Permian storm current-produced offshore bars from an ancient shelf sequence : Northwestern Karoo basin, republic of South Africa

Journal of African Earth Sciences, Vol. 9, No. 2, pp. 363-370, 1989

0899-5362/89 $3,00 + 0.00 © 1990 Pergamon Press plc

Printed in Great Britain

Permian storm current-produced offshore bars from an ancient shelf sequence : Northwestern Karoo basin, Republic of South Africa A. M. Smith and E K. Zawada Geological Survey,PrivateBag X112, Pretoria 0001, Republic of South Africa Abstract - The Ecca-Beaufort transition zone from the Karoo Basin comprises upward-coarsening sequences whichare interpretedas prograding,storm-producedoffshorebars. Eightfaciesare recognised: (A) dark-greyshale,(B) thinlyinterbeddedsiltstoneand mudstone,(C) thinlyinterbeddedsiltstoneandvery fine-grainedsandstone, (D) blue-greycoarse-grainedsihstone, (E) low-angletruncatedand fiat-laminated sandstone,(F) wave-rippledsandstone,(G)planarcross-beddedsandstone,(H) intraformationalclay-pellet conglomerate. Four sub-environments are recognised, these being: (l) the bar crest which comprises proximal tempestites,(2) the bar slope consistingof soft-sedimentdeformedsiltstone, (3) the bar fringe/ marginwhichis composedof stormlayersand offshoresiltstonesand (4) the interbar/offshoreenvironment comprisingsiltstoneand distalstormlayers.Thesebars formedinresponseto waveand stormprocessesand migrated across a muddy shelf environment. The orientation of bars was probably coast-parallel to subparallel withrespect to the inferrednorth-northwest-south-southeastcoastline.Theseproposed, stormproducedbars actedas majordepo-centreswithinthe shelfsettingof the studyarea. As shelf sedimentsare recorded from almost the entire northwestern Karoo Basin it is anticipated that bar formationwas an important sedimentaryfactor in the depositionof the sediments now referred to as the Ecca-Beaufort transition zone. INTRODUCTION This r e s e a r c h r e p o r t s o n a portion of t h e n o r t h w e s t e r n p a r t of t h e M a i n Karoo B a s i n of S o u t h e r n Africa (Fig. 1). S t r a t a d e s c r i b e d in t h i s p a p e r f o r m p a r t of t h e Karoo S e q u e n c e (Fig. I) t h a t r a n g e s in age f r o m t h e late C a r b o n i f e r o u s , D w y k a glaciation to early J u r a s s i c , D r a k e n s b e r g volcanism. The Karoo B a s i n o c c u p i e d a n i n t r a c r a t o n i c position w i t h i n t h e G o n d w a n a l a n d s u b c o n t i n e n t . As G o n d w a n a l a n d drifted o u t of polar l a t i t u d e s a n a m e l i o r a t i o n of climate followed leading to t h e progressive c h a n g e of p a l a e o e n v i r o n m e n t from glacial to fluvio-deltaic t h r o u g h to p l a y a - l a k e s e d i m e n t a t i o n . Following glacial retreat, t o w a r d s t h e close of D w y k a s e d i m e n t a t i o n , a shallow epic o n t i n e n t a l s e a (Ecca Sea) developed a c r o s s a b r o a d slowly s u b s i d i n g platform. The a r e a investigated is located b e t w e e n t h e t o w n s of De A a r a n d Philipstown. In" general, e x p o s u r e is poor b e c a u s e of t h e isolated m e s a t o p o g r a p h y b u t m a y be locally good o n hill slopes (Fig. 2). The r o c k s investigated are located in t h a t p a r t of t h e Karoo S e q u e n c e (Fig. l) w h i c h s t r a d d l e s t h e E c c a - B e a u f o r t G r o u p J u n c t i o n a n d are inform a l l y t e r m e d t h e E c c a - B e a u f o r t t r a n s i t i o n zone. This zone w a s originally defined a s a t r a n s i t i o n from b a s i n a l - t h r o u g h deltaic to fluvial deposition

(Visser a n d Loock, 1974; L e m m e r , 1977; Nel, 1977; Terblanche, 1979). T e r b l a n c h e (1979) specifically i n t e r p r e t e d t h i s u n i t w i t h i n a Mississippi-type delta model. Recent investigations of t h e t r a n s i t i o n zone in t h e n o r t h w e s t Karoo B a s i n have led to its reinterp r e t a t i o n a s a m a r i n e - s h e l f e n v i r o n m e n t (Siebrits, 1987; S m i t h a n d Z a w a d a , 1988). The latter a u t h o r s , w o r k i n g in t h e Philipstown area, docum e n t e d a t r a n s i t i o n from offshore siltstone to shelf deposition a n d f u r t h e r s u b d i v i d e d t h e latter into an outer and inner storm and wave-dominated e n v i r o n m e n t . Two h u n d r e d k i l o m e t e r s to t h e w e s t Siebrits (1987) also recognised a s t o r m a n d waved o m i n a t e d a n c i e n t s h e l f f r o m t h e n o r t h w e s t Karoo Basin. This p a p e r a t t e m p t s to refine t h e m a r i n e s h e l f m o d e l in t e r m s of fine-grained offshore b a r migration. FACIES DESCRIPTIONS AND INTERPRETATIONS F a c i e s A: d a r k - g r e y s h a l e Facies A c o m p r i s e s d a r k - g r e y s h a l e t h a t a t t a i n s a m a x i m u m t h i c k n e s s of 2 0 0 m (Nel, 1977). Polished s e c t i o n s a n d r a d i o g r a p h s reveal l e n t i c u l a r waverippled siltstone a n d very fine-grained s a n d s t o n e laminae together with a b u n d a n t bioturbation

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(Fig. 3). Flat lamination is rare and restricted to the upper portions of the facies. Smith and Zawada (1988) interpreted this facies as a basin shale. Additional evidence from polished sections and radiographs indicates periodic influxes of silt and very fine-grained sand. It is therefore now suggested that this material represents distal storm layers similar to those described by Pedersen (1985) which have been bioturbated during fairweather conditions.

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Facies B: thinly interbedded s i l t s t o n e and mudstone Facies B forms a minor part of the sequence under study. It consists of interbeds (approximately 2 cm thick)comprising light-gr~ siltstone and dark-grey mudstone. Units of facies B are characterised by unidentified vertical and horizontal burrows. Primary sedimentary structures, if originally present, may have been subsequently destroyed by bioturbation. The textural duality of siltstone and mudstone indicates alternating energy regimes. In view of the bioturbated nature o f facies B it is likely that considerable lulls in sedimentation occurred between depositional increments. The lack of tractional features and the predominantly silt- and clay-grade nature of facies B suggest that deposition occurred primarily by suspension settling.

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Facies C: t h i n l y Interbedded s i l t s t o n e and very fine-grained s a n d s t o n e Facies C is e x t e n s i v e l y b i o t u r b a t e d by Siphonichnus, Skolithos and Planolites. It comprises light-grey, very free-grained sandstone interbedded with dark-grey sfltstone (Fig, 4). The sandstone beds thicken upward (2-13 cm),

Permian storm current-produced offshore bars from an ancient shelf sequence : Republic of South Africa

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Fig. 3. Radiograph of facies A showing flat lamination (F), wave-ripple lamination (w) and a subvertlcal burrow (s).

w h e r e a s the sfltstone b e d s thin upward to partings. S a n d s t o n e beds are sharply based, fine upward, show u n d u l a t o r y lamination (cf. Allen, 1981), fiat lamination and h u m m o c k y crossstratification (HCS). Most sandstone beds have wave-rippled tops although examples of planarerosive tops and gutter-cast s t r u c t u r e s are present (Fig. 4). Minor multiple c h a n n e l s (30-63 c m across a n d 3-4 c m thick) are also present. Facies C shows a textural duality consistent with a regular c h a n g e in hydraulic regime as was the case with facies B. The highest energy regime is characterised by fiat lamination, undulating lamination a n d HCS. Undulatory lamination is a wave-formed fabric recording high oscillatory flow intensities of short duration (Allen, 1981). There is increasing agreement that HCS forms in response to storm wave activity, coupled with a coastparallel c u r r e n t (Swift e t al, 1983; Allen, 1985; Swift a n d Nummedal, 1987). The wave-rippled sandstone bed tops are interpreted as reworking during waning storm conditions. The lower energy regime is represented by the bioturbated siltstone beds. These represent fairweather suspension settling which have b e e n subjected to biogenic reworking. The small-scale c h a n n e l s resemble the sandfilled gutter casts found at the base of inferred offshore storm s a n d layers (Goldring a n d Aigner, 1982). In addition, the structure illustrated in Figure 4 is similar to the the mud-filled gutter casts formed by sand-deficient c u r r e n t s from the transition between shelf m u d and shoreface in the m o d e m North Sea (Aigner a n d Reineck, 1982). In the light of the evidence shown in Figure 4 it is believed that the s a n d s t o n e beds with planar erosive tops were also formed by s u c h currents. The presence of beds showing HCS, u n d u l a t o r y lamination, sharp bases and tops, gutter c a s t s a n d upward-fining motifs is consistent with a tempestite origin. The Interbedded s a n d s t o n e a n d siltstone testifies to alternating storm a n d fairweather regimes, respectively. Further, the two gutter-cast types indicate the

Fig. 4. Facies C. Note the presence of wave ripples (w), HCS bed (HCS) and mud-filled gutter cast (G). Lens cap i s 5 cm in diameter.

presence of erosional storm c u r r e n t s which where either sedlment-laden or deficient.

Facies D: blue-grey, coarse-grained siltstone Facies D comprises blue-grey, coarse-grained siltstone t h a t varies from massive to intensely deformed a n d m a y contain s a n d s t o n e pillows and undeformed sandstone rafts (Fig. 5). The upper portion of facies D m a y show well-developed finegrained sandstone ball-and-pillow s t r u c t u r e s and passes vertically upward, and laterally, into facies E (low-angle t r u n c a t e d a n d fiat-laminated sandstone). In one case portions of s a n d s t o n e beds, up to 12 m long a n d hinged to the capping sandstone, were seen to dip into the underlying siltstone (Fig. 5). The regions between the hinged units range from extremely distorted to chaotic sandstone blocks floating in massive siltstone. The dist u r b a n c e s p a n s avertical distance of 3 m and is up to 40 m wide. The field relationships of the hinged sandstone beds and u n d e f o r m e d s a n d s t o n e exotic blocks Facies D. Massive ILnd soft sed~nent-defot~ned siItston¢

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(Fig. 5) show that the competency of the underlying siltstone s u d d e n l y failed. The feature illustrated in Fig. 5 shows a m a r k e d similarity to the collapse depressions recorded by Coleman and Garrison (1977). According to these a u t h o r s collapse depressions are bowl-like features 1-3 m deep and approximately 100 m in diameter which form by locallsed liquifaction caused by storm-wave loading. They are usually circular features, m a y be rimmed by listric fault scarps, and possess a m a s s of isolated blocks of sediment in the central area (EMott, 1986). This interpretation is reasonable in this case because storm-wave evidence is commonly d o c u m e n t e d in this s t u d y (see facies C and E). Collapse depressions are usually recorded from distal distributary areas (EMott, 1986) however, this is not the case here where a shelf setting has been postulated (Smith and Zawada. 1988). It is concluded that the siltstone contained a high quantity of pore water, probably due to rapid deposition, and that subsequently cyclic-storm wave-loading (cf. Henkel, 1970; Dalrymple, 1979; Allen, 1982) triggered liquifaction to form these features.

F a c i e s E: l o w - a n g l e t r u n c a t e d a n d fiatlaminated sandstone Facies E coMprises erosionally-based, finegrained, fiat laminated sandstone beds displaying low-angle t r u n c a t i o n surfaces and primarycurrent lineation. These beds m a y grade upwards into Skolithos-burrowed wave ripples. Bed thicknesses vary between 10 and 60 cm and are frequently amalgamated and stacked in units up to 3.60 m thick. The beds with low-angle truncations, shown in Figure 6. have been interpreted as amalgamated HCS. The sequence in bedding style from h u m m o c k s through parallel lamination, to wave ripples indicates a storm-dominated regime characterised by strong oscillatory flow (Dott and Bourgeois, 1982). ~,

Abating storm action is indicated by the change from parallel lamination of the u p p e r flow regime through to oscillatory wave ripples formed in the lower flow regime (Chan a n d Dott, 1986). Amalgam a t e d HCS, indicates that deposition was d u e to multiple storms probably below fair-weather, yet above storm-wave base.

Facies F: wave-rlppled s a n d s t o n e Facies F units comprise wave-rippled sandstone beds that vary in thickness between 0.40 and 2.20 m. The sandstone h a s a we[I-developed waveformed fabric with bedding-plane exposures showing a north-northwest - s o u t h - s o u t h e a s t aligned crests (Fig. 7). The wave ripples have wavelengths of 2-14 cm and amplitudes of 0.50-2.0 cm. Skolithos and unidentified trails were observed on bedding planes. Symmetrical ripples are produced by oscillatory wave c u r r e n t s (Harms et a/. 1982). Facies G: planar cross-bedded s a n d s t o n e This is a unique occurrence and is discussed in view of its palaeocurrent significance. Facies G is a fine-grained s a n d s t o n e comprising a planar crossbedded coset 80 cm thick. The individual sets range from 10-20 cm thick and have foresets orientated t o w a r d s 160 °. Reineck and Singh (1975) attribute planar crossbedding to the migration of straight-crested bedforms. Facies H: i n t r a f o r m a t i o n a l clay-pellet conglomerate Facies H occurs only once in the sequence of rocks u n d e r study. It comprises a 3-12 c m thick, laterally p e r s i s t e n t , s t r u c t u r e l e s s , m a t r i x supported clay-pellet conglomerate (Fig, 8), The clasts are poorly sorted and vary in size from 0.20 cm to 8.0 cm. Clast shape varies from wellrounded to angular and subangular. Many clasts • '~

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Permian storm current-produced offshore bars from an ancient shelf sequence : Republic of South Africa

367

area. The coarsening-upward n a t u r e of facies association 2 shows t h a t the rapidly emplaced sfltstone (facies D) was deposited In shallower water t h a n that of the basIn shale (facies A). Facies D m a y succeed facies C suggesting that It also formed In shallower w a t e r t h a n the latter. The rapidly emplaced slltstone is succeeded by proximal s a n d s t o n e tempestites (facies E) and suggests that the llquffactlon resulted from cyclic stormwave-loading (see facies D).

Fig. 8. Intraformatlonad clay-pellet conglomerate (facies H]. Coin for scale is 2 cm in diameter.

are elongate slithers o f m u d r o c k (Fig. 8) orientated parallel to the base of the bed. The matrix consists of blue-grey, coarse-grained sfltstone, infrequent examples of c r u d e normal grading is present. The base of facies H is not erosional a n d the unit as a whole appears to have a sheet-like geometry w h e n traced along strike for up to 1.5 km. According to Rupke (1978) debris flows are non erosive, matrix supported and internally structureless. The clasts are poorly sorted a n d randomly orientated (although in places they m a y be aligned with their long axis parallel to the flow direction). On this basis facies H is interpreted as a debrite. FACIES ASSOCIATIONS Facies association 1 This comprises facies A, B and C and is arranged in gradational upward-coarsening sequences indicative of progradation (Fig. 2). Facies A comprises a basin shale and distal tempestites whereas B and C consists of thinly-bedded tempestites alternating with argillaceous fairweather suspension settling. It is likely that the sand-grade material was deposited during storm conditions from dense s a n d s u s p e n s i o n s (cf. De Raaf et al, 1977). The sedimentary s t r u c t u r e is a response to the reworking of this storm-produced, sand-grade fall out (cf. J o h n s o n and Baldwin, 1986). Thus it is likely that this facies association was dominated by s u s p e n s i o n deposition which comprised both storm and fairweather components. Facies association 2 This facies association comprises facies A and D, but m a y be separated by a thin facies C unit (Fig. 2). The facies Junctions are s h a r p and overall the sequence c o a r s e n s upward. This facies association occurs, stratigraphically in the middle of the Ecca-Beaufort transition zone within the s t u d y

Facies association 3 The most c o m m o n facies transition in this association is E-F, which m a y be repeated up to three times in any given s a n d s t o n e (Fig. 2). Single examples of the transitions G-F and F-H, also exist (Fig. 2). Facies association 3 is erosivelybased with a sharp top, if not welded to a n overlying unit. The lateral facies transition E-F-C h a s also b e e n noted. The alternation of facies E a n d facies F probably represents storm and storm-abatement conditions, respectlvely. The presence of facies G crossbeds (with azimuths of 160°), which replaces facies E at one point in the stratigraphic c o l u m n m a y be explained by a n intensification of a coast-parallel flow. Swift and Nummedal (1987) have suggested that if the coast-parallel c u r r e n t is too strong t h e n planar cross bedforms will develop preferentially over HCS. It is suggested t h a t this occurred locally within the sequence u n d e r study: Facies H h a s b e e n described as a debrite and because it occurs within a sequence containing storm deposits it is assigned to such a n origin. DEPOSITIONAL MODEL

The coarsening-upward sequences shown in figure 2 are not capped by fluvial or subaerially deposited sediments and occur within a shell setting at depths between storm a n d fair-weather wave base (Smith and Zawada, 1988) so an offshore b a r origin is suggested. Table 1 illustrates the environments recognised from the area u n d e r investigation. Facies association 1 (A-B-C) is similar in style to the bar fringe setting of the isolated linear bar sequences recognised by Hobson et al, (1982), (see Table 1 and Fig. 2) (ii). Facies association 3 (bar crest) caps coarsening-upward sequences and c a n be identical to the offshore b a r sequences recognised by Rice (1984) (see table 1). Facies association 2 (A-D) also coarsens upward a n d in view of the other evidence is considered to represent part of an offshore b a r deposiUonal regime (Table 1). No ancient analogue for facies D is k n o w n b u t the bar slope of the m o d e r n S u r i n a m e m u d d y bars (cf. Rine and Ginsberg, 1985) m a y be a m o d e r n equi-

368

A . M . SMITH and P.K. ZAWADA Table

Submvironment

Facies

E,F

I. S u m m a r y

Lithology Fine-grained sandstone

of the subenvironments

Sedimentary Structures Amalgamated HCS, flat lamination, wave ripples, infrequent bioturbation HCS "--3,wave tipples. Bioturbation more common on wave-tippled tops.

BAR CREST

G (unique

Planar crossbedded

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Processes Alternation of proximal tempestites and fair-weather processes Wave oscillation, coastparallel current

Masby sandstone (Rice, 1984)

Intensified coast-parallel current

Shannon Sandstone (Type I)(Tilknan and Martinsen, 1984)

occurrence) H (unique

Comparison with imbli~ed modds

Shannon Sandstone (Type I1) (Tfllman and Martinsen, 1984)

Clay-pellet conglomerate

Matrix supported

Locally developed slope (debrite)

No known analogue

Coarse siltstone

Massive siltstone, sof-sediment deformation structures, floating fine-grained sandstone blocks.

Rapid deposition, Liquifaction and plastic failure due to cyclicstorm-wave loading

Modem Suriname Coast (Pine and Ginsberg, 1985; Ginsberg and Coleman, 1981)

Interbedded mudstone, silt-stone and sandstone

Sandstone: Wave-tippled tops, HCS, fiatlamination, guttercasts, localised small channels, sharp tops and bases, can fine and coarsen upward

occurrence)

BAR SLOPE

B,C BAR FRINGE/MARGIN

Mancos Shale (Hobson et al., 1982) Alternation of tempestites and fair-weather suspension sealing

Shannon Sandstone (Tillman and Martinsen, 1984) Mosby Sandstone (Rice, 1984)

OFFSHOREINTERBAR

Dark shale ant very finegrained sandstone

Siltstone and mudstone: Normally massive, bioturbation sharply bound, forms drapes

Sediment-laden and deficient currents wave oscillation

Modem N. Sea Shelf (Aigner and Reineck, 1982; Goldring and Aigner, 1982)

Wave tippled, bioturbated, sandstone is lenticular bedded

Distal tempestites, suspensi~ settling and biogenic reworking

(Pedersen, 1985)

valent (Table i). The lee slope of the Suriname m u d d y bars are composed of a gell-like fluid m u d which s h o w s a dominantly massive texture and m a y be soft-sedlment deformed (Rine and Ginsberg, 1985). OffJhore bar m o d e l The proposed bars were 2 to 9 m thick and in excess of several kilometres long but wldths are unknown. This offshore bar reconstruction is based on the model proposed by Swift and Rice 0984) as well as H o b s o n et al, (1982). Fig. 9a illustrates a proposed bar reconstruction based on the evidence gathered from the area of study. It is suggested that irregularities in the shelf surface determined whether or not bar growth took place. Once a certain relief w a s obtained the bar became self perpetuating and developed a down-current argillaceous slope. These irregularities m a y have been created by a pre-existing shelf topography c o m p o s e d of transgressed coastal deposits. These

were t h e n r e s h a p e d into bars, a s h a s b e e n suggested for s o m e W e s t e r n C r e t a c e o u s Interior S e a w a y offshore b a r s (Kitely a n d Field, 1984). Alternatively, t h e y were a p r i m a r y f e a t u r e related to the shelf setting a s t h e Swift a n d Rice (1984) m o d e l requires. The repetition of c o a r s e n i n g - u p w a r d s e q u e n c e s (Fig. 2) m a y be d u e to offshore b a r c r e a t i o n and p r o g r a d a t i o n (cf. H o b s o n e t a / , 1982) or repetitive drowning of c o a s t a l d e p o s i t s which were r e l e a s e d t h r o u g h the littoral energy fence a n d m o u l d e d into offshore bars. At p r e s e n t t h e evidence is insufficient to distinguish b e t w e e n t h e s e two possibilities. However. it c a n b e s t a t e d t h a t t h e shelf w a s c o n s t r u c t e d t h r o u g h the action of s t o r m c u r r e n t s which developed bars. Coastal p r o g r a d a t i o n led to the stacking a n d overlapping of b a r s a n d the formation of this a n c i e n t shelf (Fig. 9b). Palaeocurrent d a t a is s p a r c e b u t a plot of the wave-ripple c r e s t s yielded a n o r t h - n o r t h w e s t s o u t h - s o u t h e a s t a l i g n m e n t (Fig. 7). This s u g g e s t s that t h e coastline w a s o r i e n t a t e d n o r t h - n o r t h - w e s t

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Fig. 9. A proposed paJaeogeographic reconstruction of offshore bars from the Ecca-Beaufort transition zone. - s o u t h - s o u t h e a s t . According to Nel (1977) fluvial i n p u t w a s from a s o u t h e r l y s o u r c e so it is c o n t e n d ed t h a t l a n d lay to t h e w e s t - s o u t h w e s t . The pres e n c e of HCS, a c c o r d i n g to s o m e a u t h o r s (Swill et aL 1983; Allen, 1985; Swift a n d N u m m e d a l 1987) s h o w s t h e p r e s e n c e of a c o a s t - p a r a l l e r c u r r e n t . P l a n a r c r o s s - b e d a z i m u t h s (N = 4) yielded a s o u t h s o u t h e a s t direction, w h i c h parallels t h e recons t r u c t e d coastline, a n d t h u s m i g h t r e p r e s e n t a coast-parallel flow. Within t h i s p a l a e o g e o g r a p h i c f r a m e w o r k it is likely t h a t w a v e a t t a c k w a s directed t o w a r d s t h e w e s t - s o u t h w e s t (Fig. 9c). Deposition t o o k place t h r o u g h t h e c o m b i n e d action ofunldirectional (coast-parallel?} a n d stormw a v e c u r r e n t a c t i o n w h i c h a l t e r n a t e d with fairw e a t h e r p r o c e s s e s . S e d i m e n t a c c u m u l a t e d via t h e settling of d e n s e s a n d a n d silt s u s p e n s i o n s . Locally b a r s w e r e f o r m e d a n d s h a p e d b y s e d i m e n t s a t u r a t e d or s e d i m e n t deficient c u r r e n t s . T h e s e f e a t u r e s p r o g r a d e d a n d c l i m b e d to give rise to a n a n c i e n t shelf deposit. CONCLUSIONS

(I) This r e s e a r c h h a s c o n f i r m e d t h a t t h e EccaB e a u f o r t t r a n s i t i o n w i t h i n t h i s a r e a is a n offshoreshelf transition as was previously postulated by S m i t h a n d Z a w a d a (1988). D e p o s i t i o n w a s controlled b y s t o r m p r o c e s s e s w h i c h a l t e r n a t e d with f a i r w e a t h e r conditions. (2) Shelf s e d i m e n t a t i o n t o o k t h e form of prograding, u p w a r d - c o a r s e n i n g , s t o r m - p r o d u c e d offs h o r e b a r s . T h e s e f e a t u r e s were 2-9 m high, kilom e t e r s in length a n d kflometres in width. (3) F o u r offshore b a r s u b - e n v i r o n m e n t s are recognised, t h e s e b e i n g : i) proximal t e m p e s t i t e s of t h e b a r c r e s t s , ii) m a s s i v e , soft s e d i m e n t d e f o r m e d b a r slope siltstones, iii) r y t h m i c a l l y b e d d e d b a r f r i n g e / m a r g i n t e m p e s tites, iv) s u s p e n s i o n d e p o s i t s a n d distal t e m p e s t i t e s of t h e i n t e r - b a r a n d offshore regions. (4) The inferred coastline w a s o r i e n t a t e d approxi-

m a t e l y n o r t h - n o r t h w e s t - s o u t h - s o u t h e a s t (land to t h e w e s t - s o u t h w e s t ) with a s o u t h - s o u t h e a s t e r l y coast-parallel current. Wave a t t a c k w a s t o w a r d s the west-southwest.

Acknowledgements - The authors would like to thank the Chief Director of the Geological Survey for permission to publish these results. We also thank Dr. J. Bredell of the Geological Survey for his critical editing of this paper. REFERENCES

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