Another good week in the field! Temperatures are getting low in the morning, but still warming up in the afternoon. We started out by finishing up the rest of Tick Creek at the north edge of the map. Got the lower Cane Hill sandstone up Files Hollow and again on the east side of Tick. Still have good channel-bedded Imo below in the creek bed with a shale unit between the two.
Tuesday and Wednesday we finished up the lower end of Indian Creek. There were several massive-bedded sandstone units that had cut down into lower units of thin- to medium-bedded sandstone interbedded with shale.
This is probably all in the Witts Springs Formation, but we won’t know for sure until we see more of the rock in the area. One of the indications that it is Witts Springs sandstone is the characteristic curved reentrant at creek level. This is caused by exfoliation or spalling of curved sheets of rock due to more rapid dissolution of the calcareous cement near the creek. The Witts Springs sandstone is typically more calcareous than overlying formations.
You never know when you might see a classic example of a sedimentary structure while hiking around. This week we saw some good examples of load casts in the creek float. A sand deposit formed an irregular bulge as it pressed down (loaded) into the mud below. Later lithification preserved the cast of that shape on the bottom of the sandstone bed.
As we approached the southwest/northeast topographic lineation along the Middle Fork at the mouth of Indian Creek, we began to see more and more signs of a major structure. Aside from a 5 to 7 degree southeast dip, there was an increasing abundance of deformation bands in the massive sandstone. Also, we began to see a lot of non-vertical joints and small faults. Took photos of a couple of fault planes with the slickensides still evident. Slickensides are the parallel grooves or scratches left behind on the fault plane caused by the abrasion of one rock surface against another. They are typically smooth in the direction of movement and rough in the opposite direction, so can indicate which way the fault moved. Unfortunately there is no way to estimate throw, or the amount of offset on the fault, without knowing which formations are on either side. That’s why we’ll have to be extra thorough in that area. There’s probably a big fault somewhere along that lineation, but we’ve yet to find it so far.
By the way, Indian Creek must have gotten its name from the number of moccasins lying around.
We’re taking a little hiatus next week so will be at least two weeks ’til the next installment.
See you on the outcrop!
Snake count: at least 4
Tick attacks: still severe