Arturo Carranza-Edwards

Sedimentary record of late-Holocene relative sea-level change and tectonic deformation from the Guerrero Seismic Gap, Mexican Pacific Coast

Studies of the coastal sedimentary record have allowed both the reconstruction of relative sea-level changes and the determination of local rates and magnitudes of tectonic deformation, particularly in tectonically active areas. Despite their successful use elsewhere, studies of this type are much less common for the Mexican Pacific coast, which parallels the Cocos-North America subduction plate boundary. Stratigraphic, geochemical and microfossil data from sediments in Laguna Mitla, the Pacific coast of Guerrero, Mexico, document late-Holocene sea-level changes induced by tectonic activity in the Mexican subduction zone. Three major events are identified. First, the formation of the lagoon by c. 4630 yr BP, as indicated by a freshwater to brackish peat. Second, a relative sea-level rise, or land subsidence, as indicated by a shift from a freshwater marginal lagoon environment to a marine setting, preceded by a marine inundation represented by a sand unit (possibly a tsunami deposit), by c. 3400 yr BP. And finally, a return to lagoonal conditions indicating a drop in relative sea level or coastal uplift by c. 2300 yr BP. The Laguna Mitla stratigraphy indicates general coastal subsidence or relative sea level rise of c. 1 mm/yr. We argue that these relative sea-level (land-level) changes have been induced by tectonic activity associated with the Mexican megathrust. A plausible explanation for the 3400 yr BP marine inundation is probably a tsunami produced by a large seismic event accompanied by coastal subsidence. Discrete fining upward, fine to coarse, sand units with an erosional basal contact, medium to poor sorting, and clay/mud rip-up clasts; an increase in Na and Sr elemental concentrations, indicative of a marine origin; and the landward extent of the sands support a tsunamigenic source for these deposits. However, these apparent tsunami deposits require further study to determine their lateral extent and to assess whether they can be correlated from one site to another. This study demonstrates the applicability of a multiproxy sedimentary approach in interpreting relative sea-level (land-level) changes and to derive data on related earthquake and tsunami events in tropical coastal lagoons.

Beach sands from Baja California Peninsula, Mexico

Abstract Fifty beach locations were sampled in Baja California Peninsula, Mexico, in order to characterize textural and compositional parameters. The western beach sands are mainly associated with a lower relief coastal plain and high energy of waves and currents with the beach sands of the eastern littoral zone. Finer, better sorted and low carbonate and rich quartz and feldspar contents are observed for the western beach sands when compared to the eastern beach sands. The mineralogical maturity and provenance index are greater for the western beach sands than for the eastern beach sands. These contrasts may be explained by differences on coastal plain relief and differences on hydrodynamic energy of waves and currents that are responsible for the rock fragment dilution by enrichment of more stable quartz debris. Finally, some distinctions were found for Na2O–K2O–CaO values. This is thought to be a result of the presence of some samples from the eastern coastline with higher values in CaO content, probably due to the presence of basaltic rocks.

Petrography and Geochemistry of Holocene Sands in the Western Gulf of Mexico: Implications for Provenance and Tectonic Setting

ABSTRACT Petrographic and geochemical analysis of recent fluvial, beach, and dune sediments derived from the same source terrane was carried out in the western Gulf of Mexico to show the usefulness of the these three environments in determining the tectonic setting of the source. Petrographic analysis showed that dunes concentrate the less heavy minerals and quartz grains by means of the selectiveness of the wind as a transport agent. In contrast, the heavier grains, such as the rock fragments remain in the beach and fluvial environments because of the high transport energy. Beach and dune sands do not define a particular tectonic setting because of the selective wind action of the backshore of the beach and the dune environments and the less intense chemical weathering. The fluvial sands are the most representative in terms of interpretation of tectonic setting because they reflect the volcanic domain of the source rocks of the Trans-Mexican Volcanic Belt. The geochemical results of all three sedimentary environments reflect the calcalkaline character of the igneous source rocks of the Trans-Mexican Volcanic Belt. The samples fall in a continental island-arc-margin field with basaltic and andesitic source rocks. A correlation and factor analysis indicates an alkali-basalt source that contributes to the composition of the beach, dune, and fluvial sediments. The geochemical analysis of the sediments more accurately reflects the tectonic setting regardless of the depositional setting of the sediments.

Petrography of quartz grains in beach and dune sands of Northland, North Island, New Zealand

Abstract Petrographic and scanning electron microscopy (SEM) analyses of quartz grains from beach and dune sands were carried out in the western and eastern Northland coasts, New Zealand, to examine variations in durability and surface texture, which are controlled by mechanical and chemical processes, in profiles across beach and dune environments. This was done through point counts of quartz grain properties based on extinction angle and crystallinity. Variations in surface texture were assessed through SEM observations of mechanical features (conchoidal fractures, smooth surfaces, groove forms) and chemical features (solution pits, etching, silica deposits). Mechanically produced grooves are associated with beach sands affected by the high energy of the surf zone. Both mechanical and chemical processes occur in the eastern dune sands. They are associated with the greater abundance of angular grains in the eastern dune sands than the western dune sands. In addition, conchoidal fractures produced by the collision of grains in aeolian environments and linear and curved grooves produced by quartz grains from the beach support the mechanical processes taking place in the dunes. Solution pits, etching, and the presence of diatoms in the quartz grains are associated with pedogenesis and high silica precipitation in the eastern beach and dune sands. The durability of coarse‐grained polycrystalline quartz relative to fine‐grained polycrystalline quartz suggests that chemical abrasion exerts control over the distribution of quartz types in the dune sands.

A reconnaissance study of carbonates in Mexican beach sands

Abstract Carbonate concentrations were measured on sand samples from the beach face of 274 Mexican beach locations. Tectonic processes apparently have an influence on the carbonate contents in the sands. Samples from the passive margin of the Gulf of Mexico have higher values, with an average of 40.52% carbonate, while the more active margin of the Pacific with higher relief, shows an average of 8.86% of carbonated sands. In general, high amounts of carbonates apparently are related to areas of low energy, dry climate and low fluvial sediment transport. The main carbonate components are fragments of pelecypods, gastropods, corals and calcareous algae.

Heavy Minerals and Rare Earth Elements in Coastal and Inland Dune Sands of El Vizcaino Desert, Baja California Peninsula, Mexico

A heavy mineral (HM) study and light and heavy rare earth elements (LREE and HREE) analysis were performed in coastal and inland dune sands, El Vizcaino Desert, Central Baja California Peninsula, Mexico. Our study shows high abundances of hornblende and apatite in the El Vizcaino dune sands, suggesting a dominance of granodiorites/intermediate plutonic rocks and marine authigenic phosphorite in the dune sands. There is a relationship between unstable heavy minerals like hornblende, pyroxenes, and sphene, and heavy rare earth elements (HREE) that suggests that unstable heavy minerals are potential carriers of HREE in the dune fields. However, there is a slight depletion of HREE in relation to LREE, especially in one locality of the inland dunes probably associated with the wind regime and weathering of unstable heavy minerals in the sands. Inland, transitional, and coastal dune fields can be observed as different dune provinces by means of grouping HM and REE data in two separate dendograms. It seems that HREE are correlated with fine-grained sand sizes and correlated with high CIA values linked to slightly weathered sands.

Unearthing earthquakes and their tsunamis using multiple proxies: the 22 June 1932 event and a probable fourteenth-century predecessor on the Pacific coast of Mexico

Tsunami deposits have been widely studied in temperate latitudes, but the intrinsic difficulties associated with tropical coastal environments, and the intensity of bioturbation in these habitats, limit the possibilities of analysing these formations. Here, we investigate the deposits on the Colima coast of Mexico, which overlies the subducting Rivera and Cocos Plates, in order to reconstruct the tsunami inundation history and related hazard. We developed a multi-proxy study aimed to recognize and date historical and palaeotsunami deposits, including historical data on the effects of a known tsunami, geomorphological mapping, stratigraphic, grain size, organic matter content, diatoms, geochemical composition, magnetic susceptibility, and anisotropy of magnetic susceptibility, together with radiometric dating (210Pb and 14C). We identified two probable tsunami deposits at Palo Verde estuary including a historical event associated with the Mw 6.9 earthquake on 22 June 1932 and a palaeotsunami most likely generated by a similar event in the fourteenth century. This work shows that it is possible to identify both historical and palaeotsunamis in the tropical environment of Mexico’s Pacific coast. These data will serve to enhance our understanding of tsunami deposits in tropical environments and of the regional tsunami hazard.

Heavy metal enrichment in surface sediments from the SW Gulf of Mexico

The south west coastal zone in the Gulf of Mexico is an area with great industrial and agricultural development, which experiences intensive prospecting and extraction of hydrocarbons. After running through industrial, agricultural, and urban areas, waters from both the Jamapa River and La Antigua River arrive here. The rivers’ discharge areas of influence were estimated considering the textural and chemical composition of the supplied sediments. The main factors that determine sediment distribution were mineralogy, heavy minerals, carbonates, and anthropic contributions. The presence of metals in excess was evaluated using various pollution indicators, such as the enrichment factor, contamination factor, modified contamination factor, and geo-accumulation indexes. Data from different used contamination indexes show metal enrichments in As, Cu, Zn, Co, Cr, and V in La Antigua; As, Cu, and Cr in Jamapa; and As, Zn, and Pb in the Continental slope area. The adverse effects of metals on aquatic organisms were assessed using sediment quality guidelines that show Ni, As, Cu, and Cr may produce adverse effects on coastal areas. There was no evidence of contamination associated to the oil industry.

Geochemical study of hydrothermal core sediments and rocks from the Guaymas Basin, Gulf of California

Abstract A geochemical study of core sediments from the Guaymas Basin was undertaken by X-ray analysis and showed minerals such as barite, gypsum, wurtzite, chalcopyrite, marcasite, galena and calcite. The random relation between metal concentration and core depth apparently reflects the chages in hydrothermal activity. Mineral grains with holes and cavities were observed by SEM photomicrography. These holes are probably due to degastification of the samples, which can be related to the presence of the Guaymas Basin is due to active extensional tectonics, the associated seismic activity in the Southern Rift Valley and the high rate of sedimentation.

Distribution of Dissolved Trace Metals Around the Sacrificos Coral Reef Island, in the Southwestern Gulf of Mexico

A reef system in the southwestern Gulf of Mexico is affected by anthropogenic activities, sourced by urban, fluvial, and sewage waters. Dissolved metals have higher concentrations during the rainy season. V and Pb, were derived from an industrial source and transported to the study area by rain water. On the other hand, Jamapa River is the main source for Cu and Ni, which carries dissolved elements from adjacent volcanic rocks. Principal Component Analysis shows a common source for dissolved nitrogen, phosphates, TOC, and suspended matters probably derived from a sewage treatment plant, which is situated near to the study area.