ALBERT

Description: The 450-m long spit that extends westward from the northwest corner of Isla San Luis Gonzaga is one of the largest and most complex constructions of unconsolidated cobbles and boulders found anywhere in Mexico’s Gulf of California. The material source derives from episodic but intense storm erosion along the island’s andesitic cliff face with steep northern exposures. A well-defined marine terrace from the late Pleistocene cuts across the same corner of the island and provides a marker for the subsequent development of the spit that post-dates tectonic-eustatic adjustments. A total of 660 individual andesite clasts from seven transects across the spit were measured for analyses of change in shape and size. These data are pertinent to the application of mathematical formulas elaborated after Nott (2003) and subsequent refinements to estimate individual wave heights necessary for lift from parent sea cliffs and subsequent traction. Although the ratio of boulders to clasts diminishes from the proximal to distal end of the structure, relatively large boulders populate all transects and the average wave height required for the release of joint-bound blocks at the rocky shore amounts to 5 m. Based on the region’s historical record of hurricanes, such storms tend to decrease in intensity as they migrate northward through the Gulf of California’s 1100-km length. However, the size and complexity of the San Luis Gonzaga spit suggests that a multitude of extreme storm events impacted the island in the upper gulf area through the Holocene time, yielding a possible average growth rate between 7 and 8 m/century over the last 10,000 years. In anticipation of future storms, a system to track the movement of sample boulders should be emplaced on the San Luis Gonzaga spit and similar localities with major coastal boulder deposits.

Description: This study reports the first example of major erosion from hurricanes degrading a rocky coastline anywhere around the Gulf of California, although other sources of evidence are well known regarding the effect of inland erosion due to catastrophic rainfall in the Southern Cape Region of the Baja California peninsula and farther north. The uplifted, 12-m terrace on the eastern shore of Isla del Carmen is the site of an unconsolidated coastal boulder deposit (CBD) consisting of large limestone blocks and boulders eroded from underlying Pliocene strata. The CBD stretches approximately 1.5 km in length, mostly set back 25 m from the lip of the terrace. The largest blocks of upturned limestone near the terrace edge are estimated to weigh between 5.8 and 28 metric tons. Waves impacting the rocky coast that peeled back slabs of horizontally-layered limestone at this spot are calculated to have been between 11.5 and 14 m in height. Analysis of sampled boulders from the CBD set back from the terrace edge by 25 m suggest that the average wave height responsible for moving those boulders was on the order of 4.3 m. Additional localities with exposed limestone shores, as well as other more common rock types of igneous origin have yet to be surveyed for this phenomenon elsewhere around the Gulf of California.

Description: This work advances research on the role of hurricanes in degrading the rocky coastline within Mexico’s Gulf of California, most commonly formed by widespread igneous rocks. Under evaluation is a distinct coastal boulder bed (CBB) derived from banded rhyolite with boulders arrayed in a partial-ring configuration against one side of the headland on Ensenada Almeja (Clam Bay) north of Loreto. Preconditions related to the thickness of rhyolite flows and vertical fissures that intersect the flows at right angles along with the specific gravity of banded rhyolite delimit the size, shape and weight of boulders in the Almeja CBB. Mathematical formulae are applied to calculate the wave height generated by storm surge impacting the headland. The average weight of the 25 largest boulders from a transect nearest the bedrock source amounts to 1200 kg but only 30% of the sample is estimated to exceed a full metric ton in weight. The wave height calculated to move those boulders is close to 8 m. Additional localities with CBBs composed of layered rock types such as basalt and andesite are proposed for future studies within the Gulf of California. Comparisons with selected CBBs in other parts of the world are made.