Oyster buildup within the dysaerobic facies mudstones (Middle Jurassic, Central Poland) – example of benthic island colonization

In the mudstone complex which is exposed in the lignite open-pit „Bełchatów” there occurs a small oyster buildup. The buildup developed on the lens of the coquina including numerous hiatus-concretions and is surrounded by mudstone. Sedimentation of mudstones took place in disaerobic conditions, where development of benthos was restricted due to low oxygen concentration. Development of oyster buildup was possible due to the presence of the stable benthic island built of coquina which originated as the redeposited, reworked horizon. The buildup formed during episodic cessation of mud deposition and increase in aeration of bottom water.