April 8, 2020
By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
Where: East Arm Morant River, on the south flank of Jamaica’s Blue Mountains
What to see: A former reservoir completely backfilled with gravel and cobbles, due to rapid sediment production
Colorful Cretaceous marble with chert nodules, folding, and tension gashes
A great slot canyon, which is an expression of active tectonic setting, tropical precipitation, and very resistant rock (in the video link further down, and what you’re here for)
The following two images give a general idea of the East Arm Morant’s setting. Faults in the top image are hastily drawn and highly generalized from several sources to give a basic idea of the restraining bend and source of uplift.
Back in 2013, I walked across the Enriquillo-Plantain Garden Fault at Hillside, Jamaica (17.9697N 76.4813W) and headed up the East Arm of the Morant River to see the sights. This stream and its gorge are a great example of how tectonic uplift and tropical precipitation patterns combine to rapidly erode and transport bedrock.
Stop 1: Reggae Falls…which isn’t even a waterfall (17.9725N 76.4819W)
This “falls” is an old dam whose reservoir has completely filled in with gravel and cobbles. I include it as a stop because it indicates just how much sediment is produced and transported out of the East Arm Morant River basin. The dam is obviously not terribly old, so its backfilling is a clear indication of just how fast rock is transported out of uplifting areas that receive tropical rainfall. The bedrock here is weathered schist. The cobbles and gravel are a variety of lithologies from upstream…check out the next stop.
Stop 2: Gravel bar formed by backfilling of the dam (17.9755N 76.4874W)
Looking at sediment accumulation like this is a good way to see what rocks are exposed further upstream in the river system. This shot shows a typical combination (in my experience, anyway) of rock coming out of rivers draining the core of the Blue Mountains. There is a mix of metasediments, apparent metavolcanics, and plutonic rocks that look like granodiorite. The rocks being uplifted to produce the Blue Mountains formed in a Cretaceous island arc setting, so the combination of rock types seen here makes sense.
If you can open the image above in its own window, you should be able to see some of the details of the different lithologies. What I interpret to be metavolcanic rock appears to have some small, white phenocrysts. I hope that I am correct about the metasediment pointed out here, but interpretation can be difficult without a thin section because of the complex tectonic origin of these rocks and the great variety of lithologies present. Just upstream of this point, there is a definite, and very beautiful, metasedimentary rock outcrop.
Stop 3: Cretaceous marble outcrops (17.9766N 76.4901W)
I have no idea how this rock type relates to the schists exposed at Reggae Falls. Their relative locations may result from presently or recently active faulting, or they may result from fault juxtaposition that occurred in the Cretaceous arc phase and is now being exposed by modern uplift. Either way, this beautiful marble shows some cool deformation features, also of uncertain age or context.
The chert nodule above probably does not deserve a “?” as that’s almost certainly what it is. I don’t know if the shape of the nodule reflects bedding, meaning that the layered appearance of the marble is an original depositional feature, or if the chert has been deformed, flattened, boudined, etc. and the marble shows a metamorphic foliation or transposed bedding.
Much of the marble is purple and green. It’s a really stunning rock to see in person. Numerous sets of tension gashes are visible. Between the yellow shear arrows, warping and rotating of the layering is visible.
Stop 4: The slot canyon (17.9867N 80.4929W)
Upstream of the marble is more weathered schist…and then things get interesting. The bedrock appears to transition to a quartzite (or some sort of very silica-rich, fine grained, extremely hard rock), and the gorge walls steepen.
Around the bend, the walls tighten into a slot canyon, and it’s time to swim if you want to keep going. Slot canyons like this don’t just form on any river; they reflect rapid downcutting. Here, rock uplift gives the river potential energy, and massive amounts of tropical rainfall take advantage of this potential energy to accomplish intense erosion.
The video below will take you into the slot canyon. Excuse the Darth Vader breathing. I was swimming while holding the GoPro in my teeth by its foam floatie. Following the video are a few more images showing points of interest within the canyon.
The image above is a closer look at the bedrock near the slot canyon entrance. When I first saw it, I concluded it was quartzite. That may or may not be correct, but it is certainly a metamorphosed, silica-rich rock that is extremely hard. Accordingly, I think most of the slot canyon’s existence results from the strength of the rock and its ability to support overhanging gorge walls that are yet to collapse. The East Arm Morant’s gorge is steep everywhere, but weathered schist cannot support the steepness of this really hard material. In other words, the slot canyon is just a localized expression of the tectonic and erosive conditions that affect the whole area and look different within weaker rock.
The slot canyon walls are undercut, with deep scours gouging out the walls. Tools (sediment pieces that impact the bedrock and erode it) collect in these areas, which would be deep, powerful eddies with cobbles banging around in them at high flow.
As with the marble, I don’t know what the parallel features in the rock represent. They may represent original sedimentary layering that has survived metamorphism, or they may be a foliation or transposition of bedding. There are other low-angle discontinuities that tilt downstream. Their origin was also not immediately obvious.
The upper opening of the slot canyon is often much narrower than river level. The image above shows one location where the gorge is at most 3 m/10 ft across. Mango trees hang over it, and mangoes constantly fall in the river. They are the little orange spots on the bottom of the pool in the video.
Both abrasion and plucking are active in the gorge. The discontinuities in the rock intersect at very small scale, so most of the surfaces appear rounded and few large block scars are obvious. This canyon would be an ugly place at high, or even typical, flows. I made my trip at unusually low water during a prolonged drought. The piece of driftwood wedged in the crack at upper right shows where the water has been. Compared with vegetation at left, I would say this level of flow occurs regularly here, and it would go much higher during storm events.
Jamaica’s Blue Mountains are loaded with gorges just like this one due to the combination of tectonics, precipitation, and rock types exposed at the surface. I hope to make it back someday to check out a few more. Special thanks to Barrett Adventures (http://www.barrettadventures.com/) for transportation and support!