Hmmm…
posted in Geology, GigaPan, Photography |Part of my ongoing research is a project aimed at some basic bedrock geologic mapping of Rooks County, Kansas. Given that this is western Kansas, deep within the North American Craton, and the rocks exposed in the area are generally flat-lying Cretaceous (Western Interior Seaway) and Tertiary aged sediments, one doesn’t expect to find too much in the way of interesting structural geology. There are some very small offset normal faults that are well exposed in roadcuts – nice for teaching about fault motion, but not particularly earth shaking (not recently, anyhow).
So it came as quite a surprise when my undergrad field assistant and I happened upon the following outcrop:
What you see in the Gigapan above is a view of a small roadcut southwest of Stockton, Kansas. On the left side of the cut you can see massively bedded, bioturbated layers of the Fort Hays Limestone member of the Upper Cretaceous Niobrara Formation in depositional contact above black shales of the Blue Hill Shale member of the Upper Cretaceous Carlile Shale Formation. Both of these units were deposited in the Western Interior Seaway and are found throughout our mapping area. Elsewhere there is another unit, the Codell Sandstone member of the Carlile Shale that is found depositionally between these two, but its thickness is variable and its distribution is not continuous. Most of the exposures of this contact in the area nearest this outcrop lack anything but a hint of sandy shale which might correspond to the Codell Sandstone at the contact, so what you see on the left side of this outcrop is not in any way out of the ordinary.
What makes me go “Hmmm…” is the right side of this roadcut. Clearly there is a fault contact that dips to the east. Slickensides visible on the underside of the Fort Hays limestone along this fault (you can see them if you zoom in) clearly indicate nearly pure dip-slip motion on the fault. What’s baffling is that the unit in the footwall of the fault appears to be Codell Sandstone! This presents a bit of a problem. Since the Codell is usually exposed as a thin layer (not usually of mappable thickness) at the contact of the Blue Hill Shale and Fort Hays Limestone it is perplexing to find a relatively thick exposure of it without any evident top or bottom. The fact that it is generally not even present in this region is part of the issue, but another issue is in trying to explain the sense of offset on the fault. If this is correctly identified as Codell Sandstone (and there are no other sandstone units in the map area that resemble it), then we have a quandary over whether this fault is in fact a normal or reverse fault, not to mention the question of how much offset there is across it. I don’t have a good answer. Any suggestions? Maybe a structural geologist has seen something like this before?
By the way, I had another pair of field assistants with me when I went out to photograph the Gigapan last week. Berti is helping out by holding the tape measure for scale, while Edi is looking on from the footwall region. If you want to make snapshots on this Gigapan and comment on them you can do so at the Gigapan website. If you’re interested in a Gigapan Opportunity for Geoscientists let me know.
Update: This post is part of the geoblog carnival Accretionary Wedge #6. Go visit the Lounge of the Lab Lemming to see what makes other geologists go “Hmmm…”
Ron, do the slickensites imply reverse or normal slip? I don’t know which area of the fault to pan to in order to find them!
What have you found in examining the sandstone?
If the joints are of any help – that is, if they are Reidel-shear-like – they could indicate down-dip movement on the fault, although I think one should look at an angle more perpendicular to the fault plane (and at the outcrop!) to confirm this guess. There are a few small joint-like features or possible shear planes (?) just below the fault in the sandstone in the area below the tape measure, which seem to be dipping more steeply than the fault plane, and also some larger planes sub-parallel to these in the sandstone. These could be Reidel shears as seen here – or not, depending on bedding and other things.
Anyway, maybe it could be a start.
First off. That is a very Nice Bob the Builder tape measure.
The slicken sides to me look like they are indicating a reverse slip (head wall up). Especially in the ‘photo of the slickensides’. Just up and left of your finger is a good example.
Is this more like a ‘fault zone’ than a straight forward ‘fault’? Whereby all the force was taken up in the sandstone creating the incompetent’ rock between the ls and more solid ss. (looking at img_7985)
There would still be slipping along the l.s. face.
IS there any foliation to the ‘incompetent’ rock? That alignment might help with stress direction.
I hope this makes any sense.
Slickenlines could just give you the direction of the last movement on the fault, too. (And since those are within the chalk, they could also be shear fractures of a different age. Not all that likely, but possible.)
What’s the sedimentary story with the sandstone? Is there a big non-layer-cake story there – are you looking at something with Brian’s favorite types of architecture? Could there be along-strike variations in the sand deposition (like channels, or delta lobes, or something that Brian would be much better at explaining)? Or is it irregular because it was deposited during deformation, and was removed from the upthrown block?
(I prefer ductile stuff; faults actually aren’t my expertise at all, though I’m getting better with them.)
[...] if I could find something scientific to do with my shiny new Gigapan unit (besides showing off confusing faults). One idea that has come to mind is the possibility of using the Gigapan images for what I’ll [...]
[...] is wondering if his fault might swing both ways, but only preserve the medial sandstone on one [...]