Alfred Hitchcock Presents: Blue Creek Canyon
The lower reaches of Blue Creek Canyon could easily be the setting of a horror film. Rock formations that are fascinating by day turn into bizarre looking, monstrous figures at night. Even during the day, hiking up the trail beneath outcrop after outcrop of the most grotesque forms you can imagine, statuesque against the sky, you get the feeling of being dissected by the piercing gaze of gargoyles from the heights of a haunted gothic cathedral. You might imagine the large boulder my brother and I dubbed "Hitchcock Rock", pictured above in the twilight, embodying the spirit of Sir Alfred, directing a terrifying drama unfolding in the ghostly terrain below.
If you're into such nonsense, that is. My brother, Randy, and I were here in the summer of 2009 to investigate these enigmatic rocks we had come across on our exhausting August mountain loop trail trek. To tell the truth, I was so wiped out the first time we hiked by these rocks in the summer of 2008, I was ready for some gargoyle to end my misery. However, I did have the wits about me to wonder if these Blue Creek Canyon rocks were correlative to some red rocks, seen in the picture below, we had previously encountered on the western side of the Dodson Trail in the Sierra Quemada. I didn't know the answer to that when I originally wrote this, but, with the advent of the the new USGS map, Scientific Investigations Map 3142, 2011, it now appears that these rocks belong to the Emory Peak Rhyolite member of the South Rim Formation. Rhyolite is a volcanic rock high in quartz and feldspar, that is, high in silica and low in iron. More about this rock later.
Blue Creek Canyon is on the southwest side of the high Chisos Mountains, accessed from Homer Wilson ranch just off Ross Maxwell Scenic Drive. The trail up the canyon takes you into the Chisos. But before I get into the geology of the Blue Creek Canyon rocks, here is a slide show - a sampling - of what you see at the beginning of your hike up Blue Creek Canyon. Be sure to have your imagination at full throttle when you examine these pictures, if you want the full experience.
It's time to get down to the geology. A chunk was broken off one of the outcrops about a mile above Homer Wilson Ranch. There has recently been a professional investigation of the volcanic rocks of Blue Creek Canyon, (USGS Circular 1327) and this may have been a result of sampling for that research. In any case, it allowed us to examine a fresh surface of this particular outcrop. The surface is shown below.
In the close-up below you can see that the rock is a volcanic tuff. Tuff is consolidated volcanic ash, and this tuff is known as a "welded tuff", as it was forged into a hard rock by the heat of the deposited ash. Ash is thrown out of a volcano during a violent eruption (think Mt. Saint Helens or Mt. Pinatubo). Most of the ash particles (tiny crystals of quartz and feldspar) are too small to be seen in the the image, but you can see some larger quartz crystals (grayish, translucent particles) and a tiny scattering of dark minerals (most likely biotite). This rock compares well with the description of the Emory Peak Rhyolite given in the new USGS pamphlet on Big Bend geology, so there is little doubt that is what it is. The fresh rock is brilliantly white. The reddish coloring is a coating of iron oxide and/or iron hydroxide due to weathering.
When I first wrote up this virtual field trip, I identified some of the rocks in these Blue Creek outcrops as sedimentary, for example, the rock in the picture below, which shows cross-bedding. However, it could be this apparent sedimentary structure is due to the flow of rhyolitic ash that occurred during the pyroclastic (that is, explosive) volcanics that produced the rock rather than water flow. Also, due to the topographical setting of these "red rocks", I assumed they must belong to the Chisos Formation, a series of volcanic and sedimentary rocks ranging in age from 47 to 33 million years. However, I did recognize that they likely were deposited between the time of the 33 million-year-old Bee Mountain Basalt member of the Chisos Formation and a 30-million-year-old trachyandesite that was formerly thought to be the 33-million-year-old Tule Mountain Trachyandesite. This would put their age between 33 and 30 million years, stratigraphically at the top of or above the Chisos Formation. Recent work, identifying them as belonging to the Emory Peak Rhyolite and not to the Chisos Formation, pins their age at 32 million years.
I measured the strike and dip of these rocks at several locations. Strike and dip are geometrical terms that describe the orientation of a flat plane, such as the top of a sedimentary bed or the side of a fault. To conceptually measure the strike, place a ruler on the flat plane such that it is horizontal. The map direction of the ruler is then the strike. The dip is the angle of the steepest direction down the plane and is always perpendicular to the strike. There was a difference in the strike and dip of the beds on either side of Blue Creek, indicating a possible fault parallel to the trend of the canyon. (The USGS map and the geologic map published with the book The Big Bend of the Rio Grande by Ross A. Maxwell only show faults perpendicular to the direction of the creek.) On the south side of the creek the strike was north 60° west with a dip of about 10° to the southwest, whereas on the north side the strike was north 6° west with a dip of 8° west. These numbers were fairly consistent from one outcrop to another on the same side of the creek, at least in the area of the rocks closest to Homer Wilson Ranch. (It was not our aim here to do a comprehensive mapping.)
One of the mysteries of these "red rocks" is, "Why do they come in wall-like outcrops, more or less equally spaced up Blue Creek Canyon and perpendicular to the canyon? See the picture below, looking southwest down the canyon.
The geological principle of continuity says the beds in each outcrop had to be continuous with their counterparts in the other outcrops in the past. That means the rocks that once existed between the outcrops had to be worn away. (A possible correlative bedding plane is shown in the above picture.) So, was there some process that made slab-like sections in the original rock resistant to erosion such that they remain today? Or was there some process that weakened the rocks that used to exist between the present-day outcrops? The latter could occur were there a bed of resistant rock, possibly completely absent now, that acted like a caprock above the rocks you now see. (A caprock is a resistant bed that protects less resistant beds below.) Joints (aka "cracks") in this caprock might have allowed erosional forces to work down into the rock below, eating away the rock on either side of the joints. If the joints were in the same direction as the outcrops now seen, these outcrops might be the remnants of the rock that was most distant from the joints. Since joints are often fairly equally spaced, this theory would explain the apparent spacing of the outcrops. However, you might not need a caprock, just joints in the rhyolite. For now, this seems to me to be the best explanation.
There is other interesting geology along the trail up the canyon. In the following image you see exfoliation of a massive tuff bed due to weathering processes. Large-scale exfoliation, such as seen in the granites of the Sierra Nevada, has often been attributed to pressure release as the rocks are lifted upward by tectonic forces and erosion removes the overburden. This is obviously not the case here as these are volcanic rocks that were deposited at the surface of the Earth. Possibly chemical changes have caused the minerals at the surface of the rock to swell, creating stress that leads to the exfoliation observed. Alternatively, fire can also cause exfoliation, although the fire-produced exfoliation I've seen doesn't look similar to that on this outcrop.
Then there is the weird-looking outcrop seen below. What's up with this? You see rather thinly-bedded deposits overlying a mass of rock with no apparent bedding. I think the unbedded mass is a welded tuff deposit, and the thin-bedded deposits may have been laid down by a series of later volcanic actions. (Originally, due to the sedimentary appearance of some of the rock in these outcrops, I thought the thin deposits where laid down by water, but, in that case, you would expect the thin beds to be truncated by the massive rock, and that is not the case.
Although, as I've said above, the maps referenced here only shows faults running perpendicular to Blue Creek Canyon, there are definitely other faults that run parallel. (It may be these faults weren't considered to be major enough to be included in the maps. After all, faults range from miles of offset to just a few millimeters.) In the picture below you can see one such fault.
Now, you might glance at this picture and immediately surmise the movement along the fault was left-right. This would make it a strike-slip fault; in particular, a left-lateral strike-slip fault, since the dike, a sheet-like igneous intrusion, on the far side of the fault appears to be shifted to the left. (If you think about it, you'll see that it doesn't matter which side of the fault you are on; it is still a left-lateral fault.) However, what about volcanic bed with the ash (white) directly below it? It appears to have been shifted up on the far side of the fault. This would make it a dip-slip fault.
There are two possibilities here. The fault could be a combination of strike-slip and dip-slip fault, in which case the motion of the rock in the foreground along the fault plane would be down and to the right. On the other hand if the dike has a dip toward the left of the image, the fault could be purely dip slip with no horizontal offset at all. Erosion would make it appear that there was a horizontal offset. What is certain is that the fault has a significant dip-slip component and the trace of the fault on the surface is parallel to the canyon.
More can be said about this picture now that the new USGS map is available. In the general area where I took this picture, the map shows an offset dike; however, the dike is offset in the map as if due to a right-lateral strike-slip fault, but no fault is indicated. There is another dike on the map to the southwest of this one that shows no offset. If this latter dike is somehow the same feature as that in the picture, the offset may not be shown because it is too small to be rendered on the map. If either dike corresponds to the feature photographed, the dark layer of rock above the white tuff bed would be the Bee Mountain Basalt member of the Chisos Formation. Unlike rhyolite, basalt contains relatively more iron and relatively less silica. This member consists of several separate basalt flows, ranging from about 34 to 33 million years before the present. The tuff bed could be part of the Mule Ear Spring Tuff member of the Chisos Formation, deposited between 33.6 and 33.7 million years ago. This tuff and the Bee Mountain Basalt are often in contact. My opinion is this feature is too small to appear on the map but is probably close to the dikes that do appear, which would make the identification of the rock units given above likely to be correct.
Arriving at Homer Wilson Ranch in the summer of 2009, preparing to hike up and camp at the "red rocks", we noticed significant changes since our last visit. Blue Creek had eroded steep sides into the alluvium of the creek bed. Obviously, a major flooding event had occurred since we were here in 2008. This event was very evident as we made our way up the canyon. The trail was washed out in many areas. In the following photo you can see the extent of the erosion of the flash flood at one location along Blue Creek. The bank on the far side, resulting from material eroded by the flood, is about seven feet tall (!). Also, notice the color difference on the rock outcrop on the right of the creek where about six or seven feet of creek bed was removed. What a flood it must have been to remove all that material! Erosional processes continue to reshape the Big Bend region, and here you can see that rare but large events can have a substantial impact.
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