LATE PLEISTOCENE STRATIGRAPHY AND SEDIMENTOLOGY OF PROTECTION ISLAND, WASHINGTON

Protection Island, Washington is best known as an important bird sanctuary and National Wildlife Refuge. It is also of interest to geologists because of a spectacular outcrop on the northern side of the island that exposes Late Pleistocene sediments. The excellent exposures allow the tracing of units laterally illustrating the complexity and diversity of Pleistocene deposits, and the challenge of correlating these units even over relatively short distances.

The exposed outcrop is nearly 2.5 km long and about 60 m high at its highest point. Fourteen stratigraphic sections were measured and a photo mosaic of the outcrop aided correlation. Samples for radiocarbon dating and palynological analysis were used for age and paleoenvironmental analyses.

The northern exposure was subdivided into the following lithologic units. Till: The oldest exposed sediment is a poorly sorted diamict with very dense matrix supported gravel (Till A). A younger till unit (Till B) is similar to Till A except that it contains a higher abundance of gravel and cobbles. Pebble Mud: A massive mud to silt matrix containing occasional pebbles. This unit forms massive vertical faces ranging from 15 to 20 m high and is an important marker unit. Sand: Cross bedded, fine to course grained sand with gravel lenses. Gravel: Gravels consist of pebble to cobble sized clasts with graded, interbedded sand. Silt/Sand/Peat: 15 to 30 m of bedded mud, silt, sand, and interbedded peat. This unit forms the vertical cliffs on the western end of the outcrop. Several peats, 0.2 to 0.3 m thick, contain matted plant debris with flattened stems and branches. This unit correlates lithologically with the Whidbey Formation exposed at nearby Point Wilson. Glaciomarine: Interbedded silt, sand, and gravel beds containing bivalve shells. Bivalve shells from this unit were carbon¹⁴dated to about 12,500 years. The glaciomarine unit correlates to the Everson Glaciomarine Drift commonly exposed in cliffs along the shorelines of the Salish Sea.

The 14 fine grained palynology samples yielded widely varying results, from a well-preserved, diverse assemblage of pollen, spores, algae, and fungi, to samples nearly devoid of in-situ palynomorphs. No in-situ marine dinoflagellates were recovered. The palynomorph assemblages yielded July paleotemperatures ranging from 13.68 to 14.32^oC.