bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxygen-related biofacies in marine strata: an overview and update
Authors: Savrda Charles E. , Bottjer David J.
Published: April 2008

Recent studies of contemporary basins and ancient strata have provided a basis for expanding and refining early oxygen-related marine biofacies models. New observations permit higher resolution in palaeoecological, palaeoceanographic, and basin analyses. Five oxygen-related biofacies are currently recognized. (1) Anaerobic biofacies: well-laminated strata lacking in situ macro- and microbenthic body fossils and microbioturbation; may contain well-preserved remains of nektonic vertebrates, epiplanktonic or otherwise transported invertebrates, and faecal material of planktonic and/or nektonic origin. (2) Quasi-anaerobic biofacies: laminated strata, subtly disrupted by microbioturbation, containing microbenthic body fossils but lacking in situ macrobenthic body fossils; allochthonous body fossils and recognizable planktonic faecal material may be common. (3) Exaerobic biofacies: laminated strata similar to that of anaerobic or quasianaerobic biofacies but containing in situ epibenthic macroinvertebrate body fossils. (4) Dysaerobic biofacies: bioturbated strata characterized by low-diversity assemblages of relatively small, poorly calcified macrobenthic body fossils or absence of body fossils altogether. (5) Aerobic biofacies: bioturbated strata (where physical processes do not dominate) containing diverse assemblages of relatively large, heavily calcified macrobenthic body fossils. Recognition of these biofacies facilitates the recognition of relative palaeo-oxygenation gradients along the seafloor, vertically across the sediment-water interface, and through time. Application of sensitive trace-fossil models permits even more detailed reconstructions of benthic oxygenation histories for bioturbated strata, particularly those that fall
within
the previously defined dysaerobic realm.
Submarine erosion on the anoxic sea floor: stratinomic, palaeoenvironmental, and temporal significance of reworked pyritebone deposits
Authors: Baird Gordon C. , Brett Carlton E.
Published: April 2008

Numerous black (organic-rich) shale units in the Palaeozoic of North America are marked by discontinuities along their bases which are characterized by acid-insoluble lag concentrations of reworked pyrite, phosphatic debris, and siliceous material. These contacts, ranging in magnitude from widespread disconformities (typically marine flooding surfaces) to local, within-outcrop-scale, scour features, were produced by submarine erosion under anoxic to lower dysoxic conditions. Dissolution of carbonate debris under conditions of carbonate undersaturation and/or low pH in bottom waters, was generally complete. Hydraulic transport coupled with dissolution led to the formation of laterally discontinuous and current-sorted lenses of pyritic grains, fish bones, condonts, quartz sand, chert nodules, and glauconite. Lags of reworked pyritic tubes and small spherules are typical of finely interbedded black and green shale successions (‘zebra facies’) observed in the Upper Devonian (Famennian). Under anoxic conditions pyrite was chemically stable, while carbonate debris was not.

Herein, we examine lag deposits associated with a variety of black-shale-roofed discontinuities ranging in age from Middle Ordovician to Mississippian. Possible erosion mechanisms for producing these discontinuities and associated lags include deep-stormwaves, density currents and internal waves. In each case erosion occurred following an interval of condensation associated with flooding surfaces, particularly those timed with the beginning of highstand intervals.
Keys to the depositional history of the Posidonia Shale (Toarcian) in the Hils Syncline, northern Germany
Authors: Littke R. , Leythaeuser D. , Rullkötter J. , Baker D. R.
Published: April 2008

The Posidonia Shale from northwest Germany displays a threefold stratigraphic subdivision: lower marlstone, middle calcareous shale with bivalve shells, and upper calcareous shale. Deposition commenced after transgression over a disconformity on the Pliensbachian. Absence of current features indicates depositional conditions below wave base. Sediment infilling to water depths coincident with wave base probably caused cessation of Posidonia Shale deposition.

Calcitic carbonate, organic matter and detrital clay are principal primary components. TOC values are 2–15%. Macerals are mostly from marine phytoplankton; terrigenous macerals are minor. HI values are high (500–800). Detailed sampling reveals a thin (4.5 m) low carbon zone within the middle shale; water column oxidation and bacterial alteration may account for this facies. Clay and organic matter relationships suggest that land-derived nutrients controlled phytoplankton productivity; carbonate constituents were a diluent to organic matter accumulation.

Absence of burrowing, abundance of organic matter, and pervasive diagenetic pyrite demonstrate continual anoxia below the sediment-water interface. However, C/S relationships suggest that a euxinic water layer did not extend far above the depositional surface. The occurrence of benthic bivalves in the middle shale indicates that bottomwater tolerable to epibenthos sometimes prevailed. Sulphate-reducing bacteria modified the primary organic matter causing a 20–30% loss.
Inorganic geochemistry of the German ‘Posidonia Shale’: palaeoenvironmental consequences
Authors: Brumsack Hans-J.
Published: April 2008

42 samples of the SW and NW German ‘Posidonia Shale’ have been analysed for their major element and sulphur isotopic composition, as well as the trace metals Ag, As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, Sr, V, and Zn. Several of these trace metals are found enriched in the TOC-rich ‘bituminous’ layers. A comparison of the enrichment of specific metals in the Toarcian shales with black shales from the Cenomanian/Turonian anoxic event and Recent TOC-rich sediments from the Black Sea and the Gulf of California reveals a strong similarity of the ‘Posidonia Shale’ with Black Sea sapropels. The origin of the trace metals found enriched in this rock type predominantly seems to be fluvial input. Seawater alone fails to explain the high Co, Mn, and Pb concentrations of the ‘Posidonia Shale’. The environment of deposition therefore seems to be characterized by less saline surface waters overlying intermittently anoxic, more saline water masses of this epicontinental sea. Anoxic conditions must have persisted during the deposition of some bituminous layers and probably during the ‘falciferum’ Subzone.
Fate of organic carbon and reduced sulphur in dysoxic-anoxic Oligocene facies of the Central Paratethys (Carpathian Mountains and Hungary)
Authors: Vetö István , Hetényi Magdolna
Published: April 2008

The fate of organic carbon and reduced sulphur has been studied in two laminated and organic-rich formations of the Oligocene Central Paratethys. The Menilite Beds (Carpathian Flysch Belt) contrast with the Tard Clay (Hungarian Paleogene Basin) and are characterized by a high rate of sedimentation and biogenic silica enrichment.

About 80 core and outcrop samples were analyzed for their TOC, total sulphur, sulphate sulphur and pyrite iron content. Rock-Eval pyrolysis was carried out on more than half the samples.

On the basis of C/S relationships, the Menilite Beds are believed to have been deposited under sulphidic bottom water, while the existence of a sulphidic water at the bottom of the ‘Tard Sea’ cannot be documented.

C/S relationships, organic maturity data, and knowledge of anaerobic bacterial processes, especially the mechanisms and extent of the early diagenetic escape of reduced sulphur from recent sediments, permit assessment of the rates of organic carbon deposition. The obtained values of 10.8 t m
−2
Ma
−1
for the Menilite Beds and 1.4–1.8 t m
−2
Ma
−1
for the Tard Clay fit reasonably with geological settings and reflect the intensity of the corresponding planktonic productivities. The significant differences observed in TOC content between the two studied Tard Clay sections were caused by difference in the intensity of sulphate supply.
Expulsion of petroleum from type III kerogen source rocks in gaseous solution: modelling of solubility fractionation
Authors: Leythaeuser D. , Poelchau H. S.
Published: April 2008

The composition of n-alkane mixtures expelled from type III kerogen-bearing source rocks at moderate maturity levels shows strong signs of fractionation according to molecular chain length. This effect is attributed to the process of expulsion in gaseous solution. The degree of fractionation varies with the pressure and temperature conditions of primary migration. Fractionation effects for C
15+
-n-alkanes have been simulated with the help of equations developed from experimental data of Price
et al.
(1983) for gaseous solution of oil. For known compositions of expelled n-alkanes our simulation model allows the estimation of pressure and temperature conditions during primary migration. Expulsion of petroleum in gaseous solution is a particularly effective migration mechanism for overpressured type III kerogen-bearing source rocks.

Two geological case histories are presented which demonstrate the compositional effects of petroleum expulsion in gaseous solution from such source beds. The observed n-alkane distributions expelled can be closely matched by simulated distributions, which were calculated for
P-T
conditions estimated from reconstructions of the burial histories of both case histories.
Numerical modelling of petroleum expulsion in two areas of the Lower Saxony Basin, Northern Germany
Authors: Düppenbecker S. J. , Dohmen L. , Welte D. H.
Published: April 2008

The process of petroleum expulsion is mainly determined by the initial quality of the source rock and its sedimentary facies and by the temperature and pressure histories during burial. A numerical model of the complex processes of expulsion was developed based on observations of expulsion phenomena, detailed petrophysical and organic geochemical investigations and data provided by basin modelling. In this model petroleum expulsion is treated as a pressure driven transport in a separate phase using the existing pore system and newly formed microfractures as migration pathways. Microfracturing predominantly parallel to the bedding plane, as it is observed in oil generating source rocks, is initiated when the pore pressure build-up exceeds the rock strength. Major sources of overpressure in source rocks are considered to be volume expansion of organic matter during petroleum generation and compaction disequilibrium. Experimental data were obtained from detailed investigations of the Posidonia Shale source rock of the Lower Saxony Basin. An application of expulsion modelling is presented for two regions of the Lower Saxony Basin, where timing and quantity of petroleum generation and expulsion from Posidonia shale was studied with respect to different heating rates. The efficiency of transport mechanisms and migration pathways through both the pore network and newly-formed fractures could be determined by this quantitative treatment. The modelling results were in good agreement with geological observations and a mass balance based on organic geochemical analysis.
Phase-controlled molecular fractionations in migrating petroleum charges
Authors: Larter Stephen , Mills Nigel
Published: April 2008

An experimental approach has been used to make a preliminary examination of the possible effects of partitioning processes distributing molecular markers between vapour and liquid petroleum phases during petroleum migration in the system gas-saturated oil/condensate-saturated gas. These effects are discussed with reference to several traditional petroleum geochemical parameters. Some source and maturity related geochemical parameters are dispersed from the inherited source values when a petroleum system undergoes a transition from a single phase gas-condensate to a two phase system and it is probable that partitioning is one of the many processes that contribute to the scattered data sets observed in natural systems particularly those associated with gas/condensates.

In addition to affecting gross composition and gas and gasoline range composition, physical partitioning of petroleum components between vapour and liquid phases in deep reservoirs containing rich gas condensate vapour and a liquid petroleum phase in equilibrium can affect even high molecular weight components, selectively partitioning lower homologues into the vapour phase. This may disperse to various degrees any source inherited signature in source facies or maturity interpreted from parameters such as aromatic hydrocarbon distribution, gasoline range parameters, or sterane carbon number distribution.

Migration history, or the frequency and extent of multiple equilibrations between vapour and liquid phases in deep high pressure reservoirs followed by selective release of one phase to a shallower position to produce a two phase petroleum system is proposed as an important variable in the geochemical interpretation of once equilibrated petroleum systems. It is concluded that phase related effects must be considered in addition to accepted source and maturity considerations as controlling the molecular composition of two-phase petroleums derived from single-phase precursors. This is particularly so for rich gas condensate systems.
A case study of migration from the West Canada Basin
Authors: Piggott N. , Lines M. D.
Published: April 2008

A regional model for hydrocarbon charge in the West Canada Basin is presented. Three discrete ‘hydrocarbon cells’: Devonian, Mississippian-Neocomian and mid Upper Cretaceous (Post-Mannville) host petroleum which was indigenously generated from five major source rocks. Source depositional environment and Columbian-Laramide basin geometry control this pattern. On a regional scale, lateral carrier bed migration dominates basin plumbing, with the mid Cretaceous Mannville Group (oil sands) acting as the ultimate gathering point for subcropping source-carrier systems.

At a play-specific level, an example of significant local vertical migration within the gas phase is presented to explain charge of Permo-Carboniferous sands in the Peace River Arch area.
The northern Oman Tethyan continental margin: stratigraphy, structure, concepts and controversies
Authors: Robertson A. H. F. , Searle M. P.
Published: April 2008

The Oman Mountains constitute one of the best areas in which to study the development and subsequent deformation of a passive continental margin. Pulsed rifting of the Arabian craton in Early and Late Permian time was followed by Middle to Late Triassic sea-floor spreading and opening of the Tethys ocean. Passive margin subsidence continued throughout the Jurassic and Early Cretaceous, interrupted by a pulse of crustal extension in Late Jurassic (Tithonian) time. The Semail ophiolite was generated during the Cenomanian (
c.
95–98 Ma), probably above a newly initiated intra-oceanic subduction zone. Mesozoic Tethyan oceanic crust was later consumed in an oceanward- (northeast-ward)-dipping subduction zone. The essential driving force was the attempted subduction of the Tethyan continental margin. Progressive collision led, in turn to the development of a peripheral bulge and foreland basin (
c.
90 Ma), thrusting and eventual emplacement of the ophiolite onto the margin during late Campanian-Maastrichtian time. Thin-skinned thrust tectonics dominate the entire northern Oman continental margin, with late-stage deeper level thrusting affecting shelf edge and platform carbonates. The basal detachment extends down into deeper levels in the pre-Permian basement in the Musandam Peninsula in the north, and the Jebel Akhdar and Saih Hatat windows further south. During the Early Tertiary, continuing convergence was accommodated along the opposing Makran active margin, while a relatively stable platform was restored in the Oman Mountains area. Renewed compression during the mid-Tertiary, related to continental collision in the Zagros Mountains to the northwest resulted in short distance thrusting further onto the foreland in the northern Oman Mountains. The whole area currently lies within a zone of diachronous continental collision between the African and Eurasian plates.
Sedimentary and structural evolution of a continental margin transform lineament: the Hatta Zone, Northern Oman Mountains
Authors: Robertson A. H. F. , Kemp A. E. S. , Rex D. C. , Blome C. D.
Published: April 2008

The Hatta Zone, Northern Oman Mountains, is interpreted as a
c.
50 km long left-lateral offset of the North Oman passive continental margin that was generated by right-lateral transform faulting during spreading of the Neo-Tethys ocean. During Early-Late Triassic time, slumps, debris flows and calciturbidites composed of redeposited upper slope and shelf carbonates accumulated at the base of a steep carbonate platform slope (Maqam Fm., Mayhah Fm.; Sumeini Group). Clasts in channelized conglomerates include basement-derived quartzose sandstone, schist and basic extrusives. Basaltic pillow lavas were erupted at the foot of the Late Triassic continental rise of the Hatta Zone transform lineament (Zulla Fm.). During Early to mid Jurassic time, oolitic and lithoclastic calciturbidites and debris flows accumulated on tectonically unstable base of slope (Mayhah Fm.) and continental rise (Guweyza Limestone Fm.) settings. During mid-Cretaceous time (Albian-Cenomanian), alkaline magmatism constructed seamounts along the inferred oceanward extension of the Hatta transform lineament. K/Ar ages of diabase sills range from 109 Ma ± 4 Ma, to 105 Ma ± 3 Ma on sills, while basalts gave ages of 104 Ma ± 4, to 93 Ma ± 4. During the Late Cretaceous ophiolite emplacement, abyssal plain and base of slope units were deformed into a major transverse culmination (Jebel Quimah), mainly controlled by collision with the Hatta Zone continental margin transform lineament. Hanging wall imbrication and overturning of structurally high thrust sheets occurred above an inferred footwall ramp. As the Semail ophiolite overrode a northfacing transform escarpment margin, footwall slope sediments were sheared and deformed into a major lateral ramp, now represented by the Jebel Raudha-Masfut ridge. Normal faulting and deep erosion ensued, followed by Maastrichtian-early Tertiary shallow marine carbonate deposition on an unstable shelf. During renewed, mid-Tertiary deformation the Hatta Zone was reactivated, giving rise to NW-SE trending open-folding at right angles to the regional W-directed compression, followed by differential erosion to form the present lineament.
The tectonic evolution of pre-Permian rocks, Central and Southeastern Oman Mountains
Authors: Mann A. , Hanna S. S.
Published: April 2008

The structural history of pre-Permian basement rocks exposed in the domal uplifts of Jebel Akhdar, Jebel Nakhl and Saih Hatat, central and southeastern Oman mountains, is dominated by Late Palaeozoic and Late Cretaceous deformation and metamorphism. In Jebel Akhdar pre-Permian rocks have been deformed by large NE-SW trending thrusts and folds and intense cleavage during Late Palaeozoic compression. Later reactivation of these structures accompanied by a major flattening deformation caused by loading may be related to thrusting and folding in the overlying Permian to Cenomanian platform carbonates. These structures are ascribed partly to Late Cretaceous deformation. The complex internal structure of Saih Hatat is attributed to overprinting of possible Proterozoroic and Late Palaeozoic tectonic events by Late Cretaceous thrusting, folding and associated blueschist metamorphism. The identification of a major basement-cover duplex involving pre-Permian rocks and platform carbonates in Saih Hatat suggests that the pre-Permian basement and platform carbonates did not remain as autochthonous units during Late Cretaceous deformation. Instead, both basement and cover have been thrust southwards during emplacement of the Tethyan thrust sheets onto the Arabian continental margin. Sedimentological evidence from the Tertiary sequence reinforces the development of the Saih Hatat dome as a Late Cretaceous culmination above a deep seated thrust which may have been rejuvenated during the Tertiary. In Jebels Akhdar and Nakhl there is no evidence of a basement-cover duplex. Nevertheless, reactivation of pre-Permian structures and imbrication of cover rocks (Mesozoic platform carbonates) in Jebel Akhdar has important implications to the timing of its structural growth. An alternative model of under-thrusting of the N Arabian margin beneath a subduction trench is also proposed to explain the formation and subsequent elevation of the Oman blueschists.
The interpretation of gravity data in Oman: constraints on the ophiolite emplacement mechanism
Authors: Shelton A. W.
Published: April 2008

The results of the gravity survey which was carried out as part of the Open University Oman Research Project and briefly reported by Lippard
et al.
are presented in detail to further constrain the emplacement mechanism of the Semail Ophiolite. The principal conclusions of the gravity (and magnetic) study are that (i) the Semail Ophiolite nappe continues for a limited distance under the Batinah coastal plain and appears to have a smoothly tapering ‘trailing edge’; (ii) the lower surface of the nappe is approximately planar; (iii) the ‘leading edge’ of the nappe comprises slices some 20% of the thickness of the block to which they are attached; (iv) beneath the coastal plain, to the east of the Hawasina Window culmination, a completely unexposed ophiolite block exists. These conclusions favour the gravity-driven emplacement of the ophiolite nappe of Oman.
The influence of hydrography, bathymetry and productivity on sediment type and composition of the Oman Margin and in the Northwest Arabian Sea
Authors: Shimmield G. B. , Price N. B. , Pedersen T. F.
Published: April 2008

The hydrography, bathymetry and sediment composition of the Oman Margin and NW Indian Ocean areas between latitudes 24°N and 13°N, have been studied. A pronounced oxygen minimum zone characterizes the water column between 200 and 1500 m depth and extends throughout the NW arabian Sea. The bathymetry of the margin is complex, resulting from tectonic movement along the Masirah Line. Small, anoxic intrashelf basins are developed south of Masirah. The Owen/Murray Ridge provides a barrier to westward progradation of terrigenous detritus from the Indus Fan. Upwelling and surface productivity result in the deposition of biogenic calcareous and siliceous sediments, the latter being restricted to the shelf and slope regions. Depositional controls, such as redistribution by bottom currents and bathymetry, greatly affect the quality of organic carbon deposited. Evidence is presented to suggest that much of the organic matter on the Oman shelf is degraded prior to burial. This has important connotations for sediment diagenesis and hydrocarbon maturation. The distribution of calcareous and siliceous sediments resulting from upwelling may well provide an index for determining the Neogene history of upwelling in the NW Arabian Sea.
The volcano-tectonic setting of oceanic lithosphere generation
Authors: Searle R. C.
Published: April 2008

The past two decades have seen a revolution in our knowledge of the detailed structure, tectonic and volcanic patterns of mid-ocean ridge spreading centres and transform faults, brought about mainly by the use of a variety of high-resolution sonars. This paper reviews those studies, and attempts to provide an up-to-date description of those aspects of mid-ocean ridge structure that are most relevant for comparison with ophiolites.
Quaternary marginal basin volcanism in the Bransfield Strait as a modern analogue of the southern Chilean ophiolites
Authors: Keller Randall A. , Fisk Martin R.
Published: April 2008

The Bransfield Strait is an actively-spreading, 65 km wide, Quaternary marginal basin at the northern tip of the Antarctic Peninsula. Across the Drake Passage from the Bransfield Strait are the Tortuga and Sarmiento ophiolites of southern Chile, which have been interpreted as uplifted, Jurassic-Cretaceous, marginal-basin curst that formed in a setting similar to the present Bransfield Strait. We present a comparison between the Bransfield Strait and the Chilean ophiolites, emphasizing new petrological and geochemical data for volcanic rocks dredged from two seamounts at the Bransfield Strait rift axis and published analyses of volcanic rocks from the marginal basin now preserved as the Chilean ophiolites.

Volcanic rocks from the seamounts are about 100 000 years old, and major element abundances in these rocks are broadly similar to enriched mid-ocean ridge basalt (E-MORB). The rocks are slightly enriched in light rare earth elements (LREE) relative to heavy rare earth elements (HREE), as in E-MORB, but they are moderately enriched in alkali, and alkali earth elements, as in arc basalts, and concentrations and ratios of diagnostic trace elements show island arc affinities. Geochemical discrimination diagrams suggest that Bransfield Strait lavas represent the initial stages of a petrogenetic transition linked to the tectonic transition from rifting in an ensialic arc to rifting in a marginal basin.

Published rare earth element data from dykes and lavas in the Chilean ophiolites range from mildey depleted in LREE in the Tortuga ophiolite (Ce/Yb = 0.5 to 0.8, normalized to chondrites) to mildly enriched in LREE in the Sarmiento ophiolite (normalized Ce/Yb = 1.6 to 2.5). The latter are strikingly similar to data from the seamounts in the Bransfield Strait both in LREE enrichment (normalized Ce/Yb = 2 to 2.5) and total REE concentrations (20–45 times chondrite). We believe these chemical similarities, as well as other geological similarities pointed out by other authors, suggest that the Bransfield Strait marginal basin is at the same stage of development as was the marginal basin now preserved as the Sarmiento ophiolite.
Deformation styles adjacent to transform faults: evidence from the Troodos ophiolite, Cyprus
Authors: Allerton Simon , Vine F. J.
Published: April 2008

In the southeast part of the Troodos ophiolite much of the tectonic fabric is demonstrably associated with its formation at a spreading ridge adjacent to the Southern Troodos Transform Fault. A combined palaeomagnetic and structural study has revealed many features of its genesis. Palaeomagnetism can be used to identify original dyke intrusion directions and any subsequent block rotations. The region can be subdivided into two distinct sub-areas on the basis of their dyke orientations. In the southeast corner of the ophiolite, dykes had an original northwest strike, and have suffered only simple tilting about strike-parallel normal faults to assume their present orientation. In contrast, further west, dykes now striking northeast or east have been rotated on average 100° clockwise on sets of strike-slip faults, from an original northwest strike.

A model for the structural processes active at ridge-transform intersections is suggested. Normal faulting dominates at the outside corner, where there is no slip across the transform. At the inside corner, oblique normal/strike-slip faulting is prevalent. Within this context, the eastern area of the ophiolite developed at the outside corner, and the area further west, at the inside corner. A ridge-jump boundary has been preserved between the areas, where a new ridge, which subsequently formed the eastern area, cut the older, western area.
Conodont colour maturation indices for the Carboniferous of west-central Scotland
Authors: Dean Mark T.
Published: April 2008

The colour alteration of conodont elements from Carboniferous samples from west-central Scotland is summarized, and most values fall within indices 1 to 1.5. This range corresponds to geothermal temperatures of <50°C to 90°C which lie within the immature (early dry gas) to mature (perhaps mid-oil window) stages of hydrocarbon generation. Where the index values are 1.5, and other necessary geological criteria are satisfied, support is offered here for oil generation through burial maturation.
Carboniferous source rocks of the Canadian Atlantic margin
Authors: Mossman David J.
Published: April 2008

Carboniferous source rocks of relatively low to moderate maturity occur onshore along the Canadian Atlantic margin in Palaeozoic basins of western Newfoundland, New Brunswick and Nova Scotia. Coals of predominantly Westphalian age are widespread and locally abundant. In the Devono-Carboniferous Albert Formation of New Brunswick, and in equivalent Horton Group strata elsewhere in the Maritimes Basin, oil-prone type-I kerogen is predominant.

Throughout the Canadian Appalachians, the similarity of Carboniferous strata in various sedimentary basins has facilitated the recognition of four major stratigraphic cycles of deposition. Bounded in nearly every instance by unconformities, these cycles are likewise recognized offshore, where in some cases they have been the focus of commercial investigation. In the Gulf of St. Lawrence region, hydrocarbon-generating potential was early confirmed by drilling in rocks of the Windsor and Canso-Riversdale sequences. These strata may extend north from the Magdalen Basin into the Anticosti Basin, northwestward into the St. Lawrence estuary, and northeast of western Newfoundland.

Carboniferous sections south and east of Newfoundland are in most cases strongly overmature. However, recognition of equivalent Horton Group strata within the oil window in a Grand Banks well indicates the potential of Upper Palaeozoic strata to contribute to the accumulation of oil, not only within the Palaeozoic section, but also in younger strata.
Dynamic stratigraphy of the Triassic and Jurassic of the Hebrides Basin, NW Scotland
Authors: Morton Nicol
Published: April 2008

The evolution of the Hebrides Basin during the late Triassic and Jurassic has been established by integrating analyses of subsidence history, sequence stratigraphy and stratigraphical architecture. Two episodes of three-phase evolution occurred from extension-related variable sequences (1A, late Triassic to early Sinemurian; 2A, latest Toarcian to late Bajocian), through sag-related laterally more uniform sequences (1B, mid-Sinemurian to earliest Toarcian; 2B, late Bajocian to late Bathonian), to stabilization—related condensed sequences (1C, Toarcian; 2C, late Bathonian to Callovian). A final third episode of subsidence in Oxfordian to early Kimmeridgian was followed by partial basin inversion in latest Jurasic or early Cretaceous, which caused faulting, tilting and uplift of basement ridges. These structures are unconformably overlain by Upper Cretaceous sediments and Palaeocene volcanics.

There was a single Hebrides Basin during the late Triassic and Jurassic subsiding as a half-graben bounded on the west by the Minch Fault. The intrabasinal Camasunary and related faults are demonstrated not to have been significant tectonic features during the Jurassic. Occasionally they were slightly active but they did not divide the area into two sub-basins. They are largely post-Jurassic. There is evidence of a less strongly subsiding central Skye block, apparently in the area of the later Palaeocene plutonic centre.
The origin of Upper Palaeozoic sedimentary basins in Northern Ireland and relationships with the Canadian Maritime Provinces
Authors: Mitchell W. I.
Published: April 2008

The evolution of Carboniferous and early Permian sedimentary basins in Northern Ireland is related to a rapidly changing pattern of tectonic episodes. Characteristic sedimentary assemblages and facies changes are associated with discrete phases of basin formation and were related to tectonic events that can be correlated with events in a wide zone of dextral strike-slip in the Canadian Maritime Provinces. Small, localized non-marine basins developed during late Asbian-early Brigantian and Westphalian B times. These data support the extension of the Canadian Maritime transform fault system into Northern Ireland in late Silesian times.
Recent developments on the classification of mineral reserves and resources
Authors: Schaffalitzky C.
Published: April 2008

The Institution of Mining and Metallurgy (IMM) and the Irish Association for Economic Geology (IAEG) have felt for a long time that the absence of usable definitions, particularly in ‘the City’ was bringing the minerals industry into disrepute. The IMM has now addressed this problem and has published draft proposals to be presented to the membership. This paper presents a brief personal review of these proposals and suggestions for further improvements.
Assaying in resource evaluation: the need for a clear and open mind
Authors: Sawyer J. B. P.
Published: April 2008

This paper is concerned mainly with assaying of precious and noble metals, gold, silver, and platinum group metals.

The determination of the amount of saleable metals (or other mineral products), in an ore is the most important step in evaluation of a mineral deposit. Most assays, even if well done, will not tell you how the material occurs in the rock or how much of it can be recovered, but if the amount that is present in the first place is unknown, it is impossible to make any comment as to the efficiency of a recovery system. This may not be important because if a recovery can be made which will provide a large profit, that which is not recovered will not seriously affect the economics of the operation. However, where the grade, and thus the operation is marginal, this is a different matter.

Fire assaying, with its origins in antiquity, is widely regarded as the best, if not the only, way to determine precious metals in naturally occurring ores. There are a lot of misconceptions about the universal applicability of fire assay methods and the more recent combination of traditional fire assay collection techniques with laboratory instrumentation has given rise to a serious, if not ludicrous, situation in the case of some materials.

There are some naturally occurring materials which cannot be successfully determined by fire assay. Some of those same materials can only be determined by instrumental techniques if their chemistry is understood.

Even the most sophisticated instruments in the world will not yield the correct answers if the chemistry of the ores being tested is not taken into consideration in designing the sample preparation and instrumental procedures to be used. Such considerations can only be made if the chemistry is understood, and such proper understanding requires the availability of a mind open to the fact that there may indeed by something new (to man) under the sun. It is important to appreciate the implications of the fact that naturally occurring materials may be in an unusual form; hence the need for a clear as well as an open mind.

The evaluation of precious metal deposits of a certain type involves an appreciation of their setting in terms of plate tectonics models.

Case histories of assaying problems with ores from south-western United States are presented which illustrate all of the above points. Attention is drawn to the possible implications of the phenomena described for mineral exploration in the UK as well as world-wide.
The evaluation of coal borehole data for reserve estimation and mine design
Authors: Whateley M. K. G.
Published: April 2008

Information derived from the evaluation of the borehole data from a potential coalfield is used in five stages of the project programme. These are geological interpretation, reserve estimation, mine design and coal preparation studies and financial evaluation. This paper will elaborate on part of the first three of these stages. The range of methods available to calculate the resource potential and the confidence of these estimates will be discussed, as well as the geological information needed to form the basis for accurate reserve estimation and mine design. The importance of extracting the maximum amount of information from each borehole is stressed. Careful logging of boreholes will, for example, provide geotechnical information which will be invaluable in the design of both surface and underground mines, as well as assist with groundwater modelling.
Evaluation of the Barapukuria coal deposit NW Bangladesh
Authors: Norman P. S.
Published: April 2008

Bangladesh is a flat, mainly agricultural country with a population exceeding 110 million (770 people per square kilometre). The country occupies a delta plain at the confluence of the two great river systems: the Ganges (Padma) and the Brahmaputra (Jamuna). The country’s complex geological history is largely obscured by a ubiquitous cover of late Tertiary to Quaternary sediments, up to several kilometres thick. This has hampered industrial development as the bulk of the country’s minerals are concealed by this thick cover, making them difficult to identify, evaluate and exploit. Recently, Bangladesh has begun to exploit natural gas from the tectonic belt in the east of the country for electricity generation. Much of the electricity in the west of the country is transmitted large distances from the power stations in the east resulting in intermittent supply and high transmission losses. Permian (Gondwana) coal was discovered in NW Bangladesh in the 1960s, and soon a substantial deposit had been evaluated in Jamalganj, near Bogra. Although these relatively deep reserves were not developed, the Geological Survey of Bangladesh continued with a programme of regional exploration, recognizing that other, shallower, deposits might exist to the north of Bogra and Jamalganj. In 1985, a ‘wildcat’ exploration borehole drilled on a gravity anomaly at Barapukuria discovered substantial thicknesses of coal. After further drilling, the Government of Bangladesh requested international technical assistance for the full evaluation of the deposit and a feasibility study to evaluate its potential. In 1987, the British Government’s Overseas Development Administration (ODA) engaged Wardell Armstrong, International Mining Consultants, to carry out the study in two phases. This paper is a brief case history of the Barapukuria Coal project, from discovery to evaluation. If developed, the coal mine and dedicated coal-fired power-station have the potential to transform NW Bangladesh economically and industrially.
The use of integrated software techniques to evaluate a bedded clay deposit
Authors: Henley S. , Palmer Q. G. , Tucker D. K.
Published: April 2008

This paper describes the use of an integrated software system in the evaluation of a layered Ball Clay deposit in the western part of the Hampshire Basin. The site has been investigated by core drilling over the past 15 years and very detailed lithological logs have been prepared together with a comprehensive set of physical and chemical test results.

A three-dimensional cellular block model was created using DATAMINE software to produce a seven layer model for the deposit. The main clay seam was divided into four horizons based on quality criteria and 14 different physical and chemical properties were interpolated into each of these layers. From this model, the reserves were then calculated on the basis of the proposed pit design. In addition, a series of grade plans were produced for seven of the more critical properties, together with contour and isopach maps of each clay horizon plus the sandy overburden. These plans may be used for the control of clay quality during production and for the prediction of changes in those clay properties which may affect product blending.

The structure and thickness of the basal non-productive clay sequence was also modelled in order to assess areas where this sequence is thin and susceptible to rupturing by high piezometric pressure in the underlying sand sequence. Isopachyte plans and cross sections were generated to demonstrate the variations in thickness of this sequence across the site.

This case history demonstrates the way in which a computer modelled database can be used to provide comprehensive structural interpretation, quality control and volumetric assessment of a bedded deposit with complex variation in both physical and chemical properties.
The use of low cost geostatistical software in reserve estimation
Authors: Nathanail C. P. , Rosenbaum M. S.
Published: April 2008

Geostatistical techniques are now well over a quarter of a century old. Despite initial scepticism caused by misuse due to poor understanding of geostatistics, the spatial correlation between sample points is now commonly incorporated in the assessment of ore reserves.

The recent appearance of several low-cost software packages, written for the ubiquitous IBM compatible personal computer, now permits geostatistics, once the exclusive domain of university researchers and large mining companies, to be more widely available. Such packages offer a choice of variogram models, incorporate anisotropy, support a number of kriging methods, enable limited three-dimensional interpolation and produce high-quality graphical output. Their relative ease of use can do nothing, however, to overcome either the inherent limitations of the PC operating system (MS-DOS) or ignorance on the part of the user of geostatistical techniques.

An understanding of the theory of geostatistics, together with a clear definition of the problem to be solved and a knowledge of the capabilities of the software to be used, are essential if the mistakes made with ore reserve assessment in the early days are not to be repeated by today’s geostatistical apprentices.

Low-cost software has been used as an aid to planning an investigation of the resource around a sand pit and for the assessment of reserves at an opencast coal site.
ONE-D: a program for one-dimensional composite optimization
Authors: Diering J. A. C.
Published: April 2008

The process of compositing sample data from drillholes or traverses for orebodies bounded by a grade cut off rather than a geological boundary can be time consuming. A method for the optimal computation of such composites is described in this paper. A computer program ONE-D has been written which maximizes the dollar value of a composite subject to constraints on minimum and maximum length and a user definable seed value. The seed value is used to force composites to be ‘close’ to a desirable mining line. For complex layered structures, interactive intervention from the user is allowed when structures bifurcate or are folded.
A personal overview of geostatistics
Authors: Royle A. G.
Published: April 2008

The need for accurate data is emphasized and a ‘safe’ in situ sampling formula is developed. Observations on sampling and sample preparation follow. Suggestions are made to ease the problems of variogram modelling.

The more recent forms of kriging are described, with emphasis being laid on their practical utilities. Other directions of development are indicated and it is suggested that professionals who do not claim to have a wide knowledge of geostatistics should nevertheless make known their views on what they require from geostatisticans. What were once regarded as peripheral, rather than core, systems are finding wider usages. Examples are database management, graphics that introduce carry-over from CAD-CAM systems, and, of course that much abused title ‘Information Technology’ and its synonyms. With regret it must be noted that many recent symposia have been mostly concerned with theoretical notions whose practical applications remain obscure.
Recoverable reserve estimation by conditional simulation
Authors: Ravenscroft Peter J.
Published: April 2008

Recoverable reserves are defined as the tonnage and grade that can be recovered above a given cut-off grade in a selective mining operation. Their estimation depends on three factors: the support, information and constraint effects. These factors are discussed, and some traditional methods for estimation of global and local recoverable reserves are reviewed. The technique of conditional simulation is proposed as a practical alternative to some of the more complex methods currently used. The broad concepts of conditional simulation are discussed, and some alternative simulation methods presented. A case study on a set of real mining data illustrates the use of the conditional simulation approach to recoverable reserve estimation, with the support and information effects being modelled very simply. Results are compared with the discrete gaussian model, and with ordinary kriging, which is shown to be unsuitable for recoverable reserve estimation.
Qualitative modelling: fractal geometry in mineral deposit evaluation
Authors: Walduck G. P.
Published: April 2008

The spacial distribution of the physical characteristics of a mineral deposit (its elevation, thickness, grade, etc.) can be estimated with measurable limits of accuracy by geostatistical techniques, but the estimates are smoothed. Local fluctuations of such variables as hanging-wall position or orebody thickness may be of great importance to the planning function, since they can influence choice of equipment or have an important and unavoidable effect on grade dilution or ore recovery. Fractal geometry can be used to generate qualitatively detailed views of these variables, with their complexity explicit. This paper describes several constructs using fractal concepts for two-dimensional image generation. The images can be derived from data sets and used for purposes such as the planning of mineral extraction systems; they are faithful to the original seed data. The constructs are analysed and compared, through their semi-variograms, with conventional models.
Can sediment geochemistry record changes in coastal upwelling palaeoproductivity? Evidence from northwest Africa and the Arabian Sea
Authors: Shimmield Graham B.
Published: April 2008

The causes and consequences of organic carbon (C
org
) burial in sediments underlying regions of coastal upwelling are of profound importance to studies of palaeoceanography and palaeoclimatology alike. The primary control on coastal upwelling, wind stress and resulting Ekman transport, is linked intimately to variation in the Earth’s insolation record via a variety of feedback mechanisms. In comparison, the nutrients required to sustain productivity, and the factors affecting the C
org
content of the sediment, are related to variation in global glacial boundary conditions. Using a variety of geochemical indicators, it is apparent that the record of upwelling off northwest Africa, driven by the northeast trade winds, is characterized by arid glacial conditions and high wind stress sustaining elevated euphotic zone productivity. The resulting C
org
accumulation record reflects higher palaeoproductivity during glacial episodes. In contrast, the Arabian Sea upwelling is seasonal. Faunal and geochemical (Ba/Al, U/Th ratios) indicators suggest strong interglacial (23 ka) cyclicity in the Southwest Monsoon. The C
org
record is spatially and temporally consistent, but in disagreement with other palaeoproductivity indicators. These results point to the decoupling of surface ocean productivity in areas of coastal upwelling from deep-ocean circulation and nutrient supply. This result has important consequences for driving atmospheric
p
CO
2
changes on timescales less than that of oceanic mixing. Further applications of geochemical tracers to areas of coastal upwelling, such as northwest Africa and Peru, are needed urgently.