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

Abstract

Previous studies on the role of hurricanes in Mexico’s Gulf of California examined coastal boulder deposits (CBDs) eroded from limestone and rhyolite sea cliffs. Sedimentary and volcanic in origin, these lithotypes are less extensively expressed as rocky shores than others in the overall distribution of gulf shores. Andesite that accumulated as serial volcanic flows during the Miocene constitutes by far the region’s most pervasive rocky shores. Here, we define a subgroup of structures called barrier boulder deposits (BBDs) that close off lagoons as a result of lateral transport from adjacent rocky shores subject to recurrent storm erosion. Hidden Harbor (Puerto Escondido) is the most famous natural harbor in all of Baja California. Accessed from a single narrow entrance, it is commodious in size (2.3 km2) and fully sheltered by outer andesite hills linked by two natural barriers. The average weight of embedded boulders in a succession of six samples tallied over a combined distance of 710 m ranges between 74 and 197 kg calculated on the basis of boulder volume and the specific gravity of andesite. A mathematical formula is utilized to estimate the wave height necessary to transport large boulders from their source. Average wave height interpreted by this method varies between 4.1 and 4.6 m. Input from fossil deposits and physical geology related to fault trends is applied to reconstruct coastal evolution from a more open coastal scenario during the Late Pleistocene 125,000 years ago to lagoon closure in Holocene time.

Upper Pleistocene and Holocene Storm Deposits Eroded from the Granodiorite Coast on Isla San Diego (Baja California Sur, Mexico)

“…The threat of hurricanes affecting the lower Gulf of California is reviewed in prior studies on storm deposits at Ensenada Almeja north of Loreto [4], Arroyo Blanco on Isla del Carmen west of Loreto [3] Puerto Escondido south of Loreto [5], and especially on Isla Cerralvo east of La Paz [23] (see Figure 1a for geographic relationships). Between September 1996 and September 2019, six named hurricanes (Fausto, Marty, Ignacio, John, Odile, and Lorena) entered the Gulf of California after originating farther south off the mainland coast of Mexico around Acapulco (Figure 1a).…”

“…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][4][5][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.…”

“…Attention to the phenomenon of coastal mega boulders and their relationship to major storms or tsunami events is a topic of growing interest [8][9][10][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][4][5][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].…”

Holocene Boulder Beach Eroded from Chromite and Dunite Sea Cliffs at Støypet on Leka Island (Northern Norway)

“…Contrast with Coastal Deposits Elsewhere Holocene deposits formed by unconsolidated cobbles and boulders are widely distributed all around the world, but studies in coastal geomorphology seldom take into account rock density as related to variability in parent rock types when investigating the range of wave heights necessary for their development. Equation (1) as derived from Nott [17] has been applied to coastal boulder deposits throughout Mexico's Gulf of California, including those formed by limestone, rhyolite, and andesite clasts [6][7][8]. Extension of this work to include Equation (2) as influenced by Pepe et al [19] also has been applied to coastal basalt deposits in the Azores [9].…”

“…The implication is that wave heights affecting coastal surge were sufficiently powerful to remove joint-bound blocks from sea cliffs that resulted in unusually dense boulders. The same mathematical equations for estimation of wave heights as applied previously to other shores with boulders derived from more common source rocks of lesser mass, including limestone, rhyolite, andesite, and basalt [6][7][8][9], are newly applied at Støypet. In addition, the present-day steering winds and wave dynamics characteristic of Norway's Arctic Circle region [10] are reviewed in the context of prominent storms during recent decades in order to appraise the likely direction of wave impact responsible for the boulder beach at Støypet.…”