Sedimentology (1983) 30,727-742

Bar development in a fluvial sandstone (Westphalian 'A'), Scotland

M A R T I N K I R K

Department of Applied Geology, Strathclyde University, Glasgow GI IXJ, U.K.

ABSTRACT

The fluvial sandstone beneath the Mill Coal in the Westphalian 'A' of Scotland erosively overlies a lake mudstone. Slightly erosive surfaces within the sandstone, traceable for over 200 m, are used to divide it into two types of major sedimentary units termed type A and type B.

Type A sand units are approximately 200 m wide, up to 7 m thick, convex upward, and lenticular in all directions. The constituent cosets overlap to the ENE and dip mainly at 1-2" downcurrent (NNW), but locally at 10-15". Where thickest, type A sand units display a vertical facies sequence commencing with trough cross-bedded and massive sandstone, overlain by a thick zone of ripple cross-lamination, a thin zone of trough cross-beds, and a variably eroded silt drape up to 0.4 m thick. Attenuated lateral margins are dominated by flat bedded sandstone with primary current lineation. Type A sand units are interpreted as deposits which were accreted on to a large fluvial bar during successive flood events. The bar is thought to have had a similar topographic significance to sand waves described from the Brahmaputra and slip face bounded bars observed in the South Saskatchewan river.

Palaeocurrents measured from trough cross-bed sets 0.3-1.0 m thick within type B sand units indicate flow to the WSW, perpendicular to the palaeoflow direction measured from type A units. In sections perpendicular to the WSW flow direction type B units are lenticular, and in ENE-WSW trending sections they can be traced for over 80 m at a constant thickness. Type B sand units are interpreted as the product of low stage channels which flowed across bar fronts and tops.

The sandstone described herein is interpreted as a braided-type river deposit but is atypical, because it is fine grained and has an internal structure dominated by ripple cross-lamination and upper phase plane beds. The palaeoriver is thought to have been of low sinuosity, 7-10 m deep, with a high suspended load and large rapidly fluctuating discharge. At low stage a braided-type flow pattern developed around submerged bars. The regime of the palaeoriver was probably controlled by the fine sediment grain size and humid tropical climate.

INTRODUCTION AND TERMINOLOGY

Bars are the principal depositional element of all rivers, and their recognition is a prerequisite to a more certain identification of river type. The use of bar type in determining fluvial style has emerged from studies of the relationship between geomorphological ele- ments and internal structure in modem river deposits (e.g. Coleman, 1969; Bluck, 1971 ; Jackson, 1976; Cant & Walker, 1978). Different types of bars are composed of different stratification sequences. As a

0037-0746/83/ 1000-0727 $02.00 0 1983 International Association of Sedimentologists

river aggrades, bars merge to form a sediment body whose geometry and internal structure depends on bar morphology and growth pattern. Information about modern river deposits has been applied in interpreta- tion of ancient high and low sinuosity fluvial sand- stone.

The terminology relating to fluvial bars is often ambiguous, and although the problem has been discussed by Smith (1978), at present no universally accepted classification exists. In the absence of a more satisfactory nomenclature the existing descriptive terminology used by Cant & Walker (1978) and

727

728 M . Kirk

Reineck & Singh (1980) is followed here. Ripples and dunes are referred to as bedforms in the universally accepted sense (e.g. Allen, 1968). The term bar described a long, low relief feature, comprised inter- nally of various stratification types produced by bedforms. Compound bar refers to larger-scale fea- tures composed of several bar and low stage channel deposits, such as the complex sand flats of Cant & Walker (1978), side bars of Collinson (1970), and mid- channel bars of Coleman (1969). Channel describes a topographic depression on the river bed in which flow becomes concentrated at low stage. River refers to the entire depositional complex between banks. In terms of the preservation of these features in ancient fluvial sandstones, bedform corresponds with set, a series of bedforms form a coset, bar and channel equate with sand unit, and compound bar corresponds to sand body.

This paper documents the facies characteristics and relationships observed in a fine to very fine grained Westphalian ‘A’ fluvial sandstone. Sets and cosets are seen to form sand units which are interpreted either as sand bars or low stage channel fills, depending on their geometry, palaeocurrents, and internal structure. Particular attention is paid to the three-dimensional form of the facies present. Interpretations are not made using vertical profile models only (Cant & Walker, 1976; Miall, 1977) because the complexity of facies in braided-type rivers makes this unrealistic (Collinson, 1978). The character of the deposits described here differs markedly from the ancient medium grained braided-type fluvial sandstones de- scribed by Haszeldine (1983).

All these studies and the present one pay particular attention to the bar types interpreted from ancient sandstones, and the implications they have concerning river type. Ancient examples studied in this way indicate previously unrecognized grain size dependant variations within the spectrum of rivers with braided- type low stage flow patterns, and could act as a guide for future interpretations of other ancient fluvial sandstones.

GENERAL GEOLOGY AND SANDSTONE GEOMETRY

The sandstone is exposed in a National Coal Board (N.C.B.)opencast site located 30 km ESE of Glasgow at Headlesscross, Strathclyde, Scotland (Fig. 1A). Excellent exposures (Fig. 2), laterally continuous for over 300m have been examined in two successive

highwalls 60 m apart, trending mainly NNE-SSW, and in an 80 m long ESE-WNW trending highwall (Fig. 1B). Sketches drawn from photo-mosaics were used to study large-scale bedding features and the facies relationships within them. The site lies in the extreme southern part of the east Central Coalfield syncline where tectonic dip is negligible. Stratigraph- ically it exposes a lower Westphalian ‘A’, lower Communis Zone interval between the Armadale Main and Mill coals (Fig. Ic). The sandstone in question occurs at the top of this interval directly beneath the Mill Coal.

Read & Dean (1976) have examined borehole cores through several intervals in the Scottish Carbonifer- ous, including an East Central Coalfield Westphalian sequence stratigraphically equivalent to that observed at Headlesscross. They interpret this sequence to be of proximal deltaic origin based on the coarsening upwards nature of cyclothems and the slope of the linear regression line between the number of cycles and total thickness of strata (Read & Dean, 1976, fig. 2). However regional sedimentological studies of the Communis Zone by the present author, have revealed that swamp, fluvial channel, levte, crevasse splay, floodplain, and lake sub-environments predaminate. Prograding delta sequences are very rare. Down the regional palaeoslope, towards the S W, channel sand bodies become finer grained, coal seams thinner, and the proportion of fine grained overbank facies in- creases. Similar regional down-paiaeoslope transitions and assemblages of sub-environments, characterize upper and lower alluvial plain coal forming environ- ments described by Flores (1979). The absence of coastal features and marine bands from the Cornmunis Zone, except for that of the base (Ramsbottom et al., 1978), also suggest an alluvial rather than delta plain environment.

The sandstone averages about 10 m thick through- out the site and rests with a slightly erosive base showing relief of 0.3-0.4 m on approximately 3 m of lake mudstone (Fig. lc). It is everywhere overlain by a planar based 0 . 2 4 4 m thick highly rooted siltstone. A schematic cross-section drawn from N.C.B. bore- hole information, perpendicular to sand body elonga- tion (Fig. 3), clearly displays its sheet geometry and overall width/thickness ratio of 250: 1. Major features of the sandstone, such as its erosive base, elongate geometry, and associated terrestrial facies, suggest it is of fluvial origin. Perpendicular to its direction of elongation both the sandstone and underlying lake mudstone thin laterally to the east and west (Fig. 3). The thinned lateral margins of the sandstone are

Westphalian ‘A’fluvial bar 729

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C O A L S E ATE ART H

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C O A L

t -- T y p e

U

50

A

Fig. 1. (A) Locality map. (B) Simplified plan of Headlesscross opencast site, showing the location of Fig. 2(C) in relation to the palaeocurrent directions of types A and B sand units. (C) Site stratigraphy, and lithological distribution in the Upper Ball to Mill Coal interval.

composed of alternating sand and silt beds, in which the proportion of sand decreases away from the thickest part of the sandstone. These marginal sediments are interpreted as levke deposits. Overall the cross-sectional shape of the sand body is similar to that of Miocene fluvial sheet sandstones described by Friend, Slater & Williams (1979). In plan (Fig. 3) the sandstone defines a palaeoriver system flowing NW, almost parallel to a syn-sedimentary anticline running from Allanton to Salsburgh. To the south, the sandstone is approximately 2-75 krn wide and 10 m thick, but further north it narrows to 1 km and thins

’

to about 5 m. All boreholes except those along the sand body margins recorded sandstone with isolated silt beds up to 0.4 m thick. Interbedded sand and silt units typical of overbank facies, were not observed in boreholes located within the thicker parts of the sandstone.

SANDSTONE INTERNAL STRUCTURE

The sandstone can be divided into major sedimentary ‘packets’ termed sand units (Fig. 4). These large-scale

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Fig. 3. (A) Trace in plan of the sand body derived mainly from N.C.B. borehole data. (B) Schematic WSW-ENE cross-section from a to a‘ to illustrate the geometry and facies relationships of the sandstone. Vertical exaggeration 50: 1.

features are bounded by surfaces traceable laterally for over 200 m. Basal surfaces are invariably concave upwards and slightly erosive on the silt layer draping the underlying sand unit. Upper surfaces may be either flat or convex upwards. The units overlap each other towards the ENE, almost perpendicular to sand body trend. Internally they comprise sets and cosets of varying aspects. Each sand units displays a vertical sequence of facies, but no overall systematic upward facies change can be recognized throughout the whole sandstone. Two distinct types of sand unit are recognized, type A trending almost parallel and type B trending almost perpendicular to sand body elon-

gation. The complete sequence of sand units from SSW to NNE in highwall 2 is, in ascending order, A l , A2, B1, A3, and B2.

TYPE A S A N D UNITS

Palaeocurrents

Palaeocurrent directions in type A sand units (Fig. 5) indicate a major flow to the NNW, parallel to the dip of the constituent cosets, and a minor WSW flow, perpendicular to the coset dip. These flow vectors are sub-parallel and sub-perpendicular to sand body trend

732 M . Kirk

A Current -pe rpend icu la r W.S.W. E.N.E.

6 2

B C u r r e n t - p a r a l l e l S.S.E. N.N.W.

5 m 0 -57

0

Fig. 4. Schematic cross-sections through the sand body: (A) perpendicular and (B) parallel to type A sand unit palaeocurrent direction. (A) shows the overall lateral migration of type A sand units to the ENE. (B) shows type B sand units cross-cutting type A. Vertical exaggeration is approximately 2: I .

respectively. The NNW vector exhibits low variabil- ity, but too few measurements were available from the WSW vector to comment on its range of values.

Geometry and internal structure

The lenticular shape of type A sand units both parallel and perpendicular to palaeocurrent directions is clearly displayed in the extensive sections at Headles- scross (e.g. Unit A2 in Fig. 2C). Type A sand units display concave upward bases and convex upward tops. Their maximum thickness is 7 m and width approximately 200m. A lack of suitable sections precluded measurements of length but it is likely to be several hundred metres.

The constituent cosets are 0.5-1.5 m thick and are up to 60 m wide. They usually have slightly erosive bases, but occasionally the base cuts down steeply and the coset merges laterally with a thicker unit (Fig. 2A). Coset boundaries dip predominantly at 1 or 2" downcurrent, but at the downstream end of each type A unit they increase in dip to 10 or 15" concomitant with slight thickening and marked downcurrent overlap (Fig. 2C and 6B). These areas of steeper dips may extend for over 120 m in current-parallel sections. The cosets within type A sand units usually also overlap to the ENE. Thinning of these units is invariably accompanied by thinning of the constituent :osets.

Where thickest, type Asandunitsexhibitatripartite

vertical facies arrangement (A3 in Fig. 7A), compris- ing basal units of massive and trough bedded sandstone, a thick zone of ripple cross-lamination, and a capping of intercalated trough cross-beds and silt layers. Attenuated type A units may either comprise a similar sequence in which the rippled zone is replaced by upper phase plane beds (A2 in Fig. 7A), or consist entirely of upper phase plane beds (A3 in Fig. 7B).

TYPE B S A N D UNITS

Palaeocurrents

Trough cross-bedding and climbing ripple lamination observed in type B sand units (Fig. 5), indicate a palaeoflow to the WSW. Although only a few measurements were made, they are thought to be representative of type B units as a whole, because many more similarly orientated large trough cross- beds were observed in inaccessable parts of the highwalls.

Geometry and internal structure

Type B sand units are up to 4 m thick and display concave upward erosive bases and flat tops. They are lenticular perpendicular to type B palaeocurrent

Westphalian ‘ A’juvial bar 133

SEDIMENTARY TROUGH FLAT R IPPLE SCOURS TOTAL CROSS - BEDS CROSS -

BEDS LAMINATION

n = 3 4 A1 t %

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Fig. 5. Palaeocurrent directions for each structure within each sand unit, and for each sand unit as a whole. Where n < 6 individual readings are represented by an arrow.

direction (Fig. 8), but unlike type A can be traced at a constant thickness for over 80 m in current-parallel sections. Unit B1 comprises twocosetsof trough cross- bedding approximately 1.5 m thick, overlain by a drape of flat bedded sandstone. In current-perpendic- ular sections the lower trough cross-bed coset is overlapped by the upper (Fig. 8A), which eventually passes laterally into the silt drape overlying sand unit A2. Cosets persist for over 80 m in current-parallel sections through sand unit B1. Unit B2 consists internally of cosets up to 0.5 m thick which locally dip steeply in current-perpendicular section (Fig. 2C), and are convoluted in the upper 1.5-2 m. Climbing ripples are developed near the margin of unit B2. Both

sand units display gradually sloping lateral margins in sections perpendicular to their palaeocurrent direc- tions (Fig. 2C).

L I T H O F A C I E S AND T H E I R D I S T R I B U T I O N

Three major facies are recognized; conglomerate, sandstone, and siltstone. The distribution of facies and subfacies is shown in Table 1, and their relationship within types A and B sand units are illustrated in Figs 6 and 8 respectively.

M. Kirk 734

A Current -DerDendicu lor

B C u r r e n t - p o r o l l e l

0

Ib” Fig. 6. Schematic cross-sections through a type A sand unit: (A) perpendicular (palaeocurrent direction into the page) and (8) parallel to palaeocurrent direction. Vertical exaggeration (A) 3: 1, (B) 6: 1. See Fig. 7 for key.

Conglomerate facies

This is a poorly developed facies of intraformational origin occurring in discontinuous layers 0.01-0.08 m thick, mainly above the erosive bases of sand units, and also locally above erosive set and coset bases. The clasts are typically discs of blue-grey micaceous siltstone up to 0.01 m thick and 0.03 m long, supported by a fine sandstone matrix. Occasional current imbrication of the clasts has been observed where the ratio of clasts :matrix is high.

Sandstone facies

The sandstones are well sorted, fine to very fine grained, white subarkoses, divisable into six subfacies on the basis of sedimentary structures.

Massive sandstone

This subfacies occurs only in type A sand units, near the base. It comprises erosive based sets up to 0.25 m thick, with occasional silt intraclasts, which may fine upwards and terminate in a 0-01-0.03 m layer of silt and comminuted plant debris.

Convolute bedded sandstone

This subfacies is locally developed at the top of the sand body, in both type A and type B sand units (e.g.

units A3 and B2 in Fig. 2C). Cosets are 0.5-1.0m thick with rare basal silt intraclasts, and may contain relict ripples or trough cross-beds. Rootlets occur within the top 0.3 m of this subfacies.

Trough cross-bedded sandstone

This sandstone subfacies is a minor component of type A sand units, but dominates type B units. Sets comprise downcurrent-fining asymptotic foresets de- fined by 1-2 mm laminae of mica and comminuted plant debris. Basal silt clasts are rare. Occasionally counter-current ripples are seen which migrated up the foresets, and a 0.02-0.03 m layer of rippled very fine sand may occur at the top of cosets. Troughs up to 5 m wide and 0.5 m thick occur near the top and bottom of type A sand units (Fig. 7A), and at the bottom of unit B2 (Fig. 7B). Similar trough cross-beds are also developed in areas of steeper coset dip within type A units (Fig. 6B). Sets occuring at the top of type A sand units display foresets less than 0.2m high which may pass laterally into ripples towards the edges of troughs.

In unit B1 troughs 0.8-1.0 m thick form cosets up to 1.5 m thick. Form sets are sometimes preserved in the upper coset of unit B1 (Fig. 2C) by a drape of flat bedded sandstone. Some sets within unit B1 show changes from concave to convex upwards foresets in current-parallel sections (Fig. 8C).

Westphalian 'A'juuial bar 135

A L L C O A L

8 2

10

4

2

O m

A 3

A 2

eer M S U F

F S S

> 2% Conglomerate

0 S a n d s t o n e

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c o a l

1 : . : . ... I

8

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L L C O A L

8 2

M S U F F S S

Trough cross-beds h\ R o o t l e t s

F l a t beds 9 P a l a e o c u r r e n t s

a Ripple cross-lomination Climbing r ipple laminat ion

Convolute b e d d i n g fir=l Scour fac ies

Fig. 7. (A) Vertical facies sequence in sand units A2, A3 and B2 at the north end of highwall 1 . (B) Vertical facies sequence in sand units BI, A3, and B2 at the north end of highwall 2.

Flat bedded sandsto fie

Cosets 0.24.3 m thick of flat or very low angle bedded sandstone with primary current lineation, are the main component of the attenuated lateral margins of type A sand units (Fig. 6A). The coset bases are

erosive but rarely overlain by silt intraclasts. Inter- nally, many cosets show an upwards decrease in the proportion of micaceous silty laminae, and occasional ripples, depending on the extent of erosion by the overlying coset.

A non-erosive based 0.2-0-5m thick coset of flat

736 M . Kirk

A Type E l -cur ren t perpendicular 8 Type 82 -cur ren t perpendicular

La tera l accretion surfaces

C Type E l - cu r ren t para l le l

0 4 m I m - 10

[ z ] S t r u c t u r e unknown Cl imbing r ipple .+. . . laminat ion

FA Trough crass-beds E{ Fla t beds . .

Fig. 8. Schematic cross-sections through type B sand units. (A) Current-perpendicular section through unit B1. (B) Current- perpendicular section through unit B2. Vertical exaggeration 1.5 : 1. (C) Current-parallel section through unit B1. Vertical exaggeration 3 : 1.

bedded sandstone drapes the large trough cross-beds seen in the upper coset of unit B1. It lacks primary current lineation and internally comprises alternating micaceous and clear sand laminae, often showing convolutions.

Ripple cross-laminated sandstone

This subfacies consists of both climbing and non- climbing ripple cross-lamination types. The non- climbing variety comprises trough shaped current ripples with asymptotic foresets 0.01-0.02 m high. They form erosive based cosets 0.3-1.5 m thick which only rarely contain a basal silt clast layer. Cosets of this type tend to occur towards the middle of thick type A sand units (Fig. 6A), and may pass downcurrent into thicker, more steeply dipping trough cross-bedded

Table 1. Facies and subfacies occurrences

cosets (Fig. 6B). Non-climbing ripples were not observed within type B sand units.

Climbing ripples (Fig. 2B) climb at angles ranging from 20 to 70". They comprise tangential foresets up to 0.01 m high with abundant thin silt drapes over their crests. Within type A sand units climbing ripples are only sporadically developed, in cosets near the tops of units. Well-developed cosets of climbing ripple lamination up to 0.4 m thick occur in very fine sand at the margin of unit B2.

Sandstone-filled scours

This sporadically developed subfacies is associated with the steeper dipping cosets occurring within type A sand units. It comprises symmetrical scours up to 0.15 m deep and 1.5 m wide, concordantly infilled by

Faciesisubfacies Sand units

A1 A2 A3 B1 B2

Conglomerate R R R R R Massive sandstone R R R - -

c - C Convolute bedded sandstone - _ Trough cross-bedded sandstone R R R C C Flat bedded sandstone C C C R - Ripple cross-laminated sandstone c c c - R Sandstone scour R R R - - Siltstone R R R R R

Westphalian ‘A’juvial bar 737

laminated fine sandstone. The scours have flat tops and in places display primary current lineation parallel to their inferred axes.

Siltstone facies

This facies comprises blue-grey, micaceous, medium to course grained faintly horizontally laminated silt, with a small amount of comminuted plant debris. It is volumetrically subordinate, but nevertheless forms a highly significant component, occurring as a drape over type A and type B sand units in variably eroded beds up to 0.4m thick. Silt drapes are generally thicker over type A units than over type B. More localized occurrences of silt are found capping sets and cosets located near the top of sand units. Locally, silts become sandy as laterally equivalent type B sand units approached ; for example the silt drape overlying the margin of unit B1 passes laterally into flat bedded sandstone over the thicker part of BI (Fig. 8A). Up to 0.8 m of rooted silty seatearth overlie the whole sand body.

INTERPRETATION

The development of levtes marginal to the sandstone, and absence of overbank deposits within it, suggests that the sand body is the deposit of a single river. The high walls examined are situated towards the outside of a bend in the palaeoriver in a similar overall setting to some of the main areas of sand bar deposition in the present-day Tana (Collinson, 1970) and South Saskatchewan (Cant, 1978; Cant & Walker, 1978) braided rivers.

The NNW directed palaeocurrents measured from type A sand units are subparallel to sand body trend. This indicates that the N N W flow direction represents the major down-river high stage current, despite the fact that most palaeocurrent measurements were made from small-scale ripples. The WSW flow component recorded from both types of sand unit, is thought to represent deposition by low stage currents flowing perpendicular to the main down-river (NN W) flow direction. With respect to river trend, similarly orientated high and low stage flow components have been observed in modern rivers by Collinson (1970), Singh(1977), Cant & Walker(1978), andBluck(1979).

Type A sand units

Type A sand units are thought to represent the deposits of a large sand bar which migrated downstream and

towards the right bank of the palaeoriver. Internally they comprise downcurrent dipping and overlapping sets and cosets, such as produced by the bar fronts described from the modern Ganga and various Scottish rivers by Singh (1977) and Bluck (1976) respectively. A similar internal structure has also been observed within sand waves in the Brahmaputra river by Coleman (1969). Overlapping of sets and cosets to the ENE indicates lateral as well as downcurrent migration of the Westphalian bar. Original relief comparable to that displayed by bars in the South Saskatchewan river (Cant & Walker, 1978), and sand waves in the Brahamaputra (Coleman, 1969), is demonstrated by the preservation of convex upward bar surfaces beneath silt suspension deposits. Type A sand units could not represent the deposits of low stage channels, because of their lenticularity in all directions. An origin as crevasse splay deposits is also rejected, because type A sand units display progressive lateral migration to the right (Fig. 4A), whereas crevasse splay deposition would produce a vertically offset arrangement of units. The absence of upcurrent thickening towards a feeder channel also negates a crevasse splay origin. The complex internal stratifica- tion of type A sand units and vertical sequence of sedimentary structures indicating an upward decrease in flow power, resemble the internal structure of sand waves described by Coleman (1969) from the Brah- maputra river. Sand waves were deposited by falling stage flood currents and overlain by silt drapes during the suceeding low stage flow period. Coleman (1969) noted that successive deposits of sand waves separated by silt drapes were preserved in cut bank sections. By analogy, each type A sand unit is thought to represent an increment of sediment which accreted on to the right hand downcurrent margin of a large bar during successive flood events. Singh (1977) has described similar but much smaller flood generated sediment accretions on to a sand bar in the Ganga river.

Type B sand units

Units B1 and B2 have been attributed to deposition by flow in dune-floored low stage channels, occurring at topographically low and high levels in the river respectively. The concave upward erosive base, palaeocurrents trending perpendicular to type A units, and internal structure dominated by large trough cross-beds in unit B1, strongly resemble the character- istics of fairly deep low stage channels in modern sandy braided rivers. The smaller bedforms and topographically higher position of unit B2, are similar

738 M . Kirk

to low stage channels seen flowing across bar tops in the South Saskatchewan river (Cant & Walker, 1978).

Lithofacies

The basal position of intraformational conglomerates within cosets indicating an upward decrease in flow power, suggests that they represent the first formed deposits of waning flood currents. Fining upward sets of massive sandstone occurring near the base of type A sand units, indicate that the bar deposits were initially developed by relatively deep, fast flowing currents bearing a high suspended load. Lower flow regime bedforms were unable to form. Convolute beds at the top of the sandstone are clearly the product of liquefaction, most likely caused by fast deposition and the shearing action of rapidly fluctuating heavily sediment laden currents on loosely consolidated sand.

Trough cross-bed development is thought to have been inhibited by the fineness of the sand (Harms et al., 1975). Asymptotic downcurrent fining foresets and occasional counter-current ripples indicate a high suspended load. Small trough cross-beds associated with silt layers at the top of type A sand units, may have been developed by minor flood currents flowing over the bar top, as in the South Saskatchewan river (Cant, 1978). Isolated trough cross-beds in areas of steeper coset dip probably formed due to vertical flow expansion at the bar front (Fig. 9A). Changes in the dip of trough cross-bed foresets within channel B1 suggests that velocity fluctuations occurred in this channel. Flat bedded sandstones with primary current lineation indicate that upper flow regime conditions existed at the bar margins. The presence of ripples at the top of some cosets is attributed to flow power fluctuations during floods. Flat bedded sandstones without primary current lineation, which drape dunes in channel B1, are probably suspension deposits, formed after channel B1 had been abandoned.

Trough-shaped current ripples with downcurrent fining asymptotic foresets, suggest deposition by heavily sediment laden fast flowing currents. The development of these ripples and climbing ripples within the same coset, is thought to reflect rapid flow fluctuations and a very high suspended load. The sandstone scour facies is interpreted as a bar front modification produced by high energy cross-flowing currents at times of lower flood discharge. Silt suspension deposits draping sand units were deposited at low stage after bar migration had ceased. Coeval development of this facies with low stage channels is indicated by lateral merging with unit Bl. The

thickness of silt deposits (up to 0.4 m) could be related to the high suspended load, and a long low stage flow period. Rooty siltstonesoverlying the whole sand body are interpreted as floodplain deposits formed after abandonment of the river.

The large-scale bedding features, facies character- istics, and their relationships, observed in the sand- stone described herein, strongly suggest it is the product of a river containing a large sand bar around which a braided type flow pattern developed at low stage.

DISCUSSION

In modern sandy braided rivers (e.g. Brahmaputra, Platte, Saskatchewan and Tana) bars form in areas where flow expands laterally and/or vertically (Cant, 1978), reducing the velocity, and causing the river to deposit some of its load. Vertical flow expansion occurs where the flow passes over a topographic depression or where two flows of unequal depth meet. Lateral flow expansion is common at bends, especially in rivers with a high width/depth ratio. A combination of these factors is thought to be responsible for the initiation and subsequent development of the bar in the Westphalian river at Headlesscross.

The bar inferred was up to 7 m high, 200 m wide, and probably hundreds of metres long. At low stage it was crossed by channels and draped by silt. At high stage sediment eroded from upstream was transported downriver in suspension. Each type A sand unit is an increment of sediment added to the downstream bar margin. It is probable that most of the accretion occurred during falling stage, when reduced flow velocities caused sand deposition. During this time bedforms infilled lenticular hollows on the bar surface, producing cosets. Aggradation occurred downcurrent and vertically simultaneously. Falling stage vertical accretion rates could have been very high; possibly as much as 5 m per day considering the rates observed for similar sized features (sand waves) in the Brah- maputra river by Coleman (1969). The bar front probably migrated downstream due to increased deposition rates as flow expanded (Fig. 9A). A similar origin has been assigned to downcurrent dipping sets in Precambrian fluvial deposits from northern Norway by Banks (1973). The low dip of the bar front (10-15") is most likely due to the fine grain size.

The distribution of stratification types seen in current perpendicular sections through type A sand

A

Separa t ion eddy

B

R i v e r bank

/ B a r c res t l i n e s

C

H.S.F. - H i g h s t a g e f l o w

L . S . F - L o w s t a g e f l o w

5 Transverse f l o w g r a d i e n t

D d d '

-Flow di rec t ion

F f f '

Fig. 9. (A) Current-parallel section through a type A sand unit showing trough cross-bed formation in a bar front separation eddy. (B) Lateral migration of the bar obliquely downcurrent. 1,2, and 3 are sand units accreted during successive floods. Note the slight asymmetry to the right of crestlines. (C) Flow directions within the river. (D) Current-parallel section through the downstream end of the compound feature produced by bar migration, showing the location of types B1 and B2 channels. (E) Reconstruction of the conditions during low stage flow period. (F) Current-perpendicular section across three sand units-unit 3 is just developing. Note shallowing above unit 2 to the left and proximity to the inferred main channel flow to the right.

740 M . Kirk

units (Fig. 6A), indicates variations in flow power across the bar perpendicular to palaeocurrent direc- tion. At the sand unit margins flat beds with primary current lineation indicate fast flowing currents. These could be due to the effects of shallowing above previous units at the left hand margins and proximity to the main channel current at the right hand margins (Fig. 9E). In the central part of the units the vertical stratification sequence (Fig. 6A) indicates that initially fast flowing currents, which deposited massive and trough cross-bedded sandstone, waned quickly result- ing in a thick deposit of ripple cross-laminated sand. Trough cross-beds at the top of type A sand units indicate an increase in flow power during late falling stage, which could be attributed to minor flood current resurgences. However, this flow power increase could also be related to shallowing caused by vertical aggradation of the bar. Fining upwards cosets within type A sand units probably represent within flood fluctuations in current velocity. The absence of rootlets, dessication cracks, or other evidence of emergence from the bar top, suggests that it was always submerged. Collinson (1970) has noted the occurrence of permanently submerged bars in very wide reaches of the Tana river.

The continual accretion of sand units on to the right hand (east) side of the bar, suggests lateral migration to the right (Fig. 9B). This was probably controlled by a weak ENE flowing transverse flow gradient (Fig. 9C), which could have also produced slight asymmetry of the bar crestline (Fig. 9B), by increasing flow power and hence sediment transport rates at the right hand bar margin. The overall bar migration direction was obliquely downcurrent towards the NNE. Its position within the river resembles that of cross-channel bars observed fringing complex sand flats in the South Saskatchewan river by Cant & Walker (1978).

The bar described here is not wholly analogous to any so far described in the literature. In terms of size, shape, and the comparatively poor development of trough cross-bedding, it most closely resembles a fine sand channel bar described from the Ganga river by Singh (1977). However, greater low stage modification of the Ganga bar indicates that discharge varies more slowly in the Ganga river, than it did in the Westphalian river. In the Westphalian river, the preservation of nearly complete sand units deposited during successive floods, suggests almost continual accretion to form a compound bar. The compound bar type is not discernable at present, but is most likejy to be either a mid-channel or side bar, of comparable topographic significance to the mid-

channel bars of Coleman (1969) and complex sand flats of Cant & Walker (1978).

At low stage the bar was modified by scours cut across the front, and channels which flowed across the front and top. Low stage channels topographically low down in the river (type B1) flowed across the bar front, and those at higher topographic levels (type B2) across the top (Fig. 9D). Within channel B1 the upper coset overlaps the lower (Fig. SB), suggesting that flow spread further up the bar front as the channel aggraded. Large dunes within this channel indicate that current velocities were probably always higher than the bar-forming currents. The presence of complete dune forms in the upper coset of channel B1 indicates rapid abandonment. After abandonment type B1 channels were subsequently overlain by later sand units and type B2 channels by floodplain silt. Both channels are thought to have been fed by a major channel flowing adjacent to the concave bank (Fig. 9E). Similar associations comprising bars migrating towards the concave river bank, and low stage channels flowing in the opposite sense towards the convex bank, have been described from modem river deposits by Singh (1977) and Bluck (1979).

The Westphalian river at Headlesscross was prob- ably of low sinuosity. At high stage sediment was mainly transported downstream in suspension with little deposition. During falling stage bars accreted at points of flow expansion, and at low stage a braided- type flow pattern developed. Bars are thought to have formed due to high discharge and high sediment load. Since bars 7 m thick were always completely sub- merged, this figure can be taken as the minimum depth of the river. The maximum depth would have been approximately 10 m, the thickness of the sand body. Discharge appears to have been fairly high, with rapid rises and falls indicated by the relatively small amount of low stage bar modification (Jones, 1977). Discharge fluctuations may have been due to weather changes within a wet, equable, humid tropical climate, similar to that deduced for the Westphalian of Britain from the palaeobotanical evidence by Scott (1979).

Correspondence between thinning of the sandstone and underlying lake mudstone, suggests the river may have formed by avulsion into a floodplain lake. Lakes occur at the lowest topographic level on alluvial plains, and are therefore an ideal course for river avulsions. Avulsions of large rivers into relatively small lakes may result in their becoming incorporated into the river system, as is thought to have been the case at Headlesscross. Modern braided-type rivers in this

Westphalian ‘A’juvial bar 74 1

setting (e.g. the Brahamaputra) are usually also abandoned by avulsion, and subsequently overlain by rooted floodplain deposits.

CONCLUSIONS

The Westphalian sandstone described in this paper represents the deposits of a large NNE migrating sand bar, developed at a left hand bend in a wide, relatively shallow, low sinuosity river, with a high suspended load. Two distinct types of sand units are recognized : type A-slightly convex upward lenticular sand units consisting internally of gently downcurrent dipping cosets, and type B-dominated by large and small trough cross-beds with palaeocurrents directed perpendicular to those in type A units. These units are interpreted as sediment accretions on to a large sand bar deposited during successive floods, and low stage channel deposits respectively. Silt drapes preserved between units indicate continuous accretion of succes- sive flood deposits. Abandoned sand units formed a presently indeterminate type of compound bar.

The regime of the generative river has been inferred from the observed stratification sequences. Permanent submergence of the bar suggests that discharge was high. Rapid discharge fluctuations are indicated by the slight amount of low stage bar modification. Counter-current ripples, massive sandstone in deeper flows, and climbing ripple lamination suggest a high suspended load. This and the fine grain size may have influenced the low angle of the bar front, and inhibited the formation of large bedforms. The bar is thought to have developed because of high discharge and inability of the flow to transport the heavy sediment load. To my knowledge there are no reports in the literature describing either modern or ancient deposits produced by a river of strictly comparable regime. The observation made here could be utilized in fluvial sedimentology, as guidelines for interpreting other ancient deposits thought to be the product of fine grained, high suspended load, sandy rivers, with a rapidly fluctuating discharge, large sand bars and a braided-type flow pattern at low stage.

ACKNOWLEDGMENTS

This research forms part of a Ph.D. thesis carried out at Strathclyde University during the tenure of a N.E.R.C. postgraduate studentship, which is grate- fully acknowledged. I thank Dr R. Anderton for his

critical reading of this paper, the National Coal Board Opencast executive and their contractors Murphy Brothers P.L.C. for access to Headlesscross opencast site and the use of borehole information, and Helen Blay for typing the manuscript.

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(Manuscript received 4 May 1982; revision receioed 15 February 1983)