ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01267.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe Pennsylvanian Pikeville, Hyden and Four Corners formations of the Breathitt Group in eastern Kentucky, USA, contain six major facies associations along with a number of subassociations. These facies associations are offshore siltstone, rhythmically bedded mouthbar heteroliths, predominantly fine‐grained floodplain deposits, minor channel fills, major distributary channels and major, stacked fluvial bodies. The stacked fluvial bodies are incised into a variety of open marine and delta plain deposits, have widths of several kilometres and exhibit a range of sandy fill types. These fluvial complexes are interpreted as incised valley fills.Parasequences and parasequence sets are not identifiable. Nonetheless, it is possible to identify systems tracts on the basis of sequential position, facies associations and systematic changes in architectural style and sediment body geometries. The studied portion of the Breathitt Group comprises stacked 4th‐order sequences, which occur in lowstand, transgressive and highstand sequence sets related to the development of a lower frequency base level cycle.In the lowstand sequence set, incision associated with successive 4th‐order sequence boundaries has commonly removed all the HST and TST of the underlying sequences, such that succeeding 4th‐order incised valley fills are amalgamated. Within the transgressive sequence set, incision is at a minimum and incised valley fills tend to stack discretely with the maximum amount of fine‐grained TST and HST between them. The highstand sequence set is transitional between the lowstand and transgressive sequence sets in terms of the amount of transgressive and highstand deposits preserved. Incised valley fills tend to stack d

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01268.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe composition of sediment samples and/or the influence of current activity on the sediment can be easily determined using the dissection of settling velocity distributions. A new dissection method for the analysis of settling velocity distributions has been developed. The initial settling velocity distribution is decomposed by means of settling velocity distributions of the main biogenic constituents, three planktic and three benthic foraminifera species. A divergence from the natural size distribution of a foraminifera species can be attributed to current influence. Thus, negative or positive skewness of current‐affected foraminifera populations indicate erosional or depositional sediment influence. The dissection of Holocene samples from the northern North Atlantic demonstrates the advantages of this method in the interpretation of sand‐size mater

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01269.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTWe report results of experiments intended to test the validity of a model for aeolian saltation and the resulting pattern of deposition on the lee side of aeolian dunes. In steady sea‐breeze conditions on a 3‐m‐tall dune at Point Año Nuevo, California, we measured simultaneously the near‐brink wind speed and the deposition on both horizontal and lee face collector platforms. We then used the details of the deposition patterns to constrain approximate values of parameters in a numerical model of the deposition rate that incorporates the essence of the saltation process. Best fits to the data constrain a parameter that controls the probability distribution of liftoff speeds. In addition, the total vertical number flux of grains is constrained to roughly 107−108grains m−2s−1at shear velocities of 0.33–0.40 m s−1. The lee side deposition pattern, which shows the expected maximum in deposition rate at a distance of several decimetres from the brink, is also well fit by the model. In addition, simultaneous collection of horizontal and lee deposition patterns, along with the numerical simulation of these patterns, strongly implies that the windfield in the lee of this particular dune is best described as a non‐recirculating wake. Grainflows on the lee face are caused by failure of grainfall depositional bumps. Our results suggest that the principal effect of increased wind speed is to increase the frequency of grainflows. rather than to increase their size, implying that very large, thick grainflows require a

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01270.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTAalenian and lower Bajocian rocks in the central and northern Swiss Jura mountains comprise a series of parasequences that mainly reflect a shallowing‐upward trend in a shallow, mixed carbonate/siliciclastic depositional environment. Within a parasequence, ooidal ironstones may occur at three specific types of horizons. These are: regressional discontinuities and transgressional discontinuities formed by sediment bypassing, and omissional discontinuities formed by starvation.Ooidal ironstones, which principally are autochthonous, accumulated during both sea‐level rises and falls in a relatively broad bathymetric and hydrodynamic spectrum. The key physical factor for ferruginous ooid genesis is non‐deposition.Ferruginous ooids and microbialites consist of goethite, chamosite and mixtures thereof, with subordinate amounts of apatite and silica. Ferruginous ooids grew stepwise on the sediment surface in an oxygenated marine environment. Ferruginous microbialites, being the product of benthic microbial communities, grew − partly in cavities − in aerated moderate‐ to high‐energy environments. Thus, chamosite evolved from a precursor substance stable under oxidizing conditions.The close mineralogical and micromorphological resemblance of ferruginous microbialites and ooids suggests a common biogenic origin. Structural rearrangement of a biologically accreted gel‐like precursor substance consisting of various amorphous hydroxides is considered a probable mode of mineral genesis in both ferruginous ooids an

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01271.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTNo counterparts to epeiric‐sea carbonate ramps are known in present‐day environments. This hinders the interpretation of the factors controlling the growth and evolution of these depositional settings. In this study we analyse the facies and geometries of two Jurassic examples both from outcrop study and through computer modelling. This analysis is constrained by two important features of these Oxfordian and Kimmeridgian ramps: firstly, they are very well exposed, allowing accurate reconstruction of a 200‐km section from proximal to distal ramp environments, and, secondly, a time framework for correlation, section reconstruction and modelling is provided by a well‐defined ammonite biostratigraphy. The modelling results in a synthetic stratigraphy which closely matches the reconstructed cross‐sections and, when integrated with the field study, constrains and provides additional quantitative data on the following aspects of carbonate ramp systems.Resedimentation by storms is an important process in maintaining the ramp profile through time. Down‐ramp transport distances of between 25 and 40 km are indicated from the distribution of storm beds and shallow‐water allochems and from model‐matching known stratigraphic thicknesses and geometries.Modelling sediment production within the time constraints from the ammonite biozones indicates that shallow‐water carbonate production was 1–2 orders of magnitude less than that predicted for present‐day open‐marine carbonate platforms. Deeper‐water production rates were reduced by lesser amounts. These proportionally higher, outer‐ramp production rates also help to maintain ramp geometries through time.The enigmatic slope crest of ramps is shown to result from a combination of higher, shallow‐water production and erosion rates, together with loss of accommodation during highstands and high‐stillstands in the modelled sea‐level curves.The most parsimonious modelling of the two ramp sequences comes from a relative sea‐level curve composed of a linear subsidence component superposed by 20‐ and 100‐kyr cycles on a third‐order cycle. The third‐order cycles and their timing do

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01272.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe Trenton Group (Late Ordovician), the youngest carbonate unit in the Taconic foreland basin of southern Quebec, is a tripartite unit with a distinctive coarse‐grained middle part, the Deschambault Formation.Lithofacies of the Deschambault Formation are dominated by coarse‐grained bioclastic/intraclastic limestones; finer‐grained lithofacies are ubiquitous but subordinate. The complete spectrum of lithofacies indicates sedimentation ranging from above fairweather‐ to below storm‐wave base. Skeletal components are indicative of the modern temperate‐waterbryomolassociation. Non‐skeletal elements are represented by peloids and intraclasts. Accretion rates from areas of continuous sedimentation were low (<14 cm/103years). From sedimentological and faunal evidence, it is proposed that the Late Ordovician Deschambault ramp was bathed by temperate waters. The model compares favourably with modern cool‐water shelves rimming the southern edge of the Australian continent.Palaeomagnetic data locate southern Quebec in a low latitudinal setting during the Late Ordovician. Upper Ordovician facies distribution in eastern Canada and progressive disappearance of some faunal provinces through Late Ordovician time are used to conclude that the initiation of the Late Ordovician glaciation that covered most of Gondwana was instrumental in easing northward movement of cold oceanic currents. This resulted in the rapid contraction of the southern hemisphere warm‐water tropical belt from a 30° latitudinal‐wide zone in the early Caradoc to a 15° zon

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01273.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe nature of flow, sediment transport and bed texture and topography was studied in a laboratory flume using a mixed size‐density sediment under equilibrium and non‐equilibrium (aggradational, degradational) conditions and compared with theoretical models. During each experiment, water depth, bed and water surface elevation, flow velocity, bed shear stress, bedload transport and bed state were continuously monitored. Equilibrium, uniform flow was established with a discharge of about 0.05 m3s−1, a flow depth of about 0.01 m, a flow velocity of about 0.81–0.88 m s−1, a spatially averaged bed shear stress of about 1.7–2.2 Pa and a sediment transport rate of about 0.005–0.013 kg m−1s−1(i.e. close to the threshold of sediment transport). Such equilibrium flow conditions were established prior to and at the end of each aggradation or degradation experiment. Pebble clusters, bedload sheets and low‐lying bars were ubiquitous in the experiments. Heavy minerals were relatively immobile and occurred locally in high concentrations on the bed surface as lag deposits.Aggradation was induced by (1) increasing the downstream flow depth (flume tilting) and (2) sediment overloading. Tilt‐induced aggradation resulted in rapid deposition in the downstream half of the flume of a cross‐stratified deposit with downstream dipping pebbles (pseudo‐imbricated). and caused a slight decrease in the equilibrium mean water surface slope and total bedload transport rate. These differences between pre‐ and post‐aggradation equilibrium flow conditions are due to a decrease in the local grain roughness of the bed. Sediment overloading produced a downstream fining and thinning wedge of sediment with upstream dipping pebbles (imbricated), whereas the equilibrium flow and sediment transport conditions remained relatively unchanged. Degradation was induced by (1) decreasing the downstream flow depth (flume tilting) and (2) cutting off the sediment feed. Tilt‐induced degradation produced rapid downstream erosion and upstream deposition due to flow convergence with little change to the equilibrium flow and sediment transport conditions. The cessation of sediment feed produced degradation and armour development, a reduction in the mean water surface slope and flow velocity, an increase in flow depth, and an exponential decrease in bedload transport rate as erosion proceeded. A bedload transport model predicted total and fractional transport rates extremely well when the coarse‐grained (or bedform trough) areas of the bed are used to define the sediment available to be transported. A sediment routing model,MIDAS, also reproduced the equilibrium and non‐equilibrium flow conditions, total and fractional bedload transport rates and changes in b

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01274.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene‐Pleistocene) dolomitization of these rocks.Diagenetic carbonates included early siderite (δ18OSMOW=+24.4 to +26.5‰δ13CPDB=−1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ18OSMOW=+18.4 to +25.8‰; δ13CPDB=−2.1 to +0.2‰), and calcite (δ18OSMOW=+21.3 to +32.6‰; δ13CPDB=−4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values.A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ18O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post‐dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ18O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01275.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTEarly diagenesis of the Upper Cretaceous (late Coniacian to early Santonian) Marshybank Formation was controlled by depositional environment (composition of depositional water, Fe and organic content of the sediment, sedimentation rate, proximity to the shoreline) and influx of meteoric water related to relative sea‐level fall.Five depositional environments, each characterized by a distinct early diagenetic mineral assemblage, have been recognized. Offshore shelf sediments that were deposited in a dysaerobic environment are characterized by abundant framboidal pyrite and rare septarian concretions, composed of ‘early’ calcite and siderite. Intense sulphate reduction, promoted by the dysaerobic depositional water, was the primary influence on early diagenesis. Offshore shelf sediments deposited under aerobic conditions are characterized by abundant concretions, composed of two generations of siderite (S1 and S2). In this environment, methanogenesis, rather than sulphate reduction, was more important. Early diagenesis of the inner shelf sands was generally limited. However, in sands deposited proximal to the shoreline, mixing of marine and meteoric waters promoted crystallization of Fe‐rich chlorite and siderite. The shoreface was characterized by dissolution of detrital minerals in the upper portion, and precipitation of kaolinite or illite/smectite in the lower portion. In the coastal plain environment, brackish water and early reducing conditions resulted in formation of abundant euhedral pyrite. Ankerite, rather than siderite, is the typical early diagenetic carbonate.The δ18O values of the earliest cements (i.e. ‘early’ calcite, siderite S1, inner shelf siderite) indicate crystallization from a low‐18O, marine‐derived porewater. Assuming crystallization at 25°C, a δ18O value of about −7‰ (SMOW) can be estimated for the seaway during Marshybank Formation time. Similar calculations for the overlying Dowling Member (Puskwaskau Formation) suggest that the δ18O value of the seaway increased to about −4% (SMOW), consistent with its transgressive nature. Very low δ18O values are exhibited by siderite S2. These results indicate crystallization during intermediate diagenesis (≥60°C) from meteoric water (≥− 15‰ SMOW) that entered the Marshybank

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb01276.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY