Time lag of syntectonic sedimentation across an alluvial basin: theory and example from the Ebro Basin, Spain   总被引：5，自引：0，他引：5  

Merren A. Jones  Paul L. Heller†  Eduard Roca‡  Miguel Garcés  Lluís Cabrera 《Basin Research》2004,16(4):489-506

We propose and test a conceptual framework for evaluating the relative timing of different types of sedimentary indicators of tectonism in alluvial foreland basin settings. We take the first occurrence of a detrital grain from a newly exposed source‐area lithology to provide the best indicator of the onset of tectonic uplift in the source area. Source‐area unroofing may lag behind initial uplift because of the type, thickness and structure of rocks in the uplifted mountain block, drainage patterns and climate. However, once exposed, advective transport disperses grains quickly throughout fluvial systems. Because of increased subsidence rate from thrust belt loading, an increase in sedimentation rate begins coincident with tectonic load emplacement within the flexural half‐width of the basin. However, farther out into the basin increased sedimentation rates lag behind the composition signal because of time lags associated with propagation of the thrust load and attendant sediment loads into the basin. The progradation of syntectonic gravel lags behind all of these signals as a direct function of the relative proportion of gravel fraction within transported sediment and rates and geometry of subsidence, which selectively traps the coarsest grain‐size fractions in the most proximal parts of the basin. We demonstrate this signal attenuation in the syntectonic Horta–Gandesa alluvial system (late Eocene–Oligocene), exposed along the southeast margin of the Ebro Basin, Spain. The results demonstrate that: (1) the time spans between the compositional signal and the progradation of the gravel front can be geologically significant, on the order of more than a million years within as little as 20 km of the thrust front; and (2) time lags between the signals increase with distance away from the deformation front. No lag time was observed between the first appearance of a new clast composition and the arrival of gravel front when the thrust front was within a few tens of metres from the depositional site. In contrast, the time lag was 0.5–1 Myr when the thrust front was about 5–6 km away and it increased to >1 Myr when the deformation front was about 8 km away. At the most extreme position, when the thrust front was 15–20 km away, the gravel front never reached the study area.  相似文献   

Spatial and temporal changes in the structure of groundwater nitrate concentration time series (1935–1999) as demonstrated by autoregressive modelling     

A.L. Jones  P.L. Smart   《Journal of Hydrology》2005,310(1-4):201-215

Autoregressive modelling is used to investigate the internal structure of long-term (1935–1999) records of nitrate concentration for five karst springs in the Mendip Hills. There is a significant short term (1–2 months) positive autocorrelation at three of the five springs due to the availability of sufficient nitrate within the soil store to maintain concentrations in winter recharge for several months. The absence of short term (1–2 months) positive autocorrelation in the other two springs is due to the marked contrast in land use between the limestone and swallet parts of the catchment, rapid concentrated recharge from the latter causing short term switching in the dominant water source at the spring and thus fluctuating nitrate concentrations. Significant negative autocorrelation is evident at lags varying from 4 to 7 months through to 14–22 months for individual springs, with positive autocorrelation at 19–20 months at one site. This variable timing is explained by moderation of the exhaustion effect in the soil by groundwater storage, which gives longer residence times in large catchments and those with a dominance of diffuse flow. The lags derived from autoregressive modelling may therefore provide an indication of average groundwater residence times. Significant differences in the structure of the autocorrelation function for successive 10-year periods are evident at Cheddar Spring, and are explained by the effect the ploughing up of grasslands during the Second World War and increased fertiliser usage on available nitrogen in the soil store. This effect is moderated by the influence of summer temperatures on rates of mineralization, and of both summer and winter rainfall on the timing and magnitude of nitrate leaching. The pattern of nitrate leaching also appears to have been perturbed by the 1976 drought.  相似文献   

An intercomparison between finite difference and finite element (TELEMAC) approaches to modelling west coast of Britain tides     

J. Eric Jones  Alan M. Davies 《Ocean Dynamics》2005,55(3-4):178-198

A finite element model (namely TELEMAC) with a range of mesh refinements and assumptions of coastal water depths is used to determine an optimal mesh for computing the M ₂ tide in a region of significant geographical extent. The region adopted is the west coast of Britain covering the Irish and Celtic Seas. The nature of the spatially varying topography and tidal distribution, together with a comprehensive set of measurements and existing accurate finite difference model makes it ideal for such a study. Calculations show that a water-depth dependent criterion for determining element size gives an optimal distribution over the majority of the region. However, local refinements in narrow channels such as the North Channel and Bristol Channel are required. Although the specification of a zero coastal water depth, leads to a fine near coastal grid, this does not yield the most accurate solution. In addition the computational cost is high. In practice in a large area model the use of a non-zero coastal water depth yields optimum accuracy at minimal computational cost. However, calculations show that accuracy is critically dependent upon nearshore water depths. Comparison with the finite difference model shows that the bias in elevation amplitude in the finite difference solution is removed in the finite element calculation.  相似文献   

Structure of the poleward wall of the trough and the inclination of the geomagnetic field above the EISCAT radar     

D. G. Jones  I. K. Walker  L. Kersley 《Annales Geophysicae》1997,15(6):740-746

A special high-resolution routine of the EISCAT radar has been used to investigate the structure and development of the poleward wall of a deep trough in electron density. The feature was tracked by the radar during a 7-hour period under very quiet geomagnetic conditions. The field-aligned nature of the structure enabled an estimate to be made of the inclination of the geomagnetic field above EISCAT that was in good agreement with the current model. Observations of narrow field-aligned enhancements in electron temperature demonstrated that the wall of this trough is a dynamic feature, reforming regularly as the electron density responds on a time scale of tens of minutes to energy input from soft-particle precipitation.  相似文献   

Devonian fluviodeltaic sedimentation in Hong Kong     

N.S. Jones  J.A. Fyfe  R.J. Sewell  K.W. Lai  C.M. Lee 《Journal of Asian Earth Sciences》1997,15(6):171

Twenty-three distinct sedimentary facies have been distinguished in the Devonian Bluff Head Formation exposed around the Tolo Channel area of northeastern Hong Kong. These facies have been combined into five facies associations: channel, aeolian, sheetflood, deltaic and palaeosol, with the bulk of the formation belonging to the channel association. The lower part of the Bluff Head Formation, around Plover Cove, consists of upwards-coarsening cycles, each typically 3 m thick. The basal parts of some cycles contain distinctive marine fossils of Early to Middle Devonian age. Sedimentation occurred on a marine-influenced delta plain, with eustatic sea level rises controlling cycle development. The middle and upper parts of the formation contrast strongly with the lower part and show a distinct change to alluvial plain sedimentation. Upward-fining cycles, each approximately 40 m thick, comprise basal multistorey, braided, conglomerate-filled channel deposits at the base passing upwards into finer-grained sheetflood, aeolian and channel sandstones at the top. Palaeosols in the higher parts of the cycles indicate periods of non-deposition under semi-arid climatic conditions. These cycles represent repeated abandonments of the fluvial system which we suggest were controlled by tectonic pulses.  相似文献   

Regional and local variations in the emerging economic landscape of the new German Länder     

Philip N. Jones  Trevor Wild 《Applied geography (Sevenoaks, England)》1997,17(4):283-299

Events in eastern Germany since 1989 pose major challenges for geographical interpretation. This paper examines the spatial impacts of the post-unification economic and social upheavals in the new Länder. Using unemployment rates as a key indicator of regional differentiation, spatial variability within the new Länder is examined in terms of two fundamental dimensions. First, the relationship with former patterns of economic specialization is explored, and secondly, the effects of basic geographical properties such as settlement structure, location and peripherality. Prominent regional polarities are described, notably between major agglomeration cores and rural residuals, and between ‘bridgehead’ locations and less-accessible districts. Within this context the paper then identifies some ‘green shoots’ of recovery and new growth. It concludes by emphasizing the importance of incorporating sub-regional variations into any new framework of regional policy.  相似文献   

Geochemical characteristics of lavas from Broken Ridge, the Naturaliste Plateau and southernmost Kerguelen Plateau: Cretaceous plateau volcanism in the southeast Indian Ocean   总被引：3，自引：0，他引：3  

J. J. Mahoney  W. B. Jones  F. A. Frey  V. J. M. Salters  D. G. Pyle  H. L. Davies 《Chemical Geology》1995,120(3-4):315-345

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, ²⁰⁷Pb/²⁰⁴Pb and isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with ε_Nd(T) = − 13 to −7, (⁸⁷Sr/⁸⁶Sr)_T=0.7090 to 0.7130 and high ²⁰⁷Pb/²⁰⁴Pb relative to ²⁰⁶Pb/²⁰⁴Pb. In addition, the low-ε_Nd(T) Naturaliste Plateau samples are elevated in SiO₂ (> 54 wt%). In contrast to “DUPAL” oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., Th/Ta, La Nb > primitive mantle values); the lowest ε_Nd(T) and highest Th/Ta and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-²⁰⁶Pb/⁰⁴Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive ε_Nd, relatively high-²⁰⁶Pb/²⁰⁴Pb plume composition, or a plume source dominated by mantle with ε_Nd of −3 to 0.  相似文献   

A quantitative methodology to extract regional magnetotelluric impedances and determine the dimension of the conductivity structure   总被引：1，自引：0，他引：1  

R. W. Groom  R. D. Kurtz  A. G. Jones  D. E. Boerner 《Geophysical Journal International》1993,115(3):1095-1118

A methodology to determine quantitatively the dimensionality of the dominant conducting structures and the resolution of the structural parameters in magnetotelluric data is presented. In addition, the method recovers the regional impedance responses when the regional structure can be characterized, at least approximately, as 1- or 2-D. The methodology is based upon three general models of the MT tensor, each of which has a distinct parameterization and physical interpretation. A weighted statistical residual describes quantitatively the fit of the model response to the data within the scatter of the measured data and hence permits: (1) tests of dimensionality, (2) determination of the appropriate strike angle, and (3) recovery of the regional responses.
The method has been tested extensively with synthetic data and proven to be successful. These synthetic studies give insight into the different physical parameterizations and the stability of the parameters determined. We describe and illustrate some of these synthetic studies. With field data, the methodology is not always as straightforward, but its application to a great many sites has proven valuable. Data from two closely spaced sites, which are both affected by strong but very different 3-D effects, are analysed to illustrate the geological significance of the results. The analyses reveal and recover regional responses within the data which indicate the presence of electrical anisotropy located deep in the crust and upper mantle. Analyses of the entire data set, of which these two form a part, confirm this finding.  相似文献   

Application of metal zoning to gold exploration in porphyry copper systems: reply to the comments of G.V. Albino     

Brian K. Jones 《Journal of Geochemical Exploration》1993,48(3)

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Shear strength analysis of grouted coal waste materials to improve mine outslope stability: a laboratory analysis     

J. R. Jones  T. E. Ackman 《Geotechnical and Geological Engineering》1990,8(3):227-240

Summary Slope stability problems exist within the Appalachian Basin as a result of the emplacement of coal mine waste materials on mine outslopes. Prevention or elimination of slope instability problems can be costly. In an attempt to test alternative methods of slope stabilization, the United States Bureau of Mines determined some physical characteristics and shear strengths of ungrouted and polyurethenegrouted samples of coal refuse, coal spoil and natural soils collected at a number of mine outslope sites in Pennsylvania and West Virginia. The particle size distribution, per cent field porosity, per cent field water content and shear strengths of the materials were determined in the laboratory. Fifty drained direct shear strength tests were performed with the sample materials using a 0.06 m³ shear box. Tests were done at field moisture, 100% saturation and grout-infused conditions. Normal loads of 103 kN m^–2, 206 kN m^–2 and 416 kN m^–2 were used. The grout-infused tests generally showed strength increases. An infinite slope model was used to demonstrate the potential effectiveness ofin situ grouting for a variety of field slope conditions. This modelling suggests thatin situ grouting has the potential of stabilizing slopes of up to 35° at depths of 14 m for refuse material and 30° at depths of 5 m for spoil and soil materials. The validity of these increases in material strength by grout injection will require field testing for confirmation.  相似文献