Abstract
Quartz geodes are spectacularly displayed at Ras En Naqab where hundreds of geodes have weathered from their host chalky limestone and sandstone beds and scattered on slope surfaces. Geodes of different sizes, shapes and fillings appear in four horizons of the shallow marine sediments of the Upper Cretaceous Wadi Umm Ghudran Formation in South Jordan. They are characterised by a wide areal extent, but limited stratigraphic distribution, and as such, they represent a distinctive stratigraphic marker horizon. Ghudran geodes are mostly milky white in colour, botryoidally and crystalline in shape and range in diameter from 3 to 30 cm or more. It is believed that the formation of geodes took place in cavities, after complete dissolution of pre-existing fossils, which left no trace of their internal microstructures but only faint appearance of external moulds. Chalcedony and microcrystalline quartz occur as cavity linings and in some samples as cavity filling. The structure of the silica geodes begins with chalcedony in its outer rim followed, internally, by microcrystalline quartz and ends with prismatic quartz crystals in the central part. Spot analysis indicates that the geodes composed totally of Si and O with traces of Al (0.05 %). The silica-rich solutions that formed the geodes were possibly derived from the weathering of the overlying Amman Silicified Limestone Formation and the infiltration of chemical products by the action of groundwater. Investigations showed that the crystallization went slowly under equilibrium conditions and formed from the same silica source.