Summary A transect of four coreholes, drilled by the Glomar Challenger across the Irish continental margin at the Goban Spur, evidences a dynamic palaeoceanographic regime during the late Mesozoic and Cenozoic. Shallow marine waters invaded the rift-stage grabens of the Goban Spur in the early Barremian. Thereafter, the margin subsided rapidly, producing a pelagic depositional regime by late Barremian time. Deep marine conditions were maintained as sea-floor spreading began in the early Albian, and chiefly pelagic deposition continued to the present. Among a series of significant post-rift oceanographic changes, one of the most notable is the familiar fluctuation of oxic and anoxic sea-floor environments during the Cenomanian and Turonian. Another marked change took place during the late Palaeocene, when cooler, oxygen-rich, northern bottom waters reached the Goban Spur as a consequence of rifting and sea-floor spreading between Greenland, Rockall Plateau, and Norway. Later during the Cenozoic, the initial production of Antarctic bottom water, several accelerations of polar icecap growth, and fluctuating eustatic sea-level produced a variety of circulatory shifts on the Goban Spur. A particularly significant sedimentological consequence of these interacting processes was the widespread creation of numerous erosional and non-depositional unconformities.
Abstract Petrological studies of the Sherwood Sandstone of the Marchwood Borehole show that the formation is made up of two units of differing primary lithological character: a lower unit with lithic sandstone and conglomerate, and an upper unit with arkosic sandstone. A combination of compaction and calcite cementation (early and late) has severely reduced porosity and permeability in the lower unit. In the upper unit compaction is again important, but several beds have largely escaped cementation, and their primary porosity has been enhanced by leaching of feldspars. The secondary porosity reaches 7% in some samples. Oxygen and carbon isotope data for the early (calcrete) cements indicate isotopic equilibrium with typical freshwater compositions. The later cements yield more variable values, indicating precipitation under a wider range of conditions.
Heavy mineral analysis is one of a group of provenance-based methods that complement traditional biostratigraphic correlation of clastic reservoirs. A variety of processes give rise to stratigraphic changes in sediment composition, including source area uplift, unroofing, changes in climatic conditions, extent of alluvial storage on the floodplain and the interplay between different depositional systems. Heavy mineral analysis is a reliable and proven technique for the correlation of clastic successions because prolonged and extensive research has provided detailed understanding of the effects of processes that alter the original provenance signal during the sedimentary cycle, such as hydrodynamics and diagenesis. The technique has been successfully applied to a wide range of clastic reservoirs, from fluvial to deep marine and from Devonian to Tertiary, using a combination of different types of parameters (provenance-sensitive mineral ratios, mineral chemistry and grain morphology). The application of heavy mineral analysis as a non-biostratigraphic correlation tool has two limitations. The first is that valid correlations cannot be made in sequences with uniform provenance and sediment transport history, but this is a problem inherent with all provenance-based methods. The other is that the technique can be applied only to coarse clastic lithologies and is not suitable for fine-grained sediments or carbonates.
The establishment of chronostratigraphic units such as geological Systems and Series depends upon an ability to equate succession in rock strata with the passage of time, and upon a pervasive Law of Superposition. These assumptions hold true at a gross scale. But, at fine scales of stratigraphic resolution, they commonly break down. Thus, bioturbation in Phanerozoic marine deposits typically homogenizes sedimentary packages spanning millennia, affecting biostratigraphic, isotopic and paleomagnetic signals, and post-burial mass transport phenomena such as large-scale sedimentary slumps and intra-stratal diapirs locally disrupt superpositional relationships on a larger scale. Furthermore: the multi-stage transport of microfossils prior to final burial complicates the relationship between depositional and biostratigraphic ages; paleomagnetic signals, imposed at shallow burial depths, may be distinct from depositional ages; and high precision zircon U-Pb dates from tuff layers determine time of crystallization in the magma, rather than depositional age. In such circumstances, depositional units cannot be unambiguously equated with time units: because they include multiple temporal components, they cannot be subdivided precisely into time-rock units. By contrast, the different phenomena which have contributed to constructing sedimentary deposits, pre-, syn- and post-depositional, may be effectively accommodated within a unitary geological time framework.
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Abstract The interpretation of the Holocene evolution of the Humber Estuary has been made possible only through integrated multidisciplinary studies involving inter alia : drilling, to obtain sedimentary records of the Holocene Estuary fill; multi-element, carbon-nitrogen-sulphur and stable carbon isotope geochemistry; heavy and clay mineralogy; palaeomagnetism; radio-carbon dating; and pollen, diatom and foraminiferal studies. Eight chemostratigraphic suites and 14 palaeo-environments have been recognized. Sediment types, environments of deposition and provenance change in response to rising sea-level, showing a range from freshwater fluvial deposition of locally derived terrestrial sediment to intertidal and subtidal deposition of sediments from marine sources. The methods used are illustrated with reference to sediment cores from inner and outer estuary locations. The results show that Holocene environmental characterization is most secure when a number of different, but complementary, techniques are used. The integration of radiocarbon dates with palaeomagnetic and geochemical data improves the understanding of the presence and significance of time breaks, which is crucial to constraining sedimentation rates and material budgets.
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