The Research Diaries #4: Faulted Friday

While between rain storms earlier this week, I was able to get out into the field to do some outcrop work near Yellville, Marion County, Arkansas. I visited a roadcut near the US Hwy 412/AR Hwy 125 junction that is made of Lower Ordovician age dolostone rock with some shale intervals. Interestingly, it has a couple thrust (reverse) faults on display:


Thrust fault through carbonate rock (Facing north. Scale: 1.5 m staff with 10 cm intervals)

A closer look at the fault in the lower-right corner of the previous image

An even closer look at the fault. Lots of fault gouge. Notice the clasts that have rotated.

Another thrust fault at the same roadcut (Facing north. Scale: 1.5 m staff)

Some weeks ago, I was visiting a roadcut near Cotter, Arkansas where I left with buckets of samples to analyze. One such sample was a large cobble that I slabbed on the rock saw and found these beautiful normal faults, with a baby graben flanked by two horsts:

Faulted rock with two long normal faults (yellow) and minor normal faults (blue) (red arrow indicates stratigraphic top)

North-south trending normal faults are, well, normal and expected in the northern part of Arkansas. I reckon that they had formed as an extensional response to the Ozark Dome uplift, or perhaps to a small degree, the older Reelfoot Rift in the Mississippi River valley. East-west trending thrust faults are common in the Ouachita Mountains region, where continental collision resulted in a large compressional, orogenic event.

This begets the question: why are there north-south trending thrust faults in northern Arkansas? I have no answer to this. The geologic map of Arkansas depicts the normal and thrust faults that have significant offset, but no thrust faults are mapped in northern Arkansas. Likely because they do not have any significant offset on a state-wide scale. While these thrust faults in Marion County have an offset of less than 0.5 meters, their origin is still very curious and remains a mystery to me.

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