FACIES OF LATE PLEISTOCENE GLACIAL MARINE SEDIMENTS ON WHIDBEY ISLAND, WASHINGTON
AbstractDetailed mapping of over 10 kms of cliff exposure on Whidbey Island, Washington has resulted in the recognition of six major lithofacies within "Everson" age glacial-marine deposits (late-Pleistocene). These lithofacies were deposited in an isostatically depressed basin, comprise up to 25 m of section, and occur in a predictable vertical succession. Proximity to both the ice margin and sources of meltwater input played a key role in the facies distribution. Stratified and convoluted beds of diamicton exhibit sedimentary characteristics indicative of mass flow processes. An ice marginal, submarine environment is assigned to the stratified diamictons since they are interbedded with marine sediments. In most exposures stratified diamictons are directly overlain by pebbly silts, muds, and massive diamictons. Locally, small (300 m wide) delta-like sequences consist of well stratified silty sand which is interbedded with pebbly mud. Turbidity current channel infills consist of normally graded, laminated sands which fine upwards into laminated sand-silt and silt-clay. Both of these facies are overlain by pebbly mud and massive diamicton. Pebbly silt and mud grade up into massive, fossilifierous diamicton. Post depositional mass movement has locally interrupted the normal stratigraphic sequence. With regard to the ice margin, the massive "till-like" diamictons are interpreted as the most distal of the glacial-marine facies. All exposures are capped by channel lags, beach and eolian sediments which represent emergence of the basin above sea level.
Erosional surfaces at the base of the sequence (glacial and/or meltwater) and above (regressive shoreline) serve to separate the isostatic glacial marine sequence from correlative nearshore, offshore, and terrestrial glacial sequences. Extensive development of meltwater and sediment flow facies suggest that the sequence was deposited in a moderate glacial climatic setting, not unlike the present Gulf of Alaska.
Correlation of the central tendency of pebble fabric and sand content for glacial marine diamictons distinguish them from tills and sediment flow diamictons. Sediment flows are distinguished from tills and glacial marine diamictons by their high and variable clast abundances, textural variability, and variable bedding character.
Deglaciation of central and northern Whidbey Island involved retreat of a highly irregular ice margin which was grounde