The puzzle of gypsum

November 15th, 2009

Our Permian sections on this Texas trip have had thick beds of gypsum only a meter or three beneath our fossiliferous limestones and shales.

An outcrop of sedimentary gypsum below the Valera Formation (Permian).

An outcrop of sedimentary gypsum below the Valera Formation (Permian).

Gypsum (calcium sulfate) is an evaporite mineral, indicating when the Permian shallow sea in this case was much saltier than normal (hypersaline).  Our fossils show a restricted nature (lower diversity than normal, and generally smaller shells), but they were still living in at least close to normal salinities.  This is especially the case with our numerous echinoids.  We even have evidence of some evaporites within our fossiliferous limestones.  It is a curious juxtaposition of depositional environments.

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