ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02121.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe 150–160 m thick lowermost Silurian (Rhuddanian) Becscie and Merrimack formations of Anticosti Island, Canada, represent continuous deposition on a shallow, open marine carbonate ramp. Several rock types are identified: (a) laminated and homogenous mudstone; (b) laminated and homogenous packstone; (c) argillaceous mudstone and packstone; (d) calcareous shale; (e) laminated calcisiltite; (f) medium‐ to fine‐grained grainstone; and (g) bio/intraclastic rudstone. These rock types are arranged into five distinct lithofacies: (LF1) calcareous mudstone‐shale; (LF2) laminated‐homogenous mudstone; (LF3) calcareous grainstone‐shale; (LF4) laminated mudstone‐grainstone; and (LF5) laminated calcisiltite‐grainstone. The sequence reflects deposition on a low‐energy, muddy, carbonate to argillaceous ramp subject to short‐lived, episodic high‐energy storms. These events produced fining‐upwards storm units 5–80 cm thick, or tempestites, consisting of: a sharp scoured base overlain by intra/bioclastic rudstone grading upwards into medium‐grained grainstone, finely laminated calcisiltite and mudstone, or shale. These are interbedded with low‐energy, fairweather mudstones and calcareous shales.Deposition progressed from a carbonate mud‐dominated ramp in the Becscie Formation to an argillaceous mud‐dominated ramp in the Merrimack Formation. Lateral tempestite proximality trends and lithofacies distribution indicate that the Anticosti Basin deepened to the south‐east into the Iapetus Ocean and shallowed towards a SW—NE‐orientated shoreline to the north‐west. Vertical tempestite proximality trends and lithofacies changes identify third‐order eustatic sea‐level changes. After an initial deepening at the base of the formation, a shallowing‐deepening event dominated the sequence. Several higher order fluctuations, defined by lithofacies and tempestite proximality trends, are superimposed on these changes. The fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02122.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTMetresa is a thick, distinctive turburdite within the Menilite Shales in the Carpathians of south‐east Poland. It was deposited by a SE‐flowing turbidity current derived from the Silesian cordillera and can be traced for 55 km downcurrent; it has a width of at least 15 km. The current contained roughly 3 km3of sediment and conforms to the definition of a seismoturbidite. Grain‐size analyses of samples from the nine localities where Metresa is exposed indicate average velocities decreasing from 2 m s−1to less than 1 m s−1over the 55 km. Massive divisions are poorly developed; instead, the turbidite, although graded, tends to be laminated throughout. Flat lamination predominates with intercalated cross‐lamination and erosion structures at various levels. Small‐scale cross‐lamination, presumably associated with ripples, occurs at some upper levels. An exceptional feature is a large‐scale wave‐like structure somewhat resembling hummocky cross‐stratification. The structures are not consistent with the inferred velocities. It is suggested that the stability fields relating structures to stream power (herein called the ‘Allen fields’) are displaced in respect to deposition from

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02123.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTQuaternary sedimentary successions are described from the Linda Valley, a small valley in western Tasmania that was dammed by ice during Early and Middle Pleistocene glaciations. Mapping and logging of exposures suggest that an orderly sequence of deposits formed during ice incursion, occupation and withdrawal from tributary valleys. Four principal sediment assemblages record different stages of ice occupation in the valley. As the glacier advanced, a proglacial, lacustrine sediment assemblage dominated by laminated silts and muds deposited from suspension accumulated in front of the glacier. A subglacial sediment assemblage consisting of deformed lacustrine deposits and lodgement till records the overriding of lake‐bottom sediments as the glacier advanced up the valley into the proglacial lake. As the glacier withdrew from the valley, a supraglacial sediment assemblage of diamict, gravel, sand and silt facies formed on melting ice in the upper part of the valley. A lacustrine regression in the supraglacial assemblage is inferred on the basis of a change from deposits mainly resulting from suspension in a subaqueous setting to relatively thin and laterally discontinuous laminated sediments, occurrence of clastic dykes, and increasing complexity of the geometry of deposits that indicate deposition in a subaerial setting. A deltaic sediment assemblage deposited during the final stage of ice withdrawal from the valley consists of steeply dipping diamict and normally graded gravel facies formed on delta foresets by subaqueous sediment gravity flows. The sediment source for the delta, which prograded toward the retreating ice margin, was the supraglacial sediment assemblage previously deposited in the upper part of the valley. A depositional model developed from the study of the Linda Valley may be applicable to other alpine glaciated areas where glaciers flowed through or terminated in medium‐ to high‐relief topog

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02124.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTNumerous methods for roundness measurement have been developed. None, however, has been generally accepted, because of conceptual and practical deficiencies. Modern image processing and Fourier grain shape analysis have eliminated the practical shortcomings, but the conceptual ones remained. Single, higher harmonics of the Fourier series, for example, fail to serve as reliable equivalents for roundness evaluation. The concept outlined in this paper recognizes three criteria for the evaluation of roundness. (1) All curvatures, convex as well as concave or plane elements, must be considered. (2) The positions of morphological elements are significant because salient parts of a particle are more easily abraded than protected ones. Consequently, the curvatures have to be weighted by their relative position on the particle. (3) Positions and curvatures of morphological elements have to be compared with the particle's ultimate abraded shape, which is assumed to be an ellipsoid. The ellipsoid reflects the aspect of form or sphericity. The distinction between sphericity and roundness is retained because there is no evidence that sphericity changes significantly during transport. The measurement is based on the outline of a particle's maximum projection plane, which is transformed to a Fourier series. Roundness data are derived from the complete amplitude spectrum. The aspect of sphericity is eliminated by subtracting the amplitude spectrum of the best approximating ellipse from the spectrum of the empirical shape. The residual amplitudes are normalized and summed. In a final step the resulting values are rescaled. This guarantees reasonable boundaries and a normal distribution of roundness values. The procedure is automated and its efficiency permits the calculation of large samples. Tests on fluvial and coastal gravel populations demonstrate that the method is sensitive to abrasional wear during all stages of roundness.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02125.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTA new method for the description and classification of subaqueous sediment gravity flow deposits is proposed. The classification scheme employs a convenient letter code and divides deposits (individual beds) into descriptive categories of two hierarchical levels: facies and subfacies. Facies, as the higher rank categories, are distinguished chiefly on the basis of sediment type (i.e. bed grain size/texture). A total of 13 facies have been distinguished: G= gravel; GS = gravel‐sand couplet; GyS = gravelly sand; S = sand; SM = sand—mud couplet; MS = mud—sand couplet; TM = silt—mud couplet; MT = mud—silt couplet; M = mud; MyS = muddy sand; SyM = sandy mud; MyG = muddy gravel; GyM = gravelly mud. Subfacies, as the lower rank categories, are distinguished within the individual facies on the basis of the bed's internal structures. The number of subfacies is unlimited, and their labelling code includes particular facies symbols (see above) preceded by lower—case letters denoting specific sedimentary structures and their vertical arrangement. Subfacies thus refer to the bed's intervals, or divisions, which are labelled as follows: m = massive (unstratified and ungraded); g = graded (unstratified and graded); s = plane‐stratified; x=cross‐stratified; 1 = parallel– and/or cross‐laminated; q = liquefied. For example, subfacies gsG (graded to plane‐stratified gravel) are gravel beds that have a lower graded interval and an upper plane‐stratified interval; subfacies xG (cross‐stratified gravel) are gravel beds that are cross‐stratified throughout; subfacies slS (plane‐stratified to laminated sand) are sand beds that have a lower plane‐stratified interval and an upper laminated (parallel‐

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02126.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTA colour Landsat Thematic Mapper image was used to identify and to map on a regional scale alluvial facies in the lower Eocene Willwood Formation. Each facies covers a distinct geographical area of several hundred square kilometres in the Bighorn Basin of Wyoming, USA. Comparison of the image with laboratory spectra from Willwood Formation rocks and field exposures shows that the image provides information regarding the colour of mudstones, their relative abundances, and the relative abundance of sandstones. Facies were further characterized from field study and published descriptions on the basis of sandbody geometry, the types of nodules present in the mudstones, and the abundance and geometry of carbonaceous mudstones. This information helps to interpret the fluvial environments represented and types of alluvial palaeosols found in the Willwood Formation.Four facies have been distinguished. The first facies is dominated by sheet sandbodies associated with immature palaeosols. The sheets mark the locations of major channels, which tend to occur in topographic lows. The second facies is characterized by more mature, moderately drained soils that required relatively lengthy periods of non‐deposition. Facies three and four dominantly consist of drab mudstones and abundant carbonaceous units, both formed in poorly drained areas. Although minor red mudstones demonstrate that drainage conditions periodically improved in facies three, facies four underwent prolonged saturation.The geographical facies distribution suggests that east‐west faults previously mapped in the Bighorn Mountains, extend westward into the Bighorn Basin and were active during early Eocene time. The lithological heterogeneity of the Willwood Formation rocks is attributed to warping associated with movement along these basement‐controlled faults. Fault activity created topographic gradients which influenced the position of major channel systems and helped produce variable drainage conditions that affected Eocene soil develo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02127.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south‐west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (Rw), maximum projection sphericity (δp), oblate‐prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δpandS/Lwhich give good results with the Zingg classification (I/Lvs.S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade‐shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment andRwis observed, hence the abundance of disc‐shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan cong

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02128.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTEvidence from fusain deposits in Lower Carboniferous rocks of western Ireland indicates that a catastrophic wildfire destroyed thousands of square kilometres of Carboniferous ‘forest’. In addition to yielding large quantities of charcoal, this wildfire event resulted in increased surface water runoff which affected sedimentation in an adjacent estuarine environment where the fusain is now preserved. This is the oldest documented evidence for a catastrophic palaeowildfire and a clear example of the sedimentological effects of large‐scale fires.The Lower Carboniferous (Visean) rocks in the Largymore Syncline of western Donegal, Ireland, are shallow marine sandstones, mudstones and limestones. The Upper Shalwy Beds are mudstones and cross‐bedded sandstones which show bi‐polar cross‐stratification and mud drapes on cross‐bed foresets indicating deposition in a tidal environment, probably a large estuary. In three coastal exposures a bed containing up to 20% fusain is found at the same stratigraphic horizon. The fusain is interpreted as fossil charcoal produced by palaeowildfire in a land area to the north‐west. The volume of fusain present in the unit can be estimated and by comparison with charcoal production in modern wildfires it has been calculated that around 95 000 km2(more than the present land area of Ireland) was burnt.Along with the fusain, other effects of the wildfire can be seen in the deposits, which are poorly sorted compared to the rest of the Upper Shalwy Beds and are characterized by inclined heterolithic stratification produced by the draping of underlying bedforms. These features are considered to be due to a considerably increased sediment load in the estuary, resulting from enhanced surface runoff and soil erosion due

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02129.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTLight is one of the major controls on reef growth and carbonate production. The growth of present reef builders depends largely upon the amount of light available for photosynthesis. As light decreases with water depth, so does reef growth.The computer model presented extends this principle by combining two functions, one for photosynthesis and the other for the extinction of light in water. The model is used to simulate the growth of Alacran Reef, Mexico, two reefs of the Great Barrier Reef and the reefs of the windward platform of St Croix, US Virgin Islands. The model also gives an accurate simulation of the growth of fore‐reef walls in Belize, in agreement with the accretion hypothesis developed for this featur

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02130.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY