Abstract

Western Mexico's second largest river apparently built its delta across the narrow continental shelf so that the river pours sediment directly onto the continental slope. As in other cases of shelf encroachment, the slope is characterized by numerous valleys, of which one with canyon-sized dimensions can be traced seaward to the Mid-America Trench. Evidence for turbidity currents was established by current-meter records and observations from scuba and vehicle diving. A dense grid of accurately located sounding and seismic lines shows the nature of deposition seaward of this delta, but in a zone off both sides of the main river mouth, seismic profiles of the slope suggest the presence of basement rock. However, it seems somewhat more likely that, despite some rock outcrops where erosion has exceeded deposition in Necesidad Canyon, the slopes are underlain mostly by an appreciable thickness of sediments that have been homogenized by extensive slumping which has destroyed any internal reflectors. Farther seaward from this zone, where slopes are gentler, there is a broad area in which good reflectors show clear evidence of natural levees, cross-bedding, valley fills, and other signs of slope progradation.

Despite all the evidence of progradation of the slope beyond the delta, we have clear indications that much sediment is being bypassed along the two main canyons of the area. Turbidity currents partly initiated by submarine landslides are eroding locally the steep canyon heads and transporting a large amount of sediment toward the Mid-America Trench. These flows are powerful enough to move coarse sediment and coarse plant debris, and to leave large flow ripples along their broad, flat floors.

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