A recent research expedition uncovers a region of extraordinary biodiversity
The coastal region from Punta Arena to Cabo Pulmo harbors two geological singularities of immense value for biological diversity. On the one hand, the formation of Punta Arena is a sandy peninsula that was formed by the accretion of longshore bars called "beach ridges" (Tanner 1995) forming a complex of sandy crests and depressions that run parallel to the coast. With the exception of El Mogote, in the Bay of La Paz, which has a similar (but not identical) origin to Punta Arena, there is no other similar formation in the Gulf of California coasts along the peninsula. In order for ridges to form, an abundant source of sand is needed with strongly variable inputs of sediments. Curray (1995) described in detail the formation of beach ridges as a result of periodic building of offshore bars to above sea level after sufficient sand had been transported into the area and during an optimal combination of oceanographic conditions. Looking at the satellite images, it is easy to see that the origin of sand for the formation of Punta Arena is the Santiago River, which runs sporadically into the Gulf after chubascos and hurricanes, depositing large pulses of sandy granitic sediment into the sea.
On the other hand, the reefs of Cabo Pulmo, only a few kilometers south of Punta Arena, form what apparently is a barrier reef. As with Punta Arena, this is not the result of true constructional activity by hermatypic corals but the product of a second unique geological accident that has contributed to the immense biological richness of this region. The whole of the Cape Region, geologically defined as the immense batholith that forms the tip of the peninsula south of the Fault of La Paz, is formed entirely by Cretaceous granite. This massive slab of the Earth's surface, which drifted from the Mexican mainland some 6 My ago to form the southern tip of the Baja California peninsula, was cracked and fragmented in its eastern coast by the tectonic forces of the deep ocean ridges in the Gulf of California. These ridges, in turn, were filled by volcanic breccia—a mixture of granite fragments embedded in a lava-like matrix that formed some 1–2 million years ago. Two volcanic pipes of breccia form the points that mark the bay —Los Fariles and El Pulmo— and a series of dykes of volcanic material run along the bay, giving the appearance of a series of coastal barriers like those of tropical barriers reefs and atolls (Squires 1959).
The reefs of Cabo Pulmo Bay consist of a series of eight long bars of extruded igneous rock, upon which coral and other animal and plant life flourishes. All eight bars begin well up on the beach and can be easily seen, resembling dark, rocky dykes protruding up and out of the beach sand and continuing on into the sea (Brusca and Thomson 1979). The outermost bar is a continuation of the north point of Punta Los Frailes. It runs nearly the entire length of the bay and forms a submerged "barrier reef" that is actually broken up into a series of alternating bars with short, sandy stretches in between. The depth of the top of this outermost reef is 25–30 feet at the southern end (near Punta Los Frailes) and about 65 feet at the northernmost end (seaward of Cabo Pulmo, a steep, rocky headland). The depth of the sandy breaks in the reef, as well as the sand patches either side of the reef, at the southern end, is about 50 feet. Although the Cabo Pulmo coral formations may not be classified as true barrier reefs, this may be really the point of greatest interest: The Cabo Pulmo lava dykes clearly function as coral reef barriers, judging by the species richness and coral-associated fauna and flora.
In short, two geologic singularities, the beach ridges of Punta Arena formed after the last glaciation, during the last 20,000 years, and the underwater dykes of Cabo Pulmo formed during the last 2 million years as a result of the Gulf's plate tectonics, have created a region of immense biological richness and inordinately high diversity.
In this page we present a detailed report of a recent biodiversity survey made by a binational research expedition to the Punta Arena and Cabo Pulmo ecosystems, and their immediate surroundings.
1. Uncovering the Dryland Biodiversity of the Cabo Pulmo Region: Downloadable book report in pdf format.
2. Web-readable report: in e-book format (click on the image below):
References
Brusca, R.C., and D.A. Thomson. 1975. Pulmo reef: The only coral reef in the Gulf of California. Ciencias Marinas 1: 37–53.
Curray, J.R. 1995. Origin of beach ridges. Marine Geology 136: 121–125.
Squires, D. 1959. Results of the Puritan-Ameritan Museum Natural History Expedition to western Mexico. 7. Corals and coral reefs in the Gulf of California. Bulletin of the American Museum of Natural History 118(7): 367–432.
Tanner, W.F. 1995. Origin of beach ridges and swales. Marine Geology 129: 149–161.