Models of aggradation versus progradation in the Himalayan Foreland

A frequent goal of decompaction analysis is to reconstruct histories of basin subsidence and tectonic loading. In marine environments, eustatic and paleobathymetric uncertainties limit the resolution of these reconstructions. Whereas in the terrestrial basins, these ambiguities are absent, it is still necessary to account for depositional slopes between localities in order to analyze three-dimensional patterns of subsidence. We define two end-members for depositional surfaces: aggradation and progradation. The relative importance of either end-member is a function of the interplay between the rate of net sediment accumulation and the rate of basin subsidence. The models predict the patterns of major drainages (transverse versus longitudinal) and the way in which provenance should be reflected within different portions of a basin. Consequently, paleocurrent and provenance data from the ancient stratigraphic record can be used to distinguish between these endmembers. The subhorizontal depositional surfaces that dominate during times of aggradation provide a well defined reference frame for regional analysis of decompacted stratigraphies and related subsidence. Depositional slopes during progradation can not be as precisely specified, and consequently yield greater uncertainties in reconstructions of subsidence. These models are applied to the Mio-Pliocene foreland basin of the northwestern Himalaya, where sequences of isochronous strata have been analyzed throughout the basin. These time-controlled data delineate a distinctive evolution from largely aggradational to largely progradational depositional geometries as deformation progressively encroaches on the foreland. Such a reconstruction of past depositional surfaces provides a well constrained reference frame for subsequent integration of subsidence histories from throughout the foreland.

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Zusammenfassung Ein häufiges Ziel der Dekompaktionsanalyse ist es die Beckenabsenkung und die tektonische Belastung zu rekonstruieren. In marinen Ablagerungsräumen limitieren eustatische und paläobathymetrische Unsicherheiten die Auflösung der Rekonstruktion. Bei terrestrischen Becken fehlen diese Zweideutigkeiten; es ist aber trotzdem notwendig, Rechenschaft über den Ablagerungshang zwischen verschiedenen Lokalitäten abzulegen, um dreidimensionale Subsidenzmuster zu analysieren. Wir definieren zwei Endglieder von Ablagerangsflächen: Aggradation und Progradation. Die relative Wichtigkeit des jeweiligen Endglieds ist eine Funktion des Zusammenspiels zwischen der Nettorate der Sedimentakkumulation und der Beckensubsidenz. Die Modelle sagen die Hauptentwässerungsmuster (quer- oder längsverlaufend) vorher, sowie den Weg in dem die Sedimentherkunft innerhalb verschiedener Bereiche des Beckens berücksichtigt werden sollte. Folglich können Paläoströmungs- und Herkunftsdaten alter stratigraphischer Überlieferungen benutzt werden, um zwischen den Endgliedern zu unterscheiden. Die subhorizontale Ablagerungsfläche welche zur Zeit der Aggradation dominant ist, liefert einen gut definierten Referenzrahmen für die regionale Analyse von dekomprimierten Formationen und der damit verknüpften Subsidenz. Ablagerangshänge während Progradation können nicht präzise spezifiziert werden und beinhalten daher größere Unsicherheiten bei der Rekonstruktion der Subsidenz. Diese Modelle wurden übertragen auf das miozäne bis pliozäne Vorgebirgsbecken des nordwestlichen Himalayas, wo Sequenzen von isochronen Schichten durch das gesamte Becken analysiert werden konnten. Diese zeitkontrollierten Daten schildern eine ganz bestimmte Entwicklung, die von einer hauptsächlich aggradierenden zu einer progradierenden Ablagerangsgeometrie verlief, während der die Deformation schrittweise in Richtung Vorland übergriff. Diese Rekonstruktion von ehemaligen Ablagerangsflächen liefert einen guten Referenzrahmen für die folgende Integration der Subsidenzgeschichte des gesamten Vorlands.

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Résumé L'analyse de décompaction a souvent pour but de reconstituer l'histoire de la subsidence d'un bassin et de la charge tectonique. Dans les milieux marins, de telles reconstitutions sont limitées par des incertitudes de caractère eustatique et paléobathymétrique. Par contre, ces ambiguïtés ne se présentent pas dans le cas des bassins continentaux, où il convient néanmoins de tenir compte de la pente de la surface de dépôt entre les divers points considérés pour établir un schéma tridimensionnel de la subsidence. Nous définissons deux situations extrêmes pour les surfaces de dépôt: l'aggradation et la progradation. L'importance relative de ces deux extrêmes est fonction de l'interaction entre le taux d'accumulation net des sédiments et le taux de subsidence du bassin. Les modèles prévoient la répartition des drainages principaux (transverse ou longitudinal) et la manière dont l'origine des sédiments peut se répercuter dans les diverses parties d'un bassin. Il en résulte que des informations fournies par les relevés stratigraphiques à propos des paléocourants et de la source des sédiments peuvent être utilisées pour faire la distinction entre les deux cas extrêmes. Les surfaces de dépôt subhorizontales, qui prédominent pendant les périodes d'aggradation, fournissent un bon cadre de référence pour les analyses régionales de formations décompactées et de la subsidence qui leur est associée. Les surfaces de dépôt inclinées qui se présentent au cours des progradations ne peuvent pas être définies de manière aussi précise et engendrent par conséquent plus d'incertitude dans la reconstitution de la subsidence. Les auteurs appliquent ces modèles au bassin mio-pliocène d'avant-pays de l'Himalaya nord-occidental, dans lequel des séquences de couches isochrones ont été suivies à travers tout le bassin. Ces données, chronologiquement définies, fournissent l'image d'une évolution nette, depuis des géométries typiques d'aggradation jusqu' à des géométries typiques de progradation, au fur et à mesure de l'emprise progressive de la déformation sur l'avant-pays. Une telle reconstitution des surfaces de dépôt anciennes fournit un bon cadre de référence en vue de l'intégration ultérieure de l'histoire de la subsidence dans l'ensemble de l'avant-pays.

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Sedimentary and faunal events revealed by a revised correlation of post‐glacial Hirnantian (Late Ordovician) strata in the Welsh Basin,UK

The discovery of a previously unrecognized unconformity and of new faunas in the type Llandovery area underpins a revised correlation of Hirnantian strata in mid Wales. This has revealed the sedimentary and faunal events which affected the Lower Palaeozoic Welsh Basin during the global rise in sea level that followed the end‐Ordovician glacial maximum and has allowed their interpretation in the context of local and global influences. In peri‐basinal shelfal settings the onset of post‐glacial deepening is recorded by an unfossiliferous, transgressive shoreface sequence (Cwm Clyd Sandstone and Garth House formations) which rests unconformably on Rawtheyan rocks, deformed during an episode of pre‐Hirnantian tectonism. In the deep water facies of the basin centre, this same sequence boundary is now recognized as the contact between fine‐grained, re‐sedimented mudstones and an underlying regressive sequence of turbidite sandstones and conglomerates; it is at a level lower than previously cited and calls into question the established lithostratigraphy. In younger Hirnantian strata, graptolites associated with the newly recognized Ystradwalter Member (Chwefri Formation) demonstrate that this distal shelf unit correlates with the persculptus graptolite‐bearing Mottled Mudstone Member of the basinal succession. Together these members record an important macrofaunal recolonization of the Welsh Basin and mark a key event in the post‐glacial transgression. Further deepening saw the establishment of a stratified water column and the imposition of anoxic bottom water conditions across the basin floor. These post‐glacial Hirnantian events are consistent with the re‐establishment of connections between a silled Welsh Basin and the open Iapetus Ocean. However, a comparison with other areas suggests that each event records a separate deepening episode within a pulsed glacio‐eustatic transgression, while also reflecting changes in post‐glacial climate and patterns of oceanic circulation and associated biotic flux. British Geological Survey © NERC 2009. All rights reserved.     

Changing patterns of population distribution in Zimbabwe

This paper critically evaluates the causes and consequences of changes in population distribution in Zimbabwe during the colonial period and since independence in 1980. Five main aspects of population geography are examined. Firstly, by way on background on contemporary patterns of settlement and land use, the history of tenure policies is outlined. Secondly, the distribution of the African population as revealed in the 1982 census is described and major changes between the census years of 1962, 1969 and 1982 are discussed. Thirdly, changing patterns of settlement and land use within the peasant farming areas (Communal Lands) are examined in the context of increasing population pressures. Fourthly, trends in the growth of the main urban centres and the process of urbanisation of the African population are described. Fifthly, post-independence development policies directed at effecting changes in the distribution of population are discussed with particular reference to the land resettlement programme. The present distribution of population is seen as the product of interacting political, economic and environmental factors, knowledge of which is essential in devising appropriate development strategies to achieve a more equable balance of population and resources.     

Structural chemistry of synthetic cordierite: Evidence for solid solutions and disordered compositional domains in Bi-flux-grown Mg-cordierites

The structure, crystal chemistry, and microstructure of disordered and ordered Mg-cordierites synthesized in a bismuth oxide flux system have been studied by a combination of x-ray and neutron powder diffraction and quantitative x-ray microanalysis using analytical electron microscopy. Microchemical data obtained on Bi-flux cordierites using energy-dispersive x-ray analysis is interpreted through comparison with data collected on stoichiometric Mg₂Al₄Si₅O₁₈ glass and α- and β-cordierite samples synthesized by subsolidus crystallization of the glass. Bi-flux cordierites crystallize in both the hexagonal and orthorhombic polymorphs and contain 5 to 10 at% occupancy of bismuth on the C1 and C2 channel sites. The microstructure of Bi-flux α-cordierite is characterized by the existence of local domains of disordered cordierite solid solutions with variable composition and significant vacancy concentrations on the octahedral site. The β-cordierites have a more homogeneous microstructure but are still Al-deficient, Si-rich solid solutions.     

Isotopic partitioning between scallop shell calcite and seawater: effect of shell growth rate

The relationship between molluscan shell growth rate and skeletal δ¹⁸O and δ¹³C was investigated in a detailed field study for the scallop, Pecten maximus. Seasonal variation in shell growth rate was found to be a governing factor influencing shell δ¹⁸O and δ¹³C. At low shell growth rates, shell δ¹⁸O were more positive (of the order +0.4‰) and δ¹³C more negative (up to −2‰) as compared with predicted values for precipitation of inorganic calcite in isotopic equilibrium with seawater. The deviations in δ¹⁸O were hypothesized as reflecting possible differences in solution carbonate chemistry at the site of mineralization in the extrapallial fluid as compared with that of the external seawater medium. The deviations in shell δ¹³C were consistent with incorporation of isotopically depleted respiratory ¹³C (i.e., a metabolic effect). A trend toward more depleted shell δ¹⁸O and δ¹³C values occurred at higher shell growth rates, with negative δ¹⁸O values as compared with predicted equilibrium at shell growth rates above 0.13 mm per day. These simultaneous negative deviations in skeletal δ¹⁸O and δ¹³C were interpreted as resulting from a kinetic effect. The implications for environmental reconstruction from molluscan isotopic records are discussed in light of a model of isotopic behavior based on the findings of the study.     

Flux and dispersion of gases from the “Drachenschlund” hydrothermal vent at 8°18' S, 13°30' W on the Mid-Atlantic Ridge

Hydrothermal emission of mantle helium appears to be directly related to magma production rate, but other processes can generate methane and hydrogen on mid-ocean ridges. In an on-going effort to characterize these processes in the South Atlantic, the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8°17.9' S, 13°30.4' W. The vent lies on the shoulder of an oblique offset in the Mid-Atlantic Ridge and discharges high concentrations of methane and hydrogen. Measurements during expeditions in 2004 and 2006 show that the ratio of CH₄ to ³He in the neutrally buoyant plume is quite high, 4 × 10⁸. The CTD stations were accompanied by velocity measurements with lowered acoustic Doppler current profilers (LADCP), and from these data we estimate the methane transport to have been 0.5 mol s^− 1 in a WSW-trending plume that seems to develop during the ebb tidal phase. This transport is an order of magnitude greater than the source of CH₄ calculated from its concentration in the vent fluid and the rise height of the plume. From this range of methane fluxes, the source of ³He is estimated to be between 0.14 and 1.2 nmol s^− 1. In either case, the ³He source is significantly lower than expected from the spreading rate of the Mid-Atlantic Ridge. From the inventory of methane in the rift valley adjacent to the vent, it appears that the average specific rate of oxidation is 2.6 to 23 yr^− 1, corresponding to a turnover time between 140 and 16 days. Vertical profiles of methane in the surrounding region often exhibited Gaussian-like distributions, and the variances appear to increase with distance from the vent. Using a Gaussian plume model, we obtained a range of vertical eddy diffusivities between 0.009 and 0.08 m²m² s^− 1. These high values may be due to tidally driven internal waves across the promontory on which the vent is located.     

Origins and evolution of rhyolitic magmas in the central Snake River Plain: insights from coupled high-precision geochronology,oxygen isotope,and hafnium isotope analyses of zircon

We present new high-precision CA-ID-TIMS and in situ U–Pb ages together with Hf and O isotopic analyses (analyses performed all on the same grains) from four tuffs from the 15?10 Ma Bruneau–Jarbidge center of the Snake River Plain and from three rhyolitic units from the Kimberly borehole in the neighboring 10?6 Ma Twin Falls volcanic center. We find significant intrasample diversity in zircon ages (ranges of up to 3 Myr) and in δ¹⁸O (ranges of up to 6‰) and ε_Hf (ranges of up to 24 ε units) values. Zircon rims are also more homogeneous than the associated cores, and we show that zircon rim growth occurs faster than the resolution of in situ dating techniques. CA-ID-TIMS dating of a subset of zircon grains from the Twin Falls samples reveals complex crystallization histories spanning 10⁴–10⁶ years prior to some eruptions, suggesting that magma genesis was characterized by the cyclic remelting of buried volcanic rocks and intrusions associated with previous magmatic episodes. Age-dependent trends in zircon isotopic compositions show that rhyolite production in the Yellowstone hotspot track is driven by the mixing of mantle-derived melts (normal δ¹⁸O and ε_Hf) and a combination of Precambrian basement rock (normal δ¹⁸O and ε_Hf down to ??60) and shallow Mesozoic and Cenozoic age rocks, some of which are hydrothermally altered (to low δ¹⁸O values) by earlier stages of Snake River Plain magmatism. These crustal melts hybridize with juvenile basalts and rhyolites to produce the erupted rhyolites. We also observe that the Precambrian basement rock is only an important component in the erupted magmas in the first eruption at each caldera center, suggesting that the accumulation of new intrusions quickly builds an upper crustal intrusive body which is isolated from the Precambrian basement and evolves towards more isotopically juvenile and lower-δ¹⁸O compositions over time.     

Initial results and progress of the Mississippi delta sediment pore water pressure experiment

Abstract

This report describes the instrumentation, initial results, and progress of an experiment designed to measure and monitor submarine sediment pore water and hydrostatic pressures in a selected area of the Mississippi Delta. The experiment also is intended to monitor significant pressure perturbations during active storm periods. Initial analysis of the data revealed excess pore water pressures in the silty clay sediment at selected depths below the mudline. Continuous monitoring of the pore water and hydrostatic pressures was expected to reveal important information regarding sediment pore water pressure variations as a function of the geological processes active in the Mississippi Delta.     

Volcanic geology and eruption frequency,lower east rift zone of Kilauea volcano,Hawaii

Detailed geologic mapping and radiocarbon dating of tholeiitic basalts covering about 275 km² on the lower east rift zone (LERZ) and adjoining flanks of Kilauea volcano, Hawaii, show that at least 112 separate eruptions have occurred during the past 2360 years. Eruptive products include spatter ramparts and cones, a shield, two extensive lithic-rich tuff deposits, aa and pahoehoe flows, and three littoral cones. Areal coverage, number of eruptions and average dormant interval estimates in years for the five age groups assigned are: (I) historic, i.e. A D 1790 and younger: 25%, 5, 42.75; (II) 200–400 years old: 50%, 15, 14.3: (III) 400–750 years old: 20%, 54, 6.6; (IV) 750–1500 years old: 5%, 37, 20.8; (V) 1500–3000 years old: <1%, 1, unknown. At least 4.5–6 km³ of tholeiitic basalt have been erupted from the LERZ during the past 1500 years. Estimated volumes of the exposed products of individual eruptions range from a few tens of cubic meters for older units in small kipukas to as much as 0.4 km³ for the heiheiahulu shield. The average dormant interval has been about 13.6 years during the past 1500 years. The most recent eruption occurred in 1961, and the area may be overdue for its next eruption. However, eruptive activity will not resume on the LERZ until either the dike feeding the current eruption on the middle east rift zone extends farther down rift, or a new dike, unrelated to the current eruption, extends into the LERZ.