earthstory:Septarian concretions.These beautiful rocks form during the transformation of sediments i
earthstory:Septarian concretions.These beautiful rocks form during the transformation of sediments into rock, as detailed in our past post on concretions (see http://tinyurl.com/pwfs2tf). The septaria are the calcite filled cracks at the centre of the rock, indicating where the centres of the concretions have shrunk, possibly during dehydration during its long transformative journey. The actual process of formation remains unclear, and also include expansion of gases from decaying organic matter and fracturing by compaction from the weight of overlying sediment or tectonic pressure.The most famous septarian nodules are the Moearaki Boulders, found weathering out of a Paleocene mudstone in South Island New Zealand (see http://tinyurl.com/lf35859). This specimen came from Utah, measures 14.5 x 12.8 x 7.5 cm and includes some beautiful blades of baryte within the concretion as well as the surrounding calciteLozImage credit: Rob Lavinsky/iRocks.comhttp://www.priweb.org/ed/concretions.htmgeology.about.com/od/more_sedrocks/ig/concretionpics/ -- source link
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