A new look at the stratigraphy, sedimentology and ammonite fauna of the Corallian Group (Oxfordian) of south Dorset

A new look at the stratigraphy, sedimentology and ammonite fauna of the Corallian Group (Oxfordian) of south Dorset J. K. Wright WRIGHT, J. K., 1986. A new look at the stratigraphy, sedimentology and ammonite fauna of the Corallian Group (Oxfordian) of south Dorset. Proc. Geol. Ass., 97 (1) 1-21. This paper presents the results of the first systematic study of the Corallian Group of south Dorset to be made since the 1930's. The Group is divided into six formations and descriptions of each formation are presented. The members of each formation are defined, and the stratigraphic nomenclature revised. The environment of accumulation of each member is discussed in relation to Corallian sedimentation in southern England. The Middle and Upper Oxfordian ammonite faunas are those of the Sub Boreal N.W. European Province. A full description of the zonal scheme is presented here, and several Upper Oxfordian ammonites figured. Department of Geology, The New College of Royal Holloway and Bedford, Egham, Surrey TW200EX

thicknesses seen when comparing the Osmington area with the Weymouth area. Where only one thickness is given, this indicates that that in the Weymouth area is unknown. All localities are shown in Fig. 1.

1. INTRODUCTION The Dorset coast offers the most complete and easily accessible sections through the Corallian beds in Britain. The exposures have attracted the attention of a considerable number of stratigraphers, sedimentologists and palaeontologists over the last 60 years and a considerable literature has amassed. For references to the more significant papers, the reader is referred to Fiirsich (1977), Wright (1980) and the 1978 edition of the Geological Survey Memoir (Arkell, 1947). Despite this considerable literature, the beds lack a systematic modern synthesis, and the aim of this paper is to place the stratigraphy of the south Dorset Corallian beds onto a similar footing to that of the rest of the British Corallian.

(b) Nothe Formation Though Buckman (1925) proposed the subdivision of the formation into three members, this has not been accepted by subsequent workers, and no subdivision is attempted here. Nothe Grit, as used originally by Blake & Hudleston (1877), included the overlying Preston Grit. This unit is now regarded as the basal member of the Redcliff Formation to be described below. Blake & Hudleston named the Nothe Grit after the '30 ft (9 m) of calcareous sandstone with hard bands of grit well exposed beneath the Nothe Fort at the end of Weymouth Pier' (SY 687788). The comment by Arkell (1936, 1947, 1963) that Blake & Hudleston's section had been 'completely covered by masonry' is misleading. The masonry was there when Blake & Hudleston visited the section, and their reference is to the large area of reefs exposed at low tide, wherein the top 5 m of the Nothe Grit is well exposed. A log of the section is given in Fig. 2 for comparison with that of the more complete exposure at Reddiff. Though it is not practicable to use identical bed numbers, there is a general correspondence between the sections. Both consist of argillaceous, very fine grained sandstone with the frequent development of calcareous concretions. An upwards-coarsening trend is reversed about 3 m from the top, but then becomes marked in the highest 2m. The lowest beds seen at Nothe come about the middle of the Grit and contain some of the largest concretions, up to 1 m in size. They are succeeded by a persistent calcareous sandstone (2) which occupies a

2. STRATIGRAPHY (a) General The Corallian Group is here divided into six formations, the Nothe, Reddiff, Osmington, Trigonia clavellata, Sandsfoot and Ringstead Formations (Table 1). The Redcliff and Ringstead Formations are new. The former is roughly equivalent to the Stour Formation of north Dorset and the Highworth Formation of Wiltshire, both of which similarly lie in between 'Lower Calcareous Grit' and Osmington Oolite. The Ringstead Formation is used here for the highest Oxfordian beds in preference to the 'Passage Beds' of Blake (1875). Neither Cope (1980), Cox & Gallois (1981) nor Birklund et al., (1983) have used the name Passage Beds, and its attempted revival by Brookfield (1978) is unlikely to be accepted. The whole scheme is given in Table 1 below. The thicknesses given in the table illustrate the range of

1

2

J . K . WRI GHT

TABLE 1. Stratigraphic subdivisions of the Corallian strata of so uth Dorset Form ation Ringstead Sandsfoot 0-

::> 0

~

0 Trigon ia clavellata Z
:J -J


~

0

Memb er Osmington Mills Ironstone Ringstea d Clay

Thickness (m) 0.15- 0.2 3.5-5 .0

Sandsfoot Grit Sandsfoot Clay

7.2-11.3 5.0- 15.5

Red Beds Clay Band Chief Shell Beds Sandy Block

2.32 0.6 1.35-2 .12 1.52- 2.27

Nodular Rub ble Shortlake Upt on

1.6-3 .9 5.9-7 .7 7.9- 13.2

U

Redcliff

Bencliff Grit Nothe Clay Preston Grit

6.7 10.0-13.5 1.8

Z

~::;

w O O-~ 0..0 ::>~

><

0

Z w
~~ 0

Z

~:5 ~ Nothe Grit

Not subdivided

large area of the wave cut platform. The bed has extensive networks of infilled burrows and contains a substantial bivalve fauna, with Lopha gregaria (J . Sowerby) , Chlamys splendens (Dollfuss) , C. fibro sus (J. Sowerby ) and Gryphaea dilatata J. Sowerby . Peltoceras sp. and Cardioceras (Vertebriceras) sp. also occur. Bed 6 is similarly highly bioturbated . The onl y complete secti on of the Nothe Grit is at Ham Cliff (SY 712817). Th e thickness is 9.3 m, though only the lowest few metres are normall y accessible. The junction with the Oxford Clay is very sharp , and examination of specimens with a binocul ar microscope shows that the Oxford Clay had been consolidated to a partially calcareous mudstone before the Nothe Grit was deposited upon it. A non- sequence at this horizo n must be assumed. The bed s seen only at H am Cliff comprise a 3 to 4 m upwards-coarsening sequence progressing from sandy, argillaceous siltstone to massive, fine grained sand stone . The remainder of the succession is well exposed in the low cliffs west of Redcliff Point. No.4 (Fig. 2), like its counterpart at Nothe , is bioturbated but less fossiliferous and with fewer concretions. No . 5 represents a lithology not de veloped at Nothe , a true Rhaxella spiculite in which scattered sept arian concretions enclose ammonites preserved in honey coloured calcite . The re is then a steady progression towards sand at the coarse end of the fine scale in beds 7 to 9. The Preston Grit form s the highest 1.55 m of the cliff and is described below . Other exposuresr- a 6 m section occurs in the cliff

9.3

WO~

~~
000-J~---0

and wave cut platform at Osmington Mills (SY 735816). It ranges from beds 3 to 8 of the Redcliff notation, though exact correlation is impossible. A not able occurrence is a concretionary bed crowd ed with lsognomon cf. cordati (Uhlig) which is 1.5 m above the base of the section . No Preston Grit is exposed here. The highest 2 m of the Nothe Gr it is exposed in the banks of the East Fleet at SY 648775.

(c) Redcliff Formation

This formation groups together Buckman's Preston Grit with Blake & Hudleston's Nothe Clay and Bencliff Grit. Arkell used the out dated term 'Berkshire Oolites' equivalently. It is named after the section at Redcliff figured by Ark ell (1947, fig. 13). Th is is the type locality for the Preston Grit , the best exposure extant of Nothe Clay, and the most fossiliferous locality for the Bencliff Grit. A rkell (1936, 1947) rejected Buckman's term Preston Grit , preferrin g to use Trigonia hudlestoni Bed. Howe ver , Arkell was always ambivalent on the matter, and in the conclusions to his monograph (Arkell , 1935-48, p. 385) only Preston Grit appears. Buckman's name has clear priority and is used here . As far as the Nothe Clay and Bencliff Grit go, th ese are exact ly as defined by Blake & Hudleston (1877). It should be noted th at the Nothe Clay is by no means all clay, but cont ains considerable beds of limestone toward s its base , more so at the type locality.

3

C O R A L LI A N OF SOUTH DORS ET ~ ~ JORDAN : 82 " '~ " " " " " "" " " ' H I L L " " + " " " " "" " " """

~

FURZY ~

:

curr :

1

6.9

ar -.

70

i

.FARM

:

-----!- Fu,zedown ~ 1.. 1 ; Clay: i EAST : FLEET : i ••,i••••••••••••••••••••••••••••••••••

j Nathe Grit 1/

: ···77·····,

:

.,,"' """

71

72

!.:

73

·· r · · · ··· ··

L

I1

: jj . ....... •••.• ...:. ..... .••...... .... .... ... .... .....

: OSMINGTON .

MILLS

Bp~~T :

···t·

RINGSTEAD BAY

·r · · · · · · j""

80'"

.

i :

~ :

i

i l

I

j

~

~STERN

SANDSFOOT CASTLE COVE Osmington j j CASTLE . 1 Oolite ~ Trigonia ~ i /. clavellata Beds : :

~ :

:

REDCLIFF

· +· ·· · ···~~~·

. .79·· · · l"~:~·~~~~~~······T·············· · · · · ···~·~r~. ~ .~.:..~

: :

+

~6

i5

i :

+

:

~':"

I

i· · · · ·

SHORTLAKE HOUSE.

+

. LEDGES

~

1

1 i /' i i PORTLAND HARBOUR ····l·································t·· ·····························1·································!······77········ :

~

~

~

~

~

!

i

;

!

!

Fig. 1. Map of the area showing the principal Oxfordian localities .

(i) Preston Grit Mem ber This member forms the highest 1.55 m of the low cliff extending from Bowleaze Cove to Redcliff Point. Though generally comprising one massive bed , this has two contrasting parts. The lowest 0.27 m consist of soft, argillaceous, fine grained sandsto ne. This rests with a shar p break on Nothe Grit (Fig. 2), and crushed ammonites can often be found resting on this surface . The bulk of the Preston Grit consists of medium grained calcareous sandstone , shelly , and sometimes pebbly, with immature ooliths visible in thin-section . On dissolving the rock in acid, tests of the small foramini fera Cribrostomoides canui (Cushman) are commonly found in the 250,u + fraction , silicified and thus not affected by the acid. Many Cora llian foramin ifera were figure d by Gordon (1965). Arkell (1936, 1947) has published a long list of the macrofa una which is dominated by Chlamys fibrosus and Pleuromya uniformis (J. Sowerby) and the trace fossils Thalassinoides and Rh izo corallium Am monites occur infrequently, but are well prese rved with the inner chambers filled with micrite.

At the southern end of Nothe Point (SY 685786), the Preston Grit is exposed in the wave cut platfor m. Bed 8 (Fig. 2) is a massive, fine to medium grained shelly sandstone with excellent Thalassinoides network s weathering out . The highest 0.45 m seen consist of medium gained sandy limestone with a layer of Myoph orella hudlestoni (Lycett) on the top surface. Much of the carbo nate fraction consists of ooliths and shell fragments, particularly Trichites sp. Fine grained , sandy , oolitic limestone almost certain ly representing the Preston Grit occurs at the top of the East Fleet section at SY 648775. However, there is no sedimentological break to enable one to separate this bed from the Nothe Grit below as has bee n done for the two main exposures . This may be due to bioturba tion carrying medium grained sand down into the Nathe Grit. (ii) Not he Clay Membe r Blake & Hudleston defined this unit as the '40 ft (12 m) of clays contai ning hard bands of calcareo us

4

J . K . WRIGHT

REDCLIFF NaTHE POINT

Nothe Clay

2m

o

. '.' ~ r ,

Nothe

I

:i': : .~ ro @.. . . . . . . . . .

I ....

Grit

CD ::\. ::::: ;f-.:'

.......

. :.:. ~.:.: .'~

.

l::::J f£d

htTI

.

.

.. .. ...~

Sandstone Sandy limestone

LOWEST BEDS NORMALLY SEEN AT REDCLIFF

4

',c--:---, . .

: ~::: ~:.

Calca reous sandstone

~~E Shaly siltstone 1=---

~====

)b

Clay Thalassinoides

.....~ Rhizocorall ium

~

Small burrow systems

1 ---------------------------

=:===::=:=:=:=:=:=:=:=:=:=:=:==

Fig. 2. Section of the Nothe Grit Format ion and Preston Gri t Member as exposed at Redcliff and Ham Cliff, com pared with a log of th e same beds see n at Nothe Point. Th e trends of the grain size analyses are given as cumulative graphs on the left hand side of eac h column.

5

CORALLIAN OF SO UTH DORSET

grit towards the base exposed in the bay south of the town' (Weymouth). Arkell (1936, 1947) reported that the section had been obliterated by the laying out of the cliff as public gardens in 1930. However, Blake & Hudleston's 'hard bands of calcareous grit'-in fact limestone-were not obliterated, but form prominent reefs along ! km of rock platform. In order to get a more complete view, however , it is best to consider the Redcliff section first. A drawing and detailed section at Redcliff were given by Ark ell (1947, pp. 51-5 2, fig. 13). Unfortunately, Arkell omitted the lowest of several prominent beds of limestone exposed here , and his numbering scheme for the limestone beds is thus of limited use . A new, revised section measured at SY 706818 is given in Fig. 3. The transition from the Preston Grit is gradual (Fig. 2), with a progression over several centimetres from argillaceous , medium grained sandstone into sandy clay. The lowest clay then becomes very fine grained, with frequent holothurian spicules. The clays in between the limestones arc sand y, with scattered ooliths , approaching a marl in compo sition. Above Limestone No.4 , the clays are uniforml y very fine grained with less than 1 per cent fine quartz sand and sporadic Lenticulina mu ensteri Roemer. Several of the limestones have very distinctive lithologies. No. 1 is fine grained, iron rich and bioclastic. No .2 is the most distinctive of all, a white weathering shelly micrite, extensively bored , with a variable oolith content. lsognomon d. subplana (Etallon) , Liostrea sp. and Pleurom ya uniformis are commonl y found in loose blocks. No.3 conta ins lenses of shelly oolite . Nos. 4, 5 and 6 are all bioclastic, iron rich, with scattered pale green chamosite ooliths. No. 7 is white and nodular. At the type locality at Rodwell (SY 684785) the major part of the lower 8 m is exposed as a gently dipping succession in the wave cut platform . As is clear from Fig. 3, almost 50 per cent of the succession is limestone. The use of the same bed numbers as at Redcliff is intended to give a suggestion of the correlation of the sections. There is the same general sequence, beginning in the lowest beds seen with impure , micritic limestones (1 and 2) , succeeded by sandy oomicrite (3 and 4) with shelly, oolitic clay in between. The highest limestone s (5 to 8) are all fine, sandy, shelly and argillaceous , but without the siderite seen at Redcliff. The sequence from No .6 through to No. 8 is exposed above the promenade, the clay in between 6 and 7 containing large pebbles of sandy, micritic limestone extensively bored by Lith ophaga sp. and encrusted with Nanogyra nana (J. Sowerby) and Serpula sp. Some 5 m of clay must be present beneath the gardens above the promenade. The highest Nothe Clay , and its junction with the Bencliff Grit , is exposed in the base of the cliff east of

RODWEll REDCLIFF 8

7

6=

~ Clay

------

§

5

Marl

~ S he lly t::::::::j Micrite

-- -- - -

~ oomicrite

4

L7L7 Siderite

- - --- -

3

\

NOT EXPOSED

\ \ \ \

\ \ \ \

\, PRESTON GRIT

Fig. 3. Section of the lower part of the Nothe Clay as seen at Redcliff compared with a log of the beds seen at Rodwell. The author's suggested correlation is indicated by equivalent bed numbers .

Osmington Mills (SY 737816). There is a gradual increase in fine quartz sand content to 5 per cent immediately bene ath the Bencliff Grit , with 10 per cent coarse silt. Crushed Modi olus bipartitus (J . Sowerby) are common at this level. Towards low tide mark , several beds of limestone are seen at SY 737815. (iii) Bencliff Grit Member Blake & Hudlest on 's type section was 'east of Sandsfoot Castle'-in fact at Rodwell at the southern end of the bay mentioned above. The cliff exposure has now been obliterated by an access road and banking, and there is no beach exposure. A new standard section is obviously needed , and the only one possible is in the cliffs east of Osmington Mills, where the following section is excellentl y displayed :

6

J. K. WRIGHT

Section 1, the Bencliff Grit Member east of Osmington Mills (SY 740814)

(Argillaceous, sandy oolite - OSMINGTON OOLITE FORMATION) - erosive junction 5. Flaggy, bioturbated, calcareous, fine grained sandstone containing oolitefilled borings. 4. Massive, fine grained, laminated sandstone showing a beautiful series of intersecting scour channels.

m 0.15

(e) Osmington Oolite Formation

1.55

- erosive junction3. Clay with fine sand lenses passing up into bioturbated, very fine grained sand with oblique, infilled burrows. Up to 0.40 2. Strongly cross-bedded, argillaceous sandy siltstone with impersistent clay bands and marked scour-and-fill crossbedding. Two rows of large calcareous concretions are developed towards the base. 3.96 - erosive junction1. Bioturbated sandy marl, weathering rapidly, but having calcareous lenses. Oblique infilled burrows 5 mm across are common and frequent Lenticulina muensteri are found in the medium seive fraction.

charcoal occur in many blocks. A shelly macrofauna is present, the distinctive cross-bedded blocks with Diplocraterion having yielded to the author Goliathiceras spp. and Perisphinctes spp. (five ammonites in all) plus Myophorella sp. On the shores of the East Fleet (SY 652772) there are several blocks of very fine grained sandy limestone again with Diplocraterion burrows.

0.68

(Black, silty clay-NOTHE CLAY MEMBER) The Bencliff Grit comprises a series of thin, bioturbated, mostly argillaceous sands and silts (1,3 and 5) separating two thicker incursions of crossbedded, silty sand (2 and 4). There is a general coarsening-upwards, with unit 2 containing 50 per cent fine quartz sand, while unit 4 contains 70 per cent. The sediment is markedly ill sorted, with the weight in the 44/-1 and 88/-1 fractions combined exceeding that in the 63/-1 fraction in unit 2 and being two thirds of it in unit 4. Numerous small intraclasts occur. The marked lamination of the sediment is due to thin films of carbonaeous debris, with scour channels spectacularly picked out. Very occasional Diplocraterion burrows can be found in the calcareous concretions which litter the beach. The strong smell of oil in many sand samples suggests that this was once a reservoir sand. Tracing the beds westwards, though no section is visible at Redcliff, many blocks of the two concretionary bands are present on the beach. Trace fossils are numerous. Many were figured by Fiirsich (1975). The burrows of Diplocraterion and the traces of Scolica and Gyrochorte are specially well preserved. Large fragments of wood and rounded fragments of

The term Osmington Oolite was first used informally by Blake & Hudleston (1877). Arkell (1933) applied the name to the 19 m succession of limestones, sandy limestones and clays excellently exposed in the cliffs between Osmington Mills and Bran Point (SY 740814 to 742813). The unit was classed as a formation by Wright (1980). Arkell (1936, 1947) divided the formation into a series of beds, mostly with informal names (Pisolite, Upper White Oolite, etc.), and these have been adopted by subsequent authors. Several of the names are used here, though 'Nodular Clay' is used in preference to 'Littlemore Clay Facies'. On a larger scale, the formation divides naturally into three units, each of distinctive character and recognisable over the whole area. These are proposed here as formal members of the Osmington Oolite Formation (Fig. 4). The Upton Member (type section in the cliff and rock platform below Upton House, Bran Point) consists of a regularly bedded succession of sandy limestones and clays, individual units being traceable over several kilometres. The Shortlake Member (type section in the rock platform between Shortlake Steps and Black Head) contains cross-bedded oolites, bioturbated oolites and clays. Correlation of individual beds is only possible over short distances. The Nodular Rubble Member was defined as the Nodular Rubble Limestone by Arkell (1936) at Bran Point. It consists of irregularly bedded limestone and marl, calcareous concretions being predominent in the west. A problem for subsequent workers has been Arkell's failure to appreciate the cross-cutting nature of the Shortlake Member. He assumed that beds of oolite appearing at differing heights above the Nodular Clay were of different ages. These beds were named the Lower, Middle and Upper White Oolites. To take one example, the Middle White Oolite at Bran Point can be traced continuously into the Lower White Oolite, resting on Nodular Clay, at the western end of the cliff. Arkell's subdivisions of the Shortlake Member are best avoided. In the east, the Bran Point bed numbers can be used as a reference system, prefixed here with an 'A'. Arkell's thicknesses are not always absolutely accurate, and remeasured thicknesses are given below, with reference to localities. (i) Bran Point (SY 742813)

CORALLIA N OF SOU T H D O R S ET

The Upton Member begins with two beds of argillaceous, immature, sandy oolite (AI , 1.75 m). Oolite-filled burrows extend for several centimetres down into the Bencliff Grit, the junction being distinctly erosive . Small pebble s of quartz and medium grained quartz sand occur in the lower bed . Above is the Chlamys qualicosta Bed (A2, 0.57 m), a coarse , shelly oosparite with little terriginous sediment. Nanogyra nana is common and Nautilus sp. an interesting rarity. A thin , calcareous clay (A3, 0.9 m) is succeeded by a classic Corallian pisolite (A4 , 0.45 m) . The argillaceous, silty matrix contains abundant 1 em algal pisoliths and numerous fragments of N. nana. A5 is a substantial clay unit, the Nodular Clay (3.8 m). Three irregularly developed rows of calcareous nodules occur in the lower half, whereas in the upper half, three prominent double rows of nodules have an irregular shape suggesting an origin in the infilling of Thalassinoides burrows. At Bran Point, the Nodular Clay is succeeded by a series of predominently clastic sediments (A6a). Sandy , bioturbated marl (40 em) is followed by alternations of calcareous sandstone and sandy micritic limestone (45 cm). The base of the Shortlake Member is taken at the first appearance of oolite with infilled Diplocraterion burrows. At Bran Point , there are two beds of oolite with Diplocraterion separated by sandy , shelly marl, 1.2 m thick in all. The higher bed of oolite takes on a strongly cross-bedded nature westwards , and cuts right through A6a to rest on Nodular Clay in the cliff section abo ve the Bencliff Grit (SY 741814, Fig. 4). The remainder of the Shortlake Member is much more regularl y bedded. A rubbly , argillaceous oolite (A7 , 0.9 m) is succeeded by a persistent clay (A8 , 2.1 m) . The subsequent shelly, oolitic incursions (A9 and All) show no sign of cross bedding. The Nodular Rubble (A12) contains beds of markedly nodular, cream coloured limestone alternating with calcareous clay in 0.5 m bands. Wilson (1968b) has shown that the nodular, calcareous structures form in areas initially occupied by Thalassinoides burrows. (ii) Black Head (SY 725819) A complete section from the Bencliff Grit to the Trigonia c1avellata Formation can be seen in the shore section here, low spring tides being necessar y to gain access to the Upton Member. The section is 3 m thicker than that at Bran Point (Fig. 4). It is possible to use Arkell's Bran Point bed numbers with certamty. Immature oolite (AI) again rests on the Bencliff Grit , but there is little development of the Chlamys qualicost a Bed (A2). In contrast, the Pisolite (A4) is more thickly developed (0.75 m) , The Nodular Clay contains fewer nodules than at other localities, irregular beds of micrite being developed in their place. The Upton Member succession is then cut short by

7

an erosive contact below the lowest Shortl ake Member oolite. After several minor cross-bedded incursions, a tremendous spread of cross-bedded oolite with westerl y dipping foresets up to 1 m high extends continuously from Osmington Mills to Shortlake Steps. A7 is substantially developed as well bedded , bioturbated oolite . A9 and All consist of shelly oosparite with no sign of cross-bedding. The Nodular Rubble contains excellent Thalassinoides networks. (iii) Redditt (SY 708819) A large, if sometimes precariously accessible section in the Shortlake Member is available above the landslip. The section demonstrates the very variable nature of this member (Fig. 4), with two sequences of cross-bedded oolite separated by delicately laminated oolitic marl. The upper cross-bedded oolite has foresets dipping predominently eastwards in contrast to the westerly dip of the lower sequence . Profuse shale pellets indicate considerable nearby scouring . Below, the Nodular Clay with its associated concretions has slipped in considerable quant ities onto the beach . Frequently, the concretions have formed inside Perisphinctes, several species of which have been found loose on the beach.

(iv) East Fleet (SY 653771 to 657770) This section in the low, slipped cliffs of the Fleet lagoon fulfills an important role now that other exposure s in the Weymouth area are no longer accessible . There are marked difference s compared with the Osmington sections (Fig. 4). A measured section was published by Arkell (1947), but this contains errors in the correlation of the higher beds and inconsistencies in thickness measurements. The East Fleet section has been remeasured carefully in the course of this work, and the author's version is as shown at top of page 8. The Nodular Clay (Bed 1) is exposed in the beach. It is a very fine grained calcareous clay containing only 0.5 per cent fine quartz sand. Lenticulina muensteri is common , also holothurian spicules and bivalve fragments. Nos. 2 and 3 represent high Upton Member beds not seen fully elsewhere due to downcutting of the Shortlake Member. The bioturbation in the se beds is remarkably well developed , being largely attributable to Teich ichn us . The Short lake Member is again dominated by oolite in its lower part , here largely non-cro ss-bedded. After a rare quartz sand incursion in the base of bed 6, there is a thick clay succeeded by oomicrites (8, 9 and 10) which correlate best with the shelly upper oolites of Osmington . The Nodular Rubble , her e as elsewhere in the Weymouth area, contains large calcareous concretions rather than the infilled Thalassinoides burrows abundant in the east.

8

J. K. WRIGHT

Section 2; the Corallian strata exposed in the cliff of the East Fleet lagoon (between SY 653771 and 657770) TRIGONIA CLAVELLATA FORMATION, CHIEF SHELL BEDS MEMBER 13. Flaggy, sandy limestone, sporadically oolitic, with Myophorella sp. and other bivalves.

m seen to 0.55

SANDY BLOCK MEMBER 12. Flaggy, fine grained, shelly sandstone with small, phosphatic pebbles consisting of internal moulds of gastropods. The rock is well cemented apart from an argillaceous band 0.3 to 0.4 m up. Sand filled burrows extend down into the Nodular Rubble.

m

1.20

OSMINGTON OOLITE FORMATION, NODULAR RUBBLE MEMBER 11. Three prominent beds of concretionary limestone separated by sandy clay. The limestones contain glauconite, finely comminuted shelly debris, and 7.5 per cent quartz sand.

1.45

SHORTLAKE MEMBER 10. Shelly oomicrite with Pleuromya sp. and numerous gastropods including Nerinea sp. and Pseudomelania sp. 9. White oomicrite with many Nucleolites scutatus (Lamarck). The bed has the appearance of a pisolite, though the 'pisoliths' are large, coated shell fragments not apparently of algal origin. 8. Tough, shelly micrite with Chlamys sp. and occasional gastropods. Ooliths are present but very scattered. 7. Pale grey clay, poorly exposed. 6. Poorly sorted, brown oosparite resting on ripple drift laminated, flaggy calcareous sandstone. 5. Pale grey clay. 4. Well bedded, flaggy weathering oolite, some beds cross-bedded towards 200 The oolite is well sorted, with some micrite remaining.

0.85 0.57 0.60 3.50 0.57 approx. 0.60

0

•

2.50

UPTON MEMBER 3. Fine grained, highly bioturbated sandy clay containing 30 per cent fine quartz sand with scattered ooliths. 2. Series of tough, highly bioturbated, concretionary limestones alternating with highly bioturbated, sandy siltstones. Scattered ooliths and finely comminuted shell debris are present. 1. Light grey clay containing regular rows of calcareous modules. (f) Trigonia c1avellata Formation

Blake and Hudleston (1877) originated this formation as the 'Trigonia-beds' of Weymouth, They specified a type section 'on the coast near Weymouth'presumably the one at Castle Cove described below. They gave a measured section of 28 ft 6 ins (8.7 m) of strata. However, this included several beds, particularly 2 m of 'blue weathering, hummocky limestone', which would now be allocated to the Nodular Rubble Member of the Osmington Oolite. They made no attempt at formal subdivision of the Formation. Arkell (1936) chose as type section that at Bran

0.88 2.50 seen to several metres

Point (SY 743813). He gave detailed measured sections of most of the coastal exposures, these also being reproduced in his monograph (Arkell, 1935-48) and in the Geological Survey Memoir (Arkell, 1947). Arkell's fourfold subdivision of the Trigonia Beds (Sandy Block, Chief Shell Beds, Clay Band and Red Beds) has been followed by almost all subsequent workers. However, Talbot (1974) used Trigonia clavellata Beds as synonymous with Chief Shell Beds, a procedure rejected here. Both Talbot and Brookfield (1978) were chiefly concerned with palaeo-environmental aspects and described the sediments in only general terms.

9

CORALLIAN OF SOUTH DORSET

BLACK HEAD EAST FLEET

BRAN POINT

CASTLE COVE ~--4J..,..J1A13

SANDY BLOCK

NODULAR RUBBLE MB.

5

m

4

3 2

o A2 Major fossiliferous beds KEY~

a:tli 001 ite

f

Gastropods

eOomicrite

~

Bivalves

~Micrite

<6S'

Echinoids

Argillaceous ~ limestone

U

Diplocraterion

o

~ Oolitic clay

~

Sandy clay

~Clay

D

Fine sand

UPTON MB.

------------------~==~~

BENCLIFF GRIT

)"" Thalassinoides

\~~~I

General bioturbation

~o Shell fragments o~o Nodules

@),OJ

lQ~ Pisolite

Fig. 4. Sedimentary logs of the principal exposures of the Osmington Oolite Formation in the Weymouth district, with the subdivision into members adopted in the present paper, and the bed numbers of Arkell (1947) on the right hand side.

The Bran Point section has deteriorated considerably in recent years, and is difficult to make out. Black Head (SY 726819) offers a complete, perfectly exposed section at the top of the wave cut platform. The strata can be studied in detail with the opportunity to collect a substantial fauna from loose blocks as the beds rise towards Shortlake Steps. The

Black Head section has thus been chosen as the standard to describe here. (i) Sandy Block Member (2.42 m) Though there is the continuation of irregularly bedded limestone very similar in lithology to that of

10

J. K. WRIGHT

the highest Nodular Rubble Member, close examination of the section shows three points of distinction. Bands of clay in between the limestone beds typify the Nodular Rubble. These are reduced to mere traces of sandy marl in the Sandy Block. Scattered Myophorella clavellata (Parkinson) make their appearance at the very base of the Sandy Block. They are preserved in grey, argillaceous limestone, distinct in colour from the cream coloured limestone of the Osmington Oolite. The position of the junction with the Nodular Rubble then becomes clear. Erosion of this member has left an irregular, undulating surface cut in nodular limestone. Burrow systems infilled with sandy limestone descend 0.4 m into the Nodular Rubble. Laminated, sandy marl drapes over the irregular surface which has a relief of up to 12 cm. There follow four or sometimes five beds of grey, argillaceous limestone containing scattered immature white ooliths and finely comminuted shell debris set in a micritic matrix. The 'Sandy Block' is markedly sandy only in the west of the area. At Black Head the average quartz sand content of the limestone beds is 5.5 per cent. Up to 30 per cent fine sand occurs in the sandy marls in between these beds however. (ii) Chief Shell Beds Member (2.07 m) The distinguishing feature here is the incoming in profusion of M. clavellata. The highest of the beds Arkell included in the Sandy Block is here transferred to the Chief Shell Beds on this basis. There are then five M. clavellata rich layers, the shells largely dissociated and concave upwards, although valves that are still united are commonly found. They are preserved in an impure oolite. The ooliths have become large (c. 1 mm), orange coloured, and are set in a soft, argillaceous matrix. The quartz sand content rarely rises above 2 per cent, consisting chiefly of medium grained quartz grains transported in the centres of ooliths. Between shell beds 3 and 5, finely disseminated siderite is present, and weathers to give a reddish colouration similar to that of the Red Beds. (iii) Clay Band Member (0.6 m) This consists of a thin incursion of silty, shelly, iron-rich clay. (iv) Red Beds Member (2.02 m) The highest member of the Trigonia Beds comprises three alternations of oolite with sideritic micrite, capped by shelly, impure limestone. The oolite layers contain large, 1 mm, white ooliths, These are set, with comminuted shell fragments, in an argillaceous matrix. The layers of tough, grey, sideritic limestone contain only scattered ooliths and fine shell debris, and weather a bright red colour. The matrix of Red Beds fossils thus ranges in hardness from very soft to extremely hard. The succession at Bran Point as given by Arkell

(1936, 1947) is similar, though poorly exposed at present. The thicknesses and lithologies of the Sandy Block, Clay Band and Red Beds are much as at Black Head, but the Chief Shell Beds are only 1.1 m thick (2.07 m at Black Head). Weymouth On the opposite side of Weymouth Bay, there are substantial exposures of the Trigona clavellata Formation in the Western Ledges and low cliffs of Castle Cove. The Sandy Block and Chief Shell Beds Members are best seen in the base of the cliff. The reefs exposed at low tide are covered with an almost impenetrable mass of seaweed. At SY 680778, the fossil wave cut platform in the Nodular Rubble Member is excellently displayed, with the limestone nodules standing proud, the laminated sands of the Sandy Block wrapping round them, and sand filled burrows cutting through. Just above the non-sequence come 15 ern of sandy clay (Fig. 4), and the Sandy Block succession is completed by two tiers of limestone totalling 1.42 m. Both tiers consist of immature, very sandy oolite. Bioturbation is marked, with infilled Thalassinoides in the lower tier. Scattered M. clavellata in life position occur throughout. The Chief Shell Beds, exposed at SY 677777, consist of 2.02 m of variably weathering, sandy limestone. Large rounded shell fragments and scattered ooliths are set in an argillaceous matrix. Bivalves are again common. Fine, silty clay of the Clay Band Member is visible above the Chief Shell Beds, but the section is then obscured. No harder bands outcrop in the beach, suggesting that the Red Beds Member may be present in a soft, easily weathered lithology. The East Fleet section again exposes only the lower part of the Trigonia clavellata Formation, numbered 13 and 14 in measured section 2. Sand filled borings descend into the Nodular Rubble from a 7 to 14 cm bed of fine quartz sand which wraps round the irregular hummocks (SY 65657695). Soft, argillaceous sand (15 em) is then succeeded by 1.2 m of flaggy, fine grained sandstone best seen at the centre of the bay (SY 658770). The highest 0.55 m seen here consists of flaggy, immature, sandy oolite with M. clavellata and Nanogyra nana, and may represent the Chief Shell Beds. (g) Sandsfoot Formation (i) Sandfoot Clay Member The type section of Blake and Hudleston (1877) was at Castle Cove, south of Weymouth. This is now largely grassed over and landscaped, and we must rely on the descriptions of the earlier workers. Blake and Hudleston described '30 ft (9 m) of very calcareous clay with a pyritic fossil bed about the middle of the succession'. Perisphinctes sp. was recorded from this bed. Woodward (1895) described 38 ft (11.7 m) of blue

COR ALLIAN OF SO U T H DORSET

clay and calc ar eou s clay with Deltoideum delta (Smith) abunda nt, plu s A starte sp . and Serpula sp . A cross section drawn thro ugh the a rea suggests that the true thickness is 15.5 m . At present , soft, silty clay can be seen beside the paths running through the gardens. Under favourable beach conditions, th e top 0.5 m of the Sandsfoot Clay ca n be seen overlain with a sha rp, eros ive contact by Sandsfoot Grit (SY 676775) . A t Black H ead (SY 725819), on ly th e lowe r 4 m are pr eserved beneath the overstepping Sand sfoot Grit. Th e section is well expos ed, but lar gely unfossiliferous. (ii) Sandsf oot Grit Mem ber Blake and Hudleston ( 1877) de scribed their type exposur e at Sa ndsfo ot Ca stl e by means of a det ailed

11

measured sectio n of 34 ft (lOA m) of stra ta. H owever , it is almos t imp ossible to equa te their sec tio n with the exposures see n at pr ese nt (SY 676775 to 67277 1). Arke ll (1936) gave a measured section of 7.5 m of stra ta, and thi s int erpretation was accepted by subs eq ue nt worker s. Brookfield (1978) described 6.5 m of stra ta , and intro duced subdivisio ns based on R om an numerals I to III. A re-measurem ent of th e expos ures by the aut ho r has show n that B rook field 's un its I to III sho uld be I to V, units IV and V , ap pearing at the so uthern end of the cliff expos ures , havin g been confused pr eviously with a re -appearance of unit s I to III . Th e tru e tot al th ickn ess is thu s 11.3 m (Fig. 5). Unit I forms a small reef at the southern end of Castl e Cove. It ca n be classed almos t as a limeston e ,

Section 3, the Sandsf oot Grit Mem ber at Sandsf oot Castle (R INGS TEA D CL AY , poorl y exposed) U nit V 9. Tough , iro n rich , fine to medium gra ine d sandsto ne with a red , iron sta ine d top surface . Scattered cha mosite oo liths ar e present. Th e be d is hea vily bio turbate d by Thalassinoides , and co nt ains Gon iom ya sp . and Liostrea sp. 8. M assive , brow n weatherin g , fine to med ium grai ne d cha mosi tic sandston e , very so ft and argillaceous. 7. Grey, fine to medium gra ine d , calcar eous sandston e , heavily bioturbated with the burrow infillings weather ing out. Fr agmentary Chlamys midas (Damon ) are common , with Liostrea sp . and Pleurom ya sp . Uni t IV 6. Yell ow wea the ring , ligh t grey, slightly sandy clay. U nit III 5. Argilla ceou s, iron rich , lar gely fine gra ine d cha mos itic sa ndsto ne. Distinctive patches of light grey micrite are present , co ntaining well preserved cha mosite ooliths . Ringsteadia spp. and Microbiplices anglicus A rkell wea the r o ut. 4. Tough , pr ominent , sideritic pebbly sands to ne with a fine or fine to medium grained matrix. Bioturbation is str ong , but the co nsiderable fauna well preserved : Ringsteadia sp., Pinna sandsfootensis (Arkell), Chlarnys midas , Ctenostreon sp ., Liostrea delta, Isognom on sp ., and Goniomya sp . 3. T ou gh , well be dded, fine or fine to med ium grained sa ndstone, iro n-rich , with po orly prese rved cha mosi te oo liths. No t par ticul arl y fossiliferous , with Pinna sandsfootensis and Trichites sp .

m 0045 1.38

0.61

approx. 1.20

1.90

0.55

1.80

U nit II 2. Soft, arg illaceous, fine to medium grain ed sa nd with clay partings. Deltoideum delta is ab unda nt , an d the un it is coarse and she lly at th e base .

approx . 2.30

U nit I 1. Well bedded, glauconit ic, sa ndy lim eston e or calcareou s sandstone . Medium quartz sa nd ranges from 2 to 4 per cent. Pleuromya uniformis is common, and th e base consists of a coquina of belemnite guards, oyster fr agm ents , etc, se t in a medium qu artz sand an d resting with a sharp junction on Sandsfoot Clay .

1.13

(SANDSFO OT CLAY MEMB E R-fine clay seen to 0.5 m) .

12

J . K . W R IG HT

BLACK HEAD

SANDSFOOT

~ :':O

RINGSTEAD BAY

0::::".

:-::0:

.. 0-:- : :=0:

00<0 :

n

Calc areou s nodules

to med ium ' .:-:-:- sand stone

o

Sider ite

::;:::r:t:l Calcareou s sandstone

o~

Chamo site oo lith s

='"""::::::) Arg ilIaceous ;;::-:~ sandstone & mar l

4

Thalass inoides

rJ~ - Clay ':-:-:-:i Fine

~

~

Figure 5. Log of the Sandsfoot Grit Member at Sandsfoot, including grain size tre nds, compared with the atte nuated exposures in the Osmington area.

having 40 per cent CaC03 . Unit II was described as a clay by the early workers. However, it contains 50 per cent fine quartz sand and 10 per cent medium, weathering back rapidly beneath th e tough sandstones of Unit III. The clastic content of beds 3 and 4 is not markedly grea ter than that of bed 2, and it is the incoming of iron- originally as chamosite, now as disseminated siderite-which has cemented the rock . Bed 4 is mark edly pebble , with 1 em pebbles of dark quartzit e . Pinna and Chlam ys comprise 90 per cent of the faun a, the Pinna always dissociated. Bed 4 is succee ded by an iron stone (bed 5) which, though softe r and weathering re adily, contains a higher proport ion of medium grained sand (8 per cent ). Unit IV is uniqu e in the Sandsfoot section- a pale grey almost insoluble shaly clay. Unit V marks a return to the deposition of sand y iron stone, two highly bioturbated beds (7 and 9) separating soft weathering

ironstone similar in lithology to bed 5. It is clear from Fig. 5 th at the sedimentary signature of Unit V is quite distinct from that of Unit III. The nearby Fleet section (SY 659767) is very similar to that at Sand sfoot. Exposure is poor at present and only the hard er bands can be seen . The Black Head and Osmington sections show mark ed atte nuation . When the y are plott ed to scale (Fig. 5) , the reason becomes clear. There has been uplift to the northeast , and the Ringste ad Clay transgresses across much of the Sandsfoot Grit . Units I , II and III only can be recognised at Black Head , and here, the author 's measured section agrees with th at of Brookfield (1978, fig. 4). The exposure at Ringstead is covered by shingle at present. Th e mea surements of Blake and Hudl eston (1877), Woodward (1895) and Arkell (1936) conflict , so that no useful column can be compiled for Fig. 5 from the ir records.

CORALLIAN OF SOUTH DORSET

13

under turbulent beach conditions. Small pebbles of fine sandy limestone imply erosion of Nothe Grit (i) Ringstead Clay Member nearby, while pebbles of coarse grained, sandy The type exposure of Arkell (1936) in Ringstead Bay limestone have no known counterpart locally. The has been covered by shingle for many years. South of occurrence of millet seed quartz grains and the Weymouth, where Arkell saw 5 m of clay in the occasional fragment of red marl indicate an origin in 1930's, coastal erosion has been inhibited, and there is uplifted Trias sands to the southwest. Subsidence continued, and with the deeper sea little to be seen both south of Sandsfoot Castle and in the East Fleet section. The only exposures at present there was the gradual change to clay deposition. Shell are between Black Head and Osmington Mills. The sand was able to transgress repeatedly from the west, junction with the Sandsfoot Grit is sharp. Brookfield however. Each of the shelly beds marks the building (1978) describes 3.5 m of fossiliferous clays containing out of high energy shell sand into a comparatively abundant orange weathering ferruginous concretions shallow clay sea. This regular alternation of clay and towards the top. These lens shaped concretions are limestone is a feature of Middle Oxfordian sedimentavery diagnostic of the member, being profusely tion in south Dorset. Subsidence continued to be faster than the scattered on the shores of the East Fleet (SY 660766) and occurring even in north Dorset (Wright, 1981). sedimentation rate, and the upper half of the Nothe Clay lacks limestone bands entirely. Only 1 per cent (ii) Osmington Mills Ironstone Member (with quartz sand is present and the clay is finer grained than Ringstead Coral Bed) the Jordan Cliff Clays. The fauna is quite limited and Following Brookfield (1978), the Ringstead Coral Bed only two ammonites have been recorded. Epifaunal of Arkell (1929-37) is regarded as a facies of the much bivalves are common but monotonous, principally more widespread Osmington Mills Ironstone. Lopha, Liostrea and Modiolus. No infaunal bivalves Brookfield gives a very full description of the have been found. Both types of bivalve are commonly Ironstone, which consists of a thin, very varied, found in the limestone bands, especially No.2, and limonite-oolite marl, occasionally well enough these are fully marine. The Bencliff Grit rests with a sharp junction on cemented to form an impure limestone. Blocks of this lithology containing Ctenostreon sp. and Ringsteadia Nothe Clay. The normal sediments being laid down sp. can be found on the shore at Black Head. There is during the Bencliff Grit period were bioturbated a small exposure on the shore of the East Fleet at foraminiferal marl and laminated sand and clay. Into SY 660766, consisting of 0.37 m of very rubbly, bored, this area of quiet, shallow sedimentation came two limonitic mudstone, very red stained and concretion- remarkable incursions of extremely fine grained and ary towards the top. The Ringstead Coral Bed, with its poorly sorted cross-bedded sands. The strength of masses of Thamnasteria arachnoides (Parkinson), can currents necessary to form the scour-and-fill and still be seen when the beach shingle is low for a cross-bedding structures would have been such that considerable distance round into Ringstead Bay medium grained sand ought to have been introduced (SY 748813 to 755813). into the area, but only fine grained sediment was available in the source area. This sediment must have 3. HISTORY OF SEDIMENTATION been introduced from the northeast, as the Bencliff Grit becomes more marine westwards, with stronger (a) Nothe Grit Formation bioturbation, ammonites and Myophorella. The logical conclusion is that there was a localised The Corallian succession begins abruptly, the Nothe uplift to the northeast of the area which made availGrit resting on an eroded surface cut in indurated able silty clay and sandy limestone from the Nothe Oxford Clay. The sands of the Nothe Grit were Clay, medium quartz sand from the Preston Grit, fine deposited during two coarsening upwards cycles sand from the Nothe Grit, and clay with vertebrate brought out by the grain size log in Fig. 2. Fine, off- remains from the Oxford Clay. Intraclasts derived shore, silty sands pass up into well sorted, subtidal sands from all these units are found in the Bencliff Grit, laid down, especially in the case of bed 9, quite close to particularly small limestone pebbles and abundant the beach environment. reworked bone fragments. The sands of the Bencliff Grit accumulated very quickly, with little time for a (b) RedditT Formation burrowing infauna to establish itself. The burrows of Sedimentation in the Preston Grit took place after a Diplocraterion show a fast sedimentation rate with the break and minor uplift. It marks the moment of the burrow raised by up to 30 em. Large rolled fragments re-invasion of the area by the sea and is a transgressive of charcoal occur at Redcliff, with logs of wood, beach deposit accumulating rapidly, with the preserva- suggesting that the origin of the surges of cross-bedded tion of delicate foraminifera and the poor sorting of the sand is to be found in the consequences of forest fires quartz sand indicating only a modicum of sorting affecting the emergent area to the northeast, and (h) Ringstead Formation

14

J . K . WRIGHT

storm s sweeping rapidly ero ded deb ris into the shallow sea to the southwest. We have thus a braide d alluvial fan extending out into the sea , with the sea returning rapidly, bringing in amm onites and all?wing colonisation by bivalves and gastro pods leaving Gyrochorte trails. (c) Osmington Oolite Formation

The initial picture prior to deposition of the Upto n Mem ber was of a clear, shallow sea floor consisting of an erosio n surface cut in Benc/iff Gr it and bored by burrowing organisms. Sed imentation began with the arrival of sandy, sometimes pebbly oolite, succeeded by fine oolite . Much clay is pr es ~~t in these lowest beds, indicating offshore cond itions away from winnow ing currents. Wilson (1968b) .has. shown that the Chlamys qualicosta Bed and the PI ~ oht e represent small, shallowing upwards cycles. Wilson .used the concept of tidal flat regre ssion to explal ~ coarse carbonate sediment buildi ng out over earhe r, finer grai ned accumulati? ns. .. With sedimentation eventua lly failing to keep pace with subsidence, the Nodul ar Clay, with its regu larly developed rows of limestone concretions, was laid down over the whole area . Am monites had free access, and forme d the sites for growth of some concretio ns. A gradual shallowing upwards sequence followed , with the layers of concretions becoming more numerous and passing into sandy, bioturbated marl. Th e intense Teichichnus bioturb ation seen in the Eas t Fleet section suggests a shallow, offshore enviro nment with periodic scouring by gentle currents . Du ring deposition of the Shor tlake Member g~ ~t1e curre nts alternate d with stro ng curre nts depos iting cross-bedded oolite . The cross- beddi ng dips largely to the west or southwest. Using the model of Evans et al.. (1973) concerning the depositio.n of oolite.s at Abu Dhabi, the cross-bedding forms In an oolite delta migrating towards the open sea . The lagoona l ar~a wherein sea water was warmed and super saturated m CaC0 3 seems to have been towards the northeast. Tid al channe ls in which the oolith s were precipitated swept their contents out int o deeper water to the southwest, giving extensive sprea ds of cross-bedded oolite . With time, the position of the lagoon changed and its delta migrated , so tha t oolite is succeeded by oolitic marl and clay, or vice versa. The Nodular Ru bble was deposited under much more settle d conditions , with quiet, deeper water favouring the growth of the sponge Rh axella. (d) Trigonia c1avellata Formation

After renewed block uplift , a marine bench was cut into the Osmington Oolite and colonised by numerous burrowing organisms. As the basin subsided ,. the marginal source areas appea r .to have ~een uphfte? , resulting in fine quartz sand being swept Into the basin

and covering the rocky sea floor. Deposition of sand was followed by that of marl as the clastic input ameliorated. A sta ble situation was set up during the accumulation of the remainder of the Sandy Block with fine quartz sand being supplied into the area from the west and immature oolite from the south . Both these constituents become less important to the east, where fine, argillaceous limestone was laid down under deeper conditions . Clastic input was slight during depositio n of the Chief Shell Beds and Red Beds. Wilson (1968a, b) and Talbot (1973, 1974) point to the p ~ese n~e of clay and micrite as indicating accumulation In water . of moderate shelf depth away from strong wave a~tlOn except during storms. The well p~~s erv~ d , hig hly varied fauna needed high energy conditions III order to sustain it, and was presumably swept in!o the offshore environment wher e it is now found dunng storms. (e) Sandsfoot and Ringstead Formation s

For detailed analyses of the palaeoenvironment of these formations , the reader is referred to Talbot (1974) and Brookfield (1978). ~he comments ?e low are intended to update information made availab le by these authors . The Sandsfoot Clay is present throughout Dorset including the Broadbench (Purbeck) boring (Lees & Tait 1946). It is a marine clay unit laid down over much of southern Eng land as a substantial clay influx terminated the clear , carbonate depositing conditions of the Trigonia Beds. Th e member bears little comparison with the cont em po raneo~s p.art of the Ampthill Clay of eastern England with .Its profu~e ammonite faunas . It is likely that the environ ment III Dorset was low salinity (Broo kfield, 1978). Only one ammonite is known from Dorset and the bivalve fauna is limited to the Ostreidae. Southern Eng land was then subject to renewed uplift, more marked in the east of t~ e p rese~t ar~a. Because of its consequent cross-cutt ing relationship , the Sandsfoot Grit can bear no relation to the Sandsfoot Clay in the manner of a shallowing upwards cycle. The time gap inbetwe en the two ~em bers appears to be of the order of one ammonite zone , probably half a million years (Table 2). The S~ndsf~o t Grit is built up of shelly beach sands altern ating WIth pebbly chamosi tic sands. Chamosite was never the predominent constituent but , as suggeste d by Ta lbot (1974), may have been derived from lagoo ns to the west. These would be situated betwee n a landmass providing the iron by leaching, and sa,nd ba!s protecting the lagoons fro m the ope n sea . I he ~hm clay (bed 6, Fig. 5) appears to have been formed In a shallow area cut off from wave action by migrati ng sand bars. Renewed uplift to the north east caused much of the Sandsfoot Grit to be removed in the Osmington area .

CORAL LIA N OF SOUT H D ORSE T

Th e final Oxfordian trans gression of the Ringstead Form ation saw quie t , offshore marin e conditions established . The Ringstead Clay contains a varied marine fauna (Brookfield , 1978) , suggesting that salinity was normal. The large quantities of iron deposited as sideritic concretions impl y sea water depleted in sulphate at some stage. A reduction in the clay input led to the accumulation of micrite with limon ite ooliths . To the east , the wate r cleared sufficiently for the brief de velopment of a small patch reef.

(0 Cyclic and rhythmic sedimentation Cyclic sedimentation occurred in south Dorset either on the scale of the formation or of the member. A consistent pattern emerges . Following a prolonged stand still and the cutting of a mar ine bench, the sea transgressed steadily. The sea floor was colonised briefly by burrowing organisms and th ese were then smothered by shallow marine sediments, largel y clastic. Succeed ing argillaceous sediment hint s at a rate of subside nce faster than sedime nta tion. Subsiden ce then slowed down and the basin began to fill with further shallow marine sediment. This pattern is see n in the Redcliff , Osmin gton O olit e and Tri gona clavellat a Form ations and in the Sand sfoot Grit. E ach cont ains fine or med ium graine d she lly qu art z sand at the base , succeeded by argillaceous sediment of varying thickness, passing up into further shallow marin e sediment. On a smaller scale during the Middle Oxfordi an , rh ythm s alternating two sediment types were common. Limestone alte rna ted with clay in the Nothe Clay and Upton Members, and marin e marl alterna ted with alluvial sand in the Bencliff Grit Member. Away fro m southern En gland , such cyclic and rhythm ic sedimenta tion cann ot be dem onstrated . In the York shire Basin , sedim entation regularly kept pace with subsidence , leading to the accumulation with in severa l members of tens of metres of uniform sedim ent. In East Greenland, Surlyk & Clemmensen (1983) have looked carefully for evidence of cyclic sedimentati on in Middle and Upper O xfordi an rocks, and found non e. An origin for the cycles and rh ythms in eustat ic changes in sea level has to be ruled out. We must look for a cause of the cycles and rhythm s which is peculiar to southern Engl and . Th e sedime nts in the Cor allian G roup in this area were laid down during a tectoni cally unstable period . In the Middle O xfordi an , sedi men ts accumulated in a series of extensional ba sins bounded by de ep seated crustal fault s. Po sitive ar eas inbet ween th e basins rose , pro viding sediment. During the Upper Oxfordi an , there was less tend ency for localised basins to develop. Mor e widespr ead , regional subsidence occurred, punctuated by localised uplift s. Th e most dr amatic basinal developm ent during the O xfordian was that in

15

the Bristol Channel, where so me 520 m of sedime nt accumulated (Evans & Thompson , 1979). None of the onshore basins can match thi s. However , south Dorset accumulated one of the thickest sequences of Middle Oxfordi an sediment seen in on shore Britain , to talling alm ost 40m . In such an area situated close to the major exten sion al basins of the English Channel and the Bristol Ch ann el, alte rna tions of differing sedime nt types would be expected . On a small scale, whe re ther e was a delicate balance bet ween low energy and higher energy sedimentatio n as in the Nothe Clay , subsidence caused by fault mo vements would leave its mark in the regular building out of shell sand being int errupted by clay sedimentation . On a broader scale , as the regi onal stresses varied with time , subsidence in the ba sins would vary with the rate of movement of the bounding faults. Widespread clay deposition would occur in exte nsional phases, followed by a progression to shallow marine sedim ent as the stres s eased. Thi s being a regional stress, eac h basin would have a similar histo ry of subsidence and a genera lly similar stra tigraphy, but the details of the stra tigrap hy would vary with the absolute subside nce and the availabi lity of clastic and carbonat e sedime nt.

4. BIOSTRATIGRAPHY Th e ammonite faunas of the so uth Dorset Corallian Bed s are of grea t int ere st as we have her e an intermingling of ammo nites of Boreal and Teth yian pro vincial affinities . It is po ssible at several levels to collect ammo nites repr esent ati ve of these very distinct faun al provinces from the same bed , and thus to provide valuable cor relation of the sepa rate zonal schemes . Sykes & Callomon (1979) pro pose d a Sub-B ore al Province to cover this area of over lapping amm onite faunas, with several new or newly defined zones and subzone s. This chapter therefore deals fairly briefly with the well established zones and subzones, providing new biostratigraphic inform ation, and consider s in more detail the proposals of Sykes & Callomon . (a) cordatum Zone (cordatum Subzone) The No the Gr it regularl y yields amm on ites indicative of this subzone . Most have been obtained loose from the beach below R edcliff with the shell repl aced by hon ey coloured calcit e . Th e matrix is a fine grained , calcareo us sandstone. Preston Gr it ammonites from th e base of the member are preser ved as decalcified casts in fine to medium gra ined , argillaceo us sandsto ne , and sho uld not be co nfused with No the G rit ammonites . From the No the Grit has been collect ed the th ree species of Cardioceras sensuo stricto most typical of the subzone , C. (C.) cordatum (J . Sower by) , C. (C.) angusticordatum Ark ell and C.

16

J. K. WRIGHT

TABLE 2. Zones and Subzones of the Oxfordian Sub-boreal Province as present in the Corallian rocks of south Dorset (following Sykes & Callomon, 1979) with the probable ranges of the strata inducated Zone

Subzone

MemberlFormation Osmington Mills Ironstone Ringstead Clay

Ringsteadia evoluta Ringsteadia pseudocordata

Sandsfoot Grit Ringsteadia pseudocordata Ringsteadia pseudoyo

Ringsteadia caledonica Perisphinctes variocostatus

?Sandsfoot Clay

Perisphinctes cautisnigrae Perisphinctes cautisnigrae

Trigonia clavellata

Amoeboceras nunningtonense Perisphinctes pumilus Perisphinctes parandieri

Nodular Rubble Perisphinctes antecedens

Shortlake Upton Perisphinctes plicatilis Cardioceras vertebrale

Cardioceras cordatum (pars)

Cardioceras cordatum

Bencliff Grit Nothe Clay Preston Grit

r

I

Nathe Grit

I

I

I

I

I

I

I

I T

(C.) persecans S. Buckman. The holotype of C. persecans came from Red Cliff, but is extremely

(b) plicatilis Zone

poorly preserved. The opportunity is taken here to figure a topotype (Fig. 6J), agreeing very closely with specimens from Yorkshire figured by Arkell (193548).

(i) vertebrale Subzone This subzone encompasses the Preston Grit, Nothe Clay and Bencliff Grit. The best fauna has come from the Preston Grit, both from the soft sandstone at the

CORALLIAN OF SOUTH D ORSET

base and the sandy limestone at the top . Again. almost all specimens have been collected from the beach below Redcliff. I have in my collection Cardioceras (Subvertebriceras) zenaidae (Ilovaisky) , C. (V .) densiplicatum Boden, Goliathiceras (Pachycardioceras) ct. nitidum Arkell transitional to G. (P.) rhodesi Arkell , Perisphinctes (A risphin ctes) sp . and Asp idoceras (Euaspidoceras) ct. vettersianum Neuman. In general , the ammonite faunas of the vertebrale Subzone are very well known and man y excellent specimens having been figured by Arkell (1935-48) and Sykes & Callomon (1979). Ammonites are found only infrequently in the Nothe Clay and Bencliff Grit. From a sandy limestone band in the Nothe Clay I have collected a fragmentary Perisphinctes sp., and Arkell records Scoticardioceras sp . From the Bencliff Grit of Red Cliff have come recently Goliathiceras (Goliathiceras) microtrypa S. Buckman , G . (Pachycardioceras) rhodesi Arkell and three fragments of Perisphinctes sp. According to Callomon (1960), the range of the large, inflated Goliathiceras extends no higher than the vertebrale Subzone (with one possible exception) . Thus , it is very likel y that the Bencliff Grit is of vertebrale Subzone age . (ii) antecedens Subzone Considering the wealth of ammonite material from this subzone found in Oxfordshire and Yorkshire, the equivalent strata on the Dorset coast (comprising the major part of the Osmington Oolite) yield disappointingly few ammonites. Combining new records with those published previously, two cardioceratids are known , C. (Maltoniceras) ct. vagum Ilovaiski from the Pisolite and C. (M .) maltonense (Young and Bird) in white , oolitic matrix (Arkell , 1935-48, p . 386) . Thirteen perisphinctids are known, many collected recently from the landslip at Redclift, and poorly preserved. In my collection except where otherwise stated are Perisphinctes (Liosphinctes) aft. cumnorensis Arkell (matrix of the lowest sandy oolite), P. (Arisphinctes) aft. cotovui Simionescu in a similar matrix, P. (Di chotomosphinctes) ct. antecedens Salfeld (J. H. Call omon collection from the Pisolite of Osmington Mills), P. (A.) aft. maximus (Young and Bird) from the Nodular Clay (two specimens), and P. (Kranaosphinctes) aft . decurrens S. Buckman from Shortlake Member oolite . From the beach at Osmington Mills came P. (D .) aft. dobrogensis Simionescu in oolitic marl matrix (A8 or AW). The lowest Osmington Oolite is already of antecedens Subzone age , the Liosphinctes it contains being one of the principal indicators of the subzone along with D. antecedens. That the Shortlake Member cannot be younger than the ante cedens Subzone is shown by th e occurrence of Maltoniceras, this subgenus definitely not occurring in the overlying pumilus Zone (Sykes & Callomon , 1979) . However . Torrens

17

(1969) and Callomon (in Wright, 1980) record ammonites from the Nodular Rubble which are indicative of th e p um ilus Zone (see bel o w). The ammo nite fauna of the Nodular Rubble is markedly different from that of the underlying two members, and a break in sedimentation is therefore shown in Table 2. (c) pumilus Zone

This zone was proposed by Sykes & Callomon (1979) to encompass strata in Britain lying in between the long established plicatilis and cautisnigrae Zones. The lower parandieri Subzone is based on the faunas of the Wheatley Limestone and Littlemore Clay Beds of the Oxford District, consisting primarily of Perisphinctes ss. and Dichotomosphinctes (Callomon, 1960). Cardioceras spp. are common in the Hebrides (Sykes & Callomon , 1979). The nunningtonense Subzone is based on the faunas of a part of the Ampthill Clay of Fenland and the Newbridge Member of the Yorkshire Upper Calcareous Grit. Amoeboceras is abundant , plus Perisphinctes s.s., Dichotomosphinctes and Arisphinctes. Pseudarisphinctes and D ecipia make their first appearance , along with perisphinctids which Arkell mistakenly allocated to Pomerania. The zone thus includes the major change from Cardioceras to Amoeboceras in the Boreal Realm, man y Tethyian perisphinctids carrying on as before. (i) parandieri Subzone The Nodular Rubble in the Osmington Area has yielded Perisphinctes (Perisphinctes) pumilus Enay (Geological Survey Museum) and P. (P .) parandieri de Loriol (J. H . Callomon collection) . The age of the Nodular Rubble is thus firmly established in this subzone .

(ii) nunningtonense Subzone Amoeboceras transitorium Spath is confined to the subzone in Yorkshire, as is the species of Decipia figured by the author (Wright, 1972, pI. 14, fig. 4). Both of these have been collected from the Trigonia clavellata Formation. However, study of the whole Trigonia Beds fauna (see below) suggests that the age of this formation is younger than the nunningtonense Subzone, and that there is a stratigraphic break beneath the Trigonia Beds. (d) cautisnigrae Zone

This zone was introduced by Arkell (1945) , its diagnostic fauna being that of the Trigonia clavellata Formation. However, beneath this formation there is a stratigraphic break , and in the unit above , no zonal ammonites have ever beeen recorded. Both bottom and top of the cautisnigrae Zone have to be defined elsewhere in continuously fossiliferous strata . The base is defined in Yorkshire some 3 to 4 m up in the U pper Calcareous Grit Formation. It is marked by the

18

J . K . WRI GHT

F

C O RA L LI AN O F SOUTH DO RSET

incoming of Decipia decipien s (J . Sowerby) , D. lintonensis Arkell , Amoeboceras glosense (Bigot & Brasil) and the more coarsely ribbed variety of A. newbridgense Sykes & Callomon. Top of the Zone: the next overl ying faun a known is th at char acterised by Perisphinctes variocostatus Buckland, and found in the Ampthill Clay of Fenland. Syke s & Callomon (1979) have ext ended the caut isnigrae Zone upwards to include this fauna , ArkeJl's original cautisnigrae Zone having become the cautisnigrae Subzone of the enlarged zone . The top of the cautisnigrae Zone is now defined by the incoming of R ingstead ia caledonica Syke s & Callomon of the pseudocordata Zone at Staffin in the Isle of Skye. (i) cautisnigrae Subzone Almost all ammonites have been coll ected from loose blocks in the Osmington area. Each of the members of the Trigonia clavellata Formation has its own distinctive matrix , and it is possible to allocate almost every specimen found loose to an individual stratigraphic unit with confidence . Ammonites are rare in the Sandy Block . One Perisphinctes (Pseudarisphinctes) aff. shortlak ensis Arkell was collected in situ at Black Head. I know of no other records from this unit. Ammonites become quite common towards the top of th e Chief Shell Beds. From thi s matrix I have the septa te inner whorls of A moebo ceras glosense (figur ed, Sykes & Callomon , 1979, pI. 15 fig. 1), Decipia lintonensis (a specimen also in the J . H. Callomon collection) , Perisphinctes (A risphinctes) sp ., P. ( Pseudarisp hinctes) damoni Arkell and P. (Pseudarisphinctes) sp. no v. From the matrix of the Red Beds have come Amoeboceras glosense, A . newbridgense, A . transitorium Spath . A . ct. damon i Spath , Decipia sp. nov. , D. aff tranchandi (Bigot & Brasil) , Perisphinctes (Perisphinctes ) uptonensis Arkell, P. (Arisphinctes) aff. ringsteadensis Arkell , P. (Ps eudarisphinctes) shortlak ensis (1. H. Callomon collection) , and various

19

fragments of Perisphinctes s.s. and A risp hinctes. Th e pre sence of such typical nunnington ense subzo ne ammonites as Amoeboceras transitorium and Decipia sp . nov . is almost certainly a carry-over of nunnington ense ammo nites int o th e lower cautisnig rae Subzone , some thing which occurs also in York shire. The presenc e of Decipia lintonensis , Amoeb oceras glosense and A. newbridgens e (large var.) confirms that the Trigon ia c1avellata Beds fall within the cautisnigrae Sub zon e as defined in Yorkshire . Six of th e specimens of A m oeboceras ar e illustrated in Fig. 6. Figs. 6A , 6B and 6C are of A. glosense, a series showing th e typical range in prominence of the ribs in the later stages of the macroconch . There is variation from strongly ribbed, with 'asharp angle at the vental margin (6A), to forms sho wing the complete loss of primary ribs (6C). Figs. 6D and 6E show the pr ominent lateral and ventrolateral tubercles characteristic of A . transitorium. Both are microconchs, Fig. 6D being the thicker, more evolute variety occurri ng at this horizon also in Yorkshire . Fig. 6E is slim whorl ed and involute . Its style of ribbing is more char acteri stic of A . dam oni , but the latter's thick whorl section is not present. Fig. 6F is of a small, o val whorled , septa te spe cime n which app ears to represent a new spec ies . (ii) variocostatus Subzone No ammonites representative of this subzo ne a re known from sout h Dorset. The most so utherly occurrence of bed s of this age reve aled by ammonites so far appears to be aro und Steeple Ashton , Wiltshire , indicated by Amoebo ceras serratum (1. Sowerby) in De vises Museum . The subzone is probabl y represe nted in so uth Dorset by the Sand sfoot Clay. (e) pseudocordata Zone (Salfeld, 1913)

The range of Perisphinctes overlaps th at of Ringsteadia , and it is the first appearance of the latter

Fig. 6. Lower and Upper Oxfordi an ammonites from the Dorset coast. All author's collection, natural size. A, B, C A m oeboceras glosense (Bigot & Brasil). cautisnigrae Subzon e, Trigonia clavellata Formation , Red Beds, Black He ad. A and B are partial body chambers. C is completely septate. D/c/2S, D/C/24, D/C/22. D. Amoeboceras transitorium Spath . cautisnigrae Subzone , Trigoni a clavellata Formation, Red Beds, beach below Upton House, Osmington Mills. Part ial body chamber , test entirely preserved . D/C/26. E. A moeboceras aff. dam oni Spath. cautisn igrae Subzone , Trigonia c1avellata Formation , Red Beds, Black He ad . Parti al body chamber , test partl y preserved. D/C/28. F. A moeboceras sp. cautisnigrae Subzone , Trigonia clavellata Formati on , Chief Shell Beds, Black Head . Test entirely preserved . D/C/29. G . Amoeboceras rosenkrantzi Spath . pseudocordata Zone , ?pse udoyo Subzone , Loose block of Sandsfoot Grit, Black Head . D/C/60. H. Microbipli ces aff. anglucus Ark ell. pseud ocordata Zone , ?pseudoyo Subzone , Loose block of Sandsfoot Grit , Black Head . D/C/69. 1. Perisph inctes sp. pseudocordata Zone, ?pseudo yo Subzone , found loose on the shore of the East Fleet in characteristic matrix . D/CI70. J. Cardioceras (Cardioceras) persecans S. Buckman. cordatum Subzone, Nathe Grit , loose block on the beach below Redcliff. D/C/S.

20

J . K . WRIGHT

which may be taken to mark the base of the zone . Thr ee subzones were reco gnised by Morris (1968) , and were formally published for the first time by Sykes & Callomon (1979). However , no sooner had this paper app ear ed than it became app arent from new evidence that the order of the lower two subzones had been given incorre ctly. The correct ord er , as distinguished by J . H. Callomon, was publi shed by the author (Wright, 1980). In addition , the early caledonica Horizon of Sykes & Callomon was recognised as a formal subzone . (i) caledonica Subzone No evidence of Ringsteadia caledon ica has been found in southern England. Beds of this age are almost cert ainly missing due to a major erosive episode prior to the deposition of the Sandsfoot Grit. (ii) pseudo yo Subzone It is quite likely that this subzone is repre sented in

units I to III of the Sandsfoot Gr it. Ark ell (1947) record s Ringsteadia d. pseudoyo Salfeld from unit III at Sandsfoot , and Morri s (1968) record s A m oeboceras rosenk rantzi Spath from the lowest Sandsfoot Grit of the East Fleet section. I have collected a fragment of a biplicate perisphinctid here . From the basal unit III at Black Head I have collected A. rosenkrantzi and Microbiplices d. anglicus Ark ell. From Ringstead Bay. E . J. Bowen has collected Perisph inctes aff. branensis Arkell (Oxford University Museum) . The author's three specimens are illustrated in Fig. 6. The Amoeboceras rosenkrantzi (6G) matche s well the coarser ribbed form figured by Sykes & Callomon (1979, pI. 120, fig. 1) . The sepa ration of the secondaries from the primaries, and the slight swing forward s of the secondaries, are very characteristic.

Th e Microbiplices (6H) is the first found in this country complete with aperture and lappets, the bod y chamber being perfectly preserved. The East Fleet perisphinctid (61) is a distinctive form , with prorsiradi ate primary ribs becoming rectiradi ate as they bifurcate, th e secondaries passing stra ight over the venter. (iii) pseudocord ata Sub zone The subzone was introduced by Morris (1968) for the ammonit e fauna of the Sandsfoot Grit of Sandsfoot. Almost all the ammonites come from the soft, chamostic sandstones at the top of unit III and in unit V. Microbiplices anglicus is abundant and fragmentary Ringsteadia spp . are common. Morri s (1968) records R. aft pseudo cordata Blake & Hudleston and R. aft. marstonense Salfeld.

(iv) evolut a Sub zone This subzone was introduced by Morris (1968) for the ammonite faun a of the Osmington Mills Ironstone . R. evoluta (Salfeld) was common at one time in the East Fleet section, though this exposure has deteriorated in recent years. Ringsteadia sp. was collected by the auth or at Black Head , and R . evoluta from Ringstead Bay (J . H . Callomon collection ).

ACKNOWLEDGEMENTS I would like to thank Professor J. H. Callomon for his help and encourag ment during the cour se of this work , and for making available his own ammonite record s. Dr. J. C. W. Cope and Mr C. King have donated ammonites collected by themselve s. Mrs C. Flood drew the figures.

References ARKELL, W. 1. 1929-37. A monograph of British Corallian Lamellibranchia . Monogr. palaeontogr. Soc. London., xxxviii + 392 pp. , 56 pis. - - 1933. The Jurassic System in Great Britain. Clarendon Press, Oxford, xii + 681 pp . , 41 pIs. - - 1935-48. A monograph on the amm onites of the English Co rallian Beds. Monogr. palaeontogr. Soc. London , l xxiv + 420 pp., 78 pIs. - - 1936. The Corallian beds of Dorset , Pt 1, the coast. Proc. Dorset Arch. Nat. Hist. Soc. 57,59-93. - - 1945. The zones of the Upp er 1urassic of York shire. Proc. Yorkshire geol. Soc., 25, 339-58. - - 1947. The geology of the country around Weymouth, Swanage , Corfe and Lulworth . Mem. Geol. Surv. U.K. - - 1963. In (W. F. Whitt ard & S. Simpson , Eds ) Lexique Stratigraphique International-Europe, Fasc. 3a, England Wales and Scotland, Pt 3ax, Jurassic. Centre National de la Reche rche Scientifique , Paris. 394 pp . BIRKELUND , T ., 1. H . CALLOMON, C. K. CLAUSEN,

H . NOHR HANSEN & I. SALINAS. 1983. The Lower Kimmer idge Clay at Westbury, Wiltshire , England . Proc. Ceol. Ass ., 94,289-309. BLAKE, 1. F. 1875. On the Kimmeridge Clay of England . Q. JI. geol. Soc. London, 31, 196--233. - - & W. H. HUDLESTON. 1877. On the Cor allian rocks of England . Q. Jl. geol. Soc. London , 32, 260-405. BROOKFIELD , M. E. 1978. The lithostratigraph y of the Upper Oxfordian and Lower Kimmeridgian Beds of south Dorset, England. Proc. Geol. Ass., 89, 1- 32. BUCKMAN, S. S. 1925. Type Ammonites, vol. V. Publ ished by the author. London & Thame. CALLOMON , 1. H. 1960. New sections in the Corall ian Beds around Oxford, and the subzone s of the plicatilis Zon e. Proc. Ceol. Ass ., 71, 177-208. COPE, 1. C. W. 1980. The Kimmeridgian Stage. In (1. C. W. Cope , Ed .) . A correlation of the Jurassic Rocks of the British Isles, Part 2, Middle and Upper Jurassic. Special Rep. No. 15, Geol. Soc. London.

CORALLIAN OF SO UT H DORSET

COX , B. M. & R . W. GA LLOI S. 1981. The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian seque nces. Rep . Ins t. Geol. Sci. London , 80/4, 44 pp . EVANS , D . J . & M. S. THOMPSON , 1979. The geology of the cent ral Bristol Channel and the Lundy are a, South Western Approaches, British Isles. Proc. Geol. A ss., 90, 1- 14. EVANS , G ., J . W. MURRAY , H. E . J. BIGGS , R . BAT E & P. R . BUSH. 1973. The oce anography , ecology. sedimentology and geomorp hology of parts of the Trucial Coast Ba rrier Island Complex, Persian Gulf. In (B . H . Purser, ed.) Th e Persian Gu lf pp 233-78 . Ber linHeidelberg, Springer Verlag. FO RS1CH. F . T . 1975. Trace fossils as environmental indicators in the Corallian of England and Norm andy . L ethaia, 8, 151- 72. - - 1977. Cora llian (Upper Jurassic) marine bent hic associations fro m England and Norm andy. Palaeon tology, 20, 337- 85. GO RDON, W. A. 1965. Foraminifera from the Corallia n Beds, Upper Jura ssic, of Dorset , England . JI. Palaeontology, 39, 828-63. LEES , G . M. & A . H . TA IT 1946. The geological results of the search for oilfields in Gr eat Britain. Q. Jl. geol. Soc. L ondon, 101, 255-317 . MORRIS, N. J . 1968. Palaeontological and stratigraphical studies in the Upper Jurassic rocks. Unpublished D. Phil. thesis, University of Oxford . SALFELD , H . 1913. Certain Upper J urassic strat a of Eng land. Q. Jl. geol. Soc. London, 69, 423-32 . SURLYK, F . & L. B. CLEMMENSEN . 1983. Rift

21

propagation and eustacy as controlling factors dur ing Jurassic inshore and she lf sedimentatio n in northe rn East Greenland. S edimentary Geo!. , 34, 119-143. SYKES, R. M. & 1. H. CALLOMON . 1979. The Amoeboceras zonation of the Bore al Upper Oxfor dian . Palaeontology, 22, 839-903. TA LBOT , M. R. 1973. Major sedi menta ry cycles in the Corallian Beds (Oxfo rdian) of southern Eng land . Palaeontogr. Palaeoclimat ol . Palaeoecol., 14,293- 317. - - 1.974. Ironstones in the Upper Oxfordia n of south ern England. Sedime nto logy , 21, 433-50. TO RRENS , H . S. (ed .) 1969. International Field Symposium 0 11 the British Jurassic. 1969. Excursion No . I, Guide for Dorset and south Somerset. pp A 1- A71. WILSON , R. C. L. 1968a . Upper Oxfordian palaeograph y of Palaeogeogr. Palaeoclimatol. southern England . Palaeoecol., 4, 5-28. - - 1968b. Carbonate facies variation within the Osmington Oo lite Ser ies in south ern England. Palaeogeogr. Palaeoclimatol. Palaeoecol., 4, 89- 123. WOO DWA RD , H . B. 1895. The Jurassic rocks of Britai n. 5. The Middle and Upper Oolitic rocks of England (Yorkshire excepted) . Me m . geo l. Sur v. U. K. WRIG HT, J . K., 1972. The stratig raphy of the Yorkshire Co rallian . Proc. Yorks . geol. So c., 39, 225- 66. - - 1980. The Oxfordi an Stage In (1. C. W. Cope , ed) . A correlation of the Jur assic rocks of the British Isles, Part 2, Middle and Upp er Ju rassic. Sp ecial Rep . N o. 15 Geo!' Soc . Londo n. - - 1981. The Cora llian rocks of north Dorset. Proc. Geol. A ss., 92, 17-32 .