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A derived Lower Jurassic clast from the Wealden Group (Lower Cretaceous) of the Isle of Wight, southern England
SHORT COMMUNICATION
A derived Lower Jurassic clast from the Wealden Group (Lower Cretaceous) of the Isle of Wight, southern England Jon D. Radley RADLEY, J. D. 1993. A derived Lower Jurassic clast from the Wealden Group (Lower Cretaceous) of the Isle of Wight, southern England. Proceedings of the Geologists' Association, 104,71-73. A clast containing Sinemurian (Lower Jurassic) ammonites is described from the Wessex Formation (Wealden Group, Barremian) of the Isle of Wight. A possible source is the eastern margin of the Cornubian Massif, rather than the Purbeck-Isle of Wight fault zone which probably shed only Upper Jurassic material in Wealden times. This is the first record of derived Lower Jurassic macrofauna in the Lower Cretaceous of the region. Museum of Isle of Wight Geology, High Street, Sandown, Isle of Wight P036 BAF.
1. INTRODUCTION
Within the Wessex Formation of the Isle of Wight, derived pebbles are occasionally found in plant debris beds (storm-generated accumulations of wood and other allochthonous fossil debris: Stewart, 1978, 1981) and more rarely in alluvial units (personal observations). The clasts comprise 'Palaeozoic-type' cherts and other quartzose rocks (previously detected in the Wessex Formation of east Dorset: Oakley, 1947), and uppermost Jurassic (Portland Stone) and possible early Cretaceous (Purbeck Formation) debris. Some of these may be dinosaur gastroliths (Osborne White, 1921; Allen, 1975), and many must have been transported in the root systems of floating trees. The lithological composition of the Wessex Formation clasts thus resembles that from the younger marine Lower Greensand Group (see above), implying a broadly similar source and geology.
Coarse detritus is locally common in the Upper Jurassic and Lower Cretaceous strata of southern England. Analysis of the clastic suites leads to useful palaeogeographic and structural conclusions (Allen, 1972, 1989; Garden, 1987, 1991). The Lower Cretaceous strata of the Isle of Wight were deposited in the Wessex (or 'North Channel') sub-basin (Allen, 1959), to the south of the Isle of Wight monocline. In early Cretaceous times this feature was part of the upthrown side of the Purbeck-Wight fault zone, with the Wessex sub-basin depocentre lying to the south (Allen, 1981; Chadwick, 1985; Sellwood, Scott & Lunn, 1986). This structural high was probably the source of Upper Jurassic clasts in the Wealden of the Isle of Wight, which are found together with early Cretaceous pebbles in the overlying marine Lower Greensand Group (Aptian-Albian) there (Garden, 1987, 1991). Palaeozoic lithologies (quartz and chert) nevertheless characterize the Lower Greensand pebble beds (Kirkaldy, 1947; Garden, 1991) and were probably derived from the eastern margin of the Welsh Massif (Garden, 1991).
3. OCCURRENCE AND DESCRIPTION OF A FOSSILIFEROUS LOWER JURASSIC CLAST The clast described here came from slightly carbonaceous sandy mudstones, approximately 3 m below the 'Black Band', about 500 m east of Grange Chine in Brighstone Bay, on the southwest coast of the Isle of Wight (SZ 4244 8149, Fig. 1). The Black Band is a conspicuous plant debris bed in the Wessex Formation. It is approximately 1 m thick and lies about 95 m below the base of the Vectis Formation (Osborne White, 1921; Stewart, 1981, 1983). The bed from which the clast was recovered probably formed part of the base of a fine-grained river point-bar sequence (Stewart, 1983). The clast is a large cobble, with a weight of 3750 g. Dimensions are approximately 165 x 150 x 85 mm. The edges and corners are well rounded and its upper and lower surfaces are polished and show fine longitudinal striae. The latter are attributed to post-depositional compaction and re-adjustment be-
2. DERIVED CLASTS IN THE WESSEX FORMATION (WEALDEN GROUP, LOWER CRETACEOUS) The exposed Wealden Group of the Isle of Wight (Barremian or possibly up to early Aptian: Kerth & Hailwood, 1988; Allen, 1989; Allen & Wimbledon, 1991) comprises the Wessex Formation and overlying Vectis Formation (Stewart, 1978, 1981). The Wessex Formation is predominantly of subtropical alluvial origin (Stewart, 1978, 1981; Allen 1981) while the Vectis Formation is essentially of coastal lagoonal origin (Allen, 1981; Stewart, Ruffell, Wach & Goldring, 1991). 71
72
JON D. RADLEY
ertsmoulh •
.. . . .
__
- r - ~ '
••
Tertiary .......-'--r-'-rl C h a Ik
Cretaceous
Gault/Greensand Wealden
~.".,
FIg. 1. Location and geological setting of Grange Chine, Isle of Wight.
tween the lithified cobble and unconsolidated matrix. Colour is medium grey (NS) to dark grey (N3) of Goddard, Trask, DeFord, Rove, Singewald & Overbeck (1963). The lithology of the cobble is hard calcareous mudstone, crossed by calcite veins. It is highly fossiliferous; the surfaces displaying small ammonites (1-3 em diameter), fragments of thinshelled bivalves and trace fossils. The ammonites are relatively uncrushed and are preserved largely as worn and corroded sparry calcite casts with cemented mudstone infilling some of the body chambers. Polished sections of the rock display cross-sections through badly-preserved ammonites, comminuted bivalve, ammonite and echinoid debris, Chondrites burrows and calcite-filled cracks. The poorly preserved ammonites belong to the genus Promicroceras Spath (B. M. Cox, pers. comm.). On the west Dorset coast of southwest England (Lang & Spath, 1926), small Promicroceras spp. characterize the Black Yen Marls (Sinemurian Stage, Lower Lias, Lower Jurassic, e.g. Getty & Ivimey-Cook, 1980). The examples in the cobble agree well in size and shape with those figured by Lang & Spath (1926). Their abundance, preservation and occurrence in cemented calcareous mudstone invites comparison with certain bands in the Black
Yen Marls such as the StellaTe Nodules (bed 88f of Lang & Spath, 1926). 4. ORIGIN AND PROVENANCE The large size of the cobble and its association with plant debris and fine-grained sediment suggests tree-root transportation, probably to a point-bar environment. This is the first published record of derived Lower Jurassic macrofossils in the outcropping Lower Cretaceous of the Wessex sub-basin. Hitherto, the oldest Jurassic clasts noted by the present author in the Wessex Formation of the Isle of Wight were of Portland Stone (Portlandian, Upper Jurassic) age. Similarly, the younger Lower Greensand pebble beds of the Isle of Wight have only revealed Upper Jurassic macrofauna (Garden, 1987, 1991; and personal observations). In the Wessex Formation of the Isle of Purbeck, east Dorset, Garden (1987, 1991) also recorded derived Upper Jurassic fossils. This strongly indicates that erosion to the north of the Purbeck-Isle of Wight fault zone only exposed Upper Jurassic strata during Lower Cretaceous times. Borehole evidence however, suggests that Middle Jurassic sediments were being locally exhumed (Chadwick, 1985).
SHORT COMMUNICATION
The source of the considerably older Lower Jurassic clast raises interesting problems. Its similarity to Sinemurian rocks on the west Dorset coast suggests derivation from further west, probably an area flanking the Cornubian landmass. Nevertheless, a closer source cannot be ruled out entirely, for example the fault blocks of the Wytch Farm oilfield structure in east Dorset (e.g. Colter & Havard, 1981).
73
ACKNOWLEDGEMENTS I thank Perce Allen and Ross Garden for encouragement and comments on an early manuscript. I am also indebted to Drs B. M. Cox and H. Ivimey-Cook of the British Geological Survey for identifying the Liassic ammonites and to Steve Hutt of the Museum of Isle of Wight Geology who discovered the derived clast.
REFERENCES ALLEN, P. 1959. The Wealden environment: Anglo-Paris basin. Philosophical Transactions of the Royal Society, London, 8242,283-346. - - 1972. Wealden detrital tourmaline: implications for northwestern Europe. Journal of the Geological Society, London, 128, 273-294. - - 1975. Wealden of the Weald: a new model. Proceedings of the Geologists' Association, 86, 389-438. - - 1981. Pursuit of Wealden models. Journal of the Geological Society, London, 138, 375-405. - - 1989. Wealden research-ways ahead. Proceedings of the Geologists' Association, 100, 529-564. - - & WIMBLEDON, W. A. 1991. Correlation of NW European Purbeck-Wealden (nonmarine Lower Cretaceous) as seen from the English type areas. Cretaceous Research, 12,511-526. CHADWICK, R. A. 1985. End Jurassic-early Cretaceous sedimentation and subsidence (late Portlandian to Barremian) and the late Cimmerian unconformity. In (Whittaker, A; ed.) Atlas of Onshore Sedimentary Basins in England and Wales: Post-Carbiniferous Tectonics and Stratigraphy, British Geological Survey, Blackie, Glasgow, 52-56. COLTER, V. S. & HARVARD, D. J. 1981. The Wytch Farm Oil Field, Dorset. In (Illing, L. V. & Hobson, G. D., eds) Petroleum Geology of the Continental Shelf of North-West Europe. Institute of Petroleum, London, 494-503. GARDEN, I. R. 1987. 'The provenance of Upper Jurassic and Lower Cretaceous coarse-grained detritus in Southern Britain and Normandy.' PhD thesis, University of Southampton. - - 1991. Changes in the provenance of pebbly detritus in southern Britain and northern France associated with basin rifting. in (Morton, A. C., Todd, S. P. & Haughton, P. D. W.; eds) Developments in Sedimentary Provenance Studies. Geological Society, London, Special Publication, 57, 273-289. GETTY, T. A. & IVIMEY-COOK, H. 1980. Hettangian and Sinemurian correlation chart. In (Cope, J. C. W., Getty, T. A, Howarth, M. K., Morton, N. & Torrens, H. S.; eds) A Correlation of Jurassic Rocks in the British Isles. Part 1. Geological Society, London, Special Report, 14, 33-47.
GODDARD, E. N. TRASK, P. D., DEFORD, R. K., ROVE, O. N., SINGEWALD, J. T. & OVERBECK, R. M. 1963. Rock Color Chart. Geological Society of America. KERTH, M. & HAlLWOOD, E. A. 1988. Magnetostratigraphy of the Lower Cretaceous Vectis Formation (Wealden Group) on the Isle of Wight, Southern England. Journal of the Geological Society, London, 145, 351-360. KIRKALDY, J. F. 1947. The Provenance of the Pebbles in the Lower Cretaceous Rocks. Proceeding of the Geologists' Association, 58, 223-241. LANG, W. D. & SPATH, L. F. 1926. The Black Marl of Black Ven and Stonebarrow, in the Lias of the Dorset Coast. Quarterly Journal of the Geological Society of London, 82, 144-187. OAKLEY, K. P. 1947. A Note on Palaeozoic Radiolarian Chert Pebbles found in the Wealden Series of Dorset. Proceedings of the Geologists' Association, 58,255-258. OSBORNE WHITE, H. J. 1921. A Short Account of the Geology of the Isle of Wight. Memoir of the Geological Survey of the United Kingdon, HMSO, London. SELLWOOD, B. W., SCOTT, J. & LUNN, G. 1986. Mesozoic basin evolution in Southern England. Proceedings of the Geologists' Association, 97,259-289. STEWART, D. J. 1978. 'The sedimentology and palaeoenvironment of the Wealden Group of the Isle of Wight, Southern England,' PhD thesis, Portsmouth Polytechnic. - - 1981. A field guide to the Wealden Group of the Hastings area and the Isle of Wight. In (Elliott, T.; ed.) Field Guides to Modem and Ancient Fluvial Systems in Britain and Spain 3.1-3.32. International Fluvial Conference, Keele University, UK. - - 1983. Possible suspended-load channel deposits from the Wealden Group (Lower Cretaceous) of Southern England. Special Publications of the International Association of Sedimentologists, 6,369-384. - , RUFFELL, A, WACH, G. & GOLDRING, R. 1991. Lagoonal sedimentation and fluctuating salinities in the Vectis Formation (Wealden Group, Lower Cretaceous) of the Isle of Wight, southern England. Sedimentary Geology, 72, 117-134.
A derived Lower Jurassic clast from the Wealden Group (Lower Cretaceous) of the Isle of Wight, southern England Jon D. Radley RADLEY, J. D. 1993. A derived Lower Jurassic clast from the Wealden Group (Lower Cretaceous) of the Isle of Wight, southern England. Proceedings of the Geologists' Association, 104,71-73. A clast containing Sinemurian (Lower Jurassic) ammonites is described from the Wessex Formation (Wealden Group, Barremian) of the Isle of Wight. A possible source is the eastern margin of the Cornubian Massif, rather than the Purbeck-Isle of Wight fault zone which probably shed only Upper Jurassic material in Wealden times. This is the first record of derived Lower Jurassic macrofauna in the Lower Cretaceous of the region. Museum of Isle of Wight Geology, High Street, Sandown, Isle of Wight P036 BAF.
1. INTRODUCTION
Within the Wessex Formation of the Isle of Wight, derived pebbles are occasionally found in plant debris beds (storm-generated accumulations of wood and other allochthonous fossil debris: Stewart, 1978, 1981) and more rarely in alluvial units (personal observations). The clasts comprise 'Palaeozoic-type' cherts and other quartzose rocks (previously detected in the Wessex Formation of east Dorset: Oakley, 1947), and uppermost Jurassic (Portland Stone) and possible early Cretaceous (Purbeck Formation) debris. Some of these may be dinosaur gastroliths (Osborne White, 1921; Allen, 1975), and many must have been transported in the root systems of floating trees. The lithological composition of the Wessex Formation clasts thus resembles that from the younger marine Lower Greensand Group (see above), implying a broadly similar source and geology.
Coarse detritus is locally common in the Upper Jurassic and Lower Cretaceous strata of southern England. Analysis of the clastic suites leads to useful palaeogeographic and structural conclusions (Allen, 1972, 1989; Garden, 1987, 1991). The Lower Cretaceous strata of the Isle of Wight were deposited in the Wessex (or 'North Channel') sub-basin (Allen, 1959), to the south of the Isle of Wight monocline. In early Cretaceous times this feature was part of the upthrown side of the Purbeck-Wight fault zone, with the Wessex sub-basin depocentre lying to the south (Allen, 1981; Chadwick, 1985; Sellwood, Scott & Lunn, 1986). This structural high was probably the source of Upper Jurassic clasts in the Wealden of the Isle of Wight, which are found together with early Cretaceous pebbles in the overlying marine Lower Greensand Group (Aptian-Albian) there (Garden, 1987, 1991). Palaeozoic lithologies (quartz and chert) nevertheless characterize the Lower Greensand pebble beds (Kirkaldy, 1947; Garden, 1991) and were probably derived from the eastern margin of the Welsh Massif (Garden, 1991).
3. OCCURRENCE AND DESCRIPTION OF A FOSSILIFEROUS LOWER JURASSIC CLAST The clast described here came from slightly carbonaceous sandy mudstones, approximately 3 m below the 'Black Band', about 500 m east of Grange Chine in Brighstone Bay, on the southwest coast of the Isle of Wight (SZ 4244 8149, Fig. 1). The Black Band is a conspicuous plant debris bed in the Wessex Formation. It is approximately 1 m thick and lies about 95 m below the base of the Vectis Formation (Osborne White, 1921; Stewart, 1981, 1983). The bed from which the clast was recovered probably formed part of the base of a fine-grained river point-bar sequence (Stewart, 1983). The clast is a large cobble, with a weight of 3750 g. Dimensions are approximately 165 x 150 x 85 mm. The edges and corners are well rounded and its upper and lower surfaces are polished and show fine longitudinal striae. The latter are attributed to post-depositional compaction and re-adjustment be-
2. DERIVED CLASTS IN THE WESSEX FORMATION (WEALDEN GROUP, LOWER CRETACEOUS) The exposed Wealden Group of the Isle of Wight (Barremian or possibly up to early Aptian: Kerth & Hailwood, 1988; Allen, 1989; Allen & Wimbledon, 1991) comprises the Wessex Formation and overlying Vectis Formation (Stewart, 1978, 1981). The Wessex Formation is predominantly of subtropical alluvial origin (Stewart, 1978, 1981; Allen 1981) while the Vectis Formation is essentially of coastal lagoonal origin (Allen, 1981; Stewart, Ruffell, Wach & Goldring, 1991). 71
72
JON D. RADLEY
ertsmoulh •
.. . . .
__
- r - ~ '
••
Tertiary .......-'--r-'-rl C h a Ik
Cretaceous
Gault/Greensand Wealden
~.".,
FIg. 1. Location and geological setting of Grange Chine, Isle of Wight.
tween the lithified cobble and unconsolidated matrix. Colour is medium grey (NS) to dark grey (N3) of Goddard, Trask, DeFord, Rove, Singewald & Overbeck (1963). The lithology of the cobble is hard calcareous mudstone, crossed by calcite veins. It is highly fossiliferous; the surfaces displaying small ammonites (1-3 em diameter), fragments of thinshelled bivalves and trace fossils. The ammonites are relatively uncrushed and are preserved largely as worn and corroded sparry calcite casts with cemented mudstone infilling some of the body chambers. Polished sections of the rock display cross-sections through badly-preserved ammonites, comminuted bivalve, ammonite and echinoid debris, Chondrites burrows and calcite-filled cracks. The poorly preserved ammonites belong to the genus Promicroceras Spath (B. M. Cox, pers. comm.). On the west Dorset coast of southwest England (Lang & Spath, 1926), small Promicroceras spp. characterize the Black Yen Marls (Sinemurian Stage, Lower Lias, Lower Jurassic, e.g. Getty & Ivimey-Cook, 1980). The examples in the cobble agree well in size and shape with those figured by Lang & Spath (1926). Their abundance, preservation and occurrence in cemented calcareous mudstone invites comparison with certain bands in the Black
Yen Marls such as the StellaTe Nodules (bed 88f of Lang & Spath, 1926). 4. ORIGIN AND PROVENANCE The large size of the cobble and its association with plant debris and fine-grained sediment suggests tree-root transportation, probably to a point-bar environment. This is the first published record of derived Lower Jurassic macrofossils in the outcropping Lower Cretaceous of the Wessex sub-basin. Hitherto, the oldest Jurassic clasts noted by the present author in the Wessex Formation of the Isle of Wight were of Portland Stone (Portlandian, Upper Jurassic) age. Similarly, the younger Lower Greensand pebble beds of the Isle of Wight have only revealed Upper Jurassic macrofauna (Garden, 1987, 1991; and personal observations). In the Wessex Formation of the Isle of Purbeck, east Dorset, Garden (1987, 1991) also recorded derived Upper Jurassic fossils. This strongly indicates that erosion to the north of the Purbeck-Isle of Wight fault zone only exposed Upper Jurassic strata during Lower Cretaceous times. Borehole evidence however, suggests that Middle Jurassic sediments were being locally exhumed (Chadwick, 1985).
SHORT COMMUNICATION
The source of the considerably older Lower Jurassic clast raises interesting problems. Its similarity to Sinemurian rocks on the west Dorset coast suggests derivation from further west, probably an area flanking the Cornubian landmass. Nevertheless, a closer source cannot be ruled out entirely, for example the fault blocks of the Wytch Farm oilfield structure in east Dorset (e.g. Colter & Havard, 1981).
73
ACKNOWLEDGEMENTS I thank Perce Allen and Ross Garden for encouragement and comments on an early manuscript. I am also indebted to Drs B. M. Cox and H. Ivimey-Cook of the British Geological Survey for identifying the Liassic ammonites and to Steve Hutt of the Museum of Isle of Wight Geology who discovered the derived clast.
REFERENCES ALLEN, P. 1959. The Wealden environment: Anglo-Paris basin. Philosophical Transactions of the Royal Society, London, 8242,283-346. - - 1972. Wealden detrital tourmaline: implications for northwestern Europe. Journal of the Geological Society, London, 128, 273-294. - - 1975. Wealden of the Weald: a new model. Proceedings of the Geologists' Association, 86, 389-438. - - 1981. Pursuit of Wealden models. Journal of the Geological Society, London, 138, 375-405. - - 1989. Wealden research-ways ahead. Proceedings of the Geologists' Association, 100, 529-564. - - & WIMBLEDON, W. A. 1991. Correlation of NW European Purbeck-Wealden (nonmarine Lower Cretaceous) as seen from the English type areas. Cretaceous Research, 12,511-526. CHADWICK, R. A. 1985. End Jurassic-early Cretaceous sedimentation and subsidence (late Portlandian to Barremian) and the late Cimmerian unconformity. In (Whittaker, A; ed.) Atlas of Onshore Sedimentary Basins in England and Wales: Post-Carbiniferous Tectonics and Stratigraphy, British Geological Survey, Blackie, Glasgow, 52-56. COLTER, V. S. & HARVARD, D. J. 1981. The Wytch Farm Oil Field, Dorset. In (Illing, L. V. & Hobson, G. D., eds) Petroleum Geology of the Continental Shelf of North-West Europe. Institute of Petroleum, London, 494-503. GARDEN, I. R. 1987. 'The provenance of Upper Jurassic and Lower Cretaceous coarse-grained detritus in Southern Britain and Normandy.' PhD thesis, University of Southampton. - - 1991. Changes in the provenance of pebbly detritus in southern Britain and northern France associated with basin rifting. in (Morton, A. C., Todd, S. P. & Haughton, P. D. W.; eds) Developments in Sedimentary Provenance Studies. Geological Society, London, Special Publication, 57, 273-289. GETTY, T. A. & IVIMEY-COOK, H. 1980. Hettangian and Sinemurian correlation chart. In (Cope, J. C. W., Getty, T. A, Howarth, M. K., Morton, N. & Torrens, H. S.; eds) A Correlation of Jurassic Rocks in the British Isles. Part 1. Geological Society, London, Special Report, 14, 33-47.
GODDARD, E. N. TRASK, P. D., DEFORD, R. K., ROVE, O. N., SINGEWALD, J. T. & OVERBECK, R. M. 1963. Rock Color Chart. Geological Society of America. KERTH, M. & HAlLWOOD, E. A. 1988. Magnetostratigraphy of the Lower Cretaceous Vectis Formation (Wealden Group) on the Isle of Wight, Southern England. Journal of the Geological Society, London, 145, 351-360. KIRKALDY, J. F. 1947. The Provenance of the Pebbles in the Lower Cretaceous Rocks. Proceeding of the Geologists' Association, 58, 223-241. LANG, W. D. & SPATH, L. F. 1926. The Black Marl of Black Ven and Stonebarrow, in the Lias of the Dorset Coast. Quarterly Journal of the Geological Society of London, 82, 144-187. OAKLEY, K. P. 1947. A Note on Palaeozoic Radiolarian Chert Pebbles found in the Wealden Series of Dorset. Proceedings of the Geologists' Association, 58,255-258. OSBORNE WHITE, H. J. 1921. A Short Account of the Geology of the Isle of Wight. Memoir of the Geological Survey of the United Kingdon, HMSO, London. SELLWOOD, B. W., SCOTT, J. & LUNN, G. 1986. Mesozoic basin evolution in Southern England. Proceedings of the Geologists' Association, 97,259-289. STEWART, D. J. 1978. 'The sedimentology and palaeoenvironment of the Wealden Group of the Isle of Wight, Southern England,' PhD thesis, Portsmouth Polytechnic. - - 1981. A field guide to the Wealden Group of the Hastings area and the Isle of Wight. In (Elliott, T.; ed.) Field Guides to Modem and Ancient Fluvial Systems in Britain and Spain 3.1-3.32. International Fluvial Conference, Keele University, UK. - - 1983. Possible suspended-load channel deposits from the Wealden Group (Lower Cretaceous) of Southern England. Special Publications of the International Association of Sedimentologists, 6,369-384. - , RUFFELL, A, WACH, G. & GOLDRING, R. 1991. Lagoonal sedimentation and fluctuating salinities in the Vectis Formation (Wealden Group, Lower Cretaceous) of the Isle of Wight, southern England. Sedimentary Geology, 72, 117-134.