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Pseudomorphs after gypsum from the Bembridge Marls
Pseudomorphs after Gypsum from the Bembridge Marls by BRIAN DALEY Received 4 February 1966; taken as read 5 November 1966.
CONTENTS I.
2. 3. 4. 5. 6.
page 319
INTRODUCTION OCCURRENCE DESCRIPTION GYPSUM FORMATION PSEUDOMORPH FORMATION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES
319
320 320 322 322 322 323
ABSTRACT: Clastic pseudomorphs after gypsum described from the non-marine part of the Bembridge Marls of the Isle of Wight, Hampshire, England, indicate a short-lived interval in which evaporite conditions existed.
1. INTRODUCTION THE BEMBRlDGE MARLS of Oligocene age which outcrop in the Isle of Wight, Hampshire, England, consist of a basal marine sequence followed transitionally upwards by non-marine beds, to give a total thickness of approximately 100 ft. The non-marine sediments have been successively described as freshwater and estuarine (Reid & Strahan, 1889, 170), deltaic and consisting of brackish and freshwater beds (Chatwin, 1960, 73), in the main freshwater (Hughes, 1922, 66), variable fluvio-marine and lacustrine (Stinton, 1963, 89) and estuarine followed upwards by essentially freshwater beds (Curry, 1965, 170). 2. OCCURRENCE The recent discovery of pseudomorphs after gypsum in the non-marine beds suggests, that, for at least one short period of time, evaporite conditions also existed, under which the precipitation of gypsum took place. The pseudomorphs occur at a horizon some 6 ft. (1.8 m.) above the base of the 4 ft. (1.2 m.) sandy grey limestone of Forbes (Reid & Strahan, 1889, 170) and some 48 ft. (14.4 m.) above the top of the Bembridge Limestone, in the low cliff of Bembridge Marls, which dip northwards from Whitecliff Bay (642863)1 at a very low angle until they disappear in Howgate Bay (648868) under a covering of Quaternary Gravel, or are obscured by slumping. 'All Grid References lie within the !OO Km . square 40 (SZ).
319
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BRIAN DALEY
3. DESCRIPTION The pseudomorphs, or clastic casts (Brooks, 1955), occur mainly as minute sole structures on the indurated calcareous siltstone bands (ranging in thickness from 2 to 10 mm.) of a thinly laminated series of silts and clays which occur at this level. Poorly preserved, but nevertheless identifiable, pseudomorphs also occur less frequently on the upper surfaces of such bands. The pseudomorphs are most readily discernible on tumbled fragments of siltstone, whose surfaces have been washed clean of adhering clay by rainwater. In addition, such surfaces show a variety of other structures. Those of the siltstone soles include elongated erosional fills and the casts of desiccation cracks which developed in the underlying clays. The pseudomorphs range in type from single crystals, lying isolated, to groups of interpenetrating crystals (Plate 12). They can be conveniently divided into four types, although the boundaries between them are transitional: (1) Simple lenticular crystals (the majority approximately 1.0 to 0.5 rom. in diameter). Each possesses two curved faces and a sharp periphery, circular to oval in outline. (2) Intergrowths of two or three crystals in a regular manner. These may be twinned and not merely interpenetrating at random. (3) Clusters of more numerous crystals which form small knobs protruding outwards from the siltstone surfaces and which vary from groups of few crystals (transitional types from (2) above) to small (1-2 rom. in diameter) circular aggregates consisting of numerous intergrown discoid crystals which resemble miniature 'desert roses' or gypsum rosettes similar to, but far smaller than, those described by Kerr & Thomson (1963, 1729-30) from the Laguna Madre. (4) Irregular patches or lines of crystals, in some cases consisting of anastomosed rosettes and, in others, of intimately associated crystals of types (1) and (2) above. 4. GYPSUM FORMATION Little doubt exists that the original mineral was gypsum, for lenticular gypsum and gypsum rosettes of similar appearance have been described from modem sediments forming in areas subjected to high rates of evaporation, such as the mud-flats of the Laguna Madre, Texas (Masson, EXPLANATION OF PLATE 12 Top: Siltstone sole showing pseudomorphs of types 1 and 2, together with casts of desiccation cracks Bottom: Siltstone sole showing rosettes and anastomosed rosettes of types 3 and 4
PROC. GEOL. ASS., VOL. 78 (1967)
PLATE 12
[To face p. 320
PSEUDOMORPHS AFTER GYPSUM
321
1955; Kerr & Thomson, 1963) and the 'sebhka' coast flats of Abu Dhabi in the Perisan Gulf (Shearman, 1963, 1965 and personal communication). Macfadyen (1950) described lenticular gypsum from a raised beach in Somalia and lists other occurrences both recent and fossil. In the cases mentioned above , the gypsum is not deposited by direct precipitation on to the bottom from a body of standing water, but as an early diagenetic mineral within the uppermost sedimentary layers, where ground water has become concentrated by evaporation. IIling (1963), however, reports the precipitation of gypsum direct from concentrating sea-water in 'sebhka' sediments near Qatar, farther north in the Persian Gulf. Posnjak (1940) reports that gypsum precipitates from water when the salinity reaches three-and-one-third times that of normal sea-water. The faunal evidence from the horizon in question does not even suggest a salinity comparable with that of normal sea-water. Potomac/is turritissima Forbes is the only invertebrate found at this level to date. Certain modern members of the Melaniidae, to which this species is assigned, have some degree of salinity tolerance, but even if the salinity tolerance of Potomac/is were known, the fact that it frequently occurs current orientated, together with other evidence of water movement, suggests that it may very likely be that of a drifted assemblage rather than that of a community from which environmental deductions can be made. It is, therefore, suggested that the gypsum originated beneath the sedimentary surface in a manner comparable with that of the early diagenetic gypsum of modern sediments, rather than from hypersaline surface water. Exposure to the air is demonstrated by the presence of desiccation cracks and it is suggested that the evaporation from such surfaces could cause high concentrations of pore water sufficient to effect the formation of gypsum. A high rate of evaporation is compatible with the climatic implications of palaeobotanical work on the Bembridge Marls (Reid & Chandler, 1926). Such a formation could result from the concentration of waters of much lower salinity such as brackish water and does not necessitate the presence of highly saline surface water. Such waters could enter the pseudomorph bearing sediments either by lateral seepage from local surface water (cf. Shearman, 1963) or by downward seepage following the temporary inundation of the area (cf. Kerr & Thomson, 1963). The second possibility is preferred in this case, for, whereas waters spreading superficially could have sunk into the upper layers of sediment, the lateral migration of water within the sediment would have been inhibited by the clay and the discontinuous nature of the more porous silt bands. The small size of the pseudomorphs suggests a gypsum formation very close to the sediment surface (Kerr & Thomson, 1963, 1729) and that PltOC . GEOL . ASS. , VOL. 78, PART 2, 1967
21
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BRIAN DALEY
growth was discontinued after an apparently short period of time due to changes in the environmental conditions. 5. PSEUDOMORPH FORMATION The retention of good crystal shape by the pseudomorphs indicates the proximity of gypsum solution and clastic infilling in time. Since the pseudomorphs occur both as sole and top surface structures, and, assuming a similar formation in each case, they must have developed within the sediment by contemporaneous infilling and solution. In the case of the sole pseudomorphs such infilling would have resulted from the gradual replacement of gypsum by silt which fell under gravity or was depressed into the space originally occupied by the crystals. The pseudomorphs of the siltstone top surfaces would have formed by an essentially similar process, but one in which the silt was pressed upwards into the moulds developing on the upper surfaces of the silt bands. Both infilling by silt and clay probably occurred, but the resulting pseudomorphs would only be apparent if the infilling were distinguishable from the host sediment. This explains the presence of pseudomorphs on the upper and lower surfaces of the siltstones, and their apparent absence within individual clay or siltstone bands. Other occurrences of pseudomorphs after lenticular gypsum have recently been described from the Purbeck rocks of Dorset (West, 1964). Unlike those under consideration in the present paper, the pseudomorphs described by West are mineralogical rather than clastic replacements of the original gypsum. 6. CONCLUSIONS Pseudomorphs after gypsum have been identified from the traditionally non-marine part of the Bembridge Marls. These formed within the sediment by the gradual piecemeal replacement of gypsum by clastic material when the silt was still mobile and uncemented. The original gypsum formed post-depositionally within the upper layers of the sediment as a result of the concentration of contained water by evaporation from sediment surfaces exposed to the air. No evidence exists for the presence at this time of surface water of high or even normal marine salinity. ACKNOWLEDGMENTS I should like to express my thanks to Mr. D. J. Shearman, Mr. N. Edwards and Mr. I. M. West for helpful discussion and suggestions, and to Dr. R. Goldring, who, in addition, criticised the manuscript. Mr. J. Mauger kindly took the photographs. This work was financed by Portsmouth College of Technology, and for this I am most grateful.
PSEUDOMORPHS AFTER GYPSUM
323
REFERENCES BROOKS, H. K. 1955. Clastic Casts of Halite Crystal Imprints from the Rome Formation (Cambrian) of Tennessee. J. sedim. Petrol., 25, 67-71. CHATWIN, C. P. 1960. The Hampshire Basin and Adjoining Areas. Br. reg. Geol., 3rd ed. CURRY, D. 1965 The Palaeogene Beds of South-East England. Proc. Geol. Ass., 76, 151-73. HUGHES, J. C. 1922. Geological Story of the Isle of Wight, London. ILLlNG, L. Y. 1963. Discussion on Shearman, D. J. (1963). Proc. geol. Soc., 1607, 64-5. KERR, S. D. & A. THOMSON. 1963. Origin of Nodular and Bedded Anhydrite in Permian Shelf Sediments, Texas and New Mexico. Bull. Am. Ass. Petrol. Geol.,47,1726-32. MASSON. 1955. An Occurrence of Gypsum in Southwest Texas. J. sedim. Petrol., 25,72-7. MACFADYEN, W. A. 1950. Sandy Gypsum Crystals from Berbera, Somaliland. Geol. Mag., 87, 409-20. POSNJAK, E. 1940. Deposition of Calcium Sulfate from Sea Water. Am. J. Sci., 35, 539-68. REID, C. & A. STRAHAN. 1889. The Geology of the Isle of Wight. Mem. geol. Survey U.K., 2nd ed. REID, E. M. & M. E. J. CHANDLER. 1926. Catalogue of Cainozoic Plants, Vol. 1. The Bembridge Flora. Brit. Mus. (Nat. Hist.), Lond. SHEARMAN, D. J. 1963. Recent Anhydrite, Gypsum, Dolomite and Halite from the Coastal Flats of the Arabian Shore of the Persian Gulf. Proc. geol. Soc., 1607,63-4. STINTON, F. C. 1963. Field Meeting in the Oligocene of North-West Isle of Wight. Proc. Geol. Ass., 75,15-31. WEST, I. M. 1964. Evaporite diagenesis in the Lower Purbeck Beds of Dorset. Proc, Yorks. geol. Soc., 35, 47-58. B. Daley Department of Chemistry and Geology Portsmouth College of Technology
CONTENTS I.
2. 3. 4. 5. 6.
page 319
INTRODUCTION OCCURRENCE DESCRIPTION GYPSUM FORMATION PSEUDOMORPH FORMATION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES
319
320 320 322 322 322 323
ABSTRACT: Clastic pseudomorphs after gypsum described from the non-marine part of the Bembridge Marls of the Isle of Wight, Hampshire, England, indicate a short-lived interval in which evaporite conditions existed.
1. INTRODUCTION THE BEMBRlDGE MARLS of Oligocene age which outcrop in the Isle of Wight, Hampshire, England, consist of a basal marine sequence followed transitionally upwards by non-marine beds, to give a total thickness of approximately 100 ft. The non-marine sediments have been successively described as freshwater and estuarine (Reid & Strahan, 1889, 170), deltaic and consisting of brackish and freshwater beds (Chatwin, 1960, 73), in the main freshwater (Hughes, 1922, 66), variable fluvio-marine and lacustrine (Stinton, 1963, 89) and estuarine followed upwards by essentially freshwater beds (Curry, 1965, 170). 2. OCCURRENCE The recent discovery of pseudomorphs after gypsum in the non-marine beds suggests, that, for at least one short period of time, evaporite conditions also existed, under which the precipitation of gypsum took place. The pseudomorphs occur at a horizon some 6 ft. (1.8 m.) above the base of the 4 ft. (1.2 m.) sandy grey limestone of Forbes (Reid & Strahan, 1889, 170) and some 48 ft. (14.4 m.) above the top of the Bembridge Limestone, in the low cliff of Bembridge Marls, which dip northwards from Whitecliff Bay (642863)1 at a very low angle until they disappear in Howgate Bay (648868) under a covering of Quaternary Gravel, or are obscured by slumping. 'All Grid References lie within the !OO Km . square 40 (SZ).
319
320
BRIAN DALEY
3. DESCRIPTION The pseudomorphs, or clastic casts (Brooks, 1955), occur mainly as minute sole structures on the indurated calcareous siltstone bands (ranging in thickness from 2 to 10 mm.) of a thinly laminated series of silts and clays which occur at this level. Poorly preserved, but nevertheless identifiable, pseudomorphs also occur less frequently on the upper surfaces of such bands. The pseudomorphs are most readily discernible on tumbled fragments of siltstone, whose surfaces have been washed clean of adhering clay by rainwater. In addition, such surfaces show a variety of other structures. Those of the siltstone soles include elongated erosional fills and the casts of desiccation cracks which developed in the underlying clays. The pseudomorphs range in type from single crystals, lying isolated, to groups of interpenetrating crystals (Plate 12). They can be conveniently divided into four types, although the boundaries between them are transitional: (1) Simple lenticular crystals (the majority approximately 1.0 to 0.5 rom. in diameter). Each possesses two curved faces and a sharp periphery, circular to oval in outline. (2) Intergrowths of two or three crystals in a regular manner. These may be twinned and not merely interpenetrating at random. (3) Clusters of more numerous crystals which form small knobs protruding outwards from the siltstone surfaces and which vary from groups of few crystals (transitional types from (2) above) to small (1-2 rom. in diameter) circular aggregates consisting of numerous intergrown discoid crystals which resemble miniature 'desert roses' or gypsum rosettes similar to, but far smaller than, those described by Kerr & Thomson (1963, 1729-30) from the Laguna Madre. (4) Irregular patches or lines of crystals, in some cases consisting of anastomosed rosettes and, in others, of intimately associated crystals of types (1) and (2) above. 4. GYPSUM FORMATION Little doubt exists that the original mineral was gypsum, for lenticular gypsum and gypsum rosettes of similar appearance have been described from modem sediments forming in areas subjected to high rates of evaporation, such as the mud-flats of the Laguna Madre, Texas (Masson, EXPLANATION OF PLATE 12 Top: Siltstone sole showing pseudomorphs of types 1 and 2, together with casts of desiccation cracks Bottom: Siltstone sole showing rosettes and anastomosed rosettes of types 3 and 4
PROC. GEOL. ASS., VOL. 78 (1967)
PLATE 12
[To face p. 320
PSEUDOMORPHS AFTER GYPSUM
321
1955; Kerr & Thomson, 1963) and the 'sebhka' coast flats of Abu Dhabi in the Perisan Gulf (Shearman, 1963, 1965 and personal communication). Macfadyen (1950) described lenticular gypsum from a raised beach in Somalia and lists other occurrences both recent and fossil. In the cases mentioned above , the gypsum is not deposited by direct precipitation on to the bottom from a body of standing water, but as an early diagenetic mineral within the uppermost sedimentary layers, where ground water has become concentrated by evaporation. IIling (1963), however, reports the precipitation of gypsum direct from concentrating sea-water in 'sebhka' sediments near Qatar, farther north in the Persian Gulf. Posnjak (1940) reports that gypsum precipitates from water when the salinity reaches three-and-one-third times that of normal sea-water. The faunal evidence from the horizon in question does not even suggest a salinity comparable with that of normal sea-water. Potomac/is turritissima Forbes is the only invertebrate found at this level to date. Certain modern members of the Melaniidae, to which this species is assigned, have some degree of salinity tolerance, but even if the salinity tolerance of Potomac/is were known, the fact that it frequently occurs current orientated, together with other evidence of water movement, suggests that it may very likely be that of a drifted assemblage rather than that of a community from which environmental deductions can be made. It is, therefore, suggested that the gypsum originated beneath the sedimentary surface in a manner comparable with that of the early diagenetic gypsum of modern sediments, rather than from hypersaline surface water. Exposure to the air is demonstrated by the presence of desiccation cracks and it is suggested that the evaporation from such surfaces could cause high concentrations of pore water sufficient to effect the formation of gypsum. A high rate of evaporation is compatible with the climatic implications of palaeobotanical work on the Bembridge Marls (Reid & Chandler, 1926). Such a formation could result from the concentration of waters of much lower salinity such as brackish water and does not necessitate the presence of highly saline surface water. Such waters could enter the pseudomorph bearing sediments either by lateral seepage from local surface water (cf. Shearman, 1963) or by downward seepage following the temporary inundation of the area (cf. Kerr & Thomson, 1963). The second possibility is preferred in this case, for, whereas waters spreading superficially could have sunk into the upper layers of sediment, the lateral migration of water within the sediment would have been inhibited by the clay and the discontinuous nature of the more porous silt bands. The small size of the pseudomorphs suggests a gypsum formation very close to the sediment surface (Kerr & Thomson, 1963, 1729) and that PltOC . GEOL . ASS. , VOL. 78, PART 2, 1967
21
322
BRIAN DALEY
growth was discontinued after an apparently short period of time due to changes in the environmental conditions. 5. PSEUDOMORPH FORMATION The retention of good crystal shape by the pseudomorphs indicates the proximity of gypsum solution and clastic infilling in time. Since the pseudomorphs occur both as sole and top surface structures, and, assuming a similar formation in each case, they must have developed within the sediment by contemporaneous infilling and solution. In the case of the sole pseudomorphs such infilling would have resulted from the gradual replacement of gypsum by silt which fell under gravity or was depressed into the space originally occupied by the crystals. The pseudomorphs of the siltstone top surfaces would have formed by an essentially similar process, but one in which the silt was pressed upwards into the moulds developing on the upper surfaces of the silt bands. Both infilling by silt and clay probably occurred, but the resulting pseudomorphs would only be apparent if the infilling were distinguishable from the host sediment. This explains the presence of pseudomorphs on the upper and lower surfaces of the siltstones, and their apparent absence within individual clay or siltstone bands. Other occurrences of pseudomorphs after lenticular gypsum have recently been described from the Purbeck rocks of Dorset (West, 1964). Unlike those under consideration in the present paper, the pseudomorphs described by West are mineralogical rather than clastic replacements of the original gypsum. 6. CONCLUSIONS Pseudomorphs after gypsum have been identified from the traditionally non-marine part of the Bembridge Marls. These formed within the sediment by the gradual piecemeal replacement of gypsum by clastic material when the silt was still mobile and uncemented. The original gypsum formed post-depositionally within the upper layers of the sediment as a result of the concentration of contained water by evaporation from sediment surfaces exposed to the air. No evidence exists for the presence at this time of surface water of high or even normal marine salinity. ACKNOWLEDGMENTS I should like to express my thanks to Mr. D. J. Shearman, Mr. N. Edwards and Mr. I. M. West for helpful discussion and suggestions, and to Dr. R. Goldring, who, in addition, criticised the manuscript. Mr. J. Mauger kindly took the photographs. This work was financed by Portsmouth College of Technology, and for this I am most grateful.
PSEUDOMORPHS AFTER GYPSUM
323
REFERENCES BROOKS, H. K. 1955. Clastic Casts of Halite Crystal Imprints from the Rome Formation (Cambrian) of Tennessee. J. sedim. Petrol., 25, 67-71. CHATWIN, C. P. 1960. The Hampshire Basin and Adjoining Areas. Br. reg. Geol., 3rd ed. CURRY, D. 1965 The Palaeogene Beds of South-East England. Proc. Geol. Ass., 76, 151-73. HUGHES, J. C. 1922. Geological Story of the Isle of Wight, London. ILLlNG, L. Y. 1963. Discussion on Shearman, D. J. (1963). Proc. geol. Soc., 1607, 64-5. KERR, S. D. & A. THOMSON. 1963. Origin of Nodular and Bedded Anhydrite in Permian Shelf Sediments, Texas and New Mexico. Bull. Am. Ass. Petrol. Geol.,47,1726-32. MASSON. 1955. An Occurrence of Gypsum in Southwest Texas. J. sedim. Petrol., 25,72-7. MACFADYEN, W. A. 1950. Sandy Gypsum Crystals from Berbera, Somaliland. Geol. Mag., 87, 409-20. POSNJAK, E. 1940. Deposition of Calcium Sulfate from Sea Water. Am. J. Sci., 35, 539-68. REID, C. & A. STRAHAN. 1889. The Geology of the Isle of Wight. Mem. geol. Survey U.K., 2nd ed. REID, E. M. & M. E. J. CHANDLER. 1926. Catalogue of Cainozoic Plants, Vol. 1. The Bembridge Flora. Brit. Mus. (Nat. Hist.), Lond. SHEARMAN, D. J. 1963. Recent Anhydrite, Gypsum, Dolomite and Halite from the Coastal Flats of the Arabian Shore of the Persian Gulf. Proc. geol. Soc., 1607,63-4. STINTON, F. C. 1963. Field Meeting in the Oligocene of North-West Isle of Wight. Proc. Geol. Ass., 75,15-31. WEST, I. M. 1964. Evaporite diagenesis in the Lower Purbeck Beds of Dorset. Proc, Yorks. geol. Soc., 35, 47-58. B. Daley Department of Chemistry and Geology Portsmouth College of Technology