Mahmoud Aref
Cairo University, Geology Department, Faculty Member
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The outcropped evaporites exposures at Ubhur and Rabigh areas, north of Jeddah city, eastern red Sea coast of Saudi Arabia are composed dominantly of secondary gypsum highly enriched in microbial fabrics. The secondary gypsum crystals are... more
The outcropped evaporites exposures at Ubhur and Rabigh areas, north of Jeddah city, eastern red Sea coast of Saudi Arabia are composed dominantly of secondary gypsum highly enriched in microbial fabrics. The secondary gypsum crystals are represented with, in a decreasing order of abundance, porphyrotopic, granular, alabastrine gypsum in addition to satin spar gypsum veins.Anhydrite crystals are recorded as relics within the porphyrotopic gypsum at the lower part of the evaporite sequence, and as replacing felted crystals at the upper part of the evaporite sequence. The microbial fabrics (gypsified laminites and stromatolites) are the only evidence that still preserve the primary depositional textures of the evaporites. The secondary gypsum crystals were formedin two diagenetic environments, the first is a stagnant phreatic zone which leads to the widespread hydration of anhydrite into porphyrotopic and granular gypsum under equilibrium conditions. The second is active phreatic cond...
The Upper Miocene (Messinian?) evaporite deposits in the Mediterranean coast of Egypt consist of a reduced thickness of gypsum in comparison to other Mediterranean evaporites. The gypsum sequence consists of three layers; lower gypsified... more
The Upper Miocene (Messinian?) evaporite deposits in the Mediterranean coast of Egypt consist of a reduced thickness of gypsum in comparison to other Mediterranean evaporites. The gypsum sequence consists of three layers; lower gypsified stromatolite, middle skeletal and grass-like gypsum and upper swallow-tail selenitic gypsum. Microscopic studies of primary fluid inclusions in the upper and lower layers are identified by their relation to growth surface of the crystals. Elongated primary fluid inclusions in gypsum occur along the (001) plane, where more equidimensional to elongate inclusions occur normal to the (í03) plane. Most fluid inclusions preserve a spike-like shape or negative crystal morphology. The tip of the spike points to the crystal growth direction. Primary fluid inclusions in the lower and upper layers are of a single phase (liquid) and two phases (liquid-solid) inclusions, arranged in arrays parallel to the growth zonation of the host crystals. The existence of su...
The Holocene evaporite sequence in the Ras Shukeir area conformably overlies marine shell banks and cross-bedded to graded-bedded beach sands and gravels. The evaporite sequence is represented by gypsum-anhydrite layers that are... more
The Holocene evaporite sequence in the Ras Shukeir area conformably overlies marine shell banks and cross-bedded to graded-bedded beach sands and gravels. The evaporite sequence is represented by gypsum-anhydrite layers that are interbedded with mudstone layers. Field and petrographic investigations of the evaporite deposits revealed two facies types, laminated evaporite facies (primary) and nodular to enterolithic anhydrite facies (diagenetic). The laminated evaporite facies is subdivided, from the bottom to top, into regular laminated evaporite, chevron gypsum-algal micrite laminations and wavy algal laminated evaporite. Based on their textures and fabrics, the regular and wavy laminated evaporite facies are interpreted as primary deposits in a coastal lagoon and salina environment. The chevron gypsum-algal micrite facies formed by the growth of chevron gypsum at the sediment-water interface within a shallow subtidal lagoonal environment that was characterized by extensive benthic...
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The Holocene evaporite sequence in the Ras Shukeir area conformably overlies marine shell banks and cross-bedded to graded-bedded beach sands and gravels. The evaporite sequence is represented by gypsum-anhydrite layers that are... more
The Holocene evaporite sequence in the Ras Shukeir area conformably overlies marine shell banks and cross-bedded
to graded-bedded beach sands and gravels. The evaporite sequence is represented by gypsum-anhydrite layers that are
interbedded with mudstone layers. Field and petrographic investigations of the evaporite deposits revealed two facies types,
laminated evaporite facies (primary) and nodular to enterolithic anhydrite facies (diagenetic). The laminated evaporite
facies is subdivided, from the bottom to top, into regular laminated evaporite, chevron gypsum-algal micrite laminations
and wavy algal laminated evaporite. Based on their textures and fabrics, the regular and wavy laminated evaporite facies
are interpreted as primary deposits in a coastal lagoon and salina environment. The chevron gypsum-algal micrite facies
formed by the growth of chevron gypsum at the sediment-water interface within a shallow subtidal lagoonal environment
that was characterized by extensive benthic algal mats. The nodular to enterolithic anhydrite facies is secondary and formed
diagenetically within a siliciclastic supratidal sediment.
Some of the laminated evaporite facies have been diagenetically altered in a supratidal sabkha environment as evidenced
by the following: (1) the partial formation of nodular evaporite instead of laminated evaporite; (2) disruption of gypsum
laminations by plant roots and rootlets as well as by precipitation of lenticular gypsum on the root wall; and (3) partial
dissolution of halite laminae and the formation of wavy anhydrite laminae.
Consequently, the Holocene evaporites in the Ras Shukeir area were deposited in a shallow semi-closed to closed basin
that was separated from the Gulf of Suez trough. Changing sea level led to progradation of the evaporite facies from
subtidal to intertidal lagoon and salina to a supratidal sabkha.
to graded-bedded beach sands and gravels. The evaporite sequence is represented by gypsum-anhydrite layers that are
interbedded with mudstone layers. Field and petrographic investigations of the evaporite deposits revealed two facies types,
laminated evaporite facies (primary) and nodular to enterolithic anhydrite facies (diagenetic). The laminated evaporite
facies is subdivided, from the bottom to top, into regular laminated evaporite, chevron gypsum-algal micrite laminations
and wavy algal laminated evaporite. Based on their textures and fabrics, the regular and wavy laminated evaporite facies
are interpreted as primary deposits in a coastal lagoon and salina environment. The chevron gypsum-algal micrite facies
formed by the growth of chevron gypsum at the sediment-water interface within a shallow subtidal lagoonal environment
that was characterized by extensive benthic algal mats. The nodular to enterolithic anhydrite facies is secondary and formed
diagenetically within a siliciclastic supratidal sediment.
Some of the laminated evaporite facies have been diagenetically altered in a supratidal sabkha environment as evidenced
by the following: (1) the partial formation of nodular evaporite instead of laminated evaporite; (2) disruption of gypsum
laminations by plant roots and rootlets as well as by precipitation of lenticular gypsum on the root wall; and (3) partial
dissolution of halite laminae and the formation of wavy anhydrite laminae.
Consequently, the Holocene evaporites in the Ras Shukeir area were deposited in a shallow semi-closed to closed basin
that was separated from the Gulf of Suez trough. Changing sea level led to progradation of the evaporite facies from
subtidal to intertidal lagoon and salina to a supratidal sabkha.
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Large amounts of evaporitic gypsum are present in sedimentary basins worldwide. When gypsum comes into contact with hydrocarbon-rich fluids (methane, petroleum) there is a potential for subsurface microbial communities of sulphate... more
Large amounts of evaporitic gypsum are present in sedimentary basins worldwide. When gypsum comes into contact with hydrocarbon-rich fluids (methane, petroleum) there is a potential for subsurface microbial communities of sulphate reducing bacteria to develop. We have investigated sulphur-bearing authigenic carbonates from Spain (Lorca basin) and Egypt (Ras Zeit and Gemsa) which are in close stratigraphical relation to Miocene evaporitic gypsum deposits and are a by-product of this microbial activity [1, 2]. We present ...