Nebraska is set in the Great Plains region of the United State,s where the topography is almost ‘flat’ and bedding dips are ‘pseudo’ horizontal in most places. It was my first time of collecting field joint data from sedimentary rocks. I had (before now) only done this on basement rocks… and the brittleness of hard rocks makes joint observation on them pretty interesting. Also, my experience in joint analysis on sedimentary rocks had been limited to seismic data and satellite remote sensing data; so, I had a great experience driving around southern Nebraska and northern Kansas, collecting and interpreting joint data on different lithologies.
Some of the outcrops I visited were deeply weathered, while some had paleosols- both of which inhibited detailed observation of joint on the rocks. However, a lot of the outcrops had interesting structural information. See photos below:
- Open joint on a siltstone outcrop.
- Open joint on cross-bedded sandstone. The joint appears refract as it propagates into the bottom layer of relatively different mechanical property (layer with lots of cross bedding).
- Joints in interbedded shale and limestone. Because of the low competence of shale, it deforms less ‘brittly’ and more ‘ductly’, thereby making it difficult to find open joints on them. On the other hand, limestone is a more competent material than shale, therefore and it is more likely to have a higher density of joints,
- Clay-filled joint plane in shale (with thin bentonite interbedding). The vertical joint also appears to have been offset along the lower bentonite bed plane.
- Road-cut covered with paleosol. The paleosol has mud cracks which exhibit popcorn texture. It is so difficult to observe any structural discontinuity on the underlying lithology.
- Observing naturally-emplaced joints on limestone with interbedded chert. The road cut was made with explosives, which introduced many un-natural joints in the rock (which can also have their own systematic orientations). So, picking and measuring a joint on this type of exposure demands some care and thinking.