Storm sediment deposit from Ko Phra Thong, Thailand

Siliciclastic storm deposits have been described from coastlines around the world over the last six decades. Most of these storm deposits are formed from storms directly or obliquely making landfall, or tangentially bypassing nearby coastal systems. In these instances, coastal water piles up on the coast as the storm approaches the coastline before breaching or overtopping the beach barrier. Seldom have storm deposits been examined from coastlines on the lee of the storm. We describe the sedimentary and stratigraphical variation of a small storm fan from three short sediment cores from Ko Phra Thong, western Thailand. The fan was formed after a tropical depression crossed the Malay-Thai peninsula in May 2007, resulting in water piling up along the coast and breaching of the small berm on the western coastline of Ko Phra Thong. Sufficient accommodation space resulted in fine to medium quartz and heavy-mineral rich sands to be deposited upon the back beach sediments. The basal contact consists of a soil horizon above beach sands with an abundance of Casuarina equisetifolia needles. Horizontal laminations of heavy minerals characterise the lowest stratigraphic unit, conformably overlain by a massive unit which is in turn overlain by a unit with vertical and sub-horizontal root structures. At the landward extremity of the fan, faint landward dipping cross beds are observed, indicative of terminal foresets. As Ko Phra Thong preserves a well-established history of past tsunami sand washover deposits, the presence of a storm overwash fan along this coast complicates the inferred overwash event history. This complication is particularly pertinent when the palaeo-shoreline was closer to the swales where smaller, lower energy events, such as storms, can overtop the palaeo-berm and deposit sandy units. This most likely occurred between 2500 and 2000 years ago when thin, structureless sandy deposits were emplaced near the palaeo-shoreline.