THE STRATIGRAPHY OF THE MIDDLE EOCENE PLEISTOCENE SEDIMENTS IN KUWAIT

The Dammam Formation and the Kuwait Group are the major aquifers conta ining useable brackish water in Kuwait. The Dammam Formation is a lime stone-dolomite sequence of Middle Eocene age. It is underlain by Middl e Eocene Rus evaporites and is overlain unconformably by the elastic s ediments of the Kuwait Group. It is subdivided into three members. The lower member, A, is a nummulitic limestone with shale interlayers at its base. The middle member, B, is a highly silicified and dolomitized fossiliferous limestone which includes lignite and silty lignite inte rlayers. The upper member, C, is a friable, white dolomite which is si licified and karstified at the top, beneath the unconformity with the overlying Kuwait Group. Similar silicification and karstification occu rs at the top of the B member, suggesting a minor but significant unco nformity between the B and C members. The upper part of the Dammam For mation is dolomite of Lutetian age with fragmented and cherty zone on top. Secondary dolomite was formed from early dolomitization of fossil iferous micrites, pelmicites, intramicrites and fossiliferous pelmicri tes. Silification was related to percolation of acidic ground-water du ring the non-depositional period in Oligocene time when the surface of the Dammam Formation was karstified. The Dammam Formation was deposit ed on a shallow marine shelf experiencing minor fluctuations from lago on to tidal flat and swamp environments. This tectonically stable peri od was interrupted by small pulses in the source land and minor fluctu ations in the sea level, which caused alternating transgressive and re gressive cycles. Stratigraphy of Miocene-Pleistocene Kuwait Group is s tudied. The work is based on cores and bore hole logs from Umm-Gudair, NW Al-Shagaya, Al-Salmi and Al-Wafra water wells, and exposures at Ja l Az-Zor escarpment and Ahmadi quarry. The Kuwait Group was subdivided into two or three formations. There is no general agreement on the bo undaries of the formations and the number of formations. The confusion has persisted until present mainly because there is no clear cut crit eria to delineate formations. There are no markers or distinct litholo gic boundaries to set formations apart. The Kuwait Group is subdivided into three formations in the present study in agreement with previous work. These are Ghar, Jal Az-Zor and Dibdibba Formations from bottom to top. No clear cut stratigraphic clue could be obtained from wells t o identify formations. The fossiliferous sandy limestone layer at the lower part of the group was selected as the boundary between Ghar and overlying Jal Az-Zor Formations. The boundary between the Jal Az-Zor a nd Dibdibba Formations is hard to locate. The ledge-making, carbonate- cemented sand layers are included in the Jal Az-Zor, and overlying poo rly-bedded and weakly-cemented gravels are included in the Dibdibba Fo rmation. The overall stratigraphic sequence of the Dammam limestone an d the Kuwait Group bear clues for a gradual regional regression and ac companying increase in elastic material influx. The Kuwait Group start s with marine clays and continues upward as fluvial, river channel san ds, and ends with alluvial fan conglomerates. Deposition of the Kuwait Group was interrupted frequently by brief periods of quicence and non -deposition. Pauses in sedimentation are marked by surfaces of carbona te and gypsum cementation in the pore spaces of sands by evaporation o f capillary water, and alteration of clays. The lower parts of the Kuw ait Group consist mostly of loose sands without cementation. The degre e of cementation and frequency of cemented layers increase towards the top of the Kuwait Group. (C) 1997 Academic Press Limited.