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Vertical variations in bassanite distribution patterns in near-surface sediments, southern Egypt
Sedimentary Geology 181 (2005) 225 – 229 www.elsevier.com/locate/sedgeo
Vertical variations in bassanite distribution patterns in near-surface sediments, southern Egypt Florias Mees a,*, Morgan De Dapper b a
b
Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium Department of Geography, Research Unit dGeomorphology and Geo-archaeology of Tropical and Mediterranean AreasT, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium Received 15 February 2005; received in revised form 7 September 2005; accepted 7 September 2005
Abstract Nile valley sediments of the El Adaı¨ma area in southern Egypt contain bassanite (CaSO4d 0.5H2O), associated with gypsum (CaSO4d 2H2O), in the upper ~1 m of the deposits. The presence of bassanite at this site, formed by dehydration of gypsum, is marked by variations in mode of occurrence with depth. In the lowest bassanite-bearing interval, bassanite occurs mainly along the contact between gypsum crystals in pores, which is related to the protection of bassanite from rehydration in those parts of the gypsum aggregates. At a higher level, bassanite occurs along the sides of lenticular gypsum crystals in the sediment matrix. Near the top of the studied sequence, bassanite mainly appears as inclusions in gypsum crystals, which represents a texture that developed by repeated dehydration and rehydration. Elsewhere in the upper part of the deposits, bassanite occurs as aggregates of small crystals in pores, without associated gypsum, which was entirely dehydrated to bassanite in these parts. These vertical variations in bassanite distribution patterns record differences in the diagenetic history of the deposits between depth intervals. D 2005 Elsevier B.V. All rights reserved. Keywords: Bassanite; Gypsum; Diagenesis; Egypt
1. Introduction Bassanite (CaSO4d 0.5H2O) commonly forms by dehydration of gypsum (CaSO4d 2H2O) at or near the Earth’s surface in arid regions (e.g., Hunt et al., 1966; Kinsman, 1969; Arakel, 1980; Mees, 1998). This transformation requires elevated ambient temperatures and is promoted by high salinities of brines in contact with the gypsum. It generally results in topotactic replacement that proceeds from the sides of the affected gypsum
* Corresponding author. Tel.: +32 9 264 4569; fax: +32 9 264 4984. E-mail address: [email protected] (F. Mees). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.09.002
crystal (e.g., Moiola and Glover, 1965; Sipple et al., 2001). In this paper, we present a unique bassanite occurrence in gypsiferous formations, at about 0.2–1 m depth, characterised by vertical variations in bassanite distribution patterns. 2. Setting El Adaı¨ma is an important archaeological site in southern Egypt (Midant-Reynes et al., 1996). It is located along the west bank of the Nile, 60 km south of Luxor (25814V56WN, 32834V43WE). The site occupies part of a rise with an elevation of up to 5 m above the surrounding floodplain. In the study area, carbonate-
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Table 1 Summary of the main features of gypsum and bassanite occurrences in deposits of the El Adaı¨ma region Interval
Depth (cm)
Gypsum
Bassanite
Eolian sand cover Linear surface structure Upper sandy loam layer
0–21/33 21/33–33 33–60
Clay layer Lower sandy loam layer
60–90 90–130
– crystals in groundmass upper part — no gypsum; main part — crystals in groundmass tabular crystals in pores large crystals, poikilotopic texture
– inclusions in gypsum crystals upper part — aggregates in pores; main part — along sides of gypsum crystals along contact between gypsum crystals no bassanite
cemented gravel and sand beds are overlain by a sandy loam deposit, representing a sequence of Middle Pleistocene fluvial deposits (De Dapper, unpublished OSL data) (see Table 1). They are covered by a clay layer, deposited during a well-documented period with extensive flooding of the Nile (12–13 ky BP; e.g., Adamson et al., 1980). The clay layer is covered in some parts by a second sandy loam layer, which is probably anthropogenic. Along the surface of this layer, some linear structures composed of more clayey material are recognised, as possible remnants of an ancient irrigation system. Throughout the El Adaı¨ma area, the surface is covered by a thin layer (30 cm) of Middle to Late Holocene aeolian sands. The region has a desert climate, with a mean annual rainfall of about 1 mm, a mean monthly maximum temperature of 41 8C during summer, and extreme temperature maxima of 49 8C (Griffiths and Soliman, 1972). The groundwater level is about 2 m below the floodplain around El Adaı¨ma. Before completion of the Aswan dams, the Nile Valley was frequently flooded, with water levels up to 1 m above the present floodplain level. These water levels are well below the base of the studied deposits, which implies an absence or limited influence of groundwater derived directly from the Nile. No sources of surface water other than irrigation are available at El Adaı¨ma. 3. Materials and methods Samples were collected for all major units between the basal gravel and sand beds and the surface layer of aeolian sand, as part of a geo-archaeological investigation (November 2001). All units were sampled at the same site in a large excavation. The upper sandy loam layer was also sampled at another location, along the top of the interval. Thin sections (6 9 cm) were prepared after impregnation of undisturbed oriented samples with a cold-setting polyester resin. Mineral identification by optical microscopy was confirmed by energy-dispersive X-ray microanalysis using uncovered thin sections.
4. Results All sampled intervals contain bassanite and/or gypsum, as described below. Their main features are summarised in Table 1. 4.1. Lower sandy loam layer In the lower sampled unit, gypsum occurs as relatively large anhedral crystals (0.5–1 mm) and coarse-grained xenotopic intergrowths. The crystals typically enclose sand grains, resulting in poikilotopic fabrics. No bassanite is associated with gypsum in this interval. 4.2. Clay layer The clay layer has the highest gypsum content of all sampled intervals. Gypsum occurs mainly in pores, as isolated and intergrown crystals (300–750 Am), partly with subhedral tabular shapes. Bassanite is commonly associated with these gypsum aggregates, as small elongated prismatic crystals with mainly random orientations. It occurs almost exclusively along the contact between neighbouring gypsum crystals in intergrowths (Fig. 1a and b). One tubular pore contains an infilling consisting of small mainly tabular gypsum crystals (50–125 Am), with some associated bassanite (Fig. 1c). 4.3. Upper sandy loam layer The main part of the upper sandy loam layer, sampled in the excavation profile, contains scattered lenticular gypsum crystals (about 500 Am long) and some gypsum intergrowths. Bassanite mainly occurs along the sides of the gypsum crystals, commonly extending to the centre (Fig. 2a). It also occurs around enclosed sediment aggregates and sand grains in larger crystals. The bassanite crystals have a parallel orientation throughout each gypsum crystal, and the relative volume of bassanite is quite large in most crystals. Besides
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subhedral bassanite crystals (10–50 Am), commonly with elongated crystal shapes. 4.4. Linear surface structure One group of lenticular gypsum crystals (1–1.5 mm) in the fine-grained sediment matrix contains a considerable amount of bassanite (10–20 vol.%). It
Fig. 1. Gypsum and bassanite occurrences in the clay layer. (a and b) Intergrowths of gypsum crystals in pores, with bassanite occurring exclusively along contacts between the gypsum crystals (crosspolarised light [XPL]); (c) Small tabular gypsum crystals in pores, with some associated bassanite (bright areas) (XPL).
the lenticular gypsum crystals and intergrowths, clusters of small tabular gypsum crystals (25–75 Am) are also present, in tubular pores and in the space between sand grains. Topotactic replacement by bassanite is recognised for some of these occurrences. Along the top of the upper sandy loam layer, sampled at a different location, only bassanite is present, without any associated gypsum. It occurs as finegrained aggregates in pores and near the sides of pores (Fig. 2b). They consist of small anhedral to
Fig. 2. Gypsum and bassanite occurrences in the upper sandy loam layer and the linear surface structure. (a) Gypsum crystals, transformed to bassanite along their sides (upper sandy loam layer, main part; cross-polarised light [XPL]); (b) Aggregate of small bassanite crystals in pores, without any associated gypsum (upper sandy loam layer, upper part; XPL); (c) Gypsum crystals, with associated bassanite occurring mainly as inclusions (linear surface structure; XPL).
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mainly occurs as inclusions and only rarely as crystals that are associated with the sides of the gypsum crystals (Fig. 2c). All bassanite crystals have a parallel orientation, parallel to the (010) cleavage direction of the gypsum host. Only few other gypsum crystals and intergrowths occur in the sediment matrix. They mainly consist of lenticular crystals and have only little associated bassanite, which occurs as inclusions, as crystals along the sides of gypsum crystals, and locally as crystals along the contact between neighbouring gypsum crystals. Gypsum is also present in wide sand-rich subvertical veins containing a very low amount of fine material, which represents cracks that are filled with sandy material of the overlying aeolian deposits. These gypsum crystals are lenticular to anhedral (0.5–1.5 mm), commonly with an irregular surface. Bassanite is only rarely associated with these gypsum occurrences, along the sides or as inclusions, always in small amounts. 5. Discussion 5.1. Gypsum formation Gypsum formed as a post-depositional precipitate at El Adaı¨ma, in view of its occurrence in pores and as scattered lenticular crystals in the sediment matrix. Its occurrence implies that the sediments contained saline brines at some stage. As described before, water was probably derived from the surface, after flooding by irrigation. In the lower sandy loam layer, gypsum formed by slow growth in moderately saline brines, producing relatively large crystals with a poikilotopic texture. Conditions that allowed this type of growth can be expected for levels near the top of a brine-saturated interval. This is compatible with the presence of secondary carbonates and Fe/Mn oxides in this part of the deposits (in greater abundance than in all higher intervals), recording a redistribution of carbonates from the underlying carbonate-cemented layers and the existence of hydromorphic conditions at some stage, above an interval with low permeability. In the overlying clay layer, gypsum occurs as tabular to lenticular crystals in pores. It formed by evaporation of brines in macropores, derived from the surface. In the main part of the upper sandy loam layer and in the linear surface structure along its surface, gypsum mainly occurs in the sediment matrix. The latter was pervaded by brines at times and its texture allowed the occurrence of evaporation within the groundmass, outside the macropore system. In the upper part of the
upper sandy loam layer, gypsum is entirely absent. The nature of bassanite occurrences in this interval (see Section 4.4) indicates that gypsum formed only in pores and not within the groundmass. The gypsum crystals in the subvertical veins with aeolian material that are recognised for the linear surface structure seem to be part of the detrital sand fraction. 5.2. Bassanite formation In all parts of the studied deposits, bassanite clearly formed by dehydration of gypsum. Transformation of gypsum to bassanite or anhydrite (CaSO4) occurs at relatively high ambient temperatures and/or high brine salinities (e.g., 45 8C at aH2O 0.88; Hardie, 1967). In the lowest sampled interval, the lower sandy loam layer, temperatures and salinities were too low for dehydration of gypsum. In the clay layer, bassanite mainly occurs along the contact between neighbouring gypsum crystals that are part of intergrowths in pores. Bassanite must originally have formed by dehydration of gypsum along all sides of the crystals. It was subsequently transformed again to gypsum by rehydration in most parts. It is only preserved along the contact between adjacent gypsum crystals, showing that it was protected there from contact with dilute brines. This can be related to the development of gypsum overgrowths during interaction with less saline brines, sealing these bassanite occurrences at all sides. Bassanite generally shows no topotactic relationship with gypsum in this interval, which suggests that a certain degree of recrystallisation took place. The initial development of bassanite around the gypsum in pores was probably caused by an interaction with saline brines derived from the surface. The depth of this bassanite occurrence is in any case too great to have formed without contact with a liquid phase, which requires high temperatures (e.g., 95 8C; Strydom et al., 1995). In the main part of the upper sandy loam layer and in the linear structure along its surface, bassanite formed by topotactic replacement of gypsum, proceeding from all sides of the gypsum crystals. The degree of transformation is generally higher in the uppermost of these intervals. In this unit, the occurrence of bassanite as inclusions that are not in contact with the sides of the gypsum crystal records a more complex evolution, with repeated dehydration and rehydration. The upper part of the upper sandy loam layer contains bassanite aggregates in pores, without associated gypsum. These aggregates formed by complete transformation of a gypsum precursor.
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Indications that bassanite developed at a recent stage include the presence of bassanite aggregates in pores, which are susceptible to transformation by water entering the macropore system from the surface. Another indication is the local association of bassanite with small tabular gypsum crystals in pores, in view of the instability of this type of gypsum occurrence. The nearabsence of bassanite associated with (detrital) gypsum in veins of aeolian material in the upper part of the studied deposits suggests that all or most bassanite in the sandy loam and clay intervals developed before deposition of the sand cover. 6. Conclusions Earlier studies of bassanite occurrences have documented textural features such as topotactic replacement (e.g., Moiola and Glover, 1965), bassanite-anhydrite transitions (e.g., Mees, 1998), and circumgranular coatings (Mees and Stoops, 2003). Bassanite in sediments of the El Adaı¨ma region shows some additional textural features. The most unique texture is represented by bassanite occurrences along contacts between gypsum crystals. Crystals enclosing or surrounding another mineral phase generally formed at a later stage, but this special bassanite texture illustrates that the surrounding crystals can largely represent the pre-existing phase in some cases. The El Adaı¨ma deposits demonstrate that the nature of bassanite textures can change with depth. These variations are determined by vertical variations in the nature of gypsum occurrences, temperature, water availability and salinity. In lower parts, a period of bassanite formation was followed by partial rehydration, whereby bassanite was only transformed along external surfaces of gypsum intergrowths. In the overlying deposits, bassanite is still present along all sides of the gypsum crystals. In higher parts, a succession of
229
dehydration and rehydration events resulted in the development of gypsum crystals with bassanite inclusions. Elsewhere near the top of the studied sequence, gypsum in pores was completely replaced by bassanite. References Adamson, D.A., Gasse, F., Street, F.A., Williams, M.A.J., 1980. Late Quaternary history of the Nile. Nature 288, 50 – 55. Arakel, A.V., 1980. Genesis and diagenesis of Holocene evaporitic sediments in Hutt and Leeman lagoons, Western Australia. J. Sediment. Petrol. 50, 1305 – 1326. Griffiths, J.F., Soliman, K.H., 1972. The Northern Desert. In: Griffiths, J.F. (Ed.), Climates of Africa. World Survey of Climatology, vol. 10. Elsevier, Amsterdam, pp. 75 – 132. Hardie, L.A., 1967. The gypsum–anhydrite equilibrium at one atmosphere pressure. Am. Mineral. 52, 171 – 200. Hunt, C.B., Robinson, T.W., Bowles, W.A., Washburn, A.L., 1966. Hydrologic basin Death Valley, California. U. S. Geol. Surv. Prof. Pap. 494-B, 1 – 138. Kinsman, D.J.J., 1969. Modes of formation, sedimentary associations, and diagnostic features of shallow-water and supratidal evaporites. Am. Assoc. Pet. Geol. Bull. 53, 830 – 840. Mees, F., 1998. The alteration of glauberite in lacustrine deposits of the Taoudenni–Agorgott basin, northern Mali. Sediment. Geol. 117, 193 – 205. Mees, F., Stoops, G., 2003. Circumgranular bassanite in a gypsum crust from eastern Algeria—a potential paleosurface indicator. Sedimentology 50, 1139 – 1145. Midant-Reynes, B., Buchez, N., Crubezy, E., Janin, T., 1996. The predynastic site of Adaima: Settlement and cemetery. In: Spencer, A.J. (Ed.), Aspects of Early Egypt. British Museum Press, London, pp. 93 – 97. Moiola, R.J., Glover, E.D., 1965. Recent anhydrite from Clayton Playa, Nevada. Am. Mineral. 50, 2063 – 2069. Sipple, E.M., Bracconi, P., Dufour, P., Mutin, J.C., 2001. Electronic microdiffraction study of structural modifications resulting from the dehydration of gypsum. Prediction of the microstructure of resulting pseudomorphs. Solid State Ionics 141/142, 455 – 461. Strydom, C.A., Hudson-Lamb, D.L., Potgieter, J.H., Dagg, E., 1995. The thermal dehydration of synthetic gypsum. Thermochim. Acta 269/270, 631 – 638.
Vertical variations in bassanite distribution patterns in near-surface sediments, southern Egypt Florias Mees a,*, Morgan De Dapper b a
b
Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium Department of Geography, Research Unit dGeomorphology and Geo-archaeology of Tropical and Mediterranean AreasT, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium Received 15 February 2005; received in revised form 7 September 2005; accepted 7 September 2005
Abstract Nile valley sediments of the El Adaı¨ma area in southern Egypt contain bassanite (CaSO4d 0.5H2O), associated with gypsum (CaSO4d 2H2O), in the upper ~1 m of the deposits. The presence of bassanite at this site, formed by dehydration of gypsum, is marked by variations in mode of occurrence with depth. In the lowest bassanite-bearing interval, bassanite occurs mainly along the contact between gypsum crystals in pores, which is related to the protection of bassanite from rehydration in those parts of the gypsum aggregates. At a higher level, bassanite occurs along the sides of lenticular gypsum crystals in the sediment matrix. Near the top of the studied sequence, bassanite mainly appears as inclusions in gypsum crystals, which represents a texture that developed by repeated dehydration and rehydration. Elsewhere in the upper part of the deposits, bassanite occurs as aggregates of small crystals in pores, without associated gypsum, which was entirely dehydrated to bassanite in these parts. These vertical variations in bassanite distribution patterns record differences in the diagenetic history of the deposits between depth intervals. D 2005 Elsevier B.V. All rights reserved. Keywords: Bassanite; Gypsum; Diagenesis; Egypt
1. Introduction Bassanite (CaSO4d 0.5H2O) commonly forms by dehydration of gypsum (CaSO4d 2H2O) at or near the Earth’s surface in arid regions (e.g., Hunt et al., 1966; Kinsman, 1969; Arakel, 1980; Mees, 1998). This transformation requires elevated ambient temperatures and is promoted by high salinities of brines in contact with the gypsum. It generally results in topotactic replacement that proceeds from the sides of the affected gypsum
* Corresponding author. Tel.: +32 9 264 4569; fax: +32 9 264 4984. E-mail address: [email protected] (F. Mees). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.09.002
crystal (e.g., Moiola and Glover, 1965; Sipple et al., 2001). In this paper, we present a unique bassanite occurrence in gypsiferous formations, at about 0.2–1 m depth, characterised by vertical variations in bassanite distribution patterns. 2. Setting El Adaı¨ma is an important archaeological site in southern Egypt (Midant-Reynes et al., 1996). It is located along the west bank of the Nile, 60 km south of Luxor (25814V56WN, 32834V43WE). The site occupies part of a rise with an elevation of up to 5 m above the surrounding floodplain. In the study area, carbonate-
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Table 1 Summary of the main features of gypsum and bassanite occurrences in deposits of the El Adaı¨ma region Interval
Depth (cm)
Gypsum
Bassanite
Eolian sand cover Linear surface structure Upper sandy loam layer
0–21/33 21/33–33 33–60
Clay layer Lower sandy loam layer
60–90 90–130
– crystals in groundmass upper part — no gypsum; main part — crystals in groundmass tabular crystals in pores large crystals, poikilotopic texture
– inclusions in gypsum crystals upper part — aggregates in pores; main part — along sides of gypsum crystals along contact between gypsum crystals no bassanite
cemented gravel and sand beds are overlain by a sandy loam deposit, representing a sequence of Middle Pleistocene fluvial deposits (De Dapper, unpublished OSL data) (see Table 1). They are covered by a clay layer, deposited during a well-documented period with extensive flooding of the Nile (12–13 ky BP; e.g., Adamson et al., 1980). The clay layer is covered in some parts by a second sandy loam layer, which is probably anthropogenic. Along the surface of this layer, some linear structures composed of more clayey material are recognised, as possible remnants of an ancient irrigation system. Throughout the El Adaı¨ma area, the surface is covered by a thin layer (30 cm) of Middle to Late Holocene aeolian sands. The region has a desert climate, with a mean annual rainfall of about 1 mm, a mean monthly maximum temperature of 41 8C during summer, and extreme temperature maxima of 49 8C (Griffiths and Soliman, 1972). The groundwater level is about 2 m below the floodplain around El Adaı¨ma. Before completion of the Aswan dams, the Nile Valley was frequently flooded, with water levels up to 1 m above the present floodplain level. These water levels are well below the base of the studied deposits, which implies an absence or limited influence of groundwater derived directly from the Nile. No sources of surface water other than irrigation are available at El Adaı¨ma. 3. Materials and methods Samples were collected for all major units between the basal gravel and sand beds and the surface layer of aeolian sand, as part of a geo-archaeological investigation (November 2001). All units were sampled at the same site in a large excavation. The upper sandy loam layer was also sampled at another location, along the top of the interval. Thin sections (6 9 cm) were prepared after impregnation of undisturbed oriented samples with a cold-setting polyester resin. Mineral identification by optical microscopy was confirmed by energy-dispersive X-ray microanalysis using uncovered thin sections.
4. Results All sampled intervals contain bassanite and/or gypsum, as described below. Their main features are summarised in Table 1. 4.1. Lower sandy loam layer In the lower sampled unit, gypsum occurs as relatively large anhedral crystals (0.5–1 mm) and coarse-grained xenotopic intergrowths. The crystals typically enclose sand grains, resulting in poikilotopic fabrics. No bassanite is associated with gypsum in this interval. 4.2. Clay layer The clay layer has the highest gypsum content of all sampled intervals. Gypsum occurs mainly in pores, as isolated and intergrown crystals (300–750 Am), partly with subhedral tabular shapes. Bassanite is commonly associated with these gypsum aggregates, as small elongated prismatic crystals with mainly random orientations. It occurs almost exclusively along the contact between neighbouring gypsum crystals in intergrowths (Fig. 1a and b). One tubular pore contains an infilling consisting of small mainly tabular gypsum crystals (50–125 Am), with some associated bassanite (Fig. 1c). 4.3. Upper sandy loam layer The main part of the upper sandy loam layer, sampled in the excavation profile, contains scattered lenticular gypsum crystals (about 500 Am long) and some gypsum intergrowths. Bassanite mainly occurs along the sides of the gypsum crystals, commonly extending to the centre (Fig. 2a). It also occurs around enclosed sediment aggregates and sand grains in larger crystals. The bassanite crystals have a parallel orientation throughout each gypsum crystal, and the relative volume of bassanite is quite large in most crystals. Besides
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subhedral bassanite crystals (10–50 Am), commonly with elongated crystal shapes. 4.4. Linear surface structure One group of lenticular gypsum crystals (1–1.5 mm) in the fine-grained sediment matrix contains a considerable amount of bassanite (10–20 vol.%). It
Fig. 1. Gypsum and bassanite occurrences in the clay layer. (a and b) Intergrowths of gypsum crystals in pores, with bassanite occurring exclusively along contacts between the gypsum crystals (crosspolarised light [XPL]); (c) Small tabular gypsum crystals in pores, with some associated bassanite (bright areas) (XPL).
the lenticular gypsum crystals and intergrowths, clusters of small tabular gypsum crystals (25–75 Am) are also present, in tubular pores and in the space between sand grains. Topotactic replacement by bassanite is recognised for some of these occurrences. Along the top of the upper sandy loam layer, sampled at a different location, only bassanite is present, without any associated gypsum. It occurs as finegrained aggregates in pores and near the sides of pores (Fig. 2b). They consist of small anhedral to
Fig. 2. Gypsum and bassanite occurrences in the upper sandy loam layer and the linear surface structure. (a) Gypsum crystals, transformed to bassanite along their sides (upper sandy loam layer, main part; cross-polarised light [XPL]); (b) Aggregate of small bassanite crystals in pores, without any associated gypsum (upper sandy loam layer, upper part; XPL); (c) Gypsum crystals, with associated bassanite occurring mainly as inclusions (linear surface structure; XPL).
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mainly occurs as inclusions and only rarely as crystals that are associated with the sides of the gypsum crystals (Fig. 2c). All bassanite crystals have a parallel orientation, parallel to the (010) cleavage direction of the gypsum host. Only few other gypsum crystals and intergrowths occur in the sediment matrix. They mainly consist of lenticular crystals and have only little associated bassanite, which occurs as inclusions, as crystals along the sides of gypsum crystals, and locally as crystals along the contact between neighbouring gypsum crystals. Gypsum is also present in wide sand-rich subvertical veins containing a very low amount of fine material, which represents cracks that are filled with sandy material of the overlying aeolian deposits. These gypsum crystals are lenticular to anhedral (0.5–1.5 mm), commonly with an irregular surface. Bassanite is only rarely associated with these gypsum occurrences, along the sides or as inclusions, always in small amounts. 5. Discussion 5.1. Gypsum formation Gypsum formed as a post-depositional precipitate at El Adaı¨ma, in view of its occurrence in pores and as scattered lenticular crystals in the sediment matrix. Its occurrence implies that the sediments contained saline brines at some stage. As described before, water was probably derived from the surface, after flooding by irrigation. In the lower sandy loam layer, gypsum formed by slow growth in moderately saline brines, producing relatively large crystals with a poikilotopic texture. Conditions that allowed this type of growth can be expected for levels near the top of a brine-saturated interval. This is compatible with the presence of secondary carbonates and Fe/Mn oxides in this part of the deposits (in greater abundance than in all higher intervals), recording a redistribution of carbonates from the underlying carbonate-cemented layers and the existence of hydromorphic conditions at some stage, above an interval with low permeability. In the overlying clay layer, gypsum occurs as tabular to lenticular crystals in pores. It formed by evaporation of brines in macropores, derived from the surface. In the main part of the upper sandy loam layer and in the linear surface structure along its surface, gypsum mainly occurs in the sediment matrix. The latter was pervaded by brines at times and its texture allowed the occurrence of evaporation within the groundmass, outside the macropore system. In the upper part of the
upper sandy loam layer, gypsum is entirely absent. The nature of bassanite occurrences in this interval (see Section 4.4) indicates that gypsum formed only in pores and not within the groundmass. The gypsum crystals in the subvertical veins with aeolian material that are recognised for the linear surface structure seem to be part of the detrital sand fraction. 5.2. Bassanite formation In all parts of the studied deposits, bassanite clearly formed by dehydration of gypsum. Transformation of gypsum to bassanite or anhydrite (CaSO4) occurs at relatively high ambient temperatures and/or high brine salinities (e.g., 45 8C at aH2O 0.88; Hardie, 1967). In the lowest sampled interval, the lower sandy loam layer, temperatures and salinities were too low for dehydration of gypsum. In the clay layer, bassanite mainly occurs along the contact between neighbouring gypsum crystals that are part of intergrowths in pores. Bassanite must originally have formed by dehydration of gypsum along all sides of the crystals. It was subsequently transformed again to gypsum by rehydration in most parts. It is only preserved along the contact between adjacent gypsum crystals, showing that it was protected there from contact with dilute brines. This can be related to the development of gypsum overgrowths during interaction with less saline brines, sealing these bassanite occurrences at all sides. Bassanite generally shows no topotactic relationship with gypsum in this interval, which suggests that a certain degree of recrystallisation took place. The initial development of bassanite around the gypsum in pores was probably caused by an interaction with saline brines derived from the surface. The depth of this bassanite occurrence is in any case too great to have formed without contact with a liquid phase, which requires high temperatures (e.g., 95 8C; Strydom et al., 1995). In the main part of the upper sandy loam layer and in the linear structure along its surface, bassanite formed by topotactic replacement of gypsum, proceeding from all sides of the gypsum crystals. The degree of transformation is generally higher in the uppermost of these intervals. In this unit, the occurrence of bassanite as inclusions that are not in contact with the sides of the gypsum crystal records a more complex evolution, with repeated dehydration and rehydration. The upper part of the upper sandy loam layer contains bassanite aggregates in pores, without associated gypsum. These aggregates formed by complete transformation of a gypsum precursor.
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Indications that bassanite developed at a recent stage include the presence of bassanite aggregates in pores, which are susceptible to transformation by water entering the macropore system from the surface. Another indication is the local association of bassanite with small tabular gypsum crystals in pores, in view of the instability of this type of gypsum occurrence. The nearabsence of bassanite associated with (detrital) gypsum in veins of aeolian material in the upper part of the studied deposits suggests that all or most bassanite in the sandy loam and clay intervals developed before deposition of the sand cover. 6. Conclusions Earlier studies of bassanite occurrences have documented textural features such as topotactic replacement (e.g., Moiola and Glover, 1965), bassanite-anhydrite transitions (e.g., Mees, 1998), and circumgranular coatings (Mees and Stoops, 2003). Bassanite in sediments of the El Adaı¨ma region shows some additional textural features. The most unique texture is represented by bassanite occurrences along contacts between gypsum crystals. Crystals enclosing or surrounding another mineral phase generally formed at a later stage, but this special bassanite texture illustrates that the surrounding crystals can largely represent the pre-existing phase in some cases. The El Adaı¨ma deposits demonstrate that the nature of bassanite textures can change with depth. These variations are determined by vertical variations in the nature of gypsum occurrences, temperature, water availability and salinity. In lower parts, a period of bassanite formation was followed by partial rehydration, whereby bassanite was only transformed along external surfaces of gypsum intergrowths. In the overlying deposits, bassanite is still present along all sides of the gypsum crystals. In higher parts, a succession of
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dehydration and rehydration events resulted in the development of gypsum crystals with bassanite inclusions. Elsewhere near the top of the studied sequence, gypsum in pores was completely replaced by bassanite. References Adamson, D.A., Gasse, F., Street, F.A., Williams, M.A.J., 1980. Late Quaternary history of the Nile. Nature 288, 50 – 55. Arakel, A.V., 1980. Genesis and diagenesis of Holocene evaporitic sediments in Hutt and Leeman lagoons, Western Australia. J. Sediment. Petrol. 50, 1305 – 1326. Griffiths, J.F., Soliman, K.H., 1972. The Northern Desert. In: Griffiths, J.F. (Ed.), Climates of Africa. World Survey of Climatology, vol. 10. Elsevier, Amsterdam, pp. 75 – 132. Hardie, L.A., 1967. The gypsum–anhydrite equilibrium at one atmosphere pressure. Am. Mineral. 52, 171 – 200. Hunt, C.B., Robinson, T.W., Bowles, W.A., Washburn, A.L., 1966. Hydrologic basin Death Valley, California. U. S. Geol. Surv. Prof. Pap. 494-B, 1 – 138. Kinsman, D.J.J., 1969. Modes of formation, sedimentary associations, and diagnostic features of shallow-water and supratidal evaporites. Am. Assoc. Pet. Geol. Bull. 53, 830 – 840. Mees, F., 1998. The alteration of glauberite in lacustrine deposits of the Taoudenni–Agorgott basin, northern Mali. Sediment. Geol. 117, 193 – 205. Mees, F., Stoops, G., 2003. Circumgranular bassanite in a gypsum crust from eastern Algeria—a potential paleosurface indicator. Sedimentology 50, 1139 – 1145. Midant-Reynes, B., Buchez, N., Crubezy, E., Janin, T., 1996. The predynastic site of Adaima: Settlement and cemetery. In: Spencer, A.J. (Ed.), Aspects of Early Egypt. British Museum Press, London, pp. 93 – 97. Moiola, R.J., Glover, E.D., 1965. Recent anhydrite from Clayton Playa, Nevada. Am. Mineral. 50, 2063 – 2069. Sipple, E.M., Bracconi, P., Dufour, P., Mutin, J.C., 2001. Electronic microdiffraction study of structural modifications resulting from the dehydration of gypsum. Prediction of the microstructure of resulting pseudomorphs. Solid State Ionics 141/142, 455 – 461. Strydom, C.A., Hudson-Lamb, D.L., Potgieter, J.H., Dagg, E., 1995. The thermal dehydration of synthetic gypsum. Thermochim. Acta 269/270, 631 – 638.