4-河流沉积体系.ppt

FLUVIAL SYSTEM（河流沉积体系）TERMINOLOGY(1)Fluvial:used for anything associated with rivers;Channel:confined to depressions or scours in land surface which contain the flows;Overbank or floodplain:area of land between or beyond channels,receiving water only when river is in flood;Catchment:area of ground that supplies water to a river systemDischarge:volume of water flowing in the river in a time period;Thalweg:the line of the deepest part of the channelTERMINOLOGY(2)Levees:ridges built on either side of a channel but commonly better developed on the outer margins of bends;mainly fine-grained sand and silts dominated by ripple cross-laminations and small-scale cross-bedding.Some levees are actually the products of laterally coalescing splay lobes.Crevasse splays:discrete lobes of silty or sandy sediments,extending down the levee from shallow crevasses on the crest and spread on to the floodplain.Braidplains:regions where braided rivers repeatedly change position on the alluvial plain,a broad,leading to an extensive region of gravelly bar deposits many times wider than the river channel.Chute channel:a short-cut over the top of a point bar during period of high stage flow.Main morphological features of meandering river Classical point-bar model in meandering river channel TERMINOLOGY(3)Avulsion:a process by which old river course is completely abandoned and a new channel is scoured into the land surface.Oxbow lakes are an example of abandonment of a section of river channel.Lateral migration:sideway growth of inner banks of rivers due to deposition at the expense of erosion at the outer banks,forming the point bars of meandering river;Autocyclic sequence:controlled by processes taking place within the basin itself,such as migration and superposition of channel systemsAllocyclic sequence:caused by variations external to the basin,such as climatic changes,tectonic activity,and sea-level fluctuations;River Types(1)(in terms of channel sinuosity)Sinuosity:calculated by dividing the distance between two points measured along a channel by the straight-line distance between those points in a downstream direction;Meandering river:having a high sinuosity(1.5),and characteristically possessing bends that change shapes with time by erosion on one side and deposition on the other.Braided river:having a low sinuosity(1.1 1.2),and possessing a variety of bars;Straight river:having a low sinuosity,and a single channel with dividing bars.Anastomosing river:consisting a number of separate channels that divide and join along the river.Forms of river channel Evolution of depositional fluvial system River Types(2)(in terms of mode of sediment transport in the channel)Bed-load fluvial system:typically have steep gradient,high width/depth ratio(40),channel patterns of low sinuosity;Mixed-load fluvial system:preserve a higher percentage of floodplain deposits consisting of silts,mud and backswamp carbonaceous mud and clays.Channels are more stable,have a low width/depth ratio than bed-load system,filled mainly with sands and minor proportion of silt and clays,and flanked by levees sand-silt and crevasse-splay sand;Suspended-load fluvial system:characterized by high-sinuosity single channels of great stability and low width/depth ratio(10).Channel fills contain high proportion of silts and muds,with well-developed silty or muddy levee deposits and overbank fines;River Types(3)(in terms of mode of discharge)Ephemeral streams:dry for long period of time and only experience flow when there is rain in catchment area;Perennial streams:features by consistent flow,especially in more humid regionsPatterns of riversTributary drainage system:small streams converge to form large trunk channel,efficient at carrying sediments;Distributary drainage system:splitting,bifurcating,or branching into smaller rivers in a downstream direction,less efficient at carrying sediments.Channels become smaller,transmit less water,and bedload tends to be deposited Tributary versus distributary river patternsand downstream change of river system A variety of BARSLongitudinal bars(also called as mid-channel bars):Elongate along channel axis,and evolved from transverse bars;Transverse bars(also termed as cross-channel bars):Wider(than they are long)or across the channel;Linguoid bars:Crescentic,with their apex pointing downstream;Bank-attached bars(also called alternate bars):Occur at channel bends;Point bars:Present at inner bank and formed by lateral-accretion deposition;Scroll bars:Elongate sand ridges on point bar,broadly parallel to the contour,and originating low on point bar and migrate upwards until they reach bank-full level;Meandering Rivers(mixed-load river)Main facies associations of meandering river Idealized model of lateral accretion cross bedding Evolutionof channelavulsionfrom acrevasseevent Crevasse-splay depositsModel of Meandering river systemBraided Rivers(bed-load river)Main morphological features of a braided river Composite facies model for sandy braided riverDepositional architecture of a braided riverlateral migration of channel and abandonment of barsled to the build-up of channel-fill successionModel for a pebbly braided river dominated by longitudinal and transverse bars,with some bars developing within-channel lateral accretion Large-scale bedforms in fluvial channelModel of braided river systemAnastomosing rivers(suspended-load river)Anastomosing riverchannel patternsChannel-fill deposits of an ancient anastomosing river system(Nadon,1994)Model of anastomosing river system“ARCHITECTURE ELEMENT”ANALYSISAn approach to study large-scale geometry and internal organization of fluvial systemsBasic architectural elements of fluvial depositstwo mainFluvialArchitecturalElements(CH&DA)defined byaccretional style,and type&scaleof associated boundingsurfaceThree separate channel-fill successionIf exposure is limited,it is difficult to distinguish them.低曲度河流水道的砂体几何形态和内部组成低曲度河流水道的砂体几何形态和内部组成Typical channel conglomerate faciesTypical channel sandstone depositional facies控制水道砂体连接控制水道砂体连接的主要因素的主要因素Main facies in fluvial sediments Ancient Fluvial SystemAncient fluvial lithofaciesCoarser sediments occurring in channel;Finer materials deposited on floodplain;Channel LithofaciesCross-bedded conglomerate:representing bar deposits in gravelly braided river channels;Thin layers of coarse debris lying on erosional scours:representing lag deposits at the bottom of the river channel;Cross-bedded sandstone:formed by subaqueous dune migration on bars in sandy braided river or on lower parts of point bars of meandering rivers;Massive and liquefied sandstone:resulting from collapse of river bank materials into channel;bar deposits in gravelly braided river channelsbar deposits in gravelly braided river channelsBar deposits in gravelly braided river channelsImbrication of GravelsConglomerate in gravelly braided river channelsChannel fill of gravelly braided riverBar deposits in gravelly braided river channelsBar deposits in sandy braided river channelsChannel fill of gravelly braided riverChannel fill of braided riverChannel fill of braided riverChannel fill of gravelly braided riverLag deposits and channel fill of meandering riverBar depositsBank collapse-related depositsBank collapse-related depositsof a meandering riverLateral accretion of a point barLateral accretion of a point barAbandoned channel(Oxbow lake deposits)Floodplain deposits1.Thin beds normally graded from sand to mud;2.Evidence from initial rapid flow(plane-parallel lamination)quickly waning and accompanied by rapid deposition(climbing ripple lamination);3.Thin sheets of sediment,often only a few centimeters thick but extending for hundreds of meters,even kilometers,away from the channels;4.Erosion at the base of sandstone beds,normally localized to areas near the channel where the flow is most vigorous5.Desiccation cracks in muds at the top of the normally graded beds;6.Evidence of soil formation;Levee depositsFloodplain depositsFloodplain depositsFloodplain depositsDesiccation cracks in Floodplain settingPaleosols in floodplain settingSomething you should bear in mind in studying fluvial systemsScale of fluvial deposits:varying in size from streams only meters in width and tens of centimeters deep to rivers tens of kilometers wide and tens of meters deep;Source area control on sediment types:determined mainly by the availability of material of different size;Paleo-currents:be aware of the variations in different portions of the river system;Recommended ReadingsAllen,J.R.L.,Studies in fluviatile sedimentation:bars,bar complexes,and sandstone sheets(low-sinuosity braided stream)in the Brownstone(L.Devonian),Welsh borders:Sedimentary Geology,1983,v.33,p.237-293.Miall,A.D.,Architectural-element analysis:a new model of facies analysis applied to fluvial deposits:Earth-science Review,1985,v.22,p.261-308.Bridge,J.S.,Description and interpretation of fluvial deposits:a critical perspective:Sedimentology,1993,v.40,p.801-810.End of this session