Fluvial and submarine morphodynamics of laminar and near‐laminar flows: a synthesis

The interaction of flow with an erodible bed in alluvial rivers and deep‐sea channels gives rise to a wide range of self‐formed morphologies, including channels, ripples, dunes, antidunes, alternate bars, multiple‐row bars, meandering and braiding. As the flow is invariably turbulent in field manifestations of these morphologies, there has been a tendency to assume that turbulence is necessary for them to form. While turbulence undoubtedly has an important influence when it is present, it is not necessary for any of these features. Indeed, all of these features can be formed by the morphodynamic interaction of purely laminar or nearly laminar flow with an erodible bed. This paper provides a survey and synthesis of a wide range of laminar or near‐laminar flow analogues of morphologies observed in the field. Laminar‐flow analogues of turbulent‐flow morphologies cannot and should not be expected to satisfy dynamic similarity in terms of all relevant dimensionless parameters. What is of more significance is the convergence of the underlying physics. It is illustrated in this paper that many existing theoretical frameworks for the explanation of turbulent‐flow morphodynamics require only relatively minor modification in order to adapt them to laminar flows.     

Sequence stratigraphic interpretation of a Pennsylvanian (Upper Carboniferous) coal from the central Appalachian Basin,USA

Peat mires retain a sensitive record of water‐table (base‐level) fluctuations throughout their accumulation. On this basis, coals provide one of the best opportunities to interpret high‐resolution base‐level change in ancient non‐marine deposits. The petrographic composition of 275 samples collected from 11 localities along a 100 km south‐west to north‐east transect across the regionally extensive (>37 000 km²) Pennsylvanian (Upper Carboniferous) Fire Clay coal of the Central Appalachian Basin, USA was analysed to determine its internal stratigraphy. The coal is positioned within the late lowstand/early transgressive systems tract of a fourth‐order depositional sequence. The results of the petrographic analyses reveal a cyclicity in the composition of the Fire Clay coal, which defines six units that are correlated over more than 100 km. Each coal cycle is characterized by a gradual upward transition from vitrinite‐dominated to inertinite‐dominated coal, which represents a ‘drying‐up’ succession. Increased concentrations of resistant peat components at the top of the drying‐up successions indicate reduced peat accumulation rates associated with slowing rate of water‐table rise, and may represent a residue of peat remaining from a phase of exposure and erosion resulting from a falling water table. These drying‐up successions are bound by surfaces that display an abrupt coal facies shift from inertinite‐rich to vitrinite‐rich coal, representing a rapid water‐table rise. Each cycle represents markedly different mire conditions with different aerial distributions, which supports the notion of temporal disconnection between each unit of coal, and suggests that considerable time may be ‘locked‐up’ in unit bounding exposure surfaces. Recognition that the rate of peat accumulation in a mire may vary considerably through time, has important implications for studies which assume that peat and coal successions provide continuous and time‐invariant records of base‐level fluctuations or palaeoecological change.     

Structure of a silica diagenetic transformation zone: the Gjallar Ridge,offshore Norway

Three‐dimensional seismic data from the Gjallar Ridge were used together with X‐ray diffraction data, scientific boreholes and wireline logs to analyse the seismic structure of a silica diagenetic transformation zone. The following features were identified: (i) an interval some 150 to 300 m thick that contains anomalously high‐amplitude reflections; (ii) a strong reflection event at the top of this interval which cross‐cuts stratigraphy, interpreted as the transformation boundary between opal‐A‐rich and opal‐CT‐rich sediment; (iii) amplitude variations on stratigraphic reflections within the interval, attributed to variations in the proportions of opal‐A and opal‐CT; and (iv) a second, deeper, cross‐cutting reflection event within the interval, interpreted as the transformation boundary between opal‐CT‐rich and quartz‐rich sediment. The base of the interval containing the anomalously high‐amplitude reflections is interpreted as a stratigraphic reflection demarcating the base of the silica‐rich strata. On a stratigraphic reflection within the interval of high amplitudes, roughly circular regions of anomalously high amplitude with diameters of 0·8 to 2·5 km are separated by lower amplitude regions. This pattern is similar to the cells previously identified at the opal‐A to opal‐CT transformation boundary and probably results from more complete transformation of opal‐A to opal‐CT. All of these observations provide the first recognition from seismic data that silica diagenetic transformations are not always narrow boundaries represented by single cross‐cutting seismic reflection events, as implied previously, but can be heterogeneous and hundreds of metres in thickness, as observed at outcrop.     

The lithostratigraphy of the Les Echets basin, France: tentative correlation between cores

Two new long sediment cores (EC1 and EC3), recovered from different locations within the infilled basin at Les Echets, France, provide a new high-resolution record of terrestrial and lacustrine responses to climatic changes during Marine Isotope Stages (MIS) 3 and 2. The lithologies of the cores are described in detail and correlated with each other by stratigraphic marker horizons, fluctuations in organic matter and AMS radiocarbon ages. The tentative correlation of the new cores to those described and analysed by de Beaulieu et al. (1980) and de Beaulieu & Reille (1984a) provides a preliminary chronostratigraphic framework. Sedimentation during MIS3 started with accumulation of sands and silts and was followed by alternating gyttja and clayey gyttja silts. Exceptionally high sedimentation rates during MIS2 led to the infilling of the basin. Alternating organic-rich and minerogenic-rich sediments appear to coincide with changes in pollen assemblages (de Beaulieu & Reille 1984a) and suggest that millennial-scale climatic changes controlled lake productivity and catchment stability during most of MIS3.     

Exposure history of the Peekskill (H6) meteorite

Abstract— The Peekskill H6 meteorite fell on 1992 October 9. We report extensive measurements of cosmic-ray produced stable nuclides of He, Ne, and Ar, of the radionuclides ²²Na, ⁶⁰Co, ¹⁴C, ³⁶Cl, ²⁶Al, and ¹⁰Be, and of cosmic-ray track densities. After correction for shielding via the ²²Ne/²¹Ne ratio, the concentrations of cosmic-ray produced ³He, ²¹Ne and ³⁸Ar give an average exposure age of 25 Ma, which is considered to be a lower limit on the true value. The ¹⁰Be/²¹Ne age is 32 Ma and falls onto a peak in the H-chondrite exposure age distribution. The activities of ²⁶Al, ¹⁴C, ³⁶Cl, and ¹⁰Be are all close to the maximum values expected for H-chondrites. Together with cosmic-ray track densities and the ²²Ne/²¹Ne ratio, these radionuclide data place the samples at a depth >20 cm in a meteoroid with a radius >40 cm. In contrast, the ⁶⁰Co activity requires a near-surface location and/or a much smaller body. Calculations show that a flattened geometry for the Peekskill meteoroid does not explain the observations in the context of a one-stage irradiation. A two-stage model can account for the data. We estimate an upper bound of 70 cm on the radius of the earlier stage of irradiation and conclude that Peekskill's radius was <70 cm when it entered the Earth's atmosphere. This size limit is somewhat smaller than the dynamic determinations (Brown et al., 1994).     

ARTIFICIAL RECHARGE OF THE LONDON BASIN

Abstract

The helium concentrations have been measured in the groundwaters of the Sabarmati basin. Gujarat, and the Jaisalmer district, Rajasthan. The observed helium concentrations show localized anomalies. The magnitude of the excess helium is shown to be approximately inversely proportional to the square of the thickness of the sedimentary strata between the sampled aquifer and the Basement Trap surface in the Sabarmati basin.     

Geochemistry of the Othris Ophiolite, Greece: Evidence for Refertilization?

The Othris peridotite massif, Greece, shows conflicting evidencefor a mid-ocean ridge and supra-subduction zone tectonic settingwith the presence of plagioclase peridotite that may representan area of either incomplete melt extraction, or melt impregnationand accumulation. To address these problems we focus on a 3km continuous section in the Fournos Kaïtsa area, consistingof layers of harzburgite, plagioclase harzburgite and plagioclaselherzolite with accurately known structural and petrographiccontrol. Refractory, Cr-rich spinel compositions and light rareearth element depleted clinopyroxenes in the harzburgites areconsistent with     

Geochemical Constraints on the Petrogenesis of Diamond Facies Pyroxenites from the Beni Bousera Peridotite Massif, North Morocco

The petrogenesis of pyroxenite layers within the Beni Bouseraperidotite massif is investigated by means of elemental andNd-Sr-Pb-O-S isotope analyses. The light rare earth element(LREE) depleted nature of many of the pyroxenites, their widevariation in composition, and lack of correlation between incompatibleelements and fractionation indices preclude them from representingcrystallized melts from a peridotitic source. The physical characteristicsof the pyroxenites and their large (greater than a factor of20) range in Ni rule out partial melting as the cause of theirpetrological and geochemical diversity. Major and compatibletrace element geochemistry is consistent with formation of mostof the pyroxenite suite via high-pressure crystal segregationin magma conduits intruding the peridotites. These magmas crystallizedclinopyroxene, orthopyroxene, and garnet. The pressure of crystallizationis constrained to be above {small tilde}45 kbar from the presenceof graphitized diamonds in pyroxenite layers. Lack of correlationbetween fractionation indices and highly incompatible elementsand the wide variation in incompatible element abundances suggestthat the suite did not form from genetically related magmas.The presence of positive and negative Eu anomalies (Eu/Eu* =0•54–2•0) in pyroxenites which crystallizedat pressures much greater than the plagioclase stability field({small tilde} 45 kbar) suggests that the parental magmas originatedfrom precursors which formed in the crust. Oxygen isotope compositionsof coexisting minerals in the pyroxenites indicate high-temperatureequilibration but ¹⁸O values vary from +4•9 to + 9•3,ruling out their derivation from the host peridotites or othernormal mantle sources. The extreme O-isotope variation, togetherwith ³⁴S values of up to + 13 in sulphides included within CPXstrongly suggests that the melts from which the pyroxenitescrystallized were derived from hydrothermally altered, subductedoceanic lithosphere. Extreme initial radiogenic isotope variationin the pyroxenites (_Nd + 26 to –9 , ⁸⁷Sr/⁸⁶Sr 0•7025–0•7110,²⁰⁶Pb/²⁰⁴Pb 18•21–19•90) support such an originbut also require a component with ancient, high U/Pb and Th/Pbin their source to explain the high 7/4 and 8/4 values of somepyroxenites. This component may be subducted hemi-pelagic sediment.Further evidence for a sediment component in the pyroxenitesis provided by isotopically light carbon in the graphite pyroxenites(¹³C–16 to – 28). Parentdaughter isotopes in thepyroxenites are strongly decoupled, making estimation of formationages speculative. The decoupling occurred recently (<200Ma), probably as a result of partial melting associated withdiapiric upwelling and emplacement of the massif into the crustfrom the diamond stability field. This late partial meltingevent further depleted the pyroxenites in incompatible elements.The variably altered nature of the subducted protolith and complexhistory of trace element fractionation of the pyroxenites haslargely obscured geochemical mixing trends. However, Nd–Pbisotope systematics indicate that incorporation of the componentwith high U/Pb–Th/Pb occurred relatively recently (<200Ma) for some pyroxenites. Other pyroxenites do not show evidencefor incorporation of such a component and may be substantiallyolder. Tectonic, geophysical, and isotopic constraints indicateformation of the pyroxenites in the mantle wedge above a subductingslab during the Cretaceous. Physical and chemical evidence forhigh-pressure fractionation seen in most of the pyroxenitesprecludes them from simply representing ancient subducted oceaniclithosphere, thinned by diffusion. However, the petrologicaland isotopic diversity of the massif support the concept ofa ‘marble cake’ mantle capable of producing theobserved geochemical diversity seen in oceanic magmas. *Present Address: Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 Present address: Department of Geological Sciences, 1066 C.C. Little Building, University of Michigan, Ann Arbor, Michigan 48109