Abstract
This project examines the role of hurricane-strength events likely to have exceeded 119 km/h in wind speed that entered the Gulf of California from the open Pacific Ocean during Late Pleistocene and Holocene times to impact the granodiorite shoreline on Isla San Diego. Conglomerate dominated by large, ellipsoidal to subspherical boulders at the islands south end were canvassed at six stations. A total of 200 individual cobbles and boulders were systematically measured in three dimensions, providing the database for analyses of variations in clast shape and size. The project’s goal was to apply mathematical equations elaborated after Nott (2003) with subsequent refinements to estimate individual wave heights necessary to lift igneous blocks from the joint-bound and exfoliated coast on Isla San Diego. On average, wave heights on the order of 3 m are calculated as having impacted the Late Pleistocene rocky coastline on Isla San Diego during storms, although the largest boulders more than a meter in diameter are estimated to weigh two metric tons and would have required waves in excess of 10 m for extraction. Described for the first time, a fossil marine biota associated with the boulder beds confirms a littoral-to-very-shallow water setting correlated with Marine Isotope Substage 5e approximately 125,000 years ago. A narrow submarine ridge consisting, in part, of loose cobbles and boulders extends for 1.4 km to the southwest from the island’s tip, suggesting that Holocene storms continued to transport rock debris removed from the shore. The historical record of events registered on the Saffir–Simpson Hurricane Wind Scale in the Gulf of California suggests that major storms with the same intensity struck the island in earlier times.
Highlights
Oriented northeast to southwest between mainland Mexico and the Baja CaliforniaPeninsula, 40 named islands in the Gulf of California spread out over a sea surface of160,000 km2
Co-ordinates are listed in Appendix A (Tables A1–A6) for each station recorded by a hand-held device for tracking by the satellite-based global positioning system (GPS)
Study of the cobble–boulder deposits from Isla San Diego in Mexico’s lower Gulf of California offers the following insights based on mathematical equations for estimation of
Summary
Oriented northeast to southwest between mainland Mexico and the Baja CaliforniaPeninsula, 40 named islands in the Gulf of California spread out over a sea surface of160,000 km. Oriented northeast to southwest between mainland Mexico and the Baja California. Island development postdates the opening of the gulf to the Pacific Ocean by rifting from the mainland more than 5 million years ago and many formed as fault blocks influenced by regional tectonics. Of the 40 islands, 8 islands fall into the category dominated by granite or closely related granodiorite, and this study looks at one of the smallest in the lower Gulf of California called Isla San Diego with an area of 60 ha [1]. Survey work conducted through satellite imagery shows that rocky shores account for nearly half the gulf’s peninsular coastline including related islands [2]. At slightly more than 23%, andesite dominates the region’s total shores, followed by granite
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