摘要:

ABSTRACTIt is demonstrated that detailed examination of the photo‐ and thermoluminescence (PL, TL) of Holocene intertidal sediments can reveal important information regarding their depositional history in the context of sea level tendency and storm surge activity. The technique is particularly applicable to thick deposits of relatively uniform composition, and is demonstrated with a 1·7‐m core of marine to brackish water silty clay taken from Cowpen Marsh in the Tees estuary, north‐east England. This sedimentary unit is intercalated with an upper and a lower terrestrial peat bed, which have been radiocarbon dated to 5250 ± 45 and 7065 ± 4514C years BP respectively. An investigation of the mineral composition and diatom assemblage of the silty clay facies reveals a marked hiatus in deposition towards the top of the minerogenic unit. Analysis of the luminescence profiles of the sediment illustrates that, below this hiatus, sedimentation was slow and continuous, with approximately 1·0 m of sediment being deposited in 1800 ± 400 years, whilst above, sedimentation was rapid, with 0·7 m of sediment being deposited in 0 ± 200 years. It is shown that variations in the luminescence of the sediments can be interpreted in terms of the onset of a positive sea level tendency period coupled with the occurrence of a low‐frequency extreme wat

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01020.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe Miocene alluvial‐lacustrine sequences of the Madrid Basin, Spain, formed in highly varied landscapes. The presence of various types of palaeosols allows assessment of the effects of local and external factors on sedimentation, pedogenesis and geomorphological development. In the northern, more arid, tectonically active area, soils were weakly developed in aggrading alluvial fans, dominated by mass flows, reflecting high sedimentation rates. In more distal parts of the fans and in playa lakes calcretes and dolocretes developed; the former were associated with Mg‐poor fan sediments while the latter formed on Mg‐rich lake clays exposed during minor lake lowstands.The north‐east part of the basin had a less arid climate. Alluvial fans in this area were dominated by stream flood deposits, sourced by carbonate terrains. Floodplain and freshwater lake deposits formed in distal areas. The high local supply of calcium carbonate may have contributed to the preferential development of calcretes on the fans. Both the fan and floodplain palaeosols exhibit pedofacies relationships and more mature soils developed in settings more distant from the sediment sources. Palaeosols also developed on pond and lake margin carbonates, and led to the formation of palustrine limestones.The spatial distributions and stratigraphies of palaeosols in the Madrid Basin alluvial fans suggest that soil formation was controlled by local factors. These palaeosols differ from those seen in Quaternary fans, which are characterized by climatically induced periods of stability and inst

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01021.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe uppermost Permian Chuenmuping reef formed on the western side of a carbonate platform situated in the eastern Sichuan Basin, southern China. Contrasts exist between this western patch reef and the better known Jiantianba barrier reef complex on the eastern platform margin in western Hubei Province, 250 km to the east. Microbial micritic carbonates form significant components of the Chuenmuping reef limestones. Three types of fabric can be identified: irregular, clotted and scalloped‐laminated. An additional, cuneiform fabric has been found only in fore‐reef facies at Jiantianba.Irregular fabric is comparable with the surface of presently forming travertines at Rapolano Terme, central Italy, which are produced by calcification of cyanobacteria. Clotted fabric shows indistinct peloids which tend to lack rims, and are amalgamated into irregular masses, some of which exhibit bush‐like arrangements where traces of filaments can be seen. Traces of filaments are observed in both dark and light layers in the scalloped‐laminated fabric, again suggesting a microbial origin. Cuneiform fabric consists of peloids and filaments, and closely resembles deposits in Quaternary travertines which may have been caused by bacterial action.Chuenmuping reef shows a shallowing sequence in normal marine conditions. The micritic fabrics interpreted here as microbial occur more commonly in the upper part of the reef. This indicates that microbial micrites, with a cementing role, formed coevally with reef growth within the upper part of the phot

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01022.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTSantonian‐Lower Campanian and Lower Maastrichtian phosphatic chalks in northern France, southern England and Belgium are Europe's largest sedimentary phosphatc deposits. The stratigraphy, sediment‐ology, petrography, mineralogy and geochemistry of the lithofacies are reviewed and new data presented. Depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high‐ and low‐productivity phosphogenic systems. It is concluded that phosphatic chalks were deposited in well‐oxygenated, current‐swept environments. Phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates, reduced bulk sediment accumulation rate and an enhanced rate of bioturbation. Precipitation of carbonate‐fluorapatite (francolite) accompanied the bacterially mediated decomposition of organic matter, occurring within centimetres of the sediment‐seawater interface, and taking place preferentially within microbial bodies and coatings. In addition to the organically derived component, pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction. Mineralization was probably a dominantly post‐oxic process, but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation. Phosphogenesis occurred predominantly on the NE margins of the Anglo‐Paris Basin where shallower sea floors and suitable palaeoceanographic conditions prevailed. Phosphogenic episodes were limited by sea level fluctuations'which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01023.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTIn many sandstones quartz cements significantly reduce porosity. The origin of these cements is often unclear. This paper investigates a possible mechanism for the generation of silica for quartz cement by pressure dissolution in interbedded mudstones.Theoretical models of quartz pressure dissolution, including the effects of silica precipitation kinetics, show that the concentration of dissolved silica in the pore fluids of a compacting sediment increases with decreasing grain size and silica precipitation constant. Quartz precipitation is strongly inhibited by the presence of small amounts of clay within a sediment, suggesting that siltstones and quartz‐rich mudstones which are undergoing pressure dissolution may act as a source of dissolved silica for export to nearby, coarser sediments.A computational model for the diagenetic modification of a sandstone‐mudstone interface due to pressure dissolution is described. Both sandstone and mudstone layers are assumed to be actively compacting by pressure dissolution, and mass transport by molecular diffusion is considered. As quartz precipitation in the mudstone layer is relatively slow compared to that in the sandstone, significant amounts of dissolved silica become available in the mudstone, and may be exported into the adjacent sandstone. In the absence of pore‐fluid advection, this may result in the formation of extensive secondary quartz within the sandstone, close to the interface.The volume of silica exported from the mudstone is limited by the length scale over which diffusion through the mudstone is effective. This is typically 3–5 m. The volume of silica available therefore suggests that extensive porosity modification within the adjacent sandstone can only occur close to the mudstone. Thus it is possible that thin sandstones could become cemented by slow diffusive transfer of silica, but that in thicker sandstones the silica may become dispersed by pore‐fluid

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01024.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTA 166 m thick Plio‐Pleistocene sequence of glacial sediments has been cored in Ferrar Fiord in the southwestern corner of the Ross Sea, Antarctica. The core has the following lithofacies: massive diamictite (33% of the core; interpreted as lodgement or waterlain till), weakly stratified diamictite (25%; waterlain till or proximal glaciomarine sediment), well‐stratified diamictite (8%; proximal glaciomarine or glaciolacustrine sediment), sandstone (25%; sand of aeolian or supraglacial origin), mudstone(7%; derived from subglacial debris and transported offshore in suspension), and minor amounts of rhythmite and tuff. The range of facies in this polar setting differs from those normally found in subpolar and temperate glacier fiord settings in the high proportion of aeolian‐derived sand and the low proportion of mudstone facies.The core can be divided into two sequences based on composition and texture. The sequence from 162 to 100 mbsf (metres below the sea floor) comprises alternations of diamictite dominated by basement lithologies and thin marine mudstone beds. It is Pliocene in age (4.9–2.0 Ma) and records several advances and retreats of ice through the Transantarctic Mountains and across the drill site from the west. The sequence from 100 mbsf to the sea floor, of Pleistocene age, consists of alternations of diamictite, interpreted as lodgement and waterlain till, and sandstone of aeolian origin deposited in a glaciolacustrine setting, similar to ice‐covered lakes in the Dry Valleys today. These sediments have a high volcanic component, and hence are thought to have been derived by the grounding and advance of the Ross Ice Shelf from the east past volcanic Ross Island. This change in source is attributed to the rising Transantarctic Mountains increasingly containing East Antarctic ice.The Pleistocene sequence above 100 mbsf clearly represents polar glacial sedimentation, with alternations of till and glaciolacustrine sand. Mudstones from the Pliocene sequence beneath include palynomorphs, indicating times when the landscape was at least partially vegetated, but contain no evidence of meltwater

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01025.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe interaction between channel geometry, flow, sediment transport and deposition associated with a midstream island was studied in a braided to meandering reach of the Calamus River, Nebraska Sandhills. Hydraulic and sediment transport measurements were made over a large discharge range using equipment operated from catwalk bridges. The relatively low sinuosity channel on the right‐hand side of the island carries over 70% of the water discharge at high flow stages and 50–60% at low flow stages. As a result, mean velocity, depth, bed shear stress and sediment transport rate tend to be greater here than in the more strongly curved left‐hand channel. The loci of maximum flow velocity, depth and bed shear stress are near the centre of the channel upstream of the island, but then split and move towards the outer banks of both channels downstream. Variations in these loci depend on the flow stage. Topographically induced across‐stream flows are generally stronger than the weak, curvature‐induced secondary circulations. Water surface topography is controlled mainly by centrifugal accelerations and local changes in downstream flow velocity. The averaged water surface slope of the study reach varies very little with discharge, having values between 0·00075 and 0·00090. As bed shear stress generally varies in a similar way to mean velocity, friction coefficients vary little, normally being in the range 0·07–0·13. These values are similar to those in straight channels with sandy dune‐covered beds. Bedload is moved mainly as dunes at all flow stages. Grain size is mainly medium sand with coarse sand moved in thalwegs adjacent to the cut banks, and with fine sand at the downstream end of the island. These patterns of flow velocity, depth, water surface topography, bed shear stress, bedload transport rate and mean grain size can be accurately predicted using theoretical models of flow, bed topography and sediment transport rate in single river bends, applied separately to the left and right channels.During high flow stages deposition occurs persistently near the downstream end of the island, and cut banks are eroded. Otherwise, erosion and deposition occurs only locally within the channel as discharge varies. Abandonment and filling of a strongly curved channel segment may occur by migration of an upstream bar into the channel entrance at

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01026.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThree peritidal carbonate crusts and associated intercrust sediments (total thickness of ∼30cm; aged<3000 years BP) on Ambergris Cay, Belize, contain 32–100% calcian dolomite (δx=72·5% dolomite) ranging in composition from 40 to 46 mol% MgCO3(δx=43·3). Dolomite replaced high Mg calcite foraminiferal muds penecontemporaneously with sedimentation, forming partially dolomitized sediments and lithified crusts. Dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present.Detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth; 0·4 μm near the top of the section to 1·0 μm near the base of the dolomitized section. This size increase is not accompanied by any significant decrease in porosity. Crystal size distributions appear to be log‐normal and become increasingly broad and flat with depth. Rietveld X‐ray pattern‐fitting structure refinements indicate increasing Ca and Mg concentrations on their respective sites (cation ordering) as a function of increasing depth. Most of the ordering occurs within the first 15 cm of the surface. Stoichiometry does not increase with depth indicating no relationship between the Ca/Mg ratio and cation ordering. Strong geochemical trends were observed down‐section in the dolomite, including: (1) increasing Mn content (44 to 274 ppm), and (2) decreasing δ13C values (−0·9 to −5·5‰ PDB). Oxygen isotope values range from δ18O = 1·3‰ PDB in the upper part of the section to 2·6‰ PDB in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend.Down‐section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy‐driven dissolution‐reprecipitation process (Ostwald ripening). The observed trends in carbon isotopes and Mn content probably result from geochemical re‐equilibration during recrystallization and reflect reducing conditions and an isotopically light, organically derived, carbon source. Oxygen isotope compositions probably reflect relict original dolomite values and are a result of dec

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01027.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTHolocene dolomite forms in the sediment of Lake Hayward, a small permanent hypersaline lake in the Clifton‐Preston Lakeland System, Western Australia. The geomorphological setting of dolomite formation in Lake Hayward is similar to the Coorong region in South Australia. Unlike in the Coorong region, dolomite in Lake Hayward does not form as a direct precipitate from the lake water, but is of diagenetic origin. This can be deduced from the following features: (1) the dolomite occurs only below 60–70 cm from the sediment‐water interface, (2) dolomite occurs as luminescing cement, and (3) dolomite has pristine well‐formed rhomb‐shaped crystals. The source of magnesium for dolomitization is probably from the concentration on inflowing groundwater by evaporation and the selective removal of calcium by chemical and biological aragonite/calcite prec

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01028.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb01029.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY