**1. Introduction**

Oriented northeast to southwest between mainland Mexico and the Baja California Peninsula, 40 named islands in the Gulf of California spread out over a sea surface of 160,000 km2. These gulf islands range between 1224 km2 and 22 ha in size [1]. 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. Most are composed of Miocene volcanic flows or from intrusive igneous rocks of yet older Cretaceous origin. 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

**Citation:** Callahan, G.; Johnson, M.E.; Guardado-France, R.; Ledesma-Vázquez, J. Upper Pleistocene and Holocene Storm Deposits Eroded from the Granodiorite Coast on Isla San Diego (Baja California Sur, Mexico). *J. Mar. Sci. Eng.* **2021**, *9*, 555. https:// doi.org/10.3390/jmse9050555

Academic Editor: Matthew Lewis

Received: 19 April 2021 Accepted: 19 May 2021 Published: 20 May 2021

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or granodiorite at 9%, and limestone at 7.5%, while rock types including other igneous rocks or metamorphic rocks are less well represented.

This contribution belongs to a series of papers focused on the erosion of coastal boulder beds from their parent rocks within the Gulf of California. Upper Pleistocene and Holocene deposits formed by boulders are commonly found along the peninsular shores of Baja California and around the gulf islands but most studies in coastal geomorphology seldom compare the results of rock density on an interregional basis as related to different parent rock types. The application of mathematical formulae to estimate storm wave height was applied previously to coastal boulder deposits throughout the Gulf of California, including those formed by limestone, rhyolite, and andesite clasts [3–6]. Extension of this program now includes the Pleistocene boulder beds eroded from the granodiorite coast of Isla San Diego, applying the same methodology of systematic size measurements to calculate volume and weight based on rock density preliminary to the estimation of wave heights derived from competing equations. The study also newly describes marine fossils preserved within the Pleistocene conglomerate of Isla San Diego that date the deposits with reasonable accuracy. Finally, development during the Holocene time of a long marine ridge off the southwestern tip of the island brings into consideration the ongoing influence of hurricanes capable of moving large boulders in a shallow, subtidal setting.

Aside from the limited statistics available on the size, geologic origins, and coastal composition of islands in the Gulf of California [1,2], barely any literature exists on the geology and geomorphology of Isla San Diego except for an early nineteenth-century appraisal that includes the only previous description of the submarine ridge off the island's southernmost end [7]. Attention to the phenomenon of coastal mega boulders and their relationship to major storms or tsunami events is a topic of growing interest [8–11]. Especially in the context of rock density, the data from Isla San Diego provide further insight on comparison of storm beds of Pleistocene and Holocene origins throughout the gulf region [3–6] with oceanic basalt in the Azores and Canary Islands of the North Atlantic [12,13], as well as rare mantel rocks from storm beds in coastal Norway on the Norwegian Sea [14].

#### **2. Geographical and Geological Setting**

Stretching for more than 1000 km in length (Figure 1a), the Gulf of California is a marginal sea seated over a tectonically active zone that entails spreading centers offset by a succession of transform faults [15]. The central spine of the adjacent Baja California Peninsula is formed by granodiorite, broadly dated to a Cretaceous origin between 97 and 90 million years ago [16]. Upfaulted granodiorite basement occurs on the Baja California Peninsula at Punta San Antonio north of Loreto, on Isla Catalina east of Loreto, as well as Isla Santa Cruz and Isla San Diego (Figure 1a). Peninsular and island development including those areas with granodiorite resulted from Miocene extensional rifting that began prior to flooding 13 million years ago and lasted for 9.5 million years when a change in dynamics initiated transform faults connected with the San Andreas Fault on the US side of the border [1,2,15]. A detachment zone was activated approximately 3.5 million years ago that resulted in half-graben structures separating the islands from the rest of the Baja California Peninsula. The detachment zone that extends from Punta San Antonio to Isla San Diego (Figure 1a) is identified as the Comondú Detachment.

**Figure 1.** Mexico's Baja California Peninsula and Isla San Diego: (**a**) map showing the boundary between the United States and Mexico as well as the boundary between the northern and southern states of Baja California and Baja California Sur (dashed lines), together with towns on the Baja California Peninsula and key spots or islands including Ángel de la Guarda (AG), Punta San Antonio (PA), Carmen (Ca), Santa Catalina (SC), Cerralvo (Ce) and San Diego (box with asterisk); (**b**) enlarged map of Isla San Diego in the lower Gulf of California, showing the location of Stations 1 to 6 where cobbles and boulders of eroded granodiorite were measured for this study.

Isla San Diego is at the southeast end of the detachment zone 20 km or 11 nautical miles due east of the closest access point on the Baja California Peninsula. The main north–south highway is too distant from the peninsula's eastern shore to make boat access to the island convenient. Compared to other islands farther to the northwest or southeast, the relative isolation of Isla San Diego meant that it received little attention from geographers and geologists. No formal topographic map by the Federal Mexican government exists for Isla San Diego and its dimensions and topography were appraised by satellite imagery [2]. The island is elongated in shape, approximately 1.5 km in length and 0.43 km in width with northeast to southwest orientation (Figure 1b). The maximum elevation is more than 160 m above mean sea level, as attained to the north, but the island's central ridgeline tapers gradually downward to the shore at the southwest end.

The island core is composed entirely of granodiorite and the conglomerate beds eroded from these basement rocks occur exclusively at the southwestern end. A prominent submarine ridge extends from the tip of the island [7], where large boulders of loosely piled granodiorite are close to the surface (Figure 2a). Granodiorite sea cliffs are well exposed along the east shore for more than 400 m from the southwest tip of the island to the northeast, where a series of closely spaced grottos are eroded as a result of spheroidal weathering between joints in the rock (Figure 2b). Additional weathering along both flanks of the island is the result of sheeted exfoliation typical of granitoid rocks.

The greater part of Mexico's Natural Protected Areas (Áreas Naturales Protegida) is taken up by the Gulf of California Biosphere Reserve protecting all islands in the Gulf of California, which accounts for roughly 19% of the nation's total conservation reserves [17]. Therefore, Isla San Diego is protected under conservation guidelines due to its biodiversity and ecological characteristics. All materials and fossils identified in this study were left in place on Isla San Diego.

**Figure 2.** South end of Isla San Diego (lower Gulf of California: (**a**) view showing part of the shallowwater ridge composed of loose cobbles and boulders of eroded granodiorite oriented S 55◦ W off the island; (**b**) southwest end of the island showing small sea caves eroded in granodiorite basement rocks overlain by Pleistocene conglomerate.

#### **3. Materials and Methods**

#### *3.1. Data Collection*

The raw data for this study were collected in March 2021 from deposits composed exclusively of granodiorite cobbles and boulders consolidated by a thin limestone matrix. Individual clasts from six stations were measured manually to the nearest half centimeter in three dimensions perpendicular to one another (long, intermediate, and short axes). Differentiated from cobbles, the base definition for a boulder adapted in this exercise was that of Wentworth [18] for an erosional clast equal or greater than 25.6 cm in diameter. Triangular plots were employed to show variations in clast shape, following the design of Sneed and Folk [19] for river pebbles. In the field, all measured clasts were characterized as subrounded, and a smoothing factor of 20% was applied uniformity to adjust for the estimated volume calculated by the simple multiplication of length from the three axes. Comparative data on maximum cobble and boulder dimensions were fitted to bar graphs to show size variations in the long and intermediate axes from one sample to the next. The rock density from a granodiorite sample yielded a value of 2.52 g/cm3.

#### *3.2. Hydraulic Model*

Granodiorite is the typical intrusive magmatic rock characteristic of several islands in the Gulf of California. Herein, two formulas were applied to estimate the size of storm waves against joint-bound blocks. Equation (1) derives from the work of Nott [20] and Equation (2) is modified from an alternative approach using the velocity equations of Nandasena et al. [21] applied to storm deposits by Pepe et al. [22].

$$Hs = \frac{\left(\frac{\rho\_s - \rho\_w}{\rho\_w}\right)a}{\mathcal{C}\_1} \tag{1}$$

$$Hs = \frac{\frac{2\left(\frac{\rho\_l - \rho\_W}{\rho\_W}\right).c.\left[\cos\Theta + (\mu\_s \cdot \sin\Theta)\right]}{c\_1}}{100} \tag{2}$$

where *Hs* = height of the storm wave at breaking point; *ρ<sup>s</sup>* = density of the boulder (tons/m3 or g/cm3); *ρ<sup>w</sup>* = density of water at 1.02 g/mL; *a* = length of the boulder on long axis in cm; θ is the angle of the bed slope at the pretransport location (1◦ for joint-bounded boulders); μ*<sup>s</sup>* is the coefficient of static friction (=0.7); and *C*<sup>l</sup> is the lift coefficient (=0.178). Equation (1) is more sensitive to the length of a boulder on the long axis, whereas Equation (2) is more sensitive to the length of a boulder on the short axis. Therefore, some differences are expected in the estimates of *HS*.

#### **4. Results**
