The stratigraphy of the Middle Eocene–Pleistocene sediments in Kuwait

Abstract

The Dammam Formation and the Kuwait Group are the major aquifers containing useable brackish water in Kuwait. The Dammam Formation is a limestone–dolomite sequence of Middle Eocene age. It is underlain by Middle Eocene Rus evaporites and is overlain unconformably by the clastic sediments 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 interlayers. The upper member, C, is a friable, white dolomite which is silicified and karstified at the top, beneath the unconformity with the overlying Kuwait Group. Similar silicification and karstification occurs at the top of the B member, suggesting a minor but significant unconformity between the B and C members. The upper part of the Dammam Formation is dolomite of Lutetian age with fragmented and cherty zone on top. Secondary dolomite was formed from early dolomitization of fossiliferous micrites, pelmicites, intramicrites and fossiliferous pelmicrites. Silification was related to percolation of acidic ground-water during the non-depositional period in Oligocene time when the surface of the Dammam Formation was karstified.The Dammam Formation was deposited on a shallow marine shelf experiencing minor fluctuations from lagoon to tidal flat and swamp environments. This tectonically stable period was interrupted by small pulses in the source land and minor fluctuations in the sea level, which caused alternating transgressive and regressive cycles.Stratigraphy of Miocene–Pleistocene Kuwait Group is studied. 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 Jal Az-Zor escarpment and Ahmadi quarry.The Kuwait Group was subdivided into two or three formations. There is no general agreement on the boundaries of the formations and the number of formations. The confusion has persisted until present mainly because there is no clear cut criteria to delineate formations. There are no markers or distinct lithologic 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 to 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 and Dibdibba Formations is hard to locate. The ledge-making, carbonate-cemented sand layers are included in the Jal Az-Zor, and overlying poorly-bedded and weakly-cemented gravels are included in the Dibdibba Formation.The overall stratigraphic sequence of the Dammam limestone and the Kuwait Group bear clues for a gradual regional regression and accompanying increase in clastic material influx. The Kuwait Group starts with marine clays and continues upward as fluvial, river channel sands, 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 carbonate and gypsum cementation in the pore spaces of sands by evaporation of capillary water, and alteration of clays. The lower parts of the Kuwait Group consist mostly of loose sands without cementation. The degree of cementation and frequency of cemented layers increase towards the top of the Kuwait Group.