摘要:

ABSTRACTAs a result of a phase of extensional tectonics in the western Tethyan region, a horst and graben topography formed during the Middle Triassic (Ladinian) in northern Italy. Horsts were sites of shallow water carbonate sedimentation, while pelagic and volcaniclastic sediments were deposited in the grabens. Two carbonate platforms approximately 500 m thick can be distinguished in the Marmolada area of the Dolomites: the Marmolada platform proper, which covered an area of 6 km2, and the Costabella platform, which extended for about 12 km in a NW‐SE direction and was about 3 km across.The facies of these isolated platforms reflect the influence of storms from the SW. Windward platform margins were characterized by a marine sand belt of skeletal and aggregate grainstones with a dominant platform directed cross‐stratification. The central portions of the platforms were occupied by supratidal sand cays which are made up of storm washovers. Leeward parts of the platforms are composed of shallow subtidal sand flat deposits. Laterally discontinuous reefs chiefly composed of various calcareous algae are developed at the outer margins of the platforms. Along windward margins, reefs may form a belt several hundred metres wide; along leeward margins their width is commonly reduced to some tens of metres. Foreslope talus breccias surround the platforms. Clinoform bedding showing basinward dips of 30°‐40° is typical of this facies belt, which is approximately 2 km wide. Basinal sediments, only some tens of metres thick, are radiolarian micrites. Abundant sediment‐gravity‐flow deposits expand the basinal sequence at the toes of windward margins and were probably triggered by storm return flows.Synsedimentary faults striking both NNE‐SSW and NW‐SE separate the bedded platform limestones from flank deposits and reefs. They account for the stationary nature of the platforms. Neptunian dykes show preferred NNE‐SSW and E‐W trends. Sinistral displacements are associated with NW‐SE trending faults. Depressions in the basins, filled with red, turbiditic pelagic sediments, show N‐S trends and are probably compressional in origin. The structural pattern may have resulted from oblique, NW‐SE oriented extension of the E‐W trending Middle Triassic graben zone of the Dolomites.In the Ladinian of the Dolomites, the stationary platform type can be distinguished from a retrograding type, whereas continuously prograding platforms

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00530.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe Bowland Basin (northern England) contains a series of carbonates and terrigenous mudstones deposited during the Ivorian to early Brigantian. Two regional depositional environments are indicated by facies and facies associations. Wackestone/packstone and calcarenite facies indicate deposition in a carbonate ramp environment, while lime mudstone/wackestone, calcarenite and limestone breccia/conglomerate facies, often extensively slumped, represent a carbonate slope environment. Stratigraphic relations suggest that the depositional environment evolved from a ramp into a slope through the Dinantian.Two main sediment sources are indicated by the sequence; an extra‐basinal terrigenous mud source and a supply of carbonate from the margins of the basin. Deposition from suspension and from sediment gravity flows,in situproduction and remobilization of sediment during sedimentary sliding were important processes operating within the basin.Periods of enhanced tectonic activity in the late Chadian to early Arundian and late Asbian to early Brigantian are indicated by basin‐wide horizons of sedimentary slide and mass flow deposits. Both intervals were marked by a decline in carbonate production resulting from inundation and uplift/emergence. The first of these intervals separates deposition on a seafloor with gentle topography (carbonate ramp) from a situation where major lateral thickness and facies variations were present and deposition took place in a carbonate slope environment. The second interval marks the end of major carbonate deposition within the Bowland Basin and the onset of regional terrigenous sedimentat

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00531.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTDolomite occurs in open‐shelf packstones and grainstones of the Vincent House Member of the Woo Dale Limestones Formation (Lower Carboniferous) in the Wye Valley area, east of Buxton, Derbyshire, Central England. Two stages of dolomitization can be recognized. Stage 1 consists of limpid, non‐luminescent dolomite lacking detectable iron and manganese. Stage 2 comprises cloudy, inclusion‐rich, orange‐red luminescing dolomite with significant iron and manganese. The contact between the two stages is irregular and suggests a hiatus between dolomite growth stages. Dolostones lacking any unreplaced limestone may contain up to 35% calcite cement rilling intercrystal porosity. This cement shows a characteristic zonal sequence and is normally in optical continuity with the surrounding dolomite crystals. In some cases, precementation compaction of the dolomite mosaic caused fracturing of the crystals. Sherds of dolomite on the floors of pore‐spaces then provided the nucleus for growth of calcite cement.The petrographic character and stratigraphic distribution of the dolomite suggest that dolomitization occurred at depth, during burial of the Woo Dale Limestones in the Late Carboniferous. Dolomitizing solutions, expelled from basinal shales, moved up‐dip along permeable limestones or along the sub‐Carboniferous unconformity. Fluids responsible for the generation of stage 2 dolomite may have been contaminated by ions such as iron and manganese released from the alteration of volcanic rocks which occur beneath the Carboniferous Limestone in the Wy

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00532.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTDuring early Carboniferous times a major sea‐level rise led to the development of an extensive carbonate ramp over what is now South Wales. Differential subsidence and sea‐level changes resulted in distinctive facies sequences in the ramp succession and a model is offered which recognizes three distinct geomorpho‐tectonic settings; inner, mid‐ and outer ramp. The inner ramp zone occurs in the more landward part of the province and was an area undergoing little or no subsidence. The sequence is dominated by oolitic grainstones and peritidal limestones representing shoal and back shoal environments. The peritidal units are transgressive deposits consisting of stacked asymmetrical shallowing‐up cycles. The sequence contains many subaerial breaks and tectonic uplift resulted in base‐level changes and fluvial incision. The mid‐ramp zone sequence is intermediate in thickness between the inner and outer ramp successions and consists mainly of bioclastic limestones deposited below fairweather wave base. Sedimentation periodically exceeded sea‐level rise and subsidence, and regressive (progradational) oolitic sand bodies developed, the thickest of which are stacked units with up to four individual sand bodies. Storm processes were of major importance in this setting. The outer ramp zone is represented by a thick sequence of muddy bioclastic limestones deposited below storm wave base and major Waulsortian reef‐mounds also developed. None of the shallowing phases seen in the other ramp zones can be detected in this sequence. Subsidence and eustatic sea‐level rise seem to have been the major controls on deposition but the recognition of eustatic sea‐level falls is difficult. The detailed facies model for ramp carbonates presented here may be applicable elsewhere in

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00533.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTDetermination of geographically dependent sedimentological variation (‘proximality’) in ancient flysch deposits formed an important part of early turbidite studies. Attempts to quantify this variation highlighted anomalies which were neatly resolved by application of vertical sequence analysis and the use of fan models. However, there are many turbidite formations, such as the Lower Cretaceous Cumberland Bay Formation (CBF) of South Georgia, which cannot be described in terms of existing fan models but show strong proximal to distal sedimentological changes. The CBF is a thick sequence of volcaniclastic sandstone turbidites deposited in a linear back‐arc basin, principally by currents flowing WNW, parallel to the basin margin. Four lithofacies associations are recognized on the basis of sandstone/shale ratio. The two finergrained associations are constant in character across the CBF outcrop. In the coarse‐grained associations there is a change in character WNW, down the palaeocurrent direction. This is brought out by decreasing sandstone bed thickness and percentage amalgamation, but these changes are not always marked or consistent. In contrast, the internal character of the sandstone beds changes strongly, with a marked proportional increase in Tband Tcdivisions within the bed downcurrent. The evidence suggests that the system was aggradational rather than progradational: tectonic control of the basin margins prevented major migration of the depositional system, and most areas remained in the same position relative to source through time. Comparison of the CBF with other turbidite formations suggests two end‐member states which will produce radically different vertical sequences. Progradational systems will produce strong vertical facies changes, where beds deposited in distal environments are overlain by beds deposited in environments progressively nearer source, however there will be no lateral change in the character of any particular facies type. In contrast in aggradational systems the major sedimentological variation will be lateral rather than

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00534.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTAs part of an overall study of sedimentation processes in a proglacial lake an effort was made to compare field results with some of the general equations for density flows. The results suggest that in relatively small glacial lakes the occurrence of underflows with lower sediment loads involves a complex interplay between thermal and sediment effects which is extremely sensitive to varying hydrologic and climatic conditions. In terms of actual transport mechanics the results: (i) indicate that a higher α value of 0·6 or 0·7 gives a closer agreement between the measured velocity values and the established equations on moderately shallow slopes; (ii) provide field support for the experimentally derived relationship of Britter&Linden (1980) for the velocity of underflows and suggest the equation may be applicable in situations below 5° slopes; and (iii) support the relationship between velocity of the front and body of a continuous underflow for moderate slope situations suggested by Middleton (1966b). Finally the velocity values measured by electromagnetic current meters stationed in the lake, the grain‐size data obtained from mapping core data, and the application of other criteria support the concept that in this environment the underflows are capable of er

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00535.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTWe have measured the mean magnitude and direction, and the rms fluctuation intensity, of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long. Downstream of reattachment, the mean skin‐friction magnitude is about 20% greater than its free‐stream value along the centre‐line and comparably reduced to either side of it. The horizontal divergence of the skin‐friction vector field is positive (divergent) along the centre‐line and negative (convergent) to either side of it. Neither of these conditions favours development of a ridge longer than the separation length along the centre line. The development of ridges or tails many obstacle heights long, commonly observed in nature, requires considerable modification of the simple sediment‐free wakes we h

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00536.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

Book review in this articleModern and Ancient Carbonate Environments of Jamaica, by W. D. Liddell, S. L. Ohlhorst and A. G. Coates.Facies Models (2nd edition), ed. by Roger G. WalkerAn Introduction to Seismic Interpretation‐Reflection Seismics in Petroleum Exploration (new revised edition), by R. McQuillin, M. Bacon and W. Barclay.Coastal Geomorphology in Australia, ed. by B. G. Thom.Terrigenous Clastic Depositional Systems‐Applications to Petroleum, Coal, and Uranium Exploration, by W. E. Galloway, and D. K. HobdayProceedings of International Symposium on Sedimentation on the Continental Shelf with Special Reference to the East China Sea, April 12–16, Hangzhou, China, ed. by Acta Oceanologica SinicaFluvial Hydrology, by S. Lawrence Dingman. W. H. FreemanWind as a Geological Process, by R. Greeley and J. D. Iv

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00537.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.tb00538.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY