10 April 2018

An outcrop showcasing a strand of the Kentucky River Fault System

Posted by Callan Bentley

Here’s the view as my colleague Kent Ratajeski drove me on Route 27, crossing the Kentucky River heading south from Lexington, headed toward a nice roadcut:

The cliffs at the site show a fault deforming the otherwise horizontally-oriented strata of the Camp Nelson Formation (oldest rocks in Kentucky) and the Lexington Limestone. Both are Ordovician in age, but the Lexington Limestone lies at the higher stratigraphic position. The Camp Nelson formation tends to be finer-grained and thicker-bedded than the Lexington Limestone.

There were some fossils in these rocks. Here’s one block I found with a few prominent brachiopods:

Close-ups of the brachs:

There were also plenty of trace fossils to be observed. In many cases, the burrows were dolomitized, turning them tan:

As noted yesterday, the Lexington limestone has some spectacular fossils, too.

If you’re more keen on primary sedimentary structures, then here is a mud-cracked bedding plane in the Camp Nelson Formation:

But we were here for some structural geology, to sniff out of a strand of the Kentucky River Fault Zone. Here, there’s one big fault and a lot of small-offset, parallel minor faults.

There was a prominent pair of “ball & pillow” layers in the Lexington Limestone that could be used as marker beds for offset:

Here’s Kent taking a look at a prominent pillow, half obscured by vines:

Close-up pillow portrait:

Here’s a small fault in the Camp Nelson Formation:

It shows a “normal” (hanging-wall-down) sense of offset.


Note the fluid flow along this one, and the attendant increase in potential for vegetation.

Eventually, we found weathered out gullies with Lexington Limestone on the south and Fort Nelson Formation on the north. In between was fault breccia, crumbled up rock produced through the grinding action of the fault’s movement.

Here’s an example of fault breccia in outcrop…

…and in hand sample:

There’s a folding story to be told here, too, but I’ll save that for another day.

…A Friday, perhaps…