Sedimentation in the northern Gulf of California after cessation of the Colorado River discharge

Abstract

Abstract The almost total elimination of sediment supply from the Colorado River has produced a condition of sediment starvation in the entire northern Gulf of California (NGC). Textural and mineralogical analyses of 114 surface sediment samples collected in the area show that this situation has promoted alternate sediment sources to become more important in supplying and controlling the compositional characteristics of the sediments in the region. By applying multivariate data analyses using Q/F ratios as well as heavy mineral and clay mineral content of samples, four sedimentary provinces are identified: Colorado River Delta Province (CRDP), Concepcion River Province (CcRP), Transitional Province (TP) and Baja-Sonora Province (BSP). The CRDP dominates the regional distribution of sediments, being mainly confined to the western side of the NGC. The sediment source for this province is the deltaic structure of the Colorado River. Sediment recycling through resuspension and reworking of the Colorado delta and the shallow shelf of the Upper Gulf of California (UGC) is an important agent for supplying sediments to the NGC. The CcRP reflects the mineralogical association of Concepcion River, while the TP contains a mineral association that is similar to CRDP and CcRP, but with intermediate values to these provinces. This situation suggests that TP is a mixing zone between these two neighboring provinces. The higher abundance of zircon and garnet in TP also suggests a clear contribution of sediment sources from the Sonora desert in NW Mexico. Clay mineral composition of these sediments indicates a greater abundance of smectite and chlorite relative to previous studies, being mainly derived from eolian dust carried from the desert region of NW-Mexico and SW-USA. Although eolian supply of fine-grained sediments is important in the NGC, Colorado River delta sediments are still the largest single source of sediments to the region. Eolian supply of fine-grained sediments is, however, more dominant in the southern half, and along the eastern margin of the NGC. The dispersion of sediments in the NGC is consistently cross-basinal with a series of transport cells that follow a cyclonic circulation. Four depocenters were identified, located all on the western side of the NGC, and mainly associated to the deep basins. The major effect that damming of the Colorado River has had on the sedimentology of the area is that sediment dispersion is now cross-basinal instead of long-basinal. This situation indicates that sediment dispersion at present is mainly controlled by oceanic forcing instead of fluvial processes, as it was in the past.