OCCURRENCE OF PHREATIC-DOLOCRETE WITHIN TERTIARY CLASTIC DEPOSITS OF KUWAIT, ARABIAN GULF

The term dolocrete is used in the present study to describe siliciclastic sediments which are partially or completely cemented and/or replaced by terrestrial phreatic dolomite. Several successive dolocrete profiles occur within the subsurface Tertiary fluviatile muddy sand and mudstone deposits in Kuwait City. Three zones were recognized within these dolocrete profiles; these are in descending order: (1) dolocrete band, (2) dolocrete nodules, and (3) incipient dolocrete. Dolocrete also occurs within the compact green mudstone layers. The studied dolocrete comprises detrital grains inherited from the host sediment, and authigenic minerals precipitated during the dolocretization processes. The detrital grains include mainly quartz, feldspars, illite, and smectite. The abundance of authigenic minerals (dolomite, calcite and palygorskite) relative to detrital grains, reflects the degree of dolocrete maturation. Mature dolocrete is chiefly composed of dolomite and palygorskite. Several dolomite fabrics and textures were recognized; however, dolocretic dolomite is generally present as incomplete rhombohedra with spiky terminations. Variation in the nature of host sediment and diagenetic conditions are responsible for the degree of dolocretization and diversity of fabrics and textures. It is suggested that the dolocrete was developed by the precipitation of dolomite from pore solution saturated with respect to dolomite within the phreatic zone. Three main sources of chemicals required for dolocretization are suggested: (1) Mg-rich groundwater from the dolomitic Dammam Formation, (2) Mg and CO2 released from clay minerals and organic matter in the host sediment mud matrix, and (3) infiltrated meteoric water. © 1990.