Tuesday, September 27, 2016

Almost Titus Canyon: Is This a Fold? And... Apparent Dip with Post-it® Notes

(This is not a fold.)
As we leave the lower Lost Canyon section of the Titus Canyon road, we're also leaving the Leadfield area. (Say goodbye!) Less than a half mile from the Leadfield parking area, the road curves rather abruptly to the left, enters a canyon with steep walls, and presents us with the view seen above. Is this a fold? It looks like one at first, and in fact that's what I thought when first rounding this same bend many years ago: "Oh, look, a neat little syncline." Beds on the south (left) are dipping to the north, and beds on the north are nearly horizontal or dipping to the south—or are they?

Actually, we're looking at a nearly uniformly dipping section of the Cambrian Bonanza King Formation; the beds are dipping about 35° to the north and look bent or folded only because of our viewpoint and a nearly right-angle bend in the canyon wall.

Let's look at this a little more closely:
Here's the non-fold with a few colored lines.
In this second photo, I've drawn a magenta horizontal line from which I've measured dip angles of the beds in the north-south section of the canyon wall. These two dip measurements of 48° and 40° to the north might be what is called the apparent dip of the beds; they definitely are not true dip measurements, which are usually taken in the field with a type of compass called a pocket transit (mine is a Brunton compass; see my first, older-model Brunton here).

NOTE: There's a technicality about apparent dip that makes me unsure what to call the inclination of beds one can see at random angles in the field or when looking at photos like the one above. True dip is measured perpendicular to strike and is always greater than apparent dip, which is measured on a vertical plane that cuts through the rock or a map at any angle other than perpendicular to strike. As we'll see below, perspective can cause the slant of beds to look shallower or steeper than the angle of true dip. I've generally called this observed or ostensible angle "apparent dip" when I know I'm not looking straight down strike (which is most of the time if I haven't lined up with the level of a compass), but it's really something else; maybe it should be called "perspective dip" "positional dip" or "POV dip".

Getting back to the photo above, I've drawn an approximate apparent dip line of 0° on the portion of the canyon wall that is running nearly east-west (on the right). Are these beds really horizontal (or even dipping back to the south, which they appear to do above the cyan line)?

In the first Google Earth (GE) image below, we're standing about at the yellow pin marked "4370 - not fold", looking southwest at the canyon wall (4370 was my original photo number). I've drawn three strike and dip symbols and one strike line on the image in magenta. I took the strike by using points of equal elevation on four individual beds that I picked out. The two symbols on the hill south of the road, herein known as DS Hill, are on individual dip-slope beds that are quite obvious in GE; these strike lines are probably the most accurate of the four. (All four lines of strike should be considered approximate.)

So, we have a strike of about N70E, or 070° azimuth. From the photo and from the dip slope on DS Hill, we know that the dip direction for three of our points is to the northwest. For the beds north of the wash, I know from driving through the canyon that these beds also dip roughly to the north or northwest; also, if they were dipping to the south, the slope would look more like the dip-slope hill south of the wash.
Our location (yellow pin) with strike-dip symbols in magenta, a set of identifiable beds in cyan, and a small fault in dark blue.
The same area, now with two dip measurements.
To measure the dip, I selected a location to the east of of the dip-slope hill that was along strike and also at the same elevation of the larger of the two strike-dip symbols on the dip slope. In the GE image below, we are standing at that point, along strike and at equal elevation, measuring the dip angle where we see the longer of the four magenta lines. Note: Only that one line really shows up in its proper orientation due to various perspective problems with Google Earth. Also note that the northwest-dipping bedding in the "fold" area—the outcrop area just above the yellow pin that corresponds to the obviously dipping strata in our photo—doesn't really look that way in GE from this angle, also due to various perspective and topographical complications.
The prominent dip slope in Bonanza King Formation beds is marked by the longer of the four magenta lines.
From the orientation and elevation I'm sitting at above, I measured a 35° dip. Doing the same thing from the west, I was able to measure a 35° dip on the dipping beds looming in front of us as we start to drive into this canyon. If we walked up to these two locations to take a strike and dip with our Brunton, we'd probably get readings within 5 degrees or so of my GE measurements, in both strike and dip. We'd get similar readings if we measured the beds on the north side of the canyon wall.

To look at this in another way, I created an analogy to our beds with a stack of Post-it® notes. The first example approximates what we see directly ahead of us, on the north-south canyon wall, as we enter the canyon (first photo).
Our multi-colored, light and dark beds are dipping about 35° to the north.
As we move around to the south and look to the north—approximating our view of the east-west part of the canyon wall—these same beds appear to be horizontal.
The apparent dip of beds is 0° as we look to the north.
If we look to the south, we see a kind of dip slope, although here I'm looking down on them from above, so they don't look as steep as they would from the canyon floor.
We look south and see a dip slope, similar to the dip slope on the south wall of the canyon (GE image #3).
It's probably worth noting that the dips I measured directly from the "fold" photo are higher than the dip I got using Google Earth. As I mentioned earlier, our perspective can cause dip of beds to vary greatly; in fact, the angle we see from changing perspectives can vary from 0 to 90 degrees, depending on our viewing angle and orientation. When looking straight into dipping beds, that is when looking perpendicular to strike, the beds will look horizontal (as we saw in Post-it® photo #2). When viewing these same beds from a point along strike, they will appear to dip at exactly true dip (Post-it® photo #1).
Another set of Post-it® beds are dipping 35° to the north (right).
As we move around in the terrain to the ESE, these same beds look like they are dipping 40° to the NNE.
Now, having moved to the SSE, the beds appear to dip 70° to the ENE.
If we look at the same stack of beds from the south, they look almost vertical. In fact they can be measured with "apparent" dips of 85 to 88° to the east.
Without having a good handle on the strike of beds while wandering around from canyon to bluff and up, over, and across the terrain, we can't really tell the dip of beds!

I did try to simulate our canyon beds with a crudely modified block of Post-it® notes. Imagine our stack of beds as above, although this stack seemingly has some thinner, almost shale-like beds in it. Now we erode a canyon or amphitheater into the beds.
Block of dipping beds with a crude canyon eroded into it.
We can easily see the dip slope on the south side of the canyon (green slope on the left). I think that because our perspective is not from within our little amphitheater or canyon, we can't really get the feel of the beds on the north wall (right) that seemingly appear nearly horizontal (photo #1). I'd have to move a tiny camera into the beds, and maybe stand it on the purple horizon lying on the thick green unit, to get the same perspective that we have in our "fold – not-fold" photo. I tried this crude Post-it® method because most 3-D block diagrams (and physical blocks) don't show a cutout that would simulate standing in a canyon with dipping beds looming all around.

Well, let's get back to our travels down the canyon!
But first... some lupine from the spring of 2009, right below the faux fold.
After we round the bend to go past the dipping beds that we saw in our first photo, it's a mere 0.2 miles to the junction with the main branch of Titus Canyon, which comes in from the north. And then, about 0.3 miles past that junction, we come to this spot, where we can see that Titus Canyon, up ahead, widens a little.

But what else can we see?
I see some wonderful rocks in a dry wash, and...
More next time!

Location map

Related Posts:
The Approach to Titus Canyon: Tan Mountain
The Approach to Titus Canyon: Up and over White Pass
The Approach to Titus Canyon: To Red Pass
The Approach to Titus Canyon: Just Below Red Pass
A Hike at Red Pass, Titus Canyon Road, Death Valley, CA
Titus Canyon Road: A Little History and a Few Maps
Down into Titus Canyon: We Leave Red Pass Behind (Finally!)
Titus Canyon: The Upper Part of Lost Canyon
Leadfield: Scams with a Side of Geology
Leadfield: Views from Old Mine Buildings
Leadfield: Geology...and a Cactus...on the Way Back to the Parking Area


Marciepooh said...

On a field trip a few years ago in a quarry, there was a great example of a combination of apparent dip (essentially vertical walls of the quarry) and "perspective" dip. The beds were dipping out of the corner of the quarry* and looking at that corner from the floor it looked like a nice little anticline. Fooled my (then) boss, but not me.

*When facing that corner, your shoulders would be aligned pretty darn close to strike. The opposite corner was where the ramp/driveway was - no 'clean' view of the rocks and no smile from them either.

Hollis said...

Good explanation ... Post-It pads did the trick. I've always thought that good field geologists must have awesome spatial skills! (not me)

Silver Fox said...

@Marciepooh It can be frustrating when you can't get a good viewing angle in the field, and it probably is fairly common in open pit mines or quarries. I had never heard of the "smile" from a fold (smile=syncline; arch/rainbow=anticline) until looking it up just now. Must be a new analogy being taught that came into vogue after I was in school (?). I'm guessing that's what you're referring to.

Silver Fox said...

Hollis, glad it worked for you! I was trying to think of ways to talk about apparent dip, when not having the best photos. Next time I'm in Titus Canyon, I'll try to take photos of the not-fold from more than one angle.