Offshore Boreholes Research Articles

Late Devensian evolution of the marine offshore environment of western Scotland

Offshore boreholes from the outer Firth of Clyde and from the Sea of the Hebrides passed through marine deposits of Holocene, Loch Lomond Stadial (LLS) and Windermere Interstadial (WI) age, but it is likely that older, probably Dimlington Stadial (DS) strata, now removed by erosion, were formerly present at the entrance to the Firth of Clyde. The late-glacial environment was one of very rapid deposition punctuated by erosive episodes. Reworking and redeposition led to the mixing of otherwise incompatible warm, shallow-water and cold, offshore faunas, particularly during a ‘warm’ event towards the end of the WI. The latter is correlated with Greenland Interstadial GI1a, but no evidence has been found for climatic ameliorations corresponding to GI1c and GI1e earlier in the Wl. A less cold interval at the end of the LLS may be coeval with climatic changes in Norway and the Shetland–Faroe Channel. Glacier ice disappeared from the outer Firth of Clyde before the close of the DS, and the Glasgow area was deglaciated across the DS/WI transition. The borehole evidence for the timing of deglaciation in the Sea of the Hebrides is inconclusive.

A multiple-isotope (δ37Cl, 14C, 3H) approach to reveal the coastal hydrogeological system and its temporal changes in western Kyushu, Japan

Multiple isotopes were used to reveal the coastal hydrogeological system and its temporal changes, caused by transgression and deposition of marine clay, in a small catchment and its offshore extension in Kyushu, Japan. Chloride concentrations in extracted pore waters from an offshore borehole were higher than 16,400 mg/L at depths shallower than 1.5 meters below sea floor (mbsf), and decreased downwards gradually to become lower than 250 mg/L below 7.7 mbsf. Stable chlorine isotopic ratios were –1.50‰ at 6.2 mbsf and –1.27‰ at 5.6 mbsf, while those from other depths showed minor fluctuation. A one-dimensional diffusion-sedimentation model was successfully applied to explain both chloride and stable chlorine isotopic ratios. Apparent residence times of groundwater below the inter-tidal zone is on the order of 100 years while that below sea bottom is about 2,000 years, indicating that groundwater below the inter-tidal zone constitutes a part of the present-day active groundwater flow system while that below sea bottom moves extremely slowly. The coincidence of the apparent residence time of groundwater below sea-bottom and the age of the start of both deposition of marine clay and diffusion processes strongly suggests that the deposition of marine clay has controlled the hydrogeological system and resulted in the reduction of the extent of groundwater discharge.

Neogene dinocyst zonation for the eastern North Sea Basin, Denmark

A dinocyst zonation for the Neogene succession in the eastern part of the North Sea Basin (Denmark) is presented. The zonation is based on an extensive database comprising data from more than fifty onshore and offshore boreholes and about twenty five outcrops. Most of the nineteen dinocyst zones described and defined herein, are new. The zonation is correlated with previously published dinocyst zonations within the North Sea Basin, in the North Atlantic and with the revised Northwest European zonation. The presented zonation gives a detailed subdivision of the Oligocene–Miocene transition, of the Lower Miocene, and of the Upper Miocene and Pliocene successions. The previous zonation of the onshore Danish Middle Miocene is reconsidered and partly redefined. The zonation is correlated with other biostratigraphic subdivisions of the Neogene succession in the Danish region in addition to litho- and sequence stratigraphy. The dinocyst zonation and the foraminifer zonation of the Danish Miocene (based upon analysis of the same boreholes) were independently calibrated with the established nannoplankton zonation. This correlation revealed a mismatch generally corresponding to one nannoplankton zone. Absolute ages of the new dinocyst zones are proposed based on correlation from the studied succession within the North Sea Basin with the international zonations and stratigraphic schemes. In addition, parts of the succession have been dated by strontium isotope analysis of mollusc shells. Inconsistencies due to first and last occurrences of some dinocyst species in previous studies and the present study, especially within the Lower Miocene, implied that new Sr-isotope datings from mollusc shells from the cored Sdr. Vium borehole and other localities in this study would be invaluable. Indeed, this method allowed the previous last occurrence of Cordosphaeridium cantharellus (one of the zonal index fossils) dated as between 17.95 My and 19.5 My in previous studies, to be assigned a new dating of 18.4 My. The timespan of the zones range from 2.6 My for the Amiculospharea umbraculum Zone to 0.4 My for the Achomosphaera andalousiensis Zone. Neogene biostratigraphy in the North Sea Basin has been problematic due to the periodically limited connection between the North Sea Basin and the North Atlantic Ocean, especially with respect to stratigraphy based on foraminifers and calcareous nannoplankton. Many of the stratigraphically most important taxa, e.g. those defining stratigraphic boundaries in type sections, have not been found within the North Sea Basin. This problem seems to be solved by correlation based on the new dinocyst stratigraphy, because stratigraphically significant taxa do occur in the North Sea deposits, even in marginal marine settings.

Late Devensian palaeoenvironmental changes in the sea area adjacent to Islay, SW Scotland: implications for the deglacial history of the island

Synopsis From a study of offshore borehole samples collected by the British Geological Survey in 1975, it is suggested that arctic marine or glaciomarine sediments of Dimlington Stadial (DS) age (>13 ka 14 C ka BP) in Loch Indaal, Islay, were laid down following the retreat of a glacier from the head of the loch, possibly only a few hundred radiocarbon years before the opening of the Windermere Interstadial (WI, 13–11 14 C ka bp ). A reworked molluscan fauna in the DS sediments, which is identical to that living in the area today, is believed to be of last interglacial age or earlier. Sediments of WI age and Holocene age (<10 4 C ka bp ) have also been intersected in the boreholes. Deposits of Loch Lomond Stadial age (11–10 14 C ka bp ) are absent from two boreholes investigated from Loch Indaal, but may be present in a borehole east of Islay. The proposal that ice of the Last Glacial Maximum reached only as far west as central Islay is unsupported by both this assessment and the published geomorphological evidence.

Rapid and major coastal subsidence during the late Miocene in south-central Chile

Sedimentological and paleontological studies, including foraminifera, ostracodes, gastropods, and trace fossils, were carried out on Neogene sedimentary successions and offshore boreholes of south-central Chile (∼33°–45°S). Sedimentology shows the occurrence of a thin, shallow marine, basal conglomerate overlain by a succession that includes the following facies: massive sandstones, conglomerates, interbedded siltstones and sandstones showing Bouma cycles, parallel-laminated sandstones, synsedimentary breccias, slides, slumps, diamictites, and massive siltstones. These facies were deposited by gravity flows, with turbidity currents and sandy debris flows as the main modes of deposition. Paleontology indicates the occurrence of trace fossils assigned to the Zoophycos ichnofacies and deep-water (∼2000 m) benthic foraminifers, ostracodes, and gastropods. Sedimentology and paleontology indicate that deposition took place on a slope apron during a period of rapid and major forearc subsidence. Planktic foraminifers indicate ages ranging from the late Miocene to the early Pliocene (zones N16–N19) for these successions. We attribute this episode of major Neogene subsidence to an important event of subduction erosion that would have removed the underside of the upper continental plate and caused its thinning.

A discrete episode of seismic and aseismic deformation of the Nankai trough subduction zone accretionary prism and incoming Philippine Sea plate

New insights into episodic deformation at the Nankai trough subduction zone are provided by data from two Ocean Drilling Program borehole hydrologic observatories drilled into the Philippine Sea plate (Site 1173) and the seaward part of the Nankai accretionary prism off southwestern Japan (Site 808), and from an array of high-sensitivity borehole accelerometer and velocity seismometers on Shikoku Island (Hi-net). Fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site, and relaxation of the accretionary prism toe. The rate of strain (ca. 10− 6 yr− 1) inferred from the rates of pressure rise at Site 1173 (up to 5 kPa yr− 1) is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide. Opposing these trends are transient pressure anomalies observed in late June/early July 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism. Concurrent with these transients, a swarm of very-low-frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust. Reverse-fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism. We suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about June 26, 2003, at or near the up-dip limit of the locked portion of the subduction thrust, and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section. Deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas. In addition to demonstrating that the prism is far from inactive during interseismic intervals, the observations may also provide a small-amplitude analog for strain and hydrologic response at the time of great subduction earthquakes.

A revised lithostratigraphy for the Palaeogene – lower Neogene of the Danish North Sea

Intense drilling activity following the discovery of the Siri Field in 1995 has resulted in an improved understanding of the siliciclastic Palaeogene succession in the Danish North Sea sector (Fig. 1). Many of the new wells were drilled in the search for oil reservoirs in sand bodies of Paleocene–Eocene age. The existing lithostratigraphy was based on data from a generation of wells that were drilled with deeper stratigraphic targets, with little or no interest in the overlying Palaeogene sediments, and thus did not adequately consider the significance of the Palaeogene sandstone units in the Danish sector. In order to improve the understanding of the distribution, morphology and age of the Palaeogene sediments, in particular the economically important sandstone bodies, a detailed study of this succession in the Danish North Sea has recently been undertaken. An important aim of the project was to update the lithostratigraphic framework on the basis of the new data. The project was carried out at the Geological Survey of Denmark and Greenland (GEUS) with participants from the University of Aarhus, DONG E&P and Statoil Norway, and was supported by the Danish Energy Agency. Most scientific results cannot be released until September 2006, but a revised lithostratigraphic scheme may be published prior to that date. Formal definition of new units and revision of the lithostratigraphy are in preparation. All of the widespread Palaeogene mudstone units in the North Sea have previously been formally established in Norwegian or British wells, and no reference sections exist in the Danish sector. As the lithology of a stratigraphic unit may vary slightly from one area to another, Danish reference wells have been identified during the present project, and the lithological descriptions of the formations have been expanded to include the appearance of the units in the Danish sector. Many of the sandstone bodies recently discovered in the Danish sector have a limited spatial distribution and were sourced from other areas than their contemporaneous counterparts in the Norwegian and British sectors. These sandstone bodies are therefore defined as new lithostratigraphic units in the Danish sector, and are assigned Danish type and reference sections. There is a high degree of lithological similarity between the Palaeogene–Neogene mudstone succession from Danish offshore boreholes and that from onshore exposures and boreholes, and some of the mudstone units indeed seem identical. However, in order to acknowledge the traditional distinction between offshore and onshore stratigraphic nomenclature, the two sets of nomenclature are kept separate herein. In recent years oil companies operating in the North Sea have developed various in-house lithostratigraphic charts for the Paleocene–Eocene sand and mudstone successions in the Danish and Norwegian sectors. A number of informal lithostratigraphic units have been adopted and widely used. In the present project, these units have been formally defined and described, maintaining their original names whenever feasible, with the aim of providing an unequivocal nomenclature for the Palaeogene – lower Neogene succession in the Danish sector. It has not been the intention to establish a sequence stratigraphic model for this succession in the North Sea; the reader is referred to the comprehensive works of Michelsen (1993), Neal et al. (1994), Mudge & Bujak (1994, 1996a, b), Michelsen et al. (1995, 1998), Danielsen et al. (1997) and Rasmussen (2004).

東北日本の白亜系―古第三系蝦夷前弧堆積盆の地質学的位置づけと層序対比

The Cretaceous to Paleocene Yezo forearc basin was developed between the western Early Cretaceous volcanic arc and the eastern Early Cretaceous to Early Eocene accretionary complexes in the northeastern margin of Eurasia Plate. Its sediments are distributed across a 150 km-wide and 1400 km-long belt from Northeast Japan to Sakhalin including off the Pacific coast in north-south direction. It is subdivided into three subbasins on the basis of geologic setting: Hokkaido, Kitakami and Joban subbasins. In the first half of this paper, the tectonic setting is reviewed not only for the better understanding of the basin, but also in order to evaluate the distribution of carbonaceous rocks as source rocks and reservoir sandstone.In the latter half, twenty-seven stratigraphic sections including 4 onshore and 8 offshore boreholes and 15 surface sections are correlated to delineate the sedimentary history and basin evolution through the temporal and spatial distributions of facies successions and depositional sequences. Two second-order shallowing-upward cycles for the Cretaceous are recognized in Hokkaido but are obscure in northern Honshu. Shallow-marine to paralic sediments of the first cycle (early Albian to Turonian) are represented by the Mikasa Formation (late Albian to Turonian) as the western marginal facies in Hokkaido. The Campanian to Maastrichtian shallow-marine and non-marine facies of the second cycle (Coniacian to Maastrichtian) are characterized by the Hakobuchi Formation in the Hokkaido subbasin and its correlatives in the Kitakami and Joban subbasins. The two formations include several third- and fourth-order depositional sequences with coarsening-upward facies successions. The uppermost Maastrichtian to Upper Paleocene interval is missing everywhere in the Yezo basin. Furthermore, two third-order depositional sequences of the late Paleocene (uppermost Hakobuchi Formation) and the uppermost Paleocene to lower Eocene (Haboro Formation) have developed with unconformities in a few areas.

The Mesoproterozoic basement in the southern Baltic Sea: insights from the G 14?1 off-shore borehole

The continuation of the Mesoproterozoic basement of the southern Fennoscandian Shield is documented in the G 14–1 off-shore borehole, northeast of the island of Rugen, where crystalline rocks of monzogranitic composition occur beneath flat-lying early Palaeozoic sediments at a depth of approximately 2,000 m. The greenish-grey, or partly reddish-grey, granites show a slightly porphyritic texture marked by plagioclase crystals or aggregates in a groundmass dominated by fresh microcline. Chloritized biotite occurs as a subordinate mafic phase. Ductile and brittle deformation is indicated by a weak foliation and the occurrence of several cataclastic zones, respectively. Major and trace element geochemistry suggest that these rocks are K-rich calc-alkaline granites and represent a restite-poor melting product of a granodioritic protolith. The low MgO, Cr, Ni and Co concentrations and relatively high content of accessory phases (apatite, zircon) point to formation of water undersaturated, high temperature (>900 °C) melts at low degrees of partial melting. Although the G 14 granite lacks hornblende, in contrast to most of the granites from Bornholm, it seems geochemically related to them. Analysed samples mostly fit an intermediate position between the Ronne and Vang granitoids, which both belong to a group of Mesoproterozoic intrusions showing partial ductile deformation. (Y+Nb) vs. Rb plots suggest that all these granitic rocks were generated in intracratonic conditions. A genetic relationship between them and contemporaneously intruded Karlshamn-group granites in Blekinge, eastern Scania and Smaland is supported by intrusion age of 1,460±3 Ma obtained from Pb-Pb isotope ratios measured on single zircons of the G 14 granite.

Distribution of moisture contents and thermoluminescence ages in an inner shelf borehole from the new Hong Kong International Airport site, China

Offshore boreholes and excavations available through coastal engineering projects in the siliciclastics-dominated inner shelf of the northern South China Sea off Hong Kong have permitted the reconstruction of Quaternary sea-level history dating back to almost 0.5 Ma. In this study, moisture content and thermoluminescence (TL) dating are utilized to help verify ages for marine and terrestrial sediments in a 39.52 m offshore borehole from the new Hong Kong International Airport site. The distribution of moisture contents in borehole samples indicated the presence of three palaeo-desiccated crusts formed by soil development when the marine deposits of Oxygen Isotope (OI) stage 5, OI stage 7 and OI stage 9 were sub-aerially exposed during the low sea-level stand(s) of OI stages 2–4, OI stage 6 and OI stage 8, respectively. Out of the six TL dated borehole samples, three from a terrestrial deposit horizon have yielded finite ages of about 250,000 yr BP suggestive of OI stage 8. In these samples, the TL signal of quartz grains was found to increase well beyond the 250 Gy saturation level with the relatively high radiation fluxes allowing for age determination for only three samples. The three remaining samples of pre-Holocene marine deposits yielded minimum ages each having reached TL saturation. The importance of the moisture content upon the final determination of these sediment ages and the collection and storage of samples for TL dating is clearly demonstrated.

Discussion of ‘Seabed imaging using a computer mapping package: an example from Dorset’ by T. D. Badman, M. A. Gravelle & G. M. Davis

D. M. McCann & M. G. Culshaw write: We welcome the publication of this interesting paper giving details of a bathymetric study of the near-shore area off Lyme Regis. This work adds to the results of major marine surveys carried out in this area between 1970 and 1975 by the Institute of Geological Sciences (IGS) (now the British Geological Survey [BGS]). As this work is not referenced by ⇓Badman et al. (2000), we feel that a brief description will complement their paper. In 1970 and 1971 two surveys were undertaken between Lyme Regis and West Bay for similar reasons to the survey described by ⇓Badman et al. (2000), namely to evaluate the immediate sub-bottom geology of the near-shore area and its possible effect on coastal landsliding (for example, ⇓Denness et al. 1975). The surveys followed earlier acoustic surveys in Weymouth Bay by ⇓Donovan & Stride (1961). Three boreholes were sunk, two to depths of 88 and 58 m near the beach at Charmouth (3.39 m above od [SY 3656 9308]; 3.43 m above od [SY 3653 9309]) and the other to a depth of 86 m at Penn Cross, between Charmouth and Lyme Regis (166.99 m above od [SY 3482 9435]). Logs of the boreholes are available in the BGS archives, together with physical property measurements on samples from the boreholes (⇓Culshaw & Whittaker 1973). Similar measurements were also made on samples from an offshore borehole and from gravity core samples (⇓Darton 1975). The physical property data were needed to aid interpretation of the …

The anomalous behaviour of properties of seams at the Plessey (M) horizon of the Northumberland and Durham coalfields

The Plessey (M) coal in the Northumberland and Durham coalfields has never had the economic significance of many other coal seams in the region because of its variable thickness and its sporadic occurrence as several thin seams. Where mined, the seam has been long known to display aberrant coking properties compared with coals above and below in the succession. Samples of Plessey coals were analysed from nine offshore boreholes, three opencast prospecting boreholes and three collieries. Vitrinite-reflectance measurements consistently suggest that the seams have an anomalously lower degree of coalification than would be expected from their position in the geological succession. Higher volatile-matter yields from “whole-coal” samples of the Plessey seam, compared with the volatile-matter contents of coals immediately above and below in the succession, confirm this view. The suppressed vitrinite reflectances of the Plessey coals cannot be attributed to particular geothermal conditions and overlying seam lithologies, as other reduced reflectances of certain seams in the Durham coalfield have been in the past. Neither is the suppression caused by the now familiar close association that sometimes occurs between vitrinites and liptinitic macerals. The suppression and the raised volatile-matter yields are more likely to be caused by the original conditions under which the Plessey coals were deposited. The behaviour of the Plessey seam parallels that of the Katharina seam of the Ruhr coalfield, which, at the time of its deposition, was marine-influenced. Although no direct evidence exists that the Plessey seam was similarily affected, the close association of leaves of the Plessey seam in several boreholes with the Harvey Marine Band suggests that marine, near-marine or calcium-rich conditions, possibly aided by a high liptinite content in the marine band, are the most likely explanation for the production of the perhydrous coals. Bitumen exudates would have been generated early during the coalification process, so causing the aberrant properties and unexpectedly strong coking behaviour of the Plessey coals.

Radiocarbon dating and the reconstruction of late Quaternary sea-level changes in Hong Kong

Abstract Radiocarbon dating is used widely for the reconstruction of late Quaternary sea-level changes. In the present study, eight case studies in Hong Kong have been chosen to highlight the difficulty in obtaining reliable radiocarbon dates. Pre-Holocene radiocarbon dates exceeding about 8100 yr BP have been found to show a young age bias. A shell sample dated by two separate radiocarbon laboratories gave last interstadial ages differing by almost 9500 years while an oxygen-isotope stage 5e age was obtained using uranium-series dating. Other evidence including stratigraphy have shown that only the last interglacial age is acceptable. The young age bias is attributed mainly to the post-depositional introduction of ‘young’ carbon into the sample. For the reconstruction of sea-level changes, it is essential to select samples immediately above of the base of the Holocene transgressive unit in offshore boreholes and excavations to satisfy the criteria of rapid burial. These samples should preferably include mud, plant matter and shells to facilitate interpretation. Furthermore, as many samples as possible from a range of water depth down to about 130 m below the present sea level should be dated to reduce error introduced by counting statistics so the true signal of sea-level changes can be reflected.

Towards a stable and agreed nomenclature for North Sea Tertiary diatom floras — the ‘ Coscinodiscus ’ problem

Abstract Diatoms are one of the most useful microfossil groups to be found in Tertiary (particularly Palaeogene) sequences of the North Sea subsurface for their biostratigraphic utility, especially where other mineralized-walled microfossil groups (e.g. foraminifera, calcareous nannoplankton, etc.) are absent. An example from one offshore borehole (Shell UK; 29/25-1) is given. However, their further use in biostratigraphic and sequence correlation is hampered by the lack of any stable nomenclature applied to their taxonomy. The numbers of so-called ‘in-house’ taxa are legion with duplication of forms almost inevitable. Few of these many and varied taxa are directly comparable between these schemes, and this leads to almost inevitable confusion for exploration, production and development geoscientists trying to correlate between the various schemes. One stratigraphically important and familiar diatom taxon ( Coscinodiscus sp. 1) is formally described in this paper as Fenestrella antiqua (Grunow) Swatman. In the example shown here it is demonstrated that four forms, previously identified as four independent taxa in open nomenclature (with distinctive stratigraphic ranges), are in fact separate manifestations of the life habitat of this one single species. It is hoped that through an awareness of the biological complexity of these forms, and their relationships to morphology, a stable taxonomy will eventually arise. It is furthermore hoped that this will lead to a stimulus in the study of the biostratigraphic and palaeoenvironmental applications of this important microfossil group to exploration, production and development geoscience.

3-D P-wave Velocity Structure in Western Greece Determined from Tomography Using Earthquake Data Recorded at the University of Patras Seismic Network (PATNET)

—The 3-D P-wave velocity structure of the upper crust in the region of western Greece is investigated by inversion of about 1500 residuals of P-wave arrival times from local earthquake data recorded in the year 1996 by the newly established University of Patras Seismic Network (PATNET). The resulting velocity structure shows strong horizontal variations due to the complicated structure and the variation of crustal thickness. Relatively low-velocity contours are observed in the area defined by Cephallonia—Zakynthos Islands and northwestern Peloponnesos. This is in addition to some well localized peaks of relatively higher values of P-wave velocity may be related to the zone of Triassic evaporites in the region and correspond to diapirism that breaks through to the uppermost layer. Finally, a low P-velocity ‘deeping’ zone extending from Zakynthos to the Gulf of Patras is correlated with Bouguer anomaly map and onshore and offshore borehole drillings which indicate that thick sediments overly the evaporites which exist there at depth greater than 2 km.

Retractable Bits Development and Application

This paper presents a historical view and present status of retractable bit design and drilling results. Despite the significant improvements in cone-sealed bearing rock bits, the special features of retractable bits provides a wider range of field applications. Scientific drilling, both continental and offshore and stratigraphic offshore boreholes, is an area for profitable application of retractable bits with a downhole motor. Another promising application is drilling and simultaneous casing where some prototypes have been successfully tested. Other applications such as horizontal and geothermal drilling look promising for future developments.

A playa deposit of pre-Yellow Sands age (upper Rotliegend/Weissliegend) in the Permian of northeast England

A complexly interbedded thin succession of sandstone, siltstone and mudstone beneath the Permian Yellow Sands Formation (upper Rotliegend/Weissliegend) was cored in a borehole 11 km off the coast of northeast England. The cored strata rest unconformably on fissured Westphalian B mudstone and mainly comprise low-dipping fine- to coarse-grained grey sandstone with unevenly spaced thinner dark-red to grey mudstone beds and laminae. Most of the sandstone beds are moderately to poorly sorted, fine- to very fine-grained subarkoses, with some sublitharenites, and are weakly to strongly cemented by dolomite (predominant), kaolinite and illite; sedimentary structures include subhorizontal plane to wavy lamination, ripple lamination, salt ridges and soft-sediment deformation. The siltstones and mudstones are more uniform in lithology and primary thickness; they contain sandstone-filled desiccation cracks, mudstone saucers and narrow sandstone dykes. Traces of possible former evaporite minerals are concentrated in the sandstones. By comparison with modem desert depositional environments, these strata are interpreted as the deposits of a playa occupying a depression on a stony, deflationary desert surface on a rock pediment or peneplain flanking the ancestral Pennines. The floor of the depression, which probably lay in the capillary fringe just above the contemporary water table, was periodically flooded so as to form a shallow playa lake. Comparable successions beneath the Yellow Sands have not been recorded from surface exposures in northeast England but mudstone beds at this stratigraphical level have been recorded in four other cored offshore boreholes and are presumed to be of similar origin to those described in this study. The stratigraphical position of these deposits suggests that they may be of pre-Yellow Sands age and coeval with or younger than the Basal Permian Breccia. Palynological analysis of mudstone samples from the core reveal the presence of plant material and indeterminate, pyrite-invested, possibly indigenous bisaccate pollen indicative of a vegetated hinterland with deposition occurring within a reducing, possibly sulphide-rich environment.

Structure and tectonic evolution of the Tornquist Zone and adjacent sedimentary basins in Scania and the southern Baltic Sea area

Southernmost Sweden, Bornholm and the surrounding Baltic Sea region are located on a large-scale releasing bend in the dextral strike-slip system of the Tornquist Zone, with its resulting pull-apart basins. The well constrained geology of Scania and Bornholm has been combined with detailed on- and offshore borehole data and three proprietary marine seismic surveys. This in conjunction with supplementary BABEL deep seismic reflection findings allows a combined 3D interpretation of sediment/structure interactions. As a result, a regional interpretation has emerged which gives a new understanding of the interplay between structural movement on a complex strike-slip fault system (Tornquist Zone) and its intrazonal depressions (Vomb Trough and Colonus Shale Trough) as well as the sedimentation history of associated areas of sediment accumulation (Rønne and Arnager Grabens, Höllviken Halfgraben, Hanö Bay Basin and Skurup Platform). Detailed sequential litho- and seismo-stratigraphic descriptions have been possible by combination of the various data sets. This resulted in the clarification or recognition of previously unknown structural limits to sub-basins and highs in the study area. A 3D chronological (4D) model for the development of the region is proposed. This model takes into account the long-lived structural history combining elements of strike-slip, extension and inversion tectonics. The deep-seated faulting controlling these structures is integrated with the deep structure as revealed by the BABEL line in this area.

Seismic sequence stratigraphy of the Palaeogene offshore of Belgium, southern North Sea

A fine-scale seismic stratigraphic model has been developed for the Palaeogene of the southern North Sea on the basis of interpretation of a dense high-resolution reflection seismic grid, covering the Belgian sector of the continental shelf and the adjacent parts of the Dutch, French and UK sectors. Classical seismic stratigraphic criteria have allowed up to 13 major units to be defined; the geometry and seismic facies characteristics of each have been analysed in detail. The seismic stratigraphy has been compared with the results of four offshore boreholes. ‘Events and trends’ identified on seismic sections and in outcrops in northern Belgium have been correlated, and offshore seismic facies have been tentatively matched with onshore lithofacies. The geological history of the study area is discussed in terms of eustatic sea level changes and regional tectonic events, and the main characteristics of the offshore Palaeogene deposits are evaluated in a sequence stratigraphic context.

C. A. M. Franks and N. W. Woods Reply

Dr Howat has raised two issues relating to the offshore sediments and proposed reclamation work in the vicinity of North Lantau. The first relates to the stratigraphy of the offshore sediments, the second to their engineering behaviour. The terminology used by Dr Howat to describe the offshore sediments is the simplified stratigraphical model current during the early 1980s. Over the past ten years considerable effort has been directed by the Hong Kong Geological Survey (HKGS) of this department into refining this early model. The stratigraphy has been defined from boreholes and mapped from a detailed study of high resolution seismic profiles. In general the offshore sequence of sediments over which reclamation is planned within this region comprises very soft to soft marine clays and clayey silts (Hang Hau Formation) unconformably overlying intercalated sands, gravels and firm to stiff silty clays (Chek Lap Kok Formation). Recent study of offshore boreholes and seismic surveys to the south-west of Chek Lap Kok has revealed another formation (Sham Wat Formation), albeit limited in extent within the area to be reclaimed, at the base of the Hang Hau Formation comprising a soft to firm clayey silt. Available evidence suggests the Chek Lap Kok Formation underlying the marine Hang Hau Formation, is predominantly alluvial in origin as shown by detailed analysis of continuously sampled boreholes (Shaw, Zhou, Gervais & Allen 1986). Recent detailed micropalaeontological analysis of sediments from off-shore boreholes has not revealed any definitive evidence of marine conditions within the Chek Lap Kok Formation. However,