Deposition of Turbidites andAgitated-Water Siltstones: a Study

of the Upper CarboniferousWestward Ho! Formation,

North Devonby ROGER G. WALKER

Received 18 July 1969; taken as read 2 January 1970

CONTENTS

1. INTRODUCTION

2. STRATIGRAPHY AND STRUCTURE ...3. SUBDIVISION OF THE WESTWARD HoI FORMATION

4. DESCRIPTION AND INTERPRETATION OF THE UNITS(a) Unit 1 .(b) Unit 2 .(c) Unit 3 .(d) Unit 4 .(e) Unit 5 .(f) Unit 6 .(g) Unit 7 .(h) Unit 8 .

5. INTERPRETATION OF THE SEQUENCE OF UNITS6. PALAEOCURRENTS .,.

7. CLOSE ASSOCIATION OF TURBIDITES AND AGITATED-WATER FACIES

ACKNOWLEDGMENTSEXPLANATION OP THE PLATES

REFERENCES

page 444547474750515254565758596062646466

ABSTRACT: The Westward HoI Formation consists dominantly of mudstones andsiltstones. Two distinct groups of facies can be contrasted. The first contains very finegrained black mudstones and parallel bedded graded siltstones and fine sandstoneswhich show all the sedimentary structures now associated with turbidites. By contrast,the second group of facies contains irregularly rippled and cross-bedded siltstones andsandstones, the sedimentary structures indicating agitated water. The sequence offacies shows that the turbidites occur only in the lower parts of coarsening-upward(shallowing) sequences, or are associated with black mudstones in very quiet water.The upper parts of the regressive sequences contain only agitated-water facies.

Two groups of siltstone-filled scours occur. In the lower group, the scours are ofwide extent and the sequence above each scour fines-upward. In the upper group,similar fining-upward sequences above scoured surfaces occur, but the upper parts ofthe sequences contain many small channels which have a fill finer than, or identicalwith, the surrounding sediment. The fine nature of the fill suggests a slump-scar originrather than channelling.

The Westward HoI Formation is unique in containing three extremely rapidvertical transitions from turbidite to agitated-water facies. Progressive increase inagitation in the area of turbidite accumulation is suggested, rather than regionalprogradation of a slope and hence more gradual transition into agitated-water facies.

43

44 ROGER G. WALKER

1. INTRODUCTIONTHE MAIN PURPOSE of this paper is to examine in detail the relationshipbetween turbidites and agitated-water sediments in the Upper Carboni­ferous Westward Ho! Formation. The formation, which is continuouslyand superbly exposed on the coast, is unique in the writer's experiencein containing three extremely rapid transitions from graded siltstone­turbidites into agitated-water siltstones (Fig. 1).

The association of turbidites and agitated-water ('coal measure') facieswas first recognised by Prentice (1960)during his mapping of the Bidefordarea, north Devon. He suggested that a turbidite sequence (Instow Beds)was followed by 'coal measure' facies (Northam and Abbotsham Beds),and finally passed back through a slumped unit (Greencliff Beds) intoturbidites (Cockington Beds). This turbidite-e-coal measure-turbiditesequence was investigated by Dr. H. G. Reading and the author for ShellInternational Research, beginning in 1961. Interest was focused on thewell-exposed Northam and Abbotsham Beds-the junction with theGreencliffBeds is faulted, and the GreencliffBeds themselves are stronglycleaved and tectonised. The Instow Beds are not exposed on the coast inthe Westward Ho! area.

Nine regressive cyclothems, each beginning with a black mudstone andending with a sandstone containing a bioturbate top, were recognised inthe Northam and Abbotsham Beds. The stratigraphical names were re­defined, placing the lower three cyclothems in the Northam Formation,and the upper six cyclothems in the Abbotsham Formation (Fig. 2). Thebase of the Northam Formation was taken at the well-defined base of theMermaid's Pool Sandstone Member, and the base of the AbbotshamFormation was taken at the top of the Raleigh Sandstone Member. TheNortham and Abbotsham Formations were put into the Bideford Group, astratigraphical unit containing nine cyclothems which could be mappedinland, and which differed from the adjacent formations. The remaining,non-cyclic part of Prentice's Northam Beds was placed in the WestwardHo! Formation.

The results of the study for Shell International Research were publishedby de Raaf, Reading & Walker in 1965,who discussed in detail the evidencefor cyclicity in the Abbotsham Formation. The cyclothems have beentaken as models for the interpretation of other Upper Carboniferoussequences (Reading, in press). A more detailed study of the lowest cyclo­them in the Northam Formation was made in the summer of 1967,emphasising the occurrence of turbidites and their relationship to otherfacies in the regressive sequence (Walker, 1969).

Until the summer of 1967, the only study of the Westward Ho! Forma­tion was a short section of the author's D.PhiI. Thesis (Walker, 1964a, b).The beds are dominantly siltstones and mudstones, with much irregular

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Fig.T, Geological map of the wave-cut platform at Westward Ho!, north Devon. All unstippled (white) areas of the Westward Ho!Formation are composed of siltstones. The units discussed in the text are numbered on the map, and designated stratigraphicalmembers are named. The map is within National Grid 100 km. square SS

[To face p, 44

THE WESTWARD HO! FORMATION 45

lamination and rippling indicative of agitated water. In parts of thesequence the mudstones contain laterally continuous graded siltstonesinterpreted as turbidites, and at one locality the turbidites can be seen tofill large channels (Walker, 1966a). At the time of the study, no sequentialorgan isation of the facies could be recognised, and in 1967 (in press),Reading has used the term 'random' to contrast the facies sequence withthose of the nine cyclothems above (Northam and Abbotsham Formations),where the facies sequence was 'predictable'. In the summer of 1967, theauthor re-mapped and re-measured the Westward Ho! Formation on thecoast, and also mapped and measured the section in the Torridge Estuary.This section is about 2tmiles (4 krn.) eastward along strike from the coastalsection at Westward Ho!, and crops out along the west side of the Estuary1 to I t miles (1.6-2 krn.) north of the roadbridge at Bideford.

Thee separate developments of turbidites are described below, andrelated to the associated agitated-water facies. Also, some prominent silt­filled channels are described in detail, and are related both to the turbiditesand to agitated-water facies. Finally, a brief review of other turbidite­agitated-water relationships is presented.

2. STRATIGRAPHY AND STRUCTURE

Full reviews of the stratigraphical nomenclature and palaeontologicalevidence are given by Prentice (1960; (965), Reading (1965) and de Raafand others (1965). The present stratigraphical column is shown in Fig. 2.No body fossils have been found in the Westward Ho! Formation, and itsage is therefore uncertain, although probably Namurian. The base of theWestward Ho! Formation is undefined because the previous definition,at the top of the Instow Beds, is no longer valid. Money (I966) arguedfrom matching facies that the Appledore Formation, which crops outimmediately to the north, grades up into the Westward Ho! Formation.However, the two formations are separated by a mile-wide strip with noexposure. A formally defined base must await further regional mapping.On the wave-cut platform at Westward Ho l, the formation base has beentaken at the base of the Low Tide Sandstone Member (430429301, Fig. I).Below this horizon, the structure on the wave-cut platform is very com­plex, with part of the sequence probably overturned. This degree ofstructural complexity is unique in the Westward Hot, Northam andAbbotsham Formations, and through this location may run one of thethrust faults postulated by Reading (1965) to separate blocks in northDevon, each with a different stratigraphical sequence.

The Westward Ho! Formation dips uniformly southward at about 70°.It is broken by prominent north-east trending sinistral strike slip faults,

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Fig. 2. Stratigraphy of the Westward Ho! area, north Devon. Thicknesses given inmetres. Numbers 1 to 9 indicate cyclothems of the Northam and Abbotsham Forma­tions, and arrows indicate palaeocurrent vector means. The number of readings in thevector means is: Low Tide Sandstone, 1; Sandstone E, 16; Mermaid's Pool Sandstone,37; turbidite of cycle 1, 14; Rock Nose Sandstone, 28; Raleigh Sandstone, 36; tur­bidites of cycle 6, 26; Cornborough Sandstone, 21.

In the age column, arrows A, Band C refer to the Carbonicola? cf. bel/ula Boltonfauna, to the non-marine lamellibranchs of Rogers, and to the Carbonicola communisZone fauna, respectively. These faunas are discussed by de Raaf and others, 1965.

The major sandstones are stippled, black shales shown by horizontal lines and tur­bidites shown by the dots-above-line graded bedding symbol. Curved lines belowbeds indicate channels. The way the beds are sketched is intended to give an idea ofgrain size and resistance to erosion.

THE WESTWARD HOI FORMATION 47

with a maximum offset of about 70 m. There is also a set of north-westtrending dextral strike slip faults , most of them with negligible offset.Minor southward dipping thrust faults occur at the top of the Low TideSandstone, and within the Patio Pool Sandstone, and predate the strikeslip faulting.

The top of the Westward Ho! Formation is drawn at the base of theMermaid's Pool Sandstone for mapping purposes, because inland, theMermaid's Pool Sandstone cannot always be distinguished from other largesandstone bodies in the Bideford Group ( = Northam Formation plusAbbotsham Formation). The Mermaid's Pool Sandstone, however,represents a sedimentological continuation of Westward Ho! Formationconditions, and a description is incorporated in this paper.

3. SUBDIVISION OF TIlE WESTWARD HO! FORMATION

In the initial study of the Westward Ho! Formation, which formed apart of the author's D.PhiI. Thesis (Walker, 1964a), fifteen different facieswere defined. The facies relationships were examined in the same way asfor the Abbotsham Formation (de Raaf & others, 1965, 41), but theresulting facies relationship diagram could not be interpreted in terms ofany pattern of sedimentation. The facies were then combined into 'Sand­stone', 'Intermediate' and 'Mudstone' groups but , again, the overallsequence was not easily interpreted (Walker, 1964a, b).

The approach used in the present study is to subdivide the sequence intoeight units, each different from the adjacent units in aspect and inferredgeological history. Each unit contains several 'facies' (in the Northam andAbbotsham Formation sense), and it is now felt that too detailed a sub­division of facies (Walker, 1964a) has previously obscured the inherentvertical sequence of the units discussed below.

In order that the paper can be used conveniently in the field, an inter­pretation is given after the description of each unit, rather than leaving allinterpretations until the end. Also for convenience, the major sandstonebodies have been lettered A to H, with some of them formally designated asMembers (Fig. 2).

4. DESCRIPTION AND INTERPRETATION OF THE UNITS

Throughout this section of the paper, the reader is referred to Fig. 3for the measured sedimentological section, sandstone identification letters,stratigraphic thicknesses, and palaeocurrent directions.

(a) Unit 1. 0 to 45.50 m,

(i) Description. Unit 1 contains two sandstone bodies (A and B) separa­ted by black shale. The Low Tide Sandstone Member (0 to 17.50 m.)

48 ROGER G. WALKER

consists of lower and upper bedded parts, with a central deformed part.The bedded parts consist of alternations of parallel, thick, sharp-basedsandstones, and very thin dark mudstones (Plate I, A). Some of the sand­stones are slightly graded, and several of the beds are composite, oramalgamated (Walker, 1966b, 96-7). The sandstone beds are mainlydevoid of sedimentary structures, but a few of the beds contain crudeparallel lamination or cross-lamination within the top 2 to 3 em. Individualbeds are very persistent along strike. The central deformed part is litho­logically identical with the bedded parts, but contains more mudstone.The sandstones are rolled into large irregular balls up to 2 m. in diameter,but their sharp bases and graded bedding can still be detected. The ballscontain one, or several, sandstone layers, and are randomly oriented withinthe mudstone matrix-they do not resemble 'ball-and-pillow' (Pettijohn &Potter, 1964, pl. lOOA to I04A). The lowest bed of the upper bedded partof the sandstone channels slightly into the deformed layer and has well­developed flute casts, indicating current flow toward 3300

• Feature mappingof the Low Tide Sandstone eastward is not possible because of Pleistocenecover, and no equivalent unit is known in the Torridge Estuary sectionnorth ofBideford.

Between Sandstones A and B there is about 25 m. offaulted, folded andtectonically squeezed black mudstone, well banded on a l-cm. scale. Thebands consist of alternations of black mudstone, and grey, slightly siltymudstone, and the facies closely resembles the facies A black mudstonesof the Abbotsham Formation (de Raaf & others, 1965,14).

Sandstone B consists of two distinct channel fill deposits which togethercut about 15 m. into the black mudstones (Fig. 4). The lower part of thefirst channel is poorly exposed, but appears to consist mainly of massive,muddy siltstones. Bedding improves upward, and outlines a local change indip and strike (80-120 m., Fig. 4) which cannot be attributed to tectoniccauses. These tilted beds are truncated by the upper channel, which cutsdown about 8 m. The fill of the upper channel consists of graded, sharp­based fine sandstones with interbedded mudstones. Some beds are mildlydeformed. Beds are mainly massive, but some parallel and cross-laminationoccurs in the top few centimetres of some beds. The sequence of sedimen­tary structures graded bedding --+ parallel lamination --+ cross-lamination,is observed in places, with the ripple crests indicating flow toward westand north-west.

(ii) Interpretation. Throughout the Westward Ho! Formation, the finesandstones are typically entirely cross-laminated, but this features is con­spicuously restricted here to the top few centimetres of some beds. Thegrading, and sequence of sedimentary structures, graded bedding --+

parallel lamination -+ cross-lamination, indicate deposition from waningdensity currents, and a turbidite origin is suggested for Sandstones A and

INTERPRETATION

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Fig. 3. Sedimentological section of the Westward Ho! Formation, measured at thebase of the cliff or along the inshore part of the wave-cut platform between 43052929and 41832902 (Fig. I). Sandstone bodies A to G are lettered, and numbers I to 8show positions of the Plates. All individual palaeocurrent readings are shown and theirsources identified. The curve at the right gives an interpretation of changes in basinagitation and deepening and shallowing conditions. The way the beds are sketched isintended to give an idea of grain size and resistance to erosion. Scale in metres

[To face p. 48

Fig. 4. Map of channel complex in Sandstone B, Unit I. Beds dip at over 80° southward except between 80 and 120 m. in lower part of channelfill. Most of the map can therefore be considered as a vertical cro ss-section of the channels. Note the orientation of the map , with north towardthe bottom so that the reader can hold the map and view the channel right-way-up simultaneously. A full explanation is given in the text

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50 ROGER G. WALKER

B. The black mudstone reveals no evidence of current activity, enhancingthe interpretation of turbidity current emplacement of sand into a quietbasin. The very thick beds of Sandstone A, and the channelling associatedwith B both suggest near-source, or proximal turbidity current deposition(Walker, 1967).

If it is assumed that the beds above the channel contact between 80 and120 m. on Fig. 4 were deposited on an almost horizontal basin floor, thebeds below the contact had an original dip of about 60° toward north­west (340°). Sea-floor tilting of this extent is extremely unlikely, and itmust therefore be assumed that this 60° dip is an original depositionaldip. Possibly these beds represent the backfilling of a channel or slump­scar. If so, it would imply a regional palaeoslope toward north-west, whichagrees with the flute cast on Sandstone B (330°) and the ripple orientationsmeasured in Unit 2 (Fig. 3).

(b) Unit 2.45.50 to 95.50 m,

(i) Description. The bulk of Unit 2 consists of massive grey muddy silt­stone, with some faint clean parallel silt laminae up to 2 mm. thick.Interbedded with these siltstones are very fine cross-laminated sandstoneswith gradational bases, in beds up to 50 em. thick. The most prominent ofthese beds (Sandstone C) can be traced along strike for at least 430 m.(Fig. 1). Also throughout Unit 2, there is good development of 'pinch-and­swell' cross-lamination (Walker, 1969) in which silty and muddy laminaecan be traced across a ripple form with an amplitude of about 10 em. andwavelength of about 50 em. (Plate I, B). Within the top to m. of the unit,there is a gradual incoming of sharp-based sandstone beds, 1 to 4 em.thick, with cross-lamination throughout. These coarsen upward intoSandstone D, the Patio Pool Sandstone Member, which consists domin­antly of fine massive sandstones with thin muddy partings (Plate 2, A). Thebeds are lenticular, and at low tide line (42512926) cross-bedding can bediscerned through the mussel beds and barnacles. The top of the sandstoneis knife-sharp (Plate 2A), but despite careful search no sign of bioturba­tion could be found (the significance of such knife-sharp contacts of darkmudstone on top of cross-bedded sandstone has been discussed by deRaaf and others, 1965,42).

Cross-lamination directions in Unit 2 indicate flow toward north-west,but there is a significant change within the top to metres, with flow morewesterly.

(ii) Interpretation. Irregular silty lamination within Unit 2 mudstonesbegins only a few em. above the last turbidite of Sandstone B, and the firstthick (18 em.) gradational based cross-laminated sandstone occurs onlyto metres above the last turbidite. Basin agitation was clearly establishedrapidly after turbidite deposition stopped, although the regional palaeoslope

THE WESTWARD HO! FORMATION 51

probably continued to dip north-westward. The first sign of extensiveshallowing and change of conditions occurs within the upper 10 metres ofUnit 2, with the gradual coarsening-upward sequence into the Patio PoolSandstone and slight change in palaeocurrent orientations. The PatioPool Sandstone itself contains the only cross-bedding (in sets greater than10 to 15 em.) in the entire Westward Ho! Formation, and, at the top of acoarsening-upward sequence, occupies the same relative position as thevarious sandy facies (E, H, J) of the Abbotsham Formation (de Raaf andothers, 1965, 37). The thin development of the sandstone, absence of achannelled base, and absence of rootlets imply near-shore rather than trulycoastal conditions.

(c) Unit 3. 95.50 to 248.00 m,

(i) Description. The unit begins with dark, spectacularly parallel-beddedmuddy siltstones on a I to 2em. scale. These occur alone in the lower 6.50m.of the unit, and act as background sediment for the thin sandstone bedswhich occur between 102.00 and 213.00 m. These sandstones are sharp­based, 1 to 4 em. thick, and usually well graded. Despite their thinness, thesequence of sedimentary structures, graded bedding -+ parallel-lamination-+ cross-lamination, is well developed in many beds (Plate 2B). The bestexposures are near the sewer inspection structure at 42422916 (Fig. 1).Individual beds 1 to 2 em. thick can be traced for up to 100 m. across thewave-cut platform, and groups of beds can be correlated across faults andpersist along strike for about 200 m. Near the sewer inspection structure,the upstream termination of several beds can be observed. The main sand­stone bed begins abruptly with a large ripple, but upstream there are verythin, clean, discontinuous silty laminae and tiny starved ripples from 1 to3 rnrn, thick. These laminae and starved ripples only occur betweenindividual beds of the muddy siltstone matrix, never within these beds.

The thin sandstone beds tend to occur in groups, with thick develop­ments of muddy siltstones between the groups. Four such groups havebeen recognised in the cliff between Patio Pool and Seafield House (Fig. 1),but they cannot be followed all the way across the wave-cut platform. Inthe muddy siltstones between the groups, starved ripples are found,suggesting that along strike the starved ripples might pass into continuousthin graded sandstone beds.

Throughout this sequence, up to 213.00 m., there is no sign of generalbasin agitation in the form of irregular cross-lamination permeating thematrix. However, above 213 m. to the limit of exposure below SeafieldHouse before faulting (42352914), at 248.00 m. on Fig. 3, there is a changeto irregular cross-laminated siltstones and mudstones (Plate 3A). Recog­nisable sharp-based sandstones quickly drop out and are replaced bygradational based cross-laminated sandstones in beds up to 20 em. One of

52 ROGER G . WALKER

these, Sandstone E, is a good marker band which establishes the correla­tions from below Seafield House (42402915) to the wave-cut platform at42102917(Fig. 1).

Cross-lamination directions throught Unit 3 indicate current flowapproximately westward.

(ii) Interpretation . The muddy siltstones immediately above the PatioPool Sandstone show no indicat ions of current agitation, and are inter­preted as quiet water deposits. The thin graded sandstone beds have mostof the characteristics of turbidites, and the laterally equivalent starvedripples probably also represent turbidity current deposits, behind and atthe sides of the main turbidity current, where there is reduced supply ofsediment. No basin agitation is present until above 213 m., where theirregular cross-lamination and slightly coarser gradational based sand­stones appear. The section from 95.50 to 248.00 m. thus shows overallcoarsening-upward and shallowing, with the passage from muddy silt­stones, via turbidites, into agitated-water siltstones and sandstones.

(d) Unit 4. 215.50 to 291.00 m,

(i) Description. Unit 4 consists of a series of channels, or scour surfaces,the lowest of which cuts down into Unit 3. The origin of the channels hasbeen the cause of controversy in the field, because of their very fine filling­in places finer than the sediment into which the channel cuts. Until thedetailed mapping in 1967 (Fig. 5), the vertical and lateral extent of thechannels was unknown, partly because the well-exposed section at thebase of the cliff (between 423291 and 422291) is broken by many smallfaults. Although partly seaweed covered, the wave-cut platform is the bestplace to observe the extent of the channels (Fig. 5).

In channel I of Fig. 5, only the eastern wall is exposed, giving a mini­mum channel width of 170 m., and a minimum channel depth of about30 m. The wall contact is sharp and clean cut, with slightly coarser sedi­ment in the channel than in the surrounding sediments. The bulk of thefill is cross-laminated siltstone similar to that in channels 2-5 (Fig. 5).Secondary scours are observed near the base of the main channel, up to 2 m.deep and also filled with sediment slightly coarser than that outside thechannel.

'Channels' 2-5 are scoured surfaces which can be mapped continuouslyacross the wave-cut platform, a distance along strike of about 200 m.(Fig. 5 and Plate 3B). The maximum relief on the scoured surfaces is about10 m., and above the scour there is a fining-upward sequence of channelfill sediments. Basally, the siltstones and very fine sandstones are cross­laminated, and contain few muddy laminae (plate 4A). Upward, the siltylaminae approach parallel alignment, muddy laminae become morecommon, until the sequence consists entirely of dark grey mudstone.

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VI....,

54 ROGER G. WALKER

Shallow minor scours occur toward the bases of these 'channels', butnever occur in the mudstones at the top of the channel fill sequences.

The fill of channel 5 grades up into about 7 m. of very dark mudstoneswithout silty laminae. On the western part of the wave-cut platform, thesedark mudstones grade into the black mudstones at the base of Unit 5. Inthe east, the two mudstones are separated by aIm. horizon of sedimentarybreccia, discussed below.

(ii) Interpretation. Because of the necessity of considering these channelsin their stratigraphical context, and because they must be contrasted withthe channels of Unit 6, the reader is referred to Section (f) of this paper,which presents a unified interpretation of Units 4, 5 and 6.

(e) Unit 5. 291.00 to 322.50 m,

(i) Description. Unit 5 consists of a breccia, and two sandstone bodies(F and G) separated by black mudstones. Sandstone F has a maximumthickness of about 13 m. The lower part forms a channel fill, with a basalflute cast indicating flow toward 265°. The lower part of the channel fillis brecciated (Plate 4B), and the rest of the sandstone consists of inter­bedded sharp-based sandstones and mudstones. The sandstones contain allthe features associated with turbidites, and will not be described furtherhere. The lowest breccia, which dies out westward across the wave-cutplatform, is identical in character with the breccia of Sandstone F, and theoriginal sediment may therefore have been turbidity current emplaced.

Sandstone G is separated from Sandstone F by black mudstones whichcontain slump blocks of unbedded siltstone. The sandstone consists ofsharp-based graded sandstones with interbedded mudstones, and formsthe fill of a channel complex (Plate 5A). This sandstone has previouslybeen described and illustrated in detail (Walker, 1966a, 1906-9). At the farwestern edge of the wave-cut platform, Sandstone G cuts down throughthe mudstones into Sandstone F. Sandstone G contains one very distinc­tive thick ungraded sandstone bed, about 1 m. thick, which is bed C inWalker (I 966a, 1907) and also bed C in Fig. 6 of this paper. Above bed C,the character of Sandstone G changes-beds become cross-laminatedthroughout and tend to have gradational bases. These cross-laminatedbeds are cut by the first channel of Unit 6 (I, Fig. 6).

(ii) Interpretation. Evidence has previously been offered (Walker, 1966a)for the turbidite origin of Sandstones F and G, and the mechanisms ofchannel cutting and filling have also been discussed. It is important tonote that the turbidites here are again associated with black mudstoneswith no indications of basin agitation. However, the gradational-basedcross-laminated sandstones above bed C of Fig. 6 do indicate a rapidreturn to agitated water after the deposition of the channel-fill turbidites.

"HIGH" - AREA OF HIGH RELII:F ONWAVE· CUT PLATFORM

DARK MUDSTONESWITHOUT SILTY LA MINAE

,~, CROSS-LAMINATED-- SANDSTONE I UNIT 5 )

2911

4204 4203 4202

10 METRES,-- I

4201 4200

El)li::;{i~I~;:;b:·:·:f MANY}

FEW

SILTY LAMINAE

IN MUDDY MATRIX

NORTH

4197

>-l:I:tT1

~tT1en>-l~~~ti:I:o"r1o~

~~>-lH

oZ

Fig. 6. Map of channels in Unit 6, also showing upper part of turbidite complex in Un it 5. Bed C in Unit 5 is the same bed as 'C' in Walker, 1966a,figs. 8 and 9. Note that north is aga in toward the bottom of the map. The steep dip allows the map to be considered as a vertical cross-section ofthe beds. The area ean be locat ed in the field by the bulge in the eliff at 42042908 (where the number 6 appears on Fig. 1)

v.v.

56 ROGER G. WALKER

(0 Unit 6. 315.50 to 342.00 m,

(i) Description. Unit 6 is composed of siltstones with many small silt­filled channels. The distribution of these channels is shown on .Fig, 6.Channels 2, 3,4 and 5 (Fig. 6) are continuous scour surfaces, identical withthose in Unit 4, with minor channels toward their base. The fill is coarserthan the surrounding sediment, and grades into fine dark mudstones up­ward. However, there is one important contrast with the scours of Unit 4,that is, in the upper parts of the graded fill sequence, there are many smallscours cutting into the dark mudstones, with fill sediments absolutelyidentical with the surrounding sediments (plate 5B). These small scoursare particularly well developed between channel surfaces 4 and 5 (Fig. 6).Although most of the minor channels have very smooth bases, some havesteep, irregular walls, with part of the wall sediment incorporated into thefill (Plate 8). One other channel has a fill finer than the surrounding sedi­ment, that is, channell, which cuts into the top of Sandstone G. Towardthe base of this channel, there are small slump deposits of sandstone, butthe bulk of the fill is fine mudstone.

(ii) Interpretation. This interpretation covers Units 4, 5 and 6, which mustfirst be put into their overall context. Unit 5 contains black shales andturbidites, and is non-agitated ('deep water') . Consequently, a deepeningoccurred somewhere between the agitated top of Unit 3 and the base ofUnit 5. Evidence of current agitation is present throughout Unit 4, exceptin the upper part of channel 5, where the mudstone becomes unusuallydark and passes rapidly into black mudstones of Unit 5 (Fig. 5, west of thepoint 42082912), and the main deepening is inferred to have taken placerapidly at this stratigraphical level (Fig. 3).

The origin of the unique, wide (up to 170 m.) and deep (up to 30 m.)'scour-surfaces' in Unit 4 is obscure. If they are considered to be slump­scars, there is no explanation for the fining-upward sequences withdiminishing current agitation which occur above each surface. If they areerosional, there is very little evidence from which to deduce how this tookplace. Although turbidity currents could have cut the surfaces, there is nosign of turbidity current action (cf. the channel in Walker, 1969), nor doesthis mechanism account for the fining-upward sequences. The bestexplanation would seem to be erosion during a period of slightly loweredsea-level, and the development of the fining-upward fill during and en­suing rise of sea-level, with the area remaining all the time beyond thereach of currents transporting abundant sand. After scouring of channel 5in Unit 4, sea-level rise continued until the water was quiet enough for thedeposition of the black mudstones and turbidites of Unit 5.

In the upper 7-8 m. of Unit 5 irregularly cross-laminated siltstones in­dicate a very rapid change upward from the non-agitated turbidites whichform the bulk of Unit 5. In Unit 6, evidence for current agitat ion persists,

THE WESTWARD HOI FORMATION 57

with alternations of scouring, and fining-upward 'fill' sequences. The mainerosion surfaces 2, 3, 4 and 5 (Fig. 6) are identical with those in Unit 4.However, toward the top of fining-upward Sequence 4 (Fig. 6), there aremany small scours with a fill identical with the surrounding sediment. Ifthese scours are the result of channelling, why did the current not introducecoarser sediment into the scour? Alternatively, was a scouring currentnecessary at all? It is suggested here that the minor channels were notscoured out, but represent the hole, or scar left behind as a mass of sedi­ment slumped downslope moving on a well-defined basal shear plane(Laird, 1968).After slumping the scar would naturally be filled in the sameway as the surrounding sediment was deposited. Evidence of soft sedimentdeformation is associated with some of the channels in Unit 6 (42032907),and slumped siltstones can be seen in the base of 'channel'l (Fig. 6), whichmay also be a slump-scar.

After formation of the scour surfaces and slump-scars, deepening tookplace and the dark mudstones of Unit 7 were deposited in water beyondthe reach of scouring currents.

(g) Unit 7.342.00 to 389.50 m.

(i) Description. Unit 7 follows Unit 6 gradationally by loss of cross­laminated siltstones. It consists dominantly of banded silty mudstones, thebands reflecting subtle changes of grain size. Some bands are massive, butothers, up to 3 em. thick, contain thin, clean, silty laminae which areslightly undulating but not cross-laminated. The best exposures are in thecliff at 42002903, where one Teichichnus burrow was observed. On thewave-cut platform, two 10 em. graded turbidites were observed, with agroove cast trending 130-310°.

On the coast, the top 7 m. consist dominantly of silty mudstones, withvery little sign of basin agitation. The Mermaid's Pool Sandstone (Unit 8)rests abruptly on these mudstones (Plate 6). However, in the TorridgeEstuary ('Snuffy Corner' on the 25-inch map, 45892867) a different faciessequence is exposed. Taking the datum line as 389.50m. (base ofMermaid'sPool Sandstone on coast), the section in the Estuary from 352.50 to366.50m. consists of graded siltstones showing most of the characteristicsof turbidites. This group of turbidites is at the same stratigraphical level asthe two turbidites of the coast section. Above the turbidites is a 23-m.coarsening-upward sequence with irregularly cross-laminated sandstonesbecoming more frequent upward. Although the base of the Mermaid'sPool Sandstone is sharp, the regressive sequence below the Sandstoneindicates a gradual change of conditions and contrasts with the abruptjunction on the coast.

(ii) Interpretation. After the deposition of the cross-laminated siltstonesand their associated scours of Unit 6, there was apparently a return to

58 ROGER G. WALKER

quieter water conditions without basin agitation. Some turbidity currentsfound their way into this environment; in the Estuary section, to within23 m. stratigraphically of the Mermaid's Pool Sandstone. The presence of afull regressive sequence in the Estuary suggests that the base of the Sand­stone is erosive on the coast, and has cut out part of the section, althoughat the outcrop there is no evidence of cutting.

(h) Unit 8. 389.50 to 419.50 m. Mermaid's Pool Sandstone

(i) Description. The type-measured section of the Mermaid's PoolSandstone Member of the Northam Formation is in the cliff at 41942904.Individual beds of the Sandstone are very persistent along strike, as can beseen particularly between the Mermaid's Pool (41842900)and the fault at41752905. The Sandstone can be divided into four parts (Fig. I). Thelowest, part 1, shows an upward passage from cross-bedded medium sand­stone (plate 6) through cross-laminated beds into siltstones with cross­laminated sandstone lenticles (plate 7A). The cross-bedded sets reach50 ern, and indicate flow approximately north-westward.

Resting sharply on the siltstones is part 2, which again shows a passagefrom trough cross-bedded sandstones into ripple cross-laminated slightlyfiner sandstones. The trough cross-bedding is better exposed on the wave­cut platform than in the cliff. The cross-lamination appears as 'rib-and­furrow' on many bedding surfaces, and indicates south-westward flow.The top 75 em. of part 2 consists of dark mudstones with thin ripplelenticles of silt, and small horizontal burrows.

Part 3 rests sharply on part 2, and at its base are several massive sand­stone beds deformed into 'ball-and-pillow' (Pettijohn & Potter, 1964,pls. lOOA, B). Crude parallel lamination and low-angle cross-bedding canbe seen in some beds. These massive sandstones pass upward into finercross-laminated silty sandstones. Burrows become common upward, andthe top 8 ern, is extensively bioturbate.

Part 4 rests sharply on part 3, but represents a different type of sequence.Part 1, 2 and 3 were fining-upward, but part 4 coarsens upward from fine,dark silty mudstone into cross-laminated silty mudstone (Plate 7B),silty streak (de Raaf & others, 1965, 17) and finally into fine cross­laminated sandstones. In the cliff, this part of the section is slightlyfautled, but it is well exposed on the beach, where the top 4 em. areseen to be totally bioturbate, and followed very abruptly by the blackmudstones of the first of the nine cyclothems, the Rock Nose Cyclothem(Walker, 1969).

In the Torridge Estuary, the Mermaid's Pool Sandstone is probablyabout 10 m. thicker than it is on the coast, although the middle part isslightly faulted. It consists of cross-bedded and cross-laminated sand­stones, but the lower three parts of the coastal sequence cannot be

THE WESTWARD HO! FORMAnON 59

individually identified. Part 4, the coarsening-upward sequence, is welldeveloped, and of about the same thickness as seen on the coast.

(ii) Interpretation. The Mermaid's Pool Sandstone represents theculmination of the regressive sequence which started in the upper 23 m. ofthe Westward Ho! Formation. It is identical in stratigraphical and sedi­mentological position with the 'major sandstones' of the AbbotshamFormation (de Raaf & others, 1965, 37), and unquestionably representsshallower water than any of the facies in the Westward Ho! Formation.Each of the three lower parts exhibits a fining-upward sequence reminis­cent of channel-fill deposition with a waning flow regime (Allen, 1963),although in the absence of rootlets, the channels might be estuarine, or adistributary cut off, rather than alluvial. It should be pointed out that thisinterpretation in terms of channel fill cannot be substantiated in the fieldby the observation of channel margins.

The fourth part of the Mermaid's Pool Sandstone represents a minortransgression with burrowing on the top of part 3, followed by depositionof a regressive sequence as the source built forward again. This could beconsidered a minor 'cyclothem' of the Northam or Abbotsham type, butdiffers from those major cyclothems in that the transgression was in­sufficient to establish truly basinal conditions and black mud deposition.The top of the Mermaid's Pool Sandstone is bioturbate, and this biologicalactivity is interpreted to have taken place during the transgressive, non­depositional conditions leading to the establishment of basinal conditionsat the base of the Rock Nose Cyclothem.

5. INTERPRETATION OF THE SEQUENCE OF UNITS

No systematic facies sequence has previously been recognised in theWestward Ho! Formation, and by comparison with the organised sequen­ces (coarsening-upward) in the Northam and Abbotsham cyclothems, theWestward Ho! sequence was considered to show 'no pattern' (Walker,1964a, quoted by Reading (in press) as 'random'). However, the presentstudy has indicated definite sequences which are interpreted in terms ofmajor long-range changes in basin agitation. These changes in agitation intum are probably related to basin-wide shallowing and deepening (Fig. 3).

The first shallowing sequence is from the turbidites (Sandstones A andB) and black mudstones upward into the Patio Pool Sandstone. The fol­lowing deepening was abrupt, with no recognisable sedimentary record.The second shallowing sequence begins with banded dark mudstones(95.50 m.), and passes up into turbidites and eventually into agitated waterdeposits (about 215 to 245 m.). Successive minor sea-level fluctuations arebelieved to have formed the scour-surfaces and fining-upward sequences inUnit 4, with a rapid deepening leading to deposition of the turbidites and

60 ROGER G. WALKER

black mudstones of Unit 5. Shallowing and sea-level fluctuation is againbelieved to account for the scour-surfaces in Unit 6, followed by rapiddeepening. Towards the top of Unit 7, a well-developedshallowing sequencebegins, and leads to the deposition of the very shallow water Mermaid'sPool Sandstone. Above this sandstone, the behaviour of the basin appearsto have changed to a more regular development of regressive sequences,with less tendency for sea-level fluctuation-nowhere in the Northam andAbbotsham formations is there a sequence comparable with Units 4, 5 and6 (220-340 m.) of the Westward Ho! Formation.

The most surprising aspect of the sequence of units is the rapidity withwhich turbidites pass into agitated-water facies, at 45 m., 213 m. and 316 m.of Fig. 3. These transitions are discussed in more detail below.

6. PALAEOCURRENTS

Many palaeocurrent readings are now available in the Westward Ho!and Northam Formations. The individual Westward Ho! readings areshown in Fig. 3, and readings for part of the Northam Formation areshown in Walker, 1969, figs. 5, 6 and 7. Vector means for the WestwardHo!, Northam and Abbotsham Formations are shown in Fig. 2.

The lithology and mineralogy of these three formations are very similar,and all were probably deposited in essentially the same basin. A palaeo­current model must therefore be sought which accounts for all three for­mations. All palaeocurrent models involve the generalisation of specificobservations; inherent assumptions will be pointed out below. The firststriking feature of Fig. 2 is that the four major sandstones, Mermaid'sPool, Rock Nose, Raleigh and Comborough, have south-westerly vectormean orientations. The south-west flow reflects the regional palaeoslopeonly if the sand bodies are channel fills (e.g. distributaries). If the sand­stones occupy some other coastal or near-shore environment, there is littlechance that cross-bedding and ripple orientation will reflect regionalpalaeoslope: they are more likely to represent long shore currents similarto those known around the British, Dutch and German coasts (Stride,1963; Houbolt, 1968).

By contrast, the turbidity current flows are consistently perpendicularto the major sandstone palaeocurrents. This can be seen in Fig. 3, whereturbidite Sandstones A, B, F and G show north-west or west flow (exceptfor 1 flute and 1 groove in Sandstones F and G). Only four sole markscould be measured, and it has been assumed that the ripples also indicatedirection of turbidity current flow, rather than the direction of flow of anocean current, although the latter interpretation is possible (see reviewsand references by Kelling, 1964, and Walker, in press). The turbidites at352 m. also show north-west flow. However, there are also turbidites inthe Northam and Abbotsham Formations. In cycle I of Fig. 3, turbidity

THE WESTWARD HO! FORMAnON 61

currents flowed consistently south-east (Walker, 1969,figs. 5, 6 and 7), andin cycle 6, south-east flow can again be demonstrated (de Raaf and others,1965, fig. 7). It is assumed that much of the spread of palaeocurrent read­ings can be explained by the fact that palaeoslopes are not planes, but areirregular surfaces with much local variability.

Putting this information together, a trough elongated north-west-south­east can be proposed, with a slope into the trough from the north-east.The floor of the trough was probably very flat, so that turbidity currentscould flow either north-west (Westward Ho! Formation) or south-east(Northam and Abbotsham Formations). At times, there was build-forwardof the slope from the north-east, resulting in major sandstones with south­west, palaeoslope controlled, current orientations. The turbidity currentsin the trough are mineralogically identical with the agitated-water sand­stones. They might have flowed south-west down the slope, and thenbecome reorientated to flow axially along the trough, although there is nodirect evidence of this. Alternatively, very similar sediment sources in thenorth-west, south-east and north-east must be proposed.

Several objections to this model must be pointed out. First, there arethe flute and groove orientations in Sandstones F and G, which indicateflow toward 2650 and 2250 respectively. However, these occur in channels,and can be interpreted as feeder channels cutting down the south-westdipping slope into the trough. A second and more serious objectionconcerns the directions recorded in the Rock Nose Cyclothem (cycle I ofFig. 3; see Walker, 1969, figs. 5, 6 and 7). Here, the basal turbiditesshow south-east flow (as deduced from sole marks) but show rippled topsindicating flow toward east-north-east, up the slope! Also, near the top ofthe cyclothem is a feeder channel plugged with turbidites. If the sourcelies in the north-east and the slope is prograding toward the south-west,the feeder channel orientation should be downslope toward the south-west:however, directions both in and above the channel are south-east, as arethe directions in five sandstone ribs which immediately underlie the Mer­maid's Pool Sandstone. This cyclothem implies a major south-east dippingpalaeoslope, with sand advance from the north-west, except at the topwhere the Mermaid's Pool Sandstone indicates south-west flow. A thirdobjection can be seen in cycle 2 of Fig. 3, where directions indicate con­sistent north-east flowing currents, up the palaeoslope of the proposedmodel.

Alternatives to this model are less acceptable. If the palaeocurrentreadings are divorced from their facies, it could be argued in the WestwardHo! Formation that the palaeoslope originally dipped north-westward,and was gradually reoriented to dip south-westward by the time ofdeposition of the Mermaid's Pool Sandstone (Fig. 3). However, this modelbears no relationship to current directions in the Northam and Abbotsham

62 ROGER G. WALKER

Formations, and implies a source area for the sediments to the south-east,east and north-east.

It is difficult to tie in any Westward Ho!-Northam-Abbotsham modelwith the deposition of correlative formations. The Cockington Beds(turbidites) which crop out about half a mile (0.8 km.) south-west of theComborough Sandstone (Prentice, 1960, pl. 23) are now correlated atleast in part with the Abbotsham Formation (Reading, 1965, table 1).However, flute cast orientations indicate consistent east-north-east flow(Prentice, 1962, 106), a direction which cannot be related to anything inthe Abbotsham Formation. This divergence of orientation supportsReading's (1965) idea of deposition in different basins, with the Cocking­ton and Abbotsham Formations now being close together due to thrustfaulting. However, regardless of the problems of the Cockington For­mation, there is still work to be done on palaeocurrents (especially detailedstudies in cycles 3 to 9 in the Northam and Abbotsham Formations) beforethe geometry of the Westward Ho l-Northam-Abbotsham basin becomesmore clearly defined.

7. CLOSE ASSOCIATION OF TURBIDITES ANDAGITATED-WATER FACIES

Most turbidites which have been described in the literature occur inthick monotonous sequences devoid of any evidence of agitated water (forexample, Wood & Smith, 1959; McBride, 1962). Where palaeontologicalevidence is available, the fauna of the shales interbedded with the turbiditesandstones can sometimes be proved to be of deep water origin (Natland& Kuenen, 1951; Crowell, Hope, Kahle, Ovenshire & Sams, 1966).Geologists have rightly been hesitant in using genetic names, such asturbidite, for formations which do not show thousands of feet of mono­tonously interbedded graded sandstones and shales. Consequently, thereare few examples in the literature of closely associated turbidite andagitated-water facies, mainly because of the uncertainty of identifyingturbidites in such environments. It is correctly observed that many so­called turbidites could be due, for example, to crevassing and suddenintroduction of sand into a back-leveearea.

The identification of 'classical' turbidites in areas where they are closelyassociated with agitated-water facies is made easier by an understandingof the facies sequence. For example, several occurrences of turbidites inregressive sequences have now been described (de Raaf and others, 1965;Reading & Walker, 1966; Walker, 1969). The regressive sequence rep­resents the building forward of a shoreline into a basin, and the verticalfacies sequence developed as a result of regression typically coarsens up­ward. As the shoreline builds forward, turbidity currents may be generated,and will flow down-slope on to the basin floor. Hence in the regressive

THE WESTWARD HO! FORMATION 63

sequence, the context of the turbidites logically will be with the basinalfloor facies but below the prograding slope facies. Examples of turbiditesin this context include the Mam Tor Sandstone and Shale Grit turbidites(Namurian of the Pennines), which rest on basinal Edale Shales and arefollowed by the Grindslow Shales slope deposits (Walker, 1966b), cyclenumber 6 of the Abbotsham Formation (Fig. 2) of north Devon (cycle 3of de Raaf and others, 1965), a Namurian coal measure cyclothem atLottringhausen, Germany (Reading, in press), and the Upper Carboni­ferous Rock Nose cyclothem ofnorth Devon (Walker, 1969).

In the Westward Ho! Formation, sandstone bodies A and B occur inassociation with very finegrained, non-agitated black mudstones, and formthe lower part of a regressive sequence which ends with the Patio PoolSandstone. The interpretation ofA and B as turbidites on sedimentologicalgrounds is thus supported by their stratigraphical context. Similarly, thegraded siltstones of Unit 3 occur in an overall regressive sequence, aboveblack mudstones and belowagitated-water facies, and again are 'in context'for turbidites. Finally, turbidites occur again in the regressive sequenceexposed in the Torridge Estuary, above dark, non-agitated mudstones andbelow the agitated-water siltstones which pass upward into the Mermaid'sPool Sandstone deposited at the time of maximum regression.

The outstanding feature of the Westward Ho! Formation is the extremelyrapid passage from turbidites into agitated-water facies (45,213, and 317m.on Fig. 3), without any development of facies representing a progradingslope (in the sense of the Grindslow Shales (Walker, 1966b». The absenceof a slope facies, and hence absence of any evidence of shallowing andbuild-forward of sediment, suggests that there was a rapid change on thebasin floor from a quiet to an agitated environment. Such a change couldbe related to a slight change of relative sea-level. If the turbidity currentscontinued to transport sediment to the area, the sediment would be re­worked to such an extent that the primary turbidite sedimentary structureswould be lost.

It is important to try to assess whether turbidity current activity didpersist after the basin floor became agitated. At the first transition (45 m.),the turbidites pass rapidly into agitated water muddy siltstones withirregular cross-lamination. The bedding thickness and bedding regularityof the turbidites does not persist into the muddy siltstone facies, and hencethere is no evidence ofcontinued turbidity current transport ofsediment.

By contrast, the bedding thickness and regularity does persist into theagitated-water facies in the transitions at 213 and 317 m. The thin gradedsiltstones (Plate 2B) of the turbidite facies below 213 m. reappear in theagitated-water facies above 213 m. (Plate 3A), where sharp-based gradedbeds containing parallel lamination appear among the irregularly cross­laminated siltstones and mudstones. Similarly at 317 m., the bedding

64 ROGER G. WALKER

thickness and sand/shale ratio of the agitated water facies is identical withthe turbidite facies below, suggesting continued sand emplacement byturbidity currents, but into an agitated environment.

In a recent review of associated turbidite and agitated-water sediments(Walker, 1969, 140-1), the author recognised small-scale transitions with­out a regional slope facies, large-scale transitions which pass from tur­bidites by a regional slope facies into agitated-water sediments, and, finally,very large scale transitions from 'flysch' to 'molasse' in which the sedi­mentary sequence is usually interrupted by tectonics. The three transitionsin the Westward Ho! Formation are excellent examples of the small-scaletype.

ACKNOWLEDGMENTS

The author has been interested in the sedimentology of north Devonfor seven years. During this time, many friends have visited the area, andall have contributed to the understanding of the facies. A brief survey ofthe Westward Ho! Formation was included in the author's D.Phil. Thesis,supervised by Dr. H. G. Reading. Detailed work on the Westward Ho!Formation during the summer of 1967 was made possible by the awardof operating and travel grants from the National Research Council ofCanada, and assistance from H. G. Reading. The cost of publication ofthe folded figures and Plates has been met from the author's N.R.C.operating grant. Helpful comments on the manuscript have been givenby H. G. Reading and G. V. Middleton.

It is a pleasure to acknowledge all of this assistance, although the resultsare the sole responsibility of the author.

EXPLANATION OF THE PLATESPLATE lA

Low Tide Sandstone Member, showing thick, massive, fine-grained parallel beddedsandstones and interbedded shales. No sedimentary structures can be seen in the thicksandstones. Stratigraphical top to left

PLATE 18

Ripple cross-lamination in Unit 2, showing continuity of silty and muddy laminaeacross the ripple system. This implies fallout of sediment from suspension duringrippling. Thickness shown, 16 em.

PLATE 2A

Patio Pool Sandstone Member (with hammer). Below the sandstone, to left, are silt­stones with many thin rippled sandstone layers implying an agitated environment.Above (right) is a series of black mudstones, very fine grained and without sandstonelayers, implying a quiet. non-agitated environment. The banding seen in the blackshales is due to very slight differences in grain size (cf. facies A of de Raaf and others.1965, fig. 3). The top of the Patio Pool Sandstone is very sharp, with no sign ofbioturbation

PROC. GEOL. ASS., VOL. 81 (1970) PLATE 1

A

B

[To face p. 64

PROC. GEOL. ASS., VOL. 81 (1970) PLATE 2

A

B

PROC. GEOL. ASS., VOL. 8 1 (1970) PLATE 3

A

B

PROC. GEOL. ASS., VOL. 81 (1970) PLATE 4

A

B

PROC. GEOL. ASS., VOL. 8 1 (1970) PLATE 5

A

B

PROC. GEOL. ASS" VOL. 8 \ (1970) PLATE 6

PROC. GEOL. ASS ., VOL. 81 (1970) P LAT E 7

A

B

PR O C. GEOL. ASS., VOL. 8 1 (1970) P L AT E 8

To face p . 65J

THE WESTWARD HO! FORMATION 65

PLATE 2B

Thin siltstones of Unit 3. Each bed has a sharp base, and is graded. Parallel laminationcan be seen in three beds, and in the uppermost, the parallel lamination grades intocross-lamination. These thin graded siltstones are interbedded only with dark mud­stones, and are interpreted as turbidites. Current flow left to right, scale in inches.Photographed near sewer inspection structure (42422916), where these beds are bestexposed

PLATE 3AIrregularly cross-laminated siltstones and mudstones, photographed about 10 m. abovePlate 2B. Note the increase in ripple cross-lamination in the finer, muddier bedsbetween the siltstones as compared with Plate 2B; this indicates general agitation ofthe basin. Note also that at least two of the siltstones (tip of hammer shaft, centre ofshaft) show parallel lamination grading upward into ripple cross-lamination. Theauthor suggests that these beds were turbidity current emplaced (as were the beds inPlate 2B), hence accounting for the high flow regimes implied by the parallel lamina­tion. However, by contrast with Plate 2B, they were emplaced into an agitated environ­ment, with reworking of most of the turbidity current-introduced silt and sand intoirregular ripple cross-lamination. These two particular beds appear to have escapedreworking, and by comparison with Plate 2B, suggest their turbidity current origin. Itis stressed that Plate 3A itself contains little or no evidence of turbidity current activity-such an interpretation is only possible by comparison with the beds below, whichhave not been reworked, and by the position of Plates 2B and 3A in the overallregressive facies sequence

PLATE 3B

Minor scour within major scour surface number 4 of Fig. 5. The facies filling the scouris identical with that outside the scour

PLATE 4A

Detailed view of laminae just above base of scour number 4. Individual laminae arewavy and discontinuous, and can be seen to swell into cross-laminated ripple lenticles.Length of pen 15em.

PLATE 4BBrecciated base of Sandstone F resting sharply on black mudstones (partly covered byrock pool in foreground). In all places there is a sharp, discontinuous sand layerseparating the breccia from the black mudstone, and the base of this sand layer hasflute casts indicating flow toward 2650

• Above the breccia is a series of graded sand­stone beds interpreted as turbidites. The breccia itself was probably formed by thesudden disruption of several turbidite layers

PLATE 5ABase of Sandstone G shown truncating the underlying banded black mudstones. Morephotographs of this same channel are given in Walker, 1966a

PLATE 5BSmall scour just below scour surface 5, Unit 6, Fig. 6. Note that the dark silty mudstonefilling the 'scour' is identical in facies with the surrounding sediment. There is no signof coarser sediment at the 'scour' margin. If cut by a current, it seems peculiar that thesame current did not introduce some coarser sediment. The alternative, that the scouris in fact a slump-scar, is discussed fully in the text

PROC. GEOL. ASS., VOL. 81, PART I, 1970 s

66 ROGER G. WALKER

PLATE 6Sharp base of Mermaid's Pool Sandstone resting on banded dark silty mudstones (onleft) at 41992902, on the coast. The progressive coarsening-upward seen in the TorridgeEstuary section is not present on the coast. The lower beds of the Mermaid's PoolSandstone show large-scale cross-bedding, and grade up into finer grained, thinner,ripple cross-laminated beds

PLATE 7ALenticular sandstones and interbedded ripple cross-laminated siltstones in the lowerpart of the Mermaid's Pool Sandstone. Stratigraphic top to right

PLATE 7B

Isolated, rather asymmetrical ripples of silt 'floating' in mud (linsenschichten). Smallburrows on the bases of the sandstones can be seen. Upper part of the Mermaid's PoolSandstone. The sandstone bed (centre) is 4 em. thick

PLATE 8Silt-filled channel, showing very irregular wall contact, with eroded silt flakes (arrowed)incorporated in channel fill. Unit 6, above scoured surface 4; see Fig. 6. Note 5 em.scale, lower right

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CROWELL, J. c., R. A. HOPE, J. E. KAHLE, A. T. OVENSHINE & R. H. SAMS, 1966. DeepWater Sedimentary Structures, Pliocene Pico Formation, Santa Paula Creek,Ventura Basin, California. Calif. Div, Mines Geol., Spec. Rept., 89.

DE RAAF, J. F. M., H. G. READING & R. G. WALKER, 1965. Cyclic Sedimentation inthe Lower Westphalian of North Devon, England. Sedimentology, 4, I-52.

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THE WESTWARD HO! FORMATION 67

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WALKER, R. G. I964a. Some Aspects of the Sedimentology of the Shale Grit andGrindslow Shales (Namurian RiC, Derbyshire) and the Westward Ho! andNortham Formations (Westphalian, North Devon). D.Phil. Thesis, OxfordUniversity.

----. 1964b. Some Aspects of the Sedimentology of the Westward Ho! andNortham Formations. Proc. Ussher Soc., 1, 85-7.

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WOOD, A. & A. J. SMITH. 1959. The Sedimentation and Sedimentary History of theAberystwyth Grits (Upper Llandoverian). Q. JI geol, Soc. Lond., 114,163-95.

Roger G. WalkerDepartment of GeologyMcMaster UniversityHamilton, OntarioCanada