*4.7. Implications of Geomorphologic Modeling*

The extent of topographic asymmetry across the outer bulwark of lands fringing Puerto Escondido (Figure 4) invites geomorphologic modeling aimed at accounting for the amount of rock volume lost due to coastal recession. This exercise targets the un-named islet between barriers #1 and #2, where the object's shape is relatively small and simple. Inherent in the model is the assumption that a body of bedrock with uniform composition starts out having more balanced proportions at the commencement of physical erosion. Three stages are depicted graphically in the model (Figure 11). First, the islet's present-day topography is laid out on a regular grid and a diagonal line is drawn such that the asymmetry is segregated to one side (Figure 11a). The area within each successive line of topography is estimated separately and thereafter, volume may be calculated through addition in discrete topographic intervals much like adding layers in a tiered wedding cake. Following this procedure, the bulk volume of the andesite islet is found to be roughly 13.85 million cubic meters. In stage 2 (Figure 11b), those topographic lines with the closest spacing are erased. In stage 3 (Figure 11c), the size of a former islet is reconstructed by redrawing topographic lines as more evenly spaced. Thereafter, the same procedure may be followed to arrive at the bulk volume of the enlarged islet. In this way, the former islet is found to have started with a bulk volume of 19.25 million cubic meters. Subtracting present-day volume from the reconstituted volume, the original islet is argued to have lost 5.4 million cubic meters. That amount is crudely equivalent to 25% of the islet's former volume due to an imbalance of coastal erosion on its exposed seaward flank.

It is essential to point out that the dividing line applied in the model (Figure 11a–c) is not a fault line. However, a hidden fault line now underwater is projected over a distance of 4 km along the outer coast but also notably parallel to the inland valley fault on the west side of Puerto Escondido (Figure 4). Separate calculations on the volume of unconsolidated materials in the two barriers is based on the length of each structure, its average width, and depth. The last is problematic to appraise but, using an assumed value of 5.5 m, can be no greater than the maximum depth of the lagoon behind. Hence, the contents entrained in the longer barrier #1 may amount to 42.5 million cubic meters. The shorter barrier #2 holds no less than 23 million cubic meters of transported pebbles, cobbles, and boulders. In effect, the barriers that insure the sheltered inner harbor at Puerto Escondido might be removed and restored many times over based on the volume of solid bedrock deleted by coastal erosion along an original fault scarp now significantly recessed.

**Figure 11.** Progression of stages in geomorphological modeling with respect to the un-named islet between barriers # and #2; (**a**) Topography of present day islet with dividing line showing disparity between the gentler lagoon side and exposed cliff face on the open sea; (**b**) Erasure of the outer cliff face; (**c**) Restoration of past topographic gradient on the seaward face showing a better match with the sheltered side.

#### **5. Discussion**

#### *5.1. Time Constraints on Barrier Origin*

Relationships drawn from paleontological and geological evidence place time limitations on the origin of the two barriers crucial to the maintenance of shelter at Puerto Escondido. Upper Pleistocene deposits denoted by a shell drape over a sizable area in the hills west of the inner harbor (Figure 4) confirm that normal seawater circulated to that spot approximately 125,000 years ago during the last interglacial epoch. Fossil mollusks including species such as the dominant *Chione californiensis* and less abundant *Turbo fluctuosus* that still live in the Gulf of California today [18], are reliably taken as evidence for intertidal conditions at that locality. Sea level stood approximately 6 m higher compared to now [16,17], but the present elevation of the shell drape also reflects additional tectonic uplift. Higher global sea level probably enhanced marine circulation at this spot. Moreover, emplacement of the Upper Pleistocene deposit must have occurred prior to development of barriers #1 and #2. The fault traced through the narrow valley from Bahía Juncalito indicates that local uplift boosted the shell drape to a higher elevation than typically found in coeval shell drapes on 12-m terraces many places elsewhere along peninsular gulf shores [15]. In fact, the nearest expression of marine terraces cut in andesite bedrock occurs within sight of Puerto Escondido only 7 km across the Carmen Passage at the southern end of Isla del Carmen (Figure 12).

No traces of marine terraces are found along the outer shores at Puerto Escondido facing Isla del Carmen. Given the amount of coastal retreat implicated by the geomorphological model, such terraces may have existed but were entirely erased by coastal erosion. If true, the earliest development of barriers #1 and #2 occurred after the end of the Pleistocene in Holocene time. However, the physical juxtaposition on opposite sides of the Carmen Passage raises the question why marine terraces survived on neighboring Isla del Carmen but not the outer coast at Puerto Escondido? The answer likely lies in the principle sources of coastal erosion still taking place in the Gulf of California, today.

**Figure 12.** View east from Puerto Escondido across the Carmen Passage showing distinct marine terraces cut in the southwest side of Isla del Carmen. Motor boat and wake for scale at left, center.

#### *5.2. Energy Sources A*ff*ecting Barrier Development*

As discussed in our earlier work [1,2], the potential range of dynamic influences capable of shore erosion in the Gulf of California includes tidal action, long-shore currents related to strong seasonal winds, alleged tsunamis, and hurricanes. Tidal influence is especially strong in the far northern part of the gulf, where maximum amplitudes of 12 m are recorded. The tidal range around the central gulf is far less, approximately 2.75 m [19]. Tides of this magnitude transport coarse sand, but have little or no effect on rocky shores. South-directed sea swells with an amplitude of 2 m and wavelength of 10 m are not unusual during episodes of strong winds in the Carmen Passage that play out episodically between November and May [6,7]. Such prevailing winter winds stimulate long-shore currents that flow parallel to the gulf shores, or otherwise result in wave refraction around obstructing islands or headlands [20]. The energy generated by such currents is capable of moving pebbles and smaller cobbles.

Sea storms of lesser intensity are expected to shift sand, pebbles, and even cobbles entrained in a natural barrier. In part, the overall decrease in boulder size from north to south from transect 1a to 1c and from transect 2a to 2b is related to littoral drift especially during the winter season when a strong north-south wind is common. It can be argued, however, that only those episodic storms of hurricane intensity generate sufficient energy to shift large boulders close to a metric ton in weight. During the Pacific Ocean hurricane season between the months of May and November, between 25 and 30 tropical depression originate off the southwest coast of mainland Mexico [8], but few diverge from an outward path to enter the Gulf of California. The incidence of hurricane activity in the gulf region increases every 6 to 8 years during El Niño events. Hurricane Odile in September 2014, for example, was filmed in action as it pounded the rocky coast near the Almeja CBD north of Loreto with waves that impacted sea cliffs at a height 8 m above normal [2]. Clocking wind speeds of 113 km/hr by the time it reached that far into the Gulf of California, wind bands rotating counter clockwise were strong enough to generate wave surge that lashed the coast initially from east to west. As the storm migrated northward, wind direction and wave surge shifted more to a direction from northeast to southwest. Such a pattern fits the predicted scenario of rocky-shore erosion and transfer of large boulders to the barrier seawalls at Puerto Escondido. In particular, oblong blocks of andesite already fallen from the unstable sea cliffs at Cerro El Chino and the un-named islet between barriers #1 and #2 would be pushed southward and

eventually entrained in those barriers (Figures 9 and 10). Marine terraces on SW Carmen (Figure 12) would be sheltered from west-moving storm bands, and therefore avoid excessive erosion.

In theory, a large tsunami with a run-up of several meters would be energetic enough to breach the barriers protecting Puerto Escondido. In addition, the return outwash of coastal sediments dislodged during a tsunami should be transported seaward. However, the probability that a tsunami struck anywhere within the Gulf of California during the Holocene is nil, even though a recent interpretation of sedimentary deposits by McCloskey et al. (2015) for the lower Gulf of California [21] suggested a geological framework and seismic mechanisms for its interpretation. In the Alfonso Basin off La Paz, Gorsline et al. (2000) described only minor discharges from tributary coastal canyons that carried a high proportion of coarse-grained sand trapped on marine shelves, but blocked from supplying turbidites at a volume of a basin-wide magnitude [22]. Hence, those sediments reaching the basin floor probably were produced by seismically generated slope failures of silty clay deposits. The distribution of the dated turbidites and a slip face in a box core from the landward slope, indicate a source on the landward depositional slope of the fault-bounded basin. Comparable discontinuities of the same age also are reported from the east side of the gulf in the Guaymas area farther north [22].

Most earthquakes in the lower Gulf of California are generated by transform faults [23]. Fletcher and Mungia (2000) indicate that such a level of seismicity falls along different strands in a major system of normal faults extending at least 300 km along strike to define the western limit of the Gulf Extensional Province [24]. The dominant normal faults controlled distribution of Neogene basins active during middle to late Miocene times. Structural analysis of secondary faults in the southern gulf segment reveals that fault populations are Pliocene to Holocene in age and represented by mixed normal and dextral-normal faults with a bulk extension direction of west-northwest–east-southeast [25]. From the perspective of regional tectonics summarized above, it is evident that a major earthquake is not responsible for the presence of turbidites within the Alfonso basin at least during recent times.

Based on the experience of the junior author (J.L.-V.), the extraneous evidence shown by McCloskey et al. (2015, their Figure 5c) relates to a kitchen midden and not a tsunami deposit. Tsunami events are well documented outside the Gulf of California far to the south on the Mexican mainland at Jalisco [26], but such events result from deep-seated earthquakes (magnitude 7.7 or greater) associated with an active subduction zone where the Rivera lithospheric plate meets the continental mainland.

#### *5.3. Comparisons with Other Coastal Boulder Deposits*

Our previous contributions on Holocene boulder accumulations within Mexico's Gulf of California conform to the normal definition of coastal boulder deposits, where CBDs occur either at the top of sea cliffs or next to sea cliffs with well-developed bedrock stratification and jointing that corresponds to the dimensions of rocks loosened by wave impact. Metric-ton blocks of Pliocene limestone sit atop 12 m high cliffs on Isla del Carmen (Figure 1b, locality 2) from which they were peeled away [2]. Similar-size rhyolite boulders at Ensenada Almeja (Figure 1b, locality 3) occur at sea level adjacent to the sea cliffs from which they were extracted [3]. Elsewhere, major CBDs occur atop sea cliffs in northern France [27] and western Ireland [28,29]. Mega-boulders left high above sea level but close to the parent bed rock from which they were eroded also are known from the Bahamas and Bermuda [30,31]. It is debated to what extent super-waves and hurricanes are responsible for these CBDs, fueling ongoing controversy over the growing threat of global warming. In the case of giant blocks derived from low limestone cliffs on Calicoan Island in the Philippines [32], the run-up of waves exceeding 15 m inland is linked directly to the impact of Super Typhoon Haiyan in November 2013. A related but somewhat different phenomenon concerns the detachment of large boulders from the seafloor and onshore transferal to low rocky shores. The recent study by Biolchi et al. (2019) fits this category in relation to movement of limestone boulders in the northern Adriatic Sea onto the Premantura (Kamenjak) Promontory in Croatia [33]. In this case, however, the coastline is formed by a low-angle rocky shore that is more like a ramp in configuration than a sea cliff.

The barriers that close off the inner harbor at Puerto Escondido are natural breakwaters formed by a mixture of pebbles, cobbles, and boulders that trace back to single sources of bed rock in exposed sea cliffs. Those sea cliffs are steeply inclined (50◦ to 55◦) and are unstable. Andesite layers within the bedrock are tilted at a high angle dipping westward and rock falls leave fresh material at the base of the cliffs. Essentially, a combination of long-shore currents and storm waves harvest the materials and carry them southward where they are entrained in the linear barriers. A more appropriate term for this kind of feature is a barrier boulder deposit (BBD). This term is a good fit with the many bars formed by andesite cobbles and boulders that close off lagoons on Isla Angel de la Guarda in the upper Gulf of California—some of which extend for as much as 1.25 km [34]. The aerial photo from Johnson et al. (2019, their Figure 10) illustrates two such barriers and others in the process of extension from the nearest bedrock source [2]. Granite is another major rock type from which rocky shores are formed in the Gulf of California. Eroded granite boulders at Bahía San Antonio (Figure 1, locality 4) are encrusted by Upper Pleistocene fossils representing an intertidal biota preserved in growth position [35]. Close to granite bedrock at Punta San Antonio, the scenario corresponds well with the BBD concept. The potential for development of a BBD appears to be less dependent on rock type than climatic patterns that bring longshore currents and wave impact from a recurrent and propitious direction.
