Mysterious out-of-place rocks in the Ordovician of Kentucky

December 15th, 2009

MAYSVILLE, KENTUCKY–Our short geological expedition to northern Kentucky today was to look at some odd blocks of limestone that sit suspended in the sediments as if they were dropped in while the sequence was accumulating.

An eroded, bored and encrusted limestone block in the Fairview Formation (Upper Ordovician) of northern Kentucky at the Route 11 outcrop (N38.61243°, W83.75575°).

An eroded, bored and encrusted limestone block in the Fairview Formation (Upper Ordovician) of northern Kentucky at the Route 11 outcrop (N38.61243°, W83.75575°).

These rocks are bored by worms and encrusted by bryozoans on their top and sides, and they often sit at high angles to the surrounding strata.

Bryozoans encrusting a side of the block above. The beautiful pinkish bryozoan on the left is the holdfast of a ptilodictyoid which in life held an erect bifoliate portion of the colony. The field of view here is about 10 cm wide.

Bryozoans encrusting a side of the block above. The beautiful pinkish bryozoan on the left is the holdfast of a ptilodictyoid which in life held an erect bifoliate portion of the colony. The field of view here is about 10 cm wide.

It is difficult to imagine a mechanism which deposited large, lithified limestone blocks in the middle of a shallow carbonate ramp. They are almost certainly related to “seismite” structures in the outcrop (see next post), but how these earthquakes would have transported such rocks is a mystery.  We also do not know how quickly the limestone had been lithified before emplacement.  We do know that the sides of these blocks were exposed on the seafloor long enough to accumulate encrusters and borers.

Plenty yet to discover in these well-studied rocks.  It is a continuing lesson for scientists: the more you see the more questions you have.

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