摘要:

ABSTRACTA thick sedimentary sequence comprising fluvial, lacustrine and volcano‐sedimentary rocks is present in the Neogene Beypazari Basin, central Anatolia. These units display considerable lateral facies variation and interfinger with alkaline volcanic rocks along the north‐eastern margin of the basin. The uppermost Miocene Kirmir Formation contains numerous evaporite horizons. The evaporite sequence is up to 250 m thick and may be divided into four lithofacies. In ascending stratigraphical order these are: (1) gypsiferous claystone facies, (2) thenardite‐glauberite facies, (3) laminar gypsum facies and (4) crystalline gypsum facies. These facies interfinger with one another laterally along a section from the margins to central parts of the basin. The lithological and sedimentological features of the Kirmir Formation indicate fluvial, saline playa mudflat, hypersaline ephemeral playa lake and very shallow subaqueous playa lake depositional environments, which probably were influenced by alternating semi‐arid and evaporative con

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01428.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Aptian to lowermost Albian carbonate platform of Castro Urdiales (Cantabria, northern Spain) was broken up by extensional tectonic movements shortly after the beginning of the Albian. Block faulting characterized this rifting episode, the effects of which waned during the Albian. In crestal locations tilting of the fault blocks caused the subaerial exposure of parts of the older platform, resulting in intense karst diagenesis. Differential subsidence of the blocks controlled the development of a crestal residual carbonate platform (Arenillas), which was surrounded by deeper water on both sides. Seven unconformities related to platform exposure and karstification are identified on the Arenillas platform, and form the basal boundaries of seven depositional sequences (S1‐S7). On the platform, lowstand systems tract deposits consist of breccias filling caves, and grainstones and debris flow deposits filling incised canyons. Transgressive plus highstand systems tracts consist of rare marls plus shallow water rudistid, coral and chondrodontid limestones. In the basin, the unconformities are erosional surfaces at the base of resedimented limestones, marls or sandy or silty siliciclastics (lowstand systems tracts). Transgressive plus highstand systems tracts in this setting consist of marls and hemipelagic marly limestones. Tectonism is believed to be the main control on sequence formation, and only a few sequence boundaries (e.g. the base of S6 in the Upper AlbianinflatumZone) can be correlated with eustatic event

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01429.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTDimensions and plan morphology of current ripples are generally considered to vary with flow velocity and grain size. Recently, however, it has been shown that for sand ofD50=0.095 and 0.238 mm the equilibrium dimensions are identical at all velocities within the stability field of ripples and that the plan form of equilibrium ripples is linguoid. On this basis, an empirical unsteady flow model has been developed and tested with flume experiments in order to predict ripple development in natural depositional environments. The model includes the development of washed‐out ripples and upper stage plane bed. The unsteady flow model explains the development and preservation of small scale bedforms in various tidal environments more accurately than previous models. Such bedforms can serve, therefore, as indicators of prevailing hydrodynamic condition

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01430.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTCoastal erosion has occurred on the beaches of the three major promontories of the Nile Delta of Egypt. The eroded sand is generally carried alongshore to the east, or locally to the west, from these promontories and is deposited in areas of beach accretion between promontories. In this study the heavy minerals of 150 samples have been investigated with the aim of differentiating between eroded and accreted beaches. Discrimination was achieved by using bivariate plots of the raw weight percentage data and Q‐mode factor analysis. Factor analysis yielded two ‘mineral assemblages’: Factor 1 is dominated by augite, hornblende and epidote, while Factor 2 consists of opaque minerals, garnet, zircon, rutile and monazite. These two mineral factors are produced by the selective sorting of heavy minerals by natural processes of waves and currents. Minerals of Factor 2 are associated with high density opaques which tend to concentrate as a lag in areas of erosion. Conversely, the lower density minerals of Factor 1 are preferentially transported away from areas of erosion and carried to the accreted shore

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01431.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTSuper bounding surfaces (super surfaces) facilitate delineation of internal genetic architecture, or sequence stratigraphy, of aeolian sandstones and help in distinguishing controls on aeolian accumulation, hiatuses in accumulation and preservation of such sandstones. The Middle Jurassic Page Sandstone is an amalgamation of such sandstone units accumulated in a dry erg system, with water table below the surface, separated by super surfaces with features indicating an arid climate but a near surface water table. The Page Sandstone accumulated episodically with periods of aeolian accumulation followed by deflation to the water table; the water table fluctuated up and down through the sediment package. This rising water table did not directly control accumulation, but placed a limit on extent of deflation by stabilizing the substrate, thereby directly controlling preservation of the aeolian unit. A combination of relative water table behaviour, sediment supply and aerodynamic conditions upwind and within the erg controlled the thickness of genetic packages within the erg. A change in the pattern of deflation associated with super surface formation late in deposition of the Page Sandstone reflects a change in tectonic regime in the basin. The number of super surfaces and features on the super surfaces with the Page Sandstone suggest that the super surfaces represent more time than do the accumulations they enclose.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01432.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Wilde Kirche reef complex (Early‐Late Rhaetian) grew as an isolated carbonate structure within the shallow Kössen Basin. At the Triassic/Jurassic boundary a single brief (c.10–50 ka) period of subaerial exposure occurred. The preserved karst profile (70 m thick) displays a vadose zone, enhanced dissolution at a possible palaeo‐watertable (5–15 m below the exposure surface), and a freshwater phreatic zone. Karst porosity was predominantly biomouldic. Primary cavities and biomoulds were enlarged and interconnected in the freshwater phreatic zone; cavity networks developed preferentially in patch reef facies. Resubmergence of the reef complex allowed minor modification of the palaeokarst surface by sea floor dissolution and Fe‐Mn crust deposition on a sediment‐starved passive margin.Fibrous calcite (FC). radiaxial fibrous calcite (RFC) and fascicular optic calcite (FOC) cements preserved as low Mg calcite (LMC) are abundant in primary and karst dissolution cavities. FC cement is restricted to primary porosity, particularly as a synsedimentary cement at the windward reef margin. FC, RFC and FOC contain microdolomite inclusions and show patchy non‐/bright cathodoluminescence. δ18O values of non‐luminescent portions (interpreted as near original) are − 1.16 to − 1.82%0 (close to the inferred δ18O of calcite precipitated from Late Triassic sea water). δ13C values are constant (+3 to + 2.2%0). These observations suggest FC, RFC and FOC were originally marine high Mg calcite (HMC) precipitates, and that the bulk of porosity occlusion occurred not in the karst environment but in the marine environment during and after marine transgression. The HMC to LMC transition may have occurred in contact with meteoric water only in the case of FC cement. The most altered (brightly luminescent) portions of RFC/FOC cements yield δ18O=−2.44 to − 5.8%0, suggesting HMC to LMC alteration at up to 34°C. in the shallow burial environment at depths of 180–250 m.Abundant equant cements with δ18O =−4·1 to −7.1%0 show crisp, uniform or zoned dull luminescence. They are interpreted as unaltered cements precipitated at 33–36°C at 200–290 m burial depth, from marine‐derived fluids under a slightly enhanced geothermal gradient. Fluids carrying the equant cements may have induced the

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01433.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTGravel size sediment beds are tested under the combined influence of simulated wave action and co‐linear currents in a laboratory flume. Critical current speed, at threshold, increases with increasing size. Superimposed wave energy causes a small reduction in the unidirectional current energy. For low values of wave‐induced near bed current velocities, the resistance to erosion increases when the wave period decreases from 10 to 6 s. Finally, combined critical shear stresses are found to be lower than those predicted using the Shields curve, as modified for oscillatory flow. Grain protrusion is suggested as a mechanism to explain this diverge

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01434.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTCarbonate frameworks secreted by phototrophic organisms within the Arctic Circle are not well documented. Underwater surveys of the inner‐shelf off Troms, northern Norway (70°N), reveal extended fixed algal build‐ups which are fringed by rhodolith belts affected by storms. Reefal growth by coralline algae under temperature and light regimes of extreme seasonality is made possible because of a decoupling of carbon fixation during summer and utilization of stored carbon during the period of winter darkness. Although the annual growth of the framework constructing algae is comparatively low, the annual carbonate production rate is similar to subtropical‐tropical counterparts because of a remarkably high standing stock. Early diagenetic alteration is restricted to intraparticle cementation processes which startin vivo.Bioerosional destruction is the dominant control on the preservation of high latitude build‐ups. Preservation of Holocene autochthonous coralline algal biostromes is enhanced by rapid burial during storm events. Redeposition during storms is the most important process in forming a distinct sedimentary facies

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01435.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTExisting proposals for converting thin section data to their sieve equivalents are all flawed in various ways, while questions concerning the significance of the grain size of spherical grains measured on a volume frequency basis in thin section using φ units, and of non‐spherical (ellipsoidal) grains using both millimetres and φ units, have not been ‐satisfactorily resolved in the literature. It can be shown mathematically that the mean thin section diameters of spherical grains, or axis lengths of a series of parallel sections through ellipsoidal grains, will underestimate the dimensions of the corresponding central section (i.e. one passing through the grain centre) by 0.2023 φ when measured on a volume frequency basis. In order to approximate the effect of measuring particle size on random cuts through ellipsoidal grains, the dimensions of a series of sections cut in 49 unique directions, symmetrically arranged and evenly spread with respect to the ellipsoidal axes, were calculated. This calculation was carried out for five different ellipsoids which between them covered the mean sphericity and thin section axial ratio values normally encountered among naturally occurring quartz grain populations. The data indicated that the mean true nominal diameters (D̄) of ellipsoidal quartz grains can be obtained in thin sections from the mean nominal sectional diameters (d̄′) and major axes (ā′) of the central sections (derived from the observed values by multiplying the millimetre means by 1·1318 and subtracting 0·2023 from the φ means) using the following equations:A rough estimate (to within c. 5%) of both the mean nominal diameters and the mean long axes of ellipsoidal quartz grains can be arrived at by applying a simple correction factor to the mean long axis lengths as measured in thin section using either millim

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01436.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThis paper reports on the structural and sedimentary evolution of the middle to late Eocene of the Prepyrenean External Sierras (southern Pyrenees, Spain). The initiation, duration and kinematics of a set of growth structures that developed in a shallow marine depositional setting is documented. The detailed analysis of the syntectonic marine sediments not only confirms the already known east to west progression of deformation, but also reveals the continued growth of the early formed structures as later ones propagate towards the foreland. The sedimentary units coevally deposited with these growth structures are arranged in four depositional sequences. Their boundaries correspond to flooding surfaces which grade basinwards into correlative conformities. They are also indicated by the presence of both angular unconformities and onlap geometries. Each depositional sequence generally consists of two systems tracts. The lower one, or transgressive systems tract, is formed by up to 400 m of azoic marls deposited in outer ramp areas. The upper one, or highstand systems tract, mainly consists of shallow siliciclastic and carbonate facies, up to 200 m thick, deposited in middle to inner ramp areas. These depositional sequences are interpreted to be controlled by regional tectonic pulses. An increase of tectonic activity resulted in the flooding of the basin and in the subsequent deposition of a thick succession of nearly azoic blue marls (i.e. transgressive systems tract). The overlying highstand systems tract developed following periods of diminished tectonism, with the consequent growth and progradation of shallow carbonate platform facies.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01437.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY