Sedimentological aspects of the Humber Group (Upper Jurassic) of the South Central Graben, UK North Sea     

JOHN A. HOWELL  STEPHEN S. FLINT  CATHERINE HUNT 《Sedimentology》1996,43(1):89-114

Complex structure, poorly understood sedimentology and poor biostratigraphic control make the Upper Jurassic Humber Group of the South Central Graben one of the least understood and most complex hydrocarbon reservoirs of the North Sea. Detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base-level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time. A new sedimentological model, based upon the distribution of facies and facies associations, illustrates that Upper Jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments. Rifting and second-order transgression controlled the back-stepping onlap patterns observed and higher frequency base-level fluctuations controlled the internal architecture of individual sandbodies. The model presented accounts for features of these deposits that were previously considered anomalous, such as the thickness of bioturbated sandstones, paucity of foreshore deposits and complex age relationships of sands.  相似文献   

Chemical studies of H chondrites 9: Volatile trace element composition and petrographic classification of equilibrated H chondrites     

STEPHEN F. WOLF  MICHAEL E. LIPSCHUTZ 《Meteoritics & planetary science》1998,33(2):303-312

Abstract— To examine the thermal history of the parent body/bodies of equilibrated H chondrites, we treated data for 11 volatile trace elements (Co, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In in order of putative volatility) in 90 falls: 15 H4; 46 H5, and 29 H6. Using univariate statistical tests, contents of few of these elements differ significantly between any two of these suites. One element, Cs, differs systematically between all three pairs of suites; Co and Tl differ between two pairs of suites. For Co and Cs, contents varied as H4 > H5 > H6; while for Tl, contents varied as H4 < H5 < H6. Using multivariate statistical tests, all three suites can be distinguished compositionally, with trace element contents in the H5 suite being intermediate to those of H4 and H6. Surprisingly, the multivariate distinguishability reflects contents of less volatile Co, Rb, Ag, Se and Cs, and not of highly volatile Te, Zn, Cd, Bi, Tl and In. The compositional trends apparently reflect heterogeneous accretion >600 K, with the suites deriving from a stratified parent body/bodies.  相似文献   

Anatomy of a submarine pyroclastic flow and associated turbidity current: July 2003 dome collapse, Soufrière Hills volcano, Montserrat, West Indies     

JESSICA TROFIMOVS  R. STEPHEN JOHN SPARKS  PETER J. TALLING 《Sedimentology》2008,55(3):617-634

The 12 to 13 July 2003 andesite lava dome collapse at the Soufrière Hills volcano, Montserrat, provides the first opportunity to document comprehensively both the sub‐aerial and submarine sequence of events for an eruption. Numerous pyroclastic flows entered the ocean during the collapse, depositing approximately 90% of the total material into the submarine environment. During peak collapse conditions, as the main flow penetrated the air–ocean interface, phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling. The bulk of the flow was submerged and rapidly mixed with sea water forming a water‐saturated mass flow. Efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth. The coarsest components (～60% of the total volume) were deposited proximally from a dense granular flow, while the finer components (～40%) were efficiently elutriated into the overlying part of the flow, which evolved into a far‐reaching turbidity current.  相似文献   

Komatiites II: Experimental and Theoretical Investigations of Post-emplacement Cooling and Crystallization   总被引：1，自引：2，他引：1  

TURNER  J. STEWART; HUPPERT  HERBERT E.; SPARKS  R. STEPHEN J. 《Journal of Petrology》1986,27(2):397-437

When komatiite lavas are emplaced on the sea floor most of theheat transfer occurs through the upper lava-seawater boundary.We have investigated the cooling and crystallization of komatiitesusing a series of analogue laboratory experiments with aqueoussolutions and by theoretical analysis. In komatiites the viscosityis sufficiently low that convection occurs in the interior ofthe flow and these motions, due both to thermal and compositionalvariations, have an important influence on the characteristicfeatures of komatiites such as the strong compositional andtextural layering. The experiments have been conducted with crystallizing aqueoussolutions which display the same overall dynamical processesas solidifying komatiites. The solutions used are simple eutecticsystems having the property that crystallization from a solutionwhich is substantially more concentrated than the eutectic compositionleaves behind residual fluid which is less dense than the originalfluid. This models the decrease in density of komatiite meltson cooling, due to the crystallization of olivine. Such solutionshave been cooled strongly through the metal roof of an otherwiseinsulated container, using a typical fluid depth of 80 mm. Dendriticcrystals grew down vertically from the roof and released lightfluid, depleted in solute, which rose to form a zone of stagnantfluid at the top of the container, while the tips of the crystalsextended just below the bottom of this light layer. A layerof solid eutectic, with a horizontal front, grew more slowlyand filled in the space between the vertically oriented crystals. The growth of the crystals and the eutectic layer were monitoredvisually, and in some experiments the temperatures at the topand in the fluid were recorded, until solidification throughoutthe layer was complete. The solid block was sampled, and themelted products analysed to give vertical concentration profiles.Both the texture and composition are strongly influenced bythe fluid conditions during crystal growth. The top concentrationis that of the original solution, rapidly quenched against theroof, and the mean concentration through the region influencedby the stable fluid layer is also close to the original. Atthe bottom the concentration is high, reflecting the in situgrowth of close-packed crystals, and there is a sharp decreasein concentration at an intermediate level, between the upperand lower crystal layers. The experiments and associated theory shed new light on theconsolidation of komatiites and the development of their characteristictextures and compositions. Since the lava is convecting withinthe interior, the early stages of cooling are characterizedby a rapid decrease in temperature. Initial cooling rates of1 to 100 °C h^–1 are calculated. At this stage thecrust remains thin, but as the spinifex zone develops, convectionprogressively decreases in vigour and the cooling rate decreases.Spinifex texture is considered to form by constitutional supercoolingwhich is controlled by compositional convection. As the spinifextexture develops, the olivine dendrites form a layer of depletedfluid. The tips of the crystals extend beyond this differentiatedlayer into a convecting lower region and grow preferentiallyto produce the characteristic vertically oriented spinifex texture.The composition of spinifex zones is shown to be close, butnot identical, to the initial liquid composition. The compositionalprofiles of the solid products of the experiments are similarto those found in komatiites, with the most evolved rock compositionsbeing found just above the cumulate zone. The experiments alsosuggest an alternative explanation to crystal settling for thecumulate zone, in which growth of the spinifex zone by compositionalconvection concentrates crystals suspended within the turbulentlyconvecting lower layer.  相似文献   

Diagenesis of aragonite from Upper Cretaceous ammonites: a geochemical case-study     

BJØRN BUCHARDT  STEPHEN WEINER 《Sedimentology》1981,28(3):423-438

Shell aragonite from ammonites collected in the Upper Cretaceous of West Greenland was investigated by means of macroscopic/microscopic visual evaluation, analyses of calcite/aragonite ratios, carbon and oxygen isotopic compositions and Sr and Mg concentrations of shell carbonate and of amino acid compositions of organic matrices. The results are: (1) Material visually classified as well preserved may have suffered diagenetic modifications of mineralogical and chemical composition. (2) Of the chemical and mineralogical parameters studied, amino acid composition, calcite/aragonite ratios and magnesium concentrations were found to be most sensitive to post-depositional modifications, while oxygen isotope composition and strontium concentrations showed detectable diagenetic modifications only after more pronounced alterations. (3) Based on the Mg/Ca ratios and calcite concentrations of the shell aragonite, a diagenetic classification has been proposed grouping the material into well preserved, moderately preserved and poorly preserved. (4) The chemical and mineralogical composition of the best preserved material suggests that the Upper Cretaceous ammonites had a shell composition similar to that of modern Nautilus and other aragonite-shelled molluscs.  相似文献   

Garnet Types from the Cazadero Area, California     

LEE  D. E.; COLEMAN  R. G.; ERD  R. C. 《Journal of Petrology》1963,4(3):460-492

This is the third in a series of papers on glaucophane schistsfrom the Franciscan Formation near Cazadero, California. Previouspapers describe three distinct types of glaucophane-bearingFranciscan metamorphic rocks near Cazadero. The purpose of this study is to investigate the garnets presentin metamorphic types III (bedrock schists) and IV (tectonicblocks) as defined by Coleman & Lee (1963). Twenty-fourgarnet analyses are presented. Sixteen of these are from (aragonite-bearing)type III glaucophane schists, and eight are from type IV glaucophaneschists. Type IV rocks include California eclogites. Type III rocks include metabasalt, metachert, metashale, meta-ironstone,and metacarbonate that were formed under high pressure and relativelylow temperature. These rocks contain garnets that display awide range of composition, but the dominant molecules representedare consistently almandine, spessartine, and grossular. Type IV rocks are mainly metabasalts that were probably formedunder higher temperatures and pressures than type III rocks.There is a distinct difference between garnets from type IIIrocks and those from type IV (including eclogites); the lattercontainless spessartine and more pyrope, and the dominant moleculesare almandine and grossular. The four analyses of garnets fromCalifornia eclogites have an average pyrope content of aboutten molecular per cent, and they extend the range of compositionreported for eclogite garnets. Quantitative spectrographic determinations of minor elementsare listed for each of the garnets described. The values determinedfor some of the minor elements have a wide range and a capriciousdistribution over a few feet of outcrop area. As a group, both the garnets from type III rocks and those fromtype IV are pyralspites with large contents (as much as 35 molecularper cent) of ugrandite. This unusual admixture of the pyralspiteand ugrandite garnet series may have resulted in part from theconditions (high pressures and relatively low temperatures)under which the enclosing rocks were recrystallized.  相似文献   

Recrystallization of dolomite: evidence from the Monterey Formation (Miocene), California   总被引：2，自引：0，他引：2  

MITCHELL J. MALONE  PAUL A. BAKER  STEPHEN J. BURNS† 《Sedimentology》1994,41(6):1223-1239

Dolomites from the upper calcareous-siliceous member of the Miocene Monterey Formation exposed west of Santa Barbara, California, were analysed for geochemical, isotopic and crystallographic variation. The data clearly document the progressive recrystallization of dolomite during burial diagenesis in marine pore fluids. Recrystallization is recognized by the following compositional and crystallographic variations. Dolomites have decreasing δ¹⁸O and δ¹³C compositions, decreasing Sr contents and increasing Mg contents with increasing burial depths and temperatures from east to west in the study area. δ¹⁸O values vary from 5·3‰ in the east to − 5·5‰ PDB in the west and are interpreted to reflect the greater extent and higher temperature of dolomite recrystallization in the west. δ¹³C values correlate with δ¹⁸O and decrease from 13·6‰ in the east to − 8·7‰ PDB in the west. Sr concentrations correlate positively with δ¹⁸O values and decrease from a mean of 750 ppm in the east to a mean of 250 ppm in the west. Mol% MgCO₃ values inversely correlate with δ¹⁸O values and increase from a minimum of 41·0 in the east to a maximum of 51·4 in the west. Rietveld refinements of powder X-ray diffraction data indicate that the more recrystallized dolomites have more contracted unit cells and increased cation ordering. The fraction of the Ca sites in the dolomites that are occupied by Ca atoms increases slightly with the approach to stoichiometry. The fraction of the Mg sites occupied by Mg atoms strongly correlates with mol% MgCO₃. Even in early diagenetic, non-stoichiometric dolomites, there is little substitution of Mg in Ca sites. During recrystallization, the amount of Mg substituting for Ca in Ca sites decreases even further. Most of the disorder in the least recrystallized, non-stoichiometric dolomites is related to substitution of excess Ca on Mg sites.  相似文献   

The geography of tyranny and despair: development indicators and the hypothesis of genetic inevitability of national inequality   总被引：2，自引：0，他引：2  

STEPHEN MORSE 《The Geographical journal》2008,174(3):195-206

Development geography has long sought to understand why inequalities exist and the best ways to address them. Dependency theory sets out an historical rationale for under development based on colonialism and a legacy of developed core and under-developed periphery. Race is relevant in this theory only insofar that Europeans are white and the places they colonised were occupied by people with darker skin colour. There are no innate biological reasons why it happened in that order. However, a new theory for national inequalities proposed by Lynn and Vanhanen in a series of publications makes the case that poorer countries have that status because of a poorer genetic stock rather than an accident of history. They argue that IQ has a genetic basis and IQ is linked to ability. Thus races with a poorer IQ have less ability, and thus national IQ can be positively correlated with performance as measured by an indicator like GDP/capita. Their thesis is one of despair, as little can be done to improve genetic stock significantly other than a programme of eugenics. This paper summarises and critiques the Lynn and Vanhanen hypothesis and the assumptions upon which it is based, and uses this analysis to show how a human desire to simplify in order to manage can be dangerous in development geography. While the attention may naturally be focused on the 'national IQ' variables as a proxy measure of 'innate ability', the assumption of GDP per capita as an indicator of 'success' and 'achievement' is far more readily accepted without criticism. The paper makes the case that the current vogue for indicators, indices and cause–effect can be tyrannical.  相似文献   

Origin and terminal architecture of a submarine slide: a case study from the Permian Vischkuil Formation,Karoo Basin,South Africa     

WILLEM C. Van Der MERWE  DAVID M. HODGSON  STEPHEN S. FLINT 《Sedimentology》2011,58(7):2012-2038

Submarine mass movement deposits exposed in the Vischkuil Formation, Laingsburg Karoo Basin, South Africa, provide a rare opportunity to analyse and interpret their emplacement history and deformation processes at a scale comparable to seismic examples. An up to 80 m thick slide deposit, continuously exposed in two 2 km long sub‐parallel sections, passes from extensionally deformed material (clastic dykes and down‐dip facing low‐angle shear surfaces) down‐dip into a compressional toe zone with large (tens of metres amplitude) folds dissected by steep, up‐dip facing thrust planes. The compressional shear planes sole out onto a highly sheared décollement and cross‐cutting relationships indicate an up‐depositional dip younging in the timing of fold dissection. Lithofacies characteristics and detailed correlation of volcanic ash and other marker beds over more than 500 km² in the bounding undeformed stratigraphy indicate a low‐gradient (<0·1°) basin floor setting. The slide is abruptly overlain by an up to 50 m thick debrite with sandy clasts supported by an argillaceous matrix. Shear loading of the debris flow is interpreted to have driven large‐scale deformation of the substrate through the generation of high shear stresses at a rheological interface due to: (i) the abrupt contact between the slide and the debrite; (ii) the coincident thickness distributions of the debrite and slide; (iii) the distribution of the most intense folding and thrusting under the thickest parts of the debrite; (iv) the preservation of fold crests with only minor erosion along fold limbs; (v) the presence of the debrite under overturned folds; (vi) the presence of laterally extensive marker beds directly above deformation units indicating minimal depositional topography; and (vii) the demonstrably local derivation of the slide as individual folded beds are mapped into undeformed strata outside the areas of deformation. The debrite is directly overlain by fine‐grained turbidite sandstone beds that show widespread vertical foundering into the debrite. This case study demonstrates that intensely deformed strata can be generated by negligible amounts of down‐dip movement in a low‐gradient, fine‐grained basin floor setting with the driver for movement and deformation being the mass imbalance resulting from emplacement of episodic debris flows. Simple interpretation of an unstable slope setting based on the presence of such deformed strata should be treated with caution.  相似文献   

Quaternary coastal lithofacies,sequence development and stratigraphy in a passive margin setting,North Carolina and Virginia,USA   总被引：3，自引：0，他引：3  

PETER R. PARHAM  STANLEY R. RIGGS  STEPHEN J. CULVER  DAVID J. MALLINSON  W. JACK RINK  KEVIN BURDETTE 《Sedimentology》2013,60(2):503-547

Lithofacies analysis is fundamental to unravelling the succession of depositional environments associated with sea‐level fluctuations. These successions and their timing are often poorly understood. This report defines lithofacies encountered within the north‐eastern North Carolina and south‐eastern Virginia Quaternary section, interprets their depositional environments, presents a model for coastal depositional sequence development in a passive margin setting and uses this understanding to develop the stratigraphy and Quaternary evolutionary history of the region. Data were obtained from numerous drill cores and outcrops. Chronology was based on age estimates acquired using optically stimulated luminescence, amino acid racemization, Uranium series and radiocarbon dating techniques. Geomorphic patterns were identified and interpreted using light detection and ranging imagery. Since lithofacies occurrence, distribution and stratigraphic patterns are different on interfluves than in palaeo‐valleys, this study focused on interfluves to obtain a record of highstand sea‐level cycles with minimal alteration by fluvial processes during subsequent lowstands. Nine primary lithofacies and four diagenetic facies were identified in outcrops and cores. The uppermost depositional sequence on interfluves exhibits an upward succession from shelly marine lithofacies to tidal estuarine lithofacies and is bounded below by a marine ravinement surface and above by the modern land surface. Older depositional sequences in the subsurface are typically bounded above and below by marine ravinement surfaces. Portions of seven depositional sequences were recognized and interpreted to represent deposition from late middle Pleistocene to present. Erosional processes associated with each successive depositional sequence removed portions of older depositional sequences. The stratigraphic record of the most recent sea‐level highstands (Marine Isotope Stage 5a and Marine Isotope Stage 3) is best preserved. Glacio‐isostatic adjustment has influenced depositional patterns so that deposits associated with late Quaternary sea‐level highstands (Marine Isotope Stages 5c, 5a and 3), which did not reach as high as present sea‐level according to equatorial eustatic records, are uplifted and emergent within the study area.  相似文献