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Portland Limestone Formation, Divided Into Three Members

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Portland Limestone Formation, Divided Into Three Members Lithostratigraphy and deposition of the type Portlandian WILLIAM GEOFFREY TOWNSON SUMMARY The "Portland Beds" of Dorset (Portlandian environment. The dolomite formed by in situ of English usage) are described in terms of a replacement of limelime mud. The middle cycle Group comprising two Formations and seven consists of cherty fine-grained limestones Members. Facies and thickness variations deposited on the outer part of a carbonate indicate the presence of a swell separating an shelf. The abundance of replaced sponge East from a West Basin. The swell may be due spicules adequately accounts for the amount to the movement of Triassic salt. The environ­environ- of chert. The upper cycle consists of cherty mental history of the Portland Group is limestones passing up into shallow-water described in terms of three cycles consisting of grainsgrainstones. tones. Ooid shoals developed over the major regressive and minor transgressive swell. These marine limestones are overlain by phases superimposed on an overall regression. stromatolites and evaporites which formed on The lower cycle consists of siliciclastics and the basin margin. dolomite deposited in a relatively deep marine OVERLYINGOVERLYINO THE UPPER JURASSIC KIMMERIDGE CLAY in southern Eng-Eng­ land and northern France is a sequence of strata known as the "Portland Beds" (for references from 1816 to 1936I936 see Arkell 1935,I935, 1947). The Dorset outcrop provides the geographical name of Portland and in this area the Portlandian of English geologists comprises three ammonite zones (Cope in Torrens 1969) correlated with the middle Volgian (Casey 1967, 1973)I973) and the upper Tithonian (Zeiss 1968):I968) : PurbeckPur beck Beds non-marine Titanites giganteus Portland Beds Glaucolithites gorei Portlandian Progalbanites albani Topmost Kimmeridge "Epipallasiceras"EpipaUasiceras sp." late Clay Pavlovia rotunda Kimmeridgian In the past the "Portland Beds" have been studied mainly for their faunal content (Cox 1925,I925, 1929i929 and references in Arkell 1935)i935) and are still poorly defined lithostratigraphically. The detailed bed measurements of 15 sections by Arkell (i935)(1935) are very useful but his rock descriptions are less satisfactory. A lithostratigraphy is presented here based on study of 233 vertical sections, io721072 hand specimens and 68o680 stained thin sections (Townson I97I1971). ). A detailed lithofaciesli thofacies correlation of all the exposures has been made (Table I, Figs. i-3).1-3). Nomenclature The "Portland Sand" (Table x)I) is redefined as the Portland Sand Formation, divided into four Members. The base is lowered to a mappable change from Jl geol. Soc. vol. x3x131,, I975,1975, pp. 6x9-63861g-638, , 5 figs. PrintedPrinted. in Northern Ireland.Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/131/6/619/4885055/gsjgs.131.6.0619.pdf by guest on 30 September 2021 620 W.W. G. Townson TTA ABLE ]3 L E I:I" Lithostratigraphy of thethe Portland Group of Dorset. Arrows indicate regressive sequences.$e( vuences . i 19351955 ARKELL 1947 ARKELL 1956 THIS PAPER ~Ouj ~ PURBECK Upper 22m Durlston Formation PURPURBECK BECK :E~EZ Z BEDS Middle 47m =EO BEDS Lulworth Formation GROUP <I:N~~ !120m+-120m Lower 51m51 m 6 ±120m+-120m e It) to FREESTONE WINSPIT 1.&.1+1 Z SERIES MEMBER ~P_ (J)(/1 15m 6-19m " r ." 0-0 o ::o~::o 'z.== DANCING ~ m -i U) et: Q)g LEDGE MEMBER !:i ,,0,,- ..J CHERTVCHERTY -Oz~ ~ m Cl)O') ...I- _~~ __________________6-15m6_-_1_5_m~Oz C -c CC:3 0::o: SERIES zm o BedJBedJ' IJJLIJ ~0 DUNGVDUNGY HEAD Z'!L "'0 t- m MEMBER - Basal Shell MEMBER 0 ~ __~Be~d~~ Bed __~-,~19.·_5_m~.- 19.5m _________________4.5-14m4_.5_-_14_m~ 22-22-40m 40r ~ WEST EASlEAST i r- E~I8:?rtla~lan Black Ssts+Ssts + GAD CLIFF MEMBER i » CD 0-:- / West Parallel i Z = ~ ~" Wre_a_re~_B_aooWeom Bands__ s __ I~_5_m~1:~5rr __________________3"5-14.5m3_·5_-_14_.5_m~1 0 ,,o,,,,, o~Z ~1%/ We =-~ z o Ssts (castIc,ICast « St. PONDPONDFIELD FIELD MEMBER 12m'2ml E' 0-6.50-6'5 m ..J ~(J) IB&~m !:~ansAlbons o Exog yro1 t­ Exog yra Head CORTON HIHILL LL MEMBER L o::: o Bedseed_._s_s _,7-5m7·5 m Marls 2-7.7m2-7·7m ~ Gl o Z ::0 13·5m ,,-i-I1 --I :0 Q. ;3 Upper 15"5m Emmit .7- 0o I--~. o e ...I-- Black Norer Emmit Hill :Oz:;0 7>. C D 0:: HIliHill .A nl~ Beds Marls i:c "'0 c;e o Beds l1li D ~ ..----JOm 9m9m-- ...... 1 f :!! B.N, Sst IOta ~ D BLACK a.. B.N.Sst Massive Bed max 'll.5m'1I·5m BLACK 0(5 = z~ o.CL Lower Hounstout o}U) NORE Hounstout.Hounstout· N ORE Z¢:Jz BlockBlack Marls Marls c ., Nore .c: >­ MEMBER tt~ et: Beds ? / 15m ]J max 18m ~ o ..J ? ~ t._ ....- o o / • Hounstout 1 Hounstout I o ? / CloyClay 6m Clay max 6mSm N 3 e~ ~' RhynchonellofRhynchoneld Rhynchonella Marls .I Marls max IOr~IOm 15-45m 22-72 m 8. (Not permanently Lingula e .... ~ II LingulaUnQula Shales Q. exposed) Shales Topmost beds of thethe :~no!es illII max 15m g Rotunda RRotundao t U~dda Clays KIMMERIDGE CLAY o ¢ Rotunda and Clays 111 nodule bed 1vii maxl6rrmaxlSm max 31m Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/131/6/619/4885055/gsjgs.131.6.0619.pdf by guest on 30 September 2021 The type Portlandian argillaceous sands and silts to shale, instead of the present j unction within a sandy or silty sequence (Arkell 1947). The top of the Formation is not changed but is in fact the top of a widespread thick dolomite and not a sandstone (Townson 197 I). The "Portland Stone" (Table I) is redefined as the Portland Limestone Formation, divided into three Members. The base is taken in east Dorset as the junction between limestone and underlying dolomite and in the west between clay and underlying dolomite. The only change is to include the "Portland Clay" (Damon 1884, Strahan 1898, Arkell 1947). The top of the Formation is not moved. The term Portland Group replaces "Portland Beds" in their enlarged and lithostratigraphically defined and correlatable form. The repetition of "Portland" is justified on historical precedence and the absence of suitable geographical names which are not already used for the Dorset J urassic. The "Purbeck Beds", comprising the Lulworth and Durlston Beds (Casey 1963), are renamed Purheck Group, Lulworth Formation and Durlston Formation (Table I). Depositional setting Thicknesses vary across Dorset as follows (see Fig. 2 for localities): Portesham Ringstead Gad Cliff & Hounstout Isle of Area Bay Kimmeridge Area Portland Lulworth Formation 35 m 30 m 45 m 55 m 35 m Portland Group 65 m 40 m 90 m I 10 m 65 m Kimmeridge Clay 350 m 260 m 500 m ? 400 m A swell divided the area into East and West Basins. This swell plunged south­ wards as seen by the increased thickness in the Isle of Portland. Depositional thinning of the upper J urassic and lower Cretaceous across this swell is described by Arkell (1938, and earlier workers referred to therein) but no trend direction or causal explanation is given. Drummond (1970) describes the 'Mid-Dorset Swell' and its control on sedimentation during Albian to Cenomanian time (after a major tectonic phase). This feature, however, trends NW.-SE., at right angles to the direction of the late Jurassic swell (Fig. 5). The SW.-NE. orientated palaeo­ graphic maps of West (1975) for the basal beds of the Lulworth Formation follow on very satisfactorily from those for the Portland Group in Townson (197 I) . Lees and Cox (1937) state that the tectonic style of south Dorset is halo kinetic and it is here suggested that subsidence in the area was influenced by the presence of a Triassic salt pillow below the swell. Portland Sand Formation (A) BLACK NORE MEMBER The name is that used by Arkell (Table I) from the section on the Isle of Portland, now spelt Blacknor (grid ref. SY 677 I). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/131/6/619/4885055/gsjgs.131.6.0619.pdf by guest on 30 September 2021 622 w.IV. G. Townson In the East Basin only two localities expose the complete succession: Hounstout-Hounstout­ St. Albans Head and Gad Cliff (Fig. 2). Four informal divisions exist (Table 1):I) : (i) Rhynchonella "Marls". Black, dolomitic, laminated argillaceous siltstones. (ii) Hounstout "Clay". Dark grey dolomitic silts and very fine sand with thin shale interbeds. (iii) Hounstout "Marls" and (iv) Emmit Hill Beds (modified term, see Table 1).I). The latter two consist of bioturbated calcareous dolomitic clay, silt and fine sandstone with thin beds or nodule layers of lime mudstone (details in Townson 1971).197 I). In the West Basin the junction with the Kimmeridge Clay is known only in a stream section near Coryates (SY 629856)629856 ) and in a temporary exposure (SY 690833)69o833 ) over Tout Downs (Samuel 1969,I969, Townson 1974).I974). In both sections clay passes up into a mixture of dolomite and sand, but the western exposure contains up to 100/0lO% glauconite (Townson 1971).I97i ). On the Isle of Portland the Member is best exposed at Arkell's type locality. Dolomitic silty clay passes up into dark grey argillaceous dolomite with thin nodular horizons oflightof light grey lime mudstone. (B) CORTON HILL MEMBER The name is taken from the exposure in the cutting to Corton Farm, over Corton Hill. Thickness and facies changes are shown in Fig. I. In the East Basin the sediment is dominantly a mixture of clay, dolomite and fine quartz sand, with thin horizons of Exogyra. Towards the swell the proportion of carbonates and beds of Exo~raExogyra increases until the Member is mostly dolomite with current bedded Exog~ra.Exogyra.
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