LILY POINT RIP-UP DEPOSIT AT POINT ROBERTS WASHINGTON: POSSIBLE ORIGIN FROM A BIG WAVE EVENT DURING THE LAST INTERGLACIAL

The rip-up deposit at Lily Point on Point Roberts, Washington is composed of an unlikely mixture of unsorted rock, sediment and organic debris that contains wood and peat fragments. The rounded cobbles and coarse sand comprising most of the matrix are similar in appearance to the rock debris on the beaches at Point Roberts. The Rip-up clasts are composed of fine-grained, light-colored silty blocks with occasional anthracite coal clasts and rimmed with embedded granitic sand and cobbles. These sediment blocks are very weak and are easily liquefied in a turbulent water flow or subsequent shaking event – representative features of rip up deposits. Radiocarbon dating was done on large pieces of wood and peat found in this unit, and support that the deposit is likely older than 50,000 years.

The features and dating suggest the rip-up unit was deposited during a higher standard of sea level such as the last interglacial and compares to a similar unit, with similar stratigraphic relationships on Whidbey Island. Although epic flooding and huge tsunamis have been reported for during the last interglacial in the Caribbean, Hawaii and Australia, a flood source is not likely based on the lack of fine-grained sediments. Current information points to a large wave of a type that has not re-occurred in this area in the last 100,000 years.

The character of this rip-up deposit and the channel it eroded, as well as distinctive cross bedding will be discussed. The term ‘rip-up’ is used when the source of the flood that eroded and deposited the material violently and quickly is unknown. Typically these deposits are attributed to a water flood or glacial outburst, a large storm surge or rogue wave, or a tsunami (from an earthquake, meteor impact or landslide source).