PRESENCE OF MID-PLEISTOCENE DEPOSITS (MIS 4 THROUGH 8) IN THE TACOMA AREA: DID THE POSSESSION GLACIER MAKE IT TO TACOMA?

For decades, researchers and practitioners have speculated that the stratigraphy described on Whidbey Island in the northern Puget Lowland is not represented in the southern Puget Lowland. One of the strongest held beliefs is that the Possession glacier, MIS 4, did not advance farther south than the latitude of Seattle. Recent geologic mapping, optical dating, and paleomagnetic studies have determined that the Possession Drift, Whidbey Formation, Double Bluff Drift, and the informally named “Defiance drift” may be present along the Puget Sound coast in the vicinity of Tacoma. These stratigraphic units have never before been documented in the Tacoma area.

Steep bluffs along a 6-km strip expose multiple glacial and interglacial packages and unconformities, capped by Vashon Drift of MIS 2 and Olympia-aged deposits, MIS 3. At Point Evans, Possession-aged till and outwash are indicated by overlying Vashon deposits and underlying Whidbey-aged deposits. Here the Whidbey Formation consists of fluvial deposits, from west flowing streams, that have an average IRSL age of 125 ka, correlating with MIS 5. MIS 5 deposits are also present beneath the east side of the Tacoma Narrows Bridge where a sequence of reversed-to-transitional paleomagnetic directions was measured in a vertical profile, bottom to top. IRSL ages and the paleomagnetic directions are consistent with the Blake chron at 125 ka (i.e. early Stage 5e). At Point Evans, the Whidbey Formation overlies a glacial drift with a deformed till, and an outwash with an average IRSL age of 150 ka, correlative with the Double Bluff Drift of MIS 6. At Point Defiance, the oldest deposit, a glaciolacustrine unit averages 225 ka, correlative with MIS 8. Deposits of this age have not previously been recognized in the Lowland and herein are informally named the Defiance drift. Even older deposits are exposed at extreme low tide at Point Evans and rise above sea level to the north in the Tacoma Fault zone. Some of these older deposits are reversely magnetized and also older than 400 ka (IRSL).

Deposits of MIS 4 through 8 have been dated elsewhere in the Tacoma area, and some can be traced for over 10 kilometers on the Gig Harbor coastline. Further chronologic studies are underway to confirm these initial ages and correlations, which are critical for evaluations of the region’s tectonic history.