Sedimentology of a Proterozoic erg: the Venkatpur Sandstone, Pranhita-Godavari Valley, south India
T A P A N C H A K R A B O R T Y
Geological Studies Unit, Indian Statistical Institute, 203 Barrackpore Trunk Road, Calcutta-700 035, India
A B S T R A C T
Reappraisal of the Late Proterozoic V enkatpur Sandstone indicates th at the bulk of the sandstone is aeolian in origin. Aeolian stratification types, namely (i) inverse graded translatent strata, (ii) adhesion laminae, (iii) grainflow strata and (iv) grainfall strata, are present throughout the outcrop belt. N ine facies have been identified that represent both aeolian and related aqueous environments within a well-developed erg. Cosets of large cross-beds at the Bellampalli section in the N W o f the study area record dune fields in the interior of the sand sea. To the SE, at the Godavari River and Ramgundam sections, a progressive increase in the relative proportion of the flat-bedded to cross-bedded facies and intercalated non-aeolian facies delineates the transition from the dune-field to sand-sheet environment. An alternating sequence of aeolian and marine sediments at Laknavaram , in the extreme SE, m arks the termination of the sand sea.
Palaeocurrent data suggest that the N W -S E trend of the sections represents a transect across the sand sea in a direction normal to the resultant prim ary palaeowind direction.
Abundant horizontally stratified units in the Vankatpur Sandstone do not always represent the interdune sediments. On the basis of the thickness and geometry of the units, nature of bounding surfaces and associated facies sequence, the facies is variously interpreted to represent interdune, inland sabkha, sand sheet and coastal sand flat deposits.
I N T R O D U C T I O N
Proterozoic sedimentary sequences in several isolated basins cover an extensive part of P eninsular In d ia (Fig. 1). The sequences closely resemble th e ‘carbon- ate-orthoquartzite facies’ in terms of lithology, com position and extensive sheet-like disposition. T he carbonate and most o f the sandstone sequences have been interpreted as deposits of shallow c ra to n ic seas (Banerjee, 1974; Chanda & B hattacharya, 1982;
Chaudhuri & Howard, 1985; Singh, 1985; N ag raja Rao etal., 1987).
Development of extensive sheet sandstones th ro u g h shallow-marine processes alone, however, is being increasingly questioned (e.g. D ott et al., 1986). Study of modem shelf processes (Swift, Stanley & C u rrary , 1971) suggests that these are less effective in d isp ersin g sands over wide areas compared to ae o lia n a n d braided fluvial processes (Dott & Byers, 1981; F uller,
1988). N o n -m arin e processes m ig h t hav e been even more im p o rta n t in pre-Silurian tim es, before the advent o f la n d vegetation, in spread in g sand sheets (see Folk, 1968).
W ith th is background, a Late P roterozoic sandstone sequence, th e V enkatpur Sandstone o f the P ranhita- G odavari V alley, south India, h as been studied.
P relim inary analysis o f stratification types indicates th at the san d sto n e, which is spread over a linear belt o f more th a n 100 km (Fig. 1), form ed essentially through ae o lia n processes (C hakraborty, 1988). This paper d escribes th e stratification a n d facies o f this sandstone a n d th u s recognizes different aeolian sub
environm ents sim ilar to those identified in modem sand seas; it also attem pts to characterize th e extensive Proterozoic sa n d sea, based on the analysis and inter
pretation o f facies assemblages on a regional scale.
□
/ A 4S$nGondwana Supergroup s V A
a
fVenkatpurSst 'SuI Iq v q i Group “
|#. |* .* | Albaka Group
|::: | Somanpalli Group
|* ,*| Yellandlapad Group
|a a| Pakhal Group Penganga Group
1111 [ Undifferentiated Proterozoics 1^51 Basem ent Complex
Fig. 1. Geological map of th e Pranhita-G odavari Valley. T he inset shows the Proterozoic sedimentary basins o f Peninsular
India.
T H E V E N K A T P U R S A N D S T O N E T h e V enkatpur Sandstone, th e top-m ost form ation o f th e Sullavai Group, occurs as an extensive sheet, exposed in two N W -S E -trending linear outcrop belts along tw o flanks o f th e P ranhita-G odavari V alley basin (Fig. 1). The form ation consists of m edium - to fine-grained, red, subarkosic sandstone and is c h a r
acterized by th e p rofuse development of large planar cross-beds. T h e san d sto n e occurs in close association with fluvial q u a r tz are n ites and arkoses (see Table 1;
C hakraborty, 1988). Extensive development o f1 salmon-red colour, h ig h t e x t u r a l maturity and presence o f considerable am o u n ts of well-rounded first-cycle sand grains (C h a u d h u ri, 1970,1 9 7 7) characterize the sandstone th ro u g h o u t th e studied area. The sandstone
rocks ofthe Pranhita-Godavari Valley(after Chaudhuri& Howard, 1985; Chaudhuri& Chakraborty, 1987and
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has previously been in terpreted as a shallow-m arine to b each deposit (Johnson, 1965; Chaudhuri, 1970).
E V ID E N C E F O R A E O L IA N D E P O S I T I O N O F T H E V E N K A T P U R
S A N D S T O N E
In addition to the abundance o f well-rounded san d grains (Chaudhuri, 1970,1977), tw o distinctive ae o lia n features are clearly recognizable throughout th e
outcrop b e lt: a d h e sio n structures (Hunter, 1973,1980;
Allen, 1982; K o c u re k & Fielder, 1982; Figs 2 & 3) and w ind-ripple strata. Two types of wind-ripple strata are re c o g n iz e d : (i) thin, inverse-graded transla- ten t strata (H u n ter, 1977a,b; Kocurek & Dott, 1981;
Fig. 4) and (ii) in terlayered lenticular, discontinuous, coarse to m edium san d layers and associated granule lag surfaces (B agnold, 1954; Sharp, 1963; Fryberger, A hlbrandt & A n d re w , 1979; Kocurek, 1981a,b).
Though not d ia g n o stic , o ther common types of aeolian
Fig. 2. Climbing adhesion ripple cross-laminae (zone below the scale) in the flat-bedded unit at Bellampalli. The upper part shows alternate finer-grained deep-red sandstone and clean w hite sandstone. Scale = 5 cm.
Fig. 3. Plan view of adhesion ripples, Bellampalli quarry. Scale = 5 cm.
features recognized in the Venkatpur S an d sto n e are grainflow and grainfall laminations (H unter, 1977a;
Kocurek & Dott, 1981). Grainflow cross-strata are typically wedge-shaped, pinch out within w in d -rip p le or grainfall strata near the base of the cross-sets (Fig.
5) and have irregular spacing between them (Fig. 6;
Hunter & Kocurek, 1985). Grainfall stra ta on the other hand are fine grained, drape p re-existing topography and thin out in the downwind d irec tio n (Fryberger & Schenk, 1981; Anderson, 1988).
F A C IE S
The V en k a tp u r Sandstone has been classified into nine facies m ain ly on the basis o f stratification and other se d im e n tary and lithological attributes (see Table 2).
Facies 1 (FI): large-scale cross-bedded sandstone Facies 1 consists o f fine- to coarse-grained sandstone with large ta b u la r o r wedge-shaped cross-beds. Cross-
Fig. 4. A low-angle section of translatent strata from the Venkatpur Sandstone showing inverse grading and occasional faint
traces of gently dipping foreset laminae. Scale = 5 cm.
% S. Wedges of grainflow strata pinching out w ithin a grainfall deposit in the lower part of a small dune cross-bed at Bellampalli; layers of flat-bedded wind-ripple strata enclose the cross-set. Scale = 5 cm.
Table 2. Summary of facies recognized in the Venkatpur Sandstone.
Facies no. Brief description Interpretation
FI Large planar cross-beds with either (a) steeply dipping (23-30°) foresets abutting at high angle to the lower bounding surface, having a significant proportion of grainflow strata or (b) long tangential foresets comprising finer-grained grainfall strata. Rare, gently convex-up low-angle strata and cross-strata consisting entirely of fine-grained wind-ripple laminae and having a radial dip pattern in adjacent sets (Fig. 8)
Aeolian dune cross-beds; convex- up laminae may be transverse section of a cresentic dune complex or in-phase cresentic dunes or dome dunesf?) F2 Very coarse- to fine-grained flat-beds mainly of wind-ripple strata; occasional
sets of low-angle (10-15°) cross-beds com prising bimodal, coarse-grained wind-ripple strata; rare adhesion or salt-related structures and small aqueous cross-beds. Generally 01-0-7 m th ick but can be several m etres in thickness
Dry interdune o r s a n d sheet with low-angle z ib a r cross-beds(?)
F3 Medium to muddy fine-grained flat-beds w ith abundant salt-related deformational features, adhesion structures, ‘D U ’ sequences, close spaced erosion surfaces w ith microrelief (Figs 10 & 11) and occasional wave ripples. Extensive tabular units of average thickness of 0-6-1 -0 m but may be up to 5 m.
Sabkha/coastal sand flats/
periodically wetted interdune or sand sheet
F4 Areally restricted, up to 1 -5 m thick, thinly lam inated mudstone; occurs in close association with other aeolian facies
Shallow pool of quiet w a te r within the sand se a
F5 Massive, poorly sorted, fine- to very coarse-grained sandstone forming F U lenticular sandbodies w ith erosional lower contact; 0-35—1 -5 m thick (Fig.
12)
10-40-cm-thick trough cross-bedded mudclast conglomerate and feldspathic sandstone; occasional large, wavy bedforms with mud-draped, low-angle accretion surfaces (Fig. 13); sharp base, gradational upper contact w ith F7
Small, isolated channel-fills
F6 Tidal channel deposits
F7 Muddy, fine-grained sandstone with sym metrical megaripples, often interference ty p e ; gradationally overlie F6
Low-energy coastal environment/
upper part of the tidal channel sequence
F8 Fine to silty sandstone with ripple-drift cross-lam ination; few centimetre thick pseudobeds, well-preserved internal foresets and lack of within- laminae grading; gradationally/sharply overlie F9
Aqueous ripple-drift cross
lamination F9 Fine- to medium-grained sandstone with plane beds and well-developed
parting lineation; extensive tabular bodies
Upper-flow-regime plane beds
Fig. 6. Foreset bedding of a large planar cross-bed. N o te th e irregular spacing and wedge-like shape of the darker, coatse- grained grainflow strata (‘a ’) alternating with lighter, finer-grained grainfall strata (‘b ’). The view represents about 2 m of the cross-bedded section.
Fig. 7. Scalloped and lenticular shape of the grainflow tongues exposed in the bedding plane section, Bellampalli quarry.
Uniform layers of wind-ripple origin are seen in th e bottom and top right com ers o f the photograph. Scale = 5 cm.
beds occur in cosets or in isolated sets, an d ra n g e in thickness from 0-3 to 3-0 m, averaging a ro u n d 1 m . Two d is tin c t types of cross-bed have been o b s e rv e d : (i) steeply dipping (23-31 °) planar, medium- to co arse
grained foresets abutting at a high angle to th e low er bounding surface; abundant grainflow strata a lte rn a te with grainfall and wind-ripple strata (Figs 5 - 7 ) ; (ii) m edium - to fine-grained foresets of grainfall stra ta that sw eep down asymptotically on the lower b o u n d in g surface fo r several m etres and t h i n dow ndip. In p la n view, foresets of the first type are generally straig h t but at places show gentle curvature. In a few sections fine-grained wind-ripple strata define som e gently dipping cross-beds, some o f which have foresets w ith a slight upward curvature (Fig. 8). The a d ja c e n t sets show divergent dip directions.
T he cross-beds are preserved remnants o f m ig ra tin g large d u n e s, either straight-crested or cresentic. T he angular foresets probably represent sm aller dunes
where grainflow s frequently reached th e bottom o f the lee faces (K o c u re k & D ott, 1981), w hereas tangential foresets m ay represent the low er p a rt of large transverse b edform s with ab u n d an t grainfall strata in the toe region (H unter, 1981; K o cu rek & D ott, 1981;
Ross, 1983).
The convex-up wind-ripple s tra ta may record m igrating d o m e dunes (Fig. 20 o f F ryberger, Al-Sari
& Clisham , 1983) o r may represent a transverse section through a cre sen tic dune complex. S im ilar convex-up lam ination m a y also be produced by in-phase cresentic dunes (G. K o cu rek , 1990, pers. com m .).
Facies 2 (F2): flat-bedded, coarse- to fine-grained sandstone
This facies is characterized by horizo n tal to subhori
zontal, coarse- an d fine-grained w ind-ripple strata.
Locally ad h e sio n laminae, small lenses o f aeolian
* 30 m 25m 20"> 15m l0m 5rn 0
Fig. 8. Field sketch of the railway cutting section, R am gundam area. Moderate and gently dipping cross-beds consist of 1-2- cm-thick translatent strata. Some of the low-angle strata show a gentle upward curvature over a long distance; adjacent sets have almost a radial dip pattern. Note lateral transition from low-angle to high-angle cross-beds. The sequence may represent a dome dune or a transverse section of barchan dunes or in-phase cresentic dunes. T he dashed unit represents a mudstone bed.
Section trend 340-160°.
cross-beds or isolated shallow troughs of aqueous origin comprise a m inor p a rt o f the facies. In d iv id u a l facies units are 0-1—0-7 m thick, but can reach several m etres in thickness. F airly persistent subhorizontal p la n a r surfaces m arked by th e concentration o f coarse sands or small pebbles occur in this facies. T h ic k e r u nits o f this facies contain low-angle (10-15°) cross
beds varying in thickness from 25 to 40 cm . T h e individual foresets o f these low-angle sets co n sist o f bim odal, coarse-grained w ind-ripple strata.
T he bulk o f the facies developed through the m igration o f wind ripples on topographically flat, dry depositional areas of eith er a n interdune or sand sheet.
Sm all aeolian dunes developed locally. W et co nditions developed in places or for sh o rt periods, giving rise to isolated trough cross-beds, a n d salt ridge and a d h e sio n lam inae. The low-angle cross-beds are in terp reted to represent zibars, the low -am plitude aeolian bedform s th a t lack avalanche slipfaces and generally develop in th e m arginal parts o f dune fields or in sand sheets (N ielson & Kocurek, 1986).
F ades 3 (F3): flat bedded, medium- to fine-grained muddy sandstone
F acies 3 is commonly m a rk e d by alternate re d a n d w hite bands (Fig. 2) an d com prises fine to m e d iu m san d w ith a subordinate am o u n t o f coarse sand. Som e o f th e red layers are finer g rain e d and m uddy. B oth
fine- and coarse-grained wind-ripple strata are the dom inant stratifica tio n style of this facies, closely followed by ad h e sio n structures of different types, wave ripples, m udcracks and profuse soft-sediment deform ation structures. The deformation, such as irregular, p ro n o u n ced ridge-like upward flexures (Fig.
9, arrows), collapse structures, water-injection struc
tures and sm all-scale reverse faults, result in irregular and wavy b ed d in g in places (Fig. 9). The common occurrence o f ad h e sio n structures, wave ripples and pervasive d efo rm a tio n differentiates this facies from the flat-bedded sa n d s o f F2. Intervals of this facies generally vary from 0-6 to 1-0 m in thickness, but may be up to several m etres. Fairly close-spaced, slightly irregular, flat erosional surfaces with microrelief of th e order o f several centim etres (Figs 10 & 11) occur w ithin the th ic k e r units of this facies and enclose laterally extensive th in sedimentation units (Figs 9 &
11). The basal p a rts o f such units are dominated by red and w hite b an d s, abundant ridge-like and other deform ational fea tu re s as well as local occurrences of wave ripples a n d m udcracks. This is followed upward by adhesion stru ctu re s and finally by wind-ripple strata and sm all ( ^ 30 cm) aeolian dune cross-beds (Fig. 11).
T he sequences are clearly drying-upward (DU) sequences (cf. K o cu rek , 1981a). Collapse structures and reverse fau lts (Fig. 9) appear to be related to the in trasedim entary gro w th and dissolution of evaporites
Fig. 9. Close up of the sabkha sequence at Bellampalli. Salt-related deformation has given an irregular and wavy appe&rtifltf to the beds (unit ‘c’). Note pronounced up-arching o f beds th a t represent actual salt-ridge microtopography (arrows). Thin translatent strata, gently dipping to the right, comprise th e interval marked ‘b \ T he bed marked ‘a’ comprises essentially adhesion cross-laminae. Scale = 5 cm.
Fig. 10, Undulating microtopography of the upper bounding surface of a dune cross-bed, resulting from early cementation and subsequent differential wind erosion of foresets. The overlying flat-bedded unit is characterized by salt-ridge and related irregular, wavy beds. Such associations are typical of deflation surfaces close to the water-table (Stoke’s surface). Scale = 30 cm.
m icrotopography may be deflation surfaces close to the ground w ater-table, where sm o o th deflation is im paired b ecau se o f the presence o f w ater, salt or early calcite c e m e n t (M cKee, 1979; K ocurek, 1981 a,b ; Simpson & L oope, 1985; Fryberger et al., 1988). As reported fro m b o th recent and a n c ie n t sequences, (see also Hunter, 1981; Kocurek, 1981a,b; F ry b e rg e r
etal., 1983, 1984; Fryberger, Schenk & K ry s tin ik , 1988) and the pronounced convex-up la m in a e are possible salt-ridge structures (cf. G lennie, 1970;
Collinson, 1978; Fryberger et al., 1983, 1984). T h e subhorizontal erosional surfaces (Figs 10 & 11) w ith
0 Cffl. 10
Fig. U. Field sketch of stacked DU sequences of F4 at Bellampalli. Salt-ridge structures, deformed strata and adhesion cross
laminae constitute the lower part and wind-ripple strata the upper part of each sequence. Note low-angle discordances between each D U sequence and broad undulation of the contact between sequences III and IV.
interdune areas that experience periodic flooding by rain or seawater or inland sabkhas or coastal san d flats are potential geom orphological sites for accu m u lation o f this type of sedim ent (Kocurek, 1981a,b;
K ocurek& F ielder, 1982; F ry b e rg e retal., 1983,1988;
H um m el & Kocurek, 1984; P ulvertaft, 1985; K o cu rek
& N ielson, 1986; Porter, 1987; K err & D ott, 1988).
Fades 4 (F4): mudstone facies
T h is facies consists of thinly bedded mudstone to fine
grained m uddy sandstone. It occurs closely associated w ith the aeolian facies, e ith er laterally interfingering or sharply overlying the la tte r (Fig. 8). The m axim um observed thickness of a facies u n it is about 1-5 m . T he facies was deposited in a qu iet subaqueous e n v iro n m en t such as a shallow pool o r lake within th e sa n d sea.
Facies 5 (F5): lenticular units o f apparently massive sandstone
F acies 5 consists of poor to m oderately sorted fine- to coarse-grained sandstone w ith mudclasts, organized in single or superposed m ultiple lens-shaped bodies (Fig. 12; Fig. 17 at 22 a n d 51 m). Individual lenses vary from 0-35 to 1-5 m in thickness, have erosional low er bounding surfaces a n d show a well-defined fining-upward trend. T he size range and roundness o f the sand grains o f this facies is the same as th e associated aeolian sediments.
T he sandbodies evidently form ed as aqueous ch a n - nel-fill deposits. The absence o f coarse extraform a- tio n al gravels distinguishes these channels fro m typical wadis, and points to th e ir intradune field origin. Preserved channel form s suggest th a t th e
ch an n els did n o t m ig ra te significantly and developed as ep h em e ral s tre a m s as a result of intense localized surface runoff (see also G lennie, 1987).
Facies 6 (F6): cross-bedded mudclast conglomerate and medium- to coarse-grained sandstone
T h is facies is m a d e u p o f m udclast conglomerate and m ed iu m - to c o a rs e -g ra in e d feldspathic sandstone, o rg an ized m a in ly in 10-40-cm -thick trough cross-sets.
F ew c e n tim e tre -lo n g m udclasts generally lie parallel to foresets. L o cally , la rg e sym metrical wavy bedforms, a b o u t 1-3 m in w a v e le n g th , occur in association with th e tro u g h c ro s s-b e d s. T h e bedforms have form- d isco rd an t, m u d - d r a p e d , low-angle internal lamina
tio n (Fig. 13). T h e facies units show well-defined fin in g -u p w ard ( F U ) tre n d s in terms of grain size, cross-set th ic k n e s s a n d clast frequency. They have sh a rp bases a n d g ra d e u p w ard into F7.
T ro u g h c ro s s -b e d d e d m udclast conglomerate and sandstone, w a v y b e d fo rm s w ith form-discordant in
te rn a l la m in a tio n , m u d -d ra p e s on foresets and close asso ciatio n w ith F 7 a re collectively interpreted to im ply a c u rre n t- a n d w ave-influenced shallow-marine e n v iro n m e n t o f d e p o s itio n for F 6 rocks.
Facies 7 (F7): wave-rippled, muddy, fine-graced sandstone
F ine-grained sa n d sto n es o f this facies show excellent developm ent o f w av e ripples (cf. Raaf et al., 1977) at various scales a n d orientation, and are locally super
posed on one a n o th e r producing a dimpled pattern (Fig. 13, foreground). Rippled surfaces are often veneered by m u d . A wave-agitated shallow-marine environm ent is in te rp re te d for this facies.
Fig. 12. Field sketch showing a view to the west of a lenticular channel-fill body in th e road cutting section, Ramgundam. The channel-fill unit cuts into the underlying dune-interdune sequence.
Fig. 13. Large wavy bedforms of F6 showing form -discordant internal accretionary surfaces (arrows). N ote the F7 wave ripples in the foreground. Scale = 30 cm.
Facies 8 (F8): line-grained sandstone with climbing- ripple cross-lamination
This facies consists of fine-grained sandstone w ith subordinate silt and mud and is characterized by climbing-ripple cross-lamination. The th ic k n ess o f the pseudobeds in the range of 2-6 cm, w ell-preserved foreset lamination and absence of reverse g rad in g suggest that the structures formed in a su b a q u eo u s environment. F8 always occurs intimately associated with the parallel beds of F9.
Facies 9 (F9): parallel-laminated fine- to medium- grained sandstone
Parallel-laminated fine- to m edium -grained sa n d stones with well-developed parting lineations o ccur as 10-50-cm-thick, laterally extensive, tabular units. It commonly underlies the clim bing-ripple-lam inated rocks of F8.
Parting lineation has been reported fro m som e aeolian deposits (M einster & Tickell, 1976; P o rte r, 1987), but alternation of parting-lineated san d sto n e with climbing ripples o f F8 clearly in d ic a te s an aqueous origin for the F9 rocks.
F A C I E S S E Q U E N C E
Bellampalli section
Detailsof a quarry section o f the V enkatpur S a n d sto n e near Bellampalli Railway Station (19°03', 79°29') are shown in Fig. 14. The lowest 4 m consists o f d une cross-beds of F 1. The cross-stratified unit is tru n c a te d
by a knife-edge erosion surface a n d is overlain by a 5- m-thick, laterally extensive, sheet-like unit of F4.
Signatures o f evaporite and aqueous deposition abound in th is unit. It consists o f a n u m b e r of stacked D U sequences (e.g. I IV in F ig. 11). Each DU sequence is separated from the o th e r, o r th e basal DU sequence from the underlying F I unit, by a subhori
zontal erosional surface with m icrotopography (Fig.
10) im plying deflation close to th e w ater-table. The erosional surfaces resemble in all th e ir essential details the ‘S toke’s surfaces’ as described from different modern sa n d seas (Fryberger et al., 1988). Sheet-like geom etry a n d th e thickness o f th e flat-bedded unit ( * 5 m ) exceeding those of the associated cross-bedded units p reclude a dune-confined, i.e. interdune, origin.
Evidence o f deposition on the deflation surfaces, ubiquitous presence of salt and th e absence of marine beds suggest a n inland sabkha o rig in for the flat-beds (K insm an, 1969). The sabkha sequence grades into a sequence o f w ind-ripple lam ination an d small cross
beds o f F 2 reflecting a com paratively dry, sand- saturated condition. The 3-m -thick sequence domi
nated by coarse-grained w ind-ripple stra ta resembles deposits o f aeo lian sand sheets (cf. F ryberger et al., 1979,1983; K ocurek & Nielson, 1986).
The succeeding 8 m is dom inated by large ( « 3 m) cross-beds (Fig. 14). The cross-beds occur in single sets alte rn a tin g w ith thin units o f w ind-ripple flat- beds (F2). T h e topm ost p art o f th e section consists of large superposed cross-beds. Superposed cross-beds represent e ith e r a compound aeo lian bedform (see H avholm & K ocurek, 1988) o r dune complexes, whereas th e wedges or thin sheets o f w ind-ripple strata are the in te rd u n e deposits. The coset o f large cross-
20m.
S u p e r p o s e d
\ o r g * d u n e s ( O r a a )
17
Fig. 14. Field sketch showing details of the stratification exposed in the cliffs of Bellampalli quarry.
b ed s in th e basal and u p p erm o st p art of th e sectio n represents the net deposit left by the m igrating d u n e com plexes in the V enkatpur sa n d sea.
Godavari River section
T h e section measured in th e south bank o f th e G o d av a ri River, 2 km so u th o f M ancheral (1 8 °5 2 \ 79°28', Fig. 1) is about 17 m th ic k and p redom inantly finer grained than the o th e r sections. Dune cross-beds a n d interlayered thin flat-beds, between 1-5-6-0 m an d 14-17 m (Fig. 15), rep rese n t m igrating d u n e - interdune complexes. S ubhorizontal w ind-ripple
strata do m in ate th e rest of the sequence. Small- to m edium -sized d u n e cross-beds, aqueous trough cross
beds, sedim ent collapse and water-injection structures, rare adhesion la m in a e and salt-crust layers occur at different levels in th e flat-bedded unit. The sequence represents se d im e n tatio n in wide low-lying sand sheets, d issected by small channels.
C om pared to th e Bellampalli section, the dune cross-beds in th is section are much thinner. The mean thickness o f 10 d u n e cross-beds at Bellampalli is 1-3 m, w hereas th a t a t G o d av a ri is only 0-6 m. The presence of a sand sheet, com prising about 40% of the sequence, and decreased heig h t of the dune bedforms are consistent w ith a setting marginal to the central erg.
The absence o f any extra-erg, non-aeolian facies either enclosing or in te rc a la te d with the aeolian unit, on the other hand, p reclu d es an erg-margin environment.
T he sequence should correctly be termed an ‘extra- dunefield d e p o sit’ as used by Fryberger et al. (1979, Fig. 12, p. 745) a n d is analogous to the ‘zone of tran sp o rta tio n ’ in th e Jafurah Sand Sea (Fryberger et al., 1983).
Ramgundam section
A t R am gundam (18°45', 79°26') several kilometresof road cuttings w ere exam ined. The section is marked by rapid facies v aria tio n , both laterally and vertically, and com pared to th e Bellampalli or Godavari sections, the sandstone h ere is m ore coarsely grained.
Cross-beds ran g in g in thickness from 0-3 to 2-8 m w ith angular foresets are common throughout the section and a lte rn a te w ith thin (< 40 cm) wind-ripple strata (Fig. 16). T h e sequence represents a migrating du n e-in terd u n e com plex. The dune-interdune se
quence is in te rru p te d a t different levels by (i) thicket units o f co a rse-g ra in ed wind-ripple strata and zibar cross-beds (7 -9 m , 11-5-13-5 m, 25-28 m, Fig. 16)that are in terpreted to rep resen t sand sheets; (ii) wavy and irregular-bedded (F3) sabkha sequences (19, 27 and 30 m) and (iii) a p p a re n tly massive channel-fill deposits (0-2-5 m , 16-19 m , 38-39 m, Fig. 16). About 2km ESE o f the ro a d section a railway cutting exposes a th in sequence o f m udstone (F4) that overlies and partly interfingers w ith convex-up wind ripple strata (Fig. 8). The m u d sto n e sequence suggests development o f shallow e p h e m e ra l lakes within the sand sea. The aggregate th ic k n e s s o f th e flat-bedded strata compris
ing 35% o f th e sequence, frequent interlayers of sand sheet, sabkha a n d fluvio-lacustrine deposits and large am ount of fa c ie s v aria tio n is thought to reflect the organization a n d architecture of the extra-dune field
Fig. 15. Sequence of sedimentary structures in the Godavari River section.
sand-sheet-dominated environm ent. F airly la rg e iso
lated dune cross-sets averaging 1 • 17 m (cf. B e lla m p alli and Godavari R iver sections) throughout th e se
quence, however, suggest the opposite. T h e fluvio- lacustrine deposits are interpreted as p a rt o f th e erg system and the absence o f extra-erg facies in d ic a te s that Ramgundam does not represent the m a rg in o f the Venkatpur sand sea. It appears th a t th e sa n d saturation level at R am gundam was enough to in itia te the dune-building processes repeatedly in tim e a n d space, but the dunes failed to develop into tr a in s o f
com pound com plex bedforms. C oarse grain size, frequent d ev e lo p m e n t o f small sab k h as and fluvio- lacustrine pro ce sses are some o f th e identifiable factors th a t in h ib ite d d u n e growth and a low-lying sandy plain w ith sc a tte re d dune complexes resulted instead.
Laknavaram section
A stream se ctio n near Lake L ak n av aram Cherevu (18°09', 80°05') exposes a 60-m-thick sequence (Fig.
17). The low est 18 m consists essentially o f a number
Fig. 16. Columnar section o f the Venkatpur Sandstone in the road cutting, Ramgundam.
o f superposed FU sequences (Fig. 17). E a c h F U sequence starts with a n erosional surface o v erla in , successively, by m udclast conglom erate, trough cro ss
bedded sandstone facies (F6) a n d mega-rippled m u d d y fine sandstone (F7). T he sequence is inferred to rec o rd th e deposition from m ig ratin g tidal channels (cf.
M oslow & Tye, 1985).
T he overlying sequence from 19 to 26 m consists o f alternating units of aeolian a n d tidal channel facies.
A eolian facies in this interv al consist p rim a rily o f w ind-ripple strata w ith subordinate adhesion stru c tures, salt-ridge structures intercalated with ra re w av e / cu rren t ripples and a few sm all aeolian cross-beds.
T he assemblage represents coastal sabkha an d c o a sta l sand-flat deposits (see K o cu rek & Fielder, 1982;
H um m el & Kocurek, 1984). T h e alternation o f m a rin e an d aeolian environm ents resulted through in te r m it
te n t transgression o f th e shallow sea up o n th e prograding coastal dunes o r coastal sabkhas.
F rom 34 m upwards, the sequence consists essen
tially o f horizontally stratified wind-ripple strata with a few small d u n e cross-beds, interspersed with several thin horizons containing wave ripples, desiccation cracks, and ad h esio n - and salt-related structures. A unit o f sta ck e d channel-fill deposits (F5) occurs between 51 a n d 54 m (Fig. 17). The topmost 4 m shows the d evelopm ent of comparatively larger ( * 1 m) sets o f aeo lian cross-beds.
The sequence m a rk s the southeastward marine term ination o f th e V enkatpur erg where a milieu of sand sheets, co a sta l sabkhas, small isolated dunes and ephem eral ch a n n e ls appeared over an extensive sand flat close to th e palaeoshoreline. The transition in the vertical sequence from shallow-marine to aeolian- m arine in te rcalatio n and finally to dominantly aeolian facies m arks th e seaw ard progradation of the aeolian environm ents.
Machchupuram section
Several k ilo m etres n orth o f Lake Laknavaram, the V enkatpur S a n d sto n e crops out in a few low-lying ridges or is exposed in a few small quarries. The sections are, o n average, about 1 m thick, and each com prises a 10-30-cm -thick basal unit of parallel- lam inated sa n d sto n e with parting lineation (F9) overlain by a 20-60-cm -thick unit of climbing ripples and small cross-beds o f F8.
The sequence p robably represents overbank sheet- flood deposits o f an ephemeral fluvial system (cf.
M cKee et al., 1967; Tunbridge, 1981) and m ark s the developm ent o f a m ajor non-aeolian sequence within the aeolian regim e. Excellent sorting and high grain roundness suggest th a t ephemeral streams had been reworking th e sa n d s o f the adjacent dune field or sand sheets.
T H E V E N K A T P U R ERG T he occurrence o f different aeolian facies representing different ae o lia n subenvironm ents through the 100- km outcrop b e lt indicates that a well-established aeolian regim e, com parable with a well-developed m odern erg, d o m in a te d the Pranhita-Godavari Valle) during th e L a te Proterozoic. The sedimentary ® quences disp lay th e following changes from NW to S E : (i) in creasin g dom inance of flat-bedded facies, &
corresponding d ecrease o f the cross-bedded facies a*
w ell as of su p e rp o se d large cross-beds and (iii) increase in non-aeolian facies intercalated with the :
L a r g e - s c a l e w c v y b e d - . f o r m s I F i g (2 3 )
A e o l ia n d u n e
W in d -rip p le &
a d h e s io n la m in a e
A q u e o u s
W in d -rip p le a n d irre g u la r sa lt-re la te d s t r a t a
A q u e o u s
W i n d - r ip p le &
a d h e s io n la m i n a e
S m a ll a e o lia n d u n e s
D u n e com plex
M a i n ly a e o lia n w it h fe w w a v e - r ip p le d a n d m u d c r a c k e d l a y e r
C h a n n e l
C h a n n e l
- W a v e rip p le w ith m u d c r a c k
A e o l i a n d u n e s ^
Fig. 17. Stiatigraphic section of the coastal erg-margin sequence at Laknavaram.
sediments. The changes can be explained in te rm s o f the changing geomorphology from an erg in te rio r to an erg margin (Fig. 18, see also Breed et al., 1979;
Ross, 1983; Porter, 1987). The large d u n e cross
bedded sequence o f the Bellampalli section in the NW extremity o f th e study area characterizes the well- developed d u n e fields of the com paratively interior part of th e sa n d sea, whereas the L ak n av aram section
T i d a l c h a n n e l i a c i c s
W a v e - r i p p l e d f a c i e s
S a b k h a f a c i e s
S a n d s h e e t f a c i e s
D u n e - i n t e r d u n e f a c i e s
F l u v i a l c h a n n e l f a c i e s
( a ) 2 0 -
16-
1 2 -
BLP
9 L P
I
GOV
RHG
G D V
i
L K V
I
(C)
6 0 - '
5 6-
5 2-
A0-
36-
32 -
2 6 -
2
2 0-
16- 12*
i*
4 -
0 LKV
Fig. 18. (a) Generalized logs of th e sequences showing lateral facies variation from N W to SE in the Venkatpur Sandstoi (i) Graphical representation showing variation in th e proportion of different facies along the outcrop belt. BLP=Bellai section; GDV = Godavari R iver section; R M G = Ram gundam section; L K V = Laknavaram section, (ii) Changii thickness of dune cross-beds in different sections, (c) A N W -S E schematic section through the reconstructed Venkatpu:
sea.
with the intercalated shallow-marine an d aeolian deposits characterizes the coastal erg m a rg in and delineates the SE limit of the sand sea. The G o d a v a ri River and Ramgundam sections represent extensive low-lying sand-sheet areas between the in te rio r d une field and the erg margin. Certain departures from the expected have been noted in the changing facies assemblage from N W to SE, and the changes in g ra in size or set thickness o f dune cross-beds (the la tte r is assumed to be a measure o f original bedform height) along the outcrop belt are non-linear. The g ra in size of the Godavari section is finer than b o th th e Bellampalli and Ramgundam areas and som e cross
beds of the Ramgundam section are as large as those of the Bellampalli section. Apparently the m a jo r tren d offacies variation (Fig. 18) reflects the tran sitio n from the erg interior to the erg margin and the d e p a rtu re s noted above might have been influenced by local topography, sand transport path, local san d supply sources and a complex lateral arrangem ent o f erg environments with respect to the N W -SE sectio n line.
The occurrence of different facies sequences in different parts of the Venkatpur sand sea allows reconstruction of different major erg e n v iro n m en ts such as (i) dune field, (ii) sand sheet, and (iii) coastal erg margin (Table 3). The facies assem blages are comparable to those in different geom orphological terrains in the modem sand seas (Breed et al., 1983;
Sweet et al., 1988), and different erg environments recognized in ancient eolianites (Kocurek, 1981b;
Ross, 1983; D o tt et al., 1986; P orter, 1987).
Dune field
Dune-field sequences o f the V en k atp u r Sandstone are characterized by large (2-4 m th ic k ) aeolian cross
beds th a t com prise about 85-90% o f the sequence.
Bedforms w ere either solitary o r fo rm ed large super
posed cross-beds o r draas (sensu H av h o lm & Kocurek, 1988), se p arate d by thin interdune sediments. The Bellampalli section, where the sequence is well developed, reveals th at water activ ity was negligible in the dune-field sequence com pared to the immedi
ately underlying inland sabkha sequence. This reflects net sedim entation above base level, w here influence of ground w ate r is minim al and th ere is abundant sand for th e developm ent of large dunes and dune complexes.
Sand sheet
This facies assem blage dom inantly com prises horizon
tal or low -angle coarse-grained w ind-ripple strata in units 0-7-8-0 m th ic k and is m a rk e d by rapid facies variation a n d intercalated sabkha o r fluvio-lacustrine sediments. I t represents extensive low-lying sandy
Table 3. Characteristics of different erg environments in the Venkatpur Sandstone.
Erg environments Distinctive sedimentary features Occurrence
Dune fields 2-4-m-thick cross-beds constitute 85-90% of the sequence Presence of large superposed cross-beds or draas
Sets or cosets of dune cross-beds separated by thin (5-30 cm) wind-rippled flat-beds (interdune deposits)
Near absence of aqueous activity
Sand sheets 0-7-7-m-thick, laterally extensive unit of flat-beds which are thicker than or subequal to the thickness o f associated cross-beds. Generally reduced height of the cross-beds
High facies variation resulting in a complex facies mosaic
incorporating different aeolian and fluvio-lacustrine environments Dry sand sheets are dom inated by coarse wind-ripple strata with rare
zibars and dome dunes
Wet sand sheets are characterized by adhesion- and salt-related features along with wind ripples, abundant soft-sediment deformations and thin D U sequences separated by deflation surfaces close to the w ater-table
Coastal erg margin Flat-bedded units of different thickness consisting o f wind-ripple strata, adhesion structures, salt-ridge structures, etc., along w ith a number of wave-rippled an d mudcracked intervals
Dune cross-beds are rare a n d when present forms thin units Aeolian units regularly alternate w ith or laterally and vertically grade
into shallow-marine deposits
Bellampalli section, 12-20 m
Godavari and Ramgundam sections
Bellampalli section, 4-9 m
Laknavaram section
plains w ith sabkhas, isolated dunes or sm all d u n e complexes traversed by small- to m edium -scale ephem eral streams. Isolated zibars and possible dom e dunes occur in the sequence.
S abkha facies (F4) are an im portant constituent o f th e sand sheets. Sabkhas occur in interior dune- dom inated areas, extra-dune field areas or in th e coastal areas. Sabkhas occurring in the V en k a tp u r sand sea are characterized by an abundance o f salt- related deformational features, erosional fea tu re s im plying deflation close to th e water-table, th in D U sequences and a few m uddy beds w ith aqueous ripples.
M ost o f the Recent an d Phanerozoic sand sheets are characterized, am ong o th e r things, by a n erg- m argin setting resulting in a typical architecture o f these sandbodies enclosed w ithin or interfingering w ith the extra-erg facies (K ocurek & Nielson, 1986;
P orter, 1987; Sweet et al., 1988). This sandbody architecture is absent in th e V ankatpur S andstone, indicating development o f sa n d sheets as m a jo r erg facies th a t extended over large areas between th e erg m arg in and the well-developed dune fields.
T he ‘zone o f transp o rtatio n ’ recognized in p a rts o f th e Jafu rah sand sea, Saudi A ra b ia (Fryberger e t al., 1984), provides a possible m odern analogue for th e sand-sheet sequence o f th e G odavari River section.
T he zone of transportation in Jafu rah fringes th e large dune complexes and is characterized by m o d e ra te w ind energy, ‘im m ature’ sand sheets, isolated dunes o r small dune complexes and sm all sabkhas. T he san d sheets o f the Bellampalli a n d R am gundam sections, on th e other hand, were essentially a zone of d eflation w ith higher wind energy. D om inance o f wind rip p les w ith a coarse, bimodal sand population, com m on lag surfaces, occurrence o f deflation surfaces close to th e w ater-table, sabkhas an d zibars characterized these assemblages.
E rg margin
Intercalation of the shallow -m arine and ae o lia n deposits characterizes th e erg-margin sequence around Laknavaram . T he coastal erg sequence co n tain s very few small dunes an d is dominated by low- angle beds consisting o f adhesion plane beds, w in d ripples, salt deposits an d desiccated, w ave-rippled m uddy sandstone. A n overall finer grain size,- w et depositional setting and interlayered m arine d ep o sits typify the Venkatpur coastal erg-m argin sequence.
T he complete scenario o f V en k atp u r sedim entation com prises a very wide sandy plain, essentially a
mosaic o f sm all pools, sabkhas, sand sheets inter
spersed w ith isolated dunes and small channels. The plain gradually m erged with shallow-marine environ
m ents through a sandy coastal plain towards the SE, whereas to th e N W , it merged with well-developed dune complexes. T h is scenario appears to be similar to th at envisaged for m any ancient sand seas (Kocurek, 1981b; Ross, 1983; D o tt etal., 1986; Porter, 1987).
S I G N I F I C A N C E O F T H IC K FLAT- B E D D E D U N IT S
In areas d o m in a te d by transverse bedforms in most m odem erg env iro n m en ts, the length of interdune areas m easured perpendicular to the dune crests is m uch less th a n th e len g th o f the dunes (Breed & Grow, 1979). M igration o f such dunes will produce deposits consisting o f th ic k d u n e cross-beds interstratified with thin interdune h o rizontal beds. Moreover, the slow rate o f deposition in the interdunes results in thin sedim entation u n its, as is displayed in many ancient dune-field sequences. Flat-bedded units with thick
nesses co m p arab le to or exceeding those of the associated d u n e cross-beds would, on the other hand, imply sedim en tatio n outside the well-developed dune fields. F lat-bedded sequences in the Venkatpur Sand
stone occur b o th as th in sheets wedged between dune cross-beds, a n d as th ic k extensive units with signatures of significant w a te r activity and abundant salt depo
sition. The fo rm e r represents the interdunes in the dune fields, w h erea s the latter has been inferred as the product of se d im e n tatio n in wide low-lying areas like sand sheets, sa b k h a s o r coastal sand flats.
H orizontally stratified sedimentary units in aeolian deposits may d evelop in a number o f e n v iro n m e n ts : for example in te rd u n e, dune apron, sabkha, sand sheet (Fryberger e ta l., 1983; Kocurek, 1986). D eposits in these e n v iro n m en ts possess many common attri
butes, and reco g n itio n o f a specific depositional regime in th e rock rec o rd m ay b e problematical. F lat-b ed d ed units in a n c ie n t aeolian sequences, however, have been generally in te rp re ted to represent interdune deposits. T he stu d y o f th e Venkatpur Sandstone shows th a t horizontally stratified units can be inferred to have been d ep o site d b o th in sand-sheet and/or sabkha environm ents a s w ell as in interdune areas. This interpretation is b a s e d mainly on the association and assemblage o f facies as w e l l as stratigraphic consider
ations, rather th a n o n textural features or s tratific a tio n styles.
P A L A E O C U R R E N T S
A limited number of palaeocurrent d a ta collected from large dune cross-beds indicate a N E p alae o w in d direction (Fig. 19). More than 200 palaeocurrent d a ta collected by earlier workers around M ancheral(18°53', 79°26') and Kistampet (18°51 79°45') (Johnson, 1965) give a NE-ENE mean palaeoflow direction. The mean palaeocurrent direction calculated from 132 measurements from exposures SE of R a m g u n d a m is toward 68° (Chaudhuri, 1970). The consistent o rien tation of cross-beds over such widely sep arated areas indicates the existence of a stable regional w ind circulation pattern. Notably, the inferred p alaeo w in d direction is nearly perpendicular to the p re se n t N E - SE trend of the outcrop belt.
COMPARISON OF T H E V E N K A T P U R SAND S E A W I T H K N O W N PHANEROZOI C ERG D E P O S I T S Despite basic similarities, the V enkatpur S andstone differs from most other documented P roterozoic and Phanerozoic ergs in several important aspects.
Fig. 19. Palaeocurrent pattern from the Venkatpur Sandstone recorded from large-scale cross-beds, (a) Ram gundam area, (b) Godavari River section and vicinity, (c) Bellampalli area, (d) total Venkatpur Sandstone. Rose diagram construction after the method by Chenney (1983).
First, th e sa n d sea is m arked by the common developm ent o f sabkhas in its m arg in al as well as interior p a rts. Close association o f coastal sabkhas and related ev aporite facies w ith th e sand seas has been rep o rted by different w orkers (K insm an, 1969;
Glennie & E vans, 1976; K ocurek, 1981b; Porter, 1987; F ry b e rg e r et al., 1988). M a rin e sabkhas can form tens o f kilom etres inland th ro u g h rare storm flooding a n d capillary processes, b u t are commonly close to th e palaeoshoreline (C han & K ocurek, 1988).
The > 5-m -thick sabkha sequence a t Bellampalli is at least 100 k m from the recorded coastal sequence, suggesting a n inland setting and non-m arine sources for the salt. P eriodic surface flooding, high water- table even in th e interior p arts, an d an overall deflationary w in d regime are some o f th e factors that might have favoured frequent sa b k h a development.
Secondly, even though the aeolian regim e covered an extensive area, the sequences are m arked by an unusual p au c ity o f large com pound bedforms. The cross-beds are mostly in the range o f 1-2-5 m, which is m uch less th a n those of m any a n c ic n t eolianites (Ross, 1983; L oope, 1984; Porter, 1987; K err & Dott, 1988 am ong m a n y others). The close association and interbedding o f the V enkatpur S andstone with a braided fluvial system (R am giri F orm ation and M ancheral Q u artzite, formerly k n o w n as Encharani Q uartzite; C hak rab o rty , 1988) a tte s t to th e presence o f potential source material in th e close vicinity.
Extensive developm ent o f sabkha, fluvio-lacustrine and related w et environm ents p ro b ab ly caused greater entrapm ent a n d /o r reworking o f sand. T his in turn m ight have reta rd e d sand tran sp o rtatio n in the interior areas favouring grow th o f small dunes an d an extensive low-lying san d y plain. A sim ilar role o f extensive wet environm ents h as been inferred fro m th e study of recent d u n e fields as well as a n c ie n t sequences (Hum mel & K ocurek, 1984; D ott e ta l., 1986;Kocurek
& N ielson, 1986; C han & K ocurek, 1988; Chan, 1989).
Thirdly, in th e V enkatpur S andstone, the trend of facies tra n sitio n is not parallel to th e resultant palaeow ind direction, as has been reported from several a n c ie n t aeolian deposits (see Ross, 1983; Dott et al., 1986; P o rte r, 1987) and m o d ern examples such as the J a fu ra h Sand Sea (Fryberger et al., 1984).
Azim uths o f large dune cross-beds collected during the study a n d earlier (Johnson, 1965; Chaudhuri, 1970, Fig. 18) from widely scattered areas suggest a very p ersistent palaeotransport directio n towards the N E. The ex a ct relationship betw een flow direction and trend o f facies variation has n o t been resolved.
T he relationship is a p p a ren tly more com plex th a n th a t for the Aztec or S t P ete r Sandstone, a n d m ay sim ulate many m odern san d seas where the re su lta n t d rift direction is oblique to th e dune crests (B reed et al., 1979, p. 324, see F igs 201,206,235).
Recognition of the V en k a tp u r Sandstone as an extensive aeolian deposit suggests th at m any o f th e m ature Proterozoic sheet sandstones th at ab o u n d in different Proterozoic b asin s o f India, earlier in te r
p rete d as shallow-marine deposits (C haudhuri &
H ow ard, 1985; Singh, 1985; N ag raja Rao et al., 1987 am ong others), could well be aeolian deposits. In th e unvegetated landscape o f th e Proterozoic tra v e rse d by widely fluctuating sandy braid ed rivers, co m b in ed fluvial and aeolian processes m ay have been m ore effective in spreading o u t large sheets o f sand th a n m arin e processes.
C O N C L U S I O N S
T he Proterozoic V enkatpur Sandstone of the P ra n h ita -G o d a v a ri Valley, in te rp re ted earlier as a shallow - m arine deposit, abounds in diagnostic aeolian featu res such as adhesion lam inae a n d inverse-graded tra n s la t e d strata. The sandstone sequence developed in a n extensive erg th at was ch aracterized by a w ide sa n d sh eet (G odavari and R a m g u n d a m sections) co m p ris
ing flat-bedded w ind-ripple strata, zibars, sa b k h as, ephem eral lakes and channels and scattered sm all dunes. The sand sheet m erged w ith the cross-bedded dune-field sequence of th e erg interior to the n o rth w est (Bellam palli section) w hereas to the southeast, it m erged w ith the erg-m argin shallow-m arine sequence (L aknavaram section). P alaeocurrent m easurem ents denote a very consistent northeasterly palaeoflow d irection over the erg w hich is a t a high angle to th e p rese n t trend o f the lin e ar outcrop belt. A shallow w ater-table, extensive developm ent o f salt crusts, rew orking by ephem eral channels and coarser g ra in size are some of the fac to rs th a t suppressed th e developm ent o f the dunes a n d favoured the g ro w th o f an extensive sand-sheet environm ent. This in tu rn w as probably responsible for th e small dune cro ss
beds, even in the interior d u n e field areas. A bu n d an ce o f flat-beds, as in th e V en k atp u r sandstone, is uncom m on in many P h anerozoic aeolian d ep o sits;
however, similar sequences have been reported from som e pre-Silurian aeolian sequences (Dott et al., 1986).
F acies assemblages and stratigraphic consideration help in differentiating different subenvironm ents such
as interdune, sa b k h a , sand sheet or coastal sand flats for the deposition o f the flat-bedded units.
A C K N O W L E D G M E N T S
This study is a p a r t o f the author’s PhD programme funded by th e I n d ia n Statistical Institute. I express my sincere g ra titu d e to Asru K. Chaudhuri of the Indian S tatistical In stitu te under whose guidance the work was ca rrie d out. I am indebted to Professor S.
K . C handa o f th e Ja d av p u r University, who intro
duced me to th e aeo lian literature and reviewed the earlier versions o f th e manuscript. On-field interaction w ith P. K. Bose o f th e J adavpur University is gratefully acknowledged. C onstructive reviews by Steven Fry- berger and G a ry K ocurek improved the manuscript considerably. I w ould also like to thank S. N. Das for field assistance, A . K . Das for drafting work and D.
K. Saha and S. K . Chakraborty for carefully typing the m anuscript.
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