Origin and paleoecology of Middle Jurassic hiatus concretions from Poland |
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Authors: | Michał Zatoń Sylwia Machocka Mark A Wilson Leszek Marynowski Paul D Taylor |
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Institution: | (1) Faculty of Earth Sciences, University of Silesia, Będzińska 60, 41-200 Sosnowiec, Poland;(2) Department of Geology, The College of Wooster, Wooster, OH 44691, USA;(3) Department of Palaeontology, Natural History Museum, Cromwell Road, London, SW7 5BD, UK |
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Abstract: | Bored and encrusted carbonate concretions, termed hiatus concretions, coming from the Middle Jurassic (Upper Bajocian and
Bathonian) siliciclastics of the Polish Jura, south-central Poland, have been subjected to detailed paleoecological investigation
for the first time. The concretions possess variable morphology and bear distinct traces of bioerosion and encrustation as
a result of exhumation on the sea floor during intervals of low sedimentation and/or erosion. The borings are dominated by
Gastrochaenolites and Entobia. Epilithozoans, represented by at least 26 taxa, are dominated by sabellid/serpulid worm tubes and bryozoans, while sponges
and corals are minor. No relationship between the concretion size and the number of encrusters has been found, suggesting
that concretion size was not the primary factor controlling diversity. Stable isotope analyses and the presence of crustacean
scratch marks and Rhizocorallium traces on many of the hiatus concretions indicate that they formed just below the sediment–water interface, within the sulfate
reduction zone. Moreover, crustacean activities may have been a prelude to their origin, as shapes of many concretions closely
resemble thalassinoidean burrow systems. It is also possible that crustacean activity around the concretions promoted their
exhumation by loosening the surrounding soft sediment. The presence of borings and encrusters on different concretion surfaces,
as well as truncated borings and a number of abraded epilithozoans, indicate that after the concretions were exhumed they
were repeatedly overturned and moved on the sea floor, probably due to episodic storm-related bottom currents in shallow subtidal
environment. |
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