Sedimentary processes on a Gilbert-type delta in Lake Llanquihue, southern Chile

Abstract

A small Gilbert-type delta on the southern shore of Lake Llanquihue, southern Chile, was studied over a period of two months. Bottom samples were taken by scuba diving along a 10 x 10 m grid to determine the distribution of sedimentary facies. The wind direction, wave conditions, underwater current directions and the orientation of sedimentary structures were recorded on a daily basis. Observations made during fair-weather conditions indicate that neither waves nor currents have any significant effect on the bottom sediments, and that the only transport is produced by bivalves causing small avalanches on the steep delta slope. This probably prevents the oversteepening and large-scale slope collapse typical of many Gilbert deltas. Observations recorded from shore during one major storm, show an excellent agreement with wave theory and empirical predictions, indicating that storm waves can affect the whole delta front and slope. These waves break about 35 m from shore where there is a clear transition from clast-supported gravel on the inner delta front to matrix-supported gravel and gravelly sand on the outer front. The storm waves are able to transport cobbles up to 40 cm in diameter. During these events, strong lakeward-directed bottom currents enhanced by the effluent plume of the river, transport some of these clasts to the edge of the delta front, where they avalanche down to the foot of the delta. During the waning stages of the storm and shortly thereafter, dense, sediment-laden bottom currents discharged from the river mouth carry plant material down the delta slope and over the pro-delta, burying the cobbles just deposited. A single cycle of delta progradation should produce two coarsening-upward cycles, which might be confused in the rock record with two distinct phases of delta progradation.