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Characteristics, evolution and distribution of Quaternary channel calcretes, southern Jordan

By S. J. McLaren


Cemented and calcretized conglomerates of Quaternary age are found within ancient river channels exposed at various heights (between +1 and +125 m) above the current bed of the Wadi Dana in southern Jordan. In cross-section, the calcrete deposits are typically lensoid in shape and are found infilling palaeochannels cut into the bedrock. The fossilized channel sediments preserve evidence of past river conditions, sediment loads, source areas, phases of river down-cutting and sediment accretion, as well as post-depositional alteration processes.\ud \ud Samples have been analysed in thin section and under the scanning electron microscope in order to determine the calcrete micromorphology, cement types and crystal sizes. The results show that the river deposits have undergone significant displacement and replacement of the detrital grains by CaCO3, and this has resulted in the cementation in places of over 4 m of river material. The cements display an alpha-type fabric with no evidence of biogenic activity. The nature of the cement fabric within and between deposits differs as a function of two key variables: (i) the specific location within the channel and (ii) height above the modern wadi floor. Firstly, in many of the deposits cement crystal size varies, with sparite dominating at depth within impermeable palaeowadi channels, whereas on the flanks of the channels and near the top of the channels micrite is the main type of cement. These variations are thought to be a result of differences in the length of residence time of calcium carbonate-rich fluids in different parts of the channel and the spatially varying influence of evaporation. Secondly, distinct changes in cement fabric can be seen in channels of different ages with a reduction in the amount of micrite and an increase in the amount of secondary recrystallized pore-filling spar over time

Year: 2004
DOI identifier: 10.1002/esp.1135
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