Dolomitization of the Devonian Swan Hills Formation, Rosevear Field, Alberta, Canada

The Swan Hills Formation (Middle-Upper Devonian) of the Western Canada Basin is host to several NW-SE-trending gas fields developed in massive replacement dolostone. One of these, the Rosevear Field, contains two major dolostone trends along opposing margins of a marine channel that penetrates into a platform-reef complex. Dolostones consist predominantly of branching and bulbous strdmatoporoid floatstones and rudstones with well-developed moldic and vuggy porosity. Replacement dolomite is coarsely crystalline (100-600 μm), inclusion-rich, composed of euhedral through anhedral crystals and has a blotchy to homogeneous red cathodoluminescence. Geochemically, replacement dolomite is characterized by (i) nearly stoichiometric composition (50.1-51.1 mol% CaCO₃), (ii) negative δ¹⁸O values (mean=-7.5‰, PDB) and (iii) variable ⁸⁷Sr/⁸⁶Sr ratios ranging from values similar to Late Devonian-Early Mississippian seawater (～0.7082) to radiogenic compositions comparable to saddle dolomite cements (>0.7100). Dolomitization began after widespread precipitation of early, equant calcite spar and after the onset of pressure solution, implying that replacement dolomite formed in a burial environment. Oxygen isotope data suggest that dolomite formed at 35-75°C, temperatures reached during burial in Late Devonian through Jurassic time, at minimum depths of 450 m. The linear NW-SE orientation of most dolomite fields in the Swan Hills Formation is suggestive of fault control on fluid circulation. Two models are proposed for fault-controlled circulation of dolomitizing fluids at the Rosevear Field. In the first, compaction-driven, updip fluid migration occurred in response to basin tilting commencing in the Late Palaeozoic. Deep basinal fluids migrating updip were focused into channel-margin sediments along fault conduits. The second model calls upon fault-controlled convective circulation of (i) warm Devonian-Mississippian seawater or (ii) Middle Devonian residual evaporitic brines. The overlap in ⁸⁷Sr/⁸⁶Sr and δ¹⁸O compositions, and similar cathodoluminescence properties between replacement and saddle dolomites provide evidence for neomorphism of some replacement dolomite. Quantitative modelling of Sr and O isotopes and Sr abundances suggests partial equilibration of some replacement dolomite with hot radiogenic brines derived during deep burial of the Swan Hills Formation in the Late Cretaceous-Palaeocene. Interaction of replacement dolomite with deep brines led to enrichment in ⁸⁷Sr while leaving δ¹⁸O similar to pre-neomorphism values.     

Cryptotephrochronology of the Eemian and the last interglacial–glacial transition in the North East Atlantic

The Eemian interglacial and the onset of the subsequent glacial period serve as the most recent analogue for the natural operation of the climate system during the current interglacial. Pronounced climatic oscillations occurred during this period, but their nature and pattern are poorly understood due to dating limitations and unknown phase relationships between different regions and archives. Tephrochronology offers considerable potential for precise correlation of disparate palaeoclimatic archives preserving evidence of these rapid climatic transitions through the tracing of common isochronous tephra horizons. We outline the identification of three previously unknown cryptotephra horizons within a marine core from the Rockall Trough, North East Atlantic. This sequence preserves a high‐resolution record of this interval and shard size, geochemical heterogeneity and the co‐variance of shard concentrations with ice‐rafted debris data are utilized to demonstrate that primary airfall was the most likely transport and depositional pathway. The main geochemical populations of these horizons have similar transitional alkali major and trace element compositions, suggesting that they were derived from a common Icelandic source, potentially the Öræfajökull volcanic system. These tephra horizons represent additions to the North Atlantic event stratigraphy for this period and tentative correlations to Icelandic terrestrial deposits are proposed for two horizons.     

WATER MANAGEMENT—A GENERAL REPORT

Abstract

This analysis was undertaken to develop appropriate extreme flood design criteria for a nuclear power plant at Halileh, near Bushehr, Iran, adjacent to the Persian Gulf. Graphical relationships presented provide a convenient means of estimating the probable maximum precipitation and the 2- to 100-year return period rainfall events with durations from 5 min to 24 h. The relationships may be applied for drainage areas up to 25 km². Probable maximum precipitation and 2- to 100-year return period rainfall events were estimated. Precipitation depth-duration relationships were derived.     

Unravelling the conundrum of river response to rising sea‐level from laboratory to field. Part II. The Fly–Strickland River system,Papua New Guinea

The most recent deglaciation resulted in a global sea‐level rise of some 120 m over ca 12 000 years. A moving boundary numerical model is developed to predict the response of rivers to this rise. The model was motivated by experiments at small scale, which have identified two modes describing the transgression of a river mouth: (i) autoretreat without abandonment of the river delta (no sediment starvation at the topset–foreset break); and (ii) sediment‐starved autoretreat with abandonment of the delta. In the latter case, transgression is far more rapid, and its effects are felt much further upstream of the river mouth. A moving boundary numerical model that captures these features in experimental deltas is adapted to describe the response of the Fly–Strickland River system, Papua New Guinea. In the absence of better information, the model is applied to the case of sea‐level rise without local climate change in New Guinea. The model suggests that: (i) sea‐level rise has forced the river mouth to transgress over 700 km since the last glacial maximum; (ii) sediment‐starved autoretreat has forced enough bed aggradation to block a tributary with a low sediment load and create the present‐day Lake Murray; (iii) the resulting aggradation was sufficient to move the gravel–sand transition on the Strickland River upstream; (iv) the present‐day Fly Estuary may be, in part, a relict river valley drowned by sea‐level rise and partially filled by tidal effects; and (v) the Fly River is presently reforming its bankfull geometry and prograding into the Fly Estuary. A parametric study with the model indicates that sediment concentration during floods plays a key role in determining whether or not, and to what extent, transgression is expressed in terms of sediment‐starved autoretreat. A sufficiently high sediment concentration can prevent sediment‐starved autoretreat during the entire sea‐level cycle. This observation may explain why some present‐day river mouths are expressed in terms of deltas protruding into the sea, and others are wholly contained within embayments or estuaries in which water has invaded landward.     

The isotopic composition of strontium in non-marine carbonate rocks: the Flagstaff Formation of Utah*

The isotopic composition of strontium in surface water in continental basins is determined primarily by the geology of the basin and to a lesser extent by climatic conditions. Consequently, the ⁸⁷Sr/⁸⁶Sr ratios of brines in such basins can change only as a result of changes in the geology or climate. This principle of isotope geology was studied by analysis of a suite of non-marine carbonate rocks from the Flagstaff Formation (Palaeocene-Eocene) of Utah. The samples were collected from a section in Fairview Canyon of Sanpete County. They include both limestone and dolomite and were selected to have low non-carbonate residues. The concentrations of strontium in calcites averages 383 ± 128 p.p.m., while those of dolomites increase from 354 ± 74 p.p.m. in the lower 43 m of section to a maximum of 2259 p.p.m. higher up. The increase in the strontium content of dolomite is interpreted as evidence for a change from steady-state to progressively more evaporitic conditions. Two dolomites have isotopic compositions of oxygen expressed as δ¹⁸O = -2.75‰ (relative to the PDB standard) and are enriched in ¹⁸O relative to two calcites whose average δ¹⁸O value is -9.9‰. The ⁸⁷Sr/⁸⁶Sr ratios of the carbonate minerals range from 0.70890 to 0.71260. These values are clearly greater than the ⁸⁷Sr/⁸⁶Sr ratio of marine carbonates of Early Eocene age which is 0.70744. The variation of the ⁸⁷Sr/⁸⁶Sr ratio in this section of the Flagstaff Formation is real and reflects the occurrence of geological events which changed the isotopic composition of Sr entering Lake Flagstaff. The non-carbonate fractions of six carbonate rocks and one sandstone fit a straight line on the strontium mixing diagram in co-ordinates of initial ⁸⁷Sr/⁸⁶Sr and 1/Sr concentration. These results suggest that the isotopic composition of strontium in Lake Flagstaff may have been modulated by periodic input of volcanogenic detritus of felsic composition.     

Mineral Chemistry of Peridotites from Paleozoic, Mesozoic and Cenozoic Lithosphere: Constraints on Mantle Evolution beneath Eastern China

Major- and trace-element data on the constituent minerals ofgarnet peridotite xenoliths hosted in early Paleozoic (457–500Ma) kimberlites and Neogene (16–18 Ma) volcanic rockswithin the North China Craton are compared with those from thepre-pilot hole of the Chinese Continental Scientific DrillingProject (CCSD-PP1) in the tectonically exhumed Triassic (220Ma) Sulu ultrahigh-pressure (UHP) terrane along its southernmargin. P–T estimates for the Paleozoic and Neogene peridotitexenoliths reflect different model geotherms corresponding tosurface heat flows of 40 mW/m² (Paleozoic) and 80 mW/m² (Neogene).Garnet peridotite xenoliths or xenocrysts from the Paleozoickimberlites are strongly depleted, similar to peridotites fromother areas of cratonic mantle, with magnesium olivine (meanFo_92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet(and spinel) peridotite xenoliths hosted in Neogene basaltsare derived from fertile mantle; they have high Al₂O₃ and TiO₂contents, low-Mg-number olivine (mean Fo_89.5), low-Cr garnetand diopside with flat rare earth element (REE) patterns. Thedifferences between the Paleozoic and Neogene xenoliths suggestthat a buoyant refractory lithospheric keel present beneaththe eastern North China Craton in Paleozoic times was at leastpartly replaced by younger, hotter and more fertile lithosphericmantle during Mesozoic–Cenozoic times. Garnet peridotitesfrom the Sulu UHP terrane have less magnesian olivine (Fo_91.5),and lower-Cr garnet than the Paleozoic xenoliths. The diopsideshave low heavy REE (HREE) contents and sinusoidal to light REE(LREE)-enriched REE patterns. These features, and their highMg/Si and low CaO and Al₂O₃ contents, indicate that the CCSD-PP1peridotites represent a moderately refractory mantle protolith.Details of mineral chemistry indicate that this protolith experiencedcomplex metasomatism by asthenosphere-derived melts or fluidsin Mesoproterozoic, and subsolidus re-equilibration involvingfluids/melts derived from the subducted Yangtze continentalcrust during UHP metamorphism in the early Mesozoic. Tectonicextension of the subcontinental lithospheric mantle of the NorthChina Craton and exhumation of the Sulu UHP rocks in the earlyMesozoic induced upwelling of the asthenosphere. Peridotitessampled by the Neogene basalts represent newly formed lithospherederived by cooling of the upwelling asthenospheric mantle inJurassic–Cretaceous and Paleogene time. KEY WORDS: garnet peridotite xenoliths; North China Craton; lithospheric thinning; Sulu UHP terrane; UHP lithosphere evolution; mantle replacement     

Sedimentary facies and geomorphic evolution of a blocked‐valley lake: Lake Futululu,northern Kwazulu‐Natal,South Africa

Blocked‐valley lakes are formed when tributaries are impounded by the relatively rapid aggradation of a large river and its floodplain. These features are common in the landscape, and have been identified in the floodplains of the Solimões‐Amazon (Brazil) and Fly‐Strickland Rivers (Papua New Guinea), for example, but their inaccessibility has resulted in studies being limited to remotely sensed image analysis. This paper documents the sedimentology and geomorphic evolution of a blocked‐valley lake, Lake Futululu on the Mfolozi River floodplain margin, in South Africa, while also offering a context for the formation of lakes and wetlands at tributary junctions. The study combines aerial photography, elevation data from orthophotographs and field survey, and longitudinal sedimentology determined from a series of cores, which were sub‐sampled for organic content and particle size analysis. Radiocarbon dating was used to gauge the rate and timing of peat accumulation. Results indicate that following the last glacial maximum, rising sea‐levels caused aggradation of the Mfolozi River floodplain. By 3980 years bp , aggradation on the floodplain had impounded the Futululu drainage line, creating conditions suitable for peat formation, which has since occurred at a constant average rate of 0·13 cm year^?1. Continued aggradation on the Mfolozi River floodplain has raised the base level of the Futululu drainage line, resulting in a series of back‐stepping sedimentary facies with fluvially derived sand and silt episodically prograding over lacustrine peat deposits. Blocked‐valley lakes form where the trunk river has a much larger sediment load and catchment than the tributary stream. Similarly, when the relative difference in sediment loads is less, palustrine wetlands, rather than lakes, may be the result. In contrast, where tributaries drain a steep, well‐connected catchment, they may impound much larger trunk rivers, creating lakes or wetlands upstream.     

Geometry and facies of Tertiary clinothems, Spitsbergen

Sandstone clinothems from the Battfjellet Formation (Palaeogene) on Spitsbergen are locally exceptionally well preserved along depositional dip-parallel mountainsides. The clinothems are more than 1 km wide and more than 100 m thick. Superposition of several sandstone clinothems separated by mudstones reflects repeated shoreline progradation and transgression. Deposition took place partly on‘post-transgressional’ depositional shelves, and partly by contributing seaward-sloping wedges, or clinothems, to a ramp progradation. Shorelines dominated both by mouth bar and shoreface environments have been identified. The clinothems are organized into an overall progradational architecture with a geometry having features in common with progradational seismic facies.     

GARDENS ARE US,WE ARE NATURE: TRANSCENDING ANTIQUITY AND MODERNITY*

ABSTRACT. The history of gardens and the history of humans are linked inextricably, especially in the context of environment. As people and their institutions have changed, so too have gardens. This brief essay illustrates some important aspects in the evolution of gardens, exploring three seemingly discrete, but actually interconnected, notions: the antiquity of gardens, combining archaeological data with ethnographic parallels; the role of gardens in the changing spatial manifestations of agriculture, from dump heaps to amber waves of grain; and the transformation of domestic space, the literal and figurative reconstruction of garden fences into house walls. Changes are discussed as inadvertent products, rather than as consequences of deliberate actions. Modern ideas about categorizing landscapes are challenged further. The nature/culture dualism is a myth even at the household scale, as is our traditional way of looking at the world.     

Geolimnology of Freefight Lake: an unusual hypersaline lake in the northern Great Plains of western Canada*

Freefight Lake, Canada's deepest salt lake, is a meromictic, hypersaline lake located in the most arid part of the northern Great Plains. The lake has a distinctive basin morphology, with a large expanse of seasonally flooded mud flats and sand flats adjacent to a deep, flat bottomed perennial waterbody. The mixolimnion, dominated by magnesium, sodium and sulphate ions, has an average salinity of 110 ppt and overlies a monimolimnion of 180 ppt total dissolved salts. The entire water column is strongly supersaturated with respect to a variety of calcium and magnesium carbonate minerals; the lower water mass is also saturated or supersaturated with respect to a number of very soluble sodium, magnesium and sodium + magnesium salts. The modern sedimentary processes operating in Freefight Lake give rise to six main sedimentary facies: (i) colluvium, (ii) mud flats and sand flats, (iii) algal flats, (iv) delta, (v) slope and debris apron, and (vi) deep basin. The colluvium, mud flats and sand flats, and delta facies are dominated by physical processes and consist mainly of detrital siliciclastic sediment. The algal flats, slope and debris apron, and deep basin facies are dominated by endogenic and authigenic sediments derived mainly by physicochemical and biologically mediated carbonate and evaporite mineral precipitation. As one of very few deep water lakes in the world in which soluble evaporite minerals are forming and being preserved, Freefight Lake occupies an important position within the realm of lacustrine sedimentology. Although many of the sedimentological and geochemical processes taking place in the basin today are unusual, the delineation and evaluation of these processes is essential in order to decipher properly the stratigraphic records of Quaternary lake sediments in this large area of North America, as well as lacustrine sequences from other arid and semi-arid regions of the world.