Geomorphology of a Holocene Hurricane Deposit Eroded from Rhyolite Sea Cliffs on Ensenada Almeja (Baja California Sur, Mexico)

Abstract

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.

Holocene Hurricane Deposits Eroded as Coastal Barriers from Andesite Sea Cliffs at Puerto Escondido (Baja California Sur, Mexico)

“…The laboratory result was subsequently applied uniformly to all further calculations using the formula cited in the methods section, above. For comparison, the specific gravity of limestone from our earlier study at nearby Isla del Carmen (Figure 1b, locality 2) was determined to be 1.86 [2] and the banded rhyolite from our study at Ensenada Almeja (Figure 1b, locality 3) was determined to be 2.16 [3].…”

“…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].…”

“…Previous work focused on a coastal boulder deposit (CBD) with metric-ton blocks of Pliocene limestone torn from the outer margin of a 12-m marine terrace on Isla del Carmen in the Gulf of California [2]. A subsequent study examined similar-size boulders pried from a rhyolite coast not far to the north at Ensenada Almeja [3]. Limestone rocky shores amount to only 7.5% of shores in the western Gulf of California and rhyolite is so uncommon, it was not part of our original satellite reconnaissance [1].…”

Coastal Boulder Deposits of the Neogene World: A Synopsis

“…Consideration of the context of its usage in the journal articles permits outlining some characteristics of such a sedimentary formation. These include, particularly, accumulation of boulders distinguished by their large size and/or huge weight [9,10,17,18,[25][26][27], angularity with certain roundness [8], high-topography and inland occurrence [8,9,25,27], high-energy coastlines [5,8,15,17], and relevance to storm and tsunami activity [9,10,18,26,27]. It is notable that the previous works focused more on boulders individually rather than on entire deposits.…”

“…On the other hand, boulders are regarded as precious evidence of present and ancient rocky shore facies and extreme events (such as storms, tsunamis, hurricanes, typhoons, and cyclones). This evidence has been examined by Autret et al [5], Bhatt et al [6], Biolchi et al [7], Cox et al [8][9][10], Dawson [11], Engel et al [12], Erdmann et al [13], Hearty and Tormey [14], Herterich et al [15], Hongo et al [16], Johnson et al [17,18], Kennedy et al [19], Kortekaas and Dawson [20], Lau et al [21], Olsen et al [22], Paris et al [23], Pepe et al [24], Scheffers et al [25], Schneider et al [26], Shah-Hosseini et al [27], Suanez et al [28], Terry and Goff [29], Terry et al [30], Trenhaile [31], Watanabe et al [32], and Weiss and Sheremet [33]. Most probably, the devastating catastrophes like the 2004 Indian Ocean tsunami [34] and the 2011 Tohoku tsunami [35] have fueled the interest of researchers in coastal sedimentology and, particularly, large clasts [36].…”