M carbonate siliciclastic sedimentation in the U C N F,

Mixed carbonate–siliciclastic sedimentationin the Upper Cretaceous Nilkanth Formation,Garhwal Himalaya, India

SHRUTI R MISHRA1,3, ADITI SHARMA

2,*, PARTHA P CHAKRABORTY2,

SARADA P MOHANTY3 and SATISH C TRIPATHI

4

1Geological Survey of India, Northern Region, Lucknow, India.

2Department of Geology, University of Delhi, Delhi 110 007, India.

3Indian Institute of Technology (ISM), Dhanbad, India.

4Geological Survey of India, Southern Region, Hyderabad, India.*Corresponding author. e-mail: [email protected]

MS received 4 December 2019; revised 8 February 2020; accepted 10 February 2020

The Upper Cretaceous Nilkanth Formation awaits a process-based depositional model despite being atopic of discussion between stratigraphers, palaeobiologists and structural geologists over the last fewdecades. Sedimentary facies analysis of a *50 m thick section along a *2.8 km long section alongRishikesh–Tal Bidhashini in Pauri Garhwal district of Uttarakhand allowed documentation of mixedcarbonate–silicicalstic facies types, dominantly consisting of sand- and pebble-sized carbonate debrismixed with siliciclastics in a proximal to distal facies tract. Ten different facies types that include matrix-rich and matrix-poor shelly conglomerate, mixed clastic-carbonate wackestone, packstone, impure cal-cirudite and calcarenite, biomicrite and ferruginous sandstone are documented. Delineation of faciesassociation and documentation of facies stacking pattern provide a post-Santonian mixed carbon-ate–siliciclastic sedimentation history of the Nilkanth Formation, deposited in the form of mass Cows ofvaried rheology on a barred low- to moderate-gradient carbonate ramp, formed at the leading edge of theIndia plate before its collision with the Kohistan–Ladakh arc. Carbonate clasts comprising bivalves,crinoids, algae, bryozoan, etc., were produced in a narrow high-energy transgressive coastline and sup-plied across shelf along with reworked siliciclastics from clastic shoreface bar. It is argued that thereworked fossils, including the bryozoa Ceriocava Nilkanthi, present within massCows may not justifyBxing of an absolute age for the formation but may definitely help in providing an age range.

Keywords. Nilkanth Formation; Upper Cretaceous; mixed carbonate–siliciclastic; bryozoan; massCows.

1. Introduction

The Nilkanth Formation (cf. Singh 1999), a para-autochthonous sedimentary succession of the LesserHimalayas bordering the Main Boundary Thrust(MBT), always remained a topic of interest forstratigraphers, paleobiologists and structural

geologists (Srikantia and Bhargava 1967; Shringar-pure and Shah 1987; Mathur and Juyal 2000a, b;Prasad and Sarkar 2002; Bhatia andBhargava 2005;Mathur et al. 2008 andmany others).Working in theMussouri area, Singh (1999) brought amajor changein perception in the stratigraphic status of the unitby recognising a prominent stratigraphic break

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between the shelly limestone of the NilkanthFormation and underlying unfossiliferous quartzitesof Proterozoic Upper Tal Formation and therebysuggests a Jurassic–Cretaceous age for the NilkanthFormation. Further, on the basis of presence ofbounding paleosols, Mathur and Juyal (2000b)considered shell limestones of the Formation sepa-rate from the underlying Tal Quartzite (cf. Singh1999) and overlying Subathu Formation withprominent stratigraphic breaks in between. Subse-quent study (Mathur et al. 2008) reBned the idea andan upper Cretaceous (Santonian; 86.3–83.6 Ma) agefor the shelly limestones of the formation was pro-posed from the presence of age-connotative bryozoaCeriocava Nilkanthi and associated echinoid radi-ole. Shringerpure and Shah (1987) reported occur-rence of fossils of widely varying ages in theformation including some exotic group, e.g.,Xenacanthid (shark teeth), akin to upper Pennsyl-vanianXenacanthid. It is interesting to note that allthese studies have reported layers of broken and ill-preserved remains of wide spectrum fossil fragmentsbelonging to different ages within the formation thatincludes bryozoan, echinoid radioles, hydrozoa,foraminifera, green algae and bivalve, but refrainedfrom suggesting any reason behind that. A generaldirty clastic-rich character of limestones and med-ium- to coarse-grained arenite and calciruditepackage at the top of the Nilkanth succession,though noticed, never been addressed in terms ofoperative depositional process/es and paleogeo-graphical settings. In fact, there is no process-basedsedimentology available from the formation todescribe the underpinning depositional controls onwidely varying siliciclastic content within the lime-stone and reason behind mixing of fossils of wideranging ages.Amongst doubly plunging synclines along Lesser

Himalayan exposure belt, the Garhwal and Mus-souri synclines provide better scope to observe well-preserved exposures of the Nilkanth Formation. InGarhwal syncline, the formation is exposed in thesouthern limb as a NW–SE striking linear beltextending from Rishikesh in the west to Dharkot inthe east and considered to be conformable withoverlying Subathu Formation (Mathur et al. 2008)though unconformable relationship between the twoformations is also claimed from some other sectionswith documentation of incipient paleosol horizons(Mathur et al. 2008). Considering the proposedUpper Cretaceous age for the Nilkanth Formationand Paleocene–Eocene age for the immediatelyoverlying Subathu Formation, it may be valid to

consider that the Nilkanth Formation may holdpotential to unravel paleogeography and operativedepositional process/(es) at the leading edge of theIndianplate at the terminal phase of its journey as anisolated island continent during the period betweenseparation from the supercontinent ‘Gondwana’ andcollision with an island arc or with parts of the Asiancontinental landmass (Srikantia and Bhargava1978; Gaetani and Garzanti 1991; Thakur 1992;Mathur and Juyal 1996; Valdiya 1998). The presentstudy aims at process-based facies and paleo-envi-ronmental analysis of the formation from Rishi-kesh–Tal Bidhashini section in Pauri Garhwaldistrict, Uttarakhand (Bgure 1) to understand thedepositional motif of mixed shelly limestone andmedium-grained clastic succession to throw lightabout the mixing of fossils as well as presence of‘exotic’ fossils. A direct implication of this study is tocritically evaluate the stratigraphic age claimed onthe basis of presence of bryozoa in the formation.

2. Geology of the studied section

Overlying the Neoproterozoic Blaini Quartzitewith an unconformity, the mixed calcareous–sili-ciclastic succession of the Nilkanth Formation isexposed in the SW part of the Garhwal syncline. Inthe study area, the formation is represented by a*50 m thick succession comprised of shell lime-stone, calcareous siltstone, calcareous sandstoneand calcarenite with thin bands of marl and earthylimestone. In general, limestones of the NilkanthFormation are impure and contain visible amountsof quartz and chert grains. Though calcareoussiltstone and shelly limestone, in combination,constitute the bulk of the rock unit, the topmostpart is coarse-grained and is represented by a *10m thick succession of thick-bedded multi-storiedcross-stratiBed calcirudite/calcarenite and sand-stone package. A *95 m thick succession of olivegreen/grey shale with intervals of ferruginousoolite and bioclastic limestone of the SubathuFormation unconformably overlies the NilkanthFormation in the study area (cf. Mathur et al.2008).

3. Depositional facies

Keeping aside the partitioned view for siliciclasticand carbonate sediments, sedimentologists haveemphasized on the importance of mixed

125 Page 2 of 14 J. Earth Syst. Sci. (2020) 129:125

carbonate–siliciclastic sedimentological study thatis common in middle- to low-latitude shelf/plat-form settings. Number of mixing modes are sug-gested for the two distinctive kinds of sediments(Mount 1984; Flood and Orme 1988; Dorsey andKidwell 1999; Tucker 2003). Mount (1984) madethe Brst attempt to classify mixing in four differentmodes. In fact, mixing of the two genetically-dis-tinctive sediments is reported from many moderndepositional settings where rivers debouch sedi-ment onto carbonate-depositing ramps (Belperioand Searle 1988; Wright et al. 2005; Ryan-Mishkinet al. 2009).In this backdrop, the mixed carbonate–silici-

clastic sediments of the Nilkanth Formation oAeran unique opportunity to study the role of paleo-geography and operative process on (i) significantmixing of clastics within carbonate; at casesexceeding 70%, (ii) variation in relative contribu-tion of siliciclastic and allochemical (ooid, bioclast)constituents in the succession, and (iii) occurrenceof broken and ill-preserved remains of wide spec-trum fossil fragments of different ages. However,the detached, poor quality of deformed exposures,alluvium cover and the general tendency of car-bonate rocks to get weatherd pose difBculty byobliteration of sedimentary structures. Hence, thefacies analysis has been supplemented by detailedmicroscopy.A *2.8 km long section along Rishikesh–Tal

Bidhashini in Pauri Garhwal district of Uttara-khand is studied for detailed outcrop, sectionmeasurement and sampling (Bgure 1). The mea-sured section has provided critical observation onthe bed geometry, internal structure, bioclastcharacter and vertical variation of lithofacies; all ofwhich are used to decipher the depositional

character (table 1). Several samples were collectedfor petrographic analysis.

4. Facies associations andpaleo-environmental settings

Four distinctive facies associations, each with acouple of facies constituents, have been recognizedfrom the Nilkanth Formation; which are describedand interpreted below:

4.1 Facies association I (FA I)

Constituted of moderate- to well-sorted medium-to coarse-grained sandstone (Bgure 2; facies A) andwackestone (facies B; after Dunham 1962). Thisfacies association (max. 4.9 m in thickness) occu-pies basal-most position of the studied succession.Whereas very high concentration of siliciclastics([90%; table 2) with moderate to well grain sort-ing result in clast-supported fabric in relict topoorly-organised beds of facies A, matrix-sup-ported wackestone beds (facies B) reveal presenceof carbonate intraclasts and skeletal fragments ofalgae and bivalve set in carbonate mud/sparmatrix with moderate (max. up to 73.55%) silici-clastic content. Contact between beds of the twofacies types varies between sharp and gradational.Quartz and feldspar grains constitute bulk of thesiliciclast population (C90%) in facies A and aresubangular to subrounded. Feldspar grains aremostly decomposed along cleavage planes anddisintegrated. In contrast, decimeter-thick mas-sive- or relict-bedded wackestone beds are withlesser volume of sand, granules, lithic clasts ofquartzite (65–75%) along with disarticulated and

Figure 1. Detailed geological map of the study area with studied transect (A–B) marked. Geological subdivisions of theHimalaya are shown on the left and broad stratigraphic disposition of the Nilkanth Formation is on the right.

J. Earth Syst. Sci. (2020) 129:125 Page 3 of 14 125

Table 1. Facies types, sedimentary structures and inferred depositional processes for mixed siliciclastic–carbonate sediments ofNilkanth Formation.

Faciescode Facies name Facies description Sedimentary structures Interpretation

A Sandstone Medium- to coarse-grainedsandstone; high content ofsiliciclasts with subangularto subrounded grains, clastsupported, moderate- towell-sorted. Present only inbottom part of the studiedsection

Massive or with incipientreverse grading

High-density granular Cow

B B1 Wackestone with highbioclast

Matrix-supported, poorlysorted with abundant shellfragments, normal size andconcentration grading;occasional outsized bioclastnear the bed top.Recrystallized mud matrixalong the margin offramework grains;interbedded with facies D.High (maximum up to 32%)bioclast content

Sharp, planar base andgradational top. Presence ofBouma Tab, Tabc, Tae cycles

Turbidity current in a muddepositing environment;akin to classical turbidites

B2 Wackestone with highsiliciclast

Beds with Cat base andconvex-up geometry.Poorly-sorted siliciclasticgrains with calcite sparswithin the interstitialspaces. High (maximum upto 97%) siliciclast content

Trough cross-stratiBcation(set thickness up to 11 cm,width 16 cm), cosetsterminated by broadundulatory erosion surface

Tractional reworking ofsubfacies B1 by waves andcurrents in shelf setting

C C1 Matrix supported shellyconglomerates

Matrix supported planar,non-erosional base, poorsorting, ungraded or withinversely graded clast fabricand predominantoccurrence of disarticulatedand broken shells

Isolated or interbedded withC2 beds; no definitestacking

Cohesive subaquous debrisCows with high matrixstrength and frictionalfreezing; derivation mostlyfrom bivalve and algalcolonies, but often also ofmixed clastic derivation

C2 Wackestone–Packstone–mudstone alternation

Tabular, parallel-sided bedwith planar non-erosionalbase except near thetransition to mudstonesubdivision; where somebioclasts are observed insubvertical fashion; clasts,in general, are bed-parallel

Sharp contact between twosubdivisions; erosional, attimes diffused. Size andconcentration gradingshown by clasts inmudstone subdivision

Transformed cohesive Cows.A viscous, non-turbulent,inertia Cow layer below anda turbulent layer above, theinterface between the twobeing an interface ofphysical discontinuity

D Biomicrite Massive to weakly laminatedbeds. Bryozoa, includingthe Cretaceous(Maaestrichtian) indexspecies Diplocava Nilkanthiand fragments of bivalveand crinoids, replacement ofmicrite with microspar

Absence of wave or currentfeature. Commonly at sharpcontact with thickerinterbeds of facies B1, C1 orC2 dispersed at differentstratigraphic levels

Low energy shelf/rampdeposition

E E1 Ferruginous sandstone Dominating siliciclasts withmonocrystalline, angular tosubrounded, moderatelysorted, medium sand sizequartz grains. Highclast:groundmass ratio,uniformly thick beddedwith broad lensoid andamalgamated character

Low-angle planar curvedcross-stratiBed (average setthickness *0.35 m) withtheir bases and tops sharpand undulatory, oscillatoryripples as dominant surfacebedform

Swash origin in a shallow,wave-dominated high-energy environment abovefair-weather wave base


broken shell fragments. On the basis of siliciclast:bioclast ratio and presence of sedimentary struc-ture, wackestones (facies B) are further subdividedinto two types, viz., (i) Sub-facies B1; with sharp,erosional-to non-erosional base, high bioclastcontent (*32%) and presence of Bouma Ta–c

subdivisions with incipient normal clast size orconcentration grading (Bgure 3), and (ii) sub-faciesB2; with relatively higher proportion (*73%) ofsiliciclastic (Bgure 4) and undoubted presence ofco-sets of trough cross-stratiBcation (Bgure 3).Recrystallisation of the mud matrix along themargin of framework grains in this subfacies B1 is acommon feature. With a diffuse, gradationalcontact subfacies B2 overlies subfacies B1. Calcitespars within the interstitial spaces of poorly-sorted

siliciclastic grains in subfacies B2 looks very dirtyand do not show any preferred crystal arrange-ment. Trough cross-stratiBcations in wackestonesof subfacies B2 measures up to 11 cm in setthickness and maximum width of trough measuredas 16 cm; trough cosets terminated by broadlyundulatory erosion surface. Bed of B2 subfacies, atplaces, has Cat base and convex-up geometry. Anincrease in carbonate groundmass (muddymatrix+cement) and decrease in framework:groundmass ratio is noticed upward through thefacies association.

Origin of decimeter-to-meter thick massive,moderate- to well-sorted sandstone beds in geo-logical record always remain a topic of discussionand attempts have been made to interpret theseunits as product of extensive liquefaction, intensebioturbation or high concentration grain Cow(Mukhopadhyay et al. 2019). The massive, relict-bedded sandstone of facies A with planar, non-erosional bed boundary and without any indepen-dent signature supporting liquefaction or biotur-bation is interpreted as the result of high-densitygranular Cow. Gradual aggradation and upwardmigration of the depositional Cow boundary due tograin hyper-concentration and hindered settling ina sustained steady, quasi-steady, high-densitycurrent possibly resulted the facies A unit (cf.Branney and Kokelaar 1992; Kneller and Branney1995). In contrast, matrix-rich shelly wackestoneunit (subfacies B1) with relatively low siliciclasticcontent, sharp erosional base and normal gradingpoint towards low-density turbidity Cow origin

Table 1. (Continued.)

Faciescode Facies name Facies description Sedimentary structures Interpretation

E2 Calcareoussandstone

Low clast:groundmass ratio,alternation between tabular/wedge-shaped thick andthin-beds

Low-angle planar curvedcross-stratiBed (average setthickness *0.35 m) with theirbases and tops sharp andundulatory

Thick, lenticular beds areproducts of shoreface barmigration and the thin beddedunits are products of interbarorigin

F F1 Calcirudite Coarse-grained carbonateclasts with siliciclastics;Bioclast, ooid, crinoid andalgal clast. Presence ofCretaceous (Maaestrichtian)index species DiplocavaNilkanthi

Plane-laminated, medium-scalevery low-angle troughcross-stratiBcation

Surf-swash condition; Upper Cowregime condition

F2 Calcarenite Sand-sized carbonate clastswith siliciclastics

Large-scale (average set thickness35 cm) multi-storeyed cross-stratiBcation deBning chevroncross-stratiBcation.Concentration of relatively coarseclasts along cross-laminae

Nearshore wave-reworked bar

Figure 2. Facies A (sandstone) and B (wackestone). Faciesboundaries are highlighted by dashed line.


Table 2. Modal analysis including abundance of different allochems (ooids, bioclasts and siliciclasts) and clast:groundmass ratiofrom different facies types of Nilkanth Formation.

Facies

code Facies name

Matrix +

cement

%

(groundmass)

Modal % of clast

Clast:

Groundmass

Skeletal component

(Bioclast; Bivalve,

Crinoid, Algae

Bryozoa, etc.) Siliciclast

Non-skeletal

component

(ooid)

A Sandstone 9 Nil 91 Nil

B B1 Wackestone with

high bioclast

26.35 32.52 40.93 Nil

B2 Wackestone with

high siliciclast

14.30 2.55 83.15 Nil

C C1 Matrix-supported

shelly

conglomerates

42.45 10.55 43.2 3.80

C2 Wackestone–

Packstone–

mudstone

alternation

31.30 9.45 56.02 3.23

D Biomicrite 81.75 9.33 7.32 1.60

E E1 Ferruginous

sandstone

12.69 2.35 84.96 Nil

E2 Calcareous

sandstone

14.70 3.52 81.78 Nil

F F1 Calcirudite 27.83 13.82 50.25 8.10

F2 Calcarenite 25.35 8.83 51.22 14.60

Clast, Groundmass.


(Dorsey and Kidwell 1999; Mutti et al. 2003; Beraet al. 2009). Occurrence of some outsized bioclast inthe upper part of this deposit is the result ofhydraulic sorting from heavier siliciclastic grains.Trough cross-stratiBed wackestone bed of subfacies

B2 is possibly product of tractional reworking ofsubfacies B1 wackestone by waves and currents inshallow-marine shelf setting.

4.2 Facies association II (FA II)

Alternation between tabular, parallel-sidedwackestone–packstone bed and carbonate mud-stone characterize this association. In close look,the association found to be an alternation betweenmatrix-rich shelly conglomerate (wackestone topackstone; facies C) and massive to weakly beddedsparse biomicrite (facies D) (Bgure 5). Beds ofshelly conglomerate are found in two differentmodes. In one variety (facies C1) conglomeratebeds are matrix-supported (av. matrix + cement42.45%; table 2), with planar, non-erosional base,poor sorting, without any current stratiBcation,internally ungraded and with predominant

Figure 3. Alternation between sub-facies B1 and B2. Notepresence of planar-curved cross-stratiBcation in B2.

Figure 4. (a) Matrix-rich wackestone of facies B. Note profuse presence of bioclasts including crinoid. (b) Moderately sortedsandstone of facies A with occasional presence of outsized clasts (arrowed).

Figure 5. Alternation between shelly conglomerates of subfacies C1 and biomicrite of facies D. Note sharp to gradationaltransition between the two facies. Also note chaotic clast orientation in facies C1.


occurrence of disarticulated and broken shells. Inaddition to the bioclasts, siliciclasts in high pro-portion and ooids constitute the clast population.Centimeters-thick beds of this unit are overlain bybeds of facies D with sharp, non-erosional contact.In contrast, shelly conglomerates of facies C2 areerosional-based and internally made up of twosubdivisions; basal calcirudite and overlying mud-stone (Bgure 6). The calcirudite subdivision iscomprised of pebble-sized bioclasts in high con-centration and carbonate lithic clasts in subordi-nate proportion, whereas overlying mudstonesubdivision is characterized by relatively sparseclast concentration. Moderate-to-well-deBned size-and concentration-grading can be tracked withinclasts of mudstone subdivision. Contact betweenthe two subdivisions of facies C2 is sharp, erosional,or, at times, diffused (Bgure 7). In calciruditesubdivision of facies C2, bryozoans (including theCretaceous (Maaestrichtian) index species Diplo-cava Nilkanthi) and fragments of bivalve and cri-noids are encountered with a limited distribution(Bgure 8). Often it becomes difBcult to differentiatemudstone subdivision of facies C2 and lime mud-stone of facies D in absence of any significantdistinction in lithology or clast concentrationbetween the two. The main diagenetic feature infacies D includes replacement of micrite withmicrospar; disarticulated bioclasts and ooids arefound Coating within both micrite mud and sparite(Bgure 9).Poorly sorted, ungraded matrix-supported shelly

conglomerates (facies C1) record deposition by cohesive subaquous debris Cows with high matrixstrength and frictional freezing (Nemec 1990; Sin-clair 1997; Dorsey and Kidwell 1999). In contrast,bipartite beds (facies C2) with a basal high-con-centration layer and turbulent rider are interpretedas products of transformed cohesive Cows. A Cowseparation possibly resulted in a viscous, inertiaCow layer below and a turbulent layer above, theinterface between the two being an interface ofphysical discontinuity (Felix and Peakall 2006). Attimes, larger clasts are found elutriated (ripped up)from the viscous, inertia Cow layer by strong tur-bulence of the upper, normal graded layer (cf. Beraet al. 2008). From low correlation coefBcients(0.15–0.3) between adjacent wackestone–packstonebed and Bner biomicrite interbed thicknesses theirindependent origin is suggested (Schwarzacher andFischer 1982); whereas shelly conglomerates (faciesC) are allogenic in origin, weakly laminatedbiomicrite beds (facies D) are interpreted as in-sitususpension-settlement product.

Figure 6. Bipartite character of subfacies C2; calcirudite isoverlain by mudstone subdivision with incipient normalgrading. Note disarticulated and broken bioclast fragmentselutriated at the boundary between the two subdivisions.

Figure 7. Recurrent alternation between two subdivisions ofsubfacies C2. Note erosional to diffused character of boundarybetween the two subdivisions.

Figure 8. Petrography of calcirudite subdivision of subfaciesC2. Note presence of bryozoan Diplocova Nilkanthi (arrowed).


4.3 Facies association III (FA III)

Centimeters to decimeter thick Bne- to medium-grained ferruginous and calcareous sandstones (av.clast:groundmass ratio 6.34:1) of this association(max. thickness *5 m) are comprised of inter-mingling between two different facies (facies E1and E2); both constituted of lenticular beds butdiffer in clast:groundmass ratio, bed thickness anddegree of lenticularity. Facies E1, ferrugineoussandstones with a high clast:groundmass ratio(6.88:1), is uniformly thick bedded (average bedthickness *2.8 m; Bgure 10) with broad lensoid(width 65–90 cm; maximum length 1.45 m) andamalgamated character. Clast population withinthese sandstones includes both siliciclasts andbioclasts with siliciclasts being dominant

volumetric contributor (*90%) and consistsprimarily of monocrystalline, angular to sub-rounded, moderately sorted, medium sand sizedquartz grains. Bioclasts (*10%) include shellfragment of bivalve and crinoids. Iron oxide cementis present as pore Bllings (Bgure 11). In contrast,facies E2, sandwiched between units of facies E1,reveal a clast: groundmass ratio 5.8:1 and com-prises of alternation between tabular/wedge-shaped thick (av. thickness 1.2 m) and thin-bedded(av. thickness 0.16 m) calcareous sandstone(Bgure 10). Thick bedded units are structureless orlow-angle planar-curved cross-stratiBed (av. setthickness *0.35 m) with their bounding surfacessharp and undulatory, and show oscillatory ripplesas surface bedforms. The ripples are symmetrical inproBle, mostly straight crested (rarely threedimensional) and occasionally have crest bifurca-tion. Average wavelength and amplitude of theseripples are 14.6 and 1.85 cm, respectively. Thinbeds within facies E2 maintain similar grain sizewith the thick-bedded ones but differ in degree ofbedding lenticularity. Ripples on the surface ofthin-bedded intervals are smaller in size (av.wavelength and amplitude 9.6 and 0.9 cm, respec-tively) and are often isolated and starved. Upwardthrough the association beds of facies E1 becomethicker and amalgamated with concomitantdecrease in the thickness of facies E2 units. Thesesandstones have been aAected by pressure solution,indicated by concavo–convex grain contacts andpressure solution seams.Fine- to medium-sized sandy character and

frequent occurrence of wave-generated bedforms

Figure 9. Disarticulated bioclasts and ooids set in micrite andsparite of facies D.

Figure 10. Alternation between subfacies E1 (thick bedded)and E2 (thin bedded) within facies E. Note upward dominanceof subfacies E1.

Figure 11. Petrography of facies E; angular to subroundedmedium sized quartz grains with spots and clusters of ironoxide cement.


indicate deposition in a shallow, wave-dominatedhigh-energy environment above fair-weather wavebase (cf. Seidler and Steel 2001). Wavelength:amplitude ratio (*8:1) for the wave ripples sug-gests their swash origin (Clifton 1969, 2006; Sarkaret al. 1996) in a shallow water domain. Beside,mutual association with other nearshore productsallowed us to interpret this association as productsof outer to middle shoreface environment (Walkerand Plint 1992; Samanta et al. 2019). While thethick, lenticular beds are interpreted as products ofshoreface bar migration, the thin bedded units withrelatively lower clast:groundmass ratio and pres-ence of small scale ripple bedforms are products ofinterbar (Tamura et al. 2007). Upward increase inthickness and degree of amalgamation of facies E1units bear indication for transition from outer tomiddle shoreface domain.

4.4 Facies association IV (FA IV)

This association represents the topmost part of thestudied section. Meters-thick tabular beds of cal-carenite to calcirudite, often in amalgamation,constitute this association. In comparison to FAIII, this association records lower values both inclast:groundmass ratio and siliciclast volume.Whereas clast:groundmass ratio is 2.59:1, amongstclast population siliciclastics contribution approx-imate maximum up to 70% with bioclast, algae andooid share nearly equal volumetric proportion inthe rest 30% (table 2; Bgure 12). Moderately sor-ted, Bne to medium grained, subangular to sub-rounded monocrystalline quartz grains representthe siliciclastic population. An alternation between

plane-laminated calcirudite, medium-scale verylow-angle trough cross-stratiBcation (facies F1)and large-scale planar-curved cross-stratiBed cal-carenite (facies F2) constitute the basic buildingmotif of this association; transition between thetwo facies types is gradational (Bgure 13). Whereasplane laminations in calcirudites of facies F1 aremm-thick and with low angle truncation, the low-angle trough cross-stratiBcations, in association,are of average set thickness 10 cm. Lateral transi-tion between the two can be observed within thewidth of outcrop. Cosets of decimeter-thick planar-curved cross-stratiBcations (av. set thickness 35cm) constitute facies F2. A tendency of decrease inset thickness up the F2 facies unit can be noticed inthe outcrop.The high maturity of sand, subangular to

subrounded quartz grains and occurrence of ooids,broken disarticulated shell fragments pointtowards deposition under moderate to high energycondition. Planar lamination with interruption oftrough cross-stratiBcation (facies F1) is interpretedas product of surf-swash condition (Sarkar et al.1996; Clifton 2006). In gradational transition, cal-carenites with co-sets of large-scale cross-stratiB-cation suggest presence of nearshore bar. Bar-trough systems are common in many moderncoastlines and many ancient open ocean succes-sions (Clifton 2006; Boggs 2011). The formation of

Figure 12. Petrography of facies F; note bioclasts, ooids andsiliciclastics set in spritic matrix.

Figure 13. Alternation between plain-laminated calcirudite(subfacies F1) and large-scale cross-stratiBed calciarenite(subfacies F2); a diffused transition between the two. Presenceof low-angle trough cross-stratiBcation within subfacies F1 isshown by dashed lines.


bars adjacent to beach-foreshore system istypically related to the breaking pattern of wavesnear the bar crests. Well-segregated grain size forfacies types F1 and F2; with F1 being rudite andF2 as arenite, clearly suggest that the developmentof bedforms in the coastline was a function of waveenergy and sediment size. StratiBcation typically isdifBcult to delineate in coarser sediment; hencewith impounding wave energy rudites responded inthe form of massive or poorly-stratiBed plane bed,whereas arenites formed in bars.

5. Depositional model

From process-based facies and paleo-environmen-tal analysis of the studied section it is inferred thatthe sediment succession of Nilkanth Formationrepresents a barred shallow ramp system.

Figure 14 illustrates facies stacking patternthrough the Nilkanth depositional history recon-structed from detached outcrops. Except for amoderate- to well-sorted sandstone bed of grainCow origin to start the succession, wackestones andpackstones, in alternation with Coatstone, withvariable amounts of siliciclasts and broken disar-ticulated shell and algal fragments constitutemajority of the succession. In alternation withmassive to poorly-stratiBed biomicrites (Coat-stones), facies B1, C1 and C2 shell conglomeratesrepresent incursion of massCows of varying rheol-ogy in low-energy middle to outer ramp (shelf)setting (Bgure 15). In contrast, lenticular beddedFA III sandstone and plane-laminated/cross-stratiBed calcarenite–calcirudite of FA IV, sepa-rated by sediments of FAII, in the upper part of thesuccession represent bar-trough setting in shallowmarine shoreface-beach set-up. Transition betweendifferent facies associations, however, often notvery clear in absence of continuity of exposure. Itmay be argued that during a relative sea level riseacross the Nilkanth coastline, the landward trans-lating coastline largely removed Bne terrigenoussediment that had accumulated during earlier sealevel lowstand and formed bars at the proximalpart of the ramp. Occurrence of oolites, abradedand rounded bioclasts of foraminifera and dasy-cladacean green algae and cosets of large-scaleplanar cross-stratiBcation in FA IV bear clearindication of relatively high energy shallow-marinedeposition. Presence of shoreline bar systems pos-sibly resulted in a restricted environment on itsshoreward side, while its seaward side remainedopen.Presence of a variety of bioclastic grains

including bivalves, algae and bryozoans (at cases)and siliciclasts in variable amount in the B1, C1and C2 type sand and/or carbonate mass Cows

Figure 14. A composite log of the studied section illustratingstacking pattern of facies associations (FA).

Figure 15. A cartoon (not to scale) to illustrate possiblepaleographic condition in course of Nilkanth sedimentation.


suggests reworking of sandy bars and/or carbonateshoals and banks present in proximal part of theramp. In the absence of storm signature, paleo-seismicity, or large-scale slope failure, it may bepresumed that bars, shoals and banks present inthe Nilkanth coastline were extensively reworkedby waves in a rising sea level stand and promptedoAshore supply of sediments in form of mass Cows.This mechanism may well account for incidence ofdisarticulated and broken bioclasts in large numberand mixing of fossils including some exotic groupswithin sediments of the Nilkanth Formation. Sincecarbonate production reaches a climax during thehighstand (Schlager et al. 1994), it is reasonable toargue that most of these carbonates are products ofpervious highstand of sea level.In this backdrop, presence of benthic Ceriocova

Nilkanthi bryozoan demands special attention sinceits presence is used by several workers as Cretaceousage marker for the formation. While Mathur et al.(2008) argued in favour of Santonian age of the for-mation citing similarity of the Nilkanth bryozoanwith bryozoan assemblage of the Bagh beds, Dhardistrict, Madhya Pradesh, Mathur and Juyal(2000a) opined for its Maaestrichtian–early Pale-ocenece age. The paleogeographic setting ofrestricted marine circulation is inferred in either oftheseworks.Detailed petrographyunder the presentstudy reveals: (i) occurrence of fenestrate bryozoanfragments in random orientation along with ooids,algae and siliciclastics, and (ii) bryozoan skeletonswith well-preserved original texture without signif-icant neomorphism. Secular variation in the form ofbryozoan colony through the geological historysuggests rarity of fenestrate form in the Cretaceous(Taylor and James 2013) and increasing tendencyfor utilization of aragonite as biomineral in place ofcalcite from the late Cretaceous (Maaestrichtian)onwards. Understandably, aragonitic skeletons areoften found reserved as moulds or partial moulds.Well-preserved skeletons of Ceriocova Nilkanthiwithin the Nilkanth Formation are suggestive of itscalcitic mineralogy and hence, prompt us to supportthe Santonian time frame, as suggested by Mathuret al. (2008). It is, therefore, argued that chrono-logically important index fossils (e.g., CeriocovaNilkanthi) in the mass Cow beds were perhapsderived fromSantonianbryozoan colonies formedonthe carbonate ramp in the preceding highstand. TheNilkanth Formation thus records a post-Santoniandepositional history andBxing an absolute age basedon reworked fossils present within clastic-carbonatemass Cow beds may not be fully justiBed. It is,

however, undeniable that occurrence of bryozoanmayhelp inmaking an agebracket for the formation.As discussed, mixing of two types of contrasting

sediments can be best achieved in (i) the transitionbetween two laterally adjacent facies, (ii) colonies ofcarbonate-secreting organisms within siliciclasticsetting, (iii) settling of air-bornedust onto carbonatemudCat or (iv) simultaneous transport of siliciclastand carbonate by the same current. Detailed docu-mentation of facies pattern from the Nilkanth For-mation prompt us to believe in the fourthmechanism whereby massCows generated by wavereworking of nearshore siliciclastic or mixed silici-clastic–carbonate (oolite–bioclast–algae) shoals in arising sea level condition. In general, mixed silici-clastic–carbonate successions are best developed attimes of high-amplitude sea level changes duringicehouse condition and hence, these are well repre-sented in the Quaternary and Permo-Carboniferous(Tucker 2003). Upper Cretaceous mixed sedimentsof the Nilkanth Formation is an aberration to thisgeneral hypothesis since the Cretaceous is a time ofgreenhouse globe with high sea level stand (Wattsand Steckler 1979) and continental Cooding(Sahagian et al. 1996).India broke oA from Madagascar during the Late

Cretaceous (*90 Ma; Chatterjee et al. 2013; Chak-raborty et al. 2019) with the spreading of CentralIndian Ridge and started its drift northward. Inorder to accommodate northward movement of theIndian plate, subduction of the Indian plate startedbeneath the Makran–Indus trench (van der Vooet al. 1999) that led to the collision of India with theKohistan–Ladakh (KL) arc around 85 Ma. In fact,several researchers (Ali andAitchison 2008; Jagoutzet al. 2009; Chatterjee and Scotese 2010; Burg 2011)believe that the collision of India with the Kohis-tan–Ladakh arc took place in late Cretaceous, i.e.,much before the initiation of its Bnal collision withAsia at around 52 Ma. Despite the absence of plu-tonic lithic clast that may suggest the Kohis-tan–Ladakh arc batholith as a possible provenancefor the Nilkanth Formation, the presence of upperCretaceous bryozoan suggests deBnitively that theNilkanthFormation records a sedimentationhistoryat the leading edge of the India plate before its col-lision with the Kohistan–Ladakh arc. It is pertinentto mention here that nearly all post-Paleozoic bry-ozoan-rich sediments are reported from mid-lati-tudes, outside the equatorial zone (Taylor andAllison 1998); most favoured latitude being 11�–20�.Also, paleomagnetic study on the KL arc allowedKhan et al. (2009) to suggest an equatorial position


for theKohistan–Ladakharc, not far from the Indianplate in the late Cretaceous time. Hence, it may bepresumed that bryozoan bearing Nilkanth sedi-ments record a tropical (possibly close to the lowerlatitudinal bound of the tropic, i.e., *11�) deposi-tional history between*86 and 85Ma at the leadingedge of northward advancing Indian plate.

6. Conclusions

From facies, facies association analysis and docu-mentation of facies stacking pattern, it is proposedthat the Nilkanth Formation records a mixed car-bonate–siliciclastic sedimentation history at theleading edge of the Indian plate in the Upper Cre-taceous time. Indeed, mixed bioclastic and silici-clastic facies in the barred Nilkanth carbonate rampwere deposited by a variety of submarine cohesiveCows and turbidity currents in a tectonically activemargin of the Indian plate before its collision withthe Kohistan–Ladakh arc. From Santonian age ofbryozoa Ceriocova Nilkanthi and absence of anylithic clast that may suggest derivation from theKohistan–Ladakh arc, it is proposed that theNilkanth Formation records post-Santonian depo-sitional history preceding collision of the Indianplate with the Kohistan–Ladakh arc.

Acknowledgements

The authors acknowledge the infrastructure helpprovided by their host institutes. AS acknowledgesUGC for extending Bnancial help in the form offellowship.

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Corresponding editor: SANTANU BANERJEE


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