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Fades Models 3. Sandy Fluvial Systems

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Fades Models 3. Sandy Fluvial Systems Geosclence Canada. Volume 3. Number 2. May. 1976 101 hydrocarbon reservoirs, themeandering builds laterally and downstream across systems depositing elongate shoestring the flood plain. sands stratigraphically bounded by The channel floor commonly has a shales, and the sandy braided systems coarse "lag" deposa of material that the forming thicker and laterally more river can only move at peak flood time. extensive sand bodies. This material would include the gravelly component of the clastic load, together handerlng Systems with water-logged plant material and The main elementsof a modern partly consolidated blocks of mud meandering system lexemplified by the eroded locallvfrom the channel wall. ~ississi~~ior~razoi exi is) ~iveis] Above the lag, sand is transported Fades Models are shown in Figure 1. Sandy deposition through the system as bedload. During IS normally restrlctea to the maln average discharge, tne typlcal bedlorm 3. Sandy Fluvial channel or to oart~alv or Comoletelv on the cnannel floor cons~sls~ ~ ~ ol slnbo~s-~ ~~~~ abandoned meander'loops; deposition crested dunes (Fig. I)ranging in height systems of fines (silt and clay) occurs on levees from about 30 cm to one metre. and in flood basins. It is surprising that Preservation of these dunes results in there are so few integrated studies of trough cross-stratification. In shallower Roger G. Walker modern meandering systems in the parts of the flow, higher on the point bar. Department of Geology laeralure of the last twenty years. The the bedform is commonly ripples McMaster University most important papers include those of (preserved as trough cross lamination; Hamilton, Ontario LBS 4Ml Sundborg (1956; River Klaralven). Fig. 1). As a broad generalization, we harms et a1 (1963. Red River). Bernara may propose that the preserve0 lntmductlon et a1 i 1970: Brazos River). McGowan oeoos~tsof the act ve~~- channel - w~lloass This article will follow the pattern ?-~-- and darner' [t 970; colorado (Texas) from trough cross-bedded coarsesands established previously in thisseries. I will and Amite Rivers] and Jackson (in to small scale, trough cross-laminated describe in general terms the press; Wabash River). Although the fine sands upward (Fig. 1). depositional environments of modern meandering river model is well The development of a plane bed fluvial systems, and attempt to formulate established. its basis is somewhat dated, (without riooles or dunes)~ ~~ is favoured bv a general model. I will then show how the , ~ ~ ~~ -- ~, Further integrated work (not just on one higher ve~bcitiesand shallower depths. model acts, 1 ) as a norm for purposesof point bar) would seem important. and deposition on the plane bed results comparison. 2) as a framework and in horizontal lamination. The particular guide for future observations, 3) as a a) The Main Channel Meandering in the combinations of depth and velocity prediclor in new geological situations. channel is maintained by erosion on the required to produce plane bed can and 4) as a basis for hydrodynamfc outer banks of meander loops, and occur at various river stages, and hence fnferprelalion. deposition on the inner parts of the parallel lamination can be formed both For reasons of space, I will consider loops. The main depositional lowand high on the point bar. It can here only the sandy systems. Gravelly environment isthe point bar, which therefore be preserved interbedded with and bouldery systems are less well understood, although their deposits are abundant in Canada and locally are economically important (e.g., the uraniterous conglomerates in the Elliot Lake area are generally considered to be fluvial). Sandy rivers can be straight, meandering or braided, although natural straight rivers are very uncommon. There would appear to be a spectrum of types from meandering to braided: meandering systems are fairly well understood, and sandy braided systems less well understood. At the moment R Seems best toconsiderthem separately, and use two facies models Comparison ol new situations with the meandering Flgum 1 norm and the braided norm should help Block djagram showrng major morphological point bar - the dunes and ripples in the toestablish the range of variation elements 01 a meanderingriver system. channel give rise to trough cross-bedding and ripple cross-lammation, respectively between the two types of system. Erosron on the oulsrde bendofmeander loops leads lo lateral accretion on the opposile (inset, lower right), which are preservedm a Sandy fluvial deposits are known from lining-upwardsequence. See text for detarls. all geological systems. Archean to recent. They form important trough cross-bedding, or small Scale I I trough cross-lamination (Figs. 2.3). BATTERY POINT The preservation of all of these WARY SEQUENCE $ 'MEANDERING" leatures is basically due to the lateral 8 FINING-UPWARD and downstream accretion of the point SEOUENCE bars. Channel-floor dunes may be driven slightly diagonally onto the lower I partsof the point bar, thus helping the BAR point bar to accrete laterally on topof the TOP* channel-floor lag. The upper part of the point bar may be composed of a series of ridges and swales (Fig. I),the swales acting to funnel flood waters across the point bar wlth a much straighter thatweg i POINT BAR lhan the normal low-stage meandering TOP thalweg. If coarse bed load is funnelled lhrough theswalesat high flow, itmay be oeposlted at the downstream end n rne I formof chL1e oars" lF~a1 McGowan POINT BAR and Garner, 1970).~heiechutebarsact to lengthen the point bar in the I downstream direction, hence constricting the llow and causing CHANNEL CHANNEL FLOOR increased erosion immediately FLOOR downstream. Pure meandering streams rarely have ngum 2 exposed bars in the middle of their Comparison of a summary sequence of from data m Allen, 1970 lmpl~cal~onsof the channels, and hence the sandy active- facies for the Devonjan Baltery Porn1 comparjson are d~scussed~nthe text Letters channel deposits can all be termed Sandstone(Gasp6, Quebec: Can1 and besrde 1heBafleryPornt sequence refer to the LATERAL ACCRETION deposits, Walker, 1976) with the meandering river facres 01 Can1 and Walker (1976) V A related to lateral migration of point bars model. The model is redrawn to correct scale rndrcates vertlcal accrellon (Fig. 1). c) Verlical Accrelion Deposits. Outside During rising flood stage,the levees b) Channel Abandonmenl. Meander the main channel, deposition in the flood can be breached, causing the formation loops can be abandoned gradually basins, ox-bows and levees takes place of a "crevasse-splay" (Fig. 1) - a wedge (chutecut-off)orsuddenly(neckcut-Off) by addition of sediment during flood of sediment suddenly washed into the (Allen, 1965, p. 118-9.156). During stage when the river overtops its banks flood basin, and commonly containing chute cut-off, Ihe river gradually re- (Fig. 1). In contrast lo the lateral some of the coarse bedload portion of occupies an old swale, and accretion within the main channel. the river sediment. The crevasse splay simultaneously flow gradually overbank deposition causes upbuilding deposit may resemble a classical decreases in the main channel. Gradual ofthe flood plain, hence the term turbidite in having a sharp base, overall abandonment thus results in gradual VERTICAL ACCRETION. Near to the graded bedding. and a sequenceof flowdecrease, and this could be main channel, where the flood walers sedimentary structures indicative Of reflected in the sediments by the sweep along as a stream, the vertical decreasing flow during deposition. development 01 a thick sequence of low- accretion deposits tend to be silty, and The only other deposits that may flow sedimentary structures - are commonly cross-laminated. Farther rarely be preserved as part of the vertical essentially ripple cross-lamination (Fig. from the river, flood waters may stagnate accretion sequence are windblown, and 4). Aner complete abandonment. and only mud is deposited. Alter retreat may be either loess, or coarser sandy forming an ox-bow lake, sedimentation of the flood, the mud and silt commonly deposits blown in as large dunes. would be restricted to fines (silt. mud) dry out, and dessication cracks are introduced into the ox-bow during formed. The flood basins and levees of d) Meandering River Facies Sequence. overbank flooding from the main most river systems (post-Silurian) tend The distillation of observations from a stream (Fig. 1). to be abundantly vegetated, and hence large number of modern meandering Neck cut-off involvesthe breaching01 the deposits contain root traces (Fig. 5). streams. and from many ancient a neck between two meanders, and the In semi-arid or arid environments, the formations interpreted as meandering- sudden cut-olf of an entire meander fluctuating water table and drying at the fluvial, allows a general facies sequence loop. Both the entrance to and exit from surface favour the formation of caliche- to be formulated. One version of this the loop tend to be rapidly plugged with like nodules within the vertical accretion sequence is shown in Figure 2; it was sand. Flow diminishes to zero very deposits. distilled by Allen (1970) and is redrawn quickly and the resulting sequence of to scale here. In its simplest form. the deposits is dominated by later, flood- sequence is FINING-UPWARD and ~ntroducedsilts and muds (Fig. 4). Geoscience Canada. Volume 3. Number 2.May. 1976 103 consists of in-channel deposits (lateral accretion), followed by overbank tines (vertical accretion) (Figs. 6.7). In this particular sequence, the facies relationships were determined statistically tor a large number of Devonian outcrops in Britain and North America, but application of the model has demonstrated that it can be used appropriately in many other areas. The lag deposits are overlain by trough cross-bedding. which is in turn overlain by small scale trough cross-lamination. Horizontal lamination can occur at several places within this sequence (Fig. 2). depending on the river stage at the time when the depthlvelocity criteria for plane bed were met.
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