Occurrences and genesis of calcrete and dolocrete in the Mio-Pleistocene fluviatile sequence in Kuwait, northeast Arabian Peninsula

Sedimentologic and petrographic investigation of the subsurface Mio-Pleistocene clastic sequence of the Kuwait Group in Umm Ghudair water field, southwest of Kuwait revealed the occurrence of successive cycles of calcrete profiles. The calcrete host sediment (Kuwait Group) is mainly represented by a sequence of muddy sands interbedded with mud and sand layers with occasional lenses of gravels. Individual calcrete profile has vertical gradational maturity pattern which is manifested by the occurrence of well-differentiated horizons arranged from bottom to top as follows: non-calcretized host sediment–incipient calcrete–nodular calcrete–concretionary calcrete–massive laminar calcrete.Abundance of organosedimentary structure (mainly plant roots) and diversity of calcite crystal fabrics suggest that the studied calcrete profiles were developed within two stages of calcretization: a pedogenic stage followed by a phreatic stage. The variation in the mineralogical composition between Umm Ghudair calcrete profiles and the dolocrete profiles that occur within the same clastic host sediment but at distal location within the Kuwait Group flood plain (about 80 km to the northeast) is mainly attributed to differences in the chemical composition of the groundwater and its flow pattern. It is suggested that the hydrogeological and hydrogeochemical settings may play a significant role in the development of cyclic pedogenic, phreatic and polygenetic calcretes and dolocretes within the same sedimentary basin. Alternation of tectonically stable and unstable periods may be responsible for the Cyclicity of calcretization/dolocretization.