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Journal of Palaeogeography, 2016, 5(4): 391e408 (00107)

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Lithofacies palaeogeography and sedimentology

A new ‘superassemblage’ model explainingproximal-to-distal and lateral facies changesin fluvial environments, based on theProterozoic Sanjauli Formation (LesserHimalaya, India)Ananya Mukhopadhyay a,*, Priyanka Mazumdar a,A.J. (Tom) van Loon b

a Department of Earth Sciences, Indian Institute of Engineering Science & Technology, Shibpur, Howrah,West Bengal 711103, India

b Geocom Consulting, Valle del Portet 17, 03726 Benitachell, Spain

* CorresE-mail aPeer revhttp://d2095-38(Beijing)

Abstract Facies analysis of fluvial deposits of the Proterozoic Sanjauli Formation in the Lesser Himalayawas combined with an architectural analysis. On this basis, a model was developed that may be applied toother fluvial systems as well, whether old or recent. The new model, which might be considered as anassemblage of previous models, explains lateral variations in architecture and facies but is not in all respectsconsistent with the standard fluvial models. The Sanjauli fluvial model is unique in that it deals with lateralfacies variations due to shifts of the base-level along with fluctuations in accommodation space owing tochanges in palaeoclimate.

Keywords Fluvial model, Braided river, Sanjauli Formation, Simla Basin, Proterozoic, Lesser Himalaya,India

© 2016 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of ChinaUniversity of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Received 8 December 2015; accepted 27 April 2016; available online 16 August 2016

ponding author.ddress: ananyageol@gmail.com (A. Mukhopadhyay).iew under responsibility of China University of Petroleum (Beijing).x.doi.org/10.1016/j.jop.2016.08.00136/© 2016 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of China University of Petroleum. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

392 A. Mukhopadhyay et al.

1. Introduction

Fluvial systems show significant differences. It ap-pears nevertheless that lateral facies associations cancommonly well be recognized and the geometry ofsand bodies can commonly well be explained. Thedifferences in characteristics of fluvial systemsrequire, however, different models for fluvial envi-ronments and fluvial sedimentation. It is commonlydifficult to attribute a specific fluvial system to one ofthe specific vertical facies models that were proposedby Miall (1978). Therefore, we combined a number ofhis suggested vertical profile models into a newbraided river system, and applied it to the ProterozoicSanjauli Formation (which belongs to the Simla Group)instead of trying to fit the Sanjauli characteristics inone of the existing models. The facies of the SanjauliFormation can be understood best if they are fit inwhat has been called a ‘superassemblage’ (Miall, 1994,1996), showing an evolution from the Scott-type (G11)to the Trollheim-type to the Platte-type (see Miall,1977, 1978). The formation incises the shallow-marine shales and siltstones of the underlyingChaossa Formation as a result of a changing base-leveldue to a regression.

1.1. Standard models vs. a new‘superassemblage’ model

Case histories from all over the world have madeclear that facies analysis of fluvial successions basedsolely on the study of vertical profiles can severelylimit the characterization of these depositional sys-tems, as similar types of bedform can develop in riversof different type (Allen, 1983; Bridge, 1985; Jackson II,1978; Jo and Chough, 2001; Miall, 1988, 1996).

To overcome these limitations, the main focus ofour study was an architectural analysis, with specialattention to large-scale sedimentary structures (cf.Allen, 1983; Miall, 1985, 1988, 1996). This fluvialarchitectural analysis led to the recognition of sixstandard vertical profile models, namely theTrollheim-type (G1), the Scott-type (G11), the Donjek-type (G111), the South Saskatchewan-type (S11), thePlatte-type (S11), and the Bijou Creek-type (S1) of Miall(1977, 1978). Miall (1977, 1978, 1994 and 1996) statedthat there are fluvial systems which do not fit into aparticular model. This implies that it is necessary insuch cases to compose a new model on the basis of acombination of characteristics from a number of thestandard models. After Miall (1978) had defined thevarious vertical models, we therefore felt the need to

create what he called a “superassemblage model”which would explain the proximal-to-distal changes influvial deposits. We approached the analysis of thefacies changes in the Sanjauli Formation on the basis ofMiall's considerations and therefore studied how thesediments and facies change laterally. We did so takinginto account the changes in discharge of the river andits base-level fluctuations due to changes in theclimate. By doing so, we found that the fluvial SanjauliFormation offers a unique possibility for establishingsuch a new superassemblage.

1.2. Objectives

The primary objective of the study is to recognizethe architectural elements and formulate a combineddepositional model, based on spatial/temporalchanges in lithofacies, lithofacies associations, ge-ometry of the fluvial channels and bars, and corre-lation of vertical profiles. Variations in the fluvialarchitecture were mapped for the purpose, withparticular attention to the changes in accommoda-tion space, as related to base-level changes. The datathus obtained were input for the second objective,viz. an attempt to establish a new superassemblagemodel that differs from the classical ones in such away that all sedimentary characteristics would fit init. This approach is consistent with Miall's opinionthat new models should be established in case actualfluvial deposits do not fit into one of the standardmodels.

2. Geological setting

The Simla Group was originally described as theInfra-Blaini (Kumar and Brookfield, 1987). It consti-tutes an important stratigraphic unit of the LesserHimalaya in Himachal Pradesh, India. The Simla Groupis bounded by the Chail Thrust in the north and theGiri Fault in the south (Fig. 1) (Kumar and Brookfield,1987). As defined by Srikantia and Sharma (1971), itconsists of a thick, coarsening-upward clastic suc-cession that unconformably overlies the Shali car-bonates, and that is also unconformably overlain bythe tillites of the Blaini Formation (Baliana Group).The Simla Group is divided into four formations(Table 1), viz. the Basantpur Formation (stromatoliticdolosiltstone and stromatolitic dolomudstone), theKunihar Formation (stromatolitic limestone andsandstone/siltstone heterolithic rocks), the ChaossaFormation (sandstone/siltstone/shale heterolithic

Fig. 1 Geological map of the Simla Group (modified after Kumar and Brookfield, 1987).

A new ‘superassemblage’ model explaining facies changes in fluvial environments 393

Table 1 Schematic stratigraphy of the Simla Group. Modifiedafter Kumar and Brookfield (1987).

394 A. Mukhopadhyay et al.

rocks), and the Sanjauli Formation (conglomeratesand pebbly sandstones).

2.1. Stratigraphic context

The Sanjauli Formation is the stratigraphic unitunder study here. Its lower part is built of shales,siltstones, coarse gritty sandstones and quartzites.The upper part of the Sanjauli Formation is thecoarsest and comprises conglomerates, pebbly sand-stones, proto-quartzites, and grey and purple shales.The detrital micas in the Simla Group rocks have a40Ar/39Ar age of 860 Ma (Frank et al., 2001), whichimplies a Neoproterozoic age of the Simla Group.

The earliest indications of a deltaic depositionalenvironment have been recognized in the middle partof the Basantpur Formation. The entire Simla Group isnow interpreted as a large muddy deltaic successionformed after the collapse of a shallow-water carbon-ate platform constituting the Shali Group. The sedi-mentation in the Proterozoic Simla Basin started in thenorthern part of the basin with the accumulation ofthe lower part of the Basantpur Formation. The basinshallowed gradually during deposition of the Kuniharand Chaossa Formations. The deposition of the con-glomerates and sandstones in the upper part of theSanjauli Formation marks the end of the sedimentationin the Simla Basin.

3. Methods

The facies of the rocks in the study area wereanalyzed in order to determine which facies and

facies associations are present. This outcrop-basedfacies analysis was based on characteristics of thelithology, primary sedimentary structures and ge-ometry of the lithological units. Vertical and lateralfacies relationships were studied from representa-tive logs prepared in the study area.

Architectural elements of the Sanjauli Formationwere examined in natural exposures and road cutsthroughout the area along traverses in and aroundKandaghat and from Kandaghat to Sadhupul (Fig. 2).The field data, in the form of sedimentary logs, fieldsketches, photomosaics and maps were interpreted inorder to unravel the 3-D relationships of the variousunits of the Sanjauli Formation.

4. Facies associations and architecturalelements

The fluvial system of the Sanjauli Formation showsa wide variety of lithofacies, which can be ascribed tothe development of Scott-type (G11), Trollheim-type(G1) and Platte-type (S11) fluvial deposits, resultingfrom base-level variations and consequently fromfluctuations in accommodation space. The occurrenceof massive gravel (lithofacies Gm) in combinationwith gravel showing planar cross-bedding (lithofaciesGp), trough cross-bedding (lithofacies Gt) and someintercalated sandy channel fills (lithofacies Se) ischaracteristic of Scott-type (G11) fluvial systems,whereas the conglomerate/sandstone facies assem-blage (Gm, Sh and Sl) indicates a Trollheim-type (G1).However, the abundant planar cross-beds (Sp), troughcross-beds (St), cross-stratifications (Sl) and intervalsof finely laminated siltstone or mudstone (Fl) matchwith the Platte-type (S11) vertical profile. This‘superassemblage’ model suggests a change in sedi-mentation style from a proximal braided river sub-jected to stream flows (Scott-type) and alluvial fandebris flows (Trollheim-type) into a sandy braidedriver (Platte-type). Three major facies associations(sheet-flood deposits, braided channels, floodplaindeposits) and three systems tracts (a lowstand sys-tems tract, a transgressive systems tract and a high-stand systems tract) have been recognized in theSanjauli Formation.

Fourteen lithology-based facies (Table 2) havebeen identified in the Sanjauli Formation. They aregrouped in three facies associations (FA-1, FA-2 and FA-3), based on lithology, structure, texture and archi-tectural elements.

Fig. 2 The major traverses along the Kandaghat-Chail and Kandaghat-Salogra road.

A new ‘superassemblage’ model explaining facies changes in fluvial environments 395

4.1. Braided channels (FA-1)

4.1.1. Description

The tabular sandstone bodies are gravel-dominated and range in thickness from 5.5 to 6.5 m.They show clearly erosive margins (Fig. 3). At severalintervals, sandstone bodies are fining upwards, simul-taneously decreasing in thickness. The coarse lowerparts wedge out laterally, ending against erosional,concave bounding surfaces. In some outcrops, thebasal bounding surfaces deeply incise the underlyingsediments. Disorganized, coarse conglomerates (lith-ofacies Gm), planar cross-stratified gravels (Gp),trough cross-stratified conglomerates (Gt), planarcross-stratified sandstones (Sp) and trough cross-stratified sandstones (St) are common in the lowerpart of the facies association. Convex upward and

large-scale inclined surfaces are prominent, and cut-and-fill structures are common.

4.1.2. Interpretation

Stacked tabular sand bodies with predominantlyparallel lamination typically indicate braided channels(Allen and Fielding, 2007; Bridge, 1985; Bristow andBest, 1993; Foix et al., 2013; Gibling, 2006). Thesubsequent fining-upward character and the upwarddecrease in the size of cross-stratifications suggest asteady decline in current energy and/or channeldepth.

The sandstone facies consisting of conglomeraticsandstones and medium- to coarse-grained sandstonesare interpreted as braided fluvial deposits with low-sinuosity channel bars originating from high-discharge phases that also caused fluvial incision (cf.

Table 2 Description and interpretation of sedimentary facies (after Einsele, 2000; Miall, 1985, 1996).

Faciescode

Lithofacies Description Outcrop Interpretation

Gm Disorganizedconglomerate

Massive, crudely-stratified, clast-supported conglomerate withrounded to subrounded clasts of10e40 cm, and a matrix of coarse,poorly sorted sandstone. Scale:length of the pen-cap: 4.5 cm

Non-cohesive debrisflow; bedloaddeposition as diffusegravel sheets or lagdeposits by high-energy floods

Gms Massive, matrix-supported gravel

Matrix-supported polymictconglomerates with clasts up to25 cm, mostly subangular quartz;subangular to rounded pebbles of1e2 cm thick; large clasts withineach unit seem floating. Scale:8 cm

Mass-flows depositsfromhyperconcentratedor turbulent flows

Gp Planar cross-stratified gravel

Clast-supported, planar,cross-stratified conglomerates;cobbles and granules, subroundedto rounded. Scale: 7 cm

Linguoid bar,transverse bar

Gt Trough cross-stratified gravel

Clast-supported trough cross-stratified conglomerates; cobbleand granules with imbrications;normal grading. Scale: 10 cm

Transverse bar,channel fill

396 A. Mukhopadhyay et al.

Table 2 e (continued )

Faciescode

Lithofacies Description Outcrop Interpretation

Gsh Gravel/sand-couplets

Gravel/sand couplets, stratified,but no internal stratification;clasts usually of subroundedquartz. Scale: length of thehammer- 33 cm

Sheet flow deposits

Sm Massivesandstone

Pebbly, massive medium to coarsesandstone; moderate to goodsorting. Scale: 8 cm

Rapid deposition bysediment gravityflow

Sp Planar cross-stratifiedsandstone

Very fine to coarse, planar, cross-stratified arkosic sandstone;occasionally pebbly; moderate togood sorting. Scale: 8 cm

Migration of low-relief ripples underupper-flow regime;simple bars,transversebedforms,sandwaves (lowerflow regime)

St Trough cross-stratifiedsandstone

Immature sandstone andconglomeratic sandstone withmedium- to small-scale troughcross-stratification; thickhomogeneous deposits with fewconglomeratic levels. Scale in thefigure: length of the pencil- 19 cm

Dune migration,lower flow regime

Sr Ripple cross-laminatedsandstone

Very fine to medium sandstone;symmetrical or asymmetricalripples on upper bedding surface.Scale: 5 cm

2-D or 3-D ripples;wave or currentripples

(continued on next page)

A new ‘superassemblage’ model explaining facies changes in fluvial environments 397

Table 2 e (continued )

Faciescode

Lithofacies Description Outcrop Interpretation

Se Erosional scourswith intraclasts

Crude cross-bedding. Scale: 6 cm Scour fills

Sl Sand, very fineto coarse

Low-angle cross-stratification.Scale: 8 cm; yellow arrowspointing to the cross-stratification

Scour fills

Sh Sand, very fineto very coarse oreven pebbly

Horizontal lamination, parting orstreaming lineation. Scale: 8 cm;blue arrows pointing to theparallel lamination

Planar bed flow

Fl Laminatedsandstones,siltstones andmudstones

Parallel laminated dark grey mud.Scale: length of the clinometer-10 cm

Suspension deposits,overbank orabandoned channel

398 A. Mukhopadhyay et al.

Table 2 e (continued )

Faciescode

Lithofacies Description Outcrop Interpretation

Fsm Massivemudstones

Sandy mudstone and muddysandstone with mica and raregranules; concentrations of mudclasts; some remnants oflamination visible. Scale: lengthof the clinometer-10 cm

Deposition of sand,silt and mud clastsby tractioncurrents, withpartial obliterationof sedimentarystructures bypaedogenesis

A new ‘superassemblage’ model explaining facies changes in fluvial environments 399

Allen, 1982; Blair, 1987). The conglomeratic sand-stones at the base of the channels represent channel-lag deposits within the braided channels (Olusola andAkande, 2012). The convex upward large-scale in-clined surfaces that show downstream migration areinterpreted as braid bars.

4.2. Sheet-flood deposits (FA-2)

4.2.1. Description

The buff and pinkish purple sand bodies show asheet-like shape. They are intercalated by planarcross-stratified sandstones (Sp) and massive sand-stones (Sm) (Fig. 4). Both the basal and the upperbounding surfaces are laterally persistent, sharp andparallel to each other, and the sandstones tend to showan erosional base. The bed thicknesses range from 0.4to 0.5 m. Sub-rounded quartz and feldspar grains andminor amounts of rock fragments make up the majorcomponents of the sandstones, which consist of par-ticles from coarse sand to gravel.

The sheet-like sandstone bodies are separatedfrom each other by a number of laterally non-persistent internal erosional surfaces. The domi-nating structure is horizontal lamination (Sh). Othercommon sedimentary structures are massive beds(Sm), low-angle cross-stratification (Sl), planar cross-stratification (Sp), couplets of gravel and sand (Gsh)and ripple cross-lamination (Sr).

The matrix-supported conglomerates (Gms) inthis facies association show occasionally cross-stratification (Gp, Gt). Sporadic occurrences of flaserand wavy bedding, mud-draped surfaces, partinglineation and imbricated rip-up mudstone clasts arelocally present.

4.2.2. Interpretation

The various characteristics indicate turbulent un-confined currents, and the sheet-like sandstones indi-cate intermittent high-energy currents in the form ofsheetfloods (cf. Blair and McPherson, 1994; Foix et al.,2013; Nichols and Fisher, 2007; Tooth, 1999). Compa-rable sheet flood deposits have been found as sanddeposits in extensive alluvial plains during unconfinedoverbank inundation (Kraus, 1996; Makaske, 2001;Nichols and Fisher, 2007; Therrien, 2006), sometimesrelated to short-lived braided rivers (Bell and Su�arez,1995). The laterally continuous, laminated sheet-likesandstone bodies are interpreted as sand-sheet ele-ments (LS) (Table 3). They form vertically stackedproducts of short-lived and high-energy depositionalepisodes in the much more commonly low-energyenvironment that is indicated by its gentle deposi-tional surfaces.

4.3. Floodplain deposits (FA-3)

4.3.1. Description

This lithofacies association consists of pinkish pur-ple and dark grey siltstones, pinkish purple sandstonesand dark grey mudstones (Fig. 5). The association ex-tends laterally for hundreds of meters and shows avariable thickness of 2.3e3.4 m. The sandstones areseparated by laterally persistent mudstones.

Sedimentary structures are uncommon, with fewmassive mudstones (Fsm), and only rare horizontallylaminated mudstones (Fl). Massive mudstone units(0.2e1.2 m thick) have sporadic intercalations ofsandstone and siltstone layers. These are medium- tovery fine-grained, and have a tabular shape. The

Fig. 3 Braided channel deposit near Tundla village, showing channel fills that wedged out laterally toward erosive basal concave-upbounding surfaces. The topmost figure shows badly sorted, coarse-grained conglomerates (Gm), representing lag deposits. Scale (upperoutcrop figure): 4 cm. Scale (lower outcrop figure): length of the clinometer-10 cm.

400 A. Mukhopadhyay et al.

interbedded strata show lateral continuity for severaltens of meters, with sharp, horizontal basal contacts,whereas their upper boundaries are commonlyirregular.

4.3.2. Interpretation

The grey siltstones and mudstones indicate poorlydrained, distal floodplains with standing water (cf. Jo,

Fig. 4 Sheet flood deposits along the Kandaghat-Chail road, showing (top photo) laterally persistent, sharp bounding surfaces (parallel toeach other). The buff and pinkish purple sandstone bodies clearly show sheet geometry (lower photo). Scale: length of the clinometer-10 cm.

A new ‘superassemblage’ model explaining facies changes in fluvial environments 401

2003). Themudstone units with an occasional presenceof sandy deposits are interpreted as overbank (OF)deposits (Table 3). The laminated grey mudstonesrepresent floodplain deposits formed in water-loggedbodies with reducing conditions (cf. Jo and Chough,2001; Scherer et al., 2015).

5. Vertical profile models

The idea of facies modelling was presented andworked out in studies such as those by Walker (1976)

and Miall (1978). The vertical profile model conceivedby Miall (1977, 1978) has now generally been acceptedand is considered as the banner model, even though ithas been and still is subjected to modifications. Ratherthan fitting into one of the basic vertical profilemodels of braided rivers proposed by Miall (1978), themain facies associations of the Sanjauli Formationrequire a new type, in the way of a ‘superassemblage’including the Scott-type (G11), the Trollheim-type (G1)and the Platte-type (S11) braided-river deposits.

The basal part of the Sanjauli Formationconsists mainly of Scott-type (G11) braided deposits,

Table 3 Facies associations and architectural elements of the Sanjauli Formation (modified after Miall, 1985, 1996).

Faciesassociation

Major lithofacies Grain size Vertical stacking pattern ofarchitectural elements

Architectural element

Braidedchanneldeposits

St, Sp, Sm and Sm,with minor Gm,Sh and Fsm

Pebble to cobbleconglomerate, fine-to coarse-grained sandstonewith mud clasts

Channel-fill complex(element CH)

Gms and Gm Matrix- to clast-supportedpebble to boulderconglomerate

Sediment gravityflow deposits(element SG)

Sheet flooddeposits

St, Sp and Sm,with minor Gm,Sh and Fsm

Pebble conglomerate,fine- to coarse-grainedsandstone

Lateral accretionmacroforms(element LA)

Sm, Sl, and Sr,with minor Gsh, Gp,Gt, Fl and Fsm, andvery rare Sp, St

Very fine- tomedium-grainedsandstone

Laminated sand sheets(element LS)

Mainly St and Sp Fine- to coarse-grainedsandstone

Sandy bedforms(element SB)

Floodplaindeposits

Fsm and rare Fl Mudstone and very-to fine-grained siltstone

Overbank fines(element OF)

402 A. Mukhopadhyay et al.

characterized by the predominance of gravel (Gm)with planar cross-bedding (Gp), trough cross-beds(Gt), couplets of gravel and sand (Gsh), troughcross-stratified sandstones (St), rippled sandstones(Sr) and some interbedded sandy channel fill deposits(Se).

The Scott-type (G11) deposits of the Sanjauli For-mation are topped by Trollheim-type (G1) sediments,which represent debris-flow deposits that reworked

the Scott-type (G11) deposits. Several comparableexamples have been described by Blair and McPherson(1992). The Trollheim-type is dominated by (1) parallellaminated pebbly sandstones (Sp), (2) intercalations offiner-grained sediments (Sh) suggestive of debris-flowdeposits reworked by subsequent currents, (3)massive matrix-supported gravel (Gms), (4) disorga-nized conglomerates (Gm), and (5) fining upwardsediments.

Fig. 5 Thinly laminated silt and shales characterizing well preserved floodplain deposits along the Kandaghat-Chail road. Scale: length of theclinometer-20 cm.

A new ‘superassemblage’ model explaining facies changes in fluvial environments 403

Overlying the Trollheim-type (G1) conglomeratesare coarse sandstones with abundant planar cross-beds (Sp), trough cross-beds (St), stratified sand-stones (Ss), massive mudstones (Fsm) and laminatedsandstones (Sh) which resemble Platte-type (S11)braided deposits. Interbedded with the sandstonesare finely laminated siltstones or mudstones (Fl)

that reach thicknesses of 0.5e1.2 m. The Platte-type (S11) braided deposits are characterized bycoarse sands with minor fine-grained deposits (Miall,1977) which are also similar to the deposits of theSanjauli Formation that overlies the Trollheim-type(G1) deposits. The Sanjauli sandstones build sim-ple, tabular sandstone sheets characterized by

404 A. Mukhopadhyay et al.

planar cross-beds (Sp) and minor ripples (Sr) thatresemble the Platte-type (S11) braided-river model.An exception is the thick siltstone beds in theSanjauli Formation, which are in contrast to thethinner siltstone beds of the Platte-type depositsspecified by Miall (1977).

Taking all the above into consideration, the fluvialsedimentation in the Sanjauli Formation may beconsidered as a new model in the way of the above-mentioned ‘superassemblages’, as the sedimentsshow in their extent characteristics that reflect anevolution from a Scott-type (G11) architecture to aTrollheim-type (G1) architecture to eventually aPlatte-type (S11) braided river (Fig. 6). The SanjauliFormation thus cannot well be fitted into any of thestandard models of Miall (1978) and yet contains manyfeatures which seem to fit in a combination of theScott-type, Trollheim-type and Platte-type verticalmodels of braided rivers.

6. Sequence stratigraphic architecture

Fig. 6 Schematic vertical log exhibiting lateral and vertical faciesrelationship and changes in architectural elements in the Sanjaulibraided fluvial system. The Scott-type is evolving to Trollheim-typeand Platte-type due to base-level fluctuation. Three different sys-tems tracts, viz., lowstand systems tract (LST), highstand systemstract (HST) and transgressive systems tract (TST), maximum floodingsurface (MFS), transgressive surface (TS) and a sequence boundary(SB) have been delineated.

The sequence stratigraphy of the Sanjauli Forma-tion has been established on the basis of its facies as-sociations, their vertical successions and their spatialand temporal relationships. Sequence-stratigraphicunits were distinguished in the present contributionthrough the recognition of flooding surfaces andsequence boundaries.

6.1. Sequence boundaries

The fluvial incision of the braided river deposits ofthe Sanjauli Formation into the topmost sediments ofthe Chaossa Formation (heterolithic sediments con-sisting of fines, sands and shales that represent a delta)marks a sequence boundary (SB) which has beenidentified as a type-1 unconformity.

The vertical and lateral facies changes and thedistinct differences between the Sanjauli Formationand the fine-grained top of the underlying ChaossaFormation imply that these two formations aregenetically different and must be considered as twodifferent sequences.

Three systems tracts have been recognized in theSanjauli Formation on the basis of the stacking patternand the facies tendency (Figs. 6 and 7). They owe theirpresence to base-level fluctuations in the fluvial sys-tem. Development of the systems tracts indicateschanges in the discharge pattern, which is reflected inthe facies associations.

Fig. 7 Schematic diagram of the Sanjauli braided-river system, showing vertical and lateral facies variations and changes in architecturalelements within the different systems tracts (LST, HST, TST).

A new ‘superassemblage’ model explaining facies changes in fluvial environments 405

6.1.1. Lowstand systems tract

The development of the Sanjauli Formation startedwith fluvial incision in response to a falling base-level.The fluvial incision (type-1 unconformity) may havebeen initiated by a decrease in accommodation space,resulting in the emergence of a lowstand systems tract(LST). The subsequent evolution of the LST in thecourse of the fall in base-level indicates a culminationof the river incision, which was followed by piling up ofsediments, building a lowstand wedge (i.e. progradingfluvial sediments overlying the Chaossa delta andonlapping the unconformity). The LST deposit ischaracterized by channel infillings, which ended withthe initiation of a transgression surface.

6.1.2. Transgressive systems tract

Subsequent to the maximum phase of the lowstandregression, initial flooding phases triggered a trans-gressive systems tract (TST). The braided river de-posits of the Sanjauli Formation pass upwards intomedium- and fine-grained channel sandstones withcross-stratification, double mud drapes and bidirec-tional current ripples.

These strata with clear tidal control are erosionallyoverlain by a thin pebble and cobble conglomerate,

interpreted as a transgression conglomerate (Figs. 6and 7). The persistent character of the transgressionover the pebble and cobble conglomerate is indicatedby an upwards-thinning succession of cross-stratifiedbeds, which eventually passed into thin dark shalebeds that indicate a condensed succession represent-ing the maximum flooding surface (Figs. 6 and 7).

Amalgamated channel deposits pass upwardsinto numerous isolated channel sandstones withincreasing numbers of fining-upward sequences oftrough cross-bedded to ripple-laminated sandstones.The increase in the amount of fine-grained overbankmaterial, as well as the sedimentary structures andthe vertical diminishing of the frequency of channelamalgamation imply tidal influence with increasingaccommodation space and elevation of the base-level.

6.1.3. Highstand systems tract

Gradual slowing down of the relative base-levelrise and a diminishing accommodation space(although continued subsidence still created someaccommodation) initiated highstand systems tract(HST) deposits. When the accommodation spacereached its minimum level, the river spread and shif-ted through the floodplains, resulting in reworking of

406 A. Mukhopadhyay et al.

sediments and in lateral accretion, accompanied byminor fine-grained vertical accretion.

The change from amalgamated to isolated channeldeposits and the accompanying increase in preservedfine-grained sediments record the transition from atransgressive to a highstand systems tract. The HST ischaracterized by a progressive increase in the volumeof overbank sediments.

7. Discussion and conclusions

The standard fluvial facies model of Miall (1977)has long been advocated as the standard in the clas-sification of braided river facies assemblages. It can bededuced from the sedimentary record of the SanjauliFormation, however, that the fluvial deposits of thisformation show characteristics that fit only in a com-bination of Scott-type, Trollheim-type and Platte-typebraided rivers. A new ‘superassemblage’ model istherefore proposed, after taking the following char-acteristics into consideration.

1) Scott-type (G11) deposits are characterized by apredominance of gravel (Gm) with planar cross-beds (Gp), trough cross-beds (Gt), couplets ofgravel and sand (Gsh), trough cross-stratifiedsandstones (St), rippled sandstones (Sr) andsome interbedded sandy channel-fill deposits (Se).The Scott-type (G11) deposits of the SanjauliFormation are topped by accumulated debris flowdeposits, which comply with the characteristics ofTrollheim-type river deposits. The debris flowscannot have travelled far from their source, sothat the presence of the Trollheim- and Scott-typeprofiles in the same braided river deposit reflectsvariations from fan-proximal to fan-distal (cf.Rust, 1978). The occurrence of a conglomerate/sandstone facies assemblage resembles theTrollheim-type deposit. Poorly sorted, matrix-supported debris flow deposits (Gms) with planarcross-beds (Sp), trough cross-beds (St), stratifiedsandstones (Sl) and intercalations of finer-grainedsediments (Sh) further characterize the Trollheim-type (G1).

2) The Trollheim-type (G1) deposits gradually evolveinto stacked sheet-like sandstone bodies, suggest-ing their deposition in a braided river setting duringflash floods. The progressive evolution of Trollheim-type to Platte-type deposits is indicated by thefining-upward tendency from conglomerates tocoarse-grained sandstones and eventually to

fine-grained sandstones and siltstones. Thesesandstone and siltstone bodies earmark the amal-gamation of the Platte (S11) deposits.

3) The features of Platte-type (S11) deposits areexpressed by the tabular sheet-like sandstones withplanar cross-beds (Sp) and minor ripples (Sr).Overbank deposits in this area are characterized bysiltstones and shales.

Studies of the Proterozoic Simla Basin have previ-ously been focused on structure, tectonics and stra-tigraphy. In contrast, sedimentological informationabout the Simla Basin is, in general, insignificant. Thepresent study now sheds light on the sedimentology ofthe Sanjauli Formation on the basis of outcrop-basedfacies analysis. The resulting new combined model il-lustrates the response of the fluvial system to changesin base-level and can be applied to similar successionselsewhere, whether recent or ancient in age.

The main tool used in our sequence stratigraphicapproach is the analysis of the stacking pattern of thevarious units and of the key surfaces that separateunits with different stacking patterns. Recognition andcorrelation of stacking patterns in the braided riverSanjauli siliciclastics, and their interpretation in termsof base-level changes jointly provide a rock-basedmodel that explains the evolution and the signifi-cance of the changes in sedimentary architecture.Based on the recognition of stacking patterns andfacies tendencies, three hierarchical types of systemstracts (lowstand systems tract, transgressive systemstract, and highstand systems tract) have been recog-nized in the Sanjauli Formation.

The prograding clastic wedge of the Sanjauli For-mation, which incises the delta deposits of the ChaossaFormation, represents a lowstand systems tract (LST)which developed in response to a high supply of clas-tics during a falling base-level. During this base-levelfall period a type-1 unconformity was formed as indi-cated by the incision of the fluvial sedimentary wedgeson the delta deposits of the Chaossa Formation.

The transgressive systems tract (TST), character-ized by tidally influenced medium- and fine-grainedchannel sandstones, was initiated by the first flood-ing event after the maximum lowstand regression. Theculmination of the transgression is marked by thedevelopment of a thin dark shale bed on top of thesandstone and has been interpreted as a condensedsection. The preservation of fine-grained overbankmaterial and sedimentary structures that indicatetidal influence reflect increased rates of base-levelrise and increasing accommodation space.

A new ‘superassemblage’ model explaining facies changes in fluvial environments 407

The highstand systems tract (HST) was depositedwhen the rate of base-level rise gradually diminishedand the accommodation space decreased (continuedsubsidence still created some accommodation). TheHST is characterized mainly by lateral accretion, withminor vertical accumulation of fine-grained sediment.

The sandy braided channel deposits (FA-1)resemble the Proterozoic fluvial styles described byRøe and Hermansen (1993) and Sønderholms andTirsgaard (1998), which lack, however, the mud con-tent, and are composed of medium- to high-energyfacies, reflecting hydrodynamic fluctuations andshowing stacked sand bodies. In spite of high dischargerates, which must be ascribed to the absence ofvegetation in the Proterozoic, no large quantities ofoverbank deposits accumulated because of strongreworking resulting from limited accommodationspace. Hence, the discharge was continuously high,giving rise to perennial, high-energy streams.

The occurrence of relatively thick overbank sedi-ments in FA-3 reflects increased vertical accretionbrought about by an increasing creation of accommo-dation space. A subsequent increase in the mud/sandratio suggests a distal shift of the fluvial system. Thelack of desiccation structures in the channel depositsand muddy floodplains suggests a perennial fluvialsystem in a relatively humid climate.

Acknowledgements

The authors thank the Oil and Natural Gas Corpo-ration, India for sanctioning the project and forproviding financial support. The first author gratefullyacknowledges Prof. Ravindra Kumar (retired) of theDepartment of Geology, Panjab University, Chandigarh,India, for introducing us to the basin and for sharing hisknowledge. We thank the university authorities forproviding the infrastructural support that is necessaryfor the work. We are grateful to Prof. Bhabani PrasadMukhopadhyay of the Department of Earth Sciences,IIEST, for his participation and support during the fieldwork.We thank Ms. Tithi Banerjee and Ms. Alono Thorie,research scholars of the Department, for their assis-tance and accompany during field and laboratory work.

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