October 24, 2018
By Philip Prince, Virginia Tech Active Tectonics and Geomorphology Lab
In January 2014, I was cruising around eastern Jamaica collecting samples for a study of rock uplift and exhumation along the Blue Mountain restraining bend. Upstream of Bath Fountain, our hike up a stream that failed to yield any usable rock ended in the small box canyon shown below. The plunge pool looked deep, so I stuck my iPhone (in a Lifeproof case) into the pool with the camera recording video to see just how deep it was. For whatever reason, I didn’t look at the video while I was at the pool…I wish I had. Later that evening, I took a look at the clip and realized the plunge pool was REALLY DANG DEEP.
I don’t know exactly how deep the pool is, but I think it certainly exceeds 10 meters. I also think it is mostly drilled into bedrock and not entirely the result of a tufa dam. Tufa is ubiquitous in these streams if they drain any carbonate, and accumulated tufa can certainly be seen in the falls itself and on the margins of the pool. That being said, inspection of the streambed immediately downstream left me thinking that streambed tufa buildup was minimal. Obviously this drainage sees some monstrously high flows, and I would expect them to remove tufa fairly efficiently.
I actually think the pool appears to be deeper than the waterfall is tall. I’m not sure how this occurs without tufa damming. The stream may source harder clastic rocks further upstream to act as tools, but it still seems like a stretch to produce such exaggerated drilling so far below the low water surface of the pool. Interestingly, this isn’t the only small stream I have seen in Jamaica that has developed such an outrageous plunge pool. I remember seeing a similar arrangement in a tributary of the Spanish River on the north slope of the Blue Mountains in 2013. If anyone cares to weigh in on the limits of pothole drilling, please do so!
Nearby, the East Arm of the Morant River passes through an equally impressive slot canyon. This canyon is often widest at and below the low-flow water line; in other words, the walls of the canyon get closer together moving upwards. High flows would send an impressively narrow and deep water column through this canyon.
I was only able to hike to and swim through the canyon due to drought conditions; I imagine going anywhere near this at more substantial flows would end up with you becoming part of the Yallahs Basin sedimentary record. The canyon appeared to be developed in a quartzite or felsic metavolcanic. The rock was mechanically distinct from schists downstream, which likely influences the extreme geometry of the channel and gorge in this area.
So, why do these tiny streams produce these features? Tropical precipitation is a big part of the answer, but tectonic setting is also a very significant factor. Both locations are located just north of the oblique Plantain Garden Fault at the east end of the Blue Mountains Restraining Bend. In this area, the normally left-lateral strike-slip fault accommodates both left-lateral and reverse slip.
Restraining bend faults can be very efficient at moving rock upward due to their dip, which can be notably steeper than pure thrusts due to the nature of the restraining bend stress field. Throw in heavy tropical precipitation, and the system becomes very efficient at moving rock upward, exposing it, and sending it off shore to a basin. The simple model cross section below gives a general idea of material movement along a restraining bend with significant erosional removal.
Uplift and exhumation are significant along the main fault; at the same elevation on the other side of the uplift, comparatively little uplift and erosional loss has occurred. The result is a structurally asymmetric pop-up.
Obviously this is a gross over-simplification, particularly for a setting like Jamaica, where pre-existing rift geometry strongly controls faulting within the restraining bend. I’ll try to deal with inherited geometry in the next post, but this basic model does successfully show fundamental features that are broadly visible in eastern Jamaica and another tropical restraining bend, the Sorong Fault system of the New Guinea “Bird’s Head” peninsula.
This post was originally published on The Geo Models blog.