Internal structure of mixed-sand-and-gravel beach deposits revealed using ground-penetrating radar

ABSTRACT Mixed‐sand‐and‐gravel beaches are a distinctive type of coarse‐clastic beach. Ground‐penetrating radar (GPR) and photographic records of previous excavations are used to investigate the stratigraphy and internal sedimentary structure of mixed‐beach deposits at Aldeburgh in Suffolk, south‐east England. The principles of radar stratigraphy are used to describe and interpret migrated radar reflection profiles obtained from the study site. The application of radar stratigraphy allows the delineation of both bounding surfaces (radar surfaces) and the intervening beds or bed sets (radar facies). The deposits of the main backshore berm ridge consist of seaward‐dipping bounding surfaces that are gently onlapped by seaward‐dipping bed sets. Good correspondence is observed between a sequence of beach profiles, which record development of the berm ridge on the backshore, and the berm ridge's internal structure. The beach‐profile data also indicate that backshore berm ridges at Aldeburgh owe their origin to discrete depositional episodes related to storm‐wave activity. Beach‐ridge plain deposits at the study site consist of a complex, progradational sequence of foreshore, berm‐ridge, overtop and overwash deposits. Relict berm‐ridge deposits, separated by seaward‐dipping bounding surfaces, form the main depositional element beneath the beach‐ridge plain. However, the beach ridges themselves are formed predominantly of vertically stacked overtop/overwash units, which lie above the berm‐ridge deposits. Consequently, beach‐ridge development in this progradational, mixed‐beach setting must have occurred when conditions favoured overtopping and overwashing of the upper beachface. Interannual to decadal variations in wave climate, antecedent beach morphology, shoreline progradation rate and sea level are identified as the likely controlling factors in the development of such suitable conditions.     

Regional policy in Great Britain

Recent policy initiatives for the inner cities and regions of Britain were reviewed in the context of the continued and deepening processes of metropolitan decentralisation of population and economic activity and of regional divergence in levels of economic well-being. The result is a policy focus on those areas considered as being on the margins of economic viability. Research evaluating past policy indicates only modest potential for the success of such a focus. Policies having a growth stimulation objective may be better centred outside these most marginal areas. Policies having a welfare objective may achieve greater efficiency if they have no particular spatial dimension.     

The application of spinel-bearing metapelites to P/T determinations: An example from South India

Metasedimentary migmatites from the Archean charnockitic terrain of South India contain the five phase equilibrium assemblage spinel-cordierite-garnet-corundum-sillimanite. The assemblages is a result of anatexis which has generated a silica-deficient anhydrous restite. Peak metamorphic conditions are defined by the intersection of two divariant reactions in the A1₂O₃-SiO₂-FeO-MgO system at which the five phases coexist. These reactions are univariant and their intersection invariant if the Fe/Mg ratio of at least one femic phase is fixed.The location of the invariant point in P/T space is derived from extracting standard stage thermodynamic data from published equilibria experiments in the system Al₂O₃-SiO₂-FeO. Microprobe analyses of coexisting spinel, almandine and cordierite specify the Fe/Mg distributions between phases and allow the computation of the five phase invariant point for =P _total (770° C, 5.9 kb) and =O (740° C, 4.8 kb). A low , implied by evidence of extreme anatexis, indicates a P/T field of T=740±20° C and P_total=4.8±0.5 kb which is consistent with the field of equilibration of interlayered charnockites computed from garnet-hypersthene and garnet-plagioclase pairs.     

Crustal accretion in the Pan African: Nd and Sr isotope evidence from the Arabian Shield

Nd and Sr isotope determinations on late Precambrian to early Palaeozoic igneous and sedimentary rocks from the Arabian Shield are used to investigate the proportion of reworked “older” crust, and the rate at which new crust was generated during the Pan African event. Eight Rb/Sr whole rock isochrons on igneous suites yield ages in the range 770?590 Ma and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7038?0.7023. These data confirm that magmatism in this area was largely restricted to the period 850-550 Ma, and the initial ratios are sufficiently low to preclude significant contributions from a long-lived upper crustal source. The initial ¹⁴³Nd/¹⁴⁴Nd ratios of a variety of lithologies, including several samples of possible “basement”, are all higher than the contemporaneous values for CHUR (ε_Nd = +1.6 to +6.9), suggesting that many were derived directly from the upper mantle, and that any inferred crustal source regions for the remainder could not have separated from likely LREE-depleted mantle reservoirs before 1200 Ma. The Arabian Shield therefore provides an example of rapid crustal growth during the Late Proterozoic, and contrasts with the Damara intracratonic belt of Namibia where Nd and Sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period.     

The nature of olefins and carboxyl groups in an Australian brown coal resin

The chemical structure of the resin from an Australian soft brown coal (Yallourn) has been investigated by cross-polarization nuclear magnetic resonance spectroscopy with magic angle spinning (¹³C CP MAS NMR). Some additional solution ¹H and ¹³C data were also obtained. Solid-state experiments were performed with and without a delay period before data acquisition. The resulting free induction decays were Fourier transformed with respect to acquisition time and delay period to produce two-dimensional solid-state spectra. Assignments made from the spectra clearly demonstrate that the gross chemical structure of the Yallourn resin is best described as a polymerized diterpenoid with one axial carboxylic group and two double bonds. One double bond is trisubstituted, the other is monosubstituted. After consideration of various mechanisms for polymerization of diterpenoid units during biogenesis and coalification, it was concluded that polymerization occurs at the C₁₅ carbon atoms in the diterpenoids without cyclization of the methylene units at C₈.     

Detrital spinels from alpinotype source rocks in middle Devonian sediments of the Rhenish Massif

Middle Devonian sandy siltstones of the Gummersbach area (Rhenish Massif) show increased Cr contents (up to 675 ppm). The chromium is mainly associated with detrital Cr-Al spinels. The spinels are characterized by a wide spectrum of chemical composition.In more than 50% of 134 analyzed spinel grains the Cr₂O₃ content exceeds 50 wt %. The A1₂O₃ content which is negatively correlated to Cr₂O₃ reaches 40 wt %. The MgO contents are, with few exceptions, lower than the FeO contents; both elements are negatively correlated. The Fe₂O₃ contents which were calculated from a quarter of the grains rarely exceed 5 wt %. Besides Cr, Al, Fe, Mg TiO₂ contents were determined.The chemical composition of the detrital spinels indicates provenance from peridotites of an ophiolite suite.Provenance analysis shows that the source area of the Middle Devonian clastic sediments was situated in the north.

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Zusammenfassung In sandigen Siltsteinen des Mitteldevons des Bergischen Landes (Rheinisches Schiefergebirge) wurden erhöhte Cr-Gehalte von maximal 675 ppm festgestellt. Das Chrom ist überwiegend an detritische Spinelle gebunden. Mikrosondenuntersuchungen erbrachten eine große Variationsbreite im Chemismus dieser Spinelle. Über die Hälfte der 134 analysierten Körner weist mehr als 50 Gew % Cr₂O₃ auf. Der Al₂O₃-Gehalt, umgekehrt proportional zum Cr₂O₃-Gehalt, erreicht 40 Gew %. Die MgO-Gehalte sind bis auf wenige Ausnahmen kleiner als die FeO-Gehalte; beide Elemente sind negativ korreliert. Die Fe₂O₃-Gehalte, die von einem Viertel der Körner berechnet wurden, liegen meist unter 5 Gew %. Neben Cr, Al, Fe, Mg treten in einigen Spinellen meßbare TiO₂-Gehalte auf. Von ihrem Chemismus her lassen sich die detritischen Cr-Al-Spinelle von Peridotiten alpinotyper Ophiolite ableiten. Sedimentologische Befunde zeigen an, daß das Liefergebiet der klastischen mitteldevonischen Sedimente im Norden des Rheinischen Schiefergebirges gelegen hat.

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Résumé Les siltites sableuses du Dévonien moyen de la région de Gummersbach (Massif Schisteux Rhénan) montrent des teneurs en Cr élevées, jusqu'à un maximum de 675 ppm. Le chrome est lié principalement à des spinelles détritiques. La microsonde révèle une grande variation du chimisme de ces spinelles.Plus de la moitié des 134 grains analysés contiennent plus de 50% en poids de Cr₂O₃. La teneur en Al₂O₃ est enversement proportionnelle à celle en Cr₂O₃ et atteint 40 % en poids. Les teneurs en MgO sont à quelques exceptions près plus petites que celles en FeO; les deux éléments ont une corrélation négative. Les teneurs en Fe₂O₃, déterminées dans un quart des grains, ne dépassent généralement pas 5 % en poids. En plus de Cr, Al, Fe, Mg, on rencontre, dans quelques spinelles, des teneurs mesurables en TiO₂.La composition chimique des spinelles détritiques permet de leur assigner comme origine des péridotites appartenant à une série ophiolitique de type alpin.Les études sédimentologiques montrent que la région d'origine des sédiments clastiques du Dévonien moyen se trouve au nord du Massif Schisteux Rhénan.

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/ / , 675 . . . . . 134 50 -% r₂₃. l₂₃ 40 .-%. MgO, , , FeO. . F₂₃, , 5 -%. , , iO. — , . , .

     

Numerical models of convection in the earth's mantle

Calculations on convection in Newtonian fluids at large Rayleigh numbers show behaviour in general agreement with geophysical observations. The calculated values of surface velocities, regional gravity and topographic anomalies all lie within the range of values found at the earth's surface. The sign of the gravity anomaly over rising fluid is positive because the gravitational effect of the surface deformation is greater than that of the density deficit. This result appears to agree with geophysical observations and should permit the flow within the mantle to be followed in some detail.