briksdal. southwest Norway, have led to the discovery of two types of tubular traces developed on the surfaces of material collected in sediment traps. The traces are described and attributed, with certainty, to the burrowing activities of chironomid midge larvae.