A geological and hydrogeological assessment of the electrical conductivity information from the HiRES airborne geophysical survey of the Isle of Wight

A recent high resolution airborne geophysical survey across the Isle of Wight (IoW) and Lymington area has provided the first electromagnetic data across the relatively young geological formations characterising much of southern England. The multi-frequency data provide information on bulk electrical conductivity to depths of the order of 100 m. A GIS-based assessment of the electrical conductivity information in relation to bedrock geological classification has been conducted for the first time. The analysis uses over 104,000 measurements across onshore IoW and has established average and statistical properties as a function of bedrock geology. The average values are used to provide baseline maps of apparent electrical conductivity and the variation with depth (measured as a function of frequency). The average conductivity as a function of depth within the main aquifer units is summarised. The data indicate that the majority of the Palaeogene is characterised by values consistently in excess of 100 mS/m and with a surprisingly high degree of spatial heterogeneity. The youngest (Oligocene) Hamstead Member displays some strong edge effects and the largest localized values in conductivity. The central Upper Chalk is associated with the lowest observed conductivity values and mineral content and/or porosity appears to increase with increasing age. The large central outcrop of the Lower Greensand Group, Ferruginous Sand Formation provides persistently low (<30 mS/m) conductivity values which imply a relatively uniform distribution of clean sand content. Non-geological (e.g. environmental) responses are contained within the data set and examples of these in relation to a closed municipal landfill and an area of potential coastal saline intrusion are discussed. In the south, the Gault clay/mudstone of the Early Cretaceous appears as a distinctive conductive unit. Cross sectional modelling of the data has been undertaken across the aquifer units of the Southern Downs. The results indicate that the Gault Formation, acting as an aquitard, can be traced as a distinct unit under the more resistive Early Cretaceous Upper Greensand and Late Cretaceous Chalk formations. The conductivity modelling should therefore allow an estimation of the subsurface configuration of the aquifer and aquitard units.     

A lithological assessment of the resistivity data acquired during the airborne geophysical survey of Anglesey,North Wales

A recent airborne geophysical survey has provided high resolution estimates of the electrical resistivity of the near-surface and deeper (bedrock) formations found across Anglesey and a portion of the coastal area of North Wales. This single small survey provides new geophysical information on both the complex configuration of the Neoproterozoic and Cambrian bedrock units and the shallow near-surface geology and glacial features.     

The transfer of organic signatures from bedrock to sediment

A case study in the Canadian Arctic demonstrates how an organic geochemical signature in the regional bedrock can be transferred by erosion and redeposition to younger geological formations and surface sediments. The hydrocarbon composition recorded in Laurentian (Lower Palaeozoic) carbonate bedrock was incorporated into overlying Miocene and Quaternary formations, and modern mass waste, alluvium, snow/ice, and proglacial deposits, and further distant in ice-rafted detritus. The retention of the original geological organic signature is reflected in consistent thermal maturities (hopane ratios) and environmental indicators (sterane distributions). In the modern sediments, the geological signature is variably mixed with a modern microbial organic signature reflected in high values of carbon preference index and diploptene. These data show that hydrocarbon occurrences in bedrock may be detected by the analysis of surface detritus, especially in carbonate terrains. However they also indicate that environmental signatures may be misleading if they are inherited from older geological units.     

A geological model of London and the Thames Valley,southeast England

Many geological survey organisations have started delivering digital geological models as part of their role. This article describes the British Geological Survey (BGS) model for London and the Thames Valley in southeast England. The model covers 4800 km² and extends to several hundred metres depth. It includes extensive spreads of Quaternary river terraces and alluvium of the Thames drainage system resting on faulted and folded Palaeogene and Cretaceous bedrock strata. The model extends to the base of the Jurassic sedimentary rocks.     

The stratigraphy of the Chalk Group (Cretaceous) of the Devon coast,UK

An almost continuous layer of Upper Cretaceous deposits up to 1000 m thick was probably deposited across much of SW England. Phases of uplift in the late Cretaceous and early Cenozoic, each of which was followed by extensive erosion and dissolution, resulted in the removal of all except a few outliers of Chalk Group that crop out in east Devon and south Somerset. Those on the Devon coast between Sidmouth and Lyme Regis are some of the best exposed Cenomanian to early Coniacian successions in NW Europe and include the most westerly chalks preserved onshore in England. They form an integral part of the Dorset and East Devon World Heritage Site. In contrast to the Chalk of much of southern England, the older formations in Devon, the Beer Head Limestone, Holywell Nodular Chalk and New Pit Chalk, show marked lateral lithological variations that result from a combination of penecontemporaneous movements on local faults and relatively shallow-water environments close to the western edge of the Chalk depositional basin. The younger parts of the succession, the Lewes Nodular Chalk and Seaford Chalk Formations, comprise chalks that do not appear to have been greatly affected by penecontemporaneous fault movements. These formations include lithological marker beds that have been correlated with marker beds in the Sussex type area. The principal sedimentary breaks in the Devon succession cannot be correlated with confidence with eustatic changes in sea level.     

Calibration of the Albian/Cenomanian Boundary by Ammonite Biostratigraphy: U.S. Western Interior

Calibration of numerical ages to the geological time scale is a long scientific pursuit that requires the integration of multiple data sets. A case study of the Albian/Cenomanian stage boundary, also the Lower/Upper Cretaceous series boundary, illustrates the calibration process. The numerical age of this boundary has shifted from 96 Ma to 99 Ma over a time span of nearly fifty years. Re-calibration resulted first from improvements in radiometric dating, and later from inferences about ammonite phylogeny, and most recently from radiometric dates of newly discovered volcanic beds interbedded with diagnostic guide fossils. However, the calibration process continues with study of cosmopolitan dinoflagellates.     

Hydrocarbon source rock variability within the Austin Chalk and Eagle Ford Shale (Upper Cretaceous), East Texas, U.S.A.

The Austin Chalk and Eagle Ford Shale are Upper Cretaceous deposits that extend across Texas from the northeast to southwest. These formations contain organic carbon enriched mudstones and chalks that were deposited during transgressions of the Cretaceous epeiric sea in North America. Recent workers in petroleum geochemistry have demonstrated that these organic enriched rocks possessed attributes common to oil source rocks. The present study of these Austin Chalk and Eagle Ford Shale rocks is from the perspective of organic petrology, and it augments the earlier geochemical work that documented source variability within units of these formations. As with the earlier work, the results of this study show that both formations contain intervals that are, when mature, capable of generating commercial quantities of liquid hydrocarbons. However, this work further revealed that Eagle Ford rocks not only exhibit greater or ganic carbon contents, but also have greater quantities of oil-prone kerogen (fluorescent amorphinite and exinite) when compared with rocks from the Austin Chalk. These source rock differences relate to levels or degrees of organic preservation. Dysaerobic to oxic depositional settings seem to be more characteristic of the Austin Chalk than of the Eagle Ford Shale. Such oxic environments do not consistently favor the preservation of organic matter. Usually, well-preserved kerogen forms under more anoxic conditions, such as those that occurred during deposition of some Eagle Ford units. These anoxic conditions suggest that the geographically more extensive Eagle Ford Shale is a more important source for oil than is the Austin Chalk.     

Central England's journey through time: the geological evolution of Warwickshire

The central English county of Warwickshire has been shaped by a 600 million‐year history of deposition, erosion, tectonism, continental drift, environmental change and biological evolution. The Neoproterozoic and Lower Palaeozoic rocks indicate island arc accretion, marine transgression and subduction‐related intrusive igneous activity. Upper Palaeozoic through to Triassic times witnessed mainly continental environments, as central England drifted north across the equator. Late Triassic marine transgression led to deposition of richly fossiliferous Jurassic sediments. The bedrock geology shows evidence for the influence of deep‐seated structural lineaments within the central English basement. Influenced by these structures, the modern landscape has been shaped largely by Palaeogene and Neogene uplift and erosion, further alteration by Quaternary erosion and weathering, and glacial and fluvial deposition.     

Geochemical soil surveys over cambrian and lower devonian formations in the belgian ardennes as a tool for geological mapping

A geochemical soil survey was made over outcropping Cambrian and Lower Devonian formations in the Belgian Ardennes. About 300 soil samples were collected in an area of 2.5 × 1.1 km².Analysis of rock samples from scarce, but stratigraphically well-known outcrops show that the Cambrian rocks are richer in copper than those of the Lower and Upper Gedinnian (Lower Devonian). The Upper Gedinnian rocks are more nickeliferous than the other horizons.In the well-drained soils, copper and lead distributions permit precise location of the boundary between Cambrian and Lower Gedinnian formations. The nickel distribution seems to be a good stratigraphic indicator for distinguishing Upper from Lower Gedinnian. Zinc distribution seems unrelated to underlying bedrock geology.Regression analysis confirms that the content of Cu, Pb and Ni is essentially a function of lithostratigraphy. However, the distribution of these elements is more influenced by pedological factors in the poorly drained soils.     

Late Cretaceous stratigraphy,sediments and structure: Gems of the Dorset and East Devon Coast World Heritage Site (Jurassic Coast), England

Extraordinary, long-distance litho-marker beds such as the Lewes and Shoreham Tubular Flints and associated marl seams and fossils, recognised in cliff exposures and cliff-fall boulders, are keys to unlocking the stratigraphy and tectonic structures in the Late Cretaceous of the Dorset and East Devon Coast World Heritage Site (Jurassic Coast). Durdle Cove is a special gem exposing the Lewes and Seaford Chalk stratigraphy where new marker beds are identified and sediments and tectonic structures provide clues to timing of movements that produced a Late Cretaceous pericline which grew into a Miocene monocline along the line of the underlying Purbeck Reverse Fault. During ‘inversion’ along this fault some Late Cretaceous Chalk formations were in part or completely ‘lost’ (e.g. Middle Turonian New Pit Chalk Formation) and others were condensed (e.g. Late Santonian and Early Campanian Newhaven Chalk Formation). Excavation of the A354 road cutting at the Lower Bincombe Farm, has greatly added to the stratigraphical records of Late Cretaceous fossils in South Dorset, especially Coniacian and Early Campanian inoceramid bivalves and the various stratigraphically specific forms of the Late Santonian to Early Campanian echinoid fossil Echinocorys scutata spp. not recorded before in this coastline. The very large bivalve fossil Platyceramus sp. provides clues to chalk sea-floor environments.     

A chalk revolution: what have we done to the Chalk of England?

Inversion tectonic episodes are identified in the Upper Turonian - Lower Coniacian, Santonian - Lower Campanian and later Lower Campanian Chalk. It is suggested that episodic tectonism created the seabed topography on which sea levels and erosional currents acted. Marked differentiation into linear belts of local basins and swells with a greater variety of sediments is present in the Santonian and Lower Campanian. During this same period the locus of sedimentation shifts westwards from the southern margin of the Weald to Wessex as Weald Basin inversion increases. Tectonic episodes also produced synsedimenary fracturing of the Chalk and evolution of vein networks and stylolytes. Upper Cretaceous tectonic and sea-level events also affected the platform of Europe, the Carpathians and the Syrian Arc where sedimento-tectonic scenarios provide analogues for the Chalk. Linking sea-level oscillations and tectonic episodes with microtectonic studies suggests a frequency of events within the range of 0.35-1.5 Ma.     

Late Cretaceous to Miocene and Quaternary deformation history of the Chalk: Channels,slumps, faults,folds and glacitectonics

Late Cretaceous Chalk sedimentation history across the British Isles included (i) fault controlled uplift and subsidence in Northern Ireland and the Inner Hebrides and (ii) uplift along the lines of en echelon folds in Southern Britain and northern France. Synsedimentary slump folds and downslope displacement structures are compared with penecontemporaneous interbed slides and later tectonic folds and faults. Compressional strike-slip tectonic processes at Flamborough Head, Yorkshire, illustrate intra-Chalk slump beds in a half-graben setting. Progressive ‘growth’ of structures characterises early downslope slump folding, interbed sliding and some listric faulting. Sheet-flints replacing slide shear planes and early fractures provide evidence for early movements. Availability of open-slopes or the depth of burial under which the range of structures developed is reflected in the degree of disruption and fragmentation of chalk and flint. Fragmentation provides clues to the timing of events and origin of the Late Campanian Altachuile Breccia (Northern Ireland) and the Coniacian Hope Gap slides (Sussex). Fragmentation and formation of sheet flints together help distinguish intra-Chalk tectonics from Quaternary glacitectonic structures.The role of marl seams, high porosity chalk beds and hardgrounds on bed-sliding, décollement zones and disruption of chalk blocks from bedrock in glacitectonics is discussed. Chalk formations with marl seams develop a special style of fracturing related to early interbed sliding and pore-fluid escape structures. Marl-seams are shown to be primary sedimentary features and not the products of post depositional pressure-solution. More than any other formation the Late Santonian – Early Campanian Newhaven Chalk contains extensive sheet-flints and shows great lateral variation in thickness and lithology across the fold belts of southern England and northern France.     

湖相碳酸盐岩烃源岩有机显微组分分类及其岩石学特征——以酒西盆地为例

酒西盆地主力烃源岩为下白垩统下沟组和赤金堡组暗色半深湖相-深湖相的白云质泥岩和泥质白云岩,是典型的湖相碳酸盐岩烃源岩。通过对酒西盆地下白垩统二百多块湖相碳酸盐岩烃源岩详细的有机岩石学研究,解剖了湖相碳酸盐岩烃源岩中有机质的赋存形式,揭示了藻纹层为有机质纹层的主要发育形式,其与富泥晶白云石纹层和富粘土纹层构成湖相碳酸盐岩烃源岩特有的"三元式纹层"沉积结构,系统总结了各显微组分的光性、成因、生烃性及分布规律,提出了以全岩和干酪根为基础的湖相碳酸盐岩有机显微组分分类方案,首次鉴定出对酒西盆地油气有贡献的主要生烃组分为腐泥无定形体、腐殖无定形体以及浮游藻类体、孢粉体、壳屑体五种有机显微组分。     

论基础地质与城市地质之关系及存在问题

城市地质是基础地质理论在城市建设中的应用领域。基础地质调查成果是城市地质研究的基础。目前，我国城市地质研究中普遍存在基础地质薄弱、灾害地质调查忽视基本的成因分析、第四系覆盖区地质调查带有明显的盲目性、基岩出露区地质调查忽视基本结构构造分析等问题。基础地质调查在国民经济建设中的地位不应该被忽视。如果城市地质在城市建设和可持续发展中处于基础地位，基础地质则是基础中的基础。它成为薄弱环节，是严重的策略性失误。     

准南前陆冲断带烃源岩地质地球化学特征

准南前陆冲断带具有丰富的油气资源,但与资源评价关系密切的烃源岩研究不足。本文应用地球化学分析方法,对准南前陆冲断带的上二叠统、中下侏罗统、下白垩统和古近系等4套烃源岩的有机质丰度、有机质类型、热演化程度进行了系统分析和综合对比。结果表明,上二叠统烃源岩和中下侏罗统烃源岩为良好的烃源岩,古近系安集海河组其次,下白垩统烃源岩相对较差。     

An example of cluster analysis applied to a large geologic data set: Aerial radiometric data from Copper Mountain, Wyoming

One objective of the aerial radiometric surveys flown as part of the U.S. Department of Energy's National Uranium Resource Evaluation (NURE) program was to ascertain the spatial distribution of near-surface radioelement abundances on a regional scale. Some method for identifying groups of observations with similar -ray spectral signatures and radioelement concentration values was therefore required. It is shown in this paper that cluster analysis can identify such groups with or without a priori knowledge of the geology of an area. An approach that combines principal components analysis with convergentk-means cluster analysis is used to classify 6991 observations (each observation comprising three radiometric variables) from the Precambrian rocks of the Copper Mountain, Wyoming area. This method is compared with a convergentk-means analysis that utilizes available geologic knowledge. Both methods identify four clusters. Three of the clusters represent background values for the Precambrian rocks of the area, and the fourth represents outliers (anomalously high²¹⁴Bi). A segmentation of the data corresponding to geologic reality as interpreted by other methods has been achieved by perceptive quantitative analysis of aerial radiometric data. The techniques employed are composites of classical clustering methods designed to handle the special problems presented by large data sets.     

How global are the Jurassic–Cretaceous unconformities?

The reality of the global‐scale sedimentation breaks remains controversial. A compilation of data on the Jurassic–Cretaceous unconformities in a number of regions with different tectonic settings and character of sedimentation, where new or updated stratigraphic frameworks are established, permits their correlation. Unconformities from three large reference regions, including North America, the Gulf of Mexico, and Western Europe, were also considered. The unconformities, which encompass the Jurassic‐Cretaceous, the Lower–Upper Cretaceous and the Cretaceous–Palaeogene transitions are of global extent. Other remarkable unconformities traced within many regions at the base of the Jurassic and at the Santonian–Campanian transition are not known from reference regions. A correlation of the Jurassic–Cretaceous global‐scale sedimentation breaks and eustatic curves is quite uncertain. Therefore, definition of global sequences will not be possible until eustatic changes are clarified. Activity of mantle plumes is among the likely causes of the documented unconformities.     

西藏江孜-浪卡子一带的侏罗-白垩纪界线地层

侏罗系/白垩系界线是显生宙所有系级界线中存在问题最多的一个。西藏南部出露有良好的侏罗-白垩纪地层,本次工作在喜马拉雅地层区的康马隆子地层分区开展了海相侏罗系/白垩系的界线研究。江孜地区的界线地层被划分为维美组和甲不拉组;浪卡子地区的甲不拉组之下发育一套含大量火山岩层的火山－沉积地层,被称为桑秀组。该地层分区的地层系统由下至上为：维美组浅灰色厚层状粗-细粒石英砂岩;桑秀组黑色页岩、安山岩和玄武岩;以及甲不拉组黑色页岩、硅质泥页岩夹砂岩和砂质灰岩。维美组中含化石稀少,仅在江孜地区发现零星菊石Haplophylloceras、Himalayites等。桑秀组下部页岩和粉砂岩中找到少量菊石化石,属于Spiticeras、Berriasella、Haplophylloceras的一些种,和富集成层的双壳类Inoceramus everesti等。江孜甲不拉组下部化石丰富,划分为Spiticeras-Berriasella下组合和Himalayaites-Haplophylloceras上组合。本研究区的生物地层可与聂拉木地区的菊石化石组合对比。通过生物地层学对比,江孜-浪卡子地区的维美组时代为晚侏罗世Tithonian期,江孜地区甲不拉组下部和浪卡子地区的桑秀组均属于下白垩统。桑秀组下部的页岩段与江孜甲不拉组的最下部地层相当,上部火山岩的同位素年龄为133 Ma。据此,桑秀组的时代为Berriasian至Hauterivian期,侏罗系/白垩系的界线位于该组之底,以Virgatosphinctes、Aulocosphinctes的消失和Spiticeras的出现为标志。侏罗纪末期西藏特提斯海区普遍形成大规模海退,表现为维美组和门卡墩组顶部砂岩的同期沉积。     

Under-representation of faults on geological maps of the London region: reasons,consequences and solutions

London lies mainly within an area of long-term tectonic stability known as the London Platform. This is characterised by relatively thin Cretaceous and Palaeogene sequences overlying Palaeozoic basement at shallow depths, less seismic activity than surrounding areas and, according to published geological maps, little faulting.