摘要:

ABSTRACTThe Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment‐water interface. The calcite is biologically precipitated as ostracod valves.In addition to the carbonates in the modern offshore (deep‐water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high‐Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units.Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Vic

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01839.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTGypsum and anhydrite fabrics observed in trenches and deep (500 m) cores from Bristol Dry Lake, California, USA, exhibit a vertical alignment of crystals similar to the fabric seen in bottom‐nucleated brine pond gypsum. However, geochemical and sedimentological evidence indicate that the gypsum formed in Bristol Dry Lake precipitated displacively within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater‐seepage gypsum). Evidence for displacive growth of gypsum is: (i) the geometry of the deposit, (ii) stable isotopic data and the water chemistry of the brine, and (iii) inclusions of matrix which follow twin planes and completely surround crystals as they grow.The bulk of the gypsum precipitated in the playa occurs around the edges of the playa in the playamargin facies and completely rings the lake. Sulphate concentrations in the groundwater increase toward the gypsum zone in the playa margin. Basinward of this zone, sulphate concentrations decrease sharply to trace element levels in the basin centre brine. Authigenic gypsum is rare in the centre of the playa. Stable (δ18O values measured for gypsum waters of crystallization (GWC) are similar to the values calculated for groundwater in the playa margin and alluvial fan sediments (˜– 6%0), whereas measured brine δ18O values range from + 0·5 to + 3·7%0. Deuterium values measured for groundwater are ˜– 70%0, GWC are ˜– 60 to – 65%0 and brine values are ˜– 57%0. The geometry of the deposit and the chemical data suggest that the water precipitating the gypsum is more closely associated with the groundwater than the brine. However, some mixing between groundwater and brine is likely.Within 100 m of the surface, the gypsum dehydrates to anhydrite, although the same vertically aligned fabric is retained through the diagenetic process. The similarity of displacive vertically aligned gypsum and anhydrite fabrics seen in Bristol Dry Lake to subaqueously deposited gypsum in modern brine ponds indicates that the criteria used to define subaqueous fabrics must

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01840.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTPorewaters from a variety of Recent, Pleistocene, and Eocene lithified marine carbonate frameworks displayed similar chemical characteristics: highly depleted concentrations of dissolved oxygen (>20 μM), elevated levels of dissolved methane (25‐5000 nM), and near‐seawater sulphate levels. These porewaters also had low pH values (7·5‐7·9), and contained elevated concentrations of sulphide (4–10 μM), dissolved inorganic carbon (2·05–2·46 mM), and inorganic nutrients. Hydrocarbon composition data indicate that the methane is biogenic, whereas the methane δ13C values (–47·4 ± 2·7%0) suggest that it has been subject to oxidation. The porewater dissolved inorganic carbon δ13C values varied from –0·6 to –39%0, suggesting input of carbon dioxide from organic matter oxidation. We conclude that anaerobic diagenesis involving bacterial degradation of organic matter is a common process in lithified marine carbonates and hypothesize that it may be an important factor controlling t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01841.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTLow‐grade sandstone‐hosted thorium mineralization is recorded in the Carboniferous Northwest Irish Basin in the Republic of Ireland and Northern Ireland. The thorium is concentrated within bitumen nodules which are replacive within the sandstones. A source of thorium was available in a Caledonian granite in the basin watershed, and the bitumen was a product of hydrocarbons generated within the basin. The thorium occurs as thorite inclusions in the bitumen, and the development of the nodules was enhanced by the polymerization of fluid hydrocarbons by irradiation from the thorium. Thorium mobility may have been enhanced by the formation of organometallic complexes. Mineralization probably therefore occurred during hydrocarbon migration, dated as Late Carboniferous to Perm

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01842.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTAirflow data, gathered over dunes ranging from 60‐m tall complex‐crescentic dunes to 2‐m tall simplecrescentic dunes, were used to develop an empirical model of dune lee‐face airflow for straight‐crested dunes. The nature of lee‐face flow varies and was found to be controlled by the interaction of at least three factors (dune shape, the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability). Three types of lee‐face flow (separated, attached and deflected along slope, or attached and undeflected) were found to occur. Separated flows, characterized by a zone of low‐speed (0–3O% of crestal speed) back‐eddy flow, typically occur leeward of steep‐sided dunes in transverse flow conditions. Unstable atmospheric thermal stability also favours flow separation. Attached flows, characterized by higher flow speeds (up to 84% of crestal speed) that are a cosine function of the incidence angle, typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions. Depending on the slope of the lee face, attached flow may be either deflected along slope (lee slopes greater than about 20°), or have the same direction as the primary flow (lee slopes less than about 20°). Neutral atmospheric thermal stability also favours flow attachment.As each of the three types of lee‐face flow is defined by a range of wind speeds and directions, the nature of lee‐face flow is intimately tied to the type of aeolian depositional process (i.e. wind ripple or superimposed dune migration, grainflow, or grainfall) that occurs on the lee slope and the resulting pattern of dune deposits. Therefore, the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune typ

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01843.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTDespite its significance for inception of grain transport by wind, the initial dislodgement of grains from a static surface by aerodynamic forces of drag and lift in the absence of grain collision has received little attention. This paper describes a series of wind‐tunnel experiments in which the erosion of narrow strips of loose grains from the roughened surface of a flat plate exposed to a range of wind speeds was examined.The progressive downwind development of the boundary layer over the plate provided a range of airflow conditions which permitted systematic evaluation of grain entrainment rates arising from purely aerodynamic forces. Use of closely graded size fractions in flat, single grain layers resting on identical, fixed grain support eliminated the effects of surface irregularities and impacts from saltation. Results show that erosion of strips of loose grains develops with time according to an inverse exponential function in which the entrainment rate time constant relates to Shields dimensionless shear stress function. An empirical expression defining aerodynamic entrainment rate in terms of rate of strip erosion is derived and comparisons are made between present and published data. The need for additional data to resolve several questions raised by the present investigation is stressed.In addition, a simple, objective technique for accurate determination of the aerodynamic entrainment threshold of any loose, granular sediment is propose

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01844.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTEstimates of palaeoflow conditions based on antidune geometries generally are obtained using equations derived from potential flow analysis. Unfortunately, in several previous studies and review articles, misunderstanding and improper substitution of density terms has resulted in two misconceptions (a third is discussed in detail elsewhere). Both concern the commonly derived ratio of antidune wavelength to critical flow depth (L/D). Previously published results which suggest that, for a given set of flow conditions in a two‐layer system, L/D increases with increasing density contrast (Δρ) are wrong. Correct substitutions indicate the opposite—there is a slightly inverse relationship between the value of L/D and Δρ. The second misconception concerns the two values reported for critical flows: L/D≥ 6·3 (or ≥ 2γ) and L/D≥ 12·6 (or ≥ 4γ). Despite insinuations that the latter is wrong by a factor of two, both are correct. In this case, what has been overlooked is that the former value is derived for antidunes on free surface flows (a ‘two‐layer’ system in which ρ2<<ρ1) and the latter for antidunes at the interface of a two‐layer system

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01845.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Quilalar Formation and correlative Mary Kathleen Group in the Mount Isa Inlier, Australia, conformably overlie rift‐related volcanics and sediments and non‐conformably overlie basement rocks. They represent a thermal‐relaxation phase of sedimentation between 1780 and 1740 Ma. Facies analysis of the lower siliciclastic member of the Quilalar Formation and the coeval Ballara Quartzite permits discrimination of depositional systems that were restricted areally to either N‐S‐trending marginal platform or central trough palaeogeographic settings.Four depositional systems, each consisting of several facies, are represented in the lower Quilalar Formation‐Ballara Quartzite; these are categorized broadly as storm‐dominated shelf (SDS), continental (C), tide‐dominated shelf (TDS) and wave‐dominated shoreline (WDS). SDS facies consist either of black pyritic mudstone intervals up to 10 m thick, or mudstone and sandstone associated in 6–12‐m‐thick, coarsening‐upward parasequences. Black mudstones are interpreted as condensed sections that developed as a result of slow sedimentation in an outer‐shelf setting starved of siliciclastic influx. Vertical transition of facies in parasequences reflects flooding followed by shoaling of different shelf subenvironments; the shoreface contains evidence of subaerial exposure. Continental facies consist of fining‐upward parasequences of fluvial origin and tabular, 0·4–4‐m‐thick, aeolian parasequences. TDS facies are represented by stacked, tabular parasequences between 0·5 and 5 m thick. Vertical arrangement of facies in parasequences reflects flooding and establishment of a tidal shelf followed by shoaling to intertidal conditions. WDS facies are preserved in 0·5–3‐m‐thick, stacked, tabular parasequences. Vertical transition of facies reflects initial flooding with wave reworking of underlying arenites along a ravinement surface, followed by shoaling from lower shoreface to foreshore conditions.Parasequences are stacked in retrogradational and progradational parasequence sets. Retrogradational sets consist of thin SDS parasequences in the trough, and C, TDS and probably WDS parasequences on the platforms. Thick SDS parasequences in the trough, and TDS, subordinate C and probably WDS parasequences on the platforms make up progradational parasequence sets.Depositional systems are associated in systems tracts that make up 40–140‐m‐thick sequences bounded by type‐2 sequence boundaries that are disconformities. Transgressive systems tracts consist of C, TDS and probably WDS depositional systems on the platforms and the SDS depositional system and suspension mudstone deposits in the trough. The transgressive systems tract is characterized by retrogradational parasequence sets and developed in response to accelerating rates of sea‐level rise following lowstand. Condensed‐section deposits in the trough, and the thickest TDS parasequences on the platforms reflect maximum rates of sea‐level rise and define maximum flooding surfaces. Highstand systems tract deposits are progradational. Early highstand systems tracts are represented by TDS and probably WDS depositional systems on the platforms and suspension mudstone deposits in the trough and reflect decreasing rates of sea‐level rise. Later highstand systems tracts consist of the progradational SDS depositional system in the trough and, possibly, thin continental facies on the platforms. This stage of sequence development is related to slow rates of sea‐level rise, stillstand and slow rates of fall. Lowstand deposits of shelf‐margin systems tracts are not recognized but may be represented by shore

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01846.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Queen City Formation (Eocene) displays an array of tide‐dominated coastal facies in the Tyler Basin of the northern Gulf of Mexico. This facies assemblage, which is atypical of the microtidal, wave‐dominated, coastal depositional complexes that characterize the Cenozoic Gulf basin, reflects tidal amplification in a generally protected embayment on the east flank of a strongly prograded delta system. The shallow embayment was confined to the east by contemporaneous uplift and shoaling across the Sabine Uplift. Fluvial, barrier (including ebb tidal delta), heterolithic tidal, estuary‐fill, and tidal point‐bar facies are all found at outcrop. These facies were projected into the three‐dimensional geometry of the tide‐dominated depositional complex. Inlet, estuary, and distributary‐fill sand bodies, which are linear and diporientated, dominate lithofacies maps.The Queen City facies assemblage in the Tyler Basin records a mixture of mesotidal to macrotidal environments that were interspersed in time and space with fluvial‐dominated lobes, which periodically prograded eastward from the deltaic depocentre into the flanking embayment. Queen City deposition terminated with regional marine flooding and deposition of glauconitic, fossiliferous shelf sands and muds of the Weches Formation. Transgression is marked by a prominent ravinement surface that truncates underlying facies of the tide‐dom

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01847.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Namurian (Upper Carboniferous) Scar House Beds of Yorkshire, northern England, are an example of a fluvial‐dominated deltaic sequence that cannot be adequately described using existing classification schemes for deltas. For substantial periods of the Scar House delta history, inertial processes and hyperpycnal mixing prevailed in the river mouth area due to repeated, frequent flooding in the distributary system. This generated voluminous density currents which deposited their sandy loads in successively stacked lobes beyond the river mouth bar in the prodelta area. The position of a lobe was directly controlled by the position of an active river mouth. Only during periods of low discharge in the distributary system did homo‐ and hypopycnal mixing take place. In these periods, frictional and buoyant forces operated, and sand was deposited from tractional sheet flow on the mouth bar while mudstone was laid down in the otherwise density‐current‐dominated prodelta.Because of the dominantly hyperpycnal mixing mode, the river effluent experienced a low lateral spread causing an elongate delta lobe to form that in geometry can be compared with some recent and ancient ‘bar finger’ sands. Important differences exist in terms of dominant depositional processes however. Most other ‘bar finger’ sands were controlled by a hypopycnal mixing mode and buoyant forces (e.g. South Pass, Mississippi), while the Scar House delta was controlled by hyperpycnal mixing and inertial forces.This study shows that similar sand‐body geometries can be generated from different river mouth processes. In the future, particularly in the field of hydrocarbon exploration, there may be a need to classify deltas both in terms of geometry and dominant river mouth processes. In that respect, the Scar House Beds represent a fluvial, inertia‐domin

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01848.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY