SEDIMENTATION FROM THE 26 DECEMBER 2004 SOUTH ASIA TSUNAMI IN NORTHERN SUMATRA, INDONESIA

The 2004 South Asia tsunami flooded coastal northern Sumatra to a depth of over 20 m, deposited a discontinuous sheet of sand up to 80 cm thick, and left mud up to 5 km inland. Where observed, the sand sheet contains complex internal stratigraphy, and may record the passage of as many as 5 individual waves.

Near shore, the deposit is either thin or absent. It thickens landward, selectively filling in low spots; some coastal stream channels were entirely filled with sand. Deposition generally attended inundation—where we studied, the tsunami deposited sand to within about 40 m of the inundation limit.

In northernmost Sumatra, the tsunami appears to have removed most of the available beach sand, and deposited it on land. The tsunami deposit contains primarily material indistinguishable from material on the beach one month after the event, but also contains grain sizes and compositions unavailable on the current beach. In at least one instance, these grains become increasingly dominant both landward and upward in the deposit, suggesting that some landward source of sediment was exposed and exploited by the passage of the waves. The deposit also contains the unabraded shells of subtidal marine organisms, suggesting that at least part of the deposit came from offshore.

The sand is deposited on the eroded surface of a dark brown sandy soil. We saw no evidence for loading structures in the sand, perhaps because the soil was competent enough or coarse-grained enough to prevent the formation of these structures. Internally, the lowermost sand is generally coarse grained and massive, but is overlain by a normally graded layer that grades smoothly into a reverse graded, plane laminated layer. Two thinner, normally graded, plane laminated units overly this package. The massive unit is only present near shore, and in general the units become fewer and harder to distinguish landward.

Grain sizes within the deposit generally fine upward and landward, although individual units within the deposit record either massive deposition, or reverse grading. Sorting generally becomes better landward, although the most landward sites generally become poorly sorted from the inclusion of soil clasts. These sites often show interlayering of sandy units and soil clast units.