Abstract
The effect of post depositional processes (e.g., diagenesis) on geotechnical properties of the studied Fatima carbonates has been evaluated on 32 oriented samples of Precambrian carbonate of Fatima Group, Jeddah, Saudi Arabia where Fatima carbonate considered as the oldest sedimentary rock unit. Fatima carbonate sequence represents the middle part of Fatima Group and attains 90-130m thick. Fatima carbonate composes mainly of alternating cycles of highly deformed, folded, and faulted stromatolitic carbonates. Algal laminates with scattered dolomite pseudomorphs are recorded within these carbonates and are shown clearly in cut-slabs. The laminae are millimetric in thickness. The Fatima carbonate has a relative abundance of nearly uniform-sized dolomite crystals. Dolomite crystal are represented by small-sized rhombs of euhedral, subhedral, and non-rhombic. The larger euhedral rhomb-shaped dolomite crystals are occurred as cavity filling dolomite. The silicification features are recognized as patches of cryptocrystalline quartz replacing laminated carbonates. It is believed that this type of silicification represents late diagenetic stage and not influenced by the depositional environment or by physicochemical conditions of water. XRD charts indicate that the Fatima carbonate is consisted mainly of dolomite, quartz, and minor constitutes of gypsum. These carbonates are mostly barren in marine fossils and it is clear that these carbonates were deposited within restricted (lagoonal) marine environment. The rock unit weight values of the studied Fatima carbonate range from 29.429 to 25.993kN/m(3). The total porosity varies from 9.147 to 6.112%. The uniaxial compressive strength (UCS, measured by Schmidt Hammer) values of Fatima carbonate range from 22.6 to 110MPa for the horizontal direction and from 19.7 to 71.8MPa for the vertical direction). The abovementioned results implicate that the studied Fatima carbonate rock samples are classified as medium to strong rocks.