Monday, June 24, 2013

Notes on Salish Sea Landscape:18,000 to 16,900 Years Ago

More bits from the talk. Again borrowing for the images produced by Haugerud and Greenburg.

The Salish Sea 18,000 years ago
The white is the Puget and Juan de Fuca ice lobes. As the ice advanced into the Puget lowlands it blocked the ouflow of water forming a large lake over the Puget lowland - the light blue on the map. Fine silts and clays were deposited on the lake bed. In the Seattle area these silts are called the Lawton Clay for excellent exposures at Fort Lawton/Discovery Park.
 
Glacial advance silts deposited in the proglacial lake prior to the arrival if the ice
 
Near the advancing ice margin the meltwater rivers and streams from the ice deposited thick layers of sand and gravel; the brown band between the ice and the lake on the above map indicates areas of active glacial outwash. This outwash is most often referred to as glacial advance outwash. Locally it may be called Esperance Sand in the Seattle area. 
 
Sand dominated outwash on shore line bluff capped with glacial drift

Glacial advance outwash with thin glacial drift cap on top

Glacial advance outwash with clastic dikes and deformation features

By 16,900 years ago the Puget lobe extended to south of Olympia (Haugerud and Greenburg)

During this time the ice left deposits behind in places and in other places the land was eroded. The direct ice deposits are glacial drift. Or a "a dog's breakfast" of very unsorted sediment ranging from clay to huge boulders.

Glacial drift. This particular drift image is from an older age age event. Glacial ice advanced into the Puget Sound area on multiple occasions.

Glacial erratic - boulder left behind by the ice in a glacial drift deposit. In this case the glacial drift was deposited at the top of the bluff. This erratic must have made a big thud when it rolled off of the eroding shoreline bluff.


LiDAR image of south Whidbey Island
The Puget ice lobe left streaks across the landscape
(Google Earth with HTML overlay)

3 comments:

Geoffrey Middaugh said...

I'm trying to correlate (as a nongeologistgeek)your lecture with Easterbrook's challenge in Walk Through Geologic Time.... He indicates that 11,700 years ago BP there was rapid rise and fall of sea level due to world wide sea level change, subsidence or upleft or all of the three. It seems like it was more local daming, outflow and iso-static rebound. What am I missing, or do we need to have coffee....

Dan McShane said...

Sea level, local isostasy, and tectoncis present challenges for interpretations of any units in the Salish Sea lowlands. On the 19,200 years ago image I failed to mention that the ice age had already started and that sea level would already have been dropping. Hence, the ice arrived at a time when sea level was lower. Throughout the puget ice lobe event sea level worldwide was lower, but one also has to consider that isostasy had lowered the local elevations. More on that as I eek out the talk a bit at a time as time permits.

Scott Schuldt said...

Aha! I always wondered about the elastomeric clay at the waters edge below Disco Park.