摘要:

ABSTRACTVariations in migration distances and shape characteristics of sandwaves in relation to flow conditions were studied on the Ossenisse intertidal shoal in the Westerschelde estuary. The purpose was to analyse bedform behaviour, to establish the threshold and the time lags involved, to find differences in two‐ and three‐dimensional sandwaves and to determine the implications for palaeoflow reconstructions.Sandwave migration is well correlated with the peak depth‐averaged flow velocity of the dominant tide. Thus the latter parameter may be estimated from the thickness of the tidal bundles. Other flow parameters such as shear velocity, ChezyCor roughness length do not show a correlation with the migration and cannot be used in palaeoflow analysis.Flow depth does not correlate with sandwave height or with length. Consequently, neither sandwave height and length nor set height and length can be used for palaeoflow depth determination.Sandwaves start moving when the peak dominant flow velocity exceeds 0.5–0.6 m s−1, and appreciable changes in shape occur at 0.75–0.8 m s−1. Complete reversal of sandwaves is accomplished if both the dominant and subordinate peak depth‐averaged current velocities exceed 0.85 m s−1.Two‐ and 3‐D sandwaves appeared to have different stability fields in the velocity‐depth diagram and in the diagram of the Froude number versus the depth‐grain‐size ratio. In addition the distinction between 2‐D and 3‐D sandwaves appeared to be related to a variability in current direction during periods of appreciable sand transport. There are also differences in sedimentary structures bet

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00742.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTPrimary sedimentary structures exhibiting the diagnostic criteria for single sets ofhummocky cross‐stratification(Harmset al.) have been found in the surf zone of a storm‐wave dominated coastline in the Canadian Great Lakes. Epoxy peels of box cores (0.45 m × 0.30 m) reveal hummocky stratification in well‐sorted, fine‐grained sands in water depths less than 2 m under conditions of wave breaking and strong longshore currents. The wavelengths of the hummocks (0.3–0.6 m) are somewhat smaller than the norm for their ancient analogues, but the ratios of length to height (8–12) are comparable. Depth of activity rods have been used to identify those hummocks that formed during sediment transport events when the near‐bed currents were recorded directly using electromagnetic flowmeters. Results from such experiments clearly identify the hummocky stratification as being produced by an actively growing bedform with little or no lateral migration. Hummocks occur under conditions close to that expected for the upper flat bed. In one vertical sequence, the hummocky cross‐stratification is underlain by subhorizontal, planar lamination and overlain by undulatory lamination which grades upward into small‐scale, trough cross‐lamination of wave ripple origin. This sequence was associated with a single storm and would appear to represent a combined‐flow regime sequence with the hummocky structure representing a post‐vortex (?) ripple bedform. At the inferred time of hummock formation, near‐bed oscillatory flows were dominant and reached maxima of 1.1 m s−1with a superimposed longshore current of 0.27 m s−1. Rapid sedimentation associated with vertical growth of the hummocky bedform was triggered by a significant reduction in the orbital currents (by 19%) and'steady'currents (by 67%) while the

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00743.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe Santonian‐Campanian Milk River Formation of Southern Alberta represents the transition from an open shelf, through a storm‐dominated shoreface into a non‐marine sequence of shales and sandstones, with coal. The open shelf deposits consist of interbedded bioturbated mudstones with sharp‐based hummocky cross‐stratified sandstones. There are no indications of fairweather reworking of the sandstones, which are therefore interpreted as having been deposited below fairweather wavebase. The shoreface sequence consists of a 28 m thick sandstone. It has a very sharp, loaded base, and is dominated by swaley cross‐stratification, a close relative of hummocky cross‐stratification. Angle of repose cross‐bedding is preserved in scattered patches only in the top 5 m of the sand body. Channels up to 180 m wide and 7 m deep are cut into this sand body, with channel margins characterized by lateral accretion surfaces. Regional dispersal trends, as well as local palaeocurrent readings suggest flow toward the NW. Within the channels there is some herringbone cross‐bedding and at least two examples of neap‐spring bundle cycles, suggesting that the channels are tidally‐influenced. Above the channels there is a sequence of carbonaceous shales within situroot casts and lignitic coal seams. No marine, brackish or lagoonal fauna was identified, and the sequence appears to represent a distal floodplain.The sequence from interbedded hummocky cross‐stratified sandstones and bioturbated mudstones into a 10–20 m thick, sharp‐based shoreface sandstone characterized by swaley cross‐stratification is uncommon. The scarcity or absence of angle of repose cross‐bedding in the shoreface, and the dominance of swaley cross‐stratification suggests that the shoreface was so storm‐dominated that almost no fairweather record was preserved. Other examples of swaley cross‐stratified

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00744.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThree major types of dolomite occur in the Trenton Formation (Mid‐Ordovician) of the Michigan Basin. These are: (1) ‘regional dolomite’ which is confined to the extreme western edge of the basin; (2) ‘cap dolomite’ which occurs in the upper portion of the Trenton and is confined to the basin's southern margin; and (3) ‘fracture‐related’ dolomite which occurs in association with both large‐ and small‐scale faults and fractures. These three dolomite types can be distinguished from one another by their major element chemistry, oxygen isotope ratios and rock texture.The regional dolomite is fine‐grained, has<0.34 mol% FeCO3, and mean δ18O of −6·8‰OPBD. The cap dolomite is texturally similar to regional dolomite but contains 3–13·0 mol% FeCO3and has a mean δ18O of −7·7‰. Fracture‐related dolomites are coarse‐grained, low in iron, and have the most depleted δ18O ratios (x̄=–9·0%PDB).Petrographic relationships imply that the regional dolomite, formed prior to the cap dolomite probably during early diagenesis. The cap dolomite formed at relatively shallow depths as a result of the interaction of the overlying Utica Shale and the Trenton Limestone. Fracture‐related dolomites post‐date the cap dolomite and formed during deeper burial. A temperature of precipitation of approximately 80°C was calculated for fracture‐related dolomites using oxygen isotope data.The distribution of the cap dolomite was controlled by the availability of Fe2−which was in turn controlled by the availability of S2−. In the centre of the basin Trenton‐Utica deposition was continuous. The upper Trenton contained relatively high concentrations of organic matter which was used by sulphate reducing bacteria to produce H2Sfrom seawater sulphate. The precipitation of iron sulphides (pyrite + iron monosulphide) followed and used up most of the available Fe2−. As a result only small amounts of ferroan dolomite formed. On the periphery of the basin, subaerial exposure resulted in the oxidation of most of the available organic matter. Sulphate reducing bacteria were therefore limited and produced limited amounts of H2S. As a result only a minor amount of iron sulphide (iron monosulphide) formed. The remaining Fe2‐was then available for the formation of the ferroan cap dolomite. This model is supported by the following: (1) In the southern margin of the basin, the contact between Trenton cap dolomite and the overlying Utica Shale is sharp and probably unconformable. In the centre of the basin the contact is gradational. (2) In the centre of the basin, the total organic carbon content in the upper Trenton is an order of magnitude higher than in the cap dolomite. (3) The whole‐rock concentration of iron is high in both the cap dolomite and in slightly dolomitized equivalent beds in the basin centre. (4) Iron sulphides are abundant in the centre of the basin and mostly in the form of pyrite. In the cap dolomite, iron

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00745.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTDetailed sampling and analysis of Jurassic pelagic limestones and marls from Italy, Hungary and Switzerland have enabled construction of an isotope stratigraphy across the Pliensbachian‐Toarcian boundary with resolution to the zonal level. The oxygen‐isotope record is unremarkable. The carbon isotopes, however, show two positive excursions: one, relatively minor, during the Pliensbachian,margaritatusZone,subnodosusSubzone, the other, more major, during the Toarcian. earlyfalciferumZone, where a maximum δ13C value of 4·52%PDBis attained. These intervals are known to be favoured periods of organic‐rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record. A comparable phenomenon has been documented from the Cenomanian‐Turonian boundary in the Cretaceous where it has been referred to the influence of an ‘Oceanic Anoxic Event’.Some Italian sections spanning this Lower Jurassic interval contain organic‐rich shales in thefalciferumZone; the isotopic signatures from their included, locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve. Carbon isotopes from the organic carbon itself are extremely negative, falling to –33δPDBand, in one section examined in detail, correlate with the calcium‐carbonate content of the shales; they may reflect a partial change to a non‐calcified planktonic biota during deposition of this lime‐poor interval, possibly responding to upwelling and increased fertility of near‐surface waters. The onset of upwelling may have been as early asspinatum‐tenuicostatumZone time, that is, at the Pli

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00746.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe Lower Jurassic East Berlin Formation exposed in the centre of the Hartford Basin can be divided into six facies: (1) laminated black mudstone is composed of very finely‐laminated, organic‐bearing clay‐stone with common millimetre‐scale lenses of dolomitic siltstone; (2) planar laminated mudstone is commonly mudcracked and composed of thickly‐laminated, red, green or grey mudstone with common centimetre‐scale lenses of sandstones; (3) disrupted mudstone has a complex, desiccation‐cracked fabric; (4) planar‐ and large‐scale trough cross‐stratified sandstones are composed of moderately well‐sorted medium‐ to coarse‐grained arkoses; (5) small‐scale, cross‐stratified silty sandstones with common climbing‐ripple structure; and (6) interbedded sandstones and mudstones which commonly carry desiccation cracks.Mudstone facies are organized into repetitive, metre‐scale facies sequences which change gradationally upwards from laminated black mudstones to planar‐laminated mudstones to disrupted mudstones. Facies sequences have sharp tops and bottoms and record increasing desiccation upwards. There are 15 such cycles in the upper 100 m of the formation in central Connecticut. They record long periods of dry playa mudflat aggradation punctuated by the rapid expansion and contraction of perennial lakes.The sandy facies occur as single, decimetre‐scale sedimentation units or as two or more stacked sedimentation units up to 1 m thick. These record shee

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00747.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe Westphalian (Upper Carboniferous) Coal Measures of the Durham coalfield in NE England were deposited in lower and upper delta plain environments. Distributary channels crossed the plain and were separated by shallow, interdistributary lakes and bays. Detailed observation of three‐dimensional (3‐D) opencast (surface) mine exposures, in collaboration with subsurface borehole analysis, has revealed the existence of five varieties of channel deposits and two associated overbank facies within the Durham Coal Measures.Major distributary channels were the major avenues of sediment transport across the Coal Measures plain, were variably sinuous, mostly 1–2 km wide and deposited elongate belts of sand mostly up to 5 km wide. Proximal, major crevasse splay channels formed by the breaching of major channel banks during flood events, were straight, sand‐filled and up to 400 m wide. Minor distributary channels formed by the sustained operation of such crevasses, varied from straight to highly sinuous, and deposited ‘shoestring’ sand/mud belts up to a few hundred metres wide. Minor crevasse channels, generally straight and up to 50 m wide, were formed through bank breaching of minor, and in a few cases major, distributaries. Distal feeder channels formed down‐palaeocurrent extensions of minor distributaries which supplied interdistributary minor delta subsystems, were generally straight and up to 200 m wide.Of the two types of channel overbank (levee) deposits recognized, one, comprising thinly interbedded fine‐grained sandstone and siltstone/claystone, is mostly, though not exclusively, associated with major distributary channels. The other, consisting of ‘massive’ siltstones with regularly spaced, thin claystone bands, is uniquely developed at the margins of minor distributary channels.The lower part of the Westphalian A succession in the northern Pennines records a change in the depositional environment upwards from a lower to upper delta plain. Through this transition, major channel deposits show evidence of having evolved from being of dominantly low sinuosity to being more variable in morphology. Channel sedimentation was profoundly influenced by regular, possibly seasonal, variations in flow stag

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00748.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTDetrital garnets from the Upper Triassic (Keuper) of southern Germany have well developed facets where they occur in calcite‐cemented sandstone. The grains belong to the pyralspite series and have a homogeneous chemical composition, without any zonation is revealed by microprobe analyses. It is concluded that the large facets are caused by etching during diagenesis and not by authigenic overgrowth. High calcium contents of the pore fluids may have been important in the formation of these garnet facet

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00749.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe construction and operation of a glove box which allows the fine‐scale vertical subsampling of sediment box cores under a low oxygen atmosphere is describe

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00750.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00751.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY