ripple marks in intertidal lower bhander sandstone (late proterozoic), central india: a...

Sedimentary Geology, 29 (1981) 241--282 241 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

R I P P L E M A R K S IN I N T E R T I D A L L O W E R B H A N D E R S A N D S T O N E ( L A T E P R O T E R O Z O I C ) , C E N T R A L I N D I A : A M O R P H O L O G I C A L

A N A L Y S I S

SOUMEN SARKAR *

Department of Geological Sciences, Jadavpur University, Calcutta, 700032 (India)

(Received February 27, 1980; revised and accepted December 12, 1980)

ABSTRACT

Sarkar, S., 1981. Ripple marks in intertidal Lower Bhander Sandstone (late Proterozoic), central India: a morphological analysis. Sediment. Geol., 29: 241--282.

The late Proterozoic, intertidal Lower Bhander Sandstone (Bhander Group, Vindhyan Supergroup) developed around Maihar, central India, is characterized by alternations of sandstone and shale in different scales and shows profuse ripple marks of widely varying morphology. Visual examination of their external morphology led to the identification of wave ripple, current ripple and others of intermediate character.

Standard deviation and average of ripple spacing and height of symmetrical and assym- metrical ripples show genetically significant differences analogous to those obtained by Harms (1969) for wave- and current-generated ripples. Different dimensionless parameters, e.g., R.I., R.S.I., S.I., etc., processed separately for the two types of ripples, show a wide variation in their range which encompasses the total spectrum of values stipulated for wave and current ripples. However, the frequency of any particular genetic type of ripple differs widely when analysed in terms of different dimensionless parameters. Sev- eral scatter plots, prepared after Tanner (1967) also indicate the presence of various genetic types of ripples, but there are ripples for which results remain inconclusive. Fur- thermore, scatter plots involving the vertical form index (ripple length/ripple height) and median grain size of a few asymmetrical ripples, following Reineck and Wunderlich (1968a), led to the discrimination between current ripple and wave ripple and the dis- t inction is grossly consistent with the results obtained by other means.

Ripple spacing, ripple index and grain-size data of a few representative samples of ripples of possible wave origin, analysed after Tanner (1971) and Allen (1979) indicate that they were generated in a shallow basin with restricted fetch.

Internally, the ripples, irrespective of their symmetry, are often characterized by unidirectional bundles of foresets consisting of rhythmically alternating sand and mud laminae. The sets of cross-laminae may be complexly organized with planar or curved erosional boundaries separating them. In many instances internal structures typical of wave ripples are also noted.

Inconsistencies, however, exist between the results obtained by application of different criteria in interpretation of these ripple marks. The limitations in applicability of

* Present address: Geological Studies Unit, Indian Statistical Institute, 203, B.T. Road, Calcutta 700035 (India).

0037-0738/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

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various criteria in the analysis of fossil ripples, as experienced during the present study, may arise from several factors, namely modification of crest profile and size of ripple marks by penecontemporaneous current and wave activities and by compaction of sediments, respectively. Thus, external morphology should not be used independently as a unique criterion for interpretation of fossil ripples, nor does their internal structure always help independently. Over and above the existence of ripples composed of very fine sand and silt or discrete silt and clay, cross-laminae may render interpretation quite difficult.

Plotting of ripple parameters in Tanner's (1967) scatter diagrams may lead to inconsistencies in typification of 'fossil' ripples, both within and between different diagrams.

In the ultimate analysis, integrated study of gross morphology and internal structure of ripples combined with the palaeogeographic background appears to be more paying than anything else in the interpretation of 'fossil' ripples.

INTRODUCTION

The study of ripples perhaps represents one of the oldest fields of geological endeavour and still continues to interest geologists. Early studies were mainly confined to the description of ripples f rom modern sediments and their ancient analogues. Ripples have also been widely used as palaeocurrent indicators. During the last two decades, however, considerable a t tent ion has been directed to the understanding of the hydrodynamics of ripple formation f rom a theoretical standpoint coupled with observations under natural and laboratory conditions (Allen, 1963, 1968, 1969, 1977, 1979; McKee, 1965; Simons et al., 1965; Reineck and Wunderlich, 1968a; Harms, 1969, 1975; Boersma, 1970; Reineck and Singh, 1973; Komar, 1974; Banks and Collin- son, 1975; Clifton, 1976; De Raaf et al., 1977). Despite a voluminous litera- ture, little progress seems to have been made until now in the application of these concepts to the understanding of the hydrodynamics of fossil ripples.

Today, sedimentologists are particularly concerned with the correlation of various types of ripples in ancient rocks in relation to their mode of genera- t ion and their environmental implications. Tanner (1967) made an a t tempt to distinguish different genetic types of ripples, e.g., wave ripples, current ripples, wind ripples, etc., in modern nearshore sediments, employing various scatter plots involving various dimensionless ripple parameters. Tanner (1971) further a t tempted estimation of ancient water wave height, wavelength, water depth and fetch values by using ripple spacing and grain-size data. Based on observations in modern and ancient sediments, supplemented by flume studies, Harms (1969) approached a system of interpretat ion of ripples involving morphological pattern in relation to hydraulic regime. Reineck and Wunderlich (1968a), Reineck and Signh (1973) and De Raaf et al. (1977)have also offered several diagnostic features of wave ripples as con- trasted f rom current ripples. But as is apparent, all these studies and the con- clusions arrived at have come primarily from ripples developed in modern sediments and under experimental conditions and to date very few worth- while a t tempts have been made to describe and examine the applicability of

243

the results in the interpretation of fossil ripples. In the area around Maihar (24 ° 16'N, 80°46'E; Fig. 1), Satna district, Mad-

hya Pradesh, India, the late Proterozoic, intertidal Lower Bhander Sand- stone shows profuse development of ripples and provides a unique opportun- ity for their detailed study. An integrated analysis of the external morphology and internal structures of ripples has been attempted here in the light of

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244

observations made on ripples formed in modern environments and under experimental conditions. This has further allowed a critical evaluation of the applicability of existing criteria for the interpretation of ripples in ancient sediments. As the study will illustrate, none of these criteria, irrespective of environmental setting, are unique by themselves in the recognition of different genetic types of ripples. Furthermore, estimation of ancient water wavelength, wave height, water depth, etc., have been a t tempted here utilizing equations of Tanner (1971), which primarily involve ripple spacing and grain-size data. Since the submission of this paper, a significant contribution has been made by Allen (1979} on the interpretation of wave ripple marks using their wavelengths, textural composit ion and shape. His interpretations, based on modern and experimental ripples have been incorporated in the present study as far as possible. Unfortunately, the field measurements in the present case were made much before the publication of Allen's (1979) paper and as such the data presented here could not be fully exploited in the light of his observations.

It must, however, be borne in mind that observations made on modern ripples may not be directly applicable to the ripples formed in the Protero- zoic epeiric seas, for Proterozoic nearshore environments were possibly radi- cally different from those of their modern analogues. In the absence of vege- tation, both wind activity and erosional processes (in turn depositional processes) were possibly far more active than they are today. The data presented here should therefore be viewed in this context .

GEOLOGIC SETTING

The Bhander Group represents the uppermost subdivision of the Vind- hyan Supergroup (Proterozoic) in central India. These shallow-water sediments were deposited in an epeiric sea embaying the western part of the Indian craton (Ahmad, 1958; Banerjee, 1964). The Vindhyan sediments of central India are tectonically undisturbed, and form a gentle, wide, down- warp with very low dips (Misra, 1969).

The s tudy area is located near the southern margin of the Vindhyan basin in central India (Fig. 1) and the beds show a very low regional northwesterly dip of 5* or even less. From south to north across the area (Fig. 1) the Bhander Group of rocks is exposed in the following order: (1) Ganurgarh Shale, (2) Bhander Limestone, (3) Lower Bhander Sandstone, (4) Sirbu Shale, and (5) Upper Bhander Sandstone. The assemblage of primary sedimentary structures, lithological association, palaeocurrent pattern, and the marginal location of the area in relation to the Vindhyan basin in the central part of India all combine to suggest that the Bhander Group of rocks was laid down in a prograding tidal flat--lagoon complex, south of (behind) a barrier bar (Sarkar, 1979).

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CHARACTERISTIC FEATURES OF THE LOWER BHANDER SANDSTONE

The Lower Bhander Sandstone is well exposed in several creeks and mounds north, west and northeast of Maihar (Fig. 1). In its major part the Lower Bhander Sandstone is characterized by alternations of sandstone/silt- stone (dominantly quartzarenite) and shale in three different scales, i.e., in the range of millimetres, centimetres, and decimetres. The lowest order of interlayering in the range of millimetre-thick laminae are mostly confined within ripples and appears to be more regular compared to the higher orders of interlayering. Megascopically, the alternations of sand and mud that occur in the range of centimetres, however, remain ~he hallmark of the Lower Bhander Sandstone. The first-order alternations of sand<lominant and mud- dominant mixed facies in the range of decimetres to metres are not as common as the other two types of interlayering.

Interlayering of sand and mud along with abundant , varied and complex types of small-scale ripple marks with evidence of late-stage emergence runoff, presence of desiccation features all throughout , and a well-marked hori- zon with salt pseudomorphs toward the upper part of this formation, clearly signify a high mid-flat to low supratidal environment for the deposition of the Lower Bhander Sandstone {Chanda and Bhattacharyya, 1974; Sarkar, 1979). Singh {1976), however, considered the Lower Bhander Sandstone as a component of the Bhander Limestone Formation and recognised that part of the sequence as a product of a high intertidal to supratidal flat environment rather than high mid-flat to low supratidal flat environment.

RIPPLE MARKS IN THE LOWER BHANDER SANDSTONE

The Lower Bhander Sandstone, as exposed in the pavements along the nalas (creeks), is replete with various types of small-scale ripples. Crestlines of ripples vary widely in their orientations between adjacent layers and also within the same layer. Rose diagrams made separately for crestline orientation of symmetrical ripples and lee-side orientation of asymmetrical ripples in different sectors of the s tudy area are shown in Fig. 2. Despite considerable dispersion, typical of tidal-flat sediments, palaeocurrent patterns as revealed by asymmetrical ripples have a dominant mode toward the NNE, i.e., almost down the palaeoslope. The unimodel pattern apparently arises from migration of bed forms during the ebb phase of the tide only. Crest- line orientation of symmetrical ripples show relatively greater dispersion compared to that of asymmetrical ripples {Fig. 2).

External morphology of ripples

Broadly two distinct morphological types of ripples, i.e., symmetrical and asymmetrical, are found in the Lower Bhander Sandstone. They vary, however, widely in their size, shape, and detailed morphology. The morphologi-

246

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cal descriptors proposed by Tanner {1967) have been adopted in the present study and they include crest spacing, height, ripple index, ripple symmetry index, parallelism index, straightness index. In addition, other morphological features such as angularity or roundness of crest profiles, bifurcation of crestlines, etc., which could be adjudged qualitatively in the field, have been widely used. Despite diversity of ripple morphology and their imperceptible gradation from one type to another (not necessarily within an exposure), they could be empirically subdivided into seven types with distinct morphological attributes {Table I).

Interference and modified ripples. In a simple case of two separate Wave- trains moving at right angles to each other, two sets of ripples of nearly equal magnitude may develop which are oriented at right angles to each other (Reineck and Singh, 1973, p. 368, fig. 526i). In ladder-back type ripple, such as shown in Fig. 10, the ripple crestlines running from top to bot- tom are slightly more pronounced as compared to those striking from right to left. In the slightly subdued crestlines running from right to left, even though lined up with one another in successive ripple troughs running from

247

TABLE I

External morphology of ripples in the Lower Bhander Sandstone and their possible origin

Ripple Morphological characteristics Possible type origin

Type I Parallel, rectilinear crestlines with marked continuity. Dominantly Crests separated by flat throughs, crest profiles wave symmetrical and moderately angular ( ' sol i tary ' profile of induced. Inman, 1857). Limited morphological variation within a set (Fig. 3).

Crestlines sinuous, bifurcating, parallelism largely maintained. Consequent to bifurcations, nonparallelism of crestlines may develop; one of the branches may be subdued or may die out. Crest profiles symmetrical, rounded; crests separated by rounded troughs ( ' t rochoidal ' profile of Inman, 1957). Variation in morphology within a set is very limited (Fig. 4).

Relatively larger in size compared to Type II. Crestlines relatively less continuous and displaying moderate sinuosity, poor parallelism. Branching and rejoining of crestlines in a braided pat tern may result in elongate dimple-shaped troughs. Moderately angular to sub- rounded, symmetrical crests separated by rounded troughs ( ' t rochoidal ' profile). Crest spacing, a widely ripple length, height may vary considerably even along a crestline (Fig. 5).

Crestlines moderately continuous, sinuous, and largely maintain parallelism; bifurcations are not rare. Crestlines in places show small beak-like projections pointing downcurrent. Crest profiles moderately angular and asymmetrical (ripple symmetry index, i.e., RSI: 1.4--3.2). Crest spacing, ripple length, height show perceptible variation within a set (Fig. 6).

Type V Crestlines rectilinear to sinuous, may show abrupt deflections accompanied by bifurcations. Deflections of successive crestlines generally occur in a narrow linear zone per- pendicular to crestlines resulting in sharp changes in morphology of ripples. Rectil inearity and parallelism of crestlines are maintained in parts, but consequent to deflection and bifurcations sinuosity and nonparallelism may develop. Crestlines may show sharp, beak-or tongue-like projections pointing downcurrent. Crest spacing shows wide variation within a set; ripple length and height vary widely even along a crestline. Crest profiles characterized by high degree of asymmetry (RSI: 1.6--4.5) and angularity ( ' sawtooth ' profile of Inman, 1957 ; Fig. 7 ).

Crestlines continuous, moderately sinuous, parallel; regular spacing of saddles and lobes; gentle upcurrent closure in lobes; sharp, tongue-like projections in saddles. Crest profiles moderately angular and strongly asymmetrical

Type II

Type III

TypeIV

Type VI

Dominantly wave induced.

Dominantly wave induced.

Combined flow type of Harms (1969).

Dominantly current induced.


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TABLE I (continued)

Ripple Morphological characteristics Possible type origin

Type VII

(RSI may be as high as 5.25). Ripples mostly belong to transverse, catenary, in-phase type of Allen (1968). Ripple length, height show wide variation even along a crestline (Fig. 8).

This type appears to be a derivative of Type VI and resembles cuspate ripples of Allen (1968). Compared to the preceeding type, crestlines are less continuous, may even be discontinuous and show more acute upcurrent closure. Crest spacing shows wide variation within a set, ripple length and height show marked variation along a crestline. Asymmetry is still greater relative to Type Vt and RSI may be as high as 5.7 (Fig. 9).


top to bot tom, off-setting of crests may be noticed. Such off-setting may be due to wind-generated shear operative on stagnant water during late-stage emergence. Interference of wave trains may give rise to complex patterns of ripples, as shown in Fig. 11. It may be noted that "cross-ripples" (Clifton, 1976, p. 136) and incipient lunate ripples have developed due to interference of wave-trains and/or unidirectional currents. This example (Fig. 11) is analogous to that described by Clifton (1976, his fig. 15, p. 136) from Recent sediments off the coast of southern Oregon and southeastern Spain.

Ripples once formed may be modified in various ways. One of the most common modified type of ripple includes those in which superimposition of a secondary set on an earlier set takes place. In many instances, subdued secondary crests (symmetrical, angular) are found to be oriented at right angles to the primary lunate ripple set (Fig. 9). Even though the secondary ripple set is generally confined within the troughs of a primary ripple set, the crestlines of a secondary set in places run across the crests of the primary set. The secondary set was presumably generated either by late-stage emergence runoff producing changes in flow direction (Klein, 1971, 1977), or by wind- induced shear operative on stagnant water within troughs of the primary ripple set (Tankard and Hobday, 1977). However, in the light of the observations on modern tidal fiats (Reineck and Singh, 1973, p. 366, fig. 526b), it also appears plausible that the secondary ripple set was produced by waves working simultaneously with the current responsible for the generation of the lunate ripple. A feature closely associated with these ripples is the faint linear ridges running parallel to the crestline of the ripples and occurring at the same elevation on both sides of ripple troughs. These are in all probabil- i ty water recession marks arising from ponding of water within ripple troughs during late-stage emergence. Flat-topped ripple marks, reported from many other modem and ancient tidal-flat deposits (Tanner, 1958, 1971;

249

Anan et al., 1969), are not uncommon in the Lower Bhander Sandstone (Fig. 12). Their generation has been attributed to late-stage wave action planing off ripple tops (Anan et al., 1969). Some other ripple marks are characterized by subdued secondary ridges within troughs, running parallel to the crestlines of pronounced or primary ripple sets (Fig. 13). These are believed to have formed by a sudden change in water wavelength following the formation of an earlier set of ripples (Evans, 1943, 1949; Komar, 1973).

Analyses of external morphology

Type I and Type II ripple marks (Figs. 3 and 4, Table I) are, in contrast to others, very evenly spaced. Evans (1949) noted that in wave-formed ripples such as those of Type I, the crestlines tend to be nearly straight and parallel for long distance, or if curved, as in Type II, the bends are gradual and uniform. According to Harms (1969), this pattern arises because wave motion reduces variability of ripple spacing much below that attained by currents. In contrast, the crestlines of current ripples may be crooked or irregular, with

Fig. 3. Type I ripple. Note (1) rectilinearity, parallelism and even spacing of crestlines, (2) moderately angular, symmetrical crest profiles.

250

Fig. 4. Type II ripple. Note (1) grossly parallel, sinuous, more or less evenly spaced crestlines with smooth bending, (2) bifurcations of crestlines, and (3) symmetrical, rounded crest profiles.

Fig. 5. Type III ripple. Note (I) relatively poor parallelism, sinuosity, branching and rejoining of crestlines, (2) wide variation in crest spacing, ripple length and ripple height, and (3) moderately angular, symmetrical crest profiles. Diameter of the coin is 2.5 cm.

251

Fig. 6. Type IV ripple. Note (1) moderately continuous, sinuous, grossly parallel crestlines, (2) moderately angular, slightly asymmetrical crest profiles (lee side point downward), and (3) small beak-like projections of crestlines toward downcurrent direction. Diameter of the coin is 2.5 cm.

Fig. 7. Type V ripple. Note (1) rectilinear to slightly sinuous crestlines with abrupt deflections in places, (2) wide variation of crest spacing, ripple length and ripple height, (3) strongly asymmetrical (lee side points toward lower left corner) and highly angular crest profile, and (4) abrupt tongue-like projections of crestlines toward downcurrent direction.

2 5 2

many sharp angular turns, as in Type V (Fig. 7), and the parallelism and regularity of spacing of ripple crestlines is much less marked than those in the wave-formed ripple marks (Evans, 1949). Because of the regularity of wave action, wave-formed ripples tend to be uniform in height throughout their length. Following this criterion, Type I and Type II ripples appear to be ideal examples of wave ripple. Current-induced ripples, on the other hand, as shown by Figs. 7, 8 and 9, display rapid change in height along their crests. Sensitive reflection of the relative magnitudes of current and oscillatory velocities is recorded by the ripple profile (Harms, 1969). Sharply angular and strongly asymmetrical profiles or 'sawtooth' profiles as depicted in Fig. 7 (Type V), are common and are characteristic features of current- induced ripples (Inman, 1957; Harms, 1969). In contrast, most of the symmetrical wave ripples have rounded to subangular crests, as shown in Figs. 4 and 5. Even though Evans (1949) and Harms (1969) noted in their laboratory experiments that wave ripples usually have rounded crests, Reineck and Singh (1973), apparently on the basis of study of modern ripples, observed that their crests are usually pointed and the troughs are rounded. In rare instances, sharp-crested symmetrical ripples are, of course, found. It was suggested by Harms (1969) that sharp-crested ripple profiles may indicate steady water levels or very uniform waves and a very narrow wave spectrum. However, symmetrical ripples with rounded crests might have resulted from reworking of ripples during the period of emergence (tteineck and Singh, 1973). Harms (1969), in his flume experiments, also observed that in current ripples, the downstream faces slope at nearly the static angle of repose, whereas in wave ripples, the steepest parts of the profile do not slope at angles greater than about 23 ° . Comparison of Fig. 5 and Fig. 7 clearly reveals that such differences can be detected in fossil ripples. In many of the slightly asymmetrical ripple sets in the present case, degree as well as directions of ripple asymmetry were found to change even along an individual crestline and this feature is believed to be typical of wave ripples (Reineck and Singh, 1973; De tLaaf et al., 1977). Newton (1968), however, suggested that symmetry reversals within a ripple train or the transformation of an asymmetrical ripple into a symmetrical one is probably due to subsequent modification. A dominantly symmetrical ripple train with asymmetry retained in parts, as shown in Fig. 14, may owe its origin to processes suggested by Newton (1968). In the light of the result obtained by Harms (1969), the ripple train as shown in Fig. 7 represents low-energy current ripples and ripples belong- ing to Type VI (Fig. 8) and Type VII (Fig. 9) illustrates high-energy current ripples. Allen (1968) noted that there is a gradual transition from straight- crested through sinuous to linguoid ripples with increasing depth and velocity of water. In the present case, however, linguoid ripples are virtually absent, instead lunate ripples appear as high-energy forms. Bifurcations of crestlines in a Y-shaped pattern are a feature typical of wave ripples (Tanner, 1967; Harms, 1969; Reineck and Singh, 1973; De Raaf et al., 1977). Such bifurcations are frequently noted both in symmetrical and asymmetrical rip-

253

Fig. 8. Type VI ripple. Note (1) continuous, sinuous crestlines and moderately angular, strongly asymmetrical crest profiles (lee sides point downward), (2) regular spacing of saddles and lobes and gentle upcurrent closure in lobes, sharp tongue like projections in saddles, and (3) wide variation in ripple length and height along a crestline. Length of the scale is 15 cm.

Fig. 9. Type VII ripple. Note gross similarity of this type with Type VI and incipient development of a ~ c o n d a r y set of ripples (running left to right) the crestlines of which in places run across the crests of the primary ripple set.

254

Fig. 10. Ladder-back ripples. Diameter o f the coin is 2.5 cm.

Fig. 11. Interference ripples; note complex pat tern of ripple crestlines. Length of the scale is 15 cm.

255

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Fig. 12. Flat-topped ripples.

Fig. 13. Ripple with subdued secondary crests medial to the primary ripple crests.

256

Fig. 14. Ripple showing variation in symmetry of crest profiles. Length of the scale is 15 cm.

ples of the Bhander Sandstone. Subdued secondary crests developed within troughs and running parallel to the crests of the relatively larger primary set (Fig. 13) are a feature typical of wave ripples (Evans, 1943, 1949; Reineck and Singh, 1973). Besides pure wave and current ripples, many are possibly products of combined-flow which are very difficult to recognise because information on their characteristics and conditions of formation is meagre (Harms, 1969). Slightly asymmetrical ripples, with moderately angular crest profiles and moderate continuity, sinuosity of crestlines (Fig. 6) presumably represents a product of combined flow, possibly current dominated. It may be noted that the crestlines also show bifurcations characteristic o f wave ripples. The asymmetry in crest profile might have resulted from (1) oscillation-related mass-transport in the direction of wave propagation (Reineck, 1961; Allen, 1979), (2) asymmetry of landward and seaward orbital velocities of waves (Reineck, 1961; Newton, 1968; Clifton et al., 1971; Reineck and Singh, 1973), (3) imposition of a small, s teady unidirectional current with a component in the direction of wave propagation (Singh, 1969; Allen, 1979).

Ripple parameters and their relationship

Ripple spacing and ripple height were measured from 70 symmetrical and 54 asymmetrical ripple sets. Distribution of logarithmically transformed

2 5 7

spacing and height data for symmetrical and asymmetrical ripples are shown separately in histograms (Fig. 15). Spacing values of both the types of ripples show roughly log-normal distribution. Even though the range of variation and modal class values are the same for both types of ripples, the average mean values and standard deviations of spacing values of asymmetrical ripples are greater than those of symmetrical ripples. Similarly, the height values of both types of ripples also show a roughly log-normal distribution. Average, modal class, and standard deviation of height values of asymmetrical ripples are greater than those of symmetrical ripples, even though the range in variation of height values are the same for both types of ripples (Fig. 15).

Furthermore, the following dimensionless parameters introduced by Tanner (1967) have been used in the present study.

Ripple Index (RI) = S / h , Ripple Symmetry Index (RSI) = l=/lb,

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Height Height

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259

Straightness Index (SI) = la/d, Parallelism Index No. 1 (PII) = Id/S" A S

Parallelism Index No. 2 (PI2) = p / S .

where S = crest spacing, h = height o f ripple crest, Io = horizontal projection of stoss side, Ib = horizontal projection of lee side, ld = distance parallel with crest along which curvature can be seen, d = departure of crest in the distance, la, from a straight line, S- = mean spacing between crests,

= S m a x / S m i n ,

p = S m a x - - S m i n . Distributions of the values of different dimensionless parameters are

shown separately for symmetrical and asymmetrical ripples (Fig. 16). It may be ment ioned here that no a t tempt was made to determine the continui ty index because most of the ripple trains are only partly exposed. Comparison of the distribution of RI values of symmetrical and asymmetrical ripples shows that bo th represent roughly normal distribution and do not differ from each other markedly {Fig. 16). Distributions of RSI values of asymmetrical ripples are strongly skewed (Fig. 16). So far as the distribution patterns of other parametric indices are concerned, they do not deviate markedly from normal distribution. No marked and significant difference exists between symmetrical and asymmetrical ripples so far as their distribution patterns of SI, PII, PI: are concerned.

In order to distinguish between various genetic types of ripple marks namely wave ripple, current ripple etc., Tanner {1967) introduced a number of scatter plots involving several dimensionless parameters of ripples. The scatter plots, which are believed by Tanner (1967) to be most useful in dis- criminating ripples as to their origin and are also used in the present study, include (1) RI vs. RSI, (2) RSI vs. SI, (3) RI vs. PII and (4) PII vs. PI2. The confidence limits for distinction of different types of ripples, however, vary between such scatter plots. Excepting the plot involving PI~ and PI2, others have reliable confidence limits {Tanner, 1967, p. 97). Before any discussion of the results of scatter plots, some clarifications regarding collection of data and their use seems desirable.

(1) The term 'set ' refers to a train of ripples confined within a bedding plane and within the limit of a cont inuous exposure and characterised by homogenei ty in their morphology, dimension, and orientation.

(2) In the case of asymmetrical ripples, for a single value of RSI, two corresponding RI values were obtained because measurement of RSI confines within a crestline whereas that of RI necessarily involves two adjacent crestlines. Consequently, two RI values have been plot ted against the corresponding single value of RSI. For symmetrical ripples, however, all the RI values were considered against the constant value of RSI, i.e. 1.00 in the scatter diagram of RI vs. RSI.

260

(3) Measurement of SI values involves a curved part of a crestline and RSI values may vary considerably within that curved part. As a result, different RSI values were obtained against a single value of SI. In scatter plots of RSI vs. SI all the RSI values obtained from a curved part of a crestline were plotted against the corresponding single value of SI. In case of symmetrical ripples different SI values of a particular set were considered against the constant value of RSI, i.e. 1.00.

(4) When PI, values are measured in a curved part of a crestline (for PI, measurement involve ld), RI values may vary considerably in that part. In such cases all the values of RI have been plotted against the single value of PI1.

(5) For a particular set of ripples all the PI, values were plotted against a single value of PI:.

(6) The values of Smax, Stain, S-, AS, p were obtained from all the measurements covering a ripple set and not from any t iny part of a set (Tanner, pers. commn., 1976).

(7) In Tanner's scatter plots many fields remained unspecified, in the present study those fields have been arbitrarily named for convenience in discussion e.g. 'A' field in the scatter plot of RI vs. RSI., 'a ' and '~' field in the plot of RSI vs. SI and 'X' and ' Y' fields in the plot of PII vs. PI2.

(8) When more than two thirds of the readings of any particular ripple set are located in any specified field, e.g., wave, current or wind in any of Tan- ner's (1967) scatter plots, then the ripple set has been termed accordingly.

Plot o f RI vs. RSI. In the construction of such scatter plots, altogether 125 ripple sets were considered, amongst which 71 are symmetrical and 54 are asymmetrical. Distribution of data points derived from asymmetrical ripples are shown in Fig. 17 and Table II, but those collected from symmetrical ripples were not plotted in diagrams for reasons stated below. The position of data points of symmetrical ripples in such a scatter plot can be easily visu- alised because all the points would be located on the vertical axis where RSI = 1.00. Consequent to this, these points get overcrowded on the ordi- nate and hence their plotting has been avoided. However, the frequency of data points in different fields has been counted and is given in Table II. In the case of symmetrical ripples, out of 1343 total readings, 1310 were concentrated in the wave field; the rest occupy the wind (17) and wave--wind (15) fields "(Table II). However, when considered in terms of set, all symmetrical ripple sets could be recognised as dominant ly wave induced on the basis of this scatter plot.

Examination of Table II and Fig. 17 as a whole reveals that for asymmetrical ripples, out of 871 readings, 428 and 223 points are located in the wave field and current field, respectively. A good number of points occupy the zone of overlapping of wave and current. A few data points are also located in the field of wind ripple (Table II). It should be mentioned, however, that no difinite wind ripples could be visually recognized in the field. It appears

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Fig. 17. Scatter plot of RI versus RSI of asymmetrical ripple• Note the area marked 'A' is not defined with respect to origin by Tanner 119671. Numbers in parentheses represent data points in respective fields•

that t h e p o p u l a t i o n o f ripple marks covers a w ide spec trum f r o m pure wave to current ripple. Moreover , w h e n the results o f scatter diagrams (Fig. 17a, b, c, d) are cons idered co l l ec t ive ly in t erms o f set, it appears that , for asymmetr ica l ripples, 23 ripple sets u n e q u i v o c a l l y b e l o n g to w a v e t y p e and 12 ripple sets b e l o n g to current type , and for t h e remain ing 19 sets results were equivocal .

Plot o f RSI vs. SI. A l t o g e t h e r 63 s y m m e t r i c a l and 53 a s y m m e t r i c a l sets were cons idered . For reasons d i scussed earlier, data po in t s o f s y m m e t r i c a l ripples

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were not plotted in diagrams. It was, however, calculated that out of 302 total readings 116 are located in the wave field, 66 in the field o f overlapping of wave--wind and another 116 in the field o f wave--wind--swash (Table II).

264

When considered in terms of set only 7 ripple sets were recognised as wave ripples, the characters of others remained undefined.

When all the readings of asymmetrical ripples are considered collectively (Table II), it was revealed that total frequency of data points which occur within the current field exceed that of the wave field by a narrow margin (Table II). Unlike the earlier plot, the maximum number of points, i.e., 170, is located in the field of wind ripple, though wind ripple could never be visually recognized in the field. A large number of points as shown in Table II and Fig. 18 fail to occur in any of the specified fields of Tanner (1967), and many other points are located in the overlapping zone of wave and wind fields. When the results were considered in terms of ripple set, only 5 sets could be recognised as wave ripple, 6 as current ripple and 4 as wind ripple. But no reliable conclusion regarding genetic type of ripple could be drawn for the remining 38 sets.

Plot o f R I vs. PI1. This plot was made with data derived :from 61 symmetrical ripple sets and 50 asymmetrical sets. For symmetrical ripples, out of 1215 total readings a good number of points (532) occupy the domain of wave ripple and another 176 points occupy the field of current ripple. A large number of points (476) is located in the field of overlapping of wave and current ripple. Frequency of points in other fields, such as wind, wind--current, etc., are insignificant (Figs. 19, 20; Table II). If considered in terms of ripple set, 18 sets could be defined as wave ripple, 4 as current ripple and the remaining 39 sets defied recognition.

In the case of asymmetricl ripples, it was revealed that out of 824 readings 308 points fell within the domain of current ripples, and 144 in the field of wave ripples. In the zone of overlapping of wave and current, 318 points are plotted (Figs. 19, 20; Table II). Amongst 50 asymmetrical ripple sets considered, 13 could be recognised as current induced, and only 3 could be defined as wave ripple. The origin of a large number of sets remained ob- scare.

Plot o f PI, vs. PI2. Out of 117 ripple sets considered, 61 are symmetrical and 56 are asymmetrical. For symmetrical ripples the total number of readings was 300, amongst which 92 fell within the zone of current ripples and, sur- prinsingly, not a single data point was located in the domain of wave ripples (Fig. 21). A large number of points (146) fell in a field not specified by Tanner (1967) and for convenience of discussion that field has been desig- nated here as the 'X' field. Frequency of points in other fields of overlap are given in Table II. Amongst 61 symmetrical ripple sets examined, only 10 could be recognised as current ripples and the rest defied any such categori- zation. It is ambiguous enough that not a single ripple set could be recognised as wave ripple by this plot.

On the other hand, 19 out of 56 asymmetrical ripple sets could be recog nized as current ripple; only one could be defined as wave ripple; the rest

265

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remained unresolved in terms of their mode of generation. When considered in terms of total reading, it was revealed that 119 data points were located in the field of current ripples, whereas only 6 occupied the field of wave ripples. Similarly to that of symmetrical ripples, here also a good number of points occupy the 'X' field (Fig. 21, Table II).

Analyses of ripple parameters and scatter plots

It may be noted that in the present study, various ripple parameters of symmetrical and asymmetrical ripples have been treated separately because distinction between symmetrical and asymmetrical ripples is in many ways

267

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268

important other than as a mere classification (Clifton, 1976; Allen, 1979). It is well known that while most symmetrical ripples are produced by wave motion, asymmetrical ripples are generally current induced (Kindle, 1917, Bucher, 1919; Evans, 1949; Menard, 1950; Sundborg, 1956, Allen, 1963; Komar, 1974). The criterion is, however, not unique as many wave ripples are found to be asymmetrical (Kuenen, 1950, p. 289; Allen, 1979} and many current ripples may be symmetrical (Van Straaten, 1953, p. 1). Neverthe- less, in an environment where oscillation-related mass-transport is negligible, wave ripples generally are nearly perfectly symmetrical (Allen, 1979). Even though Allen (1979), on the basis of experimental results (Allen, 1969, Banks and Collinson, 1975), could set up an upper limit for the degree of asymmetry of wave ripples, situations may arise where distinction between wave and current ripples may be difficult on the basis of degree of asymmetry alone (Evans, 1949; Tanner, 1967, Reineck and Wunderlich, 1968a; Newton, 1968).

Harms (1969) suggested that wave-generated ripples are very evenly spaced because wave motion reduces variability of ripple spacing much below that attained by current alone. On the other hand, the height of wave ripples is also distributed within a relatively narrow range compared to that of current ripples. It may be noted in Fig. 15 that average spacing, average height, and standard deviation values of symmetrical ripples are less than those of asymmetrical ripples. It appears that the differences in the values between symmetrical and asymmetrical ripples are genetically significant and analogous to those obtained by Harms (1969) for wave- and current-generated ripples. It may be suggested, on the basis of this analysis, that the symmetrical ripples as a group are dominantly wave-formed, whereas asymmetrical ripples are dominantly current-induced.

It has been suggested by many authors that ripple size and spacing increases with increasing grain size (Bucher, 1919, p. 159; Inman, 1958, p. 522; Allen, 1963, p. 194). On the basis of experimental study, McKee (1965) also demonstrated that at any particular water depth, a considerable increase in both height and length of ripples resulted from change to coarser grain size. An examination of Table III reveals that in the present case apparently no such relationship exists. However, the absence of any such relationship does not preclude its existence because all different ripples under s tudy were not necessarily formed at the same depth. Harms {1969) further demonstrated that sand in wave-generated ripples is bet ter sorted than in current ripples; this relationship is, however, absent in the present case (Table III). The limitations in the applicability of experimental results probably arises from the fact that uniformity of parent material is not expected in nature, which is often a precondition in experimental study.

In recent years intensive studies have been made on wave-formed ripples and at tempts have been made to find out the relationships among ripple parameters, grain size and the ambient hydraulic condition under which they formed (Tanner, 1971; Komar, 1974; Clifton, 1976, Allen 1979). For sym-

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metrical wave ripples, it was demonstrated by Clifton (1976, p. 133) that in 'orbital ripples', ripple spacing is independent of grain size but i~ directly related to the length of the orbital diameter of waves; in the case of 'sub- orbital ripples' ripple spacing is inversely related to orbital diameter, but also depends on grain size. For 'anorbital ripples', ripple spacing, though depen- dent on grain size, is largely independent of orbital diameter. As given in Table III, out of 15 ripples of wide morphological variety, only 5 could be visually identified as symmetrical wave ripples. For these five ripple sets S/D ~n (where S is ripple spacing or wavelength; D is mean grain diameter) and do/D values (where do is orbital diameter) have been determined (Table IV). The relationship i.e. S = 0.8 do (Komar, 1974) was used to derive do values (Table IV). When S/D in and do/D values of those five ripple sets were examined with reference to Clifton's diagram (1976; his fig. 9, p. 134), it came out that all these wave ripples occupy the zone specified for 'orbital ripples'. Thus, as expected, no relationship between ripple spacing and grain size for these 'orbital ripples' could be recognized (Table III). Tanner's (1971) data from experiments on asymmetrical ripple in sands of diverse sizes, under small short-period waves, have been synthesized by Clifton (1976, his fig. 46, p. 137), which clearly indicates that, unlike symmetric wave ripples, wavelengths of asymmetric ripples formed under waves of small orbital diameter, depends in part on grain size. In the present case, data on wave-formed asymmetric ripples (Table III) are too meagre to verify this.

Allen (1979) recently introduced two graphs (his figs. 1 and 2) showing how the wavelength, vertical form index (vfi) or ripple index and grain size of some wave ripples in stratigraphic record can be interpreted in terms of the near-bed water particle orbital diameter (d) and maximum orbital velocity (Umax) induced by progressive surface waves. It may be recalled that field data for the present study were collected much before the appearance of Allen's paper and consequently recasting of the data in Allen's graphs is not always straightforward. For example, ripple length (L) and vfi (i.e., ripple length/height) were used by Allen {1979) instead of crest spacing (s) and RI (i.e., crest spacing/height) as used here after Tanner (1967), Harms (1969) and Komar (1974). In the present case vfi (as deduced by Allen) in addition to RI (i.e., crest spacing/height) could be determined for the asymmetrical ripples only (Table III) whereas for symmetrical ripples only RI has been deduced. It was revealed that vfi and RI for the same set of asymmetrical ripple are close to each other. It appears that for practical purposes RI as calculated here can be used instead of vfi of Allen (1979).

Thus, utilising grain size (Dmean), crest spacing (s) and ripple index (RI) of wave ripples in the background of Allen's graph, maximum orbital velocity (Um~x) and orbital diameter (d) were determined (Table V). It may be noted that values of orbital diameter as determined from Komar's (1974) equation (Table IV), differ widely from that estimated from Allen's graph (Table V). Such discrepencies may arise from the limitations of Inman's (1957) original data, as pointed by Allen (1979). In the present study, wave period (T) for

272

T A B L E V

Wave pa rame te r s and some derived express ions for some rep resen ta t ive r ipples (Af t e r Allen, 1979)

samples of wave

Spec imen L/D d/D Orbi ta l M a x i m u m orb i t a l Wave per iod, T n u m b e r d iameter , d velocity, Urnax (s)

(m) (ms -1 )

R-5 251 550 0 .06 0 .12- -0 .42 0 .44 - - 1.57 R-8 380 2100 0.27 0 .11- -0 .47 1.8 - - 7.7 R-48 320 1250 0 .26 0 .11- -0 .54 1 .51- - 7 .42 R-53 473 3700 0 .77 0 .11- -0 .54 4 .56- -21 .9 R-55 292 750 0.14 0 .12- -0 .48 0 .91- - 3 .66 R-87 340 1050 0 .15 0 .11 - -0 .48 0 .98 - - 4 .28 R-132 344 850 0 .08 0 . 1 3 - 0 . 3 1 0 .81- - 1 .93

L = cres t spacing, D = m e a n grain size.

seven sets of wave ripples was calculated (Table V) utilising Allen's equation, i.e., T = 7rd/Umax, where d is orbital diameter (as calculated after Allen, 1979) and Umax is maximum orbital velocity. Corresponding to double values of Umax, there are two values of T for each ripple set. The lower values of T lie below 4 sec, excepting in one case, whereas the higher values range from 1.5--22 sec (Table V). As indicated by Komar (1974) and Allen (1979), wave periods (T) in the range of 10 sec suggest a setting on an open shelf bordering a large sea, whereas periods of less than approximately 4 sec point to an environment of shallow water and restricted fetch. In the present case, the lower values for wave periods seem more appropriate in view of the shallow-water features associated with these sediments.

Tanner (1971) provided some useful relationships between ripple mark spacing (s), grain diameter (g), water wave height (H), water wave length ()~), water depth (h) and fetch values (f). With ripple mark spacing (s) and grain size (g) data in hand, the following equations of Tanner (1971, p. 80, 82) were used to derive different hydraulic parameters for the generation of the wave ripples under study.

The equations used are given below:

= 33.77 + 1.72S -- 7.22 In g (1)

H = 38.52 + 1.89S -- 7.11 l n g (2 )

In h = 22.74 + 0.97S -- 3.72 l n g -- 0.41H (3)

In f = 2.968 In H -- 0.583 In h -- 0.731 (4)

The results of these analyses are given in Table IV. The k, H, h, f values and the values of the expression: 0.97 S -- 3.72 In g, when considered in the background of Tanner's palaeogeographic plot (1971, his fig. 1, p. 84),

2 7 3

appear mutually compatible and all the points are located in the 'hornshaped meaningful port ion ' o f Tanner's plot. It was suggested by Tanner (1971) that an average ripple-mark spacing of 4--5 cm separates, with good reliability, large lakes and seas with greater fetch on the one hand, from ponds, small lakes and other restricted bodies of water with restricted fetch, on the other. For the ripples in the Lower Bhander Sandstone, average spacing (Fig. 15) lies in the range of the critical value of 4--5 cm and indicates restricted fetch. It is also evident from other independent evidence that ripples in the Lower Bhander Sandstone were developed in shallow, restricted bodies of water behind a barrier bar (Sarkar, 1979). Thus the fetch values as calculated from Tanner's equation appear to be relatively greater and incompatible with the general depositional model o f the Lower Bhander Sandstone.

RI values of both symmetrical and asymmetrical ripples in the present s tudy show wide variation which encompasses the total spectrum of values stipulated for wave and current ripples. However, the maximum frequency of RI values lies below 15 which according to Tanner (1967) is a critical value separating ripples generated by waves or water currents (RI < 15) from those of wind or swash origin (RI > 15).

A scatter plot involving vertical form index (ripple length/ripple height) and median grain size has been constructed after Reineck and Wunderlich (1968a) for the asymmetrical ripples (Table III). Recognition of the genetic types of those ten asymmetrical ripples by visual examination on the one hand (Table III) and by the scatter plots on the other (Fig. 22) are mutually consistent with each other, excepting for three samples.

Variation in RSI values in the present case (Fig. 16) covers a wide range encompassing the limiting values of wave ripples (RSI < 1.5) and current ripples (RSI > 3.8) as specified by Tanner (1967) and Reineck and Wunder- lich (1968a). Recently, however, a limiting value of approximately 5.44 has

25

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Fig. 22. Scat ter p l o t o f ver t ica l f o r m i n d e x and med ian grain size. ( A f t e r Re ineek and Wunderlleh, 1968a.)

274

been suggested by Allen (1979) as the upper limit to the degree of asymmetry (RSI) of wave-related ripple marks. When the distribution of SI values of ripples under s tudy are compared with critical limiting values as suggested by Tanner (1967), it appears that only a few ripples are of definite wave or current origin and the majority of the values lie in the indeterminate zone. Ripples having PI, values ~<1 are invariably current-induced, whereas those with PI, ~ 7 are invariably wave formed (Tanner, 1967). In the present case, considering the distribution of PI~ values (Fig. 16), the existence of wave ripples becomes doubtfu l which, however, appears paradoxical in view of the gross characteristic of the ripples. When PI2 values are considered, most of the values are found to be greater than 0.4 which is a critical value attained by current ripples only (Tanner, 1967). The frequency of PI2 values lying below 0.2, the critical range of values diagnostic of wave ripples (Tanner, 1967), is very insignificant and thus wave ripples appear to be rather rare. The virtual absence or rarity of wave ripples, as indicated by distribution of PI2 values, is, however, incompatible with the gross morphological character of ripples in the Lower Bhander Sandstone.

When the results of various scatter plots are examined, the presence of various genetic types of ripples is indicated. When symmetrical ripples are considered separately, wave ripples, as indicated by scatter plots of RSI vs. SI, and RI vs. PI~, appear to be predominant over current ripples. Similarly, the frequency of asymmetrical, current-generated ripples exceeds that of asymmetrical wave ripples (Table II). But as indicated by the scatter plot of RI vs. RSI, wave ripples appear to be predominant over current ripples irrespective of symmetry (Table II). For asymmetrical ripples, the plot of RSI vs. SI indicates the presence of a large number of wind ripples, a feature incompatible with the general motif of the Lower Bhander Sandstone. Well in accordance with the observation of Tanner (1967) that the plot of PI~ vs. PI2 is the least reliable of all types of scatter plots, the presence of wave ripples in the Lower Bhander Sandstone becomes doubtful from this scatter plot, whereas on the basis of visual study such ripples appear to be quite common. When considered in terms of ripple set, the contention that most asymmetrical ripples are current induced whereas most symmetrical ripples are wave generated, holds good. However, the reverse relationship also exists in many instances.

Internal structures of ripples and their interpretation

The sandy layers of the Lower Bhander Sandstone are often internally ripple-laminated, particularly towards their top. Unlike flaser and lenticular bedding (Reineck and Wunderlich, 1968b), the ripples as stated earlier, are generally internally cross-laminated with millimetre-thick alternations of sand and mud (Figs. 23 and 24). The external geometry and internal structure of ripples are in most cases mutually conformable, but there are excep- tions. In the case of form-discordant ripples, profiles controlled by the last

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Fig. 23. Superimposed sets of cross-laminae. Note (1) alternation of sand/mud cross- laminae, (2) undulatory nature of the set boundaries, (3) offshooting of cross-laminae, and (4) ripple symmetry on the top. Length of the specimen is 7.5 cm.

phase of hydraulic action, is not necessarily a reliable criterion for the classification of ripples (Picard and High, 1968).

As expected and noted by many authors (Allen, 1963, 1968; McKee, 1965), strongly asymmetrical ripples, possibly current induced, invariably exhibit unidirectional foreset lamination (Fig. 25). Symmetrical ripples, as noted by McKee (1965), Newton (1968) and Picard and High (1968), may also show unidirectional foresets (Figs. 23, 24). The unidirectional nature of foresets in symmetrical ripples may owe its origin to the same causes stated earlier for the development of asymmetry in crest profiles or is due to round- ing off of the crests of asymmetrical ripples (Newton, 1968).

The centimetre-thick sandy interlayers of Lower Bhander Sandstone in most cases are interally composed of complexly organized bundles or sets of cross laminae (Figs. 23, 24) and resemble the product of 'multiphase ripples' of Newton (1968). The foresets usually occur as cosets formed of regularly or irregularly superimposed sets with wavy or planar bounding surfaces which may be either mutually parallel or converging and diverging from each other (Figs. 23 and 24). In many instances the internal structures of ripples display assemblages of differently structured lenses with curved lower bound-

Fig. 24. Superimposed sets'of cross-laminae. Note convergence of set boundaries and symmetrical ripple profile on the top. Length of the specimen is 16 cm.

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I Fig. 25. Ripple laminated zone (darker) overlying a massive sandy zone (lighter). Note (1) preservation of asymmetrical ripple structure immediately above the massive sand and (2) draping of the ripple by a bundle of laminae followed in turn by a zone of ripple lamination. Length of the specimen is 15.5 cm.

aries, bundlewise arrangement, offshooting and draping of foresets (Figs. 26 and 27) characteristic of typical wave ripples (Reineck and Singh, 1973; De Raaf et al., 1977). Structural dissimilarity between laterally and vertically adjacent sets is not uncommon {Figs. 26 and 28). In many instances, relatively larger ripples may be preserved due to draping of mud or bundles of laminae over them (Fig. 25) and the relatively smaller ripples may migrate over the stoss side of those preserved larger ripples.

Structures mimicking ripple-drift cross-laminations have also been observed in places. They resemble Type-A and Type-B of Jopling and Walker (1968) and have been described in detail by Chanda and Bhattacharyya (1974), and Bhattacharyya and Chanda (1976). Climbing is, however, not always monotonic upward and instead there are certain oscillations in the angle of climb within a well-defined pattern. A good many of such structures

offshoots climbing ripple Iominotion mud ~rope ]

pebble

Fig. 26. Internal laminations of superimposed combined-flow ripples. Note (1)structural dissimilarity of adjacent sets, (2) swollen lens-like set, (3) offshooting and draping of foresets, and (4) irregular undulating lower boundary. Length of the specimen is 26 cm.

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Fig. 27. Superimposed sets of ripple lamination. Note offshooting of the ripple laminae in the central set. Length of the specimen is 21.5 cm.

resembling ripple~lrift are not really such, for up-current dipping surfaces separating sets of cross-laminations are often draped with mud (Bhattachary- ya and Chanda, 1976, their fig. 3) and may be marked by angular divergence and unevenness (Fig. 24; Chanda and Bhattacharyya, 1974); even when they are planar and parallel, the sets may vary in thickness and the ripples either wholly or partly may remain frozen within a set.

Rhythmically size-differentiated foreset laminae of ripples possibly represent products of time correlative discrete episodes of sand and mud deposition of a single tidal cycle (Reineck and Wunderlich, 1967; Bhattacharyya and Chanda, 1976). The complexity in the organization of foresets of ripples presumably arises from a variable rate of deposition and erosion. The com- plete ripples had been preserved in places where mud deposition was thick enough to survive erosion (Reineck, 1960); subsequent ripples had migrated over the wavy surfaces of the preserved earlier set of ripples (Bhattacharyya and Chanda, 1976). As a result of such migration over a wavy surface, as well as because of appreciable penecontemporaneous erosion, the set boundaries may meet at variable angles (Fig. 24). Where mud deposition and erosion had been more orderly, there seems to develop a regular upward progression to a

Fig. 28. Superimposed sets of ripple laminae overlying a rippled sandy layer. Note (1) dissimilarity in the structural arrangement of the ripple laminae in adjacent sets, (2) shale pebbles (dark) defining cross-laminae in the lower sandy layer. Length of the specimen is 20.5 cm.

278

pattern similar to ripple drift structure. Angularity between different set boundaries within a coset may also arise from slight changes in flow direction of the superimposed sets of ripples.

EVALUATION

Even though ripple marks in intertidal Lower Bhander Sandstone vary widely in their morphology, a good number of them could be visually recognized either as wave or current ripples. Many of them also belong to the combined flow type of Harms {1969).

When spacing and height values of symmetrical and asymmetrical ripples are compared, it appears that there exist significant differences in average values and standard deviations between the two types of ripples. More or less analogous results were obtained by Harms (1969) for wave and current ripples formed under experimental conditions. Thus it appears that many and probably most of the symmetrical ripples are wave generated, whereas most of the asymmetrical ripples are dominantly current induced.

With ripple mark spacing and grain size data in hand for a few ripples of possible wave origin, several hydraulic and wave parameters were derived after Tanner {1971). These values when considered together are found to be mutually consistent and occupy a meaningful port ion in Tanner's (1967) palaeogeographic plot which again closely corresponds with the general environmental framework of the Lower Bhander Sandstone as deduced from other evidence. Over and above this, it can be shown that the ripples in the Lower Bhander Sandstone, with an average spacing of 4--5 cm were generated in shallow restricted bodies of water with limited fetch.

The range in variation of different dimensionless parameters, excepting PI1 values, indicates the presence of both wave and current ripples. The scatter plots made after Tanner {1967) and involving different dimensionless parameters, also indicate the presence of various genetic types of ripples. Moreover, the data points of symmetrical and asymmetrical ripples generally tend to be concentrated in the wave and current fields, respectively and thus a relationship between symmetry of ripples and their genetic types becomes apparent.

In many instances the internal structures of both symmetrical and asymmetrical ripples are characterized by unidirectional foresets consisting of highly ordered varve-like rhythmic alternations of sand and mud which may be correlated with alternate traction and suspension settlement. The sets of ripple cross-laminae display a wide variety of complexity in their organization. The arrangement of the laterally and vertically adjacent sets and the foresets within them may often show characteristics typical of wave-generated structure (De Raaf et al., 1977).

Finally, mention should be made of the limitations in applicability of the various criteria for recognition of different genetic types of fossil ripples in general and for the present case in particular. It may be noted that ripples

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once formed may suffer considerable change in their morphology, particularly in their symmetry, by subsequent impulses of waves or current and may even result in the formation of the 'composi te ripples' of Newton (1968). Asymmetrical current ripples, as noted by Harms (1969), may become rounded and develop gentle lee slopes owing to intermittent suspension set- t lement over them, as has occurred in the case of many of the ripples under consideration. Furthermore, Boersma (1970, in Reineck and Singh, 1973) and Reineck and Wunderlich (1968a) suggested that in ancient sediments ripple size and with that the RI values, would be affected by compaction. Thus ripple symmetry and RI values should be used with much caution in interpreting fossil ripples. The internal structure of wave and current ripples, as shown by Newton (1968a), may not differ appreciably in many instances and thereby may fail to distinguish wave and current ripples.

Ripples composed of very fine sand or coarse to fine silt and their inter- lamination with mud, as observed here, further complicates the issue, leaving aside the problem of inequilibrium arising from intermittent mud settlement, the mechanics of ripple generation itself in very fine sediments is yet to be resolved (Harms, 1969). Many of the inconsistencies in the present analysis might owe their origin to the factors discussed above.

As mentioned earlier, the relative frequency of different genetic types of ripples does differ widely when different dimensionless parameters are considered separately and thus the limitations of such analysis become obvious. The scatter plots made with different dimensionless parameters in various combinations show that the parameters derived from measurement in different parts of the same ripple set may be dispersed over an area covering fields specified for various genetic types of ripples in the same scatter diagram. There are again examples when the data points are located in no-man's land and consequently no conclusion could be drawn with regard to their mode of origin. The ripple field indicated for a particular ripple set in one type of scatter plot may or may not correspond with the same field in another type of plot. Moreover, the presence of wind-ripples as indicated by scatter plots appear to be incompatible with the general mot i f of the Lower Bhander Sandstone and no definite wind ripples could be visually recognized in the field. Thus it appears that so far as the interpretation of fossil ripples is concerned, the scatter plots of Tanner (1967) are of limited applicability, for the results obtained from them may be mutually inconsistent and disap- pointing relative to the efforts expended.

It seems that for the effort expended, integrated analysis of external as well as internal structures of ripples (Harms, 1969; De Raaf et al., 1977) in the contex t of the broad subenvironment in which they occur, is probably more rewarding than anything else.

ACKNOWLEDGEMENTS

The work presented in this paper is an extension o f a part of the author 's Ph.D. thesis which was carried out under the joint supervision of Drs. S.K.

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Chanda and Aj i t B h a t t a c h a r y y a . The a u t h o r r eco rds his deep g r a t i t u d e to Drs. Chanda and B h a t t a c h a r y y a fo r t he i r c o n s t a n t e n c o u r a g e m e n t , he lp fu l suggest ions , s t imula t ing d iscuss ions and cr i t ica l review of t he manusc r ip t . Dr. P.K. Bose, Mr. S o u m e n R o y and Mr. C h i r a na nda De he lped in var ious stages o f t he s tudy . Cri t ical c o m m e n t s and cons t ruc t i ve suggest ions b y t h e reviewers Drs. F. Wunder l i ch and R o b e r t Marscha lko led to i m p r o v e m e n t o f the manusc r ip t . T y p i n g of the m a n u s c r i p t and f inal p r e p a r a t i o n o f l ine d rawings was d o n e b y Mr. R.M. Kar and Mr. T.P. B h a t t a c h a r y y a , respec- t ive ly . I a m t h a n k f u l t o all o f t h e m .

F inanc ia l s u p p o r t fo r th is s t u d y by Counci l o f Sc ien t i f ic and Indus t r i a l Research (New Delhi) is g ra t e fu l ly a cknowled ge d .

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ripple marks in intertidal lower bhander sandstone (late proterozoic), central india: a...

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