Selective Sorting, Storage and Progressive Dilution of Sediment in Two Tropical Deltas, Veracruz, Mexico

Abstract

Compositional and textural attributes of the surficial sediment in deltaic settings typically differ from the sediment load carried by their source rivers. This phenomenon is evaluated here by semiquantitative study of the variability of petrologic characteristics in different delta subenvironments of the Nautla and Tecolutla deltas along Mexico's Veracruz coast. Sediment composition and texture in the different environments of both deltas are related to processes that involve selective sorting during seasonal flooding and temporary storage after deposition. Displacement of particles of different size, density and shape onto the two tropical delta plains is controlled by fluctuating fluvial discharge that responds to a highly variable seasonal rainfall pattern. These discharge variations induce changes in hydraulic equivalence and, consequently, the release of substantially different proportions of volcanic, mica, heavy mineral and grain-size fractions in the Veracruz delta settings. These phenomena, together with progressive dilution of sediment upon reaching the coast, result in significant modifications to the original petrologic attributes of fluvial loads as they are transported seaward.

Comparable variations of composition and texture in subsurface Holocene sections would likely record former large scale climate, rainfall and fluvial discharge fluctuations. Petrologic analysis of core sections recovered at Prehispanic sites on the two deltas could be used to detect past long-term flooding pulses and droughts, natural events that archaeologists suggest caused deterioration of subsistence resources, stratigraphic hiatuses, cultural discontinuities and human migrations in this region.