The Karst Geology of Puerto Rico

Karst is formally defined as "A terrain, generally underlain by limestone, in which the topography is chiefly formed by the dissolving of rock, and which is commonly characterized by Karren, closed depressions, subterranean drainage, and caves. " by the USGS in 1970. The definition mentions limestone as the common rock type in karst topography, other water soluble minerals and rock forms such as evaporites and other salts. The typical use of Karst refers primarily to dissolution formations, though the term does allow for lava tubes and caves to be included.

As seen in the map, there is a great focus on sinkholes. This is because Karst topography and geology is almost always responsible for larger or more extensive sinkholes. Think of Florida, look at how the entirety of the peninsula is underlaid by a bedrock of limestone and it is no wonder why there is such a sinkhole hotspot in the state. This is the primary impact of Karst topography on the world today. As for Puerto Rico, the type of karst there is also limestone.

Limestone karst, the dominant type of karst in Puerto Rico, forms from limestones of the Miocene and Oligocene epochs of the Tertiary period. That makes these formations and caves as old as 34 million years or as young as 5.2 million years old. The karst is most prominent on the northern (Atlantic) coast of the island and extends inland to roughly halfway across the island at one point.

These limestone formations are the result of when the northern coast was underwater and accumulated the calcium carbonate shells of microscopic organisms called coccolithophores. These shells are left behind, compacted and cemented, then later uplifted to the surface of what is now Puerto Rico. Once the elevation of the rock units are generally equal to where the water table of the island are, the dissolution of limestone can take place and the caves and caverns begin to form, though the process will take hundreds of thousands of years or more. From here these caves and caverns will remain as such unless they collapse, in which they are now a sinkhole.

Red regions among the tan region in the above image represent sinkholes greater than 30 meters deep and not the numerous small sinkholes that litter the landscape. The clustering and frequency suggest extensive dissolution and cave systems as is typical for karst regions. The sad part of knowing all of these sink holes are there and likely explorable is that they are among the jungle and more extreme karst terrain making them rather inaccessible by most standards. One cave that will be featured in the story map, Cueva Balcones, is a 30 minute trek from where the nearest road ends to the entrance of the cave.

Caverna de Camuy is an accessible and commercialized cavern and small cave system that formed as a part of the Camuy River and is squarely part of the Aymamon formation of limestone. The cavern is enormous and sees massive flooding during any given hurricane that strike Puerto Rico.

Cueva Balcones is a off-the-map cave system that is not easily accessed, but very much worth the hike to see such deep and confined spaces. It, like Caverna de Camuy, is formed by dissolution of the native limestone. This system is a cave and not a cavern because it only has one entrance, whereas a cavern will have multiple.

In understanding the geological aspects of karst topography, a look at the hydrology of the area is necessary. One such look was taken by the 2021 TAMUCC Field Camp Team. On July 20th, 2021 the team took a running sample set from a notable river in the karst region, Rio Grande de Manati. The river runs from south to north, starting in the mountain and ending in the Atlantic Ocean. Following is a map of the teams sample stations along the river.

In the legend of the above map, two main characteristics are listed, pH of the water and radon content. By color we see pH and by size we see radon content, and these two characteristics are especially telling when it comes to karst. As water dissolves calcium carbonate (limestone), the dissociated ions will increase the pH of the water, making it more basic. As for radon, limestone has trace amounts of radium in its composition that will radioactively decay to radon over time. Both of these measures are telling of the interaction of water and limestone in the subsurface and by finding spikes in these values, one can locate where underground water (the water table) comes to the surface as a spring. Further, these high values would suggest an increased amount of limestone dissolution nearby, which from what we know of karst, could in turn suggest a nearby waterlogged and developing cave or cavern.

After seeing some of the ins and outs of karst topography and how it has impacted Puerto Rico, what can be expected going forward? In terms of a few years: not much. Over time, more and more limestone and related rock formations will be dissolved and form new waterlogged caves and caverns, and existing ones may collapse any time a new stress is applied. This can mean catastrophe for cities and towns with underlying caves, or it could mean new tourist attractions for sinkholes in the jungle, away from developed areas. Puerto Rican officials would not need to do much more than perhaps some geophysical surveying to identify caves and crevices and shed light on which building site is the most secure. Puerto Rican officials of 200,000 years from now, however, may be looking at a more involved selection process and a whole lot more sinkholes.