Properties and development of channel calcretes in a mountain catchment, Tabernas Basin, southeast Spain

Nonpedogenic channel calcretes of up to 3.5 m thick occur in a number of locations across the Tabernas Basin in Almerı́a Province, southeast Spain. They represent the last major phase of late Quaternary calcium carbonate cementation to affect this semiarid drainage basin. Channel calcretes are situated within the basal parts of sequences of <12-m-thick, poorly sorted, coarse-grained and schist-dominated fluvial gravels that were deposited within confined bedrock channels. Channel calcretes occupy the full basal width of the bedrock channels within which they occur, and mostly crop out near the mouths of tributary valleys to the main Rambla de Tabernas drainage system. Six profiles from five separate channel calcrete outcrops were logged and sampled. All samples were analysed in thin section and under scanning electron microscope (SEM). From these analyses, the macro- and micromorphological properties were identified, and the mode of origin of channel calcrete profiles was assessed. Results indicate that all channel calcretes are massively cemented by calcite and appear uniform at the field profile scale. Microscale analyses reveal that detrital grains within the calcretes are initially coated by micrite, which is overlain by grain-coating and pore-filling sparite. A series of broad trends in cement type and micromorphology are recognised within individual field profiles. All profiles exhibit an increasing degree of calcite crystal size in a down-profile direction. Most field profiles predominantly contain micrite in upper parts with an increasing percentage of sparite towards the base. Some profiles are, however, dominated by sparite and show increasing crystal size and occurrence of euhedral crystals towards the profile base. Many profiles also contain evidence of postcalcite cementation diagenesis in the form of dissolution of calcite crystal faces or replacement of the calcite cement by amorphous silica, with alteration mostly occurring in the lower parts of profiles. The trends within the calcrete fabric appear to have developed in conjunction with a fluctuating water table, with the increased crystal size and occurrence of euhedral crystals towards the base of profiles arising from greater duration of wetting in basal zones. Cementation in some field profiles occurred at depths of up to 12 m within the host sediment, well below the zone of capillary rise and pedogenesis, and in the absence of significant organic activity. As such, channel calcretes in the Tabernas Basin may represent an ideal opportunity to observe the influence of groundwater upon cementation of coarse-grained sediments by carbonate.