Recognition of quartz geodes in the Upper Cretaceous Wadi Umm Ghudran Formation, Ras En Naqab, South Jordan
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ORIGINAL PAPER
Recognition of quartz geodes in the Upper Cretaceous Wadi Umm Ghudran Formation, Ras En Naqab, South Jordan I. M. Makhlouf & K. Tarawneh & K. Moumani & K. M. Ibrahim
Received: 6 May 2013 / Accepted: 16 January 2014 # Saudi Society for Geosciences 2014
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 I. M. Makhlouf (*) : K. M. Ibrahim Department of Earth and Environmental Science, Hashemite University, Zarqa, Jordan e-mail: [email protected] K. Tarawneh Department of Mining, King Hussein University, Ma’an, Jordan K. Moumani Geological Mapping Division, Natural Resources Authority, P.O. Box 7, Amman, Jordan
crystallization went slowly under equilibrium conditions and formed from the same silica source. Keywords Geode . Silica . Umm Ghudran . Quartz . Cretaceous . Jordan
Introduction Geodes are defined as spherical, hollow concretionary rock bodies containing crystalline structures that vary in chemical composition, size, colour, density, internal shape and origin (Bassler 1908; Chowns and Elkins 1974; Maliva 1987; Maliva and Siever 1988, 1989; Gao and Land 1991; Makhlouf et al. 2003). These geodes are rough, dull-looking spherical objects resembling mud balls, but they are lined with silica crystals of various types. Geodes are divided into two categories depending on whether they occur in sedimentary or igneous rocks. The first requirement for their formation in sedimentary rocks is the presence of a cavity or hollow space in limestone, marlstone or chalk when a mineral nodule or concretion dissolved, or a buried animal decayed (Bassler 1908; Chowns and Elkins 19
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