This an old paper co-written by this webpage author with Malcolm Hooper who discovered the unusual deposit in 1975. The paper is here in slightly modified and updated format, but without major changes and still based on the original of 1975. The paper fixes a date for formation of the chert of the Portland Chert Member of the Portland Stone Formation. It also shows that erosion in the Weymouth region, on the site of the Weymouth Anticline (and near the Isle of Portland) took place early, in Late Cimmerian times (Berriasian, Cretaceous). It may suggest that Weymouth Anticline is an old structure, and not the result of relay ramp tectonics in Tertiary times.]

Title:

West, I.M. and Hooper, M.J. 1969. Detrital Portland chert and limestone in the Upper Purbeck Beds [i.e. Durlston Formation of the Purbeck Group] at Friar Waddon, Dorset. Geological Magazine, 106, pp. 277-280. By Ian West and Malcolm Hooper.

Abstract:
Abstract: Pebbles of derived Portland chert and limestone have been found in the Upper Purbeck Beds [now Dulston Formation of the Purbeck Group] at Friar Waddon, Dorset [just west of Upwey and north of Weymouth]. The Unio Bed at this locality is of unusual lithology and with derived phosphatic fossils and with fish teeth. The presence of this chert in the Upper Purbeck Beds [Durlston Formation] and its apparent absence in the Lower Purbeck [Lulworth Formation] pebble beds suggest that a [the?] major phase of silicification in the in the Portland Stone [Formation, late Jurassic] took place at about Middle Purbeck times [probably correct]. Furthermore erosion of the Portland Beds occurred in the Weymouth region, probably on the crest of the Weymouth Anticline during Upper Purbeck times [Berriasian].

[Update comment - re Inversion Tectonics, October 2015 - imw. It is possible that derived materal was the result of early, local erosion that occurred on or adjacent to the north side of the Abbotsbury Fault because of Late Cimmerian uplift. The location is on the south side of the boundary of the Weymouth area inversion (separated from the main inversion, further west by the well-known relay ramp. Thus, there was early upthrow to the north. cf. the Weald Basin Inversion. On a much smaller scale it is probably evidence of a small analogue of the the famous Perce Allen - type of transport in the Weald where clastics from a northern "high" were trasported into the Weald Basin, prior to the tectonic inversion. In 1969 it was not viewed in terms of Inversion Tectonics. Subsequent finds of land-derived material, including cones and seeds, at Portesham in the Portesham Charophyte Chert nearby support this later theory. However, there are problems in applying this theory. See the details and the discussion below, and bear in mind the occurrence of the "Portland Screw" gastropod - Aptyxiella. This is a complication]

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Original Text:

"It is rarely possible to establish accurately the date of formation of chert in limestones. On petrographic evidence a diagenetic sequence involving silica has been suggested for the Portland Beds by Wilson (1966) and for the Lower Purbeck Beds [Lulworth Formation] by West (1964)[Evaporite diagenesis in the Lower Purbeck Beds of Dorset. Proc. Yorks. geol. Soc., 34, 315-330.]. Both suggested that an appreciable proportion of replacement by silica occurred at an early stage. A small excavation in the Upper Purbeck Beds [Durlston Formation] near Upwey, Dorset has recently revealed pebbles of derived chert in Purbeck sandstone and these indicate the date of origin of the chert.
The old overgrown excavations or quarries (map reference 644857), in which the unusual blocks of Upper Purbeck Unio Bed were found, lie about 100 metres to the west of Friar Waddon Farm. On the same line of strike, he Unio Bed was reported to have been seen in situ in the roadway just west of the farm by Wilson et al. (1958) and it can also be found in situ in the farmyard. These exposures are the most westerly of this horizon in Dorset. They lie jus south of a small exposure of problematical unfossiliferous sand that has been regarded as either basal Wealden Beds or decalcified Upper Purbeck Beds (Wilson et al. 1958).
2. The Unio Bed.
At Friar Waddon the Unio Bed is a hard, calcite-cemented sandstone but, in contents of Unio, Viviparus, fish teeth and reptilean bones, it resembles the equivalent limestone in the Isle of Purbeck. At both localities there is the unusual feature of glauconite present in an apparently freshwater bed, although the mineral is less abundant at Friar Waddon [added note - there is also oil in the bed at Stair Hole, Lulworth Cove]. The pebbles of limestone and chert with phosphatic nodules and derived fossils at the western locality, suggest that the bed marks a minor unconformity. It may perhaps indicate an appreciabe break in sedimentation that shortly preceded the change to a Wealden facies.
Fisher in 1856 observed that the Purbeck Beds of Dorset become more arenaceous westwards and in that direction contain more of the spoils of land. In the basal Purbeck Beds the evaporitic strata of east Dorset are interdigitated in the west with freshwater horizons (West, 1961). The pebbly and sandy nature of the Upper Purbeck Beds [Durlston Formation - part] at Friar Waddon are in accordance with the general westward facies change and the proximity of land to the west.
The brown and black phosphatic nodules of the Unio Bed are isotropic in thin-section. Phosphatic rolled fossils were submitted to Dr. Casey who kindly examined them. He found (personal communication, 1964) indeterminate perisphinctacean ammonites, ?Protocardia juv., ?Anisocardia juv. ?Eodonax juv. and the remains of a gastropod and a serpulid. He commented that the black phosphatic material reminded him of debris from the Upper Jurassic Lydite Beds in which the ammonite Pavlovia abounds.
The samples from the Unio Bed contain numerous fish teeth. Dr. Patterson who examined these, reported (personal communication, 1964) indeterminate perisphinctacean ammonites [noe added: i.e. like say the well-known Pavlovia rotunda of the Upper Kimmeridge Clay of Chapman's Pool, but not necessarily that species], ?Protocardia juv. ?Anisocardia juv. ?Eodonax juv. and the remains of a gastropod and a serpulid. He commented that the black phosphatic material reminded him of debris from the Upper Jurassic Lydite Beds in which the ammonite Pavlovia abounds. [note added. In modern terms that this is the type of reworked debris that is present at or near the Late Cimmerian Unconformity, under the Gault and UGS in the Isle of Wight, imw 29th Nov. 2015.]
The samples from the Unio Bed contain numerous fish teeth. Dr. Patterson who examined these, reported (personal communication, 1964)the presence of Hybodus sp., Lepidotes sp., Caturus tenuidens Smith Woodward, Coelodus mantelli Agassiz and other indeterminate fish teeth and scales. Patterson (1966) has since described a new species present at this horizon - Lonchiodon heterodon Patterson. Teeth of crocodiles are associated with the fish teeth [fossil crocodile remains are common in the Durlston Formation of the Purbeck Group, as at Durlston Bay, Swanage.]
Particle size analysis of the dilute acetic acid insoluble residue was undertaken. The residue included the chert pebbles, the reworked phosphatic material, the quartz sand and other clastic components. A bimodal distribution was observed with modes in the 2 to 3 (?) size range (fine sand) and the -5 to -4 (?) range (pebbles).
Petrography of the Pebbles [heading]
Most of the pebbles consist of cream coloured limestone or a weathered chert with some content of calcite. The limestone is an intramicrite (terminology of Folk, 1959 [ i.e. - with intraclasts in a micrite matrix - i.e. a carbonate breccia] of coarser grain size than any seen in the Purbeck Beds. Quartz sand is usually rare but there are fragments of thick-shelled bivalves and remains of echinoderms [both more Portland features than Purbeck]. Original siliceous spicules of Pachastrella and Rhaxella have been replaced by calcite. This phenomenon was reported by Wilson (1966) who found it in the Corallian and the Portland Beds but only associated with chert in the latter [normally abundant in the Portland Chert Member]. On independent evidence he regarded this calcitisation as an early stage of diagenesis. It is a common feature of the Portland Stone wherever micrite constitutes and appreciable portion of the rock.
The pebbles of chert consist of incompletely silicified intramicrite of similar petrography to the limestones and, significantly, with turreted gastropods [i.e Aptyxiella, well-known in the Portland Roach, but generally confined to the Isle of Portland. However, see below - there is a difference from Portland regarding the detritus]. An origin in the Portland Beds seems certain.
The thin-sections of the limestone and chert pebbles were compared to about fifty thin-sections of limestone and chert from the Portland Stone of various localities in Dorset. the pebbles resemble many specimens from the Portland Freestone of the Isle of Portland. There the high content of ooliths and the general lack of micrite is characteristic. It has not been possible to find exact comparisons. [note that the Portland Stone in the Upwey region is of a rather "chalky" facies and not in general a good oolite - see details of the Weymouth Relief Road excavation.]
Discussion.
The Friar Waddon Unio Bed demonstrates that erosion of the Portland Beds occurred in Purbeck times. The limestone pebbles suggest that the material was transported only a short distance because the calcium carbonate was not removed in solution. The presence of a turreted gastropod in chert suggests comparison with the chert containing Aptyxiella portlandica which is almost confined to the Roach at the Portland Freestone at the north end of the Isle of Portland [but Aptyxiella also occurs in the Vale of Wardour and in the old Swindon Town Gardens Quarry]. Pebbles of similar chert are present at the unconformity at the base of the Gault at Osmington ( Arkell, 1947). At the neighbouring locality of Holworth House, Ringstead, only a few kilometres from Portland the condensed rotunda sequence of the [Upper] Kimmeridge Clay resembles the Upper Lydite Bed (, Arkell, 1947, Casey 1967). Erosion of the former could have provided the derived phosphatic material.
The above evidence suggests that the material was eroded from an area that was probably near Friar Waddon near the north end of the Isle of Portland and near Ringstead [all in the Relay Ramp region]. Early erosion over the crest of the Weymouth Anticline would accord with this.
[Later comment by imw, 2015. However, note (2015) that the uplift of the Weymouth Anticline may not have happened so early, i.e. in the Late Cimmerian phase. The Weymouth Anticline is more likely to be due to the second phase of tectonics, that of the Tertiary compression folding, i.e. the later Inversion. This was not understood in 1969 when the paper was written.]
[continues]. It would account for the presence in the pebbles of features typical of both the Isle of Portland and of the Dorset mainland. Erosion probably penetrated down to the uppermost part of the Kimmeridge Clay at the centre of the anticline. Later, further uplift and erosion took place before deposition of the Gault.
The chert pebbles permit a date to be established for the major phase of silicification in the Portland Beds [Portland Stone Formation]. A careful examination was made to search for any signs of silicification occurring in situ in the Unio Bed. None was found and it clear that the pebbles were of chert before their emplacement in the Upper Purbeck Beds [Durlston Formation of the Purbeck Group]. No pebbles of Portland chert were found in the Lower Purbeck dirt beds, only authigenic nodules usually associated with the original presence of evaporites [silicification of evaporites is common in the Lulworth Formation of the Purbeck Group]. Nearly forty thin-sections of pebbles from the Great Dirt Bed of Portland and of Lulworth revealed no Portland chert, only a little silicification of microscopic dimensions in pebbles of Purbeck limestone. It would be surprising if none of the resistant Portland chert was found in any of these lower Purbeck [Lulworth Formation] pebble beds, had it existed at the time of their formation. Blake (1880) referred to some detrital Purbeck chert in a Purbeck dirt bed on Portland but this could not be confirmed and he may, in fact, have observed Purbeck authigenous silica.
Thus the major phaseof chert formation in the Portland Beds apparently took place between the deposition of the Lower Purbeck Caps and the Upper Purbeck Beds. Wilson's (1966) suggestion of two stages of silica diagenesis in the Portland Beds is supported by the presence of brecciated chert of the Chert Vein at St. Alban's [or St. Aldhelm's] Head, enclosed in a later chert matrix, and by other field evidence. It is now possible to confirm independently the early date of the major phase of silicification.
Acknowledgements. The authors are obliged to Dr. R. Casey and Dr. C. Patterson for the identification of faunal remains. They are most grateful to Professor F. Hodson [the late Professor F. Hodson] for advice and encouragement.

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References:

[Expanded versions of these references, some with abstracts, and also additional related publications are available online at:
Purbeck - Formation - Geological Bibliography - General .]

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Arkell, W.J. 1947. The Geology of the Country around Weymouth, Swanage, Corfe and Lulworth. Memoirs of the Geological Survey, U.K. London.
Blake, J.F. 1880. On the Portland Rocks of England. Quarterly Journal of the Geological Society of London, vol. 36, pp. 189-236.
Casey, R. 1967. The position of the middle Volgian in the English Jurassic. Proceedings of the Geological Society, London, No. 1640, pp. 128-133.
Fisher, O. 1856. On the Purbeck strata of Dorsetshire. Transactions of the Cambridge Philosophical Society, vol. 9, pp. 555-581.
Folk, R.L. 1959. Practical petrographic classification of limestone. Bulletin of the American Association of Petroleum Geologists, vol. 43, pp. 1-38.
Patterson, C. 1966. British Wealden Sharks. Bulletin of the British Museum (Natural History) Geology, vol. 11, pp. 281-350.
West, I.M. 1961. Lower Purbeck Beds of Swindon Facies in Dorset. [re. Portesham Quarry, nearby]. Nature, London, No. 190, p. 526 only.
West, I.M. 1964. Evaporite diagenesis in the Lower Purbeck Beds of Dorset. Proceedings of the Yorkshire Geological Society. vol. 34, pp. 315-330.
Wilson, R.C.L. 1966. Silica diagenesis in Upper Jurassic Limestones of Southern England. Journal of Sedimentary Petrology, vol. 36, pp. 1036-1049.
Wilson, V., Welch, F.B.A., Robbie, J.A. and Green, G.W. 1958. Geology of the Country around Bridport and Yeovil.

[end of original reference list]

[Authors' addresses in 1969]
M.J.Hooper,
Campbell Chibougamou Mines Ltd.
Chibougamous,
Quebec.

I.M. West,
Department of Geology,
The University
Southampton.