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Sedimentological studies of neogene evaporites in the northern Western Desert, Egypt
Sedimentary Geology, 59 (1988) 261-273
261
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Sedimentological studies of Neogene evaporites in the northern Western Desert, Egypt EL-SAYED
A.A. YOUSSEF
Geology Department, Faculty of Science, Cairo University, Giza (Egypt) Received May 12, 1987; revised version accepted May 31, 1988
Abstract Youssef, El Sayed A.A., 1988. Sedimentological studies of Neogene evaporites in the northern Western Desert, Egypt. Sediment. Geol., 59: 261-273. Neogene calcium sulphate deposits at the northern Western Desert, Egypt, are interpreted as shallow photic subaqueous evaporites. Study of the evaporite sequences at Gebel E1-Hagif and Dir E1-Biraqat areas reveals two principal structures similar to those observed in the Messinian gypsum in and around the Mediterranean Sea. The structures, arranged from base to top are: (1) cryptalgal laminites and stromatolitic structures built up by cyanobacterial (blue-green algal) communities in gypsite and gypsarenite units; and (2) selenite (specchiolino or mirror like gypsum) represented by (a) disordered selenite, and (b) vertically arranged selenite. Cyclic increase and decrease in salinity are represented by interlamination of gypsum and cryptalgal carbonate laminae. General increase of salinity is represented by the general increase in crystal size of gypsum from gypsite to gypsarenite to twinned gypsum crystals more than 5 cm in length at the top of the sequence. A regressive evaporite basin model is suggested for the deposition of the studied evaporite sequence. Dehydration of gypsum into anhydrite through an intermediate bassanite phase, and the subsequent transformation of secondary anhydrite into secondary porphyrotopic gypsum, are the main diagenetic processes affecting the studied Neogene evaporites.
Introduction This s t u d y r e p o r t s o b s e r v a t i o n s o n the d e p o s i t i o n a l a n d d i a g e n e t i c forms of s o m e N e o g e n e e v a p o r i t e s at G e b e l E1-Hagif a n d D i r E1-Biraqat areas, n o r t h e r n W e s t e r n Desert, E g y p t (Fig. 1). T h e s t u d y areas lie b e t w e e n l o n g i t u d e s 30025 ' a n d 3 0 ° 5 5 ' E , a n d l a t i t u d e s 29010 ' a n d 2 9 ° 3 0 ' N . G e b e l E1-Hagif a r e a is l o c a t e d a b o u t 50 k m to the s o u t h of the M e d i t e r r a n e a n c o a s t to the s o u t h of a g r o u p of Pleistocene g y p s u m quarries, e.g. E1G h o r b a n i a t , E 1 - H a m m a m a n d E1-Omayed a n d n e w discoveries at A l a m e i n area. D i r E1-Biraqat g y p s u m q u a r r y lies a b o u t 40 k m to the west of G e b e l E1-Hagif gypsum. This s t u d y investigates the d e p o s i t i o n a l env i r o n m e n t a n d the diagenetic h i s t o r y o f N e o g e n e 0037-0738/88/$03.50
© 1988 Elsevier Science Publishers B.V.
c a l c i u m s u l p h a t e d e p o s i t s in t e r m s of the textures, structures a n d m i n e r a l o g i c a l c o m p o s i t i o n . T h e G e b e l E1-Hagif a n d D i r E1-Biraqat areas were c h o s e n to t h r o w light o n the d e p o s i t i o n a l environm e n t a n d d i a g e n e t i c h i s t o r y o f the N e o g e n e e v a p o r i t e s of the n o r t h e r n W e s t e r n Desert, for c o m p a r i s o n w i t h o t h e r s in the M e d i t e r r a n e a n basin, as well as o n the c o a s t a l a r e a o f the R e d Sea a n d G u l f o f Suez, Egypt.
Methods of study T h i n sections were p r e p a r e d a n d s t u d i e d p e t r o graphically. S o m e s a m p l e s f r o m the selenite units were s t u d i e d b y a Jeol S c a n n i n g E l e c t r o n M i c r o scope ( S E M ) J S M - 3 5 w i t h K e v e x M x 7000 system. T h e m i n e r a l o g i c a l c o m p o s i t i o n was i d e n t i f i e d
262 Z1} o , M
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32 °
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using a Philips APD 1700 automated powder diffractometer, nickel-filtered Cu-Ka[. 2 radiation (A = 1.54056, 1.54439 ,~) generated at 45 kV and 60
mA. Representative slabs and samples were photographed. In this work the author follows Warren (1982) in using the terms gypsite and gypsarenite to describe gypsum in which the majority of crystals are in the silt and sand sizes, respectively. The term selenite (specchiolino or mirror like gypsum) is used for gypsum in which more than 50% of the sample is composed of distinct, transparent crystals with grain sizes coarser than 2 mm.
Geologic setting The studied Neogene evaporites form lenticular bodies trending nearly east-west parallel to the Mediterranean coast. They extend for tens of kilometers with a maximum thickness of 15 m and are mainly composed of calcium sulphate. Based on the exposed Dir E1-Biraqat section and bore
ONCOLITIC LIMESTONE '~ I I ~ l= J )~
CONGLOMERATIC LIMESTONE EVAPORITE LIMESTONE MARL SHALE SANDSTONE
Fig. 2. Generalized composite stratigraphic section of the Neogene deposits, northern Western Desert, Egypt.
263 hole data by E1 Shazly et al. (1976), Youssef a n d Kamel (1985a) showed that the evaporite sequence consists of two gypsum horizons interlayered by oncolitic carbonate and greenish grey sandy marl enclosing gypsum crystals. The evaporite sequence unconformably overlies the Middle Miocene limestone (Marmarica Formation of Said, 1962), and unconformably underlies the Late Pliocene pink fossiliferous limestone (Sidi-Barrani Formation of Abdallah and Yehia, 1973). The upper unconformity is represented by a 1 m thick conglomeratic pink limestone layer. Abdallah and Yehia (1973) described a similar conglomeratic limestone directly overlying the Marmarica Formation at Sidi-Barrani area and considered it of Early Pliocene. The stratigraphic section exposed at the southern scarp of Gebel E1-Hagif was measured, described and named the Hagif Formation by Omara and Sanad (1975). They showed that it is composed of 30-40 m of cross bedded, well sorted medium-grained sand, limestone, clay, crystalline gypsum, gypsiferous clay and pink hard sandy limestone with Helix quadridentata at the topmost part of the section. They showed also that the Hagif Formation unconformably overlies the Early Miocene clastics (Moghra Formation; Said, 1962). The author recognised a pink conglomeratic limestone bed between the massive crystalline gypsum and the overlying pink hard sandy limestone at Gebel E1-Hagif, which is similar to the conglomerate described by Youssef and Kamel (1985a) at Dir E1-Biraqat quarry between the pink limestone and the underlying evaporite sequence (Fig. 2). The observation that the evaporite deposits at Dir E1-Biraqat quarry and at the top part of Hagif Formation underlie the Early Pliocene pink conglomeratic limestone and unconformably overlie Middle and Lower Miocene sequences, led the present author to suggest a Late Miocene (Messinian) age to the studied evaporite sequence.
Neogene evaporite sequence Upper Miocene evaporites have long been recorded and studied in and around the Mediterranean (Hsii, 1972; Bommarito and Catalano, 1973; Braune et al., 1973; Drooger, 1973; Ryan,
Hsii et al., 1973; Dronkert, 1976; Geel, 1976; Schreiber et al., 1976, 1977; Said, 1979; Rouchy and Monty, 1981; Youssef and Kamel, 1985a; and others). Deposits of calcium sulphate (gypsum, anhydrite and bassanite) are the main constituents of the Neogene succession in many localities in Egypt. The following are the sedimentary structures and textures, depositional and diagenetic models of the studied Neogene evaporite. Sedimentary structures and textures
Two main textures are recognized in the Neogene evaporite sequences at the study areas. From base to top, they are as follows: (1) Cryptalgal laminites and stromatolitic structures built up by cyanophytic (blue-green algal) communities in gypsite and gypsarenite units (Fig. 3A). (2) Selenite units (specchiolino or mirror-like gypsum), represented by (a) disordered selenite (Fig. 3B), and (b) vertically arranged selenite (Fig.
3c). (1) Cryptalgal laminites and stromatolitic structures
The cryptalgal laminites and stromatolitic structures are clearly observed in a 7 m layer at the base of Dir E1-Biraqat evaporite sequence. It is characterized by laminations resulting from alternating gypsum and cryptalgal carbonate laminae (Fig. 3A). The cryptalgal laminites show roilliraeter-sized irregular, planar and wavy laminae, most of which have remains of algal origin. They range in thickness from 300/Lm to 2 mm, and contain irregularly oriented algal filaments, 60/~m in diameter and 500/~m long (Fig. 3D, E). The contact between the algal filaments and the enclosing gypsum crystals is sharp (Fig. 3F). The cryptalgal laminae are associated with rounded calcite crystals of about 30 /Lm in diameter and some pyrite crystals. Youssef and Kamel (1985a) believe that calcite and pyrite were formed by the action of anaerobic bacteria on calcium sulphate in the presence of organic matter (algae). The gypsum laminae range in thickness from few millimeters to 2 cm and are composed of gypsite, and gypsarenite.
264
~i~ii iiii~ii~ ~?i ~
Fig. 3. A. Vertical passage of wavy-laminated to oncofitic structure in Dir E1-Biraqat evaporite sequence. B. Lense of disordered selenite unit at Dir E1-Biraqat quarry. C. Vertically arranged twinned and split gypsum crystals at Gebel E1-Hagif. D. Algal filaments disseminated in prismatic gypsum crystals (crossed nicols). E. Algal filaments associated with biogenic calcite in gypsarenite (crossed nicols). F. Algal filaments enclosed in gypsum crystals. The contact between them is clear-cut (crossed nicols).
Gypsite. G y p s i t e occurs at the base of the sequence, overlying an oncolitic c a r b o n a t e facies (Fig. 4A) a n d u n d e r l y i n g the gypsarenite. T h e g y p s u m crystals are acicular to p r i s m a t i c a n d form felted masses (Fig. 3D). S o m e t i m e s they are a r r a n g e d with long directions r a d i a l l y u p w a r d f r o m the c r y p t a l g a l c a r b o n a t e laminae, suggesting that
they were f o r m e d as a crust o n the c r y p t a l g a l laminae. S o m e crystals are b r o k e n a n d irregularly arranged, i n d i c a t i n g that they were f o r m e d at the water surface a n d later d e p o s i t e d as detrital grains. This t y p e of g y p s u m is similar to that d e s c r i b e d b y W a r r e n (1982) in the S o u t h A u s t r a l i a n Salt Lakes a n d b y Schreiber et al. (1977) in the h y p e r s a l i n e
265
Fig. 4. A. Algal pellets and grains, or pseudo-oncoid grains in the oncobiosparite facies underlying the gypsite unit (crossed nicols). B. Rounded and angular gypsum crystals showing a mosaic texture in gypsarenite unit (crossed nicols). C. Cleavage fragments of gypsum in gypsarenite unit (crossed nicols). D. Corrosion of gypsum crystals by the biogenic calcite in gypsarenite unit (crossed nicols). E. Reverse graded bedding in gypsarenite unit (crossed nicols). F. Porphyrotopic secondary gypsum crystal with deformed discontinuous peripheral zone (crossed nicols).
266 lagoon in the Arabian Gulf (Azizyah). The laminites are principally induced by the cyclic growth of filamentous algae without indication of trapping mechanism of gypsum crystals.
Gypsarenite. Gypsum crystals in this type are either in the form of rounded and granular crystals, showing a mosaic texture (Fig. 4B) or cleavage fragments (Fig. 4C). Some granular gypsum crystals are embedded in the cryptalgal carbonate laminae (Fig. 4B) and record a trapping mechanism, whereas the cleavage fragments mostly enclose algal filaments (Fig. 3F). Corrosion by the diagenetically formed calcite (biogenic calcite) is a common feature in the gypsarenite unit (Fig. 4D). Reverse graded bedding is a common texture in the laminated gypsarenite. Fine gypsum crystals 80-200 /~m in diameter form the base of the laminae (in contact with cryptalgal laminae) and change to coarse gypsum crystals 200-800/~m in size at the top of the laminae (Fig. 4E). In some laminae the coarse gypsum crystals are followed by finer crystals before going to the second cryptalgal carbonate lamina. The density of gypsum crystals also increases upward, from isolated crystals in the basal carbonate lamina to densely interlocking gypsum crystals with a mosaic texture at the top of the lamina. Porphyrotopic secondary gypsum is observed in this unit (Fig. 4F). The origin of reverse graded bedding in gypsum was discussed by many authors. Ogniben (1955) proposed that gypsum precipitated first, followed by anhydrite when the brine became more concentrated. The next water inflow would hydrate the anhydrite to coarsly crystalline gypsum resulting in the formation of reverse grading. Hardie and Eugster (1971) suggested that graded bedding resulted from mechanical deposition during storm surges on shallow offshore banks where particles of gypsum were trapped and bound by algal mats. Schreiber et al. (1976) supported the elastic origin of the original gypsum grains proposed by Hardie and Eugster (1971) and added that cyclic dilution of the overlying water may have caused the growth of gypsum crystals along the top of each lamina. In the studied gypsarenite, it is observed that some laminae are composed of fine gypsum crystals in contact with the enclosing upper and
lower cryptalgal laminae, followed by coarser gypsum crystals in the center. This suggests that the finer gypsum crystals may be related to the cryptalgal laminae and led the present author to believe that the coarse gypsum crystals were formed at a stage of increase in salinity. The deposition of finer crystals is related to dilution of the brine which increases gypsum crystallite nucleation. The cryptalgal laminae are interpreted as having formed during the periods of lowest salinity in the brine pond. Three types of relations are observed between the cryptalgal carbonate laminae and gypsum laminae: (a) In the first type, the cryptalgal laminites are irregular and planar and the laminations are mostly continuous (Fig. 3A). (b) In the second type, the cryptalgal carbonate laminae are wavy and show a prominent stromatolitic structure resulting from the regular stacking of over 1 mm thick cryptalgal laminae overlain by centimetric gypsum laminae. The maximum height is about 4 cm for an average width of about 6 cm, being separated by few centimeter wide spaces (Fig. 5A). Logan et al. (1964), named this stromatolitic structure, in which the space between structures is less than the diameter of the structure, "close lateral linked hemispheroids" (LLH-C). It reflects deposition in protected locations of re-entrant bays and behind barrier islands and ridges where wave action is usually slight (Logan, 1961). (c) In the third type seen at the top of the gypsarenite, the cryptalgal laminae are discontinuous and suffer from deformation (Fig. 3A), mainly connected with the early growth of gypsum crystals. Most of these gypsum crystals form a nucleus surrounded by an alternation of cryptalgal and gypsarenite laminae (Fig. 5B), which results in the formation of spheroidal structures or oncolites. Such structures are mostly in the form of concentrically stacked spheroids (SS), mode C (Logan et al., 1964). They range in diameter from 2 to 10 cm and have rounded, oval or irregular outlines. This structure reflects deposition in a relatively higher-energy environment than that of the LLH type. It is probably restricted to areas continuously under water and sufficiently agitated
267
Fig. 5. A. Alternations of cryptalgal carbonate and gypsum laminae showing a close lateral-linked-hemispheroid (LLH-C) structure. B. Concentric alternations of cryptalgal and gypsum laminae showing spheroidal structure (SS) or oncolite structure. C. Symmetrical ripple marks observed on bed surface of gypsarenite. They have a pointed crest and rounded trough. D. Symmetrical ripple marks connected with the growth of LLH stromatolitic structure. They have rounded crests. E. Undulated and bifurcated symmetrical ripple marks. F. Asymmetrical ripple marks.
to p e r m i t a l m o s t c o n t i n u o u s m o t i o n o f the s p h e r o i d s ( L o g a n et al., 1964). S y m m e t r i c a l r i p p l e m a r k s are o b s e r v e d on s o m e b e d d i n g surfaces o f the g y p s a r e n i t e unit (Fig. 5C), with p o i n t e d crests a n d r o u n d e d troughs. Occa-
sionally the crests m a y b e r o u n d e d (Fig. 5D), o r d i s c o n t i n u o u s with slight u n d u l a t i o n , a n d freq u e n t l y s h o w b i f u r c a t i o n (Fig. 5E). T h e length of the t i p p l e s is a b o u t 9 cm, the height 1.5 c m a n d the t i p p l e i n d e x (L/H) 6.5. T h e a b o v e d e s c r i b e d
268 character generally fits with the wave ripples described by Reineck and Singh (1975), who related the roundness of crests to reworking of ripples during emergence. However, connection of these ripple marks with the lateral-linked-hemispheroid (LLH) stromatolitic structures may suggest that the shape of the crests is related to the growth of the cryptalgal laminites (Fig. 5D). Asymmetrical ripple marks are also recorded on other bed surfaces of the gypsarenite unit (Fig. 5F). In these ripple marks the length is about 10 cm, the height 1.5 cm and the ripple index 7. According to Reineck et al. (1971), the ripple index and bifurcation suggest that they are asymmetrical wave ripples. They are similar to the undulatory small current ripples in that they have a steep lee side and a gentle stoss side and undulation of ripple crests (Reineck and Singh, 1975). Oscillation ripples were observed by Schreiber et al. (1977) in the Messinian gypsum at Salemi (Sicily). The cryptalgal laminites with algal filaments, and the LLH stromatolitic structures observed in the studied evaporites are similar to those described by Rouchy and Monty (1981) in the Messinian gypsum of Cyprus. Other authors described them in the Messinian gypsum of the Mediterranean basin (e.g. Nesteroff, 1973; Schreiber, 1973; Vai and Ricci-Lucchi, 1977; Lo Cicero and Catalano, 1978; and others). (2) Selenite (specchiofino or mirror-like gypsum uniO: The selenite unit commonly occurs in the top part of the studied Neogene evaporite sequence. It is 4-8 m thick and represents the only exposed gypsum layer at Gebel EI-Hagif area (no quarry in this area). The selenite unit is represented by two types: (a) The first type, represented by 1.5 m thick large twinned gypsum prisms 10-20 cm in length embedded in a carbonate matrix, overlies the deformed cryptalgal laminites and oncolitic structures. Gypsum prisms have their crystal c-axis either at right angles or inclined to bedding (Fig. 3B). A similar selenite type was described by Warren (1982) on the subaqueous floor of lake Inneston, South Australia where poorly aligned
gypsum prisms grow in and on an algal mush. He believed that seasonal lowering of the water table in the saline pond during hot periods may result in reworking of some deposited gypsum crystals. This probably explains the lack of laminations and the disordered arrangement of gypsum crystals. (b) The second selenite type is composed of more than 7 m of small mounds that coalesce to form a massive bed of varying thickness. It is made up of orderly rows of vertically standing twinned and split, 20-35 cm gypsum crystals (Fig. 3C). The re-entrant angle between the arms of the chevron is about 110 ° (Fig. 6A). The junctions between the two sides of the twin are zig-zag in shape (Fig. 6B). Gypsum crystals grow from an in-situ precursor maintaining nearly vertical successions. Some of these crystals appear to divide upwards, one crystal giving rise to a vertically branching family of crystals. Selenite beds showing this orientation are said to have grown according to Mottura' rule (Ogniben, 1954). In thick domes there is no complete continuity between gypsum crystals but layering is observed. The surface between layers is marked by incorporated gypsarenite, carbonate and clay materials reflecting short-term change in salinity during deposition of the twinned gypsum crystals. The gypsarenite, carbonate and clay interlayers represent the surface on which succeeding generations of crystals nucleated. A similar selenite type was described by Warren (1982) from Quaternary salt lakes in South Australia, who ascribed them to deposition in continuously subaqueous environments. Schreiber et al. (1982) discussed the origin of similar twinned and split gypsum crystals (specchiolino) in some occurrences of Messinian evaporite in the Apennines, Sicily, Spain and Cyprus, and stated that they were deposited in shallow lagoons which remain continuously concentrated into the gypsym precipitation range. Specchiolino or mirror-like gypsum was described by Youssef and Kamel (1985b) in the Miocene evaporite sequence at Ras Malaab area, westcentral Sinai as the primary phase of calcium sulphate, deposited in a shallow embayment connected with the Gulf of Suez. Therefore, the author believes that the coarse gypsum crystals in the
269
Fig. 6. A. Twinned gypsum crystals at Dir E1 Biraqat quarry. The angle between arms of the twin is about 110 o. B. Twinned gypsum crystals at Dir El Biraqat quarry. Traces of junction between two sides of the twin is zig-zag. C. Gypsum-clay relations viewed by SEM. The gypsum crystal (G) englobes clay particles (C). D. Degradation of gypsum crystals resulting in the formation of smaller lensoidal gypsum crystals. E. Gypsum crystals growing in different orientations.
studied selenite unit have grown in a shallow photic subaqueous environment subjected to rapid salinity increases.
Most of the gypsum crystals in the selenite unit contain impurities, mainly of clay and carbonate particles, which were poikilotopically enclosed by
270
the growth of gypsum crystals (Fig. 6C). Kastner (1970) recorded that an artificially grown crystal will surround impurities if the crystal growth rate is rapid, whereas the growing gypsum crystals push aside the impurity if the growth rate is slow. The growth rate of a gypsum crystal is faster in a brine pond where the salinity of gypsum-depositing water increases rapidly, than in a brine pond where the salinity changes slowly (Warren, 1982). Therefore, as the sediment column aggraded and the volume of the brine pond decreased, the high evaporation rate caused a rapid crystal growth during deposition of the selenite unit. This resuited in the presence of clay and carbonate inclusions inside the gypsum crystals of the studied selenite unit. Lenticular gypsum crystals are observed in the selenite unit (Fig. 6D). They show dissolution of gypsum crystals along cleavage planes and redeposition of calcium sulphate in the form of lensoidal gypsum crystals. Some of the gypsum crystals in this unit have crystallized in different orientations (Fig. 6E). Depositional model The evaporite sequence begins with alternating cryptalgal carbonate laminae and gypsum laminae showing LLH-C stromatolitic structure. The cryptalgal laminae reflect deposition in a normal marine or brackish environment whereas the gypsum laminae reflect deposition in a hypersaline environment. The observation that cryptalgal carbonate laminae are associated with finer gypsum crystals in the gypsarenite unit may suggest dilution of the brine which results in an increase of crystallite nucleation and consequently deposition of fine gypsum crystals. The increase of salinity resulted in deposition of coarser gypsum crystals toward the center of laminae and formation of reverse graded bedding. In agreement with Warren (1982), the rapid salinity changes created multiple crystallite nucleation to result in gypsite and gypsarenite at the early stage of evaporite deposition. Early growth of some gypsum crystals disturbed the cryptalgal laminae and sometimes resulted in the formation of concentric cryptalgal carbonate and gypsum laminae around them forming oncolitic structure.
As the brine pond shrank into a gypsarenite depositing salina, salinity increased and multiple crystallite nucleation did not occur. This resulted in deposition of the selenite unit. An interlocking network of gypsum crystals, with cryptalgal carbonate and clay material confined to the spaces between crystals, were formed at the beginning of selenite deposition under high-energy environment. Seasonal lowering of the depositing saline water level could be another factor in reworking the deposited gypsum prisms. The decrease in volume of the brine pond, and consequently the increase of salinity, resulted in the deposition of silt and sand-size gypsum crystals at the beginning of evaporite deposition, followed upward by deposition of gypsum crystals coarser than 5 cm at the end of evaporite deposition (Fig. 2). This led the present author to suggest a regressive evaporite basin model for the deposition of the Neogene evaporites at the the northern Western Desert, Egypt. However, Sellwood and Netherwood (1984) and Youssef (1986) concluded that the Miocene evaporites on the Gulf of Suez and Red Sea coasts, respectively, are composite evaporite sequences which include deposition in shallow lagoonal as well as supratidal environments. Tectonic movements and the different rates of block movements in the Gulf of Suez and Red Sea during the Miocene period were the main factors controlling the different evaporite facies.
Diagenesis Many authors have observed that gypsum is the common calcium sulphate mineral in Recent environments. Hardie (1967) noticed that anhydrite is difficult to nucleate in the laboratory and usually forms as a very fine grained aggregate, secondary after gypsum or a bassanite precursor. Shearman (1966, 1978), Kinsman (1969), and Butler (1969), observed that the natural sabkha anhydrite of the Arabian Gulf and Baja California is very fine grained and found only in nodules growing in sediments within the supratidal capillary zone. They thought that this anhydrite is of secondary origin after gypsum which is more easily and rapidly deposited. West et al. (1979) concluded that the gypsum nodules in a modern
271 sabkha on the Mediterranean coast of Egypt are of primary origin. Gypsum is considered as the primary phase in the studied evaporites. This is evidenced by the presence of the twinned gypsum (Hardie and Eugster, 1971; Schreiber et al., 1982) as well as gypsarenite (Schreiber, 1978). The studied gypsarenite is similar to that observed by the author at E1-Ballah lake, and that in the Quaternary salt lakes of South Australia (Warren, 1982) and in present day environment in the Mannar lagoon in the northwest of Srilanka (Gunatilaka, 1975). X-ray diffraction analysis for evaporites from Gebel E1-Hagif and Dir E1-Biraqat areas, showed that they are mainly composed of gypsum with minor amounts of anhydrite, bassanite, calcite and traces of pyrite and quartz. The Quaternary evaporites of the EI-Ballah area, 21 km north of Ismailia, occur as a horizontal layer about 1 m thick composed of laminated gypsarenite and twinned gypsum crystals of about 15 cm in length. X-ray diffraction shows that gypsum is the main calcium sulphate mineral. No anhydrite or bassanite is recorded in this evaporite. Warren (1982) showed the same results for the evaporites of the Quaternary salt lakes of South Australia. This may suggest that bassanite and anhydrite in the studied evaporites were formed in sequence as a result of the dehydration of primary gypsum under arid conditions and high temperature. During the humid rainy Pleistocene period, some of the secondary anhydrite may have been transformed into porphyrotopic secondary gypsum. The Middle Miocene evaporite successions on the Red Sea and Gulf of Suez coasts were subjected to a more complicated transformation sequence than those of the Neogene (Messinian) succession studied here. Youssef (1986) concluded that the Middle Miocene calcium sulphate was deposited as primary gypsum, which was transformed into anhydrite by burial. The secondary anhydrite was in turn transformed into alabastrine, granotopic and porphyrotopic secondary gypsum. Under arid hot subaerial conditions, a crustal layer of secondary powdery anhydrite is formed as pseudomorphs after secondary gypsum. Rain water may later transform the powdery anhydrite into
secondary porphyrotopic and satin-spar gypsum. Tectonic movements in the Red Sea and Gulf of Suez areas, as well as the time factor are believed to be the main factors for the more complicated transformation sequence of the Middle Miocene evaporites.
Conclusions The sedimentary structures and textures, as well as the intercalations of oncolitic carbonate and sandy gypsiferous marl that characterizes the Neogene evaporite sequence at Gebel E1-Hagif and Dir E1-Biraqat areas, suggest that they were deposited in a shallow photic lagoonal environment subjected to brine concentrations by evaporation and the activities of blue green algae. The studied Neogene evaporites are characterized by alternations of cryptalgal carbonate and gypsum laminae at the base of the sequence followed upward by massive selenite (Specchiolino or mirror like gypsum) at the top. The laminated gypsum at Dir E1-Biraqat quarry shows three main structures: irregular lamination at the base followed by lateral linked hemispheroid (LLH-C) and spheroidal structure (oncolite). The selenite unit is subdivided into massive disordered selenite and massive vertically arranged selenite. The decreasing volume of the brine pond and consequently the increase of salinity controlled the crystal size of gypsum as well as the sedimentary textures and structures. In salinas where the whole laminated gypsum sequence is gypsarenite, the rate of salinity changes has always been rapid and the bottom brine salinity has always been unstable. As the brine pond decreased in volume and the salinity increased, the selenite unit deposited.
Acknowledgements The author is grateful to the Institute of Geology, University of Bergen, Norway, for assistance in the mineralogical analysis.
References Abdallah, A.M. and Yehia, M.A., 1973. Stratigraphy and microfacies study of Sidi-Barrani area, Western Desert, A.R.E. Bull. Fac. Sci., Cairo Univ., 46: 343-371.
272 Bommarito, S. and Catalano, R., 1973. Facies analysis of an evaporitic Messinian sequence near Ceminna (Palermo, Sicily). In: C.W. Drooger (Editor), Messinian Events in the Mediterranean. Kon. Ned. Akad. Wetensch., Geodyn. Sci. Rep. 7: 172-177. Braune, K., Fabricus, F. and Hermann, K.O., 1973. Sedimentation and facies of Late Miocene strata on Celphalonia (Ionian Islands, Greece). In: C.W. Drooger (Editor), Messinian Events in the Mediterranean. Kon. Ned. Akad. Wetensch., Geodyn. Sci. Rep., 7: 192-201. Butler, G.P., 1969. Modern evaporite deposition and geochemistry of coexisting brines, the sabkha, Trucial coast, Arabian Gulf. J. Sediment. Petrol., 39: 70-89. Dronkert, H., 1976. Late Miocene evaporites in the Sorbas basin and adjoining areas. Mem. Soc. Geol. Ital., 16: 341-361. Drooger, C.W. (Editor), 1973. Messinian Events in the Mediterranean. Kon. Ned. Akad. Wetensch. Geodyn. Sci. Rep., 7:272 pp. El Shazly, M., Salman, A.B., Someida, M.M., Morsy, M.A. and El Asy, I.F., 1976. Report on the evaluation of gypsum and clay, Dir El Biraqat-E1 Hagif area, north Western Desert, Egypt. Atomic Energy Establishment, Internal Report, 50 pp. Geel, T., 1976. Messmian gypsiferous deposits of the Lorca basin (Province of Murcia, SE Spain). Mem. Soc. Geol. Ital., 16: 369-385. Gunatilaka, A., 1975. Some aspects of the biology and sedimentology of laminated algal mats from Mannar Lagoon, north-west Ceylon. Sediment. Geol., 14: 275-300. Hardie, L.A., 1967. The gypsum-anhydrite equilibrium at one atmosphere pressure. Am. Mineral., 52: 171-200. Hardie, L.A. and Eugster, H.P., 1971. The depositional environment of marine evaporites: a case for shallow clastic accumulation. Sedimentology, 16: 187-220. Hsii, K.J., 1972. Origin of saline giants: a critical review after discovery of the Mediterranean evaporite. Earth Sci. Rev., 8: 371-396. Kastner, M., 1970. An inclusion hourglass pattern in synthetic gypsum. Am. Mineral., 55: 2128-2130. Kinsman, D.J.J., 1969. Modes of formation, sedimentary associations and diagenetic features of shallow water and supratidal evaporites. Am. Assoc. Pet. Geol. Bull., 53: 830-840. Lo Cicero, G. and Catalano, R., 1978. Facies and petrography of some Messinian evaporites of the Ciminna basin (Sicily). In: R. Catalano, G. Ruggieri and R. Sprovieri (Editors), Messinian Evaporites in the Mediterranean. Mere. Soc. Geol. Ital., XVI: 55-62. Logan, B.W., 1961. Cryptozoon and associate stromatolites from the Recent of Shark Bay. Western Australia. J. Geol., 69(5): 517-533. Logan, B.W., Rezak, R. and Ginsburg, R.N., 1964. Classification and environmental significance of algal stromatolites. J. Geol., 72: 68-83.
Nesteroff, W.D., 1973. P&rographie des 6vaporites Messiniennes de La M&:literranre. Comparalsons des forages JOIDES-DSDP et des drprts du bassin de Sicile. In: C.W. Drooger (Editor), Messinian Events in the Mediterranean. Kon. Ned. Akad. Wetensch., Geodyn. Sci. Rep., 7: 111-123. Oginben, L., 1954. La "Regola de Mottura" diorientazione del gesso. Period. Mineral., 23: 53-65. Ogniben, L., 1955. Inverse graded bedding in primary gypsum of chemical deposition. J. Sediment. Petrol., 25: 273-281. Omara, S.M. and Sanad, S., 1975. Rock stratigraphy and structural features of the area between Wadi E1 Natrun and the Moghra depression (Western Desert, Egypt). Geol. Jahrb., Beih., 16: 45-73. Reineck, H.E. and Singh, I.B. 1975. Depositional Sedimentary Environments. Springer, Berlin, 439 pp. Reineck, H.E., Singh, I.B. and Wunderlich, F., 1971. Einteilung der Rippeln und anderer mariner SandkSrper. Senckenbergiana Marit., 3: 93-101. Rouchy, J.M. and Monty, C.L., 1981. Stromatolites and cryptalgal laminites associated with Messinian gypsum in Cyprus. In: C1. Monty (Editor), Phanerozoic Stromatolites. Springer, New York, N.Y., pp. 155-180. Ryan, W.B.F., Hsii, K.J. et al., 1973. Initial Reports of the Deep Sea Drilling Project, XXIII. U.S. Government Printing Office, Washington, D.C., 1447 pp. Said, R., 1962. The Geology of Egypt. Elsevier, Amsterdam, 370 pp. Said, R., 1979. The Messinian in Egypt. Ann. Geol. Pays Helen., Tome Hors Ser., Fasc., III: 1083-1090. Schreiber, B.C., 1973. Survey of the physical features of Messinian chemical sediments. In: C.W. Drooger (Editor), Messinian Events in the Mediterranean. Kon. Ned. Akad. Wetensch., Geodyn. Sci. Rep., 7: 101-110. Schreiber, B.C., 1978. Environment of subaqueous gypsum deposition. In: W.E. Dean and B.C. Schreiber (Editors), Marine Evaporites. Soc. Econ. Paleontol. Mineral., Short Course Note, 4: 43-73. Schreiber, B.C., Friedman, G.M., Decima, A. and Schreiber, E.~ 1976. Depositional environment of Upper Miocene (Messinian) evaporite deposits of the Silician basin. Sedimentology, 23: 729-760. Schreiber, B.C., Catalano, R. and Schreiber, E., 1977. An evaporitic lithofacies continuum: latest Miocene (Messinian) deposits of Salemi basin (Sicily) and a modern analog. In: J.H. Fisher (Editor), Reef and Evaporites--Concepts and Models. Am. Assoc. Pet. Geol., Stud. Geol., 5: 169-180. Schreiber, B.C., Roth, M.S. and Helman, M.L., 1982. Recognition of primary facies characteristics of evaporites and the differentiation of these forms from diagenetic overprints. In: C.R. Handford, R.G. Loucks and G.R. Davis (Editors), Depositional and Diagenetic Spectra of Evaporites. A Core Workshop. Soc. Econ. Paleontol. Mineral. Core Workshop No. 3, Calgary, Alta, pp. 1-32. Sellwood, B.W. and Netherwood, R.E., 1984. Facies evolution
273 in the Gulf of Suez area: sedimentation history as an indicator of rift initiation and development. Modem Geol., 9: 43-69. Shearman, D.J., 1966. Origin of marine evaporite by diagenesis. Trans. Inst. Min. Metall. (Sect. B), 75: 208-215. Shearman, D.J., 1978. Evaporites of coastal sabkha. In: W.E. Dean and B.C. Schreiber (Editors), Marine Evaporites. SOc. Econ. Paleontol. Mineral., Short Course, 4: 6-42. Vai, G.B. and Ricci-Lucchi, F., 1977. Algal crust autochthonous and clastic gypsum in a cannibalistic evaporite basin: a case history from the Messinian of Northern Apennines. Sedimentology, 24: 211-244. Warren, J.K., 1982. The hydrological setting, occurrence and significance of gypsum in Late Quaternary Salt Lakes in South Australia. Sedimentology, 5: 609-637.
West, I.M., Aly, A.Y. and Hilmy, M.E., 1979. Primary gypsum nodules in a modern sabkha on the Mediterranean coast of Egypt. Geology, 7: 354-358. Youssef, E.A.A., 1986. Depositional and diagenetic models of some Miocene Evaporites on the Red Sea coast, Egypt. Sediment. Geol., 48: 17-36. Youssef, E.A.A. and Kamel, A., 1985a. Depositional environment of Dir E1 Biraqat evaporites north Western Desert, Egypt. Bull. Fac. Sci., Cairo Univ., 53: 377-396. Youssef, E.A.A. and Kamel, A.M., 1985b. Depositional and diagenetic models of the Middle Miocene evaporites on the Gulf of Suez coast, between Sudr and Wadi Gharandal, Sinai, Egypt. Abstr., 2nd Symp. on Phanerozoic and Development in Egypt, National Committee of Geological Sciences, 1 p.
261
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Sedimentological studies of Neogene evaporites in the northern Western Desert, Egypt EL-SAYED
A.A. YOUSSEF
Geology Department, Faculty of Science, Cairo University, Giza (Egypt) Received May 12, 1987; revised version accepted May 31, 1988
Abstract Youssef, El Sayed A.A., 1988. Sedimentological studies of Neogene evaporites in the northern Western Desert, Egypt. Sediment. Geol., 59: 261-273. Neogene calcium sulphate deposits at the northern Western Desert, Egypt, are interpreted as shallow photic subaqueous evaporites. Study of the evaporite sequences at Gebel E1-Hagif and Dir E1-Biraqat areas reveals two principal structures similar to those observed in the Messinian gypsum in and around the Mediterranean Sea. The structures, arranged from base to top are: (1) cryptalgal laminites and stromatolitic structures built up by cyanobacterial (blue-green algal) communities in gypsite and gypsarenite units; and (2) selenite (specchiolino or mirror like gypsum) represented by (a) disordered selenite, and (b) vertically arranged selenite. Cyclic increase and decrease in salinity are represented by interlamination of gypsum and cryptalgal carbonate laminae. General increase of salinity is represented by the general increase in crystal size of gypsum from gypsite to gypsarenite to twinned gypsum crystals more than 5 cm in length at the top of the sequence. A regressive evaporite basin model is suggested for the deposition of the studied evaporite sequence. Dehydration of gypsum into anhydrite through an intermediate bassanite phase, and the subsequent transformation of secondary anhydrite into secondary porphyrotopic gypsum, are the main diagenetic processes affecting the studied Neogene evaporites.
Introduction This s t u d y r e p o r t s o b s e r v a t i o n s o n the d e p o s i t i o n a l a n d d i a g e n e t i c forms of s o m e N e o g e n e e v a p o r i t e s at G e b e l E1-Hagif a n d D i r E1-Biraqat areas, n o r t h e r n W e s t e r n Desert, E g y p t (Fig. 1). T h e s t u d y areas lie b e t w e e n l o n g i t u d e s 30025 ' a n d 3 0 ° 5 5 ' E , a n d l a t i t u d e s 29010 ' a n d 2 9 ° 3 0 ' N . G e b e l E1-Hagif a r e a is l o c a t e d a b o u t 50 k m to the s o u t h of the M e d i t e r r a n e a n c o a s t to the s o u t h of a g r o u p of Pleistocene g y p s u m quarries, e.g. E1G h o r b a n i a t , E 1 - H a m m a m a n d E1-Omayed a n d n e w discoveries at A l a m e i n area. D i r E1-Biraqat g y p s u m q u a r r y lies a b o u t 40 k m to the west of G e b e l E1-Hagif gypsum. This s t u d y investigates the d e p o s i t i o n a l env i r o n m e n t a n d the diagenetic h i s t o r y o f N e o g e n e 0037-0738/88/$03.50
© 1988 Elsevier Science Publishers B.V.
c a l c i u m s u l p h a t e d e p o s i t s in t e r m s of the textures, structures a n d m i n e r a l o g i c a l c o m p o s i t i o n . T h e G e b e l E1-Hagif a n d D i r E1-Biraqat areas were c h o s e n to t h r o w light o n the d e p o s i t i o n a l environm e n t a n d d i a g e n e t i c h i s t o r y o f the N e o g e n e e v a p o r i t e s of the n o r t h e r n W e s t e r n Desert, for c o m p a r i s o n w i t h o t h e r s in the M e d i t e r r a n e a n basin, as well as o n the c o a s t a l a r e a o f the R e d Sea a n d G u l f o f Suez, Egypt.
Methods of study T h i n sections were p r e p a r e d a n d s t u d i e d p e t r o graphically. S o m e s a m p l e s f r o m the selenite units were s t u d i e d b y a Jeol S c a n n i n g E l e c t r o n M i c r o scope ( S E M ) J S M - 3 5 w i t h K e v e x M x 7000 system. T h e m i n e r a l o g i c a l c o m p o s i t i o n was i d e n t i f i e d
262 Z1} o , M
30 u !
E DITIrR
31 °
32 °
,
3 3u
,
RANEAN
I
S ir A
3 4a
I
/
/
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,
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,
,,.
Z Z* Fig. 1. Index map showing study area.
using a Philips APD 1700 automated powder diffractometer, nickel-filtered Cu-Ka[. 2 radiation (A = 1.54056, 1.54439 ,~) generated at 45 kV and 60
mA. Representative slabs and samples were photographed. In this work the author follows Warren (1982) in using the terms gypsite and gypsarenite to describe gypsum in which the majority of crystals are in the silt and sand sizes, respectively. The term selenite (specchiolino or mirror like gypsum) is used for gypsum in which more than 50% of the sample is composed of distinct, transparent crystals with grain sizes coarser than 2 mm.
Geologic setting The studied Neogene evaporites form lenticular bodies trending nearly east-west parallel to the Mediterranean coast. They extend for tens of kilometers with a maximum thickness of 15 m and are mainly composed of calcium sulphate. Based on the exposed Dir E1-Biraqat section and bore
ONCOLITIC LIMESTONE '~ I I ~ l= J )~
CONGLOMERATIC LIMESTONE EVAPORITE LIMESTONE MARL SHALE SANDSTONE
Fig. 2. Generalized composite stratigraphic section of the Neogene deposits, northern Western Desert, Egypt.
263 hole data by E1 Shazly et al. (1976), Youssef a n d Kamel (1985a) showed that the evaporite sequence consists of two gypsum horizons interlayered by oncolitic carbonate and greenish grey sandy marl enclosing gypsum crystals. The evaporite sequence unconformably overlies the Middle Miocene limestone (Marmarica Formation of Said, 1962), and unconformably underlies the Late Pliocene pink fossiliferous limestone (Sidi-Barrani Formation of Abdallah and Yehia, 1973). The upper unconformity is represented by a 1 m thick conglomeratic pink limestone layer. Abdallah and Yehia (1973) described a similar conglomeratic limestone directly overlying the Marmarica Formation at Sidi-Barrani area and considered it of Early Pliocene. The stratigraphic section exposed at the southern scarp of Gebel E1-Hagif was measured, described and named the Hagif Formation by Omara and Sanad (1975). They showed that it is composed of 30-40 m of cross bedded, well sorted medium-grained sand, limestone, clay, crystalline gypsum, gypsiferous clay and pink hard sandy limestone with Helix quadridentata at the topmost part of the section. They showed also that the Hagif Formation unconformably overlies the Early Miocene clastics (Moghra Formation; Said, 1962). The author recognised a pink conglomeratic limestone bed between the massive crystalline gypsum and the overlying pink hard sandy limestone at Gebel E1-Hagif, which is similar to the conglomerate described by Youssef and Kamel (1985a) at Dir E1-Biraqat quarry between the pink limestone and the underlying evaporite sequence (Fig. 2). The observation that the evaporite deposits at Dir E1-Biraqat quarry and at the top part of Hagif Formation underlie the Early Pliocene pink conglomeratic limestone and unconformably overlie Middle and Lower Miocene sequences, led the present author to suggest a Late Miocene (Messinian) age to the studied evaporite sequence.
Neogene evaporite sequence Upper Miocene evaporites have long been recorded and studied in and around the Mediterranean (Hsii, 1972; Bommarito and Catalano, 1973; Braune et al., 1973; Drooger, 1973; Ryan,
Hsii et al., 1973; Dronkert, 1976; Geel, 1976; Schreiber et al., 1976, 1977; Said, 1979; Rouchy and Monty, 1981; Youssef and Kamel, 1985a; and others). Deposits of calcium sulphate (gypsum, anhydrite and bassanite) are the main constituents of the Neogene succession in many localities in Egypt. The following are the sedimentary structures and textures, depositional and diagenetic models of the studied Neogene evaporite. Sedimentary structures and textures
Two main textures are recognized in the Neogene evaporite sequences at the study areas. From base to top, they are as follows: (1) Cryptalgal laminites and stromatolitic structures built up by cyanophytic (blue-green algal) communities in gypsite and gypsarenite units (Fig. 3A). (2) Selenite units (specchiolino or mirror-like gypsum), represented by (a) disordered selenite (Fig. 3B), and (b) vertically arranged selenite (Fig.
3c). (1) Cryptalgal laminites and stromatolitic structures
The cryptalgal laminites and stromatolitic structures are clearly observed in a 7 m layer at the base of Dir E1-Biraqat evaporite sequence. It is characterized by laminations resulting from alternating gypsum and cryptalgal carbonate laminae (Fig. 3A). The cryptalgal laminites show roilliraeter-sized irregular, planar and wavy laminae, most of which have remains of algal origin. They range in thickness from 300/Lm to 2 mm, and contain irregularly oriented algal filaments, 60/~m in diameter and 500/~m long (Fig. 3D, E). The contact between the algal filaments and the enclosing gypsum crystals is sharp (Fig. 3F). The cryptalgal laminae are associated with rounded calcite crystals of about 30 /Lm in diameter and some pyrite crystals. Youssef and Kamel (1985a) believe that calcite and pyrite were formed by the action of anaerobic bacteria on calcium sulphate in the presence of organic matter (algae). The gypsum laminae range in thickness from few millimeters to 2 cm and are composed of gypsite, and gypsarenite.
264
~i~ii iiii~ii~ ~?i ~
Fig. 3. A. Vertical passage of wavy-laminated to oncofitic structure in Dir E1-Biraqat evaporite sequence. B. Lense of disordered selenite unit at Dir E1-Biraqat quarry. C. Vertically arranged twinned and split gypsum crystals at Gebel E1-Hagif. D. Algal filaments disseminated in prismatic gypsum crystals (crossed nicols). E. Algal filaments associated with biogenic calcite in gypsarenite (crossed nicols). F. Algal filaments enclosed in gypsum crystals. The contact between them is clear-cut (crossed nicols).
Gypsite. G y p s i t e occurs at the base of the sequence, overlying an oncolitic c a r b o n a t e facies (Fig. 4A) a n d u n d e r l y i n g the gypsarenite. T h e g y p s u m crystals are acicular to p r i s m a t i c a n d form felted masses (Fig. 3D). S o m e t i m e s they are a r r a n g e d with long directions r a d i a l l y u p w a r d f r o m the c r y p t a l g a l c a r b o n a t e laminae, suggesting that
they were f o r m e d as a crust o n the c r y p t a l g a l laminae. S o m e crystals are b r o k e n a n d irregularly arranged, i n d i c a t i n g that they were f o r m e d at the water surface a n d later d e p o s i t e d as detrital grains. This t y p e of g y p s u m is similar to that d e s c r i b e d b y W a r r e n (1982) in the S o u t h A u s t r a l i a n Salt Lakes a n d b y Schreiber et al. (1977) in the h y p e r s a l i n e
265
Fig. 4. A. Algal pellets and grains, or pseudo-oncoid grains in the oncobiosparite facies underlying the gypsite unit (crossed nicols). B. Rounded and angular gypsum crystals showing a mosaic texture in gypsarenite unit (crossed nicols). C. Cleavage fragments of gypsum in gypsarenite unit (crossed nicols). D. Corrosion of gypsum crystals by the biogenic calcite in gypsarenite unit (crossed nicols). E. Reverse graded bedding in gypsarenite unit (crossed nicols). F. Porphyrotopic secondary gypsum crystal with deformed discontinuous peripheral zone (crossed nicols).
266 lagoon in the Arabian Gulf (Azizyah). The laminites are principally induced by the cyclic growth of filamentous algae without indication of trapping mechanism of gypsum crystals.
Gypsarenite. Gypsum crystals in this type are either in the form of rounded and granular crystals, showing a mosaic texture (Fig. 4B) or cleavage fragments (Fig. 4C). Some granular gypsum crystals are embedded in the cryptalgal carbonate laminae (Fig. 4B) and record a trapping mechanism, whereas the cleavage fragments mostly enclose algal filaments (Fig. 3F). Corrosion by the diagenetically formed calcite (biogenic calcite) is a common feature in the gypsarenite unit (Fig. 4D). Reverse graded bedding is a common texture in the laminated gypsarenite. Fine gypsum crystals 80-200 /~m in diameter form the base of the laminae (in contact with cryptalgal laminae) and change to coarse gypsum crystals 200-800/~m in size at the top of the laminae (Fig. 4E). In some laminae the coarse gypsum crystals are followed by finer crystals before going to the second cryptalgal carbonate lamina. The density of gypsum crystals also increases upward, from isolated crystals in the basal carbonate lamina to densely interlocking gypsum crystals with a mosaic texture at the top of the lamina. Porphyrotopic secondary gypsum is observed in this unit (Fig. 4F). The origin of reverse graded bedding in gypsum was discussed by many authors. Ogniben (1955) proposed that gypsum precipitated first, followed by anhydrite when the brine became more concentrated. The next water inflow would hydrate the anhydrite to coarsly crystalline gypsum resulting in the formation of reverse grading. Hardie and Eugster (1971) suggested that graded bedding resulted from mechanical deposition during storm surges on shallow offshore banks where particles of gypsum were trapped and bound by algal mats. Schreiber et al. (1976) supported the elastic origin of the original gypsum grains proposed by Hardie and Eugster (1971) and added that cyclic dilution of the overlying water may have caused the growth of gypsum crystals along the top of each lamina. In the studied gypsarenite, it is observed that some laminae are composed of fine gypsum crystals in contact with the enclosing upper and
lower cryptalgal laminae, followed by coarser gypsum crystals in the center. This suggests that the finer gypsum crystals may be related to the cryptalgal laminae and led the present author to believe that the coarse gypsum crystals were formed at a stage of increase in salinity. The deposition of finer crystals is related to dilution of the brine which increases gypsum crystallite nucleation. The cryptalgal laminae are interpreted as having formed during the periods of lowest salinity in the brine pond. Three types of relations are observed between the cryptalgal carbonate laminae and gypsum laminae: (a) In the first type, the cryptalgal laminites are irregular and planar and the laminations are mostly continuous (Fig. 3A). (b) In the second type, the cryptalgal carbonate laminae are wavy and show a prominent stromatolitic structure resulting from the regular stacking of over 1 mm thick cryptalgal laminae overlain by centimetric gypsum laminae. The maximum height is about 4 cm for an average width of about 6 cm, being separated by few centimeter wide spaces (Fig. 5A). Logan et al. (1964), named this stromatolitic structure, in which the space between structures is less than the diameter of the structure, "close lateral linked hemispheroids" (LLH-C). It reflects deposition in protected locations of re-entrant bays and behind barrier islands and ridges where wave action is usually slight (Logan, 1961). (c) In the third type seen at the top of the gypsarenite, the cryptalgal laminae are discontinuous and suffer from deformation (Fig. 3A), mainly connected with the early growth of gypsum crystals. Most of these gypsum crystals form a nucleus surrounded by an alternation of cryptalgal and gypsarenite laminae (Fig. 5B), which results in the formation of spheroidal structures or oncolites. Such structures are mostly in the form of concentrically stacked spheroids (SS), mode C (Logan et al., 1964). They range in diameter from 2 to 10 cm and have rounded, oval or irregular outlines. This structure reflects deposition in a relatively higher-energy environment than that of the LLH type. It is probably restricted to areas continuously under water and sufficiently agitated
267
Fig. 5. A. Alternations of cryptalgal carbonate and gypsum laminae showing a close lateral-linked-hemispheroid (LLH-C) structure. B. Concentric alternations of cryptalgal and gypsum laminae showing spheroidal structure (SS) or oncolite structure. C. Symmetrical ripple marks observed on bed surface of gypsarenite. They have a pointed crest and rounded trough. D. Symmetrical ripple marks connected with the growth of LLH stromatolitic structure. They have rounded crests. E. Undulated and bifurcated symmetrical ripple marks. F. Asymmetrical ripple marks.
to p e r m i t a l m o s t c o n t i n u o u s m o t i o n o f the s p h e r o i d s ( L o g a n et al., 1964). S y m m e t r i c a l r i p p l e m a r k s are o b s e r v e d on s o m e b e d d i n g surfaces o f the g y p s a r e n i t e unit (Fig. 5C), with p o i n t e d crests a n d r o u n d e d troughs. Occa-
sionally the crests m a y b e r o u n d e d (Fig. 5D), o r d i s c o n t i n u o u s with slight u n d u l a t i o n , a n d freq u e n t l y s h o w b i f u r c a t i o n (Fig. 5E). T h e length of the t i p p l e s is a b o u t 9 cm, the height 1.5 c m a n d the t i p p l e i n d e x (L/H) 6.5. T h e a b o v e d e s c r i b e d
268 character generally fits with the wave ripples described by Reineck and Singh (1975), who related the roundness of crests to reworking of ripples during emergence. However, connection of these ripple marks with the lateral-linked-hemispheroid (LLH) stromatolitic structures may suggest that the shape of the crests is related to the growth of the cryptalgal laminites (Fig. 5D). Asymmetrical ripple marks are also recorded on other bed surfaces of the gypsarenite unit (Fig. 5F). In these ripple marks the length is about 10 cm, the height 1.5 cm and the ripple index 7. According to Reineck et al. (1971), the ripple index and bifurcation suggest that they are asymmetrical wave ripples. They are similar to the undulatory small current ripples in that they have a steep lee side and a gentle stoss side and undulation of ripple crests (Reineck and Singh, 1975). Oscillation ripples were observed by Schreiber et al. (1977) in the Messinian gypsum at Salemi (Sicily). The cryptalgal laminites with algal filaments, and the LLH stromatolitic structures observed in the studied evaporites are similar to those described by Rouchy and Monty (1981) in the Messinian gypsum of Cyprus. Other authors described them in the Messinian gypsum of the Mediterranean basin (e.g. Nesteroff, 1973; Schreiber, 1973; Vai and Ricci-Lucchi, 1977; Lo Cicero and Catalano, 1978; and others). (2) Selenite (specchiofino or mirror-like gypsum uniO: The selenite unit commonly occurs in the top part of the studied Neogene evaporite sequence. It is 4-8 m thick and represents the only exposed gypsum layer at Gebel EI-Hagif area (no quarry in this area). The selenite unit is represented by two types: (a) The first type, represented by 1.5 m thick large twinned gypsum prisms 10-20 cm in length embedded in a carbonate matrix, overlies the deformed cryptalgal laminites and oncolitic structures. Gypsum prisms have their crystal c-axis either at right angles or inclined to bedding (Fig. 3B). A similar selenite type was described by Warren (1982) on the subaqueous floor of lake Inneston, South Australia where poorly aligned
gypsum prisms grow in and on an algal mush. He believed that seasonal lowering of the water table in the saline pond during hot periods may result in reworking of some deposited gypsum crystals. This probably explains the lack of laminations and the disordered arrangement of gypsum crystals. (b) The second selenite type is composed of more than 7 m of small mounds that coalesce to form a massive bed of varying thickness. It is made up of orderly rows of vertically standing twinned and split, 20-35 cm gypsum crystals (Fig. 3C). The re-entrant angle between the arms of the chevron is about 110 ° (Fig. 6A). The junctions between the two sides of the twin are zig-zag in shape (Fig. 6B). Gypsum crystals grow from an in-situ precursor maintaining nearly vertical successions. Some of these crystals appear to divide upwards, one crystal giving rise to a vertically branching family of crystals. Selenite beds showing this orientation are said to have grown according to Mottura' rule (Ogniben, 1954). In thick domes there is no complete continuity between gypsum crystals but layering is observed. The surface between layers is marked by incorporated gypsarenite, carbonate and clay materials reflecting short-term change in salinity during deposition of the twinned gypsum crystals. The gypsarenite, carbonate and clay interlayers represent the surface on which succeeding generations of crystals nucleated. A similar selenite type was described by Warren (1982) from Quaternary salt lakes in South Australia, who ascribed them to deposition in continuously subaqueous environments. Schreiber et al. (1982) discussed the origin of similar twinned and split gypsum crystals (specchiolino) in some occurrences of Messinian evaporite in the Apennines, Sicily, Spain and Cyprus, and stated that they were deposited in shallow lagoons which remain continuously concentrated into the gypsym precipitation range. Specchiolino or mirror-like gypsum was described by Youssef and Kamel (1985b) in the Miocene evaporite sequence at Ras Malaab area, westcentral Sinai as the primary phase of calcium sulphate, deposited in a shallow embayment connected with the Gulf of Suez. Therefore, the author believes that the coarse gypsum crystals in the
269
Fig. 6. A. Twinned gypsum crystals at Dir E1 Biraqat quarry. The angle between arms of the twin is about 110 o. B. Twinned gypsum crystals at Dir El Biraqat quarry. Traces of junction between two sides of the twin is zig-zag. C. Gypsum-clay relations viewed by SEM. The gypsum crystal (G) englobes clay particles (C). D. Degradation of gypsum crystals resulting in the formation of smaller lensoidal gypsum crystals. E. Gypsum crystals growing in different orientations.
studied selenite unit have grown in a shallow photic subaqueous environment subjected to rapid salinity increases.
Most of the gypsum crystals in the selenite unit contain impurities, mainly of clay and carbonate particles, which were poikilotopically enclosed by
270
the growth of gypsum crystals (Fig. 6C). Kastner (1970) recorded that an artificially grown crystal will surround impurities if the crystal growth rate is rapid, whereas the growing gypsum crystals push aside the impurity if the growth rate is slow. The growth rate of a gypsum crystal is faster in a brine pond where the salinity of gypsum-depositing water increases rapidly, than in a brine pond where the salinity changes slowly (Warren, 1982). Therefore, as the sediment column aggraded and the volume of the brine pond decreased, the high evaporation rate caused a rapid crystal growth during deposition of the selenite unit. This resuited in the presence of clay and carbonate inclusions inside the gypsum crystals of the studied selenite unit. Lenticular gypsum crystals are observed in the selenite unit (Fig. 6D). They show dissolution of gypsum crystals along cleavage planes and redeposition of calcium sulphate in the form of lensoidal gypsum crystals. Some of the gypsum crystals in this unit have crystallized in different orientations (Fig. 6E). Depositional model The evaporite sequence begins with alternating cryptalgal carbonate laminae and gypsum laminae showing LLH-C stromatolitic structure. The cryptalgal laminae reflect deposition in a normal marine or brackish environment whereas the gypsum laminae reflect deposition in a hypersaline environment. The observation that cryptalgal carbonate laminae are associated with finer gypsum crystals in the gypsarenite unit may suggest dilution of the brine which results in an increase of crystallite nucleation and consequently deposition of fine gypsum crystals. The increase of salinity resulted in deposition of coarser gypsum crystals toward the center of laminae and formation of reverse graded bedding. In agreement with Warren (1982), the rapid salinity changes created multiple crystallite nucleation to result in gypsite and gypsarenite at the early stage of evaporite deposition. Early growth of some gypsum crystals disturbed the cryptalgal laminae and sometimes resulted in the formation of concentric cryptalgal carbonate and gypsum laminae around them forming oncolitic structure.
As the brine pond shrank into a gypsarenite depositing salina, salinity increased and multiple crystallite nucleation did not occur. This resulted in deposition of the selenite unit. An interlocking network of gypsum crystals, with cryptalgal carbonate and clay material confined to the spaces between crystals, were formed at the beginning of selenite deposition under high-energy environment. Seasonal lowering of the depositing saline water level could be another factor in reworking the deposited gypsum prisms. The decrease in volume of the brine pond, and consequently the increase of salinity, resulted in the deposition of silt and sand-size gypsum crystals at the beginning of evaporite deposition, followed upward by deposition of gypsum crystals coarser than 5 cm at the end of evaporite deposition (Fig. 2). This led the present author to suggest a regressive evaporite basin model for the deposition of the Neogene evaporites at the the northern Western Desert, Egypt. However, Sellwood and Netherwood (1984) and Youssef (1986) concluded that the Miocene evaporites on the Gulf of Suez and Red Sea coasts, respectively, are composite evaporite sequences which include deposition in shallow lagoonal as well as supratidal environments. Tectonic movements and the different rates of block movements in the Gulf of Suez and Red Sea during the Miocene period were the main factors controlling the different evaporite facies.
Diagenesis Many authors have observed that gypsum is the common calcium sulphate mineral in Recent environments. Hardie (1967) noticed that anhydrite is difficult to nucleate in the laboratory and usually forms as a very fine grained aggregate, secondary after gypsum or a bassanite precursor. Shearman (1966, 1978), Kinsman (1969), and Butler (1969), observed that the natural sabkha anhydrite of the Arabian Gulf and Baja California is very fine grained and found only in nodules growing in sediments within the supratidal capillary zone. They thought that this anhydrite is of secondary origin after gypsum which is more easily and rapidly deposited. West et al. (1979) concluded that the gypsum nodules in a modern
271 sabkha on the Mediterranean coast of Egypt are of primary origin. Gypsum is considered as the primary phase in the studied evaporites. This is evidenced by the presence of the twinned gypsum (Hardie and Eugster, 1971; Schreiber et al., 1982) as well as gypsarenite (Schreiber, 1978). The studied gypsarenite is similar to that observed by the author at E1-Ballah lake, and that in the Quaternary salt lakes of South Australia (Warren, 1982) and in present day environment in the Mannar lagoon in the northwest of Srilanka (Gunatilaka, 1975). X-ray diffraction analysis for evaporites from Gebel E1-Hagif and Dir E1-Biraqat areas, showed that they are mainly composed of gypsum with minor amounts of anhydrite, bassanite, calcite and traces of pyrite and quartz. The Quaternary evaporites of the EI-Ballah area, 21 km north of Ismailia, occur as a horizontal layer about 1 m thick composed of laminated gypsarenite and twinned gypsum crystals of about 15 cm in length. X-ray diffraction shows that gypsum is the main calcium sulphate mineral. No anhydrite or bassanite is recorded in this evaporite. Warren (1982) showed the same results for the evaporites of the Quaternary salt lakes of South Australia. This may suggest that bassanite and anhydrite in the studied evaporites were formed in sequence as a result of the dehydration of primary gypsum under arid conditions and high temperature. During the humid rainy Pleistocene period, some of the secondary anhydrite may have been transformed into porphyrotopic secondary gypsum. The Middle Miocene evaporite successions on the Red Sea and Gulf of Suez coasts were subjected to a more complicated transformation sequence than those of the Neogene (Messinian) succession studied here. Youssef (1986) concluded that the Middle Miocene calcium sulphate was deposited as primary gypsum, which was transformed into anhydrite by burial. The secondary anhydrite was in turn transformed into alabastrine, granotopic and porphyrotopic secondary gypsum. Under arid hot subaerial conditions, a crustal layer of secondary powdery anhydrite is formed as pseudomorphs after secondary gypsum. Rain water may later transform the powdery anhydrite into
secondary porphyrotopic and satin-spar gypsum. Tectonic movements in the Red Sea and Gulf of Suez areas, as well as the time factor are believed to be the main factors for the more complicated transformation sequence of the Middle Miocene evaporites.
Conclusions The sedimentary structures and textures, as well as the intercalations of oncolitic carbonate and sandy gypsiferous marl that characterizes the Neogene evaporite sequence at Gebel E1-Hagif and Dir E1-Biraqat areas, suggest that they were deposited in a shallow photic lagoonal environment subjected to brine concentrations by evaporation and the activities of blue green algae. The studied Neogene evaporites are characterized by alternations of cryptalgal carbonate and gypsum laminae at the base of the sequence followed upward by massive selenite (Specchiolino or mirror like gypsum) at the top. The laminated gypsum at Dir E1-Biraqat quarry shows three main structures: irregular lamination at the base followed by lateral linked hemispheroid (LLH-C) and spheroidal structure (oncolite). The selenite unit is subdivided into massive disordered selenite and massive vertically arranged selenite. The decreasing volume of the brine pond and consequently the increase of salinity controlled the crystal size of gypsum as well as the sedimentary textures and structures. In salinas where the whole laminated gypsum sequence is gypsarenite, the rate of salinity changes has always been rapid and the bottom brine salinity has always been unstable. As the brine pond decreased in volume and the salinity increased, the selenite unit deposited.
Acknowledgements The author is grateful to the Institute of Geology, University of Bergen, Norway, for assistance in the mineralogical analysis.
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