Internal structures of sandwaves in a tide-storm interactive system: Proterozoic Lower Quartzite Formation, India

Sedimentary Geology, 67 (1990) 133-142

133

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Internal structures of sandwaves in a tide-storm interactive system" Proterozoic Lower Quartzite Formation, India C H A N D A N C H A K R A B O R T Y and P R A D I P K. BOSE Department of Geological Sciences, Jadavpur University, Calcutta-700 032 (India) Received August 29, 1989; revised version accepted January 23, 1990.

Abstract Chakraborty, C. and Bose, P.K., 1990. Internal structures of sandwaves in a tide-storm interactive system: Proterozoic Lower Quartzite Formation, India. Sediment. Geol., 67: 133-142. A Proterozoic sandstone sequence belonging to the Lower Quartzite Formation of Vindhyan Supergroup, India, reveals the internal structures of near-symmetrical subtidal sandwaves formed in an area of strong tidal currents, occasionally interfered by wind-induced currents of varying magnitude. Internally, the sandwaves show decimetre-scale, herring-bone cross-laminated sets with inclined and horizontal set boundaries representing accretion on the gently inclined (around 5 ° ) lee and stoss surfaces of the sandwaves respectively. The internal structures suggest oblique upbuilding of the sandwaves with almost equal contributions from the two reversing current modes of the tidal flow. Evidently, the sandwaves were maintained by bedload transport through migration of megaripples superimposed on the sandwaves. Occasional superimposition of short-lived, wind-induced currents on the tidal flow caused appreciable suspension tran.sport of sand-sized sediments and led to the development of successive low-angle, unidirectional, mud-draped, cross-laminated bundles interwoven with the tide-generated structures. However, the dominant sediment type introduced during the period of wind-induced currents was in the size range of mud as reflected in the presence of exceptionally thick exotic mud layers in juxtaposition with the cross-laminated bundles. During the periods of vigorous storm currents, significant volumes of sand-sized sediments were introduced in the form of density flows, deposition from which led to the burial of tidal sandwaves. Renewal of the fair-weather tidal regime caused the development of new sandwaves.

Introduction

Large, transverse bedforms with superimposed megaripples have been observed in many tide-influenced environments and are currently being designated as sandwaves (Allen, 1982; Allen and Homewood, 1984; Dalrymple, 1984). Oceanographical studies on present-day continental shelves have brought to light the dynamics of these sandwaves in relation to the associated tidal currents (Stride, 1982). Studies on the internal structure of modem tidal sandwaves (Boersma, 1969; Boersma and Terwindt, 1981; Kohsiek and 0037-0738/90/$03.50

© 1990 - Elsevier Science Publishers B.V.

Terwindt, 1981; Terwindt, 1981; Dalrymple, 1984), confined mainly to the intertidal zone, have revealed features that are significant with regard to construction of process-response, paleo-emdronmental models from ancient analogues. Allen (1980) proposed a comprehensive theoretical model for the development and maintenence of tidal sandwaves, and predicted the internal structures for different current speeds and velocity asymmetries. As far as the stratigraphic records are concerned, many sandstone sequences with herring-bone or complex, bundled cross-stratifications have been ascribed to sandwaves (De Raaf

134

¢ ( ' H A K R A B O R T Y A N D P.K. BOSE

and Boersma, 1971: Narayan, 1971; Banks, 1973a; Anderton, 1976; Nio, 1976; Levell, 1980; Allen and Homewood, 1984; Walker, 1985; Johnson and Baldwin, 1986). Inspite of these advances our knowledge of the internal structure of sandwaves is still inadequate, especially for systems where tidal currents are interfered by non-tidal processes. Occasional superimposition of strong alien currents on tidal currents causes significant variation in the flow pattern and is likely to bring about considerable changes in the internal structure of the sandwaves beneath such flows. Enhancement of tidal currents by storm currents have been observed in modern settings (see Stride, 1982) and inferred for many ancient sequences (see Walker, 1985; Johnson and Baldwin, 1986). However, little is known about the internal structures of sandwaves formed in an environment where tidal processes are infrequently interfered with storm currents. This paper describes the internal structures of sandwaves from a Proterozoic sandstone sequence in India that were presumably developed in an interactive tide-storm shallow-marine setting.

Geologic background The Proterozoic Vindhyan Supergroup, exposed in Central India, represents one of the many platform-type, shallow-marine sedimentary sequences of the Indian Shield (Naqvi and Rogers, 1987: Fig. 1). It consists of four groups, from

bottom to top: the Semri, Kaimur, Rewa and Bhander Groups (Chanda and Bhattacharyya, 1982). The Lower Quartzite Formation, the subject of the present study, is the lowermost formation of the Kaimur Group, which rests unconformably on the Rohtas Limestone Formation of the underlying Semri Group (Naqvi and Rogers, 1987). The formation consists dominantly of mediumgrained sandstones and is exposed as an east-west trending narrow band along the southern periphery of the outcrop of the Vindhyan Supergroup (Fig. 1). Though the formation is exposed in many places along the outcrop belt, sedimentologically useful sections are available only in Churk, and studies on the internal structures and paleocurrent directions were confined to that place. However. sections have been measured in two other places. Amiliya and Amjhore (Fig. 1). The thickness of the formation is 10 m in Churk, 8 m in Amiliya and 10 m in Amjhore. The formation is everywhere overlain by an interbedded sandstone-shale facies known as the Silicified Shale Formation (Chanda and Bhattacharyya, 1982; Fig. 1). The overall environment of deposition of the Kaimur Group has been interpreted by various workers as shallow marine (see Chanda and Bhattacharyya, 1982). The Lower Quartzite Formation was probably formed during the transgression of the Vindhyan Sea at the onset of Kaimur sedimentation, and Singh (1973) attributed the origin of the formation to a shallow subtidal environment of deposition.

80"

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Fig. 1. Generalised geological map and stratigraphy of the Vindhyan Supergroup in the Son valley area. Studied areas are shown by black dots.

INTERNAL

STRUCTURES

OF SANDWAVES

IN A TIDE-STORM

INTERACTIVE

Description of the internal structures

The sandstones of the Lower Quartzite Formation comprise the following types of structural units.

Type I: Cosets of cross-laminations with inclined set boundaries

This type of structural unit is characterised by decimetre-scale cross-laminated sets separated by gently inclined (3-5 o, with respect to the primary depositional surface) planar erosional surfaces (Fig. 2, 3a). The internal cross-laminations dip both in the direction of the inclined set boundaries and in the opposite sense in almost equal proportions, resulting in a herring-bone pattern. The cross-laminations within a set locally show reactivation surfaces (Fig. 4). The dip of the crosslaminations ranges between 25 o and 30 °. Some set boundaries are marked by the presence of mud

SYSTEM

135

clasts. At places mud drapes are preserved that are locally up to 9 cm thick (Fig. 2). In vertical section there is a discernible gradual reduction in the set thickness up the coset (Figs. 3a, 5). The azimuths of the inclined set boundaries cluster between 20 o and 80 o. Type-I structural units sometimes grade laterally, in a direction opposite to the inclination of the set boundaries, into type-II structural units described below (Fig. 2). Apart from the first-order basal surface on which the structural units developed (Fig. 2), the set boundaries within the unit itself can be classified into the following hierarchical orders: (a) the surfaces characterised by preserved mud drapes ($2), (b) the surfaces marked by mud clasts ($3), (c) the surfaces across which the set thickness gradually changes ($4), and (d) the surfaces that separate cross-laminations of opposite orientations ($5) (Fig. 6). The structure of the type I unit is qualitatively similar to that what has been termed "downcurrent-dipping cross-stratification" (Banks, 1973b), "compound cross-stratification" (Harms, 1975)

I

Fig. 2. Photograph and sketch of the longitudinal section of a sandwave in the Lower Quartzite Formation showing the different structural units (marked by rectangular outlines). Note: (1) interweaving of packages of different stratification styles; (2) the first-order basal surface on which the sandwave rests; (3) lateral gradation between type-I and type-II structural units; and (4) the preserved mud layer.

136

(

C H A K R A B O R T Y A N D P.K. BOSE

Fig. 3. Close-ups of the structural units shown in Fig. 2. (a) Type-I structural unit; note: (1) inclined set boundaries with respect to the orientation of the first-order basal surface (marked by the black fine); (2) herring-bone pattern of arrangement of the cross-laminations; and (3) upward reduction in the set thickness. (b) Type-II structural unit; note horizontal set boundaries and herring-bone pattern of arrangement of the cross-laminations. (c) Type-III structural unit; note: (1) low-angle, wedge-shaped, bundled cross-laminations; (2) thin mud partings between the bundles; (3) burial of the bundles by type-I structural unit; and (4) the thick mud layer in juxtaposition with the bundles; length of the section 3.5 m.

a n d " i n c l i n e d c r o s s - b e d d i n g " ( D a l r y m p l e , 1984). T h e structures have similarities also with the Class V and VI structures of Allen (1980).

Type 11." Cosets of cross-laminations with horizontal set boundaries This type of structural unit is similar to the type-I unit in all respects except that their set b o u n d a r i e s are n e a r l y h o r i z o n t a l or have d i p s that are h a r d l y d e t e c t a b l e in the field (Figs. 2, 3b). As

m e n t i o n e d above, locally t y p e - I I units g r a d e laterally into t y p e - I units.

Type 111." Large-scale, low-angle, bundled crosslaminations A t a few localities long, w e d g e - s h a p e d b u n d l e s of u n i d i r e c t i o n a l c r o s s - l a m i n a t i o n s are interwoven within the a b o v e types of structural units (Figs. 2, 3c). The average height o f the b u n d l e s is 42 cm a n d their m a x i m u m thickness ranges b e t w e e n 10

INTERNAL

STRUCTURES

OF SANDWAVES

IN A TIDE-STORM

INTERACTIVE

~a

~

tJ

~

lO

137

SYSTEM

'%/ Boundary marked by thick,preserved mud layer. S3

~2

Boundary marked by mucl closts.

Boundary marked by groduot change

in set thickness. 1

2

3

/* 5 6 Sef Number

7

8

9

10

11

Fig. 5. Diagram showing the pattern of vertical variation in the thickness of the cross-laminated sets.

j j / / / / ~.~~.~_~

~

Boundary marked by reversal of [aminoorientation.

Fig. 6. Diagram showing different orders of surfaces observed within the type-I and type-II structural units. and 15 cm. The bundles dip gently (10-15 o) and their azimuths cluster between 40 o and 60 o. Internally each bundle is made up of 0.5-0.8 cm thick laminations that are normally graded. Each bundle is separated from the next by thin mud partings (Fig. 3c). The toe of the bundles is tangential whereas the top is truncated. The bundles accrete preferentially on gently inclined erosional surfaces cut into the previously described types of structural units, and are in turn covered by them (Figs. 2, 3c). These crosslaminated bundles resemble Class I I I A structure of Alien (1980) except that they have very low dip and internal laminations are normally graded.

Type IV." Parallel-laminated beds Between the above types of cross-laminated structural units are occasional laterally extensive parallel-laminated beds ranging in thickness from

50 to 70 cm. The lower boundaries of these beds is invariably wavy (wavelength = 5.5 m, amplitude = 0.3 m) whereas the upper boundaries are essentially flat (Fig. 7). The basal laminations of the beds drape the undulatory erosion surfaces cut into cross-laminated units. The laminae thicken from the crest towards the trough of the undulatory erosion surface; the bed morphology, as a result, flattens upward. The lamination style resembles h u m m o c k y cross-stratification in two-dimensions. The parallel-laminated beds are, in turn, overlain by another cross-laminated bed of similar nature. Volumetrically, type-I and type-II structural units are the major constituents of the Lower Quartzite Formation, type-III and type-IV units forming only a minor part. Whereas the type-IV structural unit alone constitutes a bed (Fig. 7),

Fig. 4. Reactivation surfaces (marked by arrows) within a cross-laminated set.

138

('. ( ' H A K R A B O R T Y

A N [ ) P.K. B O S E

1.5m

Fig. 7. Photograph and sketch of the type-IV parallel-laminated unit sandwiched between the cross-laminated units. Note: (1) wavy lower boundary and flat upper boundary of the parallel-laminated bed; and (2) thickening of the laminations from the crest towards the trough of the basal undulatory surface.

type-I, -II and -III structural units are intricately interwoven within a single bed (Fig. 2).

Paleocurrents The paleocurrent data represented by the azimuths of type-I and -II cross-laminations show

a bi-polar, tri-modal distribution (Fig. 8a). The azimuths of the large-scale, wedge-shaped crosslaminated bundles show a unimodal distribution (Fig. 8b). The azimuths of the inclined set boundaries of type-I structural units also show a unimodal distribution bracketed between 020 ° and 080 o.

N

b.

N

~

1 Read mg

1 Reading

Fig. 8. Rose diagrams of the azimuths of cross-laminations of (a) type-I and type-II structural units, and (b) azimuths of the low-angle, cross-laminated bundles.

INTERNAL

STRUCTURES

OF SANDWAVES

IN A TIDE-STORM

INTERACTIVE

I0.

9. :::.?,1

8-

7-

:6'2

mi

:::'~ Northern !~:~ mode

n....

,~ iliil ~il

o3. ff.~.. :::.:t;2

oz,

..... S o u t h e r n !i~;~.~ mode

6 2.

~!~:::'

z

z

~!'i~ :'~':~i

i

~ "'~i~. 0

Set thickness in cm.

i 5

0

i

1

15 2 0

Set thickness in cm.

Fig. 9. Histograms of the thickness of the northerly and southerly directed cross-laminated sets.

Set thickness

The thickness of the cross-laminated sets of type-I and type-II structural units have been plotted in separate histograms for the northerly and the southerly directed cross-lamina populations (Fig. 9). Both the populations show a mode in the range 5-10 cm. However, the northern population shows a bias towards a greater thickness than the southern population. Interpretation The stratification styles of the type-I and type-II structural units demonstrably indicate that they are the products of tidal sandwaves with superimposed megaripples. The unimodal distribution of the azimuths of the inclined set boundaries of the type-I units indicates a slightly asymmetric nature of the sandwaves that have gently inclined stoss (around 1 ° ) and relatively steeper lee (around 5 o) slopes. That the associated tidal currents were also velocity asymmetric is reflected in the wider thickness range of the northerly directed crosslaminated sets and their coarser grain size as compared to the southerly directed sets (Table 1).

139

SYSTEM

Nevertheless, the asymmetry was not strong enough to inhibit accretion on the stoss and crestal parts of the sandwaves as reflected in the stratification style of the type-II units which did accrete on these areas. The type-I units, on the other hand, represent accretion on the gently inclined lee slope of the sandwaves, implying that the flow remained virtually unseparated at the crest of the sandwaves. Lateral gradations between type-I and type-II units suggest oblique upbuilding of the sandwaves with both translational and vertical components. The presence of oppositely oriented crosslaminations in the sets of both type-I and type-II structural units indicates that the superimposed megaripples could migrate up and down (in response to flood and ebb) the lee as well as the stoss slopes of the sandwaves. The different orders of surfaces within the type-I and type-II structural units may be related to different phases of the associated tidal cycle (Fig. 6). The S5 surfaces clearly represent the f l o o d - e b b transition of the spring tides whereas the S3 surfaces were formed during the flood-ebb transition of the neap tides if it is assumed that mud could settle only during the slack water stages of the neap tides. The S4 surfaces, across which the set thickness changes (reflecting a change in the superimposed megaripple size), possibly reflect the spring-neap transition of the tidal cycle (see also Dalrymple, 1984). The cross-laminated bundles of type-Ill structural units have similarities with the Class IliA tidal sandwave stratification of Allen (1980), but the asymmetry of the tidal flow required for the development of such structures was possibly not available in the tidal flow associated with the sandwaves of the Lower Quartzite Formation, as

TABLE 1 Grain size characteristics of different structural units Mean grain size (~m) Northerly directed cross-laminated sets Southerly directed cross-laminated sets Low-angle, cross-laminated bundles Parallel-laminated beds

750 450 375 225

140

can be inferred from the stratification styles of the type-I and type-II units. Furthermore, the rare occurrence of type-Ill units indicates that they are not regular products of the prevailing tidal flow. That type-Ill units are unrelated to the tide is also reflected in the grain-size difference between type1, II and the type-Ill units (Table 1). Thus it seems logical to believe that the type-Ill units were formed during occasional superimposition of strong, alien currents on the tidal flow (see also De RaM and Boersma, 1971). Superimposition of strong currents on the tidal flow resulted in a unidirectional but periodically unsteady flow that gave rise to successive mud-draped, crosslaminated bundles similar to the Class I l i A structure of Allen (1980). However, the presence of normally graded laminae within the bundles together with their low dip suggest that they are not the product of avalanching but they represent settlement from a flow highly charged with suspended sediments. The number of bundles present in the type-Ill structural units is always found to be very small, implying that the alien current did not persist for a long time, was not very strong and therefore was probably associated with short periods of strong winds (gales) (see also Morton, 1981). Following the abatement of the gale, the fair-weather tidal cycle resumed and the type-I and II units again started to form and bury the type-III unit (Fig. 2). Some surfaces within the cross-laminated units are marked by preserved mud drapes that locally reach 9 cm in thickness (S2 surfaces, Fig. 6). Such thick mud drapes are unlikely to form at any stage of a tidal cycle (McCave, 1969, 1970, 1971a, 1985). However, thick mud layers may form if sufficient mud is supplied to a tidal environment from outside the system (McCave, 1971b; Allen, 1980). Currents associated with gales could be likely agents for the dispersal of muddy sediments from shoreline environments causing appreciable increase in the suspended sediment concentration (Young et al., 1980; Allen, 1988). High suspended sediment concentrations coupled with neap tides following foul weather could give rise to thick mud layers (McCave, 1971a; Schieber, 1986). Thus it seems probable that the mud layers on the S2 surfaces were deposited following gales during the

('. CHAKRABORTY

A N D P.K. BOS[:.

next available slack water stage of the tidal cycle. The rare occurrence of the mud layers and their close juxtaposition with the gale-generated type-III units (Fig. 2) supports this contention. However, the 9 cm thick mud layer possibly reflects unusual influx of muddy sediments at the shoreline and may represent a longer period climatic cycle (Allen, 1988). The parallel laminations of type-IV units draping the undulatory erosion surface cut into the cross-laminated units evidently represent suspension settlement of sand from a density flow following erosion similar to hummocky cross-stratification. The well-sorted nature and finer grain-size of the parallel-laminated units (Table 1) also suggest fractionation of sediments by a non-tidal agent. It is presumed that the parallel-laminated units were formed from sediment-charged flows driven by vigorous storm currents. Sedimentation from such storm-induced density currents buried the pre-existing sandwaves and renewal of fairweather condition permitted the building up of new sandwaves (Fig. 7). Evidence of infrequent storm intervention also comes from the overlying Silicified Shale Formation, which consists of sharp-based sandstones showing wave-generated laminations, enclosed within shale representing a more distal marine facies. The lack of evidence of emergence within the sandstones of the Lower Quartzite Formation indicates that the sandwaves were formed in the subtidal part of the tidally influenced Vindhyan Sea. However, the tidal currents were affected by storm currents of varying magnitude at rare intervals. The zone of sandwaves was bordered by a dominantly muddy environment farther offshore where sand could be emplaced only during intense storms. The tidal currents in the Vindhyan Sea operated roughly along a north-south line, and the storm current flowed northward. The tidal flow was slightly velocity asymmetric and pronounced asymmetry developed only during superimposition of storm currents. The sandwaves were exclusively maintained by bedload transport; significant suspension transport occurred only during storms. The development of the sandwaves of the Lower

INTERNAL

STRUCTURES

Quartzite

Formation

transgression ning

of

OF SANDWAVES

was probably

of the Vindhyan

Kaimur

coastal erosion;

transgression,

lain by the deeper thin transgressive

related

INTERACTIVE

to the

Sea at the begin-

sedimentation, the derived

ified by tidal currents tinued

IN A TIDE-STORM

which

caused

sands were later mod-

into sandwaves. the sandwave

With con-

field was over-

facies and was preserved

as a

sand sheet.

141

SYSTEM

ascribed

to

avalanching

megaripples

collapsed

sandwaves.

In

bundles sion

contrast,

described

the

settlement

superimposed the brink of the

the laminations

here clearly of sediments

of the

represent from

suspen-

a separated

flow implying that transport of sediments in suspension was appreciable when wind-induced currents were superimposed The rare occurrence

Discussion

as

on reaching

suggests sudden

on the tidal flow. of thick

introduction

mud in an otherwise

mud

mud-deficient

also

tidal environ-

ment by occasional

sentially represent a near-symmetrical variety of the bedform type occurring in the subtidal zone. The sandwaves were built up with almost equal contribution from the two reversing flows of the tidal cycle. They were maintained by bedload transport of sediments that took place in the form of megaripples. The megaripples could migrate up and down the lee as well as the stoss slopes of the sandwaves and, under condition of net deposition, sets of cross-laminations were generated with inclined and horizontal set boundaries respectively. The lee-slope of the sandwaves is very low (around

sediments were introduced by the storm-driven density flow, sedimentation from which led to the burial of the tidal sandwaves.

5”) and evidently the flow remained virtually unseparated at the brink. Little mud was available for deposition in the zone of the sandwaves and could settle only during slack water stages of the neap tides. The superimposed megaripples varied in size during spring-neap transition. Oceanographical studies have indicated that the areas of strong tidal currents are occasionally affected by non-tidal processes. The present study identifies the signatures of such superimposed processes. The features that are interpreted to have formed due to superimposition of non-tidal agents on tidal currents are: (1) low-angle, crosslaminated bundles, (2) thick mud layers, and (3) parallel-laminated sand bodies that bury the tidal deposits. It was demonstrated earlier that the low-angle, cross-laminated bundles are the products of combined tidal and wind-induced currents. In many of the earlier studies this type of bundle has also been interpreted to represent a tidal flow with pronounced time-velocity asymmetry and the internal laminations of the bundles have been

however,

currents.

of

It appears from the foregoing analysis that the sandwaves of the Lower Quartzite Formation es-

ous storms,

storm

layers

of huge quantities

huge volumes

During

vigor-

of sand-sized

Acknowledgements

The authors gratefully acknowledge the financial assistance provided by C.S.I.R. and Jadavpur University.

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