Abstract

The tidal mud flats of the Colorado River delta are located at the head of the Gulf of California. This region is characterized by exceptionally high nutrient concentrations which are able to sustain a high primary productivity. This study documents the taxonomic composition of foraminiferal assemblages of this area and their preservation potential from the intertidal zone of the southwesternmost part of the delta plain, in surface and subsurface (down to 20 cm deep) sediments sampled north of San Felipe. Thirty-three species belonging to seventeen genera have been recognized: calcareous perforate taxa make up most of the assemblage, whereas imperforate taxa (Miliolina) are less frequent and agglutinated taxa are virtually absent. Only a small number of specimens, belonging to offshore species, were likely conveyed to the delta during storms. In general, the foraminiferal surface density seems largely controlled by burrowers and deposit feeder activity, alkalinity, degree of dissolution, and the type of sediment. Counts of total foraminifera show that subsurface samples have higher densities-by one order of magnitude-than those registered in surface samples. In the study area, which is characterized by low to absent sedimentation, rapid sediment reworking may alter surface/subsurface faunal ratios and buried specimens may repeatedly be reworked to the surface in the taphonomically active zone. This is a possible explanation for the distribution of dissolution, which is higher in subsurface than surface assemblages and which exhibits no relationship to the age of specimens.

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