POLYGONAL SHRINKAGE CRACKS FILLED WITH
GYPSUM IN THE UPPER EOCENE, FAYUM, EGYPT
MAHMOUD A. M. AREF and MORSY A. MORSY1
Geology Dept., Fac. Sciences, Cairo Univ., Giza, Egypt; 1Nuclear Materials Authority,
Cairo, Egypt.
|
Polygonal shrinkage cracks occur along the upper surface of two mudstone layers in the Upper Eocene Qasr Elsagha Fm, Fayum. The polygons are largely three to five sided, non-orthogonal to orthogonal, and range from 3 to 5 ms across. Most of the cracks are V-shaped in cross section, 15 cm wide at top to 2 cm at bottom, and less than 1.5 m deep. The crack fill is mostly gypsum with some occurrences of celestite below gypsum in the lower mudstone layer. The gypsum forms horizontal blades that extend transversally from wall to wall of the cracks and partially enclose detached wall rock materials that form "inclusion bands" and "inclusion trails". Celestite is fibrous to rosette aggregates filling the cracks centripetally which leave open cavities at their center.
The polygonal cracks are interpreted to have formed by subaqueous synaeresis mechanism either due to clay flocculation and deflocculation, or due to salinity changes. This interpretation is based on the absence of wind blown elastic materials in the cracks, and the precipitation of gypsum and celestite in aqueous environment during opening of the cracks. The horizontal alignment of the gypsum crystals are controlled by the side-way diffusion of saline water from the high formation pressure between pore spaces of the clay to the low pressure zone of the cracks. The celestite crystals were precipitated from the mixing of Sr-rich interstitial water, released during dolomitization of the aragonitic mud, and
SO42- - rich saline water.
|
|