摘要:

ABSTRACTAn autosuspension criterion that has been developed directly from the fluid dynamical equations, by taking into account the vertical structure of turbidity currents, is compared to field observations of five turbidity currents. It is found that the criterion is consistent with the motion of all five currents, which suggests that the criterion may, at least under certain circumstances, be a reasonable guide in estimating the conditions necessary for a turbidity current to be self‐sustainin

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01979.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe deposition of natural dust in an area of 53 ha, situated in the northern Negev desert, is investigated in detail both in the wind tunnel (dust storm simulations over a topographic scale model) and in the field. The wind tunnel results and the field results show a high degree of agreement, indicating that scale‐model simulation may be considered an important technique for future loess and desert research. More dust settles on windward slopes than on leeward slopes, which is in contradistinction with the widespread wind shadow concept. Air‐flow separation zones immediately downwind of steep windward slopes have an important impact on dust deposition too.In the case of dust deposition on topographic scale models, a restricted height distortion of the model will not necessarily lead to serious problems. In addition, wind tunnel blockage percentages up to 13% may be allowed in order to obtain acceptable dust deposition patterns for the scale model.A mean gross dust deposition of about 200–250 g m−2year−1is calculated for the northern Negev desert for 1987. Thus, if the settled dust can be protected against erosion in the cultivated areas in the Negev, the dust content of the top soil will markedly increase with time. However, it has to be borne in mind that cultivation activities themselves may also contribute to a higher soil erosion and, hence, to a higher dust content in the atmosphere. At any rate, a higher dust content in the top soil will unquestionably have an important positive effect on agricultural yields.From the air dust concentration data and the dust deposition data, a deposition velocity of 4.7cms−1can be calculated for

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01980.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTA 45 km long barrier island exists west of the town of Guerrero Negro, Mexico, along the western coast of the Baja California peninsula, about 720 km south of San Diego, California. This barrier has developed in a mesotidal, arid‐climate regime characterized by steady, strong, onshore winds from the NW. The barrier island W of Guerrero Negro has prograded seaward about 1·6 km in the last 1800 years while an aeolian dunefield fed by sand blown from beaches has advanced inland up to 13 km. Landward progradation of the dune system from the barrier has occurred during relative rise in sea‐level; thus, aeolian sediments exist at or below the water table over a wide area. The progradation of dunes across marshes, tidal flats, and tidal channels, as well as the repeated submergence of interdune areas by tidal waters, has created a complex suite of mixed aeolian and subaqueous sediments in the back barrier. The complexity of the suites of aeolian sedimentary structures, together with the inclusion of subaqueously formed structures such as current and oscillation ripples, would make recognition of the aeolian origin of much of the sediments difficult in ancient rocks.In addition to the scientific importance of recognizing the aeolian deposits, the sedimentation model represented by the Guerrero Negro barrier has applications in petroleum exploration and development. Currently, most preservational models for barrier islands attach little volumetric importance to aeolian deposits. This modern example suggests that volumetrically significant aeolian deposits can be preserved behind a barrier, particularly in an arid‐climate regime. If preserved and charged with oil, the resulting productive sandstone could have an extremely irregular landward edge comprised in part of onshore‐prograded aeolian dune sandstone with excellent reservoir characteristics. As with current barrier models, the reservoir would be sealed landward and above by lagoonal mudstone and silt, evaporites, or evaporitic, sandy sabkha deposits. High organic productivity occurs in lagoons immediately adjacent to the dunefields of Guerrero Negro, suggesting that organic‐rich source rock may exist near aeolian sandstone in ancient settings similar to Guer

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01981.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTChannelized bodies up to 9 m thick in the Pennsylvanian Waddens Cove Formation show low width: thickness ratios (8:1 to 14:1), up to four vertically stacked storeys, and deep incision into associated floodplain strata. The presence of lateral accretion sets up to 6 m thick indicates that the migration of bank‐attached bars was a major channel‐filling process. The progressive rise in elevation of the basal part of the accretion surfaces where the sets adjoin the steeply inclined sides of the channel bodies shows that the channel floor aggraded progressively as the bars approached the channel sides. In planform, the river system probably showed a low‐sinuosity, incised valley within which the channel followed a more sinuous and partially confined course.The alluvium surrounding the channel bodies includes sandstone sheets attributed to crevasse splay and levee accumulation. The upper strata of many sheet's and some channel bodies contain siliceous duricrusts (ganisters) which formed by pedogenic lithification of sandy material commencing shortly after the landforms became inactive. The channel bodies contain duricrust‐bearing slumpblocks and show stepped margins over duricrusts. The stepped nature of the banks suggests that duricrusts were an important factor in confining the reach, and we suggest that channel‐body geometry was influenced, although not necessarily controlled, by the presence of the lithified layers.Alternation in the Waddens Cove Formation of grey, coal‐bearing zones with red, duricrust‐rich zones is attributed to water‐table fluctuation in response to base‐level changes. The low width: thickness ratios and aggradation mode'of the channel bodies may in part reflect channel incision and subsequent aggradation resulting from these

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01982.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe upper portion of the Virgelle Member (Upper Cretaceous Milk River Formation) at Writing‐on‐Stone Provincial Park of southern Alberta preserves evidence of tidal processes along an otherwise wave‐dominated, progradational shoreline in the Cretaceous Interior Seaway of North America. The upper Virgelle Member is underlain by offshore transition to lower shoreface deposits of the Telegraph Creek Member and the lower Virgelle Member, respectively, and is overlain by the non‐marine shales and sandstones of the Deadhorse Coulee Member. The sediments of the upper Virgelle Member were deposited along a prograding shoreline and are interpreted here as those of a tidal‐inlet complex. Most inlet sections consist of an erosional base overlain by a shale‐pebble conglomerate, followed by cross‐bedded sandstones which become finer‐grained and decrease in scale upwards. Indicators of tidal processes include palaeocurrent distributions, mud couplets, tidal bundles, re‐activation surfaces and herringbone cross‐beds. The sequence through the tidal‐inlet complex can be differentiated, according to prevalent palaeoflow directions and sedimentary structures, as ebb‐dominated, flood‐dominated, or mixed‐tidal influence. Ebb‐dominated sections commonly contain lateral accretion surfaces whereas flood‐dominated sections contain tidal‐ramp deposits. Back‐barrier lagoon deposits are dominated by sandstones of an extensive flood‐tidal delta with only thin shales preserved locally at the top of the inlet complex. Deposits of ebb‐tidal deltas are absent, presumably due to the effective sediment dispersal by waves and wave‐induced longshore currents acting o

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01983.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Honna Formation, of Coniacian age, consists of several hundred metres of polymictic clast‐supported conglomerate associated with sandstone and mudstone. Five conglomerate facies are recognized: ungraded beds; inverse graded beds; normal graded beds; inverse‐to‐normal graded beds; and parallel‐stratified beds. These facies are interpreted as the deposits of subaqueous cohesionless debris flows and/or high‐density turbidity currents.The depositional environment was a deep‐water, gravelly fan that draped a fault‐controlled, basin‐margin slope. The fan is inferred to have passed upslope directly into an alluvial fan (unpreserved); hence, the namefan deltacan be applied to the overall depositional system. This type of fan delta, of which the Brae oilfield in the North Sea is an example, is defined here as adeep‐water fan delta.The lack of a shelf is in marked contrast to other types of fan delta.Three facies associations are recognized in the Honna Formation: subaqueous proximal‐fan conglomerates, distal‐fan turbiditic sandstones, and pro‐fan/interfan mudstones with thin sandy turbidites. The proximal fan is envisaged as an unchannelled gravel belt with a downslope length of at least 20 km; such a long subaqueous gravel belt lacks a known modern analogue. The distal fan was an unchannelled sandy extension of the proximal gravel belt.It is postulated that the Honna Formation accumulated in a foreland basin which migrated westwards from the Coast Mountains where the Wrangellia‐Alexander terrane was colliding with North America. In this model, the Honna fan delta was sourced by a (west‐verging) thrust sheet whose sole‐thrust was the Sandspit Fault immediately to the east.Deep‐water fan deltas appear to develop preferentially when eustatic sea‐level is relatively high, so that the‘feeder’ alluvial fan is small, and gravelly throughout. In petroleum exploration and field development, care should be taken to distinguish deep‐water fan deltas from base‐of‐slope (canyon‐fed) submarine fans, because the two systems differ significa

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01984.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTA thick, areally extensive subsurface sequence of Upper Devonian carbonates occurs on the Barbwire Terrace in the Canning Basin of Western Australia. It is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized. Slightly deeper water marls have remained as limestones. The major, regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite (up to 600 m) with no primary calcite. A small volume of evaporitic, supratidal dolomite is present at one location. This dolomite is derived from highly saline fluids developed in an arid supratidal environment.Replacement dolomite of the regional dolomite type has a xenotopic form, with undulose extinction, and irregular crystal boundaries. In addition, saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite. This suggests the regional dolomite type was precipitated at slightly elevated temperatures. Dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution.Geochemically, the synsedimentary supratidal and regional dolomite types are quite distinctive. Supratidal dolomites have δ18O values which are significantly higher (δ18O=−2 to +1‰ (PDB)) than the regional dolomite type (δ18O=−9 to −2‰ (PDB)). Assuming the lowest δ18O values for the sabkha dolomite represent replacement in marine waters, the oxygen isotopic composition for Upper Devonian Canning Basin marine dolomite would be around δ18O=−2‰ (PDB).The petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin. However, sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the Barbwire Terrace. Therefore, it is suggested that dolomitization may have taken place in a near‐surface environment with a major recrystallization event superimposed duri

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01985.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTLate Cretaceous epicontinental phosphorites, porcelanites/cherts, dark‐coloured shales, glauconitic sandstones and bioclastic and fine‐grained carbonate rocks in Egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics. Two main depositional realms are interpreted and correlated through sequence stratigraphic analysis: (1) a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon‐rich shales, biosiliceous sediments and thick phosphorites, and (2) a relatively high energy depositional regime accompanying sea‐level fall during which deltas advanced, glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis, thereby promoting solution‐collapse phenomena in associated cherts. Lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments. In eastern Egypt the phosphorites form winnowed lag layers, some of which may have been redeposited down slope in structural lows. In the west, these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression. Carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning. However, direct desorption of phosphorus to pore waters from detrital iron‐oxyhydroxide phases also may have been important in the Cretaceous setting, the iron reduced in this process being available for incorporation in glauconites. The main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters.This study suggests a model for the common coexistence of glauconites and phosphorites in the geological record. Although upwelling is often advocated as the origin of nearly all giant phosphorite deposits, we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and/or reduction in bottom waters or

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01986.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTDiagenetic alunite occurs with calcrete, gypcrete and silcrete deposits in a Mio‐Pleistocene clastic sequence at several locations in southern Kuwait, Arabian Gulf. Based on their physical properties and textural characteristics the alunite deposits were divided into (1) chalky quartzitic alunite, (2) chalky quartz‐free alunite, (3) hard pink alunitic sandstone. The chalky alunite is composed mainly of hydronium‐alunite (H3O)Al3(SO4)2(OH)6, while the hard pink alunite is composed solely of well developed potassium alunite KAl3(SO4)2(OH)6. These minerals resulted from the action of sulphuric acid on clays and K‐feldspars in the muddy sandstone and mudstone host sediments. The sulphuric acid is most probably produced by the oxidation of hydrogen sulphide that might have seeped in from the oil fields of this area. Four diagenetic stages are suggested for the genesis of the studied alunites: gypsification, sulphuratization, silicification and alunitization. In oil field areas, the occurrences of alunite would serve as an indicator for the presence at depth of hydro

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01987.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Book reviews in this article:Sedimentary Cover—North American Craton: USThe Geology of North America, Vol. D‐2, ed. by L. L. SlossThermal History of Sedimentary Basins: Methods and Case Histories, ed. by N. D. Naeser and T. H. McCullohOrigin and Migration of Subsurface Sedimentary Brines, SEPM Short Course No. 21, by J. S. HanorEvaporites and Hydrocarbons, ed. by B. C. Schreiber.Phanerozoic Ironstones, ed. by T. P. Young and W. E. G. Taylor.Green Marine Clays, ed. by G. S. OdinSiliceous Sedimentary Rock‐Hosted Ores and Petroleum, ed. by J. R. Hein.Neodymium Isotope Geochemistry, by D. J. DePaoloThe Beginning of the Age of Dinosaurs. Faunal Change across the Triassic‐Jurassic Boundary, ed. by K. Padian.Basics of Physical Stratigraphy and Sedimentology, by W. J. Fritz and J. N. Moore.Techniques in Sedimentology, ed. by Maurice TuckerThe Physics of Sediment Transport by Wind and Water, a Collection of Hallmark Papers by R. A. Bagnold, ed. by C. R. Thorne, R. C. MacArthur and J. B. BradleySiliceous Deposits of the Tethys and Pacific Regions, ed. by J. R. Hein and J. ObradovicDiagenesis I and Diagenesis II, ed. by G. V. Chilingarian and K. H. Wolf.Continental Shelves, ed. by H. Postma and J. J. Zijlstra.Modern Planktonic Foraminifera, by Ch. Hermleben, M. Spindler and O. R. AndersonSedimente und Sedimentgesteine (4th edn), ed. by H. F

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb01988.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY