PETROGRAPHIC SIGNATURE FOR THE WHIDBEY FORMATION

The interglacial, late Pleistocene Whidbey Formation of Whidbey Island, Washington, contains deltaic-floodplain sediments. In outcrop, the Whidbey Formation is accompanied by non-coeval glacial and nonglacial strata. Distinguishing the nonglacial sediments of the Whidbey Formation from those of the nonglacial Olympia beds, and from glacial outwash, has been problematic where stratigraphic sections are incomplete. The need to distinguish among these units led to a petrographic analysis on sands of the Whidbey Formation and accompanying strata. Sand-bearing stratigraphic units that were point counted include the Whidbey Formation, the Olympia beds, the Possession Drift, the Double Bluff Drift, and the Vashon Drift.

Most nonglacial sands had higher amounts of hypersthene, biotite, hornblende and augite, whereas glacial sands had higher amounts of chert and polycrystalline quartz. Felsic volcanic and greenstone lithics were the most telling sand components, the latter linked to glacial sands and the former to nonglacial sands.

Sands of the Whidbey Formation and the Olympia beds typically contain over 10% felsic lithics; however, for a few locations where felsic lithics are absent, the Whidbey Formation instead contains abundant sedimentary lithics. Nonglacial sands from northern Whidbey and Camano Islands contain significantly higher concentrations of felsic lithics than do nonglacial sands from southern Whidbey and Camano Islands, the Kitsap Peninsula, and the mainland.

The concentration of serpentine lithics exceeded 2% in Olympia beds that were sampled near the Snohomish delta and the Olympic Peninsula, and not in sands of the Whidbey Formation. Beyond this, petrographically differentiating the Whidbey Formation from the Olympia beds proved inconclusive both for a lack of coarse sand in the (local) Olympia beds and the uncertain assignment of some sands to the two units.