UAE Travertines



Jebel Dhanna is a small rocky abrupt hill formed over an active salt dome on the western shore of the Khor al Bazam.

As has been extensively described by Purser and Loreau in 1973, on the shore to the southwest of the Jebel Dhanna peninsula are a series of unusual beachrocks that cap a series of recurved spits rich in bioclastic materials that have been transported here by longshore drift from the northeast.

Many of the more enclosed spits have their shoreward eastern facing margins covered by aragonitic marine cement that forms coatings not only on the beach rocks that are forming here but on cerithid gastropod shells washed over recurved coastal spits by southward longshore transport and local westerly storms. Marine carbonate cement crusts (up to 3 cm in thickness) are present and best developed to the lee of these sand spits. Uncoated cerithid shells occur seaward of the easternmost spit. This is a low energy region where shells are cemented into beachrock (Purser and Loreau, 1973).

The beachrock occurs as cemented slabs of bioclastic beach sand coated with a micritic marine aragonite resembling the flowstones from meteoric settings. Pisolites (which are similarly coated) are present locally, usually nucleated about cerithid shells and other mollusc fragments. The beachrock crusts are locally buckled into megapolygons, most commonly on the berms of the beach ridges. The buckled margins of the megapolygons represent juvenile forms of tepees, similar to those described by Assereto and Kendall (1977). Similar cement coatings are present in the brecciated tepee sediments of the Guadalupe Mountains (Permian) and the Italian Alps (Triassic), in Holocene algal head cements from the Great Salt Lake, and in similar Tertiary Algal heads in the Green River Formation of the western U.S. This suggests that hypersaline conditions and high rates of carbonate cementation contribute to tepee generation and cavity fill.

These megapolygon slabs of beachrock have an incomplete marine travertine coating.

The beachrock shows a zonation from the low intertidal where there are often dark gray uncoated to partially coated slabs at the lowest elevations to coated beachrock at higher elevations. Here some of the travertine has an almost polished surface as do the coated cerithids on the berm of the spits. Ridge and runnel sedimentary structures are preserved in cemented beachrock along the leeward edge of the beach ridges and are also coated with micritic aragonite measuring 5 to 30 mm in thickness.

The thickness, microscopic characteristics and morphology of these cement coatings from Jebel Dhanna suggest incremental deposition of aragnite coexisting with traces of sulfate minerals. Most of these cement coatings are micritic, but the layers encrusting algal flat hardgrounds have a more radially fibrous microstructure and are composed solely of aragonite. The stable isotope composition of these coatings falls within the compositional range for modern nonskeletal aragonite. This suggests that the marine travertines precipitate from agitated, slightly hypersaline Arabian Gulf sea water through repeated immersion

The topographically higher areas tend to be colonized by the halophytes which stabilize the local dunes. Among the coated cerithids, salicornia sp. also occurs. The topographically lower areas are sites of local mangrove growth represetned by Avacennia Marina, the black mangrove. Sediment grain size ranges from fine in the lower intertidal to coarse in the upper intertidal. In the protected bay east of the coastal spits, the intertidal zone is characterized by cyanobacterial flats. These flats consist of tufted algal mats.

Monday, May 23, 2016
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