The Machar Cretaceous chalk oilfield, Central North Sea, is a structural trap of chalk folded above a diaper of Zechstein salt, and formed a regional leakoff point for overpressure from Jurassic sandstones deeper in the basin. Chalk was deposited as pelagic sediment, but re-deposition by gravity flows improves reservoir quality. Diagenesis of the Machar chalk matrix occurs during burial to 1Km. Stylolites form in the chalk at depths below 600m and cause cementation of the reservoir through pressure dissolution and precipitation in a closed system. During diagenesis of the chalk matrix the underlying salt diaper evolves and as a result four fracture sets begin to form in the chalk reservoir. Hydrocarbon charge at mid-Miocene halted matrix cementation resulting in an exceptionally porous (30%mD) reservoir. The growth and evolution of the diaper is marked by the formation of healed fractures within the chalk reservoir. Fracture filling calcite records the evolution of the Tor Formation reservoir from a closed system, rock-dominated environment to an open diagenetic system. Four different fracture types have been discovered within the Tor formation on the Machar field. Of these, the first to form was Fracture Type 1, relating to bedding stylolites formed through minor extension at the crest of the reservoir as the salt attempts to remain buoyant. This fracture is filled with Calcite 1. δ¹³C values within this calcite (+ 2.0 to + 2.3‰ PDB) are similar to the values measured in the matrix chalk (+ 1.5 to + 3‰ PDB), suggesting that like the matrix cement this formed in a rock-dominated system. ⁸⁷Sr/⁸⁶Sr (0.7078 to 70787) values within Calcite 1,also similar to the matrix values (0.70770-0.70791) supports this theory. Negative δ¹⁸O (-5 to -7‰ PDB) values measured in Calcite 1 are explained by increased burial temperature during precipitation. Fracture type 2, the second fracture to form, is filled with Calcite 2; δ¹³C values (+ 3.6 to + 5.9‰ PDB), are more positive than the matrix values (+ 1.5 to + 3.0‰ PDB), and mark the opening of the reservoir to an external fluid. Fracture type 3 is filled with Calcite 3, and forms as Palaeogene sediments begin to down build around the evolving salt diaper. Calcite 3 precipitated from an external fluid that is restricted in its migration between tectonic stylolites. Calcite 3 is replaced by saddle dolomite, celestite, barite and fluorite.

The Upper Cretaceous (Maastrichtian) to Upper Eocene Ulukisla Basin in representative of the sedimentary and tectonic evolution of extensive Early Tertiary basins in central Anatolia, including the Tuzgölü and Sarkisla Basins. These basins lie along the proposed Inner Tauride Ocean suture zone between two microcontinental units in central Anatolia (Turkey), the Tauride Block and Nigde-Kirsehir Massif. Investigation of the Ulukisla Basin shed light on processes of microcontinental collision and ocean basin suturing of Neo-Tethys, which hopefully, will be applicable to other zones of continental collision (e.g. central Tibet). The Ulukisla Basin overlies a Late Cretaceous ophiolitic mélange located between the Bolkar Carbonate Platform to the south and the Nigde-Kirsehir metamorphic massif to the north. The basin stratigraphy records successive phases of transgression, subsidence, volcanism, evaporite deposition, deformation and uplift. The nature of these events can be interpreted by assessment of the lithostratigraphy in terms of sedimentary facies, biostratigraphy, structural geology and volcanic geochemistry. Further characterisation of the basin-history was achieved by analysis of sedimentary structures, construction of subsidence curves and provenance studies. Transgression in the Ulukisla Basin, during the Maastrichtian, followed by large-scale subsidence, is evidenced by shallow-marine sedimentation and subsequent deposition of unstable slope facies and turbidites. The basin succession includes c. 2 km of Upper Palaeocene-Lower Eocene basaltic to andesitic submarine pillow lavas, lava flows, volcaniclastics and intercalated limestones. Whole-rock XRF chemical analysis indicates a within plate-origin, with a marked subduction influence (e.g. relative Nb depletion). Uplift and closure of the basin are recorded by evaporite deposition, widespread compressional deformation and a regional unconformity. The subsidence curves are consistent with an extensional (or trans-tensional) basin terminated by uplift.

Deposits of temperate water biogenic carbonate sediments occur on the inner western continental shelf of Scotland. A general survey of their extent and composition has been made and facies maps prepared for the area. One small portion of the area, the Sound of Iona, was studied in detail to ascertain the origin of the carbonates and their bed-forms and to attempt an estimate of local carbonate productivity. In Iona Sound, barnacles and molluscs are the most important bioclastic constituents with locally dominant coralline algae (maerl). Barnacles predominate over most of the Sound because they grow prolifically on the coast, are readily broken down to sand size and easily transported. Four facies are recognized in Iona Sound: 1) rippled sand, 2) sand waves, and sand ribbons, 3) in situ maerl, and 4) relict glacial drift deposits. Seismic studies indicate that 1) bioclastic sediment is thickest at the sand wave facies diminishing in thickness laterally; 2) the sand waves are draped, over a bathymetric high formed from glacial drift; 3) in the northern Sound glacial drift formed deltaic deposits. Facies are controlled as follows: the rippled sands are deposited under the influence of wind-generated waves and low-velocity tidal currents which winnow fines and create low bedforms; the sand waves are formed by the drag of tidal currents crossing the bathymetric high, sculpting a hierarchy of bedforms; an associated sand ribbon is formed by tidal current acceleration following deflection by an island; maerl forms because of 3-sided wave shelter, current deceleration, erosion resistant morphology and a greater growth rate of the algae over the ambient sedimentation rate. Glacial deposits were exhumed by scouring or lie exposed in areas of very slow sedimentation, or both. Recent carbonate sedimentation is related tc Pleistocene glacial activity which created a complex coastal and seabed form trapping terrigenous sediment (termed "basinal terrigenes") in bathymetric deeps, lochs and convolute coastlines. Shallow-water, terrigene depleted, indurated rock localities of the inner shelf are exploited by carbonate producing organisms with high productivities shedding abundant bioclastic material to form "island margin carbonates".

No description available.

An integrated sedimentological, stratigraphical, structural and geochemical study has been used to establish the tectono-sedimentary evolution of the Pelagonian and Vardar (Axios) zone in N Greece. This is a key area for understanding the evolution of the Neotethyan ocean. The Pelagonian zone is interpreted as a rifted (Gondwana derived) continental fragment, whilst the Vardar zone formed a bordering Neotethyan ocean basin to the E. This study concentrated on the W part of the Vardar zone (Almopias subzone). A major aim of this thesis is to establish the evolution of sequences along the Pelagonian continent-Almopias ocean interface, from Permo-Triassic to Tertiary time. Rifting of the Almopias zone was probably initiated in the Permo-Triassic, resulting in ocean basin formation (WPB to MORB-type crust) by at least Upper Triassic (Vryssi Unit), and deposition of a clastic sequence in the Pelagonian zone followed by the establishment of a thick sequence of algal marbles during the Mid Triassic to ?Liassic (Pelagonian platform). Thus a clear W-E proximal to distal facies change from the Pelagonian zone to the Almopias zone can be recognised. Intraplatformal basins also developed within the Pelagonian zone at this time, whilst the Likostomo/Livadia-Klissochori-Rhizarion fragment possibly represented an isolated rifted fragment (Pelagonian derived) in the Almopias zone. By at least the L-M Jurassic a ocean basin existed in the Almopias zone. Its E limited was marked by subduction beneath the Paikon arc. During the M Jurassic the Pelagonian platform underwent dramatic subsidence associated with the transition to a foreland basin ahead of emplacing ophiolite nappes and parts of a subduction-accretion complex. Geochemical and stratigraphical data suggest a Pindos derivation. MORB, WPB and BSV volcanics are present. Complete subduction of the Almopias ocean also took place at this time by a combination of subduction beneath the Paikon arc and collision between the Paikon and Likostomo-Livadia fragments with the E margin of the Pelagonian zone. This was associated with transpressional (dextral) deformation and metamorphism of the E Pelagonian zone (Eohellenic event). A transgressive cover of Oxfordian-Kimmeridgian age throughout the Pelagonian and Vardar zones dates the age of ophiolite emplacement and Eohellenic deformation.

Pre-Late Jurassic basement of the Central Pontides comprises a thick subduction-accretion complex, amalgamated since Late Palaeozoic. Detailed structural, sedimentological and geochemical studies reveal a number of major tectonic units, assembled through plate tectonics processes. Two oceanic basins are recognised, separated by two different tectonic units. The first is the <i>Devrekani Metamorphic Unit</i>, gneisses and amphibolites at the base, transgressively overlain by metamorphosed carbonates. This unit is interpreted as basement of a rifted south Eurasian margin fragment. The cover of this unit may be represented by the Palaeozoic of Istanbul and Early Mesozoic sequences of the W Pontides. The second unit is the <i>Cangaldav g Complex</i>, a 10 km-thick, imbricated pile of evolved volcanics and volcaniclastics, overlying oceanic basement, comprising sheeted dykes and basic lavas. This unit is interpreted as a Late Palaeozoic south-facing oceanic arc. The northern oceanic basin is represented by the Küe Complex, a structurally thickened wedge of siliciclastic turbidites, interleaved with a dismembered, supra-subduction zone ophiolite. The Küe Complex is interpreted as a Triassic to Early-Mid Jurassic subduction-accretion complex of southward polarity. The southerly basin is represented by the <i>Domuzdav g-Saraycikdav g Unit</i>, a Palaeozoic-Early Mesozoic subduction-accretion complex of northward polarity, comprising an ophiolitic melange in the north and an accretionary prism in the south, both metamorphosed to blueschist facies. Meta-basites are of MORB type. Structurally beneath is a collapsed Permian carbonate platform, together with its passive margin sequences, both to the north and south. Lavas associated with these passive margin sequences are of within plate-type without an identifiable subduction component. In the proposed model, Palaeotethys was subducted northwards under an active Eurasian margin during Late Palaeozoic time, giving rise to a near continental margin arc. A continental sliver was rifted off (Devrekani), related to transform and/or active margin processes, opening the Küe Basin as a back-arc basin in latest Palaeozoic-earliest Mesozoic time.

No description available.

Theoretical models for the evolution of extensional sedimentary basins make a number of simple, testable predictions for subsidence behaviour. These are that active extension (the syn-rift) will generally be accompanied by rapid subsidence on the downthrown side of normal faults. Once faulting has ceased (the post-rift) the entire basin is predicted to subside at an exponentially decreasing rate, driven by the cooling and thickening of the lithosphere. The aim of this thesis is to determine the significance of second-order departures from this predicted subsidence. Three periods of North Sea subsidence appear to violate these simple predictions: anomalously slow Late Jurassic/Early Cretaceous subsidence, which marks the syn-rift/post-rift transition, and two periods of accelerated post-rift subsidence during the Early Paleogene and Plio-Quaternary. Subsidence data from over 300 boreholes from several basins with different rifting histories on the Northwest European continental plate have been analysed to determine the spatial and temporal distribution of these and other second-order subsidence anomalies. Uncertainties and errors in the observed subsidence calculations cannot explain the anomalous behaviour recognised. The periods of apparently anomalous subsidence are, instead, shown to be the result of geological or tectonic processes that modified either the subsidence history or record of subsidence of the basins studied. Apparent slow Late Jurassic/Early Cretaceous subsidence in the North Sea was the result of well siting and sediment starvation. Analysis of wells from the downthrown side of normal faults in several basins reveals an excellent correlation between rapid subsidence (often >1000m in 10My) and active extension (as documented from other sources of geological information). This was obscured by intense sediment starvation (sedimentation rates <20m/My) in the Central North Sea during Late Jurassic times which generated the apparent subsidence anomaly. Sedimentation rate maps reveal an expansion of the area of sediment starvation during late Jurassic times and into the Early Cretaceous. Cretaceous to Recent sedimentation patterns were then dominated by the interplay between the location and erosion history of extra-basinal sediment source areas and the remnant underfilled Jurassic rift topography.

With the aim of understanding the relative role of salt mobility and other controls on depocentre development in salt basins, results obtained from strategic targeted observations of three different sedimentary basins are presented. These include the Paradox Basin of Utah/Colorado, USA, where sequences which are the stratigraphic response to Pennsylvanian-Triassic mobility of a Pennsylvanian salt sequence are exposed and investigated at the outcrop scale. The other basins are the Sole Pit/Silver Pit Basin (Southern North Sea) and Shearwater area (Central North Sea), both of which are exclusively subsurface examples that have experienced Mesozoic and Tertiary mobility of Upper Permian salt sequences. These later study areas are investigated using high-resolution 3D seismic data which allow the large-scale structural and stratigraphic geometries to be investigated well beyond the outcrop scale. Observations reveal a wide variety of complex depocentre styles with varying controls on their development. Controls include tectonics, differential sedimentation, availability of salt to move and potential of the overburden to flex or be penetrated. Despite this complexity, depocentre morphologies are considered to be predictable based on the concept that salt moves as a response to the pressure state in the salt layer exerted upon it by its overburden. As salt flows down pressure gradients, subsidence resulting from salt mobility is predictably and consequent sediment accumulation increases the load providing an intricate feedback between salt mobility and sedimentation.

At present there is a poor understanding of changes in rates of erosion over long time periods and in the link between erosion and sediment delivery in Britain. Using a catchment study approach sediment accumulation rates in cores from the three lakes Semer Water, Gormire and Hornsea Mere have been used to reconstruct changes in sediment yield over time periods of up to 10,000 years, and estimate the mean annual flux of sediment to the Humber estuary. Each of the sites lies in a catchment of differing land use, relief and geology but taken together they are representative of the Humber catchment. Sediment accumulation in three cores from Semer Water has been used to determine a mean sediment yield of 8 t km<SUP>-2</SUP> a<SUP>-1</SUP> since 1950. Unusually thick sediment sequences were identified upstream of the existing lake Semer Water in the Raydale valley. Resistivity profiles and gouge cores were used to map the extent of these deposits and <SUP>14</SUP>C and pollen analysis used to establish their chronology. The combined sediment mass of Semer Water and Raydale deposits has been calculated at 11 million tonnes. This translates into a mean Holocene sediment yield of 24 t km<SUP>-2</SUP> a<SUP>-1</SUP>. The topography of five representative gullies was used to calculate the potential volume of sediment produced from gully erosion in the catchment. This technique indicates that the entire mass of sediment deposited in Raydale during the Holocene may have been produced from gully and channel erosion.