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Date Permissions Signed
Date of Award
Master of Science (MS)
Easterbrook, Don J., 1935-
Kelsey, H. M.
Suczek, Christopher A., 1942-2014
Glacial sediments exposed in the northern Okanogan trough consist of Fraser advance stratified drift, lodgement till, and recessional stratified drift. No older Quaternary sediments were recognized in the mapped area.
The advance stratified drift unit is composed of upward-coarsening braided-stream outwash with locally intercalated lacustrine and alluvial fan sediments. An upsection change in facies within the unit suggests a gradation from distal to proximal deposition of proglacial outwash with time in Spectacle Lake Coulee. The gradation probably records the approach of Cordilleran ice into the area during the Fraser advance.
Upland regions of the study area are mantled by a lodgement till complex that typically exhibits a streamlined and sometimes drumlinoid surface morphology. Interbeds of glaciolacustrine and glaciofluvial sediment containing dropstones and inclusions of diamicton are common. Thick sections of lodgement till overlying the interbeds indicate that they were deposited in subglacial lakes and streams beneath active ice of the Okanogan lobe. Crudely stratified till subunits of differing color and texture are present at some outcrops in the central portion of the study area. These subunits may have been deposited through a process of superimposed till lodgement beneath multiple, competing ice streams of a composite Okanogan lobe.
The Okanogan Valley and coulees tributary to it contain voluminous fills of recessional stratified drift. Kame terraces at higher elevations are predominantly erosional in origin and probably signify relatively rapid lowering of the ice-sheet surface during their formation. Kame terraces comprised of ice-contact glaciolacustrine and glaciofluvial sediment are more common at lower elevations and record sedimentation in a series of local ice-marginal lakes and streams.
The most prominent set of kame terraces occurs along the Okanogan Valley and is collectively known as the "Great Terrace". In the study area, the "Great Terrace" is composed of highly deformed ice-contact deltaic and glaciolacustrine sediment overlain by a cap of outwash gravel. Sediments of the "Great Terrace" were deposited along and over stagnating ice in the Okanogan Valley, predominantly as deltas built by streams issuing from tributary coulees. The presence of numerous kettles on the upper surface of the "Great Terrace" and on younger terraces cut into it indicates that buried ice was present within the sediments of the "Great Terrace" for some time after its construction.
A kame-moraine in the Sinlahekin Valley near Loomis marks a stillstand of a late Fraser valley glacier. The kame-moraine is probably a delta that was built into a proglacial lake occupying at least a portion of the southern Sinlahekin Valley. The moraine may be approximately contemporaneous with the "Great Terrace" at Tonasket and might therefore be evidence of an approximate Sumas Stade equivalent in the Okanogan trough. Further study is needed to support this tenuous correlation.
Tephra samples collected from Holocene deposits in the northern Okanogan trough were examined petrographically and identified as Mazama ash (6,600- 7,000 yr BP) and Mount St. Helens layer Wn (about 450 yrs old). As evidenced by the absence of Glacier Peak tephra in the study area and its absence from dated pollen cores in the region, the northern Okanogan trough was probably beyond the fallout area of the 11,200 yr old tephra.
Western Washington University
Subject – LCSH
Sediments (Geology)--Washington (State)--Okanogan County; Glacial epoch--Washington (State)--Okanogan County; Geology----Washington (State)--Okanogan County
Okanogan County (Wash.)
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.
Pine, Keith A. (Keith Allen), "Glacial Geology of the Tonasket – Spectacle Lake Area, Okanogan County, Washington" (1985). WWU Graduate School Collection. 836.