摘要:

ABSTRACTThe South Alligator River, Northern Territory of Australia, has a macrotidal estuary. Tidal influence (spring tidal range 5–6 m at the mouth) extends 105 km up the channel. It is dominated by freshwater in the wet season (December‐April) with a salt wedge near the mouth, but is well mixed and becomes saline throughout the dry season. The tidal channel can be divided into four different channel types: an estuarine funnel, a sinuous meandering segment, a cuspate meandering segment (in which the inside of bends are pointed) and an upstream tidal channel. The distribution of morphologically defined land classes and morphological units within each land class on the floodplain flanking the estuary differs from one channel type to another. Several stratigraphic and morphostratigraphic units have been recognized from drill holes on the coastal and deltaic‐estuarine plains, and a model of development is proposed on the basis of extensive radiocarbon chronology and palynology.The coastal plain has prograded with most rapid sedimentation between 5000 and 3000 yr BP. A similar pattern of progradation is identified in the estuarine funnel. In the sinuous segment of the estuary the channel has migrated laterally across the floodplain. Previous channel positions are indicated by palaeochannels and the meander tract is occupied by laminated channel sediments. Within the cuspate segment there are numerous sinuous palaeochannels on the plains. In the upstream segment, the channel and palaeochannels have long straight reaches with irregular bends and discontinuous levées, and channel avulsion is indicated.Mangrove mud is a widespread stratigraphic unit throughout the plains. The initial phase of development is a transgressive phase. 8000–6800 yr BP, when mangrove forests extended landwards into a pre‐existing valley as sea‐level rose. As sea‐level stabilized, the transgressive phase was followed by a widespread mangrove phase, termed the ‘big swamp’ 6800–5300 yr BP. The mangrove forests disappeared from most of the plains as vertical accretion continued, and were replaced by grass and sedge‐covered floodplains. During the sinuous phase, about 5300–2500 yr BP, the channel migrated laterally and eroded the deltaicestuarine plain and deposited lateral accretion deposits (laminated channel sediments). Part of the channel of the South Alligator River has then progressed from sinuous to cuspate in form, and erosion of river banks has occurred. Transgressive and big swamp phases occurred under rising and stabilizing sea‐level, respectively. Later morphodynamic channel adjustments occurred under conditions of stable sea‐level. The depositional model has direct application to other estuaries in northern Australia, and may be applied to other areas where sea

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01743.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTQuantitative determination of palaeochannel geometry and hydraulics from point bar deposits requires an understanding of the interaction between channel‐bend migration, temporal and spatial variation of point bar geometry and facies, and outcrop orientation. This interaction is modelled with the aid of a computer program which predicts three‐dimensional (3‐D) geometry and grain size variation of point bars. Synthetic deposits are produced for the cases of down‐valley bend migration and/or increase in channel‐bend sinuosity. Two‐dimensional (2‐D) cross‐sections in varying orientations across these simulated deposits display lateral‐accretion bedset surface geometry, and variation in mean bedset grain size and local palaeocurrent orientation.Most cross‐sections show point bar deposits thickening away from the meander‐belt axis due to a lateral progression from thinner bend‐exit deposits to thicker bend‐apex deposits (caused by down‐valley channel translation), and/or due to a progression from thinner low sinuosity deposits to thicker high sinuosity deposits caused by channel bend expansion. In association with this lateral thickening, bedset surfaces become steeper and more convex upwards while the variation in mean grain size up bedsets commonly increases. Down‐valley point bar translation allows preservation only of deposits formed downstream of the band apex, and produces characteristic fining upwards sequences. Marked lateral and vertical variations in palaeocurrent directions due to varying channel orientation relative to a given cross‐section are also predicted. These results indicate a need in palaeochannel reconstructions, for a more detailed examination of 3‐D variations in bedset surface geometry, palaeocurrent orientation and grain size distribution within and between bedse

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01744.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Miocene marine molasse of Digne is thought to have been deposited in a structural embayment with a gulf‐like morphology. This gulf was closed toward the alpine front and opened on to the Rhodano‐Provençal gulf, part of the Tethys.The outcrop of Auribeau, located on the northern margin of the gulf of Digne, displays a conspicuous vertical record of different tidal cyclicities indicating the importance of tidal dynamics on the Miocene marine molasse deposits. For the first time these tidal cyclicities are analysed from a vertical section (deposited by vertical accretion) and these are similar to the cyclicities described from megaripples or sand waves (deposited by lateral accretion). By means of thickness measurements and time series analysis, four main tidal cyclicities have been recognized: (1) the diurnal cyclicity (as it exists in the Atlantic Ocean); (2) the semi‐lunar month cyclicity of neap to spring; (3) the lunar month cyclicity of neap, high spring, neap to low spring and (4) the semi‐annual cyclicity of solstice to

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01745.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTA significant aspect of Late Proterozoic sedimentation in the Adelaide Geosyncline, South Australia, is the presence of kilometre‐deep erosional incisions which have been termed canyons. These structures were formerly described to be of submarine origin, cut and filled in an inferred basin‐slope setting by subaqueous processes. Subsequent detailed research, particularly on a specific incision known as Patsy Springs Canyon, indicates that sedimentary structures within some of the canyon‐filling sediments are indicative of deposition above fair weather wave base. In addition, an unusual carbonate unit, which is observed to veneer upper portions of canyon shoulders and to contribute to carbonate breccias interbedded with canyon‐fill, has a stable isotope signature which may imply a non‐marine origin. The presence of the carbonate veneer, where it isin situ, suggests that at least upper portions of the canyons could have been emergent during the canyon‐filling phase. Considering these observations, and combining them with regional stratigraphical relationships, an alternative model for canyon genesis is proposed involving subaerial erosion and subsequent filling by coastal onlap. Such a model requires base‐level changes of the order of 1 km, in order to account for observed canyon cutting and filling. Vertical movements associated with halokinesis, or thermally‐induced uplift of the order of 1 km, could have resulted in the observed erosional events. Alternatively, a Messinian‐style evaporitic lowering of base‐level is currently receiving serious attention. With present knowledge this mechanism most satisfactorily explai

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01746.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Cretaceous of west James Ross Island, Antarctica represents the proximal fill of a late Mesozoic back‐arc basin that was probably initiated by oblique extension during the early development of the Weddell Sea. The succession records sedimentation in two contrasting depositional systems: a laterally persistent slope apron flanking the faulted basin margin interrupted both spatially and temporally by coarse‐grained submarine fans. Slope apron deposits are dominated by thinly interbedded turbiditic sandstones and mudstones (mudstone association), interspersed with non‐channelized chaotic boulder beds, intraformational slump sheets and isolated exotic blocks representing a spectrum of mass‐flow processes from debris flow to submarine gliding. Localized sand‐rich sequences (sandstone‐breccia association) represent sandy debris lobes at the mouths of active slope chutes. The submarine fan sediments (conglomerate association) are typified by coarse conglomerates and pebbly sandstones, interpreted as the deposits of high‐density turbidity currents and non‐cohesive debris flows. Three assemblages are recognized and are suggested to represent components of the inner channelled zone of coarse‐grained submarine fans, from major fan channels through ephemeral, marginal channels or terraces to levee or interchannel environments.The occurrence of both slope apron and submarine fan depositional systems during the Early and Mid‐Cretaceous is attributed to localized input of coarse arc‐derived sediment along a tectonically active basin margin. Periods of extensive fan development were probably linked to regional tectonic uplift and rejuvenation of the arc source region; cyclicity within individual fan sequences is attributed to migration or switching of fan channels or canyons. Slope apron sedimentation was controlled largely by intrabasinal tectonics. Local unconformities and packets of amalgamated slide sheets and debris flow deposits probably reflect episodic movement on basin margin faults. Differential subsidence across the basin margin anchored the basin slope for at least 20 Myr and precluded basinward progradation of shallo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01747.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTIntense post‐depositional alteration has profoundly affected sandstones in the volcanic portions of Early Archaean (3·5–3·3 Ga) greenstone belts. The mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism, but grain textures are commonly well preserved. Consequently, microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes.Silicified Early Archaean volcaniclastic sandstones assigned to the Panorama Formation and Duffer Formation, Warrawoona Group, eastern Pilbara Block, Western Australia, were originally composed of volcanic (VRF) and sedimentary (SRF) rock fragments, volcanic quartz, feldspar, traces of ferromagnesian minerals and pumice. Only volcanic megaquartz remained stable during alteration. All other primary components were replaced by granular microcrystalline quartz (GMC) and sericite. In most areas, the sandstones were composed of dacitic to rhyolitic VRFs, now totally replaced by sericite‐poor GMC and recognized by preserved microporphyritic textures. In a few areas, quartz‐poor dacitic to andesitic(?) VRFs dominated the detrital assemblage. Minor SRFs and mafic VRFs, now replaced by GMC, are recognized on the basis of colour, internal structures, and internal textures, including skeletal, possible spinifex textures. Detrital feldspar is represented by blocky, sericite‐rich grain pseudomorphs.A semi‐quantitative point‐count scheme, developed for the analysis of heavily altered sandstones, indicates the following primary detrital‐mode ranges for Panorama arenites: quartz, 0–28%; feldspar, 0–28%, VRFs, 58–86%, and SRFs 0–25%. In about half the point‐counted samples, feldspar could not be distinguished from rock fragments. In such cases, both were counted as one grain type, Lv', which makes up from 84 to 100% of the framework modes of these rocks.These sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments, but locally including minor mafic, ultramafic, and sedimentary rocks. Much, but not all, of the felsic volcaniclastic sand represents reworked pyroclastic debris. There is no evidence for contributions from plutonic or metamorphic sources. The Panorama modal assemblage represents a provenance that is lithologically more restricted than that of Archaean greywackes and other siliciclastic units common in the sedimentary portions of these same Early Archaean greenstone belts and younger gr

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01748.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Suwolbong pyroclastic sequence in the western part of Cheju Island, Korea, comprises partly preserved rim beds of a Quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline. The sequence consists of breccia, lapillistone, lapilli tuff and tuff. Eighteen sedimentary facies are established and organized into six lateral facies sequences (LFS) and seven vertical facies sequences (VFS).The LFS 1, 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar‐bedded (LFS 1), undulatory‐bedded (LFS 4) or climbing dune‐bedded (LFS 5) (lapilli) tuff units. They are representative of relatively ‘dry’ base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting. The LFS 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent. It results from relatively ‘wet’ base surge in which sorting is poor due to the cohesion of damp ash. The LFS 3 comprises well‐sorted lapilli tuff and stratified tuff further downcurrent, suggestive of deposition from combined fall and surge of relatively ‘dry’ hydroclastic eruption. All seven vertical facies sequences generally comprise two facies units of coarse‐grained fines‐depleted lapilli tuff and an overlying fine‐grained tuff. These sequences are suggestive of deposition from base surge that consists of a turbulent head and a low‐concentration tail.Depositional processes in the Suwolbong tuff ring were dominated by a relatively ‘dry’ base surge. The base surge comprises turbulent and high‐concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport. As the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air, it develops large‐scale turbulent eddies and is segregated into coarse‐grained bedload and overlying fine‐grained suspension forming thinly stratified units. Further downcurrent, the base surge may be either cooled and deflated or pushed up into the air, depending on its temperature. The Suwolbong tuff ring comprises an overall wet‐to‐dry cycle with several dry‐to‐wet cycles in it, suggestive of overall decrease in abundance of external

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01749.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTDuring Early Jurassic time, the Scots Bay Formation accumulated in an aerobic lake on the floor of the subtropical Fundy rift valley. A diverse biota flourished in the lake as evidenced by algal stromatolites, oncolites, charophytes, ostracods, gastropods, conchostracans, fish bones, calcispheres and logs. Carbonate wackestone and packstone were deposited in the littoral zone and lime mudstone accumulated offshore on a proximal slope. Silica‐rich hydrothermal springs and seeps around and on the floor of the lake precipitated siliceous tufa and silicified adjacent carbonate strata. The tufa is characterized by the progressive sequence: (1) open‐framework, porous substrate of goethite spheres, (2) encrusting chalcedony spherulites where goethite inclusions decrease in size and number outwards towards the surfaces of the spherulites, (3) a fringe of quartz on the chalcedony spherulites and (4) coarsening inwards mosaics of quartz that filled the remaining voids. Carbonate diagenesis included conversion of low‐Mg calcite ooze to micrite and microspar‐pseudospar, and local dissolution and reprecipitation of calcite cement. At times the lake shrank in size, concentrating silica in increasingly alkaline lake water which then silicified some of the carbonate strata and logs. The latest cements were calcite, zeolites and cel

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01750.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTIn the Deer Lake Basin of western Newfoundland, Canada, analcime has been found within fine‐grained, siliciclastic and carbonate, lake and lake‐margin sediments of the Carboniferous Rocky Brook Formation. Analcime is the only zeolite observed in these unmetamorphosed and non‐volcanogenic rocks. Microscopic analcime occurs as vug‐filling, limpid, isotropic to anisotropic crystals. Microprobe analyses show this type of analcime to be lower in silica than most analcime in sedimentary rocks. A second textural type of finer‐grained submicroscopic analcime can be detected from X‐ray diffraction spectra of Rocky Brook Formation mudstones, where it is associated with phyllosilicates (illite, interstratified chlorite/smectite, smectite, chlorite), other silicates (quartz, feldspar) and carbonates (dolomite, calcite). Results of this study suggest that the analcime in the Rocky Brook Formation formed either by direct lakewater/porewater precipitation or by reaction of these waters with one or more clay mineral types or

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01751.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Rosebrook Member of the Miocene Bunyan Formation is a highly kaolinitic clay‐rock of lacustrine origin. It is ultra‐fine grained, plastic and of high cation exchange capacity (CEC) of 20–33 c mol(+)kg−1of clay. Fabrics observed optically and by transmission electron microscopy (TEM) indicate significant post‐depositional consolidation and localized shear failure. Sedimentation and TEM analyses confirm its very fine grain size which is consistent with the relatively high CEC.Tertiary soils and weathering profiles in the catchment of Lake Bunyan developed under relatively moist and cool conditions. Some were gibbsitic with kaolinite and smectite clay minerals. Highly weathered soils (oxisols) on basalt under modern high rainfall environments in the region are dominated by gibbsite and kaolinite. These and other highly weathered soils in the region, formed on a range of parent rocks, have high proportions of very fine grained kaolinite in their subsoils. Such materials are likely to have been abundant in the catchment of Lake Bunyan during the Miocene and would have contributed very fine grained kaolinite directly to the lake by sedimentation. However, the unusually high proportion of kaolinite in the Rosebrook Member and its euhedral crystal form suggest that some of its features developed diage

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01752.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY