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Combined outcrop and forward modelling studies were employed to improve upon conceptual sequence stratigraphic models of carbonate platform facies architecture during ice-house climate periods. The studied outcrops were chosen to reflect carbonate deposition in a range of sedimentary basin types of similar age (Moscovian) the Paradox Basin (Utah, USA), the Orogrande Basin (New Mexico, USA) and the Moscow-Mezen Basin (Arkhangel'sk Oblast, Russia). Results of outcrop studies were compared and contrasted with results of a one-dimensional stratigraphic forward model, designed to incorporate and test likely controls on carbonate icehouse systems. Outcrop studies and microfacies interpretation of the Honaker Trail Section (Paradox Basin) reveals no evidence of a sedimentary hierarchy, despite previous interpretation. Existing qualitative conceptual models of a sedimentary hierarchy are found to be flawed and an improved quantitative definition of a sedimentary hierarchy is presented. Results of numerical forward modelling suggest that the existence of a rigorously identifiable sedimentary hierarchy in the stratigraphic record is highly improbable. Comparison of sedimentary stacking patterns between the Orogrande Basin, the Moscow-Mezen Basin and numerical simulations suggest that although sedimentary cyclicity is highly likely to be forced by glacio-eustatic sea-level oscillations, the stacking patterns and intra-cycle facies distributions are controlled primarily by subsidence regime of the basin. Generally, it can be said that the best reservoir facies (net-to-gross thickness of grainstone) development will occur in moderately to rapidly subsiding extensional basins and moderately subsiding foreland basins. The absence of peritidal facies within ice-house carbonate successions is a sedimentological distinction between ice-house and green-house periods. Numerical forward modelling reveals that peritidal facies are developed during ice-house periods but because of their position within accommodation cycles tend to have low preservation potential.

Outcrop and laboratory analysis of the South Wales carbonate ramp validated the model hypothesis that a high rate of sediment transport is likely a dominant control on the development of a low gradient carbonate geometry. Several other ramp examples are also shown to portray diagnostic features similar to the South Wales ramp example, implying significant magnitudes of sediment transport in each case and further supporting the hypothesis that ramps are transport-dominated systems.

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No description available.

The geology of the south-east corner of the Askrigg Block and part of the Craven Basin surrounding Nidderdale, Yorkshire is described. The succession, more than 1,200 feet thick in the north and twice this thickness south of the North Craven Fault, ranges from the base of the Millstone grit to the base of the Permian, and is of Namurian and Westphalian age. The stratigraphy of each horizon is described in detail, attention being drawn to the following new finds: (i) The continuation of the Cockhill Marine Band, containing Cravenoceras cowlingense, (ii) Tylonautilus nodiferus in the Colsterdale Marine Series, (iii) shelly fossiliferous phases in the Upper Follifoot Grit, the Libishaw Sandstone, the Beverley Shales and the First Brimham Grit, (iv) Reticuloceras circumplicatile group in the Cayton Gill Shell Bed and Reticuloceras aff. pulchellum in the overlying shales, (v) an unconformity at the base of the First Brimham Grit, south of North Craven Fault, (vi) two Lingula bands and a band with Gastrioceras cumbriense in the Winksley Shales, the position of the latter suggesting the local absence of the R2 stage (vii) the presence of the basal Coal Measures, including a lingula band near the base. The succession is correlated with neighbouring areas. Structurally the district includes a northern area with gentle eastward dips and a southern area of shallow E.N.E. folds crossed by folds of N-S trend. The North Craven Fault, which approximately coincides with the junction of the two areas, is shown to have been active during R1 times in this district, but not subsequently. Its relation to the edge of the Askrigg Block is examined. The petrography of the sediments is described, a general absence of microline and garnet from the sandstones of the E stage being noted. Aspects of the water supply and mineral products of the area are mentioned and an appendix of borehole records incorporated.

Three-dimensional seismic data from the East Atlantic Margin are used to investigate the diagenesis and deformation of siliceous sediments. Three themes were tackled. At the Gjallar Ridge, offshore Norway, seismic and well data indicate that diagenesis of siliceous sediments occurs across a zone ~300 m thick. At the top of the zone circular regions, with relief of ~200 m, are interpreted as regions of preferential diagenesis. Below these, regions with a similar size and distribution are attributed to the same cause. The chemistry of formation water expelled transiently through polygonal faults may play a role in their formation. This is the first recognition from seismic data that silica diagenetic transformation zones can be hundreds of metres thick and heterogeneous, as observed at outcrop previously. On the Mauritanian continental margin layer-bound fault systems deform a probable siliceous succession. Where this succession mantles canyons the majority of faults strike perpendicular to the canyon axes due to increased bedding dip and most are antithetic to bedding dip. Where the bedding dip is greater than ~1°, synthetic faults dip more steeply than antithetic faults, which is the exact opposite that would be expected on a dipping margin. It is hypothesised that the layer hosting the faults has been subjected to simple shear of ~20° or more, rotating the fault planes. Lastly at the Vema Dome, offshore Norway four submarine slides are identified in siliceous sediments. Each covers an area of ~30 km2, is up to 600 m thick, with an upper surface topography consisting of a series of arcuate ridges perpendicular to the dip of the slope. The slides are dominated by fold-and-thrust structures, have short transport distances, and unusually low ratios of length to thickness. These characteristics are attributed to deep detachments and the shear strength of siliceous sediments at shallow burial depths.

Carbonate platforms are increasingly being studied using sequence stratigraphic concepts and models borrowed from the study of siliciclastic shelves in passive margin settings. The direct transposition of the stratigraphic model for a siliciclastic shelf to its carbonate counterpart, the carbonate shelf, assumes that the two systems respond in a very similar way to changes of relative sea-level, the interpreted major control upon depositional stacking patterns. Current models depicting the sequence stratigraphic evolution of carbonate shelves are and have been frequently applied without regard for the differences between the siliciclastic and carbonate shelf depositional systems. It is the purpose of this study to test the current sequence stratigraphic model and its assumptions for a carbonate shelf. Carbonate shelves do differ quite fundamentally from their siliciclastic equivalents. The carbonate shelf has the capacity to respond in quite different ways to changes in relative sea-level, compared to siliciclastic systems, as a result of the strong physio-chemical control upon carbonate sedimentation and the potential high rates of carbonate production at the shelf margin in comparison to rates of relative sea-level rise. Carbonate sedimentation rates are also differential across a shelf and highly sensitive to slight environmental' changes such as nutrient upwelling and temperature increases or decreases. This can lead to abrupt changes of sedimentation rate not necessarily related to changes of relative sea-level. Because of these differences carbonate shelves can develop stratal patterns similar to siliciclastic settings, but in the majority of cases they are very different. In direct contrast to siliciclastic systems the lowstand systems tract is normally impoverished on the flanks of carbonate shelves. Two different end-members of lowstand sedimentation are distinguished for carbonate shelves and these reflect the inherited morphology of the slope: low angle, mud-dominated slopes are characterized by basin-floor slides and debrites during times of falling relative sea-level and by a relatively large volume autochthonous slope wedge. In direct contrast, high angle slopes are characterized by basin-floor megabreccias and volumetrically very small or even absent autochthonous slope wedges. The carbonate transgressive systems tract can also develop a wide variety of stratal patterns, a reflection of the often complex interplay of variable sedimentation rates and rates of relative sea-level rise. Two different types of geometric stacking pattern are distinguished: type 1 geometries, developed when sedimentation rates are less than rates of relative sea-level rise, and type 2 geometries formed when sedimentation rates are equal to or greater than rates of relative sea-level rise. The highstand systems tract is the time of maximum carbonate production potential and is normally associated with rapid basinwards progradation. For the highstand systems tract two different types of foreslope progradation are distinguished, slope aprons and toe-of-slope aprons. These differences between carbonate and siliciclastic depositional models suggest that simple application of the previously published models can lead to incorrect interpretation of systems tracts, sequences and therefore relative sea-level curves. Sequence stratigraphic models and concepts are tested by application to the spectacular seismic scale exposures of the mid-Cretaceous Urgonian platform, SE France. The platform is divided into a lower regressive' part, the Glandasse Formation and an upper 'transgressive' part the Urgonian Limestone Formation. These are dominated by progradational outer-shelf grainstone facies and aggradational shelf-lagoonal facies respectively. Criteria are developed to identify key surfaces and stratal packages upon the Urgonian platform. On the shelf sequence boundaries are readily defined and are marked by sub-aerial exposure surfaces associated with meteoric diagenesis. Lowstand sedimentation is generally absent, but can be represented by lacustrine facies. Strong erosional truncation is only developed on the shelf if siliciclastics are introduced during lowstand of sea-level. Thus, the transgressive and highstand systems tracts dominate shelf sedimentation but can only be distinguished if a clear flooding surface is developed, and this is not always the case. On the slope large-scale erosional surfaces developed by sedimentary bypass and/or slope collapse can develop at any stage of a sequence and make identification of the sequence boundary more difficult Similarly, on the basin-floor allochthonous debris derived from slope collapse and/or bypassing is not restricted to times of falling relative sea-level. From the criteria developed for identification of key stral surfaces and packages a sequence stratigraphy for the Urgonian platform is built This is placed within the time scale of Haq et al. (1987), and relative sea-level curves for the platform are constructed. These are compared to the eustatic sea-level charts from which they differ significantly. Minimum aggradation rates are also compared to other well known ancient carbonate platforms, from which the Urgonian is shown to have very high sedimentation rates.

The Stainmore area is a typographical and geological depression lying between the two upstanding block areas of the Northern Pennines. The succession includes bede of Yoredale facies of the middle and upper Limestone groups, overlain by “Millstone Grit”, the total time range extending from late P(_2) age, probably to late E(_2). Detailed mapping of marine horizons has enabled the establishment of correlations between the successions of the Alston block to the north and tho Askrigg block to the south, They show that marine conditions were more persistant to the south and east of the stainmore area. Millstone grit facies entorent a lower horizon in the east and west of the stainmore area than it does in tho centre. Tho grite on the flanko correspond to the tenhill grite of N. W. Swaledale and are considered to pass laterally into the coaleleugh transgression bods in the centre. This is demonstrated by equating the upper felltop Limestone of Alston with the Heane Beck Limestone of upper Swaledale. An upper horizon of grit facies is subjacent to Botany Limestone and is considered to be the equivalent of the first Millstone crit of Durham and the water crag Grit of Upper Swaledale, the Botany Limestone corresponding to the shunner pell Limestone. The structure consists of an asymmetrical syncline, pitching gently to the east, with two small elongated domes to the south. The fault pattern on the east is similar to that of Alston block; on the west a semi-radical pattern is apparent. Joint directions are varied. The area has been affected by both the stainmore and Teesdale glaciers, the former being dominant only at first, and showing evidence of a minor readvance and second retreat.

The reservoir quality of Middle Eocene carbonates in the intracratonic Sirt Basin (at the northern margin of the African continent) is strongly influenced by depositional facies and various diagenetic modifications. This thesis investigates the petrography, sedimentology, diagenetic evolution and hydrocarbon potential of the Middle Eocene Gialo Formation in the subsurface of the north-central Sirt Basin based on data from core samples and well logs from five boreholes in the Assumood and Sahl gas-fields. Reducing risk in exploration demands an understanding of reservoir facies development, which is governed by the type and distribution of depositional facies and their diagenetic history. Seven major carbonate facies (and 20 microfacies) have been identified in this study and are interpreted as predominantly deposited under shallow-marine conditions within the photic zone, as indicated from their richness in phototrophic fauna and flora. These include lagoon (back-bank), main bank, fore-bank and open-marine facies, all of which were deposited on a homoclinal ramp type of carbonate platform. The type and distribution of the Gialo depositional facies were influenced by basin-floor architecture and environmental controls. The basin floor was shaped through pre-Eocene structural development into a series of elevated platforms and deep troughs. Platform facies were deposited across three broad facies belts: (1) inner-ramp, dominated by dasycladacean molluscan wackestone/packstone, nummulitic-bryozoan packstone, bryozoan wackestone; (2) mid-ramp, dominated by nummulitic packstone and Discocyclina-nummulitic wackestone; and (3) outer-ramp, dominated by fragmented nummulitic packstone. Troughs were dominated by thick successions of lime mudstone containing rare fine skeletal fragments and nummulites, with deposition taking place in a deeper-marine environment, below the photic zone. Present-day reservoir characteristics of the Gialo Formation are the net result of modification to the original depositional characteristics caused by diagenesis. This diagenesis took place on the seafloor, under burial, and in the meteoric diagenetic environment. Early marine diagenetic processes affecting the Middle Eocene Gialo carbonates resulted in micritization of bioclasts. Later diagenesis in meteoric to burial environments resulted in dissolution of aragonitic bioclasts, cementation (syntaxial overgrowths on echinoid grains, and blocky to equant, non-ferroan cements), neomorphism, pressure dissolution, compaction and fracturing. δ18O and δ13C values in the Gialo Formation range between -1.06 and -4.16‰ PDB, and 0.76 and 1.89‰ PDB, respectively. These values are mostly marine values, although some alteration is likely. The more negative oxygen of the cements suggests precipitation within the shallow-burial environment under the influence of meteoric water and / or precipitation at higher temperatures during further burial. The carbon isotopic signatures are typical marine values. There is a strong relationship between porosity and the diagenetic processes that-affected the Gialo sediments. Generally the porosity in the Assumood and Sahl fields is either primary or secondary, enhanced by dissolution and fracturing of the sediments. Reduction in porosity in the investigated sediments is mainly due to cementation and compaction. The common pore-types in the Gialo Formation are intergranular, moldic, intragranular, vuggy and scattered fractures. Porosity ranges from poor to very good (<1% to ~37%) and permeability varies from low to high (<1mD to 100mD). These variations in porosity and permeability are strongly related to facies changes, which were influenced by depositional environment and diagenetic processes. Shallow-water packstones/rudstones containing both primary intergranular and secondary biomouldic porosity have the best reservoir quality. The Gialo Formation is an important gas producing reservoir in the Assumood, Sahl and other surrounding fields. The gas which is generated from the gas-prone Sirt Shale source rock of the northern Ajdabiya Trough possibly migrated onto the Assumood Ridge from the northeast through late Cretaceous, Paleocene and early Eocene carbonates, before being trapped beneath the Augila Shale (Upper Eocene) which is the principal regional seal in the area. This integrated study has helped to understand the reservoir heterogeneity and potential of the Gialo carbonates and based on this current wells are being completed appropriately, as, hopefully, will future wells too. The facies pattern is different from one well to another, which does suggest that there was a strong tectonic control, that is differential tectonic subsidence and/or fault control, or that deposition was controlled by autocyclic processes. The different vertical positions and numbers of transgressive-regressive cycles in each well make formation-wide correlation problematic. The lack of correlation in terms of cycle thickness, as well as facies, between wells, also suggests autocyclic processes. Third and fourth-order relative sea-level (RSL) changes do not appear to have been a major control on deposition during this Middle Eocene time.