Facies architecture and depositional evolution of

J. Earth Syst. Sci. (2019) 128:82 c© Indian Academy of Scienceshttps://doi.org/10.1007/s12040-019-1114-9

Facies architecture and depositional evolution ofPalaeocene–Eocene, Subathu Formation, GarhwalHimalaya, Uttarakhand, India

S R Mishra1,*, Rabisankar Karmakar2, Satish C Tripathi3,Mridul Gupta1 and Rajuram Sarswat4

1Geological Survey of India, Northern Region, Lucknow, India.2Geological Survey of India, Eastern Region, Kolkata, India.3Geological Survey of India, Southern Region, Hyderabad, India.4Geological Survey of India, Western Region, Jaipur, India.*Corresponding author. e-mail: [email protected]

MS received 6 September 2017; revised 17 July 2018; accepted 12 September 2018; published online 22 March 2019

The Late Palaeocene–Middle Eocene marine sedimentary sequence of the Himalayan foreland basin,represented by the rocks of Subathu Formation, is enriched with the presence of foraminifera, bryozoa,corals, gastropods, bivalves and so on within both carbonate and clastic dominant lithologies. Thecarbonate and the shale consist mainly of larger benthic foraminifera (LBF) and the assemblage iscomposed of Nummulites and Assilina along with the other skeletal and non-skeletal components. Thepresence of a palaeosol at the base of Subathu indicates a gap in sedimentation. The lithological andbiotic assemblages along with the bed form, bed geometry and primary sedimentary features helped us toestablish four facies associations, A, B, C and D, and have been corroborated with the shoreline migration(transgression/regression) history. The facies association-A, representing the basal horizons of SubathuFormation, indicates the onset of transgression and deposition in lagoonal condition with carbonaceousshale and oolitic ironstone, followed by the facies association-B and -C deposited in shallow marineshoreface. The uppermost unit, i.e., the medium- to coarse-grained sandstone (facies association-D) ofSubathu Formation, represents a fall in relative sea level (progradational stacking pattern), whereas theunderlying contact between the facies association-B and -C is represented by an aggradational stackingpattern between the siltstone and the shale, but certainly without exposing the shelf. Petrographicstudies based on characteristic features such as framework constituent, the percentage of matrix andgrains, nature of cementing material, textural features and fossil content help to deduce a distinctchange in depositional setting from an open marine (shelf) to shoal, a lagoon that gradually gradesto foreshore/beach environment. The study reveals that the basin has gone through a transgressive(facies association-A and -B), regressive (facies association-C) as well as distinct forced regressive (faciesassociation-D) phase of shoreline migration history.

Keywords. Subathu Formation; facies association; benthic foraminifera; Garhwal Syncline.

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82 Page 2 of 13 J. Earth Syst. Sci. (2019) 128:82

1. Introduction

In recent years, the basin scale depositionalanalysis experienced a trend shift with thespatiotemporal facies analysis with respect to extra-and intra-basinal inputs. The lesser Himalayansedimentary sequence provides an unparalleledopportunity to examine the complex ways in whichthe sedimentation has taken place. The continent–continent collisional tectonics resulted in the for-mation of peripheral foreland basin and associatedsequential changes (Ziegler et al. 1995, 1998).Thus, the architecture of the thick pile of fore-land sediments holds meaningful clues for thepast climate, tectonic and depositional activitiesoperated at the different stages of mountain build-ing processes (Allen et al. 1991; DeCelles andGiles 1996; Sinclair 1997). The foreland sedimen-tation from transitional marine to continent passesthrough several stages by means of the shallowwater to fluvial deposits (Allen et al. 1991; Sinclair1997). Although such transition and its signatureshave earlier been reported from the northwest-ern Himalayan foreland, the facies architecture ofmarine Subathu Formation, both in time and spacein the Garhwal Syncline, northwestern Himalaya ispoorly documented.

The Palaeogene succession of the Himalayanforeland basin is immensely important as it isthe period related to the suturing of the Indianplate with the Asian plate and the commence-ment of mountain-building events vis-a-vis basinevolution. During the Palaeogene, the shorelinemigration inward to the Indian plate and chang-ing depositional environment from swamp–lagoonenvironment to finally foreshore/beach environ-ment through shoreface indicates marine trans-gression and regression (Mathur 1978; Sarkar andPrasad 2000; Bera et al. 2008, 2010; Bera and Man-dal 2013; Singh et al. 2010; Singh 2012, 2013). ThePalaeogene Subathu Formation preserves the evi-dence of last marine transgression in the Himalayanfold-thrust belt and spatially disposed in differentsynclines of N–W Himalaya owing to later tectonics(Bhatia 1985; Najman et al. 1993). It is an impor-tant lithostratigraphic unit to unravel the shorelinemigration history.

The present study focuses on criterion-basedinterpretation of the depositional environment andshoreline migration history with the help of fieldevidences as well as through the petrographiccharacterisation of the individual facies types. Fur-thermore, these outcomes have been corroborated

to propose a simplified three-dimensional model toexplain the basin scale depositional history.

The Late Palaeocene–Middle Eocene strata ofSubathu Formation also preserve the wide vari-ability of larger benthic foraminifera (LBF). Theseforaminifera thrived in the shallow shelf carbon-ate platforms and have a unique character ofbeing a rock forming biota. Due to the bottom-dwelling character of foraminifera in a wide rangeof platformal environments, these biotas are influ-enced by global and local factors such as sealevel changes, tectonics of upliftment or subsi-dence, trophic sources and so on. The variationof these parameters affects the biotic compositionand the abundance of biota (Chaproniere 1975;Hottinger 1983, 1997). The LBF (Nummulite andAssilina) have been used as a major tool for petro-graphic characterisation of the Subathu sedimentsand hence the depositional history. The representa-tive sections were studied in the Garhwal Syncline,lesser Himalaya.

2. Material and methodology

Lithological sections of Subathu Formation areexposed in the core part of various synclines oflesser Himalaya. The present study is carried outin the Garhwal Syncline, Uttarakhand (figure 1).The Subathu Formation varies in its thicknessand may not be laterally persistent owing to itsstructural pinching and later tectonics. Traverseswere taken in strike parallel, NW–SE trending syn-cline at different locations to understand and studythe lateral and vertical variability/correlatabilityof the lithologies. Although measuring the truethickness of sedimentary sequences in structurallydeformed and overprinted terrain like Himalayaalways remains a challenge, utmost precautionshave been taken to avoid the repetition of a par-ticular horizon in all the measured sections. Fordelineating the facies types, genetically relatedlithologies/subfacies were clubbed under a particu-lar facies, and mutually associated facies types weregrouped under facies associations. A wide variety ofrock types ranging from shale, siltstone and sand-stone to carbonate (limestone) were characterisedon the basis of field criteria including bedding fea-tures, bed geometry, faunal composition, primarysedimentary structures (cross-bedding, wave fea-tures and burrowing) and diagenetic features.

Fifty-two thin sections were prepared and stud-ied in detail under the petrological microscope.

J. Earth Syst. Sci. (2019) 128:82 Page 3 of 13 82

Figure 1. Geological map of the area showing the Subathu Formation and associated litho successions of Garhwal Syncline,Uttarakhand.

Based upon a percentage of matrix and grain,textural characteristics, nature of cement and biotacomposition (mainly LBF), the petrographic char-acterisation of the rocks of Subathu Formation wasestablished. The composition of associated fauna(presence of echinoids, molluscs and bryozoans)and non-skeletal grains (e.g., ooids, intraclasts andpeloids) were also considered in the classificationscheme.

3. Geological setting and a brief overviewof the earlier studies

The lesser Himalayan group of rocks, irrespectiveof their age, i.e., proterozoic, palaeozoic, meso-zoic or cenozoic (palaeogene only), are thrustedover sub-Himalayan (Siwalik Group) sedimentsalong with the main boundary thrust (MBT).Lithostratigraphically, the Palaeogene successionof lesser Himalaya is subdivided into marine Sub-athu, continental molasse Dagshai and fresh waterdeposit of Kasauli Formations in the ascendingorder (Medlicott 1864). The marine Subathu For-mation comprises an essential litho- and biostrati-graphic horizon of the lesser Himalayan belt andis disposed in various synclines. The measuredthickness of Subathu Formation varies from about175 m in Jammu and Kashmir (in the northwest)and 400 m in Himachal Pradesh to about 170m in Uttarakhand (in the southeast; Medlicott1864). It rests largely on Precambrian basement,except in the Dugadda and Nilkanth localities(Uttarakhand), where it overlies Upper Cretaceous

Nilkanth Formation (Valdiya 1980), and indicatingprofuse unconformity at the base. The basementrock underlying the Subathu Formation is Maas-trichtian Nilkanth/Singtali Formation (= shell lim-estone) in Uttarakhand (Auden 1937; Bhatia 1982;Mathur and Juyal 2000).

The basal lithounits of the Subathu Forma-tion exhibit a great variation in different partsof northwestern Himalaya. It comprises carbona-ceous shale with few impure coal seams, shale,limestone and sandstone sequence in Jammu area;carbonaceous shale, limestone and marly limestonein Himachal/Shimla area; and oolitic shelly lime-stone in the Uttarakhand (Mathur 1978). Beraet al. (2010) and Bera and Mandal (2013) stud-ied the force regressive shoreface sandstone fromHimalayan foreland for the marine to continen-tal transition to imply for early Himalayan tec-tonic evolution and to establish the Oligo-Mioceneunconformity. Based on the facies association, sed-imentary structures and foraminifera, Singh (2012,2013) interpreted the depositional environment andpalaeobathymetry of the early Himalayan Fore-land basin that the Subathu sequence developed inephemeral streams, barriers, lagoons, swamps andtidal flat condition of the environment.

The present study is based on the lithounitsof Subathu Formation, exposed in the southernlimb of Garhwal Syncline as a NW–SE strikinglinear belt (∼80 km), extending from Rishikeshin the west to Ramisera in the east (figure 1). Itrests either over the quartzite of Tal Group (Cam-brian) or shell limestone of Nilkanth Formation(cretaceous) along with a regional unconformity.


Figure 2. Correlation lithology showing the variation of Subathu Formation in Garhwal Syncline, Uttarakhand.

A thin palaeosol horizon demarcates the uncon-formity plane and is well exposed near Maroraand Kandakhal, Uttarkhand. The litho-successionsof Subathu are not much extended towards thenorthern side of the southern limb in the GarhwalSyncline because of being intersected by the BijniThrust. Three best exposed litho-sectionsof Subathu Formation, namely, the Kotdwar–Dugadda–Sendikhal section, the Tal–Bidasini sec-tion and the Bhaun–Nilkanth section marked bytheir variable lithological characteristics wereselected for the detailed study (figure 2). Fourmajor subdivisions of Subathu Formation aremapped and delineated based upon the litho-characteristics, bedforms and palaentologicalinputs and named as facies association-A, -B,-C and -D in the ascending order. These faciesassociations are nothing but the member level clas-sification of Subathu Formation disposed from theperiphery to core of the Garhwal Syncline.

The overall litho-assemblage of Subathu Forma-tion from bottom to top, though not ubiquitouslyexposed, shows laterally correlatible characters.The basal part of Subathu Formation (figure 2)is represented by the black/dark grey shale, cal-careous siltstone, thin bands of marly and earthylimestone and ooidal ironstone, which is succeededby the grey nummulitic shale with lenticular

sandstone grading upward to an intercalatedsequence of the coquina limestone, grey shaleand siltstone. The third distinguishable lithopack-age upward is the intercalated sequence of greenshale, siltstone and fine-grained sandstone con-taining bivalves. However, the occasional interbed-ded limestone is also recorded as in Tal–Bidasiniand Bhaun–Nilkanth sections (figure 2). Furtherupward is the striking sequence of dominatedshale with thin intermittent bands of sandstoneand shell limestone. The Subathu Formation endswith relatively coarser, texturally and mineral-ogically mature grey coloured sandstone at thetop.

4. Facies architecture

The Subathu Formation of Garhwal Syncline issubdivided into four major facies associations,namely, facies association-A (barrier bar orlagoonal), facies association-B (lower shoreface),facies association-C (upper shoreface) and thefacies association-D (foreshore). Attempts havebeen made to simplify the facies architectureand nomenclature by clubbing all the geneticallyrelated lithounits for better understanding. Differ-ent lithounits have separately been clubbed, based


on their genetic association, sediment supply, andrise and fall of the sea level (table 1).

4.1 Facies association-A

Facies association-A is the basal or lowermost faciesassociation. It has further been divided into twofacies types, i.e., A1 and A2, by clubbing the genet-ically related subfacies/lithologies. These faciestypes grade into one another both vertically aswell as laterally within a short distance. Facies A1

comprises thinly laminated and splintery carbona-ceous shale, basal nodular sandstone, and grey siltyshale, marly and calcareous siltstones. The fissil-ity planes are very well developed. Indistinct wavefeatures are also present. The shale is dark grey toblack in colour. Facies A2 is characterised by ooidalironstone along with the chemically altered sand-stone. This is a distinct unit in the basal part ofthe Subathu Formation and acts as a marker hori-zon indicating the onset of Subathu sedimentation.It is thickly bedded to massive, dense and fine- tomedium-grained sequence.

Facies A1 is not ubiquitously exposed and isrestricted to areas such as Nilkanth, Tal Bidasiniand Marora (figure 2). In the Dugadda area, theintercalated sequence of facies A1 and A2 has beenrecorded. The thickness of facies A1 varies from8 to 20 m, whereas facies A2 is 1–4 m thick.Near Dugadda, A1 is mainly carbonaceous shalewith nodules of pyrite and grey silty shale, andfacies A2 comprises ooidal ironstone bed (figure 3aand b). The entire association is laterally trace-able up to the right bank of the Khoh Riverfrom the Dugadda–Kotdwara and Dugadda–Ramriroad junctions and reappears in the Marora roadsection, where it rests over the Nilkanth For-mation with 1.8 m thick palaeosol horizon. Thepalaeosol horizon is distinguished by the occur-rence of rootlets and pisolitic horizons. Althoughnot very distinct, the fairly convincing pisolitictexture is very much preserved in the palaeosolhorizon. In the Marora, facies A1 reduces to 2.5m, whereas A2 is 3 m thick. The thickness ofthis association reduces further in the Tal–Bidasinisection and Nilkanth area.

4.1.1 Interpretation

Black shale, plane laminated fine-grained natureof sandstone and absence of any distinct waveor current features, suggests a low-energy, poorlyoxygenated restricted lagoonal condition prevailed

during the deposition of facies A1. The ooidalironstone (A2) denotes the deposition of sedimentsin agitated high-energy condition. The two facies,A1 and A2, in association with their subfacies typesare repetitive and attributed to the reducing andoxidising environments, respectively. Intermittenthigh-energy pulses (ooidal ironstone) might possi-bly explain the concurrence of these contradictorydepositional units in the deeper and peripheral partof the lagoon.

Thus, after a prolonged period of non-deposition,revealed by the presence of palaeosol horizon, theSubathu sedimentation was initiated under lagoonto near shore conditions.

4.2 Facies association-B

The facies association-A is overlain by faciesassociation-B. This association consists of grey col-ored shale with lensoidal sandstone and foramini-feral limestone. Shell limestone occurs at the topof this association.

It is sub-divided into three facies types, namely,facies B1: grey shale with nodules of pyrite (withlensoidal sandstone/siltstone and poorly preservedfossils); facies B2: grey shale, siltstone and minorbands of red and green shale along with wacke-stone containing foraminifera (figure 3c and d; andfacies B3: grey shale with minor bands of green andred shale and packstone containing foraminifera(Assilina–Nummulite packstone).

This facies association is regionally extensiveacross the study area. Primary depositional featureincludes thin to very thin parallel, horizontal lam-inae. Thin and repetitive, fining-upward laminaesets were observed in certain sections. This associa-tion tends to coarsen upward, with upper contactsbeing gradational with overlying heterolithic silt-stone and carbonate. The gradational contact isalso observed with underlying black/carbonaceousshale. These facies types also grade one into othervertically and laterally.

The thickness of this association varies from50 m (Tal–Bidasini section; figure 2) to 34 m(Dugadda section; figure 2). Facies B1 is restrictedin the Bhaun–Nilkanth, Tal–Bidasini, Marora andDugadda sections (figure 2). Its thickness variesfrom 13 to 50 m. Laterally, it pinches out andmerges with the overlying facies B2. The fine-grained sandstone of facies B1 shows varyingthickness from 50 cm to 1 m. Facies B2 is over-lain either by facies B3 or facies association-C.Facies B2 has been recorded from all the measured


Table

1.Facies

associationsummary

forSubathuForm

ationfrom

theGarhwalSyn

cline,

Uttarakhand.

Stac

king

an

d se

a le

vel

fluct

uatio

n

Lith

o-ch

arac

ters

/ be

d Fo

rms

Faci

es

asso

ciat

ion

Sedi

men

tary

st

ruct

ure

Lith

o-D

epos

ition

al

envi

ronm

ent

Low

-ene

rgy,

poo

rly

Foss

ilsL

itho-

asso

ciat

ions

Cal

care

ous s

hale

, cal

care

ous

stra

tigra

phy

Thin

ly la

min

ated

shal

e,In

disti

nct w

ave/c

urre

nt

feat

ure

Botto

mFo

ram

inife

ra,

echi

noid

spin

essil

tsto

ne,m

arla

ndea

rthy

oxyg

enat

ed re

stri

cted

barr

ier

bar

lago

onal

co

nditi

on w

ith

inte

rmitt

ent h

igh-

en

ergy

agi

tate

d en

viro

nmen

t

fine-

grai

ned

natu

re o

fsan

dsto

ne,

FA1

limes

tone

, car

bona

ceou

ssh

ale w

elld

evel

oped

fissi

lityp

lane

soo

idal

iron

ston

eand

nodu

lar

sand

ston

eof

shal

e,pr

esen

ceof

pyri

tecu

bes

Com

pact

, fla

t to

sligh

tly u

ndul

ated

Larg

er b

enth

ic fo

ram

inife

ra,

biva

lve

and

gast

ropo

d

Gre

y sh

ale a

nd si

ltsto

ne,

inte

rbed

ded

limes

tone

w

ith g

rey

shal

e

Burr

ows,

lentic

ular

Dee

perp

art o

f sha

llow

mar

ine

shel

f (i.e

, the

lo

wer

shor

efac

e)be

d to

p, th

inly

lam

inat

ed,

lent

icul

ar a

nd w

avy

sand

bodi

es

with

in g

rey

shal

e an

d si

ltsto

ne,

lent

icul

ar b

ands

of f

ossil

ifero

us

limes

tone

andw

avys

truc

ture

s

FA2

Fini

ng se

a-le

vel

upw

ard

rise

MFS

(Coq

uino

idlst

.)G

loss

ifung

ite su

rfac

e (ho

rizo

ntal

and

vert

ical

bur

row

s)

Lim

esto

ne le

nses

or t

hin

band

s w

ithin

gre

en sh

ale

Coq

uino

id

limes

tone

, la

rger

ben

thic

fo

ram

inife

ra,

oyst

er, b

ival

ve

and

gast

ropo

d

Coq

uino

id lim

esto

ne in

juxt

apos

ition

with

th

e gre

ysha

le su

gges

tsud

den

influ

x of h

igh-

ener

gyco

nditi

onSh

allo

wer

leve

l of s

hore

face

(i.e

,the

uppe

rsh

oref

ace)

Gre

enis

h sh

ale

with

in

terb

edde

d lim

esto

neD

istin

ct w

ave/

curr

ent

feat

ures

Nor

mal

re

gres

sive

phas

eFA

3

Eros

iona

l sur

face

Inte

rmix

ing

betw

een

FA3 a

nd F

A4Se

a le

vel

fall

Mas

sive,

plan

e la

min

ated

shal

e te

xtur

ally

and

min

eral

ogic

ally

m

atur

e sa

ndst

one

Shal

low

angl

e cro

ssbe

d in

sand

ston

eV

erte

brat

e fos

silR

ed/p

urpl

e sha

lean

dFo

resh

ore

silts

tone

and

med

ium

toco

arse

Forc

ed

regr

essio

ngr

aine

dsa

ndst

one

FA4

Ove

rall

coar

seni

ng

upw

ard

sequ

ence

(p

rogr

adat

iona

l)To

pSubathu formation (Marine)

Transgression

Retrogradational

Aggradational


Figure 3. (a and b) Ooidal ironstone (OI) near Dugadda, Uttarakhand, (c) Nummulite in wackestone at Begra, Uttarakhand,(d) Asillina in wackestone near Nilkanth, (e) oyster bed, Tal–Bidasini section, (f and g) Turritela-bearing limestone in theTal–Bidasini section and (h) planar cross-bedded sandstone near Dugadda, Uttarakhand.


sections. The wackestone (facies B2) thicknessvaries from 10 cm to 1 m. The presence of Bry-ozoa has also been noticed within this facies in theBhaun–Nilkanth section. The intercalated sequenceof facies B2 and B3 has been observed in Dugadda.Facies B3 conformably overlies the facies B2. Thethickness of this facies varies from 1 to 30 m withpackstone thickness varying from 20 cm to 1 m.This facies is overlain by shell limestone or coquinabed of facies association-C in all the measuredsections.


Compact bed geometry, flat to slightly undu-lated bed top, thin laminations and a retrogra-dational stacking pattern, i.e., a fining-upwardsequence suggests the deposition of sediments ina shallow open marine condition. The bed geom-etry, structures and the stacking pattern sug-gest a lower shoreface setting of deposition. Thenummulitic shale and siltstone, in turn, are suc-ceeded by the Assilina–Nummulite packstone. TheAssilina–Nummulite packstone and the overlyingthe shell limestone/coquina limestone bed of faciesassociation-C are present in all the measured sec-tions and probably indicate the change in shorelinemigration from transgression to regression withmaximum flooding surface (MFS) defined by theAssilina–Nummulite packstone bed.

4.3 Facies association-C

The facies association-B further grades into faciesassociation-C, comprising facies types C1 and C2.Facies C1 is represented by shell limestone orcoquina bed. Indistinct hummock beds are sus-pected at places, though could not be ascertainedwith conviction due to the poor preservation ofthe primary structure. Facies C2 comprises greenshale interbedded with fine-grained sandstone andsiltstone. The shale and sandstone of C2 containLBF (reworked), oyster (figure 3e) and gastropod(turritella). The sandstone is fine grained and thethickness varies from 50 cm to 2 m. Distinct waveand current features in terms of cross beds andsmall ripples are also present.

This facies association is laterally persistentthroughout the area and thickness ranges from10 to 14 m. At places like Tal–Bidasini andBhaun–Nilkanth sections (figure 2), interbeddedlimestone is also recorded. In the Tal–Bidasinisection, this association is represented by interca-lated green shale and oyster-bearing shell limestone

beds (figure 3e). The bed geometry is defined bycentimetre to metre thick lenticular fossiliferouslimestone which pinches out laterally. The shelllimestone is succeeded by the turritella-bearingshale and sandstone intercalated sequence (figure 3fand g).


The coquina limestone of facies C1 denotes thedeposition in high-energy condition. This faciesassociation-C being represented by the gastropod-bearing green shale along with shell limestone isindicative of a relatively shallower environmentof deposition. The facies association as a wholeshows aggradation (shale and limestone inter-calated sequence) to progradational (sandstone)stacking pattern. Sedimentary structures such assmall scale ripples low angle cross beds and aggra-dation to progradational stacking pattern withthe presence of coquina bed suggest the possibleupper shoreface to foreshore setting of deposition.The interbedded limestone indicates less siliciclas-tic input from the land.

4.4 Facies association-D

Regionally, the facies association-C is overlain byred shale of facies association-D with an ero-sional contact. This facies association comprisesNummulite-bearing (reworked shells) red shale atits base, siltstone and medium- to coarse-grainedsandstone at the top. This facies association hasfurther been classified into two facies, D1 and D2.Facies D1 consists of an intercalated sequence ofred shale, siltstone and thinly bedded fine-grainedsandstone. The thickness of facies D1 varies from4 to 30 m. Facies D2 with increased sand to shaleratio is represented by red siltstone and sandstone.Sandstone is medium to coarse grain in nature. Rel-atively massive bedded coarse-grained sandstonewith occasional shale partings is present at upperpart of facies D2. Thick sandstone bodies havinglenticular geometry also present in the Dugaddasection. Cross-bedding within sandstone at theupper part of the facies is seen both in Tal–Bidasiniand Dugadda sections (figure 3h). The sandstoneunit in the Bhaun–Nilkanth section is characterisedby wave ripples.


The basal erosional contact of this facies asso-ciation suggests the exposure of the shelf due


to the rapid fall in mean sea level (MSL). Thealternate siltstone and fine sandstone with highdegree of sorting suggest a continuous reworkingof the sediment under the wave action. Overall, thecoarsening and stacking pattern indicates the depo-sition in a progradational setting. The massive bedforms, both textural and mineralogical maturity,low-angle cross-beds, distinct wave features (waveripples) also suggest the deposition under a fore-shore environment.

5. Petrography

The facies architecture is further supplemented bythe petrographic analysis which very well ascer-tains and correlates with the depositional environ-ment discussed earlier. Based on the petrographiccharacterisation of lithologies from bottom to top(figure 4), the following nomenclature is suggestedfor rocks of Subathu Formation, which very wellsubstantiates the field observations.

5.1 Ooidal ironstone

Petrographically, the non-skeletal and coated car-bonate, i.e., ooid grains are the major constituentof the ooidal ironstone (figure 5a). The coated

grains comprise a more or less well-defined nucleus,enveloped by both ferruginous and calcareousmaterial (concentric rims). The ooids are com-posed of rims of either aragonite or calcite andoriented with a preferable size range of less than2 mm. The matrix is ferruginous in nature. Theooids comprise 25–30% of the ironstone and arewell preserved. These ooids are dark grey to steelgrey in colour and most of them are of uniformsize. Ooids are mainly rounded in shape but fewof them show deformed nature through flatten-ing effect. The presence of composite ooid grainsindicates reworking.

The presence of ooids in the rocks indicatesthat the deposition was taken place in a shal-low marine agitated condition. Some of the ooidshave quartz nuclei and some consists of calcitenuclei. The ooids cortices show the typical brown-ish translucent appearance. The colour may resultfrom the included organic matter which prob-ably plays an important part in their forma-tion. Darker areas within the ooid cortices areprobably the areas of random and equant arago-nite, and perhaps are produced through micriti-sation of endolithic micro-organisms. Sphericityand sorting suggest deposition in high-energycondition.

Figure 4. A spatiotemporal facies model illustrating the depositional environment of the facies associations of SubathuFormation in correlation with the shoreline migration.


Figure 5. Photomicrographs showing (a) oolitic ironstone, the ooids of which are composed of rims of iron-rich clay mineralsurrounded by aragonite or calcite, (b) the equatorial section of Daviesina sp. from the grey shale of Subathu Formation,the Duggada section, Uttarakhand. Assilina wackestone, having the bioclast of (c) Assilina spira abradi, from the limestonebed within grey shale, (d) Nummulites burdigalensis, from the grey shale, the Nilkanth section, Uttarakhand, (e) N. obtusus(Form B) from the grey shale, Duggada and Nilkanth sections, Uttarakhand, (f) Nummulite sp. forming the Nummuliticpackstone, (g) Assilina–Nummulitic packstone, comprising bioclasts of both Nummulite and Assilina and (h) cross-polarview of sandstone of Subathu Formation (facies association-D).


5.2 Nummulitic wackestone

Under the microscope, it is mainly characterisedby quartz grains (15–25%) and bioclast (30–40%)mostly represented by large benthic foraminiferasuch as Nummulites, Assilina and Daviesina sp.(figure 5b) held in carbonate mud. Intense micriti-sation has taken place during diagenesis where themargins of carbonate grains are replaced by micriteat or just below the sediment–water interface. Theprocess involves the microbes attacking the out-side of the grain by boring small holes in them,which are later filled with micritic mud. Matrixoccupying the space between the allochem grainsis fine-grained aphanitic mud with least admix-ture of any terrigenous material. Based on thematrix percentage, composition and relative elon-gate shape of the Nummulite shells, the lithologicalassociation is believed to be of deeper part of a shal-low marine environment and corresponds to faciesassociation-B.

5.3 Assilina wackestone

Under the microscope, this lithology largely con-sists of micrite (70–80%). Quartz grains are alsopresent substantially. The allochems together con-tribute about 20% of the framework grains. Thepresence of in-situ as well as broken bioclasts ofAsillina (figure 5c), Nummulites (figure 5d and e),echinoderm and corals indicates towards high-energy marine and lacustrine environments wherecurrents and waves result in the vigorous winnow-ing and abrasion of the shells. The presence ofNummulites is very minimal. It indicates a rela-tively deep water (50–80 m) in open marine setting(Racey 1994). This is correlatable with the middleto upper part of facies association-B.

5.4 Nummulitic packstone

Nummulitic packstone is represented by the domi-nance of Nummulite (figure 5f) although numerousbroken fragments of bivalves are also found in thisparticular lithology. It comprises grains of quartzand bioclast bounded by the carbonate cement.Bioclasts (20%) are mainly of bivalve and Num-milite shell fragments. Two generations of cemen-tation are observed. Thin micrite cementation ofearlier generation present on the original grain sur-face. Later due to the effect of diagenesis the earliermicrite, in turn, transforms into sparitic cement.The Nummulite shells are relatively globular and

flattened, rather elongated. The relatively lessermatrix percentage and grain supported nature cat-egorise this lithology, which is to be deposited inthe upper part of the shallow marine shelf envi-ronment. With these characteristics, it is believedto occur at the middle to upper part of the faciesassociation-B.

5.5 Nummulite–Assilina packstone

It comprises both Assilina sp. and Nummulitesp. Under the microscope, this lithounit is highlyfossiliferous consisting of 60–70% bioclasts ofNummulites–Assilina sp. (figure 5g), bivalve, gas-tropod and other fossil groups. These bioclastsare randomly distributed. Both broken and intactshell fragments of these bioclasts are present.Quartz grains occur in lesser quality. The micriticmud forms the main orthochemical component inwhich the bioclasts are found to be embedded.The ovate-shaped Nummulites occur as in-situ,whereas Assilina is derived from the deeper leveland deposited in shallow water depth, possibly bymeans of storm condition. All these evidences alongwith the petrographical characteristics place it inthe transition zone between the facies association-B and -C.

5.6 Oyster-bearing grainstone

It is represented by the grain supported frame-work structure. The intergranular space is filledwith calcite cement. Petrographically, it consistsof both broken and intact bioclasts, more than50% in abundance and are found to be randomlydistributed (mostly they are of bivalve shells andoysters). Grains of quartz and iron oxide are dom-inantly present in this rock (10–15%). The skeletalgrains are found to be present within the micriticmatrix which is nothing but the carbonate mud.The carbonate detritus consists of fragments of pre-existing limestone, i.e., the intraclast and the fossilfragments. The percentage of the matrix is veryless due to continuous reworking. Spary calcite isthe cementing material. Under higher magnifica-tion, the crystalline growth of the sparite duringthe diagenesis is very much prominent. However,the crystal growth is restricted within the porespaces, i.e., the drusy crystallisation structure. Theabove-described characteristic features indicate arelatively higher energy condition as compared tothe previous lithologies. This lithological unit alongwith the preceding one defines the end of the facies


association-B as well as marks the lower part offacies association-C and denotes the deposition inhigh-energy condition.

5.7 Bivalve gastropod-bearing shale and limestone

Petrographically, it consists of both broken andintact bioclasts (50%). They are randomlydistributed (mostly bivalve shells). The matrixis micritic in nature. However, at places, thesemicrites are transformed into sparitic cement. Thepresence of turritella and other bivalves brings itsaffinity more towards the near shore deposits andits equivalence to facies association-C.

5.8 Upper sandstone

Petrographically, it can be described as thesubrounded–subangular grains of quartz (80%),lithic chert fragments and opaque mineral(figure 5h). The intergranular spaces between themineral constituents are filled by both the siliceousand micaceous matrices. The cementing mate-rial is ferruginous in nature. Sub-rounded natureof quartz suggests fair transportation before thedeposition. The high abundance of quartz grainsrepresents compositionally/mineralogically maturerock, so as the high degree of sorting attributes tothe textural maturity of the rock (figure 5h). Theline as well as sutured contact between the grainsat places might be the result of high compaction.These petrographic characteristics are suggestive offoreshore or beach environment of deposition whichis correlatable with the facies association-D.

6. Discussion

The facies type varies in palaeoenvironment ofdeposition from restricted lagoonal to foreshorethrough a transgressed shoreface setting (bothlower and upper shorefaces). In all the litho-sections of the Garhwal Syncline (stretching over80 km), the invariable presence of key strati-graphic horizons/levels such as the oodial iron-stone, coquina limestone and erosional surface atthe base of upper sandstone strongly establishesa relationship between the Subathu sedimenta-tion and the relative sea level changes within thebasin.

The facies analysis correlates with geneticallyrelated lithologies as facies association, describ-ing how the sedimentation has taken place withinthe basin in accordance with shoreline migration.

The facies association-A with the predominantoolitic ironstone horizon along with the carbona-ceous shale, basal nodular sandstone, grey siltyshale, marly and calcareous siltstone accountsto intermittent pulses of high-energy conditionwithin a calm and less disturbed environment ofdeposition. This facies association also marks theonset of marine transgression for the Subathu sed-imentary sequence. The gradational trend shiftsin lithology form facies association-B to faciesassociation-C with the coquina limestone at thejuxtaposition suggests that the progradation wasgradual and of basin scale.

Both extrabasinal and intrabasinal events pos-sibly contributed to the deepening-upward trendduring the period of transgression, as the mean sealevel started approaching towards the continent,the deeper water sediments started to on lap overthe shallow marine sediments along with detritussupply from the continent. This has given rise to afining-upward sequence for the facies association-Aand -B in terms of retrogradational stacking pat-tern. Further, the facies association-C and -Dreveal the shallow to transitional marine asso-ciations with an overall progradational stackingpattern. In the stratigraphic record, such coarse-ning-upward marine to coastal strata are inter-preted as a sign of progressive inward deepening(Bourgeois 1980; Emery and Myers 1996). Thisirreversible transition from the submarine to thesubaerial condition with pronounced basal andupper basin-scale discontinuity supports a regionalfall in the sea level.

A spatio-temporal facies model for the deposi-tional environment of the above facies associationsin correlation with the shoreline migration has beenproposed (figure 4). This model further explainsthe sedimentation and stacking pattern of differ-ent lithopackages of the Subathu Formation. Theonset of transgression with proximal lagoonal car-bonaceous shale/ooidal ironstone deposit is furthersupplemented by the onlapping of the deeper waterlithofacies over the shallower by means of a fining-upward stacking pattern. Hence, it depicts deposi-tion in transgression, wherein the grey and greenshale units have come over the lagoonal facies withbasin deepening. However with the normal regres-sion, where the rate of sedimentation has outpacedthe relative increase in the mean sea level and hencethe interbedded character between the grey andgreen shale units suggests that the progradationwas gradual and of basin scale with corroborativeevidence in terms of increasing sandstone, the shale


ratio documented and correlated over the measuredsections. Sedimentological studies suggest that theupper Subathu Formation represents laterally per-sistent foreshore deposit on the starved shelf.

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Corresponding editor: Pratul K Saraswati

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