Modern sands of the Gulf of Mexico: Discriminating fluvial and coastal sand composition

 A forward stepwise linear discriminant analysis, A-Nova test and R-mode factor analysis were used on modern beach, dune and river sands to discriminate the geographic distribution of detrital modes between sands sourced by volcanic rocks (Trans-Mexican Volcanic Belt (TMVB) sands) and sedimentary rocks (Veracruz Embayment (VE) sands). Thirty-three beach, dune and river sand samples were collected across beach-dune profiles, in the inner reaches of the channel and at the river mouth during the dry season. The source terranes for these sands are constituted by Miocene to Pliocene volcanites and by Tertiary to Quaternary siliciclastic rocks. To avoid the constrained character of compositional data, point-count percentages were expressed as compositional modal Log ratios (i.e., Log Qt/Ft = total quartz/total feldspars, Log Qt/Lt = total quartz/total lithics, Log Ft/Lt = total feldspars/total lithics, Log Qm/Fk = monocrystalline quartz/potash feldspars, Log Qm/Fp = monocrystalline quartz/ plagioclase, Log Fk/Fp = potash feldspars/plagioclase, and Log Lv/Ls = volcanic lithics/sedimentary lithics). The linear discriminant and factor analyses showed that the geographical dispersal of detrital modes between the TMVB sands and VE sands are statistically significant by means of the Log Lv/Ls, Log Qt/Ft, Log Qt/Lt and Log Ft/Lt ratios. The A-Nova test and post-hoc analyses showed that the mean differences of the Log Lv/Ls, Log Qt/Ft and Log Qt/Lt ratios are highly significant for the VE river sands and the TMVB dune sands. The composition of the TMVB dune sands is controlled by a longshore drift of quartz supply. The VE river sands are depleted in Ls due to the mechanical and/or chemical weathering of Ls and the influence of TMVB on the composition of the VE river sands

Scanning electron microscopy analysis of quartz grains in desert and coastal dune sands (Altar Desert, NW Mexico)

A scanning electron microscopy analysis was performed for 570 quartz grains from desert and coastal dune sands in NW Mexico. Our main goal is to present a new application in the use of ternary diagrams with logistic normal confidence region boundaries of normalized data based on quartz surface textures (i.e., constrained data) of desert and coastal dune sands. This was done to demonstrate that quartz surface textures from desert and coastal dune sands are not significantly different even though there are apparent dominant processes (mechanical, chemical) that produce different surface textures in quartz grains from both dune types. This may be associated with the fact that quartz grains deposited in the dune sands do not reflect a second cycle of transport associated with an aeolian environment because of their low textural maturity, provenance, closeness to the source rock, and little attrition process. This study indicates that quartz grains from the desert dunes display mechanical textures probably associated with the Colorado River Delta and granitic sources that do not reflect accurately the aeolian mechanisms controlling the transport of these grains. Also, some quartz grains display chemical surface textures probably linked to the hydrothermal activity near the Colorado River Delta. In general, quartz grains display conspicuous surface textures. Quartz from coastal dune sands displays chemical surface textures that indicate quartz precipitation from silica-saturated water and, to a lesser extent, it displays mechanical features. These similarities might be associated with the mixing of processes (mechanical, chemical) in quartz from the desert and coastal dune sands

Textural characterization of beach sands from the Gulf of California, Mexico: implications for coastal processes and relief

Grain size determinations were carried out for 54 beach-sand samples from the western (n = 25) and eastern (n = 29) coasts of the Gulf of California, Mexico, in order to establish the relationship among coastal processes, relief, and grain size parameters for both areas based on grain size distributions. This was done because both coastal areas are controlled by marine processes and the geomorphology of the coast, since fluvial discharges are negligible in the distribution of sands. In general, sands from the western coast are mainly coarse, moderately sorted, near-symmetrical with leptokurtic and very leptokurtic distributions. Correlations between grain-size parameters for the western coast are controlled by less selectivity in the coastal processes to concentrate fractions in a specific range of sizes, a narrow coastal plain, and compositional differences in the sands due to the heterogeneous lithology and the presence of carbonate shells. Eastern coast beach sands are medium, moderately well sorted, near-symmetrical with mesokurtic to leptokurtic distributions. Correlations between grain size parameters for the eastern coast are controlled by longshore current drifts in a northwestern direction and a wider coastal plain compared to the western coast

Sediment geochemistry of coastal environments, southern Kerala, India: implication for provenance

Late Quaternary sediments representing the floodplain, estuary and offshore environments of southern Kerala were investigated to infer provenance. The grain size reveals the dominance of sand to silty clay, clay to clayey silt and clayey silt in the floodplain, estuary and offshore sediments, respectively. The chemical index of alteration (CIA) values and A-CN-K plot attributes to high, moderate and low weathering in floodplain, estuary and offshore regions, respectively. The SiO 2 /Al 2 O 3 values lesser than Post-Archean Australian Shale indicate low to moderate maturity for the estuarine and offshore sediments. The geochemical immaturity indicates its derivation from low to moderately weathered source rocks. The major and trace elemental ratios and discriminant function diagrams attribute that the sediments were derived from intermediate to felsic source rocks. The enrichment of Cr and Ni concentration in the sediments compared to the upper continental crust, related to the contribution of orthopyroxenes, weathered from charnockite and garnets from the granulite terrain, respectively