SEDIMENTOLOGY AND FLUID INCLUSIONS CRITERIA OF
THE UPPER MIOCENE (MESSINIAN?) GYPSUM DEPOSITS
IN THE MEDITERRANEAN COAST OF EGYPT.
OSAMA E. ATTIA1, ESSAM M. EL
KHORIBY2 AND MAHMOUD A. AREF1
1Geology Department, Faculty of Science, Cairo University, Giza, Egypt.
2Geology Department, Faculty of Science, El-Mansoura University, Egypt.
|
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 ((00l) plane, where more equidimensional to elongate inclusions occur normal to the (103) 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 such primary fluid inclusions indicates that the temperature of formation never exceeded 700C. The range of final melting temperatures of ice in the primary fluid inclusions in the lower gypsified stromatolite facies shows lower salinities (0.5 to 5.8 eq. wt % NaCl), than those in the swallow-tail selenite facies (1.9 to 8.8 eq. wt % NaCl). This indicates that these crystals crystallized from evaporative seawater mixed with dilute non-marine parent water. The absence of fluid inclusions having final melting temperatures below -7.00C (evaporative seawater at gypsum saturation, 10.5 eq. wt % NaCl and more) indicates that the gypsum did not form from simple evaporative concentration of seawater. Moreover, the existence of some microbial remains within primary fluid inclusion bands indicates that the basin was flushed with low salinity brines. The wide range of eutectic temperatures in the frozen primary fluid inclusions (-29.30C to -45.30C) indicates that the parent brine is of multi-component system.
The Sr content increases upward in both gypsified stromatolite (from 632 ppm to 1190 ppm) and selenitic gypsum (from 488 ppm to 1102 ppm) indicating an increase in the salinity of brines upward. The lower parts of both gypsified stromatolite and selenitic gypsum facies have low Sr contents and were deposited in a marginal (shallow) marine environment that has been attacked by fresh continental water. Low Mg content in the selenitic gypsum compared with the other gypsum facies is due to the presence of smectite interbeds
Therefore, field, petrographic, fluid inclusions and chemical data point to an upward increase in salinity of the parent brine, deepening of the depositional basin and more restriction. This indicates a semi-restricted evaporite basin receiving water from marine and non-marine sources. |
|