**2. Geographical and Geological Setting**

The Gulf of California is a marginal sea between the Mexican mainland and the Baja California peninsula with a NW-SE axis 1100 km long and a 180-km wide opening to the Pacific Ocean at its southern end (Figure 1a). The sea's mean annual sea-surface temperature (SST) is 24◦ which is higher than the norm of 18◦ in the adjacent ocean and the mean average rainfall on the peninsula amounts to only 15.3 cm [5]. During relatively infrequent impact by hurricanes, conditions are dramatically altered especially in terms of heavy rainfall over desert terrain lacking thick plant cover. Tropical storms that immerge in the East Pacific Ocean typically form off the coast of Acapulco below 15◦ N latitude and track northward, turning outward to the west before reaching the southern tip of the Baja California peninsula at 23◦ N latitude. Storms that stray across the Baja California peninsula into the Gulf of California are called *chubascos*, only some of which amount to disturbances of hurricane strength. Other influences that contribute to the aggradation of coastal sediments include daily tides and seasonal winds that intermittently funnel down the axis of the gulf from the northwest with an average azimuth from N to S from December to March. This is shown by a tightly constrained rose diagram (Backus and Johnson, 2009, their figure 10.1 D), based on the orientation of structures in 84 coastal sand dunes throughout the region [6]. Strong winds capable of generating sea swells given sufficient fetch are known to blow persistently for days at a time, with gusts between 8 and 10 m/s not uncommon [7,8].

**Figure 1.** Locality maps showing Mexico's Baja California peninsula and Gulf of California; (**a**) Mexico and border area with the United Sates, denoting key villages or cities with dots and the study region marked by a square; (**b**) Region around Loreto in Baja California Sur, marking coastal boulder beds (\*) at Puerto Escondido (PS), Isla Coronados, Ensenada Almeja near the Boca San Junico and Punta San Antonio (PSA); (**c**) Study site at Ensenada Almeja.

The geology of the San Basilio area 30 km north of Loreto (Figure 1b) incorporates the study site around Ensenada Almeja (Figure 1c), which features a landscape dominated by igneous rocks including banded rhyolite, hyaloclastite, massive rhyolite and volcanic ash [9]. Fossil-bearing limestone beds deposited around and above massive rhyolite domes are consistent with an assignment to the Zanclean Stage within the Lower Pliocene. Pleistocene deposits of consolidated dune limestone are seated above an eroded rhyolite shelf on the east side of Ensenada Almeja and extend inland through a north-south valley connected to Ensenada San Basilio. Mapping of fault lines throughout the region suggests that the low ground occupied by the Almeja CBB is separated from an adjacent rhyolite dome by a normal fault (Figure 2) parallel to a well-defined fault crossing through part of the headland to the east with a trend of N28◦E to S28◦W [8].
