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Facies Models 2. Turbidites and Associated Coarse Clastic Deposits

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Facies Models 2. Turbidites and Associated Coarse Clastic Deposits GeoscienceCanada, Volume 3, Number 1, Februi currents were known in lakes and thousands have been measured and reservoirs,and they appeared to be used to reconstruct paleoflow competent to transport sediment for patterns in hundreds of turbidiie fairly long distances. Many of these basins. different lines of evidence were pulled 3) Within the graded sandstone beds. together by Kuenen and Migliorini in many different sedimentary 1950 when they publishedtheir structures were recorded. By thelate experimental results in a now classic 1950s, some authors were proposing paper on "Turbidity currents as a cause turbidite models, or ideal turbidiies. of graded bedding". A full review of why based upon a generalization of these and how the concept was established in sedimentary structures and the Facies Models geology has recently been published sequence in which they occurred. (Walker. 1973). This generalization is akin tothe 2. Turbidites and After its introduction in 1950, the distillation process discussed in the turbidity current interpretation was previous paper, and the final Associated applied to rocks of many different ages. distillation and publication of the Coarse Clastic in many different places. Emphasis was presently accepted model was done laid upon describing a vast and new by Arnold Bouma in 1962. A version Deposits assemblage of sedimentary structures. of the Bouma model is shown in and using those structures to interpret Figure 1 paleocurrent directions. In the absence Roger G. Walker of a turbidite lacies model (see pevious Th. BOUMT~rbldll. hCi08 MOdd Department of Geology article in this issue of Geoscience The Bouma sequence, or model(Figs. 1, McMaster University Canada),there was no norm with which 2) can be considered as a very simple Hamilfon, Ontar10L8S 4M7 lo compare individual examples, no facies model that effectively carries out framework for organizing observations. all ol the four functions of facies models no logical basis for prediction in new discussed in the previous article. Iwill lntroduetlon situations, and no basis for a consistent examine these in turn, bdh toshed light To the sedimentologist. Me turbidity hydrodynamic interpretation.Yet upon turbidites in general, and to use current concept is both simple and gradually duringthe years 1950-1960, a turbidites as an illustration of a facies elegant. Each turb~dite(defined as the relatively small but consistent set of model in operation. I have described the deposit of aturbidity current) is the result sedimentary features began to be model as very simple because it of a single, short lived event, and once associated with turbidites. These are contains relatively few descriptive deposited, it is extremely unlikely to be considered in the following list, and can elements, and because it is narrowly rewciked by other currents. The now be taken as a set of descriptors for focussed upon sandy and siltyturbidites concept is elegant because it allows the classical turbidites: only. I shall later refer to these as interpretation of thousands of graded 1 ) Sandstone beds had abrupt, sharp "classical" turbidites. sandstone beds, alternating with shales. bases, and tended to grade upward as the result of a series of similar events. into finer sand. silt and mud. Some of I. The Bouma modelas a NORM. The and it can safely be stated that nosimilar the mud was introduced into the model (Fig. 1 ) as defined by Bouma volume of clastic rock can be interpreted basin by the turbidity current (it consists of five divisions. A-E, which so simply. conta~nedshallow benthonic occur in a fixed sequence. Bouma did In this review. I will begin by studying forams), but the uppermost very fine not give normalized thicknesses for the the "classical" turbidite, and will then mud contained bathyal or abyssal divisions, and this type ol information is gradually broaden the scale to benthonic forams and represented still unavailable. In Figure 3,l have encompass turbidites and related the constant slow rain of mud onto sketched three individual turbidites coarse clastic rocks in their typical the ocean floor. which clearly contain some of the depositional environments - deep sea 2) On the undersurface (sole) of the elements of the Bouma model, yet which fans and abyssal plains. sandstones there were abundant obviously differ from the norm. They can The concept of turbidites was markings, now classified into three be characterized as AE, BCE and CE introduced to the geological profession types: tool marks, carved into the beds. Without the model. we could ask in 1950. At that time, nobody had underlying mud by rigid tools (sticks. no more questions about these three observed a modern turbidity current in stones) in the turbidity current; scour turbidites, but with the norm, we can ask the ocean, yet the evidence for density marks, cut into the underlying mud by why certain divisions of the sequence currents had become overwhelming. fluid scour: and organic markings - are missing. I will try and answer this The concept accounted for graded trails and burrows - filled in by the rhetorical question later. sandstone beds that lacked evidence of turbidity current and thus peserved shallow water reworking, and it on the sole. The tool and scour 2. The Bouma model as a framework accounted for transported shallow water markings give an accurate indication andguide lor description. The model has forams in the sandstones, yet bathyal or of local flow directions of theturbidity served as the basis for description in a abyssal benthonic forams in currents, and by now, many large number of studies, particularly in interbedded shales. Low density Canada, U.S.A. and Italy. With the BOUMA DIVISIONS INTERPRETATION ................ :... .........:.......I.. FINES IN TURBIDITY CURRENT, FOLLOWED BY PELAGIC SEDIMENTS --', 3 p-' LOWER ....................... TRACTION IN FLOW REGIME ....................... ............................................. UPPER T: 2 RAPID DEPOSITION, ? QUICK BED I Figure 1 lo ernphasfze that alln wealhered or fecton~zed Five drnsions of the Bourna model for OutcroDs ~tcannot be separared from E- ru,o orcs A-qrzoeo or mass .e wnosrone pe r c r , son [~<lrl, orpos teo 0,me Gigure 3 R-pa*a e 3mn~reosan!lsto~r C-, jp Y rum or, c,rren p~!,npm or nq r Hypothetical sequence oflhree lurbrdrles. cross~larninaredline sandstone: (DJ-fain1 lnterpretatlons of deposrtionalprocess are drsciihed as AE BCE and CE m the Bourna parallel lam~natronsof sill andrnud, bracketed grouped rnlo lhree main phases, see text model See text such quantities and at such a rate that water is forcibly expelled upward, and momentarily. the grainlwater mixture becomes fluidized (or "quick"). The flu~dizationwould destroy any possible sedimentary structures. The second phase of deposttion involves traction of grains on the bed. Flow velocities are lower, and the rate of depositton from suspension is much lower. By direct comparison wlth many experimental studies, division B represents the upper flow regime plane bed, and dtvlsion C, the lower flow regtme rippled bed. The thtrd phase of deposition lnvolves slow deposition of fines from the tail of the current. The origin of the delicate laminations in division (0)is not understood, and I preferto placedivlsion Figure 2 (D)In brackets, tmplyingthat in all butthe Com~lele'Bourna"twbldrle (see FIO 11 C D,nsrons fDI and E were broken off thrs -. , , cleanest outcrops. (D) cannot be shoi,ngpelitjc d~wsonE ofloierbed(bottorn speornen, whfch is from the Care Frechette leftl: oraded drvision A, oarallel larnrnaled roadcul. Levs Forrnatlon lCarnbrian1. separated from E. In the uppermost part drvrsron B and npple cross larnrnated drvrsron Quebec of divlsion E, there may be some true pelagic mudstone with a deep water (bathyal or abyssal) benthonic fauna frameuorn provtdea oy the mode one 3 Tnr.moor1 ;IS 3 bass 101 (forams in Tert~aryand younger rocks). rt!crod,nnrn r ir:rrr[~rttctl!u,lTne can aJlcK v loo a seawnce of t~rb~ o~ tes as AEIBCEICE etc.(as in the three existence of the Bouma model enables 4. The Bouma model as a predictor. turbidites of Fig. 3), and then add to the us to make one integrated interpretation Here, I shall show how the basic description any other features of of classical turbidites,rather than having hydrodynamic interpretation of the note. With the model as a framework, to propose different origins for each model, togetherwlth departures from the one is not only aware of the features different type of bed In Figure I,the norm, can be used on a predtctive basis. presented by any bed, but is also aware interpretation is considered in three Turbldile 1 (Fig. 3) begins with a thick of any features embodied in the model parts, Division A contains no sandy d~vision(A), and was deposited but missing in a particular bed. sedimentary structures except graded from a high velocity current. Turbldite 2 bedding. It represents very rapid settling (Fig. 3),by comparison with the norm. of grains from suspension, possibly in Geosciencecanada. Volume3, Number 1. February, 1976 27 does not contain division A. It begins with Bourna divlsion B, and was presumably deposited from a slower current. Turbidite 3 (Fig. 3) lacks divisions A and B. and presumably was deposited from an even slower current. In a caullous way, we can now make some predict~onsbased upon comparison wilh the norm, and uponthe hydrodynamic interprelations. A sequence of many tens of turbidites in which all of the beds are thick and begin with division A (Fig. 4, and, for example, the Cambrian Charny Sandstones in the St. Romuald road cut near Levis. Quebec) probably represents an environment where all of the turbidity currents were fast-flowing during deposition. Such an environment was probably close tothe source of the turbidity currents (proximal). By contrast (Fig. 5). a sequence of many tens of beds In which a1 the tdrolo tes oeg n ellher w~tnd~vlslon B or C rOroov~can Ut ca Formation at Montmorency Falls.
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