Surface sediment samples from 87 locations within the Gulf of California were analyzed in order to establish the supply and dispersal processes of the fine-grained sediments in this marine rift basin. Q-mode factor analysis of sediment bulk chemistry indicates that the surface sediments can be described in terms of four end-member sources, including terrigenous clastics, biogenic opal, biogenic carbonate, and manganese-rich components. The terrigenous factor accounts for as much as 80 to 90% of the total sedimentation on the eastern margin of the central and southern Gulf, but represents less than 40% of the total sediment on the western margin of the central and southern Gulf. Mass accumulation rates of the terrigenous component in nine laminated cores taken in the Gulf re estimated to range from 168 to 265 g/m²/yr (Guaymas Basin), 168 to 651 g/m²/yr (southeastern slopes), and 63 to 88 g/m²/yr (western slopes). Decreasing mass accumulation rates of the terrigenous component from the eastern to the western slopes of the southern and central Gulf account for the low content of biogenic components to the east (dilution by terrigenous silt and clay). The east-west trend in accumulation rates also indicates a dominant eastern (mainland) source of terrestrial sediment supply to the Gulf. A minor manganese component, of hydrothermal origin, is associated with basin-floor sediments of the Guaymas Basin in the central Gulf.

X-ray diffraction studies of two size fractions--clay (2-4 µm) and silt (16-32 µm)--identify distinct mineralogical provinces within the Gulf based on quartz:feldspar ratios and on anorthite content of the plagioclase feldspars. The clay fractions throughout the northern Gulf (north of Tiburon Island) are dominated by Colorado River origins. In contrast, Colorado River silt is restricted to only the northernmost Gulf, north of Angel de la Guarda Island, an area

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of strong tidal forcing. In the central and southern Gulf, the silt and clay fractions have similar mineralogical origins, which largely reflect the geologic provenances of the Mexican mainland. Only minor inputs of silt and clay from Baja California are found on the western shelves and upper western slopes of the southern Gulf region. Whereas the southern Gulf shows dominant northeast-southwest (across-Gulf) mixing gradients, the central Gulf generally shows southeast-northwest (along-Gulf) mixing gradients. River sediment dispersal by Gulf surface and geostrophic currents is thought to predominate in the southern Gulf, where river runoff is high along the eastern margin. By comparison, eolian transport of terrigenous sediments from the arid northern mainland might play an important ole in supplying sediment to the central Gulf basins, as suggested by Baumgartner et al. (1991b).