David H. Backus

Sequestration of Carbonate Shell Material in Coastal Dunes on the Gulf of California (Baja California Sur, Mexico)

Abstract Atmospheric and oceanographic conditions stimulate high productivity of marine organisms in the Gulf of California that are bulk producers of calcium carbonate. Due to prevailing winter winds, north-facing beaches receive vast amounts of shell debris derived from offshore clam banks above the 50-m isobath. As shells undergo mechanical abrasion, the smaller particles of carbonate material are transferred from beaches by the wind and sequestered in coastal dunes. This study reports on two dune fields from the midriff zone of the Gulf. Sieve analysis is used to describe the grain size and sorting characteristics of seven samples from dunes at Cerro El Gallo near Mulegé and 12 samples from dunes farther south near San Nicolás. Dune sediments also were impregnated with epoxy to simulate rock samples from which thin sections were made to determine composition and relative abundances of constituent grains. The dunes at Cerro El Gallo are carbonate poor (5–15%) compared with those near San Nicolás (26–51%) and possible factors contributing to regional variation are explored. Another part of this study appraises the fecundity necessary to produce any carbonate fraction integrated by coastal dunes from seashells. One of the regions more abundant clams, Megapitaria squalida, was used as a model to estimate the number of individuals of a given size and age class required to generate a cubic meter of pure carbonate sand. Satellite images of the dune fields at El Gallo and San Nicolás were viewed to map surface coverage. With input on the thickness of dune deposits and their composition, it is possible to roughly estimate the number of mature Megapitaria equal to the carbonate fraction of these dunes. The method may be applied to all coastal dunes on the Gulf of California and could be used to assess a part of the regions overall carbon budget heretofore unappreciated.

Distribution, Sediment Source, and Coastal Erosion of Fan-Delta Systems on Isla Cerralvo (Lower Gulf of California, Mexico)

Abstract BACKUS, D.H.; JOHNSON, M.E., and RIOSMENA-RODRÍGUEZ, R., 2012. Distribution, sediment source, and coastal erosion of fan-delta systems on Isla Cerralvo (lower Gulf of California, Mexico). Located near the tip of the Baja California peninsula, Isla Cerralvo is the sixth largest island in the Gulf of California. Although surrounded by some of the most productive waters in the world, field surveys show that Isla Cerralvos shelf is largely devoid of biogenic carbonates, especially rhodolith beds, which are found in abundance elsewhere within the region. In counterpoint, a series of prominent fan deltas extend from the mouths of arroyos on Isla Cerralvo, despite the fact that the island has a granitic core, suggesting that the islands bedrock is severely weakened. Field observations suggest that the fracture pattern (submeter), hydrothermal alteration, as well as the orientations of metamorphic foliation, fracture sets, and fault planes all play a role in the accelerated rate of erosion on the island. The role of hydrothermal alteration is illustrated by a principal components analysis of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Short Wave Infrared (SWIR) bands, which links heavily eroded areas at the south end of the island to areas with high concentrations of clays. A synthetic drainage system created using a 30-m-resolution digital elevation model (DEM) generated from an ASTER image was used to model the Isla Cerralvo drainage basins and stream channel networks. Analyses of basin and stream network information, including basin slope values and channel slope values, were used to identify islandwide differences in basin morphology and erosion characteristics. Stream channel profiles and slope-area data supported by limited uplift data indicate that Isla Cerralvo has not been uplifted as a single block, but it is broken into at least two major structural blocks with different uplift histories. Due to the arid climate and low annual precipitation, we find that sediment removal from the interior of Isla Cerralvo can only be accomplished by episodic, but very short (hours to days), catastrophic rainfall events caused by hurricanes or chubascos (winter storms). Subsequently, the sediment is eroded from fan deltas and transported southward by longshore currents and wind-generated waves, choking carbonate production along Isla Cerralvos shore and shelf.

Tectonic Decapitation of a Pliocene Mega-Delta on Isla Del Carmen in the Gulf of California (Mexico): And a River Ran through It

The Tiombó conglomerate on Isla del Carmen in Baja California Sur represents a mega-delta deposited mainly during the middle Pliocene (early Piacenzian age). A cross-section with the characteristically arched profile of a fan delta is exposed in sea cliffs 60 m high that extend for 2 km along the east side of the fourth-largest island in Mexico’s Gulf of California. A tidewater aspect to the delta is proven by the inclusion of fossil-rich lenses dominated by the pecten Patinopecten bakeri, which also occurs abundantly together with many other middle Pliocene fossils in the Arroyo Blanco limestone juxtaposed against the south flank of the delta construction. Inland, the conglomerate fills a broad, cross-island channel approximately 3 km2 in area that ends with cliff exposures of a lesser thickness overlooking the Carmen Passage between Isla del Carmen and the peninsular mainland of Baja California. Based on exposures that fail to reveal the full basal contact, the combined fluvial and deltaic parts of the system are conservatively estimated to exceed 200 million m3. This study finds that the Pliocene Tiombó Delta is massively too large for the island it now occupies and that the related distributary channels in the delta’s original watershed extended to the peninsular mainland and the Sierra de la Giganta in the Loreto area. The direct implication is that the Pliocene delta suffered tectonic decapitation when a reactivated halfgraben subsided and that the Carmen Passage flooded only in later Pliocene time. A useful comparison in scale is provided through examination of the modern delta at Loreto, together with its associated watershed on the peninsular mainland. Long-term changes in El Niño Southern Oscillation cycles are appraised as an influence on regional Pliocene delta construction at several other localities along the peninsular gulf coast of Baja California.

Pliocene Stratigraphy at Paredones Blancos: Significance of a Massive Crushed-Rhodolith Deposit on Isla Cerralvo, Baja California Sur (Mexico)

Abstract EMHOFF, K.F.; JOHNSON, M.E.; BACKUS, D.H., and LEDESMA-VÁZQUEZ, J., 2012. Pliocene stratigraphy at Paredones Blancos: Significance of a massive crushed-rhodolith deposit on Isla Cerralvo, Baja California Sur (Mexico). A white blaze across coastal cliffs is the hallmark of Paredones Blancos on Isla Cerralvo, Mexico. Cliff-forming strata include three roughly 10-m-thick units with a lateral coherence of only 0.75 km. The middle unit is a massive deposit of crushed rhodoliths. The other two units consist of matrix-supported conglomerate with cobbles and boulders of granodiorite, basaltic andesite, and hornblende diorite. By volume, the matrix accounts for >80% of those units, mostly composed of grus from weathered granite. Thin sections were studied from samples collected at five levels through the middle rhodolith unit to determine carbonate purity as a ratio between organic CaCO3 and inorganic minerals. The bottom and top margins of the deposit show higher levels of mixed clastics, with 37% inorganic mineral content, as compared with 11% toward the middle. Original depositional environments are interpreted as a rhodolith bank adjacent to a large fan delta built seaward from a wide canyon mouth. The stratigraphic sequence records a rise in sea level that brought rhodolith debris to the flooded canyon mouth above the basal conglomerate, and a drop that emplaced another conglomerate above the rhodolith deposit. A Middle Pliocene age is based on co-occurrence of Clypeaster bowersi and Argopecten revellei within the carbonates.

Stromatolitic mats from an uplifted Pleistocene lagoon at Punta Chivato on the Gulf of California (Mexico)

ABSTRACT Crusts identified as a stromatolitic mat are described for the first time from a small, tectonically uplifted basin associated with a marine terrace along the central gulf coast of the Baja California peninsula at Punta Chivato. Microscopic analyses of crustal laminae exhibit calcified features directly comparable to microbial processes that precipitate carbonates in modern stromatolites. Floating calcified mats recently described from extremely hypersaline, closed lagoon systems in the northern Gulf of California (Isla Angel de la Guarda) provide the best analog to the Punta Chivato locality. Preservation of the Punta Chivato stromatolites is likely due to the persistence of arid conditions through multiple glacial-interglacial cycles. Comparison with the marine terrace sequence at Punta Chivato implies an age range of 334 ka to 712 ka for the deposit. This study lends support for a dedicated survey of closed lagoons and terraces where microbialites and their fossil stromatolite representatives may be found along the roughly 3000-km length of peninsular gulf shores and related islands.